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My research group aims at the discovery of new highly-reactive transition metal compounds and their applications in coordination chemistry and homogeneous catalysis. Our work particularly focusses on the chemistry of the abundant and biorelevant 3d-metals iron, cobalt and nickel. Some of the proposed projects specialize in phosphorus chemistry, specifically the intriguing and complex transformations of the P4 molecule by low-valent metal complexes. Phosphaalkynes and phosphabenzenes are used as building blocks for the construction of new versatile phosphaorganometallic complexes. In the framework of the graduate research training group "Chemical Photocatalysis", we develop new photocatalytic oxygenation reactions using visible light as an abundant energy source and dioxygen as a terminal oxidant.

Title Slide 20


P4-ACTIVATION

White Phosphorus Activation

Low-valent alkene and polyarene complexes are used to develop new transformations of the P4 allotrope that proceed at mild conditions and enable the the selective transformation of P4 into new polyphosphide anions. The subsequent selective functionalization of the polyphosphide ligands thus produced will open up a new avenue to the synthesis of organophosphorus compounds and unprecedented polyphosphorus species.

An example for our approach is the reaction of a reactive naphthalene ferrate anion that serves as a "Cp*Fe-" source in the reaction with P4, affording unusual anionic polyphosphide iron complexes..

Fe-polyphosphides

Angew. Chem. Int. Ed. 2011, 50, 6657–6660..

Rare cyclo-P44- units are observed in a series of dinuclear complexes [{(BIAN)Co}2(μ-P4)]x (x = 0, 1-, 2-). The redox-active BIAN ligand (BIAN = 1,2-bis(2,6-diisopropylphenylimino)acenaphthene is crucial for obtaining a highly reduced character of the P4-fragment.

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Co2p4

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Chem. Eur. J. 2016 DOI:10.1002/chem.201603296

REDUCTIVE CATALYSIS

Hydrogenations

Catalytic hydrogenations constitute one of the most important operations for the synthesis of fine chemicals and pharmaceuticals. In collaboration with our colleage Prof. Axel Jacobi von Wangelin (UR) we recently discovered that bis(anthracene)cobaltate 1 and -ferrate 2 are potent precatalysts for the hydrogenation of alkenes, ketones and imines under mild conditions. The active catalyst appears to be stabilized by π-coordination of the substrate in the case of alkene hydrogenations, while the hydrogenation of ketones and imines likely involves the formation of catalytically active cobalt particles.

The goal of this project is to further explore the scope and mechanism of arene- and alkene-based catalysts. In order to identify the nature of the catalytically active species, we pursue catalyst poisoning experiments, synthesize model complexes and perform NMR monitoring and ESI-MS studies (the latter in cooperation with the group of Konrad Koszinowski at the University of Göttingen).

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Hydrogenation

Angew. Chem. Int. Ed. 2014, 53, 3722-3726.

Cross-Couplings

Well defined, low-valent anthracene iron(-I) and cobalt(-I)complexes are competent precatalysts for cross-coupling reactions. Other organoiron and -cobalt complexes that we investigated show little to no activity. The oxidation state of the metal in the catalyst precursors has little bearing on the catalytic performance, but a labile coordination environment seems crucial for high catalytic activities.

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Cross-couplings

ChemCatChem 2011, 3, 1572-1577; Chem. Sci. 2013, 4, 776-784.

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PHOTOCATALYSIS

Photocatalytic Oxygenations via Coupled Redox Photocatalysis

The selective oxidation of alkenes and aliphatic hydrocarbons represents a major challenge in modern chemistry. In nature, these oxidations are performed successfully under mild conditions by metalloenzymes such as cytochrome P450 and non-heme based oxygenases. In the lab, a plethora of successful transition metal-based catalysts have been developed. However, only few catalysts are able to use atmospheric oxygen as the terminal oxidant, and these rare examples still show poor selectivities. Moreover, few examples for photocatalytic oxyfunctionalizations have been described to date.

In this project, we develop new photocatalytic methods for the selective oxyfunctionalization of hydrocarbons. Preferably, atmospheric oxygen is used as the oxidant. The investigations are conducted in the framework of the research training group "Chemical Photocatalysis" (GRK 1626).

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Photocatalysis 1

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Photocatalytic Benzylic C-H Bond Oxidation with a Flavin Scandium Complex

The use of flavins as photoreceptors and redox cofactors by nature has inspired the development of synthetic flavin analogues as photocatalysts. Riboflavin tetraacetate (RFT) is a prominent example. The photocatalytic C-H bond oxidation of alkyl benzenes to the corresponding aldehydes is one of its particularly intriguing applications. However, the substrate scope was previously limited to substrates with strongly electron-donating substituents on the arene ring. Fukuzumi et al. reported that the reduction potential of RFT can be increased by complex formation with Lewis acidic scandium(III) ions. In this project we have now found that this RFT/Sc(OTf)3 system allows the efficient catalytic oxidation of benzylic C-H bonds by aerobic oxygen.

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Fig1

Chem. Commun. 2015, 51, 8425-8428.

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C-H Photooxygenation of Alkylbenzenes Catalyzed by Riboflavin Tetraacetate and a Non-Heme Iron Catalyst

The Sc(OTf)3/RFT system enables the oxygenation of alkylbenzenes with electron-withdrawing substituents, but this system still performs poorly for various other benzylic substrates. We developed a dual catalyst consisting of RFT and the tris(2-pyridylmethyl)amine complex [Fe(TPA)(MeCN)2](ClO4)2, which efficiently catalyzes the challenging photooxygenation of alkylbenzenes. The catalytic activity of the Fe complex for H2O2 disproportionation and alkylbenzene oxygenation contributes to the high efficiency of this catalyst combination.

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Fig2

.Angew. Chem. Int. Ed. 2016, 55, 427-430.

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Halogenase-Inspired Oxidative Chlorination Using a Flavin Photocatalysis

In collaboration with group of Prof. Burkhard König (UR), we have developed a photocatalyst system for the selective chlorination of aromatic substrate. The RFT-based system is inspired by flavin adenine dinucleotide (FAD)-dependent halogenases. Compared to the high specificity of enzymes, our photocatalyzed protocol offers a more general strategy, allowing a broader substrate scope.

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Fig3

Angew. Chem. Int. Ed. 2016, 55, 5342-5345.

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LOW-VALENT COMPLEXES

Low-valent Transition Metalate Anions

Low-valent alkene and polyarene complexes are used as sources of "Mx-"- and "CpMx-" synthons (M = transition metal, Cp = cyclopentadienyl ligand) for the synthesis of unusual new compounds and as new catalysts.  Starting with anionic synthons, we synthesize "functional" anionic complexes, metal clusters and oligonuclear complexes. The catalytic properties are investigated in reductive transformations (see below). Our investigations focus on readily availabe and relatively cheap 3d metals, in particular iron and cobalt.

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Cp_Fe_minus

Chem. Commun. 2010, 46, 2832–2834; Dalton Trans. 2010, 39, 1453–1456; Chem. Eur. J. 2010, 16, 14322–14334; Inorg. Chem. 2012, 51, 6719–6730; Organometallics 2013, 32, 6040-6052; Dalton Trans. 2014, 43, 4247-4250.

Metalloradicals

We investigate the chemistry of new mononuclear 3d metal complexes with an open-shell electron configuration (metalloradicals). Our work currently focusses on the synthesis of new cyclopentadienyl metal(I) complexes. N-heterocyclic carbenes are used as ancillary ligands to stabilize these complexes in monomeric form. Their spectroscopic and magnetochemical characterization is carried out in collaboration with Prof. Bas de Bruin (University of Amsterdam) and Dr. Serhiy Demeshko (Prof. Franc Meyer, University of Göttingen).

NHC-stabilized cyclopentadienyl nickel(I) radicals are accessible in a straightforward manner by reducing nickel(II) halide complexes [CpNi(I)(NHC)Cl]. Spectroscopic investigations reveal that the these complexes can indeed be described as nickel-centered radicals.

.Homepage Metalloradicals

Chem. Commun. 2014, 50, 7014-7016.

White phosphorus is an excellent radical trap that reacts with these cyclopentadienyl nickel(I) compounds very selectively at low temperature. The resulting transition metal phosphides may serve as starting materials for the functionalization of the P4 moiety in the coordination sphere of the metal.

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P4

Chem. Commun. 2016, 52, 6601-6604..

In collaboration with Prof. Sjoerd Harder (University of Erlangen-Nürnberg) and Prof. Manfred Scheer (UR), we prepared a series of thermally stable half-sandwich complexes of the type [CpArM(μ-Br)]2. Taking the advantage of the bulky pentaarylcyclopentadienyl ligand C5(4-EtC6H4)5) (CpAr) and the distinct electronic properties compared to alkyl-substituted cyclopentadienyl ligands, these complexes may serve as a platform for new low valent transition metal chemistry. Employing [CpArNi(μ-Br)]2, a mononuclear cyclopentadienyl nickel(I) complex stabilized by a gallanediyl ligand Ga(nacnac) was synthesized. Spectroscopic and DFT studies show that the unpaired electron of this bimetallic Ni-Ga  complex resides at the Ni center.

Figure2

Inorg. Chem. 2016, 55, 3065-3074; Inorg. Chem. 2016, 55, 3075-3078.

Reduction of [CpArNi(μ-Br)]2 with KC8 in an aromatic solvent leads to a reactive "CpArNi" surrogate, which is trapped by N-heterocyclic carbenes, yellow sulfur and grey selenium, affording new Ni(I) and Ni(II) complexes.

Figure3

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Organometallics 2016, 35, 1624-1631.

P-COMPOUNDS

1,3-Diphosphacyclobutadiene Sandwich Anions

1,3-diphosphacyclobutadiene sandwich anions of type 1 are conveniently accessible by the reaction of phosphaalkynes with low-valent metalate anions. This program aims at the use of these versatile phosphaorganometallic building blocks in coordination chemistry and catalysis. Their reactivity and coordination properties are elucidated. In recent work,  new hetero(bi)metallic complexes have been synthesized.

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Entwurf0912 Cr1

Eur. J. Inorg. Chem2016, 5, 736-742; Angew. Chem. Int. Ed. 2014, 53, 2771-2775.

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New Iron Phosphinine Complexes

This project studies the reactivity and catalytic properties of new iron phosphinine complexes. We are developing new and versatile methods for the synthesis of unprecedented phosphacyclohexadienyl complexes which are used as new phosphorus-based catalysts. Recent work has shown that a range of new phosphinine and phosphacyclohexadienyl complexes is accessible from the novel phosphinine ferrate [K([18]-crown-6)(thf)2][Cp*Fe(TPP)] (TPP = 2,4,6-triphenylphosphinine).

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Phospinin

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Dalton Trans. 2016, 45, 8875-8884; Organometallics 2015, 34, 622-635.

  1. Fakultät für Chemie und Pharmazie
  2. Institut für Anorganische Chemie

Arbeitskreis Prof. Wolf

Prof. Dr. Robert Wolf

Prof. Dr. Robert Wolf

Universitätsstr. 31
93053 Regensburg

Email

Tel. +49 941 943-4485
Fax +49 941 943-814485
Büro: CH 22.2.83