Charge-Transfer-Induced Magic Cluster Formation of Azaborine Heterocycles on Noble Metal Surfaces

We report a combined experimental and theoretical study of the adsorption and assembly of a nitrogen–boron-containing heterocycle, 1,2-dihydro-1,2-azaborine, on Au(111) and Cu(111). Despite the inherent molecular dipole moment, the self-assembly behavior is found to be highly surface dependent, with isolated molecules prevalent on Cu(111) and discrete (“magic”) clusters on Au(111). The ability to form clusters of a particular size can be understood in terms of a balance between attractive intermolecular interactions, including directional B–H···H–N dihydrogen bonding, and repulsive forces from Coulombic interactions between the charged molecules dictated by differences in the charge transfer and Pauli repulsion between the adsorbate and the surface. This work highlights the importance of metal–molecule charge transfer in the adsorption and assembly of dipolar molecules on surfaces and demonstrates that their surface-bound properties cannot be predicted a priori from gas-phase dipole moments alone.