Comparisons and Contrasts in a Complete Set of Alkali Metal Cumyl Structures

Alkali-metal benzyl complexes derived from toluene are known to vary their metal-ligand coordination mode as a function of the alkali-metal, with the metal transitioning from a typical σ bond to the anionic CH₂ for lithium toward an interaction with the delocalized pi system of the aromatic ring as the metal gets larger and softer. Here, by switching to cumene, we report the charge-localizing effect of replacing the hydrogen atoms at the formally carbanionic carbon CH₂ with electron-donating methyl groups in C(Me)₂. NMR spectroscopic studies reveal competitive ring-metalation occurs, at the meta and para positions, alongside α-metalation on using an alkyl lithium base, with the meta- and α-isomers crystallographically characterized as a solvated dimer and monomer, respectively. Using Lochmann-Schlosser type base pairs to access the heavier alkali-metal complexes unveils only α-metalation. The presence of the methyl groups limits the variation in metal-ligand bonding, their electron-donating properties forcing the delocalization of the negative charge into the ring resulting in M-Ph interactions and sp² hybridization at the formally deprotonated α-carbon regardless of the metal used. Considerable variation in aggregation state is observed with monomeric (Na), polymeric (K, Cs) and tetrameric (Rb) motifs identified in the solid-state.