Abstract

The transient titanium neopentylidyne, [(PNP)Ti (CBu)-Bu-t] (A; PNP- N[2-(PPr2)-Pr-i-4-methylphenyl](2)(-)), dehydrogenates ethane to ethylene at room temperature over 24 h, by sequential 1,2-CH bond addition and beta-hydrogen abstraction to afford [(PNP)Ti(eta(2)-H2C = CH2)((CH2Bu)-Bu-t)] (1). Intermediate A can also dehydrogenate propane to propene, albeit not cleanly, as well as linear and volatile alkanes C-4-C-6 to form isolable alpha-olefin complexes of the type, [(PNP)Ti(eta(2)-H2C = CHR)((CH2Bu)-Bu-t)] (R = CH3 (2), CH2CH3 (3), Pr-n (4), and Bu-n (5)). Complexes 1-5 can be independently prepared from [(PNP)Ti =(CHBu)-Bu-t(OTf)] and the corresponding allcylating reagents, LiCH2CHR (R = H, CH3(unstable), CH2CH3, Pr-n, and Bu-n). Olefin complexes 1 and 3-5 have all been characterized by a diverse array of multinuclear NMR spectroscopic experiments including H-1-P-31 HOESY, and in the case of the alpha-olefin adducts 2-5, formation of mixtures of two diastereomers (each with their corresponding pair of enantiomers) has been unequivocally established. The latter has been spectroscopically elucidated by NMR via C H coupled and decoupled H-1-C-13 multiplicity edited gHSQC, H-1-P-31 HMBC, and dqfCOSY experiments. Heavier linear alkanes (C-7 and C-8) are also dehydrogenated by A to form [(PNP)Ti(eta(2)-H2C = CH(n)Pentyl)((CH2Bu)-Bu-t)] (6) and [(PNP)Ti(eta(2)-H2C = CH(n)Hexyl)((CH2Bu)-Bu-t)] (7), respectively, but these species are unstable but can exchange with ethylene (1 atm) to form 1 and the free alpha-olefin. Complex 1 exchanges with D2C = CD2 with concomitant release of H2C = CH2. In addition, deuterium incorporation is observed in the neopentyl ligand as a result of this process. Cyclohexane and methylcyclohexane can be also dehydrogenated by transient A, and in the case of cyclohexane, ethylene (1 atm) can trap the [(PNP)Ti((CH2Bu)-Bu-t)] fragment to form 1. Dehydrogenation of the alkane is not rate-determining since pentane and pentane-d(12) can be dehydrogenated to 4 and 4-d(12) with comparable rates (KIE = 1.1(0) at similar to 29 degrees C). Computational studies have been applied to understand the formation and bonding pattern of the olefin complexes. Steric repulsion was shown to play an important role in determining the relative stability of several olefin adducts and their conformers. The olefin in 1 can be liberated by use of N2O, organic azides (N3R; R = 1-adamantyl or SiMe3), ketones (O = CPh2; 2 equiv) and the diazoalkane, N(2)CHtolyl(2). For complexes 3-7, oxidation with N2O also liberates the alpha-olefin.