Mechanism of the reaction of an NHC-coordinated palladium(II)-hydride with O2 in acetonitrile

Article abstract of DOI:10.1016/j.poly.2020.114501

Pd^II-hydride species are important intermediates in many Pd-catalyzed aerobic oxidation reactions, and their reaction with molecular oxygen has been the subject of considerable previous study. This investigation probes the reactivity of trans-[(IMes)₂Pd(H)(OBz)] (IMes = 1,3-dimesitylimidazol-2-ylidene) with O₂ in acetonitrile, a polar coordinating solvent that leads to substantial changes in the kinetic behavior of the reaction relative the previously reported reaction in benzene and other non-coordinating solvents. In acetonitrile, the benzoate ligand dissociates to form the solvent-coordinated complex trans-[(IMes)₂Pd(H)(NCMe)][OBz]. Upon exposure to O₂, this cationic Pd^II–H complex reacts to form the corresponding Pd^II-hydroperoxide complex trans-[(IMes)₂Pd(OOH)(NCCD₃)][OBz]. Kinetic studies of this reaction revealed a complex rate law, rate = k₁k₂[3][OBz]/(k_?1[CD₃CN] + k₂[OBz]) + k₃[3][OBz], which is rationalized by a mechanism involving two parallel pathways for rate-limiting deprotonation of the Pd^II–H species to generate the Pd⁰ complex, Pd(IMes)₂. The latter complex undergoes rapid (kinetically invisible) reaction with O₂ and BzOH to afford the Pd^II-hydroperoxide product. The results of this study are compared to observations from the previously reported reaction in benzene and discussed in the context of catalytic reactivity.

Full text of DOI:10.1016/j.poly.2020.114501

Polyhedron 182 (2020) 114501
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Polyhedron
journal homepage: www.elsevier.com/locate/poly
Mechanism of the reaction of an NHC-coordinated palladium(II)-hydride
with O2 in acetonitrile
1
⇑
Michael M. Konnick , Spring M.M. Knapp, Shannon S. Stahl
Department of Chemistry, University of Wisconsin-Madison, 1101 University Avenue, Madison, WI 53706, United States
a r t i c l e i n f o
a b s t r a c t
PdII-hydride species are important intermediates in many Pd-catalyzed aerobic oxidation reactions, and
their reaction with molecular oxygen has been the subject of considerable previous study. This investi-
gation probes the reactivity of trans-[(IMes)2Pd(H)(OBz)] (IMes = 1,3-dimesitylimidazol-2-ylidene) with
O2 in acetonitrile, a polar coordinating solvent that leads to substantial changes in the kinetic behavior of
the reaction relative the previously reported reaction in benzene and other non-coordinating solvents. In
acetonitrile, the benzoate ligand dissociates to form the solvent-coordinated complex trans-[(IMes)2Pd
(H)(NCMe)][OBz]. Upon exposure to O2, this cationic PdII–H complex reacts to form the corresponding
PdII-hydroperoxide complex trans-[(IMes)2Pd(OOH)(NCCD3)][OBz]. Kinetic studies of this reaction
revealed a complex rate law, rate = k1k2[3][OBz]/(kꢀ1[CD3CN] + k2[OBz]) + k3[3][OBz], which is rational-
ized by a mechanism involving two parallel pathways for rate-limiting deprotonation of the PdII–H spe-
cies to generate the Pd0 complex, Pd(IMes)2. The latter complex undergoes rapid (kinetically invisible)
reaction with O2 and BzOH to afford the PdII-hydroperoxide product. The results of this study are com-
pared to observations from the previously reported reaction in benzene and discussed in the context
of catalytic reactivity.
Article history:
Received 30 December 2019
Accepted 3 March 2020
Available online 6 March 2020
Dedicated to Prof. John E. Bercaw on the
occasion of his 75th birthday, in recognition
of his valued mentorship and scientific
leadership in the field of organometallic
chemistry and catalysis.
Keywords:
Palladium
Hydride
Oxygen
Oxidation
N-heterocyclic carbene
Ó 2020 Elsevier Ltd. All rights reserved.
1. Introduction
in addition to Pd, including Co [32], Rh [33–39], Ir [40–44], and Pt
[45–48]. Among the different mechanistic pathways that have
Palladium-catalyzed reactions are among the most versatile
methods for the selective aerobic oxidation of organic molecules
[1–7]. Reactions that use ancillary ligands to support Pd catalyst oxi-
dation by O2 have been the focus of extensive study over the past
couple decades, including efforts to probe fundamental Pd/O2 reac-
tivity and to characterize catalytic mechanisms [8–18]. These reac-
tions typically proceed via a PdII/Pd0 catalytic cycle in which PdII
oxidizes the organic substrate and Pd0 is then oxidized by molecular
oxygen (Scheme 1). PdII-catalyzed oxidation reactions are broadly
grouped into two classes, those that generate the product via reduc-
tive elimination (e.g., biaryl coupling reactions) and those that gen-
erate the product via b-hydride elimination (e.g., alcohol oxidation
and Wacker-type reactions of alkenes) [14]. The latter reactions gen-
erate an intermediate PdII-hydride species, and the mechanisms by
which such species react with O2 have been the focus of attention
by multiple research groups [19–31].
been characterized for these reactions, two different mechanisms
have been established for the reaction of PdII-hydride species with
O2 to form PdII-hydroperoxide species (Scheme 2). The first is an
‘‘H–X reductive elimination” (HXRE) pathway, which is initiated
by reductive elimination of HX to afford a Pd0 intermediate, fol-
lowed by addition of O2 to Pd0 and protonolysis of a PdII–O bond
to form the resulting
g
2-peroxo species (Scheme 2a). The second
is an ‘‘H-atom abstraction” (HAA) pathway initiated by abstraction
of a hydrogen atom (HÅ) by O2, followed by rearrangement of the
hydroperoxyl radical to allow Pd–O bond formation with the inter-
mediate PdI species (Scheme 2b).
Experimental studies by Goldberg, Kemp, and coworkers [21],
supported by subsequent computational studies [23,49], demon-
strated that the PCP-pincer-ligated PdII-hydride complex in Eq.
(1) reacts with O2 via an HAA pathway. This mechanism is sup-
ported by a large KIE (kPd–H/kPd–D = 5.8) and a first-order depen-
dence of the reaction rate on pO2. Independently, we reported
and investigated the reaction of trans-[(IMes)2Pd(H)(OBz)] (1)
Fundamental reactions of metal-hydride complexes with O2
have been documented for a number of different transition metals
(IMes
= 1,3-dimesitylimidazol-2-ylidene, OBz = O2CPh) with
molecular oxygen in benzene to form the PdII–OOH complex 2
(Eq. (2)).[22,24,26] Experimental and computational studies sup-
ported an HXRE pathway, initiated by rate-limiting reductive
⇑
Corresponding author.
E-mail address: stahl@chem.wisc.edu (S.S. Stahl).
Present address: Hyconix, Inc., 4575 Weaver Parkway, Warrenville, IL 60555,
1
United States.
https://doi.org/10.1016/j.poly.2020.114501
0277-5387/Ó 2020 Elsevier Ltd. All rights reserved.

2
M.M. Konnick et al. / Polyhedron 182 (2020) 114501
In the present study, we revisit the reaction of 1 with O2 in the
polar coordinating solvent acetonitrile. The previous mechanistic
studies were primarily conducted in the non-polar and non-coordi-
nating solvent benzene, which strongly disfavors ionization of the
PdII-carboxylate bond. Use of the polar non-coordinating solvents,
chlorobenzene and dichloromethane, led to significant rate accel-
eration, which was rationalized by the presence of a polar/ionic
transition state that is stabilized by the use of polar solvent [26].
Here, we show that use of a polar solvent that is capable of coordi-
nating to PdII in the reaction of 1 with O2 negates the accelerating
effect observed with polar non-coordinating solvents and signifi-
cantly changes the kinetic behavior of the reaction. These observa-
tions are shown to reflect the formation of new ground-state
species, corresponding to the solvent-coordinated, cationic PdII–H
complex trans-[(IMes)2Pd(H)(NCMe)][OBz] (3), which reacts with
O2 to afford the PdII-OOH species 4 (Scheme 3). The complex
kinetic behavior observed for the conversion of 3 into 4 is pre-
sented and analyzed herein. Overall, the data are consistent with
a stepwise HXRE pathway initiated by parallel rate-limiting depro-
tonation of three- and four-coordinate cationic PdII–H species,
[(IMes)2Pd(H)]+ and trans-[(IMes)2Pd(H)(acetonitrile)]+. Implica-
tions of these observations for catalytic aerobic oxidation reactions
are discussed.
Scheme 1. Representative catalytic cycles for Pd-catalyzed aerobic oxidation
reactions that proceed via PdII-hydride intermediates.
2. Results and discussion
2.1. Characterization of reaction of (IMes)2Pd(H)(OBz) with O2 in
acetonitrile
The PdII–hydride complex trans-(IMes)2Pd(H)(OBz), 1 (Bz = ben-
zoyl), was prepared by addition of BzOH to (IMes)2Pd0, as
described previously [22,26]. (Note: An improved protocol has been
developed for the synthesis of (IMes)2Pd0; see Section 4 for
details.) When 1 was dissolved in acetonitrile d3, the chemical
shifts for the OBz resonances were found to be identical to those
for the benzoate anion of NBu4OBz. In addition, preparation of
(IMes)2Pd(H)(BF4) (5) by addition of HBF4 to (IMes)2Pd0 and char-
acterization this complex by 1H NMR spectroscopy in acetoni-
trile d3 exhibits chemical shifts for the IMes and hydride ligand
identical to those of 1. Together, these results are consistent with
the formation of the cationic, solvent-coordinated complex trans-
[(IMes)2Pd(H)(NCCD3)]+ and free [OBz]– in acetonitrile (Eq. (3)).
This complex is similar to the solvent-coordinated, cationic PdII–
H complex bearing phosphine and DMF ligands, [(Ph3P)2Pd(H)
(DMF)]+, reported previously by Amatore, Jutand and coworkers
[50].
Scheme 2. Mechanisms established for the reaction of PdII-hydride complexes with
O2.
elimination of BzO–H from 1. Relevant data include a zero-order
dependence on pO2 and a small KIE (kPd–H/kPd–D = 1.3). A subse-
quent study showed that changing the carboxylate ligand in 1 to
a more electron-donating carboxylate (e.g., AcO– or 4-MeOC6H4-
CO–2) led to initiation of a competing HAA mechanism [29], evident
by a positive linear dependence of the rate on pO2 and a KIE of 3.1.
Computational studies indicated that this change in mechanism
arises from (at least) two factors: (1) the HXRE pathway with 1
involves nearly full ionization of the Pd–O2CR bond in the transi-
tion state, and more electron-donating carboxylates are less readily
ionized; and (2) more electron-donating carboxylates exhibit a
stronger trans influence that weakens the PdII–H bond and makes
it more susceptible to HAA by O2.
ð3Þ
The reaction of the PdII–hydride complex [(IMes)2Pd(H)
(NCCD3)][OBz] (3) and molecular oxygen in acetonitrile d3 was
evaluated by 1H NMR spectroscopy (Eq. (4) and Fig. 1). Resonances
corresponding to the ortho and para methyl groups of the IMes
ligands are intense, sharp singlets in the 1H NMR spectrum, and
they appear at 1.73 ppm (24 protons) and 2.01 ppm (12 protons),
respectively. These resonances are conveniently monitored during
the reaction. The oxygenation of 3 proceeds cleanly in the presence
of 0.23 atm O2 to generate the solvent-coordinated analog of 2, the
PdII-hydroperoxide, [(IMes)2Pd(OOH)(NCCD3)][OBz], 4 (Eq. (4))
[51], exhibiting an exponential time course throughout the reac-
tion (Fig. 1). No new resonances for the benzoate anion are
observed throughout the reaction, consistent with the benzoate
ð1Þ
ð2Þ

M.M. Konnick et al. / Polyhedron 182 (2020) 114501
3
Scheme 3. Reaction of PdII-hydride complex 1 with O2 in acetonitrile.
–
+
–
+
[OBz ]
[OBz ]
IMes
IMes
II
II
H
D CCN
Pd
OOH
D CCN
Pd
3
O2
3
CD3CN
60 °C
N
N
N
N
3
4
1
0.8
0.6
0.4
0.2
0
0
50
100
150
Time (min)
Fig. 1. Representative (A) stacked plot for the relevant portions of the 1H NMR spectra, and (B) time course for the aerobic oxygenation of [(IMes)2Pd(H)(NCCD3)][OBz], 3, to
form [(IMes)2Pd(OOH)(NCCD3)][OBz], 4, in acetonitrile d3. Conditions: [3]0 = 1.2 mM, pO2 = 0.23 atm, 0.4 mL CD3CN, 60 °C.
remaining ionized. Decomposition of 4 is apparent at later reaction
times and is more pronounced at higher O2 pressures; however,
this decomposition reaction appeared to have negligible influence
on the kinetics of the reaction of 3 with O2, and was therefore not
investigated in detail.
ð4Þ

4
M.M. Konnick et al. / Polyhedron 182 (2020) 114501
Table 1
The involvement of radical-chain autoxidation mechanisms in
Effect of radical probes on the aerobic oxidation of [(IMes)2Pd(H)(NCCD3)][OBz], 3, in
the reactions of other transition-metal hydrides with O2 [52]
prompted us to evaluate the reaction of 3 with O2 in the presence
of the radical initiator 2,20-azobisisobutyronitrile (AIBN) and the
radical inhibitor 2,6-di-tert-butyl-4-methylphenol (BHT). These
additives were found to have minor influence on the reaction of
3 with O2 (Table 1).
CD3CN.a
b
Entry
Additive
kobs (10–3/minꢀ1
)
1
2
3
None
16.0
13.0
15.4
0.1 mM AIBNc (14%)
5.3 mM BHTd (750%)
a
Reaction Conditions: [3]0 = 0.7 mM, pO2 = 1.0 atm, 61 °C, 0.4 mM CD3CN.
Uncertainty in these values is estimated at approx. 5% based on the use of NMR
b
2.2. Kinetic studies of reaction of [(IMes)2Pd(H)(NCCD3)][OBz] with O2
in acetonitrile
integrations to obtain the values.
c
AIBN = 2,20-azobisisobutyronitrile.
BHT = 2,6-di-tert-butyl-4-methylphenol.
d
The clean kinetic behavior for the conversion of 3 into 4 under
an oxygen atmosphere facilitated more thorough kinetic investiga-
tion of the oxygenation reaction. Analysis of the effect of pO2
revealed that the reaction rate exhibited no dependence on the
oxygen pressure from 0.23 to 3.29 atm (Fig. 2). Clean exponential
kinetic decay of 3 was observed even at 0.23 atm, which corre-
sponds to only 1.8 equiv of dissolved O2 relative to 3 in solution
(cf. Fig. 1) [53].
0.02
0.015
0.01
0.005
0
The rate of oxygenation of 3 in the presence of varying [OBz–]
was evaluated and found to influence the reaction rate in a com-
plex manner: a saturation dependence of the rate on [OBz–] is evi-
dent at low concentrations, while a linear dependence is evident at
higher [OBz–] (Fig. 3a). In the absence of [OBz–], with [(IMes)2Pd(H)
(NCCD3)][BF4] as the PdII–H species, no reaction with O2 is
observed under otherwise identical reaction conditions. This
observation suggests that a basic counteranion is necessary for
the reaction of the PdII–H with O2 to form the PdII–OOH product.
It was possible to achieve a good fit to the [OBz–] kinetic data in
Fig. 3a with a two-term rate law, incorporating both saturating
and first-order terms with respect to [OBz–] (Eq. (5)).
0
0.5
1
1.5
2
2.5
3
3.5
O2 pressure (atm)
Fig. 2. The effect of O2 pressure on the rate of oxygenation of 3 in acetonitrile d3.
Conditions: [3]0 = 1.2 mM, pO2 = 0.23–3.29 atm, 0.4 mL CD3CN, 60 °C.
The rates of reaction of 3 with O2 at different [OBz–] were also
monitored at four different temperatures to enable Eyring analysis
and determination of activation parameters (Fig. 5). The kobs values
obtained from time-course data for different [OBz–] and tempera-
tures in Fig. 5 were plotted (Fig. 6a), and a global fit of these data
according to Eq. (6) provided the basis for determination of activa-
tion parameters for k1 and k3 steps. The activation parameters
k1k2½3ꢁ½OBzꢁ
rate ¼
þ k3½3ꢁ½OBzꢁ
ð5Þ
ðk ½CD3CNꢁ þ k2½OBzꢁÞ
ꢀ1
The reaction of 3 with O2 was also conducted in the presence of
BzOH as an acidic additive, and an inhibitory effect was observed
(Fig. 3b). This observation, rationalized further below, contrasts
previous observations of the reaction of 1 with O2 in benzene,
which showed that addition of BzOH accelerates the oxygenation
of 1 to afford the PdII-OOH product 2. Collectively, the kinetic data
and rate law in Eq. (5) are consistent with the stepwise mechanism
in Scheme 4. This mechanism features two parallel pathways for
rate-limiting deprotonation of the PdII–H by BzO–. The first
involves dissociation of MeCN to generate a three-coordinate PdII–
H species 5 (k1/kꢀ1) followed by reaction of 5 with BzO– (k2), while
the other involves direct reaction of 3 with BzO– (k3). Both of these
deprotonation pathways lead to formation of (IMes)2Pd0 (6), which
undergoes rapid (kinetically invisible) reaction with O2 to form
(IMes)2Pd(O2) (7) and then with BzOH to afford the PdII–OOH spe-
cies 4. At low [OBz–], the two-step deprotonation pathway domi-
nates and accounts for the saturation dependence on [OBz–]. At
high [OBz–], the direct, one-step pathway dominates, as reflected
by the linear dependence on [OBz–]. The inhibitory effect of BzOH
(Fig. 3b) is attributed to a hydrogen bonding equilibrium with
BzO–, which lowers the concentration of the OBz– base.
Further assessment of this proposed mechanism was conducted
by analyzing deuterium kinetic isotope effects for the reaction of 3
with O2. The reaction of 3 proceeds more rapidly than the reaction
of the corresponding PdII–D (3-d1) (Fig. 4). Fitting of the [OBz–]-
dependence data according to Eq. (5) allowed determination of
the values k1 and k3 for the two complexes (the k-1 and k2 terms
are strongly correlated and could not be evaluated independently).
The fit revealed a small KIE for k1, 1.1(0.3), and a moderate KIE for
k3, 2.2(0.4), consistent with the expectation for secondary and pri-
mary KIEs associated with these steps in Scheme 4.
obtained in this manner are
D D
Hà1 = 28(3) kcal/mol and Sà1 = +11
(10) for k1 and
D
Hà3 = 19(1) kcal/mol and
D
Sà3 = ꢀ20(5) for k3 (cf.
Scheme 4 and Fig. 6; note: Fig. 6b reflects a reproduction of data in
a conventional Eyring plot but was not used for data fitting). The
positive
while the negative
reaction involving deprotonation of the cationic PdII–H by benzoate.
D
Sà value for k1 is consistent with a ligand dissociation step,
D
Sà value for k3 is consistent with a bimolecular
D
H1z
Sz1=R
ðeꢀ =RTeD ÞðkBT=hÞk2½OBzꢁtot
D
H3z
Sz3=R
kobs
¼
þ ðeꢀ =RTeD
Þ
k
½CD3CNꢁ þ k2½OBzꢁ
ꢀ1
tot
ꢂ ðkBT=hÞ½OBzꢁtot
ð6Þ
2.3. Mechanistic analysis, comparison to the reactivity of 1 in benzene,
and catalytic implications
The equilibrium for reductive elimination between L2PdII(H)(X)
species and L2Pd0/HX depends on the identity of the ancillary
L-
type ligands [50,54]. A previous instance of a cationic PdII–H noted
above, [(Ph3P)2Pd(H)(DMF)]+ [50], requires a large excess of the
acid (AcOH or HCO2H) to favor formation of the PdII–H species.
In contrast, trans-[(IMes)2Pd(H)(NCMe)][OBz] (3) is stable in solu-
tion (in the absence of O2) in the absence of added BzOH. Even
when 20 equiv of NBu4OBz is added to a solution of 3, no Pd(IMes)2
(6) was detected by 1H NMR spectroscopy. These observations may
be rationalized by the ability of the electron-rich IMes ligands to
stabilize the PdII–H relative to PPh3.
The results of this study highlight both similarities and differ-
ences with respect to the previously reported study of the reaction

M.M. Konnick et al. / Polyhedron 182 (2020) 114501
5
A)
B)
0.16
0.12
0.08
0.04
0
0.016
0.012
0.008
0.004
0
0
5
10
15
20
0
2
4
6
8
10
[OBz]tot (mM)
[BzOH] (mM)
Fig. 3. The effect of [NBu4OBz] (A) and [BzOH] (B) on the rate of oxygenation of 3 in acetonitrile d3. Conditions: (A) [3]0 = 0.55 mM, pO2 = 1 atm, [NBu4OBz] = 0–18.9 mM,
0.4 mL CD3CN, 61 °C. [OBz]tot = [Pd]0 + [NBu4OBz], data fit to Eq. (5). (B) [3]0 = 0.65 mM, pO2 = 1 atm, [BzOH] = 0–8.63 mM, 0.4 mL CD3CN, 61 °C.
Scheme 4. Proposed mechanism for the aerobic oxidation of 3 in acetonitrile.
zene, and dichloromethane, to the cationic MeCN-coordinated
cationic species 3 in acetonitrile. Furthermore, the rate law estab-
lished for the reaction of 1 with O2 in benzene (Eq. (7)) is quite dif-
ferent from that established for the oxygenation of 3 in acetonitrile
(Eq. (5)). Added BzO– has no effect on the reaction rate in benzene
but significantly increases the rate in acetonitrile, while BzOH
accelerates the reaction in benzene but inhibits the reaction in ace-
tonitrile (Fig. 3b). These differences highlight the complex inter-
play that can occur between solvents and acid/base additives
that are employed in catalytic reactions.
0.16
0.12
0.08
0.04
0
3
3-d1
0
5
10 15 20 25 30 35 40
rateðbenzeneÞ ¼ k1½1ꢁ þ k2½1ꢁ½BzOHꢁ
ð7Þ
[OBz]tot (mM)
The results of this study document another example of a PdII–H
complex that reacts with O2 by undergoing initial reductive elimi-
nation/deprotonation of a PdII–H species to afford a Pd0 intermedi-
ate that reacts with O2 and an acid to generate a PdII–OOH species.
Access to a facile equilibrium between PdII–H/Pd0 species appears
to be an important feature of Pd catalyst systems that promote
substrate oxidation via b-hydride elimination. As noted in a recent
review article [7], all PdII-catalyzed aerobic oxidation reactions
reported to date that proceed via PdII–H intermediates appear to
undergo catalyst oxidation by O2 via this type of HXRE pathway.
Even catalyst systems that were initially proposed to favor an
HAA pathway (on the basis of computational analysis [20]) have
been shown to have kinetically facile pathways available for depro-
tonation of the PdII–H species to generate a transient Pd0 interme-
diate [31].
Fig. 4. The effect of [NBu4OBz] on the rate of oxygenation of 3 and 3-d1 in
acetonitrile d3. Conditions: [3]0 = 0.55 mM, [3-d1] = 0.6 mM, pO2 = 1 atm, [NBu4-
OBz]3
= 0–18.9 mM, [NBu4OBz]3-d1 = 0–35.7 mM, 0.4 mL CD3CN, 61 °C.
[OBz] = [Pd]0 + [NBu4OBz], data fit to Eq. (5).
of trans-(IMes)2Pd(H)(OBz) (1) with O2. The overall mechanism is
still consistent with the HXRE pathway in Scheme 2 (cf. Scheme 4).
Both studies reveal a zero-order dependence on pO2. This observa-
tion is rationalized by rate-limiting reductive elimination of HX
(i.e., deprotonation of the PdII–H), with rapid reaction of Pd0 with
O2. We also note that the small KIE for k1 in Scheme 4, KIE = 1.1
(0.3), corresponding to the acetonitrile dissociation step, is similar
to the KIE observed for the reaction conducted in benzene, KIE = 1.3
(0.1), wherein ionization of the Pd–OBz bond is rate-limiting. In
spite of these similarities, the approximately similar rates observed
for the PdII–H oxygenation reaction in acetonitrile and benzene
was not expected. In the previous report [26], a dramatic increase
3. Conclusion
in rate was observed in more polar solvents [benzene (
chlorobenzene ( = 5.7), and dichloromethane ( = 9.1)], and these
observations suggested that the rate should be exceedingly fast in
e = 2.3),
e
e
The results described herein represent an important extension
of the previously reported investigation of the reaction of trans-
[(IMes)2Pd(H)(OBz)] (1) with O2 to afford trans-[(IMes)2Pd(OOH)
(OBz)] (2) [26]. The change from a non-coordinating solvent (ben-
zene, previous study) to a coordinating solvent (acetonitrile, pre-
acetonitrile (e = 36.6). The much slower than expected rate in ace-
tonitrile correlates with a change in the ground state PdII–H
species, from a BzO-coordinated species 1 in benzene, chloroben-

6
M.M. Konnick et al. / Polyhedron 182 (2020) 114501
69 °C
A) 0.6
0.5
0.4
0.3
0.2
0.1
0
B)
61 °C
0.6
0.5
0.4
0.3
0.2
0.1
0
0 mM
0 mM
0.45 mM
2.6 mM
10 mM
0.072 mM
0.70 mM
4.1 mM
19 mM
0
1000
2000
3000
4000
0
1000 2000 3000 4000 5000 6000 7000
Time (sec)
Time (sec)
C)
50 °C
D)
39 °C
0.6
0.5
0.4
0.3
0.2
0.1
0
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
0 mM
0 mM
0.11 mM
1.1 mM
6.9 mM
24 mM
1.1 mM
6.9 mM
23 mM
0
10000 20000 30000 40000 50000
0
50000
100000
150000
Time (sec)
Time (sec)
Fig. 5. Kinetic data for the reaction of 3 with O2 at different [NBu4OBz], together with fits of the raw kinetic data to the equation [3] = [3]0e–(kobs)(t), where kobs is defined as
presented in Eq. (6). Conditions: [3]0 = 0.5 mM (39 °C, 50 °C, 69 °C), 0.55 mM (61 °C), pO2 = 1 atm, [NBu4OBz] = 0–23.6 mM, 0.4 mL CD3CN. [OBz] = [Pd]0 + [NBu4OBz].
A) 0.28
0.24
0.2
B) -11
-12
69 °C
-13
0.16
0.12
0.08
0.04
0
-14
61 °C
-15
-16
k1
50 °C
39 °C
-17
k3
-18
0.0029
0
5
10
15
20
25
0.003
1/T (K–1
0.0031
)
0.0032
[OBz]tot (mM)
Fig. 6. Effect of temperature on the rate constants k1 and k3 for the aerobic oxidation of 3 in acetonitrile d3. Conditions: [3]0 = 0.5 mM (39 °C, 50 °C, 69 °C), 0.55 mM (61 °C),
pO2 = 1 atm, [NBu4OBz] = 0–23.6 mM, 0.4 mL CD3CN. [OBz] = [Pd]0 + [NBu4OBz]. Activation parameters were obtained by fitting the raw kinetic data directly to two
independent Eyring equations (for k1 and k3, see Fig. 5, Eq. (6)).
sent study) leads to significant changes in the kinetic behavior of
the reaction, originating from substitution of the benzoate ligand
with acetonitrile to generate the cationic complex trans-[(IMes)2-
Pd(H)(NCMe)][OBz] (3). In spite of the kinetic differences, however,
the overall mechanism still aligns with a pathway involving con-
version of the PdII–H species into a Pd0 intermediate, followed by
subsequent reaction of Pd0 with O2 and BzOH.
tion. NBu4OBz was recrystallized prior to use from pentane/ether.
Benzoic acid-d was prepared by dissolving benzoic acid in d4-
methanol and then evaporating the d4-methanol six times. All sol-
vents used were dried and deoxygenated prior to usage: diethyl
ether, pentane, and toluene were passed through a column of acti-
vated alumina and Q4. C6D6 was degassed via standard freeze/
pump/thaw methods and then dried by distillation from Na/ben-
zophenone, CD3CN was degassed via standard freeze/pump/thaw
methods and then dried by distillation from CaH2. NMR data were
recorded using a Varian INova (1H: 500 MHz or 1H: 600 MHz) spec-
trometer. Spectra recorded at elevated temperatures were cali-
brated with ethylene glycol. 1H NMR chemical shifts were
referenced to residual protons in the deuterated solvent, ben-
zene d6: 7.16 ppm, acetonitrile d3: 1.94 ppm. One kinetic time
course was obtained for each variation in reagent concentration,
temperature, or other variable. All reagents were purchased from
commercial sources and used as received unless noted otherwise.
4. Materials and methods
4.1. General considerations
All procedures, except the oxygenation reactions, were carried
out under an inert atmosphere of nitrogen in a MBraun glove
box or by using standard Schlenk techniques. Benzoic acid,
tetrafluoroboric acid solution in diethyl ether, 1,3,5-trimethoxy-
benzene, AIBN, BHT, and oxygen gas were used without purifica-

M.M. Konnick et al. / Polyhedron 182 (2020) 114501
7
Pd(IMes)2(H)(OBz), 1 [26], and Pd(IMes)2(OOH)(OBz), 2 [26], were
synthesized following published literature procedures.
(500 MHz, CD3CN): d 7.94 (m, 2H), 7.27 (m, 3H), 7.07 (s, 4H),
6.97 (s, 8H), 2.44 (s, 12H), 1.74 (s, 24H), ꢀ16.71 (s, 1H).
4.2.3. Preparation of trans-[(IMes)2Pd(OOH)(NCCD3)][OBz], 4 in
CD3CN
4.2. Synthesis and characterization of prepared compounds
Pd(IMes)2(OOH)(OBz), 2, was synthesized and characterized in
C6D6 as previously reported [26]. Dissolution of 2 in CD3CN results
in ionization of the benzoate to afford [(IMes)2Pd(OOH)(NCCD3)]
[OBz], 4, evident by 1H NMR spectroscopy. 1H NMR (500 MHz, CD3-
CN): d 7.94 (m, 2H), 7.27 (m, 3H), 7.12 (s, 4H), 7.09 (s, 8H), 4.65 (s,
1H), 2.50 (s, 12H), 1.83 (s, 24H).
4.2.1. Modified procedure for synthesis of Pd(IMes)2
The synthesis and characterization of Pd(IMes)2 has already
been established in the literature [19,55]. The established proce-
dure has many inherent drawbacks, including utilization of a large
excess of expensive phosphine, difficulty in scaling, and difficulty
in final product purification. To circumvent these difficulties, we
developed a modified procedure for the synthesis of Pd(IMes)2.
4.2.4. Preparation of trans-[(IMes)2Pd(D)(OBz)]
The round bottom flask (10 mL) and stir bar utilized for this
reaction were washed with 5 ꢂ 3 mL of fresh D2O and then with
5 ꢂ 0.75 mL portions of MeOD. Benzoic acid (5.0 mg, 40.9 mmol)
was dissolved in MeOD (0.75 mL), which was stirred for 10 min
and then evaporated; this procedure was repeated eight times.
The product BzOD was directly transferred into an inert atmo-
sphere glovebox after completion to minimize atmospheric proton
contamination. trans-Pd(IMes)2(D)(OBz), 1-d1, was prepared in a
plastic vial following the literature procedure for synthesis of 1
[26] using freshly prepared BzOD [61]. Dissolution of 1-d1, in CD3-
CN results in the clean formation of 3-d1.
4.2.1.1. Synthesis of Pd[PPh(tBu)2]2. The compounds [PdCl(C3H5)]2
and PPhtBu2 were synthesized following established literature pro-
cedures [56,57]. A variation of several literature procedures were
used in the synthesis of Pd[PPh(tBu)2]2 [58,59]. [PdCl(C3H5)]2
(0.5 g, 1.30 mmol) was dissolved into 20 mL of a 1:1 solution of
THF:toluene. LiCp (0.25 g, 3.47 mmol) was dissolved into 10 mL
of a 1:1 solution of THF:toluene. These solutions were cooled to
ꢀ30 °C in a glove box freezer. With vigorous stirring, the LiCp solu-
tion was added dropwise to the Pd solution over a period of
10 min. The immediate development of a deep brick red color indi-
cated formation of (Cp)Pd(C3H5). The solution was allowed to
warm to ambient temperature and stirred for 1 h. The solvents
were evaporated under reduced pressure to yield the crude pro-
duct (Cp)Pd(C3H5) [60]. The red solid was extracted with
3 ꢂ 15 mL portions of n-pentane and filtered. The filtrate was
allowed to flow directly into a Schlenk flask that had been charged
with a stir bar and PPhtBu2 (1.3 mL, 5.45 mmol). The flask was
removed from the glove box and allowed to stir for 24 h at ambient
temperature, forming a green-yellow precipitate. The reaction was
then heated to 45 °C for 2 h to drive it to completion. The resulting
solution was concentrated in vacuo to dryness, then the solid
washed with methanol. The crude crystals were dissolved in hot
pentane, filtered, then the filtrate concentrated in vacuo to a quar-
ter of the original volume, then placed in the glove box freezer
overnight. The product was filtered, then washed with cold pen-
tane. Yield: 1.5 g (70%). The 1H NMR spectrum matches literature
values.
4.3. Kinetics of hydride oxygenation by NMR spectroscopy
A stock solution of internal standard (5.9 mM 1,3,5-trimethoxy-
benzne in 10 mL CD3CN) was prepared in a glove box. Stock solu-
tions of 3 were prepared by dissolving 1 in the CD3CN solution
containing internal standard; stock solutions of additives were
made by dissolving them in the CD3CN solution containing internal
standard. The concentrations of each species were established by
1H NMR spectroscopy. These solutions were added to a volume-
calibrated Wilmad J-Young NMR tube (#535PP-JY) to achieve the
desired concentrations when diluted to 0.400 mL. The NMR tube
was connected to a gas manifold attached to a volume calibrated
mercury manometer. The solution was degassed and filled with
the desire pressure of O2. Elevated pressures of O2 were achieved
by cooling the tube in liquid N2. The tube was sealed and kept fro-
zen in a dry ice/acetone bath until it was inserted into the pre-
heated spectrometer probe. Multiple scans were taken for each
data point, with the delay between acquisitions >5 T1 (T1 > 6 s
under 1 atm O2).
Safety Note – Extreme caution should be used when cooling O2-
filled NMR tubes in liquid N2 to prevent formation of liquid O2.
NMR tubes should be pressure tested at a higher temperature
and pressure than will be used in the NMR spectrometer by con-
ducting a test reaction behind a blast shield in the fume hood. To
prevent catastrophic damage to the NMR probe, only use NMR
tubes that have been pressure tested. Pressurized NMR tubes
should always be transported to the NMR spectrometer inside of
appropriate secondary containment with a lid. All pressurized
samples should be handled with appropriate eye protection at all
times, even at the NMR spectrometer.
4.2.1.2. Synthesis of Pd(IMes)2. In a glove box, a solution of IMes
(2.80 g, 9.2 mmol) in 10 mL toluene was added to a solution of
Pd[PPh(tBu)2]2 (1.00 g, 4.5 mmol) in 10 mL toluene. The immediate
development of an intense yellow color indicated the formation of
Pd(IMes)2. The solution was allowed to stir for 30 min, and then
the solvent was evaporated, leaving behind a mixture of yellow
solid and oil. Addition of 5 mL of cold (ꢀ30 °C) n-pentane resulted
in the precipitation of a yellow solid. The product was collected by
vacuum filtration and washed with 3 ꢂ 5 mL portions of n-pentane
to remove excess phosphine. Yield: 3.04 g (95%). The 1H NMR spec-
trum matches literature values [19].
4.2.2. Preparation of trans-[(IMes)2Pd(H)(NCCD3)]X, (X = BzO, 3; BF4,
5) in CD3CN
Pd(IMes)2(H)(OBz), 1, was synthesized in diethyl ether by addi-
tion of BzOH to (IMes)2Pd0, followed by removal of the ether under
vacuum, as described previously [22,26]. Trans-(IMes)2Pd(H)BF4
was prepared similarly by addition of HBF4ꢃOEt2 to (IMes)2Pd0. Dis-
solution of these complexes in CD3CN results in solutions that are
identical by 1H NMR spectroscopy, with an upfield Pd–H resonance
at ꢀ16.71 ppm, assigned to the solvated complex trans-[(IMes)2Pd
(H)(NCCD3)]+, with solvent-separated anions BzO– (3) and BF4 (5).
The solution of 3 also has peaks associated with free BzO– (identi-
cal to the benzoate resonances of NBu4OBz). 1H NMR for 3
4.4. Data fitting procedure
Rate constants were determined by a non-linear least squares
fit of a set of NBu4OBz dependent data to Eq. (5), floating values
for the rate constants k1, k–1, k2, k3, [3]o (individually for each
run), [3]infinity (individually for each run), and using the solver func-
tion in Microsoft Excel. In some cases, temperature was not equili-
brated before the start of data acquisition (tube temperature
equilibration was determined to take approximately 3 min). In
such cases, the early data points were not included in the fit. Acti-

8
M.M. Konnick et al. / Polyhedron 182 (2020) 114501
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fitting of the modified Eyring equation (Eq. (6)) globally to the data
acquired, individually floating [3]o and [3]infinity (for each time
course), k–1 and k2 (for each temperature),
D D D
Hà1, Sà1, Hà3, and
D
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CRediT authorship contribution statement
[28] S. Chowdhury, I. Rivalta, N. Russo, E. Sicilia, J. Chem. Theory Comput. 4 (2008)
1283–1292.
Michael M. Konnick: Conceptualization, Methodology, Valida-
tion, Formal analysis, Investigation, Writing - original draft, Visual-
[29] M.M. Konnick, N. Decharin, B.V. Popp, S.S. Stahl, Chem. Sci. 2 (2011) 326–330.
[30] N. Decharin, S.S. Stahl, J. Am. Chem. Soc. 133 (2011) 5732–5735.
[31] N. Decharin, B.V. Popp, S.S. Stahl, J. Am. Chem. Soc. 133 (2011) 13268–13271.
[32] S. Thyagarajan, C.D. Incarvito, A.L. Rheingold, K.H. Theopold, Chem. Commun.
(2001) 2198–2199.
ization. Spring M. M. Knapp: Writing
- review & editing,
Visualization. Shannon S. Stahl: Conceptualization, Methodology,
Writing - review & editing, Supervision, Funding acquisition.
[33] M.T. Atlay, M. Preece, G. Strukul, B.R. James, Can. J. Chem. 61 (1983) 1332–
1338.
Declaration of Competing Interest
[34] A. Bakac, J. Am. Chem. Soc. 119 (1997) 10726–10731.
[35] W. Cui, B.B. Wayland, J. Am. Chem. Soc. 128 (2006) 10350–10351.
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[41] S. Chowdhury, F. Himo, N. Russo, E. Sicilia, J. Am. Chem. Soc. 132 (2010) 4178–
4190.
The authors declare that they have no known competing finan-
cial interests or personal relationships that could have appeared
to influence the work reported in this paper.
Acknowledgments
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[43] T.S. Teets, D.G. Nocera, Dalton Trans. 42 (2013) 3521–3527.
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We thank Clark Landis for helpful discussions regarding data
fitting. Funding for this work was provided by the NSF (CHE-
1665120 and CHE-0543585). NMR instrumentation was supported
by the NSF (CHE-8813550 and CHE-9629688) and the NIH (S10
RR04981-01 and S10 RR13866-01).
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