Preparation of Dimeric Monopentamethylcyclopentadienyltitanium(III) Dihalides and Related Derivatives

The synthesis, crystal structure, and reactivity of a series of half-sandwich titanium(III) dihalide complexes [Ti(eta(5)-C5Me5)X-2] (X = Cl, Br, I) and several of its Lewis base derivatives were investigated. The reaction of the trihalides [Ti(eta(5)-C5Me5)X-3] (X = Cl (1), Br (2), I (3)) with LiAlH, (>= 1 equiv) in toluene at room temperature results in the formation of the halide-bridged dimers [{Ti(eta(5)-C5Me5)X(mu-X)}(2)] (X = Cl (4), Br (5), I (6)). The treatment of 4 with [Li{N(SiMe3)(2)}] (>= 2 equiv) at room temperature affords the precipitation of the amido titanium(III) complex [{Ti(eta(5)-C5Me5)(mu-Cl){N(SiMe3)(2)}}(2)] (7), but analogous reactions of 4 with other lithium reagents [LiR] (R = Me, CH2SiMe3, NMe2) lead to disproportionation into titanium(IV) [Ti(eta(5)-C5Me5)R-3] and presumably titanium(II) derivatives. Similarly, complex 4 in solution at temperatures higher than 100 degrees C undergoes disproportionation as demonstrated by its reactions with cobaltocene and N-(4-methylbenzylidene)aniline yielding the ionic paramagnetic compound [Co(eta(5)-C5H5)(2)][Ti(eta(5)-C5Me5)Cl-3] (8) and the diamagnetic diazatitanacyclopentane [Ti(eta(5)-C5Me5) Cl{N(Ph)CH(p-tolyl)}(2)], respectively. Treatment of complex 4 with 2 equiv of 2,6-dimethylphenylisocyanide or tertbutylisocyanide in toluene at room temperature affords the paramagnetic titanium(III) dinuclear adducts [{Ti(eta(5)-C5Me5)Cl(mu-Cl)(CNR)}(2)] (R = 2,6-Me2C6H3 (9), tBu (10)). Magnetic studies for polycrystalline 9 show that it displays a weak intramolecular antiferromagnetic coupling between the Ti ions, which is consistent with the long Ti-Ti distance of 3.857(1) angstrom determined by X-ray diffraction. The isocyanide ligands in complex 10 undergo a reductive coupling reaction in toluene to give the titanium(IV) iminoacyl derivative [{Ti(eta(5)-C5Me5)Cl-2}(2) (mu-eta(2):eta(2)-tBuN=C-C=NtBu)] (11). Whereas an analogous dinuclear structure was found in the aqua titanium(III) complex [{Ti(eta(5)-C5Me5)Cl(mu-Cl)(OH2)}(2)] (12), resulting from the reaction of 4 with adventitious amounts of water, compound 4 reacts with excess ammonia to give a mononuclear adduct [Ti(eta(5)-C5Me5)Cl-2(NH3)(2)] (13) with a robust layered pattern in the solid state.