Comparison of the one-electron oxidations of CO-bridged vs unbridged bimetallic complexes: Electron-transfer chemistry of Os2Cp2(CO)4 and Os2Cp2(μ-CO)2(CO)2 (Cp = η5-C5H5, Cp = η5-C5Me5)

Derek R. Laws, R. Morris Bullock, Richmond Lee, Kuo Wei Huang, William E. Geiger

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Abstract

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 = 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 Os2Cp2(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)

Original languageEnglish
Pages (from-to)4716-4728
Number of pages13
JournalOrganometallics
Volume33
Issue number18
DOIs
Publication statusPublished - Sep 22 2014

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Organic Chemistry
  • Inorganic Chemistry

Fingerprint Dive into the research topics of 'Comparison of the one-electron oxidations of CO-bridged vs unbridged bimetallic complexes: Electron-transfer chemistry of Os<sub>2</sub>Cp<sub>2</sub>(CO)<sub>4</sub> and Os<sub>2</sub>Cp<sup>∗</sup><sub>2</sub>(μ-CO)<sub>2</sub>(CO)<sub>2</sub> (Cp = η<sup>5</sup>-C<sub>5</sub>H<sub>5</sub>, Cp<sup>∗</sup> = η<sup>5</sup>-C<sub>5</sub>Me<sub>5</sub>)'. Together they form a unique fingerprint.

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