Crown ether-like octanuclear molybdenum(V) clusters for cation binding and gas adsorption.

Octanuclear polyoxomolybdenum-based porous materials, Na8[Mo8O8(µ2-O)8(µ2-OH)8(3-apz)4]2·26H2O (1, 3-Hapz = 3-aminopyrazole), K8[Mo8O8(µ2-O)8(µ2-OH)8(3-apz)4]2·7H2O (2) and (NH4)4[Mo8O8(µ2-O)8(µ2-OH)4(3-apz)8]·20.5H2O (3), have been successfully synthesized by a hydrothermal method and fully characterized. X-ray structural analyses show that microporous materials 1-3 contain round pores formed by octanuclear molybdenum-oxygen groups connected sequentially with pore sizes of 4.0, 4.0, and 4.8 Å, respectively. Both 1 and 2 are composed of two {Mo8} rings, which are connected by strong intramolecular hydrogen bonds between bridging hydroxy groups and oxygen atoms to form dimeric structures. The central pores in 1 and 2 are occupied by Na+ and K+, respectively, while they are empty in 3. This reflects the structural expansion and contraction effects induced by different cations. Through intermolecular stacking, 1-3 also exhibit channels with sizes of 14.0 × 6.4, 4.6 × 2.6, and 5.4 × 5.4 Å, respectively, which were used for the studies of gas adsorption. The results show that 1-3 can selectively adsorb CO2 and O2, including the empty hole in 3, while they show little or no affinity for gases H2, N2, and CH4. Moreover, an additional polyoxomolybdenum-based species (Mo8O26)n·4n(3-H2apz) (4) has been obtained with protonated 3-aminopyrazole in the absence of a reducing agent, which can serve as an intermediate for the polyoxomolybdenum-based porous products.

Chiroptical Activity Amplification of Chiral Metal Nanoclusters via Surface/Interface Solidification.

It remains a grand challenge to amplify the chiroptical activity of chiral metal nanoclusters (NCs) although it is desirable for fundamental research and practical application. Herein, we report a strategy of surface/interface solidification (SIS) for enhancing the chiroptical activity of gold NCs. Structural analysis of [Au19(2R,4R/2S,4S-BDPP)6Cl2]3+ (BDPP is 2,4-bis(diphenylphosphino)pentane) clusters reveals that one of the interfacial gold atoms is flexible between two sites and large space is present on the surface, thus hampering chirality transfer from surface chiral ligands to metal core and leading to low chiroptical activity. Following SIS by filling the flexible sites and replacing chlorides with thiolate ligands affords another pair of [Au20(2R,4R/2S,4S-BDPP)6(4-F-C6H4S)2]4+, which shows a more compact and organized structure and thus an almost 40-fold enhancement of chiroptical activity. This work not only provides an efficient approach for amplifying the chiroptical activity of metal nanoclusters but also highlights the significance of achiral components in shaping chiral nanostructures.

Analyses of the electronic structures of FeFe-cofactors compared with those of FeMo- and FeV-cofactors and their P-clusters.

The electronic structures of FeFe-cofactors (FeFe-cos) in resting and turnover states, together with their PN clusters from iron-only nitrogenases, have been calculated using the bond valence method, and their crystallographic data were reported recently and deposited in the Protein Data Bank (PDB codes: 8BOQ and 8OIE). The calculated results have also been compared with those of their homologous Mo- and V-nitrogenases. For FeFe-cos in the resting state, Fe1/2/4/5/6/7/8 atoms are prone to Fe3+, while the Fe3 atom shows different degrees of mixed valences. The results support that the Fe8 atom at the terminal positions of FeFe-cos possesses the same oxidation states as the Mo3+/V3+ atoms of FeMo-/FeV-cos. In the turnover state, the overall oxidation state of FeFe-co is slightly reduced than those in the resting species, and its electronic configuration is rearranged after the substitution of S2B with OH, compatible with those found in CO-bound FeV-co. Moreover, the calculations give the formal oxidation states of 6Fe2+-2Fe3+ for the electronic structures of PN clusters in Fe-nitrogenases. By the comparison of Mo-, V- and Fe-nitrogenases, the overall oxidation levels of 7Fe atoms (Fe1-Fe7) for both FeFe- and FeMo-cos in resting states are found to be higher than that of FeV-co. For the PN clusters in MoFe-, VFe- and FeFe-proteins, they all exhibit a strong reductive character.

Protonated and deprotonated vanadyl imidazole tartrates for the mimics of the vanadium coordination in the FeV-cofactor of V-nitrogenase.

Chiral imidazole-based oxidovanadium tartrates (H2im)2[Δ,Λ-VIV2O2(R,R-H2tart)(R,R-tart)(Him)2]·Him (1, H4tart = tartaric acid, Him = imidazole) and [Λ,Λ-VIV2O2(R,R-tart)(Him)6]·4H2O (2) and their corresponding enantiomers (H2im)2[Λ,Δ-VIV2O2(S,S-H2tart)(S,S-tart)(Him)2]·Him (3) and [Δ,Δ-VIV2O2(S,S-tart)(Him)6]·4H2O (4) were obtained in alkaline solutions. Interestingly, the tartrates chelate with vanadium bidentately through α-alkoxy/α-hydroxy and α-carboxy groups and imidazole coordinates monodentately through nitrogen atom. It is worth noting that complexes 1 and 3 contain both protonated α-hydroxy and deprotonated α-alkoxy groups simultaneously, which have short V-Oα-alkoxy distances [1.976(4)av Å in 1-4] and long V-Oα-hydroxy distances [2.237(3)av Å in 1 and 2.230(2)av Å in 3]. There is an interesting strong intramolecular hydrogen bond [O(11)⋯O(1) 2.731(5) Å] between the two parts in 1 and 3. The protonated V-O distances are closer to the average bond distance in reported FeV-cofactors (FeV-cos, V-Oα-alkoxy 2.156av Å) in VFe proteins, which corresponds to the feasible protonation of coordinated α-hydroxy in R-homocitrate in V-nitrogenase, showing the homocitrate in the mechanistic model for nitrogen reduction as a secondary proton donor. Furthermore, vibrational circular dichroism (VCD) and IR spectra of 1-4 pointed out the disparity between the characteristic vibrations of the C-O and C-OH groups clearly. EPR experiment and theoretical calculations support +4 oxidation states for vanadium in 1-4. Solution 13C {1H} NMR spectra and CV analyses exhibited the solution properties for 1 and 2, respectively, which indicates that there should be a rapid exchange equilibrium between the protonated and deprotonated species in solutions.

Asymmetric dinuclear, hexanuclear and octanuclear oxovanadium citrates with triazolates: novel mixed-ligands and mixed-valence complexes.

The triazolate-assisted asymmetric dinuclear oxovanadium(IV) citrate [V2O2(cit)(Hdatrz)3]·5H2O (1, H4cit = citric acid, Hdatrz = 1H-1,2,4-triazole-3,5-diamine) and its additive salt [V2O2(cit)(Hdatrz)3][V2O2(cit)2]½·2H2datrz·9.5H2O (2) and the polymerized hexanuclear product [V6O6(µ3-O)2(cit)2(Hdatrz)4]·4H2O (3) have been isolated at different temperatures, respectively. Adduct 2 shows strong evidence for the conversion of a symmetric dinuclear oxovanadium(IV) citrate to a mixed-ligand asymmetric oxovanadium(IV) citrate. Moreover, a fully oxidized trinuclear vanadium(V) species [V3O6(µ2-OH)(µ3-O)(Hdatrz)2]·4.5H2O (4) has also been isolated as a quasi-intermediate product of 3 without the coordination of citrate. Intriguingly, an octanuclear mixed-valence oxovanadium(V/IV) citrate K2{[VIV/V2O2(cit)(Hdatrz)(datrz)]2[VIV2O2(cit)(Hdatrz)(datrz)]2}·27.5H2O (5) has been obtained with different vanadium units, where dinuclear mixed-ligands and mixed-valence oxovanadium(IV/V) citrates [VIV/V2O2(cit)(Hdatrz)(datrz)] (5a) and [VIV2O2(cit)(Hdatrz)(datrz)] (5b) have been trapped. Citrate adopts a µ2-η1:η1:η1:η2 coordination mode in 1, 2 and 5, while a µ3-η1:η1:η1:η2 fashion has been observed in 3. Unlike 1-4, complex 5 contains both protonated and deprotonated triazolates simultaneously, where four triazolates further coordinate in a µ3-η1:η1:η1 manner to construct an octanuclear unit. These different structural features in 1-5 are dominated by flexible multidentate citrates and protonated/deprotonated triazolates, showing their synergistic effects. Furthermore, 1 exhibits a rectangular channel, showing preferential adsorption of O2 and CO2 over gases N2, H2, and CH4.

Octa- and Tetradecanuclear Mixed-Valent Vanadium(III/IV) Malates with Triazoles: Gas Adsorptions, Chiral Interface Recognition, and In Situ Decarboxylation.

Spontaneous resolution has been found for a pair of cage-like octanuclear mixed-valent vanadium(III/IV) malate enantiomers [Δ-VIII4VIV4O5(R-mal)6(Hdatrz)6]·44.5H2O (ΔR-1) and [Λ-VIII4VIV4O5(S-mal)6(Hdatrz)6]·38.5H2O (ΛS-1, H3mal = malic acid; Hdatrz = 3,5-diamino-1,2,4-triazole). Their racemic reduced product K7[VIII5VIV3O5(R,S-mal)6(trz)6]·17H2O (2) and tetradecanuclear species K11[VIII3VIV11O11(R,S-mal)12(atrz)6]·45.5H2O (3) (Htrz = 1,2,3-triazole, Hatrz = 3-amino-1,2,4-triazole) have also been obtained. In situ decarboxylation happens for 3-amino-1,2,4-triazole-5-carboxylic acid (H2atrzc) to yield 3-amino-1,2,4-triazole under hydrothermal conditions. Both 1 and 2 show an interesting bicapped-triangular-prismatic {V8O5(mal)6} building block, which further decorates symmetrically with three [VIV2O2(R,S-mal)2]2- units to construct a pinwheel-like {V14}-cluster, 3. Bond valence calculation sum (BVS) analyses manifest that the oxidation states of bicapped V atoms are +3 in 1-3, while the other V atoms in the {V6O5} core are ambiguous between +3 and +4 states with strong electron delocalization. Intriguingly, the triple helical chains in 1 associate in parallel to generate an amine-functionalized chiral polyoxovanadate (POV)-based supramolecular open-framework. The diameter of the interior channel is 13.6 Å, showing preferential adsorption of CO2 over gases N2, H2, and CH4. Importantly, homochiral framework ΔR-1 is capable of performing chiral interface recognition for R-1,3-butanediol (R-BDO) through host-guest interactions, as confirmed by the structural analysis of the host-guest complex ΔR-1·3(R-BDO). There are six R-BDO molecules in the channel of ΔR-1.

Crown ether-like discrete clusters for sodium binding and gas adsorption.

Hexanuclear polyoxomolybdenum-based discrete supermolecules Nax[MoV6O6(µ2-O)9(Htrz)6-x(trz)x]·nH2O (x = 0, n = 15, 1; x = 1, n = 12, 2; x = 2, n = 10, 3; x = 2, n = 49, 4; Htrz = 1H-1,2,3-triazole) have been prepared and fully characterized with different amounts of sodium cations inside and outside the intrinsic holes. Structural analyses demonstrate that they all exist a triangular channel constructed by six molybdenum-oxygen groups with inner diameters of 2.86 (1), 2.48 (2), and 3.04 (3/4) Å, respectively. Zero, one, or two univalent enthetic guest Na+ have been hosted around the structural centers, which reflect the expansion and contraction effects at microscopic level. Water-soluble species can serve as crown ether-like metallacycles before and after the sodium binding. Diverse nanoscale pores are further formed through intermolecular accumulations with hydrogen bonding. Gas adsorption studies indicate that 2-4 can selectively adsorb CO2 and O2 but have little or even no affinities toward H2, N2, and CH4. Theoretical calculations corroborate the roles of Na+ and auxiliary ligand with different states in bond distances, molecular orbitals, electrostatic potentials, and lattice energies in these discrete clusters. The binding orders of sodium cations in 2-4 are similar with the classical crown ethers, where 2 is the strongest one with 2.226(4)av Å for sodium cation bonded to six O atoms.

Decorating an Anticuboctahedral Copper Kernel with Labile Surface Coatings for Controlling Optical and Catalytic Properties.

Manipulating the interfacial/surface structure of ligand-stabilized atomically precise metal nanoclusters (NCs) is one of the central tasks in nanoscience because surface motifs are directly related to key properties of nanomaterials. Although great progress has been made in engineering the surface of gold and silver nanoclusters, parallel studies on lighter copper analogues hitherto remain unexplored. In this work, we report the design, synthesis, and structure of a new class of copper nanoclusters featuring virtually identical kernels but different surface motifs. The four Cu29 nanoclusters share the same Cu13 kernel with unprecedented anticuboctahedral architecture. Finely modulating synthetic parameters endows the Cu13 core with diverse surface structures, thus affording the Cu29 series with labile surface coatings. More interestingly, the slight surface modification results in distinct optical and catalytic properties of the cluster compounds, highlighting the importance of the surface structure in shaping the behaviors of copper nanomolecules. This work not only exemplifies the efficiency of surface engineering for controlling properties of well-defined copper nanoclusters but also provides a new family of Cu materials with a clear molecular structure and controlled surface motifs that hold great promise in studying structure-property relationships.

Chiral copper-hydride nanoclusters: synthesis, structure, and assembly.

An effective strategy is developed to synthesize a novel and stable layered Cu nanocluster using a one-pot reduction method. The cluster, with a molecular formula of [Cu14(tBuS)3(PPh3)7H10]BF4 which has been unambiguously characterized by single crystal X-ray diffraction analysis, exhibits different structures from previously reported analogues with core-shell geometries. In the absence of chiral ligands, the cluster displays intrinsic chirality owing to the non-covalent ligand-ligand interactions (e.g., C-H⋯Cu interactions and C-H⋯π interactions) to lock the central copper core. The interlacing of chiral-cluster enantiomers forms a large cavity, which lays the foundation for a series of potential applications such as drug filling and gas adsorption. Moreover, the C-H⋯H-C interactions of phenyl groups between different cluster moieties promote the formation of a dextral helix and realization of the self-assembly of nanostructures.

An alkynyl-protected Ag13-xCu6+x nanocluster for catalytic hydrogenation.

A novel alkynyl-stabilized silver-copper alloy nanocluster with the composition of [Ag13-xCu6+x(tBuC6H4CC)14(PPh3)6](SbF6)3 was prepared by the (PPh3)2CuBH4-mediated reduction approach. The nanocluster features a centred disordered-octahedral Ag7Cu6 kernel, which is protected by hybrid alkynyl and triphenylphosphine ligands. Structural comparison of this two-electron nanocluster with other alkynyl-capped Ag/Cu ones suggested that the structure of alkynyl ligands played an important role in dictating the structures of the resulting nanoclusters. The title cluster showed high performance in the catalytic hydrogenation of 4-nitrophenol, indicative of the bright future of cluster-based catalysts.

Successive constructions of regular tetra-, hexa- and octanuclear microporous polyoxovanadates(III) for gas adsorption.

Pyrazole-assisted tetranuclear microporous polyoxovanadates(III) (POVs) (NH4)2K2[V4(µ2-OH)4(ox)4(pz)4]·9H2O (1, ox = oxalate and pz = pyrazole) and (NH4)2Na2[V4(µ2-OH)4(ox)4(4-mpz)4]·7H2O (2, 4-mpz = 4-methylpyrazole) have been constructed in reduced media, along with their triazole neutral hexa- and octanuclear products K2[V6(µ2-OH)6(ox)6(Hdatrz)6]Cl2·29.5H2O (3) and [V8(µ2-OH)8(SO3)8(Hdatrz)8]·38H2O (4, Hdatrz = 1H-1,2,4-triazole-3,5-diamine) successively. Both polyanionic structures of 1 and 2 share similar inorganic building blocks that consist of regular {V4(µ2-OH)4} skeletons with an inner diameter of 2.8 Å, while a paddle wheel-shaped cluster 3 contains a {V6(µ2-OH)6} skeleton with two chlorides encapsulated around the center of the ring, occupying a hole of 3.7 Å. An interesting isolated intrinsic polyoxometalate-based metal-organic framework (POMOF) 4 exists as an octanuclear petaloid-like skeleton {V8(µ2-OH)8(SO3)8} with an inner diameter of 5.2 Å. Bond valence sum calculations manifest that all V ions have severely reduced +3 oxidation states in 1-4, which are supported by charge balance, structural and magnetic data. Moreover, gas adsorptions indicate that 1, 2 and 4 can adsorb CO2 and O2 more favorably than N2, CH4 and H2 gases. Compared with 1 and 2, due to the functionalization of microchannels with Lewis base amino and hydroxy groups and uncoordinated azolate N-donors inside POMOF 4, it should have notable affinities toward CO2 adsorption.