Conformational change of the N,N′-bis(3-aminopropyl)oxamidate ligand in nuclearity tailoring copper(II) complexes

Abstract

N,N′-Bis(3-aminopropyl)oxamide (H₂L¹) which can adopt cis and trans conformations reacted with copper(II) salts in its deprotonated form to yield the compounds [CuL¹]1, [Cu₃L₂¹(NO₃)₂]·H₂O 2, [Cu₂L¹(NO₃)₂]3 and [Cu₂L¹(O₂CMe)₂]·2H₂O 4. The structures of 1, 3 and 4 have been determined by single-crystal X-ray diffraction methods. That of 1 consists of mononuclear [CuL¹] units where the oxamidate is in a cis conformation co-ordinated to the copper atom through its four nitrogen atoms. The copper surroundings are close to square planar. The amidate–copper bond distance [1.955(6)Å] is significantly shorter than that involving the amine group [1.999(7)Å]. The structures of 3 and 4 comprise centrosymmetric trans oxamidate-bridged copper(II) dinuclear units which are linked by asymmetric bis(monodenate) nitrate (3) and acetate (4) ligands to yield a sheet-like polymer (3) and an alternating chain (4). The co-ordination geometry around each copper atom is distorted square pyramidal: the equatorial plane comprises the oxygen and nitrogen atoms of the amide, the nitrogen atom of the amine group and an oxygen atom from nitrate (3) or acetate (4) ligands. The apical position is filled by an oxygen atom from another nitrate (3) or acetate (4) group. The copper–copper separations across the oxamidate are 5.190(1) and 5.244(1)Å for 3 and 4, respectively, and those across the nitrato and acetato groups are 5.116(1) and 3.350(1)Å. Variable-temperature magnetic susceptibility measurements show a Curie law behaviour for 1 and the occurrence of a strong antiferromagnetic coupling through the oxamidate bridge in 2–4(J=–325, –393 and –305 cm^–1, respectively). The versatility of L¹ as a ligand and its ability to mediate strong antiferromagnetic interactions is analysed and discussed.