Single-atom NCS bridging in binuclear complexes of MnII, FeII, CoII, NiII, and CuII of macrocyclic 20- and 22-membered ligands: a spectroscopic, crystallographic, and molecular mechanics study

Abstract

Using a set of macrocyclic ligands 8,12;21,25-dinitrilo-1,6,14,19-tetra-azacyclohexacosa-1 (26),6,8,10,13,19,22,24-octaene (L¹), 7,11; 19,23-dinitrilo-1,5,13,17-tetra-azacyclotetracosa-1 (24),5,7,9,12,17,20,22-octaene (L²), and 7,10;18,21 -diepoxy-1,5,12,16-tetra-azacyclodecosa-1 (22),5,7,9,11,16,18,20-octaene (L³), NCS bridged binuclear complexes of all five members of the first transition series from Mn^II to Cu^II have been obtained. The observation of strong i.r. absorption below 2 000 cm^–1 shows that the NCS^– ligand adopts the rare NCS bridging mode in binuclear tetrakis- or bis-(thiocyanato) complexes of Mn^II, Fe^II, Co^II, and Ni^II with the ligand L¹ and in complexes of Co^II and Ni^II with L². With L³, a NCS-N link between copper(II) ions is seen, the first observation of a truly symmetric Cu–N(CS)–Cu assembly. These structures have been investigated by molecular mechanics which confirms that the steric constraints of the macrocycles are responsible for the structure variations. Magnetic susceptibility measurements show that the NCS-N bridge is a poor mediator of magnetic interaction between paramagnetic centres, as compared with N₃^–-N or OH^– links. The structures of the precursor compounds [BaL¹(ClO₄)₂] and [BaL²(ClO₄)₂]·EtOH have been determined by X-ray crystallography.