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*)­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.