UCL Discovery

Abstract

The non-racemic chiral ligands 2-[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L-1) and 2,6-bis[(4R,6R)-4,6-dimethyl-1,3-dioxan-2-yl]pyridine (L-2) reacted with the [ReX(CO)(5)] compounds (X=Cl, Br, or I) to form fac-[ReX(CO)(3)(L-1)] and fac-[ReX(CO)(3)(L-2)], 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)(L-1)] and [ReBr(CO)(3)(L-2)]. There are four possible diastereoisomers, depending on the configuration at the metal and the acetal carbon atom. In the solid state [ReI(CO)(3)(L-1)] and [ReBr(CO)(3)(L-2)] exist exclusively as the RR and SR diastereoisomers, respectively. In solutions of the complexes of L-1 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 L-2, 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)(L-1)] 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 L-2 are exchanged via the tick-tock mechanism; the free energies of activation are in the range 75-79 kJ mol(-1).