Chromium Hydrides and Dihydrogen Complexes in Solid Neon, Argon, and Hydrogen: Matrix Infrared Spectra and Quantum Chemical Calculations

Activation of H2S by Atomic Cr, Mn, and Fe: Matrix Infrared Spectra and Quantum Chemical Calculations

Hydrogen sulfide is toxic and corrosive gas abundantly available in nature. The activation of hydrogen sulfide to produce hydrogen and elemental sulfur is of great significance for possible applications in toxic pollutant control and hydrogen energy regeneration. The activation of H₂S by transition metal atoms (M = Cr, Mn, and Fe) has been studied by low-temperature matrix isolation infrared spectroscopy and quantum chemical calculations. Experimental and theoretical results indicate that the reaction between ground-state M atoms and H₂S is inhibited by the repulsive interactions between the reactants. After being excited upon photolysis, the corresponding excited-state M atoms react with H₂S molecules spontaneously. The produced insertion product HMSH further decomposed to metal sulfides upon full-arc mercury lamp irradiation by the splitting of hydrogen.

Matrix infrared spectroscopy of F2BMF and FB[triple bond, length as m-dash]WF2 (M = Cr, Mo and W) complexes and quantum chemistry calculations

Laser-ablated group 6 transition metal atoms react with BF3 to yield typical transition metal inserted complexes F2B-MF (M = Cr, Mo, and W) and terminal borylene complex FB[triple bond, length as m-dash]WF2. These products are investigated by using infrared spectroscopy, isotopic substitution and theoretical frequency calculations. The inserted complexes F2B-MF (M = Cr, Mo, and W) were identified by antisymmetric and symmetric stretching modes of F-B-F. The FB[triple bond, length as m-dash]WF2 molecule has a 11B-F (10B-F) stretching frequency at 1453.2 (1505.0) cm-1 and the triple bond between boron and tungsten is confirmed by EDA-NOCV calculations, CASSCF calculation and NBO analysis. Furthermore, the bonding for tungsten complexes is compared with that of molybdenum and chromium complexes, which reveals interesting differences in their chemistries.

How molecular is the chemisorptive bond?

Scaling rules differ for early and late transition metals. Their electronic structure and topological bond analysis are shown.

Pressure-driven formation and stabilization of superconductive chromium hydrides

Chromium hydride is a prototype stoichiometric transition metal hydride. The phase diagram of Cr-H system at high pressures remains largely unexplored due to the challenges in dealing with the high activation barriers and complications in handing hydrogen under pressure. We have performed an extensive structural study on Cr-H system at pressure range 0 ∼ 300 GPa using an unbiased structure prediction method based on evolutionary algorithm. Upon compression, a number of hydrides are predicted to become stable in the excess hydrogen environment and these have compositions of Cr₂H_n (n = 2–4, 6, 8, 16). Cr₂H₃, CrH₂ and Cr₂H₅ structures are versions of the perfect anti-NiAs-type CrH with ordered tetrahedral interstitial sites filled by H atoms. CrH₃ and CrH₄ exhibit host-guest structural characteristics. In CrH₈, H₂ units are also identified. Our study unravels that CrH is a superconductor at atmospheric pressure with an estimated transition temperature (T _c) of 10.6 K, and superconductivity in CrH₃ is enhanced by the metallic hydrogen sublattice with T _c of 37.1 K at 81 GPa, very similar to the extensively studied MgB₂.

Thermodynamically neutral Kubas-type hydrogen storage using amorphous Cr(iii) alkyl hydride gels

We present chromium(iii) hydride gels that store 5.08 wt% H₂ at 160 bar and 298 K with a measured enthalpy of H₂ adsorption of +0.37 kJ mol⁻¹ H₂.

Tetrahydrometalate species VH(2)(H(2)), NbH(4), and TaH(4): matrix infrared spectra and quantum chemical calculations

Laser ablated V, Nb, and Ta atoms react with molecular hydrogen in excess neon at 4 K to give vanadium, niobium, and tantalum dihydrides that further react with H(2) to form VH(2)(H(2)), NbH(4), and TaH(4). The reaction products are identified by deuterium and deuterium hydride isotopic substitution. DFT and CCSD theoretical calculations are used to predict energies, geometries, and vibrational frequencies for these novel metal hydrides complex and molecules. The vanadium dihydride hydrogen complex, VH(2)(H(2)), is identified, while the niobium and tantalum tetrahydrides, NbH(4) and TaH(4,) with D(2d) symmetry structures are confirmed. Reactions of group 5 metal atoms with H(2) condensing in solid hydrogen gave VH(2)(H(2)) and the higher tetrahydride-hydrogen complexes NbH(4)(H(2))(4) and TaH(4)(H(2))(4).

Mechanisms of chemical generation of volatile hydrides for trace element determination (IUPAC Technical Report)

Aqueous-phase chemical generation of volatile hydrides (CHG) by derivatization with borane complexes is one of the most powerful and widely employed methods for determination and speciation analysis of trace and ultratrace elements (viz. Ge, Sn, Pb, As, Sb, Bi, Se, Te, Hg, Cd, and, more recently, several transition and noble metals) when coupled with atomic and mass spectrometric detection techniques. Analytical CHG is still dominated by erroneous concepts, which have been disseminated and consolidated within the analytical scientific community over the course of many years. The overall approach to CHG has thus remained completely empirical, which hinders possibilities for further development. This report is focused on the rationalization and clarification of fundamental aspects related to CHG: (i) mechanism of hydrolysis of borane complexes; (ii) mechanism of hydrogen transfer from the borane complex to the analytical substrate; (iii) mechanisms through which the different chemical reaction conditions control the CHG process; and (iv) mechanism of action of chemical additives and foreign species. Enhanced comprehension of these different mechanisms and their mutual influence can be achieved in light of the present state of knowledge. This provides the tools to explain the reactivity of a CHG system and contributes to the clarification of several controversial aspects and the elimination of erroneous concepts in CHG.

Sodium Hydride Clusters in Solid Hydrogen and Neon: Infrared Spectra and Theoretical Calculations

Laser-ablated sodium atom reactions with H2 have been investigated in solid molecular hydrogens and neon. The NaH molecule and (NaH)2,3,4 clusters were identified by IR spectra with isotopic substitution (HD and D2) and comparison to frequencies calculated by density functional theory and the MP2 method. The use of para-hydrogen enriched samples provides evidence for a (H2)nNaH subcomplex surrounded by the solid hydrogen matrix cage. The ionic rhombic (NaH)2 dimer is characterized by strong absorptions at 761.7, 759.1, and 757.0 cm(-1), respectively, in solid neon, para-hydrogen, and normal hydrogen matrices. The cyclic sodium hydride trimer and tetramer clusters are also observed. Although the spontaneous reaction of two Li and H2 to form (LiH)2 occurs on annealing in solid H2, the formation of (NaH)2 requires near uv photoexcitation.

Matrix Infrared Spectroscopic Studies of the MH−C2H3 and MH2−C2H2 Intermediates in the Reactions of Ethylene with Laser-Ablated Group 5 Metal Atoms

Reactions of ethylene with laser-ablated group 5 metal atoms in excess argon have been carried out during codeposition at 8 K, and the matrix infrared spectra of intermediate products have been investigated. Oxidative C-H insertion of the transition metal into a C-H bond occurs and beta-hydrogen transfer follows to form the dihydrido complexes (MH2-C2H2). In the Ta spectra, the dihydrido complex is the primary product, whereas the Nb and V spectra reveal absorptions from both the insertion (MH-C2H3) and dihydrido complexes. The insertion and dihydrido complexes identified here are in fact the reaction intermediates in the hydrogen elimination of ethylene proposed in previous reaction dynamics studies. Calculations also show that the higher oxidation-state complex becomes more stable relative to the insertion product going down the group 5 family.

Infrared Spectra of the CH3−MX and CH2−MHX Complexes Formed by Reactions of Laser-Ablated Group 3 Metal Atoms with Methyl Halides

Reactions of laser-ablated group 3 metal atoms with methyl halides have been carried out in excess of Ar during condensation and the matrix infrared spectra studied. The metals are as effective as other early transition metals in providing insertion products (CH3-MX) and higher oxidation state methylidene complexes (CH2-MHX) (X = F, Cl, Br) following alpha-hydrogen migration. Unlike the cases of the group 4-6 metals, the calculated methylidene complex structures show little evidence for agostic distortion, consistent with the previously studied group 3 metal methylidene hydrides, and the C-M bond lengths of the insertion and methylidene complexes are comparable to each other. However, the C-Sc bond lengths are 0.013, 0.025, and 0.029 A shorter for the CH2-ScHX complexes, respectively, and the spin densities are consistent with weak C(2p)-Sc(3d) pi bonding. The present results reconfirm that the number of valence electrons on the metal is important for agostic interaction in simple methylidene complexes.

Infrared Spectra and Density Functional Calculations of CH2MHX and CH⋮MH2X Complexes Prepared in Reactions of Methyl Halides with Mo and W Atoms

The simple methylidene and methylidyne complexes (CH2=MHX and CH[triple bond]MH2X; X = F, Cl, Br, and I) are prepared in reactions of laser-ablated Mo and W atoms with the methyl halides and investigated by matrix infrared spectroscopy and density functional theory calculations. These complex structures are photoreversible: visible irradiation converts the methylidene complex to the methylidyne complex, and UV irradiation reverses this effect via alpha-hydrogen migration. While the higher oxidation state complexes are readily formed regardless of halogen size, the Mo methylidyne complex is relatively less favored with increasing halogen size, and the W complex shows the opposite tendency. The group 6 metal methylidenes are predicted to have the most agostically distorted structures among the early transition-metal methylidenes. The computed carbon-metal bond shortens with increasing halogen size for both the methylidene and methylidyne complexes. Harmonic and anharmonic frequencies computed by DFT converge on the experimental values and thus provide support for the identification of these new Mo and W complexes.

Fourier transform infrared isotopic study of linear CrC3: Identification of the ν1(σ) mode

A vibrational fundamental of linear CrC3 has been detected in the products from the laser ablation of chromium and carbon rods trapped in solid Ar at approximately 10 K. Fourier transform infrared measurements of frequencies and 13C isotopic shifts are in very good agreement with the predictions of density functional theory calculations at the B3LYP6-311G+(3df) level, resulting in the identification of the nu1(sigma) stretching mode at 1789.5 cm(-1). This is the first optical detection of the linear isomer of the transition-metal carbide CrC3 for which previous photoelectron spectroscopic studies have reported evidence of both linear and cyclic isomers.

Infrared Spectra of the CH3−MX, CH2MHX, and CH⋮MH2X- Complexes Formed by Reaction of Methyl Halides with Laser-ablated Group 5 Metal Atoms

Reactions of group 5 metal atoms and methyl halides give carbon-metal single, double, and triple bonded complexes that are identified from matrix IR spectra and vibrational frequencies computed by DFT. Two different pairs of complexes are prepared in reactions of methyl fluoride with laser-ablated vanadium and tantalum atoms. The two vanadium complexes (CH(3)-VF and CH(2)=VHF) are persistently photoreversible and show a kinetic isotope effect on the yield of CD(2)=VDF. Identification of CH(2)=TaHF and CH[triple bond]TaH(2)F(-), along with the similar anionic Nb complex, suggests that the anionic methylidyne complex is a general property of the heavy group 5 metals. Reactions of Nb and Ta with CH(3)Cl and CH(3)Br have also been carried out to understand the ligand effects on the calculated structures and the vibrational characteristics. The methylidene complexes become more distorted with increasing halogen size, while the calculated C=M bond lengths and stretching frequencies decrease and increase, respectively. The anionic methylidyne complexes are less favored with increasing halogen size. Infrared spectra show a dramatic increase of the Ta methylidenes upon annealing, suggesting that the formation of CH(3)-TaX and its conversion to CH(2)=TaHX require essentially no activation energy.

Contrasting Products in the Reactions of Cr, Mo, and W Atoms with H2O2: Argon Matrix Infrared Spectra and Theoretical Calculations

Products in the reactions of H2O2 and H2, O2 mixtures have been observed by matrix infrared absorptions and identified through comparisons with vibrational frequencies calculated for these molecules. The chromium reactions are dominated by lower oxidation state products, whereas molybdenum and tungsten chemistry favors higher oxidation state products. For example chromium dihydroxide, Cr(OH)2, molybdenum hydride oxide, H2MoO2, and tungsten hydride oxide, H2WO2, were observed in laser-ablated metal atom reactions with H2O2, and calculations show that these are the most stable molecules for this stoichiometry. Chromium monohydroxide, CrOH, was identified through O-H and Cr-O stretching modes, while HWO was observed by W-H and W=O stretching modes. The metal oxyhydroxides, HMO(OH), were observed for all metals. However, reactions with two H2O2 molecules give OCr(OH)2, MoO2(OH)2, and WO2(OH)2. The relative stabilities of different structures for Cr, Mo, and W are due to different participations of occupied d orbitals. The reactivity of the cold metal atoms with H2O2 on annealing the solid argon matrix increases on going down the group.

Methane Activation by Laser-Ablated V, Nb, and Ta Atoms: Formation of CH3−MH, CH2MH2, CH⋮MH3-, and (CH3)2MH2

Methane activation by group 5 transition-metal atoms in excess argon and the matrix infrared spectra of reaction products have been investigated. Vanadium forms only the monohydrido methyl complex (CH3-VH) in reaction with CH4 and upon irradiation. On the other hand, the heavier metals form methyl hydride and methylidene dihydride complexes (CH3-MH and CH2=MH2) along with the methylidyne trihydride anion complexes (CHMH3-). The neutral products, particularly the methylidene complex, increase markedly on irradiation whereas the anionic product depletes upon UV irradiation or addition of a trace of CCl4 or CBr4 to trap electrons. Other absorptions that emerge on irradiation and annealing increase markedly at higher precursor concentration and are attributed to a higher-order product ((CH3)2MH2)). Spectroscopic evidence suggests that the agostic Nb and Ta methylidene dihydride complexes have two identical metal-hydrogen bonds.

Linear HThThH: a candidate for a Th-Th triple bond

The acetylene-like linear HThThH structure is calculated to lie almost degenerate to its rhombic alternative. Its unsupported Th-Th triple bond would be the first one of its kind.

Synthesis, Neutron Structure, and Reactivity of the Bis(dihydrogen) Complex RuH2(η2-H2)2(PCyp3)2 Stabilized by Two Tricyclopentylphosphines

Treatment of Ru(eta4-C8H12)(eta6-C8H10) with 3 bar H2 in the presence of 2 equiv of tricyclopentylphosphine (PCyp3) in pentane resulted in the isolation of the new bis(dihydrogen) complex RuH2(eta2-H2)2(PCyp3)2 (2), characterized by NMR and single-crystal X-ray and neutron diffraction. The single-crystal neutron diffraction study is the first carried out for a bis(dihydrogen) complex. The coordination geometry around the metal center is a distorted octahedron defined by the two phosphines in a trans configuration (making an angle of 168.9(1) degrees ), two cis dihydrogen ligands, and two hydrides trans to them, defining the equatorial plane. The H-H bond distances (0.825(8) and 0.835(8) A) are characteristic of two "unstretched" dihydrogen ligands. H/D exchange between the Ru-H and the C-D bonds of deuterated benzene is observed within 1 h, leading to the formation of various isotopomers RuHxD6-x(PCyp3)2 (with x = 0-6). 2 is a catalyst precursor for ethylene coupling (20 bar, 293 K) to a functionalized arene (Murai reaction). We found a 90% conversion of acetophenone to 2-ethylacetophenone within 35 min, whereas 10 h was needed in the same conditions using the analogous tricyclohexylphosphine complex, RuH2(eta2-H2)2(PCy3)2, the best catalyst precursor, at room temperature, prior to this work.

Matrix Infrared Spectra and Density Functional Theory Calculations of Molybdenum Hydrides

Laser-ablated Mo atoms react with H2 upon condensation in excess argon, neon, and hydrogen. The molybdenum hydrides MoH, MoH2, MoH4, and MoH6 are identified by isotopic substitution (H2, D2, HD, H2 + D2) and by comparison with vibrational frequencies calculated by density functional theory. The MoH2 molecule is bent, MoH4 is tetrahedral, and MoH6 appears to have the distorted trigonal prism structure.

Infrared Spectra of CH3−CrH, CH3−WH, CH2WH2, and CH⋮WH3 Formed by Activation of CH4 with Cr and W Atoms

Laser-ablated W atoms react with CH4 in excess argon to form the CH3-WH, CH2=WH2, and CH[triple bond]WH3 molecules with increasing yield in this order of product stability. These molecules are identified from matrix infrared spectra by isotopic substitution. Tungsten methylidene and methylidyne hydride molecules are reversibly interconverted by alpha-H transfers upon visible and ultraviolet irradiations. Matrix infrared spectra and DFT/B3LYP calculations show that CH[triple bond]WH3 is a stable molecule with C3v symmetry, but other levels of theory were required to describe agostic distortion for CH2=WH2. Analogous reactions with Cr gave only CH3-CrH, which is calculated to be by far the most stable product.

[CH3MoF], [CH2MoHF], and [CHMoH2F] Formed by Reaction of Laser-Ablated Molybdenum Atoms with Methyl Fluoride: Persistent Photoreversible Interconversion through α-Hydrogen Migration and Agostic Interaction

Simple molybdenum methyl, carbene, and carbyne complexes, [CH3--MoF], [CH2=MoHF], and [CH[triple chemical bond]MoH(2)F], were formed by the reaction of laser-ablated molybdenum atoms with methyl fluoride and isolated in an argon matrix. These molecules provide a persistent photoreversible system through alpha-hydrogen migration between the carbon and metal atoms: The methyl and carbene complexes are produced by applying UV irradiation (240-380 nm) while the carbyne complex is depleted, and the process reverses on irradiation with visible light (lambda>420 nm). An absorption at 589.3 cm(-1) is attributed to the Mo--F stretching mode of [CH3--MoF], which is in fact the most stable of the plausible products. Density functional theory calculations show that one of the alpha-hydrogen atoms of the carbene complex is considerably bent toward the metal atom (angle-spherical HCMo=84.5 degrees ), which provides evidence of a strong agostic interaction in the triplet ground state. The calculated C[triple chemical bond]Mo bond length in the carbyne is in the range of triple-bond values in methylidyne complexes.

Infrared Spectra of CH3−MoH, CH2MoH2, and CH⋮MoH3 Formed by Activation of CH4 by Molybdenum Atoms

Reaction of laser-ablated Mo atoms with CH(4) in excess argon forms the CH(3)-MoH, CH(2)=MoH(2), and CH(triple bond)MoH(3) molecules, which are identified from infrared spectra by isotopic substitution and density functional theory frequency calculations. These simple methyl, methylidene, and methylidyne molybdenum hydride molecules are reversibly interconverted by alpha-H transfers upon visible and ultraviolet irradiations. The methylidene dihydride CH(2)=MoH(2) exhibits CH(2) and MoH(2) distortion and agostic interaction to a lesser degree than CH(2)=ZrH(2). Molybdenum methylidyne trihydride CH(triple bond)MoH(3) is a stable C(3v) symmetry molecule.