Heats of reaction of HMo(CO)3(C5R5) (R = H, CH3) with diphenyldisulfide and of formation of the clusters [PhSMo(CO)x(C5H5)]2, X = 1,2. Thermodynamic study of molybdenum-sulfur bond strengths

The enthalpies of reaction of HMo(CO)₃C₅R₅ (R = H, CH₃) with diphenyldisulfide producing PhSMo(CO)₃C₅R₅ and PhSH have been measured in toluene and THF solution (R = H, ΔH= −8.5 ± 0.5 kcal mol⁻¹ (tol), −10.8 ± 0.7 kcal mol⁻¹ (THF); R = CH₃, ΔH = −11.3±0.3 kcal mol⁻¹ (tol), −13.2±0.7 kcal mol⁻¹ (THF)). These data are used to estimate the Mo---SPh bond strength to be on the order of 38–41 kcal mol⁻¹ for these complexes. The increased exothermicity of oxidative addition of disulfide in THF versus toluene is attributed to hydrogen bonding between thiophenol produced in the reaction and THF. This was confirmed by measurement of the heat of solution of thiophenol in toluene and THF. Differential scanning calorimetry as well as high temperature calorimetry have been performed on the dimerization and subsequent decarbonylation reactions of PhSMo(CO)₃Cp yielding [PhSMo(CO)₂Cp]₂ and [PhSMo(CO)Cp]₂. The enthalpies of reaction of PhSMo(CO)₃Cp and [PhSMo(CO)₂Cp]₂ with PPh₃, PPh₂Me and P(OMe)₃ have also been measured. The disproportionation reaction: 2[PhSMo(CO)₂Cp]₂ → 2PhSMo(CO)₃Cp + [PhSMP(CO)Cp]₂ is reported and its enthalpy has also been measured. These data allow determination of the enthalpy of formation of the metal-sulfur clusters [PhSMo(CO)_nC₅H₅]₂, N = 1,2.     

Nickel(II) and copper(II) complexes of mixed benzene-triazolehemiporphyrazines

The kinetics of substitution reactions of [η-CpFe(CO)₃]PF₆ with PPh₃ in the presence of R-PyOs have been studied. For all the R-PyOs (R = 4-OMe, 4-Me, 3,4-(CH)₄, 4-Ph, 3-Me, 2,3-(CH)₄, 2,6-Me₂, 2-Me), the reactions yeild the same product [η⁵-CpFe(CO)₂PPh₃]PF₆, according to a second-order rate law that is first order in concentrations of [η⁵-CpFe(CO)₃]PF₆ and of R-PyO but zero order in PPh₃ concentration. These results, along with the dependence of the reaction rate on the nature of R-PyO, are consistent with an associative mechanism. Activation parameters further support the bimmolecular nature of the reactions: ΔH^≠ = 13.4 ± 0.4 kcal mol⁻¹, ΔS^≠ = −19.1 ± 1.3 cal k⁻¹ mol⁻¹ for 4-PhPyO; ΔH^≠ = 12.3 ± 0.3 kcal mol⁻¹, ΔS^≠ = 24.7 ±1.0 cal K⁻¹ mol⁻¹ for 2-MePyO. For the various substituted pyridine N-oxides studied in this paper, the rates of reaction increase with the increasing electron-donating abilities of the substituents on the pyridine ring or N-oxide basicities, but decrease with increasing ¹⁷O chemical shifts of the N-oxides. Electronic and steric factors contributing to the reactivity of pyridine N-oxides have been quantitatively assessed.     

Ricochet inter-ring haptotropic rearrangement of σ-methyl-(η-indenyl) chromium tricarbonyls. Experimental kinetic and theoretical DFT study

σ-Methyl-(η⁵-indenyl) chromium tricarbonyl (III) rearranges quantitatively into η⁶-1-endo-methylindene) chromium tricarbonyl (IV) in C₆D₆ solution at 30–60°C. Methyl group attachment to the positions 2 or 3 of indenyl ligand in (III) has no influence on the activation parameters of this ricochet inter-ring haptotropic rearrangement (ΔG^#=23.6 kcal mol⁻¹; ΔH^#=18.9±0.2 kcal mol⁻¹; ΔS^#=−18.6±0.2 cal K⁻¹ mol⁻¹). (IV) undergoes further irreversible isomerization at 60–120° into (ν⁶-3-methylindene) chromium tricarbonyl (V) with a higher activation barrier (ΔG^#=28.5±0.1 kcal mol⁻¹) via two consecutive [1,5]-sigmatropic hydrogen shifts. The mechanisms of both rearrangements have been studied in detail using density functional theory (DFT) calculations with extended basis sets. Calculations show that the rearrangement (III) → (IV) proceeds in two steps. Methyl group migration from chromium into position 1 of the indenyl ligand is the rate-determining step leading to the formation of the 16-electron intermediate (VII). The calculated activation barrier (E_a=19.6 kcal mol⁻¹) is in good agreement with the experimental one. Further rearrangement (VII) → (V) proceeds via a trimethylenemethane-type transition state (XVIII) with an activation barrier 11.8 kcal mol⁻¹. The coordination of the chromium tricarbonyl group at the six-membered ring has only minor influence on the kinetic parameters of the hydrogen [1,5]-sigmatropic shift in indene.     

Rate constants and activation parameters for organo-cobalt porphyrin bond homolysis from NMR relaxation times

¹H NMR line broadening is found to be an effective complimentary method to chemical trapping for determining the rates and activation parameters for organo-metal bond homolysis events that produce freely diffusing radicals. Application of this method is illustrated by measurement of bond homolysis activation parameters for a series of organo-cobalt porphyrin complexes ((TPP)Co-C(CH₃)₂CN (ΔH^≠ = 19.5±0.9 kcal mol⁻¹, ΔS^≠ = 12±3 cal°K⁻¹ mol⁻¹), (TMP)Co-C(CH₃)₂CN (ΔH^≠ = 20±1 kcal mol⁻¹,ΔS^≠ = 13±2 cal°K⁻¹ mol⁻¹), (TAP)Co-C(CH₃)₂CO₂CH₃ (ΔH^≠ = 18.2±0.5 kcal mol⁻¹, ΔS^≠ = 12±2 cal °K⁻¹ mol⁻¹), (TAP)Co-CH(CH₃)C₆H₅ (ΔH^≠ = 22.5±0.5, ΔS^≠ = 17±2 cal °K⁻¹ mol⁻¹)). The line broadening method is particularly useful in determining activation parameters for dissociation of weakly bonded organometallics where the rate of homolysis can exceed the range measurable by conventional chemical trapping methods.     

Kinetics and mechanism of the stoichiometric oxygenation of [Cu(fla)(idpa)]ClO4 [fla=flavonolate, IDPA=3,3′-imino-bis(N,N-dimethylpropylamine)] and the [Cu(fla)(idpa)]ClO4-catalysed oxygenation of flavonol

Oxygenation of [Cu^II(fla)(idpa)]ClO₄ (fla=flavonolate; IDPA=3,3′-iminobis(N,N-dimethylpropylamine)) in dimethylformamide gives [Cu^II(idpa)(O-bs)]ClO₄ (O-bs=O-benzoylsalicylate) and CO. The oxygenolysis of [Cu^II(fla)(idpa)]ClO₄ in DMF was followed by electronic spectroscopy and the rate law −d[{Cu^II(fla)(idpa)}ClO₄]/dt=k_obs[{Cu^II(fla)(idpa)}ClO₄][O₂] was obtained. The rate constant, activation enthalpy and entropy at 373 K are k_obs=6.13±0.16×10⁻³ M⁻¹ s⁻¹, ΔH^‡=64±5 kJ mol⁻¹, ΔS^‡=−120±13 J mol⁻¹ K⁻¹, respectively. The reaction fits a Hammett linear free energy relationship and a higher electron density on copper gives faster oxygenation rates. The complex [Cu^II(fla)(idpa)]ClO₄ has also been found to be a selective catalyst for the oxygenation of flavonol to the corresponding O-benzoylsalicylic acid and CO. The kinetics of the oxygenolysis in DMF was followed by electronic spectroscopy and the following rate law was obtained: −d[flaH]/dt=k_obs[{Cu^II(fla)(idpa)}ClO₄][O₂]. The rate constant, activation enthalpy and entropy at 403 K are k_obs=4.22±0.15×10⁻² M⁻¹ s⁻¹, ΔH^‡=71±6 kJ mol⁻¹, ΔS^‡=−97±15 J mol⁻¹ K⁻¹, respectively.     

The vibrational spectrum of TpRhH2(H2): a computational and inelastic neutron scattering study

We report extensive density functional theory studies of the structures and vibrational frequencies of Tp^3,5-MeRhH₂(H₂) in its ground and various transition states as well as the first direct comparison of observed and calculated inelastic neutron scattering (INS) vibrational spectra on this type of compound. Geometry optimizations produced canted η²-dihydrogen dihydride local minima of C₁ symmetry; with HH distances for the C₁ minimum energy structure of 0.842 and 0.898 Å and barriers to rotation of 0.34 and 0.50 kcal mol⁻¹, respectively for B3LYP/BS1 and BP86/BS1 calculations of Tp^3,5-MeRhH₂(H₂). The latter results from one transition state rotated approximately 60° away (a second lower energy transition state which is a few hundreds of a kcal mol⁻¹ above the C₁ MIN is rotated approximately 30° away). With these calculated d(HH) values for the C₁ MIN the previously reported experimental data on the rotation of the dihydrogen ligand yields an experimental barrier to rotation of 1 kcal mol⁻¹ and places the torsional transition at 200 cm⁻¹ in the INS spectrum. Optimization of the Rh structure, that is analogous to the related Ir(V) C_s minimum found for TpIrH₄, generates a high-energy (>4 cal mol⁻¹) C_s transition state TpRh^IIIH₄ structure with an η³-H₃ ⁻ ligand. This transition state (C_s TSE) exchanges the hydrogen in the mirror plane between two chiral C₁ MIN structures. Comparisons between observed and computed INS spectra suggests that the experimental INS spectrum be viewed as resulting from a quantum-averaged ground state encompassing at least two of the low energy structures found in our calculations.     

Reaction heat measurements in the enzymatic hydrolysis reaction of penicillin GK to obtain 6-APA

The purpose of this study was to measure heats involved in the hydrolysis process for the industrial production of 6-aminopenicillanic acid (6-APA) using dynamic calorimetry techniques. The experimental design was planned using Hess' law. The information derived from the calorimeter was correlated mathematically to determine the heat released during enzymatic hydrolysis. This is important for temperature control systems and reactor design. The results obtained with the calorimetric measurements at 308 K and pH 7.5 are the penicillin hydrolysis, ΔH_hydrol, at 35.9 ± 5.7 kJ mol⁻¹ and phenyl acetic acid neutralization, ΔH_neut, at −47.1 ± 3.8 kJ mol⁻¹.     

Model reactions of a carbonylation catalyst: phosphite induced migratory CO insertion in [MeIr(CO)2I3]

Carbonylation of the anionic iridium(III) methyl complex, [MeIr(CO)₂I₃]⁻ (1) is an important step in the new iridium-based process for acetic acid manufacture. A model study of the migratory insertion reactions of 1 with P-donor ligands is reported. Complex 1 reacts with phosphites to give neutral acetyl complexes, [Ir(COMe)(CO)I₂L₂] (L = P(OPh)₃ (2), P(OMe)₃ (3)). Complex 2 has been isolated and fully characterised from the reaction of Ph₄As[MeIr(CO)₂I₃] with AgBF₄ and P(OPh)₃; comparison of spectroscopic properties suggests an analogous formulation for 3. IR and ³¹P NMR spectroscopy indicate initial formation of unstable isomers of 2 which isomerise to the thermodynamic product with trans phosphite ligands. Kinetic measurements for the reactions of 1 with phosphites in CH₂Cl₂ show first order dependence on [1], only when the reactions are carried out in the presence of excess iodide. The rates exhibit a saturation dependence on [L] and are inhibited by iodide. The reactions are accelerated by addition of alcohols (e.g. 18× enhancement for L = P (OMe)₃ in 1:3 MeOH-CH₂Cl₂). A reaction mechanism is proposed which involves substitution of an iodide ligand by phosphite, prior to migratory CO insertion. The observed rate constants fit well to a rate law derived from this mechanism. Analysis of the kinetic data shows that k₁, the rate constant for iodide dissociation, is independent of L, but is increased by a factor of 18 on adding 25% MeOH to CH₂Cl₂. Activation parameters for the k₁ step are ΔH^≠ = 71 (±3) kJ mol⁻, ΔS^≠ = −81 (±9) J mol⁻¹ K⁻¹ in CH₂Cl₂ and ΔH^≠ = 60(±4) kJ mol⁻¹, ΔS^≠ = −93(± 12) J mol⁻¹ K⁻¹ in 1:3 MeOH-CH₂Cl₂. Solvent assistance of the iodide dissociation step gives the observed rate enhancement in protic solvents. The mechanism is similar to that proposed for the carbonylation of 1.     

Proton nuclear magnetic resonance study of N,N-diethylformamide exchange on (N,N-diethylformamide)2,2′2″-tris(dimethylamino)triethylamine)cobalt(II) and its copper(II) analogue

Proton NMR studies of N,N-diethylformamide (def) exchange on [M(Me₆tren)def]²⁺ where M = Co and Cu yield: k_ex (298.2K) = 26.3 ± 2.2, 980 ± 70 s⁻¹; ΔH^≠ = 58.3 ± 1.7, 36.3 ± 0.9 kJ mol⁻¹; ΔS^≠= −22.2 ± 4.6, −65.9 ± 2.5 J K⁻¹ mol⁻¹; and ΔV^≠ = −1.3 ± 0.2, 5.3 ± 0.3 cm³ mol⁻¹ respectively. These data which are consistent with a and d activation modes operating when M = Co and Cu respectively are compared with data for related systems.     

3nion-independent conformational ordering in iota-carrageenan: Disorder-order equilibria and dynamics

The equilibria and dynamics of the disorder-to-order transition of the anionic polysaccharide iota-carrageenan have been studied in the presence of tetramethyl-ammonium salts. By the use of a stopped-flow polarimeter, the rate equation and temperature dependence of the observed forward rate-constant were found to accord with a co-operative dimerisation process. Activation parameters for helix nucleation were shown to be independent of the anion for solutions containing tetramethylammonium chloride and bromide, i.e., ΔH^‡ = 1 ±3 kJ.mol⁻¹, ΔS^‡ = −178 ±10 J.mol⁻¹.K⁻¹, ΔG^‡_298K = 54 ±2 kJ.mol⁻¹, and k_nuc,298K = 1880 ±80 dm³.mol⁻¹.s⁻¹. The temperature dependence of optical rotation was also shown to be independent of the anion present.     

Fuscin, an inhibitor of mitochondrial SH-dependent transport-linked functions

1. 1. Fuscin, a mould metabolite, is a colored quinonoid compound which reacts readily with −SH groups to give colorless addition derivatives.

2. 2. Binding of fuscin to mitochondria has been monitored spectrophotometrically. Fuscin binding is prevented by −SH reagents such as N-ehylmaleimide, N-Methylmaleimide, mersalyl or p-chloromercuribenzoate. Conversely, fuscin prevents the binding of −SH reagents as shown with N-[¹⁴C]ethylmaleimide. Once bound to mitochondria, fuscin is not removable by washing of mitochondria.

3. 3. High affinity-fuscin binding sites (K_d = 1 μM, N = 4–8 nmoles/mg protein) are present in whole mitochondria obtained from rat heart, rat liver, pigeon heart or yeast (Candida utilis). They are lost upon sonication but are still present in digitonin inner membrane + matrix vesicles. On the other hand, lysis of mitochondria by Triton X-100 does not increase the number of high affinity binding sites indicating that all these sites are accessible to fuscin in whole mitochondria. The number of fuscin high affinity sites appears to correlate with the glutathione content of mitochondrial preparations.

4. 4. Fuscin as well as N-ethylmaleimide and avenaciolide are penetrant SH-reagents;

5. 5. Fuscin interferes with the ADP-stimulated respiration of mitochondria on NAD-linked substrates, several functions of the mitochondrial respiratory apparatus being inhibited by fuscin in a non-competitive manner, but to various extents: (a) The electron transfer chain (K_i in the range of 0.1 mM); (b) the lipoamide dehydrogenase system (K_i = 5–10 μM); (c) the transport systems of phosphate (K_i ≈ 20 μM) and of glutamate (K_i = 3–5 μM); (d) the ADP transport, indirectly (K_i ≈ 10 μM).

6. 6. Like N-ethylmaleimide, fuscin inhibits the glutamate-OH⁻ carrier, the inhibition of that carrier bringing about an apparent increase of aspartate entry in glutamate-loaded mitochondria by the glutamate-aspartate carrier.

7. 7. The inhibition of phosphate transport by fuscin probably accounts for the inhibition of the reduction of endogenous NAD by succinate in intact pigeon heart mitochondria.

8. 8. By binding the −SH groups of mitochondrial membrane specifically unmasked by addition of micromolar amounts of ADP, fuscin, like N-ethylmaleimide, prevents the functioning of ADP translocation.

9. 9. Because of their specific and analogous effects on some well defined mitochondrial functions such as glutamate transport and ADP transport, fuscin and N-ethylmaleimide can be distinguished from other −SH reagents. The lipophilic nature of fuscin and N-ethylmaleimide which accounts for the accessbility of these compounds to hydrophobic sites in the mitochondrial membrane or on the matrix side of this membrane may be partly responsible for their characteristic inhibitory effects on mitochondrial functions.

Dihydroxamic acid complexes of iron (III): ligand pKa and coordinated water hydrolysis constants

A series of dihydroxamic acid ligands of the formula [RN(OH)C(O)]₂(CH₂)_n, (n = 2, 4, 6, 7, 8; R = CH₃, H) has been studied in 2.0 M aqueous sodium perchlorate at 25.0 °C. These ligands may be considered as synthetic analogs to the siderophore rhodotorulic acid. Acid dissociation constants (pK_a) have been determined for the ligands and for N-methylacetohydroxamic acid (NMHA). The pK_a1 and pK_a2 values are: n = 2, R = CH₃ (8.72, 9.37); N = 4, R = CH₃ (8.79, 9.37); N = 6, R = CH₃; N = 7, R = CH₃ (8.95, 9.47); N = 8, R = CH₃ (8.93, 9.45); N = 8, R = H (9.05, 9.58). Equilibrium constants for the hydrolysis of coordinated water (log K) have been estimated for the 1:1 feeric complexes of the ligands n = 2, 4, 8; R = CH₃. The N = 8 ligand forms a monomeric complex with Fe(III) while the n = 2 and 4 ligands form dimeric complexes. For hydrolysis of the n = 8 monomeric complex, log K₁ = −6.36 and log K₂ = −9.84. Analysis of the spectrophotometric data for the dimeric complexes indicates deprotonation of all four coordinated waters. The successive hydrolysis constants, log K_1–4, for the dimeric complexes are as follows: n = 2 (−6.37, −5.77, −10.73, −11.8); n = 4 (−5.54, −5.07, −11.57, −10.17). The log K₂ values for the dimers are unexpectedly high, higher in fact than log K₁, inconsistent with the formation of simple ternary hydroxo complexes. A scheme is proposed for the hydrolysis of the ferric dihydroxamate dimers, which includes the possible formation of μ-hydroxo and μ-oxo bridges.     

Analysis of equilibrium dissociation and unfolding in denaturants of soybean agglutinin and two of its derivatives

The equilibrium denaturation of tetrameric soybean agglutinin (SBA) in urea and guanidine hydrochloride (GdnHCl) has been examined by steady-state fluorescence and size-exclusion chromatography. The denaturation of SBA reveals two distinct and separable transitions: dissociation (native tetramer↔tertiary monomer) and unfolding (tertiary monomer↔unfolded monomer). The urea denaturation curves of N-dimethyl and acetyl derivatives of SBA are also similar to unmodified lectin but the midpoints, [D]_1/2, are shifted to lower denaturant concentrations. The free energy of stabilization of tertiary structure (ΔG_u,aq) of SBA is estimated to be 4.5–4.6 kcal mol⁻¹, which shows a decrease by 10–15% for both N-dimethyl SBA and acetyl-SBA. The free energy term (ΔG_{d, aq}) for the relative stability of the quaternary structure of SBA and its derivatives shows that the decrease in stability relative to SBA occurs by <10% for N-dimethyl SBA while for acetyl-SBA, this occurs by 30%. However, the m values depicting the dependence of free energy on denaturant concentration for SBA and its derivatives are similar for dissociation as well as unfolding, which suggest similar denaturation pathways of unmodified and modified SBA.     

The migratory insertion of carbon monoxide in pentamethylcyclopentadienyliridium (III) complexes. Structural effects on reactivity and mechanism for rhodium and iridium systems

The reactions of complex (C₅Me₅)Ir(Cl) (CO) (Me) (1a) with cyclohexylisocyanide and phosphines (L=CyNC, PHPh₂, PMePh₂, PMe₂Ph) give the products of alkyl migratory insertion (C₅Me₅Ir(Cl) (COMe) (L), in toluence or tetrahydrofuran at 323 K or higher temperature. The phenyl analogue (C₅Me₅)Ir(Cl)(CO)(Ph) or the iodide complexes (C₅Me₅)Ir(I) (CO) (R) (R=Me, Ph_are not reactive under the same conditions. The reaction of (C₅Me₅)Ir(Cl)(CO)(Me) with PMePh₂ and PMe₂Ph in acetonitrile yields the chloride substitution product [(C₅Me₅)Ir(CO)(L)(Me)]⁺Cl⁻. Kinetic measurements for the reactions of (C₅Me₅)Ir(Cl)(CO)(Me) in toluene are first order in the iridium complex and exhibit a saturation dependence on the incoming donors L. Analysis of the data suggests a two-step process involving (i) rapid formation of a molecular complex [(C₅Me₅)Ir(Cl)(CO)(Me), (L)], in which the structure of 1a is unperturbed within the limits of spectroscopic analysis, and (ii) rate determining methyl migration. The reaction parameters are K for the pre-equilibrium step (K = 1.5 (CyNC), 7.3 (PHPh₂), 7.1 (PMePh₂) dm³ mol⁻¹ at 323 K) and k₂ for the slow carbon---carbon bond formation (k₂ (10⁵) = 6.9 (CyNC), 1.2 (PHPh₂), 1.0 (PMePh₂) s⁻¹ at 323 K). The activation parameters for the methyl migration step in the reaction with PMePh₂ obtained between 308 and 338 K, are ΔH^≠ = 106±16 kJ mol⁻¹ and ΔS^≠ = − 14±5 J K⁻¹ mol⁻¹. The reaction of 1a with PMePh₂ proceeds at similar rates in tetrahydrofuran (K = 3.7 dm³ mol⁻¹, k₂ (10⁵) = 1.2 s⁻¹, 323 K). The crystal structure of (C₅Me₅)Ir(Cl)(COMe) (PMe₂Ph) has been determined by X-ray diffraction. C₂₀H₂₉ClOPIr: M_r = 544.1, monoclinic, P2₁/n, A = 8.084 (2), B = 9.030(2), C = 28.715 (3) Å, β = 91.41 (3)°, Z = 4, D_c = 1.71 g cm⁻³, V = 2095.5 ų, room temperatyre, Mo K, γ = 0.71069, μ = 65.55 cm⁻¹, F(000) = 1044, R = 0.037 for 2453 independent observed reflections. The complex shows a deformed tetrahedral coordination assuming the η⁵-C₅Me₅ molecular fragment as a single coordination site. The iridium-chlorine bond is staggered with respect to two adjacent C(ring)-methyl bonds, while the Ir---P and the Ir---COMe bonds are eclipsed with respect to C(ring)-methyl bonds.     

A series of heteroleptic complexes of the type fac-[MnL2] [H2L=derivatives of N-(2-hydroxybenzyl)glycine or N-(5-nitro-2-hydroxybenzyl)sarcosine] possessing unusual Mn(III) co-ordination spheres

The mononuclear manganese(III) complexes [C₅H₁₀NH₂][MnL₂] [L²⁻=a substituted N-(2-hydroxybenzyl)glycinate (hbg²⁻) viz. 3,5-dibromo- (3,5-Br-hbg²⁻), 3,5-dichloro- (3,5-Cl-hbg²⁻), 3-methyl-5-chloro- (3,5-Me,Cl-hbg²⁻), 5-bromo- (5-Br-hbg²⁻), 5-chloro- (5-Cl-hbg²⁻), 5-nitro- (5-NO₂-hbg²⁻) or N-(5-nitro-2-hydroxybenzyl)sarcosine (5-NO₂-hbs²⁻)] have been synthesised by reaction of the appropriate ligand with manganese(II) perchlorate under ambient conditions in a 2:1 molar ratio using piperidine as base. The structures of three of these complexes, [C₅H₁₀NH₂][Mn(3,5-Cl-hbg)₂] (2), [C₅H₁₀NH₂][Mn(5-NO₂-hbg)₂] (6) and [C₅H₁₀NH₂][Mn(5-NO₂-hbs)₂] (7) have been elucidated by single-crystal X-ray crystallography and each displays two similar, independent [MnL₂]⁻ ions in the asymmetric unit linked via piperidinium cations through hydrogen bonding. The ligands co-ordinate in a facial tridentate fashion with the three donor atoms being the phenolate and carboxylate oxygens and the amine nitrogen. The geometry at the Mn centres is compressed rhombic octahedral consistent with a pseudo-Jahn–Teller compression along the Mn–O(phenolate) axis. Mean bond lengths are in the ranges 1.886–1.889 Å for the Mn–O(phenolate), 2.062–2.125 Å for the Mn–O(carboxylate) and 2.091–2.184 Å for the Mn–N(amine) distances. The magnetic susceptibility and electronic and IR spectroscopic data are discussed with reference to the crystal structures.     

Biochemical and biophysical studies on cytochrome aa3 VIII. Effect of cyanide on the catalytic activity

1. 1. Cyanide inhibits the catalytic activity of cytochrome aa₃ in both polarographic and spectrophotometric assay systems with an apparent velocity constant of 4·10³ M⁻¹·s⁻¹ and a K_i that varies from 0.1 to 1.0 μM at 22 °C, pH 7·3.

2. 2. When cyanide is added to the ascorbate-cytochrome c-cytochromeaa₃−O₂ system a biphasic reduction of cytochrome c occurs corresponding to an initial K_i of 0.8 μM and a final K_i of about 0.1 μM for the cytochrome aa₃−cyanide reaction.

3. 3. The inhibited species (a²+a₃³⁺HCN) is formed when a²⁺a₃³⁺ reacts with HCN, when a²⁺a₃²⁺HCN reacts with oxygen, or when a³⁺a₃³⁺HCN (cyano-cytochrome aa₃) is reduced. Cyanide dissociates from a²⁺a₃³⁺HCN at a rate of 2·10⁻³ s⁻¹ at 22 °C, pH 7.3.

4. 4. The results are interpreted in terms of a scheme in which one mole of cyanide binds more tightly and more rapidly to a²⁺a₃³⁺ than to a³⁺a₃³⁺.

The kinetics and energetics of formation of a molecular hydrogen complex involving a dinuclear ruthenium(II) centre

The reversible equilibrium conversion under H₂ of [RuCl(dppb) (μ-Cl)]₂ (1) to generate (η²-H₂) (dppb) (μ-Cl)₃RuCl(dppb) in CH₂Cl₂ (dppb = Ph₂P(CH₂)₄PPh₂) has been studied at 0–25 °C by UV-Vis and ³¹P{¹H} NMR spectroscopy, and by stoppe kinetics; the equilibrium constant and corresponding thermodynamic parameters, and the forward and reverse rate constants at 25 °C have been determined. A measured ΔH° value of 0 kJ mol⁻¹ allows for an estimation of an exothermicity of 60 kJ mol⁻¹ for binding an η²-H₂ at an Ru(II) centre; a ΔS° value of 60 J mol⁻¹ K⁻¹ indicates that in solution 1 contain s coordinated CH₂Cl₂. The kinetic and thermodynamic data are compared to those obtained from a previously studied hydrogenation of styrene catalyzed by 1. Preliminary findings on related systems containing Ph₂P(CH₂)₃PPh₂ and (C₆H₁₁)₂P(C₆H₁₁)₂ are also noted.     

Experimental and theoretical study of a truly functional biomimetic molybdenum oxotransferase analogue system

Density functional theory (DFT) computations at the B3LYP/Lanl2DZ level were used to elucidate the oxygen atom transfer (OAT) and coupled electron proton transfer (CEPT) reaction steps involved in the biomimetic catalytic cycle performed by polymer-supported Mo^VIO₂(NN′)₂ complexes [NN′ = phenyl-(pyrrolato-2-ylmethylene)-amine] with water as oxygen source, trimethyl-phosphane as oxygen acceptor and one-electron oxidising agents. The DFT method employed has been validated against experimental data [X-ray crystal structures of a NN′ ligand and a Mo^VIO₂(NN′)₂ complex as well as kinetic data]. The rate-limiting step in the forward-OAT from [Mo^VIO₂] to PMe₃ is the attack of PMe₃ at an oxo ligand with ΔG^≠ (298 K) = 64.6 kJ mol⁻¹. Dissociation of the product OPMe₃ is facile with ΔG^≠ (298 K) = 26.3 kJ mol⁻¹ giving a mono-oxo [Mo^IVO] complex which fills its coordination sphere with a further PMe₃ substrate with ΔG^≠ (298 K) = 39.2 kJ mol⁻¹. One-electron oxidation to a Mo(V) phosphane complex precedes the coordination of water/hydroxide. Additionally, the comproportionation of [Mo^VIO₂] and [Mo^IVO] to dinuclear oxo-bridged [OMo^V–O–Mo^VO] species has been calculated as the thermodynamic sink in this system and the back-OAT from dmso to mono-oxo [Mo^IVO] to give [Mo^VIO₂] has been shown to involve an equilibrium between stereoisomeric [Mo^VIO₂] complexes with an activation barrier of ΔG^≠ (298 K) = 113.1 kJ mol⁻¹.     

Hetero-polynuclear complexes containing bridging diphenylphosphino-alkenes and -alkynes. The crystal structure of an Rh2{μ−ν:ν−P(Ph2) (CH2)3C≡CR}Co2 complex

The phosphinoalkenes Ph₂P(CH₂)_nCH=CH₂ (n= 1, 2, 3) and phosphinoalkynes Ph₂P(CH₂)_n C≡CR (R = H, N = 2, 3; R = CH₃, N = 1) have been prepared and reacted with the dirhodium complex (η−C₅H₅)₂Rh₂(μ−CO) (μ−η²−CF₃C₂CF₃). Six new complexes of the type (ν−C₅H₅)₂(Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃)L, where L is a P-coordinated phosphinoalkene, or phosphinoalkyne have been isolated and fully characterized; the carbonyl and phosphine ligands are predominantly trans on the Rh---Rh bond, but there is spectroscopic evidence that a small amount of the cis-isomer is formed also. Treatment of the dirhodium-phosphinoalkene complexes with (η−CH₃C₅H₄)Mn(CO)₂thf resulted in coordination of the manganese to the alkene function. The Rh₂---Mn complex [(η−C₅H₅)₂Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃) {Ph₂P(CH₂)₃CH=CH₂} (η−CH₃C₅H₄)Mn(CO)₂] was fully characterized. Simi treatment of the dirhodium-phosphinoalkyne complexes with Co₂(CO)₈ resulted in the coordination of Co₂(CO)₆ to the alkyne function. The Rh₂---Co₂ complex [(η−C₅H₅)₂Rh₂(CO) (μ−η¹:η¹−CF₃C₂CF₃) {Ph₂PCH₂C≡CCH₃}Co₂(CO)₂], C₃₇H₂₅Co₂F₆O₇PRh₂, was fully characteriz spectroscopically, and the molecular structure of this complex was determined by a single crystal X-ray diffraction study. It is triclinic, space group (C_i¹, No. 2) with a = 18.454(6), B = 11.418(3), C = 10.124(3) Å, = 112.16(2), β = 102.34(3), γ = 91.62(3)°, Z = 2. Conventional R on |F| was 0.052 fo observed (I > 3σ(I)) reflections. The Rh₂ and Co₂ parts of the molecule are distinct, the carbonyl and phosphine are mutually trans on the Rh---Rh bond, and the orientations of the alkynes are parallel for Rh₂ and perpendicular for Co₂. Attempts to induce Rh₂Co₂ cluster formation were unsuccessful.     

The reaction between the superoxide anion radical and cytochrome c

1. 1. The superoxide anion radical (O₂⁻) reacts with ferricytochrome c to form ferrocytochrome c. No intermediate complexes are observable. No reaction could be detected between O₂⁻ and ferrocytochrome c.

2. 2. At 20 °C the rate constant for the reaction at pH 4.7 to 6.7 is 1.4 · 10⁶ M⁻¹ · s⁻¹ and as the pH increases above 6.7 the rate constant steadily decreases. The dependence on pH is the same for tuna heart and horse heart cytochrome c. No reaction could be demonstrated between O₂⁻ and the form of cytochrome c which exists above pH ≈ 9.2. The dependence of the rate constant on pH can be explained if cytochrome c has pKs of 7.45 and 9.2, and O₂⁻ reacts with the form present below pH 7.45 with k = 1.4 · 10⁶ M⁻¹ · s⁻¹, the form above pH 7.45 with k = 3.0 · 10⁵ M⁻¹ · s⁻¹, and the form present above pH 9.2 with k = 0.

3. 3. The reaction has an activation energy of 20 kJ mol⁻¹ and an enthalpy of activation at 25 °C of 18 kJ mol⁻¹ both above and below pH 7.45. It is suggested that O₂⁻ may reduce cytochrome c through a track composed of aromatic amino acids, and that little protein rearrangement is required for the formation of the activated complex.

4. 4. No reduction of ferricytochrome c by HO₂ radicals could be demonstrated at pH 1.2–6.2 but at pH 5.3, HO₂ radicals oxidize ferrocytochrome c with a rate constant of about 5 · 10⁵–5 · 10⁶ M⁻¹ · s⁻¹

.