Kontakt

 

 

We are a group of fundamental scientists, thinking that true innovation is only possible through fundamental science. For us it is of importance to develop new ideas and concepts; we are eager to leave the established paths. We do not restrict our research to a particular element; research in our group ranges from transition metal to main group element chemistry and crosses the border between traditional inorganic and organic chemistry. Redox chemistry and bond activation reactions are research themes that especially interest us. We frequently use guanidines, either as redox-active ligands (e.g. the new class of guanidino-functionalized aromatics (GFAs) introduced by our group) or as electron-donating bridging substituents. Always starting with fundamental research, we develop novel applications, going the complete way from the basics to applications. For example, basic research on the stimulation of inter- and intramolecular electron transfer processes led to the development of new redox catalysts.

If you like to find out more about our reseach, please have a look at our publications and/or come to us.

Please contact us if you are interested in joining our group for your master or PhD thesis, or for a research practical course

 

 

Please find out more about the following topics:

• Nucleophilic diboranes and highly-charged oligoboron compounds

• Redox-active guanidines: coordination chemistry and proton-coupled electron transfer

• Bond activation processes at metal atom and metal oxide clusters, studied with the matrix isolation technique

 

 

 

 

 

Nucleophilic diboranes and highly-charged oligoboron compounds

Current projects:

1) Synthesis of s-aromatic oligoboron compounds with small HOMO-LUMO gaps

Selection of tetraboron compounds

 

                                                Left: Photo showing the bight orange color of precipitated salt of
                                                [B₄(hpp₄]⁴⁺. Right: Fluorescence of the compounds under an
                                                UV lamp. Top: reaction mixture with precipitated crystals
                                                (shiny dots), Bottom: solid material after isolation.

 

Triboron compounds (2 sceletal electrons)

 

 

 

Selected publications:

H.-J. Himmel, Electron-Deficient Triborane and Tetraborane Ring Compounds: Synthesis, Structure and Bonding, Angew. Chem. 2019, 131, 11724-11742; Angew. Chem. Int. Ed. 2019, 58, 11600-11617. (Review article)

A. Widera, H. Wadepohl, H.-J. Himmel, Angew. Chem. 2019, 131, 5957-5961, Angew. Chem. Int. Ed. 2019, 58, 5897-5901.

A. Widera, E. Kaifer, H. Wadepohl, H.-J. Himmel, Chem. Eur. J. 2018, 24, 1209-1216.

S. Litters, E. Kaifer, H.-J. Himmel, Eur. J. Inorg. Chem. 2016, 4090-4098.

S. Litters, E. Kaifer, H.-J. Himmel, Angew. Chem. 2016, 128, 4417-4420; Angew. Chem. Int. Ed. 2016, 55, 4345-4347.

S. Litters, E. Kaifer, M. Enders, H.-J. Himmel, Nature Chem. 2013, 5, 1029-1034.

 

 

2) Redox reactions making use of the combined Lewis acidity and electron donor properties of nucleophilic diboranes

The sketch below shows how the combined Lewis acidity and electron donor properties of nucleophilic diboranes could be used for reactions with s-donor / p-acceptor substrates (L_ox).

 

 

 

Example for the reduction of benzoquinones:

 

 

                                                                                                                 

Other redox reactions:

 

Diboration of acetonitrile

 

 

 

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Hydroboration of CO₂ (catalyst-free). The nucleophilic attack of the diborane at the C atom of CO₂ is sketched in the box.

 

 

Selected publications:

D. Vogler, N. Wolf, E. Kaifer, H.-J. Himmel, Dalton Trans. 2019, 48, 14354-14366.

A. Widera, D. Vogler, H. Wadepohl, E. Kaifer, H.-J. Himmel, Angew. Chem. 2018, 130, 11627-11630; Angew. Chem. Int. Ed.  2018 57, 11456-11459.

H.-J. Himmel, Eur. J. Inorg. Chem. 2018, 2139-2154. (Review article)

M. Frick, J. Horn, H. Wadepohl, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2018, 24, 16983-16986.

M. Frick, E. Kaifer, H.-J. Himmel, Angew. Chem. 2017, 129, 11804-11807; Angew. Chem. Int. Ed.   2017, 56,1645-1648.

 

3) Synthesis and chemistry of redox-active macrocycles

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A. Widera, E. Filbeck, H. Wadepohl, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2020, 26, 3435-3440.

 

 

Redox-active guanidines: coordination chemistry and proton-coupled electron transfer

 

Current projects:

 

1) Stimulation of intramolecular electron transfer in coordination compounds with redox-active guanidine ligands

Thermally stimulated IET

Synthesis of a first dinuclear copper complex showing a temperature-dependent equilibrium between two redox isomers (valence tautomerism (VT)).

 

Examples for mononuclear complexes with redox-active guanidine ligands showing VT are:

 

 

 

Stimulation of IET by oxidation

Starting with a Cu^II complex with neutral guanidine ligand, Redox-induced electron transfer (RIET) leads to a Cu^I complex with dicationic ligand unit. Seemingly paradoxically, oxidation of the complex leads to metal reduction.

 

IET could also be triggered by coordination of metals to a secondary coordination sphere, e.g. a crown ether function.

 

Selected publications:

S. Wiesner, A. Wagner, E. Kaifer, H.-J. Himmel, A Valence Tautomeric Dinuclear Copper Tetrakisguanidine Complex, Chem. Eur. J. 2016, 22, 10438–10445.

S. Haaf, E. Kaifer, H. Wadepohl, H.-J. Himmel, Use of Crown Ether Functions as Secondary Coordination Sheres for the Manipulation of Ligand-Metal Intramolecular Electron Transfer in Copper-Guanidine Complexes, Chem. Eur. J. 2020, DOI: chem.202003469.

D. F. Schrempp, E. Kaifer, H. Wadepohl, H.-J. Himmel, Copper Complexes of New Redox-Active 4,5-Bisguanidino-Substituted Benzodioxole Ligands: Control of the Electronic Structure by Counter-Ligands, Solvent, and Temperature, Chem. Eur. J. 2016, 22, 16187–16199.

D. F. Schrempp, S. Leingang, M. Schnurr, E. Kaifer, H. Wadepohl, H.-J. Himmel, Inter- and Intramolecular Electron Transfer in Copper Complexes: Electronic Entatic State with Redox-Active Guanidine Ligands, Chem. Eur. J. 2017, 23, 13607–13611.

H.-J. Himmel, Inorg. Chim. Acta, 2018, 481, 56-68. (Review article)

 

 

2) Application of complexes with redox-active guanidine ligands as catalysts

Catalytic Aerobic Phenol Homo- and Cross-Coupling with Copper Complexes Bearing Redox-Active Guanidine Ligands

 

 

 

^a) Cross-coupling selectivity was determined via 1H NMR by integratin an internal standard,  the cross-coupling product and the homo-coupling product (seeSI).

The average of two experiments is given.

 

F. Schön, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2019, 25, 8279-8288

 

 

3) Redox-active guanidines in proton-coupled electron transfer (PCET) reactions

 

 

Green and efficient recycling of the reactive, oxidized form (GFA1)²⁺ by catalytic oxidation of (GFA1+2H)²⁺ or (GFA1+4H)⁴⁺ with dioxygen (cat. = CuCl₂/[Cu(H₂O)₆](BF₄)₂ (1:1)).

 

 

 

Stoichiometric reactions without addition of acid

 

 

Stoichiometric reactions with addition of an acid

The doubly-oxidized guanidine could be protonated.

 

 

Examples:

Oxidation of benzylamine and o-phenylene-diamine with (GFA1+2H)⁴⁺.

 

 

Coupling reaction of triarylamines of various potentials with (GFA1)²⁺ in the presence of acid.

 

 

Oxidative coupling of 3,3’’,4,4’’-tetramethoxy-o-terphenyl to 2,3,10,11-tetramethoxy-triphenylene with (GFA1)²⁺.

 

Oxidative intermolecular aryl-aryl coupling of N-ethylcarbazole to N,N‘-diethyl-3,3‘-bicarbazole. Conditions: acetonitrile, 1 eq. 1(ClO₄)₂, 16 eq. HBF₄·OEt₂, 15 min at 0 °C, 45 min at r.t., 95% yield.

 

 

Catalytic reactions with dioxygen as terminal oxidant

General scheme for the oxidation of organic molecules (S_red) with dioxygen with a redox-active guanidine (GFA) as catalyst and a dioxygen-activating co-catalyst (e.g. CuCl₂/[Cu(H₂O)₆](BF₄)₂ (1:1)).

 

 

Examples for the use of GFA1 as redox catalyst for the oxidation of organic substrates with different redox-potentials.

 

Selected publications:

U. Wild, P. Walter, O. Hübner, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2020, 26, 16504-16513.

U. Wild, O. Hübner, H.-J. Himmel, Chem. Eur. J. 2019, 25, 15988-15992.

U. Wild, O. Hübner, L. Greb, M. Enders, E. Kaifer, H.-J. Himmel, Eur. J. Org. Chem. 2018, 5910-5915.

H.-J. Himmel, Synlett 2018, 29, 1957-1977. (Review article)

U. Wild, F. Schön, H.-J. Himmel, Angew. Chem. 2017, 129, 16630-16633; Angew. Chem. Int. Ed. 2017, 56, 16410-16413.

U. Wild, S. Federle, A. Wagner, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2016, 22, 11971-11976.

Selected publications:

U. Wild, P. Walter, O. Hübner, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2020, 26, 16504-16513.

U. Wild, O. Hübner, H.-J. Himmel, Chem. Eur. J. 2019, 25, 15988-15992.

U. Wild, O. Hübner, L. Greb, M. Enders, E. Kaifer, H.-J. Himmel, Eur. J. Org. Chem. 2018, 5910-5915.

H.-J. Himmel, Synlett 2018, 29, 1957-1977. (Review article)

U. Wild, F. Schön, H.-J. Himmel, Angew. Chem. 2017, 129, 16630-16633; Angew. Chem. Int. Ed. 2017, 56, 16410-16413.

U. Wild, S. Federle, A. Wagner, E. Kaifer, H.-J. Himmel, Chem. Eur. J. 2016, 22, 11971-11976.

 

 

Bond activation processes at metal atom and metal oxide clusters, studied with the matrix isolation technique

 

Matrixisolation

The electronic structure and chemical properties of reactive molecular compounds and intermediates are heavily influenced by the environment (e.g. solvation, aggregation), especially if they are open-shell species. The matrix isolation technique offers the possibility to study the electronic structure and “intrinsic” properties of these species, free of any “extrinsic” influences such as solvation or aggregation. In difference to gas-phase studies, standard spectroscopic techniques could be used for this analysis, since the compounds could be kept in the matrices for several hours or even days if demanded. The clear separation of intrinsic and extrinsic effects on a chemical reaction under consideration allows detailed insight into the reaction pathway and therefore is of key importance for the development of synthetic strategies.

 

The compounds are isolated in the matrix like the raisins in a raisins cake. Hence the matrix is a special host material, in which host-guest interactions are reduced to a minimum by choice of frozen inert gases (e.g. Ne or Ar) as host materials. The metal atoms in the following picture could not aggregate to form solid metal, and their colors arise from electronic transitions that are only slightly shifted in energy with respect to the gas-phase.

 

If metal atoms, dimers or clusters are isolated in the absence of a reaction partner, one could analyze their electronic structure in the ground state, and also in electronically excited states, which are most important for the understanding of their reactivity. On the other hand, if the inert gas is doped with a reaction partner, one could study the reactivity of these species. Tempering of the matrix material enables diffusion of the reaction partners. For reactions which are subjected to a reaction barrier, energy could be inscribed through irradiation of the matrix, making use of the almost complete transmissibility of the matrix material in the UV and visible region.

 

The matrix is deposited on a substrate which is cooled to a very low temperature (e.g. 4 K), generally by means of a closed-cycle cryostat. To avoid the deposition of air and moisture, the matrix is integrated in a high-vacuum system. The following picture shows the home-built Heidelberg matrix isolation apparatus and a schematic drawing which illustrates the possible spectroscopic techniques which could be applied.

 

 

 

 

 

Current projects:

 

 

This project is part of the SFB 1249 “N-Heteropolyzyklen als Funktionsmaterialien”

J. Thusek, M. Hoffmann, O. Hübner, O. Tverskoy, U.H.F. Bunz, A. Dreuw, H.-J. Himmel, Chem. Eur. J. 2019, 25, 15147-15154

 

2) Reactivity of ligand-free metal dimers and clusters

One important research theme in our group is the understanding of the reactivity of “naked” (ligand-free) metal atom clusters. A metal atom is surprisingly inert, which could be explained by large symmetry barriers. One would expect the reactivity to reach a maximum when the number of atoms increases. Such studies help to understand catalytic reactions involving heterogeneous catalysts, a concept which is known as “molecular surface science”.

 

O. Hübner, H.-J. Himmel, Chem. Eur. J. 2018, 24, 8941-8961. (Review article)

O. Hübner, H.-J. Himmel, Angew. Chem. 2017, 129, 12510-12514; Angew. Chem. Int. Ed. 2017, 56, 12340-12343.

O. Hübner, H.-J. Himmel, Phys. Chem. Chem. Phys. 2016, 18, 14667-14677.

L. Manceron, O. Hübner, H.-J. Himmel, Eur. J. Inorg. Chem. 2009, 595-598.

K. Navarantnarajah, J. C. Green, H.-J Himmel, New J. Chem. 2006, 30, 1253-1262.

H.-J. Himmel, M. Reiher, Angew. Chem. 2006, 118, 6412-6437; Angew. Chem. Int. Ed. 2006, 45, 6264-6288. (Review article)

H.-J. Himmel, O. Hübner, W. Klopper, L. Manceron, Phys. Chem. Chem. Phys. 2006, 8, 2000-2011.

H.-J. Himmel, O. Hübner, W. Klopper, L. Manceron, Angew. Chem. 2006, 118, 2865-2868; Angew. Chem. Int. Ed. 2006, 45, 2799-2802.

 

 

3) Generation and spectroscopic characterization of compounds with high bond orders between two metal atoms

Example: Generation of V₂H₂ (two isomeric forms) by reaction between V₂ and H₂

O. Hübner, H.-J. Himmel, Angew. Chem. 2020, 132, 12304-12310; Angew. Chem. Int. Ed. 2020, 59, 12206-12212.

O. Hübner, H.-J. Himmel, Angew. Chem. 2017, 129, 12510-12514; Angew. Chem. Int. Ed. 2017, 56, 12340-12343.