Exohedral Complexation of B40, C60and Arenes with Transition Metals: A Comparative DFT Study

Theoretical study on exohedral complexes C6H6TMB40 (TM = Sc-Ni)

Exohedral borospherene complexes comprised of 3d transition metal (TM) atoms with borospherene B₄₀ and benzene (C₆H₆) molecules, C₆H₆TMB₄₀, were systematically studied by using density functional theory (DFT). Results show that in the ground states, the bonding type between TM and B₄₀ changes from η⁷ (TM = Sc-V) to η⁶ (TM = Cr-Fe) and then to η⁷ (TM = Co, Ni) with the increasing number of d electrons. Except for C₆H₆TiB₄₀ and C₆H₆FeB₄₀ being triplets, all C₆H₆TMB₄₀ clusters have the lowest spin. Namely, the ground spin state with an even number of electrons is a singlet state, and the ground spin state with an odd number of electrons is a doublet state. The investigated C₆H₆TMB₄₀ clusters (TM = Sc-Ni) possess higher stabilities through the ionic-covalent interactions between transition metals, benzene and the B₄₀ cage.

B-B Coupling and B-B Catenation: Computational Study of the Structure and Reactions of Metal-Bis(borylene) Complexes

A detailed molecular orbital analysis of the metal-bis(borylene) complex [Fe(CO)₃ {B(Dur)B(N(SiMe₃ )₂ )}] (Dur=2,3,5,6-tetramethylphenyl) (1 a) serves as a focal point of recent developments in this area of chemistry, such as B-B coupling and B-B catenation reactions. There is strong a π delocalization between the Fe(CO)₃ and (B-Dur)(B-N(SiMe₃ )₂ ) units; the short B-B distance in 1 a is due to this π delocalization. The π-donor ligand N(SiMe₃ )₂ on the boron provides a decisive stability to the complex 1 a. The LUMO of 1 a has B-B σ-bonding character. Hence B-B coupling is facilitated by filling the LUMO. Strong σ-donating ligands, such as PMe₃ or PCy₃ , induce B-B coupling. Expulsion of one CO from 1 a followed by dimerization leads to [Fe(CO)₂ {B(Dur)B(N(SiMe₃ )₂ )}]₂ (3 a) with a short Fe-Fe distance of 2.355 Å. A detailed mechanism for the reaction of 3 a with CO to give the B-B catenation product 2 f is presented. The bonding of all intermediates is compared to their isolobal main-group analogues.

Noble gas encapsulated B40 cage

The efficacy of B₄₀ borospherene to act as a host for noble gas atoms is explored via density functional theory based computations. Although the Ng@B₄₀ complexes are thermochemically unstable with respect to dissociation into free Ng and B₄₀, it does not rule out their viability as all the systems possess a high activation free energy barrier (84.7-206.3 kcal mol^-1). Therefore, once they are formed, it is hard to take out the Ng atom. Two Ng atoms can also be incorporated within B₄₀ for the lighter Ng atoms (He and Ne). In fact, the destabilization offered by the encapsulation of one and two He atoms and one Ne atom inside B₄₀ is significantly less than that in experimentally synthesized He@C₂₀H₂₀, highlighting their greater possibility for synthesis. Although Ar₂ and Kr₂ encapsulated B₄₀ systems are very much destabilized by the repulsive interaction between Ng₂ and B₄₀, an inspection of the bonding situation reveals that the confinement can even induce some degree of covalent interaction between two otherwise non-bonded Ng atoms. Ng atoms transfer electrons towards B₄₀ which is smaller for lighter Ng atoms and gradually increases along He to Rn. Even if the electrostatic interaction between Ng and B₄₀ is the most predominant term in these systems, the extent of the orbital interaction is also considerable. However, the very large Pauli repulsion counterbalances the attractive interaction, eventually turning the interaction repulsive in nature. Ng@B₄₀ also shows dynamical behaviour involving continuous exchange between hexagonal and heptagonal holes, similar to the host cage, as understood from the very little variation in the activation barrier because of the Ng encapsulation. Furthermore, sandwich complexes like [(η⁵-C₅Me₅)Fe(η⁶-B₄₀)]⁺ and [(η⁵-C₅Me₅)Fe(η⁷-B₄₀)]⁺ are noted to be viable with the latter being slightly more stable than the former. The encapsulation of Xe slightly improves the dissociation energy associated with the decomposition into Xe@B₄₀ and [Fe(η⁵-C₅Me₅)]⁺ compared to that in the bare one.

[(Cp2M)2B9H11] (M = Zr or Hf): early transition metal 'guarded' heptaborane with strong covalent and electrostatic bonding

We have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp₂M)₂B₉H₁₁] (Cp = η⁵-C₅H₅; M = Zr or Hf).

Among the series of stable closo-borate dianions, [B_nH_n]^2–, the X-ray crystallographic structure of [B₇H₇]^2– was determined only in 2011. To explore its chemistry and stability, we have isolated and structurally characterized two new transition metal complexes of the heptaborane, [(Cp₂M)₂B₉H₁₁] (Cp = η⁵-C₅H₅; M = Zr or Hf). The structures of [(Cp₂M)₂B₉H₁₁] contain a pentagonal bipyramidal B₇ core, coordinated by two {Cp₂M} and two {BH₂} units equatorially. Structural and spectroscopic characterizations and DFT calculations show that [(Cp₂M)₂B₉H₁₁] complexes are substantially more stable than the parent dianion, in either [B₇H₇]^2– or (ⁿBu₄N)₂[B₇H₇]. Our theoretical study and chemical bonding analyses reveal that the surprising stability of the two new heptaborane metal complexes is due to multi-center covalent bonds related to the two exo-{Cp₂M} units, as well as electrostatic interactions between the {Cp₂M} units and the B₇ core. The facile syntheses of the heptaborane metal-complexes will allow further exploration of their chemistry.

Structures, stabilities and spectral properties of metalloborospherenes MB0/−40 (M = Cu, Ag, and Au)

Metalloborospherenes MB0/−40 (M = Cu, Ag, and Au) are predicted. Relative energies of these metalloborospherenes suggest that Cu, Ag and Au atoms favor the exohedral configuration.