Higher Ammoniates of BF3and SiF4: Syntheses, Crystal Structures, and Theoretical Calculations
We report on the synthesis and crystal structure of a higher ammoniate of BF3NH3, the monammine trifluorido boron(III) ammonia (1/3), [BF3NH3]·3NH3. The neutral compound crystallizes in the orthorhombic space group Pna21 with a = 11.5062(2), b = 9.9888(2), c = 6.5846(1) A, V = 756.79(2) A3, and Z = 4, forming a complex three-dimensional network through N–H···F and N–H···N hydrogen bonding, including the rare case of a trifurcated N–H···F hydrogen bond. A higher ammoniate of SiF4(NH3)2 was obtained and characterized for the first time. Diammine tetrafluorido silicon(IV) ammonia(1/2), [SiF4(NH3)2]·2NH3, was found to crystallize in the orthorhombic space group Cmca with a = 7.0595(1), b = 7.9814(1), c = 13.4595(2) A, V = 758.37(2) A3, and Z = 4. Its structure displays discrete SiF4(NH3)2 molecules, which are connected to crystal ammonia molecules via N–H···N hydrogen bonds and interconnected by N–H···F hydrogen bonds to {}^2_\infty[SiF4(NH3)2·2NH3] sheets. These layers are further interconnected by N–H···F hydrogen bonds from the crystal ammonia molecules leading to a three-dimensional network. ELF and AIM calculations and population analyses of the ELF basins were carried out for the BF3NH3 and SiF4(NH3)2 units of the two compounds. The calculations reflect the partially covalent bonding of fluorine to boron and silicon and confirm the expected trend of Lewis acidity. It is shown that ELF charges are a better descriptor for the bond covalency in these compounds than AIM charges.