Mono- and bis-ortho-chelated lithium and sodium phenolates. Molecular structures of the first lithium phenolate with a stable, trinuclear structure and of a tetranuclear sodium analog, [MOC6H2(CH2NMe2) 2-2,6-Me-4]x (M = Li, x = 3; M = Na, x = 4)

The lithium and sodium phenolates of 2-[(dimethylamino)methyl]phenol, 1a and 2a, 2-[(dimethylamino)methyl]-4,6-dimethylphenol, 1b and 2b, and 2,6-bis[(dimethylamino)methyl]-4-methylphenol, 1c and 2c, respectively, have been synthesized. The structures of these complexes in solution were studied by variable-temperature ¹H and ¹³C NMR spectroscopy and cryoscopic molecular weight measurements. The solid-state structures of complexes 1c and 2c derived from 2,6-bis[(dimethylamino)methyl]-4-methylphenol, were determined by X-ray diffraction. The complexes containing only one ortho-chelating (dimethylamino)methyl group, i.e. 1a,b and 2a,b, are soluble in weakly polar solvents, like tetrahydrofuran. Complexes 1a and 1b are tetramers in benzene. These tetramers occur in solution in two isomeric forms, i.e. as aggregates with D₂ and S₄ symmetry. The lithium "pincer-phenolate" containing two ortho-chelating (dimethylamino)methyl groups (1c) is a trimer in apolar solvents as well as in the solid-state. It is the first example of a phenolate complex with a stable, trinuclear structure. In contrast, the sodium analog (2c) is a dimer in solution and crystallizes as a tetramer with a cubic-like Na₄O₄ core. Crystals of Li₃C₃₉H₆₃N₆O₃, 1c, are monoclinic, space group P2₁/n, with unit cell dimensions a = 42.381(5) Å, b = 8.8417(11) Å, c = 11.1802(10) Å, β = 90.539(8)°, and Z = 4. The structure was refined to R = 0.068 for 4106 reflections and 610 parameters. The trimeric structure contains a Li₃O₃-ring which is almost perfectly planar (within 0.05 Å), and very short Li-O and C-O bond lengths (averages of 1.865(9) and 1.317(6) Å, respectively). As a result of intramolecular amine coordination 1c is a "propeller-like" molecule with screw-type chirality. Crystals of Na₄C₅₂H₈₄N₈O ₄·C₆H₆, 2c, are monoclinic, space group P2/c, with a = 16.021(2) Å, b = 10.275(1) Å, c = 24.190(2) Å, β = 130.35(1)°, Z = 2, and final R = 0.064 for 3476 reflections and 400 parameters. This tetramer consists of a double dimer in which the NMe₂ groups of one dimer do exclusively coordinate to the sodium atoms of the same dimer.