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A dipyrrin-supported copper complex mediates C–H bond amination and aziridination of exogenous substrates using electron-deficient arylazides, proceeding through copper-nitrene adducts.

 

Despite the myriad Cu-catalyzed nitrene transfer methodologies to form new C–N bonds (e.g.,amination, aziridination), the critical reaction intermediates have largely eluded direct characterization due to their inherent reactivity. Herein, we report the synthesis of dipyrrin-supported Cu nitrenoid adducts, investigate their spectroscopic features, and probe their nitrene transfer chemistry through detailed mechanistic analyses. Treatment of the dipyrrin CuI complexes with substituted organoazides affords terminally ligated organoazide adducts with minimal activation of the azide unit as evidenced by vibrational spectroscopy and single crystal X-ray diffraction. The Cu nitrenoid, with an electronic structure most consistent with a triplet nitrene adduct of CuI, is accessed following geometric rearrangement of the azide adduct from k1-N terminal ligation to k1-N internal ligation with subsequent expulsion of N2. For perfluorinated arylazides, stoichiometric and catalytic C–H amination and aziridination was observed. Mechanistic analysis employing substrate competition reveals an enthalpically-controlled, electrophilic nitrene transfer for primary and secondary C–H bonds. Kinetic analyses for catalytic amination using tetrahydrofuran as a model substrate reveal pseudo-first order kineticsunderrelevantaminationconditionswithafirst-orderdependenceonbothCuandorganoazide. Activation parameters determined from Eyring analysis(DH‡=9.2(2)kcalmol−1,DS‡=−42(2)calmol−1 K−1, DG‡ 298K =21.7(2) kcal mol−1) and parallel kinetic isotope effect measurements (1.10(2)) are consistent with rate-limiting Cu nitrenoid formation, followed by a proposed stepwise hydrogen-atom abstraction and rapid radical recombination to furnish the resulting C–N bond. The proposed mechanism and experimental analysis are further corroborated by density functional theory calculations. Multiconfigurational calculations provide insight into the electronic structure of the catalytically relevant Cu nitrene intermediates. The findings presented herein will assist in the development of future methodology for Cu-mediated C–N bond forming catalysis. 

The electronic structures and contrasting reactivity of [Cu(CF₃)₄]⁻and [Cu(CF₃)₃(CH₃)]⁻were probed using coupled cluster andab initiovalence bond calculations.

 

Nickel K- and L 2,3 -edge X-ray absorption spectra (XAS) are discussed for 16 complexes and complex ions with nickel centers spanning a range of formal oxidation states from II to IV. K-edge XAS alone is shown to be an ambiguous metric of physical oxidation state for these Ni complexes. Meanwhile, L 2,3 -edge XAS reveals that the physical d-counts of the formally Ni IV compounds measured lie well above the d 6 count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF 6 2− is considered using high-level molecular orbital approaches as well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d 6 Ni IV center. The reactivity of Ni IV complexes is then discussed, highlighting the dominant role of the ligands in this chemistry over that of the metal centers. 

The exposure of CrCl 2 (THF) 2 to 1 equiv. of TEMPO and 1 equiv. [TEMPO]Na afforded (η 2 -O,N-TEMPO) 2 CrCl (1, 67%); addition of [TEMPO]Na to 1 yielded (η 2 -O,N-TEMPO) 2 Cr(TEMPO) (2). Both 1 and 2 exhibit pseudo-pentagonal planar (PPP) geometry, instead of myriad alternatives. Calculations and spectral studies suggest the solid-state geometry persists in solution. 

Lithium peroxide is the crucial storage material in lithium–air batteries. Understanding the redox properties of this salt is paramount toward improving the performance of this class of batteries. Lithium peroxide, upon exposure to p –benzoquinone ( p –C 6 H 4 O 2 ) vapor, develops a deep blue color. This blue powder can be formally described as [Li 2 O 2 ] 0.3   · [LiO 2 ] 0.7   · {Li[ p –C 6 H 4 O 2 ]} 0.7 , though spectroscopic characterization indicates a more nuanced structural speciation. Infrared, Raman, electron paramagnetic resonance, diffuse-reflectance ultraviolet-visible and X-ray absorption spectroscopy reveal that the lithium salt of the benzoquinone radical anion forms on the surface of the lithium peroxide, indicating the occurrence of electron and lithium ion transfer in the solid state. As a result, obligate lithium superoxide is formed and encapsulated in a shell of Li[ p –C 6 H 4 O 2 ] with a core of Li 2 O 2 . Lithium superoxide has been proposed as a critical intermediate in the charge/discharge cycle of Li–air batteries, but has yet to be isolated, owing to instability. The results reported herein provide a snapshot of lithium peroxide/superoxide chemistry in the solid state with redox mediation.