Unsupported μ‐Oxo‐ and μ‐Hydroxo‐Iron(III) Dimers and Mononuclear Iron(III) Complexes with Pyridylbis(aminophenol) Ligands

Unsupported hydroxo‐ and oxo‐bridged diiron(III) and mononuclear iron(III) complexes with pyridylbis(aminophenol) ligands (L amine ) 2– and (L t Bu‐amine ) 2– were synthesized and characterized [H 2 L amine = 2,2′‐(2‐methyl‐2‐(pyridin‐2‐yl)propane‐1,3‐diyl)bis(azanediyl)bis(methylene)diphenol; H 2 L t Bu‐amine =6,6′‐(2‐methyl‐2‐(pyridin‐2‐yl)propane‐1,3‐diyl)bis(azanediyl)bis(methylene)bis(2,4‐di‐ tert ‐butylphenol)]. Dimeric [(FeL amine ) 2 (μ‐OH)]BPh 4 ( 1 ) and [(FeL t Bu‐amine ) 2 (μ‐O)] ( 2 ), and monomeric [FeL t Bu‐amine (OCH 3 )] ( 3 ) were synthesized from ferric perchlorate. Monomeric [FeL amine Cl] ( 4 ) and[FeL t Bu‐amine Cl] ( 5 ) were synthesized from ferric chloride. Complex 1 is comprised of two [FeL amine ] + units, where(L amine ) 2– is pentadentate, bridged by a single hydroxo ligand. Intramolecular H‐bonding between the NH groups on one [FeL amine ] + unit and the phenolato O atoms of the other [FeL amine ] + unit stabilizes the structure. Complex 2 is an oxo‐bridged dimer, but the steric constraints from the tert ‐butyl groups of (L t Bu‐amine ) 2– prevent intramolecular H‐bonding. Monomeric 4 and 5 are isostructural with pentadentate ligands (L amine ) 2– or (L t Bu‐amine ) 2– , respectively, and a chloro ligand in the sixth position of the six‐coordinate iron(III) moieties. Conversion of 2 to its protonated hydroxo‐bridged analog [(FeL t Bu‐amine ) 2 (μ‐OH)] + by treatment with mild acid was monitored spectroscopically and electrochemically, but the reaction was not reversible by treatment with base. Similarly, hydroxo‐bridged 1 could not be deprotonated to generate its oxo‐bridged analog, [(FeL amine ) 2 (μ‐O)], suggesting that the p K a values of the hydroxo groups in 1 and [(FeL t Bu‐amine ) 2 (μ‐OH)] + are quite large. The CV of 2 revealed well‐defined ligand‐based redox couples, but no metal‐based redox couples. When 2 was treated with mild acid, metal‐based redox couples for [(FeL t Bu‐amine ) 2 (μ‐OH)] + grew in. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)