Studies on spin-equilibrium iron(III) complexes. Part 1. Syntheses and magnetic properties of a new family of spin cross-over iron(III) complexes with a unidentate ligand over a wide range of the spectrochemical series and a quinquedentate ligand derived from salicylaldehyde and di(3-aminopropyl)amine. X-Ray crystal structure of [4-azaheptamethylene-1,7-bis(salicylideneiminato)](4-methylpyridine)iron(III) tetraphenylboratet

Abstract

Iron(III) complexes of general formula [Fe(X)L]ⁿ⁺(n= 0 or 1) have been prepared and characterized, where X is a unidentate ligand such as Cl^–, N₃^–, NCO^–, CN^–, pyridine (py), 3-methylpyridine (3Me-py), 4-methylpyridine (4Me-py), 4-aminopyridine (apy), 3,4- dimethylpyridine (3,4Me,-py), imidazole (Him), N-methylimidazole (mim), or 2- methylimidazole (2Me-im), and L denotes a quinquedentate Schiff base derived from salicylaldehyde and di(3-aminopropyl)amine. On the basis of the cryomagnetic data, these complexes can be classified into three groups: high spin (S=5//₂) for X = Cl, N₃, Him, mim; low spin (S=5//₂) for X = CN; spin cross-over (S =½⇌S=5//₂) for X = py, 3Me-py, 4Me-py, 3,4Me₂-py, 2Me-im. This indicates that the spin state for the series of complexes depends predominantly on the order in the spectrochemical series of the ligand X. The magnetic moments for the spin cross-over complexes increased gradually with increase of the temperature and showed no thermal hysteresis, indicating that the present complexes are of continuous spin-transition type. The spin cross-over complexes showed a thermochromism both in the solids and solutions, changing colour from dark violet to blue green, with decreasing temperature, and the thermochromism was studied by the temperature dependence of the electronic spectra in dichloromethane solution. The temperature dependence of the Mossbauer spectra for [Fe(2Me-im)L]BPh₄ and [Fe(4Me-py)L]BPh₄ has also provided evidence that the spin transition between the high- and low-spin states takes place and that the rate of spin interexchange is as fast as the inverse of the lifetime (1 × 10^–7 s) of the Mossbauer nucleus. One of the complexes, [Fe(4Me-py)L] BPh₄, was also subjected to a single-crystal X-ray analysis. The result verified the detailed structure in which the iron(III) atom assumes a pseudo-octahedral co-ordination geometry with a trans geometry for the two salicylideneiminate moieties, and the two axial positions are occupied by the secondary amine nitrogen of the di(3-aminopropyI)amine moiety of L and the nitrogen atom of the 4-methylpyridine ligand.