Photochemical conversion of CO{sub 2} to fuels or useful chemicals using renewable solar energy is an attractive solution to both the world's need for fuels and the reduction of greenhouse gases. Rhenium(I) and ruthenium(II) diimine complexes have been shown to act as photocatalysts and/or electrocatalysts for CO{sub 2} reduction to CO. We have studied these photochemical systems focusing on the identification of intermediates and the bond formation/cleavage reactions between the metal center and CO{sub 2}. For example, we have produced the one-electron-reduced monomer (i.e. Re(dmb)(CO){sub 3}S where dmb = 4,4'-dimethy-2,2'-bipyridine and S = solvent) either by reductive quenching of the excited states of fac-[Re(dmb)(CO){sub 3}(CH{sub 3}CN)]PF{sub 6} or by photo-induced homolysis of [Re(dmb)(CO){sub 3}]{sub 2}. We previously found that: (1) the remarkably slow dimerization of Re(dmb)(CO){sub 3}S is due to the absence of a vacant coordination site for Re-Re bond formation, and the extra electron is located on the dmb ligand; (2) the reaction of Re(dmb)(CO){sub 3}S with CO{sub 2} forms a CO{sub 2}-bridged binuclear species (CO){sub 3}(dmb)Re-CO(O)-Re(dmb)(CO){sub 3} as an intermediate in CO formation; and (3) the kinetics and mechanism of reactions are consistent with the interaction of the CO{sub 2}-bridged binuclear species with CO{sub 2} to form CO and CO{sub 3}{sup 2-}

FUJITA,E.

MUCKERMAN, J.T.

TANAKA, K.

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 BNL-79593-2007-CP       Photochemical CO2 Reduction by Rhenuim and Ruthenium Complexes    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 Figure 1 B3LYP calculated structure of the CO2-bridged Re dinuclear species. PHOTOCHEMICAL CO2 REDUCTION BY RHENUIM AND RUTHENIUM COMPLEXES  Etsuko Fujita,1 James T. Muckerman,1 and Koji Tanaka2   1 Chemistry Department, Brookhaven National Laboratory, Upton, NY 11973-5000, USA 2 Institute for Molecular Science, 5-1 Higashiyama, Myodaiji, Okazaki, Aichi 444-8787, Japan  Introduction Photochemical conversion of CO2 to fuels or useful chemicals using renewable solar energy is an attractive solution to both the world’s need for fuels and the reduction of greenhouse gases. Rhenium(I) and ruthenium (II) diimine complexes have been shown to act as photocatalysts and/or electrocatalysts for CO2 reduction to CO. We have studied these photochemical systems focusing on the identification of intermediates and the bond formation/cleavage reactions between the metal center and CO2.1,2 For example, we have produced the one-electron-reduced monomer (i.e. Re(dmb)(CO)3S where dmb = 4,4′-dimethy-2,2′-bipyridine and S = solvent) either by reductive quenching of the excited states of fac-[Re(dmb)(CO)3(CH3CN)]PF6 or by photo-induced homolysis of [Re(dmb)(CO)3]2. We previously found that: (1) the remarkably slow dimerization of Re(dmb)(CO)3S is due to the absence of a vacant coordination site for Re−Re bond formation, and the extra electron is located on the dmb ligand; (2) the reaction of Re(dmb)(CO)3S with CO2 forms a CO2-bridged binuclear species (CO)3(dmb)Re−CO(O)−Re(dmb)(CO)3 as an intermediate in CO formation; and (3) the kinetics and mechanism of reactions are consistent with the interaction of the CO2-bridged binuclear species with CO2 to form CO and CO32−. In photocatalytic systems with cobalt, nickel, ruthenium and rhenium complexes as CO2 reduction catalysts to produce CO, the photoproduction of the reduced catalysts is fast (< 10 µs), the rate constants for CO2 binding to the reduced metal centers depend on the catalysts and solvent, and range from 10−2 to 108 M−1 s−1.3 The detailed mechanism of CO formation from the CO2 adducts remains unclear in most cases, and the production of CO is very slow with a turnover frequency typically less than 10 h−1. Furthermore, despite substantial effort, to the best of our knowledge no one has succeeded in producing methanol from CO2 using homogeneous systems with protons and electrons or H2. Products in the homogeneous photochemical reduction of CO2 reported so far have been limited to CO and/or HCOOH. In homogeneous electrochemical reduction of CO2, a few additional C2 species have been reported as minor products.1  We believe that a very promising approach to homogeneous carbon dioxide reduction is a succession of hydride-ion and proton transfer reactions. As examples of these pathways, stoichiometric chemical conversion of a coordinated CO to methanol or methane using NaBH4 has been reported for Ru0 → [Ru−CO2] (i.e., [RuII−(CO22−)]) ⇋  [Ru−COOH]+ ⇋  [Ru−CO]2+ → [Ru−CHO]+ → [Ru−CH2OH]+ (Ru = Ru(bpy)2(CO))4 and for [Re−CO]+ → Re−CHO → Re−CH2OH → Re−CH3 (Re = Re(Cp)(NO)(CO)).5 Then can we replace NaBH4 by photochemically produced hydride donors?  While metal hydrides are good hydride donors to CO2 and CO,6 the regeneration quite often requires H2 or other hydride donors. In biological systems, nicotinamide adenine dinucleotide (NAD+) is one of the most important redox mediators and acts as a reservoir/source of two electrons and a proton (i.e., hydride). We prepared a polypyridylruthenium complex with an NAD+/NADH model ligand, [Ru(bpy)2(pbn)]2+ (bpy = 2,2´-bipyridine, pbn = 2-(2-pyridyl)-benzo[b]-1,5-naphthyridine), which acts as a catalyst in the electrochemical reduction of acetone to 2-propanol similar to the enzymatic NAD+/NADH.7 While the mechanism remains unclear, this is the first example that an NADH model complex such as [Ru(bpy)2(pbnHH)]2+ (or other intermediates in this system) works as a catalytic hydride donor for the formation of 2-propanol. We have recently reported clear evidence of photochemical and radiolytic formation of [Ru(bpy)2(pbnHH)]2+ with H+.8 These results open a new door for photocatalytic hydride (or proton-coupled-electron) transfer reactions originating from metal-to-ligand charge-transfer (MLCT) excited states of metal complexes with a bio-inspired NADH-like ligand, and for the utilization of photogenerated hydride donors for CO2 reduction.  We have prepared rhenium and ruthenium complexes with a pbn ligand, and herein we report the acid-base properties, electrochemical properties, and reactivity toward CO2 reduction using C−H hydride transfer reactions.  Experimental Section. The ligand pbn was prepared from 3-aminoquinoline by a seven-step synthesis using previously published methods. [Ru(bpy)2(pbn)](PF6)2 was prepared as previously described and characterized by NMR, UV-vis, IR, ES-MS spectroscopy. Re(pbn)(CO)3Cl and [Re(pbn)(CO)3(PCy3)]PF6 (Cy = cyclohexyl) were prepared using previously published methods with slight modifications.  UV-vis spectra were measured on a Hewlett-Packard 8452A diode array spectrophotometer. FTIR spectra were measured on a Thermo Nicolet NEXUS 670 FT-IR spectrometer or a Bruker IFS 66/s spectrometer. NMR spectra were measured on a Bruker UltraShield 400 MHz spectrometer. Transient absorption spectra were measured with home-build TA setup. The second or third harmonic of a Continuum Surelite I YAG:Nd3+ laser was used as an excitation source. The laser was operated in pulsed mode with a repetition rate of 10 Hz and energy 2 mJ per pulse. Flash photolysis experiments were performed using a sample prepared by a vacuum distillation of a solvent (CH3CN, THF, etc.) to a container equipped with an optical cell at 25 °C under vacuum or with a known pressure of CO2 and/or Ar. Transient FTIR spectra of Re complexes were measured using a Bruker IFS 66/s spectrometer with a Kolmar HgCdTe detector and a flow cell (1.0 − 2.8 mm pathlength) for step-scan (time resolution: 10 ns) or with a Graseby Infrared photoconductive HgCdTe detector and a 0.1 − 0.5 mm pathlength vacuum tight cell for rapid scan. Acetonitrile solutions containing N+NN RuH2e-, 2H+3+ 2+NNN Ru-H+H+CH3COCH3CH3CH(OH)CH3NNNNNNNN2+NNN RuNNNNH HHScheme 1 Protonation and acetone reduction by [Ru(bpy)2(pbn)]2+ BNL-79593-2007-CP Prepr. Pap.-Am. Chem. Soc., Div. Fuel Chem. 2008, 53 (1),  xxxx 1−2 mM Re complex with or without 1 M TEA were pressurized by ~2 atmospheric pressure of Ar or CO2. The sample was excited with the third harmonic (355 nm, 6 ns) of a Continuum Surelite-1 Nd:YAG laser. The excitation energy was typically 20 mJ cm-2 per pulse. Photochemical reduction of rhenium and ruthenium species was conducted in either CH3CN/Et3N (4:1 v/v) under vacuum or N2-bubbled DMF/TEOA (4:1 v/v). The solution was irradiated either with monochromated light or light through a cut-off filter using a 75 W lamp at 25 °C. Electrochemical measurements were conducted with BAS 100b electrochemical analyser from Bioanalytical Systems using solutions (1 mM) in acetonitrile. A glassy carbon electrode, SCE, and Pt wire were used with a standard H-shape cell. Gas and solution phase DFT calculations were carried out using the hybrid B3LYP method and the Hay-Wadt VDZ (n+1) ECP (LANL2DZ ECP) 5d basis set. Geometry optimizations and frequency analyses were performed using the Gaussian 03 package of programs.  Results and Discussion Thermodynamic hydricity, the ability to donate a hydride ion (H−), for metal hydrides and dihydrobenzonicotinamides in acetonitrile has been determined through systematic studies of electrochemistry and acidity by DuBois and his coworkers.9 Hydricity is defined as the free-energy change for dissociation of H−, in contrast with acidity and homolytic cleavage. Experimental estimates of hydricity of substrate DH are obtained from cycles based on combinations of the processes from the free-energy changes as ΔGoH− = ΔGoH+(D−H) − 2 Eo(D+/D−) + ΔGoH+(H2) + Eo(NHE), where ΔGoH−, ΔGoH+(D−H), Eo(D+/D−), ΔGoH+(H2), and Eo(NHE) are hydricity of DH, acidity of DH, reduction potential of DH, H2 acidity, and reduction potential of H+, respectively. Hydricities obtained in our preliminary calculations in acetonitrile solution are compared to experimental values in Table 1. The computed electronic energy hydricity differs somewhat from the hydricity free energy (comparable to that derived from experimental data) in that it does not include the contribution of zero-point and thermal energy or entropic effects, but these effects are generally small and trends can be compared. The strongest hydride donor, CHO−, will transfer a hydride to any acceptor listed below it. Several metal complexes seem capable of hydride transfer to CO2 in acetonitrile: Cp(bpy)Mo(CO)(H) from the metal and Re(bpy)(CO)3(CHO) and Re(CO)3(dcb)(CHO) from the C−H of the formyl ligand. Our preliminary calculations predict that the reduction of acetone to 2-propanol can take place in acetonitrile by the sequential transfer of a proton from solution followed by a hydride from [Ru(bpy)2(pbnHH)]2+ in two slightly exoergic steps in disagreement with the previously published proposed mechanism.7 The calculations also predict that neither [Ru(bpy)2(pbnHH)]2+ nor [Ru(bpy)2(pbn)]2+ can reduce CO2. We are therefore exploring the development of a stronger hydride donor than [Ru(bpy)2(pbnHH)]2+ with the guidance of calculations of hydricities to screen the possibilities. In order to test if this type of scenario will work, we have prepared several new complexes including [Ru(bpy)2(pbnHH)]2+, Re(pbn)(CO)3Cl and [Re(pbn)(CO)3(PCy3)]+. We examined the acid-base and electrochemical properties of the ground and excited states of Re(pbn)(CO)3Cl and [Re(pbn)(CO)3(PCy3)]+. The titration of a solution containing Re(pbn)(CO)3Cl or [Re(pbn)(CO)3(PCy3)]+ by acid shows the disappearance of the lowest MLCT absorption band and the appearance of the red-shifted band indicating the formation of the protonated species as found in [Ru(bpy)2(pbn)]2+ to form [Ru(bpy)2(pbnH)]3+. The excited-state properties under presence and absence of acid, and the reactivity of the ground and excited-state species toward CO2 activation are currently investigated using [Ru(bpy)2(pbnHH)]2+, Re(pbn)(CO)3Cl and [Re(pbn)(CO)3(PCy3)]+. The results will be presented at the ACS National Meeting in New Orleans. LA.  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) Hayashi, Y.; Kita, S.; Brunschwig, B. S.; Fujita, E. J. Am. Chem. Soc. 2003, 125, 11976-11987. (2) Fujita, E.; Muckerman, J. T. Inorg. Chem. 2004, 43, 7636-7647. (3) (a) Fujita, E. Coord. Chem. Rev. 1999, 185-86, 373-84. (b) Fujita, E.; Brunschwig, B. S., Homogeneous Redox Catalysis of CO2 Fixation. In Electron Transfer in Chemistry, Balzani, V., Ed. Wiley-VCH: 2001; Vol. IV, pp 88-126. (4) Tanaka, K.; Ooyama, D. Coord. Chem. Rev. 2002, 226, 211-218. (5) Sweet, J. R.; Graham, W. A. J. Am. Chem. Soc. 1982, 104, 2811-2815. (6) Creutz, C.; Chou, M. H. J. Am. Chem. Soc. 2007, 129, 10108-10109., (7) Koizumi, T.; Tanaka, K. Angew. Chem., Int. Ed. 2005, 44, 5891-5894. (8) (a) Polyansky, D. E.; Cabelli, D.; Muckerman, J. T.; Fujita, E.; Koizumi, T.; Fukushima, T.; Wada, T.; Tanaka, K. Angew. Chem. Int.  Ed. 2007, 46, 4169-4172. (b) Polyansky, D. E.; Cabelli, D.; Muckerman, J. T.; Fukushima, T.; Tanaka, K.; Fujita, E. Inorg. Chem. Submitted (9) (a) Ciancanelli, R.; Noll, B. C.; DuBois, D. L.; DuBois, M. R. J. Am. Chem. Soc. 2002, 124, 2984-2992. (b) Berning, D. E.; Noll, B. C.; DuBois, D. L. J. Am. Chem. Soc. 1999, 121, 11432-11447. (c) Ellis, W. W.; Ciancanelli, R.; Miller, S. M.; Raebiger, J. W.; DuBois, M. R.; DuBois, D. L. J. Am. Chem. Soc. 2003, 125, 12230-12236. (d) Curtis, C. J.; Miedaner, A.; Raebiger, J. W.; DuBois, D. L. Organometallics 2004, 23, 511-516.    Table 1.  Computed Electronic Energy Hydricities in Acetonitrile (B3LYP/LANL2DZ level of theory)  Hydride Donor  Product El. Energy Hydricity (kcal/mol) CHO− CO −61 CH3CHN− CH3CN 0 isopropoxide− acetone 24 Cp(bpy)(CO)MoH (+ CH3CN) Cp(bpy)(CO)Mo(NCCH3)+  39a [ReI(bpy)(CO)3(CHO)]0 [ReI(bpy)(CO)4]+ 45 [ReI(CO)3(dcb)(CHO)]0 [ReI(CO)4(dcb)]+  49b HCOO− CO2         51 (43) N-Me-nicH N-Me-nic+ 60 N-Bz-nicH N-Bz-nic+         61 (59) Cp(bpy)(CO)MoH Cp(bpy)(CO)Mo+   65a para-monohydroquinone− para-benzoquinone 69 [CpReI(NO)(CO)(CHO)]0 [CpReI(NO)(CO)2]+         70 (55)c H2 (+ CH3CN) CH3CNH+         80 (76) [RuII(bpy)2(pbnHH)]2+ [RuII(bpy)2(pbnH+)]3+ 92 isopropanol CH3C(OH)CH3+ 94 [IrIII(bpy)2(pbnHH)]3+ [IrIII(bpy)2(pbnH+)]4+ 104 para-hydroquinone para-monohydroquinone+ 128 a LANL2DZ/6-31G(d,p) 6D basis; b dcb=(4,4′)-dichloro-(2,2′)-bipyridine;  c Experimental value corresponds to Cp*, not Cp as calculated;  N-Me-nicH =N-methylnicotinamide, N-Bz-nicH=N-benzylnicotinamide; calculations compared to DuBois ΔGo values (in parentheses) are in blue.  

PHOTOCHEMICAL CO2 REDUCTION BY RHENUIM AND RUTHENIUM COMPLEXES.

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PHOTOCHEMICAL CO2 REDUCTION BY RHENUIM AND RUTHENIUM COMPLEXES.

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