Single-atom NCS bridging in binuclear complexes of Mn^II, Fe^II, Co^II, Ni^II, and Cu^IIof 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 MnII to CuII 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 MnII, FeII, CoII, and NiII with the ligand L1 and in complexes of CoII and NiII 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.