Structure of the Iron(III)-“Capped” Porphyrin, FeCl(C₂-Cap), [5,10,15,20-[Pyrromellitoyl-tetrakis(o-(oxyethoxy)phenyl)]porphyrinato]chloroiron(III)

The structure of the iron(III) “capped” porphyrin, [5,10,15,20-[pyrromellitoyltetrakis(o-(oxyethoxy)phenyl)]-porphyrinato]chloroiron(III), FeCl(C₂-Cap), has been determined from single-crystal, X-ray diffraction data collected at −150 °C. The material crystallizes as the trichloroform solvate, FeCl(C₂-Cap)·3CHCl₃, in space group C⁵ _2h-P2₁/c of the monoclinic system with four formula units in a cell of dimensions a = 14.473 (5) Å, b = 19.514 (9) Å, c = 22.830 (11) Å, β = 101.61 (2)°, V= 6316 ų. The structure was described by 396 variables, and at convergence of the full-matrix, least-squares refinement the values of R and R_w (on F, 9897 data having F₀ ² > 3σ(F₀ ²)) are 0.065 and 0.078. The chloro ligand is outside the “cap” and the iron atom is above the mean plane of the porphyrin core away from the “cap”. The geometry about the iron atom is typical for FeCl(porphyrin) systems, with Fe−Cl = 2.242 (1) Å and with Fe 0.46 Å above the mean N₄ plane and 0.47 Å above the mean 24-atom porphyrin plane. The porphyrin itself is slightly nonplanar, with the mean displacement from the least-squares plane being 0.076 Å and the maximum displacement being 0.208 Å. Although the conformations of the side chains that link porphyrin to “cap” are distinctly different from those found in the related free-base porphyrin, H₂(C₂-Cap), the “cap” to porphyrin distances are very similar in the two structures. The separations between the centroids of the phenyl “cap” and respectively those of the four pyrrole nitrogen atoms and the 24-atom porphyrin skeleton are 4.03 and 4.01 Å in FeCl(C₂-Cap) and 3.86 and 3.96 Å in H₂(C₂-Cap). These separations are too small to accommodate O₂ or CO within the cap and imply considerable flexibility of these “capped” porphyrins in solution where binding of O₂ or CO to Fe^II(C₂-Cap) systems occurs.