Theoretical consideration and computational simulation have been performed on the voltammetric properties of Keggin polyoxometalates (POMs), and the conversion from successive one-electron transfer in unacidified media to four-electron transfer (through two-electron transfer) in acidified media has been discussed. Perfect simulation of the cyclic voltammograms of POMs could be achieved using the standard formal potentials and the protonation constants, systematically evaluated by the equations, in which “simple (intrinsic)” and “synergistic (extrinsic)” electron-withdrawing effects of the μ4-oxygen were taken into consideration. In the proposed model, the formal potential of the one-electron redox waves for the ith reduction step is presented by Ei°(z0, s) = Ei** + 0.51(z0 – i + 1) + 1.067s (i = 1, 2, 3, 4; E1** = E2** = 0.577 V; E3** = E4** = 0.377 V), where z0 is the initial ionic charge of a Keggin POM and s is the mean bond valence of the μ4-O–W bonds in the POM. The values of Ei**s are related to the energy levels of the two lowest unoccupied molecular orbitals (LUMOs) of a hypothetical Keggin POM with null charge and null bond valence. Then it was revealed that the LUMOs have small on-site repulsion, which may be an important factor that makes multielectron transfer feasible. These findings would give a big clue in developing novel redox materials exhibiting multielectron transfers.

We found that the hexatriacontamolybdate [Mo36O112(H2O)16]8− ({Mo36}) compound of 1,3-diamino-2-propanol (βOHC3-DA) forms seven structural phases with the formula (βOHC3-DA)4{Mo36}·nH2O. They showed a range of dimensionality: three zero-dimensional (0D), two 1D, and two 2D MoO framework structures consisting of {Mo36} anions. Two of the phases have 0D framework structures crystallized in the mother solution. The remaining five phases were obtained when the crystals of these two 0D phases were aged in resin. The dense 2D framework ({Mo36}-nanosheet) of the title compound was formed via solid-phase condensation reactions under restricted dehydration conditions such as in resin-coated crystals, unlike the loose {Mo36}-nanosheet of the (C3DA)4{Mo36}·nH2O. The formation processes of the related high-dimensional MoO frameworks were guided by hydrogen-bonding contacts initially formed between {Mo36} anions in the crystal. There were two different conversion routes: the one starting from the phase consisting of {Mo36} hydrogen-bonded at their head/foot parts lead to the dense 2D nanosheet, while the other originating from the phase consisting of {Mo36} hydrogen-bonded at their trunk parts, to 1D {Mo36}-nanochain with rare triple oxygen bridges. These routes had neither branching nor intercrossing.Graphical abstractSeven structural phases with MoO framework structures of various dimensionalities were found. Two phases having 0D framework structures were formed in the mother solution. The remaining five phases were obtained when the two phases were aged in resin, indicating that the high-dimensional (1D and 2D) MoO frameworks of the compound were formed under restricted dehydration conditions such as in resin. There were two different conversion routes respectively originating from the two phases formed in the mother solution. The phase sequences in the routes showed snapshots concerning the formation processes of the high-dimensional frameworks.Highlights► Structural diversity of the compound (C3H12N2O)4[Mo36O112(H2O)16-m]·nH2O was revealed. ► Seven structural phases with MoO framework structures of various dimensionalities were found. ► Seven phases showed snapshots concerning formation processes of high-dimensional frameworks. ► Initially formed hydrogen bonds guided us to obtain the compounds with high-dimensional frameworks. ► Restricted dehydration enabled us to obtain the compounds with high-dimensional frameworks.

The tetrapropylammonium (Pr4N+) salt of [(P2O7)2W30O90]8− was prepared from a 200 mM WVI–10 mM P2O74−–0.5 M HCl–20% (v/v) CH3CN system. The X-ray structural analysis revealed that the [(P2O7)2W30O90]8− anion possessed the same framework as [(P2O7)2Mo30O90]8− isolated by Kortz. The present [(P2O7)2W30O90]8− anion exhibited consecutive one-electron redox waves in neutral CH3CN media, and the one-electron waves were merged into two-electron waves by the presence of H+. These voltammetric properties are entirely different from those of [(P2O7)2Mo30O90]8− that undergoes a two-electron reduction without the aid of H+. The difference in the voltammetric properties can be ascribed to the large pseudo Jahn–Teller (PJT) distortions of MoO6 octahedra in the [(P2O7)2Mo30O90]8− structure.Graphical abstract[(P2O7)2W30O90]8− anion, a tungsto analog of [(P2O7)2Mo30O90]8−, was first prepared. It exhibited consecutive one-electron redox waves in neutral CH3CN media. The voltammetric properties are entirely different from those of [(P2O7)2Mo30O90]8− that undergoes a two-electron reduction without the aid of H+.Highlights► Polyoxotungstate having a B-type cigar-shaped structure is newly prepared. ► The tungsten complex undergoes multi-step one-electron reductions in neutral media. ► This electrochemical behavior is in contrast to that of the molybdenum analog. ► The difference can be ascribed to the pseudo Jahn–Teller distortions.

An α-Keggin-type [ZnW12O40]6− complex was prepared and structurally characterized. Unlike [XW12O40]n− (n = 3−5) complexes that undergo one- and two-electron reductions in neutral and acidic media, respectively, [ZnW12O40]6− showed a four-electron redox wave in acidified CH3CN. The present study demonstrated that the voltammetric properties of the Keggin anions were governed by the bond valence of the μ4-O−W bond as well as the ionic charge, and the four-electron behavior was ascribed to the increase of the bond valence (the decrease of the μ4-O−W distance), owing to the greater X−μ4-O distances. For the Keggin anions with identical ionic charge, the first one-electron redox wave was situated at more positive potentials with an increase of the bond valence. It turned out that the electron density on the W atom was decreased with an increase of the bond valence, because the positive shift of the one-electron wave parallels the downfield shift of the 183W NMR chemical shift value.

The synthesis and crystal structure of NiMoO4·nH2O were investigated. The hydrate crystallized in the triclinic system with space group P−1, Z=4 with unit cell parameters of a=6.7791(2) Å, b=6.8900(2) Å, c=9.2486(2) Å, α=76.681(2)°, β=83.960(2)°, γ=74.218(2)°. Its ideal chemical composition was NiMoO4·3/4H2O rather than NiMoO4·1H2O. Under hydrothermal conditions the hydrate turned directly into α–NiMoO4 above 483 K, giving nanorods thinner than the crystallites of the mother hydrate. On the other hand, it turned into Anderson type of polyoxomolybdate via a solid-solution process in a molybdate solution at room temperature.NiMoO4·nH2O crystallized in the triclinic system with space group P-1, and its ideal chemical composition was NiMoO4·3/4H2O rather than NiMoO4·1H2O.

From an aqueous MoVI–GaIII–HCl system, a colourless complex was isolated as a K+ salt, which consists of a hexaprotonated Anderson-type [Ga(OH)6Mo6O18]3− anion. A yellow complex became kinetically stable by the presence of CH3CN at concentrations of 30–40% (v/v). The X-ray structural analysis revealed that the yellow (NPrn4)4[(GaO4)Mo12O35(OH)] crystal contains an α-Keggin structure and the oxygen atom at an edge-shared contact is protonated. The formation conditions of the Keggin complex were elucidated in relation to those of the Anderson complex by a combined 71Ga NMR and voltammetric study. Evidence was obtained of a spontaneous conversion of [Ga(OH)6Mo6O18]3− to [(GaO4)Mo12O35(OH)]4− in the MoVI–GaIII system.

Hydrothermal synthesis in the M/Mo/O (M=Co,Ni) system was investigated. Novel transition metal tetramolybdate dihydrates MMo4O13·2H2O (M=Co,Ni), having an interesting pillared layer structure, were found. The molybdates crystallize in the triclinic system with space group P−1, Z=1 with unit cell parameters of a=5.525(3) Å, b=7.058(4) Å, c=7.551(5) Å, α=90.019(10)°, β=105.230(10)°, γ=90.286(10)° for CoMo4O13·2H2O, and a=5.508(2) Å, b=7.017(3) Å, c=7.533(3) Å, α=90.152(6)°, β=105.216(6)°, γ=90.161(6)° for NiMo4O13·2H2O The structure is composed of two-dimensional molybdenum-oxide (2D Mo-O) sheets pillared with CoO6 octahedra. The 2D Mo-O sheet is made up of infinite straight ribbons built up by corner-sharing of four molybdenum octahedra (two MoO6 and two MoO5OH2) sharing edges. These infinite ribbons are similar to the straight ones in triclinic-K2Mo4O13 having 1D chain structure, but are linked one after another by corner-sharing to form a 2D sheet structure, like the twisted ribbons in BaMo4O13·2H2O (or in orthorhombic-K2Mo4O13) are.Novel transition metal tetramolybdate dihydrates MMo4O13·2H2O (M=Co,Ni), having an interesting pillared layer structure, were found. The structure was composed of two-dimensional molybdenum-oxide sheets pillared with CoO6 octahedra. Structural comparison with various tetramolybdates was also made to find a key to structural control.

A yellow [(HPO3)2(P2O7)Mo30O90]8− anion was prepared as a tetrapropylammonium (Pr4N+) salt from a 50 mM MoVI−2 mM P2O74−−4 mM HPO32−−0.95 M HCl−60% (v/v) CH3CN system at ambient temperature. The (Pr4N)8[(HPO3)2(P2O7)Mo30O90] salt crystallized in the orthorhombic space group Pnma (No. 62), with a = 30.827(2) Å, b = 22.8060(15) Å, c = 30.928(2) Å, V = 21743(3) Å3, and Z = 4. The structure contained a (P2O7)Mo12O42 fragment derived from the removal of each corner-shared Mo3O13 unit in a polar position from a [(P2O7)Mo18O54]4− structure, and each side of the (P2O7)Mo12O42 fragment was capped by a B-type (HPO3)Mo9O24 unit. The [(HPO3)2(P2O7)Mo30O90]8− anion was characterized by voltammetry and IR, UV−vis, and 31P NMR spectroscopy. Unlike the Keggin and Dawson anions and the parent [(P2O7)Mo18O54]4− anion, the [(HPO3)2(P2O7)Mo30O90]8− anion exhibited two-electron redox waves in CH3CN with and without acid.

1,4-Dimethoxytriptycene (diMeO-TP) and triptycenequinone (TPQ), non-planar donor and acceptor molecules, respectively, were found to form two types of mixed crystals with limited solubility, i.e., (diMeO-TP)x(TPQ)1−x with x = ca.0.25 and (diMeO-TP)x(TPQ)1−x with x = ca.0.97. Crystal structures of the mixed crystals suggested that their characteristic colors, which are different from those of TPQ (yellow) and diMeO-TP (colorless), are caused by local CT interactions between 1,4-benzoquinone and 1,4-dimethoxybenzene moieties in the crystals. The present mixed crystals can be regarded as non-stoichiometric quinhydrone-type CT complexes similar to that formed by TPQ and TPHQ (triptycenehydroquinone).

We have explored several structure-inheriting solid-state reactions (SISSRs) under hydrothermal conditions for syntheses in the Co–Mo–O system. And we found an interesting hydrothermal SISSR from CoMoO4·3/4H2O to high-pressure (hp-) phase of CoMoO4, which enabled us to considerably reduce the severe conditions for the synthesis of hp-CoMoO4. As similar hydrothermal SISSRs are expected to be useful tools for material syntheses, we also briefly discuss them as a means of developing novel material syntheses and designs.The present structure-inheriting solid-state reaction (SISSR) under a hydrothermal condition (ca. 10 bars and 453 K) enables us to markedly reduce the severe conditions for the usual synthesis of hp-CoMoO4 via a solid-state phase transition (50 kbars and ca. 900 K).

We determined the crystal structures of the title compounds H2N(CO)NH–(CH2)n–NH(CO)NH2 with n=4 and 5 to check an idea that the replacement of the terminal methyl group of alkylurea H2N(CO)NH–(CH2)n–CH3 by a ureido group –HN(CO)NH2 is useful to construct two and three-dimensional supramolecular assemblies with unique interlocking networks only from simple and non-rigid molecules via hydrogen bonds. The crystal structures of the two compounds were found to be quite similar to each other and the basic structure motif of each crystal was an infinite, two-dimensional network of the molecules connected via urea-chains formed by two NH⋯O hydrogen bonds (H-bonds) with graph-set C21(4)[R21(6)]. This network (single-network) comprised (2n  +18)-membered open rectangular framework with graph-set R44(2n+18) composed of two molecules as a whole and two ureido groups. Two adjacent single-networks crossed each other to form a double-network characterized by catenane-type interlocks (i.e. an interpenetrating undulating two-dimensional (6,3) double-network). Neighboring double-networks related by center of symmetry were bridged by dimer-type H-bonds with graph-set R22(8) to give infinite, three-dimensional supramolecular assembly.

Many of alkylureas (H2NCONHCnH2n+1, Cn–U) exhibited phase transitions and those found for C4,8–U had fairly large entropy of transition, indicating formation of quasi-plastic phases. X-ray structure determinations of C2,5–14–U revealed a characteristic common hydrogen bond network to result in the formation of two-dimensional, plate-like supramolecules having crossed arrangement of alkyl groups. The crystal structures of C2,5–14–U were found to be built by stacking the corresponding supramolecules along the a axis. The phase transitions of C10,13,14–U and those of C8,12–U were found to accompany shifts of supramolecules along the crystal c axis and along both the b and c axes, respectively. The transitions found for C4,8–U were proved to be of order–disorder type associated with the disordering of the alkyl groups. The transition of C4–U was found to be associated with a drastic twist of the plane of the NCON moiety. The transition temperatures of C4–U and C8–U were depressed significantly by doping C3–U and C7–U, respectively, to form corresponding mixed crystals.

The hydrothermal syntheses of the alkali metal molybdenum bronzes from starting solids (HxMoO3) with structural affinities to the desired products were investigated. Single-phase potassium blue and red bronzes were prepared by the hydrothermal treatments at around 430 K, and characterized by powder X-ray diffraction, IR spectroscopy, and SEM. The formation processes of these two bronzes during the hydrothermal treatments were found to differ. The blue bronze was formed by a structure-inheriting solid-state route from HxMoO3 with x<0.3x<0.3, whereas the red bronze was formed for x>0.3x>0.3 through a solution dissolution/deposition route via the formation of MoO3+MoO2.

We examined low-temperature synthetic route based on the amorphous nature of giant species to succeed to prepare Cs blue bronze (Cs0.3MoO3), which has never obtained by usual high-temperature methods, at ca. 680 K. Solid solutions (K1−xRbx)0.28MoO3 and (Li1−xNax)0.9Mo6O17 were also obtained at lower temperatures (ca. 670 K). For the latter system consisting of non-isostructural end members, Li0.9Mo6O17-structure type solid solution was formed even when 0.25