Ping Yang

Theoretical studies of bond-activation chemistry

The thorium(IV) and uranium(IV) alkyl complexes (C₅Me₅)₂AnR₂, where the actinide (An) is thorium (Th) or uranium (U) and the alkyl (R) is a methyl (CH₃), a benzyl (CH₂Ph), or a phenyl (Ph), have proven to be versatile starting materials for the synthesis of a diverse array of actinide organometallic systems containing actinide–nitrogen (An–N) bonds such as imido, hydrazonato, and ketimido complexes, which feature novel electronic properties.

Recently, a group led by Jaqueline Kiplinger of Condensed Matter and Thermal Physics (MPA-10) reported that these actinide alkyl complexes undergo interesting carbon-hydrogen (C-H) and carbon–nitrogen (C–N) bond cleavage chemistry with N-heterocycles, such as 2-picoline (2-methylpyridine), which possesses both sp² and sp³ hybridized C–H bonds. The sp² bond is hybridized from one s orbital and two p orbitals, resulting in a trigonal planar structure. The sp³ bond is formed as a hybrid of one s and three p orbitals, resulting a tetrahedral shape.

The thorium alkyl complex (C₅Me₅)₂Th(CH₃)₂ activates both an sp³ C–H bond on the 2-picoline methyl group and an ortho sp² C–H bond on the ring, producing the kinetic α-picolyl product, (C₅Me₅)₂Th(CH₃)[η²-(N,C)-2-CH₂-NC₅H₃], and the thermodynamic η²-pyridyl product, (C₅Me₅)₂Th(CH₃)[η²-(N,C)-6-CH₃-NC₅H₃], respectively. This is in marked contrast with the uranium system, which only reacts with an sp² C–H bond on the 2-picoline aromatic ring, producing the η²-pyridyl product (C₅Me₅)₂U(CH₃)[η²-(N,C)-6-CH₃-NC₅H₃]. Deuterium-labeling studies demonstrated that the thorium and uranium (C₅Me₅)₂An(CH₃)₂ complexes react with 2-picoline by different mechanistic reaction pathways.

The thorium alkyl complex (C5Me5)2Th(CH3)2 and 2-picoline react to give preferential sp3 C–H bond activation in the presence of a more reactive sp2 C–H bond, while the analogous uranium complex, (C5.Me5)2U(CH3)2, reacts with only the ortho 2-picoline sp2 C–H bond.

Density functional theory methods have been employed to explore actinide–ligand interactions in a variety of complexes using the current generation of hybrid functionals. The structures, thermochemistry, and spectroscopic properties revealed through density functional theory provide information to compare with available structural and spectroscopic data from experiment. By comparison, relatively little information has been available on reaction mechanisms of f-element complexes, such as in the C–H activation processes.

Working with Los Alamos colleagues Richard L. Martin and P. Jeffrey Hay, and Ingolf Warnke of the University of Saarland, Germany, I performed a computational study of the competitive sp² versus sp³ C–H activations within the same reactant molecule. Th(IV) complexes possess a closed shell electronic ground state (5f ⁰), while U(IV) systems represents a high-spin (5f ²) system with two unpaired electrons in f orbitals. The products and resulting thermochemistry in these reactions were compared and likely reaction precursors and transition states were also identified, as well the plausible reaction pathways.

The results of theoretical study are consistent with reported experimental results. Optimized geometries are in excellent agreement with X-ray crystal data. The calculated reaction energies prove that sp² C–H bond activation product is the most stable structure, i.e., thermodynamic product. Both theoretical and experimental observations point to the same conclusion: in the thorium system, the sp³ product is kinetic and the sp² product is thermodynamic; while in the uranium system, the sp² product is both kinetic and thermodynamic.

In summary, the reaction initiates from formation of a weakly bound adduct, followed by the activation of adjacent C–H activation by an actinide center leading to an agostic transition state. My colleagues and I found that the actinide atom plays a fundamental role during the hydrogen migration process from 2-picoline to the methyl-leaving group. Agostic five-centered transition structures for the actinide C–H activation reaction pathways are reported to the best of our knowledge, for the first time. The origin of the regioselectivity of these reactions rests in these highly ordered configurations of transition states. Despite many common features found between thorium and uranium systems, including the similar geometries of the products, adducts, and the agostic transition states, the calculated activation energies between sp² and sp³ activation differ slightly. For the thorium system, the sp² activation energy is higher than that for the corresponding sp³ reaction. In contrast, for the uranium system, the sp² activation energy is lower than that for the corresponding sp³ reaction.

On the basis of the combination of labeling, structural, and computational information, we proposed a general mechanism for the C–H activation of N-heterocycle by actinocene complexes. “Agostic migration” cyclomelatation is indicated as an operative mechanism.

The calculated three-dimensional structures of the transition states involved in the sp2 and sp3 C–H bond activation of 2-picoline from the most stable thorium and uranium adducts.

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