Fast Hydrocarbon Oxidation by a High‐Valent Nickel–Fluoride Complex

Abstract

In the search for highly reactive oxidants we have identified high‐valent metal–fluorides as a potential potent oxidant. The high‐valent Ni–F complex [NiIII(F)(L)] (2, L=N,N′‐(2,6‐dimethylphenyl)‐2,6‐pyridinedicarboxamidate) was prepared from [NiII(F)(L)]− (1) by oxidation with selectfluor. Complexes 1 and 2 were characterized by using 1H/19F NMR, UV‐vis, and EPR spectroscopies, mass spectrometry, and X‐ray crystallography. Complex 2 was found to be a highly reactive oxidant in the oxidation of hydrocarbons. Kinetic data and products analysis demonstrate a hydrogen atom transfer mechanism of oxidation. The rate constant determined for the oxidation of 9,10‐dihydroanthracene (k2=29 m−1 s−1) compared favorably with the most reactive high‐valent metallo‐oxidants. Complex 2 displayed reaction rates 2000–4500‐fold enhanced with respect to [NiIII(Cl)(L)] and also displayed high kinetic isotope effect values. Oxidative hydrocarbon and phosphine fluorination was achieved. Our results provide an interesting direction in designing catalysts for hydrocarbon oxidation and fluorination

Synthesis, Characterization, and Reactivity of High-Valent Carbene Dicarboxamide-Based Nickel Pincer Complexes

“…EPR spectra of this formal high-valent Ni III species 4 in CH 2 Cl 2 exhibited a rhombic signal corresponding to a low-spin ( S = 1/2) d 7 Ni III complex with g avg = 2.159 (Figure b). The spectral features are very similar to the reported Ni III EPR spectra. ,,− The total yield of Ni III in the sample was calculated to be 85 ± 5% by comparison of the double integral of the entire signal to that of a (2,2,6,6-tetramethylpiperidin-1-yl)­oxyl (TEMPO) standard. The nature of the signal and the spin quantification results are consistent with the assignment of 4 as [(L 2 )­Ni III F].…”

“…Addition to CTAN (1.0 equiv) to complex 3 in dry CH 2 Cl 2 at −20 °C (Scheme ) afforded an immediate color change (yellowish orange to deep pink), which was followed in the UV–vis by monitoring the formation of a low energy band at λ = 445 nm, and a shoulder at λ = 570 nm (Figure a), as previously observed in similar tetra-coordinated Ni III complexes. ,− …”

“…[(L 2 )­Ni II F]̅ ( 3 ) showed a peak at −315.98 ppm in the 19 F NMR spectrum, which corresponded to the presence of the Ni–F bond in the complex (Figure S13). The peak is ∼100 ppm downfield shifted than the corresponding Ni–F peak in the pyridine dicarboxamide nickel fluoride complex (−421 ppm). − …”

“…The small change in the Ni−F bond distance upon oxidation has also been observed in the corresponding Ni III −F pyridine-2-dicarboxamide complex (doublet ground state; the Ni III −F bond distance 1.810 is close to the Ni II −F bond of 1.808 Å). 57,45 In contrast, a significant elongation of the Ni III −C carbene bond length (0.03 Å) was observed upon oxidation of 3 to 4 (from 1.85 to 1.88 Å) (Table 1). The increase in Ni−C bond distance in the Ni III −F complex is indicative of the higher Ni oxidation state since the π-back bonding from Ni to carbene is expected to be weaker for the higher oxidized congener.…”

Controlling One-Electron Vs Two-Electron Pathways in the Multi-Electron Redox Cycle of Nickel Diethyldithiocarbamate

“…This signal was simulated using superhyperfine coupling to two N atoms with coupling constants A123 = 13.0, 13.0, 18.0 G giving rise to quintet splitting with a 1:2:3:2:1 intensity ratio. This superhyperfine pattern clearly indicates the coordination of two 4-MeO-py ligands to the Ni III center [45][46][47][48][49]. Furthermore, the signal produced by the second species strongly resembles EPR data collected for trans-Ni III (salen)(py)2 complexes 44.…”