The binding ability of iron bonded to porphodimethene: Structural, magnetic, and electronic relationship to iron porphyrin complexes

The availability of the parent compound, meso-hexaethylporphodimetheneiron( II), [(Et6N4)Fe] (2), of this report results from a novel synthetic methodo logy that makes [Et6N4Li2] (1) easily available. The major focus is on how the axial positions, which are the key reactive sites in metalloporphyrins, and the electronic configuration of the metal can be affected by the break down of the aromaticity of the porphyrin skeleton and by the nonplanar conf ormation of the ligand. DFT calculations indicate a B-3(1)(d(z2))(1)(d(yz)) (1) ground state for 2 versus the (3)A(2)(d(xz))(1)(d(yz))(1) ground state in the porphyrin analogue. The intermediate-spin state (S = 1) of 2 changed drastically upon addition of one or two axial ligands, as hexacoordination is preferred by iron(II). The hexacoordinate complexes [(Et6N4)Fe(L)(L')] (L = L' = THF, 3; L = L' = Py, 4; L = PhNO, L' = Py, 14) have been isolated and structurally characterized. Strong-field ligands lead to a low-spin di amagnetic state for iron(II), namely for complexes 4-7, 9, and 14, whereas 3 is a typical d(6) high-spin complex, as is the pentacoordinate [(Et6N4)Fe (CN)]Bu4N (8). The structural analysis showed common features for 6, 7, 9, and 14: i) a small displacement of the metal from the N-4 plane, and ii) an N-4 cavity, larger than that in the corresponding porphyrins, affecting th e Fe-N bond lengths. The H-1 NMR spectrum is quite diagnostic of the two-fo ld symmetry in the diamagnetic hexacoordinate complexes, revealing either a D-2h or a C-2v symmetry. The CO stretching frequency (1951 cm(-1)) in comp lex 6 probes the good electron density at the metal. The one-electron oxida tion of 2 led to pentacoordinate iron(III) derivatives [(Et6N4)Fe(Cl)] (10) , [(Et6N4)(2)Fe-2(mu -O)] (11), and [(Et6N4)(2)-Fe-2(mu -p-OC6H4-O)] (12). Complex 10 is a typical high-spin iron(III) (5.85 mu (B) at 298 K), while 1 1 and 12 behave as antiferromagnetic coupled iron(III) (J = -9.4 cm(-1), 12 , and J = -115 cm(-1), 11). In complexes 10, 11, and 12 iron is sitting in a quite distorted square pyramidal geometry, in which the ligand displays a very distorted roof conformation with different degrees of ruffling. Disti nctive structural and magnetic features have been found for the nitrosyl de rivative [(Et6N4)Fe-NO], which has a low-spin state (S = 1/2) and the follo wing structural parameters: Fe-N-O, 147%3(2)degrees; Fe-N, 1.708(2) Angstro m; N-O, 1.172(3) Angstrom. A comparative structural, magnetic, and theoreti cal analysis of the compounds listed above has been made with the analogous porphyrin derivatives. The detailed structural investigation has been mapp ed through the X-ray analysis of 2, 7, 8, 9, 11, 13, and 14.