Reactions of a Bis(pentalene)dititanium complex with alkenes; the molecular structure of the butadiene complex [Ti2(μ: ?5,?5-Pn&DAG;&DAG;)2(μ: ?2,?2-s-trans-C4H6)] (Pn&DAG;&DAG;=1,4-(SiiPr3)2-C8H4)

Reaction of the syn-bimetallic complex [Ti-2(mu:eta(5),eta(5)-Pn & DAG;& DAG;)(2)] (1) (Pn & DAG;& DAG; = 1,4-((SiPr3)-Pr-i)(2)-C8H4) with 1,3-trans-buta-diene in toluene results in the clean formation of the 1:1 adduct [Ti-2(mu:eta(5),eta(5)-Pn & DAG;& DAG;)(2)(mu: eta(2),eta(2)-s-trans-C4H6)] (2) featuring an essentially planar butadiene ligand. Complex (2) represents the first example of a bimetallic early transition metal complex where a coordinated butadiene adopts such a conformation. When (1) is reacted with propene an unexpected tuck-over alkene pi-complex (3) is formed with co-current loss of propane. Complex (3) features a coordinated eta(2),eta(1) vinylic (H2C = CMe)-(SiPr2)-Pr-i-Pn & DAG; moiety as a result of C-H activation of one of the isopropyl substituents of the (SiPr3)-Pr-i groups on the Pn & DAG;& DAG; supporting ligand. One of the hydrogens of this secondary vinylic moiety is significantly shifted upfield in the H-1 NMR spectrum of (3) and a single crystal XRD study shows an interaction between this hydrogen and one of the Ti centres. Preliminary kinetic studies of the formation of (3) show a slightly negative entropy of activation and 1st order consumption of (1) both of which suggest the involvement of a Ti-H(Pr-i) agostic interaction during the cyclometallation reaction.

Automated summary

The reaction of the syn-bimetallic complex with different alkenes results in the formation of products containing organic ligands with specific conformations, where Ti-H(Pr-i) agostic interaction plays a critical role in the reaction mechanism.