Synthesis, stability and structure of the complex of bismuth(III) with the nitrogen-donor macrocycle 1,4,7,10-tetraazacyclododecane. The role of the lone pair on bismuth(III) and lead(II) in determining co-ordination geometry

Abstract

The complex [Bi(cyclen)(H ₂ O)(ClO ₄ ) ₃ ] (cyclen = 1,4,7,10-tetraazacyclododecane) has been synthesized and its crystal structure determined. The Bi has square-antiprismatic co-ordination to the four nitrogens of cyclen (mean Bi–N 2.39 Å) with the upper square being provided by one oxygen from each of the three perchlorates and a water molecule (mean Bi–O 2.77 Å). The formation constant (log K ₁ ) of Bi ^III with cyclen has been determined as 23.45 at I = 0.5 mol dm ^-3 and 25 °C by differential pulse polarography in an out-of-cell technique, where equilibration was slow and required 6 months. This is a high log K ₁ value for a complex with cyclen, in accord with predictions made of the high affinity that Bi ^III would have for polyamines on the basis of a model of acid–base interactions in aqueous solution. The structures of some complexes of Bi ^III and the isoelectronic Pb ^II were analysed in terms of the effect that the lone pair of electrons has on the M–L bond length (M = Bi or Pb, L = ligand donor atom). A relationship was found between the M–L bond length and the distance of L from the lone pair as quantified by the l.p.–M–L (l.p. = lone pair) angle, with M–L decreasing as l.p.–M–L increases. This observation is rationalized in terms of an analogy with the linear co-ordination geometry found in many complexes of Hg ^II , where the lone pair on Bi ^III or Pb ^II occupies one of the two co-ordination sites in the analogous linear two-co-ordinate mercury(II) complex.