High- and Low-spin Interconversion in a Series of (α-picolylamine)iron(II) Complexes


The magnetic properties of [Fe(α-P)3]Br2·S (α-P = α-picolylamine; S = methanol, ethanol) and [Fe(α-P)3]Cl2·C2H5OH and the crystal structure of the latter complex have been determined. Each of the complexes is high spin at room temperature and each undergoes a similar transition to low spin in the region 130-90 K. In each case the transition is relatively sharp, being mostly complete in the region 130-110 K, and is sharper than in the analogous complex [Fe(α-P)3]Cl2·CH3OH. We noted earlier that [Fe(α-P)3]2+ has approximate threefold symmetry in the purely low-spin [Fe(α-P)3]Cl2·2H2O (fac geometry) but has one of its ligands coordinated with the two donor atoms reversed in [Fe(α-P)3]Cl2·CH3OH (mer geometry). The crystal structure of [Fe(α-P)3]Cl2-C2H5OH and the similarity of the magnetic properties indicate that the present complexes all have the same “ligand-reversed” structure of the [Fe(α-P)3]2+ ion. Removal of the included solvent S drastically alters the magnetic properties, eliminating the sharp spin transition and replacing it with a spin equilibrium which is so gradual that low spin (1A1) is not attained even at the lowest temperatures studied (4 K). Literature observations of such very gradual spin transitions can now be explained in terms of sample drying which removed solvent that had originally been held in the lattice via hydrogen bonding. The crystal structure of [Fe(α-P)3]Cl2·C2H5OH reveals hydrogen bonding between the ethanol molecule and a halogen but not with the cation. Comparison of this structure with that of [Fe(α-P)3]Cl2·2H2O reveals a large change, 0.18 Å, in the average metal-ligand bond length between the high-spin and low-spin iron(II) species, providing important support for our previous determination of 0.19 Å for this quantity. Crystal data for [Fe(α-P)3]Cl2·C2H5OH: space group P21/c, Z = 4, a = 10.937 (5) Å, b = 22.061 (9) Å, c = 11.571 (6) Å, β = 116.70(4)°, V = 2496 Å3, R = 5.3% for 1470 reflections. © 1979 American Chemical Society.



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© 1979 American Chemical Society (ACS), All rights reserved.

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01 Jan 1979