Computational study of hydrido boronium dications and comparison with the isoelectronic carbon analogs

Calculations at the CCSD(T)/cc-pVTZ level show that higher coordinate, hydrido boronium dications are viable species despite their charge-charge repulsion. Structure and bonding of the studied boronium dications are discussed and are compared with their isoelectronic carbon analogs. Although the studied boronium dications are thermodynamically unstable towards deprotonation, their computational characterization supports their transient existence


Introduction
Main group hydrido cations and dications have been a subject of numerous experimental and theoretical investigations.1-6 A while ago we have reported 7 the calculated structures of parent hexa-and heptacoordinate boronium ions, BH 6 + and BH 7 2+ , respectively, with two and three two-electron three-center (2e-3c) bonds.The structures were found to be isostructural with their isoelectronic carbon analogs, CH 6 2+ , 8,9 and CH 7 3+ , 10 respectively.We have also previously investigated the structures and energetics of BH n 2+ (n=1-8) 11 and B 2 H 2n 2+ (n=1-4) 12 dications at the MP2/aug-cc-pVTZ and QCISD(T)/6-311G** levels, respectively.In continuation of our study of higher coordinate onium ions, we have carried out a study on the structure and bonding of higher coordinate hydrido boronium dications.For comparison, all the structures were calculated at the same CCSD(T)/cc-pVTZ level.

Results and Discussion
Gaussian 09 program 13 was employed for geometry optimizations and frequency calculations.The geometries were first optimized at the MP2/cc-pVTZ, level.Vibrational frequencies at the MP2/cc-pVTZ//MP2/cc-pVTZ level were used to characterize stationary points as minima (NIMAG (number of imaginary frequency) = 0 or transition state NIMAG = 1) and to compute zero point vibrational energies (ZPE), which were scaled by a factor of 0.96.14 The MP2/cc-pVTZ geometries were further optimized at the CCSD(T)/cc-pVTZ level.CCSD(T)/cc-pVTZ optimizations have been performed with the CFOUR program.15 Computed energies are given in Table 1.

BH n 2+ (n=3-8) systems
In line with previous calculations at the MP2/aug-cc-pVTZ level, 11 our present calculations at the CCSD(T)/cc-pVTZ level also shows that the C 2v symmetric structure 1 is a viable minimum for BH 3 2+ (Figure 1).The structure 1 is characterized with a two-electron three-center (2e-3c) bond, a two-electron two-center (2e-2c) bond, resembling a complex between BH 2+ and H 2 .The B-H and H-H bond distances of the 2e-3c bond are computed to be 1.491 Å and 0.889 Å, respectively.Expectedly, both thermodynamically and kinetically the dication is unstable due to charge-charge repulsion.The dissociation of 1 into BH 2 + and H + is exothermic by 100.6 kcal/mol and the process has a kinetic barrier of 8.2 kcal/mol at the CCSD(T)/cc-pVTZ//CCSD(T)/cc-pVTZ + ZPE level (Table 2).
Previously we have reported   2).   is also propeller-shaped resembling a complex between BH 2+ with three hydrogen molecules resulting in formation of three 2e-3c bonds.Average B-H bond distances of 2e-3c interactions of the structure 7 is found to be 1.359 Å. Deprotonation of 7 into BH 6 + 5 is exothermic by only 23.2 kcal/mol and the process has a high kinetic barrier of 55.1 kcal/mol at the CCSD(T)/cc-pVTZ//CCSD(T)/cc-pVTZ + ZPE level (Table 1).Relative energies of the structures 2, 4, 5, 7 are plotted in Figure 2. We also searched for any eight coordinate energy-minimum for dication BH 8 2+ .The structure 8 was found to be a minimum on the potential energy surface of BH 8 2+ (Figure 1).The C 3v symmetrical structure 8 can be considered as a weak complex between BH 6 2+ 6 and H 2 with a long bond distance of 1.844 Å between boron atom and H 2 moiety.
Triply hydrogen-bridged structure 12 was found to be the lowest minimum.This structure 12 also contains a 2e-3c bond involving a boron atom and two hydrogen atoms.One of the borons in structure

Conclusions
Structures of higher coordinate hydrido boronium dications were calculated using CCSD(T)/cc-pVTZ level of theory and compared with their isoelectronic carbon analogs.These calculations indicate that higher coordinated doubly charged boronium dications are viable species in the gas phase despite strong chargecharge repulsion.In such small first row highly charged cations, charge-charge repulsions are clearly substantial, but the bonding interactions are strong enough to counter this.Although such hydrido boronium dications are thermodynamically unstable, their computational characterization gives support for their transient existence.Structures, stability and multicenter bonding concept of the intriguing higher coordinate boronium and carbonium (hypercarbon) 19 ions as well characterized compounds have now been calculationally established.By development of new techniques, the characterization of these intriguing species is considered possible.

Protonation of BH 4 + 2
leads to the dication BH 5 2+ 4. We previously reported 18 the structure of BH 5 2+ at the QCISD(T)/6-311G** level.CCSD(T)/cc-pVTZ optimized structure of BH 5 2+ 4 is now computed and displayed in Figure1.The structure 4 resembles a complex between BH 2+ and two H 2 molecules resulting in formation of two 2e-3c bonds with an empty p-orbital orthogonal to the plane of the molecule.The dissociation of 4 into BH 4 + 2 and H + was calculated to be exothermic by 56.6 kcal/mol but has a high kinetic barrier of 36.5 kcal/mol (Table

Table 2 .
12 is sixcoordinated and the other one is five-coordinated.Dication 11, on the other hand, is doubly hydrogen-bridged structure.Additionally it also contains a 2e-3c bond involving a boron atom and two hydrogen atoms.Structure 11 is only 0.7 kcal/mol less stable than 12. Structure 13, unlike structures 11 and 12, was characterized to be a non-hydrogen-bridged ( i n v o l v i n g b o t h b o r o n a t o m s ) structure.Dication 13 with two 2e-3c bonds can be considered as a complex between B 2 H 2 2+ and two hydrogen molecules.Structure 13 is 6.3 kcal/mol less stable than the structure 12.The dissociation of 12 into B 2 H 5 + and H + was calculated to be exothermic by 33.9 kcal/mol but has a kinetic barrier of 45.1 kcal/mol (Table2).Dissociation Energy (∆E 0 ) and Respective Kinetic Barrier at 298 K for the Selected Processes a