VALENCE LOCALIZATION IN IRON COMPLEXES WITH O-QUINONE-DERIVED LIGANDS STUDIED BY HIGH MAGNETIC-FIELD MOSSBAUER-SPECTROSCOPY

High magnetic field Mossbauer spectroscopy at liquid-helium temperatures is used to characterize the unpaired electrons in five iron complexes with o-quinone-derived ligands. The question addressed is whether the residual unpaired electrons resultant after strong antiferromagnetic exchange interactions between paramagnetic ligands and the iron ion are largely metal based or are delocalized throughout the complex. An internal Fe-57 hyperfine field of -495 kG was determined from spectra taken in 60- and 51-kG fields for the S = 5/2 complex [Fe(saloph)(catH)], where saloph2- is NN'-phenylenebis(salicylideneaminato) and catH- is the monoprotonated direduced form (i.e., catecholate) of o-benzoquinone. This clearly establishes this saloph complex as a high-spin Fe(III) complex with diamagneticligands. Thecomplex[Fe(salen)(3,5-DBSQ)], where salen2-is NN'-ethylenebis(salicylideneaminato) and 3,5-DBSQ is the semiquinonate form (S = 1/2) of 3,5-di-tert-butyl-o-benzoquinone, has been reported to have an S = 2 ground state as the result of an exchange interaction between the one S = 1/2 ligand and a high-spin Fe(III) ion. An internal Fe-57 hyperfine field of -455 kG was determined for this complex; this is consistent with four unpaired electrons localized on the Fe(III) ion. An internal Fe-57 hyperfine field of -440 kG was determined for [Fe(phenSQ)(phenCAT)(bpy)]-0.5phenQ, which verifies that this complex has an S = 2 ground state resultant from a relatively strong antiferromagnetic exchange interaction between one semiquinonate ligand and a high-spin Fe(III) ion. The ligands in this third complex are 2,2'-bipyridine (bpy) and the semiquinonate (phenSQ-) and catecholate (phenCAT2-) forms of 9,10-phenanthrenequinone (phenQ). The internal Fe-57 hyperfine fields of -365 and \310\ kG for [Fe(3,5-DBSQ)3] and [Fe(phenSQ)3].phenQ, respectively, clearly indicate S = 1 ground states that result from a relatively strong antiferromagnetic exchange interaction between three S = 1/2 semiquinonate ligands and a high-spin Fe(III) ion. The two unpaired electrons in the S = 1 complex Fe(3,5-DBSQ)3 are localized on the Fe(III) ion. A long-range intermolecular antiferromagnetic exchange interaction is present in [Fe(phenSQ)3].phenQ. This compound shows a six-line magnetic field pattern in its Mossbauer spectrum in the absence of an external magnetic field. The above results indicate that all of the complexes have relatively localized bonding between a high-spin Fe(III) ion and semiquinonate ligands.