P-P Coupling and Terminal Metal-Phosphorus Intermediates.

Terminal metal-phosphorus (M-P) complexes are of significant contemporary interest as potential platforms for P-atom transfer (PAT) chemistry. Decarbonylation of metal-phosphaethynolate (M-PCO) complexes has emerged as a general synthetic approach to terminal M-P complexes. M-P complexes that are stabilized by strong M-P multiple bonds are kinetically persistent and isolable. In the absence of strong M-P stabilization, the formation of diphosphorus-bridged complexes (i.e., M-P-P-M species) is often interpreted as evidence for the intermediacy of reactive, unobserved M-P species. Here, we demonstrate that while diphosphorus complexes can arise from reactive M-P species, P-P coupling can also proceed directly from M-PCO species without the intermediacy of M-P complexes. Photochemical decarbonylations of a pincer-supported Ni (II)-PCO complex at 77 K afford a spectroscopically observed terminal Ni-P complex, which is best described as a triplet, Ni(II)-metallophosphinidene with two unpaired electrons localized on the atomic phosphorus ligand. Thermal annealing of this transient Ni-P complex results in rapid dimerization to afford the corresponding P-bridged dinickel complex. Unexpectedly, the same P-bridged dinickel complex can also be accessed via a thermally promoted process in the absence of light. The analysis of reaction kinetics, isotope-labeling studies, and computational results indicate that the thermal P-P coupling process proceeds via a noncanonical mechanism that avoids terminal M-P intermediates. Together, these results represent the first observation of P-P coupling from characterized terminal M-P species and demonstrate that terminal M-P intermediates are not required to obtain P-P coupling products. These observations provide critical mechanistic understanding of the activation modes relevant to P-atom transfer.