Synthesis and characterization of Pyrazolate-Supported Fe₃(µ₃-O) complexes and their redox properties

Jessica M. López Plá and Raphael G. Raptis

Department of Chemistry and Biochemistry, Florida International University, Miami, Florida 33199

Trinuclear complexes with formula [Fe₃(μ₃-O)(μ-4-NO₂-pz)₆X₃]^n- (X = Cl, Br, NCS, CN, py and, 4-Me-py; n = 2 and 3) show one reversible redox process, where the potential is fully dependent on the electron donating and/or withdrawing properties of the terminal ligand. One-electron reduction of the H.S. all-ferric thiocyanate complex, [Fe₃(μ₃-O)(μ-4-NO₂-pz)₆(SCN)₃]^2-, to its mixed-valent [Fe₃]^3- trianionic analogue triggers a cascade of H.S. to L.S. electronic reorganization. It is expected that terminal ligands with similar electron donating capacity like SCN (NCO and N₃) will exhibit similar behavior. The all-ferric complexes [Fe₃(μ₃-O)(μ-4-NO₂-pz)₆(NCO)₃]^2- (A) and [Fe₃(μ₃-O)(μ-4-NO₂-pz)₆(N₃)₃]^2- (B) have been synthesized by substitution of terminal Cl ligands of the parent compound [Fe₃(μ₃-O)(μ-4-NO₂-pz)₆Cl₃]^2- using NaNCO and NaN₃. Complexes A and B were characterized by single crystal X-ray diffraction, UV-Vis-NIR, ¹H-NMR, ⁵⁷Fe-Mössbauer and cyclic voltammetry. Expected reversible reduction processes were observed for both complexes at more cathodic potential, -0.70 V,  than the thiocyanate complex (-0.36 V). Additionally, compound 2 shows a unique reversible oxidation process at 0.82 V (vs. Fc⁺/Fc) to [Fe₃(μ₃-O)(μ-4-NO₂-pz)₆(N₃)₃]^-, a mixed-valent, formally Fe₂³⁺/ Fe⁴⁺ species.