Srinivasan Natarajan

Proton Conduction in Metal–Organic Frameworks and Related Modularly Built Porous Solids

Proton-conducting materials are an important component of fuel cells. Development of new types of proton-conducting materials is one of the most important issues in fuel-cell technology. Herein, we present newly developed proton-conducting materials, modularly built porous solids, including coordination polymers (CPs) or metal-organic frameworks (MOFs). The designable and tunable nature of the porous materials allows for fast development in this research field. Design and synthesis of the new types of proton-conducting materials and their unique proton-conduction properties are discussed.

Open‐Framework Structures of Transition‐Metal Compounds

Inorganic framework solids are no longer limited to just the silicates and phosphates. Recent research has revealed that carboxylates, arsenates, sulfates, selenates, selenites, germanates, phosphites can also form such structures. One of the emerging areas combines the rich coordination chemistry of the central metal ions of many of these structures with the flexibility and functionality of the organic linkers to give rise to organic-inorganic hybrid compounds. The compounds of the transition-metals appear to provide many variations arising from their coordination preferences, ligand geometry, and the valence state. In addition, the combination of the magnetic nature of the transition metal center with the channel structure of open frameworks suggests interesting potential applications. In this Review the synthesis, structures and properties of the various transition-metal open-framework compounds are discussed.

Role of Temperature and Time in the Formation of Infinite −M−O−M− Linkages and Isolated Clusters in MOFs: A Few Illustrative Examples

The role of temperature and time of reaction in the formation of metal-organic frameworks (MOFs) has been studied in two systems of compounds, manganese oxybis(benzoate) and manganese trimellitates, and the results compared and contrasted with other similar studies in the literature. The investigation reveals the formation of six different phases in oxybis(benzoate) and three phases in trimellitate systems. The low-temperature phases in both systems of compounds possess Mn 4 cluster units linked by the carboxylate ligands, while the high-temperature phase, irrespective of the duration of the reaction, has a three-dimensional structure with -Mn-O-Mn- linkages with brucite-related layers pillared by oxybis(benzoate) and Kagome-related layers pillared by trimellitate ligands. In all of the preparation, the reactions appear to have thermodynamic control as a function of the temperature. The isolation of low-dimensional structures in manganese oxybis(benzoate) at moderate time and temperature indicates possible kinetic control. The formation of reactive low-dimensional phases has been rationalized by considering the local charge distribution around the Mn site and also invoking a possible dissolution-recrystalization mechanism.

Highly Luminescent and Thermally Stable Lanthanide Coordination Polymers Designed from 4-(Dipyridin-2-yl)aminobenzoate: Efficient Energy Transfer from Tb3+ to Eu3+ in a Mixed Lanthanide Coordination Compound

Herein, a new aromatic carboxylate ligand, namely, 4-(dipyridin-2-yl)aminobenzoic acid (HL), has been designed and employed for the construction of a series of lanthanide complexes (Eu(3+) = 1, Tb(3+) = 2, and Gd(3+) = 3). Complexes of 1 and 2 were structurally authenticated by single-crystal X-ray diffraction and were found to exist as infinite 1D coordination polymers with the general formulas {[Eu(L)(3)(H(2)O)(2)]}(n) (1) and {[Tb(L)(3)(H(2)O)].(H(2)O)}(n) (2). Both compounds crystallize in monoclinic space group C2/c. The photophysical properties demonstrated that the developed 4-(dipyridin-2-yl)aminobenzoate ligand is well suited for the sensitization of Tb(3+) emission (Φ(overall) = 64%) thanks to the favorable position of the triplet state ((3)ππ*) of the ligand [the energy difference between the triplet state of the ligand and the excited state of Tb(3+) (ΔE) = (3)ππ* - (5)D(4) = 3197 cm(-1)], as investigated in the Gd(3+) complex. On the other hand, the corresponding Eu(3+) complex shows weak luminescence efficiency (Φ(overall) = 7%) due to poor matching of the triplet state of the ligand with that of the emissive excited states of the metal ion (ΔE = (3)ππ* - (5)D(0) = 6447 cm(-1)). Furthermore, in the present work, a mixed lanthanide system featuring Eu(3+) and Tb(3+) ions with the general formula {[Eu(0.5)Tb(0.5)(L)(3)(H(2)O)(2)]}(n) (4) was also synthesized, and the luminescent properties were evaluated and compared with those of the analogous single-lanthanide-ion systems (1 and 2). The lifetime measurements for 4 strongly support the premise that efficient energy transfer occurs between Tb(3+) and Eu(3+) in a mixed lanthanide system (η = 86%).

Quasi-2D XY Magnetic Properties and Slow Relaxation in a Body Centered Metal Organic Network of [Co4] Clusters

Octahedral Co(2+) centers have been connected by mu(3)-OH and mu(2)-OH(2) units forming [Co(4)] clusters which are linked by pyrazine forming a two-dimensional network. The two-dimensional layers are bridged by oxybisbenzoate (OBA) ligands giving rise to a three-dimensional structure. The [Co(4)] clusters bond with the pyrazine and the OBA results in a body-centered arrangement of the clusters, which has been observed for the first time. Magnetic studies reveal a noncollinear frustrated spin structure of the bitriangular cluster, resulting in a net magnetic moment of 1.4 microB per cluster. For T > 32 K, the correlation length of the cluster moments shows a stretched-exponential temperature dependence typical of a Berezinskii-Kosterlitz-Thouless model, which points to a quasi-2D XY behavior. At lower temperature and down to 14 K, the compound behaves as a soft ferromagnet and a slow relaxation is observed, with an energy barrier of ca. 500 K. Then, on further cooling, a hysteretic behavior takes place with a coercive field that reaches 5 T at 4 K. The slow relaxation is assigned to the creation/annihilation of vortex-antivortex pairs, which are the elementary excitations of a 2D XY spin system.

Amino Acid Based MOFs: Synthesis, Structure, Single Crystal to Single Crystal Transformation, Magnetic and Related Studies in a Family of Cobalt and Nickel Aminoisophthales

Four new 5-aminoisophthalates of cobalt and nickel have been prepared employing hydro/solvothermal methods: [Co(2)(C(8)H(5)NO(4))(2)(C(4)H(4)N(2))(H(2)O)(2)].3H(2)O (I), [Ni(2)(C(8)H(5)NO(4))(2)(C(4)H(4)N(2))(H(2)O)(2)].3H(2)O (II), [Co(2)(H(2)O)(mu(3)-OH)(2)(C(8)H(5)NO(4))] (III), and [Ni(2)(H(2)O)(mu(3)-OH)(2)(C(8)H(5)NO(4))] (IV). Compounds I and II are isostructural, having anion-deficient CdCl(2) related layers bridged by a pyrazine ligand, giving rise to a bilayer arrangement. Compounds III and IV have one-dimensional M-O(H)-M chains connected by the 5-aminoisophthalate units forming a three-dimensional structure. The coordinated as well as the lattice water molecules of I and II could be removed and inserted by simple heating-cooling cycles under the atmospheric conditions. The removal of the coordinated water molecule is accompanied by changes in the coordination environment around the M(2+) (M = Co, Ni) and color of the samples (purple to blue, Co; green to dark yellow, Ni). This change has been examined by a variety of techniques that include in situ single crystal to single crystal transformation studies and in situ IR and UV-vis spectroscopic studies. Magnetic studies indicate antiferromagnetic behavior in I and II, a field-induced magnetism in III, and a canted antiferromagnetic behavior in IV.

Pillaring of CdCl2‐Like Layers in Lanthanide Metal–Organic Frameworks: Synthesis, Structure, and Photophysical Properties

A hydrothermal reaction of lanthanide salts, pyridine-2,3-dicarboxylic acid, benzene-1,4-dicarboxylic acid, and water gave rise to a new series of three-dimensional mixed carboxylates (homocyclic and heterocyclic) of lanthanides with the general formula [M2(H2O)4][{C5H3N(COO)2}2{C6H4(COO)2}], M=La (I), Pr (II), and Nd (III). The structure consists of M2O14N2 dimeric units connected by pyridine-2,3-dicarboxylate moieties to form two-dimensional layers that are pillared by terephthalate units. The structures also possess two co-ordinated water molecules, which are arranged to form one-dimensional helical chains and can be reversibly adsorbed. The connectivity within the layers closely resembles that of the CdCl2 layered structure with 3(6) topology. To the best of our knowledge, this is the first observation of CdCl2 topology in lanthanide metal-organic framework compounds. Partial substitution of La3+ in I by Eu3+ and Tb3+ (2 and 4 %) gives rise to characteristic red/pink or green emission, which suggests a ligand-sensitized metal-centered emission. The Nd compound III shows interesting UV and blue emission through an up-conversion process.

Transformations of low-dimensional zinc phosphates to complex open-framework structures. Part 1: zero-dimensional to one-, two- and three-dimensional structures

Zero-dimensional 4-membered zinc phosphate monomers, [C6N2H18][Zn(HPO4)(H2PO4)2], I, and [C6N4H21][Zn(HPO4)2(H2PO4)], II, transform under simple reaction conditions to one-, two- and three-dimensional structures. Monomer II, on heating with zinc acetate dihydrate (Zn(OAc)2) in aqueous solution, gives a layered phosphate [C6N4H21][NH4][Zn6(PO4)4(HPO4)2]H2O, III. A novel three-dimensional structure [C6N4H21]4[Zn7(PO4)6]3, IV, with channels comprising Zn7O6 clusters is obtained on heating II in water under hydrothermal conditions. The monomer I transforms to a one-dimensional ladder, [C3N2H5][Zn(HPO4)], V, on heating with imidazole and to a three-dimensional structure, [C4N2H12][Zn2(H2O)(PO4)(HPO4)]2, VI, on heating with piperazine under ordinary conditions. I also transforms to a layered zinc phosphate, [C6N2H18][Zn3(H2O)4(HPO4)4], VII, on heating in water. In addition to the monomer, II, compounds III–VI have been obtained for the first time. The structures formed by the transformations of the monomers also exhibit unique structural features. Thus, in the ladder structure, V, the imidazole molecule is linked to the Zn center similar to the phosphate unit in a typical ladder structure, while in the layered phosphate, III, one-dimensional tubules are linked via ZnO4 tetrahedra and the three-dimensional structure, IV, possesses Zn7O6 clusters. Isolation of several related solids encompassing a variety of architectures through the transformations of zero-dimensional monomeric phosphates demonstrates not only that the 4-membered ring is a basic structural building unit in these open-framework materials, but also sheds light on the building-up process involved in their formation.

Transformations of the low-dimensional zinc phosphates to complex open-framework structures. Part 2:one-dimensional ladder to two- and three-dimensional structures

Open-framework zinc phosphates with one-dimensional ladder structures are shown to transform, under simple reaction conditions, to two- and three-dimensional structures. Thus, the one-dimensional ladder, [C6N4H22]0.5[Zn(HPO4)2], I, on heating with piperazine in aqueous solution gives a layer phosphate, [C4N2H12][Zn2(PO4)2], III, and the three-dimensional phosphates [C2N2H10]0.5[Zn(PO4)], IV, [C6N4H22]0.5[Zn2(PO4)2], V and [C6N4H21]4[Zn21(PO4)18], VI. On heating in water in the absence of any amine, I transforms to a three-dimensional solid, [C6N4H22]0.5[Zn3(PO4)2(HPO4)], VII, with 16-membered channels. Of these, III and IV are the only new compounds. The phosphates formed by the transformations of I exhibit unique structural features. Thus, in III, the layers are formed only with 3- and 4-membered rings and have step-like features due to the presence of infinite Zn–O–Zn linkages. Compound IV has a structure similar to that of the naturally occurring aluminosilicate, gismondine, and VI possesses unusual Zn7O6 clusters. The ladder zinc phosphate, [C3N2H12][Zn(HPO4)2], II, transforms to two layered compounds, [C3N2H12][Zn4(PO4)2(HPO4)2], VIII, and [C3N2H12][Zn2(HPO4)3], IX, on heating with zinc acetate and water, respectively. II, on heating in water in the presence of other amines, forms a ladder, [C3N2H5][Zn(HPO4)], X, and a three-dimensional phosphate, [C3N2H12]2[Zn5(H2O)(PO4)4(HPO4)], XI. The syntheses and structures of VIII–XI have already been reported. What is interesting is that the majority of the transformations seem to occur through the process of deprotonation of the phosphoryl group and elimination of the –HPO4 unit. The transformations of the ladder phosphates to higher dimensional structures reported in the present study not only demonstrate the seminal role of the one-dimensional structures as basic building units, but also the likely occurrence of self-assembly of these one-dimensional units in the building-up process.

Amine Phosphates as Intermediates in the Formation of Open‐Framework Structures

Chain, ladder, layer, and three-dimensional metal phosphates were obtained by the reaction of amine phosphates with metal ions under mild hydrothermal conditions. The role of amine phosphates as intermediates in hydrothermal syntheses was corroborated by in situ NMR spectroscopy and X-ray diffraction studies. The picture shows a simple corner-shared linear chain structure formed in the reaction of piperazine phosphate with Zn(II) ions.