Interaction of 1,3-Adamantanediacetic Acid (H₂ADA) and Ditopic Pyridyl Subunits with Cobalt Nitrate under Hydrothermal Conditions: pH Influence, Crystal Structures, and Their Properties

A systematic investigation of the pH influence on the interaction between 1,3-adamantanediacetic acid (H₂ADA), 1,3-bis(4-pyridyl)propane (bpp)/4,4′-dipyridine (4,4′-bipy), and cobalt nitrate under hydrothermal conditions has resulted in a series of coordination polymers with different composition and dimensionality, namely [Co(HADA)₂(bpp)]_n (1), {[Co(ADA)(bpp)(CH₃OH)]·H₂O}_n (2), [Co₂(ADA)₂(bpp)]_n (3), [Co(HADA)₂(4,4′-bipy)(H₂O)₂]_n (4), and [Co(ADA)(4,4′-bipy)_0.5]_n (5). These neutral polymeric complexes exhibit structural and dimensional diversity due to various coordination modes of the flexible H₂ADA ligand. X-ray structural analysis reveals that 1 and 4 are one-dimensional (1-D) chains. The chain of complex 1 is directed by bpp, in which HADA located at both sides of the chain. Compared with the case of 1, the 1-D linear chain is composed of 4,4′-bipy and Co(II) ions in 4. Complexes 2, 3, and 5 show different 2-D networks. In 2, the final structure is formed via the linkage of Co-bpp chains by ADA. Complex 3 is composed of 1-D looped chains in which there are two kinds of [Co₂ADA₂] metallomacrocycles, and the chains are further connected by bpp to form the 2-D structure. The building block represents a one-dimensional chain consisting of [Co₂ADA₂] units linked by 4,4′-bipy in 5. Five new coordination modes of H₂ADA are observed; among them, the bridging modes have proven that H₂ADA can be used as an effective bridging ligand for metal−organic complexes, especially for metal organic layered structures (MOLS). It has also been found that the structure formation of the complexes is mainly governed by the pH value of the solution. The other effect is probably the flexibility of the pyridyl-containing ligands optimizing their position within the complexes. The TG, XRPD data of synthesized complexes and the magnetic properties for 3 and 5 have also been investigated.