Structure and dynamics of binary and ternary lanthanide(III) and actinide(III) tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione] (TTA)-tributylphosphate (TBP) complexes. Part 3, the structure, thermodynamics and reaction mechanisms of 8- and 9-coordinated binary and ternary Y-TTA-TBP complexes studied by quantum chemical methods

Possible mechanisms for intermolecular exchange between coordinated and solvent water in the complexes Y(TTA)(3)(OH(2))(2) and Y(TTA)(3)(TBP)(OH(2)) and intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(OH(2))(HTTA) and Y(TTA)(3)(TBP)(HTTA) have been investigated using ab initio quantum chemical methods. The calculations comprise both structures and energies of isomers, intermediates and transition states. Based on these data and experimental NMR data (Part 2) we have suggested intimate reaction mechanisms for water exchange, intramolecular exchange between structure isomers and intermolecular exchange between free HTTA and coordinated TTA. A large number of isomers are possible for the complexes investigated, but only some of them have been investigated, in all of them the most stable geometry is a more or less distorted square anti-prism or bicapped trigonal prism; the energy differences between the various isomers are in general small, less than 10 kJ mol(-1). 9-coordinated intermediates play an important role in all reactions. Y(TTA)(3)(OH(2))(3) has three non-equivalent water ligands that can participate in ligand exchange reactions. The fastest of these exchanging sites has a QM activation energy of 18.1 kJ mol(-1), in good agreement with the experimental activation enthalpy of 19.6 kJ mol(-1). The mechanism for the intramolecular exchange between structure isomers in Y(TTA)(3)(OH(2))(2) involves the opening of a TTA-ring as the rate determining step as suggested by the good agreement between the QM activation energy and the experimental activation enthalpy 47.8 and 58.3 J mol(-1), respectively. The mechanism for the intermolecular exchange between free and coordinated HTTA in Y(TTA)(3)(HTTA) and Y(TTA)(3)(TBP)(HTTA) involves the opening of the intramolecular hydrogen bond in coordinated HTTA followed by proton transfer to coordinated TTA. This mechanism is supported by the good agreement between experimental activation enthalpies (within parenthesis) and calculated activation energies 68.7 (71.8) and 35.3 (38.8) kJ mol(-1). The main reason for the difference between the two systems is the much lower energy required to open the intramolecular hydrogen bond in the latter. The accuracy of the QM methods and chemical models used is discussed.     

NMR and electron paramagnetic resonance studies of [Gd(CH3CN)9]3+ and [Eu(CH3CN)9]2+: solvation and solvent exchange dynamics in anhydrous acetonitrile

Homoleptic acetonitrile complexes [Gd(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](3) and [Eu(CH(3)CN)(9)][Al(OC(CF(3))(3))(4)](2) have been studied in anhydrous acetonitrile by (14)N- and (1)H NMR relaxation as well as by X- and Q-band EPR. For each compound a combined analysis of all experimental data allowed to get microscopic information on the dynamics in solution. The second order rotational correlation times for [Gd(CH(3)CN)(9)](3+) and [Eu(CH(3)CN)(9)](2+) are 14.5 ± 1.8 ps and 11.8 ± 1.1 ps, respectively. Solvent exchange rate constants determined are (55 ± 15) × 10(6) s(-1) for the trivalent Gd(3+) and (1530 ± 200) × 10(6) s(-1) for the divalent Eu(2+). Surprisingly, for both solvate complexes CH(3)CN exchange is much slower for the less strongly N-binding acetonitrile than for the more strongly coordinated O-binding H(2)O. It is concluded that this exceptional behavior is due to the extremely fast water exchange, whereas the exchange behavior of CH(3)CN is more regular. Electron spin relaxation on the isoelectronic ions is much slower than on the O-binding water analogues. This allowed a precise determination of the hyperfine coupling constants for each of the two stable isotopes of Gd(3+) and Eu(2+) having a nuclear spin.     

Oxygen atom transfer from a trans-dioxoruthenium(VI) complex to nitric oxide

In aqueous acidic solutions trans-[Ru(VI)(L)(O)(2)](2+) (L=1,12-dimethyl-3,4:9,10-dibenzo-1,12-diaza-5,8-dioxacyclopentadecane) is rapidly reduced by excess NO to give trans-[Ru(L)(NO)(OH)](2+). When ≤1 mol equiv NO is used, the intermediate Ru(IV) species, trans-[Ru(IV)(L)(O)(OH(2))](2+), can be detected. The reaction of [Ru(VI)(L)(O)(2)](2+) with NO is first order with respect to [Ru(VI)] and [NO], k(2)=(4.13±0.21)×10(1) M(-1) s(-1) at 298.0 K. ΔH(≠) and ΔS(≠) are (12.0±0.3) kcal mol(-1) and -(11±1) cal mol(-1) K(-1), respectively. In CH(3)CN, ΔH(≠) and ΔS(≠) have the same values as in H(2)O; this suggests that the mechanism is the same in both solvents. In CH(3)CN, the reaction of [Ru(VI)(L)(O)(2)](2+) with NO produces a blue-green species with λ(max) at approximately 650 nm, which is characteristic of N(2)O(3). N(2)O(3) is formed by coupling of NO(2) with excess NO; it is relatively stable in CH(3)CN, but undergoes rapid hydrolysis in H(2)O. A mechanism that involves oxygen atom transfer from [Ru(VI)(L)(O)(2)](2+) to NO to produce NO(2) is proposed. The kinetics of the reaction of [Ru(IV)(L)(O)(OH(2))](2+) with NO has also been investigated. In this case, the data are consistent with initial one-electron O(-) transfer from Ru(IV) to NO to produce the nitrito species [Ru(III)(L)(ONO)(OH(2))](2+) (k(2)>10(6) M(-1) s(-1)), followed by a reaction with another molecule of NO to give [Ru(L)(NO)(OH)](2+) and NO(2)(-) (k(2)=54.7 M(-1) s(-1)).     

Rates of water exchange on the [Fe4(OH)2(hpdta)2(H2O)4]0 molecule and its implications for geochemistry

The ammonium salt of [Fe(4)O(OH)(hpdta)(2)(H(2)O)(4)](-) is soluble and makes a monospecific solution of [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) in acidic solutions (hpdta = 2-hydroxypropane-1,3-diamino-N,N,N',N'-tetraacetate). This tetramer is a diprotic acid with pK(a)(1) estimated at 5.7 ± 0.2 and pK(a)(2) = 8.8(5) ± 0.2. In the pH region below pK(a)(1), the molecule is stable in solution and (17)O NMR line widths can be interpreted using the Swift-Connick equations to acquire rates of ligand substitution at the four isolated bound water sites. Averaging five measurements at pH < 5, where contribution from the less-reactive conjugate base are minimal, we estimate: k(ex)(298) = 8.1 (±2.6) × 10(5) s(-1), ΔH(++) = 46 (±4.6) kJ mol(-1), ΔS(++) = 22 (±18) J mol(-1) K(-1), and ΔV(++) = +1.85 (±0.2) cm(3) mol(-1) for waters bound to the fully protonated, neutral molecule. Regressing the experimental rate coefficients versus 1/[H(+)] to account for the small pH variation in rate yields a similar value of k(ex)(298) = 8.3 (±0.8) × 10(5) s(-1). These rates are ～10(4) times faster than those of the [Fe(OH(2))(6)](3+) ion (k(ex)(298) = 1.6 × 10(2) s(-1)) but are about an order of magnitude slower than other studied aminocarboxylate complexes, although these complexes have seven-coordinated Fe(III), not six as in the [Fe(4)(OH)(2)(hpdta)(2)(H(2)O)(4)](0)(aq) molecule. As pH approaches pK(a1), the rates decrease and a compensatory relation is evident between the experimental ΔH(++) and ΔS(++) values. Such variation cannot be caused by enthalpy from the deprotonation reaction and is not well understood. A correlation between bond lengths and the logarithm of k(ex)(298) is geochemically important because it could be used to estimate rate coefficients for geochemical materials for which only DFT calculations are possible. This molecule is the only neutral, oxo-bridged Fe(III) multimer for which rate data are available.     

Enhanced luminescence of Eu(3+) by Y(3+) in ternary complexes Eu(X)Y(1-X)(TTA)3Dipy

Rare-earth ternary complexes Eu(X)Y(1-X)(TTA)3Dipy {X=0, 0.1, 0.25, 0.5, 0.75, 0.9, 1.0, using thenoyltrifluoroacetone (TTA) as ligand and 2,2'-dipyridyl (Dipy) as synergic agent} were synthesized. Characterization with X-ray diffraction (XRD), IR and elemental analysis had also been carried out. The photophysical properties of these complexes were studied in detail with ultraviolet absorption spectra and fluorescent spectra. It is found that the enhanced luminescence of Eu(3+) ions by Y(3+) ions occurs in ternary complexes. And we monitored the spectra of Eu(X)Y(1-X)(TTA)3Dipy (PVK:Eu/BCP/AlQ/Al) at the different rate (rpm). The results showed that the Y(3+) ion acts as an energy transfer bridge that helps energy transfer from PVK to Eu(3+).     

The oxidative degradation of dibenzoazepine derivatives by cerium(IV) complexes in acidic sulfate media

The kinetics of the oxidation of imipramine and desipramine using cerium(IV) complexes were studied in the presence of a large excess of azepine derivative (TCA) in acidic sulfate media using UV-Vis spectroscopy. The reaction proceeds via dibenzoazepine radical formation, identified by EPR measurements. The kinetics of the first degradation step were studied independently of the further slower degradation reactions. Linear dependences, with zero intercept, of the pseudo-first-order rate constants (k(obs)) on [TCA] were established for both dibenzoazepine radical formation processes. Rates of reactions decreased with increasing concentration of the H(+) ion indicating that cerium(IV) as well as both reductants exist in an equilibrium with their protolytic forms. The activation parameters for the degradation of dibenzoazepine derivatives in the first oxidation stage were as follows: ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 8 J K(-1) mol(-1) for imipramine and ΔH(≠) = 39 ± 2 kJ mol(-1), ΔS(≠) = -28 ± 6 J K(-1) mol(-1) for desipramine, respectively. Imipramine and desipramine radicals dimerized leading to an intermediate radical dimer, which decayed in a first-order consecutive decay process. These two further reactions proceed with rates which are characterized by non-linear dependences of the pseudo-first-order rate constants (k(obs)) on [TCA]. The degradation reaction of the intermediate radical dimer leads to an uncharged dimer as a final product. Mechanistic consequences of all the results are discussed.     

Rates of water exchange for two cobalt(II) heteropolyoxotungstate compounds in aqueous solution

Polyoxometalate ions are used as ligands in water-oxidation processes related to solar energy production. An important step in these reactions is the association and dissociation of water from the catalytic sites, the rates of which are unknown. Here we report the exchange rates of water ligated to Co(II) atoms in two polyoxotungstate sandwich molecules using the (17)O-NMR-based Swift-Connick method. The compounds were the [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) and the larger αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-) ions, each with two water molecules bound trans to one another in a Co(II) sandwich between the tungstate ligands. The clusters, in both solid and solution state, were characterized by a range of methods, including NMR, EPR, FT-IR, UV-Vis, and EXAFS spectroscopy, ESI-MS, single-crystal X-ray crystallography, and potentiometry. For [Co(4)(H(2)O)(2)(B-α-PW(9)O(34))(2)](10-) at pH 5.4, we estimate: k(298)=1.5(5)±0.3×10(6) s(-1), ΔH(≠)=39.8±0.4 kJ mol(-1), ΔS(≠)=+7.1±1.2 J mol(-1) K(-1) and ΔV(≠)=5.6 ±1.6 cm(3) mol(-1). For the Wells-Dawson sandwich cluster (αββα-[Co(4)(H(2)O)(2)(P(2)W(15)O(56))(2)](16-)) at pH 5.54, we find: k(298)=1.6(2)±0.3×10(6) s(-1), ΔH(≠)=27.6±0.4 kJ mol(-1) ΔS(≠)=-33±1.3 J mol(-1) K(-1) and ΔV(≠)=2.2±1.4 cm(3) mol(-1) at pH 5.2. The molecules are clearly stable and monospecific in slightly acidic solutions, but dissociate in strongly acidic solutions. This dissociation is detectable by EPR spectroscopy as S=3/2 Co(II) species (such as the [Co(H(2)O)(6)](2+) monomer ion) and by the significant reduction of the Co-Co vector in the XAS spectra.     

The methanol-methanolate CH(3)OH...OCH3(-) bridging ligand: tuning of exchange coupling by hydrogen bonds in dimethoxo-bridged dichromium(III) complexes

Two bis(mu-methoxo)dichromium(III) complexes, [L(Se)(2)Cr(2)(mu-OCH(3))(2)(CH(3)OH)(2)] 1 and [L(Se)(2)Cr(2)(mu-OCH(3))(2)(CH(3)OH)(CH(3)O)](-) 2, where L(Se) represents the dianion of 2,2'-selenobis(4,6-di-tert-butylphenol), have been reported to demonstrate the effect of hydrogen bonding on the exchange coupling interactions between the chromium(III) centers. The corresponding sulfur analogue of the ligand, i.e., 2,2'-thiobis(4,6-di-tert-butylphenol), also yields the analogous [L(S)(2)Cr(2)(mu-OCH(3))(2)(CH(3)OH)(2)] 3 and [L(S)(2)Cr(2)(mu-OCH(3))(2)(CH(3)O)(CH(3)OH)](-) 4, which exhibit similar exchange coupling parameters. An acid-base dependent equilibrium between 1 and 2 or 3 and 4 has been established by electronic spectral measurements.     

pH-sensitive fluorescent sensors based on europium(III) complexes

New europium(III) complexes Eu(TTA)(2)-DSQ and Eu(TTA)(3)-DR1 were designed and synthesized as new fluorescent pH probes (where HDSQ = 5-(dimethylamino)-N-(4-(2-((8-hydroxyquinolin-2-yl)methylene)hydrazinecarbonyl)phenyl)naphthalene-1-sulfonamide, DR1 = N(1)-(4-(dimethylamino)benzylidene)-N(2)-(rhodamine-6G) lactamethylene-diamine and TTA = thiophentrifluoroacetone). Eu(TTA)(2)-DSQ exhibited high sensitivity in monitoring pH changes in neutral aqueous solution with negligible background fluorescence. Eu(TTA)(3)-DR1 comprised a green light emitting Rhodamine 6G fluorophore and a Eu(III) moiety as the origin of red light. These pH-sensitive emitter components have pK(a) values of 5.0 and 7.2 respectively, and exhibit isolated protonated steps within one molecule. Luminescence titrations demonstrate that Eu(TTA)(3)-DR1 was able to detect pH values at both near neutral pH and acidic pH ranges, and was also able to detect pH in both cultured cells and in vivo.     

Isostructural dinuclear phenoxo-/acetato-bridged manganese(II), cobalt(II), and zinc(II) complexes with labile sites: kinetics of transesterification of 2-hydroxypropyl-p-nitrophenylphosphate

Using the dinucleating phenol-based ligand 2,6-bis[3-(pyridin-2-yl)pyrazol-1-ylmethyl]-4-methylphenol] (HL(2)), in its deprotonated form, the six new dinuclear complexes [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][PF(6)] (M = Mn (2a), Co (3a), Zn (4a)) and [M(II)(2)(L(2))(μ-O(2)CMe)(2)(MeCN)(2)][BPh(4)] (M = Mn (2b), Co (3b), Zn (4b)) have been synthesized. Crystallographic analyses on 2b·2MeCN, 3b·2MeCN, and 4b·2MeCN reveal that these complexes have closely similar μ-phenoxo bis(μ-carboxylato) structures. The physicochemical properties (absorption and ESI-MS spectral data, 2a,b, 3a,b, and 4a,b; (1)H NMR, 4a,b) of the cations of 2a-4a are identical with those of 2b-4b. Each metal ion is terminally coordinated by a pyrazole nitrogen and a pyridyl nitrogen from a 3-(pyridin-2-yl)pyrazole unit and a solvent molecule (MeCN). Thus, each metal center assumes distorted-octahedral M(II)N(3)O(3) coordination. Temperature-dependent magnetic studies on Mn(II) and Co(II) dimers reveal the presence of intramolecular antiferromagnetic (J = -8.5 cm(-1)) for 2b and ferromagnetic exchange coupling (J = +2.51 cm(-1)) for 3b, on the basis of the Hamiltonian H = -JS(1)·S(2). The exchange mechanism is discussed on the basis of magneto-structural parameters (M···M distance). Spectroscopic properties of the complexes have also been investigated. The pH titration and kinetics of phosphatase (transesterification) activity on 2-hydroxypropyl-p-nirophenylphosphate (HPNP) were studied in MeOH/H(2)O (33%, v/v) with 2a-4a, due to solubility reasons. This comparative kinetic study revealed the effect of the metal ion on the rate of hydrolysis of HPNP, which has been compared with what we recently reported for [Ni(II)(2)(L(2))(μ-O(2)CMe)(2)(MeOH)(H(2)O)][ClO(4)] (1a). The efficacy in the order of conversion of substrate to product (p-nitrophenolate ion) follows the order 4a > 3a > 2a > 1a, under identical experimental conditions. Notably, this trend follows the decrease of pK(a) values of M(II)-coordinated water (7.95 ± 0.04 and 8.78 ± 0.03 for 1a, 7.67 ± 0.08 and 8.69 ± 0.06 for 2a, 7.09 ± 0.05 and 8.05 ± 0.06 for 3a, and 6.20 ± 0.04 and 6.80 ± 0.03 for 4a). In this work we demonstrate that the stronger the Lewis acidity (Z(eff)/r) of the metal ion, the more acidic is the M(II)-coordinated water and the greater is the propensity of the metal ion to catalyze hydrolysis of the activated phosphate ester HPNP. Notably, the observed k(2) values (M(-1) s(-1)) for Mn(II) (2a, 0.152), Co(II) (3a, 0.208), and Zn(II) (4a, 0.230) complexes (1a, 0.058; already reported) linearly correlate with Z(eff)/r values of the metal ion. In each case a pseudo-first-order kinetic treatment has been done. Kinetic data analysis of complexes 2a-4a were also done following Michaelis-Menten treatment (catalytic efficiency k(cat)/K(M) values 0.170 M(-1) s(-1) for 2a, 0.194 M(-1) s(-1) for 3a and 0.161 M(-1) s(-1) for 4a; for 1a the value is 0.089 M(-1) s(-1)). Temperature-dependent measurements were done to evaluate kinetic/thermodynamic parameters for the hydrolysis/transesterification of HPNP and yielded comparable activation parameters (E(a) (kJ mol(-1)): 71.00 ± 4.60 (1a; reported), 67.95 ± 5.71 (2a), 62.60 ± 4.46 (3a), 67.80 ± 3.25 (4a)) and enthalpy/entropy of activation values (ΔH(?) (kJ mol(-1)) = 68.00 ± 4.65 (1a; reported), 65.40 ± 5.72 (2a), 60.00 ± 4.47 (3a), 65.29 ± 3.26 (4a); ΔS(?) (J mol(-1) K(-1)) = -109.00 ± 13 (1a; reported), -107.30 ± 16 (2a), -122.54 ± 14 (3a), -104.67 ± 10 (4a)). The E(a) values for all the complexes are comparable, suggesting a closely similar reaction barrier, meaning thereby similar course of reaction. The ΔS(?) values are consistent with an associative process. Positive ΔH(?) values correspond to bond breaking of the activated complex as a result of nucleophilic attack at the phosphorus atom, releasing cyclic phosphate and p-nitrophenolate ion. These data have helped us to propose a common mechanistic pathway: deprotonation of a metal-bound species to form the effective nucleophile, binding of the substrate to the metal center(s), intramolecular nucleophilic attack on the electrophilic phosphorus atom with the release of the leaving group, and possibly regeneration of the catalyst.     

Nitrile hydration by thiolate- and alkoxide-ligated Co-NHase analogues. Isolation of Co(III)-amidate and Co(III)-iminol intermediates

Nitrile hydratases (NHases) are thiolate-ligated Fe(III)- or Co(III)-containing enzymes, which convert nitriles to the corresponding amide under mild conditions. Proposed NHase mechanisms involve M(III)-NCR, M(III)-OH, M(III)-iminol, and M(III)-amide intermediates. There have been no reported crystallographically characterized examples of these key intermediates. Spectroscopic and kinetic data support the involvement of a M(III)-NCR intermediate. A H-bonding network facilitates this enzymatic reaction. Herein we describe two biomimetic Co(III)-NHase analogues that hydrate MeCN, and four crystallographically characterized NHase intermediate analogues, [Co(III)(S(Me2)N(4)(tren))(MeCN)](2+) (1), [Co(III)(S(Me2)N(4)(tren))(OH)](+) (3), [Co(III)(S(Me2)N(4)(tren))(NHC(O)CH(3))](+) (2), and [Co(III)(O(Me2)N(4)(tren))(NHC(OH)CH(3))](2+) (5). Iminol-bound 5 represents the first example of a Co(III)-iminol compound in any ligand environment. Kinetic parameters (k(1)(298 K) = 2.98(5) M(-1) s(-1), ΔH(?) = 12.65(3) kcal/mol, ΔS(?) = -14(7) e.u.) for nitrile hydration by 1 are reported, and the activation energy E(a) = 13.2 kcal/mol is compared with that (E(a) = 5.5 kcal/mol) of the NHase enzyme. A mechanism involving initial exchange of the bound MeCN for OH- is ruled out by the fact that nitrile exchange from 1 (k(ex)(300 K) = 7.3(1) × 10(-3) s(-1)) is 2 orders of magnitude slower than nitrile hydration, and that hydroxide bound 3 does not promote nitrile hydration. Reactivity of an analogue that incorporates an alkoxide as a mimic of the highly conserved NHase serine residue shows that this moiety facilitates nitrile hydration under milder conditions. Hydrogen-bonding to the alkoxide stabilizes a Co(III)-iminol intermediate. Comparison of the thiolate versus alkoxide intermediate structures shows that C≡N bond activation and C═O bond formation proceed further along the reaction coordinate when a thiolate is incorporated into the coordination sphere.     

Syntheses, structure, and magnetic properties of hexanuclear Mn(III)(2)M(III)(4) (M = Y, Gd, Tb, Dy) complexes

The synthesis and characterizations of a family of isomorphous [Mn(III)(2)M(III)(4)L(2)(μ(4)-O)(2)(N(3))(2)(CH(3)O)(2)(CH(3)OH)(4)(NO(3))(2)]·2H(2)O (M = Y(1), Gd(2), Tb(3), Dy(4)) are reported, where H(4)L = N,N'-dihydroxyethyl-N,N'-(2-hydroxy-4,5-dimethylbenzyl)ethylenediamine. They were obtained from the reactions of H(4)L with M(NO(3))(3)·6H(2)O, Mn(ClO(4))(2)·6H(2)O, NaN(3) and NEt(3) in a 1?:?1?:?1?:?2?:?2 molar ratio. The core structure consists of a Mn(2)M(4) unit. The four M(III) ions that are held together by two μ(4)-bridging oxygen atoms form a butterfly M(4) moiety. The M(4) core is further connected to the two five-coordinate trigonal-bipyramidal Mn(III) ions via one μ(4)-O(2-), two alkyloxo and one methoxo triple bridges. Magnetic susceptibility measurements indicate the presence of intramolecular antiferromagnetic interactions in complex 2, and overall intramolecular ferromagnetic interactions in complexes 3 and 4. The alternating current (AC) magnetic susceptibility studies revealed that complexes 3 and 4 showed frequency-dependent out-of-phase signals, which indicates that they exhibit slow relaxation of the magnetization.     

Stabilization of the tautomers HP(OH)2 and P(OH)3 of hypophosphorous and phosphorous acids as ligands

Treatment of [CpRu(PPh(3))(2)Cl] 1 with the stoichiometric amount of H(3)PO(2) or H(3)PO(3) in the presence of chloride scavengers (AgCF(3)SO(3) or TlPF(6)) yields compounds of formula [CpRu(PPh(3))(2)(HP(OH)(2))]Y (Y = CF(3)SO(3) 2a or PF(6) 2b) and [CpRu(PPh(3))(2)(P(OH)(3))]Y (Y = CF(3)SO(3) 3aor PF(6) 3b) which contain, respectively, the HP(OH)(2) and P(OH)(3) tautomers of hypophosphorous and phosphorous acids bound to ruthenium through the phosphorus atom. The triflate derivatives 2a and 3a react further with hypophosphorous or phosphorous acids to yield, respectively, the complexes [CpRu(PPh(3))(HP(OH)(2))(2)]CF(3)SO(3) 4 and [CpRu(PPh(3))(P(OH)(3))(2)]CF(3)SO(3) 5 which are formed by substitution of one molecule of the acid for a coordinated triphenylphosphine molecule. The compounds 2 and 3 are quite stable in the solid state and in solutions of common organic solvents, but the hexafluorophosphate derivatives undergo easy transformations in CH(2)Cl(2): the hypophosphorous acid complex 2b yields the compound [CpRu(PPh(3))(2)(HP(OH)(2))]PF(2)O(2) 6, whose difluorophosphate anion originates from hydrolysis of PF(6)(-); the phosphorous acid complex 3b yields the compound [CpRu(PPh(3))(2)(PF(OH)(2))]PF(2)O(2) 7, which is produced by hydrolysis of hexafluorophosphate and substitution of a fluorine for an OH group of the coordinated acid molecule. All the compounds have been characterized by elemental analyses and NMR measurements. The crystal structures of 2a, 3a and 7 have been determined by X-ray diffraction methods.     

Proton transfer to nickel-thiolate complexes. 2. Rate-limiting intramolecular proton transfer in the reactions of [Ni(SC(6)H(4)R-4)(PhP[CH(2)CH(2)PPh(2)](2))](+) (R = NO(2), Cl, H, Me, or MeO)

The protonation of [Ni(SC(6)H(4)R-4)(triphos)](+) (triphos = PhP[CH(2)CH(2)PPh(2)](2); R = NO(2), Cl, H, Me, or MeO) by [lutH](+) (lut = 2,6-dimethylpyridine) to form [Ni(S(H)C(6)H(4)R-4)(triphos)](2+) is an equilibrium reaction in MeCN. Kinetic studies, using stopped-flow spectrophotometry, reveal that the reactions occur by a two-step mechanism. Initially, [lutH](+) rapidly binds to the complex (K(2)(R)) in an interaction which probably involves hydrogen-bonding of the acid to the sulfur. Subsequent intramolecular proton transfer from [lutH](+) to sulfur (k(3)(R)) is slow because of both electronic and steric factors. The X-ray crystal structures of [Ni(SC(6)H(4)R-4)(triphos)](+) (R = NO(2), H, Me, or MeO) show that all are best described as square-planar complexes, with the phenyl substituents of the triphos ligand presenting an appreciable barrier to the approach of the sterically demanding [lutH](+) to the sulfur. The kinetic characteristics of the intramolecular proton transfer from [lutH](+) to sulfur have been investigated. The rate of intramolecular proton transfer exhibits a nonlinear dependence on Hammett sigma(+), with both electron-releasing and electron-withdrawing 4-R-substituents on the coordinated thiolate facilitating the rate of proton transfer (NO(2) > Cl > H > Me < MeO). The rate constants for intramolecular proton transfer correlate well with the calculated electron density of the sulfur. The temperature dependence of the rate of the intramolecular proton transfer reactions shows that deltaH() is small but increases as the 4-R-substituent becomes more electron-withdrawing [deltaH = 4.1 (MeO), 6.9 (Me), 11.4 kcal mol(-)(1) (NO(2))], while DeltaS() becomes progressively less negative [deltaS = -50.1 (MeO), -41.2 (Me), -16.4 (NO(2)) cal K(-)(1) mol(-)(1)]. Studies with [lutD](+) show that the rate of intramolecular proton transfer varies with the 4-R-substituent [(k(3)(NO)2)(H)/(k(3)(NO)2)(D) = 0.39; (k(3)(Cl))(H)/(k(3)(Cl))(D) = 0.88; (k(3)(Me))(H)/(k(3)(Me))(D) = 1.3; (k(3)(MeO))(H)/(k(3)(MeO))(D) = 1.2].     

Carboxylate-free Mn(III)2Ln(III)2 (Ln = lanthanide) and Mn(III)2Y(III)2 complexes from the use of (2-hydroxymethyl)pyridine: analysis of spin frustration effects

The initial employment of 2-(hydroxymethyl)pyridine for the synthesis of Mn/Ln (Ln = lanthanide) and Mn/Y clusters, in the absence of an ancillary organic ligand, has afforded a family of tetranuclear [Mn(III)(2)M(III)(2)(OH)(2)(NO(3))(4)(hmp)(4)(H(2)O)(4)](NO(3))(2) (M = Dy, 1; Tb, 2; Gd, 3; Y; 4) anionic compounds. 1-4 possess a planar butterfly (or rhombus) core and are rare examples of carboxylate-free Mn/Ln and Mn/Y clusters. Variable-temperature dc and ac studies established that 1 and 2, which contain highly anisotropic Ln(III) atoms, exhibit slow relaxation of their magnetization vector. Fitting of the obtained magnetization (M) versus field (H) and temperature (T) data for 3 by matrix diagonalization and including only axial anisotropy (zero-field splitting, ZFS) showed the ground state to be S = 3. Complex 4 has an S = 0 ground state. Fitting of the magnetic susceptibility data collected in the 5-300 K range for 3 and 4 to the appropriate van Vleck equations revealed, as expected, extremely weak antiferromagnetic interactions between the paramagnetic ions; for 3, J(1) = -0.16(2) cm(-1) and J(2) = -0.12(1) cm(-1) for the Mn(III)···Mn(III) and Mn(III)···Gd(III) interactions, respectively. The S = 3 ground state of 3 has been rationalized on the basis of the spin frustration pattern in the molecule. For 4, J = -0.75(3) cm(-1) for the Mn(III)···Mn(III) interaction. Spin frustration effects in 3 have been quantitatively analyzed for all possible combinations of sign of J(1) and J(2).     

Bis(2-pyridylimino)isoindolato iron(II) and cobalt(II) complexes: structural chemistry and paramagnetic NMR spectroscopy

Condensation of phthalodinitrile and 2-amino-5,6,7,8-tetrahydroquinoline gave the bis(2-pyridylimino)isoindole protioligand 1 (thqbpiH) in high yield. Deprotonation of thqbpiH (1) using LDA in THF at -78 °C yields the corresponding lithium complex [Li(THF)(thqbpi)] (2) in which the lithium atom enforces almost planar arrangement of the tridentate ligand, with an additional molecule of THF coordinated to Li. Reaction of cobalt(II) chloride or iron(II) chloride with one equivalent of the lithium complex 2 in THF led to formation of the metal complexes [CoCl(THF)(thqbpi)] (3a) and [FeCl(THF)(thqbpi)] (3b). The paramagnetic susceptibility of 3a,b in solution was measured by the Evans method (3a: μ(eff) = 4.17 μ(B); 3b: μ(eff) = 5.57 μ(B)). Stirring a solution of 1 and cobalt(II) acetate tetrahydrate in methanol yielded the cobalt(II) complex 4 which was also accessible by treatment of 3a with one equivalent of silver or thallium acetate in DMSO. Whereas 3a,b were found to be mononuclear in the solid state, the acetate complex 4 was found to be dinuclear, the two metal centres being linked by an almost symmetrically bridging acetate. For all transition metal complexes paramagnetic (1)H as well as (13)C NMR spectra were recorded at variable temperatures. The complete assignment of the paramagnetic NMR spectra was achieved by computation of the spin densities within the complexes using DFT. The proton NMR spectra of 3a and 3b displayed dynamic behaviour. This was attributed to the exchange of coordinating solvent molecules by an associative mechanism which was analysed using lineshape analysis (ΔS(≠)= -154 ± 25 J mol(-1) K(-1) for 3a and ΔS(≠) = -168 ± 15 J mol(-1) K(-1) for 3b).     

Iridium(III) complexes formed by O-H and/Or C-H activation of 2-(arylazo)phenols

Reaction of 2-(arylazo)phenols with [Ir(PPh(3))(3)Cl] in refluxing ethanol in the presence of a base (NEt(3)) affords complexes of three different types, viz. [Ir(PPh(3))(2)(NO-R)(H)Cl] (R = OCH(3), CH(3), H, Cl and NO(2)), [Ir(PPh(3))(2)(NO-R)(H)(2)] and [Ir(PPh(3))(2)(CNO-R)(H)]. Structures of the [Ir(PPh(3))(2)(NO-Cl)(H)Cl], [Ir(PPh(3))(2)(NO-Cl)(H)(2)] and [Ir(PPh(3))(2)(CNO-Cl)(H)] complexes have been determined by X-ray crystallography. In the [Ir(PPh(3))(2)(NO-R)(H)Cl] and [Ir(PPh(3))(2)(NO-R)(H)(2)] complexes, the 2-(arylazo)phenolate ligands are coordinated to the metal center as monoanionic bidentate N,O-donors, whereas in the [Ir(PPh(3))(2)(CNO-R)(H)] complexes, they are coordinated to iridium as dianionic tridentate C,N,O-donors. In all three products formed in ethanol, the two PPh(3) ligands are trans. Reaction of 2-(arylazo)phenols with [Ir(PPh(3))(3)Cl] in refluxing toluene in the presence of NEt(3) affords complexes of two types, viz. [Ir(PPh(3))(2)(CNO-R)(H)] and [Ir(PPh(3))(2)(CNO-R)Cl]. Structure of the [Ir(PPh(3))(2)(CNO-Cl)Cl] complex has been determined by X-ray crystallography, and the 2-(arylazo)phenolate ligand is coordinated to the metal center as a dianionic tridentate C,N,O-donor and the two PPh(3) ligands are cis. All of the iridium(III) complexes show intense MLCT transitions in the visible region. Cyclic voltammetry shows an Ir(III)-Ir(IV) oxidation on the positive side of SCE and an Ir(III)-Ir(II) reduction on the negative side for all of the products.     

Proton exchange kinetics from the bound waters on the oxo-centered rhodium(III) trimer [Rh3(mu3-O)(mu-O2CCH3)6(OH2)3]+: a variable pH and temperature 1H NMR study

Proton exchange from the bound to the bulk waters on the oxo-centered rhodium(III) trimer, [Rh(3)(micro(3)-O)(micro-O(2)CCH(3))(6)(OH(2))(3)](+)(abbreviated as Rh(3)(+)), was investigated over the temperature range of 219.1-313.9 K using a (1)H NMR line-broadening technique. By solving the modified Bloch equations for a two-site chemical exchange, lifetimes (tau) for proton transfer at pH = 2.7, 3.6, and 7.0 ([Rh(3)(+)]= 26 mM, T= 298 K) were determined to be 0.3 (+/-.08) ms, 2 (+/-0.3) ms, and 0.2 (+/-0.2) ms, respectively. From the temperature dependence of the rate, the activation parameters were determined to be DeltaH(++)= 16.2 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 123 (+/-2) J mol(-1) K(-1), DeltaH(++)= 14.9 (+/-0.5) kJ mol(-1) and DeltaS(++)=- 141 (+/-2) J mol(-1) K(-1), and DeltaH(++)= 45 (+/-2) kJ mol(-1) and DeltaS(++)=- 22 (+/-5) J mol(-1) K(-1) for pH = 2.7, 3.6 and 7.0, respectively. All results are reported for a mixed solvent system [acetone : 250 mM NaClO(4)(aq)(3:1)], which was necessary to depress the freezing point of the solution so that the (1)H NMR signal due to bound water could be observed. The pK(a) of Rh(3)(+) was measured to be 8.9 (+/-0.2) in the mixed solvent, which is near the pK(a) for an aqueous solution (8.3 (+/-0.2)). Surprisingly, the lifetimes for protons on Rh(3)(+) are close to those observed for the Rh(OH(2))(6)(3+) ion, in spite of the considerable difference in structure, Br?nsted acidity of the bound waters and average charge on the metal ion.     

Dinuclear cyano complexes of cobalt(III) and Iron(III) containing noninnocent 1,2,4,5-tetrakis(2-pyridinecarboxamido)benzene bridging ligands

Two new dinucleating ligands 1,2,4,5-tetrakis(2-pyridinecarboxamido)benzene, H(4)(tpb), and 1,2,4,5-tetrakis(4-tert-butyl-2-pyridinecarboxamido)benzene, H(4)(tbpb), have been synthesized, and the following dinuclear cyano complexes of cobalt(III) and iron(III) have been isolated: Na(2)[Co(III)(2)(tpb)(CN)(4)] (1); [N(n-Bu)(4)](2)[Co(III)(2)(tbpb)(CN)(4)] (2); [Co(III)(2)(tbpb(ox2))(CN)(4)] (3); [N(n-Bu)(4)](2)[Fe(III)(2)(tpb)(N(3))(4)] (4); [N(n-Bu)(4)](2)[Fe(III)(2)(tpb)(CN)(4)] (5); [N(n-Bu)(4)](2)[Fe(III)(2)(tbpb)(CN)(4)] (6). Complexes 2-4 and 6 have been structurally characterized by X-ray crystallography at 100 K. From electrochemical and spectroscopic (UV-vis, IR, EPR, M?ssbauer) and magnetochemical investigations it is established that the coordinated central 1,2,4,5-tetraamidobenzene entity in the cyano complexes can be oxidized in two successive one-electron steps yielding paramagnetic (tbpb(ox1))(3)(-) and diamagnetic (tbpb(ox2))(2)(-) anions. Thus, complex 6 exists in five characterized oxidation levels: [Fe(III)(2)(tbpb(ox2))(CN)(4)](0) (S = 0); [Fe(III)(2)(tbpb(ox1))(CN)(4)](-) (S = (1)/(2)); [Fe(III)(2)(tbpb)(CN)(4)](2)(-) (S = 0); [Fe(III)Fe(II)(tbpb)(CN)(4)](3)(-) (S = (1)/(2)); [Fe(II)(2)(tbpb)(CN)(4)](4)(-) (S = 0). The iron(II) and (III) ions are always low-spin configurated. The electronic structure of the paramagnetic iron(III) ions and the exchange interaction of the three-spin system [Fe(III)(2)(tbpb(ox1))(CN)(4)](-) are characterized in detail. Similarly, for 2 three oxidation levels have been identified and fully characterized: [Co(III)(2)(tbpb)(CN)(4)](2)(-) (S = 0); [Co(III)(2)(tbpb(ox1))(CN)(4)](-) (S = (1)/(2)); [Co(III)(2)(tbpb(ox2))(CN)(4)](0). The crystal structures of 2 and 3 clearly show that the two electron oxidation of 2 yielding 3 affects only the central tetraamidobenzene part of the ligand.     

Coordination mode of nitrate in uranyl(VI) complexes: a first-principles molecular dynamics study

According to Car-Parrinello molecular dynamics simulations for [UO(2)(NO(3))(3)](-), [UO(2)(NO(3))(4)](2-), and [UO(2)(OH(2))(4-)(NO(3))](+) complexes in the gas phase and in aqueous solution, the nitrate coordination mode to uranyl depends on the interplay between ligand-metal attractions, interligand repulsions, and solvation. In the trinitrate, the eta(2)-coordination is clearly favored in water and in the gas phase, leading to a coordination number (CN) of 6. According to pointwise thermodynamic integration involving constrained molecular dynamics simulations, a change in free energy of +6 kcal/mol is predicted for eta(2)- to eta(1)-transition of one of the three nitrate ligands in the gas phase. In the gas phase, the mononitrate-hydrate complex also prefers a eta(2)-binding mode but with a CN of 5, one H(2)O molecule being in the second shell. This contrasts with the aqueous solution where the nitrate binds in a eta(1)-fashion and uranyl coordinates to four H2O ligands. A driving force of ca. -3 kcal/mol is predicted for the eta(2)- to eta(1)- transition in water. This structural preference is interpreted in terms of steric arguments and differential solvation of terminal vs uranyl-coordinated O atoms of the nitrate ligands. The [UO(2)(NO(3))(4)](2-) complex with two eta(2)- and two eta(1)- coordinated nitrates, observed in the solid state, is stable for 1-2 ps in the gas phase and in solution. In the studied series, the modulation of uranyl-ligand distances upon immersion of the complex in water is found to depend on the nature of the ligand and the composition of the complex.