Selection and Amplification of Mixed‐Metal Porphyrin Cages from Dynamic Combinatorial Libraries

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Abstract
Mixed metallo–porphyrin cages were selected and amplified from dynamic combinatorial libraries (DCLs) by using appropriate templates. The cages are composed of two bisphosphine substituted zinc(II) porphyrins as ligand donors and two rhodium(III) or ruthenium(II) porphyrins as ligand acceptors, and are connected through metal–phosphorus coordination. Ru and Rh porphyrins that display a large structural diversity were employed. The templating was achieved by using 4,4′‐bpy, 3,3′‐dimethyl‐4,4′‐bipyridine and benzo[lmn]‐3,8‐phenanthroline, and acts through zinc–nitrogen coordination. The absolute amount of amplification from the DCLs is strongly dependent on the combination of the Ru/Rh porphyrin and the template; cages with sterically demanding porphyrins can only form with smaller templates. In the case of tert‐butyl‐substituted TPP (TPP=tetraphenylporphyrin), cages are not formed at all. The formation of the cages is usually complete within 24 h at an ambient temperature; in the case of the cage containing RhIIIOEP (OEP=octaethylporphyrin) and bpy, the pseudo‐first‐order rate constant of cage formation was determined to be 2.1±0.1×10−4 s−1 (CDCl3, 25 °C). Alternatively, heating the mixtures to 65 °C and cooling to room temperature yields the cages within minutes. The 1H NMR chemical shifts of several characteristic protons show large differences upon changing the identity of the Ru/Rh porphyrin and the central metal; this is most likely to arise from variations in the geometry of the cages. The X‐ray crystal structure of a cage, which contains RhIIIOEP as a porphyrin acceptor and bpy as template, demonstrates that the cages can adopt severely distorted conformations to accommodate the relatively short templates. An extension to mixed DCLs showed that only limited selectivity is displayed by the various templates. Formation of mixed cages that contain two different rhodium porphyrins prevents effective selection, although the kinetic lability of the systems allows for some amplification. This lability, however, also prevents isolation of the individual cages. Removal of the template leads to re‐equilibration, thus the templates act as scaffolds to keep the structures intact.