Synthesis and characterization of (.eta.5-C5Me5)2Ti(R)Cl (R = Me, Et, n-Pr, CH:CH2, Ph, O-n-Pr) and their salt metathesis reactions. Thermal decomposition pathways of (.eta.5-C5Me5)2Ti(Me)R' (R' = Et, CH:CH2, Ph, CH2Ph)

Abstract

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.