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A simple “one-pot” strategy for accessing heteroleptic complexes is unlocked by selective deprotonation of a pH-sensitive N-donor ligand before in situ complexation by mixing it with a coligand and a metal salt. For a series of hydroxyl-disubstituted bis(pyrazol-3-yl)pyridines (3-bpp), it allowed almost quantitative formation of heteroleptic cobalt(II) complexes with a terpyridine coligand─even if available in minute amounts otherwise, owing to the improved solubility of 3-bpp upon conversion to its anionic form. Together with substitution and coligand effects, reversible deprotonation/protonation by a base/acid─all within the same solution containing a heteroleptic complex─induces and modulates the spin-crossover behavior through an intricate interplay of ligand-field strength, steric effects, and symmetry lowering due to the deprotonation-induced trans-effect and negative charge accumulation on the O– groups. Combined with NMR analysis of the multicomponent mixtures resulting from two ligands, a metal salt, an acid, and a base, the proposed strategy, which is transferable to other ligands with (de)protonable groups, offers a powerful tool to search for new spin-crossover compounds and guide their chemical design, e.g., as pH-sensitive probes for MRI diagnostics.
Safiullina et al. (Mon,) studied this question.