Tricarbonylrhenium(I) halide complexes of 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L1) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L2): structure and solution stereodynamics

Abstract

The non-racemic chiral ligands 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L¹) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L²) reacted with the [ReX(CO)₅] compounds (X = Cl, Br, or I) to form fac-[ReX(CO)3(L¹)] and fac-[ReX(CO)₃(L²)], respectively. The ligands co-ordinate to the metal moiety in a bidentate fashion via the N donor of the pyridine ring and one of the acetal O atoms. The bonding modes are confirmed by the molecular structures of [ReI(CO)₃(L¹)] and [ReBr(CO)₃(L²)]. There are four possible diastereoisomers, depending on the configuration at the metal and the acetal carbon atom. In the solid state [ReI(CO)₃(L¹)] and [ReBr(CO)₃(L²)] exist exclusively as the RR and SR diastereoisomers, respectively. In solutions of the complexes of L¹ all four diastereoisomers are observed in varying concentrations, with the SR species being dominant. Only two of the four possible species are observed in solutions of the complexes of L², with the SR diastereoisomer being present in ca. 96% diastereomeric excess over the RR diastereoisomer. Above ambient temperatures both series of complexes are fluxional. A reversible flip of the acetal ring in the complexes [ReX(CO)₃(L¹)] leads to the pair-wise exchange of diastereoisomers as a result of the formal inversion of configuration at the acetal carbon atom; the free energies of activation are in the range 78–82 kJ mol⁻¹. The ring flip process appears to occur via a pseudo seven-co-ordinate transition state, in which both O atoms of the acetal ring are loosely bound to the metal. The pendant and co-ordinated acetal rings in the complexes of L² are exchanged via the tick-tock mechanism; the free energies of activation are in the range 75–79 kJ mol⁻¹.