Metathesis-like Splitting Reactions of Metallated [36]Octaphyrins(1.1.1.1.1.1.1.1): Experimental and Computational Investigations

meso-Octakis(pentafluorophenyl)-substituted [36]octaphyrin(1.1.1.1.1.1.1.1) (1) is a figure-of-eight nonaromatic macrocycle that serves its a unique platform to induce a metathesis-like splitting reaction upon bis-Cu-II metalation. To get a better understanding of this splitting reaction, we examined the metalation of 1 with several metal ions. In contrast with the smooth and quantitative splitting reaction of bis-Cu-II complex 1-CuCu, free-base 1, mono-Cu-II complex 1-Cu, and bis-Zi(II) complex 1-ZnZn do not undergo the splitting reaction. Mono-Con complex 1-Co was selectively produced from metalation with the Co-II ion, from which hybrid complex 1-CoCu was synthesized. The hybrid complex 1-CoCu undergoes the splitting reaction to give 2-Co and 2-Cu quantitatively. Activation parameters of the splitting reactions were determined: E-a = 104 kJ mol(-1), Delta H double dagger 101 kJ mol(-1), Delta S double dagger = -25.0 J mol(-1) K-1, and Delta V double dagger = 18 cm(3)mol(-1) for 1-CuCu and E-a = 105 kJ mol(-1), Delta H double dagger = 102 kJ mol(-1), and Delta S double dagger = -29.9 J mol(-1) K-1 for 1-CoCu. A marked difference between the splitting reaction reactivity of 1-CuCu and 1-ZnZn has been examined by DFT calculations at the B3LYP/631SDD// B3LYP/LANL2DZ level, which revealed that the reaction proceeds through it stepwise route involving initial C1-C20 bond formation to give INT1 as the rate-determining step and subsequent C21-C40 bond formation to give a spirocyclobutane intermediate (INT2), followed by a radical reverse cycloaddition reaction to give two metalloporphyrins. Inherent instability of 1-CuCu, which may arise from its strongly distorted structure, was indicated to be a main factor in the smooth splitting reaction. Finally, a new bis-Pd-II complex (5-PdPd) was isolated in the metalation of 1 with Pd(OAc)(2) in a 9:1 mixture of 2,2,2-trifluoroethanol and methanol as a manifestation of the transannular electronic interaction in metalated octaphyrin complexes. Collectively, these results underscore the importance of the transannular electronic interactions that are enhanced by metalation, depending upon the coordinated metal ions.