Intermolecular Activation of Hydrocarbon C−H Bonds under Ambient Conditions by 16-Electron Neopentylidene and Benzyne Complexes of Molybdenum

Abstract

CpMo(NO)(CH(2)CMe(3))(2) (1), a complex with alpha-agostic C-H.Mo interactions, evolves neopentane in neat hydrocarbon solutions at room temperature and forms the transient 16-electron alkylidene complex, CpMo(NO)(=CHCMe(3)), which subsequently activates solvent C-H bonds. Thus, it reacts with tetramethylsilane or mesitylene to form CpMo(NO)(CH(2)CMe(3))(CH(2)SiMe(3)) (2) or CpMo(NO)(CH(2)CMe(3))(eta(2)-CH(2)C(6)H(3)-3,5-Me(2)) (3), respectively, in nearly quantitative yields. Under identical conditions, 1 in p-xylene generates a mixture of sp(2) and sp(3) C-H bond activation products, namely CpMo(NO)(CH(2)CMe(3))(C(6)H(3)-2,5-Me(2)) (4, 73%) and CpMo(NO)(CH(2)CMe(3))(eta(2)-CH(2)C(6)H(4)-4-Me) (5, 27%). In benzene at room temperature, 1 transforms to a mixture of CpMo(NO)(CH(2)CMe(3))(C(6)H(5)) (6) and CpMo(NO)(C(6)H(5))(2) (7) in a sequential manner. Most interestingly, the thermal activation of 6 at ambient temperatures gives rise to two parallel modes of reactivity involving either the elimination of benzene and formation of CpMo(NO)(=CHCMe(3)) or the elimination of neopentane and formation of the benzyne complex, CpMo(NO)(eta(2)-C(6)H(4)). In pyridine, these intermediates are trapped as the isolable 18-electron adducts, CpMo(NO)(=CHCMe(3))(NC(5)H(5)) (8) and CpMo(NO)(eta(2)-C(6)H(4))(NC(5)H(5)) (9), and, in hydrocarbon solvents, they effect the intermolecular activation of aliphatic C-H bonds at room temperature to generate mixtures of neopentyl- and phenyl-containing derivatives. However, the distribution of products resulting from the hydrocarbon activations is dependent on the nature of the solvent, probably due to solvation effects and the presence of sigma- or pi-hydrocarbon complexes on the reaction coordinates of the alkylidene and the benzyne intermediates. The results of DFT calculations on these processes in the gas phase support the existence of such hydrocarbon complexes and indicate that better agreement with experimental observations is obtained when the actual neopentyl ligand rather than the simpler methyl ligand is used in the model complexes.

An organometallic tetranuclear cluster with phosphine and phosphido ligands in nonclassical bonding modes: X-ray structural characterization

“…The interaction between the phosphido containing P(2) and the Pd(2) centre is very noteworthy and different to the previously proposed interaction. The Pd(2)-C(37) distance (2.391(5) Å ) is rather short [18][19][20][21][22][23][24][25], as is the Pd(2)-P(2) distance (2.272(1) Å ), in fact it is even Scheme 1. shorter than the Pd(1)-P(2) and Pt(1)-P(2) distances (2.389(1) and 2.358(1) Å , respectively). These data indicate that the P(2)-C(37) bond interacts with the Pd(2) centre giving rise to a 2e-3c bond or an agostic-like interaction of a P-C sp2 bond with an unsaturated metal atom, as it has been recently called for related phosphine complexes [26][27][28].…”

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

“…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 1–3. 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 CH bond‐activation process (a key step in many transition‐metal catalyzed reactions) 4–8…”

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

“…[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.…”