Facile Heterogeneous and Homogeneous Anion Induced Electrosynthesis: An Efficient Method for Obtaining π-Extended Porphyrins.

Two closely related electrosynthetic approaches are applied for the preparation of novel π-extended tetraphenylporphyrins from malononitrile-appended meso-β di-fused porphyrins, represented as MTPP(MN)2, where TPP = the dianion of tetraphenylporphyrin and MN = malononitrile. The first method involves application of a controlled reducing potential at a platinum electrode in CH2Cl2, while the second proceeds via cyanide anion induced electron transfer. Both methods produced the same decyanated, π-extended di-fused porphyrins represented as MTPP(VCN)2 where VCN = vinyl cyanide and M = H2, NiII, CuII, or ZnII in almost quantitative yields. The final isolated and purified porphyrin products are characterized by a split Soret band ranging from 411-497 nm and two broad intense Q bands. The new π-extended porphyrins are easier to reduce than the parent MTPP or MTPP(MN)2 compounds by 760-800 mV and 180-190 mV, respectively, and possess an electrochemical HOMO-LUMO gap ranging from 1.48 to 1.66 V. They are also characterized by two reversible one-electron ring-centered reductions in CH2Cl2 and three reversible one-electron ring-centered reductions in THF. A fourth irreversible reduction is seen in THF at more negative potentials and is assigned to one or two of the fused cyanobenzene rings of the macrocycle.