Geometric and electronic structure of [{Cu(MeAN)}2(µ-η2:η2(O2(2-)))]2+ with an unusually long O-O bond: O-O bond weakening vs activation for reductive cleavage.

Certain side-on peroxo-dicopper(II) species with particularly low ν(O-O) (710-730 cm(-1)) have been found in equilibrium with their bis-µ-oxo-dicopper(III) isomer. An issue is whether such side-on peroxo bridges are further activated for O-O cleavage. In a previous study (Liang, H.-C., et al. J. Am. Chem. Soc.2002, 124, 4170), we showed that oxygenation of the three-coordinate complex [Cu(I)(MeAN)](+) (MeAN = N-methyl-N,N-bis[3-(dimethylamino)propyl]amine) leads to a low-temperature stable [{Cu(II)(MeAN)}(2)(µ-η(2):η(2)-O(2)(2-))](2+) peroxo species with low ν(O-O) (721 cm(-1)), as characterized by UV-vis absorption and resonance Raman (rR) spectroscopies. Here, this complex has been crystallized as its SbF(6)(-) salt, and an X-ray structure indicates the presence of an unusually long O-O bond (1.540(5) Å) consistent with the low ν(O-O). Extended X-ray absorption fine structure and rR spectroscopic and reactivity studies indicate the exclusive formation of [{Cu(II)(MeAN)}(2)(µ-η(2):η(2)-O(2)(2-))](2+) without any bis-µ-oxo-dicopper(III) isomer present. This is the first structure of a side-on peroxo-dicopper(II) species with a significantly long and weak O-O bond. DFT calculations show that the weak O-O bond results from strong σ donation from the MeAN ligand to Cu that is compensated by a decrease in the extent of peroxo to Cu charge transfer. Importantly, the weak O-O bond does not reflect an increase in backbonding into the σ* orbital of the peroxide. Thus, although the O-O bond is unusually weak, this structure is not further activated for reductive cleavage to form a reactive bis-µ-oxo dicopper(III) species. These results highlight the necessity of understanding electronic structure changes associated with spectral changes for correlations to reactivity.

New dicarboxylic acid bipyridine ligand for ruthenium polypyridyl sensitization of TiO2.

An ambidentate dicarboxylic acid bipyridine ligand, (4,5-diazafluoren-9-ylidene) malonic acid (dfm), was synthesized for coordination to Ru(II) and mesoporous nanocrystalline (anatase) TiO(2) thin films. The dfm ligand provides a conjugated pathway from the pyridyl rings to the carbonyl carbons of the carboxylic acid groups. X-ray crystal structures of [Ru(bpy)(2)(dfm)]Cl(2) and the corresponding diethyl ester compound, [Ru(bpy)(2)(defm)](PF(6))(2), were obtained. The compounds displayed intense metal-to-ligand charge transfer (MLCT) absorption bands in the visible region (ε > 11,000 M(-1) cm(-1) for [Ru(bpy)(2)(dfm)](PF(6))(2) in acetonitrile). Significant room temperature photoluminescence, PL, was absent in CH(3)CN but was observed at 77 K in a 4:1 EtOH:MeOH (v:v) glass. Cyclic voltammetry measurements revealed quasi-reversible Ru(III/II) electrochemistry. Ligand reductions were quasi-reversible for the diethyl ester compound [Ru(bpy)(2)(defm)](2+), but were irreversible for [Ru(bpy)(2)(dfm)](2+). Both compounds were anchored to TiO(2) thin films by overnight reactions in CH(3)CN to yield saturation surface coverages of 3 × 10(-8) mol/cm(2). Attenuated total reflection infrared measurements revealed that the [Ru(bpy)(2)(dfm)](2+) compound was present in the deprotonated carboxylate form when anchored to the TiO(2) surface. The MLCT excited states of both compounds injected electrons into TiO(2) with quantum yields of 0.70 in 0.1 M LiClO(4) CH(3)CN. Micro- to milli-second charge recombination yielded ground state products. In regenerative solar cells with 0.5 M LiI/0.05 M I(2) in CH(3)CN, the Ru(bpy)(2)(dfm)/TiO(2) displayed incident photon-to-current efficiencies of 0.7 at the absorption maximum. Under the same conditions, the diethylester compound was found to rapidly desorb from the TiO(2) surface.

Influence of ion pairing on the oxidation of iodide by MLCT excited states.

The oxidation of iodide to diiodide, I(2)˙(-), by the metal-to-ligand charge-transfer (MLCT) excited state of [Ru(deeb)(3)](2+), where deeb is 4,4'-(CO(2)CH(2)CH(3))(2)-2,2'-bipyridine, was quantified in acetonitrile and dichloromethane solution at room temperature. The redox and excited state properties of [Ru(deeb)(3)](2+) were similar in the two solvents; however, the mechanisms for excited state quenching by iodide were found to differ significantly. In acetonitrile, reaction of [Ru(deeb)(3)](2+*) and iodide was dynamic (lifetime quenching) with kinetics that followed the Stern-Volmer model (K(D) = 1.0 ± 0.01 × 10(5) M(-1), k(q) = 4.8 × 10(10) M(-1) s(-1)). Excited state reactivity was observed to be the result of reductive quenching that yielded the reduced ruthenium compound, [Ru(deeb(-))(deeb)(2)](+), and the iodine atom, I˙. In dichloromethane, excited state quenching was primarily static (photoluminescence amplitude quenching) and [Ru(deeb(-))(deeb)(2)](+) formed within 10 ns, consistent with the formation of ion pairs in the ground state that react rapidly upon visible light absorption. In both solvents the appearance of I(2)˙(-) could be time resolved. In acetonitrile, the rate constant for I(2)˙(-) growth, 2.2 ± 0.2 × 10(10) M(-1) s(-1), was found to be about a factor of two slower than the formation of [Ru(deeb(-))(deeb)(2)](+), indicating it was a secondary photoproduct. The delayed appearance of I(2)˙(-) was attributed to the reaction of iodine atoms with iodide. In dichloromethane, the growth of I(2)˙(-), 1.3 ± 0.4 × 10(10) M(-1) s(-1), was similar to that in acetonitrile, yet resulted from iodine atoms formed within the laser pulse. These results are discussed within the context of solar energy conversion by dye-sensitized solar cells and storage via chemical bond formation.

Sulfur donor atom effects on copper(I)/O(2) chemistry with thioanisole containing tetradentate N(3)S ligand leading to µ-1,2-peroxo-dicopper(II) species.

To better understand the effect of thioether coordination in copper-O(2) chemistry, the tetradentate N(3)S ligand L(ASM) (2-(methylthio)-N,N-bis((pyridin-2-yl)methyl)benzenamine) and related alkylether ligand L(EOE) (2-ethoxy-N,N-bis((pyridin-2-yl)methyl)ethanamine) have been studied. The corresponding copper(I) complexes, [(L(ASM))Cu(I)](+) (1a) and [(L(EOE))Cu(I)](+) (3a), were studied as were the related compound [(L(ESE))Cu(I)](+) (2a, L(ESE) = (2-ethylthio-N,N-bis((pyridin-2-yl)methyl)ethanamine). The X-ray structure of 1a and its solution conductivity reveal a monomeric molecular structure possessing thioether coordination which persists in solution. In contrast, the C-O stretching frequencies of the derivative Cu(I)-CO complexes reveal that for these complexes, the modulated ligand arms, whether arylthioether, alkylthioether, or ether, are not coordinated to the cuprous ion. Electrochemical data for 1a and 2a in CH(3)CN and N,N-dimethylformamide (DMF) show the thioanisole moiety to be a poor electron donor compared to alkylthioether (1a is ∼200 mV more positive than 2a). The structures of [(L(ASM))Cu(II)(CH(3)OH)](2+) (1c) and [(L(ESE))Cu(II)(CH(3)OH)](2+) (2c) have also been obtained and indicate nearly identical copper coordination environments. Oxygenation of 1a at reduced temperature gives a characteristic deep blue intermediate [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) with absorption features at 442 (1,500 M(-1) cm(-1)), 530 (8,600 M(-1) cm(-1)), and 605 nm (10,400 M(-1) cm(-1)); these values compare well to the ligand-to-metal charge-transfer (LMCT) transitions previously reported for [{(L(ESE))Cu(II)}(2)(O(2)(2-))](2+) (2b(P)). Resonance Raman data for [{(L(ASM))Cu(II)}(2)(O(2)(2-))](2+) (1b(P)) support the formation of µ-1,2-peroxo species ν(O-O) = 828 cm(-1)(Δ((18)O(2)) = 48), ν(sym)(Cu-O) = 547 cm(-1) (Δ((18)O(2)) = 23), and ν(asym)(Cu-O) = 497 cm(-1) (Δ((18)O(2)) = 22) and suggest the L(ASM) ligand is a poorer electron donor to copper than is L(ESE). In contrast, the oxygenation of [(L(EOE))Cu(I)](+) (3a), possessing an ether donor as an analogue of the thioether in L(ESE), led to the formation of a bis(µ-oxo) species [{(L(EOE))Cu(III)}(2)(O(2-))(2)](2+) (3b(O); 380 nm, ε ∼ 10,000 M(-1) cm(-1)). This result provides further support for the sulfur influence in 1b(P) and 2b(P), in particular coordination of the sulfur to the Cu. Thermal decomposition of 1b(P) is accompanied by ligand sulfoxidation. The structure of [{(L(EOE))Cu(II)(Cl)}(2)](+) (3c) generated from the reductive dehalogenation of organic chlorides suggests that the ether moiety is weakly bound to the cupric ion. A detailed discussion of the spectroscopic and structural characteristics of 1b(P), 2b(P), and 3b(O) is presented.

Reaction of Ru(II) diazafluorenone compound with nanocrystalline TiO2 thin film.

The Ru(II) compounds [Ru(bpy)(2)(mcbH)](2+) and [Ru(bpy)(2)(dafo)](2+), bpy is 2,2'-bipyridine where mcbH is 3-(CO(2)H)-2,2'-bipyridine and dafo is 4,5-diazafluoren-9-one, were synthesized, characterized, and anchored to nanocrystalline mesoporous TiO(2) thin films for excited state and interfacial electron transfer studies. X-ray crystallographic studies of [Ru(bpy)(2)(mcbH)](PF(6))(Cl) revealed a long Ru-N distance to the unsubstituted pyridine ligand of mcbH. Reaction of [Ru(bpy)(2)(dafo)](2+) with TiO(2) thin films resulted in interfacial chemistry. The IR, (1)H NMR, UV-vis, and photoluminescence spectral data indicated a room-temperature ring-opening reaction of the dafo ligand of [Ru(bpy)(2)(dafo)](2+) that ultimately yielded a carboxylate group in the 3-position of bipyridine anchored to TiO(2). Comparative reactions of [Ru(bpy)(2)(mcbH)](2+) with TiO(2) were performed and support this conclusion. In regenerative photoelectrochemical solar cells with 0.5 M LiI/0.05 M I(2) in acetonitrile, photocurrent action spectra were observed for both sensitized materials. The incident photon-to-current efficiency (IPCE) was significantly lower for Ru(bpy)(2)(dafo)/TiO(2), behavior attributed to a lower excited-state injection yield.

Thioether S-ligation in a side-on micro-eta2:eta2-peroxodicopperii complex.

[(ANS)Cu(I)(CH(3)CN)](+) reacts with O(2) giving [{(ANS)Cu(II)}(2)(micro-eta(2):eta(2)-O(2)(2-))](2+), nu(O-O) = 731 cm(-1), shown to possess S-thioether ligation, based on comparisons with analogues having all N-ligands or a -S(Ph) group. The finding is a rare occurrence and new for side-on O(2)(2-) binding.

Copper(I)/O2 chemistry with imidazole containing tripodal tetradentate ligands leading to mu-1,2-peroxo-dicopper(II) species.

Cuprous and cupric complexes with the new imidazolyl containing tripodal tetradentate ligands {L(MIm), (1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)methanamine, and L(EIm), 2-(1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)ethanamine}, have been investigated to probe differences in their chemistry, especially in copper(I)-dioxygen chemistry, compared to that already known for the pyridyl analogue TMPA, tris(2-pyridyl)methyl)amine. Infrared (IR) stretching frequencies obtained from carbon monoxide adducts of [(L(MIm))Cu(I)](+) (1a) and [(L(EIm))Cu(I)](+) (2a) show that the imidazolyl donor is stronger than its pyridyl analogue. Electrochemical data suggest differences in the binding constant of Cu(II) to L(EIm) compared to TMPA and L(MIm), reflecting geometric changes. Oxygenation of [(L(MIm))Cu(I)](+) (1a) in 2-methyltetrahydrofuran (MeTHF) solvent at -128 degrees C leads to an intensely purple colored species with a UV-vis spectrum characteristic of an end-on bound peroxodicopper(II) complex [{(L(MIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (1b(P)) {lambda(max) = 528 nm}, very similar to the previously well characterized complex [{(TMPA)Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) {lambda(max) = 520 nm (epsilon = 12 000 M(-1) cm(-1)), in MeTHF; resonance Raman (rR) spectroscopy: nu(O-O) = 832 (Delta((18)O(2)) = -44) cm(-1)}. In the low-temperature oxygenation of 2a, benchtop (-128 degrees C) and stopped-flow (-90 degrees C) experiments reveal the formation of an initial superoxo-Cu(II) species [(L(EIm))Cu(II)(O(2)(*-))](+) (2b(S)), lambda(max) = 431 nm in THF) . This converts to the low-temperature stable peroxo complex [{(L(EIm))Cu(II)}(2)(mu-1,2-O(2)(2-))](2+) (2b(P)) {rR spectroscopy: nu(O-O) = 822 (Delta((18)O(2)) = -46) cm(-1)}. Complex 2b(P) possess distinctly reduced Cu-O and O-O stretching frequencies and a red-shifted UV-vis feature {to lambda(max) = 535 nm (epsilon = 11 000 M(-1) cm(-1))} compared to the TMPA analogue due to a distortion from trigonal bipyramidal (TBP) to a square pyramidal ligand field. This distortion is supported by the structural characterization of related ligand-copper(II) complexes: A stable tetramer cluster complex [(mu(2)-L(EIm-))(4)(Cu(II))(4)](4+), obtained from thermal decomposition of 2b(P) (with formation of H(2)O(2)), also exhibits a distorted square pyramidal Cu(II) ion geometry as does the copper(II) complex [(L(EIm))Cu(II)(CH(3)CN)](2+) (2c), characterized by X-ray crystallography and solution electron paramagnetic resonance (EPR) spectroscopy.

Photodriven spin change of Fe(II) benzimidazole compounds anchored to nanocrystalline TiO(2) thin films.

Ferrous tris-chelate compounds based on 2-(2'-pyridyl)benzimidazole (pybzim) have been prepared and characterized for studies of spin equilibria in fluid solution and when anchored to the surface of mesoporous nanocrystalline (anatase) TiO(2) and colloidal ZrO(2) thin films. The solid state structure of Fe(pybzim)(3)(ClO(4))(2).CH(3)CN.H(2)O was determined by single-crystal X-ray diffraction at 110 K to be triclinic, P-1, a = 11.6873(18), b = 12.2318(12), c = 14.723(4) A, alpha = 89.864(13) degrees , beta = 71.430(17) degrees , gamma = 73.788(11) degrees , V = 1907.1(6) A(3), Z = 2, and R = 0.0491. The iron compound has a meridional FeN(6) distorted octahedral geometry with bond lengths expected for a low-spin iron center at 110 K. The visible absorption spectra of Fe(pybzim)(3)(2+) and Fe(pymbA)(3)(2+), where pymbA is 4-(2-pyridin-2-yl-benzimidazol-1-ylmethyl)-benzoic acid, in methanol solution were dominated by metal-to-ligand charge-transfer (MLCT) bands. Variable-temperature UV-visible absorption spectroscopy revealed dramatic changes in the extinction coefficient consistent with a high-spin ((1)A) left harpoon over right harpoon low-spin ((5)T) equilibrium. Thermodynamic parameters for the temperature-dependent spin equilibrium of Fe(pymbA)(3)(2+) in methanol were determined to be DeltaH(HL) = 3270 +/- 210 cm(-1) and DeltaS(HL) = 13.3 +/- 0.8 cm(-1) K(-1). The corresponding values for Fe(pybzimEE)(3)(2+), where pybzimEE is (2-pyridin-2-yl-benzimidazol-1-yl)-acetic acid ethyl ester, in acetonitrile solution were determined to be 3072 +/- 34 cm(-1)and 10.5 +/- 0.1 cm(-1) K(-1). The temperature-dependent effective magnetic moments of Fe(pybzimEE)(3)(2+) in acetonitrile solution were also quantified by the Evans method. Pulsed 532 nm light excitation of Fe(pybzim)(3)(2+) or Fe(pymbA)(3)(2+) in solution resulted in an immediate bleach of the MLCT absorption bands. Relaxation back to the equilibrium state followed a first-order reaction mechanism. Arrhenius analysis of the (5)T --> (1)A rate constant yielded an activation energy, E(a), of 1090 +/- 20 cm(-1) and 710 +/- 10 cm(-1) for Fe(pybzim)(3)(2+) and Fe(pymbA)(3)(2+) in methanol, respectively. The compound Fe(pymbA)(3)(2+) was found to bind to colloidal TiO(2) and ZrO(2) thin films. The absorption spectra of the surface-attached compounds were quantified from 295 to 193 K. Pulsed light excitation of Fe(pymbA)(3)/TiO(2) and Fe(pymbA)(3)/ZrO(2) resulted in the immediate bleach of the MLCT absorption bands. Relaxation was nonexponential but was well described by kinetic models based on a Gaussian distribution of activation energies or a Levy distribution of lifetimes. An Arrhenius analysis of the Gaussian data yielded average activation energies of 660 +/- 80 cm(-1) and 730 +/- 40 cm(-1) for Fe(pymbA)(3)(ClO(4))(2) on TiO(2) and ZrO(2) surfaces, respectively. The Levy distribution analysis did not adequately fit the Arrhenius model.

A peroxynitrite complex of copper: formation from a copper-nitrosyl complex, transformation to nitrite and exogenous phenol oxidative coupling or nitration.

Reaction of nitrogen monoxide with a copper(I) complex possessing a tridentate alkylamine ligand gives a Cu(I)-(*NO) adduct, which when exposed to dioxygen generates a peroxynitrite (O=NOO(-))-Cu(II) species. This undergoes thermal transformation to produce a copper(II) nitrito (NO(2) (-)) complex and 0.5 mol equiv O(2). In the presence of a substituted phenol, the peroxynitrite complex effects oxidative coupling, whereas addition of chloride ion to dissociate the peroxynitrite moiety instead leads to phenol ortho nitration. Discussions include the structures (including electronic description) of the copper-nitrosyl and copper-peroxynitrite complexes and the formation of the latter, based on density functional theory calculations and accompanying spectroscopic data.

Copper-hydroperoxo-mediated N-debenzylation chemistry mimicking aspects of copper monooxygenases.

A substantial oxidative N-debenzylation reaction along with PhCHO formation occurs from a hydroperoxo-copper(II) complex that has a dibenzylamino substrate (N(CH 2Ph) 2 appended as a substituent on one pyridyl group of its tripodal tetradentate TMPA (also TPA, (2-pyridylmethyl)amine)) ligand framework. During the course of the (L (N(CH 2 ) (Ph) 2 ))Cu (II)( (-)OOH) reactivity, the formation of a substrate and a (-)OOH-derived (an oxygen atom) alkoxo Cu (II)( (-)OR) complex occurs. The observation that the same Cu (II)( (-)OR) species occurs from Cu (Iota)/PhIO chemistry suggests the possibility that a copper-oxo (cupryl) reactive intermediate forms during the alkoxo species formation; new ESI-MS data provide further support for this high-valent intermediate. A net H atom abstraction chemistry is proposed on the basis of the kinetic isotope effect studies provided here and the previously published study for a closely related Cu (II)( (-)OOH) species incorporating dimethylamine (N(CH 3) 2) as the internal substrate; the Cu (Iota)/PhIO reactivity with similar isotope effect results provides further support. The reactivity of these chemical systems closely resembles the proposed oxidative N-dealkylation mechanisms that are effected by the copper monooxygenases, dopamine beta-monooxygenase (DbetaM) and peptidylglycine- alpha-hydroxylating monooxygenase (PHM).

Reaction of a copper-dioxygen complex with nitrogen monoxide (*NO) leads to a copper(II)-peroxynitrite species.

A discrete peroxynitrite-copper(II) complex, [(TMG3tren)CuII(-OONO)]+ (3), has been generated in solution (ESI-MS, m/z = 565.15; tetragonal EPR) by reacting *NO(g) with superoxo complex [(TMG3tren)CuII(O2*-)]+ (2). Complex 3 undergoes a thermal transformation to give CuII-nitrite complex [(TMG3tren)CuII(-ONO)]+ (4) (X-ray) along with ca. 0.5 molar equiv dioxygen. A DFT calculation derived structure with cyclic bidentate k2-O,O'-OONO bound peroxynitrite moiety and dx2-y2 ground state is proposed. Experiments using 18O2 suggest that the adjacent peroxo oxygen atoms in 3 are derived from molecular oxygen. Further, 18O2 containing 3 undergoes O-O bond cleavage to form singly 18-O-labeled 4. The results suggest the viability of biological CuI/O2/(*NO) peroxynitrite formation and chemistry, that is, not coming from free superoxide plus *NO reaction.

Copper dioxygen adducts: formation of bis(mu-oxo)dicopper(III) versus (mu-1,2)Peroxodicopper(II) complexes with small changes in one pyridyl-ligand substituent.

The preference for the formation of a particular Cu 2O 2 isomer coming from (ligand)-Cu (I)/O 2 reactivity can be regulated with the steric demands of a TMPA (tris(2-pyridylmethyl)amine) derived ligand possessing 6-pyridyl substituents on one of the three donor groups of the tripodal tetradentate ligand. When this substituent is an -XHR group (X = N or C) the traditional Cu (I)/O 2 adduct forms a (mu-1,2)peroxodicopper(II) species ( A). However, when the substituent is the slightly bulkier XR 2 moiety {aryl or NR 2 (R not equal H)}, a bis(mu-oxo)dicopper(III) structure ( C) is favored. The reactivity of one of the bis(mu-oxo)dicopper(III) species, [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) (6tbp = (6- (t)Bu-phenyl-2-pyridylmethyl)bis(2-pyridylmethyl)amine), was probed, and for the first time, exogenous toluene or ethylbenzene hydrocarbon oxygenation reactions were observed. Typical monooxygenase chemistry occurred: the benzaldehyde product includes an 18-O atom for toluene/ 7- (1) (8)O 2 reactivity, and a H-atom abstraction by 7-O 2 is apparent from study of its reactions with ArOH substrates, as well as the determination of k H/ k D approximately 7 in the toluene oxygenation (i.e., PhCH 3 vs PhCD 3 substrates). Proposed courses of reaction are presented, including the possible involvement of PhCH 2OO (*) and its subsequent reaction with copper(I) complex, the latter derived from dynamic solution behavior of 7-O 2 . External TMPA ligand exchange for copper in 7-O 2 and O-O bond (re)formation chemistry, along with the ability to protonate 7-O 2 and release of H 2O 2 indicate the presence of an equilibrium between [{(6tbp)Cu (III)} 2(O (2-)) 2] (2+) ( 7-O 2 ) and a (mu-1,2)peroxodicopper(II) form.

Suggestion of an organometallic intermediate in an intramolecular dechlorination reaction involving copper(I) and a ArCH2Cl moiety.

A copper(I)-mediated reductive dechlorination reaction involving an "internal" chloromethylene substrate at the pyridyl 6-position of one TMPA arm (TMPA triple bond TPA triple bond tris(2-pyridylmethyl)amine) leads to a 1:1 ratio of the starting ligand 6ClCH2-TMPA and a new methyl-TMPA product, 6CH2H-TMPA. On the basis of observed product distributions and a kinetic study, a reaction mechanism involving intramolecular oxidative insertion of Cu(I) to the C-Cl substrate is suggested. The resulting organometallic intermediate is then protonated, leading to the observed products.

A low-spin alkylperoxo-iron(III) complex with weak Fe-O and O-O bonds: implications for the mechanism of superoxide reductase.

The synthesis of a mononuclear, five-coordinate ferrous complex [([15]aneN4)FeII(SPh)](BF4) (1) is reported. This complex is a new model of the reduced active site of the enzyme superoxide reductase (SOR), which is comprised of a [(NHis)4(Scys)FeII] center. Complex 1 reacts with alkylhydroperoxides (tBuOOH, cumenylOOH) at low temperature to give a metastable, dark red intermediate (2a: R = tBu; 2b: R = cumenyl) that has been characterized by UV-vis, EPR, and resonance Raman spectroscopy. The UV-vis spectrum (-80 degrees C) reveals a 526 nm absorbance (epsilon = 2150 M-1 cm-1) for 2a and a 527 nm absorbance (epsilon = 1650 M-1 cm-1) for 2b, indicative of alkylperoxo-to-iron(III) LMCT transitions, and the EPR data (77 K) show that both intermediates are low-spin iron(III) complexes (g = 2.20 and 1.97). Definitive identification of the Fe(III)-OOR species comes from RR spectra, which give nu(Fe-O) = 612 (2a) and 615 (2b) cm-1, and nu(O-O) = 803 (2a) and 795 (2b) cm-1. The assignments for 2a were confirmed by 18O substitution (tBu18O18OH), resulting in a 28 cm-1 downshift for nu(Fe-18O), and a 46 cm-1 downshift for nu(18O-18O). These data show that 2a and 2b are low-spin FeIII-OOR species with weak Fe-O bonds and suggest that a low-spin intermediate may occur in SOR, as opposed to previous proposals invoking high-spin intermediates.

An isolable, nonreducible high-valent manganese(V) imido corrolazine complex.

The manganese(V) imido complex [(TBP8Cz)Mn(V)(NMes)] (2) was synthesized from the Mn(III) complex [(TBP8Cz)Mn(III)] (1) and thermolysis of mesityl azide. An X-ray structure of 2 reveals a short Mn-N distance [1.595(4) A], consistent with the Mn-N triple bond expected for a manganese(V) imido species. This high-valent species is remarkably inert to one- and two-electron reductive processes such as NR group transfer to alkenes or H-atom abstraction from O-H bonds. Electrochemical studies support this lack of reactivity. In contrast, oxidation of 2 is easily accomplished by treatment with [(4-BrC6H4)3N]*+SbCl6, giving a pi-radical-cation complex.

Directed synthesis of noncentrosymmetric molybdates using composition space analysis.

A systematic investigation of the factors governing the reaction product composition, hydrogen bonding, and symmetry was conducted in the MoO3/3-aminoquinuclidine/H2O system. Composition space analysis was performed through 36 individual reactions under mild hydrothermal conditions using racemic 3-aminoquinuclidine. Single crystals of three new compounds, [C7H16N2][Mo3O10] x H2O, [C7H16N2]2[Mo8O26] x H2O, and [C7H16N2]2[Mo8O26] x 4 H2O, were grown. The relative phase stabilities for these products are dependent upon the reactant mole fractions in the initial reaction gel. This phase stability information was used to direct the synthesis of two new noncentrosymmetric compounds, using either (S)-(-)-3-aminoquinuclidine dihydrochloride or (R)-(+)-3-aminoquinuclidine dihydrochloride. [(R)-C7H16N2]2[Mo8O26] and [(S)-C7H16N2]2[Mo8O26] both crystallize in the noncentrosymmetric space group P2(1) (No. 4), which has the polar crystal class 2 (C2). The second-harmonic generation activities were measured on sieved powders. The structure-directing properties of the molybdate components in each compound were determined using bond valence sums. The structures of all five compounds were determined using single-crystal X-ray diffraction.

Triiodide quenching of ruthenium MLCT excited state in solution and on TiO2 surfaces: an alternate pathway for charge recombination.

The excited states of [Ru(bpy)2(deeb)](PF6)2, where bpy is 2,2-bipyridine and deeb is 4,4'-(CO2CH2CH3)2-2,2'-bipyridine, were found to be efficiently quenched by triiodide (I3-) in acetonitrile and dichloromethane. In dichloromethane, I3- was found to quench the excited states by static and dynamic mechanisms; Stern-Volmer analysis of the time-resolved and steady-state photoluminescence data produced self-consistent estimates for the I3- + Ru(bpy)2(deeb)2+ <==> [Ru(II)(bpy)2(deeb)2+,(I3-)]+ equilibrium, K = 51,000 M(-1), and the bimolecular quenching rate constant, kq = 4.0 x 10(10) M(-1) s(-1). In acetonitrile, there was no evidence for ion pairing and a dynamic quenching rate constant of k(q) = 4.7 x 10(10) M(-1) s(-1) was calculated. Comparative studies with Ru(bpy)2(deeb)2+ anchored to mesoporous nanocrystalline TiO2 thin films also showed efficient excited-state dynamic quenching by I3- in both acetonitrile and dichloromethane, kq = 1.8 x 10(9) and 3.6 x 10(10) M(-1) s(-1), respectively. No reaction products for the excited-state quenching processes were observed by nanosecond transient absorption measurements from 350 to 800 nm under any experimental conditions. X-ray crystallographic, IR, and Raman data gave evidence for interactions between I3- and the bpy and deeb ligands in the solid state.

Selective DNA strand scission with binuclear copper complexes: implications for an active Cu2-O2 species.

A homologous series of binuclear copper(II) complexes [Cu(II)(2)(Nn)(Y)(2)](2+) (1-3) (n = 3-5 and Y = (ClO(4))(-) or (NO(3))(-)) were studied to investigate the intermediate(s) responsible for selective DNA strand scission in the presence of MPA/O(2) (MPA = 3-mercaptopropanoic acid). While the N3 complex does not react, the N4 and N5 analogues show comparable activity with strand scission occurring at a single-strand/double-strand junction. Identical reactivity is also observed in the alternate presence of H(2)O(2). Spectroscopic and reactivity studies with [Cu(II)(2)(N4)(Y)(2)](2+) (2) and H(2)O(2) are consistent with DNA oxidation mediated by formation of a side-on peroxodicopper(II) (Cu(2)-O(2)) complex.

Second generation, orally active, antimalarial, artemisinin-derived trioxane dimers with high stability, efficacy, and anticancer activity.

In only two steps and in 63% overall yield, naturally occurring 1,2,4-trioxane artemisinin (1) was converted into C-10-carba trioxane conjugated diene dimer 4. This new dimer was then transformed easily in one additional 4 + 2-cycloaddition step into phthalate dimer 5, and further modification led to bis-benzyl alcohol dimer 7 and its phosphorylated analogues 8 and 9. Bis-benzyl alcohol dimer 7 is the most antimalarially active in vitro, 10 times more potent than artemisinin (1). Bis-benzyl alcohol dimer 7 is approximately 1.5 times more orally efficacious in rodents than the antimalarial drug sodium artesunate and is about 37 times more efficacious than sodium artesunate via subcutaneous administration. Both dimers 5 and 7 are thermally stable neat even at 60 degrees C for 24 h. Phthalate dimer 5 is very highly growth inhibitory but not cytotoxic toward several human cancer cell lines; both dimers 5 and 7 very efficiently and selectively kill human cervical cancer cells in vitro in a dose-dependent manner with no cytotoxic effects on normal cervical cells.