AbstractThe synthesis and characterization of two symmetrical FeII complexes, [FeLB4(NCS)2], 1, and [FeLB5(NCS)2], 2 {LBx =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 LB4 is planar in 1, the X‐ray structure of 2 illustrates the folded conformation of LB5. Ferrous [FeLEx(NCS)2] complexes were synthesized {LEx = N‐[(1‐N‐R2‐imidazol‐2‐yl)methylene]‐N′‐(1‐pyridin‐2‐yl‐ethylidene)‐2,2‐R1‐propane‐1,3‐diamine; 3: x = 1, R1 = H, R2 = H; 4: x = 2, R1 = H, R2 = Me; 5: x = 3, R1 = Me, R2 = H; 6: x = 4, R1 = Me, R2 = Me}. These dissymmetrical Schiff bases, possessing pyridine and imidazole rings, are obtained through the reaction of 2‐methyl‐2‐pyridin‐2‐yl‐hexahydropyrimidine (R1 = H; 3, 4) or 2,5,5‐trimethyl‐2‐pyridin‐2‐yl‐hexahydropyrimidine (R1 = Me; 5, 6) aminals with imidazole‐2‐carboxaldehyde (R2 = H: 3, 5) or 1‐N‐methylimidazole‐2‐carboxaldehyde (R2 = 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 Mössbauer 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 χMT 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 R1 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 R2 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 R1 and R2. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)