Bond Homolysis Reactions Research Articles

Excited State Bond Homolysis of Vanadium(V) Photocatalysts for Alkoxy Radical Generation.

Advancements in photocatalysis have transformed synthetic organic chemistry, using light as a powerful tool to drive selective chemical transformations. Recent approaches have focused on metal-halide ligand-to-metal charge transfer (LMCT) photoactivated bond homolysis reactions leveraged by earth-abundant elements to generate valuable synthons for radical-mediated cross-coupling reactions. Of recent utility, oxovanadium(V) LMCT photocatalysts exhibit selective alkoxy radical generation from aliphatic alcohols upon blue light (UVA) irradiation under mild conditions. The selective photochemical liberation of alkoxy radicals is valuable for applying late-stage fragmentation approaches in organic synthesis and depolymerization strategies for nonbiodegradable polymers. Steady-state and time-resolved spectroscopy were used to assign the electronic structure of three well-defined V(V) photocatalysts in their ground and excited states. We assign the excited state for this transformation at earth-abundant vanadium(V), interrogating the electronic structure using static UV-visible absorption, ultrafast transient absorption, and electron paramagnetic resonance spectroscopy coupled to computational approaches. These findings afford assignments of the short-lived excited state intermediates that dictate selective homolytic bond cleavage in metal alkoxides, illustrating the valuable insight gleaned from fundamental investigations of the molecular photochemistry responsible for light-escalated chemical transformations.

Triplet Upconversion under Ambient Conditions Enables Digital Light Processing 3D Printing.

The rapid photochemical conversion of materials from liquid to solid (i.e., curing) has enabled the fabrication of modern plastics used in microelectronics, dentistry, and medicine. However, industrialized photocurables remain restricted to unimolecular bond homolysis reactions (Type I photoinitiations) that are driven by high-energy UV light. This narrow mechanistic scope both challenges the production of high-resolution objects and restricts the materials that can be produced using emergent manufacturing technologies (e.g., 3D printing). Herein we develop a photosystem based on triplet-triplet annihilation upconversion (TTA-UC) that efficiently drives a Type I photocuring process using green light at low power density (<10 mW/cm2) and in the presence of ambient oxygen. This system also exhibits a superlinear dependence of its cure depth on the light exposure intensity, which enhances spatial resolution. This enables for the first-time integration of TTA-UC in an inexpensive, rapid, and high-resolution manufacturing process, digital light processing (DLP) 3D printing. Moreover, relative to traditional Type I and Type II (photoredox) strategies, the present TTA-UC photoinitiation method results in improved cure depth confinement and resin shelf stability. This report provides a user-friendly avenue to utilize TTA-UC in ambient photochemical processes and paves the way toward fabrication of next-generation plastics with improved geometric precision and functionality.

Pyrolysis mechanism law of β-O-4 lignin dimer model compounds: A density functional theory study

Lignin is the second abundant natural renewable after cellulose. Pyrolysis is usually transformed lignin into high value-added platform compounds. In order to fully utilized lignin, the pyrolysis mechanism needs an in-depth understanding. Herein we performed density function theory (DFT) calculation to unveil the initial mechanism of pyrolysis of 9 representative β − O− 4 lignin dimer model compounds with phenyl-1, 3-propanediol as the main body. Several cleavage pathways were investigated, including Cβ−O bond breaking（Cβ−O bond homolysis reaction, Maccoll elimination reaction and retro-ene fragmentation reaction） and Cα−Cβ bond breaking. Results show that the substituents on the B-ring have a greater effect on the pyrolysis pathway of lignin dimer than that on the A-ring. When no substituent on the ortho-position of B-ring, the retro-ene fragmentation reaction is the dominant pathway. And, its main reason is these structures have no steric hindrance in the process of hydrogen transfer. The Cβ−O bond homolysis is the dominant pathway when methoxy group exists on ortho-position of B-ring. Maccoll elimination reaction also has certain advantages due to the influence of electronic factors of methoxyl group on ortho-position of B-ring. The cleavage reaction of Cβ−O bond is always prior to that of Cα−Cβ bond. The effect of water molecules on the pyrolysis reaction has been also discussed. The results showed that H2O could promote the Maccoll elimination I reaction, but did not change the dominant pyrolysis mechanism.

Mobile loop dynamics in adenosyltransferase control binding and reactivity of coenzyme B12

Cobalamin is a complex organometallic cofactor that is processed and targeted via a network of chaperones to its dependent enzymes. AdoCbl (5'-deoxyadenosylcobalamin) is synthesized from cob(II)alamin in a reductive adenosylation reaction catalyzed by adenosyltransferase (ATR), which also serves as an escort, delivering AdoCbl to methylmalonyl-CoA mutase (MCM). The mechanism by which ATR signals that its cofactor cargo is ready (AdoCbl) or not [cob(II)alamin] for transfer to MCM, is not known. In this study, we have obtained crystallographic snapshots that reveal ligand-induced ordering of the N terminus of Mycobacterium tuberculosis ATR, which organizes a dynamic cobalamin binding site and exerts exquisite control over coordination geometry, reactivity, and solvent accessibility. Cob(II)alamin binds with its dimethylbenzimidazole tail splayed into a side pocket and its corrin ring buried. The cosubstrate, ATP, enforces a four-coordinate cob(II)alamin geometry, facilitating the unfavorable reduction to cob(I)alamin. The binding mode for AdoCbl is notably different from that of cob(II)alamin, with the dimethylbenzimidazole tail tucked under the corrin ring, displacing the N terminus of ATR, which is disordered. In this solvent-exposed conformation, AdoCbl undergoes facile transfer to MCM. The importance of the tail in cofactor handover from ATR to MCM is revealed by the failure of 5'-deoxyadenosylcobinamide, lacking the tail, to transfer. In the absence of MCM, ATR induces a sacrificial cobalt-carbon bond homolysis reaction in an unusual reversal of the heterolytic chemistry that was deployed to make the same bond. The data support an important role for the dimethylbenzimidazole tail in moving the cobalamin cofactor between active sites.

PH-Sensitive C–ON Bond Homolysis of Alkoxyamines of Imidazoline Series: A Theoretical Study

The pH-dependent kinetics of C-ON bond homolysis reactions of the four alkoxyamines (viz., the derivatives of 2-(4-(dimethylamino)-2-ethyl-5,5-dimethyl-2-(pyridin-4-yl)-2,5-dihydro-1H-imidazol-1-oxyl and 2-(2-carboxyethyl)-5,5-diethyl-2,4-dimethyl-2,5-dihydro-1H-imidazol-1-oxyl) in water solution have been scrutinized using DFT calculations (M06-2X/6-311++G(2df,p) level of theory with the PCM model). On the basis of computations, the experimental results obtained before (J. Org. Chem. 2011, 76, 5558) have been rationalized. The concentration dependence of all possible protonated forms of the four alkoxyamines was obtained from pH measurements. The contributions of particular protonated forms into the overall rate constants of thermolysis were estimated using the DFT calculated Gibbs free energies Δ(r)⁰G of C-ON bond homolysis reactions. The computations revealed that the observed rate constants of thermolysis of the two species at several pH values are dominated by decomposition reactions of two or even three protonated forms. The observed trends in reactivity of the alkoxyamines were mainly attributed to destabilization of the radical products of C-ON bond scission reactions. A linear correlation between the sum of radical stabilization energies (RSEs) of products of thermolysis and the calculated Gibbs free energies of reactions was found for various protonated forms of the species studied. Apart from this, the linear correlation exists between the relative RSE and nitrogen hyperfine constants aN of various protonated forms of the nitroxide radical products.

Unleashing radical sites in non-covalent complexes: The case of the protonatedS-nitrosocysteine/18-crown-6 complex

Introducing radicals onto gas-phase non-covalent complexes and studying their chemistry is a relatively unexplored frontier. In generating these radicals via bond homolysis reactions, it is important that the energy necessary for forming the radical does not exceed the energy required for dissociating the complex itself. Based on this consideration, new approaches for creating these radicals will probably have to involve incorporation of weak bonds that can easily undergo homolysis. The formation of a radical cation, via collision-induced dissociation, of protonated S-nitrosocysteine non-covalently bound to the crown ether 18-crown-6 is described here. The radical cation of this complex was isolated and subjected to collisional activation and ion-molecule reactions with allyl iodide. The results were compared with those of the radical cation of 'bare' cysteine. Collisional activation of the radical cation of the cysteine/crown complex led to fragmentation of cysteine as well as of the crown ether. Ion-molecule reactions of the radical cation of the complex with allyl iodide led to products arising from I and allyl abstraction. Isolation and CID of the former product ion led to the loss of iodocysteine. Cleavage of the weak S-NO bond has allowed the formation of a radical site onto a non-covalent complex. Ion-molecule reactions and collisional activation were utilized to probe the chemistry of this radical cation. The approach adopted here for incorporating a radical onto a cysteine/crown complex shows promise for the introduction of radical sites onto other biological non-covalent complexes.

A Closer Look at Photochemical Reactions of Transition‐Metal Compounds by Time‐Resolved EPR

AbstractFor Abstract see ChemInform Abstract in Full Text.

Carbon–hydrogen bond dissociation enthalpies in ethers: a theoretical study

Density functional theory calculations based on different representations of the exchange-correlation functional (BLYP, B3LYP, B3PW91, mPW1PW91, B1LYP, BHandHLYP, BHandH, and B3P86) were carried out to predict C–H bond dissociation enthalpies in selected ethers (dimethyl ether, tetrahydrofuran (thf), and 1,4-dioxane), and some related molecules. Comparison with available experimental information shows that bond dissociation enthalpies are accurately predicted by several functionals when isodesmic and isogyric reactions are considered. However, for bond homolysis reactions only the hybrid functional B3P86 has an acceptable performance, slightly underestimating bond dissociation enthalpies. DFT calculations were further compared with the multilevel CBS-Q method. The analysis of structural properties of the radical species suggests that hyperconjugation and ring strain are the main features to consider for understanding their thermodynamic stabilization. These effects are also useful to explain some trends of bond dissociation enthalpies that are observed for the compounds involved in the present study. The C–H bond dissociation enthalpies of thf (leading to β-furanyl) and 1,4-dioxane, for which no experimental information is available, are 410 and 381 kJ mol −1, respectively, at the CBS-Q level.

A closer look at photochemical reactions of transition-metal compounds by time-resolved EPR

A review is given of applications of time-resolved electron paramagnetic resonance (TREPR) in the field of photochemistry of transition-metal compounds. The two main TREPR techniques used in these studies are described. A brief overview is given of chemically induced dynamic electron polarization mechanisms that can affect TREPR spectra and that can give insights into the mechanism of photochemical reactions. Following these background sections, experimental results are presented. The discussion focuses in particular on the Fourier-transform EPR studies of photoinduced metal-alkyl bond homolysis reactions of a series of transition-metal (Co, Ru, Re, Pt) complexes carried out by the authors.

Peroxynitrate and peroxynitrite: a complete basis set investigation of similarities and differences between these NOx species.

Peroxynitric acid/peroxynitrate (PNA) rivals peroxynitrous acid/peroxynitrite (PNI) in importance as a reactive oxygen species. These species possess similar two-electron oxidative behavior. On the other hand, stark differences exist in the stability of these molecules as a function of pH and in the presence of CO(2), and also in the types of bond homolysis reactions that PNA and PNI may undergo. Using CBS-QB3 theory, we examine these similarities and differences. The activation barriers for two-electron oxidation of NH(3), H(2)S, and H(2)C=CH(2) by PNA and PNI are found to be generally similar. The O-O BDE of O(2)NOOCO(2-) is predicted to be 26 kcal/mol greater than that of ONOOCO(2-). This accounts for the insensitivity of PNA to the presence of CO(2). Likewise, the O-O BDE of O(2)NOOH is predicted to be 19 kcal/mol greater than that of ONOOH, in excellent agreement with experiment. The fundamental principle underlying the large differences in O-O BDEs between PNA and PNI species is that the NO(2) that is formed from PNI can relax from the (2)B(2) excited state to the (2)A(1) ground state, whereas no such comparable state change occurs with NO(3) from PNA. Decomposition of the anions O(x)NOO(-) is more complex, with the energetics influenced by solvation. ONOO(-) can homolyze to yield NO/O(2-); however, this pathway represents a thermodynamic "dead end" since the radical pairs generated by homolysis should mostly revert to starting material. However, decomposition of O(2)NOO(-) yields the stable products NO(2-)/(3)O(2), a couple that is more stable than starting material. This may occur either by initial formation of NO(2)/O(2-) or NO(2-)/(1)O(2), with the latter intermediates thermodynamically favored both in the gas phase and in solution. Given the extremely fast back-reaction of the homolysis products, heterolysis probably dominates the observed O(2)NOO(-) decomposition kinetics. This is in agreement with the first of two "kinetically indistinguishable" mechanistic possibilities proposed for O(2)NOO(-) decomposition (Goldstein, S.; Czapski, G.; Lind, J.; Merényi, G. Inorg. Chem. 1998, 37, 3943-3947).

IR spectroelectrochemical studies of the metal carbonyl dimers Mn2(CO)10 and [CpM(CO)3]2 (M = W, Mo). Cross-coupling and ligand substitution reactions of electrochemically generated organometallic radicals

Abstract Reflectance IR spectroelectrochemistry (IR SEC) has been used to study the electrochemical reactions of dinuclear metal carbonyl compounds. Reduction of the dinuclear metal carbonyl complexes by two electrons yields two equivalents of the corresponding monomeric metal carbonyl anions. Upon re-oxidation, the dimeric starting materials can be re-formed in almost quantitative yields. Reactions of the dinuclear metal carbonyls include metal-metal bond homolysis reactions such as electrochemical cross-coupling reactions between Mn2(CO)10 and [CpM(CO)3]2, where M = W, Mo. The ligand substitution reaction of Mn2(CO)10 and PPh3, which occurs only during the metal-metal bond homolysis reaction, is also described. The design of the anaerobic specular reflectance IR SEC cell used in these studies is also reported.

Coordination of the potentially tridentate ligands 2,6-diacetylpyridine-bis(anil) (dapa) and 2-(2-(2′-methylidenepyridyl) aminoethyl) pyridine (map) in the complexes fac-BrMn(CO) 3 L, fac-(CO) 5MM′(CO) 3L (M, M′= Mn, Re; L = dapa, map) and their photoproducts. The crystal structure of BrMn(CO) 2(N,N,N-dapa)

In complexes of the type fac-(CO) 5MM′(CO)3(N,N-L) (M, M′= Mn, Re; L = dapa, map) the potentially tridentate ligand L is bidentately (N,N) coordinated to the metal centre M′. Irradiation of these complexes into their lowest-energy absorption band resulted in metal-metal bond homolysis and/or loss of CO from the M′(CO) 3(N,N-L) fragment. Upon irradiation of fac-(CO) 5MRe(CO) 3(N,N-dapa) (M = Mn, Re) only the @etal-metal bond homolysis reaction was observed. The 16-electron Re +(CO) 3(N,N-dapa-) species thus formed was readily converted into the 18-electron radical complex Re +(CO) 3(N,N,N- dapa −.) by coordination of the third nitrogen donor atom of dapa to the Re centre, as shown by ESR spectroscopy. Addition of excess PR 3 (R = nBu, Ph) gave rise to the immediate conversion of Re +(CO) 3(N,N,N-dapa −.) into the Re +(CO) 3(N,N-dapa −.) (PR 3) adduct. The tendency of the dapa ligand to adopt a tridentate coordination mode was also reflected in the ready formation of the complexes BrMn(CO) 2(N,N,N-dapa) and (CO) 5ReMn(CO) 2(N,N,N-dapa). The X-ray structure of the former complex was determined; the dapa ligand is coordinated in-plane and the CO ligands are in a mutually cis-disposition. Irradiation of fac-(CO) 5MMn(CO) 3(N,N-map) (M = Mn, Re) at low temperature gave the CO-bridged complex (CO) 4M(μ-CO)Mn(CO) 2(N,N-map). In the absence of CO, the latter complex reacted thermally to give (CO) 5MMn(CO) 2(N,N,N-map). For M = Mn this complex was stable only below 263 K, whereas for M = Re it was obtained as a stable photoproduct at room temperature. Metal-metal bond homolysis of fac-(CO) 5ReRe(CO) 3(N,N-map) at temperatures below 203 K was followed by a recombination of the resulting · Re(CO) 5 and Re +(CO) 3(N,N-map −.) radicals, which was accompanied by a change in coordination of the map ligand.

Metal to ligand charge transfer photochemistry of Re(I)-alkyl complexes

A preliminary study has been carried out concerning the photochemistry and photophysics of a series of (bpy)Re I(CO) 3-R complexes, where bpy is 2,2′-bipyridine and R=−CH 3,−CH 2Ph and −Ph. Photolysis of the complexes into the lowest energy absorption band (436 nm) leads to a highly efficient ReC bond homolysis reaction for R=−CH 3 and −CH 2Ph. Absorption and low-temperature emission studies of the complexes suggest that the lowest excited state is based on dπ (Re)→π * (bpy) metal to ligand charge transfer (MLCT). Nanosecond transient absorption studies confirm that the MLCT stare is formed after near-UV excitation of (bpy)Re(CO) 3-Ph; however, only the reactive intermediate which is formed as a result of ReC bond homolysis is observed for (bpy)Re(CO) 3−CH 3 and (bpy)Re(CO) 3−CH 2Ph. The results are interpreted by a photochemical model in which the bond homolysis reaction occurs either directly from an unrelaxed MLCT excited state or from a spectroscopically silent state which is reached during non-radiative relaxation of the Franck-Condon MLCT state.

Metal to ligand charge-transfer photochemistry of metal-metal-bonded complexes 11. Halogen atom and electron transfer reactions of (CO) 5MM′(CO) 3(α-diimine) (M, M′Mn, Re) complexes

This article describes three types of photoreactions of the complexes (CO) 5MM′(CO) 3(α-diimine) (M,M′Mm, Re). First of all, halogen atom transfer reactions are discussed which take place in the presence of halocarbons. Reactions with CCl 4 and CBr 4 appeared to proceed by a radical chain mechanism. This catalytic behaviour was ascribed to the CX 3 radicals, formed out of CX 4 which can initiate secondary halogen atom transfer reactions. Quantum yields for the reactions with halocarbons were therefore determined for CH 2Cl 2 which did not show a catalytic behaviour because of its higher CCl bond dissociation energy. For all complexes high quantum yields were obtained (0.3–0.7) and variations of these values throughout the series of complexes were ascribed to changes in spin-orbit coupling and rates of non-radiative decay processes. The photoreactions were also studied in the presence of 9,10-phenanthrenequinone in order to confirm that these quantum yields were indeed a measure for the photolysis of the patent compounds. Quinones are known to react with the primary photoproducts of both the CO loss and MM′ bond homolysis reactions. From the agreement between the quantum yield data for the reactions with CH 2Cl 2 and 9,10-phenanthrenequinone, it was concluded that the CO loss products of the (CO) 5MM′(CO) 3(α-diimine) compounds, contrary to those of the Ph 3SnMn(CO) 3(α-diimine) complexes, also react with halocarbons. Finally, the complexes (CO) 5MRe(CO) 3(α-diimine) photodisproportionate in the presence of 2e-donors L (LTHF, PR 3) into M(CO) 5 − and Re(CO) 3(α-diimine)(L) + ions. Irradiation of the complexes in toluene at 183 K gave rise to another electron transfer reaction, viz. the formation of a contact ion-pair (CIP). The CIPs [M(CO) 5 −Re(CO) 3(α-diimine) +] showed a characteristic low frequency v(CO) band of a bridging CO ligand that was not present in the spectra of solvent-separated ion-pairs.

Spectrophotometric determination of trace amounts of copper(II) using the homolysis reaction of the copper(II)–N-p-nitrobenzyl-5,10,15,20-tetrakis(4-sulphonatophenyl)porphine complex

N-p-Nitrobenzyl-5,10,15,20-tetrakis(4-sulphonatophenyl)porphine[p-NO2Bz(Htpps)4–] was synthesized and used for the spectrophotometric determination of trace amounts of copper(II) based on the following reaction: p-NO2Bz(Htpps)4–+ CuII→p-NO2Bz[Cu(tpps)]3–+ H+ [graphic omitted] Cu(tpps)4–+p-NO2BzOH. The p-nitrobenzyl group was removed to produce the non-substituted porphyrins by cleavage of the C—N bond (homolysis reaction). The reaction caused a large change in the absorption spectra and was specific for copper(II) as this ion fitted well into the porphyrin core. Excess of zinc(II) and palladium(II) did not show the homolysis reaction. The acid dissociation constants of the porphyrin (–log Ka) were found to be 2.51 ± 0.02 and 7.70 ± 0.02 at 25 °C and an ionic strength of 0.1 mol dm–3(NaNO3). Optimum conditions for the determination of copper(II) were investigated and the method was applied to the determination of ppb levels of total copper(II) in a river water sample and in some interstitial sea-water samples from the mud at the bottom of the Ariake Sea. The molar absorptivity was 4.22 × 105 dm3 mol–1 cm–1 and the Sandell sensitivity was 0.15 ng cm–2 of copper(II).

Carbon reduction in triethylarsenic-grown GaAs films using chemically activated arsine as a co-reagent

N-type gallium arsenide films grown from triethylarsenic (Et3As) and trimethylgallium (Me3Ga) are generally of poor quality (μ77K(max) = 16,100 cm2/V-s) and are severely contaminated with carbon (>1018 cm−3), whereas films grown using a mixture of triethylarsenic and arsine (AsH3) with Me3Ga are typically of high purity (β77K(max) = 60,000 cm2/V-s) and contain significantly reduced carbon levels (~mid-1015 cm-3). These differences in film purity are due to the inherent growth chemistry of each reagent mixture. The respective growth chemistries of these reagent systems have been inferred from a series of decomposition experiments carried out under pseudo-growth conditions, and the differences in growth chemistry are consistent with the differences in corresponding epilayer purity. Triethylarsenic appears to decompose primarily via a bond homolysis reaction to generate alkyl-containing radical species, which can react with a growing GaAs epilayer to cause severe carbon contamination. In the Et3As/AsH3 coreagent system, the Et3As reagent decomposes to produce these alkyl-containing radical intermediates, but they then apparently react further with the arsine co-reagent to generate reactive arsenic hydride radicals under relatively facile conditions. These reactive arsenic-hydride radical species can contribute to the GaAs growth process without introducing carbon into the resultant films.