The ubiquitous paddle-wheel building block in two-dimensional coordination polymers with square grid structure

This work describes design of a series of new paddle-wheel binuclear clusters containing 2-D coordination polymers based on ditopic carboxylate linkers, 1,4-benzenedicarboxylate (BDC) or 2-amino,1,4-benzenedicarboxylate (Am-BDC). The strategic use of strongly coordinating base/solvent as blocking ligand to restrict the structure in 2-D space is explored, and the role of organic base on the overall structure formation is further elaborated. The isostructural [Zn(BDC)(Py)]_n (1) and [Co(BDC(Py)]_n (2) were formed by the use of strong base pyridine (Py) as a blocking ligand whereas reaction using N-methylimidazole (Mim) in place of pyridine gives [Co(BDC)(Mim)]_n (3) with similar topology and coordination environment. The use of weak/non-coordinating base such as 2-chloropyrimidine, pyrazine, and tetramethylammoniumhexafluorophosphate [(CH₃)₄ N(PF₆)] gives the DMF-coordinated 2-D frameworks, [Cu(BDC)(DMF)]_n (4), [Zn(BDC)(DMF)]_n (5), and [Zn(AmBDC)(DMF)]_n (6). All the structures crystallize in monoclinic crystal system yielding 2-D nets with square grid 4⁴ topology and solid state 3-D structure via extensive non-covalent supramolecular interactions. Surface area analysis via N₂ adsorption of three representative 2-D coordination polymers, 1, 4, and 6, indicate that 4 has a surface area of 450 m² g⁻¹ which is a signature of microporosity, while 1 and 6 have moderate (161.6 m² g⁻¹) and negligible (33 m² g⁻¹) surface areas, respectively.