Reactions of a Bis(pentalene)dititanium complex with alkenes; the molecular structure of the butadiene complex [Ti2(µ: η5,η5-Pn††)2(μ: η2,η2-s-trans-C4H6)] (Pn†† = 1,4-(SiiPr3)2-C8H4)

Abstract

Reaction of the syn-bimetallic complex [Ti2(µ:η5,η5-Pn††)2] (1) (Pn†† = 1,4-(SiiPr3)2-C8H4) with 1,3-trans-butadiene in toluene results in the clean formation of the 1:1 adduct [Ti2(µ:η5,η5-Pn††)2(μ: η2,η2-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 π-complex (3) is formed with co-current loss of propane. Complex (3) features a coordinated η2,η1 vinylic (H2C = CMe)-SiiPr2-Pn† moiety as a result of CH activation of one of the isopropyl substituents of the SiiPr3 groups on the Pn†† supporting ligand. One of the hydrogens of this secondary vinylic moiety is significantly shifted upfield in the 1H 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(iPr) agostic interaction during the cyclometallation reaction.