Aqueous Organometallic Chemistry of the Electrophilic [(η-C₅(CH₃)₅)Ru(NO)]²⁺ Fragment

Abstract

Treatment of Cp‘Ru(NO)(CH3)2 with 2 equiv of HOSO2CF3 (HOTf) leads to the formation of the ditriflate complexes Cp‘Ru(NO)(OTf)2 (1a,b) (Cp‘ = η-C5(CH3)5, Cp* (1a), η-C5(CH3)4(CH2CH3), Cp† (1b)). The complex salts [Cp†Ru(NO)(OTf)(OH2)][OTf] (2b) and [Cp†Ru(NO)(OH2)2][OTf]2 (3b) can be isolated from the hydration of 1b. The structures of 1b, 2b, and 3b are determined by single-crystal X-ray diffraction methods. In 0.1 M H2O/CH2Cl2 the equilibria 1a 2a + + OTf- 3a 2+ + 2OTf- exist, with 1a being the predominant complex and ΔH 1 = −15(3) kcal/mol and ΔS 1 = −60(30) eu (for K 1) and ΔH 2 = −9(1) kcal/mol and ΔS 2 = −40(25) eu (for K 2). For comparison, the equilibria 1a [Cp*Ru(NO)(OTf)(THF)]+ + OTf- [Cp*Ru(NO)(THF)2]2+ + 2OTf- exist in neat THF with ΔH 1 = −4.5(3) kcal/mol and ΔS 1 = −30(10) eu (for K 1) and ΔH 2 = −4.1(1) kcal/mol and ΔS 2 = −20(10) eu (for K 2). The anion exchange equilibria in CH2Cl2 1a + 2Cl- [Cp*Ru(NO)(OTf)(Cl) + OTf- + Cl- [Cp*Ru(NO)Cl2] + 2OTf- has ΔH 1 = −9(1) kcal/mol and ΔS 1 = −30(10) eu (for K 1) and ΔH 2 = −11(1) kcal/mol and ΔS 2 = −30(10) eu (for K 2). While loss of the OTf- ligands is exothermic, the displacement of OTf- from the coordination sphere carries a significant entropy cost due to the formation of ions in a more-ordered solvent cage. Complex salts 1a,b dissolve in water to give acidic red-orange solutions containing an equilibrium mixture of the diaqua complex cations [Cp‘Ru(NO)(OH2)2]2+ (3a 2+, pK a = 2.7; 3b 2+) and the dinuclear cations [Cp‘Ru(NO)(μ-OH)]2 2+ (4a 2+, pK a = 5.5; 4b 2+). The cations 4a 2+ and 4b 2+ exist as a mixture of cis (major) and trans (minor) isomers; X-ray results show 4b 2+ to be cis in the solid state. Crossover between 4a 2+ and 4b 2+ to give the mixed Cp*/Cp† dimer 4c 2+ occurs readily under acidic conditions but not under basic conditions. The pH dependence together with kinetic and van't Hoff analyses support the process 2 3a 2+ ⇄ 4a 2+ + 2H3O+. The ∠Ru−N−O values of ca. 160°, correspondingly low νNO values in the Nujol mull IR spectra, and relatively short Ru−O bonds show the H2O and OH- ligands to be significant π-donors to the electrophilic Ru center. Dissolution of 4a,b in basic D2O causes complete deuteration of the ring CH3 groups but no deuteration of the Cp†-CH2CH3 group; the CD3 groups are easily exchanged to CH3 by exposure to basic H2O conditions. Chloride substitution by H2O occurs when Cp‘Ru(NO)Cl2 is dissolved in water, giving an equilibrium mixture of undissociated [Cp‘Ru(NO)Cl2]aq together with the [Cp‘Ru(NO)(Cl)(OH2)]+ and [Cp‘Ru(NO)(μ-OH)]2 2+ ions. H/D exchange on the Cp‘ ring CH3 groups also occurs slowly when Cp‘Ru(NO)Cl2 is dissolved in D2O but not when dissolved in a D2O/DCl mixture. The present work suggests that Cp‘-ring slippage and the reversible release of H+/D+ is facilitated by the π-donor ability of the H2O and OH- ligands.

Photocatalytic Oxygenation of Substrates by Dioxygen with Protonated Manganese(III) Corrolazine

“…Coordination of OTf- provides a possible mechanism for enhancing catalytic reactivity through a proposed Mn V (O)(TBP 8 Cz) intermediate. 44,45,51 In our previous report on proton-assisted catalysis, we suggested a mechanism that involved a short-lived tripseptet excited state of the monoprotonated Mn III complex as a key intermediate. This mechanism was based on prior photochemical characterization of the parent Mn III complex.…”

Reactivity of electrophilic Cp*Ru(NO) complex towards alcohols

“…The triflate group binding and dissociation in these complexes can be monitored by 19 F NMR spectroscopy [14,20]. system, where the investigation of the triflate substitution was carried out in more polar THF solutions [19,20].…”

“…Triflate ( OTf ,OSO2CF3) complexes have been shown to possess remarkable catalytic and stoichiometric reactivity over a range of reaction conditions [13]. Among the important features of the ruthenium triflate system under consideration are the aqueous solubility and electrophilicity, resulting in water binding and activation by the cationic [Cp*Ru(NO)] 2+ fragment((Cp*= η 5 -C(CH3)5) [14]. The present study deals with the reactivity of the Cp*Ru(NO)(OTf)2 complex with alcohols, probing the thermodynamics of the triflate-alcohol coordination exchange and the kinetic aspects of the subsequent stoichiometric oxidation of alcohol.…”

(η6-Hexamethylbenzene)(mesidine)ditriflatoruthenium(II)

“…Several complexes of Ru II containing coordinated triuoromethanesulfonate have previously been characterized crystallographically (Sutter et al, 1996;Abbenhuis et al, 1998;Fong et al, 1998;Gemel et al, 1997;Burns & Hubbard, 1994;Blosser et al, 1992;Svetlanova-Larsen et al, 1996;Kraakman et al, 1992;de Klerk-Engels et al, 1994;Mauthner et al, 1997). The RuÐO distances in these complexes range from 2.125 to 2.299 A Ê .…”