Timing matters: pre-assembly versus post-assembly functionalization of a polyoxovanadate–organic cuboid

The pre-assembly and post-assembly approaches in the functionalization of a polyoxovanadate–organic cuboid, [{V6S}8(QPTC)8{V3}2]10−, are discussed. We have shown that the two pathways have led to distinctly different systems, with either an expanded or contracted interior void space, when phenylphosphonate is introduced at different stages of the self-assembly. One leaves the cuboid framework largely intact, whereas the other results in a compact, twisted cuboid. Kinetic factors will have to be considered in the equilibrium of these complex processes. Furthermore, the exceptional stability of these polyoxometalate–organic systems facilitates mass spectrometric characterization, which confirms the composition of the complexes and also indicates that the methoxide groups on the vanadium cluster nodes are labile. The results will help deepen the mechanistic understanding of the formation mechanisms of polyoxovanadate-based metal–organic cages and other functionalized polyoxovanadate clusters in general.


Instruments and Physical Measurements
General: IR spectra (KBr pellets) were collected on a Thermo Nicolet Avatar 360 FTIR spectrophotometer.
Atmosphere compensation (CO2 and H2O) and baseline corrections were carried out after spectrum collection. 1 H NMR spectra and 31 P NMR were collected on a Bruker Avance III 400 MHz NMR instrument. For base-digestion NMR experiments, the MOP samples were dissolved in 1 M NaOD and aged for 2 days (or until the solution turned colorless, indicating complete oxidation of V(IV) to V(V)) before running NMR measurements. Elemental analyses were performed on a Vario EL III analyzer (for C, H, N) and Perkin-Elmer ICP-OES (for P, S, and V). The UVvisible spectra were recorded on a Perkin-Elmer Lambda 750 spectrophotometer. Thermal gravimetric analyses were measured with a TA Instruments SDT-Q600 thermal analysis system under N2 flow with 10 °C/min heating.
High-resolution mass spectra were obtained on a quadrupole-orbitrap mass spectrometer (QExactive, ThermoFisher Scientific). The mobile phase consisted of (A) Milli-Q water and (B) HPLC-grade acetonitrile. The HPLC-MS was operated with electrospray ionization in negative polarity mode. The solvent molecules, especially those between MOP cages, are highly disordered in the crystal structures. Therefore, the numbers of the solvent molecules (CH3OH and DMF) were determined by TGA and elemental analyses.
The determination of counter ions (both their identities and numbers) in the three MOP compounds was based on collective evidence from single-crystal X-ray diffraction, NMR spectroscopy, TGA, and elemental analyses.
Solvothermal decomposition of DMF solvents has been known to generate dimethylammonium (DMA + ) and tetramethylammonium (TMA + ) ions, depending on the reaction conditions and if other solvents such as CH3OH are also present. S2, S3 In each of the MOP crystal structures, we have located multiple TMA + counter ions. Solution NMR in DMSO-d6, although no informative for identifying the ligand signals, can exclude the presence of DMA + in the crystals as there is no peaks at 2.47 and 9.08 ppm (from internal DMA + Cl − reference). The TMA + signals, however, overlap with that of the methoxide groups on the {V6S/P} SBUs (See Figs. S20 and S21). Thermal decomposition of TMA + counterions in TGA occurs in the temperature range of 375-550 °C as a well-separated weight loss step, so its number can be accurately determined.

Single-crystal X-ray diffraction:
The single-crystal X-ray diffraction data were collected using a Bruker D8 Quest X-ray diffractometer equipped with an Incoatec Microfocus Mo Source (IµS 3.0, λ = 0.71071 Å) and a PHOTON II CPAD detector. Suitable crystals were coated with Paratone N oil, suspended on a small fiber loop, and placed in a 173(2) K cooled nitrogen stream from Oxford Cryosystems Cryostream equipment. For all cases, the raw data were processed with the Bruker APEX3 software package. The data were solved by intrinsic phasing methods S4 and the refinement was done by full-matrix least squares on F 2 using SHELXL S5 (2018/3). Hydrogen atoms on the organic linkers, terminal methoxyl groups, the interior organophosphonate units and the tetramethylammonium counter ions are place geometrically and refined using a riding model; FLAT and AFIX 66 restraints were used, if necessary, to treat the disordered phenyl groups on the phosphonates and organic linkers (QPTC). The SQUEEZE option of PLATON S6 was used to model the contribution of disordered solvent molecules and counter ions to the reflection intensities.             Fig. S22. The distances between adjacent hexavanadate clusters (S/P· · · S/P separations, in Å) and those between adjacent QPTC linkers in 1(a) and 1'(b); the distances between adjacent hexavanadate clusters in 1 (c); and pi-pi stacking interactions between the phenyl rings of the phosphonate functional groups in 1'(d).

Single-Crystal X-Ray Structure Determination
S19

Bond Valence Sum (BVS) Calculations:
For determination of the oxidation states of metal centers and the protonation states of oxygen sites, BVS calculations were carried out using the method of I. D. Brown. S8 The ro values were taken from the literature S9 for calculations performed on V.