Synthesis and Structural Characterization of Di- and Tetranuclear Zinc Complexes with Phenolate and Carboxylate Bridges. Correlations between 13C NMR Chemical Shifts and Carboxylate Binding Modes

Two di- and a tetranuclear zinc−carboxylate complexes with different coordination modes, Zn2L(μ1,3-OAc)2(ClO4) (1), Zn2L(μ1,3-Pro)2(ClO4) (2), and Zn2L(μ1,1-HCO2)(μ1,3-HCO2)2(ClO4)2 (3) (where L = 2,6-bis(N-2-(2‘-pyridylethyl)formimidoyl)-4-methylphenol, OAc = acetate, and Pro = propionate) have been synthesized. Their compositions and structures have been identified by elemental analyses, IR, NMR, and X-ray single-crystal diffraction. The cations in both 1 and 2 reveal that the two zinc ions are assembled by a phenolate and a pair of syn−syn μ1,3-carboxylate bridges with metal−metal distances of 3.281 and 3.331 Å, respectively, and each polyhedron around the zinc ion is a slightly distorted trigonal bipyramid. Compound 3 is a tetranuclear complex consisting of two identical dinuclear subunits that connect to each other by the two formate groups. In each subunit, the pair of metal ions separated at 3.130(1) Å is assembled by a phenolate oxygen from L, and a monodentate and a syn−syn bidentate formate bridges. The formate group displays a novel tridentate mode, namely, monodentate and syn−anti bidentate bridges. On the other hand, the solid-state 13C NMR technique was employed to distinguish the different binding modes of acetate group in five-coordinate zinc complexes. The chemical shifts are as follows: chelating mode (ca. 184 ppm) > bidentate bridge (ca. 180 ppm) > monodentate bridge (ca. 176 ppm).