Factors influencing C-ON bond homolysis in alkoxyamines: unexpected behavior of SG1 (N-(2-methyl-2-propyl)- N-(1-diethylphosphono-2,2-dimethylpropyl)-N-oxyl)-based alkoxyamines

Alkoxyamines and persistent nitroxides are important regulators of nitroxide-mediated radical polymerization (NMP). Since the polymerization time decreases with the increasing equilibrium constant K (k(d)/k(c)), i.e., the increasing rate constant k(d) of the homolysis of the C-ON bond between the polymer chain and the nitroxide moiety, the factors influencing the cleavage rate constants are of considerable interest. SG1-based alkoxyamines have turned out to be the most potent alkoxyamine family to use for NMP of various monomers. Therefore, it is of high interest to determine the factors which make SG1 derivatives better regulators than TEMPO (2,2,6,6-tetramethylpiperidine-N-oxyl) derivatives. Contrary to what we had observed with TEMPO derivatives, we observed two relationships for the plot E(a) vs BDE(C-H), one for the nonpolar released alkyl radicals (E(a) (kJ/mol) = -133.0 + 0.72BDE) and the other one for the polar released alkyl radicals (E(a) (kJ/mol) = -137.0 + 0.69BDE). However, for both families (SG1 and TEMPO derivatives), the rate constants k(d) of the C-ON bond homolysis were correlated to the cleavage temperature T(c) (log(k(d)(s(-)(1))) = 1.51 - 0.058T(c)). Such correlations should help to design new alkoxyamines to use as regulators and to improve the tuning of NMP experiments.     

Thermal decomposition mechanisms of tert-alkyl peroxypivalates studied by the nitroxide radical trapping technique

The thermolysis of a series of tert-alkyl peroxypivalates 1 in cumene has been investigated by using the nitroxide radical-trapping technique. tert-Alkoxyl radicals generated from the thermolysis underwent the unimolecular reactions, beta-scission, and 1,5-H shift, competing with hydrogen abstraction from cumene. The absolute rate constants for beta-scission of tert-alkoxyl radicals, which vary over 4 orders of magnitude, indicate the vastly different behavior of alkoxyl radicals. However, the radical generation efficiencies of 1 varied only slightly, from 53 (R = Me) to 63% (R = Bu(t)()), supporting a mechanism involving concerted two-bond scission within the solvent cage to generate the tert-butyl radical, CO(2), and an alkoxyl radical. The thermolysis rate constants of tert-alkyl peroxypivalates 1 were influenced by both inductive and steric effects [Taft-Ingold equation, log(rel k(d)) = (0.97 +/- 0. 14)Sigmasigma - (0.31 +/- 0.04)SigmaE(s)(c), was obtained].     

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

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

Kinetic and product study of the gas-phase reactions of OH radicals, NO(3) radicals, and O(3) with (C(2)H(5)O)(2)P(S)CH(3) and (C(2)H(5)O)(3)PS

Rate constants for the reactions of OH radicals and NO3 radicals with O,O-diethyl methylphosphonothioate [(C(2)H(5)O)(2)P(S)CH(3); DEMPT] and O,O,O-triethyl phosphorothioate [(C(2)H(5)O)(3)PS; TEPT] have been measured using relative rate methods at atmospheric pressure of air over the temperature range 296-348 K for the OH radical reactions and at 296 +/- 2 K for the NO(3) radical reactions. At 296 +/- 2 K, the rate constants obtained for the OH radical reactions (in units of 10(-11) cm(3) molecule(-1) s(-1)) were 20.4 +/- 0.8 and 7.92 +/- 0.27 for DEMPT and TEPT, respectively, and those for the NO(3) radical reactions (in units of 10(-15) cm(3) molecule(-1) s(-1)) were 2.01 +/- 0.20 and 1.03 +/- 0.10, respectively. Upper limits to the rate constants for the reactions of O(3) with DEMPT and TEPT of <6 x 10(-20) cm(3) molecule(-1) s(-1) were determined in each case. Rate constants for the OH radical reactions, measured relative to k(OH + alpha-pinene) = 1.21 x 10(-11) e(436/T) cm(3) molecule(-1) s(-1), resulted in the Arrhenius expressions k(OH + DEMPT) = 1.08 x 10(-11) e(871+/-25)/T cm(3) molecule(-1) s(-1) and k(OH + TEPT) = 8.21 x 10(-13) e(1353+/-49)/T cm(3) molecule(-1) s(-1) over the temperature range 296-348 K, where the indicated errors are two least-squares standard deviations and do not include the uncertainties in the reference rate constant. Diethyl methylphosphonate was identified and quantified from the OH radical and NO(3) radical reactions with DEMPT, with formation yields of 21 +/- 4%, independent of temperature, from the OH radical reaction and 62 +/- 11% from the NO(3) radical reaction at 296 +/- 2 K. Similarly, triethyl phosphate was identified and quantified from the OH radical and NO(3) radical reactions with TEPT, with formation yields of 56 +/- 9%, independent of temperature, from the OH radical reaction and 78 +/- 15% from the NO(3) radical reaction at 296 +/- 2 K.     

Kinetic origin of the chelate effect. Base hydrolysis, H-exchange reactivity, and structures of syn,anti-[Co(cyclen)(NH3)2]3+ and syn,anti-[Co(cyclen)(diamine)]3+ ions (diamine = H2N(CH2)2NH2, H2N(CH2)3NH2)

The synthesis of syn,anti-[Co(cyclen)en](ClO4)3 (1(ClO4)3) and syn,anti-[Co(cyclen)tn](ClO4)3 (2(ClO4)3) is reported, as are single-crystal X-ray structures for syn,anti-[Co(cyclen)(NH3)2](ClO4)3 (3(ClO4)3). 3(ClO4)3: orthorhombic, Pnma, a = 17.805(4) A, b = 12.123(3) A, c = 9.493(2) A, alpha = beta = gamma = 90 degrees, Z = 4, R1 = 0.030. 1(ClO4)3: monoclinic, P2(1)/n, a = 8.892(2) A, b = 15.285(3) A, c = 15.466(3) A, alpha = 90 degrees, beta = 91.05(3) degrees, gamma = 90 degrees, Z = 4, R1 = 0.0657. 2Br3: orthorhombic, Pca2(1) a = 14.170(4) A, b = 10.623(3) A, c = 12.362(4) A, alpha = beta = gamma = 90 degrees, Z = 4, R1 = 0.0289. Rate constants for H/D exchange (D2O, I = 1.0 M, NaClO4, 25 degrees C) of the syn and anti NH protons (rate law: kobs = ko + kH[OD-]) and the apical NH, and the NH3 and NH2 protons (rate law: kobs = kH[OD-]) in the 1, 2, and 3 cations are reported. Deprotonation constants (K = [Co(cyclen-H)(diamine)2+]/[Co(cyclen)(diamine)3+][OH-]) were determined for 1 (5.5 +/- 0.5 M-1) and 2 (28 +/- 3 M-1). In alkaline solution 1, 2, and 3 hydrolyze to [Co(cyclen)(OH)2]+ via [Co(cyclen)(amine)OH)]2+ monodentates. Hydrolysis of 3 is two step: kobs(1) = kOH(1)[OH-], kobs(2) = ko + kOH(2)[OH-] (kOH(1) = (2.2 +/- 0.4) x 10(4) M-1 s-1, ko = (5.1 +/- 1.2) x 10(-4) s-1, kOH(2) = 1.0 +/- 0.1 M-1 s-1). Hydrolysis of 2 is biphasic: kobs(1) = k1K[OH-]/(1 + K[OH-] (k1 = 5.0 +/- 0.2 s-1, K = 28 M-1), kobs(2) = k2K2[OH-]/(1 + K2[OH-]) (k2 = 3.5 +/- 1.2 s-1, K2 = 1.2 +/- 0.8 M-1). Hydrolysis of 1 is monophasic: kobs = k1k2KK2[OH-]2/(1 + K[OH-1])(k-1 + k2K2[OH-]) (k1 = 0.035 +/- 0.004 s-1, k-1 = 2.9 +/- 0.6 s-1, K = 5.5 M-1, k2K2 = 4.0 M-1 s-1). The much slower rate of chelate ring-opening in 1, compared to loss of NH3 from 3, is rationalized in terms of a reduced ability of the former system to allow the bond angle expansion required to produce the SN1CB trigonal bipyramidal intermediate.     

Kinetics and Mechanism of the Formation and Acid Dissociation of Cobalt(II), Nickel(II), and Copper(II) Complexes with the Highly Enolized beta-Diketone 3-(N-Acetylamido)pentane-2,4-dione (=Hamac) in Aqueous Solution

The beta-diketone Hamac = 3-(N-acetylamido)pentane-2,4-dione was characterized by potentiometric, spectrophotometric, and kinetic methods. In water, Hamac is very soluble (2.45 M) and strongly enolized, with [enol]/[ketone] = 2.4 +/- 0.1. The pK(a) of Hamac is 7.01 +/- 0.07, and the rate constants for enolization, k(e), and ketonization, k(k), at 298 K are 0.0172 +/- 0.0004 s(-1) and 0.0074 +/- 0.0015 s(-1), respectively. An X-ray structure analysis of the copper(II) complex Cu(amac)(2).toluene (=C(21)H(28)CuN(2)O(6); monoclinic, C2/c; a = 20.434(6), b = 11.674(4), c = 19.278(6) ?; beta = 100.75(1) degrees; Z = 8; R(w) = 0.0596) was carried out. The bidentate anions amac(-) coordinate the copper via the two diketo oxygen atoms to form a slightly distorted planar CuO(4) coordination core. Rapid-scan stopped-flow spectrophotometry was used to study the kinetics of the reaction of divalent metal ions M(2+) (M = Ni,Co,Cu) with Hamac in buffered aqueous solution at variable pH and I = 0.5 M (NaClO(4)) under pseudo-first-order conditions ([M(2+)](0) > [Hamac](0)) to form the mono complex M(amac)(+). For all three metals the reaction is biphasic. The absorbance/time data can be fitted to the sum of two exponentials, which leads to first-order rate constants k(f) (fast initial step) and k(s) (slower second step). The temperature dependence of k(f) and k(s) was measured. It follows from the kinetic data that (i) the keto tautomer of Hamac, HK, does not react with the metal ions M(2+), (ii) the rate constant k(f) increases linearly with [M(2+)](0) according to k(f) = k(0) + k(2)[M(2+)](0), and (iii) the rate constant k(s) does not depend on [M(2+)](0) and describes the enolization of the unreactive keto tautomer HK. The pH dependence of the second-order rate constant k(2) reveals that both the enol tautomer of Hamac, HE, and the enolate, E(-), react with M(2+) in a second-order reaction to form the species M(amac)(+). At 298 K rate constants k(HE) are 18 +/- 6 (Ni), 180 +/- 350 (Co), and (9 +/- 5) x 10(4) (Cu) M(-1) s(-1) and rate constants k(E) are 924 +/- 6 (Ni), (7.4 +/- 0.6) x 10(4) (Co), and (8.4 +/- 0.2) x 10(8) (Cu) M(-1) s(-1). The acid dissociation of the species M(amac)(+) is triphasic. Very rapid protonation (first step) leads to M(Hamac)(2+), which is followed by dissociation of M(Hamac)(2+) and M(amac)(+), respectively (second step). The liberated enol Hamac ketonizes (third step). The mechanistic implications of the metal dependence of rate constants k(HE), k(E), k(-HE), and k(-E) are discussed.     

Organic Free Radical-Promoted Isomerization of alpha- and beta-Alkylcobinamides(1)

Anaerobic reaction of alpha- or beta-alkylcobinamides (alpha- or beta-RCbi(+)'s) with the corresponding alkyl free radical, R(*) (where R = CH(3), CH(3)CH(2), or CH(3)CH(2)OCH(2)CH(2)), generated by the Fenton reaction using Fe(2+) and an alkyl hydroperoxide, RC(CH(3))(2)OOH, causes isomerization and leads to mixtures of alpha- and beta-RCbi(+)'s. The reaction does not occur, however, under aerobic conditions or under anaerobic conditions in the presence of an excess of the free radical scavenger H-Tempo. In addition, alpha-CH(3)CH(2)Cbi(+) reacts with 50 molar equiv of tert-butyl hydroperoxide and Fe(2+) to give a mixture of alkylcobinamides that contains 6% alpha-CH(3)Cbi(+) and 94% beta-CH(3)Cbi(+), showing that multiple transalkylations occur. A Co(II)-induced isomerization and the S(H)2 mechanism are ruled out on the basis of the known reactivity of RCbi(+) and product analysis. A mechanism is proposed which involves a direct oxidative free radical displacement by an R(*) to the metal of RCbi(+) via a dialkylcobalt(IV) corrinoid species. Since the reaction leads to equilibration of the two diastereomers under mild conditions, it can be used to study the equilibria between diastereomeric RCbi(+)'s. Thus, the equilibrium for the diastereomeric ethyl-13-epicobinamides, in which the e propionamide side chain of the corrin ring has been epimerized from the alpha to the beta face of the corrinoid, lies significantly more toward the alpha diastereomer than that for the normal ethylcobinamides. This represents the most direct experimental evidence obtained to date that the corrin ring side chains control the relative steric accessibility of the two faces of the cobalt corrinoids.     

Chiral polyoxotungstates. 1. Stereoselective interaction of amino acids with enantiomers of [Ce(III)(alpha1-P2W17O61)(H2O)x]7-. The structure of DL-[Ce2(H2O)8(P2W17O61)2]14-

The ammonium salt of the 1:1complex (1) of Ce(III) with alpha(1)-[P(2)W(17)O(61)](10)(-) was prepared and characterized by elemental analysis, vibrational and NMR spectroscopy ((31)P, (183)W), cyclic voltammetry, and single-crystal X-ray analysis (P1; a = 15.8523(9) A, b = 17.4382(10) A, c = 29.3322(16) A, alpha = 99.617(1) degrees, beta = 105.450 (1) degrees, gamma = 101.132(1) degrees, V = 7460.9(7) A(3), Z = 2). The anion consists of a centrosymmetric head-to-head dimer, [[Ce(H(2)O)(4)(P(2)W(17)O(61))](2)],(14-) with each 9-coordinate Ce cation linked to four oxygens of one tungstophosphate anion and to one oxygen of the other anion. On the basis of P NMR spectroscopy, a monomer-dimer equilibrium exists in solution with K = 20 +/- 4 M(-1) at 22 degrees C. Addition of chiral amino acids to aqueous solutions of 1 results in splitting of the (31)P NMR signals as a result of diastereomer formation. No such splitting is observed with glycine or DL-proline, or when chiral amino acids are added to the corresponding complex of the achiral alpha(2)-isomer of [P(2)W(17)O(61)](10)(-). From analysis of the (31)P NMR spectra, formation constants of the two diastereomeric adducts of 1 with L-proline are 7.3 +/- 1.3 and 9.8 +/- 1.4 M(-1).     

Water-exchange study revealed unexpected substitution behavior of [(CO)2(NO)Re(H2O)3]2+ in aqueous media

The pH-dependent water-exchange rates of [(CO)2(NO)Re(H2O(cis))2(H2O(trans))]2+ (1) in aqueous media were investigated by means of 17O NMR spectroscopy at 298 K. Because of the low pK(a) value found for 1 (pK(a) = 1.4 +/- 0.3), the water-exchange rate constant k(obs)(H2O(trans/cis)) was analyzed with a two-pathway model in which k(Re)(H2O(trans/cis)) and k(ReOH)(H2O)(trans/cis)) denote the water-exchange rate constants in trans or cis position to the nitrosyl ligand on 1 and on the monohydroxo species [(CO)2(NO)Re(H2O)2(OH)]+ (2), respectively. Whereas the rate constants k(ReOH)(H2O)(trans)) and k(ReOH)(H2O)(cis)) were determined as (4.2 +/- 2) x 10(-3) s(-1) and (5.8 +/- 2) x 10(-4) s(-1), respectively, k(Re)(H2O)(trans)) and k(Re)(H2O)(cis)) were too small to be determined in the presence of the much more reactive species 2. Apart from the water exchange, an unexpectedly fast C identical with 16O --> C identical withO exchange was also observed via NMR and IR spectroscopy. It was found to proceed through 1 and 2, with rate constants k(Re)(CO) and k(ReOH)(CO) of (19 +/- 4) x 10(-3) s(-1) and (4 +/- 3) x 10(-3) s(-1), respectively. On the other hand, N identical with 16O --> N identical with *O exchange was not observed.     

Role of the alkyl fragment of initiating alkoxyamine in nitroxide mediated polymerization of styrene

The role of the alkyl fragment of alkoxyamine initiating the homopolymerization of styrene is experimentally and theoretically studied in the case of 2,2,6,6-tetramethylpiperidine-1-oxyl derivatives. The rate constants of homolysis and recombination are measured for the alkoxyamines under study. The characteristics of polymerization initiated by these compounds are experimentally examined. The kinetics of polymerization is theoretically calculated with allowance for the measured kinetic parameters and the experimentally detected side reaction of hydrogen-atom transfer to the nitroxide radical during the thermolysis of alkoxyamine 2-cyano-2-isopropyl-TEMPO.     

Direct oxidation of L-cysteine by [FeIII(bpy)2(CN)2]+ and [FeIII(bpy)(CN)4]-

The oxidation of L-cysteine by the outer-sphere oxidants [Fe(bpy)2(CN)2]+ and [Fe(bpy)(CN)4]- in anaerobic aqueous solution is highly susceptible to catalysis by trace amounts of copper ions. This copper catalysis is effectively inhibited with the addition of 1.0 mM dipicolinic acid for the reduction of [Fe(bpy)2(CN)2]+ and is completely suppressed with the addition of 5.0 mM EDTA (pH<9.00), 10.0 mM EDTA (9.010.0) for the reduction of [Fe(bpy)(CN)4]-. 1H NMR and UV-vis spectra show that the products of the direct (uncatalyzed) reactions are the corresponding Fe(II) complexes and, when no radical scavengers are present, L-cystine, both being formed quantitatively. The two reactions display mild kinetic inhibition by Fe(II), and the inhibition can be suppressed by the free radical scavenger PBN (N-tert-butyl-alpha-phenylnitrone). At 25 degrees C and micro=0.1 M and under conditions where inhibition by Fe(II) is insignificant, the general rate law is -d[Fe(III)]/dt=k[cysteine]tot[Fe(III)], with k={k2Ka1[H+]2+k3Ka1Ka2[H+]+k4Ka1Ka2Ka3{/}[H+]3+Ka1[H+]2+Ka1Ka2[H+]+Ka1Ka2Ka3}, where Ka1, Ka2, and Ka3 are the successive acid dissociation constants of HSCH2CH(NH3+)CO2H. For [Fe(bpy)2(CN)2]+, the kinetics over the pH range of 3-7.9 yields k2=3.4+/-0.6 M(-1) s(-1) and k3=(1.18+/-0.02)x10(6) M(-1) s(-1) (k4 is insignificant in the fitting). For [Fe(bpy)(CN)4]- over the pH range of 6.1-11.9, the rate constants are k3=(2.13+/-0.08)x10(3) M(-1) s(-1) and k4=(1.01+/-0.06)x10(4) M(-1) s(-1) (k2 is insignificant in the fitting). All three terms in the rate law are assigned to rate-limiting electron-transfer reactions in which various thiolate forms of cysteine are reactive. Applying Marcus theory, the self-exchange rate constant of the *SCH2CH(NH2)CO2-/-SCH2CH(NH2)CO2- redox couple was obtained from the oxidation of L-cysteine by [Fe(bpy)(CN)4]-, with k11=4x10(5) M(-1) s(-1). The self-exchange rate constant of the *SCH2CH(NH3+)CO2-/-SCH2CH(NH3+)CO2- redox couple was similarly obtained from the rates with both Fe(III) oxidants, a value of 6x10(6) M(-1) s(-1) for k11 being derived. Both self-exchange rate constants are quite large as is to be expected from the minimal rearrangement that follows conversion of a thiolate to a thiyl radical, and the somewhat lower self-exchange rate constant for the dianionic form of cysteine is ascribed to electrostatic repulsion.     

Determination of the rate constant and product channels for the radical-radical reaction NCO(X 2Pi) + C2H5(X 2A') at 293 K

The rate constant and product branching ratios for the reaction of the cyanato radical, NCO(X (2)Pi), with the ethyl radical, C(2)H(5)(X (2)A'), have been measured over the pressure range of 0.28 to 0.59 kPa and at a temperature of 293 +/- 2 K. The total rate constant, k(1), increased with pressure, P(kPa), described by k(1) = (1.25 +/- 0.16) x 10(-10) + (4.22 +/- 0.35) x 10(-10)P cm(3) molecule(-1) s(-1). Three product channels were observed that were not pressure dependent: (1a) HNCO + C(2)H(4), k(1a) = (1.1 +/- 0.16) x 10(-10), (1b) HONC + C(2)H(4), k(1b) = (2.9 +/- 1.3) x 10(-11), (1c) HCN + C(2)H(4)O, k(1c) = (8.7 +/- 1.5) x 10(-13), with units cm(3) molecule(-1) s(-1) and uncertainties of one-standard deviation in the scatter of the data. The pressure dependence was attributed to a forth channel, (1d), forming recombination products C(2)H(5)NCO and/or C(2)H(5)OCN, with pressure dependence: (1d) k(1d) = (0.090 +/- 1.3) x 10(-11) + (3.91 +/- 0.27) x 10(-10)P cm(3) molecule(-1) s(-1). The radicals were generated by the 248 nm photolysis of ClNCO in an excess of C(2)H(6). Quantitative infrared time-resolved absorption spectrophotometry was used to follow the temporal dependence of the reactants and the appearance of the products. Five species were monitored, HCl, NCO, HCN, HNCO, and C(2)H(4), providing a detailed picture of the chemistry occurring in the system. Other rate constants were also measured: ClNCO + C(2)H(5), k(10) = (2.3 +/- 1.2) x 10(-13) , NCO + C(2)H(6), k(2) = (1.6 +/- 0.11) x 10(-14), NCO + C(4)H(10), k(4) = (5.3 +/- 0.51) x 10(-13), with units cm(3) molecule(-1) s(-1) and uncertainties of one-standard deviation in the scatter of the data.     

Diastereomeric excess upon cleavage and reformation of diastereomeric alkoxyamines

The thermal reactions of several TEMPO and DEPN (N-(2-methylpropan-2-yl)-N-(1-diethylphosphono-2,2-dimethylpropyl)-aminoxyl) based alkoxyamines were studied by means of (1)H and (31)P NMR spectroscopy aiming to distinguish the contributions of diastereoselective homolysis and coupling to the total equilibrium diastereomeric distribution. The TEMPO-based compounds reveal no diastereomeric excess while DEPN based compounds show a moderate excess both upon homolysis and coupling. The diastereomeric preference of homolysis for DEPN-(propionate-like) adducts does not depend on the ester group and it is mainly affected by the size of beta-substituents. The diastereoselective coupling is sensitive to the total recombination rate constant k(c) and diastereoselectivity increases with the decrease of k(c). Small diastereoselective coupling is found in the recombination of DEPN with sec-butyl isobutyrate radical, where no prochiral centers are formed upon cleavage of corresponding alkoxyamine.     

Equilibrium and Kinetics of Bromine Hydrolysis

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

Kinetic isotope effects in cycloreversion of rhenium (V) diolates

Cycloreversion of 4-methoxystyrene from the corresponding Tp'Re(O)(diolato) complex (Tp' = hydrido-tris-(3,5-dimethylpyrazolyl)borate) was measured competitively for various isotopomers at 103 degrees C. Primary ((12)C/(13)C) and secondary ((1)H/(2)H) kinetic isotope effects were determined. The primary KIEs were k(12C)/k(13C) = 1.041 +/- 0.005 at the alpha position and 1.013 +/- 0.006 at the beta position. Secondary KIEs were k(H)/k(D) = 1.076 +/- 0.005 at the alpha position and 1.017 +/- 0.005 at the beta position. Computational modeling (B3LYP/LACVP+) located a transition state for concerted cycloreversion of styrene from TpRe(O)(OCH(2)CHPh) exhibiting dramatically different C-O bond lengths. A Hammett study on cycloreversions of substituted styrenes from a series of Tp'Re(O)(diolato) showed dichotomous behavior for electron donors and electron-withdrawing groups as substituents: rho = -0.65 for electron donors, but rho = +1.13 for electron-withdrawing groups. The data are considered in light of various mechanistic proposals. While the extrusion of 4-methoxystyrene is concluded to be a highly asynchronous concerted reaction, the Hammett study reflects a likelihood that multiple reaction mechanisms are involved.     

Kinetics and mechanism of ligand substitution in bis(N-alkylsalicylaldiminato)oxovanadium(IV) complexes

Conventional and stopped-flow spectrophotometry was used to to study the kinetics of ligand substitution in a number of bis(N-alkylsalicylaldiminato)oxovanadium(IV) complexes (=VO(R-X-sal)(2)) by 1,1,1- trifluoropentane-2,4-dione (=Htfpd) in acetone, according to the following reaction: VO(R-X-sal)(2) + 2Htfpd --> VO(tfpd)(2) + 2R-X-salH. The acronym R-X-salH refers to N-alkylsalicylaldimines with substituents X = H, Cl, Br, CH(3), and NO(2) in the 5-position of the salicylaldehyde ring and N-alkyl groups R = n-propyl, isopropyl, phenyl, and neopentyl. Under excess conditions ([Htfpd](0) > [VO(R-X-sal)(2)](0)), substitution by Htfpd occurs in two observable steps, as characterized by pseudo-first-order rate constants k(obsd(1)) and k(obsd(2)). Both rate constants increase linearly with [Htfpd](0) according to k(obsd(1)) = k(s(1)) + k(1)[Htfpd](0) and k(obsd(2)) = k(s(2)) + k(2)[Htfpd](0), with k(s(1)) and k(s(2)) describing small contributions of solvent-initiated pathways. Depending on the nature of R and X, second-order rate constants k(1) and k(2) lie in the range 0.098-0.87 M(-1) s(-1) (k(1)) and 0.022-0.41 M(-1) s(-1) (k(2)) at 298 K. For ligand substitution in the system VO(n-propyl-sal)(2)/Htfpd, the activation parameters DeltaH++ = 35.8 +/- 2.8 kJ mol(-1) and DeltaS++ = -146 +/- 23 J K(-1) mol(-1) (k(1)) and DeltaH++ = 40.2 +/- 1.3 kJ mol(-1) and DeltaS++ = -142 +/- 11 J K(-1) mol(-1) (k(2)) were obtained. The Lewis acidity of the complexes VO(n-propyl-X-sal)(2) with X = H, Cl, Br, CH(3), and NO(2) was quantified spectrophotometrically by determination of equilibrium constant K(py), describing the formation of the adduct VO(n-propyl-X-sal)(2).pyridine. The adduct VO(tfpd)(2).n-propyl-salH, formed as product in the system VO(n-propyl-sal)(2)/Htfpd, was characterized by its dissociation constant, K(D) = (3.30 +/- 0.10) x 10(-3) M. The mechanism suggested for the two-step substitution process is based on initial formation of the adducts VO(R-X-sal)(2).Htfpd (step 1) and VO(R-X-sal)(tfpd).Htfpd (step 2).     

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

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

Falloff curves for the Reaction CH3 + O2 (+ M) --> CH3O2 (+ M) in the pressure range 2-1000 bar and the temperature range 300-700 K

The reaction CH(3) + O(2) (+M) --> CH(3)O(2) (+M) was studied in the bath gases Ar and N(2) in a high-temperature/high-pressure flow cell at pressures ranging from 2 to 1000 bar and at temperatures between 300 and 700 K. Methyl radicals were generated by laser flash photolysis of azomethane or acetone. Methylperoxy radicals were monitored by UV absorption at 240 nm. The falloff curves of the rate constants are represented by the simplified expression k/k(infinity) approximately [x/(1 + x)]F(cent)(1/{1+[(log)(x)/)(N)(]2}) with x = k(0)/k(infinity) F(cent) approximately 0.33, and N approximately 1.47, where k(0) and k(infinity) denote the limiting low and high-pressure rate constants, respectively. At low temperatures, 300-400 K, and pressures >300 bar, a fairly abrupt increase of the rate constants beyond the values given by the falloff expressions was observed. This effect is attributed to a contribution from the radical complex mechanism as was also observed in other recombination reactions of larger radicals. Equal limiting low-pressure rate constants k(0) = [M]7 x 10(-31)(T/300 K)(-3.0) cm(6) molecule(-2) s(-1) were fitted for M = Ar and N(2) whereas limiting high-pressure rate constants k(infinity) = 2.2 x 10(-12)(T/300 K)(0.9) cm(3) molecule(-1) s(-1) were approached. These values are discussed in terms of unimolecular rate theory. It is concluded that a theoretical interpretation of the derived rate constants has to be postponed until better information of the potential energy surface is available. Preliminary theoretical evaluation suggests that there is an "anisotropy bottleneck" in the otherwise barrierless interaction potential between CH(3) and O(2).     

Time-resolved measurements of the photolysis and recombination of adenosylcobalamin bound to glutamate mutase

Femtosecond to nanosecond transient absorption spectroscopy is used to investigate the photolysis of 5'-deoxyadenosylcobalamin (coenzyme B12, AdoCbl) bound to glutamate mutase. The photochemistry of AdoCbl is found to be inherently dependent upon the environment of the cofactor. Excitation of AdoCbl bound to glutamate mutase results in formation of a metal-to-ligand charge transfer intermediate state which decays to form cob(II)alamin with a time constant of 105 ps. This observation is in contrast to earlier measurements in water where the photohomolysis proceeds through an intermediate state in which the axial dimethylbenzimidazole ligand appears to have dissociated, and measurements in ethylene glycol where prompt bond homolysis is observed (Yoder, L. M.; Cole, A. G.; Walker, L. A., II; Sension, R. J. J. Phys. Chem. B 2001, 105, 12180-12188). The quantum yield for formation of stable radical pairs in the enzyme is found to be phi = 0.05 +/- 0.03, and the resulting intrinsic rate constants for geminate recombination and "cage escape" are 1.0 +/- 0.1 and 0.05 +/- 0.03 ns(-1), respectively. The rate constant for geminate recombination is 30% less than that observed for AdoCbl in water or ethylene glycol. This reduction is insufficient to account for the 10(12)-fold increase in the homolysis rate observed when substrate is bound to the protein. Finally, the protein provides a cage to prevent diffusive loss of the adenosyl radical; however, the ultimate yield for long-lived radicals is determined by the evolution from a singlet to a triplet radical pair as proposed for AdoCbl in ethylene glycol.     

Linear Trinuclear Pt.Mo-Mo Complexes [Mo(2)PtX(2)(pyphos)(2)(O(2)CR)(2)](2) (X = Cl, Br, I; R = CH(3), C(CH(3))(3); pyphos = 6-(Diphenylphosphino)-2-pyridonate) with an Axial Interaction between the Quadruply Bonded Mo(2) Moiety and the Platinum Atom

An example of a direct axial interaction of a platinum(II) atom with a Mo(2) core through a uniquely designed tridentate ligand 6-(diphenylphosphino)-2-pyridonate (abbreviated as pyphos) is described. Treatment of PtX(2)(pyphosH)(2) (2a, X = Cl; 2b, X = Br; 2c, X = I) with a 1:1 mixture of Mo(2)(O(2)CCH(3))(4) and [Mo(2)(O(2)CCH(3))(2)(NCCH(3))(6)](2+) (3a) in dichloromethane afforded the linear trinuclear complexes [Mo(2)PtX(2)(pyphos)(2)(O(2)CCH(3))(2)](2) (4a, X = Cl; 4b, X = Br; 4c, X = I). The reaction of [Mo(2)(O(2)CCMe(3))(2)(NCCH(3))(4)](2+) (3b) with 2a-c in dichloromethane afforded the corresponding pivalato complexes [Mo(2)PtX(2)(pyphos)(2)(O(2)CCMe(3))(2)](2) (5a, X = Cl; 5b, X = Br; 5c, X = I), whose bonding nature is discussed on the basis of the data from Raman and electronic spectra as well as cyclic voltammograms. The linear trinuclear structures in 4b and 5a-c were confirmed by NMR studies and X-ray analyses: 4b, monoclinic, space group C2/c, a = 34.733(4) ?, b = 17.81(1) ?, c = 22.530(5) ?, beta = 124.444(8) degrees, V = 11498(5) ?(3), Z = 8, R = 0.060 for 8659 reflections with I > 3sigma(I) and 588 parameters; 5a, triclinic, space group P&onemacr;, a = 13.541(3) ?, b = 17.029(3) ?, c = 12.896(3) ?, alpha = 101.20(2) degrees, beta = 117.00(1) degrees, gamma = 85.47(2) degrees, V = 2599(1) ?(3), Z = 2, R = 0.050 for 8148 reflections with I > 3sigma(I) and 604 parameters; 5b, triclinic, space group P&onemacr;, a = 12.211(2) ?, b = 20.859(3) ?, c = 10.478(2) ?, alpha = 98.88(1) degrees, beta = 112.55(2) degrees, gamma = 84.56(1) degrees, V = 2433.3(8) ?(3), Z = 2, R = 0.042 for 8935 reflections with I > 3sigma(I) and 560 parameters; 5c, monoclinic, space group P2(1)/n, a = 13.359(4) ?, b = 19.686(3) ?, c = 20.392(4) ?, beta = 107.92(2) degrees, V = 5101(2) ?(3), Z = 4, R = 0.039 for 8432 reflections with I > 3sigma(I) and 560 parameters.