Concerted proton-electron transfer oxidation of phenols and hydrocarbons by a high-valent nickel complex

Article abstract of DOI:10.1039/c9sc05565g

The high-valent nickel(iii) complex Ni(pyalk)₂⁺ (2) was prepared by oxidation of a nickel(ii) complex, Ni(pyalk)₂ (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives were fully characterized by mass spectrometry and X-ray crystallography. Electron paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that the oxidation is metal-centered. 2 was found to react with a variety of phenolic and hydrocarbon substrates. A linear correlation between the measured rate constant and the substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon substrates. Large H/D kinetic isotope effects were also observed for both sets of substrates. These results suggest that 2 reacts through concerted proton-electron transfer (CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond dissociation free energy (BDFE) of ～91 kcal mol^-1 for the O-H bond of the bound pyalk ligand. These findings may shed light onto CPET steps in oxidative catalysis and have implications for ligand design in catalytic systems.

Full text of DOI:10.1039/c9sc05565g

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Concerted Proton-Electron Transfer Oxidation of Phenols and
Hydrocarbons by a High-Valent Nickel Complex
Katherine J. Fisher, Margalit L. Feuer, Hannah M. C. Lant,
Brandon Q. Mercado, Robert H. Crabtree*, and Gary W. Brudvig*
Department of Chemistry, Yale University,
New Haven, CT 06520, U.S.A.
*To whom correspondence should be addressed:
robert.crabtree@yale.edu, gary.brudvig@yale.edu

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Abstract
+
The high-valent nickel(III) complex Ni(pyalk)2 (2) was prepared by oxidation of
a nickel(II) complex, Ni(pyalk)2 (1) (pyalk = 2-pyridyl-2-propanoate). 2 and derivatives
were fully characterized by mass spectrometry and X-ray crystallography. Electron
paramagnetic resonance spectroscopy and X-ray photoelectron spectroscopy confirm that
the oxidation is metal-centered. 2 was found to react with a variety of phenolic and
hydrocarbon substrates. A linear correlation between the measured rate constant and the
substrate bond dissociation enthalpy (BDE) was found for both phenolic and hydrocarbon
substrates. Large H/D kinetic isotope effects were also observed for both sets of
substrates. These results suggest that 2 reacts through concerted proton-electron transfer
(CPET). Analysis of measured thermodynamic parameters allows us to calculate a bond
dissociation free energy (BDFE) of ~91 kcal/mol for the O-H bond of the bound pyalk
ligand. These findings may shed light onto CPET steps in oxidative catalysis and have
implications for ligand design in catalytic systems.
Introduction
Reactions in which protons and electrons move in a single, concerted step
(concerted proton-electron transfer, or CPET) play a significant role in many organic,
inorganic, and bioinorganic catalytic systems. CPET at high-valent metal centers has
been proposed or observed in the catalytic mechanisms ranging from enzymatic
reactions1, 2 to water-oxidation catalysis3, 4 to organic synthesis.5, 6 In many of these
systems, the proton and electron are transferred to a high-valent metal-oxo species;
however, another strategy involves the transfer of the proton to the ligand scaffold
instead. This approach is also relevant to some bioinorganic systems. For instance, in
nickel superoxide dismutase (NiSOD), a CPET step is proposed to occur at a highly
oxidized nickel(III) intermediate in which a coordinated thiol or amide moiety acts as a
proton donor.7-9
Understanding the CPET reactivity of high-valent metal centers may give some
insight into their reactivity in catalytic systems. In particular, understanding CPET steps
in systems in which the proton is transferred to the ligand may provide insight into
catalytic systems which do not or cannot go through metal-oxo intermediates, such as
copper- or nickel-containing water-oxidation catalysts or NiSOD.
In our previous studies, we found that the strongly donating ligand 2-pyridinyl-2-
propanoate (pyalk) can stabilize metal centers in high oxidation states, including Ir(V).10
In addition, the pyalk alkoxide has been suggested to act as a proton shuttle in catalytic
water oxidation.11, 12 We reasoned that a high-valent metal compound stabilized by the
pyalk ligand was a good candidate for fast CPET. We now describe the preparation of a
stable Ni(III) species capable of reacting with a variety of O-H and C-H bonds via CPET.
Results and Discussion
Characterization of Ni3+
We prepared 1, a new square-planar nickel(II) complex with two pyalk ligands
arranged in a trans orientation (Scheme 1), which was characterized by X-ray
crystallography, 1H NMR spectroscopy, and cyclic voltammetry (CV). The CV of 1
2

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shows a reversible redox feature at 0.15 V vs. Fc/Fc+, suggesting that oxidation generates
a stable Ni(III) species (Figure 1).
Figure 1. Left: X-ray crystal structure of Ni(pyalk)2 (1). Atoms are shown at the 50%
probability level. (1). Right: Cyclic voltammogram of 1 in MeCN. The reversible wave at
0.15 V vs. Fc/Fc+ is assigned as a Ni(II/III) couple.
In CH2Cl2, 1 was treated with [NO][BF4] or [NO][PF6](E0 = 0.6 V vs. Fc/Fc+)
(Scheme 1). The solution immediately changed from light green to deep blue. UV-visible
spectroscopy indicated the presence of two intense features in the absorption spectrum
(λmax = 340 nm and 610 nm, Figure S1). This change suggested the formation of an
oxidized species. Such intense absorption features in the visible and NIR regions are
consistent with the data from other Ni(III) compounds,13 which led us to suspect that the
Ni center had been oxidized. In addition, 1H NMR of 2 gave a broad paramagnetic
spectrum, in contrast to the diamagnetic spectrum of 1 (Figure S3). Measurement of the
solution magnetic susceptibility by the Evans method14 at room temperature gave a μeff of
1.87 for 2, consistent with the presence of one unpaired electron, as expected for a
square-planar Ni(III) compound.
+
Scheme 1. Synthetic route for preparation of NiII(pyalk)2 (1) and NiIII(pyalk)2 (2)
Electron paramagnetic resonance (EPR) spectroscopy showed the presence of a
single S = ½ species (Figure 2). A ligand centered oxidation would be expected to show
an isotropic signal near g = 2.0. However, the observed rhombic spectrum had g values of
gx = 2.077, gy = 2.091, gz = 2.274 (Figure 2, left). These g values, as well as gave = 2.145,
are consistent with a low-spin square-planar d7 Ni(III) complex.15
As demonstrated for a variety of iridium complexes,16-19 X-ray photoelectron
spectroscopy (XPS) provides another method for identifying a metal-centered oxidation.
For metal-centered redox events with no change to the ligand set, the binding energy is
3

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expected to increase with oxidation state. Between 1 and 2, the Ni 2p binding energy
increases by 1.4 eV, consistent with a metal-centered oxidation from Ni(II) to Ni(III)
(Figure 2, right).
Figure 2. Left: experimental (blue) and simulated (red) EPR spectra of 2 taken in
CH2Cl2/toluene at 77 K. Right: XPS spectrum of 2 (top) and 1 (bottom). The ~1 eV shift
is consistent with a one-electron oxidation of the nickel center.
Electrospray ionization (ESI) mass spectrometry confirmed the molecular formula
of 2. A solution of 2 in CH2Cl2 gave a peak at m/z = 330.09 with the expected isotopic
pattern for nickel (Figure S2a). A mass spectrum of the parent compound, 1, was also
taken, which showed a peak at m/z = 331.09 (Figure S2b), corresponding to [1 + H+].
Despite having the same elemental formula, 1 is uncharged, and thus is observed as the
positively charged protonated species, whereas 2 bears a positive charge already, and thus
is observed without any associated ions. This result thus confirms that 2 is the one-
electron oxidation product of 1. The ESI mass spectrum of 2 does show a small signal at
m/z = 331.09, which may indicate that some reduction of 2 occurs during the mass
spectral analysis or over time in CH2Cl2.
Despite our best efforts, we were unable to obtain a crystal structure of 2. While
attempting to use pyridine as a co-solvent for crystallization, however, the solution
changed from deep blue to bright orange (λmax = 420 nm, Figure S1). Crystals of this new
complex, 3, were successfully grown from CH2Cl2/pentanes (Figure 3, left). 3 proved to
be an octahedral complex, with two equatorial pyalk ligands and two axial pyridines. 3
-
contains a PF6 anion, and a comparison of Ni-O bond lengths (Table S3) indicate that the
alkoxide arms of the pyalk ligands on 3 remain deprotonated, suggesting that 3 remains
in the Ni(III) oxidation state. The EPR spectrum of 3 (Figure 3, right) exhibits an axial
signal with gx = 2.202, gy = 2.163, and gz = 2.030. A five-line hyperfine pattern is
observed on the gz turning point, indicating coupling of the unpaired electron to the
pyridyl nitrogens.20 We found that the pyridine ligands do not remain bound in solution
unless excess pyridine is present; when crystals of 3 were dissolved in CH2Cl2, 3
4

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immediately reverted to 2, as monitored by UV-visible spectroscopy (Figure S1). Upon
addition of excess pyridine, the absorbance spectrum of 3 was re-established.
Remarkably, 2 and 3 are stable at room temperature, both as solids and in
solution; this is rare for Ni(III) compounds, which tend to be stable only between -80 °C
and -20 °C.13, 21, 22
Figure 3. Left: X-ray crystal structure of 3. Atoms are shown at the 50% probability
level. A non-coordinated pyridine has been omitted for clarity. Right: experimental
(black) and simulated (red) EPR spectra of 3 in CH2Cl2/pyridine at -80 °C.
Oxidation of phenols
With the oxidation state of 2 established, we sought to test its oxidative reactivity.
We had shown in previous studies that the pyalk ligand could be reversibly protonated
while coordinated to first row transition metals;23 therefore, we hypothesized that 2 could
undergo proton-coupled electron transfer (PCET) through the reaction proposed in
Scheme 2.
Scheme 2. Proposed PCET pathway for oxidation of phenols.
To test 2 for PCET reactivity, 2 was treated with 100 equivalents of 2,4,6-tri-tert-
butylphenol (TTBP), a common substrate for PCET reactions. When the colorless
solution of TTBP was added to the deep blue solution of 2 in CH2Cl2, the color changed
immediately to bright blue. The appearance of the characteristic peaks at 383 nm and
400 nm in the absorbance spectrum of this solution (Figure 4) indicated that the tri-tert-
butylphenoxyl radical had been formed, causing the color change,24 and the
disappearance of the features at 340 nm and 610 nm indicated that 2 had been consumed.
In a similar reaction between 2 and 2,6-di-tert-butyl-phenol (DTBP), the radical coupling
product 3,3',5,5'-tetra-tert-butyl-[1,1'-bi(cyclohexylidene)]-2,2',5,5'-tetraene-4,4'-dione
was detected by gas chromatography-mass spectrometry (GC-MS, Figure S4). These
results suggested that a PCET process was occurring.
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Figure 4. UV-visible spectrum of 2 before (blue) and after (red) addition of excess tri-
tert-butylphenol. The red trace is consistent with the published UV-visible spectrum of
tri-tert-butylphenol.24
For reactions of highly oxidized metal species with phenols, proton transfer
followed by electron transfer (PT-ET), electron transfer followed by proton transfer (ET-
PT), and concerted proton-electron transfer (CPET)/hydrogen atom transfer (HAT)
mechanisms have all been observed.25-27 Analysis of kinetic measurements can be used to
differentiate between these mechanisms. Therefore, the kinetics of the reaction of 2 with
a series of para-substituted 2,6-di-t-butylphenols (4-X-2,6-DTBP) was studied by
stopped-flow spectrophotometry in order to further investigate the mechanism of
oxidation by 2. For these reactions, excess substrate was used to ensure pseudo-first order
conditions (10-100 equivalents 4-X-2,6-DTBP). Representative UV-visible spectra as a
function of time can be found in Figure 5 and representative time traces can be found in
Figures S5 and S6. Reactions with all substrates under these conditions fit well to a
single exponential decay at a single wavelength (λ = 610 nm), for which global analysis
gave kobs values. Plots of kobs vs. initial substrate concentration displayed a linear
dependence (Figures S7-S13), allowing us to determine the second-order rate constant
(k2) for each substrate (Table S1).
Figure 5. Representative UV-visible spectra as a function of time for the reaction of 2
with 4-X-2,6-DTBP in CH2Cl2.
+
Measured k2 values were plotted against the Hammett parameter σp (Figure 6,
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+
+
left). The σp constant was chosen rather than σp because σp is suggested to more
accurately represent the electronic structure of the transition state for CPET from
+
phenols28 and a strong negative correlation between σp and reaction rate is well known
for this class of reactions.29 Hammett analysis showed that k2 decreases with the electron-
withdrawing capabilities of the para-substituent of the substrate (ρ = -2.19). This result is
inconsistent with a rate-limiting proton transfer followed by electron transfer (PT-ET)
mechanism, which would show a positive linear correlation in the Hammett plot.30 The
observed strong negative association is consistent with the formation of the electron
deficient phenoxyl radical intermediate.31, 32 These results suggest a CPET mechanism for
the reaction of 4-X-2,6-DTBP substrates with 2. Similarly, the plot of log(k2) vs. BDEO-H
of the phenol substrates also demonstrated a strong linear correlation (Figure 6, right).33
In contrast, the plots of k2 vs. substrate pKa or E1/2 showed a much poorer correlation
(Figure S14). This linear dependence also suggests a CPET mechanism and is consistent
with similar plots constructed for other metal-based oxidants.21, 34, 35 It should be noted
that this mechanism could also be described as hydrogen atom transfer (HAT), which
also operates through a concerted mechanism, and is frequently referred to as such in the
literature.36
Figure 6. Left: Hammett plot for the reaction of 2 with 4-X-2,6-DTBP substrates. Right:
Plot of log(k2) vs. BDE for 4-X-2,6-DTBP substrates.
H/D kinetic isotope effects (k2(H)/k2(D), KIE) can help differentiate between the
possible mechanisms. When the phenolic proton of 2,6-DTBP was replaced with a
deuterium atom, the measured k2 value decreased dramatically, and a KIE of ~4 was
calculated (Figure 7). A primary KIE of this magnitude implies that the cleavage of an O-
H bond occurs in the rate-determining step, ruling out pathways involving rate-limiting
electron transfer. This value for the KIE is also in good agreement with KIEs reported for
the reaction of other metal-based oxidants with 2,6-DTBP that react via CPET.22, 31
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Figure 7. Left: representative time trace of the absorbance at λ = 610 nm for the reaction
of 2 with 2-6-DTBP-H (top) and 2,6-DTBP-D (bottom). Right: Plot of kobs vs.
concentration for 2,6-DTBP-H and 2,6-DTBP-D substrates.
It should be noted that pKa and BDE values used in our analysis were measured in
dimethylsulfoxide (DMSO) rather than CH2Cl2, due to the lack of an absolute pKa scale
in CH2Cl2.37 The general linear trend, however, is expected to remain the same regardless
of solvent, as relative BDEs and BDFEs (bond dissociation free energies) do not change
significantly with solvent. To test this assumption, the BDFEs in CH2Cl2 of several
phenol substrates were estimated by converting from DMSO values using Abraham’s
empirical model.38-40 Calculating BDFEs in this manner relies on a number of
assumptions, which are discussed in more detail in the supporting information (p. S20),
so the BDFE values should be treated as estimates only. Even so, we still see a strong
linear correlation between log(k2) and BDFECH2Cl2 (Figure S15). In a similar fashion, we
also used Ingold’s kinetic solvent effect relationship to calculate the expected rate
constants in DMSO,41 which we then plotted against known BDFEs and BDEs measured
in DMSO (Figure S16). Once again, we saw a strong linear correlation, further
supporting our conclusion of a CPET mechanism.
Oxidation of hydrocarbons
To further probe its CPET reactivity, 2 was also treated with a number of
hydrocarbon substrates. Initial reactions of 2 with 1,4-cyclohexadiene and 9,10-
dihydroanthracene produced the expected products of benzene and anthracene, as
determined by gas chromatography and 1H-NMR (Figure S17a). To determine the extent
of reactivity of 2 with hydrocarbons, 2 was treated with a number of substrates having a
range of C-H bond strengths.
As with phenols, the reactions were monitored by stopped-flow UV-visible
spectroscopy or by UV-visible spectroscopy for slowly reacting substrates. All reactions
were carried out under pseudo-first order conditions (10-100 equivalents of substrate).
Reactions with all substrates under these conditions gave a good fit to a single
exponential decay at a single wavelength (λ = 610 nm), except for reactions with THF,
and global analysis was used to find kobs values. For reactions with THF, the method of
initial rates was used to find kobs values. Plots of kobs vs. substrate concentration gave
good linear fits for all substrates, and the slopes of these fits were used to extract k2
8

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values for each substrate (Figures S19-S25). When appropriate, the data were statistically
corrected to account for the number of hydrogen atoms susceptible to oxidation. For
kinetic analysis of multi-proton/multi-electron reactions, such as the oxidation of 9,10-
dihydroanthracene to anthracene (Figure S17b), the first concerted proton-electron
transfer step was considered to be the rate-determining step, since the resulting radical
species tend to have significantly lower BDEs than the parent compounds.38, 42, 43
2 was found to react with hydrocarbons with C-H bond strengths that ranged from
77-92 kcal/mol. For substrates with low C-H bond strengths, 2 reacted at appreciable
rates – log k2 values of 0.89 and 0.74 were found for reactions of 2 with 1,4-
cyclohexadiene and dihydroanthracene (DHA), respectively. A plot of log(k2) vs. C-H
BDE showed a strong linear correlation (Figure 8). In contrast, plots of log(k2) vs. E01/2 or
pKa showed a very poor correlation (Figure S26). This result strongly suggests a CPET
mechanism for hydrocarbon oxidation as well.
Figure 8. Plot of log(k2) vs. bond dissociation energy for the reaction of 2 with
hydrocarbon substrates.
To further investigate the mechanism, dihydroanthracene-d4 was prepared, and an
H/D kinetic isotope effect was measured. A large H/D KIE of ~11 was observed,
indicating the involvement of a proton in the rate-determining step (Figure S27). This
result, in combination with the linear correlation between log(k2) and C-H bond strength,
further supports our assignment of a CPET mechanism.
Thermodynamic Analysis
We sought to identify the nickel-containing products of the reaction of 2 with
phenol and hydrocarbon substrates. Based on our results suggesting that a CPET
mechanism was at play, we suspected that the nickel center was being reduced and that
the pyalk ligand was accepting a proton and transforming the alkoxide ligand to an
alcohol, resulting in the formation [Ni(pyalk)(pyalkH)]+. 1H NMR of reaction products of
2 with 2,6-DTBP, however, showed only the presence of the fully deprotonated 1 in
solution. A blue precipitate was also identified. This precipitate was dissolved in water
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and extracted into CH2Cl2 using NaBArF (BArF = [B[3,5-(CF3)2C6H3]4]−). 1H NMR
analysis of the resulting product indicated the presence of [Ni(pyalkH)2][2(BArF)] (4)
(Figure 9a, Figure S28). This result suggests that, once the nickel metal center has been
reduced, the pyalkH proton is labile enough to rearrange. Attempts to crystallize 4 were
unsuccessful; however, when treating Ni(OAc)2 with the pyalkH ligand,
Ni(pyalkH)2(OAc)2 (5) was crystallized, demonstrating the binding of the pyalkH ligand
to a nickel center (Figure 9).
Figure 9. (a) Structure of [Ni(pyalkH)2]2+ (4). (b) X-ray crystal structure of
Ni(pyalkH)2(OAc)2 (5). Atoms are shown at the 50% probability level.
With the reaction products more fully understood, we constructed the square
scheme in Scheme 3 to determine the thermodynamics of the reaction. The pKa for the
first deprotonation of the pyalk ligand of 4 (pKa1, Figure S29) was estimated
spectroscopically to be ~25 in MeCN using data from the deprotonation of 4 to 1. Using
this value, along with the E01/2 value of 0.15 V vs. Fc/Fc+ in MeCN, we were able to
determine a BDFE for the bound pyalk O-H bond of ~91 kcal/mol in MeCN using the
square scheme shown in Scheme 2 and the following relationship:38
BDFE = 13.7pKa + 23.06E0 + CG,sol
(1)
Scheme 3. Thermochemical square scheme for stepwise vs. concerted proton and
electron transfer to the complexes described in this work. The lower equation shows the
proposed proton exchange, explaining the observed products of reactions of 2 with
substrates.
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It is generally considered more appropriate to use BDFEs to describe the
thermodynamics of PCET by transition metal complexes due to non-negligible entropic
contributions.38 However, many reported high-valent metal-oxo or metal-hydroxo
oxidants report only the BDE of the O-H bond formed upon the reaction with substrate.
Therefore, in order to facilitate a comparison between 2 and reported high-valent
CPET/HAT reagents, the BDE of the pyalk O-H bond was also calculated. The BDE of
the pyalk O-H bond for the CPET product of 2 was calculated using same
thermochemical parameters described above and the following equation:
BDE = 13.7pKa + 23.06E0 + CH,sol
(2)
For 2, a BDE of ~94 kcal/mol in MeCN was calculated. This can be compared with a
value of 105 kcal/mol for tBuO-H,44 taken as a model compound for free pyalkH,
suggesting a modest O-H bond weakening on binding.
By cyclic voltammetry, 4 also showed a quasi-reversible redox feature in MeCN
at E01/2 = 0.58 V vs. Fc/Fc+ (Figure 10). Using the BDFE calculated from Equation 1 and
the square-scheme relationship, we can estimate the value for pKa2 to be ~18, 7 pKa units
below the calculated pKa1 of 25.
Figure 10. Cyclic voltammogram of 4 in MeCN. The quasi-reversible couple at E0 =
0.58 V vs. Fc/Fc+ is assigned as a Ni(II/III) couple.
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Table 1. Thermodynamic and kinetic parameters of high-valent metal compounds
capable of performing CPET with hydrocarbon substrates
Compound
E0 (V vs. BDE
log(k2)
DHAa,b
0.74
log(k2)
TTBPa,b
2.48
Ref
1
Fc/Fc+) (kcal/mol)
[NiIII(pyalk)2]+ (2)
0.15
94
This
work
21
NiIII(MepyN2)(ONO2)f
NiIII(MepyN2)(Cl)f
0.43
0.56
-
-
-
-
0.91
-
-
22
45
0.394d
-0.82
[(MeAN)CuIII(μ-
O2)NiIII(MeNacnac)]+ g
[MnIII(H3buea)(O)]2- h
[MnIV(H3buea)(O)]- h
[FeIII(PY5)(OCH3)]2+ i
CuIII(iPrpyN2)(OH)j
RuIV(bpy)2(py)(O)
25
25
-2.0
-1.0
0.73
77
89
84
90
84
-0.53
-1.59
-2.25
2.27
-
-
46
-0.22e
34
-0.074
-
-
35, 47, 48
0.48c
2.09
a k2 = second-order rate constant for reaction with the designated substrate, measured at
25 °C unless otherwise noted; b DHA= 9,10-diyhdroanthracene, TTBP = tri-tert-
butylphenol (4-tBu-2,6-DTPB) c vs. SCE; d measured at -40 °C; e measured at -50 °C; f
MepyN2 = N, N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide; g MeAN =
N,N,N′,N′,N′-pentamethyl-dipropylenetriamine and MeNacnac =
[HC(CMeNC6H3(iPr)2)2]); h H3buea = tris[(N′-tert-butylureaylato)-N-ethylene]amine; i
PY5 = 2,6-bis(bis(2-pyridyl)methoxymethane)pyridine; j iPrpyN2 = N, N′-bis(2,6-
diisopropylphenyl)-2,6-pyridinedicarboxamide.
The reactivity of 2 with 2,6-DTBP and DHA compares favorably with other
reported high-valent metal-oxo and metal-hydroxo complexes capable of CPET or HAT
(Table 1). 2 reacts with DHA at a faster rate than several manganese- and iron-oxo
complexes. 2 also compares extremely well with high-valent metal alkoxide and
carboxylate compounds, including the only other reported Ni(III) systems, which tend to
react with C-H bonds at slower rates than their metal-oxo counterparts. We hypothesize
that the particularly strong BDE of the O-H bond formed and the strong oxidizing power
of 2 contribute to its high reactivity toward O-H and C-H bonds. This result demonstrates
that fast CPET can be achieved in high-valent metal systems without necessarily going
through a metal-oxo or metal-hydroxo intermediate, a result relevant to several proposed
water-oxidation mechanisms in artificial photosynthetic systems of cobalt,49 copper,11
and nickel.50 This result also demonstrates that high-valent metal-alkoxide systems are
capable of attacking strong C-H and O-H bonds, which is particularly relevant to water-
oxidation catalysis. The pyalk ligand thus proves particularly useful for catalytic
oxidations of this type.
Conclusions
We have synthesized and characterized a Ni(III)-alkoxide compound capable of
reacting with strong C-H and O-H bonds at appreciable rates. A strong linear dependence
between the second-order rate constant, k2, and substrate bond dissociation enthalpy
indicates a CPET mechanism. Large H/D kinetic isotope effects also support this
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assignment. We attribute the fast reactivity of 2 to the strong O-H BDE of the
pyalk/pyalkH supporting ligand and the high oxidizing power of the complex. This result
demonstrates that fast PCET can occur in high-valent metal oxidants without a metal-oxo
unit, which may be relevant to certain nickel-containing enzymes or water-oxidation
catalysts of cobalt, copper, or nickel. This report also provides the first full
thermodynamic analysis of CPET by a high-valent nickel complex. The value of pyalk as
a ligand for catalytic oxidations is further supported.
Acknowledgements
This work was supported the U.S. Department of Energy, Office of Science, Office of
Basic Energy Sciences, Division of Chemical Sciences, Geosciences, and Biosciences as
part of the Center for Light Energy Activated Redox Processes (LEAP) Energy Frontier
Research Center under Award Number DE-SC0001059. K.J.F acknowledges the National
Science Foundation Graduate Research Fellowship Program (Grant No. DGE1122492).
We thank the Mass Spectrometry & Proteomics Resource and Fabian Menges for
assistance with HRMS. We would also like to thank Dr. Min Li and Dr. Liam
Sharninghausen for collection of XPS data. We would also like to thank Professor Jim
Mayer and Catherine Wise for their helpful comments.
Supporting Information
Experimental details, supplementary figures, and crystallographic data can be found in
the supporting information.
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H
O
CPET from O-H
or C-H
O
N
O
N
O
NiIII
NiII
N
N
2
1,4-Cyclohexadiene
DHA
Cyclohexene
1
0
Fluorene
Diphenylmethane
-1
-2
THF
75
80
85
90
95
BDE (kcal/mol)