Coordination behavior and coligand-dependent cis/trans isomerism of calcium bis(diphenylphosphanides)

Article abstract of DOI:10.1002/ejic.201100241

The salt metathesis reaction of KPPh₂ with CaI₂ and SrI₂ in THF yields the corresponding complexes [(THF) ₄Ae(PPh₂)₂] [Ae = Ca (1), Sr]. Depending on the crystallization conditions, cis- or trans-[(THF)₄Ca(PPh ₂)₂] can be isolated. In solution, a fast equilibrium converts these isomers into each other leading to a single set of resonances in the NMR spectra. The THF ligands can be replaced by stronger Lewis bases, such as N-methylimidazole (MeIm), which leads to the formation of [(MeIm) ₄Ae(PPh₂)₂] [Ae = Ca (2a), Sr (2b)]. The use of tetradentate hexamethyltriethylenetetramine (hmteta) fixes a cis configuration yielding [(hmteta)Ca(PPh₂)₂] (3). All compounds are very air and moisture sensitive, therefore a few crystals of the partially oxidized product [(hmteta)Ca(PPh₂)(OPPh₂)] (4) were also obtained. [(L)₄Ca(PPh₂)₂] is an effective hydrophosphanylation catalyst. To predetermine and fix a cis or trans arrangement of the phosphanide anions, the neutral coligand sphere of four Lewis base donors, such as four THF molecules or one hmteta ligand, was altered. Copyright