This project focused on the molecular design and synthesis of compounds where B, Al, and Ga atoms are combined with N or P atoms via two/three electron pairs.
At the outset, we studied the target chemical bonds computationally. The origins of chemical bonds are inaccessible by experiments but can be approached theoretically. Since no exclusive approach exists to describe it unequivocally, there are continued debates on its definition. An approach to analyze it consists of classifying the electron-pair interactions as electron-sharing and dative. We demonstrated that there is a bonding situation manifested in metals, a so-called spin-polarized bond, which can also be used to describe the nature of the chemical bond in main group compounds.
Additionally, we have developed a combined molecular orbital theory-based method with topological analysis (EDA-IQA). This work provides an interesting approach for analyzing the directionality of bonding interactions such as orbital relaxation, Pauli repulsion, and electrostatic interaction.
The boron chemistry has been explored intensively from the experimental part. The isoelectronic relationship between C=C and B=N units has been exploited to prepare hybrid (in)organic polycyclic aromatic hydrocarbons (PAHs). The BN units, as traditionally incorporated, are described as electron sharing σ-bond and π-donation of the lone pair of nitrogen into an empty p-orbital of boron. We envisage that π-electronic systems of iminoborane-Lewis base adducts may provide a different kind of BN-unit. The bonding description consists of a double bond where the σ- and π-systems are electron-sharing with a strong dipolar moment. This bonding gives a twist since we obtained unprecedented thermal stability by introducing Lewis-base coordinated iminoborane adducts into pre-established cyclic geometry. Notably, the nature of the electronic situation unit differs from previous examples; having a HOMO-LUMO gap is significantly reduced. In addition, the pending N atom allows functionalization by a series of electrophiles, which can be used as a link to tune optoelectronic properties.
Analogues compounds containing B=P units are very scarce. Monomeric species need extra kinetic stabilization by blocking the P lone pair with a Lewis acid, or the B empty p-orbital with a Lewis base. We have successfully prepared new Lewis base-stabilized phosphaborenes by promoting TMSCl elimination. The attractive approach avoids the dimerization step observed in other synthetic protocols, allowing less bulky groups on the P atom. The B=P units can be transferred to organic, inorganic, and organometallic electrophiles. Unfortunately, our efforts to increase the unsaturation degree were unsuccessful.
We also pursued the formation of Al and Ga-containing analogues. These combinations bring experimental difficulties because of the big electronegativity difference in the atoms involved, which makes the species very reactive. Besides, conventional functionalization of aluminium species relies on small molecule elimination, salt elimination, and the activation of molecules with Al(I) species. Our focal point is to generate low-valent aluminium compounds with extremely rigid scaffolds with less sterical protection by side groups. Interestingly, instead of generating a nucleophilic aluminyl anion, we formed aluminium radicals which reduced benzene. We have also investigated the aluminium hydride chemistry with a counter ion (alkaline metal), envisioning application in catalysis for hydroborilation and hydrosilylation. We found particular use of boron-based substituents as pseudo-halide functionalities. This ligand is adaptable to multiple chemical environments and leads to valuable precursors for building sophisticated compounds.
Finally, the chemistry of gallium compounds leads to the preparation of monocoordinate species as a key intermediate for accessing GaN unsaturated functionalities.
