Synthesis and Physical Properties of K4[Fe(C5O5)2(H2O)2](HC5O5)2·4H2O (C5O52– = Croconate): A Rare Example of Ferromagnetic Coupling via H-bonds

The reaction of the croconate dianion (C5O5)2– with a Fe­(III) salt has led, unexpectedly, to the formation of the first example of a discrete Fe­(II)–croconate complex without additional coligands, K4[Fe­(C5O5)2(H2O)2]­(HC5O5)2·4H2O (1). 1 crystallizes in the monoclinic P21/c space group and presents discrete octahedral Fe­(II) complexes coordinated by two chelating C5O52– anions in the equatorial plane and two trans axial water molecules. The structure can be viewed as formed by alternating layers of trans-diaquabis­(croconato)­ferrate­(II) complexes and layers containing the monoprotonated croconate anions, HC5O5–, and noncoordinated water molecules. Both kinds of layers are directly connected through a hydrogen bond between an oxygen atom of the coordinated dianion and the protonated oxygen atom of the noncoordinated croconate monoanion. A H-bond network is also formed between the coordinated water molecule and one oxygen atom of the coordinated croconate. This H-bond can be classified as strong–moderate being the O···O bond distance (2.771(2) Å) typical of moderate H-bonds and the O–H···O bond angle (174(3)°) typical of strong ones. This H-bond interaction leads 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 equivalent H-bonds. From the magnetic point of view, these connections lead to an S = 2 quadratic layer. The magnetic properties of 1 have been reproduced with a 2D square lattice model for S = 2 ions with g = 2.027(2) and J = 4.59(3) cm–1. This model reproduces quite satisfactorily its magnetic properties but only above the maximum. A better fit is obtained by considering an additional antiferromagnetic weak interlayer coupling constant (j) through a molecular field approximation with g = 2.071(7), J = 2.94(7) cm–1, and j = −0.045(2) cm–1 (the Hamiltonian is written as H = –JSiSj). Although this second model might still be improved since there is also an extra contribution due to the presence of ZFS in the Fe­(II) ions, it confirms the presence of weak ferromagnetic Fe–Fe interactions through H-bonds in compound 1 which represents one of the rare examples of ferromagnetic coupling via H-bonds.