Design of Redox-Active Cs⊂{Fe₄Ni₄} Cyanide-Bridged Cage Complexes Bearing Monodentate Ligands: Synthesis, Structure, and Electronic Properties

In recent years, strong efforts have been devoted to the design and crystallization of molecular models of the cyanido-bridged inorganic polymers known as “Prussian Blue Analogues” (PBA). In this work, we more specifically focus on soluble octametallic complexes that represent an elemental unit of the PBA cubic network. The self-assembly of tris-cyanido, the fac-[FeIII(Tp)(CN)3]− (Tp = hydro-tris(pyrazol-1-yl-borate) complex, and Ni(II) salts is shown to lead to {[Fe(Tp)(CN)3]4[Ni(S)3]4} procubes, where the Ni(II) ions are coordinated to labile solvent molecules (S: DMF or CH3CN). 1H and 133Cs paramagnetic NMR studies show that (i) the cubic complex is relatively stable during several days in solution, in particular in acetonitrile and (ii) a Cs+ ion can be inserted inside the cubic cavity as revealed by the presence of a strongly shifted 133Cs paramagnetic signal near −800 ppm at 300 K. The resulting Cs⊂{[Fe(Tp)(CN)3]4[Ni(S)3]4}5+ procube shows an enhanced stability as shown by time dependent 1H and 133Cs paramagnetic NMR spectra collected during several weeks. FT-IR spectra confirm the expected Fe(III) and Ni(II) redox states, with the occurrence of a single characteristic cyanide stretching vibration near 2170 cm–1. The Fe(III) redox state is also confirmed by cyclic voltammetry experiments in acetonitrile, with the occurrence of four pseudoreversible reduction events that can be assigned to the successive reduction of the FeIII ions in the Cs procube. Although these procubes could not be isolated as single crystals, the substitution of the labile coordinated solvent of the Cs-containing procube by N-donor imidazole monodentate ligands allows the crystallization of the novel Cs⊂{[Fe(Tp)(CN)3]4-[NiII(Im)3]4}Cl3 (Im = 1H imidazole) cubic complex by slow diffusion of ether into a DMF mother solution. The analyses of this cube demonstrate that in solution in ambient condition, Fe(III) centers are gradually reduced to Fe(II). The presence of three chlorides as counterions suggests that the cube can be isolated in the FeII2FeIII2NiII4 redox state. The presence of at least two Fe(II) ions is actually confirmed by cyclic voltammetry experiment in DMF and FT-IR spectroscopy. An intermediate state FeIIFeIII3NiII4 can be isolated by isolating fresh crystals (ca. 72 h) as demonstrated by FT-IR, cyclic voltammetry, and the magnetic measurements, that also reveal the presence of paramagnetic ferromagnetic interactions between the low-spin FeIII and NiII ions.