Structural and dynamic behaviour of complexes of lead(II) with two tetra-aza macrocyclic ligands as studied by X-ray crystallography and natural-abundance carbon-13 and nitrogen-15 nuclear magnetic resonance spectroscopy

Abstract

Two complexes of the type [PbL][NO3]2, where L = La= 1,4,8,11-tetra-azacyclotetradecane or Lb= 1,4,8,11-tetramethyl-1,4,8,11-tetra-azacyclotetradecane, have been investigated by n.m.r. techniques in dimethyl sulphoxide (dmso) solution. The complex where L = La is interpreted as adopting a cis-octahedral geometry with a folded macrocyclic ligand. This geometry is confirmed by an X-ray analysis which also identifies unsymmetrical bidentate nitrate ions in cis positions. The asymmetry of these groups is probably caused by the lone pair on PbII. The complex crystallises in the monoclinic space group P21/c, Z= 4, with a= 10.326(2), b= 11.145(3), c= 14.832(4)A, and β= 96.19(2)°. 1 909 Reflections with I/σ(I) > 3.0 have been recorded on a four-circle diffractometer and least-squares refinement produced a final R of 0.049. The n.m.r. data reveal high-temperature, dynamic, conformational changes, and two strikingly different 1J(207Pb–15N) coupling constants (207.5 and 19.8 Hz) at low temperatures. The dynamic process, with ΔH‡= 110 ± 10 kJ mol–1 and ΔS‡= 105 ± 20 J K–1 mol–1, is interpreted as an intramolecular rearrangement between two identical cisoid structures, while the differing coupling constants for axial and equatorial nitrogen atoms are believed to be a consequence of the axial distortion observed in the crystal structure. When L = Lb the complex is not very stable in dmso (K= 1.31 dm3 mol–1 at 298 K) and macrocycle exchange is found to occur at ambient temperatures (ΔH‡= 38.5 ± 0.7 kJ mol–1, ΔS‡=–70 ± 3 J K–1 mol–1). The structure and behaviour of both complexes are discussed in relation to the ionic radius of the lead(II) ion.