Anomalous electric polarizations and the sub-millimetre spectra in solution of some metal acetylacetonates

Abstract

The molar electric polarizations at 29.7 cm^–1 are reported for beryllium (II) bisacetylacetonate (2,4-pentanedionate) for the tris-acetylacetonates of Al^III, Co^III, Cr^III, Fe^III and Mn^III and for Th^IV-tetrakisacetylacetonate in benzene solution. The values are all intermediate between those at radio and visible frequencies, showing that there is absorption above, at and below 29.7 cm^–1.

The spectra between 5 and 25 cm^–1 for benzene solutions of the trisacetylacetonates of Al^III and Fe^III and for Fe^III trisdipivalylmethanate (hexamethylacetylacetonate) are reported. The first two of these show a steady rise in absorption with increasing frequency, the third shows much less absorption rising slowly. Spectra in benzene solutions between 30 and 200 cm^–1 are reported for Be(acac)₂, for the trisacetylacetonates of Al^III, Co^III, Cr^III, Fe^III and Mn^III. All these show very broad absorption bands divided into two regions by a window at 100–130 cm^–1. The lower band is centred around 60–80 cm^–1.

Solution spectra for Ce^IV- and Th^IV-tetrakisacetylacetonates show little absorption below 130 cm^–1 but above this it rises rapidly. The spectra for Fe^III-trishexafluoroacetylacetonate and Fe^III-trisdipivalylmethanate show respectively a blue shift of the lower peak to ∼106 cm^–1 and very little absorption below 160 cm^–1.

These spectra, together with those observed by Larsson and Eskilsson at 300–1600 cm^–1 explain ∼50–60 % of the observed differences of polarization between radio and visible frequencies (P_RF–P_VF). For the Al^III and Cr^III compounds most of the remaining differences are explained by the absorption in the microwave region observed by Dasgupta and Smyth and by DiCarlo et al. The unusually large P_RF–P_VF values are due to almost continuous absorption over a very wide range of frequency.

The origin of the very broad absorption peaks below 100 cm^–1 is discussed. For various reasons, including comparisons with the spectra observed for the crystalline substances, it is concluded that they arise from intramolecular vibrations which are very heavily damped by collisions with solvent. The low absorption by Fe^III trisdipivalylmethanate and by the Ce^IV- and Th^IV-tetrakisacetylacetonates below 130 cm^–1 is probably due to steric interference.

The possible nature of these vibrations is considered in the light of spectroscopic and dielectric observations and further evidence from crystallographic calculations made by Prof. C. E. Pfluger, from neutron scattering spectra for the trisacetylacetonates of Al^III, Cr^III and Fe^III, the trishexadeutero- and trishexafluoro-acetylacetonates of Fe^III, Fe^III-trisdipivalylmethanate and the tris-3-bromo- and tris-3-nitroacetylacetonates of Cr^III together with the sub-millimetre spectra of the last two compounds. It seems probable that they involve bending of the chelate rings about the O … O vectors, with some ring flexing, thus causing large movements of the peripheral atoms.