Synthesis, structures and DFT study of copper coordination with bis- benzimidazole based thiones and selones: Scope for nonstoichiometric Cu2-xSe nanoparticles

The newly synthesized benzimidazole-based [E2]-donor ligands (E = S or Se) (L1 -L4) on reaction with copper(II) chloride and copper(II) acetate involved reduction of Cu(II) to Cu(I) and yielded copper(I) complexes. The ligands L1 -L3 formed stable 20-membered metallocyclic binuclear Cu(I)-complexes of stoichiometry, [Cu2Cl2(L1-L3)2] (5-7) and [Cu2(OAc)2(L1-L3)2] (9-11). Interestingly, L4 ligand yielded a stable complex, [Cu2Cl2(L4)2] (8) with a chloride anion, but with acetate it formed an unstable complex, [Cu2(OAc)2(L4)2] (12). ESI-mass of coordination compounds 5-12, supported the characteristic molecular ion peaks/ fragmented peaks for their ions. NMR study of ligands and complexes, single crystal x-ray crystallography of ligands L1, L2, L4 and complex 6, as well as DFT studies of all ligands and complexes are reported for characterization of ligands and complexes (1-12). Compound 12 with -NCH2CH2OH moiety in the ligand L4 showed very unusual behavior as compared to the compound 11 with -NCH3 moiety in the ligand L3. It degraded under mild conditions to form copper selenide (Cu2-xSe) nanoparticles in the solution, which were characterized using various techniques, such as TEM, SEM and EDX. The plausible mechanism for the instability of the complex 12 even at low temperature (at 0 degrees C), leading to the formation of Cu2-xSe nanoparticles, has been discussed in this paper.