An N-Heterocyclic Quinodimethane: A Strong Organic Lewis Base Exhibiting Diradical Reactivity.

We report the preparation of a new organic σ-donor with a C6H4-linker between an N-heterocyclic carbene (NHC) and an exocyclic methylidene group, which we term N-heterocyclic quinodimethane (NHQ). The aromatization of the C6H4-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 (E1/2=-1.71 V vs. Fc/Fc+) and exhibits a remarkable small singlet-triplet gap of ΔES→T=4.4 kcal mol-1.

Halide Effects in Reductive Splitting of Dinitrogen with Rhenium Pincer Complexes.

Transition metal halide complexes are used as precursors for reductive N2 activation up to full splitting into nitride complexes. Distinct halide effects on the redox properties and yields are frequently observed yet not well understood. Here, an electrochemical and computational examination of reductive N2 splitting with the rhenium(III) complexes [ReX2(PNP)] (PNP = N(CH2CH2PtBu2)2 and X = Cl, Br, I) is presented. As previously reported for the chloride precursor ( J. Am. Chem. Soc.2018, 140, 7922), the heavier halides give rhenium(V) nitrides upon (electro-)chemical reduction in good yields yet with significantly anodically shifted electrolysis potentials along the halide series. Dinuclear, end-on N2-bridged complexes, [{ReX(PNP)}2(µ-N2)], were identified as key intermediates in all cases. However, while the chloride complex is exclusively formed via 2-electron reduction and ReIII/ReI comproportionation, the iodide system also reacts via an alternative ReII/ReII-dimerization mechanism at less negative potentials. This alternative pathway relies on the absence of the potential inversion after reduction and N2 activation that was observed for the chloride precursor. Computational analysis of the relevant ReIII/II and ReII/I redox couples by energy decomposition analysis attributes the halide-induced trends of the potentials to the dominating electrostatic Re-X bonding interactions over contributions from charge transfer.

Photoelectrochemical Conversion of Dinitrogen to Benzonitrile: Selectivity Control by Electrophile- versus Proton-Coupled Electron Transfer.

Nitride complexes are key species in homogeneous nitrogen fixation to NH3 via stepwise proton-coupled electron transfer (PCET). In contrast, direct generation of nitrogenous organic products from N2 -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 N2 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.

Characterization of copper complexes with derivatives of the ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) and their reactivity towards oxygen.

A series of copper(I) complexes with ligands derived from the tripodal ligand (2-aminoethyl)bis(2-pyridylmethyl)amine (uns-penp) have been structurally characterized and their redox chemistry analyzed by cyclic voltammetry. While the redox potentials of most of the complexes were similar their reactivity towards dioxygen was quite different. While the complex with a ferrocene derived ligand of uns-penp reacted in solution at low temperatures in a two-step reaction from the preliminary formed mononuclear end-on superoxido complex to a quite stable dinuclear peroxido complex it did not react with dioxygen in the solid state. Other complexes also did not react with dioxygen in the solid state while some showed a reversible formation to a green compound, indicating formation of an end-on superoxido complex that unfortunately so far could not be characterized. In contrast, copper complexes with the Me2uns-penp and Et-iProp-uns-penp formed dinuclear peroxido complexes in a solid-state reaction. While the reaction of dioxygen with the [Cu(Me2uns-penp]BPh4 was quite slow an instant reaction took place for [Cu(Et-iProp-uns-penp]BPh. Very unusual, it turned out that crystals of the copper(I) complex that could be structurally characterized still were crystalline when reacted with dioxygen. Therefore, it was possible to solve the structure of the corresponding dinuclear peroxido complex directly from the same batch of crystals. The crystalline structures of the copper(I) and copper(II) complex revealed that the reason for this is the fact, that the copper(I) complex is kind of preorganized for the uptake of dioxygen and does not really change in its overall structure when being oxidized.

Cyanate Formation via Photolytic Splitting of Dinitrogen.

Light-driven N2 cleavage into molecular nitrides is an attractive strategy for synthetic nitrogen fixation. However, suitable platforms are rare. Furthermore, the development of catalytic protocols via this elementary step suffers from poor understanding of N-N photosplitting within dinitrogen complexes, as well as of the thermochemical and kinetic framework for coupled follow-up chemistry. We here present a tungsten pincer platform, which undergoes fully reversible, thermal N2 splitting and reverse nitride coupling, allowing for experimental derivation of thermodynamic and kinetic parameters of the N-N cleavage step. Selective N-N splitting was also obtained photolytically. DFT computations allocate the productive excitations within the {WNNW} core. Transient absorption spectroscopy shows ultrafast repopulation of the electronic ground state. Comparison with ground-state kinetics and resonance Raman data support a pathway for N-N photosplitting via a nonstatistically vibrationally excited ground state that benefits from vibronically coupled structural distortion of the core. Nitride carbonylation and release are demonstrated within a full synthetic cycle for trimethylsilylcyanate formation directly from N2 and CO.

Photo-Initiated Cobalt-Catalyzed Radical Olefin Hydrogenation.

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 H2 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 H2 to give a dihydrogen complex, which is a strong hydrogen atom donor, enabling the stepwise hydrogenation of both styrenes and unactivated aliphatic olefins with H2 via HAT.

Examination of Protonation-Induced Dinitrogen Splitting by in Situ EXAFS Spectroscopy.

The splitting of dinitrogen into nitride complexes emerged as a key reaction for nitrogen fixation strategies at ambient conditions. However, the impact of auxiliary ligands or accessible spin states on the thermodynamics and kinetics of N-N cleavage is yet to be examined in detail. We recently reported N-N bond splitting of a {Mo(µ2:η1:η1-N2)Mo}-complex upon protonation of the diphosphinoamide auxiliary ligands. The reactivity was associated with a low-spin to high-spin transition that was induced by the protonation reaction in the coordination periphery, mainly based on computational results. Here, this proposal is evaluated by an XAS study of a series of linearly N2 bridged Mo pincer complexes. Structural characterization of the transient protonation product by EXAFS spectroscopy confirms the proposed spin transition prior to N-N bond cleavage.

A platinum(II) metallonitrene with a triplet ground state.

Metallonitrenes (M-N) are complexes with a subvalent atomic nitrogen ligand that have been proposed as key reactive intermediates in nitrogen atom transfer reactions. However, in contrast to the common classes of nitride complexes (M≡N) and organic nitrenes (R-N), structurally and spectroscopically well defined 'authentic' metallonitrenes with a monovalent atomic nitrogen ligand remain elusive. Here we report that the photolysis of a platinum(II) pincer azide complex enabled the crystallographic, spectroscopic, magnetic and computational characterization of a metallonitrene that is best described as a singly bonded atomic nitrogen diradical ligand bound to platinum(II). The photoproduct exhibits selective C-H, B-H and B-C nitrogen atom insertion reactivity. Despite the subvalent metallonitrene character, mechanistic analysis for aldehyde C-H amidation shows nucleophilic reactivity of the N-diradical ligand. Ambiphilic reactivity of the metallonitrene is indicated by reactions with CO and PMe3 to form isocyanate and phosphoraneiminato platinum(II) complexes, respectively.

(Electro-)chemical Splitting of Dinitrogen with a Rhenium Pincer Complex.

The splitting of N2 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 N2 splitting, nitrogen activation and cleavage upon (electro)chemical reduction of [ReCl2(L2)] {L2 = N(CHCHPtBu2)2 -} is reported. The electrochemical characterization of [ReCl2(L2)] and comparison with our previously reported platform [ReCl2(L1)] {L1 = N(CH2CH2PtBu2)2 -} provides mechanistic insight to rationalize the dependence of nitride yield on the reductant. Furthermore, the reactivity of N2 derived nitride complex [Re(N)Cl(L2)] with electrophiles is presented.

The Impact of a Proton Relay in Binuclear α-Diimine-Mn(CO)3 Complexes on the CO2 Reduction Catalysis.

Herein, we describe the redox chemistry of bi- and mononuclear α-diimine-Mn(CO)3 complexes with an internal proton source in close proximity to the metal centers and their catalytic activity in the electrochemically driven CO2 reduction reactions. In order to address the impact of the two metal sites and of the proton source, we investigate a binuclear complex with phenol moiety, 1, a binuclear Mn complex with methoxyphenol unit instead, 2, and the mononuclear analogue with a phenol unit, 3. Spectroelectrochemical investigation of the complexes in dmf under a nitrogen atmosphere indicates that 1 and 3 undergo a reductive H2 formation forming [Mn2(H-1L1)(CO)6Br] and [Mn(H-1L3)(CO)3], respectively, which is redox neutral for the complex and equivalent to a deprotonation of the phenol unit. The reaction likely proceeds via internal proton transfer from the phenol moiety to the reduced metal center forming a Mn-H species. 2 dimerizes during reduction, forming [Mn2(L2)(CO)6]2, but 1 and 3 do not. Reduction of 1, 2, and 3 is accompanied by bromide loss, and the final species represent [Mn2(H-1L1)(CO)6]3-, [Mn2(L2)(CO)6]2-, and [Mn(H-1L3)(CO)3]2-, respectively. 1 and 2 are active catalysts in the electrochemical CO2 reduction reaction, whereas 3 decomposes quickly under an applied potential. Thus, the second redox active unit is crucial for enhanced stability. The proton relay in 1 alters the kinetics for the 2H+/2e- reduced products of CO2 in dmf/water mixtures. For 2, CO is the only product, whereas formate and CO are formed in similar amounts, 40% and 50%, respectively, in the presence of 1. Thus, the reaction rate for the internal proton transfer from the phenol moiety to the metal center forming the putative Mn-H species and subsequent CO2 insertion as well as the reaction rate of the reduced metal center with CO2 forming CO are similar. The overpotential with regard to the standard redox potential of CO2 to CO and the observed overall rate constant for catalysis at scan rates of 0.1 V s-1 are higher with 1 than with 2, that is, the OH group is beneficial for catalysis due to the internal proton transfer.

A Terminal Iridium Oxo Complex with a Triplet Ground State.

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.

Oxidative Coupling of Terminal Rhenium Pnictide Complexes.

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 {Pn2 }0 complexes that are stabilized by donor-acceptor interactions with the metal and a pyrazole ligand.

Interconversion of Phosphinyl Radical and Phosphinidene Complexes by Proton Coupled Electron Transfer.

The isolable complex [Os(PHMes*)H(PNP)] (Mes*=2,4,6-t Bu3 C6 H3 ; PNP=N{CHCHPt Bu2 }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.

A bio-inspired imidazole-functionalised copper cage complex.

An imidazole-functionalised cage is synthesised that can coordinate to Cu(i). X-ray analysis reveals a T-shaped coordination of copper by the imidazole ligands reminiscent of the coordination geometry found in enzymatic active sites. This cage complex can catalyse the oxidation of benzylic alcohols to benzaldehydes utilizing oxygen as the terminal oxidant.

Selectivity of tungsten mediated dinitrogen splitting vs. proton reduction.

Mo complexes are currently the most active catalysts for nitrogen fixation under ambient conditions. In comparison, tungsten platforms are scarcely examined. For active catalysts, the control of N2 vs. proton reduction selectivities remains a difficult task. We here present N2 splitting using a tungsten pincer platform, which has been proposed as the key reaction for catalytic nitrogen fixation. Starting from [WCl3(PNP)] (PNP = N(CH2CH2PtBu2)2), the activation of N2 enabled the isolation of the dinitrogen bridged redox series [(N2){WCl(PNP)}2]0/+/2+. Protonation of the neutral complex results either in the formation of a nitride [W(N)Cl(HPNP)]+ or H2 evolution and oxidation of the W2N2 core, respectively, depending on the acid and reaction conditions. Examination of the nitrogen splitting vs. proton reduction selectivity emphasizes the role of hydrogen bonding of the conjugate base with the protonated intermediates and provides guidelines for nitrogen fixation.

Metal-Ligand Cooperative Synthesis of Benzonitrile by Electrochemical Reduction and Photolytic Splitting of Dinitrogen.

Thermal nitrogen fixation relies on strong reductants to overcome the extraordinarily large N-N bond energy. Photochemical strategies that drive N2 fixation are scarcely developed. Here, the synthesis of a dinuclear N2 -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 N2 to nitrile fixation that relies on electrochemical reduction, photochemical N2 -splitting and thermal nitrogen transfer.

Hydrosilane Synthesis by Catalytic Hydrogenolysis of Chlorosilanes and Silyl Triflates.

Hydrogenolysis of the chlorosilanes and silyl triflates (triflate = trifluoromethanesulfonate, OTf-) Me3- nSiX1+ n (X = Cl, OTf; n = 0, 1) to hydrosilanes at mild conditions (4 bar of H2, room temperature) is reported using low loadings (1 mol %) of the bifunctional catalyst [Ru(H)2CO( HPNP iPr)] ( HPNP iPr = HN(CH2CH2P( iPr)2)2). Endergonic chlorosilane hydrogenolysis can be driven by chloride removal, e.g., with NaBArF4 [BArF4- = B(C6H3-3,5-(CF3)2)4-]. Alternatively, conversion to silyl triflates enables facile hydrogenolysis with NEt3 as the base, giving Me3SiH, Me2SiH2, and Me2SiHOTf, respectively, in high yields. An outer-sphere mechanism for silyl triflate hydrogenolysis is supported by density functional theory computations. These protocols provide key steps for synthesis of the valuable hydrochlorosilane Me2SiClH, which can also be directly obtained in yields of over 50% by hydrogenolysis of chlorosilane/silyl triflate mixtures.

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(CH2CH2P tBu2)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.

An iridium(iii/iv/v) redox series featuring a terminal imido complex with triplet ground state.

The iridium(iii/iv/v) imido redox series [Ir(NtBu){N(CHCHPtBu2)2}]0/+/2+ was synthesized and examined spectroscopically, magnetically, crystallographically and computationally. The monocationic iridium(iv) imide exhibits an electronic doublet ground state with considerable 'imidyl' character as a result of covalent Ir-NtBu bonding. Reduction gives the neutral imide [Ir(NtBu){N(CHCHPtBu2)2}] as the first example of an iridium complex with a triplet ground state. Its reactivity with respect to nitrene transfer to selected electrophiles (CO2) and nucleophiles (PMe3), respectively, is reported.