An Allosterically Regulated, Four-State Macrocycle

Article abstract of DOI:10.1021/acs.inorgchem.7b02745

Macrocycles capable of host-guest chemistry are an important class of structures that have attracted considerable attention because of their utility in chemical separations, analyte sensing, signal amplification, and drug delivery. The deliberate design a

Full text of DOI:10.1021/acs.inorgchem.7b02745

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Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
An Allosterically Regulated, Four-State Macrocycle
Andrea I. d’Aquino,† Ho Fung Cheng,† Joaquín Barroso-Flores,‡ Zachary S. Kean,†
Jose Mendez-Arroyo,† C. Michael McGuirk,† and Chad A. Mirkin*,†
†Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, United States
‡Centro Conjunto de Investigacion
UNAM, Unidad San Cayetano, Toluca, Estado de Mex
́
en Química Sustentable, UAEM-UNAM, Carretera Toluca-Atlacomulco Km 14.5, Personal de la
ico C.P. 50200, Mexico
́
́
S
* Supporting Information
ABSTRACT: Macrocycles capable of host−guest chemistry are
an important class of structures that have attracted considerable
attention because of their utility in chemical separations, analyte
sensing, signal amplification, and drug delivery. The deliberate
design and synthesis of such structures are rate-limiting steps in
utilizing them for such applications, and coordination-driven
supramolecular chemistry has emerged as a promising tool for
rapidly making large classes of such systems with attractive
molecular recognition capabilities and, in certain cases, catalytic
properties. A particularly promising subset of such systems are
stimuli-responsive constructs made from hemilabile ligands via
the weak-link approach (WLA) to supramolecular coordination
chemistry. Such structures can be reversibly toggled between different shapes, sizes, and charges based upon small-molecule and
elemental-anion chemical effectors. In doing so, one can deliberately change their recognition properties and both stoichiometric
and catalytic chemistries, thereby providing mimics of allosteric enzymes. The vast majority of structures made to date involve
two-state systems, with a select few being able to access three different states. Herein, we describe the synthesis of a new
allosterically regulated four-state macrocycle assembled via the WLA. The target structure was made via the stepwise assembly of
ditopic bidentate hemilabile N-heterocyclic carbene thioether (NHC,S) and phosphino thioether (P,S) ligands at PtII metal
nodes. The relatively simple macrocycle displays complex dynamic behavior when addressed with small-molecule effectors, and
structural switching can be achieved with several distinct molecular cues. Importantly, each state was fully characterized by
multinuclear NMR spectroscopy and, in some cases, single-crystal X-ray diffraction studies and density functional theory
computational models. This new structure opens the door to complex multicue switching reminiscent of multistate
chemoswitches that could be important in controlling stoichiometric and catalytic transformations as well as generating
molecular logic systems.
with deliberate control. WLA-based supramolecular systems are
attractive because they offer (1) chemical access to multiple
different states with tailorable selectivity and binding
affinities,16,17 (2) high functional group tolerance,11,18 and (3)
modularity, including access to structures with a wide variety of
metal nodes and ligand types.9,10,18 Many stimuli-responsive
systems have been developed based on this platform, through
the incorporation of catalytic,19−23 redox-active,24,25 and host−
guest recognition sites17 into the ligands in such a way that
small-molecule-induced structural changes result in marked
changes in the properties of these complexes.
INTRODUCTION
■
The rational design of supramolecular systems, which undergo
reversible structural changes, has been a long-standing goal of
chemists because of the potential for developing technologically
useful catalysts, switches, and sensors.1,2 Coordination-driven
supramolecular chemistry has emerged as a powerful means to
assemble large and complex macrocycles, cages, and capsules,
with general methods developed by several groups,3−11
including our own,12−14 for assembling such multicomponent
structures. Among the different approaches to coordination-
driven supramolecular constructs, the weak-link approach
(WLA) has emerged as a powerful means for synthesizing
complexes that can undergo reversible small-molecule-induced
structural changes and therefore be toggled between “closed”
rigid states and “open” flexible ones (Figure 1a−c).10,11,15
Through the modular and convergent assembly of metal ions
and hemilabile ligands, the WLA provides a platform for
engineering molecular selectivity and stimuli-responsiveness
The addressable nature of these structures arises from the
hemilabile phosphinoalkyl chalcoether or amine (P,X; P =
Ph2PCH2CH2− and X = O, S, Se, N) ligands typically
Special Issue: Self-Assembled Cages and Macrocycles
Received: November 1, 2017
© XXXX American Chemical Society
A
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Figure 1. WLA to coordination-driven supramolecular (a) macrocycles, (b) tweezers, and (c) triple-decker complexes. For the sake of this example,
complexes with PdII and PtII metal centers are illustrated, but the WLA works with many other metal systems including RhI, CuI, and IrI.26−29
coordinated to d8 transition metals (such as RhI, PdII, or PtII).
These complexes can reversibly access three distinct states
(fully open, semiopen, and fully closed) through the removal
and introduction of small molecules coordinating ligands (XC)
or elemental anions, which can easily displace the metal−S
bond (“weak-link”, in this example), while preserving the
metal−P bond (“strong-link”; Figure 1; for simplicity, the
semiopen macrocycle is not shown in Figure 1a and the fully
open states are omitted from Figure 1b,c). Recently, our group
reported the stepwise synthesis of heteroligated PtII WLA
tweezer and triple-decker complexes with monotopic bidentate
hemilabile N-heterocyclic carbene thioether (NHC,S) and
phosphino thioether (P,S) ligands (Scheme 1a).30 This strategy
allowed for the assembly of air-stable heteroligated structures
without the requirement of different electron-donating abilities
at the “weak-links”, a requisite for previous heteroligated PtII
WLA systems.31 This stepwise approach was demonstrated in
the synthesis of heteroligated tweezer and triple-decker
complexes.30 Recently, this approach was used to synthesize a
switchable molecular receptor that can access two distinct
structural states: flexible “open” and rigid “closed” states
(Scheme 1b).16 Although this construct was the first example of
a heteroligated PtII macrocyclic system, the limited structural
states accessible by this complex prevented switching between
multiple binding pockets, each capable of binding a different
guest molecule or exhibiting a different, but complementary
function. Developing allosterically regulated structures that can
access multiple states with different properties is a fundamental
challenge and remains an important synthetic goal. The
structures reported herein are the first step toward the
development of multistate, stimuli-responsive receptors, cata-
lysts, and chemoswitches, with fundamentally interesting
functions relating to their different chemically tunable
structures.
Herein, we apply this stepwise approach to the synthesis of a
heteroligated multistate macrocycle, possessing two different
B
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Scheme 1. (a) Stepwise Approach to the Synthesis of WLA Tweezer and Triple-Decker Complexes, (b) Stepwise Assembly of a
WLA Heteroligated PtII Macrocyclic Capsule, and (c) the Scope of This Work Applying This Approach to the Synthesis of an
a
Allosterically Regulated, Multistate, Heteroligated Macrocycle
a
MeOD = methanol-d4.
ditopic bidentate ligands bridging two PtII metal nodes, which
exhibits four-state switching behavior, considerably more
complex than previously reported for WLA systems (Scheme
1c). Experimental and computational evidence suggest that,
while the addressability of the system arises from the
coordination chemistry, the resulting complexity of the
macrocycle behavior is a function of multiple factors, including
geometry, strain, strength of coordination bonds, and
electronics. This is in contrast with previously reported WLA
systems that rely on the use of weakly chelating ligands to
achieve the assembly of heteroligated structures, which are too
unstable to form significantly strained structures.11
to confirm that the metalation reaction of 1 with Ag2O and the
subsequent transmetalation reaction with PtCl2(cod) were
successful, complex 2 was characterized by matrix-assisted laser
desorption/ionization time-of-flight mass spectrometry
(MALDI-TOF MS), with the monoisotopic mass and isotopic
distribution matching those of the molecular ion [M − Cl]+
(see the Supporting Information, SI). Because of the low
solubility of complex 2 in CH2Cl2, single crystals suitable for X-
ray diffraction were obtained by the slow diffusion of diethyl
ether (Et2O) into a solution of complex 2 in dimethyl sulfoxide
(DMSO). The solid-state structure reveals a complex with
DMSO ligands coordinated to the metal center, displacing the
weakly bound thioether groups on the NHC,S hemilabile
ligand (see the SI).
RESULTS AND DISCUSSION
■
The irreversible metalation−transmetalation reaction at the
NHC ligand allows for the subsequent use of strongly binding
P,S ligands without the occurrence of previously observed
ligand-exchange reactions.31,32 Because of the low solubility of
2 in CH2Cl2, complex 2 was dissolved in a 1:1 mixture of
CH2Cl2 and methanol (MeOH) and reacted with 1 equiv of the
P,S-aryl-S,P ligand 3 (Scheme 2). Characterization of the
WLA Macrocycle Synthesis and Characterization. In
order to synthesize the PtII heteroligated WLA macrocycle, we
utilized a previously reported stepwise approach employing a
ditopic bidentate NHC,S-based ligand in conjunction with a
traditional ditopic bidentate P,S-based ligand, both of which
bind PtII strongly enough to result in kinetically stable
structures. Benzimidazolium chloride salt 1 was treated with
2 equiv of dichloro(1,5-cyclooctadiene)platinum(II)
[PtCl2(cod)] and 1 equiv of silver(I) oxide (Ag2O) in
dichloromethane (CH2Cl2) to produce Pt2Cl4(κ2:μ:κ2-
NHC,S) (2) as an insoluble white solid (Scheme 2). In order
1
resulting product by H and 31P NMR spectroscopy revealed
that the fully open macrocycle was obtained as a mixture of
isomers (4a−4c) when synthesized under homogeneous
conditions in a 1:1 mixture of MeOH and CH2Cl2 (Scheme
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a
Scheme 2. Synthesis of Fully Open Heteroligated Macrocycles 4a−4c
a
Reaction conditions: (i) 2 equiv of PtCl2(cod); 1 equiv of Ag2O; 6:1 CH2Cl2/MeOH, 24 h, 60 °C. (ii) 1:1 MeOH/CH2Cl2, 25 °C. (iii) CH2Cl2, 25
°C.
2). Consistent with this observation, a high-resolution mass
spectrometry (HRMS) spectrum of the sample exhibited a peak
with a mass-to-charge (m/z) ratio corresponding to the
molecular ion [M − Cl]+ (see the SI).
31P NMR spectroscopy was used to further characterize the
resulting compounds because it is highly diagnostic of the
coordination environment. Additionally, the resonance shifts
and platinum−phosphorus coupling patterns were consistent
with the presence of a combination of cis-4a, trans-4b, and
combined cis/trans (4c) fully open complexes (Figure 2a).33−35
As summarized in Figure 2, the fully open trans species 4b
[trans-Pt2Cl4(κ1:μ:κ1-NHC,S)(κ1:μ:κ1-P,S)] contains two plat-
inum nodes in which the phosphine and carbene are trans to
one another; this configuration is evidenced by a resonance at
9.7 ppm with a JP−Pt coupling constant of 2300 Hz. The
presence of the cis complex 4a [cis-Pt2Cl4(κ1:μ:κ1-NHC,S)-
(κ1:μ:κ1-P,S)] is evidenced by a resonance at 1.8 ppm with a
larger JP−Pt of 3740 Hz. Downfield from each of the two major
resonances are additional sets of signals at 2.5 and 10.5 ppm,
which are of equal intensity relative to one another (Figure 2a).
We proposed that they correspond to a species that has both cis
and trans nodes in one complex, 4c [cis-PtCl2(κ1:μ:κ1-
NHC,S)(κ1:μ:κ1-P,S)-trans-PtCl2(κ1:μ:κ1-NHC,S)(κ1:μ:κ1-
P,S)]. This assignment is supported by the 31P−195Pt coupling
constants, which are comparable to that of the purely cis and
purely trans isomers (Figure 2). The relative chemical shifts
along with the 31P−195Pt coupling constants reveal important
information about the trans influence inherent to these
structures.34−36 In the case of strongly donating ligands, the
chemical shift typically appears further downfield,35 such as in
the trans complex 4b. Between isostructural complexes, larger
31P−195Pt coupling constants are indicative of a more weakly
coordinating ligand trans to the phosphine ligand, while smaller
coupling constants indicate the presence of a more strongly
coordinating ligand. The NHC is a more strongly donating
ligand than the chloride; therefore, the coupling constant of the
Figure 2. (a) 31P NMR spectrum of the complex mixture 4 in CD2Cl2,
(b) 31P NMR spectrum of single crystals of the fully open cis complex
4a dissolved in CD2Cl2, (c) 31P NMR spectrum of 4a after several
hours, indicating the reappearance of resonances attributable to the cis
(4a), trans (4b), and cis/trans (4c) species.
trans complex 4b (JP−Pt = 2300 Hz) is significantly smaller than
that of the cis complex 4a (JP−Pt = 3740 Hz). The cis/trans
species (4c) follows the same trend, with one PtII node in the
cis configuration and the other in the trans orientation, each
having a chemical shift and coupling constant in the same
region of the spectrum as the fully cis and trans counterparts,
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respectively. The resonance at 2.5 ppm has a larger coupling
constant (JP−Pt = 2943 Hz), while the resonance of equal
intensity further downfield at 10.5 ppm has a smaller coupling
constant (JP−Pt = 2270 Hz).
When the solvent from the solution of 4a−4c was slowly
evaporated, single crystals composed exclusively of 4a were
isolated. The solid-state structure of 4a is consistent with the
solution-phase spectroscopic characterization of the fully open
cis isomer, with one κ1:μ:κ1-P,S ligand and one κ1:μ:κ1-NHC,S
ligand coordinated to PtII through the phosphines and
carbenes, respectively (Figure 3). We hypothesized that the
(DFT) calculations in the gas phase. DFT calculations were
carried out on the solid-state structure of 4a with the Gaussian
09 suite of programs at the B97D/LANL2DZ level of theory.
The selected functional B97D empirically includes dispersion
effects, which allows for a better description of the electronic
structure. The geometry-optimized model of 4a provided the
basis for developing the energy-minimized models of 4b and 4c
and their respective isomers. The bond lengths and angles of
model 4a are nearly identical with those measured in the solid-
state crystal structure. All structures correspond to local minima
on the potential energy surfaces and, therefore, are chemically
accessible states. Population analysis was performed within the
natural bond orbital formalism to provide a localized, pairwise
description of the electron density.
We hypothesized that complexes 4a−4c would be relatively
close in their free energies because of the observation of rapid
and dynamic conversion from the purely cis state to the mixture
of cis, trans, and cis/trans states. Energy-minimized models
(Figure 4) were obtained for each complex (4a−4c), and their
Figure 3. Crystal structure of 4a, drawn with a 50% thermal ellipsoid
probability. Solvent molecules and H atoms have been omitted for
clarity. Selected bond lengths [Å] and angles [deg]: Pt1−Cl2
2.360(2), Pt1−Cl2 2.352(2), Pt2−Cl3 2.340(2), Pt2−Cl4 2.357(2);
Cl1−Pt1−Cl2 90.57(6), Cl3−Pt2−Cl4 89.42(6).
fully open cis structure would dynamically equilibrate back to
the 4a−4c complexes initially observed in solution, an
indication of fully open isomers that are similar in energy. To
assess the kinetic stability of 4a, its crystals were dissolved in
CD2Cl2 and the corresponding 31P NMR spectrum was
collected (Figure 2b), confirming our assignment of the 1.8
ppm shift belonging to 4a. Over time, we observed the
reappearance of resonances attributable to the trans (4b) and
cis/trans (4c) species, in addition to the cis complex 4a,
indicating that the complex undergoes isomerization in solution
(Figure 2c). Consistent with an equilibrium that involves small
energetic differences between isomers, we observed strong
solvent effects in the synthesis of 4a−4c. In an alternative
procedure, we reacted 2 with 3 in CH2Cl2 under heterogeneous
conditions (Scheme 2), resulting in the selective formation of
4a, as confirmed by 1H and 31P NMR spectroscopy and HRMS.
These results suggest that solvent effects play a key role in
lowering the kinetic barriers to establish an equilibrium
between isomers 4a−4c.
Figure 4. Energy-minimized models of complexes 4a−4c, obtained
from DFT calculations.
Solid-State Structure of 4a. The solid-state structure of
4a confirms that the fully open cis complex contains two inner-
sphere Cl atoms in the cis orientation. Complex 4a crystallized
in the triclinic centrosymmetric space group P1, and the
respective frontier orbitals were identified because they point to
the identity of the orbitals involved in WLA bonding. The
relative highest occupied molecular orbital (HOMO) and
lowest unoccupied molecular orbital (LUMO) energies were
calculated, and free energies were compared across different
structures. Table 1 summarizes the free energies calculated
from the DFT results.
Computational modeling of the fully open states suggests
that the overall free energies of the three states are close
enough that there is no strong preference to form one isomer
over the others. This is consistent with a system that has very
small energy differences between isomers, suggesting that 4a−
4c are nearly isoenergetic.
̅
asymmetric unit contains distinct Pt−Cl bond lengths and
angles (Figure 3) comparable to those found in the
literature.11,30 The PtII metal nodes adopt a square-planar
geometry [Cl1−Pt1−Cl2 = 90.57(6)°], with the phosphine
moieties cis to the carbene moieties [P1−Pt1−C1 = 94.6(2)°]
(Figure 3). The solid-state structure supports the assignments
1
made in the H and 31P NMR spectra (see the SI).
Computational Studies of Complexes 4a−4c. In order
to further investigate and understand the intramolecular
interactions and relative energies of each fully open isomer in
solution, we explored the electronic structures of 4a−4c and
their possible diastereomers with density functional theory
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phosphines).40,41 A variable-temperature (VT) 31P NMR study
revealed that the two signals (36 and 37 ppm) present at room
temperature coalesce at high temperature (330 K) in methanol-
d4, indicating rapid interconversion of the diastereomers in
solution (Figure 6). These results were further supported by
energy-minimized DFT calculations of the two diastereomers
(see the SI).
Table 1. Free Energies (kcal/mol) of 4a−4c Obtained from
DFT Calculations
compound
free energy (kcal/mol)
relative free energy (kcal/mol)
4a
4b
4c
−2811.3087
−2811.3029
−2811.3018
0.0
0.006
0.007
Formation of a New WLA Intermediate State. The
semiopen complex 5 was then reacted with 4 equiv of silver
tetrafluoroborate (AgBF4) in MeOH to abstract the four Cl
ions (two inner sphere and two outer sphere) and form the
fully closed complex with fully chelated ditopic bidentate P,S
and NHC,S ligands (Scheme 4a). Despite being a widely
reported method for closing WLA complexes, the reaction did
not result in a fully closed complex, as evidenced by 31P NMR
and MALDI-TOF MS. Instead, the signals in the 31P NMR
spectrum indicate a seemingly nonsymmetric product, display-
ing two distinct 31P resonances of equal intensity: one at 37
ppm and the other at 47 ppm (Figures 5a, 6). The signal at 37
ppm is consistent with previously reported PtII semiopen
complexes bearing NHC,S ligands,30 while the resonance
downfield at 47 ppm is characteristic of a fully closed
complex.30 Given that the peaks were of approximately equal
intensity, we hypothesized that a new WLA structural state was
obtained, namely, a macrocycle with one PtII metal center in a
fully closed state and the other in a semiopen state with
methanol-d4 bound to the Pt center rather than the expected
thioether moiety ([cis-Pt2(κ1:μ:κ2-NHC,S)(κ2:μ:κ2-P,S)-
CD3OD][BF4]4 (6); Scheme 4a). This hypothesis was
supported by the observation that the relative intensities of
the two peaks did not change when additional AgBF4 was
added, indicating that the reaction had gone to completion and
that the signal at 37 ppm does not belong to the residual
starting material. Although single crystals suitable for X-ray
Formation of the Semiopen WLA Macrocycle. Upon
exposure of the neutral, fully open state 4a (or the mixture of
4a−4c) to MeOH, the dicationic, semiopen complex [cis-
Pt2Cl2(κ1:μ:κ1-NHC,S)(κ2:μ:κ2-P,S)][Cl]2 (5) was formed via
the transfer of a chloride anion from the inner coordination
sphere of each PtII center to the outer coordination sphere
(Scheme 3). In polar protic solvents such as MeOH, outer-
sphere chloride counterions are stabilized by hydrogen
bonding, and the overall charge of the complex is stabilized
by the polar solvent, thus favoring the semiopen state. This
transformation is enthalpically driven and has been observed in
similar systems.11,37,38
The driving force of the switching from the fully open to the
semiopen state is 2-fold: (i) the two outer-sphere Cl ions and
the charged complex are stabilized by the polar solvent and (ii)
the structure is able to relieve ring strain by adopting the
semiopen configuration, as evidenced by DFT calculations (see
1
the SI). Complex 5 was thoroughly characterized via H and
31P NMR spectroscopy, diffusion ordered spectroscopy
(DOSY), two-dimensional correlation spectroscopy, and
HRMS (see the SI). The chemical shift observed in the 31P
NMR spectrum of complex 5 (δ 36 ppm) is shifted dramatically
downfield from that of the fully open PtII complexes with a
concurrent decrease in the coupling constant (JP−Pt = 3421 Hz;
Figure 5a). The downfield chemical shift and the decrease in
JP−Pt are consistent with increased back-donation from the Pt
atoms in the semiopen state in which the P,S ligand is fully
chelated with the phosphine moiety trans to the chloride.
Additionally, this downfield shift is diagnostic of phosphine-
containing ligands forming a five-membered ring.39
1
diffraction were not obtained, H and 31P NMR spectroscopy,
MALDI-TOF MS, and DFT calculations support the formation
of complex 6. MALDI-TOF MS analysis of complex 6 revealed
a peak at m/z 1767.308, which matches the simulated mass of
the molecular ion [M + H]+ (see the SI). Additionally, DFT
calculations were used to calculate the strength of the PtII−
OHCH3 bond, for comparison to the traditional PtII−S bond.
The PtII−OHCH3 interaction was calculated to be 30.70 kcal/
mol, which is comparable to that of the PtII−S bond energy in
the same compound (42.25, 57.10, and 63.77 kcal/mol),
further supporting the formation of 6 (see the SI).
A small shoulder at 37 ppm, hypothesized to be due to the
presence of diastereomers, is present just downfield of the
major resonance at 36 ppm in the 31P NMR spectrum. Previous
reports have shown that stereoisomers in WLA tweezer
complexes with two phosphinoalkyl thioether ligands, as well
as in WLA complexes bearing NHC,S and P,S ligands, can be
observed via 1H and 31P NMR spectroscopy.11,30 This is
attributed to the intramolecular inversion of the thioether
moiety trans to a ligand with a strong trans effect (such as
a
Scheme 3. Formation of the Semiopen WLA Macrocycle
a
Reaction conditions: (i) MeOH, 25 °C. (ii) CH2Cl2, 25 °C.
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Figure 5. (a) 31P NMR spectra of each distinct state accessible by the WLA macrocycle, namely, fully open (4), semiopen (5), and new intermediate
(6) states with one PtII node in the closed state and the other in a semiopen state and a fully closed state (7). (b) MALDI-TOF spectrum compared
with the ISOPRO simulation of the molecular ion [M − BF4]+ of complex 7.
solvent (nitromethane-d3) and analyzed by 31P NMR spectros-
copy (Figure 5a, 7), two major resonances at 49 ppm (JP−Pt
=
3450 Hz) and 51 ppm (JP−Pt = 3450 Hz) were observed with an
integral ratio of 2:1 (see the SI). This result is consistent with
the formation of the fully closed complex 7 with a 2:1 mixture
of diastereomers, as was the case in the semiopen complex.
Complex 7 was further investigated by a VT 31P NMR study
from room temperature to 338 K, a temperature range in which
thioether inversion was previously observed (see the SI).30,37
The coalescence of the two signals at high temperature
confirmed that the two peaks stem from the two diastereomers
of the fully closed complex 7.
To further investigate the structures and energies of the two
diastereomers, energy-minimized models were calculated by
DFT, and their relative energies were obtained (see the SI).
The energy-minimized models suggest that both diastereomers
are structurally accessible states and are comparable in their free
energies, consistent with the nearly equal proportion observed
in the 31P NMR spectrum (see the SI). The presence of
complex 7 was further confirmed by MALDI-TOF MS with a
peak at m/z 1647.070, which matches the simulated isotopic
distribution of the molecular ion [M − BF4]+ (Figure 5b).
Additionally, complex 7 can also be obtained directly from the
semiopen complex 5 via a reaction with 4 equiv of AgBF4 in the
noncoordinating solvent nitromethane (Scheme 4b).
Figure 6. 31P NMR spectra of complex 5 in methanol-d4 from 213 to
330 K. Coalescence of the major resonance (36 ppm) and the
corresponding diastereomer’s resonance (37 ppm) is observed at high
temperature, consistent with the presence of rapidly interconverting
diastereomers at elevated temperature.
Accessing Each of the Reversible States. As previously
mentioned, WLA complexes are unique in their ability to be
allosterically regulated in a reversible manner. As such, we
sought to reopen complex 7 and reform each of the previously
accessed states (Scheme 5). Complex 6 was readily regenerated
from complex 7 by dissolution in MeOH. Upon the addition of
4 equiv of a soluble chloride source, namely, bis-
(triphenylphosphoranylidene)ammonium chloride (PPNCl),
complex 6 was converted to the semiopen complex 5. The
Formation of the Fully Closed State. In order to fully
close the macrocycle, complex 6 was subjected to high vacuum
for 2 h to liberate what was hypothesized to be a methanol-d4
molecule bound to the PtII metal center and form the κ2:μ:κ2-
NHC,S and κ2:μ:κ2-P,S coordinated complex (Scheme 4b).
Characterization by 1H and 31P NMR spectroscopy (Figure 5a,
7), MALDI-TOF MS, and DFT calculations confirmed the
formation of a fully closed state. Specifically, when the fully
dried product was dissolved in a noncoordinating deuterated
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a
Scheme 4. Formation of (a) the New Intermediate WLA Macrocycle State (6) and (b) the Fully Closed Macrocycle
a
Reaction conditions: (i) 4 equiv of AgBF4, methanol-d4. (ii) (1) High vacuum to remove methanol-d4 (MeOD), 2 h; (2) CH3NO2-d3. (iii) 4 equiv
of AgBF4, CH3NO2-d3.
a
Scheme 5. Reversible Closing and Reopening of the WLA Multistate Macrocycle
a
MeOD = methanol-d4.
H
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mass spectrometry (MALDI-TOF MS) was recorded on a Bruker
Autoflex III smartbeam in reflectron and positive-ion mode.
opening was evidenced by the disappearance of the signal at 47
ppm and the concomitant appearance of signals at 36 and 37
ppm, corresponding to the two diastereomers of complex 5
(see the SI). Upon dissolution of the fully dried complex 5 in
CD2Cl2, the fully open complex 4a was regenerated, completing
the closing and reopening cycle. The 31P NMR spectrum of the
resulting product shows the disappearance of the resonances at
36 and 37 ppm and the reappearance of the signal at 1.8 ppm
(JP−Pt = 3740 Hz), consistent with the formation of the cis
complex 4a (see the SI).
Synthesis. Benzimidazolium Salt (1). In a modified procedure,30 a
solution of 1,4-bis[(chloromethyl)thio]benzene (50 mg, 0.209 mmol)
and 1-methylbenzimidazole (74.09 mg, 0.627 mmol) was dissolved in
5 mL of anhydrous dimethylformamide. The reaction mixture was
stirred and heated at 100 °C for 24 h in a Schlenk flask under N2 gas.
The solvent was reduced to ca. 1 mL in vacuo with heptane. The
product was then washed with diethyl ether (Et2O) and dichloro-
methane (CH2Cl2) and dried in vacuo to obtain a white solid (66.3
mg, 0.132 mmol, 63% yield). 1H NMR (400 MHz, DMSO-d6): δ 9.85
(s, 2H), 7.68 (dtd, J = 15.7, 7.4, and 6.3 Hz, 4H), 7.72−7.64 (m, 4H),
7.37 (s, 4H), 6.17 (s, 4H), 4.05 (s, 6H). 13C NMR (126 MHz, DMSO-
d6): δ 142.32, 132.31, 131.98, 131.67, 130.12, 126.79, 126.67, 114.06,
113.91, 49.83, 33.42.
CONCLUSIONS
■
Of the various supramolecular architectures that have been
synthesized through coordination-driven supramolecular as-
sembly, the macrocycle continues to be an important
architecture because of its defined cavity, making it useful for
host−guest chemistry, designed molecular recognition, and
catalysis.2 Additionally, macrocyclic architectures have paved
the way for the synthesis of larger, more sophisticated
structures with three-dimensional cavities. The addressable
construct described here is a potentially useful building block
for the construction of higher order, chemically addressable
complexes with multistate switching capabilities. Key to the
success of the stepwise formation of heteroligated WLA
macrocycles is the kinetic stability of the complexes where
the “weak-link” thioether is a relatively strong donor. The
consequence of this strong bonding is illustrated by the
formation of the semiopen/fully closed complex 6, in which a
MeOH molecule occupies one of the empty coordination sites.
The MeOH molecule displaces a single thioether and alleviates
some of the fully closed structure’s strain energy, which has
been investigated by DFT calculations. The ring strain present
in this system suggests that the WLA may allow us to design
systems in which strain can be exploited to enable the use of
allosteric effectors that are not anionic. The result is a
macrocyclic structure that can adopt new intermediate states
previously not observed in traditional WLA systems, making it
the first platinum-based system to require multiple cues or
stimuli to be switched between states and the only known four-
state WLA system.
[Pt2Cl4(κ1:μ:κ1-NHC,S)(C2H6OS)2] (2-DMSO). A solution of benzi-
midazolium salt (1) (100 mg, 0.199 mmol) in a solvent mixture of
CH2Cl2/MeOH at a ratio of 6:1 (4 mL) was combined with Ag2O (46
mg, 0.199 mmol), and the mixture was stirred at 60 °C until the
solution became murky and the black Ag2O powder disappeared. After
10 min, a solution of PtCl2(cod) (149 mg, 0.397 mmol) in the solvent
mixture of CH2Cl2/MeOH at a ratio of 6:1 (6 mL) was added to the
mixture, which was then stirred at 60 °C over 24 h. The suspension
was centrifuged, washed with Et2O (10 mL × 3), and dried in vacuo.
The product was obtained as an off-white powder (115 mg, 0.120
1
mmol, 60% yield). H NMR (400 MHz, DMSO-d6): δ 7.74 (dd, J =
8.2 and 3.7 Hz, 2H), 7.41 (d, J = 1.7 Hz, 4H), 7.35 (t, J = 7.7 Hz, 2H),
7.29−7.16 (m, 4H), 6.20−6.02 (m, 4H), 4.19 (d, J = 3.6 Hz, 6H). 13C
1
NMR (126 MHz, DMSO-d6): δ 155.68 (d, JC−Pt = 4.2 Hz), 133.17
(s), 133.05 (s), 132.73 (s), 132.66 (s), 124.02 (s), 123.89 (s), 111.69
(s), 111.36 (s), 111.31 (s), 51.37 (s), 34.83 (s). MALDI-TOF MS
(matrix: dithranol). Calcd for [M − Cl]+: m/z 926.961. Found: m/z
926.803.
1,4-Bis(diphenylphosphino)ethylthiobenzene (3). In a modified
procedure,42,43 benzene-1,4-dithiol (0.340 mg, 2.36 mmol) was
combined with azobis(isobutyronitrile) (catalytic amount) in a
Schlenk flask in THF (15 mL) under N2 gas. Diphenylvinylphosphine
(KPPh2; 1.0 g, 4.71 mmol) was added dropwise to the reaction
mixture, via a syringe, under N2 gas over the course of 30 min. The
solution turned from yellow to red. The reaction was refluxed for 18 h
under N2 gas. After stirring for 18 h, the reaction solution was
concentrated in vacuo to give a yellow oil. The crude product was
washed with hexanes (10 mL × 3) followed by MeOH (10 mL × 3),
filtered, and dried under high vacuum. The pure product appeared as
1
an off-white solid powder (1.18 g, 90% yield). H NMR (400 MHz,
CD2Cl2): δ 7.41−7.35 (m, 8H), 7.34−7.31 (m, 12H), 7.10 (s, 4H),
2.96−2.90 (m, 4H), 2.36−2.32 (m, 4H). 31P NMR (162 MHz,
CD2Cl2): δ −17.36.
EXPERIMENTAL SECTION
■
General Methods/Instrument Details. Commercially available
chemicals were purchased as reagent grade from Sigma-Aldrich, Acros,
and Alfa Aesar, unless otherwise noted, and used as received. Unless
otherwise stated, all solvents were purchased anhydrous and degassed
under a stream of argon prior to use. All glassware and magnetic
stirring bars were thoroughly dried in an oven (180 °C). Reactions
were monitored using thin-layer chromatography (TLC), commercial
TLC plates (silica gel 254, Merck Co.) were developed, and the spots
were visualized under UV light at 254 or 365 nm. Flash
chromatography was performed using SiO2-60 (230−400 mesh
ASTM, 0.040−0.063 mm; Fluka). Deuterated solvents were purchased
[cis-Pt2Cl4(κ1:μ:κ1-NHC,S)(κ1:μ:κ1-P,S)], [trans-Pt2Cl4(κ1:μ:κ1-
NHC,S)(κ1:μ:κ1-P,S)], and [cis-PtCl2(κ1:μ:κ1-NHC,S)(κ1:μ:κ1-P,S)-trans-
PtCl2(κ1:μ:κ1-NHC,S)(κ1:μ:κ1-P,S)] (4a−4c). A solution of 3 (81.3 mg,
0.143 mmol) in CD2Cl2/CD3OD (1:1, 2.5 mL) was added to a
suspension of complex 2 (115 mg, 0.120 mmol) in CD2Cl2/CD3OD
(1:1, 0.7 mL) in a glass vial in the glovebox. The mixture was then
stirred at room temperature for 48 h, during which the yellow murky
solution became clear and a dark precipitate formed (AgCl). The
supernatant was then dried in vacuo to obtain a pale-yellow solid
1
powder as the product (114 mg, 0.0745 mmol, 62% yield). H NMR
from Cambridge Isotope Laboratories and used as received. H, 31P,
1
(400 MHz, CD2Cl2): δ 8.06−6.95 (m, 38H), 4.27−3.82 (m, 4H),
3.35−3.10 (br, 8H), 1.26 (s, 6H). 31P NMR (162 MHz, CD2Cl2): δ
10.45 (d, 1JP−Pt = 2270 Hz); 9.73 (d, 1JP−Pt = 2300 Hz); 2.53 (d, 1JP−Pt
31P{1H}, and 19F{1H} NMR spectra were recorded on a Bruker
Avance 400 MHz spectrometer, and chemical shifts (δ) are given in
parts per million. 1H NMR spectra were referenced internally to
residual proton resonances in the deuterated solvents (dichloro-
methane-d2 = δ 5.32; nitromethane-d3 = δ 4.33; methanol-d4 = δ 3.31).
31P and 31P{1H} NMR spectra were referenced to an external 85%
H3PO4 standard (δ 0). High-resolution mass spectrometry (HRMS)
measurements were recorded on an Agilent 6120 LC-TOF instrument
in positive-ion mode. Electrospray ionization mass spectrometry (ESI-
MS) was recorded on a Micromas Quatro II triple−quadrapole mass
spectrometer. Matrix-assisted laser desorption/ionization time-of-flight
1
= 2943 Hz); 1.81 (d, JP−Pt = 3740 Hz). HRMS (ESI+). Calcd for [M
− Cl]+: m/z 1493.1039. Found: m/z 1493.1051.
[cis-Pt2Cl4(κ1:μ:κ1-NHC,S)(κ1:μ:κ1-P,S)] (4a). Crystals of complex 4a
were obtained by the slow diffusion of Et2O into a CH2Cl2 solution of
4a−4c. 1H NMR (400 MHz, CD2Cl2): δ 8.06−8.00 (m, 4H), 7.75 (s,
4H), 7.60−7.48 (m, 10H), 7.35 (s, 4H), 7.30−7.11 (m, 11H), 7.01−
6.98 (m, 9H), 6.03−5.84 (d, 13.54 Hz, 2H), 5.78−5.57 (d, 13.51 Hz,
2H), 3.82 (s, 6H), 3.45−3.22 (dt, 6.16, 6.16, 11.32 Hz, 4H), 3.19−2.92
(m, 4H). 31P NMR (162 MHz, CD2Cl2): δ 1.84 (1JP−Pt = 3740 Hz).
I
DOI: 10.1021/acs.inorgchem.7b02745
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Forum Article
[cis-Pt2Cl2(κ1:μ:κ1-NHC,S)(κ2:μ:κ2-P,S)][Cl]2 (5). A complex mixture
of 4a−4c (or single crystals of 4a) was fully dried and then dissolved
in CD3OD. 1H NMR (400 MHz, methanol-d4): δ 8.25 (s, 4H), 7.78−
7.60 (m, 12H), 7.57−7.43 (m, 12H), 7.35−7.30 (m, 4H), 7.28 (s, 4H),
7.27−7.24 (m, 2H), 5.51 (d, J = 14.0 Hz, 2H), 4.40 (d, J = 14.0 Hz,
2H), 4.10 (s, 6H), 3.63−3.52 (m, 4H), 3.28−3.11 (m, 4H). 31P NMR
(162 MHz, methanol-d4): δ 36.83(d, 1JP−Pt = 3421 Hz), 36.29. HRMS
(ESI+). Calcd for [M − 2Cl]2+: m/z 729.0677. Found: m/z 729.0680.
[cis-Pt2(κ1:μ:κ2-NHC,S)(κ2:μ:κ2-P,S)CD3OD][BF4]4 (6). A total of 4
equiv of AgBF4 was added to a solution of 5 in MeOH. The solution
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based on work supported by the National
Science Foundation under Grant CHE-1709888 and the U.S.
Army under Grant W911NF-15-1-0151. A.I.d. acknowledges a
National Science Foundation Graduate Research Fellowship,
and J.M.-A. acknowledges a fellowship from Consejo Nacional
de Ciencia y Tecnologia (CONACYT). We also thank
DGTIC-UNAM for granting access to their supercomputing
facilities known as “Miztli”.
1
was allowed to stir for 30 min. H NMR (400 MHz, methanol-d4): δ
8.54−7.17 (m, 36H), 6.19−5.94 (br, 2H), 4.08−4.02 (s, 2H), 3.68−
3.57 (m, 4H), 3.20−2.94 (br, 4H). 31P NMR (162 MHz, methanol-
d4): δ 37.16 (semiopen), 46.95 (fully closed). MALDI-TOF MS
(matrix: α-cyano-4-hydroxycinnamic acid). Calcd for [M + H]+: m/z
1767.249. Found: m/z 1767.308.
[cis-Pt2(κ2:μ:κ2-NHC,S)(κ2:μ:κ2-P,S)][BF4]4 (7). Complex 6 was
subjected to high vacuum for 2 h to remove the MeOH molecule
hypothesized to be bound to the PtII metal node. The fully dried
powder was then dissolved in deuterated nitromethane (CD3NO2). 1H
NMR (400 MHz, CD3NO2): δ 8.51 (s, 4H), 8.16−7.92 (m, 10H),
7.69 (s, 6H), 7.65−7.43 (m, 16H), 6.32−5.84 (m, 4H), 4.02−3.77 (m,
2H), 3.62 (d, J = 19.7 Hz, 6H), 3.34−3.14 (m, 2H), 2.98−2.70 (m,
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* Supporting Information
The Supporting Information is available free of charge on the
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Accession Codes
CCDC 1583493−1583494 contain the supplementary crystal-
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: chadnano@northwestern.edu.
ORCID
Andrea I. d’Aquino: 0000-0002-4204-8219
Ho Fung Cheng: 0000-0003-2580-0912
Joaquín Barroso-Flores: 0000-0003-0554-7569
C. Michael McGuirk: 0000-0002-7420-1169
Chad A. Mirkin: 0000-0002-6634-7627
Author Contributions
The manuscript was written through contributions of all
authors. All authors have given approval to the final version of
the manuscript.
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