Accessing alkyl- and alkenyl-cyclopentanes from Cr-catalysed ethylene oligomerization using 2-phosphinophosphinine ligands

Desilylation of the 2-phosphinophosphinine 2-PPh2-3-Me-6-SiMe3-PC5H2 with HCl gave 2-PPh2-3-Me-PC5H2, demonstrating the late-stage modification of this class of heterocyclic ligand, complementing previous approaches where ligand substitution is fixed at an early stage. Group 6 metal carbonyl complexes of these ligands showed κ2-binding and very small bite angles of 65.1 – 68.3°, and also demonstrated that the donor properties of 2-phosphinophosphinines can be tuned readily by the presence of the SiMe3 group which gives a more π-accepting phosphinine ligand. The properties of 2-phosphinophosphinines were compared to bidentate diphosphorus ligands computationally, contextualizing them in the Ligand Knowledge Base for bidentate P,P donor ligands (LKB-PP) and were found to occupy an area of ligand space adjacent to Ar2PN(R)NAr2 ligands that have been successfully used in ethylene oligomerization reactions, but with well-separated properties in the second principal component. Testing 2-phosphinophosphinines in Cr-catalyzed ethylene oligomerization reactions showed key differences to standard PNP ligands in that a high proportion of alkyl- and alkenyl-cyclopentanes were formed. This demonstrates that the different donor properties of 2-phosphinophosphinines influence the reactivity of the key 7-membered metallacycle postulated in the metallacyclic reaction mechanism, generating products from isomerization and subsequent ethylene insertion. Alkyl- and alkenyl-cyclopentanes represent new products for the key industrial feedstock ethylene, with the alkenes having potential as new monomers, comonomers or additives for plastics. Computational evaluation of ligand properties and the resulting property maps can play a role in suggesting future ligand developments to change the selectivity of this industrially-relevant system in the pursuit of new products generated from ethylene.