Allosteric Cooperativity and Template-Directed Synthesis with Stacked Ligands in Porphyrin Nanorings.

The link between allosteric cooperativity and template-directed synthesis has been investigated by studying complexes in which two oligopyridine ligands bind inside a zinc porphyrin nanoring in a stacked arrangement. The binding of a 6-porphyrin nanoring to two tridentate ligands (with s-triazine or benzene cores) occurs with high negative allosteric cooperativity (α ≈ 10-3-10-4). Formation constants for 1:1 and 1:2 complexes were determined by UV-vis-NIR denaturation titration, using pyridine as a competing ligand, and cooperativity factors were confirmed by NMR spectroscopy. The rate constants for formation of the 1:1 and 1:2 complexes are approximately equal, and the negative cooperativity can be attributed to faster dissociation of the 1:2 complex. These tridentate ligands are not effective templates for directing the synthesis of the 6-porphyrin nanoring, in keeping with their negative cooperativity of binding. In contrast, the binding of a 12-porphyrin nanoring to two hexadentate ligands occurs with high positive allosteric cooperativity (α > 40), and the ligand is an effective Vernier template for directing the synthesis of the 12-porphyrin nanoring. This stacked Vernier template approach creates the product in an open circular conformation, which is advantageous for preparing macrocycles that do not easily adopt a figure-of-eight geometry.

Compressing a Non-Planar Aromatic Heterocyclic [7]Helicene to a Planar Hetero[8]Circulene.

This work describes a synthetic approach where a non-planar aromatic heterocyclic [7]helicene is compressed to yield a hetero[8]circulene containing an inner antiaromatic cyclooctatetraene (COT) core. This [8]circulene consists of four benzene rings and four heterocyclic rings, and it is the first heterocyclic [8]circulene containing three different heteroatoms. The synthetic pathway proceeds via a the flattened dehydro-hetero[7]helicene, which is partially a helicene and partially a circulene: it is non-planar and helically chiral as helicenes, and contains a COT motif like [8]circulenes. The antiaromaticity of the COT core is confirmed by nucleus independent chemical shift (NICS) calculations. The planarization from a helically π-conjugated [7]helicene to a fully planar heterocyclic [8]circulene significantly alters the spectroscopic properties of the molecules. Post-functionalization of the [7]helicenes and the [8]circulenes by oxygenation of the thiophene rings to the corresponding thiophene-sulfones allows an almost complete fluorescence emission coverage of the visible region of the optical spectrum (400-700 nm).

Cooperative assembly of H-bonded rosettes inside a porphyrin nanoring.

The melamine·barbiturate H-bonded rosette motif is of comparable dimensions and symmetry to the cavity of a butadiyne-linked 6-porphyrin nanoring. Functionalisation of each of the barbiturate components and the pyrimidine components of a H-bonded rosette with a pyridine ligand leads to a self-assembled hexapyridine ligand, which binds cooperatively to the zinc porphyrin nanoring. UV-vis-NIR and 1H NMR experiments show that the 7-component assembly forms at concentrations at which neither the H-bonding interactions nor the zinc porphyrin-pyridine interactions are formed in the absence of one of the three components. The mean effective molarities of these rosette complexes are around 200 mM in chloroform at 298 K.

Template-Directed Synthesis of Molecular Nanorings and Cages.

This Account is about templates as construction tools: molecules for making molecules. A template organizes the reactants and provides information to promote formation of a specific product, but it is not part of the final product. We have developed many different strategies for using oligopyridines as templates for the synthesis of alkyne-linked π-conjugated metalloporphyrin oligomers. These compounds include some of the largest macrocycles ever synthesized, such as a 50-porphyrin ring with a diameter of 21 nm containing a ring of 750 C-C bonds. Metalloporphyrins are excellent models for exploring template directed synthesis, as they can be functionalized in many different positions and the central metal (typically Zn or Mg) provides a handle for coordination to templates. Classical template-directed macrocyclization reactions have a 1:1 complementarity between the template and the product. This strategy works well for preparing nanorings of 5-7 porphyrin units, but larger templates are laborious to synthesize. Rings of 8 or more porphyrin units are most easily prepared using "nonclassical" strategies, in which several small templates work together to direct the formation of a large ring. In the Vernier approach, a mismatch between the number of binding sites on the template and the building block leads to a mathematical amplification of the length scale: the number of binding sites in the product is the lowest common multiple of those in the template and the building block. For example, a 40-porphyrin ring can be prepared by coupling a linear decamer in the presence of an octadentate template. Linear Vernier templating opens up intriguing possibilities for self-replication. When several small radial oligopyridine templates bind inside a large nanoring they can form complexes with some vacant coordination sites that display correlated motion like the caterpillar tracks of a bulldozer. These caterpillar track complexes can be used in template-directed synthesis and they provide the most convenient route to 8- and 10-porphyrin rings. Russian doll complexes provide another strategy for template-directed synthesis: a number of specifically designed ligands bind to a central nanoring to form a template for constructing a larger concentric nanoring. The same oligopyridine templates that are used to prepare nanorings can also be used to synthesize three-dimensional nanotubes and nanoballs. Again, nonclassical approaches, in which several small templates work together cooperatively, are much simpler than creating a single large template with sufficient binding sites to define the whole geometry of the product. Oligopyridine ligands can also be used as shadow mask templates to control the demetalation of magnesium porphyrin nanorings, because metal centers that are not coordinated by the template can be selectively demetalated with acid. Thus, the template forms a permanent shadow on the porphyrin nanostructure that remains after the template has been removed. Shadow mask templates provide a simple route to heterometalated molecular architectures. The insights emerging from these studies are widely applicable, and there are many opportunities for inventing new ways of using templates to control reactions.

Shadow Mask Templates for Site-Selective Metal Exchange in Magnesium Porphyrin Nanorings.

Molecular templates can be used in many different ways to control the outcome of chemical reactions. Herein, we present a new type of template-directed synthesis. We show that templates can be used as shadow masks: The shape of the template becomes imprinted on the product because reactions only occur at sites not masked by the template. We demonstrate this effect by using oligopyridine templates to dictate the sites of demetalation when a magnesium porphyrin nanoring is treated with acid. Magnesium centers that are coordinated to the template are protected whereas uncoordinated magnesium centers are removed. After site-selective demetalation, the template can be removed, and other cations, such as zinc(II) and copper(II), can be inserted into the free-base porphyrin centers. This strategy provides a simple route to a wide range of heterometalated porphyrin arrays.

ß-Mannosylation with 4,6-benzylidene protected mannosyl donors without preactivation.

Mannosylations with benzylidene protected mannosyl donors were found to be ß-selective even when no preactivation was performed. It was also found that the kinetic ß-product in some cases anomerizes fast to the thermodynamically favored α-anomer under typical reaction conditions.