LEAD MACROCYCLIC COMPLEXES - THE SYNTHESIS, COMPLEX-FORMATION AND X-RAY CRYSTAL-STRUCTURES OF [PB(L1)(NO3)2] AND [PB(L2)(NO3)2] (L1 = 1,4,7,10-TETRAOXA-13-AZACYCLOPENTADECANE, L2 = 1,4,7,10,13-PENTAOXA-16-AZACYCLOOCTADECANE)

Lead(II) complexes of the 15- and 18-membered ring macrocycles 1,4,7,10-tetraoxa-13-azacyclopentadecane (L1) and 1,4,7,10,13-pentaoxa-16-azacyclooctadecane (L2) have been prepared. The stability constants for the 1 : 1 lead complexes [L1, log beta-6.0(1); L2, log beta-8.4(1)] have been determined potentiometrically (0.1 M NEt4ClO4, 95% methanol). The complexes [Pb(L1)(NO3)2] and [Pb(L2)(NO3)2] have been examined by C-13 NMR spectroscopy and single-crystal X-ray structural analysis. In the molecule [Pb(L1)(NO3)2] the lead(II) cation is situated 1.52 angstrom above the plane of the macrocyclic ring. The Pb-N(1) distance of 2.465(5) angstrom is the shortest bond to lead(II) in the structure which also exhibits two short [2.627(4) and 2.643(4) angstrom] and two long [2.909(4) and 2.992(5) angstrom] Pb-O(macrocycle) distances. The presence of a stereoactive lone pair of electrons on the cation is inferred from this stereochemistry. In [Pb(L2)(NO3)2] the lead(II) cation lies in the macrocyclic cavity. As for [Pb(L1)(NO3)2], the Pb-N(1) distance of 2.539(9) angstrom is the shortest bond to lead in this structure, which has in addition two short [2.694(7) and 2.697(8) angstrom] and three long [2.877(10), 2.951(6) and 2.999(9) angstrom] Pb-O(macrocycle) interactions. There is no evidence for a stereoactive lone pair of electrons in this structure.