A Boron-Fluorinated Tris(pyrazolyl)borate Ligand (FTp*) and Its Mono- and Dinuclear Copper Complexes [Cu(FTp*)2] and [Cu2(FTp*)2]: Synthesis, Structures, and DFT Calculations

Reaction of [Si(3,5-Me(2)pz)(4)] (1) with [Cu(MeCN)(4)] [BF4] (2) gave the mono-and dinuclear copper complexes [Cu-2((F)Tp*)(2)] (3) and [Cu((F)Tp*)(2)] (4). Both complexes contain the so-far unprecedented boron-fluorinated (F)Tp* ligand ([FB(3,5-Me(2)pz)(3)](-) with pz=pyrazolyl) originating from 1, acting as a pyrazolyl transfer reagent, and the [BF4](-) counter anion of 2, serving as the source of the {BF} entity. The solid-state structures as well as the NMR and EPR spectroscopic characteristics of the complexes were elaborated. Pulsed gradient spin echo (PGSE) experiments revealed that 3 retains (almost entirely) its dimeric structure in benzene, whereas dimer cleavage and formation of acetonitrile adducts, presumably [Cu((F)Tp*)(MeCN)], is observed in acetonitrile. The short Cu center dot center dot center dot Cu distance of 269.16 pm in the solid-state is predicted by DFT calculations to be dictated by dispersion interactions between all atoms in the complex (the Cu-Cu dispersion contribution itself is only very small). As revealed by cyclic voltammetry studies, 3 shows an irreversible (almost quasi-reversible at higher scan rates) oxidation process centred at E-pa = -0.23 V (E-1/2(0) = -0.27 V) (vs. Fc/Fc(+)). Oxidation reactions on a preparative scale with one equivalent of the ferrocenium salt [Fc][BF4] (very slow reaction) or air (fast reaction) furnished blue crystals of the mononuclear copper(II) complex [Cu((F)Tp*)(2)] (4). As expected for a Jahn-Teller-active system, the coordination sphere around copper(II) is strongly distorted towards a stretched octahedron, in accordance with EPR spectroscopic findings.