Alkyne Cyclotrimerization Catalyzed by Bimetallic Fe(II) Complexes with Low-Basicity Urea-Derived Ligands

Catalytic cyclotrimerization of alkynes by iron complexes would be an important reaction for the preparation of useful building blocks. It would be important to broaden the ligand scope, but the reactions can be catalyzed only when a lower-valent Fe(0) or Fe(I) is used or when Fe(II) is coordinated to strongly basic ligands. Here, we describe how bimetallic Fe(II) complexes with urea-derived ligands can catalyze the cyclotrimerization of alkyne despite the relatively lower basicity. Treatment of H2NMe2[NNNN] with a half-equivalent of {FeMes2}2 formed a bimetallic Fe(II) complex, {NMe2[NNNN]Fe}2 (Fe1), and the catalytic cyclotrimerization of phenylacetylene was achieved under mild conditions. The reaction exhibited an approximately half-order dependence on Fe1 and an apparent substrate order of ca. 1.5 under initial-rate conditions, although deviations from a simple 1.5-order rate law were observed. The Eyring plot exhibited a moderately negative ΔS‡ = −7.82 cal/mol·K, while the negative slope in the Hammett analysis implicated the electron-deficient transition state. The combined kinetic studies are consistent with the formation of a monomeric active species prior to the rate-determining step. Finally, both reaction rates and isomer ratios improved when the substituents were replaced with more donating, sterically bulkier Et or nPr groups, {NEt2[NNNN]Fe}2 (Fe2) and {NnPr2[NNNN]Fe}2 (Fe3). Overall, this study demonstrates how Fe(II) complexes with lower-basicity ligands successfully catalyze alkyne cyclotrimerization reactions and can control the isomer ratios.