Carbonyl complexes of copper(i) stabilized by bridging fluorinated pyrazolates and halide ions

Abstract

Syntheses of neutral and anionic, di- and tetra-nuclear copper carbon monoxide complexes using binary copper(I) pyrazolate precursors are reported. The reaction of {[3,5-(CF₃)₂Pz]Cu}₃ (2), {[4-Cl-3,5-(CF₃)₂Pz]Cu}₃ (3) or {[3,4,5-(CF₃)₃Pz]Cu}₃ (4) with CO in CH₂Cl₂ led to copper carbonyl complexes. They however, lose CO quite easily if not kept under a CO atmosphere. Compounds {[3,5-(CF₃)₂Pz]Cu(CO)}₂ (5) and {[3,4,5-(CF₃)₃Pz]Cu(CO)}₂ (7) were characterized by X-ray crystallography. They are dinuclear species with a Cu₂N₄ core. The reaction of {[3,5-(CF₃)₂Pz]Cu}₃ with CO in the presence of [NEt₄]Br or [NEt₄][3,5-(CF₃)₂Pz] affords relatively more stable [NEt₄][{[3,5-(CF₃)₂Pz]Cu(CO)}₄(μ₄-Br)] (8) and [NEt₄]{[3,5-(CF₃)₂Pz]₃Cu₂(CO)₂} (9). The related [NEt₄][{[4-Cl-3,5-(CF₃)₂Pz]Cu(CO)}₄(μ₄-Br)] (10) and [NEt₄][{[4-Cl-3,5-(CF₃)₂Pz]Cu(CO)}₄(μ₄-Cl)] (11) can be synthesized using {[4-Cl-3,5-(CF₃)₂Pz]Cu}₃, CO and [NEt₄]Br or [NEt₄]Cl. The X-ray structures show that 8, 10 and 11 are tetranuclear species with terminal Cu–CO groups and quadruply bridging Cl⁻ and Br⁻ ions. Compound 9 features an anionic cage of nearly D_3h symmetry formed by three bridging [3,5-(CF₃)₂Pz]⁻ ions and two terminal Cu–CO moieties. Theoretical calculations show that bonding in these 16- and 18-electron copper complexes follows Dewar–Chatt–Duncanson (DCD) model, where the CO stretching frequencies correlate well to the orbital interaction energy ΔE_orb. The major Cu–CO interaction however is electrostatic in nature. Further theoretical exploration of the role of the substituent at pyrazolyl ring 4-position between –H, –Cl, and –CF₃, shows a slight decrease in covalent character of the Cu–CO interaction and diminished π-back bonding as pyrazolate groups become more weakly donating with added electron withdrawing substituents.