Reactivtiy of Hybrid Silylene and aNHC Toward Transition Metal Complexes

Abstract

N-heterocyclic tetrylene’s (NHTs) are neutral divalent species of group 14 elements, which show ambhiphilic behaviour due to a vacant p-orbital and a lone pair of electrons. They are currently one of the most rigorously investigated groups of ancillary ligands in modern main-group chemistry. The discovery of bottleable N-heterocyclic carbenes (NHCs) by Arduengo in 1991, followed by the first N-heterocyclic silylene (NHSi) by West and Denk in 1994 and the first metal-free aNHC (abnormal N-Heterocyclic carbene) by Bertrand in 2009 sparked widespread interest. Since then, researchers across academia and industry have actively explored NHCs and their potential applications in catalysis, synthesis, stoichiometric transformations, photoemitters and OLED fabrication marking the start of a new era in low-valent main-group chemistry. While NHCs have demonstrated superior potential than their heavier congeners and aNHC in these areas, recent advancements in transition-metal complexes bearing isolable NHSi and aNHC ligands have led to new synthetic strategies with diverse properties, significantly impacting organic methodologies, particularly small-molecule activation and a very small number of organic transformation reactions. However, NHSi has only one report in the domain of photoemitters while aNHC remained completely unexplored. Therefore, through this thesis, we have expanded our understanding of NHTs (NHSi and aNHC) by exploring their versatility in transition metal chemistry, main-group reactivity, and photophysical properties. The first section of the thesis demonstrated how phosphine-based hybrid silylene opt coordination with 3d-transition metal halides. These complexes show coordination-driven redox-active and magnetic behaviour. Magnetization in an external magnetic field displays paramagnetic behaviour at room temperature and superparamagnetic at low temperatures. The phosphine-based hybrid silylenes were also utilised in isolating several coinage metal complexes and exploring their unique reactivity and bonding patterns. Furthermore, various silylenes, including phosphine-based hybrid silylene, disilene, carbazole-substituted silylene, and bis-silylene ligand backbones, were examined for their reactivity with azide derivatives, leading to the formation of silaimines. Thus, this part of the thesis investigates the reactivity of silylenes with transition metal halides, emphasizing their coordination-driven redox activity, magnetic behaviour, and diverse reactivities with azides. Driven by the untapped potential of aNHCs in photophysical applications, we explored their unique properties as a photoemitters. Unlike conventional NHCs, aNHCs donate via the C5 position, making it less thermodynamically stable but a strong σ-donor. Thus, the aNHC-coinage metal (Cu, Ag, and Au)-amide framework was synthesized, thoroughly characterized and further investigated for its photophysical properties. Our work aims to pledge new insights for designing the next generation of stable abnormal carbene-metal-amide photoemitters. However, the last part of the thesis dealt with aNHC reactivity with boron hydride and its coordination towards metal carbonyls. Overall recognizing the vital role of main group ligands in modern synthetic chemistry, this thesis highlights the unique potential of NHTs, particularly NHSis and aNHC, in real-time applications, aiming to advance the understanding of fundamental main group chemistry.