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Related papers
2006
The oxygen atom transfer reactivity of a discrete dioxo-molybdenum(VI) complexes, Tp iPr MoO 2 (OPh) (where Tp iPr = hydrotris(3-isopropylpyrazol-1-yl)borate), Tp Me2 MoO 2 (OPh), Tp Me2 MoO 2 (SPh), Tp Me2 MoO 2 (Cl), (where Tp Me2 = hydrotris(3,5dimethylpyrazol-1-yl)borate), with seven tertiary phosphines (PMe 3 , PMe 2 Ph, PEt 3 , P(n-Bu) 3 , PEt 2 Ph, PPh 2 Et and PPh 2 Me) have been investigated. The first nucleophilic step follows a second-order rate with an associative transition state in all cases. The second step of the reaction, i.e., the exchange of the coordinated phosphine oxide with acetonitrile, follows a first-order process. The reaction follows a dissociative interchange (I d) or associative interchange (I a) type mechanism, as it is substrate and compound dependent. It has been established that there are three main physico-chemical parameters that contribute to the reactivity of phosphorous (III) compounds, two of which are electronic and the third is of steric origin. It is commonly accepted that these reactions involve a σbasicity component, a π-acidity component and a steric/size component. However, there has been little investigation into the reactivity of the analogue oxo-phosphorous (V) compounds which are typically generated during oxygen atom transfer reactions (OAT) when the parent phosphorous (III) compounds act as nucleophiles toward oxygen. Here, we explore the current concepts associated with reactivity, the origin of reaction parameters and the general applicability of phosphorous (III) parameters toward reactions that evolve phosphorous (V) products. Lastly, the regeneration of a catalytically active enzyme, after formal OAT has occurred, is believed to involve two, one electron/ one proton transfer steps that result in a formal Mo(V) intermediate. However, no crystal structure exists for the enzyme in a transient 5+ state. To this end we have synthesized, and fully characterized a series of monooxo-Mo(V) complexes in attempt to model the transient Mo(V) state of the enzyme. Furthermore we show that these complexes can be isolated as two discrete isomers with respect to the position of a heteroatom donor relative to the oxo-group and have detailed the kinetics of isomerization and electronic structure of these complexes. Based upon our findings we have postulated a serine gated electron transfer hypothesis (SGET) to provide a possible explanation for the role of isomerism in the regeneration step of a catalytically active protein. v To my wife Joanna, my daughter Juliana, and mother Marilyn; you are my love, my support and my inspiration. vi
Journal of the American Chemical Society, 1988
before, most recently in those of the respective tetramethylammonium salts. 15 Crystal structure determinations of the remaining complexes of the system, with 5 to 8 mol of hydrogen fluoride per mol of drogen fluorides), too. Anions H4F5and, in one instance each, HsN.4HF, H5F6-and H7Fsare already established species in the solid ~t a t e .~J~ Supplementary Material Available: Listing of anisotropic thermal parameters for the non-hydrogen atoms (2 pages); structure factor tables (8 pages). Ordering information is given on any current masthead page. pyridine, most probably these as pyridinium ply(hy-32001-55-1; C5H,N.2HF, 87979-78-0; C5H5N.3HF, 79162-49-5; C5-(15) Mootz, D.; Boenigk, D. Abstract: The kinetics and mechanism of the oxygen atom transfer reactions Mo02(L-NS2) + (RF)3P -MoO(L-NS2)(DMF) + (RF)3P0 (1) and MoO(L-NS2)(DMF) + XO -L Md2(L-NS2) + X, with X = (RF)2S0 (2) and 3-fluoropyridine N-oxide (3), have been investigated in DMF solutions (L-NS2 = 2,6-bis(2,2-diphenyl-2-mercaptoethyl)pyridine(2-), RF = p-C,H4F). The following rate constants (297.5 K) and activation parameters were obtained: reaction 1, k2 = 9.7 (4) X M-I s-l, AH* = 11.7 (6) kcal/mol, AS* = -28.4 (1.6) eu; reaction 2, kl = 14.0 (7) X IO4 s-', AH* = 22.1 (1.3) kcal/mol, AS* = 2.6 (1.6) eu; reaction 3, kl = 16.0 (8) X lo4 s-l, AH* = 23.4 (1.4) kcal/mol, AS* = 7.2 (2.0) eu. Reactions 2 and 3 exhibit saturation kinetics, under which the rate-determining step is intramolecular atom transfer. Mechanisms and transition states are proposed. The activation parameters are the first measured for oxo transfer from substrate; the small activation entropies suggest a transition state structurally similar to the complex MoO(L-NS2)(XO) formed in a labile equilibrium prior to oxo transfer to Mo. Coupling of reaction 1 with reaction 2 or 3 affords the catalytic reaction 4, ( R F )~P + XO -(RF),PO +
Proceedings of the National Academy of Sciences, 2003
The kinetics of proton-coupled electron-transfer (pcet) reactions are reported for Mn 4O4(O2PPh2)6, 1, and [Mn4O4(O2PPh2)6] ؉ , 1 ؉ , with phenothiazine (pzH). Both pcet reactions form 1H, by H transfer to 1 and by hydride transfer to 1 ؉ . Surprisingly, the rate constants differ by only 25% despite large differences in the formal charges and driving force. The driving force is proportional to the difference in the bond-dissociation energies (BDE >94 kcal͞mol for homolytic, 1H 3 H ؉ 1, vs. Ϸ127 kcal͞ mol for heterolytic, 1H 3 H ؊ ؉ 1 ؉ , dissociation of the OOH bond in 1H). The enthalpy and entropy of activation for the homolytic reaction (⌬H ‡ ؍ ؊1.2 kcal͞mol and ⌬S ‡ ؍ ؊32 cal͞mol⅐K; 25-6.7°C) reveal a low activation barrier and an appreciable entropic penalty in the transition state. The rate-limiting step exhibits no H͞D kinetic isotope effect (k H͞kD ؍ 0.96) for the first H atom-transfer step and a small kinetic isotope effect (1.4) for the second step (1H ؉ pzH 3 1H 2 ؉ pz • ). These lines of evidence indicate that formation of a reactive precursor complex before atom transfer is rate-limiting (conformational gating), and that little or no NOH bond cleavage occurs in the transition state. H-atom transfer from pzH to alkyl, alkoxyl, and peroxyl radicals reveals that BDEs are not a good predictor of the rates of this reaction. Hydride transfer to 1 ؉ provides a concrete example of two-electron pcet that is hypothesized for the OOH bond cleavage step during catalysis of photosynthetic water oxidation.
Inorganic chemistry, 2010
The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp iPr MoO 2 (OPh) (Tp iPr = hydrotris(3-isopropylpyrazol-1-yl)borate), with a range of tertiary phosphines (PMe 3 , PMe 2 Ph, PEt 3 , PBu n 3 , PEt 2 Ph, PEtPh 2 and PMePh 2 ) has been investigated. The first step in all the reactions follows a second-order rate law indicative of an associative transition state, consistent with nucleophilic attack by the phosphine on an oxo ligand, viz. Tp iPr MoO 2 (OPh) + PR 3 → Tp iPr MoO (OPh)(OPR 3 ). The calculated free energy of activation for the formation of the OPMe 3 intermediate (Chem. Eur. J. 2006, 12, 7501) is in excellent agreement with the experimental ΔG ‡ value reported here. The second step of the reaction, i.e., the exchange of the coordinated phosphine oxide by acetonitrile, Tp iPr MoO(OPh)(OPR 3 ) + MeCN → Tp iPr MoO(OPh)(MeCN) + OPR 3 , is first-order in starting complex in acetonitrile. The reaction occurs via a dissociative interchange (I d ) or associative interchange (I a ) mechanism, depending on the nature of the phosphine oxide. The activation parameters for the solvolysis of Tp iPr MoO(OPh)(OPMe 3 ) (ΔH ‡ = 56.3 kJ mol −1 ; ΔS ‡ = −125.9 J mol −1 K −1 ; ΔG ‡ = 93.8 kJ mol −1 ) and Tp iPr MoO(OPh)(OPEtPh 2 ) (ΔH ‡ = 66.5 kJ mol −1 ; ΔS ‡ = −67.6 J mol −1 K −1 ; ΔG ‡ = 86.7 kJ mol −1 ) by acetonitrile are indicative of I a mechanisms. In contrast, the corresponding parameters for the solvolysis reaction of Tp iPr MoO(OPh)(OPEt 3 ) (ΔH ‡ = 95.8 kJ mol −1 ; ΔS ‡ = 26.0 J mol −1 K −1 ; ΔG ‡ = 88.1 kJ mol −1 ) and the remaining complexes by the same solvent are indicative of an I d mechanism. The equilibrium constant for the solvolysis of the oxo-Mo(V) phosphoryl complex, [Tp iPr MoO(OPh)(OPMe 3 )] + , by acetonitrile was calculated to be 1.9 × 10 −6 . The oxo-Mo(V) phosphoryl complex is more stable than the acetonitrile analogue, whereas the oxo-Mo(IV) acetonitrile complex is more stable than the phosphoryl analogue. The higher stability of the Mo(V) phosphoryl complex may explain the phosphate inhibition of sulfite oxidase.
Dramatic Influence of an Anionic Donor on the Oxygen-Atom Transfer Reactivity of a Mn V -Oxo Complex
Chemistry - A European Journal, 2014
Addition of an anionic donor to an Mn V (O) porphyrinoid complex causes a dramatic increase in 2-electron oxygen-atom-transfer (OAT) chemistry. The 6-coordinate [Mn V (O)(TBP 8 Cz)(CN)] À was generated from addition of Bu 4 N + CN À to the 5-coordinate Mn V (O) precursor. The cyanide-ligated complex was characterized for the first time by Mn K-edge X-ray absorption spectroscopy (XAS) and gives MnÀO = 1.53 , MnÀCN = 2.21 . In combination with computational studies these distances were shown to correlate with a singlet ground state. Reaction of the CN À complex with thioethers results in OAT to give the corresponding sulfoxide and a 2e À -reduced Mn III (CN) À complex. Kinetic measurements reveal a dramatic rate enhancement for OAT of approximately 24 000-fold versus the same reaction for the parent 5-coordinate complex. An Eyring analysis gives DH°= 14 kcal mol À1 , DS°= À10 cal mol À1 K À1 . Computational studies fully support the structures, spin states, and relative reactivity of the 5-and 6-coordinate Mn V (O) complexes.
Journal of the American Chemical Society, 2014
Addition of anionic donors to the manganese(V)-oxo corrolazine complex Mn(V)(O)(TBP8Cz) has a dramatic influence on oxygen-atom transfer (OAT) reactivity with thioether substrates. The six-coordinate anionic [Mn(V)(O)(TBP8Cz)(X)](-) complexes (X = F(-), N3(-), OCN(-)) exhibit a ∼5 cm(-1) downshift of the Mn-O vibrational mode relative to the parent Mn(V)(O)(TBP8Cz) complex as seen by resonance Raman spectroscopy. Product analysis shows that the oxidation of thioether substrates gives sulfoxide product, consistent with single OAT. A wide range of OAT reactivity is seen for the different axial ligands, with the following trend determined from a comparison of their second-order rate constants for sulfoxidation: five-coordinate ≈ thiocyanate ≈ nitrate < cyanate < azide < fluoride ≪ cyanide. This trend correlates with DFT calculations on the binding of the axial donors to the parent Mn(V)(O)(TBP8Cz) complex. A Hammett study was performed with p-X-C6H4SCH3 derivatives and [Mn(V)(...
Polyhedron, 2000
The synthesis and characterisation of (PPh 4 ) [M(CN) 4 O(pz)]·3H 2 O (M= Mo or W; pz =pyrazine) are presented. The salts are reactive towards molecular oxygen, both in solution and in the solid state, with formation of (PPh 4 ) 2 [M(CN) 4 O(O 2 )]. The X-ray crystal structure of the molybdenum compound confirmed the presence of a peroxo ligand cis to the M O bond; the O O bond distance is 1.41 A , . The IR spectra exhibit two absorption bands in the 950 -850 cm − 1 region assigned to the terminal M O group [917 (Mo) and 933 (W) cm − 1 ] and the peroxo group [893 (Mo) and 871 (W) cm − 1 ]. The possible mechanism of molecular oxygen uptake by pyrazine complexes is discussed.
Oxygen Isotope Effects as Structural and Mechanistic Probes in Inorganic Oxidation Chemistry
Inorganic Chemistry, 2010
Oxidative transformations using molecular oxygen are widespread in nature but remain a major challenge in chemical synthesis. Limited mechanistic understanding presents the main obstacle to exploiting O 2 in "bioinspired" industrial processes. Isotopic methods are presently being applied to characterize reactions of natural abundance O 2 including its coordination to reduced transition metals and cleavage of the O-O bond. This review describes the application of competitive oxygen-18 isotope effects, together with Density Functional Theory, to examine O 2 reductive activation under catalytically relevant conditions. The approach should be generally useful for probing small-molecule activation by transition-metal complexes.
Applied Catalysis A: General, 2005
Two series of metal catalysts (Rh, Pt, Pd, Ru and Ir) supported over CeO 2 and Ce 0.63 Zr 0.36 O 2 were prepared. Catalysts were pretreated at 500 8C (fresh) and further sintered either in H 2 or in air at 700-900 8C. All catalysts were characterized by H 2 chemisorption at À85 8C and/or by transmission electron microscopy (TEM). The 16 O 2 + 18 O 2 homoexchange reaction was carried out in the 200-500 8C temperature range. Rh, Ru and Ir showed the highest homoexchange rate R (per m 2 metal) while Pd and to a lower extent Pt were less active. Sintering affected the metal performances differently: while R is higher on small Rh particles (fresh catalysts), the reverse situation could be observed for Ru and Ir where the sintered catalysts were more active than the fresh ones, especially in the case of CeO 2 .
Inorganic Chemistry, 2010
The oxygen atom transfer reactivity of the dioxo-Mo(VI) complex, Tp iPr MoO 2 (OPh) (Tp iPr = hydrotris(3-isopropylpyrazol-1-yl)borate), with a range of tertiary phosphines (PMe 3 , PMe 2 Ph, PEt 3 , PBu n 3 , PEt 2 Ph, PEtPh 2 and PMePh 2) has been investigated. The first step in all the reactions follows a second-order rate law indicative of an associative transition state, consistent with nucleophilic attack by the phosphine on an oxo ligand, viz. Tp iPr MoO 2 (OPh) + PR 3 → Tp iPr MoO (OPh)(OPR 3). The calculated free energy of activation for the formation of the OPMe 3 intermediate (Chem. Eur. J. 2006, 12, 7501) is in excellent agreement with the experimental ΔG ‡ value reported here. The second step of the reaction, i.e., the exchange of the coordinated phosphine oxide by acetonitrile, Tp iPr MoO(OPh)(OPR 3) + MeCN → Tp iPr MoO(OPh)(MeCN) + OPR 3 , is first-order in starting complex in acetonitrile. The reaction occurs via a dissociative interchange (I d) or associative interchange (I a) mechanism, depending on the nature of the phosphine oxide. The activation parameters for the solvolysis of Tp iPr MoO(OPh)(OPMe 3) (ΔH ‡ = 56.3 kJ mol −1 ; ΔS ‡ = −125.9 J mol −1 K −1 ; ΔG ‡ = 93.8 kJ mol −1) and Tp iPr MoO(OPh)(OPEtPh 2) (ΔH ‡ = 66.5 kJ mol −1 ; ΔS ‡ = −67.6 J mol −1 K −1 ; ΔG ‡ = 86.7 kJ mol −1) by acetonitrile are indicative of I a mechanisms. In contrast, the corresponding parameters for the solvolysis reaction of Tp iPr MoO(OPh)(OPEt 3) (ΔH ‡ = 95.8 kJ mol −1 ; ΔS ‡ = 26.0 J mol −1 K −1 ; ΔG ‡ = 88.1 kJ mol −1) and the remaining complexes by the same solvent are indicative of an I d mechanism. The equilibrium constant for the solvolysis of the oxo-Mo(V) phosphoryl complex, [Tp iPr MoO(OPh)(OPMe 3)] + , by acetonitrile was calculated to be 1.9 × 10 −6. The oxo-Mo(V) phosphoryl complex is more stable than the acetonitrile analogue, whereas the oxo-Mo(IV) acetonitrile complex is more stable than the phosphoryl analogue. The higher stability of the Mo(V) phosphoryl complex may explain the phosphate inhibition of sulfite oxidase.