Metal-Ligand Cooperation as Key in Formation of Dearomatized Ni(II)-H Pincer Complexes and in Their Reactivity toward CO and CO(2)

The unique synthesis and reactivity of [((R)PNP*)NiH] complexes (1a,b), based on metal-ligand cooperation (MLC), are presented ((R)PNP* = deprotonated PNP ligand, R = (i)Pr, (t)Bu). Unexpectedly, the dearomatized complexes 1a,b were obtained by reduction of the dicationic complexes [((R)PNP)Ni(MeCN)](BF(4))(2) with sodium amalgam or by reaction of the free ligand with Ni(0)(COD)(2). Complex 1b reacts with CO via MLC, to give a rare case of a distorted-octahedral PNP-based pincer complex, the Ni(0) complex 3b. Complexes 1a,b also react with CO(2) via MLC to form a rare example of η(1) binding of CO(2) to nickel, complexes 4a,b. An unusual CO(2) cleavage process by complex 4b, involving C-O and C-P cleavage and C-C bond formation, led to the Ni-CO complex 3b and to the new complex [(P(i)Pr(2)NC(2)O(2))Ni(P(O)(i)Pr(2))] (5b). All complexes have been fully characterized by NMR and X-ray crystallography.