page_title: Borole
section_title: Sandwich complexes
hierarchical_section_title: Borole / Reactions / Sandwich complexes
caption_attribution_description: English: synthesis of borole sandwich
context_page_description: 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 C₅H₅⁺ 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 C₄H₄BH 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 interesting reactivities such as metal free hydrogen activation and rearrangements upon cycloaddition which are unobserved in other structural analogues like pyrrole or furan.
context_section_description: Since the borole dianion is isoelectronic to the cyclopentadienide anion, it also exhibits capability to from ferrocene-like sandwich compounds through an η⁵ coordination.
  Boroles have been experimentally shown to form stable complexes other group 13 elements, such as Aluminium and Galium. Both were synthesised from the neutral substituted borole and the corresponding metal (I)-Cp* complex in benzene as summarised in below.
  
  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 (C₄BH₅)Al(C₅H₅) 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 (C₄B) unit. However, this charge resides on the butadiene backbone as opposed to on boron (Cβ −0.24; Cα −0.99; B +1.68). In contrast, the charge accumulated on the central (C₅)‐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 Cα, bond critical points are only found for the between Al and Cα (delocalization index, DI=0.25) but not between Al and Cβ (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 η⁵‐aminoborole complex of germanium(II) 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 gemanium 4pₓ and 4py orbitals with the π orbitals of the borole ring. HOMO‐3 is the antibonding combination of mainly the germanium 4s and 4pz 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 0.31–0.42). 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 Li₂[H₂NBC₄H₄].  Ionic interactions prevail (WBI(LiC)=WBI(LiB)=0.02) in the latter complex. While the calculated charge distribution for Li₂[H₂NBC₄H₄] 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 Li₂[H₂NBC₄H₄]. 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 stabi
caption: Synthesis of the Al and Ga sandwich complexes.
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