Title : Computational studies of functionalized [Pn7] (Pn = P, As) clusters for the catalyzed hydroboration of pyridine; Exploration of the treatment of entropy
Abstract:
Zintl clusters have recently emerged as competent catalysts for a variety of organic transformations. This work reports the mechanism of functionalized Zintl-cluster-catalyzed hydroboration of pyridine and subsequent analysis of the energetic profile of this catalytic cycle, employing the energetic span model to identify key states that underpin the catalytic activity and observe how these states and other properties of the catalytic cycle change for several catalyst analogues.
We also explore the effect of entropy, and how it is computed in solution, in determining the energetics and kinetics of the target systems, highlighting the issues caused by the parameterisation of continuum solvation models (CSMs) around free energies, and how these issues can result in inaccurate Gibbs energies; currently there is variance in how entropy is accounted for, and little-to-no consensus as to what method is most appropriate. Our work shows that two technically theoretically sound methods can produce distinctly different outcomes. This is especially true in the context of a catalytic cycle involving association and dissociation steps, where large energetic discrepancies between these steps can cumulatively lead to different conclusions about which catalyst performs best and which states within the cycle influence that performance.
We use electronic energies calculated within the solution model based on density (SMD) method of solvent incorporation, partially incorporate entropic due to the aforementioned CSM parameterisation, which allows us to reframe this electronic energy as a Gibbs energy, in opposition to the more traditional method of deriving entropic contributions from the vibrational frequencies. We find this allows for the calculation of turnover frequencies that are much more catalytically viable when compared to more conventional methods, whereas obtaining a Gibbs energy from the combination of solvated electronic energies with gas-phase derived vibrational frequencies, a method we refer to as ‘double counting’, results in insurmountable energetic barriers.
We also find little substantial change in the catalytic mechanism for these analogues, in spite of substantial changes to their structure, with this being testament to the versatility of these functionalized Zintl clusters, and the complete mechanism and subsequent employment of the ESM being a substantial advancement in our understanding of the processes that underpin their catalytic activity.
