Achintesh Narayan Biswas

Modeling TauD-J: A High-Spin Nonheme Oxoiron(IV) Complex with High Reactivity toward C–H Bonds

High-spin oxoiron(IV) species are often implicated in the mechanisms of nonheme iron oxygenases, their C-H bond cleaving properties being attributed to the quintet spin state. However, the few available synthetic S = 2 Fe(IV)═O complexes supported by polydentate ligands do not cleave strong C-H bonds. Herein we report the characterization of a highly reactive S = 2 complex, [Fe(IV)(O)(TQA)(NCMe)](2+) (2) (TQA = tris(2-quinolylmethyl)amine), which oxidizes both C-H and C═C bonds at -40 °C. The oxidation of cyclohexane by 2 occurs at a rate comparable to that of the oxidation of taurine by the TauD-J enzyme intermediate after adjustment for the different temperatures of measurement. Moreover, compared with other S = 2 complexes characterized to date, the spectroscopic properties of 2 most closely resemble those of TauD-J. Together these features make 2 the best electronic and functional model for TauD-J to date.

Fluorescence signaling systems for sensing Hg(II) ion derived from A2B-corroles

Selective detection of Hg(II) ions in solution by a series of novel free base bis-(nitrophenyl) corroles (1-4) with general formula A(2)B (where A = nitrophenyl, and B = N,N-dimethylaminophenyl, thienyl, naphthyl and tridecyloxyphenyl group) is described. Among the free base corroles, 4, with a tridecyloxy long chain moiety, has been found to exhibit the highest Hg(II) sensing ability. The detection is based on the fluorescence quenching of the corroles, arising from the combined effect of static (coordination) and dynamic (exciplex formation) factors.

Modeling Non-Heme Iron Halogenases: High-Spin Oxoiron(IV)–Halide Complexes That Halogenate C–H Bonds

The non-heme iron halogenases CytC3 and SyrB2 catalyze C-H bond halogenation in the biosynthesis of some natural products via S = 2 oxoiron(IV)-halide intermediates. These oxidants abstract a hydrogen atom from a substrate C-H bond to generate an alkyl radical that reacts with the bound halide to form a C-X bond chemoselectively. The origin of this selectivity has been explored in biological systems but has not yet been investigated with synthetic models. Here we report the characterization of S = 2 [Fe(IV)(O)(TQA)(Cl/Br)](+) (TQA = tris(quinolyl-2-methyl)amine) complexes that can preferentially halogenate cyclohexane. These are the first synthetic oxoiron(IV)-halide complexes that serve as spectroscopic and functional models for the halogenase intermediates. Interestingly, the nascent substrate radicals generated by these synthetic complexes are not as short-lived as those obtained from heme-based oxidants and can be intercepted by O2 to prevent halogenation, supporting an emerging notion that rapid rebound may not necessarily occur in non-heme oxoiron(IV) oxidations.

C(naphthyl)-H bond activation by rhodium: isolation, characterization and TD-DFT study of the cyclometallates{

The C1(naphthyl)–H, C2(naphthyl)–H, C3(naphthyl)–H and C8(naphthyl)–H bonds of the naphthyl group present in a group of naphthylazo–2′–hydroxyarenes (H2L) have been activated by [Rh(PPh3)3Cl] in a toluene medium. Here the cyclometallation is accompanied by metal centered oxidation [Rh(I)→Rh(III)]. All the resulting cyclometallates [Rh(PPh3)2(L)Cl] (2–5) have been isolated in a pure form. The characterization of the cyclometallates [Rh(PPh3)2(L)Cl] have been done on the basis of spectral (IR, UV–vis, and FAB mass) data. The structures of the representative cyclometallates 2a, 3a, 4a, 4b and 5b have been determined by X-ray diffraction. In all the cyclometallates, rhodium(III) is coordinated to naphthylazo–2′–hydroxyarenes via terdentate C(naphthyl), N(diazene), O(phenolato/ naphtholato) donor centers & one chloride ion in a plane along with two axial transPPh3 molecules. Intermolecular association in the solid state is observed due to C–H⋯π and π⋯π interactions. Compounds show an oxidative response within 0.93 to 1.11 V (vs.SCE) and a reductive response at ∼ −1.0 V (vs.SCE). Both the responses are based on the coordinated diazene function and are irreversible in nature, indicating limited stability of the oxidized and reduced species. The electronic structures of selected cyclometallates have been calculated using a TD-DFT model and the simulated spectra are consistent with the observed spectra of those cyclometallates.

A non-heme cationic Fe(III)-complex intercalated in montmorillonite K-10: synthesis, characterization and catalytic alkane hydroxylation with H2O2 at room temperature

A non-heme iron complex cis-[FeIII(cyclam)Cl2]Cl (cyclam = 1,4,8,11-tetraazacyclotetradecane) has been intercalated into smectite montmorillonite K-10. The intercalated solid has been characterized using EDXRF, AAS, TGA, PXRD, IR and UV-visible analyses. The heterogeneous iron(III)–cyclam has been found to be capable of hydroxylating C–H bonds as strong as those in cyclohexane (BDE = 99.7 kcal mol−1) using benign H2O2 at room temperature. The reactivity of the heterogeneous catalytic system is found to significantly improve in comparison with that of cis-[FeIII(cyclam)Cl2]Cl/H2O2 under homogeneous conditions. The catalytic reactions are marked by a very high selectivity for alcohols which is comparable with the best known non-heme catalysts with H2O2. The results critically reflect the role of the clay matrix surrounding the cationic metal complex in tuning the catalytic activity and selectivity of alkane hydroxylation.

Synthesis and Liquid Crystalline Properties of Novel Triazene-1-oxide Derivatives

A new group of compounds, C6H5-N(O)=N-NH-C6H4-C(O)-O-C6H4-CH=N-C6H4-OR, ( 1 ), [R = n-alkyl group; 1a (R = C7H15 ) to 1h (R = C14H29)], has been designed and synthesized to develop triazene-1-oxide based liquid crystals. All the compounds have been characterized on the basis of their spectral data. Their thermal behavior has been examined and found to exhibit thermotropic liquid crystalline behavior over a wide range of temperature. All members of this family uniformly show nematic phase behavior on melting up to the clearing point during heating as well as on phase entering up to the solidification during cooling. An odd–even effect on transition temperatures has been observed during heating as well as cooling.

Novel synthetic route to liquid crystalline 4,4′‐bis(n‐alkoxy)azoxybenzenes: spectral characterisation, mesogenic behaviour and crystal structure of two new members

A simple synthetic method has been developed for the preparation of long‐chain 4,4′‐bis(n‐alkoxy)azoxybenzenes by reductive coupling of 4‐n‐alkoxynitrobenzenes using zinc powder and ammonium chloride in aqueous ethanol medium at ambient temperature. The new method was employed to synthesise known members (n = 1–12) of the 4,4′‐bis(n‐alkoxy)azoxybenzene (C n H2n+1OPhN(O) = NPhOC n H2n+1) series and also two hitherto unknown members (n = 13–14) of the series. The new compounds were characterised using spectral (IR, UV–visible, 1H NMR and FAB‐MS) data. The mesogenic behaviour of both compounds was studied by polarising optical microscopy, differential scanning calorimetry and small‐angle X‐ray diffraction techniques. The crystal structure of 4,4′‐bis(n‐tetradecyloxy)azoxybenzene was determined using single‐crystal X‐ray diffraction data. The packing of the molecules in the crystalline state is found to be a precursor to the smectic C phase structure.

Liquid crystalline aryltriazene‐1‐oxides with two ester units: synthesis, characterisation, structure and thermal properties

A new series of mesogenic triazene‐1‐oxides, C6H5–N(O) = N–NH–C6H4–C(O)–O–C6H4–O–(O)C–C6H4–OR (1, R = n‐alkyl group from CH3 to C14H29), was designed and synthesised. All members of this new series were characterised on the basis of spectral and analytical data. The thermotropic liquid crystalline behaviour of the compounds was observed over a wide temperature range using optical microscopy. The mesophase structure was confirmed by a small‐angle X‐ray diffraction study of a representative member (1k). The molecular structure of compound 1i was determined using the single crystal X‐ray diffraction method as a representative case. Dimer formation in the solid state occurs due to intermolecular N–H…O and C–H…O interactions. Intermolecular C–H…π interactions were also detected in 1i. The intermolecular hydrogen bonding and intermolecular C–H…π interactions arrange the phenyl triazene‐1‐oxide fragments of the molecules in layers within the molecular assembly.

Structure of Liquid Crystalline 1-Phenyl-3-{4-[4-(4-octyloxybenzoyloxy)phenyloxycarbonyl]phenyl}triazene-1-oxide at Low Temperature

The molecular structure of 1-phenyl-3-{4-[4-(4-octyloxybenzoyloxy)-phenyloxycarbonyl]phenyl}triazene-1-oxide, a member of newly developed liquid crystalline homologous series, has been investigated by crystal X-ray crystallography at low temperature (100 K). The title compound crystallizes in the triclinic crystal class in the space group P with cell parameters a = 5.766(5) Å, b = 12.151(10) Å, c = 21.751(17) Å, α = 79.089(13)°, β = 88.646(14)°, γ = 84.278(14)°, V = 1489(2) Å3 for Z = 2. It establishes the N-oxide form of the triazene-1-oxide moiety. The overall molecule is not planar, the dihedral angles between pairs of adjacent benzene rings are 14.00 (10), 52.36 (07), and 50.57 (07)°. Intramolecular N–H…O hydrogen-bonding is present within the triazene-1-oxide moiety of the title compound. The compound forms inversion dimer via an intermolecular N–H…O and an intermolecular C–H…O links. The dimers are then linked into chains in a parallel fashion by C–H…O hydrogen bonds. The crystal packing is further stabilized by C–H…π interactions.

1-Phenyl-3-{4-[4-(4-undecyl-oxybenzoyl-oxy)phenyl-oxycarbon-yl]phen-yl}triazene 1-oxide.

The X-ray crystallographic study of the title compound, C37H41N3O6, at 150 K establishes the N-oxide form of the triazene 1-oxide unit. There is one intramolecular N—H⋯O hydrogen-bonding interaction and the crystal packing is stabilized by one N—H⋯O, three C—H⋯O and three C—H⋯π intermolecular interactions. The dihedral angles between pairs of adjacent benzene rings are 14.9 (3), 56.3 (1) and 56.0 (1)°