Coordination Chemistry of Unsymmetrical Tripodal Ligands with an NNO2 Donor Set

Abstract

The synthesis of two new tripodal ligands N(CH2CH2NH2)(CH2CH2CH2OH)2 (H4-1) and N[2,3,5-C6H2-(OH)(SCH3)(CH3)](CH2CH2CH2NH2)(CH2CH2CH2OH) (H4-2) is reported. Both tetradentate ligands contain a central tertiary nitrogen atom, as well as two OH and one NH2 functionalized ligand arm. The tripods do not only exhibit an unsymmetrical N2O2 donor set, but also possess two C3 and one C2 chains between the central nitrogen atom and the terminal donors. On coordination of the central tertiary nitrogen atom, the ligands are capable of forming both six- and five-membered chelate rings in their metal complexes. Both ligands react with [Cu(OAc)2·H2O]2 to give the dinuclear copper(II) complexes [Cu(η4-μ-O-H3-1)2Cu](PF6)2 (3) and [(η3-H3-2)Cu(μ-OAc)2Cu(η3-H3-2)]·2CH3CN (4·2CH3CN). The molecular structures of 3 and 4·2CH3CN have been determined by X-ray diffraction. Complex 3 contains two slightly distorted square-pyramidal (τ = 0.181) copper atoms, with O-donors in the apical positions. The dinuclear complex 4, which was synthesized to model the copper site in galactose oxidase, also shows a distorted square-pyramidal coordination geometry (τ = 0.205 and τ = 0.101) around both copper(II) atoms. Complex 4 contains two uncoordinated primary alcohol functionalities of the ligands. In addition, both the ligand H4-2 and its dinuclear copper complex [(η3-H3-2)Cu(μ-OAc)2(η3-H3-2)]·2CH3CN can easily be oxidized to yield free or coordinated phenoxyl radicals, which are stable on the time scale of cyclic voltammetry.