The Stereochemical activity or non-activity of the ‘Inert’ pair of electrons on lead(II) in relation to its complex stability and structural properties. Some considerations in ligand design

Abstract

The role of the lone pair of electrons on Pb(II) in its coordination geometry and complex stability is examined. In a series of macrocyclic ligands where oxygen donors are successively replaced by nitrogen donors, it is found that when three or four nitrogens are present, there is a sudden marked increase in the rate of change of complex stability per nitrogen donor added. This is attributed to a change from a stereochemically inactive lone pair with approximately two or fewer nitrogen donors present, to an active lone pair. Below the transition point, the Pb(II) ion behaves as a large metal ion with rather ionic ML bonding. In this state it responds to added oxygen donor bearing groups as expected for such a metal ion. Thus, the size-related selectivity patterns of Pb(II) with the ligand DAK-22 (4,7,13,16-tetraoxa-1,10-diazacyclooctadecane- N,N′-diacetate) are as expected for its size. The protonation constants and formation constants of DAK-22 with several metal ions are reported. For the complexes formed by 12-aneN 4 (1,4,7,10-tetraazacyclododecane) and 12-aneN 3O (1-oxa-4,7,10-triazacyclododecane) the Pb(II) appears to have a stereochemically active lone pair. Thus, when N-(2-hydroxypropyl) groups are added to 12-aneN 4 and 12-aneN 3O to give the ligands THP-12-aneN 4 and THP-12-aneN 3O, the Pb(II) ion does not respond to the added hydroxyalkyl groups as might have been expected. It behaves as a smaller more covalent ion, and a study of the formation constants of THP-12-aneN 4 and THP-12-aneN 3 with Cu(II), Zn(II), Cd(II), Pb(II), Ca(II), Sr(II) and Ba(II) reveals lower than anticipated Pb/Zn selectivities. A crystallographic study of [Pb(C 20Hn 44N 4O 4)](NO 3) 2·C 3H 8O reveals that there is space between the O donors for a stereochemically active lone pair, but the lack of shortening of the PbN bonds suggests that the lone pair is not active. The complex crystallizes in the orthorhombic system, space group Pnma, with cell dimensions a=10.352(8), b=14.781(2), and c=21.850(4) Å, Z=4. A final conventional R=0.056 was obtained. Although the ligand THP-12-aneN 4 has four chiral carbon atoms, the crystal structure suggests that only the RRRR and SSSS enantiomers of the free ligand are obtained after recrystallisation from n-hexane. The structure indicates that the [Pb(THP-12-aneN 4)] 2+ cations are disordered, with 50% site occupancy by the RRRR and by the SSSS conformer.