Séminaire LCT | Sason Shaik "Identifying the reactive spin state..."

Séminaire professeur invité par le LCT

Sason Shaik (Institute of Chemistry, The Hebrew University, Jerusalem, Israel)

présentera une conférence intitulée

"Identifying the Reactive Spin State, the Stereochemistry and the
Constitution of the Transition State in Multi-State H-Abstraction Reactions by Oxo-Iron Complexes"

The talk outlines the interplay of theory-experiment in the quest of identifying the reactive-spin-state in chemical reactions, which possess a few spin-dependent routes. As such, these reactions involve Two-state Reactivity (TSR) or generally speaking, Multistate Reactivity (MSR).[1] TSR and MSR pose mechanistic challenges since there are no clear kinetic signatures for the reactive-spin-state in such reactions.

The talk describes how theory joins experiment in realizing this quest,[2] using a set of H-abstraction reactions which proceed with TSR/MSR mechanisms, and which possess experimentally-determined kinetic isotope effect (KIEexp) data.

As the talk demonstrates, the comparison of KIEexp results with calculated tunneling-corrected-KIE (KIETC) data leads to a clear identification of the reactive spin-state during H-abstraction reactions. This strategy provides the mechanistic chemist with a powerful capability to identify the reactive-TS not only in terms of its spin state, but also its geometry and its change, the orbital that controls this geometry, ligand-sphere constitution, as well as the manner by which steric effects shift the transition state from one spin to the other.

Since tunneling “cuts through” barriers, it serves also as a chemical selectivity factor which leads to seemingly counter-intuitive results.[2,3]

References:
[1] a) S. Shaik, D. Danovich, A. Fiedler, D. Schröder, and H. Schwarz, Helv. Chim. Acta. 1995, 78, 1393-1407;
b) D. Schröder, S. Shaik, and H. Schwarz, Acc. Chem. Res. 2000, 33, 139-145;
c) S. Shaik, H. Hirao, and D. Kumar, Acc. Chem. Res. 2007, 40, 532-542; d) S. Shaik, S.P. de Visser, F. Ogliaro, H. Schwarz, and D. Schröder, Curr.Opin. Chem. Biol., 2002, 6, 556-567.
[2] D. Mandal, D. Mallick, and S. Shaik, Acc. Chem. Res. 2018, 51, 107-117.
[3] a) D. Mandal, R. Ramanan, D. Usharani, D. Janardanan, B. Wang, and S. Shaik, J. Am. Chem. Soc. 2015, 137,
722-733;
b) J.E.M.N. Klein, D. Mandal, W.-M. Ching, D. Mallick, L. Que, Jr., and S. Shaik, J. Am. Chem. Soc. 2017, 139, 18705-18713