Sublimation behavior of tris(2,2,6,6-tetramethyl-3,5-heptanedionato)lanthanoid(III).

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Two fundamental experiments (to determine vapor pressures and vacuum sublimation-recrystallizing temperature zones) were undertaken on a series of remarkably volatile and thermally stable lanthanoid(III) chelates of 2,2,6,6-tetramethyl-3,5-heptanedione (Ln(thd)<SUB>3</SUB>) in order to obtain information about their sublimation behavior. Vapor pressures of these chelates were measured from 100 to 150 °C by means of a modified Knudsen effusion technique. Vacuum sublimation-recrystallizing temperatures were determined by using a sublimation apparatus with a continuous temperature gradient along its sublimation tube. The obtained enthalpies of sublimation were classified into three groups (La to Gd, Tb and Dy, and Ho to Lu), and the vacuum sublimation-recrystallizing temperature zones were divided into two sets (La to Tb, and Dy to Lu). The Tb and Dy chelates were also found to change thermally from a dimeric form to a monomeric form at a definite temperature. These results are explained in terms of the two different crystallizing forms of Ln(thd)<SUB>3</SUB> chelates. Furthermore, the regularity in their sublimation behavior is discussed as a function of the atomic numbers of the lanthanoid metals, using thermochemical considerations.
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