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

Abstract

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)2RuIII(µ‐L1)RuIII(acac)2 (1, acac = acetylacetonato) and (acac)2RuIII(µ‐L2)RuIII(acac)2 (2), as well as the monodeprotonated HL1‐derived mononuclear counterpart (acac)2RuIII(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 ≈ 2 along with half‐field signals at g1/2 ≈ 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 (Kc) of 102 and 106 or 107. 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 λ ≈ 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.