Mixed Donor–Acceptor-Derived N,N′-Diarylpyrazine-2,5-dicarboxamido-Bridged Diruthenium Systems: Structures, Magnetic Properties, and Electronic Forms in Multiredox States

Herein, we report diruthenium(III) complexes bridged by the deprotonated N,N-diphenylpyrazine-2,5-dicarboxamide (H2L1) and N,N-dimesitylpyrazine-2,5-dicarboxamide (H2L2) ligands in meso diastereomeric form, namely, (acac)(2)Ru-III(mu-L-1)Ru-III(acac)(2) (1, acac = acetylacetonato) and (acac)(2)Ru-III(mu-L-2)Ru-III(acac)(2) (2), as well as the monodeprotonated HL1-derived mononuclear counterpart (acac)(2)Ru-III(HL1) (3). Variable-temperature magnetic studies of 1 revealed a weak bridge-mediated intramolecular antiferromagnetic interaction with J = -7.58 cm(-1). The dinuclear complexes exhibited anisotropic electron paramagnetic resonance (EPR) signals at g approximate to 2 along with half-field signals at g(1/2) approximate to 4, whereas the mononuclear counterpart 3 displayed an EPR signal with rhombic symmetry. Complexes 1 and 2 exhibited two oxidation and reduction steps with comproportionation constants (K-c) of 10(2) and 10(6) or 10(7). Complex 3 displayed one oxidation and two reductions steps. The electrogenerated compounds 1(-) and 2(-) exhibited metal-based EPR signals and a very weak and broad near-infrared (NIR) transition at approximate to 1200 nm. The C=O bands of the bridge were monitored through IR spectroelectrochemistry, which revealed a shift of these bands to lower energies on reduction. However, the native and reduced states have the same number of bands, which suggests a similar local symmetry for both redox forms and a delocalized electronic situation in the reduced compounds. A combination of electrochemistry and spectroelectrochemistry was used to probe the electronic structures of the various redox states in these complexes.