Carboxamidate Ligand Noninnocence in Proton Coupled Electron Transfer

Article abstract of DOI:10.1021/acs.inorgchem.9b00055

Recent breakthroughs have brought into question the innocence (or not) of carboxamidate donor ligands in the reactivity of high-valent oxidants. To test the reactivity properties of high-valent carboxamidate complexes, [Ni^II(^tBu-terpy)(L)] (1, ^tBu-terpy = 4,4′,4′′-tri-tert-butyl-2,2′6′,2″-terpyridine; L = N,N′-(2,6-dimethylphenyl)-2,6-pyridinedicarboxamidate) was prepared and converted to [Ni^III(^tBu-terpy)(L)]⁺ (2) using ceric ammonium nitrate. 2 was characterized using electronic absorption and electron paramagnetic resonance spectroscopies and electrospray ionization mass spectrometry. 2 was found to be a capable oxidant of phenols and through kinetic analysis was found to oxidize these substrates via a nonconcerted or partially concerted proton coupled electron transfer (PCET) mechanism. The products of PCET oxidation of phenols by 2 were phenoxyl radical and the protonated form of 1, 1H⁺. 1H⁺ was crystallographically characterized providing convincing evidence of 1's ability to act as a proton acceptor. We demonstrate that the complex remained intact through a full cycle of oxidation of 1 to 2, PCET of 2 to yield 1H⁺, and deprotonation of 1H⁺ to yield 1 followed by reoxidation of 1 to yield 2. The N-H bond dissociation energy of the protonated amide in 1H⁺ was determined to be 84 kcal/mol. Our findings illuminate the role carboxamidate ligands can play in PCET oxidation.

Full text of DOI:10.1021/acs.inorgchem.9b00055

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
Carboxamidate Ligand Noninnocence in Proton Coupled Electron
Transfer
‡
‡
Caitilín McManus, Prasenjit Mondal, Marta Lovisari, Brendan Twamley, and Aidan R. McDonald*
School of Chemistry, Trinity College Dublin, The University of Dublin, College Green, Dublin 2, Ireland
*
S Supporting Information
ABSTRACT: Recent breakthroughs have brought into question the
innocence (or not) of carboxamidate donor ligands in the reactivity of
high-valent oxidants. To test the reactivity properties of high-valent
II t
t
carboxamidate complexes, [Ni ( Bu-terpy)(L)] (1, Bu-terpy = 4,4′,4′′-tri-
tert-butyl-2,2′;6′,2″-terpyridine; L = N,N′-(2,6-dimethylphenyl)-2,6-pyridi-
III t
+
nedicarboxamidate) was prepared and converted to [Ni ( Bu-terpy)(L)]
(
2) using ceric ammonium nitrate. 2 was characterized using electronic
absorption and electron paramagnetic resonance spectroscopies and
electrospray ionization mass spectrometry. 2 was found to be a capable
oxidant of phenols and through kinetic analysis was found to oxidize these
substrates via a nonconcerted or partially concerted proton coupled electron
transfer (PCET) mechanism. The products of PCET oxidation of phenols
+
+
by 2 were phenoxyl radical and the protonated form of 1, 1H . 1H was
crystallographically characterized providing convincing evidence of 1’s ability to act as a proton acceptor. We demonstrate that
+
+
the complex remained intact through a full cycle of oxidation of 1 to 2, PCET of 2 to yield 1H , and deprotonation of 1H to
+
yield 1 followed by reoxidation of 1 to yield 2. The N−H bond dissociation energy of the protonated amide in 1H was
determined to be 84 kcal/mol. Our findings illuminate the role carboxamidate ligands can play in PCET oxidation.
INTRODUCTION
workers reported another carboxamidate-supported complex
■
32
that facilitated the oxidation of benzyl alcohol. There is thus
a very large cohort of carboxamidate complexes that facilitate
biomimetic oxidation reactions.
The conversion of inert, often saturated, hydrocarbons to high
value products relies on our ability to activate strong C−H
1
−7
bonds.
Enzymes have evolved to employ an array of high-
The high-valent oxidants tend to react with hydrocarbon C−
H bonds through a proton coupled electron transfer (PCET)
oxidation mechanism. PCET represents a class of C−H
oxidation reactions where a proton and electron are transferred
valent transition-metal oxidants that activate strong C−H
bonds. Such species have been identified as containing
terminal metal-oxo (MO) or bridging metal-oxo entities.
Great effort has been made to mimic the structural, electronic,
3
3−36
4
,7−13
in a nonconcerted or concerted fashion.
complexes, for example, perform hydrogen atom transfer
HAT, a form of concerted PCET) of C−H bonds to yield a
MO
and reactivity properties of these metal-based oxidants.
Such complexes are designed to imbue sufficient stability into
metastable high-valent oxidants. Supporting ligands are
therefore required to be excellent sigma-donors or contain
anionic donors to stabilize metals in high oxidation states.
In this regard, anionic carboxamidate supporting ligands
have been used with great success because of their ability to
stabilize complexes with metals in high oxidation states.
Pioneering work by Collins and co-workers has demonstrated
the stabilizing effect of anionic tetraamidomacrocylic ligands
(
one-electron reduced M−O−H and a carbon-based radical
that will rebound to yield a hydroxylated product and a two-
electron reduced metal ion. The factors that determine PCET
n
n−1
oxidation are the redox potential of the M /M couple and
the pKa of the proton accepting entity. For high-valent
complexes supported by neutral donor ligands (e.g., poly-
pyridine ligands), the O-atom will always be sufficiently basic
to act as a proton acceptor. We were interested in exploring the
role that significantly more basic anionic donor ligands (e.g.,
carboxamidate, amide) may play in PCET oxidation. The
recent reports by Fukuzumi and co-workers, where a
14,15
(
TAMLs) to support MO’s.
Borovik and co-workers
have similarly employed anionic tris-urea ligands to support a
1
6−19
large array of MO’s.
Tolman and co-workers have also
demonstrated the use of the 2,6-pyridinedicarboxamidate for
20−22
carboxamidate-supported Mn−OH complex was surprisingly
supporting high-valent Cu complexes.
We and others
2
31
capable of HAT, Garcia-Bosch and co-workers, where a
carboxamidate ligand was postulated to act as a proton
found that the same ligand was ideal for trapping high-valent
2
3−30
Ni species.
Most recently, Fukuzumi and co-workers have
reported a mononuclear nonheme manganese(III)−aqua
complex supported by a carboxamidate ligand that was a
Received: January 10, 2019
3
1
capable oxidant. In a similar light, Garcia-Bosch and co-
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.9b00055
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
3
2
acceptor in PCET, and Heyduk and co-workers, who
postulated that a diarylamide ligand could act as a H-atom
elemental formula of 1 (observed mass = 831.39 m/z;
calcuated mass for 1 + H = 831.39 m/z, Figure S4). We
+
3
7
acceptor, heightened this interest. Herein, we explore the
role, if any, of the 2,6-pyridinedicarboxamidate ligand in PCET
reactions.
thus determined that complex 1 could be assigned the formula
II t
8
II
[Ni ( Bu-terpy)(L)] and contained a d S = 1 Ni ion in a
pseudo-octahedral ligand field.
II
III
The Ni complex 1 was oxidized to the Ni complex 2 in
RESULTS AND DISCUSSION
acetone at −80 °C using ceric ammonium nitrate (CAN,
■
IV
II
t
t
(
NH ) [Ce (ONO ) ], 8 equiv, Scheme 1). The reaction was
[
2
Ni ( Bu-terpy)(L)] (1, Bu-terpy = 4,4′,4′′-tri-tert-butyl-
4 2
2 6
complete within 400 s. 1 displayed weak features at λ = 420
,2′;6′,2″-terpyridine; L = N,N′-(2,6-dimethylphenyl)-2,6-
max
30
II
and 840 nm typical of octahedral Ni complexes, while 2
demonstrated an intense and broad band at λmax = 620 nm
pyridinedicarboxamidate, Scheme 1) was prepared from
a
(
Figure 2). Similar features in the visible region have been
Scheme 1. Preparation of 2 from 1
a
IV
CAN = (NH ) [Ce (ONO ) ].
4
2
2 6
II
26
t
[
Ni (NCCH )(L)] by ligand exchange with Bu-terpy in
3
tetrahydrofuran (THF) (see the Supporting Information for
II
details). The X-ray structure of 1 demonstrated a Ni ion in a
pseudo-octahedral geometry (Figure 1 and Table S1) and
Figure 2. (Top) Electronic absorption spectra of 1 (black trace, 0.4
mM, acetone) and 2 (red trace, after oxidation of 1 by CAN (8
equiv)) at −80 °C. ε calculated on the basis of EPR yield. (Bottom)
X-band EPR spectrum of 2 in a frozen acetone solution (red trace)
measured at 77 K, 6.35 mW microwave power with 0.3 mT
modulation amplitude, and the simulated spectrum of 2 (gray trace;
see the Supporting Information for simulation details).
III
reported for octahedral Ni complexes and analogous
III
23−28,30,40,41
[
Ni (X)(L)] complexes.
2 could also be generated
Figure 1. ORTEP plot of 1 with atomic displacement shown at 50%
in the same yield in acetone at −80 °C using the one-electron
oxidant magic blue (tris-(4-bromophenyl)ammoniumyl hexa-
chloroantimonate, Figure S6). ESI-MS of 2 demonstrated a
new mass peak at m/z = 830.37 (Figure S7), which can be
probability. Hydrogen atoms and three cocrystallized CH Cl2
2
molecules omitted for clarity.
II
III t
+
compared favorably with [Ni (terpy)(L)] (terpy = 2,2′;6′,2″-
attributed to the [Ni ( Bu-terpy)(L)] ion. The combined
3
0
1
terpyridine). The H nuclear magnetic resonance (NMR)
electronic absorption and ESI-MS results led us to assign 2 the
III t
+
spectrum of 1 showed broad and shifted peaks, consistent with
formula [Ni ( Bu-terpy)(L)] .
II
30,38
1
containing a paramagnetic Ni ion (Figure S1).
The
Electron paramagnetic resonance (EPR) spectroscopy
7
III
40,42−44
number of resonances observed (11) was typical of a
symmetric molecule; if there were asymmetry in the molecule,
up to 24 resonances would be expected. A magnetic moment
showed that 2 contained an S = 1/2 d Ni species.
The EPR spectrum of 2 exhibited axial symmetry, where g >
⊥
g∥ and gav = 2.11 (Figure 2), indicating an axially elongated
octahedral geometry, with the unpaired spin density localized
on the Ni ion. 2 displayed hyperfine coupling of five lines in
(
μ ) of 2.51 μ was determined using the Evan’s method,
eff B
30,38,39
indicating two unpaired electrons (Figure S2).
Electro-
spray ionization mass spectrometry (ESI-MS) confirmed the
the g component, associated with coupling to two equivalent
∥
B
DOI: 10.1021/acs.inorgchem.9b00055
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
14N-donor ligands.30
The EPR signal obtained for 2 was similar
Article
2 with 4-H-2,6-DTBP at −40 °C to allow comparison to
previously reported [Ni (X)(L)] complexes (Table 1, Figure
III
III
to those obtained for all other [Ni (X)(L)] complexes,
displaying axial symmetry and hyperfine coupling only in
2
4,26,28,30
III
g .
The yield of the Ni species 2 was calculated to be
Table 1. Phenol BDEO−H, E , and k Values
OX 2
∥
7
0 ± 20%. The obtained spectrum supported the assignment
4
-X-2,6-
BDE
(kcal/mol)
E
(V) vs
k
2
III t +
OX
Fc/Fc
of 2 as [Ni ( Bu-terpy)(L)] .
46
+
47,48
σp+
49
(M−1 s−1
)
DTBP
Complex 2 did not decay over the course of 6 h at −80 °C.
OCH3
CH3
C(CH3)3
H
78.3
81.0
81.2
82.1
82.1
0.53
0.90
0.93
1.07
−0.78
−0.31
−0.26
0
89.0
At 0 °C, 2 displayed a t1 = 2250 s. It was decided to explore
/2
0.195
0.165
0.004
0.542
the reactivity of 2 at −80 °C to ensure no influence of self-
decay on our kinetic analysis. 2 did not react with any
hydrocarbons with weak C−H bonds (e.g., cyclohexadiene),
2
,4-DTBP
III
24,26,28
unlike previous examples of [Ni (X)(L)].
In contrast,
para-substituted 2,6-di-tert-butylphenol (4-X-2,6-DTBP, X =
t
S15). A k value of 0.06 M s−1 for 2 at −40 °C was
−1
H, CH , OCH , Bu) and 2,4-DTBP reacted readily with 2,
3
3
2
allowing us to derive mechanistic insights from the kinetic
analysis of these reactions (Figures S8−S16). Substrates
containing relatively strong O−H bonds such as 4-X-2,6-
determined. The post-reaction mixtures for this substrate were
analyzed by gas chromatography mass spectrometry (GC-MS),
showing peaks corresponding to 3,3′,5,5′-tetra-tert-butyl-[1,1′-
bis(cyclohexane)]-2,2′,5,5′-tetraene-4,4′-dione (Figure S8).
This product provides a strong indication of PCET by 2,
indicating the formation of a phenoxyl radical that underwent
radical−radical coupling. A kinetic isotope effect (KIE) of 2.1
was measured for the reaction between 2 and 4-H/D-2,6-
DTBP (Figure S16), which compares favorably to other
DTBP (where X = Cl, Br, CN, NO ) did not react with 2 when
2
added in 3000-fold excess. Furthermore, TEMPO−H (1-
hydroxy-2,2,6,6-tetramethylpiperidine), containing a relatively
weak O−H bond, reacted at such a high rate (10 equiv reacted
within 2 s, Figure S17) with 2 that accurate kinetic analysis was
not possible. This demonstrated that a narrow substrate O−H
III
bond dissociation energy (BDE ) window existed to explore
examples of [Ni (X)(L)] complexes oxidatively activating O−
O−H
24,26,28,45
the reactivity of 2 under the experimental conditions
employed.
Upon reaction with 4-H-2,6-DTBP, the rate of decay of the
characteristic band of 2 (λmax = 620 nm) was monitored with
time (see Figure 3 for the typical reaction). The decay was
fitted as first order to obtain a pseudo-first order rate constant
H and C−H bonds.
These combined observations
suggest the rate-limiting step involved the phenolic O−H
hydrogen atom. The products of the reaction suggest the
formation of a phenoxyl radical in these reactions, suggesting
some form of PCET oxidation by 2.
The k values determined for the substituted 4-X-2,6-DTBP
X = H, CH , OCH , Bu) that did react with 2 spanned a very
wide range (Table 1), with 4-OCH -2,6-DTBP (containing the
weakest BDEO−H) reacting extremely rapidly, while 4-H-2,6-
DTBP (containing the strongest BDEO−H) demonstrated
2
t
(
(kobs), and a plot of kobs against [substrate] gave the second
3 3
order rate constant for the reaction (k , Figures S10−S16). A
3
2
−
1
k for the reaction between 4-H-2,6-DTBP and 2 of 0.004 M
2
−
1
s
was measured at −80 °C. We also explored the reaction of
−
1
−1
relatively slow reactivity. Furthermore, a k of 0.54 M
s
2
was measured for the reaction between 2 and 2,4-DTBP at
80 °C (Figure 3). This represents a 135-fold enhancement
for the sterically less encumbered 2,4-DTBP, with both
−
50
substrates displaying similar BDEO−H
.
A plot of log(k2)
against substrate BDEO−H of the 4-X-2,6-DTBP showed a
linear relationship with a negative slope (Figure 4, Table 1).
When Gibbs free energies from the k values were derived
2
‡
using the Eyring equation, a ΔG /Δ(BDE) value of 0.99 was
obtained (Figure S18). A linear Hammett plot was also
produced from this data (Figure 4), demonstrating a large and
negative slope of ρ = −5.5. A plot of (RT/F)ln(k ) against E
2
OX
(
R = gas constant, T = temperature, F = Faraday constant) was
also linear with a negative slope value of −0.29 (Figure 4).
Discussion on Reactivity Properties and Exploration
of Mechanism. In order to put the reactivity of 2 into
context, we explored the reaction of 2 with 2,6-DTBP at −40
III
°
C to compare it to previously reported [Ni (X)(L)] (Table
−
1
−1
−1 −1
2
: k = 0.06 M
s
for 2 at −40 °C; k = 0.13−1.96 M
s
2
2
III
for [Ni (X)(L)] at −40 °C). The rate at which 2 reacted
t
would indicate that introducing a neutral donor ( Bu-terpy),
+
unable to act as a H -acceptor, inhibited its reactivity relative
III
to the previously reported [Ni (X)(L)] complexes that had
+
−
−
−
anionic H -accepting ancillary donors ( Cl, OAc, ONO ).
2
Furthermore, at −40 °C, 2 was unreactive to hydrocarbons
with weak C−H bonds (e.g., 9,10-dihydroanthracene, 1,4-
Figure 3. (Top) Electronic absorption spectra demonstrating the
decay of 2 (red trace) over time upon reaction between 2 and 2,4-
DTBP (inset depicts decay of λmax = 620 nm feature against time).
III
cyclohexadiene), whereas the other [Ni (X)(L)] (X = Cl,
(Bottom) Plot of observed rate versus substrate concentration for the
OAc) were capable of oxidizing such substrates. This
reaction of 2 with 2,4-DTBP (green) and 2,6-DTBP (blue).
demonstrates a marked difference in reactivity properties for
C
DOI: 10.1021/acs.inorgchem.9b00055
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
2
reacted with 2,4- and 2,6-DTBP with a 135-fold
enhancement in reactivity for the sterically less encumbered
,4-DTBP substrate. Such rate enhancement has previously
been ascribed to concerted PCET reactions with phenols
2
(
HAT), although is considerably higher than previous
24,26,28,50,51
examples (values in the range of ∼30−70).
As
described below, the mechanism by which 2 reacted with
phenols may be a nonconcerted or partially concerted PCET.
The large steric effect observed for these substrates thus could
indicate alternative forms of PCET and should not be ascribed
simply to concerted PCET. The difference in relative rates
could also indicate a difference in reaction mechanism between
the two substrates, but we have no evidence for such a change.
2
4
displayed a KIE value of 2.1 for the reactions between 2 and
-H/D-2,6-DTBP. This compared favorably with many
analogous M−O−X oxidants, including all members of the
III
[
Ni (X)(L)] family, where low KIE values (<7) have been
24,26,28,45
observed in the vast majority of cases.
This
observation simply confirms PCET as rate-limiting but does
not provide any insight into whether it was concerted or not.
‡
The large ΔG /Δ(BDE) value (0.99), Hammett ρ-value
(
−5.5), and the slope of the plot of (RT/F)ln(k ) against
2
substrate E (−0.29) all suggested a nonconcerted PCET
OX
mechanism or a mechanism where electron transfer (ET)
‡
played a greater role in PCET. First, the large ΔG /Δ(BDE)
value (0.99) is well in excess of the ideal value predicted by
Marcus theory (0.5) and is beyond the range where concerted
34,36
PCET (HAT) has previously been ascribed (0.15 to 0.7).
For analogous complexes supported by L, we previously
‡
III
observed ΔG /Δ(BDE) values of 0.31 ([Ni (OAc)(L)],
III
hydrocarbon oxidation) and 0.66 ([Ni (Cl)(L)], phenol
oxidation) where concerted PCET (HAT) was implicated in
‡
both cases. We conclude that the large ΔG /Δ(BDE) value is
Figure 4. Kinetic data for the reaction between 2 and 4-X-2,6-DTBP
indicative of a mechanism of O−H bond activation that does
substrates in acetone. (Top) Plot of log(k ) versus substrate BDEO−H.
2
not involve concerted PCET, or involves partially concerted
(
Middle) Hammett plot, slope = −5.5. (Bottom) Plot of (RT/
(
partial transfer of charge simultaneous with PT), or involves
F)ln(k ) against substrate E , slope = −0.29. Note: for p-Cl/Br/CN/
2
OX
PCET toward different locations (so-called multisite
PCET).
NO -2,6-DTBP, there was no reaction with 2, whereas for TEMPO−
52,53
2
H, very high rates prevented accurate determination of k .
2
Second, the slope of the (RT/F)ln(k ) against the substrate
2
+
EOX plot (−0.29) was higher than would be expected for a
Table 2. Phenol BDE , E , σ Values and k for Their
Reaction With 2
O−H
OX
p
2
a
concerted PCET reaction. For rate limiting electron transfer
28,54−57
(
ET), a slope of −0.5 was predicted.
If proton transfer
‡
k
ΔG /
(RT/F)
ln(k )/E
(PT) was rate limiting, this slope would be closer to −1.0. If
the rate determining step was concerted PCET, a slope closer
to zero is expected. Our results suggest either concerted PCET
or rate-limiting ET for the reaction between 2 and phenols,
because the slope was closer to the ideal value for rate-limiting
2
Ni(X)(L)]+/0 (M−1 s−1
)
b
KIE
c
Δ(BDE)
ref.
this
[
2
OX
d
d
d
2
0.06
2.1
0.99
−0.29
−0.15
work
e
e
e
Cl
O CCH3
ONO2
0.18
0.13
1.96
2.4
2.1
0.66
0.31
28
26
26
e
f
2
III
ET. We previously ascribed a slope of −0.15 for [Ni (Cl)(L)]
28
to concerted PCET. It is possible that ET was not exclusively
a
b
All reactions performed in acetone. Oxidation of 2,6-DTBP at −40
rate limiting, with ET and PT demonstrating similar rates. It
has previously been suggested that, in cases where the slope is
less than −0.5, a partial transfer of charge may occur in the
c
d
°
C. For oxidation of H- or D-2,6-DTBP. Measured at −80 °C.
e
f
Measured at −40 °C. Determined for C−H bond activation at 25
°C.
47,58
rate-limiting step.
It is also possible that multisite PCET,
where the proton and electron are delivered to different
locations, resulted in intermediary (partially concerted) PCET
5
2,53
2
: 2 was unable to activate weak C−H bonds, thus rendering it
kinetic behavior.
thermodynamically less reactive than previous examples of
The mechanism by which 2 reacted with phenols appeared
to be different from previously reported [Ni (X)(L)]
oxidants, all of which displayed ΔG /Δ(BDE) values closer
to 0.5 and (RT/F)ln(k ) against substrate E slopes closer to
zero, indicating concerted PCET (Tables 1 and 2). In those
cases, anionic ancillary ligands were deemed to be acting as
proton acceptors forming acetic acid and HCl for [Ni (OAc)-
III
[Ni(X)(L)] complexes where X was an anionic donor; 2 was
‡
also a kinetically less reactive oxidant of phenols than those
complexes with anionic ancillary ligands. Thus, although 2 was
a capable oxidant, it should be emphasized that it was
thermodynamically and kinetically less reactive than analogous
complexes with ancillary proton acceptors.
2
1/2
III
D
DOI: 10.1021/acs.inorgchem.9b00055
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
III
(
L)] and [Ni (Cl)(L)], respectively. The Ni atom was
pyridine dicarboxamide was through the central pyridine
donor, one carboxamidate N-donor, and one carboxamide O-
donor. One of the carboxamidate N-donors was protonated
and no longer bound to the metal ion. The C−O and C−N
bond lengths of the reconfigured carboxamide were consistent
with an amide (1.240(3) and 1.332(3) Å, respectively). A
triflate counterion balanced the charge on the complex ion,
confirming no change in the overall oxidation state of the
complex during protonation. The X-ray crystal structure of
deemed to be accepting the electron. For 2, there are multiple
plausible proton acceptor groups (carboxamidate O- or N-
atoms, pyridine N-atoms) with the Ni atom acting as the likely
electron acceptor.
The fact that 2 was capable of oxidizing phenols indicated
that the carboxamidate ligand L may be acid/base noninnocent
in PCET reactions, with either N- or O-atoms acting as
+
potential H -acceptors. To probe this further, we explored the
acid/base chemistry of complexes 1 and 2. Ni complex 1 did
II
+
+
1H confirmed the H -acceptor capability of the 2,6-
pyridinecarboxamidate ligand in complex 1.
not react with any of the (weakly acidic) 4-X-2,6-DTBP
substrates (Figure S19), demonstrating that 2 was the only
entity reacting with these phenols. Upon exposure of 1 to two
equiv of the more acidic pyridinium triflate (PyHOTf) at −40
C, a shift in the near-infrared (near-IR) features to lower
energy was observed by electronic absorption spectroscopy
+
Critically, the protonation of 1 to yield 1H was readily
reversed by the addition of a slight excess of base (KOH
dissolved in CH OH, Figures 6 and S27). We tested the
3
°
+
(
Figures 6 and S20, new species defined as 1H ). Such near-IR
features have been attributed to a d Ni ion in an octahedral
8
II
ligand field. The preservation of these features indicated that
+
30
1
H maintains an octahedral 6-coordinate environment. The
same shift was observed when HCl (one equiv) was used as a
+
H -donor (Figure S21), indicating that the same product was
+
obtained with different H -donors. This would suggest that the
protonation reaction does not involve replacement of one of
−
−
the ligands by OTf (from PyHOTf) or Cl (from HCl).
+
Satisfyingly, we were able to synthesize and isolate 1H in
the solid state by reacting 1 with two equivalents of pyridinium
triflate in CH Cl followed by precipitation with excess THF
2
2
1
(
see the Supporting Information for details). H NMR analysis
+
comparison of 1 and 1H showed the conversion of a highly
symmetric paramagnetically shifted spectrum for 1 to another
paramagnetically shifted spectrum with much lower symmetry
for 1H (Figure S22), indicating monoprotonation of the
ligand. Fourier transform infrared (FT-IR) analysis of 1H
+
Figure 6. Complex 1 (black trace), complex 1H (red trace, 1 +
+
pyridinium (two equiv)), the reformation of 1 from the reaction
+
+
between 1H and KOH (green trace, three equiv KOH), and 2 (blue
−
1
showed a new feature at ν = 3235 cm , which can be ascribed
to an N−H vibrational mode (Figure S23). This feature was
absent in 1 (Figure S3); however, such N−H resonances were
trace) from the reaction where CAN (8 equiv) was added to the 1 +
pyridinium + KOH mixture.
observed in FT-IR spectra of LH (Figure S24). ESI-MS
+
2
reactivity of a series of bases toward 1H in CH CN and found
3
+
confirmed the elemental formula of 1H (Figure S25).
Single crystals of 1H were obtained by slow diffusion of
+
that the following bases reacted with 1H to yield 1: potassium
+
tert-butoxide, KOH, 1,8-diazabicyclo[5.4.0]undec-7-ene
+
THF into a CH Cl solution of 1H (Figure 5 and Table S1).
2
2
(DBU), triethylamine (NEt ), tert-butylamine, 4-N,N-dime-
3
+
II
1
H displayed a pseudo-octahedral geometry around the Ni
center with both pyridine dicarboxamide and Bu-terpy ligands
still bound to the Ni ion. However, the binding mode of the
thylaminopyridine (DMAP), benzylamine, and imidazole. The
latter five bases reacted slowly with respect to the former three
consistent with their relative pK values in CH CN (Table S2).
t
II
a
3
+
In contrast, the following did not react with 1H : 2,6-lutidine,
aniline, and pyridine. This allowed us to estimate the pK of
a
+
the proton in 1H to be 15.00 in CH CN (Table S2). We then
3
proceeded to more accurately measure the equilibrium
+
constant (K ) for the reaction 1 + PyHOTf → 1H + Py in
a
+
order to accurately determine pK for 1H . Titration of 1 with
a
PyHOTf showed a linear decay of the electronic absorption
features associated with 1 (Figure S28). However, a plot of
+
[
1H ][Py]/[1] against [PyHOTf] was not linear, and
therefore, the slope of the plot and thus K could not be
a
determined (Figure S29; see the Supporting Information for
details on calculations). The observed nonlinear plot suggests
+
that the reaction between 1 and PyHOTf to yield 1H was not
a simple A-to-B conversion. An isosbestic point was not
observed during the titration, confirming that this was not a
+
+
clean 1 to 1H conversion (Figure S30). Similar results were
Figure 5. ORTEP plot of 1H with atomic displacement shown at
5
+
obtained for the reverse reaction between 1H and KOH
0% probability. Hydrogen atoms (apart from the amide N−H),
trifluoromethane sulfonate counterion, and cocrystallized CH Cl and
(Figure S31). This observation is not unexpected given that
2
2
THF molecules omitted for clarity.
the N-atom in 1 must undergo protonation, dissociation from
E
DOI: 10.1021/acs.inorgchem.9b00055
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
+
the metal, and rotation around the Cketone−N bond (in no
Our kinetic analysis suggested that 1H may have reacted
through nonconcerted or partially concerted PCET, we
therefore decided to assess the driving force for the reaction
between 2 and 4-X-2,6-DTBP should the reaction in fact be
concerted PCET (homolytic O−H bond cleavage). This can
+
particular order) to yield 1H .
No reaction was observed upon the addition of CAN to
+
+
1
H , indicating that 2 could not be generated from 1H . CV
+
analysis of 1H demonstrated a chemically irreversible wave at
+
0
.93 V versus Fc/Fc (Figure S26), which is significantly
be analyzed by determining the N−H bond dissociation
enthalpy (BDE ) of 1H . We calculated the BDE
methods for metal-based oxidants in PCET pioneered by
+
+
shifted by 0.74 V with respect to 1 (0.19 V versus Fc/Fc ,
Figure S5). However, 2 was formed in 75% yield when KOH
using
N−H
N−H
+
36,53
was added to 1H followed by CAN (Figures 6 and S32).
Mayer and co-workers.
The following formula was used to
Furthermore, 2 was unreactive toward PyHOTf under the
calculate BDE: BDEN−H = 1.37(pK ) + 23.06(E1/2) +
a
+
same conditions, showing that it was not capable of acting as a
CH,CH3CN, yielding a BDEN−H for 1H of ∼84 kcal/mol (pK
a
+
H -acceptor with weak acids. 2 reacted with aqueous HCl
= 15.00; E1/2 = 0.19 V). The error in this measurement is likely
yielding a featureless electronic absorption spectrum (Figure
quite high (±3 kcal/mol) given that the pK was estimated as
described above. It is important to note that the reactivity
studies were performed in acetone while BDEN−H was
a
S33). Addition of KOH (dissolved in CH OH) resulted in the
3
reformation of 2 (Figure S34). Complex 2 was thus capable of
+
acting as a reversible H -acceptor; however, only very strong
calculated in CH CN because we could not determine the
3
II/III
acids reacted with 2, meaning 2 was unlikely capable of
Ni
E1/2 accurately in acetone. Furthermore, the nonlinear
deprotonating 4-X-2,6-DTBP substrates.
Once it was established that 1H was a stable species, it was
important to establish its role, if any, in the PCET oxidation of
behavior observed for the reaction between 1 and PyHOTf
+
+
suggested that other species may form prior to 1H in the
+
protonation of 1, meaning the BDEN−H in 1H may not be a
phenols. Analysis of the 2 + 4-OCH -2,6-DTBP post-reaction
driver for this reaction. Nonetheless, the calculated BDEN−H
value is consistent with the observed reactivity patterns, where
4-X-2,6-DTBP substrates with relatively strong O−H bonds
3
mixture displayed features that were the same as those
obtained for 1H (Figure 7). In this instance, 1H+ was
+
did not react with 2 (e.g., X = CN, NO : BDE
> 84 kcal/
2
O−H
mol) whereas those with BDEO−H < 84 kcal/mol did react with
(Table 1), demonstrating that thermodynamically the
2
reaction with substrates with strong O−H bonds may not be
favorable.
Carboxamidate complex 2 has thus been shown to be
capable of PCET oxidation, and the carboxamidate donor
group is readily protonated resulting in an amide complex
(
Scheme 2). We conclude, on the basis of the observation that
Scheme 2. Ligand Acid/Base Noninnocence in a 2,6-
Pyridinedicarboxamidate Complex
Figure 7. Electronic absorption spectrum of the reaction mixture after
the reaction between 2 (3 mM) and 4-OCH -2,6-DTBP (red trace,
3
2
0 equiv), and thatcomplex 1 (black trace, 3 mM). The red trace is
+
the same as that measured for 1H .
obtained in 70% yield with respect to 2, as determined by
electronic absorption spectroscopy. The same absorption
spectrum was obtained for all other substrates (4-X-2,6-
DTBP and TEMPO-H, Figure S35). In these spectra, the
characteristic electronic absorption features attributed to 1
1H+ was the product of the PCET oxidation by 2, that
protonation of the carboxamidate N-atom results in decoordi-
III
were absent. No evidence for Ni (no intense bands in the
1
UV−vis region, EPR silent) was present while H NMR
nation and rotation around the C
−N bond. During the
ketone
III
showed only resonances for the substrate (Figure S36). ESI-
PCET reaction, the Ni ion acted as an electron acceptor
+
II
MS analysis displayed mass ions for 1H , the substrate, the
furnishing Ni . We believe our results provide some insight to
oxidized product, and fragments of the complex (i.e., {Ni +
Bu-terpy} , {Ni + L} , Figure S37). No evidence for oxidative
recent studies by Fukuzumi and co-workers and Garcia-Bosch
and co-workers where carboxamidate complexes have been
found to, somewhat unexpectedly, mediate PCET oxida-
t
+
+
ligand degradation was observed. These observations indicate
31,32
+
that 1H was the sole product of the reaction between 2 and 4-
tions.
We postulate that in these cases the carboxamidate
+
OCH -2,6-DTBP. Addition of KOH to the post-reaction
ligand could be acting as a H -acceptor (as predicted by
Garcia-Bosch using computational methods).
3
mixture yielded absorption features attributed to 1 (Figure
S31), which could in turn be converted back to 2 again by
addition of CAN (Figure S32). These observations demon-
CONCLUSION
■
+
III
strate that 1H was the product of PCET oxidation of 4-X-2,6-
A coordinatively saturated Ni −carboxamidate complex (2)
DTBP by 2. Furthermore, it demonstrates that the complex
remained intact through a full cycle of oxidation of 1 to 2,
was synthesized and characterized. 2 reacted with phenols with
weak O−H bonds but not with hydrocarbons in contrast to
+
+
III
PCET by 2 to yield 1H , and deprotonation of 1H to yield 1
followed by reoxidation of 1 to yield 2.
analogous [Ni (X)(L)]. 2 reacted with phenols giving rate
constants 3−30 times lower than analogous complexes. Thus,
F
DOI: 10.1021/acs.inorgchem.9b00055
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
was thermodynamically and kinetically less reactive than
analogous complexes with ancillary proton acceptors. Analysis
of the kinetics of the reaction with a series of para-substituted
Article
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K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.;
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5) Crabtree, R. H. Introduction to Selective Functionalization of C-
2
,6-di-tert-butylphenol substrates showed that the reaction
9
(
likely proceeded through a nonconcerted or partially concerted
PCET mechanism. It appeared that ET was not exclusively rate
limiting, with ET and PT demonstrating similar rates. The
supporting carboxamidate ligand was found to be acid−base
(
+
noninnocent and thus capable of acting as a H -acceptor
(
+
(
forming complex 1H ). Indeed, we demonstrated that the
H Bonds. Chem. Rev. 2010, 110 (2), 575−575a.
complex remained intact through a full cycle of oxidation of 1
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to 2, PCET by 2 to yield 1H , and deprotonation of 1H to
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less than 84 kcal/mol. Our results demonstrate that a
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ASSOCIATED CONTENT
Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.9b00055.
3
■
*
S
(
2
(
Physical methods; synthesis methods; reactivity proto-
cols and results; acid/base studies; further EPR analysis;
X-ray crystallography data (PDF)
(12) Elwell, C. E.; Gagnon, N. L.; Neisen, B. D.; Dhar, D.; Spaeth, A.
Accession Codes
D.; Yee, G. M.; Tolman, W. B. Copper−Oxygen Complexes
Revisited: Structures, Spectroscopy, and Reactivity. Chem. Rev.
CCDC 1850648 and 1885722 contain the supplementary
crystallographic data for this paper. These data can be obtained
free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by
emailing data_request@ccdc.cam.ac.uk, or by contacting The
Cambridge Crystallographic Data Centre, 12 Union Road,
Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
2
(
017, 117 (3), 2059−2107.
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Carboxylate-Bridged Diiron Metalloproteins: Strategies and Tactics.
Chem. Rev. 2004, 104 (2), 987−1012.
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L.; Weintraub, S. T.; Kauffmann, K. E.; Munck, E.; Kostka, K. L.;
Uffelman, E. S.; Rickard, C. E. F.; Noon, K. R.; Collins, T. J.
Designing ligands to achieve robust oxidation catalysts. Iron based
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AUTHOR INFORMATION
Corresponding Author
E-mail: aidan.mcdonald@tcd.ie.
ORCID
■
*
(
15) Popescu, D.-L.; Chanda, A.; Stadler, M.; de Oliveira, F. T.;
Ryabov, A. D.; Munck, E.; Bominaar, E. L.; Collins, T. J. High-valent
̈
Aidan R. McDonald: 0000-0002-8930-3256
Author Contributions
C.McM. and P.M. contributed equally.
first-row transition-metal complexes of tetraamido (4N) and
diamidodialkoxido or diamidophenolato (2N/2O) ligands: Synthesis,
structure, and magnetochemistry. Coord. Chem. Rev. 2008, 252 (18−
‡
20), 2050−2071.
Notes
(16) Borovik, A. S. Bioinspired hydrogen bond motifs in ligand
The authors declare no competing financial interest.
design: the role of noncovalent interactions in metal ion mediated
activation of dioxygen. Acc. Chem. Res. 2005, 38, 54−61.
(17) Stone, K. L.; Borovik, A. S. Lessons from nature: unraveling
biological C-H bond activation. Curr. Opin. Chem. Biol. 2009, 13,
ACKNOWLEDGMENTS
■
This publication has emanated from research supported by the
European Union (ERC-2015-STG-678202). Research in the
McDonald lab is supported in part by a research grant from
Science Foundation Ireland (SFI/15/RS-URF/3307). We are
grateful to Prof. Robert Barkley for training on and use of an
EPR spectrometer.
1
(
14−118.
18) Borovik, A. S. Role of metal-oxo complexes in the cleavage of
C-H bonds. Chem. Soc. Rev. 2011, 40, 1870−4.
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(
S. Metal complexes with varying intramolecular hydrogen bonding
networks. Polyhedron 2013, 52, 261−267.
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DOI: 10.1021/acs.inorgchem.9b00055
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