Models for the Photosynthetic Reaction Center: Preparation, Spectroscopy, and Crystal and Molecular Structures of Cofacial Bisporphyrins Linked by cis-1,2- and trans-1,2-Ethene Bridges and of 1,1-Carbinol-Bridged Bisporphyrins
Abstract
The low-valent titanium induced reductive coupling of metal(II) 5-formyl-octaethylporphyrin (OEP) (2, 3) leads to the formation of ethene-bridged bisporphyrins. Structural studies on the main products of the coupling of Cu(II) (2) and Ni(II) 5-formyl-OEP (3) revealed, surprisingly, the formation of the cis-isomers 1,2-bis[5-(2,3,7,8,12,13,17,18-octaethylporphyrinato)metal(II)]-cis-ethene (4, 5). This is in contrast to other reported reductive couplings of aldehydes and ketone which usually lead to the trans-product. The cis-isomer can undergo an acid-catalyzed isomerization to the trans-isomer (6, 7) which is also formed as a side product in the coupling reaction. An explanation for the formation of the cis-ethene-bridged bisporphyrins may involve the strong aggregation tendencies of metalloporphyrins. These aggregation forces are evidenced in the crystal structures of the dsdimers which exhibit considerable overlap of the π-systems and have a cofacial macrocycle arrangement with almost coplanar macrocycles. Besides the strong intramolecular aggregation effects, intermolecular π-stacking was observed in the structures of the Cu(II) (4) and Ni(II) (5) cis-dimers, the nickel(II) trans-isomer (7) and the related structure of bis[2-(3,7,8,12,13,17,18-heptaethylpophyrinato)nickel(II)]hydroxymethane (13) which is obtained by low valent titanium coupling of Ni(II) 2-formyl-OEP (12). The synthesis of the cofacial cis-dimers is achieved in a single step from the corresponding formylporphyrins in high yields, thus presenting an improvement over other published procedures for synthesis of face-to-face bisporphyrins. Crystal data: 4, Mo Kα (λ = 0.710 69 Å) at 130 K, a = 19.208(7) Å, b= 14.672(5) Å, c = 24.170(7) Å, β = 111.15(3)°, V= 6352(4) Å3 Z = 4, space group P21/n, R = 0.056; 5, Mo Kα (β= 0.710 69 Å) at 130 K, a = 21.671(7) Å, b= 14.079(5) Å, c = 22.110(7) Å β= 108.86(2)°, V = 6384(3) Å3, Z = 4, space group P21/c, R = 0.064; 7, Cu Kα (λ = 1.541 78 Å) at 126 K, a = 11.163(3) Å, b = 12.775(4) Å, c = 14.554(3) Å, α= 69.59(2)°, β = 81.15(2)°, γ= 73.21(2)°, V = 1859(1) Å3, Z = 1, space group P1, R = 0.072; 13, Mo Kα (λ= 0.710 69 Å) at 130 K, a = 40.31(2) Å, b = 14.997(7) Å, c = 21.954(11) Å, β = 108.6(4)°, V = 12579(10) Å3, Z = 8, space group C2/c, R = 0.122. © 1994, American Chemical Society. All rights reserved.