Nickel Perfluoroalkyl Complexes Supported by Simple Acetate Coligands.

The interaction of tetramethylammonium acetate with [(MeCN)Ni(CF)], [(MeCN)Ni(CF)], and [NMe][(MeCN)Ni(CF)] was explored by F NMR spectroscopy. We show that depending on the nature of the nickel complex, one or two acetate ligands can add to the metal center and replace the nickel-bound MeCN ligands, depending on the acetate concentration. The number of acetates that could bind to nickel, and whether the resulting complex exists as a monomer or dimer, was determined to be dependent on the nature of the fluoroalkyl ligand.

Scrutinizing formally Ni centers through the lenses of core spectroscopy, molecular orbital theory, and valence bond theory.

Nickel K- and L-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-edge XAS reveals that the physical d-counts of the formally Ni compounds measured lie well above the d count implied by the oxidation state formalism.

Synthesis, structure, and electrochemical properties of [LNi(R)(CF)] and [LNi(R)] complexes.

The new anionic nickelate complexes [(MeCN)Ni(CF)(CF)], [(MeCN)Ni(CF)(CF)], [(IMes)Ni(CF)(CF)], [(IMes)Ni(CF)] (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene), and [(F-NHC)Ni(R)] (F-NHC = 1,3-bis(2,4-FPh), 2,4,6-FPh- or 3,4,5-FPh)imidazol-2-ylidene; (R = CF or CF) were synthesized and structurally characterized. The electrochemical properties of all new compounds were revealed by cyclic voltammetry studies and compared to the known CF analogue [(MeCN)Ni(CF)]. The IMes-coordinated complexes exhibited initial oxidation events that were well-separated from a second oxidation process in the cyclic voltammograms.

Solvated Nickel Complexes as Stoichiometric and Catalytic Perfluoroalkylation Agents*.

The acetonitrile-solvated [(MeCN)Ni(C F ) ] was prepared in order to compare and contrast its reactivity with the known [(MeCN)Ni(CF ) ] towards organic electrophiles. Both [(MeCN)Ni(CF ) ] and [(MeCN)Ni(C F ) ] successfully react with aryl iodonium and diazonium salts as well as alkynyl iodonium salts to give fluoroalkylated organic products. Electrochemical analysis of [(MeCN)Ni (C F ) ] suggests that, upon electro-oxidation to [(MeCN) Ni (C F ) ], reductive homolysis of a perfluoroethyl radical occurs, with the concomitant formation of [(MeCN) Ni (C F ) ].

[(MeCN)Ni(CF)] and [Ni(CF)]: Foundations toward the Development of Trifluoromethylations at Unsupported Nickel.

Nickel anions [(MeCN)Ni(CF)] and [Ni(CF)] were prepared by the formal addition of 3 and 4 equiv, respectively, of AgCF to [(dme)NiBr] in the presence of the [PPh] counterion. Detailed insights into the electronic properties of these new compounds were obtained through the use of density functional theory (DFT) calculations, spectroscopy-oriented configuration interaction (SORCI) calculations, X-ray absorption spectroscopy, and cyclic voltammetry. The data collectively show that trifluoromethyl complexes of nickel, even in the most common oxidation state of nickel(II), are highly covalent systems whereby a hole is distributed on the trifluoromethyl ligands, surprisingly rendering the metal to a physically more reduced state.

Nickel-Mediated Trifluoromethylation of Phenol Derivatives by Aryl C-O Bond Activation.

The increasing pharmaceutical importance of trifluoromethylarenes has stimulated the development of more efficient trifluoromethylation reactions. Tremendous efforts have focused on copper- and palladium-mediated/catalyzed trifluoromethylation of aryl halides. In contrast, no general method exists for the conversion of widely available inert electrophiles, such as phenol derivatives, into the corresponding trifluoromethylated arenes.

Exploiting the trifluoroethyl group as a precatalyst ligand in nickel-catalyzed Suzuki-type alkylations.

We report herein the exploitment of the partially fluorinated trifluoroethyl as precatalyst ligands in nickel-catalyzed Suzuki-type alkylation and fluoroalkylation coupling reactions. Compared with the [L Ni(aryl)(X)] precatalysts, the unique characters of bis-trifluoroethyl ligands imparted precatalyst [(bipy)Ni(CHCF)] with bench-top stability, good solubilities in organic media and interesting catalytic activities. Preliminary mechanistic studies reveal that an eliminative extrusion of a vinylidene difluoride (VDF, CH[double bond, length as m-dash]CF) mask from [(bipy)Ni(CHCF)] is a critical step for the initiation of a catalytic reaction.

Comparative profiling of well-defined copper reagents and precursors for the trifluoromethylation of aryl iodides.

A number of copper reagents were compared for their effectiveness in trifluoromethylating 4-iodobiphenyl, 4-iodotoluene, and 2-iodotoluene. Yields over time were plotted in order to refine our understanding of each reagent performance, identify any bottlenecks, and provide more insight into the rates of the reactions. Interestingly, differences in reactivity were observed when a well-defined [LCuCF] complex was employed directly or generated in situ from precursors by published reports.

Strategies of heavy metal uptake by three Armeria species growing on different geological substrates in Serbia.

This study surveyed three species of the genus Armeria Willd. from five ultramafic outcrops, two non-ultramafic (schist) soils, and one tailing heap of an abandoned iron-copper mine from Serbia. Similarities and differences among the three Armeria species growing on different geological substrates in the ability to control uptake and translocate nine metals were examined.

Nickel-Catalyzed Trifluoromethylselenolation of Aryl Halides Using the Readily Available [MeN][SeCF] Salt.

A convenient and efficient method for the construction of aryl trifluoromethyl selenoethers from the corresponding aryl halides in the presence of Ni(COD) and an appropriate ligand is reported. Various aryl iodides, bromides, and chlorides were smoothly converted in this reaction by simply varying the ligand, which afforded aryl and heteroaryl trifluoromethyl selenoethers in good to almost quantitative yields. The reaction was also applicable to the synthesis of druglike molecules.

Unsymmetrical N-Aryl-1-(pyridin-2-yl)methanimine Ligands in Organonickel(II) Complexes: More Than a Blend of 2,2'-Bipyridine and N,N-Diaryl-α-diimines?

The new organonickel complexes [(R-PyMA)Ni(Mes)X] [R-PyMA = N-aryl-1-(pyridin-2-yl)methanimine; aryl = phenyl, 2,6-Me-, 3,5-Me-, 2,4,6-Me-, 2,6-Pr-, 3,5-(OMe)-, 2-NO-4-Me-, 4-NO-, 2-CF-, and 2-CF-6-F-phenyl; Mes = 2,4,6-trimethylphenyl; X = F, Cl, Br, or I] were obtained as approximate 1/1 cis and trans isomeric mixtures or pure cis isomers depending on the PyMA ligand and X. The [(R-PyMA)Ni(Mes)X] complexes with X = Br or Cl were directly synthesized from the precursors trans-[(PPh)Ni(Mes)X], while [(PyMA)Ni(Mes)X] derivatives with X = F or I were obtained from [(PyMA)Ni(Mes)Br] through X exchange reactions. Although density functional theory (DFT) calculations show a preference for the sterically favored cis isomers, both isomers could be observed in many cases; in three cases, even single crystals for X-ray diffraction could be obtained for the trans isomers.

Direct Difluoromethylation of Aryl Halides via Base Metal Catalysis at Room Temperature.

A stable and isolable difluoromethyl zinc reagent has been prepared through the reaction of ICF2H with diethyl zinc and DMPU. This new zinc reagent is a free-flowing solid and can be used in combination with a nickel catalyst to difluoromethylate aryl iodides, bromides, and triflates at room temperature. Such mild conditions for the catalytic difluoromethylation of these substrates are unprecedented.

Versatile Route to Arylated Fluoroalkyl Bromide Building Blocks.

New difunctionalized and fluoroalkylated silyl reagents have been prepared that react with silver and copper salts to afford active catalysts that can be used to synthesize arylated fluoroalkyl bromide building blocks. It has been shown that the [(phen)Ag(CF2)nBr] intermediates are capable of transferring both the phenanthroline ligand and the fluoroalkyl bromide chain to copper iodide, eliminating the need for a preligated copper salt precursor. The methodology is compatible with various chain lengths of the fluoroalkyl halide functionality.

Triphenylphosphine-Mediated Deoxygenative Reduction of CF3 SO2 Na and Its Application for Trifluoromethylthiolation of Aryl Iodides.

We report herein a practical method for taming Langlois' reagent CF3 SO2 Na to generate CuSCF3 by a triphenylphospine-mediated deoxygenative reduction process. This chemistry highlights a novel utilization of the inherent CF3 S skeleton of Langlois' reagent as a CF3 S feedstock under mild conditions. The CuSCF3 intermediate generated by this protocol can react with a wide array of supporting ligands to furnish several air-stable [LCu(SCF3 )] complexes as valuable trifluoromethylthiolating agents.

Iron-catalyzed electrochemical C-H perfluoroalkylation of arenes.

A new iron-catalyzed reaction for the coupling of perfluoroalkyl iodides (RFI) with aromatic substrates is described. The perfluoroalkylated arene products are obtained in good to excellent yields in the presence of a [(bpy)Fe(ii)] catalyst (10%) electrochemically regenerated or generated from [(bpy)Fe(iii)] at room temperature. The development, scope, and preliminary mechanistic studies of these transformations are reported.

Accessing perfluoroalkyl nickel(II), (III), and (IV) complexes bearing a readily attached [C4F8] ligand.

The [C4F8] ligand was shown to support well-defined terpyridyl nickel complexes in the +2 and +3 oxidation states. Notably, a cyclic voltammetry study of the nickel(iii) species indicates that an additional oxidation is accessible, providing a family of related fluoroalkyl nickel complexes spanning the +2 to +4 oxidation states.

Manganese-Catalyzed Aerobic Oxytrifluoromethylation of Styrene Derivatives Using CF3SO2Na as the Trifluoromethyl Source.

A mild and practical protocol for manganese-catalyzed aerobic oxytrifluoromethylation of olefinic bonds of styrene derivatives using CF3SO2Na (Langlois' reagent) as the CF3 source is described. A distinguishing feature of this method is the generation of trifluoromethyl radicals from CF3SO2Na using the simple manganese salt/O2 system. The reaction proceeds under ambient conditions, free of added peroxide initiators, and provides moderate to good selectivities for alcohol versus ketone product.

Nanoheterogeneous catalysis in electrochemically induced olefin perfluoroalkylation.

Ni-catalyzed electroreductive olefin perfluoroalkylation affords both monomeric and dimeric products depending on the reaction media. Recycling of the catalyst can be achieved by immobilization of a (bpy)NiBr2 complex on silica nanoparticles decorated with anchoring amino-groups. Switching the homogeneous and heterogeneous catalysts is found to be one more factor to control the product ratio.

Synthesis and reactivity of new aminophenolate complexes of nickel.

New well-defined, paramagnetic nickel complexes have been prepared and characterized by X-ray crystallography. The complexes were found to be active for the cross-coupling of alkyl electrophiles (especially ethyl 2-bromobutyrate) with alkyl Grignard reagents. The ligand architecture in these new complexes could potentially be rendered chiral, opening up future possibilities for performing asymmetric cross-coupling reactions.

Lithium bromide-induced structural changes in a nickel bis-alkoxide complex.

The bis-alkoxide [(DEAMP)2Ni] (1, DEAMP = 1-(diethylamino)-2-methylpropan-2-olate) was found to react with trace amounts of lithium bromide to afford the bis-LiBr adduct 2, in which the oxygens of the DEAMP ligand coordinate to lithium to form a chiral-at-metal complex. This new complex is five-coordinate at nickel, and contains nickel and oxygen atoms which are all chiral. One diastereomer precipitates from pentane solution.

A five-coordinate nickel(II) fluoroalkyl complex as a precursor to a spectroscopically detectable Ni(III) species.

Mechanistic proposals for nickel-catalyzed coupling reactions often invoke five-coordinate alkyl- or aryl-bound Ni(II) and/or high-valent nickel(III) species, but because of their reactive nature, they have been difficult to study and fingerprint. In this work, we invoked the stabilizing properties of fluoroalkyl ligands to access such nickel species bearing ligands that are commonplace in organic coupling reactions. We show that five-coordinate Ni(II) complexes containing nickel-carbon bonds can readily be prepared given the appropriate precursor, and we also present evidence for the formation of Ni(III) species upon chemical and electrochemical oxidation of the five-coordinate complexes.

Synthesis and electronic properties of a pentafluoroethyl-derivatized nickel pincer complex.

The synthesis of a bis(amino)amide nickel pincer complex bearing a perfluoroethyl ligand was effected by reaction of the corresponding nickel chloride complex with cesium fluoride and trimethyl(pentafluoroethyl)silane. Electrochemical experiments revealed that the oxidation of the LNi-C(2)F(5) complex occurs at the same potential as the LNi-Cl derivative, but reduction of the LNi-C(2)F(5) complex occurs at slightly more positive potentials. The similarity of the electrochemical data was corroborated by density functional theory (DFT), which predicts that the energies of the HOMOs (HOMO = highest occupied molecular orbital) and LUMOs (LUMO = lowest unoccupied molecular orbital) of the LNi-C(2)F(5) and LNi-Cl complexes are equal in magnitude.

Unliganded HIV-1 gp120 core structures assume the CD4-bound conformation with regulation by quaternary interactions and variable loops.

The HIV-1 envelope (Env) spike (gp120(3)/gp41(3)) undergoes considerable structural rearrangements to mediate virus entry into cells and to evade the host immune response. Engagement of CD4, the primary human receptor, fixes a particular conformation and primes Env for entry. The CD4-bound state, however, is prone to spontaneous inactivation and susceptible to antibody neutralization.