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 = L_a= 1,4,8,11-tetra-azacyclotetradecane or L_b= 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 = L_a 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 Pb^II. 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)Å, 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 = L_b 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.