Where Are the Protons in α-[H_xW₁₂O₄₀]^(8-x)- (x = 2−4)?

Abstract

The equilibria and speciation of the proton cryptate polyoxometalate alpha-(H2)W(12)O(40)]6- (1) were examined by NMR following the phase-transfer cation metathesis of aqueous Na(6)1 with Q+Br-/CH(2)Cl(2), leading to the isolation of the (n-Bu)4N+ (Q+) salts Q(6)1 and alpha-Q5[(H3)W(12)O(40)](Q(5)2). Several groups report salts of the protonated anions H(x)1 (x = 1 and 2) with no consensus on proton numbers or locations. Reported herein, a combination of 1H and 183W NMR evidence, elemental analysis, acid titration measurements, and H/D isotopomer assignments establishes that in nonaqueous media the internal cryptand cavity of 1 reversibly accommodates only one more proton to form 2. Because an external proton must transfer across the close-packed tungsten oxide surface of 1, which should constitute a substantial activation barrier, it is significant that the transformation is instantaneous by 1H NMR (1 equiv of HBr in CH(3)CN), whereas the reverse process is slow (t1/2 approximately 17.4 h; 1 equiv of Q+OH-).

Thermal stability of mesoscopic compounds of cetyltrimethylammonium and Keggin metatungstates

“…The two protons of the [H2W12O40] 6anion are located within the central cavity of the Keggin-like structure and are relatively inaccessible. 8,9 Due to their rich suite of redox, 10 structural 11 and photo-electronic 12 characteristics, POM-based materials have received attention 13 for applications such as opto-electronic devices, 14 oxidation catalysts, 15 medicine, 16 chemical sensors 17 and mesoporous cationic conductors. 18 POMs can react with organic cations to form a wide range of hybrid organic-inorganic entities.…”

Polyoxometalates in solution: speciation under spotlight

“…were reported, with all three or four protons assigned to the internal cryptand tetrahedral cavity. [227][228][229] Although [H 3 W VI 12 O 40 ] 5À is not formed in aqueous W VI (0.05-0.50 M) solutions, the correct choice of countercation (e.g. tetrabuthylammonium) can lead to its isolation from aqueous W VI (0.05 M) solutions at pH 4.…”

Recent advances in isopolyoxotungstates and their derivatives

“…In detail, the stepwise assembly mainly includes three synthetic routes: (i) reaction of preformed iso-POT precursors with other reagents (namely, organic ligands, TM cations, RE cations and counter-cations) (Chi et al, 2014;Han et al, 2014;Liu et al, 2006;Li et al, 2002;Pang et al, 2008); (ii) introduction of one type of prefabricated mononuclear metal complex or polynuclear metal cluster into the simple tungstate system (Bi et al, 2004;Fang & Luban, 2011;Meng et al, 2013); (iii) combination of the preformed iso-POT precursors and the preformed mononuclear metal complexes (Han et al, 2001). Here, some typical stable and metastable iso-POT precursors can be listed as follows: ( (Sprangers et al, 2006). Notably, these above-mentioned iso-POT precursors can selfdecompose and reassemble into their derivative fragments, which provides numerous opportunities for synthesizing multifunctional iso-POT-based materials.…”