Synthesis and crystal structures of salen-type Cu(ii) and Ni(ii) Schiff base complexes: application in [3+2]-cycloaddition and A3-coupling reactions

Abstract

The synthesis of two new salen-type Schiff base complexes of the type [Cu(L)]·0.5H₂O, 1, and [Ni(L)], 2, from the reaction of a 6,6′-[(1E,1′E)-(cyclohexane-1,2-diylbis(azanylylidene))bis(methanylylidene)bis(3-(diethylamino)phenol)] salen-type Schiff base ligand (H₂L) with Cu(OAc)₂·H₂O and Ni(OAc)₂·4H₂O in methanol at room temperature, respectively, is described. The complexes are isolated as coloured crystalline solids and characterized by elemental analysis, FT-IR spectroscopy, UV-visible spectroscopy and single crystal X-ray diffraction studies. The paramagnetic nature of complex 1, having g_iso = 2.076, was confirmed by EPR studies, which indicated a distorted square planar geometry of the complex. In contrast to this, the nickel complex was found to be diamagnetic in nature and it was additionally characterized by ¹H NMR. The crystal structures of complexes 1 and 2 confirm the distorted square planar geometry of both the complexes. Complex 1 was found to be a better catalyst for the synthesis of a series of 5-substituted 1H-tetrazoles from nitriles and sodium azide via [3+2]-cycloaddition and for the A³-coupling reaction of aldehydes, secondary amines and terminal alkynes with a low catalyst loading (0.7 and 0.9 mol%, respectively) as compared to complex 2. Complex 1 is novel in the sense that, being a homogeneous catalyst, it can be recovered almost quantitatively in both reactions and recycled up to four times to afford good yields of the corresponding products.