Synthesis, reactivity, and properties of N-fused porphyrin manganese(I) tricarbonyl complexes

The reactions of N-fused tetraphenylporphyrin (NFTPP, 1a) and its 21-substituted derivatives, 21-Br-NFTPP (1b), 21-NO₂-NFTPP (1c), and 21-Bz-NFTPP (1d), with Mn(CO)₅Br gave the manganese(I) tricarbonyl complexes bearing N-fused tetraphenylporphyrinato ligands (2a-d), respectively, in 46-99% yields. The complexes were characterized by mass, IR, ¹H and ¹³C NMR spectroscopy, and the final structural proof was evident from the X-ray crystallographic analysis for 2a. The crystals of 2a•CH ₂Cl₂ belong to the monoclinic space group P2₁/n (#14), with a = 15.007(2) Å, b = 12.5455(19) Å, c = 21.150(3) Å, β = 102.227(4)°, and Z = 4. The lengths (Å) of three manganese-nitrogen and three manganese-carbon bonds are inequivalent respectively [Mn-N(2), 2.007(2); Mn-N(23), 2.033(2); Mn-N(24), 1.988(3); and Mn-CO, 1.798(4), 1.804(4), 1.841(3)], reflecting the asymmetric structure of the NFp ligand. The aromatic substitution reactions of 2a, such as nitration, formylation, and chlorination, proceeded without a loss of center metal to give the corresponding 21-nitro (2c), 21-formyl (2e), and 21-chloro (2f) derivatives, regioselectively. In the electrochemical measurements of 2, one reversible oxidation and two reversible reduction waves were observed. The redox potentials of 2 indicate the narrow energy gaps between the highest occupied molecular orbital and the lowest unoccupied molecular orbital (HOMO-LUMO) being consistent with the electronic absorption spectra that display the absorption edges over 1000 nm. Protonation occurred at the inner core nitrogen of 2a upon the addition of acids, which is inferred from the ¹H NMR spectra as well as theoretical calculations. By a treatment with amine N-oxides, demetalation of 2 proceeded to afford the corresponding NFP free-bases (1).