XBphos-Rh: A halogen-bond assembled supramolecular catalyst

Article abstract of DOI:10.1039/c8sc00233a

The use of halogen bonding as a tool to construct a catalyst backbone is reported. Specifically, pyridyl- and iodotetrafluoroaryl-substituted phosphines were assembled in the presence of a rhodium(i) precursor to form the corresponding halogen-bonded complex XBphos-Rh. The presence of fluorine substituents at the iodo-containing supramolecular motif was not necessary for halogen bonding to occur due to the template effect exerted by the rhodium center during formation of the halogen-bonded complex. The halogen-bonded supramolecular complexes were successfully tested in the catalytic hydroboration of terminal alkynes.

Full text of DOI:10.1039/c8sc00233a

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XBphos-Rh: A Halogen-Bond Assembled Supramolecular Catalyst†
Lucas Carreras,a Marta Serrano-Torné,a Piet W. N. M. van Leeuwen,b* and Anton Vidal-Ferrana,c
*
Received 00th January 20xx,
Accepted 00th January 20xx
The use of halogen bonding as a tool to construct the catalyst backbone is reported. Specifically, pyridyl- and
iodotetrafluoroaryl-substituted phosphines were assembled in the presence of a rhodium(I) precursor to form the
corresponding halogen-bonded complex XBphos-Rh. The presence of fluorine substituents at the iodo-containing
supramolecular motif was not necessary for halogen bonding to occur due to the template effect exerted by the rhodium
center during formation of the halogen-bonded complex. The halogen-bonded supramolecular complexes were
successfully tested in the catalytic hydroboration of terminal alkynes.
DOI: 10.1039/x0xx00000x
www.rsc.org/
respect to the halogen bonding motifs, we envisaged that the
use of a fluorinated iodoarene and a pyridine group could be
Introduction
The evolution of metal-based homogeneous catalysis has run
in parallel with the development of structurally diverse
ligands.1 Ligand design allows the behaviour of the metal
centre to be influenced and its catalytic activity and selectivity
to be modified.
suitable due to the reported complementarity of both
supramolecular motifs.8 Concerning the metal centre, we
focused on rhodium as its complexes are catalytically active in
pivotal organic transformations. In the discussion that follows,
we report our results in the preparation and full
characterisation of the first examples of halogen-bonded
rhodium(I) complexes and their catalytic performance in
alkyne hydroborations.
Supramolecular chemistry has emerged as a more efficient
tool for the construction of (enantiopure) ligand backbones of
metal complexes when compared with standard covalent
chemistry. Supramolecular strategies mainly rely on the self-
assembly of building blocks that contain both the
complementary motifs required for the assembly, and the
binding groups necessary for the desired catalysis event. This
approach has been successfully employed for preparing
catalysts by assembling supramolecularly complementary
building blocks through hydrogen-bonding, metal-ligand, or
ionic interactions.2 However, the use of halogen bonding for
constructing the backbone of a catalyst remains, to the best of
our knowledge, unexplored.3
Results and discussion
Examination of CPK models revealed that the relative
arrangements of the P-ligating groups and the halogen bond
acceptor and donor motifs in 2-pyridyldiphenylphosphine (
1)
and 2-iodo-3,4,5,6-tetrafluorophenyldiphenylphosphine (2),
respectively, were adequate for the formation of the target
halogen-bonded Rh(I) complex (see Scheme 2). Building blocks
The directionality and strength of halogen bonding,4 together
1
and 2 were successfully prepared in good yields in a
with
a
greater tolerance to solvent polarity changes,5
multigram scale by slight modification of well-established
synthetic protocols.9 We envisaged that the use of
[Rh(Cl)(CO)2]2 along with a halide scavenger would render a
putative [Rh(CO)2]X intermediate that should react with
prompted us to design and develop new halogen-bonded
metal catalysts (see Scheme 1) We speculated that the
halogen bonding-mediated assembly of two appropriate
building blocks, each bearing a ligating group, could lead to the
formation of a metal complex due to the chelation effect that
should be exhibited in the presence of the metal centre. With
regard to the ligating groups, we focused our attention on
phosphino groups due to their pre-eminence in homogeneous
catalysis6 and their tolerance to functionalisation.7 With
phosphines
1 and 2. After some experimentation, halogen-
bonded rhodium(I) complexes were obtained by first reacting
[Rh(Cl)(CO)2]2 and NaBArF,10 and then adding ligands
1 and 2
as indicated in Scheme 2.11 The structure of the complex was
confirmed by standard spectroscopic studies. The 31P{1H} NMR
Scheme 1
Supramolecular halogen-bonded catalysts (A: halogen bond acceptor; X:
halogen bond donor; M: metal centre).
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F
H
The analysis of the solid-state structures revealed a longer N
distance for than for XBphos-Rh (2.84(1) Å for and 2.757(8)
Å for ), which indicates a stronger halogen bond interaction in
−I
F
F
H
H
F
H
5
5
N
PPh2
Ph2P
Ph2P
4
I
I
the fluorine-containing complex.
1
2
3
The formation of a halogen bonding interaction between the
pyridine and iodoarene motifs is driven by an enthalpy gain
but penalised by an entropic unfavourable contribution arising
from the geometry constraints for effective bonding (i.e. linear
+
i. NaBArF (1.0 equiv.)
R
R
1
2
3
ii. (1.0 equiv.) + or (1.0 equiv.)
[Rh(Cl)(CO)2]2
0.5 equiv.
N
I
R
CH2Cl2, rt, 2h
Ph2P Rh PPh2
R
arrangement of the N···I−C atoms). Overall, the free energy
R = F, 4, XBphos-Rh (74% yield)
R = H, (57% yield)
associated with a single interaction is low.16 In order to gain a
deeper insight into the halogen bonding event, full level DFT
calculations were carried out. Geometries and stabilities of
BArF-
O
5
Scheme 2
Synthesis of halogen-bonded rhodium(I) chelates.
[Rh(CO)(1)(2 1)(3
)]+ and [Rh(CO)( )]+ and those of the starting
building blocks were computed with Gaussian 0917 with the
TPSS18-D319 density functional employing a medium-sized
triple-zeta (def2TZVP20) basis set, which is a good compromise
between the computational cost and the reliability in
describing halogen bonding.21 Selected geometrical
parameters and computed binding free energies are
summarised in Table 1. To aid comparison, the supramolecular
interaction between 4MePy and IPFB (see footnote a in Table 1
for the abbreviations) was also computed and was found to be
spectrum showed two complex multiplets, from which a high
2
value JP–P coupling constant (i.e. 276 Hz; unequivocally
assigned by 2D 31P{1H} J-resolved spectroscopy; Figure SI 52)
was calculated. According to the literature reviewed, this high
2
value JP–P coupling constant indicates
a trans-spanning
bisphosphine coordinated to the rhodium centre.12 We initially
expected a dicarbonyl complex, but ESI-MS analysis identified
complex [Rh(CO)(
the discussion that follows.
X-ray analysis confirmed the proposed halogen-bonded
structure of the rhodium complex derived from and (see
1)(2)]BArF (4
), referred to as XBphos-Rh13 in
a
slightly endergonic process with
constant value being in agreement with that experimentally
measured (i.e.; Kexp = 1 ± 1 M-1 in toluene in ref. 16 and Kcalc
0.03
-1 from the predicted ΔΔG value in entry 1 in Table 1).
The computed N I distances were in agreement with the
a calculated binding
1
2
Scheme 2 for the structure of XBphos-Rh and Fig. 1A for the
=
crystal structure). The nitrogen and iodine are aligned with a
M
N−I distance of 2.757(8) Å and with a N···I−C bond angle of
−
169.4(5)°. These structural parameters are in agreement with
those reported for halogen-bonded complexes involving
pyridine and fluorinated iodoaryl moieties.14 Interestingly, the
experimental values and their formations were calculated to
be highly exergonic processes. Computational studies
confirmed a slightly higher stability for [Rh(CO)(
[Rh(CO)( )( and 3, Table 1). Electrostatic
)]+ (entries
potential surfaces were calculated at the level of theory
described above (see Fig. 2 for and [Rh(CO)( )(
)]+ and ESI
for all information). The maximum values of the σ-hole in
(Figure 2) and fluorine-free derivative (ESI) are 25.1 and 12.5
1)(2
)]+ than for
iodine appears to be coordinated to the rhodium centre (Rh−I
1
3
2
distance = 2.5535(7) Å). Thus, the tridentate coordination of
the halogen-bonded ligands in square planar XBphos-Rh
resembles that observed in pincer-type complexes,15 and
therefore this complex can be formally considered as the first
2
1 2
2
3
reported P−I−P pincer-type complex.
Given the favourable template effect exerted by the rhodium
kcal/mol, respectively. A lower σ-hole value indicates a lower
donor character of the halogen atom to the supramolecular
bond. This observation derived from electrostatic potentials is
in agreement with previously discussed X-ray data (i.e. a longer
centre on the coordinated atoms in XBphos-Rh, we envisaged
that fluorine-free compound
halogen bond donor. Thus, complex
good yields following a synthetic protocol analogous to that
employed for XBphos-Rh (Scheme 2). Crystals from suitable
for X-ray analysis confirmed the proposed structure for the
rhodium complex derived from and (Scheme 2 and Fig. 1B).
3
could also be used as the
5
could be prepared in
N−I bond for 5 than for XBphos-Rh).
The interesting structural features of XBphos-Rh and
5 (Table
2),22 prompted us to investigate reactivity features as catalysts
5
Table 1 Selected geometrical parameters for halogen-bonded complexes (TPSS-
1
3
D3/def2TZVP, 298 K, toluene) and computed binding free energies.
N…I−C bond
ΔΔG (kcal·
Entry
1
Complex
dN−I (Å)
mol-1
)
angle (o)
4MePy·IPFBa
[Rh(CO)(1)(2)]+
2.714
2.709
179.4
+2.03b
175.2
2c
−112.83d
(2.757(8))
2.827
(169.4(5))
176.2
3c
[Rh(CO)(1)(3)]+
−111.97d
(2.84(1))
(171.2(7))
a
4MePy·PFIB
robenzene. b ΔΔG = ΔG4MePy·IPFB − ΔG4MePy − ΔGIPFB
d ΔΔG = ΔG[Rh(CO)(1)(2 or 3)]+ + ΔGCO − ΔG[Rh(CO) + − ΔG1 − ΔG2 or 3
=
complex between 4-methylpyridine and iodopentafluo-
Fig. 1
Crystal structures of XBphos-Rh (A) and 5 (B). Hydrogen atoms and the BArF
.
c X-ray values in parentheses.
unit have been omitted for the sake of clarity. Colour scheme: C: black, P: orange, Rh:
green, F: green, N: blue, I: purple, O: red. Atomic displacement ellipsoids are drawn at a
50% probability.
.
2
]
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Table 3 Rh-Catalysed Hydroboration of Alkynes.a
8+9+10
yield%
Ratio
8 : 9 : 10
Entry
Alkyne, (R2O)2BHb
Cat.
Fig. 2
Electrostatic potential surfaces at an isovalue of 0.001 a.u.
1c
2
7a, HBpin
7a, HBpin
7a, HBpin
7a, HBcat
7a, HBcat
7a, HBcat
7b, HBpin
7b, HBpin
7b, HBpin
7b, HBcat
7b, HBcat
7b, HBcat
7c, HBpin
7d, HBpin
XBphos-Rh
88
36
54
51
49
50
49
50
59
50
51
58
58
74
78 : 2 : 20d
72 : 5 : 23
94 : 3 : 3
that could complement existing catalytic tools. These
complexes were tested in the hydroboration of terminal
alkynes towards vinylboronic acid derivatives, as the outcome
of these reactions is heavily influenced by the electronic and
steric properties of the ligand.23 Methods for synthesizing
vinylboronic acid derivatives, which are important synthetic
intermediates, by an atom-economical hydroboration
reaction24 are scarce in the literature.23,25
5
3
6
4
XBphos-Rh
82 : 2 : 16
82 : 2 : 16
86 : 9 : 5
5
5
6
6
To aid comparison of the results obtained with XBphos-Rh and
7
XBphos-Rh
79 : 4 : 17
79 : 4 : 17
93 : 2 : 5
5
, we included the monodentate [Rh(CO)(PPh3)3]BArF (
6)
complex in our catalytic studies.26 A summary of the results
8
5
obtained is shown in Table 3. Both XBphos-Rh and
active hydroboration catalysts, and E-isomers
5
were
were
9
6
8
preferentially formed in all cases. Although XBphos-Rh is the
most selective hydroboration catalyst for phenylacetylene 7a
(88% overall yield for HBpin and 51% for HBcat; entries 1 and 4
in Table 3), hydroboration selectivities for 1-octyne 7b using
10
11
12
13c
XBphos-Rh
5
52 : 4 : 44
66 : 4 : 30
86 : 4 : 10
71 : 2 : 27
83 : 3 : 14
6
XBphos-Rh are slightly lower than those with
6 (entries 7–12,
XBphos-Rh
Table 3). It is important to mention that in all cases selectivity
towards branched derivatives is higher with the halogen-
14c
XBphos-Rh
bonded catalysts than with the background catalyst 6 26
For
.
a Reactions were performed in CD2Cl2 (0.5 M) under N2 and reacted for 24h at room
temperature. Yields were determined by 1H NMR using 1,2,4,5-tetramethylbenzene as
example, a ten-fold increase in yield was obtained for the
branched product 10a,pin when XBphos-Rh was used (entries
1 and 3, Table 3). It is also noteworthy that, as far as we are
aware, XBphos-Rh provides the highest reported yield for the
branched derivative 10b,cat (44% branched product with
respect to all hydroboration products, entry 10, Table 3).
Regarding electronic effects of the product distribution in the
hydroboration of aryl-substituted acetylenes, higher ratios of
the branched product were obtained for electron-deficient
arene 7c than for electron rich derivative 7d (entries 13 and
14, Table 3).
the internal standard. See ESI for details. (R2O)2BH: HBpin = pinacolborane, HBcat =
b
catecholborane. 2 equiv. of (R2O)2BH. Results obtained with bidentate complex
[Rh(CO)(Xantphos)]BArF as catalyst in the reaction of 7a and HBpin are as follows:
69% overall yield (ratio 8 : 9 : 10 = 97 : 2 : 1)
c
d
Conclusions
In summary, we have successfully prepared and fully
characterised the first halogen-bonded supramolecular
rhodium(I) complexes. To the best of our knowledge, these are
the first examples where halogen bonding is the driving force
for the assembly of the catalyst backbone. Furthermore, we
have demonstrated that the presence of fluorine groups at the
iodo-containing supramolecular motif is not necessary for
efficient halogen bonding due to the favourable template
effect exerted by the rhodium centre. The two new catalysts
are active in the hydroboration of terminal alkynes, with the
catalytic activity of XBphos-Rh rivalling that of the highest
performing catalysts. Moreover, this catalyst favours enhanced
ratios of the branched alkenyl boronic acid products. Ongoing
investigations in our group include mechanistic studies on the
Table 2 Selected bond distances (Å) and angles (°).a
P1−P2
Rh−CO
1.798(5)
1.872(8)
1.848(9)
P1−Rh−P2
164.42(4)
177.09(7)
178.6(3)
Entry
1b
Complex
[Rh(CO)(xant)]BF4
XBphos-Rh
5
4.506(2)
4.618(2)
4.632(9)
2
3c
a X-ray values. b xant = Xantphos; see ref. 22a.
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outcome of the reaction,27 application of this system to other
catalytically relevant transformations, extension of the
application of this halogen bonding approach to other
transition metals, and studies into alternative ligand designs.
Research Maatschappij B. V.), EP499328A2, 1992. For the
preparation of and , see: (b) P. G. Eller and D. W. Meek, J.
Organometal. Chem., 1970, 22, 631-636.
2
3
10 The tetrakis[3,5-bis(trifluoromethyl)phenyl]borate counter-
ion enhances the solubility and crystallinity of the resulting
derivatives, which we deemed as useful for further catalytic
studies. See: L. Carreras, L. Rovira, M. Vaquero, I. Mon, E.
Martin, J. Benet-Buchholz and A. Vidal-Ferran, RSC Adv.,
Acknowledgements
2017,
11 Attempts to prepare homocomplexes [Rh(CO)(
[Rh(CO)( )2]BArF following an analogous strategy to that
7, 32833-32841.
The authors would like to thank MINECO (CTQ2014-60256-P,
CTQ2017-89814-P and Severo Ochoa Excellence Accreditation
2014-2018 SEV-2013-0319) and the ICIQ Foundation for the
financial support. L. C. thanks MINECO for a FPI-SO pre-
doctoral fellowship (BES-2015-071872). The Université
Fédérale Toulouse et Midi-Pyrénées is thanked for an IDEX
Chaire d'attractivité. We also thank Drs. E. Martin and J. Benet-
Buchholz for X-ray crystallographic data, Dr. N. Bampos for
NMR 31P{1H} J-resolved data, Dr. J. L. Núñez-Rico for his
support with graphic design and Ms. R. Somerville for proof
reading the manuscript.
1
)2]BArF or
2
used for XBphos-Rh failed, as complex mixtures of rhodium
derivatives were obtained in all cases. Experimental reaction
conditions for the selective preparation of rhodium
complexes arising from C−I oxidative addition processes in 2
have been also developed (see p. SI-12 in ESI). It is
interesting to note that 31P NMR signals that could
correspond to the above mentioned homocomplexes, or
complexes arising from C−I oxidative additions in 2, were not
found in any spectra of the crude reaction mixtures.
12 Z. Freixa and P. W. N. M. van Leeuwen, Coord. Chem. Rev.,
2008, 252, 1755-1786.
13 XB in XBphos-Rh stands for halogen (X) bonding (B).
14 V. Vasylyeva, L. Catalano, C. Nervi, R. Gobetto, P. Metrangolo
and G. Resnati, CrystEngComm, 2016, 18, 2247-2250.
15 For representative literature reports on pincer-type metal
complexes, see: Pincer and Pincer-Type Complexes:
Applications in Organic Synthesis and Catalysis, (Eds.: K. J.
Szabó, O. F. Wendt), Wiley-VCH, Weinheim, 2014.
16 C. C. Robertson, J. S. Wright, E. J. Carrington, R. N. Perutz, C.
A. Hunter and L. Brammer, Chem. Sci., 2017, 8, 5392-5398.
17 Gaussian 09, Revision D.01, M. J. Frisch et al. Gaussian, Inc.,
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Notes and references
1
2
Homogeneous Catalysis: Understanding the Art, (Ed.: P. W.
N. M. van Leeuwen), Kluwer Academic Publishers, Dordrecht,
2004.
For selected reviews, see: (a) B. Breit, Angew. Chem., Int. Ed.,
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(e) R. Bellini, J. I. van der Vlugt and J. N. H. Reek, Isr. J. Chem.,
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and P. W. N. M. van Leeuwen, Chem. Soc. Rev., 2014, 43
,
1660-1733. (g) ibid. 1734-1787. For a selected example on
hydroborations employing supramolecular catalysts, see: (f)
S. A. Moteki and J. M. Takacs, Angew. Chem., Int. Ed., 2008,
47, 894-897.
20 (a) F. Weigend and R. Ahlrichs, Phys. Chem. Chem. Phys.,
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Phys., 2006,
7
, 3297-3305. (b) F. Weigend, Phys. Chem. Chem.
, 1057-1065.
8
3
For the application of halogen bonding as a secondary
interaction for substrate recognition in catalysis, see for
example: (a) T. Caronna, R. Liantonio, T. A. Logothetis, P.
Metrangolo, T. Pilati and G. Resnati, J. Am. Chem. Soc., 2004,
126, 4500-4501. For the use of halogen bonding in the
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transformation, see for example: (b) V. N. G. Lindsay, W. Lin
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21 R. Sure and S. Grimme, Chem. Commun., 2016, 52, 9893-
9896.
22 Examples of ligands with a bite angle close to 180° are scarce
in the literature. See, for example: (a) A. J. Sandee, L. A. Van
der Veen, J. N. H. Reek, P. C. J. Kamer, M. Lutz, A. L. Spek and
P. W. N. M. Van Leeuwen, Angew. Chem., Int. Ed., 1999, 38
,
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J. J. Carbó and E. Fernández, Chem. -Eur. J., 2012, 18, 1512-
1521.
24 These compounds can also be synthesised by hydroboration
of alkynes with diboron derivatives. For selected examples,
see the following references and those cited therein: (a) H.
Jang, A. R. Zhugralin, Y. Lee and A. H. Hoveyda, J. Am. Chem.
Soc., 2011, 133, 7859-7871. (b) D. P. Ojha and K. R. Prabhu,
Org. Lett., 2016, 18, 432-435. (c) B. M. Trost, J. J. Cregg and
N. Quach, J. Am. Chem. Soc., 2017, 139, 5133-5139.
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D. M. Khramov, E. L. Rosen, J. A. V. Er, P. D. Vu, V. M. Lynch
and C. W. Bielawski, Tetrahedron, 2008, 64, 6853-6862. (d)
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L. C. Gilday, S. W. Robinson, T. Barendt, M. J. Langton, B. R.
Mullaney and P. D. Beer, Chem. Rev., 2015, 115, 7118-7195.
M. G. Sarwar, B. Dragisic, L. J. Salsberg, C. Gouliaras and M. S.
Taylor, J. Am. Chem. Soc., 2010, 132, 1646-1653.
Phosphorus(III) Ligands in Homogeneous Catalysis: Design
and Synthesis, (Eds.: P. C. J. Kamer, P. W. N. M. van
Leeuwen), John Wiley & Sons Ltd., United Kingdom, 2012.
For examples on the design and use of functionalised
phosphorus-based catalysts, see for example: H. Fernández-
Pérez, P. Etayo, A. Panossian and A. Vidal-Ferran, Chem. Rev.,
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7
8
9
S. Tsuzuki, A. Wakisaka, T. Ono and T. Sonoda, Chem. -Eur. J.,
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For the preparation of 1, see: (a) P. A. A. Klusener, J. C. L.
Suykerbuyk and P. A. Verbrugge (Shell Internationale
4 | J. Name., 2012, 00, 1-3
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ARTICLE
N. Iwadate and M. Suginome, Org. Lett., 2009, 11, 1899-
1902.
26 The bidentate rhodium complex [Rh(CO)(Xantphos)]BArF
was also considered
transformation and incorporated to catalytic hydroboration
studies of alkyne 7a (see footnote in Table 1).
a background catalyst in this
d
[Rh(CO)(Xantphos)]BArF displayed a lower performance than
XBphos-Rh in terms of overall yield of hydroboration
products of 7a with HBpin.
27 As a detailed mechanism on Rh-catalysed hydroboration of
terminal alkynes has still to be established (see: B. M. Trost
and Z. T. Ball, Synthesis, 2005, 853-887), elementary
mechanistic steps involving XBphos-Rh cannot be
unequivocally proposed.
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Chemical Science
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XBphos-Rh constitutes the first example of halogen bonding as driving force behind the assembly
of a transition-metal catalyst for hydroborations.