Properties and reactivity of unusual diiron complexes of a linear tetradentate ligand. Crystal structures of diiron-(II) and -(III) complexes

Abstract

Dinuclear iron complexes containing the tetradentate N₄ ligand N,N′-dimethyl-N,N′-bis(2-pyridyl-methyl)-ethane-1,2-diamine (L) have been prepared. The diiron(II) complex [LFe(O₂CMe)₂FeL]-[ClO₄]₂1 has been characterized by X-ray crystallography: triclinic, space group P, with a= 13.756(3), b= 15.033(3), c= 13.637(3)Å, α= 97.82(1), β= 108.95(1), γ= 118.07(1)° and Z= 2. The structure refined to a final R value of 0.046 for 3556 reflections. The complex contains two iron(II) ions bridged by two acetate groups, Fe ⋯ Fe 4.382(2)Å. Each acetate bridges the iron atoms in an O,O′ mode using the syn lone pair of one carboxylate oxygen atom and the anti Ione pair of the other. The ligand shows a cis-α conformation about the iron(II) centres. Complex 1 is readily oxidized in solution by air and from these solutions a diiron(III) complex containing a µ-oxo-µ-acetato core, [LFeO(O₂CMe)FeL][ClO₄]₃, was recovered. Another diiron(III) complex, prepared in the absence of other possible bridging groups, is formulated as [LFeO(OH)FeL][ClO₄]₃·2H₂O, 2. The spectroscopic data for it are consistent with the unusual formulation of a doubly bridged µ-oxo-µ-hydroxo-diiron(III) core, only the second reported example of a small molecule with this core structure. In solution, 2 is sensitive towards carbon dioxide, reacting to give [LFeO(CO₃)FeL][ClO₄]₂·2H₂O, 3. The structure of 3 has been solved by X-ray crystallography and shows a dibridged µ-oxo-µ-carbonato-O,O′ core: monoclinic, space group C2/c, a= 19.565(8), b= 13.427(2), c= 13.722(9)Å, β= 94.31(3)° and Z= 8. The structure refined to a final R value of 0.066 for 4575 reflections. The average Fe–(µ-O) is 1.804(5)Å, Fe-O-Fe is 126.1(3)° and Fe ⋯ Fe 3.216(1)Å. Again L shows a cis-α conformation. Complex 2 was used to prepare other complexes containing the [LFeOFeL]⁴⁺ core: [LFeO(O₂CMe)FeL][ClO₄]₃, [LFeO(O₂CH)-FeL][ClO₄]₃ and [LFeO(SO₄)FeL][ClO₄]₂, by addition of acetic acid, formic acid and sodium hydrogensulfate respectively; and [L(Cl)FeOFe(Cl)L][ClO₄]₂ by addition of chloride ions.