Thermal dehydrochlorination in the 4-fluoroaniline-trichloroborane system: identification of reactive intermediates involved in the formation of B,B',B''-trichloro-N,N',N''-tri((4-fluoro)phenyl)borazine.

Borazines are used in chemical vapor deposition processes to produce hybrid graphene-boron nitride nanostructures. As the knowledge on the mechanism of borazine formation is scarce, we studied the mechanism of formation of B,B',B''-trichloro-N,N',N''-tri(p-fluorophenyl)borazine (3a) from p-fluoroaniline and boron trichloride employing NMR spectroscopy, X-ray single crystal structure analysis, trapping experiments, and computational chemistry methods up to the coupled cluster CCSD(T) level of theory. These studies suggest the initial formation of the 1 : 1 adduct 1a (ArNH2BCl3, Ar = 4-fluorophenyl) with a dative B-N bond that could be fully characterized including single crystal X-ray diffraction. Adduct 1a undergoes unimolecular hydrogen chloride elimination with a first-order rate constant of k1 = 3.03(7) × 10-2 min-1 in toluene at 100 °C. This rate constant is in very good agreement with the one derived (k1 = 3.18 × 10-2 min-1) from computed activation parameters (ΔH‡373.15 = 28.1 kcal mol-1, ΔS‡373.15 = 1.56 eu, ΔG‡373.15 = 27.6 kcal mol-1). The product of the first hydrogen chloride evolution is anilinodichloroborane ArNHBCl2 (2a). Compound 2a cannot be isolated in a pure form due to instability, but its presence as a transient reactive intermediate can be derived from NMR spectroscopy. Reactive intermediates other than anilinodichloroborane cannot be assigned by NMR spectroscopy. We propose that the mechanism of formation of borazine 3a involves the reaction of 2a with 4-fluoroaniline as the rate determining step.

Photoreactions of Phenylborylene with Dinitrogen and Carbon Monoxide.

Formal removal of two bonding partners from boranes, BR3, yields borylenes, RB, which have been inferred as reactive intermediates in a number of reactions. Phenylborylene (R = C6H5; 1) is accessible from phenyldiazidoborane by photochemical extrusion of dinitrogen under matrix isolation conditions. Concomitantly, the nitrene PhNBN is formed via phenyl rearrangement. Here we used a combination of UV/vis, IR, and ESR spectroscopy under cryogenic matrix isolation conditions to investigate the properties and reactivity of phenylborylene. We detected an absorption band of phenylborylene at 375 nm (S0 → S2) and tentatively assigned the S0 → S1 transition to a very weak band at 518 nm. We also show for the first time that an electrophilic borylene such as 1 can react with N2 reversibly and with CO irreversibly under photochemical conditions. The corresponding photoproducts PhBNN and PhBCO have triplet electronic ground states. Their small E values are in agreement with the linear arrangements Ph-B-N-N and Ph-B-C-O obtained by density functional theory computations. The D values decrease in the series PhNBN > PhBNN > PhBCO and approach the value for phenylcarbene (PhCH). Indeed, the boron center in PhBCO is isoelectronic with the carbene center in PhCH. The compounds are the first examples of boron analogues of diazoalkanes (R2CNN) and ketenes (R2CCO), and their formation may serve as a demonstration of the high reactivity of phenylborylene.

On-Surface Synthesis of BN-Substituted Heteroaromatic Networks.

We report on the bottom-up fabrication of BN-substituted heteroaromatic networks achieved by surface-assisted polymerization and subsequent cyclodehydrogenation of specifically designed BN-substituted precursor monomers based on a borazine core structural element. To get insight into the cyclodehydrogenation pathway and the influence of molecular flexibility on network quality, two closely related precursor monomers with different degrees of internal cyclodehydrogenation have been employed. Scanning tunneling microscopy shows that, for both monomers, surface-assisted cyclodehydrogenation allows for complete monomer cyclization and the formation of covalently interlinked BN-substituted polyaromatic hydrocarbon networks on the Ag(111) surface. In agreement with experimental observations, density functional theory calculations reveal a significantly lower energy barrier for the cyclodehydrogenation of the conformationally more rigid precursor monomer, which is also reflected in a higher degree of long-range order of the obtained heteroaromatic network. Our proof-of-concept study will allow for the fabrication of atomically precise substitution patterns within BNC heterostructures.

Construction of an Internally B3 N3 -Doped Nanographene Molecule.

The synthesis of a hexa-peri-hexabenzocoronene (HBC) with a central borazine core is described. The solid-state structure of this BN-doped HBC (BN-HBC) is isotypic with that of the parent HBC. Scanning tunneling microscopy shows that BN-HBC lies flat on Au(111) in a two-dimensional pattern.