Laser-ablated Group Research Articles

Infrared Spectroscopic and Theoretical Investigations of Group 13 Oxyfluorides OMF2 and OMF (M=B, Al, Ga, In).

Group 13 oxyfluorides OMF2 were produced by the reactions of laser-ablated group 13 atoms M (M=B, Al, Ga and In) with OF2 and isolated in excess neon or argon matrices at 5 K. These molecules were characterized by matrix-isolation infrared spectroscopy and isotopic substitution experiments in conjunction with quantum-chemical calculations. The calculations indicate that the OMF2 molecules have a 2 B2 ground state with C2v symmetry. The computed molecular orbitals and spin densities show that the unpaired electron is mainly located at the terminal oxygen atom. Oxo monofluorides OMF were only observed in solid argon matrices and exhibit a linear structure in the singlet ground state. The M-O bonding in the OMF molecules can be rationalized as highly polar multiple bonds based on the calculated bond lengths and natural resonance theory (NRT) analyses. In particular, the molecular orbitals of OBF exhibit the character of a triple bond B-O resulting from two degenerate electron-sharing π bonds and an O→B dative σ bond formed by the oxygen 2p lone pair which donates electron density to the boron empty 2p orbital.

Reactions of titanium, zirconium and hafnium atoms with hydrogen selenide: A matrix-isolation study

Laser-ablated group IV transition metal atoms have been co-deposited with hydrogen selenide at 5 K in excess of argon. The metal atoms were observed to insert into the SeH bond of hydrogen selenide to form the novel molecules HMSeH, H2MSe, H2M(SeH)2 and HMSe (M = Ti, Zr, Hf). These species were identified by argon matrix-isolation IR spectroscopy, and their assignment is supported by 1H/2D substitution as well as quantum-chemical calculations. In Particular, the molecules H2MSe and HMSe are of interest as they are rare examples of multiple bonding between a heavier main group element and a group IV transition metal.

Matrix Infrared Spectra and Electronic Structure Calculations of Linear Alkaline Earth Metal Di-isocyanides CNMNC, Ionic (NC)M(NC) Bowties, and Ionic (MNC)2 Rings.

Laser-ablated group 2 metal atoms exhibit different reactivities with (CN)2 in excess argon and neon during condensation at 4 K. UV irradiation (220-290 nm) is required to activate Be to produce the linear CNBeNC di-isocyanide molecule with a strong antisymmetric C-N stretching band at 2104.3 cm-1 and a C-N-Be-N-C stretching mode at 1265.7 cm-1. The di-isocyanide appears at lower frequency and exhibits more nitrogen and less carbon isotopic shift than the cyanide counterpart, which is confirmed by B3LYP isotopic frequency calculations. Two weak bands were observed for the cyanide NCBeCN, and three absorptions were found for the mixed ligand CNBeCN molecule, which would be difficult to synthesize and put into a bottle. Mg reacts with (CN)2 to form the CNMgNC counterpart at 2085.8 cm-1 on annealing to 25 K. Absorptions for the Ca(NC)2, Sr(NC)2, and Ba(NC)2 molecules at 2060.8, 2048.1, and 2045.9 cm-1 increase on sample annealing with these more reactive heavier alkaline earth metal atoms, which have calculated twisted bowtie side-bound (CN) structures of C2 symmetry with shorter computed M-N than M-C distances. NBO calculations for the latter molecules reveal natural charges of +1.54, 1.64, 1.71 e on the metal centers and - 0.77, 0.82, and 0.855 e on the corresponding CN subunits, with a doubly occupied sp-sp σ and two p-p π bonds on each (CN), which supports an ionic model for bonding in these molecules. A weaker band at 2056.6 cm-1 behaves nearly the same as the Ca bowtie band in the spectrum on 25 K annealing and photolysis, and it is assigned to the 6 kJ/mol higher-energy linear CNCaNC isomer. Additional similar sharp absorptions for a new Ca, Sr, and Ba reaction product at 2036.3, 2040.6, and 2036.0 cm-1 increase on annealing at the expense of adjacent broader bands: B3LYP and NBO calculations validate their assignment to ionic hexagonal C2 h (MNC)2 rings from the reaction of two M atoms with (CN)2.

Fluoroborylene Complexes FBMF2 (M = Sc, Y, La, Ce): Matrix Infrared Spectra and Quantum Chemical Calculations.

Laser-ablated group 3 transition metal and cerium atom reactions with boron trifluoride were explored in excess solid neon at 4 K through matrix isolation infrared spectroscopy and quantum chemical calculations. The fluoroborylene complexes FBMF2 (M = Sc, Y, La, Ce) were trapped in inert gas and identified by the isotopic substitutions. The observed frequencies of FBMF2 were reproduced by DFT, NEVPT2, and CASSCF calculations. From Sc to La, the observed F-11B stretching mode has been observed at 1391.9 cm-1 (Sc), 1370.8 cm-1 (Y), and 1337.1 cm-1(La); however, for Ce this mode shifts up to 1340.8 cm-1, which is due to relativistic effects. The electron localization function (ELF) analysis and the theory of atoms in molecules (AIM) were applied to investigate the character of the B-M bond in FBMF2 molecules, which favors bond order 1.5.

Splitting of Hydrogen Sulfide by Group 14 Elements (Si, Ge, Sn, Pb) in Excess Argon at Cryogenic Temperatures.

The water gas reaction C + H2O → CO + H2 has been employed for centuries; however little is known for analogous reaction M + H2S → MS + H2 (M = Si, Ge, Sn, Pb). In addition, this latter reaction is intriguing in its function of converting pollutant H2S to clean energy source H2. We report herein the reactions of laser-ablated Group 14 atom M (M = Si, Ge, Sn, Pb) with H2S using matrix isolation infrared spectroscopy as well as the state-of-the-art quantum chemical calculations. Important reactive intermediates HMSH with high active H (protonic Hδ+ and hydridic Hδ-) are identified. In addition, the reaction mechanisms are established for insertion reaction M + H2S → HMSH, and photoinduced H2 elimination reactions of HMSH → MS+ H2 and H2SiS → SiS+ H2 in low-temperature matrices.

Matrix Infrared Spectra of Insertion and Metallacyclopropane Complexes [CH3CH2-MH and (CH2)2-MH2] Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Ethane.

CH3CH2-MH and (CH2)2-MH2 were identified in the matrix IR spectra from reactions of laser-ablated group 3 metal atoms with ethane, and they were characterized via theoretical investigations. The observed products are the most stable in the proposed reaction path. Because of the small number of valence electrons, the group 3 metal high oxidation-state complexes are less stable. The C-C insertion product [(CH3)2M], which was predicted to be more stable than the observed ones, was not observed probably because of the high energy barrier and a likely slower rate for insertion into one C-C bond than one of six C-H bonds. The C-C bond of the metallacyclopropanes is the shortest among the early transition-metal analogues, and its stretching frequencies are the highest, revealing the weakest interaction between the metal dihydride and ethylidene groups. The undetected ethylidene is not agostic, parallel to the previously examined methylidene.

Observation and Characterization of CH3CH2-MH, (CH2)2-MH2, CH2═CH-MH3, and CH3-C≡MH3- Produced by Reactions of Group 5 Metal Atoms with Ethane.

The primary products in reactions of laser-ablated group 5 metal atoms with ethane were identified in argon matrix IR spectra and characterized via density functional theory computations. The second- and third-row transition metals Nb and Ta produced insertion, metallacyclopropane, vinyl trihydrido, and anionic ethylidyne complexes (CH3CH2-MH, (CH2)2-MH2, CH2═CH-MH3, and CH3C≡MH3-), while the first-row transition metal V yielded only the insertion and metallacyclopropane products. The energetically higher ethylidenes and neutral ethylidynes (CH3CH═MH2 and CH3C[Formula: see text]MH3) were not detected. The unique anionic ethylidynes are the most stable anionic species in the Nb and Ta systems. Evidently back-donation from the metal center to the C-C π* orbital is stronger than that in the group 6 metal analogue but weaker than that in the corresponding group 4 metal complex. The C-M bond for the Nb and Ta ethylidyne anions is a true triple bond.

Observation and Characterization of CH3CH2–MH, (CH2)2–MH2, and CH3–C≡MH3 Prepared in Reactions of Ethane with Laser-Ablated Group 6 Metal Atoms

Reactions of laser-ablated group 6 metal atoms with ethane have been carried out, and the products identified in argon matrix IR spectra on the basis of deuterium shifts, correlation with computation results, and previous related works. Mo and W generated the insertion, metallacyclopropane, and ethylidyne complexes [CH3CH2–MH, (CH2)2–MH2, and CH3C≡MoH3], whereas Cr gave only the insertion product. The higher oxidation-state complex from ethane becomes relatively more stable in the heavier metal system, parallel to those from methane. The unobserved ethylidene CH3CH═MH2 is energetically higher than (CH2)2–MH2, and the energy barrier from CH3CH2–MH to the ethylidene is also substantially higher than that for the observed cyclic product. NBO calculations indicate that the C–M bonds of the group 6 metal ethylidenes and ethylidynes are true double and triple bonds.

Infrared Spectra of M-η2-C2H2 and HM-C≡CH Produced in Reactions of Laser-Ablated Group 6 Metal Atoms with Acetylene

The π and C-H insertion complexes (M-η(2)-C(2)H(2) and HM-C≡CH) are identified in the matrix infrared spectra from reactions of laser-ablated Group 6 metal atoms with acetylene. In annealing, the π complex is produced, and it converts to the insertion product during photolysis with no trace of the vinylidene product. This observation is consistent with the considerably higher activation energy to H(2)CCM than that to HM-CCH in the previously proposed reaction path, whereas the three plausible products are in fact energetically comparable. The back-donations in the Group 6 metal π complexes are evidently weaker than those in the Groups 3-5 metal analogues but still stronger than those in the main group and Group 7-10 metal systems. The insertion complexes have bent CMH moieties in contrast with the linear Mn complex.

Infrared Spectra of M-η2-C2H2, HM–C≡CH, and HM–C≡CH– Prepared in Reactions of Laser-Ablated Group 3 Metal Atoms with Acetylene

The major HM-C≡CH and M-η(2)-C(2)H(2) products are observed in the matrix infrared spectra from reactions of laser-ablated group 3 metal atoms with acetylene, while the vinylidene product is not detected. These results reveal that coordination of group 3 metal atoms to the acetylene π-bond and H-migration from C to M readily occur during codeposition and photolysis afterward. The product absorption assigned to the La-vinylidene complex in a previous study is reassigned to one of the absorptions of La-η(2)-C(2)H(2)···C(2)H(2). Two strong Sc-H stretching absorptions are assigned to the free HSc-CCH(-) anion, in accord with a previous study, but a lower frequency counterpart is reassigned to HSc-CCH(-) coordinated to acetylene based on the increasing relative intensity of the low-frequency component at higher acetylene concentration. The group 3 metals evidently form weaker π-complexes than the group 4 metals. The addition of an electron to HM-CCH elongates the M-H and M-C bonds in the anionic species due to the lower ionic contributions to the bonding.

Infrared spectra and density functional calculations of the M←NCCCH3, η2–M(NC)–CH3, CH3–MNC, CH2[dbnd]M(H)NC, and CH[tbnd]M(H)2NC complexes produced by reactions of Group 6 metal atoms with acetonitrile

The M←NCCH3, η2–M(NC)–CH3, CH3–MNC, CH2M(H)NC, and HCM(H)2NC complexes are produced by reactions of laser-ablated Group 6 metal atoms with acetonitrile and identified from the matrix IR spectra through isotopic substitution and comparison to frequencies calculated by density functional theory in the harmonic approximation (B3LYP/6-311++(3df,3pd) and BPW91/6-311++G(3df,3pd), this all-electron set for Cr, and SDD for Mo and W). Good general agreement is shown in tables for all observed frequencies and these calculations. The products follow the reaction path proposed earlier for Group 4 and 5 metal reactions, which are supported by density functional energy calculations. The M←NCCH3 complex strengthens the assumption that the lone electron pair on the N-end first attracts the metal atom. The π-complexes show that the back-donation of the Group 6 metal is weaker than those of Zr and Group 5 metals. The observed products clearly show the increasing preference for the higher oxidation-state complex with increasing atomic number of the Group 6 metal, and subsequent photolysis increases the yields for the higher oxidation-state complexes. The methylidyne isocyanide HCW(H)2NC has a Cs structure similar to the HCW(H)2X complexes prepared earlier.

Infrared Spectra of HC≡C−MH and M-η2-(C2H2) Produced in Reactions of Laser-Ablated Group 5 Transition-Metal Atoms with Acetylene

The ethynyl metal hydride (HC≡C−MH) and metallacycle complexes (M-η2-(C2H2)) are identified in the matrix infrared spectra from reactions of laser-ablated group 5 metal atoms with acetylene. The observed intensity variations reveal spontaneous formation of the cyclic complex upon annealing and its conversion to the insertion product via oxidative C-H insertion reaction and also during subsequent photolysis. The less stable vinylidene complex is not identified. The high binding energies, low C-C stretching frequencies, and long C-C bonds of the group 5 metal M-η2-(C2H2) species all indicate strong bonding relative to the late-transition-metal and light-metal analogues, but they are somewhat weaker than the group 4 metal counterparts.

Infrared Spectra of CH2═M(H)NC, CH3−MNC, and η2-M(NC)−CH3 Produced by Reactions of Laser-Ablated Group 5 Metal Atoms with Acetonitrile

Methylidene isocyanides, methyl isocyanides, and eta(2)-nitrile-pi-complexes are observed in the matrix IR spectra from reactions of Group 5 metals with acetonitrile isotopomers. The primary isocyanide products with no trace of cyanide complexes are consistent with the reaction path proposed in the analogous Zr study. The major products (CH(2)=Ta(H)NC, CH(3)-NbNC, eta(2)-Nb(NC)-CH(3), and eta(2)-V(NC)-CH(3)) after codeposition and reaction of metal with CH(3)CN clearly show the increasing preference for the higher oxidation-state complex on going down the group column, and the subsequent photochemistry provides further information for molecular rearrangements. The Group 5 metal methylidene isocyanides exhibit more agostic distortion than the Zr counterparts and are comparable to the previously studied Group 5 metal methylidene hydrides and halides. The computed structures and observed frequencies indicate that the effects of metal conjugation (C=Ta-N=C:) are minor.

Reactions of Group 14 Metal Atoms with Acetylene: A Matrix Isolation Infrared Spectroscopic and Theoretical Study

Laser-ablated group 14 metal atoms have been codeposited at 4 K with acetylene in excess argon. Products, Ge(C2H2), HGeCCH, Sn(C2H2), Sn2CCH2, HSnCCH, and HPbCCH, have been formed in the present experiments and characterized using infrared spectroscopy on the basis of the results of the isotopic shifts, mixed isotopic splitting patterns, stepwise annealing, the change of reagent concentration and laser energy, and the comparison with theoretical predictions. Density functional theory calculations have been performed on these molecules. The agreement between the experimental and the calculated vibrational frequencies, relative absorption intensities, and isotopic shifts supports the identification of these molecules from the matrix infrared spectra. Plausible reaction mechanisms have been proposed to account for the formation of these molecules.

Matrix Infrared Spectra of the C−H Insertion and Dihydrido Cyclic Products from Reactions of Group 3 Metal Atoms with Ethylene

Reactions of laser-ablated Group 3 metal atoms with ethylene isotopomers have been carried out, and the primary products have been identified in the matrix IR spectra. The insertion and dihydrido cyclic products (HM-C(2)H(3) and H(2)M-C(2)H(2)) are identified from the matrix IR spectra of Sc and Y, whereas only the insertion product is observed in the La spectra along with many lanthanium hydride absorptions. The observed metal hydride absorptions suggest that the H(2)-elimination reaction becomes faster upon going down the group column, and the trend is also consistent with the energies of the H(2)-elimination products relative to those of the dihydrido cyclic products. The present results support the conclusion that C-H insertion and following H migration from C to M are in fact general phenomena in reactions of early transition metals (Groups 3-6) and actinides with ethylene. The back-donation of electron-deficient Group 3 metals to the pi-cyclic system of H(2)M-C(2)H(2) is evidently much weaker than that of the previously studied systems.

Experimental and Theoretical Investigation of Simple Terminal Group 6 Arsenide As≡MF3 Molecules

Laser-ablated group 6 metal atoms react with NF3 and PF3 to form the simple lowest energy N[triple bond]MF3 and P[triple bond]MX3 products, and this investigation has been extended to AsF3. Mo and W atoms react with AsF3 upon excitation by laser ablation or UV irradiation to form stable trigonal As[triple bond]MF3 terminal arsenides. These molecules are identified by comparison of the closely related infrared spectra of the analogous phosphide species and with frequencies calculated by density functional theory and multiconfigurational second order perturbation theory (CASSCF/CASPT2). Computed CASSCF/CASPT2 triple bond lengths for the As[triple bond]MoF3 and As[triple bond]WF3 molecules are 2.240 A and 2.250 A, respectively. The natural bond orders calculated by CASSCF/CASPT2 decrease from 2.67 to 2.60 for P[triple bond]MoF3 to As[triple bond]MoF3 and from 2.74 to 2.70 for P[triple bond]WF3 to As[triple bond]WF3 as the arsenic valence orbitals are less effective than those of phosphorus in bonding to each metal atom and the larger metal orbital size becomes more compatible with the arsenic valence orbitals. The Cr atom reaction gives the arsinidene AsF=CrF2 product instead of the higher energy As[triple bond]CrF3 molecule as the Cr (VI) state is not supported by the softer pnictides.

Infrared Spectra of Metallacyclopropane, Insertion, and Dihydrido Complex Products in Reactions of Laser-Ablated Group 6 Metal Atoms with Ethylene Molecules

Reactions of laser-ablated group 6 metal atoms with ethylene have been investigated. The insertion and dihydrido products (MH-CHCH(2) and MH(2)-C(2)H(2)) are identified from reactions of W and Mo with ethylene isotopomers, whereas products in the Cr spectra are assigned to the insertion and metallacyclopropane (M-C(2)H(4)) complexes. Our experiments with CH(2)CD(2) show that the dihydrido complex is formed by beta-hydrogen transfer in the insertion complex because the MHD-CHCD isotopic product is favored. The present matrix infrared spectra and DFT computational results support the general trend that the higher oxidation-state complexes become more stable on going down the group 6 column. Unlike the cases of group 4 and 5 metals, binary metal hydride (MH(x)) absorptions are not observed in the infrared spectra, suggesting that the H(2)-elimination reactions of ethylene by group 6 metals are relatively slow, consistent with previous gas-phase reaction dynamics studies.

A Combined Theoretical and Experimental Study of Simple Terminal Group 6 Nitride and Phosphide N≡MX3 and P≡MX3 Molecules

Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF 3, PF 3, and PCl 3 to form the simple lowest energy N[triple bond]MF 3, and P[triple bond]MX 3 products following insertion and halogen transfer, with the exception of P[triple bond]CrF3, which is a higher energy species and is not observed. The E[triple bond]MX3 pnictide metal trihalide molecules are identified from both argon and neon matrix infrared spectra and frequencies calculated by density functional theory and multiconfigurational second-order perturbation theory (CASSCF/CASPT2). These simple terminal nitrides involve strong triple bonds, which range from 2.80 to 2.77 to 2.59 natural bond order for M = W, Mo, and Cr, respectively, as computed by CASSCF/CASPT2, and the M[triple bond]N stretching frequencies also follow this order. The terminal phosphides are weaker with bond orders 2.74, 2.67, and 2.18, respectively, as the more diffuse 3p orbitals are less effective for bonding to the more compact metal valence d orbitals.

Infrared Spectra of HC≡C-MH and M−η2-(C2H2) from Reactions of Laser-Ablated Group- 4 Transition-Metal Atoms with Acetylene

Reactions of laser-ablated group 4 transition-metal atoms with acetylene have been carried out. The ethynyl metal hydrides (HC[triple bond]C-MH) and corresponding pi complexes (M-eta(2)-(C2H2)) are identified in the matrix infrared spectra. The observed M-H and C-M stretching absorptions show that oxidative C-H insertion readily occurs during codeposition and photolysis afterward. The absorptions from the pi complex, on the other hand, are relatively weak in the original deposition spectrum but increase dramatically in the process of annealing. The vinylidene complex, another plausible product, is not identified in this study. The observed spectra and DFT calculations both show that the back-donations from the group 4 metals to the antibonding pi* orbital of C2H2 are extensive such that the group 4 metals form unusually strong pi complexes. Thus, it is the formation of two Ti-C bonds in the group 4 systems than leads to the stronger bonding than that in the group 8 systems. While bonds form, the Ti atom is weakly bound to C2H2, and we still refer to it as a pi complex. Evidence of relativistic effects is also observed in frequency trends for the Ti, Zr, and Hf products.

Silylidyne, HSi≡MoH3 and HSi≡WH3, and Silyl Metal Hydride, SiH3—CrH, Products in Silane Reactions

Laser-ablated group 6 metal atoms react with silane to form inserted SiH3−MH hydride intermediates, which are identified from M−H and Si−H stretching modes. Following two successive α-H-transfers, the HSi≡MH3 (M = Mo, W) silylidyne molecules are produced. These silicon−metal triple-bonded species are identified as major products from the strong M−H stretching modes through deuterium substitution and comparison with frequencies and intensities from density functional calculations and from the analogous methylidynes. The silylidynes have calculated C3v structures and longer Si−H bonds than silane, but the C3v methylidyne analogues have shorter C−H bonds than methane. The Si≡Mo and Si≡W bonds are polarized differently and have slightly lower effective bond orders than their carbon analogues. In addition, calculations for the group 6 silylidene molecules reveal Cs structures with no evidence of agostic distortion, in contrast to the corresponding methylidene molecules.