Synthesis, Spectral, and Structural Studies of Porphyrins Having Sterically Hindered [η5‑CpCo(η4‑C4Ph4)] Cobalt Sandwich Units at the Meso Positions

Synthesis, spectral, and structural studies of the first examples of porphyrins substituted at the meso positions with sterically hindered η5-CpCo­(η4-C4Ph4) cobalt sandwich units are described. The novel dipyrromethane derived cobalt sandwich compound {η5-[(C4H4N)2CH]­C5H4}­Co­(η4-C4Ph4) 1, as well as its parent aldehyde, η5-[C5H4(CHO)]­Co­(η4-C4Ph4), were used in the synthesis of porphyrins having one or two η5-CpCo­(η4-C4Ph4) groups at their meso positions. 1,9-Diformyldipyrromethane derived η5-CpCo­(η4-C4Ph4) 2 was synthesized using dipyrromethane 1 under Vilsmeier conditions. A reaction of 2 with unsubstituted dipyrromethane under basic conditions in the presence of Pd­(C6H5CN)2Cl2 yielded an A-type palladium coordinated porphyrin 3 [where A = η5-CpCo­(η4-C4Ph4)]. A similar reaction of 2 with meso aryl and ferrocenyl-substituted dipyrromethanes yielded trans-AB type palladium coordinated porphyrins 4–6 [where A = η5-CpCo­(η4-C4Ph4) and B = 4-tert-butylphenyl 4, ferrocenyl 5, and pentafluorophenyl 6]. Reactions of 2 with 5-ferrocenyl dipyrromethane under the same reaction conditions in the presence of Ni­(acac)2 and Zn­(OAc)2 gave the trimetallic nickel­(II) and zinc­(II) complexed trans-AB type porphyrins 7 and 8 having both cobalt and iron sandwich units at the meso positions. Crystal structure of the Pd­(II) porphyrin 5 and nickel­(II) porphyrin 7 showed nonplanar structures having distinct ruffle type distortion of the porphyrin ring. Demetalation of the zinc­(II) trans-AB type porphyrin 8 in the presence of trifluoroacetic acid gave the metal free base porphyrin 9. Reactions of the cobalt sandwich aldehyde [(η5-C5H4(CHO)]­Co­(η4-C4Ph4) with sterically hindered dipyrromethane derivatives under acid-catalyzed condensation reactions gave trans-A2B2 type porphyrins [where A = η5-CpCo­(η4-C4Ph4) and B = pentafluorophenyl, 10 mesityl 11]. In contrast, reactions of [η5-C5H4(CHO)]­Co­(η4-C4Ph4) with sterically unhindered meso-4-tert-butylphenyl dipyrromethane resulted in both AB3 12 and cis-A2B2 13 type porphyrins [where A = η5-CpCo­(η4-C4Ph4) and B = (4-tert-butylphenyl] as a result of scrambling. The new porphyrin derivatives have been structurally characterized, and their spectral and electrochemical features were determined.