Effect of Spacer Atoms in the Dicarboxylate Linkers on the Formation of Coordination Architectures—Molecular Rectangles vs 1D Coordination Polymers: Synthesis, Crystal Structures, Vapor/Gas Adsorption Studies, and Magnetic Properties

Abstract

Using the Cu(II)-tpbn system, three new compounds—[Cu4(tpbn)2(glutarate)2(H2O)4](ClO4)4·xH2O (when x = 2, 1; x = 4, 1a), {[Cu2(tpbn)(diglyconate)(H2O)2](ClO4)2·solvent}n (when no solvent, 2; solvent = CH3OH/CH3CN, 2a), and [Cu4(tpbn)2(2,2′-thiodiacetate)2(H2O)4](ClO4)4·xH2O (when x = 0, 3; x = 4, 3a), where tpbn = N,N′,N″,N‴-tetrakis(2-pyridylmethyl)-1,4-diaminobutane—have been synthesized from a one-pot self-assembly reaction in a methanol–water mixture under ambient conditions. Their formation is dependent on the spacer atom in the dicarboxylate linker: molecular rectangles (1/1a and 3/3a) are obtained with spacer atoms CH2 and S, whereas a helical one-dimensional (1D) coordination polymer (2/2a) is obtained with O as the spacer atom in the linker. These are the first molecular rectangles containing the dicarboxylates used in this study. In addition to their characterization by elemental analysis and UV–vis and IR spectroscopy, their solid-state molecular structures are determined by single-crystal X-ray diffractometry. Using electrospray mass spectrometry, their structural integrity in the solution state is confirmed. Water vapor adsorption study of 1–3 corroborates well with their solid-state structures in terms of their affinity toward water: 3 shows a stepwise adsorption with a maximum water vapor intake of 16 water molecules per molecule of 3, having a small hysteresis loop, while 1 shows a continuous adsorption with a maximum water vapor intake of 10.5 water molecules per molecule of 1, having a larger hysteresis loop, and 2 has the least affinity. Similarly, adsorption of CO2 is 3 times more by 1 than 3 at 296 K, and both show selectivity for CO2 over N2. Comparison of their magnetic behavior based on variable-temperature magnetic susceptibility measurements (2–300 K) indicates weak antiferromagnetic interactions between the Cu(II) centers in 1 and 3 and a weak ferromagnetic interaction between the Cu(II) centers in 2. This is well correlated to the Cu···Cu distances in these compounds. Each compound showed a characteristic broad isotropic electron paramagnetic resonance signal for distorted square-pyramidal Cu(II) ions at 77 K.