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.[1] [2] [3] [4] 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).[4] [5] [6] However, more recent work has stabilized the triboracyclopropenyl moiety via coordination to donor ligands or transition metals, dramatically expanding the scope of its chemistry.[7] [8] [9] [10]
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.[11] B3+ displays π aromaticity associated with its a2'']-symmetric HOMO. In its singlet electronic ground state, it is a Hückel 2π electron system analogous to the cyclopropenium cation, but it is too reactive to be isolated. It is triangular, with D3h symmetry - all of its B atoms and B-B bond distances are chemically equivalent. The gas-phase adducts B3(N2)3+ and B3(CO)3+ have been computationally studied through ETS-NOCV (extended transition state - natural orbitals for chemical valence) theory, which dissects the changes in energy and electron density that result as a molecule is prepared from a reference state of noninteracting fragments.[12] ETS-NOCV energy decomposition analysis suggests that the N2 and CO adducts are primarily stabilized (by -83.6 and -112.3 kcal/mol respectively) through σ donation of the exocyclic ligands into the highly electron-deficient boron ring. As a result, each was interpreted as a B3+ moiety supported by dative bonding from N2 or CO. The electron deformation density constructed from the NOCVs of this system, together with charges derived from natural bond orbital populations, indicate electron flow from the exocyclic ligand into the ring, which induces all the equivalent bonds of the B3+ core to shorten by approximately 4 pm. π-symmetry interactions are observed with both the weak σ donor N2 and the strong π acceptor ligand CO. However, the out-of-plane π backdonation (from the π system of the B3 ring to the π acceptor orbitals of each ligand) is less stabilizing than the in-plane π backdonation, with strengths of -26.7 and -19.6 kcal/mol for the [B<sub>3</sub>(CO)<sub>2</sub><sup>+</sup> + CO] system. This suggests that the minimum-energy configuration of the molecule is one which preserves maximal π aromaticity in the B3+ core.
Just as aromatic species like the cyclopentadienyl anion and the cyclopropenium cation can coordinate to transition metals, it was recently demonstrated that the B3+ ring can bind to metal centers.[13] Laser ablation of a mixed B/Ir target produces two isomers of IrB3−, a B3+ ring coordinated to a formal Ir2- anion. These are a pseudo-planar η2 adduct and a tetrahedral η3 adduct, the latter of which contains an aromatic triboracyclopropenyl fragment. Both are nearly identical in energy and coexist in the generated cluster beam. Computations suggest that B3+ may even bind inert noble-gas atoms to form an unusual family of compounds B3(Rg)3+ (Rg = rare/noble gas), with nonnegligible bond strengths (from 15 to 30 kcal/mol) that originate from Rg p-orbital σ donicity and a significant degree of charge transfer from Rg to B3+. The possibility of new noble-gas compounds that form exothermally and spontaneously is an opportunity for experimental work.[14]
B3 possesses a singly occupied a1' HOMO (a SOMO) that consists of σ-symmetric orbitals oriented toward the core of the ring, associated with σ delocalization and slightly shorter B-B bond lengths as compared to B3+. It is paramagnetic with a doublet ground state.[15] It is nonpolar, flat and triangular, having D3h symmetry.
B3−, with a filled a1' HOMO in D3h symmetry, is considered to be "doubly" aromatic and relatively stable - it simultaneously possesses highly delocalized σ and π electrons in its HOMO and HOMO-1 respectively.[16] [17]
B3R32-, formulated with electron-sharing B−R bonds rather than dative arrows, is isoelectronic to B3+. 8 electrons are assigned to the triboracyclopenyl core, 6 in σ bonding orbitals and 2 in the π system, resulting in Hückel aromaticity. The only experimentally characterized compound of this class is Na4[B<sub>3</sub>(NCy<sub>2</sub>)<sub>3</sub>]2 • 2 DME, a dimer of stacked B3R32- units which are themselves aromatic. Natural bond orbital analysis indicates that this compound is highly stabilized (by roughly 45 kcal/mol) by a donor-acceptor interaction of localized B−B bond orbitals with corresponding B−N antibonding orbital across the ring, in addition to being bound together by electrostatic attraction to bridging Na+ cations identified in the crystal structure. DFT calculations show that the HOMO and HOMO-1 are antisymmetric and symmetric combinations of the π HOMO of an individual ring, respectively - a feature shared with metallocenes. As expected for a species with B−B bonds that have a formal MO bond order of
4/3
Triboracyclopropenyl-derived compounds were first identified by their mass-to-charge ratio, as transient species in the mass spectrometry of complex mixtures of cationic boron clusters. Reactive scattering studies with O2 soon followed, revealing the relatively strong bonding within light boron clusters. Subsequently, B3 was isolated in matrices of frozen noble gases and electron paramagnetic resonance spectra were recorded which confirmed its D3h geometry. Hyperfine coupling of the unpaired electron to the 11B nucleus provided an estimate of 15% s-orbital character for the a1' HOMO. The small and nonpolar B3 rings were able to tumble and rotate freely even when confined in the matrix.
In general, triboracyclopropenyl-containing species have been too short-lived and produced in insufficient quantity for transmission-mode infrared spectroscopy. However, dissociating B3(N2)3+ with infrared light and observing the decay of the corresponding mass-to-charge signal via mass spectrometry allowed an effective infrared spectrum of B3(N2)3+ to be recorded.[18] This vibrational photodissociation spectrum contained only a single detectable vibration with a redshift of 98 cm−1 relative to gaseous N2, suggesting a highly symmetric B3(N2)3+ adduct with slightly weakened N≡N bonding.
Negatively charged ions containing triboracyclopropenyl have proven amenable to study by photoelectron spectroscopy. By Koopman's theorem, neglecting the effects of strong electron correlation, the kinetic energies of electrons detached by X-rays can be mapped onto binding energies of individual orbitals and reveal the molecular electronic structure.[19] [20] Splitting of the resulting spectral peaks from "vibrational progression" (according to the Franck-Condon principle) indicates how ionization at different energies changes specific vibrational frequencies of the molecule, and such effects on bonding are interpreted in terms of changes to the electron configuration. In B3−, an unusually high-intensity and high energy band corresponding to a multielectron or "shake-up" transition (coupled electron detachment and electronic excitation) was observed, suggesting the strong electron correlation present in the triboracyclopropenyl fragment.[4] For IrB3−, vibrational progression from the stretching and breathing vibrations of IrB3 could be assigned in the overlaid spectra of both isomers present in the cluster beam. By comparison to computations, the minimum energy structure of IrB3 could then be formulated as a tetrahedron with an intact, aromatic B3+ moiety.