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 = CuI, AgI). 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, AuI 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 CRYSTAL 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
AuI 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.