Monotopic ligands

Balzani et al. (70) employed a strategy focusing on complexes as metals and as ligands (71,72), for developing a number of interesting supramolecular systems. Ditopic polypyridyl ligands, for example, 2,3- and 2,5-dipyridylpyrazine (2,3-dpp and 2,5-dpp), 2,2 -biquinoline (biq), tetrapyrido[3,2-fl 2, 3 -c 3",2"-/z 2", 3 "-/]-phenazine (73,74) (tppz), and 2,3,5,6-tetrakis(2-pyridyl)pyrazine (75-77) (tpypz), in combination with monotopic ligands (e.g., bpy and phen) were utilized in the preparation of a number of homo- and heteropolynuclear complexes of well-defined structures (Fig. 2). 

If K is the microscopic equilibrium constant for the association of the monotopic ligand A with B, the stepwise constants are K = 3K Ki =K Ks= K/3. The statistical factors are easily understood in the first equilibrium, three equivalent binding sites A are available for binding in the second, two equivalent binding sites A are available in the forward reaction but two equivalent bonds A—B can dissociate in the reverse reaction in the third, there are three equivalent bonds A—B which can dissociate in the reverse reaction. In general, for the interaction of a symmetrical m-topic ligand with a monotopic metalloporphyrin, the stepwise constants K( are given by Eq. 2, from which Eq. 3 is easily derived [20,29-31]. 

The simplest of these three architectures is the rack, which consists of a single, linear polytopic ligand and several monotopic ligands (Figure 3.10(a)). Naked metal ions act as linkers between the two types of ligand so that the resulting... 

In this way hosts 141-145 with both a cr-bonded Lewis acidic boron atom for complexation of anions and a conventional multidentate ligand for cations are generated. Complexation experiments of the 21-membered crown [6] boronate 142 with different potassium salts KX (X = F, Cl, Br, I, SCN, CN, OMe) indicate that there is a high specificity for the incorporation of KF, whereby F is bound covalently to the boron atom and is complexed by the crown ether (146, Fig. 39). An X-ray study has shown that the complexation of KF is heterotopic, i.e., both ions are complexed inside the same host. Some of the salts can only be bound in a monotopic way (KI and KSCN) [237]. 

---