STRUCTURE and CONFORMATION OF PHOTOSYNTHETIC PIGMENTS and RELATED COMPOUNDS. 2. NICKEL (II) METHYL PYROPHEOPHORBIDE a–A SEVERELY ISTORTED CHLOROPHYLL DERIVATIVE*

The crystal structure of Ni(II) methyl pryopheophorbide a (3) (C_MH₃₄N₄Ni0₃, M_u= 605.4) was determined by x-ray diffraction methods in order to determine the influence of the nickel-ion on the ring conformation. The crystals were blue octahedrons and crystallized in the tetragonal space group P4₁2₁,2 with a= 14.727(4) Å, c = 26.666(12) Å, V= 5781(3) ųZ= 8, D= 1.391 mg/m³. (Mo Kα) = 0.71069 Åμ= 0.713 mm^?1, F(000) = 2544, 130 K. The final R value was 0.064 for 4096 observed reflections, wR = 0.065. The molecule is severely distorted and shows a saddle-shaped ring conformation, the β-pyrrole atoms being displaced up to 0.6 A out of the plane of the nitrogens. The severe deformation restricts the use of Ni-derivatives in spectroscopic model studies on chlorophylls and provides an example for the conformational flexibility of the phorbin macrocycle.