Peter D. Frischmann

Capsule formation, carboxylate exchange, and DFT exploration of cadmium cluster metallocavitands: highly dynamic supramolecules.

A family of molecular heptacadmium carboxylate clusters templated inside [3 + 3] Schiff base macrocycles has been isolated and studied by variable temperature solution and solid-state NMR spectroscopy, single-crystal X-ray diffraction (SCXRD), and density functional theory (DFT) calculations. These metallocavitand cluster complexes adopt bowl-shaped structures, induced by metal coordination, giving rise to interesting host-guest and supramolecular phenomena. Specifically, dimerization of these metallocavitands yields capsules with vacant coordination and hydrogen-bonding sites accessible to encapsulated guests. Strong host-guest interactions explain the exceptionally high packing coefficient (0.80) observed for encapsulated N,N-dimethylformamide (DMF). The guest-accessible hydrogen-bonding sites arise from an unusual mu(3)-OH ligand bridging three cadmium ions. Thermodynamic and kinetic studies show that dimerization is an entropy-driven process with a highly associative mechanism. In DMF the exchange rate of peripheral cluster supporting carboxylate ligands is intrinsically linked to the rate of dimerization and these two seemingly different events have a common rate-determining step. Investigation of guest dynamics with solid-state (2)H NMR spectroscopy revealed 3-fold rotation of an encapsulated DMF molecule. These studies provide a solid understanding of the host-guest and dynamic properties of a new family of metallocavitands and may help in designing new supramolecular catalysts and materials.

Reversible−Irreversible Approach to Schiff Base Macrocycles: Access to Isomeric Macrocycles with Multiple Salphen Pockets

We have developed methodology for the formation of a new family of metal-free Schiff base macrocycles utilizing the differential exchange rates of aldimines and ketimines. The more robust ketimine bond is kinetically inert under the milder conditions used for aldimine bond formation. In particular, this route enables access to the first conjugated macrocycles with four unsymmetrical N2O2 salphen-like pockets.

Columnar organization of head-to-tail self-assembled Pt4 rings.

Coordination of Pt(2+) to a family of tunable Schiff base proligands directs the 12-component self-assembly of disk-shaped Pt(4) rings in a head-to-tail fashion. Aggregation of these S(4) symmetric Pt(4) macrocycles into columnar architectures was investigated by dynamic and static light scattering, NMR spectroscopy, powder X-ray diffraction, and transmission electron microscopy. Data from these experiments support the formation of columnar architectures for all of the structures studied except when bulky tris(4-tert-butylphenyl)methyl substituents were present. In this case, aggregation was limited to dimers in CHCl(3) (K(dim) = 3200 +/- 200 L mol(-1) at 25 degrees C) and a thermodynamic analysis revealed that dimerization is an entropy driven process. Columnar architectures of Pt(4) rings with branched 2-hexyldecyl substituents organize into lyotropic mesophases in nonpolar organic solvents. These new self-assembled supramolecules are promising candidates to access nanotubes with multiple linear arrays of Pt(2+) ions.

Bright Fluorescence and Host–Guest Sensing with a Nanoscale M4L6 Tetrahedron Accessed by Self-Assembly of Zinc–Imine Chelate Vertices and Perylene Bisimide Edges†

A highly luminescent Zn4L6 tetrahedron is reported with 3.8 nm perylene bisimide edges and hexadentate Zn(II)-imine chelate vertices. Replacing Fe(II) and monoamines commonly utilized in subcomponent self-assembly with Zn(II) and tris(2-aminoethyl)amine provides access to a metallosupramolecular host with the rare combination of structural integrity at concentrations <10(-7) mol L(-1) and an exceptionally high fluorescence quantum yield of Φ(em) =0.67. Encapsulation of multiple perylene or coronene guest molecules is accompanied by strong luminescence quenching. We anticipate this self-assembly strategy may be generalized to improve access to brightly fluorescent coordination cages tailored for host-guest light-harvesting, photocatalysis, and sensing.

Subcomponent Self‐Assembly of a 4 nm M4L6 Tetrahedron with ZnII Vertices and Perylene Bisimide Dye Edges

Formation of a tetrahedron with >4 nm perylene bisimide (PBI) dye edges and Zn(II) vertices in a one-pot 22 component self-assembly reaction is reported. The luminescent polyhedron equilibrates to a Zn2 L3 helicate and disassembles upon dilution. Insights into the subcomponent self-assembly of extended PBI ligands help to refine design rules for constructing large photofunctional metallosupramolecular hosts.

Spontaneous hierarchical assembly of crown ether-like macrocycles into nanofibers and microfibers induced by alkali-metal and ammonium salts.

Schiff base macrocycle 1, which has a crown ether like central pore, was combined with different alkali-metal and ammonium salts in chloroform, resulting in one-dimensional supramolecular aggregates. The ion-induced self-assembly was studied with solid-state NMR spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and atomic force microscopy (AFM). It was found that the lengths and widths of the superstructures depend on the cation and counteranion of the salts. Among the salts being used, Na(+) and NH(4) (+) ions with BF(4) (-) ions showed the most impressive fibrous structures that can grow up to 1 mum in diameter and hundreds of microns in length. In addition, the size of the fibers can be controlled by the evaporation rate of the solvent. A new macrocycle with bulky triptycenyl substituents that prevent supramolecular assembly was prepared and did not display any nanofibers with alkali-metal ions in chloroform when studied with TEM.

SCHIFF BASE MACROCYCLES: RELIABLE TEMPLATES FOR MULTINUCLEAR METALLOCAVITANDS

Recent studies of polydentate Schiff base macrocycles indicate larger diameter macrocycles often deviate from planarity upon metal complexation, taking on a concave conformation. These bowl shaped complexes are reminiscent of organic cavitands, however, their dependence on metal coordination rather than C-C bonds for curvature has led us to dub them metallocavitands. In particular, we have prepared a family of soluble [3 + 3] Schiff base macrocycles from the condensation of 3,6-diformylcatechol with 1,2-dialkoxy-4,5-diaminobenzenes and investigated their ability to template the formation of metallocavitands. When reacted with seven equivalents of Zn(OAc)2 or Cd(OAc)2, hepta-nuclear metallocavitands are formed. Dimerization of these complexes into capsules has been observed and the thermodynamics of self-association has been studied. Tuning the cavity dimensions and strength of supramolecular self-association is easily achieved by switching metals. Isolation of a tetra-zinc intermediate has provided insight into the template effect our macrocycles exhibit. These results highlight the reliability of Schiff base macrocycles as scaffolds for metallocavitand formation.

Role of Entropy and Autosolvation in Dimerization and Complexation of C60 by Zn7 Metallocavitands

The supramolecular chemistry of bowl-shaped heptazinc metallocavitands templated by Schiff base macrocycles has been investigated. Dimerization thermodynamics were probed by (1)H NMR spectroscopy in benzene-d(6), toluene-d(8), and p-xylene-d(10) and revealed the process to be entropy-driven and enthalpy-opposed in each solvent. Trends in the experimentally determined enthalpy and entropy values are related to the thermodynamics of solvent autosolvation, solvent molecules being released from the monomeric metallocavitand cavity into the bulk solvent upon dimerization. The relationship established between experimentally measured dimerization thermodynamics and autosolvation data successfully predicts the absence of dimerization in CH(2)Cl(2) and CHCl(3) and was used to estimate the number of solvent molecules interacting with the monomeric metallocavitand in solution. Host-guest interactions between heptazinc metallocavitands and fullerene C(60) have also been investigated. Interestingly, metallocavitand-C(60) interactions are only observed in solvents that facilitate entropy-driven dimerization suggesting entropy and solvent autosolvation may be important in explaining concave-convex interactions.

Family of cadmium acetate coordination networks with structurally diverse [Cd4(OAc)9(μ3-OH)]2- secondary building units.

By subtly varying crystallization conditions, four distinct cadmium acetate coordination networks with unit cell formulas Cd(87)(H(2)O)(36)(EtOH)(18)(OH)(12)(CH(3)CO(2))(162) (1), Cd(87)(H(2)O)(72)(OH)(12)(CH(3)CO(2))(162) (2), Cd(10)(H(2)O)(6)(OH)(2)(CH(3)CO(2))(18) (3), and Cd(20)(H(2)O)(20)(OH)(4)(CH(3)CO(2))(36) (4) have been isolated. The coordination networks exhibit interesting structural diversity and have been investigated by powder X-ray diffraction, elemental analysis, thermal gravimetric analysis, infrared spectroscopy, and single-crystal X-ray diffraction. All four complexes are composed of secondary building units with the general formula [Cd(4)(OAc)(9)(μ(3)-OH)](2-). Complexes 1 and 2 exhibit a remarkable three-dimensional network composed of aligned columns, each 4.5 nm long, containing three different cadmium acetate clusters. Complexes 3 and 4 extend in two-dimensions with each unit cell repeating a different linkage isomer of the [Cd(4)(OAc)(9)(μ(3)-OH)](2-) cluster.

Sterically‐Limited Self‐Assembly of Pt4 Macrocycles into Discrete Non‐covalent Nanotubes: Porous Supramolecular Tetramers and Hexamers

We report a template-free strategy based on steric repulsion for the isolation of discrete columnar aggregates of macrocycles. Specifically, introduction of sterically-demanding trityl-derived substituents at the periphery of Pt(4) Schiff base macrocycles limits the otherwise infinite one-dimensional columnar aggregation to discrete tetrameric and hexameric assemblies. Single crystal X-ray diffraction studies of these compounds reveal discrete nanotubes of finite length that pack inefficiently resulting in three-dimensional networks of interconnected void space. The discrete assemblies were studied by N(2) adsorption and show enhanced surface area when stacked. In the absence of bulky substituents the macrocycles are nonporous. This strategy for engineering discrete supramolecular macrocyclic aggregates may be generalized to other columnar assembling systems.