Nickel-Templated Replacement of Phosphine Substituents in a Tetradentate Bis(amido)bis(phosphine) Ligand

Abstract

The replacement of phosphine substituents in nickel-bound PNNP ligands is reported as an alternative method for preparing multidentate phosphine ligands with alkyl substituents. Treatment of the previously reported bis(phosphide) complex {K(THF)(x)}(2)(2Ph)[PNNP]Ni (2) with 2 equiv of MeI, (PrI)-Pr-i, and 1,3-dibromoethane formed alkyl-substituted complexes (2Ph,2Me)[PNNP] Ni (3), (2Ph,2 iPr)[PNNP]Ni (4), and (2Ph,propylene)[PNNP]Ni (5), respectively. The stereoselectivity (racemic vs meso) of these reactions can be controlled by varying the reaction temperature. The racemic mixtures of products with the new alkyl substituents in an anti configuration were favored at lower temperatures, whereas a larger proportion of meso compounds was acquired at higher temperatures. Further treatment of 3 with KH resulted in selective elimination of the remaining phenyl groups rather than the methyl substituents, affording bis(methylphosphide) complex {K(THF)(x)}(2)(2Me)[PNNP]Ni (6). Subsequent treatment of 6 with additional MeI formed (4Me)[PNNP]Ni (7), in which all four phenyl groups were replaced with methyl substituents. As a proof of concept, demetalation of the ligand from 7 was achieved using aqueous KCN to form a free dimethylphosphine-substituted ligand H-2(4Me)[PNNP] (8), and 8 was subsequently coordinated to a different metal, using PdCl2 to form (4Me)[PNNP]Pd (9). Unlike the clean elimination of phenyl substituents from 3, the reactions of KH with 4 and 5 exhibited competitive elimination of both alkyl and phenyl substituents and/or attenuated reactivity.