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

Template reaction of 3,7-diazanonane-1,9-diamine, formaldehyde and methylamine, in the presence of Ni(ClO4)2, gives the pentaazamacrocyclic low-spin complex [Ni(azacyclam)][ClO4]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-K-alpha radiation: space group P2(1)2(1)2(1) with a = 15.967(3), b = 13.497(3), c = 8.737(2) angstrom, alpha = beta = gamma = 90-degrees, 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 HCl 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 HClO4 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.