Synthesis and Properties of Calcium Tetraorganylalanates with [Me4−nAlPhn]− Anions

Abstract

Triphenylalane yields in THF or Et2O the corresponding ether complexes [(thf)AlPh3] (1a) and [(Et2O)AlPh3] (1b). The reaction of these triphenylalanes with phenylcalcium iodide in THF yielded quantitatively [(thf)5CaI][AlPh4] (2), which can be recrystallized from diethyl ether/THF mixtures without ether exchange reactions. The reaction of PhCa(thf)4I with trimethylalane in THF in an equimolar ratio leads to the formation of solvent-separated [(thf)6Ca][AlMe3Ph]2 (5), which immediately shows ligand redistribution. Therefore, a fractionated crystallization gives [(thf)6Ca][AlMe2Ph2]2 (4) at 4 °C, [(thf)4CaI2] at −20 °C, and after reduction of the volume of the mother liquor [(thf)6Ca][AlMe3Ph]2 (5) at −40 °C and [(thf)6Ca][AlMe4]2 (6) at −78 °C. The formation of (thf)4CaI2 confirms that a Schlenk equilibrium is operative besides the ligand redistribution reactions. A solution of crystalline [(thf)6Ca][AlMe2Ph2]2 (4) in THF shows 4 as the major component besides [(thf)6Ca][AlMe3Ph]2 (5) and [(thf)6Ca][AlMePh3]2 (3). With an increasing number of methyl groups the melting points decrease from 210 °C for the tetraphenylalanate 2 to 20 °C for the tetramethylalanate 6.