Synthesis and characterization of azaborepin radicals in solid neon through boron-mediated C-N bond cleavage of pyridine.

The reactivity of pyridine is a complex topic due to its unique electronic structure. The reactions of atomic boron with pyridine molecules in solid neon have been investigated using matrix isolation infrared absorption spectroscopy. Three products (marked as A, B, and C) were observed and characterized through 10B, D and 15N isotopic substitution pyridine regents as well as quantum chemical calculations. In the reaction, the ground-state boron atom can attack the lone pair electrons of the nitrogen atom in the pyridine molecule, resulting in the formation of a 1-boropyridinyl radical (A). Alternatively, addition to the aromatic π-system of pyridine can occur in a [1,4] type, leading to the formation of a B[η2(1,4)-C5H5N] complex (B). Under UV-visible light (280 < λ < 580 nm) irradiation, these two compounds can further undergo photo-isomerization to form BN-embedded seven-membered azaborepin compounds (C). The observation of species A, B, and the subsequent photo-isomerization to species C is consistent with theoretical predictions, indicating that these reactions are kinetically favorable. This research provides valuable insights into the future design and synthesis of corresponding BN heterocyclic derivatives.

The intermolecular polar bond interaction and coupling induced spectral splitting phenomenon for a binary mixture.

To explain the polarization Raman noncoincidence effect of specific polar bonds and the noncoincidence phenomenon between FT-Raman and FT-IR spectra, aggregation-induced spectral splitting theory was proposed. In this paper, the vibration splitting theory was demonstrated using two strategies: improving the spectral resolution with cryogenic matrix isolation techniques and identifying cases where the coupling splitting is large enough to be distinguishable. The monomer and dimer splitting bands of acetone were detected when cryogenically isolated by the Ar matrix. Additionally, the polarization Raman and two-dimensional infrared spectra of a ß-propiolactone (PIL)/CCl4 binary mixture were collected at room temperature, and the spectral splitting phenomenon was clearly observed. The dynamic transformation between the monomer and dimer could be achieved and detected by adjusting the PIL concentration. The observed splitting phenomenon was further confirmed by theoretical DFT calculations based on the monomer and dimer of PIL, as well as the FT-IR and FT-Raman spectra of PIL. Concentration-triggered 2D-COS synchronous and asynchronous spectra also confirmed the splitting phenomenon and the dilution kinetics of PIL/CCl4.

Spectroscopic characterization of two boron heterocyclic radicals in the solid neon matrix.

Two novel boron heterocyclic radicals, a 3,4,5-trihydroborinine radical and a 1-methyl-2-dihydro-1H-borole radical, were observed in the reaction of boron atom with cyclopentene. These radicals were trapped in solid neon and identified using matrix isolation infrared spectroscopy as well as quantum chemical calculations. The ground-state boron atom selectively inserts into the CC double bond of cyclopentene forming a 3,4,5-trihydroborinine radical (A). This radical can further isomerize to a 1-methyl-2-dihydro-1H-borole radical (B) under UV light irradiation via multiple hydrogen-atom transfer and rearrangement reactions. The observation of species A and further photo-isomerization to species B is consistent with theoretical predictions that these reactions are thermodynamically exothermic and kinetically facile. This is an exciting example of the synthesis of borole derivatives through multiple hydrogen-atom transfer and rearrangement reactions of borinine compounds induced by light irradiation. The present findings provide new insight into the future design and synthesis of the corresponding boron heterocyclic compounds.

Formation of 1-ethynyl-1H-silole from the reaction of silicon atoms with benzene: matrix infrared spectroscopy and quantum chemical calculations.

The reaction of silicon atoms with benzene molecules in solid neon is studied using matrix isolation infrared spectroscopy. Aided by carbon-13 and deuterium isotopic shifts as well as quantum-chemical calculations, the reaction intermediates and products are safely assigned. Among them, silicon atom addition to the aromatic π-system of benzene occurs in a [1,4] fashion Si[η2(1,4)-C6H6] complex (A) has been formed spontaneously on annealing, which further isomerizes to the seven-membered ring 1-silacycloheptatrienylidene (B) under UV light irradiation. A previously unconsidered five-membered ring 1-ethynyl-1H-silole (C) is generated as the final product under broadband UV light irradiation. The underlying reaction mechanism of complex A photo-isomerizing to species B and C is discussed in detail, which shows a remarkable boron-silicon diagonal relationship as compared to the reaction of boron with benzene in forming borole derivatives. The results presented herein provide new insight into the future design and synthesis of corresponding silole derivatives.

Bond dissociation energies of low-valent lanthanide hydroxides: lower limits from ion-molecule reactions and comparisons with fluorides.

Despite that bond dissociation energies (BDEs) are among the most fundamental and relevant chemical properties they remain poorly characterized for most elementary lanthanide hydroxides and halides. Lanthanide ions Ln+ = Eu+, Tm+ and Yb+ are here shown to react with H2O to yield hydroxides LnOH+. Under low-energy conditions such reactions must be exothermic, which implies a lower limit of 499 kJ mol-1 for the Ln+-OH BDEs. This limit is significantly higher than previously reported for YbOH+ and is unexpectedly similar to the BDE for Yb+-F. To explain this apparent anomaly, it is considered feasible that the inefficient hydrolysis reactions observed here in a quadrupole ion trap mass spectrometer may actually be endothermic. More definitive and broad-based evaluations and comparisons require additional and more reliable BDEs and ionization energies for key lanthanide molecules, and/or energies for ligand-exchange reactions like LnF + OH ↔ LnOH + F. The hydroxide results motivated an assessment of currently available lanthanide monohalide BDEs. Among several intriguing relationships is the distinctively higher BDE for neutral LuF versus cationic LuF+, though quantifying this comparison awaits a more accurate value for the anomalously high ionization energy of LuF.

Assessment of the Second-Ionization Potential of Lawrencium: Investigating the End of the Actinide Series with a One-Atom-at-a-Time Gas-Phase Ion Chemistry Technique.

Experiments were performed at the Lawrence Berkeley National Laboratory 88-Inch Cyclotron facility to investigate the electron-transfer reduction reaction of dipositive Lr (Z = 103) with O2 gas. Ions of 255Lr were produced in the fusion-evaporation reaction 209Bi(48Ca,2n) 255Lr and were studied with a novel gas-phase ion chemistry technique. The produced 255Lr2+ ions were trapped and O2 gas was introduced, such that the charge-exchange reaction to reduce 255Lr2+ to 255Lr1+ was observed and the reaction rate constant was determined to be k = 1.5(7) × 10-10 cm3/mol/s. The observation that this reaction proceeds establishes the lower limit on the second ionization potential of Lr to be 13.3(3) eV. This gives further support that the actinide series terminates with Lr. Additionally, this result can be used to better interpret the situation concerning the placement of Lu and Lr on the periodic table within the current framework of the actinide hypothesis. The success of this experimental approach now identifies unique opportunities for future gas-phase reaction studies on actinide and super heavy elements.

Characterization of Uranyl Coordinated by Equatorial Oxygen: Oxo in UO3 versus Oxyl in UO3.

Uranium trioxide, UO3, has a T-shaped structure with bent uranyl, UO22+, coordinated by an equatorial oxo, O2-. The structure of cation UO3+ is similar but with an equatorial oxyl, O•-. Neutral and cationic uranium trioxide coordinated by nitrates were characterized by collision induced dissociation (CID), infrared multiple-photon dissociation (IRMPD) spectroscopy, and density functional theory. CID of uranyl nitrate, [UO2(NO3)3]- (complex A1), eliminates NO2 to produce nitrate-coordinated UO3+, [UO2(O•)(NO3)2]- (B1), which ejects NO3 to yield UO3 in [UO2(O)(NO3)]- (C1). Finally, C1 associates with H2O to afford uranyl hydroxide in [UO2(OH)2(NO3)]- (D1). IRMPD of B1, C1, and D1 confirms uranyl equatorially coordinated by nitrate(s) along with the following ligands: (B1) radical oxyl O•-; (C1) oxo O2-; and (D1) two hydroxyls, OH-. As the nitrates are bidentate, the equatorial coordination is six in A1, five in B1, four in D1, and three in C1. Ligand congestion in low-coordinate C1 suggests orbital-directed bonding. Hydrolysis of the equatorial oxo in C1 epitomizes the inverse trans influence in UO3, which is uranyl with inert axial oxos and a reactive equatorial oxo. The uranyl ν3 IR frequencies indicate the following donor ordering: O2-[best donor] ≫ O•-> OH-> NO3-.

Boron-Mediated Carbon-Carbon Bond Cleavage and Rearrangement of Benzene Forming the Borepinyl Radical and Borole Derivatives.

The reaction of atomic boron with benzene in solid neon has been investigated by matrix isolation infrared spectroscopy with isotopic substitutions as well as quantum chemical calculations. The reaction is initiated by boron atom addition to benzene in forming an η2-(1, 4) π adduct (A). A borepinyl radical (B) formed by C-C bond insertion is also observed on annealing. The η2-(1,4) π adduct photoisomerizes to an unprecedented borole substituted vinyl radical intermediate (C) via ring-opening and rearrangement reactions involving an antiaromatic borole subunit. A previously unconsidered 1-ethynyl-2-dihydro-1H-borole radical (D) is generated as the final product under UV light irradiation. The results presented herein give new insight into the benzene carbon-carbon bond cleavage and rearrangement reactions mediated by a nonmetal and provide a possible route for the construction of heterocyclic borepinyl and borole species via benzene ring opening and rearrangement reactions.

Proton affinities of pertechnetate (TcO4-) and perrhenate (ReO4-).

The anions pertechnetate, TcO4-, and perrhenate, ReO4-, exhibit very similar chemical and physical properties. Revealing and understanding disparities between them enhances fundamental understanding of both. Electrospray ionization generated the gas-phase proton bound dimer (TcO4-)(H+)(ReO4-). Collision induced dissociation of the dimer yielded predominantly HTcO4 and ReO4-, which according to Cooks' kinetic method indicates that the proton affinity (PA) of TcO4- is greater than that of ReO4-. Density functional theory computations agree with the experimental observation, providing PA[TcO4-] = 300.1 kcal mol-1 and PA[ReO4-] = 297.2 kcal mol-1. Attempts to rationalize these relative PAs based on elementary molecular parameters such as atomic charges indicate that the entirety of bond formation and concomitant bond disruption needs to be considered to understand the energies associated with such protonation processes. Although in both the gas and solution phases, TcO4- is a stronger base than ReO4-, it is noted that the significance of even such qualitative accordance is tempered by the very different natures of the underlying phenomena.

Destruction of the Uranyl Moiety in a U(V) "Cation-Cation" Interaction.

A gas-phase uranyl peroxide dimer supported by three 12-crown-4 ether (12C4) ligands, [(UO2)2(O2)(12C4)3)]2+ (A), was prepared by electrospray ionization. Density functional theory (DFT) indicates a structure with two terminal 12C4 and the third 12C4 bridging the uranium centers. Collision induced dissociation (CID) of A resulted in elimination of the bridging 12C4 to yield a uranyl peroxide dimer with two terminal donor ligands, [(12C4)(UO2)(O2)(UO2)(12C4)]2+ (B). Remarkably, CID of B resulted in elimination of the bridging peroxide concomitant with reduction of U(VI) to U(V) in C, [(12C4)(UO2)(UO2)(12C4)]2+. DFT studies indicate that in C there is direct interaction between the two UO2+ species, which can thus be considered as a so-called cation-cation interaction (CCI). This formal CCI, induced by tetradentate 12C4 ligands, corresponds to destruction of the linear uranyl moieties and creation of bridging U-O-U oxo-bonds. On the basis of the structural rearrangement to achieve the structurally extreme CCI interaction, it is predicted also to be accessible for PaO2+ but is less feasible for transuranic actinyls.

Halide anion discrimination by a tripodal hydroxylamine ligand in gas and condensed phases.

Electrospray ionization of solutions containing a tripodal hydroxylamine ligand, H3TriNOx ([((2-tBuNOH)C6H4CH2)3N]) denoted as L, and a hydrogen halide HX: HCl, HBr and/or HI, yielded gas-phase anion complexes [L(X)]- and [L(HX2)]-. Collision induced dissociation (CID) of mixed-halide complexes, [L(HXaXb)]-, indicated highest affinity for I- and lowest for Cl-. Structures and energetics computed by density functional theory are in accord with the CID results, and indicate that the gas-phase binding preference is a manifestation of differing stabilities of the HX molecules. A high halide affinity of [L(H)]+ in solution was also demonstrated, though with a highest preference for Cl- and lowest for I-, the opposite observation of, but not in conflict with, what is observed in gas phase. The results suggest a connection between gas- and condensed-phase chemistry and computational approaches, and shed light on the aggregation and anion recognition properties of hydroxylamine receptors.

Uranyl/12-crown-4 Ether Complexes and Derivatives: Structural Characterization and Isomeric Differentiation.

The following gas-phase uranyl/12-crown-4 (12C4) complexes were synthesized by electrospray ionization: [UO2(12C4)2]2+ and [UO2(12C4)2(OH)]+. Collision-induced dissociation (CID) of the dication resulted in [UO2(12C4-H)]+ (12C4-H is a 12C4 that has lost one H), which spontaneously adds water to yield [UO2(12C4-H)(H2O)]+. The latter has the same composition as complex [UO2(12C4)(OH)]+ produced by CID of [UO2(12C4)2(OH)]+ but exhibits different reactivity with water. The postulated structures as isomeric [UO2(12C4-H)(H2O)]+ and [UO2(12C4)(OH)]+ were confirmed by comparison of infrared multiphoton dissociation (IRMPD) spectra with computed spectra. The structure of [UO2(12C4-H)]+ corresponds to cleavage of a C-O bond in the 12C4 ring, with formation of a discrete U-Oeq bond and equatorial coordination by three intact ether moieties. Comparison of IRMPD and computed IR spectra furthermore enabled assignment of the structures of the other complexes. Theoretical studies of the chemical bonding features of the complexes provide an understanding of their stabilities and reactivities. The results reveal bonding and structures of the uranyl/12C4 complexes and demonstrate the synthesis and identification of two different isomers of gas-phase uranyl coordination complexes.

Cleaving Off Uranyl Oxygens through Chelation: A Mechanistic Study in the Gas Phase.

Recent efforts to activate the strong uranium-oxygen bonds in the dioxo uranyl cation have been limited to single oxo-group activation through either uranyl reduction and functionalization in solution, or by collision induced dissociation (CID) in the gas-phase, using mass spectrometry (MS). Here, we report and investigate the surprising double activation of uranyl by an organic ligand, 3,4,3-LI(CAM), leading to the formation of a formal U6+ chelate in the gas-phase. The cleavage of both uranyl oxo bonds was experimentally evidenced by CID, using deuterium and 18O isotopic substitutions, and by infrared multiple photon dissociation (IRMPD) spectroscopy. Density functional theory (DFT) computations predict that the overall reaction requires only 132 kJ/mol, with the first oxygen activation entailing about 107 kJ/mol. Combined with analysis of similar, but unreactive ligands, these results shed light on the chelation-driven mechanism of uranyl oxo bond cleavage, demonstrating its dependence on the presence of ligand hydroxyl protons available for direct interactions with the uranyl oxygens.

Isocyanate Formation from Reactions of Early Lanthanide Metal Atoms with NO and CO in Solid Argon.

The reactions of early lanthanide metal atoms (Ce, Pr, and Nd) with carbon monoxide and nitric oxide mixtures are studied by infrared absorption spectroscopy in solid argon. The reaction intermediates and products are identified via isotopic substitution as well as theoretical frequency calculations. The results show that the reactions proceed with the initial formation of inserted NLnO molecules, which subsequently react with CO to form the NLnO(CO) complexes on annealing. The NLnO(CO) complexes further isomerize to the more stable isocyanate OLnNCO species under UV light excitation.

Observation of Main-Group Tricarbonyls [B(CO)3 ] and [C(CO)3 ](+) Featuring a Tilted One-Electron Donor Carbonyl Ligand.

A combined experimental and theoretical study on the main-group tricarbonyls [B(CO)3 ] in solid noble-gas matrices and [C(CO)3 ](+) in the gas phase is presented. The molecules are identified by comparing the experimental and theoretical IR spectra and the vibrational shifts of nuclear isotopes. Quantum chemical ab initio studies suggest that the two isoelectronic species possess a tilted η(1) (µ1 -CO)-bonded carbonyl ligand, which serves as an unprecedented one-electron donor ligand. Thus, the central atoms in both complexes still retain an 8-electron configuration. A thorough analysis of the bonding situation gives quantitative information about the donor and acceptor properties of the different carbonyl ligands. The linearly bonded CO ligands are classical two-electron donors that display classical σ-donation and π-back-donation following the Dewar-Chatt-Duncanson model. The tilted CO ligand is a formal one-electron donor that is bonded by σ-donation and π-back-donation that involves the singly occupied orbital of the radical fragments [B(CO)2 ] and [C(CO)2 ](+) .

Experimental and theoretical identification of the Fe(vii) oxidation state in FeO4.

The experimentally known highest oxidation state of iron has been determined to be Fe(vi) so far. Here we report a combined matrix-isolation infrared spectroscopic and theoretical study of two interconvertible iron oxide anions: a dioxoiron peroxide complex [(η2-O2)FeO2]- with a C2v-structure and a tetroxide FeO4- with a D2d tetrahedral structure, which are formed by co-condensation of laser-ablated iron atoms and electrons with O2/Ar mixtures at 4 K. Quantum chemistry theoretical studies indicate that the Jahn-Teller distorted tetroxide FeO4- anion is a d1 species with hereto the highest iron formal oxidation state Fe(vii).

Observation of Spontaneous C=C Bond Breaking in the Reaction between Atomic Boron and Ethylene in Solid Neon.

A ground-state boron atom inserts into the C=C bond of ethylene to spontaneously form the allene-like compound H2 CBCH2 on annealing in solid neon. This compound can further isomerize to the propyne-like HCBCH3 isomer under UV light excitation. The observation of this unique spontaneous C=C bond insertion reaction is consistent with theoretical predictions that the reaction is thermodynamically exothermic and kinetically facile. This work demonstrates that the stronger C=C bond, rather than the less inert C-H bond, can be broken to form organoboron species from the reaction of a boron atom with ethylene even at cryogenic temperatures.

Infrared Photodissociation Spectroscopy of the Ni(O2)n(+) (n = 2-4) Cation Complexes.

The infrared spectra of mass-selected Ni(O2)n(+) (n = 2-4) and their argon-tagged complexes are measured by infrared photodissociation spectroscopy in the gas phase. The experimental spectra provide distinctive patterns allowing the determination of their geometric and electronic structures by comparison with the simulated vibrational spectra from density functional theory calculations. The [Ni(O2)2Ar2](+) cation complex was determined to have D2h symmetry involving a Ni(O2)2(+) core ion with two equivalent superoxide ligands side-on bound to a Ni(3+) cation center. The higher Ni(O2)3(+) and Ni(O2)4(+) cation complexes were determined to have structures with a chemically bound Ni(O2)2(+) core ion that is weakly coordinated by neutral O2 molecule(s).

Infrared photodissociation spectroscopy of oxygen-rich Fe(O2)n(+) (n = 3-5) cation complexes.

Infrared spectra of mass-selected oxygen-rich iron dioxygen complexes Fe(O2)n(+) with n = 3-5 are measured via infrared photodissociation spectroscopy in the gas phase. These cation complexes are produced via a laser vaporization supersonic ion source. The structures are established by comparison of the experimental spectra with the simulated spectra derived from density functional calculations. All of the Fe(O2)n(+) complexes studied have a single IR-active band in the 1050-1100 cm(-1) region, arising from the O-O stretching vibration of the superoxo ligand(s). These complexes are determined to have structures with a chemically bound Fe(O2)2(+) core ion that is weakly coordinated by neutral O2 molecules. The Fe(O2)2(+) core ion has a planar D2h symmetry with two equivalent side-on superoxo ligands bound to an Fe(3+) cation center.

Photoinduced boron atom insertion of benzocyclobutene forming an unprecedented fused boron heterocyclic radical.

Two novel boron heterocyclic radicals, an addition bicyclo[4.2.1]octa-1,3,5-trien-1-yl-borane radical (A) and an insertion 7-1H-borolo[1,2-a]borinine radical (B), were synthesized, and characterized in the reaction of atomic boron with benzocyclobutene. Species B involving a fused boron heterocyclic was spectroscopically characterized for the first time. This work is a new approach for boron-mediated molecular editing and the synthesis of fused boron heterocyclic compounds.