Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species

Article abstract of DOI:10.1002/chem.201501841

Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a Ni^II precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., CH bonds, CC bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic OO bond cleavage of a Ni-HmCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a Ni^III-oxyl compound. A highly reactive nickel-oxygen species has been spectroscopically trapped after heterolytic OO bond cleavage during the reaction of a Ni^II precursor with meta-chloroperbenzoic acid (HmCPBA). This species is used to carry out the oxidation of different substrates, such as olefins, sulfides, and CH bonds (see scheme).

Full text of DOI:10.1002/chem.201501841

DOI: 10.1002/chem.201501841
Full Paper
&
Nickel Intermediates
Reactivity of a Nickel(II) Bis(amidate) Complex with meta-
Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species
Teresa Corona,[a] Florian F. Pfaff,[b] Ferran AcuÇa-ParØs,[a] Apparao Draksharapu,[c]
Christopher J. Whiteoak,[a] Vlad Martin-Diaconescu,[a] Julio Lloret-Fillol,[a, d]
Wesley R. Browne,[c] Kallol Ray,[b] and Anna Company*[a]
Abstract: Herein, we report the formation of a highly reac-
tive nickel–oxygen species that has been trapped following
reaction of a NiII precursor bearing a macrocyclic bis(ami-
date) ligand with meta-chloroperbenzoic acid (HmCPBA).
This compound is only detectable at temperatures below
250 K and is much more reactive toward organic substrates
(i.e., CÀH bonds, C=C bonds, and sulfides) than previously
reported well-defined nickel–oxygen species. Remarkably,
this species is formed by heterolytic OÀO bond cleavage of
a Ni–HmCPBA precursor, which is concluded from experi-
mental and computational data. On the basis of spectrosco-
py and DFT calculations, this reactive species is proposed to
be a NiIII–oxyl compound.
[NiII(tpa)(OAc)(H2O)]+ ion (tpa=tris(pyridylmethyl)amine) and
related systems that use the oxidant meta-chloroperbenzoic
acid (HmCPBA) occurs through NiIII–oxo or NiII–oxyl intermedi-
ate species.[10–12] Furthermore, experimental and theoretical
studies have indicated that a [NiIIIÀO]+ species is a potent oxi-
dant in the gas phase for the conversion of methane into
methanol.[13,14] However, the evidence available for NiIV–oxygen
intermediates is limited.[7]
Introduction
The study of high-valent nickel complexes in particular and the
redox chemistry of nickel species in general has attracted the
attention of the bioinorganic-chemistry community, thus pro-
viding models of nickel-containing enzymes that catalyze
redox processes.[1] Enzymes with redox-active nickel sites in-
clude [NiFe] hydrogenases,[2] CO dehydrogenase,[3] acetyl-coen-
zyme A (CoA) synthase,[4] and nickel superoxide dismutase.[5]
Moreover, high-valent nickel species have been frequently
postulated to be key reaction intermediates in the catalytic
cycle of oxidation reactions[6,7] and in coupling reactions.[8,9]
In the field of oxidation chemistry, nickel–oxygen species are
perceived to be formed upon homolytic or heterolytic OÀO
bond cleavage of the terminal oxidant bound to a NiII precur-
sor. For example, alkane oxidation catalyzed by the
In contrast to the large number of mononuclear Mn–oxygen
and Fe–oxygen species reported over the past decade,[15] rela-
tively few examples of such species based on late-transition
metals such as nickel have been described. Recently, Ray and
co-workers have shown that a NiII–acylperoxo species coordi-
nated to tris[2-(N-tetramethylguanidyl)ethyl]amine (TMG3tren)
is the precursor to a NiIII–oxo/hydroxo compound that can per-
form an oxo transfer and CÀH activation with a rate-determin-
ing hydrogen-atom abstraction.[16] Moreover, Hikichi and co-
workers reported the selective hydroxylation of cyclohexane
catalyzed by tris(pyrazolyl)borate-based NiII complexes with
HmCPBA.[17] For these latter systems, thermally stable NiII–acyl-
peroxo species were spectroscopically detected and crystallo-
graphically characterized.[17] Evidence for the formation of
a transient [NiIV(OH)(cyclam)]2+ species (cyclam=1,4,8,11-tet-
raazacyclotetradecane) competent to epoxidize olefins was
gathered for the reaction of the corresponding NiII precursor
with H2O2 in acidic media.[7] Most recently, McDonald and co-
workers reported the characterization and reactivity of a termi-
nal NiIII–oxygen adduct, which could perform hydrogen-atom
abstraction of weak CÀH bonds and oxygen-atom transfer to
triphenylphosphine.[18]
[a] T. Corona, F. AcuÇa-ParØs, Dr. C. J. Whiteoak, Dr. V. Martin-Diaconescu,
Dr. J. Lloret-Fillol, Dr. A. Company
Group de Química Bioinorgànica, Supramolecular i Catàlisi (QBIS-CAT)
Institut de Química Computacional i Catàlisi (IQCC)
Departament de Química, Universitat de Girona
Campus Montilivi, 17071 Girona (Spain)
Fax: (+34)972-41-81-50
E-mail: anna.company@udg.edu
[b] F. F. Pfaff, Dr. K. Ray
Humboldt Universität zu Berlin, Department of Chemistry
Brook-Taylor Strasse 2, 12489 Berlin (Germany)
[c] Dr. A. Draksharapu, Prof. Dr. W. R. Browne
Stratingh Institute for Chemistry
Faculty of Mathematics and Natural Sciences, University of Groningen
Nijenborgh 4, 9747 AG Groningen (The Netherlands)
[d] Dr. J. Lloret-Fillol
There are precedents that show that pincerlike tridentate
2,6-pyridinecarboxamidate ligands can support well-defined
Current address: Institute of Chemical Research of Catalonia (ICIQ)
Av. Pa¨ısos Catalans 16, 43007 Tarragona (Spain)
NiIII or NiIV complexes[19] (including a NiIII–OCOOH species[18]
)
Supporting information for this article is available on the WWW under
http://dx.doi.org/10.1002/chem.201501841.
and a highly reactive CuIII–OH motif, as reported by Tolman
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Figure 1. a) Schematic representation of H2L. b) X-ray structure and selected
bond lengths [Š] and angles [8] of 1·NaCF3SO3. Hydrogen atoms and cocrys-
tallized NaCF3SO3 have been omitted for clarity.
and co-workers.[20] Herein, we show that the reaction of [NiII(L)]
(1; L is a tetradentate dianionic macrocyclic ligand with two
amidate, one pyridine, and one aliphatic amine groups; Fig-
ure 1a) with HmCPBA forms a transient nickel–oxygen species
2, which has been spectroscopically characterized. Compound
2 is only stable at subambient temperatures (<250 K) and is
more reactive toward organic substrates (CÀH bonds, C=C
bonds, and sulfides) than the previously reported well-defined
nickel–oxygen species. Experimental and computational meth-
ods indicate that this species is formed by heterolytic OÀO
bond cleavage.
Figure 2. UV/Vis absorption spectroscopic changes observed upon the reac-
tion of 1 (dotted line) with 3 equivalents of HmCPBA in CH3CN at À308C to
form 2 (dashed line). Inset: kinetic trace at l=420 nm.
tion spectroscopy indicated the formation of the metastable
dark-yellow species 2 with an absorption band at l=420 nm
(e>7000mÀ1 cmÀ1)
and
a
shoulder
at
l=580 nm
(e>800mÀ1 cmÀ1; Figure 2). The half-life of this species at
À308C was 4.5 hours. Compound 2 was not detected when
the reaction was carried out at room temperature.
Compound 2 reacts with various substrate classes. Indeed,
the decay of 2 was accelerated substantially by the addition of
thioanisole, which was ascertained by UV/Vis absorption spec-
troscopic analysis (see Figure S5 in the Supporting Informa-
tion). Under conditions of excess substrate, the decay in ab-
sorbance showed pseudo-first-order behavior and could be
fitted with a monoexponential function. The value of kobs
varied linearly with the thioanisole concentration, thus afford-
ing a second-order rate constant (k) of 0.56mÀ1 sÀ1 at À308C
(see Figure S6 in the Supporting Information). Reaction rates
were dependent on the substituent at the para positon to the
sulfide group. The logarithm of the second-order rate con-
stants of a series of para-substituted methylphenyl sulfides,
namely, para-X-thioanisoles (kX; X=Me, Cl, CN), showed a corre-
lation with the Hammett parameter (sp) with a reaction con-
stant (1) of À0.86 (Figure 3a). The negative 1 value indicates
a buildup of positive charge in the transition state; hence, 2
has an electrophilic character in these reactions. Moreover,
a plot of log(kX) against the one-electron oxidation potentials
of each para-X-thioanisole species (E8ox) afforded a linear corre-
lation with a slope of À1.84 (see Figure S7 in the Supporting
Information), thus indicating that the oxidation of sulfides by 2
occurs through direct oxygen-atom transfer rather than elec-
tron-transfer oxidation.[24]
Results and Discussion
The ligand H2L was synthesized following a four-step synthetic
route (see Scheme S1 in the Supporting Information). Selective
methylation of the central amine group of the commercially
available N-(2-aminoethyl)-1,3-propanediamine required prior
protection of the two terminal amine functionalities with
phthalic anhydride. After methylation and amine deprotection
with hydrazine, [1+1] cyclization with 2,6-pyridinedicarbonyl
chloride yielded H2L. This step was the most critical due to the
formation of [2+2] macrocyclic byproducts. Thus, the reaction
was carried out under high-dilution conditions to obtain the
desired [1+1] product in at least modest yields (see the Sup-
porting Information). The reaction of equimolar amounts of
H2L and [NiII(CF3SO3)2(CH3CN)3] with two equivalents of NaH
under anaerobic conditions in acetonitrile afforded complex
[NiII(L)], which cocrystallizes with NaCF3SO3 in 72% yield as
1·NaCF3SO3. The nickel center is present in a square-planar ge-
ometry with coordination to the pyridine ring, an aliphatic ter-
tiary amine group, and two amidate units trans to each other
(Figure 1b). The bond lengths NiÀNpy, NiÀNCH , and NiÀNamide
3
(i.e., 1.80, 1.93, and 1.85–1.87 Š, respectively) are consistent
with previously reported NiII square-planar complexes with pyr-
idine, amine, or amidate-based ligands.[21–23] The geometry ren-
ders the complex diamagnetic, thus enabling characterization
by using 1D and 2D 1H and 13C NMR spectroscopy (see Fig-
ures S1–S4 in the Supporting Information). As expected, the
complex does not present C2 symmetry and the b protons of
the pyridine ring appear as two well-resolved doublets at d=
7.43 and 7.36 ppm. Analysis with high-resolution QTOF-MS
showed a major peak at m/z 341.053, with an isotopic pattern
fully consistent with {[Ni(L)]+Na}+.
Compound 2 also behaves as an electrophilic oxygen-atom
transfer reagent toward alkenes. Thus, under pseudo-first-order
reaction conditions, a second-order rate constant of k=
0.18mÀ1 sÀ1 was obtained for the oxidation of cyclooctene,
whereas this value decreased to k=0.04mÀ1 sÀ1 for 1-octene.
The reaction of 2 with a series of para-substituted styrenes,
namely, para-Y-styrenes (Y=OMe, Me, H, Cl, and NO2), further
evidences the electrophilic character of 2 as it affords a nega-
tive reaction constant of 1=À0.86 (Figure 3b). Analysis of the
final oxidation products for the reaction of 2 with alkenes at
Monitoring the reaction of 1 with three equivalents of
HmCPBA in acetonitrile at À308C by means of UV/Vis absorp-
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Figure 4. Plot of log(k’) (determined at À308C) against the CÀH BDE for the
oxidation of alkanes with activated CÀH bonds by 2.
ed, the rate constants decreased with an increase of
CÀH bond-dissociation energy (BDE). More interestingly, log(k’)
values correlated linearly with BDE with a slope of À0.23
(Figure 4). Such a linear relationship between the reaction
rates and BDE provides strong evidence for hydrogen-atom ab-
straction as the rate-determining step for the oxidation. Parallel
reactions with deuterated 9,10-dihydroanthracene ([D4]DHA)
yielded a kinetic-isotope effect (KIE) of 4 (see Figure S14 in the
Supporting Information), which is a value consistent with CÀH
bond cleavage as the rate-determining step.[27,28]
Figure 3. The Hammett plot log(kX/kH) versus the Hammett parameter sp for
the reaction of 2 in acetonitrile at À308C with a) para-X-thioanisoles and b)
para-Y-styrenes.
The reaction of 2 with substrates bearing stronger CÀH
bonds was also examined. The addition of alkanes, such as tol-
uene, ethylbenzene, or cyclohexane (300 equiv), to a solution
of 2 in CH3CN at À308C caused the decay of its characteristic
band at l=420 nm with significantly higher rates than in the
absence of these substrates (see Figure S15 in the Supporting
Information). However, the decay of 2 did not follow simple
single-exponential functions, most probably because the back-
ground catalytic reaction significantly interferes with the kinet-
ic trace. Analysis of the final organic products showed the for-
mation of oxidized products (i.e., benzaldehyde, acetophe-
none, or cyclohexanone, respectively) in yields from 21 to 47%
with respect to 1.
À308C indicates the formation of the corresponding epoxides
with yields that range from 140% for cyclooctene oxide and
styrene oxide to 50% for 1,2-epoxyoctane (with respect to the
nickel complex). Despite the fact that peracids are well known
to be capable of directly oxidizing alkenes without the media-
tion of a metal complex,[25] control experiments (in the absence
of a nickel complex) indicated that no epoxide was formed by
direct reaction of HmCPBA with the alkene substrate under the
current reaction conditions (i.e., the reaction mixture was
quenched with excess NaHSO3 after full decay of 2 at À308C).
These results suggest that the slight excess of oxidant necessa-
ry to maximize the formation of 2 triggered a catalytic reaction
as a background, which explains the yields of over 100% ob-
tained for some of the substrates tested. Thus, the combina-
tion of 1 and HmCPBA might afford an efficient catalytic
system for the oxidation of selected substrates (see below).
Compound 2 could perform hydrogen-atom abstraction
from OÀH bonds by reacting with 2,4,6-tri-tert-butylphenol to
quantitatively form the corresponding phenoxyl radical, which
manifested in the appearance of the intense absorption band
at l=626 nm that is characteristic of this radical (see Fig-
ure S11 in the Supporting Information).[26] Remarkably, 2 was
also reactive toward hydrocarbon substrates with activated
methylene CÀH bonds, such as fluorene, 1,4-cyclohexadiene,
9,10-dihydroanthracene, and xanthene, again obeying pseudo-
first-order kinetics under conditions with excess substrate and
kobs values linearly dependent on substrate concentration (see
Figure S13 in the Supporting Information). The obtained
second-order rate constants were adjusted for the reaction sto-
ichiometry to yield the corrected rate constants (k’). As expect-
The oxidizing power of 2 was compared with the previously
spectroscopically characterized NiII–acylperoxo complex
[NiII(TpCF Me)(mCPBA)]
(TpCF Me =hydrotris(3-trifluoromethyl-5-
3
3
methylpyrazolyl)borate)[29] and NiIII–hydroxo(oxo) compound
[NiIII{O(H)}(TMG3tren)]n+[16] obtained by the reaction of
the NiII precursors with HmCPBA (Table 1). Interestingly, 2
reacts more than 200 times faster with CÀH bonds than
[NiIII{O(H)}(TMG3tren)]n+ at the same temperature (À308C). The
same reaction is up to three orders of magnitude faster rela-
tive to [NiII(TpCF Me)(mCPBA)], whereas the reaction toward the
3
para-Y-styrenes proceeds about 50 times faster. However, the
much higher temperature (+708C) used for the reactivity stud-
ies with the Tp-based system indicates that differences with re-
spect to 2 are indeed much greater. Overall, compound 2 is
significantly more active than previously reported well-defined
nickel–oxygen species. A comparison with the reactivity of the
NiIII–oxygen adduct recently reported by McDonald and co-
workers[18] was hampered because studies with this last com-
pound are limited to substrates containing weaker OÀH and
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The EPR spectrum of a sample
frozen to 77 K after mixing
1 and HmCPBA at À308C shows
signals of two rhombic S=1/2
species: a major species (95%)
with g1 =2.03, g2 =2.22, and g3 =
2.24 and a minor species (5%)
with g1 =2.02, g2 =2.19,and g3 =
2.31 (Figure 5). These EPR prop-
erties, that is, gav =2.16 and 2.18
for the major and minor species,
respectively, and gjj >g? are in-
dicative of a NiIII species with dis-
Table 1. Second-order rate constants (k, mÀ1 sÀ1
)
for the oxidation of different substrates by 2,
[NiIII{O(H)}(TMG3tren)]n+, and [NiII(TpCF Me)(mCPBA)].
3
[NiIII{O(H)}(TMG3tren)]n+
(À308C)[16]
[NiIII(TpCF Me)(mCPBA)]
(+708C)[29]
3
2
(À308C)
xanthene
2.93
2.62
1.69
0.28
0.45
0.69
1.51
0.0131
0.0125
0.0073
0.0018
0.00051
0.019
9,10-dihydroanthracene
1,4-cyclohexadiene
fluorene
styrene
4-methylstyrene
4-methoxystyrene
–
–
–
–
0.0010
0.0088
0.017
0.022
CÀH bonds (i.e., 2,6-di-tert-butylphenol and 1-benzyl-1,4-dihy-
dronicotinamide, respectively; BDE=64 kcalmolÀ1) or triphe-
nylphosphine.
Given the high reactivity of compound 2 toward several sub-
strate classes, including alkanes bearing strong CÀH bonds, we
tested the ability of 1 to act as a catalyst in the oxidation of cy-
clohexane with HmCPBA as oxidant. The slow addition of
HmCPBA (150 equiv) to a solution containing 1 and cyclohex-
ane (15000 equiv) afforded a mixture of cyclohexanol (A) and
cyclohexanone (K) with a total turnover number of 100 and
a product ratio of A/Kꢀ1:1, with an overall 67% yield based
on the oxidant. Blank experiments in the absence of the nickel
catalyst showed the formation of only trace amounts of oxi-
dized products (<0.5% yield).
Further insight into the nature of the oxidizing species was
gained through the oxidation of cis-1,2-dimethylcyclohexane
and adamantane. The oxidation of this substrate (150 equiv)
by 1 (1 equiv) with HmCPBA (150 equiv) as an oxidant afforded
the corresponding tertiary alcohol product with 84% retention
of configuration (RC) of the tertiary carbon atoms. Under simi-
lar experimental conditions, adamantane was oxidized with
a high preference for the tertiary carbon atom with a tertiary/
secondary ratio of 18:1 (corrected according to the number of
equivalent secondary and tertiary CÀH bonds). Much lower RC
values and tertiary/secondary ratios would be obtained if
freely diffusing radicals (i.e., hydroxyl or alkoxyl) were in-
volved.[30] This data indicates that a metal-based oxidant, most
likely the spectroscopically detected species 2 (see below), is
mainly responsible for the observed oxidation reactions.
The characterization of 2 with cryospray ionization mass
spectrometry (CSI-MS) at À308C revealed a clean spectrum
Figure 5. Top: EPR spectrum of the reaction of 1 (1.9 mm) with 3 equivalents
of HmCPBA in acetonitrile at À308C after 60 seconds under argon. Bottom:
Simulated EPR spectrum (black line), which accounts for a major species
(95%, dotted line) g1 =2.03, g2 =2.22, and g3 =2.24 (anisotropic broadening:
H1 =200, H2 =235, and H3 =100 MHz) and a minor species (5%, dashed line)
with g1 =2.02, g2 =2.19, and g3 =2.32 (anisotropic broadening H1 =50,
H2 =180, and H3 =180 MHz).
with a major peak at m/z 318.0605, with an isotopic pattern torted-square-planar, trigonal-bipyramidal, or compressed-octa-
fully consistent with the [NiIII(L)]+ ion. Interestingly, monitoring hedral coordination and the unpaired electron in the dxy or
the reaction of 2 with 1-octene by means of CSI-MS showed dx2Ày2 orbitals.[31–34] The maximum concentration of the NiIII spe-
the progressive formation of the NiII species (m/z 355.0319). cies, however, amounts to only 16%. Moreover, the time de-
However, no NiII species was formed during the reaction with pendence of the change in the intensity of the EPR signal did
9,10-dihydroanthracene, and only a NiIII species was observed not directly correlate with that of the UV/Vis absorbance at l=
(m/z 318.0605; see Figures S20–S21 in the Supporting Informa- 420 nm (or its shoulder at l=580 nm) assigned to 2. The con-
tion). In any case, the mass spectra of the reaction mixture ob- centration of the NiIII species steadily increased over time, even
tained upon warming to room temperature, either in the pres- after the disappearance of the chromophore (see Figure S22 in
ence or absence of the substrate, only exhibited signals that the Supporting Information). These data indicate that the
corresponded to NiII species with an oxidized/dehydrogenated signal of the NiIII species corresponds to a decayed species of 2
ligand (m/z 355.0319).
that is not responsible for the observed oxidation chemistry.
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Indeed, the addition of 1 to preformed 2 causes the immediate
decay of the latter species, as followed by using UV/Vis absorp-
tion spectroscopy (see Figure S19 in the Supporting Informa-
tion). The EPR spectrum of the resultant solution shows the
formation of the NiIII species in a much higher yield of 35%
(relative to the total nickel concentration after adding
2.75 equivalents of the NiII complex to the preformed solution
of 2; note that the maximum possible yield is 53%; see Fig-
ure S23 and Scheme S2 in the Supporting Information). Analy-
sis of the reaction mixture by means of ESI-MS also shows the
presence of a NiIII species, thus further suggesting that com-
proportionation occurred between 2 (possibly a formal NiIV
species; see below) and 1 (a NiII species) to give the [NiIII(L)]+
ion (m/z 318.0617; see Figure S18 in the Supporting Informa-
tion).
To help further characterize the Ni center of 2, XAS was ap-
plied at the metal K-edge. The pre-edge of 2, associated with
1s!3d transitions, occurs at approximately 8333.5 eV and has
a normalized area of 0.16, thus indicating the presence of
a high-valence Ni species (see Figure S24 in the Supporting In-
formation). Generally for NiII complexes, the 1s!3d transitions
Figure 6. Fourier-transformed EXAFS spectra of 2 (no phase correction, FT
window=2–12 ŠÀ1): dotted line=data, black line=best fit. Inset: k3-weight-
ed unfiltered EXAFS spectra: dotted line=data, black line=best fit.
occur at around 8332 eV and are 2–3-fold less intense.[35]
A
higher oxidation state for 2 is further emphasized by a higher
rising-edge energy determined by using the half-height
method. This energy in 2 is at approximately 8343.4 eV, which
is between 1.5 and 2 eV higher in energy than reported for
a NiII species[35] and is consistent with a NiIII species.[36–38] Previ-
ous studies on Ni–oxido and Ni–cyclam derivatives show
a shift of 1.5–2 eV on going from NiII to NiIII species, whereas
a shift of approximately 4 eV would be expected for a NiIV spe-
cies.[36–38] Relative to Ni foil, this translates to a shift of approxi-
mately 4 eV for a NiIII center, whereas a NiIV species would be
expected to have a shift of approximately 6 eV to higher
energy (see Figure S24 in the Supporting Information).[37]
Therefore, the X-ray absorption near-edge structure (XANES)
spectra of 2 is most consistent with a NiIII oxidation of the
metal center. The Fourier-transformed EXAFS spectra of 2 is
shown in Figure 6. Three scattering shells are implied by the
features at 1.4, 1.8, and 2.2 Š. Single-scatter fits are consistent
with the first two peaks that correspond to two N/O scattering
shells (see Table S4 in the Supporting Information). On the
other hand, the feature at 2.2 Š is consistent with contribu-
tions from multiple scattering of the pyridine ring. Several mul-
tiple scattering models were attempted (see Table S5 in the
Supporting Information), and the EXAFS analysis converged on
a model with two longer metalÀN/O bonds of approximately
2.12 Š, and three shorter NÀO bonds of approximately 1.88 Š,
including the pyridine ligand (Table 2).
Table 2. EXAFS multiple-scattering model showing NiÀligand bond
lengths and the coordination number for complex 2.[a]
Model
Path
Dr [Š]
s2 [”103 Š2]
R [%]
cv2
N2N3(Pyr)
2 N2.0
3 N1.8
1 Pyr[b]
0.12(1)
0.08(1)
0.08(1)
1.9(6)
1.9(6)
6(2)
8.7
7.2
[a] E0 =8344.2 eV and S0 =0.9. [b] The pyridine scattering paths do not in-
clude the primary NÀNi single-scatter path.
DFT calculations were carried out to explore the possible
nature of 2. Previous reports on related systems indicate four
distinct mechanistic scenarios for the reaction of 1 with
HmCPBA (reactions A-D; Scheme 1). Pathway A involves the
formation of the NiII–acylperoxy [(L)NiII-mCPBA]À species. How-
ever, this process was considered to be kinetically and thermo-
dynamically unfeasible due to the high free-energy difference
of the [(L)NiII-mCPBA]À ion relative to the starting reactants
(DGA = +47.7 kcalmolÀ1). This difference may be rationalized
Scheme 1. Possible pathways (A–D) studied by DFT that correspond to the
reaction of 1 with HmCPBA in acetonitrile. Free energies are given in
kcalmolÀ1 at À308C.
by the acidity of HmCPBA (pKa =31), which was computed to
be much lower than that of 1ÀH+ (pKa =7) in acetonitrile, so
that proton transfer from HmCPBA to 1 is unfavorable. An al-
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ternative mechanism involves the formal oxidation of NiII to
NiIII (accompanied by the 1eÀ reduction of HmCPBA to give
the radical anion), and the subsequent coordination of another
HmCPBA or mCPBA molecule to the +3 metal center. Howev-
er, DFT calculations suggest the thermodynamic unviability of
these two processes (pathway B: DGA = +30.0 kcalmolÀ1; path-
way C: DGA = +13.5 kcalmolÀ1). This result is in agreement
with the EPR data, which indicated that the NiIII species is not
related to chromophore 2.
and GC-MS, thus indicating that an OÀO heterolysis pathway is
followed under the experimental conditions (see Figure S17 in
the Supporting Information). Thus, both experimental and the-
oretical data point toward an OÀO heterolytic pathway as the
most plausible mechanism. According to DFT calculations, the
terminal oxygen atom of the Ni product 2 formed after OÀO
heterolysis bears an interaction with the acidic proton of the
acid byproduct (i.e., HmCBA). This compound would carry out
the oxidation of the substrate.
Instead, the complexation of 1 with HmCPBA is thermody-
namically reasonable with a DGA value of +4.4 kcalmolÀ1
(pathway D, Scheme 1). Complex [NiII(L)]–HmCPBA may evolve
through two different reaction pathways as previously postu-
lated for other nickel systems (Scheme 1): 1) homolytic OÀO
bond cleavage to form a NiIII–hydroxo species and a carboxyl
radical, which decomposes to give chlorobenzene and carbon
dioxide or 2) heterolytic OÀO bond cleavage to form a NiIV–
oxo/hydroxo intermediate and the corresponding benzoic acid
(HmCBA). Computational studies indicate that the homolytic
pathway is kinetically unfavorable by +30.0 kcalmolÀ1
(Scheme 2). Instead, the OÀO heterolysis shows a lower barrier
of only +9.0 kcalmolÀ1 to afford a nickel–oxygen species. In-
terestingly, chromatographic analysis of the reaction mixture
after the self-decay of 2 did not show the presence of chloro-
benzene or CO2, but the formation of quantitative amounts of
HmCBA was ascertained instead by using NMR spectroscopy
Analysis of the Hirshfeld spin density on the nickel center
(1(Ni)=0.66) and the oxo moiety (1(O)=1.29) suggests that 2
is best described as a NiIIIÀO· species (see Tables S6 and S7 and
Figure S26 in the Supporting Information). Interestingly, inspec-
tion of the spin natural orbitals (SNOs) of complex 2 shows
two single occupied orbitals: 1) s*(dz2/pz) distributed between
the Ni and O centers and 2) py orbital centered on the terminal
oxygen atom (see Figure S27 in the Supporting Information).
This electron distribution may be responsible for the weaken-
ing of the NiÀO bond (1.95 Š) and the significant oxyl charac-
ter of the oxygen group. Moreover, the Mayer index for the
NiÀO bond is about 0.6, which is in agreement with the half
broken s bond and the lack of p bonding showed in the SNOs.
Finally, an atoms-in-molecules (AIM) analysis was performed on
2 to better understand the nature of the NiÀO bond. We
2
found negative but close to zero values of r 1(r) and H(r),
which suggests a very weak NiÀO interaction with almost no
Scheme 2. Energetic profile of pathway D for the reaction of 1 with HmCPBA in CH3CN (homolytic and heterolytic OÀO bond cleavage). Free energies are
given in kcalmolÀ1 at À308C. DEZPE values in kcalmolÀ1 are shown in parentheses (ZPE=zero-point energy).
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covalent character (see Figure S28 in the Supporting Informa-
(100 equiv) in CH3CN and in the presence of 18O-labeled water
at À308C affords 7% of the corresponding 18O-labeled epoxide
product. This data indicates that water exchange can occur
prior to reaction with substrates, as previously observed for
other metal–oxo species.[39]
The formulation of 2 as a NiIII–oxyl species is controversial.
To date, metal–oxyl species have been postulated several
times, but have been scarcely directly detected.[40,41] A possible
alternative to this mechanism would be the formulation of 2
as the [NiII(L)]ÀHmCPBA adduct, a precursor to the high-valent
nickel species (Scheme 3). However, this possibility would not
tion). Therefore, the DFT analysis of the electronic structure of
III
C
[Ni (L)(O)] reveals that the Ni and O atoms are weakly connect-
ed, thus making the terminal oxygen atom highly reactive.
The experimental extended X-ray absorption fine structure
(EXAFS) distances of 2 are consistent with the NiIII–oxyl radical
theoretical model, with three shorter and two longer NÀO
bonds. The NiIII–oxyl radical model predicts a pyridineÀNi bond
of 1.85 Š, with the two proximal NiÀN bonds of 1.86 and
1.90 Š, which is consistent with the N/O scattering shell of
1.88 Š. The NiIII–oxyl radical model also predicts two longer
bonds for NiÀO and NiÀN (1.95 and 1.96 Š, respectively), thus
corresponding to the two longer NÀO distances of 2.12 Š de-
rived from EXAFS analysis. This finding is well within the reso-
lution of the EXAFS single-scatter fits (ca. 0.14 Š) for the long
NiÀN/O distances.
Resonance Raman spectroscopic analysis of 2 in frozen ace-
tonitrile (77 K) showed enhancement of two sets of bands at
n˜ =450 and 477 cmÀ1 and n˜ =736 and 879 cmÀ1, which appear
concomitantly with the absorbance of 2 (Figure 7). Simulation
Scheme 3. Schematic representation of the nickel species formed upon the
reaction of 1 with HmCPBA.
agree with the data from X-ray absorption spectroscopy (XAS),
which supports a metal center with a higher valence. More-
over, the reaction of 1 with an aliphatic peracid, such as perno-
nanoic acid, under the same conditions as those used for the
generation of 2 (3 equivalents of peracid, CH3CN, À308C) af-
fords a UV/Vis absorption spectrum almost identical to 2, with
a characteristic absorption band centred at l=416 nm (see
Figure S25 in the Supporting Information). Given the different
nature of the two peracids (i.e., pernonanoic acid and
HmCPBA), markedly different UV/Vis absorption spectra would
be expected for both systems if the peracid unit was coordi-
nated to the nickel center in 2, which is not the case. More-
over, the formation of a NiIII–oxyl species by heterolytic OÀO
bond cleavage is in agreement with the much higher reactivity
of the present system relative to the previously reported and
well-defined NiÀmCPBA species.[16,29]
Figure 7. Resonance Raman spectra (lex =457 nm) in frozen acetonitrile
(77 K) of a) 2 formed after the reaction of 1 (0.24 mm) with 3 equivalents of
HmCPBA at À308C, b) HmCPBA (0.72 mm), c) 1 (0.25 mm), and d) decom-
posed 2.
of the Raman spectra of [NiIII(L)(O·)]ÀHmCBA by using DFT
methods predicted a NiÀO vibration at n˜ =433 cmÀ1 and
a ligand-based stretching vibration at n˜ =443 cmÀ1 (see Fig-
ure S29 in the Supporting Information). Thus, within error, the
experimental and theoretical results are in agreement for the
first set of bands at n˜ =450 and 477 cmÀ1. The bands at n˜ =
736 and 879 cmÀ1 are tentatively assigned to OÀO stretching
modes from a byproduct, most likely a NiIII–peroxy species.
The formulation of 2 as a NiIII–oxyl species is consistent with
the DFT calculations and with the fact that a heterolytic OÀO
bond cleavage is experimentally observed (i.e., the formation
of HmCBA as a reaction byproduct). This formulation would
also be in agreement with the EPR and NMR silence of 2 and
with the EXAFS data. Analysis by CSI-MS that showed the lack
of signals with intensity–time profiles similar to those observed
by UV/Vis absorption spectroscopic analysis agrees with the
neutral character of 2. Finally, the reaction of 2 with 1-octene
Conclusion
The NiII complex of the bis(amidate) macrocyclic ligand (L) has
been shown to react with HmCPBA at low temperatures to
form compound 2, which has been spectroscopically trapped.
This species is kinetically competent enough to carry out the
oxidation of different substrates, such as olefins, sulfides, and
CÀH bonds. Remarkably, the activity of 2 is much higher than
that previously established for well-defined nickel–oxygen sys-
tems, which may indicate that an alternative mechanism
occurs in the present system. A combination of experimental
and theoretical results have suggested that a heterolytic OÀO
bond cleavage in a NiÀHmCPBA adduct occurs, thus giving rise
to the formation of a high-valent nickel–oxygen species that is
best formulated as a NiIII–oxyl complex. This work suggests
that the use of a dianionic ligand may lead to alternative reac-
tion pathways relative to previous systems, thus favoring the
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tions were additionally corrected for volume errors, thus resulting
in slight differences in tube diameter. EPR simulation was per-
formed by using EASYSPIN.[44]
formation of high-valent nickel species that behave as strong
oxidants. In this line of research, work in our group is aimed at
ligand tuning and the use of alternative oxidants to increase
the reactivity of the nickel species further.
A sample of 2 (4 mm; prepared by reaction of 1 with 3 equivalents
of HmCPBA in CH3CN at À308C) was loaded into a holder (2 mm)
with Kapton tape windows and stored at liquid-nitrogen tempera-
tures until run. Data was collected at the SOLEIL synchrotron
SAMBA beamline equipped with a Si(220) double-crystal mono-
chromator and a liquid-helium cryostat (20 K). X-ray absorption
(XAS) was detected in fluorescence mode by using a Canberra 35-
element Ge detector and a Z-1 filter. An internal-energy calibration
was performed by using the first inflection point of the XANES
spectrum of nickel foil (Ecal =8331.6 eV). Data reduction and nor-
malization was performed by using the Athena software package
with the AUTOBK algorithm. To extract intensities and energy posi-
tions, the XANES pre-edge and edge were fit with pseudo-Voigt
functions and the edge jump was modeled by using a cumulative
Gaussian–Lorentzian sum function. EXAFS were extracted by using
a Rbkg value of 1.05 Š and a spline between k=1 and 13.7 ŠÀ1. The
Artemis software program with an IFEFFIT engine and FEFF6 code
was used for EXAFS analysis.[45–47] The k3-weighted data was fit in r
space over a range of k=2–12 ŠÀ1, with S0 =0.9 and a Kaiser–
Bessel window (dk 2). The spectra were not phase corrected and
a global DE0 value was employed, with the initial E0 value set to
the inflection point of the rising edge at 8344.2 eV. Single-scatter
paths for NiÀN with initial reff values of 1.8 and 2.0 Š and multiple
scattering from pyridine (initial reff =1.8 Š) were fit in terms of Dreff
and s2, as previously described.[48–50] To assess the goodness of fit
from different models, the Rfactor (%R) and the reduced c2 (cv2) were
minimized. Although the Rfactor is generally expected to decrease
with the number of adjustable parameters, c2v may eventually in-
crease, thus indicating that the model is overfitting the data.[51]
Experimental Section
Materials and methods
The reagents and solvents used are commercially available and
were purchased from Panreac, Scharlau, and Aldrich. The prepara-
tion and handling of air-sensitive materials were carried out in a N2
drybox (MBraun ULK 1000) with O2 and H2O concentrations of
<1 ppm. Commercially available 70% meta-chloroperbenzoic acid
was purified prior to use by following a reported procedure.[42] The
deuterated substrate [D4]-9,10-dihydroanthracene was prepared
from 9,10-dihydroanthracene by following previously reported pro-
cedures.[43]
Elemental analyses of C, H, and N were performed on a PerkinElmer
EA2400 series II elemental analyzer. Mass spectrometric analysis
was performed by electrospray ionization (ESI) on a high-resolution
Bruker micrOTOF QII (Q-TOF) mass spectrometer with a quadrupole
analyzer and positive and negative ionization modes. 1H NMR,
13C NMR, COSY, and HSQC spectra were performed on Bruker Ultra-
shield Avance III400 and Ultrashield DPX300 spectrometers. UV/Vis
absorption spectra were performed on a diode-array Agilent Cary
60 spectrophotometer and low-temperature control was main-
tained with a cryostat from Unisoku Scientific Instruments. X-ray
analyses were carried out on Bruker Smart Apex CCD diffractome-
ter with graphite-monochromated MoKa radiation (l=0.71073 Š)
from an X-ray tube. GC analyses were carried out on an Agilent
7820A gas chromatograph (HP5 column, 30 m) with a flame-ioniza-
tion detector. GC-MS was performed on an Agilent 7890A gas
chromatograph interfaced with an Agilent 5975c mass spectrome-
ter with a triple-axis detector. The identification of CO2 was carried
out on an Agilent 7820A GC system equipped with three columns,
washed molecular sieves (5 Š; outside diameter (OD)= 2 m”1/
8 inch, mesh 60/80 SS; Porapak Q, OD=4 m”1/8 inch, mesh 80/
100 SS), and a thermal-conductivity detector. Raman spectra were
recorded in NMR tubes (diameter=5 mm) at 77 K in a liquid-nitro-
gen-filled quartz dewar. Spectra were collected in the back-scatter-
ing mode (1358) with excitation at l=457 nm (Cobolt Lasers,
50 mW) and planoconvex lens (diameter=25 mm) to collect and
collimate the Raman scattering, which was passed through the
long pass cutoff filter (Semrock). The scattering was focused at the
entrance slits of a Shamrock 303i spectrograph with a grating of
1200 LmmÀ1 blazed at l=500 nm and a iDUS-420-BRDD CCD de-
tector (Andor Technology). Spectral calibration was carried out in
acetonitrile and toluene (1:1 v/v). The spectra were processed on
Andor Solis and Spectrum 10 (PerkinElmer). Cyclic voltammetry
(CV) was performed by using a potentiostat from CHInstruments
with a three-electrode cell. The working electrode was a glassy
carbon disk from BAS (0.07 cm2), the reference electrode was a sa-
turated KCl calomel electrode, and the auxiliary electrode was
a platinum wire. CV was carried out with nBu4NPF6 (TBAP) as a sup-
porting electrolyte (0.1m). The EPR spectra were recorded on an
ESP 300 X-Band EPR spectrometer from Bruker with a TE011 super-
high Q microwave resonator. The samples were cooled to 77 K in
a liquid-nitrogen Dewar. Spin quantifications were calculated on
the basis of double integrals of the recorded spectra relative to
a measured standard of CuII ions of a given concentration. Sample
tubes were filled higher than the cavity dimension to guarantee an
equally filled cavity for all the measured samples. Spin quantifica-
Synthesis of [NiII(CF3SO3)2(CH3CN)3]
NiCl2 (2.36 g, 0.018 mmol) was suspended in dry acetonitrile
(50 mL) in a Schlenk flask (100 mL). Me3SiOTf (7.1 mL, 0.039 mmol)
was added to the solution in an N2 atmosphere. The skin-colored
suspension was stirred vigorously at room temperature for 3 weeks
while the color darkened to deep blue. The mixture was then fil-
tered to remove the starting material, the solvent was evaporated
under reduced pressure, and a purple precipitate was formed. The
solid was collected, dissolved in acetonitrile (5 mL), and slow diffu-
sion of diethyl ether at room temperature over the resulting solu-
tion afforded a purple solid, which was dried under vacuum to
yield [NiII(CF3SO3)2(CH3CN)3] as
a
pale-purple solid (5.43 g,
0.011 mmol, 63%). Elemental analysis (%) calcd for C8H9F6N3NiO6S2:
C 20.02, H 1.89, N 8.75; found: C 19.76, H 1.97, N 8.63.
Synthesis of [NiII(L)] (1)
A solution of [NiII(CF3SO3)2(CH3CN)3] (32.09 mg, 0.057 mmol) in an-
hydrous acetonitrile (0.5 mL) in a glove box was added dropwise
to a vigorously stirred suspension of H2L (15.10 mg, 0.057 mmol) in
anhydrous acetonitrile (0.5 mL). After a few seconds, the solution
became colorless. The addition of NaH (2.71 mg, 0.11 mmol,
2 equiv) caused a further color change to orange. The reaction
mixture was stirred for 3 h, the solvent was removed, and the re-
sulting residue dissolved in methanol, filtered through celite, and
concentrated. Slow diffusion of diethyl ether over the resulting so-
lution afforded 1 in a few days as orange crystals (20.07 mg,
0.041 mmol, 72%). ESI-MS: m/z (%): 341.05 [M+Na]+ (100), 659.12
[2M+Na]+ (40); 1H NMR (CD3CN, 400 MHz, 298 K): d=7.95 (t, J=
7.6 Hz, 1H; Ha), 7.43 (dd, J=7.6 Hz, 1H; Hb), 7.36 (dd, J=7.6 Hz,
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1H; Hc), 3.43–3.35 (m, 1H; Hd), 3.33–3.20 (m, 3H; He/f/g), 2.94–2.87
(m, 1H; Hh), 2.82–2.74 (m, 1H; Hi), 2.73–2.71 (m, 1H; Hj), 2.69 (s,
3H; CH3), 2.50–2.45 (m, 1H; Hk), 1.81–1.73 ppm (m, 2H; Hl/m);
13C NMR (CD3CN, 100 MHz, 298 K): d=169.39 (C12 or C13), 166.14
(C12 or C13), 153.54 (C10 or C11), 152.53 (C10 or C11), 141.27 (C1),
121.72 (C2), 121.60 (C3), 65.38 (C4), 58.28 (C5), 41.54 (C6), 41.23 (C7),
40.90 (C8), 26.21 ppm (C9); elemental analysis (%) calcd for
C13H16N4NiO2·NaCF3SO3: C 34.24, H 3.28, N 11.61; found: C 33.98, H
3.08, N 11.23; CV (CH3CN vs. SCE): E1/2 =0.96 V.
Grimme D3 correction with Becke–Johnson damping.[57] The con-
nection between the transition states and minimum values was
verified by using intrinsic reaction coordinate (IRC) calculations.
The Hirshfeld spin densities and charges, Mayer bond-order
index,[58,59] and spin natural orbitals (SNO) were computed to ra-
tionalize the electronic structure of intermediate 2. A Bader AIM
analysis[60–62] was also conducted on 2 to elucidate the nature of
the NiÀO bond.
Analytical frequency calculations were performed to evaluate the
thermal corrections and entropic effects at 243.15 K and to charac-
terize the located stationary points in the condensed phase.
Raman spectra intensities of intermediate 2 were simulated at 77 K
and with laser excitation at l=457 nm by using the GaussSum 3.0
software.[63]
Generation of 2
In a typical experiment, a solution of 1 in acetonitrile (2.5 mL,
0.24 mm) was placed in
a cuvette (pathlength=1 cm; [1]=
0.6 mmol). The quartz cell was placed in the Unisoku cryostat of
a UV/Vis absorption spectrophotometer and cooled to 243 K. After
reaching thermal equilibrium, an UV/Vis absorption spectrum of
the starting complex was recorded. Then, a solution of HmCPBA
(3 equiv) in acetonitrile (105 mL, 17 mm) was added. The formation
of a band at lmax =420 nm (e>7000mÀ1 cmÀ1) and a shoulder at
lmax =580 nm (e>800mÀ1 cmÀ1) was observed. Compound 2 was
fully formed within 100 s.
Final Gibbs energies (G) were evaluated by using the following
equation:
G ¼ ETZVPðSMD þ D3Þ þ Gcorr
ð1Þ
where ETZVP(SMD+D3) was obtained by using single-point calcula-
tions with the TZVP basis set on equilibrium geometries, including
solvation and dispersion effects, and Gcorr is the thermal correction
obtained from a thermostatistical analysis at the B3LYP/SMD level.
Analysis of the reaction of 2 with substrates
The pKa values were computed according to:
Once 2 was fully formed, an aliquot of a solution in acetonitrile
(150 mL) containing the corresponding equivalents of the desired
substrate was added to the cuvette. The decay of the band at l=
420 nm was monitored, and the reaction was quenched after com-
plete decay by adding an excess of NaHSO3 (0.1 mL of a commer-
cially available 40% aqueous solution). Biphenyl was added as an
internal standard, and the nickel complex was removed by passing
the solution through a short plug of silica. The products were
eluted with ethyl acetate and analyzed by using a gas chromatog-
raphy flame-ionization detector (GC-FID). The organic products
were identified by comparison with authentic compounds.
DGA
ð2Þ
pka ¼
RTInð10Þ
where R is the universal gas constant and T is the temperature.[64]
The standard dissociation free-energy change (DGA) between an
acid (AH) and its conjugate base (AÀ) in the solvent phase may be
calculated by using the following equations:
DGA ¼ GðAsÀolÞ þ GðHsþolÞÀGðAHsol Þ þ DG
GðHþaqÞ ¼ GðHgþasÞ þ DG
ð3Þ
ð4Þ
*
Hþ
solv
Catalytic experiments at room temperature with HmCPBA
where G(AHsol )and G(AÀsol) are the standard free energies of the
acid and its conjugate base, respectively. The G(Hþsol) is the free
energy of the proton in acetonitrile, obtained from the solvation
In a typical reaction, a solution of HmCPBA (0.58m, 0.5 mL,
290 mmol) in acetonitrile was delivered by syringe pump over
30 min at 258C to a vigorously stirred a solution of the nickel cata-
lyst (2.0 mmol) and the substrate (1900 mmol) in acetonitrile
(2.5 mL). The final concentrations of the reagents were 0.7 mm
nickel catalyst, 97 mm HmCPBA, and 0.62m substrate. After sy-
ringe-pump addition, the resulting solution was stirred for another
30 min. For the oxidation of cyclohexane, biphenyl was added as
an internal standard and the nickel complex was removed by pass-
ing the solution through a short path of silica gel. The products
were eluted with ethyl acetate and analyzed by using a GC-FID.
The organic products were identified by comparison with authen-
tic compounds.
freeÀ1energy of
a
proton in acetonitrile (DGHsoþlv =À260.2 kcal
[56]
mol
)
and its gas-phase free energy (GðHþgasÞ=À6.3 kcal
molÀ1).[57] DG* is the standard-state thermodynamic correction as-
sociated with the conversion from a standard state of 1m in the
aqueous phase and 1 atm in the gas phase into 1m in both
phases, with a value of 1.54 kcalmolÀ1 at 243.15 K.
Acknowledgements
Financial support for this work was provided by the European
Commission (FP7-PEOPLE-2011-CIG-303522 to A.C.) and the
COST Action CM1305 (ECOSTBio) including a STSM grant
(COST-STSM-CM1305-21541) to T.C. The Spanish Ministry of Sci-
ence is acknowledged for a Ramón y Cajal contract to A.C.
Also F.A.-P. thanks the Universitat de Girona for a predoctoral
grant. K.R. thanks the Cluster of Excellence “Unifying Concepts
in Catalysis” (EXC 314/2), Berlin and the Heisenberg Pro-
gramme of the Deutsche Forschungsgemeinschaft for financial
support. J.L.-F. thanks the Cellex Foundation for financial sup-
port from the starting-career program. W.R.B. acknowledges
Computational details
All DFT calculations were carried out by using the Gaussian09 set
of programs.[52] The X-ray diffraction structure of [NiII(L)] (1) was
chosen as a starting point for geometry optimizations by using the
B3LYP exchange-correlation functional[53,54] and the TZVP basis
set.[55] Nickel species were considered in all possible spin states
without symmetry constraints. The CH3CN solvation and effects
were included in the geometry optimizations through the solvent
model D (SMD) polarizable continuum model.[56] Dispersion effects
were introduced through single-point calculations with the
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Received: May 11, 2015
Published online on August 25, 2015
Chem. Eur. J. 2015, 21, 15029 – 15038
www.chemeurj.org
15038
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