Cationic methallylnickel and (meth)allylpalladium 2-phosphinophenol complexes:: Synthesis, structural aspects, and use in oligomerization of ethylene

Reactions of methallylnickel bromide with 2-R2PC6H4OH 1a,b (R = Ph, cHex) and AgSbF6 furnish methallylnickel phosphinophenol hexafluoroantimonates 2a,b, which are not very stable and decay in solution at room temperature within an hour (2a) or days (2b) to tetranuclear nickel complexes 3a,b. The related cationic allylpalladium complex 4a is more stable. A crystal structure analysis gives evidence of the monomeric nature, the typical eta(3)-coordination of the allyl group, and the coordination of the phenolic hydroxyl group to palladium. In solution the OH group is uncoordinated and underlines the hemilabile character of the phosphinophenol ligands. The allylpalladium phosphinophenol tetrafluoroborate 5a, stable as a solid, slowly decomposes in CDCl3 solution to give a dinuclear complex 6a and in THF/water as the solvent the palladium bis(phosphinophenolate) 7a. Methallylpalladium phosphinophenol acetates 8a,b, existing as phosphinophenolate acetic acid conjugates, are uncommonly stable and lose the acid on heating in a vacuum without cleavage of the methallyl group to give neutral methallylpalladium phosphinophenolates 9a,b. The labile complexes 2a and 2b proved to be highly active single-component catalysts for the conversion of ethylene to isomer mixtures of butenes, hexenes, and lower amounts of higher olefins at ambient temperature. The C-6 fraction consists mainly of branched and internal olefins. 5a is less active but converts ethylene on heating under pressure to a mixture of butenes and smaller amounts of hexenes, while 8 and 9 are inactive.