1,2-METALLOTROPIC SHIFTS IN TRIMETHYLPLATINUM(IV) AND TRICARBONYLRHENIUM(I) HALIDE-COMPLEXES OF PYRIDAZINE AND 4-METHYLPYRIDAZINE

Pyridazine (pydz) and 4-methylpyridazine (4Me-pydz) form stable bis(monodentate) complexes of general formulae fac-[PtXMe3L2] (X = Cl, Br or l) and fac-[ReX(CO)3L2] (X = Cl, Br or l). 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 H-1 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 Pt-195 two-dimensional EXSY and total bandshape analysis. Energy barriers, DELTAG(double dagger) (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.