Synthesis and Reactivity of Nickel an Palladium Fluoride Complexes with PCP Pincer Ligands. A NMR-Based Assessment of Electron-Donating Properties of Fluoride and Other Monoanionic Ligands

Abstract

A series of complexes of the type [(iPrPCP)M-L] (M = Ni, Pd), containing the 2,6-bis(diisopropylphosphinomethyl)phenyl (iPrPCP) pincer ligand and simple monoanionic ligands L (F, Cl, Br, H, Me, Ph, NO3, and OTf), have been synthesized and characterized. The fluoride derivatives [(iPrPCP)M-F] were prepared from the halides [(iPrPCP)M-X] (M = Ni, X = Br; M = Pd, X = I) by exchange reactions with AgF or, alternatively, by protonolysis of the methyl complexes [(iPrPCP)M-Me] with Et3N·3HF (TREAT-HF). A survey of the 13C NMR data for the new complexes and previously reported ones with L = OH, OMe, and NH2 revealed significant trends that can be directly related to the electronic properties of the anionic ligands L coordinated to Ni(II) and Pd(II) centers. The chemical shift of the 13C resonance of the metal-bound ipso carbon is very sensitive to the σ-donating capacity of L, which increases in the order L = OTf > NO3 > F > OAc > OMe > Cl > OH ≈ Br > I > NH2 > Ph > Me > H. In addition, the reactivity of [(iPrPCP)M-F] complexes was investigated. These readily undergo fluoride exchange with LiOMe, affording the corresponding methoxides [(iPrPCP)M-OMe] and LiF. They also react with n-C12H13I to cleanly yield [(iPrPCP)M-I] and n-C12H13F. Comparison of the rates of the latter reactions and analogous ones involving bromide or chloride complexes showed that the fluorides are significantly more nucleophilic, particularly the palladium derivative. This property was applied in a palladium-catalyzed reaction for the synthesis of alkyl fluorides using AgF as the fluoride source.