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]­crown-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 2,2′-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>₃Fe₃(OTf)₆ (<b>1</b>) and <b>L</b>₃Zn₃(OTf)₆ (<b>2</b>), in which the three metal centers display alternating Λ and Δ configurations. Selective formation of the analogous heterometallic side-by-side helicate <b>L</b>₃Fe₂Zn­(OTf)₆ (<b>3</b>) was achieved from a mixture of <b>L</b>, Fe­(CH₃CN)₂(OTf)₂, and Zn­(OTf)₂ (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 Zn²⁺ at the central position. The formation of <b>3</b> was also achieved starting from either <b>L</b>₃Zn₃(OTf)₆ or <b>L</b>₃Fe₃(OTf)₆ by adding Fe­(CH₃CN)₂(OTf)₂ or Zn­(OTf)₂, respectively. ESI-MS and ¹H NMR studies elucidated different transmetalation mechanisms for the two cases: While a Zn²⁺-to-Fe²⁺ transmetalation occurs by the stepwise exchange of single ions on the helicate <b>L</b>₃Zn₃(OTf)₆ at room temperature, this mechanism is almost inoperative for the Fe²⁺-to-Zn²⁺ transmetalation in <b>L</b>₃Fe₃(OTf)₆, which is kinetically trapped at room temperature. In contrast, dissociation of <b>L</b>₃Fe₃(OTf)₆ at higher temperature is required, followed by reassembly to give <b>L</b>₃Fe₂Zn­(OTf)₆. The reassembly follows an interesting mechanistic pathway when an excess of Zn­(OTf)₂ is present in solution: First, <b>L</b>₃Zn₃(OTf)₆ forms as the high-temperature thermodynamic product, which is then slowly converted into the thermodynamic heterometallic <b>L</b>₃Fe₂Zn­(OTf)₆ 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 2,2′-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-metal­(II) ions. Three such new homometallic helicates L₃M₃(OTf)₆ [ M = Cu²⁺ (<b>4</b>); Ni²⁺ (<b>5</b>); Co²⁺ (<b>6</b>)], along with our recently reported helicates L₃Fe₃(OTf)₆ (<b>1</b>), L₃Zn₃(OTf)₆ (<b>2</b>), and L₃Fe₂Zn­(OTf)₆ (<b>3</b>) are taken into consideration for competitive formation and transmetalation studies. Single-crystal X-ray structures of L₃Cu₃(OTf)₆ (<b>4</b>) and L₃Ni₃(OTf)₆ (<b>5</b>) show the formation of trinuclear triple-stranded side-by-side helicates with alternating Λ and Δ chiralities at the metal ions as earlier observed in cases of L₃Fe₃(OTf)₆ (<b>1</b>), L₃Zn₃(OTf)₆ (<b>2</b>), and L₃Fe₂Zn­(OTf)₆ (<b>3</b>). ESI-FTICR mass spectrometry and UV–vis spectroscopy studies show that helicates L₃Fe₃(OTf)₆ (<b>1</b>), L₃Zn₃(OTf)₆ (<b>2</b>), L₃Fe₂Zn­(OTf)₆ (<b>3</b>), and L₃Co₃(OTf)₆ (<b>6</b>) can easily be transmetalated to helicate L₃Cu₃(OTf)₆ (<b>4</b>) in the presence of Cu­(OTf)₂. On the other hand, only a trace amount of heterometallic helicate L₃Ni₂Cu­(OTf)₆ forms even after several days, when Cu­(OTf)₂ is added to a the solution of homometallic helicate L₃Ni₃(OTf)₆ (<b>5</b>). Further, we have demonstrated the formation of a heterometallic helicate L₃Ni₂Co­(OTf)₆ (<b>7</b>) from a 1:1:1 reaction mixture of <b>L</b>, Ni­(OTf)₂, and Co­(OTf)₂, which can also be prepared from homometallic helicate L₃Co₃(OTf)₆ (<b>6</b>) by transmetalation with Ni­(OTf)₂

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-(2,2′:6′,2″-terpyridine-4′-yl)­dodecane-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^II 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^II 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 terpyridine–Pd^II 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

Polyamide–Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase

Polyamide–polyamine hybrid macrobicycle <b>L</b> is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated <b>L</b> with the series of dianions from succinate (suc^2–) through glutarate (glu^2–), α-ketoglutarate (kglu^2–), adipate (adi^2–), pimelate (pim^2–), suberate (sub^2–), to azelate (aze^2–) have shown adipate preference with association constant value of <i>K</i> = 4900 M^–1 in a H₂O/DMSO (50:50 <i>v/v</i>) binary solvent mixture. The binding constant increases from glu^2– to adi^2– 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 host–guest 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, α-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

Polyamide–Polyamine Cryptand as Dicarboxylate Receptor: Dianion Binding Studies in the Solid State, in Solution, and in the Gas Phase

Polyamide–polyamine hybrid macrobicycle <b>L</b> is explored with respect to its ability to bind α,ω-dicarboxylate anions. Potentiometric studies of protonated <b>L</b> with the series of dianions from succinate (suc^2–) through glutarate (glu^2–), α-ketoglutarate (kglu^2–), adipate (adi^2–), pimelate (pim^2–), suberate (sub^2–), to azelate (aze^2–) have shown adipate preference with association constant value of <i>K</i> = 4900 M^–1 in a H₂O/DMSO (50:50 <i>v/v</i>) binary solvent mixture. The binding constant increases from glu^2– to adi^2– 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 host–guest 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, α-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