Triol Ligands Research Articles

Impact of Surface Ligand Identity and Density on the Thermodynamics of H Atom Uptake at Polyoxovanadate-Alkoxide Surfaces.

An understanding of how molecular structure influences the thermodynamics of H atom transfer is critical to designing efficient catalysts for reductive chemistries. Herein, we report experimental and theoretical investigations summarizing structure-function relationships of polyoxovanadate-alkoxides that influence bond dissociation free energies of hydroxide ligands located at the surface of the cluster. We evaluate the thermochemical descriptors of O-H bond strength for a series of clusters, namely [V6O13-x(OH)x(TRIOLR)2]-2 (x = 2, 4, 6; R = NO2, Me) and [V6O11-x(OMe)2(OH)x(TRIOLNO2)2]-2, via computational analysis and open circuit potential measurements. Our findings reveal that modifications to the TRIOL ligand (e.g., changing from the previously reported electron withdrawing nitro-backed ligand to the electron-donating methyl variant) have limited influence on the strength of surface O-H bonds as a result of near complete thermodynamic compensation in these systems (i.e., correlated changes in redox potential and cluster basicity). In contrast, changes in surface density of alkoxide ligands via direct alkoxylation of the polyoxovanadate-alkoxide surface result in measurable increases in bond dissociation free energies of surface O-H bonds for the mixed-valent derivatives. Our findings indicate that the extent of (de)localization of electron density across the cluster core has an impact on the bond dissociation free energies of surface O-H bonds across all oxidation states of the assembly.

Organic/Inorganic Species Synergistically Supported Unprecedented Vanadomolybdates.

Vanadomolybdates (VMos), comprised of Mo and V in high valences with O bridges, are one of the most important types of polyoxometalates (POMs), which have high activity due to their strong capabilities of gaining/losing electrons. Compared with other POMs, the preparation of VMos is difficult due to their relatively low structural stability, especially those with unclassical architectures. To overcome this shortcoming, in this study, triol ligands were applied to synthesize VMos through a beaker reaction in the presence of V2O5, Na2MoO4, and organic species in the aqueous solution. The single-crystal X-ray diffraction results indicate that two VMo clusters, Na4{V5Mo2O19[CH3C(CH2O)3]}∙13H2O and Na4{V5Mo2O19[CH3CH2C(CH2O)3]}∙13H2O, with a similar architecture, were synthesized, which were both stabilized by triol ligand and {MoO6} polyhedron. Both clusters are composed of five V ions and one Mo ion in a classical Lindqvist arrangement with an additional Mo ion, showing an unprecedented hepta-nuclear VMo structure. The counter Na+ cations assemble into one-dimensional channels, which facilitates the transport of protons and was further confirmed by proton conductivity experiments. The present results provide a new strategy to prepare and stabilize VMos, which is applicable for developing other compounds, especially those with untraditional architectures.

Cations Modulated Assembly of Triol-Ligand Modified Cu-Centered Anderson-Evans Polyanions.

Counter-cations are essential components of polyoxometalates (POMs), which have a distinct influence on the solubility, stabilization, self-assembly, and functionality of POMs. To investigate the roles of cations in the packing of POMs, as a systematic investigation, herein, a series of triol-ligand covalently modified Cu-centered Anderson-Evans POMs with different counter ions were prepared in an aqueous solution and characterized by various techniques including single-crystal X-ray diffraction. Using the strategy of controlling Mo sources, in the presence of triol ligand, NH4+, Cu2+ and Na+ were introduced successfully into POMs. When (NH4)6Mo7O24 was selected, the counter cations of the produced POMs were ammonium ions, which resulted in the existence of clusters in the discrete state. Additionally, with the modulation of the pH of the solutions, the modified sites of triol ligands on the cluster can be controlled to form δ- or χ-isomers. By applying MoO3 in the same reaction, Cu2+ ions served as linkers to connect triol-ligand modified polyanions into chains. When Na4Mo8O26 was employed as the Mo source to react with triol ligands in the presence of CuCl2, two 2-D networks were obtained with {Na4(H2O)14} or {{Na2(H2O)4} sub-clusters as linkers, where the building blocks were δ/δ- and χ/χ-isomers, respectively. The present investigation reveals that the charges, sizes and coordination manners of the counter cations have an obvious influence on the assembled structure of polyanions.

Physicochemical implications of surface alkylation of high-valent, Lindqvist-type polyoxovanadate-alkoxide clusters.

We report a rare example of the direct alkylation of the surface of a plenary polyoxometalate cluster by leveraging the increased nucleophilicity of vanadium oxide assemblies. Addition of methyl trifluoromethylsulfonate (MeOTf) to the parent polyoxovanadate cluster, [V6O13(TRIOLR)2]2- (TRIOL = tris(hydroxymethyl)methane; R = Me, NO2) results in functionalisation of one or two bridging oxide ligands of the cluster core to generate [V6O12(OMe)(TRIOLR)2]1- and [V6O11(OMe)2(TRIOLR)2]2-, respectively. Comparison of the electronic absorption spectra of the functionalised and unfunctionalised derivatives indicates the decreased overall charge of the complex results in a decrease in the energy required for ligand to metal charge transfer events to occur, while simultaneously mitigating the inductive effects imposed by the capping TRIOL ligand. Electrochemical analysis of the family of organofunctionalised polyoxovanadate clusters reveals the relationship of ligand environment and the redox properties of the cluster core: increased organofunctionalisation of the surface of the vanadium oxide assembly translates to anodic shifts in the reduction events of the Lindqvist ion. Overall, this work provides insight into the electronic effects induced upon atomically precise modifications to the surface structure of nanoscopic, redox-active metal oxide assemblies.

{VMo9O31[RC(CH2O)3]}6-: the first class of triol ligand covalently-decorated Keggin-type polyoxomolybdates.

Three trivacant polyoxomolybdates were constructed, presenting the first class of triol ligand covalently-decorated Keggin-type clusters through C-O-Mo bonds. The introduction of the organic component enhanced the stability of the cluster in the solid state with the leaving of active sites at the lacunary position.

Supramolecular interaction-induced assemblies of polyanions and 2-aminopyridinium in two polyoxometalate-based hybrids.

Organic-inorganic hybrids consisting of organic cations and polyanions are promising functional materials due to their various compositions and structures. An important aspect of these materials is the interactions between the organic and inorganic components, which not only produce the final structures, but also influence the properties. Here, we investigated the interactions between organic cations and polyanions using protonated 2-aminopyridinium (Hap) as the cation, and successfully obtained two polyoxometalate-based hybrids, namely (C5H7N2)4[Mo8O26], (I), and (C5H7N2)2[NiMo6O16(OH)2{CH3C(CH2O)3}2]·4H2O, (II). In the crystal structure of (I), every Hap cation links with two polyanions by donating one or two N-H...O hydrogen bonds, and every polyanion is surrounded by eight Hap cations via terminal or bridging O atoms. Conversely, in compound (II), every Hap cation only links with one polyanion decorated by a triol ligand; this organic-inorganic component further assembles via uncoordinated water molecules. In the extended structures, Hap plays a key role, not only providing a counter charge, but also acting as `glue' linking polyanions in the role of hydrogen-bond donors. In both compounds, as the nodes of the supramolecular network, the polyanions exhibit an ordered two-dimensional arrangement due to strong hydrogen bonds and electrostatic interactions between the organic and inorganic parts. The electrochemistry of compound (I) shows that redox sourcing from polyanions is a surface-controlled process. Conversely, the magnetic behaviour of compound (II) indicates dominant antiferromagnetic properties.

Recent advances in controllable alkoxylation chemistry of Anderson-type polyoxometalates from synthetic strategies perspective

Polyoxometalates (POMs) are an exceptional family of inorganic oxide anions consisting of Mo, W, V, etc. early transition metal ions at their highest oxidization states with structural versatility. The Anderson-type hetreopolyanions are one of the most important subclass since their most flexibility benefited from the central heteroatom. Among the frontier of current POMs investigations, organic modification and functionalization of POMs stands for one of most hot topics. The organic modification of POMs can incorporate POMs with advanced functional organic moieties on their surfaces to make the structure versatility more accessible and the design of advanced functional materials more rational. In this review, recent advances in controllable alkoxylation chemistry of Anderson-type polyoxometalates are briefly summarized into 9 parts as following: (1) the synthesis strategy development of direct modification protocol for the synthesis of single-side triol-functionalized Anderson δ isomer organic hybrids. (2) Based on direct modification protocol, the synthesis strategy development of μ 2-O atom pronation and activation chemistry in Anderson cluster to afford single-side triol-functionalized Anderson χ and ψ isomer organic hybrids. The proton-controlled isomer transformation between the δ , χ and ψ isomer was discovered. This will open up a new era for developing chemical reactivity of μ 2-O atoms in the Anderson cluster. (3) By precise proton control, the synthesis strategy development to obtain diol functionalized Anderson organic hybrids. Such diol functionalization mode not only works for some specific triol ligands but also can readily be extended to the diol ligands, which will greatly enrich the species of alkoxo-derivatized Anderson POM clusters. (4) Based on direct modification protocol, extending special tripodal ligands with carboxyl group to symmetrically functionalized the Anderson cluster. Such Anderson organic hybrids cannot be obtained for the traditional reconstruction protocol. This has extended the versatility of tripodal ligands applied for Anderson-type POMs modification. (5) Through stepwise synthesis strategy, single-side δ isomer was utilized as a precursor to efficiently synthesized asymmetrically triol-functionalized Anderson organic hybrids. (6) The synthesis strategy development of direct parent Anderson alkoxylation protocol for triol-functionalized butterfly-shaped β isomers of Anderson POMs hybrids. (7) The synthesis strategy development to extend the triol-functionalized Anderson from polymolybdates(Mo6O24) to polyoxotungstates(W6O24). (8) The newly development and revised synthesis strategy of Mo8 reconstruction protocol. (9) The original concept proposed by our group, organic ligand protected and inorganic-ligand supported atomically precise Anderson nanocluster based molecular armor hybrid material design as effective catalysts for green aerobic oxidation catalysis applications. In summary, the synthetic strategies and structures of novel alkoxyl-functionalized Anderson cluster have been discussed in this review. These newly developed synthesis strategies have greatly enriched the category of organoalkoxylation derivatives of Anderson cluster, providing a reliable and convenient synthetic methodology for the controllable design of triol-functionalized Anderson organic hybrids with specific and desirable functionalization modes. The extension of triol ligands with functional groups to other POMs units are still need to be developed. One of the important application fields for POMs is catalysis, however, the exploration of catalysis of alkoxyl-functionalized Anderson nanoclusters is quite limited to the recent cases we mentioned in this review. It is worth exploring to develop more alkoxyl-functionalized Anderson nanoclusters as catalysts for useful reactions, because alkoxyl-functionalized Anderson nanoclusters have multiple active sites and special properties derived from functional groups of alkoxyl ligands as well as cooperativities. The charge transfer properties between POMs and organic moieties in alkoxyl-functionalized Anderson nanoclusters have also attracted widely attentions. Using alkoxyl ligands with conjugated groups to functionalize Anderson nanoclusters may enhance the ability of electron transfer to improve the intrinsic redox properties of Anderson nanoclusters. It can be concluded with confidence that the persistent and dedicated efforts of chemists have revealed a vast advance in alkoxylation of Anderson nanoclusters, which may in fact just be the reason why the alkoxylation chemistry of Anderson nanoclusters research field has flourished in recent years. Of course, more Anderson nanoclusters-based materials with novel structures, interesting and functional properties remain to be explored and the alkoxylation chemistry of Anderson nanoclusters still has great growing opportunities in the future.

Structure and property of a Cu(II) compound bearing isopolyanion and flexible triol ligand

The investigations of the organic-inorganic hybrid materials attract widely attentions in recent years due to their applications in many fields, such as medicine, magnetism and catalysis. As one of the important types of inorganic components, polyoxometalates (POMs) bear the features of various compositions, rich structures and tunable charges, and play key roles in the construction of hybrid materials. Since the first [Mo7O24]6- polyanion was synthesized and its structure was confirmed by single-crystal X-ray analysis, many different kinds of cations, including inorganic and organic ions, were introduced to this system. While the studies of this type of polyanion with coordination compounds are still rarely, especially for transition metal ions and oxygenated ligands. As our ongoing study in the field of POM-containing compounds, we succeeded in obtaining an organic-inorganic hybrid based on [Mo7O24]6- polyanion in aqueous solution with CuSO4·5H2O, (NH4)6Mo7O24·4H2O and a triol ligand as staring materials. The formula of the obtained compound was determined as (NH4)2[CuL(H2O)]2MO7O24∙3H2O, L=(HOCH2)3CNH(CH2)3NHC(CH2OH)3 ( 1 ) through X-ray analysis, elemental analysis, IR spectrum and TG analysis. Single-crystal X-ray analysis reveals that compound 1 belongs to the monoclinic system with the space group of P 21. The unit cell parameters is a =10.778(2) A, b =21.391(3) A, c =11.596(3) A, α =90°, β =96.82(1)°, γ =90°, V =2654.4(9) A3, Z =2, and the final R 1 is 0.0155. In the crystal structure of compound 1 , Cu2+ cation is in a six-coordination environment and combines with ligand L in the unsymmetrical chelate coordination manner, where two triol groups provide one and two O atoms respectively and the other three positions are occupied by two N atoms and one O atom from water molecule. The [CuL(H2O)]2+ subunit connects with [Mo7O24]6- polyanion through the strong O−H∙∙∙O and N−H∙∙∙O hydrogen bonds to form a supramolecular two-dimensional network. TG curve shows that compound 1 has a good thermal stability until 160°C. The behavior of temperature dependence magnetic susceptibility of compound 1 in the range of 2−300 K expresses a strong antiferromagnetic property. In this work, the flexibility of the ligand plays a key role and provides the possibility for the formation of the unsymmetrical chelate coordination style of Cu2+. In addition, the high charge number of polyanion is an important factor for the construction of the two-dimensional network through providing more interactive sites.

Ratio-Controlled Precursors of Anderson-Evans Polyoxometalates: Synthesis, Structural Transformation, and Magnetic and Catalytic Properties of a Series of Triol Ligand-Decorated {M2Mo6} Clusters (M = Cu2+, Co2+, Ni2+, Zn2+).

A series of triol ligand [CH3C(CH2OH)3] covalently decorated polyoxometalates (POMs), which could be ascribed to the primary complexes with structural formulas {M2[Mo2O4(CH3C(CH2O)3)2]3}2- (M = Cu2+, Co2+, Ni2+, Zn2+), have been synthesized in organic solvents. Single-crystal X-ray structural analysis reveals that the synthesized polyanionic clusters are comprised of three {Mo2} units and two divalent transition-metal ions connecting to each other in an alternating style, where all {Mo2} blocks were covalently decorated by two triol ligands in the trans conformation. The 1/3 molar ratio of M/Mo in the prepared complexes was higher than those ratios in typical Anderson-Evans, Wells-Dawson, and Keggin POMs. With a decrease in the M/Mo molar ratio of a Mo-contained reactant to 1/6 and/or the addition of acetic acid to the reaction solution, the primary complexes acting as precursors transformed continuously into the corresponding triol-ligand-decorated Anderson-Evans POMs. Detailed investigations were conducted by using different isopolymolybdates in various solvent environments, and several Anderson-Evans POMs in different triol-ligand-decorated fashions were obtained from the primary complexes. In addition, we also realized the transformation between the Anderson-Evans clusters in different decoration fashions by simply controlling the acidity in solution. Magnetic measurement showed a general property, but the catalytic experiments demonstrated that CoII- and Zn II-containing POMs displayed a higher efficiency for the selective oxidation of thioanisole to sulfoxide.

Triol-Ligand Modification and Structural Transformation of Anderson-Evans Oxomolybdates via Modulating Oxidation State of Co-Heteroatom.

Various covalent decorations and structural transformations of a series of R-C(CH2OH)3 triol ligands on CoII/CoIII-centered Anderson-Evans polyoxomolybdates were studied in aqueous and/or organic solution. Single-crystal X-ray structural analysis demonstrated that four manners, including (1) double-sided χ/χ-isomer and (2) double-sided δ/χ-isomer with CoII central heteroatom, and (3) single-sided δ-isomer and (4) double-sided δ/δ-isomer with CoIII central heteroatom, appeared on the decoration of the mother polyanion [CoII/III(OH)6Mo6O18]4-/3-. Through careful manipulation of the reaction conditions, a gradual oxidation process from CoII to CoIII was disclosed to happen in air, and accompanying that process, the CoII-centered double-sided δ/χ-isomer that formed in the initial stage transformed into a CoIII-centered double-sided δ/δ-isomer. When the electron-withdrawing group -NO2 substituted triol ligand was used to replace CH3C(CH2OH)3, the oxidation process accelerated spontaneously. A further investigation showed that this oxidation process can be blocked by acid. While the triol ligands' decoration on Anderson-Evans polyoxometalates (POMs) took place along with the oxidation of the central heteroatom, that codecoration of other components was also accomplished. In the presence of excess acetic acid, an unprecedented single-sided δ-isomer with an acetate ligand covalently attaching on the other side by replacing one μ3-O atom was obtained in aqueous solution. The reaction of the mother cluster [(n-C4H9)4N]3[Co(OH)6Mo6O18] and triol ligands in methanol resulted in a codecoration of methanol with the double-sided δ/δ-isomer, which has never been reported in B-type Anderson-Evans POMs. The obtained results revealed that the oxidation state of the heteroatom has a significant impact on the triol ligand decoration styles, such as the χ/χ- or δ/χ-isomer for the divalent heteroatom and the δ- or δ/δ-isomer for the trivalent heteroatom.

A closed hollow capsule structure assembled by double acetate-decorated Anderson-like polyanions

An inorganic–organic hybrid vanadotungstate, Na6[VW6O22(CH3COO)3]·11H2O (1), was synthesized in aqueous solution and characterized by infrared spectrum, elemental analysis, thermogravimetric analysis, X-ray photoelectron spectroscopy, and single-crystal X-ray diffraction. The result of the single-crystal X-ray diffraction analysis reveals that the polyanion of 1 is an acetate single-sided decorated Anderson-like polyoxotungstate with a VV in the central tetrahedral position. For better understanding of the characteristic distinction between 1 and the classical Anderson-type polyanion, we synthesized a triol ligand single-side anchored on the Anderson-type polyoxometalate, [(n-C4H9)4N]3{AlMo6O18(OH)3[(CH2O)3CNHCOC5H4N]}·8H2O (2), as a comparison. One interesting architectural feature of 1 is that two polyanions [VW6O22(CH3COO)3]6− are linked by six sodium cations on the same plane to form a closed hollow capsule structure with the inner major axis of 8.54 Å and minor axis of 3.11 Å. As basic building blocks, the closed hollow capsule moieties are assembled into 1D chain-like structures through direct Na−O bonds without additional linkers. The electrochemical behavior of 1 was investigated in pH 4.8 buffer solution to investigate its potential application in electrochemical catalysis; 1 has good catalytic activity for oxidation of thioanisole.

Controlled Triol-Derivative Bonding and Decoration Transformation on Cu-Centered Anderson-Evans Polyoxometalates.

To create new types of organic ligands covalently grafted onto polyoxometalates and identify the reaction mechanism, we selected Cu(II) as the central heteroatom for the synthesis of a series of disklike Anderson-Evans clusters bearing different triol derivatives on both their faces via one-pot and/or step-by-step routes. By using a [(n-C4H9)4N]4[Mo8O26] precursor cluster and copper acetate as the starting materials, several organically modified χ isomers with Cu(II) heteroatom centers were obtained. Starting from a [(n-C4H9)4N]2[Mo2O7] subcluster, however, a half-malposition coordination fashion of triol ligands with a δ isomer on one face and a χ isomer on the other face of the Anderson-Evans cluster was obtained. By changing the reaction solvent from acetonitrile to methanol, we realized a secondary organic modification of the triol-grafted clusters and obtained a triol ligand/methanol codecoration on the Anderson-Evans polyoxometalate. In addition, by changing the reaction environment, we succeeded in modulating the transformation of triol ligands from one site to another on the polyoxometalate cluster. Importantly, by control of the reaction condition, the methanol molecules were also taken off from the cluster.

The proton-controlled synthesis of unprecedented diol functionalized Anderson-type POMs.

By addition of excess acid into the reaction mixture, a series of organically-derived Anderson-type polyoxometalates, {[R1CR2(CH2O)2]CrMo6O18(OH)4}(3-), with diols as the ligands are reported herein. Such a diol functionalization mode not only works for some specific triol ligands but also can readily be extended to the diol ligands, which will greatly enrich the species of alkoxo-derivatized Anderson POM clusters.

Step-by-step strategy from achiral precursors to polyoxometalates-based chiral organic-inorganic hybrids.

Using two types of triol ligands, several novel asymmetrically triol-functionalized Anderson organic hybrids have been efficiently synthesized in high purity and good yields via a convenient two-step esterification reaction. These organic-inorganic hybrids are chiral and can be spontaneously resolved with suitable solvents. Their molecular and crystal structures have been confirmed by single-crystal X-ray diffraction studies. Stable solid-state chirality of the corresponding enantiopure crystals has also been confirmed definitively by CD spectra. Interestingly, these organic-inorganic hybrids possess a layer-by-layer structure, forming solvent-accessible nanoscale chiral channels via a 1D infinite helical chain substructure. TGA measurements indicated that the species of the central heteroatoms significantly effects the stability of these compounds.

XPS, FTIR-ATR, and AFM Structural Study of Silicon-Grafted Triol Monolayers for Controlled Anchoring of Single Molecule Magnets

The structures of monolayers on Si(100) made up of two acetyl-protected triol ligands, derived from 2-hydroxymethyl-propane-1,3-diol, namely, R-C(CH2OAc)3 with R = CH2═CH−(CH2)9−O−CH2− and R = CH2═CH−(CH2)7−, have been investigated. After deprotection, the two ligands are well-suited receptors for anchoring molecular functionalities, such as Fe4 single molecule magnets, to the Si surface through OH groups. Monolayers were obtained by the hydrosilylation reactions between H-terminated Si(100) and the CH2═CH− end groups and were prepared in pure form or diluted with inert 1-alkenes to achieve an accurate control over the concentration and concerted orientation of the receptors on the surface. Subsequent anchoring of a Fe4 cluster, especially designed for a vertical atom sequence, has been expedient to clarify the influence of ligand mole fraction on the ordering of molecular receptors. We have achieved insight into the related structures and morphologies by combining angle-resolved X-ray photoelectron spect...