Integrative Assembly of Heteroleptic Tetrahedra Controlled by Backbone Steric Bulk

A bent fluorenone-based dipyridyl ligand L^A reacts with Pd^II cations to a solvent-dependent dynamic library of [PdL₂] assemblies, constituted by a [Pd₃L^A₆] ring and a [Pd₄L^A₈] tetrahedron as major components, and a [Pd₆L^A₁₂] octahedron as minor component. Introduction of backbone steric hindrance in ligand L^B allows exclusive formation of the [Pd₆L^B₁₂] octahedron. Combining equimolar amounts of both ligands results in integrative self-sorting to give an unprecedented [Pd₄L^A₄L^B₄] heteroleptic tetrahedron. Key to the non-statistical assembly outcome is exploiting the structural peculiarity of the [Pd₄L₈] tetrahedral topology, where the four lean ligands occupy two doubly bridged edges and the bulky ligands span the four remaining, singly bridged edges. Hence, the system finds a compromise between the entropic drive to form an assembly smaller than the octahedron and the enthalpic prohibition of pairing two bulky ligands on the same edge of the triangular ring. The emission of luminescent L^A is maintained in both homoleptic [Pd₃L^A₆] and heteroleptic [Pd₄L^A₄L^B₄].