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

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 D3h 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 ΔEorb. 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.