Metal(ii)-directed self-assembly of amino acid functionalized polyoxometalates

Six new hybrid species based on amino acid functionalized γ-octamolybdates, [(H2O)3M]2[Mo8O26(pro)2]·2H2O (M = Cu 1, Zn 2, Co 3), [(H2O)8(lys)2Cu3][Mo8O27]·12H2O (4), and [(H2O)6M][Mo8O26(lysH)2]·4H2O (M = Zn 5, Co 6) (pro = proline; lys, lysH = lysine), have been synthesized and characterized by elemental analysis, IR and UV-vis spectroscopy, TG analysis, powder X-ray diffraction and single crystal X-ray diffraction. Compounds 1–3 are isostructural, consisting of proline functionalized γ-octamolybdates linked by metal cations (M = Cu2+, Zn2+ and Co2+) to form a 2D framework with 4-connected topology. The proline functionalized γ-octamolybdates [Mo8O26(pro)2]4− consist of two γ-type [Mo8O26]4− anions covalently bridged by two proline molecules via the carboxyl oxygen atoms. When a lysine molecule was used instead of the proline ligand, compounds 4–6 with different structures were obtained. In 4, the γ-octamolybdates are firstly linked to a 1D chain via sharing oxygen atoms, and then adjacent chains are joined together by trinuclear {Cu3(lys)2} coordination fragments to yield a 2D network as a rare (3,6)-connected net with the Schläfli symbol (3·62)(34·42·62·74·83). When Zn2+ and Co2+ cations were added to replace Cu2+, isostructural 5 and 6 were obtained, respectively, and they are constructed from lysine functionalized γ-octamolybdates [Mo8O26(lysH)2]2− and free metal–H2O coordination cations. The activities of 1–6 against two human tumor cell lines (human breast cancer MCF-7 and human liver hepatocellular cells HepG2) were investigated in vitro. Cu- and Zn-directed amino acid functionalized POMs exhibit higher activities than Co-directed POMs and the parent amino acid functionalized octamolybdates ([Mo8O26(pro)2]4− and [Mo8O26(lysH)2]2−), which opens up new ways of developing novel drugs based on amino acid functionalized molybdate clusters.