Trisubstituted 4f- and 4d tungstoantimonates as artificial phosphoesterases for nerve agent degradation

Three new trisubstituted 4f- and 4d tungstoantimonates (TA) K3Na21[(M(CH3COO))3(HPO3)(WO4)(SbW9O33)3]·nH2O {M3(HPO3)Sb3W28} (M = GdIII, YIII, YbIII, n = 35–36) were synthesized using a double-template synthetic approach. Following their characterization in the solid state employing single- and powder X-ray diffraction (XRD), IR-spectroscopy, and elemental – and thermogravimetric analyses (TGA), {M3(HPO3)Sb3W28} were subjected to a comprehensive set of solution characterization methods including UV/vis- and multinuclear 31P and 13C NMR spectroscopy. All representatives were shown to be highly active, recyclable, and stable Lewis-acid catalysts towards the nerve agent simulant O,O-dimethyl O-(4-nitrophenyl) phosphate (DMNP) at neutral pH (in Tris–HCl [125 mM] at pD 7.0 25 °C). Control experiments showing catalytic activity of the unsubstituted trilacunary TA [SbW9O33]9− suggest the non-innocence of Tris in the DMNP hydrolysis for the first time.


Experimental Procedure
{M3(HPO3)Sb3W28} were prepared employing a double-template synthetic strategy, which, in contrast to conventional template approaches exploits more than one template agent to yield more complex POM architectures exhibiting modulable properties depending on the type and number of incorporated template. 9 In the present study, a {HPO3} motif was incorporated into the POM framework in addition to the stabilizing central tetrahedral {WO4} moiety thereby allowing 31 P NMR spectroscopy experiments for speciation studies under catalytic turnover conditions.

Unit cell dimensions [Å] and [°]
30.5863 (5)   Note that differences between the simulated and the experimental PXRD patterns may be due to factors such as loss of solvent molecules as indicated by TGA measurements further leading to the collapse of the lattice giving rise to different crystalline phases at different temperatures. Additionally, the experimental PXRD pattern was obtained at room temperature (298 K), whereas the CIF file used for the simulated pattern was obtained from a single crystal measured at 112 K.

NMR spectroscopy
Pre-catalytic studies The 13 C NMR spectrum of a freshly prepared aqueous solution of {Y3(HPO3)Sb3W28} displays two peaks corresponding to the methyl -(δ = 23.20 ppm) and carboxyl group (δ = 181.46 ppm), respectively (Fig. S9). Subsequent addition of NaOAc to the {Y3(HPO3)Sb3W28} solution resulted in an increase of both 13 C NMR peaks with no additionally occurring peaks, thereby suggesting the exclusive presence of free acetate ligands in aqueous solution, hence demonstrating the facilitated accessibility of the Lewis-acid metal centers in {M3(HPO3)Sb3W28} (Fig. S10 A, B).
The 31 P NMR spectrum of {Y3(HPO3)Sb3W28} in D2O (pD = 7.0) displays a singlet peak located at 2.73 ppm which can be attributed to the central {HPO3} motif present in all isostructural TAs of the compound series ( Fig. S11 A, B). In contrast, the 31 P NMR spectrum of a freshly prepared aqueous solution of phosphorous acid displays various peaks (Fig. S11 A) thereby excluding the presence of free phosphorous acid in the case of {M3(HPO3)Sb3W28}. Ageing experiments that were performed by incubating a freshly prepared aqueous solution of {Y3(HPO3)Sb3W28} under pre-catalytic conditions (D2O, pD = 7.0, 25°C, no substrate) revealed no change of the 31 P NMR singlet peak at 2.73 ppm after one week of incubation thereby demonstrating the polyanion's pre-catalytic stability (Fig. S11 B), which is additionally strongly supported by the successful crystallographic characterization of {Y3(HPO3)Sb3W28} that can be isolated as single crystals after the aging experiments (Tables S12, S13).

Post-catalytic stability studies
The post-catalytic stability of (POM)-catalysts in homogeneous systems is a frequently encountered issue since the use of NMR-spectroscopic techniques under turnover conditions as a powerful method to prove solution stability is mostly hampered by the low solubility and sensitivity of the 183 W nucleus (14.3 % natural abundance), strong paramagnetic interactions within the polyanion, or the lack of "NMR-active" nuclei with a suitable multiplicity and abundance. Attributed to the presence of the {HPO3}-motif in all TA-catalysts and the diamagnetic nature of Y(III), the polyanions' speciation behavior under turnover conditions could be monitored employing 31 P NMR spectroscopy on catalytic solutions containing {Y3(HPO3)Sb3W28} as a representative member of the isostructural TA compound series. Upon addition of DMNP to a solution containing {Y3(HPO3)Sb3W28}, an immediate downfield -shift of the singlet from 2.73 ppm to 2.91 ppm can be observed indicating the substrate's coordination to the POM-catalyst (Fig. S12). Over the course of the reaction, the singlet exhibits an upfield -shift to 2.82 ppm close to its initial position without the occurrence of any additional peaks directly demonstrating the post-catalytic stability of the {M3(HPO3)Sb3W28} catalyst (Fig. S12, S34A), which is additionally supported by post-catalytic IR-spectroscopic measurements and energy dispersive X-ray analysis (EDX) that were conducted on the precipitated cesium-salts of the {M3(HPO3)Sb3W28} polyanions, thereby clearly showing the characteristic W-O-W bridging and terminal W=O vibrations in the tungsten fingerprint area from 300 -1000 cm -1 (Fig. S30 -S32) and a comparable Sb:W ratio between the pre-catalytic {Y3(HPO3)Sb3W28} and the post-catalytically isolated cesium salt Cs{Y3(HPO3)Sb3W28} (Fig.  S33). Subsequent control experiments with YCl3 as a non-shielded free Lewis-metal center resulted in the immediate appearance of a singlet at 1.37 suggesting the formation of an unidentifiable Y-complex, even at low concentrations of the metal center, which additionally proves the solution stability of the POM compounds under reaction conditions (Fig. S28).

Recyclability
The recyclability of {M3(HPO3)Sb3W28} and {SbW9} as catalysts for the decontamination of DMNP was tested in a consecutive experiment by reloading the reaction mixture with DMNP substrate after 31 P-NMR measurements confirmed the DMNP conversion (> 98%) to the hydrolysis product DMP (Fig. S34A). A direct comparison of the turnover frequency (TOF) values obtained for {M3(HPO3)Sb3W28} and {SbW9} in the first and the second reaction cycle indicates no significant change in the catalytic performance of the polyanions after one reaction cycle ( Fig. S35 -S38, Table S14).
Following the confirmed end of the first reaction cycle as indicated by 31 P-NMR measurements (> 98% DMNP conversion, Fig. S34 A) Fig. S35 -S38) were used to determine the amount of formed product. The TONs obtained were normalized by the number of three active metal centers in the case of {M3(HPO3)Sb3W28}. A comparison of the TOF values determined for the first and the second reaction cycle is given in Table S14.       To further explore the role of the basic terminal O-sites in the {SbW9} for the observed catalytic activity, a control experiment employing Na9[B-α-AsW9O33]·27H2O {AsW9} 2 , which apart from an As III primary hetero ion instead of Sb III , is isostructural to {SbW9} and features the same charge, was employed under elsewise identical catalytic conditions (Tris-HCl [125 mM], pD 7 at 25°C). Incubation of the reaction mixture for 24 h resulted in ~69.9% DMNP conversion corresponding to a calculated TOF value of 0.048 h -1 (Figure S41). This value is almost identical to the TOF calculated for {SbW9} of 0.047 h -1 ( Table S14) ultimately suggesting a general catalytic activity for trilacunary lone-pair containing POTs featuring accessible terminal O-sites in the presence of Tris.