Structural Diversity in Alkaline Earth Metal Complexes of a Phosphine-Borane-Stabilized 1,3-Dicarbanion

The reaction between (4-tBuC6H4CH2)2Ca and one
equivalent of PhP­(BH3)­(CH2SiMe3)2 (1) in diethyl ether gives the dimeric complex
[[PhP­(BH3)­{CH­(SiMe3)}2]­Ca­(OEt2)]2 (2) in good yield. Similar reactions
between 1 and one equivalent of either (PhCH2)2Sr­(THF) or (PhCH2)2Ba yield the
corresponding
dimers [[PhP­(BH3)­{CH­(SiMe3)}2]­Sr­(THF)1.75(OEt2)0.25]2 (3) and [[PhP­(BH3)­{CH­(SiMe3)}2]­Ba­(OEt2)1.75(THF)0.25]2 (4), respectively. Unexpectedly, an attempt to prepare 3 from a one-pot reaction between SrI2, 1,
and two equivalents of PhCH2K gave the complex [[PhP­(BH3)­{CH­(SiMe3)}2]2­Sr3K2(OEt2)­(THF)2]2·Et2O (5) in low yield. While superficially
similar, compounds 2, 3, and 4 crystallize with distinct structures, which differ either in the
chirality of the carbanion centers or in the nature of the bridging
group. Compounds 2, 3, and 4 decompose slowly in THF solution to give ethylene, Ae­(OEt)2 (or Ae­(OCHCH2)2), and the monocarbanion
derivatives [PhP­(BH3)­{CH­(SiMe3)}­{CH2SiMe3}]2Ae­(THF)n, according to NMR spectroscopy [Ae = Ca, Sr, Ba].