skip to main content
OSTI.GOV title logo U.S. Department of Energy
Office of Scientific and Technical Information

Title: Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(l) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies

Abstract

Thermodynamic and kinetic parameters for the oxidative addition of H2 to [Rh{sup II}(bpy){sub 2}]{sup +} (bpy = 2,2'-bipyridine) to form [Rh{sup III}(H){sub 2}(bpy){sub 2}]{sup +}were determined from either the UV-vis spectrum of equilibrium mixtures of [Rh{sup II}(bpy){sub 2}]{sup +} and [Rh{sup III}(H){sub 2}(bpy){sub 2}]{sup +} or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh{sup III}(H){sub 2}(bpy){sub 2}]{sup +} as a function of H{sub 2} concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol{sup -1} and -18 cal mol{sup -1} K{sup -1}, respectively. The reaction enthalpy and entropy are -10.3 kcal mol{sup -1} and -19 cal mol{sup -1} K{sup -1}, respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm{sup 3} mol{sup -1} in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm3 mol-1 in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a shortmore » H-H bond (0.823 and 0.810 Angstroms, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 Angstroms, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.« less

Authors:
; ; ; ; ; ;
Publication Date:
Research Org.:
Brookhaven National Laboratory (BNL) National Synchrotron Light Source
Sponsoring Org.:
Doe - Office Of Science
OSTI Identifier:
914053
Report Number(s):
BNL-78621-2007-JA
Journal ID: ISSN 0020-1669; INOCAJ; TRN: US0801504
DOE Contract Number:
DE-AC02-98CH10886
Resource Type:
Journal Article
Resource Relation:
Journal Name: Inorg. Chem.; Journal Volume: 45
Country of Publication:
United States
Language:
English
Subject:
10 SYNTHETIC FUELS; 43 PARTICLE ACCELERATORS; ACETONE; ENTHALPY; ENTROPY; IRRADIATION; KINETICS; METHANOL; MIXTURES; SOLVENTS; THERMODYNAMICS; NSLS; national synchrotron light source

Citation Formats

Fujita,E., Brunschwig, B., Creutz, C., Muckerman, J., Sutin, N., Szaida, D., and van Eldik, R. Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(l) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies. United States: N. p., 2006. Web. doi:10.1021/ic0515498.
Fujita,E., Brunschwig, B., Creutz, C., Muckerman, J., Sutin, N., Szaida, D., & van Eldik, R. Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(l) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies. United States. doi:10.1021/ic0515498.
Fujita,E., Brunschwig, B., Creutz, C., Muckerman, J., Sutin, N., Szaida, D., and van Eldik, R. Sun . "Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(l) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies". United States. doi:10.1021/ic0515498.
@article{osti_914053,
title = {Transition State Characterization for the Reversible Binding of Dihydrogen to Bis(2,2'-bipyridine)rhodium(l) from Temperature- and Pressure-Dependent Experimental and Theoretical Studies},
author = {Fujita,E. and Brunschwig, B. and Creutz, C. and Muckerman, J. and Sutin, N. and Szaida, D. and van Eldik, R.},
abstractNote = {Thermodynamic and kinetic parameters for the oxidative addition of H2 to [Rh{sup II}(bpy){sub 2}]{sup +} (bpy = 2,2'-bipyridine) to form [Rh{sup III}(H){sub 2}(bpy){sub 2}]{sup +}were determined from either the UV-vis spectrum of equilibrium mixtures of [Rh{sup II}(bpy){sub 2}]{sup +} and [Rh{sup III}(H){sub 2}(bpy){sub 2}]{sup +} or from the observed rates of dihydride formation following visible-light irradiation of solutions containing [Rh{sup III}(H){sub 2}(bpy){sub 2}]{sup +} as a function of H{sub 2} concentration, temperature, and pressure in acetone and methanol. The activation enthalpy and entropy in methanol are 10.0 kcal mol{sup -1} and -18 cal mol{sup -1} K{sup -1}, respectively. The reaction enthalpy and entropy are -10.3 kcal mol{sup -1} and -19 cal mol{sup -1} K{sup -1}, respectively. Similar values were obtained in acetone. Surprisingly, the volumes of activation for dihydride formation (-15 and -16 cm{sup 3} mol{sup -1} in methanol and acetone, respectively) are very close to the overall reaction volumes (-15 cm3 mol-1 in both solvents). Thus, the volumes of activation for the reverse reaction, elimination of dihydrogen from the dihydrido complex, are approximately zero. B3LYP hybrid DFT calculations of the transition-state complex in methanol and similar MP2 calculations in the gas phase suggest that the dihydrogen has a short H-H bond (0.823 and 0.810 Angstroms, respectively) and forms only a weak Rh-H bond (1.866 and 1.915 Angstroms, respectively). Equal partial molar volumes of the dihydrogenrhodium(I) transition state and dihydridorhodium(III) can account for the experimental volume profile found for the overall process.},
doi = {10.1021/ic0515498},
journal = {Inorg. Chem.},
number = ,
volume = 45,
place = {United States},
year = {Sun Jan 01 00:00:00 EST 2006},
month = {Sun Jan 01 00:00:00 EST 2006}
}
  • A planar complex, [Rh(bpy){sub 2}]{sup +} (bpy=2,2{sup {prime}}-bipyridine), was obtained from [Rh(ox)(bpy){sub 2}]{sup +} (ox=oxalato) by photoirradiation. A rate constant k for the photoreaction was evaluated as 1 x 10{sup 8} s{sup {minus}1} in simple first-order kinetics, whereas a ligand dissociation, a reorganization of the coordinated bpy, and a two-electron transfer were involved in the reaction. The process of the Rh(I) complex generation was investigated in terms of a discrete variational(DV)-X{alpha} molecular orbital calculation on [Rh(ox)(HN=CHCH=NH){sub 2}]{sup +} instead of [Rh(ox)(bpy){sub 2}]{sup +}. From the calculation, using the transition-state method, it was predicted that a transition of the ox {pi}{supmore » *} orbital to the metal 4d{sub z}{sup 2} orbital caused the ligand dissociation and the reorganization of the coordinated bpy occurred in the ox {pi} to Rh 4d{sub x{sup 2}-y{sup 2}} excited state stabilized by the ox elimination. Upon release of the ligand and a change from octahedral to square-planar geometry, the electron density on the metal increased and the Rh 4d orbital acquired a d{sup 8} electronic configuration.« less
  • Deprotonation of the photovoltaic dye sensitizers cis-(H{sub 2}-dcbpy){sub 2}RuX{sub 2} (L{sub 2}RuX{sub 2})(X= {minus}CN{sup {minus}}, {minus}NCS{sup {minus}}; H{sub 2}-dcbpy = L = 2,2{prime}-bipyridine-4,4{prime}-dicarboxylic acid) can be achieved in dimethylformamide by reductive electrolysis at platinum electrodes at 20 C, which allows the thermodynamic and spectral changes associated with deprotonation to be established. The overall reaction that occurs when a potential of {minus}2.0 V vs. Fc/Fc{sup +} (Fc = ferrocene) is applied to a platinum electrode can be summarized as (H{sub 2}-dcbpy){sub 2}Ru(NCS){sub 2} + xe{sup {minus}} {r{underscore}arrow} [(H{sub 2{minus}x/2}-dcbpy{sup x/2{minus}}){sub 2}Ru(NCS){sub 2}]{sup x{minus}} + {sup x}/{sub 2}H{sub 2}, where x ismore » always slightly less than 4. Thus, under certain experimental conditions, [(H-dcbpy{sup {minus}}){sub 2}RuX{sub 2}]{sup 1{minus}} is believed to be the major product formed by bulk electrolysis, where H-dcbpy{sup {minus}} is the singly deprotonated H{sub 2}-dcbpy ligand. The hydrogen gas formed in this electrochemically induced deprotonation can be generated heterogeneously at the electrode surface or via homogeneous redox reactions between ligand-reduced forms of L{sub 2}RuX{sub 2} and protons or water. Short time domains, reduced temperatures, and glassy carbon electrodes lead to detection of transiently stable ligand-reduced forms of L{sub 2}-RuX{sub 2}. The reversible half-wave potentials for the ligand-based reduction of electrochemically generated deprotonated L{sub 2}RuX{sub 2} are 0.65 V more negative than their protonated counterparts. In contrast, deprotonation leads to the metal-based oxidation process being shifted by only about 0.3 V. Interestingly, protonated and deprotonated forms of L{sub 2}RuX{sub 2} do not coexist in a facile acid-base equilibrium state on the voltammetric time scale. Data obtained from electrogenerated deprotonated forms of the sensitizers are compared to those found for ``salts'' used in photovoltaic cells which are prepared by reaction of L{sub 2}RuX{sub 2} with tetrabutylammonium hydroxide. Molecular orbital calculations were employed to provide theoretical insights into the effect of deprotonation on reversible potentials and electronic spectra, and results are in good agreement with experimentally obtained data. Electronic spectra, measured in situ during the course of reduction in a spectroelectrochemical cell, reveal that all bands shift to higher energies and that the absorbance decreases as deprotonation occurs. Implications of the importance of the findings related to reduction potentials and electronic spectra to the operation of photovoltaic cells that utilize deprotonated forms of sensitizers are considered.« less
  • The reaction of Rh(bpy)/sub 3//sup 3 +/ with radiation-generated e/sub aq//sup -/ and (CH/sub 3/)/sub 2/COH in deaerated aqueous solutions containing 0.1 M 2-propanol generates H/sub 2/ in the absence of any catalyst. The yield of H/sub 2/, above that produced as background from the raiolytic act, is greatest at pH 4.2; the yield of extra H/sub 2/ diminishes to zero at pH 2 and 7. The G value of extra H/sub 2/ rises from zero in the limit of zero radiation dose to a plateau at approx. 0.25 Mrd; G(H/sub 2/) is also a function of the initial concentrationmore » of Rh(bpy)/sub 3//sup 3 +/, approaching a maximum value at infinite substrate dilution. These results, coupled with the measured yield of Rh(I), give rise to a proposed mechanism for the uncatalyzed formation of H/sub 2/ in which the direct precursor to H/sub 2/ is RhH(bpy)/sub 2//sup +/. This latter species is seen to arise from the reduction of RhH(bpy)/sub 2//sup 2 +/, the form of Rh(I) stable in acidic solution, by Rh(bpy)/sub 3//sup 2 +/. The maximum turnover number for the generation of H/sub 2/ from this system upon exposure to large radiation doses is approx. 12. The data presented account for the origin of H/sub 2/ in uncatalyzed photochemical systems.« less
  • Thin, transparent films of SnO[sub 2] semiconductor have been prepared from 30-A-diameter colloids. Scanning electron microscopy and X-ray powder diffraction studies indicate a three-dimensional network of SnO[sub 2] nanocrystallites of particle diameter [le]50 [angstrom]. These thin nanocrystalline films exhibit reversible electrochromic effects. The electron trapping process in SnO[sub 2] particles has been investigated by both spectroelectrochemical and laser flash photolysis techniques. These electrodes are photoelectrochemically active in the UV region with incident photon-to-photocurrent conversion efficiency of 20% at 280 nm. The photocurrent increases with increasing film thickness but attains a limiting value at thickness greater than 0.75 [mu]m. The migrationmore » of charge across the grain boundaries is a limiting factor for the photocurrent generation in thicker films. These SnO[sub 2] films are highly porous and exhibit strong affinity for adsorption of sensitizer molecules such as bis(2,2[prime]-bipyridine) (2,2[prime]-bipyridine-4,4[prime]-dicarboxylic acid)ruthenium(II) perchlorate (Ru(bpy)[sub 2](dcbpy)[sup 2+]). SnO[sub 2] films modified with Ru(bpy)[sub 2](dcbpy)[sup 2+] exhibit excellent photoelectrochemical response in the visible with a power conversion efficiency of [approximately] 1% at 470 nm. 39 refs., 10 figs.« less