Abstract

The alkyne complexes [(PNP)Ti(eta(2)-HC CH)((CH2Bu)-Bu-t)] (2) and [(PNP)Ti(eta(2)-HC CMe)((CH2Bu)-Bu-t)] (3) have been prepared by treatment of [(PNP)Ti=(CHBu)-Bu-t(OTf)] (1) with the Grignard reagents H2C=CHMgCl and MeHC=CHMgBr, respectively. Complex 3 can be also prepared using the Grignard H2C=C(Me)MgBr and 1. The 2-butyne complex [(PNP)Ti(eta(2)-MeC CMe)((CH2Bu)-Bu-t)] (4) can be similarly prepared from 1 and MeHC=C(Me)MgBr. Complexes 2 and 3 have been characterized with a battery of multidimensional and multinuclear (H-1, C-13, and P-31) NMR spectroscopic experiments, including selectively P-31 decoupled H-1{P-31}, H-1-P-31 HMBC, H-1-P-31 HOESY, and P-31 EXSY. Variable-temperature H-1 and P-31{H-1} NMR spectroscopy reveals that the acetylene ligand in 2 exhibits a rotational barrier of 11 kcal mol(-1), and such a process has been corroborated by theoretical studies. Formation of the titanium alkyne ligand in complexes 2 and 3 proceeds via the vinyl intermediate [(PNP)Ti=(CHBu)-Bu-t(CH=CHR)] followed by a concerted, metal-mediated beta-hydrogen abstraction step that has been computed to have a barrier of 20-22 kcal mol(-1). The geometry and rotational mechanism of the alkyne ligand in 2 are presented and compared with those of the ethylene derivative [(PNP)Ti(eta(2)-H2C-CH2)((CH2Bu)-Bu-t)] (5), which does not display rotation of the bound ethylene under the same conditions.