Synthesis and Characterization of All-Alkyl-Substituted Mono-, Di-, and Trioxosapphyrins

The synthesis and characterization of the following heterosapphyrins is presented:  3,7,18,22-tetraethyl-2,8,12,13,17,23-hexamethyl-27-oxasapphyrin (5), 3,8,12,13,17,22-hexaethyl-2,7,18,23-tetramethyl-15,29-dioxasapphyrin (6), and 3,7,18,22-tetraethyl-2,8,12,13,17,23-hexamethyl-15,27,29-trioxasapphyrin (7). These macrocycles were synthesized from the hitherto unknown precursors methyl 3,4-dimethylfuran-2-carboxylate (11) and methyl 4-ethyl-3-methylfuran-2-carboxylate (12). Single-crystal X-ray diffraction structures of the bis(hydrochloric acid) salt (5a) and the bis(trifluoroacetic acid) salt (5b) of 5 and of the bis(hydrochloric acid) salt (6a) of 6 were obtained. These structures are compared to those of the parent all-aza sapphyrin 4. In this context, we report the first solid-state structural analysis of a trifluoroacetic acid salt of sapphyrin (4b). Crystals of 5a are triclinic, space group P1̄ in a cell of the dimensions a = 11.9686(13) Å, b = 12.790(2) Å, c = 16.236(2) Å, α = 80.924(10)°, β = 88.845(9)°, γ = 80.481(10)°, V = 2420.5(5) ų, F(000) = 1036, ρ = 1.38 g cm³, with Z = 2. Crystals of 5b are monoclinic, space group P2₁/c, in a cell of the dimensions a = 9.590(1) Å, b = 33.911(3) Å, c = 13.200(1) Å, β = 94.931(6)°, V = 4276.8(6) ų, F(000) = 1904, ρ = 1.42 g cm³, with Z = 4. Crystals of 6a are triclinic, space group P1̄, in a cell of the dimensions a = 12.827(3) Å, b = 13.116(3) Å, c = 17.202(7) Å, α = 82.89(2)°, β = 82.14(2)°, γ = 80.22(1)°, V = 2810(2) ų, F(000) = 1184, ρ = 1.36 g cm³, with Z = 2. Crystals of 4b are triclinic, space group P1̄, in a cell of the dimensions a = 11.431(3) Å, b = 11.563(2) Å, c = 16.367(4) Å, α = 88.07(2)°, β = 75.25(2)°, γ = 86.50(2), V = 2087.8(9) ų, F(000) = 872, ρ = 1.32, with Z = 2. The monooxasapphyrin salt 5a displays a structure very similar to that of the parent all-azasapphyrin (4a) in that one chloride anion is bound 1.936 Å above and one chloride anion 1.814 Å below the plane of the macrocycle. These distances are 1.774 and 1.877 Å, respectively, for 4a. Almost the same behavior is found for 5b. Here, the carboxylate anions are bound 1.085 Å above and 1.870 Å below the plane of the macrocycle. These distances are 1.305 and 1.444 Å, respectively, for 4b. Completely different behavior is observed in the case of 6a. In this instance, only one chloride anion is bound directly with a distance of 1.685 Å above the plane of the macrocycle, while the other chloride anion is well removed from the dioxasapphyrin and solvated by four chloroform molecules.