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, Zn(2)L(mu(1,3)-OAc)(2) (1), Zn(2)L(mu(1,3)-Pro)(2) (2), and Zn(2)L(mu(1,1)-HCO(2))(mu(1,3)-HCO(2))(ClO(4))(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 mu(1,3)-carboxylate bridges with metal-metal distances of 3.281 and 3.331 A, 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) A 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 (13)C 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).