High-valent FeIVO 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 CoIV–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 CoIV–O and CoIIIO 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 pKa 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 proton-responsive pyridinediimine ligand, (DEA)PDI (where (DEA)PDI = [(2,6-(i)PrC6H3)(N[double bond, length as m-dash]CMe)(N(Et)2C2H4)(N[double bond, length as m-dash]CMe)C5H3N]) was utilized for the reduction of NO2(-) to NO. Nitrite reduction is facilitated by the protonated secondary coordination sphere coupled with the ligand-based redox-active sites of [Fe(H(DEA)PDI)(CO)2](+) and results in the formation of the {Fe(NO)2}(9) DNIC, [Fe((DEA)PDI)(NO)2](+).
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-NO 3 ). 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 πcovalency 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 sp 3 C−H bonds.
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