Macroscopic Helical Assembly of One-Dimensional Coordination Polymers: Helicity Inversion Triggered by Solvent Isomerism.

Assembling macroscopic helices with controllable chirality and understanding their formation mechanism are highly desirable but challenging tasks for artificial systems, especially coordination polymers. Here, we utilize solvents as an effective tool to induce the formation of macroscopic helices of chiral coordination polymers (CPs) and manipulate their helical sense. We chose the Ni/R-,S-BrpempH2 system with a one-dimensional tubular structure, where R-,S-BrpempH2 stands for R-,S-(1-(4-bromophenyl)ethylaminomethylphosphonic acid). The morphology of the self-assemblies can be controlled by varying the cosolvent in water, resulting in the formation of twisted ribbons of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-2T) in pure H2O; needle-like crystals of R-,S-Ni(Brpemp)(H2O)2·1/3CH3CN (R-,S-1C) in 20 vol % CH3CN/H2O; nanofibers of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-3F) in 20-40 vol % methanol/H2O or ethanol/H2O; and superhelices of R-,S-Ni(Brpemp)(H2O)·H2O (R-,S-4H or 5H) in 40 vol % propanol/H2O. Interestingly, the helicity of the superhelix can be controlled by using a propanol isomer in water. For the Ni/R-BrpempH2 system, a left-handed superhelix of R-4H(M) was obtained in 40 vol % NPA/H2O, while a right-handed superhelix of R-5H(P) was isolated in 40 vol % IPA/H2O. These results were rationalized by theoretical calculations. Adsorption studies revealed the chiral recognition behavior of these compounds. This work may contribute to the development of chiral CPs with a macroscopic helical morphology and interesting functionalities.

Layer or Tube? Uncovering Key Factors Determining the Rolling-up of Layered Coordination Polymers.

Nanotubular materials have garnered considerable attention since the discovery of carbon nanotubes. Although the layer-to-tube rolling up mechanism has been well recognized in explaining the formation of many inorganic nanotubes, it has not been generally applied to coordination polymers (CPs). To uncover the key factors that determine the rolling-up of layered CPs, we have chosen the Co/R-, S-Xpemp [Xpemp = (4-X-1-phenylethylamino)methylphosphonic acid, X = H, F, Cl, Br] systems and study how the weak interactions influence the formation of layered or tubular structures. Four pairs of homochiral isostructural compounds R-, S-Co(Xpemp)(H2O)2 [X = H (1H), F (2F), Cl (3Cl), Br (4Br)] were obtained with tubular structures. The inclusion of 3,3'-azobipyridine (ABP) guest molecules led to compounds R-, S-[Co(Xpemp)(H2O)2]4·ABP·H2O with layered structures when X was Cl (5Cl) and Br (6Br), but tubular compounds 1H and 2F when X was H and F. Layered structures were also obtained for racemic compounds meso-Co(Xpemp)(H2O)2 [X = F (7F), Cl (8Cl), Br (9Br)] using racemic XpempH2 as the reaction precursor, but not when X = H. A detailed study on R-6Br revealed that layer-to-tube transformation occurred upon removal of ABP under hydrothermal conditions, forming R-4Br with a tubular structure. Similar layer-to-tube conversion did not occur in organic solvents. The results demonstrate that weak interlayer interactions are a prerequisite but not sufficient for the rolling-up of the layers. In the present cases, water also provides a driving force in the layer-to-tube transformation. The experimental results were rationalized by theoretical calculations.

Synthesis of the Elusive S = 1/2 Star Structure: A Possible Quantum Spin Liquid Candidate.

Materials with two-dimensional, geometrically frustrated, spin-1/2 lattices provide a fertile playground for the study of intriguing magnetic phenomena such as quantum spin liquid (QSL) behavior, but their preparation has been a challenge. In particular, the long-sought, exotic spin-1/2 star structure has not been experimentally realized to date. Here we report the synthesis of [(CH3)2(NH2)]3[CuII3(µ3-OH)(µ3-SO4)(µ3-SO4)3]·0.24H2O with an S = 1/2 star lattice. On the basis of the magnetic susceptibility and heat capacity measurements, the layered Cu-based compound exhibits antiferromagnetic interactions but no magnetic ordering or spin freezing down to 2 K. The spin-frustrated material appears to be a promising QSL candidate.

A Mixed-Valent Iron (II/III) Diamond Chain with Single-Ion Anisotropy.

The geometrically frustrated diamond spin chain system has yielded materials with a diversity of interesting magnetic properties but is predominantly limited to compounds with single-spin components. Here, we report the compound [(CH3)2NH2]6[FeIII4FeII2(µ3-O)2(µ3-OH)2(µ3-SO4)8] (1), which features the mixed-valent iron(II/III) diamond chain: ∞[FeIII-(FeIII)2-FeIII-(FeII)2]. 57Fe Mössbauer spectroscopy shows that two-thirds of the total spins in the ∞[FeIII4FeII2] diamond chain are spin-5/2 (high-spin FeIII), while the remaining one-third are spin-2 (high-spin FeII). To date, 1 is the only diamond-chain compound composed of more than one type of dimer, namely, (FeIII)2 and (FeII)2. On the basis of temperature-dependent 57Fe Mössbauer spectroscopy data, an alternating noncollinear 90° magnetic structure is proposed. Both the (FeIII)2 and (FeII)2 dimers are antiferromagnetically coupled and align in the direction along the chain axis ≈ [010], whereas the moments of the bridging FeIII monomers are oriented orthogonally. The spin canting, arising from the anisotropy of the FeII ions, leads to ferrimagnetic ordering at low temperatures.

Sulfate-Bridged, Octanuclear Chromic and Ferric Wheels.

Two new amine-templated transition metal-based sulfates, [(CH3)2NH2]17.4 [SO4]0.7 [MIII8(µ2-OH)8(µ2-SO4)16] where M = Cr and Fe, have been synthesized via mild solvothermal synthesis. The compounds are isostructural and were refined in the monoclinic space group P21/n. They feature the rare sulfate-bridged inorganic molecular wheels [CrIII8(OH)8(SO4)16]16- and [FeIII8(OH)8(SO4)16]16-. In both the octanuclear chromic (J = -2.4 cm-1 based on Hex = -J Sî · Sĵ convention) and ferric wheels (J = -38.3 cm-1), the coupling between the adjacent metal ions is antiferromagnetic giving spin-singlet ground states. The variation in the magnitude of the exchange coupling constants is due to the differences in the superexchange mechanisms, namely, a π-pathway for the Cr- and a σ-pathway for the Fe-wheel cluster.

A large spin, magnetically anisotropic, octanuclear vanadium(iii) wheel.

A large spin, magnetically anisotropic, octanuclear vanadium(iii) wheel-like cluster has been synthesized. The coupling between adjacent VIII ions is ferromagnetic giving a ground state of St = 8, the largest known for a vanadium-based complex.

Dissecting Porosity in Molecular Crystals: Influence of Geometry, Hydrogen Bonding, and [π···π] Stacking on the Solid-State Packing of Fluorinated Aromatics.

Porous molecular crystals are an emerging class of porous materials that is unique in being built from discrete molecules rather than being polymeric in nature. In this study, we examined the effects of molecular structure of the precursors on the formation of porous solid-state structures with a series of 16 rigid aromatics. The majority of these precursors possess pyrazole groups capable of hydrogen bonding, as well as electron-rich aromatics and electron-poor tetrafluorobenzene rings. These precursors were prepared using a combination of Pd- and Cu-catalyzed cross-couplings, careful manipulations of protecting groups on the nitrogen atoms, and solvothermal syntheses. Our study varied the geometry and dimensions of precursors, as well as the presence of groups capable of hydrogen bonding and [π···π] stacking. Thirteen derivatives were crystallographically characterized, and four of them were found to be porous with surface areas between 283 and 1821 m2 g-1. Common to these four porous structures were (a) rigid trigonal geometry, (b) [π···π] stacking of electron-poor tetrafluorobenzenes with electron-rich pyrazoles or tetrazoles, and

Photoinduced Strain Release and Phase Transition Dynamics of Solid-Supported Ultrathin Vanadium Dioxide.

The complex phase transitions of vanadium dioxide (VO2) have drawn continual attention for more than five decades. Dynamically, ultrafast electron diffraction (UED) with atomic-scale spatiotemporal resolution has been employed to study the reaction pathway in the photoinduced transition of VO2, using bulk and strain-free specimens. Here, we report the UED results from 10-nm-thick crystalline VO2 supported on Al2O3(0001) and examine the influence of surface stress on the photoinduced structural transformation. An ultrafast release of the compressive strain along the surface-normal direction is observed at early times following the photoexcitation, accompanied by faster motions of vanadium dimers that are more complex than simple dilation or bond tilting. Diffraction simulations indicate that the reaction intermediate involved on picosecond times may not be a single state, which implies non-concerted atomic motions on a multidimensional energy landscape. At longer times, a laser fluence multiple times higher than the thermodynamic enthalpy threshold is required for complete conversion from the initial monoclinic structure to the tetragonal lattice. For certain crystalline domains, the structural transformation is not seen even on nanosecond times following an intense photoexcitation. These results signify a time-dependent energy distribution among various degrees of freedom and reveal the nature of and the impact of strain on the photoinduced transition of VO2.

Semifluorinated Alkylphosphonic Acids Form High-Quality Self-Assembled Monolayers on Ag-Coated Yttrium Barium Copper Oxide Tapes and Enable Filamentization of the Tapes by Microcontact Printing.

A custom-designed semifluorinated phosphonic acid, (9,9,10,10,11,11,12,12,13,13,14,14,15,15,16,16,16-heptadecafluorohexadecyl)phosphonic acid (F8H8PA), and a normal hexadecylphosphonic acid (H16PA) were synthesized and used to generate self-assembled monolayers (SAMs) on commercially available yttrium barium copper oxide (YBCO) tapes. In this study, we wished to evaluate the effectiveness of these monolayer films as coatings for selectively etching YBCO. Initial films formed by solution deposition and manual stamping using a non-patterned polydimethylsiloxane stamp allowed for a comparison of the film-formation characteristics. The resulting monolayers were characterized by X-ray photoelectron spectroscopy (XPS), contact angle goniometry, and polarization modulation infrared reflection absorption spectroscopy (PM-IRRAS). To prepare line-patterned (filamentized) YBCO tapes, standard microcontact printing (µ-CP) procedures were used. The stamped patterns on the YBCO tapes were characterized by scanning electron microscopy (SEM) before and after etching to confirm the effectiveness of the patterning process on the YBCO surface and energy-dispersive X-ray spectroscopy (EDX) to obtain the atomic composition of the exposed interface.

Cyclotetrabenzoin: Facile Synthesis of a Shape-Persistent Molecular Square and Its Assembly into Hydrogen-Bonded Nanotubes.

Cyanide-catalyzed benzoin condensation of terephthaldehyde produces a cyclic tetramer, which we propose to name cyclotetrabenzoin. Cyclotetrabenzoin is a square-shaped macrocycle ornamented with four α-hydroxyketone functionalities pointing away from the central cavity, the dimensions of which are 6.9×6.9 Å. In the solid state, these functional groups extensively hydrogen bond, resulting in a microporous three-dimensional organic framework with one-dimensional nanotube channels. This material exhibits permanent-albeit low-porosity, with a Langmuir surface area of 52 m(2) g(-1) . Cyclotetrabenzoin's easy and inexpensive synthesis and purification may inspire the creation of other shape-persistent macrocycles and porous molecular crystals by benzoin condensation.

Mesoporous Fluorinated Metal-Organic Frameworks with Exceptional Adsorption of Fluorocarbons and CFCs.

Two mesoporous fluorinated metal-organic frameworks (MOFs) were synthesized from extensively fluorinated tritopic carboxylate- and tetrazolate-based ligands. The tetrazolate-based framework MOFF-5 has an accessible surface area of 2445 m(2) g(-1), the highest among fluorinated MOFs. Crystals of MOFF-5 adsorb hydrocarbons, fluorocarbons, and chlorofluorocarbons (CFCs)-the latter two being ozone-depleting substances and potent greenhouse species-with weight capacities of up to 225%. The material exhibits an apparent preference for the adsorption of non-spherical molecules, binding unusually low amounts of both tetrafluoromethane and sulfur hexafluoride.

Adsorption of fluorinated anesthetics within the pores of a molecular crystal.

Commonly used inhalation anesthetics-enflurane, isoflurane, sevoflurane, halothane, and methoxyflurane-are adsorbed within the pores of a porous fluorinated molecular crystal to the tune of up to 73.4(±0.2)% by weight. Uptake of all studied anesthetics is quite fast, typically reaching saturation in less than three minutes.

Two distinct redox intercalation reactions of hydroquinone with porous vanadium benzenedicarboxylate MIL-47.

One of the enticing features of metal-organic frameworks (MOFs) is the potential to control the chemical and physical nature of the pores through postsynthetic modification. The incorporation of redox active guest molecules inside the pores of the framework represents one strategy toward improving the charge transport properties of MOFs. Herein, we report the vapor-phase redox intercalation of an electroactive organic compound, hydroquinone (H2Q) or benzene-1,4-diol, into the channels of the host [V(IV)O(bdc)], (bdc =1,4-benzenedicarboxylate) conventionally denoted as MIL-47. The temperatures and especially the atmosphere in which the reactions took place were found to determine the products. In ambient atmosphere, quinhydrone charge-transfer complexes are formed inside the channels. Under anhydrous conditions, the framework itself was functionalized by a radical anion species derived from the pyrolysis of hydroquinone. Both cases are accompanied by the reduction of V(4+) to V(3+) via single-crystal-to-single-crystal transformations. The products were characterized by single crystal X-ray diffraction, thermogravimetric analysis, infrared spectroscopy, and electron paramagnetic resonance spectroscopy.

Thermally robust and porous noncovalent organic framework with high affinity for fluorocarbons and CFCs.

Metal-organic and covalent organic frameworks are porous materials characterized by outstanding thermal stability, high porosities and modular synthesis. Their repeating structures offer a great degree of control over pore sizes, dimensions and surface properties. Similarly precise engineering at the nanoscale is difficult to achieve with discrete molecules, since they rarely crystallize as porous structures. Here we report a small organic molecule that organizes into a noncovalent organic framework with large empty pores. This structure is held together by a combination of [N-H···N] hydrogen bonds between the terminal pyrazole rings and [π···π] stacking between the electron-rich pyrazoles and electron-poor tetrafluorobenzenes. Such a synergistic arrangement makes this structure stable to at least 250 °C and porous, with an accessible surface area of 1,159 m(2) g(-1). Crystals of this framework adsorb hydrocarbons, CFCs and fluorocarbons-the latter two being ozone-depleting substances and potent greenhouse species-with weight capacities of up to 75%.

Synthesis, crystal structures, magnetic, and thermal properties of divalent metal formate-formamide layered compounds.

A series of layered divalent metal formate compounds, [M(HCOO)2(HCONH2)2] (M = Mn (1Mn), Ni (2Ni), Cu(3Cu), Zn(4Zn), Mg(5Mg)), have been prepared by solvothermal synthesis and their room temperature (RT) and low-temperature (LT) crystal structures, and thermal and magnetic properties determined. All the compounds contain octahedral metal ions connected by four anti-anti formato ligands to form (4,4) nets with the composition of M(HCOO)2. The oxygen atoms from two coordinating formamide ligands above and below the layer complete the MO6 distorted octahedral coordination. Order-disorder phase transformations involving the formamide ligands were observed in the 1Mn, 2Ni, and 4Zn compounds. Like transitions in related formate structures with perovskite like topology, the transitions correspond to the ordering of the amine groups of the terminating formamide ligands which are disordered at ambient temperature. The magnetic properties of the three magnetic members of the series 1Mn, 2Ni, and 3Cu were investigated using microcrystalline samples, over the temperature range of 2 K-300 K under different applied fields. All compounds belong to antiferromagnetic square lattices with S = 5/2, 1, and 1/2. Exchange constants for a nearest neighbor model are presented here. Specific heat measurements indicate magnetic long-range order at lower temperatures, S = 5/2 (antiferromagnetic) and S = 1 (ferrimagnetic).

Antimony tartrate transition-metal-oxo chiral clusters.

A chiral precursor K2Sb2(L-tartrate)2 was used for the assembly of three homochiral heterometallic antimony(III)-tartrate transition-metal-oxo clusters: Mn(H2O)6[Fe4Mn4Sb6(µ4-O)6(µ3-O)2(l-tartrate)6(H2O)8]·10.5H2O (1), [V4Mn5Sb6(µ4-O)6(µ3-O)2(L-tartrate)6(H2O)13]·9.5H2O (2), and (H3O)[Ni(H2O)6]2[NiCrSb12(µ3-O)8(µ4-O)3(l-tartrate)6]·6H2O (3). In 1 and 2, the antimony tartrate dimer precursor decomposes and recombines to form Sb3(µ3-O)(L-tartrate)3 chiral trimers, which act as scaffolds to construct negative-charged [Fe4Mn4Sb6(µ4-O)6(µ3-O)2(L-tartrate)6](2-) in 1 and neutral [V4Mn5Sb6(µ4-O)6(µ3-O)2(L-tartrate)6] in 2. The scaffold is flexible and accommodates different types of transition-metal-oxo clusters due to the different possible coordination modes of the L-tartrate ligand. In 3, a two-level chiral scaffold Sb3(µ3-O)(L-tartrate)3Sb3 is formed from the precursor. Two such scaffolds are linked by three bridging oxygen atoms to form a cavity occupied by one Cr(3+) ion and one Ni(2+) ion disordered over two positions. Cr(3+) and Ni(2+) ions are located in two face-shared MO6 octahedra at the center of a negatively charged [NiCrSb12(µ3-O)8(µ4-O)3(L-tartrate)6](3-) cluster.

Monodisperse SnO2-coated gold nanoparticles are markedly more stable than analogous SiO2-coated gold nanoparticles.

This manuscript describes the synthesis of uniform monodisperse SnO2-coated gold nanoparticles and examines their colloidal stability as function of pH, with direct comparison to better known and widely used SiO2-coated gold nanoparticles. Aqueous acidic and basic colloidal SnO2-coated and SiO2-coated Au nanoparticle solutions were prepared, and their stability was monitored visually and by UV-vis spectroscopy. Notably, the SnO2-coated Au nanoparticle solutions were stable up to pH 12.5. However, at pH 13 and 14, the SnO2-coated Au nanoparticles underwent aggregation, which could be fully reversed upon neutralization of the solutions. In contrast, the SiO2-coated Au nanoparticle solutions were unstable at pH>10.5, irreversibly producing a precipitate composed of bare Au nanoparticle aggregates having little or no silica coating. Under acidic conditions, sedimentation was observed from both the colloidal SnO2-coated and SiO2-coated Au nanoparticle solutions, but the colloidal solutions could be reconstituted upon neutralization of the acidic solutions. The sedimentation at low pH coincided with the reported isoelectric pH values of SiO2 and SnO2, respectively. From an applications perspective, we are seeking to develop SnO2-coated metal nanoparticles as stable alternatives to the more widely employed SiO2-coated nanoparticles, with a particular emphasis on their use in sensor devices and solar cells.

Giant magnetoresistance and anomalous magnetic properties of highly epitaxial ferromagnetic LaBaCo2O(5.5+δ) thin films on (001) MgO.

Ferromagnetic thin films of the A-site nano-ordered double perovskite LaBaCo(2)O(5.5+δ) (LBCO) were grown on (001) MgO, and their structural and magnetic properties were characterized. The as-grown films have an excellent epitaxial behavior with atomically sharp interfaces, with the c-axis of the LBCO structure lying in the film plane and the interface relationship given by (100)(LBCO)//(001)(MgO) and [001](LBCO)//[100](MgO) or [010](MgO). The as-grown LBCO films exhibit a giant magnetoresistance (54% at 40 K under 7 T) and an anomalous magnetic hysteresis, depending strongly on the temperature and the applied magnetic field scan width.

A homochiral diamond framework constructed from Fe(III) and Mn(II) oxo-clusters supported by Sb(III) tartrate scaffolds.

A homochiral diamond framework was obtained by using a secondary building unit (SBU) comprising an oxo-cluster containing 4Fe(III) and 3Mn(II) ions supported by Sb(3)O tartrate scaffolds. A fully protonated tetratopic cation, tetrakis(4-pyridyloxymethylene)methane (TPOM), acts as a template to arrange the clusters on a diamond lattice.

[M(rac-N-benzyl Asp)(H2O)] (M = Co, Ni): noncentrosymmetric coordination polymeric chains with racemic chiral ligands.

The hydrothermal reaction of fumaric acid, benzylamine, and metal salts yielded M[(rac-N-benzyl-Asp)(H(2)O)] (M = Co, Ni), 1 and 2, and Ni[(rac-N-benzyl-Asp)(H(2)O)(3)]·H(2)O 3. Under mild hydrothermal conditions, Michael addition of benzylamine to fumaric acid led to the formation of a racemic mixture of N-benzyl aspartic acid enantiomers. The noncentrosymmetric structures of 1 and 2 consist of one-dimensional polymeric chains in which metal cations are bridged by d- and l-N-benzyl aspartate anions alternating along the chain. The centrosymmetric structure of 3 is composed of discrete Ni[(rac-N-benzyl-Asp)(H(2)O)(3)] units that are connected by hydrogen bonds into layers. The single layers are homochiral but are hydrogen bonded to similar homochiral layers that contain the N-benzyl aspartate with the opposite handedness. Compounds 1 and 2 showed second harmonic generation (SHG), and their magnetic and thermodynamic properties are described.