Han-Gook Cho

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 eta2-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 actinide metal atoms with ethane: computation and observation of new Th and U ethylidene dihydride, metallacyclopropane dihydride, and vinyl metal trihydride complexes.

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 density 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 alpha-H repulsions, which leads to a substantial decrease in the alpha-H to Th agostic interaction distance, and contributes to our understanding of agostic distortion in organometallic complexes.

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

Rhenium carbyne complexes (HC identical with ReH 3, HC identical with ReH 2X, HC identical with ReHX 2, [X = F, Cl, and Br] and CH 3C identical with ReH 3) are produced by reactions of laser-ablated Re atoms with methane, methyl halides, methylene halides, and ethane via oxidative C-H(X) insertion and alpha-hydrogen migration in favor of the carbon-metal triple bond. The stabilities of the carbyne complexes relative to other possible products are predicted by DFT calculations. The diagnostic methylidyne C-H stretching absorptions of HC identical with ReH 3 and its mono- and dihalo derivatives are observed on the blue sides of the precursor C-H stretching bands, and the frequency decreases and the bond length increases in the order of H, F, Cl, and Br, following the decreasing s character in hybridization for the C-H bond. The dihalo methylidynes have higher C-H stretching frequencies and s characters than the monohalo species. The rhenium methylidynes have C s structures, and as a result the HC identical with ReH 3 and CH 3C identical with ReH 3 complexes have two equivalent shorter and one longer Re-H bonds, as compared to the tungsten methylidyne HC identical with WH 3 with three equivalent W-H bonds.

Matrix infrared spectroscopic and computational investigations of the lanthanide-methylene complexes CH2LnF2 with single Ln-C bonds.

Laser-ablated lanthanide metal atoms were condensed with CH(2)F(2) in excess argon at 6 K or neon at 4 K. New infrared absorption bands are assigned to the oxidative addition product methylene lanthanide difluorides on the basis of deuterium substitution and vibrational frequency calculations with density functional theory (DFT). Two dominant absorptions in the 500 cm(-1) region are identified as lanthanide-fluoride stretching modes for this very strong infrared absorption. The predominantly lanthanide-carbon stretching modes follow a similar trend of increasing with metal size and have characteristic 30 cm(-1) deuterium and 14 cm(-1) (13)C isotopic shifts. The electronic structure calculations show that these CH(2)LnF(2) complexes are not analogous to the simple transition and actinide metal methylidenes with metal-carbon double bonds that have been investigated previously, because the lanthanide metals (in the +2 or +3 oxidation state) do not appear to form a π-type bond with the CH(2) group. The DFT and ab initio correlated molecular orbital theory calculations predict that these complexes exist as multiradicals, with a Ln-C σ bond and a single electron on C-2p weakly coupled with f(x) (x = 1 (Ce), 2 (Pr), 3(Nd), etc.) electrons in the adjacent Ln-4f orbitals. The Ln-C σ bond is composed of about 15% Ln-5d,6s and 85% C-sp(2) hybrid orbital. The Ln orbital has predominantly 6s and 5d character with more d-character for early lanthanides and increasing amounts of s-character across the row. The Ln-F bonds are almost purely ionic. Accordingly, the argon-neon matrix shifts are large (13-16 cm(-1)) for the ionic Ln-F bond stretching modes and small (∼1 cm(-1)) for the more covalent Ln-C bond stretching modes.

Infrared Spectra of CH2=Zr(H)NC, CH3-ZrNC, and η2-Zr(NC)-CH3 Produced by Reactions of Laser-Ablated Zr Atoms with Acetonitrile

The zirconium methylidene isocyanide, methyl isocyanide, and eta(2)-nitrile-pi-complexes are observed in the matrix IR spectra from reactions of laser-ablated Zr atoms and acetonitrile isotopomers. The methylidene CH(2)=Zr(H)NC has a C(1) agostic structure in line with simple early transition-metal methylidenes recently produced from reactions with small alkanes and methyl halides, and the extent of agostic distortion is also comparable. Formation of the isocyanide complexes from acetonitrile is interesting but not surprising according to previous studies of metal reactions with nitrile-containing compounds, and their stabilities over the cyanide species are reproduced by DFT calculations. Observation of the relatively rare nitrile pi-complex and its photodissociation suggests that the reaction proceeds in the order of Zr<--NCCH(3), eta(2)-Zr(NC)-CH(3), CH(3)-ZrNC, and CH(2)=Zr(H)NC. The intermediate transition-state structures are also examined.

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.

Matrix infrared spectroscopic and electronic structure investigations of the lanthanide metal atom-methyl fluoride reaction products CH3-LnF and CH2-LnHF: the formation of single carbon-lanthanide metal bonds.

Lanthanide metal atoms, produced by laser ablation, were condensed with CH(3)F in excess Ar at 8 K. New infrared absorption bands are assigned to the first insertion CH(3)LnF and oxidative addition methylene lanthanide hydride fluoride CH(2)LnHF products on the basis of (13)C and deuterium substitution and density functional theory calculations of the vibrational frequencies. It is also possible to observe the cationic species CH(3)LnF(+) for some Ln. For Ln = Eu and Yb, only CH(3)LnF is observed. CH(3)LnF in the Ln formal +2 state is predicted to be more stable than CH(2)LnHF with the Ln in the formal +3 oxidation state. CH(3)-LnF forms a single bond between Ln and C and is a substituted methane. Similar to CH(2)-LnF(2), CH(2)-LnHF does not form a π-bond between Ln and C and is best described as a LnHF-substituted CH(3) radical, with an unpaired p electron on C weakly interacting with the unpaired f electrons on the Ln. The calculated potential energy surface for the CH(3)F + La → CH(3)-LaF/CH(2)-LaHF shows a number of intermediates and transition states on multiple paths. The reaction mechanism involves the potential formation of LaF and LaHF intermediates.

Infrared spectra of CX2=CoX2 and CX3-CoX complexes from reactions of laser-ablated cobalt atoms with halomethanes.

Simple cobalt complexes with substantial carbon-cobalt double bond character are produced in Co atom reactions with tetra-, tri-, and dihalomethanes, whereas insertion complexes are identified only in the dihalomethane matrix infrared spectra. These complexes are identified from matrix infrared spectra and comparison with frequencies computed by density functional theory. Exclusive generation of carbenes in the tetrahalomethane systems is consistent with the computational results that the staggered allene-type conformer is the only meaningful energy minimum in the reaction path. Their short C-Co bondlengths of 1.732-1.764 A and CASSCF computed bond orders near 1.7 are also appropriate for carbon-cobalt double bonds. Hence, reactions of laser-ablated Co atoms are effective means to generate rarely reported high oxidation-state Co complexes with carbon-cobalt double bonds. Unlike the Rh and Ir cases, Co carbynes (with C-Co triple bonds) are not formed. The observation of CH(2)Cl-CoF with photoreversible intensity variation in the spectra provides unique insight on halogen migration from interconversion with CH(2)F-CoCl through the CH(2)=CoFCl carbene.

Infrared spectra of the ethynyl metal hydrides produced in reactions of laser-ablated Mn and Re atoms with acetylene.

The ethynyl metal hydride molecules (HM-C≡CH) are identified in the matrix infrared spectra from reactions of laser-ablated Mn and Re atoms with acetylene using D and (13)C isotopic substitution and density functional computed frequencies. The assignment of strong M-H as well as C≡C bond stretching product absorptions suggests oxidative C-H insertion during reagent codeposition and subsequent photolysis. The unique linear structure calculated for HMn-C≡CH is parallel to C(3v) structures found recently for Mn complexes including CH(3)-MnF.