Reactions Of Laser-Ablated Group Research Articles

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.

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.

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.

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.

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.