X-Ray structural study of [Fe(saltrien)]X (X = Br·2H2O, BPh4 or PF6). Origin of unusual magnetic behaviour of the spin-crossover complex [Fe(saltrien)]PF6

Abstract

The crystal structures of three six-co-ordinate iron(III) complexes; [Fe(saltrien)] Br·2H₂O (low-spin)(1), [Fe(saitrien)]BPh₄(spin-crossover)(2), and [Fe(saitrien)]PF₆(spin-crossover)(3) have been determined by X-ray analysis at 293 K, where H₂, saitrien denotes the Schiff base derived from salicyiaidehyde and triethylenetetra-amine. Crystals of (1) are monoclinic with a= 24.601(3), b= 10.024(2), c= 12.084(2)Åβ= 131.934(9)°, space group P2₁/c, and Z= 4. Crystals of (2) are monoclinic with a= 25.99(1), b= 10.825(7), c= 28.418(9)Å, β= 102.75(3)°, space group C2/c, and Z= 8. Crystals of (3) are monoclinic with a= 9.526(2), b= 27.646(4), c= 17.724(2)Å, β= 92.78(1)°, space group P2₁/c, and Z= 8. In all three complexes the geometry around the iron(III) ion is best described as a distorted octahedron, but substantial differences are observed for the bond distances and angles among these complexes; e.g. the Fe–N distances in (2) and (3)(2.1–2.2 Å) are much longer than those of (1)(1.94–2.00 Å), and the 12 angles subtended at the metal atom by adjacent atoms lie in the range 75ndash;105° in (2) and (3), but 84–95° in (1). Crystals of (3) contain two slightly structurally different complex cations in an asymmetric unit, and the unusual variation of magnetic moment with temperature of this compound has been attributed to the presence of two crystallographically independent cations, one of which undergoes the high-spin → low-spin transformation in the temperature range 100–200 K, whereas for the other this occurs below 100 K.