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This is a Final Technical Report of DOE AWARD NUMBER: DE-SC0009363. PROJECT TITLE: “Controlling High-Valent Reactivity in Earth Abundant Metals”. PROJECT PERIOD: AUGUST 16, 2019 TO AUGUST 15, 2022. This report provides a comprehensive summary of all work completed under this award.

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 d⁶ count implied by the oxidation state formalism. The generality of this phenomenon is explored computationally by scrutinizing 8 additional complexes. The extreme case of NiF₆^2- is considered using high-level molecular orbital approaches asmore » well as advanced valence bond methods. The emergent electronic structure picture reveals that even highly electronegative F-donors are incapable of supporting a physical d⁶ 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.« less

Nickel–sulfur bonds play key roles in catalysis and biological systems. In most metal complexes, sulfur acts as a bridge joining metal centers, rather than as a terminal sulfide ligand. The addition of phenyl isothiocyanate to Ni(dcpe)(Ph)(SH) afforded the dithiocarbamate complex Ni(dcpe)(κ²-S₂C=NPh) with the loss of benzene. The reaction was monitored by UV–visible spectroscopy under pseudo-first-order rate conditions and showed limited dependence on isothiocyanate concentration, suggesting that the reaction proceeds through an intermediate terminal sulfido nickel complex. The values of ΔH^‡ and ΔS^‡ for the reaction were found to be 18.6 ± 1.5 kcal/mol and –17.8 ± 4.7 eu, respectively. Fullmore » characterization of Ni(dcpe)(κ²-S₂C=NPh), including a single-crystal X-ray structure determination, was achieved.« less

The homoleptic trifluoromethyl nickel complex [Ni(CF₃)₄]^2- catalyzes a stereoselective trifluoromethylation of brucine with Umemoto II reagent. The trifluoromethylation process proceeds with concomitant isomerization of its alkenyl double bond leading to a trifluoromethylated neobrucine-like derivative. A minor difunctionalization product was also detected from the reaction mixture, consistent with a radical addition process that is occurring in parallel. Stoichiometric reactions between the nickel(II) catalyst precursor and brucine led to no reaction. However, a stochiometric reaction with a high-valent nickel(IV) complex formed trifluoromethylated neobrucine, implicating its intermediacy.

The new anionic nickelate complexes [(MeCN)Ni(C₄F₈)(CF₃)]^-, [(MeCN)Ni(C₄F₈)(C₂F₅)]^-, [(IMes)Ni(C₄F₈)(CF₃)]^-, [(IMes)Ni(CF₃)₃]^- (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), and [(F-NHC)Ni(Rf)₃]^- (F-NHC = 1,3-bis(2,4-F2Ph), 2,4,6-F₃Ph- or 3,4,5-F₃Ph)imidazol-2-ylidene; (R_f = CF₃ or C₂F₅) were synthesized and structurally characterized. The electrochemical properties of all new compounds were revealed by cyclic voltammetry studies and compared to the known CF₃ analogue [(MeCN)Ni(CF₃)₃]^-. The IMes-coordinated complexes exhibited initial oxidation events that were well-separated from a second oxidation process in the cyclic voltammograms. The complexes containing F-substituted NHC ligands [(F-NHC)Ni(CF₃)₃]^- are structurally quite similar to the IMes derivative and reveal also two separated oxidation waves in their cyclic voltammograms. The absolute potentials asmore » well as the separation between the two waves vary with the substitution pattern, suggesting that the NHC ligand environment (NHC = N-heterocyclic carbene) is an interesting platform for the development of new redox-triggered reactions that release trifluoromethyl and perfluoroalkyl radicals upon oxidation.« less

Here, the acetonitrile ligands in [(MeCN)₄Co(C₄F₈)][PF₆] were found to be labile toward ligand substitution reactions, allowing for the preparation of [(MeCN)(tpy)Co(C₄F₈)][PF₆], [(tpy)CoBr(C₄F₈)], [(bpy)₂Co(C₄F₈)][PF₆], and [(cis-κ²-pym-PPh₂)₂Co(C₄F₈)][PF₆] (tpy = 2,2':6',2"-terpyridine, bpy = 2,2'-bipyridine, and pym-PPh₂ = diphenyl(2-pyrimidyl)phosphine). All of the aforementioned complexes have been structurally characterized by X-ray diffraction. Additionally, an improved procedure for the preparation of [(MeCN)₄Co(C₄F₈)][X] (X = PF₆ or BF₄) is reported.

Here, reaction of the versatile tris(perfluoroethyl) cobalt(III) precursor [fac-(MeCN)₃Co(C₂F₅)₃] with [NMe₄]F and [PPh₄]Cl in THF formed the unexpected cobalt(III) bridging fluoride dimer ([(C₂F₅)₃Co(μ-F)]₂^2–. The new fluoro-organometallic cobalt(III) complex was characterized by NMR and UV-vis spectroscopies, X-ray crystallography, cyclic voltammetry, and by computational methods. In the strongly coordinating solvent MeCN, ([(C₂F₅)₃Co(μ-F)]₂^2– exhibits dynamic processes on the NMR timescale which are consistent with solvent coordination. However, in the weakly coordinating solvent CH₂Cl₂, a more static structure is observed suggesting that the dimer retains its structure in solution state. An electrochemical analysis of ([(C₂F₅)₃Co(μ-F)]₂^2– was performed, and the data are compared to previouslymore » reported cobalt(III) perfluoroethyl complexes.« less

Abstract The acetonitrile‐solvated [(MeCN)Ni(C ₂ F ₅ ) ₃ ] ⁻ was prepared in order to compare and contrast its reactivity with the known [(MeCN)Ni(CF ₃ ) ₃ ] ⁻ towards organic electrophiles. Both [(MeCN)Ni(CF ₃ ) ₃ ] ⁻ and [(MeCN)Ni(C ₂ F ₅ ) ₃ ] ⁻ successfully react with aryl iodonium and diazonium salts as well as alkynyl iodonium salts to give fluoroalkylated organic products. Electrochemical analysis of [(MeCN)Ni ^II (C ₂ F ₅ ) ₃ ] ⁻ suggests that, upon electro‐oxidation to [(MeCN) _n Ni ^III (C ₂ F ₅ ) ₃ ], reductivemore » homolysis of a perfluoroethyl radical occurs, with the concomitant formation of [(MeCN) ₂ Ni ^II (C ₂ F ₅ ) ₂ ]. Catalytic C−H trifluoromethylations of electron‐rich arenes were successfully achieved using either [(MeCN)Ni(CF ₃ ) ₃ ] ⁻ or the related [Ni(CF ₃ ) ₄ ] ²⁻ . Stoichiometric reactions of the solvated nickel complexes reveal that “ligandless” nickel is exceptionally capable of serving as reservoir of CF ₃ groups under catalytically relevant conditions.« less

Abstract The acetonitrile‐solvated [(MeCN)Ni(C ₂ F ₅ ) ₃ ] ⁻ was prepared in order to compare and contrast its reactivity with the known [(MeCN)Ni(CF ₃ ) ₃ ] ⁻ towards organic electrophiles. Both [(MeCN)Ni(CF ₃ ) ₃ ] ⁻ and [(MeCN)Ni(C ₂ F ₅ ) ₃ ] ⁻ successfully react with aryl iodonium and diazonium salts as well as alkynyl iodonium salts to give fluoroalkylated organic products. Electrochemical analysis of [(MeCN)Ni ^II (C ₂ F ₅ ) ₃ ] ⁻ suggests that, upon electro‐oxidation to [(MeCN) _n Ni ^III (C ₂ F ₅ ) ₃ ], reductivemore » homolysis of a perfluoroethyl radical occurs, with the concomitant formation of [(MeCN) ₂ Ni ^II (C ₂ F ₅ ) ₂ ]. Catalytic C−H trifluoromethylations of electron‐rich arenes were successfully achieved using either [(MeCN)Ni(CF ₃ ) ₃ ] ⁻ or the related [Ni(CF ₃ ) ₄ ] ²⁻ . Stoichiometric reactions of the solvated nickel complexes reveal that “ligandless” nickel is exceptionally capable of serving as reservoir of CF ₃ groups under catalytically relevant conditions.« less

The coligand X was varied in the organonickel complexes [Ni(Phbpy)X] (X = F, Cl, Br, I, C₆F₅) carrying the anionic tridentate C^N^N ligand 6-(phen-2-ide)-2,2'-bipyridine (Phbpy^–) to study its effect on electronic structures of these complexes and their activity in Negishi-like C–C cross-coupling catalysis. The complexes were synthesized from the precursor [Ni(COD)₂] (COD = 1,5-cyclooctadiene) by chelate-assisted oxidative addition into the phenyl C–X bond of the protoligand 6-(2-halidophenyl)-2,2'-bipyridine) and were obtained as red powders. Protoligands X–Phbpy carrying the halide surrogates X = OMe, OTf (triflate) failed in this reaction. Single-crystal XRD allowed us to add the structures of [Ni(Phbpy)Cl] and [Ni(Phbpy)I]more » to the previously reported Br derivative. Cyclic voltammetry showed reversible reductions for X = C₆F₅, F, Cl, while for Br and I the reversibility is reduced through rapid splitting of X^– after reduction (EC mechanism). UV–vis spectroelectrochemistry confirmed the decreasing degree of reversibility along the series C₆F₅ > F > Cl » Br > I, which parallels the “leaving group character” of the X coligands. This method also revealed mainly bpy centered reduction and essentially Ni(II)/Ni(III) oxidations, as corroborated by DFT calculations. The rather X-invariant long-wavelength UV–vis absorptions and excited states were analyzed in detail using TD-DFT and were consistent with predominant metal to ligand charge transfer (MLCT) character. Initial catalytic tests under Negishi-like conditions showed the complexes to be active as catalysts in C–C cross-coupling reactions but did not display marked differences along the series from Ni–F to Ni–I.« less