Manideepa Saha

Copper–organic frameworks assembled from in situ generated 5-(4-pyridyl)tetrazole building blocks: synthesis, structural features, topological analysis and catalytic oxidation of alcohols

Two new metal-organic compounds {[Cu3(μ3-4-ptz)4(μ2-N3)2(DMF)2](DMF)2}n (1) and {[Cu(4-ptz)2(H2O)2]}n (2) {4-ptz = 5-(4-pyridyl)tetrazolate} with 3D and 2D coordination networks, respectively, have been synthesized while studying the effect of reaction conditions on the coordination modes of 4-pytz by employing the [2 + 3] cycloaddition as a tool for generating in situ the 5-substituted tetrazole ligands from 4-pyridinecarbonitrile and NaN3 in the presence of a copper(ii) salt. The obtained compounds have been structurally characterized and the topological analysis of 1 discloses a topologically unique trinodal 3,5,6-connected 3D network which, upon further simplification, results in a uninodal 8-connected underlying net with the bcu (body centred cubic) topology driven by the [Cu3(μ2-N3)2] cluster nodes and μ3-4-ptz linkers. In contrast, the 2D metal-organic network in 2 has been classified as a uninodal 4-connected underlying net with the sql [Shubnikov tetragonal plane net] topology assembled from the Cu nodes and μ2-4-ptz linkers. The catalytic investigations disclosed that 1 and 2 act as active catalyst precursors towards the microwave-assisted homogeneous oxidation of secondary alcohols (1-phenylethanol, cyclohexanol, 2-hexanol, 3-hexanol, 2-octanol and 3-octanol) with tert-butylhydroperoxide, leading to the yields of the corresponding ketones up to 86% (TOF = 430 h(-1)) and 58% (TOF = 290 h(-1)) in the oxidation of 1-phenylethanol and cyclohexanol, respectively, after 1 h under low power (10 W) microwave irradiation, and in the absence of any added solvent or additive.

Greener Selective Cycloalkane Oxidations with Hydrogen Peroxide Catalyzed by Copper-5-(4-pyridyl)tetrazolate Metal-Organic Frameworks

Microwave assisted synthesis of the Cu(I) compound [Cu(µ4-4-ptz)]n [1, 4-ptz = 5-(4-pyridyl)tetrazolate] has been performed by employing a relatively easy method and within a shorter period of time compared to its sister compounds. The syntheses of the Cu(II) compounds [Cu3(µ3-4-ptz)4(µ2-N3)2(DMF)2]n∙(DMF)2n (2) and [Cu(µ2-4-ptz)2(H2O)2]n (3) using a similar method were reported previously by us. MOFs 1-3 revealed high catalytic activity toward oxidation of cyclic alkanes (cyclopentane, -hexane and -octane) with aqueous hydrogen peroxide, under very mild conditions (at room temperature), without any added solvent or additive. The most efficient system (2/H2O2) showed, for the oxidation of cyclohexane, a turnover number (TON) of 396 (TOF of 40 h−1), with an overall product yield (cyclohexanol and cyclohexanone) of 40% relative to the substrate. Moreover, the heterogeneous catalytic systems 1–3 allowed an easy catalyst recovery and reuse, at least for four consecutive cycles, maintaining ca. 90% of the initial high activity and concomitant high selectivity.

The effect of remote substitution on the formation of preferential isomers of cobalt(III)-tetrazolate complexes by microwave assisted cycloaddition

The 1,3-dipolar cycloaddition reaction of cis-[Co(N3)2(en)2]NO31 with different organonitriles (NCR) under focussed microwave irradiation produced bis-tetrazolate complexes [Co(N4CR)2(en)2](NO3). Interestingly, in the case of 3-cyano pyridine the reaction produced both cis- and trans-isomers (cis-2 and trans-2), whereas for 4-cyano pyridine the compound obtained was exclusively cis (cis-3) and for 4-bromobenzonitrile it was only the trans- (trans-4) compound which was isolated. This indicates a probable role of remote substitution of the phenyl ring in dictating the formation of the preferential isomer. When starting from the trans-isomer of the diazido complex (trans-[Co(N3)2(en)2]ClO4, 1a), upon reacting with different nitriles a mixture of cis- and trans-isomers of [Co(N4CR)2(en)2]ClO4 was produced in each case, with a greater preference towards cis-geometry [R = 4-NC5H4 (cis-5 and trans-5), 4-BrC6H4 (cis-6 and trans-6) and C6H5 (cis-7 and trans-7)]. The preferential formation of the cis-analogue of compound trans-4 when starting from the trans-precursor was quite curious. A theoretical investigation of compounds trans-4 and cis-6 reveals that the greater stability of the trans-complex 4 may arise from additional van der Waals interactions in the solid state because of the presence of an extra DMF molecule as solvent of crystallization. However, an interacting counter-anion and a probable halogen–halogen interaction may also contribute to the formation of preferential isomers for cycloaddition complexes, even in the solution state.

Coordination complexes based on 4-aminobenzonitrile with different dimensionalities

Reaction of 4-aminobenzonitrile with different transition metal ions in the presence of chloride provides metal complexes of different dimensionalities. Whereas platinum forms a 0-D mononuclear complex {[PtCl2(4-ABN)2} 1, three new coordination polymers {[Zn(4-ABN)2(H2O)2](ClO4)2} n 2, {[Cu(4-ABN)2(MeOH)2](ClO4)2} n 3, and {Cd(4-ABN)Cl2} n 4 have been synthesized and characterized.Complexes 2 and 3 crystallize in 1-D polymeric chains, whereas 4 forms a 3-D coordination network, based on fused 12-membered rings of cadmium and chloride. A topological analysis of 4 reveals a uninodal five-connected network with the point (Schlafli) symbol of (44 × 66) and the nov (5/4/08) topology. All the isolated complexes were characterized by IR, elemental, and X-ray single crystal structural analyses. Complexes 1, 2, and 4 were additionally characterized by NMR spectroscopy.