Deactivation Pathways of Neutral Ni(II) Polymerization Catalysts

• Published in: Journal of the American Chemical Society Vol. 131; no. 4; pp. 1565 - 1574
• Main Authors: Berkefeld, Andreas, Mecking, Stefan
• Format: Journal Article
• Language: English
• Published: United States American Chemical Society 04.02.2009
• ISSN: 0002-7863; 1520-5126; 1520-5126
• DOI: 10.1021/ja808855v

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)−NCH−(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(αCH2 βCH3)(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 k ins,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 [ΔH ⧧ = (57 ± 1) kJ mol−1 and ΔS ⧧ = −(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-PMe 3 ) to afford ethane. Kinetic studies show this reaction to be bimolecular [ΔH ⧧ = (47 ± 6) kJ mol−1 and ΔS ⧧ = −(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.