The Crystal and Molecular Structure of an Octaaza[14]annulene Complex of Iron(II). A Molecular Oxygen Carrier

The four-coordinate iron(II) complex of the substituted octaaza[14]annulene reported by Baldwin and Huff (J. Am. Chem. Soc., 95, 5757 (1973)) to be a molecular oxygen carrier has been characterized by a complete X-ray structural determination. The complex crystallizes with two molecules in space group C_2h ⁵-P2₁/c of the monoclinic system in a cell of dimensions, a = 11.976 (8) Å, b = 8.147 (5) Å, c = 22.567 (14) Å, and β = 98.83(4)°. A crystallographic center of inversion is imposed on each molecule. The structure has been refined anisotropically to a final R index (on F) of 0.085 for the 2651 measured intensities having F_o ² > 3σ(F_o ²). The coordination about the iron atom is approximately square-planar with two unequal Fe-N bond lengths of 1.846 (4) and 1.826 (4) Å. The bond lengths within the macrocyclic ligand are nearly identical with those observed in other octaaza[14]annulene complexes, although the ligand is distinctly nonplanar in this complex. We believe the short Fe-N bond lengths in the complex result in part from the constrictive effect of the macrocycle.