FeII Bistable Materials with Dissymmetrical Ligands: Synthesis, Crystal Structure, Magnetic and Mössbauer Properties of FeII Complexes Based on N4 Schiff Bases Possessing 2-Pyridyl and 1-R-Imidazol-2-yl Rings
✍ Scribed by Nicolas Bréfuel; Sergiu Shova; Jean-Pierre Tuchagues
- Publisher
- John Wiley and Sons
- Year
- 2007
- Tongue
- English
- Weight
- 251 KB
- Volume
- 2007
- Category
- Article
- ISSN
- 1434-1948
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✦ Synopsis
Abstract
The synthesis and characterization of two symmetrical Fe^II^ complexes, [FeL^B4^(NCS)~2~], 1, and [FeL^B5^(NCS)~2~], 2 {L^B__x__^ =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^B4^ is planar in 1, the X‐ray structure of 2 illustrates the folded conformation of L^B5^. Ferrous [FeL^E__x__^(NCS)~2~] complexes were synthesized {L^E__x__^ = N‐[(1‐N‐R^2^‐imidazol‐2‐yl)methylene]‐N′‐(1‐pyridin‐2‐yl‐ethylidene)‐2,2‐R^1^‐propane‐1,3‐diamine; 3: x = 1, R^1^ = H, R^2^ = H; 4: x = 2, R^1^ = H, R^2^ = Me; 5: x = 3, R^1^ = Me, R^2^ = 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‐hexahydropyrimidine (R^1^ = Me; 5, 6) aminals with imidazole‐2‐carboxaldehyde (R^2^ = H: 3, 5) or 1‐N‐methylimidazole‐2‐carboxaldehyde (R^2^ = Me: 4, 6) and iron(II) thiocyanate.Single‐crystal X‐ray structures were determined for [FeL^E1^(NCS)~2~] 3 [FeL^E3^(NCS)~2~] 5 and [FeL^E4^(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 χ~M~T increase between 350 and 400 K results from the onset of spin crossover for 3 and 4. The spin crossover properties in the [FeL^E__x__^(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^2^ 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^. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)