Self-assembly of metallosupramolecular rhombi from chiral concave 9,9'-spirobifluorene-derived bis(pyridine) ligands

Two new 9,9’-spirobifluorene-based bis(4-pyridines) were synthesised in enantiopure and one also in racemic form. These ligands act as concave templates and form metallosupramolecular [(dppp)₂M₂L₂] rhombi with cis-protected [(dppp)Pd]²⁺ and [(dppp)Pt]²⁺ ions. The self-assembly process of the racemic ligand preferably occurs in a narcissistic self-recognising manner. Hence, a mixture of all three possible stereoisomers [(dppp)₂M₂{(R)-L}₂](OTf)₄, [(dppp)₂M₂{(S)-L}2](OTf)₄, and [(dppp)₂M₂{(R)-L}{(S)-L}](OTf)₄ was obtained in an approximate 1.5:1.5:1 ratio which corresponds to an amplification of the homochiral assemblies by a factor of approximately three as evidenced by NMR spectroscopy and mass spectrometry. The racemic homochiral assemblies could also be characterised by single crystal X-ray diffraction.