Date Published: November 01, 2018
Publisher: International Union of Crystallography
Author(s): Nadiia I. Gumerova, Amir Blazevic, Tania Caldera Fraile, Alexander Roller, Gerald Giester, Annette Rompel.
A post-functionalization protocol was used for the synthesis of two new tris-hybrid Al-centred Anderson-type polyoxomolybdates with indometacin or cinnamic acid.
Polyoxometalates (POMs), an exceptional class of metal–oxide clusters with various compositions, exhibit an oxygen-rich surface with strong coordination potential (Pope, 1983 ▸). They have attracted much attention owing to their unique catalytic (Wang & Yang, 2015 ▸), redox (Gumerova & Rompel, 2018 ▸), magnetic (Clemente-Juan et al., 2012 ▸) and bioactive properties (Bijelic & Rompel, 2015 ▸, 2017 ▸; Molitor et al., 2017 ▸; Fu et al., 2015 ▸; Bijelic et al., 2018a ▸,b ▸) and constitute promising building blocks for advanced materials. Recently, increasing effort has been devoted to the introduction of organic and metal–organic units into the metal oxide frameworks in order to functionalize POM materials (Dolbecq et al., 2010 ▸). Among the various synthetic strategies for the organic functionalization of POMs, alkoxylation has gained much attention due to the diversity and tunability of alkoxyl ligands, especially when using the disk-shaped Anderson-type anions [Xn+HmM6O24](12–n–m)– (M = Mo6+ and W6+; X = heteroatom, e.g. Te6+ and I7+ for A-type with m = 0, or Al3+ and Ni2+ for B-type with m = 6), with a wide spectrum of central heteroatoms (Blazevic & Rompel, 2016 ▸; Zhang et al., 2018 ▸). In particular, after Hasenknopf et al. (2002 ▸) had pioneered and established the synthesis of tris-derivatives of Anderson polyoxomolybdates (POMos), this archetype has been widely used as starting materials for the attachment of various tris [tris(hydroxymethyl)methane]-based organic ligands [RC(CH2OH)3, denoted R-Tris]. If the R group itself is reactive (–NH2, –CH2OH etc.), post-functionalization with a variety of organic molecules, including ligands containing aromatic units (Al-Sayed et al., 2015 ▸) or alkyl chains (Rosnes et al., 2013 ▸) via imine, amide or ester-bond formation, is possible. The resulting hybrid materials were used in supramolecular self-assembly (Macdonell et al., 2015 ▸) or for the formation of metal–organic frameworks (MOFs; Li et al., 2016 ▸). Major application fields are bio-inorganic (Yvon et al., 2014 ▸), nano-structured (Song et al., 2009 ▸), energy storage (Ji et al., 2015 ▸), optical (Boulmier et al., 2018 ▸) and photochemical (Schaming et al., 2010 ▸) materials.
AlMo6-NH-Cin and AlMo6-NH-Indo were prepared via post-functionalization by pre-forming the hybrid cluster AlMo6-NH2 which was modified by amidation reactions (Fig. 1 ▸). The fact that single-side grafted anions were obtained supports an earlier theory claiming that the aqueous solvent is a key factor for the formation of single-sided Anderson derivatives (Wu et al., 2011 ▸; Blazevic et al., 2015 ▸; Gumerova et al., 2016 ▸).
The success in synthesizing AlMo6-NH-Cin and AlMo6-NH-Indo shows the versatility and reproducibility of the post-functionalization protocol for the alkoxylation of Anderson POMs. The attachment of bioactive ligands makes the hybrid Anderson POMos reported herein potentially superior to pure inorganic structures for antibacterial applications.