Palladium(II) Weak-Link Approach Complexes Bearing Hemilabile N-Heterocyclic Carbene-Thioether Ligands

Article abstract of DOI:10.1021/acs.inorgchem.7b00543

A new class of homoligated palladium(II) weak-link approach (WLA) complexes bearing hemilabile N-heterocyclic carbene (NHC)-thioether ligands is reported that, unlike previous tweezer-like WLA complexes, expand and contract in a linear fashion when switching between configurational states. These complexes can be chemically switched between a trans open state and a trans closed state via the addition or subsequent extraction of Cl^-. These bis(NHC) complexes also display unusual isomerization behavior. For example, an NMR spectroscopic investigation into the solution-state configuration of the open complex reveals the presence of interconverting syn,trans and anti,trans isomers, and a kinetic study shows that the barrier is large enough to isolate, store, and study the anti,trans isomer at room temperature. Notably, the linker length between the NHC and thioether moieties can be tailored with additional -CH₂- groups, which affords considerable control over the geometric changes imposed by switching. Therefore, this class of complexes may be useful in the construction of allosterically regulated supramolecular assemblies and materials.

Full text of DOI:10.1021/acs.inorgchem.7b00543

Article
pubs.acs.org/IC
Palladium(II) Weak-Link Approach Complexes Bearing Hemilabile
N‑Heterocyclic Carbene−Thioether Ligands
Yuan Liu, Zachary S. Kean, Andrea I. d’Aquino, Yashin D. Manraj, Jose Mendez-Arroyo,
and Chad. A. Mirkin*
Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, United States
*
S Supporting Information
ABSTRACT: A new class of homoligated palladium(II)
weak-link approach (WLA) complexes bearing hemilabile
N-heterocyclic carbene (NHC)−thioether ligands is reported
that, unlike previous tweezer-like WLA complexes, expand and
contract in a linear fashion when switching between configura-
tional states. These complexes can be chemically switched
between a trans open state and a trans closed state via the
−
addition or subsequent extraction of Cl . These bis(NHC)
complexes also display unusual isomerization behavior. For
example, an NMR spectroscopic investigation into the solution-
state configuration of the open complex reveals the presence of
interconverting syn,trans and anti,trans isomers, and a kinetic
study shows that the barrier is large enough to isolate, store, and study the anti,trans isomer at room temperature. Notably,
the linker length between the NHC and thioether moieties can be tailored with additional −CH − groups, which affords
2
considerable control over the geometric changes imposed by switching. Therefore, this class of complexes may be useful in the
construction of allosterically regulated supramolecular assemblies and materials.
INTRODUCTION
flexible open state and a rigid closed state by the addition or
subsequent removal of allosteric effectors (such as CO and Cl )
■
−
Advances in supramolecular chemistry have allowed chemists
to synthesize biomimetic molecular assemblies for applications
that displace the weakly coordinating X moiety in a metal−X
bond while retaining the strongly coordinating metal−P bond.
Through the rational design of ligands, the WLA has led to a
variety of different supramolecular architectures, such as
1
,2
3,4
5,6
in catalysis, molecular recognition, and drug delivery.
In particular, coordination-chemistry-based approaches have
been exploited as an efficient means to synthesize supramolecular
constructs in a predictable and modular fashion. Generally,
there are three strategies used for the assembly of such struc-
2
1,25,26
13,20,25,27
7
tweezer,
(Figure 1).
macrocycle,
and triple-decker complexes
25,28,29
8
,9
I
1
0
The initial work focused on Rh -based WLA complexes, which
tures: the directional-bonding approach, the symmetry-inter-
1
1
12,13
could be toggled between a trans open state and a cis closed state
action approach, and the weak-link approach (WLA).
3
0
(
Scheme 1a). Benefiting from dramatic geometry changes,
Among these three strategies, the WLA is unique in that it
incorporates addressable transition-metal nodes in conjunc-
tion with hemilabile ligands, resulting in systems that can be
allosterically regulated via the reversible coordination of anions
or small-molecule effectors. With the advent of the halide-
I
Rh -based WLA complexes formed the basis for allosteric
catalysts for applications in switchable polymerization,
ELISA/PCR mimicking detection and signal amplification,
and photochemical switches,
ligands. The Rh metal center, however, is air-sensitive, thus
limiting the scope of applications of those supramolecular con-
structs.
Recently, our group reported a stepwise synthesis of air-stable
2
9
31,32
33−35
when tethered to functional
I
14,15
induced ligand rearrangement reaction,
it is now possible to
make WLA complexes with two different hemilabile ligands,
providing the ability to synthesize complexes with asymmetric
reaction sites, complex recognition pockets, and more sophis-
ticated architectural parameters and reactivities.
responsive properties of these assemblies render the WLA an
emerging and important tool for preparing allosterically
addressable supramolecular constructs that mimic enzymes.
Typically, WLA complexes are assembled from d transi-
tion metals (such as Rh and Pt ) and hemilabile bidentate
25
II
24
16,17
heteroligated WLA complexes, utilizing Pt metal centers
and N-heterocyclic carbene−thioether (NHC,S) hemilabile
41,42
The stimuli-
36−40
ligands,
in combination with traditional (P,S) ligands.
I
1
8−21
In contrast to previous Rh -based WLA complexes, which can be
switched between trans open and cis closed states, Pt -based
II
8
I
II
2
2−24
phosphino−chalcoether (P,X; X = O, S, Se) ligands.
assembly, the resulting complexes can be switched between a
Upon
Received: March 1, 2017
©
XXXX American Chemical Society
A
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Article
Figure 1. Supramolecular tweezer, macrocycle, and triple-decker complexes via the WLA. Charges and anions have been omitted for clarity.
I
Scheme 1. Allosteric Regulation of (a) Classical Rh -Based
smaller changes in shape and interligand spacing than their
II
I
WLA Systems, (b) Pt -Based Heteroligated Systems, and
Rh -based counterparts, thereby restricting the possible struc-
II
a
II
(
c) Pd -Based Systems Reported Herein
tures and applications of Pt -based WLA complexes.
To address the aforementioned limitations, we decided to
II
explore the chemistry of Pd -based WLA complexes bearing
4
4,45
bis(NHC,S) hemilabile ligands.
In contrast to previously
42
reported (NHC,S)(P,S)PtX systems, bis(NHC)PdX (X =
2
2
Cl, Br) complexes typically adopt a trans geometry around the
II
Pd metal center because of steric interactions, an effect that
is amplified when bulky groups are installed on the NHC
46−49
ligands.
We hypothesized that halide extraction could result
in the reversible conversion of the trans open state to the trans
closed state, allowing for a large and linear change in the overall
shape of the complex (Scheme 1c). Furthermore, we believe that
by replacing the phosphine ligands with the NHCs, the resulting
complexes would be kinetically more stable because of a stronger
50,51
binding affinity of the NHCs to the metal center.
In addition
to their stronger σ-donating nature, NHC ligands are also
synthetically tailorable by altering the substituents of the
imidazolium salt precursors. This feature allows one to study
how the structural elements, such as the linker length between
the NHC and thioether moieties, influence the geometry and
properties of the resulting complexes.
Herein, we report the first example of homoligated bis-
II
(
NHC,S)-Pd WLA complexes with different linker lengths
between the NHC and thioether moieties. Unlike the classical
WLA complexes, this family of compounds adopts a trans
geometry in both the open and closed states, which allows for a
linear change in geometry upon the addition or subsequent
−
removal of an allosteric effector (Cl ), thus enabling the explo-
ration of a new class of allosteric complexes, flexible coordination
a
52−54
P = PPh , Ar = aryl group, and NHC = N-heterocyclic carbene.
2
polymers, or molecular muscles.
We observe dramatically
different changes in the structure upon switching between fully
open and fully closed states, when the linker length between the
NHC and thioether moieties is altered. Additionally, as a result of
complexes undergo interconversion between cis open and cis
closed states (Scheme 1b). The resulting constructs display
43
B
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our investigations into the switching behavior, we have dis-
Scheme 3. Synthesis of the Homoligated Fully Open
Complexes 4 and 5 via a Silver Transmetalation Procedure
covered, for the first time, a method for the selective preparation
II
II
of syn,trans-bis(NHC)Pd (5a) and anti,trans-bis(NHC)Pd
5b) complexes. Not only does this work expand the scope
(
of the WLA chemistry, but it also adds to the fundamentaIlI
understanding of the isomerization behavior of bis(NHC)Pd
complexes.
RESULTS AND DISCUSSION
■
Our design of the (NHC,S) hemilabile ligands was motivated
by the hypothesis that if the substituent attached to the imidazole
is of sufficient steric bulk, the trans isomer will be favored in the
resulting coordination complex. Thus, we synthesized imidazo-
lium salts substituted with a tert-butyl group (tBu) and a tethered
thioaryl group (Scheme 2), where the linker between the NHC
Scheme 2. Synthesis of (NHC,S) Hemilabile Ligands 2 and 3
the steric interaction between the two bulky substituents on the
ligands. We hypothesize that the mixture of syn (4a or 5a) and
anti (4b or 5b) isomers was obtained as a result of the decreased
steric demand of the flexible ethylene or propylene chains in our
system relative to the previously reported aryl substituents.
Furthermore, we observed the ratio of 4a to 4b to be approx-
imately 1:1 based upon 1H NMR spectroscopy (see the
Supporting Information, SI). To verify the chemical structure
of the two isomers, we investigated the solution-state
conformations, using one-dimensional nuclear Overhauser effect
(1D NOE) experiments. For the fully opened complex 4, we
and thioether moieties was either an ethylene (2) or a pro-
pylene (3) chain. The synthesis of asymmetrically substituted
imidazolium salt precursors (2 and 3) began with the preparation
of 1-tert-butylimidazole (1), which was prepared via the con-
densation reaction of tert-butylamine, paraformaldehyde, ammo-
nium carbonate, and glyoxal, according to a modified pro-
cedure. We synthesized two ligands with two different linker
lengths in order to understand how the structure of the chelating
ligand affects the geometry of the rigid, fully closed WLA com-
plexes. Therefore, 1 was subsequently reacted with either
5
5
a
utilized two diagnostic resonances at 5.05 ppm (H ) and 5.01 ppm
b
1
2
-chloroethylphenyl sulfide or 3-chloropropyl phenyl sulfide in
(H ) in the H NMR spectrum of 4 in dichloromethane-d
2
a
refluxing acetonitrile (MeCN) to yield imidazolium chlorides
(CD Cl ; Figure 2a). Upon irradiation of H , we observed no
correlation resonance from the tBu at 2.00 ppm in the 1D NOE
2
2
a
2
and 3, respectively (Scheme 2).
The homoligated fully open complexes were synthesized using
spectrum. This observation is consistent with a syn isomer
47,56
a
a
a silver transmetalation procedure,
in which the imidazolium
of 4a, where H and tBu are pointing away from each other.
b
b
salt 2 was first reacted with silver oxide (Ag O) to afford a silver
In contrast, when H was irradiated, a resonance from tBu at
2
carbene intermediate, as evidenced by the disappearance of
2.03 ppm was observed in the 1D NOE spectrum, indicating that
NHC
b
b
the diagnostic
C−H resonance in the downfield region of the
H and tBu are within a distance of 5 Å from each other, thus
supporting 4b as the anti isomer (Figure S1). The stereo-
chemistry of 5a and 5b was also determined by 1D NOE experi-
ments (Figures 2b and S2), which similarly confirmed that the
downfield and upfield resonances of the tBu groups were
attributable to the anti (5b) and syn (5a) isomers, respectively.
We next investigated the complex geometries in the solid state
via single-crystal X-ray diffraction. A solution of 4 (mixture of 4a
and 4b isomers) was subjected to crystallization by slow vapor
diffusion, resulting in the formation of crystals composed
exclusively of 4b. The solid-state structure of 4b confirms
that the two NHC ligands are anti and trans to one another
1
H NMR spectrum (δ ∼ 11 ppm, CDCl ). This carbene inter-
mediate, in turn, was transmetalated with dichloro(1,5-
3
cyclooctadiene)palladium(II) [Pd(COD)Cl , where COD =
2
1,5-cyclooctadiene] in refluxing MeCN to afford the fully open
complex 4 with an isolated yield of 36% over two steps. In an
analogous fashion, fully open complex 5 was synthesized with an
overall yield of 54% (Scheme 3).
1
13
Notably, we observed two sets of resonances in the H and C
NMR spectra for both 4 and 5, suggesting the formation of two
isomers during the reaction: syn (4a or 5a) and anti (4b or 5b)
isomers. The formation of syn and anti isomers has been studied
57
NHC
for bis(NHC)NiX complexes; however, this observation dif-
[C1−Pd1−C16 = 179.9(3)°], and the Pd− C bond length
2
fers from other reported bis(NHC)PdX complexes that adopt a
[Pd1−C1 = 2.045(4) Å; Pd1−C16 = 2.042(4) Å] values are
2
47−49,58
48,49,58
single anti,trans conformation,
a geometry that minimizes
comparable to those found in the literature (Figure 3a).
C
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X-ray diffraction study indicates that 5b adopts the anti,trans
conformation, with bond lengths of Pd−Cl [2.3259(2) Å] and
NHC
Pd− C [2.0416(7) Å] (Figure 3b). We then sought to
⧧
determine the activation barrier to isomerization (ΔG ) by
1
calculating the isomerization rate constant (k) using H NMR
spectroscopy. Complex 5b was dissolved in MeCN-d (CD CN)
3
3
and subsequently heated to 65 °C, which resulted in the
conversion of 5b to an equilibrium concentration of 5a over
⧧
approximately 4 h (Figure S3). The value of ΔG (338 K) is
−1
1
08.54 kJ mol , as calculated using the Eyring equation and
the rate constant observed at the early stages of the reaction. This
⧧
large ΔG value is consistent with the slow syn-to-anti iso-
merization of 5 observed at room temperature. Furthermore, this
elevated-temperature study underscores the thermal stability of
II
II
the NHC−Pd bond in bis(NHC,S)Pd WLA complexes, which
Figure 2. 1D NOE experiments on the fully opened complexes (a) 4 and
II
42
(
b) 5. All experiments were conducted in CD Cl at room temperature.
are comparable to their Pt -based WLA counterparts.
2
2
We next studied the switching of the fully open complex 4 with
the fully closed form 6 via halide extraction. Upon the addition of
2
equiv of silver tetrafluoroborate (AgBF ) to a solution of 4 in
4
CH Cl , the chloride atoms were extracted, generating the fully
2
2
closed complex 6 upon coordination of the pendant thioether
1
moieties to the metal center (Scheme 4). H NMR spectroscopy
Scheme 4. Reversible Conversion of the Fully Open Complex
4
to the Fully Closed Complex 6
Figure 3. Crystal structures of (a) the fully open complex 4b and (b) the
fully open complex 5b drawn with 50% thermal ellipsoid probability.
Hydrogen atoms, solvent molecules, and anions have been omitted for
clarity. Selected bond lengths [Å] and angles [deg]: (a) 4b, Pd1−Cl1
2
.315(1), Pd1−Cl2 2.313(1), Pd1−C1 2.045(4), Pd1−C16 2.042(4),
Cl1−Pd1−Cl2 179.71(7), C1−Pd1−C16 179.9(3); (b) 5b, Pd1−Cl1
.3259(2), Pd1−C1 2.0416(7).
2
When a crystal of 4b was redissolved in CD Cl , only one set of
2
2
resonances was observed, corresponding to the anti isomer (see
the SI). No isomerization was observed for either 4b or 4a at
ambient temperature, which suggested that the activation barrier
for the isomerization of 4b into 4a is relatively high with respect
to the thermal energy at room temperature. We believe that the
barrier to rotation is the result of steric interaction between the
tBu groups and chloride atoms, which must be overcome in order
for isomerization to occur.
(CD Cl at room temperature) evidence for the reaction included
2 2
the disappearance of the sharp resonances attributed to the open
complex 4 and the concomitant appearance of broad resonances
assigned to complex 6, along with a downfield shift of the aromatic
proton resonances.
The solid-state structure of 6 was verified by a single-crystal
X-ray diffraction study, showing that the two NHC ligands main-
tain their trans geometry [C9−Pd1−C24 = 171.25(5)°] with
NHC
Slowly diffusing diethyl ether into a solution of 5 in 1,2-dichlo-
roethane (DCE) resulted in the formation of crystals exclusively
composed of 5b (see the SI), while the mother liquor con-
Pd− C bond lengths [Pd1−C9 = 2.061(2) Å; Pd1−C24 =
2.038(2) Å] similar to those of the fully open complex 4, while
the two thioether moieties are now bound to the metal center
with a small amount of torsion [S1−Pd1−S2 = 169.52(1)°]
(Figure 4a). This distortion from ideal square-planar geometry is
likely due to the short length of the ethylene chain, which imparts
significant ring strain in the square-planar complex. To explore
the ability to toggle between states, we reopened the fully closed
1
tained no remaining 5a, as determined by H NMR spec-
troscopy. This indicates that either the crystallization process
mediates conversion of 5a into 5b or the isomerization is fast
enough such that the crystallization of 5b drives the consumption
of 5a in solution at equilibrium. Similar to 4b, a single-crystal
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Scheme 5. Conversion of the Fully Open Complex 5 to the
Fully Closed Complex 7
increased linker length between the NHC and thioether motifs
in 7 results in reduced ring strain, a relatively undistorted
II
square-planar geometry around the Pd metal center, and an
antiparallel orientation of the terminal phenyl moieties.
We then examined the reversibility of this complex closing
process by reacting the closed complex 7 with Cl− ions.
Intriguingly, we obtained the syn isomer 5a as the sole reaction
product (Figure 5e). Although the mechanism of the selective
formation of the syn isomer 5a is unclear, we believe it is the
result of a kinetically controlled process because it differs from
the observed ratio of the isomers obtained via direct synthesis at
the elevated temperature. Because the activation barrier for syn-
to-anti isomerization is relatively high, the syn isomer 5a can be
isolated, analyzed, and stored. As a result, we can selectively
prepare isomers 5a and 5b through chloride addition to the fully
closed complex 7 or by crystallization from the thermodynamic
mixture, respectively (Figure 5a).
Figure 4. Crystal structures of the (a) fully closed complex 6 and (b) the
fully closed complex 7, drawn with a 50% thermal ellipsoid probability.
Hydrogen atoms, solvent molecules, and anions have been omitted for
clarity. Selected bond lengths [Å] and angles [deg]: (a) 6, Pd1−S1
2
.328(1), Pd1−S2 2.357(1), Pd1−C9 2.061(2), Pd1−C24 2.038(2),
S1−Pd1−S2 169.52(1), C9−Pd1−C24 171.25(5); (b) 7, Pd1−S1
2
.324(1), Pd1−S2 2.326(1), Pd1−C10 2.058(6), Pd1−C26 2.067(6),
S1−Pd1−S2 178.37(6), C10−Pd1−C26 174.7(3).
complex 6 to complex 4 by the addition of 2 equiv of tetra-
CONCLUSION
■
1
butylammonium chloride (nBu NCl). As indicated by H NMR
4
In summary, we have reported a new class of WLA complexes
bearing trans-bis(NHC,S) hemilabile ligands. These complexes
are air-stable and exhibit dramatic changes in the shape
spectroscopy in CD Cl , a mixture of syn (4a) and anti (4b)
2
2
isomers is observed when the open complex 4 is obtained via this
route (Figure S4).
(
up to ∼9 Å) and flexibility between the fully closed and fully
Ideally, we would like to develop complexes capable of
large-amplitude coparallel linear changes, and although toggling
between the fully open complex 4 and fully closed complex 6
has been demonstrated, the distorted geometry of the closed
complex was less than ideal. We therefore wanted to study
the solid-state geometry of the closed complex 7 to determine
whether it would adopt a more favorable structure. To obtain 7,
the remaining chlorides in 5 were abstracted by the addition of
open forms, thus enabling the design and synthesis of stimuli-
responsive supramolecular systems possessing a new linear
change in geometry. In addition, by adjusting the linker length
between the NHC and thioether moieties, we probed the
influence of the linker length on the geometry of the fully closed
complex and the closed-to-open transformation and discovered
an unusual isomerization behavior and the conditions that allow
the selection of one isomer over another. This isomerization has
2
equiv of AgBF (Scheme 5). Unlike 6, no broad resonances are
II
4
not been explored for bis(NHC)Pd complexes, yet it could be
1
observed in the H NMR spectrum of 7; however, the appearance
of diastereotopic resonances assigned to the methylene protons
is indicative of the formation of a rigid fully closed complex
critical in the design of new palladium(II) catalysts, where the
isomeric state is important with regard to the stereoselectivity of
59−61
the catalysts.
(
Figure 5d). The X-ray crystal structure of 7 further confirms the
II
coordination of both thioether moieties to the Pd center in the
solid state; however, in comparison to 6, less deviation from an
ideal square-planar geometry is observed, and the two phenyl
groups are oriented in a nearly antiparallel manner [S1−Pd1−S2
EXPERIMENTAL SECTION
General Methods/Instrument Details. 3-Chloropropyl phenyl
sulfide was prepared according to a modified literature procedure.
All other chemicals and anhydrous solvents were purchased from Sigma-
Aldrich and used as received. Deuterated solvents were purchased
from Cambridge Isotope Laboratories and used as received. All NMR
■
62
1
78.37(6)°; C10−Pd1−C26 174.7(3)°] (Figure 4b). These
observations contrast with those for 6, suggesting that the
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Figure 5. (a) Selective formation of 5a and 5b from the isomeric mixture via a chloride extraction/addition sequence and crystallization, respectively.
1
H NMR spectra of the complex in the (b) fully open form 5, (c) crystallized fully open form 5b, (d) fully closed form 7, and (e) regenerated fully open
form 5a. All of the spectra were taken in CD Cl . Regions of the spectra between 1.0−4.6 and 5.2−5.6 ppm have been omitted for clarity.
2
2
spectra were recorded on a Bruker Advance 400 MHz spectrometer, a
Bruker Advance III 500 MHz spectrometer, an Agilent DD 400 MHz
spectrometer, and an Agilent DD2 500 MHz spectrometer. H NMR
CDCl ): δ 10.74 (s, 1H), 7.32−7.21 (m, 6H), 7.15−7.12 (m, 1H), 4.56
3
(t, 2H, J = 7.1 Hz), 2.96 (t, 2H, J = 6.8 Hz), 2.23 (tt, 2H, J = 6.9 and
1
13
6.9 Hz), 1.64 (s, 9H). C NMR (126 MHz, CDCl ): δ 137.01, 135.01,
3
spectra were referenced to residual protons in the deuterated solvents
129.79, 129.28, 126.70, 122.08, 119.11, 60.60, 50.69, 48.55, 30.18, 29.57.
13
+
− +
(
chloroform-d, δ 7.26; dichloromethane-d , δ 5.32) internally. C NMR
HRMS (ESI ). Calcd for C H N S ([M − Cl ] ): m/z 275.1576.
2
16 23
2
spectra were referenced to residual carbons in the deuterated solvents
chloroform-d, δ 77.16; dichloromethane-d , δ 54.00) internally.
Found: m/z 275.1578.
(
Fully Open Complex 4. Silver(I) oxide (39 mg, 0.165 mmol) was
added to a 5 mL flask containing a solution of imidazolium salt 2 (89 mg,
0.300 mmol) in anhydrous DCM (3 mL), and the resulting mixture was
stirred at room temperature for 6 h in the absence of light. The mixture
was then filtered, and the filtrate was dried under reduced pressure to
afford a silver carbene intermediate as a brown oil. In a 50 mL flask, silver
carbene was dissolved in anhydrous MeCN (25 mL), and Pd(COD)Cl2
(29 mg, 0.100 mmol) was added. The resulting mixture was heated at
reflux for 2 days. The solvent was removed under reduced pressure, and
the resulting residue was purified by flash column chromatography on
2
Electrospray ionization (ESI) mass spectrometry (MS) spectra were
recorded on an Agilent 6210 LC-TOF instrument in positive-ion mode.
Synthesis. 1-tert-Butylimidazole (1). In a modified procedure, a
solution of tert-butylamine (31.5 mL, 0.300 mol) in 40 mL of anhydrous
ethanol (EtOH) was added dropwise to an ice-bath-cooled 250 mL flask
containing a solution of paraformaldehyde (9.30 g, 0.310 mol) in
anhydrous EtOH (40 mL). After the addition and once the solution
reached 0 °C, ammonium carbonate (14.4 g, 0.150 mol) and a 40%
glyoxal solution (34.4 mL, 0.300 mol) in 80 mL of anhydrous EtOH
were added slowly. The reaction mixture was stirred overnight at room
temperature, the solvent was removed in vacuo, and the crude oil was
silica gel (CH Cl /MeOH = 10/1) to afford the desired product 4 as a
2 2
yellow solid in 37% yield (26 mg, 0.0370 mmol). The product contained
a mixture of two isomers (syn and anti). The assignment of these
isomers was performed with the aid of NOE experiments (Figure S1).
distilled under high vacuum (200 mTorr, 60 °C) to yield 1 as a yellow oil
1
in 33% yield (12.5 g, 0.101 mol). H NMR (500 MHz, CDCl ): δ 7.58
3
1
syn
anti
(
1
s, 1H), 7.03 (dd, J = 1.1 and 1.1 Hz, 1H), 7.01 (dd, J = 1.3 and 1.2 Hz,
H NMR (500 MHz, CD Cl ): δ 7.41−7.38 (m, 4H , 4H ), 7.32−
syn anti syn anti anti
2
2
H), 1.53 (s, 9H). 13C NMR (126 MHz, CDCl ): δ 134.41, 129.14,
3
7.27 (m, 4H , 4H ), 7.21−7.18 (m, 2H , 2H ), 7.02 (d, 2H , J =
syn syn anti
1
16.37, 54.79, 30.71.
Imidazolium Salt 2. 2-Chloroethyl phenyl sulfide (1.30 mL,
2.0 Hz), 7.01 (d, 2H , J = 2.0 Hz), 6.95 (d, 2H , 2H , J = 2.0 Hz),
syn
anti
syn
5.04 (t, 4H , J = 6.8 Hz), 5.01 (t, 4H , J = 6.8 Hz), 3.85 (t, 4H , J =
6.8 Hz), 3.72 (t, 4H , J = 6.8 Hz), 2.03 (s, 18H ), 2.03 (s, 18H ). C
NMR (101 MHz, CD Cl ): δ 167.57 (syn), 167.55 (anti), 135.96 (anti),
anti
anti
syn 13
8.86 mmol) was added to a 25 mL flask containing a solution of 1
(
1.00 g, 8.05 mmol) in anhydrous MeCN (8 mL). The resulting mixture
2
2
was refluxed for 2 days, at which time the solvent was removed under
reduced pressure to yield a dark-brown oil. The resulting residue
was purified by flash column chromatography on silica gel (CH Cl /
135.80 (syn), 129.67 (syn and anti), 129.65 (anti), 129.55 (syn), 126.77
(anti), 126.75 (syn), 121.47 (anti), 121.07 (syn), 119.54 (anti), 119.24
(syn), 59.30 (anti), 59.14 (syn), 51.52 (anti), 51.45 (syn), 34.54 (anti),
2
2
+
MeOH = 10/1) to afford the desired ligand 2 as a brown oil in 28% yield
34.34 (syn), 32.53 (anti), 32.32 (syn). HRMS (ESI ). Calcd for
1
− +
(
0.661 g, 2.23 mmol). H NMR (500 MHz, CDCl ): δ 11.09 (s, 1H),
C H ClN PdS ([M − Cl ] ): m/z 663.1410. Found: m/z 663.1410.
3
30 40
4
2
7
2
.34−7.27 (m, 5H), 7.22−7.18 (m, 1H), 7.16 (s, 1H), 4.72 (t, J = 6.1 Hz,
Single crystals suitable for X-ray diffraction studies were obtained by the
slow diffusion of diisopropyl ether into a solution of the fully open
complex 4 in DCE.
13
H), 3.63 (t, J = 6.1 Hz, 2H), 1.67 (s, 9H). C NMR (126 MHz,
CDCl ): δ 137.45, 133.99, 129.75, 129.54, 127.12, 123.23, 117.93, 60.62,
3
+
− +
4
9.50, 34.44, 30.22. HRMS (ESI ). Calcd for C H N S ([M − Cl ] ):
m/z 261.1420. Found: m/z 261.1416.
Fully Open Complex 5. Silver(I) oxide (127 mg, 0.550 mmol) was
added to a 10 mL flask containing a solution of imidazolium salt 3
(311 mg, 1.00 mmol) in anhydrous DCM (5 mL), and the resulting
mixture was stirred at room temperature for 6 h in the absence of light.
The mixture was then filtered, and the filtrate was dried under reduced
pressure to afford a silver carbene intermediate as a brown oil. In a 50 mL
flask, silver carbene was dissolved in anhydrous MeCN (20 mL), and
15
21
2
Imidazolium Salt 3. 1 (1.00 g, 8.05 mmol) and 3-chloropropyl
phenyl sulfide (2.26 g, 12.1 mmol) were dissolved in anhydrous MeCN
(8 mL). The resulting mixture was heated at reflux for 2 days. The
solvent was removed under reduced pressure to yield a dark-brown oil.
The resulting residue was purified by flash column chromatography on
silica gel (CH Cl /MeOH = 10/1) to afford the desired ligand 3 as a
Pd(COD)Cl (82 mg, 0.286 mmol) was added. The resulting mixture
2
2
2
1
brown oil in 75% yield (1.88 g, 6.06 mmol). H NMR (500 MHz,
was refluxed for 2 days. The solvent was removed under reduced
F
DOI: 10.1021/acs.inorgchem.7b00543
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
pressure, and the resulting residue was purified by flash column chro-
matography on silica gel (CH Cl /MeOH = 10/1) to afford the desired
product 5 as a yellow solid in 63% yield (130 mg, 0.180 mmol).
The product contained a mixture of two isomers (syn and anti). The
assignment of these isomers was performed with the aid of NOE
Crystallographic data for complex 5b in CIF format (CIF)
Crystallographic data for complex 6 in CIF format (CIF)
Crystallographic data for complex 7 in CIF format (CIF)
2
2
1
experiments (Figure S3). H NMR (500 MHz, CD Cl ): δ 7.39−7.35
2
2
AUTHOR INFORMATION
syn
anti
syn
anti
syn
(
m, 4H , 4H ), 7.32−7.27 (m, 4H , 4H ), 7.22−7.17 (m, 2H
,
■
anti
anti
anti
syn
2
2
4
7
H
H
), 7.09 (d, 2H , J = 2.0 Hz), 7.07 (d, 2H , J = 2.1 Hz), 6.93 (d,
Corresponding Author
*E-mail: chadnano@northwestern.edu.
ORCID
Yashin D. Manraj: 0000-0003-2710-6457
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.
syn
syn
, J = 2.1 Hz), 6.91 (d, 2H , J = 2.0 Hz), 5.01 (t, 4H , J = 7.0 Hz),
anti
anti
syn
.95 (t, 4H , J = 7.3 Hz), 3.05 (t, 4H , J = 7.1 Hz), 3.00 (t, 4H , J =
syn
anti
anti
syn
.0 Hz), 2.50−2.40 (m, 4H , 4H ), 2.01 (s, 18H ), 2.00 (s, 18H ).
1
3
C NMR (126 MHz, CD Cl ): δ 167.70 (syn and anti), 136.57 (anti),
2
2
1
1
1
36.47 (syn), 129.93 (syn), 129.88 (anti), 129.56 (syn), 129.55 (anti),
26.68 (syn), 126.65 (anti), 120.57 (anti), 120.10 (syn), 119.83 (anti),
19.55 (syn), 59.14 (anti), 59.07 (syn), 50.57 (anti), 50.45 (syn), 32.50
(
anti), 32.34 (syn), 31.30 (syn), 31.19 (anti), 30.45 (anti), 30.10 (syn).
HRMS (ESI ). Calcd for C H ClN PdS ([M − Cl ] ): m/z
+
− +
3
2
44
4
2
691.1727. Found: m/z 691.1728. Single crystals suitable for X-ray
diffraction studies were obtained by the slow diffusion of diethyl ether
into a solution of the fully open complex 5 in DCE.
Notes
The authors declare no competing financial interest.
Fully Closed Complex 6. Fully open complex 4 (18 mg, 0.026 mmol)
and silver tetrafluoroborate (10 mg, 0.052 mmol) were mixed in
anhydrous DCM (1 mL). The resulting mixture was stirred at room
temperature for 10 min in the absence of light. The mixture was then
filtered, and the filtrate was dried under reduced pressure to afford
ACKNOWLEDGMENTS
■
This material is based on work supported by the following
awards: U.S. Army Grant W911NF-15-1-0151, National Science
Foundation Grant CHE-1149314, and the Vannevar Bush
Faculty Fellowship program sponsored by the Basic Research
Office of the Assistant Secretary of Defense for Research and
Engineering and funded by the Office of Naval Research through
Grant N00014-15-1-0043. Z.S.K. acknowledges an International
Institute for Nanotechnology postdoctoral fellowship, 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.
the desired product 6 as a light-yellow solid in 96% yield (20 mg,
1
0
5
.026 mmol). H NMR (500 MHz, CD Cl ): δ 7.73−7.09 (m, 14H),
2
2
13
.50−4.47 (m, 4H), 3.80−3.19 (m, 4H), 1.61−1.21 (m, 18H).
NMR (126 MHz, CD Cl ): δ 132.50, 131.48, 131.28, 131.22, 130.88,
C
2
2
1
29.15, 125.04, 124.26, 123.11, 122.44, 60.75, 59.99, 31.89, 31.20, 30.25,
+
−
− +
29.99. HRMS (ESI ). Calcd for C H FN PdS ([M − 2BF + F ] ):
30
40
4
2
4
m/z 645.1713. Found: m/z 645.1719. Single crystals suitable for X-ray
diffraction studies were obtained by the slow diffusion of diethyl ether
into a solution of the fully closed complex 6 in MeCN.
Fully Closed Complex 7. The fully open complex 5 (20 mg,
.028 mmol) and silver tetrafluoroborate (11 mg, 0.056 mmol) were
0
mixed in anhydrous DCM (3 mL). The resulting mixture was stirred at
room temperature for 1 h in the absence of light. The mixture was then
filtered, and the filtrate was dried under reduced pressure to afford
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the desired product 7 as a light-yellow solid in 98% yield (23 mg,
1
0
7
5
2
2
.027 mmol). H NMR (400 MHz, CD Cl ): δ 7.73−7.60 (m, 4H),
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2
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2 2
1
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−
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(
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32
44
4
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2
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solution program using direct methods and refined with the ShelXL
refinement package using least-squares minimization.
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*
S
Supporting Information
The Supporting Information is available free of charge on the ACS
Publications website at DOI: 10.1021/acs.inorgchem.7b00543.
2
Experimental details, including 1D NOE spectra, kinetic
experiments, reversibility of the structural changes, and
NMR and mass spectra (PDF)
(
Crystallographic data for complex 4b in CIF format (CIF)
G
DOI: 10.1021/acs.inorgchem.7b00543
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
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