Acceptorless, intramolecular, alkyl dehydrogenation in the solid-state in a rhodium phosphine complex; reversible uptake of three equivalents of H 2 per molecule

Article abstract of DOI:10.1039/b718615k

Addition of H₂ to the phosphine alkene ligated complex [Rh(dppe)(PCyp₂Cyp′)][BArF4] 1 (Cyp = cyclo-C₅H ₉; Cyp′ = cyclo-C₅H₇, Ar^F = 3,5-(CF₃)₂C₆H₃) in the solid state results in hydrogenation of the alkene and uptake of two further equivalents of H₂ to afford the dihydride-dihydrogen complex [Rh(dppe)(PCyp ₃)(H)₂(η²-H₂)][BArF4] 2. Placing 2 under a vacuum or argon results in sequential loss of H₂ and dehydrogenation of one of the cyclopentyl rings to reform 1. The hydrogenation-dehydrogenation cycle has been repeated 5 times without apparent degradation and has been followed by solid-state ³¹P{¹H} NMR spectroscopy. Intermediates on this process have been trapped using acetonitrile to give stable complexes that have been characterised in solution. We have previously shown that this hydrogenation-dehydrogenation process also happens in the solution-state; and evidence is presented that shows that, apart from a subtle difference, the same overall transformation occurs in the solid-state. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique.

Full text of DOI:10.1039/b718615k

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Acceptorless, intramolecular, alkyl dehydrogenation in the solid-state in
a rhodium phosphine complex; reversible uptake of three equivalents of
H2 per molecule
Thomas M. Douglas and Andrew S. Weller*
Received (in Montpellier, France) 3rd December 2007, Accepted 18th December 2007
First published as an Advance Article on the web 21st January 2008
DOI: 10.1039/b718615k
Addition of H2 to the phosphine alkene ligated complex [Rh(dppe)(PCyp2Cyp0)][BAr4F] 1
(Cyp = cyclo-C5H9; Cyp0 = cyclo-C5H7, ArF = 3,5-(CF3)2C6H3) in the solid state results in
hydrogenation of the alkene and uptake of two further equivalents of H2 to afford the
dihydride–dihydrogen complex [Rh(dppe)(PCyp3)(H)2(Z2-H2)][BAr4F] 2. Placing 2 under a vacuum
or argon results in sequential loss of H2 and dehydrogenation of one of the cyclopentyl rings to
reform 1. The hydrogenation–dehydrogenation cycle has been repeated 5 times without apparent
degradation and has been followed by solid-state 31P{1H} NMR spectroscopy. Intermediates on
this process have been trapped using acetonitrile to give stable complexes that have been
characterised in solution. We have previously shown that this hydrogenation–dehydrogenation
process also happens in the solution-state; and evidence is presented that shows that, apart from a
subtle difference, the same overall transformation occurs in the solid-state.
sible, and placing 2 under a vacuum relatively rapidly (B1 h)
regenerates 1. Intermediates in this process have been observed
spectroscopically and also studied using computational meth-
ods.9 We now report that these reversible hydrogenation–
dehydrogenation reactions also occur in the solid-state by
solid–gas reactions. This represents not only a relatively rare
Introduction
The selective activation of C–H bonds in hydrocarbons has
been a long-standing topic of interest.1,2 Transition metal
catalyzed alkane dehydrogenation to afford the corresponding
olefin is particularly interesting as it delivers valuable unsatu-
rated functionality. Although many dehydrogenations require
a sacrificial hydrogen acceptor,3 ‘‘acceptorless’’ dehydrogena-
tion using transition metal catalysts has been achieved.4 As
well as this synthetic utility, the dehydrogenation of alkanes,5
or alkane analogs such as ammonia boranes,6 also represents a
possible vector for the storage and transportation of H2, which
is of current interest due to the potential use of hydrogen as an
energy carrier.7 Many of these elegant systems, however, are
not as yet truly reversible, or require forcing conditions (heat)
and only work in solution. Of interest, then, would be a system
that performed this transformation reversibly at ambient, or
near ambient, conditions in the solid-state. We report here
such a system, that operates by a reversible acceptorless alkyl
dehydrogenation in the solid-state to store and release up to
three equivalents of H2 per cycle.
example of an organometallic solid–gas reaction with small
10–13
molecules
but also, as far as we are aware, the first
example of reversible and acceptorless alkyl dehydrogenation
to be reported in the solid-state. Alkene hydrogenation11,14–16
and C–H activation16,17 (an early mechanistic step in alkane
dehydrogenation2) have been described in the solid-state and
in one case dehydrogenation of cyclopentene has also been
reported, although a hydrogen acceptor is required and the
reaction was not reported to be reversible.11
Results and discussion
Addition of H2 to light orange powder 1 at just above room
temperature (40 1C) results in a subtle change in color to very
pale yellow. This change can be followed spectroscopically by
31P{1H} solid-state NMR (SSNMR) spectroscopy (Scheme 2
and Fig. 1), which shows a change consistent with the com-
plete conversion of 1 to a new complex. Although we were not
successful in using 1H SSNMR to resolve a bound dihydrogen
ligand,18 rapid vacuum transfer of CD2Cl2 at 78 K onto the
pale yellow solid gave solution 31P{1H} and 1H NMR spectra
(at 180 K) that showed a B1 : 1 mixture of [Rh(dppe)
(PCyp3)(Z2-H2)(H2)][BAr4F] 2 and [Rh(dppe)(PCyp3)(L)(H2)]-
[BArF4 ] 3, the latter of which we have previously characterized
as being a dihydride complex with a weakly bound ligand
(C–H agostic or solvent) in the sixth coordination site.9 We
have demonstrated previously that the bound dihydrogen
ligand in 2 is sufficiently labile so that it is easily lost in
We have recently reported the remarkably quick acceptor-
less dehydrogenation of one alkyl ring of tris-cyclopentyl-
phosphine (PCyp3) in the complex [Rh(dppe)Cl(PCyp3)] on
addition of Na[BArF4 ], to afford [Rh(dppe)(PCyp2Cyp0)]-
[BArF4 ] 1 in quantitative yield (Scheme 1, ArF = 3,5-
(CF3)2C6H3).8,9 This generates a hybrid phosphine–olefin
ligand on a square planar Rh(I) centre. Addition of H2 to 1
in CH2Cl2 solution results in hydrogenation of the alkene and
the formation of the dihydrogen–dihydride complex 2,
[Rh(dppe)(PCyp3)(Z2-H2)(H2)][BArF4 ]. This reaction is rever-
Department of Inorganic Chemistry, University of Oxford, Oxford,
OX1 3QR. E-mail: andrew.weller@chem.ox.ac.uk
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Scheme 1
solution under an Ar atmosphere at room temperature to form
3,9 meaning that we cannot definitively state whether 2 is
formed exclusively in the solid-state or a mixture of 2 and 3 is
formed. However, the overall close similarities between solu-
tion and solid state reactivity coupled with the observation
that in the solid-state 2 loses H2 to form a product other than 3
(vide infra), suggests that 2 is most likely formed exclusively,
some of which then loses H2 to form 3 on dissolving in
CD2Cl2. Vacuum transfer of CD2Cl2 spiked with MeCN onto
the yellow solid affords the acetonitrile adduct [Rh(dppe)-
(PCyp3)(NCMe)H2][BArF4 ] 4 as the only product. This mirrors
the solution chemistry which shows that addition of MeCN to
either 2 or 3 gives 4 cleanly.9 The formation of labile dihydro-
gen complexes in the solid-state from hydrogenation of a
bound alkene has been reported previously.12,14
Addition of H2 to the red solid immediately regenerates the
yellow powder which analyses by solution 31P{1H} NMR
spectroscopy for 2 (along with 3 as observed previously). We
have not been able to obtain a SSNMR of the red intermediate
in the pure state, but monitoring the gradual loss of H2 in the
NMR probe from 2 shows the growth in and disappearance of
new peaks at d B 79 and d B 38 ppm, which we assign to this
intermediate complex, with the concomitant increase in the
signals due to the dehydrogenated product 1 (Fig. 1). These
peaks correlate reasonably well with those observed in solu-
tion for the red intermediate species (vide infra).
Solution trapping experiments suggest the identity of this
dark red intermediate present in the solid-state. Vacuum
transfer of CD2Cl2 onto the red solid results in a very broad
room temperature solution 31P{1H} NMR spectrum suggest-
ing a dynamic process (Fig. 2). Cooling to 200 K arrests this
processes and, still broad, signals are observed at d B 74, 62
and 23 that demonstrate a new product has been formed,
which we assign to [Rh(dppe)PCyp3(L)][BArF4 ] (L = agostic
C–H, solvent or adventitious water) 5. The hydride region of
the 1H NMR spectrum at low temperature is featureless
showing that loss of the hydride ligands has occurred. The
alkene region (d 6.0–3.5) is also featureless, demonstrating that
dehydrogenation of the PCyp3 ligand has not occurred. Solu-
tions of 5 slowly return to give 1 on standing at room
Placing the pale yellow, ‘‘hydrogen charged’’, solid under a
vacuum at room temperature results in a very rapid (seconds)
darkening to deep red. This same transformation can be
effected on replacing the H2 atmosphere by flushing Ar over
the solid. A slower (1 h) return to the pale orange of the
starting material occurs either under vacuum or argon (as
monitored by 31P{1H} SSNMR and solution NMR spectro-
scopies). The 31P{1H} SSNMR of the fully-cycled material
show that 1 is returned unchanged, and the cycle can be
repeated (five times) with no appreciable decomposition.
Scheme 2 Diagram showing the relationship between the final products and intermediates observed during the hydrogenation–dehydrogenation
cycle in solution and the solid-state. Solid arrows indicate reactions in CD2Cl2 solution, some of which have been previously reported.9 Dashed
arrows show the solid-state reactions. This diagram makes no comment on the detailed mechanistic steps and only connects observed complexes.
For example in solution 3 to 1 most likely proceeds through the intermediacy of 5, which is not observed.
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Fig. 2 31P{1H} NMR (202.5 MHz) spectra of yellow and red solids
after addition of CD2Cl2 or CD2Cl2/MeCN. Complex 2 (top spec-
trum) displays a tightly coupled ABMX coupling pattern and the outer
AB lines are of low intensity. See ref. 9 for more details.
Fig. 1 Solid state 31P{1H} NMR (121.4 MHz) spectra of (a) 1, (b)
1 + H2 (i.e. complex 2), (c, d) the intermediate regime demonstrating
gradual H2 loss from 2, and (e) complex 1 after a complete cycle. *
marks the peaks assigned to the intermediate species. The spin rate for
all spectra was ca. 8000 Hz. Low temperature (223 K) used in an
attempt to reduce the H2 loss from complex 2.
In conclusion we have demonstrated that a remarkably
undemanding reversible acceptorless dehydrogenation of a
cyclic alkane aids the storage and release of up to 3 equivalents
of H2 per molecule occurs in the solid-state. Although this
overall would result in a modest H2 storage uptake and release
of 0.35% (w/w), which is clearly impractical for mobile
applications,7 that this process occurs in the solid-state sug-
gests that it might offer new opportunities in this technologi-
cally important area. Very recently a similar, solution only,
process has been reported using RuH2(Z2-H2)2(PCyp3)2 that
stores 1.7% H2 but this requires an acceptor (ethene) to
promote transfer hydrogenation.20
temperature. Vacuum transfer of CD2Cl2/MeCN to the deep
red intermediate in the solid-state gives [Rh(dppe)PCyp3(NC-
Me)][BArF4 ] 6, with no 4 observed. The same product is
observed by addition of MeCN to a CD2Cl2 solution of 5 or
slow (24 h) loss of H2 from 4.9 No hydride signals were
observed in the high field region. As complex 6 would result
from addition of MeCN to a complex such as [Rh(dppe)-
PCyp3(L)][BArF4 ] 5 we suggest that placing 2 under vacuum
results in the loss of 2 equivalents of H2 in the solid-state to
give 5. This is in contrast to solution experiments that show
that the dihydride 3 is initially formed on application of a
vacuum to 2.9 Both solution and solid-state processes even-
tually give 1—the dehydrogenated product. Thus although the
solid-state and solution experiments follow the same overall
transformation, they differ in the nature of the ‘‘low-hydride’’
intermediates that are observed (5 versus 3, respectively).
Although the reasons for this are not completely clear at the
present time we speculate that the structural change associated
with stabilizing the vacant site in 3 is energetically less favor-
able in the solid state than for 5. Changes between solution
and solid-state reactivity with regard to hydrogen addition/
ligand reorientation13 and selectivity19 have been noted
previously.
Experimental
All manipulations, unless otherwise stated, were performed
under an atmosphere of argon, using standard Schlenk-line
and glove-box techniques. Glassware was oven dried at 130 1C
overnight and flamed under vacuum prior to use. CD2Cl2 was
distilled under vacuum from CaH2 and stored over 3 A
molecular sieves. [Rh(dppe)(PCyp2Cyp0)][BAr4F] 1 was pre-
pared by a published literature method.8,9 Complexes 2, 3, 4
and 6 have previously been prepared.9 1H, 31P{1H} and
13C{1H} solution NMR spectra were recorded on Bruker
Avance 400 MHz and Bruker Avance 500 MHz spectrometers.
Residual protio solvent was used as the reference for 1H NMR
spectra (CD2Cl2: d = 5.33). 13C{1H} spectra were referenced
to the perdeuterio solvent signal. 31P{1H} spectra were refer-
enced against 85% H3PO4 (external). 31P{1H} Solid State
NMR spectra were recorded on a Varian Unity Inova spectro-
meter with a 4 mm rotor (o.d.) and MAS probe. Operating
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frequency (121.4 MHz for 31P). Spectra were recorded using
cross polarization experiments and referenced against 85%
H3PO4 for 31P. Contact time = 1 ms, recycle delay = 10 s.
Chemical shifts are quoted in ppm. Coupling constants are
quoted in Hz.
particular Dr David Apperly, are thanked for acquiring the
solid-state NMR spectra.
References
Solid state NMR spectra for [(dppe)Rh(PCyp2Cyp0)][BAr4F] (1)
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ꢂ3
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was hydrogenated at B4 atm H2 pressure (298 K/77 K = 3.8)
and heated at 40 1C overnight to give 2 as a very pale yellow
solid. CD2Cl2 was vacuum transferred (at 77 K) into a sample
of the pale yellow solid and the resulting solution was char-
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giving spectra fully consistent with a mixture of 2 and 3 in a 1 : 1
ratio.9 Vacuum transfer of CD2Cl2/MeCN into a sample of the
yellow solid gave spectra consistent with 49 as the only product.
Preparation of [(dppe) Rh(PCyp3)(L)][BArF4] (5) (L = agostic
interaction, vacant site or weakly bound adventitious ligand) in
the solid state
ꢂ3
A solid sample of 2 (5 mg, 3.11 ꢁ 10
mmol) was placed
under vacuum at 298 K for 5 seconds giving a deep red solid.
CD2Cl2 was vacuum transferred (at 77 K) into this red solid
and 1H and 31P{1H} NMR spectroscopy of the resulting
solution at 200 K tentatively identified the complex as [(dppe)
Rh(PCyp3)(L)][BArF4 ] 5. Vacuum transfer of CD2Cl2/MeCN
into a sample of the red solid gave 1H and 31P{1H} NMR
spectra fully consistent with those previously reported for 6.9
1H NMR (500.1 MHz, CD2Cl2, 200 K): d 7.92–7.31 (m,
20H, ArH), 7.70 (s, 8H, BArF4 ), 7.51 (s, 4H, BArF4 ), 2.88–0.85
(m, 31H, CH2/CH). 31P{1H} NMR (202.5 MHz, CD2Cl2, 200
K): d 73.90 (br m), 61.92 (br m), 23.52 ppm (v br m).
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Acknowledgements
The Royal Society and the EPSRC for funding. The EPSRC
solid-state NMR service at the University of Durham, and in
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