Comparisons of thirteen- to sixteen-membered tetra-azacycloalkane copper(II) complexes derived from template syntheses involving nitroethane and formaldehyde. Crystal structures of (10-methyl-10-nitro-1,4,8,12-tetra-azacyclopentadecane)copper(II) and (3-methyl-3-nitro-1,5,9,13-tetra-azacyclohexadecane)copper(II) perchlorates

Abstract

Reaction of the (1,11-diamino-4,8-diazaundecane)copper(II) ion with formaldehyde and nitroethane yields the co-ordinated 16-membered macrocycle 3-methyl-3-nitro-1,5,9,13-tetra-azacyclohexadecane (L⁴). The 13- to 15-membered analogues 12-methyl-12-nitro-1,4,7,10-tetra-azacyclotridecane (L¹), 6-methyl-6-nitro-1,4,8,11-tetra-azacyclotetradecane (L²), and 10-methyl-10-nitro-1,4,8,12-tetra-azacyclopentadecane (L³) have been described, and structures reported for salts of the 13- and 14-membered complexes. Structures of compounds of the 15- and 16-membered ligands are now reported, and structural parameters for the series of copper(II) cations are compared. The [Cu(L³)]²⁺ and [Cu(L⁴)]²⁺ cations contain the copper in a distorted square plane of the macrocycle amine donors, with the pendant nitro group occupying one axial site (Cu–O 2.443 and 2.703 Å in the L³ and L⁴ compounds, respectively) and a perchlorate oxygen in the other (Cu–O ca. 2.58 Å in each case). The Cu–N distances lie in the range 2.005–2.093 Å in [Cu(L³)]²⁺ and 2.007–2.029 Å in [Cu(L⁴)]²⁺. The effect of ring size for the series on solution structures analysed by electronic and e.s.r. spectroscopy, on copper(II) reduction potentials, and on rate constants for acid hydrolysis are reported; the L² cavity appears most appropriate for accommodating copper(II).