Metallohosts with a heart of carbon

Article abstract of DOI:10.1021/ja204888w

Single-crystal X-ray diffraction studies have confirmed that Ni ₃S₃-based molecular bowls prepared in one-pot reactions capture either CH₂Cl₂ or C₆₀. The nature of the pendant substituents (naphthalen-2-ylmethyl, benzyl, or ethyl) around the rim of the bowl dictates the formation of a 1:1 (bowl host-C₆₀ guest) or 2:1 (capsule host-C₆₀ guest) architecture. In CDCl₃, the trimeric complexes were found to be in equilibrium with dimeric analogues. For the naphthalen-2-ylmethyl-substituted host, NMR spectroscopic titration data confirmed a 1:1 host-C₆₀ guest complex in 1,2-Cl₂C ₆D₄ solution.

Full text of DOI:10.1021/ja204888w

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Metallohosts with a Heart of Carbon
Edwin C. Constable,* Guoqi Zhang, Daniel H€aussinger, Catherine E. Housecroft,* and Jennifer A. Zampese
Department of Chemistry, University of Basel, Spitalstrasse 51, CH-4056 Basel, Switzerland
S Supporting Information
b
Scheme 1. One-Pot Synthesis of Compounds 1ꢀ3
ABSTRACT: Single-crystal X-ray diffraction studies have
confirmed that Ni3S3-based molecular bowls prepared in
one-pot reactions capture either CH2Cl2 or C60. The nature
of the pendant substituents (naphthalen-2-ylmethyl, benzyl,
or ethyl) around the rim of the bowl dictates the formation
of a 1:1 (bowl hostꢀC60 guest) or 2:1 (capsule hostꢀC60
guest) architecture. In CDCl3, the trimeric complexes were
found to be in equilibrium with dimeric analogues. For the
naphthalen-2-ylmethyl-substituted host, NMR spectro-
scopic titration data confirmed a 1:1 hostꢀC60 guest com-
plex in 1,2-Cl2C6D4 solution.
he assembly of metallomolecular containers allows the crea-
tion of beautiful architectures tailored to facilite specific
T
reactions or catalytic processes.1ꢀ7 The cavity size, shape, and
ability to engage in noncovalent supramolecular interactions
are of paramount importance for tuning the hostꢀguest chemistry.
Calixarene containers7 are suited to hosting fullerenes,8 and the
selective complexation and purification of C60 and C70 is a major
achievement in this area.9 Fullerene separations have been achieved
using cyclotriveratrylenes with pyrimidinone substituents capable of
self-recognition through hydrogen bonding.10 Other capsules
that host fullerenes include those assembled from porphyrins,11ꢀ13
concave aggregates of aromatic rings,14,15 Cu-based molecular
squares,16 and Ni(II) complexes of tetraazaannulenes.17 Fujita
has engineered a coordination network that undergoes single-crystal
fullerene inclusion.18 We now report the assembly of Ni3S3-
based bowls that capture a C60 molecule as either a 1:1 (bowl
hostꢀC60 guest) or 2:1 (capsule hostꢀC60 guest) species.
The reaction of 2-hydroxy-3-(naphthalen-2-ylmethoxy)-
Figure 1. (a) Structure of 1 in 2(1) CH2Cl2 with ellipsoids plotted
3
at the 30% probability level and H atoms omitted. Symmetry codes:
(i) 2 ꢀ y, 1 + x ꢀ y, z; (ii) 1 ꢀ x + y, 2 ꢀ x, z. (b) Hosting of CH2Cl2 in
the capsule formed by two molecules of 1 in 2(1) CH2Cl2.
3
The manner in which 2(1) CH2Cl2 assembles and the diameter
3
of the capsule (∼8.5 Å for the central cavity) suggested that 1
may host C60 (diameter ≈ 7 Å). A search of the Cambridge Struc-
tural Database (version 5.31 with August 2010 updates)25 gave
23 structures with C60 in a bowl-shaped host or molecular capsule.
Among these, calixarene26 and functionalized macrocyclic metal
complexes27 predominate. In addition, there are structures with
less well-defined molecular containers (e.g., porphyrins that by
virtue of a twisted conformation or pendant substituents act as
shallow vessels for C60 guests28).
benzaldehyde, 2-aminoethanethiol, and NiCl2 6H2O resulted in
3
the formation of brown crystals. Structural analysis19 revealed the
assembly of the [3 + 3] complex 1 (Scheme 1 and Figure 1a).
This crystallizes as 2(1) CH2Cl2 in the rhombohedral space
3
group R3 with one-third of the complex in the asymmetric unit.
The latter contains one-sixth of a molecule of CH2Cl2, which is
disordered about the 3 position (0,0,0), Wyckoff position a. The
assembly of 1 and the chair conformation of the central Ni3S3
core mimic observations on related systems.20ꢀ24 Each O,N,
S-donor set of the ligand binds in a tridentate mode to a square-
planar Ni2+ ion, and the S atom bridges two Ni centers. The bond
lengths in the nickel coordination sphere are Ni1ꢀO1 =
1.851(2) Å, Ni1ꢀN1 = 1.887(3) Å, Ni1ꢀS1 = 2.1870(10) Å,
and Ni1ꢀS1ii = 2.2141(10) Å. Molecule 1 has a bowl shape with
the Ni3S3 unit as its base, and the CH2Cl2 molecule is encapsu-
lated between two host molecules of 1 (Figure 1b).
Crystalline 2(1) CH2Cl2 was dissolved in 1,2-Cl2C6H4/
3
CH2Cl2 (4:1 by volume), and 1 equiv of C60 was added. After
sonication and filtration, Et2O was diffused into the filtrate, and
crystals of 1 C60 0.5(1,2-Cl2C6H4) Et2O grew in 3 days. Structural
3
3
3
analysis19 revealed that bowl-shaped 1 hosts a C60 molecule
(Figure 2). The closest separations between the host and guest
Received: May 27, 2011
Published: June 22, 2011
r
2011 American Chemical Society
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|

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Figure 2. Bowl host 1ꢀC60 guest assembly in 1 C60 0.5(1,2-Cl2C6H4)
3
3
3
Figure 3. (a) Centrosymmetric dimers of 2 supported by π stacking.
(b) One of two independent 2(2) C60 molecular capsules in 2(2) C60 1,
Et2O.
3
3
3
2-Cl2C6H4 0.25H2O.
3
involve Ni Cfullerene [3.24(1)ꢀ3.54(1) Å] and Nimine
3 3 3
3 3 3
Cfullerene [3.22(1)ꢀ3.41(2) Å] contacts. Thenaphthalenedomains
of 1 do not engage in face-to-face interactions with the C60
guest. Rather, the former are twisted in such a way that weak
CHnaphthalene πfullerene interactions develop. However, these
3 3 3
are not optimal, with the shortest contact between a CH and
the centroid of a fullerene C6 ring being 3.04 Å.
In order to investigate whether the assembly of the hostꢀ
guest complex also occurs in solution, compound 1 was dissolved
in 1,2-Cl2C6D4. In this solvent, there was no evidence for the
presence of a dimeric species (see below); no exchange peaks were
observed in an NMR excitation spectroscopy (EXSY) spectrum.
A series of NMR samples was prepared [see the Supporting
Information (SI)] in which the number of molar equivalents of
C60 with respect to 1 ranged from 0 to 7.5. A comparison of the
1H NMR spectra for the samples (Table S1 in the SI) showed
that the resonance assigned to the imine proton of 1 undergoes
the greatest perturbation (Δδimine) upon addition of C60 to the
solution of 1. This is in keeping with the proximity of the imine
CH protons to the fullerene in the solid state (see above). The
data in Table S1 also indicate that in solution, three of the
naphthalene protons (HB1, HB7, and HB8) sense the presence
of the C60 molecule. In the titration curve for addition of C60 to
1 equiv of 1 (Figure S3 in the SI), the observed chemical shift
difference Δδimine approaches the hypothetical value of 0.299
calculated for a 1:1 hostꢀguest complex (see the SI). The
Job’s plot shown in Figure S4 also confirms the formation of
Figure 4. Hosting of CH2Cl2 in the capsule formed by two independent
molecules of 3 in 2(3) CH2Cl2.
3
contains two independent molecules of 2 and two half-C60
molecules. Operation through inversion centers completes two
independent 2(2) C60 capsules (Figure 3b). In one capsule, a
3
benzyl group is disordered and was modeled over two sites. The
most important hostꢀguest interactions involve (i) short Ni
C
3 3 3
contacts [3.380(7)ꢀ3.672(13) Å] and (ii) face-to-face π inter-
actions between the fullerene and the central arene rings in 2.
In one of the independent molecules, there is an edge-to-face
contact (2.54 Å) between a CHbenzyl-ring unit and a six-membered
ring of the fullerene.
Since it was difficult to assess the importance of the pendant
aromatic substituents in facilitating the assembly of the hostꢀ
guest complexes, we prepared 3 (Scheme 1) having ethyl groups
as the substituents. The method of preparation was as for 1 and 2,
starting from 3-ethoxy-2-hydroxybenzaldehyde, and structural
characterization19 of brown crystals of 3 confirmed the formation
of a [3 + 3] complex (Figure S2). The compound crystallizes as
a 1:1 complex. The dissociation constant for 1 C60 is Kd =
3
2.1 mmol dmꢀ3 (see the SI).
We investigated the effect of changing the substituent from
naphthalen-2-ylmethyl to benzyl. Complex 2 (Scheme 1) was
prepared in a manner analogous to that for 1 starting from
2(3) CH2Cl2, with two independent molecules of 3 forming a
3-(benzyloxy)-2-hydroxybenzaldehyde. Crystals of 2 CH2Cl2
3
3
capsule around a CH2Cl2 molecule (Figure 4). The bond lengths
in the coordination sphere of atom Ni1a [Ni1aꢀO1a = 1.843(2) Å,
Ni1aꢀN1a = 1.881(3) Å, Ni1aꢀS1a = 2.1896(10) Å, Ni1aꢀS2a =
2.2265(10) Å] are representative of those for each independent
Ni. The CH2Cl2 molecule is disordered over two positions, with
the two Cl sites in common. The six closest Ni Cl host
were grown by evaporation of a CH2Cl2/MeOH solution of 2,
and the structure determination confirmed the presence of a [3 + 3]
complex (Figure S1). Unlike 1, 2 does not host a CH2Cl2 mole-
cule in a dimeric capsule but instead forms centrosymmetric dimers
by π stacking (separation of 3.50 Å) of the NiNCCCO chelate
ring and adjacent arene rings (Figure 3a). Solid 2 was dissolved
in 1,2-Cl2C6H4/CH2Cl2 (4:1 by volume), and ∼0.5 equiv of C60
was added. After sonication and filtration, Et2O was diffused into
the filtrate. Structural analysis of the black plates that grew
confirmed a formulation of 2(2) C60 1,2-Cl2C6H4 0.25H2O
3 3 3
3 3 3
guest contacts lie in the range 3.6623(17)ꢀ3.8634(17) Å.
Crystalline 2(3) CH2Cl2 was dissolved in 1,2-Cl2C6H4/
3
CH2Cl2, and 1 equiv of C60 was added. Following sonication,
the mixture was filtered, and Et2O was allowed to diffuse into the
fitrate, yielding crystals of 2(3) C60 CH2Cl2 within a few days.
3
3
3
and the assembly of molecular capsules hosting C60. The com-
3
3
Structural analysis19 revealed that 3 hosts either a C60 or CH2Cl2
pound crystallizes in the P1 space group, and the asymmetric unit
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solution of 3 is smaller in size than the major component.
Diffusion coefficients (D) of 8.38 ꢁ 10ꢀ10 and 9.46 ꢁ 10ꢀ10
m2 sꢀ1 were determined for the major and minor components
(Figure S5). The relative sizes of the species follow from the
StokesꢀEinstein equation. The ratio of the D values is 0.885,
which leads to a ratio of molecular masses for the two species
of 0.693, in reasonable agreement with a calculated value of
0.667 for M(dimer)/M(trimer). EXSY spectra were recorded for
a CDCl3 solution of 3 with mixing times of 250 ms, 500 ms, 1 s,
and 2 s, and with the approximation of initial rates, the rate
constants kf and kb in eq 1 were determined to be 0.0254 and
0.0577 sꢀ1, respectively.
Figure 5. Partial packing diagram for 2(3) C60 CH2Cl2.
3
3
kf
2½NiLꢂ3
s
3½NiLꢂ
ð1Þ
fs
r
2
kb
Attempts to separate the two species by chromatography resulted
in only one brown fraction, the 1H NMR spectrum of which was
identical to the original spectrum of the bulk sample. The ESI-
MS spectra of dissolved crystalline 2 CH2Cl2 and 2(3) CH2Cl2
3
3
revealed peak envelopes corresponding to [Ni2L2 + Na]+, [Ni3L3
+ Na]+, and [Ni4L4 + Na]+. We propose that the latter is better
formulated as [2{Ni2L2} + Na]+ rather than as a single tetrameric
complex. The matrix-assisted laser desorption ionizationꢀtime of
flight (MALDIꢀTOF) mass spectra also indicated the presence
of trinuclear and dinuclear complexes.
The presence of a dynamic equilibrium was also tested by
adding benzyl derivative 2 to a CDCl3 solution of 3. Figure 6a
shows the imine resonances of the trinuclear and dinuclear species
of 3. Upon addition of 2, seven imine signals were observed
(Figure 6b), consistent with an equilibrium mixture of homo-
and heteroleptic ethoxy and benzyloxy di- and trinuclear species.
In summary, we have shown that Ni3S3-based bowls prepared
in one-pot syntheses can capture either CH2Cl2 or C60. Although
numerous examples of hostꢀfullerene guest complexes have pre-
viously been structurally characterized, this report is a rare example
in which the size and shape of the cavity of the metallohost
complements that of the C60 molecule. The formation of 1:1
(bowl host) or 2:1 (capsule host) assemblies with the fullerene
depend upon the pendant substituents. NMR spectroscopic
titration data and a Job’s plot confirmed the formation of a 1:1
Figure 6. Part of the 400 MHz 1H NMR spectra (CDCl3) of 3 (a) alone
and (b) after addition of 2. Signals from the NdCH protons are marked
as follows: trimers, hashed arrows; dimers, solid arrows. * indicates
residual CHCl3.
molecule (Figure 5). The bowl hostꢀC60 guest assembly is similar
to that observed for 1 C60 0.5(1,2-Cl2C6H4) Et2O but contrasts
3
3
3
with the 2:1 (capsule hostꢀC60 guest) species observed in
2(2) C60 1,2-Cl2C6H4 0.25H2O. 2(3) C60 CH2Cl2 crystal-
3
3
3
3
3
lizes in the trigonal P31c space group, and the asymmetric unit
contains one-third of each of the two independent molecules of 3
and one-third of both CH2Cl2 and C60. Each complete molecule
is generated by rotation about the threefold axis passing through
the center of each Ni3S3 unit (which causes threefold disorder of
the CH2Cl2). The 3 CH2Cl2 domains stack parallel to the c axis,
3
with the Cl atoms of the guest molecule facing into the cavity of 3
hostꢀguest complex 1 C60 in 1,2-Cl2C6D4 solution.
3
[closest Ni Cl contact = 3.275(3) Å]; the CH2Cl2 H atoms
3 3 3
interact with S atoms of the adjacent bowl (shortest S
H
3 3 3
’ ASSOCIATED CONTENT
contact = 2.95 Å). The fullerene nestles into the bowl with
shortest Ni C contacts of 3.674(10) and 3.544(7) Å.
3 3 3
S
Supporting Information. Experimental details, full char-
b
Although the solid-state structures of 1, 2 and 3 all feature a
trimeric nickel(II) complex, the electrospray ionization mass
spectrometry (ESI-MS) data and NMR spectra in CDCl3 solu-
tion are consistent with more than one solution species in each
acterization data for all new compounds, Figures S1ꢀS5, Table
S1, and crystallographic data (CIF). This material is available free
of charge via the Internet at http://pubs.acs.org.
1
case. The H and 13C NMR spectra of dissolved crystals of
’ AUTHOR INFORMATION
2(3) CH2Cl2 show two sets of signals (Figure 6a; also see the
3
Corresponding Author
edwin.constable@unibas.ch; catherine.housecroft@unibas.ch
SI). The subspectra of the two components are very similar (e.g.,
for the imine H, singlets appear at 7.67 and 7.62 ppm in a
reproducible ratio of 1:3). Similarly, NMR spectra of CDCl3
solutions of 1 or 2 CH2Cl2 showed two sets of signals in a ratio
3
’ ACKNOWLEDGMENT
of 1:4, the latter being concentration-dependent. These results
are consistent with those observed for a related Ni3S3 system for
which the minor component has been proposed to be dimeric.21
Using pulsed-gradient spin-echo (PGSE) diffusion NMR spec-
troscopy, we confirmed that the minor species in a CDCl3
We thank the Swiss National Science Foundation and the
University of Basel for financial support. G.Z. thanks the Novartis
Foundation (formerly Ciba-Geigy Jubilee Foundation) for support.
Cathrin Ertl is thanked for the PGSE spectroscopic data.
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