Application of a Structure/Oxidation‐State Correlation to Complexes of Bridging Azo Ligands

Abstract

Based on data from more than 40 crystal structures of metal complexes with azo-based bridging ligands (2,2'-azobispyridine, 2,2'-azobis(5-chloropyrimidine), azodicarbonyl derivatives), a correlation between the N-N bond lengths (d(NN)) and the oxidation state of the ligand (neutral, neutral/back-donating, radical-anionic, dianionic) was derived. This correlation was applied to the analysis of four ruthenium compounds of 2,2'-azobispyridine (abpy), that is, the new asymmetrical rac-[(acac)(2)Ru1(μ-abpy)Ru2(bpy)(2)](ClO(4))(2) ([1](ClO(4))(2)), [Ru(acac)(2)(abpy)] (2), [Ru(bpy)(2)(abpy)](ClO(4))(2) ([3](ClO(4))(2)), and meso-[(bpy)(2)Ru(μ-abpy)Ru(bpy)(2)(ClO(4))(3) ([4](ClO(4))(3); acac(-) =2,4-pentanedionato, bpy=2,2'-bipyridine). In agreement with DFT calculations, both mononuclear species 2 and 3(2+) can be described as ruthenium(II) complexes of unreduced abpy(0), with 1.295(5)<d(NN) <1.320(3) Å, thereby exhibiting effects from π back-donation. However, the abpy ligand in both the asymmetrical diamagnetic compound 1(2+) (d(NN) =1.374(6) Å) and the symmetrical compound 4(3+) (d(NN) =1.360(7), 1.368(8) Å) must be formulated as abpy(·-). Remarkably, the addition of [Ru(II)(bpy)(2)](2+) to mononuclear [Ru(II)(acac)(2)(abpy(0))] induces intracomplex electron-transfer under participation of the noninnocent abpy bridge to yield rac-[(acac)(2) Ru1(III)(μ-abpy(·-) )Ru2(II)(bpy)(2)](2+) (1(2+)) with strong antiferromagnetic coupling between abpy(·-) and Ru(III) (DFT (B3LYP/LANL2DZ/6-31G*)-calculated triplet-singlet energy separation E(S=1)-E(S=0) =11739 cm(-1)). Stepwise one-electron transfer was studied for compound 1(n), n=1-, 0, 1+, 2+, 3+, by UV/Vis/NIR spectroelectrochemistry, EPR spectroscopy, and by DFT calculations. Whereas the first oxidation of compound 1(2+) was found to mainly involve the central ligand to produce an (abpy(0))-bridged Class I mixed-valent Ru1(III)Ru2(II) species, the first reduction of compound 1(2+) affected both the bridge and Ru1 atom to form a radical complex (1(+)), with considerable metal participation in the spin-distribution. Further reduction moves the spin towards the {Ru2(bpy)(2)} entity.