Zwitterionic Nickel(II) Catalyst for CO–Ethylene Alternating Copolymerization

Abstract

A zwitterionic nickel(II) catalyst has been discovered to display an initial catalytic activity comparable to that of cationic palladium catalysts for alternating copolymerization of carbon monoxide and ethylene. This demonstrates the absence of a severe dormant state in the present zwitterionic system, in contrast to the cationic nickel(II) catalysts. However, the highly active catalyst is short-lived. Stoichiometric decomposition of the catalyst under carbon monoxide suggests that the insufficient stability of the tetraphenylborate motif in the ligand framework with respect to electrophilic attack is likely a culprit for catalyst deactivation. P alladium-catalyzed alternating copolymerization of alkene and carbon monoxide is a highly efficient method for synthesis of aliphatic polyketones. 1−5 Simple members in this family, such as the terpolymer of CO, ethylene, and propylene, possess a plurality of desirable materials properties and are attractive semicrystalline engineering thermoplastics suitable for a variety of applications. 6,7 Since the initial attempts by Shell and BP in the 1990s, efforts of large-scale production of the aliphatic polyketones have continued. However, one of the shortcomings of the current technology is the necessity of using the expensive palladium metal as the catalyst.Nickel is the first metal discovered to catalyze CO−ethylene copolymerization but is generally far inferior to palladium and has not been as widely studied. The cationic palladium catalysts in the commercial processes typically have activities of ∼6000 g (g of Pd) −1 h −1 (grams of polyketone per gram of palladium per hour) and productivities of 10 6 g (g of Pd) −1 (grams of polyketone per gram of palladium). 2 The nickel analogues of the cationic palladium catalysts are less productive and less active than the latter by several orders of magnitude. 8,9 Brookhart and his co-workers have shown from a mechanistic viewpoint that the difference between cationic Ni and Pd catalysts is the involvement of five-coordinate intermediates in the nickel system. 10 The 18-electron, 5-coordinate chelate intermediate A reacts with ethylene very slowly and may be a resting state that severely impedes the catalytic process. If the 5-coordinate resting state could be eliminated, theoretical work predicted that the cationic nickel catalysts should be more active than the cationic palladium catalysts. 11,12 Neutral nickel compounds with anionic N,O-and P,Ochelating ligands are isoelectronic with the cationic compounds and should be less Lewis acidic. 13,14 These compounds as catalyst precursors are known to catalyze the alternating copolymerization of CO and ethylene. The neutral arylnickel-(II) compounds with anionic N,O-chelating ligands appear rather active, but an induction period of at least 45 min under copolymerization conditions is required before any copolymer is produced. 14a The activities of these catalysts have not been rigorously examined. Their reported productivity (∼3000 g (g of Ni) −1 ) is significantly lower than that o...

Synthesis, Properties, and Catalytic Application of a Triptycene-Type Borate-Phosphine Ligand

“…In this context, we sought to replace the Si atoms of these silicon-containing caged phosphines with a tetravalent B atom to produce anionic caged phosphine molecules , that increase the electron-donor power of the P lone pair . We also expected that such borate-phosphine hybrid ligands would produce zwitterionic organometallic species with well-defined locations of the cation and anion centers upon binding to cationic metal species …”

Zwitterionic nickel(II) complexes: Synthesis, characterization, decomposition, and stoichiometric and catalytic reactivities

“…Elemental analysis was carried out by Microanalysis, INC., Delaware. Ligand 1 was synthesized following a procedure in the literature [ 11 ] (TMEDA)MiMe 2 was in situ generated following a procedure in the literature [7].…”

“…A nickel catalyst that is somewhat less active and productive than the palladium catalysts might still have a cost advantage. We reasoned that the enhanced electron-donating ability of anionic phosphine ligand [1,2] might alleviate a possible catalyst resting state in Ni-catalyzed CO-ethylene copolymerization [ 6 ] and recently reported a zwitterionic Ni catalyst that possessed an extraordinary initial activity for catalyzing the copolymerization [7]. Our exploration along this line began with the nickel analogs of the palladium complexes reported by Peters.…”

Studies on Alternating Copolymerization of Ethylene and Carbon Monoxide Using Nickel‐Based Catalyst: Cocatalyst and the Polarity of Solvent

“…[18] Recently, several studies have been focused on zwitterionic nickel-based catalysts for the preparation of PK. [19,20] The activity of zwitterionic catalysts was very high, 15.5 × 10 3 g PK/(g Ni h), in the first 5 min, and decreased quickly later, which resulted in low productivity. [18] Li and co-workers tried to reduce the deactivation rate by adjusting the ligand of zwitterionic catalysts to improve the productivity.…”