Practical Ligand-Enabled N₂ Adsorption of Rhenium(I) Carbonyl Complexes Based on Meridionally Coordinated Terpyridine

The prerequisite for N₂ activation under mild conditions is the efficient adsorption of N₂ onto metal sites. Understanding the selectivity and environmental dependence of N₂ adsorption is essential for elucidating fundamental mechanisms and designing efficient catalysts. In this study, we synthesized the complex mer, cis-Re(tpy-κ³N)(CO)₂Cl (where tpy stands for terpyridine) and investigated its reactivity toward N₂ using electrospray ionization mass spectrometry (ESI-MS) and density functional theory (DFT) calculations. Using ESI, the species [Re(tpy)]⁺, [Re(tpy)(CO)]⁺ and [Re(tpy)(CO)₂]⁺ were generated and subsequently allowed to react with N₂. The mass spectrometry experiments revealed the reactivity order of N₂ adsorption as: [Re(tpy)]⁺ > [Re(tpy)(CO)]⁺ ≫ [Re(tpy)(CO)₂]⁺ (inert). DFT calculations indicate the presence of π back-donation between the Re atom and N₂. The observed decrease in reactivity can be attributed to two primary factors: (i) electron depletion at the Re center by CO ligands, which diminishes its π back-donation capacity, and (ii) steric hindrance from CO ligands, which hinders N₂ access to the preferred planar coordination site. This work provides key insights into the synergistic effects between organic ligands and transition metals, advancing the rational design of molecular systems for N₂ adsorption and activation.