Synthesis and Structural Diversity of Barium (N,N-Dimethylamino)diboranates

The reaction of a slurry of BaBr2 in a minimal amount of tetrahydrofuran (THF) with 2 equiv of Na­(H3BNMe2BH3) in diethyl ether followed by crystallization from diethyl ether at −20 °C yields crystals of Ba­(H3BNMe2BH3)2(Et2O)2 (1). Drying 1 at room temperature under vacuum gives the partially desolvated analogue Ba­(H3BNMe2BH3)2(Et2O)x (1′) as a free-flowing white solid, where the value of x varies from 1 or 1′ with Lewis bases that bind more strongly to barium than diethyl ether results in the formation of new complexes Ba­(H3BNMe2BH3)2(L), where L = 1,2-dimethoxyethane (2), N,N,N′,N′-tetramethylethylenediamine (3), 12-crown-4 (4), 18-crown-6 (5), N,N,N′,N′-tetraethylethylenediamine (6), and N,N,N′,N″,N″-pentamethylethylenetriamine (7). Recrystallization of 4 and 5 from THF affords the related compounds Ba­(H3BNMe2BH3)2(12-crown-4)­(THF)·THF (4′) and Ba­(H3BNMe2BH3)2(18-crown-6)·2THF (5′). In addition, the reaction of BaBr2 with 2 equiv of Na­(H3BNMe2BH3) in the presence of diglyme yields Ba­(H3BNMe2BH3)2(diglyme)2 (8), and the reaction of 1 with 15-crown-5 affords the diadduct [Ba­(15-crown-5)2]­[H3BNMe2BH3]2 (9). Finally, the reaction of BaBr2 with Na­(H3BNMe2BH3) in THF, followed by the addition of 12-crown-4, affords the unusual salt [Na­(12-crown-4)2]­[Ba­(H3BNMe2BH3)3(THF)2] (10). All of these complexes have been characterized by IR and 1H and 11B NMR spectroscopy, and the structures of compounds 1–3, 4′, 5′, and 6–10 have been determined by single-crystal X-ray diffraction. As the steric demand of the Lewis bases increases, the structure changes from polymers to dimers to monomers and then to charge-separated species. Despite the fact that several of the barium complexes are monomeric in the solid state, none is appreciably volatile up to 200 °C at 10–2 Torr.