Acetylene and terminal alkyne complexes of copper(i) supported by fluorinated pyrazolates: syntheses, structures, and transformations

Abstract

Trinuclear {μ-[3,5-(CF₃)₂Pz]Cu}₃ reacts with acetylene to produce the 2 : 1 copper(I) acetylene complex, Cu₄(μ-[3,5-(CF₃)₂Pz])₄(μ-HCCH)₂. Related Cu₄(μ-[4-Br-3,5-(CF₃)₂Pz])₄(μ-HCCH)₂ and Cu₄(μ-[4-Cl-3,5-(CF₃)₂Pz])₄(μ-HCCH)₂ have also been isolated using the corresponding copper(I) pyrazolate and acetylene. The 1 : 1 adducts Cu₂(μ-[3,5-(CF₃)₂Pz])₂(HCCH)₂ and Cu₂(μ-[4-Br-3,5-(CF₃)₂Pz])₂(HCCH)₂ are significantly less stable to the acetylene loss and can be observed in solution at low temperatures under excess acetylene. The X-ray crystal structures of 2 : 1 and 1 : 1 complexes, Cu₄(μ-[3,5-(CF₃)₂Pz])₄(μ-HCCH)₂ and Cu₂(μ-[4-Br-3,5-(CF₃)₂Pz])₂(HCCH)₂ are reported. Raman data show a reduction in _CC stretching frequency by about ∼340 and ∼163 cm⁻¹ in the 2 : 1 and 1 : 1 Cu(I)/acetylene complexes, respectively, from that of the free acetylene. Copper(I) pyrazolate complexes of the terminal alkynes, phenylacetylene, 1,8-nonadiyne, and 1,7-octadiyne are also reported. They form adducts involving one copper atom on each alkyne moiety. The {μ-[3,5-(CF₃)₂Pz]Cu}₃ is also a very versatile and competent catalyst for alkyne transformations as evident from its ability to catalyze the alkyne C(sp)–H bond carboxylation chemistry with CO₂, azide–alkyne cycloadditions leading to 1,2,3-triazoles including the use of acetylene itself as a substrate, and thiol addition to phenylacetylene affording vinyl sulfides.