Transfer hydrogenation catalysis by a N-heterocyclic carbene iridium complex on a polyoxometalate platform

Article abstract of DOI:10.1002/ejic.201402020

A divacant Keggin polyanion has been decorated with a N-heterocyclic carbene (NHC) iridium(I) organometallic complex to provide a molecular model of an Ir-based supported catalyst. The characterization of the hybrid compound has been performed by multinuc

Full text of DOI:10.1002/ejic.201402020

SHORT COMMUNICATION
DOI:10.1002/ejic.201402020
Transfer Hydrogenation Catalysis by a N-Heterocyclic
Carbene Iridium Complex on a Polyoxometalate Platform
Gloria Modugno,[a] Angele Monney,[b] Marcella Bonchio,[a]
Martin Albrecht,*[b] and Mauro Carraro*[a]
Dedicated to the memory of Elena Bertacco
Keywords: Iridium / Polyoxometalates / Transfer hydrogenation / Carbene ligands / Nanoscaffolds
A divacant Keggin polyanion has been decorated with a N-
heterocyclic carbene (NHC) iridium(I) organometallic com-
plex to provide a molecular model of an Ir-based supported
catalyst. The characterization of the hybrid compound has
been performed by multinuclear NMR spectroscopy, infrared
spectroscopy, cyclic voltammetry, and mass spectroscopy,
and the results are in agreement with a bisfunctionalization
of the polyoxometalate scaffold. The resulting supported
homogeneous complex has been successfully used to cata-
lyze the transfer hydrogenation from iPrOH to benzo-
phenone [with a turnover number (TON) of 680 and a turn-
over frequency (TOF) of up to 540 h–1].
Herein, we present a synthetic strategy for the immobili-
zation of Ir–NHC domains on a molecular polyoxometalate
(POM), which results in the vicinal arrangement of two re-
active sites (Scheme 1). POMs offer a unique nanodimen-
sional and totally inorganic platform and they are often
considered as molecular fragments of metal oxide solid sup-
ports (oxides, zeolites, and mesoporous materials). There-
fore, the use of POMs to anchor heterocatalytic sites has
been proposed to bridge the gap between homogeneous and
heterogeneous systems.[5] Moreover, POMs behave as ionic
nanotags and can impart a tunable solubility by tailored
counterion association[6] or they can be immobilized on cat-
ionic supports by electrostatic interactions.[7]
Introduction
The decoration of bulk surfaces with organometallic do-
mains is a powerful strategy to replicate single-site reactive
units into functional arrays. The resulting catalytic surfaces
can provide a combined tuning of steric and electronic fea-
tures of the reactive sites, while offering opportunities for
cooperativity, environmental assistance, and process intensi-
fication.
In this context, on-surface transfer hydrogenation cataly-
sis (THC) may represent a valuable alternative to hetero-
geneous gas-phase protocols with dihydrogen. Indeed, THC
has a high synthetic appeal that stems from the use of
alcohols as an organic hydrogen source for liquid-phase re-
duction under mild pressure and temperature conditions.[1]
Among molecular TH catalysts, iridium complexes with
N-heterocyclic carbene ligands (Ir–NHC complexes) show
high thermal robustness and stability during multiturnover
catalysis.[2,3] However, heterogeneous THC by Ir–NHC spe-
cies is limited to polymeric supports and plagued by
irreversible leaching.[4]
[a] Department of Chemical Sciences and ITM-CNR, University
of Padova,
via F. Marzolo 1, 35131 Padova, Italy
E-mail: mauro.carraro@unipd.it
www.chimica.unipd.it/NanoMolCat/
[b] School of Chemistry and Chemical Biology, University College
Dublin,
Belfield, Dublin 4, Ireland
E-mail: martin.albrecht@ucd.ie
www.ucd.ie/chem/staff/profmartinalbrecht
Supporting information for this article is available on the
WWW under http://dx.doi.org/10.1002/ejic.201402020.
Scheme 1. Synthesis of the Ir–NHC complexes.
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SHORT COMMUNICATION
The design of POM-based supports for THC is unprece-
dented and holds great potential in terms of innovative
functional nanostructures and advanced catalysis. In par-
ticular, THC protocols could benefit from metal-oxide-as-
sisted hydrogen translocation[8] and/or the possibility to
tune reaction conditions in different H-donor media.[9]
The modification of POM surfaces with organometallic
substituents can be achieved by introduction of covalent te-
thers to provide binding motifs for heterometal cen-
ters.[10–12] According to this strategy, molecular catalytic
species have been obtained through the decoration of
POMs with phosphines and NHC chelates for Rh[11] and
Pd[12] ions, respectively.
Figure 1. 29Si NMR spectra (79.5 MHz, T = 25 °C) of 3 in CD3CN
and 2 in CDCl3 (inset).
Results and Discussion
The synthetic route to introduce Ir–NHC domains on Presumably, the inorganic POM interacts electrostatically
POMs exploits the nucleophilic reactivity of the surface with the Ag+ ions as well as with the imidazolium moieties,
oxygen atoms on vacant polyoxotungstates with tailored or- thus hampering the formation of the silver carbene interme-
1
ganosilanes. Covalent grafting occurs in acetonitrile in the diate. The resulting product was characterized by H, 13C,
presence of a lipophilic ammonium salt, usually nBu4NBr 183W, and 29Si NMR spectroscopy, FTIR spectroscopy,
(tetrabutylammonium bromide, TBABr), to promote the ESI-MS, and cyclic voltammetry (Supporting Information,
solubilization of the POM by counterion metathesis Figures S5–S11).
(Scheme 1).[13] Under these conditions, the decoration of
The 183W NMR spectrum of POM–Ir hybrid 3 shows
the divacant Keggin polyanion [γ-SiW10O36]8– yields three resonances at δ = –107.4, –135.8, and –142.1 ppm in a
bisfunctionalized hybrids with two surface-anchored or- 2:1:2 ratio, in agreement with a C2v symmetry of the NHC-
ganosilyl (RSi–) groups, each one linked to two oxygen decorated polyanion (Figure S5).[14] The 29Si NMR spec-
atoms of two edge-shared WO6 octahedra.[14]
trum reveals two signals at δ = –62.8 and –88.4 ppm for the
To this end, the trialkoxysilyl-tagged imidazolium pre- organosilane and the silicotungstate residues, respectively,
cursor 1 was initially synthesized under strictly anhydrous in the expected 2:1 integration ratio (Figure 1). These re-
conditions from (3-chloropropyl)triethoxysilane and 1-but- sults are consistent with a bis-substitution of the POM sur-
ylimidazole (Scheme 1). The reaction of 1 with Ag2O, fol- face and indicate a diagnostic 14.6 ppm upfield shift for the
lowed by transmetallation with [Ir(C8H12)Cl]2 (C8H12 = 1,5- organosilicon signal upon grafting to the POM.
cyclooctadiene, COD) afforded the Ir–NHC complex 2 with
The FTIR evidence is also in agreement with the ex-
a trialkoxysilyl tag in 78% yield.[15] The identity of 2 was pected functionalization, as shown by the RSi–O vibration
1
confirmed by FTIR spectroscopy, H, 13C, and 29Si NMR band at ν = 1102 cm–1 and by the diagnostic spectral
˜
spectroscopy, and ESI-MS analyses (see Supporting Infor- pattern of the W–O bond vibrations between ν = 1000 and
˜
mation, Figures S1–S4). The absence of a resonance for the 700 cm–1 (Figure S6).[14] Upon exposure to a CO atmo-
C(2)-bound imidazolium proton (δH = 10.6 ppm for the sphere, the resulting complex 3 undergoes a displacement
imidazolium salt 1 in CDCl3) is corroborated by the ap- of the COD ligand, which provides additional evidence for
pearance of a new signal at δC = 178.7 ppm, which is char- the presence of the Ir–NHC unit. Indeed, when a solution
acteristic of an iridium-bound carbene carbon atom (Fig- of 3 in CH3CN (2 mm) was saturated with CO gas (1 atm)
ures S2 and S3).[15] Moreover, a single 29Si NMR signal at for 10 min, the FTIR spectrum revealed two new bands at
δ = –46.2 ppm is indicative of a pure product (Figure 1, ν = 2058 and 1977 cm–1 (Figure S7), which are diagnostic
˜
inset).
The ESI-MS (positive mode) analysis confirms the for- upon successful grafting of the bis(iridium) site.[15,16]
mation of the desired iridium complex bearing NHC and An ESI-MS spectrum of 3 recorded in negative mode
for CO bonding at the iridium carbene moiety, as expected
COD ligands with signals centered at m/z = 628.9 and shows a dominant cluster centered at m/z = 1723, which was
670.6, which correspond to [M – Cl]+ and [M – Cl + attributed to {[γ-SiW10O36][Ir(C8H12)(C10H17N2Si)]2O}2–,
CH3CN]+, respectively (Figure S4).
that is, [M – 2Cl – 4(nBu4N)]2– (Figure S8).[12]
The Ir complex 2 was then reacted with the decatungsto-
The successful formation of the organometallic POM hy-
silicate K8[γ-SiW10O36] in acetonitrile under phase-transfer brid was also concluded from the H and 13C NMR spec-
conditions (Scheme 1), which led to the hybrid POM tra, which confirmed the presence of the NHC ligand (het-
(nBu4N)4{[γ-SiW10O36][Ir(C8H12)Cl(C10H17N2Si)]2O} (3) in erocyclic protons at δH = 7.22 and 7.04 ppm), the COD
75% yield. A different synthetic approach that consisted of residues (diagnostic olefinic protons in the δH = 4.44–
postmetalation of the POM-attached 1 with Ag2O was also 4.24 ppm range; Figure S9), and of the iridium-bound carb-
investigated; however, no carbene formation was observed. ene carbon atom (δC = 178.7 ppm; Figure S10).
1
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SHORT COMMUNICATION
Ir-centered oxidation has been observed for NHC-based Table 1. Hydrogen transfer reaction to benzophenone by NHC–IrI
complexes.[a]
systems with potentials in the range 0.65 to 0.92 V.[17] Cyclic
voltammetry (CV) of 3 in CH3CN shows a prominent
irreversible anodic peak at Epa = 0.64 V (vs. Ag/AgCl),
which arises from IrI oxidation and displays a shift of ca.
70 mV to lower potential with respect to the POM-free
complex 2 (Figure 2). The cathodic scan is instead domi-
nated by a reversible redox process involving the tung-
sten(VI) ions of the POM framework (E1/2 = –0.84 V vs.
Ag/AgCl, Figure S11).
Entry Catalyst Base
Conversion TON[c]
[%][b]
TOF[d]
[h–1]
1
2
3
4
5
6
7
4
KOH
KOH
KOH
KOH
tBuOK
nBu4NOH
tBuOK
97
Ͻ2
97
72
99
23
68
97
Ͻ2
97
72
99
23
680
234
–
198
96
210
24
540
5
4 + 5
3
3
3
3[e]
[a] 1 mol-% catalyst (0.01 mmol), benzophenone (1.0 mmol), and
base (10 mol-%, 0.1 mmol) in iPrOH (5.0 mL), T = 80 °C. In all
reactions, the ketone was introduced after the mixture had been
1
heated for 10 min. [b] Conversion of substrate as monitored by H
NMR spectroscopy. [c] Turnover number (mol of converted sub-
strate per mol of catalyst) at t = 5 h. [d] Turnover frequency (cata-
lytic cycles per hour), calculated from initial rates at Ͻ30% conver-
sion. [e] Reaction with 0.1 mol-% Ir loading.
Figure 2. Cyclic voltammograms (anodic scan) of
2 and 3
(0.05 mm) in CH3CN (nBu4NClO4; 0.1 m); scan rate 200 mVs–1,
potentials vs. Ag/AgCl.
The POM-grafted bis-NHC iridium compound features
a suitable manifold of redox states, which can leverage the
key catalytic steps that are involved in alcohol oxidation
and hydride transfer. Thus, hybrid 3 was screened for cata-
lytic transfer hydrogenation by investigation of the re-
duction of benzophenone to diphenyl methanol in iPrOH
as a model reaction (Scheme 2).
Figure 3. Kinetics of hydrogen transfer from isopropanol to benzo-
phenone catalyzed by 3 in the presence of different bases (Table 1,
Entries 4, 5, and 6).
Both turnover number and frequency (TON and TOF)
decrease in the order tBuOKϾ KOH Ͼ nBu4NOH (Table 1,
Entries 4–6; Figure 3). Indeed, quantitative conversion and
performance, identical to that of homogeneous 4, were ob-
served after 2 h with the bulky organic base tBuOK (c.f.
Table 1, Entries 1 and 5). In this case, the possibility to
avoid aqueous bases is expected to prevent the slow deacti-
vation of the POM scaffold, for example, by irreversible hy-
drolysis. The base sensitivity of the POM-free 4 is less criti-
cal, and tBuOK and KOH provided essentially identical re-
sults. Optimization of the process efficiency was achieved at
a lower iridium loading (0.1 mol-%) with tBuOK as the
base; 70% conversion was achieved after 5 h with an overall
TON of 680 and a maximum TOF of 540 h–1 (Table 1, En-
try 7). Such high activity underpins the efficiency of the im-
mobilization strategy adopted here.
Scheme 2. Hydrogen transfer reaction from isopropanol to benzo-
phenone by Ir catalysts.
All catalyst manipulations were performed under air, as
no special precautions are needed for the handling of the
POM-based system. The initial reaction setup was designed
with 1 mol-% iridium loading.[18]
The product formation was monitored by 1H NMR spec-
troscopy to assess the overall catalytic efficiency (Table 1)
and the relevant kinetic profiles (Figure 3).
An inspection of the data in Table 1 allows us to bench-
mark catalyst 3 against the POM-free Ir–NHC complex
[Ir(C8H12)Cl(InBu)] (4, InBu = N,NЈ-dibutylimidazol-2-
ylidene), and to confirm the role of the iridium sites
(Table 1, Entry 1).
Indeed, the Ir-free POM [γ-SiW10O36]8– (5) does not con-
tribute to the reactivity on its own or in combination with
4 (Table 1, Entries 2 and 3). THC works in the presence of
Conclusions
The successful grafting of an NHC–iridium(I) complex
a base additive and, as shown in Figure 3, the nature of the onto a polyoxometalate surface has been demonstrated and
base has a major impact on the kinetics of the reaction provides a working model for heterogenized transfer hydro-
catalyzed by 3.[19]
genation catalysis. The catalytic efficiency has been pre-
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SHORT COMMUNICATION
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served when compared to that of the POM-free organome-
tallic analogue upon appropriate tailoring of reaction con-
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foster solubility/immobilization in highly polar or ionic
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Experimental Section
[3]
General: K8[γ-SiW10O36],[22] 1-butyl-3-(3-triethoxysilylpropyl)imid-
azolium bromide,[12] [Ir(C8H12)Cl]2, and [Ir(C8H12)Cl(InBu)][15]
(InBu = N,NЈ-dibutylimidazol-2-ylidene) were prepared as de-
scribed in the literature. All syntheses were performed under a ni-
trogen atmosphere with reagent grade solvents, which were used as
received without further purification.
(nBu4N)4{[γ-SiW10O36][Ir(C8H12)Cl(C10H17N2Si)2O]} (3): K8[γ-
SiW10O36] (380 mg, 0.13 mmol) was suspended in H2O (500 μL)
under nitrogen. nBu4NBr (206 mg, 65 mmol, 5 equiv.) and CH3CN
(5 mL) were added, and the mixture was stirred at room tempera-
ture for 20 min. Compound 2 (170 mg, 0.26 mmol) was dissolved
in CH2Cl2/CH3CN (1:2, 3 mL) under nitrogen. The solution of 2
and HCl (4.05 m, 190 μL) were then added to the initial mixture in
rapid sequence. Upon the addition of acid, the solution immedi-
ately became clear. The mixture was vigorously stirred overnight at
room temp. and then filtered to remove the insoluble material. The
mixture was concentrated to ca. 1 mL, and the product was precipi-
tated by the addition of deionized water (20 mL). The product was
collected by filtration through a fritted funnel, washed with a few
portions of deionized water and diethyl ether, and dried under vac-
uum; yield: 298 mg (0.07 mmol, 54% yield). The full characteriza-
[4]
[5]
1
tion data [FTIR spectroscopy, H, 13C{1H}, 29Si, and 183W NMR
[6]
[7]
spectroscopy, ESI-MS (–), and elemental analysis] are reported in
the Supporting Information.
THC Protocol: In a typical reaction, the iridium catalyst was stirred
in iPrOH (5.0 mL) with the appropriate base additive (0.1 mmol)
at reflux temperature for 10 min. The ketone (1.0 mmol) was
added, and aliquots (0.2 mL) were taken at fixed times, diluted with
hexane (2 mL), filtered through a short pad of SiO2, and analyzed
1
by H NMR spectroscopy.
Supporting Information (see footnote on the first page of this arti-
cle): Detailed experimental procedures and complete characteriza-
tion for 2 and 3.
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Acknowledgments
The European Union (ESF COST Actions CM1205-Carisma and
CM1003) (ESF COST Actions D40 and 1203, PoCheMoN) and
the European Research Council (ERC StG 208561) are gratefully
acknowledged for financial support. Johnson Matthey is thanked
for a generous loan of iridium.
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