Bonding modes, structures and fluxionality in rhodium and iridium tris(3,5-dimethylpyrazolyl)methane diene complexes

Abstract

The structures adopted by a range of hydrotris(3,5-dimethylpyrazolyl)methane complexes [ML₂{HC(pz′)₃}]⁺ (M = Rh, Ir; L₂ = diene) have been investigated. There is low steric hindrance between ligands in [Rh(η-nbd){HC(pz′)₃}]⁺ (nbd = norbornadiene) and [Rh(η-dmbd){HC(pz′)₃}]⁺ (dmbd = 2,3-dimethylbuta-1,3-diene) resulting in κ³ co-ordination of the pyrazolylmethane. The complexes [M(η-cod){HC(pz′)₃}]⁺ (cod = cycloocta-1,5-diene) (M = Rh, Ir) are κ² co-ordinated with the free pyrazolyl ring positioned above and approximately parallel to the square plane about rhodium or iridium. The HC(pz′)₃ complexes undergo fast exchange of the co-ordinated and unco-ordinated pyrazolyl rings on the NMR spectroscopic timescale. However, for [Rh(η-dmbd){HC(pz′)₃}]⁺, the fluxional process is slowed at low temperatures, so that inequivalent pyrazolyl rings may be observed. A mechanism for the fluxional process is proposed involving dynamic interconversion between isomeric forms in solution. The bonding mode of the HC(pz′)₃ ligand can be determined by ¹³C NMR spectroscopy. The ¹³C chemical shifts (for the sp³ hybridised carbon of the ligand) show the general pattern, κ³ < 71.5 ppm < κ². The electrochemical behaviour of the pyrazolylmethane complexes is related to the degree of structural change, which occurs on electron transfer and is compared with that of the pyrazolylborate analogues.