Photochemical Production of Ethane from an Iridium Methyl Complex

Article abstract of DOI:10.1021/acs.organomet.7b00175

An iridium methyl complex, [Cp?Ir(bpy)(CH₃)]⁺, was prepared by electrophilic methylation of Cp?Ir(bpy) with CH₃I and characterized electrochemically, photophysically, crystallographically, and computationally. Irradiation of the MLCT transition of [Cp?Ir(bpy)(CH₃)]⁺ in the presence of CH₃I in acetonitrile produces ethane, methane, propionitrile, and succinonitrile. A series of mechanistic studies indicates that C-C bond formation is mediated by free methyl radicals produced through monometallic photochemical homolysis of the Ir-CH₃ bond.

Full text of DOI:10.1021/acs.organomet.7b00175

Article
pubs.acs.org/Organometallics
Photochemical Production of Ethane from an Iridium Methyl
Complex
Catherine L. Pitman and Alexander J. M. Miller*
Department of Chemistry, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, United States
S
* Supporting Information
ABSTRACT: An iridium methyl complex, [Cp*Ir(bpy)(CH3)]+,
was prepared by electrophilic methylation of Cp*Ir(bpy) with
CH3I and characterized electrochemically, photophysically, crys-
tallographically, and computationally. Irradiation of the MLCT
transition of [Cp*Ir(bpy)(CH3)]+ in the presence of CH3I in
acetonitrile produces ethane, methane, propionitrile, and succino-
nitrile. A series of mechanistic studies indicates that C−C bond
formation is mediated by free methyl radicals produced through
monometallic photochemical homolysis of the Ir−CH3 bond.
Given that the Cp*Ir(bpy) scaffold facilitates efficient
bimetallic coupling for H2 evolution, we questioned whether
substitution of a methyl for the hydride could lead to C−C
bond formation. Herein we report the synthesis and character-
ization of the methyl complex, [Cp*Ir(bpy)(CH3)]+ ([2]+) and
our mechanistic investigation of its photochemical C−C bond
formation reactivity.
INTRODUCTION
■
Photocatalysts capable of mediating C−C bond formation
could play an important role in the light-driven generation of
energy-dense liquid fuels. Ethane formation from two methyl
fragments is the simplest C−C bond forming reaction leading
to an alkane, and organometallic methyl complexes have been
the subject of photophysical and photochemical inquiry.
Though photochemical ethane generation has been observed
from metal methyl complexes,1 such reactivity is notably
uncommon.2
Many approaches to integrating light absorption with C−C
bond formation have been explored. In photoredox catalysis,
photosensitizers can trigger C−C bond formation following an
excited state electron transfer.3,4 Photochemical ligand
dissociation (which opens a coordination site and triggers
migratory insertion) and M−C bond homolysis (leading to
radical reactivity) can also form C−C bonds.5 Radicals are a
common component of metal alkyl photochemistry. In the
1980s, Crabtree investigated Hg sensitization, which produces
radicals, for various alkane functionalizations.6 Platinum
complexes are also known to couple two alkyl ligands on a
single site through a radical mechanism.7
In thinking of ways to develop photochemical C−C bond
formation, we drew inspiration from our recently discovered,
bimolecular mechanism for H−H bond formation from a
monohydride.8 Quantum yields of hydrogen production
nearing unity can be achieved when irradiating [Cp*Ir(bpy)-
(H)]+ ([1]+; Cp* is η5-pentamethylcyclopentadienyl and bpy is
2,2′-bipyridine) in the presence of acids in CH3CN. The bond
formation is initiated by electron transfer between excited state
[1]+* and ground state [1]+, producing the reactive species
Cp*IrII(bpy)(H) and [Cp*IrIV(bpy)(H)]2+ that together form
H2. This mechanism allows a slightly endergonic excited state
electron transfer process to be coupled to an exergonic bond
formation.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of an Ir Methyl
Complex. Previously unreported methyl complex [Cp*Ir-
(bpy)(CH3)][I] ([2][I]) was synthesized by electrophilic
methylation of the Ir(I) precursor Cp*Ir(bpy) (3). A 50 mM
solution of CH3I in Et2O was added dropwise to a purple
solution of Cp*Ir(bpy) in Et2O, resulting in precipitation of
1
[2][I] as a fluffy yellow solid. The H NMR spectrum in
CD3CN shows the expected four aromatic resonances of bpy,
the 15H singlet of Cp*, and an upfield 3H singlet at δ −0.04.
The 13C NMR spectrum features a methyl resonance (δ
−6.35).
Vapor diffusion of Et2O into a solution of [2][I] in CH3CN
produced yellow crystals suitable for X-ray diffraction (XRD).
Complex [2][I] takes on a “piano stool” structure with a
methyl ligand and an outer sphere iodide ion (Figure 1). The
Ir−CH3 distance (2.147(5) Å) falls into the range of other
reported Cp*Ir−CH3 distances (2.03−2.22 Å; see Supporting
Information for full details).9 The Ir center, however, is not
centered below the Cp* ring: The carbons trans to methyl form
bonds to Ir (Ir−C13 2.235(5) Å and Ir−C14 2.236(4) Å)
longer than those cis to methyl (Ir−C11 2.186(4) Å, Ir−C12
2.176(4) Å, Ir−C15 2.189(4) Å). The structure of [Cp*Ir-
(bpy)(Cl)][ClO4], by contrast, features Ir−C distances that do
not vary around the cyclopentadienyl ring (2.163 Å).10 The
Received: March 6, 2017
© XXXX American Chemical Society
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(1.463(11) Å), in which the bpy is not considered to be
partially reduced.
In acetonitrile, the UV−vis spectrum of [2]+ shows an
absorbance with λmax = 418 nm (εmax = 3300 M−1 cm−1, Figure
3A). The absorbance of [2]+ is best fit by two Gaussians
Figure 1. Structural representation of [2][I] with ellipsoids drawn at
the 50% probability level. Hydrogen atoms and iodide counterion
omitted for clarity. Selected distances (Å): Ir−C21 2.147(5), C5−C6
1.458(6).
structure of [1][PF6]11 displays the same asymmetry found in
the structure of [2][I], suggesting that this asymmetry is
indicative of methyl and hydride ligands acting as strong σ-
donors in [2][I] and [1][PF6].
A structural comparison of Ir(III) complex [2]+ with reduced
Ir(I) precursor 3 was sought. A large purple block crystal
suitable for XRD was produced by slow evaporation of a
solution of 3 in THF. The report of this crystal structure
completes the group 9 Cp*M(bpy) series (M = Co, Rh, and
Ir).12−14 Like the lighter metal congeners, 3 adopts a near-
perpendicular orientation of the Cp* and bpy planes (84.68°,
Figure 2). The C−C bond connecting the pyridine rings of bpy
Figure 3. (A) Absorptivity (solid blue) and normalized emission
(dashed red) of [2]+ in CH3CN solution. Excitation at 420 nm. (B)
Cyclic voltammogram of 1 mM [2]+ in CH3CN with 0.1 M
[nBu4N][PF6] under an atmosphere of N2. Scan rate 0.25 V·s−1. (C)
Orbitals from TD-DFT involved in the lowest energy transitions at
450 and 442 nm.
Figure 2. Structural representation of 3 with ellipsoids drawn at the
50% probability level. Hydrogen atoms omitted for clarity. Selected
distance (Å): C5−C6 1.403(5).
centered at 413 and 476 nm that are estimated to have
extinction coefficients of 3300 and 1100 M−1 cm−1, respectively
(Figure S9). To explore the nature of the orbitals involved in
these transitions, we turned to time-dependent density
functional theory (TD-DFT). The singlet ground-state
structure of [2]+ was optimized using the M06 functional
(LANL2DZ ECP basis set for the Ir atom and 6-311G** for all
other atoms), modeling acetonitrile solvation with a polarized
continuum model. Absorption properties in acetonitrile were
explored using TD-DFT from the optimized ground-state
geometry (see Supporting Information for full details). The
transitions at 450 nm (f = 0.0161) and 442 nm (f = 0.0937)
reflect the observed spectrum: Both calculation and experiment
feature two transitions under the MLCT with the weaker
feature at a longer wavelength. According to TD-DFT, both
transitions arise out of mixed HOMO and HOMO−1 states to
(C5−C6 1.403(5) Å) shows the characteristic contraction
observed in these electron-rich species, attributed to electron
delocalization into bpy resulting in partial double-bond
character in the interpyridyl bond. In free bipyridine, this
distance is 1.49 Å and contracts to 1.43 Å in bpy•− and 1.39 Å
for bpy2−.15 In Ir complex 3, however, this bond is shorter than
that in either of the lighter brethren (M = Co, 1.419 Å;12 M =
Rh, 1.423 Å),13,14 suggesting that more electron density resides
on bpy when M = Ir.
The structure of [2][I] features a substantially longer C5−
C6 distance (1.458(6) Å) than that found in 3. The interpyridyl
distance in [2][I] is similar to that of [Cp*Ir(bpy)(Cl)][ClO4]
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Table 1. Comparison of Electrochemical and Photophysical Features of [1]+ and [2]+
parameter
X = CH3
X = H
E°(III/II)
E°(IV/III)
λMLCT, [Cp*IrIII(bpy)(X)]+
emission max, [Cp*IrIII(bpy)(X)]+
ΔGST
−1.82 V
0.74 V
418 nm (3300 M−1cm−1
−1.80 V8
0.50 V8
18
)
428 nm (3000 M−1 cm−1
708 nm8
)
702 nm
8
50 kcal·mol−1
0.35 V
47−52 kcal·mol−1
E°(III*/II)
E°(IV/III*)
0.37 V8
−1.42 V
−1.67 V8
the LUMO. The HOMO is a bonding interaction between a d-
orbital of Ir and the methyl C-pz orbital (z-axis along the Ir-
CH3 vector). HOMO−1, however, is an antibonding
interaction between Ir-dxz and methyl C-px orbitals (Figure
3C). The LUMO is primarily bpy π*-based.
9 kcal·mol−1 endergonic. Though a sizable barrier, it may be
surmountable when coupled to a thermodynamically favorable
chemical step, as has been observed for uphill electron transfers
with [Ru(bpy)3]2+.19 Self-quenching of [1]+ is ∼3 kcal·mol−1
endergonic and occurs with high efficiency at high concen-
tration.8
The absorbance spectrum of analogous Ir hydride [1]+ in
CH3CN has a single Gaussian MLCT transition at 428 (3000
M−1 cm−1), ascribed to a charge transfer from a σ bonding
HOMO (with Ir d-orbital and hydride s-orbital character) to a
bpy π* LUMO.16 While the LUMOs of [1]+ and [2]+ are
similar, the presence of p-orbitals in the methyl ligand account
for the spectral differences.
ΔG°ET = 23.06(E°(IV/III)) − 23.06(E°(III/II)) − ΔGST
(1)
From the comparison of electrochemical and photophysical
properties of hydride [1]+ and methyl [2]+ in Table 1 and from
the structural features discussed above, the electronic
similarities of the complexes are apparent. The similarity of
the emission features of [1]+ and [2]+ gives rise to similar
ΔGST; the value of ΔGST of 50 kcal·mol−1 determined here for
[2]+ falls within the range of 47 kcal·mol−1 < ΔGST < 52 kcal·
mol−1 previously determined for [1]+.8 While the reduction
potentials of [1]+ and [2]+ are similar, the oxidation of [2]+ is
240 mV more positive than that of [1]+. Additionally, this
oxidation is noticeably more reversible for [2]+ than that for
[1]+. Fast scan rates (200 V·s−1) were required to observe the
rereduction process for hydride [1]+,8 but for methyl [2]+, a
reduction feature is apparent even at a scan rate of 0.05 V·s−1
(Figure S13).
Upon irradiation into the MLCT absorbance of [2][I]
(excitation at 420 nm), no steady-state emission was observed
at room temperature in CH3CN. Hypothesizing that the iodide
counteranion was quenching the excited state,17 counterion
exchange was carried out by addition of excess NH4PF6 to a 9:1
H2O/MeOH solution of [2][I], resulting in precipitation of
[Cp*Ir(bpy)(CH3)][PF6] ([2][PF6]) as a light yellow powder.
Metathesized salt [2][PF6] is weakly emissive at room
temperature. In CH3CN solution, excitation at 420 nm
produces a broad emission feature at 702 nm with a
photoluminescence quantum yield of 0.04% (Figure 3A). The
excitation spectrum of [2][PF6] aligns with the MLCT feature
of the absorbance spectrum. The energy between the singlet
ground state and the triplet excited state (ΔGST) can be
estimated by extrapolation of the high-energy edge of the
emission feature to the baseline, giving ΔGST = 50 kcal·mol−1
(Figure S11).
Photochemical Production of Ethane and Methane.
Encouraged by the similarities in the electronic structures of
[1]+ and [2]+, we sought to probe for similarities in
photochemical reactivity. Inspired by the ability of [1]+ to
undergo light-induced H2 formation in the presence of a H+
source, we considered the analogous reaction for C−C bond
formation: light-induced ethane formation from [2]+ and a
The electronic structure of methyl complex [2]+ was
investigated by cyclic voltammetry. A 1 mM solution of
[2][PF6] was prepared in CH3CN containing 0.1 M [nBu4N]-
[PF6]. A reversible reduction (ΔEp = 77 mV, ip,c/ip,a = 0.94) of
[2]+ was observed at E1/2 = −1.82 V and a nearly irreversible
oxidation (ΔEp = 95 mV, ip,a/ip,c = 0.30 at 0.25 V·s−1) was
observed at E1/2 = 0.74 V (Figure 3B). The reduction, formally
Ir(III) to Ir(II), may also be described as a bpy-based reduction,
[2]+/Cp*IrIII(bpy•−)(CH3). The oxidation, formally [2]+/
[Cp*IrIV(bpy)(CH3)]2+, extends across the Cp*, Ir, and CH3
fragments according to our DFT. Excited state reduction
potentials can be calculated from the ground state potentials
and ΔGST (see Supporting Information for full details). The
excited state can be reduced at E°(III*/II) = 0.35 V, and it can
be oxidized at E°(IV/III*) = −1.42 V.
+
CH3 source.
A CD3CN solution of [2][PF6], excess CH3I, and a
mesitylene internal standard was illuminated with a 443 nm
1
LED for 3.5 h (Figure S14). H NMR spectroscopic analysis
showed the characteristic resonances of ethane, methane, and
propionitrile. Product yields were determined by measuring
gaseous concentrations by headspace GC and by measuring
1
dissolved gases by H NMR spectroscopy. Yields are reported
relative to [2]+ consumed in the reactions. Ethane, methane,
and propionitrile were produced in 19, 39, and 9% yield,
respectively. As hypothesized, photolysis of [2]+ forms C−C
bonds.
The primary Ir-containing species after photolysis was
[Cp*Ir(bpy)(I)]+ ([4]+, 95%). Iodide [4]+ was observed by
HR-MS (ESI+) in solutions after photolysis, and [4]+ was also
independently prepared by addition of 3 equiv of NaI to a
solution of [Cp*Ir(bpy)(OH2)][OTf]2 in CD3CN. The
Our initial hypothesis was that methyl [2]+ might undergo
similar excited state reactivity to hydride [1]+, which undergoes
self-quenching electron transfer from an MLCT excited state.
To determine the energetics of self-quenching of [2]+, the
driving force for electron transfer between excited state [2]+*
and ground state [2]+ (ΔG°ET) can be determined using either
an excited state and ground state potential (0.74 V − 0.35 V =
0.39 V) or according to eq 1. ΔG°ET is determined to be about
1
resulting H NMR spectrum was consistent with that of the
product of photolysis. The UV−vis spectrum of [4]+ in
CH3CN shows an absorbance at λmax = 375 nm (2500 M−1
cm−1).
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+
Weaker CH3 sources were also investigated. Samples of
was formed (eq 2) along with the distinctive 1:2:3:2:1 pentet
(JHD = 1.1 Hz) indicative of propionitrile isotopologue
CH3CD2CN (eq 3). Both results indicate D• abstraction
from the solvent CD3CN, which is a thermodynamically
[2][I] with CH3OTs (OTs is tosylate) in CD3CN were
irradiated with 443 nm light for 22 h (Figure S15). Ethane and
methane, but not propionitrile, were observed by H NMR
spectroscopy. The appearance of some of the same products
suggests that the reaction may be proceeding through the same
mechanism as with CH3I.
1
•
favorable reaction for CH3 (vide infra). Attempts were made
to examine the solvent decomposition specifically, but
photolysis in the absence of added substrate produced an
intractable mixture of products, as occurs for [1]+.
•
Mechanistic Considerations. Scheme 1 shows several
possible reaction pathways from the excited state of [2]+.
•CH3 + CD3CN → CH3D + CD2CN
(2)
•CD2CN + CH3I → I• + CH3CD2CN
Scheme 1. Possible Reaction Pathways of Metal Methyls
from the Excited State
(3)
If ethane formation follows a self-quenching mechanism,
then ethane, 3, and [Cp*Ir(bpy)(NCCH3)]2+ would likely be
the initial products, according to the top pathway of Scheme 2.
We have shown that acetic acid (AcOH) will readily protonate
reduced 3 to form [1]+.8,21 In the presence of AcOH then, a
reaction following a self-quenching pathway would be expected
to initially form ethane and then form H2 as [1]+ formed in situ
reacts. Irradiation of [2][PF6] in the presence of excess AcOH
1
produced CH4 and CH3D, observed by H NMR and GC
headspace analysis (Figure S18). Methane was detected in 50%
yield. However, only trace amounts of ethane were observed by
1H NMR spectroscopy and headspace GC analysis, and H2 was
not detected. The primary Ir-containing product was [Cp*Ir-
(bpy)(OAc)]+ (86%). The absence of propionitrile in this
reaction and in the reaction with CH3OTs suggests that
propionitrile is formed from a reaction between activated
solvent and CH3I (eq 3). Detecting neither H2 nor hydride
[1]+ suggests that Ir(I) complex 3 is not produced in situ.
Evidence for a Radical-Based Mechanism. To inves-
tigate the mechanism of ethane formation, unlabeled [Cp*Ir-
(bpy)(12CH3)][PF6] was photolyzed in the presence of labeled
Reactions that lead to bond breaking are shown in color and
electron transfers shown in black. Photochemical α-elimination
is typically observed in early metal dimethyl species (gray)20
and is thus considered unlikely in the present late metal
monomethyl system. To probe for CH3 formation, [2]+ was
+
irradiated at 443 nm in CD3CN with 7 equiv of pyridine to act
+
as a CH3 trap through the formation of 1-methylpyridinium.
No reaction was observed over 5 h of photolysis. To ensure
that the reaction was not reversed upon stopping photolysis,
Ir(I) complex 3 and 5 equiv of 1,4-dimethylpyridinium iodide
in CD3CN were allowed to mix in the dark for 24 h. No
methylation of 3 was observed; thus, the red pathway of
Scheme 1 forming methyl cation is unlikely to be operative.
To investigate the other mechanistic possibilities, we
examined the intermediates that would lead to ethane
formation for two likely reactions: a self-quenching mechanism
(Scheme 2, top) and a radical homolysis mechanism (Scheme
1
13CH3I. Methyl group parentage was tracked by H NMR
spectroscopy over the course of a 30 min photolysis of
[2][PF6] and 13CH3I with 443 nm light in CD3CN. In that
period, 20% of methyl [2]+ had reacted to form iodide [4]+.
During photolysis, 12CH3 scrambled into the methyl iodide at
approximately the same rate as [2]+ was consumed, but only a
small amount (1%) of isotopic enrichment of the methyl group
of [2]+ was observed. No isotopic scrambling was observed in
the dark.
Scheme 2. Pathways for Ethane Production from [2]+
These scrambling rates are telling of the initial steps in the
reaction (Scheme 3). If Ir(I) complex 3 were formed in the
Scheme 3. Initial Steps of Isotope Scrambling
2, bottom). In the case of self-quenching, the methyl ligands of
2 equiv of [2]+ couple in order to make ethane. Further
reaction of resultant 3 with CH3I would regenerate [2]+. With a
radical homolysis pathway, methyl radical reacts with CH3I to
produce C2H6, and the resulting iodine atom traps [Cp*Ir-
(bpy)]+ to form [4]+. Though the products of these simplified
mechanisms are indistinguishable, reactions from intermediate
species can differentiate the two pathways.
course of the reaction and methyl [2]+ were regenerated by
methyl iodide (as in Scheme 2, top), then 13CH3 would be
expected to be incorporated into [2]+ at half the rate at which
[2]+ is consumed. The observed rates fit a mechanism in which
initial cleavage of the Ir−12CH3 bond produces a radical pair
that can either separate or recombine. Once separated, the
12CH3 radical reacts with 13CH3I to form 13CH3 radical, which
becomes the dominant radical carrier because of the high
Preliminary reactions gave indications of an intermediate
methyl radical. In addition to CH4, partially deuterated CH3D
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concentration of 13CH3I. Termination of free methyl radical
with [Cp*Ir(bpy)]2+ must be rare to account for the minimal
13C enrichment of [2]+.
The dominance of 13CH3 radical as the primary radical
carrier in solution is also seen in the distribution of organic
products. Because of the complex 1H NMR splitting pattern of
12CH313CH3 and overlapping peaks from different isotopo-
logues,22 precise quantification of the ethane distribution was
untenable. Integration of the satellites gives the total
concentration of 13C incorporated in ethane either in
12CH313CH3 or 13CH313CH3 (Figure 4, closed squares) while
1
Figure 5. Concentrations from H NMR spectroscopy following the
irradiation at 443 nm of a solution of 5.6 mM [2][PF6] (black
squares), 12 mM CH3I, and 24 mM TEMPO in CD3CN. TEMPO−
CH3 (red circles) grows in at twice the rate of iodide [4]+ (blue
triangles). The decrease in [CH3I] (not shown) accounts for half of
the methyl of TEMPO−CH3. Methyl [2]+ is consumed at a slightly
faster rate than that at which [4]+ appears; a minor iridium (open
triangles, 10%) containing species grows in as evidence of a side
reaction.
quickly cyclize to form cyclopentylmethyl radical.23 To access
this radical clock, excess 6-bromo-1-hexene was added to a
purple solution of Ir(I) complex 3 in Et2O and allowed to stir
in the dark for 4 days. Over that time, a yellow solid
precipitated from solution and was isolated by filtration and
washed with ether giving [Cp*Ir(bpy)((CH2)4CHCH2)][Br]
([5][Br]).
Irradiation of a solution of [5][Br] in CD3CN gave
methylcyclopentane (confirmed by spiking with an authentic
sample), 1,5-hexadiene, and methylenecyclopentane (Scheme
5). The Ir products were 3, [Cp*Ir(bpy)(Br)]+ and a new
Figure 4. Time course of dissolved methane and ethane during
photolysis of [2]+ and 13CH3I in CD3CN, showing that incorporation
of 13C into organic products outpaces that of 12C. 13CH4 and 13CH3D
(closed red squares), 12CH4 and 12CH3D (open red squares), 13C in
ethane (closed blue circles), and 12C in ethane (open blue circles).
the center peak gives 12C concentration in 12CH313CH3 or
12CH312CH3 (Figure 4, open squares). Because this treatment is
counting carbons individually, the ethane concentration is half
the sum of the two measurements. About 5 times as much 13C
is incorporated into ethane as 12C. Consistent with this, 13CH4
and 13CH3D were generated at a faster rate than 12CH4 and
12CH3D. These results suggest that the methyl radical that
forms on irradiation undergoes nonproductive radical reactions
with the excess of 13CH3I before forming methane or ethane.
Radical probes were employed to provide further evidence
for the presence of free radicals during photolysis. Photolysis of
5.6 mM [2]+ and 12 mM CH3I in CD3CN was carried out with
443 nm light for 90 min in the presence of 24 mM radical trap
TEMPO (Scheme 4). Irradiation exclusively produced
Scheme 5. Reaction of Radical Clock Complex [5][Br]
Scheme 4. Reaction with Radical Trap TEMPO
minor species. ESI-MS of the product mixture showed a peak at
m/z = 567.23452. This is the same mass as the starting material
[5]+ (calcd m/z = 567.2351), but NMR data showed that the
starting material was fully consumed (Figure S21). The same
mass would also be consistent with ring-closed product
[Cp*Ir(bpy)(CH2C5H9)]+ (included in gray in Scheme 5
because it was not fully characterized).
The presence of methylcylopentane in the product mixture is
evidence for the homolysis of the Ir−C bond. The ring-closed
cyclopentylmethyl radical formed after homolysis could (a)
recombine with Ir(II) to form [Cp*Ir(bpy)(CH2C5H9)]+, (b)
lose H• to form methylenecyclopentane, or (c) gain H• to form
methylcyclopentane. 1,5-Hexadiene is formed by H• abstraction
by [Cp*IrII(bpy)]+ from the formed 5-hexenyl radical before
ring closure. This process produces [Cp*IrIII(bpy)(H)]+ ([1]+)
which can supply the additional H atom equivalent need to
TEMPO−CH3 to the exclusion of methane and ethane. The
iridium product was [4]+, which was formed at half the rate at
which TEMPO−CH3 appeared (Figure 5). Ir−CH3 homolysis
will produce 1 equiv of TEMPO−CH3 and [Cp*Ir(bpy)]+. To
account for rates and electrons, the Ir(II) complex must cleave
CH3I to form an additional equivalent of TEMPO−CH3 and
the final species [4]+.
An intramolecular alkyl radical clock was also synthesized to
probe radical intermediates. 5-Hexenyl radical is known to
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form methylcyclopentane. Formation of the final Ir(I) and
Ir(III) products could be formed by disproportionation of
[Cp*Ir(bpy)]+. Similar mechanisms for light-induced β-hydride
elimination have been previously proposed on Ir,24 and this is a
common mechanism in cobalt alkyl photochemistry.25,26
Differentiating Mechanisms with Excited State Life-
time and Kinetic Order. Ir−C bond homolysis could occur
through either monometallic homolysis from the excited state
(Scheme 1, blue) or following an electron transfer which
weakens the Ir−CH3 bonds (Scheme 1, black). Excited state
lifetimes and kinetic order were investigated to determine
which of these pathways was operative.
Excited state lifetimes and luminescence quantum yields were
investigated to look for self-quenching directly (see Supporting
Information for full details). No change in photoluminescent
quantum yield was observed in the range of 0.02−0.6 mM of
[2]+. The lifetime of [2]+* was found to be 50 ns by time-
correlated single photon counting, with no dependence on [2]+
in the range of 0.02−0.9 mM. Both results indicate that [2]+* is
not efficiently quenched by [2]+. Quenching by methyl iodide
(E1/2 = −2.18 V)27 was also investigated; no change in lifetime
was observed to 1 M CH3I.
overall first-order dependence on chromophore [2]+. Order in
CH3I was determined by varying the concentration of substrate
from 0.02 to 0.3 M at constant concentration of [2]+. No
dependence of quantum yield on substrate concentration was
observed, indicating that the reaction is zero-order in CH3I.
Order in Ir and substrate was similarly investigated for the
reaction of [2]+ in the presence of AcOH to probe the reaction
with a different substrate. Variation of [2]+ concentration from
0.8 to 0.21 mM in the presence of a constant concentration of
0.1 M AcOH in CH3CN resulted in no change in quantum
yield (1.7(3)%, Figure S27). To probe higher iridium
concentrations, solutions of 1−8 mM [2]+ in CD3CN and
0.1 M AcOH were irradiated in standard NMR tubes with 443
nm light and monitored by 1H NMR spectroscopy. No change
in quantum yield was observed (Figure S28). Varying [AcOH]
caused no change in quantum yield, indicating a zero-order
dependence on substrate.
Additionally, if a bimolecular reaction of two cationic iridium
species was occurring, then one might expect an increase in
quantum yield on addition of electrolyte. Quantum yields for
the disappearance of [2]+ with and without 0.1 M [nBu4N]-
[PF6] were identical in four trials with concentrations of [2]+ at
0.07 and 0.18 mM with 0.05 M AcOH acid, indicating that the
reaction does not involve the close approach of two charged
species.
From a kinetic analysis indicating that the reaction is first-
order in [2]+ and zero-order in substrate, several characteristics
of the reaction become apparent. The first is that bimolecular
self-quenching cannot play a significant role in the reaction.
The second is that reactions involving the added organic
methyl source must occur after the rate-determining step.
Additionally, the radical chain is not significantly propagated
through radical attack on [2]+ to produce either Ir(II) or
methyl radical. Such a process would result in an additional
pathway to consuming [2]+ and typically results in 3/2-order in
[2]+.28
Many of the species that would be formed to continue
propagation of methyl radical in solution, ICH2CN and I2, have
particularly weak bonds (44.7 and 36.4 kcal·mol−1, respec-
tively)29 and are unlikely to be generated in high concen-
trations. Indeed, attempts to detect ICH2CN by photolyzing a
solution of [2][PF6] with excess CH3I in 9:1 CH3CN/CD3CN
produced no iodoacetonitrile and no ethane by 1H NMR
spectroscopy. Instead, evidence for another C−C bond forming
event was observed: 0.5 equiv of succinonitrile ((CH2CN)2)
was detected per [2]+, presumably produced by the termination
of two acetonitrile radicals. Given that iodide [4]+ was formed
in this reaction as well, CH3I must be involved in other radical
reactions; therefore, short chain reactions must occur.
The reaction of [2]+ and CH3I in CH3CN during irradiation
with 443 nm light was also monitored by UV−vis spectroscopy
in quartz cuvettes. The disappearance of the MLCT feature of
[2]+ was monitored at 443 nm, and quantum yields for the
disappearance of [2]+ were calculated over the course of 10%
consumption. Samples which were photolyzed until no further
change occurred were consistent with the spectrum of [4]+
(Figure S25).
Order in Ir was determined by monitoring the disappearance
of [2][PF6] in an irradiated quartz cuvette in the presence of
0.1 M CH3I in CH3CN. The quantum yield for consumption of
starting material remained constant at 1.0(1)% from 0.08 to
0.28 mM [2]+ (Figure 6). Because calculating quantum yield
includes normalization for sample absorbance, a lack of
dependence on the chromophore concentration indicates an
To assess the presence of succinonitrile, liquid-phase GC/
MS was undertaken on the solution resulting from photolysis of
6 mM [2][PF6] and 45 mM CH3I in CH3CN and in CD3CN.
In CH3CN, 2.6 mM succinonitrile was produced, while in
CD3CN, only 0.1 mM succinonitrile-d4 was produced. The
absence of ethane in CH3CN suggests that the slightly more
inert C−D bond of CD3CN encourages ethane formation.
Given the difference in product distribution in protio and
deutero solvent, we investigated the quantum yield for the
reaction of [2]+ in CD3CN with CD3I and found no decrease.
Therefore, the isotope effect is product-determining but not
rate-determining. Either initial homolysis or solvent separation
of the radical pair is likely the rate-determining step.
Figure 6. Dependence of quantum yield on [CH3I] (blue squares) and
[[2]+] (red circles) in CH3CN photolyzed with 443 nm light. The
dashed line marks the constant concentrations of CH3I and [2]+ (0.1
M and 0.13 mM, respectively) held while the other reagent varied. The
purple point is 4 independent trials under the same conditions with
error given by one standard deviation. The lack of dependence on
CH3I indicates the reaction is zero-order in substrate, while the lack of
dependence on [2]+ indicates an overall first-order reaction as [2]+ is
the chromophore.
F
DOI: 10.1021/acs.organomet.7b00175
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
Understanding the Preference for Homolysis. The
collected data indicate that ethane and methane production
occur following monometallic Ir−CH3 bond homolysis from
the excited state. Bond dissociation enthalpies relevant to this
discussion are collected in Table 2. Deuterium atom abstraction
[1]+* and [1]+ (3 kcal·mol−1) and may, indeed, be
insurmountable even with a following reaction. Such self-
quenching reactions are often kinetically limited as well. Self-
quenching of [Ru(bpy)3]2+ with its excited state is
thermodynamically favorable, but only occurs when suitable
electron donors and acceptors are included in solution.32,33 In
the absence of a viable electron transfer pathway for [2]+,
homolysis provides a feasible pathway to dissipate the energy of
the excited state.
Table 2. Relevant Bond Dissociation Energies
molecule
ΔH (kcal·mol−1
)
ref
H−H
H−CH3
H−CH2CN
I−CH3
I−CH2CN
I−I
H3C−CH2CN
NCCH2−CH2CN
H−Ir(Cp*)(PMe3)(H)
H3C−Ir(Cp*)(PMe3)(CH3)
104.2
105
97
29
29
29
29
29
29
29
30
31
31
CONCLUSIONS
■
A new iridium methyl complex, [Cp*Ir(bpy)(CH3)]+, was
prepared and characterized. Photolysis of this complex in the
presence of CH3I forms C−C bonds; ethane, propionitrile, and
succinonitrile are all formed. The mechanism of C−C bond
formation was shown to proceed through monometallic radical
homolysis, rather than the bimetallic electron transfer
mechanism that led to H−H bond formation from [1]+. The
minor structural change of substituting a methyl ligand for a
hydride ligand, despite modifying the electrochemical and
photophysical properties of the molecule only slightly, was
sufficient to give rise to drastically different reactivity.
Comparison of the bond dissociation enthalpies of similar
complexes suggests that the mechanism change can be
explained by the weaker Ir−CH3 bond in comparison to the
Ir−H bond. The dramatic differences underscore the changes
in reactivity that can be observed on minor synthetic variation.
57
44.7
36.4
83.2
70.6
74
56
by •CH3 from CD3CN forms CH3D. Methyl radical is
thermodynamically capable of abstracting a hydrogen atom
from acetonitrile.29 The absence of propionitrile in reactions
with AcOH and CH3OTs suggests that propionitrile-d2 results
•
from the reaction of CD2CN with CH3I. Ethane could be
formed either in a reaction of •CH3 with CH3I or in a
termination step upon recombination of two methyl radicals.
Addition of I• to the primary Ir photoproduct [Cp*IrII(bpy)]+
offers an organometallic termination pathway that affords the
observed iodide complex.
EXPERIMENTAL SECTION
■
To better understand the mechanism change from electron
transfer with [1]+ to homolysis with [2]+, relevant thermody-
namic parameters were considered. One possibility is that
homolysis of an Ir−CH3 bond is thermodynamically favorable
relative to homolysis of an Ir−H bond. While the hydrogen
atom and methyl radical are almost equally stable (ΔH(H−H)
= 104 kcal·mol−1; ΔH(H3C−H) = 105 kcal·mol−1),29 the bond
dissociation enthalpies of Ir−H in [1]+ and Ir−CH3 in [2]+
may be very different. The most similar system for which M−
CH3 and M−H bond strengths have been measured is
Cp*Ir(PMe3)X2 (where X = H and CH3). In this system,
ΔH(Ir−H) = 74 kcal·mol−1, while ΔH(Ir−CH3) = 56 kcal·
mol−1. This ∼20 kcal·mol−1 decrease in bond dissociation
enthalpy from ΔH(Ir−H) to ΔH(Ir−CH3) is consistent with
other Ir systems in which both have been measured.31 This is a
considerable difference, especially when considered in light of
the overall similarity of [1]+ and [2]+.
Homolysis is also consistent with the nature of the orbitals
involved in the MLCT transition. The largest contribution to
the excitation depopulates a σ bond between Ir and CH3
(HOMO, Figure 3C). Irradiation of this transition would,
therefore, be expected to weaken the Ir−C bond.
While a more accessible homolysis pathway explains much of
the observed chemistry, the differences in quantum yields for
[1]+ and [2]+ suggest that there may be other factors. The
quantum yields for conversion of [2]+ are low, especially when
compared with quantum yields for H2 evolution for [1]+ that
approach unity. Homolysis may be out-competing electron
transfer, and low quantum yields could be a result of efficient
recombination from the radical pair. Alternatively, the
discrepancy may be explained by an inaccessible self-quenching
process. As discussed above, self-quenching for [2]+* and [2]+
lies 9 kcal·mol−1 uphill. This is a larger barrier than that for
General Considerations. Procedures were carried out under
nitrogen except where noted. All solutions containing metal methyl
species were protected from ambient light during preparation to
prevent excited state reactions. All reagents were commercially
available and used without further purification. Organic solvents
were dried and degassed with argon using a Pure Process Technology
solvent system. Deuterated solvents were purchased from Cambridge
Isotope Laboratories, Inc. and degassed with three freeze−pump−
thaw cycles before storing over sieves in a N2 glovebox. UV−vis
spectra were obtained using an Ocean Optics USB2000+ spectrometer
with a DTMINI-2GS deuterium/tungsten halogen light source
controlled by OceanView software. NMR spectra were obtained on
400, 500, or 600 MHz spectrometers. H and 13C NMR spectra were
1
referenced to the residual solvent signals. Spectra were processed using
the MestReNova software suite from Mestrelab Research S. L. Mass
spectrometry was carried out with an LTQ FT (ICR 7T)
(ThermoFisher, Bremen, Germany) mass spectrometer. Samples (in
acetonitrile solution) were introduced via a microelectrospray source
at a flow rate of 3 μL/min. Xcalibur (ThermoFisher, Breman,
Germany) was used to analyze the data. Molecular formula
assignments were determined with Molecular Formula Calculator (v
1.2.3). For ICP-MS, samples were prepared in 2% nitric acid solution
diluted with 18.2 MΩ/cm water and were analyzed with an Element
XR inductively coupled plasma (ThermoFisher, Bremen, Germany)
mass spectrometer (ICP-MS). Samples were introduced via a
peristaltic pump connected to an Elemental Scientific SC autosampler
(Omaha, Nebraska). Iridium 193 was monitored in low-resolution
mode for 30 s for each sample (∼300 scans). For succinonitrile
detection, the chromatographic separation and detection were
performed on an Exactive GC system (ThermoFisher Scientific,
Breman, Germany) equipped with a Trace 1300 SSL injector and
TriPlus RSH autosampler with a TRACE TG-5SILMS column and He
carrier gas (purity 99.999%). The mass spectrometer was operated in
full scan mode using positive mode chemical ionization (PCI, UHP
Methane gas) with a mass range of 40−600 m/z. The instrumental
control, data acquisition, and data processing were performed with
Thermo’s Xcalibur and TraceFinder software packages. All UV−vis
G
DOI: 10.1021/acs.organomet.7b00175
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
and photoluminescence samples were prepared in an N2 glovebox in 1
cm path length 4-sided quartz cuvettes, sealed with screwcaps, and
reinforced with parafilm on removing from the glovebox, unless
otherwise noted. Single-crystal XRD data were collected on a Bruker
APEX-II CCD diffractometer at 100 K with Cu Kα radiation (λ =
1.54175 Å). Using Olex2,34 the structures were solved with the
olex2.solve35 structure solution program using Charge Flipping and
refined with the XL36 refinement program using least-squares
minimization. The complex Cp*Ir(bpy) was prepared following
literature procedures.37 Elemental analyses were performed by
Robertson Microlit Laboratories (Ledgewood, NJ).
Electrochemistry. Cyclic voltammograms were measured with
glassy carbon working electrodes (polished with 0.3 and 0.05 μm
alumina powder before use), platinum wire counter electrodes, and a
Ag pseudoreference electrode. An undivided cup cell was used.
Experiments were carried out in an N2 filled glovebox using a Pine
WaveNow potentiostat controlled by Aftermath software connected to
a computer outside of the glovebox by a custom USB feed through.
Ferrocene was added at the end of data collection, and all potentials
are reported relative to the Fc0/+ couple.
Photoluminescence Studies. Steady-state emission and excita-
tion spectra were recorded at room temperature on a Photon
Technology, Inc. Quantamaster 4SE-NIR5 spectrometer PC-con-
trolled by FelixX32 software. Excitation light was provided by a 75 W
xenon light source coupled to a single monochromator outfitted with a
1200 L/mm grating blazed at 400 nm. A 300 nm long pass filter was
placed before the sample to prevent deep UV excitation from a
second-order grating effect. Emission was collected at a right angle
relative to excitation, focused into a single monochromator (grating
blazed at 500 nm with 1200 L/mm) and detected by a Hamamatsu
R928P photomultiplier tube used in single photon counting mode. Slit
widths for both emission and excitation monochromators were fixed at
0.5 mm. All spectra were corrected for system response.
Absolute photoluminescence quantum yield was measured on an
Edinburgh FLS-920 emission spectrophotometer with detection by a
Hamamatsu R2658P photomultiplier tube to count photons to 950
nm by which wavelength the phosphorescence had decayed to zero.
Photoluminescent lifetimes were measured by time-correlated single
photon counting on the Edinburgh instrument with excitation by a
443 nm, 73.3 ps width laser.
[Cp*Ir(bpy)(I)]+ ([4]+). [Cp*Ir(bpy)(I)]+ was prepared by addition
of 3 equiv of NaI to 3.9 mg of [Cp*Ir(bpy)(OH2)][OTf]2 in CD3CN.
1H NMR (500 MHz, CD3CN) δ 8.88 (d, J = 5.7 Hz, 2H), 8.46 (d, J =
8.1 Hz, 2H), 8.18 (t, J = 8.1 Hz, 2H), 7.75−7.71 (m, 2H), 1.74 (s,
15H). λabs,max (CH3CN) = 375 nm (2500 M−1 cm−1).
[Cp*Ir(bpy)((CH2)4CHCH2)][Br] ([5][Br]). To a purple stirring
solution of Cp*Ir(bpy) (28.4 mg, 0.059 mmol) in Et2O was added
10 equiv of 6-bromo-1-hexene (79 μL, 0.59 mmol). The solution was
allowed to stir for 4 days, during which a yellow solid precipitated,
which was filtered to separate and washed 3 times with ether giving
1
34.2 mg of [5][Br] (0.053 mmol, 90% yield, 96% pure by H NMR).
1H NMR (600 MHz, CD3CN) δ 8.65 (ddd, J = 5.8, 1.5, 0.8 Hz, 2H),
8.47 (dt, J = 8.0, 1.1 Hz, 2H), 8.11−8.07 (m, 2H), 7.60 (ddd, J = 7.4,
5.7, 1.4 Hz, 2H), 5.63−5.49 (m, 1H, Ir-CH2CH2CH2CH2CHCH2),
4.79−4.71 (m, 2H, Ir-CH2CH2CH2CH2CHCH2), 1.71 (tdd, J = 8.1,
6.2, 1.4 Hz, 2H, Ir-CH2CH2CH2CH2CHCH2), 1.65 (s, 15H), 0.97 (p,
J = 6.9 Hz, 2H, Ir-CH2CH2CH2CH2CHCH2), 0.83−0.72 (m, 4H, Ir-
CH2CH2CH2CH2CHCH2). 13C{1H} NMR (151 MHz, CD3CN) δ
155.93, 152.13, 139.98 (Ir-CH2CH2CH2CH2CHCH2), 138.59, 128.53,
125.00, 114.47 (Ir-CH2CH2CH2CH2CHCH2), 90.93, 33.91 (Ir-
CH2CH2CH2CH2CHCH2), 33.73 (Ir-CH2CH2CH2CH2CHCH2),
3 0 . 5 3 ( I r - C H 2 C H 2 C H 2 C H 2 C H C H 2 ) , 1 3 . 4 4 ( I r -
CH2CH2CH2CH2CHCH2), 8.19. HRMS (ESI+) [5]+ Calcd for
C26H34IrN2: 567.23522. Found: 567.23542.
1
Representative Photolysis Monitored by H NMR Spectros-
copy. In a nitrogen-filled glovebox, 4.8 mg of [2][PF6] was dissolved
in 1 mL of CD3CN. Then, 0.98 mL of this solution was transferred to
a vial containing 6 μL of CH3I, and 20 μL of a 150 mM mesitylene
solution in filtered CD3CN was added as an internal standard. The
solution was split between two screw cap NMR tubes. 1H NMR
spectra were recorded after removing the tubes from the glovebox and
reinforcing the seal with parafilm. One tube remained wrapped in
aluminum foil, while the other was irradiated with 443 nm light (Thor
Multichannel) until [2]+ had fully reacted. Using a gastight, locking
syringe, 0.3 mL of the headspace was removed and analyzed by GC to
determine yields of methane and ethane
Representative Photolysis Monitored by UV−Vis Spectros-
copy. Samples for analysis by UV−vis spectroscopy were typically
prepared in sets of 4. In a nitrogen-filled glovebox, excess CH3CN was
filtered through a pipet filter to remove sieve dust from storage. Stock
solutions of [2][PF6] and CH3I were prepared and delivered to the
cuvettes with volumetric syringes. The total volume of solution was
brought to 2 mL by addition of CH3CN. After capping with screwcap,
the samples were removed from the glovebox, parafilmed around the
cap, and stored in the dark. Samples were photolyzed sequentially.
Each was irradiated in 30 s segments (Thor Multichannel, 2.37 × 10−6
moles of photons min−1, determined by chemical actinometry)8
followed by collection of a UV−vis spectrum. Samples were
photolyzed for a total of 3 min.
Computational Methods. Geometry optimizations, frequency,
and time-dependent calculations were done using the hybrid
functional M06 as implemented in Gaussian 0938 with the LANL2DZ
ECP basis set39,40 for the iridium atom and 6-311G** for all other
atoms. The PCM implicit solvation models (CH3CN solvent) was
employed for all calculations. This strategy has been effective for Ir
TD-DFT in the literature.41,42 Calculations were analyzed using the
Chemcraft suite.
Synthesis. [Cp*Ir(bpy)(CH3)][I] ([2][I]). A slight excess of a 50 mM
solution of CH3I in Et2O (3.30 mL, 0.165 mmol) was added dropwise
to a stirring saturated purple solution of Cp*Ir(bpy) (56.6 mg, 0.117
mmol). A fluffy yellow solid precipitated over the course of several
minutes. The solid was separated by filtration and washed with Et2O
resulting in 67.6 mg of air-stable [2][I] (0.108 mmol, 92% yield, >99%
1
pure by H NMR and EA). Single crystals for XRD were grown by
vapor diffusion of Et2O into a solution of [2][I] in CH3CN.
1
Characterization obtained of the original precipitate: H NMR (600
MHz, CD3CN) δ 8.67 (ddd, J = 5.8, 1.5, 0.8 Hz, 2H), 8.43 (dt, J = 8.2,
1.1 Hz, 2H), 8.08 (ddd, J = 8.1, 7.5, 1.5 Hz, 2H), 7.60 (ddd, J = 7.4,
5.7, 1.4 Hz, 2H), 1.68 (s, 15H), −0.04 (s, 3H). 13C{1H} NMR (151
MHz, CD3CN) δ 156.01, 152.44, 139.08, 129.00, 125.32, 90.85, 8.70,
−6.35. λabs,max (CH3CN) = 418 nm. HRMS (ESI+) [2]+ Calcd for
C21H26IrN2: 499.17258. Found 499.17310. Anal. Calcd for
C21H26IIrN2: C, 40.32; H, 4.19; N, 4.48. Found: C, 40.49; H, 4.12;
N, 4.33.
[Cp*Ir(bpy)(CH3)][PF6] ([2][PF6]). In air, complex [2][I] (54.1 mg,
0.086 mmol) was dissolved in 75 mL of 9:1 H2O/CH3OH. Excess
[NH4][PF6] (160 mg) was added as a solid, resulting in the immediate
precipitation of a fine yellow solid. The solid was separated by
filtration through a fine frit and flushed with water resulting in 49.8 mg
of [2][PF6] (0.077 mmol, 89% yield, >99% pure by 1H NMR and EA).
1H NMR (600 MHz, CD3CN) δ 8.67 (d, J = 5.7 Hz, 2H), 8.43 (d, J =
8.1 Hz, 2H), 8.08 (t, J = 7.8 Hz, 2H), 7.60 (t, J = 6.6 Hz, 2H), 1.68 (s,
15H), −0.04 (s, 3H). 13C{1H} NMR (151 MHz, CD3CN) δ 156.01,
152.45, 139.07, 129.00, 125.33, 90.85, 8.70, −6.36. Anal. Calcd for
C21H26F6IrN2P: C, 39.19; H, 4.07; N, 4.35. Found: C, 38.94; H, 3.89;
N, 4.15.
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.organo-
met.7b00175.
NMR spectra, additional experimental details, crystallo-
graphic and computational details (PDF)
Text file of [2]+ computed molecule Cartesian
coordinates in a format for convenient visualization
(XYZ)
H
DOI: 10.1021/acs.organomet.7b00175
Organometallics XXXX, XXX, XXX−XXX

Organometallics
Article
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Accession Codes
CCDC 1545750 and 1545751 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12, Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail: ajmm@email.unc.edu.
(22) Alemany, L. B.; Malloy, T. B.; Nunes, M. M.; Zaibaq, N. G. J.
Mol. Struct. 2012, 1023, 176−188.
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ORCID
Catherine L. Pitman: 0000-0003-2094-8280
Alexander J. M. Miller: 0000-0001-9390-3951
Notes
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The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
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■
We gratefully acknowledge funding from the Division of
Chemical Sciences, Geosciences & Biosciences, Office of Basic
Energy Sciences, U.S. Department of Energy through Grant
DE-SC0014255. C.L.P. is a Fellow of the Royster Society. This
research made use of instrumentation funded by the UNC
EFRC: Center for Solar Fuels, an Energy Frontier Research
Center supported by the U.S. Department of Energy, Office of
Science, Office of Basic Energy Sciences, under award number
DE-SC0001011. We thank B. Ehrmann for help with GC-MS,
L. Gregor for help with ICP-MS, K. Brereton for help with TD-
DFT, and M. Chambers for helpful discussions.
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DOI: 10.1021/acs.organomet.7b00175
Organometallics XXXX, XXX, XXX−XXX