The present study demonstrates that [Ru{sup II}(NH{sub 3})(q)(trpy)]{sup +} has an ability to oxidize alcohols catalytically under very mild conditions under electrolysis at +0.35 V in MeOH. The elucidation of the reaction mechanisms in the alcohol-oxidation is underway

FUJITA, E.

MUCKERMAN, J.

TANAKA,K.

WADA, T.

UNT Digital Library

 BNL-79592-2007-CP       Reversible Conversion Between Chemical and Electrical Energies Catalyzed by Ru Complexes Aimed to Construct Sustainable Society      Presented at the 235th ACS National Meeting New Orleans, Louisiana April 6-10, 2008   November 2007     Chemistry Department  Brookhaven National Laboratory P.O. Box 5000 Upton, NY 11973-5000 www.bnl.gov       Notice: This manuscript has been authored by employees of Brookhaven Science Associates, LLC under Contract No. DE-AC02-98CH10886 with the U.S. Department of Energy. The publisher by accepting the manuscript for publication acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes.   This preprint is intended for publication in a journal or proceedings.  Since changes may be made before publication, it may not be cited or reproduced without the author’s permission.   DISCLAIMER  This report was prepared as an account of work sponsored by an agency of the United States Government.  Neither the United States Government nor any agency thereof, nor any of their employees, nor any of their contractors, subcontractors, or their employees, makes any warranty, express or implied, or assumes any legal liability or responsibility for the accuracy, completeness, or any third party’s use or the results of such use of any information, apparatus, product, or process disclosed, or represents that its use would not infringe privately owned rights. Reference herein to any specific commercial product, process, or service by trade name, trademark, manufacturer, or otherwise, does not necessarily constitute or imply its endorsement, recommendation, or favoring by the United States Government or any agency thereof or its contractors or subcontractors.  The views and opinions of authors expressed herein do not necessarily state or reflect those of the United States Government or any agency thereof.                                     Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53 (1),  xxxx Reversible Conversion between Chemical and Electrical Energies Catalyzed by Ru Complexes Aimed to Construct Sustainable Society    Koji Tanaka,1 Tohru Wada,1 Etsuko Fujita,2 and James Muckerman2  1Institute for Molecular Science, Myodaiji, Okazaki 444-8787, Japan 2Brookhaven National Laboratory, Upton, NY. 11973-5000, USA  Introduction Reversible conversion between CO2 and MeOH through six-electron redox reactions is a feasible pathway to construct sustainable societies.  Metal-oxo species involved in metal proteins work as active centers to catalyze hydroxylation, epoxidation and oxidation of various biological molecules under mild conditions.1-3 Metal-oxo complexes, therefore, would be reasonable candidates for electrode materials in methanol fuel cells.   A variety of high valent Ru-oxo polypyridyl complexes have been prepared by sequential electron and proton loss from RuII-aqua ones, 4 though redox potentials of the complexes thus obtained are too positive for use as energy converters.   On the other hand, introduction of a quinone ligand into Ru-aqua complexes allows smooth double deprotonation without using any oxidants.  For example, the pKa value of [RuII(OH2)(q)(trpy)]2+ (q = 3,5-di-tert-butyl-1,2-benzoquinone) is 5.5.  The resultant [RuII(OH)(q)(trpy)]+ dissociates the hydroxy proton in a pH range over 10 to form the first oxyl radical complex, [RuII(O-.)(sq)(trpy)]0 (eq 1) due to intramolecular electron transfer from the negatively charged oxo ligand to quinone (eq 1).5  However, the Ru-oxyl radical complex thus formed did not show distinct reactivity toward the oxidation of MeOH.  So, we prepared [RuII(NH3)(sq)(trpy)](ClO4) ([1](ClO4)) in place of Ru-aqua complexes to examine the reactivity of the Ru-aminyl radical toward the oxidation of alcohol under basic conditions.  Experimental An aqueous HClO4 solution (70%, 2 ml) was added to a methanol solution (6 ml) of [RuII(OAc)(sq)(trpy)] 6,7 (202 mg), stirred overnight.  The crude products were extracted with CH2Cl2, dried with Na2SO4 and evaporated.  To the residue was added THF (20 ml) and an aqueous NH3 solution (28%, 2 ml), and stirred overnight.  The mixture was extracted with CH2Cl2, dried, evaporated, and purified on neutral aluminum oxide column using acetone/MeOH as an eluent.  Dark purple needles of [RuII(NH3)(sq)(trpy)](ClO4) ([1](ClO4)) were crystallized from an acetone/NaClO4 aqueous solution; yield 52.8 mg (24%).  Anal. Calcd for C29H34ClN4O6Ru: C, 51.90; H, 5.11; N, 8.35%.  Found: C, 52.09; H, 5.27; N, 8.09%.   The oxidized form of [1]+ was prepared under the controlled-potential electrolysis of [1](ClO4) (50 mg) at +0.64 V in MeOH containing LiBF4 as an electrolyte with using a glassy carbon plate and a platinum foil as working and counter electrodes, respectively.  After 8.2 coulombs passed in the electrolysis, the solution was extracted with CH2Cl2, dried and evaporated.  The residue was washed with ethyl acetate and diethyl ether several times.  Deep blue needles of [1](ClO4)2 were crystallized from acetone/NH4ClO4 aqueous solution; yield 28 mg (48%).  Anal. Calcd for C29H34Cl2N4O10Ru: C, 45.20; H, 4.45; N, 7.27%.  Found: C, 44.98; H, 4.53; N, 6.99%. Cyclic voltammetric experiments were carried out in a one-compartment cell consisting of a glassy carbon working electrode, a platinum wire auxiliary electrode, and an Ag/Ag(NO3) (0.01 M) reference electrode.  All solutions were deoxygenated with a stream of Ar.    Cyclic voltammograms of the complexes under basic conditions were obtained by an addition of various amounts of a methanolic solution of CH3ONa (0.45 M) to the solution.   Bu4NClO4  (0.10 M) was used as an electrolyte.   An ALS/Chi model 660 electrochemical analyzer was used to collect the cyclic voltammetric data.  All potentials are quoted vs. SCE.   ESI-MS were obtained on a PE SCIEX API300.   EPR spectrum was measured with JEOL X-band spectrometer (JES-REIXE).  The g value was calibrated precisely with an Mn2+ marker.  Results and Discussion The addition of an aqueous HClO4 solution to [RuII(OAc)(sq)(terpy)] in MeOH gave [RuII{O(H)Me}(sq)(terpy)]+.   Treatment of the MeOH adduct with NH3 in THF produced [RuII(NH3)(sq)(terpy)]+ ([1]+), which was isolated as a ClO4- salt ([1](ClO4)).   Electrochemical oxidation of purple [1](ClO4) at +0.64 V in dry MeOH afforded blue [RuII(NH3)(q)(terpy)](ClO4)2 ([1](ClO4)2).    A broad isotropic EPR signal (g = 2.008) of [1]+ at 298 K in CH2Cl2 and the absence of any EPR signals of [1]2+ reflect the electronic structures of the RuII(sq) and RuII(q) frameworks, respectively,   The electronic absorption spectra of [1]+ and [1]2+ in CH3OH displayed a strong absorption band at 853 nm (ε 13 000 M-1cm-1) and 615 nm (ε 13 000 M-1cm-1), respectively, assignable to the charge-transfer transition within the RuII-sq and RuII-q frameworks.  It is worthy of note that [1]2+ is quantitatively reduced to [1]+ by an addition of 1 equiv of CH3ONa or t-BuOK in CH2Cl2 (Figure 1).   The cyclic voltammogram (CV) of [1]2+ showed two reversible redox waves at E1/2 = –0.46 V and E1/2 = +0.34 V (vs. SCE) assignable to the [RuII(NH3)(cat)]0 / [RuII(NH3)(sq)]+ and [RuII(NH3)(sq)]+ / [RuII(NH3)(q)]2+ couples, respectively, in CH3OH.   The addition of excess amounts of CH3ONa to [1]2+ results in the appearance of strong catalytic currents at potentials more positive than the redox couple at +0.34 V (Figure 2).   Furthermore, the [RuII(NH3)(cat)]0 / [RuII(NH3)(sq)]+ redox couple at E1/2 = –0.39 V remained unchanged even in multi-scanning CV in the presence of [RuII(OH2)(q)(trpy)]2+-H++H+[RuII(OH-)(q)(trpy)]+-H++H+[RuII(OH-•)(q)(trpy)]0 (1) Figure 1. The electron absorption spectra of [1]2+ in the presence of various amounts of t-BuOK in CH2Cl2  BNL-79592-2007-CP Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53 (1),  xxxx excess of CH3ONa, indicating smooth regeneration of [1]+ after the oxidation of MeOH by the oxidized form of [1]2+.   Indeed, 73 F / mole of electricity were consumed in the controlled potential electrolysis of [1]+ (1 mM) at +0.35 V in the presence of CH3ONa (100 equivs based on [1]+) in MeOH,8 and then the electrolysis completely stopped.  Electron spray ionization mass spectra of the final electrolyte solution revealed the complete conversion from [1]+ to [RuII(OCH3)(q)(trpy)]+ during the electrolysis.9  In fact, [RuII(OCH3)(sq)(trpy)] did not show any catalytic activity toward the oxidation of MeOH under similar reaction conditions.   In contrast to [1]+ , treatment of [RuII(NHPh-bpa)(q)]+ (NH2Ph-bpa = 2-[Bis(2-pyridylmethyl)aminomethyl]aniline) with BuOLi in DMF did not cause the reduction of  the quinone ligand, but selectively gave [RuII(.NPh-bpa)(sq)] with semiquinone and anilido radicals, because the complex showed a triplet state in the EPR spectrum composed of gxx = gyy = 2.060, gzz = 2.025, and |D| = 0.018 cm–1 in CH2Cl2 at 5 K.9   Furthermore, electrochemical reduction of [RuII(.NPh-bpa)(sq)] at –1.5 V (vs. SCE) in DMF resulted in the reduction of sq.   The resultant [RuII(•NPh-bpa)(cat)]–  displayed a rhombic pattern EPR spectrum with g1 = 2.175, g2 = 2.105, and g3 = 1.950, and the g3 component was split into three signals due to the hyperfine coupling with the N atom (I = 1) on the anilino radical with a hyperfine coupling constant, A(N) of 8.2 mT in DME at 20 K.10 Thus, deprotonation of [RuII(NHPhbpa)(q)]+ is followed by the intra-molecular electron transfer from (N-Phbpa)2- to q to generate [RuII(.NPh-bpa)(sq)] (eq 2), similar to the equilibrium of eq 1. Electrochemical oxidation of [RuIII(NH3)(bpy)(trpy)]2+ at +0.8 V (vs. SCE) in H2O has been shown to produce [RuII(NO2)(bpy)(trpy)]+ through [RuII(NH3)(bpy)(trpy)]+ and [RuIV(=NH)(bpy)(trpy)]2+  due to successive electron and proton loss of the central Ru and the NH3 ligand.11   On the other hand, the ligand based of the [sq]/[q] redox couple plays the main role in the oxidative activation of [1]+ toward the oxidation of MeOH.   The catalytic cycle of MeOH oxidation would be initiated by the dissociation of the amino proton of [RuII(NH3)(q)(trpy)]2+ followed by intramolecular electron transfer from the negatively charged NH2–  to the RuII(q) framework.  Such an electron transfer generates radical character on the nitrogen atom.   The reactivity of the radical formed through the acid-base equilibrium of amino protons is largely dependent on the substituent on the nitrogen atom.  The anilido radical complexes of both [RuII(.NPh-bpa)(Bu2sq)] and [RuII(•NPh-bpa)(Bu2cat)]– were definitely identified by EPR and resonance Raman spectra. On the other hand, the amino radical complexes of neither [RuII(.NH2)(sq)]+ nor [RuII(.NH2)(q)]2+ were detected by spectroscopic measurements due to the high reactivity of the complexes.   The distinct difference of the reactivity between the RuII(•NPh-bpa) and RuII(.NH2) groups is ascribed to the large π-electron distribution over the NPh-bpa group of the former, and the large spin density of the nitrogen atom of the latter due to the localization on the nitrogen atom.    The catalytic oxidation of MeOH, therefore, is explained by hydrogen abstraction from MeOH by the RuII(NH2) group.  Conclusions The present study demonstrates that [RuII(NH3)(q)(trpy)]+ has an ability to oxidize alcohols catalytically under very mild conditions under electrolysis at +0.35 V in MeOH.  The elucidation of the reaction mechanisms in the alcohol-oxidation is underway.  Acknowledgement.   Work performed at Brookhaven National Laboratory was funded under contract DE-AC02-98CH10886 with the U.S. Department of Energy and supported by its Division of Chemical Sciences, Geosciences, & Biosciences, Office of Basic Energy Sciences.   References (1) Holm R. H, Chem. Rev. 1987, 87, 1401; Mayer J. M,  Acc. Chem. Res. 1998, 31, 441. (2) Watanabe Y, J. Biol. Inorg. Chem. 2001, 6, 846.  (3) Fontecave M,; Menage S,; Duboc-Toia C,; Coord. Chem. Rev. 1998, 178-180, 1555.  (4) Meyer T. J,; Huynh M. H. V,; Inorg. Chem. 2003, 42, 8140. (5) Kobayashi K,;  Ohtsu H,; Wada T,; Kato T,;  Tanaka K, J. Am. Chem. Soc. 2003, 125, 6729. (6) Pierpont C. G,; Lange C. W, Prog. Inorg. Chem. 1994, 41, 331; Bhattacharya S,; Pierpont C. G,; Inorg. Chem. 1994, 33, 6038. (7) Kurihara M,; Daniele S,; Tsuge K,; Sugimoto H,; Tanaka K, Bull. Chem. Soc. Jpn. 1998, 71, 867. (8) The controlled potential electrolysis was performed under the similar conditions as the preparation of [1]2+. (9) [RuII(OCH3)(sq)(trpy)] synthesized from [RuIICl(sq)(trpy)] by the reaction with NaOMe in MeOH. (10) Miyazato Y,; Wada T,; Muckerman J. T,; Fujita E,; Tanaka K, Angew. Chem. Int. Ed. 2007, 46, 5728. (11) Thompson M. S,; Meyer T. J, J. Am. Chem. Soc. 1981, 103, 5577.       RuIINNN OOt-But-Bu+NHRuIINNN OOt-But-Bu0N-H+ (2)Figure 2. The CVs of [1]+ in the absence (a) and the presence of t-BuOK (100 equiv) (b) in MeOH  

REVERSIBLE CONVERSION BETWEEN CHEMICAL AND ELECTRICAL ENERGIES CATALYZED BY Ru COMPLEXES AIMED TO CONSTRUCT SUSTAINABLE SOCIETY.

https://digital.library.unt.edu/ark:/67531/metadc901625/m2/1/high_res_d/925819.pdf

REVERSIBLE CONVERSION BETWEEN CHEMICAL AND ELECTRICAL ENERGIES CATALYZED BY Ru COMPLEXES AIMED TO CONSTRUCT SUSTAINABLE SOCIETY.

Abstract

Similar works

Full text

Available Versions

UNT Digital Library