Pincers with diverse donors and their interconversion: application to Ni(II)

A pincer ligand composed of a pyridine with ortho positions substituted by a bulky phosphine arm and a pyrazole arm (PNNH) is installed on nickel(II) to yield the diamagnetic planar complex [(PNNH)NiCl]Cl. The chloride anion can be replaced by BPh 4 − by a metathesis. The acidic pyrazole proton can be removed with LiN(SiMe 3 ) 2 to yield the square planar neutral molecule (PNN − )NiCl. The coordinated chloride can be metathetically replaced by azide to yield diamagnetic (PNN − )Ni(N 3 ). To evaluate changing the phosphine donor for a phosphine sulfide, the corresponding pincer ligand SPNNH was synthesized and installed on NiBr 2 . The reduced steric bulk from the more distant phosphorous keeps both halides coordinated in the paramagnetic molecular species (SPNNH)NiBr 2 . Several attempts to dehydrobrominate this species result in synthesis and characterization of two unexpected products. One effort revealed that the electrophilic character of P(V) leaves the phosphorus atom in (SPNNH)NiBr 2 vulnerable to nucleophilic attack, resulting in a P/C cleavage product which was characterized.