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)

Abstract

Complexes of general formulae cis-[M(C 6F 4CF 3) 2L] (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 nitrogen 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 ( ΔG ≠ (298.15 K)) were in the ranges 74–113 (metal hurdling). 69–118 (metal 1,4-shifts) and 37–43 (pendant pyridyl rotations) kJ 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(C 6F 4CF 3) 2(mstd)] (mstd = meso-stilbenediamine) was synthesised. At low temperatures, C 6F 4CF 3 ring rotations and five-membered ring puckering in this complex were arrested, ΔG ≠ (208K) for the latter process being 42.8 kJ mol −1.