Effects of substituent and catalyst on the intramolecular Povarov reaction—synthesis of chromenonaphthyridines

2-(N-Alkenyl-N-aryl)aminochromone-3-carbaldehyde undergoes intramolecular Povarov reaction with aromatic amines in the presence of Ph₃P·HClO₄ to produce chromenonaphthyridine. The effects of substituent and catalyst have been studied. The substituent on the alkenyl part of aminochromone controls the mode of reaction as well as the stereochemistry of the product.     

Transition metals as electron traps. II. Structures,energetics and electron transfer dissociations of ternary Co,Ni and Zn–peptide complexes in the gas phase

Transition metal cations Co²⁺, Ni²⁺ and Zn²⁺ form 1 : 1 : 1 ternary complexes with 2,2′‐bipyridine (bpy) and peptides in aqueous methanol solutions that have been studied for tripeptides GGG and GGL. Electrospray ionization of these solutions produced singly charged [Metal(bpy)(peptide ? H)]⁺ and doubly charged [Metal(bpy)(peptide)]²⁺ ions (Metal = metal ion) that underwent charge reduction by glancing collisions with Cs atoms at 50 and 100 keV collision energies. Electron transfer to [Metal(bpy)(peptide)]²⁺ ions was less than 4.2 eV exoergic and formed abundant fractions of non‐dissociated charge‐reduced intermediates. Charge‐reduced [Metal(bpy)(peptide)]⁺ ions dissociated by the loss of a hydrogen atom, ammonia, water and ligands that depended on the metal ion. The Ni and Co complexes mainly dissociated by the elimination of ammonia, water, and the peptide ligand. The Zn complex dissociated by the elimination of ammonia and bpy. A sequence‐specific fragment was observed only for the Co complex. Electron transfer to [Metal(bpy)(peptide ? H)]⁺ was 0.6–1.6 eV exoergic and formed intermediate radicals that were detected as stable anions after a second electron transfer from Cs. [Metal(bpy)(peptide ? H)] neutrals and their anions dissociated by the loss of bpy and peptide ligands with branching ratios that depended on the metal ion. Optimized structures for several spin states, electron transfer and dissociation energies were addressed by combined density functional theory and Møller–Plesset perturbational calculations to aid interpretation of experimental data. The experimentally observed ligand loss and backbone cleavage in charge‐reduced [Metal(bpy)(peptide)]⁺ complexes correlated with the dissociation energies at the present level of theory. The ligand loss in ⁺CR^? spectra showed overlap of dissociations in charge‐reduced [Metal(bpy)(peptide ? H)] complexes and their anionic counterparts which complicated spectra interpretation and correlation with calculated dissociation energies. Copyright © 2009 John Wiley & Sons, Ltd.     

Selective sensing of Hg2+ via sol–gel transformation of a cholesterol-based compound

A cholesterol-based dithioacetal functionalised compound 1 has been designed, synthesised and successfully applied for the selective detection of Hg²⁺ ion. The specific Hg²⁺-induced deprotection of the thioacetal functionality of 1 resulted in sol-to-gel transition in DMF-H₂O (1:1, v/v) via the formation of precursor aldehyde 3. This Hg²⁺-prompted rapid sol to gel conversion is insensitive to the nature of Hg²⁺-salt. In the study, other metal ions did not show any change. To the best of our knowledge, this is a first time report of a chemodosimeter that functions as a selective ‘naked-eye’ Hg²⁺-detector by showing in situ sol-to-gel conversion.     

Synthesis and X-ray crystallographic characterization of copper and iron complexes with tetradentate-N4 ligands: Reactivity towards catechol oxidation

The reaction of copper chloride dihydrate and ferric chloride hexahydrate with a tripodal N₄ ligand (ntb) under mild conditions affords two novel complexes [Cu(ntb)Cl]₂[CuCl₄] · 2H₂O (1) and [Fe(ntb)Cl₂]Cl · 3H₂O (2). The reaction of ferric chloride with another N₄ ligand, bispicpn, forms an octahedral mononuclear complex, [Fe(bispicpn)Cl₂]Cl (3). Single-crystal X-ray structural studies of 1, 2 and 3 reveal the formation of hydrogen-bond sustained 3D, 2D and 1D networks, respectively, involving (N–HO) and (N–HCl) interactions. The packing arrangement in 2 further reveals the existence of hexagonal channels with helical propagation along the diagonal of the crystallographic b- and c-axes. The reactions of these complexes with 3,5-di-tert-butylcatechol have been studied in dimethylformamide. NMR techniques have been used to identify the reaction products.     

Photophysical behaviour of 4-(imidazole-1-yl) phenol and its complexation with beta-cyclodextrin in ground and excited states

This paper mainly dwells on photophysics of 4-(imidazole-1-yl) phenol (IDP) in different solvents and temperatures from the investigations of absorption, emission and laser flash photolysis and also on the nature of complexation with beta-cyclodextrin (CD) in ground and excited states. IDP makes 1:1 inclusion complex with beta-CD in ground, excited singlet and also in triplet states. The orientation of complex could be ascertained as imidazole moiety stays inside the cavity with phenol moiety stays in the bulk. A proposed energy level scheme unveils that vibronic interaction and spin-orbit interaction are found to be active differently in aprotic and protic solvents.     

New route to the mixed valence semiquinone-catecholate based mononuclear FeIII and catecholate based dinuclear MnIII complexes: first experimental evidence of valence tautomerism in an iron complex

The semiquinone-catecholate based mixed valence complex, [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] x DMF (1), and catecholate based (H2bispictn)[Mn2III(Cl4Cat)4(DMF)2] (2) (bispicen = N,N'-bis(2-pyridylmethyl)-1,2-ethanediamine, bispictn = N,N'-bis(2-pyridylmethyl)-1,3-propanediamine, Cl4Cat = tetrachlorocatecholate dianion, and Cl4SQ = tetrachlorosemiquinone radical anion) were synthesized directly utilizing a facile route. Both the complexes have been characterized by single crystal X-ray diffraction study. The electronic structures have been elucidated by UV-vis-NIR absorption spectroscopy, cyclic voltammetry, EPR, and magnetic properties. The structural as well as spectroscopic features support the mixed valence tetrachlorosemiquinone-tetrachlorocatecholate charge distribution in 1. The ligand based mixed valence state was further confirmed by the presence of an intervalence charge transfer (IVCT) band in the 1900 nm region both in solution and in the solid. The intramolecular electron transfer, a phenomenon known as valence tautomerism (VT), has been followed by electronic absorption spectroscopy. For 1, the isomeric form [FeIII(bispicen)(Cl4Cat)(Cl4SQ)] is favored at low temperature, while at an elevated temperature, the [FeII(bispicen)(Cl4SQ)2] redox isomer dominates. Infrared as well as UV-vis-NIR spectral characterization for 2 suggest that the MnIII(Cat)2- moiety is admixed with its mixed valence semiquinone-catecholate isomer MnII(SQ)(Cat)-, and the electronic absorption spectrum is dominated by the mixed charged species. The origin of the intervalence charge transfer band in the 1900 nm range is associated with the mixed valence form, MnII(Cl4Cat)(Cl4SQ)-. The observation of VT in complex 1 is the first example where a mixed valence semiquinone-catecholate iron(III) complex undergoes intramolecular electron transfer similar to manganese and cobalt complexes.     

A Soret marker band for four-coordinate ferric heme proteins from absorption spectra of isolated Fe(III)-Heme+ and Fe(III)-Heme+(His) ions in vacuo

In this work, we report the absorption spectra in the Soret band region of isolated Fe(III)-heme+ and Fe(III)-heme+(His) ions in vacuo from action spectroscopy. Fe(III)-heme+ refers to iron(III) coordinated by the dianion of protoporphyrin IX. We find that the absorption of the five-coordinate complex is similar to that of pentacoordinate metmyoglobin variants with hydrophobic binding pockets except for an overall blueshift of about 16 nm. In the case of four-coordinate iron(III), the Soret band is similar to that of five-coordinate iron(III) but much narrower. These spectra serve as a benchmark for theoretical modeling and also serve to identify the coordination state of ferric heme proteins. To our knowledge this is the first unequivocal spectroscopic characterization of isolated 4c ferric heme in the gas phase.     

Synergistic Effect of NiLDH@YZ Hybrid and Mechanochemical Agitation on Glaser Homocoupling Reaction

Herein, we report the synthesis of nickel-layered double hydroxide amalgamated Y-zeolite (NiLDH@YZ) hybrids and the evaluation of the synergistic effect of various NiLDH@YZ catalysts and mechanochemical agitation on Glaser homocoupling reactions. Nitrogen adsorption-desorption experiments were carried out to estimate the surface area and porosity of NiLDH@YZ hybrids. The basicity and acidity of these hybrids were determined by CO₂-TPD and NH₃-TPD experiments respectively and this portrayed good acid-base bifunctional feature of the catalysts. The NiLDH@YZ-catalyzed mechanochemical Glaser coupling reaction achieved best yield of 83 % for the 0.5NiLDH@0.5YZ hybrid after 60 min of agitation, which revealed the highest acid-base bifunctional feature compared to all the investigated catalysts. The developed catalyst has proven itself as a robust and effective candidate that can successfully be employed up to four catalytic cycles without significant loss in catalytic activity, under optimized reaction conditions. This work demonstrated a new strategy for C−C bond formation enabled by the synergy between mechanochemistry and heterogeneous catalysis.     

A sum-over-state scheme of analysis of hyperpolarizabilities and its application to spiroconjugated molecular system

The static first and second hyperpolarizabilities of a number of spiromolecules with varying degree of polarity have been calculated at the HF and MP2 level using the 6-31+G* basis set and the B3LYP/6-31+G* optimized geometry. The variation of mean second hyperpolarizability in these molecular systems has been explained in terms of the ground state dipole moment, mean linear polarizability and second-order polarizability. A number of relationships among these quantities have been derived in the framework of the sum-over-state scheme and the generalized Thomas–Kuhn sum rule. The spiroconjugation results in the significant increase of the mean polarizability. The appreciable enhancement of first hyperpolarizability due to the spiroconjugation between two dipolar monomer units has been accounted for the rather significant increase of the mean polarizability tensor and the ground state dipole moment. The relatively larger value of the average second hyperpolarizability of the spiroconjugated molecules compared to that of the corresponding monomers arises from the rather significant increase of the nonaxial component γ _xxyy . The replacement of spirocarbon by spirosilicon results in the enhancement of the cubic polarizability manifold. The donor–acceptor substituted spirocompounds are predicted to be the superior third-order nonlinear optical (NLO) phores. The nature of π-conjugation in the monomer units around the spirocenter shows a strong modulation of the NLO properties of spirocompounds. The influence of electron correlation on the NLO properties at the MP2 level has been found to be rather significant.     

Unwinding dynamics of double-stranded polymers

We consider the unwinding of two lattice polymer strands of length N that are initially wound around each other in a double-helical conformation and evolve through Rouse dynamics. The problem relates to quickly bringing a double-stranded polymer well above its melting temperature, i.e., the binding interactions between the strands are neglected, and the strands separate from each other as it is entropically favorable for them to do so. The strands unwind by rotating around each other until they separate. We find that the process proceeds from the ends inward; intermediate conformations can be characterized by a tightly wound inner part, from which loose strands are sticking out, with length l～t(0.39). The total time needed for the two strands to unwind scales as a power of N as τ(u)～N(2.57±0.03). We present a theoretical argument, which suggests that during this unwinding process, these loose strands are far out of equilibrium.