Electronic Structures of the Electron Transfer Series [M(bpy)3]n, [M(tpy)2]n, and [Fe(tbpy)3]n (M = Fe, Ru; n=3+, 2+, 1+, 0, 1-): A Mossbauer Spectroscopic and DFT Study

The five-membered electron-transfer series [M(bpy)3]n and [M(tpy)2]n (M = Fe, Ru; bpy = 2,2'-bipyridine, tpy = 2,2':6',2?-terpyridine) and [Fe(tbpy)3]n (tbpy = 4,4'-di-tert-butyl-2,2'-bipyridine; n = 3+, 2+, 1+, 0, 1) have been investigated and the electronic structure of the so called low-valent complexes (n = 1+, 0, 1) have been established by a combination of electro- and magnetochemistry, electron paramagnetic resonance (EPR) and Mossbauer spectroscopy, X-ray crystallography, and DFT calculations. These complexes are accessed by reduction of the d6 S = 0 dicationic starting materials [M(bpy)3]2+ and [M(tpy)2]2+ (M = Fe, Ru) and [Fe(tbpy)3]2+. The monocations [M(bpy.)(bpy0)2]1+ (S = 1/2) and [M(tpy.)(tpy0)]1+ (S = 1/2) (M = FeII, RuII) also contain a low-spin (t2g)6 divalent metal center, plus a single radical monoanion (bpy.)1 or (tpy.)1, and two or one neutral (bpy0) or (tpy0) ligands, respectively. The unpaired electron resides in a ligand p* orbital. The neutral complexes [FeII(bpy.)2(bpy0)]0 and [RuII(tpy.)2]0 were found, by DFT calculations, to possess an S = 1 ground state that is attained by weak intramolecular ferromagnetic coupling between two ligand radical anions and a singlet excited state (S = 0). In contrast, the neutral species [RuII(bpy.)2(bpy0)]0 possesses an S = 0 ground state. The metal centers of these neutral complexes retain a low-spin (t2g)6 configuration. Remarkably, the corresponding neutral complex [FeII(tpy.)2]0 contains high-spin FeII (SFe = 2). Strong intramolecular antiferromagnetic metalradical coupling yields an experimentally observed S = 1 ground state. The monoanions [MII(bpy.)3]1 (M = Fe, Ru) are composed of a low-spin metal ion [Fe, Ru; (t2g)6; SM = 0] and three (bpy.)1 radical anions.