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

Four metal halides of the ((PCN)-P-tBu) 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, F-19 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 ((PCP)-P-tBu) NiF in terms of halogen-bonding capability toward the halogen-bond donor, iodopentafluorobenzene (C6F5I, IPFB). The positive Delta S degrees 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.