Dicationic Acridinium/Carbene Hybrids as Strongly Oxidizing Photocatalysts.

A new design concept for organic, strongly oxidizing photocatalysts is described based upon dicationic acridinium/carbene hybrids. A highly modular synthesis of such hybrids is presented, and the dications are utilized as novel, tailor-made photoredox catalysts in the direct oxidative C-N coupling. Under optimized conditions, benzene and even electron-deficient arenes can be oxidized and coupled with a range of N-heterocycles in high to excellent yields with a single low-energy photon per catalytic turnover, while commonly used acridinium photocatalysts are not able to perform the challenging oxidation step. In contrast to traditional photocatalysts, the hybrid photocatalysts reported here feature a reversible two-electron redox system with regular or inverted redox potentials for the two-electron transfer. The different oxidation states could be isolated and structurally characterized supported by NMR, EPR, and X-ray analysis. Mechanistic experiments employing time-resolved emission and transient absorption spectroscopy unambiguously reveal the outstanding excited-state potential of our best-performing catalyst (+2.5 V vs SCE), and they provide evidence for mechanistic key steps and intermediates.

Pyridinium-Derived Mesoionic N-Heterocyclic Olefins (py-mNHOs).

Mesoionic polarization allows access to electron-rich olefins that have found application as organocatalysts, ligands, or nucleophiles. Herein, we report the synthesis and characterization of a series of 3-methylpyridinium-derived mesoionic olefins (py-mNHOs). We used a DFT-supported design concept, which showed that the introduction of aryl groups in the 1-, 2-, 4-, and 6-positions of the heterocyclic core allowed the kinetic stabilization of the novel mesoionic compounds. Tolman electronic parameters indicate that py-mNHOs are remarkably strong σ-donor ligands toward transition metals and main group Lewis acids. Additionally, they are among the strongest nucleophiles on the Mayr reactivity scale. In reactions of py-mNHOs with electron-poor π-systems, a gradual transition from the formation of zwitterionic adducts via stepwise to concerted 1,3-dipolar cycloadditions was observed experimentally and analyzed by quantum-chemical calculations.

Pushing the Upper Limit of Nucleophilicity Scales by Mesoionic N-Heterocyclic Olefins.

A series of mesoionic, 1,2,3-triazole-derived N-heterocyclic olefins (mNHOs), which have an extraordinarily electron-rich exocyclic CC-double bond, was synthesized and spectroscopically characterized, in selected cases by X-ray crystallography. The kinetics of their reactions with arylidene malonates, ArCH=C(CO2 Et)2 , which gave zwitterionic adducts, were investigated photometrically in THF at 20 °C. The resulting second-order rate constants k2 (20 °C) correlate linearly with the reported electrophilicity parameters E of the arylidene malonates (reference electrophiles), thus providing the nucleophile-specific N and sN parameters of the mNHOs according to the correlation lg k2 (20 °C)=sN (N+E). With 21

Gold-Catalyzed [3,3]-Sigmatropic Rearrangement of ortho-Alkynyl-S,S-diarylsulfilimines.

Highly functionalized 5H-pyrrolo[2,3-b]pyrazine cores, carrying a diaryl sulfide moiety at the C-7 position, were obtained from a gold-catalyzed reaction using easily accessible ortho-alkynyl-substituted S,S-diarylsulfilimines as intramolecular nitrene transfer reagents for the first time. The reaction proceeds under mild conditions, providing excellent yields while tolerating a large variety of different substitution patterns. We provide experimental evidence for an intramolecular reaction mechanism, likely including an unprecedented gold-catalyzed amino sulfonium [3,3]-sigmatropic rearrangement.

Room-Temperature-Stable Diazoalkenes by Diazo Transfer from Azides: Pyridine-Derived Diazoalkenes.

Recently, stable diazoalkenes have received significant attention as a new substance class in organic chemistry. While their previous synthetic access was exclusively limited to the activation of nitrous oxide, we here establish a much more general synthetic approach utilizing a Regitz-type diazo transfer with azides. Importantly, this approach is also applicable to weakly polarized olefins such as 2-pyridine olefins. The new pyridine diazoalkenes are not accessible by the activation of nitrous oxide, allowing for a considerable extension of the scope of this only recently accessed functional group. The new diazoalkene class has properties distinct from the previously reported classes, such as photochemically triggered loss of dinitrogen affording cumulenes and not C-H insertion products. Pyridine-derived diazoalkenes represent the so far least polarized stable diazoalkene class reported.

Organic Four-Electron Redox Systems Based on Bipyridine and Phenanthroline Carbene Architectures.

Novel organic redox systems that display multistage redox behaviour are highly sought-after for a series of applications such as organic batteries or electrochromic materials. Here we describe a simple strategy to transfer well-known two-electron redox active bipyridine and phenanthroline architectures into novel strongly reducing four-electron redox systems featuring fully reversible redox events with up to five stable oxidation states. We give spectroscopic and structural insight into the changes involved in the redox-events and present characterization data on all isolated oxidation states. The redox-systems feature strong UV/Vis/NIR polyelectrochromic properties such as distinct strong NIR absorptions in the mixed valence states. Two-electron charge-discharge cycling studies indicate high electrochemical stability at strongly negative potentials, rendering the new redox architectures promising lead structures for multi-electron anolyte materials.

Characterization of a Triplet Vinylidene.

Singlet vinylidenes (R2C═C:) are proposed as intermediates in a series of organic reactions, and very few have been studied by matrix isolation or gas-phase spectroscopy. Triplet vinylidenes, however, featuring two unpaired electrons at a monosubstituted carbon atom are thus far only predicted as electronically excited-state species and represent an unexplored class of carbon-centered diradicals. We report the photochemical generation and low-temperature EPR/ENDOR characterization of the first ground-state high-spin (triplet) vinylidene. The zero-field splitting parameters (D = 0.377 cm-1 and |E|/D = 0.028) were determined, and the 13C hyperfine coupling tensor was obtained by 13C-ENDOR measurements. Most strikingly, the isotropic 13C hyperfine coupling constant (50 MHz) is far smaller than the characteristic values of triplet carbenes, demonstrating a unique electronic structure which is supported by quantum chemical calculations.

N2/CO Exchange at a Vinylidene Carbon Center: Stable Alkylidene Ketenes and Alkylidene Thioketenes from 1,2,3-Triazole Derived Diazoalkenes.

We present a new class of room-temperature stable diazoalkenes featuring a 1,2,3-triazole backbone. Dinitrogen of the diazoalkene moiety can be thermally displaced by an isocyanide and carbon monoxide. The latter alkylidene ketenes are typically considered as highly reactive compounds, traditionally only accessible by flash vacuum pyrolysis. We present a new and mild synthetic approach to the first structurally characterized alkylidene ketenes by a substitution reaction. Density functional theory calculations suggest the substitution with isocyanides to take place via a stepwise addition/elimination mechanism. In the case of carbon monoxide, the reaction proceeds through an unusual concerted exchange at a vinylidene carbon center. The vinylidene ketenes react with carbon disulfide via a four-membered thiete intermediate to give vinylidene thioketenes under release of COS. We present spectroscopic as well as structural data for the complete isoelectronic series (R2C═C═X; X = N2, CO, CNR, CS) including 1J(13C-13C) data. As N2, CO, and isocyanides belong to the archetypical ligands in transition-metal chemistry, this process can be interpreted in analogy to coordination chemistry as a ligand exchange reaction at a vinylidene carbon center.

Dibenzothiophenesulfilimines: A Convenient Approach to Intermolecular Rhodium-Catalysed C-H Amidation.

A sulfilimine-based Group 9 transition-metal-catalysed C-H amidation procedure is reported. Dibenzothiophene-based sulfilimines were shown to constitute a class of novel amidation reagents which enable the transfer of a wide range of N-sulfonyl and N-acyl moieties. It was demonstrated that sulfilimines, which are easily accessible from cheap reagents, are safe-to-handle and represent broadly applicable amidation reagents. The dibenzothiophene can be recycled after use. The C-H amidation was shown to proceed with high selectivity and gave the mono-amidated products, mostly in good to excellent yields.

Stable Mesoionic N-Heterocyclic Olefins (mNHOs).

We report a new class of stable mesoionic N-heterocyclic olefins, featuring a highly polarized (strongly ylidic) double bond. The ground-state structure cannot be described through an uncharged mesomeric Lewis-structure, thereby structurally distinguishing them from traditional N-heterocyclic olefins (NHOs). mNHOs can easily be obtained through deprotonation of the corresponding methylated N,N'-diaryl-1,2,3-triazolium and N,N'-diaryl-imidazolium salts, respectively. In their reactivity, they represent strong σ-donor ligands as shown by their coordination complexes of rhodium and boron. Their calculated proton affinities, their experimentally derived basicities (competition experiments), as well as donor abilities (Tolman electronic parameter; TEP) exceed the so far reported class of NHOs.

Organic Redox Systems Based on Pyridinium-Carbene Hybrids.

New redox systems with three oxidation states are highly sought-after, for example, for redox-flow battery applications, selective reducing agents, or organic electronics. Herein, we describe a straightforward and modular synthesis of a new class of such a three-state redox system based on the intermolecular reaction of a large variety of pyridinium salts with carbenes. These hybrids represent organic (super) electron donors with tailored electrochemical properties and feature three stable oxidation states, which could be fully characterized including by X-ray diffraction. We elaborate which electronic factors either promote attainment of stable radicals through one electron transfer or instead favor 2e- processes. Indeed, based on X-ray data, a verification for a potential compression mechanism is given that originates through a large structural distortion in the first oxidation event. By geometrically locking this hybridization change, a potential expansion can be realized. The new class of stable organic radicals are highly persistent and even moderately stable toward air. Additionally, we demonstrate that our modular synthesis approach is also applicable to remarkably strong multielectron (4e-) donors by utilizing bridged pyridinium salts. Based on the stability and reversibility of the new redox system, we could demonstrate by charge-discharge experiments the use of the hybrid molecules as novel anolyte materials for nonaqueous redox-flow batteries.

P-Protected Diphosphadibenzo[ a, e]pentalenes and Their Mono- and Dicationic P-Bridged Ladder Stilbenes.

The previously elusive diphosphadibenzo[ a, e]pentalene core skeleton was assembled via a surprisingly straightforward cyclization pathway starting from R2P-substituted 2,2'-diphosphinotolanes (R = Ph, iPr). The resulting P-protected diylidic compounds 4 (R = Ph, iPr) were converted to the corresponding P-bridged ladder stilbenes via two consecutive oxidation steps: upon selective one-electron oxidation, the persistent radical monocations 5 (R = Ph, iPr) were obtained and further oxidized to afford the respective fluorescent and air-stable dications 6 (R = Ph, iPr).

1,1-Digoldallylium Complexes: Diaurated Allylic Carbocations Indicate New Prospects of the Coordination Chemistry of Carbon.

Reacting (NHC)(cyclopropenyl)gold(I) complexes with cationic gold complexes [(IPr)AuX] afforded extremely reactive allylium-1,1-diido-bridged digold intermediates. We prove the existence and constitution of this structure with FT-ICR-MS/MS, NMR, and UV-vis-NIR experiments and isolated the nucleophilic addition product [(Me)(Ph)(CCHC){Au(IPr)}2(SOMe2)]NTf2 with DMSO. Our computational investigation unveiled that the bonding situation of this µ-allylium-1,1-diido digold domain was best described as a three-center-four-electron bond with a π-backbond. The valence orbitals showed extreme delocalization and strong π-interactions between the three centers Au1-C1-Au2. The bridging carbon atom C1 was best described as trigonal planar sp-hybridized carbon in this structure. Excitation succeeded in UV-vis-NIR measurements with energies as low as near-IR radiation.

Pyrylenes: A New Class of Tunable, Redox-Switchable, Photoexcitable Pyrylium-Carbene Hybrids with Three Stable Redox-States.

A new synthetic and modular access to a large family of redox-switchable molecules based upon the combination of pyrylium salts and carbenes is presented. The redox-properties of this new molecule class correlate very well with the π-accepting properties of the corresponding carbenes. While the pyrylium moiety acts as a chromophore, the carbene moiety can tune the redox-properties and stabilize the corresponding radicals. This leads to the isolation of the first monomeric pyranyl-radical in the solid-state. The three stable oxidation states could be cleanly accessed by chemical oxidation, characterized by NMR, EPR, UV-vis, and X-ray diffraction and supported by (TD)-DFT-calculations. The new hybrid class can be utilized as an electrochemically triggered switch and as a powerful photoexcited reductant. Importantly, the pyrylenes can be used as novel photocatalysts for the reductive activation of aryl halides and sulfonamides by consecutive visible light induced electron transfer processes.

The Gold(I)-Mediated Domino Reaction to Fused Diphenyl Phosphoniumfluorenes: Mechanistic Consequences for Gold-Catalyzed Hydroarylations and Application in Solar Cells.

A domino sequence, involving a phosphinoauration and a gold-catalyzed 6-endo-dig cyclization step, was developed. Starting from modular and simple-to-prepare phosphadiynes, π-extended phosphoniumfluorenes were synthesized. The mechanistic proposal was supported by kinetic measurements and by the trapping of key intermediates. These led to important conclusions for the gold-catalyzed hydroarylation mechanism. Cyclic voltammetry (CV) and UV/Vis spectroscopy measurements indicated interesting properties for materials science. The phosphoniumfluorene structure was tested as a hole-blocking layer in perovskite solar cells of inverted architecture. Devices with the phosphoniumfluorene exhibited an efficiency of 14.2 %, which was much higher than that of devices without (10.7 %).

Benzylene-linked [PNP] scaffolds and their cyclometalated zirconium and hafnium complexes.

The benzylene-linked [PNP] scaffolds HN(CH2-o-C6H4PPh2)2 ([A]H) and HN(C6H4-o-CH2PPh2)2 ([B]H) have been used for the synthesis of zirconium and hafnium complexes. For both ligands, the dimethylamides [A]M(NMe2)3 ([A]1-M) and [B]M(NMe2)3 ([B]1-M) were prepared and converted to the iodides [A]MI3 ([A]2-M) and [B]MI3 ([B]2-M) (M = Zr, Hf). Starting from these iodides, the corresponding benzyl derivatives [A]MBn3 ([A]3-M) and [B]MBn3 ([B]3-M) (M = Zr, Hf) were obtained via reaction with Bn2Mg(OEt2)2. For zirconium, the benzylic ligand positions in [A]3-Zr and [B]3-Zr were found to cyclometalate readily, which led to the corresponding κ4-[PCNP]ZrBn2 complexes [A]4-Zr and [B]4-Zr. As these complexes failed to hydrogenate cleanly, cyclometalated derivatives with only one alkyl substituent were targeted and the mixed benzyl chlorides κ4-[PCNP]MBnCl ([B]5-M, M = Zr, Hf) were obtained in the case of ligand [B]. Upon hydrogenation of [B]5-Zr, the η6-tolyl complex [B]Zr(η6-C7H8)Cl ([B]6-Zr) was generated cleanly, but the corresponding hafnium complex [B]5-Hf was found to decompose unselectively in the presence of H2. Using a closely related carbazole-based [PNP] ligand, Gade and co-workers have shown recently that zirconium η6-arene complexes similar to [B]6-Zr may serve as zirconium(ii) synthons, namely when reacted with 2,6-Dipp-NC (L) or pyridine (py). Both these substrates were shown to react cleanly with [B]6-Zr, which led to the formation of the bis-isocyanide complex [B]ZrCl(L)2 ([B]7-Zr) and the 2,2'-bipyridine derivative [B]ZrCl(bipy) ([B]8-Zr), respectively. Upon reaction of [B]Zr(η6-C7H8)Cl ([B]6-Zr) with NaBEt3H, the cyclometalated derivative κ4-[PCNP]Zr(η6-C7H8) ([B]9-Zr) was isolated. In an attempt to synthesise terminal hydrides, complexes [A]MI3 ([A]2-M) were treated with KBEt3H, which led to the isolation of the cyclometalated hydrido complexes κ4-[PCNP]M(H)(κ3-Et3BH) ([A]10-M; M = Zr, Hf) featuring a κ3-bound triethyl borohydride moiety.