Crystal structures, mössbauer spectra and magnetic properties of two iron(III) spin-crossover complexes

Abstract

The six-co-ordinate iron(III) complexes [Fe(bzpa)₂]ClO₄(Hbzpa =(1-benzoylpropen-2-yl)(2-pyridyl-methyl)amine] and [Fe(acen)(NC₅H₃Me₂-3,4)₂] BPh₄[H₂acen = ethylenebis(acetylacetoneiminate)] have been shown by variable-temperature magnetic and Mössbauer spectroscopy measurements to be spin-crossover (S=5//₂; ⇌S=½) complexes. Magnetic moments for both complexes show a continuous variation with temperature, which parallels that of the absorption area under the Mössbauer spectra. ‘Time-averaged’ Mössbauer spectra between the high- and low-spin states were observed for [Fe(bzpa)₂]ClO₄. X-Ray crystallographic structure determinations for both complexes were done for both high- and low-spin states. Both cations possess a N₄O₂ donor-atom set forming a distorted octahedron about the metal atom, and the terminal oxygen donor atoms occupy cis positions. The average metal-ligand bond lengths for the low-spin form are shorter by 0.069 Å for the bzpa complex and 0.104 Å for the acen complex than for the appropriate high-spin form. Remarkable changes in the Fe–O–C angles for both complexes are observed with spin-state transformation. In [Fe(acen)(NC₅H₃Me₂-3,4)₂]BPh₄ the two axial positions are occupied by the nitrogen atom of the 3,4-dimethylpyridine ligand. Crystal data: [Fe(bzpa)₂]ClO₄, space group I4₁/a, Z= 16 α=β=γ= 90.00°, (at 290 K)a=b= 27.965(6), c= 15.525(4)Å, R= 0.050, 3550 reflections, (140 K)a=b= 27.456(9), c= 15.400(4)Å, R= 0.044, 3236 reflections; [Fe(acen)(NC₅H₃Me₂-3,4)₂]BPh₄ space group P2₁/a, Z= 4, α=γ= 90.00°, a= 28.021(3)(at 298 K), b= 14.313(2), c= 11.166(1)Å, β= 90.68(1)°, R= 0.038, 3546 reflections, (at 120 K)a= 27.707(4), b= 14.085(2), c= 10.998(2)Å, β= 91.04(1)°, R= 0.037, 5906 reflections. The results support a model in which flexibility of the angle Fe–O–C in the co-ordination sphere is important for change in iron-donor atom bond lengths upon spin-state interexchange.