1,2-Metallotropic shifts in trimethylplatinum(IV) and tricarbonylrhenium(I) halide complexes of pyridazine and 4-methylpyridazine

Abstract

Pyridazine (pydz) and 4-methylpyridazine (4Me-pydz) form stable bis(monodentate) complexes of general formulae fac-[PtXMe₃L₂](X = Cl, Br or I) and fac-[ReX(CO)₃L₂](X = Cl, Br or I). These complexes in above-ambient temperature solutions exhibit 1,2-fluxional shifts between the nitrogen donor pairs of each ring. All linkage isomers of the pydz complexes are equivalent whereas three NMR-distinct linkage isomers, namely (1,1), (1,2)/(2,1) and (2,2) species, occur in solutions of the 4Me-pydz complexes. Fluxional exchange kinetics have been measured by variable-temperature ¹H NMR bandshape analysis and two-dimensional exchange spectroscopy (EXSY). Platinum-195 NMR data have been obtained for the complexes [PtXMe₃(4Me-pydz)₂](X = Br or I) and their fluxional dynamics investigated by ¹⁹⁵Pt two-dimensional EXSY and total bandshape analysis. Energy barriers, ΔG^‡(298.15 K) for the 1,2-M–N fluxion are in the ranges 69–74 kJ mol^–1 for the Pt^IV complexes and 84–90 kJ mol^–1 for the Re^I complexes. The crystal structure of [PtClMe₃(pydz)₂] shows the cis pydz rings oriented in a propeller arrangement with their unco-ordinated nitrogens oriented away from the cis positioned chlorine.