A Bis-organosilyl-Functionalized Wells-Dawson Polyoxometalate as a Platform for Facile Amine Postfunctionalization.

The development of modular platforms that can undergo postfunctionalization reactions permits coupling of inorganic clusters with different organic functionalities, thereby expanding the range of key physicochemical properties that are relevant for applications in different areas of science. In this work, a novel hybrid Wells-Dawson polyoxometalate (POM) platform was developed and successfully used for postfunctionalization via a nucleophilic substitution reaction. Two new halogen-functionalized bis-organosilyl Wells-Dawson POMs TBA6[α2-P2W17O61{O(SiC3H6-X)2}] (X = Cl or I) were synthesized, and their coupling with amine substrates was explored in a one-step postfunctionalization reaction. The iodide form of the POM has proven to be much more reactive, and its reaction with a range of primary and secondary amines resulted in a series of new bis-substituted Wells-Dawson POMs with the general formula TBA6[α2-P2W17O61{O(SiC3H6-NR1R2)2}]. Coupling of 18 amines with R1 and R2 groups, which exhibited a wide variety in terms of both chemical nature and bulkiness, was achieved under mild conditions via a catalyst-free approach. Using Na2CO3 as a base in acetonitrile solutions at 55 °C resulted in hybrid products that were obtained in high purity and good yields, after a simple isolation and purification procedure.

A Simple Nucleophilic Substitution as a Versatile Postfunctionalization Method for the Coupling of Nucleophiles to an Anderson-Type Polyoxometalate.

A new postfunctionalization method was developed for the Anderson-type POM based on a nucleophilic substitution reaction occurring at an electrophilic sp3 hybridized carbon localized on the hybrid POM. Using this method, several types of different nucleophiles including primary and secondary amines, carboxylates, and thiolates were efficiently coupled to a chloride-functionalized Anderson-type POM in high yields and purity. The heterogeneous acetonitrile-Na2CO3 conditions were found to be superior over other bases and solvents for the coupling of amines and thiolates to the chloride-functionalized POM. Moreover, the addition of 1 equiv of tetrabutylammonium iodide as a catalyst drastically decreased the reaction times to 24 h for the complete coupling of amines and only a couple of hours for thiolates. In the case of carboxylic acids as substrates, using tetrabutylammonium hydroxide as the base for the reaction proved to be beneficial. This is because the resulting tetrabutylammonium carboxylates were found to be much more reactive than the corresponding sodium carboxylates and allowed homogeneous reaction conditions. Using sodium carbonate, only 25% of N-acetyl glycylglycine could be coupled after 24 h at 80 °C, while full conversion was achieved after the same reaction time when using tetrabutylammonium hydroxide as a base.

Gallium(III)-Containing, Sandwich-Type Heteropolytungstates: Synthesis, Solution Characterization, and Hydrolytic Studies toward Phosphoester and Phosphoanhydride Bond Cleavage.

The gallium(III)-containing heteropolytungstates [Ga4(H2O)10(ß-XW9O33)2](6-) (X = As(III), 1; Sb(III), 2) were synthesized in aqueous acidic medium by reaction of Ga(3+) ions with the trilacunary, lone-pair-containing [XW9O33](9-). Polyanions 1 and 2 are isostructural and crystallized as the hydrated sodium salts Na6[Ga4(H2O)10(ß-AsW9O33)2]·28H2O (Na-1) and Na6[Ga4(H2O)10(ß-SbW9O33)2]·30H2O (Na-2) in the monoclinic space group P21/c, with unit cell parameters a = 16.0218(12) Å, b = 15.2044(10) Å, c = 20.0821(12) Å, and ß = 95.82(0)°, as well as a = 16.0912(5) Å, b = 15.2178(5) Å, c = 20.1047(5) Å, and ß = 96.2(0)°, respectively. The corresponding tellurium(IV) derivative [Ga4(H2O)10(ß-TeW9O33)2](4-) (3) was also prepared, by direct reaction of sodium tungstate, tellurium(IV) oxide, and gallium nitrate. Polyanion 3 crystallized as the mixed rubidium/sodium salt Rb2Na2[Ga4(H2O)10(ß-TeW9O33)2]·28H2O (RbNa-3) in the triclinic space group P1̅ with unit cell parameters a = 12.5629(15) Å, b = 13.2208(18) Å, c = 15.474(2) Å, α = 80.52(1)°, ß = 84.37(1)°, and γ = 65.83(1)°. All polyanions 1-3 were characterized in the solid state by single-crystal XRD, FT-IR, TGA, and elemental analysis, and polyanion 2 was also characterized in solution by (183)W NMR and UV-vis spectroscopy. Polyanion 2 was used as a homogeneous catalyst toward adenosine triphosphate (ATP) and the DNA model substrate 4-nitrophenylphosphate, monitored by (1)H and (31)P NMR spectroscopy. The encapsulated gallium(III) centers in 2 promote the Lewis acidic synergistic activation of the hydrolysis of ATP and DNA model substrates at a higher rate in near-physiological conditions. A strong interaction of 2 with the P-O bond of ATP was evidenced by changes in chemical shift values and line broadening of the (31)P nucleus in ATP upon addition of the polyanion.

Ln12 -Containing 60-Tungstogermanates: Synthesis, Structure, Luminescence, and Magnetic Studies.

A new class of hexameric Ln12 -containing 60-tungstogermanates, [Na(H2 O)6 ⊂Eu12 (OH)12 (H2 O)18 Ge2 (GeW10 O38 )6 ](39-) (Eu12 ), [Na(H2 O)6 ⊂Gd12 (OH)6 (H2 O)24 Ge(GeW10 O38 )6 ](37-) (Gd12 ), and [(H2 O)6 ⊂Dy12 (H2 O)24 (GeW10 O38 )6 ](36-) (Dy12 ), comprising six di-Ln-embedded {ß(4,11)-GeW10 } subunits was prepared by reaction of [α-GeW9 O34 ](10-) with Ln(III) ions in weakly acidic (pH 5) aqueous medium. Depending on the size of the Ln(III) ion, the assemblies feature selective capture of two (for Eu12 ), one (for Gd12 ), or zero (for Dy12 ) extra Ge(IV) ions. The selective encapsulation of a cationic sodium hexaaqua complex [Na(H2 O)6 ](+) was observed for Eu12 and Gd12 , whereas Dy12 incorporates a neutral, distorted-octahedral (H2 O)6 cluster. The three compounds were characterized by single-crystal XRD, ESI-MS, photoluminescence, and magnetic studies. Dy12 was shown to be a single-molecule magnet.

A versatile and highly efficient post-functionalization method for grafting organic molecules onto Anderson-type polyoxometalates.

A new azide functionalized Anderson polyoxometalate was synthesized, fully characterized and subsequently used as a building block for further POM post-functionalization with organic compounds through a copper catalyzed azide-alkyne cycloaddition (CuAAC) reaction. Optimization of the reaction conditions led to an efficient, fast, convenient and versatile POM coupling method.

Amino acid side chain induced selectivity in the hydrolysis of peptides catalyzed by a Zr(IV)-substituted Wells-Dawson type polyoxometalate.

In this paper the reactivity of K15H[Zr(α2-P2W17O61)2]·25H2O (1), a Zr(IV)-substituted Wells-Dawson polyoxometalate, is examined towards a series of Gly-Aa, Aa-Gly or Aa-Ser dipeptides, in which the nature and the size of the Aa amino acid side chain were varied. The rate of peptide bond hydrolysis, determined by (1)H NMR experiments, in Gly-Aa dipeptides is strongly dependent on the molecular volume and the chemical structure of the Aa side chain. When the volume of the aliphatic side chain of the Aa residue in Gly-Aa increased, a clear decrease in the hydrolysis rate was observed. Replacing one α-H in the C-terminal Gly residue of Gly-Gly by a methyl group (Gly-Ala) resulted in a 6-fold reactivity decrease, pointing towards the importance of steric factors for efficient peptide bond hydrolysis. The rate constants for peptide bond hydrolysis in Gly-Aa dipeptides at pD 5.0 and 60 °C ranged from 208.0 ± 15.6 × 10(-6) min(-1) for Gly-Ser to 5.0 ± 1.0 × 10(-6) min(-1) for Gly-Glu, reflecting the influence of the different nature of the amino acid side chains on the hydrolysis rate. Faster hydrolysis was observed for peptides containing Ser and Thr since the hydroxyl group in their side chain is able to facilitate amide bond hydrolysis by promoting an N→O acyl rearrangement. Peptides containing positively charged side chains at pD 5.0 show enhanced hydrolysis rates as a result of the secondary electrostatic interactions with the negatively charged surface of the polyoxometalate, which stabilize the peptide-polyoxometalate complex. A slow hydrolysis rate was observed for Gly-Glu, because of the preferential coordination of the carboxylate group in the side chain of Glu to Zr(IV), which prevents coordination of the peptide carbonyl group and its activation towards hydrolysis.

Hydrolysis of DNA model substrates catalyzed by metal-substituted Wells-Dawson polyoxometalates.

In this study we report the first example of phosphoester bond hydrolysis in 4-nitrophenyl phosphate (NPP) and bis-4-nitrophenyl phosphate (BNPP), two commonly used DNA model substrates, promoted by metal-substituted polyoxometalates (POMs). Different transition metal and lanthanide ions were incorporated into the Wells-Dawson polyoxometalate framework and subsequently screened for their hydrolytic activity towards the cleavage of the phosphoester bonds in NPP and BNPP. From these complexes, the Zr(iv)-substituted POM showed the highest reactivity. At pD 7.2 and 50 °C a NPP hydrolysis rate constant of 7.71 × 10(-4) min(-1) (t(1/2) = 15 h) was calculated, representing a rate enhancement of nearly two orders of magnitude in comparison with the spontaneous hydrolysis of NPP. The catalytic (k(c) = 1.73 × 10(-3) min(-1)) and formation constant (K(f) = 520.02 M(-1)) for the NPP-Zr(iv)-POM complex were determined from kinetic experiments. The reaction proceeded faster in acidic conditions and (31)P NMR experiments showed that faster hydrolysis is proportional to the presence of the 1 : 1 monosubstituted Zr(iv)-POM at acidic pD values. The strong interaction of the 1 : 1 monosubstituted Zr(iv)-POM with the P-O bond of NPP was evidenced by the large chemical shift and the line broadening of the (31)P nucleus in NPP observed upon addition of the metal complex. Significantly, a ten-fold excess of NPP was fully hydrolyzed in the presence of the Zr(iv)-POM, proving the principles of catalysis. The NMR spectra did not show sign of any paramagnetic species, excluding an oxidative cleavage mechanism and suggesting purely hydrolytic cleavage.