Borole

Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation or abbreviated as and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles. The parent unsubstituted compound with the chemical formula has yet to be isolated outside a coordination sphere of transition metals. Substituted derivatives, which have been synthesized, can have various substituents at the 4 carbons and boron. The high electron deficiency leads to various reactivities such as metal free hydrogen activation and rearrangements upon cycloaddition which are unobserved in other structural analogues like pyrrole or furan. Once reduced to the dianion, the borolediide complex gains aromaticity and can then participate in similar reactions as the anion, including forming sandwich complexes.

Boroles represent a class of molecules known as metalloles, which are heterocyclic 5-membered rings. As such, they can be viewed as structural analogs of cyclopentadiene, pyrrole or furan, with boron replacing a carbon, nitrogen and oxygen atom respectively. They are isoelectronic with the cyclopentadienyl cation or abbreviated as and comprise four π electrons. Although Hückel's rule cannot be strictly applied to borole, it is considered to be antiaromatic due to having 4 π electrons. As a result, boroles exhibit unique electronic properties not found in other metalloles. The parent unsubstituted compound with the chemical formula has yet to be isolated outside a coordination sphere of transition metals. Substituted derivatives, which have been synthesized, can have various substituents at the 4 carbons and boron. The high electron deficiency leads to various reactivities such as metal free hydrogen activation and rearrangements upon cycloaddition which are unobserved in other structural analogues like pyrrole or furan. Once reduced to the dianion, the borolediide complex gains aromaticity and can then participate in similar reactions as the anion, including forming sandwich complexes. A simple Hückel model can be used to compare the spectroscopic properties and observed reactivity of boroles against the isoelectronic cyclopentadienyl cation . Unlike which has a doubly degenerate pair of HOMOs, the introduction of a boron center lifts their degeneracy by increasing the energy of the antisymmetric molecular orbital somewhat and the symmetric molecular orbital significantly. As a result, the HOMO in boroles is doubly occupied and no biradical character is observed, in line with a singlet ground state and the diamagnetic character of boroles. Furthermore, boroles exhibit a small HOMO–LUMO gap (principally consisting of a π–π* transition) and the lowest-energy electronic absorption of boroles is considerably red shifted in the UV-Vis spectra (e.g. : λ max = 560 nm). Accordingly, boroles exhibit a characteristic blue color. By contrast, introducing two electrons into the vacant LUMO either by reduction or adduct formation with Lewis bases significantly increases the HOMO–LUMO gap. Consequently, a dramatic blue shift of the lowest-energy excitation is observed (e.g. ·pyridine: λ max = 340 nm) and the resulting species are usually yellow to red in color. A qualitative drawing is presented to the left. The small HOMO-LUMO gap also makes boroles excellent participants in Diels–Alder type reactions either with themselves or with a variety of alkenes and alkynes. [[File:Resonance in borole 2-.png|center|thumb|464x464px|Resonance forms in the Borole dianion. Contributions to overall structure are 30.30%, 30.71% and 13.04% respectively going from left to right as determed using NRT. Analyses were performed using NBO7 on a [C 4 BH 5 ] 2- structure optimised using BP86-D3BJ and def2- TZVPP basis set.|alt=]] Since borole dianions are isoelectronic to the ubiquitous cyclopentadienyl anion, aromatic delocalisation of the 6π electrons should cause bond lengths assimilation within the BC 4 backbone. This is exactly what was found for several characterised structures, namely K 2 [PhBC 4 Ph 4 ], K 2 [(4-Me 3 Si-C 6 H 4 )BC 4 Ph 4 ] and K 2 [ClBC 4 Ph 4 ]. Thus, the observed B–C bonds are rather short (1.505–1.543 Å) and all C–C bond lengths lie within a narrow array (1.409–1.456 Å). Depending on the boron-bound substituent, the electron density at boron can be altered. Hence, substituents can exert strong influences on the spectroscopic properties of the whole borole system. For instance, significant π-back bonding interactions from nitrogen in [(Me 3 Si) 2 NBC 4 Ph 4 ] raises the borole LUMO energy and a resulting blue shift of the lowest energy absorption in UV-Vis spectra (λ max = 478 nm) compared to [PhBC 4 Ph 4 ] (λ max = 560 nm). In order to synthesis less sterically congested boroles, a zirconacycle transfer strategy was adopted by Fagan et al. Reaction of [Cp 2 ZrC 4 Me 4 ] with PhBCl 2 was expected to result in the formation of [PhBC 4 Me 4 ]. However, the product was too reactive and only its Diels–Alder dimer has been isolated. Evidence for the intermediate [PhBC 4 Me 4 ] before dimerisation was shown through trapping experiments with 2-butyne and reactivity studies using a variety of unactivated alkenes. In 2018, Lee et al. successfully transformed a borapyramidane into a stable halogen substituted planar borole dianion which was stabilized by Li + ions positioned above and below the plane of the borole ring, revealing a direct synthetic path to borolediides from borapyramidane. In general, such reactions are facile and proceed quantitatively, facilitating their isolation in high yield. Upon Lewis base coordination, the former vacant p orbital at boron becomes occupied and cyclic delocalization of the π electron system is no longer feasible, corresponding to the loss of antiaromaticity. However, strong bond length alternation in the BC 4 backbone is still observed and remain almost unaffected by these fundamental electronic changes. In contrast, spectroscopic measurements are much more sensitive to adduct formation. Unlike the respective borole precursors which are intensely coloured, the adducts are pale yellow solids with characteristic UV-Vis excitations at λ max = 350–380 nm which agrees with an increase in the HOMO-LUMO gap. In the Aluminium complex, an Al (III) ion is sandwiched between a Cp* − and a substituted borolediide dianion, giving an overall neutral structure. Sindlinger reported that examining the orbitals of the simpler hypothetical model complex revealed similar features to the fully substituted compound. The successful transfer of two electrons onto the borole ring becomes apparent from the borole-based HOMO essentially being identical with the LUMO in free borole. The oxidation of Al is further in line with Bader charges of +2.29 at Al and -0.78 on the borole (C 4 B) unit. However, this charge resides on the butadiene backbone as opposed to on boron (C β −0.24; C α −0.99; B +1.68). In contrast, the charge accumulated on the central (C 5 )-Cp* moiety amounting to −1.17 is equally distributed among the five carbon atoms. Topology analysis was performed using QTAIM, which revealed no bond critical point between Al and B. In line with a strong localisation of electron density on C α , bond critical points are only found for the between Al and C α (delocalization index, DI=0.25) but not between Al and C β (DI=0.11) as shown below. In contrast, the gallium analogue forms a Lewis-base adduct with a dative Ga−B bond rather than the neutral heteroleptic case for Aluminium, suggesting that Ga still retains the +1 oxidation state. Furthermore, synthetic evidence for a neutral η 5 -aminoborole germanium(II) complex half sandwich has been reported as illustrated to the right. Tholen et al. performed DFT calculations at the M06-2X/Def2-TZVP level of theory and determined that the model complex matched up closely with experimental data determined experimentally by X-Ray Diffraction (XRD), with a maximum of 1.6% deviation in atomic distances and angles. Using the model compound, its frontier orbitals were calculated and they reveal covalent bonding interactions between the apical germanium atom and the borole base. As shown in the figure to the right, the HOMO is mainly located at the nitrogen atom while HOMO-1 and HOMO-2 are bonding combinations of germanium 4p x and 4p y orbitals with the π orbitals of the borole ring. HOMO-3 is the antibonding combination of mainly the germanium 4s and 4p z orbitals and the lowest π orbital of the borole ring. Performing natural bond analysis (NBO) on the model pointed to significant orbital interactions between the apical germanium atom and the borole basis as reflected by Wiberg bond indices (WBIs) between the ring carbon and boron atoms and the apical germanium atom (between 0.31 and 0.42). For reference, these values are very similar to those of the germanium (II) Cp* cation complex (WBI(GeC)=0.29) and significantly larger than those between lithium ions and the ring atoms of the borole ring in the inverse sandwich complex Li 2 [H 2 NBC 4 H 4 ]. Ionic interactions prevail (WBI(LiC)=WBI(LiB)=0.02) in the latter complex. While the calculated charge distribution for Li 2 [H 2 NBC 4 H 4 ] has an expected highly negative charge (−1.90 a.u.) on the aminoborole unit, the same unit has a much less negative charge in the aminoborole germanium complex (−0.83 a.u.). The increased charge transfer from the aminoborole unit to the germanium atom is also reflected by the uniformly smaller WBI indices between the ring atoms compared those in Li 2 [H 2 NBC 4 H 4 ]. In line with the NBO analysis results, quantum theory of atoms in molecules (QTAIM) analysis predicted a cage-like molecular topology for the model compound, with bond paths between all ring carbon atoms and the germanium atom as shown below. Further theoretical studies have also been conducted at the M06-2X/Def2-TZVP level of theory to investigate the stability of half sandwich complexes between C 4 H 4 BNH 2 with other group 14 elements (C, Si, Sn, Pb), where the borole ring binds to the divalent metal cation in η 5 mode. AIM analysis was performed and the laplacian for C is 0. The former indicates a substantial amount of covalency, while the latter confirms the presence of a polar covalent interaction.

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