Crystal structures of low- and high-spin iron(III) complexes with quadridentate Schiff bases

Abstract

The crystal structures of three iron(III) complexes, [Fe(acen)(Him)2]BPh4(1), [Fe(salen)(Him)2]ClO4·H2O (2), and [Fe(salphen)(Him)2]BPh4(3), were determined by the X-ray diffraction method [H2acen =NN′-ethylenebis(acetylacetonylideneimine), H2salen =NN′-ethylenebis(salicylideneimine), H2salphen =o-phenylenebis(salicylideneimine), and Him = imidazole. The low-spin complex (1) crystallizes in monoclinic space group P21/n with a= 32.939(7), b= 12.705(2), c= 8.991(1)A, β= 93.63(2)°, and Z= 4. The high-spin complex (2) crystallizes in monoclinic space group P21/a with a= 16.146(8), b= 15.205(3), c= 10.042(3)A, β= 92.73(3)°, and Z= 4. The high-spin complex (3) crystallizes in orthorhombic space group Pna21 with a= 26.966(8), b= 16.969(7), c= 8.977(3)A, and Z= 4. The complex cation contains a quadridentate Schiff base in the equatorial plane and two imidazole molecules at the axial positions. The mean Fe–N (Schiff base), Fe–O, and Fe–N (imidazole) bond distances are 1.899, 1.920, and 1.990 A for (1), respectively, 2.108, 1.917, and 2.143 A for (2), and 2.125, 1.896, and 2.165 A for (3), indicating that the metal to imine nitrogen bond distances are sensitive to the spin state of iron, whereas the changes in the Fe–O bond distances are very little affected. The cause of this trend, which was also found for other iron(III) complexes, is discussed in terms of the feasibility of π bonding. No relation between the relative orientation of axial imidazole rings and spin state was detected.