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

摘要

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(mu-Bu)(2){mu-OC(=CH2)Mes}(4)(TMEDA)(2)] (6) and homoanionic [Na2Mg2{mu-OC-(=CH2)Mes}(6)(TMEDA)(2)] (4), respectively, while 4 equiv of the ketone with Na2MgBu4 affords homoanionic [Na2Mg{mu-OC(=CH2)Mes}(4)(TMEDA)(2)] (5). X-ray crystallographic studies reveal that 4 and 6 adopt similar tetranuclear Na center dot center dot center dot Mg center dot center dot center dot Mg center dot center dot center dot Na chain arrangements, held together by enolato O bridges in the former and a mixture of enolato 0 and alkyl C bridges in the latter, whereas 5 adopts a smaller trinuclear Na center dot center dot center dot Mg center dot center dot center dot 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. H-1 and C-13 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 [Na-2{OC(=CH2)Mes}(2)(TMEDA)(2)] (7), a simple enolate O-bridged dimer, is also reported.