A Mechanochemically Active Metal‐Organic Framework (MOF) Based on Cu‐Bis‐NHC‐Linkers: Synthesis and Mechano‐Catalytic Activation

Abstract

Porous coordination polymers, more commonly known as metal‐organic frameworks (MOFs), are constructed from metal ions and organic linkers which form a robust network‐like structure similar to very dense polymeric networks. In this work the mechanochemical activation of a MOF is reported, based on the previous results on the mechanochemical activation of latent Cu(I)‐based N‐heterocyclic carbenes (NHCs) embedded covalently in various polymers. A mechanochemically triggered copper‐catalyzed azide‐alkyne cycloaddition (CuAAC) is achieved using an NHC‐MOF, constructed from 1,3‐bis(4‐carboxyphenyl)imidazolium chloride as organic linker. A Cu(I) bis(NHC) is embedded into the MOF analogous to ([Zn4O{Cu(L)2}2]) using the organic bis‐NHC ligand, in turn generating a largely amorphous MOF. On activation by ultrasound, the MOF containing 4.66% of Cu shows mechanochemical activation to obtain Cu(I), subsequently triggering a CuAAC with conversions up to 26.5% as monitored by the reaction of benzylazide and phenylacetylene to yield 1‐benzyl‐4‐phenyl‐1H‐1,2,3‐triazole over a period of 20 sonication cycles. With this knowledge in solution, there are prospects to transfer this mechanochemical activity into polymer networks with embedded MOFs to detect stress in hybrid materials.

Mechanochemical Activation of a Metal–Organic Framework Embedded within a Thermoplastic Polyurethane Matrix: Probing Fluorogenic Stress‐Sensing

“…Subsequently, a waiting period of further 75 mins at 25 °C was observed to recover the properties of the TPU (Scheme S4, Supporting Information). Based on previous control experiments conducted for MOF (1) in solution only a limited thermal activation at 60 °C had occurred for a duration of 50 h. [55] We thus can exclude that MOF (1) plays a significant role in promoting a CuAAC at a temperature of 50 °C over 12.5 h (30 mins per cycle, 25 cycles) in bulk, thus justifying the chosen parameters for the measurements. Further the shape memory properties of the TPU (SMPU) containing MOF (1) were probed via a brief study of the thermally induced shape memory effect.…”

“…This is attributed to the methodology of suspending the MOF (1) by stirring (see Table S2, Supporting Information), resulting in the formation of an already activated MOF (1′) (see Scheme 3), with a visible CuAAC in the TPU solution before solidification. [55] We therefore chose another strategy for dispersion by dispersing the MOF via ultrasonication in an ultrasonication bath for times of either 10 mins (TPU50_USB1 (Table 1, Entry 2)) or 10 s (TPU50_USB2 (Table 1, Entry 3)). Shorter ultrasonication times (10 s) resulted in only a low level of this primordial activation (see Figure 1A) in the sample TPU50_USB2 (Table 1, Entry 3).…”

“…The size of the MOF (1) particles in the samples TPU50_Stir (14 nm) and TPU50_USB2 (85 nm), as observed in the AFM measurements (see Figure 1B), was attributed to the different dispersion methods, clearly indicating that TPU50_USB2 with its larger MOF (1) particles exhibited a lower initial catalytic behavior. [55] The TPU50 embedded with MOF (1), ( 5), and ( 6) was then subjected to oscillating tensile rheology, for which the samples were cut to obtain dumbbell-shaped samples using a sample cutter (see Scheme 3(V)). Oscillating tensile rheology was conducted via a "Universal Extensional Fixture (UXF)" by repeatedly applying stress over multiple cycles.…”

“…We previously have reported the use of a metal-organic framework (MOF) containing the mechanophoric Cu(bis-NHC)-complex, proving its ultrasound-mediated mechanochemical activation via a fluorogenic copper-catalyzed "click"-reaction (CuAAC)-reaction of benzylazide and phenylacetylene to yield 1-benzyl-4-phenyl-1H-1,2,3-triazole. [55] The coordination of the terminal alkyne with the now available Cu(I) of an active Cu(I) mono(NHC) forms the copper acetylide to subsequently promote the formation of the 1,2,3triazole product, [53,55] which can be easily quantified in a fluorogenic "click" reaction (see Scheme 1).…”

Mechanically Triggered Polymer Deconstruction through Mechanoacid Generation and Catalytic Enol Ether Hydrolysis

“…Examples include a self-immolative polymer whose depolymerization is initiated by mechanical scission, mechanical gating that results in chemically degradable units along the polymer main chain, − and the mechanical activation of a delayed cascade rearrangement that subsequently chops down the polymer backbone . These examples add to a growing mechanophore toolkit that already includes functionalities such as colorimetric response and painting, ,− catalysis, − molecule/drug release, ,, network physical properties modification, , and conductivity switching …”