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]nn− (3) and [Fe2O­(ox)2Cl2]n2n− (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 (Tc) being as
high as 70 K.