Synthesis and structural characterization of di- and tetranuclear zinc complexes with phenolate and carboxylate bridges. Correlations between C-13 NMR chemical shifts and carboxylate binding modes

摘要

Two di- and a tetranuclear zinc-carboxylate complexes with different coordination modes, [Zn2L(mu(1,3)-OAc)(2)](ClO4) (1), [Zn2L(mu(1,3)-Pro)(2)](ClO4) (2), and [Zn2L(mu(1,1)-HCO2)(mu(1,3)-HCO2)](2)(ClO4)(2) (3) (where L = 2,6-bis(N-2-(2'-pyridylethyl)-f ormimidoyl)-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 Angstrom, 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) Angstrom 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 C-13 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). [References: 47]