Accessing Alkyl- and Alkenylcyclopentanes from Cr-Catalyzed 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-PC5H3, demonstrating the late-stage modification
of this bidentate heterocyclic ligand. 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 those of 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 from standard PNP ligands in that
a high proportion of alkyl- and alkenylcyclopentanes were formed.
This demonstrates that the different donor properties of 2-phosphinophosphinines
influence the reactivity of the key seven-membered metallacycle postulated
in the metallacyclic reaction mechanism, generating products from
isomerization and subsequent ethylene insertion. Alkyl- and alkenylcyclopentanes
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.