Nieuwland's cuprocatalytic reaction in terms of crystal chemical analysis

Abstract

Crystallochemical treatment of the Nieuwland reaction is carried out on the basis of structural data obtained for crystalline acetylene complexes of the formulas NH₄Cu₈Cl₉·4C₂H₂·1/2HCu₂Cl₃·H₂O (I), NH₄Cu₃Cl₄·C₂H₂ (II), KCu₈Cl₉·4C₂H₂·1/2HCu₂Cl₃·H₂O (III), KCu₃Cl₄·C₂H₂ (IV), (NH₄)₂Cu₃Cl₅·4/9H₂O·(xC₂H₂) with x=0 (Va), 1/9 (Vb), and 4/9 (Vc) and divinylacetylene (DVA) copper chloride compounds 2CuCl·DVA (VI) and 3CuCl·DVA (VII). Because of the π-coordination of a copper atom, the C≡C bond of the acetylene molecule is activated, as indicated by its significant (up to 1.32 Å) stretch (complexes I and II). The zeolite-like structure of complexes Va-Vc, which form in a catalytic solution, is realized as an infinite {[Cu₁₀₈Cl₁₆₈(H₂O)₁₆]⁶⁰⁻}_n anion with discrete [Cl(NH₄)₆]⁵⁺ cations inside. In this structure, only 16 Cu(1) atoms have a trigonal-pyramidal environment with the oxygen atom of the crystallization water located in the vertex (d_Cu−O=2.79 Å). Under the liquid-phase conditions of the Nieuwland reaction, these copper atoms are active centers stimulating the reaction to the subsequent acetylene oligomerization due to the π-interaction with the C₂H₂ molecule. The mutual arrangement of the catalytically active Cu(1) atoms in structure Va serves as a matrix for the synthesis of DVA, as shown by the structure of the 2CuCl·DVA adduct.