Pressure-induced vibrational changes in [Cu(bipyridine)₂(Cl)]⁺ complex: Raman spectroscopy and DFT mode assignments

Understanding how metal-organic complexes behave under extreme conditions is crucial for designing advanced materials. Here, we present a comprehensive investigation of the [Cu(Bip)₂(Cl)]⁺ complex. Its structural properties were characterized by powder X-ray diffraction with Rietveld refinement. Additionally, the study incorporated High-pressure Raman spectroscopy (up to 7.5 GPa), which was complemented by density functional theory (DFT) calculations for accurate vibrational mode assignment, alongside Hirshfeld surface analysis. Our results reveal a gradual conformational phase transition in the [Cu(Bip)₂(Cl)]⁺ complex occurring between 1.0 and 4.0 GPa. This transition is primarily driven by changes in intermolecular interactions, including the emergence of a new CH stretching mode and subtle variations in the CuCl stretching mode. Notably, the molecular framework of the complex remains remarkably rigid, with only minimal pressure-induced shifts observed in its internal vibrational modes up to 7.5 GPa. This contrasts with pure pyridine, which undergoes significant structural changes under pressure. The enhanced rigidity of [Cu(Bip)₂(Cl)]⁺ stems from the distinct nature of its intermolecular forces: while hydrogen bonds in pyridine contribute to great mechanical strength, the van der Waals forces in [Cu(Bip)₂(Cl)]⁺ promote compressibility, enabling a progressive conformational transition rather than abrupt structural collapse. The structural reversibility and stability of the complex under pressure highlight its potential for applications in variable-pressure environments, such as sensors and optoelectronic devices.