Imidazolate- and Oxo-Bridged Metalloporphyrins

Article abstract of DOI:10.1021/ja00400a027

Urea-linked tetraphenylporphyrin dimers have been synthesized with bridging imidazolate ligands between Fe(II)/Fe(II), Mn(II)/Mn(II), and Mn(II)/Co(II) centers.Magnetic susceptibility measurements down to 4 K show that imidazolate is a relatively weak mediator of antiferromagnetic coupling in these complexes (-J = 2-5 cm^-1).The heteronuclear Mn(II)/Co(II) dimer, an S = 5/2/S = 1/2 spin model for the heme a₃/Cu_B active site of cytochrome oxidase, has -J = 5 cm^-1.Thus, there is little support for the role of histidyl imidazolate as a bridging ligand between Fe(III) and Cu(II) in cytochrome oxidase where -J>200 cm^-1.The magnetic, chemical, and X-ray structural properties of the μ-oxo ferric dimer of the face-to-face porphyrin have been investigated.Crystal data for O*H2O*2toluene follow: triclinic, a = 14.004(6) Angstroem, b = 22.995(6) Angstroem, c = 13.192(4) Angstroem, α = 101.45(2) deg, β = 93.91(2) deg, γ = 80.40(2) deg; space group P1/; Z = 2.The average Fe-N distance in the biporphyrin is 2.075 Angstroem, and the average Fe-O distance is 1.787 Angstroem.The iron atoms are displaced 0.60 and 0.65 Angstroem from the mean porphinato planes.The Fe-O-Fe angles is distinctly nonlinear with a value of 161.4 deg; a water molecule forms a hydrogen-bonding network between the oxo ligand and the hydrogen atoms of the urea link which joins the two halves of the biporphyrin.The two porphinato planes are not coplanar, having a dihedral angle of 15.8 deg.