Showing papers by "Lester Andrews published in 2008"

Infrared spectra and electronic structures of agostic uranium methylidene molecules.

Abstract: Through reactions of laser-ablated uranium atoms with methylene halides CH2XY (XY = F2, FCl, and Cl2), a series of new actinide methylidene molecules CH2UF2, CH2UFCl, and CH2UCl2 are formed as the major products. The identification of these complexes has been accomplished via matrix infrared spectra, isotopic substitution, and relativistic density functional calculations of the vibrational frequencies and infrared intensities. Density functional calculations using the generalized gradient approach (PW91) show that these CH2UXY methylidene complexes prefer highly distorted agostic structures rather than the ethylene-like symmetric structures. The calculated agostic angles ([angle]H-C-U) are around 89 degrees for all the three uranium complexes, and the predicted vibrational modes and isotopic shifts agree well with experimental values. Electronic structure calculations reveal that these U(IV) molecules all have strong C=U double bonds in the triplet ground states with 5f (2) configurations. The calculated bond lengths and bond energies indicate that the C=U double bonds are slightly weaker in the fluoride species than in the chloride species because of the radial contraction of the U (6d) orbitals by the inductive effect of the fluorine substituent. The agostic uranium methylidene complexes are compared with analogous transition metal and thorium complexes, which reveal interesting differences in their chemistries.

Infrared Spectra of Methylidynes Formed in Reactions of Re Atoms with Methane, Methyl Halides, Methylene Halides, and Ethane : Methylidyne C-H Stretching Absorptions, Bond Lengths, and s Character

Abstract: Rhenium carbyne complexes (HC≡ReH3, HC≡ReH2X, HC≡ReHX2, [X = F, Cl, and Br] and CH3C≡ReH3) are produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and e...

Preparation and Characterization of Simple Dihalomethylidene Platinum Dihalide Complexes in Reactions of Laser-Ablated Pt Atoms with Tetrahalomethanes

Abstract: Platinum atoms react with tetrachlorofluoromethanes upon laser-ablation and with ultraviolet irradiation to form dihalomethylidene platinum dihalide complexes, CX2═PtX2. These new molecules are identified from carbon-13 and chlorine isotopic shifts, displacements in functional group frequencies as chlorine is replaced with fluorine, and comparison to frequencies calculated by density functional theory. The Pt—C bond lengths calculated here, 1.810 to 1.816 A, are shorter than analogous bond lengths measured earlier for Pt(II) carbene complexes (1.943−1.950 A). The computed effective Pt−C bond orders range from 1.41 to 1.70 as chlorine is replaced by fluorine since the more electronegative halogen appears to concentrate the Pt 5d orbitals and make them bond better with carbon. These platinum methylidene complexes thus have a substantial amount of double bond character from dπ−pπ bonding.

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

Abstract: Reactions of laser-ablated group 4 transition-metal atoms with acetylene have been carried out. The ethynyl metal hydrides (HC≡C-MH) and corresponding π complexes (M−η2-(C2H2)) are identified in th...

Reactions of actinide metal atoms with ethane: computation and observation of new Th and U ethylidene dihydride, metallacyclopropane dihydride, and vinyl metal trihydride complexes.

Abstract: A combined computational and experimental investigation provides evidence that excited thorium and uranium atoms activate ethane to form the vinyl metal trihydride, metallacyclopropane dihydride, and ethylidene metal dihydride for thorium and the latter complex and the inserted ethyl metal hydride for uranium. These products are trapped in solid argon and identified through deuterium isotopic substitution and vibrational frequencies calculated by densitiy functional theory. Comparisons are made with group 4 and methane reaction products. Numerous calculations using several methods show that these simple ethylidene complexes are more distorted by the agostic interaction than the corresponding methylidene species. This enhanced agostic interaction probably arises from methyl hydrogen to α-H repulsions, which leads to a substantial decrease in the α-H to Th agostic interaction distance, and contributes to our understanding of agostic distortion in organometallic complexes.

Reactions of Thorium Atoms with Polyhalomethanes: Infrared Spectra of the CH2=ThX2, HC÷ThX3, and XC÷ThX3 Molecules

Abstract: Laser-ablated thorium atoms react with methylene fluoride to form singlet CH2=ThF2, with fluoroform to give triplet HC÷ThF3, and with CF4 to produce triplet FC÷ThF3 molecules as the major products trapped in solid argon. Infrared spectroscopy, isotopic substitution, and density functional theoretical calculations confirm the identity of these methylidene and methylidyne complexes. Parallels with the analogous chloromethane and Group 4 metal reaction products are discussed. Structure calculations show that the C=Th bond lengths decrease and the agostic distortion increases from CH2=ThF2 to CH2=ThFCl to CH2=ThCl2 for the methylidene complexes. The triplet-state HC÷ThF3 and FC÷ThF3 electron-deficient methylidyne complexes exhibit delocalized -bonding as evidenced by spin densities comparable to those calculated for the analogous zirconium complexes. Chlorine substitution for fluorine supports stronger C÷Th bonds. Thus, thorium appears to react as the early transition-metal atoms with fluoro- and chloromethanes. However, there is a substantial contribution from Th 5f orbitals in addition to 6d in the SOMO forming the weak π-bonds in these electron-deficient methylidyne complexes.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

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

Abstract: 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.

Infrared spectra of the WH4(H2)4 complex in solid hydrogen.

Abstract: The codeposition of laser-ablated tungsten atoms with neat hydrogen at 4 K forms a single major product with a broad 2500 cm-1 and sharp 1860, 1830, 1782, 1008, 551, and 437 cm-1 absorptions, which are assigned to the WH4(H2)4 complex on the basis of isotopic shifts and agreement with isotopic frequencies calculated by density functional theory. This D2d structured complex was computed earlier to form exothermically from W atoms and hydrogen molecules. Annealing the matrix allows hydrogen to evaporate and the complex to aggregate and ultimately to decompose. Comparison of the H−H stretching mode at 2500 cm-1 and the W−H2 stretching mode at 1782 cm-1 with 2690 and 1570 cm-1 values for the Kubas complex W(CO)3(PR3)2(H2) suggests that the present physically stable WH4(H2)4 complex has more strongly bound dihydrogen ligands. Our CASPT2 calculations suggest a 15 kcal/mol average binding energy per dihydrogen molecule in the WH4(H2)4 complex.

Infrared Spectra of Insertion, Methylidene, and Methylidyne Complexes in Reactions of Laser‐Ablated Ruthenium Atoms with Halomethanes and Methane

Abstract: Reactions of laser-ablated Ru atoms with halomethanes and methane have been investigated. Small carbyne and carbene complexes or insertion products are produced in the reactions of halomethanes via C–X insertion and halogen migration, whereas only the insertion complex is formed in the reactions of methane. The results are consistent with computed product stabilities. The diagnostic C–H and C–X stretching absorptions of the carbyne complexes are observed on the blue side of the corresponding precursor bands. The high C–H and C–X stretching frequencies are explained with the higher s character in hybridization and combination with the C≡Ru stretching motion, respectively. The structures of the carbyne complexes are highly distorted and show a large variation with the ligands, and the extra electron pair on the Ru atom evidently plays an important role in the carbyne. In contrast to early transition-metal analogs, the Ru carbene complexes show no agostic distortion.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)

A combined theoretical and experimental study of simple terminal group 6 nitride and phosphide N MX3 and P MX3 molecules

Abstract: Organometallic complexes containing terminal metal nitrides and phosphides are important synthetic reagents. Laser-ablated group 6 metal atoms react with NF3, PF3, and PCl3 to form the simple lowest energy N MF3, and P MX3 products following insertion and halogen transfer, with the exception of P CrF3, which is a higher energy species and is not observed. The E 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 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 ThH2, ThH4, and the hydride bridging ThH4(H2)x (x = 1-4) complexes in solid neon and hydrogen.

Abstract: Laser-ablated Th atoms react with molecular hydrogen to give thorium hydrides and their dihydrogen complexes during condensation in excess neon and hydrogen for characterization by matrix infrared spectroscopy. The ThH2, ThH4, and ThH4(H2)x (x = 1−4) product molecules have been identified through isotopic substitution (HD, D2) and comparison to frequencies calculated by density functional theory and the coupled-cluster, singles, doubles (CCSD) method and those observed previously in solid argon. Theoretical calculations show that the Th−H bond in ThH4 is the most polarized of group 4 and uranium metal tetrahydrides, and as a result, a strong attractive “dihydrogen” interaction was found between the oppositely charged hydride and H2 ligands ThH4(H2)x. This bridge-bonded dihydrogen complex structure is different from that recently computed for tungsten and uranium hydride super dihydrogen complexes but is similar to that recently called the “dihydrogen bond” (Crabtree, R. H. Science 1998, 282, 2000). Natura...

Infrared spectrum of the CH3-PtH complex in solid argon prepared in the oxidative C-H insertion of methane by laser-ablated Pt atoms.

Abstract: Reactions of laser-ablated Pt atoms with CH4 during condensation in excess argon form CH3−PtH through oxidative C−H insertion show and that the late transition-metal atom Pt is an effective methane activation reagent, in agreement with gas phase investigations. Six observed infrared absorptions correlate with the six strongest calculated harmonic frequencies. The computed C−Pt bond length is slightly shorter than those of Pt complexes with large ligands. In addition, the strongest absorption of the CH2═PtH2 methylidene is detected.

C-H activation of ethane by group 4 metal atoms: observation and characterization of the MH-CH2CH3, MH2-(CH2)2, and MH3-CH=CH2 complexes.

Abstract: Group 4 metal atoms excited in the laser ablation process activate ethane to form the C-H insertion product, the metallacyclopropane dihydride, and vinyl metal trihydride complexes as major products. These three new metal hydrides are characterized by their strong M-H stretching absorptions and other weaker modes as predicted by density functional theory vibrational frequency calculations.

Infrared spectra of metallacyclopropane, insertion, and dihydrido complex products in reactions of laser-ablated group 6 metal atoms with ethylene molecules.

Abstract: Reactions of laser-ablated group 6 metal atoms with ethylene have been investigated. The insertion and dihydrido products (MH−CHCH2 and MH2−C2H2) 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−C2H4) complexes. Our experiments with CH2CD2 show that the dihydrido complex is formed by β-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 (MHx) absorptions are not observed in the infrared spectra, suggesting that the H2-elimination reactions of ethylene by group 6 metals are relatively slow, consistent with previous gas-phase reaction dynamics studies.

Infrared Spectra, Structure, and Bonding of the GeH3—CrH, HGe≡MoH3, and HGe≡WH3 Molecules in Solid Neon and Argon

Abstract: Laser ablated chromium, molybdenum, and tungsten atoms react with germane during condensation in excess noble gases. The chromium reaction stopped at the germyl metal hydride, molybdenum gave some hydride but mostly germylidyne, and tungsten reacted spontaneously to give only the germylidyne species. These molecules were identified by isotopic shifts, density-functional theory product energy and frequency calculations, and comparison to the analogous methane and silane reaction products. Effective bond orders for the HGe≡MoH3 and HGe≡WH3 molecules are 2.82 and 2.87 using the B3LYP density functional, and are slightly lower than their silicon and carbon analogues. Our calculated Ge≡M triple bond lengths for these simple trihydride complexes are 0.05 to 0.10 A shorter than those measured for larger group 6 organometallic complexes.

Boron atom reactions with acetylene. Ab initio calculated and observed isotopic infrared spectra of the borirene radical BC2H2. A fingerprint match

Abstract: Pulsed laser evaporated boron atoms react with C2H2 to produce several new organoboron species. The strongest absorption at 1170.6 cm−1 in the B‐C2 stretching region exhibits 10B, 13C, and D shifts for 7 isotopic molecules in agreement with MP2/DZP calculations for a BC2H2 borirene radical species. This agreement provides a ‘‘fingerprint’’ match for identification of the BC2H2 ring species. Calculated bond lengths are appropriate for delocalized bonding in the BC2 ring system.

Reactions of Thorium Atoms with Polyhalomethanes: Infrared Spectra of the CH2=ThX2, HC÷ThX3, and XC÷ThX3 Molecules (Eur. J. Inorg. Chem. 7/2008)

Abstract: the cover picture shows the HC÷ThF3 complex product of the Th and CHF3 reaction. The two singly occupied SOMO orbitals on the right reveal a strong overlap with the thorium center. A portion of the DFT-predicted infrared spectrum for this complex is illustrated over the background image of a typical laser-ablation experiment in progress in our vacuum chamber. Details are discussed in the article by J. T. Lyon and L. Andrews on p. 1047 ff.