Ligand geometry-dependent coordination of alkali halide cluster ions by bipyridine isomers in the gas phase

Ligand geometry plays an important role in metal-ion coordination and influences both the structures and growth of ligand-bound alkali halide clusters. Here, we investigate the interactions of LiI cluster cations with three bipyridine isomers using electrospray ionization-ion mobility spectrometry-mass spectrometry (ESI-IMS-MS) combined with density functional theory (DFT) calculations. Ion mobility measurements together with theoretical collision cross section calculations reveal that 2,2'-bipyridine consistently adopts localized bidentate chelation, whereas 3,3'-bipyridine preferentially bridges multiple Li⁺ centers. These distinct coordination modes give rise to fundamentally different cluster growth behaviors: chelating coordination limits cluster growth, while bridging coordination stabilizes progressively larger LiI frameworks and can even alter the preferred structures of the ionic cores. In contrast, 4,4'-bipyridine exhibits only weak coordination because of its unfavorable donor geometry. Comparison with our previous study on flexible diamine ligands further reveals that ligand flexibility and donor geometry represent two complementary factors governing ligand-assisted ionic cluster assembly. These results establish a direct relationship between ligand geometry, coordination mode, and ionic cluster architecture, providing molecular-level insight into ligand-directed alkali halide cluster assembly.