Bis(diphenylphosphido) Derivatives of the Heavier Group 2 Elements

A number of heavier alkaline-earth (Ae) diphenylphosphides have been prepared by protonolysis of the corresponding
bis(trimethylsilyl)amides and completely characterized in the solid state and in solution. Through careful control of
the reaction and workup conditions, preparations performed with tetrahydrofuran (THF) as the only donor solvent
crystallize as octahedral tetrakis-solvated THF adducts [Ae(PPh2)2(THF)4] when Ae = Ca and Sr. Both structures
are similar and contain the two diphenylphosphide substituents in a trans disposition. In contrast, the analogous
reaction performed upon the barium amide yielded a one-dimensional polymeric complex containing both five- and
six-coordinated Ba centers in which the polymer is propagated via bridging diphenylphosphide units. Further strictly
mononuclear derivatives of the Ca and Ba diphenylphosphides may be straightforwardly prepared by treatment of
the THF adducts with 18-crown-6. Although the THF-solvated species are evidently labile to loss of solute, solution
(diffusion-ordered spectroscopy NMR, THF-d8) studies indicate that, under catalytically relevant conditions, a similar
mononuclear constitution is most likely retained.