Subcomponent Exchange Transforms an FeII4L4 Cage from High- to Low-Spin, Switching Guest Release in a Two-Cage System

Article abstract of DOI:10.1021/jacs.7b01478

Subcomponent exchange transformed new high-spin Fe^II₄L₄ cage 1 into previously-reported low-spin Fe^II₄L₄ cage 2: 2-formyl-6-methylpyridine was ejected in favor of the less sterically hindered 2-formylpyridine, with concomitant high- to low-spin transition of the cage's Fe^II centers. High-spin 1 also reacted more readily with electron-rich anilines than 2, enabling the design of a system consisting of two cages that could release their guests in response to combinations of different stimuli. The addition of p-anisidine to a mixture of high-spin 1 and previously-reported low-spin Fe^II₄L₆ cage 3 resulted in the destruction of 1 and the release of its guest. However, initial addition of 2-formylpyridine to an identical mixture of 1 and 3 resulted in the transformation of 1 into 2; added p-anisidine then reacted preferentially with 3 releasing its guest. The addition of 2-formylpyridine thus modulated the system's behavior, fundamentally altering its response to the subsequent signal p-anisidine.

Full text of DOI:10.1021/jacs.7b01478

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Communication
II4
4
Subcomponent Exchange Transforms a FeLCage from High-
to Low-Spin, Switching Guest Release in a Two-Cage System
Anna McConnell, Catherine Aitchison, Angela Beth Grommet, and Jonathan R. Nitschke
J. Am. Chem. Soc., Just Accepted Manuscript • Publication Date (Web): 20 Apr 2017
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Journal of the American Chemical Society
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Subcomponent Exchange Transforms a FeII L Cage from High- to
Low-Spin, Switching Guest Release in a Two-Cage System
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Anna J. McConnell,† Catherine M. Aitchison, Angela B. Grommet and Jonathan R. Nitschke*
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, U.K.
Supporting Information Placeholder
spin-crossover,13 attributed to a steric clash between methyl
groups and adjacent pyridine rings destabilizing the low-spin
ABSTRACT: Subcomponent exchange transformed new
high-spin FeII4L4 cage 1 into previously-reported low-spin
state relative to the high-spin state.14 FeII mononuclear com-
plexes are observed to preferentially incorporate
2-formylpyridine over 2-formyl-6-methylpyridine.14 Thus,
alleviation of steric clash might drive exchange of
2-formylpyridine for 2-formyl-6-methylpyridine, transform-
ing a FeII cage from high-spin to low-spin.
FeII4L4 cage 2: 2-formyl-6-methylpyridine was ejected in favor
of the less sterically hindered 2-formylpyridine, with con-
comitant high- to low-spin transition of the cage’s FeII cen-
ters. High-spin 1 also reacted more readily with electron-rich
anilines than 2, enabling the design of a system consisting of
two cages that could release their guests in response to com-
binations of different stimuli. The addition of p-anisidine to a
mixture of high-spin 1 and previously-reported low-spin
FeII4L6 cage 3 resulted in the destruction of 1 and the release
of its guest. However, initial addition of 2-formylpyridine to
an identical mixture of 1 and 3 resulted in the transformation
of 1 into 2; added p-anisidine then reacted preferentially with
3 releasing its guest. The addition of 2-formylpyridine thus
modulated the system’s behavior, fundamentally altering its
response to the subsequent signal p-anisidine.
Here we report the self-assembly of high-spin FeII4L4 cage 1
incorporating 2-formyl-6-methylpyridine as a subcompo-
nent. Cage 1 binds a variety of neutral guests in acetonitrile.
Paramagnetic 1H NMR spectroscopy provides a sensitive
means for detecting guest encapsulation and determining
guest identity due to the isotropic shifts of the paramagnetic
FeII centers.15 High-spin 1 did not transition to a low-spin
state upon lowering the temperature to 268 K; however, the
chemical stimulus 2-formylpyridine transformed high-spin 1
into previously-reported low-spin FeII4L4 cage 2,16 as a conse-
quence of aldehyde exchange. This transformation was used
to set up a system such that either one of a pair of cages
could be opened and its guest released, following the addi-
tion of a different chemical stimulus, p-anisidine.
Stimuli-responsive container molecules,1 whose uptake and
release of guests can be controlled through the application of
external signals,2 are useful building blocks for molecular
networks.3 The development of new stimuli-responsive be-
haviors1f, 4 allows for these networks to increase in complexi-
ty, as cages may be addressed individually within mixtures,5
or signals may be passed between network members in order
to construct complex responses.6 An ultimate goal is to ap-
proach the functional complexity exhibited by the signaling
pathways in biological systems.
Structures prepared via subcomponent self-assembly7 can
transform in response to external stimuli through the re-
versible reconfiguration of the dynamic covalent and coordi-
native bonds holding the structures together;8 examples in-
clude the rearrangement of a Schiff-base ligand7b and meso-
helicates7e via aldehyde exchange, and the imine exchanges
undergone by dynamic cages when an electron rich amine
substitutes an electron poor amine residue.9
Cage 1 was prepared by reaction of iron(II) triflate, tri-
amine A16 and commercially available 2-formyl-6-
methylpyridine (1:1:3) in CD3CN (Scheme 1). The [FeII4L4]
composition was confirmed by high-resolution ESI-MS (Fig-
ure S4). Unlike previously-reported low-spin 2,16 which con-
tains 2-formylpyridine residues, the FeII centers of 1 are high-
1
spin between 268 K and 318 K (Figure S5). The H NMR spec-
trum of 1 has the eight proton signals expected for a T-
symmetric structure spread over ca. 240 ppm (Figure S1),
displaying Curie-Weiss behavior above 268 K (Figure S6).17
We infer the high-spin character of 1 to be a consequence of
steric clash between methyl groups and adjacent pyridine
rings.13-14
The paramagnetic signals of 1 were assigned following the
methods employed by Raehm18 and Ward19 for paramagnetic
CoII complexes. T1 relaxation times were measured and corre-
lated to the distances between the paramagnetic center and
proton according to the Solomon equation (Table S1, Figure
S7).20 Cross-peaks observed in the COSY spectrum (Figure
S3) and comparison to the calculated chemical shifts for a
related high-spin FeII mononuclear complex21 provide addi-
In this study, we envisaged a new approach whereby a
chemical stimulus transforms a high-spin FeII4L4 cage into a
low-spin analog through exchange of a more bulky aldehyde
subcomponent for a less bulky one. Others10 and our group11
have reported FeII cages and helicates that undergo spin-
crossover12 induced by heat and light. Mononuclear FeII
complexes incorporating 2-formyl-6-methylpyridine undergo
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1
tional support for our H NMR assignments and proposed
1
solution structure of 1.
Scheme 1. Self-Assembly of High-Spin FeII L4 Cage 1
and Transformation to Low-Spin Cage 2
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Figure 1. Guests for cage 1 that a) bind strongly, b) bind
weakly and c) are not observed to bind.
encapsulated guest signals were observed alongside the ma-
jor set of peaks corresponding to bound adamantane (Figure
S14). GC-MS analysis of these guests (commercial claimed
purity of 98+%) revealed trace impurities (Figures S10-13).
We infer the minor NMR signals to correspond to these im-
purities encapsulated within cage 1. Thus, cage 1 can detect
trace quantities of strongly binding guests within mixtures
by NMR spectroscopy in a manner that would not be possi-
ble using diamagnetic hosts.
Similarly, trace cis-decalin could be sensed as an impurity
in commercial trans-decalin (Figures S9, S65). Cage 1 also
binds o-xylene over the other xylene isomers (Figure S66).
The separation of xylene isomers from mixed hydrocarbons
is costly and inefficient due to the close similarity in the
physicochemical properties of the isomers.22 Host-guest
complexation within 1 could thus enable the separation of
o-xylene.23
Host-guest studies revealed cage 1 (2 mM) encapsulates a
similar range of guests (10-15 equiv) to cage 216 in acetonitrile
at 298 K, although guest uptake was faster (approaching
equilibrium within 4 h) since high-spin FeII complexes have
more labile N→FeII bonds than their low-spin analogs. In the
1H NMR spectra each guest was bound in slow exchange with
shifts of both the cage and guest peaks observed upon encap-
sulation. Encapsulated guest signals were observed in all
cases between -10 and -20 ppm and their T1 values were of a
similar magnitude to the T1 value for proton h, reflecting the
isotropic shifts experienced by the guests within the para-
magnetic host cavity (Table S2).
Subcomponent exchange within cage 1, empty or with
bound 1-fluoroadamantane (1-FA), proceeded in anhydrous
CD3CN upon addition of 2-formylpyridine (24 equiv), result-
ing in a color change from orange to red-purple. The use of
1
excess 2-formylpyridine was found to result in sharper H
NMR spectra, rendering the process easier to follow. Re-
1
leased 2-formyl-6-methylpyridine was observed by H NMR
Prospective guests for 1 were divided into three groups
based upon the extent of host-guest complexation (Figure 1).
Complete conversion to the host-guest complex was ob-
served for guests that matched the size and shape of the cavi-
ty well, such as adamantane (Figures 1a, S8), whereas two
sets of NMR signals corresponding to the empty cage and
host-guest complex were observed for guests with a poorer
match for the cavity, such as o-xylene (Figures 1b, S9). m- and
p-Xylene bound in trace amounts (Figures S49-52) and hex-
afluorobenzene did not bind at all, which we attribute to
unfavorable interactions between this electron deficient
guest and the electron deficient cavity (Figure 1c). The rela-
tive binding affinities of the guests in Figure 1a were estimat-
ed from competition experiments (Figures S53-S66).
The paramagnetic FeII centers in cage 1 enabled the sensi-
tive detection of guest encapsulation and straightforward
discrimination of signals belonging to different isomers by 1H
NMR spectroscopy. Encapsulated NMR signals are spread
over a wider chemical shift range, thus reducing signal over-
lap and improving dispersion upon encapsulation.15b For
strongly bound guests, such as adamantane, minor sets of
spectroscopy with a concomitant decrease in the intensities
of the peaks for cage 1 and appearance of new paramagnetic
species after 16 h at 50 °C (Figures S69, S72). However, no
NMR peaks corresponding to cage 2 (Figure S70) or its host-
guest complex (Figure S73) were observed until 5% D2O
(v/v) was added, resulting in a color change to dark purple
(Figure S67). The requirement for water to complete the
transformation implies that hydrolyzed species are interme-
diates during the exchange process, as reported by Hahn7b
and Hooley.7e When no guest was present, the transfor-
mation was complete within 1 day at 50 °C (Figure S70)
whereas the equilibration process was slower for the full
cage, with kinetics depending on the amount of water added
(Figure S75). In addition to the encapsulated 1-FA within 2, a
putative intermediate encapsulated 1-FA species was ob-
served in the 19F NMR spectrum during equilibration (Figure
S74), suggesting the guest remained bound during the trans-
formation.
Selective cage breakdown and guest release could be trig-
gered by the chemical signal p-anisidine, which opens cages
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Scheme 2. Cage Disassembly and Guest Release Triggered by p-Anisidinea
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a
High-spin FeII4L4 cage 1 reacted with p-anisidine in the presence of low-spin FeII4L6 cage 3, releasing the guest
1-fluoroadamantane (1-FA) and forming mononuclear complex 4. Following the transformation from high-spin cage 1 to low-spin
cage 2, low-spin FeII4L6 cage 3 was instead broken down by p-anisidine releasing the guest BF4 and forming 5. Each transfor-
-
mation was performed in the presence of an excess of 1-FA (10 equiv per cage 1 or 2).
through imine exchange.3a, 24 As cage 1 was thermodynami-
cally unstable with respect to in the presence of
1-FA signal disappeared in the 19F NMR spectrum, while BF4
-
2
remained encapsulated within 3 (Figure S87).
2-formylpyridine, we hypothesized that 1 might react more
readily with p-anisidine and that a low-spin FeII4L6 cage 325
(Scheme 2) might react with p-anisidine at a rate intermedi-
ate between those of 1 and 2. This differential reactivity
would allow for the functioning of the system shown in
Scheme 2. A mixture of 1 and 3 would thus react with
p-anisidine to selectively liberate the guest of 1, whereas ini-
tial treatment with 2-formylpyridine would result in a mix-
ture of 2 and 3, whose reaction with p-anisidine would selec-
In contrast, the selectivity of cage disassembly was invert-
-
ed for an equimolar mixture of [1-FA ⊂ 2] and [BF4 ⊂ 3] due
to the increased thermodynamic stability of face-capped
compared with edge-bridged tetrahedra;24 only the NMR
-
signals for [BF4 ⊂ 3] disappeared following progressive addi-
tion of p-anisidine (12 equiv) and heating (Figures S100-S101).
Control experiments were also carried out for the individual
cages (Figures S102-S105). The mixture of [1-FA ⊂ 2] and
-
[BF4 ⊂ 3] resulting from aldehyde exchange reacted similarly
-
tively liberate the guest of 3. Guests containing fluorine (BF4
with stoichiometric p-anisidine (12 equiv) upon heating (Fig-
for 3 and 1-FA for 1 and 2) were chosen so that guest release
could be monitored by 19F NMR spectroscopy. Importantly,
control experiments revealed there was no aldehyde or guest
exchange between [BF4- ⊂ 3] and [1-FA ⊂ 1] and the aldehyde
exchange process was unaffected by the presence of [BF4 ⊂ 3]
(Figures S95-S97).
ure S98), resulting in disassembly of [BF4- ⊂ 3] and release of
-
BF4 , while 1-FA remained bound within the transformed
cage 2 (Figures S99).
We have demonstrated the transformation of FeII centers
in a FeII4L4 cage from high- to low-spin for the first time via
selective aldehyde exchange. In the presence of low-spin
FeII4L6 cage 3, this transformation was employed to switch
the guest release outcome following application of the chem-
ical signal p-anisidine, due to the difference in the relative
reactivities of the high- and low-spin cages. Switching the
spin state of a cage is thus a promising new strategy for sig-
nal transduction. Future work will focus upon integrating
these processes into larger signalling networks.6
-
The selectivity of cage disassembly and guest release was
first investigated for an equimolar mixture of [1-FA ⊂ 1] and
[BF4- ⊂ 3]. As p-anisidine (24 equiv) was progressively added,
the H NMR signals corresponding to [1-FA ⊂ 1] were ob-
served to disappear while those for [BF4 ⊂ 3] remained (Fig-
ures S88-S89). Control titrations with each cage separately
revealed p-anisidine to react with both cages, but more readi-
ly with [1-FA ⊂ 1] (Figures S91-S94). The addition of excess
p-anisidine (24 equiv) to [1-FA ⊂ 1] thus resulted in its com-
plete disassembly at room temperature in the presence of
[BF4- ⊂ 3], releasing 1-FA (Figures S85-S87). The encapsulated
1
-
ASSOCIATED CONTENT
Supporting Information
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Corresponding Author
jrn34@cam.ac.uk.
Present Address
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†Otto Diels-Institute of Organic Chemistry, Kiel University,
Otto-Hahn-Platz 4, Kiel D-24098, Germany.
Notes
The authors declare no competing financial interests.
ACKNOWLEDGMENT
This work was funded by the European Research Council
(695009) and EPSRC (EP/M01083X/1). We also thank the
Cambridge Trusts for PhD funding (A.B.G.), the Cambridge
Chemistry NMR staff and James Keeler for useful discussions,
and Jenifer Mizen for preliminary studies on the microwave
synthesis of a precursor of A. We also thank Cally Haynes,
Julia Guilleme, Marion Kieffer, Felix Rizzuto and the EPSRC
UK National Mass Spectrometry Facility at Swansea for col-
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>
>
≥
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≈
≈
=
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c)
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triamine A
high spin Fe4L4 cage 1
low spin Fe4L4 cage 2
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+ 12
BF4-  3
low spin Fe4L6
4
1-FA  1
high spin Fe4L4
BF4-  3
low spin Fe4L6
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low spin Fe4L4
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low spin Fe4L6
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low spin Fe4L4
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