Halogen-Bonding Interactions and Electrochemical Properties of Unsymmetrical Pyrazole Pincer Ni^II Halides: A Peculiar Behavior of the Fluoride Complex (PCN)NiF

Abstract

Four metal halides of the (tBuPCN)NiX family [X = F (1), Cl (2), Br (3), or I (4); PCN = 1-[3-[(di-tert-butylphosphino)methyl]phenyl]-1H-pyrazole] have been prepared through a direct reaction of the corresponding anhydrous NiX2 salts and the unsymmetrical PCN pincer ligand. Structural information from single-crystal X-ray data, 19F solution nuclear magnetic resonance, and combined electrochemical results [cyclic voltammetry (CV) and in situ electron paramagnetic resonance spectro-electrochemistry] reveal that the fluoride complex 1 is different from the other halides 2–4 in terms of electro-oxidation behavior (presence of three anodic peaks instead of one in the CV profile). 1 is also different from its symmetrical analogue (tBuPCP)NiF in terms of halogen-bonding capability toward the halogen-bond donor, iodopentafluorobenzene (C6F5I, IPFB). The positive ΔS° value for the formation of the 1···IPFB adduct inferred from the van’t Hoff plots is in agreement with the existence of (1·H2O) aggregates in solution, not present in 2–4. The high nucleophilicity of the fluoride ligand and the simultaneous presence of electrophilic (acidic) protons on the sidearm of the pyrazole ligand are at the origin of the observed phenomena.