Electron‐induced dissociation of doubly protonated betaine clusters: controlling fragmentation chemistry through electron energy

The [M₂₁+2H]²⁺ cluster of the zwitterion betaine, M = (CH₃)₃NCH₂CO₂, formed via electrospray ionisation (ESI), has been allowed to interact with electrons with energies ranging from >0 to 50 eV in a Fourier transform ion cyclotron resonance (FT‐ICR) mass spectrometer. The types of gas‐phase electron‐induced dissociation (EID) reactions observed are dependent on the energy of the electrons. In the low‐energy region up to 10 eV, electrons are mainly captured, forming the charge‐reduced species, {[M₂₁+2H]^{+ .} }*, in an excited state, which stabilises via the ejection of an H atom and one or more neutral betaines. In the higher energy region, above 12 eV, a Coulomb explosion of the multiply charged clusters is observed in highly asymmetric fission with singly charged fragments carrying away more than 70% of the parent mass. Neutral betaine evaporation is also observed in this energy region. In addition, a series of singly charged fragments appears which arise from C? X bond cleavage reactions, including decarboxylation and CH₃ group transfer. These latter reactions may arise from access of electronic excited states of the precursor ions. Copyright © 2009 John Wiley & Sons, Ltd.     

An ab initio MO calculation of the electronic structure and geometry of protonated methanol

Protonated methanol, CH₃ OH₂⁺, has been studied using the LCAO—MO—SCF method with a 7, 3 and 9, 5, 1 Gaussian orbital basis set on the heavy atoms and 4s on hydrogen. It is found that the ground state is non-planar around oxygen, in contrast with previous calculations, with an inversion barrier of 3 kcal mol^?1. The changes in electron distribution in the reacting systemCH₃⁺ + H₂O → CF₃OH₂⁺is also examined.     

On the Gas‐Phase Reactivity of Complexed OH+ with Halogenated Alkanes

OH⁺ is an extraordinarily strong oxidant. Complexed forms (L? OH⁺), such as H₂OOH⁺, H₃NOH⁺, or iron–porphyrin‐OH⁺ are the anticipated oxidants in many chemical reactions. While these molecules are typically not stable in solution, their isolation can be achieved in the gas phase. We report a systematic survey of the influence on L on the reactivity of L? OH⁺ towards alkanes and halogenated alkanes, showing the tremendous influence of L on the reactivity of L? OH⁺. With the help of with quantum chemical calculations, detailed mechanistic insights on these very general reactions are gained. The gas‐phase pseudo‐first‐order reaction rates of H₂OOH⁺, H₃NOH⁺, and protonated 4‐picoline‐N‐oxide towards isobutane and different halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂ have been determined by means of Fourier transform ion cyclotron resonance meaurements. Reaction rates for H₂OOH⁺ are generally fast (7.2×10^?10–3.0×10^?9 cm³ mol^?1 s^?1) and only in the cases HCF₃ and CF₄ no reactivity is observed. In contrast to this H₃NOH⁺ only reacts with tC₄H₉Cl (k_obs=9.2×10^?10), while 4‐CH₃‐C₅H₄N‐OH⁺ is completely unreactive. While H₂OOH⁺ oxidizes alkanes by an initial hydride abstraction upon formation of a carbocation, it reacts with halogenated alkanes at the chlorine atom. Two mechanistic scenarios, namely oxidation at the halogen atom or proton transfer are found. Accurate proton affinities for HOOH, NH₂OH, a series of alkanes C_nH_2n+2 (n=1–4), and halogenated alkanes C_nH_2n+1Cl (n=1–4), HCF₃, CF₄, and CF₂Cl₂, were calculated by using the G3 method and are in excellent agreement with experimental values, where available. The G3 enthalpies of reaction are also consistent with the observed products. The tendency for oxidation of alkanes by hydride abstraction is expressed in terms of G3 hydride affinities of the corresponding cationic products C_nH_2n+1⁺ (n=1–4) and C_nH_2nCl⁺ (n=1–4). The hypersurface for the reaction of H₂OOH⁺ with CH₃Cl and C₂H₅Cl was calculated at the B3 LYP, MP2, and G3_m* level, underlining the three mechanistic scenarios in which the reaction is either induced by oxidation at the hydrogen or the halogen atom, or by proton transfer.     

Photofragmentation of 2‐Deoxy‐D‐Ribose Molecules in the Gas Phase

We have measured the synchrotron‐induced photofragmentation of isolated 2‐deoxy‐D ‐ribose molecules (C₅H₁₀O₄) at four photon energies, namely, 23.0, 15.7, 14.6, and 13.8 eV. At all photon energies above the molecule′s ionization threshold we observe the formation of a large variety of molecular cation fragments, including CH₃⁺, OH⁺, H₃O⁺, C₂H₃⁺, C₂H₄⁺, CH_xO⁺ (x=1,2,3), C₂H_xO⁺ (x=1–5), C₃H_xO⁺ (x=3–5), C₂H₄O₂⁺, C₃H_xO₂⁺ (x=1,2,4–6), C₄H₅O₂⁺, C₄H_xO₃⁺ (x=6,7), C₅H₇O₃⁺, and C₅H₈O₃⁺. The formation of these fragments shows a strong propensity of the DNA sugar to dissociate upon absorption of vacuum ultraviolet photons. The yields of particular fragments at various excitation photon energies in the range between 10 and 28 eV are also measured and their appearance thresholds determined. At all photon energies, the most intense relative yield is recorded for the m/q=57 fragment (C₃H₅O⁺), whereas a general intensity decrease is observed for all other fragments— relative to the m/q=57 fragment—with decreasing excitation energy. Thus, bond cleavage depends on the photon energy deposited in the molecule. All fragments up to m/q=75 are observed at all photon energies above their respective threshold values. Most notably, several fragmentation products, for example, CH₃⁺, H₃O⁺, C₂H₄⁺, CH₃O⁺, and C₂H₅O⁺, involve significant bond rearrangements and nuclear motion during the dissociation time. Multibond fragmentation of the sugar moiety in the sugar–phosphate backbone of DNA results in complex strand lesions and, most likely, in subsequent reactions of the neutral or charged fragments with the surrounding DNA molecules.     

An Insight of the Results Provided by Color‐Tuning Studies Made on Ir(III) Complexes: A pi‐Neutral CF3 Group Viewed by Adjusting Energies of pi‐type Molecular Orbitals

The pi‐nature of a CF₃ group can be understood through analysis of its bond orbitals (BOs) mixed into the pi‐type molecular orbitals of CF₃‐substituted Ir(ppy)₂MDPA⁺ complexes (ppy=2‐phenyl‐pyridine and MDPA=methylated 2,2′‐dipyridyl amine). It has been found that, through this natural bond orbital analysis, the parent’s molecular orbitals (MOs) can be stabilized by χρ^*_CF BO via negative hyperconjugation and, simultaneously, destabilized by electron lp(F) BO. Since these two competing pi‐effects are virtually counterbalanced as indicated by the vanishing values of crystal orbital overlap populations, the chemical substitution strategy originated from lowering of HOMO by using this electron‐withdrawing CF₃ group has been found effective in color‐tuning to blue region. Based on reduced shielding effect due to de‐ creased χρ‐electron density, the reported position dependent CF₃‐substitution effects on pi‐type MOs can also be understood through HOMO/LUMO wavefunction analysis.     

Substituent effects on the Bergman cyclization of (Z)‐1,5‐hexadiyne‐3‐enes: a systematic computational study

The effects of several substituents (? BH₂, ? BF₂, ? AlH₂, ? CH₃, ? C₆H₅, ? CN, ? COCH₃, ? CF₃, ? SiH₃, ? NH₂, ? NH₃⁺, ? NO₂, ? PH₂, ? OH, ? OH₂⁺, ? SH, ? F, ? Cl, ? Br) on the Bergman cyclization of (Z)‐1,5‐hexadiyne‐3‐ene (enediyne, 3 ) were investigated at the Becke–Lee–Yang–Parr (BLYP) density functional (DFT) level employing a 6‐31G* basis set. Some of the substituents (? NH₃⁺, ? NO₂, ? OH, ? OH₂⁺, ? F, ? Cl, ? Br) are able to lower the barrier (up to a minimum of 16.9 kcal mol^?1 for difluoro‐enediyne 7rr ) and the reaction enthalpy (the cyclization is predicted to be exergonic for ? OH₂⁺ and ? F) compared to the parent system giving rise to substituted 1,4‐dehydrobenzenes at physiological temperatures. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 1605–1614, 2001     

Regioselective Sequential Additions of Nucleophiles and Electrophiles to Perfluoroalkylfullerenes: Which Cage C Atoms Are the Most Reactive and Why?

The sequential addition of CN^? or CH₃^? and electrophiles to three perfluoroalkylfullerenes (PFAFs), C_s‐C₇₀(CF₃)₈, C₁‐C₇₀(CF₃)₁₀, and C_s‐p‐C₆₀(CF₃)₂, was carried out to determine the most reactive individual fullerene C atoms (as opposed to the most reactive C?C bonds, which has previously been studied). Each PFAF reacted with CH₃^? or CN^? to generate metastable PFAF(CN)^? or PFAF(CH₃)₂^2? species with high regioselectivity (i.e., one or two predominant isomers). They were treated with electrophiles E⁺ to generate PFAF(CN)(E) or PFAF(CH₃)₂(E)₂ derivatives, also with high regioselectivity (E⁺=CN⁺, CH₃⁺, or H⁺). All of the predominant products, characterized by mass spectrometry and ¹⁹F NMR spectroscopy, are new compounds. Some could be purified by HPLC to give single isomers. Two of them, C₇₀(CF₃)₈(CN)₂ and C₇₀(CF₃)₁₀(CH₃)₂(CN)₂, were characterized by single‐crystal X‐ray diffraction. DFT calculations were used to propose whether a particular reaction is under kinetic or thermodynamic control.     

Determination of Rate Constants for Aqueous Reactions of HCFC‐123 and HCFC‐225ca with OH− Along with Henry's Law Constants of Several HCFCs

Henry's law constants of six kinds of hydrochlorofluorocarbons (HCFCs) were determined at 313–353 K by means of a phase‐ratio variation headspace method: and (K_H³⁵³ in M atm^?1, ΔH_sol in kJ mol^?1) = (0.0070 ± 0.0006, –23 ± 2), (0.0038 ± 0.0011, –22 ± 10), (0.0065 ± 0.0007, –21 ± 3), (0.0026 ± 0.0007, –23 ± 8), (0.0016 ± 0.0003, –30 ± 4), and (0.0022 ± 0.0003, –25 ± 4), respectively, for HCFC‐141b (CH₃CCl₂F), HCFC‐142b (CH₃CClF₂), HCFC‐123 (CF₃CHCl₂), HCFC‐124 (CF₃CHClF), HCFC‐225ca (CF₃CF₂CHCl₂), and HCFC‐225cb (CClF₂CF₂CHClF). Errors represent two standard deviations only for the fitting. The decay of headspace partial pressures of these HCFCs via hydrolysis was discerned only for CF₃CHCl₂ and CF₃CF₂CHCl₂ under the experimental conditions examined. Rate constants (k_OH– in M^?1 s^?1) for aqueous reactions of CF₃CF₂CHCl₂ and CF₃CHCl₂ with OH^? at 313–353 K were determined to be and , respectively, from monitoring changes in headspace partial pressure over prescribed concentrations of aqueous NaOH as a function of the headspace time duration and concentration of aqueous NaOH. The calculations performed included consideration of gas–water equilibrium and hydrolysis at both headspace and room temperatures. The calculation for CF₃CHCl₂ also included consideration of salting‐out effects: The salting coefficient of NaCl on a natural log basis was determined to be 0.36 ± 0.06 M^?1, and this value was used for consideration of the salting‐out effect of NaOH. Whereas the activation energy for CF₃CF₂CHCl₂ was greater than that for CF₃CHCl₂, the k_OH– value at 353 K of CF₃CF₂CHCl₂ was 10³ times larger than that of CF₃CHCl₂, indicating that reaction mechanisms for these two HCFCs differed from each other. The aqueous reaction of CF₃CF₂CHCl₂ with OH^? was found to proceed through dehydrofluorination on the basis of detection of CF₃CF?CCl₂ as a primary degradation product of the reaction and proportionality of the rate constants to both concentrations of CF₃CF₂CHCl₂ and OH^?.     

Excited states of gaseous ions. V. The predissociation of the h2o+ ion

The B?² state of H₂O⁺ is predissociated twice. First, by the ã⁴B₁ state, giving OH⁺ + H fragments via spinorbit coupling interaction. Secondly, by a²A state, giving H ⁺ OH fragments via spin-orbit coupling and Coriolis interactions. A vibrational analysis of the photoelectron band of the B? state of H₂O⁺ and D₂O⁺ is carried out. This provides the vibrational frequencies of the H₂O⁺, D₂O⁺ and HDO⁺ ions, as well as a vibrational assignment of the peaks. The H₂O⁺ ion in its B?²B₂ state is found to have a OH bond length of 1.12 A and a valence angie of 78°.In order to describe the unimolecular fragmentation process, a distinction is introduced between the totally symmetric, optically active vibrational modes, and the antisymmetric ones which are coupled to the continuum. The former are supplied with photon or electron impact energy, but only the latter are chemically efficient. The dynamics of the dissociation process depends therefore on the couplings among normal modes. This is studied in the framework of two models. In Model 1, it is assumed that, as a result of the anharmonicity of the potential energy surface, only even overtones of the antisymmetric vibration are excited by Fermi resonance. In Model II, excitation of the odd overtones is provided by vibronic coupling. Model II is in better agreement with experiment than Model I. Calculated and experimental results have been compared on the following points: isotopic shift on the appearance potential of OH⁺ and OD⁺ ions, shapes of the photoionization curves, fragmentation pattern with 21 eV photons, presence of a unimolecular metastable transition, production of O⁺ ions. All the vibrational levels situated above the dissociation asymptote are totally predissociated. Autoionization is shown in this case to contribute only to the formation of molecular H₂O⁺ ions, and not to that of the OH⁺ fragments. For 21 eV electrons, the contribution due to direct ionization is calculated to represent about 25% of the total cross section, the rest being due to autoionization.     

Lifetimes of the vibronic Ã2A1 states of H2O+ and of the 3Πi (υ′ = 0) state of OH+

Using the delayed coincidence technique, lifetimes have been measured for some Σ and Π vibronic òA₁ states of H₂O⁺ and for the ³Π_i (υ′ = 0) state of OH⁺ by analysing the decay curves of the òA₁(0, υ′₂, 0) ? X?²B₁ (0, υ″₂, 0) and the ³Π_i(υ′ = 0) ? ³Σ^?(υ″ = 0) emission intensities respectively. The excited molecular ionic states are produced via excitation of H₂O molecules by 200 eV electrons. For H₂O⁺(òA₁) the vibronic Σ levels with υ′₂ = 13 and 15 and the vibronic Π levels with υ′₂ = 12 and 14 have been considered. The radiative lifetimes obtained for these levels have about the same value, namely 10.5(±1) × 10^?6 s. The radiative lifetime for the OH⁺(³Π_iυ′= 0) state is 2.5(±0.3) × 10^?6 s. The lifetimes found in this work for H₂O⁺(òA₁) and OH⁺(³Π_i,υ′= 0) are about ten and three times longer respectively than the corresponding lifetimes given by other investigators [1,2]. The probable reason for this discrepancy is that in the other experiments no attention has been paid to the presence of a large space charge effect. This effect is caused by the positive ions which are created by the primary electron beam.     

Isomeric ion-neutral complexes generated from ionized 2-Methylpropanol and n-Butanol: the effect of the polarity of the neutral partner on complex-mediated reactions

Photoionization was used to characterize the energy dependence of C₃H ₇ ⁺ , C₃H ₆ ⁺ , CH₃OH ₂ ⁺ and CH₂=OH⁺ formation from (CH₃)₂)CHCH₂OH^+? (1) and CH₃CH₂CH₂CH₂OH^+? (2). Decomposition patterns of labeled ions demonstrate that close to threshold these products are primarily formed through [CH ₃ ⁺ CHCH₃ ^?CH₂OH] (bd3) from 1 and through [CH₃CH₂CH₂ ^?CH₂=OH⁺] (9) from 2. The onset energies for forming the above products from 1 are spread over 85 kJ mol^?1, and are all near thermochemical threshold. The corresponding onsets from 2 are in a 19 kJ mol^?1 range, and all except that of CH₂=OH⁺ are well above their thermochemical thresholds. Each decomposition of 3 occurs over a broad energy range (> 214 kJ mol^?1), This demonstrates that ion-permanent dipole complexes can be significant intermediates over a much wider energy range than ion-induced dipole complexes can be. H-exchange between partners in the complexes appears to be much faster than exchange by conventional interconversions of the alcohol molecular ions with their distonic isomers. The onsets for water elimination from 1 and 2 are below the onsets for the complex-mediated processes, demonstrating that the latter are not necessarily the lowest energy decompositions of a given ion when the neutral partner in the complex is polar.     

Time‐Dependent Density Functional Theory Molecular Dynamics Simulations of Liquid Water Radiolysis

The early stages of the Coulomb explosion of a doubly ionized water molecule immersed in liquid water are investigated with time‐dependent density functional theory molecular dynamics (TD–DFT MD) simulations. Our aim is to verify that the double ionization of one target water molecule leads to the formation of atomic oxygen as a direct consequence of the Coulomb explosion of the molecule. To that end, we used TD–DFT MD simulations in which effective molecular orbitals are propagated in time. These molecular orbitals are constructed as a unitary transformation of maximally localized Wannier orbitals, and the ionization process was obtained by removing two electrons from the molecular orbitals with symmetry 1B₁, 3A₁, 1B₂ and 2A₁ in turn. We show that the doubly charged H₂O²⁺ molecule explodes into its three atomic fragments in less than 4 fs, which leads to the formation of one isolated oxygen atom whatever the ionized molecular orbital. This process is followed by the ultrafast transfer of an electron to the ionized molecule in the first femtosecond. A faster dissociation pattern can be observed when the electrons are removed from the molecular orbitals of the innermost shell. A Bader analysis of the charges carried by the molecules during the dissociation trajectories is also reported.     

Negative‐ion/positive‐ion coincidence spectroscopy as a tool to identify anionic fragments: The case of core‐excited CHF3

We have studied the dissociation of the trifluoromethane molecule, CHF₃, into negative ionic fragments at the C 1s and F 1s edges. The measurements were performed by detecting coincidences between negative and positive ions. We observed five different negative ions: F^?, H^?, C^?, CF^?, and F₂^?. Their production was confirmed by the analysis of triple coincidence events (negative‐ion/positive‐ion/positive‐ion or NIPIPI coincidences) that were recorded with cleaner signals than those of the negative‐ion/positive‐ion coincidences. The intensities of the most intense NIPIPI coincidence channels were recorded as a function of photon energy across the C 1s and F 1s excitations and ionization thresholds. We also observed dissociation channels involving the formation of one negative ion and three positive ions. Our results demonstrate that negative‐ion/positive‐ion coincidence spectroscopy is a very sensitive method to observe anions, which at inner‐shell edges are up to three orders of magnitude less probable dissociation products than cations.     

Multiple-stage mass spectrometry in structural characterization of organophosphorus compounds

Multiple-stage mass spectrometry involving consecutive collision-activated dissociation reactions was used to examine the structures of fragment ions commonly formed on electron ionization of organophosphorus esters. The compounds studied include several aryl thiophosphates, some of which are analogs of common pesticides. Energy-resolved collisionactivated dissociation experiments allow the dissociation of the molecular ions of these compounds in such a manner that only a few fragment ions dominate the spectrum. An abundant fragment ion of m/z 109, formed from all of the compounds studied, can have at least four different stable structures: (CH₃O)₂PO⁺, CH₃CH₂OP(O)OH⁺, CH₂ =CHOP(H)(OH)₂ ⁺, and (CH₂O)₂P(H)OH⁺. The structure of the fragment ion of m/z 109 was found to reflect the phosphorus-containing part of the compounds studied. Another abundant fragment ion obtained for all the aryl esters studied is structurally characteristic of the aromatic moiety of the molecule. This fragment ion is the result of a complex rearrangement involving transfer of an alkylene group to the aromatic ring from the phosphoruscontaining part of the molecular ion. The utility of these fragment ions in the structural characterization of unknown organophosphorus esters is discussed.     

Preliminary theoretical study of perfluorodimethyl ether and its protonated form

3-21G RHF calculations on (CF₃)₂O and (CF₃)₂OH⁺ molecules are carried out to assess the changes induced in the molecular properties of ether links in poly (perfluoro ethers) interacting with acid sites at contact surfaces. Geometry of the species, vibrational frequencies, proton affinity, and energetics of the (CF₃)₂OH⁺ fragmentation provide a preliminary basis to understand problems raised by lubricant degradation. © 1993 John Wiley & Sons, Inc.     

UV emission of CF2 produced by electron and ion impact on CHF3

Emission spectra produced by 0–1 keV electron and 1–25 keV H⁺ impact on CHF₃ were obtained and absolute cross section for a band in the UV were determined. The latter emission between ≈ 230 and 350 nm is attributed to the CF₂ (ā → X?) transition by comparison with fluorescence spectra from photolysis of CF₂ containing species. An analysis of the energy dependence of the cross sections indicates that the corresponding excitation processes involve excited state(s) of CHF₃ in particular the (4e)^?2(3p) Rydberg state dissociating into CF₂(ā) and H, F fragments.     

Crystallographic report: Crystal structures of organometallic aqua complexes [Cp*RhIII(bpy)(OH2)]2+ and [Cp*RhIII(6,6′‐Me2bpy)(OH2)]2+ used as key catalysts in regioselective reduction of NAD+ analogues

Crystal structures of organometallic aqua complexes [Cp*Rh^III(bpy)(OH₂)]²⁺ ( 1 , Cp* = η⁵‐C₅Me₅, bpy = 2,2′‐bipyridine) and [Cp*Rh^III(6,6′‐Me₂bpy)(OH₂)]²⁺ ( 2 , 6,6′‐Me₂bpy = 6,6′‐dimethyl‐2,2′‐bipyridine) used as key catalysts in regioselective reduction of NAD⁺ analogues were determined definitely by X‐ray analysis. The yellow crystals of 1 (PF₆)₂ and orange crystals of 2 (CF₃SO₃)₂ used in the X‐ray analysis were obtained from aqueous solutions of 1 (PF₆)₂ and 2 (CF₃SO₃)₂. The Rh–O_aqua length of 2.194(4) Å obtained for 1 (PF₆)₂ is significantly different from that of 2.157(3) Å obtained for the previously reported disorder model [Cp*Rh^III(bpy)(0.7H₂O/0.3CH₃OH)](CF₃SO₃)₂·0.7H₂O in which the coordinated water is replaced by a coordinated methanol. The five‐membered ring involving the Rh atom and the 6,6′‐Me₂bpy chelating unit in 2 (CF₃SO₃)₂ is not flat, whereas the five‐membered chelate ring in 1 (PF₆)₂ is nearly flat. Such a non‐planar structure in 2 (CF₃SO₃)₂ is ascribed to the steric repulsion between the 6,6′‐Me₂bpy ligand and the Cp* ligand. Copyright © 2005 John Wiley & Sons, Ltd.     

Chiral Transformation in Protonated and Deprotonated Adipic Acids through Multistep Internal Proton Transfer

Protonated and deprotonated adipic acids (PAA: HOOC? (CH₂)₄? COOH₂⁺ and DAA: HOOC? (CH₂)₄? COO^?) have a charged hydrogen bond under the influence of steric constraint due to the molecular skeleton of a circular ring. Despite the similarity between PAA and DAA, it is surprising that the lowest energy structure of PAA is predicted to have (H₂O???H???OH₂)⁺ Zundel‐like symmetric hydrogen bonding, whereas that of DAA has H₃O⁺ Eigen‐like asymmetric hydrogen bonding. The energy profiles show that direct proton transfer between mirror image structures is unfavorable. Instead, the chiral transformation is possible by subsequent backbone twistings through stepwise proton transfer along multistep intermediate structures, which are Zundel‐like ions for PAA and Eigen‐like ions for DAA. This type of chiral transformation by multistep intramolecular proton transfers is unprecedented. Several prominent OH???O short hydrogen‐bond stretching peaks are predicted in the range of 1000–1700 cm^?1 in the Car–Parrinello molecular dynamics (CPMD) simulations, which show distinctive signatures different from ordinary hydrogen‐bond peaks. The O? H? O stretching peaks in the range of 1800–2700 cm^?1 become insignificant above around 150 K and are almost washed out at about 300 K.     

Cationic versus neutral Ru(II)--N-heterocyclic carbene complexes as latent precatalysts for the UV-induced ring-opening metathesis polymerization

A series of cationic and neutral Ru^II complexes of the general formula [Ru(L)(X) (tBuCN)₄]⁺X^? and [Ru(L)(X)₂(tBuCN)₃)], that is, [Ru(CF₃SO₃){NCC(CH₃)₃}₄(IMesH₂)]⁺[CF₃SO₃]^? ( 1 ), [Ru(CF₃SO₃){NCC(CH₃)₃}₄(IMes)]⁺[CF₃SO₃]^? ( 2 ), [RuCl{NCC(CH₃)₃}₄(IMes)]⁺Cl^? ( 3 ), [RuCl{NCC(CH₃)₃}₄(IMesH₂)⁺Cl^?]/[RuCl₂{NCC(CH₃)₃}₃(IMesH₂)] ( 4 ), and [Ru(NCO)₂{NCC(CH₃)₃}₃(IMesH₂)] ( 5 ) (IMes=1,3‐dimesitylimidazol‐2‐ylidene, IMesH₂=1,3‐dimesityl‐imidazolin‐2‐ylidene) have been synthesized and used as UV‐triggered precatalysts for the ring‐opening metathesis polymerization (ROMP) of different norborn‐2‐ene‐ and cis‐cyclooctene‐based monomers. The absorption maxima of complexes 1 – 5 were in the range of 245–255 nm and thus perfectly fit the emission band of the 254 nm UV source that was used for activation. Only the cationic Ru^II‐complexes based on ligands capable of forming μ²‐complexes such as 1 and 2 were found to be truly photolatent in ROMP. In contrast, complexes 3 – 5 could be activated by UV light; however, they also showed a low but significant ROMP activity in the absence of UV light. As evidenced by ¹H and ¹³C NMR spectroscopy, the structure of the polymers obtained with either 1 or 2 are similar to those found in the corresponding polymers prepared by the action of [Ru(CF₃SO₃)₂(IMesH₂)(CH‐2‐(2‐PrO)‐C₆H₄)], which strongly suggest the formation of Ru‐based Grubbs‐type initiators in the course of the UV‐based activation process. Precatalysts that have the IMesH₂ ligand showed significantly enhanced reactivity as compared with those based on the IMes ligand, which is in accordance with reports on the superior reactivity of IMesH₂‐based Grubbs‐type catalysts compared with IMes‐based systems.