β-Diketiminate calcium hydride complexes: the importance of solvent effects

Abstract

A series of (DIPPnacnac)CaN(SiMe₃)₂·S complexes (DIPPnacnac = HC[C(Me)N(2,6-iPr-C₆H₃)]₂; S = solvent) could be obtained by the addition of S = THF, DME or N-Me-morpholine (Morph) to (DIPPnacnac)CaN(SiMe₃)₂·OEt₂ or (DIPPnacnac)CaN(SiMe₃)₂. Crystal structures for complexes with S = DME and Morph are compared to literature-known structures with S = none, THF or Et₂O. Bulkier and weaker Lewis bases like the tertiary amines Et₃N, TMEDA and DABCO did not interact with (DIPPnacnac)CaN(SiMe₃)₂. The reaction of (DIPPnacnac)CaN(SiMe₃)₂ with PhSiH₃ gave conversion to a calcium hydride complex that dismutated in (DIPPnacnac)₂Ca and CaH₂. The reaction of (DIPPnacnac)CaN(SiMe₃)₂·S with PhSiH₃ gave [(DIPPnacnac)CaH·S]₂ for S = THF, Et₂O or N-Me-morpholine (Morph). For S = DME, high reaction temperatures were needed and dismutation into (DIPPnacnac)₂Ca and CaH₂ was observed. Extensive NMR investigations (VT-NMR and PGSE) confirm the dimeric nature of [(DIPPnacnac)CaH·THF]₂ in aromatic solvents or in THF. Thermal decomposition of [(DIPPnacnac)CaH·THF]₂ (release of H₂ at 200 °C) is compared to that of Mg and Zn analogues. Weakly coordinating Et₂O in [(DIPPnacnac)CaH·OEt₂]₂ could be replaced by THF, Morph or DABCO but not with Et₃N. The addition of TMEDA led to the formation of CaH₂ and unidentified products. The addition of DME led to the decomposition of Et₂O and complex [(DIPPnacnac)CaOEt]₂ was obtained. Crystal structures of the following compounds are presented: (DIPPnacnac)CaN(SiMe₃)₂·S (S = Morph, DME), [(DIPPnacnac)CaH·S]₂ (S = Et₂O, Morph and DABCO) and [(DIPPnacnac)CaOEt]₂. Although bulky ligands have long been thought to be the key to the stabilization of calcium hydride complexes, the presence of a polar, strongly coordinating, co-solvent is also crucial.