Selective catalytic deuteration of phosphorus ligands using ruthenium nanoparticles: a new approach to gain information on ligand coordination

Article abstract of DOI:10.1039/c5cc06984j

Phenyl rings in phenyl- or phenyl-alkylphosphines are selectively deuterated at the ortho position using Ru/PVP nanoparticles, while are fully reduced in the case of arylphosphine oxide derivatives and do not react in the case of arylphosphite. This different behavior provides information about the coordination mode of each ligand.

Full text of DOI:10.1039/c5cc06984j

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Selective Catalytic Deuteration of Phosphorus Ligands using
Ruthenium Nanoparticles. A New Approach to Gain Information
on Ligand Coordination
Received 00th January 20xx,
Accepted 00th January 20xx
Emma BresóꢀFemenia,a,b Cyril Godard,c Carmen Claver,c Bruno Chaudretb* and Sergio Castillón.a*
DOI: 10.1039/x0xx00000x
www.rsc.org/
Phenyl rings in phenyl- or phenyl-alkylphosphines are selectively (magic angle spinning) NMR spectroscopy,7 and reactivity
deuterated at the ortho position using Ru/PVP nanoparticles, studies with CO.8 All these studies contributed to better
while are fully reduced in the case of arylphosphine oxide understand the influence of the ligands on the site selective
derivatives and do no react in the case of arylphosphite. This hydrogenations. Other ligands such as secondary phosphine
different behavior provides information about the coordination oxides9 and 4ꢀ(3ꢀphenylpropyl)pyridine10 were also studied
mode of each ligand.
using these techniques.
Phosphines are ubiquitous ligands and reagents in organic and
organometallic chemistry and the availability of labelled
phosphines for mechanistic studies is of high interest. Several
methods based on homogeneous or heterogeneous catalysis for
H/D exchange have been reported.11 There is only one example
of deuteration using Ru nanoparticles stabilized by
polyvinylpyrrolidone (PVP) and D2,12 applied to the selective
deuteration of nitrogen containing compounds such as
quinolines, indoles and amines by CꢀH activation. Ru/PVP
nanoparticles are considered as “naked” particles that permit
the coordination of the substrates to their surface. Using these
NPs the exclusive deuteration of positions next to the nitrogen
atoms were achieved, even in the presence of other
electronegative elements such as oxygen atoms. Deuteration
therefore appears as a powerful technique to get insights into
the interaction between nanoparticles and molecules.
In the last decade, the application of metal nanoparticles in
catalysis has developed due to their unique electronic
properties, the mild reaction conditions required, and the high
selectivity obtained.1 Such nanocatalysts can be stabilized by
polymers, surfactants or ligands, which allow the control of
their size, shape and dispersion.2 Furthermore, the choice of the
appropriate stabilizer for the MꢀNPs is crucial since it usually
has an important effect on NPs catalytic performances. For
instance, phosphorus ligands have been employed as stabilizers
of transition metal nanoparticles due to their excellent
coordination properties.3 Ru NPs stabilized by phosphorus
ligands were shown to be excellent catalysts for various
catalytic processes such as arene reduction,4 and in particular,
Ru NPs stabilized by triphenylphosphine were outstanding
catalysts for selective hydrogenation reactions.5
The interaction of stabilizing ligands such as phosphines,
carbenes and aromatic compounds with the nanoparticle surface
has been an object of interest in the last years, with the aim of
understanding how and where it takes place and its possible
influence on NPs catalytic performance. Different techniques
have been used for this purpose such as 1H and 13C MAS6
Fig. 1 Different phosphorus derivatives employed in deuteration
reaction using Ru/NPs.
In this context, we envisioned that phosphorus ligands,
previously reported to coordinate at the surface of
nanoparticles, could be labelled using these nanoparticles as
catalysts. Herein, we report a study on the use of Ru/PVP
nanoparticles and D2 for the deuteration of various phosphorus
containing compounds, namely substituted phosphines,
triphenylphosphine oxide and triphenylphosphite (Figure 1).
riphenylphosphine was initially used as a model substrate. The
reaction was performed at 2 bar of D2 and 55°C12 (Scheme 1)
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and monitored over time. Figures 2 and 3 show the 31P{1H} and
13C{1H} NMR spectra, respectively, at different reaction times
(iiꢀv) compared with that of free triphenylphosphine (i). During
the reaction, the deuteration products 2a-2f were detected.
Fig. 2 Evolution of deuteration of PPh3 (
1
) over time by 31P-NMR
.
Scheme 1. Products detected during the deuteration of PPh3
catalysed by Ru NPs.
Thus, when the reaction was monitored by 31P NMR, after 16h,
6 new resonances located between ꢀ5.5 ppm and ꢀ6.2 ppm
appeared together with the initial signal of triphenylphosphine
at ꢀ5.55 ppm (Figure 2i, ii). At longer reaction times, the
intensity of the signals at higher fields progressively increased
(Figure 2, iii, iv). These 7 signals were shown to arise from
PPh3 and compounds 2a-f. When the reaction was carried out
for 48h at 80ºC, the signal at higher field (ꢀ6.2 ppm)
corresponding to 2f was almost exclusively observed, together
with a small resonance at ꢀ6.1 ppm corresponding to compound
2e (Figure 2v). It was therefore concluded that each added
deuterium produced an isotopic shift of 0.11 ppm at higher
fields and a broadening of the 31P signal by 1 Hz, attributed to
small PꢀD couplings. Mass spectrometry experiments
confirmed the identity of these compounds. No signals of
reduced aromatic rings were observed under these conditions.
Information about the evolution of the reaction can be also
obtained by 13C{1H} NMR. Thus, after 16h, the signal
corresponding to C2ꢀD was detected at 134.4 ppm (td, 2JC,P= 18
Fig. 3 Evolution of deuteration of PPh3 (
1
) over time by 13C NMR
´
At this point, deuteration of various paraꢀsubstituted
triphenylphosphines was explored in order to investigate the
effect of electron donating and electron withdrawing
substituents (Scheme 2).
1
Hz, JC,D=26 Hz) while C2ꢀH remained unchanged as a doublet
at 134.8 ppm (JC,P= 19 Hz) (Figure 3, 3ii). This observation was
in agreement with the partial deuteration of the ortho position
of triphenylphosphine. The ipso carbon (137.3 ppm) was
detected as a multiplet, as a result of the overlap of several
doublets from the partially deuterated species.
When the reaction was carried out for 36h and 48h, or at 80ºC
the 13C NMR spectrum of the reaction product (Figure 3iiiꢀv)
clearly displayed an increase in intensity of the C2ꢀD signal and
the decrease of that of C2ꢀH, as well as the evolution of C1 to a
single doublet, due to the progressive and selective deuteration
of ortho positions. Deuteration was >95%, measured by 31P
NMR. Only penta and hexadeuteraded phosphine were present
in a ratio 0.08:1.
Scheme 2. Deuteration of para-substituted PPh3 using Ru NPs.
In all cases, the selective deuteration of ortho positions of the
aromatic rings of these ligands was observed. However,
relevant differences in the rate of deuteration of these
compounds were observed when compared with that of PPh3.
Indeed, the deuteration of phosphines
heating of 80ºC for 88h for achieving levels of deuteration
similar to PPh3, while that of phosphine was even slower and
3, 4 and 6 required
5
only 72% of labelling was achieved in similar reaction times.
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Mono (11) and bidentate (13) phosphine ligands containing These results were surprising since previous studies determined
aromatic and aliphatic substituents were also treated under that in the presence of nanoparticles as catalysts, aromatic
similar reaction conditions. In these cases, only the selective substrates with electronꢀdonating groups are hydrogenated
deuteration of ortho positions of aromatic rings was observed to faster than compounds with electronꢀwithdrawing groups,14
give 12 and 14, respectively, whereas the methyl and methylene although it should be mentioned that in some cases, no clear
groups remained unaltered (Scheme 3.3).
effects could be observed.5 Here, despite that aryl rings of
O=PPh3 can be considered as electron poorer than those of the
corresponding PPh3, significant reduction of the aromatic rings
was observed for the former while only H/D exchange was
detected for the phosphine substrate. This fact was thus
DD
P
Ru/PVP NPs
P
D
D
D2
Me
Me
11
12
explained
considering
that
the
coordination
of
triphenylphosphine oxide to the nanoparticles involves πꢀ
interactions of the phenyl rings and, consequently, that
reduction is favoured.
D
D
D
D
P
P
Ru/PVP NPs
D2
P
P
D
D
D
D
Finally, the deuteration of triphenylphosphite was attempted
under the same reaction conditions but, surprisingly, no
reaction was observed. Even when the reaction was performed
at 80°C for 48 h, no deuteration was detected, and the starting
material was recovered unaltered.15
14
13
Scheme 3. Deuteration of alkyl-containing phosphines 11, 13
using Ru NPs.
It was therefore concluded that under these conditions, the
Ru/PVP catalysts selectively deuterate the aromatic ortho
positions and does not react with these aliphatic groups, in
contrast to what has been observed with nitrogen containing
compounds.12
Scheme 4. Deuteration of P(OPh)3 using Ru-NPs.
We tested next deuteration of triphenylphosphine oxide (15),
which was initially carried out under 2 bar of D2 at 55ºC for 36
h. On the 31P{1H} NMR spectrum, the detection of signals at
45.6 and 51.4 ppm, corresponding to the dicyclohexylꢀ
Since phosphite ligands were previously reported to efficiently
stabilise metal nanoparticles, it was concluded that the absence
of deuteration or reduction of P(OPh)3 resulted from the
disposition of the phenyl groups of this ligand, which are
probably pointing away from the surface of the Ru/PVP
catalyst.
Although the reactivity of nanoparticles takes places in specific
sites of the surface, the interest for the study of
substrate/surface interactions has been limited. So far
nanoparticles stabilized by ligands presenting very similar
structures were shown to exhibit very distinct reactivities. Here
we demonstrate that selective deuteration provides relevant
information for the understanding of the surface chemistry of
nanoparticles. The three types of structurally related ligands
studied display very different reactivities which can be
understood in terms of their coordination mode to the
nanoparticle surface.
phenylphosphine oxide13
(17) and tricyclohexyl phosphine
oxide (18), respectively (Scheme 4), indicated that reduction of
the substrate was taking place under these conditions (See
Supplementary Information, Figure 43). The 13C{1H} NMR
spectrum was much more complex than that of PPh3 and an
important number of signals were observed in the aliphatic
zone, confirming the reduction of the phenyl rings. In the
corresponding 2D NMR spectrum, the presence of several broad
signals in the aliphatic region, and the practical absence in the
aromatic region, was also observed (See Supplementary
Information, Figures 44, 46). But during the hydrogenation
phase which must occur by arene coordination some additional
incorporation of deuterium can result from H/D exchange.
Thus, the phosphines
1, 3-6 and the diphosphine 13 were
selectively deuterated at the ortho positions of their aryl rings
(but not in the alkyl groups), the phosphine oxide 15 gave
reduction products and triphenylphosphite
(19) remained
unaltered under the same reaction conditions. These results
suggest that phosphines coordinate to the nanoparticle surface
through the phosphorus atom thus placing the ortho protons
very close to the surface, which favours an H/D exchange
(Figure 4). Similarly to what was proposed for nitrogen
substrates, five memberedꢀring intermediates RuꢀPꢀCꢀCꢀRu’
Scheme 4. Deuteration of OPPh3 (15) using Ru NPs
.
In order to avoid reduction of the substrate, the reaction time formed by oxidative addition are expected to produce this
and the temperature were decreased. However, the reduced exchange.16
compounds 16 and 17 and/or starting material were again In contrast, the fact that for phosphine oxides, the phenyl rings are
observed (See Supplementary information, Figures 45, 46).
reduced can be explained considering that coordination to the
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nanoparticle takes place through the aromatic rings. The
coordination through oxygen is unlikely, so the preferred
coordination through the arenes occurs and leads to the reduction of
the aromatic rings.
Notes and references
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Fig. 4. Proposed coordination mode of the ligands studied to the
6
Ru nanoparticle surface.
2
The results of isotopic labelling described above are compatible
with the observed reduction of phosphines in catalytic
processes under hydrogenation conditions, since in the presence
of substrates and under drastic reaction conditions the mobility
of the phosphines must increase allowing the coordination
through faces and consequently the hydrogenation of the
aromatic ring. However, it is remarkable that even at 80ºC for
88h no reduction of PPh3 is observed under the 2 bar of D2
pressure.
Important differences were observed between phosphine, phosphine
oxide and phosphite. In the case of phosphine, the selective
deuteration of C2 carbons indicates that the ligand is coordinated
through the phosphorus atom, similarly to what has been previously
observed for nitrogen donors and that the CꢀH bond can approach
the NPs surface. This interaction allows, however, an exchange with
additional ligands present in the solution so that deuteration of a
macroscopic sample is possible.
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In conclusion, the H/D exchange method described here allows
the selective deuteration of phenyl rings in phenylꢀ or phenyl
alkylphosphines, including diphosphines, using Ru/PVP
nanoparticles and D2, and enables the comprehension of how
different phosphorus ligands coordinate to the nanoparticle
surface.
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conditions using Rh NPs stabilized by phosphite ligands. See ref 3c.
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We are grateful to the Spanish Ministerio de Economía
y
Competitividad (CTQꢀ2011ꢀ22872, CTQ2013ꢀ43438ꢀR, Ramón y
Cajal to CG) for financial support. E. B thanks Ministerio de
Educación Cultura y Deporte for a grant. We also thank the Serveis
de Recursos Científics (URV) for their support. BC also
acknowledges financial support from CNRS and the European Union
for ERC Advanced Grant (NANOSONWINGS 2009ꢀ246763).
Chaudret, ChemPhysChem 2005, 6, 605ꢀ607. b) C. Taglang L. M.
,
MartínezꢀPrieto, I. del Rosal, L. Maron, R. Poteau, K. Philippot, B.
Chaudret, S. Perato, A. Sam Lone, C. Puente, C. Dugave, B. Rousseau,
G. Pieters Angew. Chem. Int. Ed. 2015, 54, 10474–10477.
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Selective Catalytic Deuteration of Phosphorus
Ligands using Ruthenium Nanoparticles. A New
Approach to Gain Information on Ligand
Coordination
Emma Bresó, Cyril Godard, Carmen Claver, Bruno Chaudret*
and Sergio Castillón*
Selective deuteration of phenyl rings phenyl-alkyl
phosphines, including diphosphines, was achieved
using Ru/PVP nanoparticles and D2, and enables the
comprehension of how different phosphorus ligands
coordinate to the nanoparticle surface.
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