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 [Fe^IIpzTTP(FeDMF_1-xFeOH_x)]_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.