Attachment of a RuII Complex to a Self-Folding Hexaamide Deep Cavitand

Article abstract of DOI:10.1021/jacs.7b05458

We report the design, synthesis and characterization of a new Ru^II metallocavitand that is catalytically active in alkene epoxidation reactions. The elaboration of the resorcin[4]arene's aromatic cavity produced a self-folding, deep hexaamide cavitand featuring a single diverging terpyridine (tpy) group installed at its upper rim. The construction of the metallocavitand involved the initial chelation of a Ru^III chloride complex by the tpy ligand followed by the incorporation of 2-(phenylazo)pyridine (azpy) as an ancillary ligand. The resulting Ru^II chloro complex was converted into the catalytically active aqua counterpart by a ligand exchange process.

Full text of DOI:10.1021/jacs.7b05458

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Communication
Attachment of a RuII complex to a self-folding hexaamide deep cavitand
Saša Korom, and Pablo Ballester
J. Am. Chem. Soc., Just Accepted Manuscript • DOI: 10.1021/jacs.7b05458 • Publication Date (Web): 18 Aug 2017
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Attachment of a RuII complex to a self-folding hexaamide deep cavi-
tand.
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Saša Korom† and Pablo Ballester*,†,‡.
† Institute of Chemical Research of Catalonia (ICIQ), The Barcelona Institute of Science and Technology, Av. Països Cataꢀ
lans 16, 43007 Tarragona, Spain
‡ ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain.
Supporting Information Placeholder
ABSTRACT: We report the design, synthesis and characterization of a new RuII metallocavitand that is catalytically active in
alkene epoxidation reactions. The elaboration of the resorcin[4]arene’s aromatic cavity produced a selfꢀfolding, deep hexaamide
cavitand featuring a single diverging terpyridine (tpy) group installed at its upper rim. The construction of the metallocavitand
involved the initial chelation of a RuIII chloride complex by the tpy ligand followed by the incorporation of 2ꢀ(phenylazo)pyridine
(azpy) as an ancillary ligand. The resulting RuII chloro complex was converted into the catalytically active aqua counterpart by a
ligand exchange process.
The concept of merging molecular recognition with catalysis
constitutes an area of growing interest in chemical research.1
The synthetic availability of resorcin[4]arene cavitands that
are not symmetrically substituted2 allows the incorporation of
functional groups at their upper rims that coordinate to catalytꢀ
ic metal centers. Examples of these structural modifications
ders the RuIV=O species suitable for stereoselective epoxidaꢀ
tion reactions.14
,
,
include oxazoline3 porphyrin,4 5 salphen,6 7 and pyridine8 subꢀ
stituted resorcin[4]arene cavitands. All these molecular conꢀ
structs feature a selective binding site in close proximity to an
organometallic catalytic site. They constitute a viable strategy
to achieve new selectivity and activity in transition metal
catalysis.9
Inspired by the wide range of oxidative transformations cataꢀ
lyzed by the polypyridyl RuIV=O species,10 we decided to
investigate the feasibility of covalently attaching a heteroleptic
terpyridine (tpy) RuII aqua complex at the upper rim of a
resorcin[4]arene cavitand, transꢀ1•RuII•(azpy)•OH2 (Figure
1). It is well known that Ru is a redox metal capable of dπꢀpπ
backꢀbonding, in addition to the conventional σꢀbonding.11 In
polypyridyl complexes, the RuIV=O high oxidation state can
be achieved from the initial RuIIꢀOH2 derivative through two
successive protonꢀcoupled electron transfer (PCET) processꢀ
es.12 A sacrificial chemical oxidant, i.e. iodosobenzene diaceꢀ
tate, can be used to mediate the electron transfer processes.
Figure 1. Structures of 2ꢀ(phenylazo)pyridine (azpy), metalloꢀ
resorcin[4]arene cavitand transꢀ1•RuII•(azpy)•OH2 and olefins 2,
3 and 4 used in the epoxidation assays.
We synthesized hexaamide cavitand 1 with a 2,2’:6’,2’’ terꢀ
pyridine ligand at its upper rim by condensation of the known
diamine cavitand 517 and the terpyridine aldehyde 618 in dioxꢀ
ane solution and using air as oxidant (Scheme 1). Hexaamide
cavitands aka. selfꢀfolding cavitands closely related to 1 were
previously described by Rebek et al.19 The seam of up to seven
intramolecular hydrogen bonds at the upper rim of hexaamide
cavitands stabilizes the “vase” conformation. These sizable
hosts feature an electron rich aromatic cavity suitable for the
inclusion of neutral and cationic organic/organometallic
guests.
In RuIV=O complexes, the ligand selection plays a key role in
tuning the redox potential, based on electronic effects. Catalyꢀ
sis performance and control of the oxygen transfer mechaꢀ
,
nisms in epoxidation reactions (radical or concerted)13 14 can
be optimized by the choice of ancillary ligand. Owing to its
strong electronꢀwithdrawing properties and redoxꢀactive naꢀ
ture,15 2ꢀ(phenylazo)pyridine (azpy) was selected as the ancilꢀ
lary ligand. Llobet et al. reported that 4•RuII•(azpy)•(OH2)
complexes (7 = tpy) display a single twoꢀelectron wave for the
RuII/RuIV couple that is shifted to high positive potentials.16
That is, azpy destabilizes the Ru(III) oxidation state and renꢀ
Scheme 1. Synthesis of the hexaamide cavitand 1, the meꢀ
tallocavitands and the reference complex.
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Compound 1 lacks symmetry and shows different signals for
each proton in its H NMR spectrum in CD2Cl2 solution. The
1
benzimidazole NH signal is a sharp singlet at δ = 12.36 ppm
(H2, Figure 2a), indicating its involvement in strong hydrogen
bonding. The six amide NH protons resonate as separate sinꢀ
glets in the downfield region of the spectrum (●, Figure 2a).
The unidirectional orientation of amide groups at the upper
rim of 1 renders the entire molecule chiral. It exists in solution
as a mixture of two conformational cycloenantiomers whose
interconversion requires the tautomerization of the imidazole
unit and concomitant rotation of the amide groups.19 The racꢀ
emization process is fast on the human timescale but slow on
the EXSY timescale resulting in a pattern of exchange crossꢀ
peaks between related NH signals. We calculated an energy
barrier of 16 kcal/mol for the cycloenantiomerization process.
A total of 16 singlets are observed for the aromatic protons of
the resorcin[4]arene core and the bridging walls (● and H1,8,
Figure 2). The terpyridine substituent has five distinct proton
signals owing to its free rotation through the connecting CꢀC
bond (H3ꢀ7, Figure 2). Because CD2Cl2 is a suitable guest for
the aromatic cavity of 1, the vase conformation is adopted in
this solvent. This is shown by the appearance of signals cenꢀ
tered at δ = 5.63 ppm (●, Figure 2a) that represent different
Figure 2. a) Downfield regions of the 1H NMR spectrum of caviꢀ
tand 1 in CD2Cl2 solution. See Scheme 1 for proton assignments;
b) solid state structure of the dimer of the terpyridine substituted
cavitand 1; the six included CH2Cl2 molecules are shown as space
filling models.
We assigned these signals to the protons of the tpy unit coorꢀ
dinated to RuIII. A six coordinate RuIII complex should be in
the S = 2 spin state and thus paramagnetic.23 However, other
dynamic processes occurring in solution like aggregation (vide
infra) or conformational exchange cannot be excluded as
additional sources of the observed signal broadening. The
solidꢀstate structure of 1•RuIII•Cl3 showed the tendency to
dimerize by filling the cavity of one monomer with the
tpy•RuIII•Cl3 moiety of another (Figure 3b). This type of arꢀ
rangement induces a conformational change in two bridging
aryl walls having vicinal diamides, which become equatorially
oriented. The two adjacent remaining bridging aryl walls, one
equipped with tpy•RuIII•Cl3 moiety and the other with a vicinal
diamide, maintain an axial orientation.
A suspension of 1•RuIII•Cl3 in an ethanol/water (4:1) mixture
was refluxed for 12 h with azpy24 in the presence of Et3N and
LiCl. This reaction is known to produce the exchange of two
chloro ligands by the bidentate azpy ligand with concomitant
reduction of RuIII to RuII. After the exchange of the nonꢀ
coordinated chloride anion by the more lipophilic PF6ꢀ counꢀ
terpart, the crude reaction mixture was purified by column
,
triplets for the methyne protons.20 21
Single crystals suitable for XꢀRay diffraction were grown from
a methylene chloride solution of 1. In the solid state, cavitand
1 also adopts the vase conformation and assembles into dimerꢀ
like capsules including six methylene chloride molecules
(Figure 2b). The two monomers of 1 involved in the dimer
assembly are cycloenantiomers (M/P).19, The seam of seven
22
hydrogen bonds present at the upper rim of 1 that is responsiꢀ
ble for the “vase” stabilization is clearly seen in the solid
structure (Figure 3a). The average distance between nitrogen
and oxygen in the five C=O···HN hydrogen bonds is 2.862 Å.
Two of the three intraꢀring hydrogen bonds are 0.1 Å shorter
than the two interꢀring counterparts. The two hydrogen bonds
involving the imidazole units have distances of 2.746 Å
chromatography.
The
monoꢀchloro
metallocavitand
[1•RuII•(azpy)•Cl]+ PF6ꢀ was isolated in 88% yield as a pink
solid and characterized by a set of high resolution spectra
1
(UVꢀVis, HRMS, H, 19F and 31P NMR) and differential pulse
voltammetry (DPV). Most likely, the isolated solid is comꢀ
posed of a mixture of the cisꢀ and transꢀ isomers (relative
orientation of the pyridyl N atom with respect to the central N
of the tpy) owing to the nonꢀsymmetric nature of the bidentate
(ImNH··O=C) and 3.114
Å (NH···NIm) between doꢀ
nor/acceptor atoms. Ruthenium was coordinated to cavitand 1
with RuCl3 in ethanol solution at reflux for 12 h (Scheme 1).
The octahedral 1•RuCl3 was isolated as a brown solid in 83%
yield and characterized by a highꢀresolution spectra and Xꢀray
crystallography. The 1H NMR spectrum of 1•RuIIICl3 in
CD2Cl2 displayed complex and broad signals. Of special interꢀ
est was the observation of four signals resonating at very high
field, in the range of δ = ꢀ3 to ꢀ8 ppm (SI).
ꢀ
ligand. Treatment of [1·RuII•(azpy)•Cl]+ PF6 with excess
AgPF6 in an acetone/water mixture (3 mL : 10 drops) at 60°C
for 12 h produced a precipitate of AgCl. Addition of water to
the filtered acetone solution afforded a brown solid of the aqua
ꢀ
complex [1·RuII•(azpy)•OH2]2+ (PF6 )2, probably a mixture of
isomers deriving from the staring material.25
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band centered at ≈ 3390 cmꢀ1 that was assigned to the OH
vibration in the RuII•OH2. This band was absent in the two
parent chloro complexes. The chloro and aqua metallocaviꢀ
tands displayed an additional broad band at ≈ 3240 cmꢀ1 asꢀ
signed to the vibration of NHs in hydrogenꢀbonded amides.8
The N=N stretching frequency of the coordinated ancillary
ligand was appreciably lower than for the free azpy ligand.
This is consistent with backꢀbonding donation from the metꢀ
al.11
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We performed preliminary epoxidation assays using
ꢀ
[1•RuII•(azpy)•OH2]2+ (PF6 )2 as catalyst and styrene 2 or
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Figure 3. a) Top view of the Mꢀcycloenantiomer of 1 in the solid
state. Included CH2Cl2 molecules are not shown. The distances
(Å) between heteroatoms involved in hydrogen bonds are reportꢀ
ed; b) Xꢀray structure of the dimer aggregate assembled by
1•RuIII•Cl3. Nonꢀpolar hydrogen atoms are omitted for clarity.
Sixꢀcoordinated RuII should be in the S = 1 spin state resulting
in a diamagnetic complex, which is expected to produce wellꢀ
resolved NMR spectra. However, the 1H NMR spectra in
transꢀβꢀmethyl styrene 3 as test substrates. The reactions were
performed at 298 K in NMR tubes with a solution of CD2Cl2
(550 µL) containing 3 µmol of substrate, 6 µmol of iodosoꢀ
benzene diacetate, 150 µmol of water to ensure the generation
of the sacrificial oxidant iodosylbenzene from the diacetate
iodobenzene precursor and 5 molꢀ% of the catalyst.26 The
1
progress of the reaction was monitored using H NMR specꢀ
troscopy by acquiring a series of spectra of the reaction mixꢀ
ture at regular time intervals. After 70 min, the analysis of the
reaction mixtures revealed that ~20% of the alkenes were
converted and the yield of the epoxides was ~15 %. At this
point, the sacrificial oxidant was consumed in a larger amount
than required for the ~20% conversion of the substrate and the
reactions slowed down significantly. Most likely, after two to
three turnovers the catalyst for the epoxidation reaction is
partially destroyed. We obtained similar results using the
CD2Cl2
solution
of
both
the
aqua
complex
ꢀ
[1•RuII•(azpy)•OH2]2+ (PF6 )2 and the parent monoꢀchloro
metallocavitand [1•RuII•(azpy)•Cl]+ PF6ꢀ displayed broad and
illꢀdefined signals. Remarkably, neither spectra showed the
upfieldꢀshifted signals observed in the paramagnetic precursor
1•RuIII•Cl3. This result provides support to the diamagnetic
nature of RuII in the two heteroleptic complexes.
1
The H NMR spectra of the two heteroleptic complexes feaꢀ
tured signals in the region of 4 ppm. These signals are asꢀ
signed to methine protons that experience a shielding effect
owing to the equatorial orientation of the bridging aryl walls in
resorcinarene cavitand. Both 2D NOESY and ROESY spectra
revealed the existence of cross peaks due to chemical exꢀ
change between the signals of the methine protons resonating
at δ = 4 ppm and those at δ = 5.6 ppm (SI). These results hint
to the existence of a fluxional chemical exchange process in
solution that is slow/intermediate in the chemical shift time
scale and involves multiple conformers of the chloro and aqua
metallocavitands featuring a partialꢀkite structure and producꢀ
ing the broadening of some their proton signals. A significant
ꢀ
reference aqua complex [7•RuII•(azpy)•OH2]2+ (ClO4 )2 as
catalyst. Increasing the amount of the cavitand catalyst to 10
molꢀ% led to an increase in the yield of the epoxide to ~ 50 %
after 50 min of reaction. Encouraged by these findings, we
investigated the epoxidation of the cisꢀβꢀalkyl styrene olefin 4
equipped with a terminal trimethyl ammonium function.27
Substrates bearing a trimethylammonium ‘‘knob’’ engage in
1:1 inclusion complexes with Znꢀsalphen monofunctionalized
cavitands that can accelerate the reaction of the bound guests.
6,7
Unfortunately, the amount of the epoxide produced from 4
using the metallocavitand as catalysts was almost identical to
that obtained using the neutral olefins. Taken together, these
results indicate that the covalent attachment of the
tpy•RuII•(azpy)•OH2 moiety to a selfꢀfolding hexamine caviꢀ
tand preserves the catalytic properties of the metal center but it
does not show signs of supramolecular catalysis.28 The small
reduction in catalytic efficiency observed for the cavitand
catalyst in the epoxidation of the cationic olefin 4 compared to
the neutral counterparts, 2 and 3, is assigned to putative comꢀ
plexes displaying binding geometries not suitable for intramoꢀ
lecular oxygen transfer.
1
difference between the two H NMR spectra of the heterolepꢀ
tic metallocavitands is the presence of a broad singlet resonatꢀ
ing at δ ≈ 1.7 ppm in the case of the aqua derivative. We asꢀ
sign this signal to the hydrogen atoms of the water molecule
coordinated to the RuII metal center. Unfortunately, all our
attempts to grow single crystals of [1•RuII•(azpy)•Cl]+ PF6ꢀ
ꢀ
and [1•RuII•(azpy)•OH2]2+ (PF6 )2 were unsuccessful. Neverꢀ
theless, the high resolution MALDIꢀMS (+) spectra of the two
complexes showed ion peaks for their corresponding monoꢀ
cationic species [M ꢀ PF6]+ and [M ꢀ 2PF6 ꢀ H]+, respectively,
with isotopic patterns that nicely coincide with the theoretical
ones (SI).
In conclusion, we report the synthesis and characterization of
the terpyridine functionalized selfꢀfolding cavitand 1. We
metallated the terpyridine moiety with ruthenium and customꢀ
ized the metal center for twoꢀelectron oxygen transfer reacꢀ
tions by preparing the corresponding RuII aqua derivative with
2ꢀ(phenylazo)pyridine as ancillary ligand. A limited catalytic
activity of the aqua metallocavitand was demonstrated through
the epoxidation reaction of simple olefins. The proximity
between the catalytic center and the cavitand binding site in 1
did not produce a supramolecular catalyst for the epoxidation
of 4.
The spectroscopic properties of the two metallocavitands were
compared to the reference counterparts, [7•RuII•(azpy)•Cl]+
ꢀ
ꢀ
ClO4 and [7• RuII•(azpy)•OH2]2+ (ClO4 )2. The latter complexꢀ
es were prepared following a reported procedure and using a
similar manifold of reactions as for the former.14 The UVꢀVis
spectra of the heteroleptic metallo cavitands displayed a charꢀ
acteristic MLCT (Metal to Ligand Charge Transfer) band,
centered at 529 nm for [1•RuII•(azpy)•Cl]+ PF6ꢀ and blueꢀ
shifted (502 nm) in the case of the aqua derivative (SI). This
behavior is in total agreement with the reported spectra for the
reference complexes. The IR spectra registered for
[1•RuII•(azpy)•OH2]2+ [PF6ꢀ]2 and its reference showed a broad
ASSOCIATED CONTENT
Supporting Information
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18 Fallahpour, R. A. Synthesis 2000, 1665ꢀ1667.
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19
Hooley, R. J.; Iwasawa, T.; Rebek, J., Jr. J. Am. Chem. Soc.
2007, 129, 15330ꢀ15339.
Experimental details and characterization data for new comꢀ
pounds. 1D and 2D NMR, MS, IR, UV/Vis spectra, DPV and
HPLC data of compounds and complexes.
Xꢀray crystallography data for 1
20
Moran, J. R.; Ericson, J. L.; Dalcanale, E.; Bryant, J. A.;
Knobler, C. B.; Cram, D. J. J. Am. Chem. Soc. 1991, 113, 5707ꢀ5714.
21 Azov, V. A.; Jaun, B.; Diederich, F. Helv. Chim. Acta 2004, 87,
449ꢀ462.
Xꢀray crystallography data for 1•RuIII•Cl3
22
P/M indicates the stereochemical designation of the axial
AUTHOR INFORMATION
Corresponding Author
chirality defined by the unidirectional sense of rotation of the amide
groups viewed from above the cavity. See Figure 3a.
9
23
Novotný, J.; Sojka, M.; Komorovsky, S.; Nečas, M.; Marek, R.
*Eꢀmail: pballester@iciq.es.
ORCID
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J. Am. Chem. Soc. 2016, 138, 8432ꢀ8445.
24
Baldwin, D. A.; Lever, A. B. P.; Parish, R. V. Inorg. Chem.
1969, 8, 107ꢀ115.
25
Pablo Ballester: 0000ꢀ0001ꢀ8377ꢀ6610
Notes The authors declare no competing financial interest.
ACKNOWLEDGMENT
All attempts to achieve the epoxidation of styrene using
[1·Ru(II)·azpy·Cl]+ PF6ꢀ as catalyst were unsuccessful. We considered
that the aqua complex [1·Ru(II)·azpy·OH2]2+ was not formed under
catalysis conditions. For this reason, the aqua complex was separately
prepared.
26
Aguilo, J.; Francas, L.; Bofill, R.; GilꢀSepulcre, M.; Garciaꢀ
Gobierno de España MINECO (CTQ2014ꢀ56295ꢀR and Severo
Ochoa Excellence Accreditation 2014−2018 SEVꢀ 2013ꢀ031),
FEDER funds (Project CTQ2014ꢀ56295ꢀR) and the CERCA
Programme / Generalitat de Catalunya are acknowledge for fundꢀ
ing. We thank Eduardo C. EscuderoꢀAdán, Xꢀray Diffraction Unit
of ICIQ, for help with the analysis of the Xꢀray crystallographic
data and Dr. María A. Sarmentero, Dr. Eddy Martin, Prof. Antoni
Llobet, Dr. Lydia Vaquer, ICIQ, for experimental assistance and
helpful discussions in the initial phases of the project. S. K. thanks
ICIQ for predoctoral fellowship.
Anton, J.; Poater, A.; Llobet, A.; Escriche, L.; Meyer, F.; Sala, X.
Inorg. Chem. 2015, 54, 6782ꢀ6791.
27 The transꢀisomer of 4 was insoluble in CD2Cl2.
28
The aqua metallocavitand also failed to act as supramolecular
catalysts in the oxidation of primary alcohols.
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13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
TOC
59x44mm (300 x 300 DPI)
ACS Paragon Plus Environment