Directing the Breathing Behavior of Pillared-Layered Metal–Organic Frameworks via a Systematic Library of Functionalized Linkers Bearing Flexible Substituents

Flexible metal–organic frameworks (MOFs), also
referred to as soft porous crystals (SPCs), show reversible structural
transitions dependent on the nature and quantity of adsorbed guest
molecules. In recent studies it has been reported that covalent functionalization
of the organic linker can influence or even integrate framework flexibility
(“breathing”) in MOFs. However, rational fine-tuning
of such responsive properties is very desirable but challenging as
well. Here we present a powerful approach for the targeted manipulation
of responsiveness and framework flexibility of an important family
of pillared-layered MOFs based on the parent structure [Zn2(bdc)2(dabco)]n (bdc = 1,4-benzenedicarboxylate;
dabco = 1,4-diazabicyclo[2.2.2]­octane). A library of functionalized
bdc-type linkers (fu-bdc), which bear additional dangling side groups
at different positions of the benzene core (alkoxy groups of varying
chain length with diverse functionalities and polarity), was generated.
Synthesis of the materials [Zn2(fu-bdc)2(dabco)]n yields the respective collection of highly
responsive MOFs. The parent MOF is only weakly flexible; however,
the substituted frameworks of [Zn2(fu-bdc)2(dabco)]n contract drastically upon guest removal
and expand again upon adsorption of DMF (N,N-dimethylformamide), EtOH, or CO2, etc., while
N2 is hardly adsorbed and does not open the narrow-pored
form. These “breathing” dynamics are attributed to the
dangling side chains that act as immobilized “guests”,
which interact with mobile guest molecules as well as with themselves
and with the framework backbone. The structural details of the guest-free,
contracted form and the gas sorption behavior (phase transition pressure,
hysteresis loop) are highly dependent on the nature of the substituent
at the linker and can therefore be adjusted using our approach. Combining
our library of functionalized linkers with the concept of mixed-component
MOFs (solid solutions) offers very rich additional dimensions of tailoring
the structural dynamics and responsiveness. Implementation of two
differently functionalized linkers in varying ratios yields multicomponent
single-phased [Zn2(fu-bdc′)2x(fu-bdc″)2–2x(dabco)]n MOFs (0 < x < 1)
of increased inherent complexity, which feature a non-linear dependence
of their gas sorption properties on the applied ratio of components.
Hence, the responsive behavior of such pillared-layered MOFs can be
extensively tuned via an intelligent combination of functionalized
linkers.