Robotic Stepwise Synthesis of Hetero-Multinuclear Metal Oxo Clusters as Single-Molecule Magnets.

An efficient stepwise synthesis method for discovering new heteromultinuclear metal clusters using a robotic workflow is developed where numerous reaction conditions for constructing heteromultinuclear metal oxo clusters in polyoxometalates (POMs) were explored using a custom-built automated platform. As a result, new nonanuclear tetrametallic oxo clusters {FeMn4}Lu2A2 in TBA5[(A-α-SiW9O34)2FeMn4O2{Lu(acac)2}2A2] (IIA; A = Ag, Na, K; TBA = tetra-n-butylammonium; acac = acetylacetonate) were discovered by the installation of diamagnetic metal cations A+ into a paramagnetic {FeMn4}Lu2 unit in TBA7[(A-α-SiW9O34)2FeMn4O2{Lu(acac)2}2] (I). POMs IIA exhibited single-molecule magnet properties with the higher energy barriers for magnetization reversal (IIAg, 40.0 K; IINa, 40.3 K; IIK, 26.7 K) compared with that of the parent I (19.7 K). Importantly, these clusters with unique properties were constructed as designed by a step of the predictable sequential multistep reactions with the time-efficient platform.

Cation-Disorder-Assisted Reversible Topotactic Phase Transition between Antifluorite and Rocksalt Toward High-Capacity Lithium-Ion Batteries.

Multielectron reaction electrode materials using partial oxygen redox can be potentially used as cathodes in lithium-ion batteries, as they offer numerous advantages, including high reversible capacity and energy density and low cost. Here, a reversible three-electron reaction is demonstrated utilizing topotactic phase transition between antifluorite and rocksalt in a cation-disordered antifluorite-type cubic Li6CoO4 cathode. This cubic phase is synthesized by a simple mechanochemical treatment of conventionally prepared tetragonal Li6CoO4. It displays a reversible capacity of 487 mAh g-1, a high value because of a reversible three-electron reaction using Co2+/Co3+, Co3+/Co4+, and O2-/O22- redox, occurring without O2 gas evolution. The mechanochemical treatment is assumed to reduce its lattice distortion by cation-disordering and facilitate a reversible topotactic phase transition between antifluorite and rocksalt structures via a dynamic cation pushing mechanism.

Synthesis of unsymmetrically substituted triarylamines via acceptorless dehydrogenative aromatization using a Pd/C and p-toluenesulfonic acid hybrid relay catalyst.

An efficient and convenient procedure for synthesizing triarylamines based on a dehydrogenative aromatization strategy has been developed. A hybrid relay catalyst comprising carbon-supported Pd (Pd/C) and p-toluenesulfonic acid (TsOH) was found to be effective for synthesizing a variety of triarylamines bearing different aryl groups starting from arylamines (diarylamines or anilines), using cyclohexanones as the arylation sources under acceptorless conditions with the release of gaseous H2. The proposed reaction comprises the following relay steps: condensation of arylamines and cyclohexanones to produce imines or enamines, dehydrogenative aromatization of the imines or enamines over Pd nanoparticles (NPs), and elimination of H2 from the Pd NPs. In this study, an interesting finding was obtained indicating that TsOH may promote the dehydrogenation.

Controlled Assembly Synthesis of Atomically Precise Ultrastable Silver Nanoclusters with Polyoxometalates.

Silver nanoclusters have attracted scientific interest due to their properties and applications. However, practical synthetic methods to access these materials are still limited mainly due to the low stability. Here, we report a controlled assembly strategy for fabricating atomically precise silver nanoclusters using polyoxometalates (POMs) as structure-directing as well as functionalizing units. A trefoil-propeller-shaped {Ag27}17+ nanocluster was synthesized by assembling reactive nanoclusters supported by open-Dawson-type POMs [Si2W18O66]16-. The {Ag27}17+ nanocluster possessed 10 delocalized valence electrons and showed unprecedented ultrastability in solutions. The cluster showed unique {Ag27}-to-POM charge transfer bands in the visible light region.

Methyl-Selective α-Oxygenation of Tertiary Amines to Formamides by Employing Copper/Moderately Hindered Nitroxyl Radical (DMN-AZADO or 1-Me-AZADO).

Methyl-selective α-oxygenation of tertiary amines is a highly attractive approach for synthesizing formamides while preserving the amine substrate skeletons. Therefore, the development of efficient catalysts that can advance regioselective α-oxygenation at the N-methyl positions using molecular oxygen (O2 ) as the terminal oxidant is an important subject. In this study, we successfully developed a highly regioselective and efficient aerobic methyl-selective α-oxygenation of tertiary amines by employing a Cu/nitroxyl radical catalyst system. The use of moderately hindered nitroxyl radicals, such as 1,5-dimethyl-9-azanoradamantane N-oxyl (DMN-AZADO) and 1-methyl-2-azaadamanane N-oxyl (1-Me-AZADO), was very important to promote the oxygenation effectively mainly because these N-oxyls have longer life-times than less hindered N-oxyls. Various types of tertiary N-methylamines were selectively converted to the corresponding formamides. A plausible reaction mechanism is also discussed on the basis of experimental evidence, together with DFT calculations. The high regioselectivity of this catalyst system stems from steric restriction of the amine-N-oxyl interactions.

Selective Synthesis of Primary Anilines from NH3 and Cyclohexanones by Utilizing Preferential Adsorption of Styrene on the Pd Nanoparticle Surface.

Dehydrogenative aromatization is one of the attractive alternative methods for directly synthesizing primary anilines from NH3 and cyclohexanones. However, the selective synthesis of primary anilines is quite difficult because the desired primary aniline products and the cyclohexanone substrates readily undergo condensation affording the corresponding imines (i.e., N-cyclohexylidene-anilines), followed by hydrogenation to produce N-cyclohexylanilines as the major products. In this study, primary anilines were selectively synthesized in the presence of supported Pd nanoparticle catalysts (e.g., Pd/HAP, HAP=hydroxyapatite, Ca10 (PO4 )6 (OH)2 ) by utilizing competitive adsorption unique to heterogeneous catalysis; in other words, when styrene was used as a hydrogen acceptor, which preferentially adsorbs on the Pd nanoparticle surface in the presence of N-cyclohexylidene-anilines, various structurally diverse primary anilines were selectively synthesized from readily accessible NH3 and cyclohexanones. The Pd/HAP catalyst was reused several times though its catalytic performance gradually declined.

Self-Assembly of Anionic Polyoxometalate-Organic Architectures Based on Lacunary Phosphomolybdates and Pyridyl Ligands.

The development of novel systems for metal-organic architectures is an attractive research field because they are fascinating materials with unexplored functions. Lacunary polyoxometalates (POMs) offer structurally well-defined coordination sites with various coordination directions and numbers in addition to the designable properties; thus, lacunary POMs are ideal building blocks for inorganic-organic architectures. However, their utilization is currently limited by their low stability and difficulty in controlling the reactivity. Here, we report the successful self-assembly of anionic POM-organic architectures comprising multivacant lacunary POMs and pyridyl ligands. By introducing pyridine moieties to its vacant sites, the trivacant lacunary phosphomolybdate [A-α-PMo9O34]9- is significantly stabilized in organic solvents. Furthermore, the resultant structure can be utilized as a stable and reactive building block to synthesize a dimer pillared by 4,4'-bipyridyl and a tetramer bridged by two cofacial porphyrin ligands, which can intercalate aromatic molecules.

Ring-Shaped Polyoxometalates Possessing Multiple 3d Metal Cation Sites: [{M2(OH2)2}2{M(OH2)2}4P8W48O176(OCH3)8]16- (M = Mn, Co, Ni, Cu, Zn).

A ring-shaped polyoxometalate (POM), [P8W48O184]40- (P8W48), formed by condensation of four hexavacant lacunary α-Dawson-type [α-P2W12O48]14- units, is an attractive superlacunary compound possessing a large cavity (approximately 10 Å). Although the introduction of metal species into the cavity was attempted in aqueous media using the K+/Li+ salt of P8W48, we encountered several challenges: excess metal species are required, and K+ and/or Li+ remain in the cavity. Therefore, controlling the number and arrangement of the introduced metal species proved difficult. In this study, we overcame the aforementioned problems by developing a novel method for introducing metal species in organic media by employing the organic-solvent-soluble tetra- n-butylammonium (TBA) salt of P8W48 (I, TBA12H28[P8W48O184]·30H2O). We began by synthesizing compound I via a cation exchange reaction of the K+/Li+ salt of P8W48 with TBABr in aqueous nitric acid. Single crystals (I', TBA14H26[P8W48O184]·20H2O·2CH3NO2) were obtained by recrystallizing I from a nitromethane/toluene mixture in the presence of excess TBAOH. The bond valence sum values of I' showed that protons were located on the oxygen atoms inside the ring, and thus highly reactive sites capable of reacting with metal species were selectively generated within the ring. When I was reacted with 8 equiv of 3d metal species (Mn2+, Co2+, Ni2+, Cu2+, or Zn2+) in an acetonitrile/1,2-dichloroethane mixture in the presence of TBAOH and methanol, a series of POMs with eight metal sites (IIM, TBA14H2[{M2(OH2)2}2{M(OH2)2}4P8W48O176(OCH3)8]· nH2O· mCH3CN, where M = Mn, Co, Ni, Cu, Zn) was synthesized. In IIM, two edge-sharing bis(square-pyramidal) {(µ-O)2(M-OH2)(µ3-O)2(M-OH2)(µ-O)2} and four isolated M(OH2)2 sites were present within the cavity. During the metal introduction, two α-P2W12 units were converted into γ-P2W12 units, and unique α,γ,α,γ-type P8W48 frameworks were obtained.

Hexavacant γ-Dawson-type phosphotungstates supporting an edge-sharing bis(square-pyramidal) {O2M(µ3-O)2(µ-OAc)MO2} core (M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+).

In aqueous media, the introduction of additional metal species into polyoxometalates (POMs) with multiple vacant sites, such as a hexavacant Dawson-type phosphotungstate, which is of interest for the synthesis of novel metal oxide clusters, is generally difficult because they easily undergo self-condensation and/or structural decomposition. In this study, we succeeded in developing a novel synthetic method to obtain metal-substituted γ-Dawson-type phosphotungstate monomers by introducing metal species into an organic solvent-soluble lacunary phosphotungstate, TBA4H10[α-P2W12O48] (I) (TBA = tetra-n-butylammonium), in organic media. The reaction of I, which possessed two types of vacant sites, i.e. middle and edge sites, with divalent metal species such as Mn2+, Co2+, Ni2+, Cu2+, or Zn2+ in acetonitrile afforded a series of isostructural POMs M2 (TBA5[γ-P2W12O44M2(OAc)(CH3CONH)2]·nH2O·mCH3CN; M = Mn2+, Co2+, Ni2+, Cu2+, or Zn2+; OAc = acetate) with an edge-sharing bis(square-pyramidal) {O2M(µ3-O)2(µ-OAc)MO2} core. The bis(square-pyramidal) core was selectively placed at the middle site of the hexavacant lacunary phosphotungstate, and the two metals in the core were bridged by two phosphate units and one acetate species. Meanwhile, the edge sites were capped by acetimidate ligands, which protect the reactive lacunary POM from self-condensation. To the best of our knowledge, this is the first report describing the synthesis and characterization of metal-substituted hexavacant γ-Dawson-type POM monomers.

Porous Cubic Cesium Salts of Silicododecatungstate(molybdate)/Borododecatungstate Blends: Synthesis and Molecular Adsorption Properties.

In this work, the synthesis and molecular adsorption properties of cubic cesium salts of Keggin-type silicododecametalates (SiM; M = W or Mo) and borododecatungstate (BW) blends were reported. Pentavalent BW, which is normally packed in a monoclinic fashion, could be packed into the desired cubic fashion by being blended with tetravalent SiW or SiMo. By simply mixing aqueous solutions containing SiM and BW in different SiM:BW molar ratios [SiM:BW = N:(100 - N), where N is the percentage of SiM in the synthetic solution and equals 100, 80, 50, 30, 20, 10, or 5] with a cesium nitrate solution, we obtained a series of cubic cesium salts (CsSiMBW- N) with POM vacancy-like pores (i.e., kinetically formed interparticle pores). Notably, the yields, compositions, and BET surface areas of the obtained CsSiMBW- N were dependent on only the N values and independent of SiW or SiMo. In CsSiMBW- N, SiM and BW were well blended at the molecular (nanometer) level. The nitrogen, water, and ethanol adsorption properties of CsSiWBW- N systematically changed according to the SiW:BW molar ratios. In the case of SiW-rich salts, nitrogen, water, and ethanol were adsorbed in the pores; the amounts of nitrogen, water, and ethanol adsorbed were nearly identical for CsSiWBW-100. When the SiW:BW molar ratio was decreased, the salts selectively incorporated water and ethanol within the pores. In the case of CsSiWBW-5, barely any nitrogen was adsorbed. The plausible mechanisms for particle growth and interparticle POM vacancy-like pore formation are also discussed.

Effect of Heteroatoms on Field-Induced Slow Magnetic Relaxation of Mononuclear FeIII ( S = 5/2) Ions within Polyoxometalates.

In this paper, the synthesis and magnetic properties of mononuclear FeIII-containing polyoxometalates (POMs) with different types of heteroatoms, TBA7H10[(A-α-XW9O34)2Fe] (IIX, X = Ge, Si; TBA = tetra- n-butylammonium), are reported. In these POMs, mononuclear highly distorted six-coordinate octahedral [FeO6]9- units are sandwiched by two trivacant lacunary units [A-α-XW9O34]10- (X = Ge, Si). These POMs exhibit field-induced slow magnetic relaxation based on the single high-spin FeIII magnetic center ( S = 5/2). Combining experiment and ab initio calculations, we investigated the effect of heteroatoms of the lacunary units on the field-induced slow magnetic relaxation of these POMs. By changing the heteroatoms from Si (IISi) to Ge (IIGe), the coordination geometry around the FeIII ion is mildly changed. Concretely, the axial Fe-O bond length in IIGe is shortened compared with that in IISi, and consequently the distortion of the [FeO6]9- unit in IIGe from the ideal octahedral coordination geometry becomes larger than that in IISi. The effective demagnetization barrier of IIGe (11.4 K) is slightly larger than that of IISi (9.2 K). Multireference ab initio calculations predict zero-field splitting parameters in good agreement with experiment. Although the differences in the coordination geometries and magnetic properties of IIGe and IISi are quite small, ab initio calculations indicate subtle changes in the magnetic anisotropy which are in line with the observed magnetic relaxation properties.

CuCl/TMEDA/nor-AZADO-catalyzed aerobic oxidative acylation of amides with alcohols to produce imides.

Although aerobic oxidative acylation of amides with alcohols would be a good complement to classical synthetic methods for imides (e.g., acylation of amides with activated forms of carboxylic acids), to date, there have been no reports on oxidative acylation to produce imides. In this study, we successfully developed, for the first time, an efficient method for the synthesis of imides through aerobic oxidative acylation of amides with alcohols by employing a CuCl/TMEDA/nor-AZADO catalyst system (TMEDA = teramethylethylendiamine; nor-AZADO = 9-azanoradamantane N-oxyl). The proposed acylation proceeds through the following sequential reactions: aerobic oxidation of alcohols to aldehydes, nucleophilic addition of amides to the aldehydes to form hemiamidal intermediates, and aerobic oxidation of the hemiamidal intermediates to give the corresponding imides. This catalytic system utilizes O2 as the terminal oxidant and produces water as the sole by-product. An important point for realizing this efficient acylation system is the utilization of a TMEDA ligand, which, to the best of our knowledge, has not been employed in previously reported Cu/ligand/N-oxyl systems. Based on experimental evidence, we consider that plausible roles of TMEDA involve the promotion of both hemiamidal oxidation and regeneration of an active CuII-OH species from a CuI species. Here promotion of hemiamidal oxidation is particularly important. Employing the proposed system, various types of structurally diverse imides could be synthesized from various combinations of alcohols and amides, and gram-scale acylation was also successful. In addition, the proposed system was further applicable to the synthesis of α-ketocarbonyl compounds (i.e., α-ketoimides, α-ketoamides, and α-ketoesters) from 1,2-diols and nucleophiles (i.e., amides, amines, and alcohols).

Polyoxometalate LUMO engineering: a strategy for visible-light-responsive aerobic oxygenation photocatalysts.

We report the efficient visible-light-responsive photocatalysis of polyoxometalates (POMs) by engineering the lowest unoccupied molecular orbitals (LUMOs). By the introduction of vanadium atoms into the γ-Keggin-type phosphotungstate, a new V3d/W5d mixed LUMO appeared to afford a visible-light-responsive catalyst (I), which showed high photocatalytic activity for aerobic oxygenation of sulfides to sulfoxides.

High Proton Conduction in Crystalline Composites Based on Preyssler-Type Polyoxometalates and Polymers under Nonhumidified or Humidified Conditions.

Keggin-type polyoxometalate (POM)-based compounds have long been considered as one of the environmentally friendly candidates of solid electrolytes exhibiting high proton conductivity under high relative humidity (RH >95%). However, their application has been limited by the lack of structural stability and large decrease in conductivity with a slight decrease in RH. In order to overcome these disadvantages, we report a series of crystalline composites based on Preyssler-type POMs ([Na(H2O)P5W30O110]14-, [Bi(H2O)P5W30O110]12-), which are known to show higher acidity in comparison to Keggin-type POMs, and polymers (polyethylene glycol (PEG), polyallylamine (PAA)). Electrostatic interactions between POMs and polymers contribute to enhance the structural stability, and it has been widely known that polymer electrolytes promote cation transport via segmental motion of the polymer chain. In the crystalline composites, K+ acts as a linker to connect the POMs three-dimensionally, resulting in an all-inorganic framework, and polymers and waters of crystallization reside in the framework. The composites with PEG exhibit moderate proton conductivities of 10-4 S cm-1 under nonhumidified (RH <10%) and low-temperature (<368 K) conditions by the aid of segmental motion of PEG. The composite with PAA exhibits a high proton conductivity of 10-3 S cm-1 under humidified (RH 75%) and low-temperature (<338 K) conditions.

Formal arylation of NH3 to produce diphenylamines over supported Pd catalysts.

In the presence of supported Pd nanoparticle catalysts, e.g., Pd/Al2O3, various diphenylamines could be synthesized through acceptorless formal arylation using NH3 or its surrogates, e.g., urea, as nitrogen sources and cyclohexanones as arylation sources. The observed catalysis was truly heterogeneous, and the catalyst was reusable with retention of its high catalytic performance.

Selective Synthesis of Primary Anilines from Cyclohexanone Oximes by the Concerted Catalysis of a Mg-Al Layered Double Hydroxide Supported Pd Catalyst.

Although the selective conversion of cyclohexanone oximes to primary anilines would be a good complement to the classical synthetic methods for primary anilines, which utilize arenes as the starting materials, there have been no general and efficient methods for the conversion of cyclohexanone oximes to primary anilines until now. In this study, we have successfully realized the efficient conversion of cyclohexanone oximes to primary anilines by utilizing a Mg-Al layered double hydroxide supported Pd catalyst (Pd(OH)x/LDH) under ligand-, additive-, and hydrogen-acceptor-free conditions. The substrate scope was very broad with respect to both cyclohexanone oximes and cyclohexenone oximes, which gave the corresponding primary anilines in high yields with high selectivities (17 examples, 75% to >99% yields). The reaction could be scaled up (gram-scale) with a reduced amount of the catalyst (0.2 mol %). Furthermore, the one-pot synthesis of primary anilines directly from cyclohexanones and hydroxylamine was also successful (five examples, 66-99% yields). The catalysis was intrinsically heterogeneous, and the catalyst could be reused for the conversion of cyclohexanone oxime to aniline at least five times with keeping its high catalytic performance. Kinetic studies and several control experiments showed that the high activity and selectivity of the present catalyst system were attributed to the concerted catalysis of the basic LDH support and the active Pd species on LDH. The present transformation of cyclohexanone oximes to primary anilines proceeds through a dehydration/dehydrogenation sequence, and herein the plausible reaction mechanism is proposed on the basis of several pieces of experimental evidence.

Proton conduction in alkali metal ion-exchanged porous ionic crystals.

Proton conduction in alkali metal ion-exchanged porous ionic crystals A2[Cr3O(OOCH)6(etpy)3]2[α-SiW12O40]·nH2O [I-A+] (A = Li, Na, K, Cs, etpy = 4-ethylpyridine) is investigated. Single crystal and powder X-ray diffraction measurements show that I-A+ possesses analogous one-dimensional channels where alkali metal ions (A+) and water of crystallization exist. Impedance spectroscopy and water diffusion measurements of I-A+ show that proton conductivities are low (10-7-10-6 S cm-1) under low relative humidity (RH), and protons mostly migrate as H3O+ with H2O as vehicles (vehicle mechanism). The proton conductivity of I-A+ increases with the increase in RH and is largely dependent on the types of alkali metal ions. I-Li+ shows a high proton conductivity of 1.9 × 10-3 S cm-1 (323 K) and a low activation energy of 0.23 eV under RH 95%. Under high RH, alkali metal ions with high ionic potentials (e.g., Li+) form a dense and extensive hydrogen-bonding network of water molecules with mobile protons at the periphery, which leads to high proton conductivities and low activation energies via rearrangement of the hydrogen-bonding network (Grotthuss mechanism).

Alkoxides of Trivacant Lacunary Polyoxometalates.

Lacunary polyoxometalates (POMs) are useful rigid multidentate inorganic ligands to construct multinuclear metal oxo clusters and inorganic-organic hybrid materials. However, the use of multivacant lacunary POMs often suffers from their undesired condensation reactions at the highly reactive vacant sites. Herein, the introduction of alkoxy protecting groups into the vacant sites of lacunary POMs in organic media is reported for the first time. By reacting organic solvent-soluble trivacant lacunary Keggin-type POMs with methanol, six methoxy groups were successfully introduced into the vacant sites, resulting in the formation of the alkoxides TBA4 [A-α-XW9 O28 (OCH3 )6 ] (Ix -OMe; X=Si, Ge). These methoxy groups could protect the reactive vacant sites and suppress the undesired dimerization reaction into the corresponding α-Dawson-type POMs. Since the introduced monodentate methoxy groups could easily be dissociated by hydrolysis, ISi -OMe could be utilized as the precursor for the synthesis of metal-substituted POMs. In addition, by using pentaerythritol as the multidentate alkoxy ligand, the monomeric alkoxide TBA4 [A-α-SiW9 O28 {(OCH2 )2 C(CH2 OH)2 }3 ] (ISi -PE) was successfully synthesized. In particular, these multidentate alkoxy groups showed high resistance against hydrolysis. Moreover, by reacting ISi -PE with trivacant lacunary Keggin-type POMs, the dimeric inorganic-organic-inorganic hybrid structures TBAn H8-n [(A-α-SiW9 O28 ){(OCH2 )2 C(CH2 O)2 }3 (A-α-XW9 O28 )] (IISiX -PE; X=Si, Ge) with the same or different heteroatoms (Si and Ge) were successfully synthesized.