Photolabile ruthenium nitrosyls with planar dicarboxamide tetradentate N(4) ligands: effects of in-plane and axial ligand strength on NO release

Four ruthenium nitrosyls, namely [(bpb)Ru(NO)(Cl)] (1), [(Me(2)bpb)Ru(NO)(Cl)] (2), [(Me(2)bpb)Ru(NO)(py)](BF(4)) (3), and [(Me(2)bqb)Ru(NO)(Cl)] (4) (H(2)bpb = 1,2-bis(pyridine-2-carboxamido)benzene, H(2)Me(2)bpb = 1,2-bis(pyridine-2-carboxamido)-4,5-dimethylbenzene, H(2)Me(2)bqb = 1,2-bis(quinaldine-2-carboxamido)-4,5-dimethylbenzene; H is the dissociable amide proton), have been synthesized and characterized by spectroscopy and X-ray diffraction analysis. All four complexes exhibit nu(NO) in the range 1830-1870 cm(-)(1) indicating the [Ru-NO](6) configuration. Clean (1)H NMR spectra in CD(3)CN (or (CD(3))(2)SO) confirm the S = 0 ground state for all four complexes. Although the complexes are thermally stable, they release NO upon illumination. Rapid NO dissociation occurs when solutions of 1-3 in acetonitrile (MeCN) or DMF are exposed to low-intensity (7 mW) UV light (lambda(max) = 302 nm). Electron paramagnetic resonance (EPR) spectra of the photolyzed solutions display anisotropic signals at g approximately 2.00 that confirm the formation of solvated low-spin Ru(III) species upon NO release. The ligand trans to bound NO namely, anionic Cl(-) and neutral pyridine, has significant effect on the electronic and NO releasing properties of these complexes. Change in the in-plane ligand strength also has effects on the rate of NO release. The absorption maximum (lambda(max)) of 4 is significantly red shifted (455 nm in DMF) compared to the lambda(max) values of 1-3 (380-395 nm in DMF) due to the extension of conjugation on the in-plane ligand frame. As a consequence, 4 is also sensitive to visible light and release NO (albeit at a slower rate) upon illumination to low-intensity visible light (lambda > 465 nm). Collectively, the photosensitivity of the present series of ruthenium nitrosyls demonstrates that the extent of NO release and their wavelength dependence can be modulated by changes of either the in-plane or the axial ligand (trans to bound NO) field strength.     

High-valent nonheme iron. Two distinct iron(IV) species derived from a common iron(II) precursor

The reaction of [Fe(II)(beta-BPMCN)(OTf)2] (1, BPMCN = N,N'-bis(2-pyridylmethyl)-N,N'-dimethyl-trans-1,2-diaminocyclohexane) with tBuOOH at low-temperature yields alkylperoxoiron(III) intermediates 2 in CH2Cl2 and 2-NCMe in CH3CN. At -45 degrees C and above, 2-NCMe converts to a pale green species 3 (lambda(max) = 753 nm, epsilon = 280 M(-1) cm(-1)) in 90% yield, identified as [Fe(IV)(O)(BPMCN)(NCCH3)]2+ by comparison to other nonheme [Fe(IV)(O)(L)]2+ complexes. Below -55 degrees C in CH2Cl2, 2 decays instead to form deep turquoise 4 (lambda(max) = 656, 845 nm; epsilon = 4000, 3600 M(-1) cm(-1)), formulated to be an unprecedented alkylperoxoiron(IV) complex [Fe(IV)(BPMCN)(OH)(OOtBu)]2+ on the basis of M?ssbauer, EXAFS, resonance Raman, NMR, and mass spectral evidence. The reactivity of 1 with tBuOOH in the two solvents reveals an unexpectedly rich iron(IV) chemistry that can be supported by the BPMCN ligand.     

Oxygen-17 NMR, Electronic, and Vibrational Spectroscopy of Transition Metal Peroxo Complexes: Correlation with Reactivity

The (17)O NMR chemical shifts of several previously characterized mono- and diperoxo complexes of vanadium(V), molybdenum(VI), tungsten(VI), and titanium(IV) were measured. Compilation of NMR, electronic, and vibrational spectroscopic data and metric parameters for these and other complexes permits us to draw correlations among (17)O peroxo chemical shift, the electronic charge transfer band, the O-O vibrational frequency, and the length of the oxygen-oxygen bond. Monoperoxo complexes exhibit (17)O chemical shifts of 500-660 ppm, while those of diperoxo complexes fall in the range 350-460 ppm. The correlation of chemical shift with the inverse ligand-to-metal charge transfer energy from electronic spectra is consistent with a formalism developed by Ramsey, despite the variations in the metals, the number of peroxo ligands, and the nature of the remaining ligands in the coordination sphere. Vibrational frequency and length of the oxygen-oxygen bond also correlate with the inverse ligand-to-metal charge transfer energy. Monoperoxo complexes show values of nu(O)(-)(O) above 900 cm(-)(1) and O-O distances in the range 1.43-1.46 ?. Diperoxo complexes have values of nu(O)(-)(O) below 900 cm(-)(1) and O-O distances of 1.46-1.53 ?. The assignment of nu(O)(-)(O) = 910 cm(-)(1) for the infrared spectrum of ammonium aquaoxoperoxo(pyridine-2,6-dicarboxylato)vanadium(V), NH(4)[VO(O(2))(dipic)(H(2)O)], was made by isotopic substitution. The stretching frequency and length of the O-O bond for peroxo complexes are explained in terms of sigma-bonding between a metal d orbital and a peroxo pi orbital. A comparison of the spectroscopic properties of these complexes with their reactivity as oxidizing agents suggests that the strength of the O-O bond is an important factor. The most reactive species exhibit lambda(max) values below 400 nm, stretching frequencies below 900 cm(-)(1), and (17)O chemical shifts below 600 nm. These generalizations may permit the prediction of peroxometal reactivity from spectroscopic information.     

Determination of ruthenium and osmium in each other's presence in chloride solutions by direct and third-order derivative spectrophotometry

Ruthenium and osmium (up to 20 mug Ru(Os) ml(-1)) can be determined in chloride solutions directly after absorption of RuO(4) and OsO(4) in hydrochloric acid. In 9 M HCl, RuO(4) and OsO(4) are quantitatively converted into RuCl(6)(2-) (lambda(max) = 480.0 nm, epsilon = 4.8 x 10(3) l mol(-1) cm(-1)) and OsCl(6)(2-) (lambda(max) = 334.8 nm, epsilon = 8.4 x 10(3) l mol(-1) cm(-1)) respectively. Osmium does not interfere with the determination of ruthenium in the form of the RuCl(6)(2-) complex by direct spectrophotometry. The absorbance of the obtained solution at lambda(max) = 480.0 nm corresponds only to the concentration of ruthenium. A derivative spectrophotometric method using numerical calculation of absorption spectra of the RuCl(6)(2-) and OsCl(6)(2-) complexes has been developed for the determination of osmium in a mixture with ruthenium. The interfering effect of ruthenium on the determination of osmium can be eliminated by measuring the value of a third-order derivative spectrum of the OsCl(6)(2-) complex at 350.0 nm ("zero-crossing point" of ruthenium). Simple and rapid determination of ruthenium and osmium in a calibration standard solution of the noble metals (Ru, Rh, Pd, Os, Ir, Pt and Au) for plasma spectroscopy using the proposed methods has been achieved.     

Fe(III)Fe(III) and Fe(II)Fe(III) Complexes as Synthetic Analogues for the Oxidized and Reduced Forms of Purple Acid Phosphatases

Novel Fe(III)Fe(III) and Fe(II)Fe(III) complexes [Fe(2)(BBPMP)(&mgr;-OAc)(&mgr;-X)](n)() (1, X = OAc(-), n = 1+; 2, X = OH(-), n = 1+; 3, X = OAc(-), n = 0; 4, X = OH(-), n = 0), where BBPMP(3)(-) is the anion of 2,6-bis[(2-hydroxybenzyl)(2-pyridylmethyl)aminomethyl]-4-methylphenol, and OAc(-) is acetate, were prepared in order to provide models for the active site of purple acid phosphatases (PAPs). Complex 1 was obtained by the reaction of H(3)BBPMP with Fe(ClO(4))(2).6H(2)O in methanol and sodium acetate trihydrate under ambient conditions, while complex 3 was synthesized as described for 1, under an argon atmosphere with low levels of dioxygen. 2 was isolated from 1in acetonitrile by a substitution of the bridging acetate group by hydroxide, while 4 was generated in solution during a spectropotentiostatic experiment on 2, under argon. Complex 1, [Fe(III)(2)(BBPMP)(&mgr;-OAc)(2)]ClO(4).H(2)O, has been characterized by X-ray crystallography. Crystal data: monoclinic, space group P2(1)/n, a = 14.863(5) ?, b = 12.315(3) ?, c = 20.872(8) ?, beta = 90.83(3) degrees, Z = 4. IR, M?ssbauer, magnetic, electronic absorption, and electrochemical properties of 1-3 have been investigated, and some of these properties represent a contribution to the understanding of the dinuclear iron center of PAPs. Complexes 2, [Fe(III)(2)(BBPMP)(&mgr;-OAc)(&mgr;-OH)]ClO(4) (lambda(max) = 568 nm/epsilon = 4760 M(-)(1) cm(-)(1)), and 4 [Fe(II)Fe(III)(BBPMP)(&mgr;-OAc)(&mgr;-OH)] (lambda(max) = 516 nm/epsilon = 4560 M(-)(1) cm(-)(1)), constitute good synthetic analogues for the chromophoric site for the oxidized and reduced forms, respectively, of the enzyme.     

Copper(I) complex O(2)-reactivity with a N(3)S thioether ligand: a copper-dioxygen adduct including sulfur ligation, ligand oxygenation, and comparisons with all nitrogen ligand analogues

In order to contribute to an understanding of the effects of thioether sulfur ligation in copper-O(2) reactivity, the tetradentate ligands L(N3S) (2-ethylthio-N,N-bis(pyridin-2-yl)methylethanamine) and L(N3S')(2-ethylthio-N,N-bis(pyridin-2-yl)ethylethanamine) have been synthesized. Corresponding copper(I) complexes, [CuI(L(N3S))]ClO(4) (1-ClO(4)), [CuI(L(N3S))]B(C(6)F(5))(4) (1-B(C(6)F(5))(4)), and [CuI(L(N3S'))]ClO(4) (2), were generated, and their redox properties, CO binding, and O(2)-reactivity were compared to the situation with analogous compounds having all nitrogen donor ligands, [CuI(TMPA)(MeCN)](+) and [Cu(I)(PMAP)](+) (TMPA = tris(2-pyridylmethyl)amine; PMAP = bis[2-(2-pyridyl)ethyl]-(2-pyridyl)methylamine). X-ray structures of 1-B(C(6)F(5))(4), a dimer, and copper(II) complex [Cu(II)(L(N3S))(MeOH)](ClO(4))(2) (3) were obtained; the latter possesses axial thioether coordination. At low temperature in CH(2)Cl(2), acetone, or 2-methyltetrahydrofuran (MeTHF), 1 reacts with O(2) and generates an adduct formulated as an end-on peroxodicopper(II) complex [{Cu(II)(L(N3S))}(2)(mu-1,2-O(2)(2-))](2+) (4)){lambda(max) = 530 (epsilon approximately 9200 M(-1) cm(-1)) and 605 nm (epsilon approximately 11,800 M(-1) cm(-1))}; the number and relative intensity of LMCT UV-vis bands vary from those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {lambda(max) = 524 nm (epsilon = 11,300 M(-1) cm(-1)) and 615 nm (epsilon = 5800 M(-1) cm(-1))} and are ascribed to electronic structure variation due to coordination geometry changes with the L(N3S) ligand. Resonance Raman spectroscopy confirms the end-on peroxo-formulation {nu(O-O) = 817 cm(-1) (16-18O(2) Delta = 46 cm(-1)) and nu(Cu-O) = 545 cm(-1) (16-18O(2) Delta = 26 cm(-1)); these values are lower in energy than those for [{Cu(II)(TMPA)}(2)(O(2)(2-))](2+) {nu(Cu-O) = 561 cm(-1) and nu(O-O) = 827 cm(-1)} and can be attributed to less electron density donation from the peroxide pi* orbitals to the Cu(II) ion. Complex 4 is the first copper-dioxygen adduct with thioether ligation; direct evidence comes from EXAFS spectroscopy {Cu K-edge; Cu-S = 2.4 Angstrom}. Following a [Cu(I)(L(N3S))](+)/O(2) reaction and warming, the L(N3S) thioether ligand is oxidized to the sulfoxide in a reaction modeling copper monooxygenase activity. By contrast, 2 is unreactive toward dioxygen probably due to its significantly increased Cu(II)/Cu(I) redox potential, an effect of ligand chelate ring size (in comparison to 1). Discussion of the relevance of the chemistry to copper enzyme O(2)-activation, and situations of biological stress involving methionine oxidation, is provided.     

Electrical properties of 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene and its application for organic light emitting diodes

We found that a phenylene ethynylene derivative, 1,4-bis(4-(phenylethynyl)phenylethynyl)benzene (BPPB), provides very high photoluminescence efficiency both in solution (Phi(PL) = 95 +/- 3%) and thin films (Phi(PL) = 71 +/- 3%); further, we observed blue electroluminescence (EL) of lambda(EL(max)) approximately 470 and 510 nm with an external EL efficiency of eta(EL) approximately 0.53% and maximum luminance of approximately 70000 cd m(-2) at current density of approximately 2 A cm(-2) with BPPB as an emitter; also we identified that BPPB functions as a hole transport layer in organic light emitting diodes.     

4,4'-dithiodipyridine as a bridging ligand in osmium and ruthenium complexes: the electron conductor ability of the -S-S- bridge

The compounds [Ru(NH(3))(5)(dtdp)](TFMS)(3), [Os(NH(3))(5)(dtdp)](TFMS)(3), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](TFMS)(6), [(NH(3))(5)Os(dtdp)Ru(NH(3))(5)](TFMS)(3)(PF(6))(2), and [(NH(3))(5)Os(dtdp)Fe(CN)(5)] (dtdp = 4,4'-dithiodipyridine, TFMS = trifluoromethanesulfonate) have been synthesized and characterized by elemental analysis, cyclic voltammetry, electronic, vibrational, EPR, and (1)H NMR spectroscopies. Changes in the electronic and voltammetric spectra of the ion complex [Os(NH(3))(5)(dtdp)](3+) as a function of the solution pH enable us to calculate the pK(a) for the [Os(NH(3))(5)(dtdpH)](4+) and [Os(NH(3))(5)(dtdpH)](3+) acids as 3.5 and 5.5, respectively. The comparison of the above pK(a) data with that for the free ligand (pK(1) = 4.8) provides evidence for the -S-S- bridge efficiency as an electron conductor between the two pyridine rings. The symmetric complex, [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](6+), is found to exist in two geometric forms, and the most abundant form (most probably trans) has a strong conductivity through the -S-S- bridge, as is shown by EPR, which finds it to have an S = 1 spin state with a spin-spin interaction parameter of 150-200 G both in the solid sate and in frozen solution. Further the NMR of the same complex shows a large displacement of unpaired spin into the pi orbitals of the dttp ligand relative to that found in [Os(NH(3))(5)(dtdp)](3+). The comproportionation constant, K(c) = 2.0 x 10(5), for the equilibrium equation [Os(II)Os(II)] + [Os(III)Os(III)] right harpoon over left harpoon 2[Os(II)Os(III)] and the near-infrared band energy for the mixed-valence species (MMCT), [(NH(3))(5)Os(dtdp)Os(NH(3))(5)](5+) (lambda(MMCT) = 1665 nm, epsilon = 3.5 x 10(3) M(-)(1) cm(-)(1), deltanu(1/2) = 3.7 x 10(3) cm(-)(1), alpha = 0.13, and H(AB) = 7.8 x 10(2) cm(-)(1)), are quite indicative of strong electron delocalization between the two osmium centers. The electrochemical and spectroscopic data for the unsymmetrical binuclear complexes [(NH(3))(5)Os(III)(dtdp)Ru(II)(NH(3))(5)](5+) (lambda(MMCT) = 965 nm, epsilon = 2.2 x 10(2) M(-)(1) cm(-)(1), deltanu(1/2) = 3.0 x 10(3) cm(-)(1), and H(AB) = 2.2 x 10(2) cm(-)(1)) and [(NH(3))(5)Os(III)(dtdp)Fe(II)(CN)(5)] (lambda(MMCT) = 790 nm, epsilon = 7.5 x 10 M(-)(1) cm(-)(1), deltanu(1/2) = 5.4 x 10(3) cm(-)(1), and H(AB) = 2.0 x 10(2) cm(-)(1)) also suggest a considerable electron delocalization through the S-S bridge. As indicated by a comparison of K(c) and energy of the MMCT process in the iron, ruthenium, and osmium complexes, the electron delocalization between the two metal centers increases in the following order: Fe < Ru < Os.     

Metal-ligand charge-transfer-promoted photoelectronic Bergman cyclization of copper metalloenediynes: photochemical DNA cleavage via C-4' H-atom abstraction

Metal-to-ligand charge-transfer (MLCT) photolyses (lambda > or = 395 nm) of copper complexes of cis-1,8-bis(pyridin-3-oxy)oct-4-ene-2,6-diyne (bpod, 1), [Cu(bpod)(2)]PF(6) (2), and [Cu(bpod)(2)](NO(3))(2) (3) yield Bergman cyclization of the bound ligands. In contrast, the uncomplexed ligand 1 and Zn(bpod)(2)(CH(3)COO)(2) compound (4) are photochemically inert under the same conditions. In the case of 4, sensitized photochemical generation of the lowest energy (3)pi-pi state, which is localized on the enediyne unit, leads to production of the trans-bpod ligand bound to the Zn(II) cation by photoisomerization. Electrochemical studies show that 1, both the uncomplexed and complexed, exhibits two irreversible waves between E(p) values of -1.75 and -1.93 V (vs SCE), corresponding to reductions of the alkyne units. Irreversible, ligand-based one-electron oxidation waves are also observed at +1.94 and +2.15 V (vs SCE) for 1 and 3. Copper-centered oxidation of 2 and reduction of 3 occur at E(1/2) = +0.15 and +0.38 V, respectively. Combined with the observed Cu(I)-to-pyridine(pi) MLCT and pyridine(pi)-to-Cu(II) ligand-to-metal charge transfer (LMCT) absorption centered near approximately 315 nm, the results suggest a mechanism for photo-Bergman cyclization that is derived from energy transfer to the enediyne unit upon charge-transfer excitation. The intermediates produced upon photolysis degrade both pUC19 bacterial plasmid DNA, as well as a 25-base-pair, double-stranded oligonucleotide. Detailed analyses of the cleavage reactions reveal 5'-phosphate and 3'-phosphoglycolate termini that are derived from H-atom abstraction from the 4'-position of the deoxyribose ring rather than redox-induced base oxidation.     

Syntheses,structure, and reactivity of acyclic enetriyne and enetetrayne derivatives

Sonogashira coupling of buta-1,3-diynylbenzene with ((2-iodophenyl)ethynyl)trimethylsilane and 1,2-diiodobenzene led to the novel enetriyne, 1-ethynyl-2-(phenylbuta-1,3-diynyl)benzene, and enetetrayne, 1,2-bis(phenylbuta-1,3-diynyl)benzene, respectively. Solid state structural and thermal analyses are also described. In solution, 1-ethynyl-2-(phenylbuta-1,3-diynyl)benzene was found to undergo thermal Bergman cyclization to afford 2-(phenylethynyl)naphthalene.     

Thermal C1-C5 diradical cyclization of enediynes

Computational studies at the BLYP/6-31G(d) level (supplemented by BCCD(T)/cc-pVDZ calculations) suggest that in aryl-substituted 1,2-diethynylbenzenes, steric effects disfavor the thermal C1-C6 diradical cyclization reaction (Bergman) and electronic effects favor the regiovariant C1-C5 cyclization to the extent that the C1-C5 process should become an important reaction pathway in the thermolyses of such compounds. Experimentally, thermolyses of 1,2-bis(2,4,6-trichlorophenylethynyl)benzene, a particularly favorable case, yields only products derived from C1-C5 cyclization [specifically, 1-(2,4,6-trichlorobenzylidene)-2-(2,4,6-trichlorophenyl)-1H-indene and its hydrogenation product 3-(2,4,6-trichlorobenzyl)-2-(2,4,6-trichlorophenyl)-1H-indene], and even for the parent hydrocarbon 1,2-bis(phenylethynyl)benzene, the formation of C1-C5 cyclization products is competitive with the major Bergman reaction. Although some C1-C5 cyclization products are probably formed by transfer hydrogenation from 1,4-cyclohexadiene (commonly included in such reactions), thermolyses in the absence of 1,4-CHD as well as deuterium labeling studies confirm the existence of direct C1-C5 diradical cyclizations for diaryl-substituted enediynes.     

Triggered dye release via photodissociation of nitric oxide from designed ruthenium nitrosyls: turn-ON fluorescence signaling of nitric oxide delivery

Two new fluorescein-tethered nitrosyls derived from designed tetradentate ligands with carboxamido-N donors have been synthesized and characterized by spectroscopic techniques. These two diamagnetic {Ru-NO}(6) nitrosyls, namely, [(Me(2)bpb)Ru(NO)(FlEt)] (1-FlEt, Me(2)bpb = 1,2-bis(pyridine-2-carboxamido)5-dimethylbenzene, FlEt = fluorescein ethyl ester) and [((OMe)(2)IQ1)Ru(NO)(FlEt)] (2-FlEt, (OMe)(2)IQ1 = 1,2-bis(isoquinoline-1-carboxamido)-4,5-dimethoxybenzene), display NO stretching frequencies (ν(NO)) at 1846 and 1832 cm(-1) in addition to their FlEt carbonyl stretching frequencies (ν(CO)) at 1715 and 1712 cm(-1), respectively. Coordination of the dye ligand enhances the absorptivity and NO photolability of these two nitrosyls in the visible region (450-600 nm) of light. Exposure to visible light promotes rapid loss of NO from both {Ru-NO}(6) nitrosyls to generate Ru(III) photoproducts in dry aprotic solvents, such as MeCN and DMF. The FlEt(-) moiety remains bound to the paramagnetic Ru(III) center in such cases, and hence, the photoproducts exhibit very weak fluorescence from the dye unit. In the presence of water, the Ru(III) photoproducts undergo further aquation and loss of the FlEt(-) moiety via protonation. These steps lead to turn-ON fluorescence (from the free FlEt unit) and provide a visual signal of the NO photorelease from 1-FlEt and 2-FlEt in aqueous media.     

Synthesis and structural characterization of porphyrinic enediynes: geometric and electronic effects on thermal and photochemical reactivity

We report the preparation of [5,10,15,20-tetraphenyl-2,3,7,8,12,13,17,18-octakis(phenylethynyl)porphinato] complexes of Ni(II), H(2), Zn(II), Mg(II), and Cu(II), as well as select trimethylsilanylethynyl derivatives. The X-ray structures of the octakis(phenylethynyl) compounds show systematic deviations from planarity (Ni(II), 0.2851 A; Zn(II), 0.0304 A) as a function of the central cation. These geometric distortions are reflected in bathochromic shifts of the Soret and Q bands (Ni(II), 497, 604, and 650 nm; Mg(II), 515, 595, 642, and 705 nm) which loosely correlate with increasing planarity of the structure. Similarly, vibrational modes involving the octasubstituted porphyrin core exhibit shifts to lower frequency as a function of increasing planarity in the solution-state resonance Raman spectra (lambda(exc) = 501.7 nm) of these compounds. Analogous trends are also observed in their solid-state electronic and resonance Raman spectra, indicating that the structural distortions within the octakis(phenylethynyl) series are preserved in solution. Comparison of the saddle distortion of the octasubstituted Ni(II) compound with the ruffle/saddle distortions of the pentakis and hexakis Ni(II) derivatives reveals some influence of asymmetric peripheryl substitution on geometric structure. These Ni(II) derivatives also exhibit systematic red shifts in their electronic spectra as a function of the number of conjugated alkyne units ( approximately 13 nm/alkyne), revealing participation of the enediyne units in the electronic ground and excited states. The solid-state Bergman cyclization temperatures of the phenylethynyl compounds vary markedly as a function of planarity, and correlate loosely with alkyne termini separation (Ni(PA)(8), 4.00 A, 281 degrees C; MgP(PA)(8), 3.77 A, 244 degrees C). In solution, both thermal and photochemical activation of the free-base octakis(phenylethynyl) compound lead to formal reduction of the porphyrin backbone via H-atom addition at opposing meso-positions. Generation of a common product suggests that both thermal and photochemical pathways to Bergman cyclization in solution contain significant activation barriers to formation of the 1,4-phenyl diradical intermediate, and under these solution conditions, alternate reaction channels are more thermodynamically favorable.     

Photochromic switching of excited-state intramolecular proton-transfer (ESIPT) fluorescence: a unique route to high-contrast memory switching and nondestructive readout

Aiming at the high-contrast photochromic switching of fluorescence emission and its perfect nondestructive readout, a polymer film highly loaded with a specific photochromic compound, 1,2-bis(2'-methyl-5'-phenyl-3'-thienyl)perfluorocyclopentene (BP-BTE), and an excited-state intramolecular proton-transfer (ESIPT)-active compound, 2,5-bis(5'-tert-butyl-benzooxazol-2'-yl)hydroquinone (DHBO), was employed in this work. The special class of photochrome, BP-BTE, has negligible absorbance at 415 nm both in the open form and in the 365 nm photostationary state (PSS), and the ESIPT fluorophore, DHBO, emits large Stokes' shifted (175 nm; lambda(max)(abs) = 415 nm, lambda(max)(em) = 590 nm) and enhanced fluorescence (Phi(F)(powder) = 10%, Phi(F)(soln) = 2%). Bistability, high-contrast switching (on/off fluorescence switching ratio >290), nondestructive readout (over 125000 shots), and erasability were all together accomplished in this novel recording medium.     

Synthesis,structure, and magnetic properties of dinuclear cobalt(II) complexes with a new phenol-based dinucleating ligand with four hydroxyethyl chelating arms

A new end-off type acyclic ligand with four hydroxyethyl arms, 2,6-bis[bis(2-hydroxyethyl)aminomethyl]-4-methylphenol [H(bhmp)], formed dinuclear cobalt(II) complexes [Co(2)(bhmp)(OAc)(2)]BPh(4) (1) and [Co(2)(bhmp)(OBz)(2)]BPh(4) (2). The complex 1.2.5CH(3)CN (C(50)H(62.5)BCo(2)N(4.5)O(9)) crystallizes in the monoclinic space group C2/c with dimensions a = 25.424(5) A, b = 13.376(2) A, c = 29.913(6) A, beta = 105.930(3) degrees, and V = 9781(3) A(3) and with Z = 8. X-ray diffraction analysis revealed a mu-phenoxo-bis(mu-acetato)dicobalt(II) core structure containing two octahedral cobalt(II) ions. Electronic spectra were investigated for 1 and 2 in the range 400-1800 nm, and the data were typical for the octahedral high-spin cobalt(II) complexes. Magnetic susceptibility was measured for 1 and 2 over the temperature range 4.5-300 K, and the data were analyzed well using our theoretical method. The best fitting parameters were kappa = 0.77, lambda = -116 cm(-1), Delta = 572 cm(-1), and J = -0.44 cm(-1) for complex 1 and kappa = 0.96, lambda = -93 cm(-1), Delta = 616 cm(-1), and J = -0.33 cm(-1) for complex 2.     

Probing the ruthenium-cumulene bonding interaction: synthesis and spectroscopic studies of vinylidene- and allenylidene-ruthenium complexes supported by tetradentate macrocyclic tertiary amine and comparisons with diphosphine analogues of ruthenium and osmium

The synthesis and spectroscopic properties of trans-[Cl(16-TMC)Ru[double bond]C[double bond]CHR]PF(6) (16-TMC = 1,5,9,13-tetramethyl-1,5,9,13-tetraazacyclohexadecane, R = C(6)H(4)X-4, X = H (1), Cl (2), Me (3), OMe (4); R = CHPh(2) (5)), trans-[Cl(16-TMC)Ru[double bond]C[double bond]C[double bond]C(C(6)H(4)X-4)(2)]PF(6) (X = H (6), Cl (7), Me (8), OMe (9)), and trans-[Cl(dppm)(2)M[double bond]C[double bond]C[double bond]C(C(6)H(4)X-4)(2)]PF(6) (M = Ru, X = H (10), Cl (11), Me (12); M = Os, X = H (13), Cl (14), Me (15)) are described. The crystal structures of 1, 5, 6, and 8 show that the Ru-C(alpha) and C(alpha)-C(beta) distances of the allenylidene complexes fall between those of the vinylidene and acetylide relatives. Two reversible redox couples are observed by cyclic voltammetry for 6-9, with E(1/2) values ranging from -1.19 to -1.42 and 0.49 to 0.70 V vs Cp(2)Fe(+/0), and they are both 0.2-0.3 and 0.1-0.2 V more reducing than those for 10-12 and 13-15, respectively. The UV-vis spectra of the vinylidene complexes 1-4 are dominated by intense high-energy bands at lambda(max) < or = 310 nm (epsilon(max) > or = 10(4) dm(3) mol(-1) cm(-1)), while weak absorptions at lambda(max) > or = 400 nm (epsilon(max) < or = 10(2) dm(3) mol(-1) cm(-1)) are tentatively assigned to d-d transitions. The resonance Raman spectrum of 5 contains a nominal nu(C[double bond]C) stretch mode of the vinylidene ligand at 1629 cm(-1). The electronic absorption spectra of the allenylidene complexes 6-9 exhibit an intense absorption at lambda(max) = 479-513 nm (epsilon(max) = (2-3) x 10(4) dm(3) mol(-1) cm(-1)). Similar electronic absorption bands have been found for 10-12, but the lowest energy dipole-allowed transition is blue-shifted by 1530-1830 cm(-1) for the Os analogues 13-15. Ab initio calculations have been performed on the ground state of trans-[Cl(NH(3))(4)Ru[double bond]C[double bond]C[double bond]CPh(2)](+) at the MP2 level, and imply that the HOMO is not localized purely on the metal center or allenylidene ligand. The absorption band of 6 at lambda(max) = 479 nm has been probed by resonance Raman spectroscopy. Simulations of the absorption band and the resonance Raman intensities show that the nominal nu(C[double bond]C[double bond]C) stretch mode accounts for ca. 50% of the total vibrational reorganization energy, indicating that this absorption band is strongly coupled to the allenylidene moiety. The excited-state reorganization of the allenylidene ligand is accompanied by rearrangement of the Ru[double bond]C and Ru[bond]N (of 16-TMC) fragments, which supports the existence of bonding interaction between the metal and C[double bond]C[double bond]C unit in the electronic excited state.     

Metal-metal interactions in dinuclear d(8) metal cyanide complexes supported by phosphine ligands. Spectroscopic properties and ab initio calculations of [M(2)(mu-diphosphine)(2)(CN)(4)] and trans-[M(phosphine)(2)(CN)(2)] (M = Pt,Ni)

Structural, spectroscopic properties on the dinuclear [M(2)(dcpm)(2)(CN)(4)] (M = Pt, 1a; Ni, 2a, dcpm = bis(dicyclohexylphosphino)methane) and [M(2)(dmpm)(2)(CN)(4)] (M = Pt, 1b; Ni, 2b, dmpm = bis(dimethylphosphino)methane) and the mononuclear trans-[M(PCy(3))(2)(CN)(2)] (M = Pt, 3; Ni, 4, PCy(3) = tricyclohexylphosphine) and theoretical investigations on the corresponding model compounds are described. X-ray structural analyses reveal Pt.Pt and Ni.Ni distances of 3.0565(4)/3.189(1) A and 2.957(1)/3.209(8) A for 1a/1b and 2a/2b, respectively. The UV-vis absorption bands at 337 nm (epsilon 2.41 x 10(4) dm(3) mol(-)(1) cm(-)(1)) for 1a and 328 nm (epsilon 2.43 x 10(4) dm(3) mol(-)(1) cm(-)(1)) for 1b in CH(2)Cl(2) are assigned to (1)(5d(sigma) --> 6p(sigma)) electronic transitions originating from Pt(II)-Pt(II) interactions. Resonance Raman spectroscopy of 1a, in which all the Raman intensity appears in the Pt-Pt stretch fundamental (93 cm(-)(1)) and overtone bands, verifies this metal-metal interaction. Complexes 1a and 1b exhibit photoluminescence in the solid state and solution. For the dinuclear nickel(II) complexes 2a and 2b, neither spectroscopic data nor theoretical calculation suggests the presence of Ni(II)-Ni(II) interactions. The intense absorption bands at lambda > 320 nm in the UV-vis spectra of 2a and 2b are tentatively assigned to d --> d transitions.     

Tetraaminoperylenes: their efficient synthesis and physical properties

Trimethylsilylation of 1,8-diaminonaphthalene gave 1,8-bis(trimethylsilylamino)naphthalene (1 a), which was in turn lithiated with two molar equivalents of n-butyllithium to give the tris(thf)-solvated dilithium diamide [1,8-[(Me(3)SiN)Li(thf)](2)C(10)H(6)](thf) (2 a). Metal exchange of 2 a with TlCl was carried out in two steps, via the previously characterized mixed-metal amide [1-[(Me(3)SiN)Li(thf)(2)]-8-[(Me(3)SiN)Tl]C(10)H(6)], to give the dithallium diamide [1,8-[(Me(3)SiN)Tl](2)C(10)H(6)] (3 a). Thermolysis of 3 a cleanly gave a 1:1 mixture of the 4,9-bis(trimethylsilylamino)perylenequinone-3,10-bis(trimethylsilylimine) (4 a) and 1 a. By this route, a whole series of silylated homologues of 4 a was obtained in good yields, while the same method proved to be inefficient for the synthesis of the alkyl-substituted analogues. Compound 4 a and its tert-butyldimethylsilyl derivative 4 d were reduced with sodium amalgam to give, after protonation, the corresponding 3,4,9,10-tetraaminoperylenes 7 a and 7 d. Cyclic voltammetry showed two reversible, closely spaced reduction waves (E(red 1)=-1.39, E(red 2)=-1.59 V versus SCE) corresponding to this conversion. The perylenes 7 a and 7 d are thought to be the primary products in the reaction cascade leading to the perylene derivatives, involving the thermal demetalation of the thallium amides, possibly via Tl(II)bond;Tl(II) intermediates, first to give 7 a and its analogues. The final oxidation of the tetraaminoperylenes by one molar equivalent of 3 a and analogous thallium amides gave the quinoidal derivatives such as 4 a and 4 d, a step that could be studied by direct reaction of the isolated species. The UV/Vis absorption spectra of the 4,9-bis(silylamino)perylenequinone-3,10-bis(silylimines) are characterized by a long-wavelength absorption band with a pronounced vibrational structure (lambda(max)=639 nm, lg epsilon =4.53) attributed to a pi*<--pi and a pi*<--n absorption band at 454 nm (lg epsilon 4.83), along with intense absorption in the UV region. A weak red emission with a rather low quantum yield (Phi(fl)=0.001, lambda(max)=660 nm) is observed upon irradiation of a sample; the lifetime of the emission is only 66 ps. The low emission quantum yield is attributed to the *pi<--n transition of the amino perylene, which induces strong spin-orbit coupling, leading to a large triplet yield. The triplet state was probed by transient absorption spectroscopy and found to have a lifetime of 200 ns in air, and 1100 ns in argon-flushed solution. Treatment of 4 a with a stoichiometric amount of KF in methanol/water under phase-transfer conditions (with the cryptand [C 222]) gave an almost quantitative yield of the parent compound 4,9-diaminoperylenequinone-3,10-diimine (8). Treatment of 8 with two molar equivalents of the ruthenium complex [Ru(bpy)(2)(acetone)(2)](PF(6))(2), generated in situ, yielded the blue dinuclear ruthenium complex [(bpy)(4)Ru(2)[mu(2)-N,N':N",N"'-[[4,9-(NH(2))(2)-3,10-(NH)(2)]C(20)H(8)]]](PF(6))(4) (9), the redox properties of which were studied by cyclic voltammetry. The difference in the potentials of the two one-electron redox steps (225 mV) indicates strong coupling of the metal centers through the 4,9-diaminoperylenquinone-3,10-dimine bridging ligand and corresponds to a comproportionation constant K(c) of 6.3 x 10(3). The UV/Vis absorption spectrum of the mixed valent form, which is stable in air, has a characteristic intervalence charge-transfer (IVCT) band in the near infrared at 930 nm (lg epsilon =3.95), from which an electronic coupling parameter J of 760 cm(-1) could be estimated, placing compound 9 at the borderline between the class II and class III cases in the Robin-Day classification.     

Disulphides of dithiophosphinic acids as analytical reagents-I Extractive spectrophotometric determination of palladium with some bis(dialkylthiophosphoryl)- and bis(diarylthiophosphoryl)disulphides

The disulphides of dithiophosphinic acids (DS) with the general formula R(2)P(S)SSP(S)R(2), where R = C(2)H(5), C(3)H(7), C(5)H(11), C(6)H(5) (I-IV) form coloured complexes of 1:3 stoichiometry with Pd(II). The absorption maxima and molar absorptivities are: a lambda(I) = 302 nm, epsilon(I) = 2.04 x 10(4) 1.mole(-1).cm(-1); lambda(II) = 305 nm, epsilon(II) = 2.58 x 10(4); lambda(III) = 303 nm, epsilon(III) = 2.60 x 10(4); lambda(IV) = 315 nm, epsilon(IV) = 3.25 x 10(4). The reaction takes about 3 min at room temperature, and the colour is stable for 24 hr. The influence of time, pH, reagent concentration, organic solvents and interferences have been studied. An extractive photometric method of determination of Pd(II) is described and applied to the determination of Pd(II) in a mixture of platinum metals.     

A double arene hydroxylation mediated by dicopper(II)-hydroperoxide species

The dicopper(II) complex [Cu(2)(L)](4+) (L = alpha,alpha'-bis[bis[2-(1'-methyl-2'-benzimidazolyl)ethyl]amino]-m-xylene) reacts with hydrogen peroxide to give the dicopper(II)-hydroquinone complex in which the xylyl ring of the ligand has undergone a double hydroxylation reaction at ring positions 2 and 5. The dihydroxylated ligand 2,6-bis([bis[2-(3-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)benzene-1,4-diol was isolated by decomposition of the product complex. The incorporation of two oxygen atoms from H(2)O(2) into the ligand was confirmed by isotope labeling studies using H(2)(18)O(2). The pathway of the unusual double hydroxylation was investigated by preparing the two isomeric phenolic derivatives of L, namely 3,5-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (6) and 2,6-bis([bis[2-(1-methyl-1H-benzimidazol-2-yl)ethyl]amino]methyl)phenol (7), carrying the hydroxyl group in one of the two positions where L is hydroxylated. The dicopper(II) complexes prepared with the new ligands 6 and 7 and containing bridging micro-phenoxo moieties are inactive in the hydroxylation. Though, the dicopper(II) complex 3 derived from 6 and containing a protonated phenol is rapidly hydroxylated by H(2)O(2) and represents the first product formed in the hydroxylation of [Cu(2)(L)](4+). Kinetic studies performed on the reactions of [Cu(2)(L)](4+) and 3 with H(2)O(2) show that the second hydroxylation is faster than the first one at room temperature (0.13 +/- 0.05 s(-1) vs 5.0(+/-0.1) x 10(-3) s(-1)) and both are intramolecular processes. However, the two reactions exhibit different activation parameters (Delta H++ = 39.1 +/- 0.9 kJ mol(-1) and Delta S++ = -115.7 +/- 2.4 J K(-1) mol(-1) for the first hydroxylation; Delta H++ = 77.8 +/- 1.6 kJ mol(-1) and Delta S++ = -14.0 +/- 0.4 J K(-1) mol(-1) for the second hydroxylation). By studying the reaction between [Cu(2)(L)](4+) and H(2)O(2) at low temperature, we were able to characterize the intermediate eta(1):eta(1)-hydroperoxodicopper(II) adduct active in the first hydroxylation step, [Cu(2)(L)(OOH)](3+) [lambda(max) = 342 (epsilon 12,000), 444 (epsilon 1200), and 610 nm (epsilon 800 M(-1)cm(-1)); broad EPR signal in frozen solution indicative of magnetically coupled Cu(II) centers].