Tunable Cu-Ni phosphine-thiolate complexes as water-compatible catalyst for base-free click chemistry

Abstract

Three new transition metal complexes incorporating phosphine and thiolate ligands were synthesized and structurally characterized: [Cu2(dppe)3(dmtzsH)2], (1) [Ni(dppe)(dmtzsH)2], (2) and an unusual heterobimetallic structural motif [Ni(DMF)4(H2O)2] [Cu4(dppe)2(dmtzs)4Ni], (3). The sulfur-rich ligand 2,5-dimercapto-1,3,4-thiadiazole (dmtzsH₂) exhibits versatile coordination behaviour, binding as a monodentate thiolate anion in complexes 1 and 2, but functioning as a dianionic amido-thiolate bridging ligand in 3. The auxiliary ligand 1,2-bis(diphenylphosphino)ethane (dppe) consistently stabilises the Cu(I) and Ni(II) coordination environments across all complexes. Single-crystal X-ray diffraction confirmed the unique structural Cu4Ni core in complex 3, and natural bond orbital (NBO) analysis revealed possible electronic communication between Cu(I) and Ni(II). Catalytic performance was evaluated in the multicomponent synthesis of 1,4-disubstituted 1,2,3-triazoles, a prominent type of “click” chemistry known as an azide-alkyne cycloaddition reaction. The reaction proceeds in water, under base-free conditions with minimal catalyst loading, while maintaining high activity and quantitative yields over multiple cycles. This work presents the first application of a crystallographically defined Cu4Ni heterobimetallic system in click chemistry, highlighting how the chosen ligand promotes dual-metal structural proximity, which may facilitate interaction to enhance the efficiency of green catalytic transformations.