title: Redox chemistry and electronic properties of 2,3,5,6-tetrakis(2-pyridyl)pyrazine-bridged diruthenium complexes controlled by N,C,N'-biscyclometalated ligands.
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(wedge)C(wedge)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)(mu-tppz)](4+) (tpy = 2,2':2'',6-terpyridine) ([3](4+)) as a model compound, the complexes [{Ru(R-N(wedge)C(wedge)N)}(mu-tppz){Ru(tpy)}](3+) (R-N(wedge)C(H)(wedge)N = 4-R-1,3-dipyridylbenzene, R = H ([4a](3+)), CO(2)Me ([4b](3+))), and [{Ru(R-N(wedge)C(wedge)N)}(2)(mu-tppz)](2+), (R = H ([5a](2+)), CO(2)Me ([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 = CO(2)Me) 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.
authors.affiliation: Chemical Biology & Organic Chemistry, Utrecht University, Padualaan 8, 3584 CH Utrecht, The Netherlands.
authors.forename: Sipke H
authors.initials: SH
authors.lastname: Wadman
authors.forename: Remco W A
authors.initials: RW
authors.lastname: Havenith
authors.forename: Frantisek
authors.initials: F
authors.lastname: Hartl
authors.forename: Martin
authors.initials: M
authors.lastname: Lutz
authors.forename: Anthony L
authors.initials: AL
authors.lastname: Spek
authors.forename: Gerard P M
authors.initials: GP
authors.lastname: van Klink
authors.forename: Gerard
authors.initials: G
authors.lastname: van Koten
mesh_terms: D008024:Ligands; D008670:Metals / Q000737:chemistry*; D010084:Oxidation-Reduction; D011719:Pyrazines / Q000737:chemistry*; D017975:Ruthenium Compounds / Q000737:chemistry*; D019265:Spectroscopy, Near-Infrared
chemical_list: D008024:Ligands; D008670:Metals; D011719:Pyrazines; D017975:Ruthenium Compounds