Chelation Drives Surface Substitution in Hybrid-MXenes

Hybrid organic-inorganic MXenes (h-MXenes) offer a versatile platform for tailoring the surface chemistry of two-dimensional transition-metal carbides and nitrides through covalently bound organic ligands. Although monodentate amido/imido functionalization has been demonstrated previously, chelating ligand binding has remained unexplored. Here, we report the synthesis of Ti₃C₂(en)_x h-MXenes via substitution of Br terminations in Ti₃C₂Br₂ with deprotonated ethylenediamine (en). By varying the amount of n-BuLi used for en deprotonation, the surface coordination evolves from predominantly monodentate to bidentate binding. This transition is evidenced by a contraction of the interlayer spacing and attenuation of the ─NH₂ signal in x-ray photoelectron spectroscopy. Solid-state NMR reveals that bidentate coordination dominates when 4 equiv. of n-BuLi is employed, while inelastic neutron scattering, supported by simulated vibrational spectra, provides independent confirmation of the bidentate binding motif. Density functional theory calculations show that bidentate en is significantly more stable than monodentate configurations for replacing Br terminations. Ab initio molecular dynamics further reveal dynamic surface chemistry involving proton transfer, β-H elimination, surface imine-Ti bond formation, and partial reversion to monodentate coordination. These findings establish ligand denticity as a new design parameter for engineering MXene surface chemistry and tuning material properties.