A luminescent tricarbonylchlororhenium(I) complex featuring a flexible “crown ether” ligand. Manipulation of photoexcited state properties via binding of small cations†

Abstract

A luminescent complex, [Re(CO)₃(Cl)(LL)] where LL is 1,11-bis(4-pyridylcarboxy)-3,6,9-trioxaundecane, was prepared, and the crystal structures of two solvates were determined. The flexible polyether chain of the chelating ligand adopts an all gauche conformation about the aliphatic C–C bonds, and does not appear to introduce any appreciable constraints on the geometry at the octahedral metal center. The cavity formed by the “crown ether” has a diameter of ca. 7 Å. Binding of cations causes a decrease of the luminescence lifetime of the Re^IILL^– excited state, consistent with an “energy gap” effect and with a rapid excited-state equilibrium between bound and unbound moieties. Binding constants were determined for several alkali metal ions, and for the ammonium cation. The chemical affinity of the crown-functionalized ligand is not specific for any of the cations studied, but, instead, is similar for a range of cations. Comparative studies with a complex containing ester-functionalized pyridine ligands, but lacking the polyether linkage necessary to form the crown, also showed excited state binding of small cations, indicating that the anionic ester pyridyl fragment, rather than the polyether fragment, is responsible for the binding.