Olefin oxygenation by water on an iridium center

Article abstract of DOI:10.1021/jacs.5b03055

Oxygenation of 1,5-cyclooctadiene (COD) is achieved on an iridium center using water as a reagent. A hydrogen-bonding interaction with an unbound nitrogen atom of the naphthyridine-based ligand architecture promotes nucleophilic attack of water to the met

Full text of DOI:10.1021/jacs.5b03055

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Olefin Oxygenation by Water on an Iridium Center
Tapas Ghatak, Mithun Sarkar, Shrabani Dinda, Indranil Dutta, S. M. Wahidur Rahaman, and Jitendra K. Bera
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 07 May 2015
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Olefin Oxygenation by Water on an Iridium Center
Tapas Ghatak‡, Mithun Sarkar‡, Shrabani Dinda, Indranil Dutta, S. M. Wahidur Rahaman and
Jitendra K. Bera*, †
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† Department of Chemistry and Center for Environmental Science and Engineering, Indian Institute of Technology
Kanpur, Kanpur 208016, India. Fax: +91-512-2597436; Tel: + 91-512-2597336
Supporting Information Placeholder
gives only the oxetane analogue 3 , whereas L3 ligand bearing
ABSTRACT: Oxygenation of 1,5-cyclooctadiene (COD) is
a flexible anilinyl group afforded an oxo-irida-allyl analog 4,
but the corresponding oxetane compound could not be iso-
lated. Experiments with H2O18 unambiguously establish that
the source of oxygen in the oxygenated compounds is water
and not adventitious oxygen. Density functional theory
(DFT) calculations support a ligand assisted bifunctional
achieved on an iridium center using water as a reagent. Hy-
drogen bonding interaction with an unbound nitrogen atom
of the naphthyridine-based ligand architecture promotes
nucleophilic attack of water to the metal-bound COD. Iri-
daoxetane and oxo-irida-allyl compounds are isolated, prod-
ucts which are normally accessed from reactions with H2O2
or O2. DFT studies support a ligand-assisted water activation
mechanism.
water activation pathway as
oxidative addition.
a superior alternative to
Scheme 1. Naphthyridine based ligands employed in this
work. Simultaneous metal coordination and H-bonding
interaction of water with ligand scaffold is shown for L1.
The use of water as substrate in metal-catalyzed organic
transformations has been limited. A few examples are anti-
Markovnikov hydration of alkynes,1 nitrile hydration,2 cyclic
amines to lactams,3 water-gas shift reaction4 and aldehyde-
water shift reaction.5 Grotjahn et al. reported a variety of
bifunctional metal catalysts for alkyne hydration, where pen-
dant basic groups attached to the ligands serve as the inter-
nal base for water activation.6 Photocatalytic water oxidation
in presence of organometallic complexes is a fundamentally
important reaction.7 Milstein and coworkers demonstrated
bifunctional water splitting by dearomatized [Ru–PNN]
pincer complex and subsequent light induced formation of
O2 in a stoichiometric manner.8 The water addition proceeds
via initial metal coordination followed by proton migration
to the cooperating ligand.9 Thus, the metal-ligand interplay
provides an alternate pathway for water activation that is
kinetically more accessible than the direct oxidative addition
of water to the metal.10
Scheme 2. Syntheses of 2-iridaoxetane dimers and 5-oxo-
6-iridia(1,2,3)-allyl compounds in the water reactions.
The water addition to a metal is associated with an
entropic penalty.11 Hydrogen bonding interaction in the
secondary coordination sphere may compensate for this
entropic loss. The 1,8-naphthyridine ligands L1–L3, shown in
Scheme 1, offer prospect for simultaneous metal coordination
and hydrogen bond interaction with the ligand architecture.
Such interactions promote nucleophilic attack of water to a
metal-bound substrate and thus favor hydration reactions.2a
Water reactions have been largely carried out with Pd, Ru
and Rh, but a parallel chemistry with iridium is not known.
Herein we describe the first case of olefin oxygenation by
water on an iridium center. A prolonged reaction (24 h) of
IrI(COD) with L1 in the presence of water affords a 5-oxo-6-
irida(1,2,3)-allyl compound 1 (Scheme 2). While performing
the same reaction for 6 h, a 2-iridaoxetane intermediate 2 is
isolated as a dimer which readily rearranges to 1 in solution.
Use of L2 ligand with bulky benzyl substituent at the ortho
position to the coordinating nitrogen in the oxazoline ring
Ir(I) complexes containing COD generally show limited
reactivity with molecular O2 and moisture. Removal of
bridging chlorides in [IrCl(COD)]2 by TlOTf in acetonitrile
and subsequent treatment with L1 afforded a stable complex
[Ir(COD)(L1)](OTf) (Scheme 2).12 Subsequent reaction with 10
fold excess of degassed, deionized H2O at room temperature
for 24 hours afforded the 5-oxo-6-irida(1,2,3)-allyl compound
1. The molecular structure of 1, depicted in Figure 1, shows η1,
η3−coordination of COD. The C5 carbon is oxidized to
carbonyl with a C=O distance of 1.217(1) Å. The naphthyridine
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ligand chelates the metal and is disposed trans to the allyl
1
2
3
unit (Ir–C = 2.145(8), 2.105(8) and 2.154(8) Å). The Ir–C(alkyl)
distance is 2.056(7) Å. Coordination by a water molecule
completes octahedral geometry around the metal.
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Figure 3. Molecular structure of the dicationic unit
[Ir2(C8H12O)2(L2)2] in 3.
Both the complexes are also characterized by 1H-NMR
spectroscopy (Figures S6 and S7). For complex 2, four olefinic
CH signals and two Ir-O-CH signals of COD appear in the
range between δ = 5.07–5.96 ppm, while complex 3 shows
additional oxazoline ring protons and benzylic protons in
this region, and appear together in the range between δ =
2.90–5.85 ppm. The ESI-MS of 2 resembles that of 1,
suggesting a rapid conversion under mass spectroscopy
conditions, whereas 3 shows a mass peak at m/z 1361.293
(z=1) corresponding to [M–OTf]+ (Figure S9c).
Figure 1. Molecular structure of the cationic unit
[Ir(C8H10O)(L1)(H2O)] in 1.
The 1H-NMR spectrum of 1 (Figure S4) in deuterated
acetonitrile is broadly divided into three regions. Nine
aromatic protons of the ligand appear in the range between δ
= 7.77–9.69 ppm. Six methylene protons and one methyne
proton of COD appear in the range of δ = 1.05–2.58 ppm,
while three allylic protons show signals further upfield (δ =
4.64–5.40 ppm). Signal attributable to metal-coordinated
water was not observed indicating that CD3CN possibly dis-
places the water at the metal center. The 13C-NMR showed a
characteristic signal at δ = 220.5 ppm assigned to the
carbonyl carbon. A strong band at 1666 cm–1 in the IR
spectrum supports the presence of the carbonyl group. ESI–
MS at m/z = 522.098 is attributed to [M–H2O–OTf]+ (Figure
S9a).
With the intent to isolate a possible intermediate, the
reaction was quenched after 6 h and an iridaIII-oxetane dimer
2 was obtained. Another oxetane analog 3 was also isolated
using L2. Complex 2 has a crystallographically imposed
inversion center and only half of the molecule appears in the
asymmetric unit, whereas for complex 3, bearing chiral
ligand L2, the full dimer appears in the asymmetric unit.
The intermediacy of metallaoxetanes has been proposed in
catalytic
oxygenation processes.13 A handful of metallaoxetane and
hydroxy-metalla-allyl compounds are structurally
homogeneous
and
heterogeneous
olefin
characterized which are accessed via the reaction of metal-
bound COD with H2O2 or molecular oxygen (For selected
examples, See Scheme S1).14 Klemperer and co-workers
reported oxidation of [Ir(P3O9)(COD)]2– by molecular oxy-
gen to give a four-membered 2-iridaoxetane intermediate,
which upon thermal treatment, rearranges to more stable 5-
hydroxy-6-irida(1,2,3)-allyl complex.15 Following a similar
reaction between [Rh(PhN3Ph)(COD)] and molecular
oxygen, Tejel and coworkers were able to isolate 2-
rhodaoxetane as a dimer, which after rearrangement gives a
polymeric hydroxy-allyl complex in solution.16 The formation
and reactivity of 3-rhoda-1,2-dioxolane and 2-rhodaoxetane
were extensively studied by Gal and coworkers.17 The
simplest 2-rhodaoxetane was obtained by the reaction of
Rh(I)-ethylene complex with H2O2.17a
COD oxygenation with water, assisted by naphthyridine
ligands, appears to be a reliable reaction. The use of L3
following a protocol identical to the synthesis of 1 afforded
an oxo-irida-allyl complex 4 which is characterized by X-ray
crystallography (see Figure S17, SI) as well as other
spectroscopic techniques. The nature of products clearly
depends on the type of ligands used. Both iridaoxetane 2 and
oxo-irida-allyl 1 were isolated for L1. Iridaoxetane 3 and oxo-
irida-allyl compound 4 were obtained exclusively from L2 and
L3, respectively. Compound 2, which was isolated by careful
manipulation of the reaction conditions, further converted to
1 over time with the evolution of dihydrogen gas. The evolu-
tion of H2 was confirmed by GC.18 It is therefore prudent to
assume that the formation of the oxo-irida-allyl compound
Molecular structures (Figures
2 and 3) confirm four-
membered metallaoxetane ring formation with Ir–O
distances 2.085(4) Å for 2; 2.087(6) and 2.098(5) Å for 3. One
chelate bound ligand, the olefinic bond of COD and
metallaoxetane ring around Ir constitute each monomer.
Oxygen coordination from the metallaoxetane ring of one
monomer to the other completes the sixth coordination site
of Ir and forms the dimeric, structures (Ir–O = 2.137(5) Å for
2; 2.105(7) and 2.095(8) Å for 3).
proceeds through the metallaoxetane intermediate.
A
tentative mechanism is proposed in Scheme 3. In
coordinating solvent, the metallaoxetane dimer dissociates
into two monomers.19 Allyl proton transfer to the oxetane
oxygen gives the hydroxy-metalla-allyl complex,16,20 which
Figure 2. Molecular structure of the dicationic unit
[Ir(C8H12O)(L1)]2 in 2.
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The free nitrogen present in the ligand architecture plays a
crucial role for heterolytic water dissociation and subsequent
oxygenation process.2a Under identical reaction condition,
the bipyridine (bpy) analogue [Ir(bpy)(COD)][OTf] (Figure
S18, SI), which lacks a free nitrogen, does not react with
water to give the COD-oxygenated product. A reaction
pathway for metallaoxetane formation supported by DFT
calculations is given in Scheme 4. Optimized structures of
the intermediates and the transition states are collected in
Figure S19, and a computed energy profile in acetonitrile is
given in Figure S20. A water adduct [Ir(pyNP)(COD)(H2O)]+
[1] is calculated on the potential energy surface (PES) where
water is coordinated to the metal (Ir–O = 2.109 Å) and
simultaneously hydrogen bonded to the free nitrogen N3 of
the naphthyridine unit (N3⋅⋅⋅H1 = 1.982 Å). Migration of the
water proton (H1) to the naphthyridine nitrogen affords a
metal hydroxy species [2], (Ir–O = 2.051 Å, N3–H1 = 1.038 Å).
This is an exothermic process by –4.1 kcal mol–1 and requires
small activation energy barrier (∆Ea) of 8.0 kcal mol–1.
Subsequent proton shift from the naphthyridine to the metal
center leads to an IrIII–hydride intermediate [3] with Ir–
H/OH distances 1.546/2.049 Å, respectively. Formation of [3]
is a slightly downhill process by 0.2 kcal mol–1, and the ∆Ea is
3.7 kcal mol–1. An alternative proton shift at a later stage after
the hydroxy attack to COD is, however, an energetically
uphill process (Scheme S2, Figure S21). The next logical step
is nucleophilic attack of the hydroxide to one of the COD
double bonds. The hydroxide in [3] is not suitably disposed
for such attack, but isomer [4] conversely is. The
rearrangement of [3]→[4] can be envisioned as a multi-step
process involving rearrangement of hydroxide and hydride
around the metal center, an uphill process by 7.6 kcal mol–1.
A direct oxidative addition of H2O at the metal to yield the
species [4] is an energetically difficult process with very high
∆Ea (31.1 kcal mol–1). Ligand assisted water dissociation is
thus computed to be a favorable process compared to the
direct oxidative addition.
The nucleophilic attack of hydroxy to the metal-bound
olefinic double bond leads to the intermediate [5], featuring
a four-membered metallacycle. A normal mode analysis of TS
[4–5] confirms this transformation. The ∆Ea and relative
reaction energies are 12.9 and –0.7 kcal mol–1, respectively. A
similar reaction with D2O didn’t afford deuterium incorpo-
rated product as evidenced from the 13C NMR spectrum (Fig-
ure S5) and ESI-MS discounting the possibility of a competi-
tive hydride migration to the olefinic double bond.
For the oxetane ring formation, a proton shift from O–H to
the metal is expected which would essentially lead to an Ir-
dihydrogen complex [6′]. This transformation [5]→[6′] is
found to be a highly endothermic process and demands very
high ∆Ea= 30.7 kcal mol–1. Since the reaction includes water, a
model was designed to calculate the same proton shift step
via water cluster. The proton shuttling via the water cluster is
anticipated to release the strains of the four-membered
transition structure [5–6′] and hence lower the activation
barrier. The new barrier for [5]→[6] is reduced to 13.6 kcal
mol–1. The pertinent TS structure [5–6] includes three water
undergoes
β-hydride
elimination
followed
by
1
2
3
4
5
6
7
8
dehydrogenation to afford the oxo-metalla-allyl compound.
The proposed mechanism explains why metallaoxetane
product is obtained when allylic proton is absent.17a Fur-
thermore, use of a tris-chelate capping ligand blocks the cis
position necessary for β-hydride elimination, causing the
reaction to cease at the hydroxy-metalla-allyl stage.15 In the
present study, L2 ligand possessing
a bulky benzyl
substituent impedes structural reorganization of the COD
unit, preventing allyl proton transfer and subsequent β-
hydride elimination. The final product is thus the
metallaoxetane product 3, despite the presence of COD allyl
proton. Prolonged reaction time or heating in acetonitrile do
not convert 3 to the corresponding oxo analogue. For L1 and
L3, where no such steric hindrances arise, the corresponding
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oxo-metalla-allyl complexes
obtained.
1 and 4, respectively, are
Scheme 3. Conversion of metallaoxetane to oxo-
metalla-allyl complex.
A strict anaerobic condition was maintained to exclude the
possibility of dioxygen in the reaction mixture. Nonetheless,
to confirm the source of oxygen, synthesis of 1 was carried
out with H2O18. The product was subjected to high resolution
mass spectrometry and IR analysis. On the basis of the
masses and isotopic distribution patterns, the signals at m/z
=
524.123, 565.149 and 585.172 are assigned for
[Ir(L1)(C8H10O18)]+, [Ir(L1)(C8H10O18)(CH3CN)]+
and
[Ir(L1)(C8H10O18)(CH3CN)(H2O18)]+, respectively (Figure 4),
which reveal a two unit increase of mass for the first two
cases and four unit increase of mass for the last case,
compared to O16 analogue of 1. The CO stretching band at
1666 cm–1 for complex 1 shifted to the lower energy region at
1645 cm–1, a shift of 21 cm–1, which is attributed to isotopic
replacement (Figure S12). These studies clearly show O18
incorporation in the final product the source of which is
H2O18.
molecules. The resultant product [6] is
a dihydrogen
complex (Ir–H1/H3 = 1.782/1.754 Å, H1–H3 = 0.841 Å). Removal
of H2 from [6] is an exothermic process and leads to the
oxetane monomer [7]. Overall, the transformation of [1]→[7]
is merely an isothermal process in acetonitrile. Further
removal of H2 from [7] is an exothermic process and yields
Figure 4. ESI-MS spectrum of O18 analogue of 1
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the stable oxo complex [8]. Transformation of [7]→[8] is a
complex process as proposed in Scheme 3, and not explicitly
studied here by DFT.
for the fellowship. JKB thanks Prof. Balaji Jagirdar and Prof. R
N Mukherjee for helpful discussions.
1
2
3
4
5
6
7
8
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Scheme 4. Proposed reaction pathway for 2-iridaoxetane
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Oxidative addition of water
31.1
[1]
water dissociation
[2]
[3]
[4]
H1 Transfer
from N to Ir
-4.3
3.7
Bifunctional
rearrangement
3.3
0.0
-4.1
8.0
Nucleophilic
9
12.9
attack
10
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12
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Association of
water cluster
Proton
transfer
-H2
-H2
[6']
[8]
[7]
[6]
[5]
-4.7
0.1
13.6
2.6
30.7
13
24.3
N
N
N
N
N
N
N
N
N
H1
H1
O
H1
OH2
N
H1
N
N
OH2
IrIII
IrIII
IrI
IrI
H2
H2O
[3]
[1]
[2]
[4]
N
N
O
N
N
N
N
N
N
H1
H3
N
H
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H
H
O
O
IrIII
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r
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H
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H
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H
[5]
[7]
[6]
N
N
=
N
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N
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Activation barrier
)
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In conclusion, we present here the first report of COD
oxygenation by water on an iridium center aided by an un-
bound nitrogen atom on the naphthyridine ligand.
Metallaoxetane and oxo-metalla-allyl compounds, which are
potential intermediates in the catalytic oxygenation process,
are isolated with dihydrogen as the side product. The steric
bulk and rigidity of the ligands dictate the nature of the
products. A ligand assisted water activation pathway is pro-
posed supported by DFT studies. The challenge remains to
make this reaction catalytically viable.
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M. M.; Gal, A. W. Angew. Chem. Int. Ed. 2001, 40, 2106. (d) Krom, M.;
Coumans, R. G. E.; Smits, J. M. M.; Gal, A. W. Angew. Chem. Int. Ed.
2002, 41, 576. (e) de Bruin, B.; Brands, J. A.; Donners, J. J. J. M.; Donꢀ
ners, M. P. J.; de Gelder, R.; Smits, J. M. M.; Gal, A. W.; Spek, A. L,
Chem. Eur. J. 1999, 5, 2921.
ASSOCIATED CONTENT
Supporting Information
Detailed experimental procedures, supporting schemes and
figures, and crystallographic table. This material is available
free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
(18) Calculated amount of [IrCl(COD)]2, TlOTf and L1 were taken in
5ml acetonitrile in a sealed reaction vessel. 10 fold excess of degassed,
deionized water was added and the reaction mixture was allowed to stir at
room temperature. The evolved gas was injected into the GC instrument.
H2 was identified by comparing with the retention time of the authentic
sample.
(19) ESIꢀMS of 3 in acetonitrile reveals a major signal (100%) for
[Ir(C8H8O)(L2)]+ along with a minor signal (<2%) for dimer suggesting
dissociation in solution (see, Figure S11, SI).
Corresponding Author
jbera@iitk.ac.in
Author Contributions
‡These authors contributed equally to this work.
Notes
The authors declare no competing financial interests.
(20) del Río, M. P.; Ciriano, M. A.; Tejel, C. Angew. Chem. Int. Ed.
2008, 47, 2502.
ACKNOWLEDGMENT
Dedicated to the memory of Professor Guy Lavigne. JKB
thanks DST for the Swarnajayanti fellowship. ID thanks CSIR
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+
+
2+
N
N
OH2
N
N
N
N
O
N
O
IrIII
H2O
6h, RT
IrI
IrIII
RT
N
N
IrIII
N
N
N
O
N
24h
N
N
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
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40
41
42
43
44
45
46
47
48
49
50
51
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54
55
56
57
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60
N
=
N
N
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