Macrocyclic ligands ring contraction

Sapphyrins (39) are metallated with divalent metal aoetates in the presence of sodium acetate (Scheme 109).274 For the Zn11 and Co11 complexes, structure (44) was proposed with the dianionic macrocycle coordinating to the metal ion as a tetradentate ligand. These metallosapphyrins experience ring contraction to porphyrin complexes under mass spectral conditions. 

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... 

METAL ION-CONTROLLED SYNTHESIS AND RING CONTRACTION OF A CONJUGATED, PLANAR, HEXADENTATE, MACROCYCLIC LIGAND... 

This tendency of superphthalocyanine to contract to phthalocyanine may be attributed to the internal ring strain, the impaired macrocyclic n system, the inadequately large centre hole and the unstable coordination geometry. In (45a) the ligand ring cannot take a planar conformation but is buckled severely, and the uranium atom is centred in a distorted pentagonal bipyramidal structure with an average U—Ngp, distance of 2.52 A. 

Synthesis anti Ring Contraction of a Macrocyclic Ligand 173... 

As far as quadridentate ligands are concerned, the easiest attainment of the Ni state, denoted by the less positive redox potential, is observed with the 14-membered macrocycle, 5, known as cyclam. Both contraction and expansion of tetramine ring atomicity, to 12 and to 16, respectively, makes the potential more positive, thus destabilizing Ni with respect to Ni Moreover, the isomeric 14-membered macrocycle, 6 (isocyclam) stabilizes the Ni state to a much lesser extent than cyclam. Notice that the two macrocyclic complexes simply differ in the sequence of the five-... 

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