Cyclopentadienyl Cation Research Articles

Substituent Effects on the Electronic Ground State (Singlet versus Triplet) of Indenyl Cations: DFT and CASPT2 Studies

AbstractThe electronic ground states of cations (singlet versus triplet) attract significant attention in the discovery of novel carbocation chemistry. Although the cyclopentadienyl cation exhibits a triplet ground state because of the antiaromaticity of the closed‐shell singlet state, the ground state of the indenyl cation, which is a benzene‐fused derivative of the cyclopentadienyl cation, is reportedly a closed‐shell singlet state rather than a triplet state. In this study, in order to establish a strategy for controlling the ground state of indenyl cations, the substituent effects at the C2 and C5 positions of their structures were computed using density functional theory and complete active space multiconfiguration second order perturbation levels of theory.

Structural characterization and reactivity of a room-temperature-stable, antiaromatic cyclopentadienyl cation salt.

The singlet states of cyclopentadienyl (Cp) cations are considered as true prototypes of an antiaromatic system. Unfortunately, their high intrinsic reactivity inhibited their isolation in the solid state as a salt, and controlled reactions are also scarce. Here we present the synthesis and solid state structure of the room-temperature-stable Cp cation salt [Cp(C6F5)5]+[Sb3F16]-. Although the aromatic triplet state of the [Cp(C6F5)5]+ cation is energetically favoured in the gas phase according to quantum chemical calculations, coordination of the cation by either [Sb3F16]- or C6F6 in the crystal lattice stabilizes the antiaromatic singlet state, which is present in the solid state. The calculated hydride and fluoride ion affinities of the [Cp(C6F5)5]+ cation are higher than those of the perfluorinated tritylium cation [C(C6F5)3]+. Reactions of [Cp(C6F5)5]+[Sb3F16]- with CO, which probably yields the corresponding carbonyl complex, and of radical Cp(C6F5)5∙ with selected model substrates (Cp2Fe, (Ph3C∙)2 and Cp*Al) are also presented.

Benchmarking the semi-stochastic CC(P;Q) approach for singlet-triplet gaps in biradicals.

We recently proposed a semi-stochastic approach to converging high-level coupled-cluster (CC) energetics, such as those obtained in the CC calculations with singles, doubles, and triples (CCSDT), in which the deterministic CC(P;Q) framework is merged with the stochastic configuration interaction Quantum Monte Carlo propagations [J. E. Deustua, J. Shen, and P. Piecuch, Phys. Rev. Lett. 119, 223003 (2017)]. In this work, we investigate the ability of the semi-stochastic CC(P;Q) methodology to recover the CCSDT energies of the lowest singlet and triplet states and the corresponding singlet-triplet gaps of biradical systems using methylene, (HFH)-, cyclobutadiene, cyclopentadienyl cation, and trimethylenemethane as examples.

Singlet/Triplet State Anti/Aromaticity of CyclopentadienylCation: Sensitivity to Substituent Effect

It is well known that singlet state aromaticity is quite insensitive to substituent effects, in the case of monosubstitution. In this work, we use density functional theory (DFT) calculations to examine the sensitivity of triplet state aromaticity to substituent effects. For this purpose, we chose the singlet state antiaromatic cyclopentadienyl cation, antiaromaticity of which reverses to triplet state aromaticity, conforming to Baird’s rule. The extent of (anti)aromaticity was evaluated by using structural (HOMA), magnetic (NICS), energetic (ISE), and electronic (EDDBp) criteria. We find that the extent of triplet state aromaticity of monosubstituted cyclopentadienyl cations is weaker than the singlet state aromaticity of benzene and is, thus, slightly more sensitive to substituent effects. As an addition to the existing literature data, we also discuss substituent effects on singlet state antiaromaticity of cyclopentadienyl cation.

A Cationic NHC-Supported Borole.

This work describes the synthesis and characterization of a highly reactive cationic borole. Halide abstraction with Li{Al[OC(CF3)3]4} from the NHC‐chloroborole adduct yields the first stable NHC‐supported 1‐(MeNHC)‐2,5‐(SiMe3)2‐3,4‐(Ph*)2‐borole cation. Electronically, it features both a five‐membered cyclic conjugated 4 π‐electron system and a cationic charge and thus resembles the yet elusive cyclopentadienyl cation. The borole cation was characterized crystallographically, spectroscopically (NMR, UV/Vis), by cyclovoltammetry, microanalysis and mass‐spectrometry and its electronic structure was probed computationally. The cation reacts with tolane and reversibly binds carbon monoxide. Direct comparison with the structurally related, yet neutral, 1‐mesityl borole reveals strong Lewis acidity, reduced HOMO–LUMO gaps, and increased anti‐aromatic character.

Charge delocalization mode and change in aromaticity of protonated 7‐phenylbenzo[k]fluoranthenes studied by experimental observation and DFT calculations

Abstract7‐Phenylbenzo[k]fluoranthene (4), which is one of non‐alternant polycyclic aromatic hydrocarbons, was protonated at the C(12) position in neat CF3SO3H 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 13C 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 Δδ13C 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.

Benzenium Ion: Aromatic as the π-Complex or Antiaromatic as the σ-Complex Being Somewhat Similar to the Cyclopentadienyl Cation.

The spatial magnetic properties, through-space NMR shieldings (TSNMRSs), of the benzenium cation (C6H7+) 1 and of ±I/M-substituted analogues C6H6X+ 3-8 [X = -Me, -CF3, -NH2, -NO2, -NO, -SiH3] have been calculated using the gauge-independent atomic orbital perturbation method employing the nucleus-independent chemical shift concept, and iso-chemical-shielding surfaces of various sizes and directions have been observed. The TSNMRS values were employed to compare the spatial magnetic properties (TSNMRS) of benzene and the benzenium ion 1 and then further compared with analogues 3-8, to answer the question whether the electronic structures of 1 and 3-8 are still similar to those of aromatic species or somewhat similar to the antiaromatic cyclopentadienyl cation 2, supported by structural data and δ(13C)/ppm values.

Exploiting the Aromatic Chameleon Character of Fulvenes for Computational Design of Baird-Aromatic Triplet Ground State Compounds.

Due to the reversal in electron counts for aromaticity and antiaromaticity in the closed-shell singlet state (normally ground state, S0 ) and lowest ππ* triplet state (T1 or T0 ), as given by Hückel's and Baird's rules, respectively, fulvenes are influenced by their substituents in the opposite manner in the T1 and S0 states. This effect is caused by a reversal in the dipole moment when going from S0 to T1 as fulvenes adapt to the difference in electron counts for aromaticity in various states; they are aromatic chameleons. Thus, a substituent pattern that enhances (reduces) fulvene aromaticity in S0 reduces (enhances) aromaticity in T1 , allowing for rationalizations of the triplet state energies (ET ) of substituted fulvenes. Through quantum chemical calculations, we now assess which substituents and which positions on the pentafulvene core are the most powerful for designing compounds with low or inverted ET . As a means to increase the π-electron withdrawing capacity of cyano groups, we found that protonation at the cyano N atoms of 6,6-dicyanopentafulvenes can be a route to on-demand formation of a fulvenium dication with a triplet ground state (T0 ). The five-membered ring of this species is markedly Baird-aromatic, although less than the cyclopentadienyl cation known to have a Baird-aromatic T0 state.

Paramagnetic ring current effects in anti-aromatic structures subject to substitution/annelation quantified by spatial magnetic properties (TSNMRS)

The spatial magnetic properties, through-space NMR shieldings (TSNMRS), of the typically anti-aromatic cyclopentadienyl cation, cyclobutadiene, pentalene, s-indacene and of substituted/annelated analogues of the latter structures have been calculated using the GIAO perturbation method employing the nucleus independent chemical shift (NICS) concept and visualized as iso-chemical-shielding surfaces (ICSS) of various size and direction. The TSNMRS values were employed to visualize and quantify the dia(para)magnetic ring current effects in the studied compounds. The interplay of dia(para)magnetic ring current effects due to substitution/annelation caused by heavy exo-cyclic n,π-electron delocalization can be qualified.

Systematic design of active spaces for multi-reference calculations of singlet–triplet gaps of organic diradicals, with benchmarks against doubly electron-attached coupled-cluster data

Singlet-triplet gaps in diradical organic π-systems are of interest in many applications. In this study, we calculate them in a series of molecules, including cyclobutadiene and its derivatives and cyclopentadienyl cation, by using correlated participating orbitals within the complete active space (CAS) and restricted active space (RAS) self-consistent field frameworks, followed by second-order perturbation theory (CASPT2 and RASPT2). These calculations are evaluated by comparison with the results of doubly electron-attached (DEA) equation-of-motion (EOM) coupled-cluster (CC) calculations with up to 4-particle-2-hole (4p-2h) excitations. We find active spaces that can accurately reproduce the DEA-EOMCC(4p-2h) data while being small enough to be applicable to larger organic diradicals.

Isolation of an Antiaromatic Singlet Cyclopentadienyl Zwitterion.

The reaction of triplet tetrachlorocyclopentadienylidene with BF3 in rare gas matrices yields a zwitterion consisting of a cyclopentadienyl cation bearing a positive charge and a negatively charged BF3 unit. IR and UV-vis spectra as well as the absence of EPR signals demonstrate a singlet ground state of the zwitterion, and its calculated geometry and magnetic properties clearly reveal a strong antiaromatic character. The zwitterion is highly labile and by visible or IR irradiation rearranges via a 1,2-fluorine migration from boron to carbon. Interaction with a second molecule of BF3 stabilizes the zwitterion and suppresses the fluorine migration, thus providing a convenient and efficient synthesis of an antiaromatic molecule under very mild conditions.

Economical Doubly Electron-Attached Equation-of-Motion Coupled-Cluster Methods with an Active-Space Treatment of Three-Particle-One-Hole and Four-Particle-Two-Hole Excitations.

The previously developed active-space doubly electron-attached (DEA) equation-of-motion (EOM) coupled-cluster (CC) method with up to four-particle-two-hole (4p-2h) excitations [Shen, J.; Piecuch, P. J. Chem. Phys. 2013, 138, 194102], which utilizes the idea of applying a linear electron-attaching operator to the CC ground state of an (N - 2)-electron closed-shell system to generate ground and excited states of the N-electron open-shell species of interest, has been extended to a considerably less expensive model, in which both 3p-1h and 4p-2h terms rather than 4p-2h contributions only are selected using active orbitals. As illustrated by the calculations involving low-lying singlet and triplet states of methylene, trimethylenemethane, cyclobutadiene, and cyclopentadienyl cation and bond breaking in F2, the proposed DEA-EOMCC method with the active-space treatment of 3p-1h and 4p-2h excitations and its lower-level counterpart neglecting 4p-2h contributions are capable of accurately reproducing the results obtained using their considerably more expensive parent counterparts with a full treatment of 3p-1h and full or active-space treatment of 4p-2h excitations.

A new architecture for high spin organics based on Baird's rule of 4n electron triplet aromatics

Due to the absence of open subshells (unlike transition metal compounds), stable high spin organic molecules are rare and are mostly limited to states of low multiplicity. As an alternative to high multiplicity polyradicals and polycarbenes, with their small energetic separation of different spin isomers, it is demonstrated that Baird's rule of 4n electron aromaticity in the triplet electronic state allows, in principle, the design of polycyclic high spin organics with high spin multiplicity in the electronic ground state and a large energetic separation for other spin states. Energy spacing between spin isomers is dictated here by the aromaticity or antiaromaticity of individual cycles (taking into account all π electrons), rather than by a spin Hamiltonian alone (accounting only for unpaired spin electrons). As a proof of concept, dyads of the cyclopentadienyl cation (which has been reported to possess a triplet ground state) have been computationally found to possess a quintet electronic ground state with two ferromagnetically coupled Baird aromatic rings (with SCF-GIAO NICS(0) = -4.6 and -4.4, respectively; "NICS" is "nucleus independent chemical shift") at the CASMP2(8,10)/6-311G*//CASSCF(8,10)/6-311G* level, which is 48.3 kcal mol-1 lower in energy than the C2 open shell singlet with two antiaromatic rings (with NICS = +17.4), and 19.7 kcal mol-1 below the triplet which has one aromatic and one antiaromatic ring, with NICS = -4.8 and +45.0, respectively. Triads of the cyclopentadienyl cation in linear and branched topologies are also proposed to be ground states of maximum spin multiplicity by computations at the DFT and CCSD(T)/6-31G//UB3LYP/6-311G* levels.

Effect of Ring Size and Migratory Groups on [1,n] Suprafacial Shift Reactions. Confirmation of Aromatic and Antiaromatic Transition-State Character by Ring-Current Analysis.

Suprafacial sigmatropic shift reactions of 5-substituted cyclopentadienes, 3-substituted cyclopropenes, and 7-substituted cycloheptatrienes have been studied computationally at the MP2/6-31+G* level for structures and energetics and with the ipsocentric method at the CHF/6-31G** level to calculate current-density maps. The hydrogen shifts in cyclopentadienes have a diatropic ring current indicating aromatic, cyclopentadienide anion character. This result stands in contrast to the fluorine shift in 5-fluorocyclopentadiene which requires much more energy and has a paratropic ring current in the TS pointing to antiaromatic, cyclopentadienyl cation character. [1,3] hydrogen shifts in cyclopropenes are very difficult, passing through transition states that have an extended C-C bond. For 3-fluorocyclopropene, the [1,3] fluorine shift is much easier than the hydrogen shift. For 7-fluorocycloheptatriene, the [1,7] hydrogen shift is predicted but requires very high energy and has a paratropic ring current and antiaromatic character. The [1,7] suprafacial fluorine shift is relatively easy, having a TS with cycloheptatrienyl cation character. Patterns of currents, and the reversal for H and F migration, are rationalized by orbital analysis based on the ipsocentric method. Calculated charges and structural features for reactants and transition states support these conclusions.

ChemInform Abstract: Novel Aromatic and Antiaromatic Systems

AbstractReview: 48 refs.

Novel aromatic and antiaromatic systems.

We contributed to the field of non-benzenoid aromatic compounds by creating the cyclopropenyl cation and various of its derivatives, including cyclopropenone; it was the first aromatic system with other than six pi electrons in a single ring, and the simplest aromatic system. The pioneering work of Tetsuo Nozoe in tropolone chemistry was celebrated with the founding of ISNA, the International Symposium on Non-Benzenoid Aromatic Compounds, where I described our work in the field. It fit the prediction that aromaticity would be found in systems with 4n + 2 pi electrons, where n is an integer. I was also concerned with the properties of monocyclic systems with 4n cyclically conjugated pi electrons. They were expected not to be aromatic, but the interesting question was whether they were actually antiaromatic, especially destabilized by the cyclic conjugation in such 4n species as the cyclopropenyl anion, cyclobutadiene, and cyclopentadienyl cation. The evidence supports antiaromaticity in these cases. We also examined compounds where 4n cyclic pi systems were fused with aromatic systems, and most interestingly systems in which two 4n pi systems were fused. In these cases the periphery of the molecules had 4n + 2 pi electrons, for aromaticity, but the components were antiaromatic. Recently we have studied electrical conductivities in aromatic molecules such as thiophene and saw that aromaticity added resistance to the systems, so non-aromatic compounds are better conductors and antiaromatic compounds are predicted to be the best of all.

Infrared detection of the 1,3-dimethyl cyclopentadienyl cation, an isomer of protonated toluene

Protonated toluene (C7H9+) is a well-known stable carbocation, usually formed by σ-protonation of the parent molecule. Here, the corresponding C7H9+ ion is formed with a pulsed discharge in a supersonic expansion from a norbornene precursor. The different molecular framework of this precursor makes it possible to access different structural configurations. Infrared photodissociation spectroscopy is employed to characterize the C7H9+ ions produced. Protonated toluene is most abundant, however, new bands in the fingerprint and CH stretching regions indicate that another isomer is also present. Computational chemistry makes it possible to identify the 1,3-dimethylcyclopentadienyl cation, a less stable isomer not detected previously.

A computational study on a strategy for isolating a stable cyclopentadienyl cation.

A computational study has been carried out to examine the feasibility of generating a simple monocyclic cyclopentadienyl cation that may be sufficiently stable to isolate and handle at ambient temperatures. Using judicious placement of electron-withdrawing groups (CF3) about the ring we have identified a derivative that may approach the stability of isolobal (and isolatable) borole rings, as evaluated by HOMO-LUMO and singlet-triplet gaps. These Cp(+) derivatives may therefore be an attractive target for synthetic isolation.

High-Level ab Initio Predictions for the Ionization Energy, Electron Affinity, and Heats of Formation of Cyclopentadienyl Radical, Cation, and Anion, C5H5/C5H5+/C5H5–

The ionization energy (IE), electron affinity (EA), and heats of formation (ΔH°f0/ΔH°f298) for cyclopentadienyl radical, cation, and anion, C5H5/C5H5(+)/C5H5(-), have been calculated by wave function-based ab initio CCSDT/CBS approach, which involves approximation to complete basis set (CBS) limit at coupled-cluster level with up to full triple excitations (CCSDT). The zero-point vibrational energy correction, core-valence electronic correction, scalar relativistic effect, and higher-order corrections beyond the CCSD(T) wave function are included in these calculations. The allylic [C5H5((2)A2)] and dienylic [C5H5((2)B1)] forms of cyclopentadienyl radical are considered: the ground state structure exists in the dienyl form and it is about 30 meV more stable than the allylic structure. Both structures are lying closely and are interconvertible along the normal mode of b2 in-plane vibration. The CCSDT/CBS predictions (in eV) for IE[C5H5(+)((3)A1')←C5H5((2)B1)] = 8.443, IE[C5H5(+)((1)A1)←C5H5((2)B1)] = 8.634 and EA[C5H5(-)((1)A1')←C5H5((2)B1)] = 1.785 are consistent with the respective experimental values of 8.4268 ± 0.0005, 8.6170 ± 0.0005, and 1.808 ± 0.006, obtained from photoelectron spectroscopic measurements. The ΔH°f0/ΔH°f298's (in kJ/mol) for C5H5/C5H5(+)/C5H5(-) have also been predicted by the CCSDT/CBS method: ΔH°f0/ΔH°f298[C5H5((2)B1)] = 283.6/272.0, ΔH°f0/ΔH°f298[C5H5(+)((3)A1')] = 1098.2/1086.9, ΔH°f0/ΔH°f298[C5H5(+)((1)A1)] = 1116.6/1106.0, and ΔH°f0/ΔH°f298[C5H5(-)((1)A1')] = 111.4/100.0. The comparisons between the CCSDT/CBS predictions and the experimental values suggest that the CCSDT/CBS procedure is capable of predicting reliable IE(C5H5)'s and EA(C5H5) with uncertainties of ± 17 and ± 23 meV, respectively.

Bondpseudorotation, Jahn‐Teller, and pseudo‐Jahn‐Teller effects in the cyclopentadienyl cation and its pentahalogeno derivatives

AbstractMultireference averaged quadratic coupled cluster (MRAQCC) (4,5)/cc‐pVTZ calculations predict that bond pseudorotation (BPR) in the first excited singlet state of the cyclopentadienyl cation (CPC) proceeds with a barrier of just 0.35 kcal/mol, where five dienylic forms present the minima and five allylic forms the transition states of the pseudorotation process. Vibrational and entropic corrections revert the order of stabilities and lead to a Δ G(298) of just 0.05 kcal/mol indicating that BPR is unhindered at room temperature. The description of the CPC ring in terms of curvilinear deformation coordinates (seven for C5, seven for X5, and three coupling coordinates) make it possible to explore both the six‐dimensional (6D) Jahn‐Teller and the 8D pseudo‐Jahn‐Teller space and assess the importance of Jahn–Teller and pseudo‐Jahn–Teller deformations of the CPC ring. The latter dominate the ring deformations along the BPR path. The only somewhat larger Jahn–Teller contribution results from a E‐symmetrical CCH bending motion. For the perhalogenated CPCs, the dominance of the pseudo‐Jahn–Teller effect increases, however, the total deformation of the D5h‐symmetrical ring decreases and thereby also the stabilization of the 1A1 forms along the BPR path. This leads to a reduction of the BPR barriers to just 0.14 kcal/mol for C5I. For all pentahalogeno CPCs, the dienylic form is more stable both at the energy and free energy level. The use of curvilinear deformation coordinates facilitates the understanding of the electronic features of cyclic (pseudo‐) Jahn–Teller systems. © 2012 Wiley Periodicals, Inc.