[H₂CZrH₂]: The Simplest Carbene Hydride Complex, Agostic Bonding, and CH Activation of CH₄ to Form [(CH₃)₂ZrH₂]

Abstract

High oxidation state transition-metal complexes with a carbon-metal double bond have proven important for understanding the nature of metal coordination and for catalysts in alkene-metathesis and alkane-activation reactions. [1][2][3] Several early transition-metal alkylidenes are agostic, [1] and these compounds provide the opportunity to characterize the agostic interaction of hydrogen to a transition-metal center in a simple carbene complex (most agostic interactions involve much more complicated systems) and to help understand the important alkane CÀH bond-activation process (a key step in many transition-metal catalyzed reactions). [4][5][6][7][8] The simplest compound of this type is the methylidene complex, [H 2 C = MH 2 ], which is an ideal model system to examine substituent effects and the agostic interaction. Such Group 4 compounds have been investigated by electronicstructure calculations using small basis sets and shown to have symmetrical structures without agostic interactions. [9][10][11] Recently we have treated laser-ablated Ti, Zr, and Hf atoms with CH 3 F and prepared the fluorine substituted [H 2 C = MHF] derivatives. [12][13][14] These methylidene species result from ahydrogen migration in the initial [CH 3 MF] intermediate. Our electronic-structure calculations using basis sets with polarization functions reveal a tilted CH 2 group and provide evidence for agostic interaction in the [H 2 C=MHF] molecules.The simple [H 2 C = MH 2 ] analogues contain two intense infrared chromophores, namely the CH 2 and MH 2 groups, which are sensitive to the distortion associated with intramolecular agostic bonding. Hence the IR spectrum of [H 2 C= ZrH 2 ] can provide evidence for agostic interaction through symmetry lowering in the molecule, as can computations using large basis sets. In particular, a simple count of [CHD = ZrHD] isotopomers provides a ready method to confirm agostic interactions in this simplest possible carbene complex, as only two isotopomers (cis and trans) will be formed if there is no agostic interaction (C 2v or C s symmetry), but four isotopomers (cis and trans for H and for D in the agostic position) if there is agostic interaction (C 1 symmetry). We report herein the first experimental characterization of [H 2 C = ZrH 2 ] and demonstrate the agostic CÀH···Zr interaction from the IR spectrum in solid neon and from density functional structure and frequency calculations.The laser-ablation matrix-IR experiment has been described elsewhere. [15,16] Briefly, laser-ablated zirconium atoms (Johnson-Matthey) reacted with CH 4 (Matheson), 13 CH 4 , CD 4 , CH 2 D 2 (Cambridge Isotopic Laboratories) in excess neon (Spectra Gases) during condensation on a CsI window at 5 K. IR spectra were recorded at 0.5 cm À1 resolution on a Nicolet 750 spectrometer with HgCdTe detector. Samples were irradiated by a mercury arc lamp (175 W, globe removed), were annealed, and more spectra were recorded.Complementary density functional theory (DFT) calculations were performed using the Gaussian 98 package, [...

Metal (Ti, Zr, Hf) insertion in the C–H bond of methane: Manifestation of an agostic interaction

“…For all cases, the energetic gain when forming the first intermediate, the association of the methane molecule and the methylidene complex, is sufficient to overcome the barrier height of the transition state leading to the final product via a highly exothermic step. This is in agreement with experimental findings [2][3][4][5]; even if the C 3v methyl group axes are not in line with the C-M axes, the overall geometry of the product remains C 2v .…”

“…Taking this delocalization energy as a measure, we may expect that the agosticity is more pronounced in the case of Ti than for the two other metals. As already observed by Andrews et al [1][2][3][4][5] the geometrical parameter ( d MCH bond angle) and the a-agostic C-H vibrational frequency increase when the agosticity decreases in going from Ti to Zr and to Hf.…”

“…During the last decade, a complete experimental and DFT theoretical study of the reaction between an early transition metal and methane has been done by Andrews and Cho [1]. Using the laser-ablation matrix-infrared technique, they showed that: [2][3][4][5] • The group 4 transition metals can activate methane to give the simple methylidene dihydrides, CH 2 = MH 2 , which have been identified by some of their vibrational modes. Particularly, the experimental observation of inequivalent C-H stretching frequencies for CH 2 = MH 2 and for the isotopic species evidenced a -CH 2 distortion around of C = M axis, called ''agostic interaction''.…”

Characterization of agostic interactions in theory and computation

“…Because agostic bonds are usually slightly elongated compared to their anagostic counterparts the associated CH stretching frequency is lower than that of a CH bond with no additional interactions present. This has been used in a series of combined experimental laser ablation and spectroscopic as well as computational studies of small carbene hydride systems H 2 C=MH 2 (M = Ti, Zr, Hf, Mo, W) [91,65,92,93,94,95,96,97,98,99,100,101]. This combined approach does not only allow to probe systems for agostic interactions it also provides a valuable tool for the often difficult identification of compounds in matrix IR.…”

Rotational analysis of the $3_0^1 $31 band of the $\tilde A{}^6\Sigma ^ + \leftarrow \tilde X{}^6\Sigma ^ + $Ã6Σ+←X̃6Σ+ system of CrCCH

“…Matrix isolation infrared spectra of many CH 3 MX and CH 2 MX 2 molecules are also known. [15][16][17][18][19][20] Moving to transition metal molecules containing two carbon atoms, the strength of the carbon-carbon bond dictates that the two carbons will be chemically bound to each other. The MC 2 molecules are expected to be highly ionic due to the high electron affinity of C 2 (EA = 3.269(6) eV).…”