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 ) 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 -backbonding, whereas the short CN distances in CH3NC and the high energy (CN) stretches are attributed to appreciable -donation of CH3NC. Of particular note are the longer Rh-Rh bonds (2.76 ) that 8 exhibits and unprecedented short ax Rh-C distances in the same range as the eq Rh-C bonds (2.01-2.04 ) 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 -backbonding in 5-8 stabilizes the Rh2(*) orbitals and -donation from CH3NC destabilizes the Rh 2() orbitals, with the extreme case being 8 where Rh 2() becomes the HOMO. This fact accounts for the 20-fold increase in intensity of the HOMO Rh2() LUMO Rh 2(*) transition for 8 (370 nm) vs.4, also in accord with TD-DFT calculations. 2013 The Royal Society of Chemistry.
