Yubin M. Kwon

The recent focus on developing high-valent non-oxo-metal complexes for late transition metals has proven to be an effective strategy to study the rich chemistry of these high-valent species while bypassing the synthetic challenges of obtaining the oxo-metal counterparts. In our continuing work of exploring late transition metal complexes of unusually high oxidation states, we have obtained in the present study a formal mononuclear Ni(IV)-nitrate complex (2) upon 1-e^- oxidation of its Ni(III) derivatives (1-OH and 1-NO₃). Characterization of these Ni complexes by combined spectroscopic and computational approaches enables deep understanding of their geometric and electronic structures, bonding interactions, and spectroscopic properties, showing that all of them are square planar complexes and exhibit strong π-covalency with the amido N-donors of the N3 ligand. Furthermore, results obtained from X-ray absorption spectroscopy and density functional theory calculations provide strong support for the assignment of the Ni(IV) oxidation state of complex 2, albeit with strong ligand-to-metal charge donation. Notably, 2 is able to oxidize hydrocarbons with C-H bond strength in the range of 76-92 kcal/mol, representing a rare example of high-valent late transition metal complexes capable of activating strong sp³ C-H bonds.

High-valent Fe^IV═O intermediates with a terminal metal–oxo moiety are key oxidants in many enzymatic and synthetic C–H bond oxidation reactions. While generating stable metal–oxo species for late transition metals remains synthetically challenging, notably, a number of high-valent non-oxo–metal species of late transition metals have been recently described as strong oxidants that activate C–H bonds. In this work, we obtained an unprecedented mononuclear Co^IV–dinitrate complex (2) upon one-electron oxidation of its Co(III) precursor supported by a tridentate dianionic N3 ligand. 2 was structurally characterized by X-ray crystallography, showing a square pyramidal geometry with two coordinated nitrate anions. Furthermore, characterization of 2 using combined spectroscopic and computational methods revealed that 2 is a low-spin (S = 1/2) Co(IV) species with the unpaired electron located on the cobalt d_z2 orbital, which is well positioned for substrate oxidations. Indeed, while having a high thermal stability, 2 is able to cleave sp³ C–H bonds up to 87 kcal/mol to afford rate constants and kinetic isotope effects (KIEs) of 2–6 that are comparable to other high-valent metal oxidants. The ability to oxidize strong C–H bonds has yet to be observed for Co^IV–O and Co^III═O species previously reported. Therefore, 2 represents the first high-valent Co(IV) species that is both structurally characterized by X-ray crystallography and capable of activating strong C–H bonds.

Incorporation of the triad of redox-activity, hemilability, and proton responsivity, into a single ligand scaffold is reported. Due to this triad, the complexes Fe(^PyrrPDI)(CO)₂ (3) and Fe(^MorPDI)(CO)₂ (4) display 40-fold enhancements in the initial rate of NO₂^- reduction, with respect to Fe(^MeOPDI)(CO)₂ (7). Utilizing the proper sterics and pK_a of the pendant base(s) to introduce hemilability into our ligand scaffolds, we report unusual {FeNO}^x mononitrosyl iron complexes (MNICs) as intermediates in the NO₂^- reduction reaction. The {FeNO}^x species behave spectroscopically and computationally similar to {FeNO}⁷. an unusual intermediate-spin Fe(III) coupled to triplet NO− and a singly-reduced PDI ligand. These {FeNO}^x MNICs facilitate the enhancements in the initial rate.

The reduction of NO₂⁻ to NO is achieved with a FePDI complex containing a proton-responsive secondary coordination sphere coupled with redox-active sites.