Redox-induced κ2-κ3 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=PPh3 2, PCy3 3, L=P(NMe2)(3) 4, P(C6H4Me-p)(3) 5 or P(C6H4Me-m)(3) 6}, prepared from [Rh(CO)(2)Tp'] 1 and L, and [Rh(PPh3)(2)L'] [L'=Tp' 8, Tp 9 or B(pz)(4) 10 {Tp=HB(pz)(3), pz=pyrazolyl}] and [Rh(dppe)Tp'] 11, prepared from [{Rh(mu -Cl)(PPh3)(2)}(2)] or [{Rh(mu -Cl)(dppe)}(2)] and KL', adopt four-co-ordinate kappa (2) 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 Rh . . .N contact [2.764(2) Angstrom] 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 kappa (3) 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 kappa (2)-kappa (3) isomerisation. These structures and ESR spectroscopy are consistent with a five-co-ordinate square pyramidal geometry with the unpaired electron in a sigma* Rh-N-axial orbital.