Solid state and solution structures of rhodium and iridium poly(pyrazolyl)borate diene complexes

Abstract

The structures adopted by a range of poly(pyrazolyl)borate complexes [ML₂Tp^x] [M = Rh, Ir; L₂ = diene; Tp^x = Bp′ {dihydrobis(3,5-dimethylpyrazolyl)borate}, Tp′ {hydrotris(3,5-dimethylpyrazolyl)borate}, Tp {hydrotris(pyrazolyl)borate}, B(pz)₄ {tetrakis(pyrazolyl)borate}] have been investigated. Low steric hindrance between ligands in [Rh(η-nbd)Tp] (nbd = norbornadiene), [Rh(η-cod)Tp] (cod = cycloocta-1,5-diene) and [Rh(η-nbd)Tp′] results in κ³ coordination of the pyrazolylborate but [M(η-cod)Tp′] (M = Rh, Ir) are κ² coordinated with the free pyrazolyl ring positioned above and approximately parallel to the square plane about the metal. All but the most sterically hindered Tp^x complexes undergo fast exchange of the coordinated and uncoordinated pyrazolyl rings on the NMR spectroscopic timescale. For [Rh(η-cod){B(pz)₄}], [Rh(η-dmbd)Tp′] (dmbd = 2,3-dimethylbuta-1,3-diene) and [Rh(η-cod)Tp^Ph] {Tp^Ph = hydrotris(3-phenylpyrazolyl)borate} the fluxional process is slowed at low temperatures so that inequivalent pyrazolyl rings are observed. The bonding modes of the Tp′ ligand (but not of other pyrazolylborate ligands) can be determined by ¹¹B NMR and IR spectroscopy. The ¹¹B chemical shifts (for a series of Tp′ complexes) show the general pattern, κ³ < −7.5 ppm < κ² and the ν(BH) stretch κ³ > 2500 cm⁻¹ > κ². The electrochemical behaviour of the pyrazolylborate complexes is related to the degree of structural change which occurs on electron transfer. One-electron oxidation of complexes with Tp′, Tp and B(pz)₄ ligands is generally reversible although that of [Ir(η-cod)Tp] is only reversible at higher scan rates and that of [Ir(η-cod){B(pz)₄}] is irreversible. Of the complexes with the more sterically hindered Tp^Ph ligand, only [Rh(η-nbd)Tp^Ph] shows any degree of reversible oxidation. The ESR spectra of a range of Rh(II) complexes show coupling to both ¹⁴N and ¹⁰³Rh nuclei in most cases but what appears to be coupling to rhodium and one hydrogen atom, possibly a hydride ligand, for the oxidation product of [Rh(η-nbd)Tp^Ph].