Alkaline earth metal complexes Schiff bases

In contrast to the extensive coverage of transition metal Schiff base complexes, there is a somewhat desultory presentation of alkali and alkaline earth metal complexes of these ligands. 

The crystal structures of alkaline earth metal complexes of several (1 + 1) and (2 + 2) Schiff base macrocycles have been reported. These macrocycles are formed by the metal template-controlled condensation of the required heterocyclic dicarbonyl derivative and a, co functional diamine in alcoholic solution. (1 +1) complexes arise from the condensation reaction of one dicarbonyl with one diamine and (2 + 2) complexes from the condensation of two dicarbonyls with two diamines. 

The nitrogen atom of a Schiff base or oxime can coordinate to various metal ions (alkali metal, alkaline earth metal, transition metal, heavy metal ion). In particular, chelates such as salen (bis(salicylidene)ethylenediamine), saloph (bis(salicylidene)-o-phenylenediamine), and salamo (bis(salicylideneaminooxy)ethane) afford stable metal complexes, which are utilized for a variety of molecular functions such as catalysis of organic reactions, magnetism, luminescence, etc. °... 

Addition of acid leads to protonation of the sidearm N and the formation of a new hydrogen bonding leads to o -state in solution at relatively mild conditions - p/Cj value of 26a in 5.89 in acetonitrile [67]. The on-off process is truly reversible in ground state, showing that controlled ground state tautomerism is possible in azonaphthols and structurally similar Schiff bases [15, 66]. The same effect is observed upon alkali and alkaline earth metal ions addition when piperidine unit in 26a is replaced by an aza-15-crown-5 [68]. The o/i-state is stabilized through 1 1 complex formation and the excess of metal ion leads to deprotonation of the tautomeric ligand as well as to 2 2 complexation. 

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