Cu(II)-Based Coordination Polymers Containing 1,4-Bis(1H-imidazol-1-yl)benzene and Monovalent Fluorinated Anions

Abstract

The design of the organic linker is one of the most important steps in driving the formation of the desired crystal structure in coordination polymers. Imidazole derivatives exhibit wide and interesting coordination capabilities. This property makes these π-conjugated ligands suitable linkers in the synthesis of metal–organic frameworks (MOFs). Despite these considerations, few examples of two-dimensional (2D) materials based on the 1,4-bis(1H-imidazol-1-yl)benzene (bib) ligand, which can be seen as a topological analogue of 4,4′-bipyridine, have been reported so far, and there is a lack of literature on the exploration of their gas adsorption properties. The combination of bib ligand with Cu(BF₄)₂ led us to obtain a doubly interpenetrated three-dimensional (3D) material (UdP-1) of minimal formula [Cu(bib)_2.5(BF₄)]·(BF₄)·1.5H₂O that loses 0.5 equiv of bib upon heating and undergoes a phase transition to a new material named UdP-2. UdP-2 was obtained as a microcrystalline powder by direct synthesis, but its crystal structure could not be elucidated. Attempts to obtain single crystals of UdP-2 led us to discover four new compounds, named UdP-3, UdP-3-i, UdP-4, and UdP-5, displaying both 2D and 3D structures. Using Cu(CF₃SO₃)₂ instead of Cu(BF₄)₂ allowed us to prepare a 2D layered material of the formula [Cu(bib)₂(CF₃SO₃)₂]·2MeOH (UdP-6), where the methanol molecules are trapped in the pores of the structure as in the ELM-12 structure, which contains 4,4′-bipyridine as the organic ligand. Different from ELM-12, UdP-6 displays a Type I CO₂ adsorption isotherm, with no evidence of a gating phenomenon. This behavior is attributed to the larger number of supramolecular interactions existing between adjacent layers in UdP-6.