Copper(I) Alkynyl Clusters, [Cu_x+y(hfac)_x(CCR)_y], with Cu₁₀–Cu₁₂ Cores

Abstract

The facile syntheses and the structures of five new Cu(I) alkynyl clusters, [Cu(12)(hfac)(8)(C[triple chemical bond]CnPr)(4)(thf)(6)]xTHF (1), [Cu(12)(hfac)(8)(C[triple chemical bond]CtBu)(4)] (2), [Cu(12)(hfac)(8)(C[triple chemical bond]CSiMe(3))(4)] (3), [Cu(10)(hfac)(6)(C[triple chemical bond]CtBu)(4)(diethyl ether)]/[Cu(10)(hfac)(6)(C[triple chemical bond]CtBu)(3)(C[triple chemical bond]CnPr)(diethyl ether)] (4) and [Cu(10)(hfac)(6)(C[triple chemical bond]CtBu)(4)(diethyl ether)] (5) are reported, in which hfacH=1,1,1,5,5,5-hexafluoropentan-2,4-dione. The first independent molecule found in the crystals of 4 (4 a) proved to be chemically identical to 5. The Cu(10) and Cu(12) cores in these clusters are based on a central "square" Cu(4)C(4) unit. Whilst the connectivities of the Cu(10) or Cu(12) units remain identical the geometries vary considerably and depend on the bulk of the alkynyl group, weak coordination of ether molecules to copper atoms in the core and CuO intramolecular contacts formed between Cu-hfac units on the periphery of the cluster. Similar intermolecular contacts and interlocking of Cu-hfac units are formed in the simple model complex [Cu(2)(hfac)(2)(HC[triple chemical bond]CtBu)] (6). When linear alkynes, C(n)H(2n+1)C[triple chemical bond]CH, are used in the synthesis and non-coordinating solvents are used in the workup, further association of the Cu(4)C(4) cores occurs and clusters with more than eighteen copper atoms are isolated.

Advancements in the mechanistic understanding of the copper-catalyzed azide–alkyne cycloaddition

“…Fokin and Finn, on the other hand, point out the “complex ways” [81], in which CuAAC catalysis responds to changes in concentration, type of ligand, presence of chloride ions, type of buffer and other parameters. In objection to Donnelly’s suggestion of a mononuclear pathway, they strongly emphasize the need for a second copper centre to assist the formation of the C–N bond between the azide and the acetylide, which is in accordance with evidence from structural studies [117,161,165–185] as well as quantumchemical calculations [125,186]. Fokin and Finn suggest that more than one mechanism might be operational under the conditions studied and stress that the first order rate dependence in the presence of TBTA is an exception, as a second order dependence was found for most other ligand systems with related structures (vide infra) [80–81].…”

“…Usually, however, copper(I) acetylides are polynuclear structures in which the acetylide ligands coordinate to two copper centres either in the unsymmetric σ,π-coordination mode (μ 2 -η 1 ,η 2 ) or in the symmetric σ,σ-coordination mode (μ 2 -η 1 ,η 1 ) [117,161,165–185]. It is consensus in all mechanistic investigations that π-complexes of the alkyne substrate with the copper(I) centres need to be formed in order to enable facile deprotonation to give the corresponding σ-complexes.…”

“…The equilibrium between π-complex [Cu(HC≡CH)] + , monocopper acetylide [Cu(C≡CH)] and bisacetylide Cu 2 C 2 is greatly influenced by the pH of the medium. The presence of halide ions and other ligands such as phosphines has a strong impact on speciation and nuclearity of these acetylide complexes, but they are in all cases complex polynuclear structures [117,161,165–185]. This is why our research group chose a tetranuclear model copper(I) acetylide and a dinuclear copper(I) acetylide complex with additional phenanthroline ligands as resting states for a DFT study on the CuAAC’s mechanism [186].…”

“…Tetranuclear copper acetylide complexes as reported by Weiss (left) [176] and Tasker (middle) [185] and model of tetranuclear copper(I) acetylides as resting state for mechanistic DFT studies [186]. …”

Fluktuierender Kupferacetylid‐Cluster in der CuAAC‐Katalyse

“…[11] Eine wiederkehrende Annahme bei quantenchemischen Rechnungen auf diesem Gebiet ist die Bindung von Aquoliganden an Kupferionen mit niedriger Koordinationszahl bei CuAAC-Modellintermediaten und der Vorschlag dieser Kupfer-Aquokomplexe als Katalysator-Ruhezustand. [ [27] Wirv erwendeten die Substrate Ethylpropiolat und Benzylazid für einen kinetischen Vergleich bei Raumtemperatur. Aufgrund der hochgradig exothermen Natur der Tr iazolbildung ist eine hohe Verdünnung der Reaktionsmischung zwingend erforderlich, um eine isotherme Reaktionsführung zu gewährleisten und ein thermisches Durchgehen binnen weniger Minuten zu verhindern.…”