Bonding and Aromaticity in Electron-Rich Boron and Aluminum Clusters

Abstract

In this work bonding and aromaticity of triply bonded atoms of group 13 elements (MM, M = B and Al) in recently characterized B 2 Al 3 − , Na 3 Al 2 − , and Na 4 Al 2 are studied. Here, I show that although molecular orbital-based analyses characterize triple bonds, the electropositive nature of group 13 elements gives these bonds unique characteristics. The bond orders derived from the delocalization index, topology of the electron density, and local characteristics of (3, −1) critical points, as defined within the context of quantum theory of atoms in molecules, do not conform with those of ordinary triple bonds. In Na 3 Al 2 − and Na 4 Al 2 clusters non-nuclear attractors form between the electropositive Al atoms acting like pseudo atoms. The bond between boron atoms in B 2 Al 3 − is more similar to an ordinary triple covalent bond benefiting from the exchange−correlation component of the interatomic interaction energy as defined via interacting quantum atom theory. However, extreme electrostatic repulsion between negatively charged boron atoms attenuates this bond. Finally, current density analysis suggests that B 2 Al 3 − is a magnetic aromatic system, nearly 50% more aromatic compared to benzene.

Unraveling the effect of aromaticity for the dynamics of excited states of single benzene fluorophores

“…For instance, the NICS(0) values of benzene (‐10.3 ppm) and Kekulé benzene (‐9.3 ppm) differ only slightly, whereas for fulvene (‐0.8 ppm) and pentalene (+20.2 ppm) they differ widely, despite the fact that the former pair of molecules represent compounds with very different ‐delocalization (aromatic and nonaromatic), and the latter pair represents two molecules with similar ‐delocalization (both weakly anti‐aromatic). These observations are in line with the previously obtained results that negative/positive NICS values are not necessarily signatures of aromatic stabilization/antiaromatic destabilization 19–22 …”

“…Furthermore, characterization of aromaticity by criteria, such as NICS, based on magnetically induced current is prone to ambiguities, where the ability to sustain diamagnetic current (i.e., large negative NICS) can be mistaken for a signature of energetic stability (i.e., aromatic stabilization) 19–22 …”

The Search for Enhanced σ‐Donor Ligands to Stabilize Boron‐Boron Multiple Bonds

“…To stabilize compounds with BB multiple bonds, it is necessary to fill the empty 2p z orbital on each boron atom using a σ‐donor ligand that donates an electron pair (see Figure 1). [2,25,26,29–38] Another strategy to stabilize group 13 multiple bonding is through electronic transmutation as used by Boldyrev, Bowen, and Zhang [39–41] …”

“…[2,25,26,[29][30][31][32][33][34][35][36][37][38] Another strategy to stabilize group 13 multiple bonding is through electronic transmutation as used by Boldyrev, Bowen, and Zhang. [39][40][41] It is worth noting that the coordination of σ-donor ligands stabilizes the B=B and B�B multiple bonds in diborenes and diborynes, but it could also reduce their capability to activate small molecules. [34,35] The same is also true for diboraallenes and diboratriazoles, which have been stabilized with PMe 3 and (alkyl)(amino)carbene ligands.…”

Nuclear Magnetic Resonance

“…GIAO NMR calculations were used in studies of bonding and aromaticity in electron-rich boron and aluminum clusters. 130 Host À guest binding of b-cyclodextrin complexes was revealed by experimental NMR spectroscopy and supported by GIAO NMR calculations using the DFT approach. 131…”