The energetics and mechanism of fluxionality of 2,2′∶6′,2″-terpyridine derivatives when acting as bidentate ligands in transition-metal complexes. A detailed dynamic NMR study

Abstract

Syntheses are reported for the following transition metal complexes of derivatives of 2,2′∶6′,2″-terpyridine, [ReBr(CO)₃L] (L = mcpt, mcpmt, mmtt or bmtt), [PtIMe₃(mcpt)], [Pd(C₆F₅)₂(mcpt)] and [Pt(C₆F₅)₂(mcpt)] where mcpt = 4-methyl-4′-(4-chlorophenyl)-2,2′∶6′,2″-terpyridine, mcpmt = 4-methyl-4′-(4-chlorophenyl)-4″-methyl-2,2′∶6′,2″-terpyridine, mmtt = 4-methyl-4′-methylthio-2,2′∶6′,2″-terpyridine and bmtt = 4-tert-butyl-4′-methylthio-2,2′∶6′,2″-terpyridine. In organic solvents (CD₂Cl₂, CDCl₂·CDCl₂) they exist as bidentate chelate complexes which undergo double 1,4-metallotropic shifts (‘tick-tock’ twists). These shifts have been monitored by variable temperature one- and two-dimensional NMR methods and activation energy data obtained. These energy data are very metal-dependent, magnitudes of ΔG ^‡ (298.15 K) being in the range 66–101 kJ mol^–1 and showing the trend Pt^II  Pd^II ≈ Re^I > Pt^IV. The nature of the ligand does not greatly influence the ligand fluxionality but does affect the relative populations of the pairs of metal complexes found in solution (when L = mcpt, mmtt and bmtt). Above-ambient temperature 2-D-exchange spectroscopy (EXSY) NMR experiments provide firm evidence for the fluxions occurring by an associative mechanism, involving five-co-ordinate intermediates for palladium(II) and platinum(II) complexes, and seven-co-ordinate intermediates for platinum(IV) and, by analogy, rhenium(I) complexes.