FeII bistable materials with dissymmetrical ligands:: Synthesis, crystal structure, magnetic and Mossbauer properties of FeII complexes based on N4 Schiff bases possessing 2-pyridyl and 1-R-imidazol-2-yl rings

The synthesis and characterization of two symmetrical Fell complexes, [FeLB4(NCS)(2)], 1, and [FeLB5(NCS)(2)], 2 {L-Bx = N,N'-bis[(2-N-R-imidazol-1-yl)methylene]-2,2-dimethylpropane- 1,3-diamine ligands; 1: x = 4, R = H; 2: x = 5, R = Me} are reported. Whereas the tetradentate ligand L 14 is planar in 1, the X-ray structure of 2 illustrates the folded conformation of L-B5. Ferrous [FeLEx(NCS)21 complexes were synthesized (L-Ex = N-[(1-N-R-2 -imidazol-2-yl)methylenel-N'-(1-pyridin-2-yl-ethylidene)-2,2-R-1-propane-1,3-diamine; 3: x = 1, R-1 = H, R-1 = H; 4: x = 2, R-1 H, R-2 = Me; 5: x = 3, R1 = Me, R2 = H; 6: x = 4, R-1 = Me, R-2 Me}. These dissymmetrical Schiff bases, possessing pyridine and imidazole rings, are obtained through the reaction of 2-methyl-2-pyridin-2-yl-hexahydropyrimidine (R-1 = H; 3, 4) or 2,5,5-trimethyl-2-pyridin-2-yl-hexallydropyrimidine (R-1 = Me; 5, 6) aminals with imidazole2-carboxaldehyde (R-2 = H: 3, 5) or 1-N-methylimidazole-2carboxaldehyde (R-2 = Me: 4, 6) and iron(II) thiocyanate. Single-crystal X-ray structures were determined for [FeLE1(NCS)(2)] 3 [FeLE3(NCS)(2)] 5 and [FeLE4(NCS)(2)] 6; these complexes are in the low-spin state at 180 K. Magnetic susceptibility and Mossbauer measurements showed a smooth and incomplete spin crossover involving ca. 50% of the iron centres between 200 and 400 K for 5 and 6; 3 and 4 remain low spin up to 350 K; the slight chi MT increase between 350 and 400 K results from the onset of spin crossover for 3 and 4. The spin crossover properties in the [FeLEx(NCS)(2)] series may thus be tuned by the ligand substituents: by changing R-1 from H (for 3, 4) to Me (for 5, 6) the spin crossover is lowered from above 400 K to ca. 350 K; the effect of the imidazolyl substituent is weaker: by changing R-1 from H (for 5) to Me (for 6) the spin crossover temperature is lowered by ca. 35 K. The electronic effect of the highly conjugated Fe-N=C-C=N- imidazolyl fragment overpasses and qualitatively and quantitatively modifies the effects of R-1 and R-2.