Australian Journal of Chemistry Australian Journal of Chemistry Society
An international journal for chemical science

Magnetic, Spectral and Structural Aspects of Spin Transitions in Iron(II) Complexes of 2-(Pyrazol-3-yl)pyridine and 3-(Thiazol-2-yl)pyrazole

Lucia S. Harimanow, Kristian H. Sugiyarto, Donald C. Craig, Marcia L. Scudder and Harold A. Goodwin

Australian Journal of Chemistry 52(2) 109 - 122
Published: 1999


Tris(ligand)iron(II) complexes of 2-(pyrazol-3-yl)pyridine (3ppH) and 3-(thiazol-2-yl)pyrazole (3tpH) undergo temperature-induced singlet (1A1) ⇔ quintet (5T2) transitions. The transition in [Fe(3ppH)3] [CF3SO3]2.2H2O is continuous and centred above room temperature while that in the anhydrous triflate salt is discontinuous and is centred below room temperature. The latter transition occurs via a thermal hysteresis loop of width 12 K, Tc↓ and Tc↑ being 229 and 241 K, respectively. The displacement of the transition to lower temperature in the anhydrous salt is believed to be associated with the loss of hydrogen bonding involving the uncoordinated pyrazole >NH group and solvate water. In [Fe(3tpH)2(3tp)] [ClO4].2H2O and [Fe(3tpH)2(3tp)] [BF4].2H2O (3tp is the deprotonated ligand) continuous transitions are observed, centred below room temperature. In these instances the displacement is consistent with the intrinsically weaker field of the bidentate system containing two five-membered heterocycles. Structural data were obtained for [Fe(3ppH)3][CF3SO3]2.2H2O, [Fe(3tpH)3] [BF4]2.1·5H2O and [Ni(3tpH)3] [BF4]2.2(3tpH). The average metal–nitrogen distances in the complexes are 1·97, 2·18 and 2·09 Å, severally. The large difference in the distances for the two iron complexes arises from the different ground states: a singlet for the 3ppH complex and a quintet for the 3tpH complex. In all three salts there is extensive hydrogen bonding involving the pyrazole >NH groups, the anions and the solvate molecules. [Fe(3ppH)3] [CF3SO3]2.2H2O: monoclinic, space group P21/c, a 12·33(1), b 24·44(1), c 12·55(1) Å, β 115·27(4)°, Z 4. [Fe(3tpH)3] [BF4]2.1·5H2O: monoclinic, space group C 2/c, a 41·56(2), b 16·418(3), c 18·154(7) Å, β 106·94(2)°, Z 8. [Ni(3tpH)3] [BF4]2.2(3tpH):P bcn, a 14·928(2), b 15·310 (3), c 17·882 (3) Å, Z 4.

© CSIRO 1999

Rent Article (via Deepdyve) Export Citation Cited By (18)