Controlled Copper-Mediated Chlorination of Phenol Rings under Mild Conditions

The very unusual case of copper-mediated chlorination of phenol rings under mild conditions at room temperature is reported. Reaction of the ligand 1,7-bis(2-hydroxyphenyl)-2,6-diaza-4-hydroxylheptane (H<sub>3</sub>L1) with CuCl<sub>2</sub> in acetonitrile leads to either the formation of a tetranuclear copper(II) complex [Cu<sub>4</sub>(HL3)<sub>2</sub>(μ-Cl)<sub>2</sub>Cl<sub>2</sub>](CH<sub>3</sub>CN) (<b>1</b>) or a linear trinuclear complex [Cu<sub>3</sub>(HL1)<sub>2</sub>Cl<sub>2</sub>(CH<sub>3</sub>CN)<sub>2</sub>](CH<sub>3</sub>CN)<sub>2</sub> (<b>2</b>), depending on the reaction conditions. Both compounds have been fully characterized, including the determination of their 3D structures by X-ray diffraction. The unprecedented tetranuclear compound <b>1</b> is constituted of a dichlorido-bridged dimer of di-μ-phenoxido-dinuclear species, whereas the trinuclear complex <b>2</b> presents a linear array of copper(II) ions, held together through di-μ-phenoxido bridges of the central and external ions. The magnetic susceptibility of the two compounds was investigated, revealing either very strong (<i>J</i> < −500 cm<sup>-1</sup>) or strong (<i>J</i> value around −370(1) cm<sup>-1</sup>) antiferromagnetic dominant interactions among the Cu<sup>II</sup> ions for <b>1</b> and <b>2</b>, respectively. The tetranuclear complex <b>1</b> is obtained, under dry conditions, through the in situ formation of ligand HL3 (H<sub>3</sub>L3 = 1,7-bis(2-hydroxy-5-chlorophenyl)-2,6-diaza-4-hydroxylheptane) by oxidative chlorination of (HL1)<sup>2-</sup>. In the presence of traces of water, <b>1</b> is partially hydroxylated at the ortho position of one of the phenyl rings. The use of trimethylorthoformate as the dehydrating agent prevents the formation of hydroxylated ligands. Several partly chlorinated/hydroxylated products (identified as H<sub>3</sub>L2) have also been obtained through slight variations of the synthetic procedures (presence or absence of water and/or triethylamine in the reaction mixtures). These partially chlorinated and/or hydroxylated coordination species are mutually isomorphous to either <b>1</b> or <b>2</b>. Several “modified” ligands have been isolated and characterized by <sup>1</sup>H NMR and MS, after reaction with sodium sulfide of the complexes formed.