Fluxional rearrangements in molybdenum(>0>), tungsten(>0>) and rhenium(I) carbonyl complexes of 2,6-bis[(4S)-isopropyloxazolin-2-yl]pyridine (L). Crystal structure of [Mo(CO)4L]‡

Abstract

Treatment of the [ReX(CO)₅] (X = Cl, Br or I) and [M(CO)₄(pip)₂] (M = Mo or W, pip = piperidine) compounds with the C₂-symmetric ligand 2,6-bis[(4S)-isopropyloxazolin-2-yl]pyridine (L) yielded complexes of general formulae fac-[ReX(CO)₃L] and cis-[M(CO)₄L], in which the ligand is co-ordinated in a bidentate fashion. In solution these complexes undergo a fluxional process that exchanges the co-ordinated and pendant oxazoline rings. In the case of the M(CO)₄ complexes all permutational isomers are equivalent. However, in the rhenium(I) complexes the lower symmetry of the metal moiety leads to the formation of chemically distinct species. The different exchange pathways between these species gives rise to different magnetisation transfers, providing a spectroscopic handle on the mechanism of the ligand rearrangement. The activation parameters have been evaluated by standard one-dimensional band shape analysis and by two-dimensional exchange spectroscopy; ΔG ^‡ (298 K) ≈ 52.0 and 62.6 kJ mol^–1, respectively for the complexes of Mo⁰ and W⁰ and is in the range 78.5–80.5 kJ mol^–1 for the rhenium(I) complexes.