Comparison of the one-electron oxidations of CO-bridged vs unbridged bimetallic complexes: Electron-transfer chemistry of Os2Cp2(CO)4 and Os2Cp∗2(μ-CO)2(CO)2 (Cp = η5-C5H5, Cp∗ = η5-C5Me5)
KAUST DepartmentKAUST Catalysis Center (KCC)
Physical Sciences and Engineering (PSE) Division
Chemical Science Program
Homogeneous Catalysis Laboratory (HCL)
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AbstractThe one-electron oxidations of two dimers of half-sandwich osmium carbonyl complexes have been examined by electrochemistry, spectro-electrochemistry, and computational methods. The all-terminal carbonyl complex Os2Cp2(CO)4 (1, Cp = η5-C5H5) undergoes a reversible one-electron anodic reaction at E1/2 = 0.41 V vs ferrocene in CH2Cl2/0.05 M [NBu4][B(C6F5)4], giving a rare example of a metal-metal bonded radical cation unsupported by bridging ligands. The IR spectrum of 1+ is consistent with an approximately 1:1 mixture of anti and gauche structures for the 33 e- radical cation in which it has retained all-terminal bonding of the CO ligands. Density functional theory (DFT) calculations, including orbital-occupancy-perturbed Mayer bond-order analyses, show that the highest-occupied molecular orbitals (HOMOs) of anti-1 and gauche-1 are metal-ligand delocalized. Removal of an electron from 1 has very little effect on the Os-Os bond order, accounting for the resistance of 1+ to heterolytic cleavage. The Os-Os bond distance is calculated to decrease by 0.10 å and 0.06 å as a consequence of one-electron oxidation of anti-1 and gauche-1, respectively. The CO-bridged complex Os2Cp∗2(μ-CO)2(CO)2 (Cp∗ = η5-C5Me5), trans-2, undergoes a more facile oxidation, E1/2 = -0.11 V, giving a persistent radical cation shown by solution IR analysis to preserve its bridged-carbonyl structure. However, ESR analysis of frozen solutions of 2+ is interpreted in terms of the presence of two isomers, most likely anti-2+ and trans-2+, at low temperature. Calculations show that the HOMO of trans-2 is highly delocalized over the metal-ligand framework, with the bridging carbonyls accounting for about half of the orbital makeup. The Os-Os bond order again changes very little with removal of an electron, and the Os-Os bond length actually undergoes minor shortening. Calculations suggest that the second isomer of 2+ has the anti all-terminal CO structure. (Figure Presented) © 2014 American Chemical Society.
SponsorsD.R.L. and W.E.G. acknowledge the support of the National Science Foundation under Grant CHE-0808909. K.-W.H. acknowledges financial support from KAUST. R.M.B. thanks the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, Division of Chemical Sciences, Biosciences and Geosciences for support. Pacific Northwest National Laboratory is a multiprogram national laboratory operated by Battelle for the U.S. Department of Energy. We thank Dr. S. I. Gorelsky for the discussion on the OOP analysis.
PublisherAmerican Chemical Society (ACS)