Markus Finger

Dinitrogen Splitting and Functionalization in the Coordination Sphere of Rhenium

[ReCl3(PPh3)2(NCMe)] reacts with pincer ligand HN(CH2CH2PtBu2)2 (HPNP) to five coordinate rhenium(III) complex [ReCl2(PNP)]. This compound cleaves N2 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 N2 is demonstrated by selective C-N bond formation with MeOTf.

Conversion of Dinitrogen into Acetonitrile under Ambient Conditions

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 N2 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 N2 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.

A Terminal Osmium(IV) Nitride: Ammonia Formation and Ambiphilic Reactivity

Low-valent osmium nitrides are discussed as intermediates in nitrogen fixation schemes. However, rational synthetic routes that lead to isolable examples are currently unknown. Here, the synthesis of the square-planar osmium(IV) nitride [OsN(PNP)] (PNP=N(CH2 CH2 P(tBu)2 )2 ) is reported upon reversible deprotonation of osmium(VI) hydride [Os(N)H(PNP)](+) . The Os(IV) complex shows ambiphilic nitride reactivity with SiMe3 Br and PMe3 , respectively. Importantly, the hydrogenolysis with H2 gives ammonia and the polyhydride complex [OsH4 (HPNP)] in 80 % yield. Hence, our results directly demonstrate the role of low-valent osmium nitrides and of heterolytic H2 activation for ammonia synthesis with H2 under basic conditions.

Coupling of terminal iridium nitrido complexes

The oxidative coupling of nitride ligands (N3−) to dinitrogen and its microscopic reverse, N2-splitting to nitrides, are important elementary steps in chemical transformations, such as selective ammonia oxidation or nitrogen fixation. Here an experimental and computational evaluation is provided for the homo- and heterocoupling of our previously reported iridium(IV) and iridium(V) nitrides [IrN(PNP)]n (n = 0, +1; PNP = N(CHCHPtBu2)2). All three formal coupling products [(PNP)IrN2Ir(PNP)]n (n = 0–+2) were structurally characterized. While the three coupling reactions are all thermodynamically feasible, homocoupling of [IrN(PNP)]+ is kinetically hindered. The contributing parameters to relative coupling rates are discussed providing qualitative guidelines for the stability of electron rich transition metal nitrides.

Chemical Non-Innocence of an Aliphatic PNP Pincer Ligand.

The synthesis of the divinylamido PNP nickel(II) complex [NiBr{N(CHCHPtBu2 )2 }] is reported. This compound exhibits reversible, ligand-centered oxidation and protonation reactions. The resulting pincer chemical non-innocence can be utilized for benzylic C-H hydrogen atom abstraction. The thermochemistry and kinetics of hydrogen atom transfer were examined.

Mechanism of Chemical and Electrochemical N2 Splitting by a Rhenium Pincer Complex

A comprehensive mechanistic study of N2 activation and splitting into terminal nitride ligands upon reduction of the rhenium dichloride complex [ReCl2(PNP)] is presented (PNP– = N(CH2CH2PtBu2)2–). Low-temperature studies using chemical reductants enabled full characterization of the N2-bridged intermediate [{(PNP)ClRe}2(N2)] and kinetic analysis of the N–N bond scission process. Controlled potential electrolysis at room temperature also resulted in formation of the nitride product [Re(N)Cl(PNP)]. This first example of molecular electrochemical N2 splitting into nitride complexes enabled the use of cyclic voltammetry (CV) methods to establish the mechanism of reductive N2 activation to form the N2-bridged intermediate. CV data was acquired under Ar and N2, and with varying chloride concentration, rhenium concentration, and N2 pressure. A series of kinetic models was vetted against the CV data using digital simulations, leading to the assignment of an ECCEC mechanism (where “E” is an electrochemical step and “C” is a chemical step) for N2 activation that proceeds via initial reduction to ReII, N2 binding, chloride dissociation, and further reduction to ReI before formation of the N2-bridged, dinuclear intermediate by comproportionation with the ReIII precursor. Experimental kinetic data for all individual steps could be obtained. The mechanism is supported by density functional theory computations, which provide further insight into the electronic structure requirements for N2 splitting in the tetragonal frameworks enforced by rigid pincer ligands.

CO2-assisted synthesis of non-symmetric α-diketones directly from aldehydes via C–C bond formation

CO2-assisted various symmetric and non-symmetric α-diketones have been synthesized directly from the corresponding aldehydes using transition metal-free catalysts. This method can even be applied to synthesize pharmaceuticals directly from aldehydes. The crucial role of CO2 has been investigated in detail and the mechanism is proposed on the basis of experiments and DFT calculations.

Photochemically Driven Reverse Water‐Gas Shift at Ambient Conditions mediated by a Nickel Pincer Complex

The endothermic reverse water-gas shift reaction (rWGS) for direct CO2 hydrogenation to CO is an attractive approach to carbon utilization. However, direct CO2 hydrogenation with molecular catalysts generally gives formic acid instead of CO as a result of the selectivity of CO2 insertion into M-H bonds. Based on the photochemical inversion of this selectivity, several synthetic pathways are presented for CO selective CO2 reduction with a nickel pincer platform including the first example of a photodriven rWGS cycle at ambient conditions.