Crystal growth and physical properties of an antiferromagnetic molecule: Trans-dibromidotetrakis(acetonitrile)chromium(III) tribromide, [CrBr₂(NCCH₃)₄](Br₃)

The synthesis, crystal structure determination, magnetic properties and bonding interaction analysis of a novel 3d transition-metal complex, [CrBr2(NCCH3)4](Br3), are reported. Single-crystal X-ray diffraction results show that [CrBr2(NCCH3)4](Br3) crystallizes in space group C2/m (No. 12) with a symmetric tribromide anion and the powder X-ray diffraction results show the high purity of the material specimen. X-ray photoelectron studies with a combination of magnetic measurements demonstrate that Cr adopts the 3+ oxidation state. Based on the Curie-Weiss analysis of magnetic susceptibility data, the Néel temperature is found to be around 2.2K and the effective moment (μeff) of Cr3+ in [CrBr2(NCCH3)4](Br3) is ∼3.8μB, which agrees with the theoretical value for Cr3+. The direct current magnetic susceptibility of the molecule shows a broad maximum at ∼2.3K, which is consistent with the theoretical Néel temperature. The maximum temperature, however, shows no clear frequency dependence. Combined with the observed upturn in heat capacity below 2.3K and the corresponding field dependence, it is speculated that the low-temperature magnetic feature of a broad transition in [CrBr2(NCCH3)4](Br3) could originate from a crossover from high spin to low spin for the split d orbital level low-lying states rather than a short-range ordering solely; this is also supported by the molecular orbital diagram obtained from theoretical calculations.