1.10- Phenanthroline steric effects

Chelate complexes. Chelate rings can be formed by pyridines containing -substituents such as carboxyl or CH=NR. Important bicyclic chelating agents are 2,2-bipyridyl 68 (R = H), o-phenanthroline 69 and 8-hydroxyquinoline 70, which all form bis and tris complexes with many metals <1994CSR327>. This type of complex formation has many analytical applications. Overlap between the d-orbitals of the metal atom and the pyridine -orbitals is believed to increase the stability of many of these complexes. Steric effects can hinder complex formation as in 68 (R = Me). 

Of the aromatic amines and diamines, pyridine, 1,10-phenanthroline and neocuproine form moderate active systems with Ru3(CO)i2. Exudation of turnover frequencies in Table 7.7 reveals that a steric effect may instead be operative in the Ru3(CO)i2-amine system, since the rates follow the order pyrrolidine > piperidine, (CH3>2NCH2CH20H > (C2H5>2NCH2CH20H and pyridine > quinoline. 

First, let us discuss the shapes of the chelated complexes. In most cases they are tetrahedral, but iron (both ferrous and ferric) forms octahedral complexes. However, the most remarkable contribution to selectivity is made by cupric ions which form planar complexes. This means that copper is uniquely sensitive to a type of steric effect imposed by bulky groups. Thus, in Table 11.2, the stability constants for the cupric complexes ofbipyridyl [11,19), phenanthroline [11.18), and folic acid lie below those of the corresponding nickel complexes, which is contrary to the natural order discussed in Section 11.2. 

In these complexes, the phenanthroline is tilted out of the normal square plane because of the steric effect of the CH3 substituents, and the latter are below this plane in the structures shown in reaction (4.7). One of these substituents is distinguished by an asterisk to illustrate that they are not equivalent. In the H NMR, the signals from these CH3 groups coallesce at... 

Fig. 1.7. The interactions of optical isomers of complexes of phenanthroline, [M(phen)3 showing the differences in steric effects upon binding to DNA. From Reference 66.

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