Porous Polyrotaxane Coordination Networks Containing Two Distinct Conformers of a Discontinuously Flexible Ligand

A new divergent homopiperazine-derived
ligand N,N′-bis­(4-carboxyphenyl)-1,4-diazacycloheptane H2L has been prepared,
containing a semirigid saturated heterocyclic core which is capable
of providing multiple distinct bridging geometries. Reaction of H2L with zinc acetate
in DMSO gives a two-dimensional parallel interpenetrated polyrotaxane
structure 1 in which the loops and rods are formed by
the bent cis-(eq,ax) twist boat and trans-(ax,ax) twist chair conformers, respectively. By matching the distances
between the solvated metal sites in the structure of 1, a related material 2 can be prepared incorporating
the pillaring ligand trans-1,2-bis­(4-pyridyl)­ethylene bpe. Compound 2 displays a similar polyrotaxane
interpenetration mode, permitted by the presence of both cis and trans ligand conformers, but displays a three-dimensional
2.69 topology related to the dia diamondoid
network. The guest exchange and gas adsorption properties of both
materials were investigated; while compound 1 displays
poor stability to guest exchange and negligible gas uptake, the higher
connectivity microporous compound 2 shows facile guest
exchange and a surprisingly high CO2 capacity of 12 wt
% at 1 bar and 273 K, and a zero-loading enthalpy of adsorption of
−32 kJ mol–1. High-pressure adsorption isotherms
also show moderate physisorption of H2 and CH4 within the material.