Subrata Kundu

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.

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 High‐Valent Heterobimetallic [CuIII(μ‐O)2NiIII]2+ Core with Nucleophilic Oxo Groups

A heterobimetallic CuNi bis(μ-oxo) diamond core is shown to possess nucleophilic oxo groups, and has been demonstrated for the first time as a viable intermediate during the deformylation of fatty aldehydes by cyanobacterial aldehyde decarbonylase.

Redox Non-Innocence of Nitrosobenzene at Nickel

Nitrosobenzene (PhNO) serves as a stable analogue of nitroxyl (HNO), a biologically relevant, redox-active nitric oxide derivative. Capture of nitrosobenzene at the electron-deficient β-diketiminato nickel(I) complex [(i) Pr2 NNF6 ]Ni results in reduction of the PhNO ligand to a (PhNO)(./-) species coordinated to a square planar Ni(II) center in [(i) Pr2 NNF6 ]Ni(η(2) -ONPh). Ligand centered reduction leads to the (PhNO)(2-) moiety bound to Ni(II) supported by XAS studies. Systematic investigation of structure-reactivity patterns of (PhNO)(./-) and (PhNO)(2-) ligands reveals parallels with superoxo (O2 )(./-) and peroxo (O2 )(2-) ligands, respectively, and forecasts reactivity patterns of the more transient HNO ligand.

Copper(II) Activation of Nitrite: Nitrosation of Nucleophiles and Generation of NO by Thiols

Nitrite (NO2-) and nitroso compounds (E-NO, E = RS, RO, and R2N) in mammalian plasma and cells serve important roles in nitric oxide (NO) dependent as well as NO independent signaling. Employing an electron deficient β-diketiminato copper(II) nitrito complex [Cl2NNF6]Cu(κ2-O2N)·THF, thiols mediate reduction of nitrite to NO. In contrast to NO generation upon reaction of thiols at iron nitrite species, at copper this conversion proceeds through nucleophilic attack of thiol RSH on the bound nitrite in [CuII](κ2-O2N) that leads to S-nitrosation to give the S-nitrosothiol RSNO and copper(II) hydroxide [CuII]-OH. This nitrosation pathway is general and results in the nitrosation of the amine Ph2NH and alcohol tBuOH to give Ph2NNO and tBuONO, respectively. NO formation from thiols occurs from the reaction of RSNO and a copper(II) thiolate [CuII]-SR intermediate formed upon reaction of an additional equiv thiol with [CuII]-OH.

Mechanism of phenol oxidation by heterodinuclear Ni Cu bis(μ-oxo) complexes involving nucleophilic oxo groups

Oxidation of phenols by heterodinuclear Cu(III)(μ-O)2Ni(III) complexes containing nucleophilic oxo groups occurs by both proton coupled electron transfer (PCET) and hydrogen atom transfer (HAT) mechanisms; the exact mechanism depends on the nature of the phenol as well as the substitution pattern of the ligand bound to Cu.

Temperature Dependence of the Catalytic Two- versus Four-Electron Reduction of Dioxygen by a Hexanuclear Cobalt Complex

The synthesis and characterization of a hexanuclear cobalt complex 1 involving a nonheme ligand system, L1, supported on a Sn6O6 stannoxane core are reported. Complex 1 acts as a unique catalyst for dioxygen reduction, whose selectivity can be changed from a preferential 4e-/4H+ dioxygen-reduction (to water) to a 2e-/2H+ process (to hydrogen peroxide) only by increasing the temperature from -50 to 25 °C. A variety of spectroscopic methods (119Sn-NMR, magnetic circular dichroism (MCD), electron paramagnetic resonance (EPR), SQUID, UV-vis absorption, and X-ray absorption spectroscopy (XAS)) coupled with advanced theoretical calculations has been applied for the unambiguous assignment of the geometric and electronic structure of 1. The mechanism of the O2-reduction reaction has been clarified on the basis of kinetic studies on the overall catalytic reaction as well as each step in the catalytic cycle and by low-temperature detection of intermediates. The reason why the same catalyst can act in either the two- or four-electron reduction of O2 can be explained by the constraint provided by the stannoxane core that makes the O2-binding to 1 an entropically unfavorable process. This makes the end-on μ-1,2-peroxodicobalt(III) intermediate 2 unstable against a preferential proton-transfer step at 25 °C leading to the generation of H2O2. In contrast, at -50 °C, the higher thermodynamic stability of 2 leads to the cleavage of the O-O bond in 2 in the presence of electron and proton donors by a proton-coupled electron-transfer (PCET) mechanism to complete the O2-to-2H2O catalytic conversion in an overall 4e-/4H+ step. The present study provides deep mechanistic insights into the dioxygen reduction process that should serve as useful and broadly applicable principles for future design of more efficient catalysts in fuel cells.

Three-Coordinate Copper(II) Aryls: Key Intermediates in C–O Bond Formation

Copper(II) aryl species are proposed key intermediates in Cu-catalyzed cross-coupling reactions. Novel three-coordinate copper(II) aryls [CuII]-C6F5 supported by ancillary β-diketiminate ligands form in reactions between copper(II) alkoxides [CuII]-OtBu and B(C6F5)3. Crystallographic, spectroscopic, and DFT studies reveal geometric and electronic structures of these Cu(II) organometallic complexes. Reaction of [CuII]-C6F5 with the free radical NO(g) results in C-N bond formation to give [Cu](η2-ONC6F5). Remarkably, addition of the phenolate anion PhO- to [CuII]-C6F5 directly affords diaryl ether PhO-C6F5 with concomitant generation of the copper(I) species [CuI](solvent) and {[CuI]-C6F5}-. Experimental and computational analysis supports redox disproportionation between [CuII]-C6F5 and {[CuII](C6F5)(OPh)}- to give {[CuI]-C6F5}- and [CuIII](C6F5)(OPh) unstable toward reductive elimination to [CuI](solvent) and PhO-C6F5.

A Dinitrogen Dicopper(I) Complex via a Mixed‐Valence Dicopper Hydride

Low-temperature reaction of the tris(pyrazolyl)borate copper(II) hydroxide [(iPr2) TpCu]2 (μ-OH)2 with triphenylsilane under a dinitrogen atmosphere gives the bridging dinitrogen complex [(iPr2) TpCu]2 (μ-1,2-N2 ) (3). X-ray crystallography reveals an only slightly activated N2 ligand (N-N: 1.111(6)u2005Å) that bridges between two monovalent (iPr2) TpCu fragments. While DFT studies of mono- and dinuclear copper dinitrogen complexes suggest weak π-backbonding between the d(10) Cu(I) centers and the N2 ligand, they reveal a degree of cooperativity in the dinuclear Cu-N2 -Cu interaction. Addition of MeCN, CNAr(2,6-Me) , or O2 to 3 releases N2 with formation of (iPr2) TpCu(L) (L=NCMe, CNAr(2,6-Me2) ) or [(iPr2) TpCu]2 (μ-η(2) :η(2) -O2 ) (1). Addition of triphenylsilane to [(iPr2) TpCu]2 (μ-OH)2 in pentane allows isolation of a key intermediate [(iPr2) TpCu]2 (μ-H) (5). Although 5 thermally decays under N2 to give 3, it reduces unsaturated substrates, such as CO and HC≡CPh to HC(O)H and H2 C=CHPh, respectively.

Metal-oxo-mediated O-O bond formation reactions in chemistry and biology

Abstract O-O bond formation is one of the key reactions that ensure life on earth. Dioxygen is produced in photosystem II, as well as in chlorite dismutase. The reaction mechanisms occurring in the enzyme active sites are controversially discussed – although their structures have been resolved with less unambiguity. Artificial molecular catalysts have been developed in the last years to obtain vital insights into the O-O bond formation step. This review put together the scarce literature on the topic that helped in understanding the key steps in the O-O bond formation reactions mediated by high-valent oxo complexes of the first-row transition metals.