Cationic Niobium-Sandwich and Piano-Stool Complexes.

The syntheses of the homoleptic bis(arene) niobium cations [Nb(arene)2 ]+ (arene = C6 H3 Me3 , C6 H5 Me) with 16 valence electrons and heteroleptic arene-carbonyl cations [(CO)Nb(arene)2 ]+ (arene = C6 H3 Me3 , C6 H5 Me) and [(arene)M(CO)4 ]+ (arene = C6 H3 Me3 , C6 H6 ) obeying 18 valence electrons are described. Stabilization of these complexes was achieved by using the weakly coordinating anions [Al(ORF )4 ]- or [F{Al(ORF )3 }2 ]- (RF = C(CF3 )3 ). The limits of two synthesis routes starting from neutral Nb(arene)2 (arene = C6 H3 Me3 , C6 H5 Me) or [NEt4 ][M(CO)6 ] (M = Nb, Ta) were investigated. All compounds were analyzed by single crystal X-ray determination, vibrational and NMR spectroscopy. DFT calculations were executed to support the experimental data.

Critical role of H-aggregation for high-efficiency photoinduced charge generation in pristine pentamethine cyanine salts.

The mechanism of photoinduced symmetry-breaking charge separation in solid cyanine salts at the base of organic photovoltaic and optoelectronic devices is still debated. Here, we employ femtosecond transient absorption spectroscopy (TAS) to monitor the charge transfer processes occurring in thin films of pristine pentamethine cyanine (Cy5). Oxidized dye species are observed in Cy5-hexafluorophosphate salts upon photoexcitation, resulting from electron transfer from monomer excited states to H-aggregates. The charge separation proceeds with a quantum yield of 86%, providing the first direct proof of high efficiency intrinsic charge generation in organic salt semiconductors. The impact of the size of weakly coordinating anions on charge separation and transport is studied using TAS alongside electroabsorption spectroscopy and time-of-flight techniques. The degree of H-aggregation decreases with increasing anion size, resulting in reduced charge transfer. However, there is little change in carrier mobility, as despite the interchromophore distance increasing, the decrease in energetic disorder helps to alleviate the trapping of charges by H-aggregates.

Altering Charges on Heterobimetallic Transition-Metal Carbonyl Clusters.

The homoleptic group 5 carbonylates [M(CO)6 ]- (M=Nb, Ta) serve as ligands in carbonyl-terminated heterobimetallic Agm Mn clusters containing 3 to 11 metal atoms. Based on our serendipitous [Ag6 {Nb(CO)6 }4 ]2+ (4 a2+ ) precedent, we established access to such Agm Mn clusters of the composition [Agm {M(CO)6 }n ]x (M=Nb, Ta; m=1, 2, 6; n=2, 3, 4, 5; x=1-, 1+, 2+). Salts of those molecular cluster ions were synthesized by the reaction of [NEt4 ][M(CO)6 ] and Ag[Al(ORF )4 ] (RF =C(CF3 )3 ) in the correct stoichiometry in 1,2,3,4-tetrafluorobenzene at -35 °C. The solid-state structures were determined by single-crystal X-ray diffraction methods and, owing to the thermal instability of the clusters, a limited scope of spectroscopic methods. In addition, DFT-based AIM calculations were performed to provide an understanding of the bonding within these clusters. Apparently, the clusters 3+ (m=6, n=5) and 42+ (m=6, n=4) are superatom complexes with trigonal-prismatic or octahedral Ag6 superatom cores. The [M(CO)6 ]- ions then bind through three CO units as tridentate chelate ligands to the superatom core, giving overall structures related to tetrahedral AX4 (42+ ) or trigonal bipyramidal AX5 molecules (3+ ).

Group†6 Hexacarbonyls as Ligands for the Silver Cation: Syntheses, Characterization, and Analysis of the Bonding Compared with the Isoelectronic Group†5 Hexacarbonylates.

The syntheses of the two novel complexes [Ag{Mo/W(CO)6 }2 ]+ [F-{Al(ORF )3 }2 ]- (RF =C(CF3 )3 ) are reported along with their structural and spectroscopic characterization. The X-ray structure shows that three carbonyl ligands from each M(CO)6 fragment bend towards the silver atom within binding Ag-C distance range. DFT calculations of the free cations [Ag{M(CO)6 }2 ]+ (M=Cr, Mo, W) in the electronic singlet state give equilibrium structures with C2 symmetry with two bridging carbonyl groups from each hexacarbonyl ligand. Similar structures with C2 symmetry (M=Nb) and D2 symmetry (M=V, Ta) are calculated for the isoelectronic group 5 anions [Ag{M(CO)6 }2 ]- (M=V, Nb, Ta). The electronic structure of the cations is analyzed with the QTAIM and EDA-NOCV methods, which provide detailed information about the nature of the chemical bonds between Ag+ and the {M(CO)6 }2 q (q = -2, M = V, Nb, Ta; q = 0, M = Cr, Mo, W) ligands.

Development, Scope, and Applications of Titanium(III)-Catalyzed Cyclizations to Aminated N-Heterocycles.

The exceptionally mild conditions of a titanium(III)-catalyzed cyclization reaction paired with a convenient acid/base extraction have enabled the straightforward synthesis, isolation, and direct N-functionalization of amino heterocycles such as 3-aminoindoles and -pyrroles. The unprotected heterocycles are ideal building blocks for the installation of aminated indoles and pyrroles into target molecules, but their sensitivity has previously impeded their synthesis by modern catalytic methods. This full paper presents the development and extended scope of the new cyclization methodology. The transformation of the products into fused bis-indoles is also demonstrated along with the discovery of an unusual palladium-catalyzed reductive biphenyl coupling reaction. The titanium(III)-catalyzed cyclization has also been applied to the synthesis of substituted 3-iminoindolines, which are of potential interest for applications in natural product synthesis and exhibit tunable blue-to-green fluorescence properties.

Why Do Five Ga+ Cations Form a Ligand-Stabilized [Ga5 ]5+ Pentagon and How Does a 5:1 Salt Pack in the Solid State?

The reaction of the Ga+ source [Ga(PhF)2 ]+ [Al(ORF )4 ]- with the neutral σ-donor ligand dmap (4-Me2 N-C6 H4 N) produces the unexpectedly large and fivefold positively charged cluster cation salt [Ga5 (dmap)10 ]5+ ([Al(ORF )4 ]- )5 . It includes a regular and planar Ga5 pentagon with strong metal-metal bonding. Additionally, the compound represents the first salt in which an ionic 1:5 packing is realized. We discuss the nature of this structure which results from the conversion of the non-bonding 4s2 lone-pair orbitals into fully Ga-Ga-bonding orbitals and the solid-state arrangement of the ions constituting the lattice as an almost orthohexagonal AX5 lattice, possibly the aristotype of any 5:1 salt.

Dibenzo[a,e]pentalenophanes: Bending a Non-Alternant Hydrocarbon.

In cyclophanes, an aromatic moiety is incorporated into a (strained) cyclic structure. Of particular interest as model systems for bent carbon nanostructures are those containing polycyclic aromatic hydrocarbons. Dibenzo[a,e]pentalene (DBP) is a non-alternant polycyclic hydrocarbon with small band gap and tunable optoelectronic properties. However, changing these properties by bending of the DBP structure has yet to be investigated. Herein, we report the synthesis, optoelectronic, and structural properties of (2,7)dibenzo[a,e]pentalenophanes with four different bridge sizes and bending angles of the DBP unit, accompanied by (TD)DFT calculations. The last, strain-inducing dehydration reaction was accomplished by using Burgess' reagent. The HOMO and LUMO levels and the magnetic shielding of protons pointing inside the cyclophane cavity grew stepwise with increasing ring strain. Single-crystal X-ray structures of the smallest three derivatives revealed a near semi-circle and a bend angle of the DBP unit of almost 88° for the smallest derivative. We demonstrated the synthetic versatility of our approach by varying the substituents at the DBP unit, allowing for further tuning of optoelectronic properties. The synthetic strategy presented herein may pave the way for the synthesis of conjugated DBP nanorings.

A Unified Approach to Customized Chiral Carbon-Bridged ansa-Metallocenes.

Chiral ansa-metallocenes are privileged catalysts for a range of stereoselective transformations. Their synthesis, however, has remained a tremendous challenge, which has prevented a broad and systematic exploration for applications in synthesis and catalysis. A modular approach to such ansa-metallocenes that enables a facile modification of the ring substitution and the ligand bridge, as well as the introduction of various core metals, is described. The complexes were formed with good rac-selectivity and could be isolated with high purity. The strength of the approach was demonstrated by the synthesis of several new and previously known complexes, including a unique helical chiral ansa-metallocene. Using a chiral ligand, a moderate central-to-planar chirality transfer was observed.

Synthesis, Characterisation and Reactions of Truly Cationic NiI -Phosphine Complexes.

The recently published purely metallo-organic NiI salt [Ni(cod)2 ][Al(ORF )4 ] (1, cod=1,5-cyclooctadiene, RF =C(CF3 )3 ) provides a starting point for a new synthesis strategy leading to NiI phosphine complexes, replacing cod ligands by phosphines. Clearly visible colour changes indicate reactions within minutes, while quantum chemical calculations (PBE0-D3(BJ)/def2-TZVPP) approve exergonic reaction enthalpies in all performed ligand exchange reactions. Hence, [Ni(dppp)2 ][Al(ORF )4 ] (2, dppp=1,3-bis(diphenylphosphino)propane), [Ni(dppe)2 ][Al(ORF )4 ] (3, dppe=1,3-bis(diphenyl-phosphino)ethane), three-coordinate [Ni(PPh3 )3 ][Al(ORF )4 ] (4) and a remarkable two-coordinate NiI phosphine complex [Ni(PtBu3 )2 ][Al(ORF )4 ] (5) were characterised by single crystal X-ray structure analysis. EPR studies were performed, confirming a nickel d9 -configuration in complexes 2, 4 and 5. This result is supported by additional magnetization measurements of 4 and 5. Further investigations by cyclic voltammetry indicate relatively high oxidation potentials for these NiI compounds between 0.7 and 1.7 V versus Fc/Fc+ . Screening reactions with O2 and CO gave first insights on the reaction behaviour of the NiI phosphine complexes towards small molecules with formation of mixed phosphine-CO-NiI complexes and oxidation processes yielding new NiI and/or NiII derivatives. Moreover, 4 reacted with CH2 Cl2 at RT to give a dimeric NiII ylide complex (4 c). As CH2 Cl2 is a rather stable alkyl halide with relatively high C-Cl bond energies, 4 appears to be a suitable reagent for more general C-Cl bond activation reactions.

Dibenzo[a,e]pentalenes with Low-Lying LUMO Energy Levels as Potential n-Type Materials.

Ambipolar organic semiconductors are of high interest for organic field-effect transistors. For n-type conduction, low LUMO energies are required. Dibenzo[a,e]pentalenes (DBPs) are promising compounds; however, few derivatives exist with energetically low-lying LUMO levels. Here, we present DBP derivatives with LUMO energies down to -3.73 eV and small bandgaps down to 1.63 eV determined through cyclic voltammetry, UV/vis absorption spectroscopy, and TDDFT calculations. Single-crystal X-ray diffraction analysis revealed a 1D π-stacking mode. The addition of arylalkynyl substituents at the five-membered rings in a facile and versatile synthetic route allowed for tuning of the band gaps and LUMO energies. The synthetic route can easily be modified to access a variety of DBP derivatives. The LUMO energies of the DBP derivatives presented herein make them attractive for an application in n-type or ambipolar field-effect transistors.

Tetracationic Gallium Cluster Cations.

The univalent salt Ga(PhF)2 + [Al(ORF )4 ]- (RF =OC(CF3 )3 ) forms the strongly metal-metal bonded cluster tetracations [GaII 2 (L)4 ]4+ and [GaI 4 (L')8 ]4+ , when it reacts with innocent ligands like phenanthroline (L=phen) or t-butylisonitrile (L'=t Bu-NC). Their structures and energetics are discussed as a function of the employed ligands, supported by DFT calculations and Born-Fajans-Haber cycles.

Access to the Bis-benzene Cobalt(I) Sandwich Cation and its Derivatives: Synthons for a "Naked" Cobalt(I) Source?

The synthesis and structural characterization of the hitherto unknown parent Co(bz)2+ (bz=benzene) complex and several of its derivatives are described. Their synthesis starts either from a CoCO5+ salt, or directly from Co2 (CO)8 and a Ag+ salt. Stability and solubility of these complexes was achieved by using the weakly coordinating anions (WCAs) [Al(ORF )4 ]- and [F{Al(ORF )3 }2 ]- {RF =C(CF3 )3 } and the solvent ortho-difluorobenzene (o-DFB). The magnetic properties of Co(bz)2+ were measured and compared in the condensed and gas phases. The weakly bound Co(o-dfb)2+ salts are of particular interest for the preparation of further CoI salts, for example, the structurally characterized low-coordinate 12 valence electron Co(Pt Bu3 )2+ and Co(NHC)2+ salts.

From an Easily Accessible Pentacarbonylcobalt(I) Salt to Piano-Stool Cations [(arene)Co(CO)2 ].

The facile synthesis of a pentacarbonyl cobalt(I) salt without the need for a superacid as solvent is presented. This salt, [Co(CO)5 ]+ [Al(ORF )4 ]- {RF =C(CF3 )3 }, readily accessible on a multigram scale, undergoes substitution reactions with arenes yielding the hitherto unknown class of two-legged cobalt piano-stool complexes [(arene)Co(CO)2 ]+ with four different arene ligands. Such a substitution chemistry would have been impossible in superacid solution, as the arenes used would have been oxidized and/or protonated. Thus, the general approach described herein may have a wide synthetic use. Additionally, the thermochemistry of the piano-stool complexes is shown to be not easy to describe computationally and most of the established DFT methods overestimate the reaction energies. Only CCSD(T) calculations close to the basis set limit gave energies fully agreeing with the experiment.

Using the Lewis Acid Me3 Si-F-Al(ORF )3 To Prepare Phosphino-Phosphonium Cations with the Least-Coordinating Anion [(RF O)3 Al-F-Al(ORF )3 ].

By reaction of two equivalents of Me3 Si-F-Al(ORF )3 1 with an equimolar amount of PPh2 Cl, the salt [Ph2 P-PPh2 Cl]+ [(RF O)3 Al-F-Al(ORF )3 ]- 2 is prepared smoothly in 91 % yield (NMR, XRD). The synthesis of [Ph2 P-PPh3 ]+ [(RF O)3 Al-F-Al(ORF )3 ]- 3 is best achieved by a two-step reaction: first, two equivalents of 1 react with one PPh3 to give [Me3 Si-PPh3 ]+ [(RF O)3 Al-F-Al(ORF )3 ]- 4 (NMR, XRD), which, upon reaction with PPh2 Cl, yields pure 3 and Me3 SiCl (NMR, XRD). Typically, a stoichiometry of two equivalents of 1 with respect to one equivalent of the chloride donor should be used. Otherwise, the residual strong Lewis acidity of the [(RF O)3 Al-F-Al(ORF )3 ]- anion in the presence of the [F-Al(ORF )3 ]- anion-that forms with less than two equivalents of 1-leads to further chloride exchange reactions that complicate work-up. This route presents the easiest way to introduce the least-coordinating [(RF O)3 Al-F-Al(ORF )3 ]- anion into a system. We expect a wide use of this route in all areas, in which chloride-bond heterolysis in combination with very weakly coordinating anions is desirable. Additionally, we performed calculations on the bond dissociation mechanisms of [R2 P-PMe3 ]+ and the isoelectronic Me2 P-SiMe3 and Me2 Si-PMe3 in dependence of the solvent permittivity. These calculations show, especially for the neutral reference compounds, a heavy influence of the solvent on the dissociation mechanism, which is why we suggest investigating these properties in solution instead of gas phase.

The Parent Cyclopentadienyltin Cation, Its Toluene Adduct, and the Quadruple-Decker [Sn3 Cp4 ]2.

Truly cationic metallocenes with the parent cyclopentadienyl ligand are so far unknown for the Group 14 elements. Herein we report on an almost "naked" [SnCp]+ cation with the weakly coordinating [Al{OC(CF3 )3 }4 ]- and [{(F3 C)3 CO}3 Al-F-Al{OC(CF3 )3 }3 ]- anions. [SnCp][Al{OC(CF3 )3 }4 ] was used to prepare the first main-group quadruple-decker cation [Sn3 Cp4 ]2+ again as the [Al{OC(CF3 )3 }4 ]- salt. Additionally, the toluene adduct [CpSn(C7 H8 )][Al{OC(CF3 )3 }4 ] was obtained.

Homoleptic Gold Acetonitrile Complexes with Medium to Very Weakly Coordinating Counterions: Effect on Aurophilicity?

A series of gold acetonitrile complexes [Au(NCMe)2 ]+ [WCA]- with weakly coordinating counterions (WCAs) was synthesized by the reaction of elemental gold and nitrosyl salts [NO]+ [WCA]- in acetonitrile ([WCA]- =[GaCl4 ]- , [B(CF3 )4 ]- , [Al(ORF )4 ]- ; RF =C(CF3 )3 ). In the crystal structures, the [Au(NCMe)2 ]+ units appeared as monomers, dimers, or chains. A clear correlation between the aurophilicity and the coordinating ability of counterions was observed, with more strongly coordinating WCAs leading to stronger aurophilic contacts (distances, C-N stretching frequencies of [Au(NCMe)2 ]+ units). An attempt to prepare [Au(L)2 ]+ units, even with less weakly basic solvents like CH2 Cl2 , led to decomposition of the [Al(ORF )4 ]- anion and formation of [NO(CH2 Cl2 )2 ]+ [F(Al(ORF )3 )2 ]- . All nitrosyl reagents [NO]+ [WCA]- were generated according to an optimized procedure and were thoroughly characterized by Raman and NMR spectroscopy. Moreover, the to date unknown species [NO]+ [B(CF3 )3 CN]- was prepared. Its reaction with gold unexpectedly produced [Au(NCMe)2 ]+ [Au(NCB(CF3 )3 )2 ]- , in which the cyanoborate counterion acts as an anionic ligand itself. Interestingly, the auroborate anion [Au(NCB(CF3 )3 )2 ]- behaves as a weakly coordinating counterion, which becomes evident from the crystallographic data and the vibrational spectral characteristics of the [Au(NCMe)2 ]+ cation in this complex. Ligand exchange in the only room temperature stable salt of this series, [Au(NCMe)2 ]+ [Al(ORF )4 ]- , is facile and, for example, [Au(PPh3 )(NCMe)]+ [Al(ORF )4 ]- can be selectively generated. This reactivity opens the possibility to generate various [AuL1 L2 ]+ [Al(ORF )4 ]- salts through consecutive ligand-exchange reactions that offer access to a huge variety of AuI complexes for gold catalysis.

Mechanism of the Ti(III)-Catalyzed Acyloin-Type Umpolung: A Catalyst-Controlled Radical Reaction.

The titanium(III)-catalyzed cross-coupling between ketones and nitriles provides an efficient stereoselective synthesis of α-hydroxyketones. A detailed mechanistic investigation of this reaction is presented, which involves a combination of several methods such as EPR, ESI-MS, X-ray, in situ IR kinetics, and DFT calculations. Our findings reveal that C-C bond formation is turnover-limiting and occurs by a catalyst-controlled radical combination involving two titanium(III) species. The resting state is identified as a cationic titanocene-nitrile complex and the beneficial effect of added Et3N·HCl on yield and enantioselectivity is elucidated: chloride coordination initiates the radical coupling. The results are fundamental for the understanding of titanium(III)-catalysis and of relevance for other metal-catalyzed radical reactions. Our conclusions might apply to a number of reductive coupling reactions for which conventional mechanisms were proposed before.

Synthesis of bridged benzazocines and benzoxocines by a titanium-catalyzed double-reductive umpolung strategy.

A sequence of two titanium(III)-catalyzed reductive umpolung reactions is reported that allows the rapid construction of benzazo- and benzoxozine building blocks. The first step is a reductive cross-coupling of quinolones or chromones with Michael acceptors. This reaction proceeds with complete syn-selectivity for the quinolone functionalization while the anti-diastereomers are obtained as the major products from chromones. With different reaction conditions, the stereochemical outcome can be altered to afford the syn-chromanone products as well. A subsequent reductive ketyl radical cyclization forges the tricyclic title compounds in good yields. A stereochemical model explaining the observed stereoselectivities is provided and the product configurations were unambiguously verified by X-ray analyses and 2D NMR spectroscopic experiments.

[Ni(cod)2][Al(OR(F))4], a Source for Naked Nickel(I) Chemistry.

The straightforward synthesis of the cationic, purely organometallic Ni(I) salt [Ni(cod)2](+)[Al(OR(F))4](-) was realized through a reaction between [Ni(cod)2] and Ag[Al(OR(F))4] (cod = 1,5-cyclooctadiene). Crystal-structure analysis and EPR, XANES, and cyclic voltammetry studies confirmed the presence of a homoleptic Ni(I) olefin complex. Weak interactions between the metal center, the ligands, and the anion provide a good starting material for further cationic Ni(I) complexes.

Anagostic interactions under pressure: attractive or repulsive?

Square-planar d(8)-ML4 complexes might display subtle but noticeable local Lewis acidic sites in axial direction in the valence shell of the metal atom. These sites of local charge depletion provide the electronic prerequisites to establish weakly attractive 3c-2e M⋅⋅⋅H-C agostic interactions, in contrast to earlier assumptions. Furthermore, we show that the use of the sign of the (1)H NMR shifts as major criterion to classify M⋅⋅⋅H-C interactions as attractive (agostic) or repulsive (anagostic) can be dubious. We therefore suggest a new characterization method to probe the response of these M⋅⋅⋅H-C interactions under pressure by combined high pressure IR and diffraction studies.