Selective Guest Inclusion in Oxalate-Based Iron(III) Magnetic Coordination Polymers

The preparation and structural characterization of four novel oxalate-based iron(III) compounds of formulas {(MeNH₃)₂[Fe₂(ox)₂Cl₄]·2.5H₂O}_n (1), K(MeNH₃)[Fe(ox)Cl₃(H₂O)] (2), {MeNH₃[Fe₂(OH)(ox)₂Cl₂]·2H₂O}_n (3), and {(H₃O)(MeNH₃)[Fe₂O(ox)₂Cl₂]·3H₂O}_n (4) (MeNH₃⁺ = methylammonium cation and H₂ox = oxalic acid) are reported here. 1 is an anionic waving chain of oxalato-bridged iron(III) ions with peripheral chloro ligands, the charge balance being ensured by methylammonium cations. 2 is a mononuclear complex with a bidentate oxalate, three terminal chloro ligands, and a coordinated water molecule achieving the six-coordination around each iron(III) ion. Its negative charge is balanced by potassium(I) and methylammonium cations. 3 and 4 are made up of oxalate-bridged and either hydroxo (3)- or oxo-bridged (4) iron(III) chiral three-dimensional (3D) networks of formulas [Fe₂(OH)(ox)₂Cl₂]_n^n- (3) and [Fe₂O(ox)₂Cl₂]_n^2n- (4) with methylammonium (3 and 4) and hydronium (4) as counterions. The common point these compounds share is related to their synthetic strategy, which consists of the use of mixed alkaline/alkylammonium cations as templating agents for the growth of the 1D or 3D iron(III) motifs. Interestingly, even in the presence of any given alkaline cation in the reaction solutions, the resulting coordination polymers (1, 3, and 4) exclusively contain the methylammonium cation, revealing the highly selective character of the 1D and 3D networks. Furthermore, the isolation of the very unstable compound 1 could be only achieved in the presence of the KCl salt, suggesting a probable templating effect of the potassium(I) cations. Finally, a study of the variable-temperature magnetic properties of the 3D compounds 3 and 4 showed the occurrence of weak ferromagnetic ordering due to a spin canting, the value of the critical temperature (T_c) being as high as 70 K.