- Mechanism of dihydrogen cleavage by high-valent metal oxo compounds: Experimental and computational studies
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The oxidation of dihydrogen by metal tetraoxo compounds was investigated. Kinetic measurements of the oxidations of H2 by MnO4- and RuO4, performed by UV-vis spectroscopy, showed these reactions to be quite rapid at 25 °C (k1 ≈ (3-6) x 10-2 M-1 s-1). Rates measured for H2 oxidation by MnO4- in aqueous solution (using KMnO4) and in chlorobenzene (using nBu4NMnO4) revealed only a minor solvent effect on the reaction rate. Substantial kinetic isotope effects [(kH2/kD2 = 3.8(2) (MnO4- aq), 4.5(5) (MnO4-, C6H5C1 soln) and 1.8(6) (RuO4, CCl4 soln)] indicated that H-H bond cleavage is rate determining and that the mechanism of dihydrogen cleavage is likely similar in aqueous and organic solutions. Third-row transition-metal oxo compounds, such as OsO4, ReO4-, and MeReO3, were found to be completely unreactive toward H2. Experiments were performed to probe for a catalytic hydrogen/deuterium exchange between D2 and H20 as possible evidence of dihydrogen σ-complex intermediates, but no H/D exchange was observed in the presence of various metal oxo compounds at various pH values. In addition, no inhibition of RuO4-catalyzed hydrocarbon oxidation by H2 was observed. On the basis of the available evidence, a concerted mechanism for the cleavage of H2 by metal tetraoxo compounds is proposed. Theoretical models were developed for pertinent MnO4- + H2 transition states using density functional theory in order to differentiate between concerted [2 + 2] and [3 + 2] scissions of H2. The density functional theory calculations strongly favor the [3 + 2] mechanism and show that the H2 cleavage shares some mechanistic features with related hydrocarbon oxidation reactions. The calculated activation energy for the [3 + 2] pathway (ΔH? = 15.4 kcal mol-1) is within 2 kcal mol-1 of the experimental value.
- Collman,Slaughter,Eberspacher,Strassner,Brauman
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- Hyperfine coupling and local structures of octacyanomolibudenum magnetic complexes and related compounds as studied by solid state 13C and 2H NMR
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Three-dimensional polymeric complex of Cu2[Mo(CN) 8]·8H2O (1) is known to undergo a transition from paramagnetic to ferromagnetic state by irradiation with blue light. To elucidate the local structure of this polymeric complex, which is important for switching to ferromagnet, we studied hyperfine coupling of 13C-enriched cyanide ions of 1 and of closely related structure known polycyanide complexes of Fe2II[MoIV(13CN)8]·8H2O(2), Mn2II[MoIV(13CN)8]·8H2O(3), [CuII(bpy)2]2[MoIV( 13CN)8]·5H2O·CH3OH (4), UV-irradiated K4[MoIV(13CN) 8]·2D2O (5), Cs3[MoV( 13CN)8]·2H2O (6), [Cu2II(tren)213CN] (BF4)3(7), and [Ni2II(tetren)213CN](ClO4)3(8) by solid-state 13C NMR spectrum. We found two distinct 13C NMR signals for the cyanide ions in a paramagnetic phase of 1, i.e. high frequency and low frequency peak. Each of high- and low-frequency peak shows a positive (+19 MHz) and negative (-2 MHz) hyperfine coupling constant (hfcc), respectively. Then we assume that there are two distinct coordination structures for Cu(II) ions of 1. The peak with positive hfcc was also observed for 2 and 3, in which the cyanide ions of [MoIV(CN)8]4- coordinate to Fe(II) and Mn(II), respectively, in a square planar (SQP) structure. The negative hfcc of -0.79 MHz, which resembles the low frequency peak of 1, was observed for the 13C atom of one of the CN ions of 4, which coordinates to the Cu(II) in an equatorial plane of trigonal bipyramidal (TBP) structure. Based on these NMR results, it is suggested that there are two distinct Cu(II) coordination environments in 1, namely SQP and the structure that is largely distorted from SQP. The distortion may be important for ferromagnetic switching by photoinduced electron transfer from Mo(IV) to Cu(II) ion in a collective manner in the solid state.
- Watanabe, Ryo,Ishiyama, Hiroki,Maruta, Goro,Takeda, Sadamu
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p. 2599 - 2606
(2008/10/09)
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- Thermal decomposition of H3O- produced in reaction of OH- with H2CO
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The ion-molecule reaction OH- + H2CO H3O- + CO has been studied at 300 K with isotropic labeling of reactants.The H3O- product is only observed in small abundance because the ion dissociates into HO- + H2 upon multiple collisions in a helium buffer gas.Without isotopic labeling, the pseudo-first-order kinetics plots for the reactions of OH- with H2CO and OD- + D2CO were found to be curved as a result of the regeneration of OH- or OD- reactant.A scavenger technique was used to remove the H3O- (or D3O-) produced prior to dissociation, to reveal the true first-order attenuation of OH- (or OD-) in reaction with H2CO (or D2CO).The rate constant for the OH- + H2CO reaction is 7.6x10-10 cm3 s-1, and for OD- + D2CO is 5.7x10-10 cm3 s-1.For the isotopically mixed cases OH- + D2CO and OD- + H2CO, the rate constants are equal to 1.3x10-9 cm3 s-1, about twice as large as those for the reactions involving only a single hydrogen isotope, indicating that isotopic exchange is an important process.The rate constants for the thermal dissociation of H3O- and D3O- in helium were found to be 1.6x10-12 and 1.1x10-12 cm3 s-1, respectively, within a factor of 2.The results are discussed in terms of other thermal dissociation reactions of ions.
- Viggiano, A. A.,Miller, Thomas M.,Miller, Amy E. Stevens,Morris, Robert A.,Paulson, John F.,et al.
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p. 357 - 361
(2007/10/02)
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- The formation and destruction of H3O-
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We report the first measurements of rate constants for formation and reaction of the Ion Phys. 49, 311 (1983)>, namely, dehydrogenation of formaldehyde by hydroxide to form hydrated-hydride ion and carbon monoxide.The OD- + H2CO reaction is about 35percent efficient at 298 K, with OD-/OH- exchange occurring in about half the reactions.H3O- was observed to undergo thermal dissociation in a helium carrier gas at room temperature with a rate constant of 1.6 * 10-12 cm3 s-1.We also studied a new reaction in which H3O- is formed: The association of OH- with H2 in a He carrier gas at low temperatures.The rate coefficient for this ternary reaction is 1*10-30 cm6 s-1 at 88 K.Rate coefficients and product branching fractions were determined for H3O- reactions with 19 neutral species at low temperatures (88-194 K) in an H2 carrier.The results of ion-beam studies, negative-ion photoelectron spectroscopy, and ion-molecule reaction data allow us to specify the hydride-water bond energy D0298(H- - H2O) = 14.4+/-1.0 kcal mol-1 (0.62+/-0.04 eV).The heat of formation of H3O-, -37.5+/-1.0 kcal mol-1, and the proton affinity of H3O-, 386.0+/-1.0 kcal mol-1, are derived from these results.Dissociation of H3O- into OH- and H2 requires 4.5+/-1.0 kcal mol-1 energy.
- Miller, Thomas M.,Viggiano, A. A.,Miller, Amy E. Stevens,Morris, Robert A.,Henchman, Michael,et al.
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p. 5706 - 5714
(2007/10/02)
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