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/

Syntheses are reported for the following transition metal complexes of derivatives of 2,2': 6',2 ''-terpyridine, [ReBr(CO)(3)L] (L = mcpt, m cpmt, 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 ''-terp yridine, 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 org anic solvents (CD2Cl2, CDCl2 . CDCl2) they exist as bidentate chelate complexes which undergo double 1,4-metallotropic shifts ('tick-tock' t wists). These shifts have been monitored by variable temperature one-a nd two-dimensional NMR methods and activation energy data obtained. Th ese energy data are very metal-dependent, magnitudes of Delta G double dagger (298.15 K) being in the range 66-101 kJ mol(-1) and showing th e trend Pt-II much greater than Pd-II approximate to Re-I > Pt-IV. The nature of the ligand does not greatly influence the ligand fluxionali ty but does affect the relative populations of the pairs of metal comp lexes found in solution (when L = mcpt, mmtt and bmtt). Above-ambient temperature 2-D-exchange spectroscopy (EXSY) NMR experiments provide f irm evidence for the fluxions occurring by an associative mechanism, i nvolving five-co-ordinate intermediates for palladium(II) and platinum (II) complexes, and seven-co-ordinate intermediates for platinum(rv) a nd, by analogy, rhenium(I) complexes.