Probing the oxidation chemistry of half-sandwich iridium complexes with oxygen atom transfer reagents

Article abstract of DOI:10.1021/ic4016405

The new complexes [Ir(Cp*)(phpy)3,5-bis(trifluoromethyl)benzonitrile] ⁺ (1-NCAr⁺) and [Ir(Cp*)(phpy)(styrene)]⁺ (1-Sty⁺, Cp* = η⁵-pentamethylcyclopentadienyl, phpy = 2-phenylene-κC^1′-pyridi

Full text of DOI:10.1021/ic4016405

Article
pubs.acs.org/IC
Probing the Oxidation Chemistry of Half-Sandwich Iridium
Complexes with Oxygen Atom Transfer Reagents
Christopher R. Turlington, Daniel P. Harrison,† Peter S. White, Maurice Brookhart,
and Joseph L. Templeton*
W. R. Kenan Laboratory, Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina
27599-3290, United States
S
* Supporting Information
ABSTRACT: The new complexes [Ir(Cp*)(phpy)3,5-bis-
(trifluoromethyl)benzonitrile]+ (1-NCAr+) and [Ir(Cp*)-
(phpy)(styrene)]+ (1-Sty+, Cp* = η5-pentamethylcyclopenta-
dienyl, phpy = 2-phenylene-κC1′-pyridine-κN) were prepared
as analogues of reported iridium water oxidation catalysts, to
study their reactions with oxygen atom transfer (OAT)
reagents at low temperatures. In no case was the desired
product, an Ir(V)oxo complex, observed by spectroscopy. Instead, ligand oxidation was implicated. Oxidation of 1-NCAr+ with
the OAT reagent dimethyldioxirane (DMDO) yielded dioxygen when analyzed by GC, but formation of a heterogeneous or
paramagnetic species was simultaneously observed. This amplifies uncertainty over the actual identity of iridium catalysts in the
harsh oxidizing conditions required for water oxidation. Catalyst stability was then assessed for a reported styrene epoxidation
+
mediated by [Ir(Cp*)(phpy)(OH2)]+ (1-OH2 ). It was found that the OAT reagent iodosobenzene (PhIO) extensively oxidized
+
the organic ligands of 1-OH2 . Acetic acid was detected as a decomposition product. In addition, both the molecular structure
and the aqueous electrochemistry of 1-OH2+ are described for the first time. Oxidative scans revealed rapid decomposition of the
complex. All of the above experiments indicate that degradation of the organic ligands in catalysts built with the Ir(Cp*)(phpy)
framework are facile under oxidizing conditions. In separate experiments designed to promote ligand substitution, an unexpected
silver-bridged, dinuclear Ir(III) species with terminal hydrides, [{Ir(Cp*)(phpy)H}2Ag]+ (2), was discovered. The source of Ag+
for complex 2 was identified as AgCl.
Scheme 1. (a) Aqueous Oxidation of 3 with 58 equiv of
INTRODUCTION
■
a
CAN and (b) CAN Oxidation of a Representative Ir(Cp*)
Molecular iridium catalysts for water oxidation have received
considerable attention.1−7 These catalysts were originally
considered to be homogeneous molecular catalysts when the
oxidation was driven by ceric ammonium nitrate (CAN), and
the reactive species was postulated to be an Ir(V)oxo species.8,9
A popular ligand framework employs the η5 Cp* ligand to form
half-sandwich complexes, often in conjunction with a chelating
κ2 ligand. Many catalysts have been reported with this motif.4−8
Later work raised significant questions about catalyst
stability.10−12 Water oxidation experiments typically employ
the harsh oxidant CAN in excess of 100−10,000 equivalents.
To study catalyst evolution with fewer equivalents of oxidant,
Grotjahn et al. acquired scanning transmission electron
microscopy (STEM) images after 15 min of water oxidation
with Ir(Cp*)(phpy)Cl (3) and 58 equiv of CAN (Scheme
1a).13 The STEM images revealed significant agglomeration of
iridium- and cerium-based particles. Although nanoparticle
formation can occur during STEM sample preparation or from
electron beam impact during data collection, many representa-
tive Ir(Cp*) catalysts required 15−30 equiv of CAN before
oxygen evolution started.13 A homogeneous process requires
only 4 equiv of CAN to generate 1 equiv of oxygen, so
modification of the ligands of the catalyst may occur before the
onset of oxygen evolution.
b
Catalyst (ref 13)
a
Particles were detected in STEM images after 15 minutes.
b
Monitored by NMR spectroscopy in D2O. Complete degradation of
the catalyst was observed.
Received: June 27, 2013
© XXXX American Chemical Society
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Compelling evidence for ligand oxidation was obtained when
the catalysts were slowly treated with 15 equiv of CAN in D2O,
and the reactions were monitored by NMR spectroscopy. Even
addition of 1 equiv of CAN to a representative Ir(Cp*) catalyst
(pictured in Scheme 1b) resulted in loss of 16% of the complex
after 20 min. As 14 more equivalents of CAN were added over
an additional 3 h, the characteristic signals of the metal complex
disappeared completely, and were replaced by many signals of
low intensity. Acetic and formic acids were identified at the end
of the oxidative reaction sequence, and such oxidative
degradation was witnessed for all of the catalysts tested.13
Later NMR studies by Macchioni confirmed that Ir(Cp*)
catalysts can evolve acetic, formic, and glycolic acids when
oxidized with a large excess of CAN (>450 equiv).14,15
Evidence for homogeneous catalysis was obtained when
using sodium periodate as the sacrificial oxidant in place of
CAN. Crabtree and co-workers have employed time-resolved
dynamic light scattering (DLS) as an effective method to probe
for heterogeneous particles, and they have concluded that
molecular iridium catalysts are responsible for water oxidation
and C−H hydroxylation in the presence of excess sodium
periodate (Scheme 2a).16,17 New bidentate ligands that are
Scheme 3. Strategy for Observing an Ir(V)oxo Complex at
Low Temperatures in Organic Solvents
only two examples are known.19,20 Terminal oxo ligands on late
transition metals are uncommon since filled dπ orbitals on the
metal and filled p orbitals on a terminal oxide engender
destabilizing, repulsive four-electron interactions (Figure 1).21
Figure 1. Repulsive interaction between filled dπ orbitals on metal and
filled p orbitals that destabilize a late transition metal oxo complex.
Scheme 2. (a) Homogeneous Iridium-Catalyzed Water
Oxidation and Hydroxylation Reactions Driven by Sodium
Periodate, and (b) Sacrificial Loss of the Cp* Ligand of 4 Is
Required for Catalyst Activation
Motivation for this work included literature precedence for
platinum oxo formation. Milstein reported a Pt(IV)oxo
complex through OAT to a Pt(II)pincer complex using
dimethyldioxirane (DMDO).22 An important feature of this
experiment was the use of acetone as a highly labile ligand on
the Pt(II) precursor, which allowed the OAT reagent to react at
the metal center. Inspired by this report, we utilized analogous
conditions for our attempts to oxidize iridium complexes that
were models of water oxidation catalysts.
In addition, we desired to study the aqueous electrochemistry
+
of 1-OH2 , a proposed intermediate in a water oxidation
reaction.4 Two proton-coupled electron transfer (PCET)
events would result in the net loss of two protons and two
electrons, yielding a transient Ir(V)oxo species. PCET
oxidations can often be observed electrochemically in aqueous
solutions. For these reasons, it was believed that 1-OH2+ would
be a viable complex to probe for evidence of a high-valent
iridium species competent for water oxidation.
more stable under oxidative conditions, such as 2-(2′-pyridyl)-
2-propanolate (pictured in Scheme 2), also help stabilize
molecular catalysts. Even in systems using sodium periodate
and robust bidentate ligands, however, initial oxidation of the
complex results in sacrificial oxidation and displacement of the
Cp* ligand. Experimental evidence suggested that the
homogeneous resting state of precatalyst 4 was a dinuclear,
IrIV(bis-μ-oxo)IrIV species (Scheme 2b).18 Oxidation to Ir(V)
could yield the active species.18 These studies demonstrate that
alteration of the ligand environment (sacrificial loss of the Cp*
ligand) was still necessary to achieve homogeneous catalysis
with sodium periodate.
Although recent work has revealed oxidative alteration or
degradation of Ir(Cp*) complexes, we postulated that
stoichiometric oxidations of Ir(Cp*)(phpy) complexes with
oxygen atom transfer (OAT) reagents at low temperatures
might allow spectroscopic detection of a high-valent Ir(V)oxo
species (Scheme 3). Using OAT reagents offers the possibility
of completing the two electron oxidation to a terminal oxo in
one step, and switching from aqueous to organic solvents
would allow reactions to be run at low temperatures. Low
temperatures would improve the chance of observing oxidation
products. Iridium complexes with terminal oxo ligands are rare;
This work describes our attempts to observe an Ir(V)oxo
species using derivatives of 3, a reported water oxidation
catalyst.8,23 When stoichiometric oxidations did not yield an
observable Ir(V)oxo species, reactions with excess oxidant were
studied to investigate catalyst stability. It was found that 1-
NCAr+ reacted with multiple equivalents of DMDO, generating
oxygen. This process, however, could not be definitively
assigned to a homogeneous catalytic cycle. An epoxidation
+
reaction with 1-OH2 , styrene, and iodosobenzene (PhIO) was
also monitored. The reaction generated acetic acid, a product of
+
organic ligand degradation. The structure of 1-OH2 is
reported, but the complex is unstable when oxidized electro-
chemically. These experiments highlight the difficulties
associated with observing homogeneous high-valent iridium
species, and demonstrate that oxygen evolution or olefin
oxidation is not easily assigned to such molecular species.
Finally, an unexpected, silver-bridged dinuclear iridium species
formed in the presence of AgCl and Ir(Cp*)(phpy)H.24
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RESULTS AND DISCUSSION
■
Synthesis of Iridium Complexes. A goal of this project
was to synthesize an iridium complex with a weakly
coordinating ligand to study OAT, analogous to Milstein’s
work.22 Accordingly, we prepared [Ir(Cp*)(phpy)(3,5-bis-
(trifluoromethyl)benzonitrile)][B(ArF)4] (1-NCAr+, B(ArF)4
= tetrakis[3,5-bis(trifluoromethyl)phenyl]borate). This was
accomplished by halide abstraction from 3 with Na[B(ArF)4]
in the presence of the nitrile ligand, 3,5-bis(trifluoromethyl)-
benzonitrile (Scheme 4). The reaction was complete within 1 h
Scheme 4. Synthesis of [Ir(Cp*)(phpy)L][B(ArF)4]
Complexes
+
Figure 2. ORTEP drawing of 1-OH2 , with partial numbering scheme.
Ellipsoids are drawn at the 50% probability level. The anion and the
hydrogen atoms on the Cp* and phpy moieties are omitted for clarity.
Selected bond lengths (Å) and bond angles (deg): Ir1−O1 2.176(3),
Ir1−N1 2.089(4), Ir1−C1 2.055(4), O1−Ir1−C1 87.26(15), N1−Ir1−O1
81.88(14), C1−Ir1−N1 78.29(16).
Reaction of 1-NCAr+ with Dimethyldioxirane (DMDO).
In a reaction monitored by NMR spectroscopy in acetone-d6,
the reagent DMDO (added as a solution in acetone)30
disappeared over 50 min at −50 °C in the presence of 1
equiv of cationic iridium complex 1-NCAr+. DMDO does not
decompose at an appreciable rate in the absence of metal under
these reaction conditions.31 Less than 10% of the coordinated
nitrile ligand was displaced (Supporting Information, Figure
S5), but no other iridium species was detected over the course
of the reaction. The reaction products, therefore, were either
NMR silent or buried beneath the large acetone peak.
in dichloromethane at room temperature. Notably, the nearly
overlapping resonances of the aromatic protons of the nitrile
ligand shifted from 8.16 ppm (2H) and 8.15 (1H) in
dichloromethane-d2 to 8.15 ppm (1H) and 7.83 (2H) upon
coordination to the metal. This allowed convenient distinction
between bound or free nitrile in OAT reactions. Even though
1-NCAr+ has a weak two-electron nitrile donor ligand, the
complex is neither water sensitive nor air sensitive and can be
stored on the benchtop indefinitely.
In the reaction with DMDO we postulated that 1-NCAr+
was the resting state for a homogeneous catalytic cycle that
generates dioxygen from DMDO (Scheme 5). OAT from
DMDO to 1-NCAr+ could generate a transient [IrV(Cp*)-
A styrene derivative, [Ir(Cp*)(phpy)(styrene)][B(ArF)4] (1-
Sty+), was prepared in an analogous manner to 1-NCAr+.
Complex 1-Sty+ was studied in styrene epoxidation reactions, as
styrene readily displaced weaker ligands from Ir(Cp*)(phpy)
precatalysts to form 1-Sty+.
Scheme 5. Possible Catalytic Cycle for Homogeneous
Oxygen Evolution from DMDO Catalyzed by 1-NCAr+
+
The complex [Ir(Cp*)(phpy)(OH2)][OTf] (1-OH2 ) was
prepared according to a known procedure (OTf = trifluor-
omethanesulfonate).25 Crystals suitable for X-ray diffraction
studies were grown by layering pentane on top of a
+
concentrated solution of 1-OH2 in dichloromethane, and the
molecular structure is reported here for the first time (Figure
2).26 The aqua protons were located during refinement of the
structure, establishing that water is the sixth ligand in the
coordination sphere. Each aqua proton was within hydrogen
bonding distance to a triflate anion in the crystal lattice. The
iridium−oxygen bond length is 2.176 Å, which is within the
midrange for reported iridium aqua distances (2.136 Å−2.243
Å).27,28 For comparison, the bipyridine (bpy) derivative,
[Ir(Cp*)(bpy)(OH2)]2+, has an iridium oxygen bond length
of 2.153 Å.29 The bpy complex presumably has a shorter
iridium−oxygen bond length because it is a dication. When a
−
−
noncoordinating B(ArF)4 anion was substituted for the OTf
anion, it was found that [1-OH2][B(ArF)4] was stable in
methanol and acetone, but it decomposed within 30 min to
unidentified species in common organic solvents such as
benzene, chloroform, and dichloromethane. Even when stored
as a solid in a glovebox freezer at −25 °C, [1-OH2][B(ArF)4]
decomposed over 5 months into intractable materials. The
inherent instability of [1-OH2][B(ArF)4] was surprising.
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Figure 3. Oxygen evolution from DMDO catalyzed by 1-NCAr+ (17 mol %) at −40 °C. The overall percent yield of dioxygen (based on DMDO)
was plotted as a function of time for reactions A−D, pictured above. Reactions C and D were run side-by-side. Irreproducible results suggested that a
homogeneous mechanism was unlikely.
Figure 4. Time resolved NMR spectra of reaction of 1-NCAr+ and 6 equiv of DMDO at −40 °C. Complete disappearance of 1-NCAr+ was observed
after 2 h, but no decomposition products were observed by NMR spectroscopy. A heterogeneous species or paramagnetic species was likely the
product. DMDO continued to react over 5 h.
(phpy)O]+ complex, analogous to Milstein’s reported DMDO
oxidation of a Pt(II)pincer to form a Pt(IV)oxo complex.22
To test for the presence of oxygen, the DMDO reaction with
complex 1-NCAr+ was performed in an enclosed, degassed
vessel. All reagents were carefully degassed, and gas samples of
the reaction headspace were analyzed by GC. Traditional
oxygen probes (Clark electrodes, fluorescent probes) were not
compatible with acetone solutions. The reaction was scaled up
and the ratio of DMDO to iridium was increased from 1:1 to
6:1, to generate detectable levels of oxygen. When 6 equiv of
DMDO were added to 1 equiv of 1-NCAr+ in acetone at −40
°C, oxygen evolution started, and profiles of percent yield of
oxygen versus time for multiple reactions are shown in Figure 3.
To the best of our knowledge, only one example of oxygen
production from DMDO decay has been reported, and that was
when DMDO was treated with deprotonated peroxy acids.32 A
background reaction in the absence of iridium was also
monitored, and only trace amounts of oxygen were detected
A
terminal Ir(V)oxo species is expected to be electrophilic at
oxygen, allowing for nucleophilic attack.8 A lone pair of
electrons on a DMDO oxygen atom could serve as a
nucleophile, leading to oxygen−oxygen bond formation and a
two-electron reduction of iridium. Loss of acetone would lead
to a dioxygen complex, [Ir(Cp*)(phpy)O2]+, which could
liberate dioxygen, completing the catalytic cycle. The products
of such a catalytic cycle, oxygen and acetone, would be
1
undetected in the H NMR experiment. Acetone is already
present in such a large excess (because it is the solvent for
DMDO) that a small increase could not be reliably measured.
Our initial hypothesis was to invoke a homogeneous Ir(III)/
Ir(V) process to account for these results.
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Scheme 6. Styrene Oxidation Reactions Using PhIO as a Sacrificial Oxidant
when compared to reactions with 1-NCAr+ (Supporting
Information, Figure S6). The GC was calibrated using known
samples, and oxygen levels measured in the background
reaction were subtracted from the runs with catalyst to
determine overall yield.
oxidation products in 46% yield (Scheme 6, experiment a).25
The reactive species was proposed to be [IrV(Cp*)(phpy)O]+.
The reaction, however, took 12 h, and the mixture changed
from yellow to green upon addition of oxidant, perhaps
indicating formation of a heterogeneous component.13,16 In
addition, acetoxylation of a Cp* methyl group was observed in
a related oxidation of Ir(Cp*)(phpy)Me with PhI(OAc)2.34
Our goal was to investigate the nature of the iridium catalyst in
the epoxidation reaction in greater detail.
Oxygen was clearly produced, but each profile of percent
yield of oxygen over time showed dramatically different
behavior. The erratic data were confusing, so two reactions
were run side-by-side (Figure 3, samples C and D).
Inconsistent results were observed yet again. Some traces
showed nearly linear behavior, while other reactions exhibited
marked induction periods. Yields varied dramatically (50%−
94%, based on DMDO). It became clear that highly variable
processes were occurring during the reactions. The induction
periods observed for samples A and D also raised concerns that
oxidative degradation or particle formation from 1-NCAr+ had
to occur before oxygen evolution started. The irregular
behavior could not be attributed to the presence of oxone (a
reagent in the synthesis of DMDO),30 as DMDO was distilled
away from the reaction mixture. Note that carbon dioxide, a
decomposition product of [Ir(phpy)2(OH2)]+ in the presence
of excess CAN, was not observed in the GC analysis.1
The epoxidation reaction with 1-OH2+ was carefully repeated
in our lab, but our results differ from literature reports. Only
69% of the PhIO had reacted after 24 h (Scheme 6, experiment
b), while the previously reported reaction was complete after 12
h. Of the PhIO consumed, only 22% of the oxidative
equivalents yielded oxidized styrene derivatives, corresponding
to a turnover number of 15. Nearly 55 oxidative equivalents
(relative to iridium) were unaccounted for. The products of
styrene oxidation were 2-phenylacetaldehyde and benzalde-
hyde, in approximately equimolar ratios. No styrene epoxide
was observed, which was the major product reported in the
literature; however, the phenylacetaldehyde product likely
forms via acid-catalyzed rearrangement of styrene epoxide.35,36
Although the mechanism for formation of benzaldehyde is not
understood, it has also been observed as a product in styrene
epoxidation reactions catalyzed by manganese porphyrins.37
Iridium had to be present for styrene oxidation to occur.
After filtering off unreacted PhIO, the filtrate was loaded
onto a short silica gel plug. All of the organic compounds were
eluted with dichloromethane, and then the inorganic species
were eluted with acetone. After removing the eluent of the
metal-containing fractions in vacuo, the solids were dissolved in
dichloromethane-d2 and analyzed by NMR spectroscopy. The
When the reaction with a 6:1 ratio of DMDO to 1-NCAr+
was run in deuterated solvent (acetone-d6) and monitored by
NMR spectroscopy at −40 °C, the Cp* methyl signals and the
phpy resonances of 1-NCAr+ disappeared over 2 h at the same
time as the nitrile ligand was displaced. No minor species or
decomposition products such as glycolic, formic, or acetic acids
were observed in the NMR spectra, unlike the results of
Grotjahn13 and Macchioni14,15 in CAN oxidations of Ir(Cp*)
complexes. Meanwhile, the DMDO slowly disappeared over 5 h
(Figure 4). The results from the NMR experiment were
reproducible. The NMR data and concentration versus time
profiles suggest either ligand oxidation and particle formation
or generation of a paramagnetic iridium species (perhaps akin
to the resting state of homogeneous catalysts oxidized by
sodium periodate).18,33 It was difficult, therefore, to definitively
identify the species responsible for oxygen evolution. In view of
these results a homogeneous Ir(III)/Ir(V) mechanism, as we
originally postulated, seems unlikely. We believe that oxygen
evolution cannot be unambiguously assigned to molecular
Ir(V)oxo species in water oxidation catalysis. As this study with
DMDO and other oxidations with CAN demonstrate,11−15
harsh oxidants can lead to complex reactions and variable rates
of oxygen evolution, results which are not compatible with a
well-behaved molecular catalyst.
+
spectrum showed extensive ligand degradation, and 1-OH2
was not observed (Supporting Information, Figure S7). A host
of low intensity, sharp resonances (presumably oxidized ligand
fragments) surrounded broad resonances at 7.3 ppm, 4.1 ppm,
and 1.8 ppm. Complicated NMR spectra have also been
observed by Grotjahn in CAN oxidations of Ir(Cp*) catalysts.13
Additionally, acetic acid, observed multiple times as a
decomposition product of Ir(Cp*) catalysts in CAN oxidations,
was detected in an NMR spectrum of the crude reaction
mixture.12−15 The majority of oxidative equivalents in the
epoxidation reaction likely participate in oxidation of the
ligands, and the active iridium species could not be easily
identified.
In the absence of oxidant, styrene readily displaced the aqua
ligand of 1-OH2+ to form 1-Sty+. Complex 1-Sty+ is a potential
intermediate before the onset of catalyst decomposition. The
epoxidation experiment was repeated with independently
synthesized 1-Sty+, and similar results were obtained. After 24
Oxidation with Iodosobenzene (PhIO). Generation of
[IrV(Cp*)(phpy)O]+ has also been invoked to explain olefin
oxidations.4,9 For example, a styrene epoxidation catalyzed by
+
1-OH2 in the presence of the sacrificial oxidant PhIO gave
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Scheme 7. Proposed Electrochemical Oxidation of 1-OH2 to Generate an [IrV(Cp*)(phpy)O]+ Intermediate by Sequential
+
PCET Steps in Aqueous Solution
h, 64% of the PhIO reagent had been consumed (Scheme 6,
experiment c). Equimolar amounts of 2-phenylacetaldehyde
and benzaldehyde were observed once again, but only in a total
yield of 12% (based on PhIO consumed). Acetic acid was also
identified in the NMR spectrum of the crude reaction materials.
Isolation of the iridium species was accomplished again using a
silica gel plug, and NMR spectroscopy likewise revealed
significant ligand decomposition, along with the three
characteristic broad peaks (Supporting Information, Figure
S8). One of the minor species in both spectra included an α-
olefin pattern (presumably a bound styrene), but this was
distinct from 1-Sty+. It is clear that the iridium complexes
undergo oxidative degradation to unidentified species in the
presence of excess PhIO. The exact nature of the precatalyst has
little influence on the reaction, which would be expected if
there is significant ligand degradation. It is possible that an
Ir(V)oxo complex is the iridium species responsible for styrene
oxidation, but degradation of the ligand framework complicates
any assignment. Oxidations with PhIO, like oxidations with
CAN, lead to degradation of Ir(Cp*)(phpy) complexes and
generation of acetic acid.
Electrochemical Oxidation. When it became apparent
that OAT would not allow a definitive spectroscopic glimpse of
[IrV(Cp*)(phpy)O]+ in situ, attempts were made to detect an
Ir(V)oxo intermediate with electrochemical methods. Electro-
chemical oxidation of 1-OH2+ in an aqueous environment (Ag/
AgCl reference electrode (3 M NaCl; 0.197 V vs NHE), Pt-wire
counter electrode) could yield the Ir(V)oxo complex via
sequential PCET events (Scheme 7). Complex 1-OH2+ and the
two products of PCET processes were good targets to study
because they were originally proposed as intermediates for
homogeneous water oxidation using the precatalyst 3.4 A PCET
process is easily identified by observable pH dependent redox
processes that follow Nernstian behavior.38,39
Figure 5. (a) Cyclic voltammogram of [1-OH2][B(ArF)4] in water/
ethylene carbonate (2:1 v:v) at pH 7.0 (0.1 M phosphate buffer, 0.5 M
NaClO4) at 100 mV s−1 with a glassy carbon working electrode (0.07
cm2, solid line). (b) Differential pulse voltammogram of [1-
OH2][B(ArF)4] (dashed line, offset) at 23 3 °C.
there were no clear, reversible redox events at either pH, it was
not possible to identify PCET processes. Electrochemistry of
the precatalyst 3 in water/ethylene carbonate solutions at pH =
7.0 also did not reveal reversible waves and suggested catalyst
decomposition (Supporting Information). All electrochemical
data pointed to chemical alteration of the catalyst when
oxidized at high potentials, an outcome which is in agreement
with the chemical oxidants studied. The chemical and
electrochemical oxidations in this work demonstrate that
catalyst decomposition is facile under harshly oxidizing
conditions.
Isolation of a Silver-Bridged Dinuclear Iridium
Species. A single, unexpected, product was isolated from an
attempted one-pot synthesis of Ir(Cp*)(phpy)(OH).41 Com-
+
plex 1-OH2 was formed according to the literature procedure
Solutions of [1-OH2][B(ArF)4] in water/ethylene carbonate
(2:1 v:v) with added 0.1 M phosphate buffer and 0.5 M
supporting electrolyte (NaClO4) were prepared for electro-
chemistry. Ethylene carbonate aids solubility of [1-OH2][B-
(ArF)4] in water and is noncoordinating. Cyclic voltammo-
grams (CVs) using a freshly polished glassy carbon working
electrode at different pHs and scan rates, however, were
featureless and poorly defined. For example, a freshly prepared
0.73 mM solution of [1-OH2][B(ArF)4] in a water/ethylene
carbonate mixture at pH = 7.0 (0.1 M phosphate buffer, 0.5 M
NaClO4) passed a steadily increasing current at potentials
higher than 0.8 V (vs NHE) at a scan rate of 100 mV s−1
(Figure 5a). No reversible or clean oxidations were observed.
CVs of the water/ethylene carbonate solvent (2:1 v:v, pH = 7.0,
0.1 M phosphate buffer, 0.5 M NaClO4) showed that
background oxidation of the solvent was not significant until
potentials exceeded 1.5 V.40 Differential pulse voltammograms
(DPV) also did not show well-defined oxidations of the
complex, although there were three to four overlapping events
between 0.6 and 1.4 V (Figure 5b). Acidic solutions (pH = 2.4,
0.1 M phosphate buffer, 0.5 M NaClO4) also did not yield
useful electrochemical data (Supporting Information). Since
by mixing 3 and 1.1 equiv of AgOTf in methanol/water (30:1.3
v:v).25 After stirring for 1 h, 1.4 equiv of aqueous NaOH were
added prior to filtering. A silver-bridged dinuclear iridium
hydride species, [{Ir(Cp*)(phpy)H}2Ag][OTf], was isolated
(2, Scheme 8). The expected deprotonated product, Ir(Cp*)-
(phpy)(OH), either formed transiently or did not form at all.
The structure of 2 was unequivocally established via X-ray
diffraction studies on a suitable crystal (Figure 6),42 and the
terminal hydrides were confirmed by NMR spectroscopy. The
dinuclear complex is nearly centrosymmetric, which made
refinement using least-squares difficult. Instead, gradient
a
Scheme 8. Synthesis of 2
a
Sodium hydroxide must be added prior to filtering off AgCl.
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observed in the NMR experiment. When the reaction was
performed on a preparative scale in methanol (CH3OH), the
isolated product was the same as the product of the NMR
experiment, and it included a singlet for the hydride at −15.0
ppm.24
The source of Ag+ in 2 could not be excess AgOTf, as the 1.1
equiv of AgOTf converted to AgCl in the presence of
Ir(Cp*)(phpy)Cl (3). This suggested instead that the silver
source was silver chloride. To test the reactivity of silver
chloride, 3.5 mg of the isolated monomeric Ir(Cp*)(phpy)H
complex (7.2 μmol) was mixed with 7 mg of AgCl (48 μmol) in
0.5 mL of methanol-d4. NMR spectroscopy revealed clean
formation of the dinuclear iridium complex [{Ir(Cp*)(phpy)-
H}2Ag][Cl] (2-Cl) after 12 h. Notably, the hydride did not
exchange with deuterated solvent. The hydride resonance
shifted upfield to −11.7 ppm and appeared as a doublet, with a
JH−Ag coupling constant of 77 Hz. Silver has two isotopes with
nuclear spins of 1/2 with nearly identical gyromagnetic ratios.
The spectrum was not sufficiently resolved to observe the two
nearly equal coupling constants expected for the two silver
isotopes. Formation of 2-Cl verified that Ir(Cp*)(phpy)H
extracts Ag+ from AgCl. Synthesis of 2-Cl, therefore, is
proposed to occur via hydride elimination from methoxide to
form Ir(Cp*)(phpy)H, followed by Ag+ abstraction from AgCl,
as shown in Scheme 9. Although silver chloride is not often
Figure 6. ORTEP drawing of 2 with partial numbering scheme.
Ellipsoids are drawn at the 50% probability level. The anion and the
hydrogen atoms are omitted for clarity. The terminal hydrides were
observed by NMR spectroscopy, but were not located during
refinement of the crystallographic data. Selected bond lengths (Å)
and bond angles (deg): Ir1−Ag1 2.7176(6), Ir2−Ag1 2.7150(6), Ir1−
C11 2.055(8), Ir1−N1 2.031(7), Ag1−Ir1−C11 71.35(19), N1−Ir1−Ag1
108.23(18), C11−Ir1−N1 77.9(3).
refinement was used. Silver-bridged iridium complexes are
known, and the Ir−Ag bond lengths of 2.718 Å and 2.715 Å for
2 are as expected for such complexes when hydrides are
present.43−46 The Ir−Ag−Ir bridge is essentially linear (bond
angle of 179.35 degrees), and each Ir(Cp*)(phpy)(H)(Ag)
fragment resembles a four-legged piano stool. If one considers
each iridium to form a dative bond to Ag+, the iridium cores
would formally be seven-coordinate iridium(III), donating a
metal electron pair to silver. This provides evidence that the
Ir(Cp*)(phpy) framework is electron-donating. Oxidation
products seem to be unstable, as demonstrated in the chemical
and electrochemical oxidations reported, but stable products
with electron-donating Ir(III) centers, such as 2, may be
isolated. Hydrides are known to stabilize higher oxidation states
of iridium.47,48
Scheme 9. Proposed Mechanism for the Formation of 2-Cl,
a
+
Starting from 1-OH2
a
The individual steps were completed in two separate NMR
experiments.
considered a Ag+ source, it has found increasing use as a
reagent in the synthesis of silver complexes with coordinated N-
heterocyclic carbenes, which are useful transmetallating
reagents.49,50 Other silver sources, such as AgOTf and
AgSbF6, can be used in place of silver chloride.
The formation of 2 from 1-OH2+ upon addition of base was
investigated, as the sources of the hydrides and the silver bridge
were not readily apparent. It was postulated that the hydride
ligands were formed via hydride elimination from sodium
methoxide, generating formaldehyde. Hydroxide and meth-
oxide exist in equilibrium because of their similar pKa’s. The
ratio of methoxide to hydroxide was calculated to be 16:1 for a
mixture of 30 mL of methanol (pKa = 15.5) and 1.3 mL of
water (pKa = 15.7), the conditions of the reaction. The addition
of NaOH to methanol, therefore, predominantly yielded
NaOMe.
In solution, the two Ir(Cp*)(phpy)H units of dinuclear 2
were magnetically equivalent, so only one set of resonances for
the Cp* groups and the terminal hydrides were observed. The
aromatic region, however, showed sharp and broad resonances.
Variable temperature 1H NMR spectroscopy (Supporting
Information) revealed dynamic processes due to interconver-
sion of diastereomers of 2. At −60 °C, two diastereomers were
observed in approximately a 2:1 ratio. Two distinct hydride
signals were also seen as doublets between −11.5 and −12.0
ppm, with differing JH−Ag coupling constants of 87 and 90 Hz
for the two diastereomers.51 The two distinct coupling
constants expected for the two isotopes of silver could not be
resolved for any hydride resonance at any temperature. The
chemical shifts of the hydride signals for the isomeric
complexes were temperature dependent; the two doublets
moved together and overlapped to form a pseudotriplet at −20
°C.
To test the hypothesis that methoxide was acting as the
+
hydride source, 5 mg of 1-OH2 (7.7 μmol) was treated with
approximately 1 mg of NaOMe (19 μmol) in 0.5 mL of
methanol-d4. The added base exchanged rapidly with methanol-
d4, leading to formation of sodium methoxide-d3. Clean
conversion to a new metal species was observed by NMR
spectroscopy, and the product was identified as Ir(Cp*)-
(phpy)D.24 The formation of the deuteride complex from 1-
+
OH2 and NaOCD3 lent support to the hypothesis that
methoxide was the source of the terminal hydrides of 2.
Whether the deuteride was transferred via an outer sphere or
inner sphere mechanism is unclear. In either case, the aqua
ligand appeared to be innocent, acting strictly as a labile ligand.
Formaldehyde-d2 generated as a byproduct would not be
Few changes occurred in the alkyl and aromatic regions of
the 1H NMR spectrum between −60 °C and −20 °C. The two
Cp* peaks stayed well-resolved and sharp. There were 10
separate resonances in the aromatic region (one would expect
G
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purification. 1H and 13C NMR spectra were recorded on Bruker
16 independent resonances for two diastereomers), but
multiple peaks were clearly overlapping. Interconversion on
an NMR time scale between the two diastereomers was visible
at 0 °C. The Cp* and hydride resonances broadened and
merged, but the hydrides maintained their coupling to silver
(both JH−Ag values were 85 Hz), indicating an intramolecular
conversion between the diastereomers. Analysis of line
broadening on the Cp* signal of the major diastereomer at 0
°C allowed a rate constant for conversion of the major to the
minor isomer of 29 s−1 (ΔG⧧ = ca. 14 kcal/mol) to be
extracted. In the aromatic region, four signals were sharp
relative to at least six broad signals. The sharp signals were
identified as pyridine protons, and the position of these peaks
did not change upon warming or cooling. The pyridine
resonances, therefore, had almost identical chemical shifts in
the two diastereomers and did not noticeably broaden at the
onset of interconversion. Six of the eight broad resonances
expected for exchanging phenyl protons for the two
diastereomers were observed.
The Cp* peaks and the hydride resonances coalesced at 30
°C, and the hydrides were still coupled to silver (JH−Ag = 77
Hz). The four sharp pyridine proton resonances in the aromatic
region were complemented by four broad phenyl proton
resonances, and the phenyl proton resonances continued to
sharpen as the temperature was raised to 50 °C. At this point,
the hydride coupling to silver was lost and the resonance for the
hydrides coalesced into a singlet, indicating rapid dissociation/
reassociation of an Ir(Cp*)(phpy)H fragment. The phenyl
protons could not be fully resolved at higher temperatures
because the molecule was thermally unstable.
1
DRX400, AVANCE400, and AMX300 spectrometers. H NMR and
13C NMR chemical shifts were referenced to residual 1H and 13C
signals of the deuterated solvents. X-ray diffraction studies were
conducted on a Bruker-AXS SMART APEX-II diffractometer. Suitable
crystals were selected and mounted using a paratone oil on a MiteGen
mylar tip. Elemental analyses were performed by Robertson Microlit
Laboratories of Madison, NJ.
Electrochemistry. Electrochemical measurements were conducted
on a CH Instruments 660D potentiostat with a glassy carbon working
electrode (0.07 cm2), Pt-wire counter electrode, and Ag/AgCl
reference (saturated NaCl, 0.197 V vs NHE). E1/2 values were
obtained from the peak currents in differential pulse voltammograms
and are reported vs the normal hydrogen electrode (NHE). The glassy
carbon working electrode was polished on alumina paste, rinsed with
water, and dried before each use.
Synthesis of Ir(Cp*)(phpy)Cl (3). The compound was prepared
as reported.23
Synthesis of [Ir(Cp*)(phpy)(3,5-bis(trifluoromethyl)-
benzonitrile)][B(ArF)4] (1-NCAr+). A 100 mL Schlenk flask was
charged with 720 mg of Na[B(ArF)4] (0.81 mmol), 150 μL of 3,5-bis-
(trifluoromethyl)benzonitrile (0.89 mmol), and 20 mL of dichloro-
methane. A 360 mg portion of Ir(Cp*)(phpy)Cl (0.70 mmol) was
added, and the solution was stirred for 24 h at room temperature. The
reaction mixture was filtered, and the solvent of the filtrate was
reduced to a minimal volume. The concentrated solution was slowly
added to rapidly stirring pentanes chilled to −78 °C. The mixture was
filtered, and the bright yellow solid was heated at 50 °C overnight
under vacuum, yielding 920 mg product (82%). Anal. Calcd. (found)
for C62H38N2BF30Ir: C, 47.01 (47.10); H, 2.42 (2.70); N, 1.77 (1.78).
1H NMR (600 MHz, CD2Cl2) δ 8.65 (d, J = 5.4, 1H), 8.15 (s, 1H),
7.96 (d, J = 8.4, 1H), 7.88 (t, J = 7.8, 1H), 7.83 (s, 2H), 7.78 (t, J = 8.4,
2H), 7.72 (s, 8H), 7.55 (s, 4H), 7.34 (t, J = 7.2, 1H), 7.23−7.27 (m,
2H), 1.72 (s, 15H). 13C{1H} NMR (150 MHz, CD2Cl2) δ 167.7, 161.7
(1:1:1:1 quartet, JC−B = 50), 155.1, 151.7, 144.6, 139.7, 135.6, 134.8,
133.5, 133.3, 132.2, 128.8 (q, 2 bond JC−F = 29), 128.4, 124.6 (q, 1
bond JC−F = 271), 124.9, 124.6, 123.9, 121.9 (q, 1 bond JC−F = 269),
120.3, 117.5, 116.6, 112.2, 92.7, 8.7.
CONCLUSIONS
■
Oxidation to form observable high-valent iridium species is
challenging, especially when the target is an Ir(V)oxo species.
Oxidation using OAT reagents provided some encouraging
initial results, as oxidation of 1-NCAr+ with DMDO yielded
dioxygen. In addition, some oxidized styrene products were
Synthesis of [Ir(Cp*)(phpy)(styrene)][B(ArF)4] (1-Sty+). To a
solution of 200 mg of Ir(Cp*)(phpy)Cl (0.39 mmol) in 30 mL of
dichloromethane were added 400 mg of Na[B(ArF)4] (0.45 mmol, 1.2
equiv) and 90 μL of styrene (0.79 mmol, 2 equiv). The mixture was
filtered after stirring for 2 h, and the filtrate was reduced to minimal
volume, cooled to −78 °C, and filtered again. The concentrated
solution was added dropwise to 40 mL of rapidly stirring hexanes
cooled to −40 °C. The precipitate was collected via filtration and
+
observed in the presence of 1-OH2 and PhIO. Further
investigation, however, revealed extensive ligand oxidation or
suggested a heterogeneous component, and molecular Ir(V)oxo
complexes could not be definitively implicated in either system.
Acetic acid, a decomposition product observed for oxidation
with CAN, was also identified in oxidations with PhIO. These
results demonstrate that oxidation to high-valent iridium
species with retention of ligand identity cannot be assumed.
These conclusions are strengthened by the aqueous electro-
1
dried, yielding 266 mg of pale yellow solid (47%). The H NMR
spectrum and 13C NMR spectrum revealed the presence of a major
and minor isomer (∼4:1 ratio). Multiple resonances overlapped in the
1H NMR spectrum. Anal. Calcd. (found) for C61H43NBF24Ir: C, 50.56
(50.33); H, 2.99 (2.78); N, 0.97 (0.93). 1H NMR (500 MHz, CD2Cl2)
δ (Major isomer) 7.72 (s, 9H), 7.63 (d, J = 10.0, 2H), 7.56 (s, 4H),
7.54 (t, J = 10.5, 1H), 7.40 (m, 2H) 7.30 (t, J = 9.5, 1H), 6.98 (t, J =
9.0, 1H), 6.84 (t, J = 9.5, 1H), 6.59 (t, J = 7.5, 1H), 4.78 (dd, J = 11.0
and 14.5, 1H), 3.61 (m, 2H), 1.57 (s, 15H); δ (Minor isomer; multiple
resonances were unobserved) 8.19 (d, J = 7.0, 1H), 7.88 (t, J = 9.0,
1H), 7.13 (t, J = 9.5, 1H), 7.04 (t, J = 9.5, 1H), 6.95 (t, J = 10.5, 1H),
6.28 (t, J = 9.0, 2H), 4.51 (dd, J = 10.0 and 15, 1H), 3.77 (dd, J = 11.0
and 1.5, 1H), 3.57 (partially overlapping, 1H), 1.57 (shoulder on
major isomer, 15H). 13C{1H} NMR (125 MHz, CD2Cl2) δ (Major
isomer, one carbon merged to others) 166.0, 162.1 (1:1:1:1 quartet,
JC−B = 51), 153.2, 151.8, 143.8, 139.1, 136.7, 135.5, 135.2, 132.0, 129.2
(q, 2 bond JC−F = 31), 129.0, 128.5, 125.7, 125.5, 125.0 (q, 1 bond JC−F
= 270), 123.5, 119.8, 117.9 (pseudo t, J = 15), 100.5, 70.6, 46.6, 8.2; δ
(Minor isomer, one carbon merged to others) 167.6, 162.1 (1:1:1:1
quartet, JC−B = 51), 156.8, 152.8, 142.9, 140.3, 135.9, 135.7, 135.2,
129.2 (q, 2 bond JC−F = 31), 128.2, 128.0, 126.4, 125.6, 125.0 (q, 1
bond JC−F = 270), 124.9, 124.3, 120.6, 117.9 (pseudo t, J = 15), 100.3,
71.2, 49.0, 8.6.
+
chemistry of 1-OH2 , which was unstable when oxidized
electrochemically. While all of these results reflect the instability
of high-valent iridium species, it was found that the Cp* and
phpy ligands supported the formation of 2, a stable dinuclear
iridium species with an expanded coordination sphere at
iridium. Complex 2 presumably formed from the reaction of
Ir(Cp*)(phpy)H and AgCl. The reactivity observed with silver
chloride is a rare example of Ag+ extraction from AgCl.
EXPERIMENTAL SECTION
■
Materials and Methods. All reactions were performed under an
atmosphere of dry argon using standard Schlenk and drybox
techniques, unless noted otherwise. Argon was purified by passage
through columns of BASF R3-11 catalyst and 4 Å molecular sieves.
Methylene chloride, hexanes, and pentane were purified under an
argon atmosphere and passed through a column packed with activated
alumina. All other chemicals were used as received without further
H
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Article
+
Synthesis of [Ir(Cp*)(phpy)(OH2)][OTf] (1-OH2 ). The com-
pound was prepared as reported.25 Crystals suitable for X-ray
diffraction were obtained by layering pentane on top of a concentrated
solution in dichloromethane.
the reaction headspace were taken periodically with a gastight syringe
and analyzed by GC to determine percent yield of dioxygen.
NMR-Tube Reaction of DMDO and 1-NCAr+, 6:1. A 15%
acetone: 85% acetone-d6 (v:v) solution of DMDO and DMDO-d6
(15:85 ratio, 0.123 M, 65 μL, 8 μmol) was added slowly to an NMR
tube chilled to −80 °C and containing a solution of 1-NCAr+ (2.0 mg,
1.3 μmol) in 0.5 mL of acetone-d6. The reaction was monitored by
NMR spectroscopy at −40 °C. The resonances of 1-NCAr+
disappeared over 2 h, and the nitrile ligand was displaced. No new
iridium species was observed, and the DMDO continued to react over
5 h.
Synthesis of [Ir(Cp*)(phpy)(OH2)][B(ArF)4] ([1-OH2][B(ArF)4]).
A 100 mL Schlenk flask was charged with 376 mg of [Ir(Cp*)-
(phpy)(OH2)][OTf] (0.58 mmol), 525 mg of Na[B(ArF)4] (0.59
mmol), and 20 mL of dichloromethane. After stirring for 30 min, the
mixture was filtered, and the solvent of the filtrate was reduced to a
minimal volume. The solution was slowly added to 30 mL of rapidly
stirring pentanes at −40 °C. The precipitate was isolated by filtration
and heated under vacuum at 50 °C overnight, yielding 546 mg of a
deep red solid (69%). Anal. Calcd. (found) for C53H37NBF24IrO: C,
+
Styrene Oxidation with PhIO and 1-OH2 . A solution of 7.0 mg
+
of 1-OH2 (0.01 mmol) in 2.5 mL of dichloromethane was slowly
1
added to a mixture of 220 mg of PhIO (1.0 mmol) and 460 μL of
styrene (4.0 mmol) in 2.5 mL of dichloromethane cooled to 0 °C. The
reaction was stirred for 24 h at 0 °C, at which point the insoluble PhIO
was removed from the reaction mixture by filtration. A drop of the
46.71 (46.98); H, 2.74 (2.66); N, 1.03 (1.04). H NMR (500 MHz,
CD3OD) δ 8.94 (d, J = 5.5, 1H), 8.09 (d, J = 8.0, 1H), 8.02 (d, J = 7.5,
1H), 7.97 (t, J = 7.5, 1H), 7.83 (d, J = 7.5, 1H), 7.59 (s, 12H), 7.40 (t,
J = 6.0, 1H), 7.28 (t, J = 7.5, 1H), 7.16 (t, J = 7.5, 1H), 1.65 (s, 15H).
13C{1H} NMR (125 MHz, CD3OD) δ 169.3, 162.7 (1:1:1:1 quartet,
JC−B = 49), 162.3, 153.3, 147.5, 141.0, 136.9, 135.8, 132.6, 130.4 (q, 2
bond JC−F = 31), 125.7 (q, 1 bond JC−F = 270), 125.5, 125.4, 125.3,
120.8, 118.7, 89.6, 9.1.
1
filtrate was added to dichloromethane-d2 and analyzed by H NMR
spectroscopy to identify reaction products and determine yields. The
filtrate was then concentrated under reduced pressure and loaded onto
a short silica gel plug (10 cm). After the organic compounds were
eluted with dichloromethane, acetone was used as the eluent to
retrieve the inorganic species. The solvent was removed under reduced
pressure, and the solids were redissolved in dichloromethane-d2 and
analyzed by NMR spectroscopy.
Synthesis of [{Ir(Cp*)(phpy)H}2Ag][OTf] (2). A 100 mL Schlenk
flask was charged with 30 mL of methanol, 1.3 mL of water, 300 mg of
Ir(Cp*)(phpy)Cl (0.58 mmol), and 155 mg of AgOTf (0.60 mmol).
After stirring for 1 h in the dark, 160 μL of 5 M aqueous NaOH (0.80
mmol, 1.4 equiv) was added, and the solution immediately turned red.
After stirring for 20 min, the color had changed to a pale yellow-green.
The reaction was stirred for five additional hours and then filtered. The
solvent of the filtrate was removed under reduced pressure, and the
solid was redissolved in minimal methanol. Water was slowly added to
the solution to precipitate the product, which was isolated by filtration.
The solid was then redissolved in minimal, hot isopropanol, filtered,
and slowly added to rapidly stirring hexanes chilled to −40 °C. After
storing overnight at −30 °C, the mixture was filtered, and 169 mg of a
yellow-green solid was isolated after drying under high vacuum for 4 h
(48%). Crystals suitable for X-ray diffraction were obtained by layering
hexanes on top of a concentrated solution in 2-propanol. Anal. Calcd.
(found) for C43H48N2AgF3Ir2O3S: C, 42.26 (42.33); H, 3.96 (3.86);
N, 2.29 (2.30). 1H NMR (500 MHz, CD3OD) δ 8.72 (d, J = 5.5, 2H),
7.92 (s, 2H), 7.77 (t, J = 7.5, 2H), 7.67 (s, 2H), 7.42 (d, J = 7.5, 2H),
7.12 (t, J = 6.5, 2H), 6.90 (s, 2H), 6.63 (s, 1−2H), 1.74 (s, 30H),
−11.70 (d, JH−Ag = 77 Hz, 2H).
Synthesis of Ir(Cp*)(phpy)H. The compound can be prepared as
reported.24 An additional synthesis is given here. A solution of 100 mg
of Ir(Cp*)(phpy)Cl (0.19 mmol) and 85 mg of NaOMe (1.57 mmol,
8 equiv) in 20 mL of methanol was stirred for 7 h, at which point the
solvent was removed. Twenty-five milliliters of water were added, and
the mixture was sonicated for 20 min. The mixture was filtered, and
the pale orange solid was dried in air for 3 h, yielding 48 mg solid
(49%). The hydride exchanged for chloride in chlorinated solvents.
Anal. Calcd. (found) for C21H24NIr·H2O: C, 50.38 (50.31); H, 5.23
(4.93); N, 2.80 (2.61).
Styrene Oxidation with PhIO and 1-Sty+. The procedure for
styrene epoxidation with 1-OH2+ was followed, except that 15.3 mg of
1-Sty+ (0.01 mmol) was used.
ASSOCIATED CONTENT
■
S
* Supporting Information
NMR spectra, including those of new compounds, the
recovered metal species after epoxidation reactions, and variable
temperature spectra of 2 are provided. Also included are the
electrochemical studies of 3 and crystallographic information of
the reported structures. This material is available free of charge
via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: joetemp@unc.edu.
Present Address
†Virginia Military Institute, 403 Maury-Brooke Hall, Lexington,
VA 24450.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
NMR-Tube Reaction of DMDO and 1-NCAr+, 1:1. An acetone
solution of DMDO (70 μL, 0.057 M, 4.0 μmol) was added slowly to
an NMR tube chilled to −80 °C and containing a solution of 1-NCAr+
(6.0 mg, 3.8 μmol) in 0.5 mL of acetone-d6. The reaction was
transported at −80 °C to an NMR probe cooled to the same
temperature. The onset of the reaction occurred upon warming to −50
°C, and the reaction progress was monitored by NMR spectroscopy.
After the DMDO had reacted, at least 90% of the original 1-NCAr+
compound was intact, and no new iridium species was observed.
Reactions of DMDO and 1-NCAr+, 6:1, Monitored by GC.
Conditions of a representative reaction are described. A 10 mL round-
bottom flask was charged with 11.9 mg of 1-NCAr+ (7.5 μmol) and
1.6 mL of acetone, and then tightly capped with a serrated silicone
rubber septum. The solution was cooled to −40 °C and sparged with
nitrogen for 5 minutes. A 0.5 mL portion of a degassed acetone
solution of DMDO (0.091 M, 45.5 μmol, 6.1 equiv) was added slowly
via syringe, and the reaction was gently stirred at −40 °C. Samples of
This research was primarily supported by the UNC EFRC
“Center for Solar Fuels”, an EFRC funded by the U.S.
Department of Energy, Office of Science, Office of Basic Energy
Sciences under award DE-SC0001011 supporting D.P.H. In
addition C.R.T. acknowledges support from the National
Science Foundation Graduate Research Fellowship Program
under Grant DGE-1144081.
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52
+
crystal of 1-OH2 was kept at −173 °C. Using Olex2, the structure
was solved with the XS structure solution program using direct
methods and refined with the XL refinement package using least-
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= monoclinic; space group = P21/c; a/b/c (Å) = 16.1914(4)/
12.4570(3)/11.5310(3); α/β/γ (deg) = 90.00/107.4690(10)/90.00; Z
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