Complex of Silylene with Nitrogen: A Combined Matrix-Spectroscopic and Density Functional Theory Study

N2 cleavage by silylene and formation of H2Si(μ-N)2SiH2

Fixation and functionalisation of N2 by main-group elements has remained scarce. Herein, we report a fixation and cleavage of the N ≡ N triple bond achieved in a dinitrogen (N2) matrix by the reaction of hydrogen and laser-ablated silicon atoms. The four-membered heterocycle H2Si(μ-N)2SiH2, the H2SiNN(H2) and HNSiNH complexes are characterized by infrared spectroscopy in conjunction with quantum-chemical calculations. The synergistic interaction of the two SiH2 moieties with N2 results in the formation of final product H2Si(μ-N)2SiH2, and theoretical calculations reveal the donation of electron density of Si to π* antibonding orbitals and the removal of electron density from the π bonding orbitals of N2, leading to cleave the non-polar and strong NN triple bond.

Structure and IR spectroscopic properties of complexes of 1,2,4-triazole and 3-amino-1,2,4-triazole with dinitrogen isolated in solid argon

Complexes of 1,2,4-triazole (TR) and 3-amino-1,2,4-triazole (AT) with N₂ were studied computationally employing MP2 and B3LYPD3 methods and experimentally by FTIR matrix isolation technique. The results show that both triazoles interact specifically with dinitrogen in several different ways. For the 1:1 complexes of 1,2,4-triazole five stable minima were located on the potential energy surface. The most stable of them comprises a weak hydrogen bond formed between the NH group of the ring and the lone pair of the nitrogen molecule. The second most stable structure is bound by the N⋯π bond formed between one of the N atoms of the N₂ molecule and the triazole ring. Three other complexes are stabilized by the C-H⋯N and N⋯N van der Waals interactions. In the case of 3-amino-1,2,4-triazole, the two most stable dinitrogen complexes are analogous to those found for the 1,2,4-triazole and involve N-H⋯N and N⋯π bonds. In other structures amino or CH groups act as proton donors to the N₂ molecule. The N⋯N van der Waals interactions are also present. The analysis of the infrared spectra of low temperature matrices containing TR or AT and dinitrogen indicates that in both systems mostly 1:1 hydrogen-bonded complexes with the NH group interacting with N₂ are present in solid argon.

Matrix Isolation FTIR and Theoretical Study of Weakly Bound Complexes of Isocyanic Acid with Nitrogen

Weak complexes of isocyanic acid (HNCO) with nitrogen were studied computationally employing MP2, B2PLYPD3 and B3LYPD3 methods and experimentally by FTIR matrix isolation technique. The results show that HNCO interacts specifically with N₂. For the 1:1 stoichiometry, three stable minima were located on the potential energy surface. The most stable of them involves a weak, almost linear hydrogen bond from the NH group of the acid molecule to nitrogen molecule lone pair. Two other structures are bound by van der Waals interactions of N⋯N and C⋯N types. The 1:2 and 2:1 HNCO complexes with nitrogen were computationally tracked as well. Similar types of interactions as in the 1:1 complexes were found in the case of the higher stoichiometry complexes. Analysis of the HNCO/N₂/Ar spectra after deposition indicates that the 1:1 hydrogen-bonded complex is prevalent in argon matrices with a small amount of the van der Waals structures also present. Upon annealing, complexes of the 1:2 and 2:1 stoichiometry were detected as well.

Molecular Main Group Metal Hydrides

This review serves to document advances in the synthesis, versatile bonding, and reactivity of molecular main group metal hydrides within Groups 1, 2, and 12-16. Particular attention will be given to the emerging use of said hydrides in the rapidly expanding field of Main Group element-mediated catalysis. While this review is comprehensive in nature, focus will be given to research appearing in the open literature since 2001.

Flash Vacuum Pyrolysis: Techniques and Reactions

Flash vacuum pyrolysis (FVP) had its beginnings in the 1940s and 1950s, mainly through mass spectrometric detection of pyrolytically formed free radicals. In the 1960s many organic chemists started performing FVP experiments with the purpose of isolating new and interesting compounds and understanding pyrolysis processes. Meanwhile, many different types of apparatus and techniques have been developed, and it is the purpose of this review to present the most important methods as well as a survey of typical reactions and observations that can be achieved with the various techniques. This includes preparative FVP, chemical trapping reactions, matrix isolation, and low temperature spectroscopy of reactive intermediates and unstable molecules, the use of online mass, photoelectron, microwave, and millimeterwave spectroscopies, gas-phase laser pyrolysis, pulsed pyrolysis with supersonic jet expansion, very low pressure pyrolysis for kinetic investigations, solution-spray and falling-solid FVP for involatile compounds, and pyrolysis over solid supports and reagents. Moreover, the combination of FVP with matrix isolation and photochemistry is a powerful tool for investigations of reaction mechanism.

Chain or ring: which one is favorable in nitrogen-rich molecules N6XHm, N8XHm, and N10XHm (X = B, Al, Ga, m = 1 and X = C, Si, Ge, m = 2)?

A series of nitrogen-rich molecules N6XHm, N8XHm, and N10XHm (X = B, Al, Ga, m = 1 and X = C, Si, Ge, m = 2) consisting of N3 and N5 radicals, are systematically investigated by using B3LYP and B3PW91 DFT methods. It is found that for the nitrogen-rich molecules, the structures with N3-chains (N5-ring) are more stable than those containing a N3-ring (N5-chain). This result could be well-explained by the intrinsic stability of the N3 and N5 radicals and their charge distribution in nitrogen-rich molecules. The dissociation energies further indicate that the B-doped and C-doped structures are the most stable among the molecules with three elements of group 13 and 14, respectively. Energy decomposition analysis shows the bond of boron-nitrogen is stronger than that of carbon-nitrogen. Detailed bonding analysis demonstrates that the B-N bond is determined by σ and π interactions between the B and N atoms, whereas C-N bonds by only σ interactions. These results imply that the boron atom is more suitable than the carbon atom for building the nitrogen-rich molecules studied in this article.

Flash flow pyrolysis: mimicking flash vacuum pyrolysis in a high-temperature/high-pressure liquid-phase microreactor environment

Flash vacuum pyrolysis (FVP) is a gas-phase continuous-flow technique where a substrate is sublimed through a hot quartz tube under high vacuum at temperatures of 400-1100 °C. Thermal activation occurs mainly by molecule-wall collisions with contact times in the region of milliseconds. As a preparative method, FVP is used mainly to induce intramolecular high-temperature transformations leading to products that cannot easily be obtained by other methods. It is demonstrated herein that liquid-phase high-temperature/high-pressure (high-T/p) microreactor conditions (160-350 °C, 90-180 bar) employing near- or supercritical fluids as reaction media can mimic the results obtained using preparative gas-phase FVP protocols. The high-T/p liquid-phase "flash flow pyrolysis" (FFP) technique was applied to the thermolysis of Meldrum's acid derivatives, pyrrole-2,3-diones, and pyrrole-2-carboxylic esters, producing the expected target heterocycles in high yields with residence times between 10 s and 10 min. The exact control over flow rate (and thus residence time) using the liquid-phase FFP method allows a tuning of reaction selectivities not easily achievable using FVP. Since the solution-phase FFP method does not require the substrate to be volatile any more--a major limitation in classical FVP--the transformations become readily scalable, allowing higher productivities and space-time yields compared with gas-phase protocols. Differential scanning calorimetry measurements and extensive DFT calculations provided essential information on pyrolysis energy barriers and the involved reaction mechanisms. A correlation between computed activation energies and experimental gas-phase FVP (molecule-wall collisions) and liquid-phase FFP (molecule-molecule collisions) pyrolysis temperatures was derived.

Matrix isolation infrared observation of HxSi(N2)y (x = 0, 1, 2 and y = 1, 2) transient species using a 121-nm vacuum ultraviolet photolysis source

Vacuum ultraviolet photolysis (121.6 nm) of silane in a nitrogen matrix at 12 K leads to the observation of several transient species, which have been characterized using Fourier transform infrared spectroscopy. Four transient species containing silicon and nitrogen have been observed (SiN2, Si(N2)2, HSiN2, and H2SiN2), and one transient species containing only silicon and hydrogen has been observed. The assignment of the infrared bands due to each of these species is accomplished by performing isotopic substitution experiments (SiD4, 15N2, and mixtures with SiH4 and 14N2), matrix annealing experiments, UV-visible photolysis experiments, by comparison with previous experimental matrix isolation frequencies, where available, and for HSiN2 and H2SiN2 by comparison to B3LYP/aug-cc-pVTZ-calculated vibrational frequencies. The observation and infrared assignment of the HSiN2 and H2SiN2 molecules in these experiments is significant in that HSiN2 has not been previously reported in the matrix isolation literature, and H2SiN2 has only been reported once previously by a different route of formation. The energetics of the overall formation pathways for the molecules observed in these experiments is discussed using B3LYP/aug-cc-pVTZ calculations.