Synthesis and structural studies of a dimeric bis(oxamate) copper(II) complex

Abstract

A new series of a dimeric bis(oxamato) copper(II) complex of formula M4[Cu(oeo)]2 (where Et2H2(oeo) is diethyl ethylene-1,2-dioxamate and M = alkali metal) is described. The complex is formed by two copper(II) atoms and two ligand molecules of which each oxamato group is coordinating towards two different copper(II) ions leading to a distorted square planar environment. Due to the trans-bis(N,O) arrangement around the copper(II) ion, the dimer displays a helicoidal structure. We present here detailed structural studies of this complex with a number of counter ions. Many polymorphs and hydrates are observed and their synthesis and structures are described. A sequential strategy has been developed in which starting from the ligand precursor, Et2H2oeo, the addition of alkali metal hydroxides affords the hydrolysis of the ester leading to the formation of the salt of the ester, M2H2oeo. Then, transmetallation of the alkali metal salt is carried out with the addition of a copper(II) salt. The dimeric copper(II) complex, M4[Cu(oeo)]2, is finally achieved with the addition of the corresponding alkali metal hydroxide. Structural studies of the intermediate compounds are also described. The partial occupancy in the copper(II) site is a common feature in M4[Cu(oeo)]2. Surprisingly, the copper(II) site can be left vacant and the compound still remain with the same helicoidal structure as the non-coordinated ligand, when the amide is protonated, adopting a structurally similar H-bond arrangement. A wide range of copper(II) occupancies have been observed in the different obtained compounds: from compounds with a fully occupied site to compounds with only 10% of copper(II)present in the complex . These differences in the amount of copper(II) present in the complex has consequences in its packing in the crystal structure. The synthesis and characterisation of the copper(II) complex using tetrabutylammonium hydroxide as the counterion is also reported. As a result of the bulky nature of the cation, the mononuclear structure is observed, (n-Bu4N)4[Cu(oeoH)2]. Initial transmetallation reactions are reported with Co(II), in which partial and complete exchange from Cu(II) is observed