Supramolecular Recognition: Protonmotive-Driven Switches or Motors?

Article abstract of DOI:10.1002/chem.200305620

A dicationic molecular receptor bearing two cofacially disposed terpyridyl-Pd-Cl units forms stable 1:1 host-guest complexes with planar, neutral platinum(II) complexes. When the guest is modified to incorporate a pyridine group, the now basic guest is protonated by trifluoroacetic acid in acetonitrile solutions. The basic yellow guest forms a stable, deep red 1:1 host-guest complex with the yellow palladium receptor. Addition of trifluoroacetic acid to this host-guest complex leads to the displacement of the guest from the receptor. It is proposed that the dissociation of the guest is caused by electrostatic repulsion between the dicationic receptor and the positively charged protonated guest. Addition of base restores the host-guest complex. This protonmotive translocation of the guest from the host to the solution is discussed in terms of the mechanisms that drive molecular motors, the power stroke and the Brownian ratchet. It is concluded that the system is best described as a molecular switch that operates by the same mechanism as one stroke of a molecular motor.

Full text of DOI:10.1002/chem.200305620

FULL PAPER
Supramolecular Recognition: Protonmotive-Driven Switches or Motors?
James D. Crowley, Andrew J. Goshe, Ian M. Steele, and Brice Bosnich*[a]
Abstract: A dicationic molecular re-
ceptor bearing two cofacially disposed
terpyridyl-Pd-Cl units forms stable 1:1
host guest complexes with planar, neu-
tral platinum(ii) complexes. When the
guest is modified to incorporate a pyri-
dine group, the now basic guest is pro-
tonated by trifluoroacetic acid in aceto-
nitrile solutions. The basic yellow guest
forms a stable, deep red 1:1 host guest
complex with the yellow palladium re-
ceptor. Addition of trifluoroacetic acid
to this host guest complex leads to the
displacement of the guest from the re-
ceptor. It is proposed that the dissocia-
tion of the guest is caused by electro-
static repulsion between the dicationic
receptor and the positively charged
protonated guest. Addition of base re-
stores the host guest complex. This
protonmotive translocation of the guest
from the host to the solution is dis-
cussed in terms of the mechanisms that
drive molecular motors, the power
stroke and the Brownian ratchet. It is
concluded that the system is best de-
scribed as a molecular switch that oper-
ates by the same mechanism as one
stroke of a molecular motor
Keywords: host guest systems
¥
molecular motors molecular
¥
switches
¥
protonmotive
displacement
chemistry
¥
supramolecular
Introduction
its work for a specific task, such as locomotion. These defini-
tions imply that a molecular switch and motor can operate
in a similar manner if the switch involves molecular translo-
cation.
It has become common to refer to certain responses of
supramolecular assemblies as representing the operation of
molecular switches, motors, or even machines. How the re-
sponse of a molecular assembly to various chemical or phys-
ical stimuli is characterized can depend on the perspective
of the observer of the phenomena. Since we deal with the
operation of what are described as molecular switches or
motors herein, it is useful to define these terms. Although
there are more complicated definitions,[1] a simple molecular
switch can be defined as a molecular system that can be in-
duced to transform from one state to another by stimuli
such as light, electrochemistry, or chemical reactions.
Changes in state are, of course, associated with changes in
physical characteristics. A molecular motor is a molecular
assembly that can transform chemical energy into mechani-
cal work. Molecular motors, therefore, involve the coupling
of chemical reactions with molecular motion and usually
refer to molecular assemblies where concerted, repetitive
chemically driven ™strokes∫ cause specific molecular move-
ment. A molecular machine is a molecular motor that uses
Biological systems incorporate numerous molecular
motors that perform a variety of tasks.[2] Relevant in the
present context are the rotary molecular motors, FoF1AT-
Pase[3] and the bacterial flagella motor[4] that are driven by
the protonmotive force associated with transmembrane
proton gradients. In the former case, motor rotation leads to
ATP synthesis whereas in the latter the protonmotive force
driven rotation leads to bacterial locomotion. The mecha-
nism by which the molecular motion is driven by the proton
gradient is complex, but relies on electrostatic, hydrophobic,
and hydrophilic interactions that are switched on and off by
protonation and deprotonation.[3] The biological rotary
motors are especially complex machines so that attempts to
emulate some of their features in simple systems present
formidable challenges. Despite this, a number of reports
have appeared of systems that couple chemical energy with
molecular motion.[5] In the present context, two examples of
proton promoted translocation are notable. One of these is
a two-site rotaxane that, upon protonation of the rotaxane
thread, leads to intramolecular site translocation of the
charged, threaded macrocycle.[6] An intermolecular analogue
involves a positively charged platinum-based host bearing
dimethylaniline guests.[7] In this case the guests are released
from the positively charged host cavity by guest protonation.
In both of these examples the host guest association arises
from weak noncovalent forces[8] that are overcome by elec-
[a]J. D. Crowley, Dr. A. J. Goshe, Dr. I. M. Steele, Prof. B. Bosnich
Department of Chemistry, The University of Chicago
5735 South Ellis Avenue, Chicago, IL 60637 (USA)
Fax : (+1)773-702-0805
E-mail: bos5@uchicago.edu
Supporting information for this article is available on the WWW
under http://www.chemeurj.org/ or from the author.
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DOI: 10.1002/chem.200305620
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1944 1955
trostatic repulsion after protonation. The work described
here examines several aspects of proton-induced guest disso-
ciation from host guest complexes. The process is discussed
in terms of the work that is done.
Results andDiscussion
Host guest complexes: In previous work[9] we showed that
the palladium-based receptor 1 forms a very stable 1:1 asso-
ciation complex with the platinum complex 2. It was found
Figure 1. Structure of the guest 5 (ORTEP diagram; the thermal ellip-
soids are shown at the 50% probability).
Table 1. Crystallographic data for 5 and 1¥5CH3CN.
Compound
formula
5
1
C10H12N2O2Pt
C55H55Cl2N7Pd2 + 2PF6
+ 5C2H3N
formula weight
387.31
1713.07 (including sol-
vent)
P1
≈
space group
a [ä]12.787(3)
b [ä]4.4830(10)
c [ä]18.885(4)
a [8]90.00
P21/c
13.169(3)
13.686(3)
22.240(4)
87.57(3)
85.34(3)
72.79(3)
3815.5(13)
4
both in solution and in the solid state that the guest 2 lies
within the molecular cleft provided by the receptor 1, and is
probably stabilized by p p stacking interactions and by
weak metal metal interactions between the host and guest
metals.[10] Although the host guest complex is stable in solu-
tion in CH3CN, K=12000mꢀ1, rapid intermolecular host
guest exchange occurs.
Because the receptor 1 is positively charged, a neutral
guest in the cleft upon becoming positively charged, by
some method, will experience strong electrostatic repulsion
and consequently is expected to be repelled from the cleft
into the solvent. The simplest method of switching the guest
charge from neutral to positive is by using a neutral guest
that can be protonated. Analogous guests to 2 are the plati-
num complexes 3 5. The guests 3 and 4 are basic but 5 is
b [8]109.115(4)
g [8]90.00
V [ä3]1022.9(4)
Z
2
crystal size
[mm], color,
habit
0.68î0.60î0.80,
orange, needle
0.07î0.04î0.015, yellow,
diamond fragment
1calcd [gcmꢀ3]2.515
m [mmꢀ1]13.697
T [K]100(5)
1.491
0.663
100(5)
wavelength [ä]0.71073 (Mo
)
0.54993 (synchrotron ra-
diation)
5.52
Ka
R(F) [%][a]
3.54
9.14
R(wF2) [%][a]
14.50
[a]Quantity minimized =R(wF2)=ꢀ[w(Fo2ꢀFc2)2]/ꢀ[(wF2o)2]1/2
;
R=ꢀD/
ꢀ(Fo), D=j(FoꢀFc)j, w=1/[s2(Fo2)
(Fo, 0)]/3.
+ +
(aP)2 bP], P=[2F2c +Max
average deviation of any non-hydrogen atom from the mo-
lecular plane is 0.07 ä. The bond lengths and angles are un-
exceptional and are provided in the Supporting Information.
Suitable crystals of 1 for structure determination using
synchrotron radiation were grown from a solution in aceto-
nitrile at ꢀ208C after standing for two years in a screw-top
vial. The structure is shown in Figure 2 and the crystallo-
graphic data are provided in Table 1. It will be seen that the
terpy-Pd-Cl units of pairs of molecules interpenetrate. A
center of inversion lies between Pd(1A) and Pd(1B), and
the spacer units are in chiral conformations, the molecules
having opposite absolute configurations. The unit cell has
two molecules of 1 as shown, but ™vertical∫ stacking of
terpy-Pd-Cl units extends through the crystal. This stacking
is shown in Figure 3, in which the interplanar separations
are given and interplanar angles are shown in brackets. The
planes of the terpy-Pd-Cl units are essentially parallel and
the interplanar distances within the unit cell are similar to
those expected for p p stacking. The planar separation be-
tween unit cells (Pd(2A), Pd(2B)) is larger than that expect-
not. The 5-methyl groups were incorporated to hinder coor-
dination of the pyridine nitrogen atom to metals. In the
cases of 3 and 5, formylacetone rather than acetylacetone
was used to prepare the tridentate ligands to obtain planar
complexes. When acetylacetone is used, the inner methyl
group sterically interacts with the neighboring aromatic ring
to cause the coordinated ligand to twist.[11] Such twisting
would diminish the binding capacity of such guests in the
cleft.
Crystal structures of 1 and5 : That the guest 5 is planar was
established by its crystal structure (Figure 1, Table 1). The
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B. Bosnich et al.
FULL PAPER
ꢀ
Pd(2A) Pd(2B) 4.471, and
ꢀ
Pd(1A) Pd(2A) 6.765 ä. The
distance between Pd(1A) and
Pd(1B) is the only separation
that may indicate significant
[10]
ꢀ
Pd Pd interaction.
The interpenetrated structure
of 1 (Figure 2) must generate
strong electrostatic repulsions
between the closely spaced pos-
itively charged terpy-Pd-Cl
units. In the crystal, the nega-
Figure 2. Side view of the stacking that is present in the crystal of 1¥5CH3CN (ORTEP diagram; thermal ellip-
soids are shown at 50% probability). The solvent and counterions have been removed for clarity.
ꢀ
tively charged PF6 counterions
are found to cluster about the
terpy-Pd-Cl units to neutralize the charges. Charged poly-
pyridyl-Pd-X+ complexes are known to stack in the solid
state so that the interpenetrated structure found here is
analogous.[12]
In acetonitrile solutions, 1 is completely dissociated. Thus,
1
between 10ꢀ2 and 10ꢀ5 m the H NMR spectrum and the ab-
sorption spectrum in the ultraviolet and visible regions are
independent of concentration. Further, the conductivity of 1
in acetonitrile solutions is 235 Wꢀ1 cm2 molꢀ1 indicating that
at 10ꢀ3 m, 1 is a 2:1 electrolyte.[13] Thus, the receptor 1 is
fully dissociated with respect to itself and its counterions in
acetonitrile solutions. Consistently, neither the terpy-Pd-Cl+
nor the terpy-Pt-Cl+ complex associates with 1 in acetoni-
1
trile solutions as judged from the H NMR spectra and UV/
Vis spectra.
Synthesis: The preparation of 1 is given elsewhere.[9] The
preparation of 4 is outlined in Scheme 1. The nitro com-
pound 6 is cleanly reduced to the air-sensitive amine 7 by
the hydrazine/graphite[14] method and the Schiff×s base 8
forms readily with salicylaldehyde. The platinum compound
9 is prepared by a variation of the Pregosin method.[15] Re-
placement of the dimethyl sulfoxide (DMSO) ligand from 9
by ammonia gives the ammine complex 4 efficiently.
The condensation of 7 with 10 to give 11 proved trouble-
some [Eq. (1)]. After all of the obvious solution methods
were tried, simply heating a mixture of 7 and 10 provided
the desired product 11 in reasonable yield and purity. The
compound 11 is susceptible to ready hydrolysis. Reaction of
Figure 3. Representation of the stacking of the terpy-Pd-Cl units in the
crystal of 1¥5CH3CN. The interplanar distances are given and the inter-
planar angles are shown in parentheses.
ed for p p stacking. The terpy-
Pd-Cl units of each molecule 1
are twisted with respect to the
spacer, the planes are roughly
parallel (3.98) and are separated
ꢀ
by 6.714 ä. The Pd Pd distan-
ces depend on the interplanar
separations and on the amount
of displacement of these planes
with respect to each other.
Thus the interplanar separa-
tions do not necessarily reflect
the metal metal separations:
ꢀ
Pd(1A) Pd(1B)
3.364,
ꢀ
Pd(1B) Pd(2A)
3.453, Scheme 1. Synthesis of 4.
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Supramolecular Recognition
1944 1955
11 with [Pt(dmso)2Cl2]by the method shown in Scheme 1
gives the DMSO complex that is then converted to the
stable ammine complex 3. The preparation of 5 followed a
similar procedure except that, in this case, 10 reacted readily
with o-aminophenol.
Figure 4. Schematic representation of the important structural details
from the crystal structure of the host guest complex formed from 1
and 3.
Host guest formation: Solutions of the receptor 1 and of
the potential guests 3 5 in acetonitrile are light yellow.
Upon mixing of 1 with any one of these guests, a deep red
solution results indicating host guest formation. At 258C
the 1H NMR spectra of the three host guest complexes 1
and 3, 1 and 4, 1 and 5 are broad but become sharp at 708C.
The host guest association constants for 1 and 3 and for 1
and 4 were determined in CD3CN at 708C by the chemical
shift titration method.[16] Both systems form 1:1 host guest
association complexes. The equilibrium constant for the
host guest complex between 1 and 3 is K1,3 =21000ꢁ
Acid base reactions: For the study of the protonation of the
host guest complexes, trifluoroacetic acid (TFA) was used
because it was found that TFA readily protonated the free
guest 3 in acetonitrile solution and that excess of TFA did
not lead to decomposition of the guest nor the host over
several hours at 708C. The pKa values of the trifluoroacetic
acid and pyridine in dry CH3CN solution at 258C are report-
ed[18] to be 12.65 and 12.33, respectively. Presumably, the
free guests 3 and 4 in acetonitrile will have similar pKa
values to pyridine. The receptor 1, however, also has a pyri-
dine nitrogen atom, which could be protonated by TFA. The
analogous pyridine compound 12 is reported[19] to have a
pKa of 1.73 in 70% EtOH/H2O at 258C. The lower than ex-
4000mꢀ1 and for the complex between 1 and 4 is K1,4
=
37000ꢁ6000mꢀ1. ESI-MS (CH3CN) data for solutions con-
taining 1 and 3, 1 and 4, as well as 1 and 5 also show that
1:1 host guest complexes are formed and the mass spectra
show isotopic resolution (see Supporting Information). The
1H NMR spectra of the host guest complexes show large
chemical shifts for the Ha, Hb, and Hd protons (see 1) of the
receptor but almost no shift is observed for Hc. This indi-
cates[17] that the guests reside inside of the molecular cleft
and that they have essentially no residency time on the
™outside∫ terpy-Pd-Cl face as is observed in other host -
guest complexes.[9] This assertion is supported by the crystal
structure of the host guest complex between 1 and 3.
X-ray data were collected on the adduct formed by 1 and
3 which was isolated by vapor diffusion of methanol into a
host guest solution in methyl ethyl ketone. It was found
that the guest existed in two orientations in the ratio of
70:30 inside of the receptor cleft. Both orientations had the
ammine ligands oriented in a similar direction as the two
chloro groups of the receptor which has terpy-Pd-Cl units
twisted with respect to each other. The two orientations of
the guest are believed to arise from 1808 rotation about the
pected pKa value may be due to the restricted cavity in
which the nitrogen lone pair exists. A 10ꢀ3 m CD3CN solu-
tion of 1 at 258C gave 1H NMR spectra that were un-
changed for up to 100 equivalents of TFA, suggesting that
no protonation occurs. In addition to steric crowding, the
very low pKa of the receptor 1 in CD3CN solution is proba-
bly connected with the electrostatic repulsions that are gen-
erated between the dipositively charged receptor and a posi-
tively charged protonated nitrogen atom of the receptor.
When considering the degree of dissociation of the guests
3 and 4 from their host guest complexes in the presence of
acid, three equilibria require consideration: the host guest
association constant KHG, the dissociation constant of TFA,
Ka, and the proton dissociation constant of the free guest,
Kg. The constants are defined as shown in Scheme 2.
ꢀ
Pt NH3 bond. Because of the two orientations of the guest,
we were unable to determine accurately the coordinates of
all of the atoms of the guest molecule. The structure of the
host guest complex, as far as could be determined, is given
in the Supporting Information. Several important features
of the structure were determined with confidence, however.
The three planes provided by the guest and two terpy-Pd-Cl
units of the receptor are essentially parallel and the three
metal atoms Pd-Pt-Pd are essentially aligned. The salient
structural information is illustrated in Figure 4. The rather
Combining these three equilibrium equations, an analytic
expression for the relationship between the concentration of
the host guest complex with acid concentration can be de-
rived (see supporting information). Further, using the ex-
pression for Ka and Kg, the variation of a physical property,
such as optical density or chemical shift, with acid titration
ꢀ
short Pt Pd distances may indicate metal metal interac-
tion[10] and the interplanar separations are those expected
for p-stacked molecules.
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B. Bosnich et al.
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Figure 5. Absorption spectra for the titration of 3 with TFA. The experi-
ments were performed at 258C in dry CH3CN where the concentration of
3 was held constant at 0.477mm and the concentration of TFA was varied
between 0.927mm and 38.9mm.
Scheme 2. Definition of the dissociation constants.
of the base can be derived analytically (see Supporting In-
formation). In principle, the (real) solution of this equation
will give the values of Ka and Kg. Knowing KHG experimen-
tally, it is possible in principle to find the concentration of
all of the species in solution for a given acid concentration
from the derived equations. In the present case, the pKa
values for TFA and 3 are likely to be about 12, in acetoni-
trile solution at 258C, but the most dilute concentration of
guest that can be used for accurate physical measurements
is 10ꢀ4 m. Under these circumstances, it is not possible to
obtain accurate values of Ka and Kg from acid titration of
the guest.[20]
Given these restrictions to the formal method, a different
approach was used for assessing the amount of acid required
to fully protonate the guest and for complete acid-induced
dissociation of the guest from the host. The method involves
plotting the variation in a physical property versus acid con-
centration. At the point where no further variation in physi-
cal property with acid concentration is observed, it is as-
sumed that the guest is fully protonated or fully dissociated
from the host.
Figure 6. Plots of the absolute change in absorbance (jDAj) versus equiv-
alents TFA for the titration of 3 with TFA. The data is derived from
Figure 5.
Figure 5 shows the variation of the absorption spectrum
of 3 upon incremental additions of dry TFA in dry CH3CN
solution at 258C. As will be noted, the spectral variation in-
dicates that a clean protonation occurs. Figure 6 shows two
plots of the change in absorbance versus equivalents of TFA
derived from the data in Figure 5. Both plots indicate that
the guest is fully protonated after about 20 equivalents of
TFA are added. Figure 7 shows the variation in absorption
spectra of the host guest complex formed between 1 and 3
with added TFA. The absorption centered around 525 nm is
the ™charge transfer∫ absorption associated with host guest
formation. Upon successive additions of TFA to the host
guest complex, this band gradually decreases in intensity
and at sufficient TFA the charge transfer band is extinguish-
ed. For most of the TFA addition, an isosbestic point is ob-
Figure 7. Absorption spectra for 1 (A, 2.00mm), 3 (B, 2.00mm), a 1:1 sol-
ution of 1 and 3 (C, 2.00mm each), and the titration of the host guest
complex formed from 1 (2.00mm) and 3 (2.00mm) with TFA (varied be-
tween 4.62mm and 191mm). The experiments were performed at 258C in
dry CD3CN.
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Supramolecular Recognition
1944 1955
served. At nearly complete protonation of the guest (see
Figure 5) the protonated guest has absorption which in-
trudes into the isosbestic point region. Visually, the solution
of the host guest complex changes from a deep red color to
a light straw color as TFA is added. When triethylamine is
incrementally added to the yellow solution derived from
protonation of the guest in the host guest solution, the var-
iation of the spectrum in Figure 7 occurs in the reverse di-
rection. When the number of equivalents of triethylamine is
equal to the equivalents of TFA originally added, the host
guest spectrum is fully restored. Plots of the change in ab-
sorbance versus equivalents of TFA are shown in Figure 8
that expected for a 1:1 electrolyte in CH3CN solution (120
160 Wꢀ1 cm2 molꢀ1)[13] but the observed conductivity indi-
cates that protonated 3 and trifluoroacetate exist predomi-
nantly as separated ions in dilute CH3CN solution. This ob-
servation supports the assertion that the guest in the pres-
ence of TFA exists as a positively charged ion that is expect-
ed to be repelled by the dicationic host 1.
The guest 5 has no conventional basic elements and, con-
sistently, addition of TFA to a solution of 5 in CH3CN led to
very small changes in the absorbance at around 400 nm indi-
cating that some interaction between TFA and 5 may exist.
As expected, the host guest complex formed by 1 and 5 was
not significantly affected by the addition of 100 equivalents
of TFA as judged from absorption spectral changes.
The (basic) guest 4 forms a more stable host guest com-
plex with 1, than does 3 with 1. On this basis, alone, more
TFA would be required to fully remove the guest from the
host by protonation. More TFA would also be required if, in
addition, 4 is a weaker base than 3. This appears to be the
case as seen from TFA titration of 4, monitored by the ab-
sorption spectrum. The variation in the spectra with TFA
addition, however, did not provide stable isosbestic points,
suggesting that protonation generates more than one spe-
cies. The overall spectra of the protonated and unprotonated
guest 4, however, was similar to those found for the guest 3
(Figure 4). Plots of absorbance versus equivalents of TFA at
258C in dry CH3CN solution indicate that about 60 equiva-
lents of TFA were required to fully protonate 4, three times
the number of equivalents of TFA required to fully proto-
nate 3. Thus 4 appears to be a weaker base than 3, and 4
forms a more stable adduct with 1 than does 3. Consistently,
even after 200 equivalents of TFA were added to the host
guest complex, the guest 4 was not fully dissociated from
the host 1, as judged by spectrophotometry at 258C (Sup-
porting Information).
Figure 8. Plots of the absolute change in absorbance (jDAj) versus equiv-
alents TFA for the titration of the host guest complex formed from 1
and 3 with TFA. The data is derived from Figure 7.
where it can be seen that about 45 equivalents of TFA are
required to fully dissociate the guest 3. The greater number
of equivalents of TFA required to fully protonate the guest
3 in the presence of one equivalent of host 1 is due to com-
petition for the guest by the host and the acid as defined by
the equilibrium constants HHG, Ka, and Kg. Titration of the
guest 3 and the host guest complex between 1 and 3 with
TFA in dry CD3CN solution at 708C was also monitored by
1H NMR spectroscopy that showed shifts in the proton reso-
nance frequencies in both the host and the guest upon guest
protonation (see Supporting Information). Plots of chemical
shifts versus TFA equivalents revealed similar stoichiome-
tries as were observed from the UV data at 258C. The latter
are more accurate, however.
The changes in physical property when the guest is al-
lowed to react with TFA could arise from a guest TFA hy-
drogen bonded adduct or from the formation of a separated
protonated guest cation and trifluoroacetate anion. If the
latter is the case, conductivity measurements should detect
the formation of ions. The conductivity of the guest 3 alone
and of TFA alone in dry acetonitrile at 258C is essentially
zero. When dry TFA is added to a 10ꢀ3 m solution of 3 in dry
CH3CN solution, the molar conductivity increases until
about 20 equivalents of TFA are added, which is the same
saturation level found by spectrophotometry. At this con-
centration of TFA, the molar conductance was
110 Wꢀ1 cm2 molꢀ1. This conductance is somewhat lower than
Discussion
The proton-induced (protonmotive) dissociation of the guest
from the receptor can be described in several ways. It can
be referred to as an acid-induced shift in equilibrium, or as
a molecular switch or as a molecular motor. Each and all of
these descriptions are correct but each characterization elic-
its a different perspective on the process. The process descri-
bed here is in some ways different from a conventional shift
in equilibrium because the host guest complex is stabilized
by noncovalent forces and the guest is released from the
host by noncovalent electrostatic forces. Because the proto-
nmotive force causes the guest to move from the cleft of the
receptor to the solution and the process is accompanied by a
color change from red to yellow, the response of the system
can be called a simple molecular switch. Because acid (or
base) causes the translocation of the guest, the process can
also be regarded as a single step (stroke) of a molecular
motor. If the present system is described as one stroke of a
molecular motor, it is necessary to demonstrate that work is
done. This is shown by considering the mechanisms by
which molecular motors operate.
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B. Bosnich et al.
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Two distinct mechanisms are recognized that drive a mo-
lecular motor, the power stroke[21] and the Brownian ratch-
et.[22] Both mechanisms may operate in the same motor.
These two mechanisms are illustrated in Scheme 3 and
Scheme 4 for the present system. The power stroke is per-
haps the simplest process to comprehend because it bears
some relationship to the operation of macroscopic motors.
The Brownian ratchet mechanism, however, bears no mac-
roscopic motor analogy because it operates by the rectifica-
tion of stochastic Brownian motion, that is, thermal energy
is harnessed to do work.
leads to molecular motion and, consequently, work (force by
distance) is done on both the receptor and the guest.
The operation of the Brownian ratchet for the present
system is illustrated in Scheme 4. Brownian motion induces
(reversible) dissociation of the guest from the host guest
complex 13. The free guest is then protonated by the acid
and thus the guest is prevented from returning to the recep-
tor. An alternative possible path involves the formation of
the hydrogen-bonded host guest complex 14, followed by
the dissociation of the hydrogen bonded guest which, by dis-
sociation of trifluoroacetate, gives 1 and the protonated
guest. As for the case of the power stroke, work is done in
the Brownian ratchet mechanism because the protonated
guest is unable to return to the receptor.
The experiments described here do not speak in support
of one or other of these processes. It is probable, however,
that both mechanisms participate for the present system.
The proton induced translocation of the macrocycle in the
two-site rotaxane[6] can be described by the mechanisms pre-
sented here. The acid-induced removal of dimethylanilines
from the cationic platinum-based cage,[7] however, most
probably involves the Brownian ratchet mechanism because
strong acid in water was used. It is unlikely that protons
would enter the positively charged cage to protonate an in-
carcerated guest.
Scheme 3 outlines the potential operation of the power
stroke in the present system. The host guest complex, 13, is
stabilized by noncovalent interactions. Because TFA is es-
sentially undissociated in acetonitrile solution and because
If a molecular switch operates by translocation of a mole-
cule, as is the case for the present protonmotive removal of
the guest from the receptor, then one stroke of a switch is
identical to one stroke of a motor. Consequently, whatever
mechanism or description is ascribed to one can be ascribed
to the other. Both require work to be performed on mole-
cules.
One crucial aspect of molecular motors is that they are
driven by a repetitive succession of molecular events, each
of which does work and, in concert, leads to motion of mo-
lecular assemblies. For example, the proton driven motor,
FoF1ATPase, has a rotor that has 9 14 protein copies[23] each
of which is involved in a concerted unidirectional rotation of
the rotor and its attached drive shaft.[24] Thus, if concerted
Scheme 3. Power stroke mechanism for driving a molecular motor.
the receptor is a dication, it is
unlikely that a solvated proton
will exist to combine with the
basic guest in the molecular
cleft. It is more probable that
TFA will form the neutral hy-
drogen bonded species, 14,
from which, upon release of the
trifluoroacetate anion, the un-
stable host guest complex 15 is
generated. Electrostatic repul-
sion between the host and the
protonated guest will lead to
dissociation resulting in the for-
mation of the free receptor 1
and the free protonated guest.
The electrostatic force that op-
erates on both the dication re-
ceptor and the protonated guest Scheme 4. Brownian ratchet mechanism for driving a molecular motor.
1950
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Supramolecular Recognition
1944 1955
was flushed with argon and hydrazine monohydrate (1.6 g, 1.6 mL,
32.44 mmol) was added. Upon addition of the hydrazine, the color of the
exothermic reaction solution changed to bright yellow and the viscosity
of the suspension increased. The suspension was stirred for 5 min where-
after the suspension was refluxed for 1.75 h during which time the color
of the solution changed from bright yellow to nearly colorless. At the
end of this period, the reaction mixture was cooled to room temperature
and was filtered through celite. The solvent and any remaining hydrazine
were removed under reduced pressure. The crude product was then dis-
solved in THF and was chromatographed on basic alumina (25 g) using
THF as the eluent. The eluent solvent was removed to yield air-sensitive,
white crystals of 7 (1.69 g, 84.1%) that were stored under nitrogen. To
prevent excessive decomposition of the aminopyridine, it was used imme-
diately to generate the desired imine.
repetitive action is part of the definition of a molecular
motor, the present and previously published chemically
driven systems[5,25] are not motors. They are better described
as molecular switches. But as noted here, the mechanisms
for the operation of a molecular switch when molecular
translocation is involved and of one stroke of a molecular
motor can be the same. The difference is largely related to
the function that the molecular device performs. Devising
molecular systems that can drive a molecular assembly by
concerted, repetitive strokes is a considerable challenge[26]
but is a prerequisite to the development of molecular ma-
chines.
2-{[(2-Hydroxyphenyl)methyl]imino}-6-methylpyridin-3-ol (8): The ami-
nopyridine 7 (1.69 g, 13.6 mmol) was dissolved in anhydrous ethanol
(50 mL). Salicylaldehyde (1.66 g, 13.6 mmol) was added and the suspen-
sion was refluxed for 1 h. During this time, the color changed to red-
orange and nearly all of the starting materials dissolved. The solution
was allowed to cool to room temperature and the solvent was removed
under reduced pressure to give a red solid. This solid was dissolved again
in absolute ethanol (25 mL) and the solvent was evaporated again. This
procedure was repeated twice. The solid was dissolved into boiling meth-
anol (150 mL) and the product was crystallized by cooling to ꢀ258C. The
product deposited as red plates which were collected by filtration and
were washed with cold methanol. A second crop of the product could be
obtained from the filtrate by similar treatment. Yield: 2.0 g, 64%. 1H
NMR (CD2Cl2, 208C, 500 MHz): d=2.50 (s, 3H), 6.91 6.95 (m, 2H), 7.06
(d, J=8.14 Hz, 1H), 7.25 (d, J=6.92 Hz, 1H), 7.45 (dd, J1 =8.10, J2 =
1.61 Hz, 1H), 7.74 (d, J=6.59 Hz, 2H), 9.38 (s, 1H), 12.4 ppm (br. s.,
1H); 13C NMR ([D6]DMSO, 278C, 125 MHz): d=23.07, 116.87, 118.93,
119.26, 123.60, 125.23, 133.17, 133.49, 143.92, 145.42, 147.00, 161.07,
161.43 ppm; ESI-MS (CD2Cl2): m/z: 227 [Mꢀ1], 228 [M]; elemental anal-
ysis calcd (%) for C13H12N2O2: C 68.41, H 5.30, N 12.27; found: C 68.30,
H 5.33, N 12.13.
Conclusion
It has been shown that a dicationic receptor incorporating a
neutral basic guest will release the guest upon guest proto-
nation. Electrostatic repulsion is believed to cause host
guest dissociation, a process that is accompanied by a color
change from deep red to yellow. It is suggested that the
proton-driven dissociation of the guest can be described as a
molecular switch or one stroke of a molecular motor. Both
the switch and motor operate by similar mechanisms,
namely, an electrostatically driven power stroke and a Brow-
nian ratchet. Because molecular motors are usually defined
as systems that operate by repetitive cycles, it is concluded
that the present system is better described as a molecular
switch.
[Pt(8)DMSO] (9):
A 50-mL flask was charged with [Pt(dmso)2Cl2]
(0.925 g, 2.19 mmol) dissolved in DMSO (20 mL). K2CO3 (0.715 g,
5.17 mmol) and 8 (0.5 g, 2.19 mmol) were added and the mixture was
heated to 1408C. The reaction mixture was stirred at 1408C for 15 min
and the color changed to red-brown. The reaction mixture was then al-
lowed to cool to 908C and distilled water (25 mL) was slowly added, pre-
cipitating the product. The mixture was stirred for 15 min and was then
poured into distilled water (100 mL). The reaction mixture was stirred
until it had cooled to room temperature. The crude product was isolated
by filtration and was washed well with distilled water (2î25 mL). The
impure yellow solid was dried, and then was dissolved in hot ethyl ace-
tate (500 mL) and was passed through a filter frit. This solution was then
chromatographed on basic alumina (20 g) using ethyl acetate as the
eluent. The product is the only material that passes through this column.
The yellow solution was concentrated to 75 mL under reduced pressure
whereupon the product deposited as yellow needles. After the mixture
had been cooled to ꢀ208C, the crystals were collected by filtration. The
yellow needles of the product were washed with pentane (2î10 mL).
Yield: 0.61 g, 56%. 1H NMR ([D6]DMSO, 278C, 500 MHz): d=2.48 (s,
3H), 2.54 (s, 6H), 6.87 (t, J=6.9 Hz, 1H), 7.00 (d, J=8.3 Hz, 1H), 7.25
(d, J=8.5 Hz, 1H), 7.34 (d, J=8.3 Hz, 1H), 7.54 (td, J1 =6.8, J2 =1.6 Hz,
1H), 8.03 (dd, J1 =8.0, J2 =1.4 Hz, 1H), 9.46 ppm (s, 1H); 3J(195Pt, H-
9.46)=29.7 Hz; 13C NMR ([D6]DMSO, 278C, 125 MHz): d=22.72, 40.41,
117.10, 120.44, 121.26, 123.35, 125.29, 134.49, 135.55, 144.02, 146.46,
149.00, 156.93, 162.25 ppm; ESI-MS (CD3CN): m/z: 499 [M], 500 [M+1];
elemental analysis calcd (%) for C15H16N2O3PtS: C 36.07, H 3.23, N 5.61;
found: C 36.22, H 3.18, N 5.54.
Experimental Section
General procedures: All reagents were obtained from commercial suppli-
ers and were used without further purification. All reactions were per-
formed under an atmosphere of argon, unless otherwise specified. H and
1
13C NMR spectra were recorded using a Bruker DRX500 or a Bruker
DMX500 Fourier transform spectrometer. Proton and carbon chemical
shifts, d, are reported in ppm, and referenced to tetramethylsilane
(TMS). Coupling constants, J, are reported in hertz. Electronic absorp-
tion spectra were obtained using a Perkin Elmer Lambda 6 UV/Vis spec-
trophotometer. For a given wavelength and acid concentration, the abso-
lute change in absorbance, (jDAj), was calculated by subtracting the acid
dependent absorbance from the absorbance in the absence of acid, (j
DAjo). The absolute change in chemical shift, (jDAj0) is calculated simi-
larly. Elemental analyses were performed by Desert Analytics, Tucson,
Arizona. Conductance measurements were performed at 238C with 1î
10ꢀ3 m samples using an YSI Scientific model 35 conductance meter.
Melting points are uncorrected. Acetonitrile was dried over CaH2, tetra-
hydrofuran (THF) was dried over potassium/benzophenone ketyl, diethyl
ether was dried over sodium/benzophenone ketyl, and dichloromethane
was dried over CaH2. Dry trifluoroacetic acid (TFA) was obtained by dis-
tillation from a solution containing 5% trifluoroacetic anhydride. Thin
layer chromatography was carried out using precoated silica gel (What-
man PE SIL G/UV) or precoated aluminum oxide (J. T. Baker, alumi-
num oxide IB-F). Silica gel 60 ä (Merck, 230 400 mesh) and aluminum
oxide 58 ä (either activated, basic, Brockman I or activated, neutral,
Brockman I) were used for chromatography as indicated. Celite is J.T.
Baker Celite 503. [Pt(dmso)2Cl2]was prepared by the literature
method.[27] The preparation of the receptor complex 1 has been previous-
ly described.[9]
[Pt(8)NH3] (4): A dry 50-mL flask was charged with [Pt(8)DMSO]
(0.200 g, 0.400 mmol) dissolved into freshly distilled CH3CN (20 mL).
Ammonia, as a 7m solution in methanol, (6.60 mL, 46.2 mmol) was
added to the yellow suspension of the DMSO complex. The reaction mix-
ture was stirred at room temperature for 120 h, during which time the
color of the yellow suspension changed to orange. Periodically, more am-
monia was added to the reaction mixture. After 120 h the crystals were
dissolved by increasing the volume of acetonitrile to 60 mL and heating.
As this orange solution cooled, the product deposited as orange micro-
2-Amino-6-methylpyridin-3-ol (7): A 100-mL flask was charged with the
6 (2.5 g, 16.22 mmol) dissolved in anhydrous ethanol
(27 mL). Graphite (4.9 g) was added to this yellow solution. The flask
nitropyridine
1951
Chem. Eur. J. 2004, 10, 1944 1955
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

B. Bosnich et al.
FULL PAPER
crystals. The volume of acetonitrile was reduced to approximately 7 mL
and the resulting suspension was cooled to ꢀ208C. The orange crystals of
the product were collected by filtration and were washed with cold aceto-
nitrile (1î3 mL), diethyl ether (1î10 mL), and pentane (1î10 mL).
Yield: 0.145 g, 82.4%. 1H NMR ([D7]DMF, 278C, 500 MHz): d=2.54 (s,
3H), 4.96 (br. s, 3H), 6.75 (t, J=7.7, 1H), 6.92 (d, J=8.3, 1H), 7.05 (d,
J=8.5, 1H), 7.25 (d, J=8.2, 1H), 7.50 (td, J1 =8.4, J2 =1.4, 1H), 7.94 (d,
J1 =7.9, 1H), 9.57 ppm (s, 1H); 3J (195Pt, H-9.57)=34.1 Hz; 13C NMR
([D7]DMF, 278C, 125 MHz): d=23.09, 116.85, 122.26, 122.99, 123.23,
125.23, 133.03, 135.19, 142.47, 143.23, 152.13, 160.05, 163.35 ppm.
LM(CH3CN)=1.05 Wꢀ1 cm2 molꢀ1; ESI-MS (CD3CN): m/z: 438 [M], 439
[M+1]; elemental analysis calcd (%) for C13H13N3O2Pt: C 35.62, H 2.99,
N 9.59; found: C 35.95, H 2.98, N 9.57.
analysis calcd (%) for C10H13N3O2Pt: C 29.85, H 3.26, N 10.44; found: C
29.85, H 3.40, N 10.47.
[(2-Hydroxyphenyl)amino]but-3-en-2-one (17): A 50-mL flask was flame-
dried and was flushed with argon. The flask was charged with the 4-me-
thoxy-3-buten-2-one (0.5 g, 4.99 mmol) dissolved in THF (10 mL). This
yellow solution was cooled to ꢀ108C and o-aminophenol (0.545 g,
4.99 mmol) dissolved in THF (7 mL) was added. The reaction mixture
was stirred at ꢀ108C for 2.5 h and was then allowed to warm to room
temperature. The reaction mixture was stirred at room temperature for
2 h. The solvent was removed under reduced pressure to yield a yellow
solid. This solid was recrystallized from a minimum amount of hot THF
by cooling to ꢀ208C. The crystals were collected by filtration and were
washed with ethyl acetate (5 mL) and hexanes (5 mL). This yielded a
yellow powder of sufficient purity (~97%) for the next step. Yield:
0.445 g, 50.3%. 1H NMR ([D6]DMSO, 168C, 500 MHz): d=2.03 (s, 3H),
5.30 (d, J=7.70 Hz, 1H), 6.77 6.84 (m, 4H), 7.31 (d, J=7.75 Hz, 1H),
7.58 (dd, J1 =12.8, J2 =7.60 Hz, 1H), 10.02 (br. s, 1H), 11.53 (d, J=
12.8 Hz, 1H); 13C NMR ([D6]DMSO, 278C, 125 MHz): d=29.75, 97.59,
113.85, 115.81, 120.26, 123.43, 129.07, 142.77, 145.97, 197.53 ppm; ESI-MS
(CD2Cl2): m/z: 177 [M], 178 [M+1].
4-[3-Hydroxy-6-methylpyridin-2-yl)amino]but-3-en-2-one (11): A 200-mL
flask was charged with 7 (2.01 g, 16.2 mmol) and 4-methoxy-3-buten-2-
one (1.62 g, 16.2 mmol). The neat mixture was heated at 1208C for 2 h.
The flask was flushed with argon periodically. At the end of this period,
any methanol that had been generated was removed under reduced pres-
sure, yielding a black, resinous material. This was dissolved in THF
(10 mL) and was filtered through silica (35 g). The solvent was removed
and the resulting brown solid was slurried in diethyl ether (10 mL). The
solid was recovered by filtration and was washed with ether (10 mL),
ethyl acetate (5 mL), and pentane (10 mL). This yielded the product as a
tan solid of sufficient purity (~95%) for use in the next step. Yield:
1.28 g, 41.0%. This product is air stable, but is susceptible to hydrolysis
in wet solvents. 1H NMR ([D6]DMSO, 278C, 500 MHz): d=2.10 (s, 3H),
2.32 (s, 3H), 5.40 (d, J1 =8.0 Hz, 1H), 6.64 (d, J1 =7.9 Hz, 1H), 7.04 (d,
J1 =7.9 Hz, 1H), 7.93 (dd, J1 =8.0, J2 =12.0 Hz, 1H), 9.91 (br. s, 1H)
11.62 ppm (d, J1 =12.0 Hz, 1H); 13C NMR ([D6]DMSO, 278C, 125 MHz):
d=22.64, 29.10, 98.01, 116.77, 121.74, 138.13, 139.20, 140.35, 145.79,
197.90 ppm; ESI-MS (CH2Cl2): m/z: 191 [Mꢀ1], 192 [M].
[Pt(17)DMSO] (18): A 100-mL flask was charged with [Pt(dmso)2Cl2]
(0.477 g, 1.13 mmol) dissolved into DMSO (8 mL). K2CO3 (0.477 g,
1.13 mmol) and 17 (0.2 g, 1.13 mmol) were added and the slurry was
heated to 1408C. The slurry was stirred at 1408C for 15 min as the color
changed to yellow-brown. The reaction mixture was then allowed to cool
to 908C and distilled water (23 mL) was slowly added, precipitating the
product. This suspension was stirred for 15 min and was then poured into
distilled water (80 mL). The mixture was stirred until it had cooled to
room temperature. The crude product was isolated by filtration and was
washed well with distilled water (2î25 mL). The yellow solid was dried
and was then dissolved into hot ethyl acetate and passed through a filter
frit. This solution was then chromatographed on basic alumina (8 g)
using ethyl acetate as the eluent. The product is the only material that
passes through this column. The yellow eluent solution was concentrated
to a small volume under reduced pressure. The product deposited as
yellow needles on the sides of the flask. The product was completely pre-
cipitated by addition of ether. It was collected by filtration (0.435 g,
[Pt(11)DMSO] (16):
A
dry 200-mL flask was charged with
[Pt(dmso)2Cl2](1.10 g, 2.6 mmol) dissolved in anhydrous DMSO
(15 mL). Compound 11 (0.500 g, 2.6 mmol) and K2CO3 (0.850 g,
6.14 mmol) were added as solids to the yellow solution. The resulting
slurry was heated to 1408C over 15 min and was maintained at that tem-
perature for a further 15 min, during which time the color changed to
dark green. The solution was allowed to cool to room temperature and
water (180 mL) was added to precipitate a yellow/green powder, which
was isolated by filtration and was washed with water (3î20 mL). The dry
solid was dissolved into hot ethyl acetate and was passed through basic
alumina (20 g). The eluted solvent was removed under reduced pressure
yielding yellow needles, which were recrystallized from hot hexanes. The
solid (0.85 g, 70.3%) was collected by filtration was washed with cold
hexanes and vacuum dried. 1H NMR ([D6]acetone, 278C, 500 MHz): d=
2.15 (s, 3H), 2.39 (s, 3H), 3.44 (s, 6H), 5.77 (d, J1 =6.7 Hz, 1H), 6.72 (d,
J1 =8.2 Hz, 1H), 7.13 (d, J1 =8.2 Hz, 1H), 8.64 ppm (d, J1 =6.7 Hz, 1H);
3J(195Pt, H-8.64)=23.3 Hz; 13C NMR ([D6]acetone, 278C, 125 MHz): d=
23.11, 25.99, 42.11, 100.00, 120.97, 124.48, 141.22, 144.28, 151.31, 156.41,
178.13 ppm; ESI-MS (CH3CN): m/z: 463 [M], 464 [M+1]; elemental
analysis calcd (%) for C12H16N2O3PtS: C 31.10, H 3.48, N 6.05; found: C
31.14, H 3.44, N 5.95.
1
86.0%). H NMR ([D6]acetone, 278C, 500 MHz): d=2.12 (s, 3H), 3.30 (s,
6H), 5.65 (d, J=6.6 Hz, 1H), 6.57 6.60 (m, 1H), 6.87 6.93 (m, 2H), 7.70
(d, J=7.9 Hz, 1H), 8.18 ppm (d, J=6.6 Hz, 1H); 3J(195Pt, H-8.18)=
25.4 Hz; 13C NMR ([D6]acetone, 278C, 125 MHz): d=25.77, 42.23, 99.46,
114.92, 116.58, 117.97, 126.83, 140.40, 141.78, 166.12, 175.55 ppm; ESI-MS
(CD3CN): m/z: 448 [M], 449 [M+1]; elemental analysis calcd (%) for
C12H15NO3PtS: C 32.14, H 3.37, N 3.12; found: C 32.41, H 3.44, N 3.04.
[Pt(17)NH3] (5):
A dry 50-mL sealable flask was charged with
[Pt(17)DMSO](0.150 g, 0.335 mmol) dissolved into freshly distilled
CH3CN (10 mL). Ammonia, as a 7m solution in methanol, (1.5 mL,
10.5 mmol) was added to the yellow suspension of the DMSO complex.
The flask was sealed and the temperature was elevated to 608C. The re-
action mixture was stirred at this temperature for 24 h. The volume of
acetonitrile was then reduced to 5 mL and the product was crystallized
from the solution by vapor diffusion with ether. The orange crystals of
the product were collected by filtration and were washed with diethyl
ether (5 mL) and pentane (1î10 mL). Yield: 0.090 g, 69%. 1H NMR
([D6]acetone, 278C, 500 MHz): d=1.86 (s, 3H), 4.30 (br. s, 3H), 5.42 (d,
J=6.3 Hz, 1H), 6.47 6.50 (m, 1H), 6.79 6.85 (m, 2H), 7.64 (d, J=8.1 Hz,
1H), 8.09 ppm (d, J=6.3 Hz, 1H); 3J(195Pt, H-8.09)=30.6 Hz; 13C NMR
([D6]acetone, 278C, 125 MHz): d=25.37, 99.01, 114.85, 117.24, 125.82,
[Pt(11)NH3] (3):
A dry 50-mL sealable flask was charged with
[Pt(11)DMSO](0.150 g, 0.32 mmol) dissolved in freshly distilled CH 3CN
(20 mL). Ammonia, as a 7m solution in methanol, (2 mL, 14.0 mmol) was
added to the yellow solution of the DMSO complex. The reaction flask
was sealed and the reaction mixture was heated to 608C using an oil
bath. The reaction mixture was stirred at this temperature for 24 h,
during which time the color changed to light orange. At the end of this
period, the reaction mixture was allowed to cool and was then transfer-
red to a 100-mL flask. The solvent was removed under reduced pressure
yielding a yellow powder. The compound was recrystallized by vapor dif-
fusion of diethyl ether into a CH3CN solution of the product. The yellow
needles (0.121 g, 92.0%) were collected by filtration and were washed
with cold CH3CN (3 mL), diethyl ether (5 mL), and pentane (5 mL). 1H
NMR ([D6]acetone, 278C, 500 MHz): d=1.86 (s, 3H), 2.35 (s, 3H), 4.31
(br. s, 3H), 5.52 (d, J=6.5 Hz, 1H), 6.62 (d, J=8.1 Hz, 1H), 7.01 (d, J=
8.1 Hz, 1H), 8.58 ppm (d, J=6.5 Hz, 1H); 3J(195Pt, H-8.58)=32.5 Hz; 13C
NMR ([D6]acetone, 278C, 125 MHz): d=23.12, 25.76, 99.74, 120.12,
123.63, 137.53, 142.50, 153.65, 158.11, 174.27 ppm; LM (CH3CN)=1.36
137.47, 141.54, 167.82, 171.57 ppm; LM(CH3CN)=1.12 Wꢀ1 cm2 molꢀ1
;
ESI-MS (CD3CN): m/z: 387 [M], 388 [M+1]; elemental analysis calcd
(%) for C10H12N2O2PtS: C 31.01, H 3.12, N 7.23; found: C 31.32, H 2.98,
N 7.27.
Host guest interaction of 1 with 3: A series of 2.00 mm solutions of 1 in
dry CD3CN containing varying amounts of 3, ranging from 0.46 mm to
10.96 mm, were prepared and were examined by 1H NMR spectroscopy
(708C). The host in CD3CN solution is yellow, as is the guest. The host
guest mixtures vary in color from pale orange to deep red. The mole
ratio method[16] was applied, and the stoichiometry of the association was
found to be 1:1 (supporting information). The maximum chemical shift
change for any of the protons of the host was Dd=0.6 ppm (Hb). Several
other protons of the host had maximum chemical shift changes in the
W
ꢀ1 cm2 molꢀ1; ESI-MS (CH3CN): m/z: 402 [M], 403 [M+1]; elemental
1952
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Chem. Eur. J. 2004, 10, 1944 1955

Supramolecular Recognition
1944 1955
range, Dd=0.25 to 0.6 ppm. The maximum chemical shift change ob-
served for the guest was Dd=1.07 ppm, several other protons had chemi-
cal shifts changes of about Dd=0.51 to 1.02 ppm. The data were analyzed
by using a curve fitting procedure[9] and K1,3 was found to be 21000ꢁ
4000mꢀ1; ESI-MS (CD3CN): m/z: 609.5 [Hꢀ2PF6]2+, 810.0 [HGꢀ2PF6]2+
(see Supporting Information).
mined as P21/c based on systematic absences and intensity statistics. Pat-
terson methods were used to locate the Pt atom. Repeated difference
Fourier maps allowed recognition of all expected C, O, and N atoms. Fol-
lowing anisotropic refinement of all atoms, ideal H atom positions were
calculated. No anomalous bond lengths or thermal parameters were
noted.
Crystallographic structural determination for 1: X-ray quality crystals of
the complex 1 were grown by slow evaporation of a ꢀ208C acetonitrile
solution of the complex over two years. The yellow diamonds that result
are very fragile and they effloresce, and were stored in the mother liquor.
The small crystal size, weak diffractions, and ready loss of solvent neces-
sitated data collection using synchrotron radiation. Data collection: A
fragment of a yellow, diamond shaped crystal (0.07î0.04î0.015 mm) was
selected under a polarizing microscope while immersed in Fluorolube oil
to avoid possible loss of solvents of crystallization. The crystal was re-
moved from the oil using a tapered glass fiber that also served to hold
the crystal for data collection. The crystal was mounted and centered on
a goniometer with Kappa geometry at ChemMatCARS Sector 15 beam-
line at the Advanced Photon Source at Argonne National Laboratory.
The sample was cooled to 100 K. Still images showed the diffractions to
be sharp. Data collection consisted of 1220 frames separated by 0.3
degree rotations in phi with the detector to sample distance set to 4 cm.
Integration time for each frame was 2 s. The unit cell was obtained by
using SMART and integration of intensities and refinement of cell pa-
rameters were done using SAINT.[28] Absorption corrections were applied
using SADABS[28] based on redundant diffractions. Structure solution
Host guest interaction of 1 with 4: The host guest association constant
for 1 and 4 in CD3CN solution at 708C was determined by the 1H NMR
titration method described for the association of 1 and 3. A series of
2.00 mm solutions of 1 in dry CD3CN containing varying amounts of 4,
ranging from 0.53 mm to 13.9 mm, were used. The data were analyzed
using a curve fitting procedure[9] and K1,4 was found to be 37000ꢁ
6000mꢀ1; ESI-MS (CD3CN): m/z: 608.6 [Hꢀ2PF6]2+, 828.0 [HGꢀ2PF6]2+
(see Supporting Information).
Titration of 3 with trifluoroacetic acid(TFA) : A series of 0.477 mm solu-
tions of 3 in dry CD3CN containing varying amounts of dry TFA, ranging
from 0.927 mm to 38.9 mm, were prepared and were examined by absorp-
tion spectrometry (258C). The guest 3 in CD3CN solution is yellow, and
TFA is colorless. The acid base mixtures vary in color from yellow to
orange (Figure 5). The protonation of
3 was also studied by using
1H NMR spectroscopy (708C). A series of 2.15 mm solutions of 3 in dry
CD3CN containing varying amounts of dry TFA, ranging from 1.30 mm to
198 mm, were prepared. The data was consistent with the results obtained
by UV/Vis spectroscopy (see Supporting Information).
Titration of the host guest complex derived from 1 and 3 with trifluoro-
acetic acid: A series of solutions containing 2.00mm of 1 and 2.00 mm of
3 in dry CH3CN containing varying amounts of TFA, ranging from
4.62mm to 191mm, were prepared. These solutions were examined by ab-
sorption spectrometry at 258C giving the results shown in Figure 7. The
protonation of the host guest complex, formed by 2.00mm each of 1 and
≈
and refinement: The space group was determined as P1 based on system-
atic absences and intensity statistics. Patterson methods were used to
locate the Pd atoms as well as P and Cl atoms. Repeated difference Four-
ier maps allowed recognition of all expected C, N, and F atoms. Five ace-
tonitrile molecules were also located in the asymmetric unit. Hydrogen
atom positions were calculated. Final refinement was anisotropic for all
non-hydrogen atoms and isotropic for H atoms. No anomalous bond
lengths were noted although a few displacement ellipsoids were elongat-
ed, possibly due to disorder in tert-butyl groups.
1
3 was also monitored by H NMR spectroscopy at 708C in CD3CN in the
presence of TFA ranging from 0.64 mm to 199mm. Plots of the change in
chemical shift (Dd) verses equivalents of TFA, obtained from the 1H
NMR data (708C) were again consistent with the results obtained by
UV/Vis spectroscopy (see Supporting Information).
Crystallographic structural determination for the host guest complex
formedfrom 1 and3 : Crystals of the complex were grown by dissolving a
1:1 mixture of the host 1 and guest 3 in methyl ethyl ketone and vapor
diffusing with methanol for seven days, at ꢀ48C. Data collection: A red
diamond (0.40î0.40î0.45 mm) was selected under a stereo-microscope
while immersed in Fluorolube oil to avoid possible loss of solvent. The
crystal was removed from the oil using a tapered glass fiber that also
served to hold the crystal for data collection. The crystal was mounted
and centered on a Bruker SMART APEX system at 100 K. Rotation and
still images showed the diffractions to be sharp. Frames separated in re-
ciprocal space were obtained and provided an orientation matrix and ini-
tial cell parameters. Final cell parameters were obtained from the full
data set.
A full sphere data set was obtained using 0.3o steps in w using 30 s inte-
gration times for each frame. Data collection was made at 100 K. Integra-
tion of intensities and refinement of cell parameters were done using
SAINT[28]. Absorption corrections were applied by using SADABS[28]
based on redundant diffractions.
Titration of 4 with trifluoroacetic acid: A series of 0.456 mm solutions of
4 in dry CD3CN containing varying amounts of TFA, ranging from
0.608 mm to 77.9 mm, were prepared and were examined by absorption
spectrometry. The change in absorbance (|DA|) was plotted against the
equivalents of TFA (supporting information). This plot indicated that
60 equivalents of TFA are required to fully protonate 4. The protonation
1
of 4 was also studied by using H NMR spectroscopy (708C). A series of
2.15 mm solutions of 4 in dry CD3CN containing varying amounts of
TFA, ranging from 1.30mm to 260 mm, were prepared and examined by
1H NMR spectroscopy. The data were consistent with the results ob-
tained by UV/Vis spectroscopy (see Supporting Information).
Titration of the host guest complex derived from 1 and 4 with trifluoro-
acetic acid: These experiments were carried out in analogous way to that
described for TFA addition to the host guest complex formed by 1 and
3. The degree of dissociation was monitored by UV/Vis spectroscopy at
258C and by 1H NMR spectroscopy at 708C in CD3CN solutions (see
Supporting Information).
Crystallographic structural determination for 5: X-ray quality crystals of
5 were grown by cooling a hot saturated solution of the complex in aceto-
nitrile. Data collection: A needle-shaped crystal (0.68î0.08î0.06 mm)
was selected under a stereo-microscope while immersed in Fluorolube
oil. The crystal was removed from the oil using a tapered glass fiber that
also served to hold the crystal for data collection. The crystal was mount-
ed and centered on a Bruker SMART APEX system at 100 K. Rotation
and still images showed many of the diffractions to be sharp, however, a
few diffractions appeared doubled or amorphous on some frames.
Frames separated in reciprocal space were obtained and provided an ori-
entation matrix and initial cell parameters. Final cell parameters were ob-
tained from the full data set. A ™hemisphere∫ data set was obtained
which samples approximately 1.2 hemispheres of reciprocal space to a
resolution of 0.84 ä using 0.3o steps in w using 20 s integration times for
each frame. Data collection was made at 100 K. Integration of intensities
and refinement of cell parameters were done using SAINT.[28] Absorption
corrections were applied using SADABS[28] based on redundant diffrac-
tions. Structure solution and refinement: The space group was deter-
Structure solution andrefinement : The space group was determined as
P1 based on systematic absences and intensity statistics. Patterson meth-
≈
ods were used to locate the Pt atom, the Pd atoms, and the P atoms. Re-
peated difference Fourier maps allowed recognition of all expected C, N,
and Cl atoms located on the host (C(1) C(65), N(1) N(7), and Cl(1) and
Cl(2)). The atoms on the host molecule were located with ease and fol-
lowing anisotropic refinement of these atoms, the ideal H atom positions
were calculated. The bond distances and angles of the atoms associated
with the host are unremarkable and no unusual thermal ellipsoids were
noted. Refinement of the occupancy of the P atoms located by Patterson
methods revealed one of the P atoms to be of single occupancy and two
to be of one-half occupancy. Repeated difference Fourier maps allowed
recognition of the F atoms associated with these P atoms. The PF6 ion of
100% occupancy was refined aniostropically. The F atom octahedra of
the two one-half occupancy PF6 ions share a face and the P and F atoms
of these one-half occupancy ions were refined isotropically. The guest is
disordered over two positions in approximately a 7:3 ratio determined by
examination of C atom occupancy. The Pt atom and immediate coordina-
1953
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¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim

B. Bosnich et al.
FULL PAPER
tion sphere (N(8), N(9), O(1), and O(2)) of the two orientations of the
guest appear to be superimposed. That is, the two orientations of the
guest appear to be related by rotation about the line defined by N(8)-
Pt(1)-N(9). The C and (pyridine) N atoms of the two orientations of the
guest were located with difficulty by comparison of a Fourier contour
map of the electron density of this plane with the positions that would be
predicted from the crystal structure of 5. The occupancy of these C and
N atoms has been refined. The C and (pyridine) N atoms associated with
the 70% occupancy isomer are C(66) C(75) and N(10) and those associ-
ated with the 30% occupancy isomer are C(76) C(83) and N(11). The Pt
atom and the atoms of the immediate coordination sphere (N(8), N(9),
O(1), and O(2)) were refined anisotropically. The carbon and nitrogen
atoms of the remaining portion of the ligand were refined isotropically
and the H atoms associated with the ligand of the guest were not calcu-
lated. The bond distances and angles between Pt(1) and (N(8), N(9),
O(1), and O(2) are near expected values. Because of the two-site disor-
der and near superpositioning of many of the remaining atoms of the
guest, the bond distances and angles of the remaining portion of the
guest are distorted from expected values. Two methanol molecules were
located in the structure, one of 70% occupancy, the other 100% occu-
pancy. One 70% occupancy acetonitrile molecule was also located. These
solvent molecules were refined isotropically and the H atom positions
were not calculated. In addition to these well-defined solvent molecules
two regions of unidentifiable but structured electron density were located
in the E-map. These were assigned as C atoms (C(94) C(98) and C(88)
C(93) and C(99)) and the occupancy refined. The locations and occupan-
cies of these atoms suggest that they are associated with disordered sol-
vent. In addition to these semi-structure locations of electron density in
the E-map a number of unstructured single points of electron density
were located in the E-map. SQUEEZE was implemented to remove
these remaining points of electron density and 61 electrons were re-
moved using this routine. A check of the ™cif∫ file for this structure re-
vealed the possibility of higher symmetry (C2/c). Solution of the structure
in C2/c, however, led to less satisfactory results.
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Acknowledgement
This work was supported by the Basic Energy Sciences Division, Depart-
ment of Energy. Argonne National Laboratories are thanked for access
to the CARS facility. ChemMatCARS Sector 15 is principally supported
by the National Science Foundation/Department of Energy under grant
numbers CHE9522232 and CHE0087817 and by the Illinois board of
higher education. The Advanced Photon Source is supported by the U.S.
Department of Energy, Basic Energy Sciences, Office of Science, under
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Received: October 14, 2003
Revised: January 19, 2004 [F5620]
1955
Chem. Eur. J. 2004, 10, 1944 1955
www.chemeurj.org
¹ 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim