Controlling the reactivity of ruthenium(II) arene complexes by tether ring-opening

The closed- and open-tethered Ru(II) eta(6)-arene complexes [Ru(II)(eta(6):eta(1)-C(6)H(5)(C(6)H(4))NH(2))(en)]Cl(2) (2) and [Ru(II)(eta(6)-C(6)H(5)(C(6)H(4))NH(2))Cl(en)]Cl (3), where en is ethylenediamine, have been synthesized and their X-ray structures determined. Interconversion between 2 and 3, that is, tethered-arene ring-closure and ring-opening, in different solvents has been investigated. Complex 2 opens in dimethylsulfoxide (DMSO) by solvent-induced dissociation of the NH(2) group of the pendant arm. In methanol, however, equilibrium between 2 and 3 is reached after 12 h when both species coexist in solution in a ratio of 2:1 (open/closed). In water (pH 7), complete ring closure of 3 to 2 at 298 K occurs in less than 2 h. The tether ring of complex 2 opens at basic pH and closes at neutral pH. Complex 2 opens over time (18 h) in concentrated HCl. The opening-closing process is fully reversible in the pH range 2-12. Density Functional Theory calculations have been used to obtain insights into the electronic structure of complexes 2 and 3, their UV-vis properties, and their stability compared to their aqua derivatives. Control of tether-ring-opening can contribute toward a strategy for activation and for achieving cytotoxic selectivity of ruthenium arene anticancer drugs.