Two-electron processes are commonly invoked in transition metal mediated homogeneous catalysis of reactions including olefin polymerization, C-C bond formation, and C-H functionalization. Conventionally, these redox events have been assigned exclusively to metal centers, with couples such as Ir<sup>III/I</sup> and Pd<sup>II/0</sup> commonly invoked in catalysis. More recently, the scope of such processes has been expanded to include first-row metals. The proclivity of such metals to undergo 1-electron redox processes has been addressed via the inclusion of redoxactive or “redox non-innocent” (RNI) ligands that can themselves store oxidizing or reducing equivalents. This strategy permitted the use of first-row metals in numerous catalytic bond breaking/forming reactions. However, in some cases the RNI is “cryptic” and must be inferred via spectroscopic and/or computational means. For example, metal K-edge X-ray absorption spectroscopy (XAS) has been used to determine whether or not a change in metal physical oxidation state (effectively, the d-electron count) attends a given redox event. However, metal K-edge XAS transitions involving ligand-localized acceptor orbitals as well as spectral shifts that arise from changes in coordination environment can complicate metal K-edge analysis. In principle, redox activity at the N atom of coordinated ligands, such as in the pincer ligand of the Co complexes mentioned above, could be probed directly via ligand K-edge XAS. This approach, popularized for P, S, and Cl-donors, monitors 1s → valence/continuum excitations that can be used to quantify metal–ligand covalency valence vacancies. Extension of this approach to coordination complexes bearing period 2, light atom (C- ,N- ,O- , and F-donor) ligands has been rare. We now showcase the ability of N Kedge XAS to unmask “cryptic” RNI in a bis-phosphino diarylamido (PNP) pincer ligand coordinated to Ni. Bearing a lone N atom and hence providing only one locus for N oxidation, the PNP ligand represents an ideal case to use N K-edge XAS to study RNI. Identification of an inner-sphere aminyl radical is achieved through direct observation of a N-localized vacancy formed upon oxidation.

Direct identification of inner-sphere nitrogen radicals via nitrogen K-edge X-ray absorption spectroscopy

255th National Meeting (2018)

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