Theoretical Investigation of the Excited States of 2‐Nitrobenzyl and 4,5‐Methylendioxy‐2‐nitrobenzyl Caging Groups†

The photochemistry of caged compounds of the o‐nitrobenzyl type has been investigated thoroughly in the past. However, even recently new side reactions have been discovered. Earlier, we reported [Bley, F., K. Schaper, and H. Görner (2008), Photochem. Photobiol. 84 162–171] that we found long‐lived triplet states which do not lead to product formation for the bathochromic absorbing compounds with 4,5‐methylendioxy‐2‐nitrobenzyl caging group. Here, we report on theoretical studies which explain the special behavior of these compounds. These studies reveal that the bathochromic shift of absorption for these compounds compared with o ‐nitrobenzyl compounds themselves is not due to a shift in energy of the involved states, but due to a substantial change of oscillator strength of the respective transitions. The lack of reactivity of the triplet state of 4,5‐methylendioxy‐2‐nitrobenzyl compounds can be attributed to state switching. In the triplet manifold the lowest state is a nonreactive charge transfer state, while the lowest state in the singlet manifold is a reactive local excitation in the nitro‐group. From these results we conclude that it will be most likely not possible to create derivatives of caged compounds based on the o ‐nitrobenzyl caging group which have absorption which is shifted even more strongly to longer wavelengths.     

Coordination complexes of 2‐thienyl‐ and 2‐furyl‐mercurials

The reactions of di(2‐thienyl)mercury, 2‐thienylmercury chloride and 2‐furylmercury chloride with a variety of nitrogen‐ and phosphorus‐containing ligands have been studied. The presence of the electron‐withdrawing heteroatoms results in these mercurials being stronger acceptors than the corresponding phenylmercury compounds. The complexes have been characterized by elemental analysis, melting points, infrared, and ¹⁹⁹Hg NMR spectroscopy. 2,9‐Dimethyl‐ and 3,4,7,8‐tetramethyl‐phenanthroline form 1:1 chelate complexes, as does 1,2‐bis(diphenylphosphino)ethane, whereas ethylenediamine and 2,2′‐bipyridyl do not form complexes. Though non‐chelating ligands such as 2,4′‐ and 4,4′‐bipyridyl do not form complexes, bis(diphenylphosphino)methane forms 1:2 complexes in which the ligand bridges two mercury atoms. Monodentate ligands, such as triphenylphosphine, cause disproportionation of the organomercury chloride. 2‐Thienylmercury chloride forms a 4:1 complex with 4,4′‐dipyridyl disulfide in which it is believed that a molecule of the organomercurial is coordinated to both of the nitrogen and both of the sulfur atoms. Copyright © 2004 John Wiley & Sons, Ltd.     

Tris{2‐[N‐(diethylaminothiocarbonyl)benz(‐amidino;‐imidoxy;‐imidothio)‐N′‐yl]ethyl}amine – neue tripodale Liganden. Darstellung,Komplexstabilität und Extraktionsverhalten ihrer Silber(I)‐Komplexe

Tris{2‐[ N ‐(diethylaminothiocarbonyl)benz(‐amidino; imidoxy; ‐imidothio)‐ N ′‐yl]ethyl}amines – New Tripodal Ligands. Synthesis, Complex Stability, and Extraction Behaviour of their Silver(I) Complexes N‐(Thiocarbamoyl)‐benzimidoylchlorides react with trivalent nucleophiles to give four novel tripodal ligands. Two of them have been characterized by X‐ray methods. The ligands form with silver(I) cationic mononuclear complexes in which the three arms of the ligand are coordinated monodentately via sulfur. The results of FAB and ESI mass spectrometry as well as ESCA and NMR investigations verify this binding mode. The protonation constants of the ligands and the stability constants of silver(I) complexes have been determined potentiometrically. The novel tripodal compounds behave as powerful extractands for silver(I).     

Electrochemical Synthesis and Characterization of Zinc(II) Complexes with [(4‐Methylphenyl)Sulfonyl]‐1H‐Amido‐2‐Phenyl‐2‐Oxazolines

A series of [(4‐methylphenyl)sulfonyl]‐1H‐amido‐2‐phenyl‐2‐oxazoline ligands, HTs‐ROz, has been synthesized by the reaction of substituted 2‐(2‐aminophenyl)oxazolines and p‐toluensulfonyl chloride. The electrochemical oxidation of a sacrificial zinc anode in an acetonitrile solution of the corresponding ligand gave compounds of general formula [Zn(Ts‐ROz)₂]. All complexes have been characterized by microanalysis, IR and ¹H NMR spectroscopy and single‐crystal X‐ray diffraction. In all cases, the metal atom is coordinated by the nitrogen atoms of two monoanionic ligands.     

Dendrimer Conjugation Enables Multiphoton Chemical Neurophysiology Studies with an Extended π‐Electron Caging Chromophore

We have developed a caged neurotransmitter using an extended π‐electron chromophore for efficient multiphoton uncaging on living neurons. Widely studied in a chemical context, such chromophores are inherently bioincompatible due to their highly lipophilic character. Attachment of two polycarboxylate dendrimers, a method we call “cloaking”, to a bisstyrylthiophene (or BIST) core effectively transformed the chromophore into a water‐soluble optical probe, whilst maintaining the high two‐photon absorption of over 500 GM. Importantly, the cloaked caged compound was biologically inert at the high concentrations required for multiphoton chemical physiology. Thus, in contrast to non‐cloaked BIST compounds, the BIST‐caged neurotransmitter can be safely delivered onto neurons in acutely isolated brain slices, thereby enabling high‐resolution two‐photon uncaging without any side effects. We expect that our cloaking method will enable the development of new classes of cell‐compatible photolabile probes using a wide variety of extended π‐electron caging chromophores.     

Construction and photoluminescence properties of a three‐dimensional ZnII coordination network based on naphthalene‐1,4‐dicarboxylic acid and 1,6‐bis(pyridin‐3‐yl)‐1,3,5‐hexatriene

In recent years, coordination polymers constructed from multidentate carboxylate and pyridyl ligands have attracted much attention because these ligands can adopt a rich variety of coordination modes and thus lead to the formation of crystalline products with intriguing structures and interesting properties. A new coordination polymer, namely poly[[μ₂‐1,6‐bis(pyridin‐3‐yl)‐1,3,5‐hexatriene‐κ²N:N′](μ₃‐naphthalene‐1,4‐dicarboxylato‐κ⁴O¹,O^1′:O⁴:O^4′)zinc(II)], [Zn(C₁₂H₆O₄)(C₁₆H₁₄N₂)]_n, has been prepared by the self‐assembly of Zn(NO₃)₂·6H₂O, naphthalene‐1,4‐dicarboxylic acid (1,4‐H₂ndc) and 1,6‐bis(pyridin‐3‐yl)‐1,3,5‐hexatriene (3,3′‐bphte) under hydrothermal conditions. The title compound has been structurally characterized by IR spectroscopy, elemental analysis, powder X‐ray diffraction and single‐crystal X‐ray diffraction analysis. Each Zn^II ion is six‐coordinated by four O atoms from three 1,4‐ndc²⁻ ligands and by two N atoms from two 3,3′‐bphte ligands, forming a distorted octahedral ZnO₄N₂ coordination geometry. Pairs of Zn^II ions are linked by 1,4‐ndc²⁻ ligands, leading to the formation of a two‐dimensional square lattice ( sql ) layer extending in the ab plane. In the crystal, adjacent layers are further connected by 3,3′‐bphte bridges, generating a three‐dimensional architecture. From a topological viewpoint, if each dinuclear zinc unit is considered as a 6‐connected node and the 1,4‐ndc²⁻ and 3,3′‐bphte ligands are regarded as linkers, the structure can be simplified as a unique three‐dimensional 6‐connected framework with the point symbol 4⁴6¹⁰8. The thermal stability and solid‐state photoluminescence properties have also been investigated.     

Asymmetric Ruthenium‐Catalyzed Hydrogenation of 2,6‐Disubstituted 1,5‐Naphthyridines: Access to Chiral 1,5‐Diaza‐cis‐Decalins

The first asymmetric hydrogenation (AH) of 2,6‐disubstituted and 2,3,6‐trisubstituted 1,5‐naphthyridines, catalyzed by chiral cationic ruthenium diamine complexes, has been developed. A wide range of 1,5‐naphthyridine derivatives were efficiently hydrogenated to give 1,2,3,4‐tetrahydro‐1,5‐naphthyridines with up to 99 % ee and full conversions. This facile and green protocol is applicable to the scaled‐up synthesis of optically pure 1,5‐diaza‐cis‐decalins, which have been used as rigid chelating diamine ligands for asymmetric synthesis.     

Nickel‐Catalyzed Difluoroalkylation of (Hetero)Arylborons with Unactivated 1‐Bromo‐1,1‐difluoroalkanes

A nickel‐catalyzed cross‐coupling between (hetero)arylborons and unactivated 1‐bromo‐1,1‐difluoroalkanes has been developed. The use of two ligands (a bidentate bipyridine‐based ligand, 4,4′‐ditBu‐bpy, and a monodentate pyridine‐based ligand, DMAP) offers a highly efficient nickel‐based catalytic system to prepare difluoroalkylated arenes which have important applications in medicinal chemistry.     

Poly­[manganese(II)‐μ2‐benzidine‐κ2N:N′‐μ3‐bi­phenyl‐2,2′‐di­carboxylato‐κ4O:O′,O′′:O′′′]

The title compound, [Mn(C₁₄H₈O₄)(C₁₂H₁₂N₂)]_n, with a novel three‐dimensional framework, has been prepared by a hydro­thermal reaction at 433 K. Each Mn atom lies on a twofold axis in a slightly distorted octahedral geometry, coordinated by two N atoms from two benzidine ligands and four O atoms from three symmetry‐related biphenyl‐2,2′‐dicarboxylate (bpdc) ligands. The benzidine ligands lie about inversion centres and the bpdc ligands about twofold axes. Each bpdc ligand is bonded to three Mn ions to form a continuous chain of metal ions. The bpdc ligands are accommodated in a series of distorted holes resembling hexagonal prisms.     

Electrochemical Synthesis and Characterization of Nickel(II) Complexes with N‐2‐Pyridyl‐sulfonamide Ligands

Heteroleptic nickel(II) complexes [NiL₂L′] of a series of monoanionic and potentially bidentate N‐2‐pyridyl‐sulfonamide ligands [HL] and 2,2′‐bipyridine or 1,10‐Phenanthroline (L′) have been prepared by electrochemical oxidation of a nickel anode in an acetonitrile solution of the ligands. The complexes have been characterized by microanalysis, IR and electronic spectroscopy, magnetic measurements and LSI mass spectrometry. The crystal structure of [Ni(Ms6mepy)₂(bipy)] has been determined by x‐ray diffraction and shows the metal in an octahedral NiN₆ environment. Octahedral structures are also proposed for the other complexes with the N‐2‐pyridyl‐sulfonamide ligands acting as N,N′ or N, O bidentate systems, depending on the position of the methyl substituent on the pyridine ring.     

Scalable Synthesis of N‐Acetylated α‐Amino Acid‐Derived Oxazoline Ligands

A scalable cost‐effective synthesis of promising α‐amino acid‐derived oxazoline ligands has been developed. The advantage of the reported procedures is the use of crystallization for the purification of key intermediates and final products. The ligands obtained have recently demonstrated remarkable enantioselectivity in Pd (II) catalyzed C─H activation reactions. Hence, more rational synthetic route presented here will contribute to this rapidly growing field of chemistry.     

Macrocyclic Polyoxazoles as G‐Quadruplex Ligands

This review deals with recent progress in the synthesis and evaluation of our telomestatin‐inspired macrocyclic polyoxazoles as G‐quadruplex (G4) ligands. The hexaoxazole derivatives (6OTDs) interact with and stabilize G4‐forming oligonucleotides, depending upon the character of the side chain functional groups. Cationic functional groups are particularly effective due to their secondary interaction with phosphate in the DNA backbone. On the other hand, heptaoxazole derivatives (7OTDs) showed potent G4‐binding and stabilization activity regardless of the functional groups on the side chain. A caged G4 ligand, Y2Nv2‐6OTD ( 7 ), and a fluorescent G4 ligand, L1BOD‐7OTD ( 13 ), have been synthesized.     

Four novel lanthanide complexes with 4‐ethylbenzoic acid and 5,5′‐dimethy‐2,2′‐bipyridine: Structures,luminescent, thermal properties and bacteriostatic activities

Four new lanthanide complexes [Ln(4‐EBA)₃(5,5′‐DM‐2,2′‐bipy)]₂·2C₂H₅OH (Ln = Ho ( 1 ), Tb ( 2 ), Er ( 3 )); [Ln(4‐EBA)₃(4‐EBAH)(5,5′‐DM‐2,2′‐bipy)]₂ (Ln = Eu( 4 ); 4‐EBA =4‐ethylbenzoate; 5,5′‐DM‐2,2′‐bipy =5,5′‐dimethy‐2,2′‐bipyridine; 4‐EBAH = 4‐ethylbenzoic acid) have been synthesized and characterized by elemental analysis and IR spectra. The single crystal results reveal that complexes 1 – 3 are isostructural. It is worth noting that the mole ratios of the carboxylate ligands and neutral ligands is 4:1 in complex 4 , which is different from the former and has been rarely reported. Nevertheless, all complexes are connected to form 1D chain by π ···π wake staking interactions. Additionally, the complexes 2 (Tb(III)) and 4 (Eu(III)) exhibit characteristic luminescent properties, indicating that ligands can be used as sensitizing chromophore in these systems. The thermal decomposition mechanism of the complexes has been investigated by TG/DSC–FTIR technology. Stacked plots of the FTIR spectra of the evolved gases show complexes broken down into H₂O, CO₂, and other gaseous molecules as well as the gaseous organic fragments. The studies on bacteriostatic activities of complexes show that four complexes have good bacteriostatic activities against Candida albicans but no bacteriostatic activity on Escherichia coli , and Staphylococcus aureus . Additionally, the complexes 1 to 3 have better bacteriostatic activities on Candida albicans than complex 4 .     

Structural and spectroscopic characterization of a new luminescent NiII complex: bis{2,4‐dichloro‐6‐[(2‐hydroxypropyl)iminomethyl]phenolato‐κ3O,N,O′}nickel(II)

The design and preparation of transition‐metal complexes with Schiff base ligands are of interest due to their potential applications in the fields of molecular magnetism, nonlinear optics, dye‐sensitized solar cells (DSSCs), sensing and photoluminescence. Luminescent metal complexes have been suggested as potential phosphors in electroluminescent devices. A new luminescent nickel(II) complex, [Ni(C₁₀H₁₀Cl₂NO₂)₂], has been synthesized and characterized by single‐crystal X‐ray diffraction and elemental analysis, UV–Vis, FT–IR, ¹H NMR, ¹³C NMR and photoluminescence spectroscopies, and LC–MS/MS. Molecules of the complex in the crystals lie on special positions, on crystallographic binary rotation axes. The Ni^II atoms are six‐coordinated by two phenolate O, two imine N and two hydroxy O atoms from two tridentate Schiff base 2,4‐dichloro‐6‐[(2‐hydroxypropyl)iminomethyl]phenolate ligands, forming an elongated octahedral geometry. Furthermore, the complex exhibits a strong green luminescence emission in the solid state at room temperature, as can be seen from the (CIE) chromaticity diagram, and hence the complex may be a promising green OLED (organic light‐emitting diode) in the development of electroluminescent materials for flat‐panel‐display applications.     

Synthesis of Bis‐1,2,4‐triazines via Telescoped Condensation of [1,10]‐Phenanthroline‐2,9‐dicarbonitrile with Aromatic 1,2‐Dicarbonyls

Efficient separation of minor actinides from spent nuclear fuel remains a formidable challenge. As part of ongoing efforts to identify effective ligands for separation of toxic radionuclides, a series of bis‐1,2,4‐triazines, three novel, have been prepared from [1,10]‐phenanthroline‐2,9‐dicarbonitrile in two‐telescoped steps without additives, complicated workups, prolonged reaction times, or additional purification.     

Quinine‐Catalyzed Asymmetric Synthesis of 2,2′‐Binaphthol‐Type Biaryls under Mild Reaction Conditions

Simple quinine as an organocatalyst mediates the addition of various naphthols to halogenated quinones to afford non‐C₂‐symmetrical, axially chiral biaryl products, which are promising compounds as chiral ligands and organocatalysts. The rotational barrier required to have two distinct atropisomers has been evaluated in the products generated from the addition of naphthols to various quinones by means of DFT calculations and HPLC. The use of halogenated quinones as reagents was necessary to have configurationally stable enantiomeric products which can be obtained in good yield and stereoselectivity. These compounds have also been prepared in gram quantities and recrystallized to near enantiopurity.     

Synthesis and Anti‐tumor Evaluation of B‐ring Modified Caged Xanthone Analogues of Gambogic Acid

Gambogic acid (GA, 1 ), the most prominent member of Garcinia natural products, has been reported to be a promising anti‐tumor agent. Previous studies have suggested that the planar B ring and the unique 4‐oxa‐tricyclo[4.3.1.0^3,7]dec‐2‐one caged motif were essential for anti‐tumor activity. To further explore the structure‐activity relationship (SAR) of caged Garcinia xanthones, two new series of B‐ring modified caged GA analogues 13a – 13e and 15a – 15e were synthesized utilizing a Claisen/Diel‐Alder cascade reaction. Subsequently, these compounds were evaluated for their in vitro anti‐tumor activities against A549, MCF‐7, SMMC‐7721 and BGC‐823 cancer cell lines by MTT assay. Among them, 13b – 13e exhibited micromolar inhibition against several cancer cell lines, being approximately 2–4 fold less potent in comparison to GA. SAR analysis revealed that the peripheral gem‐dimethyl groups are essential for maintaining anti‐tumor activity and substituent group on C1 position of B‐ring has a significant effect on potency, while modifications at C‐2, C‐3 and C‐4 positions are relatively tolerated. These findings will enhance our understanding of the SAR of Garcinia xanthones and lead to the development of simplified analogues as potential anti‐tumor agents.     

Synthesis,structural characterization and computational studies of catena‐poly[chlorido[μ3‐(pyridin‐1‐ium‐3‐yl)phosphonato‐κ3O:O′:O′′]zinc(II)]

Coordination polymers are constructed from two basic components, namely metal ions, or metal‐ion clusters, and bridging organic ligands. Their structures may also contain other auxiliary components, such as blocking ligands, counter‐ions and nonbonding guest or template molecules. The choice or design of a suitable linker is essential. The new title zinc(II) coordination polymer, [Zn(C₅H₅NO₃P)Cl]_n , has been hydrothermally synthesized and structurally characterized by single‐crystal X‐ray diffraction and vibrational spectroscopy (FT–IR and FT–Raman). Additionally, computational methods have been applied to derive quantitative information about interactions present in the solid state. The compound crystallizes in the monoclinic space group C 2/c . The four‐coordinated Zn^II cation is in a distorted tetrahedral environment, formed by three phosphonate O atoms from three different (pyridin‐1‐ium‐3‐yl)phosphonate ligands and one chloride anion. The Zn^II ions are extended by phosphonate ligands to generate a ladder chain along the [001] direction. Adjacent ladders are held together via N—H…O hydrogen bonds and offset face‐to‐face π–π stacking interactions, forming a three‐dimensional supramolecular network with channels. As calculated, the interaction energy between the neighbouring ladders is −115.2 kJ mol⁻¹. In turn, the cohesive energy evaluated per asymmetric unit‐equivalent fragment of a polymeric chain in the crystal structure is −205.4 kJ mol⁻¹. This latter value reflects the numerous hydrogen bonds stabilizing the three‐dimensional packing of the coordination chains.     

Synthesis of Precursors of 4‐Phosphino‐1H‐pyrrole‐3‐carbaldehyde Derivatives

In this work, possible approaches to the synthesis of 1,2,5‐substituted 4‐phosphoryl‐3‐formylpyrroles have been considered. As a result, two methods for the synthesis of 4‐(diphenylphosphoryl)‐1‐(4‐ethoxyphenyl)‐2,5‐dimethyl‐1H‐pyrrole‐3‐carbal‐dehyde were proposed; the highest yields gives formylation of 3‐(diphenylphosphorothioyl)‐1‐(4‐ethoxyphenyl)‐2,5‐dimethyl‐1H‐pyrrole. The formyl fragment was successfully converted into a Schiff base with phenethylamine, and the phosphoryl group has been reduced to phosphine with silicochloroform, which suggests a promising approach to the synthesis of chiral bidentate phosphine ligands. © 2013 Wiley Periodicals, Inc. Heteroatom Chem 24:146–151, 2013; View this article online at wileyonlinelibrary.com . DOI 10.1002/hc.21069