Unprecedented partial paddlewheel dirhodium methyl isocyanide compounds with unusual structural and electronic properties: a comprehensive experimental and theoretical study

摘要

A series of dirhodium acetonitrile compounds, namely cis-[Rh2(DTolF)2(CH3CN)6][BF4]2 (1) cis-[Rh2(F-form)2(CH3CN)6][BF4]2 (2), cis-[Rh2(NNN)2(CH3CN)6][BF4]2 (3) ([F-form]~-: p-difluorophenylformamidinate, [NNN]~-: p-ditolyltriazenide, [DTolF]~-: p-ditolylformamidinate), cis-[Rh2[Ph2P(C6H4)]2(CH3CN)6][BF4]2 (4) and their unprecedented methyl isocyanide analogs 5-8, respectively, were synthesized and characterized by X-ray crystallography, cyclic voltammetry, NMR, electronic and infrared spectroscopies. The elongation of the Rh-Rh distances (～0.07 A) and the bonds trans to eq CH3NC in 5-8 vs. 1-4 are in accord with the strong trans influence of isocyanide. The short Rh-C (CH3NC) distances in 5-8 are attributed to π-backbonding, whereas the short CN distances in CH3NC and the high energy v(CN) stretches are attributed to appreciable a-donation of CH3NC Of particular note are the longer Rh-Rh bonds (～2.76 A) that 8 exhibits and unprecedented short ax Rh-C distances in the same range as the eq Rh-C bonds (2.01-2.04 A) for CH3NC. TD-DFT calculations predict the lowest-energy LMCT transitions in the order 1<2<5<6, 3<7 and the HOMO-LUMO energy gaps to be greater in 5-7 vs. 1-3, respectively, findings corroborated by electrochemical and electronic spectral data. This trend is attributed to the stabilization of the HOMOs and destabilization of the LUMOs in 5-7. The π-backbonding in 5-8 stabilizes the Rh2(π*) orbitals and σ-donation from CH3NC destabilizes the Rh2(a) orbitals, with the extreme case being 8 where Rh2(σ) becomes the HOMO. This fact accounts for the ～20-fold increase in intensity of the HOMO Rh2(σ)→ LUMO Rh2(σ*) transition for 8 (～370 nm) vs. 4, also in accord with TD-DFT calculations.