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)

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 E 1/2 = 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, E 1/2 = −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.

Access to Stable Metalloradical Cations with Unsupported and Isomeric Metal–Metal Hemi‐Bonds

“…+ ) was found in the previous solution studies 9a. Nonetheless, a similar equilibrium between trans ‐ and anti ‐bimetallic radical cations of osmium was observed during the anode electrochemical oxidation of Os 2 Cp*(μ‐CO) 2 (CO) 2 , but this has not been structurally identified 15…”

“…[9a] Nonetheless,asimilar equilibrium between trans-a nd anti-bimetallic radical cations of osmium was observed during the anode electrochemical oxidation of Os 2 Cp*(m-CO) 2 (CO) 2 ,b ut this has not been structurally identified. [15] Single crystal X-ray diffraction shows that 3C + is composed of two distinctly different isomers, anti-3C + and trans-3C + ,which co-crystallize ( Figure 5). Thef ormer has the anti conformation with aC o-Co bond unsupported by bridging carbonyl ligands,while the latter exhibits a trans configuration with two bridging CO groups.S imilar to 1C + and 2C + ,C o-C(O) bonds slightly lengthen while OC-Co-CO angles become wider in anti-3C + compared to neutral 3.Intrans-3C + ,Co-C(O) and C À Ob ond lengths of bridging CO are longer than those of terminal CO.The Co-Co bond length in anti-3C + (3.074(2) ) is slightly longer than that in 2C + ,while the Co-Co bond length (2.6208 (16) ) in trans-3C + is close to expected for aC o-Co single bond.…”

Controlled Dissociation of Iron and Cyclopentadienyl from a Diiron Complex with a Bridging C3 Ligand Triggered by One-Electron Reduction

“…For instance, the one-electron oxidation/reduction of the iron­(I) prototype [Fe 2 Cp 2 (CO) 4 ] gives rise to single mononuclear products [oxidation, iron­(II); reduction, iron(0)], favored by the fluxionality of the carbonyl ligands . As a useful comparison, the 4d and 5d homologues [M 2 Cp 2 (CO) 4 ] (M = Ru, Os) maintain their dinuclear structure upon one-electron oxidation.…”

Substrate-Gated Transformation of a Pre-Catalyst into an Iron-Hydride Intermediate [(NO)2(CO)Fe(μ-H)Fe(CO)(NO)2]− for Catalytic Dehydrogenation of Dimethylamine Borane

“…In addition to the partial generation of DNIC 5 when DNIC 3-K-crown reacted with 10 equiv of DMAB in THF for 1 h, a shift of the IR ν CO and ν NO stretching frequencies from 1840 and (1699, 1684) cm –1 to (1989, 1968) and (1743, 1718, 1693) cm –1 reveals the parallel conversion of DNIC 3-K-crown into an iron-hydride intermediate [(NO) 2 (CO)­Fe­(μ-H)­Fe­(CO)­(NO) 2 ] − ( A , Scheme a and Figure a), − which is further transformed into complexes [(NO) 2 Fe­(μ-CO) 2 Fe­(NO) 2 ] − ( 6 , yield 74.4%) and [Fe­(CO) 3 (NO)] − when H 2(g) evolution stopped (Scheme b, Figures b and S12–S13). Notably, the absence of H 2(g) evolution upon further addition of DMAB implicated that the iron-hydride species A , derived from reaction of pre-catalyst DNIC 3-K-crown and DMAB, served as an intermediate during catalytic dehydrogenation of DMAB (Figure S14 and Scheme c).…”