30 research outputs found
Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(I) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies
Thermodynamic and kinetic parameters for the oxidative addition of H_2 to [Rh^I(bpy)_2]^+ (bpy = 2,2‘-bipyridine) to form [Rh^(III)(H)_2(bpy)_2]^+ were determined from either the UV−vis spectrum of equilibrium mixtures of [Rh^I(bpy)_2]^+ and [Rh^(III)(H)_2(bpy)_2]^+ or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh^(III)(H)_2(bpy)_2]^+ as a function of H_2 concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol^(-1) and −18 cal mol^(-1) K^(-1), respectively. The reaction enthalpy and entropy are −10.3 kcal mol^(-1) and −19 cal mol^(-1) K^(-1), respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (−15 and −16 cm^3 mol^(-1) in methanol and acetone, respectively) are very close to the overall reaction volumes (−15 cm^3 mol^(-1) in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H−H bond (0.823 and 0.810 Å, respectively) and forms only a weak Rh−H bond (1.866 and 1.915 Å, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process
Highly stereoselective decarboxylation of (+)-1-Bromo-1-chloro-2,2,2-trifluoropropanoic acid gives (+)-1-Bromo-1-chloro-2,2,2-trifluoroethane ((+)-Halothane) with retention of configuration
The absolute configuration of the title acid (2) has been determined to be S by X-ray crystallography. Thus, decarboxylation of 2 produces (S)-(+)-halothane with 99% retention of configuration. This behavior is compared to other stereoselective decarboxylation reactions of ?-haloacids from the literature that also give high degrees of retention of configuration when in the form of their quaternary ammonium salts, which contain one proton. The proton of the ammonium salt is necessary to protonate the anionic intermediate formed from decarboxylation. In the absence of this relatively acidic proton, we had previously found that using triethylene glycol (TEG) as both solvent and proton source for the decarboxylation reaction of acid 2 caused poor stereoselectivity. This was in contrast to 1,2,2,2-tetrafluoro-1-methoxypropionic acid (6), which showed a high degree of retention of configuration in TEG. To rationalize this differing behavior we report DFT studies at PCM-B3LYP/6-31++G** level of theory (the results were additionally confirmed with 6-311++G** and aug-cc-pVDZ basis sets). The energy barrier to inversion of configuration of the anionic reaction intermediate of acid 2 (11) is 10.23 kcal/mol. However, we find that the anionic intermediate from acid 6 (10) would rather undergo ?-elimination instead of inversion of configuration. Thus the planar transition state required for inversion of configuration is never reached, regardless of the rate of proton transfer to the anion
Lability and Basicity of Bipyridine-Carboxylate-Phosphonate Ligand Accelerate Single-Site Water Oxidation by Ruthenium-Based Molecular Catalysts
A critical step in creating an artificial
photosynthesis system
for energy storage is designing catalysts that can thrive in an assembled
device. Single-site catalysts have an advantage over bimolecular catalysts
because they remain effective when immobilized. Hybrid water oxidation
catalysts described here, combining the features of single-site bis-phosphonate
catalysts and fast bimolecular bis-carboxylate catalysts, have reached
turnover frequencies over 100 s<sup>–1</sup>, faster than both
related catalysts under identical conditions. The new [(bpHc)ÂRuÂ(L)<sub>2</sub>] (bpH<sub>2</sub>cH = 2,2′-bipyridine-6-phosphonic
acid-6′-carboxylic acid, L = 4-picoline or isoquinoline) catalysts
proceed through a single-site water nucleophilic attack pathway. The
pendant phosphonate base mediates O–O bond formation via intramolecular
atom-proton transfer with a calculated barrier of only 9.1 kcal/mol.
Additionally, the labile carboxylate group allows water to bind early
in the catalytic cycle, allowing intramolecular proton-coupled electron
transfer to lower the potentials for oxidation steps and catalysis.
That a single-site catalyst can be this fast lends credence to the
possibility that the oxygen evolving complex adopts a similar mechanism
Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium Bipyridine–Dicarboxylate Complexes
In order to gain
a deeper mechanistic understanding of water oxidation by [(bda)ÂRuÂ(L)<sub>2</sub>] catalysts (bdaH<sub>2</sub> = [2,2′-bipyridine]-6,6′-dicarboxylic
acid; L = pyridine-type ligand), a series of modified catalysts with
one and two trifluoromethyl groups in the 4 position of the bda<sup>2–</sup> ligand was synthesized and studied using stopped-flow
kinetics. The additional −CF<sub>3</sub> groups increased the
oxidation potentials for the catalysts and enhanced the rate of electrocatalytic
water oxidation at low pH. Stopped-flow measurements of ceriumÂ(IV)-driven
water oxidation at pH 1 revealed two distinct kinetic regimes depending
on catalyst concentration. At relatively high catalyst concentration
(ca. ≥10<sup>–4</sup> M), the rate-determining step
(RDS) was a proton-coupled oxidation of the catalyst by ceriumÂ(IV)
with direct kinetic isotope effects (KIE > 1). At low catalyst
concentration (ca. ≤10<sup>–6</sup> M), the RDS was
a bimolecular step with <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> ≈ 0.8. The results support a catalytic mechanism
involving coupling of two catalyst molecules. The rate constants for
both RDSs were determined for all six catalysts studied. The presence
of −CF<sub>3</sub> groups had inverse effects on the two steps,
with the oxidation step being fastest for the unsubstituted complexes
and the bimolecular step being faster for the most electron-deficient
complexes. Though the axial ligands studied here did not significantly
affect the oxidation potentials of the catalysts, the nature of the
ligand was found to be important not only in the bimolecular step
but also in facilitating electron transfer from the metal center to
the sacrificial oxidant