Ligand substitution effects in uranyl β-ketoenolates

Abstract

Fifty-five uranyl β-ketoenolates have been prepared, comprising anhydrous complexes [UO 2L 2] (L = variously substituted β-ketoenolates); base adducts [UO 2L 2B] (B = water, pyridine, pyridine N-oxide); and adducts of substituted pyridines [UO 2L 2(R-C 5H 4N)]. Pure field and resonance parameters are used to derive a quantitative order of the electronic effects (Σσ) of the β-ketoenolate substituents. Values of v(UO) generally correlate well with the Σσ values. Differences between the substituent dependence of v(M-O) in copper(II) and uranyl β-ketoenolates are interpreted in terms of the relative importance of π-interaction in the uranyl complexes. Infrared evidence for resonance interaction of phenyl substituents with the β-ketoenolate ring is supported by the NMR spectra. The variation in ligand field strength produced by the electronic effects of the substituents shifts v(UO) in conformity with the relationship Δ v = -(electrostatic effect) -σ(L → U)-π(L → U) ± π (U → L) previously proposed for a series of uranyl nitrate complexes, [UO 2NO 3) 2L 2]. Evidence is cited for the transmission of the electronic effects of the β-ketoenolate substituents to the pyridine ring in the complexes [UO 2L 2py] and for the transmission of the electronic effects of the pyridine substituents to the β-ketoenolate ring in the complexes [UO 2(AA) 2(R-C 5H 4N)] (AA = acetylacetonate ion; R = 3- or 4-pyridine substituent).