Experimental and theoretical investigations of carbon-carbon, carbon-hydrogen and carbon-sulfur bond activations of nitriles using zerovalent nickel

Thesis (Ph. D.)--University of Rochester. Dept. of Chemistry, 2011. Chapters 2-3 were co-authored with Tülay A. Ateşin. Chapter 7 was co-authored with Matthew R. Grochowski.[Ni(dippe)H]2 complex has been reacted with a variety of carbon nitriles. Upon mixing with substrates, it releases H2 to generate the 14-electron fragment [Ni(dippe)], which is proposed to be the active species in bond activations. In the reaction with acetonitrile, initially the η2-nitrile complex was observed, which upon heating or photolysis leads to the C-CN bond activation product Ni(dippe)(CH3)(CN). No product from C-H bond cleavage was seen and the independently synthesized Ni(dippe)(H)(CH2CN) complex was very unstable and can only exist at low temperature. Computational studies using DFT calculation methods showed that the C-CN bond activation is favored exclusively over the C-H bond activation due to the strong thermodynamic driving force and slightly lower kinetic barrier. In reactions with aromatic nitriles, the [Ni(dippe)] fragment first coordinates η2 to the nitrile C≡N and/or C=C moiety. Rearrangement then occurs to give the C–CN oxidative addition product Ni(dippe)(Aryl)(CN). Both experimental and DFT calculation results have shown that an η2-arene complex with nickel coordinated to the C=C double bond next to the cyano substituent is the crucial intermediate leading to C–CN bond activation. Furthermore, the fluxional processes of the η2-arene species were investigated by low-temperature experiments as well as computational methods. In the cases of benzonitrile and dicyanobenzenes, a mechanism was found with the Ni(dippe) fragment rotating as it migrates around the phenyl ring through a series of η3-allyl-like transition states. For polycyclic aromatic nitriles, only certain η2-arenes were stable enough to contribute to the fluxional process, and nickel migrates via an η4-coordinated transition state. The transition states connecting the η2-nitrile complex to the η2-arene intermediate and the η2-arene intermediate to the C–CN bond activation products are at much higher energies compared to those for migration around the ring. In the reaction of 9-cyanoanthracene, the instability of the η2-arene precursor and the high energy activation barrier resulted in the absence of the C–CN oxidative addition product. The complex with 9-cyanophenanthrene only undergoes C–CN cleavage upon photolysis. The Lewis acid BEt3 disfavors the C-CN bond activation in acetonitrile, but can facilitate C-CN cleavage in aromatic nitriles. The isomerization of 2-methyl-3-butenenitrile (2M3BN) carried out by [Ni(dippe)H]2 can follow either a C-CN activation pathway to form the linear product 3-pentenenitrile (3PN), or a C-H activation pathway to give the branched olefin product 2-methyl-2-butenenitrile (2M2BN). Both pathways have been studied by DFT calculation methods and the results matched well with those observed in stoichiometric experiments. A detailed mechanism has been proposed and tested on several other model bisphosphine ligands to investigate bite angle and electronic effects on the selectivity of nickel bisphosphine catalysts. In the reaction of [Ni(dippe)H]2 with 2-cyanothiophene, the processes of C–C and C–S bond cleavage have been studied. At room temperature, cleavage of the nitrile-substituted C–S bond occurs, forming the Nimetallacycle complex (dippe)Ni(K2-S,C–SCH=CHCH=C(CN)), which was converted to the C–CN cleavage product (dippe)Ni(CN)(2-thiophenyl) when heated in solution. A kinetic product (dippe)Ni(K2-S,C-SC(CN)=CHCH=CH) was formed from cleavage of the non-substituted C–S bond, as well as a Ni(0) η2-nitrile intermediate (dippe)Ni(η2-C,N-2-cyanothiophene), and a dinuclear mixed Ni(0)-Ni(II) product. A complete DFT analysis of this system has been carried out to reveal comparative details about the two bond cleavage transition states