Oxidation of a Monomethylpalladium(II) Complex with O2 in Water: Tuning Reaction Selectivity to Form Ethane, Methanol, or Methylhydroperoxide

Article abstract of DOI:10.1021/jacs.5b12832

Photochemical aerobic oxidation of n-Pr₄N[(dpms)Pd^IIMe(OH)] (5) and (dpms)Pd^IIMe(OH₂) (8) (dpms = di(2-pyridyl)methanesulfonate) in water in the pH range of 6-14 at 21 °C was studied and found to produce, in combined high yield, a mixture of MeOH, C₂H₆, and MeOOH along with water-soluble n-Pr₄N[(dpms)Pd^II(OH)₂] (9). By changing the reaction pH and concentration of the substrate, the oxidation reaction can be directed toward selective production of ethane (up to 94% selectivity) or methanol (up to 54% selective); the yield of MeOOH can be varied in the range of 0-40%. The source of ethane was found to be an unstable dimethyl Pd^IV complex (dpms)Pd^IVMe₂(OH) (7), which could be generated from 5 and MeI. For shedding light on the role of MeOOH in the aerobic reaction, oxidation of 5 and 8 with a range of hydroperoxo compounds, including MeOOH, t-BuOOH, and H₂O₂, was carried out. The proposed mechanism of aerobic oxidation of 5 or 8 involves predominant direct reaction of excited methylpalladium(II) species with O₂ to produce a highly electrophilic monomethyl Pd^IV transient that is involved in subsequent transfer of its methyl group to 5 or 8, H₂O, and other nucleophilic components of the reaction mixture.

Full text of DOI:10.1021/jacs.5b12832

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Article
Oxidation of a Monomethylpalladium(II) Complex with O2 in Water: Tuning
Reaction Selectivity to Form Ethane, Methanol or Methylhydroperoxide
Anna V. Sberegaeva, Peter Y. Zavalij, and Andrei N Vedernikov
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.5b12832 • Publication Date (Web): 14 Jan 2016
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Journal of the American Chemical Society
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Oxidation of a Monomethylpalladium(II) Complex with O2 in Water:
Tuning Reaction Selectivity to Form Ethane, Methanol or Methyl-
hydroperoxide
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Anna V. Sberegaeva, Peter Y. Zavalij, Andrei N. Vedernikov*
The Department of Chemistry and Biochemistry, University of Maryland, College Park, MD 20742
E-mail: avederni@umd.edu
ABSTRACT: Photochemical aerobic oxidation of n-Pr4N[(dpms)PdIIMe(OH)], 5, and (dpms)PdIIMe(OH2), 8, (dpms =
di(2-pyridyl)methanesulfonate) in water in the pH range of 6 – 14 at 21 oC was studied and found to produce, in combined
high yield, a mixture of MeOH, C2H6 and MeOOH, along with water-soluble n-Pr4N[(dpms)PdII(OH)2], 9. By changing the
reaction pH and concentration of the substrate the oxidation reaction can be directed toward selective production of
ethane (up to 94% selectivity) or methanol (up to 54% selective); the yield of MeOOH can be varied in the range of 0-
40%. The source of ethane was found to be an unstable dimethyl PdIV complex (dpms)PdIVMe2(OH), 7, which could be
generated from 5 and MeI. To shed light on the role of MeOOH in the aerobic reaction, oxidation of 5 and 8 with a range
of hydroperoxo compounds, MeOOH, t-BuOOH, H2O2, was carried out. The proposed mechanism of aerobic oxidation of
5 or 8 involves predominant direct reaction of excited methylpalladium(II) species with O2 to produce highly electrophilic
monomethyl PdIV transient that is involved in subsequent transfer of its methyl group to 5 or 8, H2O and other nucleo-
philic components of the reaction mixture.
generate related PdIII and/or PdIV species has also been
INTRODUCTION
demonstrated (see, e.g., Scheme 1, c).16-18
Palladium complexes have received much attention as
Scheme 1. Examples of reactions between O2 and
PdIICH3 complexes.13,14,17
homogeneous catalysts for selective C-H oxidation of or-
ganic substrates.1-7 Among such processes oxidation of
methane1,7 is one of the least developed and one of the
most sought after transformations that can be used, in
particular, for the development of economically viable
routes for “methane upgrade” to higher hydrocarbons,8
and value-added oxygenates such as methanol. Taking
into account the low cost of some simplest methane –
derived products MeOH, C2H6, etc., oxidants that may be
practically important in such processes must be very
cheap. In this regard the use of oxygen from the air as a
virtually free oxidant is an excellent choice.7,9 While direct
oxidation of palladium(0) species with dioxygen has been
studied in some detail so allowing for substantial advanc-
es in aerobic C-H bond oxidations involving Pd0/PdII cou-
ple,4 direct aerobic oxidation of organopalladium(II)
complexes, especially chemistry involving palladium tran-
sients in higher oxidation states, is poorly explored. As a
result, the use of dioxygen in catalytic mediator-free
chemistry of PdII compounds remains very limited.10-12
There are only a few reports on the reactivity of PdIICH3
complexes toward O2.13-15 The reported reactions of O2
insertion into Pd-C bond produce methylperoxo PdII
complexes and employ either a chain radical (Scheme 1,
a)13 or a non-radical photochemical mechanism (Scheme
1, b).14 Oxidation of PdII hydrocarbyl complexes with O2 to
As a guideline for possible further development of aer-
obic organopalladium(II) chemistry one can use an analo-
gy with dioxygen – platinum(II) chemistry. There are a
number of parallels here. For instance, a homolytic19 or
non-radical photochemical14,20 dioxygen insertion into
PtII-CH3 bond is also known for platinum analogs of com-
plexes shown in Scheme 1 (a, b). In addition, dimethyl PtII
complexes are known to react with O2 in protic media to
produce corresponding dimethyl PtIV hydroxo derivatives
(Scheme 2, a).21 It is also known that the range of PtII
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Page 2 of 12
complexes which can be involved in aerobic PtII-to-PtIV
It is also important to note that aerobic functionaliza-
tion of PdII-CH3 bond in either mono- or dimethyl PdII
compounds to form MeOH has never been documented.
The only example of formation of methanol in a low yield
in a photochemical reaction of O2 with a monomethyl
PdIII complex was reported by Mirica.28 Hence, the goal of
this work was to find out if the aerobic functionalization
in water of a PdII analog of complex 2, such as 5 (Chart 1),
might lead to PdIV intermediates 6 and 7, analogs of 3 and
4, that can be highly reactive at the production of MeOH
and ethane, respectively. We were interested to see if this
type of reactivity can compete with or complement the
insertion of O2 into PdII-C(sp3) bond described in Gold-
berg or Britovsek’s works.13,14 In this paper we report re-
sults of such study revealing that the aerobic functionali-
zation of the monomethyl PdII complex 5 is viable and it
can lead to selective formation of either ethane or metha-
nol with or without concomitant formation of methylhy-
droperoxide. We also disclose some means to control the
reaction selectivity with respect to each of these three
products.
1
2
3
4
5
6
7
8
oxidation can be expanded when they are supported by
facially chelating ligands such as 1,4,7-triazacyclononane22
or 1,3,5-triaminocyclohexane.23 Importantly, the PtIV-C
bond of the resulting oxidation products can be function-
alized. Such reactions are facilitated when the supporting
facially chelating ligands contain a good leaving group, as
it is the case of di(2-pyridyl)methanesulfonate, dpms
(Scheme 2, b).24-26 The dpms–enabled aerobic PtII-C(sp3)
bond functionalization sequence can be very efficient and
selective, e.g., MeOH can be produced in >97% yield,
along with 3% Me2O, starting from neutral zwitterionic
(dpms)PtIIMe(OH2) complex 1 or its anionic derivative 2
and O2 in water via the intermediacy of methyl PtIV hy-
droxo species 3.24 Hence, considering the Pt chemistry
noted above, it would be interesting to see if a similar
aerobic PdII-CH3 bond oxyfunctionalization is possible for
Pd analogs of complexes 1 and 2. Exploring chemistry of
such Pd analogs may be even more intriguing since the
selectivity of the oxidation reaction in Scheme 2 is pH –
dependent: transformation of 2 to monomethyl PtIV com-
plex 3 is impeded at pH ≥ 12 and leads to the predominant
PtIV-to-PtII methyl group transfer resulting in the for-
mation of dimethyl PtIV hydroxo derivative 4 (Scheme 2,
c).25,26 A similar observation has also been recently made
by Goldberg using an analog of complex 2 supported by a
tripod dipyrazolylacetate ligand.27 Though complex 4 is
robust and does not eliminate ethane, the knowledge of
the mechanism of its formation may be useful for the pos-
sible development of similar palladium – based systems
for aerobic methane-to-ethane oxidative coupling. In fact,
first examples of oxidative aerobic coupling of dimethyl
PdII complexes to form ethane via intermediacy of PdIV
species were reported recently by Mirica.16,18 The same
group has described a photochemical elimination of
ethane from a monomethyl PdIII complex28 whereas Canty
and Sanford have demonstrated oxidative coupling of
monomethyl PdII species to form ethane using some other
oxidants.8 Notably, though monomethyl PdII species are
usually considered to be most relevant to methane C-H
activation by PdII complexes,1,7 oxidative aerobic coupling
of monomethyl PdII complexes to form ethane remained
so far unknown.
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Chart 1. DPMS-supported palladium analogs of com-
plexes 2-4.
RESULTS AND DISCUSSION
a. Oxidation of n-Pr4N[(dpms)PdIIMe(OH)] with O2
in water.
a1. Product distribution. The anionic monomethyl PdII
complex [(dpms)PdIIMe(OH)]- in the form of a water-
soluble tetra-n-propylammonium salt 5 was prepared us-
ing standard synthetic protocols (see SI). Similar to the
PtII analog 2, aqueous solutions of 5 are basic; the pH of a
4-7 mM solutions typically used in our experiments is
~10.6, which translates to about 6% fraction of the corre-
sponding protonated species (dpms)PdIIMe(OH2), 8, with
an estimated pKa of 9.4. Exposure of such solutions to air
or pure O2 atmosphere at 1 atm pressure in a Tef-
lon-sealed NMR tube leads to complete consumption of 5
Scheme 2. Aerobic oxidation of dimethyl PtII com-
plexes (a-c) and functionalization of PtII-CH3 bond
(b).
o
after 5-6 days at 21 C accompanied by the formation of
dihydroxo PdII complex 9 and ethane as two major prod-
ucts along with small amounts of an unidentified black
solid and methanol (eq 1; Table 1, entries 1-2). No transi-
ent Pd complexes were observed in the course of these
1
experiments by means of H NMR spectroscopy. Hence,
dpms ligand enables an unprecedented selective aerobic
oxidative coupling of a monomethylpalladium(II) species
to form ethane with 80-90% selectivity. Importantly, this
tripod ligand also allows for the formation of methanol,
albeit in a low yield, a type of reactivity which, to the best
of our knowledge, has never been observed previously in
aerobic reactions of methylpalladium(II) compounds.
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Table 1. Summary of product distribution in complete oxidation of n-Pr4N[(dpms)PdIIMe(OH)], 5, in NMR tube
under O2 atmosphere and 20 oC.a
1
2
3
4
5
6
7
8
9
C2H6, % /
CH4, %
MeOH,
%
MeO2H,
%
Entry
Conditions
Air, pH 10.6
O2, pH 10.6
Reaction time
5 days
Other Products, %
9, 83 ± 5 b
1
89 ± 3
82 ± 4
44d
9 ± 2
16 ± 3
54 ± 2
49 ± 1
0
0
0
2
3
4
5
6
6 days
9, 83 ± 5 b
O2, pH 14.0 c
O2, pH 14.0 c, e
Ar, pH 10.6 a
4 days
4 ± 1
20 ± 1
0
9, 83 ± 5 b
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9, 71 ± 1 b
2 days
31d
dpms-, 21 ± 1
55 ± 3 /
43 ± 1
63 ± 9 /
27 ± 8
7 days
dpms-, 47 b
Ar, pH 10.6,
Me-TEMPO,
7 ± 2 b
13 days
0
0
TEMPO f
a All the reactions in Table 1 were carried out under standard ambient fluorescent lamp lighting. Yields are based on initial
[PdIIMe]; b formation of small amounts of a black solid was observed, which may be the reason for about 15% lower than ex-
pected dpms ligand balance; c 1.0 M KOH solution; d Calculated assuming 100% methyl group balance. Poor solubility of ethane
in this solution precluded quantitative determination of ethane yield; e Irradiation with 26 W CFL; no stirring f 1.9 equiv. of
TEMPO additive.
It is important to note that in all of our closed-vessel
experiments where reliable integration of the alkane sig-
nals was possible (that is, all experiments except entries 3-
4 where 1M KOH was used) the methyl group balance was
reasonably close to 100%.
Based on the fact that the rate and selectivity of the ox-
a2. Factors affecting rate of oxidation of complex 5
idation of the PtII analog 2 with O2 are strongly pH de-
(eq 1) and possible mechanisms of O2 activation.
pendent,26 we hypothesized that the effect of pH may also
The effect of various factors on the rate of reaction in
be pronounced in aerobic oxidation of 5. The effect of the
reaction pH on the product distribution was probed using
1.0 M KOH (pH ~ 14.0) and some buffer solutions. The pH
change from 10.6 to 14.0 turned out to have the most
dramatic effect on the reaction (1) selectivity (Table 1,
entry 3 vs. 2): the yield of methanol has almost tripled up
amount of ethane formed could not be quantified. Hence,
to 54% after 4 days and a small amount of methylhydrop-
eq 1 was analyzed next with the goal to get some insight
into the reaction mechanism. To maintain a steady con-
centration of the oxidant in the solution the experiments
were carried out with rapid stirring under ambient pres-
o
sure of oxygen gas at 20 C. Under these conditions the
the yield of ethane was estimated assuming 100% methyl
eroxide (4%) was detected among the reaction products.
group balance as it was observed in our sealed flask exper-
All the experiments in Table 1 were conducted using
1
iments (Table 1). The oxidation was monitored by H
standard borosilicate glass vessels under standard lighting
NMR spectroscopy for ~3-4 reaction half-lives and was
conditions employing fluorescent hood lights as the only
found to follow first order kinetics in the complex 5 con-
light source. In the course of these experiments it was
centration (Table 2, entries 1-5).
noticed that the reaction rate and selectivity are changed
a) Oxygen partial pressure. A comparison of the exper-
when the mixtures are exposed to more or less light (vide
infra). Notably, when the reaction was performed under
irradiation with 26 W CFL at pH 14.0 the yield of
fold increase in the partial pressure of oxygen leads to
methylhydroperoxide could be increased up to 20% (entry
iments performed under air and pure O2 atmosphere at
pH 10.6 (entries 1 and 2, respectively) shows that an ~5-
only ~25% faster rate of the overall reaction (eq 1) and
4).
does not change significantly its selectivity. Hence, for the
Methane was not detected among reaction products in
most of the contributing processes, the rate-determining
step of the reaction does not involve O2.
either of our aerobic experiments. By contrast, when ar-
gon was used instead of air or oxygen, methane formed in
b) TEMPO additives. The reaction rate is moderately in-
43% yield after 7 days (entry 5). Notably, in the presence
of 1.9 equivalents of TEMPO formation of Me-TEMPO
adduct was detected by means of H NMR spectroscopy
ates. Formation of Me-TEMPO adduct was detected in
and electro-spray mass-spectrometry (entry 6) so suggest-
hibited by an additive of 2.4 eq of TEMPO (entry 3) point-
ing to a radical-like reactivity of the reaction intermedi-
1
those experiments by means of 1H NMR spectroscopy.
ing that methyl radicals may be involved in the anaerobic
reaction. Formation of methane in the course of decom-
position of monomethyl PdII complexes in the absence of
any oxidants was also reported by Sanford.8
c) Solution pH. The most significant change of the rate
and/or selectivity of the reaction between 5 and O2 could
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Table 2. Effect of various factors on the kinetics of oxidation of 6 mM complex 5 at 20 oC.a
1
MeOH,
2
3
4
5
6
7
8
9
Entry
Conditions
t1/2, h
C2H6,b % MeO2H, %
Other Products, %
%
1
Air, pH 10.6
O2, pH 10.6
31 ± 1
25 ± 1
7 ± 2
12 ± 2
94
88
0
0
9, 55 ± 6 c
9, 81 ± 4 c
9, 89 ± 4 c
2
O2, pH 10.6,
3
38 ± 1
11 ± 1
80
0
TEMPO d
Me-TEMPO, 9 ± 1
4
5
O2, pH 6.8 e
O2, pH 14.0 f
4.8 ± 0.1
10 ± 1
51 ± 4
90
44
0
11, 70 ± 1 c
16.0 ± 0.2
5 ± 1
9, 99 ± 5
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60
O2, pH 10.6,
1.0 M KNO3
O2, pH 14.0,f
Light g
6
7
8
15.8 ± 0.3
4.4 ± 0.1
187
9 ± 2
41 ± 1
6
91
45
94
0
14 ± 2
0
9, 69 ± 18
9, 98 ± 2 c
dpms-, 70 c
O2, pH 14.0,f
Dark h
a Yields are based on the amount of 5; typical concentration [5] 6-7 mM;b calculated assuming 100% methyl group balance; c for-
mation of black solid was observed; d 2.4 equiv. of TEMPO; e KH2PO4/K2HPO4 buffer; f 1.0 M KOH solution; g Reaction mixture
was irradiated with 26 W fluorescent lamp; h reaction flask wrapped with aluminum foil.
be achieved by changing its pH from the “native” value of
10.6 to either lower or higher side. A 5-fold acceleration
without change in the selectivity in MeOH was observed
at a lower pH 6.3 (entry 4). In turn, a 4-fold increase in
selectivity with respect to MeOH with a modest 30% in-
crease of the reaction rate was seen at pH 14.0 (entry 5 vs.
2). The effect of the ionic strength of 1.0 M KOH solution
may be responsible for the latter reaction rate increase
but not its selectivity change, as it is seen from the results
of oxidation at pH 10.6 in the presence of 1.0 M KNO3 (en-
try 6 vs. 2).
A non-radical photochemical O2 insertion into PdII-CH3
bond described by Britovsek (Scheme 1b)14 may be consid-
ered as another possible mechanism of O2 activation in
reaction (1) also leading to the formation of PdII-OOMe
species via a PdIV peroxo methyl intermediate. The Bri-
tovsek’s reaction model suggests an involvement of a di-
nuclear excited triplet species which, if applied to our
system, translates into second order rate of disappearance
in [5]. This mechanism is also unlikely to be predominant
in our system since reactions listed in Table 2 (entries 1-5)
follow first order kinetics in [5].
d) Effect of light. Finally, the photochemical nature of
the reaction in eq 1 was demonstrated by i) running it in
the presence of 26W CFL bulb placed near temperature –
controlled reaction flask; the reaction rate increased ~5-
fold (entry 7) and ii) running the oxidation in the dark
(aluminum foil wrapping; entry 8). In the latter case the
reaction was slowed down more than tenfold.
Since none of the reaction models above can account
for our experimental observations, we propose that a di-
rect photochemical oxidation of 5 to produce an electro-
philic PdIV complex 6 (Chart 1; see section f) might consti-
tute a third mechanistic possibility for O2 activation in
reaction (1). In such a case the aerobic reaction (1) would
follow a first order dependence in [5], consistent with our
observations. In this mechanism formation of MeOOH
may result from a nucleophilic attack of H2O2 at the me-
thyl ligand in complex 6 (vide infra).29
Discussing possible mechanisms of O2 activation in our
system it worth noting that the formation of MeOOH, the
effects of TEMPO additives and light on the reactivity of
complex 5 are reminiscent of a chain radical O2 insertion
into PdII-CH3 bond observed for a bipyridyl dimethyl PdII
complex (Scheme 1a).13 The methylhydroperoxide may be
released into the reaction mixture as a result of subse-
quent reaction with the solvent (eq 2).
b. Oxidation of n-Pr4N[(dpms)PdIIMe(OH)] with
ROOH in water (R = Me, t-Bu, H).
The fact that methylhydroperoxide is observed as one
of the reaction products in our aerobic experiments at pH
14.0 prompted us to consider potential involvement of
MeOOH in the oxidation reaction (eq 1) as an intermedi-
ate which is more reactive and hence “invisible” at lower
30
pH values. It is worth noting that the ability of H2O2
and some alkyl hydroperoxides16,28 to oxidize organo-
palladium(II) complexes has been documented previous-
ly.
At the same time, a modest inhibition of the reaction
(1) by TEMPO and the lack of noticeable changes in its
selectivity in the presence of TEMPO (entry 3 vs. 2) sup-
port the notion that the chain radical O2 – insertion is not
the major mechanism of O2 activation in reaction (1) un-
der ambient-light conditions.
Since MeOOH is not commercially available and is a
dangerous to work with compound, dilute solutions of
MeOOH (~0.3 mmol/L) were generated at pH 14.0 using
reaction (1) carried out under light. These solutions con-
tained no unreacted complex 5; dihydroxopalladium(II) 9
was the only soluble Pd-containing complex. The content
1
of CH3OH and CH3OOH was quantified using H NMR
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Journal of the American Chemical Society
Table 3. Results of the oxidation of 0.60 mM n-Pr4N[(dpms)PdIIMe(OH)] with MeOOH in water at 20 oC.a
1
2
3
4
5
6
Conversion, % /
Reaction time
MeOH,
equiv. b
Entry
Conditions
Other Products, %
1
pH 14.0,c MeOOH (2.5 equiv.)
pH 5.7,d MeOOH (2.0 equiv.)
100 / 16 h
1.62 ± 0.05
-
2
100 / 5 h
1.16 ± 0.09
LPdIVMe2(OH), 28 ± 1
a Yields are based on the initial [PdIIMe]; b The amount of MeOH formed with respect to that of 5; c 1.0 M KOH solution; d pH
7
8
adjusted with aqueous solution of HBF4.
9
spectroscopy using 1,4-dioxane as internal standard. Sta-
bility of the resulting solutions at pH 14.0 was tested and
no noticeable change in MeOOH concentration was ob-
served in the course of at least 2 days (Table S9). Upon
mixing of solutions containing 5 and MeOOH to allow for
2.5:1 MeOOH:5 ratio, the pH of the MeOOH solution was
adjusted by addition of 50% aqueous HBF4 when neces-
these oxidants was used in the reaction with 5 in water at
pH 5.7, 10.6 and 14.0 formation of MeOH was detected
with the highest MeOH yield, 32% for H2O2 and 61±6% for
t-BuOOH as oxidant, seen at pH 14.0. In the latter case
quantitative formation of t-BuOH (99% NMR yield) was
also observed. Notably, one more reaction product,
MeOOH, was also detected in the oxidation of 5 with
H2O2, and t-BuOOMe in the oxidation with t-BuOOH
when these reactions were carried out at pH 14 and 6 (see
SI).
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1
sary and the mixture was monitored by H NMR spectros-
copy.
In the reaction carried out at pH 14.0 complex 5 was
consumed in the course of 16 h. Intriguingly, the oxida-
tion produced 1.62±0.05 mol of methanol per mol of 5
(Table 3, entry 1). Assuming that 1 equivalent of MeOH is
liberated from MeOOH as a result of its reduction with 5
(vide infra), the additional 0.62±0.05 equivalents should
have originated from the methyl ligands of 5. We propose
that MeOH produced beyond the expected 1 equivalent is
derived from complex 6 (see section d), as shown in eqs 3
(R = Me) and 4.
Based on these observations and the proposed inter-
mediacy of monomethyl PdIV complex 6 in the oxidation
of 5 with O2 (eq 1), methanol may result from i) the reduc-
tion of transient MeOOH with 5 (eqs 3, 5; R = Me) and ii)
from nucleophilic attack by water solvent or OH- at the
monomethyl PdIV complex 6 (eq 4). A similar nucleophilic
attack at 6 involving H2O2 instead of water would also
lead to the formation of MeOOH that we observed at pH
14.0.
As in the case of the reaction with MeOOH, the reac-
tion of 5 with H2O2 (except at pH 14.0) was fast and com-
plete in the course of a few minutes at 21 oC. Formation of
the dimethyl PdIV intermediate 7 was seen in these exper-
iments in yields ranging from 15-50%. The oxidation of 5
with t-BuOOH at pH ≤ 14.0 was also fast enough to ob-
serve transient formation of 7.
Production of ethane was detected in all the oxidation
experiments above where MeOOH, t-BuOOH or H2O2
were used as oxidants. The source of ethane was found to
be the dimethyl PdIV complex 7. In contrast to its thermal-
ly robust dimethyl PtIV analog 4 (Scheme 1), the PdIV
Oxidation of 5 with MeOOH at pH 5.7 (Table 3, entry 2)
was complete in 5 h, that is noticeably and expectedly26
(using analogy with PtII complex 1) faster than at pH 14.0.
The reaction produced 1.16±0.09 mol of methanol per mol
of 5. Importantly, formation of an intermediate dimethyl
PdIV complex 7 along with dihydroxopalladium(II) species
9 (eq 5; R = Me) was also detected. The amount of 7 in-
creased during initial stages of reaction reaching a maxi-
mum yield of 28%; the complex disappeared by the end of
the reaction.
o
complex 7 eliminates ethane readily at 21 C (eq 6). This
complex was prepared and characterized independently
as described below.
c. Synthesis of (dpms)PdIVMe2(OH)
The dimethyl PdIV complex 7 could be generated by
stirring aqueous solutions of complex 5 with methyliodide
at 5 oC (eq 7). This reaction can be viewed as a formal SN2
attack of the nucleophilic methylpalladium(II) species 5
at the electrophilic MeI.31 The reaction is relatively slow
and elimination of ethane from the thermally unstable 7
(eq 6) is competitive with the formation of 7. As a result,
The reactivity observed in our experiments involving 5
and MeOOH was found to be common also for some oth-
er hydroperoxides, tert-butylhydroperoxide and hydrogen
peroxide (eqs 3-5, R = H, t-Bu; see also SI). When either of
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1
2
3
4
5
6
the fraction of 7 in the reaction mixtures increases in the
An indirect evidence for the transient formation of in-
7
8
9
course of the first 40 min and then remains steady at 40-
47% level during the following hour (Chart S4) so allow-
ing for a reliable NMR characterization of 7. According to
1H NMR spectroscopy, complex 7 is C1 – symmetric; selec-
tive NOE experiments show interaction between the axial
methyl ligand (2.91 ppm) and both ortho-C-H protons of
the dpms ligand pyridine rings (Fig. S12). An interaction
between the axial and equatorial (2.51 ppm) methyl lig-
ands is also observed.
termediate 6 can be obtained from the oxidation of 5 with
NaIO4 in D2O/CD3OD (eq 9). Analysis of the products by
1H NMR spectroscopy shows that CH3OCD3 (32% yield) is
formed along with CH3OH (32% yield). These products
may result from concurrent reactions of 6 with OCD3- and
OH- respectively (eq 10). Remarkably, the PtIV analog of 6,
complex 3, is much less electrophilic and is virtually unre-
active with respect to such nucleophiles.26,29
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53
54
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57
58
59
60
Chart 2. NOE interaction in DPMS-supported palla-
dium complex 7.
Our attempts to observe formation of 6 in reactions be-
tween 5 and 3 equiv. NaIO4 were unsuccessful even at
temperatures as low as -60 oC using D2O-CD3OD mix-
tures. Complex 5 was fully consumed within first 5-10
minutes producing the dimethyl PdIV complex 7 and two
more C1- symmetric dimethylpalladium(IV) species
(dpms)PdMe2X in a 54% combined yield (see SI); here X
was presumed to be OMe (eq 8; R = Me) and IO4. All
three (dpms)PdMe2X complexes decompose rapidly at 21
oC with elimination of ethane with a half-life ranging be-
tween 3 and 6 min.
It is also apparent that, similar to (dpms)Pt chemistry,26
a PdIV-to-PdII methyl transfer from the electrophilic me-
thyl PdIV complex 6 to a nucleophilic species 5 may be
involved (eq 11). The PdIV-to-PdII methyl transfer reaction
(11) occurs concurrently with reactions (4) and (10) where
6 reacts with hydroxide and methoxide anions, respec-
tively, acting as nucleophiles.
Notably, complex 7 does not produce any MeOH even
at pH 14.0. When temperature of the solutions containing
o
7 is raised to 21 C, ethane elimination becomes fast with
the half-life of 6 min for this first order process (Chart
S5). The short reaction half-life suggests that complex 7
can be observed at this temperature only when the rate of
its formation is much faster or comparable to that of the
ethane elimination. In fact, this condition was never met
in our aerobic oxidation experiments as well as in the oxi-
dation of 5 with H2O2 at pH 14.0 so allowing to account
for our failure to detect 7 in the corresponding reaction
mixtures.
d. Competitive formation of 7 and methanol via
proposed methylpalladium(IV) transient 6.
A much faster conversion of 5 to mixtures of dimethyl
PdIV complex 7, dihydroxo PdII derivative 9 and MeOH
can be achieved using a more efficient oxidant, NaIO4 (eq
8; R = H).
e. Mechanism of oxidation of
Pr4N[(dpms)PdIIMe(OH)] via O2 insertion into PdII-C
bond
As discussed in section (b), MeOH and ethane in reac-
tion (1) may form both via intermediacy of MeOOH and
monomethylpalladium(IV) transient 6, respectively.
Scheme 3 summarizes the hypotheses concerning a possi-
ble mechanism of such photo-induced transformations.
The first step is the photo-excitation of 5 to produce, after
The oxidation is complete after 11 minutes at pH 10.6 and
o
21 C leading to 27% yield of MeOH, 31% yield of 7 and
free ethane. Overall, this chemistry is reminiscent of the
reactions of 5 with hydroperoxides (eqs 3 and 5) and, pre-
sumably, also proceeds via intermediacy of a highly elec-
trophilic monomethyl palladium(IV) complex 6 (eq 9):
3
a spin interconversion, a triplet spin-isomer [5] (step a).
This photo-excitation should be rate-limiting consistent
with the observed first order of reaction (1) in complex 5.
3
Subsequent fast trapping of [5] with O2 leads to the O2
insertion into Pd-CH3 bond of 3[5] to form a palladium(II)
methylperoxo species (step b; see section g for more de-
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5 is (1.0 / 0.005) / 1.6 ≈ 100 times more nucleophilic than
tail). Hydrolysis of the palladium(II) methylperoxo spe-
1
2
3
4
5
6
7
cies generates free methylhydroperoxide (step c). The
MeOOH is then converted to MeOH by another equiva-
OH-. In turn, at pH 5.7 the observed MeOH : C2H6 ratio is
1.16 : 0.84 = 58% : 42% which suggests that ~5 mM com-
plex (dpms)PdIIMe(OH2), 8, is about 0.84 / 0.16 = 5.3
times more competitive than 55 M H2O in their reaction
with 6. Hence, 8 is (55 / 0.005) × 5.3 ≈ 60000 times more
nucleophilic than H2O.
lent of
5
to produce
a
highly electrophilic
monomethylpalladium(IV) transient 6 (step d). This pH –
dependent reaction is fast at pH ≤ 11 so that MeOOH does
not accumulate in the mixtures and is undetectable by
1
means of H NMR spectroscopy. By contrast, the reaction
We can now compare MeOH : C2H6 ratios in reactions
of 5 at pH 14.0 when O2 is the oxidant that range from 49 :
31 to 54 : 44 (Table 1, entries 4 and 3, respectively) vs. 80 :
20 (see above) when MeOOH is the oxidant. These ratios
at a lower pH ~7 are 10 : 90 with O2 as oxidant (Table 2,
entry 4) vs. 58 : 42 when MeOOH is the oxidant. In both
cases in the aerobic reaction the observed fraction of
MeOH is always lower than in the analogous reactions
utilizing MeOOH as oxidant. The observed dramatic dif-
ference in the reaction selectivity implies that the pre-
dominant mechanisms leading to the formation of MeOH
and ethane do not involve intermediacy of MeOOH either
at pH 14.0 or pH 6-7. Therefore, the mechanism in
Scheme 3 can have only a minor contribution to the over-
all reaction (1). Hence, we propose another mechanism
leading to complex 6 as discussed in the following sec-
tion.
8
9
is slow enough at pH 14.0 so that MeOOH is an observa-
ble product of photo-oxidation (1).
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The transient 6 may be involved in two concurrent pro-
cesses: i) a nucleophilic reaction with the solvent or hy-
droxide anion (SN2) leading to a second equivalent of
MeOH (step e) and ii) a PdIV-to-PdII methyl transfer reac-
tion leading to a dimethyl PdIV intermediate 7 (step f) that
is responsible for ethane production (step g) in this reac-
tion sequence.
If the step f fully outcompetes e, the MeOH : C2H6 ratio
is expected to be 1 : 1, whereas if step e is also competitive,
the MeOH : C2H6 ratio should be greater than 1 : 1. In the
latter case this ratio is expected also to grow as pH in-
creases since greater concentration of the nucleophile,
OH-, involved in the reaction e, becomes available. In-
deed, when ROOH are used as oxidants (R = H, Me, t-Bu)
in their reactions with complex 5 (see Table 3), both
pathways, e and f, are operational and competitive. For
instance, when MeOOH is used at pH 14.0, the observed
MeOH : C2H6 ratio is about 1.62 : 0.38 = 80% : 20%. This
ratio of products points to a 0.62 / 0.38 = 1.6 times greater
contribution of the reaction e vs. f, so that in 1.0 M KOH
solution the hydroxide anion is about 1.6 times more
competitive than ~5mM 5 in the reaction with 6. Hence,
f. Direct photochemical formation of complex 6
with O2 as oxidant
The alternative mechanism for the formation of MeOH
and ethane is given in Scheme 4 for the methylpalladi-
um(II) aqua complex 8 as an example. A similar reaction
sequence can be written for its hydroxo derivative 5.
Scheme 3. Mechanism of MeOOH and MeOH formation in reaction (1) via initial photochemical O2 insertion
into PdII-C bond.
hv
(dpms)PdIIMe(OH)-
3[(dpms)PdMe(OH) ]–
( a )
Photo-excitation (RDS):
3[5]
5
O2
3[(dpms)PdMe(OH)]-
(dpms)PdII(OH)(O2Me) –
( b )
( c )
O2 insertion:
3[5]
–
(dpms)PdII(OOMe)(OH)– + H2O
(dpms)PdII(OH)2 + MeOOH
Formation of MeOOH:
9
O
O
H
S
O
PdIV
+ MeO2H, H2O
( d )
N
Me
OH
(dpms)PdIIMe(OH)-
Reduction of MeOOH to MeOH:
N
- MeOH
- OH-
5
OH
6
O
O
H
S
Elimination of MeOH from MePdIV
(competitive at pH 14.0)
:
O
PdIV
OH- or H2O
SN2
N
Me
OH
( e )
+
MeOH
9 or 11
N
OH
6
PdIV-to-PdII methyl transfer:
(predominant at pH 6.7)
( f )
+ 5 or 8; - 9 or 11
O
O
H
S
O
PdIV
OH- or H2O
Ethane elimination from Me2PdIV
:
( g )
N
Me
OH
Me-Me
+
9 or 11
N
Me
7
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Complex 8 is expected to be the predominant (> 99%) way (c) with H2O2 as a nucleophile may serve for the
1
2
3
4
5
6
7
8
palladium - containing species in ~6 mM solution at pH
6.7; this complex should be a minor (~6%) species at pH
10.6 and a trace component at pH 14. According to the
mechanism in Scheme 4, the PdII complex 8 is photo-
excited (a formal rate-determining step of the overall oxi-
dation reaction) to produce, after a spin interconversion,
MeOOH production in our reaction mixtures at pH 14.0,
besides insertion of O2 into PdII-C bond (eq 2).
g. Time-dependent DFT analysis of 3[5] and 3[8].
To shed light on the anticipated reactivity toward O2 of
3
3
3
the triplet species [5] and [8] that are expected to result
from photo-excitation and spin interconversion involving
5 and 8, respectively, we used methods of time-dependent
Density Functional Theory (TDDFT). The geometry opti-
mization for 5 and 8 and calculations of the 30 lowest
energy singlet and triplet excited states were done for
water as a solvent (see SI for details).
a triplet spin-isomer [8] (vide infra). The triplet transient
is trapped by O2 to form
a derived hydroperoxo
9
methylpalladium(IV) species 10. In support of the viability
of the formation of PdIV intermediates in direct aerobic
oxidation of palladium(II) methyl species, precedents
involving dimethyl PdII complexes were reported earlier
by Mirica.16-18 In contrast to these previous observations,
the aerobic oxidation in Scheme 4 is photo-induced and
involves monomethyl palladium(II) species.
10
11
12
13
14
15
16
17
18
19
20
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The calculated UV-VIS spectrum of the anionic com-
plex 5 contains multiple electronic metal-to-ligand charge
transfer (MLCT) transitions in the range of wavelengths
420 – 600 nm. Most of the excitations correspond to the
transitions of the metal electrons promoted from its 4d
orbitals to the π* orbitals of the pyridine rings (see SI). In
particular, the lowest energy excitation at 549 nm leading
Scheme 4. Direct photochemical oxidation of 5 or 8
with O2 to produce complex 6.
1
to a singlet spin-isomer [5549] involves an electron transi-
tion from the metal dz2 orbital to a π* pyridine rings mo-
lecular orbital. A higher energy excitation at 355 nm lead-
1
ing to a spin-isomer [5355] corresponds to a transition
from the metal dz2 orbital to a predominantly Pd-C σ*-
orbital. Analysis of the triplet excitations of 5 allows to
find the triplet excited states with similar spin popula-
tions (see SI for details). For instance, the lowest energy
3
triplet excited state [5575] has two singly occupied molec-
ular orbitals, SOMO, which is predominantly the metal
4dz2 orbital, and SOMO+1, which is one of the pyridine
π*-orbitals (Fig. 1, a-b).
Similar analysis was performed for the aqua complex 8.
Compared to 5, the absorption bands of the MLCT are
blue-shifted spanning from 350 - 550 nm (Fig. 1). The low-
est energy excitation at 512 nm leading to a spin-isomer
1[8512] involves an electron transition from the metal dz2
orbital to a π* pyridine rings orbital (89% contribution)
and a Pd-C σ*-orbital (11%).
Since a fluorescent lamp light spectrum has greatest in-
tensity in the visible range, we may limit our analysis to
the MLCT transitions above. For both lowest energy tri-
3
3
plet excited states, [5575] and [8582] (Fig. 2), the highest
energy SOMO+1 is predominantly one of the pyridine π*-
orbitals where the unpaired electron can be “picked up”
by an O2 molecule to produce a superoxide anion-radical
and a corresponding PdIII metal-radical (Scheme 5). In the
3
case of the reaction sequence involving [8582] a subse-
quent radical pair collapse may be assisted by a proton
transfer from the nearby aqua ligand to give the Pd(IV)
intermediate 10. Such reaction sequence may be less effi-
As in the case of the monomethyl hydroxopalladium(IV)
6 (Scheme 3), several competing reactions are possible
that involve 10. Among them are: i) a nucleophilic attack
by methylpalladium(II) complex 8 (paths a, b) leading to
a PdIV-to-PdII methyl transfer product 7; H2O2 is another
expected product here (path a), or/and an OH-group
transfer via O-O cleavage to produce 6 (path b), and ii) a
nucleophilic attack by water and/or H2O2 to produce
MeOH and/or MeOOH, respectively (path c). The path-
3
cient for [5575] at pH 14.0 and the corresponding spin-
isomer 3[5575] may decay back to 5.
3
A higher-energy spin-isomer of 3[5], [5455], with the
SOMO+1 being essentially a Pd-CH3 antibonding orbital,
is expected to have a weakly bound Me ligand which may
be abstracted by O2 to form eventually a PdII
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a)
b)
c)
d)
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3
Figure 1. Singly occupied molecular orbitals, SOMO and SOMO+1, corresponding to a triplet excited spin-isomer [5575] (a, b)
and 3[8582] (c, d).
Scheme 5. Reactivity of 3[5] and 3[8].
O2
S
SO3
H
3
SO3
N
SO3
N
H
H
H
O
2+
Me
OH2
O2
N
h
ν
Me
PdIII
2+
N
PdIV
O2H
Me
OH2
Me
OH2
PdII
N
PdIII
N
N
OH
N
512 nm
O2
3[8582
]
8
10
O2
S
3
SO3
N
SO3
N
H
H
H
O
+H2O
O2
Me
OH
Me
Me
OH
PdIII
PdIII
N
PdIV
O2H
N
N
h
ν
N
OH
O2
549 nm
SO3
N
3[5575
]
H
Me
OH
PdII
N
h
ν
3
SO3
N
SO3
N
SO3
N
H
H
H
355 nm
5
MeO2
O2
Me
OH
O2Me
PdI
PdII
Pd
N
N
N
OH
OH
3[5455
]
We hypothesize that in the absence of O2 some of the
spin-isomeric triplet species [5] and 3[8] may be involved
in a homolytic PdIII-CH3 bond cleavage resulting in a hy-
drogen atom abstraction / methane formation.
3
methylperoxo complex. A similar methyl radical abstrac-
tion reaction may lead to Me-TEMPO when TEMPO is
present in reaction mixtures. Similar pathways may be
considered for 3[8582] as well.
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Table 4. Effect of [PdIIMe] on the product distribution in the oxidation of n-Pr4N[(dpms)PdIIMe(OH)] with O2 in water at pH
14.0.a,b
c
Conversion /
Reaction time
MeOH
(%)
MeOOH
(%)
C2H6
(%)
Entry
[5], mM
Other Products (%)
1
13.2
1.3
98 % / 4 days
100 % / 4 days
54 ± 2
44 ± 2
4 ± 1
42
26
9, 82 ± 5
9, 92 ± 6
2
30 ± 1
8
9
9, 69 ± 2
3
0.7
100 % / 4 days
43 ± 1
40 ± 3
17
dpms-, 27 ± 7
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a Yields are calculated based on initial [PdIIMe]; b 1.0 M KOH solution; c Calculated assuming 100% mass balance
.
h. Controlling the C2H6/MeX and MeOH/MeOOH
ratios: effect of [PdII] on product distribution
e.g., water or hydroxide anion would form methanol, hy-
drogen peroxide would form methylhydroperoxide and
another methylpalladium(II) species would produce a
dimethylpalladium(IV) intermediate. The dimethylpalla-
dium(IV) complex was detected in some reaction mix-
tures, was prepared independently and shown to be high-
ly reactive in ethane reductive elimination. The
knowledge gained in this work may be useful for the de-
velopment of selective photochemical aerobic oxidation
of methane to value-added oxygenates or ethane.
Three organic products of aerobic PdII-CH3 bond func-
tionalization resulting from reaction (1), MeOH, MeOOH
and ethane, could be produced in different ratios in our
experiments. Some results pertinent to control of the re-
action selectivity are summarized here. With the goal to
maximize the yield of the oxygenated products, MeOH
and MeOOH, resulting from reaction (1), one should per-
form the aerobic oxidation of 5 at pH 14.0 (Table 2, entry
5).
ASSOCIATED CONTENT
Since the rate of formation of ethane is expected to be
second order in [5] (Scheme 4, path a), one would expect
that at a lower [5] formation of ethane should become less
competitive as compared to MeOH (Scheme 4, path c)
which is expected to be first order in [5]. In fact, decreas-
ing concentration of the methylpalladium(II) complex 5
about 10-20 – fold from 13.2 mM (Table 4, entry 1) to 1.3
mM (entry 2) and 0.7 mM (entry 3) allows to decrease the
yield of ethane from 42% to 17% while maintaining the
yield of MeOH at the 43-54% level.
Supporting Information. Experimental details regarding
the synthesis and characterization of the ligands and metal
complexes, and TDDFT calculations. This material is availa-
ble free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*avederni@umd.edu
Notes
The authors declare no competing financial interests.
Similarly, if the major pathway for the reduction of
MeOOH to MeOH involves its reaction with 5 (eq 3), by
lowering [5] one can increase the MeOOH : MeOH ratio.
Indeed, this ratio could be raised from 0.07 (Table 4, en-
try 1) to 0.93 (entry 3) as a result of about 20-fold decrease
of [5].
ACKNOWLEDGMENT
This work was supported by the Center for Catalytic Hydro-
carbon Functionalization, an Energy Frontier Research Cen-
ter Funded by the U.S. Department of Energy, Office of Sci-
ence, Office of Basic Energy Sciences, under Award Number
DE-SC0001298, and, in part, by the National Science Founda-
tion (CHE-1112019, CHE-1464772).
CONCLUSIONS
In this work the first example of aerobic oxidation of
monomethyl palladium(II) complexes has been demon-
strated. The reaction is photochemical in nature and can
lead to the formation of three organic products, ethane,
methanol and methylhydroperoxide. By varying the solu-
tion pH and concentration of methylpalladium(II) species
the reaction selectivity can be tuned to favor the for-
mation of predominantly ethane (≤94%) or methanol
(≤54%). The fraction of methylhydroperoxide could also
be changed from 0 to 40% using solution pH and light as
key tools. A reaction mechanism is proposed that includes
photochemical oxidation of monomethyl palladium(II)
complexes to their monomethylpalladium(IV) derivatives
(major reaction path) and O2 insertion into PdII-C bond
(minor path). The methylpalladium(IV) complexes are
proposed to be highly electrophilic and responsible for
concurrent methyl group transfer to nucleophiles present,
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