Ammonia Borane Dehydrogenation Catalyzed by (κ4-EP3)Co(H) [EP3 = E(CH2CH2PPh2)3; E = N, P] and H2 Evolution from Their Interaction with NH Acids

Article abstract of DOI:10.1021/acs.inorgchem.6b02673

Two Co(I) hydrides containing the tripodal polyphosphine ligand EP₃, (κ⁴-EP₃)Co(H) [E(CH₂CH₂PPh₂)₃; E = N (1), P (2)], have been exploited as ammonia borane (NH₃BH₃, AB) dehydrogenation catalysts in THF solution at T = 55 °C. The reaction has been analyzed experimentally through multinuclear (¹¹B, ³¹P{¹H}, ¹H) NMR and IR spectroscopy, kinetic rate measurements, and kinetic isotope effect (KIE) determination with deuterated AB isotopologues. Both complexes are active in AB dehydrogenation, albeit with different rates and efficiency. While 1 releases 2 equiv of H₂ per equivalent of AB in ca. 48 h, with concomitant borazine formation as the final spent fuel , 2 produces 1 equiv of H₂ only per equivalent of AB in the same reaction time, along with long-chain poly(aminoboranes) as insoluble byproducts. A DFT modeling of the first AB dehydrogenation step has been performed, at the M06//6-311++G? level of theory. The combination of the kinetic and computational data reveals that a simultaneous B-H/N-H activation occurs in the presence of 1, after a preliminary AB coordination to the metal center. In 2, no substrate coordination takes place, and the process is better defined as a sequential BH₃/NH₃ insertion process on the initially formed [Co]-NH₂BH₃ amidoborane complex. Finally, the reaction of 1 and 2 with NH-acids [AB and Me₂NHBH₃ (DMAB)] has been followed via VT-FTIR spectroscopy (in the -80 to +50 °C temperature range), with the aim of gaining a deeper experimental understanding of the dihydrogen bonding interactions that are at the origin of the observed H₂ evolution.

Full text of DOI:10.1021/acs.inorgchem.6b02673

Article
pubs.acs.org/IC
Ammonia Borane Dehydrogenation Catalyzed by (κ4‑EP3)Co(H) [EP3
= E(CH2CH2PPh2)3; E = N, P] and H2 Evolution from Their Interaction
with NH Acids
Stefano Todisco,† Lapo Luconi,† Giuliano Giambastiani,†,§ Andrea Rossin,*,† Maurizio Peruzzini,†
Igor E. Golub,‡,∥ Oleg A. Filippov,‡ Natalia V. Belkova,‡ and Elena S. Shubina*,‡
†Istituto di Chimica dei Composti Organometallici−Consiglio Nazionale delle Ricerche (ICCOM - CNR), Via Madonna del Piano
10, 50019, Sesto Fiorentino, Italy
‡A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences (INEOS RAS), Vavilova Street 28, 119991
Moscow, Russia
§Kazan Federal University, Kremlyovskaya Street 18, 420008 Kazan, Russia
∥People’s Friendship University of Russia (RUDN University), 6 Miklukho-Maklay St., 117198 Moscow, Russia
S
* Supporting Information
ABSTRACT: Two Co(I) hydrides containing the tripodal
polyphosphine ligand EP3, (κ4-EP3)Co(H) [E(CH2CH2PPh2)3; E
= N (1), P (2)], have been exploited as ammonia borane
(NH3BH3, AB) dehydrogenation catalysts in THF solution at T =
55 °C. The reaction has been analyzed experimentally through
multinuclear (11B, 31P{1H}, 1H) NMR and IR spectroscopy,
kinetic rate measurements, and kinetic isotope effect (KIE)
determination with deuterated AB isotopologues. Both complexes
are active in AB dehydrogenation, albeit with different rates and
efficiency. While 1 releases 2 equiv of H2 per equivalent of AB in
ca. 48 h, with concomitant borazine formation as the final “spent
fuel”, 2 produces 1 equiv of H2 only per equivalent of AB in the
same reaction time, along with long-chain poly(aminoboranes) as
insoluble byproducts. A DFT modeling of the first AB dehydrogenation step has been performed, at the M06//6-311++G**
level of theory. The combination of the kinetic and computational data reveals that a simultaneous B−H/N−H activation occurs
in the presence of 1, after a preliminary AB coordination to the metal center. In 2, no substrate coordination takes place, and the
process is better defined as a sequential BH3/NH3 insertion process on the initially formed [Co]−NH2BH3 amidoborane
complex. Finally, the reaction of 1 and 2 with NH-acids [AB and Me2NHBH3 (DMAB)] has been followed via VT-FTIR
spectroscopy (in the −80 to +50 °C temperature range), with the aim of gaining a deeper experimental understanding of the
dihydrogen bonding interactions that are at the origin of the observed H2 evolution.
characterization. Nevertheless, when considering practical
applications (such as the transportation sector), these metals
are unsuitable because of their low natural abundance and high
cost. The choice of more cost-effective and earth-abundant 3d
elements (like titanium,4 chromium,5 manganese,6 iron,7 or
nickel8) represents the new frontier in this research area.
Within the 3d series, cobalt is one of the least exploited. To
date, only one example of a cobalt(I) (PBP) pincer complex
has been shown to catalyze AB dehydrogenation with
satisfactory rates at room temperature and with the production
of 1 equiv of H2 per monomer.9 Besides pincer backbones,
another class of robust ligands is that of the potentially
tetradentate E(CH2CH2PPh2)3 (EP3; E = N, P). EP3 ligands
INTRODUCTION
■
Chemical hydrogen storage, i.e., the possibility to release
molecular H2 on demand from hydrogen-rich, light, and safe
compounds, has become a challenging research area in the past
decade.1 Under this perspective, chemical compounds that are
easy-to-handle, thermally stable, and with a relatively high
hydrogen content are in the spotlight of contemporary
chemical hydrogen storage research. One of the most studied
inorganic materials is ammonia borane (NH3BH3, AB),2 with a
maximum hydrogen content of 19.3 wt %. Homogeneously
catalyzed AB dehydrogenation has become increasingly popular
in the recent organometallic chemistry literature, where H2
evolution is mediated by organometallics of the elements from
the second and third transition series.3 The exploitation of 4d
or 5d metals helps with the stabilization of reaction
intermediates, allowing for their isolation and thorough
Received: November 6, 2016
© XXXX American Chemical Society
A
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offer a diversity of coordination geometries, showing varied κ4-,
κ3-, and κ2-coordination hapticities.10 The presence of the E
donor and of ethylene spacers separating the phosphorus
donors from the bridgehead atom makes EP3 more flexible than
other tripodal polyphosphines, e.g., MeC(CH2PPh2)3 (triphos),
increasing at the same time the overall basicity at the metal
center.10,11 Following our previous investigations on NP3-
containing ruthenium polyhydrides as AB dehydrogenation
catalysts,12 two cobalt hydrides (κ4-NP3)Co(H) (1) and (κ4-
PP3)Co(H) (2) (Scheme 1) have been exploited for the same
was syringed into the NMR tube containing the hydride sample, at 25
°C. The temperature was slowly raised to 55 °C within the NMR
1
spectrometer probe head, and new sets of multinuclear H, 31P{1H},
and 11B NMR data were collected at that temperature for a total
analysis time of 24 h.
Kinetic Measurements. The H2 production during the reaction of
AB with 1 or 2 in THF was monitored through the Man of the Moon
X102 kit [see Figure S1 of the Supporting Information (SI) for
details]. In a typical experiment, 26 mg of 1 (0.036 mmol) or 28 mg of
2 (0.038 mmol) were placed under an inert atmosphere in a two-
necked round-bottomed 30 mL flask connected to a switchable three-
way valve through a Torion screw. The solid was dissolved in 3 mL of
THF; then, a total amount of 15 equiv of solid AB was added to the
catalyst solution. The as-obtained mixture was brought to 55 °C in a
water/oil bath under stirring, and then the valve was switched to the
pressure transducer which is wirelessly connected to the software
recording the kinetic profile online for a total time of 24 h. The
resulting kinetic data were corrected by the vapor pressure of pure
THF at the same temperature recorded on a blank solution; the
pressure increase caused by THF was used to calculate the correct
number of released H2 equivalents. The calculation was made by
assuming ideality for hydrogen gas, applying the PV = nRT ideal gas
law. At the end of the reaction, the supernatant was transferred to an
NMR tube and analyzed through 11B NMR spectroscopy, to identify
the soluble reaction byproducts. In the case of 2, the insoluble residue
obtained after 24 h was washed twice with degassed pentane and dried
under vacuum before recording the IR spectrum (KBr).
a
Scheme 1. Synthesis of the EP3 Co(I) Hydrides 1 and 2
a
See refs 14 and 15 for details.
task, at T = 55 °C. Their reaction with AB in catalytic regimes
has been followed experimentally [multinuclear 11B, 31P{1H},
1H NMR and IR spectroscopy, kinetic rate measurements, and
kinetic isotope effect (KIE) studies through the employment of
deuterated AB isotopologues] and modeled computationally
(DFT//M06//6-311++G** modeling of the first dehydrogen-
ation step). The interaction of 1 and 2 with AB and
Me2NHBH3 (DMAB) has been examined under stoichiometric
conditions using variable-temperature IR spectroscopy (VT-
FTIR), to gain a deeper understanding of the dihydrogen
bonding (DHB) interactions that are at the origin of the
observed H2 evolution.
Variable-Temperature Infrared (VT-IR) Measurements. Vari-
able-temperature IR studies were carried out on FTIR Shimadzu IR
Prestige-21 spectrometer using 0.04−0.22 cm CaF2 cells in the −80 to
+50 °C temperature range using a home-modified cryostat (Carl Zeiss
Jena). The accuracy of the experimental temperature was 0.5 °C.
The cryostat modification allows for transfer of the reagents (premixed
at either low or room temperature) under an inert atmosphere directly
into the cells.
Computational Details. Density functional theory (DFT)
calculations were performed using the Gaussian09 program (revision
C.01).17 Model structures were optimized with a M06 functional18
(already employed successfully in other cases for the treatment of
molecules containing dative bonds19) using the SDD/D95 pseudopo-
tential and a related basis set20 on the cobalt and phosphorus atoms, a
6-31G* basis set on the C atoms, and a 6-311++G** basis set on the
B, N, and H atoms. The introduction of diffuse functions is essential to
well-reproduce conformational equilibria and experimental electron
affinities.21 An extra d-type polarization function for P and an extra f-
type function for Co were added to the standard set.22 Gibbs energy
calculations to infer relative thermodynamic stabilities were carried out
on a model system. The initially guessed geometry for the
optimization was obtained starting from the XRD structures of 1
and 2 and by replacing the phenyl groups on the NP3 ligand with H
atoms (HNP3), in order to reach a satisfactory compromise between
model system accuracy and short computational times. IRC analysis23
was performed to find the two minima linked by the related transition
structure. When IRC calculations failed to reach the minima, geometry
optimizations from the initial phase of the IRC path were performed.
Frequency calculations were made on all the optimized structures, to
characterize the stationary points as minima or transition states (TSs),
as well for the calculation of zero-point energies, enthalpies, entropies,
and gas-phase Gibbs energies at 298 K. Evaluation of the solvent
effects was performed through a continuum modeling of the reaction
medium. Bulk solvent effects (THF, ε = 7.42) were expressed through
the SMD continuum model,24 with the same basis set used for the gas-
phase optimizations. Gibbs energy in solution was calculated according
to the following simplified equation: GTHF = Ggas + (ETHF − Egas).
EXPERIMENTAL SECTION
General Considerations. All reactions were performed using
■
standard Schlenk procedures under a dry nitrogen atmosphere. The
NP3 ligand,13 1,14 2,15 NH3BD3, ND3BH3, and ND3BD3 were
16
prepared according to the published procedures. Commercial reagents
(PP3, ammonia borane, dimethylamine borane, sodium borohydride,
cobalt tetrafluoroborate hexahydrate [Co(BF4)2·6H2O]) were pur-
chased from Aldrich and used as received, without further purification.
Tetrahydrofuran (THF) and toluene were purified by standard
distillation techniques. THF-d8 (Aldrich) was stored over 4 Å
molecular sieves and degassed by three freeze−pump−thaw cycles
before use. NMR spectra were recorded on a BRUKER AVANCE II
1
300 spectrometer. H and 13C{1H} chemical shifts are reported in
parts per million (ppm) downfield of tetramethylsilane (TMS) and
were calibrated against the residual resonance of the deuterated
solvent, while 31P{1H} chemical shifts were referenced to 85% H3PO4
with downfield shift taken as positive. 11B and 11B{1H} were
referenced to BF3·OEt2. FTIR spectra in the solid state (KBr pellets)
were recorded on a PerkinElmer Spectrum BX Series FTIR
spectrometer, in the 4000−400 cm−1 range, with a 2 cm−1 resolution.
NMR Monitoring of the Reactions of 1 and 2 with AB. A
quartz 5 mm NMR tube was loaded with 30 mg of 1 (molecular mass
= 713.65 u, 0.042 mmol) or 31 mg of 2 (molecular mass = 730.62 u,
0.042 mmol) under an inert atmosphere, and then 0.3 mL of dry and
degassed THF-d8 was transferred into the tube via cannula, under
RESULTS AND DISCUSSION
1
■
nitrogen. The solution obtained was first used to record the H and
31P{1H} NMR spectra of the catalysts. The reactions with AB were
monitored as follows: 5 equiv of AB was dissolved in 0.2 mL of THF-
d8 in a separate Schlenk flask under nitrogen, and the resulting solution
Characterization of Hydrides. The classical Co(I)
hydrides 1 and 2 were straightforwardly prepared following
the literature procedures14,15 from a mixture of cobalt(II)
B
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Figure 1. Typical 11B{1H} NMR spectra evolution over time of the 1:AB mixtures (1:3 stoichiometric ratio) in THF-d8 at T = 55 °C.
tetrafluoroborate hydrate and EP3 in the presence of an excess
(2−4 equiv) of a reducing agent (NaBH4) at T = 50 °C, in
acetone/ethanol (Scheme 1). A full and detailed multinuclear
NMR characterization has already been reported for the PP3-
containing species 2,25 while it is missing for the NP3 analogue
1. Therefore, we report it herein for the sake of completeness.
In THF-d8 at 25 °C, the bright-red compound (κ4-NP3)Co(H)
to the superposition with aromatic overtones also present in
this spectral region.
Reactions of 1 and 2 with Ammonia Borane and
Determination of the Kinetic Parameters for 1.
Complexes 1 and 2 were exploited as homogeneous catalysts
for AB dehydrogenation, and the process was followed through
multinuclear NMR spectroscopy combined with the quantita-
tive determination of the H2 produced in the process.
Dehydrogenation occurs at an appreciable rate only at 55 °C;
no H2 production is observed at ambient temperature, as
witnessed by the absence of any appreciable change in the
NMR spectra of the reagents mixture. Hydrogen evolution
1
features H and 31P{1H} NMR spectra that show a broad
quartet centered at δH = −25.7 ppm in the hydride zone (2JH−P
= 45.4 Hz, Figure S2, SI) and a singlet at δP = 54.1 ppm (Figure
S3, SI) respectively, as expected for a five-coordinated cobalt
center in a trigonal bipyramidal coordination geometry. The
three P atoms are magnetically and chemically equivalent, lying
on the equatorial plane of the bipyramid. The IR spectra of 1
and 2 dissolved in THF feature one broad νCoH band (1798
cm−1 for 1 and 1795 cm−1 for 2), which is unsymmetrical due
1
(seen as a sharp singlet at δH = 4.6 ppm in the H NMR
spectra) was recorded for both 1 and 2, albeit with different
efficiency and rates. In both cases, the starting classical hydride
1
was the only chemical species detected in the H and 31P{1H}
NMR spectra at all times. For 1, ca. 2 equiv of H2 per
C
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Scheme 2. Catalytic AB Dehydrogenation with 1 and 2 as Homogeneous Catalysts
equivalent of AB can be produced, together with the
byproducts deriving from the monomer oligomerization
observed in the 11B{1H} NMR spectra [the cyclic tetramer B-
(cyclotriborazanyl)-amine-borane (BCTB), borazine, and poly-
borazylenes; see Figure 1 and Scheme 2].1e,2b,26,7a Given the
2:1 intensity ratio of the two signals at δB = −13.2 (triplet on
the 11B spectrum, “BH2N2” boron chemical surrounding) and
−7.2 ppm (doublet on 11B, “BHN3”), the aminoborane trimer
cyclotriborazane (B3N3H12) may also be present in solution.
As for 2, the amount of H2 produced is lower (1 equiv of H2
per equivalent of AB). An insoluble BN-containing product is
the final reaction outcome, identified as linear/branched
poly(aminoboranes) of general formula [BH2NH2]n, as judged
from the typical ν(NH2)/ν(BH2)/ν(BN) bands of the IR
spectrum of the solid precipitate [Figure S4 (SI) and Scheme
2].27 In the 11B{1H} NMR spectrum of the reaction mixture
(Figure S5, SI), there are only few traces of the soluble fraction
of the poly(aminoboranes) byproduct. The difference in the
catalytic outcome of the two hydrides allows for their
classification as type II (1) and type I (2) transition-metal
catalysts, respectively, according to the criterion recently
proposed by Paul and co-workers based on previous studies
of Baker and co-workers.28 Figure 2 shows the kinetic profiles
of the two catalysts obtained through volumetric measurements
of the amount of H2 produced over time (see the Experimental
Section), for a total reaction time of 24 h. It is clear that the H2
equivalents produced by 1 are doubled when compared with
those produced by 2. In addition, in the case of 1 it was found
that the increase of H2 pressure in the closed vessel as long as
the catalysis proceeds is detrimental, causing a significant
reduction of the reaction rate. This finding is in line with the
hypothetical formation of a transient dihydrogen complex as a
catalytic intermediate (see the section on DFT analysis below).
H2 “overpressure” had no effect on the catalytic performance of
2. The amount of H2 produced and the related catalyst
efficiency of 1 is higher than that of (PBP)Co(N2),9 while that
of 2 is comparable to that of (PBP)Co(N2) (albeit 2 works at
higher temperatures and for longer reaction times). The
behavior of 1 is very similar to that reported for the ruthenium
analogue (κ4-NP3)Ru(H)2,12 where the same number of H2
equivalents was produced and the same working temperature
was recorded.
Kinetics laws and parameters for the better-performing
catalyst (1) were also calculated through volumetric measure-
ments over time. Initial rate experiments performed at different
AB concentrations and also at variable catalyst concentrations
(Figures S6 and S7, SI) showed that the overall reaction is first-
order in both [AB] and [1]: −d[AB]/dt = kobs[AB] =
k[1][AB]. A final k value of 6.9 × 10−3 M−1 s−1 was calculated
for 1 at 55 °C. Reactions of 1 with NH3BD3, ND3BH3, and
ND3BD3 were carried out, in order to shed light on the kinetic
isotope effect (KIE) (Table S1, SI) in the dehydrogenation
catalysis. The collected data showed that a normal rate decrease
with deuteration is observed, with the same extent for both N-
deuteration [ND3BH3; kH/kD = 2.2(1)] and B-deuteration
[NH3BD3; kH/kD = 2.2(7)]. In the fully deuterated compound
ND3BD3, the KIE is equal to the product of the ND and BD
values (in the limit of the experimental error): kH/kD = 4.5(2).
This result suggests that both B−H and N−H activation take
place during the rate-determining step (as confirmed by the
DFT analysis, vide infra).29 The consequent release of the
inorganic ethylene analogue BH2NH2 in the reaction
environment as an intermediate was confirmed experimentally
through the observation of H2N−BCy2 in the 11B{1H} NMR
spectrum of the reaction mixture upon dehydrogenation in the
presence of cyclohexene (Figure S8, SI), according to a well-
established trapping protocol developed by Baker and co-
workers.28 The solution retains a bright-red color during the
catalysis, and the addition of mercury does not modify the
reaction rate in the limit of the experimental error (Figure S9,
SI). This is consistent with a homogeneous process. As for 2,
no H2N−BCy2 could be found in the 11B{1H} NMR spectra
Figure 2. H2 equivalents vs time kinetic profiles of 1 and 2 in their
reaction with AB (1:3 stoichiometric ratio, THF, T = 55 °C).
D
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when the catalysis is carried out in the presence of cyclohexene
(Figure S10, SI), thus confirming Paul’s classification described
above.28
Reactions with NH Acids. VT-FTIR Monitoring of the
DHB Interaction. The interaction of complexes 1 and 2 with
AB and DMAB was investigated by VT-FTIR (−40 to 0 °C) in
toluene (DMAB) and THF (AB) solutions. In toluene, the
region of the CoH stretching vibrations is hidden by the solvent
overtones. Thus, the reaction was followed only by monitoring
the NH and BH stretching regions of the amine borane of
choice.
Initially, the stoichiometric mixture of 1 and DMAB was
taken into account. Under these conditions, a new νNHbond band
appeared at 3133 cm−1, 136 cm−1 below that of free DMAB
(Figure 3a), indicating NH···HCo dihydrogen bond forma-
Figure 4. (a) FTIR spectra in the νCoH region of 1 (0.022 M, blue
dashed−dotted line), AB (0.066 M, black dashed line), and their 1:3
mixture after 1 h of reaction (red solid line) (THF; T = 55 °C; l = 1
mm). (b) Time evolution of FTIR spectra in the νBH region of 1
(0.022 M):AB (0.066 M) = 1:3 mixture and AB (0.066 M, black
dashed line) (THF; T = 55 °C; l = 1 mm). Total reaction time: 1 h.
Spectra were recorded every 2−5 min.
and 2, respectively). The intensity of both νCoH bands remained
almost unchanged, whereas a gradual intensity decrease during
the reaction was observed for the νNH and νBH bands of free AB
(Figure 4b). These observations are in line with the
involvement of Co−H bonds of 1 or 2 in the interaction
with AB, which is further supported by DFT calculations (vide
infra).
DFT Analysis of the AB Dehydrogenation Process. The
interaction of 1 and 2 with AB has been simulated through
DFT modeling. For the sake of simplicity and shorter
computational times, the phenyl rings on the P atoms of the
real EP3 ligand on the catalysts have been replaced with simple
Figure 3. VT-FTIR spectra of DMAB at −83 °C (0.064 M, black solid
line) and 1 at 0 °C (0.064 M, black dashed line) and in the presence 1
mol equiv of DMAB in the νNH region (a) and in the νBH region (b) in
toluene (−83 to −13 °C); l = 1 mm.
H
hydrogen atoms [E(CH2CH2PH2)3, EP3]. This simplification
tion.30 Simultaneously, the desymmetrization of BHtermas bands
occurs in the region of BH stretching vibrations: two new
bands appear at 2368 cm−1 (Δν = +8 cm−1) and 2350 cm−1
does not cause any change in the Mulliken atomic charges on
the hydrides and on the cobalt atom, as confirmed by an
independent optimization carried out on the real structures of 1
and 2 with the same computational set. Therefore, it is
expected that the reactivity trend of (HNP3)Co(H) (1H) and of
(HPP3)Co(H) (2H) is similar to that of 1 and 2, respectively.
The validity of the DFT functional and basis set/
pseudopotential employed in the theoretical analysis has been
benchmarked against the Gibbs energy of TS1 in the one-step
AB dehydrogenation mediated by 1H (vide infra). For this
reaction, an overall ΔG#(THF) value of 30.2 kcal mol−1 has
been calculated, taking as the zero energy value the isolated
reagents 1H and AB in THF (see Scheme 6 below). For the
experimental validation, the reaction between 1 and AB has
been carried out in THF at four different temperatures (40/45/
50/55 °C) to infer the activation thermodynamic parameters
(Δν = −10 cm−1) instead of one BHterm band at 2360 cm−1
as
and a shoulder at 2261 cm−1 (Δν = −2 cm−1) (Figure 3b).
Such changes can be the consequence of a weak BH···Co
interaction.31 However, this spectral range is rather complicated
to analyze, due to the overlap of νBH bands with the δBH
bending vibration overtones30,31 as well as with the 10B−H
stretching vibrations32 that fall at higher wavenumbers than
those of 11B−H and induce a band broadening.
In order to reproduce the experimental catalytic conditions,
the IR spectra of complexes 1 and 2 were measured in THF at
bond
T = +55 °C in the presence of 3 equiv of AB. A new νCoH
band of the CoH···HN DHB adduct appeared at a lower
frequency30,33 (Figure 4a; ΔνCoH = −17 and −14 cm−1 for 1
E
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(ΔH# and ΔS#) with an Eyring plot showing the temperature
dependence of the kinetic rate constant (k). The data fitting is
reported in Figure S11 (SI). The experimental ΔG# value
derived from the Eyring analysis equals 21.2 kcal mol−1, in good
agreement with the calculated data. The difference in the
absolute values may be ascribed to the different substituents on
the phosphorus atoms between the real molecule and the
computational model (i.e., 1 vs 1H).
The optimized structure of 1H has been put together with
that of AB and the ensemble reoptimized. The adduct
formation from the −NH3 terminus of AB is endoergonic in
THF (ΔG = +4.6 kcal mol−1). The resulting geometry 1H·AB is
reported in Figure S12 (SI). A weakly attractive interaction
between the N−H bonds of (uncoordinated) AB and both the
hydride and cobalt atoms is present [optimized d(Co−H···H−
N) = 2.30 Å; d(Co···H−N) = 2.53 Å]. Approaching AB from
the −BH3 terminus instead leads to a different adduct where
AB is coordinated in a η1 fashion through one of its B−H bonds
to cobalt. At the same time, the N atom of the organic ligand
detaches from the metal center and the NP3 coordination mode
changes from κ4 to κ3-P,P,P: (κ3-P,P,P-HNP3)Co(H)(η1-BH-
AB), 1H_AB (Figure 5). As already observed in (κ4-NP3)Ru-
Figure 6. Optimized geometry of TS1. Selected optimized bond
lengths reported (Å). The bonds involved in the transformation are
drawn in yellow dotted lines. H atoms on NP3 omitted for clarity.
Atom color code: see caption of Figure 5.
Scheme 3. Proposed Catalytic Cycle of AB Dehydrogenation
Mediated by 1
Figure 5. Optimized geometry of (κ3-P,P,P -HNP3)Co(H)(η1-BH-AB)
(1H_AB). Selected optimized bond lengths reported (Å). DHB and
BH-agostic interaction depicted with yellow dotted lines. H atoms on
NP3 omitted for clarity. Atom color code: white, H; gray, C; purple, P;
blue, N; pink, B; light blue, Co.
H···H−N dihydrogen bond in the ensemble 2H·AB [optimized
d(Co−H···H−N) = 2.17 Å, Figure 7]. An additional (weak)
interaction between the N−H bond and the metal atom is also
present [optimized d(Co···H−N) = 2.51 Å]. The Mulliken
negative charges are higher on the cobalt center [q(H) = −0.09
e; q(Co) = −0.77 e]. No stable geometry with a coordinated
AB could be found, at the computational level used. At odds
with HNP3, HPP3 does not show any κ3-P,P,P coordination
12
(H)2 and in (κ4-NP3)Ir(H)3,34 the NP3 coordination
flexibility plays a key role in this catalysis. Similarly to the
previous case, the 1H_AB adduct formation is also endoergonic
(ΔG = +14.3 kcal mol−1). In the calculated structure, there is
an intramolecular DHB between the hydride ligand and one
N−H bond [optimized d(Co−H···H−N) = 1.88 Å]. The
partial activation of the bridging B−H bond on cobalt is
witnessed by its elongation with respect to the noncoordinated
B−H bonds [optimized d(B−H): 1.25 vs 1.20 Å, respectively].
At this stage, the H2 evolution from the DHB and
concomitant loss of the “inorganic ethylene analogue” BH2
NH2 occurs; the transition state (TS1, Figure 6) for this
transformation is located 15.9 kcal mol−1 above the reagents.
The reaction is downhill (ΔG = −4.2 kcal mol−1), and 1H is
regenerated to start a new catalytic cycle. Scheme 3 illustrates
the overall catalytic cycle; the G(THF) vs reaction coordinate
profile is reported in Scheme 6. Following this mechanism, a
simultaneous B−H and N−H activation takes place, in line with
the KIE experimental results. The presence of BH2NH2 has
also been confirmed by the trapping experiment with
cyclohexene (vide supra).
Figure 7. Optimized geometry of 2H·AB. Selected optimized bond
lengths reported (Å). DHB depicted with yellow dotted lines. H atoms
on PP3 omitted for clarity. Atom color code: see caption of Figure 5.
The same computational approach has been followed in the
case of 2H. The only interaction of 2H with AB is a simple Co−
F
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mode. The apical P atom in HPP3 is more basic than the
corresponding N atom in NP3, and no detachment from the
H2)]+, with formation of the related amidoborane complex
[(NP3)Ru(NH2·BH3)(η2-H2)]+ after H2 release.12
H
metal center is observed. Similarly to what was previously
observed for 1H, the 2H·AB adduct formation is endoergonic
(ΔG = +16.2 kcal mol−1).
The evolution of the Co−H···H−N DHB from 2H·AB
eventually leads to H2 formation and to the amidoborane
complex (HPP3)Co(NH2BH3) (3H, Figure 8). Metal proto-
From 3H, a chain growth on the coordinated amidoborane
substituent can be envisaged, since the final dehydrogenation
products observed experimentally are long-chain poly-
(aminoboranes) [BH2NH2]n. At odds with the proposed
mechanism of poly(aminoboranes) chain growth reported in
36
the literature for the iridium hydrides (POCOP)IrH2 or
[Ir(PCy3)2H2(η2-H2)2]+,37 no BH2NH2 insertion in the Co−
N bond or further AB coordination to the metal center can be
postulated here, since the experimental evidence is against this
hypothesis. A plausible chain growth mechanism in agreement
with the collected data is that proposed by Li and co-workers in
2010.38 In this paper, AB dissociation into free BH3 and NH3 is
postulated. The B−N bond cleavage is faster than the direct
intermolecular H2 production from the reaction between two
AB monomers. The AB dissociation energy in THF (calculated
as −ΔGf from NH3 and BH3 at the M06//6-311++G**
computational level) is 23.9 kcal mol−1. Following this idea, it is
reasonable to think about sequential reaction steps of 3H with
borane and ammonia. BH3 and NH3 coordination to the cobalt
center in 3H has been analyzed computationally. While
ammonia coordination is unfeasible (the metal center is highly
nucleophilic in 2H, as witnessed by its negative atomic charge;
therefore, its interaction with the nitrogen lone pair on NH3 is
repulsive), the electrophilic borane forms instead a stable
adduct: cis-(HPP3)Co(BH3)(NH2BH3) (3H_BH3, Figure 9).
Figure 8. Optimized geometry of 3H. Selected optimized bond lengths
reported (Å). H atoms on PP3 omitted for clarity. Atom color code:
see caption of Figure 5.
nation may also occur while moving along the d(Co−H···H−
N) reaction coordinate, because of the high negative charge on
the metal itself. The product would be the ionic species
[(HPP3)Co(H)2][NH2BH3] (Scheme 4). The creation of an
ion pair from two neutral reagents is always a highly
unfavorable process. The corresponding transition state cannot
be found on the potential energy surface, even if the geometry
optimizations are carried out in THF. A free energy scan made
along the d(N−H···H−Co) reaction coordinate gives a
ΔE#(THF) estimation of ca. 28 kcal mol−1 for this step, at
d(H···H) = 0.7 Å. The classical bis(hydride) quickly releases
H2, given the easy Co(H)2 ↔ Co(η2-H2) interconversion
observed for this cation.35 Finally, 3H is the stable product.
Thermodynamics for 2H·AB → 3H + H2 provides ΔG = −3.5
kcal mol−1. A similar reaction path was postulated in the
nonclassical ruthenium cationic hydride [(κ4-NP3)Ru(H)(η2-
Figure 9. Optimized geometry of 3H_BH3. Selected optimized bond
lengths reported (Å). H atoms on PP3 omitted for clarity. Atom color
code: see caption of Figure 5.
Scheme 4. Proposed Formation of Species 3 from 2 + AB: Metal Protonation, Dihydride−Dihydrogen Cationic Complex
Tautomerization, and H2 Release
G
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Article
ΔG for this step equals −7.1 kcal mol−1. The coordination
geometry on cobalt is now octahedral; the borane and
amidoborane ligands are in a cis disposition. The Co−P bond
trans to BH3 is longer than the others [optimized d(Co−P) =
2.30 vs 2.17 Å, respectively], because of the trans effect of the
electrophilic boron atom weakening the Co−P interaction.
From this species, a transition state of borane insertion in the
Co−N bond has been found (TS2, Figure 10), lying at 33.0 kcal
agostic interaction with the metal center is slightly longer than
the others [optimized d(B−H) = 1.31 vs 1.20 Å, respectively].
From 4H, further reaction of free NH3 with the “BNB” chain to
get the “NBNB” dimer NH3BH2NH2BH3 and regenerate 2H is
the last step closing the catalytic cycle (Scheme 5). The
Scheme 5. Proposed Reaction Scheme of AB
Dehydrogenation Mediated by 2: Oligomeric Chain Growth
via BH3 Insertion into the Co−N Bond (TS2) Followed by
Chain Termination and Regeneration of the Active Species
through Reaction with Free NH3 (TS3)
Figure 10. Optimized geometry of TS2. Selected optimized bond
lengths reported (Å). The bonds involved in the transformation are
drawn in yellow dotted lines. H atoms on PP3 omitted for clarity.
Atom color code: see caption of Figure 5.
corresponding transition state TS3 has been located at 16.7 kcal
mol−1 above the reagents (Figure 12). ΔG for the 4H···NH3 →
mol−1 above 3H_BH3. The reaction product is the “BNB”
complex (HPP3)Co(σ-BH-BH3NH2BH3) (4H, Figure 11). ΔG
for this step equals −15.1 kcal mol−1. In this compound, the
anionic [BH3NH2BH3]− chain is coordinated to the cobalt ion
in a σ-BH agostic fashion. The B−H bond involved in the
Figure 12. Optimized geometry of TS3. Selected optimized bond
lengths reported (Å). The bonds involved in the transformation are
drawn in yellow dotted lines. H atoms on PP3 omitted for clarity.
Atom color code: see caption of Figure 5.
2H···NBNB reaction equals −7.7 kcal mol−1. The complete
G(THF) vs reaction coordinate profile is reported in Scheme 6.
Overall, the highest energy barrier to overcome to start the
catalysis is that related to formation of the amidoborane
complex 3H, higher than that found for TS1 in the NP3 case.
This justifies the sluggish reactivity and the lower H2
production efficiency of 2 when compared with 1 under the
same experimental conditions.
CONCLUSIONS
■
Two cobalt(I) hydrides, (κ4-EP3)Co(H) (E = N, P), have been
shown to be homogeneous catalysts for AB dehydrocoupling.
The NP3 species shows a higher catalytic activity when
compared with its PP3 analogue; the catalytic performance of
Figure 11. Optimized geometry of 4H. Selected optimized bond
lengths reported (Å). σ-BH-agostic bond drawn in yellow dotted lines.
H atoms on PP3 omitted for clarity. Atom color code: see caption of
Figure 5.
H
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Article
Scheme 6. Comparison of the Gibbs Energy (THF) vs Reaction Coordinate Profiles for the Two Catalysts 1H (blue line) and 2H
(red line)
the former (in terms of H2 equivalents per monomer equivalent
produced) is the best known to date for a cobalt-containing
catalyst.9 The spent fuels obtained at the end of the process are
different for the two catalysts, and the computational analysis
has confirmed that the underpinning mechanism is different.
The coordination flexibility of the tripodal polyphosphine NP3
(switching its hapticity from κ4 to κ3-P,P,P) plays a crucial role
in tuning the catalytic performance of (κ4-NP3)Co(H) and in
lowering the activation barrier of the dehydrogenation reaction.
The PP3 ligand in (κ4-PP3)Co(H) (with a more basic apical P
atom) does not possess the same degree of flexibility, and this
mirrors the lesser extent of hydrogen production under the
same experimental conditions. The combination of spectro-
scopic and computational tools has allowed for a more detailed
description of the H2 evolution process that follows DHB
formation. New transition-metal complexes containing poly-
dentate ligands are currently being scrutinized in our
laboratory, with the aim of finding better-performing
homogeneous catalysts for H2 generation from AB and amine
boranes.
AUTHOR INFORMATION
Corresponding Authors
*A.R. e-mail: a.rossin@iccom.cnr.it.
*E.S.S. e-mail: shu@ineos.ac.ru.
ORCID
Giuliano Giambastiani: 0000-0002-0315-3286
Andrea Rossin: 0000-0002-1283-2803
Igor E. Golub: 0000-0003-4142-454X
Oleg A. Filippov: 0000-0002-7963-2806
Natalia V. Belkova: 0000-0001-9346-8208
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
The bilateral project CNR (Italy)−RFBR (Russian Federation)
2015−2017 (RFBR grant no. 15-53-78027) and the Premiale
project “Energia da fonti rinnovabili” 2010−2012 of the Italian
National Research Council are acknowledged for financial
support. CREA (Centro Ricerche Energia e Ambiente) in Colle
Val d’Elsa (Siena, Italy) is acknowledged for computational
resources. Financial support was also provided by the Russian
Foundation for Basic Research (RFBR, grant nos. 14-03-00594,
16-03-00324) and by the Ministry of Education and Science of
the Russian Federation (Agreement number 02.a03.0008).
ASSOCIATED CONTENT
■
S
* Supporting Information
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.6b02673.
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Inorganic Chemistry
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Oligomers: Importance of Dihydrogen Interactions. Inorg. Chem.
2010, 49, 7710−7720.
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DOI: 10.1021/acs.inorgchem.6b02673
Inorg. Chem. XXXX, XXX, XXX−XXX