Structure and Bonding in π-Stacked Perylenes: The Impact of Charge on Pancake Bonding.

Perylene (PER) is a prototype of polycyclic aromatic hydrocarbons (PAHs), which play a pivotal role in various functional and electronic materials due to favorable molecule-to-molecule overlaps, which enhance electronic transport. This study provides guidelines regarding the impact of molecular charge on pancake bonding, a form of strong π-stacking interaction. Pancake bonding significantly boosts interaction energies within the monopositive dimer ([(C20H12)2]•+ or PER2+), crucial for stabilizing aggregation and crystal formation. We discovered energetically feasible sliding and rotation pathways within the [(C20H12)2]•+ dimer, connecting different configurations found in the Cambridge Structural Database (CSD). The dimer's charge profoundly influences the pancake bond order (PBO) and the strength and structural preferences of pancake bonding. The most stable configuration is found in the monocationic state (PER2+), featuring a pancake bond order of 1/2 with one-electron multicenter bonding (1e/mc) with similar characteristics for charge -1. Increasing the total charge of the dimer to +2 or -2 leads to an unstable local minimum. Diverse distribution of pancake bonding types present in crystal structures is interpreted with modeling based on dimer computations with varying charges.

Continuous Topological Transition and Bandgap Tuning in Ethynylene-Linked Acene π-Conjugated Polymers through Mechanical Strain.

By variation of the chemical repeat units of conjugated polymers, only discrete tuning of essential physical parameters is possible. A unique property of a class of π-conjugated polymers, where polycyclic aromatic hydrocarbons are linked via ethynylene linkers, is their topological aromatic to quinoid phase transition discovered recently by Cirera et al. and González-Herrero et al., which is controllable in discrete steps by chemical variations. We have discovered by means of density functional theory computations that such a phase transition can be achieved by applying continuous variations of longitudinal strain, allowing us to tune the bond length alternation and bandgap. At a specific strain value, the bandgap becomes zero due to an orbital level crossing between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). Our hypothesis provides a perspective on the design of organic electronic materials and provides a novel insight into the properties of a continuous phase transition in topological semiconducting polymers.

Structure, Properties, and Reactivity of Polyoxocationic Zirconium and Hafnium Clusters: A Computational Investigation.

Tetravalent zirconium and hafnium molecular metal oxides clusters are key building blocks of many metal-organic frameworks; however, the chemical space to form all possible MOF nodes is vast, containing many potential new clusters. Our computational study provides a complete picture of the structure, properties, and reactivity of two tetrameric zirconium and hafnium [M4(µ2-η2:η2-O2)x(µ2-OH)8-2x(H2O)16]8+ polycationic clusters. The electronic structure of the studied species has characteristic polyoxometalate oxygen-based and metal-based bands in the valence region. The energetics for the evolution of pure metal clusters into mixed-metal clusters revealed that only the incorporation of zirconium into hafnium clusters is thermodynamically favorable. We confirmed that the incorporation of up to four peroxide ligands is thermodynamically favorable; however, the experimental absence of rich peroxide species with three or more peroxides is attributed to their thermal degradation. The mechanism for peroxide incorporation involves the partial dissociation of the cluster rather than complete dissociation.

Aqueous Speciation of Tetravalent Actinides in the Presence of Chloride and Nitrate Ligands.

Speciation of hexachloride tetravalent uranium, neptunium, and plutonium species in aqueous media has been investigated using density functional theory in the presence of inner sphere ligands such as chloride, nitrate, and solvent molecules. All possible structures with the formula [AnIV(Cl)x(H2O)y(NO3)z]4-x-z (An = U, Np, and Pu; x = 0-6, y = 0-8, and z = 0-6) were considered to explore the speciation chemical space of each actinide. The nature of the mixed-ligand complexes present in solution is controlled by the concentration of free ligands in solution. A low chloride concentration is suitable to drive the speciation away from the highly thermodynamically stable hexachloride species. Furthermore, the formation of dimeric species can proceed through both olation and oxolation mechanisms. Oxolation is preferred for monomers that contain fewer water ligands, while olation becomes favorable for complexes with more water ligands.

Stabilizing Porosity in Organic Cages through Coordination Chemistry.

The number of studies concerning the permanent porosity of molecular materials, especially porous organic cages (POCs) and porous coordination cages (PCCs), have increased substantially over the past decade. The work presented here outlines novel approaches to the preparation of porous molecular structures upon metalation of nonporous, amine-based organic cages. Reduction of the well-known CC3 and CC1 imine-based POCs affords nonporous, highly flexible amine cages. These materials can be endowed with significant levels of structural rigidity via post-synthetic metalation of their ethylenediamine-type binding pockets. The hybrid metal-organic cages accessed through this approach combine aspects of POC and PCC chemistry, with structures of this type providing a potentially promising new direction for the design and development of porous molecular materials with tunability in overall charge, metal cation, porosity, and solubility.

Deoxygenation of nitrosoarene by N-heterocyclic carbene (NHC): an elusive Breslow-type intermediate bridging carbene and nitrene.

N-Heterocyclic carbene (NHC) activates and deoxygenates nitrosoarene (ArNO) to afford arylnitrene (ArN), thereby portraying a fundamental route connecting two 6e- species. A combination of spectroscopic and computational studies suggests that the interaction of ArNO with NHC affords a transient 2,2'-diamino imine-N-oxide as a key intermediate in ArNO deoxygenation.

Design and synthesis of aryl-functionalized carbazole-based porous coordination cages.

A subset of coordination cages have garnered considerable recent attention for their potential permanent porosity in the solid state. Herein, we report a series of functionalized carbazole-based cages of the structure type M12(R-cdc)12 (M = Cr, Cu, Mo) where the functional groups include a range of aromatic substituents. Single-crystal X-ray structure determinations reveal a variety of intercage interactions in these materials, largely governed by pi-pi stacking. Density functional theory for a subset of these cages was used to confirm that the nature of the increased stability of aryl-functionalized cages is a result of inter-cage ligand interactions.

Design and synthesis of capped-paddlewheel-based porous coordination cages.

To leverage the structural diversity of metal-organic frameworks, the ability to controllably terminate them for the isolation of porous coordination cages is advantageous. However, the strategy has largely been limited to ligand termination methods, particularly for paddlewheel-based materials. Here, we show a paddlewheel-capping strategy can be employed to afford previously unattainable coordination cage structures that are mimetic of metal-organic framework pores.

Analysis of pseudo jahn-teller distortion based on natural bond orbital theory: Case study for silicene.

Ground state (GS) instability of nondegenerate molecules in high symmetric structures is understood through Pseudo Jahn-Teller mixing of the electronic states through the vibronic coupling. The general approach involves setting up of a Pseudo Jahn-Teller (PJT) problem wherein one or more symmetry allowed excited states couple to the GS to create vibrational instability along a normal mode. This faces two major complications namely (1) estimating the adiabatic potential energy surfaces for the excited states which are often difficult to describe in case the excited states have charge-transfer or multi-excitonic (ME) character and (2) finding out how many such excited states (all satisfying the symmetry requirements for vibronic coupling) of increasing energies need to be coupled with the GS for a particular PJT problem. An analogous alternative approach presented here for the well-known case of symmetry breaking of planar (D6h ) hexasilabenzene (Si6 H6 ) to the buckled (D3d ) structure involves identifying the second-order donor-acceptor, hyperconjugative interactions (E2 i → j ) that stabilize the distorted structure. Following the recent work of Nori-Shargh and Weinhold, one observes that the orbitals involved in the vibronic coupling between the S0 /Sn states and those for the donor (filled)-acceptor (empty) interactions are identical. In fact, deletion of any particular pair of E2 i → j interaction creates vibrational instability in the buckled structure and as a corollary, deleting it for the planar structure removes its instability. The one-to-one correlation between the natural bond orbital theory and PJT theory assists in an intuitive identification of the relevant (few) excited states from a manifold of computed ones that cause symmetry breaking by vibronic coupling. © 2019 Wiley Periodicals, Inc.

Transforming atmospheric CO2 into alternative fuels: a metal-free approach under ambient conditions.

This work demonstrates the first-ever completely metal-free approach to the capture of CO2 from air followed by reduction to methoxyborane (which produces methanol on hydrolysis) or sodium formate (which produces formic acid on hydrolysis) under ambient conditions. This was accomplished using an abnormal N-heterocyclic carbene (aNHC)-borane adduct. The intermediate involved in CO2 capture (aNHC-H, HCOO, B(OH)3) was structurally characterized by single-crystal X-ray diffraction. Interestingly, the captured CO2 can be released by heating the intermediate, or by passing this compound through an ion-exchange resin. The capture of CO2 from air can even proceed in the solid state via the formation of a bicarbonate complex (aNHC-H, HCO3, B(OH)3), which was also structurally characterized. A detailed mechanism for this process is proposed based on tandem density functional theory calculations and experiments.

Effects of Ancillary Ligands on Redox and Chemical Properties of Ruthenium Coordinated Azoaromatic Pincer.

In this work, the effect of the electronically different ancillary ligands on the overall properties of the RuIIL moiety (L = 2,6-bis(phenylazo)pyridine) in heteroleptic complexes of general formula [RuLQCl]0/+ was investigated. Four different ancillary ligands (Q) with different electronic effects were used to prepare the heteroleptic compounds from the precursor complex, [RuL(CH3CN)Cl2] (1); Q = pcp: 2-(4-chloro-phenylazo)pyridine (strong π-acceptor), [2]+; bpy: 2,2'-bipyridyl (moderate π-acceptor), [3]+; acac-: acetylacetonate (strong σ-donor), 4; and DTBCat2-: 3,5-di- tert-butyl catecholate (strong π-donor), 5. The complexes [2]+, [3]+, 4, and 5 were fully characterized and structurally identified. The electronic structures of these complexes along with their redox partners were elucidated by using a host of physical measurements: nuclear magnetic resonance, cyclic voltammetry, electronic paramagnetic resonance, UV-vis-NIR spectroscopy, and density functional theory. The studies revealed significant effects of the coligands on azo bond lengths of the RuL moiety and their redox behavior. Aerobic alcohol oxidation reactions using these Ru complexes as catalysts were scrutinized. It was found that the catalytic efficiency is primarily controlled by the electronic effect of the coligand. Accordingly, the complex [2]+ (containing a strong π-acceptor coligand, pcp) brings about oxidation efficiently, producing 86% of benzaldehyde. In comparison, however, the complexes 4 and 5 (containing electron donating coligand) furnished only 15-20% of benzaldehyde under identical reaction conditions. Investigations of the reaction mechanism suggest that an unstable Ru-H species is formed, which is transformed to a Ru-hydrazo intermediate by H-walking as reported by Hall et al. ( J. Am. Chem. Soc., 2015, 137, 12330). Aerial O2 regenerates the catalyst via oxidation of the hydrazo intermediate.

Understanding Thermal and Photochemical Aryl-Aryl Cross-Coupling by the AuI /AuIII Redox Couple.

Systematic mechanistic investigations of the gold(I)/gold(III) redox-controlled aryl-aryl cross-coupling reaction have been performed by using both a thermal and photochemical approach. Electron-deficient and electron-rich arenes were considered as the coupling partners of the reaction. Based on transition-state modeling and distortion/interaction analyses, it is shown that AuI prefers to react with electron-deficient arenes whereas AuIII likes to activate electron-rich arenes. This orthogonal reactivity of gold makes it an efficient catalyst for the aryl-aryl cross-coupling reaction. The crucial role of the carboxylate ligand in the reaction has been elucidated through analysis of the transition states. It is shown that due to the presence of two coordination sites, a carboxylate ligand can stabilize the transition state more efficiently than other monodentate ligands such as chloride (Cl- ). Moreover, carbon-boron transmetalation is shown to be favorable over direct C-H metalation, hence reactions initialized by C-B transmetalation are expected to be much faster and selective. Additionally, a dual photoredox/gold catalyst was employed to access the AuI /AuIII catalytic cycle for the cross-coupling reaction. [Ru(bpy)3 ]2+ was used as the photoredox catalyst for the reaction, which, on excitation, transfers an electron to one of the coupling partners, namely a diazonium salt (ArN2+ ), and initializes the cycle.

An Azoaromatic Ligand as Four Electron Four Proton Reservoir: Catalytic Dehydrogenation of Alcohols by Its Zinc(II) Complex.

Electroprotic storage materials, though invaluable in energy-related research, are scanty among non-natural compounds. Herein, we report a zinc(II) complex of the ligand 2,6-bis(phenylazo)pyridine (L), which acts as a multiple electron and proton reservoir during catalytic dehydrogenation of alcohols to aldehydes/ketones. The redox-inactive metal ion Zn(II) serves as an oxophilic Lewis acid, while the ligand behaves as efficient storage of electron and proton. Synthesis, X-ray structure, and spectral characterizations of the catalyst, ZnLCl2 (1a) along with the two hydrogenated complexes of 1a, ZnH2LCl2 (1b), and ZnH4LCl2 (1c) are reported. It has been argued that the reversible azo-hydrazo redox couple of 1a controls aerobic dehydrogenation of alcohols. Hydrogenated complexes are hyper-reactive and quantitatively reduce O2 and para-benzoquinone to H2O2 and para-hydroquinone, respectively. Plausible mechanistic pathways for alcohol oxidation are discussed based on controlled experiments, isotope labeling, and spectral analysis of intermediates.

Direct and Autocatalytic Reductive Elimination from Gold Complexes ([(Ph3 P)Au(Ar)(CF3 )(X)], X=F, Cl, Br, I): The Key Role of Halide Ligands.

Kinetic and thermodynamic preferences for the reductive elimination of Caryl -CF3 , Caryl -X, Caryl -P, and CF3 -X bonds and competitive phosphine dissociation from a series of AuIII complexes [(Ph3 P)Au(Ar)(CF3 )(X)] (1 X ; Ar=4-Me-C6 H4 ; X=F, Cl, Br, I) are studied computationally. Kinetically, the most favorable pathways were found to consist of an initial phosphine dissociation from complex 1 X , which furnished the respective three-coordinate AuIII complexes [Au(Ar)(CF3 )(X)] (2 X ). The computed enthalpy barriers for various reductive elimination reactions from complex 2 X by a direct (or uncatalyzed) mechanism showed that Caryl -CF3 bond formation was the most favorable fate for any X group. When the direct elimination was compared with an autocatalytic mechanism that proceeded through the formation of a mixed-valent binuclear AuIII -AuI intermediate, the preference for the formation of a Caryl -CF3 bond is dependent on the nature of the bridging halide atom and follows the order F>Cl>Br>I. Concomitantly, the selectivity for the formation of Caryl -X bonds for various X atoms follows the opposite trend. The preference for the direct and autocatalytic processes is controlled entirely by the nature of the halide ligand. The predicted mechanisms and product selectivity trends for various halides show excellent agreement with recent experimental observation. The selectivity of various reductive elimination pathways was rationalized by using molecular orbital theory and distortion-interaction model analyses. Attractive interactions between the AuI complex and complex 2X were found to reduce the activation barrier for Caryl -X elimination and critically control the selectivity of the product formation.

Metal-Free Reduction of CO2 to Methoxyborane under Ambient Conditions through Borondiformate Formation.

An abnormal N-heterocyclic carbene (aNHC) based homogeneous catalyst has been used for the reduction of carbon dioxide to methoxyborane in the presence of a range of hydroboranes under ambient conditions and resulted in the highest turnover number of 6000. A catalytically active reaction intermediate, [aNHC-H⋅9BBN(OCOH)2 ] was structurally characterized and authenticated by NMR spectroscopy. A detailed mechanistic cycle of this catalytic process via borondiformate formation has been proposed from tandem experimental and computational experiments.

Exclusively Ligand-Mediated Catalytic Dehydrogenation of Alcohols.

Design of an efficient new catalyst that can mimic the enzymatic pathway for catalytic dehydrogenation of liquid fuels like alcohols is described in this report. The catalyst is a nickel(II) complex of 2,6-bis(phenylazo)pyridine ligand (L), which possesses the above requisite with excellent catalytic efficiencies for controlled dehydrogenation of alcohols using ligand-based redox couple. Mechanistic studies supported by density functional theory calculations revealed that the catalytic cycle involves hydrogen atom transfer via quantum mechanical tunneling with significant kH/kD isotope effect of 12.2 ± 0.1 at 300 K. A hydrogenated intermediate compound, [NiIICl2(H2L)], is isolated and characterized. The results are promising in the context of design of cheap and efficient earth-abundant metal catalyst for alcohol oxidation and hydrogen storage.

Complete Transmetalation in a Metal-Organic Framework by Metal Ion Metathesis in a Single Crystal for Selective Sensing of Phosphate Ions in Aqueous Media.

A complete transmetalation has been achieved on a barium metal-organic framework (MOF), leading to the isolation of a new Tb-MOF in a single-crystal (SC) to single-crystal (SC) fashion. It leads to the transformation of an anionic framework with cations in the pore to one that is neutral. The mechanistic studies proposed a core-shell metal exchange through dissociation of metal-ligand bonds. This Tb-MOF exhibits enhanced photoluminescence and acts as a selective sensor for phosphate anion in aqueous medium. Thus, this work not only provides a method to functionalize a MOF that can have potential application in sensing but also elucidates the formation mechanism of the resulting MOF.

Strain Control: Reversible H2 Activation and H2/D2 Exchange in Pt Complexes.

Experiments have indicated that bulky ligands are required for efficient H2 activation by Pt-Sn complexes. Herein, we unravel the mechanisms for a Pt-Sn complex, Pt(Sn(t)Bu3)2(CN(t)Bu)2 (1a), catalyzed reversible H2 activation. Among a number of Pt-Sn catalysts used to model H2 activation and H2/D2 exchange reactions, only 1a with large strain was found to be suitable because the addition of H2 to 1a requires lowest distortion energy, minimal structural changes, and smallest entropy of activation. The activity of this Pt-Sn complex was compared vis-à-vis its Pt-Ge and Pt-Si analogues, and we predicted that strained Pt-Ge complex can efficiently activate H2 reversibly. Direct dynamics calculations for the rate of reductive elimination of H2, HD, and D2 from Pt(Sn(t)Bu3)(CN(t)Bu)2H3 (4a) and Pt(Sn(t)Bu3)(CN(t)Bu)2HD2 (4a([2D])) shows that H/D atom tunneling contributes significantly, which leads to an enhanced kinetic isotope effect. Strain control is suggested as a design concept in H2 activation.

Mechanistic insights into the synergistic catalysis by Au(I), Ga(III), and counterions in the Nakamura reaction.

A computational study based on density functional theory (DFT) establishes the mechanisms for synergistic Au/Ga catalyzed addition of unactivated terminal alkynes to dicarbonyls, the Nakamura reaction. The role played by each of the metal catalysts and the counterion in the reaction has been elucidated. It has been shown that the triazole (TA) ligand could specifically activate the formation of a particular regioisomer through strong non-covalent interactions. Calculated regioselectivities and activation free energies are in excellent agreement with the experimental results. Observed regioselectivities were rationalized employing a distortion interaction analysis which suggests that the interaction between metal activated reactant fragments in the transition state geometries is a major factor that contributes to the overall barrier height and selectivity. Such enhanced preference for the reaction at the alkyl/aryl substituted carbon of alkynes was strongly influenced by the additional non-covalent interactions exerted by the TA ligand. Excellent agreement between the calculations using a homogeneous gold complex as the catalyst and experimentally observed kinetics and selectivity negates the role of in situ formed gold clusters in the Nakamura reaction.

Preparation of multi-coloured different sized fluorescent gold clusters from blue to NIR, structural analysis of the blue emitting Au7 cluster, and cell-imaging by the NIR gold cluster.

Blue, green, orange-red, red and NIR emitting gold quantum clusters have been prepared in aqueous media by using a bioactive peptide glutathione (reduced) at physiological pH. Matrix-assisted laser desorption ionization-time-of-flight mass spectrometry (MALDI-TOF MS) analyses show that the core structure sizes of the five different gold clusters are Au7 (blue), Au16 (green), Au19 (orange-red), Au21 (red) and Au22 (NIR). The photo-stability and pH-stability of these quantum clusters have been measured, and these are photo-stable against continuous UV irradiation for a few hours. They also exhibit moderate to good pH-stability within the pH range of 5-12.5. A computational study reveals the organisation of gold atoms in the thiolate-protected blue quantum cluster and its several structural parameters, including the mode of interaction of ligand molecules with Au atoms in the Au7 cluster. Interestingly, it has been found that NIR emitting gold quantum cluster can easily be internalized into the adenocarcinomic human alveolar basal epithelial cell line (A549 cell line). Moreover, a MTT assay indicates that our NIR emitting gold quantum cluster show very low cytotoxicy to A549 cancer cells.