Structural investigations of a lead(iv) tetraacetate–pyridine complex

Abstract

A 1 : 1 crystalline complex of lead(IV) tetraacetate and pyridine (LTA–py) has been prepared. The single-crystal X-ray structure, at 296 and 150 K, establishes the presence of a relatively short Pb–N bond (2.307 Å) within an intriguing seven-coordinate lead inner sphere consisting of the pyridine ligand and two bidentate and two monodentate acetate ligands. The pyridine occupies a surprising amount of the available coordination space and has induced a dramatic change in coordination compared to the four chelating acetate ligands found in lead tetraacetate (LTA). Thermal measurements (TGA/DSC) indicate the de-coordination of pyridine and its loss from the solid between 360 and 380 K. ²⁰⁷Pb CP/MAS NMR spectroscopy also demonstrates the existence of the Pb–N bond through observation of ¹J(²⁰⁷Pb,¹⁴N) = 63 Hz and a ²⁰⁷Pb–¹⁴N dipolar coupling constant, of 149 Hz. The solid-state ²⁰⁷Pb NMR parameters are used to give insight into the coordination environment of Pb(IV) in LTA–py. In solution, ligand exchange is rapid on chemical shift and J-coupling time scales. A ²⁰⁷Pb NMR study of the titration of an LTA solution by pyridine yields a stability constant for LTA–py of K = 1.5 M⁻¹ and predicts it to have a ²⁰⁷Pb NMR chemical shift essentially identical to that observed by CP/MAS NMR in the solid state. This correlation between the solid state and solution indicates that the seven-coordinate LTA–py structure found in the crystalline state does persist in solution, and this could further explain why the addition of pyridine has such profound effects on lead(IV) carboxylate-mediated organic reactions. Simulations of exchange-broadened line shapes of ¹³C CP/MAS NMR spectra in the temperature regime above 280 K indicate local motion of the pyridine rings in the form of 180° jumps (activation energy 72.5 kJ mol⁻¹); these are first such ring flips reported for a coordinated pyridine ligand.