Synthesis and dynamic NMR studies of fluxionality in rhenium(I), platinum(II) and platinum(IV) complexes of ‘back-to-back’ 2,2′∶6′,2″-terpyridine ligands

Abstract

Syntheses are given for the following transition metal complexes, [{ReBr(CO)₃}₂L] (L = L¹, L² or L³), [(PtClMe₃)₂L¹], [(PtIMe₃)₂L] (L = L² or L³), [ReBr(CO)₃L¹], [Pt(C₆F₅)₂L³], [Pt(C₆F₄CF₃)₂L] (L = L¹ or L²), [{Pt(C₆F₄CF₃)₂}₂L] (L = L¹ or L²) and [ReBr(CO)₃PtIMe₃L¹] where the ligands, L, are the ‘back-to-back’ terpyridine ligands, 6′,6″-bis(2-pyridyl)-2,2′∶4′,4″∶2″,2‴-quaterpyridine (L¹), 1,4-bis(2,2′∶6′,2″-terpyridin-4′-yl)benzene (L²) and 6′,6″-bis{2-(4-methylpyridyl)}-2,2′∶4′,4″∶2″,2‴-quaterpyridine (L³). All the complexes undergo 1,4-metallotropic shifts in solution at above-ambient temperatures and restricted rotations of the pendant pyridyl rings at below-ambient temperatures. Activation energies for these processes have been computed from variable temperature one-dimensional bandshape analysis and 2D-exchange spectroscopy (2D-EXSY) NMR experiments. The metallotropic shift energies are very metal-dependent being in the order Pt^IV < Re^I < Pt^II, with ΔG ^‡ (298.15 K) values ranging from 62 to 101 kJ mol^–1. The fluxions are sensitive only to the local metal-coordination environment, there being negligible electronic interaction between the metal centres in the dinuclear complexes. In the mixed-metal dinuclear complex [ReBr(CO)₃PtIMe₃L¹] it proved possible to measure the different rates of fluxion of the Re^I and Pt^IV moieties.