Charge Separation in Ligand Design
Authors:   , Tretiakov Serhii 
Published: November 2023

As chemists, we are bound by the laws of the universe in which we exist. We do not invent new reactions, we discover them. We cannot force atoms into molecules unless they are capable of binding to one another in the first place. Rather, we find the starting points and conditions that allow things to naturally fall into a desired state – these are the only two degrees of freedom a synthetic chemist can actually use. Despite how claustrophobia-inducing it may sound, varying just these two parameters gives rise to a wealth of fields, spanning from organic, organometallic and main-group chemistry/catalysis to electro-, mechano- and photochemistry among others. All of them are concerned with harnessing chemical reactivity by means of understanding and using this knowledge to achieve a desired outcome. Ultimate reactivity control is the holy grail of synthetic chemistry, especially in the face of the ongoing environmental crisis and the quest for inexpensive and sustainable transformations.

Aside from varying conditions, chemists have come up with a number of clever ways to steer reactivity in a preferred direction. Among these are the use of elements with an incomplete electron shell, such as transition metals or main-group compounds (often in unusual oxidation states), introducing strain or charge separation (CS). The latter approach remains largely underutilised and, therefore, was chosen as the central point of this thesis. As such, some extent of CS is present in every molecule. However, large CS can be implemented by design in molecules that bear unquenched charges of opposite sign, i.e. zwitterions.

The aim of the current work is to develop a ligand with built-in CS that allows the study of its effects in the contexts of both transition metal and main-group chemistry. We introduce a novel 1,3-zwitterionic scaffold, tris-skatylmethylphosphonium (TSMP(2–)), isolated as a potassium salt TSMPK2, which is also the first known C3-symmetric dianionic homoscorpionate capable of metal exchange.

Chapter 1 of this thesis discusses the mechanisms through which CS may affect reactivity, and highlights both natural and synthetic systems with separated charges.

Chapter 2 describes the synthesis and characterization of the TSMPK2 salt followed by the extensive study its Fe(II), Ni(II) and Cu(I) coordination compounds. Of note is the bis-ligated tetrahedral complex [(TSMP)2Fe(II)]K2 that shows rich redox behavior.

Chapter 3 continues with the [(TSMP)2Fe(II)]K2 complex, exploring its one- and two-electron oxidation to the corresponding anionic Fe(III) and neutral Fe(IV) species, which demonstrates the ability of the TSMP(2–) scaffold to support catalytically-relevant high oxidation states. Electronic structure of the Fe(IV) complex is studied in great detail using a variety of spectroscopic and computational methods.

Chapter 4 explores applications of the TSMP(2–) scaffold to main-group chemistry in the context of silanes. It discusses how strain and charge separation contribute to the anomalous acidity of the Si–H bond (at least three orders of magnitude higher than O–H bond of benzoic acid).

Finally, Chapter 5 – the outlook – reflects on the work done in the previous experimental chapters, while also providing suggestions on possible improvements and future developments.