A Hybrid Combination of NHC and Fluorenyl as an Ancillary Ligand Support for Divalent Main Group Metals

Mondal, Sumana (2025) A Hybrid Combination of NHC and Fluorenyl as an Ancillary Ligand Support for Divalent Main Group Metals. PhD thesis, Indian Institute of Science Education and Research Kolkata.

Text (PhD thesis of Sumana Mondal (19RS042)) 19RS042.pdf - Submitted Version Restricted to Repository staff only Download (5MB)

Abstract

The field of chemistry has seen a rise in the use of earth abundant and nontoxic main group elements as active sites in 'transition metal-free catalysis'. Zinc and magnesium, in particular, have emerged as prime candidates due to their well-established organometallics. To ensure the success of heteroleptic molecular chemistry involving these electropositive divalent metals, it is crucial to select an appropriate ancillary ligand with 'effective kinetic protection' to prevent unwanted ligand exchanges or the deleterious Schlenk equilibrium. Additionally, N-heterocyclic carbenes (NHCs) have become widely used as a versatile ligand platform in catalysis over the past few decades. In my research, I have combined these two aspects to develop a range of versatile catalysts supported by both neutral [LH = [(Flu)H-(CH2)2- NHC]) and anionic form [L⁻ = [Flu-(CH2)2-NHC]−), which exhibit a range of homogeneous catalytic activities. My zinc catalyst was the first to demonstrate the hydroboration of N-heteroarenes, amide, and ester functionalities with any NHC-supported catalyst, regardless of the metal used. The partial reduction of N-heteroarenes is particularly interesting as controlling the 1,2- vs. 1,4- regioselectivity is challenging. To my great satisfaction, under the present catalytic conditions, various N-heteroarenes are hydroborated exclusively in 1,2-regioselective manner. From the detailed computational analyses of the catalytic cycle, it is clear that the hydride transfer is responsible for this selectivity. In this study, I have presented a series of magnesium organometallic complexes supported by both LH and L⁻. The coordination of neutral NHCs to electropositive main group and early transition metals with a d⁰ configuration, including magnesium, is known to have kinetic lability. Although there are existing NHC adducts with magnesium alkyl, aryl, silyl, amide, alkoxide, aryloxide, and hydride, their practical applications, particularly in catalysis, are limited due to this lability issue. A chelating monoanionic sidearm might be helpful in this regard by invoking more kinetic stability. But examples of such cases are rare and with not much of focus on catalysis. L⁻ appears to be an effective ancillary support for heteroleptic magnesium chemistry as evident by the thermal resilience of [(L)MgN(SiMe3)2] and [(L)MgMe(thf)] and their abilities to catalyse the intramolecular hydroamination-cyclization of a model aminoalkane. Intramolecular hydroamination/cyclization of aminoalkanes is a useful and atom-efficient route for making N-heterocycles by adding a N-H bond across a C-C unsaturation. In recent times, there has been a growing interest in studying the chemistry of heavier alkaline earth elements. However, this research comes with a potential challenge due to the increasing bias towards the Schlenk equilibrium. To address this, I have conducted a study where I have presented a series of heteroleptic half-sandwich calcium and strontium organometallic complexes supported by L⁻ ligands. These complexes have been thoroughly characterized, providing detailed information about their structures and properties. It is worth noting that only a limited number of NHC supported heavier alkaline earth complexes have been reported so far, which possess a higher kinetic stability. Among these complexes, the calcium catalyst has shown remarkable efficiency in facilitating intramolecular hydroamination reactions under ambient conditions. Germylenes are interesting due to their activity towards small molecule activation. Starting from LGeCl I have synthesised LGeMe and LGeOSiPh3. Also synthesised LGe-GeL reducing LGeCl with [MesNacnacMg]₂. These complexes are charecterised using NMR and XRD. I am yet to check the reactivity of these complexes.

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