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- 3a2+ + 2OTf- exist, with 1a being the predominant complex and ΔH1 = −15(3) kcal/mol and ΔS1 = −60(30) eu (for K1) and ΔH2 = −9(1) kcal/mol and ΔS2 = −40(25) eu (for K2). For comparison, the equilibria 1a [Cp*Ru(NO)(OTf)(THF)]+ + OTf- [Cp*Ru(NO)(THF)2]2+ + 2OTf- exist in neat THF with ΔH1 = −4.5(3) kcal/mol and ΔS1 = −30(10) eu (for K1) and ΔH2 = −4.1(1) kcal/mol and ΔS2 = −20(10) eu (for K2). The anion exchange equilibria in CH2Cl2 1a + 2Cl- [Cp*Ru(NO)(OTf)(Cl) + OTf- + Cl- [Cp*Ru(NO)Cl2] + 2OTf- has ΔH1 = −9(1) kcal/mol and ΔS1 = −30(10) eu (for K1) and ΔH2 = −11(1) kcal/mol and ΔS2 = −30(10) eu (for K2). 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+ (3a2+, pKa = 2.7; 3b2+) and the dinuclear cations [Cp‘Ru(NO)(μ-OH)]22+ (4a2+, pKa = 5.5; 4b2+). The cations 4a2+ and 4b2+ exist as a mixture of cis (major) and trans (minor) isomers; X-ray results show 4b2+ to be cis in the solid state. Crossover between 4a2+ and 4b2+ to give the mixed Cp*/Cp† dimer 4c2+ 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 3a2+ ⇄ 4a2+ + 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)]22+ 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.