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Enhanced thermodynamic stability is a fundamental characteristic of aromatic molecules, yet most previous studies of aromatic stabilization energy (ASE) have been limited to small rings with up to 18 π-electrons. Here we demonstrate that ASE can be detected experimentally in π-conjugated porphyrin nanorings with Hückel circuits of 76-108 π-electrons. This conclusion is supported by analyzing redox potentials to calculate the energy change for isodesmic reactions that convert an aromatic ring to an antiaromatic ring or vice versa. It is also supported by analyzing the energy barriers to conformational equilibria that disrupt aromaticity in the transition state. Both types of experiment indicate that cationic porphyrin nanorings display ASEs of 1-5 kJ mol-1. Density functional theory calculations reproduce the results for both types of experiment and predict ASEs in the range of 1-16 kJ mol-1. The experimental ASEs in porphyrin nanorings are compared with an experimental ASE of [18]annulene of ∼11 kJ mol-1, deduced from analysis of the energy barriers to conformational equilibria in [16], [18], and [20]annulene. Calculated energies of isodesmic reactions give an ASE of ∼37 kJ mol-1 in [18]annulene. This work contributes to a fundamental understanding of aromaticity in large macrocycles.

Mobius aromaticity was developed for twisted annulenes with electron counting rules opposite to those of Huckel aromaticity. The introduction of transition metals makes it possible for planar cyclic systems to exhibit Mobius aromaticity. Here we report the first planar monocyclic metallaboron systems with Mobius aromaticity. The structures and bonding of two rhenium-boride clusters are studied by high-resolution photoelectron imaging and ab initio calculations. The ReB3- cluster is shown to have a near-pyramidal structure, while ReB4- is found to be a planar pentagonal ring. Chemical bonding analyses show that both ReB4- and ReB4 possess four delocalized π-electrons, including two π-electrons in an orbital of Mobius topology. NICS calculations reveal strong aromatic characters in ReB4- and ReB4, consistent with the 4n electron counting rule for Mobius aromaticity.

We report the synthesis and characterization of a two-dimensional (2D) conjugated Ni(II) tetraaza[14]annulene-linked metal organic framework (NiTAA-MOF) where NiTAA is a macrocyclic MN4 (M = metal,...

Since aluminum is the most electropositive element among the p-block elements, the construction of molecules bearing a dianionic Al-Al σ-bond is inherently highly challenging. Herein, we report the first synthesis of a dianionic dialane(6) 2 based on the Al2O three-membered ring scaffold, namely, an aluminum analog of oxirane. The structure of 2 has been unambiguously ascertained by spectroscopic analysis as well as X-ray crystallography, and computational studies revealed that 2 bears a highly strained Al-Al σ-bond. 2 readily reacts with the unsaturated substrates such as isocyanide, ethylene, and ketone, concomitant with the cleavage of the Al-Al σ-bond under mild conditions, leading to the four- and five-membered heterocycles 3-5. Furthermore, the reaction of 2 with two molecules of benzonitrile (PhCN) furnishes a seven-membered heterocycle 6, resulting from the C-C coupling reaction of PhCN. We further delineate that 2 selectively activates an arene ring C-C bond of biphenylene, rendering a di-Al-substituted benzo[8]annulene derivative 7. Preliminary computational studies propose that the stepwise reaction mechanism involves the Al-Al σ-bond cleavage, dearomative Al-C bond formation, subsequent sigmatropic [1,3]shifts, and a pericyclic reaction. Ministry of Education (MOE) Nanyang Technological University Submitted/Accepted version We are grateful to Nanyang Technological University (NTU) and the Singapore Ministry of Education (MOE2018-T2-2-048(S)) for financial support.

Solution-phase synthesis of buckybowls remains a big challenge, and there are limited reports on the fragments of C70. Herein, we report a new basic subunit of C70 with C2v symmetry, the dicyclopenta[4,3,2,1- ghi:4',3',2',1'- pqr]perylene (2CP-Per). Its aryl-substituted derivative 2CP-Per-Ar was synthesized and shows a bowl-shaped geometry according to X-ray crystallographic analysis. A fast bowl-to-bowl inversion process was observed above 183 K by variable temperature nuclear magnetic resonance (NMR), with a small inversion energy barrier. 2CP-Per-Ar displays amphoteric redox behavior with a small electrochemical energy gap (1.29 eV). Different from many other aromatic buckybowls, 2CP-Per exhibits global antiaromaticity with a strong paratropic ring current associated with the 16π-electrons rim, as revealed by NMR measurements and theoretic calculations. Its dianion is aromatic, similar to its isoelectronic structure coronene. Its dication is predicted to be aromatic, with a [6]annulene-within-[14]annulene structure.

Midsized annulenes are known to undergo rapid π-bond shifting. Given that heavy-atom tunneling plays a role in planar bond shifting of cyclobutadiene, we computationally explored the contribution of heavy-atom tunneling to planar π-bond shifting in the major (CTCTCTCT, 5a) and minor (CTCTTCTT, 6a) known isomers of [16]annulene. UM06-2X/cc-pVDZ calculations yield bond-shifting barriers of ca. 10 kcal/mol. The results also reveal extremely narrow barrier widths, suggesting a high probability of tunneling for these bond-shifting reactions. Rate constants were calculated using canonical variational transition state theory (CVT) as well as with small curvature tunneling (SCT) contributions, via direct dynamics. For the major isomer 5a, the computed SCT rate constant for bond shifting at 80 K is 0.16 s–1, corresponding to a half-life of 4.3 s, and indicating that bond shifting is rapid at cryogenic temperatures despite a 10 kcal/mol barrier. This contrasts with the CVT rate constant of 8.0 × 10–15 s–1 at 80 K. ...

Stereocontrolled multilayer growth of supramolecular porous networks at the interface between graphite and a solution was investigated. For this study, we designed a chiral dehydrobenzo[12]annulene (DBA) building block bearing alkoxy chains substituted at the 2 position with hydroxy groups, which enable van der Waals stabilization in a layer and potential hydrogen-bonding interactions between the layers. Bias voltage-dependent scanning tunneling microscopy (STM) experiments revealed the diastereospecificity of the bilayer with respect to both the intrinsic chirality of the building blocks and the supramolecular chirality of the self-assembled networks. Top and bottom layers within the same crystalline domain were composed of the same enantiomers but displayed opposite supramolecular chiralities.

Alkyne cross-metathesis of molybdenum carbyne complex [TolC≡Mo(OCCH3(CF3)2)3]·DME with 2 equiv of functional ynamines or ynamides yields the primary cross-metathesis product with high regioselectivity (>98%) along with a molybdenum metallacyclobutadiene complex. NMR and X-ray crystal structure analysis reveals that ynamides derived from 1-(phenylethynyl)pyrrolidin-2-one selectively cleave the propagating molybdenum species in the ring-opening alkyne metathesis polymerization (ROAMP) of ring-strained 3,8-dihexyloxy-5,6-dihydro-11,12-didehydrodibenzo[a,e][8]annulene and irreversibly deactivate the diamagnetic molybdenum metallacyclobutadiene complex through a multidentate chelate binding mode. The chain termination of living ROAMP with substituted ethynylpyrrolidin-2-ones selectively transfers a functional end-group to the polymer chain, giving access to telechelic polymers. This regioselective carbyne transfer strategy gives access to amphiphilic block copolymers through synthetic cascades of ROAMP followe...

Regioselective carbyne-transfer reagents derived from (3,3,3-trifluoroprop-1-yn-1-yl)benzene give access to functionalized ring-opening alkyne metathesis polymerization (ROAMP) initiators [R-C6H4C≡Mo(OC(CH3)(CF3)2)3] featuring electron-donating or -withdrawing substituents on the benzylidyne. Kinetic studies and linear free-energy relationships reveal that the initiation step of the ring-opening alkyne metathesis polymerization of 5,6,11,12-tetradehydrobenzo[a,e][8]annulene exhibits a moderate positive Hammett reaction constant (ρ = +0.36). ROAMP catalysts featuring electron-withdrawing benzylidynes not only selectively increase the rate of initiation (ki) over the rate of propagation (kp) but also prevent undesired intra- and intermolecular chain-transfer processes, giving access to linear poly-(o-phenylene ethynylene) with narrow molecular weight distribution. The regioselective carbyne transfer methodology and the detailed mechanistic insight enabled the design of a bifunctional ROAMP-reversible additi...

Two-dimensional (2D) supramolecular self-assembly at liquid-solid interfaces is a thermodynamically complex process producing a variety of structures. The formation of multiple network morphologies from the same molecular building blocks is a common occurrence. We use scanning tunnelling microscopy (STM) to investigate a structural phase transition between a densely packed and a porous phase of an alkylated dehydrobenzo[12]annulene (DBA) derivative physisorbed at a solvent-graphite interface. The influence of temperature and concentration are studied and the results combined using a thermodynamic model to measure enthalpy and entropy changes associated with the transition. These experimental results are compared to corresponding values obtained from simulations and theoretical calculations. This comparison highlights the importance of considering the solvent when modeling porous self-assembled networks. The results also demonstrate the power of using structural phase transitions to study the thermodynamics of these systems and will have implications for the development of predictive models for 2D self-assembly.

A new bisdehydro[12]annulene derivative having a thiophene-fused structure has been synthesized. This highly twisted π-conjugated macrocycle with two acetylene moieties in close proximity produces a [2+2]-type alkyne cycloadduct by either photoirradiation or mild heating without any transition metals. Theoretical calculations reveal that the thermal reaction proceeds through successive 8π and 4π electrocyclic reactions, while the photochemical reaction is an asynchronous concerted [2+2] cycloaddition. The fused structure with the less-aromatic thiophene ring is crucial for achieving this reaction. The cycloadduct, thiophene-fused biphenylene, has significant potential as a new polycyclic π-scaffold for electronic applications.

Like the larger nonplanar Möbius rings, porphyrinoid aromaticity is not due primarily to the macrocyclic π conjugation of the corresponding annulene perimeters. The block-localized wave function (BLW)-derived aromatic stabilization energies (ASE) of several porphyrinoids reveal that, on a per atom basis, the appended 6π electron heterocycles of porphyrinoids confer aromaticity much more effectively than the macrocyclic 4n+2 π electron conjugations. There is no direct relationship between thermochemical stability of porphyrinoids and their macrocyclic 4n or 4n+2 π electron counts. Porphyrinoids having an "antiaromatic" macrocyclic 4n+2 π electron conjugation pathway (e.g., 4) as well as those having no macrocyclic conjugation (e.g., 9) can be stabilized by aromaticity. Computed nucleus independent chemical shifts (NICS) and the anisotropy of the induced current density (ACID) disclose the intricate local versus macrocyclic circulation interplay for several porphyrinoids.

A new strategy for the catalytic synthesis of substituted 1H-indenes via metalloradical activation of o-cinnamyl N-tosyl hydrazones is presented, taking advantage of the intrinsic reactivity of a Co(III) carbene radical intermediate. The reaction uses readily available starting materials and is operationally simple, thus representing a practical method for the construction of functionalized 1H-indene derivatives. The cheap and easy to prepare low spin cobalt(II) complex [Co(II)(MeTAA)] (MeTAA = tetramethyltetraaza[14]annulene) proved to be the most active catalyst among those investigated, which demonstrates catalytic carbene radical reactivity for a nonporphyrin cobalt(II) complex, and for the first time catalytic activity of [Co(II)(MeTAA)] in general. The methodology has been successfully applied to a broad range of substrates, producing 1H-indenes in good to excellent yields. The metallo-radical catalyzed indene synthesis in this paper represents a unique example of a net (formal) intramolecular carbene insertion reaction into a vinylic C(sp(2))-H bond, made possible by a controlled radical ring-closure process of the carbene radical intermediate involved. The mechanism was investigated computationally, and the results were confirmed by a series of supporting experimental reactions. Density functional theory calculations reveal a stepwise process involving activation of the diazo compound leading to formation of a Co(III)-carbene radical, followed by radical ring-closure to produce an indanyl/benzyl radical intermediate. Subsequent indene product elimination involving a 1,2-hydrogen transfer step regenerates the catalyst. Trapping experiments using 2,2,6,6-tetra-methylpiperidine-1-oxyl (TEMPO) radical or dibenzoylperoxide (DBPO) confirm the involvement of cobalt(III) carbene radical intermediates. Electron paramagnetic resonance spectroscopic spin-trapping experiments using phenyl N-tert-butylnitrone (PBN) reveal the radical nature of the reaction.

Molybdenum carbyne complexes [RC≡Mo(OC(CH3)(CF3)2)3] featuring a mesityl (R = Mes) or an ethyl (R = Et) substituent initiate the living ring-opening alkyne metathesis polymerization of the strained cyclic alkyne, 5,6,11,12-tetradehydrobenzo[a,e][8]annulene, to yield fully conjugated poly(o-phenylene ethynylene). The difference in the steric demand of the polymer end-group (Mes vs Et) transferred during the initiation step determines the topology of the resulting polymer chain. While [MesC≡Mo(OC(CH3)(CF3)2)3] exclusively yields linear poly(o-phenylene ethynylene), polymerization initiated by [EtC≡Mo(OC(CH3)(CF3)2)3] results in cyclic polymers ranging in size from n = 5 to 20 monomer units. Kinetic studies reveal that the propagating species emerging from [EtC≡Mo(OC(CH3)(CF3)2)3] undergoes a highly selective intramolecular backbiting into the butynyl end-group.

Synthesis and structural diversity of novel aromatic expanded isophlorins are described. Expanded isophlorins are higher analogues of 20pi isophlorin; 30pi isophlorins are the simplest examples of expanded isophlorins. They are synthesized from easy to make precursors. Owing to the sp(2) carbons along the conjugated network, they represent higher analogues of annulenes which are not realized until date. In contrast to the parent isophlorin 2, expanded isophlorins 7-9 and 11-13 are aromatic (4n + 2)pi systems and also differ in their optical properties and structural features. They exhibit ring inverted structures and the number of ring inversions is dependent on the nature of the heteroatoms present in the core of the macrocycle. This was confirmed by both (1)H NMR spectroscopy and single crystal-ray diffraction analysis. The upfield chemical shifts for the protons of the inverted rings confirmed the diatropic ring current effects as expected for (4n + 2)pi aromatic systems. Solid state analysis revealed a planar conformation for 7 and near planar conformations for other macrocycles. They also show strong intermolecular interactions through C(sp(2))-H...F-C(sp(2)) hydrogen bonds.

The geometries of coiled annulenes belonging to the chiral C2 and D(n) (n = 2,7) point groups are defined by two chiral indices, W(r) and T(w), respectively (writhe and twist), which sum to give an overall integer linking number, L(k) (the Cãlugãreanu-White-Fuller theorem). While the value of L(k) can been equated with single-twist (L(k) = 1pi), double-twist (L(k) = 2), and higher-order (L(k)2) twisted (Möbius-Listing) annulenes, we suggest that the correct Huckel molecular-orbital treatment is to use T(w) specifically in the 2p(pi)-2p(pi) overlap correction first suggested by Heilbronner, rather than L(k). Quantitatively, because many of these systems project much of the finite value of T(w) into W(r), a simple mechanism exists to increase the pi-electron resonance stabilization beyond what simple Heilbronner theory predicts. Examples of a diverse set of such chiral annulenes are dissected into W(r) and T(w) contributions, which reveals that those with the minimum value of T(w) are associated with the greater delocalized stability.

The self-assembly of a series of hexadehydrotribenzo[12]annulene (DBA) derivatives has been investigated by scanning tunneling microscopy (STM) at the liquid/solid interface in the absence and presence of nanographene guests. In the absence of appropriate guest molecules, DBA derivatives with short alkoxy chains form two-dimensional (2D) porous honeycomb type patterns, whereas those with long alkoxy chains form predominantly dense-packed linear type patterns. Added nanographene molecules adsorb in the pores of the existing 2D porous honeycomb type patterns or, more interestingly, they even convert the guest-free dense-packed linear-type patterns into guest-containing 2D porous honeycomb type patterns. For the DBA derivative with the longest alkoxy chains (OC20H41), the pore size, which depends on the length of the alkoxy chains, reaches 5.4 nm. Up to a maximum of six nanographene molecules can be hosted in the same cavity for the DBA derivative with the OC20H41 chains. The host matrix changes its structure in order to accommodate the adsorption of the guest clusters. This flexibility arises from the weak intermolecular interactions between interdigitating alkoxy chains holding the honeycomb structure together. Diverse dynamic processes have been observed at the level of the host matrix and the coadsorbed guest molecules.

5,10,15,20-Tetraaryl-21-vacataporphyrin (butadieneporphyrin, an annulene-porphyrin hybrid) which contains a vacant space instead of heteroatomic bridge acts as a ligand toward palladium(II). The metal ion of square-planar coordination geometry is firmly held via three pyrrolic nitrogen atoms where the fourth coordination place is occupied by a monodentate ligand or by an annulene part of vacataporphyrin. The macrocycle reveals the unique structural flexibility triggered by coordination of palladium. The structural rearrangements engage the C(20)C(1)C(2)C(3)C(4)C(5) annulene fragment which serves as a linker between two pyrrolic rings of vacataporphyrin albeit the significant ruffling of the tripyrrolic block is also of importance. Two fundamental modes of interactions between the palladium ion and annulene moiety have been recognized. The first one resembles an eta(2)-type interaction and involves the C(2)C(3) unit of the butadiene part. Alternatively the profound conformational adjustments allowed an in-plane coordination through the deprotonated trigonally hybridized C(2) center of butadiene. The coordinated vacataporphyrin acquires Hückel or extremely rare Möbius topologies readily reflected by spectroscopic properties. The palladium vacataporphyrin complexes reveal Hückel aromaticity or Möbius antiaromaticity of [18]annulene applying the butadiene fragment of vacataporphyrin as a topology selector. The properties of specific conformers were determined using (1)H NMR and density functional theory calculations.

The self-assembly of a series of hexadehydrotribenzo[12]annulene (DBA) derivatives has been scrutinized by scanning tunneling microscopy (STM) at the liquid-solid interface. First, the influence of core symmetry on the network structure was investigated by comparing the two-dimensional (2D) ordering of rhombic bisDBA 1a and triangular DBA 2a (Figure 1). BisDBA 1a forms a Kagomé network upon physisorption from 1,2,4-trichlorobenzene (TCB) onto highly oriented pyrolytic graphite (HOPG). Under similar experimental conditions, DBA 2a shows the formation of a honeycomb network. The core symmetry and location of alkyl substituents determine the network structure. The most remarkable feature of the DBA networks is the interdigitation of the nonpolar alkyl chains: they connect the pi-conjugated cores and direct their orientation. As a result, 2D open networks with voids are formed. Second, the effect of alkyl chain length on the structure of DBA patterns was investigated. Upon increasing the length of the alkyl chains (DBAs 3c-e) a transition from honeycomb networks to linear networks was observed in TCB, an observation attributed to stronger molecule-substrate interactions. Third, the effect of solvent on the structure of the nonpolar DBA networks was investigated in four different solvents: TCB as a polar aromatic solvent, 1-phenyloctane as a solvent having both aromatic and aliphatic moieties, n-tetradecane as an aliphatic solvent, and octanoic acid as a polar alkylated solvent. The solvent dramatically changes the structure of the DBA networks. The solvent effects are discussed in terms of factors that influence the mobility of molecules at the liquid-solid interface such as solvation.

The synthesis of the chloro- and parent cyclopentadienone-fused dihydropyrenes 10 and 7 are reported. Analysis of coupling constants, and chemical shift changes between these and the nonconjugated dihydro derivatives 11 and 8, and between the benzannulene 4 and the parent annulene 12, indicates without doubt that cyclopentadienone is behaving as an antiaromatic 4n-pi system and that in its effect on the ring current of 12, cyclopentanedienone has about 80% of the effect of benzene. This is the first time that a suitable probe has been used to estimate the relative ability of a 4n-pi system to bond localize the probe in comparison to the (4n + 2)-pi system benzene.

The low-temperature (-100 degrees C) dehydrohalogenation of bromocyclooctatetraene followed by immediate electron-transfer yields a stable solution of the [8]annulyne anion radical. If the unstable [8]annulyne is reacted with itself, cyclobutadiene, or benzyne, the respective bi-[8]annulenylene, [6]annuleno[8]annulene, or [6]-[8]annulenylene can be trapped as their anion radicals via one-electron transfer. These condensation products were all obtained from simple [2 + 2] cycloaddition reactions. B3LYP/6-31G geometry optimizations were carried out, and the calculated spin densities were compared to the EPR spectral results obtained for the anion radicals of [6]annuleno[8]annulene, [8]annulyne, bi[8]annulenylene, and [6]-[8]annulenylene, and excellent agreement has been realized. This simple "one-pot" approach should be applicable to a wide range of such systems.

The synthesis of eight new cyclobutadiene or ferrocene-fused organometallic dehydroannulenes is reported. Cadiot-type coupling of a 1-bromoethynyl-2-silylethynylbenzene derivative to an organometallic diyne (1,2-diethynyl-3,4-bis(trimethylsilyl)cyclobutadiene(cyclopentadienyl )cobalt or 1,2-diethynylferrocene) is followed by deprotection and Cu(OAc)(2)-promoted ring closure. Five of the organometallic dehydroannulenes were structurally characterized. Three of the novel cycles explode at temperatures from 196 to 293 degrees C and form insoluble carbon materials. The soot produced from 13a shows a high abundance of onion-like carbon nanostructures. The nanostructures were characterized by high-resolution transmission electron microscopy.

Ab initio and hybrid density functional quantum mechanical computations are applied to the structure and energetics of a series of two-atom-bridge annelated cyclooctatetraenes. The contribution of each annelation to the exo/endo relative energy is estimated. Key directing factors for a given type of annelation, such as strain, electronegativity, or cyclic electron count, can be sorted out by comparison of various bridge compositions. Overall, electron count and the essential components of the Clar/Robinson rule work well to predict the exo/endo preferences. Specifically, three 4-e(-) Hückel systems (CH-CH, NH-BH and NH-C(O)) display dominant exo forms whereas the three 4n + 2 Hückel counterparts (C(O)-C(O), BH-BH, and planar NH-NH) display a common preference for endo. These endo systems act like four independent four-membered "aromatic" rings linked by "single" bonds. An analysis based on the effective hybridization of carbon atoms in the annulene (Bent's rule) provides a rationale for subtle trends in their specific annulene geometry.

Electron paramagnetic resonance (EPR) studies have revealed that the steric interaction between the methyl hydrogens on a tert-butoxy substituent and the cyclooctatetraene (COT) ring system sterically induces a puckering of the eight-membered ring in the anion radical of tert-butoxy-COT. The induced nonplanarity of the COT ring system causes a large attenuation of the EPR coupling constants. Since the C-D bond length is slightly shorter than is the C-H bond length, replacement of the tert-butyl group with a tert-butyl-d(9) group results in less steric interaction and measurably larger electron proton coupling constants. The oscillation between the two close to planar alternating bond length (ABL) D(2d) conformers of the COT moiety was found to be extremely rapid (k10(12) s(-1)) and quantum mechanical tunneling is proposed to be involved.

The transannular cyclization of dehydroannulenes bearing several alkyne moieties in close proximity is a powerful synthetic method for producing polycyclic aromatic hydrocarbons. We report that the reactivity can be switched by the aromaticity of the ring skeletons fused with the dehydroannulene core. Thus, while thiophene-fused bisdehydro[12]annulene 1 was handled as a stable compound in the air at room temperature, the oxidation with m-chloroperbenzoic acid from the aromatic thiophene rings to the nonaromatic thiophene-S,S-dioxides induced the transannular cyclization, even at room temperature, which was completed within 1 day to produce the formal [2 + 2] cycloadduct 3. This is in stark contrast to the fact that the thermal cyclization of 1 itself required heating at 80 °C for 9 days for completion. Experimental and theoretical studies indicate that the oxidation of even one thiophene ring in 1 sufficiently decreases the activation barrier for the transannular cyclization that proceeds through the 8π and 4π electrocyclic reaction sequence. The thiophene-S,S-dioxide-fused biphenylene 3 thus produced exhibits a set of intriguing properties, such as a higher electron affinity (E1/2 = -1.17 V vs Fc and Fc(+)) and a stronger fluorescence (ΦF = 0.20) than the other relevant biphenylene derivatives, which have electron-donating and nonfluorescent characteristics.

Through the use of a potentially removable tether, a heavily substituted 10b,10c-dimethyl-10b,10c-dihydropyrene (DMDHP), 20, was synthesized exclusively as the cis-isomer. It exists as the major component (20:1) in an equilibrium with its valence isomer syn-[2.2]metacyclophanediene 19. An X-ray crystal structure determination of 20, a cis-(2,7)-10b,10c-dihydropyrenophane, provided the first experimental measurements of the cis-DMDHP skeleton. The observed bond alternation in the [14]annulene was found to be larger than that of the corresponding trans-DMDHP framework. Prior MMPI calculations, on which previous discussion of the structure of the cis-DMDHP system had been based, are in very good agreement with the experimental results. Our own DFT calculations predict a more symmetric and more bond equalized structure than was observed in 20.

As a method to generate all-carbon molecules having highly reactive polyyne units from stable precursors, the [2 + 2] cycloreversion of [4.3.2]propella-1,3,11-triene derivatives was developed. To test the efficiency of this method, the reaction was first applied to simple diethynyl- and dibutadiynyl-substituted propellatrienes, which produced upon UV-irradiation linear hexatriyne and decapentayne derivatives, respectively. Next, dehydro[12]-, [16]-, [18]-, [20]-, and [24]annulene derivatives annelated by the [4.3.2]propellatriene units were prepared as precursors to the corresponding cyclo[n]carbons, a monocyclic form of carbon clusters. Laser-desorption mass spectra of the dehydroannulenes exhibited, in the negative mode, peaks due to the corresponding cyclo[n]carbon anions (n = 12, 16, 18, 20, and 24) formed by successive losses of aromatic indane fragments. Solution photolysis of the dehydro[16]annulene and dehydro[18]annulene derivatives formed reactive polyyne intermediates by [2 + 2] cycloreversion ...

The molecular structures and relative energies of several of the lowest-energy conformations of [10]annulene (C10H10) have been investigated using the Hartree−Fock (HF) method, density functional theory (DFT), second-order Moller−Plesset perturbation theory (MP2), and (for the first time) coupled cluster singles and doubles with a perturbative inclusion of connected triple excitations [CCSD(T)]. For some years the HF method has been known to incorrectly favor bond-length-alternating structures for [10]annulene, and standard forms of DFT are now seen to incorrectly favor aromatic structures. For the naphthalene-like conformation, the B3LYP method is shown to require a large basis set before the geometry becomes properly bond-localized, i.e., similar to the predictions of CCSD(T) using even a modest basis set. With a basis set of 170 functions, B3LYP and BP86 predict that the aromatic heart-shaped conformation is 9.11 and 12.11 kcal mol-1, respectively, lower than the bond-alternating twist form, while with...

Site specifically functionalized dehydrobenzo[18]annulenes (DBAs) with previously inaccessible topologies (C2v and Cs symmetry) were prepared by utilizing an in situ protiodesilylation/alkynylation reaction. By application of a stepwise synthetic route, donor and/or acceptor functional groups were introduced to the annulenic core in a designed manner. The electronic absorption spectra of the DBAs revealed moderate to dramatic changes in the electronic structure of the [18]annulene core when subtle changes were made in substitution patterns and/or functional groups. Macrocycles containing C2v symmetry exhibited progressive and predictable bathochromic shifts ranging from 20 to 80 nm along with increasing intramolecular excited state change transfer (CT) behavior. Asymmetric (Cs) molecules presented more interesting absorption behavior, such as accentuation of bands in the visible region. X-ray crystal structures of [18]DBAs 20, 22, 23, and 25 showed normal bond lengths and bond angles within the planar ann...

The aromatic ring current probe, the bridged [14]annulene dimethyldihydropyrene, 1, is used to investigate the bond localization effects of the organometallic species tricarbonylchromiumbenzene, 37...

Fullerene/sulfur-bridged annulene cocrystals with a two-dimensional segregated alternating layer structure were prepared by a simple solution process. Single-crystal analysis revealed the existence of continuing π–π interactions in both the donor and acceptor layers, which serve as transport paths for holes and electrons separately. The ambipolar transport behaviors were demonstrated with single-crystal field-effect transistors and rationalized by quantum calculations. Meanwhile, preliminary photoresponsivity was observed with the transistor configuration.

2,4-Bis(bromomethyl)-3-methylthiophene was synthesised as an intermediate to prepare the strained thiophenobenzophanediene 11, which readily thermally valence isomerizes to the novel thia[13]annule...

Furan adducts, 13, 15, and 16, of dimethyldihydropyrene are prepared in order to test the postulate that bicyclic annelations are generally effective at inducing bond localization in aromatic systems. A large 2-ppm down-field shift of the 1H NMR shifts of the internal methyl signals in 15 compared to 16 provides a significant indicator of bond localization in 15. X-ray diffraction analysis of 13 displays regular bond length alternation. Ab initio computations that do not include dynamic electron correlation are found to be inadequate for modeling the molecular structure of 1. Density Functional Theory models 1 well and predicts bond localization in derivatives of 1 consistent with the observed NMR spectroscopic and X-ray diffraction results.

The synthesis of the biphenylene fused dihydropyrene 2 is described by cycloaddition of dibromobenzocyclobutadiene with the oxa[17]annulene 3 and deoxygenation of the adduct with Ti(0). Analysis of the NMR data indicates that biphenylene has about 55% of the relative bond fixing ability of benzene, which is equated to relative aromaticity. The experimentally determined Dewar resonance energy is 1.59 times that of benzene, while Dewar calculates 1.55 times benzene.

The Zr-C bond chem. and, particularly, its migration properties were studied where the metal is bonded to the dibenzotetramethyltetraaza[14]annulene (tmtaa) dianion in the model complexes cis-[Zr(tmtaa)R2]. In this respect the [Zr(tmtaa)] fragment was analyzed in comparison with the known [Cp2Zr] moiety, with the support of an extended Hueckel anal. The synthesis of [Zr(tmtaa)R2] (R = Me, 2; R = CH2Ph, 3) (shown as structure I) from [Zr(tmtaa)Cl2] (1) was achieved under highly controlled reaction conditions. Complexes 2 and 3 are thermally labile, and one of the alkyl groups undergoes a thermally-induced migration to one of the imino groups, leading to a trianionic ligand [Zr(R-tmtaa)R] (R = Me, 4; R = CH2Ph, 5). The nature of such complexes was elucidated by an x-ray anal. on the THF-solvated form of 4, [Zr(Me-tmtaa)(Me)(THF)] (6). Nucleophiles are even more effective in inducing the alkyl migration to the ligand. In the presence of THF or pyridine 5 may be easily converted to [Zr(R2-tmtaa)] (R = CH2Ph, 7), in which both alkyl groups have migrated to the imino groups of the ligand. The use of an excess of LiMe during the alkylation of 1 gave [Zr(Me2-tmtaa)¬?¬?¬?Li(THF)2] (8) via the intermediacy of 4. Complex 8 is a bifunctional carrier of LiMe. The general consequences of two electrophilic sites on the ligand, eventually, in competition with the electrophilic metal were analyzed. The net charge and electrophilic index are significantly higher for the metal in [Cp2Zr]2+ than in [Zr(tmtaa)]2+. The relatively higher thermal inertness of 3 allowed the authors to study the migration of the alkyl group toward the unsatd. incoming nucleophiles, ButNC and CO. The reaction of 3 with ButNC gave the bis(??2-iminoacyl) compd. [Zr(tmtaa)(??2-C(Me):NBut)2]. The four low-lying d orbitals can accommodate the two ??2-iminoacyl groups. The reaction of 3 with Bu'NC led to the formation of the bis(y2-iminoacyl) compound [Zr(tmtaa)(v2-C(Me)=NBut)2T]h.e four low-lying d orbitals can accommodate the two y2-iminoacyl groups. The reaction of 3 with CO gave [Zr(tmtaa)(yz-OC(CH2Ph)~)(]1 1) via the intermediacy of a very reactive oxycarbene $-acyl compound and ultimately gave 12 and 13, illustrating the reactivity of both zirconium and the iminato carbons in the [Zr(tmtaa)] fragment. The crystallographic details are as follows: 3 is orthorhombic, space group Pnma, a = 16.028(3) A, b = 20.666(4) A, c = 9.018(2) A, a = p = y = 90°, Z = 4, and R = 0.063. 6 is monoclinic, space group P2l/n, a = 19.859(5) A, b = 14.771(5) A, c = 9.072(4) A, a = y = 90°, /3 = 93.85(3)", Z = 4, and R = 0.044. 7 is monoclinic, space group C2/c, a = 23.608(6) A, b = 9.529(3) A, c = 19.050(4) A, a = y = 90", p = 99.30(2)", Z = 4, and R = 0.046. 10 is triclinic, space group P i , a = 12.800(4) A, b = 15.795(5) A, c = 11.758(1) A, a = 91.8(2)", p = 93.56(1)", y = 97.81(2)", Z = 2, and R = 0.042.

Parent cyclopropanes, annulenes, and related derivatives of C{sub 60} and C{sub 70} have been prepared and subjected to extensive spectroscopic scrutiny. Photolysis of the diazomethane adduct of C{sub 60} afforded the C{sub 61}H{sub 2} cyclopropane 2, accompanied by the isomeric annulene 4. Reaction of dimethyldiazomethane with C{sub 60} led to C{sub 63}H{sub 6} annulene 11, which upon heating was smoothly converted to the cyclopropane isomer 10 with an activation energy of 25 {+-} 1 kcal-mol{sup -1}. Addition of diazomethane to a toluene solution of C{sub 70} generated a 12:1:2 mixture of pyrazolines. Photolysis of the mixture gave the C{sub 11}H{sub 2} cyclopropanes 6 and 7, whereas thermolysis furnished the isomeric annulenes 8 and 9. Criteria have been developed for distinguishing fullerene cyclopropane derivatives from the corresponding annulenes via NMR and UV spectroscopy. 57 refs., 12 figs., 1 tab.

Despite its adherence to Huckel's rule, $LB 10$RB annulene is thought to prefer nonaromatic, nonplanar structures. This seemingly reasonable conclusion is based on experimental studies carried out a quarter century ago, but the detailed nature of the [10]annulene isomers has not been fully elucidated. A new, nearly planar aromatic monotrans [10]annulene configuration has now been characterized at the second-order perturbation (MP2) and density functional (B3LYP) levels of theory. The structure, with a symmetry plane that bisects the molecule, is not a stationary point at uncorrelated levels, but it is the most stable [10]annulene isomer at MP2(fc)/DZd and at higher levels. The inner hydrogen is only bent out of the approximate carbon plane by about 20[degree]. The estimated aromatic stabilization energy (17.7 kcal/mol, after correction for strain), the diamagnetic susceptibility exaltation ([Lambda] = -66.9 ppm cgs, [Lambda][sub Naphtalene] = -28.2), and the upfield NMR chemical shift of the inner hydrogen ([delta] = -6.1 ppm) confirm the aromaticity of this new C[sub s] [10]annulene conformation. 23 refs., 3 figs., 5 tabs.

Ab initio quantum mechanical methods, as well as molecular mechanics (MM2 and MM3) and semiempirical theoretical methods (AM1), have been used to predict the molecular structures and energetics of the plausible isomers of [10]annulene. Standard double-[zeta] (DZ) and double-[zeta] plus polarization (DZP) basis sets have been used for the ab initio self-consistent-field (SCF) studies. Full geometry optimizations have been carried out by these different methods. The harmonic vibrational frequencies and their infrared intensities are also predicted for all these isomers. The most likely observable isomers are the two structures with C[sub 2] symmetry, one of which could be called naphthalene-like and the other twisted. Isomers with C[sub s] (including a low-lying boat-like structure) and C[sub 1] (including a low-lying azulene-like structure) symmetries are also discussed. The aromatic character for the planar isomers, which are not local minima, has been studied using second-order perturbation theory. A random conformational searching procedure based on the MM3 method was also carried out to avoid losing any possible energetically low-lying conformations. The latter procedure uncovered one low-lying structure that had never been suggested previously. A comparison with the experimental conclusions of Masamune and co-workers is made. 21 refs., 4 tabs.

N,N'-Polymethylene-syn-1,6:8,13-diimino[14]annulenes, 1-7, in which the number, m, of CH 2 groups varies from 1 to 7, and N,N'-dimethyl-syn-1,6:8,13-diimino[14]annulene (8) were synthesized. The radical cations of 1-8, as well as those of several derivatives of 1-4, have been studied by ESR, ENDOR, and TRIPLE resonance spectroscopy. In contrast to the corresponding anions 1 .- -8 .- , which are bridged π-perimeter radicals, the cations 1 .+ -8 .+ must be considered as N-centered radicals with the bulk of the spin population residing at the two heteroatoms

The doubly-decked [8]annulene 11 has been prepared in 13 steps from octahydro 3a,9a-propenocyclopentacycloocten-2,11-dione. The scheme takes advantage of the annulating capacity of the Cook-Weiss reaction and the ability of semibullvalenes to undergo thermal isomerization to cyclooctatetraenes. Bis(semibullvalene) is both conformationally dynamic and subject to very rapid Cope rearrangement. The energetics of inversion within its interconnective cyclooctane ring have been elucidated by variable-temperature 1 H NMR spectroscopy

A series of alkyl- and alkoxy-substituted rhombic-shaped bisDBA derivatives 1a-d, 2a, and 2b were synthesized for the purpose of the formation of porous networks at the 1,2,4-trichlorobenzene (TCB)/graphite interface. Depending on the alkyl-chain length and the solute concentration, bisDBAs exhibit five network structures, three porous structures (porous A, B, and C), and two nonporous structures (nonporous D and E), which are attributed to their rhombic core shape and the position of the substituents. BisDBAs 1a and 1b with the shorter alkyl chains favorably form a porous structure, whereas bisDBAs 1c and 1d with the longer alkyl chains are prone to form nonporous structures. However, upon dilution, nonporous structures are typically transformed into porous ones, a trend that can be understood by the effect of surface coverage, molecular density, and intermolecular interactions on the system's enthalpy. Furthermore, porous structures are stabilized by the coadsorption of solvent molecules. The most intriguing porous structure, the Kagome pattern, was formed for all compounds at least to some extent, and the size of its triangular and hexagonal pores could be tuned by the alkyl-chain length. The present study proves that the concentration control is a powerful and general tool for the construction of porous networks at the liquid-solid interface.

We present here the results of studies of the synthesis and properties of donors and acceptors based on triangular dehydrobenzo[12]annulene ([12]DBA) system as a pi core. These studies were aimed at controlling the supramolecular crystal structure. Toward this end, the tricyano[12]DBA 2 and dodecafluoro[12]DBA ( 3) were synthesized as acceptors (A) and the tris(dialkylamino)[12]DBAs 4a-d as donors (D), and their electronic properties were determined by electronic absorption spectroscopy and electrochemical measurements. The main focus, though, was the formation of supramolecular structures in crystals. These compounds form distinct packing patterns as a result of the different intermolecular interactions. Tricyano[12]DBA 2 forms a two-dimensional (2D) sheet structure via hydrogen-bonding interactions, whereas a tilted-stack structure was found for 3 because of the lack of significant intermolecular interactions. Tris(dibutylamino)[12]DBA 4b exhibits a ladder-type 2D structure, probably because of van der Waals interactions between the butyl groups. The most significant finding is that charge-transfer interactions between donor 4a and acceptor 3 combined with their triangular molecular shapes and lateral CH...F hydrogen bonding result in the formation of a 2D rosette structure consisting of two different trimeric (DAD- and ADA-type) sandwich structures with 1:2 and 2:1 A/D ratios, respectively.

Isophlorins and its analogues bridge the structural features between porphyrins and annulenes. Invariably they are known to be unstable but can be stabilized by appropriate substituents in the core and periphery of a macrocycle. Solid-state characterization of these 20pi systems displays planarity for each macrocycle and even in their supramolecular arrangement due to C-H...F-C interactions.

High-resolution 13C NMR techniques were used in order to investigate the "arene-olefin" valence tautomerism between 11,l I-disubstituted 1,6-methano( IO)annulenes and their respective bisnorcaradienes in the solid phase. It was found that the bridgehead carbon resonances of the dimethyl and of the methylcyano derivatives shift with temperature in a manner that suggests the occurrence of an exchange process proceeding fast on the NMR time scale. In all the cases, the crystal packing forces were found to affect considerably the thermodynamic parameters of the kinetic processes. Although it was not possible to reach a low enough temperature so as to observe the resolved resonances of the aromatic and of the olefinic tautomers, the present solid-state NMR results were combined with previous solution NMR studies in order to obtain the free energies involved in the tautomeric processes. Moreover, a good correlation could be established between the solid-state NMR and the X-ray diffraction results of the compounds. Finally, some of the implications that the present work might have for the understanding of valence tautomerism in 1,6-methano( lO)annulenes are briefly discussed.

Two series of cyclobutene-annelated dehydro(n)annulenes (n = 18, 24, 30) were obtained by oxidative coupling of 1,2-diethynyl-1-cyclobutenes. The properties of these macrocycles are analyzed in an experimental and computational study and compared to those of the parent 1,3,7,9,13,15-hexadehydro(18)annulene and 1,3,7,9,13,15,19,21-octadehydro(24)annulene. The analysis of the electronic absorption and {sup 1}H NMR spectra shows that the trimers possess planar diatropic (18)annulene perimeters, while the tetramers possess planar paratropic (24)annulene perimeters. The chromophores in the pentamers are conformationally more flexible, and the diatropic character of these (30)annulenes is strongly reduced. Cyclobutene annelation stabilizes all macrocycles. In particular, the fusion of cyclobutene rings stabilizes dramatically the planar (24)annulene perimeter in the tetramers and is responsible for their preferred formation in the oxidative coupling reactions. AM1 and MM2 computational studies reproduce well the experimentally observed conformational preferences of the parent dehydroannulenes.

A series of novel cyclobutenodehydro(n)annulenes (n = 18, 24, 30) have been prepared as precursors in an organic approach to the cyclocarbons C{sub 18}, C{sub 24}, and C{sub 30}. On the way to these macrocycles, synthetic entries to three new classes of enediynes have been developed. Bis(1-propynyl)cyclopropenone was prepared in the reaction of 1-(trimethylsilyl)-1-propyne with trichlorocyclopropenylium tetrachloroaluminate. The 3,4-dialkynyl-3-cyclobutene-1,2-dionines were prepared by the reaction of 3,4-dichloro-3-cyclobutene-1,2-dione either with (tri-n-butylstannyl)alkynes in the presence of catalytic amounts of Pd(PPh{sub 3}){sub 4} or with the soluble copper (I) acetylides of (trialkylsilyl)acetylenes. The peculiar downfield resonances of the terminal acetylenic carbon atoms in the {sup 13}C NMR spectra of the 3,4-dialkynyl-3-cyclobutene-1,2-diones are discussed. The oxidative Hay coupling of the acetonide of 3,4-diethynyl-3-cyclobutene-1,2-diol or of the bis(ethylene ketal) of 3,4-diethynyl-3-cyclobutene-1,2-dione gave two series of cyclobutenodehydroannulenes with 18{pi}-, 24{pi}-, and 30{pi}-electron perimeters.

Decapentynylcorannulene (5) is prepared via aryl−alkyne coupling chemistry of decachlorocorannulene and trimethylstannylpentyne. The crystal structure of 5 is compared with the ab initio computed structure of decaethynylcorannulene (3). The relative structure and energetics of 3 and its carbon nanotube isomer C40H10 (2) are discussed. Per-alkynyl-polynuclear aromatic hydrocarbons, such as 3, constitute a high energy form of carbon-rich compounds. During the synthesis of 3, enediyne cyclization forms the diradical, which ring-opens to the biscumulenyl [10]annulene (6). Discovery of this process supports the idea that derivatives of 2 could result from corresponding derivatives of 3 through electrocyclic mechanisms and demonstrates a new carbon skeletal rearrangement for strained polynuclear aromatic networks. Derivatives of 3, like 5, warrant further investigation to find viable precursors for monodispersed single-walled carbon-based nanotubes.

For both [4n] and [4n + 2]annulenes, cases of configuration change in which the number of trans CC bonds changes by 1 must involve a Mobius π-bond shifting step. The known configuration change in [14]annulene (1, CTCTCTT → 2, CCTCTCT), for which Δtrans = 1, is thus predicted to involve a Mobius antiaromatic transition state for bond shifting. Using unrestricted density-functional theory (DFT), we have located the key transition state for this process, as well as other stationary points involved in the mechanism. CASPT2(14,14)/cc-pVDZ//(U)BH&HLYP/6-311+G** results provide a barrier of 19.3 kcal/mol for the overall process 1 → 2, in good agreement with the known experimental barrier (21.3 kcal/mol). The bond-shifting transition state has substantial singlet diradical character and is strongly antiaromatic on the basis of its large positive NICS value. [14]Annulene thus represents the first example of a concerted reaction for which a Mobius antiaromatic transition state provides the only possible pathway.