Effect of fused indanedione (IND) groups and antipodal beta-substituents on electrochemical properties of unsymmetrical metalloporphyrins

The interacting effect of fused indanedione (IND) groups and antipodal Br beta-substituents on the electroreduction of free-base and transition metal tetraphenylporphyrins (TPPs) are elucidated through the use of cyclic voltammetry. A systematic study of the reduction potentials for mono-and difused derivatives of MTPP(IND)(R) 2 and MTPP(IND) 2(R) 2 where R = Br or H and M = Cu-II, Ni-II, Zn-II or H-2 is carried out in pyridine containing 0.1 M tetrabutylammonium perchlorate (TBAP) as supporting electrolyte and demonstrates how the overall electrochemical properties of these compounds can be tuned by systematic changes in the number of meso-beta fused IND groups on the molecule and the presence or absence of electron-withdrawing Br beta-substituents. Under these solution conditions, two reversible oneelectron reductions are seen for H2TPP, while three reversible reductions are obtained for H2TPP(IND) or H2TPP(IND)Br-2 and four for H2TPP(IND)(2)Br-2. Multiple reductions also occur for the other investigated mono-and di-fused metalloporphyrins. Plots of measured E-1/2 values for the two ring-centered reductions of compounds in a given series vs. the number of fused IND groups are linear and with slopes varying in magnitude between 15 and 230 mV/IND depending upon the product of the electron transfer (anion radical or dianion), the specific central metal ion and the presence or absence of Br substituents on the pyrrole ring opposite the fused IND group(s). It is anticipated that the information from this study can be used for predicting the reduction potentials for yet to be synthesized compounds where a specific redox reactivity is desired.