Iron(III), chromium(III) and cobalt(II) complexes with squarate: Synthesis, crystal structure and magnetic properties

The preparation and variable temperature-magnetic investigation of three squarate-containing complexes of formula [Fe2(OH)2(C4O4)2(H2O)4]·2H2O (1) [Cr2(OH)2(C4O4)2(H2O)4]·2H2O (2) and [Co(C4O4)(H2O)4]n (3) [H2C4O4 = 3.4-dihydroxycyclobutene-1,2-dione (squaric acid)] together with the crystal structures of 1 and 3 are reported. Complex 1 contains discrete centrosymmetric [Fe2(OH)2(C4O4)2(H2O)4] diiron(II) units where the iron pairs are joined by a di-μ-hydroxo bridge and two squarate ligands acting as bridging groups through adjacent oxygen atoms. Two coordinated water molecules in cis position complete the octahedral environment at each iron atom in 1. The iron–iron distance with the dinuclear unit is 3.0722(6) Å and the angle at the hydroxo bridge is 99.99(7)°, values which compare well with the corresponding ones in the isostructural compound 2 (2.998 Å and 99.47°) whose structure was reported previously. The crystal structure of 3 contains neutral chains of squarato-O1,O3-bridged cobalt(II) ions where four coordinated water molecules complete the six-coordination at each cobalt atom. The cobalt–cobalt separation across the squarate bridge is 8.0595(4) Å. A relatively important intramolecular antiferromagnetic coupling occurs in 1 whereas it is very weak in 2, the exchange pathway being the same [J = −14.4 (1) and −0.07 cm−1 (2), the spin Hamiltonian being defined as H^=-JS^1·S^2]. A weak intrachain antiferromagnetic interaction between the high-spin cobalt(II) ions occurs in 3 (J = −0.30 cm−1). The magnitude and nature of these magnetic interactions are discussed in the light of their respective structures and they are compared with those reported for related systems.

Moderate (1) and weak (2 and 3) antiferromagnetic interactions through the di-μ-hydroxo (1 and 2) and μ-O1,O3-squarato (3) are observed in the structurally characterized compounds [Fe2(OH)2(C4O4)2(H2O)4]·2H2O (1), [Cr2(OH)2(C4O4)2(H2O)4]·2H2O (2) and [Co(C4O4)(H2O)4]n (3).Figure optionsDownload as PowerPoint slideHighlights
► Di-μ-dihydroxo-di-μ-O1,O2-squarato as bridges between paramagnetic trivalent 3d metal ions.
► A uniform chain of high-spin cobalt(II) ions with squarato-O1,O3-as bridge.
► Moderate to weak antiferromagnetic coupling in homodinuclear and chain compounds.