Development of homogeneous Co and Ni (pre−)catalysts: synthesis, characterization, and catalytic performance

Abstract

3d transition metal catalysis has been actively pursued in recent years, since these metals constitute attractive alternatives to precious metals of the 4d and 5d block. As a result, the development and implementation of catalysts based on inexpensive, earth abundant and low toxic 3d metals have thrived, and important progress has been made. The lower oxophilicity and presumed greater functional-group tolerance of late transition metals relative to early transition metals make them promising candidates for hydrogenation and polymerization catalysts. As a part of this endeavor, a novel synthetic methodology for the preparation of Co(II) and Ni(II) bis(alkyl) complexes was established. Subsequent ligand exchange reactions with strong  donors such N-heterocyclic carbenes or bidentate phosphines allowed the isolation of three- and four-coordinate Co(II) species, whereas in the case of Ni(II) only the four-coordinate species were obtained. Furthermore, the trigonal-planar complex [(ItBu2)Co(CH2SiMe3)2] exhibited good catalytic activity under mild reaction conditions in homogeneous olefin and alkyne hydrogenations featuring superior activity to four-coordinate derivatives which were reported in the literature. Distinguishing between homogeneous and heterogeneous catalysis is non-trivial, consequently independent and complementary methods were applied, such as reaction progress analysis and qualitative kinetic poisoning reactions. A different approach was taken in the development of Ni(II) pre-catalysts for olefin polymerization reactions. Cationic systems stabilized by bulky monodentate phosphines, either with pendant substituents at the phosphorus atom (TTMPP) or with large 1-adamantyl substituents (Ad3P) were synthesized, since these phosphines provide sufficient axial steric bulk to effectively prevent chain transfer reactions. Two organometallic systems were also successfully implemented in the polymerization of ethylene and norbornene. However, the insolubility of the isolated polymers in all commonly employed solvents rendered subsequent characterization by spectroscopic methods unfeasible. Nevertheless, the polyethylene polymers were analyzed by differential scanning calorimetry (DSC), which suggested the formation of linear low-density polyethylene.