The triboracyclopropenyl fragment is a cyclic structural motif in boron chemistry, named for its geometric similarity to cyclopropene. In contrast to nonplanar borane clusters that exhibit higher coordination numbers at boron (e.g., through 3-center 2-electron bonds to bridging hydrides or cations), triboracyclopropenyl-type structures are rings of three boron atoms where substituents at each boron are also coplanar to the ring. Triboracyclopropenyl-containing compounds are extreme cases of inorganic aromaticity. They are the lightest and smallest cyclic structures known to display the bonding and magnetic properties that originate from fully delocalized electrons in orbitals of σ and π symmetry. Although three-membered rings of boron are frequently so highly strained as to be experimentally inaccessible, academic interest in their distinctive aromaticity and possible role as intermediates of borane pyrolysis motivated extensive computational studies by theoretical chemists. Beginning in the late 1980s with mass spectrometry work by Anderson et al. on all-boron clusters, experimental studies of triboracyclopropenyls were for decades exclusively limited to gas-phase investigations of the simplest rings (ions of B3). However, more recent work has stabilized the triboracyclopropenyl moiety via coordination to donor ligands or transition metals, dramatically expanding the scope of its chemistry.
Synthesis
For gas-phase spectroscopic studies, triboracyclopropenyl-containing compounds are obtained via laser ablation of boron targets and collimation of the resulting plasma cloud in a flow of inert carrier gas such as helium. The charged molecules of interest are then mass-selected by time-of-flight mass spectrometry. Addition of gases such as N2 or CO to the gas stream affords the corresponding adducts, while addition of metals such as iridium and vanadium to the B target yields the corresponding metal-doped clusters.
The sole isolable example of a triboracyclopropenyl anion that persists in solution and in the solid state was identified by Braunschweig and coworkers, who synthesized it by reducing the aminoborane Cl2B=NCy2 (Cy = cyclohexyl) with finely dispersed sodium metal in dimethoxyethane (DME). Cooling of the resulting orange-red solution of the dimeric species Na4[B3(NCy2)3]2 • 2 DME resulted in crystals suitable for X-ray diffraction, by which the structure was determined. Although the detailed reduction mechanism is unknown, it has been suggested that subvalent "R2N−B" intermediates are involved in the formation of such boron clusters.
Structure and bonding
Due to their special status as the simplest aromatic cycles, the electronic structure of triboracyclopropenyl derivatives has been analyzed with a variety of techniques in computational chemistry. These have ranged from canonical molecular orbital theory to alternative formulations of bonding such as adaptive natural density partitioning theory, the quantum theory of atoms in molecules, natural bond orbital theory, natural orbitals for chemical valence and electron localization function analysis. NICS and ring current calculations have also been used to characterize the aromaticity in such systems by using magnetic criteria. In general, the extremely small size of these cycles implies that their bonding electrons experience substantial Coulomb repulsion, resulting in abnormally high ring strain. This effect is partially compensated for by the stabilization offered by aromatic delocalization.