Phosphidoboratabenzene-rhodium(i) complexes as precatalysts for the hydrogenation of alkenes at room temperature and atmospheric pressure

Article abstract of DOI:10.1039/c5dt03109e

The di-tert-butylphosphido-boratabenzene ligand (DTBB) reacts with [(C₂H₄)₂RhCl]₂ yielding the dimeric species [(C₂H₄)Rh(DTBB)]₂ (1). This species was fully characterized by multinuclear NMR and X-ray crystallography. Complex 1 readily dissociates ethylene in solution and upon exposure to 1 atm of H₂ is capable of carrying out the hydrogenation of ethylene. The characterization of two Rh-H species by multinuclear NMR spectroscopy is provided. The reactivity of 1 towards the catalytic hydrogenation of alkenes and alkynes at room temperature and 1 atm of H₂ is reported and compared to the activity of Wilkinson's catalyst under the same reaction conditions.

Full text of DOI:10.1039/c5dt03109e

Dalton
Transactions
View Article Online
View Journal
PAPER
Phosphidoboratabenzene–rhodium(I) complexes
as precatalysts for the hydrogenation of alkenes at
room temperature and atmospheric pressure†
Cite this: DOI: 10.1039/c5dt03109e
Viridiana Perez, Pierre Audet, Wenhua Bi and Frédéric-Georges Fontaine*
The di-tert-butylphosphido-boratabenzene ligand (DTBB) reacts with [(C
species [(C )Rh(DTBB)] (1). This species was fully characterized by multinuclear NMR and X-ray crystal-
lography. Complex 1 readily dissociates ethylene in solution and upon exposure to 1 atm of H is capable
2 4 2 2
H ) RhCl] yielding the dimeric
2
H
4
2
2
Received 12th August 2015,
Accepted 23rd October 2015
of carrying out the hydrogenation of ethylene. The characterization of two Rh–H species by multinuclear
NMR spectroscopy is provided. The reactivity of 1 towards the catalytic hydrogenation of alkenes and
DOI: 10.1039/c5dt03109e
alkynes at room temperature and 1 atm of H
lyst under the same reaction conditions.
2
is reported and compared to the activity of Wilkinson’s cata-
www.rsc.org/dalton
9
5
nation mode. Ring slippage to the η -coordination where the
Introduction
boron is away from the metal centre is however frequent,
1
10
Since the first report of a boratabenzene complex by Herberich,
this heterocyclic moiety has been coordinated to most tran- ents on boron. Other types of hapticities are more scarce and
sition metals, either as a neutral borabenzene or an anionic include η -allylic-like coordination
boratabenzene ligand (Fig. 1, I and II). Borabenzene derivatives where the substituent on boron interacts with the transition
especially when facilitated by π-donating or bulky substitu-
1
1
2
3
12
1
and η -coordination
3
and their metal complexes have been shown to be active in the metal. Two different boratabenzene moieties have been shown
4
polymerization of alkenes, the cyclotrimerization of alkynes to exhibit the latter coordination mode, including species III
5
6
7
13a
and nitriles, E–H bond activation, acting as a chirality source
(Fig. 2) having a unique Pt–Cl–B interaction. Upon addition
8
and generating interesting optoelectronic materials.
of a Lewis base, the Cl-boratabenzene anion can dissociate
1
3b
The boratabenzene framework is an isoelectronic analogue and undergo ligand exchange
or coordinate to other tran-
This atypical coordination mode is also
sition metals by its π system and usually adopts a η -coordi- observed for the diphenylphosphido-boratabenzene, first
1
3c
of the cyclopentadienyl ligand and as such, interacts with tran- sition metals.
6
1
4
reported by Fu, an anionic analogue of the ubiquitous PPh3,
where the lone pair on phosphorous remains available for
σ-donation, as observed in Rh complex IV (Fig. 2).
Our group reported in 2012 the synthesis of di-tert-butyl-
phosphido-boratabenzene (DTBB), a ligand designed to act as
a bulky and anionic phosphine. It was shown that the DTBB
1
fragment does not coordinate in a purely η -mode on electroni-
cally unsaturated metal centres. Instead, in addition to the
phosphine binding one metal, it was possible to coordinate
6
the DTBB ligand by the aromatic ring in a η -fashion on
another nickel centre in species VI. DTBB also coordinated in
2,3
Fig. 1 Neutral and anionic boron heterocycles.
3
a η allylic-like mode, through a P–B–C interaction with Pt(II)
1
2b
(
V).
It is precisely the hemilability of the DTBB and its
Département de Chimie, Centre de Catalyse et Chimie Verte (C3V), Université Laval,
045 Avenue de la Médecine, Cité Universitaire, Québec, QC G1V 0A6, Canada.
E-mail: frederic.fontaine@chm.ulaval.ca
Electronic supplementary information (ESI) available: Multinuclear NMR
donor capability as a bulky phosphine that makes it an attrac-
1
5
1
tive framework for catalytic applications. Herein we report
the synthesis and characterization of a rhodium di-tert-butyl-
phosphidoboratabenzene complex and its catalytic activity
towards alkene hydrogenation at room temperature and atmos-
pheric pressure.
†
characterization for complex 1 and hydride species 2 and 3, 2D NMR and crystal-
lographic data for 1. CCDC 1415613 for 1. For ESI and crystallographic data in
CIF or other electronic format see DOI: 10.1039/c5dt03109e
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Fig. 3 ORTEP plot of 1 with thermal ellipsoids set at the 50% probability
level. H atoms are shown as spheres with arbitrary radii. H atoms on all
methyl groups are omitted for clarity. Symmetry transformations used to
generate equivalent atoms: A: −x + 1, −y, −z + 1. Selected bond dis-
tances [Å] and angles [°]: P1–B1 1.962(4), B1–Rh1 2.510(4), Rh1–P1
2
2
.3226(9), Rh1–C1 2.435(3), Rh1–C2 2.267(4), Rh1–C3 2.259(3), Rh1–C4
.259(4), Rh1–C5 2.237(3), Rh1–C6 2.140(4), Rh1–C7 2.131(4); Rh1–P1–
C12 110.8(1), Rh1–P1–C8 118.4(1), Rh–P1–B1 112.4(1), C8–P1–C12
08.6(2), B1–P1–C8 97.2(2), B1–P1–C12 108.6(2), C7–Rh1–P1 97.8(1),
C6–Rh1–P1 100.3(1), Rh1–P1–B1 112.4(1), B1–Rh1–P1 100.63(9), C1–
B1–C5 113.9(3), B1–Rh1–C7 161.2(1), B1–Rh1–C6 133.6(1).
1
Fig. 2 Some borabenzene species related to this study: [(IMes)
2
Pt(H)-
(V),
H -
8 12
13a
14
12b
(
1-Cl-2-SiMe
Ni (DTBB)]
P(tBu) BC
B-Ph)] (IX).
3
-BC
(VI),
5
H
4
)] (III),
and
Rh(PMe
(η -(C,B,P)-C
3
)
3
(DPB) (IV), Pt(DTBB)
2
1
3
2
2
2
5
H
5
BP(tBu)
2
)(κ–η :η -C
6a
(
(
2
5
H
5
)Pt (VII), [(C
2
H
4
)
2
Rh(C B-NMe
5
H
5
2
)] (VIII) and [(C
2
H
4
)
2
Rh-
6
a
5 5
C H
The homobimetallic species has a B–Rh distance of
2
.510(4) Å, which is larger than in related π-boratabenzene
complex [(C BPh)Rh(CH CH ] (IX) which has a B–Rh dis-
tance of 2.398(4) Å. The B–Rh distance in 1 is somewhat
comparable to species [(C BNMe )Rh(CH CH (VIII),
5
H
5
2
2 2
)
1
2b
Results and discussion
5
H
5
2
2
2 2
) ]
The reaction of 2.2 equiv. of ligand DTBB with [RhCl(C
2
H
4
)
2
]
2
in a
6a
which has a B–Rh bond length of 2.521(2) Å, despite the fact
that there is no π-bonding between P–B since the lone pair on
benzene/THF mixture at room temperature, followed by removal
of volatiles and extraction with diethyl ether, provided complex 1
as a red-brown crystalline powder in 40% yield (Scheme 1).
Crystalline red needles, suitable for X-ray diffraction
studies, were obtained from the slow evaporation of a benzene-
16
phosphorous is engaged in a bond with one metal centre.
Interestingly, complex 1 displays some asymmetry in the
coordination of the borabenzene moiety since one of the Rh–
Cortho bond is significantly elongated (Rh–C(1) = 2.435(3) Å)
d
6
solution of 1. The bimetallic complex crystallized in a
compared to the other Rh–Cortho bond (Rh–C(5) = 2.237(3) Å),
monoclinic P2 /n space group (Table S2†). Species 1 was con-
1
suggesting that the heterocyclic moiety might be closer to an
firmed to be a centrosymmetric dimer where each of the two
DTBB ligands bridges two rhodium(I) centres, with the borata-
4
6
17
η -coordination rather than a typical η -coordination (Fig. 3).
The CvC bond length for the ethylene molecule is 1.370(6) Å
and the Rh–Cethylene distances are very similar (2.131(4) and
6
benzene moiety binding in a η -fashion to one Rh(I) and the
lone pair of the phosphine coordinating the other metal
2.140(4) Å). These values compare well to those observed in
2
centre. A η -ethylene molecule is completing the 18-electron
species IX (Rh–Cethylene = 2.145(4), 2.143(4) Å and CvC =
count of each rhodium (Fig. 3).
6a
1
.385(6) Å) and VIII (Rh–C
= 2.118(7), 2.133(7) Å and
ethylene
6
a
CvC = 1.376(9) Å). Finally, the Rh-bound phosphine ligand
retains its tetrahedral geometry, as expected. Whereas most of
the angles around phosphorous are close to the expected value
for a tetrahedral centre, the B–P–C(8) angle of 97.2(2)° is
smaller than 109° and diverts significantly from the related
1
2b
angle in the metal-free ligand (108.6(2)°).
The multinuclear NMR characterization confirmed the pres-
ence of 1 in solution. The similarity between the borataben-
1
zene proton resonances in the H NMR spectrum for 1 at δ =
Scheme 1 Synthesis of [(C
2
H
4
)Rh(DTBB)]
2
(1).
5.96, 5.55 and 4.48 for the ortho, meta and para positions,
Dalton Trans.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
respectively, with the ones observed for rhodium complexes 1 over a period of 3 h in an airtight J-Young NMR tube, two
VIII (δ = 5.82 ortho, 4.87 meta, 4.27 para) and IX (δ = 6.0 ortho hydride resonances, attributed to two different species, were
6
6a
and meta, 4.34 para) suggest similar η coordination modes.
observed at −13.94 (2) and −17.83 ppm (3), along with new
Both the ortho and meta protons appear each as a single reson- aromatic resonances. It was unfortunately impossible in our
1
ance in the H NMR spectrum, as expected for a symmetric hand to isolate these two compounds in the solid state. It
coordination of the heterocycle to the metal centre in 1, should be noted that even when handled under strict anhy-
suggesting that the asymmetry present at the solid-state is not drous and air free conditions, the presence of precipitate was
observed in solution at room temperature. As expected, the observed (evidenced by a modification in the integration for
3
1
tBu
1.6 Hz). Two resonances observed at 3.00 (J = 11.7, 2.2 Hz) phosphine oxide tBu
and 2.74 ppm (J = 12.0, 2.2 Hz) put in evidence the presence of (d, 18H, tBu ), 5.82 (d, 1H, tBu PH)), which might be caused
2
P-fragment is present as a doublet at 1.34 ppm ( JHP
=
the tBu moieties by H NMR) along with small amounts of the
3
1
1
1
1
2 6 6
PH(O) ( P{ H} δ = 61.6; H (C D ) = 0.99
2
1
2
2
an ethylene ligand in the coordination sphere. Regardless of by traces of water or oxygen in the molecular hydrogen. It is
the high donor capability of DTBB, which should allow the however possible to obtain selectively each hydride as main
metal to be more electron rich and engaging more efficiently species in solution depending on the reaction conditions.
into back-donation with a π-coordinated alkene ligand, the When left for a period of 3 days under a hydrogen atmosphere,
ethylene protons on complex 1 are observed to be less shielded species 2 was the main species observed in solution. However,
6
compared to those previously reported for Rh(I)-η -borataben- when the solvent of a solution containing both 2 and 3 was
zene species (δ = 2.19 for VIII and 1.22, 2.82 for IX).
removed under reduced pressure and the oily solid residue was
3
1
The broad doublet observed in the P NMR spectrum at dissolved back under a nitrogen atmosphere, only species
2
3.7 ppm and the JPRh value of 160 Hz support the coordi- 3 was observed. It proved impossible to revert from 3 to 2
nation of the phosphorous atom on a rhodium(I) centre. Only under one atmosphere of hydrogen. All attempts to obtain a
one resonance is observed because of the presence of a centre crystalline solid from solutions of 2 or 3 failed.
of symmetry on the complex. The chemical shift is signifi-
As mentioned above, a hydride resonance appearing as a
cantly different than the one observed for the nickel species doublet of doublets at δ −13.94 with two distinct coupling con-
−
VIII (−8.9 ppm). In contrast, the complexes where the (DTBB ) stants of 23.3 and 21.3 Hz is characteristic of species 2 by
3
1
ligand is η -coordinated to Pt exhibit similar resonances at
1.9 and 37.2 ppm for V and VII, respectively.
Boratabenzene species VIII and IX, having ethylene ligands, (t, para), along with a doublet at 1.35 ppm. The integration
H NMR spectroscopy (Fig. 4a). Aromatic resonances were also
3
observed at δ = 6.00 (t, meta), 5.78 (dd, ortho), and 5.38
were shown to exhibit dynamic behaviour attributed to the ratio of the hydride and of the aromatic resonances demon-
rotation of ethylene, as confirmed by variable temperature strates that there are two hydrides per boratabenzene moiety.
6
a
NMR experiments. However, it was not possible to observe The similarity of the splitting pattern of 2 with species
ethylene rotation with species 1 because of the dissociation of 1 suggests that similar coordination environments are present.
3
1
1
the ethylene molecule observed when dissolved in solution. The P{ H} NMR spectrum of species 2 shows a very broad
3
1
1
When the solution reached 40 °C, the broadening of the aro- multiplet at ≈52 ppm. The lack of intensity of the P{ H} res-
matic and ethylene resonances was observed but the rate of onance, even after an acquisition of 1 hour at −90 °C, suggests
1
1
ethylene dissociation was significant as suggested by the that in addition of the broadness caused by the B quadru-
1
03
increase in intensity of the singlet at 5.2 ppm. Traces of un- polar moment and the coupling with the
Rh nuclei, some
identified products were observed spectroscopically along with rapid fluxional process also takes place. The same problem
some precipitate, suggesting that degradation was taking
place. Therefore complex 1 was stored at −30 °C and protected
from light, since Rh ethylene species such as CpRh(C H )
2
4 2
have been shown to undergo rearrangements under photo-
1
8,19
chemical conditions.
Reactivity with H2
Whereas some preliminary studies have been done on the reac-
tivity of species IV and [(cod)Rh(C H B-L)][OTs] (L = Py, PMe ;
5
5
3
−
OTs = CH
3 6 4 3
C H SO ) with hydrogen suggesting that rhodium
hydride species have been generated, no details were given on
1
4,20
the characterization and reactivity of these compounds.
We were able to show that complex 1 reacts with 1 atm of H
2
to
generate free ethylene and ethane, observed as resonances at
1
δ = 5.20 and 0.90, respectively, by H NMR spectroscopy. As
mentioned above, it should be noted that free ethylene is
1
Fig. 4 Selected region of the H NMR spectra (benzene-d
6
, 500 MHz)
always observed when 1 is dissolved in solution, along with
for the boratabenzene and high field Rh–H resonances for hydride 2 (a)
uncharacterized species. When H2 is allowed to react with and 3 (b).
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.

View Article Online
Paper
Dalton Transactions
1
1
1
was observed by
B{ H} NMR spectroscopy where after
1
2 hours of acquisition (over 34 000 scans) no discernible reso-
nance was observed. It should be noted that the difficulty in
1
1
acquiring good B NMR spectra has been previously observed
1
3a
for borabenzene complexes. Nevertheless, it was possible to
3
1
1
confirm that the P{ H} resonance at 52 ppm was indeed for
1
31
species 2 by the acquisition of a H{ P} spectrum with selec-
tive decoupling at the latter frequency. Indeed, the hydride
resonance at −13.94 ppm was observed as a doublet of 23.4 Fig. 5 Proposed molecular structures for hydride species [(DTBB)Rh-
(
2 2 2
H) ] (2) and [(DTBB)Rh(H)] (3).
Hz, attributed to a Rh–H interaction, therefore confirming that
the 21.2 Hz coupling constant corresponds to a JP–H. It was
further confirmed that the ortho proton of the boratabenzene
moiety, previously observed as a triplet, was now a doublet of
2
(
(
3.2 Hz are very similar to those of [Cp*Rh(PMe )(H) ]
−13.65 ppm, dd, JRh–H = 29.8 Hz, JP–H = 41.8 Hz)
3 2
9
.3 Hz and that the doublet at 1.37 ppm for the tBu groups
24a,b
and [CpRh-
was now a singlet.
24c
3 2
PiPr )(H) ] (−14.73 ppm, dd, JRh–H = 27.5 Hz, JP–H = 33.5 Hz).
1
The H NMR hydride resonance at −17.83 ppm for species
It should be noted that only species 3 remains when a
3
(Fig. 4b) also exhibits two distinct coupling constants of 21.5
mixture of 2 and 3 are put under reduced pressure. Interest-
ingly, species 3 is shown to have two hydrogen atoms less than
and 14.6 Hz. However, five distinct resonances at δ = 6.25 (over-
lapped, 2H), 6.03, 4.95, and 4.80, can be observed for the bora-
tabenzene moiety, suggesting a loss of symmetry of the
complex in solution. It is noteworthy to mention that the inte-
gration ratio observed is now of 1 : 1 between the hydride and
1
1
species 2 according to H NMR spectroscopy. The H NMR
chemical shift of the hydride is more shielded and the JRh–H
coupling of 3 is significantly smaller compared to 2. Several
cyclopentadienyl rhodium hydride species have been shown to
have similar spectroscopic features when the complexes were
1
31
the boratabenzene resonances. Carrying a H{ P} NMR experi-
3
1
ment by selectively decoupling the ill-defined P resonance of
23c,25
23a,24c
cationic
or when the hydrides were bridging,
which
3
−
at 54 ppm did show that the doublet of doublets at
17.83 ppm in the H NMR spectrum now appeared as a
is not expected to occur for 3 since no ionization source was
added and a triplet would be observed in the case of a hydride
bridging two active
1
doublet of 15.9 Hz, which can be attributed to a JRh–H
.
103
Rh centres. The most likely speculation
Additionally, two resonances in the aliphatic region, attributed
as for the structure of 3 (Fig. 5) would be a dimeric Rh(II)
species having a Rh–Rh interaction and one hydride on each
metal. Because of the Rh–Rh interaction and the limited flexi-
bility one might imagine for such complex, the asymmetry
observed in the coordination mode of the boratabenzene
moiety in the crystal structure of 1 could be present in 3 and
kept “locked” in solution, something that was not observed for
the ethylene complex. Thereby, a centre of symmetry might be
present for 3 but each proton of the boratabenzene would
remain inequivalent. However, several other conformations are
possible for this species, especially since DTBB was shown to
have very rich coordination chemistry.
to the tBu groups, were observed as singlets at 1.46 and
1
.29 ppm rather than doublets.
It should also be noted that it was very difficult to obtain
good NMR spectra for elements other than proton regardless
of the concentration of the sample and the acquisition time.
1
3
1
For example, when carrying C{ H} NMR experiments with
2 hour acquisition for 2 and 3, no resonance other than the
1
one for the tBu group were observed. HSQCAD and gHMBCAD
correlation experiments also proved unsuccessful for both
hydride species.
T1 measurements were nevertheless informative regarding
the bonding mode of hydrogen to metal centres since they can
help discriminating between bridging and terminal hydrides,
2
2
Hydrogenation of alkenes
or put in evidence non-classical sigma bonded H
shows a minimum T1 of 4.49 s at 183 K in toluene-d
00 MHz spectrometer. At 283 K, the T1 value was 8.62 s. On effectively catalyse the C–H activation of octane. However the
the other hand, the minimum T1 value for 3 is 6.35 s at 183 K activity quickly decays overtime due to the low thermal stability
in toluene-d and a maximum of 6.75 s at 295 K (see Table S1† of the complex. Because of the activity of species 1 at ambient
2
.
Species 2
6
a
8
on a Boratabenzene complexes reported by Bazan were shown to
5
8
for T1 measurements at different temperatures). Dipole–dipole temperature and pressure with hydrogen, we were interested in
relaxation for classical metal hydrides lies in the range of looking at the possibility for 1 to act as an alkene hydrogen-
seconds, while dihydrogen complexes show a much shorter T1 ation catalyst (Scheme 2). Although several rhodium complexes
1
9,22,23
in the range of 4–100 ms.
On this basis, because 2 and 3
display long T1 dipole relaxation times, the existence of a H–H
bond can be ruled out.
Based on its spectral properties and on the structure of pre-
cursor 1, one can propose 2 to be a classical Rh(III) dihydride
2 2
species [(DTBB)Rh(H) ] (Fig. 5). Indeed, the spectral properties
of the hydride at −13.94 ppm with a JH–Rh coupling constant of Scheme 2 Catalytic hydrogenation of alkenes.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
are known to be efficient catalysts for the hydrogenation of species 1 as a precatalyst for the hydrogenation of alkenes and
2
6b
olefins, to our knowledge no boratabenzene complex has been alkynes follows the trend reported for Wilkinson’s catalyst.
1
studied for such transformation. Furthermore, very few studies It should be noted that according to H NMR species 2 is no
have been done in the homogeneous hydrogenation of alkenes longer present when catalysis starts whereas species 3 remains
2
6
at ambient temperature and pressure.
For comparison throughout catalysis. However, since some degradation pro-
purpose, the activity of Wilkinson’s catalyst ClRh(PPh
3
)
3
(4) ducts are present throughout catalysis, it is difficult to confirm
was also investigated in the same mild conditions.
which species is acting as catalyst.
Table 1 shows the activity of catalyst 1 and 4 for the hydro-
genation of alkenes and alkynes, at room temperature and
≈
2
1 atm of H . It can be observed that for most of the substrates
Experimental section
General considerations
the conversions obtained for the hydrogenation of alkenes are
similar when comparing 1 to 4. In the case of styrene and
cyclohexene the conversion yields are of 29 and 20%, respect-
ively, using 1 as a catalyst (entries 1 and 2). The conversion is
significantly higher with 1-methyl-cyclohexene and 1-octene,
with 84% and 73% yield, although the conversion is complete
when using Wilkinson’s catalyst in both entries (entries 3 and
All manipulations were carried under inert atmosphere of
nitrogen using either Schlenk line techniques or a MBraun
glovebox. THF and Et O were distilled under N atmosphere
2 2
over sodium/benzophenone; toluene, benzene, benzene-d6,
toluene-d
8
, alkenes, alkynes and alkanes were pre-treated with
Na/K alloy and vacuum transferred or distilled. DTBB was pre-
4
). A significant difference is observed for the hydrogenation
1
2a
pared according to a literature precedent.
lyst, alkenes and alkanes were purchased from Sigma-Aldrich.
RhCl(C was purchased from Strem chemicals. H gas
Wilkinson cata-
of cyclopentene, where the hydrogenation using 1 is half of
what is observed using 4 (43 vs. 90%, respectively; entry 5). In
the case of cyclooctene, both catalysts 1 and 4 give poor yields
[
2
H
4
)
2
]
2
2
UHP 5.0 was purchased from Praxair. NMR spectra were
(entry 6). The hydrogenation of alkynes to the corresponding
recorded in a sealed J-Young quartz NMR tube on an Agilent
alkanes is also possible, although the conversion is relatively
low (2 and 21% for phenyl- and diphenylacetylene, respect-
ively). It should be noted that it is possible to observe by NMR
spectroscopy that the alkenes are generated during the hydro-
genation of alkynes, suggesting that the hydrogenation is
occurring through sequential steps. Therefore, the reactivity of
1
Technologies NMR spectrometer,
H NMR (500 MHz),
1
3
31
11
C NMR (125.721 MHz), P NMR (202.404 MHz), B NMR
160.419 MHz). The coupling constants (J) should be con-
(
sidered as an average with standard deviation of ±0.2 Hz. 2D
NMR gHSQCAD, and gCOSY experiments were used to eluci-
1
3
1
1
1
date
C– H and H– H correlations. FTIR spectra were
obtained with a Nicolet Bomem Magna-850. The solid was sus-
Table 1 Hydrogenation of alkenes and alkynes using 1 and Wilkinson pended in Nujol previously degassed by bubbling N and the
2
catalyst (4)
sample was prepared inside an inert atmosphere glove box.
a
GC-FID was carried out using a Thermo Scientific trace GC
ultra coupled with an ITQ 900 mass spectrometer using elec-
tronic impact ionization (EI), standard curves of the corres-
ponding alkanes were used to calculate the concentration in
ppm, internal standards were used and yields were calculated
against the theoretical yield for each substrate. All species
exhibited decomposition and did not give satisfactory elemen-
tal analysis.
Entry
Substrate
Product
% Conversion
(
1)
2)
1: 29%
4: 27%
(
1: 20%
: 30%
4
(3)
1: 84%
: 100%
4
Synthesis of [Rh(C H )(DTPBB)] (1)
(
(
4)
5)
1: 73%
: 100%
2
4
2
4
DTBB (0.073 g, 0.28 mmol) was dissolved in 6 mL of THF. To
the yellow solution was added 4 mL of [ClRh(C H )] (0.050 g,
1: 43%
: 90%
2
4 2
4
0.13 mmol) solution in benzene. The resulting red solution
was stirred at room temperature for 2 h. The reaction mixture
was evaporated to dryness. The remaining red solid was
(6)
1: 12%
: 19%
4
washed with 2 × 8 mL of Et
=
2
O and dried under vacuum. Yield
0.066 g, 40%. H NMR (500 MHz, benzene-d ): δ 5.96 (m, 4H,
o-C H B), 5.55 (t, J = 6.5 Hz, 4H, m-C H B), 4.48 (t, J = 5.8 Hz,
1
(
(
7)
8)
1: 24%
: 26%
6
4
5
4
5
4
1: 2%
: 3%
2
(
H, p-C
5
H
4
B), 3.00 (dd, J = 11.7 and 2.2 Hz, 4H, C
dt, J = 12.0 and 2.2 Hz, 4H, C ), 1.34 (d, J = 11.6 Hz, 36H,
tBu2). C{ H} (125.72 MHz, benzene-d ): δ 101.4 (m-C H B),
2 4
H ), 2.74
4
1
2 4
H
1
3
1
(
9)
1: 21%
: 13%
6
5
4
4
99.6 (o-C
5
H
4
B), 91.8 (p-C
5
H
4
B), 36.4 (d, J = 12.8 Hz, C-tBu
2
),
3
3.2 (d, J = 3.2 Hz, CH
3
tBu
2
), 32.9 (dd, J = 13.7 and 1.6 Hz,
a
31
1
Conversion determined by GC-FID after 24 h of reaction.
C H ). P{ H} (202.40 MHz, benzene-d ): δ 23.7 (br d, J =
2 4 6
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.

View Article Online
Paper
Dalton Transactions
1
1
1
1
60 Hz, [Rh(C
2
H
4
)(DTPBB)]
2
). B{ H} (160.41 MHz, benzene- 0.3 mL of toluene. All liquids were measured with automated
pipets. In a typical hydrogenation reaction substrate, internal
d6): δ 31.
standard, 1 mol% of catalyst and 10 mL of toluene were
Reaction of [Rh(C H )(DTPBB)] (1) with H
2
2
4
2
loaded into a 18 mL Schlenk flask, equipped with a magnetic
In a sealed J-Young NMR tube (0.002 g, 2.8 μmol) were dis- stirring bar.
solved in 0.6 mL of benzene-d , the yellow solution was
6
degassed by 3 cycles of freeze–pump–thaw in a liquid nitrogen
bath and ≈1 atm of H was added by removing the liquid nitro- Conclusions
2
2
gen bath and opening the valve to H flow for 30 seconds. The
We have carried out the synthesis of species [(C H )Rh-
2
4
sample was then allowed to reach room temperature slowly.
The NMR spectrum recorded after 3 h displayed the signals for
two different hydrides along with some of the starting
material. After 3 days only species 2 was observed in solution.
Removal of the hydrogen atmosphere generates species 3. Only
(
2
DTBB)] where the DTBB moiety bridges two rhodium centre
6
by an η -coordination of the boratabenzene moiety and by the
lone pair on phosphorous. In the latter complex, the ethylene
ligand proves to be quite labile. Under a hydrogen atmosphere,
complex 2 transforms into complexes 2 and 3 having respect-
ively two and one hydride per metal centre, according to multi-
nuclear NMR spectroscopy. Interestingly, complex 1 can serve
as a precatalyst for the hydrogenation of alkenes and alkynes
at ambient temperature and pressure. Interestingly, the activity
observed is very similar to the one observed for Wilkinson’s
catalyst in similar conditions, even if formally the metal in
3
proved to be stable enough to undergo FT-IR analysis.
1
Hydride 2. H NMR (500 MHz, benzene-d
6
): δ 6.00 (t, J =
7
.5 Hz, 2H, m-C H B), 5.78 (dd, J = 8.9 and 4.1 Hz, 2H,
5
4
o-C
PtBu
5
H
4
B), 5.38 (t, J = 6.0 Hz, 1H, p-C
5 4
H B), 1.37 (d, J = 12.7 Hz,
1
31
2
), −13.94 (dd, J = 23.3 and 21.3 Hz, 2H, Rh–H). H{ P}
(
51.8 ppm) NMR (500 MHz, benzene-d ): δ 6.00 (dd, J = 9.3 and
6
5
.8 Hz, 2H, m-C
5
H
4
5 4
B), 5.78 (d, J = 8.6 Hz, 2H, o-C H B), 5.38 (t,
1
does not exhibit the same unsaturation observed for the ubi-
J = 6.0 Hz, 1H, p-C
5
H
4
B), 1.37 (s, PtBu
2
), −13.94 (d, J = 23.4 Hz,
3
1
1
quitous ClRh(PPh catalyst. It can be therefore proposed that
3 3
)
2
H, Rh–H). P{ H} (202.40 MHz, benzene-d ): δ 51 (br).
6
1
in order for catalysis to occur, the DTBB ligand needs to
present some hemilability in order to open up coordination
sites. Although the DTBB exhibits a rich and diverse coordi-
nation chemistry, its usefulness as a ligand for catalysis seems
to be impeded by the sensitivity of the P–B bond to hydrolysis
and or oxidation, which we are currently studying in more
detail.
6
Hydride 3. H NMR (500 MHz, benzene-d ): δ 6.25 (m, 2H),
6
1
1
{
.04 (m, 1H), 4.95 (d, J = 8.9 Hz, 2H), 4.80 (d, J23 = 9.7 Hz, 2H),
.46 (d, J = 12.8 Hz, 18H), 1.29 (d, J = 12.3 Hz, 20H), 0.99 (d, J =
1
4.5 Hz, 4H), −17.83 (dd, J = 21.32, 14.7 Hz, 1H, Rh–H). H
3
1
P} (54.2 ppm) NMR (500 MHz, benzene-d
.04 (dd, J = 8.4, 6.1 Hz, 2H), 4.95 (d, J = 8.5 Hz, 2H), 4.80 (d,
), 1.29 (s, 54H, PtBu ),
17.83 (d, J = 15.9 Hz, 1H, Rh–H). P{ H} (202.40 MHz,
6
): δ 6.25 (m, 4H),
6
J = 9.2 Hz, 1H) 1.46 (s, 36H, PtBu
−
benzene-d ): δ 54 (br). FT-IR (Nujol, cm ): 3663, 3308, 3196,
2
2
3
1
1
−
1
6
Acknowledgements
3
1
1
107, 3063, 3035, 2919, 2849, 2662, 2351, 2291, 2161, 2084,
979, 1753, 1679, 1591, 1567, 1453, 1375, 1240, 1116, 1084,
027, 968, 895, 869, 810, 796, 774.
The authors acknowledge the Natural Sciences and Engineer-
ing Research Council of Canada (NSERC), the Canada Foun-
dation for Innovation (CFI), the Fonds de Recherche Nature et
Technologies (FQRNT), the Centre de Catalyse et Chimie Verte
X-ray crystallography
3
A red crystal of 1 (0.26 × 0.10 × 0.08 mm ) was mounted on a
glass fibre using Paratone N hydrocarbon oil. Measurements
were made at 150(2) K on a Bruker APEX II area detector dif-
fractometer equipped with graphite monochromated MoKα
radiation. The program used for retrieving cell parameters and
(
C3V-Université Laval) for financial support.
References
2
7
data collection was APEX 2. Data were integrated using the
1 G. E. Herberich, G. Greiss and H. F. Heil, Angew. Chem., Int.
Ed. Engl., 1970, 9, 805–806.
2
8
program SAINT, and corrected for Lorentz and polarization
effects. The structure was solved and refined using
2 (a) D. A. Hoic, J.-R. Wolf, W. M. Davis and G. C. Fu, Organo-
metallics, 1996, 15, 1315–1318; (b) J. M. Schulman and
R. L. Disch, Organometallics, 1989, 8, 733–737;
(c) J. Cioslowski and P. J. Hay, J. Am. Chem. Soc., 1990, 112,
1707–1710; (d) X. Zheng and G. E. Herberich, Organometal-
lics, 2000, 19, 3751–3753; (e) M.-A. Légaré, G. Bélanger-
Chabot, G. De Robillard, A. Languérand, L. Maron and
F.-G. Fontaine, Organometallics, 2014, 33, 3596–3606.
3 For synthesis of boratabenzene species: (a) G. E. Herberich,
G. Greiss and H. F. Heil, Angew. Chem., Int. Ed. Engl., 1970,
9, 805–806; (b) A. J. Ashe and P. Shu, J. Am. Chem. Soc.,
1971, 93, 1804–1805; (c) D. A. Hoic, W. M. Davis and
2
9
SHELXS-97. All non-H atoms were refined anisotropically.
The hydrogen atoms were placed at idealized positions. Crys-
tallographic data have been deposited at CCDC (CCDC no.
1
415613). The crystallographic details are presented in
Table S1 (ESI†).
General procedure for hydrogenation reactions
Hydrogenation reactions were carried out in Schlenk flasks
over 24 h of continuous flow of H , at room temperature. Ali-
2
quots of 0.1 mL were withdrawn after 1, 6, 12 and 24 h of reac-
tion, filtered through a 1 cm silica plug and diluted with
Dalton Trans.
This journal is © The Royal Society of Chemistry 2015

View Article Online
Dalton Transactions
Paper
G. C. Fu, J. Am. Chem. Soc., 1995, 117, 8480–8481;
(b) B. B. Macha, J. Boudreau, L. Maron, T. Maris and
F.-G. Fontaine, Organometallics, 2012, 31, 6428–6437.
(
d) S. Qiao, D. A. Hoic and G. C. Fu, J. Am. Chem. Soc., 1996,
1
18, 6329–6330; (e) G. E. Herberich, B. Schmidt and 13 (a) A. Languérand, S. S. Barnes, G. Bélanger-Chabot,
U. Englert, Organometallics, 1995, 14, 471–480.
(a) G. C. Bazan, G. Rodriguez, A. J. Ashe, S. AL-Ahmad and
C. Muller, J. Am. Chem. Soc., 1996, 118, 2291–2292;
L. Maron, P. Berrouard, P. Audet and F.-G. Fontaine, Angew.
Chem., Int. Ed., 2009, 48, 6695–6698; (b) S. S. Barnes,
M.-A. Légaré, L. Maron and F.-G. Fontaine, Dalton Trans.,
2011, 40, 12439–12442; (c) V. Pérez, S. S. Barnes and
F.-G. Fontaine, Eur. J. Inorg. Chem., 2014, 5698–5702.
4
(
b) G. C. Bazan, G. Rodriguez, A. J. Ashe, S. AL-Ahmad and
J. Kampf, Organometallics, 1997, 16, 2492–2494;
(
c) R. W. Barnhart, G. C. Bazan and T. Mourey, J. Am. Chem. 14 D. A. Hoic, W. M. Davis and G. C. Fu, J. Am. Chem. Soc.,
Soc., 1998, 120, 1082–1083; (d) J. S. Rogers, S. Bu and 1996, 118, 8176–8177.
G. C. Bazan, J. Am. Chem. Soc., 2000, 122, 730–731; 15 (a) C. K. Sperry, G. C. Bazan and W. D. Cotter, J. Am. Chem.
(
2
e) J. S. Rogers, X. Bu and G. C. Bazan, Organometallics,
000, 19, 3948–3956; (f) Z. J. A. Komon, J. S. Rogers and
Soc., 1999, 121, 1513–1523; (b) A. J. Ashe, S. Al-Ahmad and
X. Fang, J. Organomet. Chem., 1999, 581, 92–97.
G. C. Bazan, Organometallics, 2002, 21, 3189–3195; 16 D. A. Hoic, M. DiMare and G. C. Fu, J. Am. Chem. Soc.,
(
5
g) X. Wang, X. Leng and Y. Chen, Dalton Trans., 2015, 44,
771–5776.
1997, 119, 7155–7156.
17 (a) G. E. Herberich, W. Boveleth, B. Hessner, W. Koch,
E. Raabe and D. Schmitz, J. Organomet. Chem., 1984, 265,
225–235; (b) G. E. Herberich, T. S. B. Baul and U. Englert,
Eur. J. Inorg. Chem., 2002, 43–48; (c) I. A. Cade and
A. F. Hill, Dalton Trans., 2011, 40, 10563–10567;
(d) G. Raabe and M. Baldofski, Aust. J. Chem., 2011, 64,
957–964.
5
6
H. von Bönneman, W. Brijoux, R. Brinkmann and
W. Meurers, Helv. Chim. Acta, 1984, 67, 1616–1624.
(a) D. H. Woodmansee, X. Bu and G. C. Bazan, Chem.
Commun., 2001, 619–620; (b) E. Li, Y. Yuan, Y. Chen and
W. Xia, ACS Catal., 2013, 3, 521–524.
7
8
(a) J. Tweddell, D. A. Hoic and G. C. Fu, J. Org. Chem., 1997,
6
2, 8286–8287; (b) G. E. Herberich, B. Ganter and M. Pons, 18 S. T. Belt, D. M. Haddleton, R. N. Perutz, B. P. H. Smith
Organometallics, 1998, 17, 1254–1256; (c) G. E. Herberich,
U. Englert, B. Ganter and M. Pons, Eur. J. Inorg. Chem.,
2
(a) B. Y. Lee, S. Wang, M. Putzer, G. P. Bartholomew, S. Bu
and G. C. Bazan, J. Am. Chem. Soc., 2000, 122, 3969–3970;
and A. J. Dixon, J. Chem. Soc., Chem. Commun., 1987, 1347–
1349.
000, 976–986.
19 S. B. Duckett, D. M. Haddleton, S. A. Jackson, R. N. Perutz,
M. Poliakoff and R. K. Upmacis, Organometallics, 1988, 7,
1526–1532.
(
b) U. Behrens, T. Meyer-Friedrichsen and J. Heck, Z. Anorg. 20 D. A. Hoic, Ph.D. thesis, Massachusetts Institute of Techno-
Allg. Chem., 2003, 629, 1421–1430; (c) C. A. Jaska, logy, 1998.
D. J. H. Emslie, M. J. D. Bosdet, W. E. Piers, T. S. Sorensen 21 F. Dornhaus, H.-W. Lerner and M. Bolte, Acta Crystallogr.,
and M. Parvez, J. Am. Chem. Soc., 2006, 128, 10885–10896; Sect. E: Struct. Rep. Online, 2005, 61, 0657–0658.
d) T. K. Wood, W. E. Piers, B. A. Keay and M. Parvez, 22 T1 measurements at different temperatures are re-
(
Angew. Chem., Int. Ed., 2009, 48, 4009–4012; (e) T. K. Wood,
W. E. Piers, B. A. Keay and M. Parvez, Chem. – Eur. J., 2010,
commended in order to find a minimum relaxation time.
(a) D. G. Hamilton and R. H. Crabtree, J. Am. Chem. Soc.,
1998, 110, 4126–4133; (b) C. Ammann, F. Isaia and
P. S. Pregosin, Magn. Reson. Chem., 1988, 26, 263–238;
(c) P. J. Desrosiers, L. Cai, Z. Lin, R. Richards and
J. Halpern, J. Am. Chem. Soc., 1991, 113, 4173–4184;
(d) P. G. Jessop and R. Morris, Coord. Chem. Rev., 1992,
121, 155–248.
1
6, 12199–12206; (f) U. Hagenau, J. Heck, E. Hendrickx,
A. Persoons, T. Schuld and H. Wong, Inorg. Chem., 1996,
5, 7863–7866; (g) D. J. H. Emslie, W. E. Piers and
M. Parvez, Angew. Chem., Int. Ed., 2003, 42, 1252–1255;
h) W. B. Tan, J. Hongmei, S.-W. Yang and G. Q. Xu, Nano-
scale, 2012, 4, 7557–7562.
3
(
9
Reviews on borabenzene coordination modes: (a) G. C. Fu, 23 (a) F. L. Taw, H. Mellows, P. S. White, F. J. Hollander,
Adv.
Organomet.
Chem.,
2001,
47,
101–119;
R. G. Bergman, M. Brookhart and D. M. Heinekey, J. Am.
Chem. Soc., 2002, 124, 5100–5108; (b) W. J. Oldham Jr.,
A. S. Hinkle and D. M. Heinekey, J. Am. Chem. Soc., 1997,
119, 11028–11036; (c) A. H. Janowicz and R. G. Bergman,
J. Am. Chem. Soc., 1983, 105, 3929–3939.
(
b) G. E. Herberich and H. Ohst, Adv. Organomet. Chem.,
1
986, 25, 199–236; (c) A. J. Ashe, S. Al-Ahmad and X. Fang,
J. Organomet. Chem., 1999, 581, 92–97; (d) C. W. Allen and
D. E. Palmer, J. Chem. Educ., 1978, 55, 497–500.
1
0 (a) M. A. Putzer, J. S. Rogers and G. C. Bazan, J. Am. Chem. 24 (a) R. A. Periana and R. G. Bergman, J. Am. Chem. Soc.,
Soc., 1999, 121, 8112–8113; (b) G. C. Bazan, W. D. Cotter,
Z. J. A. Komon, R. A. Lee and R. J. Lachicotte, J. Am. Chem.
Soc., 2000, 122, 1371–1380; (c) C. K. Sperry, G. Rodriguez
and G. C. Bazan, J. Organomet. Chem., 1997, 548, 1–7.
1986, 108, 7332–7346; (b) K. Isobe, P. M. Bailey and
P. M. Maitlis, J. Chem. Soc., Dalton Trans., 1981, 2003–2008;
(c) H. Werner and J. Wolf, Angew. Chem., Int. Ed. Engl.,
1982, 21, 296–297.
1
1
1 A. Mushtaq, W. Bi, M.-A. Légaré and F.-G. Fontaine, 25 D. M. Heinekey, J. M. Millar, T. F. Koetzle, N. G. Payne and
Organometallics, 2014, 33, 3173–3181. K. W. Zilm, J. Am. Chem. Soc., 1990, 112, 909–919.
2 (a) G. E. Herberich, U. Englert, A. Fischer, J. Ni and 26 For homogenous catalysts see: (a) J. A. Osborn,
A. Schmitz, Organometallics, 1999, 18, 5496–5501;
F. H. Jardine, J. F. Young and G. Wilkinson, Chem.
This journal is © The Royal Society of Chemistry 2015
Dalton Trans.

View Article Online
Paper
Dalton Transactions
Commun., 1965, 131–132; (b) J. A. Osborn, F. H. Jardine,
J. F. Young and G. Wilkinson, J. Chem. Soc. A, 1966, 1711–
732; (c) K. Kitamura, M. Shigeta, Y. Maezawa,
Y. Watanabe, D.-S. Hsu, Y. Ando, T. Matsumoto and
K. Susuki, J. Antibiot., 2013, 66, 131–139; (d) A. Jourdant,
E. Gonzáles-Zamora and J. Zhu, J. Org. Chem., 2002, 67,
A. Porta, G. Zanoni and G. Vidari, Chem. – Eur. J., 2015, 21,
791–799. For heterogeneous catalysts see: (k) N. K. Singha,
S. Sivaram and S. S. Talwar, Rub. Chem. Technol., 1995, 68,
281–286; (l) N. K. Singha, S. Sivaram and S. S. Talwar, Polym.
Sci., 1994, 1, 181–185; (m) K. V. Kovtunov, V. V. Zhivonitko,
I. V. Skovpin, D. A. Barskiy, O. G. Salnikov and I. B. Koptyug,
J. Phys. Chem. C, 2013, 117, 2287–22893.
1
3
163–3164; (e) S. B. Duckett, C. L. Newell and R. Eisenberg,
J. Am. Chem. Soc., 1994, 116, 10548–10556; (f) T. Iwami, 27 Bruker, APEX 2 Version 2.0.2, Bruker AXS Inc., Madison,
T. Yoshida and M. Sato, J. Chem. Soc. Jpn., 1976, 10, 1652– Wisconsin, USA, 2005.
653; (g) X. Guoi, P. J. Scott and G. L. Rempel, J. Mol. Catal., 28 Bruker, SAINT Version 7.07a, Bruker AXS Inc., Madison,
992, 72, 193–208; (h) A. Furstner and T. Nagano, J. Am. Wisconsin, USA, 2003.
1
1
Chem. Soc., 2007, 129, 1906–1907; (i) M. A. Mercadente, 29 G. M. Sheldrick, SHELXS-97 and SHELXL-97. Programs for
C. B. Kelly, C. Lee and N. E. Leadbeater, Org. Process Res.
Dev., 2012, 16, 1064–1068; (j) S. Bugoni, D. Boccato,
the refinement of crystal structures, University of Gottingen,
Germany, 1997.
Dalton Trans.
This journal is © The Royal Society of Chemistry 2015