Chem Soc Rev View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
TUTORIAL REVIEW
Cite this: Chem. Soc. Rev., 2014, 43, 1825
View Journal | View Issue
Transmetalation of self-assembled, supramolecular complexes Matthew E. Carnes, Mary S. Collins and Darren W. Johnson* Substituting one metal for another in inorganic and organometallic systems is a proven strategy for synthesizing complex molecules, and in some cases, provides the only route to a particular system. The multivalent nature of the coordination in metal–ligand assemblies lends itself more readily to some types of transmetalation. For instance, a binding site can open up for exchange without greatly effecting the many other interactions holding the structure together. In addition to exchanging the metal and altering the local binding environment, transmetalation in supramolecular systems can also lead to substantial changes in the nature of the secondary and tertiary structure of a larger assembly. In this tutorial review we will cover discrete supramolecular assemblies in which metals are exchanged. First we will address fully formed structures where direct substitution replaces one type of metal for another without changing the overall supramolecular assembly. We will then address systems where the disruptive exchange of one metal for another leads to a larger change in the supramolecular assembly. When possible we
Received 3rd October 2013
have tried to highlight systems that use supramolecular self-assembly in tandem with transmetalation to
DOI: 10.1039/c3cs60349k
synthesize new structures not accessible through a more direct approach. At the end of this review, we highlight the use of transmetalation in self-assembled aqueous inorganic clusters and discuss the
www.rsc.org/csr
consequences for material science applications.
Key learning points (1) Transmetalation is a useful synthetic tool in supramolecular chemistry and self-assembly. (2) Supramolecular metal–ligand assemblies can allow for direct metal exchange (transmetalation) while preserving the original structure. (3) Sometimes transmetalation can result in an entirely new supramolecular assembly, often dictated by a change in coordination preferences of the incoming metal ion. (4) Adding substoichiometric amounts of an incoming metal ion during transmetalation can lead cleanly to new heterometallic supramolecular assemblies. (5) Performing transmetalation on a preassembled, multicomponent assembly can enable the synthesis of metal–ligand assemblies which do not spontaneously self-assemble from the new metal ion and ligands alone.
1 Introduction Self-assembled, metal-containing supramolecular structures represent a diverse and growing class of functional synthetic materials. Their facile construction from simple building blocks and persistent, yet flexible architectures make them finely suited for applications in the fields of catalysis, sensing, and materials science.1–7 In contrast with purely inorganic mesoporous structures, the organic spacers and dynamic nature of metal ligand interactions make the pores and void spaces
University of Oregon Department of Chemistry & Biochemistry and Materials Science Institute, University of Oregon, Eugene, OR 97403-1253, USA. E-mail: [email protected]; Web: http://sustainablematerialschemistry.org/, http://pages.uoregon.edu/dwjlab/home.html; Tel: +1 541-346-1695
This journal is © The Royal Society of Chemistry 2014
within these materials adaptable to a variety of guests both in the solid state and solution. With the judicious choice of ligands and metals, chemists have developed an elegant array of open geometric solids containing individual coordination complexes at the vertices. The bite angle, orientation of binding groups, and denticity of the ligand function in tandem with the preferred coordination geometry of the metal ion to determine the final structure and geometry of the assembly. This has been used extensively to form squares, cubes, triangles, tetrahedra, and related prisms as well as a number of distorted enclosures and 3D arrays featuring these metal and ligand building blocks.1,5,7 Transmetalation is traditionally defined as an exchange of metals between an organometallic compound and either a metal or a different organometallic compound.8 The word derives from trans-, meaning ‘‘change, transfer’’, and -ation, meaning ‘‘action
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1825
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
or process’’, and thus could perhaps more broadly include inorganic species as well.9a In recent years it has become clear that within the context of multimetallic supramolecular structures, metal exchange at the linking metal coordination sites is an emerging synthetic technique to synthesize complexes that are not readily available by direct metal–ligand self-assembly. Therefore, for the purpose of this review we will use the term ‘‘supramolecular transmetalation’’ (Fig. 1) to refer to systems in which metals are exchanged for one another within the context
Matthew E. Carnes received his BS in Chemical Engineering at the University of California, San Diego in 2003, where he did undergraduate organic chemistry research with Prof. Jay Siegel. He then moved to Columbia University where he earned his PhD in Chemistry in 2008 working with Colin Nuckolls focusing on using ring opening polymerization to form new high carbon materials. Following his Matthew E. Carnes graduate work, he did two years of postdoctoral research with Prof. Dr E. W. Meijer at Technische Universiteit Eindhoven in the Netherlands before returning to the US to do another postdoc with Darren W. Johnson at the University of Oregon as a part of the Center for Sustainable Materials Chemistry starting in 2010.
Mary S. Collins is a doctoral researcher in the Department of Chemistry and Biochemistry at the University of Oregon. She obtained a BSc in Biochemistry from The University of Texas at Austin in 2010 where she completed undergraduate research investigating the electronic properties of bimetallic N-heterocyclic diaminocarbene–iridium complexes under the supervision of Dr Christopher Bielawski. In 2011 she joined Mary S. Collins the Darren W. Johnson laboratory as a PhD student studying the coordination chemistry and reactivity of pnictogen-containing self-assemblies. Currently, her research focuses on supramolecular cyclophane syntheses using a pnictogen directing agent.
1826 | Chem. Soc. Rev., 2014, 43, 1825--1834
Chem Soc Rev
of a supramolecular assembly (without restricting specifically to just those systems containing metal–carbon bonds).9b In this review we will concentrate primarily on discrete, selfassembled constructs in which a metal plays a principal structural role—either as a vertex, edge, or in the case of inorganic clusters comprises the majority of the structure—with a few representative examples included of lower dimension coordination polymers. Although many of the concepts discussed in this review also apply to metal organic frameworks, the topic of transmetalation in MOFs has been recently reviewed and is beyond the scope of this review.10 Many metal containing supramolecular assemblies are also spacious enough to house metal ions as guests. Such molecular cages exhibit rich host–guest chemistry, which has been reviewed extensively and not included specifically in this review.1–7 An exception will be made for the exchange of metal guests which lead to a structural transformation in their host. In addition, we will not specifically review the exchange of alkali metal ion guests in preorganized or selfassembled structures because they also have been extensively reviewed.11 Metal-directed self-assembly A key factor in arriving at a discrete self-assembled structure instead of a complicated blend of oligomers or an infinite network of interlocking complexes is that the metal–ligand interactions are under rapid dynamic exchange.1–7,12 This is usually accomplished by rapid ligand exchange with the solvent or other free ligands leading to growth and digestion of a wide array of oligomers and conformers. In this way ‘‘kinetic mistakes’’ can be edited out as the complex series of equilibria
Darren W. Johnson received his BS in Chemistry at the University of Texas at Austin in 1996, where he performed undergraduate research in Jonathan Sessler’s laboratory. He earned his PhD in Chemistry in 2000 from the University of California at Berkeley working with Kenneth Raymond, and then spent two years at the Scripps Research Institute as a National Institutes of Health post-doctoral fellow with Julius Rebek, Jr. He Darren W. Johnson joined the University of Oregon in 2003, where he is currently an Associate Professor of Chemistry and Associate Director of UO’s Materials Science Institute. He is a Cottrell Scholar and Scialog Fellow of Research Corporation for Science Advancement and a National Science Foundation CAREER awardee. Research in his group uses supramolecular chemistry as a tool to explore a variety of problems in coordination chemistry, molecule/ion recognition and inorganic cluster synthesis, much of which is investigated within the Center for Sustainable Materials Chemistry (http://sustainablematerialschemistry.org/).
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Tutorial Review
Fig. 1 Two primary forms of ‘‘supramolecular transmetalation’’ frequently occur: (i) transmetalation resulting in ‘‘direct exchange’’ of the metal ions in a preformed supramolecular assembly without changing the assembly topology and, (ii) transmetalation that results in a ‘‘disruptive exchange’’ that transforms the assembly from one structure (e.g., the tetrahedron pictured) to another (e.g., a triangle).
settles on the most stable structure.13–16 In the case of 3D assemblies, a guest is often used to favor capsular structures by filling the otherwise empty void space. The multivalent nature of these larger assemblies can further balance the entropic organization cost of such multicomponent self-assembly.17 It is this ability for ligands to substitute (at least partially) on and off reversibly while maintaining the larger ordered structure that enables substituting one metal for another in a number of cages of this type. The two primary classes of supramolecular transmetalation reviewed include (i) ‘‘direct exchange’’ in which the incoming metal does not alter the metal coordination geometry or overall structure of the original self-assembly; and (ii) ‘‘disruptive exchange’’ in which the geometry of the transmetalated complex differs from the original structure as a result of the metal exchange (Fig. 1).
2 Supramolecular transmetalation: direct exchange In some self-assembled supramolecular structures, the metal ions can be substituted with other metals stoichiometrically without fundamentally changing the topology of the original structure. In these cases, incoming metal ions that form stronger bonds with the sets of ligands holding the assembly together will typically be favored. Metal exchange, especially within coordination complexes, is an equilibrium process so the strength of the newly formed bonds will strongly influence the degree to which the equilibrium between the two metals is pushed in the direction of incorporating the new metal ion. This will vary as both a function of the ligands employed and the metals to be exchanged. In some cases, notably the transmetalation of purely inorganic group 13 clusters (Section 4), this kind of direct substitution can be run in reverse by using mass action to push the equilibrium in the reverse direction resulting in substitution of a metal that forms weaker bonds. One example of transmetalation that is likely driven by forming a stronger set of bonds within a group is the substitution of lighter pnictogens for heavier ones within benzyl dithiol-based cryptands.18 Complexes composed of three dithiolate ligands and
This journal is © The Royal Society of Chemistry 2014
Scheme 1 Representative ‘‘transmetalation’’ of an antimony cryptand with phosphorous tribromide or arsenic trichloride.
two pnictogens can be easily made using the larger members of group 15: antimony and bismuth. These complexes can then be readily displaced with arsenic and phosphorous to form nearly isostructural complexes (Scheme 1). This trend in reactivity has enabled the synthesis of homo- and heterometallic phosphorus containing complexes which were otherwise unattainable by direct self-assembly of the thiol ligands and a phosphorous source. While the new P–S bond is perhaps marginally weaker than the Sb–S bonds, a weak P–Br bond from the phosphorus source (PBr3) is broken and replaced by a stronger outgoing Sb–Br bond.19 By starting with the larger pnictogens, judicious addition of a single equivalent of a phosphorous source also results in partial substitution. In this way, heterometallic clusters can be synthesized in which only one of the larger pnictogens is replaced. The resulting heterometallic cryptands are chiral but to date have not been resolved into their respective enantiomers.18b The ability to partially substitute metals within supramolecular systems should be general and in the future may lead to new classes of functional heterometallic assemblies (Fig. 2). In some cases, well designed systems will not spontaneously self-assemble from ligands and metal salts alone as predicted. Transmetalation can be an effective technique for overcoming this potential barrier. By preassembling a larger supramolecular structure with a metal that is known to be effective for doing so, unfavorable interactions/energetics that could be hindering formation with the desired metal can be overcome. Since the dynamic framework is already established, it is perhaps easier to substitute metals without disrupting the overall structure.17 Such a strategy has enabled a pathway to form organometallic assemblies based on N-heterocyclic carbenoid ligands
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1827
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
Chem Soc Rev
Fig. 2 Stick representations of the X-ray crystal structures of heterometallic self-assemblies.
that were previously unavailable. Hahn and coworkers have showcased this approach in their use of N-heterocyclic carbene ligands anchored to a multi-functionalized central benzene core to assemble three-dimensional metallo-supramolecular silver complexes. These particular tetra- and trinuclear silver polycarbene macrocycles undergo a high yielding transmetalation with gold to give the homonuclear Au(I) complexes with retention of their former structures (Scheme 2). Silver carbene complexes are often used as starting materials in the transmetalation of single site organometallic complexes, and it follows that this transmetalation can also be completed in a multimetallic system using CuBr to yield the trinuclear Cu(I) complex in this example. This organometallic transmetalation shows the versatility of carbenes in metal-directed self-assembly.20 Hahn and coworkers further explored transmetalation in the context of dynamic covalent chemistry by incorporating sulfur donors adjacent to Schiff bases to create a chelating ligand. Such subcomponent self-assembly did not allow the formation of the desired Pd helicates due to a reduced reactivity of the Pd precursor complexed to the amines. This issue was relieved by a step-wise synthesis with an initial subcomponent self-assembly
Scheme 2 Transmetalation of the cylindrical trisilver hexacarbene complex cation to the homotrinuclear gold(I) complex. Reprinted (adapted) with permission from A. Rit, T. Pape and F. E. Hahn, J. Am. Chem. Soc., 2010, 132, 4572–4573. Copyright 2010 American Chemical Society.
1828 | Chem. Soc. Rev., 2014, 43, 1825--1834
Scheme 3 Synthesis of dinuclear 2-thiolatobenzaldehyde complexes via subsequent subcomponent self-assembly.
of a preformed Ni (or Zn) 2-thiolatobenzaldehyde complex with primary diamines (Scheme 3), followed by transmetalation of the resulting helicate with palladium acetate to yield the final dipalladium helicate (Scheme 4). It was postulated that the thermodynamic driving force for the metal exchange reaction was the preferred coordination of the soft N,S binding groups to even softer metal ions. This transmetalation can be followed by a rapid color change from dark brown (absorption of the Ni chromophore at lmax = 501 nm) to a deep orange for the dipalladium helicate (lmax = 463 nm). In addition, the formation of heterodinuclear complexes from the reaction of two homodinuclear helicates verified the intermolecular interaction and true reversibility of transmetalation in self-assembled structures.21 Schmittel et al. have synthesized a series of heteroleptic terpyridine/phenanthroline ladders by controlling dynamic multicomponent aggregation. Difunctional phenanthroline ligands were appended with steric blocking groups to prevent self-association. Due to the presence of these ‘‘steric stoppers’’, the phenanthroline ligands form tetranuclear heteroleptic 2 2 ladders in the presence of suitable terpyridine ligands and a Cu(I), Hg(II) or Zn(II) source. These components also show a tendency to self-sort producing only two discrete multicomponent nanostructures when two different metals and the two ligand types are mixed. In addition, a ‘‘two-step double transmetalation’’ occurred that was accompanied by an off–on–off fluorescent response (Scheme 5). The tetrameric Cu-ladder is
Scheme 4 Transmetalation of dinickel helicate (light green) with palladium acetate to afford the dipalladium helicate (magenta).
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Tutorial Review
Scheme 5 Cartoon of two-step transmetalation and off–on–off fluorescence response in heteroleptic phenanthroline/terpyridine-based selfassembled ladders.
directly transmetalated to the corresponding Zn-ladder upon treatment with Zn(II) and fluorescence is activated. Upon further treatment with Hg(II), the Hg-ladder forms, which quenches fluorescence.22 This system also represents an unusual example of a like metal/unlike ligand recognition within a multicomponent self-assembling system. Beer et al. have demonstrated the synthesis and transmetalation of a series of nanoscale polymetallic resorcinarene-based complexes. Depending on the metal ion chosen, these resorcinarene dithiocarbamate ligands either form hexanuclear ‘‘molecular loops’’ or octanuclear cage-like complexes. The octanuclear cages contain four resorcinarene subunits arranged in a pseudotetrahedral fashion, linked by Zn(II) or Cu(II)–dithiocarbamate complexes on each ‘‘edge’’. These assemblies transmetalate in the presence of an excess of HAuCl4 to give the octanuclear gold analog (Scheme 6). The reaction can be monitored visibly by the rapid color change from brown to a golden yellow, with no significant byproducts observed. The transmetalated octanuclear complex is essentially unchanged structurally from its parent complex. This particular example highlights the importance and specificity of the counterion relative to the metal, such that the reaction requires an excess of tetrachloroaurate anion to give the final, octanuclear gold complex. Without anions situated inside the central cavity of the host, the cage-like structure is destabilized from electronic repulsion between the positively charged Au(III) centers and the electron-rich cavity of the host.23
3 Supramolecular transmetalation: disruptive exchange In contrast with the above examples where changing one or more of the metals involved does not affect the overall supramolecular assembly, some notable examples of supramolecular transmetalation result in a new geometry after transmetalation. We refer to this type of transmetalation as disruptive exchange, and in some cases the resulting assembly is not even available by directly mixing the second metal and ligand. With notable exceptions, these situations are perhaps not as well studied as the direct substitution examples because the rearrangements can be unpredictable. This type of exchange behavior has been shown extensively in the exchange of lead for copper in molecular grids. We highlight one representative example of this extensive research area and the reader is directed to a recent review for a more thorough summary.24
This journal is © The Royal Society of Chemistry 2014
Scheme 6 Synthesis of octanuclear gold(III) complex by transmetalation. Reprinted (adapted) with permission from O. D. Fox, J. Cookson, E. J. S. Wilkinson, M. G. B. Drew, E. J. MacLean, S. J. Teat and P. D. Beer, J. Am. Chem. Soc., 2006, 128, 6990–7002. Copyright 2006 American Chemical Society.
Nitschke et al. have shown that metal-directed self-assembly and dynamic covalent synthesis can be used in tandem to generate high symmetry metal–ligand assemblies, coined ‘‘subcomponent self-assembly’’. In one example, it was shown that three different dynamic equilibria can be exploited in the selfassembly of dinuclear helicates: reversible condensation of imines from aldehydes and amines, disulfide exchange and pyridyl-imine metal coordination. In cases of transmetalation where the coordination number of the metal ion increases during the exchange, this can result in a higher denticity at the metal ion resulting in a larger structure. For example, transmetalating the tetrahedral Cu(I) complex shown in Scheme 7 with octahedral iron(II) in a dimetallic helicate leads
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1829
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
Scheme 7 Metal exchange of iron for copper leads to the change from a 2 : 2 to a 2 : 3 metal–ligand complex.
to quantitative transformation to the iron containing species. This was explained both on entropic and enthalpic grounds: the iron nitrogen bond is stronger than the copper nitrogen bond and 4 more bonds are made when switching to the octahedral metal. Additionally, more particles (from 7 to 8) are produced in solution as a result of this transmetalation.25 Many of the above transmetalation reactions deal with the exchange of metals without fundamentally changing the ligands themselves, only the way in which they are arranged. Recently Singh et al. have demonstrated a case of palladium and platinum transmetaling mercury which results in a dramatic change of the role of the donor groups in the coordination environment of the complex. In this example the mercury ions in the assembly are incorporated into the backbone of a dimetallic imine-based macrocyclic ligand. Treatment with Cu(I) or Ag(I) results in a macrocyclic Cu–Ag chelate complex coordinated between the two mercury ions and four imine nitrogens (Scheme 8, left). When Pd(II) or Pt(II) are added to the mercury containing macrocyclic ligand, however, the mercury ions vacate the imine coordination sphere to accommodate the incoming Pd–Pt. Furthermore, one Hg–C bond at each mercury is transmetalated by the incoming Pd–Pt, resulting in helical complex (Scheme 8, right). The partially detached mercury atoms take on the chloride ions from the incoming metals and form close axial interactions, further stabilizing the helical Pd–Pt complex that results.26 Transmetalation has also been employed to form a dimeric heteronuclear macrocycle from a monomeric (but dinuclear)
Scheme 8 A self-assembled dimercury macrocycle itself forms a macrocyclic chelate complex upon treatment with Cu(I) or Ag(I) (Hg shown in yellow, Cu–Ag in light gray). Transmetalation with Pd(II) or Pt(II) (shown in turquoise) results in a new helical complex in which a Hg–C bond has been transmetalated with the incoming Pd or Pt; the free Hg then coordinates to the axial sites on the incoming metal ion.
1830 | Chem. Soc. Rev., 2014, 43, 1825--1834
Chem Soc Rev
Scheme 9 1,8-Bis(trimethylstannyl)naphthalene is a versatile transmetalation synthon: treatment with InCl3 provides a diindacycle, reaction with BCl3 yields a mixed B–Sn species, and transmetalation with GaCl3 forms a heterometallic Ga–Sn dimeric macrocycle.
synthon (Scheme 9, middle reaction). Treatment of 1,8bis(trimethylstannyl)-naphthalene with gallium trichloride at 65 1C in toluene results in a macrocyclic dimer featuring two of the original stannyl naphthalene rings. In total, three trimethylstannyl groups have been removed, and two new Ga–C bonds have formed. This rare heterometallic macrocycle contains a bridging chloride linking the gallium and tin atoms. Upon addition of pyridine, both Lewis acidic metals are independently coordinated.27 In related studies, Gabbai and coworkers showed the same 1,8-bis(trimethylstannyl)naphthalene precursor can be transmetalated with indium trichloride to yield a dimeric indium macrocycle. The reported diindacycle synthesized by transmetalation is isolated in 65% yield, whereas metathesis of a 1,8-dilithionaphthalene with InCl3 and tmeda gives the diindacycle in only 25% yield by NMR spectroscopy. The metathesis reaction is further complicated by the presence of LiCl which compromises the isolation of the pure product. Here, the transmetalation reaction appears to be superior to metathesis not only by yield, but in purity of the target macrocyclic compound.28 Another way that a transmetalation approach can affect the overall structure of the complex is by causing metal migration (Scheme 10). Yonemura et al. have demonstrated that copper ions will migrate in response to metal addition within the ditopic binding pocket of a larger macrocycle. The macrocyclic ligand that they use contains two inward pointing phenolates, one binding site containing a diimine motif, and another containing a diamine motif. When a divalent metal (e.g., Zn(II), Ni(II), Co(II)) is added to the macrocyclic ligand already containing a single Cu(II) ion in the diimine binding site (Scheme 10, top left), it does not displace the copper but instead takes the open site containing the two amino groups.29 If instead, a bimetallic Pb–Cu derivative is synthesized first and then reacted with the same divalent metals (Scheme 10, bottom left), copper shifts to the amine binding site and the incoming metal is bound to the position vacated by the copper. This second arrangement (Scheme 10, bottom right), where the Cu2+ ion is bound by the amino groups, has been shown to be the thermodynamic product: heating the macrocycle with Cu2+ ion
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Scheme 10 Direct substitution of the open diamino binding site in the Cu-bound macrocycle (top) gives the expected product without rearrangement. When Pb(II) occupies that site (bottom) transmetalation with various metal sulfates, causes displacement of the Pb and migration of the Cu(II) from the imino to amino binding site to accommodate the new metal ion in the imino binding site.
bound by the imines at 70 1C provides the macrocycle with the reverse arrangement of metal ions, as shown by UV-vis spectroscopy.30 A related pyridazine–imine macrocycle from Brooker and coworkers also exhibits transmetalation that leads to transformation between two different metallosupramolecular structures (Scheme 11). When the macrocyclic ligand is treated with only Cu(I), the ligand folds to form a 2 : 4 ligand to metal grid-type
Tutorial Review
complex (Scheme 11, left). The tetrahedral coordination of Cu(I) allows for nearly right angles to form between the crossed ligand arrays to support the grid structure. When the same ligand is pretreated with Pb(II) and isolated as a dinuclear Pb(II)-bound macrocycle, transmetalation with Cu(I) affords a nearly flat mixed-valent dinuclear Cu(I)–Cu(II) macrocycle.31 A related dinuclear Cu(II) macrocycle can also be isolated upon transmetalation from the dilead species, and the resulting macrocycle is nearly identical in structure to the mixed-valent analog.32 Not all conformational changes due to transmetalation occur within discrete molecules. In an example by Fernandez, Laguna and coworkers, the Ag(I) ions of a bimetallic Au–Ag coordination polymer were transmetalated with Cu+ in the presence of pyrimidine to form a new copper–pyrimidine polymer backbone (Scheme 12). The gold(pentafluorophenyl)2 moiety remains intact and coordinates the incoming copper, creating a copper–pyrimidine polymer decorated with pendant gold complexes. In effect, the Au–Ag bond was transmetalated by Cu to create a new coordination polymer.33 Transmetalation has also been observed within the cavity of a self-assembled system. Changes in the binding pocket size and coordination geometry/number needed to ligate the incoming metal can cause lead to a complete reorganization of the supramolecular assembly. Many elegant examples of this concept have been demonstrated with the tetraoxime ligand used by Nabeshima et al. This linear ligand possesses multiple binding sites that converge on three zinc ions to form a nearly cyclical chelate pocket. The resulting conformation results in an open binding pocket which can bind three zinc ions, one in each oxime binding pocket and one within the catecholate binding pocket of the ligand backbone. This third Zn(II) ion appears to be too small to fit this spacious binding pocket, and it is readily substituted by larger cations. This large binding pocket is ideally suited for hard cations such as Sc and La, which readily transmetalate this central zinc ion. When these larger metal cations bind, the flexible ligand backbone adjusts its conformation to adopt a more favorable helical binding pocket for the incoming metal ion (bottom of Fig. 3), representing an example of disruptive transmetalation.34–37
Scheme 11 Direct metal binding of Cu+ by a dipyridazinetetraimmino macrocycle leads to a 4 : 2 metal to ligand grid complex. When the same ligand is first reacted with Pb+2 and then Cu+, a flat, mixed-valence Cu+–Cu+2 complex results.
This journal is © The Royal Society of Chemistry 2014
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1831
View Article Online
Tutorial Review
Chem Soc Rev
is referred to a recent comprehensive review on metallacrowns.38
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Self-assembled nanoscale inorganic clusters—an emerging area for transmetalation?
Scheme 12 A gold and silver-based heterometallic coordination polymer is transmetalated with copper resulting in a new heterometallic coordination polymer backbone while retaining the transferred gold-metal bond.
A wide-variety of spectacular metallacrown ethers have been reported. These self-assembled, multimetallic analogs of the organic crown ethers are self-assembled from ligands and metals to create a macrocycle capable of coordinating additional metal cations within the binding pocket. These central metal cations can be exchanged with more strongly bound cations, much like the metal exchange in crown ethers. Transmetalation in these complexes has been studied in the context of Zn(II) exchange in a Gd-bound metallacrown designed as an MRI contrast agent. The interested reader
The self-assembly of inorganic cluster compounds has a long history with recent advances showing that these materials may be useful for new applications in the realm of material science.39a In addition to benefits for commercial innovation, these insights are leading to better understanding of geological phenomena such as the potential for inclusion and transmetalation in small cluster ions produced from mine flux. Research is already underway to investigate the mechanism by which heavy metal contaminants are transmitted in suspensions known to contain these oligomeric ionic components.39b Recently it has been shown that the exterior metal ions of group 13 tridecameric aqueous clusters can be transmetalated (Scheme 13).40 The series of known clusters are isostructural and typically stable in solution.41 These flat tridecameric clusters of gallium and aluminium can be synthesized in high yields at gram scales and purified by crystallization.42 When these clusters are mixed with indium in the form of indium nitrate, the exterior metals of the disc shaped cluster are removed and replaced with indium. By adjusting the amount of indium nitrate added, the degree of substitution on the exterior of the cluster can be modulated. The reaction was also found to be reversible without destroying the core of the cluster molecules themselves. Aluminium or gallium nitrate can be added to their respective indium containing heterometallic
Fig. 3 The zinc complex of a linear tetraoxime ligand forms a dynamic binding pocket. Transmetalation of the central zinc atom with larger ions leads to helical complexes. The winding angle of the helix is inversely proportional to the size of the bound metal.
1832 | Chem. Soc. Rev., 2014, 43, 1825--1834
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Scheme 13 Representative transmetalation of a nanoscale aqueous M13 cluster (M = Al, Ga) with In(III). In3+ ions (green) displace other M3+ ions (purple) on the labile outer shell of the cluster. Counteranions have been omitted for clarity.
clusters to give their homometallic analogs. The aluminium and gallium substitution is stoichiometric whereas indium substitution requires large excesses to drive the equilibrium to the desired degree of substitution. In these examples, the degree of substitution can be readily controlled but the degree of regioselectivity has not been determined.
4 Conclusions Transmetalation of supramolecular assemblies is an emerging reaction strategy to synthesize multi-dentate metal–ligand complexes. We have found that transmetalation of self-assembled metal–ligand complexes can be categorized in at least two ways: (1) as a direct exchange event in which the incoming metal ion transmetalates the assembly without changing the overall topology of the complex; (2) as a disruptive exchange event in which the transmetalated assembly is now an entirely new structure type (Fig. 1). In some cases the resulting assemblies can be formed by direct treatment of the metal and ligand without the need for transmetalation. However, in many instances transmetalation proves to be the only viable route to form the new assembly cleanly. It seems likely that as more examples of supramolecular transmetalation are discovered, this strategy could develop into a common technique to prepare new complexes. The emergence of this technique also suggests that when designing new supramolecular metal-containing assemblies, it is important to screen a variety of metals to avoid missing possible metal bound intermediates that may later undergo a disruptive transmetalation to form new and previously unattainable (or even unpredicted) products. Transmetalation approaches could therefore represent a new, potentially powerful technique in exploratory supramolecular synthesis. In our lab transmetalation is now routinely employed on new multimetallic assemblies as a route to synthesize otherwise inaccessible complexes (both in supramolecular systems and nanoscale inorganic clusters). The potential for partial transmetalation to yield heterometallic assemblies has only been briefly explored. This may be due to the possibility of forming mixtures of many isomeric (or isostructural) products when multiple metal sites are available for exchange. It is also possible, however, that many of these heterometallic assemblies have been overlooked due to how difficult it is to analyse large supramolecular complexes
This journal is © The Royal Society of Chemistry 2014
Tutorial Review
once their high degree of symmetry has been broken. An example of partial substitution presented above involving pnictogen substitution within a thiolate cryptand maintains most of the final host’s symmetry elements and can only afford a single diastereomer. Nevertheless, the single metal exchange results in a chiral cryptand. This is perhaps an interesting route to introduce axial chirality in self-assembled systems, but also leads to more complex spectra. This tutorial review indicates that heterometallic complexes are probably best synthesized by using a transmetalation reaction on a preformed assembly. This approach is much less likely to result in a statistical distribution of products that may be difficult or impossible to separate. The well-defined products that could result from this process, especially those capable of binding a guest, could be interesting for potential catalytic applications and provides access to new functional supramolecular cavities. Single molecule precursors for inorganic materials applications such as the In–Ga cluster described above are able to achieve precise stoichiometric control over the composition of materials. Using a transmetalation approach with existing and newly developed inorganic cluster precursors offers a potential technique for fine-tuning this stoichiometry at the molecular level for specific target materials (such as tuning the In : Ga ratio in indium gallium oxide formed from heterometallic clusters). The ability to adjust the ratios of metals within precursors, if general, has the potential to allow more atom economical processing of materials with exact ratios of components.
Acknowledgements Support for a post-doctoral research assistantship (M.E.C.) from the NSF CCI Center for Sustainable Materials Chemistry is gratefully acknowledged (CHE-1102637). D.W.J. is a Scialog Fellow of Research Corporation for Science Advancement. The authors also thank the University of Oregon for partial support of this work.
Notes and references 1 A. V Davis, R. M. Yeh and K. N. Raymond, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4793–4796. 2 M. D. Pluth, R. G. Bergman and K. N. Raymond, Acc. Chem. Res., 2009, 42, 1650–1659. 3 J.-M. Lehn, Chem. Soc. Rev., 2007, 36, 151–160. 4 M. Fujita, M. Tominaga, A. Hori and B. Therrien, Acc. Chem. Res., 2005, 38, 369–378. 5 S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100, 853–908. 6 R. W. Saalfrank, H. Maid and A. Scheurer, Angew. Chem., Int. Ed., 2008, 47, 8794–8824. 7 R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev., 2011, 111, 6810–6918. 8 M. A. Fox and J. K. Whitesell, Organic Chemistry, Jones and Bartlett, Sudbury, Mass, 2nd edn, 1997. 9 (a) The American Heritage Dictionary of the English Language, 5th edn, Editors of the American Heritage Dictionaries,
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1833
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
10 11 12 13 14 15 16 17 18
19
20
21 22 23
Houghton Mifflin, Boston, MA, 2011; (b) we feel it is appropriate to make this distinction: in the normal context of mononuclear coordination complexes, when a new metal is introduced it is usually a result of complete ligand exchange and is described quite sufficiently in that way. However, in the context of a larger supramolecular assembly, ligands can potentially dissociate (or partially dissociate) without the multimetallic structure being compromised. In such cases the situation appears to be more similar to transmetalation than ligand exchange, since the metal ion is the component that is ultimately exchanged. M. Lalonde, W. Bury, O. Karagiaridi, Z. Brown, J. T. Hupp and O. K. Farha, J. Mater. Chem., 2013, 1, 5453–5468. J. W. Steed, Chem. Soc. Rev., 2001, 215, 171–221. P. T. Corbett, J. Leclaire, L. Vial, K. R. West, J.-L. Wietor, J. K. M. Sanders and S. Otto, Chem. Rev., 2006, 106, 3652–3711. V. M. Cangelosi, T. G. Carter, L. N. Zakharov and D. W. Johnson, Chem. Commun., 2009, 5606–5608. M. Fujita, S. Nagao and K. Ogura, J. Am. Chem. Soc., 1995, 117, 1649–1650. J. J. Pak, J. Greaves, D. J. McCord and K. J. Shea, Organometallics, 2002, 21, 3552–3561. S. Sato, Y. Ishido and M. Fujita, J. Am. Chem. Soc., 2009, 131, 6064–6065. J. D. Badjic´, A. Nelson, S. J. Cantrill, W. B. Turnbull and J. F. Stoddart, Acc. Chem. Res., 2005, 38, 723–732. (a) V. M. Cangelosi, L. N. Zakharov and D. W. Johnson, Angew. Chem., Int. Ed., 2010, 49, 1248–1251; (b) V. M. Cangelosi, A. C. Sather, L. N. Zakharov, O. B. Berryman and D. W. Johnson, Inorg. Chem., 2007, 46, 9278–9284. (a) T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 2nd edn, 1958; (b) B. de B. Darwent, National Standard Reference Data Series, National Bureau of Standards, no. 31, Washington, 1970; (c) S. W. Benson, J. Chem. Educ., 1965, 42, 502–518; (d) J. A. Kerr, Chem. Rev., 1966, 66, 465–500. (a) A. Rit, T. Pape and F. E. Hahn, J. Am. Chem. Soc., 2010, 132, 4572–4573; (b) A. Rit, T. Pape, A. Hepp and F. E. Hahn, Organometallics, 2011, 30, 334–347; (c) A. Rit, T. Pape and F. E. Hahn, Organometallics, 2011, 30, 6393–6401. ¨mer, J. C. Slootweg, F. Hupka, K. Lammertsma and J. Do F. E. Hahn, Angew. Chem., Int. Ed., 2010, 49, 6430–6433. ¨lfen and M. Schmittel, V. Kalsani, R. S. K. Kishore, H. Co J. W. Bats, J. Am. Chem. Soc., 2005, 127, 11544–11545. O. D. Fox, J. Cookson, E. J. S. Wilkinson, M. G. B. Drew, E. J. MacLean, S. J. Teat and P. D. Beer, J. Am. Chem. Soc., 2006, 128, 6990–7002.
1834 | Chem. Soc. Rev., 2014, 43, 1825--1834
Chem Soc Rev
24 M. Ruben, J. Rojo, F. J. Romero-Salguero, L. H. Uppadine and J.-M. Lehn, Angew. Chem., Int. Ed., 2004, 43, 3644–3662. 25 R. J. Sarma, S. Otto and J. R. Nitschke, Chem.–Eur. J., 2007, 13, 9542–9546. 26 U. Patel, S. Sharma, H. B. Singh, S. Dey, V. K. Jain, ¨user and R. J. Butcher, Organometallics, G. Wolmersha 2010, 29, 4265–4275. 27 M. Tschinkl, J. D. Hoefelmeyer, T. M. Cocker, R. E. Bachman and F. P. Gabbaı¨, Organometallics, 2000, 19, 1826–1828. 28 J. D. Hoefelmeyer, M. Schulte and F. P. Gabbaı¨, Inorg. Chem., 2001, 40, 3833–3834. 29 Y. Nakamura, M. Yonemura, K. Arimura, N. Usuki, M. Ohba ¯ kawa, Inorg. Chem., 2001, 40, 3739–3744. and H. O 30 M. Yonemura, K. Arimura, K. Inoue, N. Usuki, M. Ohba and ¯ kawa, Inorg. Chem., 2002, 41, 582–589. H. O 31 S. Brooker, S. J. Hay and P. G. Plieger, Angew. Chem., Int. Ed., 2000, 39, 1995–1997. 32 S. Brooker, R. J. Kelly, B. Moubaraki and K. S. Murray, Chem. Commun., 1996, 2579. ´ndez, A. Laguna, J. M. Lo ´pez-de-Luzuriaga, 33 E. J. Ferna M. Monge, M. Montiel and M. E. Olmos, Inorg. Chem., 2005, 44, 1163–1165. 34 S. Akine, T. Taniguchi and T. Nabeshima, J. Am. Chem. Soc., 2006, 128, 15765–15774. 35 S. Akine, T. Taniguchi, T. Saiki and T. Nabeshima, J. Am. Chem. Soc., 2005, 127, 540–541. 36 S. Akine, T. Taniguchi and T. Nabeshima, Angew. Chem., Int. Ed., 2002, 41, 4670–4673. 37 S. Akine, T. Matsumoto, T. Taniguchi and T. Nabeshima, Inorg. Chem., 2005, 44, 3270–3274. 38 G. Mezie, C. M. Zaleski and V. L. Pecoraro, Chem. Rev., 2007, 107, 4933–5003. 39 (a) M. Henry, J. P. Jolivet and J. Livage, Aqueous Chemistry of Metal Cations: Hydrolysis, Condensation, and Complexation, Chemistry, Spectroscopy, and Applications of Sol-Gel Glasses, Springer-Verlag, Berlin, 1992, pp. 152–206, and references therein; (b) G. Furrer, B. L. Phillips, K.-U. ¨thig and W. H. Casey, Science, 2002, 297, Ulrich, R. Po 2245–2247. 40 M. K. Kamunde-Devonish, M. N. Jackson, Jr., Z. L. Mensinger, L. N. Zakharov and D. W. Johnson, 2013, manuscript in preparation. 41 A. F. Oliveri, M. E. Carnes, M. M. Baseman, E. K. Richman, J. E. Hutchison and D. W. Johnson, Angew. Chem., Int. Ed., 2012, 51, 10992–10996. 42 Z. L. Mensinger, W. Wang, D. A. Keszler and D. W. Johnson, Chem. Soc. Rev., 2012, 41, 1019–1030.
This journal is © The Royal Society of Chemistry 2014
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
TUTORIAL REVIEW
Cite this: Chem. Soc. Rev., 2014, 43, 1825
View Journal | View Issue
Transmetalation of self-assembled, supramolecular complexes Matthew E. Carnes, Mary S. Collins and Darren W. Johnson* Substituting one metal for another in inorganic and organometallic systems is a proven strategy for synthesizing complex molecules, and in some cases, provides the only route to a particular system. The multivalent nature of the coordination in metal–ligand assemblies lends itself more readily to some types of transmetalation. For instance, a binding site can open up for exchange without greatly effecting the many other interactions holding the structure together. In addition to exchanging the metal and altering the local binding environment, transmetalation in supramolecular systems can also lead to substantial changes in the nature of the secondary and tertiary structure of a larger assembly. In this tutorial review we will cover discrete supramolecular assemblies in which metals are exchanged. First we will address fully formed structures where direct substitution replaces one type of metal for another without changing the overall supramolecular assembly. We will then address systems where the disruptive exchange of one metal for another leads to a larger change in the supramolecular assembly. When possible we
Received 3rd October 2013
have tried to highlight systems that use supramolecular self-assembly in tandem with transmetalation to
DOI: 10.1039/c3cs60349k
synthesize new structures not accessible through a more direct approach. At the end of this review, we highlight the use of transmetalation in self-assembled aqueous inorganic clusters and discuss the
www.rsc.org/csr
consequences for material science applications.
Key learning points (1) Transmetalation is a useful synthetic tool in supramolecular chemistry and self-assembly. (2) Supramolecular metal–ligand assemblies can allow for direct metal exchange (transmetalation) while preserving the original structure. (3) Sometimes transmetalation can result in an entirely new supramolecular assembly, often dictated by a change in coordination preferences of the incoming metal ion. (4) Adding substoichiometric amounts of an incoming metal ion during transmetalation can lead cleanly to new heterometallic supramolecular assemblies. (5) Performing transmetalation on a preassembled, multicomponent assembly can enable the synthesis of metal–ligand assemblies which do not spontaneously self-assemble from the new metal ion and ligands alone.
1 Introduction Self-assembled, metal-containing supramolecular structures represent a diverse and growing class of functional synthetic materials. Their facile construction from simple building blocks and persistent, yet flexible architectures make them finely suited for applications in the fields of catalysis, sensing, and materials science.1–7 In contrast with purely inorganic mesoporous structures, the organic spacers and dynamic nature of metal ligand interactions make the pores and void spaces
University of Oregon Department of Chemistry & Biochemistry and Materials Science Institute, University of Oregon, Eugene, OR 97403-1253, USA. E-mail: [email protected]; Web: http://sustainablematerialschemistry.org/, http://pages.uoregon.edu/dwjlab/home.html; Tel: +1 541-346-1695
This journal is © The Royal Society of Chemistry 2014
within these materials adaptable to a variety of guests both in the solid state and solution. With the judicious choice of ligands and metals, chemists have developed an elegant array of open geometric solids containing individual coordination complexes at the vertices. The bite angle, orientation of binding groups, and denticity of the ligand function in tandem with the preferred coordination geometry of the metal ion to determine the final structure and geometry of the assembly. This has been used extensively to form squares, cubes, triangles, tetrahedra, and related prisms as well as a number of distorted enclosures and 3D arrays featuring these metal and ligand building blocks.1,5,7 Transmetalation is traditionally defined as an exchange of metals between an organometallic compound and either a metal or a different organometallic compound.8 The word derives from trans-, meaning ‘‘change, transfer’’, and -ation, meaning ‘‘action
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1825
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
or process’’, and thus could perhaps more broadly include inorganic species as well.9a In recent years it has become clear that within the context of multimetallic supramolecular structures, metal exchange at the linking metal coordination sites is an emerging synthetic technique to synthesize complexes that are not readily available by direct metal–ligand self-assembly. Therefore, for the purpose of this review we will use the term ‘‘supramolecular transmetalation’’ (Fig. 1) to refer to systems in which metals are exchanged for one another within the context
Matthew E. Carnes received his BS in Chemical Engineering at the University of California, San Diego in 2003, where he did undergraduate organic chemistry research with Prof. Jay Siegel. He then moved to Columbia University where he earned his PhD in Chemistry in 2008 working with Colin Nuckolls focusing on using ring opening polymerization to form new high carbon materials. Following his Matthew E. Carnes graduate work, he did two years of postdoctoral research with Prof. Dr E. W. Meijer at Technische Universiteit Eindhoven in the Netherlands before returning to the US to do another postdoc with Darren W. Johnson at the University of Oregon as a part of the Center for Sustainable Materials Chemistry starting in 2010.
Mary S. Collins is a doctoral researcher in the Department of Chemistry and Biochemistry at the University of Oregon. She obtained a BSc in Biochemistry from The University of Texas at Austin in 2010 where she completed undergraduate research investigating the electronic properties of bimetallic N-heterocyclic diaminocarbene–iridium complexes under the supervision of Dr Christopher Bielawski. In 2011 she joined Mary S. Collins the Darren W. Johnson laboratory as a PhD student studying the coordination chemistry and reactivity of pnictogen-containing self-assemblies. Currently, her research focuses on supramolecular cyclophane syntheses using a pnictogen directing agent.
1826 | Chem. Soc. Rev., 2014, 43, 1825--1834
Chem Soc Rev
of a supramolecular assembly (without restricting specifically to just those systems containing metal–carbon bonds).9b In this review we will concentrate primarily on discrete, selfassembled constructs in which a metal plays a principal structural role—either as a vertex, edge, or in the case of inorganic clusters comprises the majority of the structure—with a few representative examples included of lower dimension coordination polymers. Although many of the concepts discussed in this review also apply to metal organic frameworks, the topic of transmetalation in MOFs has been recently reviewed and is beyond the scope of this review.10 Many metal containing supramolecular assemblies are also spacious enough to house metal ions as guests. Such molecular cages exhibit rich host–guest chemistry, which has been reviewed extensively and not included specifically in this review.1–7 An exception will be made for the exchange of metal guests which lead to a structural transformation in their host. In addition, we will not specifically review the exchange of alkali metal ion guests in preorganized or selfassembled structures because they also have been extensively reviewed.11 Metal-directed self-assembly A key factor in arriving at a discrete self-assembled structure instead of a complicated blend of oligomers or an infinite network of interlocking complexes is that the metal–ligand interactions are under rapid dynamic exchange.1–7,12 This is usually accomplished by rapid ligand exchange with the solvent or other free ligands leading to growth and digestion of a wide array of oligomers and conformers. In this way ‘‘kinetic mistakes’’ can be edited out as the complex series of equilibria
Darren W. Johnson received his BS in Chemistry at the University of Texas at Austin in 1996, where he performed undergraduate research in Jonathan Sessler’s laboratory. He earned his PhD in Chemistry in 2000 from the University of California at Berkeley working with Kenneth Raymond, and then spent two years at the Scripps Research Institute as a National Institutes of Health post-doctoral fellow with Julius Rebek, Jr. He Darren W. Johnson joined the University of Oregon in 2003, where he is currently an Associate Professor of Chemistry and Associate Director of UO’s Materials Science Institute. He is a Cottrell Scholar and Scialog Fellow of Research Corporation for Science Advancement and a National Science Foundation CAREER awardee. Research in his group uses supramolecular chemistry as a tool to explore a variety of problems in coordination chemistry, molecule/ion recognition and inorganic cluster synthesis, much of which is investigated within the Center for Sustainable Materials Chemistry (http://sustainablematerialschemistry.org/).
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Tutorial Review
Fig. 1 Two primary forms of ‘‘supramolecular transmetalation’’ frequently occur: (i) transmetalation resulting in ‘‘direct exchange’’ of the metal ions in a preformed supramolecular assembly without changing the assembly topology and, (ii) transmetalation that results in a ‘‘disruptive exchange’’ that transforms the assembly from one structure (e.g., the tetrahedron pictured) to another (e.g., a triangle).
settles on the most stable structure.13–16 In the case of 3D assemblies, a guest is often used to favor capsular structures by filling the otherwise empty void space. The multivalent nature of these larger assemblies can further balance the entropic organization cost of such multicomponent self-assembly.17 It is this ability for ligands to substitute (at least partially) on and off reversibly while maintaining the larger ordered structure that enables substituting one metal for another in a number of cages of this type. The two primary classes of supramolecular transmetalation reviewed include (i) ‘‘direct exchange’’ in which the incoming metal does not alter the metal coordination geometry or overall structure of the original self-assembly; and (ii) ‘‘disruptive exchange’’ in which the geometry of the transmetalated complex differs from the original structure as a result of the metal exchange (Fig. 1).
2 Supramolecular transmetalation: direct exchange In some self-assembled supramolecular structures, the metal ions can be substituted with other metals stoichiometrically without fundamentally changing the topology of the original structure. In these cases, incoming metal ions that form stronger bonds with the sets of ligands holding the assembly together will typically be favored. Metal exchange, especially within coordination complexes, is an equilibrium process so the strength of the newly formed bonds will strongly influence the degree to which the equilibrium between the two metals is pushed in the direction of incorporating the new metal ion. This will vary as both a function of the ligands employed and the metals to be exchanged. In some cases, notably the transmetalation of purely inorganic group 13 clusters (Section 4), this kind of direct substitution can be run in reverse by using mass action to push the equilibrium in the reverse direction resulting in substitution of a metal that forms weaker bonds. One example of transmetalation that is likely driven by forming a stronger set of bonds within a group is the substitution of lighter pnictogens for heavier ones within benzyl dithiol-based cryptands.18 Complexes composed of three dithiolate ligands and
This journal is © The Royal Society of Chemistry 2014
Scheme 1 Representative ‘‘transmetalation’’ of an antimony cryptand with phosphorous tribromide or arsenic trichloride.
two pnictogens can be easily made using the larger members of group 15: antimony and bismuth. These complexes can then be readily displaced with arsenic and phosphorous to form nearly isostructural complexes (Scheme 1). This trend in reactivity has enabled the synthesis of homo- and heterometallic phosphorus containing complexes which were otherwise unattainable by direct self-assembly of the thiol ligands and a phosphorous source. While the new P–S bond is perhaps marginally weaker than the Sb–S bonds, a weak P–Br bond from the phosphorus source (PBr3) is broken and replaced by a stronger outgoing Sb–Br bond.19 By starting with the larger pnictogens, judicious addition of a single equivalent of a phosphorous source also results in partial substitution. In this way, heterometallic clusters can be synthesized in which only one of the larger pnictogens is replaced. The resulting heterometallic cryptands are chiral but to date have not been resolved into their respective enantiomers.18b The ability to partially substitute metals within supramolecular systems should be general and in the future may lead to new classes of functional heterometallic assemblies (Fig. 2). In some cases, well designed systems will not spontaneously self-assemble from ligands and metal salts alone as predicted. Transmetalation can be an effective technique for overcoming this potential barrier. By preassembling a larger supramolecular structure with a metal that is known to be effective for doing so, unfavorable interactions/energetics that could be hindering formation with the desired metal can be overcome. Since the dynamic framework is already established, it is perhaps easier to substitute metals without disrupting the overall structure.17 Such a strategy has enabled a pathway to form organometallic assemblies based on N-heterocyclic carbenoid ligands
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1827
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
Chem Soc Rev
Fig. 2 Stick representations of the X-ray crystal structures of heterometallic self-assemblies.
that were previously unavailable. Hahn and coworkers have showcased this approach in their use of N-heterocyclic carbene ligands anchored to a multi-functionalized central benzene core to assemble three-dimensional metallo-supramolecular silver complexes. These particular tetra- and trinuclear silver polycarbene macrocycles undergo a high yielding transmetalation with gold to give the homonuclear Au(I) complexes with retention of their former structures (Scheme 2). Silver carbene complexes are often used as starting materials in the transmetalation of single site organometallic complexes, and it follows that this transmetalation can also be completed in a multimetallic system using CuBr to yield the trinuclear Cu(I) complex in this example. This organometallic transmetalation shows the versatility of carbenes in metal-directed self-assembly.20 Hahn and coworkers further explored transmetalation in the context of dynamic covalent chemistry by incorporating sulfur donors adjacent to Schiff bases to create a chelating ligand. Such subcomponent self-assembly did not allow the formation of the desired Pd helicates due to a reduced reactivity of the Pd precursor complexed to the amines. This issue was relieved by a step-wise synthesis with an initial subcomponent self-assembly
Scheme 2 Transmetalation of the cylindrical trisilver hexacarbene complex cation to the homotrinuclear gold(I) complex. Reprinted (adapted) with permission from A. Rit, T. Pape and F. E. Hahn, J. Am. Chem. Soc., 2010, 132, 4572–4573. Copyright 2010 American Chemical Society.
1828 | Chem. Soc. Rev., 2014, 43, 1825--1834
Scheme 3 Synthesis of dinuclear 2-thiolatobenzaldehyde complexes via subsequent subcomponent self-assembly.
of a preformed Ni (or Zn) 2-thiolatobenzaldehyde complex with primary diamines (Scheme 3), followed by transmetalation of the resulting helicate with palladium acetate to yield the final dipalladium helicate (Scheme 4). It was postulated that the thermodynamic driving force for the metal exchange reaction was the preferred coordination of the soft N,S binding groups to even softer metal ions. This transmetalation can be followed by a rapid color change from dark brown (absorption of the Ni chromophore at lmax = 501 nm) to a deep orange for the dipalladium helicate (lmax = 463 nm). In addition, the formation of heterodinuclear complexes from the reaction of two homodinuclear helicates verified the intermolecular interaction and true reversibility of transmetalation in self-assembled structures.21 Schmittel et al. have synthesized a series of heteroleptic terpyridine/phenanthroline ladders by controlling dynamic multicomponent aggregation. Difunctional phenanthroline ligands were appended with steric blocking groups to prevent self-association. Due to the presence of these ‘‘steric stoppers’’, the phenanthroline ligands form tetranuclear heteroleptic 2 2 ladders in the presence of suitable terpyridine ligands and a Cu(I), Hg(II) or Zn(II) source. These components also show a tendency to self-sort producing only two discrete multicomponent nanostructures when two different metals and the two ligand types are mixed. In addition, a ‘‘two-step double transmetalation’’ occurred that was accompanied by an off–on–off fluorescent response (Scheme 5). The tetrameric Cu-ladder is
Scheme 4 Transmetalation of dinickel helicate (light green) with palladium acetate to afford the dipalladium helicate (magenta).
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Tutorial Review
Scheme 5 Cartoon of two-step transmetalation and off–on–off fluorescence response in heteroleptic phenanthroline/terpyridine-based selfassembled ladders.
directly transmetalated to the corresponding Zn-ladder upon treatment with Zn(II) and fluorescence is activated. Upon further treatment with Hg(II), the Hg-ladder forms, which quenches fluorescence.22 This system also represents an unusual example of a like metal/unlike ligand recognition within a multicomponent self-assembling system. Beer et al. have demonstrated the synthesis and transmetalation of a series of nanoscale polymetallic resorcinarene-based complexes. Depending on the metal ion chosen, these resorcinarene dithiocarbamate ligands either form hexanuclear ‘‘molecular loops’’ or octanuclear cage-like complexes. The octanuclear cages contain four resorcinarene subunits arranged in a pseudotetrahedral fashion, linked by Zn(II) or Cu(II)–dithiocarbamate complexes on each ‘‘edge’’. These assemblies transmetalate in the presence of an excess of HAuCl4 to give the octanuclear gold analog (Scheme 6). The reaction can be monitored visibly by the rapid color change from brown to a golden yellow, with no significant byproducts observed. The transmetalated octanuclear complex is essentially unchanged structurally from its parent complex. This particular example highlights the importance and specificity of the counterion relative to the metal, such that the reaction requires an excess of tetrachloroaurate anion to give the final, octanuclear gold complex. Without anions situated inside the central cavity of the host, the cage-like structure is destabilized from electronic repulsion between the positively charged Au(III) centers and the electron-rich cavity of the host.23
3 Supramolecular transmetalation: disruptive exchange In contrast with the above examples where changing one or more of the metals involved does not affect the overall supramolecular assembly, some notable examples of supramolecular transmetalation result in a new geometry after transmetalation. We refer to this type of transmetalation as disruptive exchange, and in some cases the resulting assembly is not even available by directly mixing the second metal and ligand. With notable exceptions, these situations are perhaps not as well studied as the direct substitution examples because the rearrangements can be unpredictable. This type of exchange behavior has been shown extensively in the exchange of lead for copper in molecular grids. We highlight one representative example of this extensive research area and the reader is directed to a recent review for a more thorough summary.24
This journal is © The Royal Society of Chemistry 2014
Scheme 6 Synthesis of octanuclear gold(III) complex by transmetalation. Reprinted (adapted) with permission from O. D. Fox, J. Cookson, E. J. S. Wilkinson, M. G. B. Drew, E. J. MacLean, S. J. Teat and P. D. Beer, J. Am. Chem. Soc., 2006, 128, 6990–7002. Copyright 2006 American Chemical Society.
Nitschke et al. have shown that metal-directed self-assembly and dynamic covalent synthesis can be used in tandem to generate high symmetry metal–ligand assemblies, coined ‘‘subcomponent self-assembly’’. In one example, it was shown that three different dynamic equilibria can be exploited in the selfassembly of dinuclear helicates: reversible condensation of imines from aldehydes and amines, disulfide exchange and pyridyl-imine metal coordination. In cases of transmetalation where the coordination number of the metal ion increases during the exchange, this can result in a higher denticity at the metal ion resulting in a larger structure. For example, transmetalating the tetrahedral Cu(I) complex shown in Scheme 7 with octahedral iron(II) in a dimetallic helicate leads
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1829
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
Scheme 7 Metal exchange of iron for copper leads to the change from a 2 : 2 to a 2 : 3 metal–ligand complex.
to quantitative transformation to the iron containing species. This was explained both on entropic and enthalpic grounds: the iron nitrogen bond is stronger than the copper nitrogen bond and 4 more bonds are made when switching to the octahedral metal. Additionally, more particles (from 7 to 8) are produced in solution as a result of this transmetalation.25 Many of the above transmetalation reactions deal with the exchange of metals without fundamentally changing the ligands themselves, only the way in which they are arranged. Recently Singh et al. have demonstrated a case of palladium and platinum transmetaling mercury which results in a dramatic change of the role of the donor groups in the coordination environment of the complex. In this example the mercury ions in the assembly are incorporated into the backbone of a dimetallic imine-based macrocyclic ligand. Treatment with Cu(I) or Ag(I) results in a macrocyclic Cu–Ag chelate complex coordinated between the two mercury ions and four imine nitrogens (Scheme 8, left). When Pd(II) or Pt(II) are added to the mercury containing macrocyclic ligand, however, the mercury ions vacate the imine coordination sphere to accommodate the incoming Pd–Pt. Furthermore, one Hg–C bond at each mercury is transmetalated by the incoming Pd–Pt, resulting in helical complex (Scheme 8, right). The partially detached mercury atoms take on the chloride ions from the incoming metals and form close axial interactions, further stabilizing the helical Pd–Pt complex that results.26 Transmetalation has also been employed to form a dimeric heteronuclear macrocycle from a monomeric (but dinuclear)
Scheme 8 A self-assembled dimercury macrocycle itself forms a macrocyclic chelate complex upon treatment with Cu(I) or Ag(I) (Hg shown in yellow, Cu–Ag in light gray). Transmetalation with Pd(II) or Pt(II) (shown in turquoise) results in a new helical complex in which a Hg–C bond has been transmetalated with the incoming Pd or Pt; the free Hg then coordinates to the axial sites on the incoming metal ion.
1830 | Chem. Soc. Rev., 2014, 43, 1825--1834
Chem Soc Rev
Scheme 9 1,8-Bis(trimethylstannyl)naphthalene is a versatile transmetalation synthon: treatment with InCl3 provides a diindacycle, reaction with BCl3 yields a mixed B–Sn species, and transmetalation with GaCl3 forms a heterometallic Ga–Sn dimeric macrocycle.
synthon (Scheme 9, middle reaction). Treatment of 1,8bis(trimethylstannyl)-naphthalene with gallium trichloride at 65 1C in toluene results in a macrocyclic dimer featuring two of the original stannyl naphthalene rings. In total, three trimethylstannyl groups have been removed, and two new Ga–C bonds have formed. This rare heterometallic macrocycle contains a bridging chloride linking the gallium and tin atoms. Upon addition of pyridine, both Lewis acidic metals are independently coordinated.27 In related studies, Gabbai and coworkers showed the same 1,8-bis(trimethylstannyl)naphthalene precursor can be transmetalated with indium trichloride to yield a dimeric indium macrocycle. The reported diindacycle synthesized by transmetalation is isolated in 65% yield, whereas metathesis of a 1,8-dilithionaphthalene with InCl3 and tmeda gives the diindacycle in only 25% yield by NMR spectroscopy. The metathesis reaction is further complicated by the presence of LiCl which compromises the isolation of the pure product. Here, the transmetalation reaction appears to be superior to metathesis not only by yield, but in purity of the target macrocyclic compound.28 Another way that a transmetalation approach can affect the overall structure of the complex is by causing metal migration (Scheme 10). Yonemura et al. have demonstrated that copper ions will migrate in response to metal addition within the ditopic binding pocket of a larger macrocycle. The macrocyclic ligand that they use contains two inward pointing phenolates, one binding site containing a diimine motif, and another containing a diamine motif. When a divalent metal (e.g., Zn(II), Ni(II), Co(II)) is added to the macrocyclic ligand already containing a single Cu(II) ion in the diimine binding site (Scheme 10, top left), it does not displace the copper but instead takes the open site containing the two amino groups.29 If instead, a bimetallic Pb–Cu derivative is synthesized first and then reacted with the same divalent metals (Scheme 10, bottom left), copper shifts to the amine binding site and the incoming metal is bound to the position vacated by the copper. This second arrangement (Scheme 10, bottom right), where the Cu2+ ion is bound by the amino groups, has been shown to be the thermodynamic product: heating the macrocycle with Cu2+ ion
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Scheme 10 Direct substitution of the open diamino binding site in the Cu-bound macrocycle (top) gives the expected product without rearrangement. When Pb(II) occupies that site (bottom) transmetalation with various metal sulfates, causes displacement of the Pb and migration of the Cu(II) from the imino to amino binding site to accommodate the new metal ion in the imino binding site.
bound by the imines at 70 1C provides the macrocycle with the reverse arrangement of metal ions, as shown by UV-vis spectroscopy.30 A related pyridazine–imine macrocycle from Brooker and coworkers also exhibits transmetalation that leads to transformation between two different metallosupramolecular structures (Scheme 11). When the macrocyclic ligand is treated with only Cu(I), the ligand folds to form a 2 : 4 ligand to metal grid-type
Tutorial Review
complex (Scheme 11, left). The tetrahedral coordination of Cu(I) allows for nearly right angles to form between the crossed ligand arrays to support the grid structure. When the same ligand is pretreated with Pb(II) and isolated as a dinuclear Pb(II)-bound macrocycle, transmetalation with Cu(I) affords a nearly flat mixed-valent dinuclear Cu(I)–Cu(II) macrocycle.31 A related dinuclear Cu(II) macrocycle can also be isolated upon transmetalation from the dilead species, and the resulting macrocycle is nearly identical in structure to the mixed-valent analog.32 Not all conformational changes due to transmetalation occur within discrete molecules. In an example by Fernandez, Laguna and coworkers, the Ag(I) ions of a bimetallic Au–Ag coordination polymer were transmetalated with Cu+ in the presence of pyrimidine to form a new copper–pyrimidine polymer backbone (Scheme 12). The gold(pentafluorophenyl)2 moiety remains intact and coordinates the incoming copper, creating a copper–pyrimidine polymer decorated with pendant gold complexes. In effect, the Au–Ag bond was transmetalated by Cu to create a new coordination polymer.33 Transmetalation has also been observed within the cavity of a self-assembled system. Changes in the binding pocket size and coordination geometry/number needed to ligate the incoming metal can cause lead to a complete reorganization of the supramolecular assembly. Many elegant examples of this concept have been demonstrated with the tetraoxime ligand used by Nabeshima et al. This linear ligand possesses multiple binding sites that converge on three zinc ions to form a nearly cyclical chelate pocket. The resulting conformation results in an open binding pocket which can bind three zinc ions, one in each oxime binding pocket and one within the catecholate binding pocket of the ligand backbone. This third Zn(II) ion appears to be too small to fit this spacious binding pocket, and it is readily substituted by larger cations. This large binding pocket is ideally suited for hard cations such as Sc and La, which readily transmetalate this central zinc ion. When these larger metal cations bind, the flexible ligand backbone adjusts its conformation to adopt a more favorable helical binding pocket for the incoming metal ion (bottom of Fig. 3), representing an example of disruptive transmetalation.34–37
Scheme 11 Direct metal binding of Cu+ by a dipyridazinetetraimmino macrocycle leads to a 4 : 2 metal to ligand grid complex. When the same ligand is first reacted with Pb+2 and then Cu+, a flat, mixed-valence Cu+–Cu+2 complex results.
This journal is © The Royal Society of Chemistry 2014
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1831
View Article Online
Tutorial Review
Chem Soc Rev
is referred to a recent comprehensive review on metallacrowns.38
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Self-assembled nanoscale inorganic clusters—an emerging area for transmetalation?
Scheme 12 A gold and silver-based heterometallic coordination polymer is transmetalated with copper resulting in a new heterometallic coordination polymer backbone while retaining the transferred gold-metal bond.
A wide-variety of spectacular metallacrown ethers have been reported. These self-assembled, multimetallic analogs of the organic crown ethers are self-assembled from ligands and metals to create a macrocycle capable of coordinating additional metal cations within the binding pocket. These central metal cations can be exchanged with more strongly bound cations, much like the metal exchange in crown ethers. Transmetalation in these complexes has been studied in the context of Zn(II) exchange in a Gd-bound metallacrown designed as an MRI contrast agent. The interested reader
The self-assembly of inorganic cluster compounds has a long history with recent advances showing that these materials may be useful for new applications in the realm of material science.39a In addition to benefits for commercial innovation, these insights are leading to better understanding of geological phenomena such as the potential for inclusion and transmetalation in small cluster ions produced from mine flux. Research is already underway to investigate the mechanism by which heavy metal contaminants are transmitted in suspensions known to contain these oligomeric ionic components.39b Recently it has been shown that the exterior metal ions of group 13 tridecameric aqueous clusters can be transmetalated (Scheme 13).40 The series of known clusters are isostructural and typically stable in solution.41 These flat tridecameric clusters of gallium and aluminium can be synthesized in high yields at gram scales and purified by crystallization.42 When these clusters are mixed with indium in the form of indium nitrate, the exterior metals of the disc shaped cluster are removed and replaced with indium. By adjusting the amount of indium nitrate added, the degree of substitution on the exterior of the cluster can be modulated. The reaction was also found to be reversible without destroying the core of the cluster molecules themselves. Aluminium or gallium nitrate can be added to their respective indium containing heterometallic
Fig. 3 The zinc complex of a linear tetraoxime ligand forms a dynamic binding pocket. Transmetalation of the central zinc atom with larger ions leads to helical complexes. The winding angle of the helix is inversely proportional to the size of the bound metal.
1832 | Chem. Soc. Rev., 2014, 43, 1825--1834
This journal is © The Royal Society of Chemistry 2014
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Chem Soc Rev
Scheme 13 Representative transmetalation of a nanoscale aqueous M13 cluster (M = Al, Ga) with In(III). In3+ ions (green) displace other M3+ ions (purple) on the labile outer shell of the cluster. Counteranions have been omitted for clarity.
clusters to give their homometallic analogs. The aluminium and gallium substitution is stoichiometric whereas indium substitution requires large excesses to drive the equilibrium to the desired degree of substitution. In these examples, the degree of substitution can be readily controlled but the degree of regioselectivity has not been determined.
4 Conclusions Transmetalation of supramolecular assemblies is an emerging reaction strategy to synthesize multi-dentate metal–ligand complexes. We have found that transmetalation of self-assembled metal–ligand complexes can be categorized in at least two ways: (1) as a direct exchange event in which the incoming metal ion transmetalates the assembly without changing the overall topology of the complex; (2) as a disruptive exchange event in which the transmetalated assembly is now an entirely new structure type (Fig. 1). In some cases the resulting assemblies can be formed by direct treatment of the metal and ligand without the need for transmetalation. However, in many instances transmetalation proves to be the only viable route to form the new assembly cleanly. It seems likely that as more examples of supramolecular transmetalation are discovered, this strategy could develop into a common technique to prepare new complexes. The emergence of this technique also suggests that when designing new supramolecular metal-containing assemblies, it is important to screen a variety of metals to avoid missing possible metal bound intermediates that may later undergo a disruptive transmetalation to form new and previously unattainable (or even unpredicted) products. Transmetalation approaches could therefore represent a new, potentially powerful technique in exploratory supramolecular synthesis. In our lab transmetalation is now routinely employed on new multimetallic assemblies as a route to synthesize otherwise inaccessible complexes (both in supramolecular systems and nanoscale inorganic clusters). The potential for partial transmetalation to yield heterometallic assemblies has only been briefly explored. This may be due to the possibility of forming mixtures of many isomeric (or isostructural) products when multiple metal sites are available for exchange. It is also possible, however, that many of these heterometallic assemblies have been overlooked due to how difficult it is to analyse large supramolecular complexes
This journal is © The Royal Society of Chemistry 2014
Tutorial Review
once their high degree of symmetry has been broken. An example of partial substitution presented above involving pnictogen substitution within a thiolate cryptand maintains most of the final host’s symmetry elements and can only afford a single diastereomer. Nevertheless, the single metal exchange results in a chiral cryptand. This is perhaps an interesting route to introduce axial chirality in self-assembled systems, but also leads to more complex spectra. This tutorial review indicates that heterometallic complexes are probably best synthesized by using a transmetalation reaction on a preformed assembly. This approach is much less likely to result in a statistical distribution of products that may be difficult or impossible to separate. The well-defined products that could result from this process, especially those capable of binding a guest, could be interesting for potential catalytic applications and provides access to new functional supramolecular cavities. Single molecule precursors for inorganic materials applications such as the In–Ga cluster described above are able to achieve precise stoichiometric control over the composition of materials. Using a transmetalation approach with existing and newly developed inorganic cluster precursors offers a potential technique for fine-tuning this stoichiometry at the molecular level for specific target materials (such as tuning the In : Ga ratio in indium gallium oxide formed from heterometallic clusters). The ability to adjust the ratios of metals within precursors, if general, has the potential to allow more atom economical processing of materials with exact ratios of components.
Acknowledgements Support for a post-doctoral research assistantship (M.E.C.) from the NSF CCI Center for Sustainable Materials Chemistry is gratefully acknowledged (CHE-1102637). D.W.J. is a Scialog Fellow of Research Corporation for Science Advancement. The authors also thank the University of Oregon for partial support of this work.
Notes and references 1 A. V Davis, R. M. Yeh and K. N. Raymond, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4793–4796. 2 M. D. Pluth, R. G. Bergman and K. N. Raymond, Acc. Chem. Res., 2009, 42, 1650–1659. 3 J.-M. Lehn, Chem. Soc. Rev., 2007, 36, 151–160. 4 M. Fujita, M. Tominaga, A. Hori and B. Therrien, Acc. Chem. Res., 2005, 38, 369–378. 5 S. Leininger, B. Olenyuk and P. J. Stang, Chem. Rev., 2000, 100, 853–908. 6 R. W. Saalfrank, H. Maid and A. Scheurer, Angew. Chem., Int. Ed., 2008, 47, 8794–8824. 7 R. Chakrabarty, P. S. Mukherjee and P. J. Stang, Chem. Rev., 2011, 111, 6810–6918. 8 M. A. Fox and J. K. Whitesell, Organic Chemistry, Jones and Bartlett, Sudbury, Mass, 2nd edn, 1997. 9 (a) The American Heritage Dictionary of the English Language, 5th edn, Editors of the American Heritage Dictionaries,
Chem. Soc. Rev., 2014, 43, 1825--1834 | 1833
View Article Online
Published on 18 December 2013. Downloaded by National Chung Hsing University on 29/03/2014 13:01:06.
Tutorial Review
10 11 12 13 14 15 16 17 18
19
20
21 22 23
Houghton Mifflin, Boston, MA, 2011; (b) we feel it is appropriate to make this distinction: in the normal context of mononuclear coordination complexes, when a new metal is introduced it is usually a result of complete ligand exchange and is described quite sufficiently in that way. However, in the context of a larger supramolecular assembly, ligands can potentially dissociate (or partially dissociate) without the multimetallic structure being compromised. In such cases the situation appears to be more similar to transmetalation than ligand exchange, since the metal ion is the component that is ultimately exchanged. M. Lalonde, W. Bury, O. Karagiaridi, Z. Brown, J. T. Hupp and O. K. Farha, J. Mater. Chem., 2013, 1, 5453–5468. J. W. Steed, Chem. Soc. Rev., 2001, 215, 171–221. P. T. Corbett, J. Leclaire, L. Vial, K. R. West, J.-L. Wietor, J. K. M. Sanders and S. Otto, Chem. Rev., 2006, 106, 3652–3711. V. M. Cangelosi, T. G. Carter, L. N. Zakharov and D. W. Johnson, Chem. Commun., 2009, 5606–5608. M. Fujita, S. Nagao and K. Ogura, J. Am. Chem. Soc., 1995, 117, 1649–1650. J. J. Pak, J. Greaves, D. J. McCord and K. J. Shea, Organometallics, 2002, 21, 3552–3561. S. Sato, Y. Ishido and M. Fujita, J. Am. Chem. Soc., 2009, 131, 6064–6065. J. D. Badjic´, A. Nelson, S. J. Cantrill, W. B. Turnbull and J. F. Stoddart, Acc. Chem. Res., 2005, 38, 723–732. (a) V. M. Cangelosi, L. N. Zakharov and D. W. Johnson, Angew. Chem., Int. Ed., 2010, 49, 1248–1251; (b) V. M. Cangelosi, A. C. Sather, L. N. Zakharov, O. B. Berryman and D. W. Johnson, Inorg. Chem., 2007, 46, 9278–9284. (a) T. L. Cottrell, The Strengths of Chemical Bonds, Butterworth, London, 2nd edn, 1958; (b) B. de B. Darwent, National Standard Reference Data Series, National Bureau of Standards, no. 31, Washington, 1970; (c) S. W. Benson, J. Chem. Educ., 1965, 42, 502–518; (d) J. A. Kerr, Chem. Rev., 1966, 66, 465–500. (a) A. Rit, T. Pape and F. E. Hahn, J. Am. Chem. Soc., 2010, 132, 4572–4573; (b) A. Rit, T. Pape, A. Hepp and F. E. Hahn, Organometallics, 2011, 30, 334–347; (c) A. Rit, T. Pape and F. E. Hahn, Organometallics, 2011, 30, 6393–6401. ¨mer, J. C. Slootweg, F. Hupka, K. Lammertsma and J. Do F. E. Hahn, Angew. Chem., Int. Ed., 2010, 49, 6430–6433. ¨lfen and M. Schmittel, V. Kalsani, R. S. K. Kishore, H. Co J. W. Bats, J. Am. Chem. Soc., 2005, 127, 11544–11545. O. D. Fox, J. Cookson, E. J. S. Wilkinson, M. G. B. Drew, E. J. MacLean, S. J. Teat and P. D. Beer, J. Am. Chem. Soc., 2006, 128, 6990–7002.
1834 | Chem. Soc. Rev., 2014, 43, 1825--1834
Chem Soc Rev
24 M. Ruben, J. Rojo, F. J. Romero-Salguero, L. H. Uppadine and J.-M. Lehn, Angew. Chem., Int. Ed., 2004, 43, 3644–3662. 25 R. J. Sarma, S. Otto and J. R. Nitschke, Chem.–Eur. J., 2007, 13, 9542–9546. 26 U. Patel, S. Sharma, H. B. Singh, S. Dey, V. K. Jain, ¨user and R. J. Butcher, Organometallics, G. Wolmersha 2010, 29, 4265–4275. 27 M. Tschinkl, J. D. Hoefelmeyer, T. M. Cocker, R. E. Bachman and F. P. Gabbaı¨, Organometallics, 2000, 19, 1826–1828. 28 J. D. Hoefelmeyer, M. Schulte and F. P. Gabbaı¨, Inorg. Chem., 2001, 40, 3833–3834. 29 Y. Nakamura, M. Yonemura, K. Arimura, N. Usuki, M. Ohba ¯ kawa, Inorg. Chem., 2001, 40, 3739–3744. and H. O 30 M. Yonemura, K. Arimura, K. Inoue, N. Usuki, M. Ohba and ¯ kawa, Inorg. Chem., 2002, 41, 582–589. H. O 31 S. Brooker, S. J. Hay and P. G. Plieger, Angew. Chem., Int. Ed., 2000, 39, 1995–1997. 32 S. Brooker, R. J. Kelly, B. Moubaraki and K. S. Murray, Chem. Commun., 1996, 2579. ´ndez, A. Laguna, J. M. Lo ´pez-de-Luzuriaga, 33 E. J. Ferna M. Monge, M. Montiel and M. E. Olmos, Inorg. Chem., 2005, 44, 1163–1165. 34 S. Akine, T. Taniguchi and T. Nabeshima, J. Am. Chem. Soc., 2006, 128, 15765–15774. 35 S. Akine, T. Taniguchi, T. Saiki and T. Nabeshima, J. Am. Chem. Soc., 2005, 127, 540–541. 36 S. Akine, T. Taniguchi and T. Nabeshima, Angew. Chem., Int. Ed., 2002, 41, 4670–4673. 37 S. Akine, T. Matsumoto, T. Taniguchi and T. Nabeshima, Inorg. Chem., 2005, 44, 3270–3274. 38 G. Mezie, C. M. Zaleski and V. L. Pecoraro, Chem. Rev., 2007, 107, 4933–5003. 39 (a) M. Henry, J. P. Jolivet and J. Livage, Aqueous Chemistry of Metal Cations: Hydrolysis, Condensation, and Complexation, Chemistry, Spectroscopy, and Applications of Sol-Gel Glasses, Springer-Verlag, Berlin, 1992, pp. 152–206, and references therein; (b) G. Furrer, B. L. Phillips, K.-U. ¨thig and W. H. Casey, Science, 2002, 297, Ulrich, R. Po 2245–2247. 40 M. K. Kamunde-Devonish, M. N. Jackson, Jr., Z. L. Mensinger, L. N. Zakharov and D. W. Johnson, 2013, manuscript in preparation. 41 A. F. Oliveri, M. E. Carnes, M. M. Baseman, E. K. Richman, J. E. Hutchison and D. W. Johnson, Angew. Chem., Int. Ed., 2012, 51, 10992–10996. 42 Z. L. Mensinger, W. Wang, D. A. Keszler and D. W. Johnson, Chem. Soc. Rev., 2012, 41, 1019–1030.
This journal is © The Royal Society of Chemistry 2014
