Synthesis and characterisation of 1,7-dimethyl-1,4,7,10-tetra-azacyclododecane: crystal structure of the nickel(II) bromide monohydrate complex of this macrocycle; thermodynamic studies of protonation and metal complex formation

Abstract

The synthesis and characterisation of the macrocycle 1,7-dimethyl-1,4,7,10-tetra-azacyclododecane (L¹) are reported. The basicity constants of L¹ and the stability constant of the complex [CuL¹]²⁺ have been determined by potentiometry at 25 °C in 0.5 mol dm^–3 KNO₃. The macrocycle L¹ behaves as a diprotic base in the pH range investigated; pD–¹³C n.m.r. studies indicate that only the two secondary nitrogens are involved in the protonation process. Copper(II) and nickel(II) complexes with L¹ have been prepared and studied spectrophotometrically both in aqueous solution and in the solid state. The reflectance spectrum of dry yellow NiL¹(ClO₄)₂ shows that a square-planar geometry, with the nickel(II) in the low-spin state, is achievable also with this partially methylated twelve-macrocycle. The molecular structure of the complex [NiBr(H₂O)L¹]Br has been determined by single-crystal X-ray analysis. The compound crystallises in an orthorhombic unit cell (space group P2₁2₁2₁) with lattice constants a= 13.478(5), b= 11.186(4), and c= 10.585(4)Å for Z= 4. Least-squares refinement converged at R= 0.046 for 910 observed reflections. The complex shows a cis-octahedral geometry, with the macrocycle co-ordinated in a folded configuration to four sites around the central nickel atom. The chirality of each nitrogen atom of the macrocycle is the same. The bromide ion and the oxygen atom of the water molecule are cis to each other. The chelate rings are all asymmetric.