Azide-Assisted Growth of Copper Nanostructures and Their Application as a Carbon Supported Catalyst in Two-Step Three-Component Azide-Alkyne Cycloadditions.

Copper nanostructures were obtained from the reduction of Cu(I) under mild conditions in ethanol/water using sodium-l-ascorbate and sodium azide while performing an amination reaction. When the halobenzene substrate was reacted in the presence of a bulk carbon black (CB) support, clustered copper sub-micrometer particles (SMPs) and microparticles (MPs) form. The growth conditions of the MPs were optimized, and the supported nanostructures were isolated and characterized by scanning electron microscopy, energy dispersive X-ray spectroscopy, thermogravimetry, and inductively-coupled plasma mass spectrometry. The particles are mobile and supported within the CB matrix and proved to be active catalysts in the azide-alkyne cycloaddition (CuAAC). The catalytic competency of the particles was assessed in a two-step three-component azide-alkyne cycloaddition of benzyl bromide, sodium azide, and phenylacetylene as a model reaction. The reaction conditions were optimized, and the optimized conditions were applied for the synthesis of triazole compounds with varying levels of functionalization. The recyclability of the catalysts was investigated, depletion modes were discussed, and the conditions were fine-tuned to reach good recyclability. This demonstrates the broader applicability of the SMPs/MPs as CuAAC-catalyst and its limitations.

Copper(I) Chloride Mediated Amination of Halobenzenes via Azides: Scope, Mechanistic Aspects, and C-C Cleavage Reactions.

Selective azidation-amination of long-chain alkanoyl halobenzenes with sodium azide, promoted by copper(I) chloride, is reported. The protocol is, apart from CuCl and NaN3, additive free and allows the isolation of versatile amine-azides. Alkyl cleavage occurs as a side reaction through an unusual Schmidt-type azide insertion adjacent to the carbonyl group, forming alkyl nitriles possibly via radical pathways. Mechanistic studies involving 15N labeling experiments and test substrates indicate that the reaction occurs via 1-azido-4-alkanoyl benzenes. The amination is applicable for substrates with electron-withdrawing groups and proceeds under mild conditions. The mechanism of the amine formation involves nitrenes. Intermediates were trapped by carrying out the reaction in the presence of the 2,2,6,6-(tetramethylpiperidin-1-yl)oxyl stable radical and characterized by high-resolution mass spectrometry. The intermediates are consistent with earlier mechanistic proposals.

2,7- and 4,9-Dialkynyldihydropyrene Molecular Switches: Syntheses, Properties, and Charge Transport in Single-Molecule Junctions.

This paper describes the syntheses of several functionalized dihydropyrene (DHP) molecular switches with different substitution patterns. Regioselective nucleophilic alkylation of a 5-substituted dimethyl isophthalate allowed the development of a workable synthetic protocol for the preparation of 2,7-alkyne-functionalized DHPs. Synthesis of DHPs with surface-anchoring groups in the 2,7- and 4,9-positions is described. The molecular structures of several intermediates and DHPs were elucidated by X-ray single-crystal diffraction. Molecular properties and switching capabilities of both types of DHPs were assessed by light irradiation experiments, spectroelectrochemistry, and cyclic voltammetry. Spectroelectrochemistry, in combination with density functional theory (DFT) calculations, shows reversible electrochemical switching from the DHP forms to the cyclophanediene (CPD) forms. Charge-transport behavior was assessed in single-molecule scanning tunneling microscope (STM) break junctions, combined with density functional theory-based quantum transport calculations. All DHPs with surface-contacting groups form stable molecular junctions. Experiments show that the molecular conductance depends on the substitution pattern of the DHP motif. The conductance was found to decrease with increasing applied bias.

Synthesis of Long-Chain Alkanoyl Benzenes by an Aluminum(III) Chloride-Catalyzed Destannylative Acylation Reaction.

This paper describes the facile synthesis of haloaryl compounds with long-chain alkanoyl substituents by the destannylative acylation of haloaryls bearing tri-n-butyltin (Bu3Sn) substituents. The method allows the synthesis of many important synthons for novel functional materials in a highly efficient manner. The halo-tri-n-butyltin benzenes are obtained by the lithium-halogen exchange of commercially available bis-haloarenes and the subsequent reaction with Bu3SnCl. Under typical Friedel-Crafts conditions, i.e., the presence of an acid chloride and AlCl3, the haloaryls are acylated through destannylation. The reactions proceed fast (<5 min) at low temperatures and thus are compatible with aromatic halogen substituents. Furthermore, the method is applicable to para-, meta-, and ortho-substitution and larger systems, as demonstrated for biphenyls. The generated tin byproducts were efficiently removed by trapping with silica/KF filtration, and most long-chain haloaryls were obtained chromatography-free. Molecular structures of several products were determined by X-ray single-crystal diffraction, and the crystal packing was investigated by mapping Hirshfeld surfaces onto individual molecules. A feasible reaction mechanism for the destannylative acylation reaction is proposed and supported through density functional theory (DFT) calculations. DFT results in combination with NMR-scale control experiments unambiguously demonstrate the importance of the tin substituent as a leaving group, which enables the acylation.

Carbon-Rich Trinuclear Octamethylferrocenophanes.

Several trinuclear ferrocenes are obtained by Friedel-Crafts reaction of octamethylferrocene with ferrocenoyl chloride and subsequent modifications. 1,1'-Diferrocenoyloctamethylferrocene (3) is transformed to the divinyl derivative (4a) by reaction with MeLi and AlCl3. The reactive 4a cyclizes spontaneously to a [4]ferrocenophane with buta-1,3-diene handle (5) or in the presence of AlCl3 to a [3]ferrocenophane with propene handle (6). Structure assignments are supported by X-ray crystallography and NMR spectroscopy, and mechanisms are proposed. Electrochemical behavior of the compounds was investigated with cyclic voltammetry, and assignments of the redox processes were carried out with the aid of density functional theory calculations. The synthesized compounds and demonstrated transformations represent useful tools for preparation of materials for charge-transport studies in metal-molecule-metal junctions.

Supramolecular vs Electronic Structure: The Effect of the Tilt Angle of the Active Group in the Performance of a Molecular Diode.

It is important to understand how the supramolecular structure of molecular junctions affects their performance. Such studies are challenging because it is difficult to separate electronic effects from supramolecular structural effects because both depend on each other. Here we show that by changing the connector group that connects the active component (a ferrocene unit) of a molecular diode to the backbone (an alkyl chain), both the electronic and supramolecular structures of the junctions are modified. The connector group determines the tilt angle of the Fc unit which in turn affects the packing structure of the molecular diodes. In this case, the supramolecular structure dominates over the electronic structure of the molecular diodes, and junctions with loosely packed SAMs result in poorly performing molecular diodes, while stiff, densely packed SAMs result in well-performing molecular diodes.

Supramolecular structure of self-assembled monolayers of ferrocenyl terminated n-alkanethiolates on gold surfaces.

It is important to understand the structure of redox-active self-assembled monolayers (SAMs) down to the atomic scale, since these SAMS are widely used as model systems in studies of mechanisms of charge transport or to realize electronic functionality in molecular electronic devices. We studied the supramolecular structure of SAMs of n-alkanethiolates with ferrocenyl (Fc) end groups (S(CH2)nFc, n = 3 or 4) on Au(111) by scanning tunneling microscopy (STM). In this system, the tilt angle of the Fc units with respect to the surface normal (α) depends on the value of n because the Au-S-C bond angle is fixed. The ordered domains of the SAMs were imaged by STM after annealing at 70 °C at ultrahigh vacuum conditions. High resolution electron energy loss spectroscopy (HREELS) and cyclic voltammetry show that this annealing step only removed physisorbed material and did not affect the structure of the SAM. The STM images revealed the presence of row defects at intervals of 4 nm, that is, six molecules. We determined by near edge X-ray absorption fine structure spectroscopy (NEXAFS) that the Fc units of the SAMs of SC3Fc are more parallel to the Au(111) plane with a tilt angle α = 60.2° than the Fc units of SC4Fc SAMs (α = 45.4°). These tilt angles are remarkably close to the tilt angles measured by X-ray diffraction data of bulk crystals (bc-plane). Based on our data, we conclude that the molecules are standing up and the SAMs pack into lattices that are distorted from their bulk crystal structures (because of the build-up stain due to the differences in size between the Fc units and thiolate anchoring groups).

Functionalised organometallic photoswitches containing dihydropyrene units.

Functionalising organic molecular photoswitches with metal complexes has been shown to alter and enhance their switching states. These organometallic photoswitches provide a promising basis for novel smart molecular materials and molecular electronic devices. We have detailed the synthesis and characterisation of mono- and bimetallic half-sandwich ruthenium and iron complexes functionalised with alkynyl dihydropyrenes (DHP). Their electronic and photophysical properties were determined by the use of chemical, electrochemical and spectroelectrochemical techniques. The introduction of the metal alkynyl moiety allows access to additional redox and protonation states not accessible by the DHP alone. An additional metal alkynyl moiety inhibits observable photochromic switching. Analysis of the NIR and IR bands in the mixed valence complexes suggests there is a high degree of charge delocalisation across the DHP.

Ferrocenes with perfluorinated side chains and ferrocenophanes with fluorinated handles.

Trifluorovinyl groups are introduced onto the cyclopentadienyl ligands of ferrocene at the 1-, 1,1'-, and 1,2-positions by Negishi-type and Stille-type coupling reactions of trifluorovinylzinc chloride and tri-n-butyltrifluorovinyl stannane with several iodoferrocenes. Modification of the trifluorovinyl group by nucleophilic substitution and [2+2] cycloaddition make them versatile building blocks for synthetic transformations. 1,1'-Bis(trifluorovinyl) ferrocene reacts upon contact with silica or oxidizing agents and in the presence of a suitable nucleophile through a redox autocatalytic mechanism to afford ferrocenophanes with fluorinated handles. C(F)(H) and C(F)(OMe) moieties in α-positions allowed further modifications to be performed by nucleophilic substitution of the fluorine atoms. A series of ferrocenes with fluorinated side chains and ferrocenophanes with fluorinated handles were isolated and characterized. Several molecular structures were determined by single-crystal X-ray diffraction. The influence of the fluorine substituents on the redox properties of the iron center were studied by cyclic voltammetry.

Development of tethered dual catalysts: synergy between photo- and transition metal catalysts for enhanced catalysis.

While dual photocatalysis-transition metal catalysis strategies are extensively reported, the majority of systems feature two separate catalysts, limiting the potential for synergistic interactions between the catalytic centres. In this work we synthesised a series of tethered dual catalysts allowing us to investigate this underexplored area of dual catalysis. In particular, Ir(i) or Ir(iii) complexes were tethered to a BODIPY photocatalyst through different tethering modes. Extensive characterisation, including transient absorption spectroscopy, cyclic voltammetry and X-ray absorption spectroscopy, suggest that there are synergistic interactions between the catalysts. The tethered dual catalysts were more effective at promoting photocatalytic oxidation and Ir-catalysed dihydroalkoxylation, relative to the un-tethered species, highlighting that increases in both photocatalysis and Ir catalysis can be achieved. The potential of these catalysts was further demonstrated through novel sequential reactivity, and through switchable reactivity that is controlled by external stimuli (heat or light).

The QTAIM approach to chemical bonding between transition metals and carbocyclic rings: a combined experimental and theoretical study of (eta(5)-C5H5)Mn(CO)3, (eta(6)-C6H6)Cr(CO)3, and (E)-{(eta(5)-C5H4)CF=CF(eta(5)-C5H4)}(eta(5)-C5H5)2Fe2.

Experimental charge densities for (C(5)H(5))Mn(CO)(3) (2), (eta(6)-C(6)H(6))Cr(CO)(3) (3), and (E)-{(eta(5)-C(5)H(4))CF=CF(eta(5)-C(5)H(4))}(eta(5)-C(5)H(5))(2)Fe(2) (4) have been obtained by multipole refinement of high-resolution X-ray diffraction data at 100 K. The resultant densities were analyzed using the quantum theory of atoms in molecules (QTAIM). The electronic structures of these and related pi-hydrocarbyl complexes have also been studied by ab initio density functional theory calculations, and a generally good agreement between theory and experiment with respect to the topological parameters was observed. The topological parameters indicate significant metal-ring covalency. A consistent area of disagreement concerns the topology of the metal-ring interactions. It is shown that because of the shared-shell bonding between the metal and the ring carbons, an annulus of very flat density rho and very small wedge rho is formed, which leads to topologically unstable structures close to catastrophe points. This in turn leads to unpredictable numbers of metal-C bond paths for ring sizes greater than four and fewer M-C bond paths than expected on the basis of the formal hapticity. This topological instability is a general feature of metal-pi-hydrocarbyl interactions and means that a localized approach based on individual M-C(ring) bond paths does not provide a definitive picture of the chemical bonding in these systems. However, other QTAIM indicators, such as the virial paths, the delocalization indices, and the source function, clearly demonstrate that for the n-hapto (eta(n)-C(n)H(n))M unit, there is generally a very similar level of chemical bonding for all M-C(ring) interactions, as expected on the basis of chemical experience.

Stable Molecular Diodes Based on π-π Interactions of the Molecular Frontier Orbitals with Graphene Electrodes.

In molecular electronics, it is important to control the strength of the molecule-electrode interaction to balance the trade-off between electronic coupling strength and broadening of the molecular frontier orbitals: too strong coupling results in severe broadening of the molecular orbitals while the molecular orbitals cannot follow the changes in the Fermi levels under applied bias when the coupling is too weak. Here, a platform based on graphene bottom electrodes to which molecules can bind via π-π interactions is reported. These interactions are strong enough to induce electronic function (rectification) while minimizing broadening of the molecular frontier orbitals. Molecular tunnel junctions are fabricated based on self-assembled monolayers (SAMs) of Fc(CH2 )11 X (Fc = ferrocenyl, X = NH2 , Br, or H) on graphene bottom electrodes contacted to eutectic alloy of gallium and indium top electrodes. The Fc units interact more strongly with graphene than the X units resulting in SAMs with the Fc at the bottom of the SAM. The molecular diodes perform well with rectification ratios of 30-40, and they are stable against bias stressing under ambient conditions. Thus, tunnel junctions based on graphene with π-π molecule-electrode coupling are promising platforms to fabricate stable and well-performing molecular diodes.

Molecular diodes with rectification ratios exceeding 105 driven by electrostatic interactions.

Molecular diodes operating in the tunnelling regime are intrinsically limited to a maximum rectification ratio R of ∼103. To enhance this rectification ratio to values comparable to those of conventional diodes (R ≥ 105) an alternative mechanism of rectification is therefore required. Here, we report a molecular diode with R = 6.3 × 105 based on self-assembled monolayers with Fc-C≡C-Fc (Fc, ferrocenyl) termini. The number of molecules (n(V)) involved in the charge transport changes with the polarity of the applied bias. More specifically, n(V) increases at forward bias because of an attractive electrostatic force between the positively charged Fc units and the negatively charged top electrode, but remains constant at reverse bias when the Fc units are neutral and interact weakly with the positively charged electrode. We successfully model this mechanism using molecular dynamics calculations.

Control over cyclisation sequences of 1,1'-bifunctional octamethylferrocenes to ferrocenophanes.

This paper describes the facile synthesis of a number of electron rich octamethyl[1.4]ferrocenophanes with unsaturated handles from 1,1'-bis(1-chlorovinyl)octamethylferrocene. Treatment of this reactive compound with sodium hydroxide in DMF initiates a series of reactions resulting in the formation of four different ferrocenophanes. The most complex of these products arises from a cascade of cyclisations giving an unusual, unsymmetrical bis-ferrocenophane with a central fused cyclobutene. Control over the reaction outcome is achieved by manipulating the concentration of NaOH. Mechanisms are proposed, and supported by DFT calculations.

1,1'-Diacetyloctamethylferrocene: an overlooked and overdue synthon leading to the facile synthesis of an octamethylferrocenophane.

This paper describes the facile preparation of 1,1'-diacetyloctamethylferrocene (2) by acylation of octamethylferrocene (1) with acetyl chloride. Chloroformylation with POCl3/DMF of 2 affords a variety of products, including 1,1'-bis-(1-chlorovinyl)octamethylferrocene (3b) in high yield. Compound 3b cyclises in aqueous sodium hydroxide/DMF to an octamethyl[1,4]-ferrocenophane bearing a 1-dimethylaminobuta-1,3-diene-handle (4).

Controlling the direction of rectification in a molecular diode.

A challenge in molecular electronics is to control the strength of the molecule-electrode coupling to optimize device performance. Here we show that non-covalent contacts between the active molecular component (in this case, ferrocenyl of a ferrocenyl-alkanethiol self-assembled monolayer (SAM)) and the electrodes allow for robust coupling with minimal energy broadening of the molecular level, precisely what is required to maximize the rectification ratio of a molecular diode. In contrast, strong chemisorbed contacts through the ferrocenyl result in large energy broadening, leakage currents and poor device performance. By gradually shifting the ferrocenyl from the top to the bottom of the SAM, we map the shape of the electrostatic potential profile across the molecules and we are able to control the direction of rectification by tuning the ferrocenyl-electrode coupling parameters. Our demonstrated control of the molecule-electrode coupling is important for rational design of materials that rely on charge transport across organic-inorganic interfaces.

Syntheses and purification of the versatile synthons iodoferrocene and 1,1'-diiodoferrocene.

This paper describes the improved synthesis and purification of iodoferrocene (FcI) and 1,1'-diiodoferrocene (FcI2). FcI and FcI2 were prepared by mono- and dilithiation of ferrocene followed by conversion into iodoferrocenes by reaction with iodine. Purification was accomplished by a simple sublimation/distillation procedure, affording FcI and FcI2 in high yields (74 and 72%) and high purity (>99.9%). We determined the molecular structures of FcI and FcI2 by X-ray single crystal diffraction.

Autocatalytic formation of fluorinated ferrocenophanes from 1,1'-bis(trifluorovinyl)ferrocene.

1,1'-Bis(trifluorovinyl)ferrocene is obtained in excellent yields from a Negishi type coupling reaction. Under certain redox conditions and in the presence of a suitable nucleophile, the highly reactive compound undergoes a transformation to fluorinated ferrocenophanes. Intra and intermolecular [2+2]-cycloaddition products are formed upon heating.