Synthesis, metal-exchange kinetics and X-ray structure of the nickel(II) complex of the diazacyclam ligand 1,3,6,10,12,15-hexaazatricyclo [13.3.1.1^6,10]eicosane, [NiL] (ClO₄)₂

The preparation of the nickel(II) complex of the diazacyclam ligand 1,3,6,10,12,15-hexaazatricyclo [13.3.1.1^6,10]eicosane (2) by the reaction of the nickel(II) complex of N-(2-aminoethyl)-1,3-diaminopropane with formaldehyde in MeOH solution is described. The crystal structure of [NiL](ClO₄)₂ has been determined. The nickel atom is four coordinate and planar with Ni-N bond lengths of 1.969(4) and 1.928(3) Å in a centrosymmetric structure. The basic diazacyclam ring system has a trans III configuration with the two additional six-membered rings fused in a chair conformation. The kinetics of the metal exchange: [NiL]²⁺ + Cu²⁺ →^k [CuL]²⁺ + Ni²⁺ for the nickel complexes (1) and (2) have been studied in detail. Under the experimental conditions employed, with copper(II) in at least a tenfold excess, the reaction is independent of the copper(II) concentration. The copper(II) effectively scavenges the free ligand as the nickel(II) complex dissociates. For the nickel complex (1) k = 2 × 10^-4 s^-1 at 60°C and ΔH‡ = 126 ± 5 kJ mol^-1 and ΔS‡₂₉₈ = 61 ± 15 J K^-1 mol^-1. For the complex (2), k = 1.8 × 10^-4 s^-1 at 60°C and ΔH‡ = 99 ± 6 kJ mol^-1 and ΔS‡₂₉₈ = -21 ± 10 J K^-1 mol^-1.