Chromium coordination compounds with bis(3,5-dimethylpyrazol-1-yl)acetic acid or its anion

Two chromium(III) coordination compounds synthesized from a solution of the tridentate (NON) ligand bis(3,5-dimethylpyrazol-1-yl)acetic acid (HL = Hbdmpza), namely [Cr(L)2](H(NO3)2) (1) and [Cr(HL)Cl3] (2), are reported. The centrosymmetric central coordination sphere in compound 1 is fulfilled by two deprotonated ligands L− giving sterically closed coordination shell with pink-orange colored CrO2N4 chromophore. The coordination cation is accompanied by the dinitratohydrogenate(1-) counter anion. Contrary to compound 1, the compound 2 shows only one Hbdmpza ligand, which is protonated HL, though with the same three coordinate N,O,N atoms. Both title compounds 1 and 2 were isolated from the acetonitrile solution with the starting counter-anion remaining in the final product as a new counter anion H(NO3)2− in 1, or the coordinated chloride anions in 2 with green colored CrON2Cl3 chromophore. We were able to isolate the single crystals of 1, while only polycrystalline material for 2. Nevertheless, the crystal structure of 2 containing two octahedral coordination molecules in the asymmetric unit was determined via the combination of DFT calculations and the powder XRD analysis. Though both compounds reveal a mononuclear octahedral coordination motif, the catalytic accessibility of the compound 2, due to only one tridentate ligand L (HL) per one Cr(III) centre clearly promises more to this aspect.

Graphical abstractTwo new chromium(III) compounds with a neutral ligand bis(3,5-dimethylpyrazol-1-yl)acetic acid or its anionic form (HL/L−), namely [Cr(L)2](H(NO3)2) and [Cr(HL)Cl3] are presented. Both reveal a mononuclear octahedral coordination motif, but the catalytic expectations differ significantly. The neutral compound by one tridentate HL per one Cr(III) and labile chloride ligands is clearly more promising than the ionic compound with two L− fully surrounding the coordination sphere.Figure optionsDownload full-size imageDownload as PowerPoint slide