C. M. Nagaraja

Hydrodefluorination and Other Hydrodehalogenation of Aliphatic Carbon−Halogen Bonds Using Silylium Catalysis

Trialkylsilylium cation equivalents partnered with halogenated carborane anions (such as Et(3)Si[HCB(11)H(5)Cl(6)]) function as efficient and long-lived catalysts for hydrodehalogenation of C-F, C-Cl, and C-Br bonds with trialkylsilanes as stoichiometric reagents. Only C(sp(3))-halogen bonds undergo this reaction. The range of C-F bond-containing substrates that participate in this reaction is quite broad and includes simple alkyl fluorides, benzotrifluorides, and compounds with perfluoroalkyl groups attached to an aliphatic chain. However, CF(4) has proven immune to this reaction. Hydrodechlorination was carried out with a series of alkyl chlorides and benzotrichlorides, and hydrodebromination was studied only with primary alkyl bromide substrates. Competitive experiments established a pronounced kinetic preference of the catalytic system for activation of a carbon-halogen bond of a lighter halide in primary alkyl halides. On the contrary, hydrodechlorination of C(6)F(5)CCl(3) proceeded much faster than hydrodefluorination of C(6)F(5)CF(3) in one-pot experiments. A solid-state structure of Et(3)Si[HCB(11)H(5)Cl(6)] was determined by X-ray diffraction methods.

Visible-Light-Assisted Photocatalytic Reduction of Nitroaromatics by Recyclable Ni(II)-Porphyrin Metal-Organic Framework (MOF) at RT.

A microporous Ni(II)-porphyrin metal-organic framework (MOF), [Ni3(Ni-HTCPP)2(μ2-H2O)2(H2O)4(DMF)2]·2DMF, (MOF1) (where, Ni-HTCPP = 5,10,15,20-tetrakis(4-benzoate) porphyrinato-Ni(II)) has been synthesized by the solvothermal route. Single-crystal X-ray diffraction study of 1 reveals a 2D network structure constituted by Ni3 cluster and [Ni-HTCPP](3-) metalloligand having (3, 6)-connected binodal net with {4(3)}2{4(6)·6(6)·8(3)}-kgd net topology. The 2D layers are further stacked together through π-π interactions between the porphyrin linkers to generate a 3D supramolecular framework which houses 1D channels with dimension of ∼5.0 × 9.0 Å(2) running along the crystallographic a-axis. Visible-light-assisted photocatalytic investigation of MOF1 for heterogeneous reduction of various nitroaromatics at room temperature resulted in the corresponding amines with high yield and selectivity. On the contrary, the Ni(II)-centered porphyrin tetracarboxylic acid [Ni-H4TCPP] metalloligand does not show the photocatalytic activity under similar conditions. The remarkably high catalytic performance of MOF1 over [Ni-H4TCPP] metalloligand has been attributed due to cooperative catalysis involving the Ni-centered porphyrin secendary building units (SBUs) and the Ni3-oxo node. Further, the MOF1 was recycled and reused up to three cycles without any significant loss of catalytic activity as well as structural rigidity. To the best of our knowledge, MOF1 represents the first example of MOF based on 3d metal ion exhibiting visible-light-assisted reduction of nitroaromatics under mild conditions without the assistance of noble metal cocatalysts.

Construction of 2D interwoven and 3D interpenetrated metal–organic frameworks of Zn(II) by varying N,N′-donor spacers

Four new metal–organic frameworks (MOFs) of Zn(II) ions, [Zn2(muco)2(azopy)2]·3DMF·2H2O (1), [Zn(muco)(bpee)]·4H2O (2), [Zn(muco)(3bpdh)] (3), and [Zn4(muco)4(4bpdh)4]·4bpdh·2H2O (4) (where, muco = trans,trans-muconate dianion, azopy = 4,4′-bisazobipyridine, bpee = 1,2-bis(4-pyridyl)ethylene, 3bpdh = 2,5-bis(3-pyridyl)-3,4-diaza-2,4-hexadiene, and 4bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) have been synthesized using mixed ligand systems and characterized structurally by single-crystal X-ray diffraction. Compound 1 has a 2D network with 2-fold interwoven, (4,4)-connected, {44·62}-sql net topology. Compounds 2 and 3 have 3D diamondoid (dia) structures with interesting 5-fold and 3-fold interpenetrated nets, respectively, whereas, compound 4 has a 3D cubic (pcu, α-Po) structure with 2-fold interpenetrating, 6-connected, {412·63} net topology. Topological analyses of 2 and 3 reveal 4-connected nets with 66 topology. In spite of the interweaving/interpenetration, compounds 1 and 2 possess rectangular channels with dimensions of 3.9 × 4.5 A2 and 4.3 × 7.0 A2, respectively. Photoluminescence studies showed the emissions of 1–4 and the thermal stabilities of the compounds were also examined.

One-pot, template-free syntheses of spherical ZnS nanocrystals using a new S2− source and their photocatalytic study

A facile, one-pot synthesis of template-free ZnS microspheres composed of nanocrystals (NCs) has been successfully carried out using the solvothermal method with 4,4′-dibenzyldisulfide (DBDS = (C7H7)2S2)) as a new temperature controlled in situ source of S2− ions without (S1–S3) and with the use of a capping agent (S4). The powder X-ray diffraction measurements of all the four (S1–S4) samples revealed the cubic or zinc blende (ZB) structure of the ZnS microspheres. FESEM analyses showed an almost spherical morphology of the ZnS microspheres which are composed of smaller NCs. TEM analyses of samples S3 and S4 confirmed the ZnS microspheres with assembled NCs. Optical measurements of the samples (S1–S4) showed a blue-shift in the UV-vis absorption maxima compared to that of bulk ZnS due to the quantum confinement effect. Photoluminescence measurements show intense blue emission of the samples. Photocatalytic investigation of the uncapped (S3) and mercaptoethanol (MCE)-capped (S4) ZnS microspheres for degradation of methyl orange (MO) revealed higher photocatalytic activity of S3 over S4 under UV light irradiation. The lower catalytic activity of S4 has been attributed to the presence of MCE capping agents which act as barriers for the interaction of MO molecules with the ZnS NCs. The proposed mechanism for the formation of ZnS microspheres and their photocatalytic activity has also been presented.

Template-free syntheses of CdS microspheres composed of ultrasmall nanocrystals and their photocatalytic study

Template-free CdS microspheres composed of nanocrystals have been successfully synthesized by a one-pot solvothermal method using 4,4′-dipyridyldisulfide (DPDS = (C5H4N)2S2)) as a temperature controlled in situ source of S2− ions without (S1–S3) and with the use of capping agent (S4). The powder X-ray diffraction measurements of all four (S1–S4) samples revealed the cubic structure of the CdS microspheres and SEM analyses showed almost spherical morphology of the CdS microspheres with a broad size range of 0.5 to 2 μm. TEM analyses of the samples S3 and S4 revealed that the CdS microspheres are composed of assembled CdS nanocrystals of ultrasmall (2–5 nm) size. Optical investigation of the samples (S1–S4) showed blue-shift in the UV-vis absorption maxima compared to that of bulk CdS due to quantum confinement effects. Photocatalytic investigation of the uncapped (S3) and mercaptoethanol (MCE)-capped (S4) CdS microspheres for degradation of methyl orange (MO) revealed that the rate of photocatalytic activity of S3 is much higher than that of S4 under both UV and natural sunlight irradiation. The relatively lower activity of S4 has been attributed to the presence of MCE capping agents which acts as a barrier for the interaction of MO molecules with the CdS nanocrystals. The proposed mechanism for the formation of CdS microspheres and their photocatalytic activity has also been presented.

Construction of 3D homochiral metal–organic frameworks (MOFs) of Cd(II): selective CO2 adsorption and catalytic properties for the Knoevenagel and Henry reaction

Two new homochiral metal–organic frameworks of Cd(II), [{Cd2(L-glu)2(bpe)3(H2O)}·2H2O] (1) and [{Cd3(L-glu)2(bpe)3(H2O)}·2NO3·H2O] (2) (where, L-glu = L-glutamate dianion, and bpe = 1,2-bis(4-pyridyl)ethylene) have been synthesized solvothermally by employing two different temperatures. Single crystal X-ray diffraction studies revealed that both 1 and 2 are homochiral and possess a 3D pillar-layered framework structure having 4,8- and 8,10-connected binodal nets with vertex symbols of {3^2.4.5^3}{3^4.4^6.5^10.6^8} and {3^11.4^28.5^5.6}2{3^8.4^18.5.6}, respectively. Interestingly, solvothermal synthesis carried out at 100 °C resulted in a 3D framework, 1 which features large rectangular 1D channels with a dimension of ∼10.38 × 4.44 A2 decorated with pendant –NH2 groups. Whereas, increasing the temperature of the reaction to 120 °C led to a non-porous highly connected 3D framework, 2 in which the –NH2 group of the L-glu ligand is coordinated to a Cd(II) node. Gas (N2, CO2, H2 and Ar) uptake studies on the dehydrated framework of 1 revealed excellent selectivity for CO2 over other gases at 273 K with a high isosteric heat of adsorption (Qst) value of 40.8 kJ mol−1. The high selectivity for CO2 gas has been attributed to the stronger interaction of CO2 with the basic –NH2 functionalized pore surface of compound 1. Furthermore, 1 acts as a very good recyclable catalyst for the carbon–carbon bond forming reactions, such as the Knoevenagel condensation and Henry reaction of benzaldehydes. Moreover, the catalyst can be easily separated from the reaction mixture and reused in four consecutive cycles without significant loss of catalytic activity and structural rigidity.

Construction of 2D interwoven and 3D metal–organic frameworks (MOFs) of Cd(II): the effect of ancillary ligands on the structure and the catalytic performance for the Knoevenagel reaction

Three new Cd(II) metal–organic networks, [{Cd(muco)(bpa)1.5}·H2O] (1), [{Cd(muco)(bpee)1.5}·7H2O] (2) and [Cd(muco)(4bpdh)·(H2O)] (3) (where, muco = trans, trans-muconate dianion, bpa = 1,2-bis(4-pyridyl)ethane, bpee = 1,2-bis(4-pyridyl)ethylene and 4bpdh = 2,5-bis(4-pyridyl)-3,4-diaza-2,4-hexadiene) have been constructed using mixed ligand systems at room temperature and characterized by single-crystal X-ray diffraction and other physicochemical methods. Compounds 1 and 2 are isostructural featuring a 3D framework structure with a 5-connected, {66} net topology. Whereas, compound 3 possess an interesting 3-fold interwoven 2D network with a 4-connected, {44,62}-sql net topology. Photoluminescence measurements revealed emissions from all the three compounds owing to ligand based charge transfer (n → π* and π → π*) transitions. Catalytic investigations of the compounds for the Knoevenagel reaction unveiled the higher catalytic activity of 3 compared to that of 1 and 2. The higher catalytic performance of 3 has been attributed due to the presence of the basic azine-functionalized pore surface. Remarkably, the catalyst can be facilely separated from the reaction mixture and could be reused without significant degradation in the catalytic activity for five cycles. Compound 3 is a rare example of a 3-fold interwoven 2D network acting as an efficient recyclable heterogeneous catalyst for the Knoevenagel reaction.

Correlating Single Crystal Structure, Nanomechanical, and Bulk Compaction Behavior of Febuxostat Polymorphs

Febuxostat exhibits unprecedented solid forms with a total of 40 polymorphs and pseudopolymorphs reported. Polymorphs differ in molecular arrangement and conformation, intermolecular interactions, and various physicochemical properties, including mechanical properties. Febuxostat Form Q (FXT Q) and Form H1 (FXT H1) were investigated for crystal structure, nanomechanical parameters, and bulk deformation behavior. FXT Q showed greater compressibility, densification, and plastic deformation as compared to FXT H1 at a given compaction pressure. Lower mechanical hardness of FXT Q (0.214 GPa) as compared to FXT H1 (0.310 GPa) was found to be consistent with greater compressibility and lower mean yield pressure (38 MPa) of FXT Q. Superior compaction behavior of FXT Q was attributed to the presence of active slip systems in crystals which offered greater plastic deformation. By virtue of greater compressibility and densification, FXT Q showed higher tabletability over FXT H1. Significant correlation was found with anticipation that the preferred orientation of molecular planes into a crystal lattice translated nanomechanical parameters to a bulk compaction process. Moreover, prediction of compactibility of materials based on true density or molecular packing should be carefully evaluated, as slip-planes may cause deviation in the structure-property relationship. This study supported how molecular level crystal structure confers a bridge between particle level nanomechanical parameters and bulk level deformation behavior.

Green synthesis, optical and magnetic properties of a MnII metal–organic framework (MOF) that exhibits high heat of H2 adsorption

Green synthesis of a 3D, Mn(II) metal–organic framework (MOF) composed of a trimeric Mn(II) cluster and NDC linker formulated as [Mn3(NDC)3(DMA)4]n (1) (where, NDC = 2,6 napthalene dicarboxylic acid, DMA = N,N-dimethylacetamide) has been achieved by employing mechanochemical and sonochemical routes. Interestingly, MOF 1 undergoes reversible structural transformation upon desolvation and solvation of DMA molecules. The desolvated framework, 1′ containing two unsaturated Mn(II) sites exhibits interesting H2 and CO2 uptake properties with isosteric heats of adsorption (Qst) for H2 and CO2 of 11.8 and 29.2 kJ mol−1, respectively. Remarkably, the Qst of H2 estimated for 1′ is found to be the highest value amongst the Mn(II) MOFs reported so far and the high value has been attributed to the stronger interaction of H2 with the unsaturated Mn(II) centers. Further, variable temperature magnetic susceptibility measurements of 1 revealed weak antiferromagnetic coupling interactions between the adjacent Mn(II) ions. The thermal stability of 1 has also been examined and was found to be highly stable. Photoluminescence investigation revealed that the emission from MOF 1 is owed to ligand based charge transfer transitions. Furthermore, compound 1 undergoes temperature induced solid-state conversion into phase pure MnO nanocrystals of about 10–18 nm in size embedded in a carbonaceous layer to form MnO–C nanocomposite (NC). The MnO–C NC has been characterized by PXRD, FE-SEM, EDAX and TEM analyses.

Effect of differential surface anisotropy on performance of two plate shaped crystals of aspirin form I

Abstract Differential surface anisotropy of different crystals of the same API can have a significant impact on their pharmaceutical performance. The present work investigated the impact of differential surface anisotropy of two plate‐shaped crystals of aspirin (form I) on their hygroscopicity, stability and compaction behavior. These crystals differed in their predominant facets (100) and (001) and were coded as AE‐100 & E‐001. (100) facets exposed polar carbonyl groups which provided hydrophilicity to the facets. In contrast, (001) facets possessed hydrophobicity as they exposed non‐polar aryl and methyl groups. Both the samples showed different degradation behavior, at various stability conditions (i.e. 40 °C/75%RH, 30 °C/90%RH and 30 °C/60%RH) and different time intervals. Polar groups of aspirin have been reported to be prone to hydrolysis due to which AE‐100 was less stable than E‐001. Dynamic vapor sorption (DVS) analysis at different simulated stability conditions also supported this observation, wherein AE‐100 showed higher moisture sorption than E‐001. Both the samples having similar particle size, shape, surface area and hardness value, showed differences in their compactibility. However, milling narrowed down the predominance of facets and both the milled samples showed similar stability and compaction behavior. This study was also supported by surface free energy determination, molecular modeling and face indexation of unmilled and milled samples.