The Crystal and Molecular Structure of the Ferrihemochrome Bis(1-methylimidazole)(protoporphyrin IX)iron-Methanol-Water

The six-coordinate Fe(III) porphyrin complex Fe(1-Me-Im)₂(PP-IX)·CH₃OH·H₂O (1-Me-Im = 1-methylimidazole, PP-IX = protoporphyrin IX) has been prepared and has been characterized by a complete X-ray structural determination. The complex crystallizes with four formula units in space group C_2h⁵-P2₁/c in a cell of dimensions a = 9.975 (6) Å, b = 21.215 (12) Å, c = 19.241 (13) Å, and β = 92.32 (2)°. The calculated and observed densities are 1.35 and 1.36 (1) g/cm³. The structure has been refined anisotropically to a final weighted R index (on F²) of 0.14 for the 5757 measured intensities. The R index (on F) for the 3256 reflections having F_o²> 3σ(F_o²) is 0.08. The complex may be described as a ferric porphyrin, for which the counterion is an ionized carboxyl group of the porphyrin. The average Fe-N(porphyrin) bond length is 1.990 Å. The two axial Fe-N(1-Me-Im) bond lengths are perhaps unequal (1.966 (5) and 1.988 (5) Å) and show a variation similar to those observed in the Fe(imidazole)₂(TPP)⁺ ion. The porphyrin displays a significantly ruffled inner core (±0.14 Å) and has apparently unequal C_a-C_b and C_a-C_m bond lengths. The vinyl groups at the periphery of the porphyrin have rotated out of the planes of the pyrroles by 24 and 41°. The crystal consists of ribbons of the complex stretched out along the x direction because of a strong (2.41 Å) hydrogen bond between the propionate groups of adjacent molecules. The two propionate groups of a porphyrin are connected together by a bridging water molecule. The methanol molecule also hydrogen bonds to a propionate group.