Electronic Structures of the Electron Transfer Series [M(bpy)₃]<i>ⁿ</i>, [M(tpy)₂]<i>ⁿ</i>, and [Fe(^tbpy)₃]<i>ⁿ</i> (M = Fe, Ru; <i>n</i> = 3+, 2+, 1+, 0, 1–): A Mössbauer Spectroscopic and DFT Study

Abstract

Abstract 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( t bpy) 3 ] n ( t bpy = 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 Mössbauer spectroscopy, X‐ray crystallography, and DFT calculations. These complexes are accessed by reduction of the d 6 S = 0 dicationic starting materials [M(bpy) 3 ] 2+ and [M(tpy) 2 ] 2+ (M = Fe, Ru) and [Fe( t bpy) 3 ] 2+ . The monocations [M(bpy · )(bpy 0 ) 2 ] 1+ ( S = 1/2) and [M(tpy · )(tpy 0 )] 1+ ( S = 1/2) (M = Fe II , Ru II ) also contain a low‐spin (t 2g ) 6 divalent metal center, plus a single radical monoanion (bpy · ) 1– or (tpy · ) 1– , and two or one neutral (bpy 0 ) or (tpy 0 ) ligands, respectively. The unpaired electron resides in a ligand π* orbital. The neutral complexes [Fe II (bpy · ) 2 (bpy 0 )] 0 and [Ru II (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 [Ru II (bpy · ) 2 (bpy 0 )] 0 possesses an S = 0 ground state. The metal centers of these neutral complexes retain a low‐spin (t 2g ) 6 configuration. Remarkably, the corresponding neutral complex [Fe II (tpy · ) 2 ] 0 contains high‐spin Fe II ( S Fe = 2). Strong intramolecular antiferromagnetic metal–radical coupling yields an experimentally observed S = 1 ground state. The monoanions [M II (bpy · ) 3 ] 1– (M = Fe, Ru) are composed of a low‐spin metal ion [Fe, Ru; (t 2g ) 6 ; S M = 0] and three (bpy · ) 1– radical anions.