Dynamic stereochemical rearrangements in ‘chiral-at-ligand’ complexes. Tricarbonylhalogenorhenium(I) and halogenotrimethylplatinum(IV) complexes of 2,6-bis[4-(S)-methyloxazolin-2-yl]pyridine

Abstract

Enantiomerically pure 2,6-bis[4-(S)-methyloxazolin-2-yl]pyridine, L, reacted with the pentacarbonylhalogenorhenium(I) and halogenotrimethylplatinum(IV) metal moieties to afford complexes of general formulae fac-[ReX(CO) ₃ L] and fac-[PtXMe ₃ L] (X = Cl, Br or I) in moderate yield. The potentially terdentate ligand acts here in a bidentate bonding mode, and undergoes a dynamic structural rearrangement which exchanges the pendant and co-ordinated oxazole rings. The bidentate bonding mode and absolute configuration of the ligand were confirmed by the crystal structure of the complex [ReCl(CO) ₃ L]. The chiral centres on the ligand provide an excellent spectroscopic handle on the mechanism of the rearrangement, and two independent fluxional pathways are apparent. The energetics of the rearrangement in the complexes [ReX(CO) ₃ L] (X = Cl, Br or I) was measured by standard ¹ H NMR band-shape analysis; ΔG ^‡ (average) (298 K) is 74.3 kJ mol ^-1 for the ‘tick-tock twist’ mechanism and 84.0 kJ mol ^-1 for the ‘rotation’ mechanism. Platinum-195 NMR data are reported for the complexes [PtXMe ₃ L] (X = Cl, Br or I).