Introduction to the Organometallic Chemistry Meets Fluorine Issue

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Introduction to the Organometallic Chemistry Meets Fluorine Issue computational studies suggest that the reductive elimination does not require phosphine dissociation but rather occurs directly from the cis-isomer. A review by M. Crespo accounts for the role of fluorine in cyclometalated platinum compounds, either as a fluoro ligand or in fluorinated aryl building blocks. It is demonstrated that such cycloplatinated complexes are involved in the intramolecular activation of C−H or C−F bonds and also in C−C bond formation reactions. Cross-coupling reactions of fluorinated substrates continue to be an important topic in organofluorine chemistry and are captured in several articles. F.-L. Qing reports the discovery of iron-catalyzed crosscoupling reactions between arylzinc reagents and alkyl halides that bear fluorine substituents at the position β to the halide. In an interesting approach, B. Hoge uses palladium complexes that are characterized by electron-deficient phosphinous acid ligands as catalytic precursors in Heck and Suzuki C−C coupling reactions of fluorinated bromobenzenes. The contribution by M. Van der Boom presents both palladiumcatalyzed cross-couplings and hydrodehalogenation reactions of aryl halides. The latter presumably involve water as the hydrogen source. Furthermore, X. Zhang provides insight into palladium-catalyzed aerobic dehydrogenative cross-coupling reactions of polyfluoroarenes with thiophenes. These transformations proceed via 2-fold C−H activation steps and use O2 as oxidant. Polyfluoroarene−thiophene structures are of interest in functional materials. In a different spinoff, the research group of A. Brisdon describes the preparation of pentafluoropropenyl complexes of mercury, germanium, tin, and lead. The tin complex Bu3Sn(CFCFCF3) acts as a source of the pentafluoropropenyl group in palladium-catalyzed Stille reactions to generate pentafluoropropenyl-substituted aromatics. Carbon−fluorine bond activations may also provide an entry to new fluorinated building blocks. However, the C−F bond cleavage step itself is often the focus of mechanistic investigations. A review article by U. Rosenthal gives an overview of stoichiometric and catalytic C−F bond cleavage reactions by lanthanide and group 4 transition-metal complexes. Another review by A. Lledós covers transition-metalmediated transformations of C−F bonds into C−X bonds (X = H, N, O, S) that are initiated by a nucleophilic attack at coordinated ligands to cleave the C−F bonds. The ability of 3d transition metals to activate C−F bonds is showcased by several research groups. P. L. Holland reports on the C−F activation of fluorobenzenes at a diketiminate Co(I) complex to give Co(II) fluoro and Co(II) aryl complexes. Mechanistic studies indicate a binuclear oxidative addition mechanism. S. A. Johnson provides detailed mechanistic investigations of carbon−fluorine bond activation reactions of polyfluorinated pyridines at {Ni(PEt3)2}, including kinetics and NMR, EPR, and DFT studies. The hydrodefluorination of aromatics

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ver the past decade, fluorine chemistry has had an increasing impact on organometallic chemistry. This is evidenced by several areas where these two disciplines overlap. The research fields include the use of fluorinated ligands to tune the properties of metal complexes in catalysis or materials science, including photophysics and polymer chemistry. One key issue in this context concerns the bonding properties of fluorinated entities at transition and main-group metals. The bonding and reactivity of fluorinated ligands are also of fundamental importance in studies that are directed toward synthesis. Thus, an understanding of the reactivity of fluorinated ligands is essential for the catalytic derivatization of fluorinated molecules to access unique fluorinated building blocks for fluorinated pharmaceuticals, advanced materials, and agrochemicals. Furthermore, fluoro complexes often play a crucial role in fluorination or carbon−fluorine bond activation reactions and it is, therefore, of fundamental significance to explore the reactivity and bonding properties of a fluoro ligand. Hence, one can recognize several realms in which organometallic chemistry pairs up with the multifaceted fluorine chemistry. This special issue, which features fluorine chemistry in organometallic chemistry, illustrates some challenges and recent activities in that fascinating combination when the element fluorine meets organometallic compounds. It highlights a number of interesting aspects that are intended to stimulate further progress and research. An important topic involves the derivatization of fluorinated building blocks by transition metals. H. Amii and K. Uneyama describe trifluoroacetimidoyl palladium complexes that can be used to provide α-amino-α-trifluoromethylcarbene building blocks. A contribution by J. Gil-Rubio illustrates that an ethylene ligand bound at rhodium can be perfluoroalkylated or perfluoroarylated by iodoperfluorocarbons, but an oxidative addition can also occur. Another example is provided by O. Eisenstein and R. N. Perutz in a computational endeavor. They explore the mechanism of the hydroarylation of alkynes with fluoroaromatics at nickel phosphine catalysts. A ligand-toligand hydrogen transfer mechanism is suggested for the C−H activation step. V. I. Bakhmutov, V. V. Grushin, and S. A. Macgregor look at the reductive elimination of PhCF3 from [(Xantphos)Pd(CF3)(Ph)], which is a fundamental step in the trifluoromethylation of aryl chlorides. Experimental and © 2012 American Chemical Society

Special Issue: Fluorine in Organometallic Chemistry Published: February 27, 2012 1213

dx.doi.org/10.1021/om300083u | Organometallics 2012, 31, 1213−1215

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employing [Ni2(iPr2Im)4(COD)] (iPr2Im = 1,3-bis(isopropyl)imidazolin-2-ylidene, COD = cyclooctadiene), which is a source for [Ni(NHC)2], is featured in the article by U. Radius. Binuclear complexes can be employed in unique C−F and subsequent C−H activation reactions of fluorinated olefins, as underscored by M. Cowie in his ongoing studies on the reactivity of binuclear iridium complexes. In the present article the role of water in the activation process is discussed. J. A. Love presents mechanistic studies on Pt(IV)-mediated C−F activation and C−C coupling reactions of polyfluoroaryl imines. Investigations on elementary steps of a putative catalytic cycle give insight into recent catalytic discoveries. J. M. Brown and V. Gouverneur highlight a platinum-catalyzed alkylation of allyl monofluorides with malonate and N- or O-nucleophiles by fluoride displacement that occurs with overall retention of configuration. A. Vigalok found that a chelating ligand containing an 8-fluoroquinoline moiety as well as a phosphine entity, which are both coordinated at palladium, undergoes a nucleophilic substitution of the fluorine atom by a methoxy group at room temperature. Furthermore, research by G.-V. Rö s chenthaler reveals that carbene-stabilized (NHC) phosphorus(V) fluorides can be prepared by C−F oxidative additions of 2,2-difluorobis(dialkylamines) to phosphorus(III) halides. Carbon−fluorine bond activations of pentafluoropyridine at palladium to give metal fluoro complexes are reported by T. Braun. The fluoro compounds are catalysts in Suzuki crosscoupling reactions and can also be applied for the fluorination of 3-bromopropene. J. Cámpora presents nickel and palladium fluoro complexes, which are stabilized by a pincer ligand. The electronic properties of the fluoro ligands and, for comparison, other anionic ligands can be assessed by 13C NMR spectroscopy. The fluoride complexes convert dodecyl iodide to the corresonding alkyl fluoride. As indicated above, the electronic properties of a fluorinated ligand can be exploited to observe unprecedented species and transformations. The ability of fluorinated ligands to control catalytic transformations by transition metals is exemplified by the contribution of D. M. Roddick, which reports on new iridium complexes with fluorinated pincer ligands. They can be applied in catalytic hydrogen transfer reactions and decarbonylation chemistry. R. A. Michelin reviews the chemistry of platinum(II) complexes that bear electron-withdrawing fluoroalkyl or fluoroaryl ligands. The authors also describe the catalytic oxidation of olefins. Fluorinated ligands at main-group metals may also be exploited to control catalytic reactions, as demonstrated by J.-F. Carpentier and E. Kirillov. Their article illustrates that indium complexes of fluorinated dialkoxydiimino salen-like ligands can be applied in ring-opening polymerization reactions of racemic lactide. In a second paper J. F. Carpentier presents studies on aluminum complexes of fluorinated alkoxy-imino ligands and their usage in the ringopening polymerization of cyclic esters. In a revealing contribution, S. A. Macgregor and V. V. Grushin evaluate the computed geometries and electronic structures of trifluoromethyl in comparison to methyl complexes by natural bond analysis. Examples include [Rh(CX3)(PH3)3], trans-[Pt(Cl)(CX3)(PH3)2], [Mn(CX3)(CO)5], and [Pt(H)3(CX3)]2− (X = H, F). D. A. Vicic reports on Ni(II) bisperfluoroalkyl complexes. DFT calculations of the bipyridine complexes [Ni(CF3)2(bpy)] and [Ni(CH3)2(bpy)] suggest a large stabilization of the HOMO for the trifluoromethyl complex in comparison to the methyl complex, which is

Figure 1. Representatives of the next generation of co-workers in the area of organometallic fluorine chemistry, photographed at a January 26, 2012, meeting of the Berlin-based special graduate program (Graduiertenkolleg) described in the text, and the Editorial Team involved in this issue. Top row: Roland Friedemann, Bernd Schmidt, Moritz Kühnel, Holger Erdbrink, Paul Kläring, Frank Liebau, Battist Rábay, Anja Hermes. Middle row: Darina Heinrich, Lada Zámostná, Anna Lena Raza, Katharina Teinz, Mike Ahrens. Bottom row: Prof. Dr. Dieter Lentz (Freie Universität Berlin), Prof. Dr. Thomas Braun (Guest Editor and Humboldt Universität zu Berlin), Prof. Dr. John A. Gladysz (Editor in Chief and Texas A&M University).

consistent with the observed stability and oxidation potential. The bonding properties of trifluoromethyl ligands at titanium are assessed by R. P. Hughes and J. L. Kiplinger. In contrast to more electron-rich transition metals, the trifluoromethyl complexes feature a longer titanium−carbon bond in comparison to their methyl counterparts. The energetic decomposition of [Cp2Ti(CF3)(F)] provides a new entry into fluorinated materials containing −(CF2−CF2)− and − (CF2−CFH)− units. Other implications for materials chemistry are illustrated by D. Lentz, who contributes with a paper on the redox-autocatalytic formation of fluorinated ferrocenophanes from bis(trifluorovinyl)ferrocenes. Stable copper(I), silver(I), and gold(I) ethylene complexes supported by a fluorinated scorpionate ligand are presented by H. V. R. Dias. E. G. Hope and J. Kvı ́cǎ la provide in their contributions new synthetic pathways to complexes that contain fluorinated N-heterocyclic carbene (NHC) ligands. Hope reports on cyclometalation reactions of fluorinated NHC ligands at ruthenium and iridium, whereas Kvı ́cǎ la opens a window to “fluorous technology”. His new silver NHC complexes bear polyfluoroalkyl or polyfluoropolyalkoxy “ponytails”. Furthermore, fluorinated moieties can also be applied in self-assembly processes. M. Ferrer and M. Engeser use fluoroaryl gold(I) metalloligands together with palladium or platinum complexes as building blocks for the formation of heterometallic metallomacrocycles. In addition, F. Jäkle investigates the structural features, supramolecular structures, and photophysical properties of pentafluorophenylcopper [Cu(C6F5)]4 pyridine complexes. New main-group fluoroorganics with tetrafluoropyridyl ligands have been synthesized by W. Tyrra, who obtains Se(C5F4N)2 and Te(C5F4N)2 via redox transmetalations of Ag(C5F4N) and selenium or tellurium, respectively. M. Finze and J. Warneke carefully investigate carba-closo-dodecaboranyl ligands [closo-1-CB11X11]2− (X = H, F, Cl, Br, I) at Hg(II). The reactivity and properties of the 1214

dx.doi.org/10.1021/om300083u | Organometallics 2012, 31, 1213−1215

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fluorinated carborane are compared to those of the other derivatives. The contributions outlined above clearly show that fluorine is a key element, or “Schlüsselelement” as described in the title of a special graduate program (research training group), which is funded by the DFG (German National Research Foundation) (Figure 1). The school is coordinated by the Freie Universität Berlin and the Humboldt-Universität zu Berlin. Diverse aspects of fluorine chemistry are an everyday part of the curriculum and seminar program at these Universities, which are helping to train the next generation of researchers in this broad and ever important research field. One area of emphasis involves the transition-metal-mediated synthesis of fluoro-organics. Additional details can be found at http://www.bcp.fu-berlin.de/chemie/grkfluor/GRK1582/ en/index.html.

Thomas Braun* Guest Editor

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Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Straße 2, 12489 Berlin, Germany

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Corresponding Author

*E-mail: [email protected].

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dx.doi.org/10.1021/om300083u | Organometallics 2012, 31, 1213−1215