Hierarchy of Supramolecular Arrangements and Building Blocks: Inverted Paradigm of Crystal Engineering in the Unprecedented Metal Coordination of Methylene Blue

The aromatic methylene blue cation (MB⁺) shows unprecedented ligand behavior in the X-ray structures of the trigonal-planar (TP) complexes MBMCl₂ (M = Cu^I, Ag^I). The two isostructural compounds were exclusively synthesized by grinding together methylene blue chloride and MCl solids. Only in the case of AuCl did the technique lead to a different, yet isoformular, Au^I derivative with separated MB⁺ and AuCl₂^– counterions and no direct N–Au linkage. While the density functional theory (DFT) molecular modeling failed in reproducing the isolated Cu and Ag complexes, the solid-state program <i>CRYSTAL</i> satisfactorily provided for Cu the correct TP building block associated with a highly compact π stacking of the MB⁺ ligands. In this respect, the dispersion interactions, evaluated with the DFT functional, provide to the system an extra energy, which likely supports the unprecedented metal coordination of the MB⁺ cation. The feature seems governed by subtle chemical factors, such as, for instance, the selected metal ion of the coinage triad. Thus, the electronically consistent Au^I ion does not form the analogous TP building block because of a looser supramolecular arrangement. In conclusion, while a given crystalline design is generally fixed by the nature of the building block, a peculiarly efficient supramolecular packing may stabilize an otherwise unattainable metal complex