Expanding the horizons of porphyrin metal-organic frameworks via catecholate coordination: exploring structural diversity, material stability and redox properties.

Porphyrin based Metal-Organic Frameworks (MOFs) have generated high interest because of their unique combination of light absorption, electron transfer and guest adsorption/desorption properties. In this study, we expand the range of available MOF materials by focusing on the seldom studied porphyrin ligand H10TcatPP, functionalized with tetracatecholate coordinating groups. A systematic evaluation of its reactivity with M(iii) cations (Al, Fe, and In) led to the synthesis and isolation of three novel MOF phases. Through a comprehensive characterization approach involving single crystal and powder synchrotron X-ray diffraction (XRD) in combination with the local information gained from spectroscopic techniques, we elucidated the structural features of the solids, which are all based on different inorganic secondary building units (SBUs). All the synthesized MOFs demonstrate an accessible porosity, with one of them presenting mesopores and the highest reported surface area to date for a porphyrin catecholate MOF (>2000 m2 g-1). Eventually, the redox activity of these solids was investigated in a half-cell vs. Li with the aim of evaluating their potential as electrode positive materials for electrochemical energy storage. One of the solids displayed reversibility during cycling at a rather high potential (∼3.4 V vs. Li+/Li), confirming the interest of redox active phenolate ligands for applications involving electron transfer. Our findings expand the library of porphyrin-based MOFs and highlight the potential of phenolate ligands for advancing the field of MOFs for energy storage materials.

Confinement of Fe-Al-PMOF catalytic sites favours the formation of pyrazoline from ethyl diazoacetate with an unusual sharp increase of selectivity upon recycling.

The catalytic properties of a chemically stable iron porphyrin MOF were evaluated in a reaction with ethyl diazoacetate. In contrast to its homogeneous counterpart, an Fe-porphyrin-MOF features a different reaction pathway leading to the formation of pyrazoline due to the confinement of catalytic sites within the MOF network. Unexpectedly, a sharp increase of the selectivity from 35% (run 1) to 86% (run 5) occurs upon catalyst recycling.

Adaptability of the metal(iii,iv) 1,2,3-trioxobenzene rod secondary building unit for the production of chemically stable and catalytically active MOFs.

The use of a 5,10,15,20-tetrakis(3,4,5-trihydroxyphenyl)porphyrin has yielded a new MOF based on M-1,2,3-trioxobenzene chains that can be made of M = Zr(iv) or RE(iii) (RE = rare earth), showing a very high and limited chemical stability, respectively. The robust metallated Zr-analogue, Co-MIL-173(Zr), has proven to be a heme-like heterogeneous catalyst suitable for aerobic oxidation of hydrocarbons.

Tuning the iron redox state inside a microporous porphyrinic metal organic framework.

Two new 3D porphyrin-based metal organic frameworks were obtained by solvothermally reacting iron(iii) chloride, a free base (5,10,15,20-tetrakis[4-(2,3,4,5-tetrazolyl)phenyl]porphyrin) (H2TTPP) and either pyrazine or 1,4-diazabicyclo[2.2.2]octane (DABCO). Both MOFs displayed a 3-D open framework of the fry topology, where the inorganic building unit is a chain of corner-sharing FeN4O2 octahedra and the porphyrinic linker is metallated with iron during the reaction course, with the N-donor base bridging the iron of the porphyrinic cores. Through thorough structural and spectroscopic analyses of the pyrazine containing material the chemical formula [FeIIpzTTP(FeDMF1-xFeOHx)]n was inferred (x ≥ 0.25). Whereas the already reported carboxylate analogue is built up from a pure iron(iii) inorganic chain, here spectroscopic and structural studies evidenced a mixed valence iron(ii/iii) state, evidencing that, in agreement with the HSAB theory, the substitution of a carboxylate function by a tetrazolate one allows redox tuning. Finally, both materials feature one-dimensional channels of ca. 8 × 12 Å within the structures with permanent microporosity.