First paper from a Short-Term Scientific Mission (STSM)! (Apr 8, 2015)
Metalloradical catalysis is a topic of great interest in the group of Prof. Dr. Bas de Bruin in the University of Amsterdam, the Netherlands. In particular, the Co(II)porphyrin catalysts that are very efficient in carbene and nitrene transfer reactions present good examples of systems where spin transfer takes place from the metal center to the substrates, that is, carbenes or nitrenes.
The 'carbene radical' type intermediates have already been elucidated by his group earlier. To understand the nitrene-counterpart of these systems a holistic spectroscopic study was undertaken. Two interesting intermediates were identified. On using organic azides as nitrene precursors a mono-nitrene species with electron density being transferred from the cobalt to the nitrogen atom of the nitrene was obtained. However, if the precursor is more oxidizing like N-Nosyl iminoiodanes, then a bis-nitrene species is obtained with two nitrene fragments on the catalyst. In this very unusual scenario oxidation of the porphyrin ligand takes place which leads to very different electronic structure which is well reflected in the spectroscopic features.
Although EPR, UHR-ESI MS, UV-vis, IR and VCD studies revealed great details about the reactive intermediates, the coordination geometry of the intermediates was still unclear. In the absence of a crystal structure the researchers decided to carry out XAS measurements. This was done in collaboration with Kallol Ray (Humboldt university Berlin) and Serena DeBeer (MPI Chemical Energy Conversion Mülheim a/d Ruhr).
In particular, a one week STSM was availed by Monalisa Goswami who is a PhD student of Bas de Bruin. All the data that was obtained from the NLS, Brookhaven was interpreted and extremely valuable information regarding the coordination mode of the intermediates was obtained. As it turned out, the presumably five coordinate mono-nitrene species was clearly a six coordinate species. Although the nature of the sixth ligand is not quite clear, it is very much in line with the fact that additives don't have a very big influence on nitrene transfer reactions mediated by the Co(II)porphyrins.
This work is now accepted for publication in the Journal of the American Chemical Society (JACS):
Characterization of Porphyrin-Co(III)-'Nitrene Radical' Species Rele-vant in Catalytic Nitrene Transfer Reactions
Monalisa Goswami, Volodymyr Lyaskovskyy, Sérgio R. Domingos, Wybren Jan Buma, Sander Woutersen, Oliver Troeppner, Ivana Ivanovic-Burmazovic, Hongjian Lu, Xin Cui, X. Peter Zhang*, Edward J. Reijerse, Serena DeBeer, Matti M. van Schooneveld, Florian Pfaff, Kallol Ray, and Bas de Bruin*
J. Am. Chem. Soc. 2015, 137, 5468-5479 [abstract]
DOI: 10.1021/jacs.5b01197
