Redox-induced κ2–κ3 isomerisation in hydrotris(pyrazolyl)boratorhodium complexes: synthesis, structure and ESR spectroscopy of stabilised rhodium(II) species

Abstract

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 Rh ⋯ N 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–N_axial orbital.