Expanding the Family of Pyrazole-Bridged Mixed-Spin and Mixed-Valence Tetranuclear [2 x 2] Iron Grid Complexes

The new bis(tridentate) compartmental proligand 3,5-bis[6-(1-methyl-1H-imidazole-2-yl)pyrid-2-yl]pyrazole (HLim) was synthesized and used for the self-assembly of a rugged tetranuclear [2 x 2] Fe-4 grid complex [(Fe4L4im)-L-II](ClO4)(4) (1), which readily forms upon reaction of HLim with Fe(ClO4)(2)center dot 6H(2)O in the presence of a base (NaOtBu). According to cyclic voltammetry, all-ferrous 1 can be sequentially oxidized in four steps, with pronounced stability of the twice-oxidized dimixed-valent form [(Fe2Fe2L4im)-Fe-II-L-III](6+). Treatment of 1 with the oxidant AgClO4 yielded the dimixed-valent molecular grid [(Fe2Fe2L4im)-Fe-II-L-III](ClO4)(6) (2) and an unusual heterometallic grid [Fe-2(III)(Ag-2(I))(2)L-4(im)](ClO4)(6). (3), depending on reaction conditions. 3 can also be synthesized via the intermediate "corner complex" [Fe-II(HLim)(2)](ClO4)(2). Complexes 1, 2 and 3 were characterized by single-crystal X-ray diffraction analysis as well as by SQUID magnetometry and Mossbauer spectroscopy. 1 is shown to adopt a dimixed-spin trans-[HS-LS-HS-LS] configuration (HS: high-spin; LS: low-spin) while 2 features ferromagnetic coupling in a trans-[Fe-III(LS)-Fe-II(HS)-Fe-III(LS)-Fe-II(HS)] configuration. Correlations between structural parameters and magnetic properties are analyzed and discussed in comparison with previously reported Fe-4 grids based on related pyrazole-bridged ligands.