Cation-Anion Interactions and Synergistic Catalysis by Supramolecular Polyoxometalate Complexes [C₁₀H₁₈N]_n[XM₁₂O₄₀]

Abstract

A series of novel organic ammonium salts of Keggin [XM12O40]n− (X=P, Si, B; M=W, Mo) was synthesized and fully characterized using ESI‐MS and X‐ray crystallography. The solid‐state structure of the title compounds [C10H18N]n[XM12O40] was unveiled by X‐ray crystallography that hydrogen bonds are formed between the nitrogen center of amantadine and the bridging oxygen atoms of POM in addition to Coulombic attractions and by MS/MS that consecutive proton‐transfer reactions from [C10H18N]+ to [XM12O40]n− were observed in the gas‐phase fragmentations of the dimeric clusters {(C10H18N)m[XM12O40]2}(2n−m)−. A comparative study on the catalytic property of [XM12O40]n− in methyl phenyl sulfide (MPS) oxidation with aqueous hydrogen peroxide in MeCN was performed and the effect of counterions ([C10H18N]+ and TBA+, etc) on the catalytic performance has been investigated. The best catalytic activity of [C10H18N]3[PMo12O40] in MPS oxidation illustrates the explicit role of cations on promoting catalytic activity of polyoxoanions.

Complex solution chemistry behind the simple “one‐pot” synthesis of vanadium‐substituted polyoxometalates unraveled by electrospray ionization mass spectrometry

“…[14][15][16][17][18] The power of ESI-MS relies on the 4S characteristics (speed, sensitivity, specialty, and stoichiometry) summarized by McLafferty, [19] which enables MS suitable for monitoring the dynamic speciation changes that occur in the reaction solution and detecting the low-abundance, transient reaction intermediates. [20] We have used this technique to characterize a range of POMs with saturated and lacunary structures, [21][22][23][24] to study the synergetic catalysis, resulting from the specific cation-anion interaction, [25] and to probe the formation mechanism of sandwich-type POMs using [γ-SiW 10 O 36 ] 8À ({γ-SiW 10 }) as a starting material by combining ESI-MS and X-ray crystallography. [26] Herein, we present the first systematic ESI-MS investigation to show how the reaction conditions such as concentration, temperature, molar ratios, and reaction time can sensitively change the solution speciation and hence the final product via taking the binary reaction of Keggin-type lacunary silicotungstates ([SiW 11 O 39 ] 8À , [SiW 10 O 36 ] 8À , [SiW 9 O 34 ] 10À , abbreviated as {SiW 12Àx }, x = 1, 2, and 3) with vanadium salts (NaVO 3 ) as an example.…”