Cyclopropenium Research Articles

Isolation of cyclopropenylidene-lithium adducts: the Weiss-Yoshida reagent.

Isolation of cyclopropenylidene-lithium adducts: the Weiss-Yoshida reagent.

Structures and G3X energies of C3H3N+ intermediates

AbstractGeometries and energies of 15 stable C3H3N+ structures, potential intermediates in ion–molecule reactions that occur on the C3H3N+ potential energy surface (PES), have been calculated with G3X theory. Accurate energy ordering of the intermediates is needed to understand the initial stage of the reactions. The reactions of N(2D) with cyclopropenyl cation and of HCN and HNC with acetylene cation radical are of interest because they may lead to the formation of cyanoacetylene or some of its isomers in dense interstellar dust clouds. The energetics of the initial phases of these reactions are discussed in light of the calculated energies of reactants and intermediates. The variety of high‐energy intermediates optimized makes it likely that the reactions discussed involve long‐lived complexes and multiple rearrangements. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006

Ligand-Stabilized Aromatic Three-Membered Gold Rings and Their Sandwichlike Complexes

Electronic structure calculations (DFT) suggest that ligand-stabilized three-membered gold(I) rings constituting the core structure in a series of cyclo-Au3L(n)H(3-n) (L = CH3, NH2, OH and Cl; n = 1, 2, 3) molecules exhibit aromaticity, which is primarily due to 6s and 5d cyclic electron delocalization over the triangular Au3 framework (s- and d-orbital aromaticity). The aromaticity of the novel triangular gold(I) isocycles was verified by a number of established criteria of aromaticity. In particular, the nucleus-independent chemical shift, NICS(0), the upfield changes in the chemical shifts for Li+, Ag+, and Tl+ cations over the Au3 ring plane, and their interaction with electrophiles (e.g., H+, Li+, Ag+, and Tl+) are indicative for the aromaticity of the three-membered gold(I) rings. Interestingly, unlike the respective substituted derivatives of cyclopropenium cation and the bora-cyclopropene carbacyclic analogues, the aromatic Au3 rings, although exhibit comparable diatropicity, react with electrophiles in a different way affording 1:1 and 2:1 sandwichlike complexes. The bonding in the three-membered gold(I) rings is characterized by a common ring-shaped electron density, more commonly seen in aromatic organic molecules and in "all-metal" aromatics, such as the cyclo-[Hg3]4- tetraanion. Moreover, the cation-pi interactions in the 1:1 and 1:2 sandwichlike complexes formed upon reacting the Au3 rings with electrophiles, depending on the nature of the cation, are predicted to be predominantly electrostatic (Li+, Tl+) or covalent (H+, Ag+). The 1:2 complexes constitute a new class of sandwichlike complexes, which are expected to have novel properties and applications.

Triatomic EP2 Triangles (E=Ge, Sn, Pb) as μ2:η3,η3‐Bridging Ligands

Divalent Group 14 salts react with the [Na(thf)x]+ derivative of the terminal niobium phosphide anion [PNb{N(Np)Ar}3]− (Np=neopentyl, Ar=3,5-Me2C6H3) to give complexes of the form [(μ2:η3,η3-cyclo-EP2){Nb[N(Np)Ar]3}2] (E=Ge, Sn, and Pb; see structure). The bridging {cyclo-EP2} units in these complexes can be considered as neutral 2π-electron three-membered rings that are isolobal to the cyclopropenium ion.

Theoretical Study of the Reaction of HCl with C3H2+: A Possible Source of Carbon−Chlorine Compounds in Space

A theoretical study of the (ClC 3 H 3 ) + species has been carried out. Two different models, G2 and QCISD(T) at the B3LYP geometries, have been employed. Our calculations predict that the global minimum is an open-chain isomer ClHCCCH 2 +( 2 A), whereas five different open-chain structures are also quite stable, lying about 5.9-13.1 kcal/mol above the ground state. The lowest-lying cyclic isomer, with a three-membered carbon ring, lies about 18.3 kcal/mol higher in energy. These theoretical results allow the development of thermodynamic arguments about the reaction pathways of the process HCI + C 3 H 2 +. For the reaction of HCl with CCCH 2 +, formation of different chlorine-carbon compounds is exothermic and there are several mechanisms leading to these species that are barrier-free. However, production of carbocation compounds (cyclopropenyl cation, c-C 3 H 3 +, and propargyl cation, l-C 3 H 3 +) could be competitive with the formation of chlorine-carbon species. The predicted dominant channels for the reaction with both HCCCH + and c-C 3 H 2 + isomers are l-C 3 H 3 + and c-C 3 H 3 +, respectively. Therefore, only the reaction of HCl with the CCCH 2 + isomer seems to be a possible source of chlorine-carbon compounds in space.

Interaction of C 3H 3+ isomers with molecular nitrogen: IR spectra of C 3H 3+–(N 2) n clusters ( n=1–6)

The intermolecular interaction and microsolvation process of isomeric C 3H 3 + ions in molecular nitrogen are characterized by infrared (IR) photodissociation spectroscopy of C 3H 3 +–(N 2) n complexes ( n=1–6) and quantum chemical calculations ( n=0–4). The rovibrational analysis of the C 3H 3 +–N 2 spectrum unambiguously reveals the presence of (at least) two C 3H 3 + isomers in the ion source, namely the propargyl (H 2CCCH +) and the cyclopropenyl (c-C 3H 3 +) cations. Analysis of the cluster size-dependent vibrational frequency shifts and splittings, the photofragmentation branching ratios, and the results of density functional calculations provides a consistent picture of the microsolvation of c-C 3H 3 + and H 2CCCH + in inert nitrogen. In the most stable c-C 3H 3 +–(N 2) n complexes, the first three N 2 ligands form (nearly) linear and equivalent proton bonds to the three protons of c-C 3H 3 +, leading to highly symmetric planar structures with C 2v ( n=1, 2) and D 3h symmetry ( n=3). After completion of this first solvation subshell at n=3, further N 2 ligands form weaker intermolecular bonds to the C atoms of the nearly planar c-C 3H 3 +–(N 2) 3 ion core. The dissociation energies of the H-bonds and C-bonds in c-C 3H 3 +–(N 2) n are estimated as D 0(H)=900±130 cm −1 and D 0(C)=860±170 cm −1, respectively. In the most stable H 2CCCH +–N 2 complex, the N 2 ligand forms a linear ionic H-bond to the acetylenic CH group of H 2CCCH +, leading to a planar structure with C 2v symmetry. The calculations suggest that the next two ligands bind to the protons of the CH 2 group giving rise to planar structures with C s ( n=2) and C 2v symmetry ( n=3), and these structures are compatible with the observed IR spectra.

Site-specific interaction between hydrocarbon cations and inert ligands: IR spectra of isomeric C3H3+–L dimers (L = Ne,Ar,O2,N2,CO2)

Infrared (IR) photodissociation spectra of isomeric C3H3+–L dimers with the inert ligands L = Ne, Ar, O2, N2, and CO2 are recorded in the spectral range of the C–H stretch fundamentals. At least two C3H3+ isomers are generated by electron impact ionization of allene (H2CCCH2) in the cluster ion source, namely the cyclopropenyl (c-C3H3+) and propargyl (H2CCCH+) cations. Both C3H3+ ions form weakly-bound adducts with all ligands considered and the attraction in these ion–ligand complexes is dominated by induction and electrostatic forces. In general, the intermolecular interaction in both c-C3H3+–L and H2CCCH+–L increases in the order Ne < Ar < O2 < N2 < CO2, in line with the polarizabilities, quadrupole moments, and proton affinities of L. Rovibrational analysis of the ν4 vibrations of c-C3H3+–L involving molecular ligands reveals several competing structural binding motifs, including the formation of H-bonds (L = N2, CO2, O2), C-bonds (L = CO2), and π-bonds (L = O2). The preferred binding site and details of the equilibrium structure strongly depend on L. In contrast, the observed H2CCCH+–L dimers feature all H-bonds between L and the acetylenic proton. Ab initio calculations for c-C3H3+–L and H2CCCH+–L at the MP2(full)/6-311G(2df,2pd) level provide additional information about the intermolecular potential energy surface of these ion–ligand complexes and confirm the interpretation of the experimental IR spectra. Photofragmentation branching ratios observed for c-C3H3+–(CO2)n with n = 1–3 allow the dissociation energy of the intermolecular H-bonds and C-bonds to be estimated as D0(H/C) = 1300 ± 300 cm−1.

Photochemical reactions of substituted cyclopropenium salts.

Photochemical reactions of triphenylcyclopropenium tetrakis(pentafluorophenyl)gallate (TPCPGa) and diphenyl-(2-methoxy-1-naphthyl)-cyclopropenium tetrakis(pentafluorophenyl)gallate (DPMNCPGa) (Chart 1) have been investigated in acetonitrile. Traces of water were required for the photochemical reactions to proceed. The disappearance of both TPCPGa and DPMNCPGa obeys zero-order kinetics with rate constants (k) having a linear dependence on the concentration of water. Electron-transfer from water to the cyclopropenium cation is proposed as the primary process in the formation of the cyclopropenyl radical. The latter dimerizes leading to the photoproducts.

Gas-Phase Synthesis and Characterization of an Azaphosphirenium Ion: The First N,P-Analogue of the Aromatic Cyclopropenyl Cation

The 3-(dimethylamino)-1,1-dimethyl-1H-azaphosphiren-1-ium ion (3), the first member of the hitherto unknown class of azaphosphirenium ions and the first N,P-analogue of the aromatic cyclopropenyl cation, is generated in the gas phase via 70 eV electron ionization and demetalation of a ferriphosphaalkene (1) and rapid cyclization of the incipient phosphavinyl cation (2). Spontaneous cyclization of 2 to 3 is predicted by Becke3LYP/6-311++G(d,p) calculations, and the structure and reactivity of 3 is probed via collision-induced dissociation and ion−molecule reactions performed via pentaquadrupole mass spectrometry. Gaseous 3 fails to coordinate efficiently with nitrogen and oxygen nucleophiles, but it forms stable adduct ions with phosphorus and sulfur nucleophiles. The adducts of 3 with P and S nucleophiles are likely favored by the incorporation of relatively strong and rare PS and PP bonds and through extensive charge delocalization involving dimethylamino substituents. With isoprene, an adduct ion is also readily formed presumably via [4 + 2+] cycloaddition (followed by spontaneous ring opening), a reaction which is well documented for phosphenium ions both in solution and in the gas phase.

Electronic and infrared spectra of H2C3H+ and cyclic C3H3+ in neon matrices

The A 1A′←X 1A1 electronic transition of the propargyl cation H2C3H+ with the origin band at 267.8(2) nm has been identified in a neon matrix at 5 K. The frequencies of the two modes excited in the upper state are 667(50) and 1629(50) cm−1 and imply a reduction of symmetry from C2v in the ground state to Cs in the excited state. The most intense IR mode of the propargyl cation is observed at 2079.9(1.0) cm−1 and for the cyclopropenyl cation at 3130.4(1.0) cm−1. Ab initio calculations on the excited states of the two isomer cations support the assignment and explain why the electronic transition could not be observed for the cyclic species; it lies below 200 nm. The A 2A″←X 2B1 and B 2A′←X 2B1 absorptions of the neutral propargyl radical have also been observed with origin bands at 351.9(2) and 343.0(2) nm, respectively. These results provide the basis for the study of these astrophysically interesting C3H3+ species in the gas phase.

Thermochemistry of Organic and Heteroorganic Species. Part IX. Photoionization Studies of Isomerization and Fragmentation of Polysubstituted Cyclopropenes: Phenyl-Substituted Cyclopropenes

Ionization and appearance energies of some fragment ions from 1,2,3-trimethy1-3-phenyl-, 3-methyl-1,2,3-triphenyl-, 1,2-diphenyl-3-methoxycarbonyl-, 1,2,3-triphenyl-3-methoxycarbonyl- and 1,3,3-triphenyl-2-methoxycarbonyl-cyclopropenes were measured by photoionization mass spectrometry. It was shown that in none of these compounds did the fragment ions possess the expected stable substituted cyclopropenium ion structure. Accordingly, possible schemes of molecular ion isomerization are given. The enthalpies of formation of nearly 50 substituted cyclopropenium ions, and ions of related structure, were estimated using series of isodesmic reactions. This publication, together with the previous works of the authors in this Journal, demonstrates the general methodology for estimation of the enthalpy of formation for even-electron ions. It is suggested that the present methodology can provide a good alternative to other estimation or computation procedures applied to the thermochemistry of ions.

Ab Initio MO Study of Diverse Si3H3+Isomers

Structures and energies of many Si3H3+ isomers were investigated theoretically at the MP2/6-31G* level. The global minimum was the classical aromatic planar D3h structure (5). Isodesmic equations indicate the resonance stabilization energy to be half that of the analogous cyclopropenyl cation. The next lowest energy minimum, with a divalent silicon and a bridging hydrogen, also exhibits the 2π aromaticity. Five planar Si3H3+ isomers display cyclic three-center−two-electron (3c−2e) delocalization, and eight minima have 3c−2e Si−H−Si bridged bonds. The planar tetracoordinated silicon and five-coordinated silicon also are represented. Eleven other minima were found within a 46 kcal/mol range. An H-bridged C3v structure, derived from B3H6+, is 42.1 kcal/mol above the global minimum. However, for Ge, Sn, and Pb these A3H3+ forms are more stable than the classical structures (5, J. Am. Chem. Soc. 1995, 117, 11361). In contrast to Si3H3+, C3H3+ has only four isomers in the 189 kcal/mol range. The silicon analogues of the C3H3+ acyclic structures, the prop-2-en-1-yl-3-ylidene cation and the 1-propynyl cation, are not favorable.

Computational study of the singlet cyclopropenyl and triplet cyclopropynyl cations aromatic stabilization with Petersson's complete basis set ab initio approach

An extensive computational study of three-carbon hydrocarbon neutral and carbocation isomers were investigated with Petersson's complete basis set ab initio methods with the aim being to evaluate stabilization through conjugation, as well as through aromatization. Several approaches were used, varying from comparison of the MP2/6-31G(d′) computed structural properties to computing aromatization energies. Where ever possible, the computed energies are compared to experimental data to demonstrate the high reliability of the chosen computational approach. The aromatic character of the cyclopropenyl cation and the triplet cyclopropynyl cation was demonstrated through their bond order uniformity, hydride ion affinities, frontier molecular orbital energy gaps, electron affinities, relative hydrogenation energies, and through newly computed aromatization energies. New thermodynamic values, such as the enthalpy of formation for all the three-carbon carbocations are computed with a computational error of ±2–3kcal/mol.

High level ab initio study of singlet and triplet cyclopropene and diazocyclopropene carbenes stability in comparison with stability of singlet and triple cyclopropenyl cation and anion

Complete basis set ab initio computational studies were performed with the target being to evaluate the aromatic character (stability) of cyclopropene, diazocyclopropene, and diazocyclopropane carbenes. Their stability was evaluated through the computed structural parameters, the evaluated enthalpies of hydrogenation, frontier molecular orbital energy gaps, and triplet–singlet energy differences. All of these results are compared to the same study performed on the well known aromatic cyclopropenyl cation and the antiaromatic cyclopropenyl anion. It was demonstrated that both singlets, cyclopropene and diazocyclopropene carbenes are aromatic, while the diazocyclopropene hydrogenated isomer should be aromatic both as a singlet and as a triplet.

The cyclo-C 3 ligand: trimetallic cyclopropenium complexes of Group 6–8 metals, including the X-ray crystal structure of [{Fe(CO) 2(Cp)} 3(μ 3-C 3)][SbF 6

Several trimetallic cyclopropenium cations have been prepared via reactions of three equivalents of a metallate anion with [C 33Cl 3][SbF 6]. The X-ray crystal structure of [{Fe(CO) 2(Cp)} 3(μ 3-C 3)][SbF 6] ( 1) shows a nearly equilateral C 3 ring with an iron center bonded to each vertex. The cyclopropenium complexes [{Ru(CO) 2(Cp)} 3(μ 3-C 3)][SbF 6] ( 2), [{Mo(CO) 3(Cp)} 3(μ 3-C 3)][SbF 6] ( 3), [{W(CO) 3(Cp)} 3(μ 3-C 3)][SbF 6] ( 4), and [{Re(CO) 5} 3(μ 3-C 3)][SbF 6] ( 5) were spectroscopically characterized. Because of the 3-fold symmetrical nature of these complexes, their NMR spectra display a single cyclo-C 3 ring carbon resonance and a single set of resonances for the metal moiety. Compound 1 reacts with Li[BHEt 3] and LiMe to give complex reaction mixtures. We were not able to add a fourth metal center to the C 3 ring of 1. The spectroscopically characterized C 9 complex [{Fe(CO) 2(Cp)} 3{μ 3-C 3(CC) 3}][SbF 6] ( 6), prepared by the reaction of three equivalents of [Fe(CCSiMe 3)(CO) 2(Cp)] with [C 33Cl 3][SbF 6], was not stable enough to isolate in analytically pure form.

Thermochemistry of organic and heteroorganic species. Part V. Photoionization studies of isomerization and fragmentation of polysubstituted cyclopropenes: 3-methyl-3-acetyl- and related compounds

Appearance energies for molecular and some fragment ions from 3-methyl-3-acetyl-, 1,2-dimethyl-3-acetyl-, 1,2-dimethyl-3-hydroxycarbonyl- and 1,3-dimethyl-2-(1-methyl)vinylcyclopropenes and model compounds 2,3-dimethylfuran and 1-methylindene were measured by photoionization mass spectrometry. It was shown that, in all cases, the structure of the [M – Me]+ ion does not correspond to the expected structures of substituted cyclopropenium ions. Possible schemes for formation of [M – Me]+ ions are given. Isodesmic reactions were systematically applied for calculation of the heats of formation for 35 possible C5H5O+ isomers from 3-methyl-3-acetyl-cyclopropene. In addition, the heats of formation for more than 30 other unsaturated ions and free radicals were estimated with the help of series of isodesmic reactions. A new value of 160 kJ mol−1, as compared with the currently used value of 215.5 kJ mol−1, for the cyclopropylmethyl free radical is suggested. The publication represents the further development of th...

Ketone coordination chemistry of the diborane t-BuCH=C[B(C6F5)2]2

Reactions of the bifunctional borane t-BuCH=C[B(C6F5)2]2, 1, with simple Lewis bases were investigated to probe its Lewis acidity. Ethereal bases (Et2O, THF) bind very weakly, if at all, as no adducts were detectable in solution. Certain ketones, however, were observed to bind to 1. Acetone forms an adduct, 2, reversibly in solution (Keq at 293 K = 69.1), and the temperature dependence of this equilibrium yield thermodynamic parameters of H° = -59.5 ± 2.0 kJ mol-1; S° = -167 ± 4 J K-1 mol-1. Low temperature NMR spectroscopy suggests that the acetone binds to the more sterically accessible boron center trans to the t-Bu group. Ketone binding is strengthened when 2,3-diphenylcyclopropenone is reacted with 1, irreversibly forming an isolable adduct, 4, in high yield (95%). The availability of a resonance form incorporating the aromatic cyclopropenyl cation contributes to the added stability of this adduct in comparison to 2.Key words: organoboranes, diboranes, ketone complexes.

Application of the orbital deletion procedure (ODP) to planar carbocations

An orbital deletion procedure (ODP), which inactivates a critical π orbital and therefore interrupts cyclic conjugation, and the nucleus independent chemical shifts (NICS) have been employed to evaluate the aromaticity of the tropylium, cyclopropenyl and benzyl cations, as well as the antiaromaticity of the singlet cyclopentadienyl cation, compared with their reference cations: 1,3,5-heptatrienyl, allyl, 1,3,5-hexatrienyl-3-carbinyl and 1,3,5-pentadienyl. The aromatic stabilization energies, defined as the difference in the delocalization energies (deduced from ODP) between the cyclic and the acyclic reference cation, are in general agreement with the values from appropriate isodesmic equations.

Formation of 1,1,2,3,3‐pentakis(arylthio)‐1‐propenes from tris(arylthio)cyclopropenyl cations and their conversion into 1,1,2,5,6,6‐hexakis(arylthio)‐(3E)‐1,3,5‐hexatriene

The reaction of tetrachlorocyclopropene (1) with arenethiols (2a–e), followed by treatmentwith perchloric acid, gave tris(arylthio)cyclopropenylium perchlorates (3a–c and e), 1,1,2,3,3-pentakis(arylthio)-1-propenes (4a–d), and 2,3,3-tris(arylthio)propenals (5a–d). The structures of tris(phenylthio)cyclopropenylium perchlorate (3a), 1,1,2,3,3-pentakis(phenylthio)-1-propene (4a), and 2,3,3-tris(o-tolylthio)propenal (5b) were analyzed by single-crystal X-ray diffraction studies. The yields depended significantly on the electron-withdrawing property of the substituents of the arenethiols and the molar ratio of 2 to 1. The reaction with 2,6-dimethylbenzenethiol (2e) gave only tris(2,6-dimethylphenylthio)cyclopropenylium perchlorate (3e) without the formation of 4e and 5e. Compounds 5a–d were produced by acid hydrolysis of 4a–d. Pyrolysis of 4a–d gave (3R,4S)-1,1,2,3,4,5,6,6-octakis(arylthio)-1,5-hexadienes (9a–d) and 1,1,2,5,6,6-hexakis(arylthio)-(3E)-1,3,5-hexatrienes (10a–d) together with diaryl disulfides (11a–d). Compound 10a was also produced by photolysis. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:387–397, 1998

Formation of 1,1,2,3,3-pentakis(arylthio)-1-propenes from tris(arylthio)cyclopropenyl cations and their conversion into 1,1,2,5,6,6-hexakis(arylthio)-(3E)-1,3,5-hexatriene

The reaction of tetrachlorocyclopropene (1) with arenethiols (2a–e), followed by treatmentwith perchloric acid, gave tris(arylthio)cyclopropenylium perchlorates (3a–c and e), 1,1,2,3,3-pentakis(arylthio)-1-propenes (4a–d), and 2,3,3-tris(arylthio)propenals (5a–d). The structures of tris(phenylthio)cyclopropenylium perchlorate (3a), 1,1,2,3,3-pentakis(phenylthio)-1-propene (4a), and 2,3,3-tris(o-tolylthio)propenal (5b) were analyzed by single-crystal X-ray diffraction studies. The yields depended significantly on the electron-withdrawing property of the substituents of the arenethiols and the molar ratio of 2 to 1. The reaction with 2,6-dimethylbenzenethiol (2e) gave only tris(2,6-dimethylphenylthio)cyclopropenylium perchlorate (3e) without the formation of 4e and 5e. Compounds 5a–d were produced by acid hydrolysis of 4a–d. Pyrolysis of 4a–d gave (3R,4S)-1,1,2,3,4,5,6,6-octakis(arylthio)-1,5-hexadienes (9a–d) and 1,1,2,5,6,6-hexakis(arylthio)-(3E)-1,3,5-hexatrienes (10a–d) together with diaryl disulfides (11a–d). Compound 10a was also produced by photolysis. © 1998 John Wiley & Sons, Inc. Heteroatom Chem 9:387–397, 1998