Single‐Side Organically Functionalized Anderson‐Type Polyoxometalates

Abstract

Polyoxometalate (POM)-based organic–inorganic hybrid materials have attracted extensive attention in the last ten years due to their potential applications in catalysis, nonlinear optics, photovoltaic, and self-assembly research. Currently, trisalkoxo tripodal ligand (Tris)-functionalized Lindqvist-, Anderson-, and Dawson-type POMs show high stability and varied functionalities. Zubieta et al. used Tris ligands to stabilize macro-POM clusters and to functionalize hexavanadate. The functionalized hexavanadate was further developed by Hill and co-workers for the construction of metal–organic frameworks, which can function as highly efficient heterogeneous catalytic materials. Due to the high reactivity of vanadate, vanadium-containing Dawsontype POMs were observed to be capped by only one Tris ligand. In particular, after Hasenknopf et al. had established the synthesis of Tris derivatives of Anderson-type POMs in 2002, the groups of Cronin and Hasenknopf introduced different kinds of functional groups to the system through further organic reactions. The resulting materials show interesting functions including high biological activity against cells, photocatalytic properties, and interesting selfassembly behavior. Due to their high symmetry (D3d), both sides of Anderson-type POMs are easily functionalized simultaneously by Tris ligands during their synthesis, which seems to be an unquestionable fact (Figure 1), and there has never been any report on a single-sided functionalized Anderson-type POM with Tris. Herein, we report the synthesis and characterization of Anderson-type POMs with only one side functionalized by a Tris ligand, which not only afforded asymmetric POM-based hybrid materials for further research, but also uncovered more structural and reactive properties of Anderson-type POMs, and the reaction mechanism for the functionalization of POMs with Tris. In the previous protocol, to synthesize the double-sided functionalized Anderson-type POMs, octamolybdate, a salt of the central heteroatom, and organic ligands (Tris) were mixed in an organic solvent, such as acetonitrile, and the final products were obtained after heating to reflux for approximately 16 hours, which is a classical one-pot reaction. However, our new protocol to synthesize the single-side organically functionalized Anderson-type POM, [(C4H9)4N]5{H7ACHTUNGTRENNUNG[HOCH2C ACHTUNGTRENNUNG(CH2)3 ACHTUNGTRENNUNG(CrO6)Mo6O18]2} (1), is a two-step aqueous reaction with high yield through the direct functionalization of the parent Anderson-type POM, [CrMo6O18(OH)6] 3 . Compound [CrMo6O18(OH)6] 3 was first synthesized according to Perloff s method reported in 1970. Pentaerythritol was then added to the resulting aqueous solution of [CrMo6O18(OH)6] 3 , and the mixture was refluxed at once for a prolonged time to finish the esterification. Interestingly, the reaction is of high yield and high selectivity for the mono Tris-functionalized product 1, probably due to the hierarchical synthesis protocol and aqueous reaction media (see the Experimental Section). In the preparation reaction, a single-side-functionalized Anderson-type POM is obtained in high yield (based on Mo), even when the ratio of Tris to the parent Anderson-type POM is two or much higher than two. Therefore, the selectivity for producing such single-sided species might be related to the aqueous environment. [a] P. Wu, P. Yin, Dr. J. Zhang, Dr. J. Hao, Dr. Z. Xiao, Prof. Dr. Y. Wei Department of Chemistry, Tsinghua University Beijing, 100084 (P.R. China) Fax: (+86)10-6277-1149 E-mail : yinpanchao@tsinghua.org.cn yonggewei@tsinghua.edu.cn [b] P. Wu Institute of POM-based Materials Key Laboratory of Fermentation Engineering (Ministry of Education) Hubei University of Technology Wuhan, 430068 (P.R. China) [c] Prof. Dr. Y. Wei Key Laboratory of Cluster Science (Beijing Institute of Technology) Ministry of Education of China Beijing, 100081 (P.R. China) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201101552. Figure 1. Molecular structures of: a) unfunctionalized and b) bifunctionalized Anderson-type POMs. Color code: Mo: large black sphere; Cr: central gray sphere; O: small black sphere; C: large gray sphere; H: small gray sphere; X= functional group: -CH2OH, -NH2, -CH3, or -NO2.