A low temperature synthesis of barium organometallics via mixed ammonia-ethereal solvents

The reaction of barium metal granules in mixed ammonia-ethereal solutions at -40°C with a range of organic ligands (triphenylmethanePh₃CH, fluoreneC₃H₁₀, and indeneC₉H₈) yields the organometallic complexes [Ba(R)₂(L)₂] [where R CPh₃, L THF (1) and HMPA (2); R C₃H₉, L Et₂O (3), THF (4) and HMPA (5), L₂ DME (6); R C₉H₇, L THF (7) and HMPA (8)l (Et₂O diethylether, DME dimethoxyethane, THF tetrahydrofuran and HMPA hexamethylphosphoramide). All compounds were characterized by IR, NMR (¹H, ¹³C), mass spectroscopy and elemental analysis. Multinuclear (¹H and ¹³C) NMR studies on these species in d₆-benzene, d₈-THF and d₅-pyridine, and UV-vis measurements on 4 and 5 show that contact ion-pairs (CIPs) are the main species present in solution at room temperature. The formation of solvent separated ion pairs (SSIPs) is found to be favoured by increasing the polarity/cation solvating power of the solvent, or by alternatively lowering the solution temperature. Mass spectroscopic studies, FAB (fast atom bombardment in positive ion mode), have shown that these complexes are monomeric molecular ions in the gas phase.