Purak Das

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.

Synthesis and Characterization of a Series of Structurally and Electronically Diverse Fe(II) Complexes Featuring a Family of Triphenylamido-Amine Ligands

A family of triphenylamido-amine ligands of the general stoichiometry L(x)H(3) = [R-NH-(2-C(6)H(4))](3)N (R = 4-t-BuPh (L(1)H(3)), 3,5-t-Bu(2)Ph (L(2)H(3)), 3,5-(CF(3))(2)Ph (L(3)H(3)), CO-t-Bu (L(4)H(3)), 3,5-Cl(2)Ph (L(5)H(3)), COPh (L(6)H(3)), CO-i-Pr (L(7)H(3)), COCF(3) (L(8)H(3)), and i-Pr (L(9)H(3))) has been synthesized and characterized, featuring a rigid triphenylamido-amine scaffold and an array of stereoelectronically diverse aryl, acyl, and alkyl substituents (R). These ligands are deprotonated by potassium hydride in THF or DMA and reacted with anhydrous FeCl(2) to afford a series of ferrous complexes, exhibiting stoichiometric variation and structural complexity. The prevalent [(L(x))Fe(II)-solv](-) structures (L(x) = L(1), L(2), L(3), L(5), solv = THF; L(x) = L(8), solv = DMA; L(x) = L(6), L(8), solv = MeCN) reveal a distorted trigonal bipyramidal geometry, featuring ligand-derived [N(3,amido)N(amine)] coordination and solvent attachment trans to the N(amine) atom. Specifically for [(L(8))Fe(II)-DMA](-), a N(amido) residue is coordinated as the corresponding N(imino) moiety (Fe-N(Ar) horizontal lineC(CF(3))-O(-)). In contrast, compounds [(L(4))Fe(II)](-), [(L(6))(2)Fe(II)(2)](2-), [K(L(7))(2)Fe(II)(2)](2)(2-), and [K(L(9))Fe](2) are all solvent-free in their coordination sphere and exhibit four-coordinate geometries of significant diversity. In particular, [(L(4))Fe(II)](-) demonstrates coordination of one amidato residue via the O-atom end (Fe-O-C(t-Bu) horizontal lineN(Ar)). Furthermore, [(L(6))(2)Fe(II)(2)](2-) and [K(L(7))(2)Fe(II)(2)](2)(2-) are similar structures exhibiting bridging amidato residues (Fe-N(Ar)-C(R) horizontal lineO-Fe) in dimeric structural units. Finally, the structure of [K(L(9))Fe](2) is the only example featuring a minimal [N(3,amido)N(amine)] coordination sphere around each Fe(II) site. All compounds have been characterized by a variety of physicochemical techniques, including Mossbauer spectroscopy and electrochemistry, to reveal electronic attributes that are responsible for a range of Fe(II)/Fe(III) redox potentials exceeding 1.0 V.

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.

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.

CCDC 650028: Experimental Crystal Structure Determination

Related Article: Achintesh Narayan Biswas, Debatra Narayan Neogi, Purak Das, Amitava Choudhury, Pinaki Bandyopadhyay|2014|J.Organomet.Chem.|761|147|doi:10.1016/j.jorganchem.2014.03.023

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)°

(E)-4-{[2-(Methylsulfanyl)phenyl]diazenyl}phenol

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