Ruthenium(II) pyridylamine complexes with diimine ligands showing reversible photochemical and thermal structural change

Ruthenium(II)-TPA-diimine complexes, [Ru(TPA)(diimine)]2+ (TPA=tris(2-pyridylmethyl)amine; diimine=2,2'-bipyridine (bpy), 2,2'-bipyrimidine (bpm), 1,10-phenanthroline (phen)) were synthesized and characterized by spectroscopic and crystallographic methods. Their crystal structures demonstrate severe steric hindrance between the TPA and diimine ligands. They exhibit drastic structural changes on heating and photoirradiation at their MLCT bands, which involve partial dissociation of the tetradentate TPA ligand to exhibit a facially tridentate mode accompanied by structural change and solvent coordination to give [Ru(TPA)(diimine)(solvent)]2+ (solvent=acetonitrile, pyridine). The incoming solvent molecules are required to have pi-acceptor character, since sigma-donating solvent molecules do not coordinate. The thermal process is irreversible dissociation to give the solvent-bound complexes, which takes place by an interchange associative mechanism with large negative activation entropies. The photochemical process is a reversible reaction reaching a photostationary state, probably by a dissociative mechanism involving a five-coordinate intermediate to afford the same product as obtained in the thermal reaction. Quantum yields of the forward reactions to give dissociated products were lower than those of the backward reactions to recover the starting complexes. In the photochemical process, the conversions of the forward and backward reactions depend on the absorption coefficients of the starting materials and those of the products at certain wavelength, as well as the quantum yields of those reactions. The reversibility of the motions can be regulated by heating and by photoirradiation at certain wavelength for the recovery process. In the bpm system, we could achieve about 90 % recovery in thermal/photochemical structural interconversion.     

Excited state (formal) intramolecular proton transfer (ESIPT) in p-hydroxyphenyl ketones mediated by water

Evidence is presented that show p-hydroxyphenyl ketones 6–8 undergo excited state intramolecular proton transfer (ESIPT, via the singlet excited state), mediated by water, which formally transfers the phenol proton to the carbonyl oxygen of the ketone. ESIPT was not observed in neat CH₃CN. The ESIPT process in aqueous media generates the corresponding p-quinone methides 9–11 (and the corresponding conjugate bases (phenolate ions) 12–14), as detected by laser flash photolysis (LFP). It competes effectively with intersystem crossing to the excited triplet state. The respective p-methoxyphenyl ketones 15 and 16 failed to undergo the reaction consistent with the expected lack of proton transfer in these systems. Results for the biphenyl ketone 8 indicate that formal ESIPT can also take place over an extended range, suggesting that the process is likely general for all p-hydroxyaromatic ketones which opens up the possibility for designing photoswitchable processes based on this general phenomenon.     

Synthesis, characterization, and dioxygen reactivity of tetracarboxylate-bridged Diiron(II) complexes with coordinated substrates

The synthesis and characterization of [Fe(2)(micro-O(2)CAr(Tol))(4)L(2)] complexes, where L is benzylamine or 4-methoxybenzylamine (BA(p)()(-)(OMe)), are described. The reaction of the latter diiron(II) complex with dioxygen at -78 degrees C affords a metastable mixed-valent Fe(II)Fe(III) green intermediate. When O(2) is introduced at ambient temperature, N-dealkyation occurs to yield anisaldehyde, eliminating N-oxidation as a viable pathway for the reaction. Use of [Fe(2)(micro-O(2)CAr(T)(omicron)(l))(4)(alpha-d(1)-BA(p)()(-)(OMe))(2)] allowed a deuterium kinetic isotope of approximately 3 to be determined.     

Excited-state proton transfer in complexes of poly(methacrylic acid) with dodecyltrimethylammonium chloride

Proton-transfer reactions in aqueous solutions of poly(methacrylic acid) (PMA) were studied using a fluorescent probe and Fourier transform infrared (FTIR) spectroscopy. Protolytic photodissociation of 1-hydroxypyrene (HP) in water was found to be very slow. The PMA polyanion appeared to be very inefficient as a proton acceptor in the excited-state reaction with HP. However, a drastic increase in the deprotonation efficiency was observed in PMA solutions with the same pH values close to neutral when dodecyltrimethylammonium chloride (DTAC) was added. The protonated form of HP, as well as its anion, was shown to be solubilized in polyion-covered micelles. Time-resolved fluorescence data suggested at least two localization sites with different reactivities toward PMA. FTIR spectroscopy was used to quantify the degree of ionization of PMA in PMA-DTAC mixtures. The IR data indicated that protolytic dissociation of PMA could be well described by the Henderson-Hasselbach equation with an apparent pK of 6.6. In contrast, the fluorescent data revealed cooperative protonation of the PMA groups interacting with HP localized within surfactant assemblies. This selective protonation at a pH close to neutral may be associated with a conformational transition in the polymer-surfactant complex.     

Amine mediated proton transfer reaction and C-Cl bond activation of solvent chloroform by a trinuclear copper(II) complex of a glucopyranosylamine derived ligand

An amine mediated C-Cl bond activation process of the solvent chloroform has been explored by a coordinatively labile trinuclear Cu(II) complex, [Cu3(L1)2(MeOH)(H2O)] (1), derived from N-(3-tert-butyl-2-hydroxybenzylidene)-4,6-O-ethylidene--D-glucopyranosylamine (H3L1). The effect of activation is extremely high with methylamine, resulting in the formation of [Cu(MeNH2)5]Cl2 (2) and [Cu(L2)2] (3; HL2 = 2-tert-butyl-6-[(methylimino)methyl]phenol), however, under identical conditions it is moderate with ethylamine resulting in the isolation of crystals of the intermediate amine bound trinuclear copper(II) complex, [Cu3(L1)2(EtNH2)2(MeOH)2] (5), which was further converted into the mononuclear complex, [Cu(HL1)(EtNH2)] (6), in a novel crystal-to-crystal transformation. The successive isolation of the ethylamine-bound tri- and mononuclear complexes, 5 and 6, supported the occurrence of proton transfer reactions, which might be a key step in C-Cl bond activation. The primary and secondary amines, 2-aminomethylpyridine, N,N-dimethylethylenediamine, and 1,4,7-triazacyclononane, also having chelating features further enhance the rate of activation. No activation has been noted in the case of triethylamine and N,N,N,N-tetramethylethylenediamine. Formation of a carbene-trapped compound, 2,6-xylyl isocyanide, was confirmed in the reaction of complex 1 with 1,4,7-triazacyclononane and 2,6-xylidine in CHCl3, suggesting that the C-Cl bond cleavage led to the generation of dichlorocarbene. In addition, the mononuclear complex 6 has been transformed into a homotrinuclear complex [Cu3(L1)2(MeOH)2] by treatment with Cu(II) ions in MeOH/CHCl3, suggesting the possibility that the former could be regarded as a suitable metalloligand for heterotrimetallic complex synthesis.     

Carbon-sulfur bond cleavage in Bis(N-alkyldithiocarbamato)cadmium(II) complexes: heterolytic desulfurization coupled to topochemical proton transfer

Three bis(N-alkyldithiocarbamato)cadmium(II) complexes [Cd(S(2)CNHR)(2)] (1, R = n-C(3)H(7); 2, R = n-C(5)H(11); 3, n-C(12)H(25)) were prepared by metathesis of the corresponding lithium salt, Li[S(2)CNHR], with cadmium chloride. The crystal structures of 2 and 3 consist of planar molecular units of [Cd(S(2)CNHR)(2)] connected by intermolecular Cd.S interactions to give a one-dimensional chain. The chains are connected by a network of intermolecular N-H.S hydrogen bonds between the dithiocarbamato nitrogen atom and bridging sulfur atoms in neighboring chains. In solution, the (113)Cd NMR spectrum of 2 is dependent on concentration and temperature, indicative of a dimerization equilibrium mediated by similar Cd.S intermolecular bridging interactions. In the solid state, thermal gravimetric analyses show that all three complexes decompose smoothly via a heterolytic C-S bond cleavage reaction to give the corresponding alkyl isothiocyanate and cadmium sulfide as the primary products, with the formation of primary amine and CS(2) as coproducts. These products can result only from the net transfer of protons between N-alkyldithiocarbamato ligands in the solid state. Thus, the C-S bond cleavage reaction is interpreted in terms of the topochemical arrangement of molecular units in the crystalline state, which provides a pathway for proton transfer between ligands via N-H.S hydrogen bonds. Decomposition was also initiated by addition of a tertiary amine to a solution of [Cd(S(2)CNHR)(2)]. This confirms that C-S bond cleavage must be coupled to deprotonation of the -NH group, and explains why dialkylated derivatives [Cd(S(2)CNR(2))(2)] are inert to this particular mode of C-S bond cleavage. This system thus constitutes an unusual example of heterolytic, nonoxidative C-S bond cleavage that appears to proceed by a topochemical transfer of protons, which has implications for C-S bond cleavage processes in single-source precursors for II-VI semiconductor materials.     

The reversible addition-fragmentation chain transfer （RAFT） miniemulsion polymerization of vinyl acetate mediated by xanthate

The reversible addition-fragmentation chain transfer(RAFT) miniemulsion polymerization of vinyl acetate(VAc) mediated by methyl(methoxycarbonothioyl) sulfanyl acetate(MMSA) was carried out.The results showed that polymerizations initiated by AIBN and KPS proceeded in a controlled way.The RAFT miniemulsion polymerization of VAc initiated by KPS showed the shorter inhibition period,higher propagation rate coefficient and final conversion than those in experiment initiated by AIBN.When the monomer conversio...     

Hydride ion transfer from ruthenium(II) complexes in water: kinetics and mechanism

Reactions of hydride complexes of ruthenium(II) with hydride acceptors have been examined for Ru(terpy)(bpy)H(+), Ru(terpy)(dmb)H(+), and Ru(η(6)-C(6)Me(6))(bpy)(H)(+) in aqueous media at 25 °C (terpy = 2,2';6',2'-terpyridine, bpy = 2,2'-bipyridine, dmb = 4,4'-dimethyl-2,2'-bipyridine). The acceptors include CO(2), CO, CH(2)O, and H(3)O(+). CO reacts with Ru(terpy)(dmb)H(+) with a rate constant of 1.2 (0.2) × 10(1) M(-1) s(-1), but for Ru(η(6)-C(6)Me(6))(bpy)(H)(+), the reaction was very slow, k ≤ 0.1 M(-1) s(-1). Ru(terpy)(bpy)H(+) and Ru(η(6)-C(6)Me(6))(bpy)(H)(+) react with CH(2)O with rate constants of (6 ± 4) × 10(6) and 1.1 × 10(3) M(-1) s(-1), respectively. The reaction of Ru(η(6)-C(6)Me(6))(bpy)(H)(+) with acid exhibits straightforward, second-order kinetics, with the rate proportional to [Ru(η(6)-C(6)Me(6))(bpy)(H)(+)] and [H(3)O(+)] and k = 2.2 × 10(1) M(-1) s(-1) (μ = 0.1 M, Na(2)SO(4) medium). However, for the case of Ru(terpy)(bpy)H(+), the protonation step is very rapid, and only the formation of the product Ru(terpy)(bpy)(H(2)O)(2+) (presumably via a dihydrogen or dihydride complex) is observed with a k(obs) of ca. 4 s(-1). The hydricities of HCO(2)(-), HCO(-), and H(3)CO(-) in water are estimated as +1.48, -0.76, and +1.57 eV/molecule (+34, -17.5, +36 kcal/mol), respectively. Theoretical studies of the reactions with CO(2) reveal a "product-like" transition state with short C-H and long M-H distances. (Reactant) Ru-H stretched 0.68 ?; (product) C-H stretched only 0.04 ?. The role of water solvent was explored by including one, two, or three water molecules in the calculation.     

Mechanistic Studies of the Formation and Decay of Diiron(III) Peroxo Complexes in the Reaction of Diiron(II) Precursors with Dioxygen

Mechanistic studies of the reactions of three analogous alkoxo-bridged diiron(II) complexes with O(2) have been carried out. The compounds, which differ primarily in the steric accessibility of dioxygen to the diiron(II) center, form metastable &mgr;-peroxo intermediates when studied at low temperature. At ambient temperatures, these intermediates decay to form (&mgr;-oxo)polyiron(III) products. The effect of ligand steric constraints on the O(2) reactivity was investigated. When access to the diiron center was unimpeded, the reaction was first-order with respect to both [Fe(II)(2)] and [O(2)] and the activation parameters for O(2) addition were similar to those for O(2) reacting with the dioxygen transport protein hemerythrin. When the binding site was occluded, however, reduced order with respect to [O(2)] was observed and a two-step mechanism was required to explain the kinetic results. Decay of all three peroxide intermediates involves a bimolecular event, implying the formation of tetranuclear species in the transition state.     

Visible light induced oxygenation of cyclohexene with activation of water sensitized by dihydroxy coordinated tetraphenyloprphyrinatotin(IV)

Visible light irradiation of a reaction mixture containing dihydroxy coordinated tetraphenylporphyrinatotin(IV), cyclohexene and potassium hexachloroplatinate induced oxygenation of the cyclohexene under degassed conditions. In the reaction system, a water molecule served as the oxygen donor. Cyclohex-2-enol, 1,2-dichlorocyclohexane and 2-chlorocyclohexanol were the major oxidation products and the quantum yield was around 0.1. An experiment using H₂ ¹⁸O revealed that an ¹⁸O atom was quantitatively incorporated into the oxygenated products. The reaction was initially induced by an electron transfer from an excited triplet porphyrin to potassium hexachloroplatinate producing a cation radical of the porphyrin. Metal-oxo type complexes formed through deprotonation of the hydroxy group of the porphyrin cation radical were key reactive intermediates reacting with cyclohexene. Two kinds of the metal-oxo type complex reactive intermediate were kinetically demonstrated to be involved in the reaction system, producing different oxidation products from cyclohexene.     

Supramolecularly assembled water layers stabilized by sebacic anions in complexes of Zn(II) and Co(II)

Two three-dimensional supramolecular water architectures, [Zn(phen)₃]₂·[Zn(C₁₀H₁₆O₄)·(H₂O)₃]·(C₁₀H₁₆O₄)₂·20H₂O (1) and [Co(phen)₃]₂·[Co(H₂O)₆]·(C₁₀H₁₆O₄)₃·30H₂O (2) [phen = 1,10-Phenanthroline, C₁₀H₁₆O₄ = sebacic dianion], have been synthesized and characterized by IR, elemental analysis, thermogravimetric analysis, and single-crystal X-ray diffractions. The two structures both contain extensive hydrogen bonding between water molecules as well as between water molecules and sebacic anions. The water molecules and sebacic acid O atoms assembled 2D supramolecular corrugated sheets with different morphology in the two complexes.     

Reaction of dichloromanganese(IV) Schiff-base complexes with water as a model for water oxidation in photosystem II

The dichloromanganese(IV) Schiff-base complexes Mn(N-R-3-NO₂ sal)₂Cl₂ (R = n-C₃H₇, n-C₄H₉, n-C₆H₁₃,     

Characterization of Copper(II), nickel(II), cobalt(II) and palladium(II) complexes of vicinal oxime-imine ligands; induced chelate isomerism in the same molecule of the nickel(II) complex

Summary Metal complexes of the title ligands were characterized in order to determine the factors influencing the stability of chelate isomerism in the same molecule. The ligands were prepared by 1:1 condensation of isonitrosoacetylacetone (Hiso) with eithero-aminophenol (H₂ isoaph),p-aminophenol (H₂ isopph), or aniline (Hisoanil). The following complexes have been synthesized: [(isoaph)Cu]₄, (Hisoaph)₂Co, (Hisopph)₂ M·nH₂O (M=Ni(II), n=2;M=Pd(II), n=0;M=Co(II), n=2), [(isopph) Cu·H₂O]₂, and (isoanil)₂ M (M=Ni(II), Cu(II), Co(II), or Pd(II)). Both chelate rings in these metal complexes are five-membered. Transimination of one –C=N–C₆H₅ group to –C=NH in (isoanil)₂Ni produced a six-membered chelate ring in (isoim)Ni(isoanil). The induced chelate isomerism is ascribed to intermolecular hydrogen bonding of the imino-hydrogen and the basic nitrogen of the same six-membered chelate ring of an adjacent square planar molecule. Other types of hydrogen bondings with the oximato oxygen (intra- or intermolecular) favour the formation of five-membered chelate rings. Analytical, spectroscopic, and magnetic moment data are in accordance with the suggested formulations.Part of the Ph.D. thesis of Sana M. Imam     

Inversion of DNA helicity induced by zinc (II)-macrocyclic polyamine complexes

Abstract

Exposure of synthetic polynucleotide poly(dG-dC)·poly(dG-dC) to Zn^II cyclen, 2 (cyclen = 1,4,7,10-tetraazacyclododecane), produces a dramatic change in its circular dichroism (CD) spectrum in H₂O at pH 7.2, 24°C: the CD spectrum of the initial B form changes to that of the Z form (or a non-Z structure with a left-handed helix) at very low concentrations ([Zn^II]/[base pair] in molar basis ≤ 1). By contrast, Zn^II-[12]aneN₃, 1 ([12]aneN₃ = 1,5,9-triazacyclododecane), and Zn^II-cyclam, 3 (cyclam = 1,4,8,11-tetraazacyclo-tetradecane), do not significantly have such a topological affect on the polynucleotide even at much higher concentrations. An increase in Na⁺ ionic strength nullified the effect of 2 on the CD spectrum, indicating an outside interaction (electrostatic and/or hydrogen bonding) of the DNA model. This study illustrates the significance of the macrocyclic ligand structure around the Zn^II ion for specific interaction with DNA.     

Large pKa shifts of alpha-carbon acids induced by copper(II) complexes

A series of synthetic receptors (4-6) incorporating metal ions, specifically copper(II), were examined for their ability to enhance the acidity of active methylene compounds. The copper(II) complexes were observed to reduce the pKa of 1,3-diketone carbon acids in acetonitrile by as much as 12 pKa units. The relatively large pKa reduction achieved by the complex is attributed to the electrostatic interaction between the anionic pi system of the enolate and the copper(II) ions. The cage structure and hydrogen bonding sites in receptors 4 and 5 lead to a very modest further enhancement of the acidity relative to that with 6. This study provides insight into the way in which metalloenzymes stabilize an enolate intermediate.     

Systematic study of the tautomerism of uracil induced by proton transfer. Exploration of water stabilization and mutagenicity

To systematically investigate all the possible tautomerisms from uracil (U) and its enol form (U) induced by proton transfer, we describe a study of structural tautomer interconversion in the gas phase, in a continuum solvent, and in a microhydrated environment with 1 or 2 explicit water molecules, using density functional theory (DFT) calculations by means of the B3LYP exchange and correlation functions. A total of 62 geometries including 25 transition states were optimized, and the geometrical parameters have been discussed. Some rules of the configuration variation in tautomerization were summarized. The relative stabilities of all the tautomers were established. When a proton transfers from the di-keto form to the keto-enol form, water molecules in different regions show absolutely opposite effects: some assist, whereas others hinder the tautomerization. However, when a proton transfers from the keto-enol form to the di-enol form, water molecules in different regions show similar effects: the Gibbs free energy always increases and the activation energy always decreases. Additionally, some important factors that obviously affect the activation energy and Gibbs free energy were found and discussed in detail. The reasons that water molecules can assist or prevent the proton transfer were given. Furthermore, on the basis of our calculated results, we explain why it is hard to detect the di-enol form of uracil in general experiments.