An Alternative Mechanistic Concept for Homogeneous Selective Ethylene Oligomerization of Chromium‐Based Catalysts: Binuclear Metallacycles as a Reason for 1‐Octene Selectivity?

Abstract

An alternative concept for the selective catalytic formation of 1-octene from ethylene via dimeric catalytic centers is proposed. The selectivity of the tetramerization systems depends on the capability of ligands to form binuclear complexes that subsequently build up and couple two separate metallacyclopentanes to form 1-octene selectively. Comparison of existing catalytic processes, the ability of the bis(diarylphosphino)amine (PNP) ligand to bridge two metal centers, and the experimental background support the proposed binuclear mechanism for ethylene tetramerization.

Towards Selective Ethylene Tetramerization

“…This observation suggests that a truly selective ethylene-tetramerization catalyst may indeed exist. As a working hypothesis, we concur with the mechanistic concept recently proposed by Rosenthal and co-workers [10] that a dinuclear monovalent species, possibly without a CrÀCr multiple bond, might be formed with the aid of a non-anionic bidentate ligand and promote the formation of 1-octene through a bimetallic reductive elimination.…”

A Highly Selective Ethylene Tetramerization Catalyst

“…If the sevenmembered ring is capable of expanding readily into the ninemembered ring, it is hard to imagine why additional expansion should not occur equally fast. [9] In the end, a distribution of oligomers is to be expected and 1-octene may be a dominant product.Rosenthal and co-workers [10] first emphasized this problem and postulated an alternative mechanism for the highly selective formation of 1-octene. According to their hypothesis, a dimetallic system with two low-valent chromium centers that are not linked with each other may independently form two five-membered metallacycles.…”

A Chromium Ethylidene Complex as a Potent Catalyst for Selective Ethylene Trimerization

“…Deprotonation of 2,5-di-tert-butylpyrrole followed by reaction with [CrCl 3 (thf) 3 ] afforded [{h 5 -(tBu) 2 C 4 H 2 N}CrCl 2 -(thf)] (1), which was isolated as dark violet-blue crystals. [13] The pseudo-tetrahedral piano-stool arrangement of 1 was elucidated by an X-ray crystal structure (Figure 1).…”