Optosensing behavior of the first Ru(II)-compound of di-2-pyridylketone-p-nitrophenylhydrazone (dpknph), [Ru(bpy)2(dpknph)]Cl2

The reaction between Ru(bpy)(2)Cl(2) (bpy=2,2'-bipyridine) and di-2-pyridylketone-p-nitrophenylhydrazone (dpknph) in refluxing ethanol gave [Ru(bpy)(2)(dpknph)]Cl(2) in good yield. Optical measurements on [Ru(bpy)(2)(dpknph)]Cl(2) in non-aqueous media revealed the presence of two interlocked electronic states due to conformational changes associated with the hydrazone moiety of [Ru(bpy)(2)(dpknph)]Cl(2). The equilibrium distribution of the high-energy beta-conformation associated with the high-energy electronic state and the low-energy alpha-conformation associated with the low-energy electronic state is solvent and solute dependent controlled by the solvent-solute and solute-solute interactions. The interplay between the alpha- and beta-conformations of [Ru(bpy)(2)(dpknph)]Cl(2) allowed calculations of the extinction coefficients of electronic states by forcing the equilibrium to shift to one conformation using chemical stimuli. Extinction coefficients of 56000+/-2000 and 48500+/-2000 M(-1) cm(-1) were calculated in DMSO for the beta- and alpha-conformations of [Ru(bpy)(2)(dpknph)]Cl(2), respectively, using excess HgCl(2) in DMSO. Thermo-optical measurements on [Ru(bpy)(2)(dpknph)]Cl(2) in DMSO confirmed the interconversion between the alpha- and beta-conformations of [Ru(bpy)(2)(dpknph)]Cl(2) and gave changes in enthalpy (DeltaH(?)) of -35.5+/-4.0 and 13.0+/-0.5 kJ mol(-1), entropy (DeltaS(?)) of -126.9+/-20 and 45.2+/-4.5 kJ mol(-1), and free energy (DeltaG(?)) of 2.31+/-0.2 and -0.48+/-0.2 kJ mol(-1) in the absence and presence of NaBH(4) at 295 K. The high values for the extinction coefficients and low values and sensitivity of the activation parameters for the interconversion between the alpha- and beta-conformations of [Ru(bpy)(2)(dpknph)]Cl(2) in DMSO to solution composition allowed for the use of this system ([Ru(bpy)(2)(dpknph)]Cl(2) and surrounding solvent or solute molecules) as a spectrophotometric sensor for a variety of chemical stimuli that include metal ions. Group 12 metal ions in concentrations as low as 1.00x10(-8) M can be detected and determined using [Ru(bpy)(2)(dpknph)]Cl(2) in DMSO in the presence and absence of NaBH(4).     

Rates and mechanism of fluoride and water exchange in UO(2)F(5)(3-) and [UO(2)F(4)(H(2)O)](2-) studied by NMR spectroscopy and wave function based methods

The reaction mechanism for the exchange of fluoride in UO(2)F(5)(3-) and UO(2)F(4)(H(2)O)(2-) has been investigated experimentally using (19)F NMR spectroscopy at -5 degrees C, by studying the line broadening of the free fluoride, UO(2)F(4)(2-)(aq) and UO(2)F(5)(3-), and theoretically using quantum chemical methods to calculate the activation energy for different pathways. The new experimental data allowed us to make a more detailed study of chemical equilibria and exchange mechanisms than in previous studies. From the integrals of the different individual peaks in the new NMR spectra, we obtained the stepwise stability constant K(5) = 0.60 +/- 0.05 M(-1) for UO(2)F(5)(3-). The theoretical results indicate that the fluoride exchange pathway of lowest activation energy, 71 kJ/mol, in UO(2)F(5)(3-) is water assisted. The pure dissociative pathway has an activation energy of 75 kJ/mol, while the associative mechanism can be excluded as there is no stable UO(2)F(6)(4-) intermediate. The quantum chemical calculations have been made at the SCF/MP2 levels, using a conductor-like polarizable continuum model (CPCM) to describe the solvent. The effects of different model assumptions on the activation energy have been studied. The activation energy is not strongly dependent on the cavity size or on interactions between the complex and Na(+) counterions. However, the solvation of the complex and the leaving fluoride results in substantial changes in the activation energy. The mechanism for water exchange in UO(2)F(4)(H(2)O)(2-) has also been studied. We could eliminate the associative mechanism, the dissociative mechanism had the lowest activation energy, 39 kJ/mol, while the interchange mechanism has an activation energy that is approximately 50 kJ/mol higher.     

Structural characterization and reactivity of UO2(salophen)L and [UO2(salophen)]2: dimerization of UO2(salophen) fragments in noncoordinating solvents (salophen = N,N'-disalicylidene-o-phenylenediaminate, L = N,N-dimethylformamide, dimethyl sulfoxide)

The molecular structures of UO2(salophen)L (L = DMF, DMSO) and a uranyl-salophen complex without any unidentate ligands (L) in solid and solution were investigated using single-crystal X-ray analysis and IR, 1H NMR, and UV-visible absorption spectroscopies. As a result, it was found that the uranyl-salophen complex without L is a racemic dimeric complex, [UO2(salophen)]2, in which the UO2(salophen) fragments are held together by bridging between one of the phenoxide oxygen atoms in salophen and the uranium in the other UO2(salophen) unit. Furthermore, it was spectrophotometrically demonstrated that [UO2(salophen)]2 retains its dimeric structure even in the noncoordinating solvents such as CH2Cl2 and CHCl3 and is in equilibrium with UO2(salophen)L {2UO2(salophen)L right arrow over left arrow [UO2(salophen)]2 + 2L}. The equilibrium constants and thermodynamic parameters of this equilibrium were evaluated from UV-visible absorption and 1H NMR spectral changes; log Kdim = -2.51 +/- 0.01 for L = DMF and solvent = CH2Cl2, log Kdim = -1.68 +/- 0.02 for L = DMF and solvent = CHCl3, log Kdim = -4.23 +/- 0.01 for L = DMSO and solvent = CH2Cl2, and log Kdim = -3.03 +/- 0.02 for L = DMSO and solvent = CHCl3. The kinetics of L-exchange reactions in UO2(salophen)L and enantiomer exchange of [UO2(salophen)]2 in noncoordinating solvents were also studied using NMR line-broadening method. As a consequence, it was suggested that the DMF-exchange reaction in UO2(salophen)DMF proceeds through two pathways (dissociative and associative paths) and that the predominant path of DMSO exchange in UO2(salophen)DMSO is the dissociative one. A sliding motion of the UO2(salophen) fragments was considered to be reasonable for the enantiomer-exchange mechanism of [UO2(salophen)]2. On the basis of the kinetic information for UO2(salophen)L and [UO2(salophen)]2, reaction mechanisms including the L-exchange reaction in UO2(salophen)L, the formation of [UO2(salophen)]2 from UO2(salophen)L, and the enantiomer exchange of [UO2(salophen)]2 are proposed.     

Stoichiometries and thermodynamic stabilities for aqueous sulfate complexes of U(VI)

The formation constants of UO2SO4 (aq), UO2(SO4)2(2-), and UO2(SO4)3(4-) were measured in aqueous solutions from 10 to 75 degrees C by time-resolved laser-induced fluorescence spectroscopy (TRLFS). A constant enthalpy of reaction approach was satisfactorily used to fit the thermodynamic parameters of stepwise complex formation reactions in a 0.1 M Na(+) ionic medium: log 10 K 1(25 degrees C) = 2.45 +/- 0.05, Delta r H1 = 29.1 +/- 4.0 kJ x mol(-1), log10 K2(25 degrees C) = 1.03 +/- 0.04, and Delta r H2 = 16.6 +/- 4.5 kJ x mol(-1). While the enthalpy of the UO2(SO4)2(2-) formation reaction is in good agreement with calorimetric data, that for UO2SO4 (aq) is higher than other values by a few kilojoules per mole. Incomplete knowledge of the speciation may have led to an underestimation of Delta r H1 in previous calorimetric studies. In fact, one of the published calorimetric determinations of Delta r H1 is here supported by the TRLFS results only when reinterpreted with a more correct equilibrium constant value, which shifts the fitted Delta r H1 value up by 9 kJ x mol(-1). UO2(SO 4) 3 (4-) was evidenced in a 3 M Na (+) ionic medium: log10 K3(25 degrees C) = 0.76 +/- 0.20 and Delta r H3 = 11 +/- 8 kJ x mol(-1) were obtained. The fluorescence features of the sulfate complexes were observed to depend on the ionic conditions. Changes in the coordination mode (mono- and bidentate) of the sulfate ligands may explain these observations, in line with recent structural data.     

Structure and bonding in solution of dioxouranium(VI) oxalate complexes: isomers and intramolecular ligand exchange

Structural isomers of [UO(2)(oxalate)(3)](4-), [UO(2)(oxalate)F(3)](3-), [UO(2)(oxalate)(2)F](3-), and [UO(2)(oxalate)(2)(H(2)O)](2-) have been studied by using EXAFS and quantum chemical ab initio methods. Theoretical structures and their relative energies were determined in the gas phase and in water using the CPCM model. The most stable isomers according to the quantum chemical calculations have geometries consistent with the EXAFS data, and the difference between measured and calculated bond distances is generally less than 0.05 A. The complex [UO(2)(oxalate)(3)](4-) contains two oxalate ligands forming five-membered chelate rings, while the third is bonded end-on to a single carboxylate oxygen. The most stable isomer of the other two complexes also contains the same type of chelate-bonded oxalate ligands. The activation energy for ring opening in [UO(2)(oxalate)F(3)](3-), deltaU++ = 63 kJ/mol, is in fair agreement with the experimental activation enthalpy, deltaH++ = 45 +/- 5 kJ/mol, for different [UO(2)(picolinate)F(3)](2-) complexes, indicating similar ring-opening mechanisms. No direct experimental information is available on intramolecular exchange in [UO(3)(oxalate)(3)](4-). The theoretical results indicate that it takes place via the tris-chelated intermediate with an activation energy of deltaU++ = 38 kJ/mol; the other pathways involve multiple steps and have much higher activation energies. The geometries and energies of dioxouranium(VI) complexes in the gas phase and solvent models differ slightly, with differences in bond distance and energy of typically less than 0.06 A and 10 kJ/mol, respectively. However, there might be a significant difference in the distance between uranium and the leaving/entering group in the transition state, resulting in a systematic error when the gas-phase geometry is used to estimate the activation energy in solution. This systematic error is about 10 kJ/mol and tends to cancel when comparing different pathways.     

Rates and mechanisms of water exchange of UO2(2+)(aq) and UO2(oxalate)F(H2O)2-: a variable-temperature 17O and 19F NMR study

This study consists of two parts: The first part comprised an experimental determination of the kinetic parameters for the exchange of water between UO2(H2O)5(2+) and bulk water, including an ab initio study at the SCF and MP2 levels of the geometry of UO2(H2O)5(2+), UO2(H2O)4(2+), and UO2(H2O)6(2+) and the thermodynamics of their reactions with water. In the second part we made an experimental study of the rate of water exchange in uranyl complexes and investigated how this might depend on inter- and intramolecular hydrogen bond interactions. The experimental studies, made by using 17O NMR, with Tb3+ as a chemical shift reagent, gave the following kinetic parameters at 25 degrees C: kex = (1.30 +/- 0.05) x 10(6) s(-1); deltaH(not equal to) = 26.1 +/- 1.4 kJ/mol; deltaS(not equal to) = -40 +/- 5J J/(K mol). Additional mechanistic indicators were obtained from the known coordination geometry of U(VI) complexes with unidentate ligands and from the theoretical calculations. A survey of the literature shows that there are no known isolated complexes of UO2(2+) with unidentate ligands which have a coordination number larger than 5. This was corroborated by quantum chemical calculations which showed that the energy gains by binding an additional water to UO2(H2O)4(2+) and UO2(H2O)5(2+) are 29.8 and -2.4 kcal/mol, respectively. A comparison of the change in deltaU for the reactions UO2(H2O)5(2+)--> UO2(H2O)4(2+) + H2O and UO2(H2O)5(2+) + H2O --> UO2(H2O)6(2+) indicates that the thermodynamics favors the second (associative) reaction in gas phase at 0 K, while the thermodynamics of water transfer between the first and second coordination spheres, UO2(H2O)5(2+) --> UO2(H2O)4(H2O)2+ and UO2(H2O)5(H2O)2+ --> UO2(H2O)6(2+), favors the first (dissociative) reaction. The energy difference between the associative and dissociative reactions is small, and solvation has to be included in ab initio models in order to allow quantitative comparisons between experimental data and theory. Theoretical calculations of the activation energy were not possible because of the excessive computing time required. On the basis of theoretical and experimental studies, we suggest that the water exchange in UO2(H2O)5(2+) follows a dissociative interchange mechanism. The rates of exchange of water in UO2(oxalate)F(H2O)2- (and UO2(oxalate)F2(H2O)2- studied previously) are much slower than in the aqua ion, kex = 1.6 x 10(4) s(-1), an effect which we assign to hydrogen bonding involving coordinated water and fluoride. The kinetic parameters for the exchange of water in UO2(H2O)52+ and quenching of photo excited *UO2(H2O)5(2+) are very near the same, indicating similar mechanisms.     

Reactivity of the "yl"-bond in uranyl(VI) complexes. 1. Rates and mechanisms for the exchange between the trans-dioxo oxygen atoms in (UO2)2(OH)2 2+ and mononuclear UO2(OH)n 2-n complexes with solvent water

The stoichiometric mechanism, rate constant, and activation parameters for the exchange of the "yl"-oxygen atoms in the dioxo uranium(VI) ion with solvent water have been studied using 17O NMR spectroscopy. The experimental rate equation, (-->)v= k(2obs)[UO2(2+)]tot2/[H+]2, is consistent with a mechanism where the first step is a rapid equilibrium 2U(17)O2(2+) + 2H2O<==>(U(17)O2)2(OH)2(2+) + 2H+, followed by the rate-determining step (U(17)O2)2(OH)2(2+) + H2O<==>(UO2)2*(OH)2(2+) + H2(17)O, where the back reaction can be neglected because the (17)O enrichment in the water is much lower than in the uranyl ion. This mechanism results in the following rate equation (-->)v= d[(UO2)2(OH)2(2+)]/dt = k(2,2)[(UO2)2(OH)2(2+)] = k(2,2*)beta(2,2)[UO2(2+)]2/[H + ]2; with k(2,2) = (1.88 +/- 0.22) x 10(4) h(-1), corresponding to a half-life of 0.13 s, and the activation parameters DeltaH++ = 119 +/- 13 kJ mol-1 and DeltaS++ = 81 +/- 44 J mol(-1) K(-1). *Beta(2,)2 is the equilibrium constant for the reaction 2UO2(2+) + 2H2O<==>(UO2)2(OH)2(2+) + 2H+. The experimental data show that there is no measurable exchange of the "yl"-oxygen in UO2(2+), UO2(OH)+, and UO2(OH)4(2-)/ UO2(OH)5(3-), indicating that "yl"-exchange only takes place in polynuclear hydroxide complexes. There is no "yl"-exchange in the ternary complex (UO2)2(mu-OH)2(F)2(oxalate)2(4-), indicating that it is also necessary to have coordinated water in the first coordination sphere of the binuclear complex, for exchange to take place. The very large increase in lability of the "yl"-bonds in (UO2)2(OH)2(2+) as compared to those of the other species is presumably a result of proton transfer from coordinated water to the "yl"-oxygen, followed by a rapid exchange of the resulting OH group with the water solvent. "Yl"-exchange through photochemical mediation is well-known for the uranyl(VI) aquo ion. We noted that there was no photochemical exchange in UO2(CO3)3(4-), whereas there was a slow exchange or photo reduction in the UO2(OH)4(2-) / UO2(OH)5(3-) system that eventually led to the appearance of a black precipitate, presumably UO2.     

Mechanism of CO exchange on cis-[M(CO)2X2]- complexes (M=Rh,Ir;X=Cl, Br, or I)

The CO exchange on cis-[M(CO)2X2]- with M = Ir (X = Cl, la; X = Br, 1b; X = I, 1c) and M = Rh (X = Cl, 2a; X = Br, 2b; X = I, 2c) was studied in dichloromethane. The exchange reaction [cis-[M(CO)2X2]- + 2*CO is in equilibrium cis-[M(*CO)2X2]- + 2CO (exchange rate constant: kobs)] was followed as a function of temperature and carbon monoxide concentration (up to 6 MPa) using homemade high gas pressure NMR sapphire tubes. The reaction is first order for both CO and cis-[M(CO)2X2]- concentrations. The second-order rate constant, k2(298) (=kobs)[CO]), the enthalpy, deltaH*, and the entropy of activation, deltaS*, obtained for the six complexes are respectively as follows: la, (1.08 +/- 0.01) x 10(3) L mol(-1) s(-1), 15.37 +/- 0.3 kJ mol(-1), -135.3 +/- 1 J mol(-1) K(-1); 1b, (12.7 +/- 0.2) x 10(3) L mol(-1) s(-1), 13.26 +/- 0.5 kJ mol(-1), -121.9 +/- 2 J mol(-1) K(-1); 1c, (98.9 +/- 1.4) x 10(3) L mol(-1) s(-1), 12.50 +/- 0.6 kJ mol(-1), -107.4 +/- 2 J mol(-1) K(-1); 2a, (1.62 +/- 0.02) x 10(3) L mol(-1) s(-1), 17.47 +/- 0.4 kJ mol(-1), -124.9 +/- 1 J mol(-1) K(-1); 2b, (24.8 +/- 0.2) x 10(3) L mol(-1) s(-1), 11.35 +/- 0.4 kJ mol(-1), -122.7 +/- 1 J mol(-1) K(-1); 2c, (850 +/- 120) x 10(3) L mol(-1), s(-1), 9.87 +/- 0.8 kJ mol(-1), -98.3 +/- 4 J mol(-1) K(-1). For complexes la and 2a, the volumes of activation were measured and are -20.9 +/- 1.2 cm3 mol(-1) (332.0 K) and -17.2 +/- 1.0 cm3 mol(-1) (330.8 K), respectively. The second-order kinetics and the large negative values of the entropies and volumes of activation point to a limiting associative, A, exchange mechanism. The reactivity of CO exchange follows the increasing trans effect of the halogens (Cl < Br < I), and this is observed on both metal centers. For the same halogen, the rhodium complex is more reactive than the iridium complex. This reactivity difference between rhodium and iridium is less marked for chloride (1.5: 1) than for iodide (8.6:1) at 298 K.     

Platinum(II) and palladium(II) complexes of bisphosphine ligands bearing o-N,N-dimethylanilinyl substituents: a hint of catalytic olefin hydration

Platinum(II) and palladium(II) complexes of the potentially hexadentate P,N-donor ligand family Ar2P-X-PAr2 (X = (CH2)2 [dmape], cyclic-C5H8 [dmapcp]; Ar = o-N,N-dimethylanilinyl) are described. In CH2Cl2, the dmape complexes exist as equilibrium mixtures of MCl2(P,P'-dmape) and [MCl(P,P',N-dmape)]Cl isomers (M = Pd, Pt), governed by deltaH(o) = -19 +/- 4 kJ mol(-1) and deltaS(o) = -100 +/- 30 J mol(-1) K(-1) for M = Pt, and deltaH(o) = -11 +/- 7 kJ mol(-1) and deltaS(o) = -60 +/- 20 J mol(-1) K(-1) for M = Pd. The water-soluble dmapcp complexes exist solely in the [MCl(P,P',N-dmapcp)]Cl form, but the free and coordinated anilinyl rings in these complexes are in slow diastereoselective exchange. X-ray crystal structures for MCl2(P,P'-dmape) (M = Pd, Pt), and the [PdCl(P,P',N-dmape)]+ and [PtCl(P,P',N-dmapcp)]+ cations, are presented. Some of the complexes show marginal activity in water for the catalyzed hydration of maleic to malic acid, giving about 6-7% conversion in 24 h at 100 degrees C and substrate:catalyst loadings of 100:1. Attempts to synthesize a PdCl(P,P',N-dmapm)+ species led instead to isolation of [Pd(mu-Cl)(P,P'-dmapm)]2[PF6]2 (dmapm = Ar2PCH2Ar2).     

Oxidations of hydrocarbons by manganese(III) tris(hexafluoroacetylacetonate)

Mn(hfacac)(3) is an easily prepared and reactive oxidant (hfacac = hexafluoroacetylacetonate). It forms stable solutions in benzene and methylene chloride but is rapidly reduced in acetonitrile, DMSO, acetone, and ethers. It is reduced by ferrocene to give the Mn(II) complex [Cp(2)Fe][Mn(hfacac)(3)], which has been structurally characterized. Mn(hfacac)(3) also rapidly oxidizes 1-acetylferrocene, 1,1'-diacetylferrocene, and tris(4-bromophenyl)amine. Based on an equilibrium established with tris(2,4-dibromophenyl)amine, a redox potential of 0.9 +/- 0.1 V vs Cp(2)Fe(+/0) is calculated. Mn(hfacac)(3) oxidizes 9,10-dihydroanthracene (DHA) cleanly to anthracene, with a bimolecular rate constant of 6.8 x 10(-4) M(-1) s(-1) at 25 degrees C in benzene solution. In the presence of small amounts of water, the manganese(II) product is isolated as cis-Mn(hfacac)(2)(H(2)O)(2), which has also been structurally characterized. Mn(hfacac)(3) also oxidizes xanthene to 9,9'-bixanthene, 1,4-cyclohexadiene to benzene, and 2,4-di-tert-butylphenol to the phenol dimer. Toluene and substituted toluenes are oxidized to tolylphenylmethanes. Product analyses and relative rates--for instance that p-methoxytoluene reacts much faster than toluene--indicate that the more electron rich substrates react by initial electron transfer to manganese. For the less electron rich substrates, such as 1,4-cyclohexadiene, a mechanism of initial hydrogen atom transfer to Mn(hfacac)(3) is suggested. The ability of Mn(hfacac)(3) to abstract H* is reasonable given its high redox potential and the basicity of [Mn(hfacac)(3)](-). In CH(2)Cl(2) solution, oxidation of DHA is catalyzed by chloride ion.     

The mechanism for water exchange in [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-), as studied by quantum chemical methods.

The mechanisms for the exchange of water between [UO(2)(H(2)O)(5)](2+), [UO(2)(oxalate)(2)(H(2)O)](2)(-)(,) and water solvent along dissociative (D), associative (A) and interchange (I) pathways have been investigated with quantum chemical methods. The choice of exchange mechanism is based on the computed activation energy and the geometry of the identified transition states and intermediates. These quantities were calculated both in the gas phase and with a polarizable continuum model for the solvent. There is a significant and predictable difference between the activation energy of the gas phase and solvent models: the energy barrier for the D-mechanism increases in the solvent as compared to the gas phase, while it decreases for the A- and I-mechanisms. The calculated activation energy, Delta U(++), for the water exchange in [UO(2)(H(2)O)(5)](2+) is 74, 19, and 21 kJ/mol, respectively, for the D-, A-, and I-mechanisms in the solvent, as compared to the experimental value Delta H(++) = 26 +/- 1 kJ/mol. This indicates that the D-mechanism for this system can be ruled out. The energy barrier between the intermediates and the transition states is small, indicating a lifetime for the intermediate approximately 10(-10) s, making it very difficult to distinguish between the A- and I-mechanisms experimentally. There is no direct experimental information on the rate and mechanism of water exchange in [UO(2)(oxalate)(2)(H(2)O)](2-) containing two bidentate oxalate ions. The activation energy and the geometry of transition states and intermediates along the D-, A-, and I-pathways were calculated both in the gas phase and in a water solvent model, using a single-point MP2 calculation with the gas phase geometry. The activation energy, Delta U(++), in the solvent for the D-, A-, and I-mechanisms is 56, 12, and 53 kJ/mol, respectively. This indicates that the water exchange follows an associative reaction mechanism. The geometry of the A- and I-transition states for both [UO(2)(H(2)O)(5)](2+) and [UO(2)(oxalate)(2)(H(2)O)](2-) indicates that the entering/leaving water molecules are located outside the plane formed by the spectator ligands.     

Potentiometric and multinuclear NMR study of the binary and ternary uranium(VI)-L-fluoride systems, where L is alpha-hydroxycarboxylate or glycine

Equilibria, structures, and ligand-exchange dynamics in binary and ternary U(VI)-L-F- systems, where L is glycolate, alpha-hydroxyisobutyrate, or glycine, have been investigated in 1.0 M NaClO4 by potentiometry and 1H, 17O, and 19F NMR spectroscopy. L may be bonded in two ways: either through the carboxylate end or by the formation of a chelate. In the glycolate system, the chelate is formed by proton dissociation from the alpha-hydroxy group at around pH 3, indicating a dramatic increase, a factor of at least 10(13), of its dissociation constant on coordination to uranium(VI). The L exchange in carboxylate-coordinated UO2LF3(2-) follows an Eigen-Wilkins mechanism, as previously found for acetate. The water exchange rate, k(aq) = 4.2 x 10(5) s(-1), is in excellent agreement with the value determined earlier for UO2(2+)(aq). The ligand-exchange dynamics of UO2(O-CH2-COO)2F3- and the activation parameters for the fluoride exchange in D2O (k(obs) = 12 s(-1), deltaH(double dagger) = 45.8 +/- 2.2 kJ mo(-1), and deltaS(double dagger) = -55.8 +/- 3.6 J K(-1) mol(-1)) are very similar to those in the corresponding oxalate complex, with two parallel pathways, one for fluoride and one for the alpha-oxocarboxylate. The same is true for the L exchange in UO2(O-CH2-COO)2(2-) and UO2(oxalate)2(2-). The exchange of alpha-oxocarboxylate takes place by a proton-assisted chelate ring opening followed by dissociation. Because we cannot decide if there is also a parallel H+-independent pathway, only an upper limit for the rate constant, k1 < 1.2 s(-1), can be given. This value is smaller than those in previously studied ternary systems. Equilibria and dynamics in the ternary uranium(VI)-glycine-fluoride system, investigated by 19F NMR spectroscopy, indicate the formation of one major ternary complex, UO2LF3(2-), and one binary complex, UO2L2 (L = H2N-CH2COO-), with chelate-bonded glycine; log beta(9) = 13.80 +/- 0.05 for the equilibrium UO2(2+) + H2N-CH2COO- + 3F- = UO2(H2N-CH2COO)F3(2-) and log beta(11) = 13.0 +/- 0.05 for the reaction UO2(2+) + 2H2N-CH2COO- = UO2(H2N-CH2COO)2. The glycinate exchange consists of a ring opening followed by proton-assisted steps. The rate of ring opening, 139 +/- 9 s(-1), is independent of both the concentration of H+ and the solvent, H2O or D2O.     

Vapor species over cerium and samarium trichlorides, enthalpies of formation of (LnCl(3))(n) molecules and Cl-(LnCl(3))(n) ions

A Knudsen effusion cell mass spectrometric technique was used to study vapor species over CeCl(3) and SmCl(3). Monomer, dimer, and trimer (Sm(3)Cl(9)) molecules, and LnCl(4-), Ln(2)Cl(7-), Ln(3)Cl(10-) (Ln = Ce, Sm) negative ions, were observed in saturated vapor in the temperature range 958-1227 K. Partial vapor pressures of neutral constituents were determined and the enthalpies of sublimation (Delta(s)H, 298 K, kJ.mol(-1)) to monomers and associated molecules obtained: 328 +/- 6 (CeCl(3)), 306 +/- 6 (SmCl(3)), 453 +/- 16 (Ce(2)Cl(6)), 408 +/- 12 (Sm(2)Cl(6)), and 468 +/- 40 (Sm(3)Cl(9)). Equilibrium constants for various chemical reactions were measured and the enthalpies of reactions obtained using the second and third laws of thermodynamics. The enthalpies of formation (Delta(f)H, 298 K, kJ.mol(-1)) of molecules and ions have been calculated as follows: -730 +/- 6 (CeCl(3)), -722 +/- 6 (SmCl(3)), -1663 +/- 16 (Ce(2)Cl(6)), -1649 +/- 13 (Sm(2)Cl(6)), -2617 +/- 40 (Sm(3)Cl(9)), -1250 +/- 15 (CeCl(4)(-)), -1252 +/- 15 (SmCl(4-)), -2184 +/- 35(Ce(2)Cl(7-)), -2172 +/- 26 (Sm(2)Cl(7-)), -3183 +/- 43 (Ce(3)Cl(10-)), and -3147 +/- 43 (Sm(3)Cl(10-)).     

Kinetics of Water Exchange on the Dihydroxo-Bridged Rhodium(III) Hydrolytic Dimer

()()Conventional (18)O isotopic labeling techniques have been used to measure the water exchange rates on the Rh(III) hydrolytic dimer [(H(2)O)(4)Rh(&mgr;-OH)(2)Rh(H(2)O)(4)](4+) at I = 1.0 M for 0.08 < [H(+)] < 0.8 M and temperatures between 308.1 and 323.1 K. Two distinct pathways of water exchange into the bulk solvent were observed (k(fast) and k(slow)) which are proposed to correspond to exchange of coordinated water at positions cis and trans to bridging hydroxide groups. This proposal is supported by (17)O NMR measurements which clearly showed that the two types of water ligands exchange at different rates and that the rates of exchange matched those from the (18)O labeling data. No evidence was found for the exchange of label in the bridging OH groups in either experiment. This contrasts with findings for the Cr(III) dimer. The dependence of both k(fast) and k(slow) on [H(+)] satisfied the expression k(obs) = (k(O)[H(+)](tot) +k(OH)K(a1))/([H(+)](tot) + K(a1)) which allows for the involvement of fully protonated and monodeprotonated Rh(III) dimer. The following rates and activation parameters were determined at 298 K. (i) For fully protonated dimer: k(fast) = 1.26 x 10(-)(6) s(-)(1) (DeltaH() = 119 +/- 4 kJ mol(-)(1) and DeltaS() = 41 +/- 12 J K(-)(1) mol(-)(1)) and k(slow) = 4.86 x 10(-)(7) s(-)(1) (DeltaH() = 64 +/- 9 kJ mol(-)(1) and DeltaS() = -150 +/- 30 J K(-)(1) mol(-)(1)). (ii) For monodeprotonated dimer: k(fast) = 3.44 x 10(-)(6) s(-)(1) (DeltaH() = 146 +/- 4 kJ mol(-)(1) and DeltaS() = 140 +/- 11 J K(-)(1) mol(-)(1)) and k(slow) = 2.68 x 10(-)(6) s(-)(1) (DeltaH() = 102 +/- 3 kJ mol(-)(1) and DeltaS() = -9 +/- 11 J K(-)(1) mol(-)(1)). Deprotonation of the Rh(III) dimer was found to labilize the primary coordination sphere of the metal ions and thus increase the rate of water exchange at positions cis and trans to bridging hydroxides but not to the same extent as for the Cr(III) dimer. Activation parameters and mechanisms for ligand substitution processes on the Rh(III) dimer are discussed and compared to those for other trivalent metal ions and in particular the Cr(III) dimer.     

The dimerization of SnCl2(g): mass spectrometric and theoretical studies

The vaporization of SnCl2(s) was investigated in the temperature range between 382 and 504 K by the use of Knudsen effusion mass spectrometry. The Sn+, SnCl+, SnCl2+, Sn2Cl3+, and Sn2Cl4+ ions were detected in the mass spectrum of the equilibrium vapor. The SnCl2(g) and Sn2Cl4(g) gaseous species were identified, and their partial pressures were determined. The structure and vibrational properties of both species and corresponding fragmentation products were studied applying density functional theory and second-order M?ller-Plesset perturbation theoretical approaches. Molecular parameters yielded thermodynamic functions by the use of statistical thermodynamics. The sublimation enthalpies of SnCl2(g) and Sn2Cl4(g) at 298 K resulting from the second- and third-law methods are evaluated as 130.9 +/- 6.2 kJ mol(-1) and 155.8 +/- 7.3 kJ mol(-1), respectively. The enthalpy changes of the dissociation reactions Sn2Cl4(g) = 2 SnCl2(g) were obtained as delta(d)H degrees(298) = 106.8 +/- 6.2 kJ mol(-1). The corresponding theoretical value amounts to 103.4 kJ mol(-1). The change of monomer properties due to the dimerization reaction is also discussed.     

Iron(III) complex of a crown ether-porphyrin conjugate and reversible binding of superoxide to its Iron(II) form

The synthesis and characterization of the Fe(III) complex of a novel crown ether-porphyrin conjugate, 52-N-(4-aza-18-crown-6)methyl-54,104,154,204-tetra-tert-butyl-56-methyl-5,10,15,20-tetraphenylporphyrin (H2Porph), as well as the corresponding hydroxo, dimeric, Fe(II), and peroxo species are reported. The crystal structure of [FeIII(Porph)Cl].H3O+.FeCl4-.C6H6.EtOH is also reported. [FeIII(Porph)(DMSO)2]+ and K[FeIII(Porph)(O22-)] are high-spin species (M?ssbauer data: delta = 0.38 mm s(-1), DeltaEq = 0.83 mm s(-1) and delta = 0.41 mm s(-1), DeltaEq = 0.51 mm s(-1), respectively), whereas in a solution of reduced [FeIII(Porph)(DMSO)2]+ complex the low-spin [FeII(Porph)(DMSO)2] (delta = 0.44 mm s(-1), DeltaEq = 1.32 mm s(-1)) and high-spin [FeII(Porph)(DMSO)] (delta = 1.27 mm s(-1), DeltaEq = 3.13 mm s(-1)) iron(II) species are observed. The reaction of [FeIII(Porph)(DMSO)2]+ with KO2 in DMSO has been investigated. The first reaction step, involving reduction to [FeII(Porph)(DMSO)2], was not investigated in detail because of parallel formation of an Fe(III)-hydroxo species. The kinetics and thermodynamics of the second reaction step, reversible binding of superoxide to the Fe(II) complex and formation of an Fe(III)-peroxo species, were studied in detail (by stopped-flow time-resolved UV/vis measurements in DMSO at 25 degrees C), resulting in kon = 36 500 +/- 500 M(-1) s(-1), koff = 0.21 +/- 0.01 s(-1) (direct measurements using an acid as a superoxide scavenger), and KO2- = (1.7 +/- 0.2) x 10(5) (superoxide binding constant kinetically obtained as kon/koff), (1.4 +/- 0.1) x 10(5), and (9.0 +/- 0.1) x 10(4) M(-1) (thermodynamically obtained in the absence and in the presence of 0.1 M NBu4PF6, respectively). Temperature-dependent kinetic measurements for kon (-40 to 25 degrees C in 3:7 DMSO/CH3CN mixture) yielded the activation parameters DeltaH = 61.2 +/- 0.9 kJ mol(-1) and DeltaS = +48 +/- 3 J K(-1) mol(-1). The observed reversible binding of superoxide to the metal center and the obtained kinetic and thermodynamic parameters are unique. The finding that fine-tuning of the proton concentration can cause the Fe(III)-peroxo species to release O2- and form an Fe(II) species is of biological interest, since this process might occur under very specific physiological conditions.     

Energetics of C-Cl, C-Br, and C-I bonds in haloacetic acids: enthalpies of formation of XCH2COOH (X=CI, Br, I) compounds and the carboxymethyl radical

The standard molar enthalpies of formation of chloro-, bromo-, and iodoacetic acids in the crystalline state, at 298.15 K, were determined as deltafH(o)m(C2H3O2Cl, cr alpha)=-(509.74+/- 0.49) kJ x mol(-1), deltafH(o)m(C2H3O2Br, cr I)-(466.98 +/- 1.08) kJ x mol(-1), and deltafH(o)m (C2H3O2I, cr)=-(415.44 +/- 1.53) kJ x mol(-1), respectively, by rotating-bomb combustion calorimetry. Vapor pressure versus temperature measurements by the Knudsen effusion method led to deltasubH(o)m(C2H3O2Cl)=(82.19 +/- 0.92) kJ x mol(-1), deltasubH(o)m(C2H3O2Br)=(83.50 +/- 2.95) kJ x mol(-1), and deltasubH(o)m-(C2H3O2I) = (86.47 +/- 1.02) kJ x mol(-1), at 298.15 K. From the obtained deltafH(o)m(cr) and deltasubH(o)m values it was possible to derive deltafH(o)m(C2H3O2Cl, g)=-(427.55 +/- 1.04) kJ x mol(-1), deltafH(o)m (C2H3O2Br, g)=-(383.48 +/- 3.14) kJ x mol(-1), and deltafH(o)m(C2H3O2I, g)=-(328.97 +/- 1.84) kJ x mol(-1). These data, taken with a published value of the enthalpy of formation of acetic acid, and the enthalpy of formation of the carboxymethyl radical, deltafH(o)m(CH2COOH, g)=-(238 +/- 2) kJ x mol(-1), obtained from density functional theory calculations, led to DHo(H-CH2COOH)=(412.8 +/- 3.2) kJ x mol(-1), DHo(Cl-CH2COOH)=(310.9 +/- 2.2) kJ x mol(-1), DHo(Br-CH2COOH)=(257.4 +/- 3.7) kJ x mol(-1), and DHo(I-CH2COOH)=(197.8 +/- 2.7) kJ x mol(-1). A discussion of the C-X bonding energetics in XCH2COOH, CH3X, C2H5X, C2H3X, and C6H5X (X=H, Cl, Br, I) compounds is presented.     

Comparative EXAFS investigation of uranium(VI) and -(IV) aquo chloro complexes in solution using a newly developed spectroelectrochemical cell

The coordination of the U(IV) and U(VI) ions as a function of the chloride concentration in aqueous solution has been studied by U L(III)-edge extended X-ray absorption fine structure (EXAFS) spectroscopy. The oxidation state of uranium was changed in situ using a gastight spectroelectrochemical cell, specifically designed for the safe use with radioactive solutions. For U(VI) we observed the complexes UO2(H2O)5(2+), UO2(H2O)4Cl+, UO2(H2O)3Cl2(0), and UO2(H2O)2Cl3- with [Cl-] increasing from 0 to 9 M, and for U(IV) we observed the complexes U(H2O)9(4+), U(H2O)8Cl3+, U(H2O)(6-7)Cl2(2+), and U(H2O)5Cl3+. The distances in the U(VI) coordination sphere are U-Oax = 1.76+/-0.02 A, Oeq = 2.41 +/- 0.02 A, and U-Cl = 2.71 +/- 0.02 A; the distances in the U(IV) coordination sphere are U-O = 2.41 +/- 0.02 A and U-Cl = 2.71 +/- 0.02 A.     

Solvent tuning of the substitution behavior of a seven-coordinate iron(III) complex

A detailed kinetic study of the substitution behavior of the seven-coordinate [Fe(dapsox)(L)2]ClO4 complex (H(2)dapsox = 2,6-diacetylpyridine-bis(semioxamazide), L = solvent or its deprotonated form) with thiocyanate as a function of the thiocyanate concentration, temperature, and pressure was undertaken in protic (EtOH and acidified EtOH and MeOH) and aprotic (DMSO) organic solvents. The lability and substitution mechanism depend strongly on the selected solvent (i.e., on solvolytic and protolytic processes). In the case of alcoholic solutions, substitution of both solvent molecules by thiocyanate could be observed, whereas in DMSO only one substitution step occurred. For both substitution steps, [Fe(dapsox)(L)2]ClO4 shows similar mechanistic behavior in methanol and ethanol, which is best reflected by the values of the activation volumes (MeOH DeltaV(I) = +15.0 +/- 0.3 cm(3) mol(-1), DeltaV(II) = +12.0 +/- 0.2 cm(3) mol(-1); EtOH DeltaV(I) = +15.8 +/- 0.7 cm(3) mol(-1), DeltaV(II) = +11.1 +/- 0.5 cm(3) mol(-1)). On the basis of the reported activation parameters, a dissociative (D) mechanism for the first substitution step and a D or dissociative interchange (I(d)) mechanism for the second substitution step are suggested for the reaction in MeOH and EtOH. This is consistent with the predominant existence of alcoxo [Fe(dapsox)(ROH)(OR)] species in alcoholic solutions. In comparison, the activation parameters for the substitution of the aqua-hydroxo [Fe(dapsox)(H2O)(OH)] complex by thiocyanate at pH 5.1 in MES were determined to be DeltaH = 72 +/- 3 kJ mol(-1), DeltaS = +38 +/- 11 J K(-1) mol(-1), and DeltaV = -3.0 +/- 0.1 cm(3) mol(-1), and the operation of a dissociative interchange mechanism was suggested, taking the effect of pressure on the employed buffer into account. The addition of triflic acid to the alcoholic solutions ([HOTf] = 10(-3) and 10(-2) M to MeOH and EtOH, respectively) resulted in a drastic changeover in mechanism for the first substitution step, for which an associative interchange (Ia) mechanism is suggested, on the basis of the activation parameters obtained for both the forward and reverse reactions and the corresponding volume profile. The second substitution step remained to proceed through an I(d) or D mechanism (acidified MeOH DeltaV(II) = +9.2 +/- 0.2 cm(3) mol(-1); acidified EtOH DeltaV(II) = +10.2 +/- 0.2 cm(3) mol(-1)). The first substitution reaction in DMSO was found to be slowed by several orders of magnitude and to follow an associative interchange mechanism (DeltaS = -50 +/- 9 J K(-1) mol(-1), DeltaV(I) = -1.0 +/- 0.5 cm(3) mol(-1)), making DMSO a suitable solvent for monitoring substitution processes that are extremely fast in aqueous solution.     

Synthesis and crystal structure of ionic multicomponent complex: ([Cr(I)(PhH)(2)].+))(2)[Co(II)TPP(C(60)(CN)(2))]-[C(60)(CN)(2)](.-).3(o-C(6)H(4)Cl(2)) containing C(60)(CN)(2).- radical anion and sigma-bonded diamagnetic Co(II)TPP(C(60)(CN)(2)) -anion

The ionic multicomponent complex complex: ([Cr(I)(PhH)(2)].+))(2)[Co(II)TPP(C(60)(CN)(2))]-[C(60)(CN)(2)](.-).3(o-C(6)H(4)Cl(2)) (Co(II)TPP: cobalt (II) tetraphenylporphyrin; Cr(PhH)(2): bis(benzene)chromium; o-C(6)H(4)Cl(2): o-dichlorobenzene) containing CoTPP(C(60)(CN)(2)- anion and C(60)(CN)(2).- radical anion was obtained. The complex has the cage structure with channels, which accommodate Cr(I)(PhH)(2)(.+) and o-C(6)H(4)Cl(2) molecules. For the first time the sigma-bonding of Co(II)TPP to the fullerene radical anion with the essentially shortened Co.C(C(60)(CN)(2)) contact of 2.282 A is observed. The sigma-bonding results in the diamagnetism of Co(II)TPP(C(60)(CN)(2))(-) anion. The nonbonded C(60)(CN)(2)(.-) radical anion retains both the C(2)(v)symmetry and the shape of the molecule. The length of the C(triple bond)N bonds is 1.141 and 1.152 A.