Revisiting Experimental and Computational Bond Dissociation Energies in Copper(I) Oxazoline Complexes through IRMPD Spectroscopy

Abstract

Gas-phase ligand dissociation energies of Cu(I) bis(oxazoline) complexes exhibit large and persistent discrepancies between experiment and theory. These deviations have been interpreted as fundamental limitations of density-functional and correlated wave function approaches, despite the absence of direct structural validation of the experimentally probed ions. Here, we show, using infrared multiple-photon dissociation (IRMPD) spectroscopy, trapped ion mobility spectrometry, and electronic-structure calculations, that the mass-selected (S,S-Ph-BOX)2Cu+ ion is not a single, well-defined species. Instead, the IRMPD spectrum previously attributed to (S,S-Ph-BOX)2Cu+ arises from a composite of at least two Cu-containing ions that fall within the same nominal isolation window. Trapped ion mobility spectrometry resolves these contributions and enables acquisition of IR spectra for each species individually. The mobility-selected spectra demonstrate that only one ion corresponds to a genuine bis-chelated Cu(I)–BOX complex, whereas the second species differs in composition by one hydrogen atom and has substantially higher bond dissociation energy. Its presence in the original precursor population would have raised, not lowered, the apparent experimental threshold, confirming that the discrepancy between experiment and theory is genuine. This discrepancy is attributed to systematic overestimation of noncovalent interligand interactions by dispersion-corrected DFT, consistent with the behavior documented for structurally related organometallic systems. The present study underscores the necessity of explicit precursor characterization before thermochemical benchmarks are accepted, and identifies the treatment of noncovalent interactions as the key remaining challenge for computational methods applied to this class of complexes.