Theoretical Study of Main-Group Metal−Borazine Sandwich Complexes

B3H3Be ← F-: A Case of Extremely Strong Dative Bond

Chemical bonding has attracted chemists since its inception. Dative bonding between a donor and acceptor moiety is also an important phenomenon, which results in stabilization of many chemical compounds. Herein, we show that an extremely strong dative bond is possible between a fluoride ion and a beryllium center which is a part of a half-sandwich complex, B3H3Be. Quantum chemical calculations have shown the possibility of formation of a half-sandwich complex of Be with a B3H32- ring. Calculations reveal that the complex is stable toward dissociation. The half-sandwich complex features a very low-lying lowest unoccupied molecular orbital (LUMO) concentrated on the Be atom, thereby indicating a Lewis acidic character of the complex. This lower-lying LUMO at the beryllium center is responsible for forming an extremely strong dative bond with the fluoride donor.

Discovery of low energy pathways to metal-mediated B[double bond, length as m-dash]N bond reduction guided by computation and experiment

This manuscript describes a combination of DFT calculations and experiments to assess the reduction of borazines (B-N heterocycles) by η6-coordination to Cr(CO)3 or [Mn(CO)3]+ fragments. The energy requirements for borazine reduction are established as well as the extent to which coordination of borazine to a transition metal influences hydride affinity, basicity, and subsequent reduction steps at the coordinated borazine molecule. Borazine binding to M(CO)3 fragments decreases the thermodynamic hydricity by >30 kcal mol-1, allowing it to easily accept a hydride. These hydricity criteria were used to guide the selection of appropriate reagents for borazine dearomatization. Reduction was achieved with an H2-derived hydride source, and importantly, a pathway which proceeds through a single electron reduction and H-atom transfer reaction, mediated by anthraquinone was uncovered. The latter transformation was also carried out electrochemically, at relatively positive potentials by comparison to all prior reports, thus establishing an important proof of concept for any future electrochemical B[double bond, length as m-dash]N bond reduction.

Donor-acceptor complexes of borazines

Donor-acceptor complexes of borazine (BZ) and its substituted derivatives with Lewis acids (A = MCl(3), MBr(3); M = B, Al, Ga) and Lewis bases (D = NH(3), Py) have been theoretically studied at the B3LYP/TZVP level of theory. The calculations showed that complexes with Lewis bases only are unstable with respect to dissociation into their components, while complexes with Lewis acids only (such as aluminum and gallium trihalides) are stable. It was shown that formation of ternary D→BZ→A complexes may be achieved by subsequent introduction of the Lewis acid (acceptor A) and the Lewis base (donor D) to borazine. The nature of substituents in the borazine ring, their number, and position were shown to have only minor influence on the stability of ternary D→BZ→A complexes due to the compensation effect. Much weaker acceptor properties of borazine are explained in terms of large endothermic pyramidalization energy of the boron center in the borazine ring. In contrast to borazine, binary complexes of the isoelectronic benzene were predicted to be weakly bound even in the case of very strong Lewis acids; ternary DA complexes of benzene were predicted to be unbound. The donor-acceptor complex formation was predicted to significantly reduce both the endothermicity (by 70-95 kJ mol(-1)) and the activation energy (by 40-70 kJ mol(-1)) for the borazine hydrogenation. Thus, activation of the borazine ring by Lewis acids may be a facile way for the hydrogenation of borazines and polyborazines.

A DFT study on the dimerization of C62 , H2 C62 , and F2 C62

On the basis of calculations using the density functional theory, we show that C(62), a recently synthesized nonclassical fullerene, will presumably undergo dimerization with various isomers at elevated temperatures. This is shown by calculating the dimerization energy and the activation barrier of the dimerization. Eight possible isomers of the dimer were identified, all of which are more stable than the two isolated monomers. The relative stability of various isomers depends upon the kind of C=C bonds within the four-membered carbon ring involved in the dimerization. In addition, similar calculations were performed for the monomers and dimers of H(2)-C(62) and F(2)-C(62). Six isomers were identified for each of the dimers. Although less pronounced than the case of the C(62) dimer, all isomers of the H(2)-C(62) dimer are appreciably more stable than the individual monomers. Although a large steric repulsion due to F atoms significantly reduces the stability of F(2)-C(62) dimer, its two isomers are still more stable than separate monomers.

X-ray Photoelectron Spectroscopy and First Principles Calculation of BCN Nanotubes

Multiwalled boron carbonitride (BCN) nanotubes with two different structures were synthesized via thermal chemical vapor deposition; one has 10% C atoms homogeneously doped into BN nanotubes (B0.45C0.1N0.45 NTs), and the other has BN layers sheathed with 5-nm-thick C outerlayers (BN-C NTs). The electronic structures of the B, C, and N atoms were thoroughly probed by synchrotron X-ray photoelectron spectroscopy and the X-ray absorption near-edge structure method. The B0.45C0.1N0.45 NTs contain a significant amount of B-C and C-N bonding with a pyridine-like structure (hole structure), which reduces the pi bonding states of the B and N atoms. From the XPS valence band spectrum, the band gap was estimated to be about 2.8 eV. In the BN-C NTs, the C and BN domains are separated without forming the pyridine-like structure. Using the first principles method, we investigated the relative stabilities and electronic structures of the various isomers of the double-walled (12,0)@(20,0) BCN NTs. The C-outerlayer BN nanotube structure is the most stable isomer, when there exist no defects in the tubes with B/N = 1.0 (i.e., graphite-like structure). In addition, a reasonable model, which is characterized by the motives consisted of three pyridine-like rings around a hollow site, is presented for the local structure of C atoms in the B0.45N0.45C0.1 NTs. A considerable decrease of the band gap due to the 10% C doping was predicted, which was consistent with the experimental results.

DFT Study on the Stabilities of the Heterofullerenes Sc3N@C67B, Sc3N@C67N, and Sc3N@C66BN

On the basis of calculations using density functional theory, we investigated the relative stabilities of all isomers of Sc3N@C67B and Sc3N@C67N as well as those of stable isomers of Sc3N@C66BN. As a result, we predict that Sc3N@C68 can be doped substitutionally with a boron atom much better than C60. This effect can be ascribed to the favorable electrostatic attraction between the encased Sc3N cluster and the polar C-B bonds of the fullerene cage, which show the important role played by the encapsulated atoms in stabilizing the fullerene. A difference in the interaction also determines the regiospecificity of Sc3N@C67B. On the contrary, N-doping of the fullerenes forming Sc3N@C67N is much less favorable than that in C60 or C70. A judicious choice of stable isomers of Sc3N@C66BN among a vast number of possible isomers indicates that Sc3N@C68 can also be doped with a pair of B and N atoms better than C60 under the simultaneous existence of B and N sources. Relative stabilities of various isomers of the BN-substituted fullerenes can be understood in terms of the combined electrostatic effects in the B- and N-substitutions of Sc3N@C68 complemented by a specific local preference in the N-substitution and the formation of a B-N bond.

Density Functional Study of Chemical Stability and Nitrogen Encapsulation of C48N12 and C58N12

On the basis of calculations using density functional theory, we show that C58N12, just as C48N12, can be a stable N-dopant of C70. By considering many different isomers of the product, we find that the chemical stability of C48N12 and C58N12, with respect to oxygenation, is not significantly different from that of C70, thereby indicating that the N-dopant would not easily be oxygenated in air under normal conditions. In both C48N12O and C58N12O, many different isomers are expected, in which oxygenation occurs at different C-N bonds as well as at C-C bonds, among which specific C-N bonds are the most amenable to the reaction. Investigation of their hydrogenations shows that C48N12 is slightly more easily hydrogenated than C60, while C58N12 is less easily hydrogenated. In addition, we expect a regiospecificity in the hydrogenated products of C58N12, which prefers to react at equatorial sites, while C70 prefers reaction at polar sites. Meanwhile, comparison of the encapsulation energy of a nitrogen atom (=N en) in C60, C48N12, C70, and C58N12 shows that the N-doped fullerenes, particularly C58N12, can encase the atom much better than the undoped ones, allowing us to expect the existence of N@C48N12 and N@C58N12. Spin multiplicities are doublet for most of their stable structures. These observations correlate with the formation of N en-C bonds, which are not found in N@C60 and N@C70. Various isomers of the N-encapsulating fullerenes were identified. The relative stability of these isomers heavily depends on the number of substitutional nitrogen atoms around N en-C bonds.

Theoretical Study of Endohedral C36 and Its Dimers

It is found that atoms of lithium and carbon can be encapsulated in C(36) on the basis of the calculation of their encapsulation energies using density functional theory. Specifically, they can be encapsulated in C(36) better than in C(60) despite the smaller (70%) cavity size of the former. In C@C(36), the encapsulated carbon atom forms covalent bonds with the carbon atoms of the cage, which is in contrast with the case of N@C(60.) Two isomers are expected to be in an equilibrium which involves spin quenching and generation. Li@C(36) and C@C(36) are expected to exist in the form of dimers with nonendohedral fullerenes, i.e., as Li@C(36)-C(36) and C@C(36)-C(36). Three stable isomers were found for the former (A, B, and C). Equilibrium between A and C as well as that between B and C is accompanied by spin transfer between two fullerene units, while that between A and B is not. The two stable isomers in C@C(36)-C(36) form an equilibrium accompanied by spin quenching and generation, allowing the dimer to be potentially useful for molecular devices.

A theoretical study of fullerene–ferrocene hybrids

Using density functional theory within the generalized gradient approximation, I analyzed the electronic structure of a C(60)-ferrocene hybrid [= C(60) (*) FeCp] around HOMO in comparison with that of ferrocene, where C(60) (*) and Cp denote C(60)(CH(3))(5) and a cyclopentadienyl ring. HOMO-LUMO gap is significantly smaller than that of ferrocene because of the intervention of pi(C(60) (*)) states below LUMO. In addition, geometrical and electronic structures of N@C(60) (*) FeCp are also investigated. I find that there are two isomers with the energy difference of 0.13 eV. In one of the two, the encased nitrogen atom is located at the center of the fullerene cage. The Fe atom is eta(5)-coordinated to both Cp and R*, where R* is a five-membered ring of C(60) (*) cage. On the other hand, the atom is coordinated to R* with eta(4)-hapticity, and the nitrogen atom is bonded to a carbon atom of the R* ring in the other isomer. Upon the isomerization between the two isomers, there occurs a partial transfer of spin density between the nitrogen and Fe atoms as well as the creation and breaking of a C-N bond.

Theoretical Study of Binding of Metal-Doped Graphene Sheet and Carbon Nanotubes with Dioxin

Using density functional theory, we have theoretically studied dioxin binding on a graphene sheet or carbon nanotubes (CNT), finding that they can be effective adsorbents for dioxin in the presence of calcium atoms. This is due to a cooperative formation of sandwich complexes of graphene sheet or (5,5) CNT through the interaction pi-Ca-pi with the total binding energy of more than 3 eV. This correlates with the band structure analysis, which indicates charge transfer from the carbon systems and calcium atoms to dioxin when the molecule binds to the metal-doped carbon systems. For CNT with small radii, the relative strength of CNT-dioxin interaction is dependent on their chiralities. Upon dioxin binding, a large increase in the electronic density of states near the Fermi level also suggests that they can be used for dioxin sensing. Fe-doped CNT is also found to bind dioxin strongly, revealing an important role played by remnants of metallic catalysts in the chemical properties of CNT.