Isolation of Single‐Component Trimerization and Polymerization Chromium Catalysts: The Role of the Metal Oxidation State

Abstract

One more trimer: A series of chromium complexes of different oxidation states are employed as catalysts for the polymerization/oligomerization of ethylene. The results suggest a link between chromium oxidation state and catalytic performance, with CrIII leading to oligomerization, CrII to polymerization, and CrI to selective trimerization.

A survey of pendant donor-functionalised (N,O) phosphine ligands for Cr-catalysed ethylene tri- and tetramerisation

“…19,23 At this stage most of the evidence points towards a coordination arrangement such as shown in IV, in which the chloro group attached to aluminium acts as a third weak donor. Such an arrangement has been demonstrated by Gambarotta and co-workers in the case of Cr/pyrrole/ triethylaluminium (Phillips type) trimerisation catalysts, 42,43 as well as in other cases. 45 The highly selective nature of the PNPNH systems in conjunction with their long catalyst lifetime and the low cost of the co-catalyst make them of interest to industry, while their well-defined nature make them ideal for structural and mechanistic studies.…”

“…Interestingly, in this report combinations of Cr(acac) 3 /PNPNH/AlEt 3 were shown to be completely inactive for catalysis, and as such the authors conclude that the active species most likely contains chloride; a result which is further supported by a subsequent structural study and is analogous to previously reported ethylene trimerisation systems. 33,34,[41][42][43][44] However, while there is no doubt that addition of a chloride source to the reaction mixture yields a positive result, previous reports by Rosenthal and co-workers have also demonstrated that catalytic mixtures containing no chloride can readily give high trimerisation activity and selectivity and as such the exact role of chloro groups in the catalytic mechanism remains unclear. 19,23 At this stage most of the evidence points towards a coordination arrangement such as shown in IV, in which the chloro group attached to aluminium acts as a third weak donor.…”

A Highly Selective Ethylene Tetramerization Catalyst

“…[5] Unlike Sasols PNP-based and SK Energys PC 2 P-based catalysts, [3a,b] the current system shows a strong solvent and co-catalyst dependence. Selective ethylene tetramerization has only been observed in MeCy.…”

“…For example, when highly reactive monovalent species are provided with a sufficiently long lifetime, selective oligomerization is initiated by the so-called redox reaction/ring-expansion mechanism. [2,5,7] As mentioned above, selective ethylene tetramerization remains exceedingly rare. With a maximum of around 77 % in the case of the process developed by SK Energy,…”

“…In the presence of alkyl aluminum activators, these species are reduced to the divalent state (responsible for nonselective oligomerization and/or polymerization) or the monovalent state (responsible for selectivity). [5] To complicate the scenario, there is also the possibility for these monovalent and divalent species to undergo disproportionative redox processes that give inactive zero-valent chromium together with higher-valent species that are readily available for further cycles of reduction/reoxidation. [6] Because of the presence of such a redox dynamism in the catalytic cycle, the ancillary ligand system determines the selectivity by preferentially stabilizing one particular oxidation state.…”