Metallocycles - Rings, Squares and Related Species

A considerable number of large ring assemblies incorporating metal atoms as part of their ring structures are now known.  

With respect to the above, it has been recognised that enthalpy will tend to favour formation of a cyclic species over a corresponding linear product. This is because 

Other metallocycles assembled from two ligand molecules and two metal atoms have been reported by Harding et aV Bimetallic [2+2] macrocycles from the bis(bipyridyl) derivatives 3-7 and nickel(II), copper(I), silver(I), zinc(II) and cad-mium(II) have been reported. Products of this type may have either chiral, helical 

An extension of this work has resulted in the synthesis of related 6,6-disubsti-tuted (bipyridyl) ligand derivatives containing pyromellitimide spacers connected with propynyl, butynyl and propynyloxy linkers.The first of these ligands was shown to yield a chiral di-zinc(II) cationic complex of type [Zn2L2] . This species is stabilised by the presence of an aromatic guest p- or o-dimethoxybenzene) in its cavity. Related dinuclear species were also shown to form with cadmium(II) and nickel(II). 

Many trinuclear metallocyclic species are known. Amongst these, examples 

---