Abstract

Whereas reaction of [(η5-Cp*)TiIVCl3]0 (1) with 2 equiv of neutral 2,2′-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η5-Cp*)TiIII(bpy•)2]0 (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(η5-Cp*)Ti(μ-Cl)(bpy•)}2]0 (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(η5-Cp*)Ti(μ-Cl)(phen•)}2]0 (4). The structures of 2, 3, and 4 have all been determined by high-resolution X-ray crystallography at 153 K; the Cpy–Cpy distances of 1.420(3) and 1.431(4) Å in the N,N′-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy•)1– ligands, rather than neutral (bpy0). The electronic spectra (300–1600 nm) of these two complexes are similar in form, and contain intense π → π* transitions associated with the (bpy•)1– radical anion. Temperature dependent magnetic susceptibility measurements (4–300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 μB, which is indicative of an S = 1/2 ground state. Broken symmetry density functional theory (BS-DFT) calculations yield a picture consistent with the experimental findings, in which the central Ti atom possesses a +3 oxidation state and is coordinated by a η5-Cp* ligand and two (bpy•)1–. Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the TiIII center and on the two (bpy•)1– ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear 3 and 4 display weak but significant ferromagnetic coupling, and indicate that these complexes possess S = 1 ground states. The mechanism of the spin coupling phenomenon that yields the observed behavior was analyzed using BS-DFT calculations, and it was discovered that the tight π-stacking of the N,N′-coordinated (bpy•)1–/(phen•)1– ligands in these two complexes results from direct overlap of their SOMOs and formation of a two-electron multicentered bond. This yields a diamagnetic {(bpy)2}2–/{(phen)2}2– bridging unit whose doubly occupied HOMO is spread equally over both ligands. The two remaining unpaired electrons, one at each TiIII center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states.