Unprecedented partial paddlewheel dirhodium methyl isocyanide compounds with unusual structural and electronic properties: a comprehensive experimental and theoretical study
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A series of dirhodium acetonitrile compounds, namely cis-[Rh 2(DTolF)2(CH3CN)6][BF 4]2 (1) cis-[Rh2(F-form)2(CH 3CN)6][BF4]2 (2), cis-[Rh 2(NNN)2(CH3CN)6][BF 4]2 (3) ([F-form]-: p- difluorophenylformamidinate, [NNN]-: p-ditolyltriazenide, [DTolF]-: p-ditolylformamidinate), cis-[Rh2[Ph 2P(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 Rh 2(σ) orbitals, with the extreme case being 8 where Rh 2(σ) becomes the HOMO. This fact accounts for the ∼20-fold increase in intensity of the HOMO Rh2(σ) → LUMO Rh 2(σ*) transition for 8 (∼370 nm) vs.4, also in accord with TD-DFT calculations. © 2013 The Royal Society of Chemistry.
author list (cited authors)
Li, Z., Chifotides, H. T., & Dunbar, K. R.