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

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 1–3 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.