Angelo Maspero

High thermal and chemical stability in pyrazolate-bridged metal–organic frameworks with exposed metal sites

Reactions between the tritopic pyrazole-based ligand 1,3,5-tris(1H-pyrazol-4-yl)benzene (H3BTP) and transition metal acetate salts in DMF afford microporous pyrazolate-bridged metal–organic frameworks of the type M3(BTP)2·xsolvent (M = Ni (1), Cu, (2), Zn (3), Co (4)). Ab-initioX-ray powder diffraction methods were employed in determining the crystal structures of these compounds, revealing 1 and 2 to exhibit an expanded sodalite-like framework with accessible metal cation sites, while 3 and 4 possess tetragonal frameworks with hydrophobic surfaces and narrower channel diameters. Compounds 1–4 can be desolvated without loss of crystallinity by heating under dynamic vacuum, giving rise to microporous solids with BET surface areas of 1650, 1860, 930 and 1027 m2 g−1, respectively. Thermogravimetric analyses and powder X-ray diffraction measurements demonstrate the exceptional thermal and chemical stability of these frameworks. In particular, 3 is stable to heating in air up to at least 510 °C, while 1 is stable to heating in air to 430 °C, as well as to treatment with boiling aqueous solutions of pH 2 to 14 for two weeks. Unexpectedly, 2 and 3 are converted into new crystalline metal–organic frameworks upon heating in boiling water. With the combination of stability under extreme conditions, high surface area, and exposed metal sites, it is anticipated that 1 may open the way to testing metal–organic frameworks for catalytic processes that currently employ zeolites.

Cation‐Exchange Porosity Tuning in Anionic Metal–Organic Frameworks for the Selective Separation of Gases and Vapors and for Catalysis

The outperforming adsorptive properties of the so-called open metal–organic frameworks (MOFs) or porous coordination polymers (PCPs) rely on their fully accessible porous structure and the easy tuning of the shape, size, and chemical nature of their pores. The ability of some of these systems to mimic the structure and properties of zeolites has also been realized. There are, however, unsolved problems related to the general lower thermal and chemical stability (hydrolysissensitive nature) of MOFs compared to their zeolite counterparts. Consequently, the search for highly robust MOFs capable of withstanding the working conditions typically found in industrial processes is a highly desirable challenge. In this regard, the robustness of the metal–nitrogen(heterocycle) coordinative bonds leads to the formation of MOF materials with enhanced chemical and thermal stabilities. It should also be noted that, in contrast to the wellknown cation-exchange features of zeolites, the zeomimetic coordination polymers generally possess neutral or cationic frameworks and consequently do not usually give rise to cation-exchange processes. Herein, we report the synthesis, structural characterization, thermal/chemical stability, and adsorptive, separation, and catalytic properties of the anionic MOF NH4[Cu3(m3-OH)(m3-4-carboxypyrazolato)3] (NH4@1). In addition, we have examined the plausible modulation of its porous network by means of ion-exchange processes of the extraframework cations. The results show that the ion-exchange processes on these systems lead to profound changes in the textural properties of their porous surface and in the adsorption selectivity of different separation processes of gases and vapors. The crystal structure of NH4@1 [10] is based on an anionic 3D porous framework built up of trinuclear Cu3(m3-OH) clusters connected to another six through m3-4-carboxypyrazolato bridges (Figure 1). In this way, tetrahedral cages with

Tuning the Adsorption Properties of Isoreticular Pyrazolate-Based Metal–Organic Frameworks through Ligand Modification

Two isoreticular series of pyrazolate-based 3D open metal-organic frameworks, MBDP_X, adopting the NiBDP and ZnBDP structure types [H(2)BDP = 1,4-bis(1H-pyrazol-4-yl)benzene], were synthesized with the new tagged organic linkers H(2)BDP_X (X = -NO(2), -NH(2), -OH). All of the MBDP_X materials have been characterized through a combination of techniques. IR spectroscopy proved the effective presence of tags, while X-ray powder diffraction (XRPD) witnessed their isoreticular nature. Simultaneous TG/DSC analyses (STA) demonstrated their remarkable thermal stability, while variable-temperature XRPD experiments highlighted their high degree of flexibility related to guest-induced fit processes of the solvent molecules included in the channels. A structural isomer of the parent NiBDP was obtained with a sulfonate tagged ligand, H(2)BDP_SO(3)H. Structure solution from powder diffraction data collected at three different temperatures (room temperature, 90, and 250 °C) allowed the determination of its structure and the comprehension of its solvent-related flexible behavior. Finally, the potential application of the tagged MOFs in selective adsorption processes for gas separation and purification purposes was investigated by conventional single component adsorption isotherms, as well as by advanced experiments of pulse gas chromatography and breakthrough curve measurements. Noteworthy, the results show that functionalization does not improve the adsorption selectivity (partition coefficients) for the resolution of gas mixtures characterized by similar high quadrupole moments (e.g., CO(2)/C(2)H(2)); however, the resolution of gas mixtures containing molecules with highly differentiated polarities (i.e., N(2)/CO(2) or CH(4)/CO(2)) is highly improved.

Highly Hydrophobic Isoreticular Porous Metal-Organic Frameworks for the Capture of Harmful Volatile Organic Compounds

The release of toxic pollutants into the environment, which includes oil spills, leaks of harmful industrial products, and the deliberate emission of chemical warfare agents is a risk of growing concern. Worthy of note, oil spill cleanups amount to over 10 billion dollars annually. Remediation of these environmental problems involves the use of large amounts of adsorbents such as sand, activated carbons, or zeolites. However, the effectiveness of such adsorbents is often limited by their affinity to moisture. Consequently, the search for highly hydrophobic porous materials to be used as suitable stopgap of harmful organics spills has become of paramount importance. In the past years, porous metal–organic frameworks (MOFs) have been extensively studied to explore their possible applications in near future technologies for the safe storage of energetically and environmentally relevant gases. The tunable nature of their pores might be beneficial also in cushioning environmental problems caused by the release of harmful volatile organic compounds (VOCs). A remarkable example of the design amenability of MOFs is the well-known isoreticular [Zn4OL3] series (L= arene-dicarboxylate), wherein the size and the functionality of the pores can be modulated in a highly rational and systematic way. Nevertheless, the advantageous structural features of this family of MOFs are readily hampered by its high sensitivity to moisture, which limits its practical applications. A similar size-scaling approach has been applied by Lillerud and coworkers on the isoreticular [Zr6O4(OH)4L6] series, [9] evidencing that a significant improvement in the stability of the material can be achieved with an appropriate combination of dicarboxylate linkers and oxophylic metal fragments. Alternately, it is possible to take advantage of the enhanced stability imparted by polyazolate-containing ligands in combination with borderline metal ions. Accordingly, we designed and isolated an isoreticular series of porous MOFs, the pore size and polarity of which was modulated by coupling stiff bi-pyrazolate or mixed pyrazolate/carboxylate linkers (Scheme 1) to Ni hydroxo clusters acting as 12-connected

Cubic Octanuclear Ni(II) Clusters in Highly Porous Polypyrazolyl-Based Materials

Two highly porous coordination polymers, containing rare octanuclear hydroxo-nickel clusters and long bis-pyrazolyl spacers, are shown to possess, after mild thermal treatment, lattice cavities up to 72% of the total crystal volume.

Synthesis and characterisation of new polynuclear copper(I) pyrazolate complexes and their catalytic activity in the cyclopropanation of olefins

Abstract The reaction of [Cu(CH3CN)4](BF4) with racemic pyrazole-3,5-dicarboxylic acid di-sec-butyl ester (3,5-dicarbo-sec-butoxypyrazole, Hdcsbpz) or with pyrazole-3,5-di-ter-butyl (3,5-di-ter-butylpyrazole, Hdtbpz) quantitatively yields the new [Cu(dcsbpz)]4 and [Cu(dtbpz)]4 complexes, respectively. Crystals of [Cu(dcsbpz)]4 are triclinic, P 1 , a=10.9748(7), b=11.8399(8), c=26.5575(17) A, α=100.605(2), β=90.783(2), γ=105.362(2)°; [Cu(dtbpz)]4·CH2Cl2 is monoclinic, P21/n, a=10.902(3), b=19.200(3), c=25.772(4) A, β=93.86(2)°. Both species contain cyclic tetrameric molecules, with the heterocyclic ligands binding in the common N,N′-exo-bidentate mode; however, the shape and geometry of the inner Cu4 moiety is remarkably different, as highlighted, for example, by the absolute values of the 1,2 and 1,3 (non-bonding) Cu⋯Cu interactions. These polynuclear copper(I) pyrazolate complexes catalyse the conversion of alkenes into the corresponding cyclopropane derivatives with interesting diastereomeric excesses. Aiming at the evaluation of their catalytic activities, a systematic study of the cyclopropanation reactions in the presence of ethyl diazoacetate has been performed.

UNIQUE FORMATION OF A CRYSTAL PHASE CONTAINING CYCLIC OLIGOMERS AND HELICAL POLYMERS OF THE SAME MONOMERIC FRAGMENT

The yellow, microcrystalline compound [Cu(pymo)] (Hpymo=2-hydroxypyrimidine) has been characterized with the newly emerging technique of ab initio X-ray powder diffraction. A unique and unprecedented crystal phase containing cyclic oligomers and infinite helical polymers (see picture) of the same monomeric fragment is selectively formed upon reaction of [Cu(CH3 CN)4 ][BF4 ] and Hpymo with NEt3 .

When long bis(pyrazolates) meet late transition metals: structure, stability and adsorption of metal–organic frameworks featuring large parallel channels

A family of bis(pyrazolato)-based metal-organic frameworks (MOFs) was isolated by reacting 1,4-bis(1H-pyrazol-4-ylethynyl)benzene (H2BPEB) with a number of transition metal ions. Special attention was dedicated to their structural features, their thermal and chemical stability, as well as their spectroscopic and adsorption properties. The rod-like ligands, connecting Zn(II), Ni(II) and Fe(III) nodes, fabricate 3-D networks containing 1-D pervious channels. The combination of thermal analysis and variable-temperature XRPD demonstrated the remarkable thermal robustness of the three materials, which are stable in air up to at least 410 °C, and showed their structural response to increasing temperature. Specific experiments permitted us to test the chemical stability of the three species toward water as well as moderately acidic and basic solutions, the Ni(II) derivative being stable and hydrophobic in all the conditions assayed. The electronic transitions of both the ligand and the MOFs were investigated by solid-state UV-Vis absorption as well as by steady-state and time-resolved fluorescence analysis, which showed that the high fluorescence of the linker is perturbed in the three MOFs, suggesting high sensitivity to environmental changes. N2 adsorption measurements at 77 K allowed to estimate promising Langmuir specific surface areas, peaking at 2378 m2 g−1 in the case of the Ni(II) derivative. The best CO2 and CH4 uptake performances were achieved with the Fe(III)-based MOF. Indeed, adsorption experiments with CO2 revealed that a considerable amount, up to 40% wt, is adsorbed by the Fe(III) derivative under the mild conditions of 298 K and 10 bar.

Alkyne oligomerization catalyzed by molybdenum(0) complexes

Abstract Three new molybdenum(0) complexes, [Mo(CO)3(Hpz)3] (1), [Mo(CO)2(Hpz)2(DMAD)2] (2), (DMAD=dimethyl acetylenedicarboxylate) and [Mo(CO)3(1-Me-imidazole)3] (3) were synthesized and characterized. Their activity and selectivity in alkyne cyclotrimerization and co-trimerization reactions was investigated. The molecular structures of 1 and 2 have been determined by unconventional powder and standard single-crystal diffraction methods, respectively. 1 consists of a pseudo-octahedral complex of C3 symmetry, with the ligands in fac disposition; complex 2, of idealized C2 symmetry, is obtained by substitution of one CO and one pyrazole in 1 by two DMAD ligands, and shows the rare trans configuration of π-bound acetylenic moieties.

Thermally robust metal coordination polymers: The cobalt, nickel, and zinc pyrimidin-2-olate derivatives

A number of coordination polymers of the pymo ligand (Hpymo = 2-hydroxypyrimidine) have been prepared and fully characterized by chemical, spectroscopic, and thermal analyses. Their complete crystal structures have been solved ab initio from laboratory X-ray powder diffraction data and ultimately refined by the Rietveld method. The M(pymo)2 species (M = Co, Ni, Zn) consist of structurally related threedimensional frameworks of very high thermal stability (decomposing under N2 only at T . 550 °C), with the metal atoms, linked by μ2-η-η (N;N9) (Co, Zn) or μ2-η-η (N,O;N9)