Divalent Heterometal Doped Titanium-Oxide Cluster Polymers: Structures, Photoresponse, and Photocatalysis.

Five cluster polymers based on heterometal-doped titanium-oxide cluster (TOC) monomers are reported. The monomers feature Ti10-oxide cluster cores and are connected to the divalent closed-shell heterometal anchors by salicylate ligands. The Sr2+, Ba2+, and Pb2+ dopants cause the monomers to bind head-to-head and generate linear chains, while the Ca2+ and Cd2+ lead to head-to-tail connections and zigzag chains. The cluster polymers are responsive to visible-light up to 565 nm and photo-catalytically active in both H2 evolution and CO2/epoxide cycloaddition reactions. The photo-absorption, photo-charge separation, and photocatalytic properties of the cluster polymers are dependent on the heterometal dopants in order Cd > Pb > Ba > Sr > Ca. Heterometals serve as the catalytic sites in the cluster polymers, which depending on the contribution of the pCB bottom, facilitate photo-charge separation and interfacial charge transfer, further enhancing catalytic activity. The tunable compositions and topologies of the cluster polymers shown herein may inspire the design and synthesis of more multidimensional functional metal-oxide cluster materials for a variety of applications in the future.

Ligation of Titanium-oxide and {Mo2} Units for Solar CO2 Storage.

Two Mo-Ti-mixed oxide clusters, Ti6Mo4 and Ti4Mo4, which contain the {Mo2V} unit commonly observed in many polyoxomolybdates, were successfully synthesized. The introduction of a {Mo2V} dopant into a titanium-oxide cluster (TOC) results in a red shift of the absorption edge, hence leading to a substantial enhancement of visible-light absorption. The band gap electron transition mainly involves the ligand-to-metal charge transfer (LMCT, benzoate-to-Mo) and MoV d-d transition. Both clusters show favorable visible-light responsiveness and charge-separation efficiency. Both serve as heterogeneous photocatalysts and exhibit excellent catalytic activity in CO2/epoxide cycloadditions under very mild conditions. The mechanism study suggests that the catalytically active sites are mainly MoV, and the photogenerated electrons and holes are both involved. Ti6Mo4 exhibits better photocatalytic activity than Ti4Mo4, demonstrating the crucial role of the titanium-oxide core, which corresponds to improved light absorption and charge-separation efficiency. Our findings highlight the potential of the {Mo2V} unit in constructing Mo-Ti-mixed oxide clusters with interesting topologies and excellent solar-light-harvesting activity.

Complexed Semiconductor Cores Activate Hexaniobate Ligands as Nucleophilic Sites for Solar-Light Reduction of CO2 by Water.

Although pure and functionalized solid-state polyniobates such as layered perovskites and niobate nanosheets are photocatalysts for renewable-energy processes, analogous reactions by molecular polyoxoniobate cluster-anions are nearly absent from the literature. We now report that under simulated solar light, hexaniobate cluster-anion encapsulated 30-NiII -ion "fragments" of surface-protonated cubic-phase-like NiO cores activate the hexaniobate ligands towards CO2 reduction by water. Photoexcitation of the NiO cores promotes charge-transfer reduction of NbV to NbIV , increasing electron density at bridging oxo atoms of Nb-µ-O-Nb linkages that bind and convert CO2 to CO. Photogenerated NiO "holes" simultaneously oxidize water to dioxygen. The findings point to molecular complexation of suitable semiconductor "fragments" as a general method for utilizing electron-dense polyoxoniobate anions as nucleophilic photocatalysts for solar-light driven activation and reduction of small molecules.

Soluble Complexes of Cobalt Oxide Fragments Bring the Unique CO2 Photoreduction Activity of a Bulk Material into the Flexible Domain of Molecular Science.

The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co3O4 can serve as tractable yet well-defined functional analogues of bulk cobalt oxide. Notably, the energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the molecular complexes, 1, closely match the valence- and conduction-band (VB and CB) energies of the parent bulk oxides. Use of 1 as a molecular analogue of the parent oxides is demonstrated by its remarkably simple deployment as a cocatalyst for direct Z-scheme reduction of CO2 by solar light and water. Namely, evaporation of an aqueous solution of 1 on TiO2-coated fluorinated tin oxide windows (TiO2/FTO), immersion in wet acetonitrile, and irradiation by simulated solar light under an atmosphere of CO2 give H2, CO, and CH4 in ratios nearly identical to those obtained using 20 nm spinel-Co3O4 nanocrystals, but 15 times more rapidly on a Co basis and more rapidly overall than other reported systems. Detailed investigation of the photocatalytic properties of 1 on TiO2/FTO includes confirmation of a direct Z-scheme charge-carrier migration pathway by in situ irradiated X-ray photoelectron spectroscopy. More generally, the findings point to a potentially important new role for coordination chemistry that bridges the conceptual divide between molecular and solid-state science.

Self-Assembly and Ionic-Lattice-like Secondary Structure of a Flexible Linear Polymer of Highly Charged Inorganic Building Blocks.

Among molecular building blocks, metal oxide cluster anions and their countercations provide multiple options for the self-assembly of functional materials. Currently, however, rational design concepts are limited to electrostatic interactions with metal or organic countercations or to the attachment and subsequent reactions of functionalized organic ligands. We now demonstrate that bridging µ-oxo linkages can be used to string together a bifunctional Keggin anion building block, [PNb2Mo10O40]5- (1), the diniobium(V) analogue of [PV2Mo10O40]5- (2). Induction of µ-oxo ligation between the NbV═O moieties of 1 in acetonitrile via step-growth polymerization gives linear polymers with entirely inorganic backbones, some comprising over 140 000 repeating units, each with a 3- charge, exceeding that of previously reported organic or inorganic polyelectrolytes. As the chain grows, its flexible µ-oxo-linked backbone, with associated countercations, coils into a compact 270 nm diameter spherical secondary structure as a result of electrostatic interactions not unlike those within ionic lattices. More generally, the findings point to new options for the rational design of multidimensional structures based on µ-oxo linkages between NbV═O-functionalized building blocks.

Titanium-Oxide Host Clusters with Exchangeable Guests.

A novel family of water-soluble, polyoxocationic titanium-oxide host-guest clusters are reported herein. They exhibit an unprecedented hexagonal prismatic core structure for hosting univalent cationic guests like K+, Rb+, Cs+ and H3O+. Guest exchange has been studied using 133Cs NMR, showing the flexible pore of a host permits passage of a comparatively larger cation and giving an equilibrium constant of ca. 13 for displacing Rb+ by Cs+. Attractive ion-dipole interaction, depending on host-guest size complementarity, plays a dominant role for the preferential encapsulation of larger alkali-metal cationic guests.

Aqueous Isolation of 17-Nuclear Zr/Hf Oxide Clusters during the Hydrothermal Synthesis of ZrO2 /HfO2.

Novel 17-nuclear Zr/Hf oxide clusters ({Zr17 } and {Hf17 }) are isolated from aqueous systems. In the clusters, Zr/Hf ions are connected through µ3 -O, µ3 -OH, and µ2 -OH linkages into a pinwheel core which is wrapped with SO4 2- , HCOO- , and aqua ligands. Octahedral hexanuclear Zr/Hf oxide clusters ({Zr6 }oct and {Hf6 }oct ) are also isolated from the same hydrothermal system by decreasing the synthesis temperature. Structures, synthetic conditions, vibrational spectra, and ionic conductivity of the clusters are studied. Structural studies and synthesis inspection suggest that formation of {Zr6 }oct and {Zr17 } involves assembly of the same transferable building blocks, but the condensation degree and thermodynamic stability of the products increase with hydrothermal temperature. The role of {Zr6 }oct and {Zr17 } in the formation of ZrO2 nanocrystals are then discussed in the scenario of nonclassical nucleation theory. In addition, the Zr oxide clusters exhibit ionic conductivity owing to the mobility of protons. This study not only adds new members to the Zr/Hf oxide cluster family, but also establishes a connection from Zr4+ ions to ZrO2 in the hydrothermal preparation of zirconium oxide nanomaterials.

CDK5 activator protein p25 preferentially binds and activates GSK3ß.

Glycogen synthase kinase 3ß (GSK3ß) and cyclin-dependent kinase 5 (CDK5) are tau kinases and have been proposed to contribute to the pathogenesis of Alzheimer's disease. The 3D structures of these kinases are remarkably similar, which led us to hypothesize that both might be capable of binding cyclin proteins--the activating cofactors of all CDKs. CDK5 is normally activated by the cyclin-like proteins p35 and p39. By contrast, we show that GSK3ß does not bind to p35 but unexpectedly binds to p25, the calpain cleavage product of p35. Indeed, overexpressed GSK3ß outcompetes CDK5 for p25, whereas CDK5 is the preferred p35 partner. FRET analysis reveals nanometer apposition of GSK3ß:p25 in cell soma as well as in synaptic regions. Interaction with p25 also alters GSK3ß substrate specificity. The GSK3ß:p25 interaction leads to enhanced phosphorylation of tau, but decreased phosphorylation of ß-catenin. A partial explanation for this situation comes from in silico modeling, which predicts that the docking site for p25 on GSK3ß is the AXIN-binding domain; because of this, p25 inhibits the formation of the GSK3ß/AXIN/APC destruction complex, thus preventing GSK3ß from binding to and phosphorylating ß-catenin. Coexpression of GSK3ß and p25 in cultured neurons results in a neurodegeneration phenotype that exceeds that observed with CDK5 and p25. When p25 is transfected alone, the resulting neuronal damage is blocked more effectively with a specific siRNA against Gsk3ß than with one against Cdk5. We propose that the effects of p25, although normally attributed to activate CDK5, may be mediated in part by elevated GSK3ß activity.

The roles of Cdk5-mediated subcellular localization of FOXO1 in neuronal death.

Deficiency of cyclin-dependent kinase 5 (Cdk5) has been linked to the death of postmitotic cortical neurons during brain development. We now report that, in mouse cortical neurons, Cdk5 is capable of phosphorylating the transcription factor FOXO1 at Ser249 in vitro and in vivo. Cellular stresses resulting from extracellular stimulation by H2O2 or ß-amyloid promote hyperactivation of Cdk5, FOXO1 nuclear export and inhibition of its downstream transcriptional activity. In contrast, a loss of Cdk5 leads to FOXO1 translocation into the nucleus: a shift due to decreased AKT activity but independent of S249 phosphorylation. Nuclear FOXO1 upregulates transcription of the proapoptotic gene, BIM, leading to neuronal death, which can be rescued when endogenous FOXO1 was replaced by the cytoplasmically localized form of FOXO1, FOXO1-S249D. Cytoplasmic, but not nuclear, Cdk5 attenuates neuronal death by inhibiting FOXO1 transcriptional activity and BIM expression. Together, our findings suggest that Cdk5 plays a novel and unexpected role in the degeneration of postmitotic neurons through modulation of the cellular location of FOXO1, which constitutes an alternative pathway through which Cdk5 deficiency leads to neuronal death.

Water-Soluble Pentagonal-Prismatic Titanium-Oxo Clusters.

By using solubility control to crystallize the prenucleation clusters of hydrosol, a family of titanium-oxo clusters possessing the {Ti18O27} core in which the 18 Ti(IV)-ions are uniquely connected with µ-oxo ligands into a triple-decked pentagonal prism was obtained. The cluster cores are wrapped by external sulfate and aqua ligands, showing good solubilities and stabilities in a variety of solvents including acetonitrile and water and allowing their solution chemistry being studied by means of electrospray ionization mass spectroscopy, (17)O NMR, and vibrational spectroscopy. Furthermore, this study provides new titanium oxide candidates for surface modifications and homogeneous photocatalysis.

Small Titanium Oxo Clusters: Primary Structures of Titanium(IV) in Water.

For sol-gel synthesis of titanium oxide, the titanium(IV) precursors are dissolved in water to form clear solutions. However, the solution status of titanium(IV) remains unclear. Herein three new and rare types of titanium oxo clusters are isolated from aqueous solutions of TiOSO4 and TiCl4 without using organic ligands. Our results indicate that titanium(IV) is readily hydrolyzed into oxo oligomers even in highly acidic solutions. The present clusters provide precise structural information for future characterization of the solution species and structural evolution of titanium(IV) in water and, meanwhile, are new molecular materials for photocatalysis.

Alkali Halide Cubic Cluster Anions ([Cs8X27]19-, X = Cl, Br) Isolated from Water.

Herein we report the syntheses and the X-ray structure of [Cs8X27]19- (X = Cl, Br) clusters, the first binary cluster anions isolated in bulk crystal structures. They were obtained by electrostatic capture and face-directed recognition of the prenucleation [CsmCln](n-m)- clusters from water solutions, using [M4(OH)8(OH2)16]8+ (M = ZrIV or HfIV) as the counter cations. These compounds have been thoroughly characterized with a variety of techniques including vibrational spectroscopy and superionic conductivity analysis. This work not only provides structural models for a better understanding of the nucleation of binary materials but also shows that magic number binary clusters adopting a cubic lattice structure do form, in agreement with the time-honored theoretical and spectroscopic predictions.

A Post-Functionalizable Iso-Polyoxotitanate Cage Cluster.

During solvothermal alcoholysis of a mixture of TiI4 and Ti(O(i)Pr)4, a {I@Ti22} cage cluster encapsulating an OH and iodide guests is crystallized. The {I@Ti22} host-guest cluster surface is postfunctionalizable with catecholate and carboxylate ligands. The synthetic details, structural characterization, spectroscopic properties of the obtained cages clusters are provided. The present study provides candidates for modeling ligand exchange and electron-hole transfer at the titanate nanoparticle surface, and meanwhile offers new opportunities for understanding the TiO2 nanocrystalline formation in solvothermal processes.

Counteranion-Stabilized Titanium(IV) Isopolyoxocationic Clusters Isolated from Water.

A novel titanium(IV) oxo cluster comprised solely of Ti, O, and H atoms, [Ti6(Oµ)8(OtH2)20](8+) (Ti6) was synthesized in high yield via controlled hydrolysis and condensation of TiX4 (X = Cl, Br) in the presence of TBAX (TBA = tetrabutylammonium; X = Cl, Br) from water, while reactions of TiI4 and TBAI yielded [Ti8O12(OH2)24](8+) (Ti8). The structures and compositions of the clusters were determined by single-crystal X-ray crystallography, powder X-ray diffraction, inductively coupled plasma atomic emission spectrometry, and energy-dispersive spectrometry. Ti6 is comprised of six-coordinated titanium(IV) atoms bridged with µ2-O atoms, structurally similar to a typical Lindqvist polyoxometalate. On the basis of a structural comparison of Ti6 and Ti8, density functional theory calculations, and spectroscopic analysis, it is evident that both clusters are stabilized by halide counteranions via the formation of hydrogen bonds. This study not only presents the second example of a titanium(IV) isopolyoxocationic cluster isolated from water but also suggests that counteranions are generally important for synthesizing molecular fragments of titanium oxides.

Effects of Substitutional Dopants on the Photoresponse of a Polyoxotitanate Cluster.

In this paper, using a simple method, 17 isostructural polyoxotitanates (POTs) were synthesized, including the pristine [Ti12O16(O(i)Pr)16], the monodefected [Ti11O13(O(i)Pr)18], and the heterometal-doped [Ti11O14(O(i)Pr)17(ML)] (M = Mg, Ca, Zn, Cd, Co, or Ni; L = Cl, Br, I, or NO3). The electronic structures of these POTs were determined by UV-vis spectroscopy and DFT calculations. Upon UV irradiation of the POTs, electron spin resonance showed the formation of Ti(III) under anaerobic conditions and superoxide (O2(•-)) in the presence of O2. The photoactivities of the POTs were then probed with Ti(III) production and short-circuit photocurrent experiments. The photophysical processes were studied using steady-state and transient photoluminescence. The results show that within the very similar structures, the deexcitation processes of the photoexcited POTs can be greatly affected by the dopants, which result in enhanced or decreased photoactivities. Co and Ni doping enhances the absorption of the visible light accompanied by serious loss of UV photoactivities. On the other hand, a Ti vacancy (in [Ti11O13(O(i)Pr)18]) does not reduce the band gap of a POT but improves the UV photoactivities by serving as surface reaction site. The POTs were then used as molecular models of titanium oxide nanoparticles to understand some important issues relevant to doped titanate, i.e., coordination environment of the dopant metal, electronic structure, photoactivities, and photophysical processes. Our present findings suggest that for solar energy harvesting applications of titanium oxides like photocatalysis and solar cells substitution of titanium atoms by transition metal ions (like Co and Ni) to extend the absorption edges may not be an efficient way, while loading of Ti vacancies is very effective.

A cryptand-like Ti-coordination compound with visible-light photocatalytic activity in CO2 storage.

A cryptand-like Ti-coordination compound, namely Ti12Cs, comprising two Ti6-salicylate cages and hosting two Cs+ ions, was synthesized by the solvothermal method. It exhibits strong visible-light absorption with an absorption band edge of 652 nm, attributed to the electron transition from salicylate ligands to Ti ions. Electrochemical impedance, visible-light transient photocurrent response, and photoluminescence spectra confirm that Ti12Cs has excellent visible-light response and charge-separation properties. Ti12Cs can be used as a heterogeneous and recyclable photocatalyst for CO2/epoxide cycloaddition, with high utilization efficiency of visible-light under mild conditions. The mechanism investigation points to a synergistic effect of photocatalysis and Lewis acid catalysis.

Enhancing the photocatalytic performance of a rutile unit featuring a titanium-oxide cluster by Pb2+ doping.

Titanium-oxide clusters (TOCs) are well-defined molecular models for TiO2 materials and provide the opportunity to study the structure-activity relationships of TiO2. Here, we report a new Pb-doped TOC, Ti12Pb2, which resembles a two-layer decker of the {TiTi6} structural units of rutile TiO2 with two Ti4+ ions replaced by two Pb2+ ions. Its electronic structure, photoresponse, and photocatalytic performances were investigated and compared with those of the Ti14 cluster, which is isostructural to Ti12Pb2. Our results indicate that Pb2+ does not affect the electronic structure, but it greatly enhances the photocatalytic activity by improving the charge-separation and interfacial charge-transfer properties of the TOC. The successful synthesis of Ti12Pb2 highlights the roles of closed-shell heterometal ions in the construction of new TOCs. Our mechanism may be an inspiration for understanding the structure-activity relationships of closed-shell heterometal-doped TiO2.

A butterfly-like lead-doped titanium-oxide compound with high performance in photocatalytic cycloaddition of CO2 to epoxide.

The cycloaddition reaction of CO2 to epoxides is quite promising for CO2 capture and storage as well as the production of value-added fine chemicals. Herein, a novel atomically precise lead-doped titanium-oxide cluster with the formula Ti10Pb2O16(phen)4(Ac)12(DMF)2 (denoted as Ti10Pb2; phen = 1,10-phenanthroline; Ac = acetate; DMF = dimethylformamide) was synthesized through a facile solvothermal process, and is a molecular photocatalyst with surface-anchored main-group metal active sites. Its structure was characterized by single-crystal X-ray diffraction and other complementary techniques. Ti10Pb2 showed high photo-response and charge-separation efficiency under simulated sunlight irradiation. Ti10Pb2 was successfully used in the cycloaddition reaction of CO2 with epoxides under solvent-free conditions. While its catalytic activity due to the Lewis acidity was moderate, simulated solar light irradiation further enhanced the reaction rate, demonstrating the synergistic effect of photocatalysis and Lewis-acid thermocatalysis.

New picolinate-functionalized titanium-oxide clusters: syntheses, structures and photocatalytic H2 evolution.

Two nanosized titanium-oxide clusters (TOCs), Ti12(µ2-O)14(µ3-O)4PA16 (1; PA = 2-picolinate) and Ti12(µ2-O)18PA18 (2) were synthesized by using 2-picolinic acid and Ti(OiPr)4 in one-pot reactions. Their structures were determined using single-crystal X-ray diffractometry. Although both have the same core composition of Ti12O18, 1 exhibited superior H2 evolution activity of up to 180 µmol h-1 g-1, which is nearly eight times faster than 2. Mechanism studies revealed that 1 could induce the assembly of 2.3 nm PtNPs into 10-30 nm supra-nanoparticle structures, which contributed to the increased H2 evolution rate.

Effects of organic ammonium cations on the isolation of {Ti4} cyclic clusters from water: an 17O NMR study.

For the preparation of aqueous Ti-oxo clusters (TOCs) without organic ligands, organic ammonium cations (OACs) are important, but the mechanism is obscure. Here, the Ti4+/SO42- system was chosen as a model to investigate the roles of OACs. Three {Ti4} clusters were isolated from this system with different cationic additives. Structural analysis revealed a structural role of OACs: the hierarchically ordered array of the OACs in the organic layers was responsible for the stability and formation of crystalline precipitates. 17O NMR spectroscopy was used to study the interactions of OACs and SO42- with the Ti-oxo species in water under highly acidic conditions. SO42- enhanced the condensation of Ti4+ by binding to the TOC species. OACs could not enhance the condensation of Ti4+ but could change the solution speciation and especially promote the formation of {Ti18O27} clusters. We speculate that OACs interact with the cationic/neutral TOCs through anionic bridges to change the speciation. Under the synthetic conditions of {Ti4}, {Ti18O27} is the native TOC in solutions but {Ti4} is not.