Synthesis and Physical\nProperties of K₄[Fe(C₅O₅)₂(H₂O)₂](HC₅O₅)₂·4H₂O (C₅O₅^2– = Croconate): A Rare Example\nof Ferromagnetic Coupling via H-bonds

Abstract

The reaction of the croconate dianion (C₅O₅)^2– with a Fe­(III) salt has led, unexpectedly,\nto the formation of the first example of a discrete Fe­(II)–croconate\ncomplex without additional coligands, K₄[Fe­(C₅O₅)₂(H₂O)₂]­(HC₅O₅)₂·4H₂O (<b>1</b>). <b>1</b> crystallizes in the monoclinic <i>P</i>2₁/<i>c</i> space group and presents discrete octahedral\nFe­(II) complexes coordinated by two chelating C₅O₅^2– anions in the equatorial plane and two trans\naxial water molecules. The structure can be viewed as formed by alternating\nlayers of <i>trans</i>-diaquabis­(croconato)­ferrate­(II) complexes\nand layers containing the monoprotonated croconate anions, HC₅O₅^–, and noncoordinated water\nmolecules. Both kinds of layers are directly connected through a hydrogen\nbond between an oxygen atom of the coordinated dianion and the protonated\noxygen atom of the noncoordinated croconate monoanion. A H-bond network\nis also formed between the coordinated water molecule and one oxygen\natom of the coordinated croconate. This H-bond can be classified as\nstrong–moderate being the O···O bond distance\n(2.771(2) Å) typical of moderate H-bonds and the O–H···O\nbond angle (174(3)°) typical of strong ones. This H-bond interaction\nleads to a quadratic regular layer where each [Fe­(C₅O₅)₂(H₂O)₂]^2– anion is connected to its four neighbors in the plane through four\nequivalent H-bonds. From the magnetic point of view, these connections\nlead to an <i>S</i> = 2 quadratic layer. The magnetic properties\nof <b>1</b> have been reproduced with a 2D square lattice model\nfor <i>S</i> = 2 ions with <i>g</i> = 2.027(2)\nand <i>J</i> = 4.59(3) cm^–1. This model\nreproduces quite satisfactorily its magnetic properties but only above\nthe maximum. A better fit is obtained by considering an additional\nantiferromagnetic weak interlayer coupling constant (<i>j</i>) through a molecular field approximation with <i>g</i> = 2.071(7), <i>J</i> = 2.94(7) cm^–1,\nand <i>j</i> = −0.045(2) cm^–1 (the\nHamiltonian is written as <i>H</i> = –<i>JS_iS_j</i>). Although this second model might\nstill be improved since there is also an extra contribution due to\nthe presence of ZFS in the Fe­(II) ions, it confirms the presence of\nweak ferromagnetic Fe–Fe interactions through H-bonds in compound <b>1</b> which represents one of the rare examples of ferromagnetic\ncoupling via H-bonds.