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We report the preparation of a new organic σ-donor with a C 6 H 4 -linker between an N-heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N-heterocyclic quinodimethane (NHQ). The aromatization of the C 6 H 4 -linker provides a decisive driving force for the reaction of the NHQ with an electrophile and renders the NHQ significantly more basic than analogous NHCs or N-heterocyclic olefins (NHOs), as shown by DFT computations and competition experiments. In solution, the NHQ undergoes an unprecedented dehydrogenative head-to-head dimerization by C-C coupling of the methylidene groups. DFT computations indicate that this reaction proceeds via an open-shell singlet pathway revealing the diradical character of the NHQ. The product of this dimerization can be described as conjugated N-heterocyclic bis-quinodimethane, which according to cyclic voltammetry is a strong organic reducing agent (E 1/2 =-1.71 V vs. Fc/Fc + ) and exhibits a remarkable small singlet-triplet gap of ΔE S→T =4.4 kcal mol -1 ., (© 2023 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Nitride complexes are key species in homogeneous nitrogen fixation to NH 3 via stepwise proton-coupled electron transfer (PCET). In contrast, direct generation of nitrogenous organic products from N 2 -derived nitrides requires new strategies to enable efficient reductive nitride transfer in the presence of organic electrophiles. We here present a 2-step protocol for the conversion of dinitrogen to benzonitrile. Photoelectrochemical, reductive N 2 splitting produces a rhenium(V) nitride with unfavorable PCET thermochemistry towards ammonia generation. However, N-benzoylation stabilizes subsequent reduction as a basis for selective nitrogen transfer in the presence of the organic electrophile and Brønsted acid at mild reduction potentials. This work offers a new strategy for photoelectrosynthetic nitrogen fixation beyond ammonia-to yield nitrogenous organic products., (© 2022 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH.)

Outer-sphere radical hydrogenation of olefins proceeds via stepwise hydrogen atom transfer (HAT) from transition metal hydride species to the substrate. Typical catalysts exhibit M-H bonds that are either too weak to efficiently activate H 2 or too strong to reduce unactivated olefins. This contribution evaluates an alternative approach, that starts from a square-planar cobalt(II) hydride complex. Photoactivation results in Co-H bond homolysis. The three-coordinate cobalt(I) photoproduct binds H 2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H 2 via HAT., (© 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH.)

The splitting of N 2 into well-defined terminal nitride complexes is a key reaction for nitrogen fixation at ambient conditions. In continuation of our previous work on rhenium pincer mediated N 2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl 2 ( L 2)] { L 2 = N(CHCHP t Bu 2 ) 2 - } is reported. The electrochemical characterization of [ReCl 2 ( L 2)] and comparison with our previously reported platform [ReCl 2 ( L 1)] { L 1 = N(CH 2 CH 2 P t Bu 2 ) 2 - } provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N 2 derived nitride complex [Re(N)Cl( L 2)] with electrophiles is presented., (© 2020 The Authors. Published by Wiley‐VCH Verlag GmbH & Co. KGaA.)

A terminal iridium oxo complex with an open-shell (S=1) ground state was isolated upon hydrogen atom transfer (HAT) from the respective iridium(II) hydroxide. Electronic structure examinations support large spin delocalization to the oxygen atom. Selected oxo transfer reactions indicate the ambiphilic reactivity of the iridium oxo moiety. Calorimetric and computational examinations of the HAT revealed a bond dissociation free energy for the IrO-H bond that is sufficient for hydrogen atom abstraction towards C-H bonds and small contributions from entropy and spin-orbit coupling to the HAT thermochemistry., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The isolation of rhenium(V) complexes with terminal phosphide and arsenide ligands was achieved upon decarbonylation of rhenium(III) pnictaethynolates. One-electron oxidation of the pnictide complexes yielded Pn-Pn (Pn=P, As) coupling products, which were spectroscopically and crystallographically characterized. Computational bond analysis suggests that these complexes are best described as {Pn 2 } 0 complexes that are stabilized by donor-acceptor interactions with the metal and a pyrazole ligand., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The isolable complex [Os(PHMes*)H(PNP)] (Mes*=2,4,6- t Bu 3 C 6 H 3 ; PNP=N{CHCHP t Bu 2 } 2 ) exhibits high phosphinyl radical character. This compound offers access to the phosphinidene complex [Os(PMes*)H(PNP)] by P-H proton coupled electron transfer (PCET). The P-H bond dissociation energy (BDE) was determined by isothermal titration calorimetry and supporting DFT computations. The phosphinidene product exhibits electrophilic reactivity as demonstrated by intramolecular C-H activation., (© 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.)

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N-N bond energy. Photochemical strategies that drive N 2 fixation are scarcely developed. Here, the synthesis of a dinuclear N 2 -bridged complex is presented upon reduction of a rhenium(III) pincer platform. Photochemical splitting into terminal nitride complexes is triggered by visible light. Clean nitrogen transfer with benzoyl chloride to free benzamide and benzonitrile is enabled by cooperative 2 H + /2 e - transfer of the pincer ligand. A three-step cycle is demonstrated for N 2 to nitrile fixation that relies on electrochemical reduction, photochemical N 2 -splitting and thermal nitrogen transfer., (© 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The endothermic reverse water-gas shift reaction (rWGS) for direct CO 2 hydrogenation to CO is an attractive approach to carbon utilization. However, direct CO 2 hydrogenation with molecular catalysts generally gives formic acid instead of CO as a result of the selectivity of CO 2 insertion into M-H bonds. Based on the photochemical inversion of this selectivity, several synthetic pathways are presented for CO selective CO 2 reduction with a nickel pincer platform including the first example of a photodriven rWGS cycle at ambient conditions., (© 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The coupling of electron- and proton-transfer steps provides a general concept to control the driving force of redox reactions. N 2 splitting of a molybdenum dinitrogen complex into nitrides coupled to a reaction with Brønsted acid is reported. Remarkably, our spectroscopic, kinetic, and computational mechanistic analysis attributes N-N bond cleavage to protonation in the periphery of an amide pincer ligands rather than the {Mo-N 2 -Mo} core. The strong effect on electronic structure and ultimately the thermochemistry and kinetic barrier of N-N bond cleavage is an unusual case of a proton-coupled metal-to-ligand charge transfer process, highlighting the use of proton-responsive ligands for nitrogen fixation., (© 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

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., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

An experimental and theoretical study of the base-stabilized disilene 1 is reported, which forms at low temperatures in the disproportionation reaction of Si2 Cl6 or neo-Si5 Cl12 with equimolar amounts of NMe2 Et. Single-crystal X-ray diffraction and quantum-chemical bonding analysis disclose an unprecedented structure in silicon chemistry featuring a dative Si→Si single bond between two silylene moieties, Me2 EtN→SiCl2 →Si(SiCl3 )2 . The central ambiphilic SiCl2 group is linked by dative bonds to the amine donor and the bis(trichlorosilyl)silylene acceptor, which leads to push-pull stabilization. Based on experimental and theoretical examinations a formation mechanism is presented that involves an autocatalytic reaction of the intermediately formed anion Si(SiCl3 )3 (-) with neo-Si5 Cl12 to yield 1., (© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

The iridium dihydride [Ir(H)2 (HPNP)](+) (PNP=N(CH2 CH2 PtBu2 )2 ) reacts with O2 to give the unusual, square-planar iridium(III) hydroxide [Ir(OH)(PNP)](+) and water. Regeneration of the dihydride with H2 closes a quasi-catalytic synthetic oxygen-reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate-limiting O2 coordination followed by H-transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent MH bonds of a mononuclear metal complex., (© 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

Mixing CuCl2⋅2 H2O with benzylamine in alcoholic solutions led to an extremely colorful chemistry caused by the formation of a large number of different complexes. Many of these different species could be structurally characterized. These include relatively simple compounds such as [Cu(L(1))4Cl2] (L(1) = benzylamine) and (HL(1))2[CuCl4]. Most interestingly is the easy formation of two cluster complexes, one based on two cluster units Cu4OCl6(L(1))4 connected through one [Cu(L(1))2Cl2] complex and one based on a cubane-type cluster ([Cu4O4](C11H14)4Cl4). Both clusters proved to be highly reactive in a series of oxidation reactions of organic substrates by using air or peroxides as oxidants. Furthermore, it was possible to isolate and structurally characterize ([Cu(L(1))Cl]3 and [Cu(benz2mpa)2]CuCl2 (benz2mpa = benzyl-(2-benzylimino-1-methyl-propylidene)-amine), two copper(I) complexes that formed in solution, demonstrating the high redox activity of the cluster systems. In addition, it was possible to solve the molecular structures of the compounds Cu4OCl6(MeOH)4, [Cu(MeOH)2Cl2], [Cu(aniline)2Cl2], and an organic side product (HC13 H19 NOCl). In fact all determined structures are of a known type but the chemical relation between these compounds could be explained for the first time. The paper describes these different compounds and their chemical equilibria. Some of these complexes seem to be relevant in catalytic oxidation reactions and their reactivity is discussed in more detail., (Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.)

We have studied the formation of several N-acetyl-4-(dimethylamino)pyridine (DMAP) salts (with Cl(-), CH(3)COO(-), and CF(3)COO(-) counterions), which are considered to be the catalytically active species in DMAP-catalyzed acetylation reactions of alcohols. Combined crystal structure analyses, variable temperature matrix IR and NMR spectroscopy as well as computational techniques at the UAHF-PCM-B3LYP/6-311+G(d,p)//B3LYP/6-31G(d) level were utilized to examine the structures and dynamics of salt formation. We found clear evidence for the formation of tight ion pairs that are stabilized by dynamic hydrogen-bonding interactions. In nonpolar solvents, the nucleophilicity of acetate in its N-acetyl-DMAP salt only allows a steady-state concentration smaller 1% at room temperature. Thus, we propose additional hydrogen-bonding interactions with alcohols to be the key stabilization factor in subsequent acetylations.

Detailed mechanistic studies on the ligand hydroxylation reaction mediated by a copper bis(imine) complex are presented. Starting from a structural analysis of the CuI complex and the CuII product with a hydroxylated ligand, the optical absorption and vibrational spectra of starting material and product are analyzed. Kinetic analysis of the ligand hydroxylation reaction shows that O2 binding is the rate-limiting step. The reaction proceeds much faster in methanol than in acetonitrile. Moreover, an inverse kinetic isotope effect (KIE) is evidenced for the reaction in acetonitrile, which is attributed to a sterically congested transition state leading to the peroxo adduct. In methanol, however, no KIE is observed. A DFT analysis of the oxygenation reaction mediated by the micro-eta2:eta2 peroxo core demonstrates that the major barrier after O2 binding corresponds to electrophilic attack on the arene ring. The relevant orbital interaction occurs between the sigma* orbital of the Cu2O2 unit and the HOMO of the ligand. On the basis of the activation energy for the rate-limiting step (18.3 kcal mol(-1)) this reaction is thermally allowed, in agreement with the experimental observation. The calculations also predict the presence of a stable dienone intermediate which, however, escaped experimental detection so far. Reasons for these findings are considered. The implications of the results for the mechanism of tyrosinase are discussed.