Magnetocaloric Properties in Rare-Earth-Based Metal-Organic Frameworks: Influence of Magnetic Density and Hydrostatic Pressure.

Magnetic refrigeration based on the magnetocaloric effect (MCE) in metal-organic frameworks (MOF) is regarded as an attractive approach to create more sustainable cooling systems with higher efficiency than traditional ones. Here, we report a study of the MCE in a series of rare-earth-based MOFs. We have considered the selection of the rare-earth cation by investigating materials belonging to the α-rare-earth polymeric framework-4 (α-RPF-4) MOF family, synthesized with different rare-earth cations, and observed that paramagnetic moment and saturation magnetization play an important role in enhancing the magnetic entropy change ΔSM. The effect of structural parameters has also been considered by investigating three classes of metal-organic Gd materials built up from different types of inorganic secondary building units, including clusters (as in Gd-UiO-66), one-dimensional (as in α-RPF-4), and layered (as in Gd-LRH) conformations. Moreover, the analysis of the hydrostatic pressure influence reveals a significant increase in the -ΔSM and relative cooling power (RCP) with values between 4.3 and 16.3 and 121-509 J/kg. Specifically, the RCPmax found was ∼683 J/kg for Gd-UiO-66, which is higher than the one recently observed for Gd2SiO5 (649.5 J/kg). The present study demonstrates that the engineering of metal-organic framework systems based on high Gd densities may favor enhancing of magnetocaloric responses even at low pressures, thus promoting a new design strategy for efficient cooling devices.

Heterometallic Molecular Complexes Act as Messenger Building Units to Encode Desired Metal-Atom Combinations to Multivariate Metal-Organic Frameworks.

A novel synthetic approach is described for the targeted preparation of multivariate metal-organic frameworks (MTV-MOFs) with specific combinations of metal elements. This methodology is based on the use of molecular complexes that already comprise desired metal-atom combinations, as building units for the MTV-MOF synthesis. These units are transformed into the MOF structural constituents through a ligand/linker exchange process that involves structural modifications while preserving their originally encoded atomic combination. Thus, through the use of heterometallic ring-shaped molecules combining gallium and nickel or cobalt, we have obtained MOFs with identical combinations of the metal elements, now incorporated in the rod-shaped secondary building unit, as confirmed with a combination of X-ray and electron diffraction, electron microscopy, and X-ray absorption spectroscopy techniques.

Study of the Addition Mechanism of 1H-Indazole and Its 4-, 5-, 6-, and 7-Nitro Derivatives to Formaldehyde in Aqueous Hydrochloric Acid Solutions.

The reaction of NH-indazoles with formaldehyde in aqueous hydrochloric acid has been experimentally studied by solution and solid-state nuclear magnetic resonance (NMR) and crystallography. The mechanism of the formation of N1-CH2OH derivatives was determined. For the first time, 2-substituted derivatives have been characterized by multinuclear NMR. Theoretically, calculations with gauge-invariant atomic orbitals (GIAOs) at the Becke three-parameter (exchange) Lee-Yang-Parr B3LYP/6-311++G(d,p) level have provided a sound basis for the experimental observations. The first X-ray structures of four (1H-indazol-1-yl)methanol derivatives are reported.

Building a Green, Robust, and Efficient Bi-MOF Heterogeneous Catalyst for the Strecker Reaction of Ketones.

In this work, we present the new [Bi14(µ3-O)9(µ4-O)2(µ3-OH)5(3,5-DSB)5(H2O)3]·7H2O, BiPF-4 (bismuth polymeric framework─4) MOF, its microwave hydrothermal synthesis, as well as its behavior as a heterogeneous catalyst in the multicomponent organic Strecker reaction. The BiPF-4 material shows a three-dimensional (3D) framework formed by peculiar inorganic oxo-hydroxo-bismutate layers connected among them through the 3,5-dsb (3,5-disulfobenzoic acid) linker. These two-dimensional (2D) layers, built by junctions of Bi7 polyhedra SBU, provide the material of many Lewis acid catalytic sites because of the mixing in the metal coordination number. BiPF-4 is a highly robust, green, and stable material that demonstrates an excellent heterogeneous catalytic activity in the multicomponent Strecker reaction of ketones carried out in one-pot synthesis, bringing a reliable platform of novel green materials based on nontoxic and abundant metal sources such as bismuth.

Functionalized Crystalline N-Trimethyltriindoles: Counterintuitive Influence of Peripheral Substituents on Their Semiconducting Properties.

Three crystalline N-trimethyltriindoles endowed with different functionalities at 3, 8 and 13 positions (either unsubstituted or with three methoxy or three acetyl groups attached) are investigated, and clear correlations between the electronic nature of the substituents and their solid-state organization, electronic properties and semiconductor behavior are established. The three compounds give rise to similar columnar hexagonal crystalline structures; however, the insertion of electron-donor methoxy groups results in slightly shorter stacking distances when compared with the unsubstituted derivative, whereas the insertion of electron-withdrawing acetyl groups lowers the crystallinity of the system. Functionalization significantly affects hole mobilities with the triacetyl derivative showing the lowest mobility within the series in agreement with the lower degree of order. However, attaching three methoxy groups also results in lower hole mobility values in the OFETs (0.022 vs. 0.0014 cm2 V-1 s-1) in spite of the shorter stacking distances. This counterintuitive behavior has been explained with the help of DFT calculations performed to rationalize the interplay between the intramolecular and intermolecular properties, which point to lower transfer integrals in the trimethoxy derivative due to the HOMO wave function extension over the peripheral methoxy groups. The results of this study provide useful insights into how peripheral substituents influence the fundamental charge transport parameters of chemically modified triindole platforms of fundamental importance to design new derivatives with improved semiconducting performance.

Framework Adaptability and Concerted Structural Response in a Bismuth Metal-Organic Framework Catalyst.

Bismuth metal-organic frameworks (MOFs) as heterogeneous catalysts are scarce, and there is little knowledge on the influence of the MOF features on their resulting activity and behavior. Here, we present the synthesis, characterization, and catalytic activity in the one-pot multicomponent Strecker reaction with ketones of three new MOFs prepared with the combination of indium or bismuth and 4,4',4'',4'''-methanetetrayltetrabenzoic acid. One of them, denoted BiPF-7, is very robust and chemically stable, and demonstrates a high activity in the formation of the desired α-aminonitriles. The interaction of the catalytic substrates with the metal centers in this MOF has been crystallographically characterized, showcasing a concerted framework adaptability process that involves structural changes in framework components that are not directly involved in the binding of the guests.

The Effect of Auxiliary Nitrogenated Linkers on the Design of New Cadmium-Based Coordination Polymers as Sensors for the Detection of Explosive Materials.

Three new cadmium-based coordination polymers, denoted [Cd(hfipbb)(4,4'-bipy)] (CdPF-1), [Cd(hfipbb)(2,2'-bipy)] (CdPF-2), and [Cd(hfipbb)(1,10-phen)] (CdPF-3), have been hydrothermally synthesized by using the well-known V-shaped organic linker 4,4'-(hexafluoroisopropylidene)bis(benzoic acid) (H2 hfipbb), together with different nitrogenated auxiliary linkers. Considering the d10 configuration of the transition metal selected, the luminescent properties for these CdPF-n materials were explored, finding that materials CdPF-2 and CdPF-3 act as excellent sensors in the detection of explosive nitro aromatic compounds. The photoluminescence properties of CdPF-2 and CdPF-3 revealed that significant and sensitive fluorescence quenching was observed toward NP (nitrophenol) for CdPF-2 and PA (picric acid) for CdPF-3 in MeOH suspensions.

Untangling the Mechanochromic Properties of Benzothiadiazole-Based Luminescent Polymorphs through Supramolecular Organic Framework Topology.

Two new luminophore polymorphs of 4-bromo-7-(4-nonylphenyl)benzo[c][1,2,5]thiadiazole (1α and 1ß) exhibiting different color emissions, which switch into each other in response to shear force and solvent vapors, are presented and their X-ray structure is determined. Supramolecular organic framework topology (SOFT) studies on the two polymorphic structures led us to conclude that the mechanochromic phase transformation can be explained on the basis of modifications in their respective topological nets: mab and pcu for 1α and 1ß, respectively, as a result of the breaking and restoration of a number of weak supramolecular interactions. The color changes accompanying this transformation have been rationalized with the help of time-dependent density functional theory. We firmly believe that our findings will inspire future research on the design of novel stimuli-responsive organic materials with switchable properties based on their supramolecular interactions by establishing clear SOFT-property relationships.

Three-Dimensional Phthalocyanine Metal-Catecholates for High Electrochemical Carbon Dioxide Reduction.

The synthesis of a new anionic 3D metal-catecholate framework, termed MOF-1992, is achieved by linking tetratopic cobalt phthalocyanin-2,3,9,10,16,17,23,24-octaol linkers with Fe3(-C2O2-)6(OH2)2 trimers into an extended framework of roc topology. MOF-1992 exhibits sterically accessible Co active sites together with charge transfer properties. Cathodes based on MOF-1992 and carbon black (CB) display a high coverage of electroactive sites (270 nmol cm-2) and a high current density (-16.5 mA cm-2; overpotential, -0.52 V) for the CO2 to CO reduction reaction in water (faradaic efficiency, 80%). Over the 6 h experiment, MOF-1992/CB cathodes reach turnover numbers of 5800 with turnover frequencies of 0.20 s-1 per active site.

Encoding Metal-Cation Arrangements in Metal-Organic Frameworks for Programming the Composition of Electrocatalytically Active Multimetal Oxides.

In the present contribution, we report how through the use of metal-organic frameworks (MOFs) composed of addressable combinations of up to four different metal elements it is possible to program the composition of multimetal oxides, which are not attainable by other synthetic methodologies. Thus, due to the ability to distribute multiple metal cations at specific locations in the MOF secondary building units it is possible to code and transfer selected metal ratios to multimetal oxides with novel, desired compositions through a simple calcination process. The demonstration of an enhancement in the electrocatalytic activity of new oxides by preadjusting the metal ratios is here reported for the oxygen reduction reaction, for which activity values comparable to commercial Pt/C catalysts are reached, while showing long stability and methanol tolerance.

Efficient Rare-Earth-Based Coordination Polymers as Green Photocatalysts for the Synthesis of Imines at Room Temperature.

Five new rare-earth coordination polymers (CPs) were designed in order to offer a remarkable platform that contains light-harvesting antennas and catalytic active centers to achieve solar-energy conversion as green alternatives in the synthesis of imines. These five new spirobifluorene-containing Ln-CPs, named [Er3(Hsfdc)3(sfdc)3(H2O)]· xH2O (RPF-30-Er), [Ln(Hsfdc)(sfdc)(EtOH)]·S (RPF-31-Ln, where Ln = La, Nd, and Sm and S = H2O or EtOH), and [Ho(Hsfdc)(sfdc)(H2O)] (RPF-32-Ho) (RPF = rare-earth polymeric framework and H2sfdc = 9,9'-spirobi[9 H-fluorene]-2,2'-dicarboxylic acid), have been solvothermally synthesized, and their structural features can be described as follows: (i) RPF-30-Er shows a 3D framework in which the inorganic trimers (secondary building units) are cross-linked by Hsfdc- and sfdc2- linkers displaying a pcu topology. (ii) The isostructural RPF-31-Ln series of materials, together with RPF-32-Ho, exhibit a 1D network of chains growing along the a axis with a ribbon-of-rings topology type. The photocatalytic activity of the RPF- n materials was tested in the oxidative coupling of amines using molecular oxygen and air as oxidizing agents under warm light. Among the materials investigated, RPF-31-Nd was chosen to further investigate the approach in the selectivity of different amine derivates.

Principles of Designing Extra-Large Pore Openings and Cages in Zeolitic Imidazolate Frameworks.

We report three design principles for obtaining extra-large pore openings and cages in the metal-organic analogues of inorganic zeolites, zeolitic imidazolate frameworks (ZIFs). Accordingly, we prepared a series of 15 ZIFs, members of which have the largest pore opening (22.5 Å) and the largest cage size (45.8 Å) known for all porous tetrahedral structures. The key parameter allowing us to access these exceptional ZIFs is what we define as the steric index (δ), which is related to the size and shape of the imidazolate linkers employed in the synthesis. The three principles are based on using multiple linkers with specific range and ratios of δ to control the size of rings and cages from small to large, and therefore are universally applicable to all existing ZIFs. The ZIF with the largest cage size (ZIF-412) shows the best selectivity of porous materials tested toward removal of octane and p-xylene from humid air.

A Mesoporous Indium Metal-Organic Framework: Remarkable Advances in Catalytic Activity for Strecker Reaction of Ketones.

With the aim of developing new highly porous, heterogeneous Lewis acid catalysts for multicomponent reactions, a new mesoporous metal-organic framework, InPF-110 ([In3O(btb)2(HCOO)(L)], (H3btb = 1,3,5-tris(4-carboxyphenyl)benzene acid, L = methanol, water, or ethanol), has been prepared with indium as the metal center. It exhibits a Langmuir surface area of 1470 m(2) g(-1), and its structure consists of hexagonal pores with a 2.8 nm aperture, which allows the diffusion of multiple substrates. This material presents a large density of active metal sites resulting in outstanding catalytic activity in the formation of substituted α-aminonitriles through the one-pot Strecker reaction of ketones. In this respect, InPF-110 stands out compared to other catalysts for this reaction due to the small catalyst loadings required, and without the need for heat or solvents. Furthermore, X-ray single crystal diffraction studies clearly show the framework-substrate interaction through coordination to the accessible indium sites.

Synchronizing Substrate Activation Rates in Multicomponent Reactions with Metal-Organic Framework Catalysts.

A study on the influence of the cation coordination number, number of Lewis acid centers, concurrent existence of Lewis base sites, and structure topology on the catalytic activity of six new indium MOFs, has been carried out for multicomponent reactions (MCRs). The new indium polymeric frameworks, namely [In8 (OH)6 (popha)6 (H2 O)4 ]⋅3 H2 O (InPF-16), [In(popha)(2,2'-bipy)]⋅3 H2 O (InPF-17), [In3 (OH)3 (popha)2 (4,4'-bipy)]⋅4 H2 O (InPF-18), [In2 (popha)2 (4,4'-bipy)2 ]⋅3 H2 O (InPF-19), [In(OH)(Hpopha)]⋅0.5 (1,7-phen) (InPF-20), and [In(popha)(1,10-phen)]⋅4 H2 O (InPF-21) (InPF=indium polymeric framework, H3 popha=5-(4-carboxy-2-nitrophenoxy)isophthalic acid, phen=phenanthroline, bipy=bipyridine), have been hydrothermally obtained by using both conventional heating (CH) and microwave (MW) procedures. These indium frameworks show efficient Lewis acid behavior for the solvent-free cyanosilylation of carbonyl compounds, the one pot Passerini 3-component (P-3CR) and the Ugi 4-component (U-4CR) reactions. In addition, InPF-17 was found to be a highly reactive, recyclable, and environmentally benign catalyst, which allows the efficient synthesis of α-aminoacyl amides. The relationship between the Lewis base/acid active site and the catalytic performance is explained by the 2D seven-coordinated indium framework of the catalyst InPF-17. This study is an attempt to highlight the main structural and synthetic factors that have to be taken into account when planning a new, effective MOF-based heterogeneous catalyst for multicomponent reactions.

Saddle-Shaped Cyclic Indole Tetramers: 3D Electroactive Molecules.

We present a joint theoretical and experimental study of a series of cyclic indole tetramers aimed at understanding the fundamental electronic properties of this 3D platform and evaluating its potential in the construction of new semiconductors. To this end, we combined absorption and Raman spectroscopy, cyclic voltammetry, and spectroelectrochemistry with DFT calculations. Our results suggest that this platform can be easily and reversibly oxidized. Additionally, it has a HOMO that matches very well with the workfunction of gold, therefore charge injection from a gold electrode is expected to occur without significant barriers. Interestingly, the cyclic tetraindoles allow for good electron delocalization in spite of their saddle-shaped structures. The steric constraints introduced by N-substitution significantly inhibits ring inversion of the central cyclooctatetraene unit, whereas it only barely affects the optical and electrochemical properties (a slightly higher oxidation potential and a blueshifted absorption upon alkylation are observed).

Tunable catalytic activity of solid solution metal-organic frameworks in one-pot multicomponent reactions.

The aim of this research is to establish how metal-organic frameworks (MOFs) composed of more than one metal in equivalent crystallographic sites (solid solution MOFs) exhibit catalytic activity, which is tunable by virtue of the metal ions ratio. New MOFs with general formula [InxGa1-x(O2C2H4)0.5(hfipbb)] were prepared by the combination of Ga and In. They are isostructural with their monometal counterparts, synthesized with Al, Ga, and In. Differences in their behavior as heterogeneous catalysts in the three-component, one pot Strecker reaction illustrate the potential of solid solution MOFs to provide the ability to address the various stages involved in the reaction mechanism.

Unusual magnetic behaviors and electronic configurations driven by diverse Co(II) or Mn(II) MOF architectures.

Five novel metal organic frameworks were obtained by hydro-solvothermal reactions using the hexafluorisopropylidenebis(benzoic) acid (H2hfipbb) as linker and Co(II) or Mn(II) ions as connectors. [Co2(Hhfipbb)(TEA)], compound 1 (TEA = triethanolamine trianion) with a three-dimensional (3D) framework, and a tpu net; [Co1.5(hfipbb)2]·HN(CH2CH3)3 and [Co3(hfipbb)2]·2{HN(CH2CH3)3}, compounds 2 and 2-a, respectively, both with two-dimensional structure, sql topologies, and different layer packings. Compounds 3 and 4, having the general formula [M2(hfipbb)2]·C7H8, where M = Co (3) or Mn (4), have 3D frameworks with an sqc topology. A deep analysis of the magnetic measurements reveals different striking magnetic behaviors resulting from diverse secondary building unit and framework architectures. Compound 1 presents canted antiferromagnetic chains, compound 2 contains ferromagnetic linear trimeric clusters, and compound 3 exhibits ferromagnetic chains. For the three compounds, a 3D canted antiferromagnetic structure takes place at ∼8 K by means of weak magnetic interactions between the mentioned magnetic units. Such long-range magnetic order is precluded with the application of a high enough magnetic field. Compound 4 evidenced intrachain antiferromagnetic interactions.

Reactivity of coordinatively unsaturated bis(N-heterocyclic carbene) Pt(II) complexes toward H(2). Crystal structure of a 14-electron Pt(II) hydride complex.

The reactivity toward H2 of coordinatively unsaturated Pt(II) complexes, stabilized by N-heterocyclic carbene (NHC) ligands, is herein analyzed. The cationic platinum complexes [Pt(NHC')(NHC)](+) (where NHC' stands for a cyclometalated NHC ligand) react very fast with H2 at room temperature, leading to hydrogenolysis of the Pt-CH2 bond and concomitant formation of hydride derivatives [PtH(NHC)2](+) or hydrido-dihydrogen complexes [PtH(H2)(NHC)2](+). The latter species release H2 when these compounds are subjected to vacuum. The X-ray structure of complex [PtH(IPr)2][SbF6] revealed its unsaturated nature, exhibiting a true T-shaped structure without stabilization by agostic interactions. Density functional theory calculations indicate that the binding and reaction of H2 in complexes [PtH(H2)(NHC)2](+) is more favored for derivatives bearing aryl-substituted NHCs (IPr, 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene and IMes = 1,3-dimesityl-1,3-dihydro-2H-imidazol-2-ylidene) than for those containing tert-butyl groups (I(t)Bu). This outcome is related to the higher close-range steric effects of the I(t)Bu ligands. Accordingly, H/D exchange reactions between hydrides [PtH(NHC)2](+) and D2 take place considerably faster for IPr and IMes* derivatives than for I(t)Bu ones. The reaction mechanisms for both H2 addition and H/D exchange processes depend on the nature of the NHC ligand, operating through oxidative addition transition states in the case of IPr and IMes* or by a σ-complex assisted-metathesis mechanism in the case of I(t)Bu.

An anthraquinone-based bismuth-iron metal-organic framework as an efficient photoanode in photoelectrochemical cells.

Metal-organic frameworks (MOFs) are appealing candidate materials to design new photoelectrodes for use in solar energy conversion because of their modular nature and chemical versatility. However, to date there are few examples of MOFs that can be directly used as photoelectrodes, for which they must be able to afford charge separation upon light absorption, and promote the catalytic dissociation of water molecules, while maintaining structural integrity. Here, we have explored the use of the organic linker anthraquinone-2, 6-disulfonate (2, 6-AQDS) for the preparation of MOFs to be used as photoanodes. Thus, the reaction of 2, 6-AQDS with Bi(iii) or a combination of Bi(iii) and Fe(iii) resulted in two new MOFs, BiPF-10 and BiFePF-15, respectively. They display similar structural features, where the metal elements are disposed in inorganic-layer building units, which are pillared by the organic linkers by coordination bonds through the sulfonic acid groups. We show that the introduction of iron in the structure plays a crucial role for the practical use of the MOFs as a robust photoelectrode in a photoelectrochemical cell, producing as much as 1.23 mmol H2 cm-2 with the use of BiFePF-15 as photoanode. By means of time-resolved and electrochemical impedance spectroscopic studies we have been able to unravel the charge transfer mechanism, which involves the formation of a radical intermediate species, exhibiting a longer-lived lifetime by the presence of the iron-oxo clusters in BiFePF-15 to reduce the charge transfer resistance.

H3O2 bridging ligand in a metal-organic framework. Insight into the aqua-hydroxo↔hydroxyl equilibrium: a combined experimental and theoretical study.

A metal-organic framework (MOF) bearing the aqua-hydroxo species (O2H3)(-) in the framework, as well as the processes that govern the equilibrium aqua-hydroxo (O2H3)(-)↔hydroxyl (OH) in Sc-MOFs, are studied experimentally and theoretically. Computational studies were employed to determine the relative energies for the two compounds that coexist under certain hydrothermal conditions at pH < 2.8. The thermodynamically more stable [Sc3(3,5-DSB)2(µ-O2H3)(µ-OH)2(H2O)2] (from now on, (O2H3)Sc-MOF; 3,5-DSB = 3,5-disulfobenzoic acid) was obtained as a pure and stable phase. It was impossible to isolate [Sc3(3,5-DSB)2(µ-OH)3(H2O)4] as a pure phase, as it turned out to be the precursor of (O2H3)Sc-MOF. Additionally, a third compound that appears at pH between 3.5 and 4, [Sc3(3,5-DSB)(µ-OH)6(H2O)] and a fourth, [Sc(3,5-DSB)(Phen)(H2O)](H2O), in whose formula neither OH groups nor H3O2(-) anions appear, are reported for comparative purposes. A study of the (O2H3)Sc-MOF electronic structure, and some heterogeneous catalytic tests in cyanosilylation of aldehydes reactions, are also reported.