C2 O2 Research Articles

Deoxygenative Coupling of CO with a Tetrametallic Magnesium Hydride Complex.

Addition of CO to a tetrametallic magnesium hydride cluster results in both carbon-carbon bond formation and deoxygenation to generate an acetaldehyde enolate [C2OH3]- which remains coordinated to the cluster. To the best of our knowledge, this is the first example of formation of an isolable complex containing an [C2OH3]- fragment from reaction of CO with a metal hydride, and the first example of CO homologation and deoxygenation at a main group metal. DFT studies suggest that key steps in the mechanism involve nucleophilic attack of an oxymethylene on a formyl ligand to generate an unstable [C2O2H3]3- fragment, which undergoes subsequent deoxygenation.

Mass spectrometric study of low energy Cs+ ion-induced sputtered fragmentation of PADC polymer.

In this study, low-energy cesium (Cs+ ) ion-induced sputtered fragmentation of poly allyl diglycol carbonate (PADC) was investigated using mass spectrometry. The collision-induced dissociation mechanism revealed emission of various fragments, including monoatomic (H- , C1 - , O1 - ), diatomic (C2 - ), and multiatomic (C3 - , CO2 - , C2 O2 - , C3 O2 - ) species within the Cs+ ion energy range of 1-5keV. The anion current of these fragments exhibited a linear increase with rising incident Cs+ ion energy, indicating a corresponding rise in fragment abundance. Analysis of normalized yield indicated that at 1keV incident energy, the dominant fragment was monoatomic hydrogen (H- ), followed by diatomic carbon (C2 - ), monoatomic carbon (C1 - ), and monoatomic oxygen (O1 - ). Although C2 - remained dominant up to 5keV, other fragments exhibited varying normalized yields at different ion energy steps. The sputter yield estimation revealed that monoatomic hydrogen (H- ) and diatomic carbon (C2 - ) exhibited the highest yields, increasing exponentially beyond 3keV, while multiatomic fragments like C3 - , CO2 - , C2 O2 - , and C3 O2 - displayed the lowest yields. The sputter dissociation mechanism pointed to dehydrogenation, chain scission, and bond breakage as the primary processes during low-energy Cs+ ion impact. Postsputtering Scanning Electron Mircoscope (SEM) micrographs show craters, pits, and micropores on the PADC surface, indicating significant surface degradation. X-ray Diffraction (XRD) spectra exhibited reduced diffraction intensity, while Fourier Transform Infrared Spectroscopy (FTIR) analysis indicated the absence of molecular bands in the IR spectrum, confirming extensive surface damage due to Cs+ ion-induced sputtering.

PartialDeoxygenative CO Homocoupling By a Diiron Complex.

One route to address climate change is converting carbon dioxide to synthetic carbon-neutral fuels. Whereas carbon dioxide to CO conversion has strong precedent in homo- and heterogeneous catalysis, deoxygenative coupling of CO to products with C-C bonds-as in liquid fuels-remains challenging. Here, we report coupling of two CO molecules by a diiron complex. Reduction of Fe2(CO)2L (2), where L2- is a bis(β-diketiminate) cyclophane, gives [K(THF)5][Fe2(CO)2L] (3), which undergoes silylation to Fe2(CO)(COSiMe3)L (4). Subsequent C-OSiMe3 bond cleavage and C=C bond formation occurs upon reduction of 4, yielding Fe2(μ-CCO)L. CO derived ligands in this series mediate weak exchange interactions with the ketenylidene affording the smallest J value, with changes to local metal ion spin states and coupling schemes (ferro- vs. antiferromagnetism) based on DFT calculations, Mossbauer and EPR spectroscopy. Finally, reaction of 5 with KEt3BH or methanol releases the C2O2- ligand with retention of the diiron core.

3-(2-Chloro-5-methylphenoxy)propane-1,2-diol

3-(2-Chloro-5-methylphenoxy)propane-1,2-diol 1 is a possible precursor in the chiral beta blocker Bupranolol synthesis. Both racemic and single-enantiomeric samples of 1 were synthesized and characterized by single crystal XRD. The absolute configuration of an (S)-1 sample was determined by data refinement (the value of the Flack parameter is 0.03(4)). Hydrogen-bonded supramolecular synthons (SMS) were identified for both crystals. In rac-1, as well as in (S)-1 crystals, one of the two main SMS is the homochiral chain C11(5):21 (⋯O1–C1–C2–O2–H2⋯). The second of the two SMSs changes is the change of chiral environment. For rac-1, this is the heterochiral chain C33(9):{12/12/21}; (⋯O2–C2–C1–O1–H1⋯O′2–H′2⋯O″1–H″1⋯), in which molecules with different configurations alternate. In (S)-1 crystals, this is the homochiral chain C11(2):11 (⋯O1–H1⋯). The results obtained once again confirm the influence of the chiral environment on the crystallization of scalarly identical molecules.

Two-bond 13C-13C spin-coupling constants in saccharides: dependencies on exocyclic hydroxyl group conformation.

Seven doubly 13C-labeled isotopomers of methyl β-D-glucopyranoside, methyl β-D-xylopyranoside, methyl β-D-galactopyranoside, methyl β-D-galactopyranosyl-(1→4)-β-D-glucopyranoside and methyl β-D-galactopyranosyl-(1→4)-β-D-xylopyranoside were prepared, crystallized, and studied by single-crystal X-ray crystallography and solid-state 13C NMR spectroscopy to determine experimentally the dependence of 2JC1,C3 values in aldopyranosyl rings on the C1-C2-O2-H torsion angle, θ2, involving the C2 carbon of the C1-C2-C3 coupling pathway. Using X-ray crystal structures to determine θ2 in crystalline samples and by selecting compounds that exhibit a relatively wide range of θ2 values in the crystalline state, 2JC1,C3 values measured in crystalline samples were plotted against θ2 and the resulting plot compared to that obtained from density functional theory (DFT) calculations. For θ2 values ranging from ∼90° to ∼240°, very good agreement was observed between the experimental and theoretical plots, providing strong validation of DFT-calculated spin-coupling dependencies on exocyclic C-O bond conformation involving the central carbon of geminal C-C-C coupling pathways. These findings provide new experimental evidence supporting the use of 2JCCC values as non-conventional spin-coupling constraints in MA'AT conformational modeling of saccharides in solution, and the use of NMR spin-couplings not involving coupled hydroxyl hydrogens as indirect probes of C-O bond conformation. Solvomorphism was observed in crystalline βGal-(1→4)-βGlcOCH3 wherein the previously-reported methanol solvate form was found to spontaneously convert to a monohydrate upon air-drying, leading to small but discernible conformational changes in, and a new crystalline form of, this disaccharide.

Convenient Preparation and Structure Determination of Air- and Moisture-Tolerant Difluoromethylborates.

Convenient and reliable synthetic methods for difluoromethylborates have been established. The intermediary generated difluoromethylsilicate species from TMSCF2 H (TMS=trimethylsilyl) and potassium tert-butoxide were allowed to react with pinBPh (Me4 C2 O2 BPh) in the presence of 18-crown-6 to give the corresponding borate compound [pinB(Ph)CF2 H]- K+ (18-crown-6) as an air- and moisture-tolerant solid. The unambiguously determined crystal structure of [pinB(Ph)CF2 H]- K+ (18-crown-6) revealed that the difluoromethylborate unit partially coordinated on the potassium ion. Reaction of [pinB(Ph)CF2 H]- K+ (18-crown-6) with potassium difluoride (KHF2 ) in acetic acid enabled substitution of the pinacol unit and phenyl group with fluorides, and gave (difluoromethyl)trifluoroborate [F3 BCF2 H]- K+ (18-crown-6) in a good yield. The crystal structure of air- and moisture-tolerant [F3 BCF2 H]- K+ (18-crown-6), which would be a promising reagent for synthesis of various difluoromethylboron species, showed a polyrotaxane-like polymeric structure based on the K⋅⋅⋅F interactions between the K+ (18-crown-6), CF2 H, and BF3 units.

Optical spectroscopy of cryogenic metalated flavins: The O2(+) isomers of M+lumiflavin (M=Li–Cs)

Flavin complexes are used by nature in many photobiological processes, because their photochemical response in the visible range can be strongly modulated by their environment. Herein, we report optical spectra of mass-selected metal complexes of lumiflavin (LF) with alkali metal ions (M=Li–Cs) recorded in the visible range by photodissociation (VISPD) at cryogenic temperatures (T<20 K). VISPD spectra are measured in a tandem mass spectrometer coupled to a cryogenic ion trap and an electrospray ionization source. The vibrationally-resolved VISPD spectra obtained in the 420–450 nm range are assigned to the S0←S1 (ππ*) transition of the O2(+) isomers of M+LF by comparison to time-dependent density functional theory calculations (PBE0/cc-pVDZ) coupled to multidimensional Franck–Condon (FC) simulations. The preferred binding motif and interaction strength strongly depend on the size of M+. The spectra of M+LF with the smaller M+ ions (M=Li and Na) are attributed to the most stable O2+ isomers characterized by a N1–M–O2 chelate binding motif in which M+ can interact with the lone pairs of both N1 and O2 of LF. Because of steric interaction with the CH3 group at N10, the tricyclic aromatic ring is slightly bent and the VISPD spectra feature low-frequency out-of-plane bending modes of LF. Such an O2+ structure is sterically repulsive for the larger M+ ions (M=Rb and Cs), which instead form planar O2 minima with nearly linear C2–O2–M bonds. Their VISPD spectra are characterized by low-frequency in-plane bending modes (β) describing the M+…LF interaction. The VISPD spectrum of K+LF with the intermediate-size K+ ion is more complex and features very low-frequency modes resulting from a very shallow potential along the O2↔O2+ isomerization coordinate, which cannot be described well by harmonic FC simulations. Electronic S1 excitation slightly weakens the M+…LF interaction (6–9%), as deduced from the small ΔS1 blueshifts upon metalation at the O2(+) binding site, consistent with the charge reorganization deduced from the molecular orbitals involved in ππ* excitation. Overall, the effects of O2(+) complexation of LF are drastically different from those of the previously studied O4+ isomers, which are characterized by large ΔS1 redshifts based on the strong increase in the M+…LF interaction (up to 20%) upon the same ππ* excitation of the LF chromophor. The drastic site-specific variation in the photophysical response of the O2(+) and O4+ isomers arises mainly from the different effect of electronic excitation on the M+…LF bond strength rather than from the metal-induced change of the electronic structure and molecular orbitals of LF responsible for ππ* excitation.

Reduction of Carbon Oxides by an Acyclic Silylene: Reductive Coupling of CO.

Reactions of a boryl-substituted acyclic silylene with carbon dioxide and monoxide are reported. The former proceeds through oxygen atom abstraction, generating CO (with rearrangement of the putative silanone product through silyl-group transfer). The latter is characterized by reductive coupling of CO to give an ethynediolate fragment, which undergoes formal insertion into the Si-B bond. The net conversion of carbon dioxide with two equivalents of silylene offers a route for the three-electron reduction of CO2 to [C2 O2 ]2- .

Computational Mutation Study of the Roles of Catalytic Residues in Coenzyme B12-Dependent Diol Dehydratase

Abstract The His143, Glu170, and Asp335 residues at the substrate-binding site of diol dehydratase, a calcium–metalloenzyme, are shown by a computational mutation study to play important roles in OH group migration (the second step in the enzymatic reaction). The reaction is accelerated by the synergetic interplay of the heterolysis of the C2–O2 bond of 1,2-diol radical and the partial deprotonation of the spectator OH group by Glu170. The His143 residue works as a donor to the migrating OH group through a hydrogen bond, which contributes to the C2–O2 bond heterolysis and resultant resonance stabilization. The Glu170 residue activates the spectator OH group to energetically stabilize the transition state in the OH group migration. The resonance stabilization of the transition state in the OH group migration is observed in the wild-type enzyme while not in the His143Ala mutant. Since the cleavage of the C2–O2 bond of 1,2-diol radical proceeds in a more homolytic manner in the His143Ala mutant, Glu170 cannot effectively deprotonate the spectator OH group in the transition state. As a result, the activation energy of the OH group migration in the His143Ala mutant is increased compared to that in the wild-type enzyme. The spectator OH group is not fully activated in the Glu170Gln and Glu170Ala mutants during the OH group migration, and thus the activation energies in the Glu170Gln and Glu170Ala mutants are higher than that in the wild-type enzyme. In contrast, the OH group migration is accelerated in the Asp335Ala mutant, due to the absence of the electric repulsion between Asp335 and the migrating OH group. The computed relative activity of the His143Ala, Glu170Gln, and Glu170Ala mutants successfully reproduces the experimentally determined catalytic activity, indicating that a computational mutation study offers a useful methodology in enzyme research.

Electronic excitation of gaseous acetic acid studied by K-shell electron energy loss spectroscopy and ab initio calculations

Ab initio Configuration Interaction calculations have been carried out in order to assign the bands observed in the carbon and oxygen K-shell spectra of gaseous acetic acid, measured using the inner-shell electron energy loss spectroscopy (ISEELS) method with better energy resolution than in previous studies. The good agreement between the theoretical and the measured spectra allows us to assign precisely most of the peaks, especially for the Rydberg states. Some of them have been shown to have strong valence character. The lowest energy band at the carbon edge is assigned to the transitions 1sC1→3pπ/σ*(C1–H) and 1sC1→π*(C2–O2). Simple Franck–Condon calculations, based on the linear coupling approximation, were performed in order to reproduce the vibrational structure observed for the first time in the oxygen and carbon core excited species as well as in the previously measured X-ray photoelectron spectroscopy spectra of the core ionised molecule. Finally, the calculated structural parameters of the core states of acetic acid match well those of the corresponding valence states of their Z+1 molecules, as predicted by the equivalent core approximation. However, significant differences between the geometry of the 1sC1→π*(C2–O2) state and the CH3NOOH ground state are obtained.

New insights into the conformational properties of α-C-glucosides

A series of alkyl α-d-C-glucopyranosides were synthesized and their conformational properties analyzed by CD and NMR spectroscopy. The conformational analysis revealed that the hydroxymethyl group populations (torsion angle ω, O1–C1–C2–O2) and those around the C-glucopyranosidic bond (torsion angle Φ; O2–C6–C7–C8) depend on the structural nature of the C-aglycon. The gt and the exo–syn populations increased as the C-aglycon became more substituted. Linear correlations between these rotational populations and proton chemical shifts versus the Taft’s steric parameters revealed the significant role of the C-aglycon in the overall conformation of C-glucosides. The stereoelectronic exo-deoxoanomeric effect, affecting the rotation of the hydroxymethyl group, becomes more important as the steric hindrance of motion increases. A pseudo-anomer rotational comparison study was also performed.

Revealing regioselectivity in hydrogenation of 1-phenyl-1,2-propanedione on Pt catalysts

Adsorption of 1-phenyl-1,2-propanedione ( A), a widely studied molecule in heterogeneously catalyzed enantioselective hydrogenation, and 2,3-hexanedione on a Pt(111) surface was studied using density functional theory. A cluster consisting of two slabs and 31 Pt atoms was used as a model for the catalyst. The results revealed the origin of observed regioselectivity in the hydrogenation of carbonyl groups C1 O1 and C2 O2 of A and the lack of regioselectivity in the case of 2,3-hexanedione on Pt catalysts. The adsorption modes of A in which the C1 O1 carbonyl group next to the phenyl ring is activated toward hydrogenation ( η C 1 O 1 2 adsorptions) are more stable than the corresponding adsorption modes where the C2 O2 group is activated ( η C 2 O 2 2 adsorption). This indicates that the catalyst surface is covered mainly by reaction intermediates leading to the hydrogenation of the carbonyl group C1 O1 and eventually regioselectivity, presuming that the hydrogenation rates of C1 O1 and C2 O2 are of the same order of magnitude. The adsorption energy of 2,3-hexanedione does not depend on which of the carbonyl groups is adsorbed as η 2 -mode, and thus the hydrogenation on Pt is not regioselective, as has been observed experimentally. Thus the regioselectivity in the hydrogenation of A and 2,3-hexanedione can be explained by the interactions between the substrate and the metal surface.

Electronic and vibrational circular dichroism spectroscopic study of non-covalent interactions of meso-5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin with (dG-dC) 10 and (dA-dT) 10

The non-covalent interactions of (dG-dC) 10 and (dA-dT) 10 with 5,10,15,20-tetrakis(1-methylpyridinium-4-yl)porphyrin (TMPyP) were studied using the combination of electronic circular dichroism (ECD), vibrational circular dichroism (VCD) spectroscopy, and UV–vis and IR absorption spectroscopy at different ratios of both components r = [oligonucleotide]/[TMPyP] = 2/1–10/1 where [oligonucleotide] and [TMPyP] are the amount concentrations of oligonucleotide per base-pair and TMPyP, respectively. It was shown that TMPyP with (dG-dC) 10 provided hemiintercalative binding mode for r = 4/1 that is manifested in vibrational spectra: The absorption band assigned to the C6 O6 stretching vibration of guanine is shifted from 1683 to 1672 cm −1, the corresponding VCD couplet from 1694(−)/1674(+) to 1684(−)/1663(+) cm −1 and its intensity decreases. The absorption band assigned to the C2 O2 stretching vibration of cytosine is shifted from 1652 to 1644 cm −1 and its intensity increases. TMPyP with (dA-dT) 10 provided three binding modes: (i) external binding to the phosphate backbone, (ii) external minor groove binding for the ratios >6/1 and (iii) external major groove binging associated with the partial B- to Z-transition for the ratios <4/1. The major groove binding is manifested in VCD spectra by the intensity decrease of the bands 1655 and 1638 cm −1 assigned to the thymine vibrations while the bands assigned to the adenine vibrations are unchanged. In the (dA-dT) 10–TMPyP complexes, the external binding to the phosphate backbone accompanied by self-stacking of porphyrins along the phosphate backbone chain is preferred at temperatures higher than 40 °C.

Parallel DNA double helices incorporating isoG or m 5isoC bases studied by FTIR, CD and molecular modeling

FTIR spectroscopy has been used to follow the formation of parallel stranded DNA duplexes incorporating isoG or m 5isoC bases and determine their base pairing scheme. The results are discussed in comparison with data concerning anti-parallel duplexes with comparable base composition and sequence. In duplexes containing A–T and isoG–C or m 5isoC–G base pairs shifts of the thymine C2 O2 and C4 O4 carbonyl stretching vibrations (to lower and higher wavenumbers, respectively, when compared to their positions in classical cis Watson–Crick (WC) base pairs) reflect the formation of trans Watson–Crick A–T base pairs. All carbonyl groups of cytosines, m 5isocytosines, guanines and isoguanines are found to be involved in hydrogen bonds, indicative of the formation of isoG–C and m 5isoC–G base pairs with three hydrogen bonds. Molecular modeling shows that both structures form regular right handed helices with C2′endo sugar puckers. The role of the water content on the helical conformation of the parallel duplexes has been studied by FTIR and CD. It is found that a conformational transition similar to the B → A transition observed for anti-parallel duplexes induced by a decrease of the water content of the samples can occur for these parallel duplexes. Their helical flexibility has been evidenced by FTIR studies on hydrated films by the emergence of absorption bands characteristic of A type geometry, in particular by an S-type → N-type repuckering of the deoxyribose. All sugars in the parallel duplex with alternating d(isoG–A)/d(C–T) sequence can adopt an N-type geometry in low water content conditions. The conformational transition of the parallel hairpin duplex with alternating d(isoG–A)/d(C–T) sequence was followed by circular dichroism in water/trifluoroethanol solutions and its free energy at 0 °C was estimated to be 6.6 ± 0.3 kcal mol −1.

A combined experimental and theoretical study of 1-phenylpropane-1,2-dione hydrogenation over heterogeneous cinchonidine-modified Pt catalyst

The enantioselective hydrogenation of vicinal diketones over cinchonidine-modified Pt resulted in enantiomeric excess of structurally similar ( R)-enantiomers. Furthermore, the kinetic resolution was caused due to faster reaction of ( S)-hydroxyketone further to diols, resulting in an increase of ee. The diastereoselectivities in diols were similar. The ( R,S) or ( S,R) diols were always the main products whereas considerably less ( R,R) of ( S,S) were formed. For the first time in 1-phenylpropane-1,2-dione ( A) hydrogenation enantiomeric excesses of both C1O1 and C2O2 group have been reported. The ee 1-OH and ee 2-OH were 50 and 25%, respectively, at 50% conversion of A. Based on batch and continuous reactor experiments it could be concluded that the source of enantioselectivity is an increased formation rate of ( R)-enantiomer and decreased formation rate of ( S)-enantiomer. Theoretical calculations revealed that in the substrate-modifier diastereomeric complex the reactant forms a nonplanar s- cis conformation and bonds to the protonated cinchonidine either via a bifurcated hydrogen bond or with two hydrogen bonds where the OH group is involved also. Optimized diastereomeric complexes were equal in energy. The calculated proton affinity of CD was high, 1000 kJ mol −1, indicating that protonation is feasible under typical experimental conditions.

Synthesis and Thermal Decomposition of Mixed 2,4'-bipyridine-oxalato Complexes With Mn(II), Co(II), Ni(II) and Cu(II)

The mixed 2,4'-bipyridine-oxalato complexes of the formulae M(2,4'-bipy)2 C2 O4 2H2 O (M (II)=Mn, Co, Ni, Cu; 2,4'-bipyridine=2,4'-bipy or L ; C2 O2–4 =ox) have been prepared and characterized. IR data show that the 2,4'-bipy coordinated with these metals(II) via the least hindered (4')N atom; that oxalate group acts as bidentate chelating ligand. Room temperature magnetic moments are normal for the orbital singlet states. The thermal decomposition of these complexes was investigated by TG, DTA and DTG in air. The endothermic or exothermic character of the decomposition of ML2 (ox)2H2 O was discussed.

Tris{[tris(2-hydroxymethyl)methyl]ammonium}D-3-Phosphoglycerate at 150K and Tris(cyclohexylammonium)D-3-Phosphoglycerate Ethanol Solvate at 85K

The structures of the d-3-phosphoglycerate trianion in the tris{[tris(2-hydroxymethyl)methyl]ammonium} salt, 3C4H12NO3+.C3H4O7P3−, (I), and in the tris(cyclohexylammonium) ethanol solvate 3C6H14N+.C3H4O7P3−.C2H5OH, (II), has been determined by X-ray analyses at 150 and 85 K, respectively. A notable feature of (I) is the shortest P—O(ester) bond [1.607 (2) Å] found so far for the completely ionized phosphate ester group. In contrast, in (II) this bond length is 1.627 (2) Å, similar to that observed in other doubly ionized phosphate ester groups. In (II) the carboxyl­ate group is twisted by −23.5 (4)° relative to the α-hydroxyl group, as indicated by the O1—C1—C2—O2 torsion angle, whereas in (I), the O2—C2—C1—O1 system is almost planar. Both crystal structures are stabilized by hydrogen bonds utilizing all N and O atoms.

Hexakis(urea-O)magnesium Dichlorate

In the title compound, [Mg{OC(NH2)2}6](ClO3)2, Mg atoms are octahedrally coordinated by the O atoms of six urea molecules. Urea1 is in-plane coordinated, while Urea2 and Urea3 are bonded with N21—C2—O2—Mg and N31—C3—O3—Mg torsion angles of 22.2 (4) and 27.3 (4)°, respectively. The structure is built up of [Mg(urea)6]2+ and ClO3− ions held together by hydrogen bonds.

1,2-Ethanediol (Ethylene Glycol) at 130K

A single-crystal of the most commonly used anti-freezing agent, ethylene glycol (mp 261.6 K), was grown in situ at 220 K with the help of a miniature zone melting procedure utilizing an ir-laser beam (Boese & Nussbaumer, 1994). The structure solution and refinement followed standard procedures, in which all hydrogen atoms were treated without constraints. The molecules have a synclinal conformation (torsion angle O1—C1—C2—O2 − 64.4 °) and the packing consists of catamer-like, two-dimensional networks with hydrogen bridges parallel to the [100] plane, O⋯O distances 2.711 (1) and 2.730 (2) Å and O-H⋯O angles 176 (2), 171 (2)°.

A Cycloboronated Nickelaundecaborane: [(PhCOS)2NiB10H8(PPh3)

The title compound 7,8:7,11-di(μ-thiobenzoato-S:O)-10-(triphenylphosphine-P)-8:9-μH– 7-nickela-nido-undecaborane, C32H33B10NiO2PS2, has been synthesized and characterized by X-ray diffraction analysis. The cluster is a nido eleven-vertex {NiB10} cage with the Ni atom in the open NiB4 face. Cyclizations resulting in two five-membered rings, Ni7—S1—C1—O1—B8 and Ni7—S2—C2—O2—B11, have occurred.