Alok Ch. Kalita

A schiff base and its copper(II) complex as a highly selective chemodosimeter for mercury(II) involving preferential hydrolysis of aldimine over an ester group

The syntheses of a new Schiff base, diethyl-5-(2-hydroxybenzylidene)aminoisophthalate (HL), and a copper complex, [Cu(L2)] (1), imparting L(-), have been described. Both the ligand HL and complex 1 have been thoroughly characterized by elemental analyses, electrospray ionization mass spectrometry, FT-IR, NMR ((1)H and (13)C), electronic absorption, and emission spectral studies and their structures determined by X-ray single-crystal analyses. Distinctive chemodosimetric behavior of HL and 1 toward Hg(2+) has been established by UV/vis, emission, and mass spectral studies. Comparative studies further revealed that the chemodosimetric response solely originates from selective hydrolysis of the aldimine moiety over the ester group and 1 exhibited greater selectivity toward Hg(2+) relative to HL while the sensitivity order is reversed. Further, these followed different hydrolytic pathways but ended up with the same product analyzed for diethyl-5-aminoisophthalate (DEA). Hg(2+)-induced displacement of Cu(2+) and subsequent hydrolysis of the -HC═N- moiety in 1 affirmed the identity of the actual species undergoing hydrolysis as HL. The occurrence of Cu(2+) displacement and Hg(2+) detection via hydrolytic transformation has been supported by various physicochemical studies.

Ab Initio Chemical Synthesis of Designer Metal Phosphate Frameworks at Ambient Conditions

Stepwise hierarchical and rational synthesis of porous zinc phosphate frameworks by predictable and directed assembly of easily isolable tetrameric zinc phosphate [Zn(dipp)(solv)]4 (dippH2 = diisopropylphenyldihydrogen phosphate; solv = CH3OH or dimethyl sulfoxide) with D4R (double-4-ring) topology has been achieved. The preformed and highly robust D4R secondary building unit can be coordinatively interconnected through a varied choice of bipyridine-based ditopic spacers L1-L7 to isolate eight functional zinc phosphate frameworks, [Zn4(dipp)4(L1)1.5(DMSO)]·4H2O (2), [Zn4(dipp)4(L2)1.5(CH3OH)] (3), [Zn4(dipp)4(L1)2] (4), [Zn4(dipp)4(L3)2] (5), [Zn4(dipp)4(L4)2] (6), [Zn4(dipp)4(L5)2] (7), [Zn4(dipp)4(L6)2] (8), and [Zn4(dipp)4(L7)2] (9), in good yield. The preparative procedures are simple and do not require high pressure or temperature. Surface area measurements of these framework solids show that the guest accessibility of the frameworks can be tuned by suitable modification of bipyridine spacers.

An anionic two-dimensional indium carboxylate framework derived from a pseudo C (3)-symmetric semi-flexible tricarboxylic acid

AbstractHydrothermal treatment of indium(III) nitrate with a flexible pseudo C3-symmetric tricarboxylic acid at 115∘C for 5 days in DMF yields a new layered anionic indium carboxylate framework, [(CH3)2 NH 2)][In(L)(HCOO)(DMF)]n (1) (L = 2,4,6-tris[(4′-carboxyphenoxy)methyl]-1,3,5-trimethylbenzene), existing as two-dimensional sheets. The framework solid has been characterized by elemental analysis, FT-IR spectroscopy, TGA, PXRD and single crystal X-ray diffraction studies. DMF undergoes cleavage to dimethyl ammonium and formate ions, which are incorporated in the framework. A slipped stacking of the two dimensional sheets along a–axis in 1 results in a drastic decrease in the anticipated large porosity of the framework. Graphical AbstractA two-fold interpenetrated 2-D In(III)-MOF has been synthesized using hydrothermal treatment of flexible tris-acid ligand H3L, indium nitrate and DMF. The MOF features an anionic uninodal 6-connected sql/Shubnikov plane net (3,6) with (36.46.53) topology. DMF decomposes to (CH3)2NH2+ cations which bridge the 2-D sheets through hydrogen bonds in a staggered fashion.

Elusive Double-Eight-Ring Zeolitic Secondary Building Unit

The double-eight-ring (D8R), an elusive secondary building unit of zeolites, has been stabilized for the first time, both in solution and solid-state. The present study further establishes that any of the three double-ring building blocks of zeolites, viz. D4R, D6R and D8R ([ArPO3Zn(L)]n (n = 4, 6 or 8)), can be preferentially isolated (over the other two) through a careful choice of metal source, aryl phosphate and ancillary ligand, apart from maintaining a meticulous control on the reaction conditions.

Dependence of the SBU length on the size of metal ions in alkaline earth MOFs derived from a flexible C3-symmetric tricarboxylic acid

Four new alkaline-earth metal-based metal–organic frameworks, [Mg4(TCMTB)2(OAc)2(DMA)2(H2O)3]n (Mg-TCMTB), {[Ca4(TCMTB)2(OH)(DMF)2(H2O)5]·Cl}n (Ca-TCMTB), [Sr4(TCMTB)2(OH)(OAc)(DMA)6(H2O)]n (Sr-TCMTB) and [Ba9(TCMTB)4(NO3)6(DMA)14]n (Ba-TCMTB) (H3TCMTB = 2,4,6-tris[(4′-carboxyphenoxy)methyl]-1,3,5-trimethylbenzene), have been synthesized and structurally characterized. Structural analysis of the MOFs reveals the presence of diverse structures and topologies in these systems due to the conformational flexibility and multiple coordination sites in H3TCMTB. Coordination polymers Mg-TCMTB, Ca-TCMTB and Ba-TCMTB MOFs are three-dimensional frameworks exhibiting 2-fold interpenetration and one-dimensional hexagonal channels, while Sr-TCMTB is a 2-fold interpenetrated layered MOF. The 2D layers in Sr-TCMTB are interconnected through H–O⋯H hydrogen bonds. Increasing ionic radii and coordination number on moving down the group results in the formation of bi-, tetra- and nona-nuclear M–O–M connected inorganic building units. Owing to its smaller size and lower coordination number, framework Mg-TCMTB gives rise to a moderate surface area of 33.0 m2 g−1 (SABET) and 93.8 m2 g−1 (SALang) which is the highest observed among all the four MOFs. Emission studies of the new MOFs reveal the presence of strong photoluminescence at 380 nm.

Pseudopolymorphism leading and two different supramolecular aggregations in a phosphate monoester: role of a rare water-dimer

Whereas 2,6-diisopropyl- and 2,6-dimethylphenyl-phosphate forms a 1-D chain, 2,6-diisopropyl-4-amino-phenylphosphate crystallizes as two different 3-D supramolecular aggregates. The first pseudopolymorph 1a is a porous framework built around a rare water-dimer template that undergoes single-crystal to single-crystal transformations to yield desolvated form 1b. The second-pseudopolymorph 1c, obtained by re-crystallisation of 1a in methanol, is a dense 3-D framework.

A Solvent Switch for the Stabilization of Multiple Hemiacetals on an Inorganic Platform: Role of Supramolecular Interactions.

Reaction of Zn(OAc)2 ⋅2 H2 O with 2,6-diisopropylphenyl phosphate (dippH2 ) in the presence of pyridine-4-carboxaldehyde (Py-4-CHO) in methanol resulted in the isolation of a tetrameric zinc phosphate cluster [Zn(dipp)(Py-4-CH(OH)(OMe))]4 ⋅4 MeOH (1) with four hemiacetal moieties stabilized on the double-4-ring inorganic cubane cluster. The change of solvent from methanol to acetonitrile leads to the formation of [Zn(dipp)(Py-4-CHO)]4 (2), in which the coordinated Py-4-CHO retains its aldehydic form. Dissolution of 1 in CD3 CN readily converts it to the aldehydic form and yields 2. Similarly 2, which exists in the aldehyde form in CD3 CN, readily converts to the hemiacetal form in CD3 OD/CH3 OH. Compound 1 is an unprecedented example in which four hemiacetals have been stabilized on a single molecule in the solid state retaining its stability in solution as revealed by its (1) H NMR spectrum in CD3 OD. The solution stability of 1 and 2 has further been confirmed by ESI-MS studies. To generalize the stabilization of multiple hemiacetals on a single double-four-ring platform, pyridine-2-carboxaldehyde (Py-2-CHO) was used as the auxiliary ligand in the reaction between zinc acetate and dippH2 , leading to isolation of [Zn(dipp)(Py-2-CH(OH)(OMe))]4 (3). Understandably, recrystallization of 3 from acetonitrile yields the parent aldehydic form, [Zn(dipp)(Py-2-CHO)]4 (4). Single-crystal X-ray diffraction studies reveal that supramolecular bonding, aided by hydrogen-bonding interactions involving the hemiacetal functionalities (C-OH, C-OMe, and C-H), are responsible for the observed stabilization. The hemiacetal/aldehyde groups in 1 and 2 readily react with p-toluidine, 2,6-dimethylaniline, and 4-bromoaniline to yield the corresponding tetra-Schiff base ligands, [Zn(dipp)(L)]4 (L=4-methyl-N-(pyridin-4-ylmethylidene)aniline (5), 2,6-dimethyl-N-(pyridin-4-ylmethylene)-aniline (6), and 4-bromo-N-(pyridin-4-ylmethylene)aniline (7)). Isolation of 5-7 opens up further possibilities of using 1 and 2 as new supramolecular synthons and ligands.

Role of 4,4′-bipyridine versus longer spacers 4,4′-azobipyridine, 1,2-bis(4-pyridyl)ethylene, and 1,2-bis(pyridin-3-ylmethylene)hydrazine in the formation of thermally labile metallophosphate coordination polymers

One dimensional metallophosphate coordination polymers {[M(dtbp)2(azopy)(H2O)2]·(azopy)}x (M = Mn (1); Co (2); Cu (3); Cd (4)) have been synthesized from the reaction of a suitable metal precursor with di-tert-butylphosphate (dtbp-H) in the presence of ditopic linker 4,4′-azobipyridine (azopy) in a 1 : 2 : 2 stoichiometric ratio. Isostructural compounds 1–4 have been characterized by analytical and spectroscopic methods and single crystal X-ray diffraction studies. Single crystal X-ray diffraction measurements further reveal that compounds 1–3 (all C2/c) and 4 (P) are linear 1D coordination polymers. The uncoordinated 4,4′-azobipyridine in the lattice is responsible for the conversion of these 1D coordination polymers into 3D supramolecular assemblies through O–H⋯N hydrogen bonding interactions between coordinated water and N-centers of an azopy ligand. Similar reactions carried out using 1,2-bis(4-pyridyl)ethylene (bpe) as the linker yielded compounds having the formula {[M(dtbp)2(bpe)(H2O)2]·(bpe)}x (M = Mn (5); Co (6); Cu (7); Ni (8)). Compounds 5–8 have been characterized by analytical and spectroscopic methods. Preliminary single crystal X-ray diffraction studies carried out on poorly diffracting crystals of 5 and 6 establish their isostructural nature to 1–4, also displaying a similar supramolecular aggregation behaviour. A longer ditopic N,N′-donor ligand, 1,2-bis(pyridin-3-ylmethylene)hydrazine (bph), has been used in place of bpe/azoby to synthesize 1-dimensional coordination polymers [{M(bph)(H2O)4}{(dtbp)2}] (M = Ni (9) and M = Co (10)) and [Cd(bph)3(dtbp)2]n (11) which have completely different structural motifs when compared to 1–4.

Five different pseudo-polymorphs of 4-aminoarylphosphate: supramolecular aggregation in organophosphates

2,6-Diisopropyl-4-nitrophenylphosphate (A), synthesized in two steps from 2,6-diisopropyl-4-nitrophenol, serves as a convenient starting point to prepare several N-functionalized monoaryl phosphates via 2,6-diisopropyl-4-aminophenylphosphate (adippH2) (1), which quite interestingly exhibits five different pseudo-polymorphic structures 1a–1e. Several of these five polymorphic forms are inter-convertible to one another through single crystal to single crystal (SC–SC) transformation, aided by evacuation and solvent soaking protocols. Crystallization of crude 1 in methanol results in the isolation of the first polymorph [adippH2·CH3OH·1/3H2O]3 (1a), while evacuation of 1a results in the loss of solvent molecules from the crystal lattice to yield a new polymorph [adippH2·1/3H2O]3 (1b). Polymorph 1b can be converted back to 1a by soaking the single crystals of 1b in methanol. Similarly, soaking single crystals of 1a in DMF or in water yields two more new polymorphs [adippH2·1/3CH3OH·1/3DMF·1/3H2O]3 (1c) and [adippH2·1/2H2O]2 (1d), respectively. Polymorph 1c can once again be re-transformed to the original 1a by soaking its single crystals in methanol, whereas the SC–SC conversion of 1a to 1d is found to be irreversible. Polymorph 1e is obtained by crystallization of 1a in dry methanol. Interaction of adippH21 with triflic acid and 1,10-phenanthroline yields the corresponding salts 2 and 3, respectively. Acetylation of 1a results in the isolation of acetylamino derivative 4. Phosphate diesters 5 and 6 featuring both aryloxy and alkoxy substituents on the central phosphorus have also been synthesized by a sequential reaction starting from (4-NO2-2,6-iPr2C6H2O)P(O)Cl2. All new compounds have been exhaustively characterized by analytical and spectroscopic methods apart from establishing their crystal structures by single crystal X-ray diffraction studies. Owing to the simultaneous presence of phosphate ester groups (featuring P–OH and PO functionalities), para amino or nitro group on the aryl substituent, and solvent molecules such as methanol and water, the crystal structures of 1a–1e and 2–6 exhibit interesting supramolecular aggregation that is primarily aided by H-bond networks.

Complex Structural Landscape of Titanium Organophosphonates: Isolation of Structurally Related Ti4, Ti5, and Ti6 Species and Mechanistic Insights

[Ti(acac)2(OiPr)2] reacts with tert-butylphosphonic acid to yield a series of titanium organophosphonates such as tetranuclear [Ti4(acac)4(μ-O)2(μ-tBuPO3)2(μ-tBuPO3H)4]·2CH3CN (1), pentanuclear [Ti5(acac)5(μ-O)2(OiPr)(μ-tBuPO3)4(μ-tBuPO3H)2] (2), hexanuclear [Ti6(acac)6(μ-O)2(OiPr)2(μ-tBuPO3)6] (3), or [Ti6(acac)6(μ-O)3(OiPr)(μ-tBuPO3)5(μ-tBuPO3H)]·2CH3CN (4). The isolation of each of these products in pure form depends on the molar ratio of the reactants or the solvent medium. Among these, 3 is obtained as the only product when the reaction is conducted in CH2Cl2. The structural analysis reveals that a simple cluster growth route relates the clusters 1-4 to each other and that a reactive cyclic single-4-ring titanophosphonate [Ti(acac)(OiPr)2(tBuPO3H)]2 is the fundamental building block. While the tetranuclear 1 has structural resemblance to the D4R building block of zeolites, the hexanuclear clusters 3 and 4 have the shape of zeolitic D6R building blocks. The presence of adventitious water in the phosphonic acid (arising from small quantities of hydrogen-bonded water) results in the formation of μ-O2- bridges across an adjacent pair of titanium centers in clusters 1-4. To further verify the stability of the hexanuclear cluster over other structural forms, the reaction of tBuPO3H2 was performed with [Ti(acac)2(O)], instead of Ti(acac)2(OiPr)2, in CH3CN to yield [Ti6(acac)6(μ-O)4(μ-tBuPO3)4(μ-tBuPO3H)2]·2CH3CN (5). Compound 5 exhibits a core structure similar to those of 3 and 4 with small variations in the intracluster Ti-O-Ti linkage. Compound 3 is an efficient and selective catalyst for olefin epoxidation under both homogeneous and heterogeneous conditions.