Green carbon–carbon homocoupling of terminal alkynes by a silica supported Cu(ii)-hydrazone coordination compound

Abstract

A new heterogeneous catalytic system based on a silica-supported Cu( ii) hydrazone complex has been prepared and used for the green homocoupling of terminal alkynes.

Abstract

A Cu( ii) complex, [Cu(HL)(NO ₃)(CH ₃OH)]·CH ₃OH (1), was obtained by the reaction of Cu(NO ₃) ₂·3H ₂O and H2L in methanol solvent (H2L is ( E)-4-amino- N′-(2-hydroxy-3-methoxybenzylidene)benzohydrazide). H2L and compound 1 were characterized by various spectroscopic analyses and the molecular structure of [Cu(HL)(NO ₃)(CH ₃OH)]·CH ₃OH was determined by single-crystal X-ray analysis. The results indicated the product is a mononuclear Cu( ii) complex and contains a free NH ₂ functional group on the structure of the ligand. [Cu(HL)(NO ₃)(CH ₃OH)]·CH ₃OH was used for the preparation of a heterogeneous catalyst by supporting it on functionalized silica gel. The heterogeneous catalyst (Si-Cu) was prepared by an amidification reaction of [Cu(HL)(NO ₃)(CH ₃OH)]·CH ₃OH with functionalized silica gel. The resulting silica-supported catalyst (Si-Cu) was characterized by TGA, FT-IR, EPR, DRS, EDS, XRD, SEM and XPS analyses. Si-Cu was employed in a carbon–carbon coupling reaction and the effects of the amount of Si-Cu and temperature were investigated in the catalytic coupling. The structure of one of the products of the catalytic reactions (C ₁₆H ₂₂O ₂, CP1) was determined by single-crystal X-ray analysis, which proved the formation of a C–C bond and the production of di-acetylene by homocoupling of terminal alkyne. This catalytic system is stable and it can be reused for a coupling reaction without a significant change in its catalytic activity.