Unraveling the mechanisms of strong acid and superacid stability of an unsaturated multiply-bonded main group compound and ultrastability of the resulting materials: The marriage of hydrogen bonding & multiple bonding in main group chemistry
The study of compounds containing multiply bonded heavier main group (MG) elements (e.g. E=E, E=C, etc.; E= MG atom) is challenging and interesting at the fundamental and applied level (Ref. 1).
The main goal of this project is to rationalize the energetics and mechanisms by which a system featuring a multiply-bonded phosphorus unit, which in its native state is extremely sensitive to oxygen and water even in atmospheric amounts, is unreactive towards much harsher species, in this case strong acids such as HCl, etc. and superacids like TfOH, and HPF6, leaving the exotic phosphorus functionality intact. This is fundamentally interesting from a stabilization perspective as explained briefly below, because all previous work done combining strong acids with compounds featuring multiply bonded main group (i.e. MG) elements, which are inherently sensitive species, focused on studying their reactivity via decomposition pathways (Ref. 2). Remarkably, the reactions with these highly acidic molecules, which result in materials containing unperturbed multiply bonded phosphorus units as mentioned above, become indefinitely stable in both solution and solid state in the presence of oxygen and water. Crystallographically and spectroscopically, there is sufficient evidence leading us to believe these compounds are able to achieve stability via an unprecedented mechanism that is ubiquitous in nature (Ref. 3), that is, supramolecular chemistry via hydrogen bonding.
The basic approach to stabilize multiple bonds in MG-compounds is based on using sterically crowded aryl/alkyl substituents and "kinetically" preventing-decomposition pathways of the double bond (e.g. polymerization, disproportionation, etc.). Other strategies like creating MG transition metal complexes, as well as more novel tactics such as using N-heterocyclic carbenes, delocalization of electrons via π-system, special conjugative effects, and donor-acceptor strategies, aim to decrease the high reactivity of these molecular main group compounds containing multiply bonded MG elements in their backbone; such different approaches also demonstrate the renowned interest and importance of these types of species (Ref. 4).
The use of computational chemistry will allow us to investigate the fundamental nature (e.g. thermodynamic, kinetic, etc.) of the processes by which these molecular systems are predisposed not to react with strong and superacids. More interestingly, we will also be able to shine light on the energetic contributions, and other factors, which lead to the ultrastability of the products. Also, we plan to extend the concept to novel systems offering the same, or different architectural advantages.
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2. A) R. West, Angew. Chem. Int. Ed. 1987, 26, 1201. B) T. C. Klebach et al. J. Am. Chem. Soc. 1978, 100, 4886.
3. A) D. Braga, et al. Chem. Rev. 1998, 98, 1375.
4. A) D. J. Liptrot, P. P. Power, Nat. Rev. Chem. 2017, 1, 0004. B) Z. Dong, et al Angew. Chem. Int. Ed. 2016, 55, 15899.