1-1. Background
With the industrial revolution in the 19th century, carbon dioxide concentrations increased rapidly due to increased use of fossil fuels, resulting in significant increase in the greenhouse effect, which had a major harmful impact on the environment.
Nowadays, the rate of increase in atmospheric carbon dioxide is 100 times faster than the ice age. In the 1950s, the carbon dioxide content in the atmosphere increased by 0.7 ppm annually; by the year 2000, the annual rate of increase had gone up to 2.1 ppm.
Currently, the annual rate of increase in atmospheric CO2 is 400 ppm, and will easily
reach 450 ppm in the near future.1
Figure 1-1. Atmospheric CO
2 at Mauna Loa Observatory2
The increasing concentration of atmospheric carbon dioxide and shortage of fossil fuels are major concerns of recent times. Therefore, the efficient capture2 and conversion of carbon dioxide into other chemicals like methanol or liquid fuel3,4,5 has been a major research goal for modern scientists. Currently, carbon dioxide can be converted to other chemicals by six different techniques6: chemical7,8, photochemical9, electrochemical10,11,12, biological13,14, reforming15, and inorganic16. Electrochemical methods have the advantage of multi-electron reduction of carbon dioxide; however, the effective and accurate control of the position of the electron transfer is a major challenge. To this end, we expect that an organometallic complex can be used as an electron storage device that can help in stepwise multi-electron reduction.
1-2. Electrocatalytic reduction
6,11Figure 1-2. Electrocatalysis with electron source.
24The catalyst does not usually exist in its crystalline form, but exists in its ionic form. Therefore, the electrode must reduce the catalyst first to its neutral crystalline form, before it can reduce carbon dioxide or other derivatives. The reduction potential of the catalyst should be lower than that of carbon dioxide for carbon dioxide reduction to take place. Moreover, the applied potential should be able to break through
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intermediate energy or activation energy barrier at every step of the reaction for the reaction to proceed successfully. However, if the potential of a side reaction is less than the applied energy, the side reaction may also occur. Therefore, to effectively control the reaction pathway is also a challenge.
1-3. Proton-coupled electron transfer (PCET)
17Scheme 1-3. PCET
PCET is a common reaction mechanism for redox reactions.18 It involves the concerted transfer of an electron and a proton to the substrate. PCET is found in many enzymatic reactions19, water oxidation, carbon dioxide reduction20, and photosynthesis.21,22 In 1984, Sutin reported a series of investigation on electron transfer in chemistry and biology.23 In 2011, Savéant reported the concerted transfer of an electron and a proton in a single energy transfer process demands attention. For example17:
PhOH→ PhO˙+ e–+ H+
The above-mentioned phenol oxidation reaction can proceed via three different
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pathways: EPT (proton transfer followed by electron transfer), PET (electron transfer followed by proton transfer), and CPET (concerted proton-electron transfer).
1. EPT: This is a two-step process: first, phenol must be oxidized by losing an electron to generate an intermediate "PhOH˙", which then loses a proton to obtain the final product "PhO˙".
2. PET: This is also a two-step process, opposite to EPT. In this, phenol undergoes a proton transfer first to obtain the intermediate "PhO–", which then undergoes single electron transfer to obtain the final product "PhO˙".
The free energy from the starting reactant to the product should be low enough and the pka should be higher than that of the proton acceptor for the reaction will undergo any of the two above-mentioned pathways.
3. CPET: It involves concerted proton-electron transfer, i.e. it involves a single-step transfer of a proton and an electron without forming any intermediate. This pathway has the advantage of a lower activation energy than both EPT and PET pathways.24
It’s the alternative name of PCET, and we use PCET for this pathway of the following discussion.
1-4. Ruthenium complexes in electrocatalytic reduction
Electrocatalytic reduction of CO2 using metal catalysts that can be separated into three classes based on the ligand types: (1) macrocyclic ligands; (2) bipyridine ligands;
and (3) phosphine ligands.6
Scientists usually utilized bipyridine (bpy) ligands coordinated to Ru(II) for catalyzing CO2 reduction.It was reported by Koji Tanaka et al. that Ru(bpy)(CO)22+
and Ru(bpy)(CO)Cl+ could catalyze the conversion of CO2 to CO, HCOO-, and H2. Though the catalysts in electrocatalytic reduction of CO2 have low turnover numbers
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and low sensitivity, identifying the key intermediates in the reduction is still useful.25 In 1993, Tanaka used [Ru(bpy)(trpy)CO]2+ as a low-potential catalyst to produce four-electron products (HC(O)H, H(O)CCOOH) as well as six-four-electron products (CH3OH, HOCH2COOH).26 Tanaka et al. also predicted the mechanism in 1994.27 In 1991, Hoffman et al. reported ten Ru(II)-diimine complexes of the formula Ru(bpy) 3-m-z(bpm)m(bpz)z2+ (bpy =2, 2'-bipyridine, bpm = 2,2'-bipyrimidine, bpz = 2,2'-bipyrazine,
m and z = 0,1,2,3 and m + z
≤ 3) for one-electron reduction;they also reported that for good reduction activity, the catalyst should have the tendency localize electrons on the most easily reduced ligand (bpz > bpm > bpy).28 In 2016, Sascha Ott utilized the bpy ligand of [Ru(tBu3tpy)(bpy)(NCCH3)]2+ with a simple methyl substituent and discovered that having a simple methyl substituent on different positions on the bpy ligand can develop a new mechanistic pathway for reductive disproportionation.29In 2007, Peter G. Pickup reported that Ru(bpy)2(2-(2-pyridyl)benzothiazole)2+ as a potential catalyst for CO2 reduction. The acitivities of the benzothiazole and benzimidazole analogues have a large difference on the CO2 reduction. Bithiazole complexes require multiple reduction to become active. Involving of the -s- as the position for CO2 activation is an important factor in increasing the activities.30
In 2016, Huang et al. reported that a PN3-Ru pincer complex bearing a redox-active bpy ligand with an aminophosphine is an effective catalyst for CO2 reduction to CO and HCOOH at high FEs with negligible formation of H2 in a H2O/MeCN mixture, and proposed the overall mechanism.31 Under the same conditions, without the PN3-Ru pincer complex, H2 is the major product of CO2 reduction. Two of the most important steps involved are: (1) the bpy performs a ligand-to-metal charge transfer and leads to loss of Cl-, and (2) insertion of CO2 insertion in the complex.
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1-5. A series of investigations on CO
2reduction by Koji Tanaka
Figure 1-5. Cyclic voltammograms of [Ru(bpy)(trpy)CO]
2+ and plots of amounts for productsKoji Tanaka is one of the few scientists who use an organometallic complex as a catalyst for direct reduction of CO2 to methanol under an applied negative potential via multi-electron-proton transfer. In 1993, Koji Tanaka used [Ru(bpy)(trpy)CO]2+ as catalyst at −20 oC, using an applied potential of −1.7V for reducing carbon dioxide saturated in a methanol–water mixture (v/v 4:1).26 Carbon dioxide was successfully reduced to produce not only CO and HCOOH but also HCHO and C2H5OH; products involving formation of C–C bonds were also formed.
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Scheme 1-5. Mechanism predicted by Tanaka
Tanaka predicted the reaction mechanism of carbon dioxide reduction in 1994.27 [Ru-CO]2+ took up two electrons and one proton to form an intermediate ‘[Ru-CHO]+’.
It may obtain a proton to form HCHO in acidic solution or can react with CO2 to form a C2-product H(O)CCOOH. The reduction may continue further to form Ru-CH2OH, and finally CH3OH and HOCH2COOH. On removing the product, the vacancy of the catalyst will be replaced by solvent or CO2, forming Ru-CO2. After dehydration, the catalyst will return to its initial form [Ru-CO]2+ to complete a catalytic cycle. However, every step of the conversion process was not well understood; we propose to use a computational method to obtain more details of every step of the reaction mechanism
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1-6. Non-innocent ligand
Scheme 1-6. catecholate/o-quinone redox system
32A non-innocent ligand means a ligand that can undergo redox reactions in a catalytic system, e.g. o-quinone (Scheme 1-6). In the example shown, o-quinone can accept two electrons to generate o-semiquinone or catechol,33,34 which helps disperse the charge density of the central metal. Such electron transfers between the metal center and a ligand can maintain a low oxidation state at the metal center. Complexes containing non-innocent ligands undergo active reduction at moderate potentials because of its electrons are delocalized between the metal and the ligands.
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