title: Redox-rich spin-spin-coupled semiquinoneruthenium dimers with intense near-IR absorption.
abstract: Using the [RuCl(μ-tppz)ClRu](2+) [tppz = 2,3,5,6-tetrakis(2-pyridyl)pyrazine] platform for bridging two o-quinone/catecholate two-step redox systems (unsubstituted, Q(n), or 3,5- di-tert-butyl-substituted, DTBQ(n)), we have obtained the stable complexes [(Q(•-))Ru(II)Cl(μ-tppz)ClRu(II)(Q(•-))] (1) and the structurally characterized [(DTBQ(•-))Ru(II)Cl(μ-tppz)ClRu(II)(DTBQ(•-))] (2). The compounds exhibit mostly quinone-ligand-based redox activity within a narrow potential range, high-intensity near-IR absorptions (λ(max) ≈ 920 nm; ε > 50,000 M(-1) cm(-1)), and variable intra- and intermolecular spin-spin interactions. Density functional theory calculations, electron paramagnetic resonance (EPR), and spectroelectrochemical results (UV-vis-near-IR region) for three one-electron-reduction and two one-electron-oxidation processes were used to probe the electronic structures of the systems in the various accessible valence states. EPR spectroscopy of the singly charged doublet species showed semiquinone-type response for 1(+), 2(+), and 2(-), while 1 exhibits more metal based spin, a consequence of the easier reduction of Q as compared to DTBQ. Comparison with the analogous redox series involving a more basic N-phenyliminoquinone ligand reveals significant differences related to the shifted redox potentials, different space requirements, and different interactions between the metals and the quinone-type ligands. As a result, the tppz bridge is reduced here only after full reduction of the terminal quinone ligands to their catecholate states.
authors.affiliation: Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India.
authors.forename: Tanaya
authors.initials: T
authors.lastname: Kundu
authors.forename: Biprajit
authors.initials: B
authors.lastname: Sarkar
authors.forename: Tapan Kumar
authors.initials: TK
authors.lastname: Mondal
authors.forename: Shaikh M
authors.initials: SM
authors.lastname: Mobin
authors.forename: Francisco A
authors.initials: FA
authors.lastname: Urbanos
authors.forename: Jan
authors.initials: J
authors.lastname: Fiedler
authors.forename: Reyes
authors.initials: R
authors.lastname: Jiménez-Aparicio
authors.forename: Wolfgang
authors.initials: W
authors.lastname: Kaim
authors.forename: Goutam Kumar
authors.initials: GK
authors.lastname: Lahiri
mesh_terms: D016227:Benzoquinones / Q000737:chemistry*; D018360:Crystallography, X-Ray; D019281:Dimerization; D004578:Electron Spin Resonance Spectroscopy; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010084:Oxidation-Reduction; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*; D013056:Spectrophotometry, Ultraviolet; D019265:Spectroscopy, Near-Infrared; D013237:Stereoisomerism
chemical_list: D016227:Benzoquinones; D009942:Organometallic Compounds; C025232:semiquinone radicals; D012428:Ruthenium