Unravelling the Urea-Route to Boron Nitride: Synthesis and Characterization of the Crucial Reaction Intermediate Ammonium Bis(biureto)borate

Abstract

Ammonium bis­(biureto)­borate, NH4[B­(biu)2] (biu = NH–(CO)–NH–(CO)–NH), was synthesized as a phase pure powder from urea and triethylborate in a sophisticated solvothermal process. Its crystal structure was determined via single-crystal XRD and it crystallizes in its own structure type in space group P2/c (a = 9.2855(3) Å, b = 4.8019(2) Å, c = 10.9008(4) Å, β = 110.757(2)°, Z = 2). NH4[B­(biu)2] was characterized with FT-IR and UV/vis spectroscopy as well as thermogravimetry. It was identified as a crucial reaction intermediate during the boron nitride synthesis from urea and boric acid. Mixtures of boric acid and urea were studied via TGA and DSC, and furthermore, three series of solid state reactions varying calcination temperature, time, and ratio of starting materials were conducted in order to study the formation and thermal decomposition of NH4[B­(biu)2]. Based on these experiments under careful comparison to our studies on the properties of phase pure, solvothermally synthesized NH4[B­(biu)2] we suggest a reasonable reaction sequence for the formation of boron nitride starting from urea and boric acid.

Characterization of boron nitride nanosheets synthesized by boron-ammonia reaction

“…Similar structure was reported by Thomas et al [43] and Gross et al [44]. However, peaks marked with asterisk in Fig.…”

“…These complexes dissociate at temperature higher than 1050 °C leading to crystallization of BN phase. Gross et al [44] reported that the reaction of urea and boric oxide results in the formation of various intermediate precursors such as ammonium bis(biureto)borate. On the basis of thermogravimeteric analysis they found that this complex converts to BN by decomposition at about 850 °C.…”

Biuret—A Crucial Reaction Intermediate for Understanding Urea Pyrolysis To Form Carbon Nitrides: Crystal‐Structure Elucidation and In Situ Diffractometric, Vibrational and Thermal Characterisation

“… [58] The molecule features one intramolecular hydrogen bond between one carbonyl and one amide group (O−H distance: 1.92 Å), as well as two intermolecular hydrogen bonds between amide and carbonyl groups towards each of the two adjacent, coplanar biuret molecules, forming two sets of skew ribbons ubiquitous in the crystal chemistry of ureas (Figure 1 b). [51, 59–62] The ribbons within a set are stacked antiparallel offset with a inter‐ribbon distance of about 3.3 Å, roughly corresponding to the sum of the van der Waals radii of N/O and C. One set of these ribbons traverses the unit cell roughly parallel to [10 ], the other parallel to [14 11 ], with a dihedral angle of about 53.5° between the two ribbon planes, resulting in a criss‐cross pattern with rhombical channels along (11 0 6) (Figure 1 c). The diameter of the resulting channel corresponds to the inter‐ribbon distance, so the crystal structure can be regarded as close packed.…”

Boron in Prebiological Evolution

“…This reaction was carried out in two steps (the last one at pH 9.5) and the overall process takes less than one day (Figure 3), [14] conditions compatible with the stabilization of ribose by borate mentioned in the previous section. One of urea derivatives, biuret, [14] is obtained at lower temperatures in the presence of borate [54] . These results suggest that borate can increase the stability of ribose as well as activate urea, which is a plausible prebiological reagent, since it is found in interstellar space [55] .…”