A mechanistic study of the alkaline hydrolysis of diaryl sulfate diesters

Nearly all of the reported studies of reactions of sulfate diesters are for dialkyl or alkyl aryl diesters, which undergo reaction by carbon-oxygen bond fission. Sulfuryl transfer reactions of sulfate diesters (RO-SO(2)-OR') proceeding by attack at sulfur have been little explored. When both ester groups are aryl groups the hydrolysis reaction (sulfuryl transfer to water) occurs by way of attack at sulfur. The alkaline hydrolysis of diaryl sulfate diesters was shown to obey first-order kinetics with respect to [(-)OH] and proceed through S-O bond fission, in a mechanism that is most likely concerted. Activation parameters for 4-chloro-3-nitrophenyl phenyl sulfate and 4-nitrophenyl phenyl sulfate gave the following respective values: Delta H(++) = 88.0 +/- 0.1 and 84.83 +/- 0.06 kJ mol(-)(1) and Delta S(++) = -37 +/- 1 and -50.2 +/- 0.5 J mol(-)(1) deg(-)(1). The dependence of the second-order rate constant for hydrolysis on leaving group pK(a) was analyzed giving a beta(lg) slope of -0.7 +/- 0.2 and a Leffler alpha parameter value of 0.36. A (15)k kinetic isotope effect (KIE) for the hydroxide attack on 4-nitrophenyl phenyl sulfate of 1.0000 +/-0.0005 and an (18)k(lg) KIE value of 1.003+/-0.002 were obtained.     

Hydroxylation of phenolic compounds by a peroxodicopper(II) complex: further insight into the mechanism of tyrosinase

The dicopper(I) complex [Cu2(MeL66)]2+ (where MeL66 is the hexadentate ligand 3,5-bis-{bis-[2-(1-methyl-1H-benzimidazol-2-yl)-ethyl]-amino}-meth ylbenzene) reacts reversibly with dioxygen at low temperature to form a mu-peroxo adduct. Kinetic studies of O2 binding carried out in acetone in the temperature range from -80 to -55 degrees C yielded the activation parameters DeltaH1(not equal) = 40.4 +/- 2.2 kJ mol(-1), DeltaS1)(not equal) = -41.4 +/- 10.8 J K(-1) mol(-1) and DeltaH(-1)(not equal) = 72.5 +/- 2.4 kJ mol(-1), DeltaS(-1)(not equal) = 46.7 +/- 11.1 J K(-1) mol(-1) for the forward and reverse reaction, respectively, and the binding parameters of O2 DeltaH degrees = -32.2 +/- 2.2 kJ mol(-1) and DeltaS degrees = -88.1 +/- 10.7 J K(-1) mol(-1). The hydroxylation of a series of p-substituted phenolate salts by [Cu2(MeL66)O2]2+ studied in acetone at -55 degrees C indicates that the reaction occurs with an electrophilic aromatic substitution mechanism, with a Hammett constant rho = -1.84. The temperature dependence of the phenol hydroxylation was studied between -84 and -70 degrees C for a range of sodium p-cyanophenolate concentrations. The rate plots were hyperbolic and enabled to derive the activation parameters for the monophenolase reaction DeltaH(not equal)ox = 29.1 +/- 3.0 kJ mol(-1), DeltaS(not equal)ox = -115 +/- 15 J K(-1) mol(-1), and the binding parameters of the phenolate to the mu-peroxo species DeltaH degrees(b) = -8.1 +/- 1.2 kJ mol(-1) and DeltaS degrees(b) = -8.9 +/- 6.2 J K(-1) mol(-1). Thus, the complete set of kinetic and thermodynamic parameters for the two separate steps of O2 binding and phenol hydroxylation have been obtained for [Cu2(MeL66)]2+.     

Insights into Sonogashira cross-coupling by high-throughput kinetics and descriptor modeling

A method is presented for the high-throughput monitoring of reaction kinetics in homogeneous catalysis, running up to 25 coupling reactions in a single reaction vessel. This method is demonstrated and validated on the Sonogashira reaction, analyzing the kinetics for almost 500 coupling reactions. First, one-pot reactions of phenylacetylene with a set of 20 different meta- and para-substituted aryl bromides were analyzed in the presence of 17 different Pd-phosphine complexes. In addition, the temperature-dependent Sonogashira reactions were examined for 21 different ArX (X=Cl, Br, I) substrates, and the corresponding activation enthalpies and entropies were determined by means of Eyring plots: ArI (DeltaH(not equal)=48-62 kJ mol(-1); DeltaS(not equal)=-71--39 J mol(-1) K; NO(2)-->OMe), ArBr (DeltaH(not equal)=54-82 kJ mol(-1), DeltaS(not equal)=-55-11 J mol(-1) K), and ArCl (DeltaH(not equal)=95-144 kJ mol(-1), DeltaS(not equal)=-6-100 J mol(-1) K). DFT calculations established a linear correlation of DeltaH( not equal) and the Kohn-Sham HOMO energies of ArX (X=Cl, Br, I) and confirmed their involvement in the rate-limiting step. However, despite different C--X bond energies, aryl iodides and electron-deficient aryl bromides showed similar activation parameters.     

Influence of terpyridine as pi-acceptor ligand on the kinetics and mechanism of the reaction of NO with ruthenium(III) complexes

The kinetics of the unusually fast reaction of cis- and trans-[Ru(terpy)(NH3)2Cl]2+ (with respect to NH3; terpy=2,2':6',2"-terpyridine) with NO was studied in acidic aqueous solution. The multistep reaction pathway observed for both isomers includes a rapid and reversible formation of an intermediate Ru(III)-NO complex in the first reaction step, for which the rate and activation parameters are in good agreement with an associative substitution behavior of the Ru(III) center (cis isomer, k1=618 +/- 2 M(-1) s(-1), DeltaH(++) = 38 +/- 3 kJ mol(-1), DeltaS(++) = -63 +/- 8 J K(-1) mol(-1), DeltaV(++) = -17.5 +/- 0.8 cm3 mol(-1); k -1 = 0.097 +/- 0.001 s(-1), DeltaH(++) = 27 +/- 8 kJ mol(-1), DeltaS(++) = -173 +/- 28 J K(-1) mol(-1), DeltaV(++) = -17.6 +/- 0.5 cm3 mol(-1); trans isomer, k1 = 1637 +/- 11 M(-1) s(-1), DeltaH(++) = 34 +/- 3 kJ mol(-1), DeltaS(++) = -69 +/-11 J K(-1) mol(-1), DeltaV(++) = -20 +/- 2 cm3 mol(-1); k(-1)=0.47 +/- 0.08 s(-1), DeltaH(++)=39 +/- 5 kJ mol(-1), DeltaS(++) = -121 +/-18 J K(-1) mol(-1), DeltaV(++) = -18.5 +/- 0.4 cm3 mol(-1) at 25 degrees C). The subsequent electron transfer step to form Ru(II)-NO+ occurs spontaneously for the trans isomer, followed by a slow nitrosyl to nitrite conversion, whereas for the cis isomer the reduction of the Ru(III) center is induced by the coordination of an additional NO molecule (cis isomer, k2=51.3 +/- 0.3 M(-1) s(-1), DeltaH(++) = 46 +/- 2 kJ mol(-1), DeltaS(++) = -69 +/- 5 J K(-1) mol(-1), DeltaV(++) = -22.6 +/- 0.2 cm3 mol(-1) at 45 degrees C). The final reaction step involves a slow aquation process for both isomers, which is interpreted in terms of a dissociative substitution mechanism (cis isomer, DeltaV(++) = +23.5 +/- 1.2 cm3 mol(-1); trans isomer, DeltaV(++) = +20.9 +/- 0.4 cm3 mol(-1) at 55 degrees C) that produces two different reaction products, viz. [Ru(terpy)(NH3)(H2O)NO]3+ (product of the cis isomer) and trans-[Ru(terpy)(NH3)2(H2O)]2+. The pi-acceptor properties of the tridentate N-donor chelate (terpy) predominantly control the overall reaction pattern.     

Triggering water exchange mechanisms via chelate architecture. Shielding of transition metal centers by aminopolycarboxylate spectator ligands

Paramagnetic effects on the relaxation rate and shift difference of the (17)O nucleus of bulk water enable the study of water exchange mechanisms on transition metal complexes by variable temperature and variable pressure NMR. The water exchange kinetics of [Mn(II)(edta)(H2O)](2-) (CN 7, hexacoordinated edta) was reinvestigated and complemented by variable pressure NMR data. The results revealed a rapid water exchange reaction for the [Mn(II)(edta)(H2O)](2-) complex with a rate constant of k(ex) = (4.1 +/- 0.4) x 10(8) s(-1) at 298.2 K and ambient pressure. The activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) are 36.6 +/- 0.8 kJ mol(-1), +43 +/- 3 J K(-1) mol(-1), and +3.4 +/- 0.2 cm(3) mol(-1), which are in line with a dissociatively activated interchange (I(d)) mechanism. To analyze the structural influence of the chelate, the investigation was complemented by studies on complexes of the edta-related tmdta (trimethylenediaminetetraacetate) chelate. The kinetic parameters for [Fe(II)(tmdta)(H2O)](2-) are k(ex) = (5.5 +/- 0.5) x 10(6) s(-1) at 298.2 K, DeltaH(double dagger) = 43 +/- 3 kJ mol(-1), DeltaS(double dagger) = +30 +/- 13 J K(-1) mol(-1), and DeltaV(double dagger) = +15.7 +/- 1.5 cm(3) mol(-1), and those for [Mn(II)(tmdta)(H2O)](2-) are k(ex) = (1.3 +/- 0.1) x 10(8) s(-1) at 298.2 K, DeltaH(double dagger) = 37.2 +/- 0.8 kJ mol(-1), DeltaS(double dagger) = +35 +/- 3 J K(-1) mol(-1), and DeltaV(double dagger) = +8.7 +/- 0.6 cm(3) mol(-1). The water containing species, [Fe(III)(tmdta)(H2O)](-) with a fraction of 0.2, is in equilibrium with the water-free hexa-coordinate form, [Fe(III)(tmdta)](-). The kinetic parameters for [Fe(III)(tmdta)(H2O)](-) are k(ex) = (1.9 +/- 0.8) x 10(7) s(-1) at 298.2 K, DeltaH(double dagger) = 42 +/- 3 kJ mol(-1), DeltaS(double dagger) = +36 +/- 10 J K(-1) mol(-1), and DeltaV(double dagger) = +7.2 +/- 2.7 cm(3) mol(-1). The data for the mentioned tmdta complexes indicate a dissociatively activated exchange mechanism in all cases with a clear relationship between the sterical hindrance that arises from the ligand architecture and mechanistic details of the exchange process for seven-coordinate complexes. The unexpected kinetic and mechanistic behavior of [Ni(II)(edta')(H2O)](2-) and [Ni(II)(tmdta')(H2O)](2-) is accounted for in terms of the different coordination number due to the strong preference for an octahedral coordination environment and thus a coordination equilibrium between the water-free, hexadentate [M(L)](n+) and the aqua-pentadentate forms [M(L')(H2O)](n+) of the Ni(II)-edta complex, which was studied in detail by variable temperature and pressure UV-vis experiments. For [Ni(II)(edta')(H2O)](2-) (CN 6, pentacoordinated edta) a water substitution rate constant of (2.6 +/- 0.2) x 10(5) s(-1) at 298.2 K and ambient pressure was measured, and the activation parameters DeltaH(double dagger), DeltaS(double dagger), and DeltaV(double dagger) were found to be 34 +/- 1 kJ mol(-1), -27 +/- 2 J K(-1) mol(-1), and +1.8 +/- 0.1 cm(3) mol(-1), respectively. For [Ni(II)(tmdta')(H2O)](2-), we found k = (6.4 +/- 1.4) x 10(5) s(-1) at 298.2 K, DeltaH(double dagger) = 22 +/- 4 kJ mol(-1), and DeltaS(double dagger) = -59 +/- 5 J K(-1) mol(-1). The process is referred to as a water substitution instead of a water exchange reaction, since these observations refer to the intramolecular displacement of coordinated water by the carboxylate moiety in a ring-closure reaction.     

Thermodynamic and kinetic studies on the reaction between the vitamin B12 derivative beta-(N-methylimidazolyl)cobalamin and N-methylimidazole: ligand displacement at the alpha axial site of cobalamins

The equilibria and kinetics of substitution of the 5,6-dimethylbenzimidazole at the alpha site of beta-(N-methylimidazolyl)cobalamin by N-methylimidazole have been investigated, and the product, bis(N-methylimidazolyl)cobalamin, has been characterized by visible and 1H NMR spectroscopies. The equilibrium constant for (N-MeIm)Cbl+ + N-MeIm right harpoon over left harpoon (N-MeIm)2Cbl+ was determined by 1H NMR spectroscopy (9.6 +/- 0.1 M(-1), 25.0 degrees C, I = 1.5 M (NaClO4)). The observed rate constant for this reaction exhibits an unusual inverse dependence on N-methylimidazole concentration, and it is proposed that substitution occurs via a base-off solvent-bound intermediate. Activation parameters typical for a dissociative ligand substitution mechanism are reported at two different N-MeImT concentrations, 5.00 x 10(-3) M (DeltaH++ = 99 +/- 2 kJ x mol(-1), DeltaS++ = 39 +/- 5 J x mol(-1) x K(-1), DeltaV++ = 15.0 +/- 0.7 cm3 x mol(-1), and 1.00 M (DeltaH++ = 109.4 +/- 0.8 kJ x mol(-1), DeltaS++ = 70 +/- 3 J x mol(-1) x K(-1), DeltaV++ = 16.8 +/- 1.1 cm3 x mol(-1)). According to the proposed mechanism, these parameters correspond to the equation of (N-MeIm)2Cbl+ and the ring-opening reaction of the alpha-DMBI of (N-MeIm)Cbl+ to give the solvent-bound intermediate in both cases, respectively.     

Nitrogen-14 NMR Study on Solvent Exchange of the Octahedral Cobalt(II) Ion in Neat 1,3-Propanediamine and n-Propylamine at Various Temperatures and Pressures. Tetrahedral-Octahedral Equilibrium of the Solvated Cobalt(II) Ion in n-Propylamine As Studied by EXAFS and Electronic Absorption Spectroscopy

Solvated cobalt(II) ions in neat 1,3-propanediamine (tn) and n-propylamine (pa) have been characterized by electronic absorption spectroscopy and extended X-ray absorption fine structure (EXAFS) spectroscopy. The equilibrium between tetrahedral and octahedral geometry for cobalt(II) ion has been observed in a neat pa solution, but not in neat diamine solutions such as tn and ethylenediamine (en). The thermodynamic parameters and equilibrium constant at 298 K for the geometrical equilibrium in pa were determined to be DeltaH degrees = -36.1 +/- 2.3 kJ mol(-1), DeltaS degrees = -163 +/- 8 J mol(-1) K(-1), and K(298) = 6.0 x 10(-3) M(-2), where K = [Co(pa)(6)(2+)]/{[Co(pa)(4)(2+)][pa](2)}. The equilibrium is caused by the large entropy gain in formation of the tetrahedral cobalt(II) species. The solvent exchange of cobalt(II) ion with octahedral geometry in tn and pa solutions has been studied by the (14)N NMR line-broadening method. The activation parameters and rate constants at 298 K for the solvent exchange reactions are as follows: DeltaH() = 49.3 +/- 0.9 kJ mol(-1), DeltaS() = 25 +/- 3 J mol(-1) K(-1), DeltaV() = 6.6 +/- 0.3 cm(3) mol(-1) at 302.1 K, and k(298) = 2.9 x 10(5) s(-1) for the tn exchange, and DeltaH() = 36.2 +/- 1.2 kJ mol(-1), DeltaS() = 35 +/- 6 J mol(-1) K(-1), and k(298) = 2.0 x 10(8) s(-1) for the pa exchange. By comparison of the activation parameters with those for the en exchange of cobalt(II) ion, it has been confirmed that the kinetic chelate strain effect is attributed to the large activation enthalpy for the bidentate chelate opening and that the enthalpic effect is smaller in the case of the six-membered tn chelate compared with the five-membered en chelate.     

Guest exchange dynamics in an M4L6 tetrahedral host

Guest exchange in an M(4)L(6) supramolecular assembly was previously demonstrated to proceed through a nonrupture mechanism in which guests squeeze through apertures in the host structure and not through larger portals created by partial assembly dissociation. Focusing on the [Ga(4)L(6)](12-) assembly [L = 1,5-bis(2',3'-dihydroxybenzamido)naphthalene], the host-guest kinetic behavior of this supramolecular capsule is defined. Guest self-exchange rates at varied temperatures and pressures were measured to determine activation parameters, revealing negative DeltaS and positive DeltaV values [PEt(4)(+): DeltaH = 74(3) kJ mol(-1), DeltaS = -46(6) J mol(-1) K(-1), k(298) = 0.003 s(-)); NEt(4)(+): DeltaH = 69(2) kJ mol(-1), DeltaS = -52(5) J mol(-1) K(-1), k(298) = 0.009 s(-1); NMe(2)Pr(2)(+): DeltaH = 52(2) kJ mol(-1), DeltaS = -56(7) J mol(-1) K(-1), DeltaV = +13(1) cm(3) mol(-1), k(298) = 4.4 s(-1); NPr(4)(+): DeltaH = 42(1) kJ mol(-1), DeltaS = -102(4) J mol(-1) K(-1), DeltaV = +31(2) cm(3) mol(-1), k(298) = 1.4 s(-1)]. In PEt(4)(+) for NEt(4)(+) exchange reactions, egress of the initial guest (G1) is found to be rate determining, with increasing G1 and G2 (the displacing guest) concentrations inhibiting guest exchange. This inhibition is explained by the decreased flexibility of the host imparted by exterior, or exohedral, guest interactions by both the G1 and G2 guests. Blocking the exohedral host sites with high concentrations of the smaller NMe(4)(+) cation (a weak endohedral guest) enhances PEt(4)(+) for NEt(4)(+) guest exchange rates. Finally, guest displacement reactions also demonstrate the sensitivity of guest exchange to thermodynamic endohedral guest binding affinities. When the initial guest (G1) has a weaker affinity for the host, G2 concentration dependence is observed in addition to dependence on the G2 binding strength.     

Adsorption and thermodynamic characteristics of Hg(II)-SCN complex onto polyurethane foam

The sorption of Hg(II) in the presence of sodium thiocyanate solution onto polyurethane (PUR) foam, an excellent sorbent, has been investigated in detail. Maximum sorption of Hg(II) is achieved from 0.1 M hydrochloric acid solution containing 7.5x10(-2) M sodium thiocyanate in 5 min. The sorption data followed both Freundlich and Langmuir adsorption isotherms. The Freundlich constants 1/n and sorption capacity, C(m), are evaluated to be 0.44+/-0.02 and (3.86+/-0.89)x10(-3) mol g(-1). The saturation capacity and adsorption constant derived from Langmuir isotherm are (6.88+/-0.28)x10(-5) mol g(-1) and (5.6+/-0.37)x10(4) dm(3) mol(-1) respectively. The mean free energy (E) of Hg(II)-SCN sorption onto PUR foam computed from D-R isotherm is 12.4+/-0.3 kJ mol(-1) indicating ion-exchange type mechanism of chemisorption. The variation of sorption with temperature yields thermodynamic parameters of DeltaH=-30.7+/-1.2 kJ mol(-1), DeltaS=-70.1+/-4.1 J mol(-1) K(-1) and DeltaG=-9.86+/-0.77 kJ mol(-1) at 298 K. The negative value of enthalpy and free energy reflects the exothermic and spontaneous nature of sorption. On the basis of the sorption data, sorption mechanism has been proposed.     

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.     

Mechanistic information from low-temperature rapid-scan and NMR measurements on the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex

A detailed kinetic study of the protonation and subsequent reductive elimination reaction of a (diimine)platinum(II) dimethyl complex was undertaken in dichloromethane over the temperature range of -90 to +10 degrees C by stopped-flow techniques. Time-resolved UV-vis monitoring of the reaction allowed the assessment of the effects of acid concentration, coordinating solvent (MeCN) concentration, temperature, and pressure. The second-order rate constant for the protonation step was determined to be 15200 +/- 400 M(-1) s(-1) at -78 degrees C, and the corresponding activation parameters are DeltaH = 15.2 +/- 0.6 kJ mol(-1) and DeltaS = -85 +/- 3 J mol(-1) K(-1), which are in agreement with the addition of a proton that results in the formation of the platinum(IV) hydrido complex. The kinetics of the second, methane-releasing reaction step do not show an acid dependence, and the MeCN concentration also does not significantly affect the reaction rate. The activation parameters for the second reaction step were found to be DeltaH = 75 +/- 1 kJ mol(-1), DeltaS = +38 +/- 5 J mol(-1) K(-1), and DeltaV = +18 +/- 1 cm(3) mol(-1), strongly suggesting a dissociative character of the rate-determining step for the reductive elimination reaction. The spectroscopic and kinetic observations were correlated with NMR data and assisted the elucidation of the underlying reaction mechanism.     

Formation and reactivity of Ir(III) hydroxycarbonyl complexes

Kinetic studies show that the reaction of [TpIr(CO)2] (1, Tp = hydrotris(pyrazolyl)borate) with water to give [TpIr(CO2H)(CO)H] (2) is second order (k = 1.65 x 10(-4) dm(3) mol(-1) s(-1), 25 degrees C, MeCN) with activation parameters DeltaH++= 46+/-2 kJ mol(-1) and DeltaS++ = -162+/-5 J K(-1) mol(-1). A kinetic isotope effect of k(H2O)/k(D2O) = 1.40 at 20 degrees C indicates that O-H/D bond cleavage is involved in the rate-determining step. Despite being more electron rich than 1, [Tp*Ir(CO)2] (1*, Tp* = hydrotris(3,5-dimethylpyrazolyl)borate) reacts rapidly with adventitious water to give [Tp*Ir(CO2H)(CO)H] (2*). A proposed mechanism consistent with the relative reactivity of 1 and 1* involves initial protonation of Ir(I) followed by nucleophilic attack on a carbonyl ligand. An X-ray crystal structure of 2* shows dimer formation via pairwise H-bonding interactions of hydroxycarbonyl ligands (r(O...O) 2.65 A). Complex 2* is thermally stable but (like 2) is amphoteric, undergoing dehydroxylation with acid to give [Tp*Ir(CO)2H]+ (3*) and decarboxylation with OH- to give [TpIr(CO)H2] (4*). Complex 2 undergoes thermal decarboxylation above ca. 50 degrees C to give [TpIr(CO)H2] (4) in a first-order process with activation parameters DeltaH++ = 115+/-4 kJ mol(-1) and DeltaS++ = 60+/-10 J K(-1) mol(-1).     

Mechanistic diversity covering 15 orders of magnitude in rates: cyanide exchange on [M(CN)(4)](2-) (M = Ni, Pd, and Pt)

Kinetic studies of cyanide exchange on [M(CN)(4)](2-) square-planar complexes (M = Pt, Pd, and Ni) were performed as a function of pH by (13)C NMR. The [Pt(CN)(4)](2-) complex has a purely second-order rate law, with CN(-) as acting as the nucleophile, with the following kinetic parameters: (k(2)(Pt,CN))(298) = 11 +/- 1 s(-1) mol(-1) kg, DeltaH(2) (Pt,CN) = 25.1 +/- 1 kJ mol(-1), DeltaS(2) (Pt,CN) = -142 +/- 4 J mol(-1) K(-1), and DeltaV(2) (Pt,CN) = -27 +/- 2 cm(3) mol(-1). The Pd(II) metal center has the same behavior down to pH 6. The kinetic parameters are as follows: (k(2)(Pd,CN))(298) = 82 +/- 2 s(-1) mol(-1) kg, DeltaH(2) (Pd,CN) = 23.5 +/- 1 kJ mol(-1), DeltaS(2) (Pd,CN) = -129 +/- 5 J mol(-1) K(-1), and DeltaV(2) (Pd,CN) = -22 +/- 2 cm(3) mol(-1). At low pH, the tetracyanopalladate is protonated (pK(a)(Pd(4,H)) = 3.0 +/- 0.3) to form [Pd(CN)(3)HCN](-). The rate law of the cyanide exchange on the protonated complex is also purely second order, with (k(2)(PdH,CN))(298) = (4.5 +/- 1.3) x 10(3) s(-1) mol(-1) kg. [Ni(CN)(4)](2-) is involved in various equilibrium reactions, such as the formation of [Ni(CN)(5)](3-), [Ni(CN)(3)HCN](-), and [Ni(CN)(2)(HCN)(2)] complexes. Our (13)C NMR measurements have allowed us to determine that the rate constant leading to the formation of [Ni(CN)(5)](3-) is k(2)(Ni(4),CN) = (2.3 +/- 0.1) x 10(6) s(-1) mol(-1) kg when the following activation parameters are used: DeltaH(2)() (Ni,CN) = 21.6 +/- 1 kJ mol(-1), DeltaS(2) (Ni,CN) = -51 +/- 7 J mol(-1) K(-1), and DeltaV(2) (Ni,CN) = -19 +/- 2 cm(3) mol(-1). The rate constant of the back reaction is k(-2)(Ni(4),CN) = 14 x 10(6) s(-1). The rate law pertaining to [Ni(CN)(2)(HCN)(2)] was found to be second order at pH 3.8, and the value of the rate constant is (k(2)(Ni(4,2H),CN))(298) = (63 +/- 15) x10(6) s(-1) mol(-1) kg when DeltaH(2) (Ni(4,2H),CN) = 47.3 +/- 1 kJ mol(-1), DeltaS(2) (Ni(4,2H),CN) = 63 +/- 3 J mol(-1) K(-1), and DeltaV(2) (Ni(4,2H),CN) = - 6 +/- 1 cm(3) mol(-1). The cyanide-exchange rate constant on [M(CN)(4)](2-) for Pt, Pd, and Ni increases in a 1:7:200 000 ratio. This trend is modified at low pH, and the palladium becomes 400 times more reactive than the platinum because of the formation of [Pd(CN)(3)HCN](-). For all cyanide exchanges on tetracyano complexes (A mechanism) and on their protonated forms (I/I(a) mechanisms), we have always observed a pure second-order rate law: first order for the complex and first order for CN(-). The nucleophilic attack by HCN or solvation by H(2)O is at least nine or six orders of magnitude slower, respectively than is nucleophilic attack by CN(-) for Pt(II), Pd(II), and Ni(II), respectively.     

Complexation of Nickel(II) with Guanosine 5'-Monophosphate and Inosine 5'-Monophosphate: A Potentiometric and Calorimetric Study

The constants (K(s)) and enthalpies (DeltaH(s)) for stacking interactions between purine nucleoside monophosphates were determined by calorimetry; the values thus obtained were guanosine as follows: K(s) = 2.1 +/- 0.3 M(-)(1) and DeltaH(s) = -41.8 +/- 0.8 kJ/mol for adenosine 5'-monophosphate (5'AMP); K(s) = 1.5 +/- 0.3 M(-1) and DeltaH(s) = -42.0 +/- 1.5 kJ/mol for guanosine 5'-monophosphate (5'GMP); and K(s) = 1.0 +/- 0.2 M(-1) and DeltaH(s) = -42.3 +/- 1.1 kJ/mol for inosine 5'-monophosphate (5'IMP). The interaction of nickel(II) with purine nucleoside monophosphates was studied using potentiometric and calorimetric methods, with 0.1 M tetramethylammonium bromide as the background electrolyte, at 25 degrees C. The presence in solution of the complexes [Ni(5'GMP)(2)](2)(-) and [Ni(5'IMP)(2)](2)(-) was observed. The thermodynamic parameters obtained were log K(ML) = 3.04 +/- 0.02, log K(ML2) = 2.33 +/- 0.02, DeltaH(ML) = -18.4 +/- 0.9 kJ/mol and DeltaH(ML2) = -9.0 +/- 1.9 kJ/mol for 5'GMP; and log K(ML) = 2.91 +/- 0.01, log K(ML2) = 1.92 +/- 0.01, DeltaH(ML) = -16.2 +/- 0.9 kJ/mol and DeltaH(ML2) = -0.1 +/- 2.3 kJ/mol for 5'IMP. The relationships between complex enthalpies and the degree of macrochelation, as well as the stacking interaction between purine bases in the complexes are discussed in relation to previously reported calorimetric data.     

The rate of O2 and CO binding to a copper complex, determined by a "flash-and-trap" technique, exceeds that for hemes

The observation and fast time-scale kinetic determination of a primary dioxygen-copper interaction have been studied. The ability to photorelease carbon monoxide from [Cu(I)(tmpa)(CO)](+) in mixtures of CO and O(2) in tetrahydrofuran (THF) between 188 and 218 K results in the observable formation of a copper-superoxide species, [Cu(II)(tmpa)(O(2)(-))](+) lambda(max) = 425 nm. Via this "flash-and-trap" technique, temperature-dependent kinetic studies on the forward reaction between dioxygen and [Cu(I)(tmpa)(thf)](+) afford activation parameters DeltaH = 7.62 kJ/mol and DeltaS = -45.1 J/mol K. The corresponding reverse reaction proceeds with DeltaH = 58.0 kJ/mol and DeltaS = 105 J/mol K. Overall thermodynamic parameters are DeltaH degrees = -48.5 kJ/mol and DeltaS degrees = -140 J/mol K. The temperature-dependent data allowed us to determine the room-temperature second-order rate constant, k(O2) = 1.3 x 10(9) M(-1) s(-1). Comparisons to copper and heme proteins and synthetic complexes are discussed.     

Probing the stereointegrity of Tröger's base--a dynamic electrokinetic chromatographic study

Under acidic conditions the enantiomers of Tr?ger's base 1 (2,8-dimethyl-6 H,12 H-5,11-methanodibenzo[b,f][1,5]diazozine) are subject to enantiomerization. During enantioselective dynamic electrokinetic chromatography using 10 mM hydroxypropyl-beta-cyclodextrin as the chiral mobile phase additive in 50 mM tris/phosphate buffer at pH 2.2, enantiomerization of Tr?ger's base gives rise to characteristic elution profiles featuring plateau formation and peak broadening. Introduction of a permanent positive charge attributed to quaternization in the monobenzylated derivative of Tr?ger's base 2 (5-benzyl-2,8-dimethyl-6 H,12 H-5,11-methanodibenzo[b,f][1,5]diazozinium bromide) decreases the enantiomerization barrier significantly. To determine the rate constants of enantiomerization the experimental chromatograms were evaluated by a direct calculation method and by using the computer simulation program ChromWin. From temperature-dependent measurements the Eyring activation parameters for 1 and 2 were determined: 1: DeltaG( not equal ) (298 K)=100.9+/-0.5 kJ mol(-1), DeltaH( not equal )=89.5+/-2.0 kJ mol(-1), DeltaS( not equal )=-42+/-10 J K(-1) mol(-1); 2: DeltaG( not equal ) (298 K)=90.2+/-0.5 kJ mol(-1), DeltaH( not equal )=91.4+/-2.0 kJ mol(-1), DeltaS( not equal )=9.8+/-10 J K(-1) mol(-1).     

Seven-coordinate iron complex as a ditopic receptor for lithium salts: study of host-guest interactions and substitution behavior

Interactions between the seven-coordinate tweezerlike [Fe(dapsox)(H2O)2]ClO4 complex (H2dapsox = 2,6-diacetylpyridine-bis(semioxamazide)) with different lithium salts (LiOTf, LiClO4, LiBF4, and LiPF6) in CH3CN have been investigated by electrochemical, spectrophotometric, 7Li and 19F NMR, kinetic, and DFT methods. It has been demonstrated that this complex acts as ditopic receptor, showing spectral and electrochemical ion-pair-sensing capability for different lithium salts. In general, the apparent binding constants for lithium salts increase in the order LiOTf < LiClO4 < LiBF4. From the electrochemical measurements, the apparent lithium salt binding constants for the Fe(III) and Fe(II) forms of the complex have been obtained, suggesting a stronger host-guest interaction with the reduced form of the complex. In the presence of LiPF6, the solution chemistry is more complex because of the hydrolysis of PF6-. The kinetics of the complexation of [Fe(dapsox)(CH3CN)2]+ by thiocyanate at -15 degrees C in acetonitrile in the presence of 0.2 M NBu4OTf shows two steps with the following rate constants and activation parameters: k(1) = 411 +/- 14 M(-1) s(-1); DeltaH(1) not equal = 9 +/- 2 kJ mol(-1); DeltaS1 not equal = -159 +/- 6 J K(-1) mol(-1); k(2) = 52 +/- 1 M(-1) s(-1); DeltaH(2) not equal = 4 +/- 1 kJ mol(-1); DeltaS(2) not equal = -195 +/- 3 J K(-1) mol(-1). The very negative DeltaS not equal values are consistent with an associative (A) mechanism. Under the same conditions but with 0.2 M LiOTf, k1Li and k2Li are 1605 +/- 51 and 106 +/- 2 M(-1) s(-1), respectively. The increased rate constants for the {[Fe(dapsox)(CH3CN)2] x LiOTf}+ adduct are in agreement with an associative mechanism. Kinetic and spectrophotometric titration measurements show stronger interaction between the lithium salt and the anion-substituted forms, [Fe(dapsox)(CH3CN)(NCS)] and [Fe(dapsox)(NCS)2]-, of the complex. These experiments demonstrate that in nonaqueous media lithium salts cannot be simply used as supporting electrolytes, since they can affect the kinetic behavior of the studied complex. DFT calculations revealed that the negatively charged alpha-oxyazine oxygen atoms are responsible for cation binding (electrostatic interactions), whereas the two terminal amide groups bind the anion via hydrogen bonding.     

Phosphate triester hydrolysis promoted by an N2S(thiolate)Zn complex: mechanistic implications for the metal-dependent reactivity of peptide deformylase

The zinc(II) complex (PATH)ZnOH, where PATH is an N2S(thiolate) ligand, has been investigated for its ability to promote the hydrolysis of the phosphate triester tris(4-nitrophenyl) phosphate (TNP). The hydrolysis of TNP was examined as a function of PATH-zinc(II) complex concentration, substrate concentration, and pH in a water/ethanol mixture (66:33 v/v) at 25 degrees C. The reaction is first order in both zinc(II) complex and substrate, and the second-order rate constants were derived from linear plots of the observed pseudo-first-order rate constants versus zinc complex concentration at different pH values. A pH-rate profile yielded a kinetic pK(a) of 8.52(5) for the zinc-bound water molecule and a pH-independent rate constant of 16.1(7) M(-1) s(-1). Temperature-dependent studies showed linear Eyring behavior, yielding the activation parameters DeltaH++ = 36.9(1) kJ mol(-1) and DeltaS++ = -106.7(4) J mol(-1) K(-1). Interpretation of the kinetic data leads to the conclusion that hydrolysis of TNP takes place through a hybrid mechanism, in which the metal center plays a dual role of providing a nucleophilic hydroxide and activating the substrate through a Lewis acid effect. The synthesis and structural characterization of the related nickel(II) and iron(II) complexes [(PATH)2Ni2]Br2 (2) and (PATH)2Fe2Cl2 (3) are also described. Taken together, these data suggest a possible explanation for the low reactivity of the zinc(II) form of peptide deformylase as compared to the iron(II) form.     

Equilibria between alpha- and beta-agostic stabilized rotamers of secondary alkyl niobium complexes.

The isopropyl chloro complex Tp(Me2)NbCl(i-Pr)(PhC&tbd1;CMe) (2) [Tp(Me2) = hydrotris(3,5-dimethylpyrazolyl)borate] exhibits a beta-agostic structure in the crystal. The conformation of the alkyl group is such that the agostic methyl group lies in the Calpha-Nb-Cl plane and the nonagostic one, in a wedge formed by two pyrazole rings. As observed by solution NMR spectroscopy, restricted rotation about the Nb-C bond allows the observation of an equilibrium between this species, 2beta, and a minor alpha-agostic rotamer 2alpha. A putative third rotamer which would have the secondary hydrogen in the wedge is not observed. Similar behavior is observed for related Tp'NbCl(i-Pr)(R(2)C=CMe) [Tp' = Tp(Me2), R(2) = Me (3); Tp' = Tp(Me2,4Cl), R(2) = Ph (4)]. The two diastereomers of the sec-butyl complex Tp(Me2)NbCl(sec-Bu)(MeC=CMe) (5) have been separated. In the crystal, 5CR-AS has a beta-agostic methyl group with the ethyl group located in the wedge formed by two pyrazole rings. The same single beta-agostic species is observed in solution. The other diastereomer, 5AR-CS has a beta-agostic methylene group in the solid state, and the methyl group sits in the wedge. In solution, an equilibrium between this beta-agostic methylene complex 5AR-CSbeta and a minor alpha-agostic species 5AR-CSalpha, where the ethyl substituent of the sec-Bu group is located in the wedge between two pyrazole rings, is observed. NMR techniques have provided thermodynamic parameters for these equilibria (K = 2beta/2alpha = 4.0 +/- 0.1 at 193 K, DeltaG(o)(193) = -2.2 +/- 0.1, DeltaH(o) = -7.4 +/- 0.1 kJ mol(-)(1), and DeltaS(o) = -27 +/- 1 J K(-)(1) mol(-)(1)), as well as kinetic parameters for the rotation about the Nb-C bond (at 193 K, DeltaG(2)= 47.5 +/- 2.5, DeltaH= 58.8 +/- 2.5 kJ mol(-)(1), and DeltaS = 59.0 +/- 10 J K(-)(1) mol(-)(1)). Upon selective deuteration of the beta-methyl protons in Tp(Me2)NbCl[CH(CD(3))(2)](PhC=CMe) (2-d(6)), an expected isotope effect that displaces the equilibrium toward the alpha-agostic rotamer is observed (K = 2-d(6)beta/2-d(6)alpha = 3.1 +/- 0.1 at 193 K, DeltaG(o)(193) = -1.8 +/- 0.1, DeltaH(o) = -8.3 +/- 0.4 kJ mol(-)(1) and DeltaS(o)= -34 +/- 2 J K(-)(1) mol(-)(1)). The anomalous values for DeltaH(o) and DeltaS(o) are discussed. Hybrid quantum mechanics/molecular mechanics calculations (IMOMM (B3LYP:MM3)) on the realistic model Tp(Me2)NbCl(i-Pr)(HC=CMe) have reproduced the energy differences between the alpha- and beta-agostic species with remarkable accuracy. Similar calculations show that Tp(Me2)NbCl(CH(2)Me)(HC=CMe) is alpha-agostic only and that Tp(5)(-)(Me)NbCl(CH(2)Me)(HC=CMe), which has no methyl groups at the 3-positions of the pyrazole rings, is beta-agostic only. Analysis and discussion of the computational and experimental data indicate that the unique behavior observed for the secondary alkyl complexes stems from competition between electronic effects favoring a beta-agostic structure and steric effects directing a bulky substituent in the wedge between two pyrazole rings of Tp(Me2). All of the secondary alkyl complexes thermally rearrange to the corresponding linear alkyl complexes via a first-order reaction.     

Effects of protonation of alkyldimethylamine oxides on the dissolution temperature in aqueous media

The effects of protonation (ionization) of hexadecyldimethylamine oxides on the dissolution temperature in aqueous media were investigated by differential scanning calorimetry. Only one endothermic peak was reproducibly observed at all the degrees of ionization alpha examined that were assigned to the transition from the solid (the gel phase) to the solution containing micelles. The dissolution temperature versus alpha curves showed a maximum at alpha=0.5, strongly suggesting the formation of a stable complex of 1-to-1 composition of the nonionic and cationic species through the proposed hydrogen bond. From the shape of the dissolution curve as well as the composition analysis of the solid phase, the solid solution was found to be formed over all alpha values. Effects of alkylchain length on the dissolution temperature for a homologous series of octadecyl- (C18DAO), hexadecyl- (C16DAO), and tetradecyldimethylamine oxide (C14DAO) were also examined for alpha=0.5 and alpha=1. Both the transition temperature and the associated thermodynamic quantities DeltaH and DeltaS increased systematically with the chain length, but for alpha=0.5 smaller increases in DeltaH and DeltaS values with the chain length were observed [DeltaH/CH2 (kJ mol(-1))=7.2+/-0.2 and 2.2+/-0.5 for alpha=1 and alpha=0.5, respectively, and DeltaS/CH2 (J mol(-1) K(-1))=21.9+/-1.8 for alpha=1 and 4.6+/-1.9 for alpha=0.5]. By annealing procedures, the metastable nature of the gel phase was demonstrated for the C16DAO (alpha=1) solid.