A high field and frequency electron paramagnetic resonance study of the S = 2 chromium(II) complex trans-[Cr(NCCH3)4(FBF3)2] and investigation of its reaction with dioxygen: Crystal structure of [(CH3CN)5CrOCr(NCCH3)5](BF4)4·2CH3CN

From high field and frequency electron paramagnetic resonance (EPR) powder spectra of the high spin chromium(II) complex trans-[Cr(NCCH3)4(FBF3)2] (1) recorded at T = 5–40 K the EPR parameters were found to be D = −2.19 cm−1, E ≈ 0 cm−1, and g‖ = g⊥ = 1.9833. The reaction between a MeCN solution of 1 and dioxygen gave dark green crystals of [(CH3CN)5CrOCr(NCCH3)5](BF4)4·2CH3CN (2). From the X-ray crystal structure of 2 at T = 122 K a Cr–O–Cr angle of 180° and a Cr–O distance of 1.7556(5) Å was found. This is the shortest among the known Cr–O distances in dinuclear complexes containing a CrOCr4+ core, and this can partly explain the strong antiferromagnetic coupling between the two chromium(III) centers in 2.