High-Valent d7 NiIII versus d8 CuIII Oxidants in PCET

Article abstract of DOI:10.1021/acs.inorgchem.9b03101

Oxygenases have been postulated to utilize d⁴ Fe^IV and d⁸ Cu^III oxidants in proton-coupled electron transfer (PCET) hydrocarbon oxidation. In order to explore the influence the metal ion and d-electron count can hold over the PCET reactivity, two metastable high-valent metal-oxygen adducts, [Ni^III(OAc)(L)] (1b) and [Cu^III(OAc)(L)] (2b), L = N,N′-(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamidate, were prepared from their low-valent precursors [Ni^II(OAc)(L)]^- (1a) and [Cu^II(OAc)(L)]^- (2a). The complexes 1a/b-2a/b were characterized using nuclear magnetic resonance, Fourier transform infrared, electron paramagnetic resonance, X-ray diffraction, and absorption spectroscopies and mass spectrometry. Both complexes were capable of activating substrates through a concerted PCET mechanism (hydrogen atom transfer, HAT, or concerted proton and electron transfer, CPET). The reactivity of 1b and 2b toward a series of para-substituted 2,6-di-tert-butylphenols (p-X-2,6-DTBP; X = OCH₃, C(CH₃)₃, CH₃, H, Br, CN, NO₂) was studied, showing similar rates of reaction for both complexes. In the oxidation of xanthene, the d⁸ Cu^III oxidant displayed a small increase in the rate constant compared to that of the d⁷ Ni^III oxidant. The d⁸ Cu^III oxidant was capable of oxidizing a large family of hydrocarbon substrates with bond dissociation enthalpy (BDE_C-H) values up to 90 kcal/mol. It was previously observed that exchanging the ancillary anionic donor ligand in such complexes resulted in a 20-fold enhancement in the rate constant, an observation that is further enforced by comparison of 1b and 2b to the literature precedents. In contrast, we observed only minor differences in the rate constants upon comparing 1b to 2b. It was thus concluded that in this case the metal ion has a minor impact, while the ancillary donor ligand yields more kinetic control over HAT/CPET oxidation.

Full text of DOI:10.1021/acs.inorgchem.9b03101

Article
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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
High-Valent d7 NiIII versus d8 CuIII Oxidants in PCET
̂
̂
̂
Duenpen Unjaroen,† Robert Gericke,†,§ Marta Lovisari,†,§ Daniel Nelis,†,§ Prasenjit Mondal,†
Paolo Pirovano,† Brendan Twamley,† Erik R. Farquhar,‡ and Aidan R. McDonald*,†
†School of Chemistry, Trinity College Dublin, The University of Dublin, College Green, Dublin 2, Ireland
‡Case Western Reserve University Center for Synchrotron Biosciences, National Synchrotron Light Source II, Brookhaven National
Laboratory II, Upton, New York 11973, United States
S
* Supporting Information
ABSTRACT: Oxygenases have been postulated to utilize d4
FeIV and d8 CuIII oxidants in proton-coupled electron transfer
(PCET) hydrocarbon oxidation. In order to explore the
influence the metal ion and d-electron count can hold over the
PCET reactivity, two metastable high-valent metal−oxygen
adducts, [NiIII(OAc)(L)] (1b) and [CuIII(OAc)(L)] (2b), L
= N,N′-(2,6-diisopropylphenyl)-2,6-pyridinedicarboxamidate,
were prepared from their low-valent precursors [NiII(OAc)-
(L)]− (1a) and [CuII(OAc)(L)]− (2a). The complexes 1a/
b−2a/b were characterized using nuclear magnetic resonance, Fourier transform infrared, electron paramagnetic resonance, X-
ray diffraction, and absorption spectroscopies and mass spectrometry. Both complexes were capable of activating substrates
through a concerted PCET mechanism (hydrogen atom transfer, HAT, or concerted proton and electron transfer, CPET). The
reactivity of 1b and 2b toward a series of para-substituted 2,6-di-tert-butylphenols (p-X-2,6-DTBP; X = OCH3, C(CH3)3, CH3,
H, Br, CN, NO2) was studied, showing similar rates of reaction for both complexes. In the oxidation of xanthene, the d8 CuIII
oxidant displayed a small increase in the rate constant compared to that of the d7 NiIII oxidant. The d8 CuIII oxidant was capable
of oxidizing a large family of hydrocarbon substrates with bond dissociation enthalpy (BDEC−H) values up to 90 kcal/mol. It
was previously observed that exchanging the ancillary anionic donor ligand in such complexes resulted in a 20-fold enhancement
in the rate constant, an observation that is further enforced by comparison of 1b and 2b to the literature precedents. In contrast,
we observed only minor differences in the rate constants upon comparing 1b to 2b. It was thus concluded that in this case the
metal ion has a minor impact, while the ancillary donor ligand yields more kinetic control over HAT/CPET oxidation.
There still exists minimal insight into the influence of the
specific metal ion in such HAT reactions, that is, how
isostructural Mn vs Fe vs Ni vs Cu oxidants should fare in
performing HAT oxidation of the same substrates. For
example, nonheme iron oxygenases are postulated to employ
a mono- or dinuclear d4 FeIV oxidant to facilitate hydrocarbon
oxidation, while copper hydroxylases are often postulated to
oxidize via a mono- or dinuclear d8 CuIII oxidant.3−8 Both Fe-
and Cu- containing metalloenzymes and zeolite supported Fe
and Cu catalysts have been demonstrated to be capable of
activating the strong C−H bond of methane (C−H bond
dissociation enthalpy (BDEC−H) = 104.9 kcal/mol).3−8,15,16
This suggests that both harder, low d-electron count, and
softer, high d-electron count, transition metal oxidants are
capable of driving the oxidation of the most thermodynami-
cally challenging of substrates. This was addressed in part by
Mayer who performed a direct comparison of Fe, Co, and Ru
oxidants in HAT reactions, showing that the ground-state
entropic effects influenced HAT.15−18 We have posed the
INTRODUCTION
■
Oxidative activation of inert hydrocarbon C−H bonds
represents a very powerful transformation with the potential
for large-scale production of functionalized hydrocarbons.1,2
The ability to readily convert natural gas and petroleum
derived hydrocarbons into higher-value oxidized products
(alcohols, aldehydes, olefins) is both environmentally and
economically much sought after. To this end, catalysts based
on cheap and abundant first-row transition metals capable of
oxidative activation of saturated hydrocarbons under ambient
conditions are highly desirable.
Mn-, Fe-, Ni-, and Cu-containing metalloenzymes perform
oxidative activation of hydrocarbons, yielding oxygenated and
desaturated products.3−8 For many of these metalloenzymes,
activation of the saturated C−H bond occurs through
hydrogen atom transfer (HAT) by a high-valent oxidant,
normally a metal−oxygen adduct.3,9−14 The hydrogen atom is
transferred from the substrate to a high-valent metal−oxo,
−hydroxide, or −bis-μ-oxo adduct, yielding the respective
hydroxide, aquo, or hydroxide products, a reaction that falls
under the category of proton-coupled electron transfer
(PCET).
Received: October 22, 2019
© XXXX American Chemical Society
A
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question, should there be situations where one would choose
one metal over another?
was prepared via a two-step synthesis comparable to a
previously reported procedure for the synthesis of [NiII(OAc)-
(L*)]Et4N (L* = N,N′-(2,6-dimethylphenyl)-2,6-pyridinedi-
carboxamidate, see Supporting Information for details).23
NiCl2 was reacted with LH2 in the presence of NaOCH3
and CH3CN to form the [NiII(NCCH3)(L)] complex (3, 70%
yield).26 The CH3CN ligand was then replaced by the addition
of tetraethylammonium acetate (Et4NOAc, 1.1 equiv) to yield
1a in 73% yield. 2a was prepared from the previously reported
[CuII(NCCH3)(L)]27 in a similar fashion (60% yield). 1H and
13C nuclear magnetic resonance (NMR) spectroscopy, electro-
spray ionization mass spectrometry (ESI-MS), Fourier trans-
form infrared (FT-IR), and electron paramagnetic resonance
(EPR) spectroscopies and X-ray crystallography data sup-
ported the formation of complexes 1a and 2a (Figure 1,
Figures S1−S5).
The magnitude of the O−H bond dissociation free energy
(BDFEO−H) in the respective hydroxide, aquo, or hydroxide
products governs the thermodynamic driving force for C−H
activation. The redox potential of the high-valent oxidant and
the pKa of the O−H entity in the product (Scheme 1)
Scheme 1. HAT and CPET by High-Valent Metal-
Carboxylate Complexes
19−21
determine the magnitude of the BDFEO−H
.
We postulate
that harder, early transition metal entities are likely to display
relatively high redox potentials and relatively low pKa values
due to their relatively low electron affinity and electro-
negativity. Softer late transition metal entities may display
relatively high pKa values and low redox potentials, due to their
high electron affinity and electronegativity. This led us to
consider the effect different transition metal ions may hold
over BDFEO−H and the possibility of a direct like-for-like
comparison of isostructural high-valent oxidants with different
metal ions in oxidation reactions.
Figure 1. ORTEPs (at 50% probability level) of the X-ray
crystallographically determined structures of 1a and 2a. Hydrogen
atoms and counterions have been omitted for clarity.
We recently reported the preparation and characterization of
a family of terminal NiIII−oxygen adducts (Scheme 2),
Crystals of 1a and 2a suitable for X-ray diffraction
measurements were obtained by layering CH3CN solutions
of the complexes with diethyl ether (Et2O). The crystal
structures of both molecules showed the central metal ion
coordinated in a square planar environment with three N
donors from the 2,6-pyridinedicarboxamidate ligand and a
monodentate O atom donor from the OAc anion (Tables S1
and S2). The τ4 parameters for 1a and 2a were 0.18 and 0.26,
respectively, which indicated that the complexes were in
slightly distorted square planar geometries (τ4 = 0 for pure
square planar, τ4 = 1 for pure tetrahedral).28 The geometric
parameters of 1a and 2a were similar to those of previously
reported [NiII(OX)(L*)]− (OX = OH, OCO2H, OAc, ONO2)
and [CuII(OH)(L)]− complexes.22,23,25,27,29
Scheme 2. Generation of d7 NiIII 1b and d8 CuIII 2b by
Oxidation of 1a/2a with Tris(4-bromophenyl)
Ammoniumyl Hexachloroantimonate (Magic Blue)
Preparation and Characterization of 1b and 2b.
[NiIII(OAc)(L)] and [CuIII(OAc)(L)] (1b and 2b) were
generated using the one-electron oxidant magic blue (tris(4-
bromophenyl)ammoniumyl hexachloroantimonate) as an
oxidant. Solutions of 1a (0.3 mM) and 2a (0.25 mM) in
acetone were cooled to −40 °C, followed by the addition of
magic blue (exactly 1 equiv, as a 7.5 mM CH3CN solution)
resulting in an immediate reaction (complete within 2 s for
both complexes, Figure 2). Titration of magic blue
demonstrated a maximum yield of 1b/2b with 1 equiv of
oxidant (Figure 2). ESI-MS showed the expected elemental
formula for the high-valent species (Figure S6). At −40 °C the
complexes showed no substantial decay over the course of an
hour (1b, t1/2= 4 h; 2b, t1/2= 22 h). Complex 2b could also be
generated in high yield at 25 °C, using magic blue,
demonstrating a t1/2 = 70 min at 25 °C. Compound 1b was
thermally unstable upon preparation at 25 °C with magic blue,
[NiIII(OX)(L*)] (OX = OCO2H, O2CCH3 (OAc), and
ONO2), supported by N,N′-(2,6-dimethylphenyl)-2,6-pyridi-
nedicarboxamidate (L*).22−25 We observed a 20-fold enhance-
ment in the rate constant for HAT oxidation upon exchanging
ONO2 for OAc. Having gained an understanding of the
anionic ancillary ligand (OX in [NiIII(OX)(L*)]) in HAT
oxidations, and herein we explore the role of the metal ion, d7
NiIII versus d8 CuIII, in driving PCET oxidation of a variety of
substrates.
RESULTS AND DISCUSSION
■
Synthesis and Characterization of 1a and 2a.
[NiII(OAc)(L)]Et4N (1a, OAc = O2CCH3; L = N,N′-(2,6-
diisopropylphenyl)-2,6-pyridinedicarboxamidate, Scheme 2)
B
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630, 805 nm (Figure 2). Tolman’s [CuIII(OH)(LX)]
complexes displayed a single intense visible band at λmax
=
540 nm,34−38 whereas 2b showed two absorption bands at
lower energies, presumably as a result of the weaker-field donor
ancillary ligand (OAc for 2b, versus OH in [CuIII(OH)(LX)]).
On the basis of the electronic absorption properties of 1b and
2b, we assign them the respective structures [NiIII(OAc)(L)]
and [CuIII(OAc)(L)].
Electron paramagnetic resonance (EPR) spectroscopy
analysis of 1b showed the presence of two sets of signals
that can be attributed to two S = 1/2 d7 NiIII ions (Figure 3,
Figure 3. EPR spectra of frozen solutions of 1b (black line), obtained
by the oxidation of 1a with NaOCl/AcOH (15 equiv) or magic blue
(1 equiv), and simulated EPR spectra of 1b (gray line). The
simulation is made up of two S = 1/2 signals: blue trace, rhombic
component; red trace, axial component. The spectrum was acquired
from a frozen acetone solution, measured at 77 K, 2.02 mW
microwave power, with 0.3 mT modulation amplitude.
Figure 2. Electronic absorption spectra in acetone at −40 °C. (Top)
1a (black trace, 0.3 mM) and 1b (red trace); (bottom) 2a (black
trace, 0.25 mM) and 2b (blue trace). The insets show the titration of
magic blue with 1a (top) and 2a (bottom) monitored by plotting the
intensity of the λ = 550 nm for 1a and λ = 630 nm for 2a versus
equivalents of magic blue. The continued increase in absorbance
beyond 1 equiv is attributed to the magic blue oxidant.
see Supporting Information for details). One set displayed an
axial signal (g⊥ = 2.24, g∥ = 2.02, gav = 2.17) and the other a
more rhombic signal (gx = 2.40, gy = 2.28, gz = 2.00, gav = 2.23).
The relative ratio of the signals were 70% for the axial
component and 30% for the rhombic component. The overall
yield of NiIII in the reaction mixture was 80
20% (with
respect to [1a]), determined by the comparison of the double
integral of the entire signal to that of a TEMPO ((2,2,6,6-
tetramethylpiperidin-1-yl)oxyl) standard. The axial component
was remarkably similar to those identified for previously
reported [NiIII(OX)(L*)] complexes (OX = OCO2H, OAc,
ONO2).23 This led us to assign the axial signal to that of
[NiIII(OAc)(L)]. The rhombic component as of yet cannot be
identified. The previously prepared [NiIII(O2CC6H4Cl)(L)]
displayed a remarkably similar rhombic EPR signal.26 The
different signals may represent different binding modes of the
OAc ligand, monodentate versus bidentate.
The EPR spectrum of 2a displayed a signal typical of
[CuII(L)] complexes (Figure S9).27,34,36,37 The EPR spectrum
of 2b was found to be almost silent, with a residual presence of
∼10% of S = 1/2 CuII ion, as found by comparison to the
doubly integrated spectrum of 2a. This observation is
consistent with the conversion of a d9 CuII complex (2a) to
a d8 CuIII complex (2b), with Cu in a square planar
environment.
it was obtained in 50% yield and decayed within 120 s after the
addition of magic blue. Furthermore, when NaOCl/AcOH was
employed as an oxidant, 1b was unstable (decay within 20 s at
25 °C, Figure S7). This contrasts with the previously reported
[NiIII(OAc)(L*)] complex which was thermally stable at 25
°C when prepared with NaOCl/AcOH.23 The instability may
be attributed to ligand oxidation (at the isopropyl group)
which we previously observed for L,26 but not for L*.22,23,25
The electronic absorption spectrum of 1b showed two
intense features at λmax = 550, 760 nm (Figure 2) which were
very similar to the features observed for the previously
reported [NiIII(OAc)(L*)] (λmax = 510 nm, 760 nm, Figure
S8).23 The red shift in the higher-energy band of 1b is
presumably a result of the different substituents on the aryl
amidate donors (L = 2,6-diisopropylphenyl; L* = 2,6-
dimethylphenyl). Such features are a hallmark of NiIII
complexes supported by 2,6-pyridinedicarboxamidate donors23
and are also generally typical of NiIII complexes.22−26,30−33
Compound 2b also displayed two intense bands in the visible
Compound 1b is a member of a now large family of
[NiIII(X)(L*)] complexes that have been explored using X-ray
absorption spectroscopy (XAS) analysis.22−24 Given that the
and NIR regions of its electronic absorption spectrum at λmax
=
C
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other spectral properties associated with 1b were remarkably
similar to those previously reported, we did not believe XAS
analysis of 1b was warranted. Cu K-edge X-ray absorption near
edge structure (XANES) analysis was carried out on 2a and 2b
to explore the structural and electronic properties of the
metastable CuIII complex. Compound 2a displayed a 1s → 3d
pre-edge transition centered at 8978.1 eV (Figure 4), while 2b
Figure 5. Representative best fit (fit 7 in Table S5) to k3-weighted
EXAFS data of 2b. Experimental data is shown as a black trace, while
the best fit is represented as a dotted trace.
Figure 4. Normalized Cu K-edge spectrum of 2a (solid trace) and 2b
(dashed trace). Inset: expanded pre-edge region.
monodentate coordination of the OAc ion, a 1 C scatterer at
2.51 Å, from the bound OAc, and 4 C scatterers from the
carboxamide backbone at 2.85 Å (Table S4, Figure S13). This
data matches well with the X-ray crystallography data of the
complex (Table S2). Similarly, the best fit of the data for 2b
was obtained with 4 N/O scatterers at 1.88 Å, suggesting a
monodentate coordination of the OAc ion, 1 C/O scatterer at
2.38 Å, and 4 C scatterers at 2.76 Å (Table S5). When
comparing 2a to 2b, the average bond lengths of the first-
coordination sphere metal−scatterer distances decreased by
0.08 Å (Tables S4 and S5). This can be attributed to an
increase in the oxidation state, and thus positive charge, of the
copper atom. From the XANES and EXAFS data, 2b can be
assigned as the one-electron oxidized equivalent of 2a with
retention of the square planar geometry around the absorbing
nucleus.
had an equivalent peak at 8979.4 eV, approximately 1.3 eV
higher in energy (Figure S10).39−44 These pre-edge transition
energies fall within the expected range for CuII and CuIII
complexes, respectively. DuBois et al. showed a shift of 1.0−2.0
eV for CuII to CuIII pairs and demonstrated that this was the
only reliable predictor of oxidation state for Cu XANES.41 Our
results are fully consistent with this. The pre-edge features have
the same normalized intensity for both 2a and 2b, suggesting
that there has not been a change in geometry or coordination
number. Altering the geometry and so the degree of 3d and 4p
mixing would change the intensity of this feature.45,46 The 1.3
eV blue shift suggested an integer change in the oxidation state
of the metal (thus CuII to CuIII).39−44
Along the rising edge, there were 1s → 4p plus ligand-to-
metal-charge-transfer (LMCT) “shakedown” transitions visible
at 8984.3 and 8986.7 eV for 2a and 2b, respectively (Figure
S11, Table S3).39,41,46 For 2a, an additional 1s → 4p “main”
transition was found at 8992.3 eV, which was not seen in 2b
(Figure S12). The ratio of the main or shakedown second
derivative peaks was 4.4 and ∼0 for 2a and 2b, respectively.
Compound 2b does not have an observable 1s → 4p main
peak; the numerator approaches zero in the fraction, leading to
an overall value of ∼0. It was previously observed that, for a
CuII/CuIII pair, the ratio of the intensities of the second
derivative peaks for the 1s → 4p main or 1s → 4p plus
shakedown transitions was found to be ≥1 for CuII complexes
and then reversed to be ≤1 for the corresponding CuIII
complexes, as was observed for 2a/2b, supporting the
assignment of 2b as containing a CuIII ion.41 The rising edge
maximum energy was blue-shifted due to the increase in the
oxidation state of 2b relative to 2a; however, the profile was
unaffected. The shape of the rising edge maximum has been
postulated to be a consequence of multiple scattering (MS)
effects which are dependent on the local geometry around the
scattering atom, thus suggesting a retention of geometry upon
oxidation of 2a to 2b.47
Quantum chemical calculations at the DFT uM06L cc-
pVTZ (C, H, N, O)/SDD (Ni, Cu) level of theory were
performed to give theoretical support to the XAS-measured
structure of 2b and to predict a structure for 1b (Figures S55−
S59, Tables S8−S12). The DFT optimized structures of the
low-valent complexes 1a and 2a showed very minor deviations
of the Ni−N/O bond distances (1.880 Å) by about 0.02 Å and
Cu−N/O distances (1.991 Å) by approximately 0.03 Å when
compared to those of the molecular structures obtained from
single crystal X-ray diffraction, demonstrating our DFT
methods to be effective for predicting the structures of 1b
and 2b. For 1b, two possible coordination modes of the OAc
ligand were predicted using DFT, whereby the chelating
motive (1b-chelate) was more stable than the monodentate
structure (1b-mono) by ∼6 kcal/mol. In 1b-chelate the spin
density was mainly at the Ni core (1.07) with predominantly
2
dz character and slightly scattered to the apical O atom (0.15),
while in 1b-mono the Mulliken spin density (Figure S60) was
mainly at the Ni core (0.67) with predominantly dxz character
and scattered over the amide N atoms (0.11). This observation
is consistent with previous DFT predicted electronic structures
for [NiIII(OAc)(L*)].23 The average Ni−N/O bond lengths in
the basal plane are elongated from 1b-mono (1.87 Å) to 1b-
chelate (1.92) by about 0.05 Å as expected, and the apical Ni···
O separation is shortened by about 0.55 Å from the
Extended X-ray absorption fine structure (EXAFS, Figure 5)
analysis was carried out for both 2a and 2b. The best fit for 2a
was found with 4 N/O scatterers at 1.96 Å, suggesting a
D
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Table 1. Spectroscopic and Reactivity Properties of 1b, 2b, and [NiIII(OAc)(L*)]
a
a
II/III
λmax (nm)
g⊥, g∥, gx, gy, gx (gav)
M
E
1/2
vs Fc+/Fc (V)
M−O (Å)
k2 for 2,6-DTBP
k2 for xanthene
b
1b
550, 760
510, 760
630, 805
2.24, 2.02 (2.17)
2.24, 2.02 (2.17)
0.32
0.35
0.17
1.92
1.92
1.88
0.24
0.125
0.10
0.003
0.004
0.012
[NiIII(OAc)(L*)]
2b
a
b
All performed at −40 °C in acetone. Predicted using DFT.
monodentate motive (2.63 Å) toward the chelating motive
(2.08 Å).
(Figure S24). The measured KIE is consistent with O−H bond
cleavage being rate-determining.
For 2b, the OAc ligand was found to bind in a monodentate
fashion at Cu. The average Cu−N/O bond lengths were
shortened by about 0.08 Å with respect to 2a, which is in
excellent agreement with the EXAFS findings. The Cu···O
separation to the apical O atom of the OAc ligand was found
to be 2.46 Å, which is too long to be defined as a bond. We
have also calculated the highest occupied MO for 1b and 2b.
On average the HOMO of 2b-mono was 2.5 kcal/mol lower
than that for 1b-chelate. Overall, DFT calculations support our
XAS measurements with regard to metal−ligand bond lengths
in 2b and the coordination geometry and number at CuIII in
2b.
Reactivity Studies. Complexes 1b and 2b were examined
for their ability to oxidatively activate phenolic O−H and
hydrocarbon C−H bonds. The oxidation reactions were
monitored using electronic absorption spectroscopy by
following the decay of the absorption bands at λ = 550 nm
for 1b and λ = 630 nm for 2b against time. In the presence of
excess substrate (>10 equiv), pseudo-first-order rate constants
(kobs) were obtained by exponential fitting of the decay plots.
This allowed for the determination of second-order rate
constants (k2) by plotting kobs against the substrate
concentration and calculating the slope of the resultant linear
plot.
Reactivity of 1b and 2b with Phenol Substrates. In order
to gain insight into the kinetic and thermodynamic reactivity
properties of 1b and 2b, the oxidation of para-X-2,6-di-tert-
butylphenol substrates (p-X-2,6-DTBP, X = OCH3, C(CH3)3,
CH3, H, Br, CN, NO2) was explored at −40 °C. Upon the
addition of 2,6-di-tert-butylphenol (2,6-DTBP, 160 equiv in
acetone) to 1b at −40 °C, the bands at λ = 550 and 760 nm
decayed within 400 s (Figure S14). The radical coupling
product 3,3′,5,5′-tetra-tert-butyl-[1,1′-bis(cyclohexylidene)]-
2,2′,5,5′-tetraene-4,4′-dione was detected in approximately
10% yield along with 2,6-di-tert-butyl-1,4-benzoquinone
(Figures S15−S17). A k2 = 0.24 M−1 s−1 value was determined
(Table 1). Further second-order rate constants (Xk2) for the
oxidation of various other p-X-2,6-DTBP substrates were
subsequently determined (Table S6, Figures S18 and S19). For
p-X-2,6-DTBP, X = OCH3, C(CH3)3, H, Br, CN, NO2 the
radical coupling product 3,3′,5,5′-tetra-tert-butyl-[1,1′-bis-
(cyclohexylidene)]-2,2′,5,5′-tetraene-4,4′-dione was detected
along with 2,6-di-tert-butyl-4-quinone (Figures S21 and S22).
For p-CH3-2,6-DTBP the oxidized product was identified as
2,6-di-tert-butyl-4-methylene-3,5-cyclohexadien-1-one (Figure
S20). The reaction between 1b and p-OCH3-2,6-DTBP was
very rapid, and therefore, k2 was calculated under stoichio-
metric conditions (Figure S23, see Supporting Information for
details). p-NO2-2,6-DTBP reacted with 1b displaying a zeroth-
order decay, and accurate kinetic analysis was not possible; we
therefore excluded it from our analysis. A kinetic isotope effect
(KIE) value of 4.4 was obtained from the reactions of 1b with
[H]-p-CH3-2,6-DTBP and [D]-p-CH3-2,6-DTBP substrates
To gain further understanding of the mechanism of the
reaction between 1b and p-X-2,6-DTBP substrates, we plotted
the [log(Xk2/Hk2)] value against the Hammett σ+-parameter.
The Hammett analysis gave a good linear correlation with a
negative Hammett ρ value of −2.73 (Figure 6). The slope
obtained from this analysis is close to the Hammett correlation
slope (ρ = −2.5) for the reaction of [NiIII(Cl)(L*)] with p-X-
2,6-DTBP substrates where HAT was ascribed.24 A plot of the
log(k2) against BDEO−H displayed a linear relationship (Figure
S25). A plot of Gibbs energies (ΔG⧧) of activation from the k2
values against BDEO−H displayed a slope of 0.78 (Figure 6,
Figure S26). BDFEO−H values for all of these substrates in
acetone were not available. For ease of comparison, we have
plotted BDEO−H (an enthalpy) against ΔG⧧ (a free energy, the
differences in BDE and BDFE values are sufficiently small
enough that the comparison is valid). The ideal ΔG⧧/
ΔG°(BDFE) value is 0.5 for HAT.19−21 The obtained value
was comparable, if a little large, with the previously reported
examples of metal-based oxidants (0.15 to 0.7) that performed
HAT, and close to that for [NiIII(Cl)(L*)] (0.66) where HAT
was ascribed for the same family of substrates.24 In contrast, in
an example where nonconcerted PCET was identified, the
slope of a comparable plot for the same family of substrates
was 0.99.48 These results indicated that a concerted PCET
mechanism was at hand, either HAT or concerted proton and
electron transfer (CPET, where the H+ and e− travel
simultaneously but arrive at different locations).
A Marcus-type plot of (RT/F)ln(k2) versus the oxidation
potentials (EOX) of the phenols showed a linear correlation
which decreased with an increasing of EOX. A slope of −0.18
was obtained (Figure 6). This also indicated a HAT or CPET
reaction mechanism.49−55 If the reaction involved a rate-
limiting electron transfer from phenols followed by a fast
proton transfer, the slope of the Marcus plot would be
expected to be −0.5. If the proton transfer was rate limiting,
the slope is predicted be −1.0. A HAT mechanism has been
predicted for a slope closer to 0.00. The observed slope thus
rules out the nonconcerted PCET mechanisms. Indeed,
[NiIII(Cl)(L*)] displayed a slope = −0.15 with the same
family of substrates.24 The combination of the Hammett, Bell−
Evans−Polanyi, and Marcus relationships indicated that the
reaction of 1b with phenol substrates occurred via HAT or
CPET.
The reactivity of 2b with phenol substrates was examined
under the exact same conditions employed for 1b (Figures
S27−S29). Upon the addition of the phenol substrates to 2b,
the electronic absorption spectrum revealed the decay of the
bands at λ = 630, 805 nm, concomitant with an appearance of
new bands around λ ∼ 400 nm (Figure S27). A comparison of
the k2 values for the oxidation of electron-rich p-X-2,6-DTBP
(X = OCH3, CH3, H) showed almost the same values (1b:
343, 0.63, 0.24; 2b: 403, 1.18, 0.09, respectively, units: M−1
s−1) (Figure 6, Table S6). The radical coupling product
E
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Article
butylphenoxyl radical was identified to form in approximately
100% yield according to electronic absorption spectroscopy
(Figure S33). A KIE value of 3.2 was obtained from the
reactions of 2b with [H]-p-CH3-2,6-DTBP and [D]-p-CH3-
2,6-DTBP substrates (Figure S35). The measured KIE is
consistent with the O−H bond cleavage being rate-
determining.
Using the same Hammett, Bell−Evans−Polanyi, and Marcus
treatments employed for 1b, we see a divergence between
electron-rich and electron-poor phenol substrates for 2b
(Figure 6, Figure S36). For electron-rich substrates, there is
a linear correlation when comparing the rate constants
obtained with the respective σ+, BDEO−H, and E1/2 values.
Based on this and the respective slopes of the plots in Figure 6
(1b: −2.72, 0.78, −0.18; 2b: −3.48, 0.89, −0.27), we surmise
that there is an HAT or CPET mechanism for electron-rich
substrates that were oxidized by 2b. We also conclude that
there is little to no difference in the reactivity properties of 1b
and 2b toward this subset of phenol substrates.
In contrast, for p-X-2,6-DTBP (X = Br, CN, NO2) the
reactions were unexpectedly more rapid than for 1b. We also
obtained a poor trend line for the series p-X-2,6-DTBP (X = H,
Br, CN, NO2). We have therefore not fit these data points with
a trendline. Previously reported [CuIII(OH)(L)] and
[CuIII(O2)(L)]− complexes displayed similarly divergent
kinetics of phenol oxidation, where electron-poor phenolic
substrates displayed higher than expected rates of oxidation of
phenols.38,56 This was ascribed to a change in mechanism of
phenol oxidation, where a HAT was postulated for electron-
rich substrates, which demonstrated a linear correlation and
negative slope in the Hammett plot. A nonconcerted PCET
mechanism (separate proton and electron transfer) was
postulated for electron-poor substrates, displaying a linear
correlation and positive slope in the Hammett plot. The lack of
a reliable correlation for the electron-poor substrates in Figure
6 leads us not to predict a mechanism of oxidation by 2b for
these substrates, but to tentatively attribute this observation to
a change in mechanism that is possibly driven by rate-limiting
proton transfer or a more complicated partial transfer of charge
at rates faster than a H+ transfer.
Oxidative Activation of Hydrocarbon C−H Bonds by 1b
and 2b. Both 1b and 2b reacted readily with xanthene (200−
400 equiv) in acetone at −40 °C (Figures S37−S38).
Xanthone was identified as a product of these reactions
(Figure S39), presumably formed from the reaction of the
formed xanthyl radical and solvated dioxygen or adventitious
water. The second-order rate constants (k2) for the reaction of
xanthene with 1b and 2b were 0.003 and 0.012 M−1 s−1,
respectively (Figure 7, Table 1). This demonstrated a small
kinetic difference (4-fold) in the reactivity of the high-valent d8
(CuIII, 2b) oxidant compared to that of the d7 (NiIII, 1b)
oxidant under the same conditions with the exact same
supporting ligands. Further insight was not possible because
the natural decay of 1b at −40 °C was on the same rate order
as its reaction with hydrocarbon substrates with greater
BDEC−H than xanthene. We were, therefore, unable to probe
its reactivity toward hydrocarbons with stronger C−H bonds at
−40 °C. 1b was unstable at 25 °C, preventing exploration of its
reactivity toward substrates with greater BDEC−H at higher
temperatures.
Figure 6. (Top) Hammett correlation plots; (middle) plots of ΔG⧧
against the BDEO−H of the substrates. ΔG⧧ was determined from k2
via the Eyring equation. Note: BDFEO−H values for all of these
substrates in acetone were not available. For ease of comparison, we
have plotted BDEO−H (an enthalpy) against ΔG⧧ (a free energy, the
differences in BDE/BDFE values are sufficiently small for those
available that the comparison is valid). BDEO−H values in references
57−59. (Bottom) plots of (RT/F)ln(k2) against the oxidation
potential (EOX) of p-X-2,6-DTBP substrates; all for the reactions of
1b (red) and 2b (blue) with p-X-2,6-DTBP substrates (X = OCH3,
CH3, C(CH3)3, H, Br, CN, NO2). For 2b, X = CN, NO2 substrates
were not included in trend lines (substrates in blue boxes).
3,3′,5,5′-tetra-tert-butyl-[1,1′-bis(cyclohexylidene)]-2,2′,5,5′-
tetraene-4,4′-dione was detected in approximately 12% yield
along with 2,6-di-tert-butyl-1,4-benzoquinone (Figures S30−
S32). For p-X-2,6-DTBP, X = OCH3, C(CH3)3, H, Br, CN,
NO2, the radical coupling product 3,3′,5,5′-tetra-tert-butyl-
[1,1′-bis(cyclohexylidene)]-2,2′,5,5′-tetraene-4,4′-dione was
detected along with 2,6-di-tert-butyl-1,4-benzoquinone (Fig-
ures S30−S32). For p-CH3-2,6-DTBP, the oxidized product
was identified as 2,6-di-tert-butyl-4-methylene-3,5-cyclohexa-
dien-1-one (Figure S31). For 2,4,6-TTBP, the 2,4,6-tri-tert-
Compound 2b was sufficiently stable at 25 °C to probe its
reactivity with a series of hydrocarbons with varying BDEC−H
values (75.5−90 kcal/mol, Table S7, Figures S40 and S41).
F
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Article
In order to compare NiIII and CuIII further, we have
attempted to compare 2b and the previously reported
[NiIII(OAc)(L*)] (Figure S49). We observed that 2b
consistently displayed k2 values approximately 3- to 10-fold
less than that of [NiIII(OAc)(L*)]. This contrasts with the
observation that 2b displayed a k2 value 4-fold greater than that
of 1b in xanthene oxidation. To understand this and the
influence of the ligand (L* in [NiIII(OAc)(L*)], versus L in 1b
and 2b), we measured the rate of xanthene oxidation by
[NiIII(OAc)(L*)] under the exact same conditions as for 1b
and 2b (Table 1, Figure S50). The measured k2 value (0.004
M−1 s−1) was very close to that obtained for 1b (0.003 M−1
s−1). The comparison in Figure S49 is problematic because 2b
was prepared with magic blue in acetone at 25 °C, while
[NiIII(OAc)(L*)] was prepared with NaOCl/H2O/HO2CCH3
in acetone at 25 °C. This precludes us from making a direct
comparison of the relative rates of oxidation by 2b and
[NiIII(OAc)(L*)] because we cannot gauge the effect of H2O/
HO2CCH3 on the rates of HAT oxidation. We assume the
presence of these protons donors will cause differences in k2
values. On the basis of the data currently on hand, we conclude
that the metal ion has only a minor influence on the rate of
HAT oxidation of hydrocarbons.
In order to understand the driving force for oxidation by 1b
and 2b and any effect thereof on the k2 values, we have
attempted to calculate the BDFEO−H of the conjugate acids of
1a and 2a ([NiII(HOAc)(L)] and [CuII(HOAc)(L)]), which
are the presumed products formed by HAT/CPET oxidation
by 1b and 2b, respectively (Scheme 1). At present, we do not
have experimental evidence to show that ([NiII(HOAc)(L)]
and [CuII(HOAc)(L)] are the products. The addition of
Et4NOAc to the post-reaction mixture results in an electronic
absorption spectra typical of 1a and 2a, separately (Figure S46
and S47). This demonstrates that the [M(L)] unit remains
intact during the oxidation reactions and supports our
postulate that ([NiII(HOAc)(L)] and [CuII(HOAc)(L)] are
the reduced products.
Figure 7. Plots of kobs versus [xanthene] determined for the reactions
between 1b (red, k2 = 0.003 M−1 s−1) or 2b (blue, k2 = 0.012 M−1
s−1) and xanthene at −40 °C in acetone.
For the oxidation of DHA, we have identified anthracene as the
product. The yield of anthracene was 48% according to GC-
FID (Figure S42). For CHD, benzene was identified as the
product of oxidation, while for benzylalcohol, benzoic acid was
identified in the post-reaction mixture (Figure S43). These
products are typical of what is expected of a PCET oxidation of
these substrates. C−H bond cleavage was predicted to be rate
limiting because the measured KIE for the oxidation of 9,10-
dihydroanthracene (DHA) and its deuterated analogue (Figure
S45) was found to be KIE = 5.5. A plot of the log(k2) versus
substrate BDEC−H was linear (Figure S44). Such correlation of
the decrease in rate constants (k2) with an increase in BDEC−H
is indicative of a HAT/CPET mechanism in the rate-limiting
step. ΔG⧧ was calculated from the measured k2 values and
plotted against BDEC−H. BDFEO−H values for all of these
substrates in acetone were not available. For ease of
comparison, we have plotted BDEO−H (an enthalpy) against
ΔG⧧ (a free energy, the differences in BDE/BDFE values are
sufficiently small that the comparison is valid). A slope of 0.38
was measured (Figure 8), which was comparable to the value
of 0.5 predicted by the Marcus theory for HAT.19−21,60
The calculation of BDFEO−H would be achieved through
measuring the pKa of the conjugate acids of 1a and 2a, and
II/III
E1/2
for 1a and 2a, and solving the following equation:
BDFEO−H = 1.37 (pKa) + 23.06 (E1/2II/III) + CG.20,60 The
E1/2II/III has been measured for both 1a and 2a (Table 1, Figure
S5), showing values of 0.32 and 0.17 V versus Fc+/Fc,
respectively. In an attempt to synthesize ([NiII(HOAc)(L)]
and [CuII(HOAc)(L)], we reacted 1a and 2a with H+ donors,
and stable species were obtained. However, these were
identified as the solvent adducts [MII(NCCH3)(L)] (M =
Ni, Cu) (Figure S48).26,27 In alternative solvents (acetone and
1
THF) we could not isolate a discrete complex. H NMR
indicated mixtures of at least 3 species. We therefore could not
assess the pKa of the conjugate acids of 1a and 2a.
Unfortunately, this prevents us from measuring the BDFEO−H
of the conjugate acids of 1a and 2a and thus assessing the role
the BDFEO−H plays in comparing 1b and 2b.
For the reaction of 1b and 2b with 4-CH3-2,6-DTBP, we
have determined activation energy parameters (Figure S51).
For 1b, an activation enthalpy of ΔH⧧ = 4.5 kcal/mol was
determined, while for 2b ΔH⧧ = 7.6 kcal/mol. These values are
reasonably close to one another indicating little to no
difference in the activation enthalpy. In contrast, an activation
entropy value for 1b was determined to be ΔS⧧ = −43 cal
mol−1 K−1, and for 2b, ΔS⧧ = −30 cal mol−1 K−1. The
magnitude of these values indicates that entropic factors play a
Figure 8. Plot of ΔG⧧ against the BDEC−H for the reactions of 2b
with hydrocarbon substrates, measured at 25 °C (slope = 0.38).
BDFEO−H values for all of these substrates in acetone were not
available. For ease of comparison, we have plotted BDEO−H (an
enthalpy) against ΔG⧧ (a free energy). BDEC−H values in references
61 and 62.
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Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
significant role in the HAT/CPET reactions, as has been
reported previously by Mayer and co-workers.15,16 This was
attributed to substantial contributions from vibrational energy,
which varies between metals and is not commonly observed for
organic radicals. This demonstrates that 1b and 2b behave
similarly to previously reported oxidants but does not provide
rationale for the lack of difference in their relative reactivity,
which according to the above study should have been greater
given the differences in redox potentials for their MII/III
couples.
Discussion on Reactivity Properties. The goal of our
reactivity studies was to compare the reactivity properties of
high-valent NiIII and CuIII oxidants under the exact same
conditions. Specifically, we were interested in how the metal
ion and d-electron count can be utilized to modulate the
reactivity in PCET oxidations.
The mechanism of oxidation of phenolic substrates by 1b
and 2b followed a HAT or CPET mechanism, with both the
Bell−Evans−Polanyi and Marcus treatments being in align-
ment with a HAT/CPET reaction (Figure 6). A direct
comparison of the kinetic results for phenol oxidation by
HAT/CPET by 1b and 2b indicated little to no difference. We
observed that 2b performed the oxidation of hydrocarbon
substrates through a HAT/CPET mechanism. We could not
assess the mechanism of hydrocarbon oxidation by 1b,
although we previously showed [NiIII(OAc)(L*)] to perform
hydrocarbon oxidation through HAT. We conclude that for
both hydrocarbons and phenolic substrates there is little
difference in the rate or mechanism of oxidation by these
complexes.
with DHA when compared to 2b.34 This would again suggest
that the ancillary ligand has more influence in defining the
kinetics of hydrocarbon oxidation.
From the perspective of how strong a bond the oxidants are
capable of activating, under the conditions applied herein,
there was no difference in the capabilities of 1b, 2b, and
[NiIII(OX)(L*)]. Both 2b and [NiIII(OX)(L*)] were capable
of activating the C−H bond of toluene, but no other substrates
containing higher BDEC−H. Likewise, all three complexes were
capable of activating the O−H bond in p-NO2-2,6-DTBP, with
the highest BDEO−H in the p-X-2,6-DTBP series.
We have observed little difference in the rate constants for
1b and 2b. We previously identified that exchanging the ONO2
for OAc on NiIII resulted in rate enhancements that could be
attributed to a lower kinetic barrier or greater thermodynamic
driving force. We postulate that the product of the HAT
oxidation by these complexes is their MII-conjugate acid
complex, and the magnitude of the BDFEO−H in this complex
could drive the oxidation reaction. Unfortunately, we cannot
assess the difference in the magnitude of the BDFEO−H of the
conjugate acids for 1a, 2a, [NiII(OAc)(L*)]−, and [NiII-
(ONO2)(L*)]−. The BDEO−H values for HOAc, HNO3, and
H2O have been measured (112, 102, 119 kcal/mol,
respectively),68 and if we exclude from consideration the
influence of the metal ion, allow us to suggest that M−OH
complexes should demonstrate the highest thermodynamic
driving force for HAT and ONO2 the lowest. This is supported
by the observation that [CuIII(OH)(L)]34 displayed some of
the highest rates of substrate oxidation but contradicted by
[NiIII(ONO2)(L*)] ≫ [NiIII(OAc)(L*)]. This leads us to
surmise that the ancillary ligand, in some cases, may lower the
kinetic barrier to the PCET activation of C−H and O−H
bonds, while in this case the metal has little influence on the
kinetics of substrate oxidation.
A comparison of [NiIII(OAc)(L*)] and [NiIII(ONO2)(L*)]
yielded k2 values in the oxidation of p-H-2,6-DTBP of 0.125
and 1.96 M−1 s−1, respectively (a 20-fold difference).23 These
reactivity studies were performed under the exact same
conditions as employed for 1b and 2b (0.24 and 0.09 M−1
s−1, respectively). This represents a wide range of k2 values
upon simply exchanging the ancillary ligand and encourages us
to make the postulate that the ancillary ligand will provide a
greater degree of kinetic control, while exchanging the metal
ion had almost no influence on the rate of reaction.
In the oxidation of DHA at 25 °C, the k2 values for
[NiIII(OAc)(L*)] and 2b were 8.11 and 2.46 M−1 s−1,
respectively. When compared to a large family of other high-
valent oxidants, it places these M−O−X complexes at the more
reactive end of the scale (Table 2). We note with interest the
(estimated) superior rate at which [CuIII(OH)(L)] reacted
CONCLUSIONS
■
We have synthesized and characterized two high-valent
species: a d7 NiIII complex (1b) and a d8 CuIII complex
(2b). Both complexes were characterized through electronic
absorption, EPR, and XAS spectroscopies, mass spectrometry,
and DFT calculations. The reactivity of 1b and 2b toward a
series of para-substituted phenols (p-X-2,6-DTBP; X = OCH3,
C(CH3)3, CH3, H, Br, CN, NO2) was studied, showing similar
rates of reaction for both complexes and a HAT/CPET
mechanism of substrate oxidation. In hydrocarbon oxidation,
the d8 CuIII oxidant displayed a small (4-fold) increase in the
rate constant compared to that of the d7 NiIII oxidant in the
oxidation of xanthene. The d8 CuIII oxidant was capable of
oxidizing a large family of hydrocarbon substrates through a
HAT/CPET mechanism. Overall, it was concluded that the
metal ion had a small impact on the rate of HAT/CPET
oxidation, while, in this instance, the nature of ancillary donor
ligand appears to yield considerably more kinetic control over
HAT/CPET oxidation. Our findings provide insight into high-
valent transition metal oxidants in biology and synthetic
oxidation catalysis.
Table 2. Comparison of k2 for the Oxidation of DHA by
a
High-Valent Oxidants
ΔG⧧/ΔG°
k2 (M−1 s−1
)
KIE
5.5
3.0
24
(BDFE)
2b
2.46
8.11
∼190
0.12
∼0.03
18
0.0049
125
0.38
0.31
0.37
0.69
0.21
0.15
0.17
0.47
[NiIII(OAc)(L*)]
[CuIII(OH)(L)]34
63,64
[MnVIIO4]−
[MnIV(Me2EBC)(O)2]65
3.78
20
5.5
50−100
66
[FeIV(O)(N4Py)]2+
67
[FeIII(OCH3)(PY5)]2+
61
[RuIV(O)(bpy)2(py)]2+
ASSOCIATED CONTENT
■
S
a
* Supporting Information
For [MnIV(Me2EBC)(O)2] and [CuIII(OH)(L)], data at 25 °C was
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.9b03101.
not available: Values were derived from the experimentally
determined activation parameters via the Eyring equation.
H
DOI: 10.1021/acs.inorgchem.9b03101
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Inorganic Chemistry
Physical methods, cyclic voltammetry, EPR spectrosco-
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AUTHOR INFORMATION
Corresponding Author
*E-mail: aidan.mcdonald@tcd.ie.
■
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Aidan R. McDonald: 0000-0002-8930-3256
Author Contributions
̂
§R.G., M.L., and D.N. contributed equally.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This publication has emanated from research supported by the
European Research Council (ERC-2015-STG-678202). Re-
search in the McDonald lab is supported in part by a research
grant from Science Foundation Ireland (SFI/15/RS-URF/
3307). XAS experiments were conducted at SSRL beamline 7-
3 (SLAC National Accelerator Laboratory, USA), with support
from the DOE Office of Science (DE-AC02-76SF00515),
DOE Office of Biological and Environmental Research, and
NIH (P41-GM-103393 and P30-EB-009998). We are grateful
to COST Action CM1305 (ECOSTBio) for networking
support. We thank Prof. Robert Barkley for training on and
use of an EPR spectrometer. We wish to acknowledge the Irish
Centre for High-End Computing (ICHEC) for the provision
of computational facilities and support.
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