Synthesis and Crystal Structures of Cofacial Bisoctaethylchlorins as Structural Models for the Special Pair

Covalently linked bistetrapyrrole systems can be used to mimic structurally the arrangement of the two special pair bacteriochlorophylls in the bacterial photosynthetic reaction center. Such models require a cofacial arrangement of the two tetrapyrrole units with some overlap of the π systems and intramolecular stabilization by aggregation of the two macrocycles. Using the McMurry reaction for the coupling of metallo formyloctaethylchlorins, covalently linked cofacial bischlorins were prepared and characterized by spectroscopy and single crystal X-ray crystallography. Coupling of Ni(II)5-formyloctaethylchlorin yielded a (Z)-ethene-bridged bischlorin with cofacial orientation of the subunits stabilized by intramolecular π-π interactions that structurally resembles the situation found for the special pair in bacterial reaction centers. In addition, two different atropisomers of the respective (E)-ethene-bridged bischlorin were formed. Use of Cu(II)5-formyloctaethylchlorin yielded similar products, however a crystal structure showed a (Z)-ethene-bridged bischlorin without intramolecular stabilization via aggregation effects. The bischlorins are easily oxidized to mixed chlorin-porphyrin species.