The Puzzling Monopentamethylcyclopentadienyltitanium(III) Dichloride Reagent: Structure and Properties

Following the track of the useful titanocene [Ti­(η5-C5H5)2Cl] reagent in organic synthesis, the related half-sandwich titanium­(III) derivatives [Ti­(η5-C5R5)­Cl2] are receiving increasing attention in radical chemistry of many catalyzed transformations. However, the structure of the active titanium­(III) species remains unknown in the literature. Herein, we describe the synthesis, crystal structure, and electronic structure of titanium­(III) aggregates of composition [{Ti­(η5-C5Me5)­Cl2} n ]. The thermolysis of [Ti­(η5-C5Me5)­Cl2Me] (1) in benzene or hexane at 180 °C results in the clean formation of [{Ti­(η5-C5Me5)­Cl­(μ-Cl)}2] (2), methane, and ethene. The treatment of 1 with excess pinacolborane in hexane at 65 °C leads to a mixture of 2 and the paramagnetic trimer [{Ti­(η5-C5Me5)­(μ-Cl)2}3] (3). The X-ray crystal structures of compounds 2 and 3 show Ti–Ti distances of 3.267(1) and 3.219(12) Å, respectively. Computational studies (CASPT2//​CASSCF and BS DFT methods) for dimer 2 reveal a singlet ground state and a relatively large singlet–triplet energy gap. Nuclear magnetic resonance spectroscopy of 2 in aromatic hydrocarbon solutions and DFT calculations for several [{Ti­(η5-C5Me5)­Cl2} n ] aggregates are consistent with the existence of an equilibrium between the diamagnetic dimer [{Ti­(η5-C5Me5)­Cl­(μ-Cl)}2] and a paramagnetic tetramer [{Ti­(η5-C5Me5)­(μ-Cl)2}4] in solution. In contrast, complex 2 readily dissolves in tetrahydrofuran to give a green-blue solution from which blue crystals of the mononuclear adduct [Ti­(η5-C5Me5)­Cl2(thf)] (4) were grown.