Highly Rigid, Yet Conformationally Adaptable, Bisporphyrin <i>sp</i> ² -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 (Zn^II)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 <i>sp</i>²-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 (<i>EM</i>) values reaching record values over 10³ M. However, we discovered that the cages can still adapt their structure to a more <i>compact</i> 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 <i>EM</i> 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.