| Abstract
| - The condensation reaction between 2-pyridinecarboxaldehyde and diethylenetriamine, 3-[(2-aminoethyl)amino]propylamine, and 3,3‘-iminobis(propylamine) in a 2:1 molar ratio yields ligands that may be isolated exclusively inthe dissymmetric (cyclic) isomeric forms LA, LB/LB*, and LC. The template effect of a metal center (FeII, NiII,and ZnII)results in the ring opening of LC including one hexahydropyrimidine ring and one (long) propylene bridge. Theresulting symmetric bis-Schiff base isomeric form LC‘ is stabilized through pentacoordination, yielding [FeIILC‘(NCS)](NCS) (3), [NiIILC‘(NCS)](NCS) (6), and [ZnIILC‘(NCS)](NCS) (9). The same metal centers are too bulky to exert atemplate effect on LA including one imidazolidine ring and one (short) ethylene bridge. LA acts as a tetradentateligand yielding [FeIILA(NCS)2] (1), [NiIILA(NCS)2] (4), and [ZnIILA(NCS)2] (7). The template effect of the metal centeris selective toward the ligand LB/LB* including a hexahydropyrimidine (imidazolidine) ring and the shorter ethylene(longer propylene) bridge. The FeII cation is small enough to exert a template effect, resulting in the ring openingof LB/LB*. The resulting bis-Schiff base LB‘ is stabilized through pentacoordination, yielding [FeIILB‘(NCS)](NCS) (2).NiII is too bulky to promote the ring opening of LB/LB*: LB acts as a tetradentate ligand, yielding [NiIILB(NCS)2] (5)(the LB* isomer is totally converted to LB). The coordinative requirements and stereochemical preference of thebulkier ZnII cation allow neither the ring opening of LB/LB* nor the tetracoordination of LB or LB* but stabilize thenovel tetradentate dissymmetric form LB° in [ZnIILB°(NCS)2]·H2O (8) (LB° results from MeOH addition across theimine bond of LB). Density functional theory calculations performed for NiII and ZnII complexes of the LB/LB*/LB° setof ligands allowed one to compare the relative stabilities of all possible isomers, showing that the most stable onescorrespond to those experimentally obtained: isomerization, or methanol addition across the imine bond, of thetetradentate ligand depends on the relative stabilities of all possible isomeric complexes.
- FeII, NiII, and ZnII do not open the imidazolidine ring of LA, which acts as a tetradentate ligand. FeII, NiII, and ZnII open the imidazolidine ring of LC, which acts as a pentadentate ligand. FeII opens the hexahydropyrimidine ring of LB, while NiII does not: NiII is tetracoordinated to LB; the bis-Schiff base LB‘ is pentacoordinated to FeII; ZnII promotes MeOH addition across the LB imine bond, yielding LB°, which tetracoordinates ZnII.
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