Oxidation For “Functionalized” C-H Compounds
The concept of C-H functionalization is a truly hot topic for contemporary chemistry and related sciences. This concept implies replacing hydrogen atoms in carbon-hydrogen compounds with functional groups, allowing for valuable “functionalized” C–X compounds.
This procedure is especially important from a practical point of view as a method of transforming inert saturated organic compounds (alkanes, “the noble gases of organic chemistry”). Alkanes, widely-used components of raw oil, are required to be smoothly transformed into more reactive chemicals, e.g., oxygen- or nitrogen-containing ones. The latter then could serve as functional reagents for numerous syntheses of plastic, medical, and other high-demand materials.
It has been proven that different types of metal complexes effectively catalyze these essential alkane transformations (as well as aromatic hydrocarbons and other carbon-hydrogen compounds). These reactions are carried out under mild “green chemistry” conditions (often temperature does not exceed 100 °C) and could be selective. This is a great advantage in comparison to classical methods of raw oil component transformations (note – cracking or pyrolysis reactions usually require heating up to 1000 °C and provide a huge number of products).
It is noteworthy that some metal complex-assisted reactions of C-H activation functionalize carbon–hydrogen bonds stereospecifically. This makes the metal complexes of some analogs very active and selective metal-containing enzymes, performing C-H activation reactions in living organisms. While investigating the features of C-H compound functionalization, catalyzed by metal complexes, the chemists revealed a great role of the organic surrounding (ligands) of catalyst’s metal ions. Varying the ligands’ nature allowed for a dramatic increase in the efficiency of catalytic reactions, giving in some cases more than a thousandfold profit in catalyst activity parameters, even when compared to metal ions of the same nature. In short, adjustment of ligands influence on (i) the possibility of the existence of multimetallic catalytic compounds, (ii) the stability of catalytically-active species, and (iii) changes in the mechanism of the reaction itself.
Recently, in the context of further research of catalytically-prospective metal compounds, a series of multimetallic compounds, based on bulky silicon- or germanium sesquioxane ligands, were synthesized. The use of these specific ligands, bearing one organic group and three oxygen atoms at silicon (or germanium) center, allowed researchers to isolate numerous products. Currently, complexes including 4 to 13 metal centers were described. Their particular feature is an ability to form unique molecular structures, depending on the number and nature (transition/alkaline) of metal ions and involvement of additional organic (nitrogen or phosphorous-containing) ligands, along with Si- or Ge-matrices.
According to expectations that multi-metallic metal complexes often have higher catalytic activity in comparison with mononuclear compounds, it was found that these precatalysts exhibited high catalytic activity in the oxidation of benzene, alkanes, and alcohols with peroxides. In some cases, an additives of acids (nitric, trifluoroacetic) were used as cocatalysts.
It is important to emphasize that activity of tested metallacompounds was found to be dependent from the molecular topology of complexes (Si12Cu4Na4-based compounds of Globule- or Sandwich-like structure revealed principally different activity). Then, using of cobalt-containing complex, prismatic Si10Co5-sesquioxane, allowed to perform a highly stereoselective oxidation of 1,2-dimethylcyclohexane the oxidation of trans-1,2-dimethylcyclohexane with tert-butylhydroperoxide catalyzed by “Cooling Tower” complex, Si10Cu2Na2, proceeds stereoselectively with the inversion of configuration. Hexairon compounds (both Si20-“Asian Lantern” and Ge15-“Three-pointed star”) catalyzed the oxidation of cyclohexane affording the products in over 45% yield, which is very high for oxidation of “lazy” alkanes.
These findings are described in the article entitled Oxidation of C‒H compounds with peroxides catalyzed by polynuclear transition metal complexes in Si- or Ge-sesquioxane frameworks, recently published in the Journal of Organometallic Chemistry. This work was conducted by Mikhail M. Levitsky from INEOS RAS, Alexey N. Bilyachenko from INEOS RAS and RUDN University, and Georgiy B. Shul’pin from the Semenov Institute of Chemical Physics RAS and Plekhanov Russian University of Economics.