Bifunctional activation of amine-boranes by the W/Pd bimetallic analogs of “frustrated Lewis pairs”

Article abstract of DOI:10.1039/d0sc06114j

The reaction between basic [(PCP)Pd(H)] (PCP = 2,6-(CH₂P(t-C₄H₉)₂)₂C₆H₄) and acidic [LWH(CO)₃] (L = Cp (1a), Tp (1b); Cp = η⁵-cyclopentadienyl, Tp = κ³-hydridotris(pyrazolyl)borate) leads to the formation of bimolecular complexes [LW(CO)₂(μ-CO)?Pd(PCP)] (4a,4b), which catalyze amine-borane (Me₂NHBH₃^tBuNH₂BH₃) dehydrogenation. The combination of variable-temperature (¹H,³¹P{¹H},¹¹B NMR and IR) spectroscopies and computational (ωB97XD/def2-TZVP) studies reveal the formation of an η¹-borane complex [(PCP)Pd(Me₂NHBH₃)]⁺[LW(CO₃)]^?(5) in the first step, where a BH bond strongly binds palladium and an amine group is hydrogen-bonded to tungsten. The subsequent intracomplex proton transfer is the rate-determining step, followed by an almost barrierless hydride transfer. Bimetallic species4are easily regenerated through hydrogen evolution in the reaction between two hydrides.

Full text of DOI:10.1039/d0sc06114j

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Bifunctional activation of amine-boranes by the W/
Pd bimetallic analogs of “frustrated Lewis pairs”†
Cite this: Chem. Sci., 2021, 12, 3682
Elena S. Osipova,a Ekaterina S. Gulyaeva,a Evgenii I. Gutsul,a Vladislava A. Kirkina,a
All publication charges for this article
have been paid for by the Royal Society
of Chemistry
a
b
Alexander A. Pavlov,
Maurizio Peruzzini,
and Elena S. Shubina
Yulia V. Nelyubina,a Andrea Rossin,
b Lina M. Epstein,a Natalia V. Belkova,
*
a Oleg A. Filippov
a
*
*
a
The reaction between basic [(PCP)Pd(H)] (PCP ¼ 2,6-(CH2P(t-C4H9)2)2C6H4) and acidic [LWH(CO)3] (L ¼ Cp
(1a), Tp (1b); Cp ¼ h5-cyclopentadienyl, Tp ¼ k3-hydridotris(pyrazolyl)borate) leads to the formation of
bimolecular complexes [LW(CO)2(m-CO)/Pd(PCP)] (4a, 4b), which catalyze amine-borane (Me2NHBH3,
tBuNH2BH3) dehydrogenation. The combination of variable-temperature (1H, 31P{1H}, 11B NMR and IR)
spectroscopies and computational (uB97XD/def2-TZVP) studies reveal the formation of an h1-borane
complex [(PCP)Pd(Me2NHBH3)]+[LW(CO3)]ꢀ (5) in the first step, where a BH bond strongly binds
palladium and an amine group is hydrogen-bonded to tungsten. The subsequent intracomplex proton
transfer is the rate-determining step, followed by an almost barrierless hydride transfer. Bimetallic
species 4 are easily regenerated through hydrogen evolution in the reaction between two hydrides.
Received 5th November 2020
Accepted 15th January 2021
DOI: 10.1039/d0sc06114j
rsc.li/chemical-science
bimetallic catalysis, wherein two metal sites demonstrate
cooperativity in fundamental catalytic reactions.6
Introduction
The species featuring an H2 molecule poised between the
Lewis acid and Lewis base centers have been found by us in
previous work as the intermediate of dihydrogen evolution in
the reaction of two neutral transition metal hydrides of different
polarities. Dihydrogen bonding (DHB, M–Hdꢀ/d+H–M0; M–H ¼
(PCP)NiH, (PCP)PdH; M0–H ¼ CpWH(CO)3 (1a); PCP ¼ 2,6-C6-
H3(CH2PtBu2)2) precedes the concerted proton and hydride
transfer,7 eventually yielding the bimetallic isocarbonyl
bridging M/W bimetallic complex 4 (Scheme 1).8,9 Up to date,
there are only a few examples of the attempted use of such
systems in catalysis. CpMoH(CO)3 has been used as a proton
source in Rh-catalyzed hydroformylation.10 More recently, the
switch of selectivity between migratory insertion versus C–H
activation has been shown for the reaction of [(iPrPOCOP)Ni]+
Catalytic processes occurring under the action of “frustrated
Lewis pairs” (FLPs) have been intensively sought aer during
the past decade.1,2 Heterolytic cleavage of H2 by main group
intermolecular FLPs has been proposed to occur through an
‘encounter complex’ where the Lewis acid and Lewis base are in
close proximity, but non-bonding, and H2 is accommodated
between the two centers prior to heterolysis. DFT metadynamics
studies for the prototypical FLP system P(Mes)3/B(C6F5)3 gave
evidence for the H2 polarization followed by the rate-
determining hydride transfer to B and proton transfer to P.3
The heterolytic cleavage of H2 into a proton and a hydride is
a crucial step in many chemical and biochemical processes
such as, e.g., hydrogen oxidation by hydrogenases, transition
metal-catalyzed hydrogenation of C]O bonds, or metal-
catalyzed hydrogen oxidation in energy conversion reactions.
From this point of view, the nomenclature of FLPs has many
parallels in metal–ligand cooperation (bifunctional) catalysis,
where the two sites of cooperation are typically metal- and
ligand-based.1,4,5 Closely related to this concept is also an area of
(
iPrPOCOP ¼ 2,6-C6H3(OPiPr2)2) with phenylacetylene in the
presence of [CpW(CO)3]ꢀ presumably acting as a proton
shuttle.11 However, this reaction was performed only in a stoi-
chiometric regime.
In the eld of chemical hydrogen storage, lightweight B/N-
containing inorganic hydrides (ammonia borane, amine-
boranes, and hydrazine boranes) have gained increasing
attention in the last few years as sources of carbon-free and
high-purity H2 for renewable energy applications.12–14
Metal-catalyzed dehydrogenation became a well-developed
route for H2 production from amine-boranes15 and new
studies keep appearing in the literature aiming to optimise the
reaction conditions and to control the selectivity in product
distribution as dehydrogenative coupling not only produces H2
aA.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of
Sciences (INEOS RAS), Vavilova Str. 28, 119991 Moscow, Russia. E-mail: shu@
ineos.ac.ru; nataliabelk@ineos.ac.ru
bIstituto di Chimica dei Composti Organometallici – Consiglio Nazionale delle Ricerche
(ICCOM – CNR), Via Madonna del Piano 10, 50019, Sesto Fiorentino, Italy. E-mail:
maurizio.peruzzini@iccom.cnr.it
† Electronic supplementary information (ESI) available. CCDC 2020560 (4a) and
2020559 (4b). For ESI and crystallographic data in CIF or other electronic
format see DOI: 10.1039/d0sc06114j
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Scheme 1 Reaction between two metal hydrides, (PCP)PdH and 1.
as an energy source but also B–N-containing materials.16,17 The Rh(Xantphos-iPr)H (1500 hꢀ1
)
for BH3NMeH2 dehydrogena-
20
metal-free catalysts including FLPs are also known but far less tion, whereas
a cationic zirconocene–phosphinoaryloxide
numerous.18 Although direct comparison between different complex [Cp2ZrOC6H4PtBu2]+ which can be described as an
catalysts is somewhat ambiguous due to the difference in the early transition-metal-containing FLP gives TOF 600 hꢀ1 for
reaction conditions, RuH(PMe3){N(CH2CH2PiPr2)2} can be BH3NMe2H dehydrogenation.21
considered as the best catalyst for BH3NH3 dehydrogenation
The simultaneous presence of protic and hydridic H atoms
(TOFmax 72 000 hꢀ1
)
and (POCOP)IrH2 (TOF 2400 hꢀ1
)
and within the same amine-borane molecule makes them
19
12
Fig. 2 Molecular structures of bimetallic products 4a (top) and 4b
(bottom). Atom color code: white, H; gray, C; yellow, B; red, O; blue, N;
Fig. 1 IR spectra (top) and the corresponding kinetic curves (bottom) navy, W; pink, P; and cyan, Pd. Thermal ellipsoids are given at 50%
of a mixture of TpWH(CO)3 (1a, c ¼ 0.003 M) and (PCP)PdH (c ¼ 0.045 probability level; hydrogen atoms except for the BH group in 4b are
M) at 240 K in THF.
omitted for clarity.
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Table 1 nCO vibrations (cmꢀ1) of the metal complexes in THF and
toluene at 298 K
equilibrium 1 4 3 to the le. At the same time, the nCO intensity
of complex 4b increases due to the hydrogen evolution and right
shi of the 3 4 4 equilibrium (Fig. S2†).25 The IR monitoring of
the reaction kinetics showed that the proton transfer is the rate-
determining step of the overall process and the rate constant
values are larger for TpWH(CO)3 (1b) than those with
CpWH(CO)3 (1a): at 240 K kobs ¼ 0.202 molꢀ1 sꢀ1 for 1b and
0.140 molꢀ1 sꢀ1 for 1a. This gives Gibbs free energy values of
DG2‡98 K ¼ 16.5 ꢁ 0.1 kcal molꢀ1 for 1b and 16.9 ꢁ 0.1 kcal molꢀ1
for 1a. Complexes 4 reversibly capture molecular hydrogen
similarly to FLPs. In a closed system, the reverse cooling from
290 K to 240–200 K restores the original spectroscopic picture.
The crystal structures of both bimetallic complexes 4 (Fig. 2)
appeared to be very similar to those reported for the related Ni-
containing complexes.8,9,11 The larger steric volume of the Tp
ligand leads to a different orientation of the [TpW(CO)3] frag-
ment: rotation around the Pd–O bond places terminal CO
ligands on one side of the [(PCP)Pd] plane.
nCO, cmꢀ1
nCO, cmꢀ1
Complex
in THF
in toluene
1a
1b
CpWH(CO)3
2018, 1924
2003, 1910, 1888 2006, 1914, 1891
1891, 1775
1884, 1754
2020, 1926
TpWH(CO)3
3a, 5a [CpW(CO)3]ꢀ
1887, 1782
1862, 1767
3b, 5b [TpW(CO)3]ꢀ
4a
4b
[CpW(CO)2(m-CO)/Pd(PCP)] 1910, 1819, 1660 1907, 1814, 1656
[TpW(CO)2(m-CO)/Pd(PCP)] 1901, 1796, 1650 1904, 1798, 1644
alternative reagents in transfer hydrogenation which attracts
increased research interest.22 It also triggers an extended dihy-
drogen bonding (DHB) throughout their solid-state structure.23
DHB is preliminary to H2 evolution occurring aer a simple
thermal treatment or in a catalytic fashion.15,24 Given the
analogy between protic and hydridic H atom co-existence and
heterolytic H2 cleavage, the latter is intimately related to B/N
inorganic hydride activation. Herein, we show for the rst
time that the bimetallic systems reported above act as homo-
geneous amine-borane dehydrogenation catalysts and explore
the impact of hydride and proton transfer steps in the reaction
mechanism using 1a in comparison with its more acidic analog
TpWH(CO)3 (1b; Tp ¼ k3-hydridotris(pyrazolyl)borate) as proton
donating hydrides (Scheme 1).
Interaction with dimethylamine-borane (DMAB)
Since the bifunctional ionic pairs [LW(CO)2(m-CO)/M(PCP)] (4)
reversibly bind hydrogen, we hypothesised that they could
interact with amine-boranes which are typical bifunctional
t
molecules (RR0NHBH3; R ¼ H, Me; R0 ¼ H, Me, Bu). We used
substituted amine-boranes for these studies because they
produce boron-containing reaction products and intermediates
soluble in organic solvents.26 Besides, Me2NHBH3 usually reacts
slower than ammonia borane (NH3BH3).27
Results and discussion
Pairwise interaction of two metal hydrides
The addition of excess Me2NHBH3 (DMAB, 3.3 equiv.) to the
bimetallic complexes 4 in THF at 270 K disrupts the inter-ion
The reaction of TpWH(CO)3 (1b) with (PCP)PdH follows the interaction due to the simultaneous coordination of DMAB
same mechanism (Scheme 1) as that of CpWH(CO)3 (1a). The IR molecules between the two metals. The decrease of nCO bands of
spectra obtained for the mixture of 1b with a 1.5-fold excess of 4 and the appearance of two new nCO bands of a new ionic
(PCP)PdH in pure THF at 200 K show two new nCO bands of the complex 5 (Table 1) are observed in the IR spectra (Fig. S3†).
intermediate 3b at 1884 and 1754 cmꢀ1, which disappear These changes are reversible; the equilibrium shis toward 5
gradually upon temperature increase (Scheme 1 and Fig. S1†). upon cooling. Changing the solvent to less polar toluene allows
At the same time, the bands at 1650, 1796, and 1901 cmꢀ1 of the the reaction to move one step forward: the bands of LWH(CO)3
H2 evolution product – isocarbonyl complex 4b – increase in (1) and (PCP)PdH (nPdH 1717 cmꢀ1) appear in the IR spectrum
intensity. In a closed system, the reaction goes to equilibrium (Fig. S4†), conrming the occurrence of proton and hydride
(Fig. 1). When the reaction mixture was allowed to reach equi- transfer and the formation of neutral molecules. The hydride
librium at, e.g., 240 K and then warmed to 290 K, the intensity transfer from the boron atom to palladium is conrmed by the
decrease of the nCO bands of the ionic intermediate 3b and synchronous decrease of the DMAB band intensity (nBH
a slight increase of the nCO bands of the initial hydride 1b are 2364 cmꢀ1) and the nPdH growth on going from 260 K to 190 K
observed. This indicates the shi of the proton transfer (Fig. S5†). Warming the mixture from 190 to 260 K restores the
Scheme 2 Reaction between the ionic pair 4 and amine-borane.
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original spectral picture, further conrming the existence of the [LW(CO)3]ꢀ anion. Interestingly, neither (PCP)PdH, LWH(CO)3,
equilibrium shown in Scheme 2. Denitely, the neutral trimo- [LW(CO)3]ꢀ (obtained by the reaction of LWH(CO)3 with
lecular complex should be unfavorable due to the high entropy NaHMDS) or [(PCP)Pd]+ (ref. 27) taken individually shows any
effect (see DFT calculations below), but it is conserved at low activity in DMAB dehydrogenation. This conrms the crucial
temperatures when TDS contribution to the Gibbs energy is role played by the bimetallic complex in the process.
diminished.
The behavior of two ionic complexes 4a and 4b is qualita-
tively the same. However, the proton transfer from DMAB to 4a
Catalytic DMAB dehydrogenation
At ambient temperature in the presence of up to 50-fold excess
of DMAB over 4, the catalytic dehydrogenation reaction occurs.
To deeper understand the mechanism, the reaction was fol-
lowed spectroscopically and volumetrically. IR monitoring
shows the progressive decrease of nBH (2360–2260 cmꢀ1) and
nNH (3200 cmꢀ1) bands of DMAB in the presence of 4a or 4b
until their complete disappearance (Fig. S6†). This observation
provides strong conrmatory evidence for the cleavage of B–H
and N–H bonds. A decrease of the typical DMAB signal (dB
ꢀ15.3) and the accumulation of the dehydrogenation product –
i.e. the cyclic dimer (Me2NBH2)2 (dB 3.4), as well as the appear-
ance of the steady-state intermediate – aminoborane Me2N]
BH2 (dB 35.8, Fig. 3) – was observed in the 11B{1H} NMR spectra,
evidencing the off-metal dimerization mechanism.30
is easier; the tungsten hydride 1a is formed in a larger quantity,
in agreement with the higher basicity of the [CpW(CO)3]ꢀ anion:
pKa of the conjugated acid LWH(CO)3 in CH3CN is 16.1 (ref. 28)
and 14.4 (ref. 29) for 1a and 1b, respectively.
Thus, at low temperature (below 270 K), the DMAB molecule
becomes “arrested” between the two metal ions of 4. The
bifunctional nature of 4 allows the proton transfer from the
N–H moiety to the Lewis base center on tungsten and the
hydride transfer from the B–H moiety to the Lewis acid center
on palladium. The position of the equilibrium (Scheme 2)
depends on the solvent, temperature, and basicity of the
Under these conditions, 1H NMR spectroscopy shows the
initial appearance of metal hydride resonances (dWH and dPdH
,
Fig. S7 and S8†). Then, the tungsten hydride singlet (dWH ꢀ7.4
and ꢀ2.3 for 1a and 1b, respectively) disappears, but the
palladium hydride triplet (dPdH ꢀ4.2) still accumulates. 31P{1H}
NMR spectra show the corresponding growth of the (PCP)PdH
Fig. 3 11B{1H} NMR spectra (128.3 MHz, THF-d8) of DMAB (5 equiv.) in Fig. 4 Kinetic curves obtained from the IR spectra of DMAB (1 equiv.
the presence of 4a (top) and the corresponding changes in molar (top) and 5 equiv. (bottom)) dehydrogenation catalyzed by 4b (c ¼
fractions in time (bottom).
0.003 M). 298 K, THF.
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Fig. 7 Kinetic curves of DMAB dehydrogenation by [CpW(CO)2(m-
CO)/Pd(PCP)] (4a, c ¼ 0.003 M) in THF at 313 K. (Red) c(DMAB) ¼
0.015 M, (5 eq.); (blue) c(DMAB) ¼ 0.15 M, (50 eq.).
Fig. 5 Kinetic curves of DMAB (c ¼ 0.06 M) dehydrogenation by
reused [CpW(CO)2(m-CO)/Pd(PCP)] (4a, 5 mol%, c ¼ 0.003 M) in THF
at 313 K.
the initial reaction rate. It should be noted that the kinetic
curves obtained by the integration of proton NMR spectra have
very similar behavior to the IR curves (Fig. S13†). According to
the spectral data obtained, the bimetallic species [LW(CO)2(m-
CO)/Pd(PCP)] (4) are regenerated at the end of the catalytic
reaction and can be reused for the next substrate loading
(Fig. 5). The initial reaction rates n0 are similar when DMAB is
added to the mixture of two neutral hydrides or to the pre-
generated bimetallic complex 4, revealing that the reagents'
mixing sequence does not affect the reaction rate (Fig. S11 and
S12†).
The H2 production in the reaction of DMAB with
[CpW(CO)2(m-CO)/Pd(PCP)] (4a) in THF was monitored using
the Man on the Moon X103 kit at ambient temperature and 313
K (Fig. 5 and S14†). At 313 K and 1 : 5 catalyst : DMAB ratio,
complete DMAB conversion is achieved in 4 h (Fig. 7 and Table
S3†) with an initial reaction rate of 2.5 ꢂ 10ꢀ6 M sꢀ1. The
increase of substrate loading to 50 equivalents at the same
concentration of 4a (c ¼ 0.003 M, 2 mol%; T ¼ 313 K) leads to
complete DMAB conversion in less than 3 hours (Fig. 7), and the
initial reaction rate is 2.1 ꢂ 10ꢀ5 M sꢀ1. Under these conditions,
the TOF value reaches 26 hꢀ1 at a half-conversion time (Table
S3†). The initial reaction rate at a catalyst : DMAB ratio of 1 : 50
is higher than that at 1 : 5, since the increase of DMAB amount
at a constant catalyst concentration shis the equilibrium of
complex 5 formation (Scheme 2 and Fig. 6).
signal (dP 92.0), whereas the bimetallic complex (dP 75.6) is rst
consumed and then restored (Fig. S9†). Changes in the IR
spectra show the same trend: the ionic complex 5 is formed at
the expense of 4, whereas the nPdH band (1718 cmꢀ1) increases
in intensity (Fig. S10 and S11†). The bands (nCO, nPdH) of 4, 5 and
(PCP)PdH restore their initial intensity aer the complete
conversion of DMAB (Fig. 4, top). An increase in DMAB loading
increases the length of the quasi-stationary stage of the reaction
when the concentration of both the catalyst and reaction
intermediates is nearly constant (Fig. 4, bottom).
The reaction kinetics was measured following the decrease
of nBH bond intensity of DMAB. The observed initial rate
corresponds to the substrate disappearance (Fig. S6†). In the
presence of excess DMAB, the kinetic curves (c(DMAB) vs. t) are
linear (Fig. S12†). DMAB dehydrogenation occurs faster in the
presence of the bimetallic complex 4a with the Cp-ligand: the
initial reaction rate n0 is 5.8 ꢂ 10ꢀ6 M sꢀ1 for 4a and 1.7 ꢂ
10ꢀ6 M sꢀ1 for 4b at 20 mol% catalyst loading. Keeping the
substrate concentration constant, the increase of the catalyst 4a
concentration from 0.003 M to 0.006 M leads to the doubling of
Mechanism of R0RNHBH3 dehydrogenation by bimetallic ion
pairs
According to the experimental data, we suggest the reaction
mechanism shown in Scheme 3 involving three important
steps: (1) amine-borane molecule insertion between the two
units of the bimetallic complex, (2) proton and hydride transfer,
yielding aminoborane (Me2N]BH2) and neutral metal
hydrides, (3) dihydrogen release as a result of the reaction
between palladium and tungsten hydrides via the DHB
complex. The insertion of the amine-borane molecule into the
bimetallic complex 4 yields the h1-borane complex 5 (Scheme
Fig. 6 Dependence of the initial reaction rate on the substrate (DMAB)
concentration (c ¼ 0.015 M, 0.03 M, 0.06 M, 0.15 M; THF; 313 K).
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Scheme 3 Mechanism of R0RNHBH3 dehydrogenation by bimetallic ion pairs 4.
3). The latter is also an ionic pair with the tungsten anion the cationic Pt-catalyst32 where additional stabilization of
[LW(CO)3]ꢀ, which could explain the accumulation of [Me2NHBH2]+ by a strong nucleophile was required. For the
LW(CO)3ꢀ in the reaction mixture (Fig. 4). In the next stage, the DMAB molecule, the proton transfer from nitrogen to tung-
proton transfer from the NH group of coordinated amine- sten occurs with a barrier DG‡ ¼ 21.8 kcal molꢀ1 and the
borane to tungsten atom gives the neutral hydride 1 and the formation of the zwitterionic complex 6 and LW(CO)3H
zwitterionic complex 6. The hydride transfer from the BH group (13.0 kcal molꢀ1 above complex 5). B–H bond cleavage in
to the palladium atom inside complex 6 leads to Me2N]BH2 complex 6 is almost barrierless (<2 kcal molꢀ1) (Fig. S15†). As
elimination and the formation of (PCP)PdH. To close the cata- a result, the hydride transfer goes faster than the proton
lytic cycle, the two neutral metal hydrides react with each other, transfer (k3 [ k2); therefore, complex 6 could not be observed
regenerating the catalytically active complex 4.
experimentally. The ensemble of the reaction products ((PCP)
PdH, LW(CO)3H and H2B]NR2) is only slightly higher in free
energy than the starting adduct 5 (+1.0 kcal molꢀ1), and the
reaction is still reversible. However, since H2 evolution from
the two hydrides (k1, Scheme 3)8,9,33 and the off-metal B]N
dimerization34,35 are featured with a comparable barrier, the
overall reaction of dehydrogenative DMAB coupling becomes
irreversible.
DFT calculations
This mechanism was supported by DFT calculations at the
uB97XD/def2-TZVP theory level taking the real system for the
structure optimization in toluene, which was introduced
within the SMD model. Complex 5 is 7.7 kcal molꢀ1 above 4 on
the Gibbs free energy scale (Fig. 8); however, its formation is
possible
(electronic
energy
difference
DE(4–5)
¼
Catalytic TBAB dehydrogenation
ꢀ7.5 kcal molꢀ1). The formation of 5 from 1 + Pd + DMAB is
nearly ergoneutral (DG(1–5) ¼ +1.9 kcal molꢀ1) and thus this Dehydrogenation of mono-substituted tert-butylamine-borane
equilibrium can be effectively shied toward 5 in the presence (tBuNH2BH3, TBAB) was also studied using complex 4a as
of excess DMAB. The key bonds in complex 5 (Fig. 9) are a catalyst. However, in this case, the IR spectroscopic picture
˚
˚
elongated (r(Pd–HB) ¼ 1.89 A and r(B–HPd) ¼ 1.28 A) relative to and kinetic curves are not similar to those with DMAB
˚
the B–H bond in free DMAB (1.21 A) and Pd–H in (PCP)PdH (Fig. S16†). On the quasi-stationary stage, the concentration of
(1.64 A). The amine group of the dimethylamine fragment is neutral tungsten hydride 1a is higher, and that of the h1-borane
˚
˚
hydrogen-bonded to the tungsten anion (HN–W 3.09 A, :N– complex 5a is lower than in DMAB dehydrogenation. Measured
H–W 162.6ꢃ). Despite the B–HPd bond elongation, the relaxed 60 min aer TBAB addition (dB ꢀ21.8), the 11B NMR spectrum
potential energy surface (PES) scan for BH bond distance in shows the formation of the intermediate tBuNH]BH2 (dB 36.1,
the complex 5 did not lead to any TS, continuously going up to t) and cyclotriborazane [tBuNH–BH2]3 (dB ꢀ3.7, t) – the product
50 kcal molꢀ1. Hydride transfer does not occur at this step, of the evolution of the rst H2 equivalent (Fig. S17†). At the end
due to the instability of NR3BH2+, in agreement with the rather of the reaction, there is no initial TBAB, and the single product
low hydride-donating ability of amine-boranes.31 This is of the evolution of the second H2 equivalent – borazine [tBuN–
consistent with the suggested hydride transfer from DMAB to BH]3 (dB 28.1, d) – is observed. The formation of two equiv. H2
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than H2B]NMe2; its neutral form is preferred by
3.8 kcal molꢀ1. That suggests a lower deprotonation extent of
LWH(CO)3 when H2B]NHtBu is formed (Fig. S19 and S20†).
Based on these computational data, we could expect the
equilibrium side process of LW(CO)3H deprotonation, yielding
the ion pair CpW(CO)3ꢀ/H2B–NMe2H+ with CO/B interaction
for DMAB dehydrogenation in toluene. When THF is used as
a solvent, this ion pair is likely converted into the THF stabilized
one. Consequently, the IR spectra in the nCO range in THF
resemble those of the tungsten anion, while the IR spectra in
toluene would rather resemble those of the ion pair 4
(Fig. S21†). Thus, this side process (eqn (1)) should affect the
solution composition at the quasi-stationary stage of DMAB
ꢀ
dehydrogenation, increasing the relative amount of LW(CO)3
.
Fig. 8 Computed (DFT/uB97XD) energy profile for the DMAB dehy-
drogenation by [TpW(CO)2(m-CO)/Pd(PCP)] (4b) in toluene (SMD
model).
Switching DMAB to TBAB diminishes the impact of LW(CO)3H
deprotonation leading to the presence of both metals in
hydridic forms in the reaction mixture (Fig. S16†).
Reaction kinetics
Both DFT calculations and experimental data suggest that the NH-
bond cleavage is the rate-determining step of the catalytic reaction.
Under these conditions, the overall reaction rate is determined by
the rate of proton transfer in complex 5 (5 / 6) r2 ¼ k2$[5]. As
shown by DFT calculations, the activation energy values for step-
wise proton and hydride transfer (DG‡PT ¼ 21 kcal molꢀ1 and
DG‡HT < 2 kcal molꢀ1) indicate that k3 [ k2 (Scheme 3). Thus, the
hydride transfer goes much faster than proton transfer and
conversion of 6 to products has no inuence on the reaction rate.
When DMAB is in excess, the reaction is pseudo-zero order in
DMAB. Taking into account the pre-kinetic step of DMAB coordi-
nation to bimetallic complex 4 in the presence of excess DMAB, the
reaction rate is r ¼ k2Kc0(DMAB)$[4] ¼ keff$[4], K ¼ [5]/([4][DMAB]).
Thus, analysis of the experimental data gives the k2 values of the
rate-limiting step equal to 0.17 sꢀ1 and 0.03 sꢀ1 for 4a and 4b,
respectively (Table S4†). These values correspond to DG‡298 K 18.5 ꢁ
0.1 and 19.5 ꢁ 0.1 kcal molꢀ1, in reasonable agreement with DFT
calculations. For TBAB dehydrogenation by complex 4a, the k2
value is 0.10 sꢀ1 (DG2‡98 K 18.8 ꢁ 0.1 kcal molꢀ1), indicating faster
proton transfer. The overall activation free energy DG‡298 K for the
conversion of 4 to 6 is ca. 25 kcal molꢀ1 that is in good agreement
with DFT calculations (Fig. 8).
Fig. 9 DFT/uB97XD-optimized structure of the complex 5b. Key
bond lengths are reported in A˚.
t
per BuNH2BH3 molecule was also conrmed by volumetric
measurements (Fig. S18†).
To explain the different ratios of ionic and molecular forms of
catalyst units in the reaction with DMAB and TBAB, we considered
the behavior of the rst dehydrogenation product that accumu-
lates in an unexpectedly high amount in the case of TBAB.
Monomeric aminoborane H2B]NR2 is potentially basic36 and
appears to be able to deprotonate the tungsten hydride (eqn (1)).
4
LWðCO3ÞH þ H2B ¼ NR2%½H2B ꢀ NHR2ꢄ þ LWðCOÞ
.
3
(1)
The use of deuterated amine-boranes NDMe2BH3 and
NDMe2BD3 proves that the rate-limiting stage is the N–H bond
cleavage, as is oen for AB dehydrogenation.38 The rate
The isolated [H2B–NHR2]+ cation is not stable, but it can be
stabilized by interacting with nucleophilic atoms like the
carbonyl or the THF's oxygen atoms. Indeed, the neutral
CpWH(CO)3/H2B]NMe2 complex appears to be almost ergo-
neutral relative to the ion pair CpW(CO)3ꢀ/H2B–NMe2H+ (DE ¼
ꢀ1.3 kcal molꢀ1 in favor of the neutral form, Fig. S19†) stabi-
lized by B/OCW interactions. Introducing a THF molecule into
the system also allows stabilizing the ion pair via B/OTHF
interactions (DE ¼ +0.2 kcal molꢀ1; Fig. S19†), whereas CO
constant keff for NDMe2BH3 dehydrogenation catalyzed by 4a
ND
(Fig. S22 and S23†) is substantially lower (k ¼ 2.8 ꢂ 10ꢀ7
M
eff
sꢀ1) than that for DMAB (k ¼ 5.8 ꢂ 10ꢀ6 M sꢀ1) giving the
NH
eff
kinetic isotope effect (KIE ¼ kNH/kND) of 20.6 ꢁ 0.3. The use of
the fully deuterated analogue NDMe2BD3 does not lead to
a further decrease in the reaction rate, giving KIE ¼ kH/kD of
20.6 ꢁ 0.7. The KIE obtained is substantially higher than
typical KIEs in metal-catalyzed amine-borane dehydrogena-
tion.39–43 This value also exceeds the KIEs for self-exchange in
CpW(CO)3H/[CpW(CO)3]ꢀ or for proton transfer of
CpW(CO)3H to aniline.44,45 Such high KIE values suggest
a proton tunneling that is likely to occur when there is
ꢀ
groups of the CpW(CO)3 anion appearꢀunbounded, having
a geometry similar to that of the CpW(CO)3 /R3NH+ ion pair.37 It
should be noted that H2B]NHtBu appears to be a weaker base
3688 | Chem. Sci., 2021, 12, 3682–3692
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a minimal geometry distortion along the reaction coordi-
nate.46 Our computations partially reproduce the observed
magnitude of the isotope effect predicting increasing of the
Experimental section
All reactions were performed using standard Schlenk proce-
dures under a dry argon atmosphere. Commercial reagents
barrier upon NH to ND substitution by DDG2‡98
¼
1.5 kcal molꢀ1 (KIE ¼ kNH/kND ¼ 13). One of the reasons for
large KIE magnitudes (>10) is long (large acid–base separa-
tions) and strong H-bonded complexes.47
(dimethylamine-borane,
tert-butylamine-borane)
were
purchased from Aldrich and used aer preliminary sublima-
tion. Tetrahydrofuran (THF) and toluene were dried over Na/
benzophenone and distilled under an argon atmosphere.
˚
THF-d8 (Aldrich) was stored over 4 A molecular sieves and
Conclusions
degassed before use by three freeze–pump–thaw cycles.
NDMe2BH3 and NDMe2BD3 were prepared as described in the
literature.40,53 Variable-temperature (VT) NMR spectra were
recorded on Bruker AVANCE II and Varian Inova FT-NMR
spectrometers operating at 300 and 400 MHz in the 200–320
K temperature range. 1H chemical shis are reported in parts
per million (ppm) downeld of tetramethylsilane (TMS) and
were calibrated against the residual resonance of the deuter-
ated solvent, while 31P{1H} chemical shis were referenced to
In summary, bimetallic complexes [LW(CO)2(m-CO)/Pd(PCP)]
(4) act as “metallic-analogs” of typical main group FLPs. The
presence of acidic and basic metal centers in these ionic pairs
triggers the cooperative BH/NH bond activation in amine-
boranes. In the rst reaction stage, the h1-borane complex
[(PCP)Pd-(s1-HBH2–NR2H)/W(CO)3L] (5) is formed, in which
BH is strongly bound to the palladium atom and the amine
group is hydrogen-bonded to the tungsten atom. The step-wise
proton transfer to W and hydride transfer to Pd yield the
unsaturated B]N fragment and neutral metal hydrides.
Molecular hydrogen evolution is the result of two metal
hydrides interacting, regenerating bimetallic species 4. One
equivalent of H2 can be produced from Me2NHBH3, whereas
dehydrogenation of tBuNH2BH3 gives two equivalents of
hydrogen per monomer at room temperature. The catalytic
system can be easily generated through direct mixing of
LWH(CO)3, (PCP)PdH and amine-borane, without preliminary
synthesis of the ionic catalyst 4. The presence of ionic inter-
mediates during the dehydrogenation cycle requires the use of
polar solvents to achieve effective catalysis. The mechanistic
study showed that proton transfer is the rate-determining step;
therefore, the reaction is accelerated by a more basic anion
([CpW(CO)3]ꢀ > [TpW(CO)3]ꢀ) while the hydride transfer is
almost barrierless. The dehydrogenation process starts only in
the presence of excess amine-borane due to the shi of the pre-
kinetic equilibrium (H3BNHR2 + 4 ¼ 5) to the right, which in
turn causes the increase of the initial reaction rate. Yet another
85% H3PO4 with downeld shi taken as positive. 11B and 11
B
{1H} were referenced to BF3$OEt2. IR spectra were recorded at
different temperatures (190–293 K) using a home-modied
cryostat (Carl Zeiss Jena) with a Nicolet 6700 spectrometer
using 0.05–0.2 cm CaF2 cells. The accuracy of the experimental
temperature was ꢁ0.5 ꢃC. The cryostat modication allows
transferring the reagents (premixed at either low or room
temperature) under an inert atmosphere directly into the
cells.
Elemental analyses were carried out in the Laboratory of
Microanalysis of INEOS RAS. The classic manual technique was
used. The sample was burned in a platinum crucible in a stream
of oxygen at 950 ꢃC followed by trapping CO2 and water with
Ascaris (asbestos impregnated with NaOH) and Anhydrone
(anhydrous magnesium perchlorate), respectively, and the
analysis of the mass changes.
Synthesis of [LW(CO)2(m-CO)/Pd(PCP)] L ¼ Cp (4a), Tp (4b)
notable observation is that the dimethylaminoborane monomer Solid LW(CO)3H (0.03 mmol) was placed in a Schlenk ask lled
H2B]NR2 is able to deprotonate tungsten hydride in competi- with argon (10 ml) together with 1 ml of tetrahydrofuran. Then,
tion with H2 evolution from two hydrides and H2B]NR2 olig- 1 ml of a (PCP)PdH THF solution (0.03 mmol of hydride in 2 ml
omerization. Different basicities of dehydrogenated DMAB and of the solvent) was added to this solution. The resulting mixture
TBAB monomers lead to a different impact of this side process of two colorless hydrides instantly became yellow colored. Aer
to the overall catalytic reaction.
an hour of stirring, the solvent was concentrated to ca. 0.1 ml
Prior studies have shown that bimetallic systems featuring under vacuum. Then, the Schlenk ask was lled with argon
a metal–metal bond48 can indeed operate in a concerted way and le for several days standing at ambient temperature until
activating H2, C–H, B–H, and other bonds.49–52 These complexes a crystalline precipitate was obtained. The resulting solid
activate the ONE bond, splitting it between two metals, oen in (yellow needle-shaped crystals) was washed with a small
an oxidative addition fashion. In our case, two transition metal- amount of cold THF (2 ꢂ 0.2 ml) and dried in a vacuum. Yield:
based building units of a bimetallic complex do not interact 90%.
directly but act cooperatively as a Lewis acid and a Lewis base,
splitting the N–H and B–H bond without changing the metals' THF-d8, ppm): d 6.93–6.85 (m, 3H, ArH); 5.10 (s, 5H, Cp–H), 3.28
4a. IR (THF, nCO, cmꢀ1): 1907, 1815, 1656. 1H NMR (400 MHz,
2
3
oxidation state. So far, such reactivity has been reported only for (t, JP–H ¼ 4.12 Hz, 4H, –CH2–); 1.43 (t, JP–H ¼ 7.0 Hz; 36H,
[Cp2ZrOC6H4PtBu2]+ which can be described as an early C(CH3)3). 31P{1H} (161.9 MHz, THF-d8, ppm): d 75.3. Anal. calc.
transition-metal-containing linked FLP.21 This behavior is (%) for C32H47O3P2PdW (%): C 46.20, H 5.69. Found: C, 46.00;
similar to that of Stephan's main group FLPs18 and can be H, 5.78.
exploited for other catalytic conversions; these studies are
underway in our laboratories.
4b. IR (THF, nCO, cmꢀ1): 1901, 1796, 1650. 1H NMR (600 MHz,
THF-d8, ppm): d 7.66 (s, 3H, Tp CH), 7.59 (s, 3H, Tp CH), 5.98 (s,
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2
3H, Tp CH), 6.82–6.76 (m, 3H, ArH), 3.19 (pt, JP–H ¼ 4.12 Hz, The mixture of 1 (c ¼ 0.003–0.01 M, 1 eq.) and amine-borane (5–
3
4H, –CH2–), 1.24 (pt, JP–H ¼ 7.0 Hz, 36H, C(CH3)3). 31P{1H} 50 equiv.) in THF was injected with a syringe.
(161.9 MHz, THF-d8, ppm): d 76.8. Anal. calc. (%) for C36H53
-
The resulting mixture was stirred at 295 K. Data from
BN6O3P2PdW (%): C 44.08, H 5.45. Found: C, 43.95; H, 5.53.
a pressure sensor connected via a wireless network to
a computer were recorded as a function of pressure versus time
for 3–20 hours. The values accumulated were referenced by the
pressure of THF in a blank experiment at 295 K and used for
General procedure for DMAB dehydrogenation by 4
For NMR kinetic studies. (PCP)PdH (c ¼ 0.01 M) and 1a or 1b calculations of the H2 equivalents evolved. The calculations
complexes (c ¼ 0.01 M) and DMAB (5, 10 eq.) were mixed were performed in the ideal gas approximation (pV ¼ nRT).
simultaneously at 290 K in THF-d8.
For variable temperature IR studies. Complex 4a or 4b was
Computational details
generated in situ by the reaction of (PCP)PdH (c ¼ 0.003–0.0039
Calculations were performed with the Gaussian 09 (ref. 54)
M) with 1a or 1b (c ¼ 0.003 M), respectively. The reagents were
package at the DFT/uB97XD55 level without any ligand simpli-
dissolved in THF or toluene at 270 K and allowed to react for
cation. For all atoms, the Def2-TZVP56 basis set was applied,
30 min. Then, a chosen amount of DMAB was added, and the
supplemented with an effective core potential57 in the case of Pd
obtained mixture was monitored in the temperature range 190–
and W. The structures of all complexes and transition states
290 K.
were fully optimized in toluene (3r ¼ 2.3741) described by the
For kinetic IR studies. Reagents were prepared via three
SMD model,58 without any symmetry restrictions. The nature of
all the stationary points on the potential energy surfaces was
conrmed by vibrational analysis. Transition state (TS) struc-
methods.
Method I. A portion of isolated complex 4a or 4b was dissolved
in THF (c ¼ 0.003 M). Then, a chosen amount of DMAB (5
tures showed only one negative eigenvalue in their diagonalized
equiv.) was added and the IR spectra were monitored until full
force constant matrices, and their associated eigenvectors were
catalyst regeneration.
conrmed to correspond to the motion along the reaction
Method II. Complex 4a or 4b was generated in situ by the
coordinate under consideration using the Intrinsic Reaction
Coordinate (IRC) method.59
reaction of (PCP)PdH (c ¼ 0.003–0.0036 M) with 1a or 1b (c ¼
0.003 M), respectively. The reagents were dissolved in THF at
290 K and allowed to react for 20 min. Then, a chosen amount of
DMAB (1–5 eq.) was added, and the mixture obtained was
monitored until full catalyst regeneration.
Method III. (PCP)PdH (c ¼ 0.003–0.0036 M), 1a or 1b
complexes (c ¼ 0.003 M) and DMAB (5 eq.) were mixed simul-
taneously at 290 K in THF. The reaction mixture was monitored
until full catalyst regeneration.
Volumetric studies of amine-borane dehydrogenation.
Hydrogen evolution during dehydrogenation of amine-boranes
was monitored using the Man on the Moon X103 kit. The
volume of the system is 32 ml (two-necked round-bottom ask –
30 ml, three-way valve – 2 ml). The monitored solutions were
prepared via three methods.
Method I. A portion of the isolated complex 4a or 4b (0.006
mmol) was dissolved in THF (1 ml) in an argon-lled ask of
the device connected to a three-way valve. Then the ask was
tightly closed with a septum cap, and the valve was opened to
the pressure sensor. The chosen amount of DMAB (5–50
equiv.) in 1 ml of THF was injected with a syringe through
a septum cap.
X-ray crystallography
For crystals of 4a and 4b, X-ray diffraction data were collected at
120 K with a Bruker ApexII DUO diffractometer using graphite
monochromatic Cu-Ka and Mo-Ka radiation, respectively.
Using Olex2,60 the structures were solved with the ShelXT61
structure solution program using intrinsic phasing and rened
with the XL62 renement package using least-squares minimi-
zation. Hydrogen atom of the BH group in 4b was found in the
difference Fourier synthesis while positions of other hydrogen
atoms were calculated, and they all were rened in the isotropic
approximation within the riding model. Crystallographic data
and structure renement parameters are given in Table S1.†
CCDC 2020560 and 2020559 contain the supplementary crys-
tallographic data for 4a and 4b, respectively.†
Conflicts of interest
There are no conicts to declare.
Method II. Complex 4a or 4b (c ¼ 0.003–0.01 M) was generated
in situ by mixing the solutions of (PCP)PdH (1–1.2 eq.) and 1 (c ¼
0.003–0.01 M, 1 eq.) in THF in an argon-lled ask of the device
Acknowledgements
connected to a three-way valve. Then, the ask was tightly This work was nancially supported by the Russian Science
closed with a septum cap, and the valve was opened to the Foundation (project no. 19-13-00459). The NMR and X-ray data
pressure sensor. A chosen amount of DMAB (5–50 eq.) in THF were collected using the equipment of the Center for Molecular
was injected with a syringe through a septum cap.
Composition Studies of INEOS RAS with the support from the
Method III. The solution of (PCP)PdH in THF (1–1.2 eq.) was Ministry of Science and Higher Education of the Russian
prepared in an argon-lled ask of the device connected to Federation. Volumetric studies of dihydrogen evolution were
a three-way valve. Then the ask was tightly closed with performed in the frame of the bilateral project CNR (Italy) –
a septum cap, and the valve was opened to the pressure sensor. RFBR (Russian Federation) 2018–2020 (RFBR no. 18-53-7818).
3690 | Chem. Sci., 2021, 12, 3682–3692
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