2,2′:6′,2″-Terpyridine (terpy) acting as a fluxional bidentate ligand. Part 4. cis-[M(C6F5)2(terpy)](M = Pd or Pt): nuclear magnetic resonance studies of their solution dynamics and crystal structure of cis-[Pd(C6F5)2(terpy)]

Abstract

2,2′:6′,2″-Terpyridine reacted with trans-[M(C₆F₅)₂(diox)₂](M = Pd or Pt, diox = 1,4-dioxane) to form the square-planar complexes cis-[M(C₆F₅)₂(terpy)] in which the terpyridine acts as a bidentate chelate ligand. In solution these complexes are fluxional with the terpyridine oscillating between equivalent bidentate modes by a mechanism consisting of a ‘tick-tock’ twist of the metal moiety through an angle equal to the N–M–N angle of the metal centre. Rates of this fluxion were measured by NMR spectroscopy from the exchange effects on the ¹H signals of the aromatic hydrogens and in the ¹⁹F signals of two C₆F₅ groups. The ΔG^‡ values for the fluxion were ca. 71 and 94 kJ mol^–1 for the complexes of Pd^II and Pt^II respectively. At below-ambient temperatures further changes in the ¹⁹F NMR spectra of both complexes were interpreted in terms of varying rates of rotation of the unco-ordinated pyridine ring, with the rates of rotation of the C₆F₅ rings being substantially slower at all temperatures and not separately measurable. The lowest-temperature spectra suggested the presence of a pair of degenerate rotamers each having the planes of both C₆F₅ rings and the unco-ordinated pyridine ring closely parallel and orthogonal to the remainder of the ligand ring system. The crystal structure of [Pd(C₆F₅)₂(terpy)] confirms the bidentate chelate bonding of terpy with a N–Pd–N angle of 77.9°, and the pendant ring oriented at an angle of 46° to the adjacent co-ordinated ring.