Modulation of electronics and thermal stabilities of photochromic phosphino-aminoazobenzene derivatives in weak-link approach coordination complexes

Article abstract of DOI:10.1021/ja407148n

A series of d⁸ transition-metal (Pt(II) and Pd(II)) coordination complexes incorporating phosphine-functionalized aminoazobenzene derivatives as hemilabile phosphino-amine (P,N) ligands were synthesized and studied as model weak-link approach (WLA) photoresponsive constructs. The optical and photochemical properties of these complexes were found to be highly influenced by various tunable parameters in WLA systems, which include type of metal, coordination mode, type of ancillary ligand, solvent, and outer-sphere counteranions. In dichloromethane, reversible chelation and partial displacement of the P,N coordinating moieties allow for toggling between aminoazobenzene- or pseudostilbene- and azobenzene-type derivatives. The reversible switching between electronic states of azobenzene can be controlled through either addition or extraction of chloride counterions and is readily visualized in the separation between π-π* and n-π* bands in the complexes' electronic spectra. In acetonitrile solution, the WLA variables inherent to semiopen complexes have a significant impact on the half-lives of the corresponding cis isomers, allowing one to tune their half-lives from 20 to 21000 s, while maintaining photoisomerization behaviors with visible light. Therefore, one can significantly increase the thermal stability of a cis-aminoazobenzene derivative to the extent that single crystals for X-ray diffraction analysis can be grown for the first time, uncovering an unprecedented edge-to-face arrangement of the phenyl rings in the cis isomer. Overall, the azobenzene-functionalized model complexes shed light on the design parameters relevant for photocontrolled WLA molecular switches, as well as offer new ways of tuning the properties of azobenzene-based, photoresponsive materials.

Full text of DOI:10.1021/ja407148n

Article
pubs.acs.org/JACS
Modulation of Electronics and Thermal Stabilities of Photochromic
Phosphino−Aminoazobenzene Derivatives in Weak-Link Approach
Coordination Complexes
Jung Su Park,†,§,∥ Alejo M. Lifschitz,†,§ Ryan M. Young,†,‡ Jose Mendez-Arroyo,†
Michael R. Wasielewski,†,‡ Charlotte L. Stern,†,‡ and Chad A. Mirkin*,†
†Department of Chemistry and International Institute for Nanotechnology, Northwestern University, 2145 Sheridan Road, Evanston,
Illinois 60208, United States
‡Argonne-Northwestern Solar Energy Research (ANSER) Center, Northwestern University, Evanston, Illinois 60208, United States
S
* Supporting Information
ABSTRACT: A series of d8 transition-metal (Pt(II) and
Pd(II)) coordination complexes incorporating phosphine-
functionalized aminoazobenzene derivatives as hemilabile
phosphino−amine (P,N) ligands were synthesized and studied
as model weak-link approach (WLA) photoresponsive
constructs. The optical and photochemical properties of
these complexes were found to be highly influenced by
various tunable parameters in WLA systems, which include type of metal, coordination mode, type of ancillary ligand, solvent,
and outer-sphere counteranions. In dichloromethane, reversible chelation and partial displacement of the P,N coordinating
moieties allow for toggling between aminoazobenzene- or pseudostilbene- and azobenzene-type derivatives. The reversible
switching between electronic states of azobenzene can be controlled through either addition or extraction of chloride counterions
and is readily visualized in the separation between π−π* and n−π* bands in the complexes’ electronic spectra. In acetonitrile
solution, the WLA variables inherent to semiopen complexes have a significant impact on the half-lives of the corresponding cis
isomers, allowing one to tune their half-lives from 20 to 21000 s, while maintaining photoisomerization behaviors with visible
light. Therefore, one can significantly increase the thermal stability of a cis-aminoazobenzene derivative to the extent that single
crystals for X-ray diffraction analysis can be grown for the first time, uncovering an unprecedented edge-to-face arrangement of
the phenyl rings in the cis isomer. Overall, the azobenzene-functionalized model complexes shed light on the design parameters
relevant for photocontrolled WLA molecular switches, as well as offer new ways of tuning the properties of azobenzene-based,
photoresponsive materials.
lived cis isomers.15,16 Even though the AB family as a whole
INTRODUCTION
■
offers a wide span of photoexcitation energies and cis thermal
Azobenzene and its analogues have long been scrutinized
within the context of artificial photochromic systems capable of
stabilities,1−16 there is still a major challenge to developing new
synthetic approaches that allow for forward- and back-
reversibly transforming between two isomeric forms possessing
different electronic, structural, and functional properties.1,2 This
isomerization of AB with visible light and thermal stabilities
of the corresponding cis isomers within a wide range of time
scales. Among several synthetic strategies employed to control
photoactive synthon has thus been exploited in photo-
responsive molecular switches3−6 in a variety of research
areas ranging from industrial dyes7,8 to actuators,9 molecular
the photochemical properties given above, azo-conjugated
machines,10 liquid crystals,11 nonlinear optical devices,12
transition-metal complexes have been shown to provide new
catalysts,13 and biological applications.14 These extensive
advanced molecular functions via combinations of the
remarkable photoswitching properties of AB and the magnetic,
studies have led to the general categorization of azobenzene
electrochemical, or coordination properties of the transition-
derivatives into three types, namely azobenzene (AB), amino-
azobenzene (aAB), and pseudostilbene types (pAB), on the
basis of the relative energetic order of their n−π* and π−π*
metal complexes.17−19
We are particularly interested in exploiting the photo-
transition bands.15,16 pAB- and aAB-type materials, obtained
chemistry of AB moieties in the context of the weak-link
approach (WLA), a coordination chemistry based method for
through attachments of electron-accepting and/or -donating
the construction of allosterically controlled supramolecular
moieties to the parent AB, display the desired red-shifted
complexes.20−22 This approach allows for modulating the
photoexcitation energies for isomerization in comparison to
AB-type materials. However, both aAB and pAB exhibit
significantly increased rates of thermal cis to trans isomerization,
Received: July 12, 2013
which may limit their use in applications that require longer-
Published: November 5, 2013
© 2013 American Chemical Society
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structure and properties of functional moieties embedded into
the supramolecular architecture via coordination chemistry at
distal sites, and it has been extensively employed in catalytic
switches,23,24 chemical sensors,25,26 and signal amplification
applications.27,28 We envisioned that AB could bestow WLA
systems with multiple new functionalities, since its photo-
isomerization can allow for the toggling of properties intrinsic
to the behavior of catalytic switches, such as reactive cavity sizes
and substrate binding affinities.29,30 Furthermore, we hypothe-
sized that AB could provide signaling for coordination changes
that directly relate to the activity of a WLA system through the
modulation of AB’s electronics upon changes in coordination to
the structural regulatory centers.
also being able to modulate the half-lives of their cis conformers
over a 3 order of magnitude range, reaching values typical of
AB-type moieties. Such control over their half-lives allows one
to significantly increase the lifetime of cis isomers of aAB
derivatives to the extent that single crystals can be grown at
ambient temperature and structural characterization in the solid
state is, for the first time, possible. The model complexes
studied herein thus represent general photoactive building
blocks that can be incorporated into new WLA supramolecular
switches with unparalleled photochemical properties.
RESULTS AND DISCUSSION
■
Synthesis. In many known cases, the azo group of AB can
chelate to transition metals, thus hampering photoinduced trans
to cis isomerization processes.31 In order to avoid such a
situation, it was hypothesized that the attachment of a
phosphine moiety to aAB derivatives would not only result in
P,N-based hemilabile ligands for WLA model complexes but
could also prohibit the coordination of the azo group to soft
WLA metal centers. Under this assumption, two photo-
isomerizable ligands possessing a P,N donor set, 1 and 2,
were prepared (see Scheme 2 and the Supporting Information).
These azobenzene derivatives can be classified into aAB type
for 1 and pAB type for 2 according to the Rau classification.15,16
Toward this end, we have synthesized a series of model WLA
d8 transition-metal complexes (3−12) that incorporate hemi-
labile phosphino−amine (P,N) ligands 1 and 2 modified with
two representative AB-conjugated moieties (Scheme 1). In
Scheme 1. Modulation of Photoisomerization Excitation
Energies and cis Thermal Stabilities of Model WLA
a
Complexes
Scheme 2. Synthetic Routes to Aminoazobenzene-
Conjugated WLA Complexes
a
The numbering of the compounds corresponds to the trans isomers
unless otherwise noted.
doing so, we have discovered that the WLA provides a
promising new platform for tailoring AB’s photochemical
properties through changes in programmable WLA parameters
such as type of metal, coordination mode, type of ancillary
ligand, solvent, and outer-sphere counteranions. For instance, it
was determined that the switching between coordination
modes via abstraction or addition of chloride (closed 9−12
vs semiopen 3−8) induces dramatic electronic changes in
parent AB chromophores, resulting in the toggling between two
distinct types of AB derivatives in dichloromethane solution
(AB vs aAB or pAB). While this toggling can be used,
expectedly, to modulate the excitation energies for forward- and
back-isomerization, we also determined that the WLA platform
enables photochemical properties that are unique to the
supramolecular ensemble. Most remarkably, we are able to
generate semiopen complexes that retain the ability to be
photoisomerized with visible light characteristic of aAB, while
Using compounds 1 and 2, a series of d8 transition-metal
complexes were successfully synthesized in high yields via well-
established standard WLA protocols (Scheme 2).32,33 The array
of complexes as a whole introduces variables such as the nature
of the metal cation (Pt(II) or Pd(II)), type of ancillary
phosphino−thioether (P,S) ligand (P,S-Me or P,S-Ph) and
−
inner- or outer-sphere anions (Cl− and BF4 ), which will be
scrutinized later in terms of their effect on the photochemistry
of the AB moieties. The model complexes were first isolated as
semiopen complexes (3−8) in which the P,N-AB ligands are
coordinated through the phosphorus to the transition-metal
center and a second ancillary P,S ligand is chelated through
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both phosphorus and sulfur moieties. The use of ancillary
ligands illustrates the ability of the model complexes in question
to be incorporated into larger functional systems, since these
ligands have been shown to be easily replaced with isoelectronic
moieties containing catalytic and substrate binding groups.21,24
The semiopen complexes underwent quantitative chloride
abstraction with AgBF4 in CH2Cl2 to yield their closed
counterparts (9−12) in which both the P,N-AB and ancillary
ligands are chelated to the transition-metal center in a cis
fashion. Reversible toggling between the closed and semiopen
coordination modes is possible via the reintroduction of a
chloride counterion in the form of N(n-Bu)4Cl (TBACl). A
complete account of synthetic procedures and associated
characterization data (1H, 13C{1H}, and 31P{1H} NMR and
HRMS spectra) for compounds 1−12 is included in the
Supporting Information.
shortening of the N−C bond. Conversely, the nitrogen lone
pair of electrons in 10 participates in the Pt−N coordination
bond, which results in the lengthening of the N−C bond.
Changes in the N−C bond length are highly indicative that
metal coordination induces significant changes in the
electronics of the aAB core.
Electronics and Photochemistry. UV−vis spectra of
complexes 1−12 were measured in CH2Cl2 solutions in order
to evaluate the effects of the metal cation and coordination
mode on the electronic structures of the AB units. As
exemplified in Figure 2, the electronic spectra of 1−8 display
Solid-State Characterization. To gain further insight into
the details of structural parameters of AB-functionalized
semiopen and closed complexes, X-ray crystallographic analyses
of 1, 3−5, 7, and 10 were carried out. As depicted in Figure 1,
Figure 1. Crystal structures of 1 (a), 3 (b), 4 (c), 5 (d), 7 (e), and 10
(f) drawn with 50% probability thermal ellipsoids. Hydrogens and
counteranions are omitted for clarity. Color scheme for the atoms:
platinum, magenta; palladium, black; chloride, green; sulfur, yellow;
phosphorus, orange; nitrogen, blue; carbon, gray.
these solid-state structural studies, in conjunction with solution-
based characterization, serve to confirm our initial hypothesis
that the novel aAB derivatives 1 and 2 would act as P,N-based
hemilabile ligands which, significantly, do not undergo
coordination of the azo groups to Pt(II) or Pd(II). A detailed
inspection of these crystal structures reveals that the bond
length parameters regarding the aAB moiety in semiopen
complexes 3−5 are similar to those found in free ligand 1.
However, there is a significant difference in bond length
parameters between the closed complex 10 and 1 or 3−5. In
particular, the distance of 1.48 Å between the nitrogen in the
amino group and the 4-AB carbon in 10 is significantly longer
than those found in either 1 or the semiopen complexes 3−5
(1.37 0.01 Å). This can be rationalized upon consideration of
the resonance structures of the aAB moiety. That is, the lone
pair of electrons on the amino nitrogen engages in resonance
structures that impart a partial double-bond character in ligand
1 and semiopen complexes 3−5 and, thus, contributes to the
Figure 2. (a) Isomolar UV−vis spectra in CH2Cl2 of free ligand 1
(black), semiopen complex 4 (red), and closed complex 9 (blue). (b)
Isomolar UV−vis spectra in CH2Cl2 of free ligand 2 (black), semiopen
complex 8 (red), and closed complex 12 (blue).
a single broad absorption band in the visible region as a
common feature. Only small blue shifts were observed in the
λmax values of semiopen complexes 3−8 as compared to those
in the corresponding ligands 1 or 2 (see Table 1), suggesting
that the formation of semiopen complexes does not alter the
electronic structure of the aAB or pAB moieties significantly.
Indeed, such large spectral overlap between π−π* and n−π*
bands found in 1−8 is a key characteristic of either aAB- or
pAB-type moieties. The electronic spectra of closed complexes
9−12, however, are significantly different from those of 1−8 in
terms of the separation between the π−π* and n−π* bands and
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effectively varying the electron-donating ability of the nitrogen
lone pair of electrons.
Table 1. λmax Values of trans Forms of 1−12 Measured in
CH2Cl2 and CH3CN
The toggling between electronic states of AB can also be
exploited as a signaling tool for coordination changes. As
mentioned above, the reversible switching between rigid, closed
and flexible, open complexes by their interaction with
appropriate allosteric effectors, a staple of the WLA, is also
observed upon addition of a chloride ion source to closed
complexes 9−12 in CH2Cl2. For instance, progressive additions
of N(n-Bu)4Cl to complex 9 results in near-quantitative
conversion to the corresponding semiopen complex 4.
31P{1H} NMR spectra of the coordination complex mixture
throughout the titration reveal that, indeed, each fraction of
chloride added displaces the Pt−N bond selectively. This is
supported by the decrease in the 31P NMR signals
corresponding to a closed heteroligated complex (δ 37.4, JP−P
= 14 Hz, JP−Pt = 3298 Hz and δ 30.4, JP−P = 14 Hz, JP−Pt = 3216
Hz) and the concomitant increase in signals corresponding to a
semiopen heteroligated complex comprised of chelated P,S (δ
43.9, JP−P = 14 Hz, JP−Pt = 3523 Hz) and phosphorus-bound
P,N (δ 6.3, JP−P = 15 Hz, JP−Pt = 3124 Hz) ligands.34 The
titration process was also visualized by following the progressive
UV−vis spectral variations produced from the changes in
electronic structure of the AB moieties. As shown in Figure 3a,
the titration results in the formation of an isosbestic point at
362 nm, thus establishing the signature of a two-state
equilibrium between closed complex 9 and semiopen complex
4, consistent with the 31P{1H} NMR measurements.
Furthermore, changes in the absorption intensity of the
complex mixture as a function of the respective N(n-Bu)4Cl
concentrations were used to construct a binding isotherm and
calculate the association constant (Ka = 9.2 × 106 M−1). The
high affinity of closed complexes for chloride anions in tandem
with the large spectral changes that result from toggling
between closed and semiopen complexes makes the AB
moieties an excellent signaling unit for coordination changes
in WLA complexes.
In order to further investigate the effects of WLA variables on
the photochemical properties of AB in polar coordinating
solvents, the electronic spectra of 1−12 were also measured in
CH3CN solutions. As summarized in Table 1, the λmax values of
trans isomers for 1−8 in CH3CN are somewhat similar to those
measured in CH2Cl2 solutions; the absorption properties in
CH3CN of semiopen complexes 3−8 and their corresponding
free ligands 1 or 2 were also very similar to one other,
displaying slightly blue shifted absorption maxima (Δλmax = 7
nm for 3−6 and Δλmax = 14 nm for 7 and 8). It should be
noted, however, that the cleanly closed complexes 9−12 cannot
be made in CH3CN solution, due to CH3CN displacing the
weak Pt−N coordination bond. Indeed, dissolving closed Pt(II)
complexes 9, 10, and 12 in CH3CN results in severe
broadening of their 31P{1H} NMR signals, which suggests a
fast dynamic exchange between closed and semiopen
complexes caused by coordination of the solvent. In the case
of Pd(II) complex 11, however, two clean 31P{1H} NMR
signals were observed (δ 64.0, JP−P = 8 Hz and δ 29.2, JP−P = 8
Hz), assigned to the CH3CN-coordinated semiopen complex
(11b). In all cases, coordination of CH3CN is also supported
by high-resolution mass spectroscopy measurements. The
displacement of the coordinated amino group by CH3CN is
likely responsible for the significant red shifts observed in their
λmax values of 9−12.
λmax (nm)
compound
in CH2Cl2
in CH3CN
1
418
461
410
408
408
408
444
446
330
331
321
334
418
459
411
411
411
411
445
445
409
409
407
440
2
3
4
5
6
7
8
9
10
11
12
their respective λmax values (Figure 2). In particular, the UV−vis
spectra of all closed complexes 9−12 display a high-intensity
π−π* band in the UV region which is well separated from a
low-intensity n−π* band in the visible region, overall a
distinctive characteristic of AB-type materials.
As summarized in Table 1, the degree of blue shift in the
absorption maxima of the closed complexes 9−12, in
comparison to 1 or 2, depends on the type of metal cation
and type of AB, regardless of solvent. For instance, chelation of
the P,N group to Pd(II) induces a larger blue shift (11, Δλmax
=
97 nm) than in Pt(II) complexes (9, Δλmax = 88 nm) for an
identical ligand configuration. An even larger blue shift (Δλmax
= 127 nm) was observed in the closed Pt(II) complex 12 from
its parent pAB-type ligand 2, in comparison to the 1 vs 9 pair
(see Scheme 2). This latter finding can be rationalized from the
cancellation of the push−pull character in 2 once the highly
electron-donating ability of the amino group is diminished in
12 upon coordination to Pt(II).
Computational modeling further suggests that the spectral
differences between semiopen and closed complexes result
from the sequestration of the amino N lone pair of electrons
upon coordination to a WLA metal center. Indeed, single-point
calculations of semiopen complex 3 and closed complex 9 show
that the azobenzene-centered orbitals involved in the
transitions studied herein do not show an electronic
contribution from the amino N lone pair in the closed Pt(II)
complex 9 but do have a contribution in the azobenzene n and
π* orbitals of semiopen Pt(II) complex 3 (see the computa-
tional section in the Supporting Information).
The changes in electronic structures of AB chromophores
induced by coordination of the P,N moieties result in dissimilar
operation of the photoswitches between 1−8 and 9−12. That
is, closed complexes 9−12 undergo trans to cis isomerization
upon irradiation at their λmax values in the UV region and the
corresponding back-isomerization process is achieved upon
excitation of the n−π* band in the visible region (410−550
nm). On the other hand, 1−8 undergo reversible photo-
isomerization triggered by irradiation at two visible light
wavelengths, wherein forward-isomerization occurs upon
irradiation at the λmax of trans isomers 1−8 and back-
isomerization occurs upon excitation at λ ≥525 nm for cis-1
and cis-3−6 or at λ ≥ 550 nm for cis-2 and cis-7−8. The overall
spectral changes described above indicate that the changes in
coordination to Pt(II) or Pd(II) allow one to reversibly toggle
between distinct electronic states of the azobenzene moieties by
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Figure 4. UV−vis spectral changes upon photoisomerization of 1 (a)
and 4 (b) in CH3CN before (black line) and after (red line)
photoirradiation at the λmax value of the corresponding trans isomer.
Figure 3. (a) Plot of UV−vis spectral changes of a 25 μM solution of
closed complex 9 in CH2Cl2 arising as a function of increasing
amounts of N(n-Bu)4Cl and concomitant formation of semiopen
complex 4. (b) Binding curve analysis of chloride to complex 9 based
on data in Figure 3a.
nonlinear least-squares curve fit analyses, and the resulting
values are summarized in Table 2.
Upon irradiation at each complex’s λmax, all semiopen
complexes undergo trans to cis photoisomerization. These
photochromic effects are best exemplified by the corresponding
changes in the measured UV−vis spectra of semiopen
complexes 3−6 and 11 in CH3CN; as shown in Figure 4b,
the spectrum of cis-4 in CH3CN exhibits two distinct
absorption bands centered at 367 and 441 nm following
irradiation at λex 418 nm, which are assigned to high-intensity
π−π* and low-intensity n−π* excitation bands with the
characteristic separation of the cis-aAB isomers. Similar changes
in UV−vis spectra upon isomerization of free ligand 1 in
CH3CN and other corresponding semiopen complexes cis-3−6
and cis-11 were also obtained (see Figure 4a and the
Supporting Information).
Thermal cis to trans Isomerization. As mentioned above,
the thermal stability of cis conformers toward thermal
isomerization is strongly influenced by the electronic structure
of the AB derivative. In order to investigate the effects different
WLA variables have on the stabilities of the cis isomers of 1−12,
rates of the thermal cis to trans isomerization were monitored in
both CH2Cl2 and CH3CN via UV−vis spectroscopy. Plots of
the associated changes in absorption intensity as a function of
time were then used to estimate first-order rate constants by
In CH2Cl2 solution, improved cis thermal stabilities for both
semiopen complexes cis-3−8 and closed complexes cis-9−12
were observed in general, in comparison to their free ligand
counterparts, cis-1 or cis-2; for a given combination of Pt(II)
and the corresponding free ligands, the sequence closed >
semiopen > free ligand was observed in terms of thermal
stability of their cis isomers. We rationalize this order, in part,
on the basis of considering that the closed Pt(II) complexes 9
and 10 behave as AB-type materials, which display greater cis
half-lives than aAB-type materials across the board. As expected
from the cancellation of push−pull character in the closed
complex 12, an even greater degree of stability of its cis isomer
was observed in comparison to that of ligand 2. Finally, it is
worth noting that the stability of the cis conformers is higher in
Pt(II) complexes than in Pd(II) complexes, given an identical
set of ligands and coordination modes (cis-4 vs cis-5 and cis-9 vs
cis-11).
Significant solvent effects were also observed on the rates of
thermal cis to trans isomerization of model WLA complexes.
Most remarkably, it was found that rates of thermal cis to trans
relaxation in acetonitrile are strongly influenced by variables
associated with semiopen complexes such as type of metal,
ancillary ligands, and outer-sphere counteranions. For instance,
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a
Table 2. Rate Constants of cis to trans Thermal
Isomerization in CH2Cl2 and CH3CN
k/s‑1 (half-life/s)
compound
cis-1
in CH2Cl2
in CH3CN
8.1 × 10−2 (8.5)
9.6 × 10−2 (7.2)
1.3 × 10−2 (54)
1.8 × 10−2 (39)
6.0 × 10−2 (12)
7.7 × 10−2 (9.1)
2.3 × 10−2 (30)
2.1 × 10−2 (33)
6.9 × 10−3 (100)
7.3 × 10−3 (95)
6.7 × 10−2 (10)
2.8 × 10−3 (252)
9.2 × 10−4 (750)
1.4 × 10−2 (50)
cis-2
cis-3
5.4 × 10−5 (13000)
3.3 × 10−5 (21000)
7.6 × 10−5 (9100)
8.1 × 10−5 (8500)
2.1 × 10−3 (330)
2.1 × 10−3 (330)
cis-4
cis-5
cis-6
cis-7
cis-8
b
cis-9
7.4 × 10−3 (94)
Figure 5. Crystal structure of cis-4 drawn with 50% probability thermal
ellipsoids. The hydrogen atom in close contact with one of the phenyl
rings in the aAB moiety is shown. All other hydrogens and the
counteranion are omitted for clarity. Color scheme for the atoms:
platinum, magenta; chloride, green; sulfur, yellow; phosphorus,
orange; nitrogen, blue; carbon, gray; hydrogen, white.
b
cis-10
cis-11/ cis-11b
cis-12
1.2 × 10−3 (56)
c
3.4 × 10−2 (20)
b
1.6 × 10−2 (44)
a
All k values were calculated from the first-order plots of absorption
b
change at 300 K in CH2Cl2 and CH3CN. The structure of the
complex is part of a fast dynamic interchange between closed and
semiopen coordination modes. CH3CN-coordinated semiopen
calculated cis isomers of AB and its analogues.38−40 In
particular, unlike the tilted face-to-face orientation of two
phenyl rings found in all other reported cis structures, cis-4
adopts an edge-to-face (T-shaped) structure. The significantly
short distance between a β-positioned C−H moiety and a α-
positioned carbon atom (2.35 Å) indicates an intermolecular
CH−π interaction (H to ring plane distance of 2.38 Å).
Although this kind of attractive intramolecular edge-to-face
aromatic interaction is found in a variety of types of materials
and plays an important role in such diverse areas as protein
folding,41,42 host−guest binding in supramolecular assemblies,43
crystal packing and engineering,44 drug−receptor interactions,
and other molecular recognition processes,45 this is the first
observation of an intramolecular edge-to-face aromatic
interaction for azobenzene derivatives.
Rates of Forward trans to cis Photoisomerization.
After establishing that various parameters in the WLA system
significantly influence the thermal cis to trans isomerization
process, we went on to evaluate their influence on trans to cis
photoisomerization quantum yields. Toward this end, samples
of 1−8 in CH2Cl2 and CH3CN were irradiated with a 440 nm
laser diode and trans to cis isomerization rate constants were
calculated from the time-dependent UV−vis spectral changes
(see the Experimental Section for a full description of the
setup). The quantum yields (Φ) were obtained by comparison
to methyl orange standard (Φ = 0.47 in acetone). As
summarized in Table 3, the quantum yields of trans to cis
isomerization for all semiopen complexes 3−8 in both CH2Cl2
and CH3CN solution were found to be lower than those of the
c
complex.
complexes cis-3−8, having a coordinated chloride anion and
semiopen configuration, generally have significantly longer half-
lives than cis-1 or cis-2. Furthermore, with a particular ligand
combination, the stability of the cis conformers of Pt(II)
complexes is higher than that for the Pd(II) counterparts (cis-3
vs cis-5). The systems containing P,S-Me as an ancillary ligand
show an enhanced thermal stability relative to those having P,S-
−
Ph (cis-4 vs cis-6). In addition, the complexes with BF4 as an
outer-sphere counterion display cis stability higher than those
with chloride. Finally, when a coordinated amine group in
complexes 9−12 is replaced by CH3CN, leading to the
corresponding semiopen complexes, markedly decreased cis
thermal stabilities are observed. These observations suggest that
the overall WLA architecture, including the counterions and
solvent molecules loosely associated with it, may affect the
energetics and/or mechanism of the rearrangements undergone
throughout thermal back-isomerization, effectively enabling a
wide range of thermal stabilities of the cis conformer especially
in polar coordinating solvents. Most notably, we can utilize the
WLA architecture to access long-lived cis isomers of aAB-type
materials while maintaining photoisomerization with visible
light.
Crystal Structure of the Stabilized cis-aAB 4. To the
best of our knowledge, there have been no reports of X-ray
crystal structures of cis-aAB or cis-pAB-type molecules, although
there are simulated molecular structures in the literature.35−37
This is likely because of the fast thermal back-isomerization to
the corresponding trans states, which makes it difficult to grow
single crystals by conventional crystallization methods. The
WLA platform, however, allows one to increase the thermal
stability of cis-aAB moieties to the extent that, for the first time,
crystallization is possible. Judging from thermal back-isomer-
ization rates of cis-1−8 in acetonitrile, complex cis-4 possesses
the highest thermal stability of the cis isomers across the board.
We therefore attempted to grow diffraction-quality crystals of
cis-4 from an acetonitrile solution into which ether vapor was
allowed to diffuse slowly after illumination with 411 nm band-
filtered light. The resulting analysis of the crystals formed via
this approach (Figure 5) reveals that the molecular structure of
cis-4 is significantly different from those of reported or
Table 3. Quantum Yields of trans to cis Photosomerization in
CH2Cl2 and CH3CN
Φtrans−cis
compound
in CH2Cl2
in CH3CN
1
2
3
4
5
6
7
8
0.36
0.97
0.23
0.20
0.16
0.18
0.20
0.17
0.44
0.28
0.26
0.25
0.08
0.21
0.20
0.20
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Article
of each sample was irradiated at the appropriate λmax value in a Horiba
Jovin-Yvonne Fluorolog fluorimeter with constant stirring and power
outputs of ∼4 mW for 10 min. The samples were then placed in a
UV−vis spectrophotometer, and their absorbance spectral changes
were measured as a function of time at room temperature. Quantum
yields of trans to cis photoisomerization were measured by exciting
2.50 mL samples of 25 μM solutions of 1−8 in CH2Cl2 or CH3CN
with a 440 nm laser diode (PicoQuant). Spectral changes in the
solution upon excitation were monitored with a Shimadzu 1601 UV−
vis spectrophotometer. The measurements were carried out with
vigorous stirring and constant irradiation at 440 nm and power outputs
of 0.85 mW. trans to cis isomerization rate constants were calculated
from the time-dependent UV−vis spectral changes, and the values
were used to calculate quantum yields of trans to cis photo-
isomerization as previously reported.46 Methyl orange was used as a
standard.47,48
Synthesis. 2-[N-[4-(Phenylazo)phenyl]-N-ethylamino]-1-metha-
nesulfonate−Ethanol (13). 2-[N-Ethyl-N-[4-[2-phenyldiazenyl]-
phenyl]amino]ethanol49 (8.00 g, 29.7 mmol) was dissolved in 100
mL of CH2Cl2 under an inert atmosphere, and triethylamine was
added (20 mL). Methanesulfonyl chloride (10.2 g, 89.1 mmol, 3
equiv) in CH2Cl2 solution was slowly added, and the reaction mixture
was stirred for 3 h. The solution was washed with water and dried over
Na2SO4. The solvent was then evaporated, and the crude product was
purified via silica gel column chromatography using CH2Cl2 as eluent,
which afforded a dark red solid (9.91 g, 96% yield). 1H NMR (400.16
MHz, 25 °C, CD2Cl2): δ 7.92 (d, JH−H = 9 Hz, 2H), 7.86 (d, JH−H = 9
Hz, 2H), 7.50 (m, 2H), 7.41 (t, JH−H = 7 Hz, 1H), 6.84 (d, JH−H = 9
Hz, 2H), 4.40 (t, JH−H = 6 Hz, 2H), 3.80 (t, JH−H = 6 Hz, 2H), 3.56 (q,
JH−H = 7 Hz, 2H), 2.98 (s, 3H), 1.26 (t, JH−H = 7 Hz, 3H). 13C{1H}
NMR (100.63 MHz, 25 °C, CD2Cl2): δ 150.5 (s), 144.1 (s), 130.5 (s),
130.1 (s), 129.4 (s), 126.0 (s), 122.5 (s), 112.3 (s), 67.0 (s), 49.9 (s),
46.5 (s), 37.9 (s), 12.4 (s). MS (ESI+): m/z calcd for [M + H]+ 348,
found 348.
corresponding ligands as a common feature. We postulate that
the decreases in quantum yields of trans to cis isomerization for
semiopen WLA complexes with respect to the corresponding
photochromic ligands arise in part due to the increase in the
rotor volume for the motion of isomerization. It is worth
noting, however, that the differences in quantum yields of
photoinduced trans to cis isomerization between the semiopen
complexes and their respective free ligands are far less than
those observed for thermal cis to trans isomerization. Thus, the
results described above indicate that changes in parameters in
the WLA system can be used to modulate thermal stabilities of
the cis-AB moieties, while roughly maintaining their trans to cis
photoisomerization efficiency. Overall, the model complexes
studied herein highlight the design considerations to be taken
in the future construction of photoresponsive supramolecular
devices via the WLA.
CONCLUSION
■
In summary, we have shown that the WLA architecture can be
exploited to modulate the electronics and photophysical
properties of AB moieties though various tunable WLA
variables (type of metal cation, coordination mode, ancillary
ligand, counteranions, and solvent). Coordination chemistry
can be used to toggle between different types of AB, resulting in
large changes in spectral features and photochromic behavior,
which makes P,N-AB ligands potential signaling units for WLA
sensors and signal amplification systems. Furthermore, the fact
that WLA architecture and the counterions and solvent
molecules loosely associated with it can impart aAB and pAB
moieties with enhanced stabilities of their cis conformers
suggests that thermally stable photoswitches operated solely
with visible light can be generated by integrating different AB
moieties into larger supramolecular systems. These photoactive
moieties can thus be incorporated into WLA catalytic switches,
and variables in the WLA system can then be defined so that
the photoactive units exhibit the optimal cis state thermal
stability for a particular reaction. Taken together, the findings in
this study should pave the way to designing and synthesizing
advanced photoswitchable supramolecular architectures with
photoisomerization-dependent properties for catalysis and
sensing applications.
2-[N-[4-(4-Cyanophenylazo)phenyl]-N-ethylamino]-1-methane-
sulfonate−Ethanol (14). The compound was prepared from 4-[2-[4-
[ethyl(2-hydroxyethyl)amino]phenyl]diazenyl]benzonitrile49 (9.00 g,
30.6 mmol) following the same mechanism as outlined above for 14,
which afforded an orange solid (10.8 g, 95% yield). 1H NMR (400.16
MHz, 25 °C, CD2Cl2): δ 7.94 (m, 4H), 7.77 (d, JH−H = 8 Hz, 2H),
6.84 (d, JH−H = 9 Hz, 2H), 4.40 (t, JH−H = 6 Hz, 2H), 3.82 (t, JH−H = 6
Hz, 2H), 3.57 (q, JH−H = 7 Hz, 2H), 2.98 (s, 3H), 1.27 (t, JH−H = 7 Hz,
3H). 13C{1H} NMR (100.63 MHz, 25 °C, CD2Cl2): δ 192.2 (s), 151.6
(s), 144.2 (s), 134.4 (s), 133.6 (s), 126.8 (s), 123.0 (s), 119.2 (s),
112.33 (s), 66.8 (s), 49.9 (s), 46.6 (s), 37.9 (s), 12.4 (s). MS (ESI+):
m/z calcd for [M + H]+ 373, found 373.
P,N-AB (1). A sample of 13 (2.00 g, 5.76 mmol) was dissolved in 50
mL of tetrahydrofuran under an inert atmosphere, and the solution
was cooled to 0 °C. A 0.5 M solution of KPPh2 (11.5 mL, 1.00 equiv)
was added in a dropwise fashion, and the mixture was stirred overnight
at room temperature. The reaction was quenched by adding 10 mL of
water, and the tetrahydrofuran was evaporated. The crude product was
dissolved in CH2Cl2, and the solution was washed with water and
dried over MgSO4. Silica gel column chromatography using hexanes/
CH2Cl2 (2/3, v/v) as eluent afforded the product as a dark red solid
(0.504 g, 20% yield). 1H NMR (400.16 MHz, 25 °C, CD2Cl2): δ 7.81
(d, JH−H = 8 Hz, 2H), 7.77 (d, JH−H = 9 Hz, 2H), 7.60−7.35 (m, 13H),
6.57 (d, JH−H = 9 Hz, 2H), 3.49 (m, 2H), 3.43 (q, JH−H = 7 Hz, 2H),
2.42 (m, 2H), 1.19 (t, JH−H = 7 Hz, 3H). 13C{1H} NMR (100.63 MHz,
25 °C, CD2Cl2): δ 153.6 (s), 150.2 (s), 143.5 (s), 138.5 (d, JC−P = 13
Hz), 133.2 (d, JC−P = 19 Hz), 129.7 (s), 129.6 (s), 129.4 (s), 129.3 (s),
125.5 (s), 122.4 (s), 111.5 (s), 47.7 (d, JC−P = 26 Hz), 45.6 (s), 26.6
(d, JC−P = 15 Hz), 12.8 (s). 31P{1H} NMR (161.98 MHz, 25 °C,
CD2Cl2): δ −20.8 (s). HRMS (ESI+): m/z calcd for [M + H]+
438.2021, found 438.2094.
EXPERIMENTAL SECTION
■
General Methods. P,N and P,S ligands were synthesized and
stored using standard Schlenk line conditions, under an inert nitrogen
atmosphere. The synthesis of semiopen and closed complexes was
performed under ambient conditions. All solvents were purchased as
HPLC grade and degassed under a stream of argon previous to use.
Deuterated solvents were purchased from Cambridge Isotope
Laboratories and used as received. All other chemicals were purchased
from Aldrich Chemical Co. and used as received or synthesized
according to literature procedures. NMR spectra were recorded on a
Bruker Avance 400 MHz instrument. H and 13C{1H} NMR spectra
1
were referenced to residual proton and carbon resonances in the
deuterated solvents. 31P{1H} NMR spectra were referenced to an 85%
H3PO4 aqueous solution. All chemical shifts are reported in ppm.
High-resolution electrospray ionization (ESI) mass spectra measure-
ments were recorded on an Agilent 6120 LC-TOF instrument in
positive ion mode. Electrospray ionization mass spectra (ESI-MS)
were recorded on a Micromas Quatro II triple quadrapole mass
spectrometer.
UV−vis absorption measurements were performed with a Varian
Cary 50 Bio spectrophotometer utilizing 10 mm cell-path quartz
cuvettes (VWR). Measurements of thermal cis to trans isomerization
rates were performed using a 25 μM solution of 1−12. A 3 mL portion
P,N-AB-CN (2). The compound was prepared from 14 (2.00 g, 5.37
mmol) following the using the same procedure as outlined above for 1.
Silica-gel column chromatography using CH2Cl2 as eluent afforded the
1
product as an orange solid (124 mg, 5% yield). H NMR (400.16
MHz, 25 °C, CD2Cl2): δ 7.88 (d, JH−H = 8 Hz, 2H), 7.79 (d, JH−H = 8
16994
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Hz, 2H), 7.75 (d, JH−H = 8 Hz, 2H), 7.60−7.35 (m, 10H), 6.55 (d,
JH−H = 8 Hz, 2H), 3.50 (m, 2H), 3.44 (q, JH−H = 7 Hz, 2H), 2.42 (m,
2H), 1.19 (t, JH−H = 7 Hz, 3H). 13C{1H} NMR (100.63 MHz, 25 °C,
CD2Cl2): δ 155.8 (s), 151.1 (s), 143.6 (s), 138.3 (d, JC−P = 13 Hz),
133.1 (d, JC−P = 19 Hz), 129.4 (s), 129.1 (s), 129.0 (s), 122.9 (s),
119.3 (s), 112.2 (s), 111.6 (s), 47.8 (d, JC−P = 26 Hz), 45.8 (s), 26.6
(d, JC−P = 15 Hz), 12.7 (s). 31P{1H} NMR (161.98 MHz, 25 °C,
CD2Cl2): δ −20.9 (s). HRMS (ESI+): m/z calcd for [M + H]+
463.2046, found 463.2046.
stirred in the dark for 15 min. The mixture was then filtered through a
pad of Celite, and the product was recrystallized from a CH2Cl2/
pentane mixture to afford the corresponding closed complex (in situ
31P{1H} NMR yields were quantitative, isolated yields were >95%).
[Pt(κ2-P,S-Me)(κ2-P,N-AB)](BF4)2 (9). 1H NMR (400.16 MHz, 25
°C, CD2Cl2): δ 8.13 (d, JH−H = 7 Hz, 2H), 7.97 (d, JH−H = 7 Hz, 2H),
7.85−7.35 (m, 24H), 4.51 (m, 2H), 4.00−1.02 (m, 14H). 31P{1H}
NMR (161.98 MHz, 25 °C, CD2Cl2): δ 37.4 (d, JP−P = 15 Hz, JP−Pt
=
3308 Hz, 1P), 30.4 (d, JP−P = 15 Hz, JP−Pt = 3215 Hz, 1P). HRMS
(ESI+): m/z calcd for [M − BF4]+ 980.2488, found 980.2485.
General Procedure for the Generation of Semiopen
Complexes. Semiopen complexes were prepared as dichloride salts
via dropwise addition of a solution of the ancilliary ligand (P,S-Me or
P,S-Ph, 0.100 mmol) in 3 mL of CH2Cl2 to a suspension of
dichloro(1,5-cyclooctadiene)platinum(II) (37.4 mg, 0.100 mmol) in 3
mL of CH2Cl2. After the mixture was stirred for 5 min, a solution of
the corresponding AB-functionalized ligand (1 or 2, 0.100 mmol) in 3
mL of CH2Cl2 was added in a dropwise fashion. The solution volume
was reduced to approximately 1 mL, and the product was precipitated
with pentane. The product was collected via vacuum filtration and
washed with pentane to afford the semiopen complex (in situ 31P{1H}
NMR yields were quantitative, isolated yields were >95%). Semiopen
1
[Pt(κ2-P,S-Ph)(κ2-P,N-AB)](BF4)2 (10). H NMR (400.16 MHz, 25
°C, CD2Cl2): δ 820−6.55 (m, 34H), 4.40−0.90 (m, 13H). 31P{1H}
NMR (161.98 MHz, 25 °C, CD2Cl2): δ 37.8 (d, JP−P = 15 Hz, JP−Pt
=
3374 Hz, 1P), 30.9 (d, JP−P = 15 Hz, JP−Pt = 3188 Hz, 1P). HRMS
(ESI+): m/z calcd for [M − BF4]+ 1041.2643, found 1041.2647.
1
[Pd(κ2-P,S-Me)(κ2-P,N-AB)](BF4)2 (11). H NMR (400.16 MHz, 25
°C, CD2Cl2): δ 8.09 (d, JH−H = 8 Hz, 2H), 7.95 (d, JH−H = 7 Hz, 2H),
7.70−7.00 (m, 25H), 4.42 (s, 2H), 3.75−2.52 (m, 7H), 1.85 (s, 3H),
1.64 (s, 1H), 1.48 (t, JH−H = 7 Hz, 3H). 31P{1H} NMR (161.98 MHz,
25 °C, CD2Cl2): δ 58.4 (d, JP−P = 8 Hz, 1P), 53.6 (d, JP−P = 6 Hz, 1P).
HRMS (ESI+): m/z calcd for [M − BF4]+ 891.1907, found 891.1916.
[Pd(CH3CN)(κ2-P,S-Me)(P,N-AB)](BF4)2 (11b). 1H NMR (400.16
MHz, 25 °C, CD3CN): δ 7.88−7.30 (m, 27H), 6.77 (d, JH−H = 8 Hz,
2H), 3.75−2.52 (m, 7H), 3.477−2.47 (m, 12H), 2.11 (s, 3H), 1.28−
0.87 (m, 4H). 31P{1H} NMR (161.98 MHz, 25 °C, CD3CN): δ 63.9
(d, JP−P = 6 Hz, 1P), 29.2 (d, JP−P = 6 Hz, 1P). HRMS (ESI+): m/z
calcd for [M − BF4]+ 932.2172, found 932.2175.
−
complexes containing both Cl− and BF4 counterions were prepared
by dissolving the corresponding semiopen dichloride complex in 3 mL
of CH3OH in the presence of AgBF4 (1.0 equiv). The mixture was
stirred in the dark for 4 h, followed by solvent evaporation and
dissolution in CH2Cl2. The mixture was filtered through a pad of
Celite and recrystallized from a CH2Cl2/pentane mixture to afford the
corresponding semiopen complex (in situ 31P{1H} NMR yields were
quantitative, isolated yields were >95%).
1
[Pt(κ2-P,S-Me)(κ2-P,N-AB-CN)](BF4)2 (12). H NMR (400.16 MHz,
25 °C, CD2Cl2): δ 8.18 (d, JH−H = 8 Hz, 2H), 8.04 (d, JH−H = 8 Hz,
2H), 7.85 (d, JH−H = 8 Hz, 2H), 7.73−6.95 (m, 22H), 4.51 (m, 2H),
3.94−2.53 (m, 8H), 2.12−0.85 (m, 6H). 31P{1H} NMR (161.98 MHz,
25 °C, CD2Cl2): δ 37.3 (d, JP−P = 14 Hz, JP−Pt = 3324 Hz, 1P), 30.6 (d,
JP−P = 13 Hz, JP−Pt = 3222 Hz, 1P). HRMS (ESI+): m/z calcd for [M
− BF4]+ 1005.2440, found 1005.2449.
X-ray Crystallography. Crystallographic data are displayed in the
Supporting Information (Table S1). Single crystals were mounted
using oil (Infineum V8512) on a glass fiber. All measurements were
made with a CCD area detector with graphite-monochromated Cu Kα
radiation. Data were collected using a Bruker APEXII detector and
processed using APEX2 from Bruker. All structures were solved by
direct methods and expanded using Fourier techniques. The non-
hydrogen atoms were refined anisotropically. Hydrogen atoms were
included in idealized positions but not refined. Their positions were
constrained relative to their parent atom using the appropriate HFIX
command in SHELXL-97.
[PtCl(κ2-P,S-Me)(P,N-AB)]Cl (3). 1H NMR (400.16 MHz, 25 °C,
CD2Cl2): δ 8.00−7.05 (m, 27H), 6.47 (m, 2H), 4.05−2.40 (m, 12H),
1.82, (m, 1H) 1.16 (m, 3H). 31P{1H} NMR (161.98 MHz, 25 °C,
CD2Cl2): δ 43.9 (d, JP−P = 13 Hz, JP−Pt = 3523 Hz, 1P), 6.3 (d, JP−P
=
14 Hz, JP−Pt = 3123 Hz, 1P). HRMS (ESI+): m/z calcd for [M − Cl]+
928.2146, found 928.2155.
1
[PtCl(κ2-P,S-Me)(P,N-AB)]BF4 (4). H NMR (400.16 MHz, 25 °C,
CD2Cl2): δ 7.95−7.15 (m, 27H), 6.52 (m, 2H), 3.85−2.45 (m, 13H),
1.13 (t, JH−H = 7 Hz, 3H). 31P{1H} NMR (161.98 MHz, 25 °C,
CD2Cl2): δ 43.5 (d, JP−P = 13 Hz, JP−Pt = 3510 Hz, 1P), 6.2 (d, JP−P
=
14 Hz, JP−Pt = 3155 Hz, 1P). HRMS (ESI+): m/z calcd for [M − Cl]+
928.2146, found 928.2148.
1
[PdCl(κ2-P,S-Me)(P,N-AB)]BF4 (5). H NMR (400.16 MHz, 25 °C,
CD2Cl2): δ 7.81 (d, JH−H = 8 Hz, 2H), 7.74 (d, JH−H = 8 Hz, 2H),
7.59−7.35 (m, 23H), 6.49 (m, 2H), 3.65 (s, 2H), 3.37 (m, 2H), 3.18
(s, 2H), 2.95 (s, 3H), 2.80 (m, 2H), 2.63 (m, 2H), 1.14 (t, JH−H = 7
Hz, 3H). 31P{1H} NMR (161.98 MHz, 25 °C, CD2Cl2): δ 62.9 (d,
JP−P = 5 Hz, 1P), 20.6 (d, JP−P = 5 Hz, 1P). HRMS (ESI+): m/z calcd
for [M − Cl]+ 840.1533, found 840.1544.
ASSOCIATED CONTENT
■
1
S
* Supporting Information
[PtCl(κ2-P,S-Ph)(P,N-AB)]BF4 (6). H NMR (400.16 MHz, 25 °C,
Figures and tables giving UV−vis spectra of all species before
and after irradiation at their respective photostations,
absorbance vs time plots for thermal cis to trans isomerization
and trans to cis photoisomerization, crystallographic data for 1,
3, 4, cis-4, 5, 7, and 10, and NMR spectra for 1−12. This
material is available free of charge via the Internet at http://
pubs.acs.org.
CD2Cl2): δ 8.20−7.10 (m, 32H), 6.50 (m, 2H), 3.87−2.55 (m, 9H),
1.75−0.85 (m, 4H). 31P{1H} NMR (161.98 MHz, 25 °C, CD2Cl2): δ
44.2 (d, JP−P = 15 Hz, JP−Pt = 3497 Hz, 1P), 6.3 (d, JP−P = 15 Hz, JP−Pt
= 3201 Hz, 1P). HRMS (ESI+): m/z calcd for [M − Cl]+ 990.2304,
found 990.2310.
[PtCl(κ2-P,S-Me)(P,N-AB-CN)]Cl (7). 1H NMR (400.16 MHz, 25 °C,
CD2Cl2): δ 8.05−6.40 (m, 28H), 4.25−1.05 (m, 16H). 31P{1H} NMR
(161.98 MHz, 25 °C, CD2Cl2): δ 43.6 (d, JP−P = 13 Hz, JP−Pt = 3492
Hz, 1P), 6.0 (d, JP−P = 13 Hz, JP−Pt = 3121 Hz, 1P). HRMS (ESI+):
m/z calcd for [M − Cl]+ 953.2099, found 953.2097.
AUTHOR INFORMATION
■
1
[PtCl(κ2-P,S-Me)(P,N-AB-CN)]BF4 (8). H NMR (400.16 MHz, 25
Corresponding Author
°C, CD2Cl2): δ 7.88 (d, JH−H = 8 Hz, 2H), 7.79−7.15 (m, 23H), 6.51
(m, 3H), 3.71 (m, 2H), 3.38 (m, 2H), 3.32−2.40 (m, 9H), 1.16 (t,
JH−H = 7 Hz, 3H). 31P{1H} NMR (161.98 MHz, 25 °C, CD2Cl2): δ
43.9 (d, JP−P = 15 Hz, JP−Pt = 3507 Hz, 1P), 6.3 (d, JP−P = 15 Hz, JP−Pt
= 3125 Hz, 1P). HRMS (ESI+): m/z calcd for [M − Cl]+ 953.2099,
found 953.2109.
C.A.M.: e-mail, chadnano@northwestern.edu; fax, (1) 847-467-
5123.
Present Address
∥Department of Chemistry, College of Science, Sookmyung
Women’s University, Cheongpa-dong 2-ga, Yonsan-gu, Seoul
140-742, Korea.
General Procedure for the Generation of Closed Complexes.
−
A sample of semiopen complex containing one BF4 and one Cl−
Author Contributions
counterion (0.100 mmol) was dissolved in 3 mL of CH2Cl2, and
AgBF4 (21.7 mg, 1.10 equiv, 0.110 mmol) was added. The mixture was
§These authors contributed equally.
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Journal of the American Chemical Society
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Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This material is based upon work supported by DoD/NSSEFF/
NPS awards N00244-09-1-0012 and N00244-09-1-0071, U.S.
Army award W911NF-11-1-0229, NSF award CHE-1149314,
and Basic Science Research Programs through the National
Research Foundation of Korea (NRF) funded by the Ministry
of Science, ICT and Future Planning (NRF-
2013R1A1A1011508). M.R.W. was supported by the Chemical
Sciences, Geosciences, and Biosciences Division, Office of Basic
Energy Sciences, DOE, under Grant No. DE-FG02-
99ER14999. R.M.Y. was supported as part of the ANSER
Center, an Energy Frontier Research Center funded by the U.S.
Department of Energy (DOE), Office of Science, Office of
Basic Energy Sciences, under award number DE-SC0001059.
R.M.Y. also thanks the Camille and Henry Dreyfus
Postdoctoral Program in Environmental Chemistry for support.
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