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

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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 PtII PdII ≈ ReI > PtIV. 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.

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