Microwave-assisted synthesis of organically functionalized hexa-molybdovanadates

The microwave-assisted reaction of (TBA)4[β-Mo8O24] and (TBA)3[H3V10O28] with pentaerythritol or tris(hydroxymethyl)aminomethane yields polyanions with the general formula (TBA)2[V3Mo3O16(O3–R)] (R: C5H8OH – 1; R: C4H6NH2 – 3). Post-synthetic esterification of 1 yields the acylated derivative 2, with all compounds being characterized in the solid and solution state.

Isopolyoxometalates are oligomeric molecular metal oxides with notable structural diversity, while incorporation of heteroatoms into the metal oxide framework in the form of organic, 1 organometallic, 2 main group, 3 transition metal, 4 lanthanoid 5,6 and actinoid 7 components yield heteropolyoxometalates. Consequently, the properties of polyoxometalates (POMs) are equally extensive, resulting in their investigation in research areas encompassing molecular magnetism, 8 catalysis, [9][10][11] nano-materials, 12,13 energy storage, 14,15 and sensing applications. 16,17 Modification of POM surfaces by covalently grafting organic ligands has been a long-standing strategy to generate hybrid molecular materials via one-pot or post-synthetic methodologies. 18,19 The selection of synthetic approach is driven by the most efficient method to yield analytically pure products due to difficulties associated with the separation of POM mixtures. 20 Post-synthetic modification is therefore most commonly utilized when targeting the introduction of sophisticated organic components that are challenging to cleanly install in one step. 18 23 Overall these compounds can generally be prepared in good yield and relatively high purity.
Recently, we reported the initial findings of our investigation targeting the microwave-assisted synthesis of molybdovanadates. 24 As part of that work we discussed the facile preparation of the alkoxo-functionalized molybdovanadate (TBA) 2 24 As a continuation of that study, we established that the methodology could be extended to yield more synthetically useful derivatives.
Herein, we report the microwave-assisted synthesis and characterisation of alcohol, acetyl and amine terminated molybdovanadates of general formula (TBA) 2 22 This non-random distribution of V and Mo results in the polyanion metal-oxo core having C 3v -1A-3A, C s -1B-3B or C 1 -1C-3C point symmetry (Fig. 1). Despite the inability to distinguish isomers due to the averaging nature of X-ray diffraction, assignment of three distinct crystallographic metal sites with the following occupancies (site A -V, 0.75; Mo, 0.25, site B -V, 0.5; Mo, 0.5, site C -V, 0.25; Mo, 0.75) is possible (Scheme 1). Correspondingly, the metal-oxo bond lengths within the hexametallates can be clearly differentiated for the three crystallographically defined sites A-C (Fig. 1 (Table S7).
In addition to the crystallographic comparisons, NMR has proven very informative, with excellent correlation between the experimentally observed and theoretically predicted distribution of structural isomers for [V 3 Mo 3 O 16 (O 3 C 5 H 9 )] 2À , with no structural re-arrangement in MeCN at room temperature. We anticipated that the same distribution and stability would be observed for 1-3, however, our findings reveal a more complicated situation (Fig. 1). Assignment of the 1 H NMR spectrum of freshly prepared samples of 1 and 2 in D 3 -acetonitrile revealed the same preferential substitution pattern (A 4 B 4 C) as for [V 3 Mo 3 O 16 {C 5 H 9 O 3 }] 2À with slightly higher populations of isomers B and C ( Fig. 1 and Fig. S1, S2, ESI †). Unexpectedly this trend is reversed for the amine terminated polyanion (3C 4 3B 4 3A) with the relative populations determined by integration of 1 H peak areas ( Fig. 1 and Fig. S3, ESI †). The 1 H NMR spectra of 1-3 resemble that of the previously reported [V 3 Mo 3 O 16 {C 5 H 9 O 3 }] 2À , with deshielded methylene proton resonances between 4.50 and 5.25 ppm being due to their proximity to the electron deficient V V or Mo VI ( Fig. 1 and Tables S4-S6, ESI †). The C 3v symmetry of isomers 1A-3A results in a single resonance for the methylene À(CH 2 ) 3 -R protons of the grafted ligand, meanwhile a broad singlet and two sets of broad doublets-are observed for the C s symmetrical 1B-3B isomers -(CH A2 )(CH BC ) 2 -R (Fig. S1-S3 and Tables S4-S6, ESI †). The singlet is observed for the two equivalent protons bisected by the mirror plane with the pair of doublets ( J = 11.5 Hz) due to coupling between the two diastereotopic geminal protons H B and H C (Fig. 2). The meridional arrangement of the V atoms in isomers 1-3C/C 0 results in chiral polyanions, where each of the methylene protons are inequivalent. Significant peak overlap complicates the unambiguous assignment of the methylene protons, nonetheless two doublets of doublets with coupling constants of ( J = 11.5 Hz and 1.5 Hz) attributed to geminal and 4-bond ''W'' coupling are clearly identifiable in 2C/C 0 . Integration of peak areas associated with the methylene (3.9-4.02 ppm) and methyl (2.83-3.01 ppm) resonances of acetyl functionalised 2 assists with the assignment and relative population of isomers A-C.
Cyclic voltammetric analysis of compounds 1-3 in MeCN were conducted to gain some understanding of their   25 Furthermore, the insensitivity of the UV-Vis spectra to modifications of the organic ligand is in agreement with this observation, with the lowest lying electronic transition (for 2, 365 nm, e = 0.7670 Â 10 4 M À1 cm À1 ) being assigned as an LMCT band (Fig. 3).
In conclusion, we report the extension of our previously established microwave-assisted synthesis of the hexamolybdovanadate [V 3 Mo 3 O 16 {C 5 H 9 O 3 }] 2À to include alcohol 1 and amine 3 terminated derivatives. Post-synthetic derivatization of 1 to yield acetylated 2 verifies that this POM platform holds significant potential as a new building block that can be utilized in a wide range of functional materials. Attempts to separate the isomeric mixtures of these compounds is ongoing.
We thank the Australian Research Council (DE130100615) and the University of Melbourne for financial support.

Conflicts of interest
There are no conflicts to declare.