Synthesis and thermolysis of Cp*(C5Me4CH2)TiR complexes

Substitution of the chloride in Cp*FvTiCl with MR (Fv = C5Me4CH2; R = Me, CH2SiMe3, CH2CMe3, CH = CH2, M = Li; R = CH2Ph, M = K; R = C3H5, M = MgCl; R = Ph, M = Na . NaCl) gives Cp*FvTiR. NMR spectroscopic evidence points towards a series of structurally related compounds with a bent-sandwich geometry. The substituent R is positioned in the wedge, midway below the exocyclic methylene group and a neighbouring methyl group of the fulvene. Thermolysis of Cp*FvTiR gives, dependent on the substituent R, reduction to Cp*FvTi (R = CH2Ph) or the double ring metallated Cp*[C5Me3(CH2)(2)]Ti (R = CH2XMe3, X = C, Si) or Cp*FvTiCH=CHMe (R = eta(3)-C3H5)

SYNTHESIS AND CHARACTERIZATION OF (MU-5-C5ME5)2TI(R)CL (R = ME, ET, NORMAL-PR, CH=CH2, PH, O-NORMAL-PR) AND THEIR SALT METATHESIS REACTIONS - THERMAL-DECOMPOSITION PATHWAYS OF (MU-5-C5ME5)2TI(ME)R' (R' = ET, CH=CH2, PH, CH2PH)

Complexes Cp*2Ti(R)Cl (Cp* = eta-5-C5Me5; R = Me (1), Et (2), n-Pr (3), CH=CH2 (4), Ph (5), O-n-Pr (6)) have been prepared by oxidation Of CP*2TiR with lead dichloride. Not every compound Cp*2Ti(R)Cl was accessible and for R = CH2CMe3 and CH2Ph reduction to Cp*2TiCl and R. was observed. Homolysis of the Ti-R bond appears to be the general decomposition mode for compounds CP*2Ti(R)Cl. Attempts to prepare CP*2Ti(Et)R by salt metathesis between 2 and MeLi, KCH2Ph, or LiCH=CH2 yielded CP*2Ti(eta-2-C2H4) and RH. Isotope labeling experiments showed that RH is formed by transfer of a beta-H atom of the ethyl ligand to R. The complex Cp*2Ti(Me)CH=CH2 (from 4 and MeLi) undergoes unimolecular thermolysis (DELTA-H double dagger = 87.9 (5) kJ.mol-1, DELTA-S double dagger = -21 (4) J.mol-1.K-1) to yield the fulvene vinyl compound Cp*FvTiCH=CH2 (Fv = eta-6-C5Me4CH2) via a vinylidene intermediate Cp*2Ti=C=CH2, formed after a rate-limiting vinylic alpha-hydrogen abstraction (k(H)/k(D) = 5.1 for the thermolysis of CP*2Ti(CD=CD2)Me). Cp*FvTiCH=CH2 was also obtained from the reaction of 4 with KCH2Ph or LiCH2PMe2, indicating the formation of thermally unstable CP*2Ti(R)CH=CH2. The formation Of CP*2TiCH2CH2C=CH2 from 4 and LiCH=CH2 can be explained by insertion of CH2=CH2 formed on thermolysis of a transient bis(vinyl) compound Cp*2Ti(CH=CH2)2 into the generated vinylidene Cp*2Ti=C=CH2. Reaction of the phenyl compound Cp*2Ti(Ph)Cl (5) with RM (R = CH=CH2, n-Bu, M = Li; R = CH2Ph, M = K) gave CP*2TiPh and R2, RH and R(-H) via radical decomposition of the intermediate Cp*2Ti(Ph)R. The methyl compound CP*2Ti(Me)Ph (from 5 and MeLi) decomposes thermally to Cp*FvTiPh and methane (DELTA-H double dagger = 96.4 (7) kJ.mol-1, DELTA-S double dagger = -41 (9) J.mol-1.K-1. Labeling experiments and kinetic studies show that thermolysis occurs via a rate-determining phenyl ortho hydrogen abstraction (k(H)/k(D) = 5.7 for the thermolysis of CP*2Ti-(Me)(Ph-d5)) giving an o-phenylene intermediate. The intermediate can be trapped by CO2 to yield CP*2Ti(o-C6H4)C(O)O. The benzyl complex CP*2Ti(Me)CH2Ph (from 1 and KCH2Ph) decomposes by homolysis of the Ti-CH2Ph bond, and in the methyl alkoxide CP*2Ti(O-n-Pr)Me (from 6 and MeLi) homolysis of the Ti-Me bond occurs

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)