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Species generally exhibit one‐state aromaticity either in the lowest singlet state (S0) or the lowest triplet state (T1) according to the Hückel's and Baird's rules. Hence, it is rare for species exhibit two‐state aromaticity in both the S0 and T1 states (termed as adaptive aromaticity), let alone adaptive σ aromaticity. Here, we report adaptive σ aromaticity in unsaturated rhenacyclopropene rings via density functional theory (DFT) calculations. Various aromaticity indices including NICS, ACID, EDDB together with isodesmic reactions support the adaptive σ aromaticity in these rhenacyclopropene rings. As the T1 state of these species is formed by the ππ* excitation, the σ‐aromaticity of these three‐membered rings in the S0 state could hold in the T1 state. In addition, the aromaticity effect of the fused rings is also examined. Our findings expand the family of adaptive σ aromaticity, enriching the metallaaromatic chemistry. [ABSTRACT FROM AUTHOR]

According to Hückel's and Baird's rules, cyclic species are generally aromatic only either in the lowest singlet state (S0) or in the lowest-lying triplet ππ* excited state (T1). Thus, species with aromaticity both in S0 and T1 states (termed as adaptive aromaticity) are particularly rare. Herein, we carry out density functional theory (DFT) calculations to examine the aromaticity of 16e metallapentalenes containing heteroatoms (N, O). Interestingly, metallazapentalenes show adaptive aromaticity whereas metalloxapentalenes display nonaromaticity in the S0 and T1 states, which is supported by structural, magnetic, and electronic indices. In addition, a series of metallazapentalenes containing strong σ- or π-donor ligands are predicted to achieve adaptive aromaticity. Our findings expand the family of adaptive aromatics significantly, inviting experimental chemists to realize more hetero-metallapentalenes with adaptive aromaticity. [ABSTRACT FROM AUTHOR]

Molecular nitrogen (N2) is abundant in the atmosphere and, found in many biomolecules, an essential element of life. The Haber–Bosch process, developed over 100 years ago, requires relatively harsh conditions to activate N2 on the iron surface and generate ammonia for use as fertilizer or to produce other chemicals, leading to consumption of more than 2 % of the world's annual energy supply. Thus, developing "green" approaches for N2 activation under mild conditions is particularly important and urgent. Here we demonstrate that a metal‐free N2 activation could be favorable both thermodynamically and kinetically (with an activation energy as low as 9.1 kcal mol−1) by using a carbon‐boron formal frustrated Lewis pair, which is supported by high‐level coupled cluster calculations. Mechanistic studies reveal that aromaticity plays a crucial role in stabilizing both the transition state and the product. Our findings highlight the importance of a combination of an N‐heterocyclic carbene with a methyleneborane unit in metal‐free N2 activation, providing conceptual guidance for experimental realization. [ABSTRACT FROM AUTHOR]

The new 6π‐electron four‐membered ring compound 3‐fluoro‐1λ2,2,4,3λ3‐thiadiazaphosphetidine, FP(μ‐N)2S, has been generated in the gas phase through high‐vacuum flash pyrolysis (HVFP) of thiophosphoryl diazide, FP(S)(N3)2, at 1000 K. Subsequent isolation of FP(μ‐N)2S in cryogenic matrices (Ar, Ne, and N2) allows its characterization with matrix‐isolation IR and UV‐vis spectroscopy by combination with 15N‐isotope labeling and computations at the CCSD(T)‐F12a/VTZ‐F12 level of theory. Upon visible‐light irradiation at 550 nm, this cyclic compound undergoes ring‐opening to the thiazyl isomer FPNSN, followed by dissociation to FP and SN2 under subsequent UV‐irradiation at 365 nm. In sharp contrast to the square planar structure for the isolobal four‐membered ring S2N2, a puckered structure with significant biradical character has been found for FP(μ‐N)2S. [ABSTRACT FROM AUTHOR]

Aromaticity, one of the central topics in chemistry, has attracted continuing interest of both experimentalists and theoreticians. Given that the original definition of aromaticity could be derived from the fact that the cyclic aromatic species are thermodynamically more stable than the corresponding acyclic conjugated ones, here we propose a direct evaluation of the aromatic stabilization energy (ASE) by following this definition to compute the energy difference between the cyclic compound and its corresponding acyclic one via a homodesmotic equation. Our results indicate that this method is able to produce a reasonable result in both the singlet ground state (S 0 ) and the lowest triplet state (T 1 ), evidenced by the correlations (R 2  = 0.947 and 0.769 in the S 0 and T 1 states for organic cyclics and R 2  = 0.851 in the S 0 state for organometallics) between this method and the isomerization stabilization energy (ISE) method, and the magnetic indicator NICS(1) zz (R 2  = 0.754 and 0.882 for organics in S 0 and T 1 states, respectively), thus providing an alternative to evaluate the aromaticity in the S 0 and T 1 states. [ABSTRACT FROM AUTHOR]

Dinitrogen (N2) activation is particularly challenging under ambient conditions because of its large highest occupied molecular orbital‐lowest unoccupied molecular orbital (HOMO‐LUMO) gap (10.8 eV) and high bond dissociation energy (945 kJ mol−1) of the N≡N triple bond, attracting considerable attention from both experimental and theoretical chemists. However, most effort has focused on metallic systems. In contrast, nitrogen activation by frustrated Lewis pairs (FLPs) has been initiated recently via theoretical calculations. Here we perform density functional theory (DFT) calculations to screen a series of experimentally viable FLPs for small‐molecule activation including N2, O2, CO, CO2, CS2, H2O and CH4. In addition, aromaticity is found to play an important role in most of these small‐molecule activation. The particularly thermodynamic stabilities of the activation products and low reaction barriers could be a step forward for the development of FLP towards small‐molecule activation including N2, inviting experimental chemists' verification. [ABSTRACT FROM AUTHOR]

Discovery of species with adaptive aromaticity, being aromatic in both the lowest‐lying singlet and triplet states (S0 and T1), is a significant challenge because cyclic conjugated complexes are commonly aromatic in one state only according to Hückel's and Baird's rules. On other hand, the carbone ligands containing two lone pair electrons at the carbon(0) atom have attracted considerable attention from both theoretical and experimental chemists recently. Here, we carry out density functional theory (DFT) calculations on osmapentalene and osmapyridinium complexes with carbone ligands to examine their aromaticity in both the S0 and T1 states. It is found that these complexes can possess adaptive aromaticity, supported by various aromaticity indices including the harmonic oscillator model of aromaticity (HOMA), nucleus‐independent chemical shift (NICS), anisotropy of the induced current density (ACID), electron density of delocalized bonds (EDDB), and the change of heat of hydrogenation (∆∆H). Our findings expand the scope of the concept of adaptive aromaticity, inviting experimental chemists' verification to enrich aromatic organometallic chemistry. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations were carried out to investigate the 1,2-migration in metallasilabenzenes. The results suggested that the chloride migration of metallabenzenes is unfavorable due to the loss of aromaticity in the nonaromatic analogues. In sharp contrast, such a migration in metallasilabenzenes is favorable due to the reluctance of silicon to participate in π bonding. The migration of hydride and methyl group from the metal center to the silicon atom in metallasilabenzenes is computed to be also feasible. In addition, the π donor ligand and the third row transition metal can stabilize metallasilabenzenes. Thus, such a migration becomes less favorable thermodynamically and kinetically. These findings could be very helpful for synthetic chemists to realize the first metallasilabenzene. [ABSTRACT FROM AUTHOR]

Aromaticity, one of the most important concepts in chemistry, has attracted considerable interest from both experimentalists and theoreticians. According to Baird's rule, triplet annulenes with 4 n π electrons are aromatic. However, the approach to evaluate the magnitude of the triplet aromaticity is less developed. Herein we apply the indene-isoindene isomerization stabilization energy (ISE) method to evaluate the aromaticity in the triplet state. The reliability of this approach can be demonstrated by the strong correlation of these indene-isoindene ISE values with nucleus-independent chemical shifts [NICS(1) zz] as well as methyl-methylene ISE values. Large [4 n]annulenes have the tendency to be planar to achieve aromaticity in the T1 state. Steric effects play an important role in the stabilities of large [4 n]annulene isomers. [ABSTRACT FROM AUTHOR]

A density functional theory study on olefins with five-membered monocyclic 4 n and 4 n+2 π-electron substituents (C4H3X; X=CH+, SiH+, BH, AlH, CH2, SiH2, O, S, NH, and CH−) was performed to assess the connection between the degree of substituent (anti)aromaticity and the profile of the lowest triplet-state (T1) potential-energy surface (PES) for twisting about olefinic CC bonds. It exploited both Hückel's rule on aromaticity in the closed-shell singlet ground state (S0) and Baird's rule on aromaticity in the lowest ππ* excited triplet state. The compounds CH2CH(C4H3X) were categorized as set A and set B olefins depending on which carbon atom (C2 or C3) of the C4H3X ring is bonded to the olefin. The degree of substituent (anti)aromaticity goes from strongly S0-antiaromatic/T1-aromatic (C5H4+) to strongly S0-aromatic/T1- antiaromatic (C5H4−). Our hypothesis is that the shapes of the T1 PESs, as given by the energy differences between planar and perpendicularly twisted olefin structures in T1 [Δ E(T1)], smoothly follow the changes in substituent (anti)aromaticity. Indeed, correlations between Δ E(T1) and the (anti)aromaticity changes of the C4H3X groups, as measured by the zz-tensor component of the nucleus-independent chemical shift ΔNICS(T1;1) zz, are found both for sets A and B separately (linear fits; r2=0.949 and 0.851, respectively) and for the two sets combined (linear fit; r2=0.851). For sets A and B combined, strong correlations are also found between Δ E(T1) and the degree of S0 (anti)aromaticity as determined by NICS(S0,1) zz (sigmoidal fit; r2=0.963), as well as between the T1 energies of the planar olefins and NICS(S0,1) zz (linear fit; r2=0.939). Thus, careful tuning of substituent (anti)aromaticity allows for design of small olefins with T1 PESs suitable for adiabatic Z/ E photoisomerization. [ABSTRACT FROM AUTHOR]

The many manifestations of aromaticity have long fascinated both experimentalists and theoreticians. Due to their degenerate half-filled MOs, triplet [n]annulenes with 4nπ-electrons are also aromatic, but the degree of their stabilization has been difficult to quantify. The isomerization stabilization energy (ISE) method has been applied to evaluate the triplet aromaticity. The reliability of this approach is indicated by the strong correlation of the ISE results with NICS(1)zz, a magnetic indicator of triplet state aromaticity. [ABSTRACT FROM AUTHOR]

While sp2-hybridized carbon atoms in hydrocarbons typically contribute only one electron to their aromaticity, metals have more electrons from d or f orbitals available for participating in conjugation in organometallics, complicating the electron counting as well as analysis of their aromaticity. Here, the author comments on the challenges towards understanding aromaticity in organometallics and outlines several remaining questions that have yet to be answered. [ABSTRACT FROM AUTHOR]

As a fundamental concept in chemistry, aromaticity has been extended from traditional organics to organometallics. Similarly, hyperconjugative aromaticity (HCA) has also been developed from main group to transition metal systems through the hyperconjugation of the substituents. However, it remains unclear that how the oxidation state of transition metal in the substituents affects the HCA. Herein, we demonstrate via density functional theory calculations that HCA could disappear in indoliums when the Au(I) substituents are changed to the Au(III) ones. By tuning the ligand or cis–trans isomerization, HCA could be regained or enhanced in indoliums containing Au(III) substitutents. [ABSTRACT FROM AUTHOR]

The isolated‐pentagon rule (IPR) is a determining structural feature that accounts for hollow fullerene stabilization and properties related to Cn (n≥60) cages. The recent characterization of an unprecedented non‐IPR hydrofullerene, C2v C66H4, bearing two heptagons with adjacent fused‐pentagon motifs, largely dismisses this feature. Herein, employing DFT calculations, the 13C NMR spectroscopy and aromatic behavior of C2v C66H4 are explored. The results show the presence of three π‐aromatic circuits at the bottom boat section of C66H4, indicating the unique features of this hydrofullerene in comparison to those of pristine C60. In addition, under specific orientations of the external field, certain π‐aromatic circuits are enabled, resulting in a more aromatic fullerene than that of C60, but lower than that of the spherical aromatic C606− fulleride. Notably, under a field aligned with the saturated carbon atoms, nonaromatic characteristics are exposed. This reveals that spherical‐like cages can involve a complex magnetic response that heavily depends on the orientation of the applied field. [ABSTRACT FROM AUTHOR]

Based on Hückel's and Baird's rules, species are aromatic either in the lowest singlet state (S0) or the lowest triplet state (T1) only. Thus, species with adaptive aromaticity (with aromaticity in both the S0 and T1 states) is particularly rare. On the other hand, σ‐aromaticity in the T1 state has been underdeveloped, let alone adaptive σ‐aromaticity. Herein, via various aromaticity indices including NICS, ACID and EDDB, we demonstrate adaptive σ‐aromaticity in an unsaturated three‐membered ring, which is a traditional area dominated by π‐aromaticity. The origin of adaptive σ‐aromaticity could be attributed to the excitation mode of the T1 state formed from out‐of‐plane π molecular orbital to the π* orbitals. Thus the σ‐aromaticity of the three‐membered ring in the S0 state could hold in the T1 state. Our findings extend the concept of adaptive σ‐aromaticity into three‐membered rings and could be useful to further develop the concept of both σ‐aromaticity and adaptive aromaticity. [ABSTRACT FROM AUTHOR]

Baird's rule has been applied to a large scope of organic molecular systems for rationalizing the aromaticity reversal in the lowest-lying triplet state. In this study, we demonstrate that Baird's rule can also be extended to all-metal systems with σ- and π-aromaticity. [ABSTRACT FROM AUTHOR]

Species with adaptive aromaticity are aromatic in the ground and lowest‐lying triplet excited states and they have normally intermediate singlet‐triplet gaps. Few examples of compounds with adaptive aromaticity are known to date, including 16‐valence‐electron (16e) metallapentalenes. A sweeping search could be conducted to discover new members of this group, but efficient designs with an explicit strategy would facilitate the quest for new members of this elusive family. Density functional theory calculations and aromaticity evaluations have been performed to reveal the nature of triplet‐state aromaticity in 16e metallapentalenes. Our results show that coordination of strong σ‐ or π‐donor ligands helps achieving adaptive aromaticity of 16e metallapentalenes by means of a spin delocalization mechanism. These results have important implications for understanding the unusual properties of the organometallic adaptive aromatics, leading the way to efficient design of new compounds with tunable singlet‐triplet gaps. [ABSTRACT FROM AUTHOR]

Discovery of species with adaptive aromaticity (being aromatic in both the lowest singlet and triplet states) is particularly challenging as cyclic species are generally aromatic either in the ground state or in the excited state only, according to Hückel's and Baird's rules. Inspired by the recent realization of cyclo[18]carbon, here we demonstrate that cyclo[10]carbon possesses adaptive aromaticity by screening cyclo[n]carbon (n=8−24), which is supported by nucleus‐independent chemical shift (NICS), anisotropy of the current‐induced density (ACID), π contribution of electron localization function (ELFπ) and electron density of delocalized bonds (EDDB) analyses. Further study reveals that the lowest triplet state of cyclo[10]carbon is formed by in‐plane ππ* excitation. Thus, the major contribution to the aromaticity from out‐of‐plane π molecular orbitals does not change significantly in the lowest singlet state. Our findings highlight a crucial role of out‐of‐plane π orbitals in maintaining aromaticity for both the lowest singlet and triplet states as well as the aromaticity dependence on the number of the carbon in cyclo[n]carbon. [ABSTRACT FROM AUTHOR]

An [8]cyclo‐para‐phenylmethine derivative ([8]CPPM‐Mes) was synthesized. X‐ray analysis revealed a tub‐shaped geometry similar to the cyclooctatetraene, with alternating benzenoid/quinoid structure. Variable‐temperature NMR measurements disclosed a slow valence tautomerization process with an interconversion energy barrier of about 11.7 kcal mol−1 at coalescence temperature (273 K), and two more lower‐barrier dynamic processes involving flipping of the 1,4‐phenyl rings on the backbone and rotation of the mesityl substituents. Its dication ([8]CPPM‐Mes2+) adopts a bowl‐like geometry with a smaller depth of the cavity, and a slow bowl inversion process was observed by dynamic NMR. The bond lengths of the benzenoid/quinoid rings are more averaged via electron delocalization and the molecule shows global aromaticity, which was further validated by NMR and theoretical analysis. [8]CPPM‐Mes2+ exhibits open‐shell diradical character with a small singlet‐triplet energy gap (−1.8 kcal mol−1). [ABSTRACT FROM AUTHOR]

An [8]cyclo‐para‐phenylmethine derivative ([8]CPPM‐Mes) was synthesized. X‐ray analysis revealed a tub‐shaped geometry similar to the cyclooctatetraene, with alternating benzenoid/quinoid structure. Variable‐temperature NMR measurements disclosed a slow valence tautomerization process with an interconversion energy barrier of about 11.7 kcal mol−1 at coalescence temperature (273 K), and two more lower‐barrier dynamic processes involving flipping of the 1,4‐phenyl rings on the backbone and rotation of the mesityl substituents. Its dication ([8]CPPM‐Mes2+) adopts a bowl‐like geometry with a smaller depth of the cavity, and a slow bowl inversion process was observed by dynamic NMR. The bond lengths of the benzenoid/quinoid rings are more averaged via electron delocalization and the molecule shows global aromaticity, which was further validated by NMR and theoretical analysis. [8]CPPM‐Mes2+ exhibits open‐shell diradical character with a small singlet‐triplet energy gap (−1.8 kcal mol−1). [ABSTRACT FROM AUTHOR]

Dinitrogen (N2) activation is particularly challenging due to the significantly strong N≡N bond, let alone the catenation of two N2 molecules. Recent experimental study shows that cyclic (alkyl)(amino)carbene (CAAC)‐stabilized borylenes are able to tackle N2 activation and coupling below room temperature. Here we carry out density functional theory calculations to explore the corresponding reaction mechanisms. The results indicate that the reaction barrier for the dinitrogen activation by the first borylene is slightly higher than that by the second borylene. In addition, replacing the CAAC moiety of the borylenes with cyclic diaminocarbenes (CDACs) could make such dinitrogen activation and coupling more favorable thermodynamically. The reaction mechanisms of the intramolecular C−H bond activation of borylene have also been discussed, which is found to be favorable both thermodynamically and kinetically in comparison with N2 activation. Thus, adequate attention should be paid to the design of borylenes aiming at N2 activation. In addition, our calculations suggest that the CDAC moiety of the borylene could lead to a different product in terms of intramolecular C−H bond activation. All these findings could be useful for the development of dinitrogen activation as well as C−H bond activation by main group species. [ABSTRACT FROM AUTHOR]

Aromaticity, an old but still fantastic topic, has long attracted considerable interest of chemists. Generally, π aromaticity is described by π-electron delocalization in closed circuits of unsaturated compounds whereas σ-electron delocalization in saturated rings leads to σ aromaticity. Interestingly, our recent study shows that σ aromaticity can be dominating in an unsaturated three-membered ring (3MR) of cyclopropaosmapentalene. An interesting question is raised: Can the s aromaticity, which is dominant in the unsaturated 3MR, be extended to other cyclopropametallapentalenes? If so, how could the metal centers, ligands, and substituents affect the σ aromaticity? Here, we report a thorough theoretical study on these issues. The nucleus-independent chemical shift calculations and the anisotropy of the current-induced density plots reveal the dominant σ aromaticity in these unsaturated 3MRs. In addition, our calculations show that substituents on the 3MRs have significant effects on the s aromaticity, whereas the ligand effect is particularly small. [ABSTRACT FROM AUTHOR]

Osmapentalyne and osmapentalene complexes, now termed as carbolong species, have attracted considerable attention due to their novel structures, reactivities, and chelating properties as well as Möbius and adaptive aromaticity. On the other hand, boron monofluoride (BF), a 10-electron diatomic molecule isoelectronic to carbon monoxide (CO), is unstable below 1800 °C in the gas phase, and preparation of its metal complex is particularly challenging. Although the iron complex featuring a terminal BF ligand was synthesized recently, the structure, bonding and aromaticity of metallapentalyne and metallapentalene complexes containing BF ligands remain unclear. Herein, we carry out density functional theory (DFT) calculations to address this issue. The Os–B bond could be considered as a single, double or triple bond depending on the ligands attached to the metal center according to principal interacting orbital (PIO) analysis. The similarity on aromaticity is revealed between CO complexes and BF ones. All these species display aromaticity in the S0 state. For the T1 state, they could be aromatic, nonaromatic or antiaromatic depending on the excitation mode. All these findings could be useful to develop the concept of adaptive aromaticity as well as BF chemistry. [ABSTRACT FROM AUTHOR]

Density functional theory calculations (B3LYP) have been carried out to investigate the 4π-electron systems of 2,4-disila-1,3-diphosphacyclobutadiene (compound 1) and the tetrasilacyclobutadiene dication (compound 2). The calculated nucleus-independent chemical shift (NICS) values for these two compounds are negative, which indicates that the core rings of compounds 1 and 2 have a certain amount of aromaticity. However, deep electronic analysis reveals that neither of these two formal 4π-electron four-membered ring systems is aromatic. Compound 1 has very weak, almost negligible antiaromaticity, and the amidinate ligands attached to the Si atoms play an important role in stabilizing this conjugated 4π-electron system. The monoanionic bidentate ligand interacts with the conjugated π system to cause π-orbital splitting. This ligand-induced π-orbital splitting effect provides an opportunity to manipulate the gap between occupied and unoccupied π orbitals in conjugated systems. Conversely, compound 2 is nonaromatic because its core ring does not have a conjugated π ring system and does not fulfill the requirements of a Hückel system. [ABSTRACT FROM AUTHOR]

• The methods of NPA, LOL, ACID, CDD were examined to probe the origin of the chameleonic reactivity on the carbyne carbon atoms. • The thermodynamics and kinetics of the nucleophilic reactivity of cyclic conjugated complexes were investigated by DFT calculations. • The ring strain, aromaticity, and the phosphonium substituent determine the ambiphilicity of the same carbyne carbon. Ambiphilic reactivity is a fascinating topic in chemical reactions, attracting considerable interest because ambiphilic reagents can display properties of both nucleophilicity and electrophilicity. However, most of the previous attentions have been focused on the characterization of the ambiphilic reactivity, whereas the origin is less understood. Here we carry out thorough density functional theory (DFT) calculations to probe the origin of the ambiphilic reactivity of the carbyne atom in osmapentalynes, observed previously in experiment. A series of osmium carbyne complexes ranging from acyclic to cyclic, non-conjugated to conjugated, non-aromatic to aromatic, unsubstituted to substituted were designed for comparable studies. Our results indicate that the origin of ambiphilic reactivity in osmapentalyne complexes should be attributed to an interplay of ring strain, aromaticity, and phosphonium substituent. [Display omitted] [ABSTRACT FROM AUTHOR]

Disilene has attracted considerable interest due to the trans-bending geometry which is significantly different from the planar alkene. However, the equilibrium between disilene and isomeric silylsilylene has not been fully understood. Here, we report a density functional theory (DFT) study on this equilibrium. Calculations reveal significant effects of substituent, aromaticity and base. Specifically, the parent disilene is thermodynamically more stable than the isomeric silylene. When the methoxy substituent is introduced, the corresponding silylene becomes thermodynamically more stable, which could be rationalized by the Bent's rule. Interestingly, disilabenzene becomes thermodynamically more stable than the isomeric silylene when the concept of aromaticity is taken into account. Finally, once the base is introduced, the silylene could become thermodynamically more stable than the isomeric disilabenzene. The kinetic effect of the tautomerization with several typical substituents (F, Me and OMe) has also been investigated. Some species with a bridged form have been found to have a higher thermodynamic stability over the nonbridged ones. All these findings could be particularly useful to develop the chemistry of disilenes and silylenes. [ABSTRACT FROM AUTHOR]

Aromaticity is one of the most basic concepts in organic chemistry. The planar Möbius aromatic metallapentalynes and metallapentalenes have attracted considerable attention in the past few years. However, the aromaticity of metallapentalenes containing heteroatoms (such as B, N, and O), termed as hetero‐metallapentalenes, is rarely studied. Herein, the stability and aromaticity of a series of hetero‐metallapentalenes are theoretically investigated. The results reveal lower aromaticity in metallaborapentalene, comparable aromaticity in metallazapentalene, and nonaromaticity in metalloxapentalene relative to that of metallapentalene. Moreover, the effect of Lewis bases on the aromaticity and stability of metallaborapentalene is discussed. These results provide useful information for experimental chemists to realize more hetero‐metallapentalenes. [ABSTRACT FROM AUTHOR]

Adaptive aromaticity in the lowest singlet and triplet states is a rare property found among molecular systems. So far, only osmapentalene and osmapyridinium have been found to possess the adaptive aromaticity. Although it has been confirmed that the pattern of electron excitation is a key factor to achieve the adaptive aromaticity, further investigation of the metal center effect has not yet been made. Ruthenium, another Group 8 transition metal, can form metallacycles similar to the osmium counterparts. Here, we perform density functional theory calculations for two sets of ruthenacycles and analyze their aromaticity with multiple indices, revealing a 16-valence-electron ruthenapentalene being aromatic in both the singlet ground state (S0) and the lowest triplet state (T1) and extending the adaptive aromaticity to the second-row transition metal complexes. [ABSTRACT FROM AUTHOR]

Carbon dioxide (CO2, a common combustion pollutant) releasing continuously into the atmosphere is primarily responsible for the rising atmospheric temperature. Therefore, CO2 sequestration has been an indispensable area of research for the past several decades. On the other hand, the concept of aromaticity is often employed in designing chemical reactions and metal‐free frustrated Lewis pairs (FLPs) have proved ideal reagents to achieve CO2 reduction. However, considering FLP and aromaticity together is less developed in CO2 capture. Here we report theoretical investigations on the aromaticity‐promoted CO2 activation, involving heterocyclopentadiene‐bridged P/N‐FLPs. The calculations reveal that furan‐ and pyrrole‐bridged P/N‐FLPs can make CO2 capture both thermodynamically and kinetically favorable (with activation energies of 5.4–7.7 kcal mol−1) due to the aromatic stabilization of the transition states and products. Our findings could open an avenue to the design of novel FLPs for CO2 capture. [ABSTRACT FROM AUTHOR]

Unusual 1,2‐migration reactions of N‐heterocyclic carbene (NHC) on transition metals were investigated using density functional theory calculations. Our results reveal that the electronic properties, ring strain of the four‐membered ring, and aromaticity of NHC play crucial roles in the thermodynamics of such a 1,2‐migration. Further studies show that changing the methylene on the metal center in the reactant with a more electronegative group (NH or O) will lead to the formation of products with nitrogen coordinating to the metal center, whereas other groups (BH, CF2, and SiH2) will make such a 1,2‐migration reverse. In addition, the reversed rearrangement of 1,2‐boron, silyl migration could be thermodynamically and kinetically favorable. [ABSTRACT FROM AUTHOR]

The [1,5]‐migration reaction has attracted considerable attention from experimentalists and theoreticians for decades. Although it has been extensively investigated in various systems, studies on pyrrolium derivatives are underdeveloped. Herein, a theoretical study on the reaction mechanism of [1,5]‐migration in both pyrrolium and pyrrole derivatives is presented. The results reveal lower activation barriers in [1,5]‐migration of electropositive groups (AuPMe3 and SnH3) in pyrrolium derivatives, although the bond dissociation energies of the Au−N bond (98.8 kcal mol−1) and Sn−N bond (81.7 kcal mol−1) are larger than that of the N−F bond (57.6 kcal mol−1). The unexpectedly lower activation barriers (4.5 and 4.9 kcal mol−1 for AuPMe3 and SnH3, respectively) for [1,5]‐migration of electropositive groups, in comparison with the [1,5]‐fluorine shift, can be attributed to aromaticity stabilizing the transition states, as revealed by significantly negative nucleus‐independent chemical shift (NICS) values. Further studies indicate that charge distribution and frontier molecular orbitals also play some roles in [1,5]‐migration of pyrrolium derivatives. [ABSTRACT FROM AUTHOR]

Invited for the cover of this issue is Jun Zhu, Miquel Solà and co‐workers at Xiamen University, Jagiellonian University and the University of Girona. The image highlights the main roles in this study: ligands, metal centers, and adaptive aromaticity. Read the full text of the article at10.1002/chem.202001830. [ABSTRACT FROM AUTHOR]

Fluorine is the most electronegative element in the periodic table. Thus, activation of the carbon–fluorine (C−F) bond, the strongest single bond to carbon, has attracted considerable interest from both experimentalists and theoreticians. In comparison with numerous approaches to activate C−F bonds, the aromaticity‐promoted method is less developed. Herein, we demonstrate that the C−F bond activation could be achieved by a facile tautomerization, benefitting from aromaticity, which can stabilize both the transition states and products. Our findings highlight an important application of aromaticity in the C−F bond activation, providing experimentalists with an alternative approach to activate C−F bonds. [ABSTRACT FROM AUTHOR]

Aromaticity is one of the most fundamental and fascinating chemical topics, attracting both experimental and theoretical chemists owing to its many manifestations. Both σ‐ and π‐aromaticity can be classified depending on the character of the cyclic electron delocalization. In general, σ‐aromaticity stabilizes fully saturated rings with σ‐electron delocalization whereas the traditional π‐aromaticity describes the π‐conjugation in fully unsaturated rings. Here, we demonstrate a strong correlation between nucleus‐independent chemical shift (NICS) values and extra cyclic resonance energies (ECREs), which are used to evaluate the σ‐aromaticity in an unsaturated three‐membered ring (3MR) of cyclopropene, which were computed by molecular orbital (MO) theory and valence bond (VB) theory, respectively. Further study shows that the fully unsaturated ring in methylenecyclopropene and its metallic analogy is σ‐aromatic. Our findings revolutionize the fundamental knowledge of the concept of σ‐aromaticity, thus opening an avenue to design σ‐aromaticity in other fully unsaturated systems, which are traditionally reserved as the domain of π‐aromaticity. σ‐Aromaticity dominating in a fully unsaturated ring is reported, revolutionizing the fundamental scope of the concept of σ‐aromaticity, thus opening an avenue to design σ‐aromaticity in other fully unsaturated systems, which were traditionally reserved as the domain of π‐aromaticity. [ABSTRACT FROM AUTHOR]

Abstract: Hyperconjugation, a weak interaction in organic chemistry, can have a strong effect on aromaticity, leading to the concept of hyperconjugative aromaticity, which was first proposed by Mulliken in 1939. However, most studies are limited to main group chemistry. Here we report the most aromatic and antiaromatic pyrrolium ring by maximizing the hyperconjugation caused by transition metal fragments and the push–pull effect. Our calculations reveal that the origin of the outperformance of transition metal substituents over main group ones on hyperconjugative aromaticity could be attributed to their higher highest occupied molecular orbitals (HOMOs). Among the group 11 transition metals, a silver substituent results in the best performance. All these findings highlight the magic of the transition metal (silver) and could be particularly helpful for the design of other aromatic and antiaromatic counterparts based on a nonaromatic parent species. [ABSTRACT FROM AUTHOR]

Abstract: Isolation of the simplest 4π three‐membered heterocycles (1H‐azirine, oxirene, thiirene, and selenirene) remains a big challenge due to their π‐antiaromaticity and significant ring strain. Here we demonstrate that the incorporation of a transition‐metal fragment could stabilize the antiaromatic selenirene and pentalene frameworks simultaneously by density functional theory (DFT) calculations. Experimental verification leads to the Se‐containing metallapolycycles, osmapentaloselenirenes, with remarkable thermal stability. The osmaselenirene unit in the metallapolycycle is determined to be the first example of σ‐aromaticity dominating in an unsaturated Se‐containing ring. Our results not only highlight a remarkable stabilization by the transition‐metal but also widen the scope of σ‐aromaticity in unsaturated rings, which is traditionally reserved for the domain of π‐aromaticity. [ABSTRACT FROM AUTHOR]

Polycyclic complexes containing a bridgehead transition metal are interesting species because the transition metal is shared by all the rings simultaneously. In this study, we present a novel osmium-bridgehead system with three fused five-membered rings. This novel framework can be viewed as a 10-atom carbon chain coordinating to the osmium center. In sharp contrast to the nonplanar organic analogue, this unique metallacycle exhibits good planarity, which was unambiguously verified by means of X-ray diffraction. Interestingly, preliminary DFT calculations show that the aromaticity in the three 5MRs of these osmatricycles can be easily tuned by the ligand substitution. Finally, the broad UV/Vis absorption spectra of these novel polycyclic complexes were also reported. [ABSTRACT FROM AUTHOR]

Metalla-aromatics are attractive species because they exhibit the properties of both organometallics and aromatics. Reported metal-bridged polycyclic aromatic complexes, as well as Möbius aromatic species, are still rare. Herein, we present the construction of two new metal-bridged polycyclic aromatic frameworks, α-metallapentalenofurans and lactone-fused metallapentalynes, by the reactions of osmapentalyne with terminal aryl alkynes in the presence of H2O or HBF4/H2O, respectively. DFT calculations show that the α-metallapentalenofurans possess rare planar Craig-type Möbius aromaticity derived from the 11-center-12-electron dπ-pπ π-conjugation. This study offers a new route for the construction of polycyclic metalla-aromatics as well as planar Möbius aromatic species. [ABSTRACT FROM AUTHOR]

The synthesis of small cyclic metal carbynes is challenging due to the large angle strain associated with the highly distorted nonlinear triple bonds. Herein, we report a general route for the synthesis of five-membered cyclic metal carbyne complexes, osmapentalynes, by the reactions of an osmapentalene derivative with allene, alkyne, and alkene. Experimental observations and theoretical calculations document the aromaticity in the fused five-membered rings of osmapentalynes. The realization of transforming osmapentalene to osmapentalyne through this general route would not only allow further exploration of metallapentalyne chemistry but also show promising applications of this novel aromatic system with broad absorption band and high molar absorption coefficient. [ABSTRACT FROM AUTHOR]

The synthesis of small cyclic metal carbynes is challenging due to the large angle strain associated with the highly distorted nonlinear triple bonds. Herein, we report a general route for the synthesis of five-membered cyclic metal carbyne complexes, osmapentalynes, by the reactions of an osmapentalene derivative with allene, alkyne, and alkene. Experimental observations and theoretical calculations document the aromaticity in the fused five-membered rings of osmapentalynes. The realization of transforming osmapentalene to osmapentalyne through this general route would not only allow further exploration of metallapentalyne chemistry but also show promising applications of this novel aromatic system with broad absorption band and high molar absorption coefficient. [ABSTRACT FROM AUTHOR]

In general, aromaticity can be clarified as π- and σ-aromaticity according to the type of electrons with major contributions. The traditional π-aromaticity generally describes the π-conjugation in fully unsaturated rings whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. Reported herein is an example of σ-aromaticity in an unsaturated three-membered ring (3 MR), which is supported by experimental observations and theoretical calculations. Specifically, when the 3 MR in cyclopropaosmapentalene is cleaved by ethane through two isodesmic reactions, both of them are highly endothermic (+29.7 and +35.0 kcal mol−1). These positive values are in sharp contrast to the expected exothermicity, thus indicating aromaticity in the 3 MR. Further nucleus-independent chemical shift and anisotropy of the current-induced density calculations reveal the nature of σ-aromaticity in the unsaturated 3 MR. [ABSTRACT FROM AUTHOR]

In general, aromaticity can be clarified as π- and σ-aromaticity according to the type of electrons with major contributions. The traditional π-aromaticity generally describes the π-conjugation in fully unsaturated rings whereas σ-aromaticity may stabilize fully saturated rings with delocalization caused by σ-electron conjugation. Reported herein is an example of σ-aromaticity in an unsaturated three-membered ring (3 MR), which is supported by experimental observations and theoretical calculations. Specifically, when the 3 MR in cyclopropaosmapentalene is cleaved by ethane through two isodesmic reactions, both of them are highly endothermic (+29.7 and +35.0 kcal mol−1). These positive values are in sharp contrast to the expected exothermicity, thus indicating aromaticity in the 3 MR. Further nucleus-independent chemical shift and anisotropy of the current-induced density calculations reveal the nature of σ-aromaticity in the unsaturated 3 MR. [ABSTRACT FROM AUTHOR]

Density functional theory (DFT) calculations were performed to examine the substituent effects on the interconversion of silabenzenes and their monocyclic non-aromatic isomers. A previous study suggested that aromaticity is the driving force for this process. Interestingly, our systematic calculations reveal that the contribution from aromaticity can be evaluated quantitatively (ca. 30 kcal mol −1 ). Thus it is the interplay of aromaticity and Bent's rule that determines their relative stabilities. In addition, strong correlations of reaction energies as well as reaction barriers have been identified against the s character of Si in the Si-X bonds, providing an important guide to the synthesis of silabenzenes. [ABSTRACT FROM AUTHOR]

Aromaticity is one of the most important concepts in organic chemistry. A variety of metalla-aromatic compounds have been recently prepared and in most of those examples, the metal participates only in a monocyclic ring. In contrast, metal-bridged bicyclic aromatic molecules, in which a metal is shared between two aromatic rings, have been less developed. Herein, we report the first metal-bridged tricyclic aromatic system, in which the metal center is shared by three aromatic five-membered rings. These metalla-aromatics are formed by reaction between osmapentalyne and arene nucleophiles. Experimental results and theoretical calculations reveal that the three five-membered rings around the osmium center are aromatic. In addition, the broad absorption bands in the UV/Vis absorption spectra of these novel aromatic systems cover almost the entire visible region. This straightforward synthetic strategy may be extended to the synthesis of other metal-bridged polycyclic aromatics. [ABSTRACT FROM AUTHOR]

Aromaticity is one of the most important concepts in organic chemistry. A variety of metalla-aromatic compounds have been recently prepared and in most of those examples, the metal participates only in a monocyclic ring. In contrast, metal-bridged bicyclic aromatic molecules, in which a metal is shared between two aromatic rings, have been less developed. Herein, we report the first metal-bridged tricyclic aromatic system, in which the metal center is shared by three aromatic five-membered rings. These metalla-aromatics are formed by reaction between osmapentalyne and arene nucleophiles. Experimental results and theoretical calculations reveal that the three five-membered rings around the osmium center are aromatic. In addition, the broad absorption bands in the UV/Vis absorption spectra of these novel aromatic systems cover almost the entire visible region. This straightforward synthetic strategy may be extended to the synthesis of other metal-bridged polycyclic aromatics. [ABSTRACT FROM AUTHOR]

Invited for the cover of this issue is Jun Zhu at Xiamen University. The image depicts the aromaticity of hetero‐metallapentalenes with different metal (Fe, Co, Ni, Ru, Rh, Re, Os, Ir) centers and heteroatoms (B, N, and O) obtained by computational investigation. Read the full text of the article at10.1002/chem.202000148. [ABSTRACT FROM AUTHOR]

Keywords: aromaticity; density functional calculations; Lewis bases; metallacycles; metallapentalenes EN aromaticity density functional calculations Lewis bases metallacycles metallapentalenes 5303 5303 1 04/25/20 20200424 NES 200424 B Aromaticity has attracted b continuous attention from both experimentalists and theoreticians for many decades. The present study reveals an interesting aromaticity dependence on the heteroatoms in these heteroatom-containing metallapentalens by a DFT investigation. The present study reveals an interesting aromaticity dependence on the heteroatoms in these heteroatom-containing metallapentalens by a DFT investigation. [Extracted from the article]

σ‐Aromaticity dominating in a fully unsaturated ring (the three‐membered ring of methylenecyclopropene and its metallic analog) is reported, revolutionizing the fundamental scope of the concept of σ‐aromaticity, thus opening an avenue to design σ‐aromaticity in other fully unsaturated systems (illustrated by the upper right of a Tai Chi circle), which are traditionally reserved for the domain of π‐aromaticity. More information can be found in the Full Paper by Jun Zhu et al. on page 3691 in Issue 23, 2018 (DOI: 10.1002/asia.201801279). [ABSTRACT FROM AUTHOR]