MOSSBAUER AND ELECTRON-PARAMAGNETIC-RESONANCE STUDIES ON SOME NEW BIS-(LIGATED) PORPHYRINATOIRON(III) COMPLEXES WITH ALIPHATIC-AMINES - MODELS FOR CYTOCHROME-B

A series of bis(axially ligated) complexes [Fe(III)L(1)(L')(2)](+) (H( 2)L(1) = 3,7,12,17-tetramethyl-8, 13-divinylporphyrin-2,18-dipropanoic acid; L' = ethyl-, n-propyl-, n-butyl-, n-octyl-, n-decyl-, n-dodecyl -, n-octadecyl-amine, 1,2-diaminoethane, morpholine or piperidine) hav e been prepared In solution and their electronic absorption, EPR and M ossbauer spectra recorded in solution or frozen solution. The electron ic spectra indicate that new porphyrinato complexes have been prepared . The Mossbauer spectroscopic data show these to be low-spin iron(III) complexes, with smaller quadrupole splittings than those previously r eported for similar N-heterocyclic complexes. The Mossbauer data when compared to those of the iron(Ir) analogues as well as to those of pyr idine, imidazole and histidine complexes [both iron-(II) and -(III)] f acilitate a more comprehensive understanding of the bonding of all the compounds. [This work allowed insight into the a-bonding component of the axial ligands in bis(ligated) imidazole and histidine complexes o f (protoporphyrinato IX)iron(III).] The pi-donor ability of imidazole and histidine ligands and its effect on iron(III) porphyrins is manife st. The EPR spectra for the aliphatic amine complexes at 77 K are char acteristic of low-spin Iron(III). In several cases two similar sets of x, y and z features were obtained, indicative of two alternative conf ormers The intermediate g values were all close to 2.20. The maximum v alues were in the range 2.51-2.77 and the minimum values ill the range 1.79-1.90. These g shifts are the smallest on record for complexes of this type, which requires some mechanism for strongly lifting the d(x z,yz) orbital degeneracy. Conventionally, this splitting is discussed in terms of rr bonding involving the fifth and sixth ligands, but the amine ligands here are not able to participate in pi bonding. A latera l movement of these ligands is tentatively invoked such that one (say d(yz)) of the d orbitals is involved in sigma bonding.