V. K. Kochnev

Theoretical study of molecular hydrogen elimination from the undecahydrodecaborate monoanion [B10H11]−. Exopolyhedral substitution intermediates: [B10H9]− monoanion and neutral [B10H10] cluster

The elementary reaction of molecular hydrogen elimination from the [B10H11]− anion, which is presumably the rate-limiting stage of acid-catalyzed reactions of substitution of exopolyhedral H atoms in the [B10H10]2− decahydro-closo-decaborate anion, has been calculated by the density functional theory method (in the B3LYP/6-311++G** approximation). Specific transition states of H2 elimination in which vacancies form near the boron atoms have been localized. It has been demonstrated that regioselectivity of substitution reactions can be related to the significant difference between the activation barriers for the pathways of H2 elimination from boron atoms with different coordination numbers (CN 6 and 5). The electron density of the [B10H9]− anion that forms after hydrogen molecule elimination has a characteristic shape of the lowest unoccupied molecular orbital for the interaction with nucleophilic reagents; in acid-catalyzed reactions, different anions, for example, a carboxylic acid residue, can act as such. The direct reaction of the [B10H9]− intermediate with nucleophilic anions is hindered by the Coulomb charge repulsion. To overcome this hindrance, the possibility of [B10H9]− protonation to form the neutral [B10H10] system has been considered. It has been shown that the proton affinity of the [B10H9]− anion is ∼280–290 kcal/mol. For the [B10H10] cluster, the lowest-lying and low-lying isomers have been considered. For all the systems under consideration, the electronic chemical potential and Pearson hardness have been evaluated.

Theoretical study of dodecahydro-closo-decaborane B10H12, the diprotonated boron cluster B10H102−

The electronic and geometric structures of different isomers of the closo-B10H12 boron cluster have been calculated by the density functional theory method (in the B3LYP/6-311++G**//B3LYP/6-31G* approximation). The compound is considered to be the diprotonated (H*) analogue of the well-studied B10H102− anion and serves as a model system. The increase in the relative energies of isomers and the preferred location of the extra H* protons near the opposite B(1) and B(10) “poles” are consistent with the charge separation (in the framework of the Mulliken population analysis) between B(1) and B(10). The reactions of migration of one or simultaneously two H* protons in B10H12 over the boron polyhedron have been considered, and the corresponding energies of elementary events E and activation barriers h have been estimated. The elementary events have been predicted in which both H* protons simultaneously move along the trajectories near the opposite B(1) and B(10) poles of the B10H102− polyhedron with the same or opposite changes in the angles determining the H* position with respect to the B(1)–B(10) axis. The activation barrier to the “opposite” migration of the H* protons has been assessed to be h ∼ 1.2 kcal/mol, whereas for the migration of the H* protons in the same direction, h ∼ 1.4 kcal/mol. The H* proton transfer from the position near the B(1) pole to the position near the opposite B(10) pole is hindered, and higher activation barriers on the order of h ∼ 13–15 kcal/mol should be overcome for this transfer to occur.

Theoretical study of H2 elimination from [BnHn + 1]− monoanions (n = 6–9, 11)

Transition states of elementary reactions of H2 molecule elimination from [BnHn + 1]− anions (n = 6–9, 11) in which nucleophilic/electrophilic vacancies form at boron atoms have been localized by the density functional theory method (in the B3LYP/6-311++G** approximation). For a series of [BnHn + 1]− anions (n = 6–12), the activation barriers to H2 elimination have been compared to consider the possibility of substitution for exopolyhedral hydrogen atoms by the mechanism with the first rate-limiting stage of formation of [BnHn − 1]− (n = 6–12) intermediates with a vacant “bare” vertex of the boron cluster. For the [BnHn]2−, [BnHn + 1]−, and [BnHn − 1]− anions (n = 6–12), the electronic chemical potential μ and Pearson hardness η have been evaluated since these characteristics make it possible to assess the propensity of different reagents to react with each other in terms of the empirical HSAB principle (soft with soft and hard with hard). The application of this principle is exemplified by the interaction of the [B10H9]− and [B12H11]− anions with acetonitrile CH3CN, furan C4H4O, and 18-crown-6.

The undecahydrodecaborate anion B10H11− as the starting reagent in exopolyhedral substitution and complexation: Theoretical and experimental prerequisites

Using the electron density functional theory (B3LYP approximation) with the 6-31G* basis set, the potential energy surface of the undecahydrodecaborate anion B10H11− was calculated and the activation energies and the activation barriers for the elementary reactions of proton H* migration around the boron polyhedron were estimated. Analysis of the calculation results in comparison with the experimental data accumulated recently implies that the salts of the B10H11− anion represent a new type of starting compounds for exopolyhedral substitution and complexation involving decaborate anions. Of particular interest is the targeted preparation of isomers of metal complexes containing a decaborate anion depending on the use of B10H102− or B10H11¨- as the starting reagent. Certain trends in the reactivity of B10H10− and B10H11− anions can be explained in terms of the simple analysis of Mulliken charge distribution on atoms.

Theoretical study of the structures of [M(18C6)](HFA)2 complexes (M = Ba, Sr, Pb, Cd, Mn; 18C6 = 18-crown-6; HFA = hexafluoroacetylacetonate anion)

The structures of the [M(18C6)]2+ cations (M = Ba, Sr, Pb, Cd, Mn) and their salts [M(18C6)](HFA)2 and [M(18C6)](NO3)2 have been calculated by the density functional theory method (in the B3LYP/6-311++G** + LanL2Dz approximation). Upon geometry optimization, the gas-phase structures of compounds of different composition have been calculated; for them, the strength of binding of the central cation to the crown ether (18C6) and the degree of structural similarity have been evaluated. The structure of the [NH4(18C6)]+ cation identified in a practical synthesis has also been considered. For metal cations acting as a central atom, NH4+ and [M(18C6)]2+ complex cations, as well as for intermediate and ultimate products [M(18C6)](NO3)2 and [M(18C6)](HFA)2 (M = Ba, Sr, Pb, Cd, Mn), the electronic chemical potential and Pearson hardness, which enables the consideration of the propensity of various reagents to interact with each other in terms of the empirical HSAB principle (hard with hard and soft with soft), have been evaluated. Comparison of the estimates with the properties of the synthesized compounds with M = Ba, Sr, and Pb makes it possible to preliminarily verify the applicability of this principle to the systems under consideration and predict some properties of isostructural analogues important in the search for methods of synthesis of [M(18C6)](HFA)2, where M = Cd and Mn. The possibility of establishing a correlation between the electron density of the system, stability, and hydrolytic activity of complexes has been shown.

Theoretical study of the redox reactivity of complex boron hydrides K2[B12H12], Cs2[B12H12], and Tl2[B10H10] and their mixed salts K2[B12H12] • KCl, Cs2[B12H12] • CsCl, and Tl2[B10H10] • KNO3

The ability of K2[B12H12], Cs2[B12H12], and Tl2[B10H10] molecules to act as the oxidant of n-octane in the gas phase has been considered in comparison with the O2, HNO3, and KNO3 molecules. Calculations have been performed at the B3LYP/6-31G*//6-311+G* + LanL2Dz level. Notwithstanding the fact that model calculations of isolated K2[B12H12], Cs2[B12H12], and Tl2[B10H10] molecules only approximately reflect the properties of solid K2[B12H12], Cs2[B12H12], and Tl2[B10H10], such a consideration makes it possible to reveal the molecular analogue of the “salt” effect: the oxidative ability of mixed salts K2[B12H12] • KCl, Cs2[B12H12] • CsCl, and Tl2[B10H10] • KNO3, in terms of the difference of the electronic chemical potentials of the oxidant and reductant, as well as of estimated electron density transfer, turns out to be similar to the oxidative ability of pure K2[B12H12], Cs2[B12H12], and Tl2[B10H10].

Theoretical study of exopolyhedral substitution in the hexahydro-closo-hexaborate anion

The proximity of values of the electronic chemical potential µ of the [B6H6]2– and OH– anions allows us to assume the possibility of participation of [B6H6]2–, instead of OH–, in chemical reactions proceeding in a basic medium. A theoretical study of reactions of the [B6H6]2– anion with halogenoalkanes has been carried out with chloromethane and 2-chlorobutane as examples using the B3LYP/6-311G* approach. The interaction of [B6H6]2– with CH3Cl in a certain sense is similar to the reaction of formation of primary alcohols from primary halogenoalkanes under the influence of an alkali and results in substituted product [B6H6CH3]–, i. e. the protonated form of anion [B6H5CH3]2–. At the same time, interaction of [B6H6]2– with 2-chlorobutane has to proceed similar to the reactions of elimination of secondary and tertiary halogenoalkanes in an alkaline medium to form unsaturated compounds and according to the calculations has to result in 2-butene. Transition states (TSs) have been located and the energy profiles of reactions have been calculated. In the course of the interaction with CH3Cl, the exopolyhedral substitution in the [B6H6]2– anion can proceed practically without the activation barrier (h ~ 1.8 kcal/mol), whereas the elimination reaction of 2-chlorobutane under the influence of [B6H6]2– has to overcome an insignificant threshold of h ~ 4.2 kcal/mol.

General electronegativity profile of a hydrogen molecule

The general method for studying the equilibrium state of a system through finding the extremum of the appropriate characteristic function can be applied to an isolated molecule, since its state is determined by averaging the stochastic electron motion in the nuclear electric field. Selection of electron energy as the function of state of a hydrogen molecule H2 depending on interatomic distance R(H-H) yields the classical attraction-type potential curve. The use of electronic chemical potential of a hydrogen molecule (μ(H2)) probably also enables one to determine the equilibrium state as there is an advantage in this case that the electronic chemical potential value μ not only can be calculated but also can be experimentally assessed. It was demonstrated that the numerically determined μ(H2) versus interatomic distance dependence is also of the attraction type and has an extremum at the equilibrium R(H-H) value of ∼ 0.74–0.75 Å for a certain calculation procedure. This dependence for hydrogen anion H2− does not have a well-defined extremum, while the potential energy curve for hydrogen anion H2− is of the repulsion type.

Theoretical study of the structure and water affinity of [M(18C6)(HFA) 2 ] complexes for M = Zn, Cu, Hg, Co, Ni, and Pt

The structures of the [M(18C6)]2+ cations, where M = Zn, Cu, Hg, Ni, Co, and Pt, and cis- and trans-[M(18C6)(HFA)2]/[M(18C6)(NO3)2] molecules in the gas phase have been calculated by the density functional theory method in the B3LYP/6-31G*//6-311++G** + LanL2Dz approximation. Geometry optimization has been performed, and the strength of binding of the central cation to the crown ether (18C6) and the degree of structural similarity of the [M(18C6)(HFA)2] compounds for different central atoms M have been evaluated. For all [M(18C6)(NO3)2]/[M(18C6)(HFA)2] molecules (M = Zn, Cu, Hg, Ni, Co, Pt), the vertical ionization potential and the vertical electron affinity have been calculated. These parameters are of interest for analysis of the stability of volatile compounds [M(18C6)(HFA)2] to donor–acceptor interactions with other components of the gas phase, for example, with water vapor, which is usually a Lewis base with respect to the systems in question and can donate electron density in the course of complexation with the central atom. The propensity of the [M(18C6)(NO3)2]/[M(18C6)(HFA)2] molecules to react with water is considered for a wider range of metals M2+ = Ba2+, Sr2+, Pb2+, Mn2+, Cd2+, Zn2+, Cu2+, Hg2+, Co2+, Ni2+, and Pt2+, with taking into account the degree of matching between the ionic radii of M2+ cations and the 18C6 cavity size.