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In situFTIR spectroscopy can measureK_avalues across different classes of host–guest complexes involving hydrogen and halogen bonding. This approach requires minimal sample preparation and can track host or guest vibrational changes. 

Binding studies are ubiquitous in chemistry, but their extensive usefulness is undermined by false positive and false negative results. Centering on the G-protein mini-Gs, we present a thorough study with both simulated and experimental spectrophotometric titration data to diagnose the validity of both binding and non-binding models. Without the use of statistical tests like Bayesian Information Criterion (BIC) and data reconstruction fractions, spurious binding models may go undetected. Furthermore, if the signal change upon binding is too minute, false negatives can also result. Delineating such issues is paramount to effective science. 

This paper portrays the solution-phase dynamics as copper(II) and ethylenediamine explore a multitude of different complexes. The data from five spectrophotometric titrations were globally analysed, evidencing four predominant species ([Cu]2+, [Cu(en-N,N’)]2+, [Cu(en-N,N’)2]2+, [Cu(en-N)4]2+) along with their molar absorptivity curves and associative binding constants. The data also seem to support a fifth species, [Cu2(µ-en-N,N’)]4+, in which ethylenediamine bridges two Cu(II) centres. The thermodynamic stability of all five species is corroborated by ab initio computational calculations. The potential existence of [Cu(en-N)4]2+ highlights the suprachelate effect – going beyond the chelate effect – where multidenticity is overtaken by monodenticity. Such dangling multidentate ligands are available to bind to additional metal centres and thus build towards self-assembling supramolecules. 

Binding constants (K) are foundational to supramolecular chemistry and quantified by modelling spectroscopic (NMR, UV-vis) titration data according to chemical equilibria. Spurred by growth in data science, the tools and methods for determining K values have accelerated in recent years. To share these advances, we provided a Workshop on Quantifying Binding Constants at ISMSC 2023 in Iceland and herein share the objectives, processes, and recommendations. We framed this short course in terms of learning to drive, from the basics ‘under the hood’, to ‘behind the wheel’, and navigating ‘the open road’. These steps are crucial in the ‘drive to K-town’, where participants appreciate the importance of building, analysing, and comparing models. K-town is where they assess the hazards of incomplete models, inaccurate K values, and incorrect uncertainty assessment. We conclude with the Supramolecular Chemist’s Pledge as a starting point for considering quality control in determining K values.