Solvent-mediated isomerization of Fe^II complexes from a 3D 6⁵·8 framework to a 2D 4⁴ rhombus-grid network

The assembly of 1,4-bis(pyrid-4-yl)benzene (bpb) and Fe(NCS)₂ affords three supramolecular isomeric frameworks formulated as [Fe(bpb)₂(NCS)₂]·X (X = 3EtOH for 1, 2.5H₂O for 2, and 0 for 3, respectively). Crystallographic studies reveal that 1 belongs to the monoclinic space group C2/c and crystallizes into a threefold self-interpenetrated 3D 6⁵·8 CdSO₄-type framework, whereas 2 crystallizes in the monoclinic space group P2/c and features a 2D 4⁴ square-grid network. The immersion of red crystals of 1 into a methanol/ethanol (1:3) solvent led to the formation of yellow crystals of 3, showing structural transformation via crystal disassembly and reassembly process. 3 crystallizes in the triclinic space group P1¯ and is a 2D 4⁴ rhombus-grid network, but showing nonporous feature (compared with 2) because of more compact network stacking. The most interesting feature is that crystals of 3 are not able to be synthesized by the direct reaction of Fe(NCS)₂ with ligand bpb. We suppose that this structural isomerization is a solvent-mediated rearrangement of coordination bonds from a kinetically favored isomer (1) to the thermodynamically stable form (3) of the system. It is generally believed that the framework of higher dimensionality is a more stable phase than that of lower dimensionality. While conversions between supramolecular isomers from 1D to 2D, 1D to 3D, and 2D to 3D have been observed, examples showing 3D to 2D or 3D to 1D isomerization are rarely reported. The structural transformation herein represents a rather scarce case from higher dimensional framework (3D) to lower one (2D).