Open-shell Electronic Structure Research Articles

Suzuki-Miyaura Cross-Coupling Reaction Catalyzed by Al12M (M = Be, Al, C, and P) Superatoms with Different Numbers of Valence Electrons.

The exploration of low-cost, efficient, environmentally safe, and selective catalysts for the activation of carbon-halogen bonds has become an important and challenging topic in modern chemistry. With the help of density functional theory (DFT), it is found that phenyl bromide (PhBr) can be efficiently chemisorbed by the Al12M (M = Be, Al, C, and P) superatoms via forming highly polarized Al-Br covalent bonds, where the C-Br bonds of PhBr can be effectively activated through the electron transfer from Al12M. The different electronic structures of these four Al12M superatoms pose a substantial effect on their performances on the activation of PhBr and the catalytic mechanisms of the Suzuki-Miyaura (SM) reaction. Among them, the alkali-metal-like superatom Al12P exhibits the best performance for the activation of PhBr. In particular, Al13 and Al12P with open-shell electronic structures exhibit catalytic performances comparable to those of previously reported catalysts for this coupling reaction. Hence, it is highly expected that Al13 and Al12P could be used as novel superatom catalysts for C-C coupling reactions and, therefore, open up new possibilities to use nonprecious superatoms in catalyzing the activation and transformation of carbon-halogen bonds.

Theoretical study on open-shell electronic structures of through-bond/through-space hybrid conjugated ladder graphs

In this study, we theoretically investigated open-shell electronic structures of face-to-face π-stacked n-mers of diradicaloids with through-bond/through-space (TB/TS) hybrid conjugation structure of n-ladder graphs. Based on the Hückel theory and the valence bond (VB) picture of diradical character (y), we considered y for the local TB and TS interactions, yTB and yTS, and derived sets of conditions between yTB and yTS when y of the entire system, y(n), takes the maximum for a given n. The approximate relations between yTB, yTS, and y(n) help predict the open-shell electronic structures of face-to-face π-stacked n-mers of realistic diradicaloids with different yTB.

Regulation of quantum spin conversions in a single molecular radical.

Free radicals, generally formed through the cleavage of covalent electron-pair bonds, play an important role in diverse fields ranging from synthetic chemistry to spintronics and nonlinear optics. However, the characterization and regulation of the radical state at a single-molecule level face formidable challenges. Here we present the detection and sophisticated tuning of the open-shell character of individual diradicals with a donor-acceptor structure via a sensitive single-molecule electrical approach. The radical is sandwiched between nanogapped graphene electrodes via covalent amide bonds to construct stable graphene-molecule-graphene single-molecule junctions. We measure the electrical conductance as a function of temperature and track the evolution of the closed-shell and open-shell electronic structures in real time, the open-shell triplet state being stabilized with increasing temperature. Furthermore, we tune the spin states by external stimuli, such as electrical and magnetic fields, and extract thermodynamic and kinetic parameters of the transition between closed-shell and open-shell states. Our findings provide insights into the evolution of single-molecule radicals under external stimuli, which may proof instrumental for the development of functional quantum spin-based molecular devices.

Theoretical Study on the Open-Shell Electronic Structure and Electron Conductivity of [18]Annulene as a Molecular Parallel Circuit Model.

Herein, the electron conductivities of [18]annulene and its derivatives are theoretically examined as a molecular parallel circuit model consisting of two linear polyenes. Their electron conductivities are estimated by elastic scattering Green's function (ESGF) theory and density functional theory (DFT) methods. The calculated conductivity of the [18]annulene does not follow the classical conductivity, i.e., Ohm's law, suggesting the importance of a quantum interference effect in single molecules. By introducing electron-withdrawing groups into the annulene framework, on the other hand, a spin-polarized electronic structure appears, and the quantum interference effect is significantly suppressed. In addition, the total current is affected by the spin polarization because of the asymmetry in the coupling constant between the molecule and electrodes. From these results, it is suggested that the electron conductivity as well as the quantum interference effect of π-conjugated molecular systems can be designed using their open-shell nature, which is chemically controlled by the substituents.

PyBEST: Improved functionality and enhanced performance

PyBEST: Improved functionality and enhanced performance

Radical Single-Molecule Junctions.

Radicals are unique molecular systems for applications in electronic devices due to their open-shell electronic structures. Radicals can function as good electrical conductors and switches in molecular circuits while also holding great promise in the field of molecular spintronics. However, it is both challenging to create stable, persistent radicals and to understand their properties in molecular junctions. The goal of this Perspective is to address this dual challenge by providing design principles for the synthesis of stable radicals relevant to molecular junctions, as well as offering current insight into the electronic properties of radicals in single-molecule devices. By exploring both the chemical and physical properties of established radical systems, we will facilitate increased exploration and development of radical-based molecular systems.

Single-Molecule Magnetoluminescence from a Spatially Confined Persistent Diradical Emitter.

Luminescent radicals are an emerging class of materials that exhibit unique photofunctions not found in closed-shell molecules due to their open-shell electronic structure. Particularly promising are photofunctions in which radical's spin and luminescence are correlated; for example, when a magnetic field can affect luminescence (i.e., magnetoluminescence, ML). These photofunctions could be useful in the new science of spin photonics. However, previous observations of ML in radicals have been limited to systems in which radicals are randomly doped in host crystals or polymerized through metal complexation. This study shows that a covalently linked luminescent radical dimer (diradical) can exhibit ML as a single-molecular property. This facilitates detailed elucidation of the requirements for and mechanisms of ML in radicals and can aid the rational design of ML-active radicals based on synthetic chemistry.

Cold Ion-Molecule Chemistry: The Very Different Reactions of He⁺ with CO and NO.

The ion-molecule reactions He+ + CO → He + C+ + O and He+ + NO → He + N+ + O have been measured at collision energies between 0 and kB · 10 K. Strong variations of the rate coefficients are observed below kB · 5 K. The rate of the He+ + CO reaction decreases by ~30% whereas that of the He+ + NO reaction increases by a factor of ~1.5. These observations are interpreted in the realm of an adiabatic-channel capture model as arising from interactions between the ion charge and the dipole and quadrupole moments of CO and NO. We show that the different low-energy behavior of these reactions originates from the closed- vs. open-shell electronic structures of CO and NO.

Variation from closed-shell to open shell electronic structures in oligothiophene bis(dioxolene) complexes.

A series of oligothiophene bis(dioxolene) complexes, SQ-Thn-SQ (SQ = S = ½TpCum,MeZnII(3-tert-butyl-orthosemiquinonate); TpCum,Me = tris(5-cumenyl-3-methylpyrazolyl)borate anion) have been synthesized, structurally characterized, and studied as a function of the number of thiophene bridging units, n (n = 0-3) using a combination of variable-temperature (VT) electronic absorption and EPR spectroscopies, and VT magnetic susceptibility measurements. The thiophene bridge bond lengths determined by X-ray crystallography display dramatic differences across the SQ-Thn-SQ series. Bridge bond deviation values (Σ|Δi|) display a progressive change in the nature of the bridge fragment bonding as the number of thiophene groups increases, with quinoidal bridge character for n = 1 (SQ-Th-SQ) and biradical character with "aromatic" bridge bond lengths for n = 3 (SQ-Th3-SQ). Remarkably, for n = 2 (SQ-Th2-SQ) the nature of the bridge fragment is intermediate between quinoid and biradical aromatic, which we describe as having open-shell character as opposed to biradicaloid since the open-shell biradical configuration does not have the correct symmetry to mix with the quinoidal ground-state configuration. This bridge bonding character is reflected in the energies of the lowest lying open-shell states for these three molecules. The SQ-Th-SQ molecule is diamagnetic at all temperatures studied, and we provide evidence for SQ-SQ antiferromagnetic exchange coupling and population of triplet states in SQ-Th2-SQ and SQ-Th3-SQ, with JSQ-SQ(ave) = -279 cm-1 (VT EPR/electronic absorption/magnetic susceptibility) and JSQ-SQ = -117 cm-1 (VT EPR/electronic absorption/magnetic susceptibility), respectively. The results have been interpreted in the context of state configurational mixing within a simplified 4-electron, 3-orbital model that explicitly contains contributions of a bridge fragment. Variable-temperature spectroscopic- and magnetic susceptibility data are consistent with two low-lying open-shell states for SQ-Th3-SQ, but three low-lying states (one closed-shell and two open-shell) for SQ-Th2-SQ. This model provides a simple symmetry-based framework to understand the continuum of electronic and geometric structures of this class of molecules as a function of the number of thiophene units in the bridge.

Hydrogen Bonding Promotes Alcohol C-C Coupling.

Photocatalytic C-C bond formation coupled with H2 production provides a sustainable approach to producing carbon-chain-prolonged chemicals and hydrogen energy. However, the involved radical intermediates with open-shell electronic structures are highly reactive, experiencing predominant oxidative or reductive side reactions in semiconductors. Herein, we demonstrate that hydrogen bonding on the catalyst surface and in the bulk solution can inhibit oxidation and reverse reaction of α-hydroxyethyl radicals (αHRs) in photocatalytic dehydrocoupling of ethanol over Au/CdS. Intentionally added water forms surface hydrogen bonds with adsorbed αHRs and strengthens the hydrogen bonding between αHRs and ethanol while maintaining the flexibility of radicals in solution, thereby allowing for αHRs' desorption from the Au/CdS surface and their stabilization by a solvent. The coupling rate of αHR increases by 2.4-fold, and the selectivity of the target product, 2,3-butanediol (BDO), increases from 37 to 57%. This work manifests that nonchemical bonding interactions can steer the reaction paths of radicals for selective photocatalysis.

First-principles kinetic Monte Carlo modeling of propylene epoxidation on the Ti2CuO6/Cu(111) and Cu2O(111) surfaces

Direct propylene epoxidation by molecular oxygen is one of the most important commercial selective oxidation reactions. Although the mixed metal oxide (Ti2CuO6/Cu(111)) with highly dispersed and stabilized Cu+ has high selectivity toward propylene oxide (PO), the reaction mechanism and the physical nature of stable Cu+ involved in Ti2CuO6/Cu(111) remain unclear. To better understand the key role of stabilized Cu+ active sites, the systematic kinetic Monte Carlo (kMC) studies of the propylene epoxidation reaction on Ti2CuO6/Cu(111) and Cu2O(111) surfaces are carried out. It was found that O2 is weakly bound to Ti2CuO6/Cu(111) due to the less active of Cu+ ions compared to that of Cu+ ions in Cu2O(111), which can exist stably. The current simulation demonstrates that the relatively stable Ti2CuO6/Cu(111) mixed oxide exhibits better PO selectivity and activity than Cu2O(111). The reasons for the different catalytic performance are revealed: For the primary chemistry of propylene epoxidation mechanism, Cu2O(111) favors the first allylic hydrogen stripping, while Ti2CuO6/Cu(111) has an excellent open-shell electronic structure character, which makes it more reactive toward the propylene double bond to form the oxametallacycle (OOMMP or OMMP). For the secondary chemistry of propylene epoxidation mechanism, the energy barrier of PO formation is smaller than that of propanal(PA)/acetone(AC) due to the larger difference in bonding affinity toward the O and C atoms. In contrast, the opposite trend holds for Cu2O(111). In addition, the surface species can be balanced quickly on Ti2CuO6/Cu(111). The catalyst surface is not poisoned because the surface species coverage is moderate, which means that Ti2CuO6/Cu(111) remains highly stable during the reaction processes. The Cu2O(111) has fewer active sites due to higher surface species coverage (i.e., catalyst poisoning). One may rationally design an efficient propylene epoxidation catalyst based on the present work, i.e., doping Cu into the stable spinels or perovskites such as Cu-modified LaCoO3 perovskites (LaCoxCu1−xO3−δ) may show the similar catalytic properties like that of Ti2CuO6/Cu(111).

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

Topological Defects Induced High-Spin Quartet State in Truxene-Based Molecular Graphenoids

UREMP, RO-REMP, and OO-REMP: Hybrid perturbation theories for open-shell electronic structure calculations.

An accurate description of the electron correlation energy in closed- and open-shell molecules is shown to be obtained by a second-order perturbation theory (PT) termed REMP. REMP is a hybrid of the Retaining the Excitation degree (RE) and the Møller-Plesset (MP) PTs. It performs particularly encouragingly in an orbital-optimized variant (OO-REMP) where the reference wavefunction is given by an unrestricted Slater determinant whose spin orbitals are varied such that the total energy becomes a minimum. While the approach generally behaves less satisfactorily with unrestricted Hartree-Fock references, reasonable performance is observed for restricted Hartree-Fock and restricted open-shell Hartree-Fock references. Inclusion of single excitations to OO-REMP is investigated and found-as in similar investigations-to be dissatisfying as it deteriorates performance. For the non-multireference subset of the accurate W4-11 benchmark set of Karton et al. [Chem. Phys. Lett. 510, 165-178 (2011)], OO-REMP predicts most atomization and reaction energies with chemical accuracy (1 kcalmol-1) if complete-basis-set extrapolation with augmented and core-polarized basis sets is used. For the W4-11 related test-sets, the error estimates obtained with the OO-REMP method approach those of coupled-cluster with singles, doubles and perturbative triples [CCSD(T)] within 20%-35%. The best performance of OO-REMP is found for a mixing ratio of 20%:80% MP:RE, which is essentially independent of whether radical stabilization energies, barrier heights, or reaction energies are investigated. Orbital optimization is shown to improve the REMP approach for both closed and open shell cases and outperforms coupled-cluster theory with singles and doubles (CCSD), spin-component scaled Møller-Plesset theory at second order (SCS-MP2), and density functionals, including double hybrids in all the cases considered.

Analysis of the Geometric and Electronic Structure of Spin-Coupled Iron-Sulfur Dimers with Broken-Symmetry DFT: Implications for FeMoco.

The open-shell electronic structure of iron–sulfur clusters presents considerable challenges to quantum chemistry, with the complex iron–molybdenum cofactor (FeMoco) of nitrogenase representing perhaps the ultimate challenge for either wavefunction or density functional theory. While broken-symmetry density functional theory has seen some success in describing the electronic structure of such cofactors, there is a large exchange–correlation functional dependence in calculations that is not fully understood. In this work, we present a geometric benchmarking test set, FeMoD11, of synthetic spin-coupled Fe–Fe and Mo–Fe dimers, with relevance to the molecular and electronic structure of the Mo-nitrogenase FeMo cofactor. The reference data consists of high-resolution crystal structures of metal dimer compounds in different oxidation states. Multiple density functionals are tested on their ability to reproduce the local geometry, specifically the Fe–Fe/Mo–Fe distance, for both antiferromagnetically coupled and ferromagnetically coupled dimers via the broken-symmetry approach. The metal–metal distance is revealed not only to be highly sensitive to the amount of exact exchange in the functional but also to the specific exchange and correlation functionals. For the antiferromagnetically coupled dimers, the calculated metal–metal distance correlates well with the covalency of the bridging metal–ligand bonds, as revealed via the corresponding orbital analysis, Hirshfeld S/Fe charges, and Fe–S Mayer bond order. Superexchange via bridging ligands is expected to be the dominant interaction in these dimers, and our results suggest that functionals that predict accurate Fe–Fe and Mo–Fe distances describe the overall metal–ligand covalency more accurately and in turn the superexchange of these systems. The best performing density functionals of the 16 tested for the FeMoD11 test set are revealed to be either the nonhybrid functionals r2SCAN and B97-D3 or hybrid functionals with 10–15% exact exchange: TPSSh and B3LYP*. These same four functionals are furthermore found to reproduce the high-resolution X-ray structure of FeMoco well according to quantum mechanics/molecular mechanics (QM/MM) calculations. Almost all nonhybrid functionals systematically underestimate Fe–Fe and Mo–Fe distances (with r2SCAN and B97-D3 being the sole exceptions), while hybrid functionals with >15% exact exchange (including range-separated hybrid functionals) overestimate them. The results overall suggest r2SCAN, B97-D3, TPSSh, and B3LYP* as accurate density functionals for describing the electronic structure of iron–sulfur clusters in general, including the complex FeMoco cluster of nitrogenase.

The Magnetic Behaviour of CoTPP Supported on Coinage Metal Surfaces in the Presence of Small Molecules: A Molecular Cluster Study of the Surface trans-Effect.

Density functional theory, combined with the molecular cluster model, has been used to investigate the surface trans-effect induced by the coordination of small molecules L (L = CO, NH3, NO, NO2 and O2) on the cobalt electronic structure of cobalt tetraphenylporphyrinato (CoTPP) surface-supported on coinage metal surfaces (Cu, Ag, and Au). Regardless of whether L has a closed- or an open-shell electronic structure, its coordination to Co takes out the direct interaction between Co and the substrate eventually present. The CO and NH3 bonding to CoTPP does not influence the Co local electronic structure, while the NO (NO2 and O2) coordination induces a Co reduction (oxidation), generating a 3d8 CoI (3d6 CoIII) magnetically silent closed-shell species. Theoretical outcomes herein reported demonstrate that simple and computationally inexpensive models can be used not only to rationalize but also to predict the effects of the Co–L bonding on the magnetic behaviour of CoTPP chemisorbed on coinage metals. The same model may be straightforwardly extended to other transition metals or coordinated molecules.

A 59-Electron Non-Magic-Number Gold Nanocluster Au99(C≡CR)40 Showing Unexpectedly High Stability.

An atomically resolved gold nanocluster Au99(C≡CC6H3-2,4-F2)40 (Au99) with an unusual 59 valence electrons has been synthesized. Single-crystal X-ray diffraction reveals that its Au79 kernel is a Au49 Marks decahedron capped by two Au15 units. The surface structure of Au99 consists of 20 linear Au(C≡CR)2 staples. Intercluster interactions are observed between these D5 symmetric clusters. The existence of an unpaired electron is verified by magnetic measurement. Interestingly, this open-shell gold cluster Au99 stays intact in toluene solution at 80 °C for more than a week, and it has good charging-discharging capability under electrochemical conditions. The compact ligand shell protection around the symmetric core accounts for the high stability. This work suggests that geometric factors may play a crucial role in determining the stability of a metal nanocluster, even though the cluster has an open-shell electronic structure.

The taming of Clar's hydrocarbon.

Triangulene is the smallest non-Kekulé graphene fragment known as Clar's hydrocarbon. Due to its open-shell electronic structure, triangulene is a promising molecular building block of carbon-based organic materials for spintronics and quantum molecular science. It comprises six benzenoid rings arranged in a triangular shape with two unpaired electrons delocalized over the entire conjugated core, making this molecule highly reactive. A triplet ground state is predicted for this hydrocarbon by Ovchinnikov's rule, or Lieb's theorem, in accord with Hund's rule. The pioneering work on triangulene was performed almost 70 years ago by Erich Clar, who attempted to prepare the pristine compound. Since then, several synthetic approaches to prepare this molecule have been exploited. The extreme reactivity of triangulene can be circumvented using on-surface techniques or by installation of sterically demanding substituents, which kinetically stabilize the diradical core against oligomerization in solution. The first two examples of a persistent derivative of triangulene were simultaneously and independently developed last year. This article presents a historical development in the synthesis of triangulene and its derivatives and outlines possible future applications in ferromagnetic materials, electrically conductive polymers or quantum computing.

Pyridine dicarbanion-bonded Ag13 organometallic nanoclusters: synthesis and on-surface oxidative coupling reaction.

Highly reactive organometallic nanoclusters in situ generated in metal-catalyzed reactions are pivotal in the comprehension of catalytic mechanisms. Herein, we develop a two-step synthetic method to achieve three unprecedented aryl dicarbanion-bonded Ag13 nanoclusters by using protective macrocyclic ligands. Firstly, various aryl dicarbanion–Ag4 cluster intermediates are acquired via a silver-mediated annulation reaction within a macrocyclic ligand. These Ag4 cluster precursors are released from the surrounding macrocycle by protonation, and further undergo an inter-cluster coupling to generate bipyridine products and low-valence silver atoms. The remaining resurgent diide–Ag4 clusters assemble with low-valence silver atoms to yield a series of organometallic Ag13 nanoclusters. These Ag13 nanoclusters feature a unique open-shell electronic structure as well as a chiral cluster architecture due to the asymmetric arrangements of surrounding aryl dianion ligands. Furthermore, the pyridyl diide ligands on the surface of the nanocluster further experience an intra-cluster oxidative coupling to produce bipyridine coupling products and large nanoparticles. The coupling reaction-driven cluster-to-cluster transformation is comprehensively tracked by high resolution mass spectroscopy. This work is not only reminiscent of the detailed evolution of cluster species upon the occurrence of coupling reactions, but also reproduces novel inter- and intra-cluster coupling steps at different reaction stages.

Evolution of the electronic structure in open-shell donor-acceptor organic semiconductors

Most organic semiconductors have closed-shell electronic structures, however, studies have revealed open-shell character emanating from design paradigms such as narrowing the bandgap and controlling the quinoidal-aromatic resonance of the π-system. A fundamental challenge is understanding and identifying the molecular and electronic basis for the transition from a closed- to open-shell electronic structure and connecting the physicochemical properties with (opto)electronic functionality. Here, we report donor-acceptor organic semiconductors comprised of diketopyrrolopyrrole and naphthobisthiadiazole acceptors and various electron-rich donors commonly utilized in constructing high-performance organic semiconductors. Nuclear magnetic resonance, electron spin resonance, magnetic susceptibility measurements, single-crystal X-ray studies, and computational investigations connect the bandgap, π-extension, structural, and electronic features with the emergence of various degrees of diradical character. This work systematically demonstrates the widespread diradical character in the classical donor-acceptor organic semiconductors and provides distinctive insights into their ground state structure-property relationship.

The reactivity of O2 with copper cluster anions Cun− (n = 7−20): Leveling effect of spin accommodation

The activation of molecular oxygen is an important step in metal-catalyzed oxidation reactions and a hot subject for the research of gas-phase metal clusters. It is known that the Ag and Au clusters readily react with O2 when they have open shell electronic structures. Distinct from this, here we observed Cun− (n = 7−20) clusters of both open and closed shells possess high reactivity with O2 with few exceptions. In a combination with ab initio calculations, we demonstrate that the activation of O2 on the even- and odd-sized Cun− clusters follows the single and double electron transfer models, respectively. Such phenomenon of metal clusters with different basicity to activate oxygen is enabled by the leveling effect of spin accommodation. The activity of Cun− clusters is correlated to the HOMO level, and for the close-shell clusters is also governed by the vertical spin excitation energy (VSE). In encountering the attack of dioxygen, the activity of the copper cluster anions not only depends on their basicity to donate electrons, but also closely associated with the cluster sizes. Small copper clusters Cun− (n = 7−13) can dissociate O2 spontaneously, while large clusters require extra energies and display close relationship between the reaction rates and electronic vertical detachment energies (VDE). Our work illuminates a novel reaction mechanism between Cun− clusters and O2, which sheds light in manipulating the activity and stability of coinage clusters by controlling the spin and charge states.