Bifunctional Co(II)-Ag(I) and Ni(II)-Ag(I) frameworks: modulation of magnetic property and Co₂ uptake based on organic pillars

Abstract

This report articulates synthesis, characterization, adsorption, and magnetic properties of four bimetallic Co(II)– or Ni(II)–Ag(I) 3D porous frameworks based on mixed-ligand systems. The cyanide-bridged M(II)–Ag(I) bimetallic compounds with the general formula [M^II(L){Ag(CN)₂}₂·xH₂O]_n [1, L = piperazine, M = Co; 2, L = piperazine, M = Ni; 3, L = 1,4-diazabicyclo[2.2.2]octane (dabco), M = Co; 4, L = pyrazine, M = Co] have been synthesized by liquid phase diffusion method at room temperature. Structure determination revealed that all these compounds have α-polonium type structural topology. The Ag(CN)₂^- metallo-ligand has been used to generate 2D −M(II)–CN–Ag(I)–CN–M(II)– layers, which are further linked by different organic pillars to construct a 3D bimetallic Co(II)– or Ni(II)–Ag(I) porous pillared-layered structure. The magnetic and adsorption properties of these systems have been tuned by systematic variation of the pillars such as piperazine, pyrazine, and dabco. Temperature dependent magnetic study reveals that at low temperature, magnetized states exist for 1, 2, and 3 and spin canting behavior is evident; while 4 exhibits dominant antiferromagnetism. The degree of spin canting/antiferromagnetism depends on the organic spacers. These compounds contain water filled channels, and desolvated frameworks show high thermal stability and structural rigidity. Compounds 1, 2, and 4 exhibit permanent porosity as established by gas adsorption studies whereas 3 does not adsorb any gas unveiling that pore size could be modulated by changing the organic pillars. In the case of 3, the larger pillar dabco reduces pore size significantly resulting in a nonporous structure. Furthermore, compound 1 reveals selective Co₂ uptake properties at 195 K as other gases (N₂, H₂, O₂, and Ar) show only surface adsorption, suggesting that quadrupolar Co₂ molecules interact effectively with the pore surfaces decorated with polar −CN groups.