Redox-induced κ 2 –κ 3 isomerisation in hydrotris(pyrazolyl)boratorhodium complexes: synthesis, structure and ESR spectroscopy of stabilised rhodium( II ) species

The complexes [Rh(CO)LTp′] {Tp′ = HBR3, R = 3,5-dimethylpyrazolyl; L = PPh32, PCy33, L = P(NMe2)34, P(C6H4Me-p)35 or P(C6H4Me-m)36}, prepared from [Rh(CO)2Tp′] 1 and L, and [Rh(PPh3)2L′] [L′ = Tp′ 8, Tp 9 or B(pz)410 {Tp = HB(pz)3, pz = pyrazolyl}] and [Rh(dppe)Tp′] 11, prepared from [{Rh(μ-Cl)(PPh3)2}2] or [{Rh(μ-Cl)(dppe)}2] and KL′, adopt four-co-ordinate κ2 structures, confirmed in the cases of 2–4, 6 and 8 by X-ray structural studies. By contrast, complex [Rh(CO){P(OPh)3}Tp′] 7 has a distorted five-co-ordinate square pyramidal structure with a long RhN contact [2.764(2) Å] in the apical site and an essentially planar Rh(CO)PN2 basal plane. Each complex undergoes fluxional processes on the NMR timescale. One-electron oxidation of 1–11 gives the κ3 rhodium(II) cations 1+++–11++; the crystal structures of salts of 2+ and 8+ confirm stabilisation of the unusual rhodium(II) oxidation state by axial co-ordination of the third pyrazolyl ring as a result of oxidatively induced κ2–κ3 isomerisation. These structures and ESR spectroscopy are consistent with a five-co-ordinate square pyramidal geometry with the unpaired electron in a σ* Rh–Naxial orbital.