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Co-complexation Syntheses, Structural Characterization, and DFT Studies of a Novel Series of Polymeric Alkali-Metal Tetraorganogallates

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posted on 28.01.2013, 00:00 by David R. Armstrong, Elanor Brammer, Thomas Cadenbach, Eva Hevia, Alan R. Kennedy
Exploring the co-complexation reactions between the gallium alkyl Ga­(CH2SiMe3)3 and alkali-metal alkyl MCH2SiMe3 (M = Li, Na, or K) using an arene/hexane solvent mixture has allowed the isolation of solvent-free alkali-metal tetraorganogallates [{MGa­(CH2SiMe3)4}] (M = Li, 1; Na, 2) and related benzene adduct [{(C6H6)2KGa­(CH2SiMe3)4}] (3). By combining X-ray crystallography, NMR spectroscopy, and DFT calculations, this study sheds new light on the constitution of these mixed-metal species. X-ray crystallographic studies reveal that all gallates exhibit novel polymeric arrangements, with 1 and 2 sharing the same linear chain structure, made up exclusively of M–C and Ga–C bonds, whereas 3 displays a significantly more open structural motif, where the K and Ga atoms are connected by a single alkyl bridge and propagation occurs via weaker K···Me electrostatic interactions of a methyl from a SiMe3 group of an alkyl ligand from one monomer to the potassium from a neighboring monomeric unit. Multinuclear NMR spectroscopic studies suggest that in deuterated benzene solutions 13 exist as discrete solvent-separated ion-pair species where the alkali-metal is solvated by the arene solvent. DFT calculations show that while the infinite aggregation of these polymeric structures is key for thermodynamically favoring the formation of 1 and 2, in the case of 3 the solvation of unsaturated potassium by two molecules of benzene, via π-electrostatic interactions, appears to be the major contributor to its overall stability.

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