Structure of the Free-Base “Capped” Porphyrin, 5,10,15,20-[Pyrromellitoyl(tetrakis-o-oxyethoxyphenyl)]porphyrin

The structure of the free-base, “capped” porphyrin 5,10,15,20-[pyrromellitoyl(tetrakis-o-oxyethoxyphenyl)]porphyrin (H₂Cap) has been determined by three-dimensional, single-crystal, X-ray diffraction methods using diffractometer data collected with Mo Ka radiation. The material crystallizes as the pentachloroform, monomethanol solvate, H₂Cap. ⁵CHCl₃•CH₃OH, in space group C_2h ⁵ - P2₁/n with four formula units in a cell of dimensions (at -150° C) a = 19.947 (6) Å, h = 16.569 (5) Å, c = 22.602 (7) Å, β = 104.57 (1)°, V = 7230 Å3. The structure was described by 495 variable parameters and at convergence the values for R and R_w (on F, 8552 data having F_o ² > 3φ(F_o ²)) were 0.073 and 0.094. In contrast to most other free-base porphyrins, the 24-atom porphyrin skeleton is markedly nonplanar (mean displacement from the least-squares plane 0.135 Å; maximum displacement 0.354 Å), but conforms closely to idealized D_2d symmetry. The considerable steric strain involved in attaching a pyrromellitoyl(tetrakisoxyethoxy) “cap” to the ortho positions of the tetraphenylporphyrin core is released most obviously through buckling of the porphyrin skeleton. The transannular separation between aminopyrrole nitrogen atoms is 4.182 (6)Å; between imino nitrogen atoms it is 4.038 (5)Å. 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 3.86 and 3.96 Å- results of considerable interest in relation to the ligand binding properties of iron(II) and cobalt(II) derivatives of H₂Cap.