Interaction of Chromium(III) with a <i>N</i>,<i>N</i>′‑Disubstituted Hydroxylamine-(diamido) Ligand: A Combined Experimental and Theoretical Study

Reaction of hydroxylamine hydrochloride with prop-2-enamide in dichloromethane in the presence of triethylamine resulted in the isolation of the <i>N</i>,<i>N</i>′-disubstituted hydroxylamine-(diamido) ligand, 3,3′-(hydroxyazanediyl)­dipropanamide (Hhydia). The ligand Hhydia was characterized by multinuclear NMR, high-resolution electrospray ionization mass spectrometry (ESI-MS), and X-ray structure analysis. Interaction of Hhydia with <i>trans</i>-[Cr^IIICl₂(H₂O)₄]­Cl·2H₂O in ethanol yields the ionization isomers [Cr^III(Hhydia)₂]­Cl₃·2H₂O­(<b>1</b>·2H₂O) and <i>cis/trans-</i>[Cr^IIICl₂(Hhydia)₂]­Cl·2H₂O (<b>2</b>·2H₂O). The X-ray structure analysis of <b>1</b> revealed that the chromium atom in [Cr^III(Hhydia)₂]³⁺ is bonded to two neutral tridentate <i>O</i>,<i>N</i>,<i>O</i>-Hhydia ligands. The twist angle, <i>θ,</i> in [Cr^III(Hhydia)₂]³⁺ is 54.5(6)⁰, that is, very close to an ideal octahedron. The intramolecular hydrogen bonds developed between the N–OH group of the first ligand and the amidic oxygen atom of the second ligand and vice versa contribute to the overall stability of the cation [Cr^III(Hhydia)₂]³⁺. The reaction rate constant of the formation of Cr­(III) complexes <b>1</b>·2H₂O and <b>2</b>·2H₂O was found to be 8.7(±0.8) × 10^–5 M^–1 s^–1 at 25 °C in methyl alcohol and follows a first-order law kinetics based on the biologically relevant ligand Hhydia. The reaction rate constant is considerably faster in comparison with the corresponding water exchange rate constant for the hydrated chromium­(III). The modification of the kinetics is of fundamental importance for the chromium­(III) chemistry in biological systems. Ultraviolet-visible and electron paramagnetic resonance studies, both in solution and in the solid state, ESI-MS, and conductivity measurements support the fact that, irrespective of the solvent used in the interaction of Hhydia with <i>trans</i>-[Cr^IIICl₂(H₂O)₄]­Cl·2H₂O, the ionization isomers­[Cr^III(Hhydia)₂]­Cl₃·2H₂O (<b>1</b>·2H₂O) and <i>cis/trans-</i>[Cr^IIICl₂(Hhydia)₂]­Cl·2H₂O (<b>2</b>·2H₂O) are produced.The reaction medium affects only the relevant percentage of the isomers in the solid state. The thermodynamic stability of the ionization isomers <b>1</b>·2H₂O and <i>cis/trans-</i><b>2</b>·2H₂O, their molecular structures as well as the vibrational spectra and the energetics of the Cr^III– Hhydia/hydia^– were studied by means of density functional theory calculations and found to be in excellent agreement with our experimental observations