Current Inorganic Chemistry (Discontinued) - Volume 7, Issue 1, 2017

Background: Polyoxometalates (POMs) represent a large class of polynuclear oxo-bridged early transition metal compounds with extraordinary rich variations of building modes and versatile chemical and physical properties. However, the antimonate-based Anderson-Evans POMs have received scarce attention. Herein we report easy transformation of [SbW9O33]9- into [SbW6O24]7- by oxidation of Sb3+ to Sb5+ with bromine water. Objectives: The main objective of this study was to find an easy way to [SbW6O24]7- Anderson-Evans type anion. Methods: Na9[SbW9O33]·19.5H2O was dissolved in distilled water in a beaker, then bromine was added to the solution for oxidation. After one hour heating final solution was allowed to evaporate in the hood. Two days later large colorless crystals of Na7[SbW6O24]·16H2O together with a small amount of Na7[SbW6O24]·24H2O were collected and characterized by XRD, XRPD and IR. Results: The oxidation of SbIII in α-B-[SbIIIW9O33]9- by bromine water leads to transformation of this trilacunary Tourné anion into [SbVW6O24]7- Anderson-Evans type anion with elimination of three tungstate units. Slow evaporation of the solution gives large colorless crystals of Na7[SbW6O24]·16H2O (1a). XRPD analysis also revealed the presence of a small amount (ca. 5 %) of additional phase Na7[SbW6O24]·24H2O (1b). In both structures [SbW6O24]7- anion is not protonated and the Sb-O bond distances are nearly equal, rendering the coordination sphere around SbV nearly perfectly octahedral. In 1a d(Sb1-O1) = 1.989(4) A, d(Sb1-O2) = 1.983(4) A, and in 1b d(Sb1-O1) = 1.978(4) A, d(Sb1-O9) = 1.984(4) A, and d(Sb1-O10) = 1.976(4) A. Our preparation of [SbVW6O24]7-, taking into account that starting [SbW9O33]9- can be generated in one-pot reaction from sodium tungstate and a Sb(III) source, offer advantages of using more readily available Sb(III) without the necessity of making [Sb(OH)6]-, in a simple procedure and acceptable yield. Conclusion: Oxidation of [SbW9O33]9-·with bromine water gives Anderson-Evans type polyoxometalate [SbW6O24]7- which was isolated and characterized as a mixture of Na7[SbW6O24]·16H2O (main phase) and Na7[SbW6O24]·24H2O (minor product). The main structural difference between these is in the arrangement of sodium cations. In the main phase each Anderson-Evans anion surrounded with four type cations forming 3d network, but in the minor phase POM anions can be defined as guest molecules in a sodium-water 3d framework.

Background: Polyoxometalate (POM) clusters are molecular metal-oxide that possess advantages in integrating functionalities at the molecular level. Redox activity is key characteristic of the POMs where the electron-withdrawing ability is changed according to environments that surroundings POMs. Achieving switching phenomena of electronic state of POMs are of great interest for various applications. Objective: It is of fundamental interest to develop switching of electronic state of POMs by outer stimulus. For this purpose, we investigate the solvent effect to electron-withdrawing ability of POM so as to coupling with macroscopic electric property of the POM based salts. Method: We have developed these characteristics of POMs to design CT system by hybridizing with pyridyl-tetrathiafulvalene (TTFPy) molecule. TTFPy was reacted with Ni(II) and [α-SiW12O40]4-, yielding a single crystal with composition of [NiII(DMSO)5(TTFPy)]2[α-SiW12O40] (1). And we have revealed that a single crystal of the compound turns crystal color from orange (1) to black (1') by immersing crystals in methanol. In this paper, we newly analyses a structure of the single crystal 1 by X-ray diffraction analysis in addition to investigation of this chemo-chromic characteristics using varieties of organic solvent. Results: It was revealed that solvent acceptor number was important to consider the chromic in which transition from 1 to 1' was induced with organic solvents whose acceptor number was larger than 18. In addition, electrical properties of both crystals and revealed that conductivity was increased in 1'. Conclusion: Our results show that solvent-induced chemo-chromic characteristics were triggered by immersing crystals in a solvent. Other chemo-chromic characteristics will be explored by surrounding crystal in more ion rich environment tuned by Li+ ions toward electrode materials for Li ion batteries, for example. In addition, our findings reported here emphasize interest and importance of POM-based hybrid in which grand state is not in CT state.

Background: Polyoxometalates, especially those derived from the Kegin structure are able to exchange a large number of electrons through quasi-reversible redox processes. However, little is known about the redox behaviour of reduced bicapped Keggin ions. Thereby, this work represents the first electrochemical investigation of this Keggin-type ion. Objectives: The main objective of this study was to analyze the influence of the two capping {Sb3+} groups upon the electrochemical response of the reduced Keggin unit {SiMo12O40}. Methods: The synthesis of the bicapped [Sb2SiMo12O40]4- has been achieved via hydrothermal method using elemental antimony, ammonium molybdate and silicate in acidic aqueous solution. The crystallographic structure of the resulting single-crystals has been determined by X-Ray diffraction analysis. Furthermore, the complete formula was established by elemental analysis and thermal analysis (TGA). Electrochemical studies were carried out in aqueous solution, using glassy carbon as working electrodes and varying either pH or scan rate. Results: The [Sb2SiMo12O40]4- anion has been synthesized as its ammonium salt, which was obtained as pure crystals in high yield under controllable conditions. The structure of the six-electron reduced diantimony capped Keggin compound (NH4)4[Sb2SiMo12O40].11H2O has been determined by singlecrystal X-ray diffraction and shows two capping groups grafted onto the reduced {SiMo12O40} central core. A 2D solid network results from interactions between the outer Sb caps and oxygen atoms of POM neighbors. The delocalization of six electrons over the 12 Mo centers modifies both the IR and UVvisible spectra. The bicapped polyoxometalate (POM) exhibits two quasi-reversible multi-electron transfers in the range 0.0 to 0.3 V/SCE in aqueous solution. Conclusion: Our results indicated unambiguously the striking redox properties of the reduced capped Keggin anion which is able to deliver, in this case, six electrons in a narrow potential range with the concomitant partial hydrolysis of the two outer Sb3+ capping groups. This gives new opportunities in the field of electrochemical-based devices for energy storage or energy conversion.

Background: Lacunary (vacant) heteropolyanions are a subclass of polyoxometalates (POMs) and are known to act as inorganic, polydentate ligands with a multitude of oxophilic electrophiles such as 3d metal ions. The family of Weakley-type POM sandwich structures is rather large, as they are known for Keggin- and Dawson-based capping units, for a multitude of 3d metal ions, as well as different tetrahedral heteroatoms. Moreover, several structural modifications of the ‘sandwich core’ in terms of metal nuclearity have been achieved, in particular two ‘inner’ transition metal ions and two sodium ions in the ‘outer’ positions. Objectives: Herein we report on the synthesis and characterization of a di-nickel(II)-containing Weakleytype heteropolytungstate based on the Dawson POM. Methods: Reaction of Ni2+ ions with the trilacunary Dawson ion [P2W15O56]12- in sodium acetate buffer (pH 4.6, Method 1) or slightly basic medium (pH 8, Method 2) resulted in the dinickel(II)-containing sandwichtype 30-tungsto-4-phosphate [Ni2Na2(H2O)2(P2W15O56)2]18-, which crystallized as the hydrated sodium salt Na18[Ni2Na2(H2O)2(P2W15O56)2]·70H2O. We also decided to compare the electrochemical properties of the polyanion with those of [Ni4(H2O)2(P2W15O56)2]16-, which was reported previously. The stability of both polyanions in aqueous solution as a function of pH and time was studied by monitoring the evolution of their UV-visible spectra and their cyclic voltammograms (CVs). Both are sufficiently stable at pH 3 and pH 5 to allow for characterization by cyclic voltammetry (CV). The title compound was also studied by thermal analysis (TGA), FT-IR spectroscopy and elemental analysis. Results: As based on single-crystal X-ray diffraction, the structure of [Ni2Na2(H2O)2(P2W15O56)2]18- corresponds to a Weakley-type sandwich dimer with a pair of nickel(II) ions in the ‘inner’ positions and a pair of sodium ions in the ‘outer’ positions, encapsulated by two [P2W15O56]12- POM units. The two nickel ions in the rhombic {Ni2Na2} core are structurally and crystallographically equivalent, and are octahedrally coordinated by six oxo ligands. The synthesis can be accomplished by two different procedures, either in sodium acetate buffer at pH 4.6 (Method 1) or in slightly basic aqueous medium by adding oxalate (Method 2). The CV of [Ni2Na2(H2O)2(P2W15O56)2]18- has a composite 2e-reduction wave, These redox waves are due to the reduction of the W centers. As far as the CV of [Ni4(H2O)2(P2W15O56)2]16- is concerned, only two distinct reduction waves are observed. As expected, the entire CV shifts towards negative potentials upon increasing the pH. Conclusion: We have synthesized and structurally characterized the dinickel(II)-containing 30-tungsto-4- phosphate [Ni2Na2(H2O)2(P2W15O56)2]18-, which was prepared by a one-pot reaction of NiII ions with the trilacunary Dawson ion [P2W15O56]12- via two different procedures, either in acetate buffer at pH 4.6 or in water at pH 8.0, both leading to the same hydrated sodium salt in crystalline form. The structure of the polyanion corresponds to a Weakley-type sandwich dimer with two nickel(II) ions occupying the ‘inner’ positions and two sodium ions the ‘outer’ positions of the rhombic core of the polyanion structure.

Background: Sandwich-type polyoxometalates (POMs) of the Dawson family result from the reaction of the lacunary species [X2W15O56]12- (where X = As or P) with M, a "d" or "f" transition metal, yielding the compound [M4(H2O)2(X2W15O56)2]y- in which the metallic cluster M4O14(H2O)2 is sandwiched between two lacunary fragments [X2W15O56]12-. We have made significant progress as far as the determination of the X-ray diffraction structures and the electrochemical and electro-catalytic characterisations of these compounds are concerned. The presence of the metallic cluster M4O14(H2O)2 (with M = MnII, FeIII, CoIII, NiII, CuII, ZnII or CdII), imparts interesting properties to these complexes for potential applications in the domains of catalysis, electrocatalysis, magnetism and materials science. Objectives: The main purpose of this study was to propose a reliable synthesis method of sandwichtype, Dawson POMs containing "d" metal centres of different natures (CuII/FeIII) and to describe and understand their redox behaviours and electro-catalytic properties. Methods: Four POM molecules based on mixed copper(II)-iron(III) tetra-nuclear clusters [Fe2Cu2O14(OH2)2)] and the Dawson fragments [X2W15O56]12- (with X = As or P) have been prepared. These compounds have been characterised by several techniques (elemental analysis, UV-Visible and IR spectroscopies, cyclic voltammetry, electrochemical quartz crystal microbalance (EQCM), thermal gravimetric analysis …). They were found to be iso-structural of previous sandwich-type species described in the literature, though single crystal X-ray diffraction turned out to be incapable of making the difference between the [(CuOH2)2Fe2(X2W15O56)2]14- species with the CuII centres in external sites from the [(FeOH2)2Cu2(X2W15O56)2]14- species with the CuII centres in internal sites. Results: The electrochemical behaviour of these four compounds in solution turns out to be almost identical and no obvious differences seem to arise from changes in the relative positions occupied by the CuII and the FeIII centres. Cyclic voltammetry coupled to the quartz crystal microbalance revealed that the reduction of the FeIII centres always precedes that of the CuII centres, irrespective of their relative positions in the POM structure. Also, by comparison with their related compounds, [(NaOH2)2Fe2(X2W15O56)2]16- and [(FeOH2)2Fe2(X2W15O56)2]12-, the presence of copper markedly improves the electro-catalytic efficiency of these species towards the reduction of nitrogen oxides and dioxygen. The efficiency of these two processes decreases upon increasing the pH, but remains observable up to pH 5. Surprisingly, during the electro-catalytic reduction of nitrite, the formation of a deposit of Cu0 on the working electrode surface is not detected by EQCM. This is a crucial observation, since it is the first time that a real synergy between the FeIII and the CuII centres towards the electro-catalytic reduction of NO2 - has been demonstrated without the formation of Cu0. Conclusion: Our results reveal that the combination of CuII and FeIII centres in the same POM remarkably improves its electro-catalytic efficiency towards the reduction of dioxygen and nitrogen oxides such as the nitrite ion and nitric oxide.

Background: Keplerate-type capsules represent unique supramolecular objects displaying a tuneable spatially-restricted environment and are promising in many domains. In particular, some recent reports demonstrate that chemical transformations can occur specifically within the cavity, then acting as a nano-reactor. Some authors evidenced that the porous capsule provides internal active sites for catalytic transformation. In this idea, we recently succeeded to synthetize Keplerate capsules incorporating dicarboxylate anions, offering available coordination site for transition metals inside the Mo132 nanocapsule. Objectives: The aim of this study was to introduce a chiral ligand potentially relevant for organocatalysis inside the porous Keplerate nanocapsule, i.e. L-proline, which is known for promoting various activation modes in asymmetric aminocatalysis. Methods: The work described herein first focuses on the internal functionalization of the [(MoVI 6O21)12(MoV 2O4)30(H2O)72L30]n- nanocapsule with cyclopentanecarboxylate ligand which can be viewed as a model of L-proline in terms of size. FT-IR, X-ray diffraction and solution and solid-state NMR studies were performed in order to unambiguously evidence the formation of new Mo132 derivatives. In a second part, different strategies were attempted in solution to functionalize Mo132 capsules by L-proline. NMR studies in solid state and in solution were employed to monitor this process. Results: In this report, new cyclopentanecarboxylate-containing Keplerate compounds have been synthesized and characterized in the solid state and in solution. The 1D and DOSY 1H NMR spectra supported by X-ray crystallographic analysis are consistent with 22 to 25 ligands coordinated over the 30 {Mo2O4} linkers. Solid state 1H and 13C MAS NMR performed on the new described compound (NH4)50[Mo132O372(H2O)72(CPC)22(SO4)8]. 120H2O provided further characterizations of ligand substitution within the capsule by carboxylate ions. In a second part, the encapsulation of L-proline in Mo132 was investigated by different strategies. In solution, no substitution of sulfate or acetate ligands in the Mo132 precursors was observed in aqueous solution. DOSY 1H NMR studies evidenced that the cationic part of this molecule interacts with the surface of Mo132. A direct synthesis method was then attempted by mixing molybdate, L-proline and hydrazine in water at pH = 4. It led to an insoluble dark brown powder, for which the FT-IR spectrum, the elemental analysis and the solid state NMR are in agreement with a compound of formula (NH4)12[Mo132O372(H2O)132](L-proline)60 in which the L-proline are located outside the capsule and probably allow the formation of a network between L-proline molecules and ligand-free [Mo132O372(H2O)132]12- capsules. Conclusion: A new Keplerate compound has been discovered with cyclopentanecarboxylate ligands. Attempts for encapsulation of L-proline ligands within the cavity of Mo132 revealed unsuccessful but we isolated a powder with formula (NH4)12[Mo132O372(H2O)132](L-proline)60 in which the L-proline are located outside the capsule. It probably interacts with the surface of Mo132 capsule through the cationic part of this molecule, a phenomenon which surely competes with the encapsulation of the latter. Moreover, the low pKa value (1.95) of the carboxylate function of L-proline compared to that of the cyclopentanecarboxylate (4.90) appears as a second key criteria for the encapsulation or the non-encapsulation of ligands within the cavity of Keplerates.

Background: The chemistry of nickel(II) clusters continues to attract the intense interest of many inorganic groups around the world for a variety of reasons, including its relevance to Molecular Magnetism. Objective: The primary objective of this study was to employ the polydentate Schiff base Nsalicylidene- 4-chloro-o-aminophenol in nickel(II) carboxylate chemistry for the synthesis of coordination clusters. Methods: The influence of a variety of synthetic parameters on product identity was investigated. The solid-state structures of the complexes were determined by single-crystal X-ray crystallography. The nickel(II) clusters were characterized by microanalyses and infrared spectra. Variable-temperature magnetic susceptibility studies were performed on two selected nickel(II) clusters. Results: Tetranuclear, hexanuclear and decanuclear nickel(II) clusters were synthesized. In the tetranuclear complexes, the four metal ions are located at four vertices of a planar defective dicubane. The core of the hexanuclear cluster consists of four fused defective cubanes. The metal topology of the decanuclear cluster comprises ten edge-sharing nickel(II) triangles. One of the tetranuclear clusters has a diamagnetic spin ground state. The acetate-containing tetranuclear cluster is characterized by ferromagnetic nickel(II)···nickel(II) exchange interactions and possesses a total spin of 4 in the ground state. Conclusion: The prepared complexes have interesting molecular structures and magnetic properties. A comparison of the nickel(II) cluster chemistry with N-salicylidene-4-chloro-o-aminophenol with that of related tridentate Schiff bases reveals similarities and differences.

Background: The considerable interest in iron(III) oxo/hydroxo brigded polynuclear complexes stems from their relevance to diverse fields, from bioinorganic chemistry to molecular magnetic materials. The trinuclear oxo-centered carboxylato iron complexes have been extensively used as simple models in order to understand the magnetic interactions which have been interpreted in terms of the Heisenberg-Dirac-van Vleck irotropic spin Hamiltonian with antiferromagnetic interactions as well as non-Heisenberg interactions such as antisymmetric exchange. Objectives: The main objective of this work was to study the magnetic interactions in triangular non oxocentered iron(III) complexes, a class of complexes which has been scarcely investigated so far. Methods: Reactions of Schiff base ligands, OH-C10H6-CH=NC(CH2OH)3, H4L1, and OH-C6H4- CH=NC(R)(CH2OH)2 (R = CH3, H3L2; R = C2H5, H3L3) with 'basic' iron(III) benzoate salts afforded two trinuclear complexes, [Fe3(H2L1)3(O2CPh)3]136;™2CH2Cl2 (1136;™2CH2Cl2) and [Fe3(HL2)3(O2CPh)3] 136;™0.45CH2Cl2136;™0.40- H2O (2136;™0.45CH2Cl2136;™0.40H2O), and four octanuclear clusters, [Fe8(L3)8(HO2CPh)2]136;™2.7CH2Cl2136;™7.2H2O (3136;™2.7CH2Cl2136;™7.2H2O), [Fe8(L3)6(HL3)2 (O2CPh)2]136;™3THF (4136;™3THF), [Fe8(L3)6(HL3)2(O2CPh)2]136;™6Me2CO (5136;™6Me2CO) and [Fe8(L3)6(HL3)2 (O2CPh)2]136;™4dmf (6136;™4dmf). The crystal structures of 1-6 were determined by single-crystal X-ray crystallography. Variable-temperature magnetic susceptibility and magnetization measurements as well as Mossbauer spectra of 1-4 were undertaken. The study of the magnetic interactions within the trinuclear 1-2 was completed by X-band EPR spectroscopy. Results: The molecular structure of the trinuclear complexes 1 and 2 possesses a [FeIII 3(μ2-OR)3(μ2-O2CPh)3]3+ core with the three FeIII atoms located at the vertexes of a scalene triangle; this triangular topology missing a central μ3-oxo bridge is rare in the literature. The octanuclear complexes 3-6 consist of seven corner-sharing [Fe2(μ2-OR)2] units, alternatively described as two sets of three corner-sharing [Fe2(μ2-OR)2] units in cisorientation linked through two μ2-OR atoms; this metal topology is novel in FeIII chemistry. The 57Fe Mössbauer spectra of 1-4 are consistent with the presence of high spin ferric ions in octahedral environments. The magnetic studies of the trinuclear 1 and 2 revealed antiferromagnetic interactions with isotropic exchange parameters consistent with literature values. EPR spectroscopy revealed that the ST = 1/2 state is highly anisotropic which is interpreted considering the non-Heisenberg terms zero field splitting and antisymmetric exchange. The magnetic studies of the octanuclear 3 and 4 revealed dominant antiferromagnetic interactions. Conclusion: This work presents the use of Schiff base ligands in iron(III)/carboxylato chemistry which afforded two trinuclear non oxo-centered (1, 2) and four octanuclear (3-6) complexes. A detailed magnetic and spectroscopic (EPR, Mössbauer) study of the extremely rare triangular non oxo-centered iron(III) complexes 1 and 2 revealed the role of non Heisenberg interactions.