Monday, December 5, 2016

Carbon–Carbon Bond-Forming Reductive Elimination from Isolated Nickel(III) Complexes

James R. BourNicole M. CamassoElizabeth A. MeucciJeff W. KampfAllan J. Canty¥, and Melanie S. Sanford*


http://pubs.acs.org/doi/abs/10.1021/jacs.6b10350

Abstract

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This manuscript describes the design, synthesis, characterization, and reactivity studies of organometallic NiIII complexes of general structure TpNiIII(R)(R1) (Tp = tris(pyrazolyl)borate). With appropriate selection of the R and R1 ligands, the complexes are stable at room temperature and can be characterized by cyclic voltammetry, EPR spectroscopy, and X-ray crystallography. Upon heating, many of these NiIII compounds undergo C(sp2)–C(sp2) or C(sp3)–C(sp2) bond-forming reactions that are challenging at lower oxidation states of nickel.

Bimetallic C−C Bond-Forming Reductive Elimination from Nickel



Abstract

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Ni-catalyzed cross-coupling reactions have found important applications in organic synthesis. The fundamental characterization of the key steps in cross-coupling reactions, including C–C bond-forming reductive elimination, represents a significant challenge. Bimolecular pathways were invoked in early proposals, but the experimental evidence was limited. We present the preparation of well-defined (pyridine-pyrrolyl)Ni monomethyl and monophenyl complexes that allow the direct observation of bimolecular reductive elimination to generate ethane and biphenyl, respectively. The sp3–sp3 and sp2–sp2 couplings proceed via two distinct pathways. Oxidants promote the fast formation of Ni(III) from (pyridine-pyrrolyl)Ni-methyl, which dimerizes to afford a bimetallic Ni(III) intermediate. Our data are most consistent with the subsequent methyl coupling from the bimetallic Ni(III) to generate ethane as the rate-determining step. In contrast, the formation of biphenyl is facilitated by the coordination of a bidentate donor ligand.


http://pubs.acs.org/doi/abs/10.1021/jacs.6b00016

Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts


Selective oxidative dehydrogenation of propane to propene using boron nitride catalysts 
 

J. T. Grant,1 C. A. Carrero,1 F. Goeltl,1 J. Venegas,2 P. Mueller,1 S. P. Burt,2 S. E. Specht,1 W. P. McDermott,1 A. Chieregato,1 I. Hermans1,2*
1University of Wisconsin—Madison, Department of Chemistry, 1101 University Avenue, Madison, WI 53706, USA. 2University of Wisconsin—Madison, Department of Chemical and Biological Engineering, 1415 Engineering Drive, Madison, WI 53706, USA.
*Corresponding author. E-mail: hermans@chem.wisc.edu
 
http://science.sciencemag.org/content/early/2016/11/30/science.aaf7885.full

Abstract
The exothermic oxidative dehydrogenation of propane reaction to generate propene has the potential to be a game-changing technology in the chemical industry. However, even after decades of research, selectivity to propene remains too low to be commercially attractive because of overoxidation of propene to thermodynamically favored CO2. Here, we report that hexagonal boron nitride (h-BN) and boron nitride nanotubes (BNNTs) exhibit unique and hitherto unanticipated catalytic properties resulting in great selectivity to olefins. As an example, at 14% propane conversion, we obtain selectivity of 79% propene and 12% ethene, another desired alkene. Based on catalytic experiments, spectroscopic insights and ab initio modeling, we put forward a mechanistic hypothesis in which oxygen-terminated armchair BN edges are proposed to be the catalytic active sites.