Spectroscopic Studies on Hydrazine-Boranes, Key Compounds for Chemical Hydrogen Storage

Article abstract of DOI:10.1021/acs.jpca.9b04266

Hydrazine-boranes (H₂NNH₂·BH₃ and H₃B·NH₂NH₂·BH₃) have been proposed for the storage of hydrogen, but these compounds have not created scope for extensive research works as ammonia- A nd methylamine-boranes have made these last decades. In the exciting research devoted to energy storage with environmentally friendly processes, hydrazine-borane, hydrazine-bisborane, and their simply substituted derivatives could provide a satisfactory response for hydrogen production and recyclability of the formed products. To date, knowledge of the physical and chemical properties of these compounds is still scarce. In this paper, the electronic structure of various hydrazine-boranes complexes is studied by ultraviolet-photoelectron spectroscopy (UV-PES), which is the experimental technique giving direct access to the energy of occupied molecular orbitals. Thus, UV-PE spectra were registered and first ionization energies were determined. Understanding of different types of interactions between nitrogen lone pairs and their variations by complexation has been our essential goal in these studies. In particular, clear stabilization of all molecular orbital energies is noted when complexation with borane takes place. Evolution of the σ_BN bond during the hydrogen release process upon thermal activation has also been studied experimentally by UV-PES and supported by quantum chemical calculations.

Full text of DOI:10.1021/acs.jpca.9b04266

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A: Spectroscopy, Molecular Structure, and Quantum Chemistry
Spectroscopic Studies on Hydrazine-Boranes,
Key Compounds for Chemical Hydrogen Storage
Javier Torres Escalona, Jean-Claude Guillemin, Clovis Darrigan, and Anna Chrostowska
J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.9b04266 • Publication Date (Web): 23 Jun 2019
Downloaded from http://pubs.acs.org on July 8, 2019
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Spectroscopic Studies on Hydrazine-Boranes, Key Compounds for
Chemical Hydrogen Storage
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Javier Torres Escalona1,2, Jean-Claude Guillemin2*, Clovis Darrigan1, Anna Chrostowska1*
1 CNRS / Université de Pau et des Pays de l’Adour / E2S UPPA, Institut des Sciences Analytiques et de
Physicochimie pour l’Environnement et les Matériaux, UMR5254, 64000, Pau, France.
e-mail: anna.chrostowska@univ-pau.fr
2 Univ Rennes, Ecole Nationale Supérieure de Chimie de Rennes, CNRS, ISCR – UMR6226,
F-35000 Rennes, France.
e-mail: jean-claude.guillemin@ensc-rennes.fr
Abstract
Hydrazine-boranes (H2NNH2·BH3 and H3B·NH2NH2·BH3) have been proposed for the
storage of hydrogen but these compounds have not generated extensive researches as
ammonia- and methylamine-boranes have made these last decades. In the exciting
research devoted to energy storage with environmentally friendly processes, hydrazine-
borane, hydrazine-bisborane and their simply substituted derivatives, could provide a
satisfactory response for hydrogen production and recyclability of the formed products.
To date, knowledge of the physical and chemical properties of these compounds is still
scarce. In this paper, the electronic structure of various hydrazine-boranes complexes is
studied by the UV-photoelectron spectroscopy, which is the experimental technique
giving direct access to the energy of occupied molecular orbitals. Thus, the UV-PE spectra
were registered and the first ionization energies determined. Understanding of different
types of interactions between nitrogen lone pairs and their variations by complexation
has been our essential goal in these studies. In particular, a clear stabilization of all
molecular orbitals energies is noted when complexation with borane takes place.
Evolution of the σBN bond during the hydrogen release process upon thermal activation
has been also studied experimentally by UV-PES and supported by quantum chemical
calculations.
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INTRODUCTION
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Equilibrium between technological development and conservation of our environment is
one of the principal challenges in research. Many studies are devoted to the so-called
"energies of the future" since actual reserves are reducing while the demand for fuel is
increasing. One of the most attractive option in actual research is a fuel cell in
combination with hydrogen.1 Nevertheless, efficient processes for hydrogen production
and hydrogen storage need to be developed.
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In this field, ammonia-borane has a huge gravimetric hydrogen storage potential (19.6
wt% of H2).2 During the reaction between ammonia and borane molecules, the nitrogen
atom becomes less negative and the boron atom becomes less positive. Haaland studied
the nature of the BN bond concluding that the electron pair in this  bond is closer to the
nitrogen atom than to the boron one.3 The substitution of ammonia-borane on the
nitrogen atom by organic chains with an electron donating inductive effect rises up the
negative charge on the hydrogen atoms located on boron atom.4 However, such
substitution strongly decreases the hydrogen percentage stored in the molecule. The
electronic structures of ammonia-borane5 and N-substituted-amine-boranes6 have been
determined by UV-photoelectron spectroscopy (UV-PES). The formation of the BN
covalent sigma bond (σBN) by the reaction between the BH3 and NH3 can be interpreted
using the experimental ionization energy (IE) of the σBN (13.92 eV)6 and the nitrogen lone
pair (nN) IE in NH3 (10.84 eV).7 Nguyen et al.8,9 on the basis of calculations at the
CCSD(T)/CBS level showed how BH3 can act as a catalyst in the hydrogen release
process.10 On the other hand, in a particularly efficient approach, ammonia-borane gave
an extent of hydrogen release (>2.5 equiv. or 18 wt%) per mole of H3N-BH3 with a nickel
complex as catalyst.11 The hydrogen release reaction enthalpies for NH3·BH3, MeNH2·BH3
and Me2NH·BH3 at 25 °C studied by quantum chemical calculations at the coupled cluster
methods at the CCSD(T) level gave -5.1 kcal/mol,12 -3.5 kcal/mol13 and -1.8 kcal/mol13,
respectively. Dehydrogenation experiments of amine-borane by thermogravimetric
analysis (TGA), differential thermal analysis (DTA) and thermomanometry were carried
out noting the first hydrogen release at 120 °C.14,15 Dihydrogen released from ammonia-
borane has been also evidenced by UV-PES.16 It should be potentially possible to
regenerate the dehydrogenated products.17
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Hydrazine-boranes can be considered for the same goal that amine-boranes since a great
hydrogen percentage is stored in hydrazine-bisborane: 16.9 wt%.19-21 H3B·NH2NH2·BH3
has four protic hydrogen atoms (Hδ+) bonded to the nitrogen atoms and six hydridic
hydrogen atoms (Hδ-) bonded to the boron atoms. The capacity for hydrogen storage of a
molecule is related with the gas-phase acidity and basicity properties. Adamson et al.20
have studied the dehydrogenation of hydrazine-borane and hydrazine-bisborane,
allowing to understand that the dehydrogenation will start more probably at the N-H than
the B-H moiety and to conclude that hydrazine complexes can produce hydrogen more
easily than amine-boranes. In hydrazine-borane, there is an electron-donor effect from
NH2 to NH2·BH3, following to the stabilization the BN bond, but the experimental study of
the electronic structure of the BN bond and its nature have not been fully explored.22–26
The characteristic bond between nitrogen and boron atom has been however the
principal topic of several studies.27–31 Hydrazine-bisborane was characterized by XRD,
NMR, FTIR, Raman, TGA, and DSC by Demirci et al.32 Recent studies have been carried out
at different temperatures for hydrogen release from hydrazine-boranes and also from
lithium hydrazinoborohydride.21 The reaction enthalpies between hydrazine-borane and
LiH, NaH and KH were measured by Pylypko et al.33 showing very exothermic reactions,
but the lack of boron containing compounds was noticed in the volatile by-products.21,32,34
For monosubstituted hydrazine-boranes, only some information can be found, and
sometimes contradictions of which nitrogen atom shares its electron lone pair for the
formation of 1:1 adducts with borane are noted.35,36 Understanding interactions between
both lone pairs37 and their changes caused by complexation have been our essential goal
in these studies. UV-photoelectron spectroscopy assisted by theoretical calculations was
used to fully clarify these effects. Evolution of the σBN bond during the hydrogen release
process has been also studied experimentally by UV-PES and supported by quantum
chemical calculations. The studied compounds are drawn on Chart 1.
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Chart 1
EXPERIMENTAL AND COMPUTATIONAL METHODS
UltraViolet-PhotoElectron Spectroscopy (UV-PES). The UV-PES spectra were recorded
on a home-built (IPREM/ECP), three part spectrometer equipped with: i) main body
device, ii) He−I radiation source (21.21 and/or 48 eV) and iii) a 127° cylindrical analyzer.
The spectrometer works at constant analyzer energy under 5 × 10−6 hPa working
pressure and ≤10−7 hPa for channeltron (X914L) pressure. The monitoring is done by a
microcomputer supplemented by a digital−analogue converter (AEI spectrum). The
spectra resulting from a single scan are built from resolution and accuracy of 2048 points
and 0.05 eV, respectively. The internal calibration of spectrum is done with helium (peak
at 4.99 eV). Spectra are calibrated also with lines of xenon (12.13 and 13.44 eV) and of
argon (15.76 and 15.94 eV). The accuracy of the IEs is ±0.03 eV for sharp peaks and
±0.05 eV for broad and overlapping signals. The samples were slowly vaporized under
low pressure (10−6 hPa) inside a handmade three-valve injector (3/4 in. diameter; 10 cm
length; working temperature: −190 ≤ T ≤ +300 °C), and the gaseous flow was then
continuously analyzed by UV-photoelectron. UV-Photoelectron spectroscopy measures
the binding energies of electrons in molecules in the gas phase upon irradiation with
monoenergetic He (I) photons of 21.21 eV energy. Valence electrons are ejected and the
PE spectra are displayed as plots of the number of ionized electrons versus their
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ionization energies (IEs). Thus, PE bands correspond to direct measurement of the
accessible energy states of the molecular ion, that is, the energy of occupied molecular
orbitals.
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Flash Vacuum Thermolysis (FVT): FVT experiments were performed in a temperature
gradient range from 30 °C to 300 °C with high vacuum conditions (10-3 hPa) with a
heating device (furnace) inside the UV-photoelectron (PE) spectrometer. FVT experiments
were carried out in tandem with the UV-PES, measuring the molecular orbitals energies
and following the release of hydrogen while high temperatures are applied. A supporting
study by quantum chemical calculations for hydrogen release is included to the
experimental results.
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Computational Methods. All calculations of compound structures have been optimized
using the Gaussian 0938 software at the CAM-B3LYP/6-311G(d,p) level. A conformational
search was made to obtain the global minima for each structure and was followed by
frequency calculations in order to verify that the stationary points obtained were true
energy minima. For the reliable assignment of PE bands, density functional theory
[ΔSCF+TD-DFT (CAM-B3LYP)]39–44 and ab initio (OVGF,45–50 P349 and SAC-CI50)
calculations of ionization energies using the 6-311G(d,p) basis set51 on the ground state
have been carried out on optimized geometrical structures of all studied compounds. The
advantages of the most frequently employed ΔSCF-DFT method of calculations of the first
IEs have been demonstrated previously.30,52,53 The TD-DFT approach provides a first-
principal method for the calculation of excitation energies within a density functional
context taking into account the low-lying ion calculated by the ΔSCF method. The vertical
IEs were also calculated at the ab initio level according to OVGF (in this case the effects of
electron correlation and reorganization are included beyond the Hartree−Fock
approximation and the self-energy part was expanded up to third order) and SAC−CI
(symmetry adapted cluster/configuration interaction methods of Nakatsuji and co-
workers which describes accurately and efficiently the electronic structures of the excited,
ionized and electron-attached states of molecules) methods. Avogadro software54 was
used as a visualization tool for all MOs.
A NBO study was performed for hydrazine derivatives. To study the dehydrogenation
reaction, the transition state approach was applied; a scan mode was used, finding just
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one imaginary frequency (between -1300 cm-1 and -1500 cm-1), checking that this
vibration mode corresponds to the hydrogen release.
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RESULTS AND DISCUSSION
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Synthesis. Unsubstituted 1a and methyl substituted hydrazines 1b,1c as well as borane
(BH3) and borane-dimethylsulfide are low boiling compounds while complexes are high
boiling compounds. Hydrazine-monoboranes 2a-2c were simply synthesized by slow
addition of borane complex on an excess of hydrazine in dichloromethane. Purification
was performed by evaporation of low boiling compounds in vacuo and compounds 2a-2c
were obtained in very good yields based on borane. Similarly, hydrazine-bis-boranes 3a-
3c were prepared in very good yields by slow addition of more than 2 equiv. of borane
complex on hydrazine followed by the removing of the low boiling compounds. In the
particular case of the phenyl derivative, stoichiometric amounts of hydrazine and borane
complex were used to obtain a product with a purity higher than 97%. (see the
1
Supporting Information section). On the basis of the H NMR spectra, the presence of the
BH3 group on the nitrogen bearing one or two methyl groups (compounds 2b and 2c,
respectively) is clearly evidenced as well as the selective complexation of the NH2 group
by BH3 for phenylhydrazine 2d (Scheme 1 and NMR data in the Supporting Information).
Scheme 1
UV-photoelectron spectroscopy. The experimental UV-photoelectron spectra (UV-PES)
of the monoborane and bisborane derivatives of hydrazine, methylhydrazine and 1,1-
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dimethylhydrazine were recorded (Figures 1-3), as well as UV-PES of the free systems
which have been already reported55,56,57 (see Figures S1,S2 in the Supporting Information
Section). Phenylhydrazine and phenylhydrazine-monoborane UV-PES are also illustrated
in Figures S3 and 4 respectively. To optimize the spectrum, complexes were progressively
heated from 90 to 150 °C to be vaporized and the PE spectra were continuously
registered. Only the intensity of the spectrum changes with the increase of the
temperature whereas decomposition would have modified the spectrum.
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10.7
11.0
a)
b)
11.3
13.1
11.8
11.5
11.1
15.9
13.0
IE (eV)
IE (eV)
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Figure 1: Photoelectron spectra of a) hydrazine-borane 2a (90 °C) and b) hydrazine-
bisborane 3a (140 °C), all values in eV.
b)
H2O
THF
9.74
a)
THF
11.52
THF
12.52
13.0
11.2
10.15
THF
13.2
10.95
11.3
11.8
13.0
IE (eV)
IE (eV)
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Figure 2: Photoelectron spectra of a) methylhydrazine-borane 2b (90 °C) and b)
methylhydrazine-bisborane 3b (110 °C), all values in eV.
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10.35
10.75
10.05
10.85
13.5
12.9
a)
b)
11.5
12.05
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IE (eV)
IE (eV)
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Figure 3: Photoelectron spectra of a) 1,1-dimethylhydrazine-borane 2c (120 °C) and b)
1,1-dimethylhydrazine-bisborane 3c (150 °C), all values in eV.
12.2
9.7
8.4
11.1
10.8
IE (eV)
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Figure 4: Photoelectron spectrum of phenylhydrazine-
borane 2d (115 °C), all values in eV.
The comparison of the theoretically predicted IEs and experimental data are summarized
in Tables 1-5. It appears that the DFT calculations (CAM-B3LYP) best model the
experimentally determined IEs. 58,59
In all cases, the first PE band of non complexed hydrazines corresponds to the nitrogen
lone pair (nN) and appears at 9.91 eV for NH2-NH2, at 9.36 eV for CH3NH-NH2, at 8.85 eV
for (CH3)2N-NH2 and at 7.86 eV for C6H5NH-NH2, but, in the last case, the contribution of
-
the aromatic ring is also noteworthy (nN - 1). The donor inductive effect of methyl
substituent is experimentally evidenced and corresponds to 0.55 eV destabilization for
one methyl group and to 1.06 eV destabilization if two methyl substituents are present on
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the substituted nitrogen atom. Additional mesomeric effects of π-electron conjugation in
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the case of phenylhydrazine decrease the first IE of 2.05 eV.
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The first IE of H3B·NH2NH2 is located at 10.7 eV, for CH3NH(BH3)NH2 at 10.15 eV, for
(CH3)2N(BH3)-NH2 at 10.05 eV and at 8.4 eV for C6H5NH-NH2·BH3. The second PE bands
range respectively as follow: H3B·NH2-NH2 at 11.0 eV, CH3NH(BH3)NH2 at 10.5 eV,
(CH3)2N(BH3)-NH2 at 10.35 eV and C6H5NH-NH2·BH3 at 9.7 eV. For bisborane derivatives
the two first PE bands are located at 11.1 and 11.5 eV for H3B·NH2-NH2·BH3, 10.95 and
11.3 eV for CH3NH(BH3)-NH2·BH3 and at 10.85 and 11.5 eV, for (CH3)2N(BH3)-NH2·BH3.
The HOMO (9.91 eV) and the HOMO-1 (10.64 eV) in hydrazine 1a (Table 2) correspond to
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―
the two lone pairs of electrons ( + , and
푛푁
, respectively). The HOMO-2 (15.61 eV) is
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푛푁
and HOMO-3 (16.66 eV) is the pseudo
―
the
, while the IE of HOMO-4, which has
휎푁푁
휋
푁퐻2
not been reported before, but our calculations estimate it at 16.97 eV, should correspond
+
to the pseudo
.
휋
푁퐻2
The UV-PES of hydrazine-borane 2a shows a first band (HOMO) at 10.7 eV attributed to
the totally antibonding combination of nitrogen lone pair and NN, BN and BH bonds (
휎퐵퐻
bond
). The second band (HOMO-1) at 11.0 eV is attributed to the
― 휎퐵푁 ― 휎푁푁 ― 푛푁
휎퐵퐻
off-plane ionization. The third band at 11.3 eV (HOMO-2) is mainly due to the newly
formed BN bond, as a result of the delocalization of the HOMO-1 (10.64 eV) from the
hydrazine molecule ( ― ) to the boron 2pz vacant orbital. The HOMO-3 at 13.0 eV and
푛푁
―
HOMO-4 at 15.9 eV are the
and
orbitals, respectively.
휎푁푁,휎퐵퐻
휋푁퐻
In the UV-PES of hydrazine-bisborane 3a the first band at 11.1 eV contains two ionizations
and is attributed to the HOMO and HOMO-1. The HOMO (11.1 eV) and the HOMO-3 (11.8
+
eV) correspond to the bonding and antibonding interactions of sigma BH bonds (
and
휎퐵퐻
―
, respectively). The HOMO-1 (11.1 eV) is attributed to the antibonding interaction of
휎퐵퐻
휎퐵퐻
and
, while the HOMO-2 (11.3 eV) is more localized on
휎퐵푁
part of the molecule.
휎퐵퐻
When the first reaction takes place, the non-complexed lone pair is stabilized by 0.79 eV
(from 9.91 eV to 10.7 eV). When the second borane moiety is added, the first BN bond is
stabilized by 1.8 eV (from 11.3 eV to 13.1 eV).
The interpretations of these results supported by theoretical evaluation of ionization
energies are presented below in Tables 1-5.
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Table 1: Molecular orbital energies for hydrazine 1a and borane: experimental and
calculated by Kohn-Sham energies of MOs (-εK-S) and ΔSCF+TD-DFT at the CAM-B3LYP/6-
K–S
K–S
311G(d,p) level, OVGF, P3 and SAC-CI.
. All values in
Δ(HOMO ― LUMO) = εLUMO ― εHOMO
eV. MO's visualizations from Avogadro software.
Borane (*)
Hydrazine (*)
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Method
C2
D3h
CAM-B3LYP
6-311g(d,p)
Δ(HOMO-LUMO) = 10.615
Δ(HOMO-LUMO) = 10.705
11.466
-εK-S
ΔSCF+TD-DFT
OVGF
8.375
9.902
9.766
9.649
13.237
13.209
12.993
P3
푛푁+
a
SAC-CI
9.683
13.210
휎퐵퐻
e'
EXP.
9.91
8.514
10.273
9.970
9.841
9.875
10.64
12.24
11.466
13.237
13.209
12.993
13.210
12.24
-εK-S
ΔSCF+TD-DFT
OVGF
P3
SAC-CI
EXP.
푛푁−
b
휎퐵퐻
e'
*: Experimental values from references 60,61
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Table 2: Molecular orbital energies for hydrazine 1a, hydrazine-borane 2a and
hydrazine-bisborane 3a: experimental and calculated by Kohn-Sham energies of MOs (-εK-
S) and ΔSCF+TD-DFT at the CAM-B3LYP/6-311G(d,p) level, OVGF, P3 and SAC-CI. All
values in eV. MO's visualizations from Avogadro software.
10
11
12
13
14
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21
22
23
24
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32
33
34
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36
37
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50
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60
Hydrazine monoborane
Hydrazine bisborane
Hydrazine (*)
Method
C2
C2h
C1
-εK-S
ΔSCF+TD-DFT
OVGF
8.375
9.902
9.766
9.390
10.601
10.785
10.101
11.046
11.462
P3
9.649
9.683
10.529
10.634
11.201
11.486
푛푁+
휎퐵퐻− 휎퐵푁 − 휎푁푁 − 푛푁
SAC-CI
휎퐵+퐻
EXP.
-εK-S
9.91
8.514
10.273
9.970
9.841
9.875
10.64
14.034
a
10.7
a'
11.1
au
9.465
10.183
11.092
11.491
11.254
11.505
11.1
ΔSCF+TD-DFT
OVGF
P3
10.670
10.835
10.594
10.731
11.0
휎퐵퐻
푛푁−
휎퐵퐻 − 휎퐵푁 , σNN
SAC-CI
EXP.
-εK-S
b
a''
ag
10.007
10.350
ΔSCF+TD-DFT
OVGF
15.660
15.556
15.645
15.666
15.61
11.203
11.322
11.159
11.223
11.3
11.312
11.692
11.440
11.717
11.5
P3
휎퐵푁
휎푁푁
휎퐵퐻 − 휎퐵푁
SAC-CI
EXP.
-εK-S
a
a'
bu
15.073
16.800
16.841
16.784
16.911
16.66
11.508
12.630
12.756
12.602
12.676
13.0
10.422
11.733
11.793
11.527
11.838
11.8
ΔSCF+TD-DFT
OVGF
P3
−
휎퐵−퐻
휋
(pseudo)
b
휎푁푁, 휎퐵퐻
푁퐻2
SAC-CI
EXP.
-εK-S
a'
bg
15.438
17.088
17.143
17.111
17.241
(16.66)
14.640
15.810
16.298
16.196
16.327
15.9
12.048
12.947
13.158
13.016
13.271
13.1
ΔSCF+TD-DFT
OVGF
P3
휋푁−퐻
휋푁+퐻 (pseudo)
2
휎퐵푁
SAC-CI
EXP.
a
a''
ag
*: Experimental values from reference 61
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Table 3: Molecular orbital energies for methylhydrazine 1b, methylhydrazine-borane 2b
and methylhydrazine bisborane 3b: experimental and calculated by Kohn-Sham energies
of MOs (-εK-S) and ΔSCF+TD-DFT at the CAM-B3LYP/6-311G(d,p) level, OVGF, P3 and SAC-
CI. All values in eV. MO's visualizations from Avogadro software.
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
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30
31
32
33
34
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36
37
38
39
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41
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45
46
47
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49
50
51
52
53
54
55
56
57
58
59
60
Methylhydrazine bisborane
Methylhydrazine borane
Methylhydrazine (*)
Method
C1
C1
C1
-εK-S
ΔSCF+TD-DFT
OVGF
8.005
9.079
10.006
9.259
9.168
10.166
10.368
10.151
10.107
10.15
10.927
11.287
11.031
P3
9.023
휎퐵퐻− 휎퐵푁 − 휎푁푁 − 푛푁
푛푁휋
SAC-CI
8.983
11.033
휎퐵퐻
EXP.
-εK-S
9.36
a
a
10.95
10.039
11.023
11.332
11.070
11.136
10.95
a
8.516
9.444
ΔSCF+TD-DFT
OVGF
9.639
10.414
10.751
10.499
10.641
10.5
9.753
푛푁휎
P3
9.621
휎퐵퐻
휎퐵퐻
SAC-CI
EXP.
-εK-S
9.623
10.28
a
a
a
12.118
13.213
13.635
13.483
13.582
13.38
9.949
10.180
11.148
11.475
11.213
11.261
11.3
ΔSCF+TD-DFT
OVGF
11.078
11.017
10.870
11.022
11.2
P3
휎푁푁, 휋
퐶퐻2
휎퐵푁
휎퐵퐻
SAC-CI
EXP.
-εK-S
a
a
a
12.920
14.150
14.501
14.322
14.437
14.32
12.097
13.209
13.439
13.224
13.302
13.0
10.251
11.440
11.540
11.279
11.335
11.8
ΔSCF+TD-DFT
OVGF
P3
휎퐵푁
휋
휎푁푁, 휎퐵퐻
퐶퐻2
SAC-CI
EXP.
-εK-S
a
a
a
14.387
15.583
15.644
15.690
15.737
15.87
13.218
13.882
14.670
14.494
14.552
13.2
11.618
12.415
12.615
12.480
12.338
13.0
ΔSCF+TD-DFT
OVGF
P3
휋푁∗ 푁, 휋퐶∗푁
휎푁푁, 휋
SAC-CI
EXP.
퐶퐻2
휎퐵푁
a
a
a
*: Experimental values from reference 61
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Compared to the unsubstituted hydrazine, in methylhydrazine, (Table 3) the inductive
donor effect of methyl substituent is experimentally evidenced by the lower IE (higher
HOMO) of methylhydrazine (9.36 eV) versus hydrazine (9.91 eV). HOMO-1 energies of 1a
and 1b are located at 10.64 eV and 10.28 eV respectively, HOMO-2 (13.38 eV) in
9
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methylhydrazine 1b is the
as it is
(15.61 eV) in the hydrazine molecule, but
휎푁푁,휋
휎푁푁
퐶퐻2
due to the presence of the methyl substituent, the contribution of a pseudo
is also
휋
퐶퐻2
noticed. HOMO-3 (14.32 eV) corresponds to the
and HOMO-4 (15.87 eV) to the
휋
퐶퐻2
∗
∗
antibonding MOs (
휋푁푁 푎푛푑 휋퐶푁
), as described by Mayer et al.62 When a borane molecule
reacts with 1b, the HOMO of methylhydrazine-borane appears at 10.15 eV (
휎퐵퐻
(off-plane), while
휎퐵퐻
). HOMO-1 in 2b (10.5 eV) corresponds to the
― 휎퐵푁 ― 휎푁푁 ― 푛푁
HOMO-2 (11.2 eV) is mainly due to the
bond. HOMO-3 (13.0 eV) and HOMO-4 (13.2
휎퐵푁
eV) are attributed to the
and
, respectively. In the UV-PES of hydrazine
휎푁푁,휎퐵퐻
휎푁푁,휋
퐶퐻2
bisborane 3b the first and the second bands correspond to the HOMO (10.95 eV), HOMO-1
(10.95 eV) and HOMO-2 (11.3 eV), and have a nature. The HOMO-3 (11.8 eV) and the
휎퐵퐻
HOMO-4 (13.0 eV) are attributed to the two
bonds. When the reaction between
휎퐵푁
methylhydrazine and borane takes place, the new BN bond appears at 11.2 eV, and the
lone pair on the other nitrogen is stabilized by 0.79 eV (from 9.36 eV to 10.15 eV). When
methylhydrazine monoborane reacts with another borane molecule, the new BN bond
appears at 11.8 eV and the firstly formed BN bond is stabilized by 1.8 eV (from 11.2 eV to
13.0 eV).
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Table 4: Molecular orbital energies for 1,1-dimethylhydrazine 1c, 1,1-dimethylhydrazine-
borane 2c and 1,1-dimethylhydrazine-bisborane 3c: experimental and calculated by
Kohn-Sham energies of MOs (-εK-S) and ΔSCF+TD-DFT at the CAM-B3LYP/6-311G(d,p)
level, OVGF, P3 and SAC-CI. All values in eV. MO's visualizations from Avogadro software.
7
8
9
1,1-Dimethylhydrazine
borane
1,1-Dimethylhydrazine
bisborane
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
1,1-Dimethylhydrazine (*)
Method
C1
Cs
Cs
-εK-S
7.688
8.947
9.858
ΔSCF+TD-DFT
OVGF
P3
8.675
8.652
8.510
9.960
10.166
9.948
10.791
11.095
10.838
10.966
10.85
휎퐵퐻
푛푁휋
휎퐵퐻− 휎퐵푁 − 휎푁푁 − 푛푁
SAC-CI
EXP.
-εK-S
8.328
9.889
8.85
a
10.05
a'
a'
8.622
9.356
9.865
ΔSCF+TD-DFT
OVGF
P3
9.625
10.239
10.614
10.359
10.449
10.35
10.813
11.158
10.876
11.043
10.85
9.744
9.583
푛푁휎
휎퐵퐻
휎퐵퐻
SAC-CI
EXP.
-εK-S
9.550
10.07
a
a''
a''
11.696
12.604
13.045
12.883
12.912
12.65
9.878
10.102
10.873
11.345
11.082
11.211
10.85
ΔSCF+TD-DFT
OVGF
P3
10.851
10.844
10.707
10.829
10.75
휎퐵푁
휎푁퐶
휎퐵퐻
SAC-CI
EXP.
-εK-S
a
a'
a''
12.134
13.047
13.470
13.318
13.315
13.00
11.843
12.865
13.070
12.853
12.958
12.9
10.145
11.222
11.381
11.121
11.263
11.5
ΔSCF+TD-DFT
OVGF
P3
휋
휎푁푁
퐶퐻2
휎퐵푁
SAC-CI
EXP.
-εK-S
a
a'
a'
12.501
13.561
13.980
13.786
13.898
13.50
12.683
13.213
13.944
13.793
13.876
13.5
11.168
11.797
12.026
11.930
11.856
12.05
ΔSCF+TD-DFT
OVGF
P3
휎푁퐶
휋
퐶퐻2
휎퐵푁
SAC-CI
EXP.
a
a''
a'
*: Experimental values from reference 56
As previously described for hydrazines, the HOMO (8.85 eV) and HOMO-1 (10.07 eV) of
1,1-dimethylhydrazine 1c (Table 4) correspond to the two nitrogen lone pairs π and σ
푛푁
푛푁휎
휋
interactions (
and
, respectively).56 The HOMO-2 (12.65 eV) is the
, while the
휎푁퐶
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HOMO-3 (13.0 eV) in 1c is linked with the
. Finally, the HOMO-4 (13.5 eV) corresponds
휎푁푁
5
6
to the
molecular orbital ionization. The HOMO in 1,1-dimethylhydrazine-borane 2c
휋
퐶퐻2
7
8
(10.05 eV) corresponds to the totally antibonding combination of nitrogen lone pair (
)
푛푁
9
and the sigma BH, BN and NN MOs (
푁). The HOMO-1 (10.35 eV)
휎퐵퐻 ― 휎퐵푁 ― 휎푁푁 ― 푛
10
11
12
13
14
15
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comes from the off-plane
orbital of borane molecule. The HOMO-2 (10.75 eV) is
휎퐵퐻
휎
attributed to the ionization of the
bond formed from the
orbital in 1c (HOMO-1)
휎퐵푁
푛푁
and the empty 2pz boron orbital. The HOMO-3 and HOMO-4 (12.9 and 13.5 eV) in 2c
correspond to the and orbital ionizations, respectively. For the 1,1-
휋
퐶퐻2
휎푁퐶
dimethylhydrazine-bisborane 3c the first PE intense band at 10.85 eV contains three
ionizations: the HOMO, HOMO-1 and HOMO-2; all are attributed to the orbitals. The
휎퐵퐻
bond ionization corresponds to the HOMO-3 (11.5 eV) and HOMO-4 (12.05 eV).
휎퐵푁
Figure 5: Diagram of selected molecular orbital interactions of hydrazine, 1a-1c and
their mono and bisborane derivatives. All values in eV. Figure 5 shows the energetic
evolution of the MO energy corresponding to the nitrogen lone pair before complexation
and how this energy is lowered after interaction with p empty B orbital. Beyond, the second
complexation allows another energetic stabilization.
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To summarize these studies, a general diagram (Figure 5) of molecular orbital
5
6
7
interactions of hydrazines 1a-1c and their mono and bisborane derivatives is presented,
+
showing how the inductive effects [+I] of the substituents destabilize the
orbital. The
푛푁
8
9
energies of molecular orbitals corresponding to the lone pairs of nitrogen atoms in 1,1-
dimethylhydrazine56 are 8.85 eV (nN) and 10.07 eV (nN), respectively, and they have an
energetic gap between them of 1.22 eV. This difference is less important in the cases of
10
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56
57
58
59
60
+
methylhydrazine61 (0.92 eV; nN 9.36 eV and nN 10.28 eV) and hydrazine61 (0.73 eV; nN
-
9.91 eV and nN 10.64 eV). The experimental measures of the inductive effects of two
methyl groups in 1,1-dimethylhydrazine (1.06 eV) or one methyl group in
methylhydrazine (0.55 eV), respectively, in comparison with the corresponding orbitals in
hydrazine are thus nicely evidenced (destabilization of the HOMO and HOMO-1 orbitals).
The sigma BN bond ionization is located at 11.3 eV for hydrazine-monoborane, at 11.2 eV
for methylhydrazine-monoborane and at 10.75 eV for dimethylhydrazine-monoborane.
The second complexation by BH3 shifts the first sigma BN bonds IEs to 13.1 eV for
hydrazine bisborane, to 13.0 eV for methylhydrazine bisborane and to 12.05 eV for 1,1-
dimethylhydrazine bisborane. Furthermore, we can observe how for all hydrazine
systems the molecular orbitals are stabilized when one or two molecules of borane are
complexed to the systems with the
bond formation (figure 5). We should notice that
휎퐵푁
the complexation in these systems occurs on the nitrogen with more methyl substituents,
but when the substituent is a phenyl group, the complexation take place on the less
substituted nitrogen because of the mesomeric effects from the aromatic ring, which
strongly decrease the nucleophilicity of the lone pair of the NH group. This effect is
opposite of that we observed with methyl substituents. Table 5 shows the experimental
IE's values obtained for phenylhydrazine by Streets et al.57 and reported for HOMO to
HOMO-3. For the sake of consistency of all our results we also recorded the
phenylhydrazine photoelectron spectrum and have complemented by values of HOMO-4
and HOMO-5.
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Table 5: Molecular orbital energies for phenylhydrazine 1d and phenylhydrazine
monoborane 2d: experimental and calculated by Kohn-Sham energies of MOs (-εK-S) and
ΔSCF+TD-DFT at the CAM-B3LYP/6-311G(d,p) level, OVGF, P3 and SAC-CI. All values in
eV. MO's visualizations from Avogadro software.
Phenylhydrazine (*)
Phenylhydrazine monoborane
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
Method
Xexp=0.823
Xexp=0.459
Corrected
7.860
7.037
7.766
7.540
7.798
7.146
7.86
9.233
8.410
9.237
9.006
9.137
8.717
9.19
9.951
9.128
9.954
10.182
10.004
9.836
10.09
10.868
10.045
10.739
10.580
10.537
10.265
10.64
11.845
11.022
11.853
11.852
12.011
11.664
11.85
12.361
11.538
12.377
12.368
12.477
12.176
12.4
8.400
7.941
8.609
8.438
8.687
7.929
8.4
9.477
9.018
9.808
9.608
9.724
9.235
9.7
9.984
9.525
10.103
10.823
10.528
10.615
10.8
10.104
9.645
10.276
10.910
10.641
10.740
10.8
11.336
10.877
11.468
11.401
11.346
10.975
11.1
12.088
11.629
12.228
12.522
12.484
12.191
12.2
-εK-S
ΔSCF+TD-DFT
OVGF
P3
SAC-CI
EXP.
푛푁− − 휋1
푛푁 − 휋1
a
a
Corrected
-εK-S
ΔSCF+TD-DFT
OVGF
P3
SAC-CI
EXP.
휋2
a
휋2
a
a
Corrected
-εK-S
ΔSCF+TD-DFT
OVGF
P3
SAC-CI
EXP.
푛푁+
휎퐵퐻
a
Corrected
-εK-S
ΔSCF+TD-DFT
OVGF
P3
SAC-CI
EXP.
휎퐵퐻
a
휎퐶푁 − 휋3
a
Corrected
-εK-S
ΔSCF+TD-DFT
OVGF
P3
SAC-CI
EXP.
휋4∗
휎퐶푁 − 휋3
a
a
Corrected
-εK-S
ΔSCF+TD-DFT
OVGF
P3
SAC-CI
EXP.
휋5∗
휎퐵푁 − 휋퐶퐶
a
a
* Experimental values from reference 57. HOMO-4 and HOMO-5 IEs experimental from this
work.
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The lone pair of the NH2 group in the phenylhydrazine molecule corresponds to the
푛푁― ― 휋1
HOMO-2 (10.09 eV), while the HOMO of phenylhydrazine (7.86 eV) is the
antibonding combination. The π aromatic ring matches with HOMO-1, HOMO-3, HOMO-4
and HOMO-5 at 9.19 eV, 10.64 eV, 11.85 eV and 12.4 eV. In phenylhydrazine-borane, the
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BN bond appears as the HOMO-5 (12.2 eV), while the HOMO (
) and HOMO-1 ( )
휋2
푛푁 ― 휋1
are located at 8.4 eV and 9.7 eV, respectively.
―
The HOMO (
) and HOMO-1 ( ) of 1d are shifted by 0.54 eV and
휋2;9.19 eV
푛푁 ― 휋1;7.86 eV
0.51 eV, respectively upon complexation and appears at 8.4 eV and 9.7 eV in 2d,
respectively. HOMO-2 and HOMO-3 are orbitals (10.8 eV) and HOMO-4 (
) at
휎퐶푁 ― 휋3
휎퐵퐻
11.1 eV in 2d corresponds to the stabilization of HOMO-3 of 1d (10.64 eV).
NBO Analysis. The theoretical studies of the electronic structure of hydrazines 1a-1d and
their corresponding borane 2a-2d and bisborane derivatives 3a-3c were carried out
applying the NBO analysis62 (Figure 6). (We have identified by theoretical calculation the
regioisomers of all hydrazine derivatives. Computational details on optimized geometry of
the thermodynamically most stable isomers are reported in the Supporting Information).
The BN and NN bonds contain almost two electrons (single bond); the boron contribution
to the BN bond (%B), not exceeding the 20% and decreasing when a second borane
moiety or more methyl groups are present, with discrepancies noticed for
tetramethylated derivatives certainly due to the steric hindrance. The pyramidal shape of
nitrogen atom can be correlated with the sigma parameter (Σ), which is the sum of α, β
and γ angles as shown in Figure 7, describing a planar (sp2) nitrogen when Σ=360°. In
order to compare the monoborane and bisborane hydrazine complexes, the most
substituted nitrogen atom N1 and the attached boron atom B1 are taken into account.
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Figure 6: Natural bonding orbital (NBO) analysis of hydrazine derivatives when
complexed by one or two borane moieties. Numbers on bonds correspond to NBO's
occupation (single bond close to 2). Contribution of boron atom to the BN bond (%B) and
pyramidal geometry of nitrogen atom (Σ) are also given.
H
H
H
B1
R1
R3
R4
α
N1
N2
β
R2
γ
훴 = 훼 + 훽 + 훾
Figure 7: Representation of considered angles for hydrazine derivatives. R1, R2, R3 and R4
= H or CH3.
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The Σ parameter increases when more methyl groups are present and decreases when the
first borane molecule reacts with the more substituted nitrogen atom in hydrazine. After
complexation, the boron atom is less positively charged (and nitrogen less negatively),
corresponding to the charge transfer into the 2pz originally empty boron orbital. The
consequences of this charge transfer are related to the BN bond length elongation when
more methyl or borane substituents are present as shown in Figure 8. The presence of
methyl groups in the structure creates interactions of CH3···CH3, CH3···BH3 and BH3···BH3,
the CH3···BH3 being the favored ones, giving the gauche-like conformers.
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Figure 8: Hydrazine 1a-1c, hydrazine-monoborane 2a-2c and hydrazine-bisborane 3a-3c
derivatives. Bond length in angstroms (Å) and angles in degrees.
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Dehydrogenation. The capacity of these complexes to store and release hydrogen should
be considered. The hydrogen elimination reaction has been studied by the transition state
of a four membered ring as presented on Figure 9. Reaction pathway for 1,2-elimination
of H2 from hydrazine-borane (Figure 10) and methylhydrazine-borane (Figure 11) show
the dehydrogenation product formation considering a more exergonic reaction, and the
hydrogen release carried out with a negative Gibbs free energy, meaning that it is a more
spontaneous process when a higher temperature is applied. Computational details of
optimized geometry of the dehydrogenated compounds are reported in the Supporting
Information.
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Figure 9: Transition states of hydrazine-bisborane for
a) 1,2-elimination and b) for 1,3-elimination reactions.
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Figure 10: Calculated (CAM-B3LYP/6-311G(d,p) reaction pathway for hydrogen release
of hydrazine-monoborane 2a at 25 °C (pink) and 200 °C (red). All ΔG values in kcal/mol.
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Figure 11: Calculated (CAM-B3LYP/6-311G(d,p) reaction pathway for hydrogen release
of methylhydrazine-monoborane 2b at 25 °C (light blue) and 200 °C (dark blue). All ΔG
values in kcal/mol.
When the hydrogen is released from hydrazine-borane derivatives, the electronic
structure of molecules changes and, subsequently, the IE of
bond is modified. The
휎퐵푁
study of the evolution of the ionization energy of the
bonds is difficult to follow by UV-
휎퐵푁
PES due to the considerable overlapping of the bands in this area, also the very narrow
temperature zone for selective elimination limits our experimental set-up for that
purpose. The UV-PE spectra are so complex due to the mixture of formed products that
the reliable bands attribution is risky. However, the presence of IE = 15.98 eV with the
vibrational progression corresponding to dihydrogen (PE spectrum in Figure 12)
demonstrates the formation of dihydrogen as shown in Figures 13-15, with the exception
of 1,1-dimethylhydrazine-borane 2c for which a 1,2-dehydrogenation cannot occur.
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Figure 12: Photoelectron spectrum of dihydrogen.63
a)
IE (eV)
H2
15.98
b)
c)
H2
15.98
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IE (eV)
IE (eV)
Figure 13: FVT photoelectron spectra of a) hydrazine-borane 2a at 120 °C, b) hydrazine-
bisborane 3a at 190 °C, c) 3a at 190 °C, corrected baseline; all values in eV.
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a)
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IE (eV)
b)
IE (eV)
Figure 14: FVT photoelectron spectra of a) methylhydrazine-borane 2b at 233 °C, b)
methylhydrazine-bisborane 3b at 150 °C; all values in eV.
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IE (eV)
b)
IE (eV)
Figure 15: FVT photoelectron spectra of a) 1,1-dimethylhydrazine-borane 2c
at 300 °C, b) 1,1-dimethylhydrazine-bisborane 3c at 290 °C; all values in eV.
The theoretical values of IEs issues of ΔSCF+TD-DFT calculations have been used to track
the evolution of
bonds to double or triple BN bonds during the dihydrogen release
휎퐵푁
process. Hereafter the diagrams (Figure 16) with the energetic variation of
bonds are
휎퐵푁
showed when one, two, three or four moles of dihydrogen are released from hydrazine-
bisborane 3a and for hydrazine-borane 2a (Tables S1-S3 in the Supporting Information).
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It should be noted that formation of more conjugated systems follows to bigger general IE
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i.e. its energy lowering (stabilization).
휎퐵푁
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Figure 16: MO energy diagrams for hydrazine mono (red) and bisborane (brown) for
hydrogen release. TD-DFT values extracted from Tables S1-S3. All values in eV.
Similar studies were performed with methylhydrazine (Table S4) and 1,1-
dimethylhydrazine-borane (Table S6) and -bisborane derivatives (Tables S5,S7), leading
to the energetic stabilization of BN bond when one, two, three or four dehydrogenation
reactions occur. For 1,1-dimethylhydrazine-borane -2c) the BN bond is destabilized after
1,3-elimination reaction, while for 1,1-dimethylhydrazine-bisborane (3c) as for the other
derivatives (2a, 3a, 2b and 3b) the 1,2-elimination stabilizes the BN bond. These results
support the reason why the PE spectrum of dihydrogen was not observed in the
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experimental measurements of 2c, corroborating that dihydrogen is released by 1,2-
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CONCLUSION
Syntheses of complexes between hydrazines and borane have been performed and
products were characterized by NMR and UV-PES spectroscopies. The complexation
occurs by the lone pair of the more alkyl substituted nitrogen atom (as methyl with a
donor inductive effect). In the case of phenylhydrazine, the complexation occurs with the
lone pair of the less substituted nitrogen atom because of the presence of a mesomeric
effect (nitrogen lone pair directly involved in the electronic delocalization with the phenyl
substituent). UV-PE spectra of complexes have been recorded and the analysis was
supported by theoretical calculations of IEs. Results show a general stabilization of
molecular orbitals when complexation with borane takes place. Presence of inductive
effect when a methyl group is attached to the hydrazine structure has been highlighted by
the higher energies of HOMO and HOMO-1 (lower IEs).
FVT experiments have evidenced for the first time by UV-PES the presence of hydrogen
(15.98 eV) released from hydrazine-borane derivatives. However, the process of
dihydrogen elimination was not selective enough to allow direct characterization of
dehydrogenated products (too complex mixture of present compounds) and a reliable
attribution of PE bands present. The experimentally measured theoretically evaluated IEs
provided unique information about
bond stabilization when dihydrogen is released
휎퐵푁
from complexes and stabilization increase by conjugation effects when double or triple
bonds are formed.
Optimization of hydrogen release conditions and regeneration processes of these
compounds should be continued with the aim to demonstrate the potentiality related to
these compounds as a regenerative spent fuel.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at
DOI: ***
9
List of compounds; Experimental details on hydrazine-borane, hydrazine-
bisborane derivatives; NMR data; Computational details of optimized geometry of
hydrazines, hydrazine-borane and hydrazine-borane derivatives; Transition
states; Calculated MO energies (Tables).
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AUTHOR INFORMATION
Corresponding authors
*E-mail: jean-claude.guillemin@ensc-rennes.fr. Telephone: +33 (0)223238073
*E-mail: anna.chrostowska@univ-pau.fr. Telephone: +33 (0)559407580
ORCID
Jean-Claude Guillemin: 0000-0002-2929-057X
Clovis Darrigan: 0000-0001-5197-8293
Anna Chrostowska: 0000-0003-0371-4546
Note
The authors declare no competing financial interest.
Acknowledgments
J.E.T. acknowledges the French Ministry of Research and Higher Education for a PhD
fellowship. J.-C. G. thanks the Centre National d’Etudes Spatiales (CNES) for financial
support.
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