Factors influencing the structural and magnetic properties of octahedral cobalt(II) and iron(II) complexes of terdentate N_3 schiff base ligands

摘要

The influence of the length of the linking alkyl spacer and of the presence of either a proton or a methyl group, in four related terdentate N_3 Schiff base ligands, on the structures and properties of the resulting iron(II) and cobalt(II) complexes has been investigated. The four ligands were prepared in situ by condensation of 2-(2-aminoethyl)pyridine or 2-(aminomethyl)pyridine with 2-acetylpyridine (L1 vs, L3) or 2-formylpyridine (L2 vs. L4). Hence they comprised a mixture of a relatively rigid chelate with a 2-iminopyridyl moiety, comparable to bipyridine coordination, and a more flexible chelate containing the -(CH_2)_n- spacer. Four iron(II) complexes, [Fe(L1) _2](BF_4)_2 (1), [Fe(L2)_2](BF _4)_2 (2), [Fe(L3)_2](BF_4)_2 (3), [Fe(L4)_2](BF_4)_2 (4), were obtained whereas only in the case of the two ethylene (i.e. not methylene) spaced ligands could pure cobalt(II) complexes, [Co(L1)_2](BF_4)_2 (5), [Co(L2)_2]-(BF_4)_2 (6), be obtained. The ~1H NMR spectra confirmed that in MeCN 1-4 are diamagnetic whereas 5 and 6 are paramagnetic. X-ray structure determinations of the ethylene-linked complexes, 1, 5 and 6, revealed distorted octahedral geometries due to chelate ring restrictions. The M-N distances were typical for high-spin cobalt(II) (5 and 6) and for low-spin iron(II) (1). The magnetic data on 5 and 6 are typical of those expected for distorted octahedral high-spin d~7 species; fitting attempts have yielded zero-field splitting and low symmetry ligand field parameters. A metal-centred M~(2+/3+) redox wave and ligand-based reduction processes were observed for 1-6 in MeCN. The metal-centred redox potential (Fe: 1 0.59, 2 0.68, 3 0.58, 4 0.70 V; Co: 5 0.03, 6 0.09 V vs. Fc/Fc~+) was influenced much more strongly by the presence of the proton vs. methyl group (Fe: shift of 0.09-0.12 V, Co: shift of 0.06 V) than by the bridging methylene vs. ethylene group (Fe: shift of 0.01-0.02 V).