STRUCTURE AND CONFORMATION OF PHOTOSYNTHETIC PIGMENTS AND RELATED COMPOUNDS 7. ON THE CONFORMATION OF THE METHYL ESTER OF (20-METHYLPHYTOCHLORINATO)NICKEL(II)-A BAaERIOCHLOROPHYLL c MODEL COMPOUND

Abstract The crystal and molecular structure of the title compound 4 have been determined by single crystal X-ray crystallography. The structure shows the typical S,-ruffled conformation observed for Ni(II) tetrapyrroles. Compared to the structure of the methyl ester of (13²-demethoxycarbonyl-pheophorbidato a)nickel(II), 4 shows a smaller C_a-C_m-C_m angle and a higher degree of conformational distortion at the methyl-substituted C20 position. This local distortion of the macrocycle might account for the bathochromic shifted absorption spectra of the bacteriochlorophylls c compared to the d-series. Crystal data: C₃₅H₃₈N₄NiO₃; tetragonal, P4₃2₁2, a = 15.335(7) Å, c = 25.11(2) Å, V = 5904, Z = 8, λ(Mo Kα) = 0.71069 Å, μ= 0.701 mm⁻¹, P(000) = 2624, 130 K, R = 0.058 for 5700 reflections with F > 4.0σ(F).