Activating a Peroxo Ligand for C?O Bond Formation

Article abstract of DOI:10.1002/anie.201808840

Dioxygen activation for effective C?O bond formation in the coordination sphere of a metal is a long-standing challenge in chemistry for which the design of catalysts for oxygenations is slowed down by the complicated, and sometimes poorly understood, mechanistic panorama. In this context, olefin–peroxide complexes could be valuable models for the study of such reactions. Herein, we showcase the isolation of rare “Ir(cod)(peroxide)” complexes (cod=1,5-cyclooctadiene) from reactions with oxygen, and then the activation of the peroxide ligand for O?O bond cleavage and C?O bond formation by transfer of a hydrogen atom through proton transfer/electron transfer reactions to give 2-iradaoxetane complexes and water. 2,4,6-Trimethylphenol, 1,4-hydroquinone, and 1,4-cyclohexadiene were used as hydrogen atom donors. These reactions can be key steps in the oxy-functionalization of olefins with oxygen, and they constitute a novel mechanistic pathway for iridium, whose full reaction profile is supported by DFT calculations.

Full text of DOI:10.1002/anie.201808840

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Accepted Article
Title: Activating a Peroxo Ligand for C-O Bond Formation
Authors: M. Pilar del Río, Paula Abril, José A. López, Mariona Sodupe,
Agustí Lledós, Miguel A. Ciriano, and Cristina Tejel
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To be cited as: Angew. Chem. Int. Ed. 10.1002/anie.201808840
Angew. Chem. 10.1002/ange.201808840
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10.1002/anie.201808840
Angewandte Chemie International Edition
COMMUNICATION
Activating a Peroxo Ligand for C–O Bond Formation
M. Pilar del Río,[a] Paula Abril,[a] José A. López,[a] Mariona Sodupe,[b] Agustí Lledós,*[b] Miguel A.
Ciriano,[a] and Cristina Tejel*[a]
Abstract: Dioxygen activation for effective C−O bond formation in
the coordination sphere of a metal is a long standing challenge in
chemistry for which the design of catalysts for oxygenations is
slowed down by the complicated −sometimes poorly understood−
mechanistic panorama. In this context, olefin-peroxide complexes
could be valuable models for the study of such reactions. Herein, we
showcase the isolation of rare ‘Ir(cod)(peroxide)’ complexes (cod
=1,5-cyclooctadiene) from reactions with oxygen, and then the
activation of the peroxide ligand for O−O bond cleavage and C−O
bond formation by transfer of a hydrogen atom through PT/ET
reactions to give 2-iradaoxetane complexes and water. 2,4,6-
trimethylphenol, 1,4-hydroquinone, and 1,4-cyclohexadiene were
used as hydrogen atom donors. These reactions can be key steps in
the oxy-functionalization of olefins with oxygen and they constitute a
novel mechanistic pathway disclosed for iridium whose full reaction
profile is supported by DFT-calculations.
carbon monoxide or formic acid are the only reactions studied.[10]
Moreover, dioxygen compounds with an olefin as co-ligand are
very scarce with very few isolated,[11] or 'in situ' characterized
examples.[12,13] These complexes that display both coordination
of oxygen and olefin to the metal center can be key species to
reveal details for the interplay of these ligands leading to the
functionalization of olefins through C−O bond formation. In this
sense, inner-sphere mechanisms are commonly accepted for
late transition metals.[1c,d]
Herein we showcase the isolation and full characterization of
unusual Ir(cod)(peroxide) complexes (cod =1,5-cyclooctadiene)
and an unprecedented further activation of the peroxide ligand
leading to O−O bond cleavage and C−O bond formation through
proton transfer/electron transfer reactions.
Exposure of a benzene red suspension of [Ir(pic)(cod)] (1,
Hpic = picolinic acid)[ 14 ] to an oxygen atmosphere in the
presence of 4-methylpyridine (Mepy) gave an orange-brown
solid, further identified as the peroxide(olefin) complex
[Ir(pic)(cod)(O2)] (2), in 79 % isolated yield (Scheme 1).
Dioxygen-coordination, O=O bond cleavage, and C−O bond
formation represent an attractive sequency of reactions to yield
useful oxygenated organic compounds from raw-materials in an
environmentally friendly way.[1] Moreover, this strategy is one of
nature’s choices, in which reductive activation of molecular
oxygen at iron(II) cofactors generates high-valent iron-oxo
intermediates (Fe=O) able to insert into unactivated C−H
bonds.[ 2 ] While this strategy requires the managing of four
electrons (from M+O2 to M=O), a smoother one involves
reduction of O2 in one-electron to superoxo intermediates. In this
case, a further hydrogen atom abstraction (HAA) −to give an
hydroperoxide group− provides the thermodynamic force for the
O−O bond cleavage, as found in several iron[3] and copper[4]
enzimes and in reactions of cobalt superoxide complexes with
chatecols[5] and TEMPO-H.[6] However, the potentiality of HAA
O
O
N
O
O
O2
Mepy
O
O
N
Ir
Ir
Ir
N
O
N
O
O
1
2
3
Scheme 1. Synthesis of the peroxide(olefin) complex 2 and the 2-iridaoxetane
complex 3.
7
]
−or alternatively the most general PCET[
reactions− in
To our delight, single monocrystals of 2 suitable for X-ray
diffraction studies[15] resulted from oxygen diffusion into diluted
solutions of 1 in benzene. According to a formal description of
the complex as a Ir(III)-peroxide complex, the structure can be
described as an iridium center in a highly distorted octahedral
geometry (Figure 1, left). Indeed, the O−O bond distance
(1.472(5) Å) lies well within the metal-peroxide category.[16]
'reactivating' the two-electron reduced M−O2 adducts (i.e.
peroxide compounds) for O−O bond excision and further C−O
bond formation remains still unexplored. In this regard, iridium(I)
complexes could be suitable candidates to explore this
possibility knowing the high tendency of such complexes to
undergo two-electron redox events in their reactions with oxygen.
Indeed, a large number of iridium peroxide complexes (IrIII-O2)
−most of them having phosphanes, diphosphanes or NHCs as
ancillary ligands− have been reported since the outstanding
finding of Vaska,[8,9] for which the oxidation of phosphanes and
In addition, the IR-ATR spectrum exhibits a ν(O−O) band at
863 cm−1, assignable to a peroxide ligand,[16] which shifts to 815
cm−1 for the isotopologue [Ir(pic)(cod)(18O2)] (2*) (calculated
value 814 cm−1). Alternatively, the complex can be also
described as five-coordinate trigonal bipyramid (TBP) if the
dioxygen ligand is assumed to occupy a single coordination site.
In agreement with this proposal, the equatorial C1=C2 bond
distance is longer than the axial C5=C6, while the metal-centroid
Ct1 and Ct2 bond distances follow the opposite trend, as
expected for a larger back-donation from the metal in the
equatorial plane in TBP-geometries.[17] Accordingly, the olefinic
carbons C1 and C2 (δ = 79.3 y 78.5 ppm) were found to be high-
field shifted relative to C5 and C6 (δ = 107.4, 99.7 ppm, Figure 1,
right).
[a]
[b]
Dr. M. P. del Río, P. Abril, Dr. López, Prof. Dr. M. A. Ciriano, Dr. C.
Tejel
Departamento de Química Inorgánica, Instituto de Síntesis Química
y Catálisis Homogénea (ISQCH), CSIC-Universidad de Zaragoza
Pedro Cerbuna 12, 50009-Zaragoza (Spain)
E-mail: ctejel@unizar.es
Prof. Dr. A. Lledós, Prof. Dr. M. Sodupe
Departament de Química, Universitat Autónoma de Barcelona
Cerdanyola del Vallés, 08193-Barcelona (Spain)
E-mail: agusti@klingon.uab.es
Supporting information for this article is given via a link at the end of
the document.
This article is protected by copyright. All rights reserved.

10.1002/anie.201808840
Angewandte Chemie International Edition
COMMUNICATION
The structure of 3 shown in Figure 2 is maintained in solution
according to NMR spectra. Thus, the signals corresponding to
carbons C1 and C2 were found at δ = 0.4 and 89.7 ppm in the
13C{1H} NMR spectrum, consistently with their bonding to iridium
and oxygen, respectively. In addition, the close proximity of the
H1 and H2 protons to H9 was evidenced from the corresponding
1
cross-peaks in the H,1H-noesy spectrum, whereas the olefinic
protons H5 and H6 were found to be in close-proximity to the
ortho protons of the Mepy ligand (H15 and H19, Figure 2).
Although 2-metallaoxetanes of iridium or rhodium have been
known for a long time,[1d, 18 , 19 ] those arising from effective
coupling of oxygen with olefins have been rarely observed and
represented only by three notable examples: the mononuclear[20]
[Ir(OC8H12)(P3O9)]2− and the dinuclear[1a,21] [{Rh(OC8H12)(L)}2] (L
Figure 1. Left: Structure (ORTEP at 50% level) of the complex
[Ir(pic)(cod)(O2)] (2). Selected bond distances (Å) and angles (°): Ir−N,
2.065(4); Ir−O1, 1.988(3); Ir−O2, 2.041(4); Ir−O3, 2.098(3); Ir−Ct1, 2.030(5);
Ir−Ct2, 2.078(5); O1−O2, 1.472(5); C1−C2, 1.405(6); C5−C6, 1.375(7);
N−Ir−Ct2, 176.3(2); O3−Ir−Ct1, 101.4(2); O3−Ir−Ct3, 122.6(2); Ct1−Ir−Ct3,
135.2(2); Ct3 is the middle point of the O1,O2 bond. Right: Selected regions of
the 1H,13C-hsqc (top) and of the 1H NMR and selnOe spectra (bottom) in
CD2Cl2 upon irradiation of the proton H9, indicated with a ray.
=
diphenyltriazenide, 5,7-dimethyl-1,8-naphthyridin-2-onate)
complexes. DFT-calculations indicated that the rhodium(I)
precursors react with oxygen to give superoxide complexes
(RhII-O2 ), which are more stable than the corresponding
•
peroxides (RhIII-O2), and they initiate a dinuclear mechanism for
O−O bond cleavage.[1a] A similar mechanism was proposed by
Klemperer for the iridium(I) complex. This pathway is unrealistic
for the peroxide complex [Ir(pic)(cod)(O2)] (2), because the
corresponding superoxide redoxomers [IrII(pic)(cod)(η1-O2)] are
quite higher in energy, and consequently, a different mechanism
is operative.
DFT-calculations
(TPSSh-D3/BS2
in
CH2Cl2)
on
[IrIII(pic)(cod)(O2)] (2) indicate that the peroxide closed-shell
singlet species is much more stable than the superoxide
redoxomers [IrII(pic)(cod)(η1-O2)] in +13.8 and +17.1 kcal mol−1
in the triplet and open-shell singlet spin sates, respectively. The
calculated (ΔG) binding of O2 to 1 is −4.2 kcal mol−1.
Further information about the unprecedented transformation
of a peroxide complex into a 2-metallaoxetane compound was
gathered with the isotopologue [Ir(pic)(cod)(18O2)] (2*), which
gave [Ir(pic)(18OC8H12)(Mepy)] (3*) and H218O in the above
reaction in CH2Cl2 and, in turn, disclosed the abstraction of two
hydrogen atoms, most likely from the solvent.
Dichloromethane solutions of complex 2 decompose to give
insoluble solids and other unidentified products. Nonetheless, a
clean transformation of 2 into the 2-iridaoxetane complex
[Ir(pic)(OC8H12)(Mepy)] (3) occurs if 4-methylpyridine (Mepy) is
added (Scheme 1, 86 % isolated yield). Noticeably, complex 2
remained unaltered in a more inert solvent such as benzene
even in the presence of Mepy. The X-ray molecular structure of
3 shows an uncommon four-membered 2-iridaoxetane ring with
an intact C=C bond of the former 1,5-cyclooctadiene ligand
(Figure 2, left).
Suspecting the involvement of free-radicals in this reaction, a
definitive evidence for the participation of PT/ET reactions in the
O−O bond cleavage of the peroxo ligand was achieved by using
2,4,6-trimethylphenol (Mesityl-OH) as hydrogen atom donor.
Reaction of 2 with Mesityl-OH in benzene or dichloromethane
and in the presence of Mepy gave the 2-iridaoxetane complex
[Ir(pic)(OC8H12)(Mepy)] (3) as the sole organometallic product. In
addition, the clean formation of 3 was accompanied by the
detection and identification of water, 4-((mesityloxy)methyl)-2,6-
dimethylphenol (4), and 4-hydroxy-3,5-dimethylbenzaldehyde (5)
as by-products (Scheme 2). Furthermore, we verified that the
oxygen in 3, water, and 5 came from the peroxide ligand by 18O–
labelled experiments with [Ir(pic)(cod)(18O2)] (2*).
O
2
[Ir(pic)(cod)( 2)] ( )
H
O
O
+
Mepy
O
+
+
O
H2
[Ir(pic)( C8H12)(Mepy)]
+
OH
3
OH
OH
4
5
Scheme 2. Reaction of [Ir(pic)(cod)(O2)] (2) with Mesityl-OH and Mepy.
Figure 2. Left: Structure (ORTEP at 50% level) of the complex
[Ir(pic)(OC8H12)(Mepy)] (3). Selected bond distances (Å) and angles (°): Ir−N1,
2.060(4); Ir−N2, 2.224(4); Ir−O1, 2.015(3); Ir−O2, 2.090(3); Ir−C1, 2.046(5);
Ir−Ct2, 2.063(5); C1−C2, 1.546(6); C2−O1, 1.456(6); C5−C6, 1.380(7);
N2−Ir−C1, 167.3(2); O1−Ir−O2, 166.4(2); N1−Ir−Ct2, 171.6(2). Right: Selected
regions of the 13C{1H}-apt (top) and of the 1H,1H-noesy (bottom) NMR spectra
in CD2Cl2.
Formation of 4, 5 and water in the above reaction strongly
supports the participation of Mesityl-O• radicals. Thus, the dimer
4 results from the coupling between isomers I and IV of the
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10.1002/anie.201808840
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COMMUNICATION
Mesityl-O• radical, while the aldehyde 5 and water would require
the participation of the methylendienone (V) and hydroxyl
radicals (see below), according to the sequence of reactions
shown in Scheme 3.
The first step corresponds to the excision of the O−O bond
−through a very low-energy transition state (TS1•)− to give the
intermediate [B]•, in which the spin density is evenly located on
the iridium and one of the oxygen atoms. A further shift of the
unpaired electron from the iridium to the coordinated oxygen
bond in TS2• generates an iridium(III)-oxo species, allowing thus
the interaction of this oxo oxygen with one carbon of the C=C
bond. This step ends up with the formation of the C−O bond in
the intermediate [C]•. Spin density on this intermediate was
found to be located only on the oxygen atom of the OH group
(1.00), so that [C]• is indeed the diamagnetic complex
[Ir(pic)(OC8H12)(Mepy)] (3) and an hydroxyl radical (HO•). The
end of this hydroxyl radical to give water and the aldehyde 5 has
been already shown in Scheme 3.
CH3
OH
CH3
CH3
CH3
CH2
−H
O
O
O
OH
I
II
III
IV
OH
H2O
H
OH
H2O
H
H
CH3
H
OH
H
H
H
H
OH
OH
O
H
H
OH
We have computationally studied several alternative
pathways for the formation of the (hydroperoxide)iridium radical
([A]•). The most favourable pathway involves a stepwise proton
transfer (PT)/electron transfer (ET), with mechanistically distinct
PT and ET steps.[22] PT and Mepy coordination occur in the first
O
O
III
O
O
OH
OH
5
V
Scheme 3. Canonical forms and isomers of the Mesityl-O• radical (top) and a
plausible pathway for the formation of 5 and water (bottom).
step,
which
yields
the
diamagnetic
species
{[Ir(pic)(cod)(O2H)(Mepy)]+}{Mesityl-O−} ([A]+[Mesityl-O]−), with a
barrier of 16.1 kcal mol−1. This ion pair was found to be only 4.2
kcal mol−1 above the starting materials (2+Mepy+Mesityl-OH).
The second step is the electron transfer from the Mesityl-O− to
the iridium complex to give the radical-pair [A]•(Mesityl-O)•,
which is 22.9 kcal mol−1 above the ion pair. This step entails spin
crossing from the singlet to the triplet potential energy surfaces.
We have also analysed the functional dependence of the PT and
ET barriers. Computed barriers range from 13.5 to 18.9 kcal
mol−1 for the first one (PT), whereas the second one (ET)
−related with the change of the spin state− is more dependent
on the functional (from 15.8 to 23.3 kcal mol−1, see Supporting
Information). Overall, the stepwise PT/ET appears to be a very
plausible mechanism for the formation of the [A]• radical.
Moreover, DFT-calculations (TPSSh-D3/BS2 in CH2Cl2)
indicated that the formation of both, the dimer 4 and the
aldehyde 5 and water are exergonic, with ΔG = −13.2 and
−169.3 kcal mol−1, respectively.
With experimental confirmation of the participation of
Mesityl-O• radicals in hand, it is quite reasonable to propose the
radical-pair {[Ir(pic)(cod)(O2H)(Mepy)]•}{Mesityl-O•} ([A]•Mesityl-
O•) as an intermediate resulting from the formal transfer of a
hydrogen atom from Mesityl-OH to [Ir(pic)(cod)(O2)] (2)
followed/preceded by the coordination of 4-methylpyridine. The
transformation
of
the
(hydroperoxide)iridium
radical
[Ir(pic)(cod)(O2H)(Mepy)]• ([A]•) into the products has been
studied by DFT methods (TPSSh-D3/BS2 in CH2Cl2) in the
doublet potential energy surface (Figure 3).
Notice that oxygen activation mediated by metals requires
two steps of different (mainly opposite) nature. In a first instance
the dioxygen becomes coordinated to the metal, and thus its
electrophilic character strongly diminishes becoming quite inert.
Then, a further activation is required for O−O bond scission and
C−O bond formation. In our case, the reactivation is achieved by
the formal transference of a hydrogen atom through a PT/ET
reactions, which represents a novel pathway for O–O bond
activation and C–O bond formation.
This approach is not exclusive of Mesityl-OH. A similar
transformation of 2 into the 2-iradoxetane 3 was observed by
using p-hydroquinone as hydrogen atom donor. In this case, the
reaction was found to be accomplished with the formation of
equimolar amounts of p-quinone and water. Moreover, other H-
atom donors such as 1,4-cyclohexadiene also promotes the
reaction, to give complex 3 and benzene.
Furthermore, this protocol can be extended to other iridium-
peroxide complexes. To substantiate this point, the peroxo
formamidinate complex [Ir(PhNC(Ph)NPh)(cod)(O2)] (7, 66 %
isolated yield) was prepared and isolated as an orange solid by
reaction of [Ir(PhNC(Ph)NPh)(cod)] (6) with oxygen, and it was
fully characterized by analytical and spectroscopic methods.
Figure 3. Gibbs energy profile (TPSSh-D3/BS2 in CH2Cl2) in the doublet
potential surface for the evolution of the (hydroperoxide)iridium radical [A]•.
Values of ΔG are given in kcal mol−1. Numbers in green and red denote the
spin density on the iridium and oxygen atoms, respectively.
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10.1002/anie.201808840
Angewandte Chemie International Edition
COMMUNICATION
Complex 7 reproduces well the new protocol, giving the 2-
iridaoxetane complex [Ir(PhNC(Ph)NPh)(OC8H12)(Mepy)] (8,
61 % by 1H NMR) by reaction with Mesityl-OH in the presence of
Mepy in benzene (Scheme 4).
Keywords: Oxygenation • dioxygen cleavage • 2-iridaoxetanes •
iridium • peroxide complexes
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a) L. Vilella-Arribas, M. García-Melchor, D. Balcells, A. Lledós, J. A.
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Ph
Ph
N
N
Mepy
Ir
Ph
Ph
Ir
O
Mesityl-OH
N
N
N
Ph
O
O
Ph
[2]
a) S. C. Peck, C. Wang, L. M. K. Dassama, B. Zhang, Yi. Guo, L. J.
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8
7
Scheme 4. Synthesis of the 2-iridaoxetane complex 8.
Selective nOe experiments (selnOe) indicate that complex 8
was obtained as the stereoisomer shown in Scheme 4, with the
labile nitrogen of the formamidinate ligand placed trans to the
carbon σ-bonded to iridium. Indeed, DFT calculations on
complex 8 and the related isomer similar to 3 (with Mepy trans to
the σ-C-Ir bond) revealed that the former is 3.9 kcal mol−1 more
stable than the later.
[3]
[4]
See for example: a) E. Tamanaha, B. Zhang, Y. Guo, W.-C. Chang, E.
W. Barr, G. Xing, J. St. Clair, S. Ye, F. Neese, J. M. Bollinger, Jr., C.
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12040.
In summary, the results reported here show that iridium
peroxide complexes can be activated by proton transfer/electron
transfer reactions that yield iridium hydroperoxide radicals. If an
olefin is coordinated to the metal center such species easily
undergo O−O bond cleavage and C−O bond formation to give 2-
iridaoxetane complexes. This is a novel mechanistic pathway
disclosed for iridium in the oxygenation of olefins. Indeed, DFT-
calculations (TPSSh-D3/BS2 in CH2Cl2) of the reaction pathways
for other complexes with olefin different from cod, such as 1,4-
norbornadiene and ethylene give very similar reaction profiles
(see Supporting Information).
[5]
[6]
See for example: C. W. Anson, S. Ghosh, S. Hammes-Schiffer, S. S.
Stahl, J. Am. Chem. Soc. 2016, 138, 4186-4193 and references therein.
C.-C. Wang, H.-C. Chang, Y.-C. Lai, H. Fang, C.-C. Li, H.-K.i Hsu, Z.-Y.
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2012, 112, 4016-4093.
[7]
[8]
See for example: a) H. Baumgarth, G. Meier, T. Braun, B. Braun-Cula,
Eur. J. Inorg. Chem. 2016, 4565-4572; b) M. Feller, E. Ben-Ari, Y.
Diskin-Posner, R. Carmieli, L. Weiner, D. Milstein, J. Am. Chem. Soc.
2015, 137, 4634−4637; c) C. Schiwek, J. Meiners, M. Förster, C.
Würtele, M. Diefenbach, M. C. Holthausen, S. Schneider, Angew.
Chem. 2015, 127, 15486–15490; Angew. Chem. Int. Ed. 2015, 54,
15271-15275; d) A. V. Polukeev, O. F. Wendt, Organometallics 2015,
34, 4262-4271; e) H. Baumgarth, T. Braun, B. Braun, R. Laubenstein, R.
Herrmann, Eur. J. Inorg. Chem. 2015, 3157-3168.
The reported reactions herein can be key steps in the oxy-
functionalization of olefins with air. Also relevant is the
information on the destiny of the second oxygen atom of the
coordinated dioxygen, which is intercepted by the H-atom donor
(Mesityl-OH, p-hydroquinone, and 1,4-cyclohexadiene) to give
water, avoiding thus reactive oxygen species and unselective
side-reactions in aerobic oxygenations. The reported findings
highlight the potentiality of peroxide intermediates in
oxygenation, providing innovative possibilities to enable the
design of new and more selective and/or reactive oxygenation
catalysts.
[9]
L. Vaska, Science 1963, 140, 809-810.
[10] See for example: a) H. Baumgarth, T. Braun, B. Braun, R. Laubenstein,
R. Herrmann, Eur. J. Inorg. Chem. 2015, 3157-3168; b) S. Dürr, B.
Zarzycki, D. Ertler, I. Ivanović-Burmazović, U. Radius, Organometallics
2012, 31, 1730-1742; c) W. I. Dzik, J. M. M. Smits, J. N. H. Reek, B. de
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Acknowledgements
The generous financial support from MINECO/FEDER (Projects
CTQ2014-53033-P and CTQ2017-83421-P, C.T.; CTQ2017-
87889-P, A.L. and CTQ2017-89132-P, M.S.), and Gobierno de
Aragón (GA/FEDER, Inorganic Molecular Architecture Group
E08_17R; C.T.) is gratefully acknowledged. M.P.d.R. and P.A.
thank MINECO/FEDER for a JdC contract and a FPI fellowship,
respectively.
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Entry for the Table of Contents
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Taming oxygen for effective cleavage
of the O−O bond and a further C−O
bond formation has been achieved
through proton transfer/electron
transfer protocols. A free radical
mechanism accounts for the
transformation of peroxide complexes
into water and 2-iradaoxetane
complexes as shown in the Figure.
Code color: Ir (green), O (red), N
(blue), C (gray).
M. Pilar del Río, Paula Abril, José A.
López, Mariona Sodupe, Agustí Lledós,*
Miguel A. Ciriano, and Cristina Tejel*
Page No. – Page No.
Activating a Peroxo Ligand for C–O
Bond Formation
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