Crystal and molecular structure and solution behaviour of low-spin (3-methyl-1,3,5,8,12-pentaazacyclotetradecane)κ4N1,N5,N8,N12)nickel(II) diperchlorate

Abstract

Template reaction of 3,7-diazanonane-1,9-diamine, formaldehyde and methylamine, in the presence of Ni(CIO₄)₂, gives the pentaazamacrocyclic low-spin complex [Ni(azacyclam)][CIO₄]₂(azacyclam = 3-methyl-1,3,5,8,12-pentaazacyclotetradecane), whose crystal and molecular structure was determined from single-crystal X-ray diffraction data collected with the use of Cu-Kα radiation: space group P2₁2₁2₁ with a= 15.967(3), b= 13.497(3), c= 8.737(2)Å, α=β=γ= 90°, Z= 4 (R= 0.0705). Only the four secondary amine nitrogen atoms of azacyclam are bound to the metal, according to a regular square-planar stereochemistry, the lone pair of the tertiary nitrogen atom, N(1), being exposed. In particular, the N(1)C₃ group is flattened, probably due to a long-range electrostatic interaction between N(1) and the metal. As a consequence, N(1) displays a very small affinity towards aqueous H⁺(pK_a < 2). The [Ni(azacyclam)]²⁺ complex in aqueous solution displays the high spin–low spin (blue-to-yellow) interconversion, typical of complexes of cyclam. However, in contrast, addition of HCI favours formation of the high-spin form. This unusual behaviour is ascribed to protonation of the N(1) atom, at acid concentrations of 0.5–1.0 mol dm^–3, and to axial co-ordination of a hydrogen-bonded chloride ion. Addition of HCIO₄ first favours formation of the high-spin species, then that of the low-spin complex: in this case, the hydrogen-bond-facilitated axial co-ordination of a water molecule is hypothesized.