@misc{oai:ir.soken.ac.jp:00000164,
 author = {豊原, 清綱 and トヨハラ, キヨツナ and TOYOHARA, Kiyotsuna},
 month = {2016-02-17, 2016-02-17},
 note = {Carbon dioxide is a potential Cl source for the synthesis of future fuels and chemicals.  A variety of metal complexes have proven to be active as precursors to CO and/or HC(O)OH generations in electro- and photochemical reductions of CO2.  From the viewpoint of the utilization of CO2, however, multi-electron reduction of C02 accompanied with C-C bond formation is much more important than the two electron reduction of CO2.  Recently, highly reduced products containing C-C bonds are obtained in a few photo- and electrochemical CO2 reductions mediated by transition metal complexes in both homogeneous and heterogeneous reactions. Elucidation of the reaction mechanism including M-C bond formation of CO2 to metals would serve to construct a strategy for the design of catalysts, which effectively catalyze the reduction of CO2 accompanied with C-C bond formation.  In this connection, knowledge concerning the binding modes of CO2 to metals is a fundamental importance to understand the reactivity of CO2 ligated to metals.  Among a various metal CO2 complexes reported so far, metal complexes with an η1 -CO2 ligand are generally accepted as reaction intermediates for CO generation in photo- and electrochemical CO2 reductions, since metal-η1 -C02 complexes ([M-η1 -CO2 ]) are subject to protonation in protic media to form metal-hydroxycarbonyls ([M-C(O)OH]) and -carbonyls ([M-CO]) as precursors to HC(O)O-  and CO, respectively.  The molecular structures of M-η1 -CO2 and the conjugated acid M-η1 -C(O)OH, therefore, are of interest in the elucidation of the C02 /CO conversion on metals because of the lack of the data concerning the structural difference in those two complexes.  Furthermore, the elucidation of the role of an M-C(O)H in the reduction of CO2 is also interested on the basis of the proposal as the key intermediate in the multi-electron reduction of CO2 accompanying by C-C bond formation.
　The purpose of this study is to elucidate the main factor which controls the selectivity of two- and multi-electron reduction of CO2.  This would afford a strategy for designing of effective catalysts directed toward multi-electron reduction of CO2 accompanied by C-C bond formation.
　lnterconversion of CO2 and CO on metals requires an intramolecular 2-electron transfer between metals and CO2 or CO through the metal-C bond.  The two electron transfer from and to metal centered orbitals may cause serious configurational changes of metal complexes, which would strongly hamper the smooth CO2/CO conversion.  Such the structural change of complexes in the metal centerd redox reaction would be effectively depressed by participation of m-orbitals of ligands as well as d-orbitals of metals in the intramolecular electron transfer, and that would accelerate the C02/CO conversion.  Hydroxycarbonyl complexes (M-η1 -C(O)OH) as the intermediate in the CO2/CO conversion, therefore, may afford the fundamental knowledge about the intramolecular electron transfer by the comparison of the deprotonated complexes, M-η1 -CO2.  In the Chapter 2, the molecular structure of [Ru(bpy)2 (CO)(η1 -C(O)OH)]+  is determined as the first example of a hydroxycarbonyl intermediate in CO2 reduction.  The comparison in the molecular structures between [Ru(bpy)2 (CO)(η1 -C(O)OH)]+  and [Ru(bpy)2 (CO)(η1 -CO2 )] revealed that not only the RU-CO2 but also the Ru-N (trans to CO2) bond distance is largely influenced by a protonation of [Ru(bpy)2 (CO)(η1 -CO2)].  Thus, σ-donor and π-acceptor abilities of bpy play the role in the electron reservoir to facilitate the smooth interconversion among [Ru(bpy)2 (CO)(η1 -CO2)], [Ru(bpy)2 (CO)(η1 -C(O)OH)]+  and [Ru(bpy)2 (CO)2 ]2  in protic media.
　The adjustment of the electron density of the CO2 moiety in M-η1 -CO2  complexes would serve the control of the reactivities in the reduction of CO2.  In the Chapter 3, stabilization of [Ru(bpy)2 (CO)(η1 -CO2 )] in non protic media is described.  In solid state, [Ru(bpy)2 (CO)(η1 -CO2 )] forms three hydrogen bonds between the oxygens of the η1 -CO2 moiety and three hydrate water molecules.  The complex dissociates CO2 to form [Ru(bpy)2 (CO)H]+ in CH3CN in the presence of a small amount of H2O.  Furthermore, [Ru(bpy)2 (CO)(η1 -CO2 )] also reacted with O2, with evolving CO2 in dry CH3CN to give [Ru(bpy)2(O2CO)] possibly through an O2 adduct intermediate ([Ru(bpy)2 (CO)(O2)]).  On the other hand, [Ru(bpy)2 (CO)(η1 -CO2)] is quite stable in alcohol even in air.  Thus, hydrogen bondings formed between [Ru(bpy)2 (CO)(η1 -CO2)] and alcohol effectively stabilizes the complex by depression of the accumulation of excess electrons in the η1 -C02 moiety. Otherwise [Ru(bpy)2 (CO)(η1 -CO2)] dissociates CO2, and the resulting [Ru(bpy)2 (CO)]0 undergoes an electrophilic attack of a proton and O2.  Stabilization of [Ru(bpy)2 (CO)(η1 -CO2)l in dry CH3CN is also achieved by the presence of Li+.  The 13CNMR of an CD3CN solution of the carbon-13 enriched [Ru(bpy)2(CO)(η1 -CO2)l in the presence of Li+ showed a broad and a sharp signal at σ 215 and 203 ppm assignable to CO2 and CO groups, respectively.  Furthermore a low νasym(CO2) band appeared at 1467 cm-1 in the solution IR spectra in CD3CN in the presence of Li+.  The observations that the shift of the 13CO2 signal from δ210 ppm in CH3OH to δ215 in CH3CN and the missing of the νasym(CO2) band in the IR spectra in CD3CN in the presence of Li+, are reasonably explained by the formation of a carbene like structure (I) in the medium .  (figure I)  Thus, the bond character of a M-η1 -CO2 complex is controlled by the intermolecular bondings.  Hydrogen bonding and association of Li+ to the oxygen atoms of the η1 -CO2 group cause inclination of electrons to the CO2 moiety and stabilize [Ru(bpy)2 (CO)(η1 -CO2 )].  Destruction of those intermolecular bondings results in the shift of electrons to metal centers, which gives rise to dissociation of CO2 to form unstable [Ru(bpy)2(CO)]0.
　In the Chapter 4, reactivities of [Ru(bpy)2(CO)(CHO)]+ and [Ru(bpy)2 (CO)(CH20H)]+ as models of reaction intermediates in multi-electron reduction of C02 are studied.  Electrochemical reduction of CO2 by [Ru(bpy)(trpy)(CO)]2+ in EtOH/H2O produces not only CH2O and CH3OH but also HO(O)CCHO and HO(O)CCH2OH together with HC(O)OH as the main product.  Both [Ru(bpy)(trpy)(CHO)]+ and [Ru(bpy)(trpy)(CH2OH)]+, which are generated by two- and four-electron reduction of [Ru(bpy)(trpy)(CO)]2+, are proposed as the precursors to those highly reduced products.  In order to elucidate the role of [Ru(bpy)(trpy)(CHO)]+ in the multi-electron reduction of C02, the reactivity of [Ru(bpy)2 (CO)(CHO)]+ instead of [Ru(bpy)(trpy)(CHO)]+ toward CO2 was examined due to the extremely thermal lability of the latter.  In CH3CN, [Ru(bpy)2(CO)(CHO)]+ is stable below -20℃.  Even such a low temperature, [Ru(bpy)2 (CO)(CHO)]+ smoothly reacted with CO2 to form HC(O)O- with generation of [Ru(bpy)2 (CO)2]2+.  A strong hydride donor ability of [Ru(bpy)2 (CO)(CHO)]+ well explains the generation of HC(O)OH as the main product in the multi-electron reduction of CO2 by [Ru(bpy)(trpy)(CO)]2+.  Taking into account of the formation of HC(O)O- and [Ru(bpy)2 (CO)2]2+ in the reaction of [Ru(bpy)2 (CO)(CHO)]+ with CO2, where the reaction pass strongly interferes the multi-electron reduction, a decrease in a hydride ability of a formyl complex would overcome the difficulty in multi-electron reduction of CO2 by metal complexes.  Thus, a metal-formyl complex is the key intermediate as for the two- and multi-electron reductions of CO2.  Furthermore, the observation that protonation and carboxylation of [Ru(bpy)2 (CO)(CH2OH)]+ as a model of [Ru(bpy)(trpy)(CH2OH)]+ produced CH3OH and HO(O)CCH2OH under electrolysis conditions reasonably elucidates the function of Ru-CH2OH in the six-electron reduction of CO2.
　The multi-step conversion from Ru-CO2, Ru-C(O)OH, Ru-CO, Ru-C(O)H to Ru-CH2OH is inevitably accomplished by variation in the carbon orbital of the Ru-C bonds (sp2, sp, and sp3), which would also give crucial influence on the formation energy of HC(O)OH, CO, CH2O, CH3OH, and CH4 in multi-electron reduction of CO2 by metal complexes.  Vibrational spectroscopy may provide useful information about the Ru-C bond characters in the conversion from Ru-CO2 to Ru-CH2OH.  In the Chapter 5, comparisons of raman spectra of a series of cis-[Ru(bpy)2 (CO)X]n+ (X = CO, C(O)OH, C(O)OCH3, CO2, CHO, and CH2OH;  n = 0, 1, 2) and their 18O or deuterium substituted analogs permit reasonable assignments of ν(Ru-X) and ν(Ru-CO) bands around 500 and 470 cm-1, respectively.  The validity of the assignments of those bands led to identification of two configurational isomers of cis-[Ru(bpy)2(CO)(CH2OH)]+ with respect to the orientation of the CH2-OH bond.  The ν(Ru-X) bands shift to higher wavenumbers with lengthening the Ru-X bond distances (d(Ru-X)).  Such unusual dependence of ν(Ru-X) upon d(Ru-X) may be associated with multi-bond characters of the C=O, C=O and C-O bonds in the Ru-X moieties., application/pdf, 総研大甲第184号},
 title = {Elucidation of Metal-Carbon Bond Characters in CO2Multi-Electron Reduction on Metals},
 year = {}
}