Crystal Clear Structural Evidence for Electron Delocalization in 1,2-Dihydro-1,2-azaborines
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The first examples of “pre-aromatic” 1,2-dihydro-1,2-azaborine heterocycles have been structurally characterized, enabling the direct comparison of delocalized bonds of 1,2-dihydro-1,2-azaborines to their corresponding formal double and single bonds in nonaromatic systems. The crystallographic data provide an unprecedented look into the structural changes that occur in six-membered BN-heterocycles on their road to aromaticity, and they establish with little ambiguity that 1,2-dihydro-1,2-azaborines possess delocalized structures consistent with aromaticity.Keywords:
Electron Delocalization
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Reduction of a 4,5,6-triphospha[3]radialene gave the corresponding isolable dianionic species. Despite having two deltate dianion-like π-electrons delocalized over the central framework, its small NICS(0)(πzz) value (only -2.2 ppm) indicates only limited aromaticity.
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The first examples of “pre-aromatic” 1,2-dihydro-1,2-azaborine heterocycles have been structurally characterized, enabling the direct comparison of delocalized bonds of 1,2-dihydro-1,2-azaborines to their corresponding formal double and single bonds in nonaromatic systems. The crystallographic data provide an unprecedented look into the structural changes that occur in six-membered BN-heterocycles on their road to aromaticity, and they establish with little ambiguity that 1,2-dihydro-1,2-azaborines possess delocalized structures consistent with aromaticity.
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Cyclic delocalization of mobile electrons in two or three dimensions is probably one of the key aspects that characterize aromatic compounds. Although electron delocalization measures dates back to the 1970s, only until recently these measures have been used to quantify aromaticity. In this chapter, we discuss the different existent descriptors of aromaticity that are based on electron delocalization properties. Before, we briefly introduce the most commonly employed methods to quantify electron delocalization.
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Although the unique cyclo[18]carbon (C18) realized by recent experiments has been greatly concerned, it has so far remained elusive. In contrast, its precursors C18-(CO)n (n = 6, 4, and 2), which can be separated stably, are of more practical significance. In this paper, the bonding character, electron delocalization, and aromaticity of the C18-(CO)n (n = 6, 4, and 2) with out-of-plane and in-plane dual π systems (πout and πin) perpendicular to each other are studied by combining quantum chemical calculations and wavefunction analyses. These cyclocarbon oxides exhibit alternating long and short C-C bonds and extensive electron delocalization, and a significant diatropic induced ring current under the action of external magnetic field is therefore observed, which reveals the aromatic characteristic in the molecules. The global electron delocalization and significant influence of the number of intramolecular carbonyl (-CO) on the two sets of π conjugated systems have been focused on, and the essential reason for the distinct difference in the overall aromaticity of the molecules was also clarified. It seems that the substituent -CO groups hinders the electron delocalization of the πin system but has relatively small effect on the πout system, resulting in the molecules with less -CO group showing greater aromaticity.
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Cycloparaphenylenes (CPPs) exhibit selective host capabilities, featuring the ability to incorporate smaller CPPs to form double-walled host-guest complexes. Moreover, CPPs can also be stabilized by global aromaticity under twofold oxidation or reduction, involving electronic conjugation along with the overall structural backbone. Herein we explore the structural modifications, bonding, electron delocalization and magnetic properties of doubly reduced double-walled CPP complexes with DFT methods, in the isolated and aggregate [n + 5]CPP⊃[n]CPP2- (n = 5-8) species. Our results show that the hosts undergo structural, bonding and delocalization deformations towards quinoidal configurations and exhibit global long-ranged shielding cones similar to global aromatic free dianionic CPPs, accounting for charge delocalization on the outer nanohoops, whereas the guests preserve local aromatic benzenoid configurations, resulting in global and local aromatic circuits within the host-guest aggregate. This observation suggests that in multi-layered related species electronic delocalization will be retained at the outer structural surface. The aromaticity of the hosts is manifested in the strong upfield shifts of the guests 1H-NMR signals. Hence, CPP complexes can be extended to doubly reduced species stabilized by global host aromaticity expanding our understanding of doubled-walled nanotubes at the nanoscale regime.
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Abstract 7‐Phenylbenzo[ k ]fluoranthene ( 4 ), which is one of non‐alternant polycyclic aromatic hydrocarbons, was protonated at the C(12) position in neat CF 3 SO 3 H to give a persistent carbocation ( 4aH + ). The cation was successfully observed by NMR measurements at rt. The most deshielded protons were observed as doublet signals at 8.92 and 8.71 ppm for H(1) and H(3). The most deshielded 13 C signals appeared at 184.4 and 176.6 ppm for C(12a) and C(7). The positive charge was found to be delocalized to four carbons in a 1‐naphthalenium unit and to two other carbons according to Δδ 13 C from 4 to 4aH + . DFT calculations indicated that 4aH + was the most stable among possible protonation carbocations. The NICS(1) zz values of 4aH + suggested that the five‐membered π‐ring is nonaromatic and that the other benzenoid rings are aromatic. The influence of the 7‐phenyl group on aromaticity and on positive charge delocalization was limited. The next most stable cations were computed to be cations ( 4bH + and 4cH + ) that were generated by the protonation at the C(3) and C(4) positions, respectively. Their Gibbs energies were similar within 0.7 kcal/mol to that of 4aH + . Their five‐membered π‐rings were proved to be anti‐aromatic according to the NICS(1) zz criteria. 4bH + and 4cH + can be depicted by canonical structures with a cyclopentadienyl cation unit, which is suggested to be responsible for their anti‐aromaticity.
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Although the unique cyclo[18]carbon (C18) realized by recent experiments has been greatly concerned, it has so far remained elusive. In contrast, its precursors C18-(CO)n (n = 6, 4, and 2), which can be separated stably, are of more practical significance. In this paper, the bonding character, electron delocalization, and aromaticity of the C18-(CO)n (n = 6, 4, and 2) with out-of-plane and in-plane dual π systems (πout and πin) perpendicular to each other are studied by combining quantum chemical calculations and wavefunction analyses. These cyclocarbon oxides exhibit alternating long and short C-C bonds and extensive electron delocalization, and a significant diatropic induced ring current under the action of external magnetic field is therefore observed, which reveals the aromatic characteristic in the molecules. The global electron delocalization and significant influence of the number of intramolecular carbonyl (-CO) on the two sets of π conjugated systems have been focused on, and the essential reason for the distinct difference in the overall aromaticity of the molecules was also clarified. It seems that the substituent -CO groups hinders the electron delocalization of the πin system but has relatively small effect on the πout system, resulting in the molecules with less -CO group showing greater aromaticity.
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Abstract Gas‐phase clusters are deemed to be σ‐aromatic when they satisfy the 4 n +2 rule of aromaticity for delocalized σ electrons and fulfill other requirements known for aromatic systems. While the range of n values was shown to be quite broad when applied to short‐lived clusters found in molecular‐beam experiments, stability of all‐metal cluster‐like fragments isolated in condensed phase was previously shown to be mainly ascribed to two electrons ( n =0). In this work, the applicability of this concept is extended towards solid‐state compounds by demonstrating a unique example of a storable compound, which was isolated as a stable [K([2.2.2]crypt)] + salt, featuring a [Au 2 Sb 16 ] 4− cluster core possessing two all‐metal aromatic AuSb 4 fragments with six delocalized σ electrons each ( n =1). This discovery pushes the boundaries of the original idea of Kekulé and firmly establishes the usefulness of the σ‐aromaticity concept as a general idea for both small clusters and solid‐state compounds.
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Cyclopropane
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