C—H bonds are difficult to activate due to their inherent bond strength and non-polar properties. Early examples of C—H activation were achieved with noble metals. However, noble metals have a low natural abundance and high toxicity. The four-coordinate base metal complex Fe(depe)₂[depe =1,2-bis(diethylphosphino)ethane], is proposed to be an intermediate for the activation C—H bonds of aryl and alkyl substrates. While the reaction was synthetically characterized, the mechanism remains unexplored. To understand the steps, computational techniques were utilized. Density Functional Theory (DFT) was used to optimize geometries and verify minima on the potential energy surface. DLPNO-CCSD(T) was used to get more accurate energies.

Our results found that the Fe(II) hydride reactant ¹B undergoes two-electron reductive elimination to form the coordinately unsaturated, triplet intermediate ³A. Reaching the intermediate on the potential energy surface requires crossing a high energy transition state (20.72 kcal/mol). After crossing the transition state, the complex is further stabilized via an a-agostic interaction with the ethylene substituents on the depe ligand, which forms a five-coordinate complex, ¹A_ag. To proceed to the intermediate ³A, spin crossover (SCO) from the singlet surface to the triplet surface is required. The minimum energy crossing point (MECP) was slightly lower in energy than the five-coordinate agostic complex, ¹A_ag. After crossing over to the triplet surface, the intermediate ³A is formed. ³Awas calculated to be isoenergetic with the starting material ¹B, which suggests that the intermediate is relatively stable. Our future work will explore the mechanism for C—H activation of the intermediate ³A with substrates such as benzene to elucidate the mechanism of intermolecular C—H activation.

Sean Lutz

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