The titanium(II) alkyl trans-TiMe2(dmpe)2, where dmpe is 1,2-bis(dimethylphosphino)-ethane, reacts with 1,3-butadiene and trans,trans-1,4-diphenyl-1,3-butadiene at -20 °C to produce the titanium(II) butadiene complexes TiMe2(η4-C4H4R 2)(dmpe), where R is H or Ph. NMR spectra are consistent with structures in which the methyl groups are mutually cis, and this has been verified crystallographically for the 1,4-diphenylbutadiene complex. These molecules are fluxional on the NMR time scale, and the activation parameters for exchange are ΔH‡ = 9.1 ± 0.2 kcal mol-1 and ΔS‡ = 3 ± 1 eu for the 1,4-diphenylbutadiene complex. The process that exchanges the two Ti-Me groups, the two ends of the dmpe ligand, and the two ends of the butadiene ligand is proposed to be a trigonal twist, although we cannot entirely rule out the possibility that the exchange involves five-coordinate intermediates generated by dissociation of one "arm" of a chelating ligand. If the reaction of TiMe2(dmpe)2 and 1,3-butadiene is allowed to proceed at -20 °C for prolonged periods (>12 h), a second titanium "butadiene" complex is formed, which has been identified as the titanium(IV) η3,η1-octa-1,6-diene-1,8-diyl complex TiMe2(η3,η1-C8H 12)(dmpe). Warming a solution of TiMe2-(dmpe)2 and 1,3-butadiene to 25 °C results in the catalytic dimerization of butadiene to the Diels-Alder dimer 4-vinylcyclohexene at rates of 5 turnovers/h. A mechanism for the catalytic dimerization is proposed, which involves coupling of two butadiene ligands to form a divinyltitanacyclopentane species, allylic rearrangement to a vinyltitanacycloheptene intermediate, and reductive elimination to form the cyclic product. Treatment of TiMe2-(dmpe)2 with 1,3-butadiene in the presence of AlEt3 results in reduction to the titanium(0) complex Ti(η4-C4H6)2(dmpe), which has also been crystallographically characterized. Unlike the behavior seen for certain other early transition metal butadiene complexes, in both Ti-(η4-C4H6)2(dmpe) and TiMe2(η4-C4H4Ph 2)(dmpe) the butadiene ligands are bound like true dienes. We propose that the preferred bonding mode for butadiene complexes of the lower valent early transition metals is the π,η4 mode and that increasing σ2,π character is introduced only when there are significant steric repulsions between the ancillary ligands and the meso butadiene substituents.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry