Ethylene Reactions of a [ReH(η2-BH4)(NO)(PPh3)2] Complex: Reductive Elimination of Ethane and Oxidative Coupling to Butadiene

Abstract

The borohydride complex [Re+IH(η2-BH4)(NO)(PPh3)2] (1ph) reacts with ethylene to yield [Re+IH2(η2-C2H4)(NO)(PPh3)2] (2ph) and triethylborane formed by ethylene hydroboration. Subsequent ethylene insertion into the Re–H bond of 2ph and uptake of another 1 equiv of ethylene led to the kinetically stable cis-hydrido–ethyl complex [Re+IH(Et)(η2-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 (ΔH⧧ = 68.7 kJ mol–1, ΔS⧧ = −94 J mol–1 K–1; 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–I(η2-C2H4)(NO)(PPh3)2] could not be traced, since it rapidly reacted further with ethylene, furnishing the stable butadiene complex [Re–I(η2-C2H4)(η4-C4H6)(NO)(PPh3)] (4ph...