Alkali Metal Control of Triplet-Mediated C-H Activation in Iron-Mediated Coupling of Dinitrogen and Benzene.

Direct coupling of N2 with abundant feedstocks like benzene to form N-containing organic compounds is a promising strategy for N2 fixation pathways. The challenge of coupling N2 activation and C-H bond oxidative addition was recently solved by introducing a reversible benzene C-H bond activation process mediated by a low-valent Fe(0) complex, which gave an organometallic product that could couple with partially reduced N2. Interestingly, the energetics of the C-H oxidative addition/reductive elimination step depends on the choice of alkali metal. However, the reason why the alkali metal influences the C-H bond activation remained elusive. Herein, we present a comprehensive study on this Fe(0)-mediated reversible C-H activation. Through density functional theory combined with high-level coupled cluster calculations, we discovered that the intermediate-spin triplet (S = 1) controls the energy of the transition state for C-H cleavage, while the high-spin quintet (S = 2) controls the position of the equilibrium. Na+ drives the equilibrium toward oxidative addition due to an electrostatic effect, while K+ and Rb+ are dominated by a steric effect that favors the iron(0) species. The key role played by nonbonding interactions in the Fe-mediated C-H activation provides a conceptual model for alkali control over organometallic transformations.