Deactivation pathways of neutral Ni(II) polymerization catalysts.
The novel dimethyl sulfoxide (DMSO)-coordinated complex [(N,O)Ni(CH(3))(DMSO)] {1-DMSO; (N,O) = kappa(2)-N,O-(2,6-(3,5-(F(3)C)(2)C(6)H(3))(2)C(6)H(3))-N=CH-(3,5-I(2)-2-OC(6)H(2))} was found to be a well-defined, very reactive precursor that enables direct observation of the activation and deactivation of neutral Ni(II) catalysts. Preparative reaction with ethylene afforded the ethyl complex [(N,O)Ni((alpha)CH(2)(beta)CH(3))(DMSO)] (2-DMSO). 2-DMSO is subject to interconversion of the (alpha)C and (beta)C moieties via an intermediate [(N,O)Ni(II)H(ethylene)] complex (this process is slow on the NMR time scale). Exposure of 1-DMSO to ethylene in DMSO solution at 55 degrees C results in partial reaction to form propylene (pseudo-first-order rate constant k(ins,Me) = 6.8 +/- 0.3 x 10(-4) s(-1) at an ethylene concentration of 0.15 M) and conversion to 2-DMSO, which catalyzes the conversion of ethylene to butenes. A relevant decomposition route of the catalyst precursor is the bimolecular elimination of ethane [DeltaH(double dagger) = (57 +/- 1) kJ mol(-1) and DeltaS(double dagger) = -(129 +/- 2) J mol(-1) K(-1) over the temperature range 55-80 degrees C]. This reaction is specific to the Ni(II)-Me complex; corresponding homocoupling of the higher Ni(II)-alkyls of the propagating species in catalytic C-C linkage of ethylene was not observed, but Ni(II)-Me reacted with Ni(II)-Et to form propane, as concluded from studies with 2-DMSO and its analogue that is perdeuterated in the Ni(II)-Et moiety. Under the reaction conditions of the aforementioned catalytic C-C linkage of ethylene, additional ethane evolves from the reaction of intermediate Ni(II)-Et with Ni(II)-H. This is independently supported by reaction of 2-DMSO with the separately prepared hydride complex [(N,O)NiH(PMe(3))] (3-PMe(3)) to afford ethane. Kinetic studies show this reaction to be bimolecular [DeltaH(double dagger) = (47 +/- 6) kJ mol(-1) and DeltaS(double dagger) = -(117 +/- 15) J mol(-1) K(-1) over the temperature range 6-35 degrees C]. In contrast to these reactions identified as decomposition routes, hydrolysis of Ni(II)-alkyls by added water (D(2)O; H(2)O) occurred only to a minor extent for the Ni(II)-Me catalyst precursor, and no clear evidence of hydrolysis was observed for higher Ni(II)-alkyls. The rate of the aforementioned insertion of ethylene in 1-DMSO and the rate of catalytic ethylene dimerization are not affected by the presence of water, indicating that water also does not compete significantly with the substrate for binding sites.
Berkefeld
Andreas
A
University of Konstanz, Department of Chemistry, Universitatsstrasse 10, D-78457 Konstanz, Germany.
Mecking
Stefan
S