Controlling metal–ligand–metal oxidation state combinations by ancillary ligand (L) variation in the redox systems [L₂Ru(μ-boptz)RuL₂]ⁿ, boptz=3,6-bis(2-oxidophenyl)-1,2,4,5-tetrazine, and L=acetylacetonate, 2,2'-bipyridine, or 2-phenylazopyridine

Abstract

The new compounds [(acac)₂Ru(μ-boptz)Ru(acac)₂] (1), [(bpy)₂Ru(μ-boptz)Ru(bpy)₂](ClO₄)₂ (2-(ClO₄)₂), and [(pap)₂Ru(μ-boptz)Ru(pap)₂](ClO₄)₂ (3-(ClO₄)₂ were obtained from 3,6-bis(2-hydroxyphenyl)-1,2,4,5-tetrazine (H₂boptz), the crystal structure analysis of which is reported. Compound 1 contains two antiferromagnetically coupled (J=-36.7 cm^-1) Ru^III centers. We have investigated the role of both the donor and acceptor functions containing the boptz_2- bridging ligand in combination with the electronically different ancillary ligands (donating acac^-, moderately π-accepting bpy, and strongly π-accepting pap; acac=acetylacetonate, bpy=2,2'-bipyridine pap=2-phenylazopyridine) by using cyclic voltammetry, spectroelectrochemistry and electron paramagnetic resonance (EPR) spectroscopy for several in situ accessible redox states. We found that metal–ligand–metal oxidation state combinations remain invariant to ancillary ligand change in some instances; however, three isoelectronic paramagnetic cores Ru(μ-boptz)Ru showed remarkable differences. The excellent tolerance of the bpy co-ligand for both Ru^III and Ru^II is demonstrated by the adoption of the mixed-valent form in [L₂Ru(μ-boptz)RuL₂]³⁺, L=bpy, whereas the corresponding system with pap stabilizes the Ru^II states to yield a phenoxyl radical ligand and the compound with L=acac^-contains two Ru^III centers connected by a tetrazine radical-anion bridge.