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

The synthesis and characterization of a mononuclear iron( III ) complex [Fe(Hphpz)Cl(CH 3 OH)] ( 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 (H 2 phpz) 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 C 2 ‐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 Fe III ions of the [Fe(phpz) 2 (OH)] 2 dimer ( J = −2.8 cm −1 ). (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)