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.