Understanding Precatalyst Activation and Speciation in Manganese-Catalyzed C-H Bond Functionalization Reactions

Jonathan B. Eastwood, L. Anders Hammarback, Thomas J. Burden, Ian P. Clark, Michael Towrie, Alan Robinson, Ian J.S. Fairlamb*, Jason M. Lynam

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review


An investigation into species formed following precatalyst activation in Mn-catalyzed C-H bond functionalization reactions is reported. Time-resolved infrared spectroscopy demonstrates that light-induced CO dissociation from precatalysts [Mn(C^N)(CO)4] (C^N = cyclometalated 2-phenylpyridine (1a), cyclometalated 1,1-bis(4-methoxyphenyl)methanimine (1b)) in a toluene solution of 2-phenylpyridine (2a) or 1,1-bis(4-methoxyphenyl)methanimine (2b) results in the initial formation of solvent complexes fac-[Mn(C^N)(CO)3(toluene)]. Subsequent solvent substitution on a nanosecond time scale then yields fac-[Mn(C^N)(CO)31-(N)-2a)] and fac-[Mn(C^N)(CO)31-(N)-2b)], respectively. When the experiments are performed in the presence of phenylacetylene, the initial formation of fac-[Mn(C^N)(CO)3(toluene)] is followed by a competitive substitution reaction to give fac-[Mn(C^N)(CO)3(2)] and fac-[Mn(C^N)(CO)32-PhC2H)]. The fate of the reaction mixture depends on the nature of the nitrogen-containing substrate used. In the case of 2-phenylpyridine, migratory insertion of the alkyne into the Mn-C bond occurs, and fac-[Mn(C^N)(CO)31-(N)-2a)] remains unchanged. In contrast, when 2b is used, substitution of the η2-bound phenylacetylene by 2b occurs on a microsecond time scale, and fac-[Mn(C^N)(CO)31-(N)-2b)] is the sole product from the reaction. Calculations with density functional theory indicate that this difference in behavior may be correlated with the different affinities of 2a and 2b for the manganese. This study therefore demonstrates that speciation immediately following precatalyst activation is a kinetically controlled event. The most dominant species in the reaction mixture (the solvent) initially binds to the metal. The subsequent substitution of the metal-bound solvent is also kinetically controlled (on a ns time scale) prior to the thermodynamic distribution of products being obtained.

Original languageEnglish
Number of pages8
Early online date3 Apr 2023
Publication statusE-pub ahead of print - 3 Apr 2023

Bibliographical note

Funding Information:
We are grateful to Syngenta, the EPSRC, and the Department of Chemistry at the University of York (iCASE Studentship to L.A.H. EP/N509413/1 and studentships for J.B.E. and T.J.B.) as well as the Royal Society of Chemistry (Research Enablement Grant E21-8424864227 to support J.B.E.) and the EPSRC (Grant EP/W031914/1) for funding. We thank the STFC for program access to the ULTRA facility (Grant 1813). J.M.L. and I.J.S.F. are both supported by Royal Society Industry Fellowships (INF\R1\221057 and INF\R2\202122 respectively). The computational work in this project was undertaken on the Viking Cluster, which is a high-performance computer facility provided by the University of York. We are grateful for computational support from the University of York High Performance Computing service, Viking and the Research Computing team.

Publisher Copyright:
© 2023 The Authors. Published by American Chemical Society.

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