This document is the accepted manuscript version of a published work that
appeared in final form in Chem. Sci., 2018, 9, 3644-3648. DOI:
10.1039/c8sc00233a
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*
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.

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.
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 selfassembly 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
The directionality and strength of halogen bonding, 4 together
with a greater tolerance to solvent polarity changes, 5
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
catalysis 6 and their tolerance to functionalisation.7 With

respect to the halogen bonding motifs, we envisaged that the
use of a fluorinated iodoarene and a pyridine group could be
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.

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
1 and 2 were successfully prepared in good yields in a
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
phosphines 1 and 2. After some experimentation, halogenbonded 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

aInstitute

of Chemical Research of Catalonia (ICIQ) & The Barcelona Institute of
Science and Technology, Av. Països Catalans 16, 43007 Tarragona, Spain.
bLPCNO, INSA, 135 Avenue de Rangueil, F-31077 Toulouse, France.
cICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain.
E-mail: avidal@iciq.cat
† Electronic Supplementary Information (ESI) available: Experimental details,
spectroscopic and crystallographic data (CCDC 1815870-1815871). For ESI and
crystallographic
data
in
CIF
or
other
electronic
format
see
DOI: 10.1039/c8sc00233a

Scheme 1
Supramolecular halogen-bonded catalysts (A: halogen bond acceptor; X:
halogen bond donor; M: metal centre).

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Scheme 2

Synthesis of halogen-bonded rhodium(I) chelates.

spectrum showed two complex multiplets, from which a high
value 2JP–P coupling constant (i.e. 276 Hz; unequivocally
assigned by 2D 31P{1H} J-resolved spectroscopy; Fig. SI 52 ESI†)
was calculated. According to the literature reviewed, this high
value 2JP–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)(1)(2)]BArF (4), referred to as XBphos-Rh13 in
the discussion that follows.
X-ray analysis confirmed the proposed halogen-bonded
structure of the rhodium complex derived from 1 and 2 (see
Scheme 2 for the structure of XBphos-Rh and Fig. 1A for the
crystal structure). The nitrogen and iodine are aligned with a
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
iodine appears to be coordinated to the rhodium centre (RhI
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
reported PIP pincer-type complex.
Given the favourable template effect exerted by the rhodium
centre on the coordinated atoms in XBphos-Rh, we envisaged
that fluorine-free compound 3 could also be used as the
halogen bond donor. Thus, complex 5 could be prepared in
good yields following a synthetic protocol analogous to that
employed for XBphos-Rh (Scheme 2). Crystals from 5 suitable
for X-ray analysis confirmed the proposed structure for the
rhodium complex derived from 1 and 3 (Scheme 2 and Fig. 1B).

The analysis of the solid-state structures revealed a longer NI
distance for 5 than for XBphos-Rh (2.84(1) Å for 5 and 2.757(8)
Å for 4), which indicates a stronger halogen bond interaction in
the fluorine-containing complex.
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
arrangement of the N···IC atoms). Overall, the free energy
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
[Rh(CO)(1)(2)]+ and [Rh(CO)(1)(3)]+ and those of the starting
building blocks were computed with Gaussian 09 17 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
a slightly endergonic process with a calculated binding
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 M-1 from the predicted ΔΔG value in entry 1 in Table 1).
The computed NI distances were in agreement with the
experimental values and their formations were calculated to
be highly exergonic processes. Computational studies
confirmed a slightly higher stability for [Rh(CO)(1)(2)]+ than for
[Rh(CO)(1)(3)]+ (entries 2 and 3, Table 1). Electrostatic
potential surfaces were calculated at the level of theory
described above (see Fig. 2 for 2 and [Rh(CO)(1)(2)]+ and ESI
for all information). The maximum values of the σ-hole in 2
(Figure 2) and fluorine-free derivative 3 (ESI) are 25.1 and 12.5
kcal·mol-1, 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
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
Table 1 Selected geometrical parameters for halogen-bonded complexes (TPSSD3/def2TZVP, 298 K, toluene) and computed binding free energies.

Entry

Complex

dNI (Å)

N…IC bond
angle (o)

ΔΔG (kcal·

1

4MePy·IPFBa

2.714

179.4

2.03b

2c

[Rh(CO)(1)(2)]+

2.709

175.2

(2.757(8))

(169.4(5))

2.827

176.2

(2.84(1))

(171.2(7))

3c

Fig. 1
Crystal structures of XBphos-Rh (A) and 5 (B). Hydrogen atoms and the BArF
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|

a

[Rh(CO)(1)(3)]+

mol-1)

112.83d
111.97d

4MePy·IPFB = complex between 4-methylpyridine and iodopentafluorobenzene. b ΔΔG = ΔG4MePy·IPFB  ΔG4MePy  ΔGIPFB. c X-ray values in parentheses.
d ΔΔG = ΔG[Rh(CO)(1)(2 or 3)]+  ΔGCO  ΔG[Rh(CO) ]+  ΔG1  ΔG2 or 3.
2

Table 3

Fig. 2

Entry

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
To aid comparison of the results obtained with XBphos-Rh and
5, we included the monodentate [Rh(CO)(PPh3)3]BArF (6)
complex in our catalytic studies.26 A summary of the results
obtained is shown in Table 3. Both XBphos-Rh and 5 were
active hydroboration catalysts, and E-isomers 8 were
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
XBphos-Rh are slightly lower than those with 6 (entries 7–12,
Table 3). It is important to mention that in all cases selectivity
towards branched derivatives is higher with the halogenbonded catalysts than with the background catalyst 6.26 For
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).
Selected bond distances (Å) and angles (°). a

Entry

Complex

P1P2

RhCO

P1RhP2

1b

[Rh(CO)(xant)]BF4

4.506(2)

1.798(5)

164.42(4)

2

XBphos-Rh

4.618(2)

1.872(8)

177.09(7)

3

5

4.632(9)

1.848(9)

178.6(3)

a X-ray

values. b xant = Xantphos; see ref. 22a.

Alkyne, (R2O)2BHb

Cat.

8+9+10
yield%

Ratio
8 : 9 : 10

1c

7a, HBpin

XBphos-Rh

88

78 : 2 : 20d

2

7a, HBpin

5

36

72 : 5 : 23

3

7a, HBpin

6

54

94 : 3 : 3

4

7a, HBcat

XBphos-Rh

51

82 : 2 : 16

5

7a, HBcat

5

49

82 : 2 : 16

6

7a, HBcat

6

50

86 : 9 : 5

7

7b, HBpin

XBphos-Rh

49

79 : 4 : 17

8

7b, HBpin

5

50

79 : 4 : 17

9

7b, HBpin

6

59

93 : 2 : 5

10

7b, HBcat

XBphos-Rh

50

52 : 4 : 44

11

7b, HBcat

5

51

66 : 4 : 30

12

7b, HBcat

6

58

86 : 4 : 10

13c

7c, HBpin

XBphos-Rh

58

71 : 2 : 27

14c

Electrostatic potential surfaces at an isovalue of 0.001 a.u.

Table 2

Rh-Catalysed Hydroboration of Alkynes.a

7d, HBpin

XBphos-Rh

74

83 : 3 : 14

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
the internal standard. See ESI for details. b (R2O)2BH: HBpin = pinacolborane, HBcat =
catecholborane. c 2 equiv. of (R2O)2BH. d 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)

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

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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.

8
9

Conflicts of interest
There are no conflicts to declare.

10

Acknowledgements
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 predoctoral fellowship (BES-2015-071872). The Université
Fédérale Toulouse et Midi-Pyrénées is thanked for an IDEX
Chaire d'attractivité. We also thank Dr E. Martin and J. BenetBuchholz 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.

Notes and references
1
2

3

4
5
6
7

4|

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11

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23

24

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derivatives were obtained in all cases. Experimental reaction
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complexes arising from C–I oxidative addition processes in 2
have been also developed (see p. SI-12 in ESI†). It is
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25 For selected examples, see the following references and
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26 The bidentate rhodium complex [Rh(CO)(Xantphos)]BArF
was also considered a background catalyst in this
transformation and incorporated to catalytic hydroboration
studies of alkyne 7a (see footnote d in Table 1).
[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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