A Fluorescent Molecular Probe for the Detection of Hydrogen Based on Oxidative Addition Reactions with Crabtree-Type Hydrogenation Catalysts

Article abstract of DOI:10.1002/anie.201506918

A Crabtree-type Ir^I complex tagged with a fluorescent dye (bodipy) was synthesized. The oxidative addition of H₂ converts the weakly fluorescent Ir^I complex (Φ=0.038) into a highly fluorescent Ir^III species (Φ=0.51). This fluorogenic reaction can be utilized for the detection of H₂ and to probe the oxidative addition step in the catalytic hydrogenation of olefins.

Full text of DOI:10.1002/anie.201506918

Angewandte
Chemie
International Edition: DOI: 10.1002/anie.201506918
German Edition: DOI: 10.1002/ange.201506918
Fluorescence Probes Very Important Paper
A Fluorescent Molecular Probe for the Detection of Hydrogen Based
on Oxidative Addition Reactions with Crabtree-Type Hydrogenation
Catalysts
Pavlo Kos and Herbert Plenio*
Dedicated to Professor Herbert W. Roesky on the occasion of his 80th birthday
Abstract: A Crabtree-type IrI complex tagged with a fluores-
cent dye (bodipy) was synthesized. The oxidative addition of
H2 converts the weakly fluorescent IrI complex (F = 0.038)
into a highly fluorescent IrIII species (F = 0.51). This fluoro-
genic reaction can be utilized for the detection of H2 and to
probe the oxidative addition step in the catalytic hydrogenation
of olefins.
enables the modulation of the fluorescence intensity.[17] The
brightness of a fluorescent dye attached to organometallic
complexes based on Au, Rh, or Ir turned out to be highly
sensitive to changes in the electron density at the transition
metals. For example, the substitution of a 1,5-cycloctadiene
(cod) ligand by CO (Scheme 1) converts nonfluorescent
complexes (F < 0.01) into highly fluorescent ones (F > 0.5).
Alternatively, the substitution of a halide (F > 0.5) by
H
ydrogen is a colorless, odorless, and tasteless gas. The
a strongly electron-donating thiolato group transforms
highly fluorescent gold complexes into partially quenched
ones (F < 0.2; Scheme 1).
chemistry of hydrogen (H2) is dominated by its highly
exothermic reaction with oxygen. This poses risks when
handling hydrogen, but also offers chances for energy
production under controlled reaction conditions. As a conse-
quence of the dwindling resources of fossil fuels and excessive
CO2 emissions, a hydrogen-based economy appears to be
a desirable target.[1] The increased use of hydrogen in energy
production will inevitably create demand for hydrogen
sensing to properly address safety issues. Other applications
for hydrogen sensing concern the food industry,[2] the
detection of certain bacteria,[3] leaks in flexible food pack-
aging,[4] as well as diagnostics for lactose intolerance[5] and
hydrogen-metabolizing organisms.[6]
Electrical sensors for hydrogen are well-established,[7] but
offer the potential of spark-induced explosions; furthermore,
miniaturization is limited. Optochemical sensors for hydrogen
have distinct advantages in this respect.[7,8] For example,
fluorescent molecular probes are very convenient, since the
fluorescence is turned on upon a specific reaction with an
analyte.[9] Only a few probes are known for simple gases, such
as CO,[10] CO2,[11] H2S,[12] NO,[13] NO2,[14] and ethene,[15] which
are important in a biological and chemical context. To the best
of our knowledge, a fluorescent molecular probe for hydrogen
(H2) has so far not been reported.[16]
Scheme 1. Modulation of fluorescence intensity as a consequence of
ligand-exchange reactions. bdp=pentamethylbodipy.
We set out to test the potential of oxidative addition
reactions for the detection of molecules. Such elementary
organometallic reactions are highly important steps in various
catalytic transformations.[18] Many transition-metal com-
plexes are able to activate H2 and transfer individual hydro-
gen atoms to unsaturated substrates.[19]
We recently synthesized transition-metal complexes with
appended fluorescent tags and realized that the manipulation
of the electron density in certain transition-metal complexes
Iridium-based Crabtree-type complexes are amongst the
most popular catalysts[20] for asymmetric hydrogenation
reactions.[21] A very important property of Crabtree-type
complexes is the lack of reactivity towards oxygen or water.
The oxidative addition of H2 to a transition metal is often the
first step in hydrogenation and formally converts an electron-
rich IrI complex into an electron-deficient IrIII one.[22]
[*] M. Sc. P. Kos, Prof. Dr. H. Plenio
Organometallic Chemistry
Technische Universität Darmstadt
Alarich-Weiss-Strasse 12, 64287 Darmstadt (Germany)
E-mail: plenio@tu-darmstadt.de
We report here on the synthesis of an IrI Crabtree
complex with an appended 4,4-difluoro-4-bora-3a,4a-diaza-s-
indacene (bodipy) fluorophore and on the effect of the
oxidative addition of H2 on the fluorescence intensity. Bodipy
and its numerous derivatives are highly useful fluorophores
that are characterized by high extinction coefficients, excel-
Supporting information for this article (including synthetic proce-
dures, NMR spectra, MS data, fluorescence spectra, cyclic voltam-
metry) is available on the WWW under http://dx.doi.org/10.1002/
anie.201506918.
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lent fluorescence quantum yields, narrow emission bands,
high photostability, and good solubility in many organic
solvents.[23]
lmax(em) = 524 nm. The IrI complexes 7 and 8 are weakly
fluorescent (fluorescence quantum yields F(7) = 0.022, F-
(8b) = 0.038). Avery pronounced increase in the fluorescence
intensity was observed in the course of the reactions of
solutions of 7 or 8b or 8c with hydrogen in methanol
(Figure 1). The fluorescence quantum yield of isolated 9c was
determined to be F = 0.51. This increase in brightness
denotes the formation of the respective IrIII complex 9
following oxidative addition of H2.[30] This reaction appears
to be complete within a few minutes.
A simple paper strip method was employed to demon-
strate the hydrogen-sensing properties of the iridium com-
plexes. A filter paper was impregnated with a solution of 8b in
propylene carbonate (b.p. 2428C; c = 1.0 ” 10À3 molLÀ1). The
solution of complex 8b in this solvent is highly stable in an
ambient atmosphere and the fluorescence of the paper strip
does not change in the presence of ambient air. One of the
paper strips was exposed to an atmosphere of pure hydrogen.
Within a few minutes the highly fluorescent IrIII complex is
generated (Figure 2). Using this simple filter paper assay it is
To establish a stable linker between the fluorophore and
the transition metal, we chose N-heterocyclic carbenes
(NHCs), which are amongst the most prominent ligands in
transition-metal chemistry.[24] The synthesis of an (NHC)Ir
complex with an appended bodipy fluorophore is outlined in
Scheme 2. Trimethylsilylacetylene is coupled in a Sonogashira
reaction to monobromo-N,N’-diaryldiamine (1),[25] removal of
the trimethylsilyl-protecting group, and another Sonogashira
reaction with iodo-bodipy provides the fluorophore-tagged
diamine 4. Hydrogenation of the alkyne group gives diamine
5, in which the fluorophore is electronically decoupled from
the potential metal coordinating site. The cyclization of the
diamine with C6F5CHO provides imidazolidine 6. This com-
pound is a useful NHC transfer reagent,[26] whose reaction
with [{IrCl(cod)}2] affords complex 7 in excellent yield
(Scheme 2). The electrochemical properties of
7 were
probed by cyclic voltammetry. Two reversible redox processes
were observed: E1/2 =+ 0.77 V is a typical IrI/IrII redox
potential,[27] while E1/2 =+ 1.13 V corresponds to the bodipy
oxidation.[28] Exchange of the chloride group by weakly
coordinating anions (triflate or BArF) provides the respective
“cationic” complexes 8b and 8c. A solution of complexes 8
and 100 equiv of pyridine in methanol readily reacts with
hydrogen to form the respective cationic dihydride complex 9
within a few minutes. The triflate complex 9b was isolated
and characterized. It displays the characteristic hydride signal
at d À22.4 ppm, but is prone to decomposition in solution.
The same highly characteristic hydride shift was observed for
the closely related complex [(IMes)Ir(H)2(py)3]+ClÀ (d
À22.5 ppm), which was synthesized by Duckett and co-
workers and used in NMR hyperpolarization experiments.[29]
The fluorescence properties of the bodipy-containing
compounds 5–9 were studied and all found to absorb and
emit in a narrow range close to lmax(exc) = 510 nm and
Figure 1. Fluorescence–time plots for the reaction of 8b (gray) and 8c
(black; 1.0”10À6 molLÀ1) in methanol containing pyridine
(1.0”10À4 molLÀ1) with hydrogen.
Scheme 2. Synthesis of bodipy-tagged (NHC)Ir complexes. Reagents and conditions: a) HCCSiMe3, Na2PdCl4, CuI, Cy2PtBu·HBF4, HNiPr2, 808C,
12 h; b) NBu4F·3H2O, THF, RT, 1 h; c) iodo-pentamethyl-bodipy, Na2PdCl4, CuI, Cy2PtBu·HBF4, HNiPr2, 808C, 12 h; d) Pd/C, 5 bar H2, MeOH, RT,
4 h; e) C6F5CHO, acetic acid, RT, 12 h; f) [{IrCl(cod)}2], toluene, 708C, 2 h; g) 8b [Ag(CF3SO3)], pyridine, CH2Cl2, 60 min; 8c [Na(BArF)], pyridine,
CH2Cl2, 60 min; h) H2, MeOH, 100 equiv pyridine. BArF=[B{3,5-(CF3)2C6H3}4]À, Mes=2,4,6-trimethylphenyl, py=pyridyl.
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Figure 2. Four filter paper strips (left to right: a–d) impregnated with
a solution of 8b in propylene carbonate and irradiated with broadband
UV light. Strip (a) exposed to ambient air; strip (b) exposed to 1%
(vol) H2; strip (c) exposed to 4% (vol) H2 (lower explosion limit), and
strip (d) exposed to pure H2.
Figure 3. Emission–time plots for the reaction of 8b (gray) and 8c
(black; 1.0”10À6 molLÀ1) in 1,2-dichloroethane with hydrogen (fluores-
cence gain 11.7 and 9). Excitation wavelength 510 nm.
The time-dependent fluorescence intensity of the bodipy
was monitored during the catalytic conversion of different
substrates (allylbenzene, styrene, cyclohexene, a-terpinene).
The increase in the fluorescence intensity during these
reactions is virtually the same as observed for the iridium
dihydride complex in 1,2-dichloroethane in the absence of an
olefinic substrate (Figure 4).[34] One interpretation is that the
steps in the catalytic cycle following the oxidative addition of
hydrogen do not lead to significant changes in the fluores-
cence intensity of the iridium complex. Considering the high
sensitivity of the fluorescence intensity to changes in the
electron density at the metal, this explanation is less likely.[17]
The similar fluorescence intensities are more easily explained
by assuming that the iridium dihydride species is the
dominant one in the catalytic reaction. Previously, a dihydride
complex has been suggested to be the resting state of the
catalytic reaction, which is in accordance with our data.[35]
Notably, the formation of the fluorescent iridium dihydride
species occurs faster than substrate conversion (Figures S28,
and S29).[36] It is, therefore, less likely that the fluorescent
product observed is the decomposition product of the catalyst
following substrate conversion.
possible to detect hydrogen at a concentration corresponding
to the lower explosion limit (4% vol) and to distinguish this
from a lower hydrogen concentration (1% vol).
Next the catalytic properties of the new iridium com-
plexes were studied. Catalytic studies were limited to 8b and
8c containing weakly coordinating anions, as coordinating
anions are detrimental for substrate conversion.[31] The
reactions with hydrogen were carried out in 1,2-dichloro-
ethane as solvent, since the donor solvents used for the
hydrogen-sensing experiments are known to inhibit olefin
hydrogenation, probably by impeding coordination of the
olefin to the iridium center. The room-temperature reaction
of the test substrate allylbenzene with hydrogen in the
presence of 8c provides the fully hydrogenated product
within 10 min in nearly quantitative yield at 0.5 mol%
catalyst loading.
Next, the time-dependent fluorescence intensity of 8b and
8c during the catalytic hydrogenation reaction with H2 and
allylbenzene in 1,2-dichloroethane as solvent was monitored.
For this reaction, a lower loading (0.1 mol%) as well as
a
much lower concentration of precatalyst 8b (1.0 ”
In conclusion, we have shown that the oxidative addition
of hydrogen to an IrI complex tagged with a bodipy fluo-
rophore gives rise to a substantial increase in the fluorescence
10À6 molLÀ1) was chosen to ensure that the active species is
fully consumed during the catalytic reaction. Consequently,
the conversion of allylbenzene into the hydrogenation
product propylbenzene was only approximately 30%. The
catalytic reaction with hydrogen and an olefin is characterized
by a pronounced increase in the fluorescence intensity
(Figure 3). However, the final level of fluorescence in 1,2-
dichloroethane is lower than that observed in MeOH/pyridine
(Figure 1), thus indicating the formation of an IrIII species
different from complex 9. This is not surprising, as it is known
that, in nonpolar solvents in the absence of the stabilizing cod
ligand and after consuming the olefins in the hydrogenation
reaction, trimeric[32] as well as higher nuclearity iridium
hydride clusters are formed.[33] A hydrogenation experiment
carried out in an NMR tube using 8c and allylbenzene shows
the formation of numerous iridium hydride species, as evident
by the appearance of a large number of resonances at 0 to
Figure 4. Emission–time plot for the catalytic hydrogenation of 0.1m
allylbenzene (gray) with 8b (black; 1.0”10À5 molLÀ1) in 1,2-dichloro-
ethane; same reaction conditions, but without olefin (black).
1
À30 ppm in the H NMR spectrum, thus confirming findings
by Brown and co-workers for related complexes.[33]
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intensity of the formed IrIII dihydride complex. This signal
enables the detection of hydrogen and is useful for the
analysis of catalytic hydrogenation reactions. We, therefore,
propose fluorescence spectroscopy to be a useful tool for the
analysis of transition-metal-catalyzed reactions, since changes
in the electron density at the metal center which translate into
changes in the fluorescence intensity occur in numerous
metal-catalyzed reactions.
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This work was supported by the DFG through grant Pl 178/18-1.
Keywords: fluorescence · homogeneous catalysis · hydrogen ·
iridium · N-heterocyclic carbene
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Published online: September 11, 2015
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