A potentiometric and spectroscopic study of copper(II) diamidodiamino complexes

Abstract

Five diamidodiamino compounds, N,N′-bis[2-(dimethylamino)ethyl]propanediamide dihydrochloride L²·2HCl, N,N′-bis[2-(diethylamino)ethyl]propanediamide dihydrochloride (L⁵), N,N′-bis[2-(dimethylamino)ethyl]- and N,N′-bis[2-(diethylamino)ethyl]-ethanediamide (L¹ and L⁴) and N,N′-bis[2-(dimethylamino)propyl]ethanediamide (L³), have been synthesized and characterized. Their acid–base behaviour and complexation with copper(II) ions in aqueous solution at 25 °C and I= 0.15 mol dm^–3 Cl^– have been investigated by potentiometric and spectrophotometric techniques. Relatively small differences observed between the protonation constants may be explained in terms of electronic induction effects of the alkyl side chains. The primary and tertiary amine groups show differences in basicity which conform to the general pattern of behaviour found with amines and attributable to changes in solvation of the reacting species in the protonation reactions. Copper(II) ions have been found to form several soluble complexes with L¹–L⁴. On the other hand, L⁵ precipitates in the presence of copper(II) ions. The stoichiometries of the mononuclear copper(II) complexes are proposed as CuL, CuLH_–1 and CuLH_–2 with L³ also forming CuLH_–3. Dinuclear complexes, Cu₂LH_–2 are proposed for L¹, L³ and L⁴. In addition, Cu₂LH_–3 is proposed for L¹. The co-ordination sites and structures are suggested to be as follows: CuL, two contiguous chelate rings with the copper(II) co-ordinated to one amine N, one amide N and one carboxylate O; CuLH_–1, similar to CuL but with co-ordination to both amide N and one of the amine N; CuLH_–2, three contiguous chelate rings with co-ordination to all four N; CuLH_–3, the same as CuLH_–2 but with a proton removed from a water molecule axially co-ordinated to the copper; Cu₂LH_–2(L = L¹ or L⁴) and Cu₂LH_–3(L = L¹), similar to CuL but incorporating two sets of co-ordination sites, one amine N plus one amide N plus one carboxylate O, per copper, giving four contiguous chelate rings; in Cu₂LH_–3 a proton is removed from a water molecule co-ordinated to one of the coppers; Cu₂LH_–2(L = L³), one of the coppers is co-ordinated to all four N and the other to the two carboxylate O.