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 [(η⁵-Cp*)­Ti^IVCl₃]⁰ (<b>1</b>) with 2 equiv of neutral 2,2′-bipyridine (bpy) and 1.5 equiv of magnesium in tetrahydrofuran affords the mononuclear complex [(η⁵-Cp*)­Ti^III(bpy^•)₂]⁰ (<b>2</b>), 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 (<b>3</b>). 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 (<b>4</b>). The structures of <b>2</b>, <b>3</b>, and <b>4</b> have all been determined by high-resolution X-ray crystallography at 153 K; the C_py–C_py distances of 1.420(3) and 1.431(4) Å in the <i>N</i>,<i>N</i>′-coordinated bpy ligands of <b>2</b> and <b>3</b>, 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 <b>2</b> possesses a temperature independent magnetic moment of 1.73 μ_B, which is indicative of an <i>S</i> = 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 <i>antiferromagnetic</i> coupling of these three unpaired spins, one each on the Ti^III center and on the two (bpy^•)^1– ligands, affords the experimentally observed doublet ground state. The magnetic susceptibility measurements for dinuclear <b>3</b> and <b>4</b> display weak but significant <i>ferromagnetic</i> coupling, and indicate that these complexes possess <i>S</i> = 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 <i>N</i>,<i>N</i>′-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 Ti^III center, couple weakly in a ferromagnetic fashion to yield the experimentally observed <i>S</i> = 1 ground states