| Abstract
| - The crystal and molecular structure and the stability of lead and calcium complexes of two chelates containingpicolinate chelating groups in different geometries have been investigated in order to relate the ligand affinity andselectivity for lead over calcium with the ability of the ligand to accommodate a stereochemically active lone pair.The crystal structures of the lead complexes of the diprotonated and monoprotonated tripodal ligand tpaa2- showthat the three picolinate arms of the tripodal ligand coordinate the lead in an asymmetric way leaving a gap in thecoordination sphere to accommodate the lead lone pair. As a consequence of this binding mode, one picolinatearm is very weakly bound and therefore can be expected to contribute very little to the complex stability. Conversely,the geometry of the dipodal ligand H2dpaea is designed to accommodate the lead lone pair; in the structure of the[Pb(dpaea)] complex the donor atoms of the ligand occupy only a quarter of the coordination sphere, reducing thesterical interaction between the lead lone pair with respect to the H3tpaa complexes. As a result, in the leadstructures of H2dpaea all the ligand donor atoms are strongly bound to the metal ion leading to increased stability.The high value of the formation constant measured for the lead complex of the dipodal dpaea2- (log β11(Pb) =12.1(3)) compared to the lower value found for the one of the tripodal tpaa3- (log β11(Pb) = 10.0(1)) providesdirect evidence of the influence of the stereochemically active lead lone pair on complex stability. As a result, sincethe ligand geometry has little effect on the stability of the calcium complex, a remarkable increase in the Pb/Caselectivity is observed for dpaea-(106.6) compared to tpaa3- (101.5), making the dipodal ligand a good candidatefor application as extracting agent for the lead removal from contaminated water.
- The crystal structures of the lead complexes of the tripodal ligand tpaa3- and of the dipodal ligand dpaea2- show that the geometry of dpaea2- allows the accommodation of the lead lone pair without creating sterical interaction between the donor atoms and the lone pair. The higher affinity for lead and the higher Pb/Ca selectivity of the dipicolinate ligand compared to the trispicolinate suggest that the lone pair may play an important role in the selective complexation of lead.
|
