30 research outputs found
Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired ChromophoreâImidazoleâPhenol Complex: Stepwise Oxidation and Concerted Reduction
Proton-coupled electron
transfer (PCET) reactions were studied
in acetonitrile for a Photosystem II (PSII)-inspired [RuÂ(bpy)<sub>2</sub>(phen-imidazole-PhÂ(OH)Â(<sup><i>t</i></sup>Bu)<sub>2</sub>)]<sup>2+</sup>, in which RuÂ(III) generated by a flashâquench
sequence oxidizes the appended phenol and the proton is transferred
to the hydrogen-bonded imidazole base. In contrast to related systems,
the donor and acceptor are strongly coupled, as indicated by the shift
in the Ru<sup>III/II</sup> couple upon phenol oxidation, and intramolecular
oxidation of the phenol by RuÂ(III) is energetically favorable by both
stepwise and concerted pathways. The phenol oxidation occurs via a
stepwise ET-PT mechanism with <i>k</i><sub>ET</sub> = 2.7
Ă 10<sup>7</sup> s<sup>â1</sup> and a kinetic isotope
effect (KIE) of 0.99 Âą 0.03. The electron transfer reaction was
characterized as adiabatic with Îť<sub>DA</sub> = 1.16 eV and
280 < <i>H</i><sub>DA</sub> < 540 cm<sup>â1</sup> consistent with strong electronic coupling and slow solvent dynamics.
Reduction of the phenoxyl radical by the quencher radical was examined
as the analogue of the redox reaction between the PSII tyrosyl radical
and the oxygen-evolving complex. In our PSII-inspired complex, the
recombination reaction activation energy is <2 kcal mol<sup>â1</sup>. The reaction is nonadiabatic (<i>V</i><sub>PCET</sub> â 22 cm<sup>â1</sup> (H) and 49 cm<sup>â1</sup> (D)) and concerted, and it exhibits an unexpected inverse KIE =
0.55 that is attributed to greater overlap of the reactant vibronic
ground state with the OD vibronic states of the proton acceptor due
to the smaller quantum spacing of the deuterium vibrational levels
OâO Radical Coupling: From Detailed Mechanistic Understanding to Enhanced Water Oxidation Catalysis
A deeper mechanistic
understanding of the key OâO bond formation step of water oxidation
by the [RuÂ(bda)Â(L)<sub>2</sub>] (bdaH<sub>2</sub> = 2,2â˛-bipyridine-6,6â˛-dicarboxylic
acid; L is a pyridine or isoquinoline derivative) family of catalysts
is reached through harmonious experimental and computational studies
of two series of modified catalysts with systematic variations in
the axial ligands. The introduction of halogen and electron-donating
substituents in [RuÂ(bda)Â(4-X-py)<sub>2</sub>] and [RuÂ(bda)Â(6-X-isq)<sub>2</sub>] (X is H, Cl, Br, and I for the pyridine series and H, F,
Cl, Br, and OMe for the isoquinoline series) enhances the noncovalent
interactions between the axial ligands in the transition state for
the bimolecular OâO coupling, resulting in a lower activation
barrier and faster catalysis. From detailed transition state calculations
in combination with experimental kinetic studies, we find that the
main contributor to the free energy of activation is entropy due to
the highly organized transition states, which is contrary to other
reports. Previous work has considered only the electronic influence
of the substituents, suggesting electron-withdrawing groups accelerate
catalysis, but we show that a balance between polarizability and favorable
ĎâĎ
interactions is the key, leading to rationally devised improvements.
Our calculations predict the catalysts with the lowest Î<i>G</i><sup>⧧</sup> for the OâO coupling step to
be [RuÂ(bda)Â(4-I-py)<sub>2</sub>] and [RuÂ(bda)Â(6,7-(OMe)<sub>2</sub>-isq)<sub>2</sub>] for the pyridine and isoquinoline families, respectively.
Our experimental results corroborate these predictions: the turnover
frequency for [RuÂ(bda)Â(4-I-py)<sub>2</sub>] (330 s<sup>â1</sup>) is a 10-fold enhancement with respect to that of [RuÂ(bda)Â(py)<sub>2</sub>], and the turnover frequency for [RuÂ(bda)Â(6-OMe-isq)<sub>2</sub>] reaches 1270 s<sup>â1</sup>, two times faster than
[RuÂ(bda)Â(isq)<sub>2</sub>]
Inverse Kinetic Isotope Effect in the Excited-State Relaxation of a Ru(II)âAquo Complex: Revealing the Impact of Hydrogen-Bond Dynamics on Nonradiative Decay
Photophysics
of the MLCT excited-state of [RuÂ(bpy)Â(tpy)Â(OH<sub>2</sub>)]<sup>2+</sup> (<b>1</b>) and [RuÂ(bpy)Â(tpy)Â(OD<sub>2</sub>)]<sup>2+</sup> (<b>2</b>) (bpy = 2,2â˛-bipyridine
and tpy = 2,2â˛:6â˛,2âł-terpyridine) have been investigated
in room-temperature H<sub>2</sub>O and D<sub>2</sub>O using ultrafast
transient pump-probe spectroscopy. An inverse isotope effect is observed
in the ground-state recovery for the two complexes. These data indicate
control of excited-state lifetime via a pre-equilibrium between the <sup>3</sup>MLCT state that initiates H-bond dynamics with the solvent
and the <sup>3</sup>MC state that serves as the principal pathway
for nonradiative decay
Manipulating the Rate-Limiting Step in Water Oxidation Catalysis by Ruthenium BipyridineâDicarboxylate Complexes
In order to gain
a deeper mechanistic understanding of water oxidation by [(bda)ÂRuÂ(L)<sub>2</sub>] catalysts (bdaH<sub>2</sub> = [2,2â˛-bipyridine]-6,6â˛-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>2â</sup> ligand was synthesized and studied using stopped-flow
kinetics. The additional âCF<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. âĽ10<sup>â4</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. â¤10<sup>â6</sup> M), the RDS was
a bimolecular step with <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> â 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 âCF<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
Electrocatalysis on Oxide-Stabilized, High-Surface Area Carbon Electrodes
A procedure is described for preparing
and derivatizing novel,
high surface area electrodes consisting of thin layers of nanostructured
ITO (SnÂ(IV)-doped indium tin oxide, <i>nano</i>ITO) on reticulated
vitreous carbon (RVC) to give RVC|<i>nano</i>ITO. The resulting
hybrid electrodes are highly stabilized oxidatively. They were surface-derivatized
by phosphonate binding of the electrocatalyst, [RuÂ(Mebimpy)Â(4,4â˛-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpy)Â(OH<sub>2</sub>)]<sup>2+</sup> (Mebimpy = 2,6-bisÂ(1-methylbenzimidazol-2-yl)Âpyridine; bpy = 2,2â˛-bipyridine)
(<b>1-PO</b><sub><b>3</b></sub><b>H</b><sub><b>2</b></sub>) to give RVC|<i>nano</i>ITO-Ru<sup>II</sup>-OH<sub>2</sub><sup>2+</sup>. The redox properties of the catalyst
are retained on the electrode surface. Electrocatalytic oxidation
of benzyl alcohol to benzaldehyde occurs with a 75% Faradaic efficiency
compared to 57% on <i>nano</i>ITO. Electrocatalytic water
oxidation at 1.4 V vs SCE on derivatized RVC|<i>nano</i>ITO electrode with an internal surface area of 19.5 cm<sup>2</sup> produced 7.3 Îźmoles of O<sub>2</sub> in 70% Faradaic yield
in 50 min
Nonaqueous Electrocatalytic Oxidation of the Alkylaromatic Ethylbenzene by a Surface Bound Ru<sup>V</sup>(O) Catalyst
The catalyst [RuÂ(Mebimpy)Â(4,4â˛-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpyÂ(OH<sub>2</sub>)]<sup>2+</sup>, where
Mebimpy
is 2,6-bisÂ(1-methylbenzimidazol-2-yl)Âpyridine and 4,4â˛-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpy is 4,4â˛-bis-methlylenephosphonato-2,2â˛-bipyridine,
attached to nanocrystalline SnÂ(IV)-doped In<sub>2</sub>O<sub>3</sub> (nanoITO) electrodes (nanoITO|Ru<sup>II</sup>âOH<sub>2</sub><sup>2+</sup>) has been utilized for the electrocatalytic oxidation
of the alkylaromatics ethylbenzene, toluene, and cumene in propylene
carbonate/water mixtures. Oxidative activation of the surface site
to nanoITO|Ru<sup>V</sup>(O)<sup>3+</sup> is followed by hydrocarbon
oxidation at the surface with a rate constant of 2.5 Âą 0.2 M<sup>â1</sup> s<sup>â1</sup> (<i>I</i> = 0.1 M
LiClO<sub>4</sub>, <i>T</i> = 23 ¹ 2 °C) for the
oxidation of ethylbenzene. Electrocatalytic oxidation of ethylbenzene
to acetophenone occurs with a faradic efficiency of 95%. H/D kinetic
isotope effects determined for oxidation of ethylbenzene point to
a mechanism involving oxygen atom insertion into a CâH bond
of ethylbenzene followed by further 2e<sup>â</sup>/2H<sup>+</sup> oxidation to acetophenone
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>â1</sup>, faster than both
related catalysts under identical conditions. The new [(bpHc)ÂRuÂ(L)<sub>2</sub>] (bpH<sub>2</sub>cH = 2,2â˛-bipyridine-6-phosphonic
acid-6â˛-carboxylic acid, L = 4-picoline or isoquinoline) catalysts
proceed through a single-site water nucleophilic attack pathway. The
pendant phosphonate base mediates OâO 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 BipyridineâDicarboxylate Complexes
In order to gain
a deeper mechanistic understanding of water oxidation by [(bda)ÂRuÂ(L)<sub>2</sub>] catalysts (bdaH<sub>2</sub> = [2,2â˛-bipyridine]-6,6â˛-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>2â</sup> ligand was synthesized and studied using stopped-flow
kinetics. The additional âCF<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. âĽ10<sup>â4</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. â¤10<sup>â6</sup> M), the RDS was
a bimolecular step with <i>k</i><sub>H</sub>/<i>k</i><sub>D</sub> â 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 âCF<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
Selective Electrocatalytic Oxidation of a ReâMethyl Complex to Methanol by a Surface-Bound Ru<sup>II</sup> Polypyridyl Catalyst
The
complex [RuÂ(Mebimpy)Â(4,4â˛-((HO)<sub>2</sub>OPCH<sub>2</sub>)<sub>2</sub>bpy)Â(OH<sub>2</sub>)]<sup>2+</sup> surface bound
to tin-doped indium oxide mesoporous nanoparticle film electrodes
(nanoITO-Ru<sup>II</sup>(OH<sub>2</sub>)<sup>2+</sup>) is an electrocatalyst
for the selective oxidation of methylrhenium trioxide (MTO) to methanol
in acidic aqueous solution. Oxidative activation of the catalyst to
nanoITO-Ru<sup>IV</sup>(OH)<sup>3+</sup> induces oxidation of MTO.
The reaction is first order in MTO with rate saturation observed at
[MTO] > 12 mM with a limiting rate constant of <i>k</i> =
25 s<sup>â1.</sup> Methanol is formed selectively in 87% Faradaic
yield in controlled potential electrolyses at 1.3 V vs NHE. At higher
potentials, oxidation of MTO by nanoITO-Ru<sup>V</sup>(O)<sup>3+</sup> leads to multiple electrolysis products. The results of an electrochemical
kinetics study point to a mechanism in which surface oxidation to
nanoITO-Ru<sup>IV</sup>(OH)<sup>3+</sup> is followed by direct insertion
into the rheniumâmethyl bond of MTO with a detectable intermediate
Structure and Electronic Configurations of the Intermediates of Water Oxidation in Blue Ruthenium Dimer Catalysis
Catalytic O<sub>2</sub> evolution with <i>cis</i>,<i>cis</i>-[(bpy)<sub>2</sub>(H<sub>2</sub>O)ÂRu<sup>III</sup>ORu<sup>III</sup>(OH<sub>2</sub>)Â(bpy)<sub>2</sub>]<sup>4+</sup> (bpy
is
2,2-bipyridine), the so-called blue dimer, the first designed water
oxidation catalyst, was monitored by UVâvis, EPR, and X-ray
absorption spectroscopy (XAS) with ms time resolution. Two processes
were identified, one of which occurs on a time scale of 100 ms to
a few seconds and results in oxidation of the catalyst with the formation
of an intermediate, here termed [3,4]â˛. A slower process occurring
on the time scale of minutes results in the decay of this intermediate
and O<sub>2</sub> evolution. Spectroscopic data suggest that within
the fast process there is a short-lived transient intermediate, which
is a precursor of [3,4]â˛. When excess oxidant was used, a highly
oxidized form of the blue dimer [4,5] was spectroscopically resolved
within the time frame of the fast process. Its structure and electronic
state were confirmed by EPR and XAS. As reported earlier, the [3,4]â˛
intermediate likely results from reaction of [4,5] with water. While
it is generated under strongly oxidizing conditions, it does not display
oxidation of the Ru centers past [3,4] according to EPR and XAS. EXAFS
analysis demonstrates a considerably modified ligand environment in
[3,4]â˛. Raman measurements confirmed the presence of the OâO
fragment by detecting a new vibration band in [3,4]Ⲡthat undergoes
a 46 cm<sup>â1</sup> shift to lower energy upon <sup>16</sup>O/<sup>18</sup>O exchange. Under the conditions of the experiment
at pH 1, the [3,4]Ⲡintermediate is the catalytic steady state
form of the blue dimer catalyst, suggesting that its oxidation is
the rate-limiting step