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

Abstract

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.