Beatrice Braun

Lewis Acid Trapping of an Elusive Copper–Tosylnitrene Intermediate Using Scandium Triflate

High-valent copper-nitrene intermediates have long been proposed to play a role in copper-catalyzed aziridination and amination reactions. However, such intermediates have eluded detection for decades, preventing the unambiguous assignments of mechanisms. Moreover, the electronic structure of the proposed copper-nitrene intermediates has also been controversially discussed in the literature. These mechanistic questions and controversy have provided tremendous motivation to probe the accessibility and reactivity of Cu(III)-NR/Cu(II)N(•)R species. In this paper, we report a breakthrough in this field that was achieved by trapping a transient copper-tosylnitrene species, 3-Sc, in the presence of scandium triflate. The sufficient stability of 3-Sc at -90 °C enabled its characterization with optical, resonance Raman, NMR, and X-ray absorption near-edge spectroscopies, which helped to establish its electronic structure as Cu(II)N(•)Ts (Ts = tosyl group) and not Cu(III)NTs. 3-Sc can initiate tosylamination of cyclohexane, thereby suggesting Cu(II)N(•)Ts cores as viable reactants in oxidation catalysis.

A Reduced β-Diketiminato-Ligated Ni3H4 Unit Catalyzing H/D Exchange

An investigation concerning the stepwise reduction of the β-diketiminato nickel(II) hydride dimer [LNi(μ-H)(2)NiL], 1 (L = [HC(CMeNC(6)H(3)(iPr)(2))(2)](-)), has been carried out. While the reaction with one equivalent of potassium graphite, KC(8), led to the mixed valent Ni(I)/Ni(II) complex K[LNi(μ-H)(2)NiL], 3, treatment of 1 with two equivalents of KC(8) surprisingly yielded in the trinuclear complex K(2)[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 4, in good yields. The Ni(3)H(4) core contains one Ni(II) and two Ni(I) centers, which are antiferromagnetically coupled so that a singlet ground state results. 4 represents the first structurally characterized molecular compound with three nickel atoms bridged by hydride ligands, and it shows a very interesting chemical behavior: Single-electron oxidation yields in the Ni(II)(2)Ni(I) compound K[LNi(μ-H)(2)Ni(μ-H)(2)NiL], 5, and treatment with CO leads to the elimination of H(2) with formation of the carbonyl complex K(2)[LNi(CO)](2), 6. Beyond that, it could be shown that 4 undergoes H/D exchange with deuterated solvents and the deuteride-compound 4-D(4) reacts with H(2) to give back 4. The crystal structures of the novel compounds 3-6 have been determined, and their electronic structures have been investigated by EPR and NMR spectroscopy, magnetic measurements, and DFT calculations.

Dinuclear copper complexes based on parallel β-diiminato binding sites and their reactions with O2: evidence for a Cu-O-Cu entity.

Investigations concerning the system β-diketiminato-Cu(I)/O(2) have revealed valuable insights that may be discussed in terms of the behavior of mononuclear oxygenases containing copper. On the other hand nature also employs dinuclear Cu enzymes for the activation of O(2). With this background the ligand system [(Me(2))(C(6)H(3))Xanthdim](2-) containing two parallel β-diiminato binding sites linked by a xanthene backbone with 2,3-dimethylphenyl residues at the diiminato units was investigated with respect to its copper coordination chemistry. The diimine [(Me(2))(C(6)H(3))Xanthdim]H(2) was treated with CuOtBu in the presence of acetonitrile, PPh(3), and PMe(3) to yield the corresponding complexes [(Me(2))(C(6)H(3))Xanthdim](Cu(L))(2) (L = CH(3)CN, 1, PPh(3), 2, and PMe(3), 3) that proved to be stable and were fully characterized. Single crystal X-ray diffraction analyses performed for the three complexes showed that considerable steric crowding within the binding pockets of 2 leads to a very long Cu-Cu distance while the structures of 1 and 3 are relaxed. Compounds 2 and 3 are relatively robust toward air, whereas 1 is very sensitive and quantitatively reacts with O(2) at room temperature (r.t.) within less than 2 min to give intractable compounds. At low temperatures the formation of a green intermediate was observed that was identified as a Cu(II)-O-Cu(II) species spectroscopically and chemically. This finding is of relevance also in the context of the results obtained testing 1 as a catalyst for phenol oxidation using O(2): 1 efficiently catalyzes phenol coupling, while there was no evidence for any oxygenation reactions occurring.

OO Bond Formation Mediated by a Hexanuclear Iron Complex Supported on a Stannoxane Core

In recent years, much attention has been focused on the incorporation of redox-active transition-metal complexes into the dendrimer structure owing to their potential applications in various fields. Also, the antenna-like structure of the dendrimers, in many cases, was found to provide an ideal organization for these chromophores and redox centers to work in synergistic ways in carrying out a number of important transformations. For example, an extensive cooperative effect between the Cu centers was observed during the cleavage of supercoiled DNA catalyzed by a hexanuclear Cu-porphyrin complex, supported on a stannoxane core. The above-mentioned hexaporphyrin assembly was synthesized in high yields and in a single step utilizing the orACHTUNGTRENNUNGganostannoxane approach, whereby n-butyl stannoic acid was made to react with the corresponding porphyrin carboxylic acid in 1:1 stoichiometry in benzene; the molecular structure of the ligand was established on the basis of Sn NMR and DFT calculations. In the present paper we report the synthesis of a non-heme hexanucleating ligand (1) supported on a drum-like stannoxane central core utilizing the same organostannoxane approach (Scheme 1). 1 is characterized by X-ray diffraction, Sn NMR, and infrared methods. Most importantly, we show that the Fe-metalated hexa non-heme assembly, 2, in the presence of 2-(tert-butylsulfonyl)-iodosylbenzene (PhIO), performs a rare O O bond formation reaction, thereby generating a Fe-(O2 · ) Fe superoxo unit. Such a metal mediated O O bond formation step is considered to be the most critical part of dioxygen evolution in photosystem II and hence plays a vital role in the context of attaining a clean renewable energy source. The condensation reaction (Scheme 1) of equimolar amounts of n-butyl stannoic acid and 4-(1,3-bis(2-pyridylmethyl)-2-imidazolidinyl)benzoic acid (L1) in toluene afforded a pale yellow solid 1, whose molecular structure (Scheme 1) shows a giant-wheel arrangement of the six non-heme ligand units with a drum-like stannoxane central core serving as the structural support for the hexanucleating assembly. The general features of the stannoxane framework are found to be similar to the other structurally characterized drum-shaped molecules and have a crystallographic S6 symmetry, so that six tin atoms are crystallographically and chemically equivalent. Sn NMR spectrum of 1 exhibits a sharp singlet at 482.4 ppm (Figure S1 top), which is the characteristic signature for a hexameric organostannoxane [a] S. Kundu, F. F. Pfaff, F. Heims, B. R bay, Dr. B. Braun, Dr. K. Ray Humboldt-Universit t zu Berlin, Institut f r Chemie Brook-Taylor-Strasse 2, 12489 Berlin (Germany) Fax: (+49) 3020937387 E-mail : kallol.ray@chemie.hu-berlin.de [b] Dr. E. Matito Institute of Physics, University of Szczecin Wielkopolska 15, 70451 Szczecin (Poland) [c] Dr. S. Walleck, Prof. Dr. T. Glaser Lehrstuhl f r Anorganische Chemie I Fakult t f r Chemie, Universit t Bielefeld Universit tstr. 25, 33615 Bielefeld (Germany) [d] Dr. J. M. Luis Institut de Qu mica Computacional Department de Qu mica, Facultat de Ci ncies Universitat de Girona, 17071 Girona (Spain) [e] Dr. A. Company Institute of Chemistry: Metalorganic materials Technische Universit t Berlin Strasse des 17. Juni 135, 10623 Berlin (Germany) Supporting information for this article is available on the WWW under http://dx.doi.org/10.1002/chem.201102326. Scheme 1. Scheme showing the synthesis of the complexes. Hydrogen atoms and the n-butyl groups on tin have been omitted for clarity. Molecular structures of the hexanuclear ligand 1 and the complex [Fe(L2)ACHTUNGTRENNUNG(CH3CN)2]2+ are determined by X-ray crystallography. Structure of 2 is proposed based on ICP-MS, Sn-NMR, F NMR, IR, Mçssbauer and DFT methods (see the Supporting Information for a color version of Scheme 1).

A Dinuclear Molecular Iron(II) Silicate with Two High-Spin Square-Planar FeO4 Units†

For a given d metal ion surrounded by a given number of ligand donor atoms, various situations are conceivable for the relative energetic positioning of the d frontier orbitals. These depend on the nature of the ligands and their geometric arrangement. Filling the x electrons into the different orbital schemes, considering both high-spin and low-spin configurations, results in ligand-field stabilization energies, and their comparison provides a rationale as to why a certain structure and electron configuration is stable and others are not. For the coordination number four in a transition-metal complex, two structural motifs usually have to be distinguished: tetrahedral and square-planar ligand geometries. The latter brings about a large separation of the dx2 y2 orbital from the residual d orbitals, which is beneficial if only those are filled, in particular if the ligand field is strong. Consequently, for d electron counts higher than four, squareplanar structures are usually accompanied by low-spin configurations; corresponding compounds with high-spin configuration are extremely rare. Until 2011, only four high-spin d Fe complexes with planar N4 [1–3] and O4 [4] coordination spheres, provided by macrocyclic or chelating ligands, had been known. These are complemented by a few examples from solid-state science, which interestingly includes the rare mineral gillespite (BaFeSi4O10). [11] This mineral contains an isolated square-planar high-spin FeO4 chromophore where the oxygen atoms represent O-functions of extended cyclotetrasilicate anions (see Figure 1). Ceramics, such as SrFeO2, contain such FeO4 units linked by vertices. Precedent cases with edge-sharing FeO4 structural motifs have yet not been reported. Very recently, the first two molecular compounds 14] containing high-spin, square-planar FeO4 arrangements which are not enforced by a macrocyclic ligand were reported by Kl fers et al. and Doerrer et al. (see Scheme 1). These very unusual structures have been rationalized by a Jahn–Teller flattening due to steric repulsion of the dz2 b-spin electron

Photochemical CO2 Reduction Catalyzed by Phenanthroline Extended Tetramesityl Porphyrin Complexes Linked with a Rhenium(I) Tricarbonyl Unit

A series of heterodinuclear complexes (M-1-Re) based on a phenanthroline (phen) extended tetramesityl porphyrin ligand (H2-1) has been prepared. The phen moiety of this ligand selectively coordinates a Re(I) tricarbonyl chloride unit, whereas the metal in the porphyrin moiety has been varied: namely, Cu, Pd, Zn, Co, or Fe was used. These dinuclear complexes were fully characterized by standard analytical methods. Additionally, a crystal structure of Cu-1-Re·5.5(C7H8)·0.5(C6H6) could be obtained, and extended time-resolved emission lifetime measurements were conducted. Furthermore, their ability to catalyze the photochemical reduction of CO2 to CO was investigated. Light-driven CO2 reduction experiments were performed in dimethylformamide (DMF) using triethylamine (TEA) as the sacrificial electron donor. The TONs (turnover numbers) of CO were determined and revealed a surprising catalytic activity that is obviously independent from the redox activity of the porphyrin metal. We have recently shown that the parent M-1 compounds are active photocatalysts, but the catalytic activity was dependent on the redox activity of the porphyrin metal. In the case of the new heterodinuclear complexes M-1-Re reported in this study, the catalytic active center seems to be the Re(I) moiety and not the porphyrin. Surprisingly, Zn-1-Re proved to be the most active compound in this series showing a TONCO of 13 after 24 h of illumination using a >375 nm cutoff filter while all other compounds showed minimal activity under this condition.

SF and SC Activation of SF6 and SF5 Derivatives at Rhodium: Conversion of SF6 into H2S

The degradation of SF6 and SF5 organyls by S-F and S-C bond-activation reactions at [{Rh(μ-H)(dippp)}2] under mild conditions is reported. Fluorido and thiolato species were identified as products or intermediates, and were characterized by X-ray diffraction analysis and multinuclear NMR spectroscopy. An unprecedented cyclic process for the conversion of the potent greenhouse gas SF6 into H2S was developed.

Three-coordinate nickel(II) and nickel(I) thiolate complexes based on the β-diketiminate ligand system.

Mononuclear nickel(II) thiolate complexes [L(tBu)Ni(SEt)] (1) and [L(tBu)Ni(aet)] (2, aet = (-)S(CH2)2NH2) (L(tBu) = [HC(C((t)Bu)NC6H3((i)Pr)2)2](-)), supported by a bulky nacnac ligand, were synthesized by treatment of the nickel(II) bromide precursor [L(tBu)Ni(Br)] (I) with the potassium salts of ethanethiol and cysteamine, respectively. The nickel atom in 1 features a planar T-shaped environment, while the Ni ion within 2 shows a distorted square planar coordination geometry, as the aminoethanethiolate (aet) is coordinated as a chelating ligand. In 2 the β-diketiminate ligand binds in a rarely observed κ(2)C,N coordination mode. Reduction of complex 1 or its benzenethiolate analogue [L(tBu)Ni(SPh)] (II) by KC8 resulted in the formation of dinuclear Ni(I) thiolates (K·OEt2)(K)[L(tBu)Ni(SEt)]2 (3) and (K·OEt2)2[L(tBu)Ni(SPh)]2 (4), respectively. In these compounds [L(tBu)Ni(SR)](-) units are held together by potassium cations produced in the reduction process. All compounds mentioned were structurally characterized by single-crystal X-ray crystallography.

Cationic Zinc Organyls as Precatalysts for Hydroamination Reactions

The cationic zinc triple-decker complex [Zn2 Cp*3 ](+) [BAr(F) 4 ](-) (BAr(F) 4 =B(3,5-(CF3 )2 C6 H3 )4 ) exhibits catalytic activity in intra- and intermolecular hydroamination reactions in the absence of a cocatalyst. These hydroaminations presumably proceed through the activation of the C-C multiple bond of the alkene or alkyne by a highly electrophilic zinc species, which is formed upon elimination of the Cp* ligands. The reaction of [Zn2 Cp*3 ](+) [BAr(F) 4 ](-) with henylacetylene gives the hydrocarbonation product (Cp*)(Ph)CCH2 , which might be formed via a similar reaction pathway. Additionally, several other structurally well-defined cationic zinc organyls have been examined as precatalysts for intermolecular hydroamination reactions without the addition of a cocatalyst. These studies reveal that the highest activity is achieved in the absence of any donor ligands. The neutral complex [ZnCp(2S) 2 ] (Cp(2S) =C5 Me4 (CH2 )2 SMe) shows a remarkably high catalytic activity in the presence of a Brønsted acid.

A Heterobimetallic Superoxide Complex formed through O2 Activation between Chromium(II) and a Lithium Cation

The reaction of 1,1,3,3-tetraphenyl-1,3-disiloxandiol (LH2) with n-butyllithium and CrCl2 results in a mononuclear chromium(II) complex (1) that further reacts with O2 at low temperatures to yield a mononuclear chromium(III) superoxide complex [L2CrO2(THF)][Li2(THF)3] (2). The crystal structure revealed that the chromium superoxido entity is stabilized by the coordination to an adjacent lithium cation. Complex 2 thus contains an unprecedented heterobimetallic [Cr(III)(μ-O2)Li(+)] core; beyond this it is the first chromium superoxide for which a temperature-dependent magnetic characterization could be achieved, and the first structurally characterized representative with chromium in an exclusive O-donor environment.