Synthesis, Characterisation, and Application of the Isoelectronic Heavier Main Group Compounds: Germylene, Stannylene, Bismuth Cation

Abstract

Carbenes, especially N-heterocyclic carbenes (NHCs), are well-recognised as strong σ-donor ligands for both transition metals and main-group elements. These donor ligands have significantly advanced and transformed the field of transition metal catalysis. In line with this achievement, the heavier group 14 carbene analogues known as tetrylene E(II) (E=Si, Ge, Sn, Pb) are gaining prominence because they can mimic transition metals due to the matching of frontier orbitals with d-orbitals of transition metals. These divalent compounds, which have a lone pair of electrons and an empty p- orbital, can be either Lewis acidic or Lewis basic. The chemical properties of higher tetrylenes can be tuned by the design of the supporting ligand. Although the activation of small molecules such as H2, CO2, CO, NH3, alkene, and alkyne is well established, their stability, the limited availability of viable synthetic routes, and the limited library of known compounds hamper the development of this field. In this context, we have focused on developing accessible synthetic routes and stabilised low-valent group 14 and heavier group 15 cationic compounds, with appropriate characterisation. We further investigated its reactivity and catalytic application. The background study will be included in Chapter 1. Chapter 2, (section A) comprises the stabilisation of NHC-stabilised bis-alkynyl germylene species and exploring their reactivities. Its coordinating ability towards coinage metal forming ionic complex and its reactivity with elemental sulphur were studied. It also finds application in small molecule activation, forming a frustrated Lewis Pair (FLP) with tris-pentafluoro phenyl borane. In the presence of tetrahydrofuran, it forms an activated product, thus showing FLP-type reactivity. (section B) The catalytic applications in cyanosilylation and hydroboration of carbonyls have been studied. The catalytic reactions were carried out under mild conditions using 1 mol% of catalyst loading. Catalysis was carried out for a wide range of aromatic aldehydes and ketones. In Chapter 3, NHC-stabilised bis-alkynyl stannylene chemistry has been studied. The synthesis and characterisation of the compound have been done. The catalytic application of this bis-alkynyl stannylene has been investigated. Similarly, for the higher analogue of germylene, that is, stannylene, the catalytic applications in the cyanosilylation and hydroboration of carbonyl compounds have been investigated. The reactions were performed under mild conditions using 1 mol% of the catalyst and were successfully applied to a broad range of aromatic aldehydes and ketones. To further explore the reactivity of the stannylene, reactions were conducted between the stannylene and various substrates in stoichiometric ratios. Chapter 4 (Section A) focuses on the heavier group 15 cationic compounds in the +III oxidation state. The inherent Lewis acidity of Sb(III) and Bi(III) was enhanced by installing positive charges on them. The intentional conversion of neutral bismuth (III) compound into well-defined cationic derivatives has emerged as a promising strategy to explore compounds with fascinating bonding characteristics and reactivity patterns. This work is aimed at the stabilisation of bismuthenuim mono-cation and di-cations using a non-innocent alpha imino-pyridine ligand. (section B) The efficiency of the Bi(III) mono-cationic compound in homogeneous catalysis has been explored. The reactions were carried out under mild conditions using 1 mol% of the catalyst and were successfully applied to a wide range of aromatic aldehydes. Additionally, the Gutmann–Beckett method was employed to estimate the Lewis acidity.