Macrocyclic ligands steric constraints

More complex organic anions and polyanions (oxalate ", malonate ", succinate ", etc.), and phosphates (ATP , ADP , AMP ", etc.) also form complexes with macrocyclic ligands. Steric constraints imposed by incorporation of the larger, more complex ions into macropolycyclic ligands lead to marked selectivity patterns. - "-" ... 

Chelate ring formation may be rate-limiting for polydentate (and especially macrocyclic) ligand complexes. Further, the rates of formation of macrocyclic complexes are sometimes somewhat slower than occur for related open-chain polydentate ligand systems. The additional steric constraints in the cyclic ligand case may restrict the mechanistic pathways available relative to the open-chain case and may even alter the location of the rate-determining step. Indeed, the rate-determining step is not necessarily restricted to the formation of the first or second metal-macrocycle bond but may occur later in the coordination sequence. 

Steric constraints dictate that reactions of organohalides catalysed by square planar nickel complexes cannot involve a cw-dialkyl or diaryl Ni(iii) intermediate. The mechanistic aspects of these reactions have been studied using a macrocyclic tetraaza-ligand [209] while quantitative studies on primary alkyl halides used Ni(n)(salen) as catalyst source [210]. One-electron reduction affords Ni(l)(salen) which is involved in the catalytic cycle. Nickel(l) interacts with alkyl halides by an outer sphere single electron transfer process to give alkyl radicals and Ni(ii). The radicals take part in bimolecular reactions of dimerization and disproportionation, react with added species or react with Ni(t) to form the alkylnickel(n)(salen). Alkanes are also fonned by protolysis of the alkylNi(ii). 

This is another result that reveals the peculiarity of the corrole ligand with respect to other macrocycles similarly substituted porphyrins have very different properties. Despite the presence of a direct pyrrole-pyrrole link then, corrole seems to be less sensitive to steric constraints than porphyrin. The crystal structure of Co(OMTPC)PPh3 (see Sect. 4.1) confirms this analysis. 

---