Cu(II) MOFs Based on Bipyridyls: Topology, Magnetism, and Exploring Sensing Ability toward Multiple Nitroaromatic Explosives

In
this work, a series of three copper­(II) metal-organic frameworks
(MOFs), [Cu­(4,4′-DP)­Cl]n (1), [Cu­(4,4′-DP)0.5Cl]n (2), and [Cu­(4,4′-TMDP)­Cl]n (3) (4,4′-DP = 4,4′-dipyridyl,
4,4′-TMDP = 4,4′-trimethylenedipyridyl), is designed
and synthesized under solvothermal conditions. Crystallographic investigations
reveal that 1 and 2 have tetrahedral and 3 has octahedral environment around the Cu­(II) ion. By varying
the solvent conditions and ligand derivatives, the topology can be
interestingly tuned. TOPOS Pro provides topological conclusions that 1 is stabilized by unusual 2D + 2D → 3D polycatenation
of layers lying in (110) and (11̅0) planes with dihedral angle
of 90° showing altogether fes, hcb, and sql topologies.
On the other hand, 2 exhibits a bey (3,4-c net) topology and 3 shows 4-fold interpenetration
with the dia topology. The dc measurements
for 1–3 performed on polycrystalline
samples in a 0.1 T field confirm strong ferromagnetic behaviors for 1 and 2 and moderate antiferromagnetic behavior
for 3. To examine the sensing properties of the three
MOFs, various hazardous nitroaromatic compounds (NACs) were used as
analytes. While 1 is a potent fluorescence sensor for
highly sensitive detection of multiple NACs, 2 selectively
detects meta-dinitrobenzene (m-DNB)
with KSV = 5.73 × 105 M–1 and a remarkably lower limit of detection (LOD) value
of 1.23 × 10–7 M. 3 does not show
sensing ability toward any NAC probably due to the coordination environment
being different from those in 1 and 2. The
work demonstrates fine-tuning of the topology and in turn magnetic
and sensing properties by changing the reaction conditions.