Most Read
Early Alert
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
Abstract
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.
Australian Journal of Chemistry 52(2) 109 - 122 (1999) doi:10.1071/C98083
