Macrocyclic ligands corrin

Macrocyclic Ligands (Corrins and Corroles) (see also Table 17.10)... 

A macrocyclic ligand called the corrin ring, which bears various substituents. It resembles the porphine ring, but differs in various ways, notably in the absence of one methine, =CH, bridge between a pair of pyrrole rings. 

Porphyrins, Phthalocyanines, Corroies and Related Macrocyclic Ligands 21.1.3 CORROLES AND CORRINS... 

Several macrocyclic ligands are shown in Figure 2. The porphyrin and corrin ring systems are well known, the latter for the cobalt-containing vitamin Bi2 coenzymes. Of more recent interest are the hydroporphyrins. Siroheme (an isobacteriochlorin) is the prosthetic group of the sulfite and nitrite reductases which catalyze the six-electron reductions of sulfite and nitrite to H2S and NH3 respectively. The demetallated form of siroheme, sirohydrochlorin, is an intermediate in the biosynthesis of vitamin Bi2, and so links the porphyrin and corrin macrocycles. Factor 430 is a tetrahydroporphyrin, and as its nickel complex is the prosthetic group of methyl coenzyme M reductase. F430 shows structural similarities to both siroheme and corrin. 

Another peculiarity of corrole is that, among tetradentate N4 macrocyclic ligands, it completes the series corrin, porphyrin, corrole, where one, two or three amino-like nitrogens are present in the chelating system. 

In such a large subject, this article can only focus on certain aspects, namely those that involve complexation with inorganic substrates. We only consider the synthetic macrocycles, with emphasis on transition metal complexation. Aza, oxa, and, to a lesser extent, thia and phospha macrocycles are also covered. The naturally occurring porphyrins, corrins, corphins, chlorins, and phthalocyanins, as well as the cyclodextrins, are not included. Because of the general complexity of macrocyclic systems and the resulting complicated systematic names, commonly used abbreviations or simplified names will be employed. This review will encompass the synthesis, thermodynamics, structure, and applications of macrocyclic ligands. 

Macrocyclic ligands are defined as cyclic molecules generally consisting of organic frames into which heteroatoms, capable of binding to substrates, have been interspersed. Some reports of synthetic macrocycles (as opposed to the naturally occurring species such as porphyrins, corrins, and... 

There are many other ways, other than the cis effect, that auxiliary ligands can affect the rates of ligand-replacement reactions. Many of these effects are not well imderstood, however. For example, the normally stable Co(III) center m vitamin undergoes fast substitutions that are influenced by the axial 5,6-dimethylbenzimidazole base appended to the tetradentate corrin macrocyclic ligand. Replacement of an axial HjO by CN" in (HjO)Bj, occurs on a millisecond time scale, " and these substitutions are even faster than those of the well-studied B model complexes. " ... 

The macrocycle types discussed so far tend to form very stable complexes with transition metal ions and, as mentioned previously, have properties which often resemble those of the naturally occurring porphyrins and corrins. The complexation behaviour of these macrocycles contrasts in a number of ways with that of the second major category of cyclic ligands - the crown polyethers. 

Initial cyclizadons were effected by the addition of an enamine to an imidate ester, both groups being suitably located by ligand coordination.263 An analogous process can be carried out on a thioimidate but a sulfide is formed and removal of sulfur with consequent ring contraction yields the corrin (100).264 These two complementary routes can be effected with different metal ions, nickel(II), palladium(II) and cobalt(III) in the first route, zinc(II) in the second. Removal of zinc ions easily provides the free corrin macrocycle. These two routes are summarized in Scheme 64. The sulfide contraction route was used in the Eschenmoser-Woodward total synthesis of vitamin Bn-265... 

It is of interest to compare the porphyrin and corrin macrocycles, and to consider the reactivities of porphyrin- and corrin-bound metal ions. These may be related to the size of the cavity. The corrin macrocycle provides a smaller cavity, into which cobalt fits tightly in the B12 coenzymes. Thus the cobalt does not move relative to the corrin plane, and remains coplanar with the four nitrogen donor atoms. The cobalt remains low-spin, and reactivity is controlled by the loss of the axial ligands, giving five- and four-coordinate species. 

Despite the similarities existing among natural tetrapyrrolic macrocycles, e.g. they are all tetradentate equatorial N4 ligands, structural variations cause a fine modulation of the reactivity of the central metal atom. Thus it is most probably the corrin skeleton that enables the cobalt atom of vitamin B12 to carry out reactions impossible for a similar cobalt porphyrinate. 

The size of the macrocycle coordination hole has a profound influence on its coordination chemistry thus in the weak ligand field of hydroporphyrinoid systems Ni2 + assumes the high spin state and binds extra axial ligands, while in the stronger ligand field of the corrin macrocycle the coordination sphere of the metal is saturated and diamagnetic complexes are generated [82]. 

As tetradentate chelate ligands that, after deprotonation, carry a single (corrin) or double negative charge, the tetrapyrrole can bind even highly labile metal ions. The complex can dissociate only if all metal ligand bonds are broken at the same time (cf. chelate and macrocyclic effects). 

Corroles are tetrapyrrole macrocycles that are closely related to porphyrins, with one carbon atom less in the onter periphery and one NH proton more in their inner core. They may also be considered as the aromatic version (identical skeleton) of the only partially conjugated corrin, the cobalt-coordinating ligand in Vitamin B. Two potential application of corroles are in tumor detection and their use in photovoltaic devices. Selective snbstitntion of corroles via nitration, hydroformylation, and chlorosulfonation for the gallinm were studied in detail and the respective mechanistic pathways and spectroscopic data were reported, (an example is shown in Fignre 27). Overall, over 139 varions corroles were synthesized and the effect of various metal complexation pertaining to their selective reactivity examined. ... 

---