A mild nickel system turns some of chemistry’s toughest bonds into stable, tunable complexes whose next reaction depends on how they are activated.
Paper: Direct Access to Chemodivergent Ni(II) Diaryl Complexes via C–F Activation of Aryl Fluorides. Image Credit: RHJPhtotos / Shutterstock
Researchers have developed a method to activate strong carbon-fluorine (C-F) bonds in polyfluorinated compounds.
Their study, published in Dalton Transactions, presented a generally room-temperature, additive-free approach that converts commercially available polyfluorinated starting materials into air- and moisture-stable nickel(II) diaryl complexes.
Compared with salt-metathesis routes using preformed organometallic transfer reagents, the method starts from parent aryl fluorides and shows that heat or light can alter product distributions.
Challenges in fluorinated compound modification
Fluorine can significantly change organic compounds' properties, making fluorinated materials valuable in pharmaceuticals, agrochemicals, and materials chemistry. Selectively modifying fluorinated compounds remains challenging because aryl C-F bonds are highly stable, and their strength and reactivity vary with the degree and pattern of fluorination.
Conventional methods often depend on preactivated organometallic reagents, which can react unfavorably with functional groups such as esters and nitriles. Earlier transition-metal systems can cleave C-F bonds, but many compete with C-H activation, require fluorophilic reductants or silane additives, or are limited to products from a single C-F activation event.
Nickel offers another route; earlier phosphine- and carbene-supported nickel systems have shown low yields, long reaction times, incomplete selectivity, or ligand-cooperativity effects in which the ligand participates directly in C-F activation rather than serving only as a spectator bound to nickel.
Developing a straightforward nickel-based method to selectively activate C-F bonds while preserving sensitive functional groups could reduce reliance on prefunctionalized aryl-transfer reagents.
Dual aryl group installation via nickel coordination
To develop a general method for C-F bond activation, researchers investigated the reactions of bis(1,5-cyclooctadiene) nickel(0) with various polyfluoroarenes in the presence of nitrogen-donor bipyridine ligands.
Under the selected conditions, most reactions were conducted in acetonitrile at room temperature without additives. Using 2,2'-bipyridine ligands, the method installed two polyfluoroaryl groups onto a single nickel center, forming stable nickel(II) diaryl complexes.
The study examined a diverse range of polyfluoroarenes, including hexafluorobenzene and substrates bearing trifluoromethyl, ester, methoxy, methyl, vinyl, nitrile, and ketone groups. The substrate scope also included pentafluorobenzene, octafluoronaphthalene, and decafluorobiphenyl. This range tested C-F activation across varied electronic and steric settings.
The reaction products were analyzed using multinuclear NMR spectroscopy and ultraviolet-visible (UV-vis) absorption spectroscopy. Quantitative fluorine-19 NMR was used to determine substrate-scope yields. Crude-reaction proton NMR and positive-ion ESI-MS were also used to probe nickel-containing byproducts, but no single coproduct could be identified. Single-crystal X-ray diffraction was used to determine the solid-state structures of 15 complexes.
In addition to the bipyridine-assisted method, nickel reactions without a preinstalled bipyridine ligand were examined. The nickel(0) precursor reacted directly with pentafluoropyridine in acetonitrile, giving a solvent-coordinated nickel(II) diaryl building-block complex in 12% isolated yield that could then undergo ligand exchange with substituted bipyridines.
Structural outcomes and selectivity in reactions
Single-crystal X-ray diffraction confirmed the structures of 15 complexes across the substrate and ligand series. The bipyridine-supported complexes adopted slightly distorted square-planar geometries.
Across this series, nickel-carbon and nickel-nitrogen bond lengths varied only slightly, indicating that changes in substituent electronics had little effect on the core solid-state geometry.
Larger, extended aryl groups showed larger deviations from planarity, with planar-twist angles of 14.1° to 20.2°. The authors linked the greater distortion to steric interactions, conformational constraints, and solid-state packing.
All isolated complexes were stable to air and moisture. NMR spectroscopy showed that the solids remained unchanged after weeks of air exposure.
The reaction exhibited high regioselectivity, generally activating the C-F bond located para to a non-fluorine substituent or ring heteroatom under the selected acetonitrile conditions.
Solvent choice mattered during condition screening: benzene and toluene favored ortho activation of pentafluoropyridine, whereas acetonitrile gave exclusively the para product.
In substrates containing both C-F and C-H (carbon-hydrogen) bonds, such as pentafluorobenzene, the reaction selectively cleaved the C-F bond without detectable C-H activation. Hydrodefluorination also competed.
Yields across the complex series ranged from 28% to 83%, with isolated crystalline yields reaching up to 78% for the decafluorobiphenyl-derived complex.
The isolated nickel complexes reacted differently to the applied energy source, but they did not follow a single heat-versus-light rule.
For two pentafluoropyridine-derived complexes, heating at 100 °C favored hydrodefluorination, replacing fluorine with hydrogen. At room temperature, irradiation with approximately 370 nm light favored carbon-carbon homocoupling, which joins two aryl groups, for those two complexes, with one giving 64% homocoupled product.
The ester-substituted aryl complex showed the opposite preference: light mainly produced the hydrodefluorinated product, while heating favored homocoupling. One tetrafluoropyridine assignment remained tentative because the product was not isolated, and its hydrogen source was unresolved. Researchers found that both the aryl group and the bipyridine ligand affected the direction and extent of this response.
Possible implications for materials chemistry
The study does not test coatings, polymer synthesis, organic semiconductors, or finished materials. The method preserves sensitive functional groups, including the vinyl group in pentafluorostyrene, during C-F bond activation, and the authors note that styrene derivatives are useful substrates for radical polymerization. Polymerization itself was not tested.
Heat- and light-triggered reactions altered downstream chemistry without changing the solvent or added reagents the authors tested, but they tested only three representative compounds. The approach is compatible with functional groups such as esters, nitriles, ketones, and vinyl substituents, broadening the range of fluorinated substrates that can be used in the reaction. Materials applications remain prospective.
Conclusions and future questions
This study demonstrates that zero-valent nickel can activate C-F bonds in polyfluoroarenes under generally mild conditions, producing well-defined, air- and moisture-stable nickel(II) diaryl complexes. The additive-free method bypasses preformed perfluoroaryl organometallic transfer reagents, including organolithium and Grignard reagents. It also tolerates several reactive functional groups.
The findings indicate that heat and 370 nm light can shift product distributions, but neither stimulus yields a single universal product class across the tested complexes. The response changes with both the aryl and bipyridine substituents. The authors report that mechanistic work is underway, including time-resolved spectroscopy and computational analysis of the relevant excited states and their wavelength-dependent reactivity.
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