Quinone-capped porphyrins: n.m.r. studies of static and dynamic stereochemistry and co-ordination properties
1 H N.m.r, spectroscopy has been used to investigate the shape, motion, and flexibility of two of the title compounds, and their magnesium derivatives, and to probe the co-ordinative properties of the latter. Coupling constant and n.O.e. results for the porphyrin moiety are analysed in terms of two possible conformations. The cap moiety for simple alkyl-capped porphyries appears to have a well defined shape. In the title compounds, however, the quinone ring is flipping rapidly and a temperature dependent ‘breathing’ is observed; the quinone is closer on average to the porphyrin in the compound with additional methylene groups in the cap. At ambient temperature molecular tumbling is severely hindered in these propellor-shaped molecules leading to rapid relaxation and broad lines, but resolution improves substantially on warming. Pyridine titration data for the magnesium derivatives indicate that the metal ion is out-of-plane 5-co-ordinate, bound intramolecularly to a quinone carbonyl, and that it can accept a 6th ligand from the unhindered side of the porphyrin, moving into the plane and pulling the quinone towards the porphyrin. The average orientation of the quinone ring appears to be perpendicular to the porphyrin.