Search

lp-holes definitely exist, but do they really interact with nucleophiles to form lp-hole interactions?

The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co3O4 can serve as tractable yet well-defined functional analogues of bulk cobalt oxide. Notably, the energies of the highest-occupied and lowest-unoccupied molecular orbitals (HOMO and LUMO) of the molecular complexes, 1, closely match the valence- and conduction-band (VB and CB) energies of the parent bulk oxides. Use of 1 as a molecular analogue of the parent oxides is demonstrated by its remarkably simple deployment as a cocatalyst for direct Z-scheme reduction of CO2 by solar light and water. Namely, evaporation of an aqueous solution of 1 on TiO2-coated fluorinated tin oxide windows (TiO2/FTO), immersion in wet acetonitrile, and irradiation by simulated solar light under an atmosphere of CO2 give H2, CO, and CH4 in ratios nearly identical to those obtained using 20 nm spinel-Co3O4 nanocrystals, but 15 times more rapidly on a Co basis and more rapidly overall than other reported systems. Detailed investigation of the photocatalytic properties of 1 on TiO2/FTO includes confirmation of a direct Z-scheme charge-carrier migration pathway by in situ irradiated X-ray photoelectron spectroscopy. More generally, the findings point to a potentially important new role for coordination chemistry that bridges the conceptual divide between molecular and solid-state science.

The Breslow intermediate is key to several NHC-organocatalyzed reactions. However, accessing the Breslow intermediate via a direct intramolecular hydrogen atom transfer (HAT) is unlikely and can on...

Secondary-sphere interactions are often harnessed to control reactivity and selectivity in organometallic and enzymatic catalysis. Yet, such strategies have only recently been explicitly applied in the context of organocatalytic systems. Although increased stability, reproducibility, and selectivity were obtained in previous work using this approach, the precise mechanistic pathway promoted by secondary-sphere modification in organocatalysis remained unclear. Herein, we report a comprehensive mechanistic study on the origin of the unique reactivity patterns and stereocontrol observed with boronic acids (

Recently, a tellurium-based chalcogen-bond-catalyzed nitro-Michael reaction was reported ( Angew. Chem. Int. Ed. 2019, 58, 16923), taking advantage of the strong Lewis acidity of the catalyst. This species was found to be more effective than an analogous iodine-based halogen bond organocatalyst. Herein, we present a detailed mechanistic and kinetic analysis of these catalytic cycles including the influence of the solvent (and the performance of different intrinsic solvation models). While the chalcogen bonding interaction is fundamental to activate the C-C bond formation, we found that the presence of a two-water molecular bridge is critical to allow the following, otherwise high-energy proton transfer step. Even though the iodine-based halogen bonding interaction is stronger than the tellurium-based chalcogen bonding one, which makes the former a stronger Lewis acid and hence in principle a more efficient catalyst, solvation effects explain the smaller energy span of the latter.

A trifluoromethyl sulfur-chelated ruthenium benzylidene, Ru-S-CF3 -I, was synthesized and characterized. This latent precatalyst provides a distinct activity and selectivity profiles for olefin metathesis reactions depending on the substrate. For example, 1,3-divinyl-hexahydropentalene derivatives were efficiently obtained by ring-opening metathesis (ROM) of dicyclopentadiene (DCPD). Ru-S-CF3 -I also presented a much more effective photoisomerization process from the inactive cis-diiodo to the active trans-diiodo configuration after exposure to 510 nm (green light), allowing for a wide scope of photoinduced olefin metathesis reactions. DFT calculations suggest a faster formation and enhanced stability of the active trans-diiodo species of Ru-S-CF3 -I compared with Ru-S-Ph-I, explaining its higher reactivity. In addition, the photochemical release of chloride anions by irradiation of Cl-BODIPY in the presence of DCPD derivatives with diiodo Ru benzylidenes, led to in situ generation of chloride complexes, which quickly produced the corresponding cross-linked polymers. Thus, novel selective pathways that use visible light to guide olefin metathesis based synthetic sequences is presented.

The 21-homododecahedryl cation is a unique system in terms of its complete fluxionality based on two different rearrangements. In this work, we report the quantum tunneling effects that drive the reactions at temperatures where the semi-classical kinetics are impossible. We postulate that the tunnel effect in this system can serve to create a refrigerator that may operate at arbitrarily low temperatures.

A combined experimental and density functional theory (DFT) investigation was employed in order to examine the mechanism of electrochemical CO2 reduction and H2 formation from water reduction in neutral aqueous solutions. A water soluble cobalt porphyrin, cobalt [5,10,15,20-(tetra-N-methyl-4-pyridyl)porphyrin], (CoTMPyP), was used as catalyst. The possible attachment of different axial ligands as well as their effect on the electrocatalytic cycles were examined. A cobalt porphyrin hydride is a key intermediate which is generated after the initial reduction of the catalyst. The hydride is involved in the formation of H2 and formate and acts as an indirect proton source for the formation of CO in these H+ -starving conditions. The experimental results are in agreement with the computations and give new insights into electrocatalytic mechanisms involving water soluble metalloporphyrins. We conclude that in addition to the porphyrin's structure and metal ion center, the electrolyte surroundings play a key role in dictating the products of CO2 /H2 O reduction.

Oxidation of methane at ambient conditions to useful oxygenates at a bilayer-coated electrode is demonstrated. The composition of the coating, a Mn porphyrin mediator layer on top of a N(OH)

Herein we show the design and synthesis of an electron rich, sulfoxide-chelated, ruthenium benzylidene. In contrast to previously reported sulfoxide-chelated ruthenium benzylidenes, this complex is...

The uncertainty on XeF6 geometry persisted for over five decades, with elusive multiple C3v equivalent minima as an outcome of the Jahn-Teller effect. Herein we theoretically prove that XeF6 shows a genuine fluorine quantum mechanical tunnelling rearrangement, rapidly "jumping" between isomers even close to 0 K. The isoelectronic anions IF6- and TeF62- behave in the same way as XeF6, but with a lower and shorter energetical barrier they have a faster tunnelling rate. A complete isotopic analysis revealed a large k(18F)/k(19F) kinetic isotope effect. We speculate that it would be possible to experimentally observe the tunnel effect in XeF6 by means of cryogenic NMR or IR.

The degenerate rearrangement on Jahn–Teller distorted metal complexes is a promising reaction for the observation of significant heavy atom quantum mechanical tunnelling. Herein, a family of Cu(ii)–N6 complexes are theoretically proven to exhibit rapid dynamical Jahn–Teller tunneling even close to the absolute zero. The manifestation of our predictions apparently appeared in solid state EPR experimental measurements on [Cu(en)3]SO4 more than 40 years ago, without the authors realizing that it was a quantum outcome., The degenerate rearrangement on Jahn–Teller distorted metal complexes is a promising reaction for the observation of significant heavy atom quantum mechanical tunnelling.

Cyclo[18]carbon (C18 ), a recently synthesized carbon allotrope, was found to have a polyynic ground-state structure with D9h symmetry and formally alternating single and triple bonds. Yet, under less influencing experimental conditions this molecule might undergo an automerization reaction between its two degenerate geometries through a cumulenic (non-alternating, adjacent double bonds) D18h transition state. Herein, we discuss the role of quantum mechanical tunneling (QMT) in this degenerate reaction. Our computations predict that at the experimental temperature (5 K) the reaction in the gas phase is completely driven by an extremely rapid heavy atom tunneling (k=2.1×108 s-1 ). Even when approaching room temperature, the QMT rate is still an order of magnitude faster than the semi-classical one. We propose an experimental test to support our prediction, by measuring a characteristic tunneling energy splitting within the radio wave region. Additionally, we examine the role of QMT in other hypothetical C4n+2 carbon clusters.

Dative Bond (IUPAC): "The coordination bond formed upon interaction between molecular species, one of which serves as a donor and the other as an acceptor of the electron pair to be shared in the complex formed… The distinctive feature of dative bonds is that their minimum-energy rupture in the gas phase or in inert solvent follows the heterolytic bond cleavage path." This definition encompasses an immense number of molecules such as Lewis adducts, transition-metal complexes, supposedly hypervalent or hypovalent systems, and many molecules with multifaceted Lewis structures. Still, there is a large reticence to include dative bonds in the regular depiction of molecules, and even a larger unawareness of the dative bond arrow in many chemical circles. Herein we will discuss in simple chemical terms the past, present and future of such bonds. In addition, we will try to provide cleaner options to represent intricate molecules without sacrificing physical accuracy.

The deposition of metal oxides is essential to the fabrication of numerous multicomponent solid-state devices and catalysts. However, the reproducible formation of homogeneous metal oxide films or of nanoparticle dispersions at solid interfaces remains an ongoing challenge. Here we report that molecular hexaniobate cluster anion complexes of structurally and electronically distinct fragments of cubic-spinel and monoclinic Co

Some nitrile-boron halide adducts exhibit a double-well potential energy surface with two distinct minima: a “long bond” geometry (LB, a van der Waals interaction mostly based on electrostatics, but including a residual charge transfer component) and a “short bond” structure (SB, a covalent dative bond). This behavior can be considered as a “weak” form of bond stretch isomerism. Our computations reveal that complexes RCN−BX3 (R=CH3, FCH2, BrCH2, and X=Cl, Br) exhibit a fast interconversion from LB to SB geometries even close to the absolute zero thanks to a boron atom tunneling mechanism. The computed half-lives of the meta-stable LB compounds vary between minutes to nanoseconds at cryogenic conditions. Accordingly, we predict that the long bond structures are practically impossible to isolate or characterize, which agrees with previous matrix-isolation experiments.

Hole interactions are known by different names depending on the key atom of the bond (halogen bond, chalcogen bond, hydrogen bond, etc.), and the geometry of the interaction (σ if in line, π if perpendicular to the Lewis acid plane). However, its origin starts with the creation of a Lewis acid by an underlying covalent bond, which forms an electrostatic depletion and a virtual antibonding orbital, which can create non-covalent interactions with Lewis bases. In this (maybe subjective) perspective, we will claim that hole interactions must be defined via the molecular orbital origin of the molecule. Under this premise we can better explore the richness of such bonding patterns. For that, we will study old, recent and new systems, trying to pinpoint some misinterpretations that are often associated with them. We will use as exemplars the triel bonds, a couple of metal complexes, a discussion on convergent σ-holes, and many cases of anti-electrostatic hole interactions.

To predict barrier heights at low temperatures, it is not enough to employ highly accurate electronic structure methods. We discuss the influence of quantum tunnelling on the comparison of experimental and theoretical activation parameters (E

Anthracene can be used as a scaffold for intramolecular SN2 degenerate reactions of the “bell clapper” type, where a central boron atom or its isoelectronic carbocation bonds alternatively towards one or the other lateral Lewis bases at the first and eight anthracene positions. This ping-pong bond-switching reaction possesses a symmetrical double-well potential with low activation barrier and relatively narrow barrier width. Herein we show by computational means the active role played by heavy atom quantum tunneling in this degenerate rearrangement reaction at cryogenic temperatures. At these conditions the thermal “over the barrier” reaction is forbidden, whereas the tunneling effect enhances the rate of reaction up to an experimentally measurable half-life. Kinetic isotope effects and cryogenic NMR spectroscopy can, in principle, experimentally demonstrate the tunneling mechanism.

ϕ for philosophy, χ for chemistry. Is there a connection? Is there any interaction? Do chemists have something useful to learn from philosophers? Do philosophers from chemists? How can we define if chemistry is a science, or at least trustworthy? Which disciplines, styles and personalities are progressive in chemistry? Which ones are truth-seekers, progress-seekers or profit-seekers? Is there a point in asking these questions? Is there even an answer to them?

While the influence of intramolecular electric fields is a known feature in enzymes, the use of oriented external electric fields (EEF) to enhance or inhibit molecular reactivity is a promising topic still in its infancy. Herein we will explore computationally the effects that EEF can provoke in simple molecules close to the absolute zero, where quantum tunnelling (QT) is the sole mechanistic option. We studied three exemplary systems, each one with different reactivity features and known QT kinetics: π bond-shifting in pentalene, Cope rearrangement in semibullvalene, and cycloreversion of diazabicyclohexadiene. The kinetics of these cases depend both on the field strength and its direction, usually giving subtle but remarkable changes. However, for the cycloreversion, which suffers large changes on the dipole through the reaction, we also observed striking results. Between the effects caused by the EEF on the QT we observed an inversion of the Arrhenius equation, deactivation of the molecular fluxionality, and stabilization or instantaneous decomposition of the system. All these effects may well be achieved, literally, at the flick of a switch., Adding an external electric field to reactions driven by quantum mechanical tunneling brings a whole new dimension to the idea of switch chemistry.

Reactive intermediates are key to halting and promoting chemical transformations, however due to their elusive nature, they are seldom harnessed for reaction design. Herein, we describe studies aimed at stabilizing reactive intermediates in the N-heterocyclic carbene (NHC) catalytic cycle, which enabled fully shutting down the known benzoin coupling pathway, while rerouting its intermediates toward deuteration. The reversible nature of NHC catalysis and the selective stabilization of reaction intermediates facilitated clean hydrogen-deuterium exchange reactions of aromatic aldehydes by D2O, even for challenging electron withdrawing substrates. The addition of catalytic amounts of phenyl boronic acid was used to further stabilize highly reactive intermediates and mitigate the formation of benzoin coupling by-products. The mechanistic understanding at the foundation of this work resulted in unprecedented mild conditions with base and catalyst loadings as low as 0.1 mol%, and a scalable deuteration reaction applicable to a broad substrate scope with outstanding functional group tolerance. More importantly, adopting this approach enabled the construction of a machine-learning derived guideline for identifying the appropriate catalyst and conditions for different substrates based on a logistic regression classification model. Experimental studies combined with machine learning and computational methods shed light on the non-trivial mechanistic underpinnings of this reaction.

The uncertainty on XeF

Triplet cyclopentane-1,3-diyl diradical (T-DR) was generated via photolysis of 2,3-diazabicyclo[2.2.1]hept-2-ene (AZ) under low-temperature matrix conditions. Temperature independency of T-DR decay...

Pentafulvalene is a symmetrical unsaturated hydrocarbon built from two five-membered rings connected by an exocyclic double bond, where each ring is one electron short of being a 6π-electron aromatic system. Here, we show computationally that by selectively introducing electron withdrawing and donating substituents, we can design pentafulvalene derivatives that exhibit tunable aromaticity properties. Pentafulvalene can be shaped into a species with connected aromatic-antiaromatic rings, which can also achieve π-bond shifting by carbon tunneling. We propose an NMR technique that can experimentally prove such tunneling mechanism. In addition, we devised a doubly aromatic fulvalene involving both Hückel and Baird aromaticities. These results can open possibilities to create novel molecules in terms of spin state, aromaticity, and reactivity by quantum tunneling.

Sulfur-chelated ruthenium olefin metathesis precatalysts that possess cyclic (alkyl)(amino)carbenes (CAAC) can benefit from the synergetic effect of both ligands. Changing the steric bulk of the CAAC ligand by using different substitution patterns was shown to affect the geometry of the complexes produced and determined whether the complexes could be catalytically dormant. The cis-dichloro latent catalysts could be activated both by heat or light, even in the visible region, for representative acyclic diene metathesis and ring-opening metathesis polymerization reactions, olefin cross-metathesis, and ring-closing metathesis without isomerization byproducts. Thus, these complexes were shown to combine the uniqueness of CAAC-containing Ru olefin metathesis catalysts with the advantage of the thermal and photolatency imposed by sulfur chelation of the benzylidene.

The commercially available catalyst cis-Caz-1 is extremely robust, especially when reactions are performed in an atmosphere of air. This phosphite-containing ruthenium complex only shows olefin met...

By means of molecular orbital theory and the analysis of charge transfer and electrostatic forces, we discuss the features of non-covalent hole interactions of the halogen, chalcogen and pnictogen bond families. The use of MOs allows us to explain and predict the location of holes, and to design novel interactions such as systems with σ and π holes on the same or opposite sides. In view of the orbital origin of the hole interactions, we suggest that many chalcogen and pnictogen bonds are largely based on π holes and not on the commonly accepted σ holes. In addition, a new type of hole interaction based on δ holes is found on the sextuply bonded dimolybdenum.

Efficient light- and thermal-activated metathesis reactions of tetra-substituted olefins were obtained by the S-chelated ruthenium precatalyst Tol-SCF3. Its reactivity in a series of benchmark olefin metathesis reactions was compared to previously reported Mes-SCF3 and a novel sterically congested S-chelated complex, Dipp-SCF3. Tol-SCF3 is thus the first latent catalyst proven to be capable of promoting olefin metathesis of demanding substrates upon light stimulation at room temperature.

Several olefin metathesis reactions are studied, namely, jojoba oil oligomerization, methyl oleate self-metathesis, ring-closing metathesis (RCM) to form a nitrogen heterocycle, and 1,5-hexadiene acyclic diene metathesis polymerization (ADMET). The catalyst containing the Bertrand–Grubbs cyclic alkyl amino carbene (CAAC) ligand showed high selectivity by diminishing isomerization reactions; this was especially clear at high temperatures where the more widely used nitrogen heterocyclic carbene (NHC)-based catalysts show side reactions. Experimental and computational studies determined that it is much more difficult to produce ruthenium hydrides with CAAC, a property that can explain the improved observed activity. This finding opens a pathway for the development of even more selective olefin metathesis catalysts for reactions that require harsh conditions.

Efficient and highly selective catalytic conditions for the aerobic autoxidation of methylarenes to benzaldehydes, based on N-hydroxyphthalimide (NHPI) and cobalt(II) acetate in 1,1,1,3,3,3-hexafluoropropan-2-ol (HFIP), were developed. The sustainable conditions enable a multigram scale preparation of benzaldehyde derivatives in high efficiency and with excellent chemoselectivity (up to 99 % conversion and 98 % selectivity).

Halogen bonds (XB) are no longer newcomers in the chemistry family. However, XB in hypervalent halogens has not been thoroughly studied. We provide a molecular orbital explanation of the shape and strength of XBs in hypervalent halogens and other species, focusing on the charge transfer and electrostatic aspects of these bonds. Our results show that σ-holes (and subsequently the XBs associated with them) can be easily divided and bent by the influence of equatorial substituents. The inductive effect of both the equatorial and axial groups can affect these distortions, but also the angle between the equatorial ligands has a large influence on the shape of the σ-holes and the molecular orbitals acting as electron acceptor. Although the observation of these warped XB can be hindered by other noncovalent interactions, they may be ubiquitous in crystal structures of hypervalent species, where multiple XB can appear as secondary interactions on each halogen. We propose what can be considered the archetypal hypervalent halogen donor (a pincer type iodosodilactone) and a Lewis dot structure that includes the σ-holes.

We report the synthesis of ‘hangman-type’ PNP pincer ligands and their associated Ru complexes. Upon deprotonation, the complexes undergo sp2-sp3 C-C bond cleavage and subsequent C-C formation processes to form a new, one carbon unit longer backbone motif. Unlike the starting geometry, the products are completely inactive in alcohol dehydrogenation catalysis. DFT and mechanistic studies suggest an intramolecular isomerization process via a spyrocycle cyclopropane intermediate, dependent upon arm pyridine coordination and ligand sterics.

Transition-metal-carbon (CTM) composites show ample activity in many catalytic reactions. However, control of composition, distribution, and properties is challenging. Now, a straightforward path for the synthesis of transition-metal nanoparticles engulfed in crystalline carbon is presented with excellent control over the metal composition, amount, ratio, and catalytic properties. This approach uses molten monomers that coordinate metals ions at high temperature. At high temperatures, strong coordination bonds direct the growth of carbon material with homogeneous metals distribution and with negligible losses, owing to the liquid-like reaction compared to the traditional solid-state reaction. The strength of the approach is demonstrated by the synthesis of mono, binary, and trinary transition-metal-crystalline-carbon composites with tunable and precise elemental composition as well as good electrochemical properties as oxygen evolution reaction electrocatalysts.

In homogeneous catalysis, the turnover frequency (TOF) and turnover number (TON) are the most commonly used quantities that experimentally describe catalytic activity. Computational studies, on the other hand, generally yield the ubiquitous free energy profile, which only provides the relative heights of different intermediates and transition states for a given reaction mechanism. This information, however, can be converted into a theoretical TOF through use of the energy span model. Clearly, directly computing turnover frequencies not only allows easy comparison of the activity of different catalysts, but also provides a means of directly comparing theory and experiment. Nonetheless, obtaining detailed free energy profiles for many catalysts is computationally costly. To overcome this and accelerate the rate at which prospective catalysts can be screened, here we use linear scaling relationships in tandem with the energy span model to create volcano plots that relate an easily and quickly computed energetic descriptor variable with a catalyst’s turnover frequency. As a demonstration of their ability, we use these “TOF volcanoes” to rapidly screen prospective transition metal/pincer-ligand catalysts based on activity in facilitating the hydrogenation of CO2to formate.

High molecular weight polybutadienes and rhodium complexes were used to produce single chain organometallic nanoparticles. Irradiation of high cis-polybutadiene in the presence of a photosensitizer isomerised the double bonds to produce differing cis/trans ratios within the polymer. Notably, a higher cis percentage of carbon–carbon double bonds within the polymer structure led to faster binding of metal ions, as well as their faster removal by competing phosphine ligands. The experimental results were supported and rationalized by DFT computations.

In a recent article (Chem. Phys. Lett. 749 (2020), 137440), Kumar and Mallick predict by computations that a sulfur atom tunneling mechanism leads to a ring flipping reaction of S4N4 below 70 K. Herein, on the basis of their tunneling computations, it is shown that the sulfur atom tunneling is not viable, and therefore impossible to be probed experimentally. Based on this, we discuss the prerequisites for a chemical reaction to be driven by heavy atom tunneling.