Molecular and Electronic Structures of Mononuclear and Dinuclear Titanium Complexes Containing π-Radical Anions of 2,2′-Bipyridine and 1,10-Phenanthroline: An Experimental and DFT Computational Study

Whereas reaction of [(eta(5)-Cp*)(TiCl3)-Cl-IV]0 (1) with 2 equiv of neutral 2,2'-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(eta(5)-Cp*)Ti-III(bpy(center dot))(2)](0) (2), performing the same reaction with only 1 equiv each of magnesium and bpy provides the dinuclear complex [{(eta(5)-Cp*)Ti(mu-Cl)(bpy(center dot))}(2)](0) (3). Conducting the latter reaction using 1,10-phenanthroline (phen) in place of bpy resulted in formation of dinuclear [{(eta(5)-Cp*)Ti(mu-Cl)(phen(center dot))}(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) angstrom in the N,N'-coordinated bpy ligands of 2 and 3, respectively, are indicative of the presence of (bpy(center dot))(1-) ligands, rather than neutral (bpy(0)). The electronic spectra (300-1600 nm) of these two complexes are similar in form, and contain intense pi -> pi* transitions associated with the (bpy(center dot))(1-) radical anion. Temperature dependent magnetic susceptibility measurements (4-300 K) show that mononuclear 2 possesses a temperature independent magnetic moment of 1.73 mu(B), 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 eta(5)-Cp* ligand and two (bpy(center dot))(1-). Strong intramolecular antiferromagnetic coupling of these three unpaired spins, one each on the TiIII center and on the two (bpy center dot)(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 p-stacking of the N,N'-coordinated (bpy(center dot))(1-)/(phen(center dot))(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 Ti-III center, couple weakly in a ferromagnetic fashion to yield the experimentally observed S = 1 ground states.