Marcus Klahn

Catalytic and Kinetic Studies of the Dehydrogenation of Dimethylamine Borane with an iPr Substituted Titanocene Catalyst

In recent years, amine borane adducts have been widely discussed as promising candidates for the storage of hydrogen, owing mainly to the high gravimetric capacity (ideally up to 19.6 % in ammonia borane) and the facile release of the energy carrier under mild conditions. A significant amount of work has been reported towards the catalytic dehydrogenation of such compounds using transition metal complexes, as well as in the efficient regeneration of spent fuels. However, some of the main drawbacks for general applicability remain, such as the production of amines and boranes using environmentally benign processes and the fact that most amine borane adducts are solid and thus cannot serve as a liquid “fuel” without dilution and loss of hydrogen storage capacity. In this context, Baker et al. reported on the use of amine borane fuel blends, which remain liquid throughout the catalytic process. Moreover, addition of an ionic liquid hindered the formation of insoluble dehydrogenation products such as linear poly(aminoborane). The use of early transition metal complexes, especially of group 4 metals for the catalytic dehydrogenation of dimethylamine borane (1) was demonstrated by Manners and Chirik and co-workers for titanocene and zirconocene compounds. A study of dimethylamine borane dehydrogenation using cationic zirconocene-phosphinoaryloxide complexes was published by Wass et al. and revealed high activities (TOF up to 600 h ) and reaction times of only several minutes. Very recently, our group reported on the efficient dehydrogenation of dimethylamine borane using well-defined metallocene bis(trimethylsilyl)acetylene complexes of the type [Cp’2M(L)(h Me3SiC2SiMe3)] [M = Ti, Cp’= cyclopentadienyl (Cp) or pentamethylcyclopentadienyl (Cp*), no L; M = Zr, Cp’= Cp, L = pyridine; M = Zr, Cp’= Cp*, no L] , which eliminate the alkyne under mild conditions and thus generate the catalytically active 14 electron fragment “Cp2M”. [7] It became evident that complexes bearing Cp ligands are active whereas fully methylated Cp* species are completely inactive for dehydrogenation reactions. However, a full study of the influence of steric and electronic parameters of substituents at the Cp ligands is not available to date. The titanocene bis(trimethylsilyl)acetylene complex [(h-iPrC5H4)2Ti(h -Me3SiC2SiMe3)] (2) appeared to be worth investigating for the catalytic dehydrogenation of dimethylamine borane (Scheme 1), as it possesses the iPr group as a single sterically demanding yet moderately electron donating substituent. Complex 2 was first described by our group several years ago and reported to be isolable as a dark yellow oily liquid.

Photoassisted TiO Activation in a Decamethyltitanocene Dihydroxido Complex: Insights into the Elemental Steps of Water Splitting†

One of the major challenges for mankind concerns power supply. Renewable forms of energy are being investigated intensively as an alternative to the fossil resources commonly used nowadays. As all types of such renewable energies (except geothermal and tidal power) originate in the solar radiation, direct utilization of this ubiquitous sustainable energy source appears to be most reasonable. Apart from photovoltaic electricity production, for example, by Gr tzel cells, the conversion of sunlight into chemical energy is a very promising research topic. After the discovery of the Honda–Fujishima effect, which describes the photoassisted generation of dihydrogen and dioxygen from water using a TiO2/Pt electrode array, [3]

An Intermolecular Heterobimetallic system for Photocatalytic Water Reduction

Teamwork: A new intermolecular heterobimetallic system for photocatalytic water reduction, consisting of a photosensitizer of the type [Ru(bpy)(2)(L)](PF(6))(2) (L=bidentate ligand), a dichloro palladium complex PdCl(2)(L) serving as the water reduction catalyst, and triethyl amine as electron donor, is presented. Variations of the ligand as well as of the palladium source results in a significant improvement of the performance of the catalyst system.

Organometallic water splitting – from coordination chemistry to catalysis

Abstract This review gives an overview on the recent developments in the field of coordination chemistry of water at transition metal centres, which could give implications for a better understanding of the elementary steps of light-driven overall water splitting. Additionally, selected examples for homogeneous catalyst systems that are capable of producing hydrogen and/or oxygen from water are presented, focussing on the mechanistic aspects of water reduction and water oxidation.

Tri-tert-butyl­phospho­nium hy­droxy­tris­(penta­fluoro­phen­yl)borate

The ionic title compound, C12H28P+·C18HBF15O−, was obtained by the stoichiometric reaction of tBu3P, B(C6F5)3 and water in toluene. A weak P—H⋯O hydrogen bond is observed in the crystal structure.

2-Vinyl-pyridine-tris-(penta-fluoro-phen-yl)borane hexane monosolvate.

The title compound, C7H7N·B(C6F5)3·C6H14, was obtained by the stoichiometric reaction of 2-vinylpyridine and tris(pentafluorophenyl)borane in toluene. The formed adduct exhibits a restricted rotation along the B—N bond resulting in an asymmetry, which can be also observed in the 19F NMR spectra. The B—N distance is equivalent to the distances found for 2-methylpyridine and 2-ethylpyridine B(C6F5)3 adducts. For the final refinement, the contributions of disordered solvent molecules were removed from the diffraction data with SQUEEZE in PLATON [van der Sluis & Spek (1990). Acta Cryst. A46, 194–201; Spek (2009). Acta Cryst. D65, 148–155].