1,3-Adamantane−3,13-Porphyrin−6,6-Cyclophane: Crystal Structure of the Free Base and Steric Effects on Ligation of the Iron(II) Complex

The coupling of 1,3-adamantanediacetyl chloride to 8,18-bis(2-carbobenzoxyethyl)-3,13-bis(o-aminopropyl)- 2,7,12,17-tetramethylporphyrin affords l,3-adamantane−3,13-porphyrin−6,6-cyclophane. The material crystallizes as the bis(dichloromethane) solvate, C₆₄H₇₂N₆O₆·2CH₂Cl₂, in space group C^l_i-P of the triclinic system with two formula units in a cell of dimensions a = 15.858 (28) Å, b = 17.779 (32) Å, c = 12.970 (24) Å, α = 109.62 (6)°, β = 97.88 (7)°, γ = 114.18 (7)°, and V = 2977 ų. The structure was described by 742 variables, and at convergence of the full-matrix least-squares refinement the values of R and R_w(on F², 7749 unique data) are 0.093 and 0.168. The conventional R index on F for 5489 reflections having F_°²> 3σ(F²) is 0.065. The porphyrin macrocycle is approximately planar, with the mean displacement from the 24-atom plane being 0.038 Å and the maximum displacement being 0.101 (4) Å. As a whole, the molecule is relatively unstrained. In the solid state the adamantane bridge has an offset orientation relative to the plane of the porphyrin and short intramolecular contacts that indicate no free cavity between the strap and the porphyrin. NMR spectroscopic results show that in solution the adamantane flips rapidly between two asymmetric orientations. The (dicyclohexylimidazole)iron(II) complex of the porphyrin binds CO, O₂, and isocyanides with greatly reduced affinities compared with open heme compounds. A small steric differentiation between CO and O₂is demonstrated, and variations in the magnitude of steric effects are found to depend upon the nature of the hindering group.