Tuning the electronic properties in ruthenium-quinone complexes through metal coordination and substitution at the bridge.

DOI: 10.1002/chem.201204379

Full Paper

& Dinuclear Ruthenium Complexes

Tuning the Electronic Properties in Ruthenium–Quinone Complexes through Metal Coordination and Substitution at the Bridge Hari Sankar Das,[a] David Schweinfurth,[a, b] Jan Fiedler,[c] Marat M. Khusniyarov,[d] Shaikh. M. Mobin,[e] and Biprajit Sarkar*[a, b]

Abstract: A rare example of a mononuclear complex [(bpy)2Ru(L1H)](ClO4), 1(ClO4) and dinuclear complexes [(bpy)2Ru(m-L12H)Ru(bpy)2](ClO4)2, 2(ClO4)2, [(bpy)2Ru(mL22H)Ru(bpy)2](ClO4)2, 3(ClO4)2, and [(bpy)2Ru(m-L32H)Ru(bpy)2](ClO4)2, 4(ClO4)2 (bpy = 2,2’-bipyridine, L1 = 2,5-di-(isopropyl-amino)-1,4-benzoquinone, L2 = 2,5-di-(benzyl-amino)1,4-benzoquinone, and L3 = 2,5-di-[2,4,6-(trimethyl)-anilino]1,4-benzoquinone) with the symmetrically substituted p-quinone ligands, L, are reported. Bond-length analysis within the potentially bridging ligands in both the mono- and dinuclear complexes shows a localization of bonds, and binding to the metal centers through a phenolate-type “O” and an immine/imminium-type neutral “N” donor. For the mononuclear complex 1(ClO4), this facilitates strong intermolecular hydrogen bonding and leads to the imminium-type character of the noncoordinated nitrogen atom. The dinuclear

Introduction Quinones are a well-known class of molecules that take part in various electron-transfer processes.[1] The ability of quinones and related ligands to exist in various redox states in their metal complexes has led to the use of the term “non-innocent [a] Dr. H. S. Das,+ Dipl.-Chem. D. Schweinfurth,+ Prof. Dr. B. Sarkar Institut fr Anorganische Chemie, Universitt Stuttgart Pfaffenwaldring 55, 70550, Stuttgart (Germany) [b] Dipl.-Chem. D. Schweinfurth,+ Prof. Dr. B. Sarkar Institut fr Chemie und Biochemie, Anorganische Chemie Freie Universitt Berlin, Fabeckstraße 34-36, 14195, Berlin (Germany) E-mail: [email protected] [c] J. Fiedler J. Heyrovsky´ Institute of Physical Chemistry, v.v.i., Academy of Sciences of the Czech Republic, Dolejsˇkova 3, 18223 Prague (Czech Republic) [d] Dr. M. M. Khusniyarov Department Chemie und Pharmazie, Friedrich-Alexander Universitt Egerlandstraße 1, 91058, Erlangen (Germany) [e] S. M. Mobin National Single-Crystal X-ray Diffraction Facility Indian Institute of Technology Bombay, Powai, Mumbai, 400076 (India) [+] These authors contributed equally to this work. Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201204379. Chem. Eur. J. 2014, 20, 4334 – 4346

complexes display two oxidation and several reduction steps in acetonitrile solutions. In contrast, the mononuclear complex 1 + exhibits just one oxidation and several reduction steps. The redox processes of 11 + are strongly dependent on the solvent. The one-electron oxidized forms 23 + , 33 + , and 43 + of the dinuclear complexes exhibit strong absorptions in the NIR region. Weak NIR absorption bands are observed for the one-electron reduced forms of all complexes. A combination of structural data, electrochemistry, UV/Vis/ NIR/EPR spectroelectrochemistry, and DFT calculations is used to elucidate the electronic structures of the complexes. Our DFT results indicate that the electronic natures of the various redox states of the complexes in vacuum differ greatly from those in a solvent continuum. We show here the tuning possibilities that arise upon substituting [O] for the isoelectronic [NR] groups in such quinone ligands.

ligands” for such molecules.[2] The interaction between ruthenium centers and non-innocent ligands has fascinated chemists because of the valence ambiguity arising in such complexes as a result of the close proximity of the metal dp and ligandbased p orbitals. Such proximity of the metal- and ligandbased orbitals in these complexes makes the exact description of electronic states challenging both experimentally and theoretically.[3] The non-innocent character of ligands has led to the concept of their use as electron reservoirs, leading to the activation of small molecules and catalysis.[4] This concept has been employed in ruthenium complexes of non-innocent ligands, and such complexes have been used as catalysts for water oxidation and in dye-sensitized solar cells.[5] Our group has been developing synthetic methodologies for potentially bridging, substituted quinone ligands. The coordination abilities and electronic properties of these ligands have also been investigated.[3g,i, 6] In this context, we have recently reported two different synthetic routes for the preparation of substituted p-quinone ligands.[6b,c] One of the routes makes use of the well-known 2,5-dihydroxy-1,4-benzoquinone (DHBQ),[6c] and the other proceeds through transamination of 2,5-diamino-1,4benzoquinone (DABQ).[6b] The doubly deprotonated form (DHBQ2, Scheme 1) of 2,5-dihydroxy-1,4-benzoquinone has been used extensively as a bridging ligand in coordination

4334

2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

Full Paper purified by column chromatography (see the Experimental Section). The purity of the complexes was verified by using 1H NMR spectroscopy, elemental analysis, and mass spectrometry. The 1H NMR spectra of 2(ClO4)2–4(ClO4)2 showed a mixture of the rac and meso diastereomers. No attempts were made to separate these isomers, and all further studies were made with a mixture of diastereomers. All the isolated solids were diamagnetic, and did not display any EPR signals at either room temperature or low temperatures.

Scheme 1. [O,O,O,O] and [O,N,O,N] donor-containing quinonoid ligands.

chemistry.[7] However, the tuning of the steric and electronic properties in that ligand is limited to substitution at the C3 and C6 positions of the six-membered ring.[2n] Therefore, we have focused further on the synthesis of ligands in which [O] is substituted with the isoelectronic [NR] groups. Such substitution makes tuning of the steric and electronic properties of the complexes possible through the R groups of [NR].[6b,c] In the following, we present the synthesis of the mononuclear complex [(bpy)2Ru(L1H)](ClO4), 1(ClO4) and dinuclear complexes [(bpy)2Ru(m-L12H)Ru(bpy)2](ClO4)2, 2(ClO4)2, [(bpy)2Ru(mL22H)Ru(bpy)2](ClO4)2, 3(ClO4)2, and [(bpy)2Ru(m-L32H)Ru(bpy)2](ClO4)2, 4(ClO4)2 (bpy = 2,2’-bipyridine, L1 = 2,5-di-(isopropylamino)-1,4-benzoquinone, L2 = 2,5-di-(benzyl-amino)-1,4-benzoquinone, and L3 = 2,5-di-[2,4,6-(trimethyl)-anilino]-1,4-benzoquinone). Mononuclear complexes such as 1(ClO4) are extremely rare in the literature because the symmetric nature of the L ligands leads to comparable acidities of both the NH protons. A consequence of this is the difficulty of the targeted synthesis and isolation of mononuclear complexes with such ligands. In the present case, the mononuclear complex 1(ClO4) is used as a standard to discuss the properties of the dinuclear complexes. Structural data of 1(ClO4) and 2(ClO4)2 are presented, and these data are used to elucidate the bonding situation within the ligands in these complexes. A combination of cyclic voltammetry, UV/Vis/NIR and EPR spectroelectrochemistry, and DFT calculations is used to elucidate the electronic structure of the compounds and determine the site of electron transfer. The solvent dependence of the electrochemical properties of 1 + is reported. The complexes reported here are compared with the parent compound [(bpy)2Ru(m-DHBQ2)Ru(bpy)2]2 + , 52 + ,[7a] and with the compounds [(bpy)2Ru(m-DABQ2)Ru(bpy)2]2 + , 62 + ,[3e] [(tmpa)2Ru(m-L32H)Ru(tmpa)2]2 + , 72 + (tmpa = tris(2-pyridylmethyl)amine),[3i] and [(acac)2Ru(m-L12H)Ru(acac)2], 8 (acac = acetylacetonato).[6b] Particular stress is put on the DFT calculations of these complexes, through which we show the absolute necessity of incorporating solvent interactions to describe the real electronic situation in complexes with such closely spaced electronic states.

Scheme 2. Ruthenium complexes described in this work.

The complexes 1(ClO4) and 2(ClO4)2 could be crystallized through slow diffusion of a dichloromethane solution layered with n-hexane (1/2) at ambient temperatures. 1(ClO4) crystallizes in the monoclinic C2/c space group. The ruthenium center in 1(ClO4) is in a distorted octahedral environment, and is coordinated by an oxygen and a nitrogen atom from L1H and four nitrogen atoms of the two bpy ligands (Figure 1). The RuN and RuO bond lengths are in the expected range (Table 1). Bond-length analyses within the L1H ligand show relatively long C1O1 and C4O2 bond lengths of 1.280(4) and 1.257(4) , respectively. The C6N1 and C3N2 bond lengths of 1.328(4) and 1.309(4) , on the other hand, are relatively short. The CC bonds, particularly C4C5 at 1.387(5) and C5C6 at 1.413 , show an alternation of one short and one

Results and Discussion Synthesis and crystal structure The complexes 1(ClO4) and 2(ClO4)2–4(ClO4)2 (Scheme 2) were synthesized from [Ru(bpy)2(EtOH)2](ClO4)2 and the ligands L1–L3, respectively, in refluxing ethanol in the presence of a base, and Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

Figure 1. ORTEP plot of 1(ClO4). Protons (except NH) and perchlorate ions have been omitted for clarity. Ellipsoids are drawn at 50 % probability.

4335


Full Paper Table 1. Selected bond lengths [] and bond angles [8] for 1(ClO4). Bond C1C2 C2C3 C3C4 C4C5 C5C6 C6C1 O1C1 C4O2 N1C6 N2C3 Ru1O1 Ru1N1

Bond length [] 1.387(4) 1.388(4) 1.519(5) 1.387(5) 1.413(4) 1.491(4) 1.280(4) 1.257(4) 1.328(4) 1.309(4) 2.050(2) 2.103(3)

Bond

Bond angle [8]

N1Ru1N6 N4Ru1N6 N5Ru1N6 O1Ru1N6 N1Ru1N3 N3Ru1N4 N3Ru1N5 O1Ru1N3 N1Ru1O1 N1Ru1N4 N4Ru1N5 O1Ru1N5 N1Ru1N5 O1Ru1N4 N3Ru1N6

95.97(10) 95.73(11) 78.77(10) 87.94(10) 87.41(10) 78.69(11) 99.09(10) 97.48(10) 77.83(9) 106.18(10) 88.41(10) 87.89(9) 165.00(10) 174.19(10) 174.13(10)

Figure 2. Intermolecular hydrogen bonding in 1(ClO4) in the solid state. Hydrogen-bonding parameters given in . DonorH 0.880(.003); donor···acceptor 2.617(.004); H···acceptor 2.188(.003); donorH···acceptor 109.54(0.19); N2H2···O2 (0) (0) x,y,z.

long bond. The C1C6 and C3C4 bond lengths of 1.491(4) and 1.519(5) , respectively, lie in the range of authentic CC single bonds. Thus, of the various plausible localized structures (Scheme 3), the bond-length analysis seems to support the formulation shown in Scheme 3 a, with the mono-deprotonated

Scheme 3. Three possible descriptions of complex 1 + .

ligand acting as a bis(phenolate)-p-diimine-type donor. The ruthenium center is then coordinated through an O and a neutral imine donor from L1H. This type of preferred coordination has been observed previously for dinuclear ruthenium complexes with substituted p-quinone ligands.[3e,i, 6b,c] We have presented here structural evidence for a mononuclear complex with such ligands. This structural motive is additionally stabilized through strong intermolecular hydrogen bonding between the noncoordinated, formally iminium-type nitrogen and the non-coordinated, formally phenolate-type O (Figure 2 and data in the caption). An alternative description would be a coordination-induced p-quinone-to-o-quinone tautomerism, as shown in Scheme 3 b. However, the data discussed above as well as the RuN bond lengths favor the formulation depicted in Scheme 3 a. The third alternative, shown in Scheme 3 c, requires shorter C=O and longer CN bonds, and can therefore be ruled out in the present case. The dinuclear complex 22 + could also be crystallized. However, despite the many attempts made, the crystal quality was not improved beyond a certain point, so unfortunately, the diffraction data is not of very high quality. This precludes a detailed discussion of bond lengths within this complex. HowevChem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

er, the connectivity pattern is seen clearly (Figure S1, Supporting Information). As expected, each ruthenium center is coordinated through four N atoms of the two bipyridine rings and through the oxygen and nitrogen atoms of the bridging ligand L12H. Of the two diastereomers observed in solution through 1 H NMR spectroscopy, the meso isomer crystallizes preferentially, as seen from the ORTEP plot in Figure S1. Given the literature precedence of related compounds[3e,i, 6b,c] and the mononuclear complex 1 + discussed above, it is to be expected that the bridging ligand L12H binds to the metal centers through O and a neutral imine nitrogen. Hence, the structure of the complexes in solution, as determined by 1H NMR spectroscopy, matches well with the structure in the solid, as determined by singlecrystal X-ray diffraction. Therefore, the isolated solid has essentially the same chemical composition and structure in solution. Cyclic voltammetry Cyclic voltammetric studies were made to investigate the redox processes of the synthesized complexes. The mononuclear complex 1 + shows one oxidation (0.09 V) and several reduction steps, the first of which occurs at 1.62 V in CH3CN/ 0.1 m Bu4NPF6 at 295 K (Figure 3 and Table 2). Electron counts confirmed the number of electrons in each of the waves discussed below. The mononuclear complex has free NH and O groups on the noncoordinated side of the ligand L1H and shows strong intermolecular hydrogen bonding in the solid state, so we decided to study the solvent dependence of the redox potentials of 1 + . The redox potentials do not show very significant shifts on changing the polarity of the solvent (from dichloromethane to acetonitrile to dimethylformamide), as seen in Figure 4 and Table 3. However, on using a protic solvent such as methanol, the first reduction potential shifts significantly to 1.35 V. The effect of the protic solvent on the second reduction potential of 1 + is comparatively less significant (Table 3). Reduction of the ruthenium center can be ruled out because of the instability of the ruthenium(I) state with the ligands reported herein.[8] The OH group of methanol is likely to participate in hydrogen

4336


Full Paper Table 3. Oxidation and reduction potentials of complex 1(ClO4) in different solvents.[a] Solvent

E1/2ox1 [b]

E1/2red1 [b]

E1/2red2 [b]

DCM DMF MeCN MeOH

0.03 (irr) 0.00 (irr) + 0.09 (irr) 0.09 (irr)

1.67 1.68 1.62 1.35

2.13 2.07 2.03 1.97

(rev) (rev) (rev) (irr)

(rev) (rev) (rev) (irr)

[a] Electrochemical potentials in V vs. Fc + /0 from cyclic voltammetry in different solvents/0.1 m Bu4NPF6 at 298 K. Scan Rate: 100 mV s1. [b] irr = irreversible, rev = reversible.

This is probably because methanol changes the chemical composition of 1 + by participating in hydrogen bonding with it. Figure 3. Cyclic voltammogram of 1 + , 22 + , 32 + , and 32 + in CH3CN/0.1 m Compared with the first reduction potential, the first oxidation Bu4NPF6 at 295 K. potential shows only a marginal shift on changing the solvent from acetonitrile to methanol (Figure S2, Table 3). Such a small Table 2. Redox potentials of the complexes.[a] shift is a first indication that the first oxidation step is not cenox2 ox1 [c] red1 red2 red3 red4 red5 Complex E1/2 E1/2 Kc E1/2 E1/2 E1/2 E1/2 E1/2 tered exclusively on the L1H [b] [b] [b] [b] [b] [b] [b] (DEp) (DEp) (DEp) (DEp) (DEp) (DEp) (DEp) ligand in 1 + (vide infra). 1+ + 0.09 (92) – 1.62 (69) 2.03 (91) 2.32 (104) – – The dinuclear complex 22 + + 0.34 (78) 0.14 (68) 1.3 108 1.96 (73) 2.12 (68) 2.27 (63) 2.40 (68) 2.69 (130) 22 + 2+ 8 [d] displays two oxidation processes + 0.53 (95) + 0.00 (84) 9.8 10 1.76 (75) 2.05 (150) 2.31 (85) – – 3 + 0.35 (95) -0.23 (78) 6.3 108 1.92 (73) 2.32 (135)[d] 2.59 (90) 2.75 (95) – 42 + at 0.14 and 0.34 V in CH3CN/ 0.1 m Bu4NPF6 at 295 K (Figure 3 [a] Electrochemical potentials in V vs. Fc + /0 from cyclic voltammetry in CH3CN/0.1 m Bu4NPF6 at 298 K. Scan and Table 2). Thus, the coordinaRate: 100 mV s1. [b] DEp : difference between peak potentials in mV. [c] Kc = 10DE/59, DE = E1/2ox2E1/2ox1 in mV. [d] Overlap of two one-electron transfer waves. tion of a second Ru(bpy)2 center to 1 + and removal of a proton from L1H results in a negative 1 shift of the first oxidation potential and the emergence of bonding with the NH and O (noncoordinating) groups of L H a second oxidation step within the solvent window of acetoniin 1 + . Such a phenomenon is responsible for the large shift in trile. The removal of a proton and coordination of a second the first reduction potential of 1 + on moving from acetonitrile metal center shifts the HOMO of the complex to higher to methanol, and also indicates that the first reduction step is energy. The difference between the two oxidation potentials based on L1H. The shift of the second reduction potential is translates to a comproportionation constant (Kc)[9] value of the much lower on changing the solvent from acetonitrile to order of 108 (Table 3) for the odd-electron 23 + form. The first methanol, so this step is likely to be centered on the bpy ligand. The use of methanol as a solvent also renders the first reduction potential is shifted from 1.62 V in 1 + to 1.96 V in 22 + . Similarly to the oxidation potential, the reduction potenreduction step electrochemically irreversible, with a peak-totials also show a negative shift on moving from the mononupeak difference of more than 500 mV (Figure 4 and Table 3). clear to the dinuclear complex. Additionally, several other reduction steps appear for 22 + in comparison to 1 + , because of the reduction of the additional bpy ligands that are bound to the second Ru(bpy)2 center in 22 + . A change in the substituents on the nitrogen atoms of the bridging ligands from isopropyl (22 + ) to benzyl (32 + ) results in a positive shift of all the redox potentials (Table 2). This is a result of changing the isopropyl groups with their positive inductive effect with the aromatic phenyl ring. On moving further to 42 + , which has mesityl substituents on the nitrogen atoms of the bridge, a reverse trend is observed. All the redox potentials of 42 + are shifted negatively compared with both 22 + and 32 + . The effect of the introduction of a phenyl substituent directly on the nitrogen donor atoms seems to be overcompensated by the three methyl substituents on the Figure 4. Solvent dependence of the cyclic voltammogram (reduction side) phenyl ring with their strong positive inductive effect. The of [1] + with 0.1 m Bu4NPF6 at 295 K. Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

4337


Full Paper order of the Kc values for the one-electron oxidized forms 33 + and 43 + are in a range comparable to that of 23 + (Table 2). Whereas the bpy-based reductions are all well-resolved oneelectron transfer processes in 22 + , the second and third reductions for 32 + and 42 + merge owing to the overlap of two potentially close one-electron steps (Figure 3 and Table 2). Thus, the electrochemical coupling between the bpy units seems to be larger in 22 + than in 32 + or 42 + . The literature-known parent compound 52 + containing the all-oxygen [O,O,O,O] donor set in the bridge has its oxidation potentials at 0.36 and 0.70 V and the first reduction potential at 1.05 V.[7a] On moving from the [O,O,O,O] donor set in the bridge of 52 + to the [O,N,O,N] donor set for 22 + –42 + in the present case, the redox potentials are all significantly shifted in the negative direction. The oxidation steps thus occur at less positive potentials for 22 + –42 + than for 52 + , and the reduction processes occur at more negative potentials. This effect is related to the higher electronegativity of the [O] donors compared with their [NR] counterparts, which results in the stabilization of the orbitals in 52 + compared with those in 22 + –42 + . However, it should be noted that the shift in the reduction potentials on moving from 52 + to 22 + –42 + is much larger than the shift in the oxidation potentials. This suggests a dominant participation of the bridging ligand in the reduction, in contrast to the rather metal-centered oxidation (vide infra). The Kc value for 53 + is only of the order of 105,[7a] as compared with the range of 108 for 23 + –43 + . Thus, the thermodynamic stability of the one-electron oxidized forms of the complexes increases significantly on moving from an [O,O,O,O] donor set to an [O,N,O,N] donor set. On comparing 22 + –42 + with 62 + , which contains the doubly deprotonated form of DABQ as a bridging ligand, it is seen that the addition of a substituent on the nitrogen atoms of the bridge (other than H) leads to a cathodic shift of all the redox potentials.[3e] The Kc value for the mixedvalent forms, however, remains of the order of 108 in all cases. Changing the strongly p-accepting bpy coligand in 42 + with tmpa in 72 + , but keeping the bridging ligand the same, leads to a cathodic shift of all redox potentials and a destabilization of the RuII-based HOMO.[3i] An extreme case of this destabilization occurs in 8, which has the strongly donating acac as coligands. In 8, ruthenium is stabilized in the + 3 state in comparison to + 2 for 22 + –72 + .[6b] The complete reversibility of the redox waves, which is vital for a discussion of the spectroscopic properties of the various redox states, was further established in spectroelectrochemical experiments (see below).

Table 4. EPR data of one-electron reduced forms of the complexes.[a] Complex

giso[b]

A (99,101Ru)[c]

A (14N)[c]

1C 2+ 3+ 4+

2.000 (2.005) 2.000 (2.004) 2.003 1.998 (2.001)

n.o.[d] 2.6 2.5 2.4

n.o.[d] 5.1 5.2 4.8

[a] EPR data of species generated by in situ electrolysis in CH3CN/0.1 m Bu4NPF6. The spectra were recorded at 295 K. [b] DFT-calculated g values are given in parentheses. [c] Hyperfine coupling constant in Gauss obtained from simulation. [d] Not observed.

vent dependence of the cyclic voltammetry data, the one-electron reduced form 1C is thus best formulated as [(bpy)2RuII(L1H)2C]C. The expected hyperfine coupling to the nitrogen atoms of the L1H ligand is in all likelihood not well resolved because of unfavorable ratios of line width to hyperfine coupling constant. Such a phenomenon has precedence in the literature for related ruthenium complexes with quinone ligands.[6b] Although the possible generation of a weakly coupled dinuclear system through hydrogen bonding cannot be ruled out completely, we consider this unlikely. The starting complex displays a mononuclear structure in solution, as has been proven by 1H NMR spectroscopy. Hence, in analogy, we favor a mononuclear structure for the one-electron reduced form in solution. The one-electron reduced forms of the dinuclear complexes 2 + –4 + generated in situ electrochemically in CH3CN/0.1 m Bu4NPF6 exhibit well-resolved EPR signals at 295 K. Thus, the signal of 4 + is a well-resolved quintet centered at g = 1.998 owing to the hyperfine coupling of the unpaired electron with the two equivalent 14N atoms (I = 1) of (L12H)C (Figure 5). Additionally, ruthenium satellites (99Ru, 101Ru, I = 5/2, combined natural abundance of about 30 %)[11] are observed at both the extremities of the main signal. The spectrum could be simulated with parameters of 4.8 G for 14N and 2.4 G for 103,105Ru (Figure 5). The other two dinuclear complexes show similar spectra. The data for the one-electron reduced forms of all the

EPR spectroscopy The one-electron oxidized and the reduced states of all the complexes were probed by EPR spectroscopy to shed light on their electronic structures. The in-situ-generated one-electron reduced form 1C of the mononuclear complex shows a narrow signal in CH3CN/0.1 m Bu4NPF6 at 295 K, with a peak-to-peak separation of about 18 G (Figure S3, Table 4). The signal is centered at g = 2.000. The narrow line width, appearance of the signal in fluid solution at 295 K, and the g value are all indicative of a ligand-centered reduction.[10] Together with the solChem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

Figure 5. X-band EPR spectrum of 4 + generated in situ in CH3CN/0.1 m Bu4NPF6 at 295 K together with simulation. Simulation parameters: g = 1.998, A(99,101Ru) = 2.4 G, A(14N) = 4.8 G, line width = 3.5 G. Microwave frequency = 9.42 GHz, modulation amplitude = 0.5 G.

4338


Full Paper complexes are summarized in Table 4. The EPR data thus point clearly to a reduction of the bridging ligand in these dinuclear complexes, and the one-electron reduced forms are thus best formulated as [(bpy)2RuII(m-L2H)3CRuII(bpy)2] + . The g value of 5 + is comparable to the values for 2 + –4 + obtained here.[7a] However, because 5 + contains an all-oxygen donating bridge, no hyperfine coupling was detected in its EPR spectra.[7a] Surprisingly, for 6 + , which contains only H as substituents on the nitrogen atoms of the bridge, no hyperfine coupling was observed in the EPR spectrum.[3e] This is probably related to an unfavorable ratio of line width to hyperfine coupling constant, just as in the case of 1C discussed above. The g value for 6 + is, however, in the same range as that discussed above for 2 + –4 + .[3e] For 7 + , which also contains a mesityl substituent on the bridge, the EPR signal is almost identical to the ones observed here, and hyperfine coupling to both the nitrogen atoms of the bridge is visible.[3i] Thus, it is seen that in such systems, the most information-rich EPR results for the one-electron reduced species are obtained with a bridge containing an [O, N, O, N] donor with aromatic or aliphatic substituents on the nitrogen atoms. In contrast to the one-electron reduced forms, the one-electron oxidized forms of the dinuclear complexes 23 + –43 + in CH3CN were EPR-silent at 295 K. This is because of the fast relaxation in fluid solution, and is already an indication of possible ruthenium participation in the SOMO.[10, 12] A rhombic signal was observed on cooling the samples to 110 K (Figure 6, Table 5). The spectrum obtained for 43 + could be simulated with the parameters g1 = 2.315, g2 = 2.105, and g3 = 1.880. The gav value is 2.100, and the g-anisotropy value, Dg, is 0.435. Such a large deviation of the g value from the free-electron value of 2.0023 and the large anisotropy observed are indications of a substantial metal contribution to the SOMO.[10] The complexes 23 + and 33 + show very similar spectra and parameters (Table 5). The one-electron oxidized forms of these complexes are thus best described as [(bpy)2RuII(m-L2H)2RuIII(bpy)2]3 + or more appropriately [(bpy)2Ru2.5(m-L2 H)2Ru2.5-

Figure 6. X-band EPR spectrum of 43 + generated in situ in CH3CN/0.1 m Bu4NPF6 at 110 K together with simulation. Simulation parameters: g1 = 2.315, g2 = 2.105, g3 = 1.880, line width = 300 G. Microwave frequency = 9.42 GHz, modulation amplitude = 4 G. Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

Table 5. EPR data of one-electron oxidized forms of the complexes.[a] Complex

g1[b]

g2[b]

g3[b]

gav[b]

23 + 33 + 43 +

2.280 (2.301) 2.210 2.315 (2.216)

2.070 (2.130) 2.100 2.105 (2.167)

1.900 (2.009) 1.900 1.880 (2.001)

2.083 2.070 2.100

[a] EPR data of one-electron oxidized species generated in situ. [b] Data for the one-electron oxidized species obtained from simulation, gav = p 2 (g1 +g22+g32)/3. DFT-calculated g values are given in parentheses.

(bpy)2]3 + (vide infra). The g values are less extreme than for the “pure” RuIII case because of an orbital reduction factor (< 1), which leads to partial electron delocalization onto the ligand.[12a] The EPR signatures of 23 + –43 + are comparable to those of 63 + and 73 + , with the amount of anisotropy varying from system to system.[3e,i] The complex 53 + , with the alloxygen-donor-containing bridge, shows a different kind of signal, which is visible at room temperature in fluid solution.[7a] For complexes such as 8, which already contain two exchangecoupled spins on the two low-spin d5 RuIII centers, oxidation and reduction lead to complicated spin systems, the interpretation of which is often not straightforward. In contrast, for complexes 22 + –72 + the presence of low-spin, diamagnetic d6 RuII centers in the starting complex facilitates the interpretation of the EPR parameters of the oxidized and reduced complexes. UV/Vis/NIR spectroelectrochemistry The complete reversibility of the redox states of the various species discussed here was proved by the 100 % regeneration of the spectrum of the starting complex on setting the potential back to the region in which the starting complex exists. Hence, all kinds of chemical decomposition reactions within the measurement cells and timescales can be ruled out. The mononuclear complex 1 + displays two low-energy bands at 560 and 528 nm in CH3CN/0.1 m Bu4NPF6, which can be assigned to dp (RuII) to p* (L1H) and dp (RuII) to p* (bpy) metalto-ligand charge-transfer (MLCT) transitions, respectively (Figure 7). The higher-energy bands are probably ligand-centered. Upon one-electron reduction to the 1C form using an optically transparent thin-layer electrochemical (OTTLE) cell, the Vis bands shift and new bands appear in the NIR region at 1520 and 845 nm. These bands are tentatively assigned to SOMO (L1H) to LUMO (bpy) ligand-to-ligand charge-transfer (LLCT) transitions in the one-electron reduced form 1·. The second reduction to the 1 form leads to a slight shift of the LLCT bands and an increase in their intensity (Figure 7, Table 6). Further reduction processes for 1 + could not be investigated through spectroelectrochemical methods because of the instability of the highly reduced forms on the spectroelectrochemical timescale. The oxidation of 1 + has been found to be irreversible in both cyclic voltammetry and spectroelectrochemistry. The dinuclear complex 22 + shows two strong bands in the visible region at 615 and 552 nm in CH3CN/0.1 m Bu4NPF6.

4339


Full Paper duction potentials for 52 + than for 22 + –42 + (Table 2). The complexes 32 + and 42 + display three low-energy bands (Figure 8 and Table 6), which are all ascribed to MLCT. Multiple MLCT bands can be expected because of the presence of various empty target orbitals (L2H and bpy ligands) in these compounds (see “Cyclic voltammetry” section). The lowest-energy bands for 62 + and 72 + appear at 632[3e] and 680 nm,[3i] respectively. Thus, it is seen that the position of the MLCT bands can be tuned by substituting [O] for [NR], as well as by changing the R groups on the nitrogen atoms. Upon one-electron oxidation, the MLCT bands are shifted to lower energies, and a new band appears at around 1400 nm in the NIR region for all three dinuclear complexes. The position of this band is similar for all three compounds (Figure 8 and S4 and Table 7). However, the intensity of this band is much higher for 33 + than for 23 + or 43 + , indicating a higher oscillator strength and better orbital overlap for the NIR bands in 33 + than in the other two complexes. The experimental band width at half height is much smaller than that calculated using the Hush formulation (Table 7).[13] These results together with those obtained from EPR spectroscopy (metal-centered spin) show that the one-electron oxidized forms of the dinuclear complexes can be classified as strongly coupled class-III mixedvalent species, and the NIR bands as p–p* transitions in the strongly coupled class-III mixed-valent state. Another parameter that has been used recently in the literature to describe the extent of metal–metal coupling is the G parameter.[14] MixedFigure 7. Changes in the UV/Vis/NIR spectra of 1 + in CH3CN/0.1 m Bu4NPF6 valent systems with G > 0.5 are said to belong to the strongly during the first (top) and second (bottom) reduction steps. coupled class-III group. As seen from Table 7, all three dinuclear complexes reported herein (23 + –43 + ) have G values above 0.7, and hence belong to class III. The more intense NIR band for 33 + is also narrower than the NIR bands of 23 + or 43 + These are assigned to dp (RuII) to p* (L12H) and dp (RuII) to p* (bpy) MLCT transitions, respectively (Figure S4). Additional (Figure 8 and S4 and Table 7). The NIR bands at about 1400 nm bands at higher energies are ligand-centered in origin. For disappear on further oxidation to the 24 + –44 + forms for all the 2+ comparison, the lowest-energy MLCT band in the case of 5 dinuclear complexes, as would be expected for a homovalent appears at 721 nm in CH3CN.[7a] The low-energy shift of this complex. The complexes 24 + –44 + show a low-energy band at 2+ 2+ 2+ band for 5 compared with those of 2 –4 is consistent about 1100 nm, which is assigned to a ligand-to-metal chargewith the lower difference between the first oxidation and retransfer (LMCT) transition from L2H to RuIII. The NIR band observed here for the one-electron oxidized form was absent in the case of 53 + , highlighting the im[a] Table 6. UV/Vis/NIR data of the complexes. portance of incorporating NR Compound lmax [nm] 103 e [m1 cm1] groups into the bridging ligands. Furthermore, for 63 + and 73 + , 1+ 560 (sh), 528 (25.2), 337 (22.4), 301 (23.8) 1C 1520 (0.9), 845 (3.0), 578 (15.4), 439 (18.5), 363 (19.1), 300 (21.5), which display an NIR band, the 1530 (1.6), 814 (5.7), 629 (10.9), 522 (13.0), 460 (18.7), 426 (sh), 352 (28.1), 303 (21.8) 1 intensity is different from the 4+ 1112 (2.0), 610 (8.5), 468 (5.9), 355 (sh), 288 (20.3), 245 (21.1) 2 cases discussed here.[3e,i] Hence, 3+ 2 1386 (5.3), 770 (sh), 721 (9.7), 627 (sh)), 456 (4.4), 291 (32.5), 242 (23.0) 2+ the intensity of the NIR band, 615 (12.6), 552 (13.4), 386 (6.2), 341 (7.8), 294 (36.3), 240 (23.8) 2 ~ 1900 (0.6), 818 (32.7), 666 (9.2), 536 (sh), 466 (7.1), 430 (7.4), 370 (8.2), 294 (31.0), 241 (22.3) 2+ which is critically dependent on 1145 (5.2), 1095 (sh), 982 (4.3), 616 (31.3), 470 (12.5), 366 (sh), 308 (29.6) 34 + orbital overlap, can be influ3+ 1402 (16.7), 724 (27.7), 446 (9.0), 299 (31.1) 3 enced by changing the R sub2+ 666 (29.0), 633 (28.3), 553 (26.0), 390 (11.6), 327 (sh), 299 (37.1) 3 stituents in the [NR] groups of 1855 (1.9), 840 (10.2), 618 (18.4), 442 (17.4), 366 (16.4), 300 (31.0) 3+ 1870 (0.7), 1176 (3.1), 1006 (4.1), 626 (28.9), 362 (sh), 296 (35.9), 242 (47.1) 44 + the bridge. 1870 (2.8), 1390 (9.1), 1176 (1.9), 737 (27.0), 431 (9.9), 290 (51.0), 240 (45.7) 43 + On reduction of the dinuclear 2+ 702 (27.8), 654 (25.5), 534 (18.5), 360 (sh), 294 (58.5), 240 (45.6) 4 complexes to the 2 + –4 + forms, + 4 1796 (1.7), 860 (10.7), 596 (14.6), 456 (sh), 418 (16.3), 371 (17.8), 295 (49.2), 238 (43.5) two distinct bands appear in the [a] Obtained from OTTLE spectroelectrochemistry in CH3CN/0.1 m Bu4NPF6. NIR region for all three comChem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

4340


Full Paper DFT calculations For verification of the assignment of the spectroscopic data for the various redox forms of the complexes 1n–4n, structurebased DFT calculations were performed on these systems. For 1 + , the structure optimization at the BP86 level delivered bond lengths that matched the experimental data quite well (Table S2). Structural optimization of 22 + delivered equally good results. Calculations were also performed for 42 + by taking the structure of 22 + as a starting point. The next step involved the calculation of the EPR parameters and spin-density distribution for the reduced (1C) and oxidized (12 + ) species with the B3LYP functional. Attempting these calculations for 1C, we noticed that calculations performed “in vacuum” delivered results in which most of the spin density is localized on the bpy ligands. These results were in contrast to those obtained experimentally for related systems in the literature.[3g] Complex 1C contains a free NH and an O group in the ligand L1H, and is likely to have several energetically close orbitals in the valence region, so we argued that solvent polarity could have a significant effect on the orbital composition and spin-density distribution. Therefore, we re-ran our calculations including electrostatic solvent interactions by using the conductor-like screening model (COSMO). Gratifyingly, the spin density was seen to shift from a completely bpy-centered scenario in vacuum to an almost L1H-centered distribution in highly polar solvents such as water and acetonitrile (Figure 9). For solvents with intermediate polarity, the spin density was found to be distributed between the bpy and L1H ligands. The EPR measurements were performed in acetonitrile, and the Figure 8. Changes in the UV/Vis/NIR spectra of 32 + in CH3CN/0.1 m Bu4NPF6 during the oxidation (top, middle) and reduction (bottom) processes.

Table 7. Analysis of the NIR bands of one-electron oxidized forms of dinuclear complexes. Complex

l [nm]

Dn1/2(exptl)[a]

Dn1/2(calcd)[b]

G[c]

23 + 33 + 43 +

1386 1402 1390

1060 874 1004

4082 4059 4077

0.74 0.78 0.75

[a] From OTTLE spectroelectrochemistry in CH3CN/0.1 m Bu4NPF6. [b] Dn1/ p = (2310nmax). [c] G = 1Dn1/2(exptl)/Dn1/2(calcd).

2(calcd)

plexes (Figure 8 and S4 and Table 6). These bands are reminiscent of the NIR bands observed for the one-electron reduced form of the mononuclear complex 1C. Similarly to the mononuclear complex, these NIR bands can also be assigned to an LLCT transition from SOMO (L12H) to LUMO (bpy). The presence of multiple target orbitals makes multiple LLCT transitions possible. With the exception of the first reduction step, all the reduction processes for 22 + –42 + were found to be irreversible on the spectroelectrochemical timescale, and hence, cannot be discussed further. Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

Figure 9. a) Spin-density plot of 1 in vacuum; b) spin-density plot of 1 calculated in water using the COSMO model; c) spin population localized at the quinonoid skeleton (C6N2O2) versus the relative permittivity of the solvent used in the COSMO model.

4341


Full Paper theoretical calculations with acetonitrile as the solvent match the experimental data very well. Thus, the g value was reproduced with reasonable accuracy (giso(exptl) = 2.000, giso(calcd) = 2.005). Our results here show the absolute necessity of incorporating solvent effects into DFT calculations when dealing with systems that are capable of undergoing weak intermolecular interactions or those that may possess many energetically close electronic states. Such a strong solvent dependence of the DFT-calculated spin density has also been reported recently for ruthenium–amine complexes containing cyanamide bridges.[15] The irreversible nature of the first oxidation step of 1 + precluded the characterization of 12 + through EPR spectroscopy. However, we performed DFT calculations on 12 + , and found that the spin density is localized predominantly on the ruthenium center in this case (Figure S5). The DFT results thus show good qualitative agreement with the experimental data. However, quantitatively, the agreement is not perfect, as has been observed previously for related systems.[3e] Calculations at the B3LYP level also reproduced the UV/Vis/ NIR spectrum of 1 + with reasonable accuracy. Thus, the lowenergy broad bands for 1 + are attributed to mixed HOMO1!LUMO, HOMO2!LUMO + 1, and HOMO2! LUMO + 2 transitions (Figure S6). As discussed in the UV/Vis/ NIR section, the origin orbitals are predominantly rutheniumbased, and the target orbitals are either L1H or bpy-based, justifying the assignment of these bands as MLCT transitions. The low-energy NIR band of the one-electron reduced form 1 is a SOMO!SOMO + 1 transition (Figure S7). These orbitals are L1H and bpy-based, respectively, so the assignment of this transition as LLCT is justified. As seen from the origin and target orbitals of the dinuclear complex 22 + , the lowest-energy bands can be assigned as an MLCT transition from ruthenium to the L12H or bpy ligands (Figure S8). Similar results were also obtained for the 42 + dication. For the one-electron oxidized form 23 + , the main band observed in the visible region has MLCT character (Figure S9). In addition, the NIR band observed in this case has origin and target orbitals that are delocalized over the two ruthenium centers and the bridging quinone (Figure S9), as would be expected for a p–p* transition in a strongly coupled class-III mixed-valent system.[13] Calculation of the spin-density distribution for this species showed about 39 % spin on each of the ruthenium centers (Figure 10), confirming the mixed-valent nature of 23 + . DFT calculations reproduced the anisotropic g tensors for this species with reasonable accuracy (Table 5), and similar results were also obtained in the calculations for 43 + (Figure S10). For the one-electron reduced forms 2 + and 4 + , the calculation of spin-density distribution were fraught with the same problems as for 1C. Hence, here too, we resorted to the incorporation of solvent effects through the COSMO model. Just as in the case of 1C, the inclusion of polar solvents for 2 + and 4 + shifted the spin density from a completely delocalized situation (on the ligands) to one in which the spin density is localized predominantly on the bridging quinone ligand (Figure 10 and S11). The g values for these species were also reproduced with reasonable accuracy by using the COSMO model. The Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

Figure 10. Spin-density plot of 23 + (top, calculated g1 = 2.301, g2 = 2.130, g3 = 2.009) and [2] + (bottom, calculated giso = 2.004).

low-energy NIR band observed for the reduced species can be assigned to an LLCT transition from L2H to bpy, as seen from the origin and target orbitals for 4 + shown in Figure S12. There is good qualitative agreement between the experimental and DFT-calculated UV/Vis/NIR spectra.

Conclusion Using the isoelectronic analogy of [O] and [NR], we have reported here a rare example of a mononuclear ruthenium complex containing these symmetrically substituted p-quinone ligands and various dinuclear complexes. The substituted p-quinone ligands bind to the metal centers through a phenolate O and an imine-type neutral nitrogen donor. The consequence of this is the appearance of strong hydrogen bonding in the solid state for the mononuclear complex. The ruthenium complexes undergo various redox processes. The redox potentials can be changed by altering the substituents on the bridge. For the dinuclear complexes, the one-electron oxidized forms show absorptions in the NIR region, the position and intensity of which can be tuned by substituting [O] for [NR] in the bridging ligands. Such absorptions are absent in the homovalent forms of these complexes, as well as in the oxidized form of the mononuclear complex. The EPR spectroscopy results support the mixed-valent assignment through the large g-anisotropy of the signals. The one-electron reduced forms, on the other hand, show narrow, well-resolved EPR signals, as expected for metal-bound organic radicals. The one-electron reduced forms of the mono- and dinuclear complexes display absorptions in the NIR region, which are LLCT in nature. Thus, the complexes presented here show switchable NIR absorptions. The NIR bands can be switched on and off by a simple one-electron transfer. The positions, shapes, and intensities of the NIR bands can be varied by changing the substituents on the nitrogen atoms of the bridge, or by choosing the redox potentials (oxidation or reduction). Comparison of the dinuclear complexes with previously reported systems has shown

4342


Full Paper that the redox potentials, Kc values, positions of the MLCT bands, and EPR properties can be influenced by isoelectronic [O] for [NR] substitution, and by changing the R groups on N. The DFT results presented here highlight the importance of incorporating solvent interactions in calculating the properties of metal complexes with many energetically close orbitals. Only with the appropriate solvent interactions do these calculations reproduce experimental data correctly and provide new insights with reasonable accuracy. The electrochemical, structural, and spectroscopic data presented for the mononuclear complex provide an important basis for a discussion of similar data for substituted p-quinone-bridged dinuclear complexes. Considering the huge interest in p-quinone bridged dinuclear complexes, these data are likely to provide important benchmarks for discussing other results.

Experimental Section Materials and physical methods cis-Ru(bpy)2Cl2 was obtained from ABCR and AgClO4 was obtained from STREM. The ligands L1–L3 were synthesized according to the published procedure from 2,5-dihydroxybenzoquinone.[6c] 2,5-Dihydroxybenzoquinone, 2,4,6-trimethylaniline, benzylamine, and isopropylamine were obtained from Sigma–Aldrich. All the reagents were used as supplied. The solvents used for metal-complex synthesis were dried and distilled by using standard techniques. The 1H NMR spectra were recorded on a Bruker AC 250 spectrometer. Electronic absorption studies were recorded on J&M TIDAS and Shimadzu UV 3101 PC spectrophotometers. Cyclic voltammetry was performed in 0.1 m Bu4NPF6 solutions using a three-electrode configuration (glassy-carbon working electrode, Pt counter electrode, Ag/AgCl reference) and a PAR 273 potentiostat and function generator. The ferrocene/ferrocenium (Fc/Fc + ) couple served as an internal reference. Spectroelectrochemistry was performed with an optically transparent thin-layer electrode (OTTLE) cell.[16] Elemental analyses were performed with a Perkin–Elmer Analyzer 240 instrument. Mass spectrometry was performed on a BRUKER Daltronics Microtof Q mass spectrometer. EPR spectra in the X-band were recorded with a Bruker System EMX. EPR simulations were performed by using the Simfonia program of Bruker. The measurement parameters for the various complexes are given in Table 8. Spin quantification experiments were performed by measuring the EPR spectrum of an equimolar amount of the spin-label TEMPO in solution. Double integration of the signals was performed to obtain quantitative results. Efforts were made to perform the measurements of the sample and TEMPO under identical condi-

Table 8. Measurement parameters for the EPR spectra in the X-band of the various complexes. Complex

Microwave frequency [GHz]

Modulation amplitude [G]

1C 2+ 23 + 3+ 33 + 4+ 43 +

9.69 9.48 9.43 9.47 9.42 9.42 9.42

1 0.5 4 0.5 4 0.5 4

Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

tions, as far as the design of the single-cavity instrument permitted. Compounds 1C, 2 + , 3 + , and 4 + displayed signals, the area of which was within 10 % of the area of the equimolar TEMPO signal. Compounds 23 + , 33 + , and 43 + did not show a very good quantitative match with the signals of TEMPO measured under identical conditions. This problem is probably related to the very different nature of the organic radical TEMPO, and the predominantly metal-centered signals of 23 + , 33 + , and 43 + . A prerequisite of the determination of spin concentration is similar or identical EPR properties of the sample and the standard. For the reduced forms 1C, 2 + , 3 + , and 4 + , in which the spin is localized entirely on the ligands, the EPR properties of the samples match those of TEMPO, and hence, a good quantitative correlation could be obtained. In terms of the EPR properties, the oxidized forms 23 + , 33 + , and 43 + show very different intensity responses from that of TEMPO. For 23 + , 33 + and 43 + , which have a predominantly metalcentered spin, the use of equimolar amounts of Cu(BF4)2 (hexahydrate) as a standard provided good results. In these cases, both the standard and the sample were measured at 110 K, because the samples do not display EPR signals at ambient temperatures. The area under the signals of the samples and the standard matched within 15 %, pointing to the existence of one spin/molecule for these one-electron oxidized complexes.

Syntheses [Ru(bpy)2L1H](ClO4), 1(ClO4): [Ru(bpy)2Cl2] (0.068 g, 0.14 mmol), AgClO4 ( 0.0725 g, 0.35 mmol), and ethanol (25 mL) were added to a Schlenk flask under an argon atmosphere and heated at reflux for 3 h. This was then filtered under argon to another Schlenk flask through a G4 crucible containing a celite bed. Then, L1 (0.031 g, 0.14 mmol) and NaOMe (0.2 mL) was added to the reaction mixture, which was heated at reflux for 5 h under an argon atmosphere, resulting in a color change to purple–red. The mixture was then reduced to 7–8 mL, and an excess saturated aqueous solution of NaClO4 was added. The solid precipitate thus obtained was filtered off and dried in vacuum, and purified on an alumina (neutral) column. The purple–red product was eluted with CH2Cl2/CH3CN (1:1). Evaporation of the solvent under reduced pressure afforded the pure complex. Yield: 0.038 g (37 %); 1H NMR (250 MHz, CDCl3) d = 0.24 (3 H, d, 3J = 6.0HZ, CH3-CH-NH), 0.78 (3 H, d, 3J = 6.6HZ, CH3CH-NH), 1.17 (3 H, d, 3J = 7.0HZ, CH3-CH-N), 1.19 (3 H, d, 3J = 7.0HZ, CH3-CH-N), 4.11 (1 H, m, CH3-CH-NH), 4.29 (1 H, sept, 3J = 6.8HZ, CH3CH-N), 5.65 (1 H, s, N-C-CH), 5.77 (1 H, s, NH-C-CH), 7.13–7.27 (2 H, m, bpy), 7.4 (1 H, d, br, 3J = 8.1HZ, NH), 7.56 (2 H, t, 3J = 6.6HZ bpy), 7.65–7.81 (4 H, m, bpy), 8.06 (2 H, m, bpy), 8.36 (3 H, m, bpy), 8.49 ppm (3 H, m, bpy); ES-MS: m/z calcd for C32H33N6O2Ru: 635.17 [M] + ; found: 635.17; elemental analysis calcd (%) for C32H33N6O6RuCl (734.16 g mol1): C 52.35, H 4.53, N 11.45; found: C 51.92, H 4.38, N 10.84. [{Ru(bpy)2}2(m-L12H)](ClO4)2, 2(ClO4)2 : The ligand L1 (0.020 g, 0.09 mmol) and excess NaH (60 %) (0.011 g, 3 mmol) were dissolved in THF (20 mL), and the reaction mixture was stirred for 3 h at room temperature under an argon atmosphere to give a reddish precipitate insoluble in THF. The solvent was then removed under reduced pressure. In another Schlenk flask, [Ru(bpy)2Cl2] (0.094 g, 0.18 mmol), AgClO4 ( 0.093 g, 0.45 mmol), and ethanol (30 mL) were added under an argon atmosphere and the mixture was heated at reflux for 3 h. It was then filtered under argon to the former reaction mixture through a G4 crucible containing a celite bed, and the mixture was heated at reflux overnight under an argon atmosphere, which resulted in a color change to blue. The solvent volume was then reduced to 7—8 mL, and an excess of a saturated aqueous solution of NaClO4 was added. The solid pre-

4343


Full Paper cipitate thus obtained was filtered off and dried in vacuum. It was then purified on an alumina (neutral) column. The deep blue product was eluted with CH2Cl2/CH3CN (2:1). Evaporation of the solvent under reduced pressure afforded the pure complex. Yield: 0.036 g (32 %); elemental analysis calcd (%) for C52H48N10O10Ru2Cl2 (1246.04 g mol1): C 50.12, H 3.88, N 11.24; found: C 49.97, H 3.74, N 11.03; ES-MS: m/z calcd for C52H48N10O2Ru2 : 524.1025 [M-2ClO4]2 + ; found: 524.1041; 1H NMR (400 MHz, CD3CN) d = 0.01 (6 H, d, 3J = 6.5HZ, CH3-CH-N), 0.58 (6 H, d, 3J = 6.5HZ, CH3-CH-N), 4.01 (2 H, m, CH3-CH-N), 5.75 (2 H, s, N-C-CH), 6.91—7.00 (4 H, m, bpy), 7.13 (2 H, m, bpy), 7.43 (2 H, m, bpy), 7.50—7.68 (8 H, m, bpy), 7.92 (4 H, m, bpy), 8.18 (4 H, m, bpy), 8.29—8.43 (6 H, m, bpy), 8.50 (1 H, m, bpy), 8.60—8.67 ppm (1 H, m, bpy). Two sets of signals in the 1H NMR spectrum clearly indicate the formation of two diastereomers. Analysis of the d = 0.58 and 0.64 peaks points to a composition of about 2:1.

X-ray crystallography Single-crystal X-ray structural studies of 1(ClO4) and 2(ClO4)2 were performed on a CCD Oxford Diffraction XCALIBUR-S diffractometer equipped with an Oxford Instruments low-temperature attachment. Data were collected at 150(2) K using graphite monochromated MoKa radiation (la = 0.71073 ). The strategy for the data collection was evaluated with the CrysAlisPro CCD software. The data were collected by using standard ’phi-omega scan techniques, and were scaled and reduced using the CrysAlisPro RED software. The structures were solved by direct methods with SHELXS-97, and refined by full-matrix least-squares with SHELXL-97, refining on F2.[17] The positions of all the atoms were obtained by direct methods. All non-hydrogen atoms were refined anisotropically. The remaining hydrogen atoms were placed in geometrically constrained positions and refined with isotropic temperature factors, generally 1.2 Ueq of their parent atoms. The A-Alert in the check-cif report for 2(ClO4)2 is related to the disorder of one of the perchlorate anions. The crystal and refinement data are summarized in Table 9.

[{Ru(bpy)2}2(m-L22H)](ClO4)2, 3(ClO4)2 : The compound was prepared following the procedure for 2(ClO4)2 by using [Ru(bpy)2Cl2] (0.094 g, 0.18 mmol) and ligand L2 (0.029 g, 0.09 mmol). The compound was purified on an alumina (neutral) column by using a CH2Cl2/CH3CN (2:1) solvent mixture. Yield: 0.042 g (35 %); elemental analysis calcd (%) for Table 9. Crystallographic data for 1(ClO4) and 2(ClO4)2. C60H48N10O10Ru2Cl2 (1342.13 g mol1): Compound 1 2 C 53.69, H 3.60, N 10.44; found: C 53.38, H 3.72, N 10.13; ES-MS: m/z chemical formula C32H33N6O6ClRu C52H48N10O10Cl2Ru2 Mr 734.16 1246.04 calcd for C60H48N10O2Ru2 : 572.10 [Mcell setting, space group Monoclinic, C2/c Monoclinic, P21/n 2ClO4]2 + ; found: 572.11; 1H NMR T [K] 150(2) 150(2) (250 MHz, CD3CN) d = 4.35 (2 H, d, 2 a, b, c [] 22.3438(8), 17.7934(5), 17.2914(7) 11.9202(8), 25.997(3), 17.5434(10) J = 14.4HZ, CH2-Ph), 5.39 (2 H, d, 2J = a, b, g [8] 90.00, 108.698(4), 90.00 90.00, 99.753(7), 90.00 14.4HZ, CH2-Ph), 6.03 (2 H, d, 2J = 6511.7(4) 5357.9(7) V [3] 7.4HZ, Ph), 6.12 (2 H, s, CH-C-N), 6.17 Z 8 4 2 (2 H, d, J = 7.5HZ, Ph), 6.64 (2 H, t, 1.498 1.545 Dx [Mg m3] 3 3 J = 7.6HZ, Ph), 6.75 (2 H, t, J = 7.7HZ, MoKa radiation type MoKa Ph), 6.90 (1 H, t, 3J = 7.5HZ, Ph), 6.97 m [mm1] 0.617 0.731 crystal size [mm] 0.33 0.28 0.23 0.33 0.28 0.23 (1 H, t, 3J = 7.4HZ, Ph), 7.10–7.18 (4 H, meas., indep. and obsvd reflns 22568, 5721, 4865 41215, 9405, 2646 m, bpy), 7.23–7.29 (2 H, m, bpy), 0.0385, 0.1178, 1.087 0.0951, 0.3034, 0.2526 R[F2>2s(F2)], wR(F2), S 7.61–7.89 (16 H, m, bpy), 8.09 (2 H, F000 3008 2528 m, bpy), 8.31 (2 H, m, bpy), 8.43 (2 H, 0.0317 0.1546 Rint 3 m, bpy) 8.54 (2 H, t, J = 5.4HZ, bpy), 25.00 25.00 qmax [8] 3 J = 5.0HZ, bpy), 8.64 (1 H, d, 1.056, 0.583 0.838, 0.518 D1max, D1min [e 3] 8.64 ppm (1 H, d, 3J = 5.7HZ, bpy). 1 Two sets of signals in the H NMR spectrum clearly indicate the formation of two diastereomers. Analysis CCDC-864292 and 864293 contain the supplementary crystalloof the d = 5.39 and 5.37 peaks points to a composition of about graphic data for this paper. These data can be obtained free of 1:1. charge from The Cambridge Crystallographic Data Centre via 3 [{Ru(bpy)2}2(m-L 2H)](ClO4)2, 4(ClO4)2 : The compound was prewww.ccdc.cam.ac.uk/data_request/cif. pared following the procedure for 2(ClO4)2 by using [Ru(bpy)2Cl2] 3 (0.094 g, 0.18 mmol) and ligand L (0.034 g, 0.09 mmol). The compound was purified on an alumina (neutral) column by using Details of DFT calculations a CH2Cl2/CH3CN (2:1) solvent mixture. Yield: 0.048 g (38 %); eleThe program package ORCA 2.9.1 was used for all calculations.[18] mental analysis calcd (%) for C64H56N10O10Ru2Cl2 (1398): C 54.98, H The geometry optimization and single-point calculations were per4.04, N 10.02; found: C 54.67, H 3.83, N 9.57; ES-MS: m/z calcd for formed through the DFT method with the BP86 and B3LYP funcC64H56N10O2Ru2 : 600.13 [M-2ClO4]2 + ; found: 600.14; 1H NMR (250 MHz, CD3CN) d = 3.61 (18 H, s, CH3), 5.29 (2 H, s, CH-C-O), 6.23 tionals, respectively,[19] including relativistic effects in zero-order (2 H, s, aryl), 6.45 (2 H, s, aryl), 6.92 (2 H, m, bpy), 7.10 (2 H, m, bpy), regular approximation (ZORA).[20] Convergence criteria for the geometry optimization were set to default values (OPT) and “tight” 7.37 (2 H, m, bpy) 7.52 (2 H, m, bpy), 7.59 (2 H, m, bpy), 7.69 (4 H, m, convergence criteria were used for SCF calculations (TIGHTSCF). bpy), 7.79 (4 H, m, bpy), 7.98–8.20 (6 H, m, bpy), 8.39 (2 H, m, bpy), The triple-z basis sets with one set of polarization functions[21] 8.50 (2 H, m, 3J = 6.5HZ, bpy), 8.85 ppm (4 H, m, bpy). Two sets of (TZVP) were used for transition-metal, oxygen, and nitrogen atoms, signals in the 1H NMR spectrum clearly indicate the formation of two diastereomers. Analysis of the d = 5.29 and 5.18 peaks points and the double-z basis sets with one set of polarization functo a composition of about 6:1. tions[22] (SVP) were used for all other atoms. For all spin-density calChem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

4344


Full Paper culations, triple-z basis sets with one set of polarization functions[23] (def2-TZVP) were used for all atoms. The resolution of the identity approximation (RIJCOSX) was employed[24, 25] with matching auxiliary basis sets.[25] All single-point calculations were performed by using the conductor-like screening model (COSMO).[26] During the TD-DFT calculations, the first 100 states were calculated, and the maximum dimension of the expansion space in the Davidson procedure (MAXDIM) was set to 1000. All spin densities were calculated according to Lçwdin population analysis.[27] MOs and spin densities were visualized with the program Molekel.[28]

Acknowledgements We are extremely grateful to the Baden-Wrttemberg Stiftung and Fonds der Chemischen Industrie (Chemiefondsstipendium for DS) for financial support of this work. J.F. thanks RVO:61388955 for institutional support. Alexa Paretzki is acknowledged for EPR measurements. We are thankful to the bw GRID for access to computer clusters. Keywords: NIR absorptions · non-innocent ligands · redox chemistry · ruthenium · spectroelectrochemistry [1] a) S. Patai, Z. Rappoport, The Chemistry of Quinonoid Compounds, Vol. 1, Wiley, New York, 1988; b) M. Oh, G. B. Carpenter, D. A. Sweigart, Acc. Chem. Res. 2004, 37, 1. [2] a) W. Kaim, Inorg. Chem. 2011, 50, 9752; b) M. D. Ward, J. A. McCleverty, J. Chem. Soc. Dalton Trans. 2002, 275; c) C. G. Pierpont, Coord. Chem. Rev. 2001, 216 – 217, 99; d) P. Zanello, Coord. Chem. Rev. 2006, 250, 2000; e) B. de Bruin, D. G. H. Hetterscheid, A. J. J. Koekkoek, H. Grtzmacher, Prog. Inorg. Chem. 2007, 55, 247; f) A. I. Poddel’sky, V. K. Cherkasov, G. A. Abakumov, Coord. Chem. Rev. 2009, 253, 291; g) S. N. Brown, Inorg. Chem. 2012, 51, 1251; h) P. Chaudhuri, C. N. Verani, E. Bill, T. Weyhermller, F. Neese, K. Wieghardt, J. Am. Chem. Soc. 2001, 123, 2213; i) S. Bhattacharya, P. Gupta, F. Basuli, C. G. Pierpont, Inorg. Chem. 2002, 41, 5810; j) S. Mandal, N. Paul, P. Banerjee, T. K. Mondal, S. Goswami, Dalton Trans. 2010, 39, 2717; k) B. Sarkar, R. Huebner, R. Pattacini, I. Hartenbach, Dalton Trans. 2009, 4653; l) N. Deibel, D. Schweinfurth, J. Fiedler, S. Zalis, B. Sarkar, Dalton Trans. 2011, 40, 9925; m) A. Paretzki, H. S. Das, F. Weisser, T. Scherer, D. Bubrin, J. Fiedler, J. E. Nycz, B. Sarkar, Eur. J. Inorg. Chem. 2011, 2413; n) D. Schweinfurth, F. Weisser, B. Sarkar, Nach. Chem. 2009, 57, 862. [3] a) J. Maurer, M. Linseis, B. Sarkar, B. Schwederski, M. Niemeyer, W. Kaim, S. Zalis, C. Anson, M. Zabel, R. F. Winter, J. Am. Chem. Soc. 2008, 130, 259; b) M. Fabre, J. Bonvoisin, J. Am. Chem. Soc. 2007, 129, 1434; c) D. Kumbhakar, B. Sarkar, S. Maji, S. M. Mobin, J. Fiedler, R. J. Aparicio, W. Kaim, G. K. Lahiri, J. Am. Chem. Soc. 2008, 130, 17575; d) T. E. Keyer, R. J. Forster, P. M. Jayaweera, C. G. Coates, J. J. McGarvey, J. G. Vos, Inorg. Chem. 1998, 37, 5925; e) S. Kar, B. Sarkar, S. Ghumaan, D. Janardanan, J. van Slageren, J. Fiedler, V. G. Puranik, R. B. Sunoj, W. Kaim, G. K. Lahiri, Chem. Eur. J. 2005, 11, 4901; f) H. Masui, A. L. Freda, M. C. Zerner, A. B. P. Lever, Inorg. Chem. 2000, 39, 141; g) H. S. Das, A. K. Das, R. Pattacini, R. Huebner, B. Sarkar, P. Braunstein, Chem. Commun. 2009, 4387; h) F. A. Cotton, J.-Y. Jin, Z. Li, C. A. Murillo, J. H. Reibenspies, Chem. Commun. 2008, 211; i) F. Weisser, R. Huebner, D. Schweinfurth, B. Sarkar, Chem. Eur. J. 2011, 17, 5727; j) J. Maurer, R. F. Winter, B. Sarkar, J. Fiedler, S. Zalis, Chem. Commun. 2004, 1900; k) W. Y. Man, J.-L. Xia, N. J. Brown, J. D. Farmer, D. S. Yufit, J. A. K. Howard, P. J. Low, Organometallics 2011, 30, 1852; l) J. L. Xia, W. Y. Man, X. Zhu, C. Zhang, G.-J. Jin, P. A. Schauer, M. A. Fox, G.-A. Yu, P. J. Low, S. H. Liu, Organometallics 2012, 31, 5321; m) J.-Y. Shao, W.-W. Yang, J. Yao, Y.-W. Zhong, Inorg. Chem. 2012, 51, 4343; n) C.-J. Yao, J. Yao, Y.-W. Zhong, Inorg. Chem. 2011, 50, 6847; o) S. D. J. McKinnon, B. O. Patrick, A. B. P. Lever, R. G. Hicks, Chem. Commun. 2010, 46, 773. Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

[4] a) P. J. Chirik, K. Wieghardt, Science 2010, 327, 794; b) M. R. Ringenberg, S. Kokatam, Z. M. Heiden, T. B. Rauchfuss, J. Am. Chem. Soc. 2008, 130, 788; c) V. K. K. Praneeth, M. R. Ringenberg, T. R. Ward, Angew. Chem. 2012, 124, 10374; Angew. Chem. Int. Ed. 2012, 51, 10228; d) W. I. Dzik, J. I. van der Vlugt, J. N. H. Reek, B. de Bruin, Angew. Chem. 2011, 123, 3416; Angew. Chem. Int. Ed. 2011, 50, 3356; e) A. I. Nguyen, K. J. Blackmore, S. M. Carter, R. A. Zarkesh, A. F. Heyduk, J. Am. Chem. Soc. 2009, 131, 3307; f) N. Deibel, D. Schweinfurth, S. Hohloch, J. Fiedler, B. Sarkar, Chem. Commun. 2012, 48, 2388; g) D. Zhang, G.-X. Jin, Organometallics 2003, 22, 2851; h) S. Scheuermann, T. Kretz, H. Vitze, J. W. Bats, M. Bolte, H. W. Lerner, M. Wagner, Chem. Eur. J. 2008, 14, 2590; i) S. Hohloch, P. Braunstein, B. Sarkar, Eur. J. Inorg. Chem. 2012, 546; j) J.-P. Taquet, O. Siri, P. Braunstein, R. Welter, Inorg. Chem. 2004, 43, 6944. [5] a) M.-K. Tsai, J. Rochford, D. E. Polyansky, T. Wada, K. Tanaka, J. T. Muckerman, Inorg. Chem. 2009, 48, 4372; b) T. Wada, K. Tsuge, K. Tanaka, Angew. Chem. 2000, 112, 1539; Angew. Chem. Int. Ed. 2000, 39, 1479; c) H. C. Zhao, J. P. Harney, Y.-T. Huang, J.-H. Yum, Md. K. Nazeeruddin, M. Grtzel, M.-K. Tsai, J. Rochford, Inorg. Chem. 2012, 51, 1. [6] a) P. Braunstein, D. Bubrin, B. Sarkar, Inorg. Chem. 2009, 48, 2534; b) H. S. Das, F. Weisser, D. Schweinfurth, C.-Y. Su, L. Bogani, J. Fiedler, B. Sarkar, Chem. Eur. J. 2010, 16, 2977; c) D. Schweinfurth, H. S. Das, F. Weisser, D. Bubrin, B. Sarkar, Inorg. Chem. 2011, 50, 1150; d) A. Paretzki, R. Pattacini, R. Huebner, P. Braunstein, B. Sarkar, Chem. Commun. 2010, 46, 1497; e) N. Deibel, D. Schweinfurth, R. Huebner, P. Braunstein, B. Sarkar, Dalton Trans. 2011, 40, 431. [7] a) M. D. Ward, Inorg. Chem. 1996, 35, 1712; b) M. Leschke, M. Melter, H. Lang, Inorg. Chim. Acta 2003, 350, 114; c) S. Kitagawa, S. Kawata, Coord. Chem. Rev. 2002, 224, 11; d) C. Carbonera, A. Dei, J.-F. Letard, C. Sangregorio, L. Sorace, Angew. Chem. 2004, 116, 3198; Angew. Chem. Int. Ed. 2004, 43, 3136; e) J. S. Miller, K. S. Min, Angew. Chem. 2009, 121, 268; Angew. Chem. Int. Ed. 2009, 48, 262; f) B. Therrien, G. Sss-Fink, P. Govindaswamy, A. K. Renfrew, P. J. Dyson, Angew. Chem. 2008, 120, 3833; Angew. Chem. Int. Ed. 2008, 47, 3773. [8] B. Sarkar, W. Kaim, J. Fiedler, C. Duboc, J. Am. Chem. Soc. 2004, 126, 14706. [9] a) C. Creutz, Prog. Inorg. Chem. 1983, 30, 1; b) R. J. Crutchley, Adv. Inorg. Chem. 1994, 41, 273. [10] S. Patra, B. Sarkar, S. M. Mobin, W. Kaim, G. K. Lahiri, Inorg. Chem. 2003, 42, 6469. [11] J. A. Weil, J. R. Bolton, J. E. Wertz, Electron Paramagnetic Resonance, Wiley, New York, 1994. [12] a) A. Dei, D. Gatteschi, L. Pardi, Inorg. Chem. 1990, 29, 1442; b) F. Fabrizi de Biani, A. Dei, C. Sangregorio, L. Sorace, Dalton Trans. 2005, 3868. [13] a) N. S. Hush, Prog. Inorg. Chem. 1967, 8, 391; b) K. M. Demadis, C. M. Hartshorn, T. J. Meyer, Chem. Rev. 2001, 101, 2655; c) D. M. D’Alessandro, F. R. Keene, Chem. Soc. Rev. 2006, 35, 424; d) W. Kaim, G. K. Lahiri, Angew. Chem. 2007, 119, 1808; Angew. Chem. Int. Ed. 2007, 46, 1778; e) W. Kaim, B. Sarkar, Coord. Chem. Rev. 2007, 251, 584. [14] N. Vila, Y.-W. Zhong, J. C. Henderson, H. D. Abruna, Inorg. Chem. 2010, 49, 796. [15] M. L. Naklicki, S. I. Gorelsky, W. Kaim, B. Sarkar, R. J. Crutchley, Inorg. Chem. 2012, 51, 1400. [16] M. Krejcik, M. Danek, F. Hartl, J. Electroanal. Chem. 1991, 317, 179. [17] a) G. M. Sheldrick, Acta Crystallogr. Sect. A 2008, A64, 112 – 122; b) G. M. Sheldrick, Program for Crystal Structure Solution and Refinement, University of Gçttingen: Gçttingen, Germany, 1997. [18] Neese, F. ORCA - an Ab Initio, Density Functional and Semiempirical Program Package, version 2.9.1, Department of molecular theory and spectroscopy, Max Planck Institute for Bioinorganic Chemistry, Mlheim/Ruhr, Germany, January 2012. [19] a) A. D. Becke, J. Chem. Phys. 1993, 98, 5648; b) C. T. Lee, W. T. Yang, R. G. Parr, Phys. Rev. B 1988, 37, 785. [20] C. van Wllen, J. Chem. Phys. 1998, 109, 392. [21] A. Schfer, C. Huber, R. Ahlrichs, J. Chem. Phys. 1994, 100, 5829. [22] A. Schfer, H. Horn, R. Ahlrichs, J. Chem. Phys. 1992, 97, 2571. [23] F. Weigend, R. Ahlrichs, Phys. Chem. Chem. Phys. 2005, 7, 3297. [24] a) E. J. Baerends, D. E. Ellis, P. Ros, Chem. Phys. 1973, 2, 41; b) B. I. Dunlap, J. W. D. Connolly, J. R. Sabin, J. Chem. Phys. 1979, 71, 3396; c) O. Vahtras, J. Almlof, M. W. Feyereisen, Chem. Phys. Lett. 1993, 213, 514.

4345


Full Paper [25] a) K. Eichkorn, O. TGreutler, H. Ohm, M. Haser, R. Ahlrichs, Chem. Phys. Lett. 1995, 242, 652; b) K. Eichkorn, F. Weigend, O. Treutler, R. Ahlreichs, Theor. Chem. Acc. 1997, 97, 119. [26] A. Klamt, G. Schrmann, J. Chem. Soc. Perkin Trans. 2 1993, 799. [27] a) P. O. Lçwdin, J. Chem. Phys. 1950, 18, 365; b) P. O. Lçwdin, Adv. Quantum Chem. 1970, 5, 185.

Chem. Eur. J. 2014, 20, 4334 – 4346

www.chemeurj.org

[28] Portmann, S. Molekel, version 5.4.0.8, CSCS/UNI Geneva, Switzerland, 2009. Received: December 9, 2012 Revised: January 8, 2014 Published online on March 6, 2014

4346


Tuning the luminescent properties of Pt(II) acetylide complexes through varying the electronic properties of N-heterocyclic carbene ligands.

Tuning the electronic and phosphorescence properties of blue-emitting iridium(III) complexes through different cyclometalated ligand substituents: a theoretical investigation.

Tuning the coordination geometries and magnetic dynamics of [Ln(hfac)4](-) through alkali metal counterions.

Tuning steric and electronic effects in transition-metal β-diketiminate complexes.

Metal coordination study at Ag and Cd sites in crown thioether complexes through DFT calculations and hyperfine parameters.

Tuning the optical properties of 2-thienylpyridyl iridium complexes through carboranes and anions.

Tuning the electronic coupling in cyclometalated diruthenium complexes through substituent effects: a correlation between the experimental and calculated results.

trans isomerism.

Tuning the Electronic Structure of Anatase Through Fluorination.

Tuning reactivity and electronic properties through ligand reorganization within a cerium heterobimetallic framework.

Effective Tuning of Ketocyanine Derivatives through Acceptor Substitution.

Tuning thiophene with phosphorus: synthesis and electronic properties of benzobisthiaphospholes.

Water bridge coordination on the metal-rich facets of Gd2O3 nanoplates confers high T1 relaxivity.

Tuning the mesomorphic properties of phenoxy-terminated smectic liquid crystals: the effect of fluoro substitution.

Conformational Tuning of the Intramolecular Electronic Coupling in Molecular-Wire Biruthenium Complexes Bridged by Biphenyl Derivatives.

Electronic properties of some protein--methylglyoxal complexes.

Two-Component Polymeric Materials of Fullerenes and the Transition Metal Complexes: A Bridge between Metal-Organic Frameworks and Conducting Polymers.

Tuning the formation of discrete coordination nanostructures.

Electronic structure predictions of the properties of non-innocent P-ligands in group 6B transition metal complexes.

Electronic properties of transition-metal-decorated silicene.

Coordination-induced spin-state change in manganese(V) complexes: the electronic structure of manganese(V) nitrides.

Tuning the electronic and magnetic properties of graphene-like AlN nanosheets by surface functionalization and thickness.

Metal ion coordination, conditional stability constants, and solution behavior of chelating surfactant metal complexes.

Metal coordination core of bleomycin : comparison of metal complexes between bleomycin and its biosynthetic intermediate.