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

The P-stereogenic nickel complex {2,6-[( t -Bu)(Ph)PO] 2 C 6 H 3 }NiCl ( 2 ) has been synthesized via cyclometalation of the POCOP-pincer ligand 1,3-[( t -Bu)(Ph)PO] 2 C 6 H 4 ( 1 ) with NiCl 2 . 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] 2 C 6 H 3 }NiOTf ( 3 ) or the nickel formate complex {2,6-[( t -Bu)(Ph)PO] 2 C 6 H 3 }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.