Effect of Sulfonamide and Carboxamide Ligands on the Structural Diversity of Bimetallic RhII-RhIICores: Exploring the Catalytic Activity of These Newly Synthesized Rh2Complexes

Article abstract of DOI:10.1021/acs.inorgchem.1c00127

A new class of dirhodium(II) complexes with tethered sulfonamide and carboxamide ligands was synthesized and characterized. A new type of coordination mode was found for the quinoline moiety containing a sulfonamide ligand, which afforded the axially coordination-free bimetallic dirhodium complexes. Studies were conducted on the catalytic properties of these complexes for cyclopropanation reactions, and the findings indicate that a free axial coordination site is crucial for achieving a high degree of reactivity.

Full text of DOI:10.1021/acs.inorgchem.1c00127

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Effect of Sulfonamide and Carboxamide Ligands on the Structural
II
II
Diversity of Bimetallic Rh −Rh Cores: Exploring the Catalytic
Activity of These Newly Synthesized Rh Complexes
2
Supriya Rej and Naoto Chatani*
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ABSTRACT: A new class of dirhodium(II) complexes with tethered sulfonamide and carboxamide ligands was synthesized and
characterized. A new type of coordination mode was found for the quinoline moiety containing a sulfonamide ligand, which afforded
the axially coordination-free bimetallic dirhodium complexes. Studies were conducted on the catalytic properties of these complexes
for cyclopropanation reactions, and the findings indicate that a free axial coordination site is crucial for achieving a high degree of
reactivity.
irhodium(II,II) paddlewheel complexes are a well-known
class of bimetallic cluster compounds and are extensively
used as effective catalytic systems for promoting various
D
transformations through the decomposition of diazo com-
1
pounds. In reactions of dirhodium(II,II) complexes with a
diazo compound, a reactive dirhodium carbenoid species is
formed, which then participates in a variety of transformations
such as cyclopropanation, cycloaddition, C−H functionaliza-
tion, N−H, O−H, and Si−H insertion reactions, and ylide-
2
type transformation. The presence of various carboxylate,
carboxamide, and formamidinate bridging ligand systems has
led to access to a diverse window of dirhodium complexes that
can participate in various catalytic reactions, depending on
3
their unique electronic properties. The electrophilicity of
these types of dirhodium complexes largely depends on the
nature of the bridging ligand. Because the axial sites of
dirhodium complexes serve as electrophilic sites, weakly
coordinated solvent molecules frequently interact with these
sites (Figure 1a), but they can easily be replaced by the
reactant. In fact, various studies have shown that the axial
coordination of various coordinating moieties, such as solvent
molecules or additives, can have an impact on achieving high
4
,5
chemoselectivity, regioselectivity, or enantioselectivity. To
achieve a smooth reaction or to suppress the reactivity of
dirhodium complexes, instead of using an external ligand, a
tethering Lewis basic moiety can be incorporated in the
bridging carboxylate or carboxamide ligand, which can
coordinate with one of the Rh centers via axial coordination
6
(
Figure 1b). This axial tethered coordination can effectively
decrease the Lewis acidity of the other Rh center through
electronic communication between the two Rh atoms.
7
Figure 1. Various types of ligand-ligated dirhodium complexes.
Along with the dirhodium paddlewheel complexes, some
new types of heteroleptic dirhodium complexes were prepared
by the substitution of two bridging ligands with two neutral
bidentate chelating ligands, in which two axial positions were
coordinated with two anionic ligands (Figure 1c). In these
types of dirhodium complexes, strong coordination of the axial
Received: January 14, 2021
Published: March 3, 2021
8
,9
©
2021 American Chemical Society
https://dx.doi.org/10.1021/acs.inorgchem.1c00127
3
534
Inorg. Chem. 2021, 60, 3534−3538

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ligand decreases its electrophilicity to a sufficient extent.
Hence, outer-sphere anionic ligands have been examined in
attempts to increase the reactivity of these types of complexes.
These complexes were recently shown to act as active catalysts
Scheme 1. Synthesis of New Bimetallic Dirhodium
Complexes: Solid-State Structures of cis-1−3 and trans-4
a
Complexes
+
9
for H evolution from H . Herein we report on the synthesis
2
of structurally similar types of neutral dirhodium complexes in
which two coordination sites can remain free (Figure 1d-III).
The electron density of the dirhodium core in these complexes
can be altered, which can enhance the catalytic reactivity of
these bimetallic complexes in cyclopropanation reactions. In
this regard, the tethering moieties containing sulfonamide and
carboxamide ligands were used, which hypothetically can have
two modes of coordination to give a new class of dirhodium
complexes (Figure 1d).
To elucidate the catalytic reactivity of these complexes, open
coordination sites having half-paddlewheel-like dirhodium
complexes 1 and 2 (CCDC 2054821; as shown in Figure
1
d-III) were synthesized by ligand substitution reactions of
Rh (OAc) and Rh (OCOCF ) with the sulfonamide ligand
2
4
2
3 4
N-(quinolin-8-yl)benzenesulfonamide (L; Scheme 1). To
accomplish this, we employed a sulfonamide-type ligand that
II
we previously used as a substrate in Rh -catalyzed C−H bond
10
functionalization reactions. We realized that the choice of
ligand is crucial for obtaining an open coordination site in
paddlewheel-like dirhodium complexes. Two similar types of
carboxamide ligands, N-(quinolin-8-yl)benzamide (L′) and N-
(
2-(methylthio)phenyl)benzamide (L″), were employed in the
ligand substitution reactions of Rh (OAc) . Using 2 equiv of
2
4
ligand L′, the predominantly formed product was the cis-3
isomer (CCDC 2054823) along with minor amounts of the
trans-3 isomer, which could easily be separated (Scheme 1).
Along with them, a substantial amount of a singly
carboxamide-bridged triacetate dirhodium complex 3′ was
also isolated, which was characterized by spectroscopic data.
Similarly, the reaction of 2 equiv of ligand L″ and Rh (OAc)
2
4
resulted in the formation three separable complexes, namely,
the cis-4, trans-4 (CCDC 2054824), and 4′. It was anticipated
that the differences in complexation with the sulfonamide and
carboxamide ligands would be mainly due to the tetrahedral
geometry of the sulfonamide moiety. As anticipated, the
sulfonamide ligand L functioned better as a monoanionic N,N-
chelating ligand compared to the bridging mode. Conversely,
because of the planar geometry of the carboxamide ligands,
they function more efficiently as bridging ligands.
a
ORTEP diagram of the dirhodium complexes cis-1−3 and trans-4.
Thermal ellipsoids are drawn at the 30% probability level. A total of 2
equiv of acetic acid (byproduct) was coordinated as a neutral ligand
to complex 1 via axial coordination. A total of 2 equiv of acetonitrile
10c,d
Crystals of the 1,
obtained and analyzed by single-crystal X-ray diffraction
Scheme 1). The crystal structures of the cis-3 and trans-4
2, cis-3, and trans-4 isomers were
(crystallizing solvent) was coordinated to complex 2 via axial
coordination.
(
isomers serve to confirm that both of the Rh centers are
occupied by axial coordination of the N(sp ) atom of quinoline
sulfonamide ligands. Notably, the L ligand in complex 5 has
both bridging and monoanionic chelation modes. The solid-
state structure of complex 5 indicates that one of the Rh
2
in the cis-3 isomer and a thioether donor group in the trans-4
isomer. The Rh−Rh bond lengths in the doubly bridged
dirhodium complexes 1 [2.5178(7) Å] and 2 [2.5412(6) Å]
are longer compared to paddlewheel complexes cis-3
2
centers is axially chelated to the N(sp ) atom of the quinoline
ligand. As expected, the Rh−Rh bond length [2.5641(5) Å] in
complex 5 is slightly longer than that in complex 1. A possible
reason for this is that the axial coordination of a donor atom
could affect the electronic communication between two Rh
centers.
Because the structural orientations of complexes 1 and 5 are
similar, we hypothesized that complex 1 is involved as an
intermediate in the formation of complex 5. To confirm this,
we reacted complex 1 with 1 equiv of the sulfonamide ligand L.
As expected, complex 5 was obtained in good yield via a ligand
substitution reaction (Scheme 2-II). Complex 5 exhibited a
high stability and further substitution of acetate by a
8a,c
[2.4011(3) Å] and trans-4 [2.3944(10) Å].
The average
axial Rh−N bond length [Rh1−N2 = 2.185(2) Å and Rh2−N4
=
2.207(2) Å] in complex cis-3 is shorter than the axial Rh−S
bond [Rh1−S1 = 2.475(3) Å and Rh2−S2 = 2.472(3) Å] in
complex trans-4 because of the stronger chelating ability of a
quinoline moiety than that of a thiomethyl group.
Inspired by the coordination mode of the ligand L, we
further reacted Rh (OAc) with 3 equiv of L (Scheme 2-I). In
2
4
this reaction, a new type of complex (5; CCDC 2054822) was
formed, in which three acetate ligands are replaced by three
3
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Scheme 2. Synthesis of Complex 5 from Rh (OAc) and
Complex 1
We examined the issue of whether a correlation exists
between the electrochemical properties of the bimetallic
dirhodium complexes and their catalytic reactivity (see the
SI for details). The cyclic voltammograms of all of the
2
4
a
4
+/5+
complexes displayed reversible Rh2
couples versus
ferrocenium/ferrocene, and the redox potential varied depend-
9c
ing upon the structural coordination of the ligand moieties.
An axially coordinated ligand had a notable effect on the redox
potential. With a strong σ-donor ligand, cis-3 displayed a
greater cathodic shift compared to trans-4.
To examine the effect of axial coordination for the bimetallic
dirhodium complexes, a study of the cyclopropanation reaction
of p-methylstyrene (6) with the diazo compound 7 was carried
out (Table 1). It should be noted that the reactivities of
Table 1. Use of the Newly Synthesized Dirhodium Complex
in Catalyzed Cyclopropanation Reactions
a
ORTEP diagram of dirhodium complex 5. Thermal ellipsoids are
time
(min)
yield (%) of 8
TOF
a
−1
drawn at the 30% probability level.
entry
catalyst (XX mol %)
(d.r.)
(h
)
1
2
Rh (OAc) (1.0 mol %)
60
60
51 (58:42)
19 (53:47)
51
19
2
4
sulfonamide ligand failed to proceed, even when an excess
amount of sulfonamide ligand was used, which can be
attributed to steric interactions. By monitoring the progress
of this ligand displacement reaction (see the Supporting
Information, SI), we elucidate the activation parameters
Rh (OCOCF ) (1.0
2
3 4
mol %)
3
4
5
6
7
8
9
1
1
1
1 (1.0 mol %)
1 (0.1 mol %)
2 (1.0 mol %)
cis-3 (1.0 mol %)
trans-3 (1.0 mol %)
cis-4 (1.0 mol %)
trans-4 (1.0 mol %)
3′ (1.0 mol %)
60
10
60
60
60
60
60
60
60
60
60% (65:35)
22 (63:37)
31 (69:31)
6 (67:33)
4 (66:34)
3 (66:34)
5 (67:33)
38 (68:32)
41 (68:32)
23 (70:30)
60
1320
31
6
4
3
5
38
41
23
⧧
−1
⧧
(
ΔG
= 31.74 ± 2.3 kJ·mol , ΔH = 74.84 ± 1.3 kJ·
−1 −1
2
98 K
−
1
⧧
mol , and ΔS = −144.65 ± 3.3 J·K ·mol ) for the ligand
substitution reaction (Figure 2). The large negative value of
ΔS suggests that an ordered transition state is involved in the
⧧
11
0
1
2
ligand substitution reaction.
4′ (1.0 mol %)
5 (1.0 mol %)
a
1
Yields were determined by H NMR spectroscopy using mesitylene
as an internal standard. The diastereomeric ratio (d.r.) was
1
determined by H NMR. The presented yields are the averages of
two runs. Turnover frequency (TOF) = turnover number/h.
complexes 1 and 2 are comparable to those for the precursor
paddlewheel complexes Rh (OAc) (entry 1 vs 3) and
2
4
Rh (OCOCF ) (entry 2 vs 5), respectively. The reaction of
2
3 4
complexes 1 and 2 with the diazo compound 7 was fast, and an
immediate intense color change from yellowish green to brown
was observed upon the addition of 7. Complex 1 functioned as
a catalyst even at low loading and in a short reaction period
Figure 2. Eyring plot for the ligand-exchange reaction from 1 to 5.
(
entry 4). The high reactivity of complex 1 suggests that,
although the Rh−Rh bond is much longer than that for
12
To collect more information regarding the electronic
transitions in these structurally divergent dirhodium com-
plexes, UV−vis spectroscopic studies were carried out (see the
SI for details), and the results showed that the doubly bridged
dirhodium complexes 1, 2, and 5 exhibited a blue shift in the
UV−vis spectra due to an increase in the π*-to-σ* highest
occupied molecular orbital−lowest unoccupied molecular
orbital transition caused by the long Rh−Rh bond length.
Axial coordination also affects the electronic state of the
Rh (OAc) , electronic communication still favors a faster
2 4
diazo decomposition. Notably, complex 5 with one axial site
occupied by a strong quinoline ligand exhibited a lower
reactivity compared to complex 1 (entry 12). The importance
of the free axial coordination site for cyclopropanation
reactions was further realized by the fact that the strongly
coordinating quinoline and the thiomethyl group tethered
complexes cis-3, trans-3, cis-4, and trans-4 showed a much
lower reactivity (entries 6−9). Notably, the singly carbox-
amide-bridged dirhodium complexes 3′ and 4′ showed a much
higher reactivity compared to their doubly bridged precursor
because of the presence of a free axial coordination site.
6a
dirhodium complexes. Because of the strong σ donation for
the quinoline ligand compared to the thiomethyl group, cis-3
showed a blue shift compared to trans-4.
3
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Communication
Overall, these initial results demonstrate that the role of a
ligand and its coordination mode is much more significant than
previously thought in terms of creating a higher reactivity for a
bimetallic dirhodium-complex-catalyzed reaction. Further
explorations regarding the catalytic properties of these types
of doubly bridged neutral dirhodium complexes are currently
underway in our laboratory.
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We acknowledge support from a Grant in Aid for Specially
Promoted Research by MEXT (Grant 17H06091). S.R.
acknowledges support from the Japan Society for the
Promotion of Science for a research fellowship (P19330).
In summary, new bimetallic dirhodium complexes were
synthesized and fully characterized. It was anticipated that,
because of the tetrahedral geometry of the sulfonamide moiety,
two coordination modes would be expected in the case of a
quinoline moiety that contained a sulfonamide ligand, which
afforded a new generation of doubly bridged dirhodium
complexes possessing reactive axial coordination sites. The
initial findings regarding the use of these catalysts in
cyclopropanation reactions suggested that they exhibit a high
level of reactivity for diazo decomposition reactions.
Conversely, similar types of coordinating moieties containing
doubly bridged carboxamide ligands experienced only one
coordination mode and performed poorly in catalytic reactions
because of inaccessibility of the axial coordination site.
Furthermore, singly carboxamide-bridged dirhodium com-
plexes displayed a much higher reactivity compared to doubly
carboxamide-bridged dirhodium precursors, suggesting the
importance of the presence of a free axial coordination site in
diazo decomposition reactions. The nature of modification of
our designed ligands allows a variety of sulfonamide-based
chelating ligands to be prepared in which the electronic
properties of the bimetallic dirhodium core can be altered. We
conclude that these initial results will help us to understand the
phenomena associated with this new generation of bimetallic
dirhodium complexes in terms of their properties as well as
their catalytic activities.
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ASSOCIATED CONTENT
sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acs.inorgchem.1c00127.
■
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, and 5 (PDF)
Accession Codes
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tallographic data for this paper. These data can be obtained
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emailing data_request@ccdc.cam.ac.uk, or by contacting The
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5
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AUTHOR INFORMATION
Corresponding Author
Naoto Chatani − Department of Applied Chemistry, Faculty of
Engineering, Osaka University, Suita, Osaka 565-0871,
Japan; orcid.org/0000-0001-8330-7478;
Email: chatani@chem.eng.osaka-u.ac.jp
■
Author
Supriya Rej − Department of Applied Chemistry, Faculty of
Engineering, Osaka University, Suita, Osaka 565-0871,
Japan
3
(
70−383.
5) For selected papers of importance on axial coordination in
Complete contact information is available at:
https://pubs.acs.org/10.1021/acs.inorgchem.1c00127
dirhodium-catalyzed transformations, see: (a) Gomes, L. F. R.;
Trindade, A. F.; Candeias, N. R.; Gois, P. M. P.; Afonso, C. A. M.
3
537
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https://dx.doi.org/10.1021/acs.inorgchem.1c00127
Inorg. Chem. 2021, 60, 3534−3538