Synthesis, Characterization, and Catalytic Activity of Bimetallic Ti/Cr Complexes

We report herein the reactions of the Rosenthal complexes, Cp2Ti(BTMSA) and Cp*2Ti(BTMSA) (BTMSA: bis(trimethylsilyl)acetylene), with CpCr(CO)3H. Those complexes are known to dissociate their alkyne ligands and to exhibit the reactivities of the putative titanocenes "Cp2Ti" and "Cp*2Ti". Our reactions appear to proceed through the addition of CpCr(CO)3H to these titanocenes, giving rise to TiIV-H intermediates that lose H2 to form the TiIII–Cr complexes 1 and 2. In the solid state, 1 and 2 both adopt a dimeric geometry, involving a 12-membered "Ti2Cr2" ring held together with Ti–O–C–Cr bridging carbonyls. The terminal carbonyls in 1 and 2 are trans. DFT calculations confirm that the loss of H2 from the TiIV-H intermediate—forming the TiIII-Cr dimer—is exergonic by 14.2 kcal mol–1 in the gas phase. The Bercaw compound Cp* (C5Me4CH2)TiCH3 reacts with CpCr(CO)3H to form a TiIV-CH3 species with a coordinated CpCr(CO)3– anion (3). The Ti-Cr dimer 1 is capable of catalyzing the hydrogenation of an epoxide to an anti-Markovnikov alcohol, avoiding the use of Cp2TiX2 (X = Cl– or mesylate), NaCpCr(CO)3, and HCpCr(CO)3 separately as in our previous catalyst system.