Hemi-metallocene Ti(IV) η3-allyl-type complexes: Structure, dynamics in solution and exploration of reactivity

The cationic complexes [Cp*(tBu3P=N)Ti(η3-CH2C(C5H11)CH2)]+ (Ti-Alla+) and [Cp*(tBu3P=N)Ti(η3-CH2C(CH3)CH(C4H9))]+ (Ti-Allb+), bearing η3-allyl-type fragments coordinated to a Ti(IV) center with Cp* and phosphinimide ancillary ligands, were easily synthesized from the corresponding cationic Ti-benzyl complex by reaction with 2-methyl-1-heptene. The unusual stability of these allyl species at room temperature allowed detailed NMR spectroscopic investigation of their structure and dynamic behavior in solution. The spectroscopic results provided supporting experimental evidence for η3-coordination mode of the allyls and allowed to determine their preferential configuration. Furthermore, quantitative analysis of the exchange kinetics between syn and anti allylic protons was carried out by 1H EXSY NMR spectroscopy. The activation parameters for the latter process indicate that the faster exchange observed with Ti-Alla+ (ΔH‡ ≈ 16 kcal mol−1; ΔS‡ ≈ 0 cal mol−1 K−1) compared to Ti-Allb+ (ΔH‡ ≈ 11 kcal mol−1; ΔS‡ ≈ −19 cal mol−1 K−1) is primarily of entropic origin. A tentative interpretation for the higher stability of Ti-Alla+ and Ti-Allb+, compared to their bis-Cp titanocene analogues reported in the literature, is proposed based on steric considerations. Finally, an exploratory reaction, using benzaldehyde as representative substrate, was carried out to demonstrate the capability of Ti-Alla+ and Ti-Allb+ to serve as allylating agents for carbonyl compounds.