Quantum-Chemical Ab Initio Calculations on Borabenzene (C5H5B) and its Adducts with Ne, Ar, Kr, and N2. Could Free Borabenzene be Observed in Rare Gas Matrices?

Abstract

Quantum-chemical calculations employing different theoretical methods and basis sets have been performed on borabenzene (C 5 H 5 B) as well as on its adducts to dinitrogen (N 2 ) and the rare gases Ne, Ar, and Kr. In agreement with previous calculations, the ground state of borabenzene was found to be a planar singlet with six electrons in molecular orbitals of p symmetry and a wide C-B-C bond angle (142.28). Depending on the method (PUMP2, SAC-CI, CASPT2 (8,8)), the lowest triplet state was found to be 28 to 46 kcal mol À1 (1 kcal mol À1 ¼ 4.186 kJ mol À1 ) higher in energy. The energies associated with the formation of the adducts with N 2 , Ne, Ar, and Kr were calculated as À14.9, À0.5, À1.4, and À3.5 kcal mol À1 respectively. Our calculated spectrum of the normal modes as well as the electronic excitation spectrum of the N 2 adduct reproduce qualitatively the characteristic features of the IR and the UV-vis spectra described by experimentalists. The corresponding calculated spectra (normal modes, UV-vis) of the rare gas adducts were found to be very similar to those of free borabenzene. A Enthalpy change associated with the reaction 3 Á ethylene þ borabenzene-2 Á trans-butadiene þ 2-borabutadiene.

Corrigendum to: Quantum-chemical Ab Initio Calculations on the Donor–Acceptor Complex Pyridine–Borabenzene (C5H5N–BC5H5)

“…The reaction energy amounts to À52.2 kcal mol À1 and is, therefore, more than three times higher than the energy associated with the formation of C 5 H 5 B-N 2 from borabenzene and dinitrogen. [12] According to NBO analyses, the complex might either be described by a Lewis structure with a strongly polar B-N bond or by a non-bonded structure with a donor lone pair at N and an acceptor orbital at B. The results of our gas-phase calculations on the UV-vis spectra are best compared with the data obtained by other authors [2] in an Ar matrix.…”

“…[2] Moreover, at 120.68, the C-B-C angle is also quite similar to the experimental value of 119.1(2)8 in the solid state. Like the C-B-C angle calculated at the same level of theory for the most stable C 5 H 5 B-N 2 isomer (123.18 [12] ), this angle is much closer to the value for an ideal hexagon than the one in free singlet borabenzene (142.28, [12] TZ2P plus thermal correction/6-311þþG**). The experimental bond lengths in both rings are slightly but consistently shorter than their calculated counterparts, and in general, the partly delocalized C¼C bonds are longer in the C 5 H 5 B than in the C 5 H 5 N moiety of 1.…”

“…[11] All these energies of reaction are similar in that the change of energy on formation of the borabenzenepyridine complex from borabenzene and pyridine slightly exceeds three times the value of À14.9 kcal mol À1 calculated for the formation of C 5 H 5 B-N 2 from borabenzene and dinitrogen (CCSD(T)aug-cc-pVTZ//TZ2P plus thermal correction/-311þþG**). [12] The calculated reaction energies for the formation of 1 from its constituents are, therefore, comparable with the negative of the bond energy of Cl 3 Al-NMe 3 (À49.3 kcal mol À1 ), z the most strongly bonded donor-acceptor complex studied by Jonas et al [16] According to a natural population analysis, formation of 1 from borabenzene and pyridine goes in parallel with a charge transfer of 0.33 e from the C 5 H 5 N to the C 5 H 5 B unit. This is essentially identical to the charge transfer from NH 3 to BH 3 on formation of H 3 B-NH 3 , which, however, is associated with a change of energy of only À26.1 kcal mol À1 .…”

“…Borabenzene (C 5 H 5 B) has been the subject of some experimental [1][2][3] and an even higher number of theoretical [4][5][6][7][8][9][10][11][12] studies; however, attempts to generate the free compound have met with failure so far. At least in two experimental studies, borabenzenze apparently occurred as a transient that was trapped either by dinitrogen [1] or pyridine.…”

The Key Role of Orbital Interaction in Cooperativity between B⋅⋅⋅N and Hydrogen/Lithium Bonding: An ab initio Study

“…Besides, it seems that the addition of the ligand in part relief of strain within C 5 H 5 B which might favor the formation of C 5 H 5 B‐ligand complexes. For example, Raabe and Baldofski have performed a detailed theoretical study on the bonding properties of C 5 H 5 B complexes with rare gas atoms (Ne, Ar and Kr). According to their results, the strong Lewis acid property of C 5 H 5 B may cause reactions even with relatively inert matrix.…”