Heterolytic Dissociation of Water Demonstrated by Crystal-to-Crystal Core Interconversion from (μ-Oxo)divanadium to Bis(μ-hydroxo)divanadium Substituted Polyoxometalates

Crystal-to-crystal polymerisation of monosubstituted [PW11O39Cu(H2O)]5- Keggin-type anions

The reaction between neutral bis(picolinate)copper(II) complexes and copper(II)-monosubstituted Keggin-type phosphotungstate anions formed in situ leads to the formation of the hybrid [C(NH2)3]10[{PW11O39Cu(H2O)}2{Cu(pic)2}]·10H2O compound (1, pic = picolinate) in the presence of structure-directing guanidinium cations. Single-crystal X-ray diffraction studies demonstrate that 1 contains dimeric {PW11O39Cu(H2O)}2{Cu(pic)2} molecular species constituted by two Keggin-type anions linked by one {Cu(pic)2} octahedral complex through axial coordination to their terminal oxygen atoms. The extensive hydrogen-bonding network established by guanidium cations and Keggin clusters plays a key role in retaining the crystallinity of the system throughout dehydration to allow a single-crystal-to-single-crystal (SCSC) transformation into the anhydrous [C(NH2)3]10[{PW11O39Cu}2{Cu(pic)2}] (2a) at 170 °C. Structural modifications involve the re-orientation, shifting in ca. 1.5 Å and condensation of all the {PW11O39Cu} units to result in {PW11O39Cu}n chains in an unprecedented solid-state polymerisation. This phase transition also implies the cleavage of Cu-O bonds induced by the rotation and translation of Keggin-type anions, in such a way that hybrid dimeric units in 1 are dismantled and {Cu(pic)2} complexes become square planar. The irreversibility of the phase transition has been confirmed by combined thermal and diffractometric analyses, which evidence that the anhydrous phase adsorbs only one water molecule per cluster to become the [C(NH2)3]10[{PW11O39Cu}2{Cu(pic)2}]·2H2O (2h) hydrated derivative without any significant alteration in its cell parameters, nor in its crystalline structure. Phase transformations have been monitored by electron paramagnetic resonance spectroscopy.

Consecutive Single-Crystal-to-Single-Crystal Isomerization of Novel Octamolybdate Anions within a Microporous Hybrid Framework with Robust Water Sorption Properties

The 3D hybrid framework [{Cu(cyclam)}3 (κ-Mo8 O27 )] ⋅ 14H2 O (1) (cyclam=1,4,8,11-tetraazacyclotetradecane) undergoes sequential single-crystal-to-single-crystal transformations upon heating to afford two different anhydrous phases (2 a and 3 a). These transitions modify the framework dimensionality and enable the isomerization of κ-octamolybdate (κ-Mo8 ) anions into λ (2 a) and μ (3 a) forms through metal migration. Hydration of 3 a involves condensation of one water molecule to the cluster to afford the γ-Mo8 isomer in 4, which dehydrates back into 3 a through the 6 a intermediate. In contrast, 2 a reversibly hydrates to form 5, exhibiting the same Mo8 cluster as that of 1. It is remarkable that three of the Mo8 clusters (κ, λ and μ) are new and that up to three different microporous phases can be isolated from 1 (2 a, 3 a, and 6 a). Water vapor sorption analyses show high recyclability and the highest uptake values for POM-based systems. The isotherms display an abrupt step at low humidity level desirable for humidity control devices or water harvesting in drylands.

Single-Crystal-to-Single-Crystal Cluster Transformation in a Microporous Molybdoarsenate(V)-Metalorganic Framework

The hybrid compound [Cu(cyclam)(H₂O)₂]_0.5[{Cu(cyclam)}_1.5{B-H₂As₂Mo₆O₂₆(H₂O)}]·9H₂O (1) (cyclam = 1,4,8,11-tetraazacyclotetradecane) was synthesized in aqueous solution by reacting the {Cu(cyclam)}²⁺ complex with a mixture of heptamolybdate and an arsenate(V) source. Crystal packing of 1 exhibits a supramolecular open-framework built of discrete covalent molybdoarsenate/metalorganic units and additional [Cu(cyclam)(H₂O)₂]²⁺ cations, the stacking of which generates squarelike channels parallel to the z axis with an approximate cross section of 10 × 11 Ų where all the hydration water molecules are hosted. Thermal evacuation of solvent molecules yields a new anhydrous crystalline phase, but compound 1 does not preserve its single-crystalline nature upon heating. However, when crystals are dehydrated under vacuum, they undergo a structural transformation that proceeds via a single-crystal-to-single-crystal pathway, leading to the anhydrous phase [{Cu(cyclam)}₂(A-H₂As₂Mo₆O₂₆)] (2). Total dehydration results in important modifications within the inorganic cluster skeleton which reveals an unprecedented solid-state B to A isomerization of the polyoxoanion. This transition also involves changes in the Cu^II bonding scheme that lead to covalent cluster/metalorganic layers by retaining the open-framework nature of 1. Compound 2 adsorbs ambient moisture upon air exposure, but it does not revert back to 1, and the hydrated phase [{Cu(cyclam)}₂(A-H₂As₂Mo₆O₂₆)]·6H₂O (2h) is obtained instead. Structural variations between 1 and 2 are reflected in electron paramagnetic resonance spectroscopy measurements, and the permanent microporosity of 2 provides interesting functionalities to the system such as the selective adsorption of gaseous CO₂ over N₂.

A supramolecular open framework built of copper(II)-complexes of a macrocyclic tetraaza ligand and arsenomolybdate anions undergoes a single-crystal-to-single-crystal transformation upon dehydration that implies an unprecedented solid state cluster isomerization. The permanent porosity of the system allows the selective adsorption of CO₂ over N₂.

Thermostructural Behavior in a Series of Lanthanide-Containing Polyoxotungstate Hybrids with Copper(II) Complexes of the Tetraazamacrocycle Cyclam: A Single-Crystal-to-Single-Crystal Transformation Study

A series of 14 isostructural [Cu(cyclam)]₂[{Cu(cyclam)}₄{(α-GeW₁₁O₃₉)Ln(H₂O)(OAc)}₂]·18H₂O (1-Ln, where Ln = La-Lu; cyclam = 1,4,8,11-tetraazacyclotetradecane) polyoxometalate-based hybrids reported herein represent (i) the first example of a two-dimensional covalent hybrid lattice involving the [{(α-XW₁₁O₃₉)Ln(H₂O)(OAc)}₂] ^n- archetype and (ii) the first structural characterization of such a dimeric polyoxotungste for Ln = La and Pr as well as for the combination of X = Ge and Ln = Ce, Nd, Sm, or Lu. Compounds 1-Ln have been characterized by elemental analyses, infrared spectroscopy, and thermogravimetric analysis, and their thermostructural behavior has been monitored by powder and single-crystal X-ray diffraction. The title compounds undergo two single-crystal-to-single-crystal transformations triggered by thermal dehydration leading to the [{Cu(cyclam)}₆{(α-GeW₁₁O₃₉)Ln(H₂O)(OAc)}₂]·4H₂O intermediate (2-Ln, where Ln = Eu or Er) and [Cu(cyclam)]_0.5[{Cu(cyclam)}_5.5{(α-GeW₁₁O₃₉)Ln(OAc)}₂] (3-Ln, where Ln = Ce or Eu) final anhydrous phases, the latter evidencing a coordinatively unsaturated derivative of the dimeric archetype for the first time. These transitions involve formation and disruption of Cu-O_POM bonds that result in different {Cu(cyclam)}²⁺ moieties grafting onto and being released from Keggin surfaces, which reduces the dimensionality of 1-Ln to one-dimensional covalent assemblies for 2-Ln and 3-Ln. While all 3-Ln phases rehydrate fully upon exposure to air for 24 h, the kinetics governing the crystal transitions back toward 1-Ln through 2-Ln depend on the nature of Ln. Under ambient moisture, the anhydrous structures fully revert back to the parent framework for Ln = La-Sm, while the samples containing Eu to Lu afford mixtures of 1-Ln and 2-Ln and require immersion in water for the structural reversion to reach completion. Single-crystal X-ray diffraction analyses of the rehydrated 1R-Ln samples (Ln = Ce, Eu, and Er) support these observations.

Single-crystal-to-single-crystal transformations triggered by dehydration in polyoxometalate-based compounds

Single-crystal-to-single-crystal transformations are solid-state phase transitions between different crystalline states in which the crystal integrity and the long-range structural order are retained through the whole transformation process. Such a phenomenon constitutes the structural response that some compounds afford when being exposed to a given external stimulus (temperature, pressure, light, etc.) and, therefore, its study has become a relevant focus of interest within crystal engineering because it allows for monitoring how certain properties (colour, magnetism, luminescence, porosity) of the stimuli-responsive material are modified as the structure evolves into the activated form. A range of organic, inorganic and hybrid systems have been found to undergo such phase transitions, but these examples only include a small number of compounds that incorporate polyoxometalate anions, among which the removal of guest solvent molecules (dehydration) stands out as the most common external stimulus able to induce the occurrence of a single-crystal-to-single-crystal transformation. This feature article compiles the examples of dehydration-triggered single-crystal-to-single-crystal transformation studies that have been reported to date for polyoxometalate-based compounds and reviews some of their most relevant structural aspects.

Hydrogen-bonding cyclodiphosphazanes: superior effects of 3,5-(CF3)2-substitution in anion-recognition and counter-ion catalysis

Syntheses, X-ray characterization and employment of new cyclodiphosph(v)azane hydrogen-bonding catalysts in experimental and computational anion recognition and the N-acyl-Mannich reaction.

The chemistry of high valent tungsten chlorides with N-substituted ureas, including urea self-protonation reactions triggered by WCl6

N-Substituted ureas are oxidized by WCl₆in dichloromethane, resulting in protonated urea saltsviaactivation of C–H and/or N–H bonds.

In situ recrystallization of lanthanide coordination polymers: from 1D ladder chains to 1D linear chains

A rare SCSC transformation for lanthanide coordination polymers is triggered by solvent-mediated isomerization.

Coordination polymer flexibility leads to polymorphism and enables a crystalline solid-vapour reaction: a multi-technique mechanistic study

Despite an absence of conventional porosity, the 1D coordination polymer [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)3 ] (1; TMP=tetramethylpyrazine) can absorb small alcohols from the vapour phase, which insert into AgO bonds to yield coordination polymers [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)3 (ROH)2 ] (1-ROH; R=Me, Et, iPr). The reactions are reversible single-crystal-to-single-crystal transformations. Vapour-solid equilibria have been examined by gas-phase IR spectroscopy (K=5.68(9)×10(-5) (MeOH), 9.5(3)×10(-6) (EtOH), 6.14(5)×10(-5) (iPrOH) at 295 K, 1 bar). Thermal analyses (TGA, DSC) have enabled quantitative comparison of two-step reactions 1-ROH→1→2, in which 2 is the 2D coordination polymer [Ag4 (O2 C(CF2 )2 CF3 )4 (TMP)2 ] formed by loss of TMP ligands exclusively from singly-bridging sites. Four polymorphic forms of 1 (1-A(LT) , 1-A(HT) , 1-B(LT) and 1-B(HT) ; HT=high temperature, LT=low temperature) have been identified crystallographically. In situ powder X-ray diffraction (PXRD) studies of the 1-ROH→1→2 transformations indicate the role of the HT polymorphs in these reactions. The structural relationship between polymorphs, involving changes in conformation of perfluoroalkyl chains and a change in orientation of entire polymers (A versus B forms), suggests a mechanism for the observed reactions and a pathway for guest transport within the fluorous layers. Consistent with this pathway, optical microscopy and AFM studies on single crystals of 1-MeOH/1-A(HT) show that cracks parallel to the layers of interdigitated perfluoroalkyl chains develop during the MeOH release/uptake process.

Organometallic synthesis, reactivity and catalysis in the solid state using well-defined single-site species

Acting as a bridge between the heterogeneous and homogeneous realms, the use of discrete, well-defined, solid-state organometallic complexes for synthesis and catalysis is a remarkably undeveloped field. Here, we present a review of this topic, focusing on describing the key transformations that can be observed at a transition-metal centre, as well as the use of well-defined organometallic complexes in the solid state as catalysts. There is a particular focus upon gas-solid reactivity/catalysis and single-crystal-to-single-crystal transformations.

The solution chemistry and reactivity of lacunary Keggin silicotungstates monitored in real-time by a combination of mass spectrometry and electrochemistry

The relationship between the lacunary Keggin silicotungstate precursors and their vanadium-substituted products.

Sequential single-crystal-to-single-crystal transformations promoted by gradual thermal dehydration in a porous metavanadate hybrid

The porous hybrid metavanadate [{Cu(cyclam)}(VO₃)₂]·5H₂O undergoes a series of sequential and reversible transformations upon thermally-triggered gradual dehydration that have been monitored by single-crystal X-ray diffraction.

Synthesis of a poly-pendant 1-D chain based on ‘trans-vanadium’ bicapped, Keggin-type vanadtungstate and its photocatalytic properties

A vanadtungstate cluster-based organic–inorganic hybrid material [NiL₄V^IVW^VI₁₀W^V₂O₄₀(V^IVO)₂] was synthesized, which not only serves as an active photocatalyst for the degradation of dye molecules, but also exhibits selective photocatalytic degradation of cationic dyes in aqueous solution.

Structure, properties and reactivity of polyoxometalates: a theoretical perspective

In the thematic review dedicated to polyoxometalate (POM) chemistry published in Chemical Reviews in 1998, no contribution was devoted to theory. This is not surprising because computational modelling of molecular metal-oxide clusters was in its infancy at that time. Nowadays, the situation has completely changed and modern computational methods have been successfully applied to study the structure, electronic properties, spectroscopy and reactivity of POM clusters. Indeed, the progress achieved during the past decade has been spectacular and herein we critically review the most important papers to provide the reader with an almost complete perspective of the field.

Reactive N-protonated isocyanate species stabilized by bis(μ-hydroxo)divanadium(IV)-substituted polyoxometalate

O- or N-protonated? The bis(μ-hydroxo)divanadium(IV)-substituted γ-Keggin-type polyoxometalate (see picture, left) (TBA)(4)[γ-SiV(IV)(2)W(10)O(36)(μ-OH)(4)] (TBA = tetra(n-butyl)ammonium) was synthesized and characterized by X-ray crystallography. Its reaction with phenyl isocyanate gave (TBA)(4)[γ-SiV(IV)(2)W(10)O(38)(μ-OH)(2)(PhNHCO)(2)], which contains two N-protonated phenyl isocyanate species and catalyzes the cyclotrimerization of phenyl isocyanate.

From 1D to 3D single-crystal-to-single-crystal structural transformations based on linear polyanion [Mn4(H2O)18WZnMn2(H2O)2(ZnW9O34)2]4-

A 1D anionic polyoxometalate, [Mn(4)(H(2)O)(18)WZnMn(2)(H(2)O)(2)(ZnW(9)O(34))(2)](4-), undergoes 1D to 3D single-crystal-to-single-crystal structural transformations that are induced by transition-metal cations (Co(2+) and Cu(2+)) and solvent molecules. These solid materials present interesting catalytic activity for the oxidative aromatization of Hantzsch 1,4-dihydropyridines that is dependent on the inserted heterogeneous metal cations.

Divacant polyoxotungstates: reactivity of the gamma-decatungstates [γ-XW10O36]8-(X = Si, Ge)

The dilacunary, Keggin-based gamma-decatungstate ions [γ-XW(10)O(36)](8-) (X = Si, Ge) {XW(10)} exhibit an exciting and versatile solution chemistry, which is probably unmatched by any other lacunary polytungstate. The reactivity of {XW(10)} in the presence, and even absence, of electrophiles, includes loss/gain of tungsten, isomerization, and dimerization. Ever since the syntheses and structures of {XW(10)} were reported, many research groups around the world have investigated the reactivity of these polyanions towards nucleophiles (mostly d-block metal ions) and different products with various shape, size and composition were obtained. Here we provide an overview of the state-of-the-art in this subarea of polyoxometalate chemistry, with a focus on synthetic and structural aspects.

Toward functional carboxylate-bridged diiron protein mimics: achieving structural stability and conformational flexibility using a macrocylic ligand framework

A dinucleating macrocycle, H(2)PIM, containing phenoxylimine metal-binding units has been prepared. Reaction of H(2)PIM with [Fe(2)(Mes)(4)] (Mes = 2,4,6-trimethylphenyl) and sterically hindered carboxylic acids, Ph(3)CCO(2)H or Ar(Tol)CO(2)H (2,6-bis(p-tolyl)benzoic acid), afforded complexes [Fe(2)(PIM)(Ph(3)CCO(2))(2)] (1) and [Fe(2)(PIM)(Ar(Tol)CO(2))(2)] (2), respectively. X-ray diffraction studies revealed that these diiron(II) complexes closely mimic the active site structures of the hydroxylase components of bacterial multicomponent monooxygenases (BMMs), particularly the syn disposition of the nitrogen donor atoms and the bridging μ-η(1)η(2) and μ-η(1)η(1) modes of the carboxylate ligands at the diiron(II) centers. Cyclic voltammograms of 1 and 2 displayed quasi-reversible redox couples at +16 and +108 mV vs ferrocene/ferrocenium, respectively. Treatment of 2 with silver perchlorate afforded a silver(I)/iron(III) heterodimetallic complex, [Fe(2)(μ-OH)(2)(ClO(4))(2)(PIM)(Ar(Tol)CO(2))Ag] (3), which was structurally and spectroscopically characterized. Complexes 1 and 2 both react rapidly with dioxygen. Oxygenation of 1 afforded a (μ-hydroxo)diiron(III) complex [Fe(2)(μ-OH)(PIM)(Ph(3)CCO(2))(3)] (4), a hexa(μ-hydroxo)tetrairon(III) complex [Fe(4)(μ-OH)(6)(PIM)(2)(Ph(3)CCO(2))(2)] (5), and an unidentified iron(III) species. Oxygenation of 2 exclusively formed di(carboxylato)diiron(III) compounds, a testimony to the role of the macrocylic ligand in preserving the dinuclear iron center under oxidizing conditions. X-ray crystallographic and (57)Fe Mössbauer spectroscopic investigations indicated that 2 reacts with dioxygen to give a mixture of (μ-oxo)diiron(III) [Fe(2)(μ-O)(PIM)(Ar(Tol)CO(2))(2)] (6) and di(μ-hydroxo)diiron(III) [Fe(2)(μ-OH)(2)(PIM)(Ar(Tol)CO(2))(2)] (7) units in the same crystal lattice. Compounds 6 and 7 spontaneously convert to a tetrairon(III) complex, [Fe(4)(μ-OH)(6)(PIM)(2)(Ar(Tol)CO(2))(2)] (8), when treated with excess H(2)O.