A bis(terpyridine)ruthenium complex with three redox-active amine sites: Electrochemical, optical, and computational studies

Two ruthenium complexes, [Ru(NN)(ttpy)] ²⁺ and [Ru(NN)(daatpy)] ²⁺, have been designed and prepared, where NN is bis(amine) ligand 4′-tolyl-5,5″-bis(di-p-anisylamino)-2,2′: 6′,2″-terpyridine, ttpy is 4′-tolyl-2,2′:6′, 2″-terpyridine, and daatpy is 4′-di-p-anisylamino-2,2′: 6′,2″-terpyridine. Complex [Ru(NN)(daatpy)] ²⁺ contains three redox-active amine groups and has been characterized by single-crystal X-ray analysis. These two complexes display much-enhanced light absorption capabilities with respect to the prototype compound [Ru(tpy) ₂] ²⁺ (tpy = 2,2′:6′,2″-terpyridine), which has been rationalized on the basis of time-dependent density functional theory calculations. Electrochemical and optical studies showed that there was little electronic coupling between two amine sites in complex [Ru(NN)(ttpy)] ²⁺. On the other hand, a ligand-to-ligand (N → N′ ^•+) charge-transfer band has been observed at 1430 nm for singly and doubly oxidized forms of [Ru(NN)(daatpy)] ²⁺, and an electronic coupling parameter of 1000 cm ^-1 was derived using the Hush formula. This band is interpreted as a charge transfer from the neutral amine of the daatpy ligand to oxidized aminium units in the NN ligand.