Reactions of phenylacetylene with nickel POCOP-pincer hydride complexes resulting in different outcomes from their palladium analogues

Abstract

Nickel POCOP-pincer hydride complexes [2,6-(R₂PO)₂C₆H₃]NiH (R = ⁱPr, 4a; R = ^cPe = cyclopentyl, 4b) react with phenylacetylene to generate [2,6-(R₂PO)₂C₆H₃]NiC(Ph)CH₂ (5a–b) as the major product and (E)-[2,6-(R₂PO)₂C₆H₃]NiCHCHPh (6a–b) as the minor product. The 2,1-insertion is more favorable than the 1,2-insertion and both pathways involve cis addition of Ni–H across the CC bond. Unlike the palladium case, alkynyl complexes [2,6-(R₂PO)₂C₆H₃]NiCCPh (7a–b) and H₂ are not produced in the nickel system. The more bulky hydride complex [2,6-(^tBu₂PO)₂C₆H₃]NiH (4c) shows no reactivity towards phenylacetylene. Catalytic hydrogenation of phenylacetylene with 4a–b takes place at an elevated temperature (70–100 °C) and proves to be heterogeneous. The structures of 5b, 6a, 7a and 7b have been studied by X-ray crystallography.