Separation and characterization of the various stereoisomers of a dinuclear tricarbonylrhenium(I) complex using a theoretical and experimental approach

Background: The development of robust chiral transition metal complexes is of paramount importance for advanced asymmetric catalysis, diagnostic and therapy, as well as for the emerging field of solid-state luminescent materials. Tricarbonylrhenium(I) complexes are intrinsically chiral, and their association as dinuclear molecules introduces additional layers of diastereoisomerism and enantiomerism, enhancing their versatility for potential applications. Up to now, separation of dinuclear complexes has made it possible to isolate diasteroisomers or enantiomers bearing a second chiral center other than rhenium. However, binuclear complexes carrying only double octahedral chirality have not been isolated in enantiopure form so far. Results: In the present work, a dinuclear complex featuring two bidentate ligands, specifically two 3-(2-pyridyl)-1,2,4-triazole (pyta) ligands bearing C8-alkyl chain, merged via a phenyl linker, was considered. DFT calculations was first used to predict the existence of three distinct isomers, which differ by the position of the ancillary ligands with respect to the organic ligand. Subsequently, the three stereoisomers were successfully isolated using chiral HPLC. Their absolute configuration, i.e. (C_Re,C_Re), (A_Re,A_Re) or meso (C_Re,A _Re), was determined by circular dichroism. For each stereoisomer, evidence was given for a mixture of two rotamers (syn and anti), differing by the relative orientation of the C8-alkyl chain. The structure of all chiral species was perfectly assigned by NMR TOCSY and NOESY experiments, circular dichroism and supported by DFT calculations. Significance: This work marks the first successful separation and complete characterization of enantiomeric species of a binuclear tricarbonylrhenium complex in which chirality arises solely from the two metal centers connected by a linker. This achievement opens up new possibilities of applications where precise control of chirality is crucial.