Marked influence of the bridging carbonylligands on the photo- and electrochemistry of the clusters [Ru3(CO)8(μ-CO)2(α-diimine)] (α-diimine = 2,2′-bipyridine, 4,4′-dimethyl-2,2′-bipyridine and 2,2′-bipyrimidine)

Abstract

Bonding, photochemical and electrochemical properties of the clusters [Ru₃(CO)₈(μ-CO)₂(α-diimine)] (α-diimine = 2,2′-bipyridine (1), 4,4′-dimethyl-2,2′-bipyridine (2) and 2,2′-bipyrimidine (3)) are strongly influenced by the presence of bridging carbonyl ligands. Irradiation at 471 nm initially results in the population of a σ(Ru₃)π*(α-diimine) excited state. From this state, fast decay takes place to the optically hardly directly accessible π(Ru/μ-CO)π*(α-diimine) lowest excited state. These assignments agree with theoretical (TD-DFT) results, resonance Raman and picosecond time-resolved infrared spectra. The involvement of the bridging carbonyl ligands in the electron transfer increases the energetic barrier for the formation of open-structure photoproducts such as biradicals and zwitterions. Zwitterions were therefore only obtained in strongly coordinating media such as pyridine at 250 K. The bridging carbonyl ligands also stabilize the radical anions produced upon one-electron reduction of the clusters [Ru₃(CO)₈(μ-CO)₂(α-diimine)] and observed with cyclic voltammetry, EPR and IR spectroelectrochemistry (for α-diimine = 2,2′-bipyrimidine). In contrast, open-triangle intermediates formed along the reduction path to [Ru(CO)₂(α-diimine)]_n and [Ru₂(CO)₈]²⁻ are more reactive than their triosmium analogues.