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

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 NN 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.