Heinz Berke

Transfer Hydrogenation of Imines with Ammonia–Borane: A Concerted Double-Hydrogen-Transfer Reaction†

Ammonia–borane (H3N-BH3, AB) is considered a feasible material for chemical hydrogen storage owing to its ideally very high storage capacity (19.6 weight% H) and thus has attracted much attention. Dehydrogenations of AB were accomplished either thermally or by transition metal catalysis. Considering AB as a significantly polarized molecule, we reasoned that it could be dehydrogenated by direct reaction with a similarly polarized unsaturated compound by the rarely explored reaction mode of double H transfer (Scheme 1).

The invention of blue and purple pigments in ancient times

This tutorial review examines manmade blue and purple pigments appearing in antiquity. They were obtained by chemical synthesis from mineral starting materials and refer to chemical compounds: Egyptian Blue (CaCuSi4O10), Han Blue (BaCuSi4O10) and Han Purple (BaCuSi2O6), Maya Blue (x.indigo.(Mg,Al)4Si8(O,OH,H2O)24) and Ultramarine Blue (Na,Ca)8(AlSiO12)(S, SO4,Cl). The Egyptian and Chinese copper-based pigments are assumed to have been developed independently and are presumably an outcome of the historical developments in glazing techniques. A technology transfer from Egypt into China cannot be fully excluded but, based on the facts acquired up to now, looks less probable.

Catalytic CO2 Activation Assisted by Rhenium Hydride/B(C6F5)3 Frustrated Lewis Pairs—Metal Hydrides Functioning as FLP Bases

Reaction of 1 with B(C6F5)3 under 1 bar of CO2 led to the instantaneous formation of the frustrated Lewis pair (FLP)-type species [ReHBr(NO)(PR3)2(η(2)-O═C═O-B(C6F5)3)] (2, R = iPr a, Cy b) possessing two cis-phosphines and O(CO2)-coordinated B(C6F5)3 groups as verified by NMR spectroscopy and supported by DFT calculations. The attachment of B(C6F5)3 in 2a,b establishes cooperative CO2 activation via the Re-H/B(C6F5)3 Lewis pair, with the Re-H bond playing the role of a Lewis base. The Re(I) η(1)-formato dimer [{Re(μ-Br)(NO)(η(1)-OCH═O-B(C6F5)3)(PiPr3)2}2] (3a) was generated from 2a and represents the first example of a stable rhenium complex bearing two cis-aligned, sterically bulky PiPr3 ligands. Reaction of 3a with H2 cleaved the μ-Br bridges, producing the stable and fully characterized formato dihydrogen complex [ReBrH2(NO)(η(1)-OCH═O-B(C6F5)3)(PiPr3)2] (4a) bearing trans-phosphines. Stoichiometric CO2 reduction of 4a with Et3SiH led to heterolytic splitting of H2 along with formation of bis(triethylsilyl)acetal ((Et3SiO)2CH2, 7). Catalytic reduction of CO2 with Et3SiH was also accomplished with the catalysts 1a,b/B(C6F5)3, 3a, and 4a, showing turnover frequencies (TOFs) between 4 and 9 h(-1). The stoichiometric reaction of 4a with the sterically hindered base 2,2,6,6-tetramethylpiperidine (TMP) furnished H2 ligand deprotonation. Hydrogenations of CO2 using 1a,b/B(C6F5)3, 3a, and 4a as catalysts gave in the presence of TMP TOFs of up to 7.5 h(-1), producing [TMPH][formate] (11). The influence of various bases (R2NH, R = iPr, Cy, SiMe3, 2,4,6-tri-tert-butylpyridine, NEt3, PtBu3) was studied in greater detail, pointing to two crucial factors of the CO2 hydrogenations: the steric bulk and the basicity of the base.

Electronic Communication in C4‐Bridged Binuclear Complexes with Paramagnetic Bisphosphane Manganese End Groups

Building blocks for conducting polymers or NLO materials are the linear, unsaturated carbon chain bridged manganese complexes 1(n+) (n=0-2). All oxidation states were investigated spectroscopically and by X-ray structure determinations. The analytical data confirm a communication of the electrons over the C(4) chain-a prerequisite for electrical conductivity and NLO properties of oligo- or polymeric materials.

Amino olefin nickel(I) and nickel(0) complexes as dehydrogenation catalysts for amine boranes

A rare paramagnetic organometallic nickel(I) olefin complex can be isolated using the ligand bis(5H-dibenzo[a,d]cyclohepten-5-yl)amine. This complex and related nickel(0) hydride complexes show very high catalytic activity in the dehydrogenation of dimethylamino borane with release of one equivalent of dihydrogen.

Azulene based metal–organic frameworks for strong adsorption of H2

Two Zn MOFs, MOF-645 and MOF-646, comprised of polarized 1,3-azulenedicarboxylate were synthesized. The guest free MOF-646 showed strong MOF-H(2) interactions (7.8-6.8 kJ mol(-1)), which revealed the significant impact of internally polarized azulene backbone to stabilized H(2) molecules in the framework.

Triptycene based luminescent metal–organic gels for chemosensing

We report a novel luminescent Al-based metal-organic gel G1 comprising 1,4,5,8-triptycenetetracarboxylic acid, which exhibits highly sensitive detection towards nitro aromatic compounds particularly picric acid. Furthermore, under identical reaction conditions, using a Co(II) salt instead, a novel 3D framework material, trip-MOF-1, was isolated and its sensitivity towards picric acid was also evaluated.

Field-induced conductance switching by charge-state alternation in organometallic single-molecule junctions

Charge transport through single molecules can be influenced by the charge and spin states of redox-active metal centres placed in the transport pathway. These intrinsic properties are usually manipulated by varying the molecules electrochemical and magnetic environment, a procedure that requires complex setups with multiple terminals. Here we show that oxidation and reduction of organometallic compounds containing either Fe, Ru or Mo centres can solely be triggered by the electric field applied to a two-terminal molecular junction. Whereas all compounds exhibit bias-dependent hysteresis, the Mo-containing compound additionally shows an abrupt voltage-induced conductance switching, yielding high-to-low current ratios exceeding 1,000 at bias voltages of less than 1.0 V. Density functional theory calculations identify a localized, redox-active molecular orbital that is weakly coupled to the electrodes and closely aligned with the Fermi energy of the leads because of the spin-polarized ground state unique to the Mo centre. This situation provides an additional slow and incoherent hopping channel for transport, triggering a transient charging effect in the entire molecule with a strong hysteresis and large high-to-low current ratios.

Structural characterization of Group 4 transition metal halide bis- Arduengo carbene complexes MCl4L2: X-ray crystal structure analyses and DFT calculations

Abstract The Arduengo carbenes 1,3-diisopropyl-, 1-methyl-3-(1-methylpropyl)-, and 1-methyl-3-(2,4,6-trimethylbenzyl)imidazol-2-ylidene (3a–c) were reacted with MCl4(thf)2 (M=Zr, Hf) to yield the respective trans-(imidazol-2-ylidene)MCl4 complexes 4a–c (M=Zr) and 5a (M=Hf), respectively. These four Arduengo carbene-Group 4 metal halide complexes were characterized by X-ray diffraction. The pairs of carbene ligands are trans-positioned in a pseudo-octahedral coordination geometry at the Group 4 metals, and adopt a conformational orientation in the solid state where the two five-membered heterocycles are arranged coplanar and bisecting the ClMCl angle. DFT calculations have shown that the Arduengo carbenes serve as pure σ-donor ligands in these complexes and that the preferred conformational ligand orientation is based on steric reasons. The complexes 4a–c form moderately active ethene polymerization catalysts when activated with a large excess of methylalumoxane in toluene.

Facile metal free regioselective transfer hydrogenation of polarized olefins with ammonia borane

Transfer hydrogenation of polarized olefins bearing strongly electron-withdrawing groups on one side of the double bond was achieved with ammonia borane under mild conditions without using a catalyst. Mechanistic studies proved the character of the direct H transfers proceeding stepwise with a unique hydroboration intermediate and hydride before proton transfer.