Sensitive oxidation state ambivalence in unsymmetrical three-center (M/Q/M) Systems [(acac)₂Ru(μ-Q)Ru(acac)₂]ⁿ, Q = 1,10-phenanthroline-5,6-dione or 1,10-phenanthroline-5,6-diimine (n = +, 0, -, 2-)

Abstract

The new redox systems [(acac)₂Ru(μ -Q¹)Ru(acac)₂]ⁿ (1ⁿ) and [(acac)₂Ru(μ -Q²)Ru(acac)₂]ⁿ (2ⁿ) with Q¹ = 1,10-phenanthroline-5,6-dione and Q² = 1,10-phenanthroline-5,6-diimine were studied for n = +, 0, -, and 2- using UV-Vis-NIR spectroelectrochemistry and, in part, EPR and susceptometry. The ligands can bind the first metal (left) through the phenanthroline nitrogen atoms and the second metal (right) at the o-quinonoid chelate site. The neutral compounds are already different:∅ Compound 1 is formulated as a Ru^II(μ -Q¹)·- Ru^III species with partially coupled semiquinone and ruthenium(III) centers. In contrast, a Ru^III(μ -Q²)^2- RuIII structure is assigned to 2, which shows a weak antiferromagnetic spin-spin interaction (J = -1.14 cm^-1) and displays an intense half-field signal in the EPR spectrum. The one-electron reduced forms are also differently formulated as Ru^II(μ-Q¹)^2- Ru^III for 1^- with a Ru^III-typical EPR response and as Ru^II(μ-Q²)^{° -} Ru^II for 2^- with a radical-type EPR signal at g = 2.0020. In contrast, both 1^2- and 2^2- can only be described as Ru^II(μ-Q)²-Ru^II species. The monooxidized forms 1⁺ and 2⁺ show very similar spectroscopy, including a Ru^III-type EPR signal. Although no unambiguous assignment was possible here for the alternatives Ru^II(μ-Q) 0Ru^III,Ru^III(μ-Q)²-Ru^IV orRu^III (μ-Q)^·-Ru^III, the last description is favored. The reasons for identical or different oxidation state combinations are discussed.