Structures of Lithium Salts of 2, 3, 3-Trimethylindolenine and Its 5-Methoxy Derivative in Solution and the Solid State

An X-ray crystal structure of lithium 2, 3, 3-trimethylindolenide etherate shows it to be a disolvated dimer having a η³-azaallyl-type structure. The structures of the salt in several solvents have been established by studies of ¹³C chemical shifts, ⁶Li, ¹⁵N spin-spin splitting, ⁷Li quadrupole splitting constants, and apparent degrees of aggregation determined by vapor pressure barometry. It is the η³-azaallyl dimer disolvate in diethyl ether, a tetrasolvated dimer in dioxolane, a mixture of monomer and tetrasolvated dimer in tetrahydrofuran, and a monomer in pyridine. The species are characterized by ⁷Li quadrupole splitting constants of 230, 156, 180–190 (0.27-0.75 M), and 217 kHz, respectively. In diethyl ether with 4 equiv of hexamethylphosphoric triamide, the salt is a mixture of monomeric and triple ion species. Lithium 5-methoxy-2, 3, 3-trimethylindolenide forms similar species except that, in tetrahydrofuran, the tendency for dimer formation is enhanced, which leads to a higher proportion of C- to N-methylation in its reaction with methyl chloride in that solvent.