X-ray absorption and emission spectroscopy of NS Cu()/() complexes

• Published in: Dalton transactions : an international journal of inorganic chemistry 2024-05, Vol.53 (18), p.7828-7838
• Main Authors: Geoghegan, Blaise L, Bilyj, Jessica K, Bernhardt, Paul V, DeBeer, Serena, Cutsail, George E
• Format: Article
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/d4dt00085d

This study investigates the influence of ligand charge on transition energies in a series of CuN 2 S 2 complexes based on dithiocarbazate Schiff base ligands using Cu K-edge X-ray absorption spectroscopy (XAS) and Kβ valence-to-core (VtC) X-ray emission spectroscopy (XES). By comparing the formally Cu( ii ) complexes [ Cu II (HL1) ] (HL1 2− = dimethyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and [ Cu II (HL2) ] (HL2 2− = dibenzyl pentane-2,4-diylidenebis[carbonodithiohydrazonate]) and the formally Cu( iii ) complex [ Cu III (L2) ], distinct changes in transition energies are observed, primarily attributed to the metal oxidation state. Density functional theory (DFT) calculations demonstrate how an increased negative charge on the deprotonated L2 3− ligand stabilizes the Cu( iii ) center through enhanced charge donation, modulating the core transition energies. Overall, significant shifts to higher energies are noted upon metal oxidation, emphasizing the importance of scrutinizing ligand structure in XAS/VtC XES analysis. The data further support the redox-innocent role of the Schiff base ligands and underscore the criticality of ligand protonation levels in future spectroscopic studies, particularly for catalytic intermediates. The combined XAS-VtC XES methodology validates the Cu( iii ) oxidation state assignment while offering insights into ligand protonation effects on core-level spectroscopic transitions. The competing influences of both oxidation state and ligand protonation state in Cu( ii ) and Cu( iii ) Schiff base complexes are explored through copper K-edge X-ray absorption and valence-to-core emission spectroscopies along with DFT calculations.