Complexation to Fe^II, Ni^II, and Zn^II of Multidentate Ligands Resulting from Condensation of 2-Pyridinecarboxaldehyde with α,ω-Triamines: Selective Imidazolidine/Hexahydropyrimidine Ring Opening Revisited

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 in the 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. The resulting 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 a template effect on LA including one imidazolidine ring and one (short) ethylene bridge. LA acts as a tetradentate ligand yielding [FeIILA(NCS)2] (1), [NiIILA(NCS)2] (4), and [ZnIILA(NCS)2] (7). The template effect of the metal center is 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 opening of 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 the bulkier ZnII cation allow neither the ring opening of LB/LB* nor the tetracoordination of LB or LB* but stabilize the novel tetradentate dissymmetric form LB° in [ZnIILB°(NCS)2]·H2O (8) (LB° results from MeOH addition across the imine bond of LB). Density functional theory calculations performed for NiII and ZnII complexes of the LB/LB*/LB° set of ligands allowed one to compare the relative stabilities of all possible isomers, showing that the most stable ones correspond to those experimentally obtained: isomerization, or methanol addition across the imine bond, of the tetradentate ligand depends on the relative stabilities of all possible isomeric complexes.