CO<sub>2</sub> capture by Mn(<scp>i</scp>) and Re(<scp>i</scp>) complexes with a deprotonated triethanolamine ligand
CO 2 capture at low concentration by catalysts is potentially useful for developing photocatalytic and electrocatalytic CO 2 reduction systems. We investigated the CO 2 -capturing abilities of two complexes, fac-Mn(X 2 bpy)(CO) 3 (OCH 2 CH 2 NR 2 ) and fac-Re(X 2 bpy)(CO) 3 (OCH 2 CH 2 NR 2 ) (X 2 bpy ¼ 4,4 0 -X 2 -2,2bipyridine and R ¼ -CH 2 CH 2 OH), which work as efficient catalysts for CO 2 reduction. Both complexes could efficiently capture CO 2 even from Ar gas containing only low concentration of CO 2 such as 1% to be converted into fac-M(X 2 bpy)(CO) 3 (OC(O)OCH 2 CH 2 NR 2 ) (M ¼ Mn and Re). These CO 2 -capturing reactions proceeded reversibly and their equilibrium constants were >1000. The substituents of X 2 bpy strongly affected the CO 2 -capturing abilities of both Mn and Re complexes. The density functional theory (DFT) calculation could be used to estimate the CO 2 -capturing abilities of the metal complexes in the presence of triethanolamine.Scheme 1 Ligand substitution and CO 2 capture reactions of the Re(I) complex and the n CO values of the Re complexes.
Introduction<!>Results and discussion<!>Comparison of the CO 2 -capturing abilities of the Mn(I), Re(I) and W(0) complexes<!>Gibbs free energy change and DFT calculation<!>General procedures<!>Calculation of equilibrium constants<!>CO 2 -capturing reactions<!>Synthesis<!>DFT calculations of changes in Gibbs free energy (DG 0 )<!>Conclusions<!>Conflicts of interest
<p>The catalytic conversion of CO 2 into useful carbon resources by using sustainable energy such as sun light has attracted much attention as one of the technologies that address the issues of both global warming and the shortage of carbon-based resources. Transition metal complexes with ions such as Re(I), 1 Mn(I), 2 Ru(II), 3 Ir(III), 4 Ni(II), 5,6 Fe(II), [7][8][9][10][11] and Co(II) 12,13 have been reported as catalysts for reducing CO 2 to CO, formic acid, and/or methane. For example, fac-[Re(N^N)(CO) 3 (L)] n+ (N^N ¼ diimine ligand and L ¼ monodentate ligand) catalyses the selective conversion of CO 2 into CO in a N,N-dimethylformamide (DMF) solution containing triethanolamine (TEOA) via photocatalytic reaction or a DMF solution containing H 2 O via electrocatalytic reaction. 1,14 fac-Mn(N^N)(CO) 3 Br has also been reported as an efficient CO 2 reduction catalyst selectively giving CO or formic acid in electrocatalytic and photocatalytic systems, respectively. 2,[15][16][17] We recently found that fac-Re(bpy)(CO) 3 (OCH 2 CH 2 NR 2 ) (2Re-bpy: bpy ¼ 2,2 0 -bipyridine and R ¼ -CH 2 CH 2 OH), which has a deprotonated triethanolamine ligand, can capture a CO 2 molecule even at low concentration of CO 2 by CO 2 insertion into the Re(I)-oxygen bond (Scheme 1). 18 Since this CO 2 insertion is a reversible reaction with a large equilibrium constant, more than 93% of the Re complex is converted into a CO 2 adduct, i.e., fac-Re(bpy)(CO) 3 (OC(O)OCH 2 CH 2 NR 2 ) (4Re-bpy), when bubbled with Ar gas containing only 10% CO 2 (1.6 Â 10 À2 M in solution). This CO 2 capture reaction by the Re(I) complex was successfully applied to photocatalytic CO 2 reduction at low concentration of CO 2 . 19 A Ru(II)-Re(I) photocatalyst consisting of a Ru photosensitiser unit and fac-Re(BL)(CO) 3 (OCH 2 CH 2 NR 2 ) (BL ¼ bridging ligand) as a catalyst unit shows almost same the photocatalytic efficiency and selectivity for CO 2 reduction in an atmosphere of 10% CO 2 as that in a 100% CO 2 (0.13 M in solution) atmosphere. Even at 0.5% CO 2 concentration, its photocatalytic efficiency was about 60% of that at 100% CO 2 concentration. These results suggest that the ability of metal complex catalysts to capture CO 2 using the deprotonated TEOA ligand potentially offers an effective method for reducing low-concentration CO 2 atmospheres without the need for condensation.</p><p>Although some similar CO 2 insertion reactions into the M-OR bond of other alkoxide metal complexes, such as M ¼ Mn(I) 20,21 and Re(I) 22,23 and R ¼ -CH 3 and -C 2 H 5 , have been reported,  systematic and quantitative research, especially into the effects of different ligands and/or different central metal ions on the CO 2 capture reactions, has not yet been reported to the best of our knowledge.</p><p>Here we report the CO 2 capture abilities of Mn(I) 4,4 0 -X 2 -bpy tricarbonyl complexes, where X is any substituent of the corresponding Re(I) complexes, i.e., X ¼ H, Br, and MeO; fac-Mn(X 2 bpy)(CO) 3 (OCH 2 CH 2 NR 2 ) efficiently captured CO 2 in the same manner as the Re(I) complexes but the corresponding W(0) bpy tricarbonyl complex did not. The abilities of the Mn(I) complexes to capture CO 2 were different from those of the corresponding Re(I) complexes and the abilities of both the Mn(I) and Re(I) complexes were strongly dependent on the substituents on the diimine ligand. We successfully claried the reasons for these dependences of the CO 2 capture abilities by using density functional theory (DFT) calculations.</p><!><p>Ligand substitution of fac-[M(X 2 bpy)(CO) 3 (MeCN)] + in a DMF-TEOA mixed solution As a typical example, fac-[Mn(bpy)(CO) 3 (MeCN)] + (5.0 mM) was added to an Ar-saturated DMF solution containing TEOA (1.3 M), and the changes in the vibrational bands of the CO ligands (n CO ) were followed by FT-IR. In Fig. 1, the peaks at n CO ¼ 2046 and 1970 cm À1 were attributed to the starting complex and their intensity gradually decreased and new absorption bands appeared over time. Aer 30 min, there were no more changes in the IR spectrum and the main bands were observed at n CO ¼ 2040, 1943, and 1936 cm À1 , which are attributed to fac-[Mn(bpy)(CO) 3 (DMF)] + (1Mn-bpy) based on the similarity of the highest wavenumber peaks to those of the starting complex in the case of the Re(I) complex. Some shoulder peaks were also observed at n CO ¼ 2020 and 1900 cm À1 . Fig. 2a shows the IR spectra measured several hours aer dissolving the same complex into DMF solutions containing different concentrations of TEOA. At higher concentration of TEOA, the intensity of the peaks at n CO ¼ 2020 and 1900 cm À1 increased. We focussed on the totally symmetric vibrational bands around 2050-2010 cm À1 for understanding how many complexes were produced in the DMF-TEOA mixed solution. We could not reasonably t these spectra by using curve-tting with two components, but by using three components, we were able to identify three bands at n CO ¼ 2040, 2030, and 2017 cm À1 (Fig. S1 †). The peak at n CO ¼ 2040 cm À1 was attributed to 1Mn-bpy. The peak at n CO ¼ 2017 cm À1 can be attributed to fac-Mn(bpy)(CO) 3 (OCH 2 CH 2 NR 2 ) (2Mn-bpy) because it became larger with increased TEOA concentration (Fig. 2a). The difference in the IR bands of 2Mn-bpy and 1Mnbpy (Dn ¼ 22 cm À1 ) was very similar to the difference between fac-Re(bpy)(CO) 3 (OCH 2 CH 2 NR 2 ) and fac-[Re(bpy)(CO) 3 (DMF)] + (Dn ¼ 23 cm À1 , Scheme 1). This kind of shi to a lower energy of n CO could be caused by an increase in p back donation from the Mn centre to the CO ligand because of the anionic alkoxide ligand instead of the neutral DMF. Fig. 2b shows the relationship between the peak area ratio of these complexes and the concentration of TEOA. The ratio of 1Mn-bpy decreased with increasing TEOA concentration; however, 2Mn-bpy showed a reverse correlation. From these results, it was concluded that 1Mn-bpy and 2Mn-bpy should be in equilibrium with each other in DMF-TEOA. On the other hand, the small peak at n CO ¼ 2030 cm À1 did not strongly depend on the TEOA concentration. This was probably due to a Mn(I) complex with an imidate ester ligand, i.e., fac-[Mn(bpy)(CO) 3 (-NH]C(CH 3 )-OCH 2 CH 2 NR 2 )] + (3Mn-bpy), which is produced by the addition of deprotonated TEOA to the MeCN ligand (Fig. 1, right scheme). The difference in the totally symmetric vibrational band from fac-[Mn(bpy)(CO) 3 (MeCN)] + was Dn CO ¼ 16 cm À1 , and a similar peak shi was observed when fac-[Mn(bpy)(CO) 3 (MeCN)] + was dissolved in a MeCN-TEOA (5 : 1, v/v) solution (Dn CO ¼ 14 cm À1 , Fig. S2 †). This identication is also supported by the fact that the difference in n CO between fac-[Re(bpy)(CO) 3 (MeCN)] + and fac-[Re(bpy)(CO) 3 (-NH]C(CH 3 )-OCH 2 CH 2 NR 2 )] + was Dn CO ¼ 13 cm À1 . This reaction was much slower in the case of the Mn(I) complex than that of the corresponding Re(I) complex. To further clarify the identity of this minor product, the following experiment was performed. fac-[Mn(bpy)(CO) 3 (MeCN)] + was dissolved in a DMF solution containing 1.3 M TEOA. Aer 60 min, additional TEOA or DMF was added to this solution, i.e., the concentration of TEOA in the solution was changed from 1.3 to 3.9 or 0.65 M. The changes in the concentration of the solvent did not affect the concentration of the minor product in either case (Fig. S2 †). This suggests that the minor product was 3Mnbpy, which was only produced by the reaction between fac-[Mn(bpy)(CO) 3 (MeCN)] + and TEOA, was stable in the solution. In other words, the presence of 3Mn-bpy in the solution should not affect the equilibrium between 1Mn-bpy and 2Mn-bpy. Therefore, in the following discussion, we consider only the equilibrium between the DMF and TEOA complexes.  There are two possible equilibrium equations (eqn (1) and ( 3)) for the conversion of 1Mn-bpy into 2Mn-bpy. In the mechanism described in eqn (3), one TEOA molecule works as both a nucleophile and a base. In the case of eqn (1), two TEOA molecules contribute to the ligand substitution reaction, where proton capture from the TEOA interacting with the metal centre by the other TEOA molecule is considered. In this ligand substitution, the main mechanism is probably described in eqn (1) because the solution contained a very high concentration of TEOA (>1.3 M). Therefore, we chose eqn (1) for the DFT calculation as described below. ‡</p><p>In eqn (2), the concentration of the protonated TEOA ([H-TEOA + ]) was assumed to be the same as that of 2Mn-bpy because the proton that originated in the TEOA ligand should be captured by another TEOA molecule as described above. The concentrations of the Mn(I) complexes were calculated by curve-tting of the IR spectra and were used to determine the equilibrium constant, i.e., K 1 (Mn-bpy) ¼ (0.22 AE 0.03) Â 10 À3 , by using eqn (2).</p><p>(1)</p><p>(2)</p><p>(3)</p><p>When the Mn complexes with substituents at the 4,4 0 -position of the bpy ligand, i.e., fac-[Mn(X 2 bpy)(CO) 3 (MeCN)] + (X ¼ Br and OMe), were dissolved in the DMF-TEOA mixed solution, similar IR spectral changes were observed in both cases (Fig. S4 †). However, the equilibrium constants were very different from K 1 (Mn-bpy). Electron-withdrawing substituents gave a larger constant, i.e., K 1 (Mn-Br 2 bpy) ¼ (0.64 AE 0.03) Â 10 À3 . On the other hand, electron-donating substituents gave a smaller constant, i.e., K 1 (Mn-(MeO) 2 bpy) ¼ (0.12 AE 0.01) Â 10 À3 . These results strongly suggest that stronger electronwithdrawing substituents on the X 2 bpy ligand give rise to higher stability of 2-Mn-X 2 bpy.</p><p>In the case of the corresponding Re(I) complex, fac-[Re(X 2bpy)(CO) 3 (MeCN)] + was converted into fac-[Re(X 2 bpy)(CO) 3 -(DMF)] + (1Re-X 2 bpy) by rst dissolving in DMF § and then into fac-Re(X 2 bpy)(CO) 3 (OCH 2 CH 2 NR 2 ) (2Re-X 2 bpy) by the addition of TEOA to the solution because this procedure could suppress the formation of [Re(bpy)(CO) 3 (-NH]C(CH 3 )-OCH 2 CH 2 NR 2 )] + (3Re-bpy). 18 { The equilibrium constants between 1Re-X 2 bpy and 2Re-X 2 bpy are summarised in Table 2. The values of K 1 (Re-X 2 bpy) are consistent with those of the corresponding Mn complexes as described above, i.e., the electron-withdrawing substituents on the X 2 bpy ligand yielded a larger equilibrium constant. The equilibrium constant of the Re complexes was much larger than that of the corresponding Mn complexes, for example, K 1 (Rebpy) ¼ (71 AE 1) Â 10 À3 and K 1 (Mn-bpy) ¼ (0.64 AE 0.03) Â 10 À3 , that is, the formation of 2Mn-X 2 bpy was thermodynamically less favourable compared to that of the corresponding Re(I) complex in the DMF-TEOA mixed solution.</p><p>Introduction of CO 2 into the DMF-TEOA solution containing the equilibrium mixture of 1Mn-bpy and 2Mn-bpy caused rapid disappearance of the n CO bands attributed to 1Mn-bpy and 2Mnbpy and the appearance of three new n CO bands at 2028, 1936, and 1913 cm À1 (Fig. 3a). When this CO 2 -saturated solution was bubbled with Ar for 30 min, the original n CO bands of 1Mn-bpy and 2Mn-bpy were fully recovered (Fig. S5 †). These IR spectral changes are very similar to those of the corresponding Re(I) complexes, where both 1Re-bpy and 2Re-bpy were converted into the CO 2 adduct complex fac-Re(bpy)(CO) 3 (OC(O)OCH 2 -CH 2 NR 2 ) (4Re-bpy) upon bubbling with CO 2 . 18 These results strongly suggest that CO 2 capture by the Mn(I) complex proceeded well and that the reaction was reversible. Identication of the CO 2 adduct was conrmed by 1 H and 13 C NMR experiments in a DMSO-d 6 solution containing TEOA.</p><p>DMF-d 7 was not used because the signal of a carbonate carbon (M-OC(O)O-R) is expected to be observed at a similar magnetic eld to that of the amide carbon of DMF. We conrmed that even in a DMSO-TEOA mixed solution, similar IR spectral changes occurred to those in the DMF-TEOA solution (Fig. 3b). The 1 H NMR spectra of the solution containing the Mn complexes were rst measured under an Ar atmosphere and then measured again aer bubbling with CO 2 for 3 min (Fig. S6 †). The 1 H NMR signals attributed to the bpy ligands changed completely before and aer bubbling with CO 2 ; the proton peaks attributed to the bpy ligands of 1Mn-bpy and 2Mn-bpy, which were observed under an Ar atmosphere, disappeared and four new proton signals were observed at a higher magnetic eld under a CO 2 atmosphere. Fig. 4a shows the 13 C NMR spectrum with proton decoupling; a singlet peak attributable to the carbonate carbon (M-OC(O)O-R) at 158.7 ppm was observed under the CO 2 atmosphere. This signal was drastically enhanced by using 13 CO 2 (99% 13 C content) instead of CO 2 (Fig. 4b). Without proton decoupling, the signal at 158.7 ppm became a triplet with J C-H ¼ 3.8 Hz (Fig. 4c). This is attributable to long-range coupling with the methylene group in the deprotonated TEOA moiety of the carbonate ester ligand (Fig. 4) because a similar signal was reported in the 13 C NMR spectrum of 3Re-bpy (158.4 ppm, 3.6 Hz). 18 These results strongly suggest that the insertion reaction of CO 2 into the Mn-O bond in 2Mn-bpy gives the complex fac-[Mn(bpy)(CO) 3 (OC(O)OCH 2 CH 2 -NR 2 )] (4Mn-bpy) (eqn ( 5)).</p><p>As shown in Fig. 5a, the ratios of the peaks changed with the CO 2 concentration in the solution. We successfully conducted curve-tting to obtain the concentrations of 1Mn-bpy, 2Mn-bpy and 4Mn-bpy as shown in Fig. 5b. Direct determination of K 2 (Mn-bpy) from the peak area of 2Mn-bpy showed a large experimental error because the peak of 2Mn-bpy was very small. Therefore, K 2 (Mn-bpy) was calculated by using eqn (7) with K 1 (Mn-bpy) and K 3 (Mn-bpy), which is the equilibrium constant between 1Mn-bpy and 4Mn-bpy (eqn ( 6) and ( 7)). In eqn (7), the concentration of the protonated TEOA ([H-TEOA + ]) was assumed to be the same as the total concentrations of 2Mn-bpy and 4Mn-bpy because of the same reason in the case of eqn (2). This calculation method gave much lower experimental error; the value of K 2 (Mn-bpy) between 2Mn-bpy and 4Mn-bpy was obtained to be (61</p><p>The other Mn(I) complexes with the substituted bpy ligand also showed similar IR spectral changes during a similar experiment in the DMF-TEOA mixed solution (Fig. S7 †). Therefore, these complexes also have the ability to capture CO 2 . The equilibrium constants are summarised in Table 1. It is noteworthy that the values of K 2 (Mn-X 2 bpy) are not drastically different from those of K 1 (Mn-X 2 bpy) of the three Mn complexes. The CO 2 insertion between the Mn-O bond might be a concerted reaction between the nucleophilic attack of the oxygen atom in the ligand to the carbon in CO 2 and the electrophilic attack of the central metal to the oxygen atom in CO 2 (Scheme 2). 22,31,47 Since a higher electronic density of the central metal should induce acceleration of the former process while obstructing the latter one, these conicting effects might cancel each other out.</p><p>All of the three 2Re-X 2 bpy complexes also efficiently captured CO 2 even from gases containing low concentrations of CO 2 (eqn (5)). The equilibrium constants for K 2 (Re-X 2 bpy) are summarised in Table 2. K 2 (Re-Br 2 bpy) was the largest of the three Re(I) complexes, while the difference between K 2 (Re-bpy) and K 2 (Re-(MeO) 2 bpy) was small.</p><!><p>As described in the previous section, both series of the Mn(I) and Re(I) complexes have high CO 2 -capturing abilities (Scheme 3). The CO 2 insertion reactions into 2Mn-X 2 bpy were more favourable than those into 2Re-X 2 bpy, equilibrium constants of which are dened as K 2 (M-X 2 bpy) (M ¼ Mn and Re) (Process 2). However, the CO 2 -capturing ability of 1Mn-X 2 bpy in the presence of TEOA   was lower than that of 1Re-X 2 bpy, and the equilibrium constants are dened as K 3 (M-X 2 bpy) (Process 3). This is because the formation of the TEOA adducts (2Mn-X 2 bpy and 2Re-X 2 bpy), i.e., Process 1, is much more favourable in the case of the Re complexes, the equilibrium constants of which are K 1 (M-X 2 bpy). 1Re-Br 2 bpy as the starting complex most efficiently captured CO 2 from gases containing low concentrations of CO 2 in DMF including 1.3 M TEOA where the volume ratio between DMF and TEOA is 5 : 1; bubbling air containing only 400 ppm of CO 2 (1.7</p><p>Â 10 À4 M in solution) into the DMF-TEOA mixed solution containing 1Re-Br 2 bpy and 2Re-Br 2 bpy converted 31% of the Re complexes into the corresponding CO 2 adduct, i.e., 4Re-Br 2 bpy. This conversion ratio increased by changing the solvent to DMSO. In a DMSO solution containing 1.3 M TEOA, 4Re-Br 2 bpy formed at a 47% ratio under air.</p><p>Since the W(0) complex with a structure similar to that of the Re(I) and Mn(I) complexes has been reported, we checked the ligand substitution and CO 2 -capturing ability of fac-W(bpy)(CO) 3 (MeCN). This complex was synthesised by dissolving W(bpy)(CO) 4 into an MeCN solution, which was reuxed under an Ar atmosphere overnight. Evaporation of the solvent gave fac-W(bpy)(CO) 3 (MeCN) as a brown solid containing a small amount of W(bpy)(CO) 4 . Since fac-W(bpy)(CO) 3 (MeCN) was air-sensitive and W(bpy)(CO) 4 did not affect the following experiments, we used this solid.k</p><p>The FT-IR spectrum of fac-W(bpy)(CO) 3 (MeCN) in an MeCN solution showed carbonyl vibration bands at n CO ¼ 1898 and 1782 cm À1 (Fig. S8 †), which are consistent with a previous report. 48 This complex was dissolved in a DMF solution, and a CO vibration band at n CO ¼ 1887 cm À1 attributable to fac-W(bpy)(CO) 3 (DMF) was observed (Fig. S9 †). The other n CO bands overlapped with the carbonyl vibrational band of the DMF solvent and small absorption bands at n CO ¼ 2005, 1875, and 1831 cm À1 were attributed to fac-W(bpy)(CO) 4 , which had the same wavelength and strength as those observed in the MeCN solution (Fig. 6). Addition of TEOA into the DMF solution containing W(bpy)(CO) 3 (DMF) did not affect the n CO band at all under an Ar atmosphere. In addition, the CO vibration of fac-W(bpy)(CO) 3 (DMF) in the IR spectra did not change aer bubbling CO 2 through this solution (Fig. 6). These results clearly indicate that fac-[W(bpy)(CO) 3 (OCH 2 CH 2 NR 2 )] À did not form from fac-W(bpy)(CO) 3 (DMF) even in the presence of 1.3 M TEOA, and the CO 2 -capturing reaction also did not proceed under these reaction conditions (refer the equation in Fig. 6).</p><!><p>The Gibbs free energy (G f ) changes in Processes 1-3 were calculated by using the equilibrium constants (K 1 , K 2 , and K 3 ) and are summarised in Table 3 (values in parentheses).   The observed abilities of the metal tricarbonyl complexes in the CO 2 -capturing reactions involving TEOA were examined based on the DFT calculations. Selected geometrical parameters calculated at the def2-SVP/PBE1PBE level including solvent effects are depicted in Table S2. † All of the Mn-N(bpy) and Mn-C(CO) bond lengths were calculated for 1Mn-bpy, 2Mn-bpy and 4Mn-bpy with the singlet spin state and were in good agreement with those of the X-ray structure determined for fac-Mn(CO) 3 (bpy)I, 49 supporting the accuracy of the DFT calculation. The geometrical optimisation of 1Re-bpy, 2Re-bpy and 4Re-bpy also gave molecular structures with geometrical parameters similar to those of the X-ray structure of fac-Re(CO) 3 (bpy)OC 5 H 11 . 23 The CO stretching vibrational frequencies of the tricarbonyl complexes were calculated for the DFT-optimised structures and were corrected using the reported scaling factor (Tables S3  and S5 †). 50 While all of the calculated n CO values were slightly higher than the observed ones, the differences in n CO are almost constant for each series of the tricarbonyl complex: 31-50 cm À1 for Mn(I) complexes and 16-23 cm À1 for Re(I) complexes. The constant deviation in the calculated n CO supports the experimental identication of the species involved in the CO 2capturing reactions.</p><p>The changes in Gibbs free energy (DG 0 ) of each process for the Mn(I) and Re(I) complexes were then calculated based on the DFT calculations (Table 3). For the overall reaction (Process 3), these calculated DG 0 values are in good agreement with those obtained by the experiments with an error of 5 kJ mol À1 ($1 kcal mol À1 ).</p><p>For CO 2 capture by the Mn complexes, the calculated DG 0 for reaction 1 is about 9-14 kJ mol À1 lower than the observed value, while those for Process 2 are 13-17 kJ mol À1 higher. This trend is also seen in the Re complexes even though the differences are smaller compared to those of the Mn complexes. These results are probably caused by an overestimation of the free energy of formation for the M-TEOA complexes (2M-X 2 bpy). The TEOA moiety with three ethanol groups has several conformation isomers. The geometrical optimisation of 2Mn-X 2 bpy and 4Mn-X 2 bpy gave somewhat compact structures due to intramolecular hydrogen bonding between the metal-coordinating O atom and the two remaining OH groups in the TEOA moiety.</p><p>The lengths of the hydrogen bonds in 4Mn-X 2 bpy are around 1.76-1.81 Å. On the other hand, in the case of 2Mn-X 2 bpy, the hydrogen bonds between the anionic O atom, which is attached to the metal ion, and the OH groups are considerably short (1.50-1.59 Å) (Fig. S10 †). The same effects were observed in the Re complexes as well. Such strong intramolecular hydrogen bonding in 2M-X 2 bpy is one of the reasons for the overestimation of their stabilisation energies because, in the DMF-TEOA mixed solution, intermolecular hydrogen bonds with other TEOA molecules could break such a compact conformation and several other stable conformations would be formed. A more accurate prediction of DG 0 for the CO 2 insertion reaction therefore requires the evaluation of the TEOA conformational distribution including explicit interactions with other TEOA molecules. It should be noted that this uncertainty should not affect the DG 0 values of Process 3 because 2Mn-X 2 bpy and 2Re-X 2 bpy do not contribute to this process as shown in the equation for determining K 3 (Scheme 3). In other words, the errors based on the overestimation of the stabilisation energy of 2Mn-X 2 bpy and 2Re-X 2 bpy can be cancelled between Processes 1 and 2.</p><p>As described in the previous section, there are two possible equilibrium equations [eqn (1) and ( 3)] for the ligand substitution of DMF by TEOA. However, in solutions containing high concentration of TEOA (1.3 M), the reaction with one TEOA molecule acting as both a nucleophile and a base is considered unfavourable. The DFT calculation also supports this consideration because the observed v CO values (1902, 1918 and 2017 cm À1 ) are close to those calculated for 2M-bpy rather than for 2M-bpy-H + (see Tables S3 and S4 †). In addition, the calculated DG 0 corresponding to eqn (3) (14.5 kJ mol À1 ) is higher than the DG 0 value for Process 1 (10.1 kJ mol À1 ), which also indicates the favourability of eqn (1).</p><p>For the CO 2 -capturing reaction of the corresponding W(CO) 3 (bpy)X species, the DG 0 values for Processes 1, 2, and 3 are calculated to be +21.8, À9.0, and +12.8 kJ mol À1 , respectively. The calculated large positive DG 0 value of Process 3 is consistent with the results showing that the W(0) complex was less reactive for the CO 2 -capturing reaction in the presence of TEOA compared to the Re(I) and Mn(I) complexes.</p><p>As shown for the three series of metal complexes consisting of different metal ions and different ligands, DFT was able to calculate the DG 0 values for the overall CO 2 -capturing reaction (Process 3) with good accuracy ($5 kJ mol À1 ). We can use this method for estimating the CO 2 -capturing abilities of other metal complexes and to examine the mechanism of CO 2capturing reactions.</p><!><p>FT-IR spectra were measured using a JASCO FT/IR-610 or 6600 spectrometer at 1 cm À1 resolution. 1 H NMR and 13 C NMR spectra were measured in acetonitrile-d 3 , DMF-d 7 or DMSO-d 6</p><!><p>Preparation procedures of the solutions for the measurements were performed in a glove box (UNICO, UN-650F) with a gas circulation dehydration device (DGE-05) under Ar (water content was less than 376 ppm). Concentrations of the complexes in solutions were obtained by the peak areas of n CO at the highest wavenumber (2000-2060 cm À1 ) in the IR spectra.</p><p>The peak areas attributed to each complex were obtained by curve-tting using a linear combination of the Gaussian and the Lorentzian functions. 18,19 Ligand substitution reaction</p><p>The Mn(I) and W(0) diimine tricarbonyl acetonitrile complexes were dissolved in DMF solutions containing different concentrations of TEOA. The solutions were kept at room temperature in the dark for several hours and then their IR spectra were measured. In the case of the Re(I) complexes, the complexes with an acetonitrile ligand were dissolved in DMF to avoid the formation of 3Re-X 2 bpy. The solutions were kept at room temperature in the dark for several hours (>3 h), when all of the complexes were converted into 1Re-X 2 bpy, and then TEOA was added to these solutions. Aer keeping the solutions at room temperature in the dark for 2 h, the IR spectra of the equilibrium mixtures were recorded.</p><!><p>Various amounts (0.020-0.025 mL) of a CO 2 -saturated DMF solution (the concentration of CO 2 is 0.20 M) 18,19 were added to DMF solutions (1.8 mL) containing TEOA (1.3 M) and the equilibrium mixture of the metal complexes in a shielded sample tube by using a micro-syringe in a glove box. Aer the solutions were kept at room temperature for several hours, the IR spectra were measured.</p><!><p>fac-Re(X 2 bpy)(CO) 3 Br was synthesised according to a reported method. 1,2 fac-Mn(X 2 bpy)(CO) 3 Br was synthesised according to a reported method with a small difference indicated as follows. 15 All procedures for the synthesis of the Mn(I) complexes were performed in a dark room with red light. W(bpy)(CO) 4 and fac-W(bpy)(CO) 3 (MeCN) were synthesised according to a reported method. 51,52 fac-[Mn(bpy)(CO) 3 (MeCN)](PF 6 ). fac-Mn(bpy)(CO) 3 Br (1.0 g, 2.7 mmol) and AgPF 6 (0.69 g, 2.7 mmol) were dissolved in 350 mL of Ar-saturated acetonitrile and this solution was heated at 40 C for 1 h. Aer cooling to room temperature, the white precipitate of AgBr was removed by ltration with a Celite phase, and the solvent was evaporated. A yellow residue was dissolved in a small amount of CH 2 Cl 2 . n-Hexane was added to this solution to yield the target compound as a yellow precipitate, which was ltered and washed with a small amount of ether. Yield: 1.2 g (96%). 1  The following Mn complexes were synthesised by a similar method to that of [Mn(bpy)(CO) 3 (MeCN)](PF 6 ) except for using the corresponding Br complexes as the starting complexes.</p><p>fac-[Mn(4,4 0 -dibromo-2,2-bipyridine)(CO) 3 (MeCN)] (PF 6 ). Yield: 0.45 g (95%). 1</p><!><p>The geometry optimisations of chemical species involved in CO 2 -capturing were carried out using a hybrid density functional with 25% exchange and a 75% Perdew, Burke, and Ernzerhof correlation functional (PBE1PBE) 53 using a smaller def2-SVP basis set with an effective core potential for Re and Br atoms. The solvent DMF was modelled as a dielectric continuum using a polarisable continuum model. 54 The electronic energies were then obtained as the SCF energy of a singlepoint calculation on the optimised geometries using a larger basis set of def2-TZVP. The solvent effect of DMF was considered using the SMD method, a parametrised SCRF-based solvation model developed to predict the free energy of solvation. 55 The thermal correction terms to evaluate the standard Gibbs energy of formation (G f ) were computed by vibrational frequency analysis of the stationary structures using the same calculation level as for the geometry optimisations. An appropriate scaling factor of 0.9817 was used for the computed zeropoint energy. 50 Changes in the Gibbs free energy (DG 0 ) were calculated as the difference in G f between the reactants and the products. For the DG 0 values corresponding to K 2 and K 3 with the unit of M À1 , the calculated G f values were corrected by À7.9 kJ mol À1 for the change in the reference states from 1 bar to 1 M. 56 All DFT calculations were executed using Gaussian 16 packages. 57</p><!><p>We found that the CO 2 -capturing reactions of Mn(I) diimine tricarbonyl complexes with a deprotonated TEOA ligand were similar to those of the corresponding Re(I) complexes. The equilibrium constants of both the Mn(I) and Re(I) diimine complexes for the ligand substitution (Process 1) and for CO 2 capture (Process 2) in DMF-TEOA solutions were determined. From the DFT calculations, in addition to these equilibrium constants, we systematically and quantitatively claried the effects of both the metal and the substituents on the bpy ligand. This kind of metal affected both Processes 1 and 2. On the other hand, ligand substituents had a greater inuence on Process 1 than Process 2. The DFT calculation can be applied to estimate the DG 0 of the total reaction containing both the ligand substitution and CO 2 capture (Process 3), and the DG 0 values were in good agreement with those calculated from the experimental data. The CO 2 -capturing reaction of W(bpy)(CO) 3 (DMF) in the TEOA-DMF solution did not proceed at all because of the large DG 0 value for Process 3. These calculation approaches can give a good approximation to judge whether the total CO 2capturing phenomenon (Process 3) proceeds or not with various metal complexes.</p><!><p>There are no conicts of interest to declare.</p>