What have we learnt about heavy carbenes through laser flash photolysis studies?

Implementation of laser flash photolysis for radical-induced reactions and environmental implications

Confronted with the imperative crisis of water quality deterioration, the pursuit of state-of-the-art decontamination technologies for a sustainable future never stops. Fitting into the framework of suitability, advanced oxidation processes have been demonstrated as powerful technologies to produce highly reactive radicals for the degradation of toxic and refractory contaminants. Therefore, investigations on their radical-induced degradation have been the subject of scientistic and engineering interests for decades. To better understand the transient nature of these radical species and rapid degradation processes, laser flash photolysis (LFP) has been considered as a viable and powerful technique due to its high temporal resolution and rapid response. Although a number of studies exploited LFP for one (or one class of) specific reaction(s), reactions of many possible contaminants with radicals are largely unknown. Therefore, there is a pressing need to critically review its implementation for kinetic quantification and mechanism elucidation. Within this context, we introduce the development process and milestones of LFP with emphasis on compositions and operation principles. We then compare the specificity and suitability of different spectral modes for monitoring radicals and their decay kinetics. Radicals with high environmental relevance, namely hydroxyl radical, sulfate radical, and reactive chlorine species, are selected, and we discuss their generation, detection, and implications within the frame of LFP. Finally, we highlight remaining challenges and future perspectives. This review aims to advance our understandings of the implementation of LFP in radical-induced transient processes, and yield new insights for extrapolating this pump-probe technique to make significant strides in environmental implications.

Time-resolved, Gas-phase Kinetic and Quantum Chemical Studies of the Reaction of Germylene with Hydrogen Chloride

Time-resolved studies of germylene, GeH₂ , generated by laser flash photolysis of 3,4-dimethyl-1-germacyclopent-3-ene at 193 nm and monitored by laser absorption, have been carried out to obtain rate constants for its bimolecular reaction with HCl. The reaction was studied in the gas phase, mainly at a total pressure of 10 Torr (in SF₆ bath gas) at five temperatures in the range 295-558 K. Experiments at other pressures showed that these rate constants were unaffected by pressure. The second-order rate constants at 10 Torr (SF₆ bath gas) fitted the Arrhenius equation: log(k/cm³  molecule^-1  s^-1 )=(-12.06±0.14)+(2.58±1.03 kJ mol^-1 )/RTln10 where the uncertainties are single standard deviations. Quantum chemical calculations at G4 level support a mechanism in which an initial weakly bound donor-acceptor complex is formed. This can then rearrange and decompose to give H₂ and HGeCl (chlorogermylene). The enthalpy barrier (36 kJ mol^-1 ) is too high to allow rearrangement of the complex to GeH₃ Cl (chlorogermane).

Gas-Phase Preparation of 1-Germavinylidene (H2CGe; X1A1), the Isovalent Counterpart of Vinylidene (H2CC; X1A1), via Non-adiabatic Dynamics through the Elementary Reaction of Ground State Atomic Carbon (C; 3P) with Germane (GeH4; X1A1)

1-Germavinylidene (H₂CGe; X¹A₁), the germanium analogue of vinylidene (H₂CC; X¹A₁), was prepared via a directed gas-phase synthesis through the bimolecular reaction of ground state atomic carbon (C; ³P) with germane (GeH₄; X¹A₁) under single-collision conditions. The reaction commences with the barrierless insertion of carbon into the Ge-H bond followed by intersystem crossing from the triplet to singlet surface and migration of atomic hydrogen to germylene (H₂GeCH₂), which predominantly decomposes via molecular hydrogen loss to 1-germavinylidene (H₂CGe; X¹A₁). Therefore, the replacement of a single carbon atom in the acetylene-vinylidene system by germanium critically impacts the chemical bonding, molecular structure, and thermodynamic stability of the carbene-type structures favoring 1-germavinylidene (H₂CGe) over germyne (HGeCH) by 160 kJ mol^-1. Hence, the carbon-germane system represents a benchmark in the exploration of the chemistries of main group 14 elements with germanium-bearing systems showing few similarities with the isovalent carbon system.

Gas Phase Formation of Methylgermylene (HGeCH3)

The methylgermylene species (HGeCH₃ ; X¹ A') has been synthesized via the bimolecular gas phase reaction of ground state methylidyne radicals (CH) with germane (GeH₄ ) under single collision conditions in crossed molecular beams experiments. Augmented by electronic structure calculations, this elementary reaction was found to proceed through barrierless insertion of the methylidyne radical in one of the four germanium-hydrogen bonds on the doublet potential energy surface yielding the germylmethyl (CH₂ GeH₃ ; X² A') collision complex. This insertion is followed by a hydrogen shift from germanium to carbon and unimolecular decomposition of the methylgermyl (GeH₂ CH₃ ; X² A') intermediate by atomic hydrogen elimination leading to singlet methylgermylene (HGeCH₃ ; X¹ A'). Our investigation provides a glimpse at the largely unknown reaction dynamics and isomerization processes of the carbon-germanium system, which are quite distinct from those of the isovalent carbon system thus providing insights into the intriguing chemical bonding of organo germanium species on the most fundamental, microscopic level.

Tuning Philicity of Dichlorosilylene: Nucleophilic Behavior of the Dichlorosilylene-NHC Complex Cl2Si-IPr

Novel unsaturated four-membered ring disilene, 3,3-dichloro-1,2,4,4-tetrakis[di-tert-butyl(methyl)silyl]-¹Δ-1,2,3,4-trisilagermetene, was synthesized by the ring expansion reaction of the three-membered ring 1-disilagermirene with the silylene-NHC complex Cl₂Si-IPr. The mechanism of the unexpected formation of this compound was verified by high-level density functional theory computations, which revealed n_Si:(HOMO)-π_Si=Si(LUMO) ^* as the dominant orbital interaction.

Fast kinetics studies of the Lewis acid-base complexation of transient stannylenes with σ- and π-donors in solution

The Lewis acid-base complexation chemistry of dimethyl- and diphenylstannylene (SnMe2 and SnPh2, respectively) in hexanes solution has been studied by laser photolysis methods. Complexation of the two stannylenes with a series of nine O-, S-, and N-donors (including cyclic and acyclic dialkyl ethers and sulfides, a primary, secondary, and tertiary amine, ethyl acetate and acetone), two alkenes, and an alkyne proceeds rapidly and reversibly to generate the corresponding stannylene-donor Lewis pairs, which have been detected in each case by time-resolved UV-vis absorption spectroscopy. The complexes exhibit UV-vis absorption maxima in the range of λmax ∼ 310-405 nm depending on the donor and substitution at tin. Bimolecular rate constants for complexation (kC), which could be determined for 14 of the 24 Lewis pairs that were studied, were found to fall within a factor of four of the diffusional limit in all cases, with SnMe2 showing consistently higher reactivity than SnPh2. Equilibrium constants for complexation (KC) could be measured for all but one of the stannylene-π- and O-donor pairs, the values corresponding to (gas phase) binding free energies in the range of +1.1 to -3.9 kcal mol-1. Comparison of the experimental equilibrium constants for complexation of SnMe2 and SnPh2 with methanol and diethyl ether to those measured previously for the homologous silylenes and germylenes indicates that Lewis acidity decreases in the order SiR2 > SnR2 > GeR2 for both the dimethyl- and diphenyltetrylene series, the diphenyl derivatives exhibiting significantly stronger Lewis acidity in all three cases. The experimental trends in the binding energies and UV-vis spectra of the complexes are reproduced well by density functional theory (DFT) calculations, which have been carried out at the (TD)ωB97XD/def2-TZVP level of theory. The experimental data also show evidence of a reaction between tetramethyldistannene (Me2Sn[double bond, length as m-dash]SnMe2, 4a) and amine donors, which is suggested to afford the corresponding amine-stabilized stannylidenestannylene structure. The mechanistic proposal is supported by DFT calculations of the complexation of 4a and SnMe2 with model O-, S- and N-donors.

Time-Resolved Gas-Phase Kinetic, Quantum Chemical, and RRKM Studies of the Reaction of Silylene with 2,5-Dihydrofuran

Time-resolved kinetics studies of silylene, SiH2, generated by laser flash photolysis of phenylsilane, were performed to obtain rate coefficients for its bimolecular reaction with 2,5-dihydrofuran (2,5-DHF). The reaction was studied in the gas phase over the pressure range of 1-100 Torr in SF6 bath gas, at five temperatures in the range of 296-598 K. The reaction showed pressure dependences characteristic of a third body assisted association. The second-order rate coefficients obtained by Rice-Ramsperger-Kassel-Marcus (RRKM)-assisted extrapolation to the high-pressure limit at each temperature fitted the following Arrhenius equation where the error limits are single standard deviations: log(k/cm(3) molecule(-1) s(-1)) = (-9.96 ± 0.08) + (3.38 ± 0.62 kJ mol(-1))/RT ln 10. End-product analysis revealed no GC-identifiable product. Quantum chemical (ab initio) calculations indicate that reaction of SiH2 with 2,5-DHF can occur at both the double bond (to form a silirane) and the O atom (to form a donor-acceptor, zwitterionic complex) via barrierless processes. Further possible reaction steps were explored, of which the only viable one appears to be decomposition of the O-complex to give 1,3-butadiene + silanone, although isomerization of the silirane cannot be completely ruled out. The potential energy surface for SiH2 + 2,5-DHF is consistent with that of SiH2 with Me2O, and with that of SiH2 with cis-but-2-ene, the simplest reference reactions. RRKM calculations incorporating reaction at both π- and O atom sites, can be made to fit the experimental rate coefficient pressure dependence curves at 296-476 K, giving values for k(∞)(π) and k(∞)(O) that indicate the latter is larger in magnitude at all temperatures, in contrast to values from individual model reactions. This unexpected result suggests that, in 2,5-DHF with its two different reaction sites, the O atom exerts the more pronounced electrophilic attraction on the approaching silylene. Arrhenius parameters for the individual pathways were obtained. The lack of a fit at 598 K is consistent with decomposition of the O-complex to give 1,3-butadiene + silanone.

A nitrogen-base catalyzed generation of organotin(ii) hydride from an organotin trihydride under reductive dihydrogen elimination

Amine bases are shown to induce reductive elimination of dihydrogen from terphenyltin trihydride.

Since their first description over a decade ago, organotin(ii) hydrides have been an iconic class of compounds in molecular main group chemistry. Among other approaches they have been accessed from the hydrogenation of distannynes. We herein report their accessibility from the other direction by dehydrogenation of organotin trihydride. On reacting pyridine and amine bases with the bulky substituted organotin trihydride Ar*SnH₃ (Ar* = 2,6-trip₂(C₆H₃)–, trip = 2,4,6-triisopropylphenyl) hydrogen evolution was observed. In case of catalytic amounts of base the dehydro-coupling product diorganodistannane Ar*H₂SnSnH₂Ar* was obtained quantitatively whilst for excessive amounts (>4 eq.) the monomeric base adduct to known Ar*SnH was obtained almost exclusively. The base adducts were found to be remarkably thermally robust. They readily react with polar fulvenic C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> C-bonds in hydro-stannylenylation reactions. The resulting half-sandwich complex Ar*SnCp* was structurally characterized. Moreover, on application of less nucleophilic amine bases, the uncoordinated, in solution dimeric [Ar*SnH]₂ is formed. NMR spectroscopic studies on the kinetics of the DMAP-catalysed reductive elimination of dihydrogen were performed. The activation energy was approximated to be 13.7 kcal mol^–1. Solvent dependencies and a kinetic isotope effect KIE of k_H/k_D = 1.65 in benzene and 2.04 in THF were found and along with DFT calculations support a polar mechanism for this dehydrogenation.

A combined kinetic and computational study of the reactions of transient germylenes with oxiranes and thiiranes in solution

The reactions of dimethyl- and diphenylgermylene (GeMe2 and GePh2, respectively) with cyclohexene oxide (CHO) and propylene sulfide (PrS) have been studied in hydrocarbon solvents at 25 °C by laser flash and steady-state photolysis methods using appropriately substituted germacyclopent-3-ene derivatives as germylene precursors. GeMe2 reacts with CHO and PrS with rate constants in the range of 1.2–1.7 × 1010 M−1 s−1 in hexanes at 25 °C to form new transient products that are assigned to the corresponding Lewis acid-base complexes of the germylene with the substrates. The complexation reactions were found to be reversible and are characterized by equilibrium constants of KC = (3.7 ± 0.8) × 103 M−1 and (3 ± 1) × 104 M−1 for complexation of GeMe2 with CHO and PrS, respectively. The complexes decay over approximately 10 μs with the concomitant formation of tetramethyldigermene (Ge2Me4), identifiable by its characteristic UV-vis spectrum centered at λmax = 370 nm. Diphenylgermylene behaves analogously, reacting rapidly and reversibly with the two substrates to form the corresponding Lewis acid-base complexes (λmax ≈ 355 nm) that decay over several tens of microseconds with the concomitant growth of the characteristic UV-vis spectrum of tetraphenyldigermene (Ge2Ph4) (λmax = 440 nm). Steady-state photolysis of the germylene precursors in the presence of CHO afforded germanium-containing oligomers but showed no evidence of oxygen abstraction or the formation of substrate-derived product(s). Similar photolyses in the presence of PrS also afforded germanium-containing oligomers, but as well yielded propene in 20%–30% yield and (in the case of the GePh2 precursor) minor amounts of low molecular weight compounds that appear to be derived from the corresponding germanethione. Density functional theory calculations of the chalcogen abstraction reactions of GeMe2 with oxirane and thiirane in the gas phase have been carried out at the B3LYP/6-311+G(d,p) level of theory and are in good qualitative agreement with the experimental data.

Reaction of silylene with sulfur dioxide: some gas-phase kinetic and theoretical studies

Time-resolved kinetic studies of the reaction of silylene, SiH2, with SO2 have been carried out in the gas phase over the temperature range 297-609 K, using laser flash photolysis to generate and monitor SiH2. The second order rate coefficients at 1.3 kPa (SF6 bath gas) fitted the Arrhenius equation: log(k/cm(3) molecule(-1) s(-1)) = (-10.10 ± 0.06) + (3.46 ± 0.45 kJ mol(-1))/RT ln 10 where the uncertainties are single standard deviations. The collisional efficiency is 71% at 298 K, and in kinetic terms the reaction most resembles those of SiH2 with CH3CHO and (CH3)2CO. Quantum chemical calculations at the G3 level suggest a mechanism occurring via addition of SiH2 to one of the S=O double bonds leading to formation of the three-membered ring, thione-siloxirane which has a low energy barrier to ring expansion to yield the four-membered ring, 3-thia-2,4-dioxasiletane, the lowest energy adduct found on the potential energy (PE) surface. RRKM calculations, however, show that, if formed, this molecule would only be partially stabilised under the reaction conditions and the rate coefficients would be pressure dependent, in contrast with experimental findings. The G3 calculations reveal the complexity of possible intermediates and end products and taken together with the RRKM calculations indicate the most likely end products to be H2SiO + SO ((3)Σ(-)). The reaction is compared and contrasted with that of SiH2 + CO2.

Unusual isotope effect in the reaction of chlorosilylene with trimethylsilane-1-d. Absolute rate studies and quantum chemical and Rice-Ramsperger-Kassel-Marcus calculations provide strong evidence for the involvement of an intermediate complex

Time-resolved studies of chlorosilylene, ClSiH, generated by the 193 nm laser flash photolysis of 1-chloro-1-silacyclopent-3-ene, have been carried out to obtain rate constants for its bimolecular reaction with trimethylsilane-1-d, Me(3)SiD, in the gas phase. The reaction was studied at total pressures up to 100 Torr (with and without added SF(6)) over the temperature range of 295-407 K. The rate constants were found to be pressure independent and gave the following Arrhenius equation: log[(k/(cm(3) molecule(-1) s(-1))] = (-13.22 ± 0.15) + [(13.20 ± 1.00) kJ mol(-1)]/(RT ln 10). When compared with previously published kinetic data for the reaction of ClSiH with Me(3)SiH, kinetic isotope effects, k(D)/k(H), in the range from 7.4 (297 K) to 6.4 (407 K) were obtained. These far exceed values of 0.4-0.5 estimated for a single-step insertion process. Quantum chemical calculations (G3MP2B3 level) confirm not only the involvement of an intermediate complex, but also the existence of a low-energy internal isomerization pathway which can scramble the D and H atom labels. By means of Rice-Ramsperger-Kassel-Marcus modeling and a necessary (but small) refinement of the energy surface, we have shown that this mechanism can reproduce closely the experimental isotope effects. These findings provide the first experimental evidence for the isomerization pathway and thereby offer the most concrete evidence to date for the existence of intermediate complexes in the insertion reactions of silylenes.

Kinetic and mechanistic studies of the reactions of diarylgermylenes and tetraaryldigermenes with carbon tetrachloride

The mechanisms of the reactions of diphenylgermylene (GePh2) with CCl4 in hydrocarbon solvents and in THF solution have been studied, employing 3,4-dimethyl-1,1-diphenylgermacyclopent-3-ene (6a) and 1,1-diphenylgermacyclobutane (17) as photochemical precursors to GePh2. In hydrocarbon solvents, the reaction produces Ph2GeCl2 (10) and Ph2Ge(Cl)CCl3 (12) in a ratio of 10:12 ≈ 7, along with a variety of other radical-derived products and small amounts of Ph2GeH(D)Cl (11), which is formed partly by reaction of GePh2 with adventitious HCl. The reaction is much cleaner in THF, where 12 is formed as the major product (10:12 ≈ 0.8); a similar product distribution is obtained in hexanes containing 0.05 mol/L THF, while 12 is the exclusive product in hexanes containing 3 mmol/L NEt3. Rate constants for the reactions of CCl4 with GePh2 and five ring-substituted derivatives were determined by laser flash photolysis, as well as Arrhenius parameters for reaction of the parent (GePh2), in the two solvents. The reactions of GePh2 with CCl4 and CHCl3 have also been studied in 3-methylpentane solution at 78–90 K. Different reaction mechanisms are clearly operative in hydrocarbon and complexing solvents, but both involve modest charge donation from germanium to the substrate in the transition state for the rate-determining step. For the reaction in hydrocarbon solvents, the data are consistent with inner-sphere electron transfer following or in concert with weak Lewis acid–base complexation. A similar mechanism is proposed for the reaction in THF solution, in competition with a second involving nucleophilic attack of the germylene–THF complex at a chlorine atom of the substrate. Rate constants were also determined for reaction of CCl4 with the corresponding tetraaryldigermenes at low halocarbon concentrations in hexanes, along with Arrhenius parameters for the parent (Ge2Ph4). These reactions also proceed via initial Cl-atom abstraction, based on the identity of the products formed in the reaction of CCl4 with tetramesityldigermene.