Highly Rigid, Yet Conformationally Adaptable, Bisporphyrin sp2-Cage Receptors Afford Outstanding Binding Affinities, Chelate Cooperativities, and Substrate Selectivities

If we aim to develop efficient synthetic models of protein receptors and enzymes, we must understand the relationships of intra- and intermolecular interactions between hosts and guests and how they mutually influence their conformational energy landscape so as to adapt to each other to maximize binding energies and enhance substrate selectivities. Here, we introduce a novel design of cofacial (ZnII)bisporphyrin cages based on dynamic imine bonding, which is synthetically simple, but at the same time highly robust and versatile, affording receptors composed of only sp2-hybridized C and N atoms. The high structural rigidity of these cages renders them ideal hosts for ditopic molecules that can fit into the cavity and bind to both metal centers, leading to association constants as high as 109 M-1 in chloroform. These strong binding affinities are a consequence of the remarkable chelate cooperativities attained, with effective molarity (EM) values reaching record values over 103 M. However, we discovered that the cages can still adapt their structure to a more compact version, able to host slightly smaller guests. Such a conformational transition has an energy cost, which can be very different depending on the direction of the imine linkages in the cage skeleton and which results in EM values 2-3 orders of magnitude lower. This interplay between cooperativity and conformational adaptability leads to strong and unusual selectivities. Not only these metalloporphyrin receptors can choose to bind preferably to a particular guest, as a function of its size, but also the guest can select which host to bind, as a function now of the host's conformational rigidity. Such highly cooperative and selective associations are lost, however, in related flexible receptors where the imine bonds are reduced.

Influence of solvent and axial coordination on self-assembly of a heteroditopic porphyrin derivative

A novel heteroditopic porphyrin and its zinc complex with four long aliphatic chains on the same side of the porphyrin ring were synthesized and used for controllable self-assembly. A variety of aggregation morphologies, including nanosheets, nanospheres, films, leaves, trunks, nanorods, and disks, were furnished by using different pyridyl ligands to coordinate with the Zn-porphyrin or selecting different solvents.

Synthesis, characterization and modeling of self-assembled porphyrin nanorods

Porphyrin nanorods were prepared by ion-association between free-base meso 5,10,15,20-tetrakis-(4-[Formula: see text]-methylpyridinium)porphyrin cations and tetraphenylborate anions. The nanorods have variable lengths (up to a few micrometers long) and diameters ([Formula: see text]50–500 nm). Their structure at the molecular level was elucidated by combining multinuclear solid state NMR spectroscopy, synchrotron X-ray powder diffraction and DFT calculations.

Metalloligand Strategies for Assembling Heteronuclear Nanocages – Recent Developments

The use of metalloligands as building blocks for the assembly of metallo-organic cages has received increasing attention over the past two decades or so. In part, the popularity of this approach reflects its stepwise nature that lends itself to the predesigned construction of metallocages and especially heteronuclear metallocages. The focus of the present discussion is on the use of metalloligands for the construction of discrete polyhedral cages, very often incorporating heterometal ions as structural elements. The metalloligand approach uses metal-bound multifunctional ligand building blocks that display predesigned structural properties for coordination to a second metal ion such that the rational design and construction of both homo- and heteronuclear metal–organic cages are facilitated. The present review covers published literature in the area from early 2015 to early 2019.