Comprehensive analysis of chemical bonding in boron clusters

We present a comprehensive analysis of chemical bonding in pure boron clusters. It is now established in joint experimental and theoretical studies that pure boron clusters are planar or quasi-planar at least up to twenty atoms. Their planarity or quasi-planarity was usually discussed in terms of pi-delocalization or pi-aromaticity. In the current article, we demonstrated that one cannot ignore sigma-electrons and that the presence of two-center two-electron (2c--2e) peripheral B--B bonds together with the globally delocalized sigma-electrons must be taken into consideration when the shape of pure boron cluster is discussed. The global aromaticity (or global antiaromaticity) can be assigned on the basis of the 4n+2 (or 4n) electron counting rule for either pi- or sigma-electrons in the planar structures. We showed that pure boron clusters could have double (sigma- and pi-) aromaticity (B3-, B4, B5+, B6(2+), B7+, B7-, B8, B(8)2-, B9-, B10, B11+, B12, and B13+), double (sigma- and pi-) antiaromaticity (B6(2-), B15), or conflicting aromaticity (B5-,sigma-antiaromatic and pi-aromatic and B14, sigma-aromatic and pi-antiaromatic). Appropriate geometric fit is also an essential factor, which determines the shape of the most stable structures. In all the boron clusters considered here, the peripheral atoms form planar cycles. Peripheral 2c--2e B--B bonds are built up from s to p hybrid atomic orbitals and this enforces the planarity of the cycle. If the given number of central atoms (1, 2, 3, or 4) can perfectly fit the central cavity then the overall structure is planar. Otherwise, central atoms come out of the plane of the cycle and the overall structure is quasi-planar.     

Aromaticity and antiaromaticity in transition-metal systems

Aromaticity is an important concept in chemistry primarily for organic compounds, but it has been extended to compounds containing transition-metal atoms. Recent findings of aromaticity and antiaromaticity in all-metal clusters have stimulated further research in describing the chemical bonding, structures and stability in transition-metal clusters and compounds on the basis of aromaticity and antiaromaticity, which are reviewed here. The presence of d-orbitals endows much more diverse chemistry, structure and chemical bonding to transition-metal clusters and compounds. One interesting feature is the existence of a new type of aromaticity-delta-aromaticity, in addition to sigma- and pi-aromaticity which are the only possible types for main-group compounds. Another striking characteristic in the chemical bonding of transition-metal systems is the multi-fold nature of aromaticity, antiaromaticity or even conflicting aromaticity. Separate sets of counting rules have been proposed for cyclic transition-metal systems to account for the three types of sigma-, pi- and delta-aromaticity/antiaromaticity. The diverse transition-metal clusters and compounds reviewed here indicate that multiple aromaticity and antiaromaticity may be much more common in chemistry than one would anticipate. It is hoped that the current review will stimulate interest in further understanding the structure and bonding, on the basis of aromaticity and antiaromaticity, of other known or unknown transition-metal systems, such as the active sites of enzymes or other biomolecules which contain transition-metal atoms and clusters.     

Are Hetero-metallapentalenes Aromatic or Not? A DFT Investigation

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.     

Circuit resonance energy: a key quantity that links energetic and magnetic criteria of aromaticity

Energetic and magnetic criteria of aromaticity are different in nature and sometimes make different predictions as to the aromaticity of a polycyclic pi-system. Thus, some charged polycyclic pi-systems are aromatic but paratropic. We derived the individual circuit contributions to aromaticity from the magnetic response of a polycyclic pi-system and named them circuit resonance energies (CREs). Each CRE has the same sign and essentially the same magnitude as the corresponding cyclic conjugation energy (CCE) defined by Bosanac and Gutman. Such CREs were found to play a crucial role in associating the energetic criteria for determining the degree of aromaticity with the magnetic ones. We can now interpret both energetic and magnetic criteria of aromaticity consistently in terms of CREs. Ring-current diamagnetism proved to be the tendency of a cyclic pi-system to retain aromatic stabilization energy (ASE) at the level of individual circuits.     

Electronic structures and thermochemical properties of the small silicon-doped boron clusters B(n)Si (n=1-7) and their anions

We perform a systematic investigation on small silicon-doped boron clusters B(n)Si (n=1-7) in both neutral and anionic states using density functional (DFT) and coupled-cluster (CCSD(T)) theories. The global minima of these B(n)Si(0/-) clusters are characterized together with their growth mechanisms. The planar structures are dominant for small B(n)Si clusters with n≤5. The B(6)Si molecule represents a geometrical transition with a quasi-planar geometry, and the first 3D global minimum is found for the B(7)Si cluster. The small neutral B(n)Si clusters can be formed by substituting the single boron atom of B(n+1) by silicon. The Si atom prefers the external position of the skeleton and tends to form bonds with its two neighboring B atoms. The larger B(7)Si cluster is constructed by doping Si-atoms on the symmetry axis of the B(n) host, which leads to the bonding of the silicon to the ring boron atoms through a number of hyper-coordination. Calculations of the thermochemical properties of B(n)Si(0/-) clusters, such as binding energies (BE), heats of formation at 0 K (ΔH(f)(0)) and 298 K (ΔH(f)([298])), adiabatic (ADE) and vertical (VDE) detachment energies, and dissociation energies (D(e)), are performed using the high accuracy G4 and complete basis-set extrapolation (CCSD(T)/CBS) approaches. The differences of heats of formation (at 0 K) between the G4 and CBS approaches for the B(n)Si clusters vary in the range of 0.0-4.6 kcal mol(-1). The largest difference between two approaches for ADE values is 0.15 eV. Our theoretical predictions also indicate that the species B(2)Si, B(4)Si, B(3)Si(-) and B(7)Si(-) are systems with enhanced stability, exhibiting each a double (σ and π) aromaticity. B(5)Si(-) and B(6)Si are doubly antiaromatic (σ and π) with lower stability.     

Theoretical study of aromaticity in inorganic tetramer clusters

Ground state geometry and electronic structure of M ₄ ^2- cluster (M = B, Al, Ga) have been investigated to evaluate their aromatic properties. The calculations are performed by employing the Density Functional Theory (DFT) method. It is found that all these three clusters adopt square planar configuration. Results reveal that square planar M ₄ ^2- dianion exhibits characteristics of multifold aromaticity with two delocalised π-electrons. In spite of the unstable nature of these dianionic clusters in the gas phase, their interaction with the sodium atoms forms very stable dipyramidal M₄Na₂ complexes while maintaining their square planar structure and aromaticity.     

Planar,Stair-Stepped,and Twisted: Modulating Structure and Photophysics in Pyrene- and Benzene-Fused N-Heterocyclic Boranes

Because of their rigidity, polycyclic aromatic hydrocarbons (PAHs) have become a significant building block in molecular materials chemistry. Fusion or doping of boron into PAHs is known to improve the optoelectronic properties by reducing the LUMO energy level. Herein, we report a comprehensive study on the syntheses, structures, and photophysical properties of a new class of fused N-heterocyclic boranes (NHBs), pyrene- and benzene-linked in a “Janus-type” fashion ( 2 – 4 , 6 – 9 , and 11 ). Remarkably, these examples of fused NHBs display fluorescent properties, and collectively their emission spans the visible spectrum. The pyrene-fused NHBs all display blue fluorescence, as the excitations are dominated by the pyrene core. In notable contrast, the emission properties of the benzene-fused analogues are highly tunable and are dependent on the electronics of the NHB fragments (i.e., the functional group directly bound to the boron atoms). Pyrene-fused 2 – 4 and 11 represent the only molecules in which the K-region of pyrene is functionalized with NHB units, and while they exhibit distorted (twisted or stair-stepped) pyrene cores, benzene-fused 6 – 9 are planar. The electronic structure and optical properties of these materials were probed by computational studies, including an evaluation of aromaticity, electronic transitions, and molecular orbitals.     

Aromaticity of benzenoid hydrocarbons with inserted –B=B– and –BH–BH– groups: a comparison

Structures of selected polycyclic conjugated hydrocarbons with –B=B– and –BH–BH– moieties inserted in different places were calculated at the B3LYP/6-311++G** level and their aromatic properties evaluated. HOMA, NICS(0), NICS(1)zz, Λ and PDI indices were used for studying their aromatic properties. Both optimized planar (as in parent hydrocarbons) and non-planar structures were taken into account. It is shown that insertion of both types of boron groups disturbs and decreases the aromaticity of the corresponding hydrocarbons. The decreasing effect of the –BH–BH– group is much stronger. What is quite intriguing is that it appears that non-planar structures of the studied compounds have a little higher aromaticity than the strictly planar ones. Mutual correlations between results obtained by different aromaticity indices are calculated and thoroughly discussed.     

B14: an all-boron fullerene

Experiments revealed that small boron cluster anions and cations are (quasi-)planar. For neutral boron cluster, (quasi-)planar motifs are also suggested to be global minimum by many theoretical studies, and a structural transformation from quasi-planar to double-ring tubular structures occurs at B(20). However, a missing opportunity is found for neutral B(14), which is a flat cage and more stable than the previous quasi-planar one by high level ab initio calculations. The B(14) cage has a large HOMO-LUMO gap (2.69 eV), and NICS values reveal that it is even more aromatic than the known most aromatic quasi-planar B(12) and double-ring B(20), which indicates a close-shell electronic structure. Chemical bonding analysis given by AdNDP reveals that the B(14) cage is an all-boron fullerene with 18 delocalized σ-electrons following the 2(n+1)(2) rule of spherical aromaticity. The geometry and bonding features of the B(14) cage are unique denying conversional thinking.     

Electron delocalization and aromaticity in low-lying excited states of archetypal organic compounds

Aromaticity is a property usually linked to the ground state of stable molecules. Although it is well-known that certain excited states are unquestionably aromatic, the aromaticity of excited states remains rather unexplored. To move one step forward in the comprehension of aromaticity in excited states, in this work we analyze the electron delocalization and aromaticity of a series of low-lying excited states of cyclobutadiene, benzene, and cyclooctatetraene with different multiplicities at the CASSCF level by means of electron delocalization measures. While our results are in agreement with Baird's rule for the aromaticity of the lowest-lying triplet excited state in annulenes having 4nπ-electrons, they do not support Soncini and Fowler's generalization of Baird's rule pointing out that the lowest-lying quintet state of benzene and septet state of cyclooctatetraene are not aromatic.     

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.     

The role of the 5f orbitals in bonding, aromaticity, and reactivity of planar isocyclic and heterocyclic uranium clusters

The molecular and electronic structures, stabilities, bonding features and magnetic properties of prototypical planar isocyclic cyclo-U n X n ( n = 3, 4; X = O, NH) and heterocyclic cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH) clusters as well as the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] molecules including a planar tetracoordinate element E (ptE) and pentacoordinate U (ppU) at the ring centers, respectively, have been thoroughly investigated by means of electronic structure calculation methods at the DFT level. It was shown that 5f orbitals play a key role in the bonding of these f-block metal systems significantly contributing to the cyclic electron delocalization and the associated magnetic diatropic (magnetic aromaticity) response. The aromaticity of the perfectly planar cyclo-U n X n ( n = 3, 4; X = O, NH), cyclo-U n (mu 2-X) n ( n = 3, 4; X = C, CH, NH), E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters was verified by an efficient and simple criterion in probing the aromaticity/antiaromaticity of a molecule, that of the nucleus-independent chemical shift, NICS(0), NICS(1), NICS zz (0) and the most refined NICS zz (1) index in conjunction with the NICS scan profiles. Natural bond orbital analyses provided a clear picture of the bonding pattern in the planar isocyclic and heterocyclic uranium clusters and revealed the features that stabilize the ptE's inside the six- and eight-member uranacycle rings. The ptE's benefit from a considerable electron transfer from the surrounding uranium atoms in the E@[ c-U 4(mu 2-C) 4], (E = H (+), C, Si, Ge) and U@[ c-U 5(mu 2-C) 5] clusters justifying the high occupancy of the np orbitals of the central atom E.     

The structure of small gallium nitride clusters

The geometry, vibrational frequencies and stability of the structural isomers of small gallium nitride clusters (n = 2–4) have been investigated using density functional theory. The lowest energy structures are cyclic. The ground electronic state of the cyclic forms for n > 2 is the singlet state. All of the cyclic structures have D_nh symmetry. The caged structures for Ga₄N₄ lie higher in energy than the planar cumulenic monocyclic ring. The Ga‐N bond dominates the structures for many isomers, so that one dissociation channel is loss of a GaN monomer. However, unlike the corresponding boron and aluminum clusters, dissociation into larger fragments is energetically favored. The structural properties of the gallium nitride clusters are similar to those of the analogous AIN (and BN) clusters. © 2000 John Wiley & Sons, Inc. Heteroatom Chem 11:281–286, 2000     

Planar tetra-, penta-, hexa-, hepta-, and octacoordinate silicons: a universal structural pattern

A universal structural pattern has been presented at density function theory level to incorporate planar tetra-, penta-, hexa-, hepta-, and octacoordinate silicons in C2v B(n)E2Si series (E = CH, BH, or Si; n = 2-5) and D8h B8Si. The equivalence in valence electron counts and one-to-one correspondence of the delocalized pi and sigma valence orbitals with small boron clusters strongly support the optimized structures containing planar coordinate silicons. Planar B(n)E2Si series are predicted to be aromatic in nature, and the vertical detachment energies of their anions are presented to facilitate future photoelectron experiments. This structural pattern can be applied to form other planar coordinate nonmetals including Ge, P, As, Al, and Ga and needs to be confirmed in experiments to open a new branch of chemistry on planar coordinate main group elements.     

Hückel's Rule of Aromaticity Categorizes Aromatic closo Boron Hydride Clusters

A direct connection is established between three‐dimensional aromatic closo boron hydride clusters and planar aromatic [n]annulenes for medium and large boron clusters. In particular, the results prove the existence of a link between the two‐dimensional Hückel rule, as followed by aromatic [n]annulenes, and Wade–Mingos’ rule of three‐dimensional aromaticity, as applied to the aromatic [B_nH_n]^2? closo boron hydride clusters. The closo boron hydride clusters can be categorized into different series, according to the n value of the Hückel (4 n+2) π rule. The distinct categories studied in this work correspond to n=1, 2, and 3. Each category increases in geometrical difficulty but, more importantly, it is possible to associate each category with the number of pentagonal layers in the structure perpendicular to the main axis. Category 1 has one pentagonal layer, category 2 has two, and category 3 has three.     

Valence isoelectronic substitution in the B8(-) and B9(-) molecular wheels by an Al dopant atom: umbrella-like structures of AlB7(-) and AlB8(-)

The structures and the electronic properties of two aluminum-doped boron clusters, AlB(7)(-) and AlB(8)(-), were investigated using photoelectron spectroscopy and ab initio calculations. The photoelectron spectra of AlB(7)(-) and AlB(8)(-) are both broad, suggesting significant geometry changes between the ground states of the anions and the neutrals. Unbiased global minimum searches were carried out and the calculated vertical electron detachment energies were used to compare with the experimental data. We found that the Al atom does not simply replace a B atom in the parent B(8)(-) and B(9)(-) planar clusters in AlB(7)(-) and AlB(8)(-). Instead, the global minima of the two doped-clusters are of umbrella shapes, featuring an Al atom interacting ionically with a hexagonal and heptagonal pyramidal B(7) (C(6v)) and B(8) (C(7v)) fragment, respectively. These unique umbrella-type structures are understood on the basis of the special stability of the quasi-planar B(7)(3-) and planar B(8)(2-) molecular wheels derived from double aromaticity.     

Aromaticity and conformational flexibility of five-membered monoheterocycles: pyrrole-like and thiophene-like structures

Aromaticity and conformational flexibility of the series of five-membered monoheterocycles with group 14–16 heteroatoms, having one or two lone pairs, were studied with ab initio methods using NICS, ASE and I ₅ indices. For non-planar molecules like phosphole, aromaticity of their planar transition states was also studied, and a special modification of ASE index was proposed to that end. It was found that the presence of two lone pairs is generally preferable for aromaticity of all heterocycles except CPD and silolyl dianions. Heterocycles with group 16 heteroatoms have consistently lower aromaticity compared to other groups. A lot of structures should be classified as moderate aromatic and non-aromatic. Energies of out-of-plane deformation do not correlate with other indices studied, but reveal the same qualitative trends. Generally, aromaticity of five-membered monoheterocycles depends strongly on both heteroatom type and number of lone pairs on it.     

Neural networks as a tool to classify compounds according to aromaticity criteria

Aromaticity is a fundamental concept in chemistry, with many theoretical and practical implications. Although most organic compounds can be categorized as aromatic, non-aromatic, or antiaromatic, it is often difficult to classify borderline compounds as well as to quantify this property. Many aromaticity criteria have been proposed, although none of them gives an entirely satisfactory solution. The inability to fully arrange organic compounds according to a single criterion arises from the fact that aromaticity is a multidimensional phenomenon. Neural networks are computational techniques that allow one to treat a large amount of data, thereby reducing the dimensionality of the input set to a bidimensional output. We present the successful applications of Kohonen's self-organizing maps to classify organic compounds according to aromaticity criteria, showing a good correlation between the aromaticity of a compound and its placement in a particular neuron. Although the input data for the training of the network were different aromaticity criteria (stabilization energy, diamagnetic susceptibility, NICS, NICS(1), and HOMA) for five-membered heterocycles, the method can be extended to other organic compounds. Some useful features of this method are: 1) it is very fast, requiring less than one minute of computational time to place a new compound in the map; 2) the placement of the different compounds in the map is conveniently visualized; 3) the position of a compound in the map depends on its aromatic character, thus allowing us to establish a quantitative scale of aromaticity, based on Euclidean distances between neurons, 4) it has predictive power. Overall, the results reported herein constitute a significant contribution to the longstanding debate on the quantitative treatment of aromaticity.