Self-assembly of lanthanide helicate coordination polymers into 3D metal-organic framework structures

Abstract

Pyridine-2,6-dicarboxylic acid (pdcH₂) reacts with nitrate salts of La(III), Ce(III), and Nd(III) under hydrothermal conditions to form three-dimensional network structures, 1-4. All the compounds crystallize in the monoclinic space group P2₁/c with the following lattice parameters: {La₂(pdc)₃·3H₂O} 1, a = 10.966(3) Å, b = 17.534(4) Å, c = 13.578(2) Å, β = 100.23(3)°, V= 2569.3(9) ų, Z = 4, R1 = 0.052, wR2 = 0.143, S = 1.06; {Ce₂(pdc)₃·3H₂O} 2, a = 12.701(3) Å, b = 9.979(1) Å, c = 19.401(4) Å, β = 97.73(2)°, V= 2436.6(8) Å3, Z = 4, R1 = 0.039, wR2 = 0.117, S = 1.30; {Ce₂(pdc)₃·3H₂O} 3, a = 10.961(5) Å, b = 17.523(5) Å, c = 13.505(2) Å, β = 100.89(3)°, V= 2547.2(8) Å3, Z = 4, R1 = 0.040, wR2 = 0.103, S = 1.08; {Nd₂(pdc)₃·3H₂O} 4, a = 10.944(3) Å, b = 17.448(5) Å, c = 13.397(2) Å, β = 101.19(3)°, V= 2569.3(9) ų, Z = 4, R1 = 0.052, wR2 = 0.143, S = 1.06. Compounds 1, 3, and 4 form infinite single helical chains with large widths and pitches containing four metal ions per turn while 2 forms a different helix with smaller width and pitch containing two metal ions per turn. Each helix is further bonded on either side via carboxylate bridging producing three-dimensional metal-organic framework structures.