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

Oxidovanadium(IV/V) Complexes as New Redox Mediators in Dye-Sensitized Solar Cells: A Combined Experimental and Theoretical Study

Corrosiveness is one of the main drawbacks of using the iodide/triiodide redox couple in dye-sensitized solar cells (DSSCs). Alternative redox couples including transition metal complexes have been investigated where surprisingly high efficiencies for the conversion of solar to electrical energy have been achieved. In this paper, we examined the development of a DSSC using an electrolyte based on square pyramidal oxidovanadium­(IV/V) complexes. The oxidovanadium­(IV) complex (Ph₄P)₂[V^IVO­(hybeb)] was combined with its oxidized analogue (Ph₄P)­[V^VO­(hybeb)] {where hybeb^4– is the tetradentate diamidodiphenolate ligand [1-(2-hydroxybenzamido)-2-(2-pyridinecarboxamido)­benzenato}­and applied as a redox couple in the electrolyte of DSSCs. The complexes exhibit large electron exchange and transfer rates, which are evident from electron paramagnetic resonance spectroscopy and electrochemistry, rendering the oxidovanadium­(IV/V) compounds suitable for redox mediators in DSSCs. The very large self-exchange rate constant offered an insight into the mechanism of the exchange reaction most likely mediated through an outer-sphere exchange mechanism. The [V^IVO­(hybeb)]^2–/[V^VO­(hybeb)]⁻ redox potential and the energy of highest occupied molecular orbital (HOMO) of the sensitizing dye N719 and the HOMO of [V^IVO­(hybeb)]^2– were calculated by means of density functional theory electronic structure calculation methods. The complexes were applied as a new redox mediator in DSSCs, while the cell performance was studied in terms of the concentration of the reduced and oxidized form of the complexes. These studies were performed with the commercial Ru-based sensitizer N719 absorbed on a TiO₂ semiconducting film in the DSSC. Maximum energy conversion efficiencies of 2% at simulated solar light (AM 1.5; 1000 W m^–2) with an open circuit voltage of 660 mV, a short-circuit current of 5.2 mA cm^–2, and a fill factor of 0.58 were recorded without the presence of any additives in the electrolyte