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- Title
Application of a Structure/Oxidation-State Correlation to Complexes of Bridging Azo Ligands.
- Authors
Das, Amit; Scherer, Thomas Michael; Mobin, Shaikh M.; Kaim, Wolfgang; Lahiri, Goutam Kumar
- 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 NN bond lengths ( dNN) 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)2Ru1(μ-abpy)Ru2(bpy)2](ClO4)2 ([ 1](ClO4)2), [Ru(acac)2(abpy)] ( 2), [Ru(bpy)2(abpy)](ClO4)2 ([ 3](ClO4)2), and meso-[(bpy)2Ru(μ-abpy)Ru(bpy)2](ClO4)3 ([ 4](ClO4)3; acac−=2,4-pentanedionato, bpy=2,2′-bipyridine). In agreement with DFT calculations, both mononuclear species 2 and 32+ can be described as ruthenium(II) complexes of unreduced abpy0, with 1.295(5)< dNN<1.320(3) Å, thereby exhibiting effects from π back-donation. However, the abpy ligand in both the asymmetrical diamagnetic compound 12+ ( dNN=1.374(6) Å) and the symmetrical compound 43+ ( dNN=1.360(7), 1.368(8) Å) must be formulated as abpy.−. Remarkably, the addition of [RuII(bpy)2]2+ to mononuclear [RuII(acac)2(abpy0)] induces intracomplex electron-transfer under participation of the noninnocent abpy bridge to yield rac-[(acac)2Ru1III(μ-abpy.−)Ru2II(bpy)2]2+ ( 12+) with strong antiferromagnetic coupling between abpy.− and RuIII (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 12+ was found to mainly involve the central ligand to produce an (abpy0)-bridged Class I mixed-valent Ru1IIIRu2II species, the first reduction of compound 12+ 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.
- Publication
Chemistry - A European Journal, 2012, Vol 18, Issue 35, p11007
- ISSN
0947-6539
- Publication type
Article
- DOI
10.1002/chem.201201049