Osmium ester-hinged chlorin dimers

Abstract

A bis(dihydroxychlorin) osmate ester is formed as a side product in the osmylation of meso-tetraphenylporphyrin. The product can also be formed in good yield along well-known reaction pathways of osmate esters by treatment of the monomeric chlorin diol osmate ester bispyridine complex with acid. The dimer was spectroscopically characterized. The free base dimer can be metalated with zinc(II). The bis([dihydroxychlorinato]zinc) osmate ester dimer was spectroscopically and structurally characterized, showing its clamshell architecture with a 42[Formula: see text] angle between the mean planes of the two metallochlorin chromophores and a 9.11 Å zinc-to-zinc separation. Upon reaction with dimethylaminopyridine (DMAP), the clamshell structure pivots around the osmium center and arranges the two chromophores, each zinc center coordinated to DMAP, in maximum separation from each other, as shown by structural analysis. Preliminary 1H NMR titration data are presented that show the bis-zinc clamshell structure to form a 1:1 supramolecular complex with diethylenetriamine (DETA), expressing a three-point recognition motive: the terminal amine functionalities coordinate to the zinc ions in each half-shell, thus bridging both chromophores, and the central amine functionality was deduced to be H-bonded to the osmium oxo group pointing toward the inside of the host. The UV-vis spectroscopic response upon DETA binding shows greater complexity in the interaction at higher ligand:host ratios, suggesting multiple binding modes. A similarly complex binding situation is also observed for the interactions of the chlorin osmate ester monomer with DETA.