Formate-driven catalysis and mechanism of an iridium-copper complex for selective aerobic oxidation of aromatic olefins in water

Article abstract of DOI:10.1039/d0sc06634f

A hetero-dinuclear Ir^III-Cu^IIcomplex with two adjacent sites was employed as a catalyst for the aerobic oxidation of aromatic olefins driven by formate in water. An Ir^III-H intermediate, generated through formate dehydrogenation, was revealed to activate terminal aromatic olefins to afford an Ir-alkyl species, and the process was promoted by a hydrophobic [Ir^III-H]-[substrate aromatic ring] interaction in water. The Ir-alkyl species subsequently reacted with dioxygen to yield corresponding methyl ketones and was promoted by the presence of the Cu^IImoiety under acidic conditions. The Ir^III-Cu^IIcomplex exhibited cooperative catalysis in the selective aerobic oxidation of olefins to corresponding methyl ketones, as evidenced by no reactivities observed from the corresponding mononuclear Ir^IIIand Cu^IIcomplexes, as the individual components of the Ir^III-Cu^IIcomplex. The reaction mechanism afforded by the Ir^III-Cu^IIcomplex in the aerobic oxidation was disclosed by a combination of spectroscopic detection of reaction intermediates, kinetic analysis, and theoretical calculations.

Full text of DOI:10.1039/d0sc06634f

Showcasing research from Dr. Hong’s research group at
Innovative Oxidation Team, Interdisciplinary Research
Centre for Catalytic Chemistry, National Institute of
Advanced Industrial Science and Technology (AIST),
Tsukuba, Japan.
As featured in:
Formate-driven catalysis and mechanism of an
iridium–copper complex for selective aerobic oxidation
of aromatic olefins in water
We report a hetero-dinuclear IrIII-CuII complex with two
adjacent sites that successfully catalyses the aerobic
oxidation of aromatic olefins driven by formate to produce
corresponding methyl ketones, and the reactions are
promoted by a hydrophobic interaction between an Ir(H)-Cu
intermediate and the substrate aromatic ring in water.
See Dachao Hong et al.,
Chem. Sci., 2021, 12, 5796.
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Formate-driven catalysis and mechanism of an
iridium–copper complex for selective aerobic
oxidation of aromatic olefins in water†
Cite this: Chem. Sci., 2021, 12, 5796
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
Yoshihiro Shimoyama, a Yasutaka Kitagawa, b Yuji Ohgomori,a Yoshihiro Kon
ac
ac
*
and Dachao Hong
A hetero-dinuclear IrIII–CuII complex with two adjacent sites was employed as a catalyst for the aerobic
oxidation of aromatic olefins driven by formate in water. An IrIII–H intermediate, generated through
formate dehydrogenation, was revealed to activate terminal aromatic olefins to afford an Ir-alkyl species,
and the process was promoted by a hydrophobic [IrIII–H]-[substrate aromatic ring] interaction in water.
The Ir-alkyl species subsequently reacted with dioxygen to yield corresponding methyl ketones and was
promoted by the presence of the CuII moiety under acidic conditions. The IrIII–CuII complex exhibited
cooperative catalysis in the selective aerobic oxidation of olefins to corresponding methyl ketones, as
evidenced by no reactivities observed from the corresponding mononuclear IrIII and CuII complexes, as
the individual components of the IrIII–CuII complex. The reaction mechanism afforded by the IrIII–CuII
complex in the aerobic oxidation was disclosed by a combination of spectroscopic detection of reaction
intermediates, kinetic analysis, and theoretical calculations.
Received 4th December 2020
Accepted 15th March 2021
DOI: 10.1039/d0sc06634f
rsc.li/chemical-science
oxidations of organic substrates.19–23 Biomimetic metal
complexes were, in fact, reported to possess a hydrophobic
Introduction
environment around their reaction sites capable of suppressing
over-oxidations. In addition, water used as a reaction solvent
can promote hydrophobic interactions between organic
substrates and reactive intermediates of metal complexes in the
Molecular oxygen (O2) is an ideal oxidant for the catalytic
oxidation of organic substrates as water is generated in the
reactions without producing any waste products.1–3 Construct-
ing a system that enables regio/stereoselective or partial
oxidations of substrates by O2 for ne chemical and pharma-
ceutical productions is particularly desirable.4,5 In nature,
regioselective oxidations are accomplished by cytochrome
P450,6–8 MMO (methane monooxygenase),9,10 and DbM (dopa-
mine b-monooxygenase)11,12 that function as catalysts to mon-
ohydroxylate steroid derivatives, methane, and dopamine under
air, respectively. These enzymes accomplish such selective
oxidations by promoting interactions between the organic
substrates and the enzyme active site which oen includes
hydrophobic cavities or non-covalent interaction centers.13–18
Inspired by these enzymes, articial catalysts such as metal
complexes have been utilized for selective and partial
reaction centres, playing
reactivity.24–26
a critical role in enhancing
For the aerobic oxidations performed in aqueous solutions,
Wacker-type catalysts have been widely studied in basic and
applied synthesis over the past decades.27,28 The Wacker-type
systems are oen composed of a palladium catalyst and an
electron mediator such as copper(II) ions,27 polyoxometalates,29
or quinone derivatives.30 Metal–hydride (M–H) species have
been receiving increasing attention in the Wacker-type oxida-
tions because, by using these species, in contrast to the palla-
dium case, the catalysis exhibits unique behaviour and
proceeds without the need of an additional electron medi-
ator.31,32 This approach, however, is limited by the difficulty of
generating M–H species by reactive hydride sources, which are
reductants not usually adopted in an aqueous solutions for
oxidation reactions. Thus, a method that allows us to generate
M–H species in aqueous solution and that allows us to build
a hydrophobic environment suitable for selective oxidations of
organic substrates would be highly desirable.
aInterdisciplinary Research Center for Catalytic Chemistry, National Institute of
Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki
305-8565, Japan. E-mail: hong-d@aist.go.jp
bDepartment of Materials Engineering Science, Graduate School of Engineering
Science, Osaka University, 1-3 Machikaneyama-cho, Toyonaka, Osaka 560-8531,
Japan
cGlobal Zero Emission Research Center, National Institute of Advanced Industrial
The formation of a M–H species in aqueous solutions has
been previously reported by using a mild hydride source such as
formic acid.33–36 To the best of our knowledge, there have been
no reports on selective aerobic oxidation of olens to methyl
Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan
† Electronic supplementary information (ESI) available: NMR and UV-vis
absorption spectra, electrochemical measurements, table and Cartesian
coordinates of Ir–Cu species. See DOI: 10.1039/d0sc06634f
5796 | Chem. Sci., 2021, 12, 5796–5803
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ketones in water catalysed by M–H species that can exhibit the Wacker-type oxidation of StyreneS was quantied by 1H
a hydrophobic interaction with substrates. Mechanistic insights NMR spectroscopic analysis using DSS (sodium 4,4-dimethyl-4-
into selective aerobic oxidations performed by M–H species in silapentanesulfonate) as an internal standard (Fig. S1†). The
water have yet to be gained. Herein, we report that an IrIII–CuII selectivity for methyl-ketone reached to almost 100% and only
hetero-dinuclear complex37 successfully catalyses Wacker-type a negligible formation of aldehyde was observed from the
oxidation of aromatic olens driven by formate to produce reaction on the basis of the 1H NMR spectral analysis. The
corresponding methyl ketones in water. We found that results for the selective aerobic oxidation of StyreneS to Ace-
a hydrophobic interaction between an Ir–H intermediate and tophS in formate buffer are summarized in Table 1. The use of 1
the substrate aromatic ring in water promoted the selective as a catalyst successfully affords AcetophS with yield and TON
aerobic oxidation of aromatic olens. We have revealed that the values of 11 and 112, respectively (Table 1, entry 1). Control
iridium centre worked as the organometallic activation site of experiments were conducted to investigate the cooperative
formate and olens, while the copper centre serves as the catalysis of 1 using the mononuclear metal complexes (2–4),
electronic modulating site by improving the overall reactivity which constitute the moiety of 1, as catalysts. The IrIII and CuII
toward dioxygen under acidic conditions. We discuss the complexes used as mononuclear catalysts, however, did not
mechanistic insights into the catalytic Wacker-type oxidation of show any reactivities towards StyreneS oxidation (Table 1,
water-soluble styrene derivatives performed by the IrIII–CuII entries 2–5). The results highlight the exceptional catalysis of 1
complex based on spectroscopic detection of reaction inter- obtained for the selective aerobic oxidation of StyreneS to
mediates, kinetic analysis, and theoretical calculations.
AcetophS.
In addition, no reactivity was observed in the absence of
a catalyst and formic acid or under an Ar atmosphere (entries 6–
8), showing that both oxygen and formic acid are needed for the
aerobic oxidation of StyreneS to generate AcetophS. As previ-
ously reported,37 hetero-dinuclear metal complexes, IrIII–CoII
and IrIII–NiII, were also employed in the oxidation reactions but
did not catalyse the Wacker-type oxidation of StyreneS to Ace-
tophS (Table S1†). This result is ascribed to be the higher H2
evolution rates obtained by using Ir–Ni and Ir–Co complex as
compared to that of the Ir–Cu complex shown in our previous
report.37 Additionally, another possible explanation can be that
the CoII and NiII centres of IrIII–CoII and IrIII–NiII complexes are
allowed from their sixth coordination to form an octahedral
Results and discussion
The IrIII–CuII complex (1) and reference catalysts, [(Cp*)
IrIIICl(Hbpp)](BF4) (Ir-Hbpp, 2), [CuII(dipic)(OH2)2] (Cu-dipic, 3)
and [(Cp*)IrIIICl(Mebpp)](BF4) (Ir-Mebpp, 4) as shown in Fig. 1a,
were synthesized and characterized according to literature
procedures.37 The aerobic oxidation reactions afforded by 1–4
were conducted using p-styrenesulfonate (StyreneS) as a water-
soluble model substrate (Fig. 1b). Acetophenonesulfonate
(AcetophS) as the corresponding methyl ketones obtained from
Table 1 Yields and TONs obtained from the catalytic aerobic oxidation
of StyreneS in aqueous solutionsa
AcetophS
Entry
Cat.
Yield, %
TON
1
2
3
4
5
6
7
8
Ir–Cu (1)
Ir-Hbpp (2)
11
112
n/a
n/a
n/a
n/a
n/a
n/a
n/a
263
232
580
n/a
n/a
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
n.d.
26
23
58
n.d.
n.d.
Cu-dipic (3) + Ir-Mebpp (4) (1 : 1)
Ir-Mebpp (4)
Cu-dipic (3)
—
Ir–Cu (1)b
Ir–Cu (1)c
9
Ir–Cu (1)d
10
11
12
13
Ir–Cu (1)e
Ir–Cu (1)f
Cu-dipic (3) + Ir-Mebpp (4)f
Ir–Cu (1)g
a
Conditions: [cat.] ¼ 0.10 mM, [HCOOH] ¼ 0.20 M, [StyreneS] ¼ 0.10 M,
solvent: D2O (1.0 mL), reaction temp.: 70 ꢀC. Reaction time: 24 h. Under
air. Yield (%) ¼ 100 ꢁ ([product]/[StyreneS]), TON ¼ [product]/[cat.].
b
c
d
e
Under Ar. Without HCOOH. With excess of 3 (5.0 mM). With
f
excess of 3 (10 mM). With excess of 3 (5.0 mM) in formate buffer
g
Fig. 1 (a) Catalysts 1–4 used in this work. (b) Selective aerobic
oxidation of p-styrenesulfonate (StyreneS) to acetophenonesulfonate
(AcetophS) in water.
(0.60 M, [HCOOH] : [HOCONa] ¼ 3 : 1). With excess of 3 (5.0 mM) in
DMA/D2O (5 : 1, v/v) (0.60 M formate buffer, [HCOOH] : [HOCONa] ¼
3 : 1).
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geometry that may interfere with the access of StyreneS as StyreneS, as shown in Fig. 2 and S4.† The absorption band with
compared to a pseudo-square-pyramidal geometry of the CuII a shoulder at 380 nm, which can be assigned to the IrIII–H
center in the IrIII–CuII complex. These results demonstrate that species in 1 (Ir(H)-Cu),37 was observed in formate buffer under
the copper centre in 1 plays a signicant role in providing Ar in the absence of StyreneS (Fig. S4a†). The absorption band
a specic reaction environment suitable for the selective aerobic showed almost no changes by introducing O2 into the solution
oxidation of StyreneS to AcetophS.
without StyreneS (Fig. S4b†). The result suggests that no reac-
As evidenced by ESI-TOF-MS measurements (Fig. S2†), tions were taking place between Ir(H)-Cu and O2. On the other
complex 1 was found to dissociate to complexes 2 and 3 during hand, the further absorption growth at 405 nm was observed
the oxidation reactions, resulting in its deactivation. The aer formation of Ir(H)-Cu in the presence of StyreneS under Ar
dissociation may go through hydrolysis of 1 with an equilibrium (Fig. 2a). When Ar was replaced with O2, the absorption band
between 1 and the corresponding mononuclear Ir and Cu immediately decreased and saturated within 1500 seconds
complexes under high proton concentrations at 70 ꢀC. The (Fig. 2b). These results indicate that Ir(H)-Cu in formate buffer
addition of an extra amount of complex 3 (5.0–10 mM) into the selectively reacted with StyreneS followed by reaction with
reaction solutions containing 1 signicantly enhanced both the molecular oxygen. The excess of 3 added did not interfere with
reaction yields and TONs (Table 1, entries 9 and 10). The pres- the aerobic oxidation reaction although its absorption appeared
ence of an excess of 3 in the reaction mixtures may, thus, shi around 780 nm due to the d–d transitions of 3 (Fig. S5†).
the equilibrium towards 1 maintaining the structure of the Additionally, the continuous absorption growth at 405 nm was
active catalyst to afford the high catalytic activity observed here not observed in the absence of excess of 3 (Fig. S6†), probably
for the aerobic oxidation of StyreneS. The reaction conditions due to the dissociation of the Cu-dipic moiety from 1 as evi-
were, then, optimized by altering the formic acid/sodium denced by Fig. S2.† The UV-vis absorption spectral changes
formate concentration ratios as well as their absolute concen- observed here are consistent with the catalytic results that the
trations. The highest TONs for 1 were obtained at the 3 : 1 (mol addition of an excess of 3 improves the yield of AcetophS due to
molꢂ1) [HCOOH] : [HCOONa] ratio (Fig. S3a†) and at a formate preservation of the Ir(H)-Cu active structure.
concentration of 0.60 mM (Fig. S3b†). Using the optimized
Based on the UV-vis spectral changes of 1 observed in
conditions, the yield of AcetophS and the reaction TON for 1 formate buffer under Ar, the Ir(H)-Cu can be assumed to react
reached 58% and 580, respectively, in the selective aerobic with StyreneS to form an Ir-alkyl complex, [(Cp*)Ir(C8H8SO3)(-
oxidation of StyreneS (Table 1, entry 11). Additionally, no bpp)Cu(dipic)]ꢂ (Ir(EtBnS)-Cu) (Scheme 1). Olen insertion into
products were obtained from the reaction solution of 4 with the M–H bond of mononuclear metal complexes has been
excess of 3 under the same optimized conditions (Table 1, entry previously reported to form iridium-alkyl species.38–41 The
12), suggesting the crucial role played by the presence of Cu(II) formation of Ir(EtBnS)-Cu was supported by our electron-spray
adjacent to Ir(III) to afford the aerobic oxidation. Furthermore, ionization mass spectrometry (ESI-MS) measurements in
the use of a less polar solvent, N,N-dimethylacetamide (DMA)/ which a peak cluster at m/z ¼ 1006.96, assigned to be [[(Cp*)
D2O (5 : 1, v/v), resulted in no production of AcetophS in the Ir(C8H8SO3)(bpp)Cu(dipic)]ꢂ + HCOOH]ꢂ (calcd. for [M]ꢂ: m/z ¼
Wacker-type oxidation, even under the optimized conditions 1007.12), was observed (Fig. S7a†). The peak cluster was not
(Table 1, entry 13). This result suggests that the use of water may obtained in the absence of StyreneS under Ar (Fig. S7b†). We
promote a hydrophobic interaction between 1 and StyreneS to have attempted to detect Ir(EtBnS)-Cu by 1H NMR spectroscopy;
enable the selective aerobic oxidation shown here.
however, the paramagnetic nature of the CuII center did not
To reveal the reaction mechanism for the selective aerobic allow for a satisfactory peak assignment. When using DMA/H2O
oxidation of StyreneS afforded by 1, we measured UV-vis (5 : 1, v/v) as a solvent, the absorption growth at 405 nm derived
absorption spectral changes of 1 in the presence/absence of from the formation of Ir(EtBnS)-Cu was not observed (Fig. S8†).
Considering that the Wacker-type oxidation did not proceed in
DMA/H2O solvent (Table 1, entry 13), the results clearly indicate
that Ir(EtBnS)-Cu is formed only in net water, which is a key
nding of this work.
Since metal-hydroperoxide (M–OOH) species have so far
been proposed as intermediates in olen oxidations to produce
Fig. 2 UV-vis absorption spectral changes of 1 (0.10 mM) with 3 (5.0
mM) in formate buffer (0.60 M, [HCOOH] : [HCOONa] ¼ 3 : 1) in the
presence of 0.10 M StyreneS (a) under Ar (0–9000 s) and (b) under O2
(9000–10500 s) at 70 ꢀC. O2 gas was introduced into the solution at Scheme 1 Schematic representation of Ir(EtBnS)-Cu formation from
9000 s.
the reaction between Ir(H)-Cu and StyreneS in water.
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corresponding ketones using late-transition metal catalysts, we
A p–p stacking of aromatic substrates with catalysts as a pre-
have employed H2O2 as an oxidant instead of O2 and HCOOH to equilibrium process has been proposed before.47 The occur-
examine the selective oxidation of StyreneS by 1.42–45 Remark- rence of the p–p interaction between substrates and the bpp
ably, AcetophS was not produced in the oxidation of StyreneS ligand of 1 was demonstrated experimentally by oxidation of
when using H2O2 in the place of O2 and HCOOH (Table S2†). other terminal olens with/without an aromatic ring including
Therefore, we conclude that the oxidation of StyreneS per- 4-vinylbenzoic acid, 4-vinylpyridine, allylphenylethersulfonate
formed by 1 involved the formation of Ir(EtBnS)-Cu generated and allylsulfonate (Table 2); only the substrates having a vinyl
by the reaction between Ir(H)-Cu and StyreneS through olen group next to the aromatic ring were oxidized by 1 to produce
insertion into an IrIII–H bond instead of an Ir–OOH species.
the corresponding methyl ketones. Although the allylpheny-
In order to further support the formation of Ir(EtBnS)-Cu, we lether derivative can in principle engage in a p–p interaction
conducted theoretical calculations on the optimized structures with Ir(H)-Cu as StyreneS, the vinyl group lies too far from the
of Ir(EtBnS)-Cu (Fig. 3). The calculations showed a clear p–p Ir–H bond as a reactive site in this situation. The substrate
interaction occurring between the benzene ring of StyreneS and limitation observed clearly reects the requirement for
46
ꢀ
the bpp ligand (ca. 3.1 A) in Ir(EtBnS)-Cu (Fig. 3a). This a hydrophobic p–p interaction between the substrate aromatic
hydrophobic p–p interaction corroborates the hypothesis of an rings and 1 in water, shedding light on the mechanism of this
important water solvent contribution to the driving force for the reaction.
formation of Ir-alkyl species. The energy of the hydrophobic p–
In order to gain more insights into the formation of
p interaction of Ir(EtBnS)-Cu was roughly estimated to be Ir(EtBnS)-Cu, we performed kinetic studies under Ar, as shown
32.3 kcal molꢂ1 by the energy difference between single-point in Fig. 4. The absorption spectral changes observed at 405 nm,
calculations at the UB3LYP and UB3LYP-D3BJ levels of theory derived from Ir(EtBnS)-Cu, were analysed based on pseudo-rst-
(see ESI† for details on the computation of this energy). It order kinetics, which exhibited an apparent rate constant (kobs
)
should be noted that the energy values relative to the p–p of 3.7 ꢁ 10ꢂ4 sꢂ1 (Fig. 4a). The kobs (sꢂ1) obtained was plotted
interactions can be overestimated by the calculations and, thus, against the StyreneS concentrations and the kinetics exhibited
they should be taken into account for a qualitative rather than a saturation behaviour (Fig. 4b). Both the rst-order rate
a quantitative analysis. The results from our TD-DFT calcula- constant (kH1 ) and the equilibrium constant (KH) were deter-
tions also indicate that the oscillator strength of Ir(EnBnS)-Cu mined to be 4.7 ꢁ 10ꢂ4 sꢂ1 and 3.6 ꢁ 10 Mꢂ1, respectively,
exhibits good consistency in the region of the observed based on a pre-equilibrium between the benzene ring of
absorption band at 405 nm, which can be assigned to the metal- StyreneS and the bpp ligand (Fig. 4c). The p–p stacked structure
to-ligand charge transfer (MLCT) band from dp orbitals of IrIII of the adduct (Ir(H)-CujjjStyreneS) was obtained from DFT
(208A, 209A, 207B and 208B) to a p* orbitals of bpp and dipic calculations (Fig. S9†) and the stabilization energy derived from
ligands (211A and 211B) (Fig. 3b and c).
the p–p interaction was roughly estimated to be
Table 2 Yields and TONs obtained from the selective aerobic oxida-
tion of aromatic olefins by 1 in watera
Substrates
Products
Yield, %
TON
58
580
3
33
17
169
Fig. 3 (a) Optimized structures of Ir(H)-Cu (left) and Ir(EtBnS)-Cu
(right) as the reaction intermediates calculated at the UB3LYP-D3BJ/
LanL2TZ(f) (Ir), 6-31G* (Cu, H, C, O, N), 6-31+G* (S) level of theory. (b)
UV-vis absorption spectra of Ir(H)-Cu (black) and Ir(EtBnS)-Cu (red)
formed at 30 s and 9000 s respectively after addition of 1 under Ar
overlaid with TD-DFT calculations on the optimized structures of
Ir(H)-Cu (black trace) and Ir(EtBnS)-Cu (red trace). (c) Contributing
molecular orbitals of Ir(EtBnS)-Cu (208A, 209A, 211A, 207B, 208B, and
211B) to the absorption at 410.22 nm and 375.14 nm obtained from the
structural optimization.
n.d.
n.d.
n/a
n/a
a
Conditions: [1] ¼ 0.10 mM, [3] ¼ 5.0 mM, solvent: 0.60 M formate
buffer ([HCOOH] : [HCOONa] ¼ 3 : 1) in D2O (1.0 mL), [substrate] ¼
0.10 M, reaction temp.: 70 ꢀC. Reaction time: 24 h. Under air. Yield
(%) ¼ 100 ꢁ ([product]/[StyreneS]), TON ¼ [product]/[cat.].
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calculations on Ir(EtBnS)-Cu and Ir(EtBnS)-bpp (both have the
same charge) as shown in Fig. S12 and Table S3.† Virtually no
¨
difference in the Mulliken charges have been found on both the
ligands and the iridium center (Table S3†). It is notable that the
O2 insertion into iridium-alkyl complexes occurs only when
copper(II) is present in the moiety. Although the Ir-bpp complex,
which is the deprotonated form of 2, cannot form under the
acidic catalytic conditions, it is stabilized by the presence of the
Cu(II) moiety in 1. The Cu(II) center would help to modulate the
electronic properties of the Ir(EtBnS)-Cu intermediate, allowing
a more favorable O2 insertion in contrast to Ir(EtBnS)-Mebpp by
lowering the redox potential. It should be noted that our DFT
calculations were carried out only for the olen insertion step to
form Ir(EtBnS)-Cu.; thus, a possible inuence of electronic
effects derived from Cu(II) in other steps of the catalytic cycle
cannot be ruled out.
Fig. 4 (a) Time-profile of absorbance at 405 nm (black dots) and the
fitting curve (red line) obtained from 1 (0.10 mM) with 3 (5.0 mM) in
formate buffer (0.60 M, [HCOOH] : [HCOONa] ¼ 3 : 1) in the presence
of 0.10 M StyreneS under Ar. (b) The determined kHobs (red trace) and
kDobs (blue trace) values against StyreneS concentration (0.050–0.20 M)
and the fitting curve (red line). (c) Schematic representation of the pre-
equilibrium process.
On the basis of the combined experimental and theoretical
evidence, the catalytic cycle of the dioxygen-coupled oxidation
of terminal olens performed by 1 is proposed as shown in
Scheme 2. First of all, the Ir–hydride complex (Ir(H)-Cu),
generated by the reaction of 1 and formate, forms an adduct
(Ir(H)-CujjjStyreneS) due to a p–p interaction between the
23.1 kcal molꢂ1. The use of D2O as a solvent gave kD1 and KD substrate aromatic rings and the bpp ligand, in a pre-
values of 3.5 ꢁ 10ꢂ4 sꢂ1 and 3.7 ꢁ 10 Mꢂ1, respectively, and the equilibrium process. An iridium-alkyl complex (Ir(EtBnS)-Cu)
kinetic isotope effect (KIE, kH1 /kD1 ) was determined to be 1.4. On is then formed by the insertion of the terminal olen in the Ir–H
the other hand, KH/KD was determined to be 1.0, indicating no
bond as the R. D. S. of the catalytic cycle. Molecular oxygen
isotope effect on the pre-equilibrium between the Ir–H species inserts into the IrIII-alkyl bond to produce a putative iridium-
and StyreneS. Considering that the iridium-hydride species alkylperoxo complex (Ir(OOEtBnS)-Cu). Finally, the heterolysis
undergoes H/D exchange with the solvent as evidenced previ- of the O–O bond gives the corresponding methyl ketone and the
ously,37 the KIE of k1H/kD1 indicates that the olen insertion into initial Ir(H)-Cu complex is regenerated from formic acid. The
the Ir–H bond is involved in the rate-determining step (R. D. S.). metal-alkylperoxido species formed by the O2 insertion into the
In comparison to the StyreneS oxidation performed by 1, metal-alkyl bond has been previously reported49,50 and was also
complex 4 showed no reactivity with the same substrates proposed as a reaction intermediate in Wacker-type oxidations
of olens.31,32 The DFT-optimized structure and the electronic
despite the formation of the iridium-alkyl complex (Ir(EtBnS)-
Mebpp), which was observed by ESI-TOF-MS measurements properties of Ir(OOEtBnS)-Cu are shown in Fig. S13 and Table
(Fig. S10†). The difference in reactivity between Ir(EtBnS)-Cu S4.† According to the TD-DFT calculations performed on
and Ir(EtBnS)-Mebpp was evaluated by conducting electro-
chemical measurements on 1 and 4 in formate buffer in the
presence of StyreneS. The oxidation waves of Ir(EtBnS)-Cu and
Ir(EtBnS)-Mebpp were observed at +0.34 V and +0.82 V (vs. Ag/
AgCl), respectively (Fig. S11†), indicating that Ir(EtBnS)-Cu is
readily oxidized compared to Ir(EtBnS)-Mebpp. Metal-alkyl
species with a lower ionization potential (i.e. negative redox
potential) have been reported to exhibit fast O2 insertion into
a metal–carbon bond without changing metal valency.48 Thus,
the negative redox potential of Ir(EtBnS)-Cu explains its higher
reactivity toward O2 insertion, as compared to that of Ir(EtBnS)-
Mebpp, without involving a redox event of the Ir(III/IV) center.
As the bpp ligand bears a formal negative charge in complex
1 while it is neutral in complex 4, the charge difference could
offer a possible explanation for the negative shi in redox
potentials observed between the p–p interacting complexes
Ir(EtBnS)-Cu and Ir(EtBnS)-Mebpp. To understand the role of
the Cu(II) center and the difference in electronic properties, we
have postulated the Ir(EtBnS)-bpp species by removing the
Cu(II)-dipic moiety from Ir(EtBnS)-Cu and conducted DFT
Scheme 2 Proposed catalytic cycle for the selective aerobic oxidation
of StyreneS by 1.
5800 | Chem. Sci., 2021, 12, 5796–5803
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Ir(OOEtBnS)-Cu,
a
characteristic oscillator strength was sodium formate, sodium 4-acetylbenzenesulfonate, sodium
observed at 434.47 nm. On the other hand, no absorption allylsulfonate, and 4-vinylbenzoic acid were purchased from TCI
growth around 434 nm was experimentally observed in the Chemical Co. Ltd. Formic acid and 4-vinylpyridine were
reaction of Ir(EtBnS)-Cu and O2 due to the fast cleavage of an purchased from Fujilm Wako Co. D2O was purchased from
O–O bond in Ir(OOEtBnS)-Cu to produce AcetophS.51 In addi- ISOWATER. All chemicals were used without further
tion, Ir(EtBnS)-Cu was not observed in the UV-vis spectral purication.
changes of the reaction solution under air, but Ir(H)-Cu
(Fig. S14†). The results are consistent with the observation that
the R. D. S. of the catalytic cycle under air is the olen insertion
into Ir(H)-Cu.
General procedure for catalytic oxidations of olens
An olen substrate and DSS (internal standard) were dissolved
into a formate buffer (D2O). A certain amount (10–30 mL) of Ir–
Cu (1) in 2,2,2-triuoroethanol solution was added to the
substrate solution, and then the reaction mixture was stirred at
70 ꢀC to start the reaction. Aer each reaction time, the reaction
Conclusions
1
In summary, we have demonstrated the exceptional catalytic
activity of 1 in the selective aerobic oxidation of StyreneS to
AcetophS as compared to that of the corresponding individual
mononuclear Ir and Cu complexes derived from 1 and the
mixture of these complexes in water. The formation of iridium-
alkyl species from the StyreneS insertion into the iridium-
hydride species in 1 was the key step (R. D. S.) of the Wacker-
type oxidation of aromatic olens in water observed here,
where a hydrophobic p–p interaction between the aromatic
ring of olens and the bpp ligand of 1 engages in a pre-
equilibrium process, based on evidence from DFT calcula-
tions and kinetic analysis. Only olen substrates with an alkene
group located next to the aromatic ring were selectively oxidised
into corresponding methyl ketones, which highlights the
signicance of the occurrence of hydrophobic p–p interactions
between substrates and 1 in water. Electrochemical measure-
ments performed on the iridium-alkyl species indicate that the
Cu moiety in 1 enhances the reactivity of the iridium-alkyl
species toward O2 in acidic conditions. This work will open
the possibility of expanding the use of hetero-dinuclear metal
complexes possessing structurally adjacent sites in molecular
catalysts that exploit catalyst–substrate hydrophobic interac-
tions for efficient and selective oxidation reactions in water.
mixture was analysed with H NMR spectroscopy to probe for
products generation.
Determination of kobs, k1, and K values for the reaction
between 1 and StyreneS under Ar
The UV-vis absorption changes at 405 nm under Ar were ana-
lysed based on pseudo-rst kinetics to obtain apparent rate
constants (kobs) by the curve-tting based on eqn (1). The kobs
values obtained were plotted against StyreneS concentration
using eqn (2) to determine the rst-order rate constant (k1) and
the equilibrium constant.
Abs ¼ Abs0 + DAbs (1 ꢂ exp(ꢂkobs/t))
kobs ¼ k1K[StyreneS]/(1 + K[StyreneS]).
(1)
(2)
DFT calculations
DFT calculations were carried out for the geometry optimization
and frequency calculations using the unrestricted B3LYP-D3BJ
functional to consider van der Waals interactions.53–55 We
used the basis sets of LanL2TZ(f)56 for Ir, 6-31G*57,58 for Cu, H,
C, N, and O atoms, and 6-31+G57,58 for the S atom. The total spin
state of the complex including the Cu atom was xed to be
a doublet. The optimized geometries were conrmed that they
did not have any imaginary frequencies. TD-DFT calculations59
were performed for 30 excited states on the basis of the opti-
mized structures. All the calculations were performed in water
solvent using a polarizable continuum model (PCM) adopting
the integral equation formalism variant (IEFPCM).60 The
Gaussian 09 (Revision D.01) program package61 was used for all
DFT calculations.
Experimental
General
1H NMR spectral measurements were performed on a JEOL
JNM-ECX
400
spectrometer
using
4,4-dimethyl-4-
silapentanesulfonic acid sodium salt (DSS) as an internal
standard. UV-vis spectral measurements at 70 ꢀC were per-
formed using SCINCO UV-visible spectrophotometer S-3100
equipped with a cryostat (CoolSpeK UV USP-203, UNISOKU
Co., Ltd.). LC-MS measurements were performed on a SHI-
MADZU LCMS-2020 spectrometer. Puried water (18.2 MU cm)
was obtained from a Milli-Q system (Direct-Q3 UV, Millipore).
Electrochemical measurements were carried out using HZ-7000
HOKUTO DENKO.
Author contributions
Y. O. synthesized and characterized the catalysts. Y. S. con-
ducted most of the catalytic tests, intermediate detection, and
kinetic analysis. Y. Kitakawa performed the DFT calculations. Y.
S., Y. Kon, and D. H. conceived the ideas and designed the work.
Chemicals
Ir–Cu (1),37 Ir-Hbpp (2),37 Cu-dipic (3),52 Ir-Mebpp (4),37 Ir–Co,37 All authors contributed to the discussion on the mechanism. Y.
and Ir–Ni,37 complexes were synthesized according to the liter- S. and D. H. wrote the dra with data from all authors, and all
ature procedures. Sodium p-styrenesulfonate (StyreneS), authors edited and reviewed the dra. D. H. supervised all
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aspects of the work. All authors approved the nal version of the 18 T. Fujishiro, O. Shoji and Y. Watanabe, Tetrahedron Lett.,
dra.
2011, 52, 395–397.
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Conflicts of interest
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There are no conicts to declare.
Acknowledgements
D. H. gratefully acknowledges support from the Japan Society
for the Promotion of Science (JSPS) by the Leading Initiative for
Excellent Young Researchers (LEADER). Y. S. also gratefully
acknowledges support from Grants-in-Aid (20K15328) from
JSPS.
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