Ion 1b Research Articles

Transition Metal Carbonyl Complexes of an N-Heterocyclic Carbene Stabilized Silyliumylidene Ion.

Silyliumylidene ions-Si(II) cations with a stereochemically active lone pair of electrons-can act as ligands in transition metal complexes comparable to silylenes. However, no investigations concerning their donor abilities have been carried out. Carbonyl complexes lend themselves exceptionally well to determine the donor/acceptor strength of various ligand systems. We now report the synthesis of novel group 6 and group 8 transition metal carbonyl complexes 2-5 of N-heterocyclic carbene (NHC) stabilized silyliumylidene ion 1b, including the first Cr, Mo, and Fe complexes of a Si(II) cation. The complexes were fully characterized by multinuclear NMR and IR spectroscopy and SC-XRD studies. A combination of experimentally determined IR bands together with theoretical calculations revealed weak σ-donor properties and a negligible π-acceptor ability of NHC-stabilized Si(II) cation 1b.

The Ion‐Like Supersilylium Compound tBu3Si–F–Al[OC(CF3)3]3

AbstractThe ion‐like silylium compounds tBu3Si–F–Al[OC(CF3)3]3 and Me3Si–F–Al[OC(CF3)3]3 were prepared by degradation of the halonium salts [R3Si–X–SiR3][Al{OC(CF3)3}4] {1a(X): R = tBu, X = Br, I; 1b(X): R = Me, X = Br, I}. The bromonium and iodonium salts 1a(Br), 1a(I), 1b(Br), and 1b(I) were quantitatively obtained from R3SiX (R = Me, tBu) and [Ag][Al{OC(CF3)3}4] in dichloromethane at –50 °C. However, the related fluoronium and chloronium salts, [R3Si–X–SiR3][Al{OC(CF3)3}4] {1a(X): R = tBu, X = F, Cl; 1b(X): R = Me, X = F, Cl}, could not be generated under these conditions. Generally, at low temperatures (< –50 °C) the halonium salts 1a(Br), 1a(I), 1b(Br), and 1b(I) are stable compounds. However, at higher temperatures 1a(Br), 1a(I), 1b(Br), and 1b(I) undergo R3SiX (R = Me, tBu; X = Br, I) elimination to form the highly reactive silyl cations [R3Si]+ (R = Me, tBu). Two different decomposition pathways were observed in the thermolysis of halonium compounds 1a(Br), 1a(I), 1b(Br), and 1b(I): (1) the silylium cations [R3Si]+ reacted with dichloromethane, forming 1a(Cl) as well as 1b(Cl); (2) the silylium cations [R3Si]+ degraded the counteranion to give tBu3Si–F–Al[OC(CF3)3]3 and Me3Si–F–Al[OC(CF3)3]3 along with epoxide C4F8O. Both ion‐like silylium compounds could be isolated, and single crystals of tBu3Si–F–Al[OC(CF3)3]3 (orthorhombic, Pnma) as well as Me3Si–F–Al[OC(CF3)3]3 (orthorhombic, P212121) were grown from dichloromethane at room temp. Supersilylium [tBu3Si]+ has higher Lewis acidity than [Me3Si]+, as demonstrated by the reaction of 1a(I) with Me3SiF. Thereby the fluoronium ion 1b(F), along with tBu3SiF and tBu3SiI, was formed.

Direct Detection of a Transient Oxenium Ion in Water Generated by Laser Flash Photolysis

Laser flash photolysis of the quinol ester 2b in O2-saturated aqueous phosphate buffer at pH 7.1 with excitation at 266 nm generates a transient intermediate with lambda(max) 460 nm that decays in a first-order manner with an aqueous solution lifetime of (170 +/- 10) ns at 22 degrees C. This intermediate is not affected by O2, but reacts rapidly with N3- with an apparently diffusion-limited rate constant of (6.6 +/- 0.2) x 10(9) M-1 s-1. Steady state photolysis of 2b yields the quinol 3b as a major reaction product with a yield of ca. 30-35% after correction for photolytic decomposition of 3b. This is the same product that is quantitatively produced by hydrolysis of 2b in the dark. Photolysis of 2b in the presence of 40 mM N3- completely suppresses the yield of 3b The photolytic intermediate is identified as the aryloxenium ion 1b, that was previously indirectly detected by N3--trapping during the hydrolysis of 2b, based on the chemical behavior of the transient and the quantitative agreement of the N3-/solvent selectivity ratio, kaz/ks, measured directly during the flash photolysis experiment, and indirectly by the azide clock procedure during the hydrolysis reaction. Other, as of yet unidentified, transients are produced during the photolysis reaction. A strong transient absorbance band observed at 360 nm decays in a biphasic manner with two first-order rate constants, neither of which are affected by O2 or N3-. The lifetimes of the two intermediates of ca. 12 and 75 mus are considerably longer than that of 1b. Another very short-lived species can be detected at early reaction times (<or=20 ns) in Ar-saturated buffer. This species is suppressed, but not completely eliminated, in O2-saturated buffer.

Unimolecular reactions of halogeno phenylarsenium ions: Kinetic energy release during the elimination of halogen hydride

The four dihalogeno phenylarsanes C 6H 5AsF 2, 1, C 6H 5AsCl 2, 2, C 6H 5AsBr 2, 3, and C 6H 5AsI 2, 4, produce in the 70 eV-EI mass spectra by loss of a halogen atom abundant halogeno phenylarseniun ions C 6H 5As +–X, 1a +– 4a +. The further fragmentation reactions of ions 1a +– 4a + are elimination of a molecule halogen hydride HX and/or loss of a halogen atom X . The preferred route of fragmentation depends clearly on the strength of the As–X bond. The metastable fluoro ion 1a + and chloro ion 2a +, respectively, fragment only by loss of HF and HCl, the metastable bromo derivative 3a + exhibits losses of HBr and Br of about equal intensity, and the metastable iodo ion 4a + fragments only by loss of an I atom. The loss of HX is associated with a large kinetic energy release (KER) which yield a dish-topped peak in the MIKE spectrum of 1a + (〈 T〉 = 845 meV) and 2a + (〈 T〉 = 550 meV) and a broad round-topped peak on the MIKE spectrum of 3a + (〈 T〉 = 369 meV). Theoretical calculations (UBHLYP/6-311 + G(2d,p)/-/UBHLYP/6-31 + G(d)) confirm that the elimination of HX requires an enthalpy of activation Δ H #, and that in the case of 3a + this Δ H # and the reaction enthalpy for loss of Br are of similar size. The Δ H # of HX elimination is also responsible for the reverse enthalpy of activation Δ H rev # and the KER during this process. The observed 〈 T〉 amounts to 78% ( 1a +), 55% ( 2a +), and 40% ( 3a +) of the calculated value of Δ H rev # . Thus, the KER as well as the distribution of the KER (KERD) and the competition between elimination of HX and loss of X are very specific for the halogen ligand at the As atom. Metastable (4-fluorophenyl) arsenium ions 1b +, F–C 6H 4As +–H, and (4-chlorophenyl) arsenium ions 2b +, Cl–C 6H 4As +–H +, eliminate HF or HCl with virtually identical KERD as metastable 1a + or 2a +, C 6H 5As +–Cl, proving an identical transition state for both isomers. Accordingly, 1b + and 2b + rearrange to 1a + and 2a + prior to HCl elimination. Such a rearrangement by a reductive elimination/oxidative insertion of As + into C H and C X bonds has been observed before and appears to be typical of arylarsane radical cation and arylarsenium cations.

Protonated silanoic acid HSi(OH)2+ and its neutral counterpart: a tandem mass spectrometric and CBS-QB3 computational study

Protonated silanoic acid, HSi(OH)2+, 1a+, is cleanly generated by the dissociative electron ionization of triethoxysilane, HSi(OC2H5)3, and tetraethoxysilane, Si(OC2H5)4. This follows from tandem mass spectrometric experiments and CBS-QB3 model chemistry calculations. The calculations predict that 1a+ (ΔHf(298 K) = 205 kJ mol−1) is separated by high barriers from its isomers HOSiOH2+, 1b+ and HSi(O)OH2+, 1c+. Low-energy (metastable) ions 1a+ dissociate by loss of H2O via the pathway 1a+ → 1b+ → SiOH+ + H2O. Analysis of the metastable peak for this process confirms that the isomerization step 1a+ → 1b+ is rate determining. The calculations further predict that the incipient ions 1b+ communicate via a low barrier with the proton-bound dimer SiO···H···OH2+, 1d+. This dimer ion is much lower in energy than its counterpart OSi···H···OH2+, 1e+, which is calculated to be only marginally stable. A comparison of the potential energy diagram for the silicon-containing ions 1a+–1e+ with that of their carbon analogues reveals that the dissociation chemistries of HSi(OH)2+ and HC(OH)2+ are only superficially similar. Neutralization–reionization experiments confirm the theoretical prediction that the HSi(OH)2• radical (ΔHf(298 K) = −455 kJ mol−1) is a stable species in the rarefied gas phase. However, owing to a mismatch of Franck–Condon factors a large fraction of the neutralized ions dissociates by loss of H• yielding Si(OH)2. Copyright © 2004 John Wiley & Sons, Ltd.

The remarkable decarbonylation of CH 3OPO [rad]+ and its distonic isomer CH 2OPOH [rad]+: an experimental and CBS-QB3 computational study

The dissociation chemistry of the title ions was investigated using tandem mass spectrometry-based experiments in conjunction with isotopic labeling and computational quantum chemistry. Upon collisional activation, ions CH 3OPO + ( 1a) and CH 2OPOH + ( 1b) readily lose CH 2O. This reaction is completely suppressed in the low-energy (metastable) ions, which instead abundantly lose CO. The product ion generated in this decarbonylation reaction is the ylide ion HPOH 2 +, rather than its more stable isomer H 2POH +. Remarkably, the oxygen atoms become equivalent in this reaction: ions CH 3 18 O P O + and CH 2 O P 18 OH + lose C 18 O and C 16 O in a 1:1 ratio. The experiments further show that the dissociating ions have a fairly large internal energy content: up to 40 kcal/mol for the distonic ion 1b, which, along with the “enol” ion CH 2P(OH)O +, represents the global minimum of the CH 3O 2P + potential energy surface. The mechanism of this intriguing decarbonylation was studied, using the CBS-QB3 model chemistry to probe the structure and energetics of potential intermediates and their interconversion barriers. It appears that metastable ions 1a and 1b have sufficient internal energy to communicate with a great many isomers. Of these, the cyclic ion P[OCH 2O(H)] + and ion [CH 2OH⋅⋅⋅OP] +, a hydrogen-bridged radical cation (HBRC), play a key role in the oxygen equilibration. The HBRC’s [HOP⋅⋅⋅HC(H)O] + and [HPO(H)H⋅⋅⋅CO] + are key intermediates in the actual mechanism for the decarbonylation. The latter ion may lose CO to produce HPOH 2 + or else isomerize into the ion–dipole complex [H 2O⋅⋅⋅P(H)CO] +, whose dissociation into H 2O+HPCO + accounts for the observed minor loss of water.

Tautomerization and dissociation of ethylene phosphonate ions [OCH 2CH 2O]P(H)O [rad]+: an experimental and CBS-QB3 computational study

The unimolecular gas-phase chemistry of the cyclic title ion, [OCH 2CH 2O]P(H)O +, 1a +, and its tautomer ethylene phosphite, [OCH 2CH 2O]POH +, 1b +, was investigated using mass spectrometry-based experiments in conjunction with isotopic labelling and computational quantum chemistry, at the CBS-QB3 level of theory. A facile tautomerization of the “keto” ion 1a + into its more stable (by 34 kcal/mol) “enol” isomer 1b + is prevented by a substantial 1,2-H shift barrier (14 kcal/mol relative to 1a +). In line with this, the collision-induced dissociation (CID) and neutralization-reionization (NR) spectra of the two isomers are characteristically different. Unlike the corresponding acyclic dimethyl phosphonate/phosphite tautomers, (CH 3O) 2P(H)O +/(CH 3O) 2POH +, where the phosphonate isomer rapidly loses its structure identity by a facile distonicization into CH 2O(CH 3O)P(H)OH +, the barrier for this reaction in 1a + is prohibitively high and the cyclic distonic 1,2-H shift isomer [OCH 2CH 2O(H)]PO +, 1c +, is not directly accessible. The 1,2-H shift barrier separating 1a + and 1b + is calculated to lie close to the thermochemical threshold for the formation of C 2H 4 ++HOP(O) 2. This reaction dominates the closely similar metastable ion (MI) spectra of these tautomers. At these elevated energies, the “enol” ion 1b + can undergo ring-opening by CH 2O or CH 2CH 2 cleavage, yielding ion–dipole complexes of the type [C 2H 4] +/HOP(O) 2, 1e +, and H-bridged radical cations CH 2 O ⋯[ H O P OCH 2] + , 1f +, respectively. Moreover, communication of 1b + with the distonic ion 1c + now also becomes feasible. These computational findings account for the similarity of the MI spectra and provide a rationale for the observation that in the losses of CO, HCO and C 2H 3O from metastable ions [ OCH 2 CH 2 O] P( H) 18 O + and [ OCH 2 CH 2 O] P 18 OH + , the 18 O -atom loses its positional identity. Theory and experiment yield a consistent potential energy profile for the cyclic phosphonate/phosphite system showing that non-dissociating ions 1a + retain their structure identity in the microsecond time-frame. However, the interaction of 1a with a benzonitrile (BN) molecule in a chemical ionization type experiment readily yields the more stable “enol” type ion 1b +. Experiments with benzonitrile-d 5 support the proposal that this interaction does not involve the lowering of the 1,2-H shift barrier between the tautomers, via a proton-transport catalysis type mechanism. Rather, a “quid-pro-quo” mechanism is operative, analogous to that proposed for the benzonitrile-assisted enolization of acetamide [Int. J. Mass Spectrom. 210/211 (2001) 489].

Exploration of isomeric bis(phosphanyl)-substituted carbenium ions: identification and synthesis of the most stable isomer

Isomeric bis(phosphanyl) carbenium ions have been investigated by theoretical [MP2/6-31G(d,p) level of theory] and experimental means. On the potential energy surface (PES) of phosphorus cations of the formula [P2CH5]+, five stable isomers are found. Their structures follow the stability order (ΔErel/kcal mol−1): P-protonated diphosphiranium, [(H2P)(HP)CH2]+ 1a (0.0) > C-phosphonio phosphaalkene, [H3P–HCPH]+ 2a (4.5) ≫ bis(phosphanyl) carbenium, [CH(PH2)2]+ 3a (18.5) > C-phosphanyl methylenephosphonium, [H2P–CHPH2]+ 4a (22.6) ≫ diphosphirenium, 5a (47.1). The stability order is very different for the family of the four related stable [P2CH(NH2)4]+ ions 1b, 2b, 4b, 5b in which the phosphorus-bound hydrogens (and one carbon-bound hydrogen in the case of 1b) have been replaced by NH2 groups: 2b (−39.6) ≫ 1b (0.0) > 5b (2.2) ≥ 4b (3.7). None of these ions has a structure comparable to the classical amidinium ion [HC(NH2)2]+. While all amino carbenium ions have a positively charged carbon atom, all phosphorus analogues carry a negative charge on carbon. In some, even the p(π)-population exceeds unity suggesting that these should be considered as cationic methanides. Experimentally, a derivative of the diphosphiranium ion 1a could be prepared by methylation of 1,2-bis-(2,4,6-tri-tert-butylphenyl)diphosphirane with MeO3SCF3, and this new compound 8 was characterized by NMR methods. Although the most stable member in this family, it could only be observed under stable ion conditions (T < −40 °C, five-fold excess of MeO3SCF3) and decomposes at higher temperatures.

Influence of Chain Length on the Electrophilic Reactivity of Carbocations

The electrophilic reactivities of the bis(p-methylphenyl)carbenium ion 1b+ and its macromolecular analogue 1a+ have been compared in slow reactions with allylsilanes and in fast reactions with sily...

Kinetics of the Reactions of Flavylium Ions with π-Nucleophiles

The kinetics of the reactions of the flavylium ion 1a and the 4′-methoxyflavylium ion 1b with various π-nucleophiles and tributylstannane were investigated photometrically in dichloromethane. Electrophilicity parameters E(1a) = −3.46 and E(1b) = −4.96 were derived from the equation log k (20 °C) = s(E + N); these allow the prediction of potential reaction partners of the flavylium ions 1a and 1b.

Kinetics of the Reactions of Flavylium Ions with π-Nucleophiles

The kinetics of the reactions of the flavylium ion 1a and the 4′-methoxyflavylium ion 1b with various π-nucleophiles and tributylstannane were investigated photometrically in dichloromethane. Electrophilicity parameters E(1a) = −3.46 and E(1b) = −4.96 were derived from the equation log k (20 °C) = s(E + N); these allow the prediction of potential reaction partners of the flavylium ions 1a and 1b.

Electrophilic Reactivity of a 2-Azaallenium and of a 2-Azaallylium Ion

The rate constants for the reactions of the 2-azaallenium ion 1b+, the 2-azaallylium ion 2a+ and the iminium ion 3+ with different nucleophiles were determined by 1H NMR spectroscopy. By correlation with the Linear Free Enthalpy Relationship (LFER) lg k20°C = s (E + N), developed by Mayr and Patz, the electrophilicity parameters E(1b+) = -3.7, E(2a+) ≈ -16 and E(3+) = -10.43 were obtained. They show that the relative reactivities of these ions are approximately 1012:1:106. Quantum chemical calculations (ab initio, DFT) of the methyl anion affinities for the ions 1b+,2a+ and3+ are in agreement with the experimental E values. The X-ray structure of 3+·CF3SO is reported for the first time; it shows no strong interaction between the cation and the anion.

Comparison between a Diarylferrocenylmethylium Ion and Its Isolobal Cobalt Species: Similarities and Differences

Reaction of {(1,2,8,9-η)-tricyclo[7.5.0.01,7]tetradeca-1,8-diene}{η5-(methoxycarbonyl)cyclopentadienyl}cobalt (9) with either 2 mol of arylmagnesium bromide or aryllithium and subsequent hydrolysis yields the alcohols {(1,2,8,9-η)tricyclo[7.5.0.01,7]tetradeca-1,8-diene}{η5-(diarylhydroxymethyl)cyclopentadienyl}cobalt (13a−c). The treatment of the yellow colored alcohols 13a−c with HBF4 in ether affords the tetrafluoroborates of the violet colored carbocations 14a−c. Investigations of single crystals of 14b by means of the X-ray technique show a displacement of the C1−Cexo bond by α = 10.8° toward the metal and a pronounced bond alternation in the fulvene moiety. The bending of C1−Cexo is considerably larger than in the corresponding alcohol 13b (α = 2.5°). The interaction between the exo-carbon and the metal in 14b is further substantiated by comparison of the 13C-NMR chemical shift of 14b with 13b (Δδ = −65 ppm). The comparison of the data obtained for 14b with those of diphenylferrocenylcarbenium ion 1b suggests a stronger bonding between metal and fulvene moiety in 1b as compared to 14b. These results are substantiated by the results of ab initio calculations on 1b and 4a which show a smaller positive charge at the exo-carbon in 4a than in 1b and a stronger bond index between the exo-carbon and the metal in 1b as compared to 4a.