Structural insights into exceptional layer expansion and o-xylene selectivity of a 2D switching coordination network

Flexible coordination networks (CNs) that undergo guest-induced phase switching offer new opportunities for challenging hydrocarbon separations as they can possess both high selectivity and large adsorption capacity. However, mechanistic understanding of guest selectivity is often compromised by reliance on low-temperature structural data to interpret adsorption processes occurring under ambient conditions. Here, we report a combined crystallographic and computational study of the prototypical 2D square lattice (sql) CN [Co(4,4′-bipyridine)₂(NCS)₂]_n, sql-1-Co-NCS, which exhibits selective xylene adsorption at room temperature. Single-crystal X-ray diffraction at 303 K reveals that o-xylene (OX) and m-xylene (MX) induce a high-symmetry tetragonal I4/mmm open phase, whereas p-xylene (PX) stabilizes a lower-symmetry monoclinic C2/c open phase despite identical guest loading. The interlayer distance expands in a clay-like manner by more than twofold, from 4.46 Å in the closed phase to 9.46–9.61 Å in the xylene-loaded phases, representing one of the largest layer expansions observed in 2D CNs. Molecular simulations further show that average isosteric heats of adsorption follow the trend OX (35.51 kJ mol⁻¹) > PX (34.11 kJ mol⁻¹) > MX (33.29 kJ mol⁻¹), consistent with the experimentally observed vapor-phase selectivity. These findings rationalize the exceptional layer expansion and OX selectivity of sql-1-Co-NCS through guest-specific, temperature-dependent stabilization of distinct open phases.