Cyclodextrins as first and second sphere ligands for Rh(I) complexes of lower-rim PTA derivatives for use as catalysts in aqueous phase hydrogenation

Article abstract of DOI:10.1016/j.catcom.2014.11.030

The rhodium complex [Rh(cod)Cl(N-tBuBzPTA)]PF₆ (2) was obtained by reacting the appropriate Rh(I) precursor with the lower-rim PTA derivative [N-tBuBzPTA]PF₆ (tBuBz = 4-tert-butylbenzyl; PTA = 1,3,5-triaza-7-phosphaadamantane). The solubility and stability in water of 2 were increased in the presence of native-β-cyclodextrin (β-CD). The interaction of 2 with mono-amino β-cyclodextrin (β-CDNH₂, 2 equiv.) led to a supramolecular Rh assembly (3), identified by ³¹P, ¹H and 2D T-ROESY NMR experiments. The catalytic activity of 3 was evaluated in the water-phase hydrogenation of unsaturated and allylic alcohols and preliminary results are presented here.

Full text of DOI:10.1016/j.catcom.2014.11.030

CATCOM-04152; No of Pages 5
Catalysis Communications xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Communications
journal homepage: www.elsevier.com/locate/catcom
Cyclodextrins as first and second sphere ligands for Rh(I) complexes of
lower-rim PTA derivatives for use as catalysts in aqueous
phase hydrogenation
Jonathan Potier a, Antonella Guerriero b, Stéphane Menuel a, Eric Monflier a, Maurizio Peruzzini b,
b,
Frédéric Hapiot a, , Luca Gonsalvi
⁎
⁎
a
Université Lille Nord de France, CNRS UMR 8181, Unité de Catalyse et de Chimie du Solide – UCCS UArtois, Faculté Jean Perrin, rue Jean Souvraz, SP18, F-62307 Lens Cedex, France
Consiglio Nazionale delle Ricerche, Istituto di Chimica dei Composti Organometallici, ICCOM-CNR, Via Madonna del Piano 10, 50019 Sesto Fiorentino, Firenze, Italy
b
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 31 July 2014
Received in revised form 1 November 2014
Accepted 23 November 2014
Available online xxxx
The rhodium complex [Rh(cod)Cl(N-tBuBzPTA)]PF6 (2) was obtained by reacting the appropriate
Rh(I) precursor with the lower-rim PTA derivative [N-tBuBzPTA]PF6 (tBuBz = 4-tert-butylbenzyl; PTA =
1,3,5-triaza-7-phosphaadamantane). The solubility and stability in water of 2 were increased in the presence
of native-β-cyclodextrin (β-CD). The interaction of 2 with mono-amino β-cyclodextrin (β-CDNH2, 2 equiv.)
led to a supramolecular Rh assembly (3), identified by 31P, 1H and 2D T-ROESY NMR experiments. The catalytic
activity of 3 was evaluated in the water-phase hydrogenation of unsaturated and allylic alcohols and preliminary
results are presented here.
Keywords:
Cyclodextrins
Hydrosoluble phosphines
Aqueous catalysis
Rhodium
© 2014 Elsevier B.V. All rights reserved.
Hydrogenation
1. Introduction
nitrogen and the phosphorus atoms on the same side of the supramo-
lecular edifice.
Self-assembly has recently emerged as a very effective tool to access
new organometallic catalysts by means of host–guest interactions. Reek
et al. [1] exploited the coordination ability of a nitrogen donor group on
Zn(II) porphyrins to build a library of supramolecular phosphorus-
containing bidentate ligands. Breit and Seiche [2] focused on self-
assembly of complementary species by hydrogen bonding in aprotic
solvents. Both strategies led to very efficient catalysts whose perfor-
mances have been demonstrated in numerous reactions [3–9]. Other
supramolecular strategies have also been exploited, usually in organic
phase, via metal-directed self-assembly [10–12], acid–base interactions
[13,14], or via a pseudorotaxane molecule [15]. We recently demon-
strated that supramolecular ligands are also accessible in aqueous
media via the hydrophobic interaction between the cavity of nitrogen-
containing β-cyclodextrins (CDs) and an appropriate water soluble
phosphine [16–18]. Inclusion of the phosphine within the CD cavity by
the NH2-containing face resulted in the formation of a rigid chelating
bidentate ligand (for example to Pt and Rh precursors) with the
We extended the scope of the supramolecular approach in aqueous
media following on our interest on the application of water soluble li-
gands derived from tailored N-quaternizations of the cage-like
aminophosphine PTA (PTA = 1,3,5-triaza-7-phosphaadamantane) and
their interactions with CDs [19,20]. Hereby we report on the stabiliza-
tion effects of mono-amino β-CD (β-CDNH2, Fig. 1) on a Rh complex
of [1-(4-tert-butyl)-benzyl-1-azonia-3,5-diaza-7-phosphaadamantyl]
hexafluorophosphate (1). The corresponding supramolecular P,N,N
heterotridentate-stabilized complex was tested in the proof-of-
concept hydrogenation of unsaturated and allylic alcohols in water
and the preliminary results are here presented.
2. Experimental section
2.1. General methods
All synthetic manipulations were carried out under a purified at-
mosphere of nitrogen using standard Schlenk techniques unless
otherwise noted. [N-tBuBzPTA]Br [20] and [RhCl(cod)]2 (cod =
1,5-cyclooctadiene) [21] were prepared as described in the litera-
ture. The details of characterizations are reported in the Supporting
information section.
⁎
Corresponding authors.
E-mail addresses: frederic.hapiot@univ-artois.fr (F. Hapiot), l.gonsalvi@iccom.cnr.it
(L. Gonsalvi).
http://dx.doi.org/10.1016/j.catcom.2014.11.030
1566-7367/© 2014 Elsevier B.V. All rights reserved.
Please cite this article as: J. Potier, et al., Cyclodextrins as first and second sphere ligands for Rh(I) complexes of lower-rim PTA derivatives for use
as catalysts in aqueous..., Catal. Commun. (2014), http://dx.doi.org/10.1016/j.catcom.2014.11.030

2
J. Potier et al. / Catalysis Communications xxx (2014) xxx–xxx
(45 mL) was added and the reaction was left stirring for 1 h 30 min at
room temperature. The mixture was then filtered and the resulting yel-
low solution concentrated to about 10 mL volume. Cold dried pentane
(25 mL) was added and the resulting yellow precipitate was collected
on a G3 sintered-glass filter, washed with cold pentane (2 × 4 mL)
and dried under vacuum ((yield: 0.46 g, 61%), C25H39N3ClP2F6Rh
(695.89 g mol−1).
2.4. Formation of supramolecular complexes [Rh(cod)Cl(N-tBuBzPTA)]
PF6·(β-CDNH2)2 (3)
A Schlenk tube was charged with 2 (5.0 mg, 7.0 × 10−3 mmol) and
β-CDNH2 (16 mg, 1.4 × 10−2 mmol). Degassed deuterium oxide (2 mL)
was added and the reaction was left stirring for 1 h at room tempera-
ture. A clear yellow solution was obtained.
Fig. 1. β-CDNH2 and the lower rim PTA derivative [N-tBuBzPTA]PF6 (1).
2.2. Synthesis of 1-(4-tert-butyl)-benzyl-1-azonia-3,5-diaza-7-
phosphaadamantyl hexafluorophosphate (1)
2.5. Catalytic hydrogenation tests
Typically, the unsaturated alcohol (5.5 mmol) and 1-butanol
(1.4 mmol, GC internal standard) were added in a steel autoclave
to an aqueous solution (10 mL) containing the rhodium complex 2
(5.5 μmol) and CD (11.0 μmol). The reactor was then pressurized
with H2, and heated to 30 °C. The catalytic run was monitored during
time by withdrawing aliquots of the reaction mixture which were
analysed by GC. For 2-methyl-3-buten-2-ol, recycling tests were car-
ried out by recovering the phase containing the catalyst and repeat-
ing the hydrogenation tests in the presence of fresh substrate.
For the synthesis of ligand 1, 1-(4-tert-butyl)-benzyl-1-azonia-3,5-
diaza-7-phosphaadamantyl bromide (0.50 g, 1.3 mmol) was placed in
a Schlenk tube and dissolved in dried and degassed dichloromethane
(20 mL). In another Schlenk tube, ammonium hexafluorophosphate
(0.23 g, 1.43 mmol) was dissolved in dried THF (6 mL) and the solution
obtained was added dropwise to the solution of the ligand, causing the
precipitation of a white solid. The reaction mixture was left stirring at
room temperature for 1 h. The precipitate was then separated by filtra-
tion and the clear colourless solution dried under vacuum to obtain a
white powder ((yield: 0.40 g, 68%), C17H27N3P2F6 (449.35 g mol−1).
3. Results and discussion
2.3. Synthesis of complex [Rh(cod)Cl(N-tBuBzPTA)]PF6 (2)
3.1. Synthesis of rhodium complexes
A Schlenk tube was charged with [RhCl(cod)]2 (0.27 g, 0.55 mmol)
Rh–PTA complexes have received attention due to their catalytic ac-
and 1 (0.49 g, 1.09 mmol). Dried and degassed dichloromethane
tivity both in hydrogenation and hydroformylation reactions [22,23].
Scheme 1. Syntheses of ligand 1 and complex 2.
Fig. 2. Diagnostic regions of 31P{1H} NMR spectrum of a 2:1 mixture of β-CDNH2 and 2 in D2O at 25 °C after 1 h.
Please cite this article as: J. Potier, et al., Cyclodextrins as first and second sphere ligands for Rh(I) complexes of lower-rim PTA derivatives for use
as catalysts in aqueous..., Catal. Commun. (2014), http://dx.doi.org/10.1016/j.catcom.2014.11.030

J. Potier et al. / Catalysis Communications xxx (2014) xxx–xxx
3
Scheme 2. Proposed supramolecular species formed between Rh-complex 2 and β-CDNH2.
The novel PTA derivative [N-tBuBzPTA]PF6 (1) and its corresponding
Rh(I) complex, namely [Rh(cod)Cl(N-tBuBzPTA)]PF6 (2) (Scheme 1)
were synthesized and characterized in solution by NMR and ESI–MS
techniques (see SI). Anion exchange from known [N-tBuBzPTA]Br [20]
was easily accomplished by reaction with NH4PF6 in a CH2Cl2/THF mix-
ture at room temperature, in order to avoid the presence of bromide
that is a competing ligand to Rh.
160 Hz) and a heptet centred at −143.8 ppm (1JPF = 716 Hz) due
to PF6 anion. The ESI–MS spectrum of 2 exhibited the peaks at
304.25 m/z corresponding to [N-tBuBzPTA]+ and at 552.08 due to
[Rh(cod)Cl(N-tBuBzPTA)]+ + 2.
Complex 2 showed low solubility in water at room temperature
(b5 mg/mL). By heating to 80 °C for 30 min, it was possible to dis-
solve completely a 5 mg sample of 2 in 5 mL of D2O. However, exten-
sive decomposition to phosphine oxide was evidenced by 31P{1H}
NMR [24]. A different approach was needed to obtain stable aqueous
solutions of 2 at 25 °C. Thus, we studied the effect of added CDs and
Complex 2 was obtained as shown in Scheme 1 and characterized
by elemental analysis, ESI–MS and NMR in solution. The 31P{1H}
NMR spectrum in CDCl3 showed a doublet at −33.1 ppm (1JRhP
=
Please cite this article as: J. Potier, et al., Cyclodextrins as first and second sphere ligands for Rh(I) complexes of lower-rim PTA derivatives for use
as catalysts in aqueous..., Catal. Commun. (2014), http://dx.doi.org/10.1016/j.catcom.2014.11.030

4
J. Potier et al. / Catalysis Communications xxx (2014) xxx–xxx
Scheme 3. Possible dipolar interactions resulting from inclusion of 1 within the β-CDNH2
cavity from the primary (left) or the secondary (right) face.
of 2 was found unchanged in solution either after 24 h at room temper-
ature (ca. 60%) or after 5 h heating to 80 °C (ca. 42%), respectively.
A different behaviour was observed upon addition of β-CDNH2 (2
equiv.) to a solution of 2, as witnessed by the corresponding 31P{1H}
NMR spectrum. The initial doublet centred at δ −33.1 ppm (1JRhP
=
160 Hz) corresponding to 2 was replaced by 6 doublets falling in the
1
range between −28 and −37 ppm, with JRhP in the range 129–
Fig. 3. Partial 2D T-ROESY NMR spectrum (tBu protons) of a 2:1 mixture of β-CDNH2 and 2
at room temperature in D2O.
158 Hz (Fig. 2). Attempts to separate the different components of the
mixture or grow crystals suitable for X-ray diffraction data collection
failed to date.
as expected, both the solubility and the stability in water of the com-
plex were increased by the addition of either native β-CD or β-
CDNH2.
Different Rh-phosphine species can be hypothesized to form in solu-
tion, including chloride complexes (higher 1JRhP) and aquo complexes
(lower 1JRhP), keeping a coordinated Rh(η4-cod) moiety, with mixed
first and second sphere coordination giving P,N and P,N,N complexes,
derived from various substitution degrees on the metal centre, as
shown in Scheme 2.
3.2. In-situ formation of supramolecular complexes
In order to enhance water solubility, inclusion of 2 in cyclodextrins can
be expected upon recognition of the tBu-benzyl group, which is known to
be well recognized by the β-CD cavity [25,26]. At first, addition of 1 equiv.
of native β-CD (8.0 mg, 7.0 × 10−3 mmol) to a suspension of 2 (5 mg, 7.0 ×
10−3 mmol) in D2O (2 mL) was tested, showing however that this ratio
was clearly insufficient to solubilise 2 completely at 25 °C. Conversely,
when 2 equiv. of native β-CD (16 mg, 1.4 × 10−2 mmol) were used
under the same conditions, a clear yellow solution was obtained. We
reasoned that under these conditions both the hydrophobic character
of cod ligand and tBu-benzyl group might be masked into the hydro-
phobic CD cavity. Interestingly, also the stability in water of complex 2
was increased. 31P{1H} NMR spectra showed that a significant amount
To confirm that the tBu-phenyl group was to a large extent included
into the β-CDNH2 cavity, 2D T-ROESY NMR experiments have been car-
ried out. This particular NMR sequence, commonly used to identify the
recognition process between the CD “host” and the phosphine ligand
“guest”, evidenced the existence of dipolar contacts between the pro-
tons of the tBu-phenyl group of 2 and the β-CDNH2 inner protons H-3
and H-5, hence the expected inclusion within the β-CD cavity (Figs. 3
and 4). Moreover, weak correlations were also observed between the
aromatic protons and the H-6 and H-5 CD protons (Fig. 4), indicative
of a weak penetration of the ligand into the CD cavity. Accordingly, in-
clusion of 2 took place through the β-CDNH2 primary face confirming
that the nitrogen and the phosphorus atoms were located on the same
side of the CD. As no cross-peaks were observed between the aromatic
protons and H-3, inclusion through the CD secondary face can be ex-
cluded (Scheme 3).
3.3. Catalytic unsaturated and allylic alcohol hydrogenation tests
The possible use of the supramolecular β-CD/rhodium complex as-
semblies as homogeneous aqueous-phase catalysts was at first evaluat-
ed in the proof-of-concept hydrogenation of a water soluble substrate,
namely 2-methyl-3-buten-2-ol (Scheme 4). The choice of this model
substrate was motivated not only by its high solubility in water, but
also because no C_C isomerization can take place, thus limiting the
number of products to analyse. The effect of the CD nature on the activ-
ity and stability of the catalytic system was assessed, comparing the
combination of 2/native β-CD and 2/β-CDNH2 under 10 and 20 bar H2
Fig. 4. Partial 2D T-ROESY NMR spectrum (aromatic protons) of a 2:1 mixture of β-CDNH2
and 2 at room temperature in D2O.
Scheme 4. Rh-catalysed hydrogenation of 2-methyl-3-buten-2-ol.
Please cite this article as: J. Potier, et al., Cyclodextrins as first and second sphere ligands for Rh(I) complexes of lower-rim PTA derivatives for use
as catalysts in aqueous..., Catal. Commun. (2014), http://dx.doi.org/10.1016/j.catcom.2014.11.030

J. Potier et al. / Catalysis Communications xxx (2014) xxx–xxx
5
Table 1
Rh-catalysed hydrogenation of unsaturated alcohols in the presence of a 2:1 mixture of β-
CDNH2 and 2.
Run Substrate
Product
Conversion Selectivity
(%)
(%)
1
2-Methyl-3-buten-2-ol
2-Methylbutan-2-ol
2-Methylbutan-2-ol
2-Methylbutan-2-ol
Propan-1-ol
100
99
99
100
93
100
100
100
100
100
100
2a
3b
4
2-Methyl-3-buten-2-ol
2-Methyl-3-buten-2-ol
Propargylic alcohol
3-Methyl-2-buten-1-ol
2-Methyl-2-propen-1-ol 2-Methylpropan-1-ol 100
5
6
3-Methylbutan-1-ol
Conditions: 2, 5.5 × 10−3 mmol; β-CDNH2, 11.0 × 10−3 mmol; substrate, 5.5 mmol;
H2O, 10 mL; pH2, 20 bar; 30 °C; 80 min.
a
Performed with the recovered aqueous phase of run 1.
Performed with the recovered aqueous phase of run 2.
b
Fig. 5. Hydrogenation of 2-methyl-3-buten-2-ol in the presence of i) native β-CD/2 (2:1),
pH2, 10 bar (\■\), ii) β-CDNH2/2 (2:1), pH2, 10 bar (–●–) and iii) β-CDNH2/2 (2:1), pH2,
20 bar (\▲\), respectively. Reaction conditions: 2, 5.5 × 10-3 mmol; CDs, 11.0 × 10-3
mmol; substrate, 5.5 mmol; H2O, 10 mL; 30 °C.
Project GDRE “Catalyse Homogène pour le Développement Durable
(CH2D)” is also thanked for the support. Roquette Frères (Lestrem,
France) is gratefully acknowledged for generous gifts of cyclodextrins.
pressure. Homogeneous solutions were invariably observed without
any metal particle deposition, and could be recovered at the end of
each test. However, the formation of catalytically active Rh nanoparti-
cles stabilized by β-CDNH2 could not be ruled out.
Appendix A. Supplementary data
Supplementary data to this article can be found online at http://dx.
doi.org/10.1016/j.catcom.2014.11.030.
In the presence of native β-CD and 2 (1:1 or 2:1 ratios) under 10 bar
H2, no reduction of allylic alcohol could be observed (Fig. 5). The situa-
tion was very different when the catalytic tests were carried out in the
presence of 2/β-CDNH2. Under 10 bar H2, the reaction was completed
within 90 min. Increasing the H2 pressure from 10 to 20 bar allowed de-
creasing the reaction time to 80 min. The catalytic system could be effi-
ciently recycled without any loss either in catalytic activity or selectivity
(Table 1, runs 1–3). Other unsaturated alcohols were also hydrogenated
in very good yields within 80 min (Table 1, runs 4–6). Note that, what-
ever the substrate, no isomerization of the C_C double bond took place
during the course of the reaction.
In order to prove further that the presence of the amino group on the
CD was of crucial importance to stabilize the catalyst in water, a mixture
of native β-CD and decylamine, which is also recognized and included in
the CD cavity, was also tested as a blank run. Rhodium aggregation took
place suggesting that the inclusion of 2 in the CD cavity alone is not
enough to guarantee adequate catalyst stability under these conditions.
Thus, the formation of a supramolecular complex by the inclusion of
lower-rim modified PTA into the amino-modified CD, endowed with
two donor atoms (P from the phosphine and N from the CD) capable
of efficiently binding to Rh(I) seems to offer an adequate approach
and provide a more stable and efficient catalytic system for water
phase C_C bond hydrogenation under mild conditions. Work is in
progress to extend the scope of the catalytic systems to more challeng-
ing substrates.
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In this work, we reported the synthesis of supramolecular rhodium
complexes bearing a lower rim PTA derivative as phosphine ligand
and β-CDNH2 as first and second sphere ligands. These Rh-complexes
showed stability in water phase, allowing their use as precursors in
the catalytic hydrogenation of a small library of unsaturated alcohols
under mild conditions of temperature and H2 pressure in neat water.
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This work was financially supported by CNR and MIUR through pro-
jects EFOR and PREMIALE 2011 which are gratefully acknowledged.
Please cite this article as: J. Potier, et al., Cyclodextrins as first and second sphere ligands for Rh(I) complexes of lower-rim PTA derivatives for use
as catalysts in aqueous..., Catal. Commun. (2014), http://dx.doi.org/10.1016/j.catcom.2014.11.030