Richard G. Finke

Thermolysis of the CoC bond in adenosylcobalamin (coenzyme B12)—IV. Products, kinetics and CoC bond dissociation energy studies in ethylene glycol

Abstract Following an introduction outlining the key questions surrounding the adenosylcobalamin (AdoB 12 ) bond homolysis problem, the full details of product, kinetic and Coue5f8C5′ bond dissociation energy studies of the thermolysis of AdoB 12 in ethylene glycol are presented. The anaerobic thermolysis of AdoB 12 in the absence of the nitroxide radical trap TEMPO proceeds with four isosbestic points to yield 100±2% Co(II)B 12 and two nucleoside products, 8,5′-anhydroadenosine and 5′-deoxyadenosine. In the presence of >10 −2 M TEMPO, the TEMPO-trapped Ado . product (T-Ado) and Co(II)B 12 account quantitatively for the starting AdoB 12 . From HPLC studies of the concentration of the nucleoside products vs [TEMPO], absolute rate constants at 110°C for Ado . cyclization and its H . abstraction from glycol solvent are obtained for the first time, k c (110°C) ⋍ 5 × 10 5 s −1 and k a (110°C) ⋍ 7 × 10 3 M −1 s −1 . Kinetic studies in the presence of added, authentic Co(II)B 12 show an inverse, linear dependence of 1/ k obs vs [Co(II)B 12 ], thereby providing quantitative evidence for the long sought demonstration of the AdoB 12 ⇌ Ado . +Co(II)B 12 equilibrium outside of the enzyme-cofactor complex. The Co(II)B 12 dependence data also provide the previously unavailable rate constant for recombination of Co(II)B 12 and Ado . in ethylene glycol of k t ⋍ 3 × 10 8 M −1 s −1 . Next, the assumption used previously of slow homolysis by the base-off form, k h ,off ⪡ k h ,on , an assumption necessary to simplify the AdoB 12 thermolysis kinetics, is tested directly by the synthesis and thermolysis of adenosylcobinamide + OH − and found to be valid. The base-on homolysis rate constants ( k h ,on ) were measured from 110 to 80°C. When combined with independently measured temperature-dependence parameters for the off ⇌gon axial base equilibrium, Δ H = −7.6±0.2 kcal mol −1 and Δ S = −20.2±0.7 e.u., values for the base-on homolysis activation parameters of ethylene glycol are obtained, Δ H h ,on ‡ = 34.5±0.8 kcal mol −1 , and Δ S ‡ = 14±1 e.u. Use of the Δ H h ,on ‡ data to provide an estimate of the AdoB 12 Coue5f8C5′ BDE in ethylene glycol is discussed, as are a number of additional points and conclusions that result from the present work. Key considerations that led to the nitroxide radical trapping method, and the desirable features of this method, are then discussed, with an eye towards aiding future kinetic studies and BDE determinations of Coue5f8R and other Mue5f8R compounds.

Polyoxoanion-supported, atomically dispersed transition metals: The catalytic oxidation of cyclohexene with dioxygen by the catalyst precursors [(n-C4H9)4N]5Na3[(1,5-COD)Ir·P2W15Nb3O62], [(n-C4H9)4N]5Na3[(1,5-COD)Rh·P2W15Nb3O62], and [(n-C4H9)4N]4.5Na2.5[(C6H6)Ru·P2W15Nb3O62]

Abstract [( n -C 4 H 9 ) 4 N] 5 Na 3 [(1,5-COD)Ir·P 2 W 15 Nb 3 O 62 ], 1 , [( n -C 4 H 9 ) 4 N] 5 Na 3 [(1,5-COD)Rh·P 2 W 15 Nb 3 O 62 ], and [( n -C 4 H 9 ) 4 N] 4.5 Na 2.5 [(C 6 H 6 )Ru·P 2 W 15 Nb 3 O 62 ] have been shown to catalyze the oxygenation of cyclohexene with molecular oxygen. The polyoxoanion-supported iridium(I) complex, 1 , shows the highest activity of this group with a turnover frequency of 2.9 h −1 at 38°C in CH 2 Cl 2 (540 total turnovers), which is 100-fold greater than its parent iridium compound, [(1,5-COD)IrCl] 2 . Additional experiments using H 2 /O 2 mixtures and H 2 O 2 are also discussed. The apparent rate law for the oxidation of cyclohexene by O 2 by 1 is - d [cyclohexene]/ dt = k 2 obsd · [ 1 ] 1 [cyclohexene] 1 P(O 2 ) 1→0 . These compounds constitute the first examples of oxygenation catalysis using molecular oxygen and a polyoxoanion- supported transition-metal precatalyst.

A novel triperoxyniobium-containing polyoxoanion, SiW9(NbO2)3O377−: synthesis, characterization, catalytic allylic epoxidations with H2O2 and preliminary kinetic studies

Abstract The synthesis and characterization of the previously unknown SiW9(NbO2)3O377−, a novel composition of matter containing three adjacent NbvO2 peroxide groups, is described. Also reported is a survey of the catalytic epoxidation of seven olefinic substrates with 30% aqueous H2O2 using this heteropolyanion as a precatalyst. Additionally, the preliminary kinetic evidence presented indicates that SiW9(NbO2)3O377− serves as a catalyst precursor to the true catalyst, a catalytically active W, Nb or polyoxoanion fragment.

Cage effects in organotransition metal chemistry: Their importance in the kinetic estimation of bond dissociation energies in solution

Abstract Following a brief review of the literature of radical cage effects in solution, the available evidence for the operation of solvent cage effects in organotransition metal chemistry is summarized. Kinetic determination of Mue5f8L bond dissociation energies (BDEs) in solution are examined within this context, emphasizing a comparison of the current gas phase reaction coordinate model vs a model more appropriate for solution work that includes solvent cage effects. The temperature dependence of the cage effect is presented in its proper mathematical form which shows that solution kinetics cannot be connected to BDEs without knowledge of the cage efficiency factor (Fc), except in special cases. It is pointed out that the observed activation enthalpy in solution, ΔHobs‡(soln), is a composite containing variable amounts of the difference between the activation enthalpy for the cage combination (ΔHc‡) and that for diffusive separation of the cage pair (ΔHd‡), depending on the cage efficiency factor (Fc). No constant correction to ΔHobs‡(soln) can be expected; a better first approximation would be to use the activation enthalpy for viscous flow of the solvent, ΔHη‡, although this includes the implicit assumption that Fc = 1. The text also attempts to analyse critically the available, relevant literature and to note areas requiring further attention. A short section in which the equations are applied to Ni(CO)4 and PhCH(CH3)Co(DMG)2(Base) (and thus Niue5f8CO and Coue5f8C BDEs, respectively) is included. A brief list of major points is also provided in the Summary.

Organoactinide electrochemistry. A cyclic voltammetric and coulometric study of (C5Me5)2UCl2, [(C5Me5)2UCl2· THF]−Na+, (C5Me5)2UCl · THF and (C5Me5ThCl2

Abstract (C5Me5)2UCl2 exhibits a one-electron, reversible reduction to (C5Me5)2UCl2− without detectable Cl− loss, E 1 2 (CH3CN) = −1.30 V and E 1 2 (THF) = −1.22 V vs. SCE, which is shown to correspond to the one-electron, reversible oxidation of isolated [(C5Me5)2UCl2 · THF]− Na+ and to be distinct from the irreversible oxidation of (C5Me5)2UCl · THF (Epa (THF) = −0.71 V, 50 mV/sec scan the related (C5Me5)2ThCl2 is not reduced even out to −2.7 V.

Structure of sodium bis(pentadecatungstodiphosphato)diaquatetrazincate hydrate (16∶1∶50)

AbstractCrystals of Na16[Zn4(H2O)2(α-P2W15O56)2]·∼50H2O are triclinic,n

An assessment of the cis-influence in coenzyme B12 Models. The structure of the mixed schiff-base/oxime complex: Aquo(3,9-dimethyl-2,10-diethyl-1,4,8,1 1-tetra-azaundeca-1,3,8,10-tetraen-11-ol-1-olato)(methyl)cobalt(III)hexafluorophosphate

Pbar 1

Coenzyme B12 model compounds: A quantitative electrochemical comparison

n, witha+14.670(5),b+14.661(5),c+19.817(3)Å, α=84.95(3), β=81.51(3), γ=65.73(3)o. Least-squares refinement converged atR+0.053 for 5906 independent data withl>3σ(I). The centrosymmetric anion consists of two α-P2W15O5612− ligands attached to a planar group of four Zn2+, two of which also carry water molecules. The distortions in the ZnO6 octahedra and in the W15 framework of the ligands are smaller than the corresponding distortions in the otherwise similar [Cu4(H2O)2(P2W15O56)2]16− anion.