Synthesis, Crystal Structures, and Redox Potentials of 2,3,12,13-Tetrasubstituted 5,10,15,20-Tetraphenylporphyrin Zinc(II) Complexes

Zinc(II) complexes of antipodal β-tetrasubstituted meso-tetraphenylporphyrin with trifluoromethyl (Zn(TPP(CF3)4) (1a)), bromine (Zn(TPPBr4) (2a)), and methyl groups (Zn(TPP(CH3)4) (3a)) were synthesized in order to examine the steric and the electronic effects of trifluoromethyl groups on the macrocycle. The analysis of X-ray crystal structures of the five-coordinate complexes Zn(TPP(CF3)4)(EtOH)3 (1b), Zn(TPPBr4)(MeOH)(DMF) (2b), and Zn(TPP(CH3)4)(THF)1.6(CHCl3)0.4 (3b) revealed distorted macrocyclic cores where significant differences in the Zn−N distance between the β-substituted and the non-β-substituted side were observed. The difference was significant in 1b due to the strong steric interactions among the peripheral substituents and the electronic effects of trifluoromethyl groups. The macrocycles of 1b−3b are saddle-distorted and slightly ruffled due to the five-coordination of zinc(II) and the peripheral substitution. Distortion of the macrocycles of 2b and 3b were modest. On the other hand, distortion in 1b was severe due to the peripheral strain. Cyclic voltammetric measurements of the four-coordinate complexes Zn(TPP) and 1a−3a were performed and their redox potentials were analyzed together with previously reported potentials of Zn(TPP(CN)4). The oxidation potential of 1a did not gain as much as expected from the electron-withdrawing effect of the four trifluoromethyl groups. The HOMO−LUMO gap of 1a was very small (1.5 V) and cannot just be explained by macrocyclic distortion. The magnitude of this gap is very similar to that of Zn(TPP(CN)4). Compound 2a also exhibited a modest gap contraction. Compound 3a was easier to oxidize and harder to reduce than Zn(TPP), even though the HOMO−LUMO gap of 3a was similar to that of Zn(TPP).