REACTIVITY OF TERVALENT TITANIUM COMPOUNDS (ETA-5-C5ME5)2TIR (R = ME, ET) - INSERTION VERSUS BETA-HYDROGEN TRANSFER AND OLEFIN EXTRUSION - PREPARATION OF THE PARAMAGNETIC TITANIUM ALKOXIDE, IMINOACYL, ACYL, VINYL, AND AZOMETHIDE COMPLEXES (ETA-5-C5ME5)2TIX AND OXIDATION OF THESE WITH PBCL2 TO THE DIAMAGNETIC TETRAVALENT DERIVATIVES (ETA-5-C5ME5)2TI(X)CL

The paramagnetic, tervalent titanium alkyls Cp*2TiR (1, R = Me; 2, R = Et) were compared in their behavior toward a range of reactive molecules. These 15-electron, d1 systems appear to be weak Lewis acids, reluctant to form adducts. Only for 1 and Me3CC=N could an instable adduct Cp*2TiR.L be isolated. With active-hydrogen-containing substrates HX (X = O2C(H)Me2, OEt, C=CMe), Cp*2TiX and RH were produced. Polar, unsaturated molecules like Me3CN=C, CO, and paraformaldehyde inserted to give Cp*2Ti{eta-2-C(R)=NCMe3}, Cp*2Ti{eta-2-C(O)R}, and Cp*2TiOCH2R for both 1 and 2. Apolar unsaturated substrates did not insert into the Ti-C bond with the exception of MeC=CMe, which reacted with 1 to produce the vinyl compound Cp*2TiC(Me)=CMe2. A striking difference between 1 and 2 was found in their reaction with CO2, Me3CC=N, Me2C=O, and RC=CR1 (R, R1 = Me, Ph). While 1 either gave a normal insertion (CO2 and MeC=CMe) or adducts (Me2CO, Me3CC=N) or did not react (PhC=CPh), 2 lost ethene and gave compounds that appared to be products of insertion into a Ti-H bond, Cp*2TiO2CH, Cp*2TiN=C(H)CMe3, Cp*2TiOC(H)Me2, and Cp*2TiC(R)=C(H)R1. Facile beta-hydrogen transfer from the ethyl ligands was also observed in the reaction of 2 with olefins CH2=CHR (R = Me, Ph) to give ethene and Cp*2TiCH2CH2R. This reaction is reversible and equilibrium constants could be determined. Ground-state differences between 2 and Cp*2TiCH2CH2R were found to be 9 (2) (R = Me) and (2) kJ.mol-1 (R = Ph). Isotope-labeling experiments showed that liberation of ethene and formation of the new insertion products proceed via an intermediate hydride, Cp*2TiH. The kinetic preference for insertion of an unsaturated substrate into the Ti-H bond relative to insertion into the Ti-C bond, in combination with a rapid equilibrium between ethene elimination and reinsertion causes 2 to react in most cases as a hydride, and explains the striking difference in reactivity between 1 and 2. The products of 1 and 2 with various substrates were also characterized as their monochloride derivative Cp*2Ti(R)Cl after quantitative oxidation with PbCl2. Comparison of spectroscopic data gives information about the specific coordination of ligand R in both Cp*2TiR and Cp*2Ti(R)CI and shows that the Lewis acidity of the metal center increases substantially on oxidation to Ti(IV).