الملخص الإنجليزي
The condensation reaction of either 2-acetylpyridine or 22-hydroxyacetophenone with the appropriate primary amine in stoichiometric amounts afforded the following Schiff bases: phenyl-N-[1-(pyridine-2-yl)ethylidene]methanamine (apbi), 2-phenyl1-[1-(pyridine-2-yl)ethylidene]hydrazine (apph), N*-[1-(pyridin-2yl)ethylidene]ethane-1,2-diamine (apen), N-methyl-N-[1-(pyridine-2yl)ethylidenesethane-1,2-diamine (apmeen),
2-[1-{2(methylamino)ethylimino}ethyl]phenol (Hhapmeen) and bis(22hydroxyacetophenone)ethylenediimine (Hzhapen). As a means to probe into the coordination intricacies of iron, the stabilisation of the oxidation and spin states of iron, and the iron-based spin transformations, a diverse range of Schiff-base iron compounds have been synthesised, isolated and characterised by a variety of pertinent physicochemical techniques: [Fe"(apph)]] X2 (X = 1, C104, BF4C, BPha or PF6), [Fe(apph)3]2C1-(PF6)3, [Fe"(apph)2(NCS)2], [Fe"(apbi)3]X2 (X = r, C104, BF47, BPha or PF6), [Fe"(apen)2]X2, [Fe"(apmeen)2]X2 (X = C104, BF4, BPha or PF6), [Fe(hapmeen)2]C104 and [{Fel"(happen)}2(4-0)].
Crystallographic analyses of the complex cations [Fe" (apph)]]2+, [Fe"(apen)2]2+ and [Fe"(apmeen)2]2+ in tandem with their magnetic susceptibility measurements demonstrated that the relevant all-N-donor Schiff bases have a preference for the low-spin ferrous state on account of the LFSEs of the resultant complexes. The Mössbauer spectrum of [Fe"(apen)2]C104 corroborated the structural and magnetic observations. A comparative systematic study of the coordination of iron by the analogous pyridyl and phenolic ligands apmeen and Hhapmeen, respectively, revealed crystallographically, magnetically and spectroscopically that not only does the [N402]2-donor set provided by Hhapmeen stabilise the ferric state, but that it also supports the A1 + 2T2 crossover. Reaction of the crystallographically characterised tetradentate enolimine ligand Hyhapen with iron(II) or iron(III) yielded the u-oxomonobridged diiron(III) complex [{Fe'l(hapen)}2(4-0)] which exhibits considerable antiferromagnetic coupling (J=-99 cm) consistent with the Fe-Ou-oxo distances and internal angle of the somewhat unsymmetrical [Fe-O-Fe]44 structural core. A direct comparison of the structures of [{Fe"(hapen)}2(1-0)] and [{Fe"(salen)}2(4-0)] confirms that steric encumbrance does indeed cause the Fe-0-Fe bridge to open out.
