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 (LtBu-amine)(2-) were synthesized and characterized [H2Lamine = 2,2'-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)diphenol; H2Ltbu-amine = 6,6'-(2-methyl-2-(pyridin-2-yl)propane-1,3-diyl)bis(azanediyl)bis(methylene)bis(2,4-di-tert-butylphenol)]. Dimeric [(FeLamine)(2)(mu-OH)]BPh4 (1) and [(FeLtBu-amino)(2)(mu-O)] (2), and monomeric [FeLtBu-amine(OCH3)] (3) were synthesized from ferric perchlorate. Monomeric [(FeLCl)-Cl-amine] (4) and [FeLtBu-amineCl] (5) were synthesized from ferric chloride. Complex 1 is comprised of two [(FeLCl)-Cl-amine](+) units, where (L-amine)(2-) is pentadentate, bridged by a single hydroxo ligand. Intramolecular H-bonding between the NH groups on one [Fe-Lamine](+) unit and the phenolato O atoms of the other [FeLamine](+) unit stabilizes the structure. Complex 2 is an oxobridged dimer, but the steric constraints from the tert-butyl groups of (LtBu-amine)(2-) prevent intramolecular H-bonding. Monomeric 4 and 5 are isostructural with pentadentate ligands (L-amine])(2-) or (LtBu-amine)(2-), respectively, and a chloro ligand in the sixth position of the six-coordinate iron(III) moieties. Conversion of 2 to its protonated hydroxa-bridged analog [(FeLtBu-amine)(2)(mu-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, [(FeLamine)(2)(mu-O)], suggesting that the pK(a) values of the hydroxo groups in 1 and [(FeLtBu-amine)(2)(mu-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 [(FeLtBu-amine)(2)(mu-OH)](+) grew in. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)