Synthesis and dynamic NMR studies of fluxionality in palladium(II) and platinum(II) complexes of 2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPT) and 2,4,6-tris(2-pyridyl)-pyrimidine (TPP)

Complexes of general formulae cis-[M(C6F4CF3)(2)L] (M=Pd-II, Pt-II; L=2,4,6-tris(2-pyridyl)-1,3,5-triazine (TPT) and 2,4,6-tris(2-pyridyl)-pyrimidine (TPP)) were isolated as air stable solids. In all cases cis square-planar complexes were formed with the nitro en ligands acting as bidentate chelates towards each metal moiety. The complexes exhibited various modes of fluxionality in solution, namely 1,4-metallotropic shifts, a new 'metal hurdling' fluxion and, at below ambient temperatures, restricted rotation of the pendant pyridyl ring adjacent to the metal chelate ring. Dynamic NMR experiments (one-dimensional bandshape analysis and two-dimensional EXSY experiments) provided activation energy data for these processes, Gibbs free energy values (Delta G(not equal) (298.15 K)) were in the ranges 74-113 (metal hurdling), 69-118 (metal 1,4-shifts) and 37-43 (pendant pyridyl rotations) kT mol(-1). Energies of any of these fluxions were considerably higher in the Pt-II complexes than in the Pd-II complexes. To aid understanding of the low temperature fluxionality of the TPT complexes, the complex [Pd(C6F4CF3)(2)(mstd)] (mstd=meso-stilbenediamine) was synthesised. At low temperatures, C6F4CF3 ring rotations and five-membered ring puckering in this complex were arrested, Delta G(not equal) (208 K) for the latter process being 42.8 kJ mol(-1). (C) 1997 Elsevier Science S.A.