Redox Chemistry and Electronic Properties of 2,3,5,6-Tetrakis(2-pyridyl)pyrazine-Bridged Diruthenium Complexes Controlled by N,C,N′-BisCyclometalated Ligands

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Abstract
To investigate the consequences of cyclometalation for electronic communication in dinuclear ruthenium complexes, a series of 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) bridged diruthenium complexes was prepared and studied. These complexes have a central tppz ligand bridging via nitrogen-to-ruthenium coordination bonds, while each ruthenium atom also binds either a monoanionic, N,C,N′-terdentate 2,6-bis(2′-pyridyl)phenyl (R-N∧C∧N) ligand or a 2,2′:6′,2′′-terpyridine (tpy) ligand. The N,C,N′-, that is, biscyclometalation, instead of the latter N,N′,N′′-bonding motif significantly changes the electronic properties of the resulting complexes. Starting from well-known [{Ru(tpy)}2(μ-tppz)]4+ (tpy = 2,2′:2′′,6-terpyridine) ([3]4+) as a model compound, the complexes [{Ru(R-N∧C∧N)}(μ-tppz){Ru(tpy)}]3+ (R-N∧C(H)∧N = 4-R-1,3-dipyridylbenzene, R = H ([4a]3+), CO2Me ([4b]3+)), and [{Ru(R-N∧C∧N)}2(μ-tppz)]2+, (R = H ([5a]2+), CO2Me ([5b]2+)) were prepared with one or two N,C,N′-cyclometalated terminal ligands. The oxidation and reduction potentials of cyclometalated [4]3+ and [5]2+ are shifted negatively compared to non-cyclometalated [3]4+, the oxidation processes being affected more significantly. Compared to [3]4+, the electronic spectra of [5]2+ display large bathochromic shifts of the main MLCT transitions in the visible spectral region with low-energy absorptions tailing down to the NIR region. One-electron oxidation of [3]4+ and [5]2+ gives rise to low-energy absorption bands. The comproportionation constants and NIR band shape correspond to delocalized Robin-Day class III compounds. Complexes [4a]3+ (R = H) and [4b]3+ (R = CO2Me) also exhibit strong electronic communication, and notwithstanding the large redox-asymmetry the visible metal-to-ligand charge-transfer absorption is assigned to originate from both metal centers. The potential of the first, ruthenium-based, reversible oxidation process is strongly negatively shifted. On the contrary, the second oxidation is irreversible and cyclometalated ligand-based. Upon one-electron oxidation, a weak and low-energy absorption arises.

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