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DOI: 10.1039/A904026I (Paper) Reference Section for: J. Chem. Soc., Dalton Trans., 1999, 4495-4501

A detailed NMR study of the solution stereodynamics in tricarbonylrhenium(I) halide complexes of the non-racemic chiral ligand 2,6-bis[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L1) and the molecular structure of fac-[ReBr(CO)3(L1)][hair space]†

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Peter J. Heard, Paul M. King, Alex. D. Bain, Paul Hazendonk and Derek A. Tocher

Abstract

Tricarbonylrhenium(I) halide complexes of the non-racemic chiral ligand 2,6-bis[(4R,5R)-dimethyl-1,3-dioxan-2-yl]pyridine (L1), namely fac-[ReX(CO)3(L1)] (X = Cl, Br or I), have been prepared. In these complexes the ligand is bound in a bidentate fashion, with the N atom of the pyridine ring and an O atom of one of the acetal rings co-ordinated to the octahedral metal centre. The bidentate mode is confirmed by the crystal structure of fac-[ReBr(CO)3(L1)]. There are four possible diastereoisomers, depending on the configuration at the metal centre and at the acetal-carbon atom of the co-ordinated ring; the crystal structure of fac-[ReBr(CO)3(L1)] shows that the SR diastereoisomer is present in the solid-state. In solution, three of the four possible diastereoisomers are observed, namely SR, RR and SS; their relative populations are in the order SR > RR > SS. Above ambient temperature the complexes are stereochemically non-rigid. The fluxional kinetics have been measured by a combination of standard band shape analysis and selective inversion experiments. Two distinct processes are present: an acetal ring flip and exchange of the pendant and co-ordinated acetal rings. The latter process occurs via two independent mechanisms, namely tick-tock and rotation pathways. The activation energies for the stereodynamics are in the ranges 72–73 kJ mol–1 (tick-tock), 77–78 kJ mol–1 (acetal ring flip) and 83–90 kJ mol–1 (rotation) at 298 K.

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