Structure, Electrochemistry, and Magnetism of the Iron(III)-Substituted Keggin Dimer, [Fe₆(OH)₃(A-α-GeW₉O₃₄(OH)₃)₂]^11-

The iron(III)-substituted tungstogermanate [Fe₆(OH)₃(A-α-GeW₉O₃₄(OH)₃)₂]^11- (1) has been synthesized and characterized by IR, elemental analysis, SQUID magnetometry, electron paramagnetic resonance (EPR), and electrochemistry. Single-crystal X-ray analysis was carried out on Cs₄Na₇[Fe₆(OH)₃(A-α-GeW₉O₃₄(OH)₃)₂]·30H₂O, which crystallizes in the monoclinic system, space group C_2/m, with a = 36.981(4) Å, b = 16.5759(15) Å, c = 16.0678(15) Å, β = 95.311(3)°, and Z = 4. Polyanion 1 consists of two (A-α-GeW₉O₃₄) Keggin moieties linked via six Fe³⁺ ions, leading to a double-sandwich structure. The equivalent iron centers represent a trigonal prismatic Fe₆ fragment, resulting in virtual D_3h symmetry for 1. Electrochemistry studies revealed that 1 is stable in solution from pH 3 to at least pH 7. In pH = 3 media the reduction of the six Fe³⁺ centers was featured by a single voltammetric wave for most supporting electrolytes used. In that case, whatever the scan rate from 1000 mV·s^-1 down to 2 mV·s^-1, no splitting of the single Fe-wave of 1 was observed. The acetate medium induced a partial splitting of the wave, and this separation is enhanced with increasing pH. Remarkable efficiency of 1 in the electrocatalytic reduction of nitrite, nitric oxide, and nitrate is demonstrated. Magnetic susceptibility (χ) measurements indicate a diamagnetic (S_T = 0) ground state, with an average J = −12 cm^-1 and g = 2.00. EPR studies confirm that the ground state is indeed diamagnetic, since the EPR signal intensity steadily decreases without any line broadening as the temperature is lowered and becomes unobservable below about 50 K. The signal is a single broad peak at all frequencies (90−370 GHz), ascribed to the thermally accessible excited states. Its g_iso is 1.992 51, as expected for a high-spin Fe³⁺-containing species, and supports the χ data analysis.