Boron Bridges Research Articles

Colorless Near-Infrared Absorbing Dyes Based on B-N Fused Donor-Acceptor-Donor π-Conjugated Molecules for Organic Phototransistors.

The introduction of a colorless function to organic electronic devices allows responses to light in the near-infrared (NIR) region and is expected to broaden the applications of these devices. However, the development of a colorless NIR dye remains a challenge due to the lack of a rational molecular design for controlling electronic transitions. In this study, to suppress the π-π* transitions in the visible region, polycyclic donor-acceptor-donor π-conjugated molecules with boron bridges (Py-FNTz-B and IP-FNTz-B) are designed and synthesized, which contain pyrrole or indenopyrrole as donor units with fluorinated naphthobisthiadiazole (FNTz) as an acceptor unit. The pyrrole end-capped Py-FNTz-B shows an absorption band in the NIR region without distinct visible-light absorption, which has led to the establishment of colorless characteristics. The indenopyrrole end-capped IP-FNTz-B shows a narrow optical energy gap of 0.87eV in films. Time-resolved microwave conductance and field-effect transistors demonstrate the semiconducting characteristics of these molecules, and Py-FNTz-B-based devices function as NIR phototransistors. Theoretical analyses indicate that the combination of a polyene-like electronic structure with orbital symmetry is important to obtain NIR wavelength-selective absorption. This study suggests that a molecular design based on electronic structures can be effective in the development of colorless NIR-absorbing dyes for organic electronics.

Effect of Boron-Doping on Gate-Opening CO2 Adsorption in Zinc-Benzimidazolate Coordination Networks.

Flexible metal-organic frameworks (MOFs) have attracted much attention as selective gas adsorption and storage. This report describes boron doping in zeolitic imidazolate framework-7 (B-ZIF-7), which exhibits reversible phase transition during CO2 adsorption/desorption. We have successfully prepared B-ZIF-7 coordination networks using boron-bridged benzimidazolate (B(bim)4-) as organic ligands. Powder X-ray diffraction (PXRD) measurements and infrared spectroscopy revealed that B-ZIF-7 has a crystal structure similar to that of ZIF-7 while containing boron bridging in the coordination network. Since B-ZIF-7 forms a cationic coordination network, the guest anions are encapsulated within the pore. CO2 adsorption/desorption measurements at 300 K showed that B-ZIF-7(NO3), which contains nitrate ions (NO3-) as guest anions in its pores, exhibits a S-shaped CO2 adsorption/desorption isotherm, which is characteristic of gate-opening type MOFs. Compared with ZIF-7, B-ZIF-7(NO3) has superior CO2 adsorption capacity in the low-pressure and superior CO2 storage capacity. The CO2 adsorption and desorption behavior of B-ZIF-7(NO3) was analyzed by in situ temperature-controlled PXRD measurements and thermogravimetric analysis under a CO2 atmosphere, and a reversible phase transition was observed. We have also successfully prepared B-ZIF-7(Cl) and B-ZIF-7(OTf) (OTf = CF3SO3-) with different guest anions. The CO2 adsorption/desorption behaviors of B-ZIF-7(Cl) and B-ZIF-7(OTf) were significantly different from those of B-ZIF-7(NO3) and ZIF-7. B-ZIF-7(Cl) showed gate opening at a higher pressure than ZIF-7, and B-ZIF-7(OTf) did not show S-shaped CO2 adsorption isotherm and showed adsorption behavior in micropores. These results indicate that the CO2 adsorption behavior of B-ZIF-7 depends on the interaction between the guest anions and CO2 molecules or the cationic framework and the bulkiness of the guest anions. Boron doping in a coordination network with boron-bridged imidazolate ligands is a promising strategy to increase the gas adsorption capability of porous materials.

Arabinogalactan-Proteins as Boron-Acting Enzymes, Cross-Linking the Rhamnogalacturonan-II Domains of Pectin

Most pectic rhamnogalacturonan-II (RG-II) domains in plant cell walls are borate-bridged dimers. However, the sub-cellular locations, pH dependence, reversibility and biocatalyst involvement in borate bridging remain uncertain. Experiments discussed here explored these questions, utilising suspension-cultured plant cells. In-vivo pulse radiolabelling showed that most RG-II domains dimerise extremely quickly (<4 min after biosynthesis, thus while still intraprotoplasmic). This tallies with the finding that boron withdrawal causes cell wall weakening within 10–20 min, and supports a previously proposed biological role for boron/RG-II complexes specifically at the wall/membrane interface. We also discuss RG-II monomer ↔ dimer interconversion as monitored in vitro using gel electrophoresis and a novel thin-layer chromatography method to resolve monomers and dimers. Physiologically relevant acidity did not monomerise dimers, thus boron bridge breaking cannot be a wall-loosening mechanism in ‘acid growth’; nevertheless, recently discovered RG-II trimers and tetramers are unstable and may thus underpin reversible wall loosening. Dimerising monomers in vitro by B(OH)3 required the simultaneous presence of RG-II-binding ‘chaperones’: co-ordinately binding metals and/or ionically binding cationic peptides. Natural chaperones of the latter type include highly basic arabinogalactan protein fragments, e.g., KHKRKHKHKRHHH, which catalyse a reaction [2 RG-II + B(OH)3 → RG-II–B–RG-II], suggesting that plants can ‘enzymically’ metabolise boron.

Strong ferromagnetism of g-C3N4 achieved by atomic manipulation

Two-dimensional (2D) metal-free ferromagnetic materials are ideal candidates to fabricate next-generation memory and logic devices, but optimization of their ferromagnetism at atomic-scale remains challenging. Theoretically, optimization of ferromagnetism could be achieved by inducing long-range magnetic sequence, which requires short-range exchange interactions. In this work, we propose a strategy to enhance the ferromagnetism of 2D graphite carbon nitride (g-C3N4), which is facilitating the short-range exchange interaction by introducing in-planar boron bridges. As expected, the ferromagnetism of g-C3N4 was significantly enhanced after the introduction of boron bridges, consistent with theoretical calculations. Overall, boosting ferromagnetism of 2D materials by introducing bridging groups is emphasized, which could be applied to manipulate the magnetism of other materials.

Tris(boranil) Columnar Liquid Crystalline Fluorophores: pseudo-Triphenylene Boron(III) Complexes with Peripheral N-B-O Linkages.

The borate complexes derived from salicylaldimine ligands, called boranils, possess a wide range of photophysical and electronic characteristics intrinsically. The unique combination of molecular rigidity, rendered by four-coordinate boron bridges, and extended π-conjugation enable them to serve as technically feasible fluorescent materials (dyes). The incorporation of liquid crystallinity in these boron(III) complexes, especially the columnar (Col) mesomorphism, which is overlooked hitherto, would provide a new dimension to these complexes. Herein, we report the first examples of tris(boranil) discotic liquid crystal (LC) dyes that have been readily synthesized by treating tris(N-salicylideneaniline)s, (TSAN)s, with BF3 .Et2 O in the presence of an acid quencher. These C3 -symmetric borate complexes self-assemble into the Col phase, existing over a wide thermal span including room temperature. The 2D periodic order of the Col phases shows dependence on the length of the peripheral tails. The photophysical measurements reveal the fluorescence emission in their two condensed states viz., solid and Col phase, and in solution. Their electrochemical, two-step oxidation process coupled with the aforesaid features upholds their significance in applied research.

Making and breaking of boron bridges in the pectic domain rhamnogalacturonan-II at apoplastic pH invivo and invitro.

Cross-linking of the cell-wall pectin domain rhamnogalacturonan-II (RG-II) via boron bridges between apiose residues is essential for normal plant growth and development, but little is known about its mechanism or reversibility. We characterized the making and breaking of boron bridges invivo and invitro at 'apoplastic' pH. RG-II (13-26μm) was incubated in living Rosa cell cultures and cell-free media with and without 1.2mm H3 BO3 and cationic chaperones (Ca2+ , Pb2+ , polyhistidine, or arabinogalactan-protein oligopeptides). The cross-linking status of RG-II was monitored electrophoretically. Dimeric RG-II was stable at pH 2.0-7.0 invivo and invitro. In-vitro dimerization required a 'catalytic' cation at all pHs tested (1.75-7.0); thus, merely neutralizing the negative charge of RG-II (at pH 1.75) does not enable boron bridging. Pb2+ (20-2500μm) was highly effective at pH 1.75-4.0, but not 4.75-7.0. Cationic peptides were effective at approximately 1-30μm; higher concentrations caused less dimerization, probably because two RG-IIs then rarely bonded to the same peptide molecule. Peptides were ineffective at pH 1.75, their pH optimum being 2.5-4.75. d-Apiose (>40mm) blocked RG-II dimerization invitro, but did not cleave existing boron bridges. Rosa cells did not take up d-[U-14 C]apiose; therefore, exogenous apiose would block only apoplastic RG-II dimerization invivo. In conclusion, apoplastic pH neither broke boron bridges nor prevented their formation. Thus boron-starved cells cannot salvage boron from RG-II, and 'acid growth' is not achieved by pH-dependent monomerization of RG-II. Divalent metals and cationic peptides catalyse RG-II dimerization via co-ordinate and ionic bonding respectively (possible and impossible, respectively, at pH 1.75). Exogenous apiose may be useful to distinguish intra- and extra-protoplasmic dimerization.

An Arabidopsis thaliana arabinogalactan-protein (AGP31) and several cationic AGP fragments catalyse the boron bridging of rhamnogalacturonan-II

Rhamnogalacturonan-II (RG-II) is a complex pectic domain in plant primary cell walls. In vivo, most RG-II domains are covalently dimerised via borate diester bridges, essential for correct cell-wall assembly, but the dimerisation of pure RG-II monomers by boric acid in vitro is extremely slow. Cationic ‘chaperones’ can promote dimerisation, probably by overcoming the mutual repulsion between neighbouring anionic RG-II molecules. Highly effective artificial chaperones include Pb2+ and polyhistidine, but the proposed natural chaperones remained elusive. We have now tested cationic peptide fragments of several Arabidopsis thaliana arabinogalactan-proteins (AGPs) as candidates. Fragments of AGP17, 18, 19 and 31 were effective, typically at ∼25 µg/ml (9–19 µM), promoting the boron bridging of 16–20 µM monomeric RG-II at pH 4.8 in vitro. Native AGP31 glycoprotein was also effective, and hexahistidine was moderately so. All chaperones tested interacted reversibly with RG-II and were not consumed during the reaction; thus they acted catalytically, and may constitute the first reported boron-acting enzyme activity, an RG-II borate diesterase. Many of the peptide chaperones became less effective catalysts at higher concentration, which we interpret as due to the formation of RG-II–peptide complexes with a net positive charge, as mutually repulsive as negatively charged pure RG-II molecules. The four unique AGPs studied here may serve an enzymic role in the living plant cell, acting on RG-II within Golgi cisternae and/or in the apoplast after secretion. In this way, RG-II and specific AGPs may contribute to cell-wall assembly and hence plant cell expansion and development.

Boron bridging of rhamnogalacturonan-II in Rosa and arabidopsis cell cultures occurs mainly in the endo-membrane system and continues at a reduced rate after secretion.

Rhamnogalacturonan-II (RG-II) is a domain of primary cell-wall pectin. Pairs of RG-II domains are covalently cross-linked via borate diester bridges, necessary for normal cell growth. Interpreting the precise mechanism and roles of boron bridging is difficult because there are conflicting hypotheses as to whether bridging occurs mainly within the Golgi system, concurrently with secretion or within the cell wall. We therefore explored the kinetics of RG-II bridging. Cell-suspension cultures of Rosa and arabidopsis were pulse-radiolabelled with [14C]glucose, then the boron bridging status of newly synthesized [14C]RG-II domains was tracked by polyacrylamide gel electrophoresis of endo-polygalacturonase digests. Optimal culture ages for 14C-labelling were ~5 and ~1 d in Rosa and arabidopsis respectively. De-novo [14C]polysaccharide production occurred for the first ~90 min; thereafter the radiolabelled molecules were tracked as they 'aged' in the wall. Monomeric and (boron-bridged) dimeric [14C]RG-II domains appeared simultaneously, both being detectable within 4 min of [14C]glucose feeding, i.e. well before the secretion of newly synthesized [14C]polysaccharides into the apoplast at ~15-20 min. The [14C]dimer : [14C]monomer ratio of RG-II remained approximately constant from 4 to 120 min, indicating that boron bridging was occurring within the Golgi system during polysaccharide biosynthesis. However, [14C]dimers increased slightly over the following 15 h, indicating that limited boron bridging was continuing after secretion. The results show where in the cell (and thus when in the 'career' of an RG-II domain) boron bridging occurs, helping to define the possible biological roles of RG-II dimerization and the probable localization of boron-donating glycoproteins or glycolipids.

Pyrrolylquinoline-BF2 and BPh2 BODIPY-Type Analogues: Synthesis, Structural Analysis and Photophysical Properties

Two new pyrrolylquinoline-substituted heteroaromatic-containing compounds bearing a central boron bridge have been prepared by a short, high-yielding sequence consisting of Suzuki-coupling of 8-bromoquinoline and N-Boc 2-pyrroleboronic acid, thermolytic tert-butyloxycarbonyl deprotection, and subsequent boron chelation (either using boron trifluoride or triphenylborane). Both derivatives display longer wavelength absorption maxima (λabsmax) than a previously reported indolopyridine-BPh2 analogue, in agreement with the smaller HOMO-LUMO energy gap predicted by DFT quantum chemical calculations. Both of the pyrrolylquinoline-boron chelates display weak emission (quantum yields 0.3–0.9%) and the BPh2 complex displays a very broad, long-wavelength emission (λemmax = 715 nm, MeCN), which may be due to dimer emission and results in a large pseudo-Stokes’ shift (7753 cm−1) for this compound.

Biochemical differences in the skin of two blueberries (Vaccinium corymbosum) varieties with contrasting firmness: Implication of ions, metabolites and cell wall related proteins in two developmental stages

The pursuit of firmer and better-quality blueberries is a continuous task that aims at a more profitable production. To this end it is essential to understand the biological processes linked to fruit firmness, which may diverge among tissues. By contrasting varieties with opposing firmness, we were able to elucidate events that, taking place at immature stage, lay the foundation to produce a firmer ripe fruit. A deep analysis of blueberry skin was carried out, involving diverse comparative approaches including proteomics and metabolomics coupled to immunolocalization assays. In‘O'Neal’ (low firmness) enhanced levels of aquaporins, expansins and pectin esterases at the green stage were found to be critical in distinguishing it from ‘Emerald’ (high firmness). The latter featured higher levels of ABA, low methyl esterified pectins in tricellular junctions and high levels of catechin at this stage. Meanwhile, in ‘Emerald’ ‘s ripe fruit epicarp, several mechanisms of cell wall reinforcement such as calcium and probably boron bridges, appear to be more prominent than in ‘O'Neal’. This study highlights the importance of cell wall reorganization and structure, abundance of specific metabolites, water status, and hormonal signalling in connection to fruit firmness. These findings result particularly valuable in order to improve the fertilization procedures or in the search of molecular markers related with firmness.

Tailoring the Molecular Skeleton of Aza‐BODIPYs to Design Photostable Red‐Light‐Emitting Laser Dyes

AbstractIn this article the design and characterization of a set of novel red‐light‐emitting laser aza‐BODIPY dyes is reported. The applied synthetic method allows an exhaustive and versatile functionalization of both the dipyrrin core and the boron bridge. From the analysis of the photophysical and laser signatures, we determine the suitable modifications of the chromophoric backbone necessary to modulate the emission spectral region, efficiency and photostability under a strong irradiation regime. These dyes are endowed with efficient fluorescence and laser emission, and are particularly outstanding in terms of their high photostability, a key parameter to guarantee long‐lasting emission in any (bio)technological application. The herein‐reported results support, for the first time, the viability of aza‐BODIPYs as tunable red laser dyes. In fact, the laser performances of some of the tested aza‐BODIPYs surpass those of commercially available laser dyes in the same spectral region.

Active proton efflux, nutrient retention and boron-bridging of pectin are related to greater tolerance of proton toxicity in the roots of two Erica species

Background and aimsTolerance to soil acidity was studied in two species of Ericaceae that grow in mine-contaminated soils (S Portugal, SW Spain) to find out if there are interspecific variations in H+ tolerance which might be related to their particular location. MethodsTolerance to H+ toxicity was tested in nutrient solutions using seeds collected in SW Spain. Plant growth and nutrient contents in leaves, stems and roots were determined. Viability tests and proton exchange were studied in roots exposed, short-term, to acidic conditions. Membrane ATPase activity and the cell-wall pectic polysaccharide domain rhamnogalacturonan-II (RG-II) were analysed to find out interspecific differences. ResultsVariation in survival, growth and mineral composition was found between species. The H+-tolerant species (Erica andevalensis) showed greater concentration of nutrients than E. australis. Very low pH (pH 2) produced a significant loss of root nutrients (K, P, Mg) in the sensitive species. Root ATPase activity was slightly higher in the tolerant species with a correspondingly greater H+ efflux capacity. In both species, the great majority of the RG-II domains were in their boron-bridged dimeric form. However, shifting to a medium of pH 2 caused some of the boron bridges to break in the sensitive species. ConclusionsVariation in elements linked to the cell wall-membrane complex and the stability of their components (RG-II, H+-ATPases) are crucial for acid stress tolerance. Thus, by maintaining root cell structure, active proton efflux avoided toxic H+ build-up in the cytoplasm and supported greater nutrient acquisition in H+-tolerant species.

Thermally evolved and boron bridged graphene oxide (GO) frameworks constructed on microporous hollow fiber substrates for water and organic matters separation

For the first time, ultrathin boron bridged graphene oxide (GO) membranes have been successfully constructed on the modified microporous hollow fiber substrates using borates as the cross-linker. Nanochannels within GO layers can be molecularly engineered by manipulating the annealing process and the chemistry of boron bridges. GO framework composite membranes with proper charge properties and size exclusion functionalities have been developed to allow fast water transport and high rejections. Comparing to the pristine GO membrane, boron bridged GO membranes show not only less variations in terms of flux and rejection with annealing temperature, but also high stability during the long term test. Both pore size distribution and X-ray photoelectron spectroscopy were employed to elucidate the fundamental sciences about the evolution of nanochannels and surface chemistry of GO composite membranes with annealing temperature. The newly fabricated boron bridged GO membranes annealed at 65 °C exhibit a water permeability of 6.64–11.66 L m−2 h−1 bar−1 and high rejections of >97% against dyes with molecular weights of 300–1000 Da. The boron bridged GO membranes may have great potential to treat hot and harsh wastewater as well as for water reuse and dye separation industries.

Boron bridging of rhamnogalacturonan-II is promoted in vitro by cationic chaperones, including polyhistidine and wall glycoproteins.

Summary Dimerization of rhamnogalacturonan‐II (RG‐II) via boron cross‐links contributes to the assembly and biophysical properties of the cell wall. Pure RG‐II is efficiently dimerized by boric acid (B(OH)3) in vitro only if nonbiological agents for example Pb2+ are added. By contrast, newly synthesized RG‐II domains dimerize very rapidly in vivo. We investigated biological agents that might enable this.We tested for three such agents: novel enzymes, borate‐transferring ligands and cationic ‘chaperones’ that facilitate the close approach of two polyanionic RG‐II molecules. Dimerization was monitored electrophoretically.Parsley shoot cell‐wall enzymes did not affect RG‐II dimerization in vitro. Borate‐binding ligands (apiose, dehydroascorbic acid, alditols) and small organic cations (including polyamines) also lacked consistent effects. Polylysine bound permanently to RG‐II, precluding electrophoretic analysis. However, another polycation, polyhistidine, strongly promoted RG‐II dimerization by B(OH)3 without irreversible polyhistidine–RG‐II complexation. Likewise, partially purified spinach extensins (histidine/lysine‐rich cationic glycoproteins), strongly promoted RG‐II dimerization by B(OH)3 in vitro.Thus certain polycations, including polyhistidine and wall glycoproteins, can chaperone RG‐II, manoeuvring this polyanionic polysaccharide domain such that boron‐bridging is favoured. These chaperones dissociate from RG‐II after facilitating its dimerization, indicating that they act catalytically rather than stoichiometrically. We propose a natural role for extensin–RG‐II interaction in steering cell‐wall assembly.

Rhamnogalacturonan-II cross-linking of plant pectins via boron bridges occurs during polysaccharide synthesis and/or secretion

Rhamnogalacturonan-II (RG-II), a domain of plant cell wall pectins, is able to cross-link with other RG-II domains through borate diester bridges. Although it is known to affect mechanical properties of the cell wall, the biochemical requirements and lifecycle of this cross-linking remain unclear. We developed a PAGE methodology to allow separation of monomeric and dimeric RG-II and used this to study the dynamics of cross-linking in vitro and in vivo. Rosa cells grown in medium with no added boron contained no RG-II dimers, although these re-appeared after addition of boron to the medium. However, other Rosa cultures which were unable to synthesize new polysaccharides did not show dimer formation. We conclude that RG-II normally becomes cross-linked intraprotoplasmically or during secretion, but not post-secretion.

Boron bridging of rhamnogalacturonan‐II, monitored by gel electrophoresis, occurs during polysaccharide synthesis and secretion but not post‐secretion

The cell-wall pectic domain rhamnogalacturonan-II (RG-II) is cross-linked via borate diester bridges, which influence the expansion, thickness and porosity of the wall. Previously, little was known about the mechanism or subcellular site of this cross-linking. Using polyacrylamide gel electrophoresis (PAGE) to separate monomeric from dimeric (boron-bridged) RG-II, we confirmed that Pb2+ promotes H3BO3-dependent dimerisation in vitro. H3BO3 concentrations as high as 50 mm did not prevent cross-linking. For in-vivo experiments, we successfully cultured ‘Paul's Scarlet’ rose (Rosa sp.) cells in boron-free medium: their wall-bound pectin contained monomeric RG-II domains but no detectable dimers. Thus pectins containing RG-II domains can be held in the wall other than via boron bridges. Re-addition of H3BO3 to 3.3 μm triggered a gradual appearance of RG-II dimer over 24 h but without detectable loss of existing monomers, suggesting that only newly synthesised RG-II was amenable to boron bridging. In agreement with this, Rosa cultures whose polysaccharide biosynthetic machinery had been compromised (by carbon starvation, respiratory inhibitors, anaerobiosis, freezing or boiling) lost the ability to generate RG-II dimers. We conclude that RG-II normally becomes boron-bridged during synthesis or secretion but not post-secretion. Supporting this conclusion, exogenous [3H]RG-II was neither dimerised in the medium nor cross-linked to existing wall-associated RG-II domains when added to Rosa cultures. In conclusion, in cultured Rosa cells RG-II domains have a brief window of opportunity for boron-bridging intraprotoplasmically or during secretion, but secretion into the apoplast is a point of no return beyond which additional boron-bridging does not readily occur.

N]Borametalloarenophanes (n = 1, 2): Strained Systems with Uncommon Reactivity Patterns

AbstractThe chemistry of ansa‐complexes is of increasing importance in organometallic chemistry and materials science, a development that is closely related to the discovery of the first strained [1]silaferrocenophane in 1975 and their high value as efficient monomers in the ring‐opening polymerization to prepare high‐molecular‐weight metallopolymers. By contrast, interest in boron‐bridged derivatives has sparked only recently, which is rather surprising given the anticipated high degree of molecular ring strain imposed by the small covalent radius of the boron nucleus. Thus, it was not until 1997 that boron was successfully incorporated into an ansa‐bridge. Since then, the chemistry of strained [n]borametalloarenophanes has been studied systematically with respectto synthesis, electronic/structural properties and reactivity patterns. As a consequence, a rather large variety of [n]borametalloarenophanes based on different sandwich complex systems such as homoleptic [Fe(η5‐C5H5)2] and [M(η6‐C6H6)2] (M = V, Cr), as well as heteroleptic [Mn(η5‐C5H5)(η6‐C6H6)] and [M(η5‐C5H5)(η7‐C7H7)] (M = Ti, V, Cr) are now available. The structural and electronic consequences of the small mono‐ or diatomic boron bridges have been evaluated by different spectroscopic methods and X‐ray diffraction studies. Most importantly, these systems were shown to possess an enhanced reactivity. Uncommon and exciting reaction pathways could be encountered, which are evidently driven by the release of molecular ring strain. This Microreview is intended to highlight the consequences of molecular ring strain on the properties of [n]borametalloarenophanes. We begin with a comprehensive overview on the experimental approaches applied to their synthesis, and consider their unique properties in great detail. Subsequently, we will recognize that the highly strained character of the [n]bora‐metalloarenophanes enables unprecedented reaction pathways.

The Role of Bridging Group of Cyclopentadienyl Ligands for the Ziegler–Natta Catalysis: Theoretical Study

The potential energy surfaces of the initial reactions of ethylene insertion for the Ziegler–Natta catalysis with bridging groups of Cp ligands were studied by ab initio MO and density functional methods. Three metals (Ti, Zr, and Hf) in the Zeigler-Natta catalysis and eight bridging groups (BH, CH2, NH, O, AlH, SiH2, PH, and S) were treated. For the complex formation between ethylene and metallocenes, two type structures (vertical and horizon) were found. The vertical type structures are more stable in energy than the horizon types. The formation energy of the complex between ethylene and the metallocenes by incorporation of bridging atom or group is related to the geometrical hindrance and the bond interaction as shown in the case of boron bridging system.

Electronic communication in oligonuclear ferrocene complexes with anionic four-coordinate boron bridges

The di- and trinuclear ferrocene species Li[Fc-BPh(2)-Fc] (Li[]) and Li(2)[Fc-BPh(2)-fc-BPh(2)-Fc] (Li(2)[]) have been investigated with regard to their electrochemical properties and the degree of intervalence charge-transfer after partial oxidation. Li[] shows two distinct one-electron redox waves for its chemically equivalent ferrocenyl substituents in the cyclic voltammogram (E(1/2) = -0.38 V, -0.64 V; vs. FcH/FcH(+)). The corresponding values of Li(2)[] are E(1/2) = -0.45 V (two-electron process) and -1.18 V. All these redox events are reversible at r. t. on the time scale of cyclic voltammetry. X-ray crystallography on the mixed-valent Fe(II)(2)Fe(III) complex Li(12-c-4)(2)[] reveals the centroid-centroid distance between the cyclopentadienyl rings of each of the terminal ferrocenyl substituents (3.329 A) to be significantly smaller than in the central 1,1'-ferrocenediyl fragment (3.420 A). This points towards a charge-localized structure (on the time scale of X-ray crystallography) with the central iron atom being in the Fe(III) state. Mössbauer spectroscopic measurements on Li(12-c-4)(2)[] lend further support to this interpretation. Spectroelectrochemical measurements on Li[] and Li(2)[] in the wavelength range between 300-2800 nm do not show bands interpretable as intervalence charge-transfer absorptions for the mixed-valent states. All data accumulated so far lead to the conclusion that electronic interaction between the individual Fe atoms in Li[] and Li(2)[] occurs via a through-space pathway and/or is electrostatic in nature.

A Boron Bridge

A Boron Bridge