Rhenium(IV)-Copper(II) Heterobimetallic Complexes with a Bridge Malonato Ligand. Synthesis, Crystal Structure and Magnetic Properties

The Re(IV) complex [ReCl (mal)]2-, in the form of two slightly different salts, (AsPh4)1.5(HNEt3)0.5[ReCl4(mal)] (1a) and (AsPh4)(HNEt3)[ReCl4(mal)] (1b), and the Re(IV)-Cu(II) bimetallic complexes [ReCl4(m-mal)Cu(phen)2]‚CH3CN (2), [ReCl4(Ì-mal)Cu(bpy)2] (3), and [ReCl4(m-mal)Cu(terpy)] (4) (mal = malonate dianion, AsPh4 = tetraphenylarsonium cation, HNEt3 = triethylammonium cation, phen = 1,10-phenanthroline, bpy = 2,2'-bipyridine and terpy = 2,2':6',2''-terpyridine) have been synthesized and the structures of 1a, 1b, 2, and 3 determined by single-crystal X-ray diffraction. The structures of 1a and 1b are made up of discrete [ReCl4(mal)]2- anions and AsPh4+ and HNEt3+ cations, held together by electrostatic forces and hydrogen bonds. The Re(IV) atom is surrounded by four chloride anions and a bidentate malonate group, in a distorted octahedral environment. The structure of 2 consist of neutral dinuclear units [ReCl4(m-mal)Cu(phen)2], with the metal ions united through a bridge carboxilato. The environment of Re(IV) is nearly identical to that in the mononuclear complex, and Cu(II) is five coordinate, being surrounded by four nitrogen atoms of two bidentate phen ligands and one oxygen atom of the malonato ligand. In 3, there are also dinuclear units, [ReCl4(m-mal)Cu(bpy)2], but the Cu(II) ions complete a distorted octahedral coordination by binding with the free malonato oxygen atom of a neighbor unit, resulting in an infinite chain. The magnetic properties of 1-4 were also investigated in the temperature range 2.0-300 K. The magnetic behavior of 1a and 1b is as expected for a Re(IV) complex with a large value of the zero-field splitting (2D ca. 110 cm-1). For the bimetallic complexes, the magnetic coupling between Re(IV) and Cu(II) is antiferromagnetic in 2(J = -0.39 cm-1), ferromagnetic in 4(J = +1.51 cm-1), and nearly negligible in 3 (J = -0.09 cm-1).