Structure and redox tuning of gas adsorption properties in calixarene-supported Fe(ii)-based porous cages

Abstract

We describe the synthesis of Fe(ii)-based coordination cages whose stability and gas adsorption properties can be tuned through structural modifications and redox reactivity.

Gas Storage in Porous Molecular Materials

“…In the IRMOP series of coordination cages, an increase in the free pore diameter from 6.4 to 9.4 Å was observed when the ligand was changed from a biphenyl dicarboxylate bridge to a terphenyl dicarboxylate linker [49] . An increase in surface area and resultant gas uptakes was described in the study of gas storage in octahedral Fe calixarene cages upon expansion of the tritopic bridging linker, which our group has recently described [178] . Similar ligand expansion strategies have been used to synthesize isoreticular families of cages, albeit without exploring their utility for gas storage applications, most notably for paddlewheel‐based systems, calixarene‐capped cages, and Cp‐capped Zr‐based cages [196–199] …”

“…A number of porous cage structures have been described that incorporate tetranuclear metal cluster nodes, most commonly with nickel or cobalt, capped by thia‐ (tc4a) or sulfonylcalix[4]arene (sc4a) ligands [176, 177] . Our group recently reported a series of Fe‐based calixarene‐capped cages and explored their utility for hydrocarbon adsorption [178] . These include octahedral structures linked along the cage faces by tritopic trimesate (btc 3− ) or an expanded triazine trisbenzoate (tatb 3− ) ligand, as well as structures linked along the edges of an octahedron by terephthalate bridges.…”

“…In fact, many permanently porous molecular cages have known MOF structural analogues for which this approach may be applicable (Figure 4). [217] For instance, the octahedral pore structures of calixarene‐capped cages with square, tetranuclear metal nodes linked by tritopic, carboxylate‐based ligands (btc 3− , btb 3− , tatb 3− ) are directly analogous to the octahedral pore structures in the PCN‐9 series of frameworks [176, 178, 218–220] . Similarly, Cp‐capped trimetallic Zr‐based metal nodes linked by bridging linear ditopic carboxylate ligands generate tetrahedral cage structures that are direct analogues of the pores in the canonical UiO series of frameworks [199, 221] .…”

Stabilizing Porosity in Organic Cages through Coordination Chemistry

“…Gas adsorption analysis revealed a comparable CO 2 accessible surface area to the aforementioned Fe- and Co-based materials, with a BET (Langmuir) surface area of 108 (281) m 2 /g at 195 K. This represents a greater than 20-fold increase in CO 2 uptake relative to the unmetalated CC1-amino cage structure. We speculate that the somewhat limited porosity accessible for this cage may be attributable, in part, to the presence of nonporous counterions that diminish the accessible void space in these materials, with 12 chloride ions incorporated per cage. , The presence of these counterions may also play a role in blocking N 2 from entering the material’s pores, as this structure was found to be completely nonporous toward N 2 . Additional examples of Ru-based PCCs have been previously reported. , …”

“…These uptakes are somewhat lower than most of the reported structurally related tethered organic cages, , but this is consistent with the added steric bulk of the M–Cl bonds and higher mass of the incorporated transition metal-based tethers. Structurally diverse PCCs have previously been described that incorporate Fe , or Co-based − metal nodes.…”