Study on Metal-Ligand Interactions: Infrared Ion Spectroscopy of Coordination Compounds

Coordination compounds are essential to a vast array of important areas of chemistry. They play catalytic and structural roles in biochemistry, perform industrial functions as catalysts, and participate in atmospheric reactions across the globe. The ubiquity of coordination chemistry is due to the ability of nearly all metals to form coordinated complexes with organic ligands, which allows chemical access to over half of the periodic table. Metal-ligand interactions govern many of these compounds’ functions and influence the structure and charge distribution of the complex, making them vital to understand on a molecular level in order to progress our comprehension and development of coordination chemistry. Infrared (IR) spectroscopy offers a powerful probe into these molecular details. In solutions, analysis is complicated by interaction with the chemical environment and the speciation present in most applications. Mass spectrometric preparation of cold ionic complexes bypasses many of the solution-induced challenges in connecting IR spectroscopic information to detailed knowledge about complex molecular systems.  

Bipyridines are a well-known class of CO₂ reduction catalysts that consist of a transition metal center surrounded by bipyridine-based ligands. We investigated how the addition of formate (a product of CO₂ reduction catalysis) to the complex changed the overall molecular structure, and how different metal centers bound to the formate adduct. This work elucidated the formate-metal binding motif and how it differs across metal centers. The charge distribution throughout each complex was found to be significantly influenced by the local electronic geometry and consequent coordination chemistry of the metal, which illuminated the ligands’ role as charge reservoirs. 

EDTA is a prototypical chelator of metal cations and serves as a molecular model for protein binding pockets. We investigated EDTA bound to a series of alkaline earth metal dications and found that the size of the bound ion determines the geometry of the EDTA framework, impacting the dynamics and selectivity of the binding pocket. Upon successive hydration from one water molecule to full solvation, the binding pockets of each complex opened, further exposing the bound ion to the chemical environment. We then carried out measurements on a series of transition metal-EDTA complexes, in which we characterized the influence of d-electrons on the geometry of the EDTA chelator, spin state of each complex, and changes in EDTA spectral signatures. 

The symmetric (ν_s) and antisymmetric (ν_as) OCO stretching modes of carboxylate containing compounds encode structural information that is both difficult to decipher and desirable to model due to the sensitivity of these spectral features to small shifts in charge distribution and structure, as well as the anharmonicities of these two vibrational modes. We have developed a relation between the frequency of these modes and the geometry of the carboxylate group, showing that the splitting between ν_s and ν_as can be accurately predicted based only on the OCO bond angle obtained from quantum chemical calculations (± 58 cm^-1, R² = 0.992). The relationship is shown to hold for IR spectra of carboxylato groups in a variety of molecules measured in vacuo.  

We have collected and interpreted IR spectra on a variety of coordination compounds, identifying fundamental characteristics of their chemical abilities and developing a new model for spectral analysis. These advances will lead to more understanding and informed design of coordination compounds from biology to industry. 

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