Crystal and Molecular Structure of the Free Base Porphyrin, Protoporphyrin IX Dimethyl Ester

The structure of the free base porphyrin, protoporphyrin IX dimethyl ester, has been determined from three-dimensional x-ray diffraction data collected from a crystal of calculated weight 0.98 μg.The porphyrin crystallizes with two molecules per unit cell in the triclinic space group Cl1,A-P⥘ with a=1 1.303 (5), b=22.553 (10), c=6.079 (3) Å, α=91.38 (2), β=94.08 (2), γ=81.96 (1)°, V=1530 Å3. The structure has been refined anisotropically by full-matrix least-squares methods to a final unweighted R index (on F2) of 0.108 for 397 variables and 3889 observations. The R index (on F) for the 1982 observations having Fo2 ≥3σ(Fo2) is 0.073. Bond lengths and bond angles within the porphyrin core have been determined to estimated standard deviations of ±0.006 A and ±0.5°. The structural results for the porphyrin core do not differ significantly from those found in the free base porphyrin, mesoporphyrin IX dimethyl ester. In the present structure the vinyl groups approach planarity with the porphyrin core more than do the ethyl groups in mesoporphyrin IX dimethyl ester, but these vinyl groups fail to achieve coplanarity owing to steric constraints. The replacement of vinyl groups with ethyl groups results in some significant changes in spectral properties, in the basicity of amide nitrogen atoms, and in the binding of ligands to a central metal without causing a detectable change in the stereochemistry of the porphyrin ring.