Fluorinated ligands and their effects on physical properties and chemical reactivity

We are pleased to welcome Dalton Transactions readers to this spotlight collection on Fluorinated Ligands. Fluorine is the most electronegative element in the periodic table (3.98 on the Pauling scale) and has a remarkably high reduction potential (2.87 V). The van der Waals and covalent radii of fluorine are quite short (ranked third, only after the first-row elements hydrogen and helium), making it unusually small for its atomic number. Although the homonuclear bond in F₂ is particularly reactive, some heteronuclear bonds involving fluorine such as B–F, Si–F, and C–F are remarkably stable. The combined effects of these features on fluorine-containing molecules, especially those involving fluorinated ligands and anions, are often unmatched by any other element. In fact, metal complexes of fluorinated supporting ligands in comparison to their non-fluorinated, hydrocarbon counterparts usually display relatively high thermal and oxidative stability, volatility, as well as unique reactivity profiles. They are also ideal for applications in fluorous-biphase media and supercritical CO₂. Fluorine-containing anions are among the weakest donors known. Our aim is to illustrate the unique effects fluorinated ligands often have on physical properties and chemical reactivity of metal complexes through contributions by leading researchers to Dalton Transactions. Assembled in this spotlight collection are a potpourri of papers that show the exciting s, p, d, and f-block chemistry involving fluorine currently underway in laboratories around the world.

In this collection, the s-block chemistry focuses on the heavier group 2 elements. Fluorinated arenes interact with neutral Mg and Ca complexes generating isolable complexes as demonstrated by Hicks et al. (https://doi.org/10.1039/d1dt01532j). Sarazin et al. describes macrocyclic Ca, Sr, and Ba complexes with bis(phenolate) and bis(fluoroalkoxide) functionalization and the effects of Lewis acidity on structures (https://doi.org/10.1039/d0dt02573a). Two alkaline earth-based MOFs with a fluorinated tris(benzoate)-benzene linker perfluorinated in the central ring have a unique topology and unusual properties as reported by Ruschewitz and co-workers (https://doi.org/10.1039/d3dt00422h).

An exceptional range of chemistries are visible in the p-block, from trying to activate strong bonds with F, to using that inertness to support other chemistries. Catalytic arylation of benzylic C–F bonds utilizing boranes and silanes was the focus of work reported by Stephan et al. (https://doi.org/10.1039/d2dt03588j). The synthesis and Lewis acidity of eight new fluorinated triaryl borates have been achieved by Kaehler, Melen and co-workers (https://doi.org/10.1039/d2dt04095f). An imidazolium source of “naked” fluoride, [IPrH][F], was used to prepare [Ph₃GeF₂] and other organofluorosilicate species by Tavčar et al. (https://doi.org/10.1039/d3dt00421j). Two perfluorinated alkoxy silanes were prepared and their exceptional resistance to deprotonation was explored with DFT by a team led by Ketkov, Hupf, Grabowsky, and Beckmann (https://doi.org/10.1039/d3dt00299c). Sundermeyer et al. synthesized Lewis base adducts of the three-coordinate Al(N(C₆F₅)₂)₃ Ga(N(C₆F₅)₂)₃ and investigated their structures and thermochemistry (https://doi.org/10.1039/d2dt00003b). A very weakly coordinating anion, [Al(OC₁₀F₁₅)₄]⁻, pfAd, with perfluorinated adamantyl substituents has been prepared by Krossing and his team (https://doi.org/10.1039/d3dt00199g). The superbasic Verkade's base (proazaphosphatrane) has been combined with fluorinated and non-fluorinated ketones to explore unusual FLPs by Stephan et al. (https://doi.org/10.1039/c9dt04588k).

The tremendous breadth of chemistry in the d-block is found associated with F as well. From activating substrates to preventing ligand oxidation, tracking species in biology or lighting up reaction mixtures, fluorinated ligands are everywhere. Rabuffetti et al. report the preparation of bimetallic Mn fluorides thermally from group 1 trifluoroacetate derivatives and their reactions with quartz at higher temperatures (https://doi.org/10.1039/d2dt02822k). Que et al. describe the use of Fe(II), Co(II), and Ni(II) complexes of tetradentate macrocylces with pendant amides and two hexafluoro-t-butoxy groups as dual ¹⁹F and PARACEST imaging agents (https://doi.org/10.1039/c9dt01852b). Fluorinated triazole ligands drive spin crossover in Fe(II) and Co(II) complexes (https://doi.org/10.1039/d2dt01005d) as reported by van Slageren, Sarkar and co-workers, which have been investigated for their structure–property relationships as the degree of ligand fluorination changes (https://doi.org/10.1039/d1dt03535e). Barreca, Tabacchi et al. show the use of a heteroleptic cobalt complex bearing fluorinated diketonate ligands [Co(tfa)₂·TMEDA] as a precursor to high-quality Co₃O₄ films (https://doi.org/10.1039/d1dt01650d). The chemistry of five new NHC Ni complexes with perfluorinated alkyl groups have been investigated by Klein, Vicic and co-workers (https://doi.org/10.1039/d2dt00511e). Dias et al. reveal a new fluorinated tris(pyridyl)borate ligand and its utility in the stabilization of group 11 metal ethylene complexes (https://doi.org/10.1039/d1dt04136c). The relativistic modulation of supramolecular halogen/copper interactions and phosphorescence in copper(I) pyrazolate cyclotrimers was studied by Dias, Omary and co-workers (https://doi.org/10.1039/d2dt03725d). A {Cu(I)(CO)} complex with a pefluorinated β-diketiminate ligand has been prepared and compared with less fluorinated analogs by Schulz et al. (https://doi.org/10.1039/d0dt01943g). Erb, Hurvois, Low and co-workers investigated how different degrees of fluorination on a single Cp ring in ferrocene change the redox, electronic absorption and NMR properties (https://doi.org/10.1039/d1dt03430h). Substitution of perfluoro Cp*, [C₅(CF₃)₅]⁻ by pyridine was investigated by Malischewski et al. (https://doi.org/10.1039/d3dt00425b), while C–H and C–F bond activation of pentafluorostyrene at rhodium complexes was the focus of Braun et al. (https://doi.org/10.1039/c9dt03371h). Four different fluoroarene complexes of Rh(III) investigated by Chaplin and co-workers show preference for η² coordination (https://doi.org/10.1039/d0dt01137a). A new PONOP pincer ligand with flanking CF₃ groups has been studied with Ir(I) and Ir(III) by Chaplin and co-workers as well (https://doi.org/10.1039/d2dt03608h). Fluorinated pyrazolate complexes of Cu(I) supported acetylene and terminal alkyne complexes illustrating different coordination modes have been reported by Dias et al. (https://doi.org/10.1039/c9dt03350e). Rhenium complexes with partially fluorinated terphenyl-isocyanide complexes were prepared by Figueroa, Abram and their team (https://doi.org/10.1039/d3dt00446e) to probe the reactivity and solubility. Mitzel et al. describe the synthesis of two Au(I) dimers with fluorinated and non-fluorinated phosphino-phenylene linkers (https://doi.org/10.1039/d1dt03658k). Both transition metal and p-block phthalocyanine complexes with 3,5-bis(trifluoromethyl)phenoxy groups were investigated for their photophysical and biological properties by Şaki, Erdoğmuş, Koçak and co-workers (https://doi.org/10.1039/d0dt04351f).

Both lanthanide and actinide chemistry is supported by fluorinated ligands, and at different ends of the reactivity spectrum. The chemistry of praseodymium with Bi(C₆F₅)₃ has been investigated by Junk et al. (https://doi.org/10.1039/d3dt00534h). Frank and co-workers report polymeric Eu(II) and hexanuclear Eu(III) complexes obtained from europium and perfluorocarboxylates in liquid NH₃ (https://doi.org/10.1039/d1dt04204a). Luminescent dysprosium single-molecule magnets with fluorinated alkoxide and aryloxide ligands obtained by Long, Trifonov, et al. showed high magnetization reversal barriers (https://doi.org/10.1039/d1dt01319j). Volkringer et al. demonstrated that the ionic liquid [Bmim]PF₆ directly converts UO₂ to UF₄ (https://doi.org/10.1039/c9dt04327f).

Overall, we believe that this informative collection of manuscripts on fluorinated ligands not only demonstrates the breadth of current research activities on this topic but also their potential for further developments. We are grateful to all the authors who have contributed to this collection. We also express our gratitude to Dr Samantha Apps, Dr Debora Giovanelli and the team in the Editorial Office at Dalton Transactions for supporting us in this endeavor.

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