Small Molecule Activation by POCsp3OP-Nickel Complexes

This contribution describes the reactivities of CO2, CO, O-2, and ArNC with the pincer-type complexes [(kappa(P),kappa(C),kappa(P)'-POCsp3OP)NiX] (POCsp3OP=(R2POCH2)(2)CH; R=iPr; X=OSiMe3, NArH; Ar=2,6-iPr(2)C(6)H(3)). Reaction of the amido derivative with CO2 and CO leads to a simple insertion into the Ni-N bond to give stable carbamate and carbamoyl derivatives, respectively, the pincer ligand backbone remaining intact in both cases. In contrast, the analogous reactions with the siloxide derivative produced kinetically labile insertion products that either revert to the starting material (in the case of CO2) or react further to give the mixed-valent, dinickel species [(POCsp3OP)Ni-II{mu,kappa(O),kappa(P),kappa(P')-OCOCH(CH2CH2OPR2)(2)}Ni-0(CO)(2)]. The zero-valent center in the latter compound is ligated by a new ligand arising from transformation of the POCsp3OP ligand backbone. The carbonylation and carboxylation of the siloxido derivative also produced minor quantities of a side-product identified as the trinickel species, [{(eta(3)-allyl)-Ni(mu(O),kappa(P)-R2PO)(2)}(2)Ni], arising from total dismantling of the POCsp3OP ligand. Similar reactivities were observed with isonitrile, ArNC: reaction with the siloxido derivative resulted in a complex sequence of steps involving initial insertion, a 1,3-hydrogen shift, and an Arbuzov rearrangement to give [Ni(CNAr)(4)] and a methacrylamide based on fragments of the POCsp3OP ligand. Oxygenation of the amido and siloxido derivatives led to the phosphinate derivative, [(POCsp3OP)Ni(OP(O)R-2)], arising from oxidative transformation of the original ligand frame; the reaction with the Ni-NHAr derivative also gave ArHNP(O)R-2 through a complex N-P bond-forming reaction.