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 BPh4− by a metathesis. The acidic pyrazole proton can be removed with LiN(SiMe3)2 to yield the square planar neutral molecule (PNN−)NiCl. The coordinated chloride can be metathetically replaced by azide to yield diamagnetic (PNN−)Ni(N3). To evaluate changing the phosphine donor for a phosphine sulfide, the corresponding pincer ligand SPNNH was synthesized and installed on NiBr2. The reduced steric bulk from the more distant phosphorous keeps both halides coordinated in the paramagnetic molecular species (SPNNH)NiBr2. 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)NiBr2 vulnerable to nucleophilic attack, resulting in a P/C cleavage product which was characterized.