title: Mechanism elucidation of the cis-trans isomerization of an azole ruthenium-nitrosyl complex and its osmium counterpart.
abstract: Synthesis and X-ray diffraction structures of cis and trans isomers of ruthenium and osmium metal complexes of general formulas (nBu4N)[cis-MCl4(NO)(Hind)], where M = Ru (1) and Os (3), and (nBu4N)[trans-MCl4(NO)(Hind)], where M = Ru (2) and Os (4) and Hind = 1H-indazole are reported. Interconversion between cis and trans isomers at high temperatures (80-130 °C) has been observed and studied by NMR spectroscopy. Kinetic data indicate that isomerizations correspond to reversible first order reactions. The rates of isomerization reactions even at 110 °C are very low with rate constants of 10(-5) s(-1) and 10(-6) s(-1) for ruthenium and osmium complexes, respectively, and the estimated rate constants of isomerization at room temperature are of ca. 10(-10) s(-1). The activation parameters, which have been obtained from fitting the reaction rates at different temperatures to the Eyring equation for ruthenium [ΔH(cis-trans)‡ = 122.8 ± 1.3; ΔH(trans-cis)‡ = 138.8 ± 1.0 kJ/mol; ΔS(cis-trans)‡ = -18.7 ± 3.6; ΔS(trans-cis)‡ = 31.8 ± 2.7 J/(mol·K)] and osmium [ΔH(cis-trans)‡ = 200.7 ± 0.7; ΔH(trans-cis)‡ = 168.2 ± 0.6 kJ/mol; ΔS(cis-trans)‡ = 142.7 ± 8.9; ΔS(trans-cis)‡ = 85.9 ± 3.9 J/(mol·K)] reflect the inertness of these systems. The entropy of activation for the osmium complexes is highly positive and suggests the dissociative mechanism of isomerization. In the case of ruthenium, the activation entropy for the cis to trans isomerization is negative [-18.6 J/(mol·K)], while being positive [31.0 J/(mol·K)] for the trans to cis conversion. The thermodynamic parameters for cis to trans isomerization of [RuCl4(NO)(Hind)]-, viz. ΔH° = 13.5 ± 1.5 kJ/mol and ΔS° = -5.2 ± 3.4 J/(mol·K) indicate the low difference between the energies of cis and trans isomers. The theoretical calculation has been carried out on isomerization of ruthenium complexes with DFT methods. The dissociative, associative, and intramolecular twist isomerization mechanisms have been considered. The value for the activation energy found for the dissociative mechanism is in good agreement with experimental activation enthalpy. Electrochemical investigation provides further evidence for higher reactivity of ruthenium complexes compared to that of osmium counterparts and shows that intramolecular electron transfer reactions do not affect the isomerization process. A dissociative mechanism of cis↔trans isomerization has been proposed for both ruthenium and osmium complexes.
authors.affiliation: Université Claude Bernard Lyon 1, Laboratoire des Multimatériaux et Interfaces (UMR 5615), Campus de la Doua, 69622 Villeurbanne Cedex, France.
authors.forename: Anatolie
authors.initials: A
authors.lastname: Gavriluta
authors.forename: Gabriel E
authors.initials: GE
authors.lastname: Büchel
authors.forename: Leon
authors.initials: L
authors.lastname: Freitag
authors.forename: Ghenadie
authors.initials: G
authors.lastname: Novitchi
authors.forename: Jean Bernard
authors.initials: JB
authors.lastname: Tommasino
authors.forename: Erwann
authors.initials: E
authors.lastname: Jeanneau
authors.forename: Paul-Steffen
authors.initials: PS
authors.lastname: Kuhn
authors.forename: Leticia
authors.initials: L
authors.lastname: González
authors.forename: Vladimir B
authors.initials: VB
authors.lastname: Arion
authors.forename: Dominique
authors.initials: D
authors.lastname: Luneau
mesh_terms: D001393:Azoles / Q000737:chemistry*; D018360:Crystallography, X-Ray; D008958:Models, Molecular; D015394:Molecular Structure; D009603:Nitroso Compounds / Q000737:chemistry*; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D009992:Osmium / Q000737:chemistry*; D011789:Quantum Theory; D012428:Ruthenium / Q000737:chemistry*; D013237:Stereoisomerism
chemical_list: D001393:Azoles; D009603:Nitroso Compounds; D009942:Organometallic Compounds; D009992:Osmium; D012428:Ruthenium