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Klein BP, Ruppenthal L, Hall SJ, Sattler LE, Weber SM, Herritsch J, Jaegermann A, Maurer RJ, Hilt G, Gottfried JM. Topology Effects in Molecular Organic Electronic Materials: Pyrene and Azupyrene*. Chemphyschem 2021; 22:1065-1073. [PMID: 33768634 PMCID: PMC8251946 DOI: 10.1002/cphc.202100222] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 03/24/2021] [Indexed: 12/02/2022]
Abstract
Pyrene derivatives play a prominent role in organic electronic devices, including field effect transistors, light emitting diodes, and solar cells. The flexibility in the desired properties has previously been achieved by variation of substituents at the periphery of the pyrene backbone. In contrast, the influence of the topology of the central π‐electron system on the relevant properties such as the band gap or the fluorescence behavior has not yet been addressed. In this work, pyrene is compared with its structural isomer azupyrene, which has a π‐electron system with non‐alternant topology. Using photoelectron spectroscopy, near edge X‐ray absorption fine structure spectroscopy, and other methods, it is shown that the electronic band gap of azupyrene is by 0.72 eV smaller than that of pyrene. The difference of the optical band gaps is even larger with 1.09 eV, as determined by ultraviolet–visible absorption spectroscopy. The non‐alternant nature of azupyrene is also associated with a more localized charge distribution. Further insight is provided by density functional theory (DFT) calculations of the molecular properties and ab initio coupled cluster calculations of the optical transitions. The concept of aromaticity is used to interpret the major topology‐related differences.
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102
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Elucidating the Aromatic Properties of Covalent Organic Frameworks Surface for Enhanced Polar Solvent Adsorption. Polymers (Basel) 2021; 13:polym13111861. [PMID: 34205141 PMCID: PMC8199954 DOI: 10.3390/polym13111861] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/28/2021] [Revised: 05/08/2021] [Accepted: 05/09/2021] [Indexed: 11/17/2022] Open
Abstract
Covalent organic frameworks (COFs) have a distinguished surface as they are mostly made by boron, carbon, nitrogen and oxygen. Many applications of COFs rely on polarity, size, charge, stability and hydrophobicity/hydrophilicity of their surface. In this study, two frequently used COFs sheets, COF-1 and covalent triazine-based frameworks (CTF-1), are studied. In addition, a theoretical porous graphene (TPG) was included for comparison purposes. The three solid sheets were investigated for aromaticity and stability using quantum mechanics calculations and their ability for water and ethanol adsorption using molecular dynamics simulations. COF-1 demonstrated the poorest aromatic character due to the highest energy delocalization interaction between B–O bonding orbital of sigma type and unfilled valence-shell nonbonding of boron. CTF-1 was identified as the least kinetically stable and the most chemically reactive. Both COF-1 and CTF-1 showed good surface properties for selective adsorption of water via hydrogen bonding and electrostatic interactions. Among the three sheets, TPG’s surface was mostly affected by aromatic currents and localized π electrons on the phenyl rings which in turn made it the best platform for selective adsorption of ethanol via van der Waals interactions. These results can serve as guidelines for future studies on solvent adsorption for COFs materials.
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103
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Electrostatic Potential Topology for Probing Molecular Structure, Bonding and Reactivity. Molecules 2021; 26:molecules26113289. [PMID: 34072507 PMCID: PMC8198923 DOI: 10.3390/molecules26113289] [Citation(s) in RCA: 48] [Impact Index Per Article: 16.0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 04/10/2021] [Revised: 05/16/2021] [Accepted: 05/25/2021] [Indexed: 11/18/2022] Open
Abstract
Following the pioneering investigations of Bader on the topology of molecular electron density, the topology analysis of its sister field viz. molecular electrostatic potential (MESP) was taken up by the authors’ groups. Through these studies, MESP topology emerged as a powerful tool for exploring molecular bonding and reactivity patterns. The MESP topology features are mapped in terms of its critical points (CPs), such as bond critical points (BCPs), while the minima identify electron-rich locations, such as lone pairs and π-bonds. The gradient paths of MESP vividly bring out the atoms-in-molecule picture of neutral molecules and anions. The MESP-based characterization of a molecule in terms of electron-rich and -deficient regions provides a robust prediction about its interaction with other molecules. This leads to a clear picture of molecular aggregation, hydrogen bonding, lone pair–π interactions, π-conjugation, aromaticity and reaction mechanisms. This review summarizes the contributions of the authors’ groups over the last three decades and those of the other active groups towards understanding chemical bonding, molecular recognition, and reactivity through topology analysis of MESP.
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Manrique-de-la-Cuba MF, Leyva-Parra L, Inostroza D, Gomez B, Vásquez-Espinal A, Garza J, Yañez O, Tiznado W. Li 8 Si 8 , Li 10 Si 9 , and Li 12 Si 10 : Assemblies of Lithium-Silicon Aromatic Units. Chemphyschem 2021; 22:906-910. [PMID: 33779015 DOI: 10.1002/cphc.202001051] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/30/2020] [Revised: 03/18/2021] [Indexed: 11/12/2022]
Abstract
We report the global minima structures of Li8 Si8 , Li10 Si9 , and Li12 Si10 systems, in which silicon moieties maintain structural and chemical bonding characteristics similar to those of their building blocks: the aromatic clusters Td -Li4 Si4 and C2v -Li6 Si5 . Electron counting rules, chemical bonding analysis, and magnetic response properties verify the silicon unit's aromaticity persistence. This study demonstrates the feasibility of assembling silicon-based nanostructures from aromatics clusters as building blocks.
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105
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Impact of Deuteration and Temperature on Furan Ring Dynamics. MOLECULES (BASEL, SWITZERLAND) 2021; 26:molecules26102889. [PMID: 34068118 PMCID: PMC8152745 DOI: 10.3390/molecules26102889] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Download PDF] [Figures] [Subscribe] [Scholar Register] [Received: 04/05/2021] [Revised: 04/28/2021] [Accepted: 05/10/2021] [Indexed: 11/17/2022]
Abstract
Despite significant progress in conformational analysis of cyclic molecules, the number of computational studies is still limited while most of that available in the literature data have been obtained long time ago with outdated methods. In present research, we have studied temperature driven conformational changes of the furan ring at three different temperatures. Additionally, the effect of deuteration on the ring dynamics is discussed; in addition, the aromaticity indices following the Bird and HOMA schemes are computed along all trajectories. Our ab initio molecular dynamic simulations revealed that deuteration has changed the furan ring dynamics and the obvious consequences; in addition, the shape and size of molecule are expected to be different.
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106
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Arasu NP, Vázquez H. Origin of the Electron Transport Properties of Aromatic and Antiaromatic Single Molecule Circuits. Chemphyschem 2021; 22:864-869. [PMID: 33656792 DOI: 10.1002/cphc.202100010] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/08/2021] [Revised: 02/12/2021] [Indexed: 11/10/2022]
Abstract
Antiaromatic molecules have been predicted to exhibit increased electron transport properties when placed between two nanoelectrodes compared to their aromatic analogues. While some studies have demonstrated this relationship, others have found no substantial increase. We use atomistic simulations to establish a general relationship between the electronic spectra of aromatic, antiaromatic, and quinoidal molecules and illustrate its implications for electron transport. We compare the electronic properties of a series of aromatic-antiaromatic counterparts and show that antiaromaticity effectively p-dopes the aromatic electronic spectra. As a consequence, the conducting properties of aromatic-antiaromatic analogues are closely related. For similar attachment points to the electrodes, an interference feature is expected in the HOMO-LUMO gap of one whenever it is absent in the other one. We demonstrate how the relative conductance of aromatic-antiaromatic pairs can be tuned and even reversed through the choice of chemical linker groups. Our work provides a general picture relating connectivity, (anti)aromaticity, and quantum interference and establishes new design rules for single molecule circuits.
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107
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Markert G, Paenurk E, Gershoni-Poranne R. Prediction of Spin Density, Baird-Anti aromaticity, and Singlet-Triplet Energy Gap in Triplet-State Polybenzenoid Systems from Simple Structural Motifs. Chemistry 2021; 27:6923-6935. [PMID: 33438296 DOI: 10.1002/chem.202005248] [Citation(s) in RCA: 6] [Impact Index Per Article: 2.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/08/2020] [Revised: 01/08/2021] [Indexed: 12/15/2022]
Abstract
Triplet-state aromaticity has been recently proposed as a strategy for designing functional organic electronic compounds, many of which are polycyclic aromatic systems. However, in many cases, the aromatic nature of the triplet state cannot be easily predicted. Moreover, it is often unclear how specific structural manipulations affect the electronic properties of the excited-state compounds. Herein, the relationship between the structure of polybenzenoid hydrocarbons (PBHs) and their spin-density distribution and aromatic character in the first triplet excited state is studied. Although a direct link is not immediately visible, classifying the PBHs according to their annulation sequence reveals regularities. Based on these, a set of guidelines is defined to qualitatively predict the location of spin and paratropicity and the singlet-triplet energy gap in larger PBHs, using only their smaller tri- and tetracyclic components, and subsequently tested on larger systems.
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108
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Lingas R, Charistos ND, Muñoz-Castro A. Aromaticity of ortho and meta 8-Cycloparaphenylene and Their Dications: Induced Magnetic Field Analysis with Localized and Delocalized Orbitals in Strained Nanohoops. Chemphyschem 2021; 22:741-751. [PMID: 33620136 DOI: 10.1002/cphc.202100057] [Citation(s) in RCA: 0] [Impact Index Per Article: 0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/27/2021] [Revised: 02/21/2021] [Indexed: 11/06/2022]
Abstract
Dications of cycloparaphenyles ([n]CPPs) are known to exhibit in-plane global aromaticity, contained in a nanobelt structure. Recently synthesized ortho and meta isomers of [n]CPPs break the radial symmetry of π structure incorporating perpendicular oriented π orbitals. Herein we set to explore the aromaticity of neutral and dicationic ortho and meta isomers of [8]CPP by dissecting the induced magnetic field to contributions of the twofold radial/perpendicular π system using delocalized canonical molecular orbitals (CMO), and introducing the natural localized molecular orbitals (NLMO) analysis with DFT methods. The dications sustain a reduced global aromatic character of the radial π system under a perpendicular orientation of the external field which declines from ortho to meta isomer and reinforces local aromaticity of ortho ring while it destroys aromaticity of meta ring. Aromaticity variations are determined by symmetry governed rotational excitations of frontier π orbitals. The parallel orientation reveals a substantial reduction of local aromaticity verified with NICSπ analysis and electron delocalization indices.
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109
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Crumbach M, Bachmann J, Fritze L, Helbig A, Krummenacher I, Braunschweig H, Helten H. Dithiophene-Fused Oxadiborepins and Azadiborepins: A New Class of Highly Fluorescent Heteroaromatics. Angew Chem Int Ed Engl 2021; 60:9290-9295. [PMID: 33522053 PMCID: PMC8252115 DOI: 10.1002/anie.202100295] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 01/07/2021] [Indexed: 12/15/2022]
Abstract
Access to dithiophene-fused oxadiborepins and the first azadiborepins attained via a modular synthesis route are presented. The new compounds emit intense blue light, some of which demonstrate fluorescence quantum yields close to unity. Cyclic voltammetry (CV) revealed electrochemically reversible one-electron reduction processes. The weak aromatic character of the novel 1,2,7-azadiborepin ring is demonstrated with in-depth theoretical investigations using nucleus-independent chemical shift (NICS) scans and anisotropy of the induced current density (ACID) calculations.
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110
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Bartkowski K, Pawlicki M. (Aza)Acenes Share the C2 Bridge with (Anti)Aromatic Macrocycles: Local vs. Global Delocalization Paths. Angew Chem Int Ed Engl 2021; 60:9063-9070. [PMID: 33406296 DOI: 10.1002/anie.202011848] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/30/2020] [Indexed: 12/28/2022]
Abstract
A strong conjugation present in fused systems plays a crucial role in tuning of the properties that would be showing a dependence on the efficiency of π-electrons coupling. The π-cloud available in the final structure can be drastically influenced by a side- or a linear fusion of unsaturated and conjugated hydrocarbons. The linear welding of naphthalene/anthracene or quinoxaline/benzo[g]quinoxaline with triphyrin(2.1.1) gives structures where the competition between local and global delocalization is distinguished. The aromatic character observed in skeletons strongly depends on the oxidation state of the macrocyclic flanking and is either extended over the whole system or kept as a composition of local currents (diatropic and paratropic) of incorporated units. The hybrid systems show the properties derived from the π-conjugations that interlace one another but also show a significant independence of (aza)acene subunits reflected in the observed spectroscopic properties.
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111
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Böhmdorfer S, Rosenau T. Oxidation with a "Stopover" - Stable Zwitterions as Intermediates in the Oxidation of α-Tocopherol (Vitamin E) Model Compounds to their Corresponding ortho-Quinone Methides. ChemistryOpen 2021; 10:421-429. [PMID: 33666351 PMCID: PMC8015729 DOI: 10.1002/open.202000306] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/09/2020] [Revised: 01/12/2021] [Indexed: 11/25/2022] Open
Abstract
As a prominent member of the vitamin E group, α-tocopherol is an important lipophilic antioxidant. It has a special oxidation chemistry that involves phenoxyl radicals, quinones and quinone methides. During the oxidation to the ortho-quinone methide, an intermediary zwitterion is formed. This aromatic intermediate turns into the quinone methide by simply rotating the initially oxidized, exocyclic methyl group into the molecule's plane. This initial zwitterionic intermediate and the quinone methide are not resonance structures but individual species, whose distinct electronic structures are separated by a mere 90° bond rotation. In this work, we hindered this crucial rotation, by substituting the affected methyl group with alkyl or phenyl groups. The alkyl groups slowed down the conversion to the quinone methide by 18-times, while the phenyl substituents, which additionally stabilize the zwitterion electronically, completely halted the conversion to the quinone methide at -78 °C, allowing for the first time the direct observation of a tocopherol-derived zwitterion. Employing a 13 C-labeled model, the individual steps of the oxidation sequence could be observed directly by NMR, and the activation energy for the rotation could be estimated to be approximately 2.8 kcal/mol. Reaction rates were solvent dependent, with polar solvents exerting a stabilizing effect on the zwitterion. The observed effects confirmed the central relevance of the rotation step in the change from the aromatic to the quinoid state and allowed a more detailed examination of the oxidation behavior of tocopherol. The concept that a simple bond rotation can be used to switch between an aromatic and an anti-aromatic structure could find its use in molecular switches or molecular engines, driven by the specific absorption of external energy.
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112
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Hagspiel S, Arrowsmith M, Fantuzzi F, Vargas A, Rempel A, Hermann A, Brückner T, Braunschweig H. Highly Colored Boron-Doped Thiazolothiazoles from the Reductive Dimerization of Boron Isothiocyanates. Angew Chem Int Ed Engl 2021; 60:6446-6450. [PMID: 33492727 PMCID: PMC7986239 DOI: 10.1002/anie.202015508] [Citation(s) in RCA: 19] [Impact Index Per Article: 6.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/20/2020] [Revised: 01/04/2021] [Indexed: 01/08/2023]
Abstract
Reduction of (CAAC)BBr2 (NCS) (CAAC=cyclic alkyl(amino)carbene) in the presence of a Lewis base L yields tricoordinate (CAAC)LB(NCS) borylenes which undergo reversible E/Z-isomerization. The same reduction in the absence of L yields deep blue, bis(CAAC)-stabilized, boron-doped, aromatic thiazolothiazoles resulting from the dimerization of dicoordinate (CAAC)B(NCS) borylene intermediates.
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113
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Roy DK, Tröster T, Fantuzzi F, Dewhurst RD, Lenczyk C, Radacki K, Pranckevicius C, Engels B, Braunschweig H. Isolation and Reactivity of an Antiaromatic s-Block Metal Compound. Angew Chem Int Ed Engl 2021; 60:3812-3819. [PMID: 33210400 PMCID: PMC7898526 DOI: 10.1002/anie.202014557] [Citation(s) in RCA: 17] [Impact Index Per Article: 5.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 10/30/2020] [Indexed: 12/27/2022]
Abstract
The concepts of aromaticity and antiaromaticity have a long history, and countless demonstrations of these phenomena have been made with molecules based on elements from the p, d, and f blocks of the periodic table. In contrast, the limited oxidation-state flexibility of the s-block metals has long stood in the way of their participation in sophisticated π-bonding arrangements, and truly antiaromatic systems containing s-block metals are altogether absent or remain poorly defined. Using spectroscopic, structural, and computational techniques, we present herein the synthesis and authentication of a heterocyclic compound containing the alkaline earth metal beryllium that exhibits significant antiaromaticity, and detail its chemical reduction and Lewis-base-coordination chemistry.
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114
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Rottschäfer D, Neumann B, Stammler H, Sergeieva T, Andrada DM, Ghadwal RS. Isolation of a 16π-Electrons 1,4-Diphosphinine-1,4-diide with a Planar C 4 P 2 Ring. Chemistry 2021; 27:3055-3064. [PMID: 33080114 PMCID: PMC7898681 DOI: 10.1002/chem.202003617] [Citation(s) in RCA: 14] [Impact Index Per Article: 4.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/04/2020] [Revised: 10/05/2020] [Indexed: 02/02/2023]
Abstract
Herein, we report the first 1,4-diphosphinine-1,4-diide compound [(ADCPh )P]2 (5-Ph) (ADCPh =PhC{(NDipp)C}2 ; Dipp=2,6-iPr2 C6 H3 ) derived from an anionic dicarbene (ADCPh ) as a red crystalline solid. Compound 5-Ph containing a 16π-electron planar fused-tricyclic ring system was obtained by the 4e reduction of [(ADCPh )PCl2 ]2 (4-Ph) with Mg (or KC8 ) in a quantitative yield. Experimental and computational results imply that the central 8π-electrons C4 P2 ring of 5-Ph, which is fused between two 6π-electrons C3 N2 aromatic rings, is antiaromatic. Thus, each of the phosphorus atoms of 5-Ph has two electron-lone-pairs, one in a p-type orbital is in conjugation with the C=C bonds of the C4 P2 ring, while the second resides in a σ-symmetric orbital. This can be shown with the gold complex [(ADCPh )P(AuCl)2 ]2 (6-Ph) obtained by reacting 5-Ph with (Me2 S)AuCl. A mixture of 5-Ph and 4-Ph undergoes comproportionation in the presence of MgCl2 to form the intermediate oxidation state compound [(ADCAr )P]2 (MgCl4 ) (7-Ph), which is an aromatic species.
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115
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Greßies S, Ito M, Sakai M, Osaki H, Kim JH, Gensch T, Daniliuc C, Ando N, Yamaguchi S, Glorius F. Twofold C-H Activation Enables Synthesis of a Diazacoronene-Type Fluorophore with Near Infrared Emission Through Isosteric Replacement. Chemistry 2021; 27:2753-2759. [PMID: 33085826 DOI: 10.1002/chem.202004080] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/08/2020] [Revised: 10/20/2020] [Indexed: 12/19/2022]
Abstract
The synthesis and photophysical properties of a soluble amide-embedded coronene is reported. The key step in this synthesis is the twofold C-H activation of diazaperylene by a rhodium(III)Cp* catalyst. This unprecedented structural motif shows intense fluorescence in the near infrared region with a small Stokes shift and a distinct vibronic structure, which exhibits a slight extent of negative solvatochromism. Comparison of this compound with some relevant compounds revealed the importance of the amide incorporation in the peripheral concave region including an angular position to retain high aromaticity reflecting that of parent coronene. Treatment of this compound with Lewis acid B(C6 F5 )3 formed a bis-adduct, which exhibited enhanced aromaticity as a consequence of the increased double bond character of the amide C-N bonds.
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116
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Sharma MK, Rottschäfer D, Glodde T, Neumann B, Stammler HG, Ghadwal RS. An Open-Shell Singlet Sn I Diradical and H 2 Splitting. Angew Chem Int Ed Engl 2021; 60:6414-6418. [PMID: 33460280 PMCID: PMC7986611 DOI: 10.1002/anie.202017078] [Citation(s) in RCA: 28] [Impact Index Per Article: 9.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 12/23/2020] [Indexed: 11/17/2022]
Abstract
The first SnI diradical [(ADCPh)Sn]2 (4) based on an anionic dicarbene (ADCPh={CN(Dipp)}2CPh; Dipp=2,6‐iPr2C6H3) scaffold has been isolated as a green crystalline solid by KC8 reduction of the corresponding bis‐chlorostannylene [(ADCPh)SnCl]2 (3). The six‐membered C4Sn2‐ring of 4 containing six π‐electrons shows a diatropic ring current, thus 4 may also be regarded as the first 1,4‐distannabenzene derivative. DFT calculations suggest an open‐shell singlet (OS) ground state of 4 with a remarkably small singlet–triplet energy gap (ΔEOS–T=4.4 kcal mol−1), which is consistent with CASSCF (ΔES–T=6.6 kcal mol−1 and diradical character y=37 %) calculations. The diradical 4 splits H2 at room temperature to yield the bis‐hydridostannylene [(ADCPh)SnH]2 (5). Further reactivity of 4 has been studied with PhSeSePh and MeOTf.
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117
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Patra SG, Mondal T. Interplay of Hückel and Möbius Aromaticity in Metal-Metal Quintuple Bonded Complexes of Cr, Mo, and W with Amidinate Ligand: Ab initio DFT and Multireference Analysis*. Chemphyschem 2021; 22:298-311. [PMID: 33252161 DOI: 10.1002/cphc.202000923] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/07/2020] [Revised: 11/29/2020] [Indexed: 11/10/2022]
Abstract
The aromaticity of metal-metal quintuple bonded complexes of the type M2 L2 (M=Cr, Mo, and W; L=amidinate) are studied employing gauge including magnetically induced ring current (GIMIC) analysis and electron density of delocalized bonds (EDDB). It is found that the complexes possess two types of aromaticity: i) Hückel aromaticity through delocalization of ligand π electrons with metal-metal δ-bond-forming 6 conjugated electrons (4π and 2δ) ring; ii) Craig-Möbius aromaticity through delocalization of π electrons of both the ligands with metal d-orbitals in Craig type orientation forming 10π electrons ring with a double twist. Extended transition state natural orbital chemical valence (ETS-NOCV) and canonical molecular orbital natural chemical shielding (CMO-NCS) analysis confirm the Craig-Möbius type arrangement of the orbitals. Furthermore, the unprecedented Hückel and Möbius type aromaticity is confirmed from the plot of the current pathways using 3D line integral convolution (3D-LIC) plots. The metal-metal bond order also increases down the group as justified from the complete active space self-consistent field (CASSCF) analysis. Due to an increase in the π and δ electron conjugation, both the Hückel and Möbius aromaticity increase down the group.
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118
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Effect of Substitution of Hydrogen Atoms in the Molecules of Anthrone and Anthraquinone. Molecules 2021; 26:molecules26020502. [PMID: 33477805 PMCID: PMC7832899 DOI: 10.3390/molecules26020502] [Citation(s) in RCA: 3] [Impact Index Per Article: 1.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 11/28/2020] [Revised: 01/08/2021] [Accepted: 01/14/2021] [Indexed: 11/17/2022] Open
Abstract
The geometry of anthrone and anthraquinone-natural substances of plant origin-was investigated under the substitution of hydrogen atoms in side aromatic ring and, for anthrone, also in the central ring. A significant influence of substitution on geometry expressed by the angle between the side rings was shown. The geometry changes are connected with the changes of electron density and aromaticity of the anthrone and anthraquinone rings. The flexibility of the investigated compounds was confirmed by comparison of the optimized molecules and the molecules in the crystal state where the packing forces can influence the molecular geometry.
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119
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Zhang Y, Zhang C, Mo Y, Cao Z. Planar Tetracoordinate Silicon in Organic Molecules As Carbenoid-Type Amphoteric Centers: A Computational Study. Chemistry 2021; 27:1402-1409. [PMID: 33103283 DOI: 10.1002/chem.202004298] [Citation(s) in RCA: 9] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/30/2020] [Indexed: 11/11/2022]
Abstract
Designing and synthesizing a stable compound with a planar tetracoordinate silicon (ptSi) center is a challenging goal for chemists. Here, a series of potential aromatic ptSi compounds composed of four conjugated rings shared by a centrally embedded Si atom are theoretically designed and computationally verified. Both Born-Oppenheimer molecular dynamics (BOMD) simulations and potential energy surface scannings verify the high stability and likely existence of these compounds, particularly Si-16-5555 (SiN4 C8 H8 ) with 16 π electrons, under standard ambient temperature and pressure. Notably, the Hückel aromaticity rule, which works well for single rings, is inconsistent with the high stability of Si-16-5555 where the 16 p electrons are spread over four five-membered rings fused together. Bonding analyses show that the strong electron donation from the peripheral 12-membered conjugated ring with 16 π electrons to the vacant central atomic orbital Si 3pz leads to the stabilization for both the ptSi coordination and planar aromaticity. The partial occupation of Si 3pz results in the peculiar carbenoid-type behaviors for the amphoteric center. By modulating the electron density on the ring with substituent groups, we can regulate the nucleophilic and electrophilic properties of the central Si.
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Bresien J, Michalik D, Schulz A, Villinger A, Zander E. Azadiphosphaindane-1,3-diyls: A Class of Resonance-Stabilized Biradicals. Angew Chem Int Ed Engl 2021; 60:1507-1512. [PMID: 33038288 PMCID: PMC7839750 DOI: 10.1002/anie.202011886] [Citation(s) in RCA: 8] [Impact Index Per Article: 2.7] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Download PDF] [Figures] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/31/2020] [Revised: 10/06/2020] [Indexed: 01/12/2023]
Abstract
Conversion of 1,2-bis(dichlorophosphino)benzene with sterically demanding primary amines led to the formation of 1,3-dichloro-2-aza-1,3-diphosphaindanes of the type C6 H4 (μ-PCl)2 N-R. Reduction yielded the corresponding 2-aza-1,3-diphosphaindane-1,3-diyls (1), which can be described as phosphorus-centered singlet biradical(oid)s. Their stability depends on the size of the substituent R: While derivatives with R=Dmp (2,6-dimethylphenyl) or Ter (2,6-dimesitylphenyl) underwent oligomerization, the derivative with very bulky R=tBu Bhp (2,6-bis(benzhydryl)-4-tert-butylphenyl) was stable with respect to oligomerization in its monomeric form. Oligomerization involved activation of the fused benzene ring by a second equivalent of the monomeric biradical and can be regarded as formal [2+2] (poly)addition reaction. Calculations indicate that the biradical character in 1 is comparable with literature-known P-centered biradicals. Ring-current calculations show aromaticity within the entire ring system of 1.
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Muya JT, Isamura BK, Patouossa I, Nguyen MT, Ceulemans A. Structure, stability and bonding of the leapfrog B 24 0 ,±1,±2. J Comput Chem 2021; 42:72-80. [PMID: 33063884 DOI: 10.1002/jcc.26434] [Citation(s) in RCA: 2] [Impact Index Per Article: 0.7] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 06/12/2020] [Revised: 08/16/2020] [Accepted: 09/29/2020] [Indexed: 01/16/2023]
Abstract
Two new structural motifs of the B24 clusters are constructed by use of the leapfrog transformation. The resulting leapfrog B24 has either a bowl shape with a square vacancy or a quasi-planar 2D close-packed triangular boron sheet. The neutral and ionic forms of the latter are found to be more stable than their homologous leapfrog bowl clusters, with the exception of the dicationic B24 +2 . While the leapfrog isomer is less stable than the tubular double ring in the neutral state, it becomes competitive in some ionic states. The nucleus independent chemical shift, electron localization function, ring current maps and the electronic structure of leapfrog B24 clusters reveal them to behave as aromatics.
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Chen D, Szczepanik DW, Zhu J, Muñoz-Castro A, Solà M. Aromaticity Survival in Hydrofullerenes: The Case of C 66 H 4 with Its π-Aromatic Circuits. Chemistry 2021; 27:802-808. [PMID: 33089913 DOI: 10.1002/chem.202004322] [Citation(s) in RCA: 4] [Impact Index Per Article: 1.3] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 09/24/2020] [Revised: 10/19/2020] [Indexed: 11/08/2022]
Abstract
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 C66 H4 , bearing two heptagons with adjacent fused-pentagon motifs, largely dismisses this feature. Herein, employing DFT calculations, the 13 C NMR spectroscopy and aromatic behavior of C2v C66 H4 are explored. The results show the presence of three π-aromatic circuits at the bottom boat section of C66 H4 , 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 C60 6- 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.
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Gil-Guerrero S, Melle-Franco M, Peña-Gallego Á, Mandado M. Clar Goblet and Aromaticity Driven Multiradical Nanographenes. Chemistry 2020; 26:16138-16143. [PMID: 32893901 DOI: 10.1002/chem.202003713] [Citation(s) in RCA: 6] [Impact Index Per Article: 1.5] [Reference Citation Analysis] [Abstract] [Key Words] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Received: 08/10/2020] [Revised: 08/26/2020] [Indexed: 11/10/2022]
Abstract
The Clar Goblet, the first radical bowtie nanographene proposed by Erich Clar nearly 50 years ago, was recently synthesized. Bowtie nanographenes present quasi-degenerate magnetic ground states, which make them so elusive as unique. A thorough analysis is presented of the spin-state energetics of Clar Goblet and bowtie nanographenes by a battery of existing and novel ab initio procedures ranging from density functional theory to complete active space Hamiltonians. With this, it was proven that π radicals of bowtie nanographenes sit on BP (Benzo[cd]Pyrene) moieties driven by their local aromaticity, a purely chemical concept, which confers global stability to the whole structure. Besides, a novel Pauli energy densities analysis provided a visual intuitive explanation for this preference. These findings allow envisioning that analogous bowtie nanographenes with arbitrary polyradical character are not only feasible at the molecular scale but will share Clar Goblet's peculiar properties.
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Haags A, Reichmann A, Fan Q, Egger L, Kirschner H, Naumann T, Werner S, Vollgraff T, Sundermeyer J, Eschmann L, Yang X, Brandstetter D, Bocquet FC, Koller G, Gottwald A, Richter M, Ramsey MG, Rohlfing M, Puschnig P, Gottfried JM, Soubatch S, Tautz FS. Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization. ACS NANO 2020; 14:15766-15775. [PMID: 33186031 PMCID: PMC7690051 DOI: 10.1021/acsnano.0c06798] [Citation(s) in RCA: 12] [Impact Index Per Article: 3.0] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Received: 08/13/2020] [Accepted: 11/03/2020] [Indexed: 05/08/2023]
Abstract
We revisit the question of kekulene's aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene's highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C-C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.
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Haags A, Reichmann A, Fan Q, Egger L, Kirschner H, Naumann T, Werner S, Vollgraff T, Sundermeyer J, Eschmann L, Yang X, Brandstetter D, Bocquet FC, Koller G, Gottwald A, Richter M, Ramsey MG, Rohlfing M, Puschnig P, Gottfried JM, Soubatch S, Tautz FS. Kekulene: On-Surface Synthesis, Orbital Structure, and Aromatic Stabilization. ACS NANO 2020; 14:15766-15775. [PMID: 33186031 DOI: 10.26434/chemrxiv.12771254.v1] [Citation(s) in RCA: 1] [Impact Index Per Article: 0.3] [Reference Citation Analysis] [Abstract] [Key Words] [Grants] [Track Full Text] [Subscribe] [Scholar Register] [Indexed: 05/23/2023]
Abstract
We revisit the question of kekulene's aromaticity by focusing on the electronic structure of its frontier orbitals as determined by angle-resolved photoemission spectroscopy. To this end, we have developed a specially designed precursor, 1,4,7(2,7)-triphenanthrenacyclononaphane-2,5,8-triene, which allows us to prepare sufficient quantities of kekulene of high purity directly on a Cu(111) surface, as confirmed by scanning tunneling microscopy. Supported by density functional calculations, we determine the orbital structure of kekulene's highest occupied molecular orbital by photoemission tomography. In agreement with a recent aromaticity assessment of kekulene based solely on C-C bond lengths, we conclude that the π-conjugation of kekulene is better described by the Clar model rather than a superaromatic model. Thus, by exploiting the capabilities of photoemission tomography, we shed light on the question which consequences aromaticity holds for the frontier electronic structure of a π-conjugated molecule.
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