Strain engineering of single-site Cu on SWCNTs for highly efficient diene cyclopropanation

Cyclopropane-based aerospace fuels exhibit the excellent volumetric calorific value, leading to broad application prospects in aircrafts. However, due to the highly rigid structure of the cyclopropane (compared to cyclobutane or cyclopentane, etc.) and the difficulty in forming metal-carbene intermediates, there are still significant challenges in the precise synthesis of multi-cyclopropane-based high-energy fuels ( ≥ 2 cyclopropane structures). Herein, via the introduction of bending strain on metal organic complex, we report a single-site Cu/single-walled carbon nanotube catalyst (Cu/SWCNT), which possesses 2.6-fold conversion (up to 77.4%) and 4-fold selectivity (up to 45.7%) as that without bending strain for the preparation of bi-cyclopropane-based fuels from dienes. XAFS and DFT calculations show that bending strain enhances the upward shift of d-band center of β electrons (-4.370 to -4.366), thus promotes the adsorption and activation of CH₂N₂ and alkenes. Moreover, bending strain elongates Cu-O bonds (1.91 to 1.96 Å), making it easier for CH₂N₂ to insert and form Fischer carbene intermediates. These two aspects obviously reduce the reaction energy barrier from 40.3 to 33.7 kcal·mol^-1 for the cyclopropanation process. Our work provides a method of strain engineering to regulate C-C coupling reactions at the molecular level, and achieve highly efficient cyclopropanation from alkenes.