Synthesis and physical properties of K 4[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.