Relating Structural and Thermodynamic Effects of the Pb(II) Lone Pair: A New Picolinate Ligand Designed to Accommodate the Pb(II) Lone Pair Leads to High Stability and Selectivity

The crystal and molecular structure and the stability of lead and calcium complexes of two chelates containing
picolinate chelating groups in different geometries have been investigated in order to relate the ligand affinity and
selectivity 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- show
that the three picolinate arms of the tripodal ligand coordinate the lead in an asymmetric way leaving a gap in the
coordination sphere to accommodate the lead lone pair. As a consequence of this binding mode, one picolinate
arm 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 the
sterical interaction between the lead lone pair with respect to the H3tpaa complexes. As a result, in the lead
structures 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)) provides
direct evidence of the influence of the stereochemically active lead lone pair on complex stability. As a result, since
the ligand geometry has little effect on the stability of the calcium complex, a remarkable increase in the Pb/Ca
selectivity is observed for dpaea-(106.6) compared to tpaa3- (101.5), making the dipodal ligand a good candidate
for application as extracting agent for the lead removal from contaminated water.