Catalytic ethylene oligomerization reactions of terpyridine (NNN)-based nickel(II) complexes – Studies on catalytical intermediates

[Display omitted] •This work describes the synthesis and structural properties of new terpyridine ligands and their Ni(II) complexes.•Tpy-Ni(II) complexes perform the catalytic oligomerization of ethylene and form 1-hexene (C6) and 1-octene (C8) products.•The oligomerization reactions depend on temperature and the nature of the substituent group on the 4′ position of the terpyridine ring.•Studies demonstrate that platinacycloalkanes are true model catalytic intermediates in ethylene oligomerization reactions. The present work demonstrates the synthesis of four new terpyridine (tpy) ligands (4′-(4-(pent-4-en-1-yloxy)phenyl)-2,2′:6′,2″-terpyridine (3), 4-((3-([2,2′:6′,2″-terpyridin]-4′-yl)phenoxy)methyl)benzonitrile (4), 4′-(3-(pent-4-en-1-yloxy)phenyl)-2,2′:6′,2″-terpyridine (5), and 4-([2,2′:6′,2″-terpyridin]-4′-yl)phenyl 5-chlorothiophene-2-carboxylate (6)), the corresponding Ni(II) complexes (3C–6C), and their usage as catalysts in ethylene oligomerization reactions. The purity of all the ligands and the complexes employed in the present study was assessed by various analytical and spectroscopic techniques. One of the tpy-Ni(II) complexes (6C) was structurally characterized by single-crystal XRD analysis. Notably, the nature of ligands as well as temperature strongly influence the organic product distribution, i.e. elevated temperatures facilitate C6 product in significant amounts. Among all the complexes, the Ni(II)-tpy (6C) substituted with 5-chloro-thiophenyl group at 4′ position showed high selectivity for 1-hexene (C6, 80%) at room temperature and 1-octene (C8, 91%) at elevated temperatures. Though both mechanistic pathways were proved to be feasible, the present study is also intended to identify and analyze the key intermediate species of the reactions. In order to shed more light on mechanistic aspects, ethylene insertion reactions were compared with selected platinacycloalkane complexes in a high-pressure autoclave reactor, and the formation of large metallacycloalkane species was observed in the mass spectral data.