The third harmoni c of an Nd-YAG laser (LOTIS TII; 30 mJ, 10 ns , 10Hz) was flashed on the reaction mixture containing [Re(dmbpy) (CO)3Cl][1] (0.40mM)in DMSO/TEA (15/1). The pulse energy of the laser flashwas adjusted at 10mJ . After examining the optimum conditions to detect FT-IR signals, 150 l aser pulses was flashed to start the IR monitoring by means of a rapid scan mode for the range, 400cm- 1
～4000cm- 1. The delay time domain in the laser -induced FT-IR measurement was carefull y examined on the basis of previous report on IR studies and was set in the time scale of seconds, since the one-electron reduced fo rm of the starting Re -complex as the primary reaction product was reported b y Fujita’s group to have m uch longer lifetime than 10s and Ishitan’s group reported it to be even in second time scale. When the time domain was taken as 0 s~700 s after the laser-flash, beautiful transient spectra were observed as shown in Figure 1. The spectral changes are shown in a three dimensional axes of wavenumber, del ay time, and intensity.
62
Figure 1 Transi ent IR spectra of the reaction mixture upon laser flash under the atmosphere of 1 2CO2. The spectra are expressed as the difference spectra compared with it before the laser flash. ([1] = 0.4 mM in DMSO/TEA (15/1))
The absorption at around 1750 ~ 2050 cm- 1 (the ligand carbonyl (C≡O)) and 1500 ~ 1750 cm- 1(carbox yl carbonyl (C=O bond) exhibited clear spectral changes. According to the careful inspection and assignment, as described later, about the peak position and the dynamic charact eristics on whether or not it exhibited a depletion or an appearance behavior aft er the laser flash, the spectral changes can be due to the following five kinds of transi ent signals, Transient -I (2017, 1913, 1890 cm- 1), -II (1998, 1875, 1835, 1798 cm- 1), -III (1682 cm- 1), -IV (1733 cm- 1), and –V (1631 cm- 1). The Transient -I and –IV exhibited immediate depletion during the laser flash, while the Transient-II, and –III showed rise and deca y profi le. The Transient-V (1631 cm- 1) having monotonic deca y profile was observed even onl y in DMSO without [1], indicating that it should be cau sed b y a
18351798
1682 1631
1733
2017 1913
1875 1998
1890
ΔAbs
63
side reaction of DMSO by laser irradiation. The Transient -IV (1733 cm- 1) on the other hand was onl y observed in the reaction mixture with TEA and it showed sudden depletion upon laser flash and subsequentl y recovered. Both the Transient -IV and –V were not well characterized and their appearance were i gnored in anal yzing other transi ent species.
Assignment of the transi ent
Transient-I: The three absorption peaks at 2017, 1913, and 1890 cm- 1 are exactl y due to the ligand carbonyl (C≡O) of the starting [Re(dmb)(CO)3Cl] [1]. The facial-Re complex has three carbon yls well characterized as the symmetric -(2017cm- 1), and anti-symm etric-(1913, 1890 cm- 1) vibrational modes, respecti vel y. Their sudden depletion upon laser flash is reasonable because the laser flash induces the formation of the 3MLCT state of [1] and the one-electron reduced form of [1] is subsequentl y formed through an electron transfer from TEA to the excited [1] in its 3MLCT state. The processes should be completed before starting the rapid scanning of IR monitoring.
Transient-II: The signals at around 1998, 1875, 1835, and 1798 cm- 1 all showed similar rise and deca y time profile each other. The peak positions situatein smaller wave num ber region than those of the original ligand carbonyls (C≡O), strongl y suggesting that they are corresponding carbonyl (C≡O) peaks of the one -electron reduced forms of the Re-complex. Since the one -electron deposited either on bipyridine ring or Re center would enhance electron back -donation to the ligand carbon yl s, the peak positions would be expected to shift to the smaller wave number. The DFT calculation also predicts this. (vide infra)
Transient-III: The signal at 1682 cm- 1 is a t ypical carbox yl carbon yl (C=O bond) vibration as clearl y predi cted by DFT calculation. The detection of this peak should be the hi ghlight of this study, since it
64
clearl y indicates that CO2 is trapped within the Re -com plex to be transformed into carb ox ylic group as a reduced form of CO2. Transient-III can be assigned as [3’] in Scheme 1.
Dynamic characteristics of the transients
Transient-I: The time profile of each transient was shown in Figure 2.
The Transient -I(2017, 1913, 1890 cm- 1: carbon yl (C≡O) of t he starting [1]) exhibited sudden depletion upon laser flash and further decrease until ca. 150 s and recovered graduall y within ca. 700 s (Figure 2 a)).
It should be noted here that the d ynamic behavior of Transi ent -I looks to be in a mirror image with that of Transient -II. Both transients are considered to be closel y correlated with each other as discussed later.
Transient-II: Bei ng correlated with the Transient -I, the Transient -II (1998, 1875, 1835, 1798 cm- 1) appeared upon laser flash and furthe r increase until ca. 150 s and graduall y deca y within ca. 700 s (Fi gure 2 b)). Among the signals, the 1998 and 1875 cm- 1 ones could be assigned as either the one -electron reduced form of t he starting Re-complex, [Re(dmbpy)(CO3)Cl]. - ([1’]) or the DMF coor dinated Re-complex, [Re(dmbpy)(CO3)DMF]. ([2’]). Fujita group reported that [1’] in acetonitrile had IR absorption at 1998, 1880, and 1866
cm- 1 4 ), while [1’] in DMF was observed by Johnson group to have
signals at 1994, 1880, and 1862 cm- 1.6 )The one-electron reduced form of tetrahydrofurane (THF) coordinated complex, [Re(dmbpy)(CO3)THF]., was reported by Kleverlaan to have absorption at 1997, 1875 cm- 1.2 3 ) The DFT calculation predi cts the IR absorption of [1’] and [2’] as tabulated in Table 1. The two facial-t ype species of the one -electron reduced form of the Re -complex, [1’] and [2’] was calculated to have IR absorption at ver y similar wave number with each other. One -electron reduced Re -complex was reported to hav e prett y long lifetime in second time scale,3 )however,
65
five-coordination Re -complex (1835, 1798 cm- 1) was sim ultaneousl y observed as assigned below. This means that an elimination of chloride ion from the one-electron reduced form of Re -complex, [1’] is fast enough to form the five -coordinati on complex. The one -electron reduced Re-complex observed here, thus, might be assigned as the DMF-coordianted R e -complex, [2’].
Transient-II’: On the other hand, the signals at 1835, 1798 cm- 1 have similar rise and decay behavior with [2’] ((1998, 1875 cm- 1), while the peak positions are in far smaller wave number region than those expected for carbonyl (C≡O) of facial-t ype. The y should be one-electron reduced form of Re -complex, but might have different coordination pattern than facial-t ype. These are thus classified here as Transient -II. One of the possible t ypes would be a meri dional-t ype complex, where the two ax ial ligands are the carbon yls (C≡O) and one C≡O ligand is in the equatorial position. We have already found that
1 3C uptake into the ligand CO within the Re -complex has been graduall y induced during the photoreduction through an observationby means of CS I-MS method.1 6 )A half of CO formed in one cycle of the photoreduction was observed to be incorporated as the ligand CO within the complex.1 6 )This means that a replacement of the CO ligands in equatorial position is induced during the photoreaction, suggesti ng a possibilit y that in the elimination process of Cl- from the one-electron reduced Re-complex ([1’]) form a five-coordination Re -com plex, which allows a formation of both one -elect ron reduced meridional-t ype solvent coordinated species through t he attack on the equatorial position and facial -type one through the axial position, may be formed.
The reaction with CO2 at the equatori al position in the former case would result in CO formation at the equatorial position. Another possibilit y of the 1 3C uptake would be due to a scrambling among the four carbon yl ligands within a Re -tetracarbon yl complex
66
([Re(dmbpy)(CO4)]) which might be once form ed from the Re-carbox ylic com plex ([Re(dmbpy)(CO3)COOH]) on the reaction pathway as described later. As regard s the two possible cases of a five coordination species and a meridional-t ype solvent coordinated species, both of which have not yet been reported on their synthesis, DFT calculations were carried out as summarized in Table 2. The five coordination comple x (axiall y vacant species), mer-[Re(dmbpy)(CO3)]., was predicted to have three peaks at 1937 (ver y weak), 1823, 1806 cm- 1 were predicted, while the mer-[Re(dmbpy)(CO3)DMSO]. and mer-[Re(dmbp y)(CO3)TEA]. exhibited different positions. The observed signals at 1835, 1798 cm- 1 thus could be assi gned as those of the one-electron reduced form of mer-[Re(dmbpy)(CO3)] and 1937 cm- 1 (ver y weak) was supposed to be hidden below other signals. It should be noted here that these signals (1835, 1798 cm- 1) were observed for rather long time peri od of several tens of seconds. It indicates that the five coordination complex (axiall y vacant species), mer-[Re(dmbp y)(CO3)]., is prett y stable to be observed, though facial-t ype five coordination complex is readil y transformed into solvent coordinated complex.4 )
Transient-III: Transient -III(1682 cm- 1: [3’]) exhibited rise and deca y time profile as shown in Figure 2 c). At around 250 s, the signal showed a maximum intensit y followed by a gradual decay. The peak time (ca. 250 s) of the rise is evidentl y delayed than those of the rises of Transient -II,-II’ (ca. 150 s). The difference between the rise times (ca. 100 s) strongl y suggests that the one -electron reduced Re-complex es (Transient -II, -II’) are the precursor for Transient-III, that is, Transient -II, -II’ react with CO2 to form Transient -III (1682 cm- 1: [3’]).
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Figure 2 Time profile of the transients observed in laser flash induced FT-IR specrroscopy under 1 2CO2 atmosphere: a) Transient -I;
The starting Re-complex, [1], b) Transient -II, -II’; The one-electron reduced form of Re -complex, c) Transient -III; The CO2 adduct with Re-complex, [Re(dmbpy)(CO3)COOH] ([3’])
Table 1 Prediction of IR absorption by means of DF T cal culation for one-electron reduced form of facial-t ype Re-complexes.
s ym. as y. 1 as y. 2 dmb [ Re( dm bpy) ( CO )3Cl]•-[ 1’] 1995 1864 1851 1602 [ Re( dm bpy) ( CO )3DMSO ]• [ 2’] 1999 1868 1860 1583
Table 2 Prediction of IR absorption by means of DFT cal culation for one-electron reduced form of meridional-t ype Re-complexes.
s ym. as y. 1 as y. 2 dmb mer- [ Re( dm b py) ( CO )3]• 1937 1823 1806 1626 mer- [ Re( dm bpy) ( CO )3DMSO ]• 2040 1888 1847 1601 mer- [ Re( dm b py) ( CO )3T EA]• 2040 1893 1855 1603
3.5 1 3C isotope effect on the transient IR absorption :
To get a deeper insight into the assignment of each transi ent, a 1 3C isotope effect, on how 1 3CO2 would affect the transient behavior
-0.025 -0.02 -0.015 -0.01 -0.005 0 0.005 0.01 0.015
0 200 400 600 800 1000
ΔAbs
time (s)
(b)
(a)
(c)
time / s
68
especiall y on the peak positions of the transient IR absorption, was examined. Three atmospheric conditions under nitrogen, 1 2CO2, and
1 3CO2 were compared (Figure 3).
Figure 3 Transient FT-IR absorption spectra under three kinds of atmosphere of a) nit rogen, b) 1 2CO2, and c) 1 3CO2 at dela y t i me of 300 s after the laser flash.
Under nitrogen atmosphere, an appearance of the one -elect ron reduced species (Transient -II, -II’) along with the depletion of the starting Re-complex [1] (Transient -Ⅰ) wasonl y observed upon laser flash excitation. On the other hand, 1 2CO2 atmosphere induced an appearance of 1682 cm- 1 species(Transient -III: [3’]) in addition to Transient -I, -II, -II’ (Fi gure 1, 3). Ver y interestingl y, under 1 3CO2 atmosphere, the signal at 1682 cm- 1 clearl y shifted by 39 cm- 1 down to 1643 cm- 1. The shift well coincides with the prediction of the vibrational frequenc y (1645 cm- 1) on assumption of 1 3C introduction into C=O, indicating that the signal is actuall y due to C=O carbox yl ic vibration
0 0.02 0.04 0.06 0.08
1550 1650
1750 1850
1950 2050
ΔAbs
wave number / cm-1
(a) (b) (c)
69
and Transient -III is the CO2-adduct with Re-complex, [Re(dmbpy)(CO3)COOH]([3’]), which was detected by CS I -MS study.1 6 ) When the t ransient spectra in Figure 3 are carefull y compared among the three conditions under nitrogen, 1 2CO2, and 1 3CO2 at the delay time of 300 s after laser flash, the spectral shapes for Transient -I, -II, -II’ in the region of 2050 ~ 1750 cm- 1are evidently different between nitrogen and CO2 atmosphere. Typicall y, the relative intensit y ratios of 1895 cm- 1 band against 1798 cm- 1 one are larger under both 1 2CO2 and 1 3CO2 than nitrogen atmosphere. This is compelling us to convince that signals due to other speci es than the one-electron reduced Re -complexes (Transient -II, -II’) are superimposing over the region. To inspect carefull y the possibilit y, difference spectra be tween under1 2CO2 and nitrogen, 1 3CO2 and nitrogen were compared in Fi gure 4, respectivel y.
Figure 4 Difference spectra for the conditions under 1 2CO2 (red broken line) and 1 3CO2 (green line) against nitrogen atmosphere at the dela y time of 300 s after t he laser flash.
-0.01 0 0.01 0.02
1550 1650
1750 1850
1950 2050
ΔAbs
wave number / cm-1
70
The difference spectra for both 1 2CO2/nitrogen and 1 3CO2/nitrogen showed at least three peaks at 2006, 1904, and 1879 cm- 1 along with the carbox ylic C=O at 1682 and 1643 c m- 1. The former three peaks were compared with the carbox yl C=O (1682 or 1643 cm- 1) in Figure 5.
Though the time profiles of the two peaks of 1879 cm- 1 and C=O vibration are not necessaril y exactl y t he same for the case of 1 2CO2
(Figure 5 a)), the y are similar for 1 3CO2 (Figure 5 b)). This suggests that the three peaks at 2006, 1904, and 1879 cm- 1and carbox ylic C=O band are partl y correlated with each other but not necessaril y so in part.
We thus leave here the conclusion about w hether or not the peaks at 2006, 1904, and 1879 cm- 1 are directl y correlated as the absorption of ligand CO with the Re -COOH, [Re(dmbpy)(CO3)COOH](Transient -III;
1682 cm- 1 under 1 2CO2 and 1643 cm- 1 under 1 3CO2).
Figure 5 Compari son between the time profile of the difference absorption in the transient FT-IR: a) 1 2C O2- N2, b) 1 3CO2- N2.
Though the correlation between the three CO bands and the carbox ylic C=O band were not clear, the assignment of the carbox ylic C=O band can be discussed further in detail. The CS I -MS study have revealed that the CO2 adduct (m/z = 570 -574) detected in the reacti on mixture has different MS pattern than ReI-COOH complex
-0.002 0 0.002 0.004 0.006 0.008 0.01
0 200 400 600
ΔAbs
time / s
1682 1879 1904 2006
0 0.002 0.004 0.006 0.008 0.01 0.012
0 200 400 600
ΔAbs
time / s
1643 1879 1904 2006
71
([ReI(dmbpy)(CO3)COOH]) and ReI-format e complex ([ReI(dmbpy)(C O3)OCHO]). To reconfirm further whether or not the carbox ylic C=O species (Transient -III: [3’]) detected here by transient FT-IR is different from the ReI-COOH and ReI-formate complexes, these two Re-compl exes were s ynthesized and their IR spectra were measured as tabulated in Table 3 .
Table 3 IR absorption of [ReI(dmbp y)(CO3)COOH] and [ReIdmbp y)(CO3)OCHO]
solvent COsym COasy1 COasy2 COO Dmb
[ReⅠ(dmb)(CO)3COOH] KBr 2008 1889(br) 1592(br) 1622
DMSO+TEA 2003 1890(br) 1605 1620
[ReⅠ(dmb)(CO)3OCOH] KBr 2017 1890 1866 1636 1620(br)
DMSO+TEA 2016 1908 1886 1625 1625
The IR absorption bands observed here well coincide with the reported data.4 , 6 , 1 4 ) ReI-COOH (1605 cm- 1) and ReI-formate com plex (1625 cm- 1) have evidently different absorptions in DMSO/TEA from that of Transient-III (1682 cm- 1) and they are thus again removed from the candidates. Supposing the CO2 adduct (Transient -III: [3’]) being formed through the reaction with the one -electron reduced Re -complex, the process can be considered to be a two -electron oxidative addition of CO2 to the Re center to result in a formation of ReI I-COOH complex, [ReI I(dmbpy)(CO3)COOH]. Since ReI I-COOH has not yet been reported to be synt hesized, a DFT calculation was carried out on the ReI I-COOH complex along with ReI-com plexes (Table 4).
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Table 4 Prediction of IR absorptions by m eans of DFT calculation for the CO2 adduct with Re -complexes.
solvent COsym COasy1 COasy2 COO dmbpy facial-type
[ReⅠ(dmb)(CO)3COOH] DMSO 2013 1896 1876 1648 1627
[ReⅠ(dmb)(CO)3OCOH] DMSO 2023 1894 1886 1628 1624
[ReⅡ(dmb)(CO)3COO]・ DMSO 2018 1898 1887 1783 1626
[ReⅡ(dmb)(CO)3COOH]・+ DMSO 2062 1987 1964 1685 1624
meridional-type
[ReⅡ(dmb)(CO)3COO]・ DMSO 2048 1923 1905 1637 1627
[ReⅡ(dmb)(CO)3COOH]・+ DMSO 2046 1913 1863 1614 1626
The DFT calculation indicates that ReI I species have IR absorption at larger wavenumber than those for ReI species due to a decreased back-donation effect of the central Re as anticipated. Though the deprotonated form of the CO2 adduct with Re-complex, [ReI I(dmbpy)(CO3)COO]., is predicted to have absorption at much larger wavenumber (1783 cm- 1), the carbox ylic R e -complex, [ReI I(dmbpy)(CO3)COOH]. +, is predicted to have very similar wave number (1685 cm- 1) with the observed one (1682 cm- 1). Transient -III (1682 cm- 1) can be t hus assigned as [ReI I(dmbp y)(CO3)COOH]. +, while the CO ligands are predicted to have absorption rather different than the three signals at 2006, 1904, and 1879 cm- 1 characterized above (Figures 4, 5). The a pparent discrepancy on the CO absorption between the observed data and the DFT prediction is not clear here, but is supposed that the CO absorption of Transient -III are hidden under those of the one-electron reduced species.
Effect of proton on the transie nt IR absorption:
Though the assignm ent of Transient -III (1682 cm- 1) is rationalized as discussed above in 3.5, a further experiment to verif y the assignment was carried out. In the CS I -MS study, we have alread y postulated the reaction mechanism involving a formation of CO2 adduct with Re-complex, [ReI I(dmbpy)(CO3)COO]. followed by a subsequent
73
protonation to form [ReI I(dmbp y)(C O3)COOH]. + (Transient -III) as shown in Scheme 1. The proton donor in the process was supposed to be the radical cation of triethyl amine (TEA. +), which was formed by an electron transfer to the 3MLCT state of the starting [1] and / or the solvent coordinated Re -complex, [2]. The radical cation of TEA is well known to be deprotonated through equation (1).2 4 )
If the proton transfer is involved in the rate determining step, an addition of proton donor except for TEA. + would be expect ed to affect the rise behavior of [ReI I(dmbp y)(CO3)COOH]. + (Transient -III).
With this viewpoint, ammonium chloride (NH4+Cl-) in 20 times equivalent vs [1] was added to the reaction mixture and then the photoreaction was carried out. Very i nterestingl y, the absorbance at 1682 cm- 1 substantiall y increased as shown in Figure 6, while the one-electron reduced species observed at 1798 cm- 1 remained unchanged. The spectrum in other region also showed delicate changes. The absorption around 1650~1550 cm- 1 disappeared by the addition of NH4+Cl- , suggesting that other species was involved in the spectrum (Figure 6 (a)).
These results clearl y indicate that the 1682 cm- 1 species (Transient -III) is at least convincingl y the carbox yl ic Re -complex ([ReI I(dmbpy)(CO3)COOH] ([3’]). The difference spectrum obtained by subtraction of Fi gure 6 b) from Fi gure 6 a) showed some species having absorption at around 1700, 1650~ 1550 cm- 1 is involved. One of the possible species may be mer-[ReI I(dmbpy)(CO3)COO], since the one-electron reduced forms of Re -complex may involve meridional-t ype five coordination complex as discussed above. We can remember here that the CS I -MS study al so showed signals for the CO2
adduct, [Re(dmbpy)(CO)3COO].1 6 ) TEA
e
- TEA・+(C2H5)2NCHCH
・
3 (1)-H+
74
Figure 6 Effect of addition of NH4+Cl- on the transient absorption at the delay time of 300 s after the laser flash: a) under 1 2CO2 in DMSO/TEA (15/1) without NH4+Cl-, b) under 1 2CO2 in DMSO/TEA (15/1) with NH4+Cl- (in 20 times equivalent vs [1]), c) difference spectrum obtained by subtract ion of b) from a).
In addition to the spectral characteristics, the dynamic behavior afforded ver y interesting insight into the mechanism. As sh own in Figure 7, the rise of 1682 cm- 1 (Transient -III, [3’], Fi gure a)) was evidentl y accelerat ed upon addition of NH4+Cl- and the transient absorption reached i ts maximum at around ca. 150 s after the laser flash much shorter than it without NH4+Cl- (ca. 250 s, Figure 2). This strongl y suggests that the protonation process is the rate -determining step for the formation of [3’] in the photoreduction of CO2. The
0 0.02 0.04 0.06 0.08 0.1
1550 1650
1750 1850
1950 2050
ΔAbs
wave number / cm-1
(a) (b)
-0.02 -0.01 0 0.01 0.02
1550 1650
1750 1850
1950 2050
ΔAbs
wave number / cm-1
(c)
75
dynamic behavior of the one -el ectron reduced form of Re -complex (Transient-II, II’) and the starting Re-complex ([1]), on the other hand, were not affected at all by NH4+Cl- with their peak maxima remaining at around ca. 150 s (Fi gure 7 b), c)).Especiall y, the recover y curve of [1] was not affected by the addition of NH4+Cl- (Fi gure 7 c)). Leh n group had reported that addition of NH4+Cl- improved the quantum yield of the photoreduction of CO2 and the effect could be due to chloride ion through a coordination to Re center to recover the starting Re -Cl complex ([ReI(bp y)(CO3)Cl], driving the reaction continuousl y.2 ) However, this is not the case as described above. The improvement of the reactivit y b y t he addition of NH4+Cl- should be due to the acceleration of the rate -determining step of protonation to [3] to form [3’].
Figure 7 Time profiles of the IR transient signals upon addition of NH4+Cl- in the laser flash induced photoreduction of CO2 catal yz ed by [1] in DMSO/TEA (15/1).
0 0.01 0.02 0.03 0.04 0.05
0 200 400 600 800
ΔAbs
Time / s
-0.07 -0.05 -0.03 -0.01
ΔAbs
0 0.1 0.2 0.3 0.4
ΔAbs
(a) 1682cm-1
(b) 1890cm-1
(c) 1798cm-1
76