Alkaline Earth Metal Complexes of a Phosphine-Borane-Stabilized Carbanion: Synthesis, Structures, and Stabilities

The reaction between either MgI2 or CaI2 and 2 equiv of [(Me3Si)2{Me2(H3B)P}C]K (2) in toluene gives the
corresponding organo-alkaline earth metal compounds [(Me3Si)2{Me2(H3B)P}C]2M in moderate to good yields [M
= Mg (3), Ca (4)]. Compound 3 crystallizes solvent-free, whereas X-ray quality crystals of 4 could not be obtained
in the absence of coordinating solvents; crystallization of 4 from cold methylcyclohexane/THF gives the solvate
[(Me3Si)2{Me2(H3B)P}C]2Ca(THF)4 (4a). The corresponding heavier alkaline earth metal complexes [(Me3Si)2{Me2(H3B)P}C]2M(THF)5 [M = Sr (7), Ba (8)] are obtained from the reaction between MI2 and 2 equiv of 2 in THF,
followed by recrystallization from cold methylcyclohexane/THF. Compound 3 degrades over a period of several
weeks at room-temperature both in the solid state and in toluene solution to give the free phosphine-borane (Me3Si)2{Me2(H3B)P}CH (5) as the sole phosphorus-containing product. In addition, compounds 3, 4, and 4a react
rapidly with THF in toluene solution, yielding 5 as the sole phosphorus-containing product; in contrast, compounds
7 and 8 are stable toward this solvent.