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

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 analogue