Synthesis and formation mechanism of hydrogenated boron clusters B12Hn with controlled hydrogen content

Planar Elongated B12 Structure in M3B12 Clusters (M = Cu-Au)

Here, it is shown that the M₃B₁₂ (M = Cu-Au) clusters' global minima consist of an elongated planar B₁₂ fragment connected by an in-plane linear M₃ fragment. This result is striking since this B₁₂ planar structure is not favored in the bare cluster, nor when one or two metals are added. The minimum energy structures were revealed by screening the potential energy surface using genetic algorithms and density functional theory calculations. Chemical bonding analysis shows that the strong electrostatic interactions with the metal compensate for the high energy spent in the M₃ and B₁₂ fragment distortion. Furthermore, metals participate in the delocalized π-bonds, which infers an aromatic character to these species.

Bowl-shaped CuB12- Cluster. A viable Global Minimum with Twofold Aromaticity

A low-lying structure is revealed for the CuB 12 - cluster, which is bowl-shaped. It consists of a triangular CuB 2 base and a B 10 rim. Molecular dynamics simulations evidence its structural robustness; at an elevated temperature (600 K), the base rotates reversibly within the B 10 perimeter. Chemical bonding analysis detects 2σ- and 3π-delocalized bonds, suggesting double aromaticity, which is confirmed by two diatropic and concentric ring currents under an external magnetic field.

Why Does a B12H12 Icosahedron Need Two Electrons to be Stable: A First-Principles Electron-Correlated Investigation of B12Hn (n = 6, 12) Clusters

In this work, we present large-scale electron-correlated computations on various conformers of B₁₂H₁₂ and B₁₂H₆ clusters to understand the reasons behind the high stability of dianion icosahedron (I_h) and cage-like B₁₂H₆ geometries. Although the B₁₂ icosahedron is the basic building block in some structures of bulk boron, it is unstable in its free form. Furthermore, its H-passivated entity, i.e., a B₁₂H₁₂ icosahedron, is also unstable in the free form. However, dianion B₁₂H₁₂ has been predicted to be stable as a perfect icosahedron in the free-standing form. To capture the correct picture for the stability of B₁₂H₁₂^2- and B₁₂H₆ clusters, we optimized these structures by employing the coupled-cluster singles and doubles (CCSD) approach and the cc-pVDZ basis set. We also performed the vibrational frequency analysis of the isomers of these clusters using the same level of theory to ensure the stability of the structures. For all of the stable geometries obtained from the vibrational frequency analysis, we additionally computed their optical absorption spectra using the time-dependent density functional theory (TDDFT) approach at the B3LYP/6-31G* level of theory. Our calculated absorption spectra could be probed in future experiments on these clusters.

Boranes: The Boron Subhydride B104.67H3 with a Distorted β-Boron Crystal Structure

A single crystal of the boron subhydride B_104.67(4)H₃ was serendipitously obtained while attempting to synthesize β-boron. An accurate crystal structure analysis revealed a distorted β-boron framework with the noncentrosymmetric space group R3m. We have found one interstitial site occupied by boron. The site related by inversion remains empty. The distortions of the framework result in ideal environments for the interstitial boron atom, and for the three hydrogen atoms at bridging positions between icosahedral B₁₂ groups, they result in ideal B-H distances of 1.33 Å. B_104.67(4)H₃ is a borane with the lowest amount of hydrogen recorded to date, and it is the first compound with a noncentrosymmetrically distorted β-boron framework.

Overlap of Radial Dangling Orbitals Controls the Relative Stabilities of Polyhedral B nH n- x Isomers ( n = 5-12, x = 0 to n - 1)

The removal of H atoms from polyhedral boranes results in the formation of dangling radial orbitals with one electron each. If there is a requirement of electrons for skeletal bonding to meet the Wade's rule, these are provided from the exohedral orbitals. Additional electrons occupy a linear combination of the dangling orbitals. Stabilization of these molecular orbitals depends on their overlap. The lateral (sideways) overlap of dangling orbitals decreases with the decreasing cluster size from 12 to 5 boron atoms as the orbitals become more and more splayed out. Thus, as the number of dangling orbitals increases, the destabilization of their combinations increases at a higher rate for smaller polyhedral boranes, leading to flat structures with the removal of a fewer number of hydrogens. Though exohedral orbitals form better overlap in larger polyhedral clusters, the increase of electrons with the removal of H atoms results in occupancy of antibonding skeletal orbitals (beyond Wade's rules) and leads to flat structures. The reverse happens when hydrogens are added to a flat cluster. Substitution of BH by Si does not change structural patterns.

B12Fn0/- (n = 1-6) series: when do boron double chain nanoribbons become global minima?

We present an extensive density-functional and wave function theory study of partially fluorinated B₁₂F_n^0/- (n = 1-6) series, which show that the global minima of B₁₂F_n^0/- (n = 2-6) are characterized to encompass a central boron double chain (BDC) nanoribbon and form stable BF₂ groups at the corresponding BDC corner when n ≥ 3, but the B₁₂F^0/- system maintains the structural feature of the well-known quasi-planar C_3v B₁₂. When we put the spotlight on B₁₂F₆^0/- species, our single-point CCSD(T) results unveil that albeit with the 3D icosahedral isomers not being their global minima, C₂ B₁₂F₆ (6.1, ¹A) and C₁ B₁₂F₆^- (12.1, ²A) as typical low-lying isomers are 0.60 and 1.95 eV more stable than their 2D planar counterparts D_3h B₁₂F₆ (6.7, ¹A') and C_2v B₁₂F₆^- (12.7, ²A₂), respectively, alike to B₁₂H₆^0/- species in our previous work. Detailed bonding analyses suggest that B₁₂F_n^0/- (n = 2-5) possess ribbon aromaticity with σ plus π double conjugation along the BDC nanoribbon on account of their total number of σ and π delocalized electrons conforming the common electron configuration (π²⁽ⁿ⁺¹⁾σ²ⁿ). Furthermore, the simulated PES spectra of the global minima of B₁₂F_n^- (n = 1-6) monoanions may facilitate their experimental characterization in the foreseeable future. Our work provides new examples for ribbon aromaticity and powerful support for the F/H/Au/BO analogy.

Investigation on the neutral and anionic BxAlyH2 (x + y = 7, 8, 9) clusters using density functional theory combined with photoelectron spectroscopy

The photoelectron experimental spectra measured at 266 nm and simulated spectra of B₂Al₅H₂⁻ and B₂Al₆H₂⁻ clusters.

B12Hn and B12Fn: planar vs icosahedral structures

Using density functional theory and quantum Monte Carlo calculations, we show that B12Hn and B12Fn (n = 0 to 4) quasi-planar structures are energetically more favorable than the corresponding icosahedral clusters. Moreover, we show that the fully planar B12F6 cluster is more stable than the three-dimensional counterpart. These results open up the possibility of designing larger boron-based nanostructures starting from quasi-planar or fully planar building blocks.