Synthesis and characterization of new mixed-metal sodium-magnesium enolates derived from 2,4,6-trimethylacetophenone

The reactivity of the stoichiometric variant monosodium-monomagnesium trialkyl NaMgBu3 and disodium-monomagnesium tetraalkyl Na2MgBu4 complexes toward the sterically demanding ketone 2,4,6-trimethylacetophenone has been investigated and led to the synthesis of a series of mixed sodium-magnesium enolate complexes. Thus, 2 and 3 molar equiv of the ketone with NaMgBu3 produce heteroanionic [Na2Mg2(-Bu)2{-OC(=CH2)Mes}4(TMEDA)2] (6) and homoanionic [Na2Mg2{-OC(=CH2)Mes}6(TMEDA)2] (4), respectively, while 4 equiv of the ketone with Na2MgBu4 affords homoanionic [Na2Mg{-OC(=CH2)Mes}4(TMEDA)2] (5). X-ray crystallographic studies reveal that 4 and 6 adopt similar tetranuclear Na···Mg···Mg···Na chain arrangements, held together by enolato O bridges in the former and a mixture of enolato O and alkyl C bridges in the latter, whereas 5 adopts a smaller trinuclear Na···Mg···Na chain arrangement with all enolato O bridges. The metal coordination in all these structures is distorted tetrahedral, while the sodium cations at the ends of the mixed-metal chains carry terminal TMEDA ligands. 1H and 13C NMR spectral data recorded in C6D6 are also reported for 4-6. For comparative purposes, the synthesis and X-ray crystal structure of the homometallic sodium enolate [Na2{OC(=CH2)Mes}2(TMEDA)2] (7), a simple enolate O-bridged dimer, is also reported.