DOSY NMR, X-ray Structural and Ion-Mobility Mass Spectrometric Studies on Electron-Deficient and Electron-Rich M6L4 Coordination Cages

Article abstract of DOI:10.1021/acs.inorgchem.5b01082

A novel modular approach to electron-deficient and electron-rich M₆L₄ cages is presented. From the same starting compound, via a minor modulation of the synthesis route, two C₃-symmetric ligands L1 and L2 with different electronic properties are obtained in good yield. The trifluoro-triethynylbenzene-based ligand L1 is more electron-deficient than the well-known 2,4,6-tri(4-pyridyl)-1,3,5-triazine, while the trimethoxy-triethynylbenzene-based ligand L2 is more electron-rich than the corresponding benzene analogue. Complexation of the ligands with cis-protected square-planar [(dppp)Pt(OTf)₂] or [(dppp)Pd(OTf)₂] corner-complexes yields two electron-deficient (1a and 1b) and two electron-rich (2a and 2b) M₆L₄ cages. The single crystal X-ray diffraction study of 1a and 2a confirms the expected octahedral shape with a ca. 2000 ?³ cavity and ca. 11 ? wide apertures. The crystallographically determined diameters of 1a and 2a are 3.7 and 3.6 nm, respectively. The hydrodynamic diameters obtained from the DOSY NMR in CDCl₃:CD₃OD (4:1), and diameters calculated from collision cross sections (CCS) acquired by ion-mobility mass spectrometry (IM-MS) were for all four cages similar. In solution, the cage structures have diameters between 3.3 to 3.6 nm, while in the gas phase the corresponding diameters varied between 3.4 to 3.6 nm. In addition to the structural information the relative stabilities of the Pt₆L₄ and Pd₆L₄ cages were studied in the gas phase by collision-induced dissociation (CID) experiments, and the photophysical properties of the ligands L1 and L2 and cages 1a, 1b, 2a, and 2b were studied by UV-vis and fluorescence spectroscopy.

Full text of DOI:10.1021/acs.inorgchem.5b01082

Article
pubs.acs.org/IC
DOSY NMR, X‑ray Structural and Ion-Mobility Mass Spectrometric
Studies on Electron-Deficient and Electron-Rich M6L4 Coordination
Cages
Pia Bonakdarzadeh,† Filip Topic,† Elina Kalenius,† Sandip Bhowmik,† Sota Sato,‡ Michael Groessl,§
́
Richard Knochenmuss,§ and Kari Rissanen*,†
†University of Jyvaskyla, Department of Chemistry, Nanoscience Center, P.O. Box 35, FI-40014, University of Jyvaskyla, Finland
̈
̈
̈
̈
‡AIMR, Department of Chemistry, and JST ERATO, Tohoku University, Aoba-ku, Sendai 980-8578, Japan
§Tofwerk AG, Uttigenstr. 22, 3600 Thun, Switzerland
S
* Supporting Information
ABSTRACT: A novel modular approach to electron-deficient and electron-
rich M6L4 cages is presented. From the same starting compound, via a minor
modulation of the synthesis route, two C3-symmetric ligands L1 and L2 with
different electronic properties are obtained in good yield. The trifluoro-
triethynylbenzene-based ligand L1 is more electron-deficient than the well-
known 2,4,6-tri(4-pyridyl)-1,3,5-triazine, while the trimethoxy-triethynylben-
zene-based ligand L2 is more electron-rich than the corresponding benzene
analogue. Complexation of the ligands with cis-protected square-planar
[(dppp)Pt(OTf)2] or [(dppp)Pd(OTf)2] corner-complexes yields two
electron-deficient (1a and 1b) and two electron-rich (2a and 2b) M6L4
cages. The single crystal X-ray diffraction study of 1a and 2a confirms the
expected octahedral shape with a ca. 2000 Å3 cavity and ca. 11 Å wide
apertures. The crystallographically determined diameters of 1a and 2a are 3.7
and 3.6 nm, respectively. The hydrodynamic diameters obtained from the DOSY NMR in CDCl3:CD3OD (4:1), and diameters
calculated from collision cross sections (CCS) acquired by ion-mobility mass spectrometry (IM-MS) were for all four cages
similar. In solution, the cage structures have diameters between 3.3 to 3.6 nm, while in the gas phase the corresponding diameters
varied between 3.4 to 3.6 nm. In addition to the structural information the relative stabilities of the Pt6L4 and Pd6L4 cages were
studied in the gas phase by collision-induced dissociation (CID) experiments, and the photophysical properties of the ligands L1
and L2 and cages 1a, 1b, 2a, and 2b were studied by UV−vis and fluorescence spectroscopy.
tetrakis-monodentate,7 or tetrakis-bidentate8 ligands with
suitable metal cations.
INTRODUCTION
■
Over the last 20 years, coordination-driven self-assembly has
been demonstrated to be a highly efficient method to produce
three-dimensional supramolecular architectures. Rational de-
sign of such discrete structures is based on structurally
predesigned building blocks, viz. ligands and the known
coordination geometries of specific metal cations. Conse-
quently the coordination geometries of the metal ions (Mn+)
and the number of the binding sites of ligands (L) and overall
structure of the ligand define the geometry of the assembly.1
The simplest of “platonic solid” coordination cages are
tetrahedron, cube, and octahedron.1 Also “higher order”
coordination polyhedra are known.2 Tetrahedral coordination
cages can be prepared by edge-directed self-assembly of C2-
symmetric bis-bidentate ligands or their subcomponents with
suitable, usually octahedrally coordinated metal cations, leading
to the M4L6 complex.1c,d,3 An alternative route, face-directed
self-assembly, uses C3-symmetric tris-bidentate ligands which
upon metal coordination lead to M4L4 tetrahedra.1e,4 The cubic
coordination cages can also be obtained by edge or face-
directed self-assembly of bis-monodentate,5 bis-bidentate,6
The pioneering work by Fujita9 and Stang10 has provided the
basis for the M6L4 cages. Fujita’s molecular paneling approach
defines trigonal C3-symmetric tris-monodentate N-donor
ligands as “paneling ligands”, which upon complexation with
a proper metal cation, leads to M6L4, whereas Stang’s approach
uses “directional bonding”, which under similar conditions
leads to analogous M6L4 cages. The generally used metal
cations are the cis-protected square-planar Pd(II) and Pt(II)
“corner”-complexes with two labile anionic ligands (typically
nitrates or triflates) at 90° angle available for N-ligand binding.
The Fujita’s approach uses mostly ethylenediamine as a
protective group for the metal, whereas Stang’s approach uses
the more robust bisphosphino complex.11 The directional
bonding approach (Stang) defines the M6L4 cage as a truncated
tetrahedron, where the ligands act as the faces of a tetrahedron,
while the ditopic metals are placed in the middle of the edges,
Received: May 14, 2015
© XXXX American Chemical Society
A
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whereas the molecular paneling approach (Fujita) refers to
M6L4 as an octahedron with the paneling ligands as the faces
and the metal ions as the vertices.
triiodobenzene (Scheme 1) by adapting a known procedure.16
The ligand L1, 1,3,5-tris[(4-pyridyl)ethynyl]-2,4,6-trifluoroben-
Until now, eight trigonal C3-symmetric tris-monodentate N-
donor ligands with either pyridine or cyano groups as the
donors have been used in the construction of M6L4 cages
(Figure 1).9,10,12 Additionally, there are other ligands, mostly
Scheme 1. Syntheses of Ligands and Self-Assembly of M6L4
Cages 1a, 1b, 2a, and 2b
a
a
Reaction conditions for ligand L1: (i) 4-ethynylpyridine, [Pd-
(PPh3)2Cl2], CuI, i-Pr2NH, 70 °C, 48 h, 41% and for ligand L2: (ii)
(a) NaOCH3, 1,3-dimethyl-2-imidazolidinone, r.t., 18 h, 69%, (b)
same conditions as in (i) 59%. (iii) L1 or L2 (4 equiv),
[(dppp)M(OTf)2] (M = Pt, Pd; 6 equiv), CHCl3:CH3OH (4:1), 60
°C, 2 h.
Figure 1. Similar ligands (A−H) previously used in M6L4 cages.9,10,12
nonplanar, used in M6L4 cages as well.13 A common
modification of the well-known electron-deficient triazine
ligand (Figure 1, A)9 toward larger-sized M6L4 cages has
been the addition of aromatic ring(s) between the central
zene, was prepared via Sonogashira coupling with 4-
ethynylpyridine, while the ligand L2, 1,3,5-tris[(4-pyridyl)-
ethynyl]-2,4,6-trimethoxybenzene, was synthesized by nucleo-
philic aromatic substitution followed by Sonogashira coupling
(Scheme 1). The ethynyl spacers between the pyridine rings
and the trifluoro- or trimethoxybenzene core make the ligands
L1 and L2 and the resulting M6L4 cages larger than the
corresponding M6L4 cages containing the ligand A, and at the
same time allowing the ligand to adopt planar conformation
needed for the successful cage formation. A small change in the
synthesis protocol (Scheme 1) from the same starting material
(1,3,5-trifluoro-2,4,6-triiodo-benzene) results in a modulation
of the electron density of the benzene core. The trifluoro ligand
L1 is strongly electron-deficient, while the trimethoxy analogue
L2 is electron-rich. Using these ligands with cis-protected
Stang’s corner-complex11 [(dppp)M(OTf)2] (M = Pd or Pt)
leads to four new large M6L4 octahedral cages 1a, 1b, 2a, and
2b (Scheme 1).
triazine ring and the donor groups (Figure 1, B−D).9,12a
A
benzene core has been used together with ethynyl or ethenyl
moieties (Figure 1, E and G).10,12b,c Cyano groups as N-donors
have been used instead of pyridine in smaller (Figure 1, F)12d
or larger (Figure 1, H)12b versions. Also, changing the
protecting groups around the metal cation10,12b,c,14 or the
metal itself12d,13c,15 has been another way of modifying the
cages. Surprisingly the modification of the central benzene core
(Figure 1, E)10,12b,c has not been explored. Herein we report
the synthesis and characterization of two new C3-symmetric
ligands L1 and L2, their self-assembly with Stang’s Pt or Pd-
corner-complexes,11 [(dppp)Pt(OTf)2] and [(dppp)Pd-
(OTf)2] (dppp = 1,3-[bis(diphenylphosphino)]-propane),
into M6L4 cages 1a, 1b, 2a, and 2b and their full character-
ization by NMR spectroscopy, single-crystal X-ray diffraction,
mass spectrometry, ion-mobility mass spectrometry, UV−vis
and fluorescence spectroscopy. To the best of our knowledge,
this is the first time the size of coordination cages has been
studied in three different states of matter as thoroughly,
including also information by techniques such as DOSY NMR
and IM-MS. The concerted use of three complementary
structural techniques allows for the comparison of the sizes of
the coordination cages in solution, solid state, and gas phase.
The electrostatic potential surfaces were calculated17 for
ligands L1, L2, triazine ligand A, and tris-ethynylbenzene ligand
E (Figure 2). The strongly electron-withdrawing fluorine atoms
at 2-, 4- and 6-positions, in addition to electron-withdrawing
ethynyl moieties at 1-, 3-, and 5-positions, substantially decrease
the electron density of the benzene core of ligand L1 making it
more electron-deficient than the triazine ligand A (Figure 2).
The trimethoxy ligand L2, on the other hand, has more
electron-rich benzene core compared to the ligand E (Figure
2), due to the electron donating effect of methoxy groups
(Figure 2). In addition to the molecular M6L4 cages, Fujita et al.
have utilized the electron-deficiency of triazine ligand A in
trapping electron-rich guests inside 3D coordination net-
works18 and pillared coordination cages.19
RESULTS AND DISCUSSION
■
Ligand Design and Self-Assembly of M6L4 Cages.
Inspired by Stang et al.’s10,12b,c work on M6L4 coordination
cages and the possibility to tune the properties of the central
benzene ring, two C3-symmetric target ligands, either electron-
deficient with a trifluoro-triethynylbenzene core or electron-
rich with a trimethoxy-triethynylbenzene core, were prepared
starting from commercially available 1,3,5-trifluoro-2,4,6-
The complexation of Pt- or Pd-corner complex (6 equiv)
with ligand L1 or L2 (4 equiv) in CHCl3:CH3OH (4:1) at 60
°C for 2 h, followed by precipitation with hexane, afforded
B
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and 19F NMR spectra (Figures S8 and S13) also support the
formation of a single, highly symmetrical product.
Crystallography. Despite the 20-year history of M6L4
cages, surprisingly few good quality crystal structures have
been published. Several crystal structures of complexes with the
triazine ligand A were reported by Fujita et al. in 2002.20 The
electron-deficient Pd6L4 (L = the triazine A) cage with the
cavity volume of ca. 750 Å3 exhibits rich host−guest chemistry
due to the suitably sized cavity for complete encapsulation of
one, two, or four electron-rich guest molecules, depending on
the size of the guest, viz. one tetrabenzylsilane (molecular
volume = MV = 458 Å3) or tri-tert-butylbenzene (MV = 320
Å3), two diphenylmethane (MV = 202 Å3) or 1,2-bis(4-
methoxyphenyl)-1,2-ethanedione (MV = 279 Å3) or four o-
carborane (MV = 172 Å3) or 1-adamantanol molecules (MV =
172 Å3). A smaller version of the M6L4 cage with the cavity
volume of ca. 500 Å3 using tricyanobenzene (Figure 1, ligand
F) as the ligand has been reported by Lusby et al.12d Their cage
encapsulates four triflate anions (MV = 86 Å3) symmetrically
arranged around the center of the cavity. The largest M6L4 cage
with 1,3,5-tris(4-pyridylethynyl)benzene (Figure 1, ligand E)
and 1,3,5-tris(4-pyridyl-trans-ethenyl)benzene (Figure 1, ligand
G) as the ligands has been reported by Stang et al.12c Because
of the very large size of the cavity, large apertures (windows),
and moderate crystal quality, no solid state host−guest
complexation was observed. The overall size of the cavities of
M6L4 cages made by Stang et al. is comparable to the cages 1a
and 2a reported in this work; only the windows are smaller in
1a and 2a (see below).
Since only weak diffraction was observed with crystals of 1a
and 2a (Figure 4), obtained by slow diffusion of hexane (1a) or
acetone (2a) into a CHCl3:CH3OH (4:1) solution, the data
collections were done using synchrotron radiation (see
Supporting Information). In spite of numerous attempts with
several different crystallization conditions, the corresponding
Pd6L4 cages 1b and 2b did not form single crystals, and only
powdery precipitate was obtained.
The Pt6L4 complexes 1a and 2a are discrete self-assembled
supramolecular cages with an octahedral overall structure. The
four pseudotrigonal ligands (L1 or L2) form a large
octahedrally shaped cavity with a radius of 8 Å (measured
from the center of the cavity to the centroids of the central
benzene ring of the paneling ligand) and volume of ca. 2000 Å3.
Figure 2. Electrostatic potential surfaces of A, E, L1, and L2. The
structures are optimized at the B3LYP level of theory using the 6-
31G* basis set in vacuum.17 Red and blue represent negative and
positive charge densities, respectively.
white solids 1a−b and 2a−b (Scheme 1). The 1H NMR
analysis shows one set of ligand proton signals indicating a
highly symmetrical assembly (Figure 3a). The PyHα signals of
fluorine substituted cages are shifted downfield by (Δδ) 0.36
(1a) and 0.30 ppm (1b), whereas the PyHβ signals move
upfield by (Δδ) −0.31 (1a) and −0.35 (1b) ppm as a result of
the metal−ligand complexation. Only one band is observed in
1
the H DOSY NMR spectra for fluorine cages 1a and 1b with
diffusion coefficients (D) of 2.35 × 10−10 m2s−1 (1a, Figure 3b)
and 2.29 × 10−10 m2 s−1 (1b, Figure 3c), which correspond to
spherical hydrodynamic diameters of 3.5 (1a) and 3.6 nm (1b).
Similarly, for the methoxy substituted cages, 2a and 2b,
complexation was confirmed by 1H NMR (Figures S4, S16, and
S21). Also, the hydrodynamic diameters obtained by 1H DOSY
NMR were similar in size, viz. 3.3 nm (2a) and 3.6 nm (2b)
with the corresponding fluorine-containing cages (Figures S20
and S25). The hydrodynamic diameters are fully consistent
with crystal structures of 1a and 2a as well as with the diameters
calculated from collision cross sections (CCS) obtained from
the ion-mobility mass spectrometry (IM-MS) experiments (see
below, Table 1). The 31P NMR (Figures S9, S14, S19, and S24)
1
1
Figure 3. (a) Partial H NMR (300 MHz, CD2Cl2, 303 K) spectra of ligand L1, cages 1a and 1b (top to bottom). Partial H DOSY NMR (400
MHz, CDCl3:CD3OD 4:1, 298 K) spectra of cages (b) 1a and (c) 1b.
C
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Table 1. CCSexp Values of [M6L4(OTf)8]4+ Ions and Comparison of Diameters Obtained by IM-MS, 1H DOSY NMR and X-ray
Diffraction
a
b
c
d
e
cage
m/z
ion
CCSexp (Å2)
IM-MS d (nm)
DOSY d (nm)
X-ray d (nm)
1a
1645
1512
1681
1548
[Pt6L14(OTf)8]4+
926 (mi)
945 (ma)
962 (mi)
982 (ma)
965 (ma)
988 (mi)
989 (ma)
974 (mi)
3.4
3.5
3.5
3.5
3.5
3.5
3.6
3.5
3.7
1b
2a
2b
[Pd6L14(OTf)8]4+
[Pt6L24(OTf)8]4+
[Pd6L24(OTf)8]4+
3.6
3.3
3.6
3.6
3.5
a
b
c
Most abundant m/z. ma = major conformer, mi = minor conformer in ion mobilogram. Calculated on the basis of CCS by assuming a spherical
d
e
shape of the metal cage. Calculated using Stokes−Einstein equation. The average of the distance of the opposite H atoms +1.2 Å.
structure resulting in a coordination cage, which only slightly
deviates from a regular octahedron. The four ligands and the
four windows have all nearly the same dimensions, viz. ca. 14.5
Å, with N···N distances within the ligand varying from 13.73 to
14.64 Å for 1a and 13.73 to 14.77 Å for 2a. The trigonal angles
in the cages 1a and 2a approach the ideal values, now being
111.7−127.6° for 1a and 115.1−128.2° for 2a. The lack of
short contacts in the crystal structures of cages 1a and 2a allows
a small twisting of the pyridine rings with respect to the central
benzene ring, the twist angles varying from −12.3° to 5.13° for
1a and −13.5° to 11.00° for 2a.
Mass Spectrometry. The electrospray ionization mass
spectra (ESI-MS) measured from 50 μM solutions of 1a, 1b,
2a, and 2b in CH2Cl2 show discrete peaks corresponding to
n+
ions [M6L4(OTf)12‑n
]
(n = 3−5) (Figures 5a, S32, S33).
Figure 4. Crystal structures of 1a (top) and 2a (below), ball-and-stick
(left), and space-filling models (right).
The cages have four large similar trigonal apertures, “windows”,
with a diameter of ca. 11 Å (Figure S26), the size of the
windows being slightly smaller in 2a due to the methoxy
substituents. Nine out of the 12 methoxy substituents in the
central benzene rings point toward the interior of the cavity 2a,
two pointing outside and one parallel with the benzene ring.
The X-ray structure of the uncomplexed ligand L1 (Figures
S27−S28) reveals the ligand itself to have a pseudotrigonal
structure, showing marked deviations from the optimal 120°
angles. The angles between the central benzene ring and the
pyridine rings (Bz-Py, Figure S29) vary from 105.1 to 131.1°.
This is also reflected in the N···N distances which vary from
13.09 to 15.00 Å (Figure S30), whereas the calculated structure
of L1 (obtained using SPARTAN17) shows N···N distances of
14.30 Å with a perfect trigonal symmetry. In addition to the
deviation from the trigonal angles, also small deviations from
planarity are observed (Figure S31), with the twist angle
between the pyridine ring and the central 2,4,6-trifluoroben-
zene ring varying from −0.6 to +14.75°, the planarity being
enhanced by quite strong π−π interactions observed in the
crystal lattice of L1. Upon complexation with the Pt- or Pd-
corner complexes the ligands L1 and L2 adopt a more trigonal
Figure 5. (a) ESI-TOF MS spectrum of 1a (50 μM in CH2Cl2). Inset
shows the fit of theoretical isotopic distribution (red solid line) (b) ion
mobilogram (green line) for ion m/z 1644 corresponding to
[(dppp)Pt6(L1)4(OTf)8]4+.
Intact M6L4 cages were observed as major products in each
spectrum, although minor peaks for M5L4, M4L4, and M4L3
were evident, especially in samples of more labile Pd-cages.
These smaller ions do not necessarily originate from
fragmentation, but they may result from the instability of
cages in μM-regime concentration used in the ESI-MS
experiments, which is a common outcome with coordination
complexes.21
The drift tube ion-mobility mass spectrometry (IM-MS)
n+
experiments confirmed [M6L4(OTf)12‑n
]
ions to have CCS
values consistent with the crystal structures and the hydro-
1
dynamic diameters obtained by H DOSY NMR (Table 1).
Assuming a spherical shape for M6L4 cages, the diameters
D
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obtained from the crystal structures, hydrodynamic diameters
(DOSY NMR) and the CCS-derived diameters (IM-MS) were
all within 0.1−0.3 nm of each other. The values obtained with
different techniques and in different states of matter are similar,
indicating that any of these methods will give reliable results
when carefully executed.22
2b, such a stability difference was not observed (Figure S36).
This behavior can be rationalized through stability differences
between the metal corners. Because of the electron-with-
drawing nature of ligand L1, the properties of metal−nitrogen
bonds are more reflected in the overall gas phase stability of the
cage. The differences between the two metal centers are also
reflected in the experimental CCS values; the Pt-cages showing
smaller CCS values (shorter coordinative bonds) compared to
the corresponding Pd-cages. In addition, the difference between
the CCS values in Pd- and Pt-cages with ligand L1 is evidently
larger compared to same difference between cages with L2
(Table 1).
The mixed ligand experiment (both ligands present in
equimolar ratio) further confirmed the ability of the both
ligands to form a cage with Pt(II), with the formation of all
three possible heterometallic cages [M6L1nL24‑n] (n = 0−4)
observed experimentally (Figure S37). The formation of cages
with ligand L2 was only slightly favored over statistical
distribution. A similar mixed-metal experiment (both metals
present in equimolar ratio with one ligand) showed also close-
to-statistical distribution of heterometallic cages (Figure S38),
although the abundance of the cages was drastically lower
compared to ones formed with only one metal. In the
heterometallic experiment there was also increased abundance
of the M5L4 fragments. This indicates a decreased stability of
heterometallic cages, most probably due to slight distortion of
cage. In addition, the observation of close-to-statistical
distribution implies kinetic control in the formation of cages.
Photophysics. Distinct differences were observed in the
absorption spectra of ligands L1 and L2 (Figure 7a). Electron-
deficient L1 showed two sharp absorption bands of similar
As the drift tube of the IM-MS instrument is operated at
atmospheric pressure, it allows high resolution separations and
therefore accurate determination of CCS values without the
need for cumbersome calibration, which often severely
complicates the analysis done by using more common
traveling-wave ion mobility MS (TWIM-MS).22b Consequently,
the drift tube IM-MS is an ideal method to study gas phase
shape and CCS values of metal cages with high resolution. It
should be emphasized that IM-MS has not yet been widely
applied to study gas-phase conformations of metal−organic
cages or metal−organic structures.22,23 Therefore, the compar-
ison of CCS values with the DOSY NMR and crystal structures
underlines the potential of IM-MS and verifies its applicability
to structural studies of metal−organic cages. Especially when
characterization through traditional methods (X-ray and NMR
spectroscopy) is challenging or impossible, the IM-MS appears
to be a highly valuable alternative.
For each [M6L4(OTf)8]4+ ion, two conformers were
observed in ion mobilograms, one major and one minor
(Figure 5b, S39). In most cases, the major conformer
constituted ca. 80% of ion population, and the difference in
CCS values between the conformers was only ca. 20 Å2 (ca.
4−5 Å in diameter). It is, therefore, reasonable to assume that
the conformations arise from variation in locations of the two
triflate anions which exceed the number of metal centers in
[M6L4(OTf)8]4+. This is also supported by the mobilogram of
[M5L4(OTf)6]4+ (Figure S39) which shows only a single
conformer (only one triflate exceeding the number of metal
corners).
The isolated [M6L4(OTf)8]4+ ions were further studied in the
gas phase by collision induced dissociation (CID) experiments
(Figure 6, S36). In the qualitative sense, all metal cages
Figure 6. CID dissociation curves of isolated ions [M6L4(OTf)8]4+ for
cages 1a and 1b.
dissociated similarly and produced structurally characteristic
fragment ions through eliminations of [M(OTf)]+, [ML-
(OTf)]+, and [M2L(OTf)2]2+. The elimination of two metal
corners at most was in every case followed by elimination of a
ligand, which is well in line with the cage-like structure. The
experiments as a function of increased collisional activation
revealed a clear stability difference between cages 1a and 1b,
the former being more stable (Figure 6). Between cages 2a and
Figure 7. (a) Absorption and (b) emission spectra of ligands (4 μM)
and cages (1 μM) in CH2Cl2.
E
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intensity at 284 and 301 nm (Table 2). A relatively broad
shoulder was observed around 269 nm with a slightly lower
octahedral with a large open cavity punctured by four apertures.
The host−guest chemistry studies of the cages will be
presented in a future publication. In gas phase, collision cross
section-based diameters for all cages were obtained by drift tube
ion-mobility mass spectrometry (IM-MS). Moreover, using the
collision-induced dissociation (CID) of the parent cage ions,
the relative stabilities of the complexes in the gas phase could
be established and correlated with the electronic properties of
the ligands as well as the nature of the metal centers.
Detailed solution studies by NMR proved the highly
symmetrical structure of the M6L4 cages and gave the
hydrodynamic diameters in very good agreement with the
values obtained in solid state and gas phase studies.
Table 2. Photophysical Properties of Ligands and Cages
λmax Abs
(nm)
molar extinction coefficient
λmax Em
(nm)
quantum
yield Φ (%)
(ε) (cm−1 mol−1
)
L1
269
284
301
303
313
306
67600
93700
87900
88400
86500
367000
350
360
375
380
11.5
L2
1a
45.3
8.0
358, 375,
385
The photophysical properties of the ligands and the
corresponding cages were shown to be critically dependent
on the substituents on the core benzene ring. The ligands,
especially electron-rich L2 and the corresponding Pt-cage (2a),
were shown to be strongly emissive and could be used in the
future to develop capsules that act as fluorosensors upon guest
encapsulation.
324
305
322
341
337
397000
294000
336000
398000
521000
a
1b
nop
2a
2b
438
36.3
a
nop
a
No observable peak.
Our results highlight the importance of the combined
structural studies of metallosupramolecular assemblies in the
solid state, solution, and gas phase, particularly emphasizing the
applicability and reliability of the ion-mobility mass spectrom-
etry (IM-MS) in the structural studies of supramolecular
coordination complexes.
intensity. The more electron-rich system L2 showed broader,
low-energy, almost equally intense absorption bands at 303 and
313 nm. A starker contrast was observed in the emission
spectra of the two ligands (Figure 7b). The observed Stokes
shifts were to a similar extent of 70−80 nm for both of the
ligands. However, L2 had a typical broad spectra centered
around 380 nm, whereas L1 showed a structured emission with
three peaks at 350, 360, and 375 nm. This was distinctly
different from the modestly broad, unstructured spectra
observed for analogous compounds.16 The quantum yield of
L2 was found to be significantly higher than L1 (Table 2),
which was in good agreement with reported values for similarly
substituted 1,3,5-tris(ethynylphenyl)benzene analogues.16
The corresponding Pt- and Pd-cages (M6L4) for ligands L1
and L2 showed distinctly broader absorption spectra with
characteristic red shifts to that of the free ligands (Figure 7a).
Both Pt and Pd-cages of L2, 2a, and 2b, showed a broad
absorption peak at 341 and 337 nm, respectively, with markedly
higher intensity than the free ligands (Table 2). However, the
corresponding cages of L1, 1a, and 1b, had similar absorption
profiles characterized by two almost equally intense peaks. The
1a showed two absorption bands at 306 and 324 nm, while the
corresponding bands appeared at 305 and 322 nm for 1b.
It can be argued that the absorbance spectra of the cages are
ligand-centric and are weakly affected by coordinated metals.
However, the emission spectra of the cages were critically
influenced by the metal. While Pt-cages showed moderate to
high emission intensity, especially in case of 2a, Pd
coordination drastically quenched any fluorescence from the
ligands (Figure 7b). Cage 1a showed a broad spectrum with
vibrational structure with peaks at 358, 375, and 385 nm. The
emission spectrum of 2a was devoid of any vibrational structure
and was centered around 438 nm.
ASSOCIATED CONTENT
* Supporting Information
■
S
Experimental procedures, synthesis, and characterization data
for ligands and M6L4 cages, and their MS, IM-MS, and X-ray
diffraction data. The Supporting Information is available free of
charge on the ACS Publications website at DOI: 10.1021/
acs.inorgchem.5b01082.
AUTHOR INFORMATION
Corresponding Author
*E-mail: kari.t.rissanen@jyu.fi.
■
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
The Academy of Finland (K.R.: nos. 265328 and 263256; E.K:
nos. 284562 and 278743). University of Jyvaskyla (K.R.:
■
̈
̈
Academy professorship support) Japanese Ministry of Educa-
tion, Culture, Sports, Science, and Technology (MEXT) (SS.:
nos. 25102007, 24685010 and. 2013G640). The authors kindly
acknowledge the Academy of Finland (K.R.: nos. 265328 and
263256; E.K.: nos. 284562 and 278743), the University of
Jyvaskyla and the Japanese MEXT KAKENHI (S.S.: nos.
̈
̈
25102007 and 24685010). The synchrotron X-ray crystallog-
raphy was performed at the BL1A beamline in KEK Photon
Factory (S.S.: no. 2013G640).
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CONCLUSIONS
■
■
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