1,2,4-Triazolyl-Carboxylate-Based MOFs Incorporating Triangular Cu(II)-Hydroxo Clusters: Topological Metamorphosis and Magnetism

Bifunctional
1,2,4-triazole-carboxylate ligands, an achiral 1,2,4-triazol-4-yl-acetic
acid (trgly-H) and a chiral (d)-2-(1,2,4-triazol-4-yl)-propionic acid (d-trala-H), derived from the corresponding α-amino acid precursors revealed unique binding abilities in the construction
of Cu­(II)-coordination polymers composing discrete triangular [Cu3(μ3-OH)] clusters. A related series of MOFs,
[Cu3(μ3-OH)­(trgly)3(SO4)]·2H2O (1a), [Cu3(μ3-OH)­(trgly)3(H2O)3]­SO4·16H2O (1b), Cu3(μ3-OH)­(d-trala)3(ClO4)0.5]­(ClO4)1.5·1.5H2O (2), was prepared, and their crystal structures were determined by
means of X-ray diffraction. Being singly deprotonated, the organic
ligands act as multidentate μ3- or μ4-donors using tr and −COO– moieties. The generated [Cu3(μ3-OH)­(tr)3] cluster core is primarily supported by
three [-N-N-] triazole heterocycles in a basal plane and tripodal-assisted
μ3-anions (SO42–: 1a; ClO4–: 2) capping
the axial faces. The carboxylate groups join the units into either
two-dimensional (2D) layer (1a, 2) or 3D
zeolite-like networks (1b). Compound 1b represents
the topology of α-Po (pcu: 412.63) and
crystallizes in the noncentrosymmetric space group I4̅3m, in which the six-connected [Cu3(μ3-OH)] clusters and trgly self-assemble
in an open-channel cubic array possessing ∼56% solvent-accessible
volume. Upon slight thermal treatment (∼60 °C), the structure
irreversibly shrinks to the nonporous 2D motif 1a that
belongs to a uninodal (3,6) network type. In structure 2 (space group R32), due to the [-N-N-]
triazole and 1,3-bidentate carboxylate binding mode, each organic
ligand bridges three metal clusters affording cross-linking of two
adjacent layers with the same (3,6) topology. The resultant 3,9-c
net is novel and can be categorized as two-nodal with point symbol
{418.618}­{42.6}3. Spin
frustration and antisymmetric exchange effects, resulting in abnormally
low g values in the S = 1/2 states, were observed
in the magnetic properties and the EPR spectra.