Metal-organic coordination frameworks based on mixed methylmalonate and 4,4′-bipiridine ligands: synthesis, crystal structure and magnetic properties

Abstract

Five new complexes of formulae [M₂(4,4′-bpy)(Memal)₂X₂]_n [M = Fe(III) (2), Mn(II) (3), Co(II) (4), Ni(II) (5) and Zn(II) (6), and X = Cl⁻/OH⁻ (2) and H₂O (3–6); 4,4′-bpy = 4,4′-bipyridine and Memal = methylmalonate dianion] have been synthesized by following the previously reported procedure for [Cu₂(4,4′-bpy)(Memal)₂(H₂O)₂]_n (1). Moreover, two new phases of the Cu(II)/Memal/4,4′-bpy system, namely {[Cu(4,4′-bpy)₂][Cu(4,4′-bpy)₂(Memal)(NO₃)(H₂O)]}_n·nNO₃·3.5nH₂O (7) and [Cu(4,4′-bpy)₂(Memal)(H₂O)]_n·nH₂O (8), were obtained by varying the synthetic conditions. They were all structurally characterized by single crystal X-ray diffraction, and the magnetic properties of 2–5, 7 and 8 were investigated in the temperature range 1.9–295 K. 1–6 are isomorphous compounds whose structure consists of square grids of metal ions linked through anti–syn carboxylate bridges that grow in the crystallographic ac plane. These layers are pillared along the b axis by bis-monodentate 4,4′-bpy ligands to afford a [4⁴6⁶]-sqp three-dimensional net. Ferro- (1 and 5) and antiferromagnetic (2–4) interactions between the metal ions are mediated by the carboxylate bridge in the anti–syn conformation, the bis-monodentate 4,4′-bpy ligand being unable to transmit a significant magnetic coupling. The values of the magnetic coupling (J) for 2–5 are −0.269(3), −0.225(2), −0.05 and +0.272(3) cm⁻¹ respectively, the isotropic spin Hamiltonian being . Complexes 7 and 8 exhibit quite a different structure, as driven by the 4,4′-bpy groups. A square-grid of [Cu(4,4′-bpy)₂]²ⁿ⁺_n occurs in 7, which grows in the ab plane and is pillared through anti–syn carboxylate bridges from [Cu(Memal)(4,4′-bpy)₂(NO₃)(H₂O)] units along the c axis to build up a [4¹²6³]-pcu net. Analysis of the magnetic data for this compound shows an overall antiferromagnetic behaviour with the coexistence of ferro- and antiferromagnetic interactions. The structure of 8 consists of linear chains of copper(II) running along the c axis, where aquabis(4,4′-bipyridine)copper(II) units are connected by bis(monodentate) methylmalonate ligands. A significant intrachain antiferromagnetic interaction is observed in 8 through the extended Cu–OCCCO–Cu exchange pathway [J = −1.38(1) cm⁻¹]. The assembling role of the 4,4′-bpy coligand in 1–8 and in previous malonate-containing complexes is analyzed and discussed.