Synthesis and structural diversity of barium (N,N-dimethylamino)diboranates

Abstract

The reaction of a slurry of BaBr 2 in a minimal amount of tetrahydrofuran (THF) with 2 equiv of Na(H 3BNMe 2BH 3) in diethyl ether followed by crystallization from diethyl ether at -20 °C yields crystals of Ba(H 3BNMe 2BH 3) 2(Et 2O) 2 (1). Drying 1 at room temperature under vacuum gives the partially desolvated analogue Ba(H 3BNMe 2BH 3) 2(Et 2O) x (1′) as a free-flowing white solid, where the value of x varies from <0.1 to about 0.4 depending on whether desolvation is carried out with or without heating. The reaction of 1 or 1′ with Lewis bases that bind more strongly to barium than diethyl ether results in the formation of new complexes Ba(H 3BNMe 2BH 3) 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(H 3BNMe 2BH 3) 2(12-crown-4)(THF) ·THF (4′) and Ba(H 3BNMe 2BH 3) 2(18-crown-6) ·2THF (5′). In addition, the reaction of BaBr 2 with 2 equiv of Na(H 3BNMe 2BH 3) in the presence of diglyme yields Ba(H 3BNMe 2BH 3) 2(diglyme) 2 (8), and the reaction of 1 with 15-crown-5 affords the diadduct [Ba(15-crown-5) 2][H 3BNMe 2BH 3] 2 (9). Finally, the reaction of BaBr 2 with Na(H 3BNMe 2BH 3) in THF, followed by the addition of 12-crown-4, affords the unusual salt [Na(12-crown-4) 2][Ba(H 3BNMe 2BH 3) 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. © 2012 American Chemical Society.