Ethylene Reactions of a [ReH(畏2-BH4)(NO)(PPh3)2] Complex: Reductive Elimination of Ethane and Oxidative Coupling to Butadiene
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- 作者：Balz Dudle ; Olivier Blacque ; Heinz Berke
- 刊名：Organometallics
- 出版年：2012
- 出版时间：March 12, 2012
- 年：2012
- 卷：31
- 期：5
- 页码：1832-1839
- 全文大小：359K
- 年卷期：v.31,no.5(March 12, 2012)
- ISSN：1520-6041
文摘
The borohydride complex [Re+IH(畏2-BH4)(NO)(PPh3)2] (1ph) reacts with ethylene to yield [Re+IH2(畏2-C2H4)(NO)(PPh3)2] (2ph) and triethylborane formed by ethylene hydroboration. Subsequent ethylene insertion into the Re鈥揌 bond of 2ph and uptake of another 1 equiv of ethylene led to the kinetically stable cis-hydrido鈥揺thyl complex [Re+IH(Et)(畏2-C2H4)(NO)(PPh3)2] (3ph). 3ph was found to slowly reductively eliminate ethane. The rate of this process was determined by quantitative NMR spectroscopy in the temperature range from 293 to 338 K, enabling calculation of the activation parameters (螖H = 68.7 kJ mol鈥?, 螖S = 鈭?4 J mol鈥? K鈥?; half-life time 1.8 h at 303 K). The reaction was found to follow first-order kinetics in c(3ph) and is zeroth order in c(C2H4) and c(PPh3), ruling out preceding ligand dissociation. The presumptive intermediate [Re鈥揑(畏2-C2H4)(NO)(PPh3)2] could not be traced, since it rapidly reacted further with ethylene, furnishing the stable butadiene complex [Re鈥揑(畏2-C2H4)(畏4-C4H6)(NO)(PPh3)] (4ph) in 88% yield. This transformation of dehydrogenative ethylene coupling is suggested to involve the elementary steps of rhenacyclopentane formation from two coordinated ethylene ligands and then double C鈥揌 activation via 尾-hydride shifts to generate the butadiene unit and formal H2 elimination from the rhenium dihydride with concomitant triphenylphosphine elimination. An X-ray crystallographic study confirmed the spectroscopically derived pentacoordinate structure of 4ph.