Scorpionate‐Type Coordination in MFU‐4l Metal–Organic Frameworks: Small‐Molecule Binding and Activation upon the Thermally Activated Formation of Open Metal Sites

Abstract

Postsynthetic metal and ligand exchange is a versatile approach towards functionalized MFU-4l frameworks. Upon thermal treatment of MFU-4l formates, coordinatively strongly unsaturated metal centers, such as zinc(II) hydride or copper(I) species, are generated selectively. Cu(I)-MFU-4l prepared in this way was stable under ambient conditions and showed fully reversible chemisorption of small molecules, such as O2, N2, and H2, with corresponding isosteric heats of adsorption of 53, 42, and 32 kJ mol(-1), respectively, as determined by gas-sorption measurements and confirmed by DFT calculations. Moreover, Cu(I)-MFU-4l formed stable complexes with C2H4 and CO. These complexes were characterized by FTIR spectroscopy. The demonstrated hydride transfer to electrophiles and strong binding of small gas molecules suggests these novel, yet robust, metal-organic frameworks with open metal sites as promising catalytic materials comprising earth-abundant metal elements.

Postsynthetic Metal and Ligand Exchange in MFU‐4l: A Screening Approach toward Functional Metal–Organic Frameworks Comprising Single‐Site Active Centers

“…The presence of Cu I centers in this product is confirmed by oxygen adsorption isotherms showing clear chemisorption (the Supporting Information, Figure S46, left). The heat of oxygen chemisorption (50 kJ mol −1 , the Supporting Information, Figure S75) is very close to the one previously described for Cu I ‐MFU‐4 l (52 kJ mol −1 ) 15. Additionally, the formation of Cu I centers is supported by the IR spectrum after CO adsorption (the Supporting Information, Figure S31): A characteristic CO stretch at 2090 cm −1 is observed (2081 cm −1 in a previously described Cu I ‐MFU‐4 l prepared through path d ).…”

“…As shown by us in a recent report, the thermal decomposition of formate moieties at peripheral Zn II centers in MFU‐4 l ‐HCOO leads to the formation of ZnH species, whereas decomposition of Cu II ‐HCOO moieties gives Cu I species 15. Thus, thermal decomposition of formate ligands can be considered as a method for the in situ generation of reactive or low‐valent metal sites.…”

“…We have previously reported on the complete exchange of peripheral Zn 2+ ions in MFU‐4 l by Co 2+ at 140 °C in DMF 14. Metal exchange with Cu 2+ , however, can only be conducted at 60 °C (due to the formation of amorphous impurities at higher temperatures) and leads to a partial (≈50 %) substitution 15. Similarly, other 3d transition‐metal ions (Mn 2+ , Fe 2+ , and Ni 2+ ) can be introduced into the SBU by dispersing MFU‐4 l crystals into a solution containing a large excess of metal(II) chloride in DMF or DMA at 60 °C.…”

“…Solvent‐free 5 is red–brown and shows very intensive absorption bands at 417, 523, and 997 nm (Figure 5). 15…”

“…Unfortunately, up to now, we were not able to prepare these derivatives. It seems that tetrahedral and fivefold coordination of divalent metal centers in MFU‐4 l frameworks comprising one additional side ligand is less favorable for binding small molecules if compared with Cu I ‐MFU‐4 l featuring trigonal‐pyramidal Cu I sites and showing quite strong O 2 binding 15. Although MFU‐4 l derivatives seem rather inactive in terms of dioxygen binding from the gas phase, they show catalytic activity in liquid‐phase oxidation reactions.…”