Florian Felix Pfaff

Status of Reactive Non-Heme Metal–Oxygen Intermediates in Chemical and Enzymatic Reactions

Selective functionalization of unactivated C-H bonds, water oxidation, and dioxygen reduction are extremely important reactions in the context of finding energy carriers and conversion processes that are alternatives to the current fossil-based oil for energy. A range of metalloenzymes achieve these challenging tasks in biology by using cheap and abundant transition metals, such as iron, copper, and manganese. High-valent metal-oxo and metal-dioxygen (superoxo, peroxo, and hydroperoxo) cores act as active intermediates in many of these processes. The generation of well-described model compounds can provide vital insights into the mechanisms of such enzymatic reactions. This perspective provides a focused rather than comprehensive review of the recent advances in the chemistry of biomimetic high-valent metal-oxo and metal-dioxygen complexes, which can be related to our understanding of the biological systems.

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.

Spectroscopic Capture and Reactivity of a Low‐Spin Cobalt(IV)‐Oxo Complex Stabilized by Binding Redox‐Inactive Metal Ions

High-valent cobalt-oxo intermediates are proposed as reactive intermediates in a number of cobalt-complex-mediated oxidation reactions. Herein we report the spectroscopic capture of low-spin (S=1/2) Co(IV)-oxo species in the presence of redox-inactive metal ions, such as Sc(3+), Ce(3+), Y(3+), and Zn(2+), and the investigation of their reactivity in C-H bond activation and sulfoxidation reactions. Theoretical calculations predict that the binding of Lewis acidic metal ions to the cobalt-oxo core increases the electrophilicity of the oxygen atom, resulting in the redox tautomerism of a highly unstable [(TAML)Co(III)(O˙)](2-) species to a more stable [(TAML)Co(IV)(O)(M(n+))] core. The present report supports the proposed role of the redox-inactive metal ions in facilitating the formation of high-valent metal-oxo cores as a necessary step for oxygen evolution in chemistry and biology.

Reactivity of a Nickel(II) Bis(amidate) Complex with meta-Chloroperbenzoic Acid: Formation of a Potent Oxidizing Species

Herein, we report the formation of a highly reactive nickel-oxygen species that has been trapped following reaction of a Ni(II) precursor bearing a macrocyclic bis(amidate) ligand with meta-chloroperbenzoic acid (HmCPBA). This compound is only detectable at temperatures below 250 K and is much more reactive toward organic substrates (i.e., C-H bonds, C=C bonds, and sulfides) than previously reported well-defined nickel-oxygen species. Remarkably, this species is formed by heterolytic O-O bond cleavage of a Ni-HmCPBA precursor, which is concluded from experimental and computational data. On the basis of spectroscopy and DFT calculations, this reactive species is proposed to be a Ni(III) -oxyl compound.

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 a N-Heterocyclic Nitrenium Cation Bound to a Nickel–Cyclam Core

The redox properties of Ni complexes bound to a new ligand, [DMC-nit](+), where a N-heterocyclic nitrenium group is anchored on a 1,4,8,11-tetraazacyclotetradecane backbone, have been examined using spectroscopic and DFT methods. Ligand-based [(DMC-nit)Ni](2+/+) reduction and metal-based [(DMC-nit)Ni](2+/3+) oxidation processes have been established for the [(DMC-nit)Ni](+/2+/3+) redox series, which represents the first examples of nitrenium nitrogen (N(nit))-bound first-row transition-metal complexes. An unprecedented bent binding mode of N(nit) in [(DMC-nit)Ni](2+) is observed, which possibly results from the absence of any N(nit)→Ni σ-donation. For the corresponding [(DMC-nit)Ni(F)](2+) complex, σ-donation is dominant, and hence a coplanar arrangement at N(nit) is predicted by DFT. The binding of the triazolium ion to Ni enables new chemistry (formate oxidation) that is not observed in a derivative that lacks this functional group. Thus the N-heterocyclic nitrenium ligand is a potentially useful and versatile reagent in transition-metal-based catalysis.