CONFORMATIONAL LANDSCAPES OF NONPLANAR PORPHYRINS - SUPERSTRUCTURE, LIGATION, BINDING POCKETS AND OXIDATION EFFECTS IN CU(II) PORPHYRINS

The crystal structure of the porphyrin pi cation radical Cu(II)OETPP()pyl(3)(-). py is reported (OETPP = 3,17,18-octaethyl-5,10,15,20-tetra phenylporphyrin; py = pyridine). The compound is readily prepared by o xidation of Cu(II)OETPP in CH,CI, with excess I, in the presence of py . The structure exhibits the following features: (1) The porphyrin is oxidized, not the Cu(II). (2) The Cu(II) is pentacoordinated with one py as axial ligand. (3) The sterically crowded macrocycle is saddle sh aped. This distortion and the peripheral substituents combine to form superstructural, orthogonal 'trenches' above and below the porphyrin p lane. (4) The trenches impose a specific orientation on the ligated ax ial py along the N-Cu-N axis of the porphyrin and also form a 'distal' binding pocket that traps a second molecule of py. (5) The Cu(II)OETP P+ py radical is diamagnetic because of antiferromagnetic coupling bet ween the unpaired electrons of the Cu d(x2-y2) and the porphyrin pi or bitals which is favored by the nonplanarity of the macrocycle. (6) The radical is more distorted than its nonplanar parent and provides addi tional support for the thesis that nonplanar porphyrins can easily und ergo additional structural excursions. Conformational changes of the t ype reported here may also influence the reactivities of the rr cation radicals formed in the (photo)catalytic cycles of photosynthetic and hemoproteins also comprised of nonplanar porphyrins. Crystallographic data Cc (No. 9), a=21.706(2) Angstrom, b=16.077(2)Angstrom, c=19.993(2 ) Angstrom, beta=103.34(1)degrees, V=6789(1) Angstrom(3), Z=4. (C) 199 7 Elsevier Science S.A.