Monometallic and dimetallic ruthenium(II)-terpyridine complexes employing the tetradentate ligands dipyridylpyrazolyl,dipyridyloxadiazole, and their dimethyl derivatives

The tetradentate ligands, 2,2'-(1H-pyrazole-3,5-diyl)bis(4- methylpyridine) (4,4'-Me2dppzH), 2,2'-(1H-pyrazole-3,5-diyl)bis(6-methylpyridine) (6,6'-Me2dppzH), 3,5-di(pyrid-2-yl)pyrazole (dppzH), and dipyridyloxadiazole (dpo) react with either Ru(trpy)Cl3 or trans-Ru(trpy)Cl2(NCCH3), where trpy is 2,2',2'-terpyridine, to form a variety of Ru(II) complexes. Among these are the symmetrical chloro-bridged Ru(II) dimer and the "in" and "out" geometric isomers of the monometallic Ru(II) containing species where "in" and "out" refer to the orientation of the Ru-Cl vector relative to the centroid of the ligand backbone. Thirteen complexes were prepared and painstakingly purified by careful recrystallization and/or exhaustive column chromatography. These complexes were characterized by 1H and 13C NMR, electronic absorption, and infrared spectroscopy. Additionally, [Ru2(tryp)2(6,6'-Me2dppz)mu-Cl](BF4)2 (3b(BF4)2), [Ru2(trpy)2(4,4'-Me2dppz)mu-Cl](PF6)2.0.5MeOH (3c), [Ru2(trpy)2(6,6'-Me2dppz)(CH2C(O)CH3)](PF6)2.0.5(CH3)2CO (9b), "in"-[Ru(trpy)(4,4'-Me2dppz)Cl](PF6).(CH3)2CO (1c), and "out"-[Ru(trpy)(dpo)Cl](PF6).(CH3)2CO (2d) were characterized by X-ray crystallography. Several ligand substitution reactions were attempted. For example, [Ru2(trpy)2(6,6'-Me2dppz)mu-Cl](BF4)2 (3b) was reacted with hydroxide ion to produce [Ru2(trpy)2(6,6'-Me2dppz)mu-OH](PF6)2 (6b). Complex 6b reacts with benzyl bromide to produce [Ru2(trpy)2(6,6'-Me2dppz)mu-Br](PF6)2 (7b) or with (CH3)3Sil to produce [Ru2(trpy)2(6,6'-Me2dppz)mu-I](PF6)2 (8b). of 6b with acetone forms the methyl enolate complex [Ru2(trpy)2(6,6'-Me2dppz)(CH2COCH3)](PF6)2 (9b) while, analogously to a Cannizarro reaction, the reaction with benzaldehyde forms the bridging benzoate complex [Ru2(trpy)2(6,6'-Me2dppz)(C6H4CO2)](PF6)2 (11b). The bridging azide complex [Ru2(trpy)2(6,6'-Me2dppz)mu-N3](PF6)2 (10b) is formed by reaction of 6b with (CH3)3-SiN3. Additionally, the chloride ligands of the monometallic complexes of "in"-[Ru(trpy)(dpo)Cl](PF6) (1d), "in"-[Ru(trpy)(4,4'-Me2dpo)Cl](PF6)] (1e), and "out"-[Ru(trpy)(dpo)Cl](PF6) (2d) were substituted with water to form their respective aqua complexes, 4d, 4e, and 5d. All of the complexes exhibit broad unsymmetrial absorption bands in the visible portion of the electromagnetic spectrum. The dimetallic complexes 3b and 3c exhibit two, 1e- reversible oxidation waves at +0.72 and +1.15 V, and at +0.64 and +1.13 V, respectively. These complexes were not emissive.     

One-electron reduction of methanesulfonyl chloride. The fate of MeSO2Cl(.)(-) and MeSO2(.) intermediates in oxygenated solutions and their role in the cis-trans isomerization of mono-unsaturated fatty acids

The one-electron reduction of methanesulfonyl chloride (MeSO2Cl) leads, in the first instance, to an electron adduct MeSO2Cl(.)(-) which lives long enough for direct detection and decays into sulfonyl radicals MeSO2(.) and Cl(-), with k = 1.5 x 10(6) s(-1). Both, MeSO2Cl(.)(-) and MeSO2(.) showed a similar absorption in the UV with lambdamax of 320 nm. In the presence of oxygen, MeSO2Cl(.)(-) transfers an electron to O(2) and establishes an equilibrium with superoxide. The rate constant for the forward reaction was measured to 4.1 x 10(9) M(-1) s(-1), while for the back reaction only an interval of 1.7 x 10(5) to 1.7 x 10(6) M(-1) s(-1) could be estimated, with a somewhat higher degree of confidence for the lower value. This corresponds to an equilibrium constant in the range of 2.4 x 10(3) to 2.4 x 10(4). With reference to E degrees (O2/O2(.)(-)) = -155 mV, the redox potential of the sulfonyl chloride couple, E degrees (MeSO2Cl/MeSO2Cl(.)(-)), thus results between being equal to -355 and -414 mV (vs NHE). MeSO2Cl(.)(-) reduces (besides O2) 4-nitroacetophenone. The underlying electron transfer took place with k = 1.5 x 10(9) M(-1) s(-1), corroborating an E degrees for the sulfonyl chloride couple significantly exceeding the above listed lower value. The MeSO2(.) radical added to oxygen with a rate constant of 1.1 x 10(9) M(-1) s(-1). Re-dissociation of O2 from MeSO2OO(.) occurred only very slowly, if at all, that is, with k < 10(5) s(-1). MeSO2(.) radicals can act as the catalyst for the cis-trans isomerization of several Z- and E-mono-unsaturated fatty acid methyl esters in homogeneous solution. The effectiveness of the isomerization processes has been addressed, and in the presence of oxygen the isomerization is completely suppressed.     

Intercalation of fac-[(4,4'-bpy)ReI(CO)3(dppz)]+, dppz = dipyridyl[3,2-a:2'3'-c]phenazine, in polynucleotides. On the UV-vis photophysics of the Re(I) intercalator and the redox reactions with pulse radiolysis-generated radicals

The intercalation of fac-[(4,4'-bpy)Re(I)(CO)3(dppz)]+ (dppz = dipyridyl[3,2-a:2'3'-c]phenazine) in polynucleotides, poly[dAdT]2 and poly[dGdC]2, where A = adenine, G = guanine, C = cytosine and T = thymine, is a major cause of changes in the absorption and emission spectra of the complex. A strong complex-poly[dAdT]2 interaction drives the intercalation process, which has a binding constant, Kb approximately 1.8 x 10(5) M(-1). Pulse radiolysis was used for a study of the redox reactions of e(-)(aq), C*H(2)OH and N3* radicals with the intercalated complex. These radicals exhibited more affinity for the intercalated complex than for the bases. Ligand-radical complexes, fac-[(4,4'-bpy*)Re(I)(CO)3(dppz)] and fac-[(4,4'-bpy)Re(I)(CO)3(dppz *)], were produced by e(-)(aq) and C*H(2)OH, respectively. A Re(II) species, fac-[(4,4'-bpy)Re(II)(CO)3(dppz)](2+), was produced by N3* radicals. The rate of annihilation of the ligand-radical species was second order on the concentration of ligand-radical while the disappearance of the Re(II) complex induced the oxidative cleavage of the polynucleotide strand.     

Direct DNA photocleavage by a new intercalating dirhodium(II/II) complex: comparison to Rh2(mu-O2CCH3)4

Transition metal complexes possessing the intercalating dppz ligand (dppz = dipyrido[3,2-a:2',3'-c]phenazine) typically bind ds-DNA through intercalation (K(b) approximately 10(5)-10(6) M(-1)), and DNA photocleavage by these complexes with visible light proceeds through the generation of a reactive oxygen species. The DNA binding and photocleavage by [Rh(2)(mu-O(2)CCH(3))(2)(eta(1)-O(2)CCH(3))(CH(3)OH)(dppz)](+) (2) is reported and compared to that of Rh(2)(mu-O(2)CCH(3))(4) (1). Spectral changes and an increase in viscosity provide evidence for the intercalation of 2 to double stranded DNA with K(b) = 1.8 x 10(5) M(-1). DNA photocleavage by 2 is observed upon irradiation with lambda(irr) > 395 nm both in air and deoxygenated solution. DNA photocleavage is not observed for 1 or free dppz ligand under these irradiation conditions. The coupling of a single dppz ligand to a dirhodium(II/II) bimetallic core in 2 provides a means to access oxygen-independent DNA photocleavage with visible light.     

Photohydroxylation of 1,4-benzoquinone in aqueous solution revisited

In water, photolysis of 1,4-benzoquinone, Q gives rise to equal amounts of 2-hydroxy-1,4-benzoquinone HOQ and hydroquinone QH(2) which are formed with a quantum yield of Phi=0.42, independent of pH and Q concentration. By contrast, the rate of decay of the triplet (lambda(max)=282 and approximately 410 nm) which is the precursor of these products increases nonlinearly (k=(2-->3.8) x 10(6) s(-1)) with increasing Q concentration ((0.2-->10) mM). The free-radical yield detected by laser flash photolysis after the decay of the triplet also increases with increasing Q concentration but follows a different functional form. These observations are explained by a rapid equilibrium of a monomeric triplet Q* and an exciplex Q(2)* (K=5500+/-1000 M(-1)). While Q* adds water and subsequent enolizes into 1,2,4-trihydroxybenzene Ph(OH)(3), Q(2)* decays by electron transfer and water addition yielding benzosemiquinone (.)QH and (.)OH adduct radicals (.)QOH. The latter enolizes to the 2-hydroxy-1,4-semiquinone radical (.)Q(OH)H within the time scale of the triplet decay and is subsequently rapidly (microsecond time scale) oxidized by Q to HOQ with the concomitant formation of (.)QH. On the post-millisecond time scale, that is, when (.)QH has decayed, Ph(OH)(3) is oxidized by Q yielding HOQ and QH(2) as followed by laser flash photolysis with diode array detection. The rate of this pH- and Q concentration-dependent reaction was independently determined by stopped-flow. This shows that there are two pathways to photohydroxylation; a free-radical pathway at high and a non-radical one at low Q concentration. In agreement with this, the yield of Ph(OH)(3) is most pronounced at low Q concentration. In the presence of phosphate buffer, Q* reacts with H(2)PO(4) (-) giving rise to an adduct which is subsequently oxidized by Q to 2-phosphato-1,4-benzoquinone QP. The current view that (.)OH is an intermediate in the photohydroxylation of Q has been overturned. This view had been based on the observation of the (.)OH adduct of DMPO when Q is photolyzed in the presence of this spin trap. It is now shown that Q*/Q(2)* oxidizes DMPO (k approximately 1 x 10(8) M(-1) s(-1)) to its radical cation which subsequently reacts with water. Q*/Q(2)* react with alcohols by H abstraction (rates in units of M(-1) s(-1)): methanol (4.2 x 10(7)), ethanol (6.7 x 10(7)), 2-propanol (13 x 10(7)) and tertiary butyl alcohol ( approximately 0.2 x 10(7)). DMSO (2.7 x 10(9)) and O(2) ( approximately 2 x 10(9)) act as physical quenchers.     

Synthesis and structure of organic-soluble binuclear molecular phosphonates of tantalum,molybdenum, and tungsten

The reaction of (eta 5-C5Me5)TaMe4 with tert-butylphosphonic acid leads to the formation of a mixture of compounds: [[(eta 5-C5Me5)TaMe][t-BuP(O)(OH)][t-BuP(O)(OH)2]]2(t-BuPO3)2 (1) and [[(eta 5-C5Me5)Ta][t-BuP(O)(OH)2]]2(t-BuPO3)2(mu-O)2 (2). Compound 2 was also obtained by recrystallization of 1 from a THF/hexane mixture. Reaction of (eta 5-C5Me5)MCl4 (M = Mo, W) with PhP(O)(OH)2 yields the binuclear phosphonates [[(eta 5-C5Me5)M][PhP(O)(OH)2]]2(PhPO3)2(mu-O)2 (M = Mo (3); M = W (4)). Compounds 2.THF and 3(.)2.5THF were characterized by single-crystal X-ray studies. The tantalum and molybdenum phosphonates 2.THF and 3(.)2.5THF have different structures as compared to those of the previously reported titanophosphonate cages.     

Quantum yields of the initiation step and chain propagation turnovers in S(RN)1 reactions: photostimulated reaction of 1-iodo-2-methyl-2-phenyl propane with carbanions in DMSO

Neophyl radicals were generated by photoinduced electron transfer (PET) from a suitable donor to the neophyl iodide (1, 1-iodo-2-methyl-2-phenylpropane). The PET reaction of 1 with the enolate anion of cyclohexenone (2) afforded mainly the reduction products tert-butylbenzene (5) and the rearranged isobutylbenzene (6), arising from hydrogen abstraction of the neophyl radical (15) and the rearranged radical 16 intermediates, respectively. The photostimulated reaction of 1 with 2 in the presence of di-tert-butylnitroxide, as a radical trap, afforded adduct 10 in 57% yield. The photoinduced reaction of the enolate anion of acetophenone (3) with 1 gave the substitution products 11 (50%) and 12 (16%), which arise from the coupling of 3 with radicals 15 and 16, respectively. The rate constant obtained for the addition of anion 3 to radical 15 was 1.2 x 10(5) M(-)(1) s(-)(1), by the use of the rearrangement of this radical as a clock reaction. The anion of nitromethane (4) was almost unreactive at the initiation step, but in the presence of 2 under irradiation, it gave high yields (67%) of the substitution product 13 and only 2% of the rearranged product 14. When the ratio of 4 to 1 was diminished, it was possible to observe both substitution products 13 and 14 in 16% and 6.4% yields, respectively. These last results allowed us to estimate the coupling rate constant of neophyl radicals 15 with anion 4 to be at least of the order of 10(6) M(-)(1) s(-)(1). Although the overall quantum yield determined (lambda = 350 nm) for the studied reactions is below 1, the chain lengths (Phi(propagation)) for the reaction of 1 with anions 3 and 4 are 127 and 2, respectively.     

Model studies of DNA C5' radicals. Selective generation and reactivity of 2'-deoxyadenosin-5'-yl radical

The reaction of hydrated electrons (e(aq)(-)) with 8-bromo-2'-deoxyadenosine has been investigated by radiolytic methods coupled with product studies and addressed computationally by means of DFT-B3LYP calculations. Pulse radiolysis revealed that this reaction was complete in approximately 0.3 mus, and, at this time, no significant absorption was detected. The spectrum of a transient developed in 20 mus has an absorbance in the range 300-500 nm (epsilon(max) congruent with 9600 M(-1) cm(-1) at 360 nm), and it was assigned to aromatic aminyl radical 3. Computed vertical transitions (TD-UB3LYP/6-311+G) are in good agreement with the experimental observations. Radical 3 is obtained by the following reaction sequence: one-electron reductive cleavage of the C-Br bond that gives the C8 radical, a fast radical translocation from the C8 to C5' position, and an intramolecular attack of the C5' radical at the C8,N7 double bond of the adenine moiety. The rate constant for the cyclization is 1.6 x 10(5) s(-1). On the basis of the theoretical findings, the cyclization step is highly stereospecific. The rate constants for the reactions of C5' and aminyl 3 radicals with different oxidants were determined by pulse radiolysis methods. The respective rate constants for the reaction of 2'-deoxyadenosin-5'-yl radical with dioxygen, Fe(CN)(6)(3)(-), and MV(2+) in water at ambient temperature are 1.9 x 10(9), 4.2 x 10(9), and 2.2 x 10(8) M(-1) s(-1). The value for the reaction of aminyl radical 3 with Fe(CN)(6)(3-) is 8.3 x 10(8) M(-1) s(-1), whereas the reaction with dioxygen is reversible. Tailored experiments allowed the reaction mechanism to be defined in some detail. A synthetically useful radical cascade process has also been developed that allows in a one-pot procedure the conversion of 8-bromo-2'-deoxyadenosine to 5',8-cyclo-2'-deoxyadenosine in a diastereoisomeric ratio (5'R):(5'S) = 6:1 and in high yield, by reaction with hydrated electrons in the presence of K(4)Fe(CN)(6).     

Absolute Rate Constants for the Reactions of Primary Alkyl Radicals with Aromatic Amines(1)

Hydrogen abstraction from diarylamines (4-X-C(6)H(4))(2)NH [X = H, CH(3), C(8)H(17), CH(3)O, and Br] by the 2-methyl-2-phenylpropyl radical in n-dodecane solution was investigated by thermolysis of 3-methyl-3-phenylbutanoyl peroxide in the presence of various concentrations of the amines. The reaction is a non-chain process in which the 2-methyl-2-phenylpropyl radical and its rearrangement product, the 2-benzylpropan-2-yl radical, abstract hydrogen from both the solvent and the amine. Cross-disproportionation reactions of the rearranged radical led to the formation of significant amounts of beta,beta-dimethylstyrene. Rate constants for hydrogen abstraction by the unrearranged, primary alkyl radical from n-dodecane (k(373K) = 3.5 x 10(3) M(-)(1) s(-)(1)), diphenylamine (k(373K) = 1.3 x 10(6) M(-)(1) s(-)(1)), and the substituted diarylamines were determined from the product yields and the known rate constant for the radical rearrangement. From kinetic experiments with N-deuteriodiphenylamine the deuterium kinetic isotope effect,k(NH)/k(ND), was found to be 2.3 at 373 K.     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Metal vs ligand reduction in complexes of dipyrido[3,2-a:2',3'-c]phenazine and related ligands with [(C5Me5)ClM]+ (M = Rh or Ir): evidence for potential rather than orbital control in the reductive cleavage of the metal-chloride bond

Complexes between the chlorometal(III) cations [(C5Me5)ClM]+, M = Rh or Ir, and the 1,10-phenanthroline-derived alpha-diimine (N--N) ligands dipyrido[3,2-a:2',3'-c]phenazine (dppz), 1,4,7,10-tetraazaphenanthrene (tap), or 1,10-phenanthroline-5,6-dione (pdo) were investigated by cyclic voltammetry, EPR, and UV-vis-NIR spectroelectrochemistry with respect to either ligand-based or metal-centered (and then chloride-dissociative) reduction. Two low-lying unoccupied molecular orbitals (MOs) are present in each of these three N wedge N ligands; however, their different energies and interface properties are responsible for different results. Metal-centered chloride-releasing reduction was observed for complexes of the DNA-intercalation ligands dppz and tap to yield compounds [(N--N)(C5Me5)M] in a two-electron step. The separation of alpha-diimine centered optical orbitals and phenazine-based redox orbitals is apparent from the EPR and UV-vis-NIR spectroelectrochemistry of [(dppz)(C5Me5)M](0/*-/2-). In contrast, the pdo complexes undergo a reversible one-electron reduction to yield o-semiquinone radical complexes [(pdo)(C5Me5)ClM]* before releasing the chloride after the second electron uptake. The fact that the dppz complexes undergo a Cl(-)-dissociative two-electron reduction despite the presence of a lowest lying pi* MO (b1(phz)) with very little overlap to the metal suggests that an unoccupied metal/chloride-based orbital is lower in energy. This assertion is confirmed both by the half-wave reduction potentials of the ligands (tap, -1.95 V; dppz, -1.60 V; pdo, -0.85 V) and by the typical reduction peak potentials of the complexes [(L)(C5Me5)ClM](PF6) (tap, -1.1 V; dppz, -1.3 V; pdo, -0.6 V; all values against Fc(+/0)).     

Synthesis, DNA binding, photo-induced DNA cleavage and cytotoxicity studies of europium(III) complexes

Two Eu(III) complexes, [Eu(acac)(3)(dpq)] (1) and [Eu(acac)(3)(dppz)] CH(3)OH (2) {viz. acetylacetonate (acac), dipyrido[3,2-d:20,30-f]quinoxaline (dpq), dipyrido[3,2-a:20,30-c] phenazine (dppz)}, have been synthesized and their DNA binding, photo-induced DNA cleavage activity and cell cytotoxicity are studied. The complexes display significant binding propensity to the calf thymus DNA in the order: 2(dppz) >1(dpq). Cleavage experiments using pBR322 supercoiled DNA suggest major groove binding for 2 and minor groove binding for 1. The mechanistic aspects on natural light (natural light in room during the day) and UV-A (365 nm) irradiation are via a mechanistic pathway involving formation of singlet oxygen and hydroxyl radical as the reactive species. The photo-induced DNA cleavage activity of 2 is also stronger than 1. The cytotoxicity of 1 and 2 against HeLa (cervical) cancer cells show that the IC(50) value of 19.11 ± 3.56 μM and 17.95 ± 5.47 μM, respectively.     

Bis(dipyridophenazine)copper(II) complex as major groove directing synthetic hydrolase

The copper(II) complex [Cu(dppz)(2)Cl]Cl () has been prepared, structurally characterized and its DNA binding and cleavage properties studied (dppz, dipyridophenazine). Crystal structure of 1xdppzxH(2)O shows the presence of the monocationic copper(II) complex containing two dppz ligands and one chloride in the five coordinate structure. While one bidentate chelating dppz ligand occupies the basal plane, the other dppz ligand shows an axial/equatorial mode of bonding. The chloride ligand binds at the basal plane. The complex crystallizes with dppz and water as lattice molecules. The dppz moieties in the metal-bound and free forms are involved in pi-pi stacking interactions. The one-electron paramagnetic complex shows a visible spectral d-d band at 707 nm in DMF and displays quasireversible cyclic voltammetric response for the Cu(II)/Cu(I) couple near 0.1 V vs. SCE in DMF-0.1 M TBAP. The complex which is an avid binder to calf thymus DNA giving a binding constant (K(b)) value of 2.0 x 10(4) M(-1) in DMF-Tris buffer, cleaves supercoiled pUC19 DNA in an oxidative manner in the presence of mercaptopropionic acid (MPA) as a reducing agent or on photo irradiation at 312 nm. Control experiments show major groove binding and DNA cleavage via the formation of hydroxyl radical in the presence of MPA and by singlet oxygen in the photocleavage reaction. The complex exhibits significant hydrolytic cleavage of DNA in the dark in the absence of any additives at a rate of approximately 3.0 h(-1). The hydrolytic nature of the DNA cleavage is evidenced from the T4 ligase experiments converting the nicked circular form to its original supercoiled form quantitatively. Complex presents a rare example of copper-based major groove directing efficient synthetic hydrolase.     

Photooxidation of guanine by a ruthenium dipyridophenazine complex intercalated in a double-stranded polynucleotide monitored directly by picosecond visible and infrared transient absorption spectroscopy

Transient species formed by photoexcitation (400 nm) of [Ru(dppz)(tap)2]2+ (1) (dppz = dipyrido[3,2-a:2',3'-c]phenazine; tap=1,4,5,8-tetraazaphenanthrene) in aqueous solution and when intercalated into a double-stranded synthetic polynucleotide, [poly(dG-dC)]2, have been observed on a picosecond timescale by both visible transient absorption (allowing monitoring of the metal complex intermediates) and transient infrared (IR) absorption spectroscopy (allowing direct study of the DNA nucleobases). By contrast with its behavior when free in aqueous solution, excitation of 1 when bound to [poly(dG-dC)]2 causes a strong increase in absorbance at 515 nm due to formation of the reduced complex [Ru(dppz)(tap)2]+ (rate constant=(2.0+/-0.2) x 10(9) s(-1)). The subsequent reformation of 1 proceeds with a rate constant of (1.1+/-0.2) x 10(8) s(-1). When the process is carried out in D2O, the rates of formation and removal of [Ru(dppz)(tap)2]+ are reduced (rate constants (1.5+/-0.3) x 10(9) and (0.7+/-0.2) x 10(8) s(-1) respectively) consistent with proton-coupled electron transfer processes. Picosecond transient IR measurements in the 1540-1720 cm(-1) region in D2O solution confirm that the reduction of 1 intercalated into [poly(dG-dC)]2 is accompanied by bleaching of IR ground-state bands of guanine (1690 cm(-1)) and cytosine (1656 cm(-1)), each with similar rate constants.     

Equilibrium and Kinetics of Bromine Hydrolysis

Equilibrium constants for bromine hydrolysis, K(1) = [HOBr][H(+)][Br(-)]/[Br(2)(aq)], are determined as a function of ionic strength (&mgr;) at 25.0 degrees C and as a function of temperature at &mgr; approximately 0 M. At &mgr; approximately 0 M and 25.0 degrees C, K(1) = (3.5 +/- 0.1) x 10(-)(9) M(2) and DeltaH degrees = 62 +/- 1 kJ mol(-)(1). At &mgr; = 0.50 M and 25.0 degrees C, K(1) = (6.1 +/- 0.1) x 10(-)(9) M(2) and the rate constant (k(-)(1)) for the reverse reaction of HOBr + H(+) + Br(-) equals (1.6 +/- 0.2) x 10(10) M(-)(2) s(-)(1). This reaction is general-acid-assisted with a Br?nsted alpha value of 0.2. The corresponding Br(2)(aq) hydrolysis rate constant, k(1), equals 97 s(-)(1), and the reaction is general-base-assisted (beta = 0.8).     

New insight into solvent effects on the formal HOO. + HOO. reaction

The 2,2'-azobis(isobutyronitrile)(AIBN)-induced autoxidation of gamma-terpinene (TH) at 50 degrees C produces p-cymene and hydrogen peroxide in a radical-chain reaction having HOO* as one of the chain-carrying radicals. The kinetics of this reaction in cyclohexane and tert-butyl alcohol show that chain termination involves the formal HOO. + HOO. self-reaction over a wide range of gamma-terpinene, AIBN, and O2 concentrations. However, in acetonitrile this termination process is accompanied by termination via the cross-reaction of the terpinenyl radical, T., with the HOO. radical under conditions of relatively high [TH] (140-1000 mM) and low [O2] (2.0-5.5 mM). This is because the formal HOO. + HOO. reaction is comparatively slow in acetonitrile (2k approximately 8 x 10(7) M(-1) s(-1)), whereas, this reaction is almost diffusion-controlled in tert-butyl alcohol and cyclohexane, 2k approximately 6.5 x 10(8) and 1.3 x 10(9) M(-1) s(-1), respectively. Three mechanisms for the bimolecular self-reaction of HOO. radicals are considered: 1) a head-to-tail hydrogen-atom transfer from one radical to the other, 2) a head-to-head reaction to form an intermediate tetroxide, and 3) an electron-transfer between HOO. and its conjugate base, the superoxide radical anion, O2-.. The rate constant for reaction by mechanism (1) is shown to be dependent on the hydrogen bond (HB) accepting ability of the solvent; that by mechanism (2) is shown to be too slow for this process to be of any importance; and that by mechanism (3) is dependent on the pH of the solvent and its ability to support ionization. Mechanism (3) was found to be the main termination process in tert-butyl alcohol and acetonitrile. In the gas phase, the rate constant for the HOO. + HOO. reaction (mechanism (1)) is about 1.8 x 10(9) M(-1) s(-1) but in water at pH< or =2 where the ionization of HOO. is completely suppressed, this rate constant is only 8.6 x 10(5) M(-1) s(-1). The very large retarding effect of water on this reaction has not previously been explained. We find that it can be quantitatively accounted for by using Abraham's HB acceptor parameter, beta(2)(H), for water of 0.38 and an estimated HB donor parameter, alpha(2)(H), for HOO. of about 0.87. These Abraham parameters allow us to predict a rate constant for the HOO. + HOO. reaction in water at 25 degrees C of 1.2 x 10(6) M(-1) s(-1) in excellent agreement with experiment.     

From Monomers to π Stacks, from Nonconductive to Conductive: Syntheses, Characterization, and Crystal Structures of Benzidine Radical Cations

Salts that contain radical cations of benzidine (BZ), 3,3',5,5'-tetramethylbenzidine (TMB), 2,2',6,6'-tetraisopropylbenzidine (TPB), and 4,4'-terphenyldiamine (DATP) have been isolated with weakly coordinating anions [Al(OR(F) )(4) ](-) (OR(F) =OC(CF(3) )(3) ) or SbF(6) (-) . They were prepared by reaction of the respective silver(I) salts with stoichiometric amounts of benzidine or its alkyl-substituted derivatives in CH(2) Cl(2) . The salts were characterized by UV absorption and EPR spectroscopy as well as by their single-crystal X-ray structures. Variable-temperature UV/Vis absorption spectra of BZ(.) (+) [Al(OR(F) )(4) ](-) and TMB(.) (+) [Al(OR(F) )(4) ](-) in acetonitrile indicate an equilibrium between monomeric free radical cations and a radical-cation dimer. In contrast, the absorption spectrum of TPB(.) (+) SbF(6) (-) in acetonitrile indicates that the oxidation of TPB only resulted in a monomeric radical cation. Single-crystal X-ray diffraction studies show that in the solid state BZ and its methylation derivative (TMB) form radical-cation π dimers upon oxidation, whereas that modified with isopropyl groups (TPB) becomes a monomeric free radical cation. By increasing the chain length, π stacks of π dimers are obtained for the radical cation of DATP. The single-crystal conductivity measurements show that monomerized or π-dimerized radicals (BZ(.) (+) , TMB(.) (+) , and TPB(.) (+) ) are nonconductive, whereas the π-stacked radical (DATP(.) (+) ) is conductive. A conduction mechanism between chains through π stacks is proposed.     

Determination of the rate constants for the NH2(X2B1) + NH2(X2B1) and NH2(X2B1) + H Recombination reactions with collision partners CH4, C2H6, CO2, CF4, and SF6 at low pressures and 296 K. Part 2

The recombination rate constants for the reactions NH2(X2B1) + NH2(X2B1) + M → N2H4 + M and NH2(X2B1) + H + M → NH3 + M, where M was CH4, C2H6, CO2, CF4, or SF6, were measured in the same experiment over presseure ranges of 1-20 and 7-20 Torr, respectively, at 296 ± 2 K. The NH2 radical was produced by the 193 nm laser photolysis of NH3. Both NH2 and NH3 were monitored simultaneously following the photolysis laser pulse. High-resolution time-resolved absorption spectroscopy was used to monitor the temporal dependence of both species: NH2 on the (1)2(21) ← (1)3(31) rotational transition of the (0,7,0)A2A1 ← (0,0,0)X2B1 electronic transition near 675 nm and NH3 in the IR on either of the inversion doublets of the qQ3(3) rotational transition of the ν1 fundamental near 2999 nm. The NH2 self-recombination clearly exhibited falloff behavior for the third-body collision partners used in this work. The pressure dependences of the NH2 self-recombination rate constants were fit using Troe’s parametrization scheme, k(inf), k(0), and F(cent), with k(inf) = 7.9 × 10(-11) cm3 molecule(-1) s(-1), the theoretical value calculated by Klippenstein et al. (J. Phys. Chem. A113, 113, 10241). The individual Troe parameters were CH4, k(0)(CH4) = 9.4 × 10(-29) and F(cent)(CH4) = 0.61; C2H6, k(0)(C2H6) = 1.5 × 10(-28) and F(cent)(C2H6) = 0.80; CO2, k(0)(CO2) = 8.6 × 10(-29) and F(cent)(CO2) = 0.66; CF4, k(0)(CF4) = 1.1 × 10(-28) and F(cent)(CF4) = 0.55; and SF6, k(0)(SF6) = 1.9 × 10(-28) and F(cent)(SF6) = 0.52, where the units of k0 are cm6 molecule(-2) s(-1). The NH2 + H + M reaction rate constant was assumed to be in the three-body pressure regime, and the association rate constants were CH4, (6.0 ± 1.8) × 10(-30); C2H6, (1.1 ± 0.41) × 10(-29); CO2, (6.5 ± 1.8) × 10(-30); CF4, (8.3 ± 1.7) × 10(-30); and SF6, (1.4 ± 0.30) × 10(-29), with units cm6 molecule(-1) s,(-1) and the systematic and experimental errors are given at the 2σ confidence level.     

Activation of the sulfhydryl group by Mo centers: kinetics of reaction of benzyl radical with a binuclear Mo(micro-SH)Mo complex and with arene and alkane thiols

This paper provides evidence from kinetic experiments and electronic structure calculations of a significantly reduced S-H bond strength in the Mo(micro-SH)Mo function in the homogeneous catalyst model, CpMo(micro-S)(2)(micro-SH)(2)MoCp (1, Cp = eta(5)-cyclopentadienyl). The reactivity of 1 was explored by determination of a rate expression for hydrogen atom abstraction by benzyl radical from 1 (log(k(abs)/M(-)(1) s(-)(1)) = (9.07 +/- 0.38) - (3.62 +/- 0.58)/theta) for comparison with expressions for CH(3)(CH(2))(7)SH, log(k(abs)/M(-)(1) s(-)(1)) = (7.88 +/- 0.35) - (4.64 +/- 0.54)/theta, and for 2-mercaptonaphthalene, log(k(abs)/M(-)(1) s(-)(1)) = (8.21 +/- 0.17) - (4.24 +/- 0.26)/theta (theta = 2.303RT kcal/mol, 2sigma error). The rate constant for hydrogen atom abstraction at 298 K by benzyl radical from 1 is 2 orders of magnitude greater than that from 1-octanethiol, resulting from the predicted (DFT) S-H bond strength of 1 of 73 kcal/mol. The radical CpMo(micro-S)(3)(micro-SH)MoCp, 2, is revealed, from the properties of slow self-reaction, and exclusive cross-combination with reactive benzyl radical, to be a persistent free radical.     

Synthesis, structure, spectroscopic properties, and electrochemical oxidation of ruthenium(II) complexes Incorporating monocarboxylate bipyridine ligands

[Ru(bpy)(2)(Mebpy-COOH)](PF(6))(2).3H(2)O (1), [Ru(phen)(2)(Mebpy-COOH)](ClO(4))(2).5H(2)O (2), [Ru(dppz)(2)(Mebpy-COOH)]Cl(2).9H(2)O (3), and [Ru(bpy)(dppz)(Mebpy-COOH)](PF(6))(2).5H(2)O (4) (bpy = 2,2'-bipyridine, Mebpy-COOH = 4'-methyl-2,2'-bipyridine-4-carboxylic acid, phen = 1,10-phenanthroline, dppz = dipyrido[3,2,-a;2',3-c]phenazine) have been synthesized and characterized spectroscopically and by microanalysis. The [Ru(Mebpy-COOH)(CO)(2)Cl(2)].H(2)O intermediate was prepared by reaction of the monocarboxylic acid ligand, Mebpy-COOH, with [Ru(CO)(2)Cl(2)](n), and the product was then reacted with either bpy, phen, or dppz in the presence of an excess of trimethylamine-N-oxide (Me(3)NO), as the decarbonylation agent, to generate 1, 2, and 3, respectively. For compound 4, [Ru(bpy)(CO)Cl(2)](2) was reacted with Mebpy-COOH to yield [Ru(bpy)(Mebpy-COOH)(CO)Cl](PF(6)).H(2)O as a mixture of two main geometric isomers. Chemical decarbonylation in the presence of dppz gave 4 also as a mixture of two isomers. Electrochemical and spectrophotometric studies indicated that complexes 1 and 2 were present as a mixture of protonated and deprotonated forms in acetonitrile solution because of water of solvation in the isolated solid products. The X-ray crystal structure determination on crystals of [Ru(bpy)2(MebpyCOO)][Ru(bpy)(2)(MebpyCOOH)](3)(PF(6))(7), 1a, and [Ru(phen)(2)(MebpyCOO)](ClO(4)).6H(2)O, 2a, obtained from solutions of 1 and 2, respectively, revealed that 1a consisted of a mixture of protonated and deprotonated forms of the complex in a 1:3 ratio and that 2a consisted of the deprotonated derivative of 2. A distorted octahedral geometry for the Ru(II) centers was found for both complexes. Upon excitation at 450 nm, MeCN solutions of the protonated complexes 1-4 were found to exhibit emission bands in the 635-655 nm range, whereas the corresponding emission maxima of their deprotonated forms were observed at lower wavelengths. Protonation/deprotonation effects were also observed in the luminescence and electrochemical behavior of complexes 1-4. Comprehensive electrochemical studies in acetonitrile show that the ruthenium centers on 1, 2, 3, and 4 are oxidized from Ru(II) to Ru(III) with reversible potentials at 917, 929, 1052, and 1005 mV vs Fc(0/+) (Fc = ferrocene), respectively. Complexes 1 and 2 also exhibit an irreversible oxidation process in acetonitrile, and all compounds undergo ligand-based reduction processes.