Conformer bimetallic complex

The main path to the formation of the bimetallic complex is reaction (4.8). A variable-temperature thermodynamic study of the equilibria involved in the complex formation demonstrates that the energetic drive for the formation of both 1 1 and 2 1 complexes is in fact provided by triethylamine which captures the protons released in the process. Taking the presence of NEt3 into account leads to log Kj, = A (AG (3) = —23 kJmol-1) and to log K = 5 (AG (5) = —29 kJ mol-1). The larger value of K with respect to K points to a cooperative effect induced by the change in the conformation of the macrocycle. 

The most famous example of a quadruply bound bimetallic complex is [Re2(Cl)8] , whose structure inspired Cotton to propose, for the first time, the existence of a 5 bond, in addition to a and tt bonds, between two metallic centres. The Re-Re distance is very short (2.24 A), a value close to that found in the complex [Re2(Cl)8(H20)2] already studied above most strikingly, the complex is observed to have an eclipsed structure (4-49a) rather than the staggered conformation (4-49b) that would have been expected. It is also important to note that this complex is diamagnetic. 

Fig. 3 Conformational flexibility of bimetallic complexes containing the bis(l-pyrazolyl) methane ligand donor...

The crown ether bridge in calix[4]arene 19 freezes the calixarene stmcture in a rigid flattened-cone conformation also pushing the two metal ions apart. The bimetallic complex 19—Zu2 cleaves HPNP eight times less effectively than the flexible 2—Zu2 [17], thus confirming that a certain degree of flexibility is beneficial and is an important requisite in a supramolecular catalyst. 

If a molecule has only an inversion centre present it has Q symmetry. Examples of this point group are less common, but substituted ethanes such as (15,2/ )-l,2-dichloro-1,2-difluoroethane (Figure 3.5a) can take on conformations with C, symmetry, and some bimetallic complexes, such as the general structure shown in Figure 3.5b, also fall into this classification. 

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