Synthesis of alkaline earth metal bis(2-phosphaethynolates)

The reaction of dimethylcarbonate with magnesium bis(triisopropylsilyl phosphanide) or the alkaline earth metal bis[bis(trimethylsilyl)phosphanide] in 1,2-dimethoxyethane yields the corresponding bis(2-phosphaethynolate) of magnesium (1), calcium (2), strontium (3) and barium (4). These compounds decompose upon isolation and have to be stored in an etheral solution at low temperatures. The structure of (dme)3Ca(OCP)2 shows bent CaOC moieties and rather long CP triple bonds of 1.575 Å. In order to ensure a quantitative reaction an excess of dimethylcarbonate has to be used. For the strontium derivative a side reaction of a 1:3 stoichiometry of M[P(SiMe3)2]2 to (MeO)2CO gave a tridentate ligand which bridges two strontium atoms. The molecular structure of dimeric (dme)2Sr{OC[PC(O)OMe]2} (5) showed the delocalization of the anionic charge within the OCPCPCO fragment.