9 research outputs found
Thermodynamic Analysis of Allosteric and Chelate Cooperativity in Di- and Trivalent Ammonium/Crown-Ether Pseudorotaxanes
A detailed thermodynamic
analysis of the axle-wheel binding in
di- and trivalent secondary ammonium/[24]Ācrown-8 pseudorotaxanes is
presented. Isothermal titration calorimetry (ITC) data and double
mutant cycle analyses reveal an interesting interplay of positive
as well as negative allosteric and positive chelate cooperativity
thus providing profound insight into the effects governing multivalent
binding in these pseudorotaxanes
Formation and Transmetalation Mechanisms of Homo- and Heterometallic (Fe/Zn) Trinuclear Triple-Stranded Side-by-Side Helicates
A novel linear hybrid
tris-bidentate neutral ligand having 2,2ā²-bipyridine
and two terminal triazolylpyridine coordination sites (<b>L</b>) was efficiently synthesized and explored in the synthesis of trinuclear
triple-stranded homometallic side-by-side helicates <b>L</b><sub>3</sub>Fe<sub>3</sub>(OTf)<sub>6</sub> (<b>1</b>) and <b>L</b><sub>3</sub>Zn<sub>3</sub>(OTf)<sub>6</sub> (<b>2</b>), in which the three metal centers display alternating Ī and
Ī configurations. Selective formation of the analogous heterometallic
side-by-side helicate <b>L</b><sub>3</sub>Fe<sub>2</sub>ZnĀ(OTf)<sub>6</sub> (<b>3</b>) was achieved from a mixture of <b>L</b>, FeĀ(CH<sub>3</sub>CN)<sub>2</sub>(OTf)<sub>2</sub>, and ZnĀ(OTf)<sub>2</sub> (1:1:1) in acetonitrile at room temperature. Various analytical
techniques, i.e., single-crystal X-ray diffraction and NMR and UV/vis
spectroscopy, were used to elucidate the sequence of the metal atoms
within the heterometallic helicate, with the Zn<sup>2+</sup> at the
central position. The formation of <b>3</b> was also achieved
starting from either <b>L</b><sub>3</sub>Zn<sub>3</sub>(OTf)<sub>6</sub> or <b>L</b><sub>3</sub>Fe<sub>3</sub>(OTf)<sub>6</sub> by adding FeĀ(CH<sub>3</sub>CN)<sub>2</sub>(OTf)<sub>2</sub> or ZnĀ(OTf)<sub>2</sub>, respectively. ESI-MS and <sup>1</sup>H NMR studies elucidated
different transmetalation mechanisms for the two cases: While a Zn<sup>2+</sup>-to-Fe<sup>2+</sup> transmetalation occurs by the stepwise
exchange of single ions on the helicate <b>L</b><sub>3</sub>Zn<sub>3</sub>(OTf)<sub>6</sub> at room temperature, this mechanism
is almost inoperative for the Fe<sup>2+</sup>-to-Zn<sup>2+</sup> transmetalation
in <b>L</b><sub>3</sub>Fe<sub>3</sub>(OTf)<sub>6</sub>, which
is kinetically trapped at room temperature. In contrast, dissociation
of <b>L</b><sub>3</sub>Fe<sub>3</sub>(OTf)<sub>6</sub> at higher
temperature is required, followed by reassembly to give <b>L</b><sub>3</sub>Fe<sub>2</sub>ZnĀ(OTf)<sub>6</sub>. The reassembly follows
an interesting mechanistic pathway when an excess of ZnĀ(OTf)<sub>2</sub> is present in solution: First, <b>L</b><sub>3</sub>Zn<sub>3</sub>(OTf)<sub>6</sub> forms as the high-temperature thermodynamic
product, which is then slowly converted into the thermodynamic heterometallic <b>L</b><sub>3</sub>Fe<sub>2</sub>ZnĀ(OTf)<sub>6</sub> product at
room temperature. The temperature-dependent equilibrium shift is traced
back to significant entropy differences resulting from an enhancement
of the thermal motion of the ligands at high temperature, which destabilize
the octahedral iron terminal complex and select zinc in a more stable
tetrahedral geometry
Competitive Transmetalation of First-Row Transition-Metal Ions between Trinuclear Triple-Stranded Side-by-Side Helicates
A hybrid tris-bidentate neutral ligand
(<b>L</b>) composed of a central 2,2ā²-bipyridine and
two terminal triazolyl-pyridine chelating units connected by methylene
spacers is employed to synthesize trinuclear triple-stranded side-by-side
helicates of first-row transition-metalĀ(II) ions. Three such new homometallic
helicates L<sub>3</sub>M<sub>3</sub>(OTf)<sub>6</sub> [ M = Cu<sup>2+</sup> (<b>4</b>); Ni<sup>2+</sup> (<b>5</b>); Co<sup>2+</sup> (<b>6</b>)], along with our recently reported helicates
L<sub>3</sub>Fe<sub>3</sub>(OTf)<sub>6</sub> (<b>1</b>), L<sub>3</sub>Zn<sub>3</sub>(OTf)<sub>6</sub> (<b>2</b>), and L<sub>3</sub>Fe<sub>2</sub>ZnĀ(OTf)<sub>6</sub> (<b>3</b>) are taken
into consideration for competitive formation and transmetalation studies.
Single-crystal X-ray structures of L<sub>3</sub>Cu<sub>3</sub>(OTf)<sub>6</sub> (<b>4</b>) and L<sub>3</sub>Ni<sub>3</sub>(OTf)<sub>6</sub> (<b>5</b>) show the formation of trinuclear triple-stranded
side-by-side helicates with alternating Ī and Ī chiralities
at the metal ions as earlier observed in cases of L<sub>3</sub>Fe<sub>3</sub>(OTf)<sub>6</sub> (<b>1</b>), L<sub>3</sub>Zn<sub>3</sub>(OTf)<sub>6</sub> (<b>2</b>), and L<sub>3</sub>Fe<sub>2</sub>ZnĀ(OTf)<sub>6</sub> (<b>3</b>). ESI-FTICR mass spectrometry
and UVāvis spectroscopy studies show that helicates L<sub>3</sub>Fe<sub>3</sub>(OTf)<sub>6</sub> (<b>1</b>), L<sub>3</sub>Zn<sub>3</sub>(OTf)<sub>6</sub> (<b>2</b>), L<sub>3</sub>Fe<sub>2</sub>ZnĀ(OTf)<sub>6</sub> (<b>3</b>), and L<sub>3</sub>Co<sub>3</sub>(OTf)<sub>6</sub> (<b>6</b>) can easily be transmetalated to
helicate L<sub>3</sub>Cu<sub>3</sub>(OTf)<sub>6</sub> (<b>4</b>) in the presence of CuĀ(OTf)<sub>2</sub>. On the other hand, only
a trace amount of heterometallic helicate L<sub>3</sub>Ni<sub>2</sub>CuĀ(OTf)<sub>6</sub> forms even after several days, when CuĀ(OTf)<sub>2</sub> is added to a the solution of homometallic helicate L<sub>3</sub>Ni<sub>3</sub>(OTf)<sub>6</sub> (<b>5</b>). Further,
we have demonstrated the formation of a heterometallic helicate L<sub>3</sub>Ni<sub>2</sub>CoĀ(OTf)<sub>6</sub> (<b>7</b>) from a
1:1:1 reaction mixture of <b>L</b>, NiĀ(OTf)<sub>2</sub>, and
CoĀ(OTf)<sub>2</sub>, which can also be prepared from homometallic
helicate L<sub>3</sub>Co<sub>3</sub>(OTf)<sub>6</sub> (<b>6</b>) by transmetalation with NiĀ(OTf)<sub>2</sub>
Intermixed Terpyridine-Functionalized Monolayers on Gold: Nonlinear Relationship between Terpyridyl Density and Metal Ion Coordination Properties
Aiming at the functionalization of surfaces with terpyridine
anchors
for the coordinative deposition of additional layers, mixed self-assembled
monolayers (SAMs) were prepared from binary solutions of 12-(2,2ā²:6ā²,2ā³-terpyridine-4ā²-yl)Ādodecane-1-thiol
(TDT) and 1-decanethiol (DT). The SAMs and the order of the constituting
molecules were analyzed by X-ray photoelectron spectroscopy (XPS),
near-edge X-ray absorption fine structure spectroscopy (NEXAFS), and
time-of-flight-secondary ion mass spectrometry (ToF-SIMS). The composition
of the (TDT/DT)-SAMs and with it the surface density of terpyridyl
groups correlates linearly with the relative concentrations of the
two compounds in the solution used for depositing them. In marked
contrast, the amount of terpyridine-coordinated Pd<sup>II</sup> ions
significantly deviates from this trend with an optimum at a 1:3 ratio
of TDT/DT. This indicates a major fraction of the terpyridines in
TDT-rich SAMs not to be accessible for Pd<sup>II</sup> ion coordination.
In agreement, NEXAFS spectroscopy reveals the alkyl backbones in TDT-rich
SAMs not to be ordered, while they are preferentially upright oriented
in the optimal 1:3-(TDT/DT)-SAMs. We interpret this in terms of terpyridine
backfolding in TDT-rich SAMs, while they are located in accessible
positions on top of the SAM in the 1:3-(TDT/DT)-SAM. While the alkyl
backbones in the 1:3-(TDT/DT)-SAM are ordered, NEXAFS spectroscopy
shows the terpyridyl groups not to have a preferential orientation
in this SAM and thus retain enough flexibility to adjust to molecules
that are deposited on top of the mixed SAM. In conclusion, the novel
SAM does not undergo phase separation and consists predominantly of
intermixed phases with adjustable surface density of quite flexible
terpyridine anchor groups. The terpyridineāPd<sup>II</sup> anchors
are not only available for a future deposition of the next layer,
but the metal ions also represent a sensitive probe for the accessibility
of the terpyridyl groups
Deposition of Ordered Layers of Tetralactam Macrocycles and Ether Rotaxanes on Pyridine-Terminated Self-Assembled Monolayers on Gold
The deposition of tetralactam macrocycles and the corresponding
benzyl ether rotaxanes on gold substrates is investigated for the
first time exploiting metallo-supramolecular chemistry. Two pyridine-terminated
self-assembled monolayers (SAMs) are developed that are used as well-ordered
template layers. The two SAMs differ with respect to the rigidity
of the terminal pyridines as shown by angle-resolved near-edge X-ray
absorption fine structure (NEXAFS) spectroscopy. The template layers
are then used for the metal-mediated self-assembly of macrocylces
and rotaxanes on solid supports. The SAM with the more rigid terminal
pyridine shows a higher coverage with the macrocycles and is therefore
preferable. Angle-resolved NEXAFS spectroscopy also shows the deposited
supramolecules to be oriented preferentially upright. This order is
only achieved for the macrocycles through the deposition on the more
rigid SAM template, whereas rotaxanes form oriented layers on both
SAMs. Time-of-flight secondary-ion mass spectrometry analysis was
used to determine the deposition time required for the self-assembly
process
Sequence-Programmable Multicomponent Multilayers of Nanometer-Sized Tetralactam Macrocycles on Gold Surfaces
Multicomponent multilayers have been
deposited on gold surfaces
by metal-ion-mediated layer-by-layer self-assembly of differently
functionalized tetralactam macrocycles. The layer stack can be programmed
with respect to the sequences of metal ions and macrocycles by the
deposition sequence
Chelate Cooperativity and Spacer Length Effects on the Assembly Thermodynamics and Kinetics of Divalent Pseudorotaxanes
Homo- and heterodivalent crown-ammonium pseudorotaxanes
with different
spacers connecting the two axle ammonium binding sites have been synthesized
and characterized by NMR spectroscopy and ESI mass spectrometry. The
homodivalent pseudorotaxanes are investigated with respect to the
thermodynamics of divalent binding and to chelate cooperativity. The
shortest spacer exhibits a chelate cooperativity much stronger than
that of the longer spacers.
On the basis of crystal structure, this can be explained by a noninnocent
spacer, which contributes to the
binding strength in addition to the two binding sites. Already very
subtle changes in the spacer length, i.e., the introduction of an
additional methylene group, cause substantial changes in the magnitude
of cooperative binding as expressed in the large differences in effective
molarity. With a similar series of heterodivalent pseudorotaxanes,
the spacer effects on the barrier for the intramolecular threading
step has been examined with the result that the shortest spacer causes
a strained transition structure and thus the second binding event
occurs slower than that of the longer spacers. The activation enthalpies
and entropies show clear trends. While the longer spacers reduce the
enthalpic strain that is present in the transition state for the shortest
member of the series, the longer spacers become entropically slightly
more unfavorable because of conformational fixation of the spacer
chain during the second binding event. These results clearly show
the noninnocent spacers to complicate the analysis of multivalent
binding. An approximate description which considers the binding sites
to be connected just by a flexible chain turns out to be more a rough
approximation than a good model. The second conclusion from the results
presented here is that multivalency is expressed in both the thermodynamics
and the kinetics in different ways. A spacer optimized for strong
binding is suboptimal for fast pseudorotaxane formation
PolyamideāPolyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase
Polyamideāpolyamine
hybrid macrobicycle <b>L</b> is
explored with respect to its ability to bind Ī±,Ļ-dicarboxylate
anions. Potentiometric studies of protonated <b>L</b> with the
series of dianions from succinate (suc<sup>2ā</sup>) through
glutarate (glu<sup>2ā</sup>), Ī±-ketoglutarate (kglu<sup>2ā</sup>), adipate (adi<sup>2ā</sup>), pimelate (pim<sup>2ā</sup>), suberate (sub<sup>2ā</sup>), to azelate
(aze<sup>2ā</sup>) have shown adipate preference with association
constant value of <i>K</i> = 4900 M<sup>ā1</sup> in
a H<sub>2</sub>O/DMSO (50:50 <i>v/v</i>) binary solvent
mixture. The binding constant increases from glu<sup>2ā</sup> to adi<sup>2ā</sup> and then continuously decreases with
the length of the anion chain. Further, potentiometric studies suggest
that hydrogen bonding between the guest anions and the amide/ammonium
protons of the receptor also contributes to the stability of the associations
along with electrostatic interactions. Negative-mode electrospray
ionization of aqueous solutions of hostāguest complexes shows
clear evidence for the selective formation of 1:1 complexes. Single-crystal
X-ray structures of complexes of the receptor with glutaric acid,
Ī±-ketoglutaric acid, adipic acid, pimelic acid, suberic acid,
and azelaic acid assist to understand the observed binding preferences.
The solid-state structures reveal a size/shape complementarity between
the host and the dicarboxylate anions, which is nicely reflected in
the solution state binding studies
PolyamideāPolyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase
Polyamideāpolyamine
hybrid macrobicycle <b>L</b> is
explored with respect to its ability to bind Ī±,Ļ-dicarboxylate
anions. Potentiometric studies of protonated <b>L</b> with the
series of dianions from succinate (suc<sup>2ā</sup>) through
glutarate (glu<sup>2ā</sup>), Ī±-ketoglutarate (kglu<sup>2ā</sup>), adipate (adi<sup>2ā</sup>), pimelate (pim<sup>2ā</sup>), suberate (sub<sup>2ā</sup>), to azelate
(aze<sup>2ā</sup>) have shown adipate preference with association
constant value of <i>K</i> = 4900 M<sup>ā1</sup> in
a H<sub>2</sub>O/DMSO (50:50 <i>v/v</i>) binary solvent
mixture. The binding constant increases from glu<sup>2ā</sup> to adi<sup>2ā</sup> and then continuously decreases with
the length of the anion chain. Further, potentiometric studies suggest
that hydrogen bonding between the guest anions and the amide/ammonium
protons of the receptor also contributes to the stability of the associations
along with electrostatic interactions. Negative-mode electrospray
ionization of aqueous solutions of hostāguest complexes shows
clear evidence for the selective formation of 1:1 complexes. Single-crystal
X-ray structures of complexes of the receptor with glutaric acid,
Ī±-ketoglutaric acid, adipic acid, pimelic acid, suberic acid,
and azelaic acid assist to understand the observed binding preferences.
The solid-state structures reveal a size/shape complementarity between
the host and the dicarboxylate anions, which is nicely reflected in
the solution state binding studies