Expanded campestarene hosts for tetra- and dinuclear uranyl(VI) complexes

Article abstract of DOI:10.1039/c8cc07269h

The first expanded campestarenes based on a keto-enamine scaffold have been constructed using uranyl acetate as a template. Macrocycles with six and eight aryl rings in their backbone host dinuclear and tetranuclear uranium(vi) species, respectively. The complexes were characterized by SCXRD, FT-IR, UV-vis, and NMR spectroscopy, and DFT calculations corroborated their structures.

Full text of DOI:10.1039/c8cc07269h

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Marilyn M. Olmstead, Alan L. Balch, Josep M. Poblet, Luis Echegoyenet al.
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Expanded Campestarene Hosts for Tetra- and Dinuclear Uranyl(VI)
Complexes†
Mohammad T. Chaudhry,a Francesco Lelj,*b and Mark J. MacLachlan*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
The first expanded campestarenes based on a keto-enamine ketimines) are markedly less prone to hydrolysis than
scaffold have been constructed using uranyl acetate as a template. aldimines.18,19 However, the preparation of ketimines often
Macrocycles with six and eight aryl rings in their backbone host requires harsh conditions, a catalyst, and/or azeotropic removal
dinuclear and tetranuclear uranium(VI) species, respectively. The of water.20
complexes were characterized by SCXRD, FT-IR, UV-Vis, and NMR
spectroscopy, and DFT calculations corroborated their structures.
Macrocycles have played a pivotal role in the development of
supramolecular chemistry and continue to inspire new
research. Schiff-base macrocycles in particular have been
intensely explored owing to their ease of preparation and their
ability to chelate metals.1 They are usually prepared from the
reaction of a diamine with an aromatic dialdehyde, and have
been investigated for host-guest chemistry,2–4 multimetallic
cluster growth,5–10 sensing,11–13 and self-assembly.13-16
Fig 1. General structure of a) generic 6-nitrosalicyl aldehyde
monomer, and the campestarene
enamine tautomers.
1 in the b) enol-imine c) keto-
In 2011, we reported campestarenes, flat macrocycles that
have 5-fold symmetry.16 Campestarenes
1
are formed
selectively in high yield via an in situ reduction and
condensation approach of single precursor, 6-
The linear uranyl dication is the most common form of
uranium(VI) in water.21 Its large radius, oxophilic nature, and
prevalence for high coordination numbers make uranyl an
attractive choice for templating large macrocycles. Although
the chemistry of the uranyl ion is well-developed, there are few
studies of U in macrocyclic chemistry.22–28
Here we report the first expanded campestarene
macrocycles based on hemi-salphen ketimine ligands
containing tetranuclear and dinuclear µ-OH uranyl(VI) ions in
the cavity. These complexes are very stable and illustrate the
use of uranyl as a template for macrocycle chemistry, with
potential applications in uranium sequestration and as
metallohosts.
a
a
nitrosalicylaldehyde derivative (Figure 1a). These macrocycles
show solvent- and temperature-dependent tautomerization
between a keto-enol and enol-imine form, as shown in Figure
1b-c.17
The internal cavity of campestarenes, with hydroxyl and
aldimine groups, appears capable of binding to ions and forming
multimetallic complexes.5,8 Unfortunately, our efforts to
develop coordination chemistry with campestarenes have been
frustrated by the sensitivity of the imine bonds to hydrolysis.
Replacing the aldehyde with a ketone may improve the stability
of the macrocycle since imines derived from ketones (i.e.,
a. Department of Chemistry, University of British Columbia, 2036 Main Mall,
Vancouver, British Columbia, V6T 1Z1, Canada.
b. La.M.I. and LaSCAMM INSTM Sezione Basilicata, Dipartimento di Chimica,
Università della Basilicata, via dell’Ateneo Lucano 10, 85100 Potenza, Italy.
†
Electronic Supplementary Information (ESI) available: Crystallographic and
spectroscopic data for 2a 2b
,
,
3a, and 3b are given. Select 1H NMR experiments, UV-
Vis studies and DFT computational data and analyses. CCDC: 1862463, 1862464,
1862465, 1862466. See DOI: 10.1039/x0xx00000x
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Monomers 6a-b were prepared in three steps from the of 2.592(4) Å. Axial U-O bonds are 1.769(4)–1.782V(4iew) ÅAr,ticnleoOrmnlinael
corresponding phenol derivative (Scheme S1). Bulky p-t-octyl for [UO2]2+ complexes .32
DOI: 10.1039/C8CC07269H
and p-t-butyl groups were selected to prevent aggregation and
enhance solubility in the target macrocycle. Although aldehyde
analogues rapidly condense to give campestarenes, ketones
6
are stable in solution and as solids. We explored the use of Lewis
acid catalysts (e.g., TiCl419,29 & Sc(OTf)330,31) to condense 6a into
macrocycles, but only low yields of oligomeric compounds were
obtained. Brønsted acid catalysis (e.g., p-toluenesulfonic acid in
n-BuOH) showed only trace amounts of macrocycle by mass
spectrometry (MS) after several weeks. Similar results were
obtained using toluene as a solvent with azeotropic removal of
water. Unable to isolate the ketimine analogue of macrocycle
we turned to metal templating.
1,
Figure 2. Solid-state molecular structure of 2a determined by
SCXRD. a) Top view and b) side view. Colors: white (H), black (C),
blue (N), red (O), and green (U). Solvent molecules and non-
hydrogen bonding hydrogen atoms omitted for clarity. Thermal
ellipsoids are shown at the 50% probability level.
From the crystal structure, the macrocycle in 2a adopts a
chair-like conformation with vertices of ca. 6.5 Å (Figure S22),
likely due to repulsion between the phenyl rings. This leads it to
have approximately Ci symmetry in the solid state. There is
intramolecular hydrogen bonding within the cavity (Figure 2a),
Scheme 1. Preparation of complexes 2 and 3 from monomer 6
Marks showed that condensation of 1,2-dicyanobenzene between O2…H2N2 and O3…H3N1. The long distances between
with UO2Cl2 did not form the expected uranyl phthalocyanine O3 and H3 imply localized hydrogen bonding, unlike
complex, but instead resulted in a pentameric complex, with the campestarenes, which have a continuous network of hydrogen
uranium(VI) center adopting
a pentagonal bipyramidal bonds. Interestingly, the arene groups of the macrocycle are in
geometry inside a superphthalocyanine ligand.27 Inspired by the keto-enamine tautomeric form, in which the proton is
this work, we investigated uranyl templation with our ligand situated on the nitrogen atom rather than on the oxygen atom
system.
Reaction of 6a with of 0.2 eq of UO2(OAc)2·2 H2O (Scheme
and aromaticity is lost in the aminophenol group.
C-O and C-N bond lengths (1.292(6)–1.315(6) Å and
1) in ethanol resulted in a dark solution. Analysis of the reaction 1.312(6)–1.232(7) Å, respectively) are intermediate between
mixture by matrix-assisted laser desorption ionization time-of- single and double bonds, which implies the negative charge of
flight (MALDI-TOF) MS showed the presence of two major the O,N,O chelate is delocalized along the arene system,
products with m/z = 2413.2 and 3531.4. The two products, common for the hemi-salphen motif.32,33 The uncoordinated C-
deep-purple 2a and red 3a, were successfully separated by O and C-N bond lengths correspond to C-O double and C-N
column chromatography. Single crystals of both 2a and 3a were single bonds, suggesting the keto-enamine form of 2a 33
This
.
obtained and were analyzed by SCXRD. By increasing the was unexpected because previous investigations of
proportion of UO22+ used from 0.2 to 0.5 equiv, yields improved campestarenes showed crystallization in the enol-imine form.17
from <1% to ca. 7% and ca. 20% for 2a and 3a, respectively.
Importantly, high concentrations (ca. 150 mM) are required to (R = H, phenyl substituents removed) and 2d (R = H) at different
form the macrocyclic complexes.
levels of theory in vacuo . Calculations on 2d confirmed that the
DFT calculations were undertaken on simplified models 2c
SCXRD data revealed that 2a is comprised of a ketimine complex belongs to the Ci point group, is µ-OH bridged rather
macrocycle with six arene rings in the backbone (Figure 2) than µ-O, and is not planar, though the extent of the chair-like
hosting a dimeric [(UO2)2(µ-OH)2]2+ species in the center, bound geometry is reduced compared to the SCXRD results. On the
by two O,N,O chelating groups from the macrocycle. The other hand, 78MWB/AVTZ/6-31g/M06 calculations on
a
compound is neutral with four ketone and two amino ligands complex including the meso-phenyl groups suggest deviation
coordinating the metal centers with two bridging µ-OH ligands. from planarity is due mainly to steric repulsion of the phenyl
µ-OH protons could not be observed in the diffraction data, but rings. Additionally, 2a is more tautomerically inert than
1
they appear at 11.32 ppm in the H NMR spectrum (DCM-d2). campestarenes, and exists only in the keto-enamine form.
Equatorial U-O bond lengths for the macrocyclic ligand are
Although 2a is not flat in the solid state, it is fluxional in
1
2.433(4) and 2.304(3) Å, similar in length to the bridging Oµ solution; H NMR spectroscopy (DCM-d2) shows C2h symmetry.
ligands. The uranium(VI) atoms adopt a slightly distorted The most diagnostic signals are for the meta protons on the
pentagonal bipyramidal geometry, with a long N-U bond length monomeric units, which appear as four doublets (4JHH = 2.2 Hz)
that integrate for 2 protons each, and a singlet integrating for 4
2 | J. Name., 2012, 00, 1-3
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protons. Resonances at 18.59 and 17.13 ppm in DCM-d2 are conducted. The eight arene rings in the backVbieownAerticolefOntlhinee
assigned to the ketimine NH protons. The singlet at 11.32 ppm macrocycle adopt a bent boat-like motif,DwOhI:i1c0h.1i0s3s9i/mC8ilCaCr0t7o26th9He
is assigned to the µ-OH protons. These assignments were antiaromatic compound cyclooctatetraene (Figure S23).
1
confirmed by H/D exchange with D2O (Figure S19). H NMR
While complex 3 seems to have approximate S4 symmetry in
spectroscopy of 2a at elevated temperatures (in 1,1,2,2- the solid state, DFT computations at the 78MWB/AVTZ/6-
tetrachloroethane-d2) gave improved resolution that allowed 31g/M06 and ωB97x levels of theory in vacuo confirmed this.
additional NMR signals to be assigned (Figure S21). A singlet at This model with R=H and lacking the meso-phenyl groups
6.87 ppm split into two doublets, consistent with the splitting showed
pattern expected for 2a experimental data, suggesting that the geometry of complex
. Furthermore,
a saddle-shaped geometry very close to the
.
3
The IR spectrum of 2a shows a strong [UO2]2+ νas band at 907 is less sensitive to peripheral substitution than
2
cm-1 which has significant contribution from a µ-OH bending computations on a model complex with µ-O bridging lacks the
mode. The two NH bending vibrations observed at 1530 and S4 symmetry and has a very different geometry compared to the
1515 cm-1, respectively, and the absence of a C=N νas at ca. 1600 solid-state structure of
3
(see SI). These results confirm that the
bridging groups are µ-OH.
We had difficulty interpreting the 1H NMR data of 3a
cm-1 also confirm the keto-enamine tautomeric form of 2a
UV-Vis spectroscopy of 2a in DCM showed an intense band
.
,
at 500 nm that is reminiscent of the intense absorption band possibly because the bulky t-octyl groups led to slow
observed at ~550 nm in campestarenes.17 Electronic interconversion of multiple conformations. Thus, we discuss the
spectroscopy in an array of solvents with varying dielectric 1H NMR data for 3b with t-butyl substituent. The 1H NMR
constants did not result in drastic changes to the spectra. This spectrum of 3b is simpler than that of 2b owing to the 4-fold
lack of solvatochromism seems to indicate the tautomeric symmetry of the macrocyclic complex in solution (Figure 4). The
inertness of 2a, in agreement with the calculations.
Complex 2b was also prepared and structurally because of intramolecular hydrogen bonding.
characterized; it showed similar properties to complex 2a resonance at 12.18 ppm is assigned to the four µ-OH ligands.
NH protons (δ 18.45 ppm in DCM-d2) are highly deshielded
A
broad
.
SCXRD of 3a revealed that it is a tetranuclear uranium The meta protons of the aryl groups in the macrocycle appear
complex captured inside a ketimine macrocycle having eight as four doublets (4JHH = 2.3 Hz) and the t-butyl groups appear as
aryl rings (Figure 3). The complex adopts a “saddle-like” two singlets at 0.79 and 0.74 ppm. The 1H NMR spectrum of 3a
conformation with approximate S4 symmetry. Each of the four in DMSO-d6 (Figure S20) has similar resonances to those shown
uranyl(VI) groups in 3a is chelated by an O,N,O group of the in Figure 4 for 3b in DCM-d2, but the low solubility of complexes
macrocycle, similar to the bonding mode observed in
2
and
3 in DMSO made analyses difficult.
compounds 2a and 2b. The uranyl groups are bridged by four µ-
FT-IR spectroscopy of 3a/b shows very similar patterns to
OH groups, the H atoms of which were able to be modeled for 2a/b, except with a stronger νas [UO2]2+ at 925 cm-1. Based on
complex 3a. The U-Oax, U-Oµ, U-N, and hydrogen bond lengths the strong N-H stretch and the absence of C=N stretches, 3a and
are all similar to those observed in 2a (Table S2).
3b are also in the keto-enamine tautomeric form. The UV-Vis
Complex 3b with terminal t-butyl groups was also spectra of 3a/b are red shifted by about 10 nm relative to 2a/b
;
structurally characterized by SCXRD. The solid-state structure of absorption maxima for 3a and 3b are at 515 and 510 nm,
3b is very similar to 3a, with similar bond lengths and angles. respectively.
Figure 3. Crystal structure of 3a, a) top view and b) side view.
Thermal ellipsoids are shown at the 50% probability level.
Figure 4. 1H NMR spectrum of complex 3b (DCM-d2, 25 °C, 400
MHz). * = DCM-d2
The “saddle-like” structure of
3 allows for the formation of
two binding pockets within the complex, the external
hydrophobic one being ~1.1 nm in diameter. The internal cavity
resembles a metallocrown,34 with four oxygen atoms in
alternating up-down geometry. Long U-Oµ bond lengths
(2.315(5)–2.328(5) Å) and Oµ-Oµ distances (2.984(8) Å) creates
a cavity that can potentially host a small cation, such as Li+ or
Na+. With this in mind, host guest studies are currently being
The [(UO2)4(OH)4]4+ motif found in
3 is rare, and only a few
compounds in the CCSD have similar binding modes, most of
which are purely inorganic salts of [UO2]2+. The only related
example of a U4 cluster was published by Thuéry, where a
calix[8]arene was reacted with UO2(NO3)2·6H2O to give a
macrocyclic U4 cluster with O,O,O-tridentate environments.35,36
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The geometry of that complex is similar to
3
, with six phenoxo
Nabeshima, Inorg. Chem., 2007, 46, 2959–2961.
View Article Online
ligands coordinating to a (UO2)4(µ-O/OH)4 cluster. In their case, 11
the macrocycle templated the formation of the (UO2)4 cluster,
in contrast with our results where the macrocycle is templated 12
by the uranyl(VI) cation.
S. J. Malthus, S. A. Cameron and S. DBOroI:o1k0e.1r0, I3n9o/Crg8.CCCh0e7m26.9, H
2018, 57, 2480–2488.
Y. Sakata, C. Murata and S. Akine, Nat. Commun., 2017, 8,
16005.
Complexes
2
and
3
are very stable and form under mild 13
M. Nakamura, Y. Kaneko, E. Nishibori and T. Nabeshima,
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conditions, suggesting that the macrocycles themselves are
promising candidates for uranyl sequestering agents. Attempts 14
to demetallate 2a or 3a to liberate the free macrocycles were 15
unsuccessful. The complexes are stable for several months in
DCM-d2 or DMSO-d6.
16
S. Guieu, A. K. Crane and M. J. MacLachlan, Chem.
Commun., 2011, 47, 1169–1171.
In conclusion, we have discovered expanded campestarene
macrocycles templated by uranyl(VI). The macrocycles host 17
interesting di- and tetranuclear [UO2]2+ complexes in their
interior. Held together with µ-OH bridges, these complexes are 18
highly chemically and thermally stable. The smaller dinuclear
Z. Chen, S. Guieu, N. White, F. Lelj and M. J. MacLachlan,
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2
is housed in a hexameric macrocyclic ligand. The larger 19
is bound within the cavity of an
U4 macrocyclic complex
3
octameric macrocycle with a saddle-like geometry and could 20
have interesting host-guest properties due to its large
hydrophobic binding pocket along with its highly 21
electronegative internal cavity.
We thank NSERC for funding (CGSM Fellowship to MTC;
Discovery Grant and NSERC CREATE NanoMat to MJM). MTC
thanks Dr. M. Soto for general discussions, and Dr. B. Patrick for 22
discussion on SCXRD. FL thanks INSTM for financial support
during his leave to UBC. We thank WestGrid and 23
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TOC IMAGE
Expanded campestarene macrocycles with keto-enamine
structures host dinuclear and tetranuclear hydroxo-bridged
uranyl(VI) clusters in their interiors.
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