IRON(III) PORPHYRIN COMPLEXES WITH AXIAL ALKYL AND ACYL LIGANDS - STRUCTURES AND REACTIVITY OF THE ACYL COMPLEX TOWARD DIOXYGEN

Two highly reactive, low-spin (S = 1/2) iron(III) porphyrin complexes, one with an axial alkyl ligand and the other with an axial acyl ligan d, have been isolated as crystals suitable for single-crystal X-ray di ffraction. Red prisms of (TAP)Fe(III)(CH3).THF (TAP is the dianion of tetra-p-anisylporphyrin) crystallize in the triclinic space group P1BA R with a = 12.129(2) angstrom, b = 12.263(3) angstrom, c = 15.263(2) a ngstrom, alpha = 71.93(2)degrees, beta = 83.91(2)degrees, and gamma = 73.92(2)degrees at 130 K with Z = 2. Refinement of 586 parameters with 4858 reflections gave R = 0.069 and R(w) = 0.076. The complex consist s of a five-coordinate iron which is 0.146 angstrom out of the N4 plan e with an average Fe-N distance of 1.967 angstrom and an Fe-C distance of 1.979(9) angstrom. To accommodate the short Fe-N distances, the po rphyrin core is ruffled. Brown plates of (TAP)Fe(III)(C(O)(n-C4H9)) cr ystallize in the monoclinic space group P2(1)/c with a = 16.548(15) an gstrom, b = 18.699(9) angstrom, c = 15.774(10) angstrom, and beta = 11 8.41(4)degrees at 130 K with Z = 4. Refinement of 1774 reflections and 258 parameters yielded R = 0.060, R(w) = 0.050. The iron ion is again five-coordinate with the average Fe-N distance of 1.974 angstrom and the Fe-C distance of 1.965(12) angstrom. The iron ion is 0.19 angstrom out of the N4 plane. The porphyrin core is ruffled, and the acyl liga nd is coordinated in eta1 fashion through carbon. The reaction of seve ral acyl complexes of this type with dioxygen have been explained by H -1 and H-2 NMR spectroscopy in order to see whether reactive intermedi ates form. Oxidation is accompanied by the formation of PFe(III)OC(O)R and PFe(III)OFe(III)P (P is a generic porphyrin dianion). No evidence for the formation of previously characterized ferryl complexes in thi s process has been found.