Synthesis and Characterization of Strontium Bis(N-isopropylanilide) and Comparison with the Lighter Magnesium and Calcium Congeners

Dedicated to Professor Heinrich Nöth on the occasion of his 85th birthday The metathesis reaction of strontium diiodide [(thf)5SrI2] with K[N(Ph)iPr] in THF yields [(thf)4Sr{N(Ph)iPr}2] (1). Ligand exchange reactions with 1,2-dimethoxyethane (DME), tetramethylethylenediamine (TMEDA), and pentamethyldiethylenetriamine (PMDETA) allow the isolation of the corresponding adducts [(dme)2Sr{N(Ph)iPr}2] (2), [(tmeda)(thf)Sr{N(Ph)iPr}2] (3) and [(pmdeta)Sr{N(Ph)iPr}2] (4), respectively. Magnesiation of N-isopropylaniline with dibutylmagnesium in THF leads to the formation of [(thf)2Mg{N(Ph)iPr}2] (5). A similar reaction in TMEDA gives nearly insoluble crystalline [(tmeda)Mg{N(Ph)iPr}2] (7), whereas the mother liquor contains heteroleptic [(tmeda)(nBu)Mg{N(Ph)iPr}] (6). Magnesiation of N-isopropylaniline in 2,2,5,5-tetramethyltetrahydrofuran (Me4thf) yields [(Me4thf)Mg{N(Ph)iPr}2] (8) with a threecoordinate metal center. In hydrocarbons this complex loses the bulky ether base, and the solvent-free dimer [Mg{N(Ph)iPr}2]2 (9) can be isolated. Reaction of this complex with 1,2dimethoxyethane or metalation of N-isopropylaniline with dibutylmagnesium in DME yield the dme adduct, [(dme)Mg{N(Ph)iPr}2] (10). The crystal structures show that the nitrogen atoms of the magnesium-bound N-isopropylanilide ions are in planar environments whereas strontium-bound N-isopropylanilide ions show rather short contacts between the alkaline earth metal and the ipsocarbon atoms of the phenyl groups leading to a pyramidalization of the coordination of the nitrogen atoms.