Computational Studies of Heavier Main Group Elements: Control of Lewis Acidity of Bi and Sb by Charge and Ligand Design, and Structural Variations of [Fe(CO)4]2E2 and E2CH2 Isomers (E = C–Pb)

Abstract

Traditional main group compounds demonstrate Lewis acidity due to the presence of vacant p- orbitals. Hydrides of lighter main group elements, such as NH3, function as Lewis bases. However, introducing highly electronegative substituents X (e.g., halides) to a main group element M generates vacant M—X σ* acceptor orbitals. These σ* orbitals serve as Lewis acidic acceptor sites, interacting with Lewis bases and forming secondary interactions trans to the primary M—X bonds. The strength of these acceptor sites increases with heavier atoms, as the electronegativity difference between the main group element and the substituent becomes larger down the group. Various design strategies for controlling the energies and extension in space (shape) of these acceptor orbitals to enhance the Lewis acidity of Sb and Bi compounds form the first part of the thesis. These strategies include introducing highly electronegative substituents, generating positive and di-positive charges, tuning geometrical parameters such as the bite angle of chelating ligands, and transitioning from Bi to Sb analogues. These modified Lewis acids are then applied as catalysts in reactions such as ring-opening polymerization and catalytic hydrosilylation of olefins. The rules of structural chemistry set by the first row of the main group elements, largely by carbon, are broken while going down the Table. For instance, in the case of group 14 diatomic molecules, acetylene (H–C≡C–H) is the most stable isomer of C2H2, while the next most stable isomer, vinylidene (:C=CH2), is much higher in energy. However, in the silicon analogue Si2H2, the most stable isomer adopts a doubly bridged structure reminiscent of bridged boranes. In recent years, efforts have been made to stabilize group 14 diatomic species using donor- acceptor substituents like N-heterocyclic carbenes (NHCs). Additionally, attempts have been made to stabilize group 14 diatomic molecules using transition metal fragments as Lewis acids. We present ways of stabilizing diatomics E2 of the group 14 with a Lewis Acid Fe(CO)4. The structural variety available for [(CO)4Fe]2E2 (E = C, Si, Ge, Sn, and Pb)] extends the bonding possibilities in the main group. Bonding concepts from these unusual structural characteristics also extend to three-membered rings. When two carbon atoms in cyclopropenylidene (C3H2) are systematically replaced by group 14 heavier atoms, a variety of unusual stable isomers emerge. We predict the most stable isomer of CH2E2 (E = C–Sn) using computational methods where conventional chemistry wisdom fails. Although there have been qualitative explanations for the stability of these unusual structures before, isolobal analogy between trivalent boron (BH3) and divalent silicon (SiH2, and heavier analogues) anticipates the possibility of such unusual structures.