Spontaneous ligand loss by soft landed [Ni(bpy)₃]²⁺ ions on perfluorinated self-assembled monolayer surfaces

Transition metal (TM) complexes are widely used in catalysis, photochemical energy conversion, and sensing. Understanding factors that affect ligand loss from TM complexes at interfaces is important both for generating catalytically-active undercoordinated TM complexes and for controlling the degradation pathways of photosensitizers and photoredox catalysts. Herein, we demonstrate that well-defined TM complexes prepared on surfaces using ion soft landing undergo substantial structural rearrangements resulting in ligand loss and formation of both stable and reactive undercoordinated species. We employ nickel bipyridine (Ni-bpy) cations as a model system and explore their structural reorganization on surfaces using a combination of experimental and computational approaches. The controlled preparation of surface layers by mass-selected deposition of [Ni(bpy)₃]²⁺ cations provides insights into the chemical reactivity of these species on surfaces. Both surface characterization using mass spectrometry and electronic structure calculations using density functional theory (DFT) indicate that [Ni(bpy)₃]²⁺ undergoes a substantial geometry distortion on surfaces in comparison with its gas-phase structure. This distortion reduces the ligand binding energy and facilitates the formation of the undercoordinated [Ni(bpy)₂]²⁺. Additionally, charge reduction by the soft landed [Ni(bpy)₃]²⁺ facilitates ligand loss. We observe that ligand loss is inhibited by co-depositing [Ni(bpy)₃]²⁺ with a stable anion such as closo-dodecaborate dianion, [B₁₂F₁₂]^2-. The strong electrostatic interaction between [Ni(bpy)₃]²⁺ and [B₁₂F₁₂]^2- diminishes the distortion of the cation due to interactions with the surface. This interaction stabilizes the soft landed cation by reducing the extent of charge reduction and its structural reorganization. Overall, this study shows the intricate interplay of charge state, ion surface interactions, and stabilization by counterions on the structure and reactivity of metal complexes on surfaces. The combined experimental and computational approach used in this study offers detailed insights into factors that affect the integrity and stability of active species relevant to energy production and catalysis.