Synthesis, structural characterization and catalytic application of zinc and cadmium sulfur complexes with imidazol-2-ylidene-N’-phenylthiourea ligand scaffold

Abstract

We report the synthetic and structural studies of a series of zinc metal complexes, [(ImRNCSN(H)Ph)2ZnX2] (R = Mesityl (Mes), X = Cl (1a), Br (1b), I (1c); R = 2,6-Diisopropylphenyl (Dipp), X = Cl (2a), Br (2b), I (2c)) and the cadmium metal complexes [(ImRNCSN(H)Ph)2CdX2] (R = Mes, X = Cl (3a), I (3b)] and [(ImRNCSN(H)Ph)CdX]2 [R = Dipp, X = Cl (4a), I (4b)). The solid-state structures of metal complexes 1c, 2c, 3a, 4a, and 4b were established by single-crystal X-ray diffraction analysis. The zinc complexes 1c, 2c, and cadmium complex 3a are isostructural in solid-state, and in each case, the metal ion is ligated with two ligand moieties in a monodentate fashion via the sulfur atom. Despite similar coordination features, they exhibited a marked variation in the orientation of phenylthiourea scaffold as well as hydrogen-bonding interactions. The solid-state structures of cadmium complexes 4a and 4b involve a centrosymmetric dimer containing four-membered ring –[Cd-X−]2, formed by μ2-coordination of the halide group on each cadmium atom. In all the complexes, the metal ion (Zn2+/Cd2+) adopts a distorted tetrahedral MS2X2 environment. Additionally, the catalytic activity of [(ImMesNCSN(H)Ph)2ZnI2] (1c) as an active pre-catalyst is reported in the NH addition of amines to readily available carbodiimides to produce a series of substituted guanidine derivatives in an excellent yield. The zinc complex 1c showed very good conversion and tolerance of a broad and valuable substrate scope during the guanidine formation.