Mössbauer and electron paramagnetic resonance studies on some new bis-(ligated) porphyrinatoiron(III) complexes with aliphatic amines. Models for cytochromes b

Abstract

A series of bis(axially ligated) complexes [Fe^IIIL¹(L′)₂]⁺(H₂L¹= 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) have been prepared in solution and their electronic absorption, EPR and Mössbauer spectra recorded in solution or frozen solution. The electronic spectra indicate that new porphyrinato complexes have been prepared. The Mössbauer spectroscopic data show these to be low-spin iron(III) complexes, with smaller quadrupole splittings than those previously reported for similar N-heterocyclic complexes. The Mössbauer data when compared to those of the iron(II) analogues as well as to those of pyridine, imidazole and histidine complexes [both iron-(II) and -(III)] facilitate a more comprehensive understanding of the bonding of all the compounds. [This work allowed insight into the σ-bonding component of the axial ligands in bis(ligated) imidazole and histidine complexes of (protoporphyrinato IX)iron(III).] The π-donor ability of imidazole and histidine ligands and its effect on iron(III) porphyrins is manifest. The EPR spectra for the aliphatic amine complexes at 77 K are characteristic of low-spin iron(III). In several cases two similar sets of x, y and z features were obtained, indicative of two alternative conformers. The intermediate g values were all close to 2.20. The maximum values were in the range 2.51–2.77 and the minimum values in 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_xz,yz orbital degeneracy. Conventionally, this splitting is discussed in terms of π bonding involving the fifth and sixth ligands, but the amine ligands here are not able to participate in π bonding. A lateral movement of these ligands is tentatively invoked such that one (say d_yz) of the d orbitals is involved in σ bonding.