Metallatriangles and metallasquares: the diversity behind structurally characterized examples and the crucial role of ligand symmetry

Abstract

Reports on spontaneous self-assembly processes between metal fragments and organic ligands frequently tend to ignore the fact that the product isolated and structurally characterized in most cases is only one out of a more or less large series of feasible ones. This is true even for rings containing as few as three or four metal ions. Here we shall review metallatriangles and metallasquares containing predominantly cis-square-planar metal entities and a range of bidentate bridging ligands. The most significant features contributing to the number of possible stereoisomers appear to be ligand symmetry and flexibility, viz. rotation of two halves of a ligand about a single bond, or rotation of the whole ligand about the metal–donor atom bonds. With low-symmetry bidentate ligands the number of isomers increases dramatically with ring size as a consequence of an increase in possible connectivity patterns, hence linkage isomers, and an increase in possible rotamer states of the bridging ligands. In this tutorial review it is demonstrated how complexity increases as the symmetry of the bridging ligands is lowered from D_∞h and D_2h to C_∞v, C_2v, C_2h and C_s. Special attention will be paid to cyclic tri- and tetranuclear complexes of substituted pyrimidine ligands (C_2v and C_s symmetries) as well as the flexible 2,2′-bipyrazine, which can adopt states of either C_2v or C_2h symmetry. Uses of these complexes and ways to reduce the number of isomers will be pointed out.