Cyclopropenium Research Articles

Novel Polar Intermetallic π-Systems along the Zintl Border

AbstractStudies have shown that complex Zintl phases exhibit a rich diversity of crystal structures. These have also revealed a remarkable success of the Zintl concept in rationalizing stoichiometry, crystal structure and chemical bonding of many main group intermetallics. Still there are unresolved questions about the usefulness of the concept in explaining structure-property relationships in intermetallics near the Zintl border, and as a rational tool in designing new materials. Limitations of the concept are represented by violations often associated with “electron-deficient” phases that contain Group 13 metalloids. Recent investigations on “electron-deficient” Zintl phases containing post transition metals have led to the synthesis of a number of novel inorganic-intermetallic π-systems. Since unique structures and properties are already apparent in normal Zintl phases, it is anticipated that the exploratory synthesis and characterization of conjugated and multiple-bonded inorganic systems will produce not only unusual crystal chemistry but interesting physical properties as well. We report on new complex Zintl phases that include the semiconducting SrCa2In2Ge - which features [In2Ge]6- chains and represents a novel inorganic conjugated π-system analogous to a polyallyl chain with In-In double bonds, and Ca5In9Sn6 - which contains In trimers, [In3]5- analogous and isoelectronic with the aromatic cyclopropenium cation, [C3H3]+. These unusual materials, Zintl π-systems, represent a promising class of electronic materials with a range of potential applications.

Thermochemistry of organic and heteroorganic species. Part IV. Photoionization studies of isomerization and fragmentation of poly-substituted cyclopropenes. 3-Methyl-3-ethynyl- and tetrachlorocyclopropene

Appearance energies of [M – Me]+ ions from p-ethylphenylacetylene, 1-methyl-1-phenylcyclopropane and 3-methyl-3-ethynylcyclopropene and of [M – Cl]+ ions from tetrachlorocyclopropene were measured by photoionization mass spectrometry. The heats of formation 1040.5, 864, 1268 and 1041 kJ mol−1 were obtained for ethynyl- and vinyltropylium and ethynyl- and trichlorocyclopropenium ions, respectively. Using these and some literature data and the series of isodesmic reactions, the heats of formation of seven substituted cyclopropenium ions and four propargylic ions were derived.

The Remarkably Stabilized Trilithiocyclopropenium Ion, C3Li3+, and Its Relatives

The structures and energies of lithiated cyclopropenyl cations and their acyclic isomers (C3H3-nLin+, n = 0−3) have been calculated employing ab initio MO (HF/6-31G*) and density functional theory (DFT, Becke3LYP/6-311+G*) methods. The cyclic isomers (4, 6, 10, and 14) are always favored, but when lithium is substituted sequentially along the C3H3+, C3H2Li+, C3HLi2+, and C3Li3+ series, the acyclic forms (5, 7, 11, 16) become progressively less competitive energetically. A triply bridged c-C3(μ-Li)3+ geometry, 14, was preferred over the classical form 3 by 8.7 kcal/mol. A single lithium substituent results in a very large (67 kcal/mol) stabilization of the cyclopropenyl cation. The favorable effects of further lithium substitution are attenuated but are still large: 48.2 and 40.5 kcal/mol for the second and third replacements, respectively. Comparison with polyamino-substituted cyclopropenyl cations suggest c-C3Li3+ (3 and 14) to be a good candidate for the thermodynamically most stable carbenium ion. The stabilization of the cyclopropenyl cation afforded by the excellent π-donor substituent NH2 (42.8, 33.4, and 23.7 kcal/mol for the first, second and third NH2 groups, respectively) is uniformly lower than the corresponding values for Li substitution. The total stabilization due to two NH2 groups, and a Li (128.2 kcal/mol) is higher than that due to three NH2 groups (99.8 kcal/mol). All the lithiated cyclopropyl radicals are computed to have exceptionally low adiabatic ionization energies (3.2−4.3 eV) and even lower than the ionization energies of the alkali metal atoms Li−Cs (4.0−5.6 eV). The ionization energy of C3Li3• is the lowest (3.18 eV), followed by C3(μ-Li)3• (3.35 eV). The 1H, 6Li, and 13C NMR data of cyclopropenyl cation and its lithium derivatives indicate the carbon, lithium, and hydrogen chemical shifts to increase with increasing lithium substitution on the ring. The computed 1H chemical shifts and the magnetic susceptibility anisotropies as well as the nucleus independent chemical shifts (NICS, based on absolute magnetic shieldings) reveal enhanced aromaticity upon increasing lithium substitution. The B3LYP/6-311+G*-computed vibrational frequencies agree closely with experiment for cyclopropenyl cation and, hence, can be used for the structural characterization of the lithiated and amino species.

Ring currents and magnetic properties of the cyclopropenyl cation and isoelectronic triangular 2π electron systems

Ring currents and magnetic properties of the cyclopropenyl cation and isoelectronic triangular 2π electron systems

Ring currents and magnetic properties of the cyclopropenyl cation and isoelectronic triangular 2π electron systems

A coupled Hartree-Fock method is used to compute the magnetizability tensors and carbon and hydrogen shielding tensors of the isoelectronic series of cyclic molecules consisting of the cations C3 H3 + (cyclopropenyl), C2 H2 N+ (azirinyl), CHN2 + (diazirinyl) and N3 + (triazirinyl) and the neutral molecules C2 H3 B (borirene), CH2 NB (borazirene), N2 BH (boradiazirine), and NB2 H3 (azadiboridine). A distributed-origin method is also used to map the electron current densities induced in these by an external magnetic field. Comparisons are made with the benzene molecule. The computations show that whereas the anisotropy of the magnetizability of benzene is dominated by the contribution of the π current, that in the cyclopropenyl cation is due almost entirely to the valence σ current. The current density maps of the two molecules also differ sharply. The maps for benzene show a localized circulation of σ charge in each of the bonds, and a simple circulation of π charge around the principal axis with maximum density on the carbons of the ring: a π ring current. The maps for cyclopropenyl show a weak π ring current and a circulation of σ charge around the perimeter of the molecules that can be interpreted as a global σ ring current. The corresponding maps for the heterocyclic systems show that the π and σ ring currents are maintained with little change when CH in C3 H3 + is replaced by N, but that the introduction of B results in partial localization of these currents.

Generation of Tris(dialkylamino)cyclopropenyl Radical Dications by Pulse Radiolysis and Redox Potential Determination for the C3(NEt2)3•2+/C3(NEt2)3+ and C3(NC5H10)3•2+/C3(NC5H10)3+ Couples

Pulse radiolysis experiments were carried out with neutral aqueous solutions of cyclopropenyl cations C3(NR2)3+, (R2 = Et2, C5H10) also containing bromide, iodide, or azide, to study possible one-electron transfer rates and equilibria involving C3(NR2)3•2+/C3(NR2)3+ couples. Br2•- oxidized C3(NR2)3+ completely to the corresponding radical dication with rate constants of 2.1 × 108 (R2 = Et2) and 7.4 × 108 L mol-1 s-1 (R2 = C5H10), whereas no reaction was detected between Br- and C3(NR2)3•2+. In contrast, the C3(NR2)3•2+ radical was found to oxidize I-, although no reaction was observed between I2•- and C3(NR2)3+. With azide, however, the system reached equilibrium, N3• + C3(NR2)3+ ⇄ N3- + C3(NR2)3•2+. From the equilibrium constants K = 90 (R2 = Et2) and K = 280 (R2 = C5H10) and the known redox potential E(N3•/N3-) = 1.35 V, we calculate E(C3(NR2)3•2+/C3(NR2)3+) = 1.23 ± 0.04 (R2 = Et2) and 1.20 ± 0.03 V (R2 = C5H10) versus NHE.

Methyl Loss Kinetics of Energy-Selected 1,3-Butadiene and Methylcyclopropene Cations

The kinetics of the metastable dissociation of energy-selected 1,3-butadiene and 3-methylcyclopropene cations to form C3H3+ (cyclopropenyl cation) and CH3 have been investigated by threshold photoelectron photoion coincidence (TPEPICO) time-of-flight mass spectrometry, ab initio molecular orbital calculations, and RRKM statistical theory. Both the experimental results and the molecular orbital calculations indicate that 1,3-butadiene ions lose CH3 by first isomerizing to a higher energy structure (3-methylcyclopropene cation) which can rapidly lose CH3 or isomerize back to the 1,3-butadiene cation. A complete kinetic model of the two-well potential and its three rate constants is necessary to account for the measured dissociation rates as a function of the ion internal energy. At low energies, the dissociation rate is limited by the bond cleavage step, while at higher energies, the bottleneck shifts to the lower energy isomerization step. A fit of calculated RRKM rate constants to the experimental data yields a 0 K isomerization barrier (relative to the 1,3-butadiene ion) of 2.02 ± 0.03 eV and an activation entropy at 600 K of −4 cal K-1 mol-1. The entropy of activation for the dissociation step from 3-methylcyclopropene was found to be +7 cal K-1 mol-1. 3-Methylcyclopropene was found to have an adiabatic ionization energy of 9.28 ± 0.05 eV and a neutral heat of formation of 273 ± 2 kJ mol-1 at 0 K (257 ± 2 kJ mol-1 at 298 K). This is the first experimental determination of this value.

A Free Cyclotrigermenium Cation with a 2pi-Electron System

The reaction of tetrakis(tri-tert-butylsilyl)cyclotrigermene with trityl tetraphenylborate in benzene produces tris(tri-tert-butylsilyl)cyclotrigermenium tetraphenylborate [(tert-Bu3SiGe)3+BPh4-], which can be isolated as a yellow solid that is stable in the absence of air. The crystal structure of the cyclotrigermenium ion reveals a free germyl cation with a 2pi-electron system. The three germanium atoms form an equilateral triangle similar to the carbon analog, the cyclopropenium ion.

A Cyclopropenylidene Approach to Tricarbide Complexes: Synthesis and Structure of [M(CO)5{μ2-C3(OCH2CH3)}Fe(CO)2(Cp)] (M = Cr, Mo, W)

The diethoxycyclopropenylidene complexes [M(CO)5{C3(OCH2CH3)2}] (M = Cr, Mo, W) react with K[Fe(CO)2(Cp)] to give the bimetallic cyclopropenylidene complexes [M(CO)5{μ2-C3(OCH2CH3)}Fe(CO)2(Cp)], bu...

Facile Synthesis of Thiophene Derivatives Using a Cyclopropenyl Cation

Facile Synthesis of Thiophene Derivatives Using a Cyclopropenyl Cation

Thermochemical Assessment of the Aromatic and Antiaromatic Characters of the Cyclopropenyl Cation, Cyclopropenyl Anion, and Cyclopropenyl Radical: A High-Level Computational Study

The aromatic stabilization energy of the cyclopropenyl cation (CH)3+ is assessed with G2 theory by calculating its homodesmotic stabilization energy (247.3 kJ mol-1) and by comparing the ionization energies of the cyclopropenyl radical (6.06 eV) and the cyclopropyl radical (8.24 eV). These data indicate substantial stabilization of the two π-electron system in what is considered the archetypal aromatic cation. The calculated enthalpy of formation of the cyclopropenyl cation is 1074.0 kJ mol-1 and agrees with the experimental estimate of 1075 kJ mol-1. The small stabilization energy of the cyclopropenyl radical (37.4 kJ mol-1) suggests that this radical should not be classified as aromatic, in contrast to earlier suggestions. Our G2-calculated enthalpy of formation of the cyclopropenyl radical (ΔHf 298 = 487.4 kJ mol-1) and its ionization energy are different from experimental estimates and suggest that the experimental values may need to be revised. The most stable structure for the cyclopropenyl anion is...

Determination of the proton affinity and absolute heat of formation of cyclopropenylidene

The proton affinity and absolute heat of formation of cyclopropenylidene (c-C 3H 2) have been derived from the translational energy threshold for endothermic proton transfer from c-C 3H 3 + to ammonia in a flowing afterglow triple quadrupole instrument: c-C 3H 3 + + NH 3 → c-C 3H 2 + NH 4 +. The cyclopropenium cation C 3H 3 + was prepared in a helium flow reactor at room temperature by the reaction of ionized ethylene with acetylene, and from dissociative electron ionization of bromocyclopropane. The kinetic energy dependence of the cross-sections for proton transfer from this ion to ammonia and other neutral amines was characterized in a triple quadrupole mass analyzer. The endothermicity for the reaction with ammonia was determined to be 23.3 ± 1.8 kcal mol −1. Combining this with the known proton affinity (PA) of ammonia (204.0 ± 1.0 kcal mol −1) gives a value for PA(c-C 3H 2) of 227.3 ± 2.1 kcal mol −1. From the measured proton affinity and the known heats of formation of c-C 3H 3 + and the proton, the 298 K heat of formation of cyclopropenylidene is determined to be 119.5 ± 2.2 kcal mol −1. This value is slightly higher than a previous experimental estimate, but is in good agreement with the 298 K heat of formation predicted by high level molecular orbital calculations.

Group 14 Analogs of the Cyclopropenium Ion: Do They Favor Classical Aromatic Structures?

Group 14 Analogs of the Cyclopropenium Ion: Do They Favor Classical Aromatic Structures?

The aromatic diboracyclopropenyl (diboriranyl) anion; CB2H3 −: An ab initio study

The most stable structure of CB2H3−, as established computationally, is the aromatic diboracyclopropenyl (diboriranyl) anion (5), while open-chainC2v, isomer H2BCBH (7) is only 3 kcal/mol higher in energy at the QCISD(T)/6-311 +G**//MP2/6-31+G*+ZPE (HF/6-31 +G*). The 47-kcal/mol barrier between cyclic,5, and open-chain,7, structures suggests that both of them may be observed. The aromatic stabilization energy of the diboriranyl anion (18 kcal/mol) is half the value in the isoelectronic cyclopropenium ion, C3H3+. The computed, by IGLO method (5a), and experimental (6a) chemical shifts,δ(13C) andδ(11B), agree within 4 ppm range. The theoretical vibrational frequencies of the most stable isomers,5 and7, are presented for experimental verification of these species.

An electronic and vibrational study of the cyclopropenyl ion and its fluoroderivatives

This article presents the results of ab initio SCF-MO calculations on both the electronic structure and vibrational spectra of the cyclopropenyl cation (C 3H 3 +) and its fluoroderivatives, C 3H 2F +, C 3HF 2 + and C 3F 3 +. A very simple and unambiguous criterion for choosing the combination of diffuse and polarization functions which, together with the 6311G basis set, best describes the electron distribution in these ions is presented. The electronic structures of the cations are analysed in detail; particular emphasis is given to the analysis of the electronic effects due to successive hydrogen-by-fluorine replacements. The results of vibrational normal mode analysis carried out for all hydrogen-deuterium isotopomers of the studied ions are presented and compared with the available experimental data. The theoretical results are used both to review some band assignments previously proposed for the fluorosubstituted molecules and to give a stronger theoretical foundation to the general interpretation of the vibrational spectra of these compounds.

Synthesis and characterization of 3‐aryl‐2‐(polystyryl)cyclopropenones via cyclopropenium ion substitution on polystyrene

AbstractElectrophilic substitution of cyclopropenium ions on aromatic polymers offers a unique opportunity to introduce polar functionality in a controlled manner to conventional, nonpolar polymers. Phenylcyclopropenone substituted polystyrene with predictable chemical composition and narrow molecular weight distribution were prepared. Size exclusion chromatography (SEC) analysis demonstrated the absence of branching or crosslinking in these functionalized polystyrenes during electrophilic substitution of the parent homopolymer. 13C‐NMR confirmed that the degree of phenylcyclopropenone substitution was both highly efficient and predictable over a broad compositional range. The glass transition temperature (Tg) of the polymers was found to vary linearly with mole % phenylcyclopropenone substitution of the polystyrene. Thermal gravimetric analysis (TGA) indicated that thermal decarbonylation of the appended cyclopropenones occurred at approximately 180°C. Weight loss vs. temperature profiles correlated reasonably well with levels of substitution based on 13C‐NMR analysis, confirming that decarbonylation of the calculated cyclopropenone substituents was the predominant thermal decomposition pathway. © 1995 John Wiley & Sons, Inc.

Boron-Nitrogen Analogues of Cyclobutadiene, Benzene and Cyclooctatetraene

Abstract Iminoboranes RB [tbnd] NR′, isoelectronic with alkynes, cyclooligomerize to give B-N rings (RBNR′)n that are either analogues of cyclobutadienes (n = 2) or benzenes or Dewar benzenes (n = 3) or cyclooctatetraenes (n = 4), depending on the ligand set R/R′ and on conducting the oligomerization either thermally or catalytically. Cyclodimers and cyclotetramers may either undergo reversible interconversions or cyclotetramers are formed irreversibly from cyclodimers. The (4+2) cycloaddition of cyclodimers and iminoboranes yields cyclotrimers. Cyclotrimers may thermally be converted into cyclodimers, and poly(iminoboranes), isoelectronic with polyalkynes, can thermally be depolymerized into cyclotrimers. nido-Cluster derivatives of N2B4H6, arachno-cluster derivatives of N2B3H7, four-membered rings N2B2R3R′, or five-membered rings N2B3R4R′ are isolated, when three-membered rings NB2R3, isoelectronic with cyclopropenyl cations, either dimerize or add aminoboranes H2B=NR′2 or insert nitrenes R′N (from R′N3) or insert iminoboranes R′B [tbnd] NR into the B-B bond, respectively.

ChemInform Abstract: Intramolecular Transfer of Sulfonyl Oxygen to Vinylcarbene Generated in the Reaction of Tris(isopropylthio)cyclopropenyl Cation with Arylsulfinate Salts.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Allyl, amidinium and cyclopropenyl cations from the reactions of primary and secondary amines with pentachlorocyclopropane

The action of secondary amines on c–C3HCl5 is a simple new route to aminocyclopropenyl ions, whereas primary amines cause ring opening to an allyl cation, e.g.[(ButHN)2CCHC(NHBut)2]+ identified crystallographically, which is reversibly protonated at the central carbon to become a bis(amidinium) dication.

Transfer of the carbyne ligand to the CC bond versus alkyne–carbyne–CO coupling in the reactions of [(η5-C5H5)(CO)2MnCPh]+ with electron-rich alkynes

The carbyne cation [(η5-C5H5)(CO)2MnCPh]+1 reacts with bis(diorganylamino)acetylenes by transfer of the carbyne ligand to the C–C bond of the alkynes to give cyclopropenyl cations; in contrast, the reaction with diorganylaminopropyne affords novel η4-carbene complexes by regioselective coupling of the carbyne and one CO ligand with the alkyne.