Fluoro and Trifluoromethyl Derivatives of POCOP-Type Pincer Complexes of Nickel: Preparation and Reactivities in SN2 Fluorination and Direct Benzylation of Unactivated Arenes

This report describes the synthesis, characterization, and reactivities of the pincer-type complexes kappa(P),kappa(C),kappa(P)-(R-POCOP)NiX (R-POCOP = 2,6-(i-Pr2PO)(2),4-R-C6H3; R = H, X = F (2a), CF3 (3a); R = OMe, X = F (2b), X = CF3 (3b)). These complexes were prepared by treating their NiBr analogues with excess AgF at rt (2a and 2b) or excess AgF and Me3SiCF3 (3a and 3b) at 45 degrees C. The NiF derivative 2a was shown to be an intermediate for the formation of the NiCF3 derivative 3a. Complexes 2a, 3a, and 3b were fully characterized by multinuclear NMR spectroscopy, cyclic voltammetry, and X-ray diffraction analysis. Variable temperature, multinuclear NMR analysis of 2a helped elucidate interaction of the fluoride ligand with hydrofluoric acid present in the reaction medium. The bromo complexes 1a or 1b catalyzed fluorination of benzyl halides at 90 degrees C in the presence of excess AgF, whereas the NiCF3 complexes 3 failed to promote trifluoromethylation of benzyl bromide, leading instead to a direct benzylation of the aromatic reaction solvent (benzene, toluene, xylene, or bromobenzene). Possible mechanisms for this benzylation reaction have been probed and discussed.