Result and discussion

3-1. Background

Theoretical computations in terms of feasibility of reactions are used to confirm the mechanism predicted by Koji Tanaka. Electron and proton transfer processes were studied to determine whether CO2 reduction followed EPT, PET, or PCET

mechanism. Potential and pKa data were used discuss the feasibility of the reaction.

The energy values obtained via geometry optimization and vibrational frequency analysis were used to confirm the minimum-energy structures. The standard free energy was obtained through a thermodynamic cycle, and was substituted in

appropriate formulas to obtain the necessary potential and pKa values. The potential calculated from pH = 0 to pH = 7 at −20^oC in C2H5OH/H2O (8:2 v/v) was corrected experimentally.

The computed pKa of ethanol is 31. If the pKa value of the complex is larger than 31, it means that the deprotonated form of this complex has an ability to catch a proton from the solvent. The computed potential energy was compared with the external potential applied by Koji Tanaka at −1.70V. If our computed reduction potential energy is less negative than −1.70V, it implies that the reaction is feasible.

17

3-2. The reaction of [Ru(bpy)(trpy)CO]

²⁺

(Innocent Ligand)

3-2-1. [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO]

Scheme 3-2-1. Calculated reduction potentials for converting [Ru(bpy)(trpy)CO] to

[Ru(bpy)(trpy)CHO]. The green values represent the potentials for ET or PCET, and red values represent the pKa for PT.

For [Ru(trpy)(bpy)CO]²⁺ (referred to as [Ru-CO]²⁺ in the following discussion) to undergo a PCET process to become [Ru-CHO]²⁺, a large negative potential of −2.58V and a pKa of −15.9 are required. Hence [Ru-CO]¹⁺ cannot spontaneously accept a proton from a solvent molecule. However, a potential of only −1.05V is needed for direct reduction by an electron. Thus, we assume that [Ru-CO]²⁺ will undergo an ET process to generate [Ru-CO]¹⁺.

There are two possible pathways after [Ru-CO]¹⁺ is formed: one is ET and another is PCET. The required potentials are −1.13V and −0.96V, respectively, with pKa = 15.1.

The potential of PCET is lower than that of ET, but the pKa of [Ru(CHO)¹⁺ is lower than ethanol, and thus the reaction does not occur. It likely forms [Ru-CO]⁰⁺ by ET rather than form [Ru-CHO]¹⁺ by PCET.

When generating [Ru-CO]⁰⁺, the reaction still cannot proceed via the PCET process because of the low pKa of [Ru-CHO]⁰, although the respective potential for the PCET process is −0.97V. The pKa value of this intermediate is 23. We assumed that it will gain one electron via ET and form [Ru-CO]^1- rather than undergo a PCET to

[Ru-18

CHO]⁰. The potential of the one-electron reduction is −1.61V.

After a series of single-electron reductions to [Ru-CO]^1-, [Ru(CO)]^1- may be able go to [Ru(CHO)]^1- because of the suitable potential (−0.88V) and since pKa (30.6) of [Ru(CHO)]^1- is close to 31. The intermediate [Ru-CHO]^1- could be generated by the PCET reaction. Due to the redox couple of Ru(CO)]^-1/[Ru(CHO)]^-1 = -0.88V is larger than [Ru(CO)]⁰/[Ru(CO)]^-1 = -1.61V, the equilibrium of [Ru(CO)]^-1 with its neighboring redox species may not be established. [Ru(CO)]^-1 could undergo a disproportionation reaction to decompose to [Ru(CO)]⁰ and [Ru(CHO)]^-1; hence the 2e-1proton PCET step from [Ru(CO)]⁰ to [Ru(CHO)]^-1 at -1.25V could be the dominate path.

19

Table 3-2-1. Calculated values of pK

a and E⁰ at the Density Functional Theory Level of [Ru(bpy)(trpy)CO] to [Ru(bpy)(trpy)CHO].

20

3-2-2. [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH

2

OH]

Scheme 3-2-2. Calculated reduction potentials for converting [Ru(bpy)(trpy)CHO] to

[Ru(bpy)(trpy)CH2OH]. The green and blue values represent the potentials for ET or PCET, and red values represent the pKa for PT.

In the present study, we followed the criteria of applying external potential -1.70V with higher priority. [Ru-CHO]^1- cannot be reduced to [Ru-CHO]^2- and this step required -1.93V. There are two possible PCET pathways for [Ru-CHO]^1- to form [Ru(CH2O)]^-1 or [Ru(CHOH)]^-1 the former path forms new CH bond and the later forms new OH bond. Despite the pKa of both [Ru(CH2O)]^-1 and [Ru(CHOH)]^-1 are not strong enough for the deprotonated form to extract proton from the solvent, the proton tunneling effect or thermal electron redistribution may possibly enhance the proton extract. Thus, the catalytic cycle can go to the next step. The subsequence PCET for both [Ru(CH2O)]^-1 and [Ru(CHOH)]^-1 is straightforward, since both pKa

values are greater than 31, to form [Ru(CH2OH)]^-1 species.

21

22

[Ru-(trpy)(bpy)(CHO)]^1- (T) + H⁺ → [Ru-(trpy)(bpy)(CH2O)]⁰⁺ (S) 18.0 [Ru-(trpy)(bpy)(CHO)]^1- (T) + H⁺ → [Ru-(trpy)(bpy)(CH2O)]⁰⁺ (T) 13.5 [Ru-(trpy)(bpy)(CHO)]^1- (T) + H⁺ → [Ru-(trpy)(bpy)(CHOH)]⁰⁺ (S) 10.2 [Ru-(trpy)(bpy)(CHO)]^1- (T) + H⁺ → [Ru-(trpy)(bpy)(CHOH)]⁰⁺ (T) 11.2 [Ru-(trpy)(bpy)(CHO)]^2- (D) + H⁺ → [Ru-(trpy)(bpy)(CH2O)]^1- (D) 19.7 [Ru-(trpy)(bpy)(CHO)]^2- (D) + H⁺ → [Ru-(trpy)(bpy)(CHOH)]^1- (D) 16.8 [Ru-(trpy)(bpy)(CHO)]^3- (T) + H⁺ → [Ru-(trpy)(bpy)(CH2O)]^2- (S) 27.2 [Ru-(trpy)(bpy)(CHO)]^3- (T) + H⁺ → [Ru-(trpy)(bpy)(CH2O)]^2- (T) 29.1 [Ru-(trpy)(bpy)(CHO)]^3- (T) + H⁺ → [Ru-(trpy)(bpy)(CHOH)]^2- (S) 21.3 [Ru-(trpy)(bpy)(CHO)]^3- (T) + H⁺ → [Ru-(trpy)(bpy)(CHOH)]^2- (T) 21.2 [Ru-(trpy)(bpy)(CHO)]^4- (D) + H⁺ → [Ru-(trpy)(bpy)(CH2O)]^3- (D) 32.0 [Ru-(trpy)(bpy)(CHO)]^4- (D) + H⁺ → [Ru-(trpy)(bpy)(CHOH)]^3- (D) 27.1 [Ru-(trpy)(bpy)(CH2O)]⁰⁺ (S) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]¹⁺ (S) 27.8 [Ru-(trpy)(bpy)(CHOH)]⁰⁺ (T) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]¹⁺ (S) 34.7 [Ru-(trpy)(bpy)(CH2O)]^1- (D) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]⁰⁺ (D) 37.9 [Ru-(trpy)(bpy)(CHOH)]^1- (D) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]⁰⁺ (D) 40.7 [Ru-(trpy)(bpy)(CH2O)]^2- (T) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]^1- (T) 41.0 [Ru-(trpy)(bpy)(CHOH)]^2- (S) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]^1- (T) 49.8 [Ru-(trpy)(bpy)(CH2O)]^3- (D) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]^2- (D) 50.2 [Ru-(trpy)(bpy)(CHOH)]^3- (D) + H⁺ → [Ru-(trpy)(bpy)(CH2OH)]^2- (D) 55.1

Table 3-2-2.Calculated values of pK

and E⁰ at the Density Functional Theory Level of [Ru(bpy)(trpy)CHO] to [Ru(bpy)(trpy)CH2OH].

23

3-2-3. [Ru(bpy)(trpy)CH

2

OH] to [Ru(bpy)(trpy)CO]

Scheme 3-2-3. Calculated reduction potentials for converting [Ru(bpy)(trpy)CH

2OH]

to [Ru(bpy)(trpy)CO]. The green values represent the potentials for ET or PCET, red values represent the pKa for PT, and blue values represent for the gibbs free energy.

The Gibbs free energy from [Ru-CH2^αOH^β]^1- to form [Ru-OEt]^1- is 3 kcal/mol, and the Gibbs free energy from [Ru-OEt]^1- to form [Ru-CO2]^0- is -2.48 kcal/mol. It is possible to form [Ru-CO2]^0- and [Ru-OEt]^1-. We predicted the major reaction is one-electron transfer to [Ru-OEt]^1-, because of decreasing the coulomb attraction.

The reaction of [Ru-OEt]^2- to form [Ru-CO2]^1- is considered nonspontaneous, the Gibbs energy is 4.77 kcal/mol. According to scheme 3-2-3., the Gibbs free energy is lower while the more electrons reduced on the complex. It means [Ru-CH2αOH^β] can accept one proton to form CH3OH, which is removed immediately, and the vacancy is replaced by ethanol or CO2 instantaneously with high charge density on complex.

The potential of [Ru-CO2]^1- is −0.51V and its pKa is 33.7; therefore, it easily

24

undergoes PCET to form [Ru-COOH]^1- or undergoes ET to form [Ru-CO2]^2-. In the final step, the dehydration from [Ru-COOH]^2- to form [Ru-CO]^1-, which carries on performing all the above-mentioned reactions.

25

Table 3-2-3-1. Calculated values of pK

and E⁰ at the Density Functional Theory Level of [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)COOH].

26

Reaction Gibbs free energy

Spin Spin

[Ru-(trpy)(bpy)(CH2OH)]⁰⁺ (D) + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]⁰ (D) 0.94

[Ru-(trpy)(bpy)(CH2OH)]^1- (T) + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]^1- (S) 3

[Ru-(trpy)(bpy)(CH2OH)]^1- (T) + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]^1- (T) 3.14

[Ru-(trpy)(bpy)(CH2OH)]^2- (D) + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]^2- (D) -18.44

[Ru-(trpy)(bpy)(CH2OH)]^3- (T) + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]^3- (S) -19.4

[Ru-(trpy)(bpy)(CH2OH)]^3- (T) + [EtOH] → CH3OH + [Ru-(trpy)(bpy)(OEt)]^3- (T) -19.97

[Ru-(trpy)(bpy)(OEt)]⁰ (D) + CO2 → OEt^- + [Ru-(trpy)(bpy)(CO2)]¹⁺ (D) 15.85

[Ru-(trpy)(bpy)(OEt)]^1- (S) + CO2 → OEt^- + [Ru-(trpy)(bpy)(CO2)]⁰ (S) -2.48

[Ru-(trpy)(bpy)(OEt)]^2- (D) + CO2 → OEt^- + [Ru-(trpy)(bpy)(CO2)]^1- (D) 4.77

[Ru-(trpy)(bpy)(OEt)]^3- (T) + CO2 → OEt^- + [Ru-(trpy)(bpy)(CO2)]^2- (T) -12.4

Table 3-2-3-2.

Calculated values of Gibbs free energy at the Density Functional Theory Level of [Ru(bpy)(trpy)CH2OH] to [Ru(bpy)(trpy)CO2].

27

3-2-4. The mechanism of [Ru(bpy)(trpy)CO]

²⁺

The following Scheme 3-2-4. shows the mechanism of [Ru(bpy)(trpy)CO]²⁺.

Scheme 3-2-4. The mechanism of [Ru(bpy)(trpy)CO]

²⁺

28

3-3. The reaction of [Ru(trpy)(OBQ)CO]

²⁺

(Non-Innocent Ligand)

3-3-1. [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO]

Scheme 3-3-1. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CO] to

[Ru(trpy)(OBQ)CHO]. The green values represent the potentials for ET or PCET, and red values represent the pKa for PT.

If the pKa of a species is higher than that of the solvent, it can extract a proton from the solvent. Therefore, if there is a low-energy PCET pathway but the associated pKa is lower than that of the solvent, the pathway is impossible to execute.

Accordingly, [Ru-CO]²⁺ does not undergo PCET pathways because of its high reduction potential energy (−2.11V) and low pKa (−43.2). It accepts an electron to form [Ru-CO]¹⁺ with low potential energy, which undergoes a direct four-electron reduction to yield [Ru-CO]^2-. Fortunately, the pKa (31.8) is higher than that of the solvent when generating [Ru-CO]^2-; thus, it might carry out the PCET process to form [Ru-CHO]^2- and the associated potential energy is only −1.04V. The other pathway is disproportionation of [Ru-CO]^2- to form [Ru-CHO]^2- and [Ru-CO]^1-.

29

Table 3-3-1. Calculated values of pK

and E⁰ at the Density Functional Theory Level of [Ru(trpy)(OBQ)CO] to [Ru(trpy)(OBQ)CHO].

30

3-3-2. [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH

2

OH]

Scheme 3-3-2. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CHO] to

[Ru(trpy)(OBQ)CH2OH]. The green and blue values represent the potentials for ET or PCET, and red values represent the pKa for PT.

There are two PCET pathways for [Ru-CHO]^2-, one involving proton capture on carbon to form [Ru-CH2O]^2- and the other proton transfer to the oxygen [Ru-CH^αOH]^2-. It can undergo PCET to form [Ru-CH2O]^2-, but cannot form [Ru-CHOH]^2-, because of the respective pKa values (31.7 and 22.2). As-formed [Ru-CH2O]^2- probably undergoes a further PCET step to generate [Ru-CH2^αOH^β]^2-. In these two PCET pathways, the potential energy does not exceed −1.70V, making the processes feasible.

31

32

[Ru-(trpy)(Obq)(CHO)]^1- (S) + H⁺ → [Ru-(trpy)(Obq)(CH2O)]⁰⁺ (S) 6.7 [Ru-(trpy)(Obq)(CHO)]^1- (S) + H⁺ → [Ru-(trpy)(Obq)(CH2O)]⁰⁺ (T) -2.3 [Ru-(trpy)(Obq)(CHO)]^1- (S) + H⁺ → [Ru-(trpy)(Obq)(CHOH)]⁰⁺ (S) 9.3 [Ru-(trpy)(Obq)(CHO)]^1- (S) + H⁺ → [Ru-(trpy)(Obq)(CHOH)]⁰⁺ (T) -4.4 [Ru-(trpy)(Obq)(CHO)]^2- (D) + H⁺ → [Ru-(trpy)(Obq)(CH2O)]^1- (D) 22 [Ru-(trpy)(Obq)(CHO)]^2- (D) + H⁺ → [Ru-(trpy)(Obq)(CHOH)]^1- (D) 13.7 [Ru-(trpy)(Obq)(CHO)]^3- (S) + H⁺ → [Ru-(trpy)(Obq)(CH2O)]^2- (S) 31.7 [Ru-(trpy)(Obq)(CHO)]^3- (S) + H⁺ → [Ru-(trpy)(Obq)(CH2O)]^2- (T) 10.1 [Ru-(trpy)(Obq)(CHO)]^3- (S) + H⁺ → [Ru-(trpy)(Obq)(CHOH)]^2- (S) 22.2 [Ru-(trpy)(Obq)(CHO)]^3- (S) + H⁺ → [Ru-(trpy)(Obq)(CHOH)]^2- (T) 20.4 [Ru-(trpy)(Obq)(CHO)]^4- (D) + H⁺ → [Ru-(trpy)(Obq)(CH2O)]^3- (D) 17.6 [Ru-(trpy)(Obq)(CHO)]^4- (D) + H⁺ → [Ru-(trpy)(Obq)(CHOH)]^3- (D) 28.2 [Ru-(trpy)(Obq)(CH2O)]⁰⁺ (S) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]¹⁺ (T) -0.7 [Ru-(trpy)(Obq)(CHOH)]⁰⁺ (S) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]¹⁺ (T) 10.4 [Ru-(trpy)(Obq)(CH2O)]^1- (D) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]⁰⁺ (D) 15.3 [Ru-(trpy)(Obq)(CHOH)]^1- (D) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]⁰⁺ (D) 23.6 [Ru-(trpy)(Obq)(CH2O)]^2- (S) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]^1- (S) 33.5 [Ru-(trpy)(Obq)(CHOH)]^2- (S) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]^1- (S) 43 [Ru-(trpy)(Obq)(CH2O)]^3- (D) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]^2- (D) 47.7 [Ru-(trpy)(Obq)(CHOH)]^3- (D) + H⁺ → [Ru-(trpy)(Obq)(CH2OH)]^2- (D) 37.1

Table 3-3-2. Calculated values of pK

and E⁰ at the Density Functional Theory Level of [Ru(trpy)(OBQ)CHO] to [Ru(trpy)(OBQ)CH2OH].

33

3-3-3. [Ru(trpy)(OBQ)CH

2

OH] to [Ru(trpy)(OBQ)CO]

Scheme 3-3-3. Calculated reduction potentials for converting [Ru(trpy)(OBQ)CH

2OH]

to [Ru(trpy)(OBQ)CO]. The green values represent the potentials for ET or PCET, red values represent the pKa for PT, and blue values represent for the gibbs energy.

[Ru-CH2^αOH^β]^2- can accept a proton to remove CH3OH, and the resultant vacancy will be instantaneously replaced by ethanol or CO2 because the Gibbs free energy of the former is negative (−34.13 kcal/mol) and that of the latter is positive (21.22 kcal/mol). This means that the former reaction is spontaneous, but the latter reaction can be achieved only by supplying more energy. In contrast to [Ru-(trpy)(bpy)OEt]^2-, the vacancy of proton exaction from [Ru-(trpy)(Obq)OEt]^2- is hard to replace by CO2. It considers that a non-innocent ligand as a good π -acceptor ligand with a mild potential. The stronger π-back donation should make the charge density retain in the ligand, which make Ru less electron rich and inhibit CO2 coordination.³⁰

The intermediate is [Ru-CO2]^1- that undergoes PCET to form [Ru-COOH]^1- with

34

low potential energy (−0.15V) and suitable pKa (33.7) or undergoes ET to form [Ru-CO2]^2- with potential (-1.35V) then undergoes PCET to form [Ru-COOH]^2-.

In the final step, the dehydration from [Ru-COOH]^2- to form [Ru-CO]^1-, which carries on performing all the above-mentioned reactions.

35

Table 3-3-3-1. Calculated values of pK

and E⁰ at the Density Functional Theory Level of [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)COOH].

36

Reaction Gibbs free energy

[Ru-(trpy)(Obq)(CH2OH)]⁰⁺ (D) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]⁰ (D) -11.47

[Ru-(trpy)(Obq)(CH2OH)]^1- (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]^1- (S) -13.20

[Ru-(trpy)(Obq)(CH2OH)]^1- (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]^1- (T) 2.81

[Ru-(trpy)(Obq)(CH2OH)]^2- (D) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]^2- (D) -34.13

[Ru-(trpy)(Obq)(CH2OH)]^3- (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]^3- (S) -32.45

[Ru-(trpy)(Obq)(CH2OH)]^3- (T) + [EtOH] → CH3OH + [Ru-(trpy)(Obq)(OEt)]^3- (T) -36.13

[Ru-(trpy)(Obq)(OEt)]⁰ (D) + CO2 → OEt^- + [Ru-(trpy)(Obq)(CO2)]¹⁺ (D) 43.18

[Ru-(trpy)(Obq)(OEt)]^1- (S) + CO2 → OEt^- + [Ru-(trpy)(Obq)(CO2)]⁰ (S) 35.69

[Ru-(trpy)(Obq)(OEt)]^2- (D) + CO2 → OEt^- + [Ru-(trpy)(Obq)(CO2)]^1- (D) 21.22

[Ru-(trpy)(Obq)(OEt)]^3- (T) + CO2 → OEt^- + [Ru-(trpy)(Obq)(CO2)]^2- (S) 2.25

Table 3-3-3-2.

Calculated values of Gibbs free energy at the Density Functional Theory Level of [Ru(trpy)(OBQ)CH2OH] to [Ru(trpy)(OBQ)CO2].

37

3-3-4. The mechanism of [Ru(trpy)(OBQ)CO]

²⁺

The following Scheme 3-3-4. shows the mechanism of [Ru(trpy)(OBQ)CO]²⁺.

Scheme 3-3-4. The mechanism of [Ru(trpy)(OBQ)CO]

²⁺

38

3-4. Charge and Spin distribution

From the electron distribution in each orbital, the data of charge and spin density are known to be reduced to which part of the metal complexes, and the electron configurations of the complexes of this metal are predictable. The complexes discussed are divided into four structural parts: metal (Ru), bidentate ligand (bpy), tridentate ligand (trpy), and monodentate ligands (CO derivatives).

3-4-1. [Ru(bpy)(trpy)CO]

²⁺

39

According to the above Tables 3-4-1-1 and 3-4-1-2, the distributions of charge and spin do not change visibly over Ru. From [Ru-CO]²⁺ to [Ru-CO]^1-, the charge density decreases only slightly in the CO part, but bpy and trpy parts have significantly reduced charge distributions, that the electron distributions of 3 electrons in the trpy and bpy parts are 1.78 (0.31-(-1.47)) and 1 (0.21-(-0.79)). Otherwise, it reveals that the first electron is reduced on trpy, the second is on bpy, and the third is on trpy.

When [Ru-CO]^1- was converted to [Ru-CHO]^1- by PCET, the charge and spin distributions of bpy remain unchanged; however, the charge density of trpy increases and its spin density decreases, implying that the trpy ligand is oxidized. The charge density of CHO decreased because in addition to undergoing an electron transfer and a protonation, trpy electron density is transferred to CHO as well.

After the PCET process, [Ru-CHO]^1- is converted to [Ru-CH2O]^1-, increasing the charge distribution in CHO with part of its charge transfers to trpy.

40

When [Ru-CH2O]^1- becomes [Ru-CH2OH]^1- via PCET, charge density of [Ru-CH2OH]^2- is slightly decreased. Charge density increases sharply on accepting a proton, but that did not happen in this case. A possible explanation is that electron transfer on CH2OH in order to receive a proton.

When [Ru-CH2OH]^1- gets converted to [Ru-OEt]^1-, the charge distribution of trpy increases. This implies that after CH2OH obtained a proton from the solvent, OEt^- binds to the complex and forms [Ru-OEt]^1-, while trpy provides one electron to CH2OH.

Scheme 3-2-3 shows that the Gibbs free energy is 3 kcal/mol when [Ru-CH2OH]^1- is replaced by the solvent to form [Ru-OEt]^1-.

The conversion of [Ru-OEt]^2- to [Ru-CO2]^1- shows that the charge distributions of bpy and trpy have increasing tendencies but the spin distribution of trpy is decreased.

Therefore, we can assume that trpy provides an electron to OEt^- to abstract a proton that makes it back to the solvent. In the meantime, CO2 fills the cavity.

As [Ru-CO2]^1- is converted to [Ru-CO2]^2- by one-electron reduction, the charge density is reduced in trpy apparently.

In the end, when [Ru-CO2]^2- undergoes PCET to form [Ru-COOH]^2-, there is no obvious change in charge distribution but on the spin distribution in trpy is increasing.

Accordingly, we can assume that H⁺ is reduced in CO2 and the electron is reduced in trpy. At this moment, COOH will obtain a proton and remove a H2O to form [Ru-CO] 1-and the charge on trpy will decrease again. Thus, we can assume that trpy provides electron to COOH, which then obtains a proton and undergoes dehydration.

Above all, this reaction reduces two electrons reduced in trpy and one in bpy, following with a series of electron transfer on trpy. In the end, when [Ru-CO2]^1- is formed, trpy has two electrons and bpy has one electron.

41

Table 3-4-2-2. Spin distribution of [Ru(trpy)(OBQ)CO]

²⁺

42

According to Tables 3-4-2-1 and 3-4-2-2, when [Ru-CO]²⁺ is reduced to [Ru-CO]^2-, the charge and spin distributions of Ru decrease slightly, but the tendency is more obvious than [Ru(bpy)(trpy)CO]²⁺. The charge density is lowered slightly for CO too.

For OBQ and trpy, the charge distribution decreases sharply, implying that four electrons are reduced in OBQ and trpy. Compared with [Ru(bpy)(trpy)CO]²⁺, [Ru(bpy)(OBQ)CO]²⁺ involves reduction of two electrons in OBQ first, followed by two electrons in trpy. It reveals that the charge density is more delocalized between the metal and the ligand, which means that it can carry out a reduction better. Among trpy and OBQ, the data shows trpy is 2.08 (0.48-(-1.6)) while obq is around 1.4 (0.02-(-1.38)).

In the next step, [Ru-CO]^2- undergoes a PCET process to form [Ru-CHO]^2- after trpy and OBQ accept two electrons each. When [Ru-CO]^2- is converted to [Ru-CHO]^2-, the charge distributions of trpy and CO decrease. If there is a proton transfer in CO, the charge density must increase; however, the charge density decreases in this case as well.

According to this speculation, apart from one electron reduction in CO, the charge density of trpy is partly transferred to CO.

When [Ru-CH2O]^2- is formed, charge density of Ru increases, and that of trpy decreases. It reveals a proton-coupled electron transfer process occurs on CHO, and the charge of Ru is fed back to trpy.

When [Ru-CH2O]^2-is converted to [Ru-CH2OH]^2-, charge density of Ru decreases.

It reveals a proton-coupled electron transfer process occurs on CH2O. It’s worth mentioning that the charge density is delocalized between the metal and the ligand at the same time, which enhance the removal of methanol.

When [Ru-OEt]^2-is converted to [Ru-CO2]^1-, the charge density of trpy increases;

therefore, we can assume that trpy provides electrons to OEt^- to obtain a proton, which

43

then makes it back to the solvent while CO2 fills the cavity simultaneously.

As [Ru-CO2]^1- is converted to [Ru-CO2]^2- by one-electron reduction, the charge density is reduced in trpy apparently.

As [Ru-CO2]^2- is converted to [Ru-COOH]^2- via PCET, the charge and spin distributions of each part do not change much. The obtained data suggest that PCET occurs on CO2. If COOH obtains a proton and loses a H2O to form [Ru-CO]^1-, the charge density of trpy increases again. We can still say that trpy provides electron to COOH to obtain proton and undergo dehydration.

Overall, the first step is to reduce two electrons each in OBQ and in trpy; next, OBQ provides no electrons to the reaction, but trpy provides one electron to reduce CO2. When [Ru-CO2]^1- is formed, two electrons are reduced in OBQ, and one electron is reduced in trpy.

44

在文檔中 利用理論計算探討釕金屬之Innocent和Non-innocent ligand在電催化還原二氧碳反應機制上的差異 (頁 25-53)