Configurational Stability and Stereochemistry of P‑Stereogenic Nickel POCOP-Pincer Complexes

The P-stereogenic
nickel complex {2,6-[(t-Bu)­(Ph)­PO]2C6H3}­NiCl (2) has been
synthesized via cyclometalation of the POCOP-pincer ligand 1,3-[(t-Bu)­(Ph)­PO]2C6H4 (1) with NiCl2. The initially isolated 2 consists of a 1:1 mixture of racemic and meso isomers that are separable
through repeated crystallization and is configurationally stable even
at 110 °C. Upon mixing with t-BuOK, the meso
isomer (2-meso) displays
a higher ligand substitution rate than the racemic isomer (2-rac), likely because its nickel center
is sterically more accessible. Complex 2, as either pure 2-rac or a 2-rac/2-meso mixture, can be converted to the nickel triflate complex
{2,6-[(t-Bu)­(Ph)­PO]2C6H3}­NiOTf (3) or the nickel formate complex {2,6-[(t-Bu)­(Ph)­PO]2C6H3}­NiOCHO
(7) without epimerization at the phosphorus centers.
Under a dynamic vacuum at 90 °C, decarboxylation of 7-meso is faster than that of 7-rac, suggesting that in the transition
state the formato hydrogen approaches the nickel center from the axial
site rather than the equatorial site. The structure of 2-rac has been studied by X-ray crystallography.