7 research outputs found
Proton-Coupled Electron Transfer in a Strongly Coupled Photosystem II-Inspired Chromophore–Imidazole–Phenol 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 flash–quench
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
× 10<sup>7</sup> s<sup>–1</sup> and a kinetic isotope
effect (KIE) of 0.99 ± 0.03. The electron transfer reaction was
characterized as adiabatic with λ<sub>DA</sub> = 1.16 eV and
280 < <i>H</i><sub>DA</sub> < 540 cm<sup>–1</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>–1</sup>. The reaction is nonadiabatic (<i>V</i><sub>PCET</sub> ≈ 22 cm<sup>–1</sup> (H) and 49 cm<sup>–1</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
Tetra- and Heptametallic Ru(II),Rh(III) Supramolecular Hydrogen Production Photocatalysts
Supramolecular
mixed metal complexes combining the trimetallic
chromophore [{(bpy)<sub>2</sub>RuÂ(dpp)}<sub>2</sub>RuÂ(dpp)]<sup>6+</sup> (<b>Ru</b><sub><b>3</b></sub>) with [RhÂ(bpy)ÂCl<sub>2</sub>]<sup>+</sup> or [RhCl<sub>2</sub>]<sup>+</sup> catalytic fragments
to form [{(bpy)<sub>2</sub>RuÂ(dpp)}<sub>2</sub>RuÂ(dpp)ÂRhCl<sub>2</sub>(bpy)]Â(PF<sub>6</sub>)<sub>7</sub> (<b>Ru</b><sub><b>3</b></sub><b>Rh</b>) or [{(bpy)<sub>2</sub>RuÂ(dpp)}<sub>2</sub>RuÂ(dpp)]<sub>2</sub>RhCl<sub>2</sub>(PF<sub>6</sub>)<sub>13</sub> (<b>Ru</b><sub><b>3</b></sub><b>RhRu</b><sub><b>3</b></sub>) (bpy = 2,2′-bipyridine and dpp = 2,3-bisÂ(2-pyridyl)Âpyrazine)
catalyze the photochemical reduction of protons to H<sub>2</sub>.
This first example of a heptametallic Ru,Rh photocatalyst produces
over 300 turnovers of H<sub>2</sub> upon photolysis of a solution
of acetonitrile, water, triflic acid, and <i>N</i>,<i>N</i>-dimethylaniline as an electron donor. In contrast, the
tetrametallic <b>Ru</b><sub><b>3</b></sub><b>Rh</b> produces only 40 turnovers of H<sub>2</sub> due to differences in
the excited state properties and nature of the catalysts upon reduction
as ascertained from electrochemical data, transient absorption spectroscopy,
and flash-quench experiments. While the lowest unoccupied molecular
orbital of <b>Ru</b><sub><b>3</b></sub><b>Rh</b> is localized on a bridging ligand, it is Rh-centered in <b>Ru</b><sub><b>3</b></sub><b>RhRu</b><sub><b>3</b></sub> facilitating electron collection at Rh in the excited state and
reductively quenched state. The Ru → Rh charge separated state
of <b>Ru</b><sub><b>3</b></sub><b>RhRu</b><sub><b>3</b></sub> is endergonic with respect to the emissive Ru →
dpp <sup>3</sup>MLCT excited and cannot be formed by static electron
transfer quenching of the <sup>3</sup>MLCT state. Instead, a mechanism
of subnanosecond charge separation from high lying states is proposed.
Multiple reductions of <b>Ru</b><sub><b>3</b></sub> and <b>Ru</b><sub><b>3</b></sub><b>Rh</b> using sodium amalgam
were carried out to compare UV–vis absorption spectra of reduced
species and to evaluate the stability of highly reduced complexes.
The <b>Ru<sub>3</sub></b> and <b>Ru<sub>3</sub>Rh</b> can
be reduced by 10 and 13 electrons, respectively, to final states with
all bridging ligands doubly reduced and all bpy ligands singly reduced
Push or Pull? Proton Responsive Ligand Effects in Rhenium Tricarbonyl CO<sub>2</sub> Reduction Catalysts
Proton responsive ligands offer control
of catalytic reactions
through modulation of pH-dependent properties, second coordination
sphere stabilization of transition states, or by providing a local
proton source for multiproton, multielectron reactions. Two <i>fac</i>-[Re<sup>I</sup>(α-diimine)Â(CO)<sub>3</sub>Cl]
complexes with α-diimine = 4,4′- (or 6,6′-) dihydroxy-2,2′-bipyridine
(4DHBP and 6DHBP) have been prepared and analyzed as electrocatalysts
for the reduction of carbon dioxide. Consecutive electrochemical reduction
of these complexes yields species identical to those obtained by chemical
deprotonation. An energetically feasible mechanism for reductive deprotonation
is proposed in which the bpy anion is doubly protonated followed by
loss of H<sub>2</sub> and 2H<sup>+</sup>. Cyclic voltammetry reveals
a two-electron, three-wave system owing to competing EEC and ECE pathways.
The chemical step of the ECE pathway might be attributed to the reductive
deprotonation but cannot be distinguished from chloride dissociation.
The rate obtained by digital simulation is approximately 8 s<sup>–1</sup>. Under CO<sub>2</sub>, these competing reactions generate a two-slope
catalytic waveform with onset potential of −1.65 V vs Ag/AgCl.
Reduction of CO<sub>2</sub> to CO by the [Re<sup>I</sup>(4DHBP–2H<sup>+</sup>)Â(CO)<sub>3</sub>]<sup>−</sup> suggests the interaction
of CO<sub>2</sub> with the deprotonated species or a third reduction
followed by catalysis. Conversely, the reduced form of [ReÂ(6DHBP)Â(CO)<sub>3</sub>Cl] converts CO<sub>2</sub> to CO with a single turnover
Electrocatalytic H<sub>2</sub> Evolution by Supramolecular Ru<sup>II</sup>–Rh<sup>III</sup>–Ru<sup>II</sup> Complexes: Importance of Ligands as Electron Reservoirs and Speciation upon Reduction
The supramolecular water reduction
photocatalysts [{(Ph<sub>2</sub>phen)<sub>2</sub>RuÂ(dpp)}<sub>2</sub>RhX<sub>2</sub>]Â(PF<sub>6</sub>)<sub>5</sub> (Ph<sub>2</sub>phen = 4,7-diphenyl-1,10-phenanthroline, dpp =2,3-bisÂ(2-pyridyl)Âpyrazine
X = Cl, Br) are efficient electrocatalysts for the reduction of CF<sub>3</sub>SO<sub>3</sub>H, CF<sub>3</sub>CO<sub>2</sub>H, and CH<sub>3</sub>CO<sub>2</sub>H to H<sub>2</sub> in DMF or DMF/H<sub>2</sub>O mixtures. The onset of catalytic current occurs at −0.82
V versus Ag/AgCl for CF<sub>3</sub>SO<sub>3</sub>H, −0.90 V
for CF<sub>3</sub>CO<sub>2</sub>H, and −1.1 V for CH<sub>3</sub>CO<sub>2</sub>H with overpotentials of 0.61, 0.45, and 0.10 V, respectively.
In each case, catalysis is triggered by the first dpp ligand reduction
implicating the dpp as an electron reservoir in catalysis. A new species
with <i>E</i><sub>pc</sub> ∼ −0.75 V was observed
in the presence of stoichiometric amounts of strong acid, and its
identity is proposed as the RhÂ(H)<sup>III/II</sup> redox couple. H<sub>2</sub> was produced in 72–85% Faradaic yields and 95–116
turnovers after 2 h and 435 turnovers after 10 h of bulk electrolysis.
The identities of RhÂ(I) species upon reduction have been studied.
In contrast to the expected dissociation of halides in the RhÂ(I) state,
the halide loss depends on solvent and water content. In dry CH<sub>3</sub>CN, in which Cl<sup>–</sup> is poorly solvated, a [Ru]
complex dissociates and [(Ph<sub>2</sub>phen)<sub>2</sub>RuÂ(dpp)ÂRh<sup>I</sup>Cl<sub>2</sub>]<sup>+</sup> and [(Ph<sub>2</sub>phen)<sub>2</sub>RuÂ(dpp)]<sup>2+</sup> are formed. In contrast, for X = Br<sup>–</sup>, the major product of reduction is the intact trimetallic
RhÂ(I) complex [{(Ph<sub>2</sub>phen)<sub>2</sub>RuÂ(dpp)}<sub>2</sub>Rh<sup>I</sup>]<sup>5+</sup>. Chloride loss in CH<sub>3</sub>CN is
facilitated by addition of 3 M H<sub>2</sub>O. In DMF, the reduced
species is [{(Ph<sub>2</sub>phen)<sub>2</sub>RuÂ(dpp)}<sub>2</sub>Rh<sup>I</sup>]<sup>5+</sup> regardless of X = Cl<sup>–</sup> or
Br<sup>–</sup>
Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO<sub>2</sub> Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH<sub>3</sub>)]<sup>2+</sup> (6DHBP = 6,6′-(OH)<sub>2</sub>bpy)
Ruthenium
complexes with proton-responsive ligands [RuÂ(tpy)Â(<i>n</i>DHBP)Â(NCCH<sub>3</sub>)]Â(CF<sub>3</sub>SO<sub>3</sub>)<sub>2</sub> (tpy = 2,2′:6′,2″-terpyridine; <i>n</i>DHBP = <i>n</i>,<i>n</i>′-dihydroxy-2,2′-bipyridine, <i>n</i> = 4 or 6) were examined for reductive chemistry and as
catalysts for CO<sub>2</sub> reduction. Electrochemical reduction
of [RuÂ(tpy)Â(<i>n</i>DHBP)Â(NCCH<sub>3</sub>)]<sup>2+</sup> generates deprotonated species through interligand electron transfer
in which the initially formed tpy radical anion reacts with a proton
source to produce singly and doubly deprotonated complexes that are
identical to those obtained by base titration. A third reduction (i.e.,
reduction of [RuÂ(tpy)Â(<i>n</i>DHBP–2H<sup>+</sup>)]<sup>0</sup>) triggers catalysis of CO<sub>2</sub> reduction; however,
the catalytic efficiency is strikingly lower than that of unsubstituted
[RuÂ(tpy)Â(bpy)Â(NCCH<sub>3</sub>)]<sup>2+</sup> (bpy = 2,2′-bipyridine).
Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical
infrared experiments suggest the reactivity of CO<sub>2</sub> at both
the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl
formed by the intermediacy of a metallocarboxylic acid is stable against
reduction, and mass spectrometry analysis of this product indicates
the presence of two carbonates formed by the reaction of DHBP–2H<sup>+</sup> with CO<sub>2</sub>
Noninnocent Proton-Responsive Ligand Facilitates Reductive Deprotonation and Hinders CO<sub>2</sub> Reduction Catalysis in [Ru(tpy)(6DHBP)(NCCH<sub>3</sub>)]<sup>2+</sup> (6DHBP = 6,6′-(OH)<sub>2</sub>bpy)
Ruthenium
complexes with proton-responsive ligands [RuÂ(tpy)Â(<i>n</i>DHBP)Â(NCCH<sub>3</sub>)]Â(CF<sub>3</sub>SO<sub>3</sub>)<sub>2</sub> (tpy = 2,2′:6′,2″-terpyridine; <i>n</i>DHBP = <i>n</i>,<i>n</i>′-dihydroxy-2,2′-bipyridine, <i>n</i> = 4 or 6) were examined for reductive chemistry and as
catalysts for CO<sub>2</sub> reduction. Electrochemical reduction
of [RuÂ(tpy)Â(<i>n</i>DHBP)Â(NCCH<sub>3</sub>)]<sup>2+</sup> generates deprotonated species through interligand electron transfer
in which the initially formed tpy radical anion reacts with a proton
source to produce singly and doubly deprotonated complexes that are
identical to those obtained by base titration. A third reduction (i.e.,
reduction of [RuÂ(tpy)Â(<i>n</i>DHBP–2H<sup>+</sup>)]<sup>0</sup>) triggers catalysis of CO<sub>2</sub> reduction; however,
the catalytic efficiency is strikingly lower than that of unsubstituted
[RuÂ(tpy)Â(bpy)Â(NCCH<sub>3</sub>)]<sup>2+</sup> (bpy = 2,2′-bipyridine).
Cyclic voltammetry, bulk electrolysis, and spectroelectrochemical
infrared experiments suggest the reactivity of CO<sub>2</sub> at both
the Ru center and the deprotonated quinone-type ligand. The Ru carbonyl
formed by the intermediacy of a metallocarboxylic acid is stable against
reduction, and mass spectrometry analysis of this product indicates
the presence of two carbonates formed by the reaction of DHBP–2H<sup>+</sup> with CO<sub>2</sub>
Photocatalytic CO<sub>2</sub> Reduction by Trigonal-Bipyramidal Cobalt(II) Polypyridyl Complexes: The Nature of Cobalt(I) and Cobalt(0) Complexes upon Their Reactions with CO<sub>2</sub>, CO, or Proton
The
cobalt complexes Co<sup>II</sup>L1Â(PF<sub>6</sub>)<sub>2</sub> (<b>1</b>; L1 = 2,6-bisÂ[2-(2,2′-bipyridin-6′-yl)Âethyl]Âpyridine)
and Co<sup>II</sup>L2Â(PF<sub>6</sub>)<sub>2</sub> (<b>2</b>;
L2 = 2,6-bisÂ[2-(4-methoxy-2,2′-bipyridin-6′-yl)Âethyl]Âpyridine)
were synthesized and used for photocatalytic CO<sub>2</sub> reduction
in acetonitrile. X-ray structures of complexes <b>1</b> and <b>2</b> reveal distorted trigonal-bipyramidal geometries with all
nitrogen atoms of the ligand coordinated to the CoÂ(II) center, in
contrast to the common six-coordinate cobalt complexes with pentadentate
polypyridine ligands, where a monodentate solvent completes the coordination
sphere. Under electrochemical conditions, the catalytic current for
CO<sub>2</sub> reduction was observed near the CoÂ(I/0) redox couple
for both complexes <b>1</b> and <b>2</b> at <i>E</i><sub>1/2</sub> = −1.77 and −1.85 V versus Ag/AgNO<sub>3</sub> (or −1.86 and −1.94 V vs Fc<sup>+/0</sup>),
respectively. Under photochemical conditions with <b>2</b> as
the catalyst, [RuÂ(bpy)<sub>3</sub>]<sup>2+</sup> as a photosensitizer,
tri-<i>p</i>-tolylamine (TTA) as a reversible quencher,
and triethylamine (TEA) as a sacrificial electron donor, CO and H<sub>2</sub> were produced under visible-light irradiation, despite the
endergonic reduction of CoÂ(I) to Co(0) by the photogenerated [RuÂ(bpy)<sub>3</sub>]<sup>+</sup>. However, bulk electrolysis in a wet CH<sub>3</sub>CN solution resulted in the generation of formate as the major
product, indicating the facile production of Co(0) and [Co–H]<sup><i>n</i>+</sup> (<i>n</i> = 1 and 0) under electrochemical
conditions. The one-electron-reduced complex <b>2</b> reacts
with CO to produce [Co<sup>0</sup>L2Â(CO)] with ν<sub>CO</sub> = 1894 cm<sup>–1</sup> together with [Co<sup>II</sup>L2]<sup>2+</sup> through a disproportionation reaction in acetonitrile, based
on the spectroscopic and electrochemical data. Electrochemistry and
time-resolved UV–vis spectroscopy indicate a slow CO binding
rate with the [Co<sup>I</sup>L2]<sup>+</sup> species, consistent with
density functional theory calculations with CoL1 complexes, which
predict a large structural change from trigonal-bipyramidal to distorted
tetragonal geometry. The reduction of CO<sub>2</sub> is much slower
than the photochemical formation of [RuÂ(bpy)<sub>3</sub>]<sup>+</sup> because of the large structural changes, spin flipping in the cobalt
catalytic intermediates, and an uphill reaction for the reduction
to Co(0) by the photoproduced [RuÂ(bpy)<sub>3</sub>]<sup>+</sup>