Browsing by Author "Klopsch, Isabel"

About 20% of the ammonia production is used as the chemical feedstock for nitrogen-containing chemicals. However, while synthetic nitrogen fixation at ambient conditions has had some groundbreaking contributions in recent years, progress for the direct conversion of N-2 into organic products remains limited and catalytic reactions are unknown. Herein, the rhenium-mediated synthesis of acetonitrile using dinitrogen and ethyl triflate is presented. A synthetic cycle in three reaction steps with high individual isolated yields and recovery of the rhenium pincer starting complex is shown. The cycle comprises alkylation of a nitride that arises from N-2 splitting and subsequent imido ligand centered oxidation to nitrile via a 1-azavinylidene (ketimido) intermediate. Different synthetic strategies for intra- and intermolecular imido ligand oxidation and associated metal reduction were evaluated that rely on simple proton, electron, and hydrogen-atom transfer steps.

[ReCl3(PPh3)(2)(NCMe)] reacts with pincer ligand HN(CH2CH2PtBu2)(2) (HPNP) to five coordinate rhenium(III) complex [ReCl2(PNP)]. This compound cleaves N-2 upon reduction to give rhenium(V) nitride [Re(N)Cl(PNP)], as the first example in the coordination sphere of Re. Functionalization of the nitride ligand derived from N-2 is demonstrated by selective C N bond formation with MeOTf.

Borane-amine adducts have received considerable attention, both as vectors for chemical hydrogen storage and as precursors for the synthesis of inorganic materials. Transition metal-catalyzed ammonia borane (H3N-BH3, AB) dehydrocoupling offers, in principle, the possibility of large gravimetric hydrogen release at high rates and the formation of B-N polymers with well-defined microstructure. Several different homogeneous catalysts were reported in the literature. The current mechanistic picture implies that the release of aminoborane (e.g., Ni carbenes and Shvo's catalyst) results in formation of borazine and 2 equiv of H-2, while 1 equiv of H-2 and polyaminoborane are obtained with catalysts that also couple the dehydroproducts (e.g., Ir and Rh diphosphine and pincer catalysts). However, in comparison with the rapidly growing number of catalysts, the amount of experimental studies that deal with mechanistic details is still limited. Here, we present a comprehensive experimental and theoretical study about the mechanism of AB dehydrocoupling to polyaminoborane with ruthenium amine/amido catalysts, which exhibit particularly high activity. On the basis of kinetics, trapping experiments, polymer characterization by B-11 MQMAS solid-state NMR, spectroscopic experiments with model substrates, and density functional theory (DFT) calculations, we propose for the amine catalyst [Ru(H)(2)PMe3{HN-(CH2CH2PtBu2)(2)}] two mechanistically connected catalytic cycles that account for both metal-mediated substrate dehydrogenation to aminoborane and catalyzed polymer enchainment by formal aminoborane insertion into a H-NH2BH3 bond. Kinetic results and polymer characterization also indicate that amido catalyst [Ru(H)PMe3{N(CH2CH2PtBu2)(2)}] does not undergo the same mechanism as was previously proposed in a theoretical study.

The activation of elemental sulfur and selenium with electrophilic iridium nitride [Ir(N){N(CHCHPtBu2)(2)}](+) allowed the synthesis of thionitrosyl- and selenonitrosyliridium complexes. The crystallographic and spectroscopic characterization of this rare series of chalcogenonitrosyl complexes [Ir(NE){N(CHCHPtBu2)(2)}](+) (E = none, O, S, Se) is discussed in the context of bonding models. The experimental results indicate strong Ir-N-E multiple-bond character, in agreement with the previously proposed N-E donor-acceptor interaction as an appropriate description for the heavier chalcogens.

The activation of elemental sulfur and selenium with electrophilic iridium nitride [Ir(N){N(CHCHPtBu2)2}]+ allowed the synthesis of thionitrosyl- and selenonitrosyliridium complexes. The crystallographic and spectroscopic characterization of this rare series of chalcogenonitrosyl complexes [Ir(NE){N(CHCHPtBu2)2}]+ (E = none, O, S, Se) is discussed in the context of bonding models. The experimental results indicate strong Ir-N-E multiple-bond character, in agreement with the previously proposed N-E donor-acceptor interaction as an appropriate description for the heavier chalcogens.