Oxyfunctionalization with CpIrIII(NHC)(Me)L complexes

Article abstract of DOI:10.1021/om5007352

A series of monomethyl CpIr^III complexes were synthesized and studied for the formation of methanol in water. Methanol yields of 75(4)% in the presence of O₂ were obtained. From isotope labeling studies, it was determined that O₂ is the source of the oxygen atom in the product. From kinetic studies, oxyfunctionalization appears to proceed by dissociation of an L-type ligand followed by O₂ binding and insertion.

Full text of DOI:10.1021/om5007352

Communication
pubs.acs.org/Organometallics
Oxyfunctionalization with Cp*IrIII(NHC)(Me)L Complexes
Matthew C. Lehman, Paul D. Boyle, Roger D. Sommer, and Elon A. Ison*
Department of Chemistry, North Carolina State University, 2620 Yarbrough Drive, Raleigh, North Carolina 27695-8204, United
States
S
* Supporting Information
ABSTRACT: A series of monomethyl Cp*IrIII complexes were
synthesized and studied for the formation of methanol in water.
Methanol yields of 75(4)% in the presence of O2 were obtained.
From isotope labeling studies, it was determined that O2 is the
source of the oxygen atom in the product. From kinetic studies,
oxyfunctionalization appears to proceed by dissociation of an L-type
ligand followed by O2 binding and insertion.
Scheme 1. Oxyfunctionalization with Cp*IrIII Complexes
eveloping homogeneous catalytic systems that result in
the selective oxidation of saturated hydrocarbons remains
D
a great challenge.1 In recent years soluble transition-metal
complexes have been utilized to achieve two steps that are
critical for this transformation: (1) C−H activation to form a
metal−carbon bond and (2) C−O bond formation (oxy-
functionalization) to form a functionalized hydrocarbons. The
two most prominent examples in the field of homogeneous
catalysis remain the Shilov system2 and the Catalytica system.
In both systems PtII catalysts are utilized; however, their wide-
scale implementation has not been realized because of the
requirement for expensive PtIV oxidants (Shilov) and highly
acidic conditions (Catalytica).2,3
Much of the research effort in the field over the last four
decades has been devoted to the development of transition-
metal systems that can selectively activate C−H bonds.4 These
studies have resulted in a thorough understanding of the
mechanisms of C−H activation, and several classes of
complexes have been shown to be effective at this trans-
formation. Particularly noteworthy are a series of Cp*IrIII(L)
complexes reported by Bergman and co-workers (where L is a
PR3 ligand), which have been reported to perform C−H
activation reactions at or below room temperature.4a,h In
contrast, an understanding of the factors that lead to effective
oxyfunctionalization has been much less developed, although
these studies are critical in order to design catalytic systems for
the selective oxidation of hydrocarbons. While there have been
examples of complexes that react with oxygen atom donors5
and in some cases dioxygen itself in a stoichiometric fashion,
examples of complexes that utilize a framework that are also
effective in the activation of C−H bonds are rare.6 Thus, we
sought to investigate the oxyfunctionalization of an M−CH3
bond, in a system (Cp*IrIII(L)) that has been shown to be
effective at C−H activation (Scheme 1).
heterocyclic carbene ligand results in Ir complexes that are as
electron rich as the analogous phosphine complexes; however,
NHC complexes should be more compatible with oxidizing
conditions. Several groups have synthesized Cp*IrIII(NHC)
complexes and shown their ability to catalytically activate C−H
bonds.7 However, there has been little work focused on the C−
O bond-forming step with these systems.8
In this communication, a series of [Cp*IrIII(NHC)Me(L)]-
[OTf] complexes have been synthesized (L = DMSO, CO,
pyridine), and their ability to form methanol in the presence of
O2 in water has been demonstrated (Scheme 2). The data
obtained suggest that methanol is produced from these
complexes with O2 as the source of oxygen in the product.
Scheme 2. Formation of Methanol from Cp*IrIII Complexes
While the PR3-containing complexes have shown the best
activity for the activation of C−H bonds, the easily oxidizable
nature of the phosphine ligand makes these complexes
unsuitable for further oxyfunctionalization (see the Supporting
Information, Figure S3). Replacing the PR3 ligand with an N-
Received: July 17, 2014
Published: September 16, 2014
© 2014 American Chemical Society
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a
A series of monomethyl Cp*IrIII complexes were synthesized
(see the Supporting Information). All three complexes
incorporated an NHC ligand (NHC = 1,3,-dimethylimidazol-
2-ylidene) along with a “labile” L-type ligand (L = CO, DMSO,
pyridine). Complex 2 is similar to a complex reported by
Heinekey and co-workers, differing only in the nature of the
counterion.7a Complexes 1 and 3 are unique to this study and
have been fully characterized by 1H and 13C NMR spectroscopy
as well as elemental analysis. X-ray-quality crystals of 1 were
obtained by slow diffusion of pentane into a concentrated
solution of 1 in CH2Cl2 (see the Supporting Information).
Initial studies were focused on the formation of methanol
from complex 1 in the presence of O2 (60 psi), water (0.5 mL),
and 5 equiv of a NaCl additive over 1 h at 100 °C. The crude
Scheme 3. Isotope Labeling Studies
a
Asterisks indicate that the product was determined by GC/MS
analysis of the crude reaction mixture.
The stability of the ligands was also examined under the
reaction conditions, as Crabtree and co-workers have recently
shown that substitution of the Cp* ligand can occur for similar
Ir(III) complexes under oxidizing conditions.9 In addition,
work by Mayer and co-workers suggests that methyl groups on
the Cp* ligand can be oxidized to form C−O bonds in the
presence of PhI(OAc)2.10 In order to establish that the methyl
in the methanol product originates from the Ir−Me fragment, a
CD3 analog of 3, 3-CD3, was synthesized and its oxidation was
examined (Scheme S2, Supporting Information). Analysis of
the 2H NMR spectrum revealed CD3OH production only when
3-CD3 was used, suggesting that the methyl group in the
product comes directly from the Ir−Me group and not from
oxidation of either the Cp* or NHC ligand.
1
reaction mixture was analyzed and quantified by H NMR
spectroscopy, with the results summarized in Table 1. GC-MS
Table 1. Methanol Formation with Complexes 1−3
While this system allowed for analysis of the methanol
product, the identity of the iridium complex formed in the
reaction remained elusive. In order to identify the Ir product
following oxidation, the reaction conditions were adjusted.
After a variety of additives and reaction conditions were
considered, it was found that by replacement of NaCl with 1
equiv of HCl and adding 10 equiv of pyridine, an Ir species was
identified as Cp*Ir(NHC)PyCl (4) (Scheme 4) and charac-
a
entry
complex
amt of gas, atm
amt of methanol, %
49(1)
1
2
3
4
1
2
3
3
O2
O2
O2
N2
b
ND
75(4)
b
ND
a
Complexes 1−3 were added to a 0.5 mL solution of 0.03 M NaDSS
(DSS = 4,4-dimethyl-4-silapentane-1-sulfonate) and 0.075 mmol of an
additive (NaCl) in D2O in a 3 mL Schlenk flask equipped with a
magnetic stirrer. After three freeze−pump−thaw cycles the reaction
vessel was pressurized with O2 (60 psi) or N2 (20 psi). The reaction
mixture was then placed in a temperature-controlled oil bath set to the
appropriate temperature and stirred. After 1 h the reaction mixture was
removed from the bath and cooled to room temperature. The crude
Scheme 4. Generation of [Cp*Ir(NHC)PyCl][OTf] (4)
1
reaction mixture was then transferred to an NMR tube for H NMR
b
analysis. No methanol detected.
analysis of the crude reaction mixture was also used to verify
methanol formation. It is also important to note that, when the
reaction was performed under an N2 atmosphere, no methanol
formation was observed (entry 4).
1
terized by H and 13C NMR spectroscopy, X-ray crystallog-
raphy, and elemental analysis (see the Supporting Information).
The addition of pyridine slows the rate of the reaction
significantly; however, a reaction yield of 60(2)% was obtained
after 5 h and is comparable to that under the previous
conditions, where a maximum yield of 75(4)% after 1 h was
observed. When the crude reaction mixture was analyzed after 1
h by ESI-MS, 3 and [Cp*Ir(NHC)PyCl][OTf] (4) were
observed.11 When the reaction mixture was analyzed after 5 h,
significant Ir black formation was observed and peaks for 3 and
4 were significantly attenuated.
Complex 2 (L = CO) was synthesized and its reaction with
O2 in water was examined. Not surprisingly, when 2 is reacted
under optimized reaction condition,s no methanol product is
observed because CO presumably forms a stronger bond to the
iridium center than does DMSO (Table 1, entry 2). However,
when reactions were performed with complex 3 (L = pyridine),
improved reactivity was observed, with a 75(4)% yield of
1
methanol by H NMR spectroscopy (entry 3). Since it was
apparent that higher reactivity was obtained with 3, further
mechanistic and kinetic studies were performed with this
complex.
Isotope-labeling studies were undertaken to determine the
source of the oxygen atom in the methanol product (Scheme
3). When H218O was employed as the solvent, no CH318OH
was observed by GC-MS. However, when the reaction mixture
was pressurized with 18O2, CH318OH was detected. This
suggests that the source of the oxygen atom in the methanol
product is O2.
1
By H NMR spectroscopy, mass balance was observed for
the reaction up to 2 h (Figure 1a), with 3 being converted to 4
at rates comparable to those of methanol formation. Catalyst
decomposition is observed at reaction times longer than 2 h
with the observed formation of Ir black.12 The rates within the
first 1 h of the reaction for the formation of 4 ([1.7(1)] × 10−3
mM/s), the disappearance of 3 ([1.9(2)] × 10−3 mM/s), and
the formation of methanol ([1.7(1)] × 10−3 mM/s) were the
same within error (Figure 1b). These data strongly suggest that
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Communication
proposed in Scheme 5. In this mechanism, where L = pyridine,
pyridine dissociates from 3 to form a 16-electron intermediate.
Scheme 5. Proposed Mechanism for the Formation of
Methanol from [Cp*IrIII(NHC)MeL][OTf] Complexes
Figure 1. Reaction profile for the reaction of 3 with O2, according to
Scheme 4: (a) time profile for the reaction after 5 h (the vertical black
line occurs at the 2 h time point for the reaction); (b) time profile for
the reaction over 1 h with rates for formation of 4 and methanol and
the rate of consumption of 3. Legend: red triangles, [Ir]total = 3 + 4;
green diamonds, 3; blue squares, 4; black circles, CH3OH. Reactions
were performed with [3] and HCl = 0.03 M and [pyridine] = 0.30 M
at 100 °C in 0.5 mL of D2O. Yields for methanol and 4 were
Dioxygen then interacts with this intermediate, ultimately
resulting in insertion into the Ir−Me bond to form an Ir−
OOMe species. In the presence of HCl and pyridine, this Ir−
OOMe species oxidizes an Ir−Me species to form 2 equiv of
MeOH and 4.13 The observed first-order dependence on 3 and
O2 suggests that O2 insertion into the Ir−Me bond in 3 is slow
relative to the reaction of the Ir−OOMe species with another 1
equiv of 3. Work is ongoing to understand the intimate details
of the mechanism of the oxyfunctionalization step, which will
be key in determining ways to increase complex stability and
reaction rates.14
In this work a rare example of oxyfunctionalization to
produce methanol from an Ir−methyl bond is described.
Isotope labeling experiments suggests that the oxygen atom in
the product originates from O2, presumably via an oxygen
insertion mechanism. Kinetic studies suggest that the
mechanism for the reaction proceeds by ligand dissociation of
an L-type ligand to form a 16-electron intermediate, followed
by O2 binding and functionalization. The evidence for this is as
follows. (1) When the ancillary DMSO ligand was replaced by
pyridine, yields of up to 75(4)% were observed. (2) From
kinetic experiments, the reaction rate was found to be first
order in iridium and dioxygen and inversely proportional to the
concentration of pyridine. When the reaction conditions were
1
determined by H NMR analysis of the crude reaction mixture with
NaDSS added as an internal standard. Yields for 3 were determined by
1H NMR analysis after the solvent was removed in vacuo and dissolved
in CD2Cl2 with ferrocene added as an internal standard.
during the first 2 h of the reaction, 3 is converted to 4 directly
as methanol is formed, and the equation for this reaction can be
depicted as shown in Scheme 4.
Kinetic studies were then performed to determine the order
of the reaction with respect to 3, O2, and pyridine. Initial rate
plots were obtained (Figures S5−S7, Supporting Information),
and the corresponding log−log plots are shown in Figure 2.
Slopes close to 1 for O2 pressure (slope = 1.2(1)) and the
concentration of 3 (slope = 1.0(2)) suggest that the rate is first
order in both O2 and 3. An inverse first-order dependence was
observed for pyridine with a slope of −1.0(1) obtained from
the log−log plot. Thus, the experimentally determined rate
expression for the reaction is given by eq 1.
k1[Ir][O2]
rate =
k−1[L]
(1)
On the basis of the observed data, a mechanism for the
formation of methanol from [Cp*Ir(NHC)L][OTf] species is
Figure 2. (a) Plot of log (initial rate) vs log (pO ). Initial rates were determined at O2 pressures of 20, 40, 50, and 60 psi. Conditions: [3] = 0.03 M;
2
[HCl] = 0.03 M; [pyridine] = 0.03 M at 100 °C in 0.5 mL of D2O. (b) Plot of log (initial rate) vs log ([3]). Initial rates were determined at [3] =
22.5, 30, 45, and 60 mM. Conditions: pO = 60 psi; [HCl] = 1 equiv with respect to [3]; [pyridine] = 10 equiv with respect to 3 at 100 °C in 0.5 mL
2
of D2O. (c) Plot of log (initial rate) vs log ([pyridine]). Initial rates were determined at [pyridine] = 0.15, 0.225, 0.3, and 0.45 M. Conditions: pO
60 psi; [3] = 30 mM; [HCl] = [pyridine] = 10 M at 100 °C in 0.5 mL of D2O.
=
2
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achieved with these complexes without initial ligand decom-
position. As a result, this work provides important implications
for the development of catalytic systems for the selective
oxidation of saturated hydrocarbons. Future work will involve a
detailed study to understand the mechanism as well as the
exploration of these complexes as catalysts for the oxidation of
saturated hydrocarbons.
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ASSOCIATED CONTENT
* Supporting Information
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Text, figures, tables, and CIF files giving full experimental
procedures for 1−4, X-ray crystallographic data for 1 and 4,
experimental procedures for methanol formation, isotopic
labeling, and kinetic studies, and GC-MS data. This material
is available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*E-mail for E.A.I.: eaison@ncsu.edu.
■
Author Contributions
́
́
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The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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Chem. Soc. 2013, 135, 10837−10851. (b) Grotjahn, D. B.; Brown, D.
B.; Martin, J. K.; Marelius, D. C.; Abadjian, M.-C.; Tran, H. N.;
Kalyuzhny, G.; Vecchio, K. S.; Specht, Z. G.; Cortes-Llamas, S. A.;
Miranda-Soto, V.; van Niekerk, C.; Moore, C. E.; Rheingold, A. L. J.
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(11) A small amount of the complex [Cp*Ir(NHC)(Py)2][(OTf)2]
(5) was detected by ESI-MS.
(12) The formation of iridium hydrides is observed for both
complexes 3 and 4 under the reaction conditions with excess
methanol. These iridium hydrides are not stable under the reaction
conditions. This could explain the decomposition of the system after 2
h.
(13) Treatment of 3 under an N2 atmosphere with t-BuOOH under
identical conditions results in the formation of methanol in 41(5)%
yield (see the Supporting Information). This suggests that peroxy
species are capable of oxidizing Ir−Me bonds.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by the NSF as part of the Center for
Enabling New Technologies through Catalysis (CENTC),
Grant Nos. CHE-0650456 and CHE-1205189. We thank Karen
I. Goldberg, D. Michael Heinekey, William D. Jones, Maurice
Brookhart, Alan Goldman, and Melanie S. Sanford for helpful
discussions.
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(14) Some precedent has been established for a radical mechanism
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