Acid‐Base Self‐Assembly Chemistry and Hydrogen Bonding Interactions Resulting in the Formation of a Tetranuclear Aggregate Containing Four Crystallographically Non‐Equivalent FeIII Centers

The synthesis and characterization of a mononuclear iron(III) complex [Fe(Hphpz)Cl(CH3OH)] (1) and a tetranuclear assembly of a bis(μ‐hydroxo)‐bridged dinuclear iron(III) complex [{Fe(Hphpz)2(OH)}2][Fe(Hphpz)3]2 (2) with the ligand 3(5)‐methyl‐5(3)‐(2‐hydroxyphenyl)pyrazole (H2phpz) is reported. Compound 1 is converted into 2 in a methanol solution and in the presence of a base. The asymmetric unit of 2 consists of two independent [Fe(Hphpz)3] molecules and one dinuclear complex [Fe(Hphpz)2(OH)]2 containing two crystallographically non‐equivalent iron(III) ions; the three units are linked by N−H···O and O−H···O hydrogen bonding and stacking interactions to form a cluster with an approximate C2‐symmetry. The Mössbauer spectra of [{Fe(Hphpz)2(OH)}2][Fe(phpz)3]2 have been measured at 295 and 78 K and indicate the presence of four crystallographically distinct, but chemically similar iron(III) sites. The magnetic properties of this compound originate from the weak antiferromagnetic interaction between the FeIII ions of the [Fe(phpz)2(OH)]2 dimer (J = −2.8 cm−1). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)