Towards near-IR emissive rhenium tricarbonyl complexes: Synthesis and characterisation of unusual 2,2′-biquinoline complexes

Diethyl-2,2′-biquinoline-4,4′-dicarboxylate (debq) reacts with pentacarbonylbromorhenium in toluene to give fac-[Re(CO)3(debq)Br]; subsequent reaction with AgOTf in MeCN affords the cationic complex fac-[Re(CO)3(debq)(MeCN)]OTf. X-ray crystallographic data on fac-[Re(CO)3(debq)Br] show that the debq ligand is coordinated in a bidentate fashion at an angle of 32° from the equatorial plane of the complex: DFT calculations confirm this as the lowest energy conformation. fac-[Re(CO)3(debq)Br] and fac-[Re(CO)3(debq)(MeCN)]OTf both possess 1MLCT absorptions in the visible region, the former most red-shifted to ca. 465 nm. Both complexes demonstrated unusual luminescent properties: in solution, emission appears to be dominated by ligand-centred processes, whereas only 3MLCT emission, tailing into the near-IR, was observed in the solid state at 671 (τ = 59 ns) and 711 nm (τ = 19 ns) for fac-[Re(CO)3(debq)(MeCN)]OTf and fac-[Re(CO)3(debq)Br], respectively. However, the debq ligand is not labile in these complexes, which are robust to competing ligands and coordinating solvents.