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)

Article abstract of DOI:10.1016/s0020-1693(97)05670-3

Complexes of general formulae cis-[M(C₆F₄CF₃),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 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≠ (298.15K)) 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(C₆F₄CF₃)₂(mstd)] (mstd=mesostilbenediamine) was synthesised. At low temperatures, C₆F₄CF₃ ring rotations and five-membered ring puckering in this complex were arrested, ΔG≠ (208 K) for the latter process being 42.8 kJ mol^-1.