Dehydrogenation of amine–boranes catalyzed by a PCsp3P pincer iridium complex

Article abstract of DOI:10.1016/j.mencom.2020.05.004

Dibenzobarrelene-based PCsp3P pincer iridium complex bearing dangling CH2OH groups, ((PCsp3PCH2OH)IrH(Cl), catalyzes the dehydrogenation of amine–boranes at the reaction rate changing counter intuitively in the order: Me2NH·BH3 > > ButNH2·BH3 > NH3·BH3. T

Full text of DOI:10.1016/j.mencom.2020.05.004

Mendeleev
Communications
Mendeleev Commun., 2020, 30, 276–278
Dehydrogenation of amine–boranes catalyzed
by a PCsp3P pincer iridium complex
Vladislava A. Kirkina,a Elena S. Osipova,a Oleg A. Filippov,a Gleb A. Silantyev,a
Dmitri Gelman,b Elena S. Shubinaa and Natalia V. Belkova*a
a A. N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 119991 Moscow,
Russian Federation. E-mail: nataliabelk@ineos.ac.ru
b Institute of Chemistry, The Hebrew University, Edmond Safra Campus, Givat Ram, 91904 Jerusalem, Israel
DOI: 10.1016/j.mencom.2020.05.004
ꢃMꢄ
Dibenzobarrelene-based PCsp3P pincer iridium complex
n BꢀꢁꢂNRR'ꢀ
ꢃRR'NꢅBꢀ ꢄ ꢆ
n ꢀ2
2 n
bearing dangling CH2OH groups, (PCsp3PCH OH)IrH(Cl),
ꢀ
O
2
ꢀ
O
2.0ꢇꢋ
ꢀ
catalyzes the dehydrogenation of amine–boranes at the reaction
rate changing counter intuitively in the order: Me2NH·BH3 >
> ButNH2·BH3 > NH3·BH3. The spectral (IR and NMR) data
and DFT/M06 calculations have revealed that the binding of
amine–boranes to the dangling OH group leads to an additional
stabilization of the Ir···OH bond, thus hampering the
dehydrogenation reaction, whereas the amine–borane
coordination to iridium entails a fac- to mer-transformation
of the complex and initiates the catalytic H2 evolution.
C
C
C
2.ꢊꢋꢉ
O
C
C
Cl
C
C
C
C
C
2.ꢇ02
ꢀ
ꢇ.ꢋꢈꢇ
ꢀ
C
C
C
ꢀ
2.ꢋꢇꢍ
C
ꢀ
C
C
C
C
C
C
P
O
ꢇ.22ꢍ
B
C
C
C
C
C
P
Ir
2.ꢁꢋꢁ
C
C
2.ꢋꢉꢁ
N
C
N
ꢀ
2.ꢁ0ꢋ
C
ꢇ.ꢊꢋꢌ
ꢀ
C
C
C
C
C
P
ꢇ.ꢈꢉꢉ
C
C
C
B
ꢀ
C
ꢀ
ꢀ
P
C
Ir
C
ꢇ.2ꢉ0
ꢀ
Cl
Keywords: iridium pincer complexes, catalytic dehydrogenation, amine–boranes, molecular spectroscopy, DFT calculations.
Amine–boranes, RR'NH·BH3, attract an attention of the researchers
as compounds for the hydrogen storage systems owing to a
theoretically high H2 content1,2 and also as promising objects for
the design of BN ceramics and new polymeric materials, the so-
called inorganic polymers.3 These applications rely on the catalytic
dehydrogenation of amine–boranes, so new catalytic studies keep
appearing in the literature. A variety of recently developed
(de)hydrogenation catalysts operate according to different
metal–ligand cooperation mechanisms, and new ‘non-innocent’
ligands and their complexes are under ongoing investigations.4–6
A reversible switching between the different coordination modes
observed in these compounds have revealed new patterns of
practical reactivity in the non-oxidative (i.e., alternative to the
conventional oxidative addition/reductive elimination sequence)
activation and formation of polar and non-polar bonds. At this
end, many of such systems contain stereochemically rigid pincer
ligands and preserve their geometry during the catalytic cycles.
Dibenzobarrelene-based PCsp3P pincer complexes of iridium
have already been reported as efficient catalysts in the transfer
(de)hydrogenation of polar and non-polar substrates.7–11 Dibenzo-
barrelene-based PCsp3P pincer iridium complex 1 is catalytically
active in acceptorless dehydrogenation of alcohols (Scheme 1).
The intramolecular interaction of iridium hydride ligand with the
dangling CH2OH group of 1, which leads to a facile hydrogen
release was hypothesized as the origin of its high catalytic
activity.12 Its catalytic performance is comparable to or even
exceeds the state of the art catalysts operating under the neutral
conditions.13,14
t
Among the benzene-based pincer Iriii complexes, (Bu POCOP)IrH2
t
[
Bu POCOP is k3-2,6-(Bu2t P–O)2C6H3] appeared to be highly efficient
not only in the dehydrogenation of alkanes, but also in the dehydro-
t
genation of NH3·BH3.15 Our works on related (Bu PCP)IrH(Cl)
t
t
and (Bu PCN)IrH(Cl) (Bu PCP is k3-2,6-(But2PCH2)2C6H3 and
t
Bu PCN is 1-{3-[(di-tert-butylphosphino)methyl]phenyl}-1H-
pyrazole) have revealed an important influence of the steric
properties of pincer ligand on the catalytic activity of this series
of iridium hydrides and unveiled the dehydrogenation mechanism
for amine–boranes.16–18 The hydrogen NH···Cl bond and BH
coordination to the iridium appeared to be important initial steps
in the hydridochloride precatalyst activation [Figure 1(a)], whereas the
dihydrogen NH···H–Ir bond and BH···Ir interaction [Figure 1(b)]
initiate the hydride (from B–H to Ir) and proton (from NH to Ir–H)
transfers in the catalytic cycle. In view of the different properties
of sp2 and sp3 PCP scaffolds and the presence of dangling functional
group in the latter, the present work was aimed at testing the
catalytic activity of dibenzobarrelene-based PCsp3P complex 1 in
amine–boranes dehydrogenation.† Figure 1 shows the structures
of considered intermediates.16–18
ꢀ
ꢀ
ꢀꢁꢀ
Oꢀ
Oꢀ
ꢀ
ꢀ
ꢀO
O
ꢀ
Ir
Cl
Ir
Ph2P
Ph2P
†
PPh2
PPh2
All the manipulations were performed under an argon atmosphere using
O
Oꢀ
Cl
the standard Schlenk technique. The H2 production during the reaction of
amine–boranes with complex 1 was monitored by measuring the volume of
hydrogen gas released. In a typical experiment, DMAB (8.8 mg, 0.246 mmol)
ꢀ
Scheme 1
© 2020 Mendeleev Communications. Published by ELSEVIER B.V.
on behalf of the N. D. Zelinsky Institute of Organic Chemistry of the
Russian Academy of Sciences.
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Mendeleev Commun., 2020, 30, 276–278
0.2ꢀ
(a)
(b)
H
H
L
H
L
Cl
Ir
NR2
B
NR2
B
0.20
0.ꢁꢀ
0.ꢁ0
0.0ꢀ
0.00
Ir
H
PBu2t
H
PBu2t
H
H
H
H
H
H
Figure 1 Structures of the key hexacoordinated intermediates in the
(a) precatalyst activation and (b) catalytic cycle of DMAB dehydrogenation
catalyzed by (PCL)IrH(Cl) complexes.
2ꢀ00
2ꢃ00
nꢆcmꢇꢁ
2ꢁ00
ꢁꢂ00
Volumetric tests demonstrated that ca. 1 mol% of complex 1
readily dehydrogenated amine–boranes yielding 0.6 equiv. of H2
within 2–3 h at room temperature (Figure 2, Table S1, see Online
Supplementary Materials), whereas 1 equiv. of H2 could be obtained
at prolonged reaction times (e.g., in 6 h in the case of ButNH2·BH3).
A higher degree of self-association in low-polar media19 hampers
theMe2NH·BH3(dimethylaminoborane,DMAB)dehydrogenation
in C6H5F, where the reaction is slower (kobs = 3.9×10–5 s–1) than
that in dimethoxyethane (DME) and CH2Cl2 (kobs = 1.8×10–4 s–1)
(Figure S1). An analysis of the initial reaction rate for different
DMAB and catalyst concentrations (Figure S2) revealed that the
overall reaction obeys the first order law in both the components:
–d[DMAB]/dt = kobs and [DMAB] = k[1][DMAB].
The structure of complex 1 differs from that of many other
catalysts containing ‘non-innocent’ or bifunctional ligands, i.e.,
the hydroxymethyl group is formally far away from the metal.
Results of the variable temperature IR and NMR (1H and 31P)
analysis for 1 and its analogue, the COOEt substituted complex,
in different media (CH2Cl2, toluene, DMSO, and mixed solvents)
in combination with the quantum chemical calculations (DFT/M06)
have revealed a flexibility of the dibenzobarrelene-based scaffold20
unprecedented for conventional pincer ligands. These complexes
prefer the facial configuration of PCP ligand with P–Ir–P angle
of ca. 100° (fac-isomers: fac-1-I and fac-1-II, Figure S3). Such
geometries arise due to the stabilizing Ir···OH interaction and differ
by the mutual arrangement of H and Cl ligands. The fac-isomers
can be transformed into the meridional ones with almost linear
P–Ir–P arrangement (mer-isomer: mer-1, see Figure S3) in the
presence of coordinating additives (MeCN, DMSO, or pyridine).20,21
In the case of 1, this causes the shift of Ir–H stretching vibration
band n(IrH) in the IR spectra from 2038 to 2225 cm–1 and the
appearance in 1H NMR spectra of hydride triplet resonance at dH
from –19 to –22 instead of the doublets of doublets at ca.
–9 ppm.20,21 Upon the addition of DMAB to the solution of 1 in
CH2Cl2 at 275 K, several new bands appear in the IR spectra
evidencing the DMAB coordination to iridium (Figure 3). The
Figure 3 IR monitoring of the DMAB (c = 0.01 mol dm–3) dehydrogenation
in the presence of equimolar amount of complex 1 in CH2Cl2 at 275 K,
l = 0.01 cm.
band at 2230 cm–1 was assigned to the n(IrH) band in this new
complex 1·DMAB (Scheme 2). Its position is similar to the bands
of hexacoordinated adducts of 1 with bases possessing the mer-
configuration of dibenzobarrelene ligand.20,21 In a similar way, a
new n(IrH) band at 2205 cm–1 appears in the IR spectra in the
presence of Me3N·BH3 (TMAB) at low temperatures (Figure S5).
The DFT/M06 calculations have revealed the stronger binding of
DMAB relative to TMAB (DG298 = –11.7 and –4.0 kcal mol–1,
respectively) and predicted a lower frequency of n(BHbridge) in
1·DMAB as compared to that in 1·TMAB, which appears close to
the n(IrH) band of 1·DMAB (Figure S4, Table S2).
The amine–borane coordination to iridium also causes the
appearance of new n(BH) bands in the IR spectra. A higher
frequency band at 2474 cm–1 was assigned to the stretching
vibration of terminal (non-bonded) B–H bond in 1·DMAB,
whereas the stretching vibration of bridging BH group seems to
be overlapped with the n(IrH) band of the starting iridium complex,
resulting in the wide band with its maximum at 2010 cm–1 (see
Figure 3). The sharp intense n(BH) band at 2338 cm–1 is typical
of dihydrogen bonded BH moiety19 and suggests an amine–
borane interaction with the dangling OH group (see Scheme 2).
This type of dihydrogen bonded complexes leads to the significant
increase of oxygen nucleophilicity (due to the increase of negative
charge) and consequently, to a stronger Ir···OH interaction as
wasconfirmedbytheDFTcalculations(Figure S4). Suchcomplexes
should be more stable in the case of ButNH2·BH3 and NH3·BH3,
which is a possible reason for the lower reaction rate and noticeable
induction period for the dehydrogenation of these amine–boranes
(see Figure 2).
1H NMR measurements have confirmed the formation of
hexacoordinated mer-complex upon the DMAB addition to 1.
At low temperatures (200–250 K), a new triplet appears in the
high-field region at d(IrH) –23.15 (2JP–H 12 Hz). When the H2
0.ꢅ
0.ꢄ
0.ꢃ
0.ꢂ
OH
HO
H
OH
H
Cl
0.ꢁ
0.2
0.ꢀ
0.0
R2N
NꢈꢁꢌBꢈꢁ
ButNꢈ2ꢌBꢈꢁ
Me2NꢈꢌBꢈꢁ
R2HN·BH3
OH
Ir
Ir
Ph2P
H2B
Ph2P
PPh2
P
H
H
Cl
Ph2
1
0
2000 ꢂ000 ꢄ000 ꢆ000 ꢀ0000 ꢀ2000
tꢇs
–H2
+ R2HN·BH3
Figure 2 Kinetic curves for the H2 evolution from amine–boranes
(c = 0.07 mol dm–3) in the presence of complex 1 (1.2 mol%) at 298 K in DME.
–1/2(R2NBH2)2
R2
N
OH
HO
H
BH2
H
H
O
was dissolved in a solvent (3.0 ml) in a round-bottomed flask (V = 10 ml),
and the flask was closed with a tight-fitting rubber septum. The required
amount of 1 (dissolved in 0.5 ml of the same solvent) was transferred via
syringe to the stirred amine–borane solution. Timing was started upon the
catalyst injection. The hydrogen gas was collected in a water-filled,
upturned burette through a Teflon tube. The volume of hydrogen gas
collected was recorded periodically.
HO
H
Ir
Ir
Ph2P
Ph2P
P
P
Ph2
Cl
Cl
H
Ph2
mer-1
Scheme 2 Transformations of complex 1 in the presence of amine–boranes.
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Mendeleev Commun., 2020, 30, 276–278
Online Supplementary Materials
Supplementary data associated with this article can be found
in the online version at doi: 10.1016/j.mencom.2020.05.004.
ꢊꢂ min
ꢉ0 min
ꢈꢂ min
ꢇ0 min
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ꢀ2ꢇ.2ꢂ
ꢀꢁ.ꢂ
ꢀꢃ.0
ꢀꢃ.ꢂ
ꢀꢄ0.0
dꢅꢆꢆm
Figure 4 1H NMR monitoring of the DMAB dehydrogenation in the
presence of complex 1 (1 equiv.) in CD2Cl2 at 272.5 K.
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evolution begins above 250 K, thissignal is gradually transformed
into the yet another triplet at –9.95 ppm (2JP–H 19 Hz, Figure 4).
This suggests that the remaining iridium hydride catalyst preserves
the mer-configuration of ligand, but is formally pentacoordinated
(mer-1, see Scheme 2).20,21 There are another two hydride
resonances in the spectra, which appear upon the mixing and
transform one into another during the reaction. They are broad
signals at –2.1 and –8.6 ppm (Figure S6), which belong to
complexes 1·DMAB and 1·(H2B·NMe2), respectively.16–18,22
Complex mer-1 remains catalytically active, so the addition of
new portion of DMAB to the reaction mixture after the first run
initiates another dehydrogenation cycle, which proceeds at the
same rate (Figure S7).
In conclusion, the peculiarities of complex 1, which promote
dehydrogenation of alcohols, also lead to its decreased efficiency
in the dehydrogenation of amine–boranes. The salient
bifunctionality of amine–boranes results in their binding to the
dangling OH group, thus providing the additional stabilization to
the Ir···OH bond. That, in turn, hampers the BH coordination to
iridium, necessary for the dehydrogenation to proceed. This explains
the unexpected decrease in the reaction rate on going from
Me2NH·BH3 to ButNH2·BH3 and NH3·BH3.
21 G. A. Silantyev, E. M. Titova, O. A. Filippov, E. I. Gutsul, D. Gelman
and N. V. Belkova, Russ
Nauk, Ser. Khim., 2015, 2806).
. Chem. Bull., Int. Ed., 2015, 64, 2806 (Izv. Akad.
22 C. J. Stevens, R. Dallanegra, A. B. Chaplin, A. S. Weller, S. A. Macgregor,
B. Ward, D. McKay, G. Alcaraz and S. Sabo-Etienne, Chem. – Eur. J.,
2011, 17, 3011.
This work was supported by the Russian Science Foundation
(grant no. 19-13-00459). NMR spectroscopic data were acquired
using the equipment of Center for Molecular Composition Studies
at theA. N. Nesmeyanov Institute of Organoelement Compounds
of the Russian Academy of Sciences with the financial support
from the Ministry of Science and Higher Education of the Russian
Federation.
Received: 6th December 2019; Com. 19/6081
– 278 –