A Metal-Assembled, pH-Dependent, Resorcinarene-Based Cage Molecule

199

182

An important problem in designing any large network is the assembly of systems that are resilient to change. From a chemical point of view, an analogy can be used where one requires supramolecular assemblies to maintain their dimensionality combined with limited structural perturbation in response to variation in its intermolecular framework. The identification of hydrogenbonded framework patterns within experimentally known supramolecular assemblies that are structurally robust to disruption and selective hydrogen substitution are envisioned to act as a supramolecular blueprint or template for metal-ion retroinsertion. Here, we report the formation of a large neutral discrete pseudo-spherical coordination capsule assembled from 6 pyrogallol[4]arene ligands and 24 Cu(II) metal ions. Amazingly, this coordination capsule is structurally analogous to its hydrogenbonded counterpart. This result shows a robust ability of pyrogallol[4]arene molecules to self-assemble into large hexameric cage structures from either the hydrogen-bonding or metal-ligand coordination process. The identification of robust supramolecular assemblies that conserve their structure in response to interchangeability between hydrogen-bonded networks for metal coordination, or inversely, represents an important advancement in supramolecular design.coordination ͉ self-assembly ͉ hexamer I nspired by the architectural beauty and robustness found within nature, the design of large multicomponent spheroid cage assemblies on the nanometer scale is becoming increasing feasible. These large supramolecular architectures often resemble and conform to the simple geometrical shapes described by Platonic or Archimedean solids (1). Indeed, such structural morphologies have attracted tremendous interest because of their practical applications for encapsulating various guest molecules (2). Synthetic chemists have principally relied on various self-assembly methods in their construction, such as hydrogen bonds (3) or metal-ligand coordination (4), although some notable covalent methods have recently been reported (5).A popular strategy for designing large multicomponent cage assemblies is the metal-directed self-assembly approach, advocated chiefly by Fujita and coworkers (6-9) and Stang and coworkers (10-12). This method has mainly focused on the geometrical permutation of various triangular pyridine-like bridging ligands driven by coordination to cis-protected Pd metal centers. Nevertheless, for the construction of discrete assemblies with large molecular cavities, the molecular paneling approach by Fujita and coworkers (13) has received the most success. Indeed, an amazingly large discrete functionalized spherical coordination network assembled from 12 metal ions and 24 exo-multidentate ligands was recently reported (14). However, an inherent complication of using n-dimensional pyridine-like spacer multidentate ligands in coordination is the resultant high positive charge combined with porous surfaces within these supramolecular assemblies. The counterions us...

Section: Resultsmentioning

confidence: 99%

“…Although there are numerous reports of metal-bound resorcin [4]arene-like cage assemblies, most are dimeric and highly charged in nature (15)(16)(17)(18)(19)(20)(21)(22). Indeed, the construction of larger coordination cage structures involving the assembly of resorcin [4]arene-like ligands greater than two is still in its infancy (23,24).…”

mentioning

confidence: 99%

Supramolecular blueprint approach to metal-coordinated capsules

McKinlay

Cave

Atwood

2005

199

182

“…Rebek and colleagues (11)(12)(13)(14) have pioneered the use of self-assembly to produce a variety of entities seamed together by hydrogen bonds. Multicomponent systems also have been assembled by means of transition metal-based coordinate bonds (15)(16)(17)(18)(19)(20). The synthesis of hosts capable of housing a wide variety of guests has further been achieved by the use of the principles of crystal engineering (21,22).…”

mentioning

confidence: 99%

Organization of the interior of molecular capsules by hydrogen bonding

Atwood

Barbour

Jerga

2002

210

The enclosure of functional entities within a protective boundary is an essential feature of biological systems. On a molecular scale, free-standing capsules with an internal volume sufficiently large to house molecular species have been synthesized and studied for more than a decade. These capsules have been prepared by either covalent synthesis or self-assembly, and the internal volumes have ranged from 200 to 1,500 Å 3 . Although biological systems possess a remarkable degree of order within the protective boundaries, to date only steric constraints have been used to order the guests within molecular capsules. In this article we describe the synthesis and characterization of hexameric molecular capsules held together by hydrogen bonding. These capsules possess internal order of the guests brought about by hydrogen bond donors within, but not used by, the framework of the capsule. The basic building blocks of the hexameric capsules are tetrameric macrocycles related to resorcin[4]arenes and pyrogallol[4]arenes. The former contain four 1,3-dihydroxybenzene rings bridged together by -CHR-units, whereas the latter contain four 1,2,3-trihydroxybenzene rings bridged together. We now report the synthesis of related mixed macrocycles, and the main focus is on the macrocycle composed of three 1,2,3-trihydroxybenzene rings and one 1,3-dihydroxybenzene ring bridged together. The mixed macrocycles self-assemble from a mixture of closely related compounds to form the hexameric capsule with internally ordered guests.

“…Specifically, surface-controlled self-assembly is rapidly emerging as a valuable tool for the generation of complex structures directly on solid supports (9-11). Unlike the covalent approach used for carcerands synthesis (12), the thermodynamic control of the process conveys interesting features, among which reversibility (13), selection (14), and self-repairing properties are the most interesting.Cavitand-based coordination cages are receiving increasing attention because of the versatility of cavitand platforms in terms of preorganization and synthetic modularity (15)(16)(17)(18)(19)(20). Strong metal-ligand interactions are necessary to operate cage selfassembly (CSA) in a wide range of solvents, to extend further these self-assembly protocols to solid-liquid and liquid-liquid interfaces.…”

mentioning

confidence: 99%

“…Cavitand-based coordination cages are receiving increasing attention because of the versatility of cavitand platforms in terms of preorganization and synthetic modularity (15)(16)(17)(18)(19)(20). Strong metal-ligand interactions are necessary to operate cage selfassembly (CSA) in a wide range of solvents, to extend further these self-assembly protocols to solid-liquid and liquid-liquid interfaces.…”

mentioning

confidence: 99%

Design and self-assembly of wide and robust coordination cages

Pirondini

Bertolini

Cantadori

et al. 2002

The self-assembly of a new class of coordination cages formed from two tetrapyridyl-substituted cavitands connected through four square-planar palladium or platinum complexes is reported. The shape of the internal cavity resembles an ellipsoid with a calculated volume of 840 Å 3 . The four lateral portals have a diameter of about 6 Å, large enough to allow the fast entrance͞ egress of counterions in solution. The platinum cages 3a,e cannot be disassembled using triethylamine as competitive ligand and they are kinetically stable at room temperature, whereas the palladium cages 3b-d, 3f-h are disassembled and kinetically labile under the same conditions. The different solubility properties of the cage components have allowed the extension of this selfassembly protocol to the liquid-liquid interface. cavitands M etal-directed self-assembly has been widely used to construct three-dimensional structures presenting internal cavities of molecular dimensions, capable of trapping ions and neutral molecules (1, 2). The ability of such container molecules to confine (3) and stabilize (4) their guests makes them particularly attractive for many potential applications ranging from catalysis (5, 6) to nanotechnology (for self-assembly in nanotechnology, see ref. 7; ref. 8). Specifically, surface-controlled self-assembly is rapidly emerging as a valuable tool for the generation of complex structures directly on solid supports (9-11). Unlike the covalent approach used for carcerands synthesis (12), the thermodynamic control of the process conveys interesting features, among which reversibility (13), selection (14), and self-repairing properties are the most interesting.Cavitand-based coordination cages are receiving increasing attention because of the versatility of cavitand platforms in terms of preorganization and synthetic modularity (15)(16)(17)(18)(19)(20). Strong metal-ligand interactions are necessary to operate cage selfassembly (CSA) in a wide range of solvents, to extend further these self-assembly protocols to solid-liquid and liquid-liquid interfaces.For this purpose we designed and synthesized new cavitands bearing pyridines (for references on cages derived from multidentate pyridine ligands, see ref. 21; refs. 22 and 23) instead of nitriles (24) at the upper rim as ligands for the coordination to the metal centers. CSA requires a cavitand with four pyridines preorganized in a diverging spatial orientation, pointing outward (o) the cavity, being designated the oooo isomer (for in͞out isomerism in cavitands, see ref. 25). Materials and MethodsGeneral. All commercial reagents were ACS reagent grade and used as received. All solvents were dried over 3-and 4-Å molecular sieves. Resorcinarenes 1a,b were prepared according to the literature (26). Metal precursors were prepared from the corresponding dichlorobis derivatives following established procedures (27-29). 1 H NMR were recorded on Bruker (Billerica, MA) AC300 (300 MHz) and AMX400 (400 MHz) spectrometers and all chemical shifts (␦) were reported in parts per milli...