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

Abstract

Following the track of the useful titanocene [Ti( 5 -C5H5)2Cl] reagent in organic synthesis, the related half-sandwich titanium(III) derivatives [Ti( 5 -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( 5 -C5Me5)Cl2}n]. The thermolysis of [Ti( 5 -C5Me5)Cl2Me] (1) in benzene or hexane at 180 ºC results in the clean formation of [{Ti( 5 -C5Me5)Cl(-Cl)}2] (2), methane and ethene. The treatment of 1 with excess pinacolborane in hexane at 65 ºC leads to a mixture of 2 and the paramagnetic trimer [{Ti( 5 -C5Me5)(-Cl)2}3] (3). The Xray 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 (NMR) spectroscopy of 2 in aromatic hydrocarbon solutions and DFT calculations for several [{Ti( 5 -C5Me5)Cl2}n] aggregates are consistent with the existence of an equilibrium between the diamagnetic dimer [{Ti( 5 -C5Me5)Cl(-Cl)}2] and a paramagnetic tetramer [{Ti( 5 -C5Me5)(-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( 5 -C5Me5)Cl2(thf)] (4) were grown.

Low‐Valent Titanium Species Stabilized with Aluminum/Boron Hydride Fragments

“…The κ 2 ‐coordinated BH 4 groups agree with the long Ti⋅⋅⋅B distances of 2.389(4) and 2.402(3) Å when compared with that found (2.220(9) Å) for the κ 3 ‐BH 4 ligands of complex 4 [32] . The Ti−O bond length of 2.111(2) Å compares well with those found in other titanium(III) tetrahydrofuran adducts such as [Ti(η 5 ‐C 5 Me 5 )Cl 2 (thf)] (Ti−O=2.083(2) Å) [11,37] …”

Preparation of Dimeric Monopentamethylcyclopentadienyltitanium(III) Dihalides and Related Derivatives

“…Computational studies for the chloride-bridged dimer 4 showed a singlet ground state and a relatively large singlet–triplet energy gap . In accord with this strong antiferromagnetic interaction, diamagnetism is expected in the NMR spectra of complex 4 in a fashion similar to those suggested by NMR and electron paramagnetic resonance (EPR) spectroscopies in solution for the analogous dimeric complexes [{Ti­(η 5 -C 5 Me 4 Ph)­Cl­(μ-Cl)} 2 ] and [{Ti­(η 5 -C 5 (CH 2 Ph) 5 )­Cl­(μ-Cl)} 2 ], which show slightly longer Ti–Ti distances (3.335(1) and 3.374(2) Å, respectively) .…”

“…Compounds 4 – 6 were isolated after workup in 40–66% yields as highly air-sensitive green solids, which exhibit a good solubility in aromatic hydrocarbon solvents but are poorly soluble in hexane. We recently described the synthesis of crystals of the chloride complex 4 in higher yield (90%) by thermolysis of [Ti­(η 5 -C 5 Me 5 )­Cl 2 Me] in hexane . Crystals of compounds 5 and 6 were isolated after recrystallization in hot toluene, and those samples were used for X-ray crystal structure determinations and magnetic measurements.…”

“…In a seminal contribution, Mashima and co-workers used electron-rich organosilicon compounds as salt-free reductants for generating group 4–6 species, including [Ti­(η 5 -C 5 Me 5 )­Cl 2 ] and [Ti­(η 5 -C 5 Me 5 )­Cl 2 (thf)] . We recently reported that thermolysis or hydrogenolysis of [Ti­(η 5 -C 5 Me 5 )­Cl 2 Me] in hydrocarbon solvents leads to the clean formation of half-sandwich titanium­(III) dichloride species [Ti­(η 5 -C 5 Me 5 )­Cl 2 ] along with volatile byproducts . Dimeric [{Ti­(η 5 -C 5 Me 5 )­Cl­(μ-Cl)} 2 ] and trimeric [{Ti­(η 5 -C 5 Me 5 )­(μ-Cl) 2 } 3 ] complexes were characterized by means of X-ray diffraction determinations, and the electronic structures and stabilities of several aggregates were established by computational calculations.…”

From Dialkyltitanium Species to Titanacyclopropanes: An Ab Initio Study

“…Calculations were carried out with the Gaussian 16 package, using PBE0 as the density functional . This functional was already used with similar systems. , All atoms were represented with a def2-tzvp basis set with added D3 Grimme empirical dispersion . Geometry optimizations were performed without symmetry constraint, and vibrational analysis was used to confirm the presence of a local minimum .…”