Synthesis and crystal structures of cofacial bischlorin. Octaethylchlorin-based structural models for the special pair in photosynthesis

Mathias O. Senge, Werner W. Kalisch, Karin Ruhlandt-Senge
1996 Chemical Communications  
McMurry coupling of metalloformyloctaethylchlorins yields the first cofacial bischlorin with structural characteristics mimicking those of the special pair in photosynthesis. Owing to their structural similarity to the 'special pair' in the photosynthetic reaction centre, cofacial chlorin dimers serve as biomimetic models to investigate the primary steps of photosynthesis, charge separation and electron transfer. In order to fully understand the photophysical behaviour of such compounds,
more » ... h compounds, detailed information on their conformation and the relative spatial arrangement of the subunits is necessary; ideally the geometry of such dimers should be as close as possible to the situation in vivo, i.e. cofacial structure with parallel ring orientation and partial overlap of the n systems. Several bistetrapyrrole systems showing such features have been described,24 however, detailed information is available only for cofacial bisporphyrins while most bischlorin dimers exhibit an extended linear conformation.~--7 Here we report on the synthesis and first crystal structures of cofacial bischlorins based on octaethylchlorin (OEC). A versatile synthesis of bisporphyrins through McMurry coupling8 of formylporphyrins was developed by Smith and coworkers.4~7.9 The expected product of the reaction was the trans ethene-linked bisporphyrin but subsequent studies showed that this method gives convenient access to cofacial bisporphyrins since intramolecular n-n-aggregation leads to the predominant formation of the cis ethene product.4 However, la M = Ni / steric demand plays an important role as shown by the coupling of formylphytochlorins which led only to the formation of trans ethene-linked bis~hlorins.~ This prompted us to investigate the possibility if structural and functional chlorin-based models of the special pair are accessible via a similar route using transoctaethylchlorin (OEC) as starting material. The McMurry products of metallo 5-formyloctaethylchlorinssa prepared via Vilsmeier formylation of the respective metallo octaethylchlorins were expected to be quite different compared to the corresponding porphyrins since sp3 hybridization in the reduced pyrrole ring makes the chlorin subunits more sterically demanding. Nevertheless, low-valent titaniuminduced coupling of l a yielded three green fractions in a combined yield of 90%. The compounds were filtered through ALOX, evaporated under high vacuum and purified by column chromatography on neutral Aluminia Brockmann grade 111 using hexane-1 % thf as eluent. Spectroscopic and crystallographic analyses revealed the products to be a single cis ethene bischlorin 2t (Fig. 1) [ A , , , (CH2C12) 41 1,463,665 nm] and two different atropisomers of trans ethene bis-OEC 3 [ A, , , (CH2C12) 412,459,654 nm] and 4 [ A, , , (CH2C12) 412,464,661 nm]. Compound 2 clearly shows a cis-ethene linkage and the cofacial arrangement of the two macrocycles, which are tilted against each other by 22.4'. The two chlorin rings show an ipterplanar separation of 3.900 A, an Ni-..Ni separation of 6.15 1 A and a lateral shift of the metal centres of 4.77 A (parameters were calculated using the method given by Scheidt and Lee).lo b M = C u 4 5 Chem. Commun., 1996 2149
doi:10.1039/cc9960002149 fatcat:hrnuf7b4gjcxll6dke73kcnfm4