Formation of CH3TiX, CH2=TiHX, and (CH3)2TiX2 by reaction of methyl chloride and bromide with laser-ablated titanium atoms: photoreversible alpha-hydrogen migration

The simple methylidene (CH2=TiHX) and Grignard-type (CH3TiX) complexes are produced by reaction of methyl chloride and bromide with laser-ablated Ti atoms and isolated in a solid Ar matrix, and they form a persistent photoreversible system via alpha-hydrogen migration between the carbon and titanium atoms. The Grignard-type product is transformed to the methylidene complex upon UV (240 nm < lambda < 380 nm) irradiation and vice versa with visible (lambda > 530 nm) irradiation. More stable dimethyl dihalide complexes [(CH3)2TiX2] are also identified, whose relative concentration increases upon annealing and at high methyl halide concentration. The reaction products are identified with three different groups of absorptions on the basis of the behaviors upon broadband photolysis and annealing, and the vibrational characteristics are in a good agreement with DFT computation results.     

Infrared spectrum and bonding in uranium methylidene dihydride, CH2=UH2

Uranium atoms activate methane upon ultraviolet excitation to form the methyl uranium hydride CH3-UH, which undergoes alpha-H transfer to produce uranium methylidene dihydride, CH2=UH2. This rearrangement most likely occurs on an excited-quintet potential-energy surface and is followed by relaxation in the argon matrix. These simple U+CH4 reaction products are identified through isotopic substitution (13CH4, CD4, CH2D2) and density functional theory frequency and structure calculations for the strong U-H stretching modes. Relativistic multiconfiguration (CASSCF/CASPT2) calculations substantiate the agostic distorted C1 ground-state structure for the triplet CH2=UH2 molecule. We find that uranium atoms are less reactive in methane activation than thorium atoms. Our calculations show that the CH2=UH2 complex is distorted more than CH2=ThH2. A favorable interaction between the low energy open-shell U(5f) sigma orbital and the agostic hydrogen contributes to the distortion in the uranium methylidene complexes.     

Infrared spectra and density functional calculations of CH2 = MHX and CH[triple bond]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.     

Infrared spectra of CH(3)-MoH, CH(2)=MoH(2), and CH(triple bond)MoH(3) formed by activation of CH(4) 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.     

Infrared spectra of the CH3-MX, CH2=MHX, and CH[triple bond]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.     

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.     

Group 4 transition metal CH2=MF2, CHF=MF2, and HC/MF3 complexes formed by C-F activation and alpha-fluorine transfer

Group 4 transition metal methylidene difluoride complexes (CH2=MF2) are formed by the reaction of methylene fluoride with laser-ablated metal atoms and are isolated in an argon matrix. Isotopic substitution of the CH2F2 precursor and theoretical computations (B3LYP and CCSD) confirm product identifications and assignments. Our calculations indicate that the CH2=MF2 complexes have near C2v symmetry and are considerably more stable than other possible products (CH2(mu-F)MF and CHF=MHF). The primary reaction exothermicity provides more than enough energy to activate the initial bridge-bonded CH2(mu-F)MF products on the triplet potential energy surface to complete an alpha-F transfer to form the very stable CH2=MF2 products. Analogous experiments with CHF3 produce CHF=TiF2, which is not distorted at the C-H bond, whereas the heavier group 4 metals form lower-energy triplet HC/MF3 complexes, which contain weak degenerate C(p)-M(d) pi-bonding interactions. Comparisons are made with the CH2=MHF methylidene species, which showed considerable agostic distortions.     

Ln4(CH2)4 cubane-type rare-earth methylidene complexes consisting of "(C5Me4SiMe3)LnCH2" units (Ln = Tm, Lu)

Tetranuclear cubane-type rare-earth methylidene complexes consisting of four "Cp'LnCH(2)" units, [Cp'Ln(μ(3)-CH(2))](4) (4-Ln; Ln = Tm, Lu; Cp' = C(5)Me(4)SiMe(3)), have been obtained for the first time through CH(4) elimination from the well-defined polymethyl complexes [Cp'Ln(μ(2)-CH(3))(2)](3) (2-Ln) or mixed methyl/methylidene precursors such as [Cp'(3)Ln(3)(μ(2)-Me)(3)(μ(3)-Me)(μ(3)-CH(2))] (3-Ln). The reaction of the methylidene complex 4-Lu with benzophenone leads to C═O bond cleavage and C═C bond formation to give the cubane-type oxo complex [Cp'Lu(μ(3)-O)](4) and CH(2)═CPh(2), while the methyl/methylidene complex 3-Tm undergoes sequential methylidene addition to the C═O group and ortho C-H activation of the two phenyl groups of benzophenone to afford the bis(benzo-1,2-diyl)ethoxy-chelated trinuclear complex [Cp'(3)Tm(3)(μ(2)-Me)(3){(C(6)H(4))(2)C(O)Me}] (6-Tm).     

Synthesis and reactivity of the hydrido- and alkylrhenium methylidene complexes Cp*(PMe3)2(R)Re=CH2 (R = H, CH3)

Protonolysis of the dimethylrhenium(III) compound Cp(PMe(3))(2)Re(CH(3))(2) (3) led to formation of the highly reactive hydridorhenium methylidene compound [Cp(PMe(3))(2)Re(CH(2))(H)][OTf] (4), which was characterized spectroscopically at low temperature. Although 4 decomposed above -30 degrees C, reactivity studies performed at low temperature indicated it was in equilibrium with the coordinatively unsaturated methylrhenium complex [Cp(PMe(3))(2)Re(CH(3))][OTf] (2). Methylidene complex 4 was found to react with PMe(3) to afford [Cp(PMe(3))(3)Re(CH(3))][OTf] (6) and with chloride anion to give Cp(PMe(3))(2)Re(Me)Cl (7). When BAr(f) anion was added to 4, the thermally stable methylrhenium methylidene complex [Cp(PMe(3))(2)Re(CH(2))(CH(3))][BAr(f)] (8) was isolated upon warming to room temperature. The mechanisms of formation of both 4 and 8 are discussed in detail, including DFT calculations. The novel carbonyl ligated complex Cp(CO)(2)Re(CH(3))OTf (12) was prepared, isolated, and found to not undergo migration reactions to form methylidene complexes.     

Infrared spectrum and structure of CH2=ThH2

The actinide methylidene CH2=ThH2 molecule is formed in the reaction of laser-ablated thorium atoms with CH4 and trapped in a solid argon matrix. The five strongest infrared absorptions computed by density functional theory (two ThH2 stretches, C=Th stretch, CH2 wag, and ThH2 bend) are observed in the infrared spectrum. The computed structure shows considerable agostic bonding distortion of the CH2 and ThH2 subunits in the simple actinide methylidene dihydride CH2=ThH2 molecule, which is similar to the transition metal analogue, CH2=HfH2.     

Formation and characterization of thorium methylidene CH2=ThHX complexes

Laser-ablated thorium atoms react with methyl fluoride to give the CH2=ThHF molecule as the major product observed and trapped in solid argon. Infrared spectroscopy, isotopic substitution, and density functional theoretical frequency calculations confirm the identification of this methylidene complex. The four strongest computed absorptions (Th-H stretch, Th=C stretch, CH2 wag, and Th-F stretch) are the four vibrational modes observed. The CH2=ThHCl and CH2=ThHBr species formed from methyl chloride and methyl bromide exhibit the first three of these modes in the infrared spectra. The computed structures (B3LYP and CCSD) show considerable agostic interaction, similar to that observed for the Group 4 CH2=MHX (M = Ti, Zr, Hf) methylidene complexes, and the agostic angle and C=Th bond length decrease slightly in the CH2=ThHX series (X = F, Cl, Br).     

Formation and characterization of the uranium methylidene complexes CH2 = UHX (X = F, Cl, and Br)

The reactions between uranium atoms and CH3X (X = F, Cl, and Br) molecules are investigated in a solid argon matrix. The major products formed on ultraviolet irradiation are the CH2=UHX methylidene complexes. DFT calculations predict these triplet ground-state structures to be stable and to have significant agostic interactions. Parallels between the uranium and analogous thorium methylidene complexes are discussed.     

Reactions of methane with titanium atoms: CH3TiH, CH2=TiH2, agostic bonding, and (CH3)2TiH2

Laser-ablated titanium atoms react with methane to form the insertion product CH3TiH, which undergoes a reversible photochemical alpha-H transfer to give the methylidene complex CH2=TiH2. On annealing a second methane activation occurs to produce (CH3)2TiH2. These molecules are identified from matrix infrared spectra by isotopic substitution (CH4, 13CH4, CD4, CH2D2) and comparison to DFT frequency calculations. The computed planar structure for singlet ground-state CH2=TiH2 shows CH2 distortion and evidence for agostic bonding (H-C-Ti, 91.4 degrees), which is supported by the spectra for CHD=TiHD.     

Nucleophilic de-coordination and electrophilic regeneration of "hemilabile" pincer-type complexes: formation of anionic dialkyl, diaryl, and dihydride Pt(II) complexes bearing no stabilizing pi-acceptors

Novel anionic dialkyl, diaryl, and dihydride platinum(II) complexes based on the new "long-arm" hemilabile PCN-type ligand C6H4[CH2P(tBu)2](CH2)2N(CH3)2 with the general formula Li+[Pt(PCN)(R)2]- (R=Me (4), Ph (6) and H (9)) were prepared by reaction of [Pt(PCN)(R)] complexes (obtained from the corresponding chlorides) with an equivalent of RLi, as a result of the opening of the chelate ring. Alkylating agents based on other metals produce less stable products. These anionic d8 complexes are thermally stable although they bear no stabilizing pi acceptors. They were characterized by 1H, 31P[1H], 13C, and 7Li NMR spectroscopy; complex 9 was also characterized by single crystal X-ray crystallography, showing that the Li+ ion is coordinated to the nitrogen atom of the open amine arm and to the hydride ligand (trans to the P atom) of a neighboring molecule (H--Li=2.15 A), resulting in a dimeric structure. Complexes 4 and 9 exhibit high nucleophilic reactivity, upon which the pincer complex is regenerated. Reaction of 4 with water, methyl iodide, and iodobenzene resulted in the neutral complex [Pt(PCN)(CH3)] (3) and methane, ethane, or toluene, respectively. Labeling studies indicate that the reaction proceeds by direct electrophilic attack on the metal center, rather than attack on the alkyl ligand. The anionic dihydride complex 9 reacted with water and methyl iodide to yield [Pt(PCN)(H)] (8) and H2 or methane, respectively.     

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

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 CH3-CrH, CH3-WH, CH2=WH2, and CH[triple bond]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[triple bond]MoH2F] formed by reaction of laser-ablated molybdenum atoms with methyl fluoride: persistent photoreversible interconversion through alpha-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.     

Persistent photo-reversible transition-metal methylidene system generated from reaction of methyl fluoride with laser-ablated zirconium atoms and isolated in a solid argon matrix

A photoreversible transition-metal methylidene system has been formed for the first time by reaction of methyl fluoride and laser-ablated Zr atoms, isolated in solid argon, and investigated by means of infrared spectroscopy. Four different groups of absorptions are characterized on the basis of behaviors upon broad-band irradiation and sample annealing. Growth of Group I is accompanied by demise of Group II on irradiation with visible light (lambda > 530 nm) and vice versa with UV light (240 < lambda < 380 nm). The methylidene complex CH(2)=ZrHF is responsible for Groups I and II either in different singlet-triplet spin states or argon matrix packing configurations. The ground singlet state is stabilized by an agostic interaction. On the other hand, Group III, which arises from the Grignard type compound CH(3)-ZrF, disappears upon irradiation of UV light (lambda > 380 nm), increasing the concentration of CH(2)=ZrHF by alpha-H elimination. Fragments of methyl fluoride such as the CH(2)F radical comprise Group IV. Theoretical calculations are carried out for the alkylidene complex and other plausible products, and the results are compared with the experimental frequencies.     

Infrared spectra of CH3-MH, CH3-M, and CH3-MH- prepared via methane activation by laser-ablated Au, Ag, and Cu atoms

Methane activation by laser-ablated, excited Group 11 metal atoms has been carried out, leading to generation of CH(3)-MH, CH(3)-M, and CH(3)-MH(-), which are identified in the product infrared spectra on the basis of isotopic shifts and correlation with DFT calculated frequencies. The products reveal that C-H insertion by excited Au, Ag, and Cu readily occurs, and subsequent hydride-detachment or electron addition also follows. Each type of product has similar photochemical properties regardless of the metal. DFT computed energies reveal facile hydride dissociation and high electron affinities for the insertion complexes. The methyl metal species have the shortest C-M bonds, consistent with their highest calculated effective bond order, and the CH(3)-MH complexes have higher electron affinities than the metal atoms.     

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.