Hydrated hexacyanometallate(III) salts of triaqua(18‐crown‐6)lanthanoid(III) and tetraaqua(18‐crown‐6)lanthanoid(III) cations containing nine‐ and ten‐coordinate lanthanoids

Tetraaqua(18‐crown‐6)cerium(III) hexacyanoferrate(III) dihydrate, [Ce(C₁₂H₂₄O₆)(H₂O)₄][Fe(CN)₆]·2H₂O, and tetraaqua(18‐crown‐6)neodymium(III) hexacyanoferrate(III) dihydrate, [Nd(C₁₂H₂₄O₆)(H₂O)₄][Fe(CN)₆]·2H₂O, are isomorphous and isostructural in the C2/c space group, where the cations, which contain ten‐coordinate lanthanoid centres, lie across twofold rotation axes and the anions lie across inversion centres. In these compounds, an extensive series of O—H...O and O—H...N hydrogen bonds links the components into a continuous three‐dimensional framework. Triaqua(18‐crown‐6)lanthanoid(III) hexacyanoferrate(III) dihydrate, [Ln(C₁₂H₂₄O₆)(H₂O)₃][Fe(CN)₆]·2H₂O, where Ln = Sm, Eu, Gd or Tb, are all isomorphous and isostructural in the P space group, as are triaqua(18‐crown‐6)gadolinium(III) hexacyanochromate(III) dihydrate, [Gd(C₁₂H₂₄O₆)(H₂O)₃][Cr(CN)₆]·2H₂O, and triaqua(18‐crown‐6)gadolinium(III) hexacyanocobaltate(III) dihydrate, [Gd(C₁₂H₂₄O₆)(H₂O)₃][Co(CN)₆]·2H₂O. In these compounds, there are two independent anions, both lying across inversion centres, and the lanthanoid centres exhibit nine‐coordination; in the crystal structures, an extensive series of hydrogen bonds links the components into a three‐dimensional framework.     

Immobilization of palladium in mesoporous silica matrix: preparation, characterization, and its catalytic efficacy in carbon-carbon coupling reactions

Palladium(0) has been immobilized into the silica-based mesoporous material to develop catalyst Pd(0)-MCM-41, which is found to be highly active in carbon-carbon coupling reactions. [Pd(NH3)4]2+ ions have been incorporated into the mesoporous material during synthesis of MCM-41 and subsequently upon treatments with hydrazine hydrate Pd2+ ions present in mesoporous silica matrix were reduced to Pd(0) almost instantaneously. The catalyst has been characterized by small-angle X-ray diffraction, N2 sorption, and transmission electron microscopy (TEM). TEM and surface area measurements clearly demonstrate that the immobilization of Pd(0) into the mesoporous silica has a significant effect on pore structure of the catalyst. Nevertheless, after immobilization of palladium the meso-porosity of the material is retained, as evidenced in the nitrogen sorption measurement. The TEM micrograph shows that both MCM-41 and Pd(0)-MCM-41 have similar types of external surface morphology; however, Pd(0)-MCM-41 was less ordered. Pd(0)-MCM-41 showed high catalytic activity toward carbon-carbon bond formation reactions like Heck and Sonogashira coupling, as evidenced in high turn-over numbers. In contrast to many other Pd-based catalysts reported so far, Pd(0)-MCM-41 acts as a truly heterogeneous catalyst in C-C coupling reactions. Notably, the new heterogeneous catalyst is found to be efficient in the activation of arylchloride to give impressive conversion in cross coupling (15-45% for Heck and 30% for Sonogashira) reactions under mild conditions.     

Selective sensing of citrate by a supramolecular 1,8-naphthalimide/calix[4]arene assembly via complexation-modulated pKa shifts in a ternary complex

A water-soluble supramolecular sensing assembly, composed of an imidazolium-substituted calix[4]arene and a fluorescent aminodiacetate derivative of 1,8-naphthalimide, was studied. Addition of citrate led to a large fluorescence enhancement, while tartrate, acetate, as well as selected inorganic anions gave smaller effects. The sensing principle and selectivity for citrate rely on the formation of a ternary fluorophore-host-anion complex and complexation-induced pKa shifts of an amino group attached to the fluorophore. The complexation of citrate induces a protonation of the amino group, which switches off intramolecular photoinduced electron transfer as the fluorescence quenching pathway, leading to an enhancement of the optical output signal. The intricate sensor principle was corroborated by pH titrations, binding constants, and structural information as obtained by 1H NMR spectroscopy.     

Kinetic solvent effects on hydrogen abstraction reactions

Kinetic solvent effects on hydrogen abstractions, namely, acceleration in nonpolar solvents, have been presumed to be restricted to O-H hydrogen donors. We demonstrate that also abstractions from C-H and even Sn-H bonds by cumyloxyl radicals and n,pi*-excited 2,3-diazabicyclo[2.2.2]oct-2-ene are fastest in the gas phase and nonpolar solvents but slowest in acetonitrile. Accordingly, solvent effects on hydrogen abstractions are more general, presumably due to stabilization of the reactive oxygen or nitrogen species in polar solvents.     

MCM-41-supported oxo-vanadium(IV) complex: a highly selective heterogeneous catalyst for the bromination of hydroxy aromatic compounds in water

An ecofriendly solid catalyst has been synthesized by anchoring vanadium(IV) into organically modified MCM-41. First, the surface of Si-MCM-41 was modified with 3-aminopropyl-triethoxysilane (3-APTES), the amine group of which upon condensation with ortho-hydroxy-acetophenone affords a N(2)O(2)-type Schiff base moiety in the mesoporous matrix. The Schiff base moieties were used to anchor oxo-vanadium(IV) ions. The prepared catalyst has been characterized by UV-vis, IR spectroscopy, small-angle X-ray diffraction (SAX), nitrogen sorption, and transmission electron microscopy (TEM) studies. It is observed that the mesostructure has not been destroyed in the multistep synthesis procedure, as evidenced by SAX and TEM measurements. The catalyst has shown unprecedented high conversion as well as para selectivity toward the bromination of hydroxy aromatic compounds using aqueous 30% H(2)O(2)/KBr in water. The reaction proceeds according to the stoichiometric ratio, and the monobrominated product was obtained as the major product using a stoichiometric amount of the bromine source. The immobilized complex does not leach or decompose during the catalytic reactions, showing practical advantages over the free metal complex.     

2-Aminopyridine derivative as fluorescence ‘On–Off’ molecular switch for selective detection of Fe3+/Hg2+

2-Amino-6-methyl-4-phenyl-nicotinonitrile 1, a 2-aminopyridine-based fluorescent compound, was found to be a fluorescent chemosensor for the detection of Fe³⁺ and Hg²⁺ ions over a number of other metal ions. Compound 1 was synthesized in one step using a multicomponent reaction, and characterized using common spectroscopic tools. During Fe³⁺/Hg²⁺ sensing the compound 1 followed a ‘switch-off’ mechanism. Further, compound 1 could sense Fe³⁺ over Hg²⁺ by its distinct absorption and fluorescence quenching behaviors. 1:1 complex formation of 1 with Fe³⁺ and Hg²⁺ was clearly understood from Job’s plot. The present work brings additional evidence on the importance of multicomponent reactions which could lead to the development of fluorescence chemosensor in one step for the selective detection of biologically important metal ions.     

Modulation of Spectrokinetic Properties of o‐Quinonoid Reactive Intermediates by Electronic Factors: Time‐Resolved Laser Flash and Steady‐State Photolysis Investigations of Photochromic 6‐ and 7‐Arylchromenes

A variety of differently substituted 6‐ and 7‐arylchromenes such as that depicted undergo photoinduced C? O bond cleavage to yield colored o‐quinonoid intermediates. A combined analysis of μs–ms (laser flash) and real‐time kinetic data show that the o‐quinonoid intermediates decay faster when the C2‐aryl and C6‐/C7‐aryl rings contain electron‐donating and electron‐accepting groups, respectively. Similarly, the decay occurs slowly for the reversed scenario, while intermediate decay rates are observed when both substituents are electron donating.

     

A novel tetranuclear copper(II) complex with alternating mu 1,1-azido and phenoxo bridges: synthesis,structure, and magnetic properties of [Cu4(mu-salen)2(mu 1,1-N3)2(N3)2

A novel tetranuclear copper(II) complex containing alternating mu(1,1)-azido and monophenoxo bridges has been synthesized and characterized by spectroscopic methods, X-ray single-crystal analysis, and variable-temperature magnetic measurements. The magnetic behavior, investigated in the temperature range 2-300 K, indicates that the interactions between copper ions are antiferromagnetic in nature for both azido and phenoxo bridges. The temperature dependence of the magnetic susceptibility was fitted with J(1) = -12.8 cm(-1), J(2) = -10 cm(-1), g = 2.171, 2.1% paramagnetic component, and negligible temperature-independent paramagnetism (5 x 10(-8)). At variance with the earlier reports of these types of complexes containing a mu(1,1)-azido group, the end-on double-azido-bridged copper(II) center in this complex shows an antiferromagnetic interaction.     

Thermally induced isomerisation and decomposition of diethylenetriamine complexes of nickel(II) in the solid state

Summary Complexes [NiL₂]X₂·nH₂O (L=diethylenetriamine; n=O when X=CF₃CO₂ or CCl₃CO₂; n=1 when X=Cl or Br, and n=3 when X=0.5SO₄ or 0.5SeO₄) and NiLX₂·nH₂O (n=1 when X=Cl or Br; n=3 when X=0.5SO₄ or 0.5SeO₄) have been synthesised and investigated thermally in the solid state. NiLSO₄ was synthesised pyrolytically in the solid state from [NiL₂]SO₄·[NiL₂]X₂ (X=Cl or Br) undergo exothermic irreversible phase transitions (242–282° C and 207–228° C; H=–11.3 kJ mol^–1 and –1.9 kJ mol^–1 for [NiL₂]Cl₂ and [NiL₂]Br₂, respectively). [NiL₂]-phenomenon (158–185° C; H=2.0 kJ mol^–1). NiLX₂· nH₂O (n=1 or 3) undergo simultaneous deaquation-isomerisation upon heating. All the complexes possess octahedral geometry.