Dynamic covalent and supramolecular direction of the synthesis and reassembly of copper(I) complexes

Ionic Liquid Crystals Based on Loop-Shaped Copper(I) Complexes

This paper describes the synthesis and characterization of liquid crystals based on loop-shaped cationic copper(I) complexes of a multidentate ligand. Their synthesis involves the one-pot reaction of an alkyloxy-decorated pyridine-aldehyde unit with a diamine (2,2'-(ethylenedioxy)bis(ethylamine)) spacer to form in situ a pyridine-imine quadridentate-N4-donor ligand, L, which is able to chelate a copper(I) center associated with various noncoordinating anions. All of these compounds were characterized by NMR, IR, and electronic absorption spectroscopy, and more particularly by X-ray diffraction and mass spectroscopy, enabling unambiguous assignment of the [ML]+ mononuclear nature of the cationic components. The presence of six flexible alkyloxy chains at each end of the ligand associated with the rigidity of the core complex causes induction of a liquid crystal state with a columnar self-organized architecture, where the columns are packed in a hexagonal two-dimensional network.

Narcissistic self-sorting in anion-coordination-driven assemblies

Three tris-bis(urea) ligands with triphenylamine-based C₃-symmetric spacers were synthesized, which assembled with sulfate or phosphate to form anionic A₃L₂ pinwheel helices (A = anion and L = ligand) and A₄L₄ tetrahedra, respectively. Interestingly, narcissistic self-sorting was observed in both structures from the mixture of the ligands, wherein each assembly contains only one type of ligand with no detectable mixed-ligand product as confirmed by the NMR and MS studies.

Enzyme-responsive chiral self-sorting in amyloid-inspired minimalistic peptide amphiphiles

Self-sorting is a spontaneous phenomenon that ensures the formation of complex yet ordered multicomponent systems and conceptualizes the design of artificial and orthogonally functional compartments. In the present study, we envisage chirality-mediated self-sorting in β-amyloid-inspired minimalistic peptide amphiphile (C₁₀-l/d-VFFAKK)-based nanofibers. The fidelity and stereoselectivity of chiral self-sorting was ascertained by Förster resonance energy transfer (FRET) by the judicious choice of a pyrene (Py)-hydroxy coumarin (HOCou) donor-acceptor pair tethered to the peptide sequences. Seed-promoted elongation of the homochiral peptide amphiphiles investigated by AFM image analyses and Thioflavin-T (ThT) binding study further validated the chiral recognition of the l/d peptide nanofibers. Moreover, direct visualization of the chirality-driven self-sorted nanofibers is reported using super-resolution microscopy that exhibits enantioselective enzymatic degradation for l-peptide fibers. Such enantioselective weakening of the hydrogels may be used for designing stimuli-responsive orthogonal compartments for delivery applications.

Narcissistic self-sorting in self-assembled cages of rare Earth metals and rigid ligands

Highly selective, narcissistic self-sorting can be achieved in the formation of self-assembled cages of rare earth metals with multianionic salicylhydrazone ligands. The assembly process is highly sensitive to the length of the ligand and the coordination geometry. Most surprisingly, high-fidelity sorting is possible between ligands of identical coordination angle and geometry, differing only in a single functional group on the ligand core, which is not involved in the coordination. Supramolecular effects allow discrimination between pendant functions as similar as carbonyl or methylene groups in a complex assembly process.

Multiaddressable molecular rectangles with reversible host-guest interactions: modulation of pH-controlled guest release and capture

A series of multiaddressable platinum(II) molecular rectangles with different rigidities and cavity sizes has been synthesized by endcapping the U-shaped diplatinum(II) terpyridine moiety with various bis-alkynyl ligands. The studies of the host-guest association with various square planar platinum(II), palladium(II), and gold(III) complexes and the related low-dimensional gold(I) complexes, most of which are potential anticancer therapeutics, have been performed. Excellent guest confinement and selectivity of the rectangular architecture have been shown. Introduction of pH-responsive functionalities to the ligand backbone generates multifunctional molecular rectangles that exhibit reversible guest release and capture on the addition of acids and bases, indicating their potential in controlled therapeutics delivery on pH modulation. The reversible host-guest interactions are found to be strongly perturbed by metal-metal and π-π interactions and to a certain extent, electrostatic interactions, giving rise to various spectroscopic changes depending on the nature of the guest molecules. Their binding mode and thermodynamic parameters have been determined by 2D NMR and van't Hoff analysis and supported by computational study.

Introducing a static receptor to compete with a dynamic combinatorial library in template binding

Association constants can be obtained from HPLC analysis of a system comprising a dynamic combinatorial library and a static host.

Toward metal complexes that can directionally walk along tracks: controlled stepping of a molecular biped with a palladium(II) foot

We report on the design, synthesis, and operation of a bimetallic molecular biped on a three-foothold track. The "walker" features a palladium(II) complex "foot" that can be selectively stepped between 4-dimethylaminopyridine and pyridine ligand sites on the track via reversible protonation while the walker remains attached to the track throughout by means of a kinetically inert platinum(II) complex foot. The substitution pattern of the three ligand binding sites, together with the kinetic stability of the metal-ligand coordination bonds, affords the two positional isomers a high degree of metastability, meaning that altering the chemical state of the track does not automatically instigate stepping in the absence of an additional stimulus (heat in the presence of a coordinating solvent). The use of metastable metal complexes for foot-track interactions offers a promising alternative to dynamic covalent chemistry for the design of small-molecule synthetic molecular walkers.

A dynamic covalent, luminescent metallopolymer that undergoes sol-to-gel transition on temperature rise

The condensation of linear diamine and dialdehyde subcomponents around copper(I) templates in the presence of bulky trioctylphosphine ancillary ligands gave a linear, conjugated polymeric material in DMSO solution. This polymer solution was observed to undergo sol-to-gel transition as the temperature was raised to 140 °C, in contrast with the behavior of most gel-forming polymers, which do so upon cooling. We attribute the sol-to-gel transition to the formation of Cu(I)N(4) cross-links as the equilibria 2[Cu(I)N(2)P(2)] ⇄ [Cu(I)N(4)] + [CuP(n)](+) + (4 - n)P favor the right-hand side at higher temperatures. The material was also observed to exhibit thermochromism and photoluminescence, with the color and intensity of both absorption and emission exhibiting temperature dependence. This material thus responds predictably to combinations of stimuli (heat, light, mechanical shear) in an interconnected way, as is required to generate complex function.

Photophysical characteristics and reactivity of bis(2,9-di-tert-butyl-1,10-phenanthroline)copper(I)

The recently synthesized sterically constrained copper(I) complex [Cu(dtbp)(2)](+) (1), where dtbp is 2,9-di-tert-butyl-1,10-phenanthroline, exhibits unique photophysical and reactivity properties. Complex 1 (lambda(abs), 425 nm; epsilon, 3100 L M(-1) cm(-1); lambda(emission), 599 nm) has the longest metal-to-ligand charge-transfer (MLCT) emission lifetime (tau, 3260 ns) and largest quantum yield (varphi, 5.6%) of all [Cu(R(2)phen)(2)](+) complexes. Complex 1 also exhibits a large positive reduction potential for the [Cu(2+)(dtbp)(2)]|[Cu(+)(dtbp)(2)] couple (E(1/2) = 0.70 V vs Fc(+/0)) and a large negative excited-state reduction potential for the [Cu(2+)(dtbp)(dtbp(-*))]|[Cu(2+)(dtbp)(2)] couple (E(1/2) = -1.66 V vs Fc(+/0)), indicating that this complex is a potent photoreductant in the excited state. The steric constraint imposed by the t-butyl substituents in 1 enables unusual ligand replacement reactivity. Either CH(3)CN or CO replaces one of the dtbp ligands, a reaction that is readily followed by loss of the unique emission signature of 1. Monodentate CH(3)CN binds to the copper(I) center with an affinity 2 orders of magnitude greater than that of the displaced dtbp, despite the fact that the displaced ligand is bidentate. CO-induced displacement of dtbp from 1 is reversible, but only in the presence of 1 equiv of unbound dtbp. The exceptionally strong donor ligand CH(3)NC displaces both dtbp ligands from 1. In contrast to the facile ligand displacement reactivity with good donor ligands, 1 does not react readily with O(2), by either a ligand displacement or an oxidative pathway. Rather, O(2) induces partial quenching of emission via an outer-sphere interaction with 1.

Self-organization in coordination-driven self-assembly

Self-assembly allows for the preparation of highly complex molecular and supramolecular systems from relatively simple starting materials. Typically, self-assembled supramolecules are constructed by combining complementary pairs of two highly symmetric molecular components, thus limiting the chances of forming unwanted side products. Combining asymmetric molecular components or multiple complementary sets of molecules in one complex mixture can produce myriad different ordered and disordered supramolecular assemblies. Alternatively, spontaneous self-organization phenomena can promote the formation of specific product(s) out of a collection of multiple possibilities. Self-organization processes are common throughout much of nature and are especially common in biological systems. Recently, researchers have studied self-organized self-assembly in purely synthetic systems. This Account describes our investigations of self-organization in the coordination-driven self-assembly of platinum(II)-based metallosupramolecules. The modularity of the coordination-driven approach to self-assembly has allowed us to systematically study a wide variety of different factors that can control the extent of supramolecular self-organization. In particular, we have evaluated the effects of the symmetry and polarity of ambidentate donor subunits, differences in geometrical parameters (e.g., the size, angularity, and dimensionality) of Pt(II)-based acceptors and organic donors, the influence of temperature and solvent, and the effects of intermolecular steric interactions and hydrophobic interactions on self-organization. Our studies have shown that the extent of self-organization in the coordination-driven self-assembly of both 2D polygons and 3D polyhedra ranges from no organization (a statistical mixture of multiple products) to amplified organization (wherein a particular product or products are favored over others) and all the way to the absolute self-organization of discrete supramolecular assemblies. In many cases, inputs such as dipolar interactions, steric interactions, and differences in the geometric parameters of subunits, used either alone or as multiple factors simultaneously, can achieve absolute self-organization of discrete supramolecules. We have also observed instances where self-organization is not absolute and varies in its deviation from statistical results. Steric interactions are particularly useful control factors for driving such amplified self-organization because they can be subtly tuned through small structural variations. Having the ability to fully understand and control the self-organization of complex mixtures into specific synthetic supramolecules can provide a better understanding of analogous processes in biological systems. Furthermore, self-organization may allow for the facile synthesis of complex multifunctional, multicomponent systems from simply mixing a collection of much simpler, judiciously designed individual molecular components.

Reversible Ligand Pairing and Sorting Processes Leading to Heteroligated Palladium(II) Complexes with Hemilabile Ligands

Halide-induced ligand pairing and sorting processes have been observed in the context of Pd(II) complexes with hemilabile P,S and P,O ligands. Mixing of the ligands Ph₂PCH₂CH₂SMe (7) and Ph₂PCH₂CH₂SPh (8) with a Pd(II) precursor in CH₂Cl₂ results in a mixture of [(7)₂ClPd]Cl, [(8)₂Cl₂Pd], and [(7)(8)ClPd]Cl complexes at 20 °C. This equilibrium can be driven toward the heteroligated structure [(7)(8)ClPd]Cl by (1) cooling the mixture or (2) precipitation with hexanes, leading to the exclusive formation of semiopen heteroligated complex cis-[κ ²-(7)-κ ¹-(8)ClPd]Cl (9a), as confirmed by a single-crystal X-ray diffraction study and solid state CPMAS ³¹P{¹H} NMR spectroscopy. Dissolution of 9a in CH₂Cl₂ leads to the original mixture of complexes, which illustrates the reversible nature of this ligand pairing and sorting process. Similar processes occur when a combination of P,S and P,O ligands is used. The semiopen heteroligated complexes can be chemically manipulated in a reversible fashion to form closed complexes, allowing for control of the relative position and flexibility between neighboring substituents in these "tweezer"-like structures. Control experiments suggest these ligand sorting and pairing processes occur via a halide-induced ligand rearrangement (HILR) reaction.

Second-order self-organization in coordination-driven self-assembly: exploring the limits of self-selection

Self-organization during the self-assembly of a series of functionalized bispyridyl organic donors with complementary di-Pt(II) acceptors into supramolecular rhomboids and rectangles is explored. The connectivity and location of functional groups on the organic donors ensures that they do not interfere sterically or electronically with their respective binding sites. Carefully controlled reaction conditions are employed so that the only means of self-organization during self-assembly is through "second-order" effects arising from the distal functional groups themselves. With the selection of functionalized systems studied, the extent of second-order self-organization varies from essentially zero to quite pronounced.

A dynamic tricopper double helicate

The reaction between 8-aminoquinoline, 1,10-phenantholine-2,9-dicarbaldehyde, and copper(I) tetrafluoroborate gave a quantitative yield of a tricopper double helicate. The presence of dynamic covalent imine (C=N) bonds allowed this assembly to participate in two reactions not previously known in helicate chemistry: 1) It could be prepared through subcomponent substitution from a dicopper double helicate that contained aniline residues. An electron-poor aniline was quantitatively displaced; a more electron-rich aniline competed effectively with the aminoquinoline, setting up an equilibrium between dicopper and tricopper helicates that could be displaced towards the tricopper through the addition of further copper(I). 2) Both dicopper and tricopper helicates could be prepared simultaneously from a mixture of phenanthroline dialdehyde, aniline, and aminoquinoline, which contained all possible imine condensation products in equilibrium. Following the addition of copper(I), thermodynamic equilibration on both covalent and coordinative levels eliminated all partially-formed and mixed imine ligands from the mixture, leaving the helicates as exclusive products.

Kinetic and Thermodynamic Selectivity in Subcomponent Substitution

Within assemblies prepared by metal-templated imine condensation, one amine residue (subcomponent) may be replaced with another through substitution reactions. Proton transfer from a more to a less acidic amine may be used as the driving force for substitution. Herein, we detail the development of a set of selectivity rules to predict the outcome of subcomponent substitution reactions when several different substrates are present. When both iron and copper complexes were present, substitution occurred preferentially at imines bound to copper. This preference was kinetic in nature in the absence of a chelating amine subcomponent: The different amine residues were found to scramble between the copper and iron complexes following an initial clean substitution at the copper-bound imine. When both chelating and nonchelating amine subcomponents were present, the preference became thermodynamic in nature. Only the nonchelating amine was substituted and no evidence of scrambling was found after the reaction mixture was heated to 50 degrees C for several days. This thermodynamic selectivity, based on the chelate effect, operated in mixtures of Cu(I) and Fe(II) complexes, and in systems containing only Fe(II) complexes.

Synthetic selectivity through avoidance of valence frustration

A series of di-copper(I) complexes has been prepared via the reaction of copper(I) tetrafluoroborate, 2,6-diformylpyridine, 8-aminoquinoline, and a series of aliphatic diamines and 4-substituted anilines. To avoid a "valence-frustrated" state, involving a mismatch between the number of ligand donor atoms and the number of metal acceptor sites, the product structures formed selectively: One of the formyl groups of the diformylpyridine reacted specifically with the aminoquinoline, whereas the other formyl group reacted with the diamine or aniline. The observed selectivity was demonstrated to be thermodynamic in nature: When two dicopper complexes that were stable yet "valence-frustrated" were mixed, an imine metathesis reaction was observed to occur spontaneously to generate a "valence-satisfied" structure. In addition to control over the constitution of the ligands, we were able to exercise control over their relative orientations within the complex. Diamines exclusively gave structures in which the ligand exhibited a head-to-head orientation along the copper-copper axis to avoid stretching. Anilines gave predominantly head-to-tail structures, with the proportion of head-to-head isomer decreasing in complexes that incorporate more electron-deficient anilines and disappearing in less polar solvents. We also demonstrated the removal of the metals and the hydrogenation of the imine bonds to generate a molecule containing nonexchanging secondary amines, suggesting potential uses of this technique in the domain of organic synthesis.

Helicate, Macrocycle, or Catenate: Dynamic Topological Control over Subcomponent Self-Assembly

The aqueous reaction between equimolar amounts of 2-(2-(2-aminoethoxy)ethoxy)ethanamine, 1,10-phenanthroline-2,9-dialdehyde and copper(I) produced a dimeric helical macrocycle in quantitative yield. This ring could also be generated by the addition of two equivalents of the diamine to an acyclic helicate containing four mono-imine residues: A transimination occurred, the chelate effect being implicated as a driving force. In the case of a helicate containing mono-imines derived from anilines, the substitution of diamine for monoamine was reversible upon lowering the pH. The aliphatic diamine was protonated at a higher pH than the arylamine, which left the arylamine free for incorporation instead of the alkyl diamine. This reaction thus opened the possibility of switching between closed macrocyclic and open helicate topologies by changing the pH. An additional closed topology became accessible through the use of a diamine that incorporates two rigid phenylene spacer groups between a flexible chain and the imine-forming nitrogen atoms. The resulting catenate consists of a pair of topologically interlinked macrocycles. The presence of the phenylene groups appeared to dictate the topology of the final product, making the formation of a single macrocycle energetically disfavoured.

How to Adapt Scatchard Plot for Graphically Addressing Cooperativity in Multicomponent Self-Assemblies

A graphical method has been developed for the reliable detection of cooperativity in polymetallic complexes involving intra- and intermolecular complexation processes. The method relies on the determination of the partial occupancy r(AL)n, which represents the average number of metals bound per preassembled receptor AL(n) made up of n ligands bound to a linker A. We observe nonlinear, i.e., nonstatistical, Scatchard-like plots (r(AL)n/[M] vs r(AL)n) for metal-binding in double-stranded helicates. The present concept is extended to a virtual, pre-organized receptor L(n), in which no specific linker is involved. Applications to several polymetallic helicates reveal the presence of negatively cooperative processes attributed mainly to intermetallic repulsions, in agreement with recent thermodynamic models.

Tunable Fluorene-Based Dynamers through Constitutional Dynamic Chemistry

Dynamic covalent iminofluorene-based oligomers and polymers have been generated. They undergo constitutional recomposition under the effect of two parameters, acidity and ZnII metal ions. As a result, marked changes in physical properties take place. The results illustrate the response of such a dynamic system to chemical effectors (H+ nd ZnII), thus demonstrating the adaptive behavior of the system under the pressure of external factors. They also point to the possibility of modulating optical properties (UV-visible absorption and fluorescence) by constitutional recomposition in response to a specific trigger. Such features allow the development of dynamic materials, the functional properties of which may respond to external stimuli.

Chiral spiro Cu(i) complexes. Supramolecular stereocontrol and isomerisation dynamics by the use of TRISPHAT anions

Association of enantiopure TRISPHAT anion (1) with chiral spiro [Cu(LL')2] complexes (LL' = 2-R-phen, 2, 6-R-bpy, 3, and 2-iminopyridine, 4) leads to an efficient NMR enantiodifferentiation. Variable temperature 1H NMR spectroscopy has been used to determine the isomerisation kinetics of these pseudo-tetrahedral complexes and to evaluate their configurational stability; the latter depending on the structure of the diimine ligands. In the case of the 2-anthracenyl-phen derivative, a decent level of supramolecular stereocontrol was noted (d.e. up to 45%); the configuration of the complex being determined by electronic circular dichroism (ECD).