Ethylene Reactions of
a [ReH(η<sup>2</sup>-BH<sub>4</sub>)(NO)(PPh<sub>3</sub>)<sub>2</sub>] Complex: Reductive Elimination
of Ethane and Oxidative Coupling to Butadiene
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Abstract
The borohydride complex [Re<sup>+I</sup>H(η<sup>2</sup>-BH<sub>4</sub>)(NO)(PPh<sub>3</sub>)<sub>2</sub>] (<b>1ph</b>) reacts
with ethylene to yield [Re<sup>+I</sup>H<sub>2</sub>(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)(NO)(PPh<sub>3</sub>)<sub>2</sub>] (<b>2ph</b>) and triethylborane formed by ethylene hydroboration.
Subsequent ethylene insertion into the Re–H bond of <b>2ph</b> and uptake of another 1 equiv of ethylene led to the kinetically
stable <i>cis</i>-hydrido–ethyl complex [Re<sup>+I</sup>H(Et)(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)(NO)(PPh<sub>3</sub>)<sub>2</sub>] (<b>3ph</b>). <b>3ph</b> 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
(Δ<i>H</i><sup></sup><sup>⧧</sup> = 68.7 kJ
mol<sup>–1</sup>, Δ<i>S</i><sup></sup><sup>⧧</sup> = −94 J mol<sup>–1</sup> K<sup>–1</sup>; half-life time 1.8 h at 303 K). The reaction was found to follow
first-order kinetics in <i>c</i>(<b>3ph</b>) and is
zeroth order in <i>c</i>(C<sub>2</sub>H<sub>4</sub>) and <i>c</i>(PPh<sub>3</sub>), ruling out preceding ligand dissociation.
The presumptive intermediate [Re<sup>–I</sup>(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)(NO)(PPh<sub>3</sub>)<sub>2</sub>] could
not be traced, since it rapidly reacted further with ethylene, furnishing
the stable butadiene complex [Re<sup>–I</sup>(η<sup>2</sup>-C<sub>2</sub>H<sub>4</sub>)(η<sup>4</sup>-C<sub>4</sub>H<sub>6</sub>)(NO)(PPh<sub>3</sub>)] (<b>4ph</b>) 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–H activation via β-hydride
shifts to generate the butadiene unit and formal H<sub>2</sub> elimination
from the rhenium dihydride with concomitant triphenylphosphine elimination.
An X-ray crystallographic study confirmed the spectroscopically derived
pentacoordinate structure of <b>4ph</b>
