Radicalizing CO by Mononuclear Palladium(I).

A mononuclear, T-shaped palladium(I) d9 metalloradical (3), stabilized by a bulky carbazole-based PNP-ligand, was obtained by reduction of palladium chloride or thermal Pd-C bond homolysis of the corresponding neopentyl complex. Pressurizing with CO gave the Pd(I) carbonyl complex, which was structurally characterized by X-ray diffraction. Delocalization of the unpaired electron to the carbonyl carbon was detected by EPR spectroscopy and theoretically modeled by DFT and ab initio methods. The partially reduced and radicalized CO slowly reacts with di(tert-butyl) disulfide under homolytic S-S cleavage and C-S bond formation to give the corresponding metallathioester.

Chiral Bay-Alkynylated Tetraazaperylenes: Photophysics and Chiroptical Properties.

Fully bay-alkynylated octaazaperopyrene dioxide (OAPPDO) derivatives were accessible through Stille cross coupling reaction of the corresponding bay-chlorinated derivatives. This steric congestion of the bay area led to helically chiral fluorophores, and chiral resolution of two derivatives allowed the investigation of their chiroptical properties as well as their kinetics of enantiomerization and the related thermodynamic parameters depending on the size of the terminal alkynyl substituent. An increase of the latter resulted in stable OAPPDO atropisomers at room temperature. The dynamics of the photoexcited states of two of the OAPPDO derivatives were investigated by transient absorption (TA) and time-resolved photoluminescence (tr-PL) spectroscopy.

Zipping up tetraazaperylene: synthesis of tetraazacoronenes via double coupling in the bay positions.

Substituted tetraazacoronene fluorophores have been obtained selectively by double Suzuki-Miyaura cross coupling of symmetrically substituted 1,2-bis(pinacolatoboryl)alkenes with a bay-substituted octaazaperopyrenedioxide (OAPPDO). Subsequent Scholl reaction of the dimethoxyphenylated derivative allowed further π-extension of the azaperylene core, yielding a highly redox-active bis(phenanthro)tetraazacoronene.

Tetraazacoronenes and Their Dimers, Trimers and Tetramers.

Tetraazacoronenes were synthesized from bay-functionalized tetraazaperylenes by Zr-mediated cyclization and four-fold Suzuki-Miyaura cross coupling. In the Zr-mediated approach, an η4 -cyclobutadiene-zirconium(IV) complex was isolated as an intermediate to cyclobutene-annulated derivatives. Using bis(pinacolatoboryl)vinyltrimethylsilane as a C2 building block gave the tetraazacoronene target compound along with the condensed azacoronene dimer as well as higher oligomers. The series of extended azacoronenes show highly resolved UV/Vis absorption bands with increased extinction coefficients for the extended aromatic cores and fluorescence quantum yields of up to 80 % at 659 nm.

Peri-Functionalized Tetraazaperylenes: Strong Emitters for Molecular Polaritonics.

The functionalization of the peri position of the bay chlorinated tetraazaperylene was investigated by varying the donor behavior of the substituents to assess the resulting photophysical and electrochemical properties. To accomplish this, electron donating alkyl- and arylsulfido substituents, methoxy and methyl groups were selectively introduced into the peri position via a nucleophilic aromatic substitution of the perchlorinated tetraazaperylene. Both the alkylated and benzylated thioethers displayed high fluorescence quantum yields of up to 80 %. Compounds from the latter group were integrated in resonant optical microcavities to achieve strong light-matter coupling. The formation of exciton-polaritons was observed by angle-dependent reflectivity and photoluminescence that could be tuned by variation of the concentration of the fluorophores and of the thickness of the cavity.

Induced Fit and Mobility of Cycloalkanes within Nanometer-Sized Confinements at 5 K.

Host-guest architectures provide ideal systems for investigating site-specific physical and chemical effects. Condensation events in nanometer-sized confinements are particularly interesting for the investigation of intermolecular and molecule-surface interactions. They may be accompanied by conformational adjustments representing induced fit packing patterns. Here, we report that the symmetry of small clusters formed upon condensation, their registry with the substrate, their lateral packing, and their adsorption height are characteristically modified by the packing of cycloalkanes in confinements. While cyclopentane and cycloheptane display cooperativity upon filling of the hosting pores, cyclooctane and to a lesser degree cyclohexane diffusively redistribute to more favored adsorption sites. The dynamic behavior of cyclooctane is surprising at 5 K given the cycloalkane melting point of >0 °C. The site-specific modification of the interaction and behavior of adsorbates in confinements plays a crucial role in many applications of three-dimensional porous materials as gas storage agents or catalysts/biocatalysts.

Peri-Decoration of a Tetraazaperylene with Urea Units: Chiral Octaazaperopyrenedioxides (OAPPDOs) and Their Optical and Chiroptical Properties.

Octaazaperopyrenedioxides (OAPPDOs) are a new class of fluorescent polycyclic aromatic hydrocarbons based on a tetraazaperylene core that is formally condensed with N-substituted urea units in the two opposite peri positions. Here, we report the synthesis of series of substituted OAPPDO derivatives with different N-substitution patterns (H, alkyl, benzyl) in the peri positions, including bay-chlorinated OAPPDOs. Starting from the latter, a series of bay-arylated OAPPDOs was synthesized by Suzuki cross coupling, which resulted in the formation of helically chiral OAPPDO derivatives. The electrochemical and photophysical properties were investigated by UV/Vis and fluorescence spectroscopy as well as cyclic voltammetry. The P and M enantiomers of a phenylated OAPPDO were separated by semipreparative HPLC and further analyzed by CD spectroscopy. The frontier orbital energies, the mechanism of the isomerization, the electronic excitation and the CD spectrum (TD-DFT) were computed and compared to the experimental data. The reversible 1e- oxidation of the OAPPDOs generates the corresponding radical cations, one of which was characterized by EPR spectroscopy. The reversible oxidation process was also systematically investigated by spectro-electrochemistry.

Towards Nitrogen-Rich N-Heteropolycycles: Synthesis of Octaazaperopyrenes (OAPP).

Ortho substituted octaazaperopyrenes (OAPPs) are a new class of functional dyes characterized by their strong electron-accepting behavior. Herein, the synthesis, as well as the electrochemical and photo physical properties of an OAPP dye, is reported. The OAPP target was prepared via selective nucleophilic substitution at the peri position of a bay chlorinated tetraazaperylene by introduction of four amino-substituents. The resulting tetraminoperylene was reacted with different acyl chlorides and anhydrides to give the twisted bay chlorinated OAPP derivatives which were isolated in their reduced dihydro-form. The OAPP target could be obtained via a palladium catalyzed dehalogenation and a subsequent oxidation. The eightfold isosteric [CH→N] replacement within the peropyrene core structure results in a large decrease of the frontier orbital energies, rendering the target compound a potent oxidant while preserving the planarity of the aromatic core. The radical anion was obtained by reduction of the OAPP with KC8 and characterized by EPR spectroscopy. A general discussion of the number and location of [CH→N] replacements in peropyrene structures and their frontier orbital energies is provided.

3d Metal Complexes in T-shaped Geometry as a Gateway to Metalloradical Reactivity.

ConspectusLow-valent, low-coordinate 3d metal complexes represent a class of extraordinarily reactive compounds that can act as reagents and catalysts for challenging bond-activation reactions. The pursuit of these electron-deficient metal complexes in low oxidation states demands ancillary ligands capable of providing not only energetic stabilization but also sufficiently high steric bulk at the metal center. From this perspective, pincer ligands are particularly advantageous, as their prearranged, meridional coordination mode scaffolds the active center while the substituents of the peripheral donor atoms provide effective steric shielding for the coordination sphere. In a T-shaped geometry, the transition metal complexes possess a precisely defined vacant coordination site, which, combined with the often observed high-spin electron configuration, exhibits unusually high selectivity of these compounds with respect to one-electron redox chemistry. In light of the intractable reaction pathways typically observed with related electronically unsaturated 3d transition metal complexes, the pincer coordination mode enables the isolation of low-valent compounds with more controlled and unique reactivity. We have thus investigated a series of T-shaped metal(I) complexes using three different types of pincer ligands, which may be regarded as "metalloradicals" due to their selectively exposed unpaired electrons.These compounds display remarkably high thermal stability and represent rarely observed "naked" monovalent metal species featuring both monomeric and dimeric structures. Extensive reactivity studies using various organic substrates highlight a strong tendency of these paramagnetic compounds to undergo one-electron oxidation, leading to the isolation of a plethora of metal(II) species with reduced organic ligands as unusual structural elements. The exploration of C2 symmetric T-shaped Ni(I) complexes as asymmetric catalysts also shows success in enantioselective hydrodehalogenation of geminal dihalogenides. In addition, this specific class of low-valent, low-coordinate complexes can be further diversified by introducing redox-active pincer ligands or building homobimetallic systems with two T-shaped units.This Account focuses on the discussion of selected examples of iron, cobalt, and nickel pincer complexes bearing a [P,N,P] or [N,N,N] donor set; however, their electronic structure and radical-type reactivity can be broadly extended to other pincer systems. The availability of various types of pincer ligands should allow fine-tuning of the reactivity of the T-shaped complexes. Given the unprecedented reactivity observed with these compounds, we expect the studies of T-shaped 3d metal complexes to be a fertile field for advancing base metal catalysis.

Electronic Properties of Tetraazaperopyrene Derivatives on Au(111): Energy-Level Alignment and Interfacial Band Formation.

N-heteropolycyclic aromatic compounds are promising organic electron-transporting semiconductors for applications in field-effect transistors. Here, we investigated the electronic properties of 1,3,8,10-tetraazaperopyrene derivatives adsorbed on Au(111) using a complementary experimental approach, namely, scanning tunneling spectroscopy and two-photon photoemission combined with state-of-the-art density functional theory. We find signatures of weak physisorption of the molecular layers, such as the absence of charge transfer, a nearly unperturbed surface state, and an intact herringbone reconstruction underneath the molecular layer. Interestingly, molecular states in the energy region of the sp- and d-bands of the Au(111) substrate exhibit hole-like dispersive character. We ascribe this band character to hybridization with the delocalized states of the substrate. We suggest that such bands, which leave the molecular frontier orbitals largely unperturbed, are a promising lead for the design of organic-metal interfaces with a low charge injection barrier.

Trapping an unusual pentacoordinate carbon atom in a neutral trialuminum complex.

A neutral trialuminum complex incorporates a pentacoordinate carbon through a methylidene bridge linking the three metal atoms. The rigid electron-deficient Al3 core stabilizes the hypercoordinate carbon atom resulting in the shortest equatorial Al-C distance reported for such an Al3-(µ3-CH2) unit.

Observability of Paramagnetic NMR Signals at over 10 000 ppm Chemical Shifts.

We report an experimental observation of 31 P NMR resonances shifted by over 10 000 ppm (meaning percent range, and a new record for solutions), and similar 1 H chemical shifts, in an intermediate-spin square planar ferrous complex [tBu (PNP)Fe-H], where PNP is a carbazole-based pincer ligand. Using a combination of electronic structure theory, nuclear magnetic resonance, magnetometry, and terahertz electron paramagnetic resonance, the influence of magnetic anisotropy and zero-field splitting on the paramagnetic shift and relaxation enhancement is investigated. Detailed spin dynamics simulations indicate that, even with relatively slow electron spin relaxation (T1 ≈10-11  s), it remains possible to observe NMR signals of directly metal-bonded atoms because pronounced rhombicity in the electron zero-field splitting reduces nuclear paramagnetic relaxation enhancement.

Reactivity of a T-shaped cobalt(I) pincer-complex.

The reactivity of a paramagnetic T-shaped cobalt(i) complex, [(iPrboxmi)Co], stabilised by a monoanionic bis(oxazolinylmethylidene)-isoindolate (boxmi) NNN pincer ligand is described. The exposure to carbon monoxide as an additional neutral ligand resulted in the square-planar species [(iPrboxmi)Co(CO)], accompanied by a change in the electronic spin state from S = 1 to S = 0. In contrast, upon treatment with trimethylphosphine the formation of the distorted tetrahedral complex [(iPrboxmi)Co(PMe3)] was observed (S = 1). Reacting [(iPrboxmi)Co] with iodine (I2), organic peroxides (tBu2O2, (SiMe3)2O2) and diphenyldisulphide (Ph2S2) yielded the tetracoordinated complexes [(iPrboxmi)CoI], [(iPrboxmi)Co(OtBu)], [(iPrboxmi)Co(OSiMe3)] and [(iPrboxmi)Co(SPh)], respectively, demonstrating the capability of the boxmi-supported cobalt(i) complex to homolytically cleave bonds and thus its distinct one-electron reactivity. Furthermore, a square-planar cobalt(ii) alkynyl complex [(iPrboxmi)Co(CCArF)] was identified as the main product in the reaction between [(iPrboxmi)Co] and a terminal alkyne, 4-fluoro-1-ethynylbenzene. Putting such species in the context of the previously investigated hydroboration catalysis, its stoichiometric reaction with pinacolborane revealed its potential conversion into a cobalt(ii) hydride complex, thus confirming its original attribution as off-cycle species.

Metalloradical Reactivity, Charge Transfer, and Atom Abstractions in a T-Shaped Iron(I) Complex.

A study of the reactivity of a T-shaped iron(I) complex supported by a carbazole-based PNP pincer ligand (1) has established its strongly reducing character and propensity to bind small molecules with concomitant transfer of charge and spin density. Metalloradical reactivity was observed in the reaction with the stable radical 2,2,6,6-tetramethylpiperidinyloxyl (TEMPO) yielding the corresponding FeII hydroxylaminato complex 2. Complex 1 reacted rapidly and selectively with two molecules of carbon monoxide to give the corresponding low-spin dicarbonyl complex 3. When reacted with phenylacetylene, the alkyne complex 4 was isolated in which the alkyne was found to bind side-on as an η2-ligand, which adopts radical anion character as a result of charge transfer from the metal center. Reaction with diphenyl diazomethane generated an adduct 5 in which the diazoalkane also appears to be partially reduced and acquires radical character. Upon exposure to carbon dioxide, oxygen atom abstraction was observed, ultimately leading to the isolation of the monocarbonyl iron(I) complex 6 and a dinuclear carboxylato ferrous complex 7. Furthermore, reaction with chalcogen atom transfer reagents resulted in the formation of the corresponding dinuclear ferrous chalcogenido compounds (E = S (8), Se (9)), which were found to display strong antiferromagnetic coupling (8, JAFC = -68 cm-1; 9: JAFC = -58 cm-1).

Band Formation at Interfaces Between N-Heteropolycycles and Gold Electrodes.

Efficient charge injection at organic semiconductor/metal interfaces is crucial for the performance of organic field effect transistors. Interfacial hybrid band formation between electronic states of the organic compound and the metal electrode facilitates effective charge injection. Here, we show that a long-range ordered monolayer of a flat-lying N-heteropolycyclic aromatic compound on Au(111) leads to dispersing occupied and unoccupied interfacial hybrid bands. Using angle-resolved two-photon photoemission we determine their energy level alignment and dispersion relations. We suggest that band formation proceeds via hybridization of a localized occupied molecular state with the d-bands of the Au substrate, where the large effective mass of the d-bands is significantly reduced in the hybrid band. Hybridization of an unoccupied molecular state with the Au sp-band leads to a band with an even smaller effective mass.

Toward a Neutral Single-Component Amidinate Iodide Aluminum Catalyst for the CO2 Fixation into Cyclic Carbonates.

A new iodide aluminum complex ({AlI(κ4-naphbam)}, 3) supported by a tetradentate amidinate ligand derived from a naphthalene-1,8-bisamidine precursor (naphbamH, 1) was obtained in quantitative yield via reaction of the corresponding methyl aluminum complex ({AlMe(κ4-naphbam)}, 2) with 1 equiv of I2 in CH2Cl2 at room temperature. Complexes 2 and 3 were tested and found to be active as catalysts for the cyclic carbonate formation from epoxides at 80 °C and 1 bar of CO2 pressure. A first series of experiments were carried out with 1.5 mol % of the alkyl complex 2 and 1.5 mol % of tetrabutylammonium iodide (TBAI) as a cocatalyst; subsequently, the reactions were carried out with 1.5 mol % of iodide complex 3 as a single-component catalyst. Compound 3 is one of the first examples of a nonzwitterionic halide single-component aluminum catalyst producing cyclic carbonates. The full catalytic cycle with characterization of all minima and transition states was characterized by quantum chemistry calculations (QCCs) using density functional theory. QCCs on the reaction mechanism support a reaction pathway based on the exchange of the iodine contained in the catalyst by 1 equiv of epoxide, with subsequent attack of I- to the epoxide moiety producing the ring opening of the epoxide. QCCs triggered new insights for the design of more active halide catalysts in future explorations of the field.

Reaction Pathways and Redox States in α-Selective Cobalt-Catalyzed Hydroborations of Alkynes.

Cobalt(II) alkyl complexes supported by a monoanionic NNN pincer ligand are pre-catalysts for the regioselective hydroboration of terminal alkynes, yielding the Markovnikov products with α:ß-(E) ratios of up to 97:3. A cobalt(II) hydride and a cobalt(II) vinyl complex appear to determine the main reaction pathway. In a background reaction the highly reactive hydrido species specifically converts to a coordinatively unsaturated cobalt(I) complex which was found to re-enter the main catalytic cycle.

Single or Paired? Structure and Reactivity of PNP-Chromium(II) Hydrides.

The preparation and reactivity of a range of novel paramagnetic chromium(II) complexes supported by a carbazole-based PNP pincer ligand is reported. Deprotonation of the ligand precursors R(PNP)H (1R) and subsequent reaction with chromium(II) chloride led to the formation of square-planar chlorido complexes R(PNP)CrCl (2R). Further reaction with various alkylating agents resulted in the isolation of chromium alkyl complexes R(PNP)CrR' (3R-R') which were then hydrogenated to yield two rare examples of paramagnetic chromium(II) hydrides 4iPr and 4tBu. Both compounds were characterized by X-ray diffraction and paramagnetic NMR spectroscopy supported by a comprehensive DFT-supported assignment of the resonances. While the di(tert-butyl)phosphino PNP substituted complex 4tBu was found to exhibit a monomeric square-planar molecular structure, its isopropyl-substituted analog 4iPr forms a dimer, also indicated by a strong antiferromagnetic coupling of the chromium centers. The pronounced reactivity of these compounds toward C═X double bonds was demonstrated by reaction with benzophenone, N,N'-dicyclohexylcarbodiimide, and carbon dioxide, which gave the corresponding insertion products. The alkoxido complex 5iPr, the amidinato complex 6iPr, and the formato compound 7tBu were also characterized by X-ray diffraction.

Ultrafast Singlet Fission and Intersystem Crossing in Halogenated Tetraazaperopyrenes.

Charge carrier multiplication via singlet fission into two triplet states has the potential to increase efficiencies of photovoltaics by one-third due to the reduction of thermalization losses. In the present work, we investigate tetraazaperopyrenes, a class of N-heteropolycyles, as suitable singlet fission candidates. Using a combined experimental and theoretical approach, fundamentally different mechanisms for triplet formation in solution and thin film are identified. In solution, an ultrafast intersystem crossing process is observed, which is accelerated for heavier halide substituents not only due to enhanced spin-orbit coupling but also due to the energy tuning between the S1 and T2 states. In thin films, a correlated triplet pair is formed coherently upon photoexcitation. Subsequently, an excimer formation is observed, which competes with the electronic decorrelation of the triplet pair. The comparison with peropyrene shows that aza-substitutions within the aromatic core can be a powerful strategy for tuning the energy levels of the states important to singlet fission.

Tackling N-Alkyl Imines with 3d Metal Catalysis: Highly Enantioselective Iron-Catalyzed Synthesis of α-Chiral Amines.

A readily activated iron alkyl precatalyst effectively catalyzes the highly enantioselective hydroboration of N-alkyl imines. Employing a chiral bis(oxazolinylmethylidene)isoindoline pincer ligand, the asymmetric reduction of various acyclic N-alkyl imines provided the corresponding α-chiral amines in excellent yields and with up to >99 % ee. The applicability of this base metal catalytic system was further demonstrated with the synthesis of the pharmaceuticals Fendiline and Tecalcet.