DOI: 10.1002/chem.201405400

Concept

& Aluminum(III) Chemistry

Catalysis by Aluminum(III) Complexes of Non-Innocent Ligands Louise A. Berben*[a]

Chem. Eur. J. 2015, 21, 2734 – 2742

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Concept between substrate and metal catalyst. The cases where oxidative addition is involved in the activation of hydrogen for hydrogenation chemistry will not be discussed in this work. Metal–ligand cooperation in bond activation: Basic ligands that can accept protons from the polar bonds in very weakly acidic substrates have the potential to participate in metal– ligand cooperative bond activation (Figure 1). Shvo and co-

Abstract: Non-Innocent ligand complexes of aluminum are described in this Concept article, beginning with a discussion of their synthesis, and then structural and electronic characterization. The main focus concerns the ability of the ligands in these complexes to mediate proton transfer reactions. As examples, aluminum–ligand cooperation in the activation of polar bonds is described, as is the importance of hydrogen bonding to stabilization of a transition state for b-hydride abstraction. Taken together these reactions enable catalytic processes such as the dehydrogenation of formic acid.

Introduction Proton transfer reactions lie at the heart of chemical reactivity including, for example, processes designed for energy storage and harvesting, or for chemical synthesis. Ideally, these largescale processes would operate using sustainable materials that are cheap and abundant, and for chemical transformations generally, to be performed with minimal generation of waste products. Goals such as these are currently inspiring the development of new fundamental chemistry of non-precious metals.[1] Transition elements are prized as a group for their versatile chemistry that largely stems from an ability to mediate redox transformations. However, there are large classes of catalytic reactions that utilize transition elements without drawing on the redox capabilities of the metal center. Instead, in these instances it is usually the acidic properties of the metal ion that are important and this observation implies that non-precious acidic metal ions, such as Al, could be used in an appropriately designed catalyst.[2] Lewis acidic Al is the most abundant metallic element on earth, comprises ~ 8 % of the earth’s crust,[3] and costs just ~ $2 kg. It is also widely available from convenient sources worldwide and this provides security of supply. A challenge with Al chemistry is the design of appropriate ligands that can facilitate the desired chemistry, and also provide catalysts that are stable under the appropriate reaction conditions. An example class of catalytic reactions where transition element complexes are often employed for their acidic nature is hydrogenation and dehydrogenation chemistry.[4] These catalytic reactions include many examples in which both the metal and ligand cooperate in the movement of protons and hydride equivalents to and from substrates. When metal–ligand cooperation is involved in the mechanisms for hydrogenation and dehydrogenation, the transfer of protons to and from a supporting ligand can serve more than one function, including 1) the activation of X H bonds (X = O, N(H)), and 2) the stabilization of transition states involving proton or hydride transfer [a] Prof. L. A. Berben Department of Chemistry, University of California Davis, CA 95616 (USA) E-mail: [email protected] Chem. Eur. J. 2015, 21, 2734 – 2742

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Figure 1. Various scenarios for the activation (deprotonation) of polar X H bonds mediated by an acidic metal center and basic ligand. A = Lewis acid, B = base, L–L = multidentate ligand, R = organic functional group, X = O, N(H).

workers established the importance of metal–ligand cooperativity in Ru complexes that could dehydrogenate alcohols to provide a clean source of H2.[5] Further examples of metal– ligand cooperation in bond activation have been well documented in a series of Ru, Re, and Fe complexes presented by Milstein and Grtzmacher, amongst others, where the ligand backbone can accept protons from weakly acidic substrates to trigger bond activation reactions and catalysis.[4c, d] In addition, recently reported catalysts for the dehydrogenation of methanol activate O H bonds to initiate catalysis, and this occurs across the metal, Ru or Fe, and the donor atom in the cooperating ligand.[6] In a more general sense, the concept of acid and base working together to initiate a catalytic reaction also plays a role in Lewis acid/Brønsted base catalysis where a counteranion initially coordinated to a metal catalyst dissociates upon protonation (intramolecular-type deprotonation) to activate a substrate.[7] The additional scenario, where the donor arm of a multidentate hemilabile ligand dissociates from the metal center, upon protonation by a weakly acidic X H bond, has also been investigated in some transition metal systems.[8] In this Concept article we will describe amido complexes of AlIII in which the amido donor is part of a reduced, tridentate bis(imino)pyridine ligand. We abbreviate the ligands as RI2P, where R is methyl or phenyl, I is imino, and P is pyridine. In two-electron reduced complexes of these RI2P ligands, denoted RI2P2 , one donor arm of the ligand is formally an amido donor. The cleavage of polar N H and O H bonds across the Al amido bond will be discussed. Metal–ligand cooperative stabilization of transition states for hydrogenation and dehydrogenation chemistry: The utility of

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Concept metal–ligand cooperative effects in catalysis extends beyond bond activation. Protonated ligands also have the potential to participate in hydrogen bonding which can influence the reaction pathway during a catalytic transformation. Examples of this behavior include cases where hy- Scheme 1. Ligands that have been employed in the synthesis of reduced ligand complexes of aluminum(III). drogen bonding directs the binding mode of an incoming substrate, or where hydrogen bonding stabilizes a transition donor ligands, and these anionic ligands have the potential to state for a bond-making or bond-breaking step. A specific inaccept and release protons and facilitate dehydrogenation stance of hydrogen bond stabilization of a transition state is in chemistry. Thus, the synthesis of reduced-ligand complexes of “dissociative b-hydride abstraction” reactions.[9] In these situaAlIII over the preceding 30 years has laid the groundwork for tions, a hydrogen-bonded six-membered ring generally defines the use of AlIII complexes in promoting ligand-based proton the transition state for hydride transfer. A further report of hytransfer reactivity (Scheme 1).[15–17] Indeed, proton transfer from drogen bonding involved in transition state stabilization is in a-iminopyridine derivatives to Rh-bound O2 has been previoushydrogenation chemistry. Often mediated by acidic transition ly documented.[18] Many of these ligands have also been emmetal ions, the transition state for hydrogenation of organic ployed to enhance the redox reactivity of transition elements, substrates includes a six-membered cyclic transition state including applications in electrocatalytic hydrogen evolution, formed by hydrogen bonding between substrate and metal in hydrosilylation catalysis, and the synthesis of Pd–Ir dimeric catalyst.[10] Finally, various systems for the catalytic dehydrocomplexes.[19] genation of formic acid have been shown to depend on the In principle, an aluminum complex of the form (L)2AlX (L = protonation state of the ligand, or on the pKa of the reaction bidentate redox-active ligand, X = anionic monodentate ligand) solution when aqueous media is involved.[11, 12] These observacould give access to five successive redox states if each ligand tions point to important contributions from hydrogen bonding L can access three redox states, for example L, L , and L2 . during the catalytic cycles. Following on from the discussion of With a tridentate ligand, L’, a complex of the form (L’)AlX2 (L’ = Al–ligand cooperation in mediating polar bond activation, this could exist in four oxidation states: L’, L’1 , L’2 , and L’3 . By juConcept article will describe the role of the ligand protonation dicious choice of redox-active ligand, these redox states could state and metal–ligand cooperation in promoting b-hydride be accessed at a wide range of potentials. abstraction reactions during a catalytic cycle. Current underThis work will describe the use of the iminopyridine ligand standing of our ability to tune the reactivity of aluminum– (henceforth denoted IP) which can exist in three redox states, amido complexes for various catalytic dehydrogenation reacand the bis(imino)pyridine ligand (RI2P) which can exist in four tions is also discussed. redox states (Scheme 2). In addition, the two-electron reduced R Directly concerning Al complexes, there have been prior I2P ligand (RI2P2 ) can access multiple electronic states. Comdemonstrations of ligand-based protonation and deprotonaplexes of the transition series ions have been previously studtion chemistry. Bertrand and co-workers reported cooperative ied using these ligands, and many insights into electronic activation of HCl across an Al amido bond,[13] while Fedushkin structure and reactivity were learned from those studies.[20] and co-workers have reported several examples of reduced Moreover, IP and I2P ligands fall into the class of ligands BIAN (BIAN = bis[N-(2,6-diisopropylphenyl)imino]acenaphthene) known as a-imino pyridines and more broadly a-diimines. complexes of Group 13 elements that can activate acidic subPrior to, and concurrent with, our own work in this area, restrates through metal–ligand cooperation.[14] Building on this duced a-diimine complexes of Al,[16] and bis(imino)pyridine work, we describe herein a new concept in aluminum(III) complexes of Al,[17] have been synthesized and characterized chemistry in which ligand–mediated proton transfer facilitates (Scheme 1). In addition, van Koten and co-workers have exthe dehydrogenation of substrates such as anilines and formic plored the reactivity of IP analogues and substituted a-diiacid. In this chemistry, metal–ligand cooperative interactions mines with trialkyl aluminum precursors.[21] promote polar bond cleavage, support insertion reactions, and Complexes (IPn )2AlX were isolated in four oxidation states: facilitate hydrogen bonding interactions that promote b-hy[(IP)(IP1 )AlX] + , (IP1 )2AlX, [(IP1 )(IP2 )Al(OH)], and [(IP2 )Al] dride abstraction reactions. Each of these elementary steps (X = anionic, monodentate ligand, Scheme 3).[22] Understanding have important roles in many other catalytic reaction cycles the electronic structures of the complexes has aided us in unand so Al-mediated catalysis could find broader application. derstanding some of the reactivity presented later in this Concept Article. Al complexes of the IP ligand are paramagnetic when one or two of the ligands has the 1 oxidation state. Reduced-Ligand Complexes of Aluminum The electronic structure is most interesting in complexes which have two radical ligands, such as (IP2 )2AlX (X = Cl, SR, AlIII complexes that contain reduced redox-active ligands provide a synthetic route to access Al compounds with anionic CCPh, Me). Using SQUID magnetometry we observed that the Chem. Eur. J. 2015, 21, 2734 – 2742

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Scheme 4. Magnetic and electronic coupling parameters are dependent on the exchange coupling parameter q. The magnitude of coupling increases with decreasing q.

Scheme 2. Common oxidation states and electronic structures of IP and RI2P, pictured as observed when bound to AlIII.

Scheme 3. Examples of complexes (IPn )2AlX and (PhI2Pn )AlX that have been synthesized in different ligand oxidation states. Ligand oxidation states are noted below each figure.

magnetic moments of these complexes fall within the range of 2.0–2.6 mB at room temperature, which is consistent with two unpaired electrons. The magnetic moments decrease significantly at low temperature. This evidence for antiferromagnetic exchange coupling between the two IP1 ligands suggested that the ground state of the molecules could be described as a singlet biradical. Of further note, we observed that the Chem. Eur. J. 2015, 21, 2734 – 2742

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strength of the coupling was dependent on the dihedral angle between the IP1 ligand planes (Scheme 4).[23] The electronic coupling, determined by cyclic voltammetry measurements, displayed a similar dependence on the dihedral angle between the IP ligands. In the absence of accessible dorbitals on the AlIII center, an alternative pathway for the existence of exchange coupling between ligand-based radicals is needed to explain the observed data. Based on the experimental evidence collected, it was proposed that the pathway for exchange coupling is through the s* orbital of the Al X bond. We have also observed evidence for covalent contributions in the bonding in the complexes. More polarizable, less electronegative X ligands afforded complexes in which the IP1 ligands are comparatively more reduced (according to the bond lengths derived from crystal structures). Complexes with general formulation (PhI2Pn )AlX2 were isolated in two oxidation states, where n = 1, 2.[24] The most notable feature of the electronic structure of complexes of the twoelectron reduced tridentate I2P ligand, (PhI2P2 )AlX, is the ability of the 2 oxidation state to support multiple resonance forms of I2P. The most commonly observed resonance form is asymmetric with reduction localized at the pyridyl and one of the imino donor ligands (Scheme 2). Less common, and observed by us only in four-coordinate complexes of AlIII, is a ligand structure in which each of the C N imino bonds is equally reduced, and the ligand has mirror symmetry. Also unusual, the four-coordinate complexes we have reported with the chelating (PhI2P2 ) ligand are square planar. (PhI2P2 )AlCl has t4 = 0.22, and (PhI2P2 )AlH has a crystallographically imposed t4 = 0.13.[25] The rest of Group 13 provides only one other example of a square-planar MIII ion with [(bpy)InCl] + , where bpy = 2,2’-bipyridine.[26] These square-planar Al complexes are acidic and this is evidenced by their ability to bind Lewis bases such as 2,6-lutidine and triethylphosphine. The planar arrangement of the ligand donors around Al is likely correlated with the highly delocalized electronic structure for PhI2P2 in these complexes. However, we have no appropriate tetrahedral molecules that we could use for comparison of bond length and angle metrics. It is clear that the Al–ligand bond lengths in the square-

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Concept planar Al complexes are significantly shorter than those we observed in five-coordinate Al complexes, but this could be due simply to a larger ionic attraction between metal and ligands when there are four, as compared with five, ligands coordinated to one Al center. For example the Al Cl bond length in four-coordinate (PhI2P2 )AlCl is 2.1452(9) , and in five-coordinate (PhI2P2 )Al(THF)Cl is 0.032  longer: 2.1772(5) .

Table 1. Comparison of the pKa values of various substrates with their observed reactivity with (PhI2P2 )Al(THF)H. pKa values are obtained from measurements reported in DMSO.[28]

Ligand-Based Proton Transfer at Aluminum Complexes We have recently shown that AlIII complexes of the PhI2P ligand in the oxidation state 2 can activate polar bonds. Modestly acidic substrates, such as anilines and water (henceforth denoted generally as RX H), protonate the I2P2 ligand of (PhI2P2 )Al(THF)H at the amido donor of the ligand and result in heterolytic cleavage of the X H bonds and formation of (PhHI2P1 )Al(XR)H complexes (Scheme 5).[23, 27] The subsequent reactivity of the X H bond activated complexes varies depending on the substrate (Scheme 6), and the

Scheme 5. Heterolytic cleavage of polar bonds by (PhI2P2 )Al(THF)H.

Scheme 6. Various reaction pathways observed following polar bond activation by (PhI2P2 )Al(THF)H. + 1 H, + 2 H, and + 3 H labels denote the number of times the product has been protonated with respect to the starting (PhI2P2 )Al(THF)H: this nomenclature is used in Table 1.

reaction pathway appears dependent on the pKa value of the substrate, and a closely correlated property, the electron-donating ability of the conjugate base coordinated to the AlIII center following X H bond activation (Table 1). Weakly acidic substrates such as anilines (where the conjugate base, the anilido ligand, is relatively donating) can support H2 liberation when the N H bond activated product is heated. In these thermolysis reactions, the Al-containing product of bond activation Chem. Eur. J. 2015, 21, 2734 – 2742

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Substrate

pKa

Protonated[a]

tert-butylamine tert-butanol water aniline octanol acetone phenol tosylamine acetic acid lutidinium anilinium

~ 40 32.2 31.4 30.6 ~ 29 26.5 18 16 4.6 4.5 3.8

+ 1H + 1H + 2H + 1H + 1H + 1H + 1H + 2H + 3H + 3H + 3H

H2 with heat? yes no no yes no no no no no no no

[a] See Scheme 6 for pictures of the protonated products that define the + 1 H, + 2 H, and + 3 H nomenclature.

is five-coordinate with an asymmetric ligand electronic structure on the PhHI2P1 complex. Upon heterolytic loss of H2, the resulting four-coordinate AlIII complex has the symmetric ligand electronic structure (Scheme 6). The PhI2P2 ligands in the four-coordinate complexes are reduced by two electrons, are diamagnetic, and the bond lengths obtained from X-ray crystallography experiments indicate that the two extra electrons are distributed over the entire ligand: both of the imino functional groups, and the pyridyl ring are partially reduced to afford a ligand with mirror symmetry. The significant deviation from square planar in the structure of (PhI2P2 )Al(N(H)iPr2Ph) appears to be a result of the sterically bulky diisopropylanilido ligand. Sufficiently acidic substrates (pKa ~ 16) will liberate H2 upon protonolysis of the Al–hydride when a second equivalent of substrate is added.[29] This second reaction pathway has been observed by us with water, tosylamine, and formic acid, amongst others. When the substrate is even more acidic (pKa < 5), a third equivalent of substrate will protonate the PhI2P2 ligand a second time, this time at the imine carbon atom. This step affords a complex of the twice-protonated ligand PhH2I2P (Scheme 6). In the case of water as a substrate, our observation of the twice protonated product ( + 2 H) has been characterized crystallographically. In the other examples listed in Table 1, 1H NMR spectroscopy has provided confirmation of the ligand protonation. Our observations of thrice protonated complexes ( + 3 H) have been limited to NMR experiments which definitively identify the resonances corresponding to protonated ligand. The thrice protonated products from HOAc and H2NTs addition are stable at room temperature.

Catalysis Mediated by Ligand-Based Proton Transfer Bond activation is well-established as an elementary reaction step that can lead to catalytic chemical transformations, and we have observed that metal–ligand cooperative bond activation can initiate catalytic dehydrogenation of a number of substrates. Initially with amines, we observed that substrates con-

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Concept

Scheme 7. Dehydrogenative coupling of benzylamine.

taining b hydrogen atoms proceed past the N H bond cleavage step. Benzylamine, in the presence of 25 % loading of (PhI2P2 )Al(THF)H, is converted with 75 % efficiency into the homocoupled imine product, and both H2 and NH3 were observed as byproducts in the reaction (Scheme 7). Catalysis in this reaction was fast, but short-lived and we identified the cause of catalyst death as sensitivity of (PhI2P2 )Al(THF)H to the NH3 formed during catalysis: just four turnovers were observed in these systems.[23] Based on the stoichiometric reactions we have observed with anilines, we presumed that this reaction proceeded via an initial N H activation step, loss of H2 heterolytically, and then a b-hydride abstraction so that benzylimine was formed as an intermediate (Scheme 8). However, no experimental proof was

via direct b-hydride abstraction.[32] In the case of amine boranes, however, it is a weaker B H bond, rather than a C H bond that is cleaved. Thus, the idea that a discreet Al–hydride intermediate is formed during the dehydrogenative coupling of benzylamine is unusual. When the 5:2 formic acid–triethylamine adduct is used as the source of formic acid in THF at 65 8C, we observed catalytic conversion of formic acid selectively into CO2 and H2 with an initial turnover frequency for the reaction of 5200 turnovers per hour.[29] The reaction proceeded for over 2000 turnovers. When anhydrous formic acid is used then the turnover number is just 100, and the initial rate of reaction also decreased and was 200 turnovers per hour. Our studies on the catalytic dehydrogenation of formic acid, initiated by O H bond activation of the carboxylic acid group, have provided the most detailed insight into the mechanism of dehydrogenation catalysis promoted by (PhI2P2 )Al(THF)H. To elucidate the initial steps in the catalytic cycle a series of experiments were performed at low temperature. Low-temperature NMR experiments were used to interrogate the initial products of reaction between (PhI2P2 )Al(THF)H and formic acid. Reaction of (PhI2P2 )Al(THF)H with one equivalent of formic acid results in protonation of the amido ligand (Scheme 6). A second and third equivalent of formic acid added at low temperature afforded H2 and a complex that is protonated twice at the ligand: [(PhH2I2P)Al(OOCH)2] + . The doubly protonated ligand has two diastereomers and the aromatic pyridine ring is also very distinctive by 1H NMR spectroscopy. This same product, from reaction of (PhI2P2 )Al(THF)H with three equivalents of acid, was also detected by using 1H NMR spectroscopy when an aliquot was removed from a catalytic run. Based on these observations we believe that the doubly protonated complex: [(PhH2I2P)Al(OOCH)2] + , is the active catalyst in solution. Following from these studies on the stoichiometric reactions that form the active catalyst, we also investigated the proposed b-hydride abstraction step where CO2 is liberated and the Al–hydride bond is formed (Scheme 9). We found that in the absence of protons, insertion of CO2 at 1 atm and room

Scheme 8. Proposed mechanism for dehydrogenative coupling of benzylamine.

obtained for this reaction mechanism, and the proposal that an AlIII complex could affect b-hydride abstraction to afford an Al–hydride intermediate is a little unusual. Many Al-based catalysts perform b-hydride transfer to a hydride acceptor such as in the Meerwein–Ponndorf–Verley (MPV) reaction for ketone hydrogenation, and in the chain termination step in ethylene polymerization where hydride transfer to monomer occurs.[30] Based on these several precedents, the case for direct b-hydride abstraction by AlIII to afford a distinct Al–hydride intermediate warranted further investigation. Prior to this work, examples of sterically promoted b-hydride abstraction reactions at AlIII were known,[31] and the dehydrogenation of amine boranes using bulky Al complexes had been proposed to proceed Chem. Eur. J. 2015, 21, 2734 – 2742

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Scheme 9. Stoichiometric reactions that demonstrated the dependence of proposed outer sphere b-hydride abstraction in the presence of protons. The red H was labeled with D to confirm movement of the hydrogen during stoichiometric experiments.[29]

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Concept temperature occurred rapidly into the Al H bond of (PhI2P2 )Al(THF)H to afford (PhI2P2 )Al(OOCH): this reaction is the microscopic reverse of the proposed b-hydride abstraction step in the catalytic cycle for formic acid dehydrogenation. Upon addition of one equivalent of H2NTs, rapid elimination of CO2 was observed and (PhI2P2 )Al(HNTs)H was identified as the Al-containing product. To confirm that the Al–hydride in (PhI2P2 )Al(HNTs)H originated from the C H bond of formate, (PhI2P2 )Al(THF)D was prepared and then we could access (PhI2P2 )Al(OOCD). Following addition of H2NTs, we obtained (PhI2P2 )Al(HNTs)D. The deuterium-labeled products in this study were all identified by using 1H NMR spectroscopy and their labels were confirmed by comparison of the IR absorption bands to those of the unlabeled products. The requirement for protons to initiate the b-hydride abstraction reactions suggested to us an “outer sphere” b-hydride abstraction pathway in which the transition state for b-hydride abstraction is stabilized by a hydrogen bonding interaction with the protonated ligands to give a six-membered ring (Scheme 9). Additional support for this proposal is the rate acceleration we have reported in polar, donating solvents: these types of solvents help to stabilize the proposed, hydrogen-bonded transition states, but would decelerate the reaction by coordination to the metal center if traditional b-hydride elimination were at play.[33–35] Outer sphere b-hydride abstraction pathways have been discussed in transition metal-mediated catalytic reactions for many years. Areas where this chemistry can play a role include methanol dehydrogenation,[36] catalytic formic acid dehydrogenation,[8, 9] and in studies of stoichiometric metal formate decarbonylation.[37] The latter stoichiometric reactions are of interest partly because of proposals that metal formates might play a role as intermediates in various catalytic cycles including the water–gas shift reaction,[38] transfer hydrogenations from formate anion,[39] and CO2 hydrogenation/formic acid dehydrogenation. Recent computational studies also support this experimental work on outer sphere b-hydride abstraction pathways.[11b] The results of these stoichiometric experiments and labeling studies are consistent with a mechanism for formic acid dehydrogenation in which an outer sphere b-hydride abstraction step accounts for cleavage of the C H bond in formic acid and release of CO2. Subsequent rapid protonation of the Al–hydride to afford the observed H2 and regenerate the active catalyst, [(PhHI2P)Al(OOCH)2] + closes the cycle (Scheme 10). It should be noted that the stoichiometric reactions we performed included just 1–3 equivalents of formic acid substrate and so this differs from the catalytic reaction conditions where far greater amounts of formic acid substrate are present.

Summary and Future Outlook The use of cooperative ligands to augment the reactivity of transition metal catalysts is a vibrant and promising field where mechanistic understanding, new catalytic pathways for inorganic and organic chemical transformation, and insights into biological proton transfer pathways are being uncovered. The recent reports describing the use of Al in tandem with coChem. Eur. J. 2015, 21, 2734 – 2742

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Scheme 10. Proposed mechanism for dehydrogenation of formic acid.[29]

operative ligands add yet another dimension to the future of metal–ligand cooperation. In this Concept article we described possibilities for mediating proton transfer via ligand-based processes in reduced bis(imino)pyridine aluminum complexes and the potential for this metal–ligand cooperative behavior to promote dehydrogenation catalysis. The AlIII ion plays a very important role in this chemistry: modulating the pKa of the ligand donor atoms, binding the conjugate base following X H bond activation of substrates, and participating in the transition state for b-hydride abstraction during dehydrogenation catalysis. However, both metal and ligand, and the cooperation between them are essential for the observed proton transfer reactivity and it is the ligand that mediates the necessary proton transfer chemistry for bond activation, and provides hydrogen bond stabilization of the transition state for b-hydride abstraction. The opportunities for future development of the chemistry of proton transfer mediated by non-innocent ligands in Al complexes are rich. Even with the systems already developed, such as the dehydrogenation of formic acid by [(I2P2 )Al(THF)H], the full details of the catalytic cycle need to be uncovered and this may involve computational work. Undoubtedly, opportunities for new ligand design will evolve from these efforts. Additional efforts in understanding the thermochemistry of this reactivity, including the pKa values of the amido ligand, could also enable future catalyst design and modification. Moving forward, the chemistry of [(I2P2 )Al(THF)H] could be extended to include the reverse reactions: the hydrogenation of various substrates. And even further ahead the chemistry could evolve to a system where both proton and electron transfer are mediated by the non-innocent ligand, and this development would open the applications of the Al catalysts to a wider array of chemical transformations.

Acknowledgements This work has been partially supported by the University of California, Davis, by an Alfred P. Sloan Foundation Fellowship to L.A.B., by the United States Department of Education

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Concept through fellowships to graduate students, and by the National Science Foundation Center for Chemical Innovation (CHE1240194). I also thank the co-workers who contributed to this chemistry, in particular Cody Carr, Alexandra Holmes, Nasrin Kazem, Thomas Myers, Emily Thompson, and Toby Sherbow; and Dr. Karla Erickson for comments on the text.

[16]

[17]

Keywords: aluminum · bond activation · dehydrogenation reactions · main group chemistry · non-innocent ligands [18] [1] a) P. P. Power, Nature 2010, 463, 171; b) P. Spies, G. Erker, G. Kehr, K. Bergander, R. Froehlich, S. Grimme, D. W. Stephan, Chem. Commun. 2007, 47, 5072; c) D. W. Stephan, Top. Curr. Chem. 2013, 332, 1. [2] Another class of non-redox reactions catalyzed by transition metal ions (d0) is sigma bond metathesis: For example: a) R. Waterman, Organometallics 2013, 32, 7249; b) R. Waterman, Chem. Soc. Rev. 2013, 42, 5629; c) R. J. Less, R. L. Melen, D. S. Wright, RSC Adv. 2012, 2, 2191. [3] F. A. Cotton, G. Wilkinson, C. A. Murillio, M. Bochmann, Advanced Inorganic Chemistry, 6th ed., Wiley, New York, 1999, p. 208. [4] For example: a) R. Noyori, S. Hashiguchi, Acc. Chem. Res. 1997, 30, 97; b) G. E. Doberiner, R. H. Crabtree, Chem. Rev. 2010, 110, 681; c) C. Gunanathan, D. Milstein, Acc. Chem. Res. 2011, 44, 588. [5] a) Y. Blum, Y. Shvo, J. Organomet. Chem. 1985, 282, C7; b) H. Grtzmacher, Angew. Chem. Int. Ed. 2008, 47, 1814; Angew. Chem. 2008, 120, 1838. [6] a) R. E. Rodrguez-Lugo, M. Trincado, M. Vogt, F. Tewes, G. Santiso-Quinones, H. Grutzmacher, Nat. Chem. 2013, 5, 342; b) E. Alberico, P. Sponholz, C. Cordes, M. Nielsen, H.-J. Drexler, W. Baumann, H. Junge, M. Beller, Angew. Chem. Int. Ed. 2013, 52, 14162; Angew. Chem. 2013, 125, 14412. [7] a) B. M. Trost, Science 1991, 254, 1471; b) S. Kobayashi, R. Matsubara, Chem. Eur. J. 2009, 15, 10694; c) Y. Yamashita, S. Kobayashi, Chem. Eur. J. 2013, 19, 9420. [8] For example: a) M. D. Fryzuk, M. J. Petrella, B. O. Patrick, Organometallics 2005, 24, 5440; b) M. D. Fryzuk, P. A. MacNeil, Organometallics 1983, 2, 355; c) M. D. Fryzuk, P. A. MacNeil, S. J. Rettig, J. Am. Chem. Soc. 1987, 109, 2803. [9] S. E. Clapham, A. Hadzovic, R. H. Morris, Coord. Chem. Rev. 2004, 248, 2201. [10] R. Noyori, M. Yamakawa, S. Hashiguchi, J. Org. Chem. 2001, 66, 7931. [11] a) M. Grasemann, G. Laurenczy, Energy Environ. Sci. 2012, 5, 8171; b) E. Fujita, J. T. Muckerman, Y. Himeda, Biochim. Biophys. Acta 2013, 1827, 1031; c) R. Tanaka, M. Yamashita, K. Nozaki, J. Am. Chem. Soc. 2009, 131, 14168; d) Y. Himeda, Green Chem. 2009, 11, 2018; e) S. Fukuzumi, T. Kobayashi, T. Suenobu, J. Am. Chem. Soc. 2010, 132, 11866; f) R. Tanaka, M. Yamashita, L. W. Chung, K. Morokuma, K. Nozaki, Organometallics 2011, 30, 6743; g) J. F. Hull, Y. Himeda, W. H. Wang, B. Hashiguchi, R. Periana, D. J. Szalda, J. T. Muckermann, E. Fujita, Nat. Chem. 2012, 4, 383; h) J. H. Barnard, C. Wang, N. G. Berry, J. Xiao, Chem. Sci. 2013, 4, 1234; i) Y. Manaka, W. H. Wang, Y. Suna, H. Kambayashi, J. T. Muckerman, E. Fujita, Y. Himeda, Catal. Sci. Technol. 2014, 4, 34. [12] a) A. Boddien, D. Mellmann, F. Grtner, R. Jackstell, H. Junge, P. J. Dyson, G. Laurenczy, R. Ludwig, M. Beller, Science 2011, 333, 1733; b) T. Zell, B. Butscheke, Y. Ben-David, D. Milstein, Chem. Eur. J. 2013, 19, 8068. [13] N. Emig, H. Nguyen, H. Krautsheid, R. Rau, J. B. Cazaux, G. Bertrand, Organometallics 1998, 17, 3599. [14] a) I. L. Fedushkin, N. M. Khvoinova, A. A. Skatova, G. K. Fukin, Angew. Chem. Int. Ed. 2003, 42, 5223; Angew. Chem. 2003, 115, 5381; b) I. L. Fedushkin, A. A. Skatova, G. K. Fukin, M. Hummert, H. Schumann, Eur. J. Inorg. Chem. 2005, 2332; c) I. L. Fedushkin, A. G. Morozov, O. V. Rssasin, G. K. Fukin, Chem. Eur. J. 2005, 11, 5749. [15] For example: a) S. Herzog, K. Geisler, H. Praekel, Angew. Chem. Int. Ed. Engl. 1963, 2, 47; Angew. Chem. 1963, 75, 94; b) A. Flamini, N. Poli, Inorg. Chim. Acta 1988, 150, 149; c) R. Kçster, G. Benedikt, H. W. Schrçtter, Angew. Chem. Int. Ed. Engl. 1964, 3, 514; Angew. Chem. 1964, 76, 649; d) H. Lehmkuhl, G. Fuchs, R. Koster, Tetrahedron Lett. 1965, 6, 2511; e) W. Kaim, J. Organomet. Chem. 1980, 201, C5; f) W. Kaim, Chem. Ber. 1981, 114, 3789; g) W. Kaim, Z. Naturforsch. B 1982, 37, 783; h) J. T. Leman, A. R. Barron, J. W. Ziller, R. M. Kren, Polyhedron 1989, 8, 1909; Chem. Eur. J. 2015, 21, 2734 – 2742

www.chemeurj.org

[19]

[20] [21] [22] [23] [24] [25] [26] [27] [28]

[29] [30] [31]

[32] [33] [34] [35]

[36] [37] [38]

2741

i) G. Szigethy, A. F. Heyduk, Dalton Trans. 2012, 41, 8144; j) H. Schumann, M. Hummert, A. N. Lukoyanov, I. L. Fedushkin, Chem. Eur. J. 2007, 13, 4216. a) F. G. N. Cloke, C. I. Dalby, M. J. Henderson, P. B. Hitchcock, C. H. L. Kennard, R. L. Lamb, C. L. Raston, J. Chem. Soc. Chem. Commun. 1990, 1394; b) F. G. N. Cloke, C. I. Dalby, J. Daff, J. Green, Dalton Trans. 1991, 181; c) B. E. Cole, J. P. Wolbach, W. G. Dougherty, N. A. Piro, W. S. Kassel, C. R. Graves, Inorg. Chem. 2014, 53, 3899. a) T. Jurca, K. Dawson, I. Mallov, T. Burchell, G. P. A. Yap, D. S. Richeson, Dalton Trans. 2010, 39, 1266; b) J. Scott, S. Gambarotta, I. Korobkov, Q. Knijnenburg, B. de Bruin, P. H. M. Budzelaar, J. Am. Chem. Soc. 2005, 127, 17204. C. Tejel, M. A. Ciriano, M. Pilar del Rio, F. J. van den Bruele, D. G. H. Hetterscheid, N. Tsichlis i Spithas, B. de Bruin, J. Am. Chem. Soc. 2008, 130, 5844. For example: a) P. Chaudhuri, M. Hess, T. Weyhermuller, K. Wieghardt, Angew. Chem. Int. Ed. 1999, 38, 1095; Angew. Chem. 1999, 111, 1165; b) C. C. H. Atienza, C. Milsmann, E. Lobkovsky, P. J. Chirik, Angew. Chem. Int. Ed. 2011, 50, 8143; Angew. Chem. 2011, 123, 8293; c) S. K. Russell, E. Lobkovsky, P. J. Chirik, J. Am. Chem. Soc. 2011, 133, 8858; d) W. R. McNamara, Z. Han, P. J. Alperin, W. W. Brennessel, P. L. Holland, R. Eisenberg, J. Am. Chem. Soc. 2011, 133, 15368 – 15371; e) A. I. Nguyen, R. A. Zarkesh, D. C. Lacy, M. K. Thorson, A. F. Heyduk, Chem. Sci. 2011, 2, 166; f) C. Tejel, L. Asensio, M. Pilar del Ro, B. de Bruin, J. A. Lopez, M. A. Ciriano, Angew. Chem. Int. Ed. 2011, 50, 8893; Angew. Chem. 2011, 123, 9001. For example: V. C. Gibson, C. Redshaw, G. A. Solan, Chem. Rev. 2007, 107, 1754. G. Van Koten, T. B. H. Jastrrzebski, K. Vrieze, J. Organomet. Chem. 1983, 250, 49. T. W. Myers, N. Kazem, S. Stoll, R. D. Britt, M. Shanmugam, L. A. Berben, J. Am. Chem. Soc. 2011, 133, 8662. a) T. W. Myers, A. L. Holmes, L. A. Berben, Inorg. Chem. 2012, 51, 8997; b) C. D. Cates, T. W. Myers, L. A. Berben, Inorg. Chem. 2012, 51, 11891. T. W. Myers, L. A. Berben, J. Am. Chem. Soc. 2013, 135, 9988. E. J. Thompson, T. W. Myers, L. A. Berben, Angew. Chem. Int. Ed. 2014. DOI: 10.1002/anie.201407098. Y-J. Song, P.-C. Zhao, P. Zhang, Z.-B. Han, Z. Anorg. Allg. Chem. 2009, 635, 1454. T. W. Myers, L. A. Berben, Organometallics 2013, 32, 6647. a) F. G. Bordwell, G. E. Drucker, H. E. Fried, J. Org. Chem. 1981, 46, 632; b) W. N. Olmstead, Z. Margolin, F. G. Bordwell, J. Org. Chem. 1980, 45, 3295; c) F. G. Bordwell, D. Algrim, N. R. Vanier, J. Org. Chem. 1977, 42, 1817; d) F. G. Bordwell, Acc. Chem. Res. 1988, 21, 456; e) F. G. Bordwell, R. J. McCallum, W. N. Olmstead, J. Org. Chem. 1984, 49, 1424; f) F. G. Bordwell, H. E. Fried, D. L. Hughes, T. Y. Lynch, A. V. Satish, Y. E. Whang, J. Org. Chem. 1990, 55, 3330; g) F. G. Bordwell, D. Algrim, J. Org. Chem. 1976, 41, 2507; h) M. R. Crampton, A. I. Robotham, J. Chem. Res. 1997, 22. T. W. Myers, L. A. Berben, Chem. Sci. 2014, 5, 2771. Group 13 Chemistry III Industrial Applications (Eds.: H. W. Roesky, D. A. Atwood), Springer, Berlin, 2003. For example: a) W. Uhl, Z. Anorg. Allg. Chem. 1989, 570, 37; b) W. Uhl, A. Vester, Chem. Ber. 1993, 126, 941; c) W. Uhl, L. Marcus, Z. Anorg. Allg. Chem. 1994, 620, 1427. M. M. Hansmann, R. L. Melen, D. S. Wright, Chem. Sci. 2011, 2, 1554. P. J. Chirik, J. E. Bercaw, Organometallics 2005, 24, 5407. J. Spielmann, G. Jansen, H. Bandmann, S. Harder, Angew. Chem. Int. Ed. 2008, 47, 6290; Angew. Chem. 2008, 120, 6386. a) C. Fellay, P. J. Dyson, G. Laurenczy, Angew. Chem. Int. Ed. 2008, 47, 3966; Angew. Chem. 2008, 120, 4030; b) T. Koike, T. Ikariya, Adv. Synth. Catal. 2004, 346, 37; c) Y. Gao, J. K. Kuncheria, H. A. Jenkins, R. J. Puddephatt, G. P. A. Yap, Dalton Trans. 2000, 3212; d) W. Gan, C. Fellay, P. J. Dyson, G. Laurenczy, J. Coord. Chem. 2010, 63, 2685. a) M. Nielsen, E. Alberico, W. Baumann, H.-J. Drexler, H. Junge, S. Gladiali, M. Beller, Nature 2013, 495, 85. J. H. Merrifield, J. A. Gladysz, Organometallics 1983, 2, 782. a) T. Yoshida, Y. Ueda, S. Oteuka, J. Am. Chem. Soc. 1978, 100, 3941; b) A. D. King Jr., R. B. King, D. B. Yang, J. Am. Chem. Soc. 1981, 103, 2699; c) T. Yoshida, T. Okano, Y. Ueda, S. Otauka, J. Am. Chem. Soc. 1981, 103, 3411; d) W. A. R. Slegeir, R. S. Sapienza, R. Rayford, L. Lam, Organometallics 1982, 1, 1728; e) D. J. Darensbourg, M. B. Fischer, R. E.  2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Concept Schmidt Jr., B. J. Baldwin, J. Am. Chem. Soc. 1981, 103, 1297; f) D. J. Darensbourg, A. Rokicki, M. Y. Darensbourg, J. Am. Chem. Soc. 1981, 103, 3223; g) A. Rokicki, J. Am. Chem. Soc. 1982, 104, 349; h) D. J. Darensbourg, A. Rokicki, Organometallics 1982, 1, 1685.

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www.chemeurj.org

[39] R. Bar, Y. Sasson, J. Blum, J. Mol. Catal. 1982, 16, 175. Received: September 25, 2014 Published online on November 27, 2014

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Catalysis by aluminum(III) complexes of non-innocent ligands.

Non-Innocent ligand complexes of aluminum are described in this Concept article, beginning with a discussion of their synthesis, and then structural a...
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