Synthetic and equilibrium studies on copper(II), nickel(II), and palladium(II) complexes of N,N′,N″,N‴-tetrakis(2-hydroxyethyl)-1,4,8,11-tetra-azacyclotetradecane (L); kinetics of the acid dissociation of the copper(II) complex and X-ray structural characterisation of the nickel(II) complex [NiLH–1][ClO4]

Abstract

The preparation of the macrocyclic ligand N,N′,N″,N‴-tetrakis(2-hydroxyethyl)-1,4,8,11-tetra-azacyclotetradecane (L) by reaction of ethylene oxide with cyclam (1,4,8,11-tetra-azacyclotetradecane) is described. The pK_a values of L at 25 °C and I= 0.1 mol dm^–3(NaClO₄) are pK₁= 8.83, pK₂= 8.30, pK₃= 2.65, pK₄ < 2. The ligand forms very stable complexes with copper(II) and palladium(II) of the type [ML]²⁺ with log β₁₁₀= 16.20 for copper(II) and 18.32 for palladium(II). The equilibria with nickel(II) can be represented by (i)–(iii), as follows. Ni²⁺+ L ⇌[NiL]²⁺; log β₁₁₀= 7.45(1)(i) Ni²⁺+ L ⇌[NiLH_–1]⁺+ H⁺; log β_11–1=–1.42(1)(ii) Ni²⁺+ L ⇌[NiLH_–2]+ 2H⁺; log β_11–2=–12.39(2)(iii) The pK values of [NiL]²⁺ are 8.87 and 10.97 at 25 °C. The complex [NiLH_–1][ ClO₄](C₁₈H₃₉ ClN₄NiO₈) has been characterised in the solid state and its crystal structure determined. In 0.1 mol dm^–3 sodium hydroxide solution E_½ for the nickel(III)–nickel(II) couple is +0.28 V, one of the lowest values recorded for a macrocyclic complex. Unlike complexes of cyclam, complexes of L are rapidly formed and dissociate readily in acidic solution. Stopped-flow studies of the dissociation of the copper(II) complex in the acidity range 0.015–0.50 mol dm^–3 HClO₄ establish that rate =k_H[CuL²⁺][H⁺] with k_H= 3.44 dm³ mol^–1 s^–1 at 25 °C and I= 1.0 mol dm^–3. The copper(II) and nickel(II) complexes [ML][ClO₄]₂ have been characterised in the solid state and their spectral properties are reported.