A stereodynamic and redox-switchable encapsulation-complex containing a copper ion held by a tris-quinolinyl basket

Article abstract of DOI:10.1039/c2cc30339f

We investigated the coordination of Cu(i)/Cu(ii) ions to chiral basket (S₃)-1. The results of both experimental and computational studies suggest the formation of a copper redox-switchable system capable of entrapping CH₃CN.

Full text of DOI:10.1039/c2cc30339f

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COMMUNICATION
A stereodynamic and redox-switchable encapsulation-complex containing
a copper ion held by a tris-quinolinyl basketw
a
a
b
b
´
Sandra Stojanovic, Daniel A. Turner, Andrew I. Share, Amar H. Flood,
Christopher M. Hadad and Jovica D. Badjic*
a
a
´
Received 15th January 2012, Accepted 19th March 2012
DOI: 10.1039/c2cc30339f
We investigated the coordination of Cu(I)/Cu(II) ions to chiral
basket (S3)–1. The results of both experimental and computa-
tional studies suggest the formation of a copper redox-switch-
able system capable of entrapping CH3CN.
Improving the characteristics of transition-metal catalysts1
may require placement of the metal ion within a concave
host.2 Despite the potential of such encapsulation catalysis,
there has been a paucity of functional cavitands3 available for
investigating the concept. We recently introduced a family of
cavitands, gated molecular baskets,3b–f for controlling the
kinetics of molecular encapsulation3b and promoting chemical
reactions in gated environments.3d These hosts comprise a
bowl-shaped platform with revolving aromatic rings as gates
at the rim of the basket (Fig. 1). The gates were designed to (a)
interact via hydrogen bonding3f or (b) coordinate to Cu(I)/
Ag(I) cations.3c,e In particular, the coordinated cation resides
in either tetrahedral or trigonal ligand fields whereby the
propeller-like gates are constrained to assume either left- (L)
or right-handed (D) orientations (Fig. 1). We recently demon-
strated3e that the helicity in this stereodynamic coordination
environment4 is controllable: a stereogenic center (with R or S
configuration) at the ‘‘hinge’’ position directs the twisting of
the gates at the rim, thereby ensuring the preponderance of
one diastereomeric form (Fig. 1). The current study builds on
these observations and we initiated it to (a) investigate the
coordination of Cu(II) to basket (S3)–1 (Fig. 1),5 (b) evaluate
the scope of static-to-dynamic chirality transfer as monitored
by exciton-coupled circular dichroism (ECCD),6a,b and (c)
examine the Cu(I)/Cu(II) conversion in this coordination
system.4 Furthermore, the impetus for the work is ultimately
to learn about the structure and dynamics of copper-containing
molecular baskets in order to facilitate studies of their catalytic
function, e.g., the activation of molecular oxygen7 in a dynamic
confined space.
Fig. 1 Chemical structure of (S3)–1 basket and its 1H NMR spectrum
(400 MHz, 298.0 K) in CDCl3 (top). Top view of energy-minimized
Cu(I)–(S3)–1 (MMFF, Spartan) with quinolinyl gates, assuming D
handedness.
On the basis of Karlin and co-workers’ systematic study of
cupric ion coordinating to tetradentate tripodal ligands,5 we
envisaged that basket (S3)–1 should bind to Cu(II) to form a
complex with three, or perhaps two, quinolines coordinating
to the metal. In this scenario, acetonitrile (as the solvent)
would occupy the remaining coordination sites relative to
the complex’s coordination number (CN) of 5–6. Importantly,
the quinoline chromophore has a strong transition electric
moment (l = 233 nm, e = 57 000 MÀ1 cmÀ1) polarized along
the long axis of the aromatic ring (Fig. 2A),6c which should give
rise to bisignate ECCD6a,b spectra for these chiral complexes.
First, we used an established synthetic methodology for
preparing enantiomerically pure (S)-1-(quinolin-3-yl)ethan-
amine 8 (Scheme S1, ESIw). This amine was condensed with
a previously reported tris-anhydride 93c in toluene to give
1
basket (S3)–1 (Scheme S2, ESIw). H NMR analysis of (S3)–1
(Fig. 1) reveals a C3 symmetric molecule with gates revolving
at a sufficiently high rate (198–300 K, Fig. S15, ESIw) about
the asymmetric H–C(CH3) unit. On the basis of our previous
study,3e we hypothesized that the coordination of Cu(II) to the
quinoline moieties in (S3)–1 would drive these chromophores
into a right-handed (D) propeller. Thus, in each Cu(II)–(S3)–1
quinoline arm, the C–H bond should become eclipsed with the
a Department of Chemistry, The Ohio State University,
100 W. 18th Avenue, Columbus, OH 43210, USA.
E-mail: badjic@chemistry.ohio-state.edu; Tel: +1 11 614 247 8342
b Department of Chemistry, Indiana University, Bloomington,
IN 47405, USA
w Electronic supplementary information (ESI) available: Synthetic
procedures, spectroscopic and computational studies. See DOI:
10.1039/c2cc30339f
c
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Fig. 2 (A) The interaction of the transition electric moments from the
quinoline arms should give a negative ECCD couplet6 in Cu(II)–(S3)–1.
(B) Circular dichroism spectra of (S3)–1 (2.63 mM) obtained upon an
incremental addition (0.1–4.0 molar equivalents) of Cu(BF4)2Á6H2O.
adjacent OQC–N bond for, perhaps, relieving some steric
strain3e and a favourable alignment for the CQO group3c,e
(Fig. 1). Under these circumstances, the transition electric
moments (l E 233 nm) of the quinolines would interact
through space (Fig. 2A) and give rise to a bisignate ECCD curve:
the counter-clockwise disposition of the projected exciton axes
should result in a negative coupling, hence with the appearance of
a positive Cotton effect (CE) at shorter and a negative CE at
longer wavelengths.6b Indeed, incremental addition of Cu(BF4)2Á
6H2O to (S3)–1 in acetonitrile revealed the formation of a strong
and negative excitonic couplet centered at 239 nm (Fig. 2B);
specifically, the CD spectrum of such formed Cu(II)–(S3)–1
Fig. 3 (A) Five-coordinate complexes 2 and 3 (MQCu2+) were
energy minimized with density functional theory (RI-BHLYP/SV(P),
TZVP) to reveal a greater thermodynamic stability of the square-
pyramidal 3 (see ESIw). (B) CPK representation of square-pyramidal 3
(top-view) showing three juxtaposed hydrogens Hf (in green)
at position
8 of the quinolinyl rings. (C) Energy-minimized
RI-BHLYP/SV(P),TZVP complex 4 (MQCu+).
comprised a positive CE at 230 nm (De1 = 467 MÀ1 cmÀ1
)
followed by a negative CE at 249 nm (De2 = À373 MÀ1 cmÀ1).
Importantly, the large A (|De1| + |De2|) value of 840 indicates
a through-space interaction of the quinoline chromophores
with a contribution from all three ECCD couplets.4 The changes
in the UV-vis spectrum of (S3)–1, obtained upon the addition of
Cu(BF4)2Á6H2O, were much less dramatic (Fig. S16, ESIw and/or
Fig. 5). In particular, a broad band appearing at 234 nm is likely a
composite of p–p* transitions corresponding to the phthalimide
platform (B225 nm, e E 2.5 Â 105 MÀ1 cmÀ1 and B245 nm, e E
(Fig. 3A). Importantly, the quinolines in 3 are tilted (f = 421,
Fig. 3A), forming a small hydrophobic pocket at the top of Cu(II)
with the quinolines’ Hf atoms (Fig. 3B) being separated by only
2.4–2.8 A. Placing an acetonitrile ligand at the top position would,
however, require a greater separation of the Hf atoms5 and thereby
additional tilting of the aromatic rings, relative to what was
obtained for trigonal bipyramidal 2 (d = 4.4 A and f = 691,
Fig. 3A). It is worth mentioning that the Cu–Nquin bonds in 3 are
shorter (2.076–2.103 A) than in 2 (2.181 A), which perhaps, among
other factors, contributed to the computed difference in stability.
The pattern and the intensity of d–d absorptions (650–1000 nm
region), in the electronic spectrum of Cu(II) complexes, are
diagnostic for the coordination geometry around the metal
center.5 These bands were, however, absent in the UV-vis
spectrum of our Cu(II)–(S3)–1 (Fig. S19, ESIw). Alternatively,
we anticipated that the electron paramagnetic resonance
(EPR) spectrum of Cu(II) (d9 electronic state, S = 1/2), within
the Cu(II)–(S3)–1 complex, should report on the nature of the
coordination sphere.9 Indeed, if the geometry around the
cupric ion is square pyramidal, then the spin–orbit interaction
between its ground dx2Ày2 and excited states gives rise to a
normal/axial EPR signature with equivalent x and y axes and
two g tensor values: gII = gz 4 2.1 4 g> = gx, gy 4 2.0.9c As
a result of the unpaired electron in Cu(II) interacting with the
nuclear magnetic spin of Cu(II) (I = 3/2), there should also
appear four hyperfine lines (2I + 1) with the coupling constant
(AII) in the range of 158–200 G. Clearly, our EPR experimental
data (Fig. 4A) suggest the sole formation of complex 3 in
solution (gII = 2.31 4 g> = 2.07 and AII = 166 G).9b
1.7 Â 105 MÀ1 cm
)
À1 3e and the quinolinyl groups (B233 nm).6c
Importantly, the presence of an isosbestic point at 234 nm
(CD titration, Fig. 2B) is a good indication of two chiral species
contributing to the equilibrium. Indeed, the binding isotherm
(Fig. S17, ESIw) is in line with the formation of a 1 : 1 complex:
multivariate factor analysis of the CD data (ReactLab software)
suggests a strong affinity of Cu(II) for complexation by (S3)–1
(Ka 4 107 MÀ1, Fig. S17, ESIw). The stoichiometric 1 : 1 ratio of
Cu(II) and (S3)–1 was also confirmed by mass spectrometric
measurements (ESI-TOF, Fig. S18, ESIw) whereby the highest
intensity peak appeared at 577.63 amu corresponding to the
doubly-charged Cu(II)–(S3)–1 without any CH3CN.
What is the coordination number of the Cu(II) cation within
the Cu(II)–(S3)–1 complex? With all three quinolines coordinated
to the metal, one anticipates Cu(II)–(S3)–1 to correspond to either
trigonal bipyramidal 2 (CN = 5) or square-pyramidal 3 (CN = 5,
Fig. 3A).5 We performed some geometry minimizations,
with density functional theory (DFT)8 at the RI-BHLYP/
SV(P),TZVP8 level of theory, for the C3 symmetric 2 and C1
symmetric 3 (Fig. 3A) and noted that the square-pyramidal
complex has a greater thermodynamic stability (2.0 kcal molÀ1).
In square-pyramidal 3, the basal plane is comprised of three
quinolinyl groups and one acetonitrile, while another encapsulated
CH3CN completes the coordination sphere around the copper
The affinity of soft Cu(I) for coordinating (S3)–1 is comparatively
1
small: a nonlinear least-squares analysis of the H NMR titration
data revealed Ka of 4.1 Æ 0.3 Â 104 MÀ1 (Fig. S21, ESIw) for the
formation of 4 (Fig. 3C). As expected,3c the cuprous ion resides in a
c
4430 Chem. Commun., 2012, 48, 4429–4431
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Finally, we computed the UV-vis and CD spectra of DFT-
minimized 3 (Fig. 5) using time-dependent density functional
theory (TD-BHLYP/SV(P),TZVP).8 There is excellent agree-
ment between the experimental and computed spectra of 3,
thereby corroborating the complex’s absolute configuration as
predicted by the exciton chirality method.6a In particular, the
accurate prediction of the optical rotatory strengths of this
large (145 atoms) and C1 symmetric complex indicates a high
quality of the wave function generated with this level of
theory11 as well as conformational stability of the complex.
Given the proclivity of baskets for forming both Cu(I) and Cu(II)
complexes, there is a potential for examining these compounds as
encapsulation catalysts.12 Our studies are now directed toward
investigating the activation of O27b in a controllable and confined
environment that these baskets provide.
Fig. 4 (A) EPR spectrum of square-pyramidal complex 3 in CH3CN at
77 K. (B) Cyclic voltammograms (two subsequent scans at a rate of
0.2 V sÀ1) of complex 3 (1.0 mM) in degassed CH3CN; each measurement
was conducted with 0.1 n-BuNPF6 and a glassy carbon working electrode.
This work was financially supported with funds obtained
from the National Science Foundation (CHE-1012146 to JDB and
CHE-084441 to AHF) and by the Department of the Defense,
Defense Threat Reduction Agency (HDTRA1-11-1-0042). The
content of the information does not necessarily reflect the position
or the policy of the federal government, and no official endorse-
ment should be inferred. We thank Dr R. P. Pandian (OSU) for
assisting with EPR spectroscopic measurements.
Fig. 5 Experimental (black) and computed TD-BHLYP/SV(P),TZVP
(red)8 UV-vis (left) and CD (right) spectra of compound 3. The blue sticks
are computed electronic transitions that were subjected to Gaussian
broadening (0.3 eV) and wavelength shift (À0.7 eV) for generating the
theoretical spectra (see ESIw).
Notes and references
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tetrahedral ligand field with a coordinated molecule of
acetonitrile pointing to the basket’s interior (Fig. S23 and
S24, ESIw). The chirality transfer is operating in this system as
well (f = 571, Fig. 3C), with a strong and negative excitonic
couplet centered at 239 nm (Fig. S22, ESIw).
Redox-driven interconversion between complexes 3 and 4
(Fig. 3) should involve a reorganization of the coordination
sphere about the copper ion10 and, perhaps, the formation of two
unstable complexes square-pyramidal-Cu(I) and tetrahedral-
Cu(II) in accordance with a square-scheme mechanism.10b The
cyclic voltammetry (CV) of square-pyramidal 3 (1.0 mM,
CH3CN) showed four primary waves a–d (0.2 V sÀ1, Fig. 4B);
for more details, see primary copper-based redox processes and
switching between 3 and 4 in ESI.w The phthalimide moieties are
reduced/oxidized at waves b/c to their radical anions (Fig. 4B),10c
and the peak intensities and shapes do not differ from CVs of the
empty basket (Fig. S25, ESIw). Waves a/d are, however, associated
with the copper ions, as seen in the titration of Cu(II) into a solution
of the cage (Fig. S25w). Peak a is assigned to a Cu(II) reduction9b of
complex 3 while peak d corresponds to the oxidation of the
cuprous ion of complex 4. The peak potentials (Eapc = À0.96 V and
Edpa = À0.27 V) are similar to Cu(I) complexes of tris-
(2-pyridyl)methanamine9b and are more cathodic than for funnel-
like complexes.10a The large difference in redox potentials of
690 mV between peaks a and d is, however, consistent with the
changes in geometry and coordination number seen for other
4-coordinate Cu(I) and 5-coordinate Cu(II) complexes.10a The
formation of intermediate complexes10b was not observed at faster
scan rates (0.5–20 V sÀ1, Fig. S28, ESIw). The a/d peak intensities
are less than 1/3 those of the b/c peaks, suggesting slower hetero-
geneous electron transfer to and from the copper ions than the walls
of the cage, nevertheless, the first and second cycles of the CVs are
identical (Fig. 4B), indicating a chemically reversible process.
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c
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Chem. Commun., 2012, 48, 4429–4431 4431