R. Vaidhyanathan

Hydrogen bonded structures in organic amine oxalates

Oxalates of n-propylamine, n-butylamine, ethylenediamine, 1,4-butanediamine, piperazine, guanidine and 1,4-diazabicyclo[2,2,2]octane (DABCO) have been synthesized and characterized by single crystal X-ray diffraction and other techniques. The amine oxalates show different types of hydrogen bonded networks, linear hydrogen bonded chains characterizing the oxalates of the first five amines. Guanidinium oxalate has a sheet like structure while DABCO oxalate has dimeric hydrogen bonded rings. Hydrogen bonded structures of these oxalates are discussed in detail, besides relating their thermal stability to the strengths of the networks.

Synthons and design in metal phosphates and oxalates with open architectures

We briefly describe the structures of open-framework metal phosphates with different dimensionalities, such as the one-dimensional linear-chain and ladder structures, two-dimensional layer structures and three-dimensional structures with channels. We demonstrate the role of the zero-dimensional four-membered ring monomer and of the one-dimensional ladder structure as the starting building units or synthons involved in the formation of the complex architectures. Thus, we show how the one-dimensional ladder structure transforms to two- and three-dimensional structures under mild conditions. The two-dimensional layer structures also transform to three-dimensional structures, while the zero-dimensional monomer transforms to layered and three-dimensional structures under ordinary reaction conditions. These transformations provide an insight into the possible pathways involved in the building up of the complex structures of metal phosphates. The isolation of amine phosphates during the hydrothermal synthesis of metal phosphates and also the facile reactions between amine phosphates and metal ions to yield a variety of open-framework materials have thrown light on the mechanism of formation and design of these structures. The existence of a hierarchy of open-framework metal oxalates and their ready formation by employing amine oxalates as intermediates provides additional support to the observations made earlier with regard to the phosphates.

Synthesis of a hierarchy of zinc oxalate structures from amine oxalates

A hierarchy of novel zinc oxalates including monomers and dimers has been prepared by reaction of amine oxalates with zinc ions, the amine oxalates having been characterized for the first time. In most of the amine oxalates one of the carboxyl groups transfers a proton to the amino nitrogen, leaving the other carboxyl group free to form hydrogen bonds. The zinc oxalates obtained are composed of a network of ZnO6 octahedra and oxalate units, and possess zero-, one-, two- and three-dimensional structures. The monomer, dimer, and chain zinc oxalates are the first members of the hierarchy of structures. Relationships amongst these various oxalate structures are noteworthy and give indications as to the manner in which these structures are formed. Thus, the three-dimensional structure can be formed by the linking of layers, and the layer structure by condensation of linear chains. The isolation and characterization of a hierarchy of zinc oxalates of differing dimensionalities assumes significance in the light of the recent finding that low-dimensional structures transform to higher, more complex structures in the phosphate family.

New open-framework layered tin(II) phosphates intercalated with amines

Two new Sn(II) phosphate materials, I and II, have been synthesized hydrothermally using 1,3-diaminopropane and 1,3-diamino-2-hydroxypropane as structure-directing organic amines. The solids I and II have layered architectures and are isostructural. The structures consist of vertex sharing trigonal-pyramidal SnO 3 and tetrahedral PO 4 moieties forming infinite layers possessing 4- and 8-membered apertures. The interlamellar space is occupied by the protonated amine molecules which interact with the framework through hydrogen bonding. Crystal data for compound I: [NH 3 (CH 2 ) 3 NH 3 ] 2+ 2[SnPO 4 ] – , M=501.9, monoclinic, space group C2/c, a=18.097(1), b=7.889(1), c=9.151(1) A β=111.84(1)°, V=1212.6(2) A 3 , Z=4, R=0.033 and R w =0.061 [791 observed reflections with I=2σ(I)]; compound II [NH 3 (CH 2 CHOHCH 2 )NH 3 ] 2+ 2[SnPO 4 ] – , M=517.9, monoclinic, space group C2/c, a=18.133(1), b=7.858(1), c=9.344(1) A, β=111.3(1)°, V=1240.5(2) A 3 , Z=4, R=0.040 and R w =0.11 [867 observed reflections with I=2σ(I)].

Hydrothermal synthesis of an open-framework manganese oxalate incorporating KCl chains

A manganese oxalate of the composition, K 2 [Mn 2 (C 2 O 4 ) 3 ]2[KCl].2H 2 O, containing metal oxalate layers connected by monodentate oxalate units has been synthesized hydrothermally. An unique feature of this material is that it contains K-Cl chains with a K-Cl distance of ∼3.2 A, close to the value in bulk KCl.

A layered zinc oxalate possessing a 12-membered honeycomb aperture, stabilized by an amine and an alkali cation

A new layered anionic zinc oxalate of the formula, K[C 6 N 2 H 13 ][Zn 2 (C 2 O 4 ) 3 ].4H 2 O containing 12-membered honeycomb apertures has been synthesized hydrothermally in the presence of DABCO and K + . A noteworthy feature of the structure is the presence of 12-membered apertures, aligned in such a way as to create pseudo one-dimensional tunnels as in zeolitic structures. The amine, K + and water molecules are present in the tunnels.

Aliphatic dicarboxylates with three-dimensional metal–organic frameworks possessing hydrophobic channels

Two three-dimensional open-framework metal dicarboxylates possessing channels with a hydrophobic environment have been synthesized. One is a malonate of the formula, [Cd(O2C-CH2-CO2)(H2O)]·H2O, I, and the other a glutarate of the formula [Mn(O2C-(CH2)3-CO2)], II. The three-dimensional structure of I is attained through infinite Cd–O–Cd corner-linkages. On the other hand, II, contains Mn–O–Mn layers cross-linked by MnO6 octahedra and glutarate moieties.

A layered chlorophosphate, Na3[Cd4Cl3(HPO4)2(H2PO4)4], containing Na+ ions in the interlamellar space

Reaction between CdCl2·H2O and NaH2PO4·H2O under hydrothermal conditions gives rise to a new cadmium chlorophosphate of the formula Na3[Cd4Cl3(HPO4)2(H2PO4)4], I. This material crystallizes in the orthorhombic system with space group Fmm2 (no. 42). I has macroanionic layers of [Cd4Cl3(HPO4)2(H2PO4)4]3− with Na+ ions in the interlamellar space. The discovery of such compounds suggests that metathetic reactions carried out under hydrothermal conditions may provide a novel route for the synthesis of new open-framework structures.