Luminescent, enantiopure, phenylatopyridine iridium-based coordination capsules

Article abstract of DOI:10.1021/ja309031h

The first molecular capsule based on an [Ir(ppy)₂]⁺ unit (ppy = 2-phenylatopyridine) has been prepared. Following the development of a method to resolve rac-[(Ir(ppy)₂Cl)₂] into its enantiopure forms, homochiral Ir₆L₄ octahedra where obtained with the tritopic 1,3,5-tricyanobenzene. Solution studies and X-ray diffraction show that these capsules encapsulate four of the six associated counteranions and that these can be exchanged for other anionic guests. Initial photophysical studies have shown that an ensemble of weakly coordinating ligands can lead to luminescence not present in comparable mononuclear systems.

Full text of DOI:10.1021/ja309031h

Communication
pubs.acs.org/JACS
Luminescent, Enantiopure, Phenylatopyridine Iridium-Based
Coordination Capsules
Oleg Chepelin,† Jakub Ujma,† Xiaohua Wu,† Alexandra M. Z. Slawin,‡ Mateusz B. Pitak,§ Simon J. Coles,§
Julien Michel,† Anita C. Jones,† Perdita E. Barran,† and Paul J. Lusby*,†
†EaStCHEM School of Chemistry, University of Edinburgh, The King’s Buildings, West Mains Road, Edinburgh EH9 3JJ, U.K.
‡EaStCHEM School of Chemistry, University of St. Andrews, Purdie Building, St. Andrews, Fife KY16 9ST, U.K.
§UK National Crystallography Service, School of Chemistry, University of Southampton, Southampton, SO17 1BJ, U.K.
S
* Supporting Information
It was initially envisaged that [Ir(ppy)2]+ could be used in an
ABSTRACT: The first molecular capsule based on an
[Ir(ppy)2]+ unit (ppy = 2-phenylatopyridine) has been
prepared. Following the development of a method to
resolve rac-[(Ir(ppy)2Cl)2] into its enantiopure forms,
homochiral Ir6L4 octahedra where obtained with the
tritopic 1,3,5-tricyanobenzene. Solution studies and X-ray
diffraction show that these capsules encapsulate four of the
six associated counteranions and that these can be
exchanged for other anionic guests. Initial photophysical
studies have shown that an ensemble of weakly
coordinating ligands can lead to luminescence not present
in comparable mononuclear systems.
analogous fashion to cis-protected square planar complexes,
such as [Pd(en)]2+ or [Pt(dppp)]2+,9 and that the ppy ligands,
arranged in a C,C-cis-N,N-trans orientation, would sufficiently
labilize the exchangeable sites to facilitate self-assembly.10
Targeting an M6L4 octahedron,11 at first we chose to explore
the reaction of tris(4-pyridyl)triazine with the known iridium
compound, [Ir(ppy)2(CH3CN)2]OTf. As no evidence could be
gathered for the formation of the intact architecture, we
decided to instead investigate the self-assembly of the same Ir
starting material with 1,3,5-tricyanobenzene (tcb). While it was
anticipated that acetonitrile substitution from [Ir-
(ppy)2(CH3CN)2]OTf by tcb to generate the M6L4 assembly
should be entropically, if not enthalpically, favored, initial
experiments suggested only partial displacement. Instead, what
is presumed to be [(Ir(ppy)2OTf)2],12 prepared by treating
[(Ir(ppy)2Cl)2] with silver triflate in C2H4Cl2, was reacted with
tcb in the same solvent (Scheme 1, step a). While the product
gave a relatively complex 1H NMR spectrum (see the
Supporting Information, Figure S1a), nanoelectrospray ioniza-
tion mass spectrometry (n-ESI-MS) showed an intense 2+ peak
at 2106 m/z (see Figure S2a), which matched the predicted
isotope pattern for [1·4OTf]2+. A 1H DOSY NMR spectrum of
the product also showed that the multiple aromatic signals
possessed similar diffusion coefficients (see Figure S3a). This
led us to postulate that 1·6OTf is formed as a complex mixture
of diastereoisomers13 when the synthesis commences from
racemic [(Ir(ppy)2Cl)2] (Scheme 1, route 1), and thus tcb does
not efficiently transmit stereochemical information between
adjacent metal centers.14
he combination of transition metal ions and geometrically
Tcomplementary, multitopic bridging organic ligands has
led to the preparation of numerous molecular capsules and
cages.1 These assemblies possess well-defined internal cavities
that promote the ingress of guest molecules so that interesting
functions, such as catalysis or the stabilization of reactive
species, may be observed.1,2 By and large, the transition metal
ions within these systems have played solely a structural role,
offering advantages such as predictable, well-defined coordina-
tion preferences and bond strength.3 However, transition
metals and their complexes often possess many other notable
features, such as interesting photophysical properties; arguably
the most well-known class are the poly(pyridyl) complexes of
ruthenium(II),4 while more recently, analogous cyclometalated
C^N iridium(III) complexes have found widespread use as
luminescent biological probes5 and as dopants in organic light-
emitting devices.6 Although Newkome has described the
synthesis of several very elegant metallocycles which feature
[Ru(terpy)2]2+ connections,7 the use Ru poly(pyridyl) or
cyclometalated Ir complexes as structural components in
metallosupramolecular assemblies, in particular polyhedral
architectures, remains rare.7,8 Herein, we report the first
molecular capsule based on an [Ir(ppy)2]+ (ppy = 2-
phenylatopyridine) motif and demonstrate that incorporation
of this unit into a multimetallic array leads to a significant
luminescence enhancement with respect to a comparable
mononuclear complex, thus paving the way to capsules with
light-harvesting function and the development of devices with
emergent luminescent properties.
To eliminate the problem of mixed stereoisomer formation,
we have developed a preparative method to resolve rac-
[(Ir(ppy)2Cl)2] into its enantiopure forms,15 which involves
chromatographic resolution of serine complexes (Scheme 1,
steps b and c).16 We have found that the most efficient method
to obtain both enantiomers (which, as expected, exhibit mirror
image circular dichroism (CD) spectra; see Figure S4) is to
resolve with both D- and L-serine, as the similarity in Rf values
on silica ensures that only the faster-running diastereoisomer
can be obtained in significant quantities. Both enantiomers
crystallize in the chiral space group P212121 upon diethyl ether
Received: September 11, 2012
Published: November 8, 2012
© 2012 American Chemical Society
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Communication
EXSY NMR experiments have shown that the exo and endo
triflates are in slow exchange and that the activation barrier for
this process is 18 kcal mol−1 (see Figure S9).
Scheme 1. Synthesis of Heterochiral and Homochiral
Hexanuclear Iridium Assemblies
a
Corroboration of the capsule’s structure (Figure 1) was
obtained from X-ray diffraction (XRD) using single crystals
Figure 1. X-ray crystal structures of [4OTf⊂Δ-1]2+. Color code: Ir,
magenta; C(ppy), dark blue; C(tcb), dark green; C(OTf), gray; N,
light blue; O, red; S, yellow; F, light green.
a
Conditions: (a) (i) AgOTf, C2H4Cl2, RT, 2 h; (ii) tcb, C2H4Cl2, RT,
4 h, 48% (starting from rac-[(Ir(ppy)2Cl)2]). (b) (i) D-Serine,
NaOMe, MeOH, 313 K, 16 h; (ii) 1 M HCl, MeOH, RT, 10 min, 50%
(from rac-[(Ir(ppy)2Cl)2]). (c) L-Serine, NaOMe, MeOH, 313 K, 16
h; (ii) 1 M HCl, MeOH, RT, 10 min, 37% (from rac-[(Ir(ppy)2Cl)2]).
(d) (i) AgOTf, C2H4Cl2, RT, 2 h; (ii) tcb, C2H4Cl2, RT, 5 h, 57%
(starting from Λ-[(Ir(ppy)2Cl)2]). (e) (i) AgOTf, C2H4Cl2, RT, 2 h;
(ii) tcb, C2H4Cl2, RT, 4 h, 68% (starting from Δ-[(Ir(ppy)2Cl)2]).
grown from benzene diffusion into a saturated dichloroethane
solution of Δ-1·6OTf.17,18 The solid-state structure supports
the solution structure, [4OTf⊂Δ-1]2+, showing triflates located
in each of the octahedron “windows”. For two encapsulated
triflates (with S labels 3 and 4), the S−C axis points toward the
center of these vacant windows (or, if viewed as a truncated
tetrahedron, toward the vertex), forming a series of close
contacts between the triflate oxygens and the hydrogen atoms
attached to tcb and the ortho pyridyl positions. The other
encapsulated anions (with S labels 1 and 6) are positioned in a
slightly more “side-on” fashion, with the cage forming short
contacts between two of the oxygen atoms and one of the
fluorines. It seems probable that, in solution, these two distinct
triflate co-conformations participate in a low-energy exchange
process, thus explaining the presence of a single “encapsulated”
signal in the 19F NMR spectrum at room temperature.
To explore whether the structure and, in particular, the
encapsulated anions are preserved under MS conditions, ion-
mobility measurements have been undertaken.19 For the ions at
2106 m/z, both samples gave superimposable arrival time
distributions, consistent with a single species/conformation
(see Figure S10). The observed rotationally averaged collision
cross sections for Λ- and Δ-1 were found to be 551 and 543 Å2,
respectively, virtually within error of each other. To validate
these, ion-mobility simulations have been carried out using
structures derived from the XRD data. The values calculated
using either the exact hard-spheres scattering or trajectory
method, 555 and 551 Å2, respectively, agree remarkably well
with the experimental data, suggesting that the counteranions
remain within the capsule upon ionization into the gas phase.
Examples of metallocapsules which encapsulate multiple
species of the same charge are very rare.20 Clearly, the strong
Coulombic attraction between the positively charged Ir vertices
and negative counteranions, alongside the series of CH···X (X
diffusion into saturated dichloromethane solutions; the solid-
state structures (see Figure S5) reveal that the compound
isolated following resolution with D-serine is Λ-[(Ir(ppy)2Cl)2],
and that from L-serine is Δ-[(Ir(ppy)2Cl)2] (Scheme 1, steps b
and c).17
When the self-assembly was commenced from either Λ- or
Δ-[(Ir(ppy)2Cl)2] (Scheme 1, routes 2 and 3), a precipitate
was observed shortly after the addition of tcb to what are
presumed to be Δ- and Λ-[(Ir(ppy)2OTf)2], which was then
isolated after a few hours of stirring at room temperature. We
1
were delighted that the H NMR spectra of the two products
not only were indistinguishable (see Figure S1b,c) but also
clearly showed single species with the number and ratio of tcb-
to-ppy signals consistent with the sole formation of Λ- and Δ-1.
CD confirmed that the two compounds were enantiomers (see
Figure S4). The mass spectra also showed essentially a single
species at 2106 m/z, which corresponds to the doubly charged
[1·4OTf]2+ (see Figure S2b,c). The surprising lack of other
charge states in the mass spectra of Λ- and Δ-1·6OTf prompted
us to look at the 19F NMR spectra (see Figure S6a), which
showed two distinct signals at −77.45 and −79.04 ppm in a
ratio of 1:2, respectively. Addition of excess NBu4OTf resulted
in an increase in the intensity of the peak at −77.45 ppm (see
Figure S6b), strongly suggesting that the peak at −79 ppm is
due to encapsulated triflates. Other NMR techniques (HOESY
and DOSY; see Figures S7 and S8) confirm this assignment and
thus support the solution structure [4OTf⊂Δ-1]·2OTf. Further
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Communication
= O, F) hydrogen bonds (vide supra), is enough to overcome
the repulsion between same-charge ions. It could also be that
the CF3 groups, which are “meshed” together within the center
of the cavity (C−C distances between encapsulated OTf
counteranions range from 4.93 to 6.05 Å, with an average
distance of 5.55 Å), act to insulate this charge. To probe
further, we have calculated the capsule’s occupancy. Based on
an irregular hexadecahedron model (see Supporting Informa-
tion for details), the volume of the empty cavity has been
calculated to be 506 Å3, of which the triflate counteranions
occupy 280 Å3, which equates to 55% filled cavity space, in line
with previous observations made by Rebek.21
samples are thoroughly degassed, in contrast to many other
such Ir complexes,23 suggesting that the metallosupramolecular
architecture may also in some way inhibit collisional O2
quenching.23b Alternatively, the presence of four desolvated
anions located within the cavity of the capsule may provide
such an efficient, alternative nonradiative decay pathway (e.g.,
via electron transfer) that the capsule is insensitive to the
presence of O2.
A preliminary investigation into the host−guest chemistry of
[4OTf⊂Δ/Λ-1]2+ indicates that the encapsulated triflates can
be exchanged for other anionic species. When NBu4PF6 is
titrated into a sample of [4OTf⊂Δ-1]·2OTf in C2D2Cl4, 19F
NMR spectroscopy (see Figure S12) shows multiple OTf and
PF6 signals at low [PF6], indicative of mixed PF6−OTf host−
guest complexes. At equimolar anionic concentrations,
encapsulated OTf has mostly been displaced. This process
has also been monitored by 1H NMR spectroscopy (see Figure
S13), which shows that while the overall symmetry is preserved,
the proton signals closest to the anion binding sites (the tcb
hydrogen atom and the ortho and meta pyridyl sites) are
noticeably shifted. Similar experiments with NBu4BF4 and
NBu4CF3BF3, and additional titrations into Λ-1·6BF4 (see
Figures S14−S23), have ascertained that the overall affinity of
perfluorinated anions for the cavity of [Δ/Λ-1]6+ follows the
sequence PF6 > OTf ≈ CF3BF3 > BF4.21c While at present we
are unable to rationalize this affinity, due to the fast exchange of
some of the anionic species (even at low temperatures),
additional DOSY measurements show that the cage remains
intact following guest exchange. We have also ascertained that
there is communication between the luminescent properties of
[Δ/Λ-1]6+ and the cavity-bound guests, as evidenced by
changes to the emission spectra during anion titration
experiments (see the Supporting Information). As well as
showing that [Δ/Λ -1]6+ acts as a luminescent anion sensor,
this also suggests that it may be possible to tune luminescent
properties of multimetallic Ir assemblies through supra-
molecular means rather than via conventional covalent
modification. Our initial attempts to expand host−guest
chemistry to neutral organic species have so far been hampered
by the low solubility/stability of Λ/Δ-1 in more polar solvents.
In summary, the first molecular capsule based on an
[Ir(ppy)2]+ unit has been assembled using a multitopic nitrile
ligand and enantiopure Ir starting materials. This incorporation
leads to luminescence not present in a comparable mono-
nuclear complex. In addition, we have shown that the same
capsule is capable of binding multiple same-charge species and
that these can be swapped for other anionic guests. We are
currently exploring further aspects of this system, in particular
how guest-binding features and photophysical properties relate,
and whether the light-harvesting properties can be exploited in
relation to potential substrate binding.
Held under a standard long-wavelength UV lamp, tetra-
chloroethane (TCE) solutions of Λ- and Δ-1·6OTf luminesce
orange (Figure 2, inset). As most literature Ir complexes are
Figure 2. Absorption and emission (excitation at 413 nm) spectra of
Λ- and Δ-1·6OTf. Λ = black line; Δ = red line; TCE, 2 μM. For
comparison, the spectra of the mononuclear [Ir(ppy)2(PhCN)2]OTf
are also shown (Ir only = green line; Ir + 3000 equiv PhCN = blue
line; TCE, 12 μM). Inset: Photographs of (i) Λ-1·6OTf, (ii) Δ-
1·6OTf, and (iii) [Ir(ppy)2(PhCN)2]OTf in NMR tubes held (a) in
ambient lighting and (b) under a long-wavelength UV lamp.
luminescent, the significance of the capsules’ photophysical
properties only becomes apparent when compared against the
representative mononuclear bis(benzonitrile) complex, [Ir-
(ppy)2(PhCN)2]OTf.22 Placed next to the samples of Λ- and
Δ-1 under UV light (Figure 2, inset), there is no visible
luminescence from [Ir(ppy)2(PhCN)2]OTf in the same solvent
(at equivalent [Ir]). The absorption and emission spectra of
these samples confirm these visual observations (Figure 2).
Even with a vast excess of benzonitrile, [Ir(ppy)2(PhCN)2]OTf
is only weakly luminescent (Figure 2). The emission of Λ- and
Δ-1·6OTf is also broadened and red-shifted with respect to
[Ir(ppy)2(PhCN)2]OTf (in the presence of excess benzoni-
trile), signifying increased charge-transfer character of the
emitting state. While the quantum yields for Λ- and Δ-1·6OTf,
found to be 0.04 in air-equilibrated TCE (see the Supporting
Information), are comparable to those for many cyclometalated
Ir complexes under similar ambient conditions,23 these often
utilize either bidentate or strongly coordinating (and high
ligand strength) monodentate ligands.24 As far as we are aware,
this is the first time a collection of weakly coordinating ligands
has been used to inhibit nonradiative ligand dissociation
pathways.25 Interestingly, the emission intensity of Λ- and Δ-
1·6OTf increases only marginally (by a factor of 1.1) when the
ASSOCIATED CONTENT
■
S
* Supporting Information
Full synthetic details and compound characterization, crystallo-
graphic results in CIF fomat, details of IMMS calculations, and
volume calculations. This material is available free of charge via
the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
Paul.Lusby@ed.ac.uk
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Communication
(12) Carlise, J. R.; Wang, X.-Y.; Weck, M. Macromolecules 2005, 38,
9000.
Notes
The authors declare no competing financial interest.
(13) There are a possible four enantiomeric pairs plus two meso
compounds. This, and the lower symmetry of all but the homochiral
assemblies, leads to the complex 1H NMR spectrum. Prolonged
heating of the sample does not significantly alter the complex pattern
of signals.
ACKNOWLEDGMENTS
■
This work was supported by the EPSRC and the Royal Society.
P.J.L. is a Royal Society University Research Fellow.
(14) Meng, W.; Clegg, J. K.; Thoburn, J. D.; Nitschke, J. R. J. Am.
Chem. Soc. 2011, 133, 13652.
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