Comparison of the One-Electron Oxidations of CO-Bridged vs Unbridged Bimetallic Complexes: Electron-Transfer Chemistry of Os2Cp2(CO)4 and Os2Cp*2

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• 作者：Derek R. Laws ; R. Morris Bullock ; Richmond Lee ; Kuo-Wei Huang ; William E. Geiger
• 刊名：Organometallics
• 出版年：2014
• 出版时间：September 22, 2014
• 年：2014
• 卷：33
• 期：18
• 页码：4716-4728
• 全文大小：707K
• ISSN：1520-6041

文摘

The 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 = 畏⁵-C₅H₅) undergoes a reversible one-electron anodic reaction at E_1/2 = 0.41 V vs ferrocene in CH₂Cl₂/0.05 M [NBu₄][B(C₆F₅)₄], giving a rare example of a metal鈥搈etal 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^{鈥?/sup> 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鈥搇igand delocalized. Removal of an electron from 1 has very little effect on the Os鈥揙s bond order, accounting for the resistance of 1+ to heterolytic cleavage. The Os鈥揙s bond distance is calculated to decrease by 0.10 脜 and 0.06 脜 as a consequence of one-electron oxidation of anti-1 and gauche-1, respectively. The CO-bridged complex Os₂Cp*₂(渭-CO)₂(CO)₂ (Cp* = 畏5-C₅Me₅), trans-2, undergoes a more facile oxidation, E_1/2 = 鈭?.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鈥搇igand framework, with the bridging carbonyls accounting for about half of the orbital makeup. The Os鈥揙s bond order again changes very little with removal of an electron, and the Os鈥揙s bond length actually undergoes minor shortening. Calculations suggest that the second isomer of 2+ has the anti all-terminal CO structure.}