Small Changes Have Consequences: Lessons from Tetrabenzyltitanium and ‐zirconium Surface Organometallic Chemistry

Homoleptic benzyl derivatives of titanium and zirconium have been grafted onto silica that was dehydroxylated at 200 and 700 °C, thereby affording bi‐grafted and mono‐grafted single‐site species, respectively, as shown by a combination of experimental techniques (IR, MAS NMR, EXAFS, and elemental analysis) and theoretical calculations. Marked differences between these compounds and their neopentyl analogues are discussed and rationalized by using DFT. These differences were assigned to the selectivity of the grafting process, which, depending on the structure of the molecular precursors, led to different outcomes in terms of the mono‐ versus bi‐grafted species for the same surface concentration of silanol species. The benzylzirconium derivatives were active towards ethylene polymerization in the absence of an activator and the bi‐grafted species displayed higher activity than their mono‐grafted analogues. In contrast, the benzyltitanium and neopentylzirconium counterparts were not active under similar reaction conditions.     

One‐Dimensional Arsenic Allotropes: Polymerization of Yellow Arsenic Inside Single‐Wall Carbon Nanotubes

The pnictogen nanomaterials, including phosphorene and arsenene, display remarkable electronic and chemical properties. Yet, the structural diversity of these main group elements is still poorly explored. Here we fill single‐wall carbon nanotubes with elemental arsenic from the vapor phase. Using electron microscopy, we find chains of highly reactive As₄ molecules as well as two new one‐dimensional allotropes of arsenic: a single‐stranded zig‐zag chain and a double‐stranded zig‐zag ladder. These linear structures are important intermediates between the gas‐phase clusters of arsenic and the extended sheets of arsenene. Raman spectroscopy indicates weak electronic interaction between the arsenic and the nanotubes which implies that the formation of the new allotropes is driven primarily by the geometry of the confinement. The relative stabilities of the new arsenic structures are estimated computationally. Band‐gap calculations predict that the insulating As₄ chains become semiconducting, once converted to the zig‐zag ladder, and form a fully metallic allotrope of arsenic as the zig‐zag chain.     

Two‐Dimensional Ketone‐Driven Metal–Organic Coordination on Cu(111)

Two‐dimensional metal–organic nanostructures based on the binding of ketone groups and metal atoms were fabricated by depositing pyrene‐4,5,9,10‐tetraone (PTO) molecules on a Cu(111) surface. The strongly electronegative ketone moieties bind to either copper adatoms from the substrate or codeposited iron atoms. In the former case, scanning tunnelling microscopy images reveal the development of an extended metal–organic supramolecular structure. Each copper adatom coordinates to two ketone ligands of two neighbouring PTO molecules, forming chains that are linked together into large islands through secondary van der Waals interactions. Deposition of iron atoms leads to a transformation of this assembly resulting from the substitution of the metal centres. Density functional theory calculations reveal that the driving force for the metal substitution is primarily determined by the strength of the ketone–metal bond, which is higher for Fe than for Cu. This second class of nanostructures displays a structural dependence on the rate of iron deposition.     

Two‐, One‐, and Zero‐Dimensional Elemental Nanostructures Based on Ge9‐Clusters

We investigated the structural principles of novel germanium modifications derived by oxidative coupling of Zintl‐type [Ge₉]^4?clusters in various ways. The structures, stabilities, and electronic properties of the predicted {²_∞[Ge₉]_n} sheet, {¹_∞[Ge₉]_n} nanotubes, and fullerene‐like {Ge₉}_n cages were studied by using quantum chemical methods. The polyhedral {Ge₉}_n cages are energetically comparable with bulk‐like nanostructures of the same size, in good agreement with previous experimental findings. Three‐dimensional structures derived from the structures of lower dimensionality are expected to shed light on the structural characteristics of the existing mesoporous Ge materials that possess promising optoelectronic properties. Furthermore, 3D networks derived from the polyhedral {Ge₉}_n cages lead to structures that are closely related to the well‐known LTA zeolite framework, suggesting further possibilities for deriving novel mesoporous modifications of germanium. Raman and IR spectra and simulated X‐ray diffraction patterns of the predicted materials are given to facilitate comparisons with experimental results. The studied novel germanium modifications are semiconducting, and several structure types possess noticeably larger band gaps than bulk α‐Ge.     

Two‐Dimensional Noble‐Metal Chalcogenides and Phosphochalcogenides

Noble‐metal chalcogenides, dichalcogenides, and phosphochalcogenides are an emerging class of two‐dimensional materials. Quantum confinement (number of layers) and defect engineering enables their properties to be tuned over a broad range, including metal‐to‐semiconductor transitions, magnetic ordering, and topological surface states. They possess various polytypes, often of similar formation energy, which can be accessed by selective synthesis approaches. They excel in mechanical, optical, and chemical sensing applications, and feature long‐term air and moisture stability. In this Minireview, we summarize the recent progress in the field of noble‐metal chalcogenides and phosphochalcogenides and highlight the structural complexity and its impact on applications.     

1,2‐Dimethoxyethane Degradation Thermodynamics in Li−O2 Redox Environments

The reaction thermodynamics of the 1,2‐dimethoxyethane (DME), a model solvent molecule commonly used in electrolytes for Li?O₂ rechargeable batteries, has been studied by first‐principles methods to predict its degradation processes in highly oxidizing environments. In particular, the reactivity of DME towards the superoxide anion O₂^? in oxygen‐poor or oxygen‐rich environments is studied by density functional calculations. Solvation effects are considered by employing a self‐consistent reaction field in a continuum solvation model. The degradation of DME occurs through competitive thermodynamically driven reaction paths that end with the formation of partially oxidized final products such as formaldehyde and methoxyethene in oxygen‐poor environments and methyl oxalate, methyl formate, 1‐formate methyl acetate, methoxy ethanoic methanoic anhydride, and ethylene glycol diformate in oxygen‐rich environments. This chemical reactivity indirectly behaves as an electroactive parasitic process and therefore wastes part of the charge exchanged in Li?O₂ cells upon discharge. This study is the first complete rationale to be reported about the degradation chemistry of DME due to direct interaction with O₂^?/O₂ molecules. These findings pave the way for a rational development of new solvent molecules for Li?O₂ electrolytes.     

Mechanistic Investigations of the Stereoselective Rare Earth Metal‐Mediated Ring‐Opening Polymerization of β‐Butyrolactone

Poly(3‐hydroxybutyrate) (PHB) is produced by numerous bacteria as carbon and energy reserve storage material. Whereas nature only produces PHB in its strictly isotactic (R) form, homogeneous catalysis, when starting from racemic (rac) β‐butyrolactone (BL) as monomer, can in fact produce a wide variety of tacticities. The variation of the metal center and the surrounding ligand structure enable activity as well as tacticity tuning. However, no homogeneous catalyst exists to date that is easy to modify, highly active, and able to produce PHB with high isotacticities from rac‐β‐BL. Therefore, in this work, the reaction kinetics of various 2‐methoxyethylamino‐bis(phenolate) lanthanide (Ln=Sm, Tb, Y, Lu) catalysts are examined in detail. The order in monomer and catalyst are determined to elucidate the reaction mechanism and the results are correlated with DFT calculations of the catalytic cycle. Furthermore, the enthalpies and entropies of the rate‐determining steps are determined through temperature‐dependent in situ IR measurements. Experimental and computational results converge in one specific mechanism for the ring‐opening polymerization of BL and even allow us to rationalize the preference for syndiotactic PHB.     

Highly Stereocontrolled Ring‐Opening Polymerization of Racemic Alkyl β‐Malolactonates Mediated by Yttrium [Amino‐alkoxy‐bis(phenolate)] Complexes

Yttrium [amino‐alkoxy‐bis(phenolate)]amido complexes have been used for the ring‐opening polymerization (ROP) of racemic alkyl β‐malolactonates (4‐alkoxycarbonyl‐2‐oxetanones, rac‐MLA^Rs) bearing an allyl (All), benzyl (Bz) or methyl (Me) lateral ester function. The nature of the ortho‐substituent on the phenolate rings in the metal ancillary dictated the stereocontrol of the ROP, and consequently the syndiotactic enrichment of the resulting polyesters. ROP promoted by catalysts with halogen (Cl, Br)‐disubstituted ligands allowed the first reported synthesis of highly syndiotactic PMLA^Rs (P_r ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho‐substituted ligands proved largely ineffective. All polymers have been characterized by ¹H and ¹³C{¹H} NMR spectroscopy, MALDI‐ToF mass spectrometry and DSC analyses. Statistical and thermal analyses enabled the rationalization of the chain‐end control mechanism. Whereas the stereocontrol of the polymerization obeyed a Markov first‐order (Mk1) model for the ROP of rac‐MLA^Bz and rac‐MLA^All, the ROP of rac‐MLA^Me led to a chain end‐control of Markov second‐order type (Mk2). DFT computations suggest that the high stereocontrol ability featured by catalysts bearing Cl‐ and Br‐substituted ligands does not likely originate from halogen bonding between the halogen substituent and the growing polyester chain.     

Two‐Dimensional Boron Monolayers Mediated by Metal Substrates

Two‐dimensional (2D) materials, such as graphene and boron nitride, have specific lattice structures independent of external conditions. In contrast, the structure of 2D boron sensitively depends on metal substrate, as we show herein using the cluster expansion method and a newly developed surface structure‐search method, both based on first‐principles calculations. The preferred 2D boron on weaker interacting Au is nonplanar with significant buckling and numerous polymorphs as in vacuum, whereas on more reactive Ag, Cu, and Ni, the polymorphic energy degeneracy is lifted and a particular planar structure is found to be most stable. We also show that a layer composed of icosahedral B₁₂ is unfavorable on Cu and Ni but unexpectedly becomes a possible minimum on Au and Ag. The substrate‐dependent 2D boron choices originate from a competition between the strain energy of buckling and chemical energy of electronic hybridization between boron and metal.     

3,4‐Dithiaphosphole and 3,3′,4,4′‐Tetrathia‐1,1′‐Biphosphole π‐Conjugated Systems: S Makes the Impact

Conjugated systems based on phospholes and 1,1′‐biphospholes bearing 3,4‐ethylenedithia bridges have been prepared using the Fagan–Nugent route. The mechanism of this organometallic route leading to intermediate zirconacyclopentadienes has been investigated by using theoretical calculations. This study revealed that the oxidative coupling leading to zirconacyclopentadienes is favored over oxidative addition within the S? C≡C bond both thermodynamically and kinetically. The impact of the presence of the S atoms on the optical and electrochemical behavior of the phospholes and 1,1′‐biphospholes has been systematically evaluated both experimentally and theoretically. A comparison with their “all‐carbon” analogues is provided. Of particular interest, this comparative study revealed that the introduction of S atoms has an impact on the electronic properties of phosphole‐based conjugated systems. A decrease of the HOMO–LUMO separation and a stabilization of the LUMO level were observed. These general trends are also observed with 1,1′‐biphospholes exhibiting σ–π conjugation. The P atom of the 3,4‐ethylenedithiaphospholes can be selectively oxidized by S₈ or O₂. These P modifications result in a lowering of the HOMO–LUMO separation as well as an increase of the reduction and oxidation potentials. The S atoms of the 3,4‐ethylenedithia bridge of the 2,5‐phosphole have been oxidized using m‐chloroperoxybenzoic acid. The resulting 3,4‐ethylenesulfoxide oxophosphole was characterized by an X‐ray diffraction study. Experimental and theoretical studies show that this novel chemical manipulation results in an increase of the HOMO–LUMO separation and an important decrease of the LUMO level. The electropolymerization of 2‐thienyl‐capped 3,4‐ethylenedithiathioxophosphole and 1,1′‐biphosphole is reported. The impact of the S substituents on the polymer properties is discussed.     

Metal‐Free Hydrogen Atom Transfer from Water: Expeditious Hydrogenation of N‐Heterocycles Mediated by Diboronic Acid

A hydrogenation of N‐heterocycles mediated by diboronic acid with water as the hydrogen atom source is reported. A variety of N‐heterocycles can be hydrogenated with medium to excellent yields within 10 min. Complete deuterium incorporation from stoichiometric D₂O onto substrates further exemplifies the H/D atom sources. Mechanism studies reveal that the reduction proceeds with initial 1,2‐addition, in which diboronic acid synergistically activates substrates and water via a six‐membered ring transition state.     

The Electronic Nature of the 1,4‐β‐Glycosidic Bond and Its Chemical Environment: DFT Insights into Cellulose Chemistry

The molecular understanding of the chemistry of 1,4‐β‐glucans is essential for designing new approaches to the conversion of cellulose into platform chemicals and biofuels. In this endeavor, much attention has been paid to the role of hydrogen bonding occurring in the cellulose structure. So far, however, there has been little discussion about the implications of the electronic nature of the 1,4‐β‐glycosidic bond and its chemical environment for the activation of 1,4‐β‐glucans toward acid‐catalyzed hydrolysis. This report sheds light on these central issues and addresses their influence on the acid hydrolysis of cellobiose and, by analogy, cellulose. The electronic structure of cellobiose was explored by DFT at the BB1 K/6‐31++G(d,p) level. Natural bond orbital (NBO) analysis was performed to grasp the key bonding concepts. Conformations, protonation sites, and hydrolysis mechanisms were examined. The results for cellobiose indicate that cellulose is protected against hydrolysis not only by its supramolecular structure, as currently accepted, but also by its electronic structure, in which the anomeric effect plays a key role.     

Surface composition of materials used as catalysts for methanol steam reforming: a theoretical study.

PdZn (1:1) alloy is assumed to be the active component of a promising catalyst for methanol steam reforming. Using density functional calculations on periodic supercell slab models, followed by atomistic thermodynamics modeling, we study the chemical composition of the surfaces PdZn(111) and, as a reference, Cu(111) in contact with water and hydrogen at conditions relevant to methanol steam reforming. For the two surfaces, we determine similar maximum adsorption energies for the dissociative adsorption of H(2), O(2), and the molecular adsorption of H(2)O. These reactions are calculated to be exothermic by about -40, -320, and -20 kJ mol(-1), respectively. Using a thermodynamic analysis based on theoretically predicted adsorption energies and vibrational frequencies, we determine the most favorable surface compositions for given pressure windows. However, surface energy plots alone cannot provide quantitative information on individual coverages in a system of coupled adsorption reactions. To overcome this limitation, we employ a kinetic model, from which equilibrium surface coverages of H, O, OH, and H(2)O are derived. We also discuss the sensitivity of our results and the ensuing conclusions with regard to the model surfaces employed and the inaccuracies of our computational method. Our kinetic model predicts surfaces of both materials, PdZn and Cu, to be essentially adsorbate-free already from very low values of the partial pressure of H(2). The model surfaces PdZn(111) and Cu(111) are predicted to be free of water-related adsorbates for a partial H(2) pressure greater than 10(-8) and 10(-5) atm, respectively.     

苯在Ag(100)表面吸附的周期性密度泛函理论研究

 采用量子化学中的密度泛函理论结合平板周期模型方法,研究了苯在Ag(100)面上的吸附方式和相对稳定性. 通过对不同吸附位置的吸附能和几何构型参数的比较发现,苯在Ag(100)表面的吸附属于较强的化学作用,穴位吸附的稳定性优于桥位,顶位吸附最不稳定. 吸附的苯分子的平衡构型发生扭曲, C-C键有较大程度的伸长; C-H键的键长基本不变,但是偏离苯环平面,并背离Ag(100)表面. 在吸附过程中,电子由苯向表面银原子转移. 本文给出了详细的轨道示意图和电荷布居分析,并且与相关的实验和理论研究结果进行了比较.     

Mechanistic Investigations on the Photoisomerization Reactions of 1,2‐Dihydro‐1,2‐Azaborine

The mechanisms of the photochemical isomerization reactions were investigated theoretically by using a model system of 1,2‐dihydro‐1,2‐azaborine with the CAS(6,6)/6‐311G(d,p) and MP2‐CAS‐(6,6)/6‐311++G(3df,3pd)//CAS(6,6)/6‐311G(d,p) methods. Three reaction pathways, which lead to three kinds of photoisomers, have been examined. The structures of the conical intersections, which play a decisive role in such photorearrangements, were obtained. The thermal (or dark) reactions of the reactant species have also been examined by using the same level of theory to assist in providing a qualitative explanation of the reaction pathways. The model investigations suggest that the preferred reaction route for 1,2‐dihydro‐1,2‐azaborine, which leads to the Dewar 1,2‐dihydro‐1,2‐azaborine photoproduct, is as follows: reactant→Franck–Condon region→conical intersection→photoproduct. The results obtained allow a number of predictions to be made.     

Pseudo‐Bridging Silanols as Versatile Brønsted Acid Sites of Amorphous Aluminosilicate Surfaces

Amorphization tunes acidity : Pseudo‐bridging silanols, suggested as versatile Brønsted acid groups by molecular modeling studies, are obtained by shifted hydrolysis of Si? O? Al bridges formed by the thermal treatment of silica deposited on γ‐Al₂O₃ (100), and appear under given pretreatment conditions. Demixing of part of the silica from the aluminosilicate phase is predicted upon excess water adsorption.