Sandwich complexes based on the "all-metal" Al4 2- aromatic ring

We report on novel sandwichlike structures [Al(4)MAl(4)](q-) (q=0-2 and M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta and W) based on the recently synthesized all-metal aromatic Al(4)(2-) square ring. The sandwichlike structures have two aromatic tetraaluminum square rings which trap a transition-metal cation from either the first, second, or third row. The stability of the anionic sandwichlike complexes towards electron detachment is discussed, and addition of alkali cations is found to stabilize the 2- charged complexes, preventing spontaneous electron detachment. Once the sandwichlike complexes are formed, the Al(4)(2-) square properties remain nearly unchanged; this fact strongly supports the hypothesis that in these complexes the Al(4)(2-) square rings remain aromatic.     

Sandwich-like compounds based on the all-metal aromatic unit Al(4)2- and the main-group metals M (M=Li, Na, K, Be, Mg, Ca)

Inspired by the pioneering experimental characterisation of the all-metal aromatic unit Al(4)2- in the bimetallic molecules MAl4- (M=Li, Na, Cu) and by the very recent theoretical design of sandwich-type transition-metal complexes [Al4MAl4]q- (q=0-2; M=Ti, V, Cr, Zr, Nb, Mo, Hf, Ta, W), we used density functional theory (DFT) calculations (B3LYP/6-311+G(d) to design a series of novel non-transition-metal sandwich complexes based on the all-metal aromatic unit Al4(2-) and the main-group metals M (M=Li, Na, K, Be, Mg, Ca). The traditional homo-decked sandwich compounds [Al4MAl4]q- (without counterions) and (nM)q+[Al4MAl4]q- (with counterions M) (q=2-3, M=Li, Na, K, Be, Mg, Ca), although some of them are truly energy minima, have a much higher energy than many fused isomers. We thus concluded that it seems unlikely for Al4(2-) to sandwich the main-group metal atoms in the homo-decked sandwich form. Alternatively, we proposed a new type of sandwich complex, namely hetero-decked sandwich compounds [CpMAl4]q-, that are the ground-state structures for each M both with and without counterions. It was shown that with the rigid Cp- partner, the all-metal aromatic unit Al(4)2- might indeed act as a "superatom". These new types of all-metal aromatic unit-based sandwich complexes await future experimental verification.     

Cluster-assembled compounds comprising an all-metal subunit Li3Al4-

The recent, experimentally-discovered, all-metal antiaromatic Li3Al4- has attracted great interest and extensive investigations due to its unique chemical bonds and exotic properties. Although a very recent theoretical study demonstrated that the all-metal species Li3Al4- can be effectively stabilized by complexation with 3d transition metals, unfortunately such stabilization is at the expense of losing antiaromaticity (rectangular Al4) to become aromatic (square Al4). Here, we predict theoretically a series of cluster-assembled compounds [DM(Li3Al4)]q- (D=Li3Al4-, Cp-; M=Li, Na, K, Be, Mg, Ca). The assembled species are ground states containing the all-metal antiaromatic Li3Al4- subunits. Many fusion isomers are energetically lower than the homo-decked cluster-assembled compounds, thus, the homo-decked assembly species [M(Li3Al4)2]q- are less likely due to their thermodynamic instability. In addition, the well-retained all-metal antiaromaticity is mainly ascribed to the ionic electrostatic interactions and the protections of rigid organic aromatic Cp-deck avoiding the fusion of Li3Al4-. Our results represent the first example that the all-metal antiaromaticity is well retained in assembled compounds as that in the free Li3Al4- cluster. Sufficiently large interaction energies make the realization of all-metal antiaromatic Li3Al4--incorporated compounds very promising.     

On the resonance energy in new all-metal aromatic molecules

We have recently advanced the aromaticity concept into all-metal molecules containing Al(4)(2-), XAl(3)(-), Ga(4)(2-), In(4)(2-), Hg(4)(6-), Al(3)(-), and Ga(3)(-) aromatic units. All these systems are electron deficient species compared to the corresponding aromatic hydrocarbons. The electron deficiency results in an interesting new feature in all-metal aromatic systems, which should be considered as having both pi- and sigma-aromaticity, and that should result in their additional stability. In this work, we obtain crude evaluations of the resonance energies for Na(2)Al(4) and Na(2)Ga(4) all-metal aromatic molecules. The resonance energies were found to be unusually high: 30 kcal/mol (B3LYP/6-311+G*) and 48 kcal/mol (CCSD(T)/6-311+G(2df)) for Na(2)Al(4) and 21 kcal/mol (B3LYP/6-311+G*) for Na(2)Ga(4) compared to 20 kcal/mol in benzene. We believe that the high resonance energies in Na(2)Al(4) and in Na(2)Ga(4) are due to the presence of three completely delocalized bonds, one pi-bond and two sigma-bonds, thus confirming the presence of pi- and sigma-aromaticity.     

All-metal aromatic complexes show high reactivity in the oxidation reaction of methane and some hydrocarbons

The C-H activations of methane, ethane, propane, and propene catalyzed by all-metal aromatic complexes Al(4)Fe were investigated. The results reveal that the rate-determining barrier of methane activation reaction with Al(4)Fe is lower than that of both some well-known inorganic catalysts and some metal organic catalysts. It was found that the all-metal aromatic complexes have high reactivity for the C-H activation of ethane, propane, and propene. Further research showed that the ability of all-metal aromatic complex to accept an electron and the degree of electron delocalization on its aromatic plane had obvious influences on the reactivity of Al(4)Fe. The present work predicts a new kind of catalyst for the alkyl C-H activation reaction: all-metal aromatic catalyst.     

Stable transition metal complexes of an all-metal antiaromatic molecule (Al4Li4): role of complexations

We propose for the first time a few examples of stable transition metal complexes of an all-metal antiaromatic molecule Al4Li4. We demonstrate that these all-metal species can be stabilized by complexation with 3d transition metals very similar to their organic counterpart, C4H4. Complexation to transition metal ions reduces the frontier orbital energies and introduces aromatic charactersitics. We consider a series of such complexes: [eta4-(Al4Li4)-Fe(CO)3, eta2,sigma2-(Al4Li4)-Ni, and (Al4Li4)2Ni] and compare and contrast their energetics with their organometallic counterparts. Fragmentation energy, orbital correlation energy analysis, and the nucleus-independent chemical shift (NICS) values support the complexation-induced stabilities in these systems.     

Aromatic superclusters from all-metal aromatic and antiaromatic monomers, [Al4]2- and [Al4]4-

Calculations on the structures of dimers of all-metal aromatic and anti-aromatic molecules such as (Al4(2-)) and (Al4(4-)) reveal that, unlike their organic counterparts such as benzene and cyclobutadiene which form pi-stacked complexes, these molecules form new clusters with no reminiscence of the original units. These clusters have a very large binding energy and can be further stabilized through charge-balance by counterions and solvents.     

Stability and reactivity of all-metal aromatic and antiaromatic systems in light of the principles of maximum hardness and minimum polarizability

It is demonstrated that among various possible isomers of all-metal aromatic compounds such as Al(4)(2-) and their complexes the most stable isomer with the minimum energy is the hardest and the least polarizable. A similar situation is observed for different isomers of all-metal antiaromatic compounds such as Al(4)(4-) and their complexes. It is shown that linear Al(4)(4-) is energetically more stable than its cyclic isomer. The reaction energies associated with the complexation processes highlight the stability of those complexes. The difference in energy, hardness, and polarizability between a cyclic molecule and its linear counterpart convincingly shows that an aromatic molecule exhibits negative changes in energy and polarizability but positive changes in hardness as expected from the principles of minimum energy, minimum polarizability, and maximum hardness. Although the aromaticity of Al(4)(2-) is unequivocally established through this study, the antiaromaticity picture in the case of Al(4)(4-) is shown to be poorly understood;however, the present analysis sheds light on this controversy.     

All-metal aromatic clusters M4(2-) (M = B, Al, and Ga). Are π-electrons distortive or not?

The π-electrons in benzene, the quintessential aromatic molecule, were previously shown to be distortive, i.e., they prefer localized double bonds alternating with single bonds. It is the σ-electrons that force the double bonds to delocalize, leading to a regular, D(6h) geometry. Herein, we computationally investigate the double-bond localizing or delocalizing propensities of σ- and π-electrons in the archetypal all-metal aromatic cluster Al(4)(2-) and its second- and fourth-period analogs B(4)(2-) and Ga(4)(2-), using Kohn-Sham molecular orbital (MO) theory at BP86/TZ2P in combination with quantitative bond energy decomposition analyses (EDA). We compare the three all-metal aromatic clusters with the structurally related organic species C(4)H(4)(2+), C(4)H(4), and C(4)H(4)(2-). Our analyses reveal that the π-electrons in the group-13 M(4)(2-) molecules have a weak preference for localizing the double bonds. Instead, the σ-electrons enforce the regular D(4h) equilibrium geometry with delocalized double bonds.     

Electron affinities of Al(n) clusters and multiple-fold aromaticity of the square Al4(2-) structure

The concept of aromaticity was first invented to account for the unusual stability of planar organic molecules with 4n + 2 delocalized pi electrons. Recent photoelectron spectroscopy experiments on all-metal MAl(4)(-) systems with an approximate square planar Al(4)(2-) unit and an alkali metal led to the suggestion that Al(4)(2-) is aromatic. The square Al(4)(2-) structure was recognized as the prototype of a new family of aromatic molecules. High-level ab initio calculations based on extrapolating CCSD(T)/aug-cc-pVxZ (x = D, T, and Q) to the complete basis set limit were used to calculate the first electron affinities of Al(n)(), n = 0-4. The calculated electron affinities, 0.41 eV (n = 0), 1.51 eV (n = 1), 1.89 eV (n = 3), and 2.18 eV (n = 4), are all in excellent agreement with available experimental data. On the basis of the high-level ab initio quantum chemical calculations, we can estimate the resonance energy and show that it is quite large, large enough to stabilize Al(4)(2-) with respect to Al(4). Analysis of the calculated results shows that the aromaticity of Al(4)(2-) is unusual and different from that of C(6)H(6). Particularly, compared to the usual (1-fold) pi aromaticity in C(6)H(6), which may be represented by two Kekulé structures sharing a common sigma bond framework, the square Al(4)(2-) structure has an unusual "multiple-fold" aromaticity determined by three independent delocalized (pi and sigma) bonding systems, each of which satisfies the 4n + 2 electron counting rule, leading to a total of 4 x 4 x 4 = 64 potential resonating Kekulé-like structures without a common sigma frame. We also discuss the 2-fold aromaticity (pi plus sigma) of the Al(3)(-) anion, which can be represented by 3 x 3 = 9 potential resonating Kekulé-like structures, each with two localized chemical bonds. These results lead us to suggest a general approach (applicable to both organic and inorganic molecules) for examining delocalized chemical bonding. The possible electronic contribution to the aromaticity of a molecule should not be limited to only one particular delocalized bonding system satisfying a certain electron counting rule of aromaticity. More than one independent delocalized bonding system can simultaneously satisfy the electron counting rule of aromaticity, and therefore, a molecular structure could have multiple-fold aromaticity.     

Instability of the Al4(2-) "all-metal aromatic" ion and its implications

Al42 - is a prototype structural unit of a new class of "all-metal aromatic" molecules. Without stabilizing counterions this species is unstable with respect to electron autodetachment in the gas phase. We estimated the height of the repulsive Coulomb barrier to approximately 2.7 eV and calculated a lifetime of 9 fs. This is a short lifetime: The only way to study the isolated dianion experimentally is to use electron scattering techniques. Investigations of the validity of bound-state quantum chemical calculations on the isolated species show that the results suffer from significant admixture of continuum states to the bound-state wave function depending on the basis set. Calculations of molecular properties can therefore give essentially arbitrary results for this ill-defined system, as is demonstrated for the energy and nuclear magnetic shieldings. This substantiates that results from calculations on the isolated dianion should be approached with caution.     

DFT study of “all-metal” aromatic compounds

An overview of recent quantum chemical studies on all-metal aromatic compounds is presented. Novel classes of inorganic molecules containing bonds that are characterized by a common ring-shaped electron density are reviewed. The mechanistic insight gained for the aromatic character of all-metal aromatic molecules is discussed and the predictive nature of the electronic structure calculation methods particularly those based on density functional theory (DFT) is highlighted. The indicators of aromaticity (aromaticity criteria) - structural, magnetic, energetic and reactivity-based measures - many of which are accessible through quantum chemical calculations are also outlined herein.     

全金属负离子团簇Ga42-、In42-芳香性的磁特征与多重芳香性

The optimized geometries, frequencies, and total electronic energies of two all-metal dianionic clusters Ga42- , In42- are calculated at the B3LYP, B3PW91, and MP2 levels of theory. There are two stable structures for each Ga42- , In42- species. For Ga42- , In42- species the square isomers are the most stable. On the basis of these computed stable structures we focus on two magnetic properties: magnetic susceptibility anisotropy and nucleus-independent chemical shift (NICS) for the square planar Ga42- , In42- isomers, which are calculated with B3LYP and HF methods. The computed results of NICS show that the square planar Ga42- , In42- isomers possess strong aromaticity. The detailed molecular orbital analysis for the two isomers further reveals that the two square planar Ga42- , In42- isomers have multiple-fold aromaticity: one delocalized π MOs and two delocalized σ MOs, which play important role in explaining the special stability of these all-metal square clusters.     

Do all-metal antiaromatic clusters exist?

As shown by detailed nucleus-independent chemical shift (NICS) analyses of the contributions of each molecular orbital, the very recently reported gas-phase all-metal Al4Li3- anion and its relatives (Kuznetsov, A.E.; Birch, K.A.; Boldyrev, A.I.; Li, X.; Zhai, A.I.; Wang, L.S. Science 2003, 300, 622) are aromatic rather than antiaromatic. The paratropic (antiaromatic) four-pi-electron contribution is overcome by the predominating diatropic effects of sigma aromaticity. However, true antiaromatic all-metal clusters, such as Sn62- (Schiemenz, B.; Huttner, G. Angew. Chem., Int. Ed. Engl. 1993, 32, 297), do exist.     

The reactivity of all-metal aromatic complexes: a theoretical investigation on the methane activation reaction

The recent finding of all-metal aromaticity enlightens us to explore the reactivity of all-metal aromatic complexes. The methane activation reaction was chosen for this purpose. The whole catalysis process of all-metal aromatics was investigated by theoretical calculation. The results reveal that the reaction barriers with all-metal aromatic complexes are far lower than that without any catalysts. All-metal aromatic complexes are predicted to have potential to be used as catalysts for the first time.     

Stability of N4(2-)-based sandwich-like energetic complexes [N4TiN4]2-: effect of spins and counterions

Design and assembly of new aromatic units has received growing attention due to its fundamental and application interests. Recently, a series of singlet sandwich-like complexes [N(4)MN(4)](q) (M = Ti, V, Cr, Fe, Co, Ni) based on the energetic all-nitrogen 6pi-aromatic species N(4)(2-) have been devised. However, how the electronic spins and counterions influence the kinetic stability of sandwich-like complexes has been very little understood, though it is very important to assess their potential use. In this article, we report our DFT study on the stability of the charged [N(4)TiN(4)](2-) and [N(4)TiN(4)TiN(4)](2-) systems as well as the neutral [N(4)TiN(4)]Li(2) system in both singlet and triplet electronic states. We found that the ground state structures of di-deckered [N(4)TiN(4)](2-) and [N(4)TiN(4)]Li(2) as well as the tri-deckered [N(4)TiN(4)TiN(4)](2-) are all in triplet state, rather than the previously reported singlet ones. Therefore, the N(4)(2-) and Ti(2+)-based sandwich-like complexes should be in high spins and may have potential use for new paramagnetic materials. Moreover, our calculations indicate that although the counterions can induce the electronic stabilization, they on the other hand can lead to the considerable kinetic destabilization of the N(4)(2-)-based sandwich-like complexes because the counterions can structurally destroy the perfectness of aromatic N(4)(2-). Thus, in study of the sandwich-like complexes, the effect of counterions cannot be neglected for assessment of the kinetic stability.     

Peculiar antiaromatic inorganic molecules of tetrapnictogen in Na+Pn4- (Pn = P,As, Sb) and important consequences for hydrocarbons

Although aromaticity has been observed in inorganic and all-metal species, the concept of antiaromaticity has not been extended beyond organic molecules. Here, we present theoretical and experimental evidence that the 6 -electron tetrapnictogen dianions in Na+Pn42- (Pn = P, As, Sb) undergo a transition from being aromatic to antiaromatic upon electron detachment, yielding the first inorganic antiaromatic Na+Pn4- molecules. Two types of antiaromatic structures were characterized, the conventional rectangular species and a new peculiar quasiplanar rhombus species. Aromaticity and antiaromaticity in the tetrapnictogen molecules were derived from molecular orbital analyses and verified by experimental photodetachment spectra of Na+Pn42-. On the basis of our findings for the tetrapnictogen clusters, we predicted computationally that the organic C4H4- anion also possesses two antiaromatic structures: rectangular and rhombus. Moreover, only the rhombus antiaromatic minimum was found for the radical NC3H4, thus extending the peculiar rhombus antiaromatic structure first uncovered in inorganic clusters into organic chemistry.     

Interplay among aromaticity, magnetism, and nonlinear optical response in all-metal aromatic systems

All-metal aromatic molecules are the latest inclusion in the family of aromatic systems. Two different classes of all-metal aromatic clusters are primarily identified: one is aromatic only in the low spin state, and the other shows aromaticity even in high-spin situations. This observation prompts us to investigate the effect of spin multiplicity on aromaticity, taking Al(4)(2-), Te(2)As(2)(2-), and their copper complexes as reference systems. Among these clusters, it has been found that the molecules that are aromatic only in their singlet state manifest antiaromaticity in their triplet state. The aromaticity in the singlet state is characterized by the diatropic ring current circulated through the bonds, which are cleaved to generate excess spin density on the atoms in the antiaromatic triplet state. Hence, in such systems, an antagonistic relationship between aromaticity and high-spin situations emerges. On the other hand, in the case of triplet aromatic molecules, the magnetic orbitals and the orbitals maintaining aromaticity are different; hence, aromaticity is not depleted in the high-spin state. The nonlinear optical (NLO) behavior of the same set of clusters in different spin states has also been addressed. We correlate the second hyperpolarizability and spin density in order to judge the effect of spin multiplicity on third-order NLO response. This correlation reveals a high degree of NLO behavior in systems with excess spin density. The variance of aromaticity and NLO response with spin multiplicity is found to stem from a single aspect, the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO), and eventually the interplay among aromaticity, magnetism, and NLO response in such materials is established. Hence, the HOMO-LUMO energy gap becomes the cornerstone for tuning the interplay. This correlation among the said properties is not system-specific and thus can be envisaged even beyond the periphery of all-metal aromatic clusters. Such interplay is of crucial importance in tailoring novel paradigm of multifunctional materials.     

Aluminum avoids the central position in AlB9- and AlB10-: photoelectron spectroscopy and ab initio study

The structures and the electronic properties of two Al-doped boron clusters, AlB(9)(-) and AlB(10)(-), were investigated via joint photoelectron spectroscopy and high-level ab initio study. The photoelectron spectra of both anions are relatively broad and have no vibrational structure. The geometrical structures were established by unbiased global minimum searches using the Coalescence Kick method and comparison between the experimental and calculated vertical electron detachment energies. The results show that both clusters have quasi-planar structures and that the Al atom is located at the periphery. Chemical bonding analysis revealed that the global minimum structures of both anions can be described as doubly (σ- and π-) aromatic systems. The nona-coordinated wheel-type structure of AlB(9)(-) was found to be a relatively high-lying isomer, while a similar structure for the neutral AlB(9) cluster was previously shown to be either a global minimum or a low-lying isomer.