Co-facial dimer

The final category of binucleating ligands are those which incorporate more than one linkage between the cyclic subunits [see (118)]. The additional linkages between rings will tend to reduce the flexibility of the system and, for example, usually enable the relative positions of the metal centres to be controlled more precisely. The most common examples in this category have been termed co-facial dimers. 

Co-facial dimers consist of two simple macrocyclic systems connected by linkages such that the respective cyclic portions are constrained to stack one above the other. Such molecules define three cavities two within the macrocyclic subunits and a third between these subunits. 

The synthesis of a further group of co-facial dimers containing all-nitrogen donor atoms have been described (Busch, 1980). For example, Ni(n) species of type (142 with R = -(CH2)2-,-(CH2)2,-(CH2)4, m-xylene, or Cy-fluorene) have been isolated (Busch et al., 1981 Herron et al., 1983) these nickel dimers contain the metal ions in square-planar environments. 

Fig. 7.34 (a) Shish-kebab conducting polymers L-L = 4,4 -bipyridine or pyrazine. (b) Porphyrin co-facial dimers and possible discotic array of metal-metal bonding causing conduction. [From Collman et al. 1990.]... 

Figure 13 Molecular structure of a co-facial zinc porphyrin dimer with a xanthene spacer.784...

Homo- and bishomoallylic secondary and tertiary amines reacted with the Rh(l) complexes [Rh(/i-Cl)(CO)2]2, [Rh(/t-Gl)(CO)(CH2=CH2)]2, [Rh(/t-Cl)(CH2=CH2)2]2 to yield bidentate mono- or dimeric complexes. The facial selectivity of binding is influenced by the presence of stereogenic centers on the tether between the alkene and amine, as seen for complexes 337 and 338. ... 

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