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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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journal contribution
posted on 06.07.2009, 00:00 by Sipke H. Wadman, Remco W. A. Havenith, František Hartl, Martin Lutz, Anthony L. Spek, Gerard P. M. van Klink, Gerard van Koten
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-NCN) 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-NCN)}(μ-tppz){Ru(tpy)}]3+ (R-NC(H)N = 4-R-1,3-dipyridylbenzene, R = H ([4a]3+), CO2Me ([4b]3+)), and [{Ru(R-NCN)}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.