Structural studies of sparsely substituted synthetic chlorins and phorbines establish benchmarks for changes in the ligand core and framework of chlorophyll macrocycles

Understanding the changes in molecular structure of tetrapyrrole macrocycles upon derivatization of the organic framework is essential for diverse studies ranging from metal complexation to formation of supramolecular assemblies. New, sparsely substituted free base chlorin, 17-oxochlorin, phorbine and 13¹-oxophorbine macrocycles provide benchmarks for naturally occurring hydroporphyrins and have been examined here by X-ray crystallography, resonance Raman spectroscopy, and density functional theoretical (DFT) calculations. The macrocycles contain no substituents other than a geminal-dimethyl group attached to the reduced, pyrroline ring. The X-ray studies indicate that the benchmark compounds exhibit only slight distortion from planarity, which increases along the series porphine < chlorin < oxochlorin < phorbine < oxophorbine. The elongated C_β- C_β bond distance due to sp³ versus sp² hybridization in the pyrroline ring (ring D) of the (oxo)chlorins and (oxo)phorbines (1.52-1.54 Å) versus that of porphine (1.35 Å) is accompanied by altered bond angles in ring D. Introduction of ring E (exocyclic ring) in a chlorin to give the phorbine or oxophorbine causes alteration of the bond angles at many sites in the framework of the macrocycle; for example, the bond angles of N3-C14-C15 in the (oxo)phorbine are widened by ∼11° compared to those of porphine or the analogous sparsely substituted chlorin. As a result, the shape of the macrocycle core changes along the series of porphine (nearly square), (oxo)chlorin (kite-shaped), and (oxo)phorbine (right-angled trapezoid), and the core size increases in the order porphine < phorbine ∼ oxophorbine < oxochlorin ∼ chlorin. Comparison of the bond distances and angles in ring E of the phorbine versus oxophorbine indicates that the shortening of the C13-C13¹ bond owing to the presence of the oxo-group is quite small, only 0.024 Å; thus, the unsymmetrical structure of ring E does not appear to be due to conjugation with the C13¹=O group but may be a characteristic feature of the (oxo)phorbine framework. The X-ray data further indicate that the lengths of the oxochlorin C17=O and oxophorbine C13¹=O groups are essentially identical, a result also predicted by DFT calculations. Regardless, the observed frequencies for the stretching vibrations of the C17=O (1721 cm^-1) and C13¹=O (1701 cm^-1) groups are different and suggest that conjugation of the latter group with the π-system of the macrocycle is greater than that of the former group. Collectively, the studies provide new insights into the individual factors that give rise to the overall structural characteristics of various macrocycles.