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

Abstract

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 Å) and the bonds trans to eq CH₃NC in 5–8vs.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 ν(CN) stretches are attributed to appreciable σ-donation of CH₃NC. Of particular note are the longer Rh–Rh bonds (∼2.76 Å) that 8 exhibits and unprecedented short ax Rh–C distances in the same range as the eq Rh–C bonds (2.01–2.04 Å) 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 5–7vs.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(σ) 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.