Synthesis, Photophysical Properties, and Computational Analysis of Di- and Tetranuclear Alkyne Complexes of Copper(I) Supported by a Highly Fluorinated Pyrazolate

This work describes the reaction of a cyclic trinuclear complex, {[3,5-(CF3)2Pz]­Cu}3, with internal alkynes to construct di- and tetranuclear copper­(I) complexes with short cuprophilic contacts and the study of their intriguing bonding and photophysical properties, both experimentally and theoretically. For comparison, we have also investigated the related copper­(I) trifluoroacetate 3-hexyne complexes so as to deduce and isolate the role played by the supporting pyrazolate ligand on the luminescence of these and related complexes. Some adducts reported herein feature bridging (i.e., μ2–η2,η2-) alkyne coordination modes, which is rare for copper­(I)–alkyne complexes documented in the literature. Raman data show red shifts in the average υ̅CC stretching frequency from 2260 cm–1 in free 3-hexyne to 2050 cm–1 in the terminal η2-/2e-donor alkyne adduct Cu2(μ-[3,5-(CF3)2Pz])2(EtCCEt)2 (1) and, more drastically, to 1874 cm–1 in the bridged μ2–η2,η2-/4e-donor adduct Cu4(μ-[3,5-(CF3)2Pz])4(μ-EtCCEt)2 (2). The tetranuclear 2-butyne adduct Cu4(μ-[3,5-(CF3)2Pz])4(μ-MeCCMe)2 (3) also shows a large reduction in υ̅CC relative to that of free 2-butyne. The effects of strengthened copper-alkyne interactions in the bridging relative to terminal alkynes are also reflected in the CCC bending back angles. Photophysical studies in the solid state reveal the significance of the pyrazolate supporting ligand to bestow bright photoluminescence, which is found only for the pyrazolate-supported compounds 1–3 but not the trifluoroacetate copper-alkyne complexes Cu2(μ-CF3CO2)2(EtCCEt)2 (4) and Cu4(μ-CF3CO2)4(μ-EtCCEt)2 (5). The orange phosphorescence with ∼50 μs lifetime at room temperature in 1–3 thermochromically changes to generate additional green/yellow bands at cryogenic temperatures due to suppression of internal conversion from the respective T2 → T1 states in each solid, whereas solvent effects lead to additional blue-shifted bands in glassy solvent media. Computational analyses explains the bonding and spectral results and suggest that the major excited-state distortion is due to significant (≥20°) rotation of the alkyne ligands on Cu­(I) in the di- or tetranuclear adducts, as opposed to excimeric contraction of cuprophilic Cu­(I)···Cu­(I) distances. X-ray crystallographic data of 1–3 and 5 are also presented