Amitava Choudhury

Organically Templated Mixed-Valent Iron Sulfates Possessing Kagomé and Other Types of Layered Networks

We have been interested in developing strategies for designing new open-framework architectures. One such strategy is the case of the sulfate tetrahedron and its utilization as a primary buildingunit, in place of the silicate or the phosphate tetrahedron commonly employed for the purpose.

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.

An Unusual Open‐Framework Cobalt(II) Phosphate with a Channel Structure That Exhibits Structural and Magnetic Transitions

An Unusual Open-Framework Cobalt(II) Phosphate with a Channel Structure That Exhibits Structural and Magnetic Transitions

Hybrid open-framework iron phosphate--oxalates demonstrating a dual role of the oxalate unit

New inorganic-organic hybrid open-framework materials of the phosphate-oxalate family, [Fe2(H2O)2-(HPO4)2(C2O4)].H2O (I), [Fe2(H2O)2-(HPO4)2(C2O4)].2H2O (II), [C3N2H12]-[Fe2(HPO4)2(C2O4)1.5]2 (III), and [C3N2OH12][Fe2(HPO4)2(C2O4)1.5]2 (IV) have been synthesized hydrothermally in the presence of structure-directing amines. The amine molecules are incorporated in III and IV, whereas I and II are devoid of them. The oxalate units act as a bridge between the layers in all the compounds. The layers in I and II are entirely inorganic, being formed by FeO6 and PO4 units, whereas in III and IV oxalate units constitute the inorganic layers and act as the bridge between these layers. Such a dual role of the oxalate unit is unique and noteworthy. The formation of two types of inorganic layers in I and II consisting of four-, six-, and eight-membered rings, indicates the interconversions between the various rings in the phosphate--oxalates to be facile. All the phosphate--oxalates show antiferromagnetic ordering at low temperatures.

A Versatile Tripodal Cu(I) Reagent for C–N Bond Construction via Nitrene-Transfer Chemistry: Catalytic Perspectives and Mechanistic Insights on C–H Aminations/Amidinations and Olefin Aziridinations

A Cu(I) catalyst (1), supported by a framework of strongly basic guanidinato moieties, mediates nitrene-transfer from PhI═NR sources to a wide variety of aliphatic hydrocarbons (C-H amination or amidination in the presence of nitriles) and olefins (aziridination). Product profiles are consistent with a stepwise rather than concerted C-N bond formation. Mechanistic investigations with the aid of Hammett plots, kinetic isotope effects, labeled stereochemical probes, and radical traps and clocks allow us to conclude that carboradical intermediates play a major role and are generated by hydrogen-atom abstraction from substrate C-H bonds or initial nitrene-addition to one of the olefinic carbons. Subsequent processes include solvent-caged radical recombination to afford the major amination and aziridination products but also one-electron oxidation of diffusively free carboradicals to generate amidination products due to carbocation participation. Analyses of metal- and ligand-centered events by variable temperature electrospray mass spectrometry, cyclic voltammetry, and electron paramagnetic resonance spectroscopy, coupled with computational studies, indicate that an active, but still elusive, copper-nitrene (S = 1) intermediate initially abstracts a hydrogen atom from, or adds nitrene to, C-H and C═C bonds, respectively, followed by a spin flip and radical rebound to afford intra- and intermolecular C-N containing products.

Sulfates of organic diamines: hydrogen-bonded structures and properties

Abstract In order to investigate the supramolecular hydrogen-bonded networks and other structural features exhibited by compounds containing an organic cation and an inorganic anion, sulfates of the organic diamines, ethylenediamine (I), 1,3-diaminopropane (II), piperazine (III), and 1,4-diazabicyclo[2.2.2]octane (DABCO) (IV) have been prepared investigated by X-ray crystallography. While II, III, and IV crystallize in the centrosymmetric space group, Pbca, P21/n, Pbcn, respectively, I crystallizes in the non-centrosymmetric space group, P41 exhibiting chirality and weak NLO properties. I–IV exhibit different types of supramolecular H-bonded networks involving the organic cation and the SO2−4 anion. The nature and strength of the H-bonding network vary from one compound to another, with the strongest network found in piperazinium sulfate, III, and the weakest in II. While in III, water molecules form part of the H-bonded network, they are present as guest molecules in the channels of IV. Thermal stability of the compounds as well as the infrared spectra reflect the stabilities of these H-bonded solids.

Inorganic hybrid open-framework structures: synthesis and structure of a cobalt phosphate-oxalate, [C4N2H12]0.5[Co2(HPO4)(C2O4)1.5]

: A new open-framework inorganic-organic hybrid structure involving cobalt, [N2C4H12](0.5)[Co-2(HPO4)(C2O4)(1.5)], has been synthesized for the first time by hydrothermal methods. The three-dimensional structure is formed by the edge-shared CoO6 and CoO5 polyhedra forming a [Co2O9] dimer, that are connected with the PO4 polyhedra and the oxalate units forming one-dimensional channels. The amine molecules are located within these channels. The oxalate units have a dual functionality of connecting within the plane of the layer as well as out of the plane. The material is thermally stable up to 350 degrees C and orders anti-ferromagnetically at low temperatures. Crystal data: a = 7.494(1), b = 7.726(1), c = 17.852(3) Angstrom, beta = 97.7(1)degrees; V = 1024.2(2) Angstrom(3): space group = P2(1)/c (no. 14); Z = 4; M = 389.9; D-calc = 2.529 g cm(-3), lambda(Mo-K-alpha) = 3.451 mm(-1); lambda = 0.71073 Angstrom: R-1 = 0.04; wtR(2) = 0.10 and S = 1.13

The first organically templated linear metal selenate

Abstract An organically templated cadmium selenate of the formula [enH 2 ][Cd(H 2 O) 2 (SeO 4 ) 2 ], I , has been synthesized by carrying out the reaction between CdO and H 2 SeO 4 in the presence of the amine in aqueous medium at 80°C. The structure of I involves isolated infinite [Cd(H 2 O) 2 (SeO 4 ) 2 ] 2− chains running along the c -axis, with the protonated ethylenediamine molecules between them. Crystal data for I are as follows: Monoclinic, space group= C 2/ c , a =10.5059(7), b =10.3605(7), c=10.6028(7) A , β =94.666(2)°, V=1150.25(13) A 3 , z =8, R 1 =0.0237, wR 2 =0.0500 (for all data).

Transformations of two-dimensional layered zinc phosphates to three-dimensional and one-dimensional structures

Transformations of the layered zinc phosphates of the compositions [C6N4H22]0.5[Zn2(HPO4)3], I, [C3N2H12][Zn2(HPO4)3], II and [C3N2OH12][Zn2(HPO4)3], III, containing triethylenetetramine, 1,3-diaminopropane, and 1,3-diamino-2-hydroxypropane, respectively, have been investigated under different conditions. On heating in water, I transforms to a one-dimensional (1-D) ladder and a three-dimensional (3-D) structure, while II gives rise to only a two-dimensional (2-D) layered structure. In the transformation reaction of I with zinc acetate, the same ladder and 3-D structures are obtained along with a tubular layer. Under similar conditions II gives a layered structure formed by the joining of two ladder motifs. III, on the other hand, is essentially unreactive when heated with water and zinc acetate, probably because the presence of the hydroxy group in the amine which hydrogen bonds to the framework. In the presence of piperazine, I, II and III give rise to a four-membered, corner-shared linear chain which is likely to be formed via the ladder structure. In addition, 2-D and 3-D structures derived from the 1-D linear chain or ladder structures are also formed. The primary result from the study is that the layers produce 1-D ladders, which then undergo other transformations. It is noteworthy that in the various transformations carried out, most of the products are single-crystalline.