pH-dependent absorption and emission properties of a ReI complex working as a carboxylate ligand for Cu2+

The fac-Re(CO)3(bipy-COOH)Cl complex (bipy=2,2′-bipyridine) bears a carboxylic function in the 5-position of the bipy ligand: due to the kinetic inertness of the ReI center, the complex behaves as a R-COOH carboxylic acid. In aqueous solutions, it coordinates Cu2+, according to pH-dependent equilibria, to give the dimeric species Cu[fac-ReI(CO)3(bipy-COO)Cl]2 and {Cu[fac-Re(CO)3(bipy-COO)Cl]2(OH)2}2−, whose formation constants have been determined by means of potentiometric titration experiments. The fac-Re(CO)3(bipy-R)Cl core is luminescent, and interestingly, its emitting properties are pH-dependent. In particular, its luminescence intensity increases with the formation of the {Cu[fac-Re(CO)3(bipy-COO)Cl]2(OH)2}2− species, visually signaling the formation of this hydroxo-containing multicenter complex.