Metal–Ligand Cooperation as Key in Formation of Dearomatized Ni^II–H Pincer Complexes and in Their Reactivity toward CO and CO₂

Abstract

The unique synthesis and reactivity of [(^RPNP*)­NiH] complexes (<b>1a</b>,<b>b</b>), based on metal–ligand cooperation (MLC), are presented (^RPNP* = deprotonated PNP ligand, R = ⁱPr, ^tBu). Unexpectedly, the dearomatized complexes <b>1a</b>,<b>b</b> were obtained by reduction of the dicationic complexes [(^RPNP)­Ni­(MeCN)]­(BF₄)₂ with sodium amalgam or by reaction of the free ligand with Ni⁰(COD)₂. Complex <b>1b</b> reacts with CO via MLC, to give a rare case of a distorted-octahedral PNP-based pincer complex, the Ni(0) complex <b>3b</b>. Complexes <b>1a</b>,<b>b</b> also react with CO₂ via MLC to form a rare example of η¹ binding of CO₂ to nickel, complexes <b>4a</b>,<b>b</b>. An unusual CO₂ cleavage process by complex <b>4b</b>, involving C–O and C–P cleavage and C–C bond formation, led to the Ni–CO complex <b>3b</b> and to the new complex [(PⁱPr₂NC₂O₂)­Ni­(P­(O)ⁱPr₂)] (<b>5b</b>). All complexes have been fully characterized by NMR and X-ray crystallography