Research 

I. Small Molecule Activation and Functionalization

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Small molecules such as N2, O2, and CO2 are ubiquitous and frequently take part in element cycles and various metabolic processes. Selective functionalization (conversion or utilization) of these molecules (H2, O2, CO2, N2O) into value-added products holds promises for addressing current sustainability issues. Our ongoing research focus of H2 activation and its implication in CO2 reduction emphasizes developing a new class of innovative main-group (MG) compounds. The idea of designing such MG-compounds originates mainly from preliminary experimental findings, which are further analyzed by theoretical methods. Some seemingly very simple main group compounds (e. g., non-quenched Lewis acid-base pairs, MG-compounds bearing a formal positively charged boron or silicon) have shown to be very effective in activating small molecules. While some remarkable advances have been made during the past few years, the field is still in its infancy. More work is required to develop a further understanding of the underlying concept of bond activation by MG-compounds. New molecular systems, which activate inherently unreactive small molecules in a catalytic fashion with an appreciable TON/ TOF, are therefore highly desired. Our research addresses fundamental questions related to the small molecule activation by MG-compounds supported by un-conventional carbon-donor ligands (cf. III). 

Chem. Commun. 2013, 49, 59875989.

Chem. Commun. 2013, 49, 94409442.

Nature, 2010, 463, 171177.

 

II. Low-valent Main Group Chemistry

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Compounds featuring a low-valent main-group element have attracted immense interests during the past few years. Application of such compounds for small molecule activation is, however, so far limited to stoichiometric reactions. Similarly,  systematic investigation of these exotic species as building-blocks to new molecules as well as materials is yet to be done. Our research particularly emphasizes compounds featuring a low-valent Group 13-15 element(s). A special emphasis is given to radical, biradical and radicaloid compounds.

Chem. Eur. J.  2018, 24, 380387

Chem. Eur. J.  201723, 90449047.

Eur. J. Inorg. Chem., 2014, 49214926. 

Acc. Chem. Res., 2013, 46, 444456.

 

III. Ligand Design and Exploration

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Advancement of new catalytic structures (pre-catalysts) and functional materials is directly linked to the development of a diverse class of innovative ligands.  NHCs rank among the most powerful ligands for transition metal (TM) catalyzed chemical transformations as well as very efficient organocatalysts on their own right. Similarly, a number of striking compounds featuring a low-valent main group (MG) element, which are generally considered as highly reactive transient species, have been stabilized using NHCs. The success of NHCs in TM-catalysis as well as in taming extremely reactive MG-species is largely due to their remarkably strong electron donor ability. NHCs, in general, coordinate to a metal at the C2 position, which are termed as classical NHCs. A rather new class of carbenes is 1,3-imidazol-derived abnormal-NHCs (also known as Mesoionic Carbenes, MICs). Abnormal-NHCs (aNHCs) coordinate to metals at the unusual (C4 or C5) position and are perhaps the strongest NHCs known so far. Interestingly, most of the aNHC-metal complexes reported so far were the results of serendipitous discoveries and their rational synthesis routes are yet to be developed. This part of our research program aims at developing rational synthesis protocols to aNHCs and their metal complexes as well as to study the impact of the donor strength of aNHCs on the property, stability and activity of TM-complexes. Designing conceptually new carbon-donor ligands, incorporating the non-innocence in carbon-donor ligands, and developing other hybrid ligands equally highlight our research activities in this direction.

 Chem. Eur. J.  2018, 24,  doi: 10.1002/chem.201800260

Dalton Trans201645, 1608116095. Perspective

Chem. Eur. J. 201521, 42474251.

 

IV. Organometallic Catalysis

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The metal-ligand cooperativity has been applied to enhance, tune, and control the reactivity of TM-complexes. NHCs have emerged as versatile ligands in organometallic and transition metal catalysis. It would not be exaggerate to state that NHC-metal complexes have conquered almost all areas of catalysis research beyond initially recognized cross-coupling and metathesis reactions. While classical NHCs have gained enormous significance as ancillary ligands in TM-catalysis many challenges still remain. Some notable issues are (i) developing sustainable catalysts featuring earth-abundant base (Fe, Co, Ni) metals, (ii) expanding the substrate scope, (iii) milder reactions conditions, (iv) and establishing new reactivity. Due to the superior donor ability, enormous potential of aNHCs in sustainable catalysis and other applications can be envisioned. Our main focus in this direction is to establish rational protocols to  aNHC-TM complexes and to explore their catalytic activity for challenging chemical transformations. 

Catalysts 20177, 262. doi:10.3390/catal7090262 (Special Issue "Tailor-Made NHC Ligands").

Dalton Trans.  201746, 1202712031.

Dalton Trans. 2017, 46, 7664–7667.

Organometallics 201635, 34213429.

Chem. Eur. J. 201521, 42474251.

 

V. Molecular Materials 

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Over the last several decades, hydrogen has emerged as an ideal successor to fossil fuels because of its lightweight and clean combustion properties. Storage of dihydrogen is nevertheless highly demanding and poses a serious safety threat. One of the milestones for hydrogen storage was the development of lightweight solids that could reversibly release H2. Among chemical hydrogen storage materials, special emphasis has been given to those composed of the lighter elements such as carbon, nitrogen, and boron. In these materials, hydrogen is “released” by a chemical reaction and the hydrogen is “recaptured” by a chemical processing pathway. Ammonia-borane (AB) (NH3BH3) is a white solid, which releases hydrogen upon heating is considered to be a promising hydrogen storage material due to its stability and high gravimetric content of hydrogen (19.6 wt%). There are, however, a number of issues that need to be addressed. We aim at developing and investigating new molecular systems as potential chemical hydrogen storage materials.

Eur. J. Inorg. Chem., 2014, 49214926. 

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Anorganische Chemie und Strukturchemie (ACS)

Fakultät für Chemie, Universität Bielefeld

E4-124, Universiätsstr. 25

D-33615 Bielefeld

Tel: 0049-521-106-6167 (Off.)

Fax:0049-521-106-6026 

E-mail: rghadwal@uni-bielefeld.de

© Ghadwal Research Group 2015