Highly efficient structurally characterised novel precatalysts: di- and mononuclear heteroleptic Cu(i) dixanthate/xanthate–phosphine complexes for azide–alkyne cycloadditions

Abstract

Novel heteroleptic dinuclear [Cu₂(L)(PPh₃)₄] (L = 2,6-pyridinedimethyldixanthate L1 1, 1,4-benznedimethyldixanthate L2 2, 1,4-cyclohexanedixanthate L3 3) and mononuclear [Cu(L4)(PPh₃)₂] 4 (L4 = piperonylxanthate) and [Cu(L5)(dppf)] 5 (L5 = methylxanthate, dppf = 1,1′-bis(diphenylphosphino)ferrocene) complexes have been synthesised and characterised by elemental (C, H, and N) analysis, high resolution mass spectrometry, and IR, UV-vis, ¹H, ¹³C{¹H} and ³¹P{¹H} NMR spectroscopy. Single crystal X-ray diffraction revealed S,S-bifunctional coordination of the dixanthate group in dinuclear complexes 1–3 while 4 and 5 are mononuclear. Complexes 1–5 adopt tetrahedral coordination geometry about the copper atom. These precatalysts having two and one copper(I) centres in a single molecule have been applied in copper catalysed azide–alkyne cycloaddition (CuAAC) reactions for the synthesis of a variety of glycoconjugate triazoles using the Click approach. Particularly the dinuclear catalyst 1, formed on the pyridyl linker based dixanthate ligand, displayed outstanding and reusable catalytic activity for this reaction. Full optimization of the reaction conditions demonstrated a noteworthy Click catalytic system with low catalyst loading under mild reaction conditions.