(C-H bond activation) The C-H bond is strong with 375–440 kJ · mol^–1; in general, any reaction in which a metal ▸▸▸complex weakens (“activates”) a C-H bond within a ▸▸▸ligand (usually classified as ▸▸▸agostic interaction), e.g., acc. to C-H + R-X-TMcat. → C-R (with C as an sp² or sp³ C atom an heteroatom; X = functional group or H). C-Ha. can result in C-H bond ▸▸▸cleavage, which is best known from intramolecular decomposition pathways such as ▸▸▸cyclometallation (e.g., ▸▸▸orthometallation in under-coordinated tris(aryl)phosphine complexes of the late ▸▸▸transition metals) or α- and β-▸▸▸hydrogen elimination reactions (e.g., in alkyl complexes of ▸▸▸Lewis acidic metal centers). Cat. relevant intermolecular C-Ha. can occur acc. to five pathways. 1: A ▸▸▸radical pathway in the presence of very strong oxidants or at high temps. (e.g., the ▸▸▸enzymatic oxidation of alkanes in ▸▸▸cytochrome P-450, the ▸▸▸Gif system, and the mercury-sensitized photochemical ▸▸▸dimerization of alkanes, cf. ▸▸▸photocatalysis). 2: The ▸▸▸oxidative addition of hcs. to a low-valent metal center to form an (organo) (hydrido)metal complex [e.g., the reaction of CpIrH(PMe₃) · (CH₂Cl₂) with ▸▸▸methane to give CpIr(H₂)(PMe₃)CH₃ or the two-centered oxidative addition of alkanes to the tetramesitylporphyrin-Rh dimer]. 3: An electrophilic pathway with a number of late transition metal ions (e.g., the ▸▸▸oxidation of C-H bonds of alkanes in water by a PtCl₄^2–/PtCl₆^2– system). A related ligand-assisted four-centered electrophilic activation is possible (e.g., in lanthanide and actinide hydride or alkyl complexes). 4: The Pt-cat. nucleophilic attack acc. to the ▸▸▸Shilov reaction (cf. ▸▸▸Pt as cat. metal). 5: A newly developed activation with Rh ▸▸▸sandwich complexes couples linear alkanes with borane reagents to form linear alkyl-boranes and thus alkanes with functional groups. An example is the conv. of ▸▸▸acetals to functionalized hydroxyalkanes, thus activating the terminal methyl groups (Figure).

6: For the cat. carbene ▸▸▸insertion into C-H bonds see the article in Chem.Rev. 2010, 110, 704. Drawbacks of the cat. C-Ha. are low activities due to the high C-H bond energy, the lack of inertness of common ligands and ▸▸▸solvents, poor ▸▸▸selectivities due to comparable reactivities of different C-H bonds within the substrate molecules, and the difficulties in suppressing the activation of the usually more reactive C-H bonds of the prod. molecules. The economically most exciting problem of CHa. is the manuf. of ▸▸▸methanol directly from methane and the conv. to ▸▸▸acetic acid (see ▸▸▸functionalization; ▸▸▸Periana reaction, ▸▸▸Shilov reaction). 7: Various C-Has. have been described under ▸▸▸photocatalytic conds. A Pd-cat., ketone directed C-Ha. and sp³ C–H functionalization can be demonstrated with [60]fullerene-fused tetralones: