Deactivation Pathways of Neutral Ni(II) Polymerization Catalysts

Abstract

The novel dimethyl sulfoxide (DMSO)-coordinated complex [(N,O)Ni(CH3)(DMSO)] {1-DMSO; (N,O) ) κ2-N,O-(2,6-(3,5-(F3C)2C6H3)2C6H3)-NdCH-(3,5-I2-2-OC6H2)} 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(αCH2βCH3)(DMSO)] (2-DMSO). 2-DMSO is subject to interconversion of the αC and βC 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 °C results in partial reaction to form propylene (pseudo-first-order rate constant kins,Me ) 6.8 ( 0.3 × 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 precusor is the bimolecular elimination of ethane [ΔHq ) (57 ( 1) kJ mol-1 and ΔSq ) -(129 ( 2) J mol-1 K-1 over the temperature range 55-80 °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(PMe3)] (3-PMe3) to afford ethane. Kinetic studies show this reaction to be bimolecular [ΔHq ) (47 ( 6) kJ mol-1 and ΔSq ) -(117 ( 15) J mol-1 K-1 over the temperature range 6-35 °C]. In contrast to these reactions identified as decomposition routes, hydrolysis of Ni(II)-alkyls by added water (D2O; H2O) 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.