Application of a Structure/Oxidation‐State Correlation to Complexes of Bridging Azo Ligands |
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| Authors: | Amit Das Dipl.‐Chem. Thomas Michael Scherer Dr. Shaikh M. Mobin Prof. Dr. Wolfgang Kaim Prof. Dr. Goutam Kumar Lahiri |
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| Affiliation: | 1. Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai‐400076 (India);2. Institut für Anorganische Chemie, Universit?t Stuttgart, Pfaffenwaldring 55, D‐70550, Stuttgart (Germany) |
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| 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 (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 3 2+ 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 1 2+ (dNN=1.374(6) Å) and the symmetrical compound 4 3+ (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+ ( 1 2+) with strong antiferromagnetic coupling between abpy.? and RuIII (DFT (B3LYP/LANL2DZ/6‐31G*)‐calculated triplet–singlet energy separation ES=1?ES=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 (abpy0)‐bridged Class I mixed‐valent Ru1IIIRu2II 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. |
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| Keywords: | azo compounds electrochemistry EPR spectroscopy noninnocent ligands ruthenium |
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