Synthesis and crystal structure of the iron(II)–iron(III) complex [FeL][FeCl₄]Cl [L = 1,4,7-tris(pyrazol-3-ylmethyl)-1,4,7-triazacyclononane]

Abstract

The pyrazole-functionalized macrocycles 1,4,7-tris(pyrazol-3-ylmethyl)-1,4,7-triazacyclononane, L, and 1,4,7-tris(pyrazol-1-ylmethyl)-1,4,7-triazacyclononane, L′, gave mixed-valence iron complexes of formulae [FeL][FeCl₄]Cl and [FeL′][FeCl₄]Cl, which are considered to contain the high-spin iron(II) [FeL]²⁺ and, respectively, [FeL′]²⁺ cations, besides [FeCl₄]^- and Cl^- anions. The diamagnetic compound [FeL′][ClO₄]₂ has also been obtained. There is evidence for the iron(II) contained in [FeL][FeCl₄]Cl being formed in the reduction of iron(III), accomplished by primary alcohols in the presence of L·6HCl. The assignment of the [FeL][FeCl₄]Cl formula to the L derivative is supported by the results of an X-ray structural determination. The iron(II) in the [FeL]²⁺ cation is in a six-co-ordinate environment formed by the donor nitrogen atoms of the L ligand. The Fe–N bond distances are 2.274(4) Å (macrocycle nitrogens) and 2.120(4) Å (pyrazole nitrogens). Comparison with the much shorter distances detected for the diamagnetic compound [FeL′][ClO₄]₂ by a (poor-quality) structure determination supports the assignment of the iron(II) high-spin nature to the metal ion in the [FeL]²⁺ cation.

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