MOCs employing metal centres based on M2 paddlewheel nodes (e.g. M = CuII or RhII) are prevalent within the field. Often existing as charge-neutral structures unencumbered by counterions, their open axial metal sites dictate the interior and exterior chemical properties. Indeed, these characteristics establish M2 paddlewheel MOCs as ideal platforms for post-assembly modifications and for the development of functional materials by linking strategies. Notably, the intrinsic structural properties of these cages not only impact their cavities, but dictate the potential coordination sites available. The topology of the prepared MOC thus has significant implications when it is employed as a building block for functional materials.
Previous literature and computational modelling of MOCs formed from rigid ligands with distinct ligand bite angles (the intersection of two vectors running parallel to the ligand donor groups) has produced a suitable guide to predict the likely assembly. For M2 paddlewheel-based MOCs, the most frequent topologies reported are primarily the lantern, octahedral, and cuboctahedral structures (MnLn, where n = 4, 12, and 24) based on ligands with bite angles of 0°, 90° and 120° respectively. For ligands with 60° bite angles, the self-assembly outcome lies at the intersection of three possible structures: a D2d-symmetric tetrahedron, a D4h-symmetric square, and a D3h-symmetric triangle. For M2 paddlewheel-based MOCs, these structures remain either unreported or their selective formation is poorly understood.
Recently, our group has prepared a series of three bis-monodentate ligands derived from a rigid, 3,6-disubstituted phenanthrene backbone. The ligands vary in the length and rigidity of their carboxylate donors but bear the same native bite angle of 60°. In this work, we demonstrate the selective assembly of Cu and Rh paddlewheel MOCs based on the length and rigidity of the ligands, including a D2d-symmetric tetrahedron, a D4h-symmetric square, and a D3h-symmetric triangle.