Spin crossover complex [FeL21](ClO4)2·H2O (L1 = 4-methyl-2,6-bis(1H-pyrazol-1-yl)pyrimidine): Synthesis and properties

The complex [FeL21](ClO4)2·H2O (1a) (L1 = 4-methyl-2,6-bis(1H-pyrazol-1-yl)pyrimidine) was synthesized by the reaction of Fe(ClO4)2·6H2O with new NNN-tridentate pyrazolylpyrimidine ligand L1 in MeOH/EtOH mixture. It is the first example of iron(II) spin crossover complexes with 2,4(6)-di(1H-pyrazol-1-yl)pyrimidines and manifests gradual spin transition centered around 150 K on the χMT versus T curve without hysteresis. A broad heat capacity anomaly with the maximum at 150 K was observed on the Cp(T) dependence, ΔtrsH = 5.50 kJ/mol and ΔtrsS = 42.0 J/(mol K). Surprisingly, the syntheses of iron(II) perchlorate compound with L1 performed in EtOH and i  PrOH afford another modification of the complex, [FeL21](ClO4)2·H2O (1b), being high spin in the 78–300 K temperature range. The complexes with 2,4-bis(3,5-dimethyl-1H-pyrazol-1-yl)-6-methylpyrimidine (L2), [FeL22](ClO4)2·EtOH (2), [FeL22](CF3SO3)2 (3), [FeL2Cl2] 0.5H2O (4) and [FeL2(NCS)2] (5) were synthesized in EtOH solutions. They are in the high spin state within 78–300 K temperature range.

The complex [FeL21](ClO4)2·H2O (L1 = 4-methyl-2,6-bis(1H-pyrazol-1-yl)pyrimidine) was synthesized as the first example of iron(II) spin crossover complexes with 2,4-di(1H-pyrazol-1-yl)pyrimidines. It manifests gradual spin transition centered around 150 K on the μeff(T) curve. A heat capacity anomaly with the maximum at 150 K was observed, ΔtrsH = 5.50 kJ/mol and ΔtrsS = 42.0 J/(mol K).Figure optionsDownload as PowerPoint slideHighlights
► Iron(II) complexes with 2,4-bis(1H-pyrazol-1-yl)pyrimidines.
► Spin crossover behavior and thermochromic effect.
► Heat capacity anomaly associated with spin crossover.
► How do synthetic conditions affect the magnetic behavior?