Efficient Homogeneous Hydridoirida-β-Diketone-Catalyzed Methanolysis of Ammonia-Borane for Hydrogen Release in Air. Mechanistic Insights

Article abstract of DOI:10.1002/ejic.202100318

The hydridoirida-β-diketone [(IrH{(PPh₂(o-C₆H₄CO))₂H})₂(μ-Cl)][BF₄] (2) has been used as a homogeneous catalyst for the methanolysis of ammonia-borane to release up to 3 equivalents of hydr

Full text of DOI:10.1002/ejic.202100318

European Journal of Inorganic Chemistry
Accepted Article
Title: Efficient Homogeneous Hydridoirida-b-diketone-Catalyzed
Methanolysis of Ammonia-Borane for Hydrogen Release in Air.
Mechanistic Insights
Authors: Itxaso Bustos, Zoraida Freixa, Ariadna Pazos, Claudio
Mendicute-Fierro, and Maria Angeles Garralda
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.202100318
Link to VoR: https://doi.org/10.1002/ejic.202100318
01/2020

10.1002/ejic.202100318
European Journal of Inorganic Chemistry
1
Efficient Homogeneous Hydridoirida--diketone-Catalyzed Methanolysis of
Ammonia-Borane for Hydrogen Release in Air. Mechanistic Insights.
Itxaso Bustos,a Zoraida Freixa,a,b Ariadna Pazos,a Claudio Mendicute-Fierro*a and
María A. Garralda*a
a
Facultad de Química de San Sebastián, Universidad del País Vasco (UPV/EHU),
b
Apdo. 1072, 20080 San Sebastián, Spain, Ikerbasque, Basque Foundation for
Science, 48011, Bilbao, Spain.
ABSTRACT
The dihydridoirida--diketone [(IrH{(PPh2(o-C6H4CO))2H})2(-Cl)][BF4] (2) has been
used as a homogeneous catalyst for the methanolysis of ammonia-borane to release up to 3
equivalents of hydrogen in the presence of air. With catalyst loadings as low as 0.2 mol%,
ammonia-borane undergoes methanolysis within 6 min at 30 ºC, with TOF50% of 320
molH ·molIr1·min1, or within 80 s at 60 ºC, with an excellent TOF50% of 1991
2
molH ·molIr1·min1, and maintains its catalytic activity in consecutive runs. Triethylamine-
2
borane fails to undergo methanolysis. Kinetic studies indicate first-order dependence on
substrate and on catalyst concentration and suggest cleavage of the solvent OH bond being
involved in the rate determining step of the reaction. In methanol solution 2 forms cationic
[IrH(MeOH){(PPh2(o-C6H4CO))2H}]+ (3) and reacts with Et3NBH3 to afford a
hydridoirida--diketone [IrH(Me3NBH3){(PPh2(o-C6H4CO))2H}]+ (4), with the borane
adduct 1-coordinated to iridium. Compound [4][BArF ] shows dynamic behaviour in
4
solution due to exchange of bridging and terminal BH bonds. A multinuclear NMR study
of the catalyzed reaction shows the formation of two ammonia-methoxyborane adduct
 Corresponding authors. E-mail: mariaangeles.garralda@ehu.es. http://www.ehu.es;
claudio.mendicute@ehu.eus
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10.1002/ejic.202100318
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intermediates, H3N–BH2(OCH3) and H3N–BH(OCH3)2, and of iridium species proposed of
the hydridodiacyl type [IrH(H3NBH3x(OCH3)x)(PPh2(o-C6H4CO))2] with a coordinated
borane adduct. On account of experimental evidence, a simplified catalytic cycle is
suggested for the methanolysis of AB to release hydrogen.
INTRODUCTION
Increasing energy requirements and environmental concerns are promoting an
intensive reseach on renewable energy sources. Hydrogen, with high energy content,
abundant and clean as fuel leading only to water as by-product, is considered a
sustainable candidate. Nevertheless, its safe storage and release upon demand under
mild reaction conditions is still a challenge.[1] The catalysed release of hydrogen
from chemical materials appears suitable for this purpose and the ammonia-borane
adduct (AB, H3N–BH3) is considered a promising candidate due to its high hydrogen
content, air stability both in solid and solution at room temperature and portability.[2]
Homogeneous transition metal-based catalyzed dehydrocoupling of AB, containing
protons and hydrides, very often affords up to one equivalent of hydrogen requiring
inert atmosphere and a variety of Ni, Pd, Mo or Ru complexes allow higher H2
release, up to 2.8 hydrogen equivalents.[3] Depending on the type of catalyst, the
activation of AB has been proposed to occur through activation of a B–H bond,
activation of a N–H bond or of both sites simultaneously.[3-4] Dehydrocoupling has
also been achieved using nanoparticles, obtaining more than 2 equivalents of H2.[5]
Metal-assisted hydrolysis or methanolysis are also efficient for hydrogen release
from AB. In this case, combining hydridic B–H and protons from the solvent can
afford up to three equivalents of H2 per mole of starting material.[3, 5-6] Mechanisms
proposed for the heterogeneous transition-metal catalysed hydrolysis reactions to
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10.1002/ejic.202100318
European Journal of Inorganic Chemistry
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release H2 include interaction of AB with the catalyst surface, followed by concerted
cleavage of the B−N bond and hydrolysis of the resulting BH3;[7] or formation of a
+
NH4 BH3(OH)− intermediate, which reacts with adjacent H2O to release H2.[8]
Boron-to-metal hydrogen transfer and attack of H2O on a transient MH bond in
–
analogy to BH4 hydrolysis has also been proposed.[9, 10] Cleavage of the O−H bond
in water has been recently suggested to be involved in the rate-determining step of
these reactions leading to hydrogen release and formation of H3NB(OH)3 that upon
11]
hydrolysis produces [NH4][B(OH)4].[10,
Catalytic heterogeneous methanolysis
reactions (see Equation 1) usually allow slower hydrogen evolution than hydrolysis
reactions, whereas advantages of the former include higher stability of AB in
methanol solution and possibility of hydrogen release below 0 ºC.[5] Also, the easy
regeneration of AB from the methanolysis product, ammonium tetramethoxyborate
[NH4][B(OCH3)4], by a room temperature reaction has been reported.[12] As in the
hydrolysis reactions, noble metal nanoparticles are among the most active catalysts
for the methanolysis of AB,[13] and recently homogenized heterogeneous metal
nanoparticle catalysts have proved useful to achieve enhanced catalytic performance
on the methanolysis of AB.[14]
+
H3NBH3 + 4 MeOH  NH4 + [B(OMe)4]– + 3 H2 (Eq. 1)
The efficient homogeneous version of the hydrolysis reaction, was first reported in
2010, using a hydridoirida--diketone, hydrido-PNP-iridium complexes or
dicarbonylruthenacyclic compounds as catalysts.[15] Other iridium unsaturated
carbene,[16] or saturated proton responsive hydroxy-bipyridine complexes,[17]
acylhydrido-rhodium or –iridium derivatives[18] or ruthenium-bipyridine-p-cymene
complexes[19, 20] have also proved efficient for this hydrolysis reaction. Proposed
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mechanisms of the homogeneous hydrolysis reactions catalysed by Ru[20, 21] or Ir[17]
complexes include an initial interaction of the hydridic H–B in AB with the metal
centre and hydrolysis through formation of H3N–BHx(OH)3-x intermediates.
Efficient homogeneous hydrogen release from AB catalysed by Ir[15c] or Ru[19a]
i
complexes using PrOH/H2O mixtures have also been reported. Our group has
recently reported on the first alcoholysis homogeneous version, using a half
sandwich ruthenium complex containing a proton responsive 6,6’-dihydroxy-2,2´-
bipyridine ligand and with an excelent activity represented by an initial TOF10% of
448 molH ·molIr1·min1 or TOF50% of 120 molH ·molIr1·min1 at 60 ºC, showing an
2
2
initial activity that surpases that of any other system known for the alcoholysis of
AB, though the system suffered from deactivation at extended conversions.[22]
We report now on extraordinary efficiency of hydridoirida--diketones
[IrHCl{(PPh2(o-C6H4CO))2H}] (1) and [(IrH{(PPh2(o-C6H4CO))2H})2(-Cl)][BF4]
(2), shown in Scheme 1, to catalyze the ammonia- or amine-borane methanolysis to
release hydrogen, the unprecedented detection of intermediates in the catalyzed
reaction and studies related to mechanistic aspects of the reaction.
BF4
H
Ph2P
Ph2
P
PPh2
Ph2
P
O
H
O
H
H
Ir
Ir
Cl
Ph2P
H
PPh2
O
O
H
O
O
Cl
1
2
Scheme 1
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10.1002/ejic.202100318
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RESULTS AND DISCUSSION
The hydridoirida--diketone 1 is an efficient catalyst for the release of hydrogen by
methanolysis of ammonia-borane in the presence of air. When using initial AB
concentration of 0.46 M and a 0.4 mol% catalyst loading, at 30 ºC, release of up to
2.7 equivalents of hydrogen per mol of borane adduct within 14 min is obtained. As
shown in Figure 1, the appearance of an induction period prior to fast turnover is
observed, most likely related to the low solubility of complex 1 in methanol. When
performing the reaction at a higher temperature, namely 60 ºC, the induction period
almost disappears (10 s), allowing the release of up to three equivalents of hydrogen
per mole of borane adduct, which according to equation 1 represents 100% of the
maximum hydrogen content that can be produced, within only 2 min, with TOF50%
of 865 molH ·molIr1·min1, computing time as that elapsed post-induction. Addition
2
of excess Hg during productive turnover leads to similar results suggesting the
homogeneity of the catalytic reaction[23] (SI, Figure S1). No darkening of the clear
yellow solution was observed. It is also remarkable that the present iridium catalyst
remains active for at least six successive runs as shown in SI, Figure S2.
3
2,5
2
3
2,5
2
1,5
1
1,5
1
1
2
60 ºC
30 ºC
0,5
0
0,5
0
0
500
1000
1500
0
50
100
150
200
Time (s)
Time (s)
(a)
(b)
Figure 1.- Hydrogen release from 0.46 M MeOH solution of AB.with complexes 1
and 2. (a) Using 0.4 mol% of 1 as catalyst, at 30 ºC (□, orange) or 60 ºC (◊, blue)
(b) Using 0.4 mol% of 1 (◊, blue) or 0.2 mol% of 2 (Δ, green) as catalyst, at 60 ºC.
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10.1002/ejic.202100318
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The kinetic profile obtained in the methanolysis of AB catalysed by complex 1 at 60 ºC
can be considered to follow a pseudo-first-order reaction rate model with respect to the
substrate, as shown by the linear plots in SI, Figure S3, which was applied to determine
the overall rate constants, kobs. The rate of the hydrogen release depends also on the
catalyst loading (SI, Figure S3). Assuming a first order dependence with respect to the
catalyst the rate law agrees with: vexp = kcat[catalyst]0[substrate], where kcat[catalyst]0 =
kobs. A plot of the pseudo-first-order rate constant (kobs) versus [catalyst]0 in the 1.86 x
103 to 0.46 x 103 M range (SI, Table S1 and Figure S4) allows the proposal of a first-
order dependence on catalyst and kcat = 16.0 ± 0.6 M1 s−1.
Our search for a methanol more soluble hydridoirida--diketone than 1 led us to study
the catalytic activity of the ionic dimer [(IrH{(PPh2(o-C6H4CO))2H})2(-Cl)][BF4] (2)
(see Scheme 1). Complex 2, with two hydridoirida--diketone fragments bonded by a
chloride bridge, shows higher solubility in MeOH than 1 and certainly allows faster
hydrogen release from methanolysis of AB (see Figure 1). When using initial AB
concentration of 0.46 M and a 0.2 mol% loading of 2, which represents a 0.4 mol%
loading of iridium, at 30 ºC, the appearance of a now short induction period (40 s) is
observed, and the release of up to 2.8 equivalents of hydrogen per mol of borane adduct
within 6 min is obtained, with TOF50% of 320 molH ·molIr1·min1 (SI, Figure S5). When
2
carrying the reaction at 60 ºC, the induction period is no longer observed and the release
of up to three equivalents of hydrogen per mole of borane adduct, according to equation
1, occurs within 80 s, with an excellent TOF50% of 1991 molH ·molIr1·min1. It thus
2
appears that the induction period, most likely arising out of an active species formation,
becomes markedly shorter upon higher solubility of the precatalyst. No darkening of the
clear yellow solution, neither appearance of any insoluble material was observed during
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10.1002/ejic.202100318
European Journal of Inorganic Chemistry
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the course of the reaction. The Hg test (SI, Figure S6) supports the homogeneity of the
reaction.[23] We were pleased to observe that catalyst 2 remains active for at least six
successive runs, as shown in SI, Figure S7.
The linear plots shown in SI, Figure S8 allow a pseudo-first-order reaction rate model
with respect to [AB] to calculate the overall rate constants, kobs. The rate of the
hydrogen release depends also on the catalyst loading (SI, Table S2). A plot of the
pseudo-first-order rate constants (kobs) versus [Ir]0, in the 1.86 × 10−3 to 0.46 × 10−3 M
range (0.4 to 0.1 mol%), agrees with a first order dependence on iridium. From this plot,
a value of kcat = 42.0 ± 3.5 M−1 s−1 was obtained for compound 2.
Complex 2 is also able to release hydrogen by methanolysis of amine-borane
adducts such as dimethylamine-borane (DMAB) or tert-butylamine-borane (TBAB),
while it is unable to promote the methanolysis of triethylamine-borane (TEAB) as
shown in SI, Figure S10. This observation suggests that, as in the hydrolysis
reactions, the presence of a NH functionality in the amine-borane substrate is a
requirement for an efficient catalyzed methanolysis. At 60 ºC, with initial 0.46 M
substrate and using a 0.2 mol% loading of 2 (0.4 mol% of Ir), DMAB requires a
short, 10 s, activation period and releases up to 2.9 equivalents of hydrogen per mole
of borane adduct within 100 s, with TOF50% of 848 molH ·molIr1·min1. The more
2
encumbered TBAB requires also this short activation period and releases up to 2.6
equivalents of hydrogen per mole of borane adduct within 4 min, with TOF50% of
271 molH ·molIr1·min1.
2
Deuteration and NMR studies on the catalytic reaction. In order to obtain further
insights on the course of the catalytic methanolysis of AB we have undertaken
deuteration and multinuclear “in situ” NMR studies in CD3OD. Performing the
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methanolysis of H3NBH3 catalysed by 2 at 60 ºC in CH3OH results in kobs = (73.2 ±
1.6)x103 s1 for the hydrogen release, while the methanolysis of H3NBH3 with
CD3OD (kobs = (28.2 ± 0.2)x103 s1) or with CH3OD (kobs = (30.0 ± 0.5)x103 s1),
following similar rates, were slower than that observed when using CH3OH, as
shown in SI, Figure S11. Calculated KIE of 2.60 ± 0.08 (kCH OH/CD OD) or 2.44 ±
3
3
0.09 (kCH OH/CH OD), suggest that cleavage of the solvent OH bond is involved in
3
3
the rate determining step of the catalysed reaction. On the other hand, the
methanolysis of H3NBD3 catalysed by 2 at 60 ºC in CH3OH results in kobs = (80.4 ±
2.1)x103 s1 for the hydrogen release, leading to an inverse KIE of 0.91 ± 0.03
(kH3NBH /H NBD3). This inverse KIE would be consistent with a rate determining step
3
3
involving protonation of a IrH (or IrD) species to form HH, when H3NBH3 is
used, or the more strongly bound HD for H3NBD3..[24]
The multinuclear NMR study of the methanolysis of AB catalysed by 2 was carried
out in CD3OD (SI, Figure S12). Due to the reaction being extremely fast, the 11B
NMR spectrum shows the presence of only a singlet at 9.3 ppm, corresponding to
the methoxyborate species in [NH4][B(OCH3)4],[12] which indicates the quantitative
1
transformation of AB (–23.5 {q, JH,B = 93 Hz} ppm). Consequently, the H NMR
spectrum shows no signal due to AB at 1.45 ppm and the release of HD (4.55 ppm
{t, JD,H = 42.6 Hz}) is observed. The appearance of a multiplet at high field (–21.4
1
ppm) in the H NMR spectrum and of a signal at 19.3 ppm in the 31P{1H} NMR
spectrum suggest the formation of new iridium species (F, vide infra) containing a
hydride cis to Ir-coordinated phosphorus atoms.
The AB methanolysis reaction using complex 1 as catalyst is markedly slower than
that catalysed by 2, therefore we carried out the corresponding multinuclear NMR
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European Journal of Inorganic Chemistry
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study using 1 as catalyst and were able to observe some intermediates formed in the
11
catalysed reaction. The B NMR spectrum of a freshly prepared solution in CD3OD
shows the presence of the H3N–BH3 substrate and of the anionic reaction product
[B(OCH3)4]. More interestingly minor amounts of two ammonia-methoxyborane
adduct intermediates, up to our knowledge previously undetected, were now
observed in the coupled spectrum shown in Figure 2: a triplet at –13.9 {t, JH,B = 100
Hz} ppm and a doublet at 5.9 {d, JH,B = 120 Hz} ppm, which can be attributed to the
H3N–BH2(OCH3) and H3N–BH(OCH3)2 adducts, respectively, considering the
[25]
reported sequence for the different anions in Na[BHx(OCH3)4–x
]
and the recently
reported adduct formed by pinacolborane and a NNNN macrocycle.[26]
11
Figure 2.- B NMR spectrum of the freshly prepared AB (0.65 mmol) / 1 (0.006
mmol) solution in 0.5 mL CD3OD.
The consecutive 11B NMR spectra (SI, Figure S13a) show the disappearance of the
signal at higher field due to the H3N–BH3 substrate and an increasing amount of
[B(OCH3)4], while the small resonances due to the intermediates remain throughout
the course of the reaction and finally disappear, to appear again upon addition of a
1
new amount of AB to the NMR tube. The H NMR spectra of the methanolysis of
H3N–BH3 catalysed by 1 show the gradual disappearance of AB, the release of
hydrogen and the formation of new hydridoiridium species also observed in the
31P{1H} NMR spectra (SI, Figure S13b-c). The initial appearance of broad
1
31
resonances at  = –9.15 ppm in the H NMR and  = 5.5 ppm in the P{1H} NMR
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10.1002/ejic.202100318
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spectra, similar to those reported in the hydrolysis of AB catalysed by 1,[15b]
suggests the formation of an iridium species (B) containing a hydride trans to the
carbon atom of an acyl group and cis to phosphorus atoms, which disappears rapidly
allowing the appearance of new species F, (1H –21.40 ppm and 31P{1H}, 19.3
ppm), already observed along with the ammonium tetramethoxyborate final product
in the methanolysis reaction catalysed by 2. The addition of a new amount of AB to
the NMR tube, restarts the hydrogen evolution and the immediate appearance of B,
which is observed in the 1H NMR spectrum.
Reactivity of 2 towards amine-boranes in MeOH. We have also undertaken
studies on the behaviour of complex 2 in methanol and towards amine-borane
adducts. The NMR spectra of a freshly prepared solution of complex 2 in a
CD3OD/CDCl3 mixture indicates that methanol is able to cleave the chlorine bridge
in complex 2. As shown in Scheme 2i, this unsymmetrical cleavage affords a
cationic
complex
with
coordinated
methanol
[IrH{(PPh2(o-
C6H4CO))2H}(MeOH)][BF4] ([3]BF4), along with complex 1. Complex 3 was
identified by NMR[27] (SI, Figure 14) showing spectroscopic features similar to
1
those of related [IrH{(PPh2(o-C6H4CO))2H}(acetone)]+.[28] According to the H
NMR spectrum a mixture of all three complexes, in a 2/3/1 = 3.5:1:1 ratio is formed
and the equimolar 3/1 ratio confirms the proposed cleavage. Attempts to isolate
[3][BF4] proved unsuccessful.
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Scheme 2. Cleavage of the chloride bridge in 2. (i) CD3OD/CDCl3. (ii) Me3N–BH3
in CD3OD.
This observation prompted us to perform the reaction of complex 2 with the Me3N–
BH3 adduct, unable to undergo the methanolysis reaction, in CD3OD to determine
the ability of borane adducts to cleave the chlorine bridge and coordinate to the
iridium atom. Late transition metals are known to bind Lewis base-borane adducts to
afford borane-bonded complexes.[29] Indeed, according to the NMR spectra (SI,
Figure S15) the reaction shown in Scheme 2ii afforded a mixture of two species,
namely a new cationic complex 4 in compound [4][BF4], containing a coordinated
borane, and complex 1, which precipitated readily from the methanol solution.
Complex 4 was isolated as compound [4][BArF ] by reacting 1 with Me3N–BH3 in
4
dichloromethane in the presence of the halide scavenger Na[BArF ], as shown in
4
Scheme 3.
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10.1002/ejic.202100318
European Journal of Inorganic Chemistry
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Scheme 3. Formation of compound [IrH(Me3NBH3){(PPh2(o-C6H4CO))2H}]BArF .
4
Compound [4][BArF ] is stable and was characterized in solution by NMR
4
31
spectroscopy (SI, Figures S16-17) and ESI-MS (SI, Figure S19). The P{1H} NMR
1
spectrum shows a singlet at 23.1 ppm, due to equivalent phosphorus atoms. The H
NMR spectrum shows a triplet in the high field region, at 18.39 ppm, which agrees
with a hydride being trans to a BH bond,[30] and cis to both phosphines (JP,H = 14.6
… …
Hz) and a low field singlet at 22.61 ppm in agreement with a fairly strong O H O
hydrogen bond and complex 4 being of the irida--diketone type containing a
formally tetradentate ligand PCCP. At room temperature the BH3 fragment is
observed as a very broad resonance, almost indistinguishable from the base line, at
ca. 2.4 ppm. As other complexes containing base-stabilized 1-H3BL ligands,[23b-
c, 29, 31] 4 shows dynamic behavior in solution so that by 213 K two broad resonances
due to the bridging (10.50 ppm, 1 H relative integral) and terminal (+1.40 ppm, 2
H relative integral) BH protons of the coordinated adduct are observed, while the
… …
hydride and O H O resonances remain unaltered. This behaviour suggests
coordination of borane in an end-on 1-fashion, through a BHM three-center two
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10.1002/ejic.202100318
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electron bond and fluxionality being due most likely to exchange between terminal
and bridging BH protons or to dissociation of the ligand. In the IR spectrum (SI,
Figure S18) two bands at 2504 and 2444 cm1, in the range expected for terminal
boron hydrides, are observed. The broad signal at 1792 cm1, attributed to ν(Ir-H),
may include bridging BH stretching. These results show coordination of the borane
adduct to iridium, provided the chloride is removed from the coordination sphere of
the iridium atom.
Our experimental results suggest that the present methanolysis of AB is a
homogeneous metal-catalyzed intermolecular process with release of hydrogen
occurring in succesive steps for a particular AB to afford H3NBH2(OMe),
H3NBH(OMe)2 and H3NB(OMe)3, and in a parallel fashion for the whole
substrate. MeOH assisted BN bond cleavage in the trimethoxy substituted adduct
H3NB(OMe)3 leads to the observed [NH4][B(OMe)4] final product as in the related
AB hydrolysis.[11, 17] Taking into account i) that coordinated boranes are liable to
22]
undergo H2O nucleophilic attack;[20,
ii) the ability of methanol solutions of
transient dihydridobis(acyldiphenylphosphine)iridate(III) species to release
hydrogen with formation of hydride derivatives by O-to-Ir hydrogen transfer from a
hydroxyl fragment;[32] and iii) the present experimental evidences, we may propose a
simplified mechanism for the methanolysis of AB catalysed by irida--diketones,
shown in Scheme 4, which represents a general catalytic cycle that can be applied to
all the three succesive steps leading to the hydrogen release.
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European Journal of Inorganic Chemistry
14
H
H
H3N
B
H
X
Ph2
P
Ph2
P
Ir
1 or 2
H
O
O
H
H
B
NH3
H
H
A
S
Ph2
P
Ph2
P
H+
Ir
O
O
O
H
Ph2
P
F
PPh2
H
Ir
O
H2
MeOH
B
NH3
R
R'
H
B
H
B
NH3
R
R'
O
O
Ph2
P
Ph2
P
PPh2
PPh2
H
Ir
Ir
H
O
O
H
B
NH3
Me
R
R'
H
H
E
O
H
TS-1
MeO
B
NH3
R
R'
O
Ph2
P
PPh2
H
Ir
Me
B
H
O
O
+
H
NH3
R
C
R'
D
Scheme 4.- Simplified mechanism for the methanolysis of ammonia-borane with
hydridoirida--diketones. First step: R = R’ = H; Second step: R = H, R’ = OMe;
Third step: R = R’ = OMe. [S = Solvent and AB = ammonia-borane]
In methanol solution AB may react with 1 or 2 to afford a cationic irida--diketone
(A) with coordinated AB. Irida--diketones are known to undergo deprotonation in
the presence of bases and also in methanolic basic solution, which can result in loss
of the planar Ir(PCCP) arrangement and isomerisation.[33] In the present case, the
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deprotonation of A most likely gives neutral species that may undergo isomerisation,
leading to species B, with hydride trans to acyl. The early observation of B (by “in
situ” NMR analysis) in such a fast process suggests it could be the resting state.
Nucleophilic attack of MeOH to the boron atom in B may lead, via TS-1, to the
formation of a dihydridoiridate(III) (C) and a methanol-stabilized boronium cation
(D). The O-to-Ir hydrogen transfer, with OH cleavage, results in the dihydrogen
species E and release of the observed intermediate ammonia-methoxyborane adducts
H3NBH2(OMe), H3NBH(OMe)2 or H3NB(OMe)3. On account of the OH/OD and
H3NBH3/H3NBD3 KIEs we propose the C to E step as the rate determinig step.
Finally, coordination of a borane adduct displaces hydrogen from E and affords
species B, able to restart the hydrogen release. At high conversions, the solvent may
presumably become competitive as ligand affording species F, with coordinated
solvent. Species F would thus be the only observable species in solution upon
consumption of the substrate. The reaction of the fully dehydrogenated adduct
H3NB(OMe)3, with MeOH affords the tetramethoxyborate final product. Our
experimental findings have allowed for the proposal of a simplified mechanism.
Nevertheless, alternative mechanisms for this complex process involving at least
three successive methanolysis reactions and agreeing with our experimental data
cannot be excluded.
CONCLUSIONS
The cationic hydridoirida--diketone [(IrH{(PPh2(o-C6H4CO))2H})2(-Cl)][BF4] is a
very efficient homogeneous catalyst for the methanolysis of ammonia-borane in air to
release hydrogen. The measured activities for methanolysis of ammonia-borane using
compounds 1 and 2 (TOF50% of 865 molH ·molIr1·min1 and 1991 molH ·molIr1·min1 at
2
2
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16
60 ºC respectively) surpass those ever reported previously for this process. Namely,
compound 2 is ten times faster than the only homogeneous system known to be active
in this reaction up to date (TOF50% of 120 molH ·molIr1·min1 at 60ºC).[22] Amine-
2
borane adducts may coordinate to iridium to afford fluxional borane-coordinated
cationic
hydridoirida--diketones
[IrH(R3NBH3){(PPh2(o-C6H4CO))2H}]+.
A
multinuclear NMR study allows a proposal for the catalyzed reaction to occur in
successive and parallel steps for the whole substrate. Adduct H3N–BH3, H3N–
BH2(OCH3) or H3N–BH(OCH3)2 coordination to iridium via the borane fragment in
hydridodiacyl [IrH(H3NBH3x(OCH3)x)(PPh2(o-C6H4CO))2] species, followed by
nucleophilic attack of MeOH to boron and O-to-Ir hydrogen transfer results in the
release of hydrogen. Cleavage of the OH bond in MeOH appears involved in the rate
determining step of the catalyzed reaction.
EXPERIMENTAL SECTION
General Procedures: Synthetic procedures were carried out at room temperature under
nitrogen by standard Schlenk techniques. [IrHCl{(PPh2(o-C6H4CO))2H}] (1)[34] and
[(IrH{(PPh2(o-C6H4CO))2H})2(-Cl)]BF4 (2)[28] were prepared as previously reported.
All other reagents were purchased from commercial sources and used without further
purification. Microanalysis was carried out with a Leco Truspec Micro microanalyzer.
IR spectra were recorded with a Nicolet FTIR 510 spectrophotometer in the range 4000-
1
400 cm–1 using KBr pellets. H (TMS internal standard), 31P{1H} (H3PO4 external
standard) and 11B (BF3·Et2O external standard) NMR spectra were recorded with Bruker
Avance DPX 300 or Bruker Avance 500 spectrometers. ESI-MS spectra were recorded
on a Bruker MicrOTOF-Q Instrument. Good fit to both the principal molecular ion and
the overall isotopic distribution were obtained.
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17
Preparation of [IrH(Me3NBH3){(PPh2(o-C6H4CO))2H}][BArF ] ([4]BArF ). To a
4
4
solution of complex [IrHCl{PPh2(o-C6H4CO))2H}] (0.037mmol, 30 mg) in
dichloromethane trimethylamine borane (0.037 mmol, 2.7 mg) and sodium tetrakis[3,5-
bis(trifluoromethyl)phenyl]borate salt (0.037 mmol, 32.8 mg) were added. The resultant
yellow solution was stirred for 30 min and then washed three times with distilled water
and dried over magnesium sulphate. Filtration and solvent removal under low pressure
afforded an off-yellow solid that was collected. Yield 46 mg (72% ). IR (KBr, cm-1):
2504 (w), ν(B-Ht); 2444 (w), ν(B-Ht); 1792 (br), ν(Ir-H); 1609 (m), ν(C=O). Anal.
Calcd. For IrC74H57P2O2NB2F24·(CH2Cl2)0.5 C 50.66, H 3.31, N 0.79; found C 50.59, H
3.22, N 0.58. ESI-MS (CH3OH): calcd For C41H42BNO2P2Ir 846.2; observed 846.2
1
{M}+. H NMR (CDCl3, 298 K, 500 MHz): δ 18.39 (t, 2JP,H= 14.6 Hz, 1H, H-Ir); 2.4
(br, 3H, H-B); 1.83 (s, 9H, H3C); 78.5 (28H, Aromatics); 22.61 (br, 1H, O--H--O)
2
ppm. (CDCl3, 213 K, 500 MHz): δ 18.09 (t, JP,H= 14.6 Hz, 1H, H-Ir); 10.50 (s, 1H,
H-B); 1.40 (br, 2H, H-B); 1.83 (s, 9H, H3C); 78.5 (28H, Aromatics); 22.75 (s, 1H, O--
H--O) ppm. 31P{1H} NMR (CDCl3, 298 K, 162 MHz)): δ 23.1 (s) ppm.
Methanolysis of H3NBH3 with Complexes 1–2. A solution of 1.16 mmol of the
desired amine-borane adduct in 2 mL of methanol was prepared in a round bottom 40
mL flask fitted with a gas outlet and with a side arm sealed with a tight-fitting septum
cap. The flask was connected via the gas outlet to a gas burette filled with water. The
amine-borane adduct solution was immersed in a thermostated water bath to reach the
desired temperature under atmospheric pressure (1 atm) and in the presence of air. A
solution of the selected precatalyst, 3.75 mg, 4.64x103 mmol of 1 or 3.87 mg,
2.32x103 mmol of 2, in 0.5 mL of methanol was syringed through the septum into the
reaction flask, magnetic stirring connected and timing started. Gas evolution began
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10.1002/ejic.202100318
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18
immediately and the released gas was measured by determining periodically the volume
of water displaced in the burette.
ACKNOWLEDGMENTS
Partial financial support by Ministerio de Economía
y
Competitividad
MINECO/FEDER (CTQ2015−65268-C2−1-P and PID2019-111281GB-I00), Gobierno
Vasco (GIC 18/143 and IT1180-19) Universidad del País Vasco (UPV/EHU) and
Diputación Foral de Gipuzkoa are gratefully acknowledged. I. B. acknowledges support
by UPV/EHU.
KEYWORDS
Ammoniaborane, hydrogen release, irida--diketones, methanolysis
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Table of Contents Graphic
Hydridoirida--diketones promote methanolysis of ammonia-borane by end-on 1-
fashion borane coordination of H3NBH3 (see graph) and of intermediates H3NBH3-
x(OMe)x (X = 1, 2) in a similar manner, followed by nucleophilic attack of MeOH to
boron, and release of hydrogen.
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