Structures and spin states of mono- and dinuclear iron(II) complexes of imidazole-4-carbaldehyde azine and its derivatives
Mononuclear [Fe(H 2L R) 2]X 2 (R = H, 2-Me, 5-Me, 2-Et-5-Me; X = ClO 4, BF 4) and dinuclear [Fe 2(H 2L R) 3]X 4 complexes containing imidazole-4-carbaldehyde azine (H 2L H) and its derivatives prepared by condensation of 4-formylimidazole, 2-methyl- or 5-methyl-4-formylimidazole, or 2-ethyl-4-methyl...
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Published in | Coordination chemistry reviews Vol. 254; no. 15; pp. 1871 - 1881 |
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Main Authors | , , , , , , , , |
Format | Journal Article |
Language | English |
Published |
Elsevier B.V
01.08.2010
|
Subjects | |
Online Access | Get full text |
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Summary: | Mononuclear [Fe(H
2L
R)
2]X
2 (R
=
H, 2-Me, 5-Me, 2-Et-5-Me; X
=
ClO
4, BF
4) and dinuclear [Fe
2(H
2L
R)
3]X
4 complexes containing imidazole-4-carbaldehyde azine (H
2L
H) and its derivatives prepared by condensation of 4-formylimidazole, 2-methyl- or 5-methyl-4-formylimidazole, or 2-ethyl-4-methyl-5-formylimidazole, with hydrazine in a 2:1 mole ratio in methanol, were prepared and their magnetostructural relationships were studied. In the mononuclear complexes, H
2L
R acts as an unsymmetrical tridentate ligand with two imidazole nitrogen atoms and one azine nitrogen atom, while in the dinuclear complexes, H
2L
R acts as a dinucleating ligand employing four nitrogen atoms to form a triple helicate structure. At room temperature, [Fe
2(H
2L
H)
3](ClO
4)
4 and [Fe
2(H
2L
2-Me)
3](ClO
4)
4 were in the high-spin (HS) and low-spin (LS) states, respectively. The results are in accordance with the ligand field strength of H
2L
2-Me with electron-donating methyl groups being stronger than H
2L
H, with the order of the ligand field strengths being H
2L
2-Me
>
H
2L
H. However, in the mononuclear [Fe(H
2L
H)
2](ClO
4)
2 and [Fe(H
2L
2-Me)
2](ClO
4)
2 complexes, a different order of ligand field strengths, H
2L
H
>
H
2L
2-Me, was observed because [Fe(H
2L
H)
2](ClO
4)
2 was in the LS state while [Fe(H
2L
2-Me)
2](ClO
4)
2 was in the HS state at room temperature. X-ray structural studies revealed that the interligand steric repulsion between a methyl group of an H
2L
2-Me ligand and the other ligand in [Fe(H
2L
2-Me)
2](ClO
4)
2 is responsible for the observed change in the spin state. The same is true for [Fe(H
2L
2-Et-5-Me)
2](ClO
4)
2, while [Fe(H
2L
5-Me)
2](ClO
4)
2 does not involve such a steric congestion and stays in the LS state over the temperature range 5–300
K. Two kinds of crystals (polymorphs) were isolated for [Fe
2(H
2L
H)
3](BF
4)
4 and [Fe
2(H
2L
2-Et-5-Me)
3](ClO
4)
4, and they exhibited different magnetic behaviors. |
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ISSN: | 0010-8545 1873-3840 |
DOI: | 10.1016/j.ccr.2009.11.016 |