Structural Diversity of Cadmium(II) Coordination Polymers Induced by Tuning the Coordination Sites of Isomeric Ligands

When the coordination
sites of ligands were shifted, the solvothermal reactions of four
positional isomeric asymmetrical pyridyldicarboxylatic acids with
Cd­(NO3)2 generated four new coordination polymers,
[Cd­(L1)­(DMF)3]·DMF·H2O (1), [H2N­(CH3)2]2[Cd­(L2)2]·3DMF·H2O (2), [Cd­(L3)­(H2O)2] (3), and [Cd­(L4)]·1.5DMF
(4), where DMF = N,N-dimethylformamide, H2L1 = 2-(3′-carboxylphenyl)­isonicotinic
acid, H2L2 = 2-(4′-carboxylphenyl)­isonicotinic acid,
H2L3 = 5-(3′-carboxylphenyl)­nicotic acid, and H2L4 = 2-(3′-pyridyl)­terephthalic acid. 1 shows a rare 2D fabric structure. 2 discloses a grid-layer
structure with heterochiral helical chains and in which three sets
of layers stack in different directions, affording an unprecedented
2D + 2D + 2D → 3D polycatenating framework with 3D intersecting
porous systems. 3 also displays a 2D layer possessing
strong intralayer π···π interactions and
interlayer hydrogen bonds. 4 contains a rare Cd2(COO)4 paddle-wheel unit and forms a 3D framework with
1D open channels. The carboxyl and pyridyl groups of the positional
isomeric H2L1–H2L4 ligands show distinct
bridging fashions, which leads to the production of versatile architectures
of 1–4, and their effects on the
crystal structures are discussed. 1–4 reveal solid-state photoluminescence stemming from intraligand charge
transfer. 2 and 4 show high selectivity
for CO2 over CH4 but with different CO2 adsorption enthalpies. Grand canonical Monte Carlo simulations identified
the multiple adsorption sites in 2 for CO2.