Consequences of Oxidation in Nonplanar Porphyrins: Molecular Structure and Diamagnetism of the π Cation Radical of Copper(II) Octaethyltetraphenylporphyrin
Crystal structures are reported for the sterically crowded porphyrin Copper(II) 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (Cu(OETPP), 1) and its π cation radical Cu(OETPP).+ClO4- (2). 1 was chosen to assess the consequences of oxidation in a nonplanar porphyrin on the expectation that its multiple peripheral substituents not only induce an S4 saddle conformation on the macrocycle but should also prevent the dimerizations in the solid that have complicated several previous crystallographic studies of porphyrin π cation radicals. Interest in the consequences of oxidation arises from the presence of nonplanar bacteriochlorophylls in photosynthetic reaction centers in which the chromophores lie in van der Waals contact so that even small structural changes induced by electron transfer would alter the electronic coupling between the π cation and anion radicals generated by the primary photochemical charge separation. Oxidation of 1 does indeed result in further conformational changes in 2: an additional ruffling is imposed on the original saddle shape of 1 in which the pyrrole rings twist, the meso carbons move alternately up and down out of the porphyrin plane by ~0.2 Å, and the phenyl groups rotate further into that plane by more than 10°. The additional distortions are attributed to changes in electronic configuration due to the oxidation and to the low-energy barriers between nonplanar conformers with different degrees of nonplanarity predicted by previous molecular mechanics calculations. OETPPs retain their saddle conformations in solution because of the steric crowding of the multiple substituents. 2 thus provides a test of the proposal by Reed, Scheidt, and co-workers (e.g., J. Am. Chem. Soc. 1987, 109, 2644) that the conformations of porphyrin π cation radicals comprised of paramagnetic metals control magnetic coupling with the metals and that nonplanar macrocycles exhibit antiferromagnetic coupling. 2 displays optical and FT-IR spectral signatures diagnostic of a π cation radical in solution, but it is EPR-silent, and its NMR spectrum clearly indicates a diamagnetic species. The Cu(II) and the nonplanar x radical spins in 2 are thus indeed antiferromagnetically coupled, in accord with the above proposal. The saddle conformation of 1 destabilizes the π system of the macrocycle and causes the molecule to be readily oxidized with molecular iodine, a mild oxidizing agent. Unexpectedly, the radical crystallized in the presence of excess I2 carries a discrete I7- counterion. We report here the first example and structure of such a large polyiodide anion to be stabilized by a porphyrin cation: Cu(0ETPP),+I7- (3). Crystallographic data. CuN4C60H60 (1): triclinic space group PI, a = 13.888(4) Å, b = 16.820(3) Å, c = 13.222(3) Å, α = 97.33(2)°, β = 107.97(2)°, γ = 103.52(2)°, V= 2843.3 Å3, Z = 2, RF = 0.057 and RwF = 0.086 based on 7391 reflections with F0 > 3σF0, T = 298 K. CuN4C60H60.+ClO4--CH2Cl2 (2): monoclinic space group P21/n, a = 11.896(1) Å, b = 24.242(5) Å, c = 19.090(3) Å, β = 100.73(1)°, V = 5408.8 Å3, Z = 4, RF = 0.048 and RwF = 0.049 based on 4570 reflections with F0 > 3σF0, T = 298 K. CuN4C60H60.+I7- (3): monoclinic space group P21, a = 13.108(17) Å, b = 18.332(14) Å, c = 13.683(10) Å, β = 107.88(8)°, V= 3129.2 Å3, Z = 2,RF = 0.101 and RwF = 0.117 based on 2269 reflections with F0 > 4σF0, T = 200 K. © 1994, American Chemical Society. All rights reserved.
Publication Source (Journal or Book title)
Journal of the American Chemical Society
Rentier, M., Barkigia, K., Zhang, Y., Medforth, C., Smith, K., & Fajer, J. (1994). Consequences of Oxidation in Nonplanar Porphyrins: Molecular Structure and Diamagnetism of the π Cation Radical of Copper(II) Octaethyltetraphenylporphyrin. Journal of the American Chemical Society, 116 (19), 8582-8592. https://doi.org/10.1021/ja00098a019