Hydrogen atom transfer by a high-valent nickel-chloride complex

Article abstract of DOI:10.1021/jacs.7b11953

Oxo-metal-halide moieties have often been implicated as C-H bond activating oxidants with the terminal oxo-metal entity identified as the electrophilic oxidant. The electrophilic reactivity of metal-halide species has not been investigated. We have prepared a high-valent nickel-halide complex [Ni^III(Cl)(L)] (2, L = N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide) by one-electron oxidation of a [Ni^II(Cl)(L)]^? precursor. 2 was characterized using electronic absorption, electron paramagnetic resonance, and X-ray absorption spectroscopies and mass spectrometry. 2 reacted readily with substrates containing either phenolic O-H or hydrocarbon C-H bonds. Analysis of the Hammett, Evans-Polanyi, and Marcus relationships between the determined rate constants and substrate pK_a, X-H bond dissociation energy, and oxidation potential, respectively, was performed. Through this analysis, we found that 2 reacted by a hydrogen atom transfer (HAT) mechanism. Our findings shine light on enzymatic high-valent oxo-metal-halide oxidants and open new avenues for oxidative halogenation catalyst design.

Full text of DOI:10.1021/jacs.7b11953

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Article
Hydrogen atom transfer by a high-valent nickel-chloride complex
Prasenjit Mondal, Paolo Pirovano, Ankita Das, Erik R. Farquhar, and Aidan R. McDonald
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b11953 • Publication Date (Web): 02 Jan 2018
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Hydrogen atom transfer by a high-valent nickel-chloride complex
Prasenjit Mondal,a Paolo Pirovano,a Ankita Das,a Erik R. Farquhar,b Aidan R. McDonald*a
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aSchool of Chemistry and CRANN/AMBER Nanoscience Institute, Trinity College Dublin, The University of Dublin,
College Green, Dublin 2, Ireland; bCase Western Reserve University Center for Synchrotron Biosciences, National
Synchrotron Light Source II, Brookhaven National Laboratory II, Upton, NY 11973, USA
Abstract: Oxo-metal-halide moieties have often been implicated as C–H bond activating oxidants with the terminal oxo-
metal entity identified as the electrophilic oxidant. The electrophilic reactivity of metal-halide species has not been inves-
tigated. We have prepared a high-valent nickel-halide complex [NiIII(Cl)(L)] (2, L = N,N’-(2,6-dimethylphenyl)-2,6-
pyridinedicarboxamide) by one-electron oxidation of the [NiII(Cl)(L)]– precursor. 2 was characterized using electronic
absorption, electron paramagnetic resonance, and X-ray absorption spectroscopies, and mass spectrometry. 2 reacted
readily with substrates containing either phenolic O–H or hydrocarbon C–H bonds. Thorough kinetic analysis of these
reactions was performed. Analysis of the Hammett, Evans-Polanyi, and Marcus relationships between the determined rate
constants and substrate pKa, X-H bond dissociation energy, and oxidation potential, respectively, was performed.
Through this analysis we found that 2 reacted by a hydrogen atom transfer (HAT) mechanism. Our findings shine light on
enzymatic high-valent oxo-metal-halide oxidants and open new avenues for oxidative halogenation catalyst design.
Introduction: The selective functionalization of inert C–
H bonds to a more versatile functional group has far
reaching practical applications in many chemical indus-
tries.1-10 Nature has evolved a variety of remarkably effi-
cient metalloenzymes that functionalise inert hydrocar-
bons through oxidative hydroxylation, desaturation, C–X
bond formation (X = O, N, S), and halogenation.4, 7-10 To
facilitate these transformations, hydrogen atom transfer
(HAT) activation of the inert C–H bond is postulated to
occur via a high-valent metal-based oxidant (often a met-
al-oxo (M=O) moiety).4, 7-10 For such transformations cata-
lysed by man-made catalysts, a M=O oxidant is also most
often implicated.11-17 More recent efforts have focused on
the capability of metal–OX (OX = OH, OR, O2C-R, ONO2)
and metal-imido entities in oxidative C–H bond activa-
tion,18-28 demonstrating that a terminal M=O is not a pre-
requisite for hydrocarbon oxidation.
co-workers have demonstrated remarkably efficient oxi-
dative hydrocarbon fluorination using manganese-
porphyrin catalysts.41-42 The proposed oxidant was an F–
MnV=O species that performed HAT, with rebound of the
fluoride ligand with the carbon-centred radical yielding
the fluorinated product. Groves has similarly demonstrat-
ed oxidative chlorination of hydrocarbons using manga-
nese-porphyrin catalysts in conjunction with hypo-
chlorous acid.42 In that case, however, after initial HAT by
MnV=O, a ClO-MnIV species was proposed to react with
the resulting carbon-centred radical to form the C–Cl
bond. Recently Costas and Browne demonstrated the
chlorination of alkanes mediated by a nickel catalyst and
NaOCl,43 where a chlorine radical was postulated to bring
about hydrocarbon chlorination. Overall, metal-mediated
oxidative halogenation is well-established, however little
insight into the HAT oxidising entity has been obtained.
Interestingly, in all of these biological, biomimetic, and
catalysis studies, the HAT reactivity of the metal-bound
halide in C–H activation was not considered. To the best
of our knowledge, no examples of high-valent metal-
halide complexes performing HAT have been reported.
This is not surprising given the high electronegativity of
the halogens, seemingly excluding metal-halides from
performing electrophilic HAT reactions. However, we
believe such species have great potential as HAT oxidants,
as they could potentially eliminate oxygenated products
(hydroxylation, epoxidation) from the hydrocarbon oxida-
tion process (thus yielding solely halogenated products).
Furthermore, through halide selection, simple tuning of
the potency of the high-valent oxidant would be achieva-
The α-ketoglutarate dependent non-heme iron halo-
genases (such as CytC3 and SyrB2) constitute a class of
important enzymes that perform the biological halogena-
tion of un-activated C–H bonds.29-32 A non-heme halo-
FeIV=O moiety is postulated to perform the initial HAT
from a substrate to give a halo-FeIII–OH species and a
carbon-centered radical. Rebound of the halide ligand
with the carbon-centered radical is proposed to result in
the halogenated product.32-34 Biomimetic halo-FeIV=O
model compounds have been prepared and displayed the
ability to perform oxidative halogenation of hydrocarbons
in some cases.35-40 As in the enzymatic systems, the
FeIV=O unit was postulated to perform HAT followed by
radical rebound with the metal-bound halide. Groves and
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those obtained for analogous [NiIII(OX)(L)] complexes.25,
Page 2 of 9
ble. Herein, we describe the preparation and characteriza-
tion of a NiIII–Cl complex (2) that is capable of performing
HAT oxidation of phenolic O–H and hydrocarbon C–H
bonds.
45
The yield of NiIII was calculated to be 80% ± 20% by
1
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double integration of the signal obtained for 2 and com-
parison against the radical standard (2,2,6,6-
tetramethylpiperidin-1-yl)oxyl (TEMPO). EPR spectrosco-
py also confirmed the presence of the same species in the
preparation of 2 with magic blue (Figure S7). The ob-
tained spectrum with Cl-hyperfine interactions supported
the assignment of 2 as [NiIII(Cl)(L)].
Scheme 1. Preparation of 1 by chloride ligand exchange
(Et = C2H5) and 2 by oxidation with (NH4)2[CeIV(NO3)6]
(CAN, -40 °C) or tris(4-bromophenyl)ammoniumyl hexa-
chloroantimonate (magic blue, -80 °C).
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Results and discussion: Et4N[NiII(Cl)(L)] (1, Scheme 1, L
=
N,N’-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamide,
Et = ethyl), previously prepared by Holm and co-
workers,44 was synthesized using an alternative method
by simple chloride ligand exchange with the previously
prepared [NiII(NCCH3)(L)] (see supporting information
for details).25 1 was obtained in 85% yield and was charac-
terized by nuclear magnetic resonance (NMR), Fourier
transform infra-red (FT-IR), and electrospray ionization
mass spectrometry (ESI-MS, Figures S1-S4).
400
600
800
1000
Wavelength (nm)
1
(0.3
mM,
acetone)
was
reacted
with
g-factor
(NH4)2[CeIV(NO3)6] (CAN, 2 equiv., Scheme 1) in the pres-
ence of Et4NCl (1 equiv.) at -40 °C, resulting in an imme-
diate reaction (complete in 30 s) as evidenced by a colour
change from pale orange to blue/green. Electronic ab-
sorption spectroscopy showed the appearance of a broad
and intense feature in the visible region (λmax = 600 nm,
Figure 1) that reached a maximum after 30 s, which we
assigned to a new species, 2. In the absence of excess
Et4NCl a lower yield of 2 was obtained. 2 could also be
prepared using the one-electron oxidant tris(4-
bromophenyl)ammoniumyl hexachloroantimonate (mag-
ic blue). The addition of 1 equivalent of magic blue to a
0.3 mM solution of 1 (acetone, -80 °C) caused the imme-
diate formation of the band at λmax = 600 nm (Figure S6).
The observation of the same product by the one-electron
oxidation of 1, would suggest that 2 was [NiIII(Cl)(L)]. 2
displayed electronic absorption features similar to those
reported for analogous NiIII complexes supported by L
2.5 2.4 2.3 2.2 2.1
2
1.9
260
280
300
320
340
360
B (mT)
45
([NiIII(OX)(L)], OX = OCO2H, O2CCH3, ONO2),25, and
Figure 1. Top: Electronic absorption spectrum of 1 (red trace,
0.3 mM acetone solution, -40 °C) and of the oxidation prod-
uct 2 (blue trace) obtained after the addition of CAN (2
equiv.). Bottom: X-band EPR spectrum of 2 in a frozen ace-
tone solution (blue trace) measured at 77 K, 2 mW micro-
wave power, with 0.7 mT modulation amplitude, and the
simulated spectrum of 2 (grey trace, see supporting infor-
mation for simulation details).
other high valent Ni complexes,23, 26-27, 46-49 leading us to
conclude a change in the metal oxidation state, and that 2
was likely [NiIII(Cl)(L)].
Electron paramagnetic resonance (EPR) spectroscopy
analysis of 2 showed a metal based rhombic signal (gx =
2.32, gy = 2.23, gz = 2.00, Figure 1) with hyperfine splitting
(gz quartet), due to the Cl atom (I = 3/2, Figure 1)50-51 lying
in close proximity to the unpaired spin density. The aver-
age g-value (2.18) alongside the rhombic signal (gx > gy >>
gz) was consistent with the unpaired spin being localized
on a low-spin d7 NiIII ion in a square planar or tetragonally
distorted octahedral coordination environment.51-58 Fur-
thermore, the EPR spectrum compared favourably to
The elemental formula of 2 was confirmed using ESI-
MS. A peak at m/z = 465 corresponding to [2 + H+]+ with
the expected isotopic pattern was observed (Figure S8). It
is important to note that solutions of 2 in ESI-MS positive
mode always displayed this peak, however, solutions of 1
in ESI-MS positive mode did not. The NiII precursor 1
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could only be detected in the ESI-MS negative mode (as
Extended X-ray absorption fine structure (EXAFS) anal-
ysis of 2 showed a best fit with a first coordination sphere
with 3 O/N scatterers at 1.88 Å, and critically, a Cl scatter-
er at 2.17 Å (Figure 2, Table S1). Fitting the EXAFS data
with an O/N-only first coordination sphere did not pro-
vide reliable fits of the data, with inclusion of a Cl scatter-
er producing a dramatic improvement in fit quality. The
fit is completed with a set of single and multiple scatter-
ing paths arising from atoms of the ligand framework.
The NiIII–Cl bond length obtained for 2 was similar to the
X-ray crystallographically determined NiII–Cl bond dis-
tance (2.174 Å) in 1.44 Likewise the mean Ni–N bond
lengths are similar to those obtained for 1 (1.912 Å (x2)
and 1.796 Å) indicating little-to-no change in the first co-
the [NiII(Cl)(L)]– fragment), consistent with previous ob-
servations.23, 25
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ordination sphere upon conversion of
1 to 2. In
[NiII/III(OX)(L)]–/0, a change in Ni oxidation state had simi-
larly negligible influence on the metal-ligand bond
lengths in the first coordination sphere in this family of
complexes, which was further supported by DFT calcula-
tions.25, 45 Importantly, the identification of a larger (Cl)
scatterer in the first coordination sphere confirmed the
presence of the Cl atom in 2, alongside the ESI-MS and
EPR analyses.
Reactivity Studies: A survey of the stability and oxida-
tive reactivity of 2 in phenolic O–H and hydrocarbon C–H
bond activation reactions was performed. The half-life
(t1/2) determined for 2 was approximately 6 h at -40 °C.
Kinetic analysis of the reaction between 2 and substrates
was followed by electronic absorption spectroscopy by
monitoring the decrease in the absorbance at λmax = 600
nm upon addition of substrates to 2 (Figure 3). In all cases
the reactions obeyed pseudo-first-order kinetics. Pseudo-
first order rate constants (kobs) were found to be linearly
dependent on the substrate concentration, allowing for
determination of second-order rate constants (k2) by
measuring the slope of the resulting linear plot (Figure 3).
Figure 2. Top: normalised XANES spectra of 1 (solid line)
and 2 (dashed line). The inset depicts the second derivative
of both spectra in the pre-edge region. Bottom: best fit to k3-
weighted EXAFS data of 2. Experimental data is shown as a
dotted line, while the best fit is shown as a solid line.
Oxidation of Phenols. Phenols have been employed as
mechanistic probes for the reactivity of 2. At -40 °C, 2,6-
di-tert-butylphenol (2,6-DTBP, 65 equiv. dissolved in ace-
tone) was added to 2 (0.3 mM, acetone) resulting in an
immediate reaction that was complete within 150 s (Fig-
ure 3). A k2 value of 0.176 M-1s-1 for 2,6-DTBP was deter-
mined (Figure S9). Product analysis of the reaction mix-
ture revealed the formation of a radical coupling product
3,3′,5,5′-tetra-tert-butyl-[1,1′-bis(cyclohexane)]-2,2′,5,5′-
tetraene-4,4′-dione, as detected by ESI-MS (Scheme S1).
The product likely derived from coupling of two phenoxyl
radicals. The same reaction when performed with 2,4-di-
tert-butylphenol (2,4-DTBP) resulted in a rate constant
value of k2 = 4.77 M-1s-1 (Figure S10). Again, a radical cou-
2 was not stable enough to allow growth of crystals
suitable for X-ray crystallography, we therefore performed
Ni K-edge X-ray absorption spectroscopy (XAS) on a fro-
zen solution of 2 (Figure 2). The Ni K-edge energy (8345
eV) for 2 was similar to those previously reported for NiIII
complexes,59-61 including values obtained for several
[NiIII(OX)(L)] complexes.25, 45 A comparison of the Ni K-
edge X-ray absorption near-edge spectra (XANES) of 1
and 2 demonstrated almost no shift in the Ni K-edge. This
45
was also observed for [NiIII(OX)(L)] complexes,25, and
62-63
has been seen previously for other NiII/III couples.59,
Nonetheless, the shape and profile of the XANES spectra
are markedly different confirming a change in the proper-
ties of 1 and 2. We ascribe the change in the profile to
contributions to the edge from 1s-to-4p + shakedown ab-
sorption features that distort the edge shape ( 8340 eV).
The weak 1s-to-3d pre-edge transitions showed a modest
0.5 eV blue shift (Figure 2) upon conversion of 1 to 2,
providing evidence for a modified ligand field, consistent
with an increase in Ni oxidation state.
pling
product
(3,3′,5,5′-tetra-tert-butyl-[1,1′-
bis(cyclohexane)]-4,4′,6,6′-tetraene-2,2′-dione, Scheme S1)
was identified, again presumably formed from the reac-
tion of two phenoxyl radicals. Both substrates have simi-
lar O–H bond dissociation energies (~82 kcal mol-1),64-67
although 2,4-DTBP is expected to react considerably fast-
er in HAT because of the decreased steric bulk around the
O–H group.25, 68-71 Differing reaction rates between these
two sterically different substrates by the same oxidant has
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Page 4 of 9
previously been considered to imply a HAT mechanism.68
3
2
OMe
1
2
3
The ~30-fold difference in relative reactivity for 2 is con-
sistent with a HAT mechanism, but is slightly small com-
pared to previous reports (>50 fold).25, 68-71 Based on the
products identified, and the difference in reactivity of
these substrates, the mechanism of phenol oxidation thus
can be ascribed to HAT.
4
5
Me
tBu
1
6
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H
2.0
Br
2.0
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1.5
NO2
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-1
-2
1.5
1.0
0.5
0.0
90
180 270 360 450
Time (s)
1.0
0.5
0.0
-0.9 -0.6 -0.3 0.0 0.3 0.6 0.9
+
p
3
2
OMe
400
600
800
1000
1
Wavelength (nm)
tBu
Me
0.024
0.020
0.016
0.012
0.008
0.004
0.000
0
H
-1
-2
-3
Br
NO2
77 78 79 80 81 82 83 84 85
BDEO-H (kcal/mol)
Figure 4. Top: Hammett correlation plot for the reaction of
2 with p-X-2,6-DTBP substrates; Bottom: plot of log(k2) ver-
sus BDEO–H for p-X-2,6-DTBP.
0.00
0.03
0.06
0.09
[Substrate] (M)
0.12
0.15
To gain a better understanding of the mechanism of
phenol oxidation by 2, a series of para-substituted 2,6-di-
tert-butylphenol substrates (p-X-2,6-DTBP, X = OCH3,
C(CH3)3, CH3, Br, NO2) were reacted with 2 (Figures S11-
S15). All substrates reacted readily with 2. GC-MS analyses
indicated the formation of 2,6-di-tert-butyl-1,4-
benzoquinone in the cases of p-OCH3- and p-CH3-2,6-
DTBP (Scheme S1), presumably as a result of oxidation of
the generated phenoxyl radical by dissolved O2. All sub-
strates displayed a linear dependence of kobs against [p-X-
2,6-DTBP] allowing for k2 values for all substrates to be
determined (Table S2). We have plotted the obtained k2-
values against the substrate Hammett σp+-parameter and
Figure 3. Top: changes in the electronic absorption spec-
trum during the decay of 2 (blue trace) in the presence of 65
equivalents of 2,6-DTPB, at -40 °C. Inset: time trace of the
absorbance at λmax = 600 nm. Bottom: plots of kobs versus
[H/D-2,6-DTBP] determined for the reactions between 2 and
[H]2,6-DTBP (black) and [D]2,6-DTBP (red), at -40 °C in
acetone.
In order to probe the oxidation mechanism further, the
reaction of 2 with deuterated 2,6-DTBP was explored. [D]-
2,6-DTBP yielded a k2 value of 0.072 M-1s-1 (Figure 3, 0.176
M-1s-1 for [H]-2,6-DTBP). The obtained kinetic isotope
effect (KIE) value of 2.4 indicated that O–H bond cleavage
was involved in the rate-determining step, and was con-
sistent with a HAT mechanism. The KIE was rather small
compared to M=O oxidants which generally display KIE
values >7 for HAT.72-75 Importantly, however, the obtained
KIE value closely matched KIE values (range 2-3) deter-
mined for other high-valent Ni oxidants that were shown
to perform HAT.24-28
the substrate O–H bond dissociation free energy (BDEO–H
,
Figure 4).
The Hammett analysis showed a decrease in the k2 val-
ue with more electron-poor substrates (thus k2(p-OCH3-
2,6-DTBP) >> k2(p-NO2-2,6-DTBP), Figure 4). A slope of
ρ
= -2.5 was obtained for the Hammett plot. Such a linear
relationship in the Hammett plot with a negative slope
indicated that concerted PCET (thus HAT) was the most
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likely mechanism in phenol oxidation.68, 76-81 The plot of
40 °C, 1,4-cyclohexadiene (CHD) reacted slowly with 2
yielding a k2 = 5.5 x 10-4 M-1s-1 (Figures S16-S17). The oxi-
dized product benzene was identified from the reaction
with CHD, which is consistent with HAT initiated oxida-
tion of the CHD. Likewise, xanthene was oxidized by 2,
with a k2 value of 1.04 x 10-3 M-1s-1 (Figure S18) yielding
xanthone. The slightly higher rate constant for xanthene
oxidation is consistent with xanthene (BDE = 74 kcal mol-
1) containing a weaker C–H bond than CHD (BDE = 76
kcal mol-1), according to the Polanyi correlation.88, 101 The-
se combined observations suggest that 2 (a high-valent
metal-halide) will mediate the oxidation of hydrocarbon
substrates, suggesting that enzymatic high-valent metal-
halides may also be capable of such reactivity.
1
2
3
4
5
6
7
8
the log of k2 against the phenol BDEO–H also demonstrated
a linear relationship (Figure 4). As extensively document-
ed by Mayer for metal-based oxidants,82-83 this is expected
for a HAT mechanism, in which BDE’s govern the ΔH0 of
the reaction, and is thus a strong indication of a HAT
rate-determining step. Deriving Gibbs energies of activa-
tion from the measured k2 values, a ΔG‡/Δ(BDE) slope of -
0.66 was determined. We previously observed that
[NiIII(O2CCH3)(L)] displayed a slope of -0.31 for hydrocar-
bon C–H HAT activation.25 These values compare favour-
ably with the ideal ΔG‡/ΔG° value of -0.5 for HAT pre-
dicted by Marcus theory,84-85 and with previously reported
experimental values in the range of -0.15 to -0.7 for other
metal-based oxidants that perform HAT.74, 86-92
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Mechanism Discussion: Our results demonstrate the
capability of 2 in the activation of both O–H and C–H
bonds through HAT, yielding oxygen- or carbon-based
radicals, respectively. These organic radicals decay further
yielding the identified oxygenated, desaturated, or cou-
pled products. Halide incorporation (chlorinated prod-
ucts) was not observed for any of the hydrocarbon or
phenolic oxidation products even if the oxidation was
performed in the presence of excess Et4NCl. Interestingly,
however, the rate of the oxidation reaction by 2 was influ-
OMe
0.10
0.05
tBu
Me
0.00
H
–
-0.05
-0.10
Br
enced by the presence of free Cl anion (Figure S19). The
NO2
oxidation of p-CH3-2,6-DTBP demonstrated a first order
–
dependence on [–Cl]. This would suggest that Cl com-
petes with the phenolic substrate for interaction with 2
during the oxidation reaction.
0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.8
An important consideration is the fate of the chloride
ligand after HAT (Scheme 2). We propose that HAT re-
sults in the formation of HCl and [NiII(L)]. Post reaction
analysis of the 2 + 2,4,6-TTBP reaction showed an elec-
tronic absorption spectrum (Figure S20) that displayed
features similar to 1, but at much lower intensity. It is
E1/2 (V)
Figure 5. Plot of (RT/F)ln(k2) against the oxidation potential
(E) of p-X-2,6-DTBP substrates for the reactions of 2 with p-
X-2,6-DTBP substrates in acetone.
–
worth recalling that there is a slight excess of Cl present
A plot of (RT/F)ln(k2) versus the oxidation potentials
(Eox) of the phenols (Figure 5) showed a good linear corre-
lation with ln(k2) decreasing with decreasing Eox affording
a slope of -0.15. Such plots have been employed to under-
stand the mechanisms of proton coupled electron transfer
(PCET, HAT is a member of this family) reactions. If the
reaction involved slow (rate-limiting) electron transfer
from phenols followed by a fast proton transfer, the slope
of the Marcus plot ((RT/F)ln(k2) versus Eox)93 would be
expected to be -0.5.94 If proton transfer was rate limiting
and electron transfer was in equilibrium, the slope is pre-
dicted be -1.0. For coupled non-concerted PCET mecha-
nisms (thus non-HAT) where electron transfer and pro-
in the solution (from the preparation of 2). We surmise a
small quantity of 1 forms in the reaction mixture from the
–
interaction between [NiII(L)] and this residual Cl. The
post-reaction mixture was EPR silent. Addition of Et4NCl
to this reaction mixture gave almost quantitative re-
formation of 1 according to electronic absorption spec-
troscopy (Figure S20). This would suggest that [NiII(L)] is
present in the reaction mixture after 2 oxidised the phe-
nolic substrate. Fascinatingly, addition of 4 equiv. of CAN
to this solution yielded the formation of 2 again (Figure
S21), albeit in lower yield, presumably because of the
presence of a large excess of the 2,4,6-TTBP substrate.
These observations confirm that the [NiII(L)] core remains
intact during the preparation of 2, and after the substrate
oxidation reaction mediated by 2. Furthermore, it demon-
strates that 2 can be re-generated by simple addition of a –
Cl source and CAN, meaning the system has catalytic ca-
pabilities.
ton transfer display similar rates, a slope is predicted to
94-96
lie between -0.5 and -1.0.65,
The observed near-zero
slope of -0.15 obtained for 2 (Figure 5), thus clearly rules
out these PCET reaction profiles. Critically, however, for
HAT (concerted PCET) a slope of close to 0.00 is predict-
ed and has been previously observed.65, 97-100 These com-
bined observations confirm the reaction of the high-
valent nickel-halide 2 in oxidative O–H bond activation is
by a HAT mechanism.
More challenging to probe is the presence and form of
the HCl product (as dissolved/gaseous HCl or as a salt).
The fact that the [NiII(L)] core remains intact suggests
that HCl has been released. Addition of NH3 or NEt3 to
the post-reaction mixture resulted in a precipitate form-
Oxidation of Hydrocarbons. We investigated further the
reactivity of 2 in hydrocarbon C–H bond activation. At -
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Journal of the American Chemical Society
Page 6 of 9
ing, indicating the formation of ammonium chloride salts.
more critically, open new avenues for oxidation catalyst
design through the development of halide-only oxygen-
free oxidants. Work continues in our lab to explore the
mechanism of HAT by metal-halides and to probe the
potential of such oxidants in oxidation catalysis.
1
2
3
4
These observations simply demonstrate salt formation,
and don’t provide insight into the mechanism of HCl
formation. Nonetheless, they indicate HCl has been re-
leased during the oxidation of phenol by 2.
5
6
7
8
We previously demonstrated that the ancillary (OX)
ligand has a large effect on the relative reactivity of
[NiIII(OX)(L)] complexes (OX = O2CCH3, ONO2, OCO2H),
Acknowledgements: This publication has emanated from
research supported by the European Union (FP7-333948,
ERC-2015-STG-678202). Research in the McDonald lab is
supported in part by research grants from Science Founda-
tion Ireland (SFI/12/RC/2278, SFI/15/RS-URF/3307). XAS ex-
periments were conducted at SSRL beamline 2-2 (SLAC Na-
tional Accelerator Laboratory, USA), with support from the
DOE Office of Science (DE-AC02-76SF00515 and DE-
SC0012704) and NIH (P30-EB-009998). We are grateful to
COST Action CM1305 (ECOSTBio) for networking support.
We are grateful to Prof. Robert Barkley for training on and
use of an EPR spectrometer.
where [NiIII(ONO2)(L)] displayed a ~20-fold greater k2-
value than the other complexes.25,
These complexes
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were originally all prepared under different conditions
than were used to prepare 2, therefore a fair comparison
to 2 of their reactivity properties was not possible.
[NiIII(ONO2)(L)] can be prepared, however, under the
exact same conditions as the preparation of 2 (CAN as
45
oxidant).25,
In the oxidation of p-CH3-2,6-DTBP,
[NiIII(ONO2)(L)] displayed a k2 value of 5.12 M-1s-1 (Figure
S22), whereas 2 gave a k2 value of 2.71 M-1s-1 (Figure S14).
This demonstrates that simple tuning of the ancillary lig-
and can allow tuning (Cl versus ONO2) of the HAT reac-
tivity of the high-valent oxidant, presumably as a result of
tuning the strength of the X–H bond (NiII(H–ONO2) for
[NiIII(ONO2)(L)]; H–Cl for 2) in the product.
ASSOCIATED CONTENT
Supporting Information
Included in supporting information file (.pdf): physical
methods; synthesis methods; reactivity protocols and results;
further EPR analysis; X-ray absorption spectroscopy methods
and data.
Scheme 2. Mechanism of HAT by 2 in C–H activation by
HAT.
The Supporting Information is available free of charge on the
ACS Publications website.
AUTHOR INFORMATION
Corresponding Author
Aidan R. McDonald, aidan.mcdonald@tcd.ie
Previous reports have demonstrated the importance of
the magnitude of BDEO–H in Mn-1–O–H products formed
as a result of HAT by Mn=O oxidants in defining the po-
tency of an oxidant.85, 102 For 2, thus, the products of the
reaction are [NiII(L)] and HCl (Scheme 2). We therefore
propose that the thermodynamic driving force for the
reaction of 2 with O–H or C–H bonds (BDEO/C–H = 76-82
kcal/mol) is the relative strength of the H–Cl bond (103
kcal mol-1).103 We postulate that this opens a unique
pathway to consider hydrocarbon oxidation catalysis us-
ing high-valent metal halides, where one could simply
tune the properties of the halide (F, Cl, Br, I) to en-
hance/tune the oxidative reactivity. Furthermore, our
findings demonstrate the potential to remove the O-atom
from oxidation catalysis, thus eliminating unwanted hy-
droxylation or epoxidation outcomes in oxidative halo-
genation and desaturation reactions.
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