Reversible hydrogen adsorption at room temperature using a molybdenum-dihydrogen complex in the solid state

Article abstract of DOI:10.1039/d1dt01404h

Reversible H₂storage under mild conditions is one of the most important targets in the field of materials chemistry. Dihydrogen complexes are attractive materials for this target because they possess moderate adsorption enthalpy as well as adsorption without cleavage of the H-H bond. In spite of these advantages, H₂adsorption studies of dihydrogen complexes in the solid state are scarce. We herein present H₂adsorption properties of the 16-electron precursor complex ([Mo(PCy₃)₂(CO)₃]) in the solid state synthesized by two procedures. One is the direct synthesis under an Ar atmosphere (1), and the other is removal of the N₂-adduct under vacuum (2).2showed ideal Langmuir type reversible ad/desorption of H₂above room temperature, whereas1showed irreversible adsorption. The adsorption enthalpy of2was larger than that in THF solution. Using DFT calculation, this difference was explained by the absence of the agostic interaction in the solid state.

Full text of DOI:10.1039/d1dt01404h

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Reversible hydrogen adsorption at room
temperature using a molybdenum–dihydrogen
complex in the solid state†
Cite this: DOI: 10.1039/d1dt01404h
Kaiji Uchida,a Naoki Kishimoto,a Shin-ichiro Noro, b Hiroaki Iguchi a and
a
Shinya Takaishi
*
Reversible H2 storage under mild conditions is one of the most important targets in the field of materials
chemistry. Dihydrogen complexes are attractive materials for this target because they possess moderate
adsorption enthalpy as well as adsorption without cleavage of the H–H bond. In spite of these advan-
tages, H2 adsorption studies of dihydrogen complexes in the solid state are scarce. We herein present H2
adsorption properties of the 16-electron precursor complex ([Mo(PCy3)2(CO)3]) in the solid state syn-
thesized by two procedures. One is the direct synthesis under an Ar atmosphere (1), and the other is
removal of the N2-adduct under vacuum (2). 2 showed ideal Langmuir type reversible ad/desorption of
H2 above room temperature, whereas 1 showed irreversible adsorption. The adsorption enthalpy of 2 was
larger than that in THF solution. Using DFT calculation, this difference was explained by the absence of
the agostic interaction in the solid state.
Received 28th April 2021,
Accepted 2nd August 2021
DOI: 10.1039/d1dt01404h
rsc.li/dalton
uptake under ambient conditions due to low adsorption
Introduction
enthalpy (|ΔH°| < 10 kJ mol−1), while the interaction with
Due to growing interest toward energy and environmental pro- hydrogen is reversible and kinetics is fast. On the other hand,
blems, the pursuit of clean energy sources for a sustainable chemisorptive materials can adsorb H2 under ambient con-
society is intense around the world. Dihydrogen H2 has an ditions by forming a strong chemical bond with hydrogen, but
extremely large energy density (120 MJ kg−1 vs. 44.5 MJ kg−1 at the same time, this process is accompanied by the cleavage
for gasoline) and environment-friendly (CO2-free) power pro- of the H–H bond, which leads to irreversible adsorption and
duction is possible by using fuel cells. Because of these charac- extremely slow kinetics at room temperature. So the explora-
teristics, H2 is one of the best candidates as a future energy tion of the intermediate of these two mechanisms is necessary
source. On the other hand, it is quite difficult to store H2 for developing reversible H2 ad/desorption materials under
under mild conditions due to the extremely low density, the mild conditions.
small number of intermolecular interactions, and the resultant
low boiling point.
Metal–dihydrogen complexes are appealing candidates
because of their moderate adsorption enthalpy which ranges
To solve this problem, many kinds of materials including from −25 to −50 kJ mol−1. First a metal–dihydrogen complex
metal–organic frameworks (MOFs),1–11 covalent organic frame- was reported by Kubas and coworkers in 1984,17 and hundreds
works (COFs),12 chemical hydrides13–15 and hydrogen-absorb- of compounds have been reported so far.18–27 Orbital inter-
ing alloys16 have been studied intensely for H2 storage. These action of the metal–dihydrogen complex is described in the
solid-state H2 storage materials are roughly divided into physi- same manner as the Dewar–Chatt–Duncanson model, namely,
sorptive and chemisorptive mechanisms, and each has pros the η2 type M–H2 bond is formed by σ-donation from H2 to
and cons. Physisorptive materials have extremely low H2 metal and π-back-donation from metal to H2. In the metal–
dihydrogen complex, a chemical bond is formed between the
metal and dihydrogen molecule, whereas the H–H bond is
aDepartment of Chemistry, Graduate School of Science, Tohoku University, Sendai
980-8578, Japan. E-mail: shinya.takaishi.d8@tohoku.ac.jp
bFaculty of Environmental Earth Science, Hokkaido University, Sapporo 060-0810,
Japan
kept albeit it is elongated and weakened. This can be regarded
as an intermediate of physisorption and chemisorption, and
therefore, large adsorption enthalpy and fast ad/desorption
kinetics could be achieved. Actually, the solution calorimetry
†Electronic supplementary information (ESI) available: Experimental details,
adsorption measurements, kinetic analyses and DFT calculations. See DOI:
10.1039/d1dt01404h
study of [W(PCy3)2(CO)3] (PCy3
= tricyclohexylphosphine)
revealed that this complex has moderate enthalpy (≈−40 kJ
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mol−1) of H2 binding in THF or toluene solution.28 Also about therms were fully reversible in the measured pressure range.
the other metals, variable-temperature spectroscopic studies These data show that the H2 adsorption of 2 occurred by site-
were also performed and their thermodynamic properties were specific metal–dihydrogen complex formation in the solid
thoroughly investigated in the solution state.29–32
state.
On the other hand, 1 showed an irreversible isotherm. This
Recently, the η2 coordination motif has been widely utilized
in solid-state hydrogen storage materials.5,9,33–41 As for the irreversible isotherm and increase of adsorbed amount in the
conventional dihydrogen complexes, on the other hand, there pressure-reducing process indicates that the ad/desorption was
is only one example of the H2 adsorption study on [Mn(CO) quite slow and that equilibrium was not reached. The iso-
(dppe)2][BArF4] (ArF = 3,5-(CF3)2C6H3)42 in the solid state therms at other temperatures also showed irreversible adsorp-
though several papers reported the dihydrogen complex tion behaviour (Fig. S4†). Due to the non-equilibrium adsorp-
formation,43–45 to the best of our knowledge.
In this study, we studied the H2 adsorption of [Mo dynamic properties of 1.
tion behaviour, it was impossible to analyse the thermo-
(PCy3)2(CO)3], a 16-electron precursor of one of the convention-
In order to clarify the origin of this difference, we measured
al dihydrogen complexes, in the solid state. We synthesized the N2 adsorption of 1 and 2 at 77 K (Fig. S6 and S7†). The
[Mo(PCy3)2(CO)3] in two different methods, namely a direct BET surface area of 2 (3.6 m2 g−1) was three times larger than
synthesis under an Ar atmosphere (1) and the removal of the that of 1 (1.2 m2 g−1). In addition, the adsorption amount of 2
N2-adduct under vacuum (2). Details are shown in the ESI.† H2 largely increased at P/P0 > 0.95, suggesting that 2 was highly
adsorption isotherms of each sample were measured at pulverized probably during the N2 removal process, and it
various temperatures. Isotherms changed dramatically enabled the H2 molecule to approach the Mo centre at the
depending on the synthetic procedures, and a large difference polycrystalline surface.
of adsorption enthalpy was found between that in the solid
To investigate the thermodynamic properties of 2, variable-
state (this work) and in THF or toluene solution (ref. 28). temperature H2 adsorption isotherms were measured. The
According to the density functional theory (DFT) calculation of data between 313 and 373 K are shown in Fig. 3 (see Fig. S1†
[Mo(PR3)2(CO)3] (R = methyl, isopropyl and cyclohexyl), the for all the data). Saturation values of the adsorbed amount are
enthalpy difference between them was reasonably explained by in the range of 1.14–1.24 cm3 (STP) g−1 at all temperature,
the absence of the agostic interaction in the solid state. In this corresponding to ∼0.04 H2 per Mo centre. This small value is
paper, we report the synthesis, H2 adsorption measurements attributed to the nonporous nature of [Mo(PCy3)2(CO)3], which
and DFT calculation of [Mo(PCy3)2(CO)3].
was confirmed by the N2 adsorption isotherm at 77 K
(Fig. S6†). The Ad/desorption cycle was repeated 4 times and
no significant decrease of the adsorbed amount was observed
(Fig. S3†), which indicates that [Mo(PCy3)2(CO)3] is stable up
to 373 K in the solid state.
Results and discussion
The H2 ad/desorption isotherm at 333 K of 1 and 2 is shown in
Fig. 1. The adsorption behaviours of 1 and 2 are quite
different. 2 showed a type-I adsorption isotherm with a good
linearity in a Langmuir plot (Fig. S2†), indicating the existence
of site-specific interaction between the coordinatively unsatu-
rated Mo atom and dihydrogen molecule. Ad/desorption iso-
The thermodynamic properties of H2 adsorption were
studied by analysing the adsorption isotherms. First, the
adsorption enthalpy (ΔH°) evaluated from the isosteric differ-
ential heat of adsorption was calculated from the adsorption
isotherms, and it was almost constant around −49 kJ mol−1
regardless of the surface coverage (Fig. 2). This indicates that
H2 adsorption in 2 is a single site adsorption and each site is
independent of each other, which well obeys the Langmuir
model. All isotherms were accurately fitted (R2 > 0.998) by the
Langmuir adsorption isotherm,
Fig. 1 H2 adsorption isotherms of 1 and 2 at 333 K. Red diamond rep-
resents the data of 1 and black square represents the data of 2. Filled
and hollow symbols represent adsorption and desorption, respectively.
Fig. 2 (a) H2 adsorption isotherms of 2 at 293, 333 and 373 K. (b)
Adsorption enthalpy (ΔH) of 2 estimated using data at 293, 333, and
373 K.
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atoms to form [Mo(η2-H2)(PCy3)2(CO)3], as well as the relatively
free rotational motion of the η2-H2 ligand. It is noteworthy that
the absolute value of ΔH° is significantly larger than the
reported value of ΔH° = −27.2 kJ mol−1 in the variable-temp-
erature spectroscopic study in THF solution.28 To clarify the
origin of the energy difference between the solid state and
solution state, DFT calculation was performed.
DFT calculation was performed to evaluate the energy
difference between [Mo(PR3)2(CO)3] (R = alkyl group) with and
without agostic interaction. It has been reported that [Mo
(PCy3)2(CO)3] shows Mo–P–C–C–H⋯Mo type agostic inter-
action, in which β-hydrogen interacts with the Mo centre
(β-agostic interaction).46 We selected R = Cy and isopropyl (iPr)
as a molecule with the agostic interaction, while R = methyl
(Me) as a model without agostic interaction because PMe3 has
no β-hydrogen. The details of the calculation method are
shown in the Experimental section (see the ESI†). The opti-
mized structures of [Mo(PCy3)2(CO)3] (Mo–PCy3), [Mo
(PiPr3)2(CO)3] (Mo–PiPr3) and [Mo(PMe3)2(CO)3] (Mo–PMe3) are
shown in Fig. S13–S15.† As can be seen from the figure, Mo–
PCy3 (and Mo–PiPr3) have a highly distorted structure with a
P–Mo–P angle of 166.2° (166.4°), and the nearest C–H bond
exists in the distance of 3.06 Å (3.09 Å) from the Mo atom. This
structural distortion occurred to relieve the instability of the
coordinatively unsaturated Mo centre (16-electron complex) by
forming an agostic interaction. This structural distortion was
directly observed by single crystal X-ray diffraction measure-
ment for various metals including Cr, Mn, Re and W.27,47–49 In
the case of Mo–PMe3, on the other hand, structural distortion
is not observed and the P–Mo–P angle is 175°. The nearest C–
H bond is too far (3.90 Å) to interact with the Mo vacant site,
so there is no agostic interaction to stabilize.
Fig. 3 Variable-temperature H2 adsorption isotherms of 2. Symbols
represent the measured data and lines represent the fitting curves by
the Langmuir adsorption isotherm. Vertical axis is normalized to the
surface coverage of the adsorbent.
Va
KP
θ ¼
¼
ð1Þ
Vm 1 þ KP
where θ is the adsorbed fraction of the adsorbent, Vm is the
mono-layer adsorption capacity and K is the Langmuir equili-
brium constant. The obtained fitting parameters are listed in
Table S1.†
van’t Hoff plot was made by using these values and fitted
by the following equation,
ΔH° 1 ΔS°
ln K ¼ ꢀ
þ
R
ð2Þ
R
T
where ΔH° is the standard molar enthalpy change of adsorp-
tion, ΔS° is the standard molar entropy change of adsorption
and R is the ideal gas constant. Fitting of the van’t Hoff plot
gave the following parameters; ΔH° = −48.1 kJ mol−1 and ΔS°
= −113.5 J K mol−1 (Fig. 4).
The absolute value of ΔS° is slightly smaller than the stan-
dard molar entropy of the free dihydrogen molecule (130.7 J K
mol−1), which indicates that most of the entropic terms derive
from the suppression of translational and rotational motion of
hydrogen, and the small deviation is due to the newly
appeared 6 vibrational modes by addition of the two hydrogen
Thermodynamic parameters obtained by the DFT calcu-
lation are listed in Table S2.† From these data, the strength of
H2 adsorption is in the following order; PMe3 > PCy3 ≈ PiPr3.
This result seems counterintuitive because the order of the
electron donating ability of the phosphine ligand is opposite;
PCy3 ≈ PiPr3 > PMe3 (Tolman electronic parameter).50 Usually
a stronger electron donating ability of the phosphine ligand
leads to stronger back donation and stronger binding of the
dihydrogen ligand. On the basis of experimental and calcu-
lation results, the energy diagram of H2 adsorption is shown
Fig. 4 van’t Hoff plot of 2. Black square represents the Langmuir equili-
brium constant obtained from fitting and the red line represents the
fitted line by eqn (2).
Fig. 5 Energy diagram of H2 adsorption in the solution state and solid
state (2). The value of ΔH°1 was quoted from ref. 28.
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in Fig. 5. In the solution state, molecular motion is relatively
free and agostic interaction occurs to stabilize the 16-electron
Acknowledgements
complex, which lowers the energy gap between the 16-electron This work was partially supported by a JSPS KAKENHI Grant
complex and metal–dihydrogen complex (ΔH1° in Fig. 5). In the (B) 19H02729, Institute for Quantum Chemical Exploration
case of 2, on the other hand, the coordinatively unsaturated (IQCE), Adaptable and Seamless Technology Transfer Program
site, which is formed by N2 removal, should be fully exposed through Target-Driven R and D (A-STEP, JST) and Tohoku uni-
due to the restricted motion of the PCy3 ligand in the solid versity molecule & material synthesis platform in nanotechno-
state. Therefore, there is no stabilization of the 16-electron logy platform project sponsored by the Ministry of Education,
complex, which leads to a significantly large energy gap Culture, Sports, Science and Technology (MEXT), Japan.
between the 16-electron complex and metal–dihydrogen
complex (ΔH2° in Fig. 5).
Here we discuss the difference in the H2 adsorption iso-
Notes and references
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2
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Conflicts of interest
There are no conflicts to declare.
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