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.

本文报道了四种新型草酸盐基三价铁（iron(III)）化合物的合成与结构表征，其化学式分别为{(MeNH3)2[Fe2(ox)2Cl4]·2.5H2O}n（1）、K(MeNH3)[Fe(ox)Cl3(H2O)]（2）、{MeNH3[Fe2(OH)(ox)2Cl2]·2H2O}n（3）以及{(H3O)(MeNH3)[Fe2O(ox)2Cl2]·3H2O}n（4），其中MeNH3+为甲胺阳离子（methylammonium cation），H2ox为草酸（oxalic acid）。
化合物1为草酸根桥联三价铁离子构成的阴离子波浪链，外围辅以氯配体，电荷由甲胺阳离子平衡。
化合物2为单核配合物，含一个双齿草酸根、三个端基氯配体与一个配位水分子，每个三价铁离子均为六配位结构；其负电荷由钾(I)阳离子与甲胺阳离子共同平衡。
化合物3与4均为草酸根桥联且分别以羟桥（3）、氧桥（4）连接三价铁离子的手性三维（3D）骨架网络，其化学式分别为[Fe2(OH)(ox)2Cl2]nn−（3）与[Fe2O(ox)2Cl2]n2n−（4）；抗衡离子分别为甲胺阳离子（3和4）以及水合氢离子（hydronium，4）。
上述四种化合物的合成策略具有共性：均采用混合碱金属/烷基铵阳离子作为模板剂，用于调控一维（1D）或三维三价铁结构基元的生长。
值得注意的是，即便反应体系中存在其他碱金属阳离子，最终得到的配位聚合物（1、3与4）仅包含甲胺阳离子，这体现出该类一维与三维骨架网络具有极高的结构选择性。
此外，极不稳定的化合物1仅能在氯化钾（KCl）存在下分离得到，这表明钾(I)阳离子可能起到了模板作用。
最后，对三维化合物3与4的变温磁性质研究表明，由于自旋倾斜效应，二者呈现弱铁磁有序，其临界温度（Tc）高达70 K。