On the stability of a POCsp3OP-type pincer ligand in nickel(II) complexes.

We describe the results of a study on the stabilities of pincer-type nickel complexes relevant to catalytic hydroalkoxylation and hydroamination of olefins, C-C and C-X couplings, and fluorination of alkyl halides. Complexes [(POCsp3 OP)NiX] are stable for X=OSiMe3 , OMes (Mes=1,3,5-Me3 C6 H2), NPh2, and CC-H, whereas the O(tBu) and N(SiMe3)2 derivatives decompose readily. The phenylacetylide derivative transforms gradually into the zero-valent species cis-[{κ(P),κ(C),κ(C')-(iPr2 POCH2 CHCH2 )}Ni{η(2),κ(C),κ(C')-(iPr2 P(O)CCPh)}]. Likewise, attempts to prepare [(POCsp3 OP)NiF] gave instead the zwitterionic trinuclear species [{(η(3) -allyl)Ni}2-{μ,κ(P),κ(O)-(iPr2 PO)4 Ni}]. Characterization of these two complexes provides concrete examples of decomposition processes that can dismantle POCsp3 OP-type pincer ligands by facile C-O bond rupture. These results serve as a cautionary tale for the inherent structural fragility of pincer systems bearing phosphinite donor moieties, and provide guidelines on how to design more robust analogues.