Structure, spectroscopy, stability, and bridge exchange in the M3O4 incomplete-cubane complexes, [M(III)3(Sal-AHA)3(μ-OR)]− (M = Fe, Ga)

The [Ga(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]− anion crystallizes with one K+ and one H3O+ counter ion between each pair of anions. The structure is analogous to the previously reported [Fe(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]−, with each containing a M3O4 incomplete cubane at the base of a phenyl-derived pocket that surrounds a triply bridging methoxy group. The EPR spectrum of [Fe(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]− at 4.5 K is complex with features near geff = 2, 4.3, and above 5. Only the 4.30 feature maintains intensity much above 30 K, and it persists to above 150 K. Room temperature magnetic susceptibility measurements indicate an average spin of S = 5/2 per trimer in both solid and solution. Both the Ga and Fe clusters are stable in methanol. However, the Ga complex slowly decomposes in 40% water or in the presence of EDTA, while the Fe cluster does not. Experiments with a mixed Fe/Ga solution show that the order of stability of the [M(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]−, clusters in the presence of EDTA is M(III)3 = Fe3 > Fe2Ga > FeGa2 > Ga3. Metal or chelate exchange occurs very slowly (weeks to months) if at all. However, the μ3-methoxy bridge can be replaced by deuterated methoxy or a μ3-hydroxy bridge within hours to days.

The structures and stability of the analogous [M(III)3(3,5-diCl-Sal-AHA)3(μ-OCH3)]− complexes with Fe and Ga are compared. The Fe(III)3O4 cubane has an S = 1/2 ground state based on the EPR spectra. The clusters are stable to metal or chelate exchange, while the μ3-methoxy bridge can be exchanged with hydroxide.Figure optionsDownload as PowerPoint slide