Raman Spectroscopy as a Method to Investigate Catalytic

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Raman Spectroscopy as a Method to Investigate Catalytic Intermediates: the CO Reducing [Re(Cl)(bpy-R)(CO)] Catalyst 2

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Sabrina Imke Kalläne, and Maurice van Gastel J. Phys. Chem. A, Just Accepted Manuscript • DOI: 10.1021/acs.jpca.6b07246 • Publication Date (Web): 31 Aug 2016 Downloaded from http://pubs.acs.org on September 5, 2016

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The Journal of Physical Chemistry

Raman Spectroscopy as a Method to Investigate Catalytic Intermediates: the CO2 Reducing [Re(Cl)(bpy-R)(CO)3] Catalyst Sabrina I. Kalläne and Maurice van Gastel*

Max-Planck-Institut für Chemische Energiekonversion, Stiftstrasse 34-36, D-45470 Mülheim an der Ruhr, Germany

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Abstract Complexes of the type [Re(Cl)(bpy-R)(CO)3] (1, bpy = bipyridine, R = tBu, H, CF3) show high catalytic activity for electrochemical CO2 reduction. Application of Raman spectroscopy to these complexes as well as to the doubly reduced species [Re(bpy-R)(CO)3]- (3), which are the postulated active species, and the mono-reduced complex [Re(Cl)(bpy-CF3)(CO)3]- (2) and comparison with state-of-the-art quantum chemical calculations allows accurate investigation of electronic structures as well as geometries. For doubly reduced complexes, calculations point out a formal closed-shell singlet state only compatible with a formal {ReI(bpy-R)2−} moiety. In contrast, based on molecular orbital analysis and the change of the actual charge distribution during the overall two-electron reduction, the system is better described as {Re0(bpy-R•)−}. Interestingly, the Raman spectra of the mono-reduced and doubly reduced complexes with the CF3-substituted bpy ligand are virtually identical, which points to the same overall electronic structure of the bpy species in both complexes. Additional Raman experiments and calculations of [Re(COOH)(bpy)(CO)3] (4) and [Re(bpy)(CO)4]OTf (5), which are proposed to be intermediates of the catalytic cycle for CO2 reduction, confirm the presence of neutral bpy showing that the reducing equivalent stored at the bidentate ligand is involved in the activation of CO2. As such, Raman spectroscopy combined with quantum chemical calculations is an ideal tool to investigate catalysts with redox active ligands, since the spectra give local information about the electronic and geometric structure of the molecule.


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Introduction Small molecules such as O2, N2, H2, and CO2 play a critical role in various biochemical cycles, and, in this role, they are involved in elementary reactions relevant to the efficient and reversible storage and release of bond energy. Owing to the present exploration of alternatives for fossil fuels to cover energy demand, massive efforts are being made to chemically convert those small molecules into high-value chemical feedstocks and fuels on a large scale.1 Of particular interest is CO2 as a C1 building block,2,3 since it is a naturally abundant molecule and mainly produced as waste product in industry where it contributes to the greenhouse effect. CO2 is kinetically inert and its chemical transformation is thermodynamically challenging4,5 and usually involves multielectron redox processes, coupled with proton transfers via complicated reaction pathways.2,3 The fac-[Re(X)(bpy-R´)(CO)3] system (X = anionic ligand or neutral ligand with a counter anion; bpy-R´ = R´-substituted 2,2´-bipyridine), first reported by Lehn et al. in the 1980s,6,7 is one of the most robust and efficient CO2 reduction catalysts known to date8 and affords CO under both photochemical and electrochemical conditions.7 The properties of these and related Re compounds as well as of possible intermediates of the electrocatalytic cycle have been the subject of extensive experimental and computational9–11 investigations. The former includes visible absorption and emission spectroscopy,6,11–22 Fourier transform infrared (FTIR) spectroscopy,11,15,16,18–20,23–25

photochemistry,16,18,19

EPR

spectroscopy,26–28

X-ray

crystallography,9,10,24,29,30 X-ray absorption spectroscopy,31 cyclic voltammetry9,23,24,32 and spectroelectrochemistry,

14,23,24,33–35

stopped-flow measurements10 as well as kinetic and

labeling studies.6,10,32



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Based on mechanistic studies, hypotheses concerning the intermediates and active species involved in the mechanism of CO2 reduction have been published.17,23,24,34,36–39 In general, either the singly reduced7,40 or a doubly reduced37–39 rhenium complex has been proposed as the catalytically relevant compound for CO2 binding, whereby the assumption of the latter one is generally accepted. Theoretically the mechanisms of action of catalysts with respect to the followed reaction pathways in particular related to the order of protonation and reduction steps have as such been proposed.37 Kubiak et al. succeeded in the isolation of the doubly reduced anion [Re(bpy-R´)(CO)3]−, which was obtained by two-fold reduction of [Re(X)(bpy-R´)(CO)3].10 With the availability of this complex, its electronic structure was the focus of investigations, since the bpy ligand is assumed to play a redox non-innocent role. On the one hand, XRD studies, which reveal a bond length alteration in the ring system and a short ring-ring (C2-C2’) bond,9,29 point to a formal charge distribution of {ReI(bpy-R)2−}. Furthermore, Wieghardt and cowokers suggested a dianionic ligand based on the difference of the C2-C2’ and C2/2´-N bond lengths, which correlates in a linear fashion with the formal charge of the bpy ligand according to crystal data.41 On the other hand, XAS data indicate that the doubly reduced, diamagnetic anions are best described as {Re0(bpy-R•)−}.31 The latter electronic configuration might explain the preference of an interaction with CO2 over binding a proton.10 It was experimentally shown that the anion reacts with CO2 to give [Re(COOH)(bpy-R)(CO)3].39 One further protonation as well as two reduction processes are suggested as subsequent reaction steps in the catalytic cycle to furnish CO and H2O along with the regeneration of the doubly reduced compound.37–39 According to the postulated mechanism the theoretically less favored protonation-first-pathway leads to the formation of [Re(bpy-R)(CO)4]+, whereas an initial reduction forms [Re(COOH)(bpy-


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R)(CO)3]-. The rate of electrochemical reduction of CO2 catalyzed by Re complexes can be enhanced by the addition of Brønsted acids.32 Furthermore, a study in which the substituent in 4- and 4´-position of bpy was varied revealed that the derivative bearing the sterically demanding and electron-donating tert-butyl group exhibits greater catalytic activity than the unsubstituted bipyridine complex [Re(Cl)(bpy)(CO)3], whereas the complexes with a bipyridine ligand substituted with an electron-withdrawing group (R`= COOH, CF3), show no or little catalytic activity.9,33 Nevertheless, for the purpose of elucidating the catalytic mechanism, the use of CF3-substituted bipyridine is advantageous, since it allows the synthesis and the X-ray analytical characterization of a mono-reduced monomeric complex.9 Resonance Raman spectroscopy has already been applied to the system, although the strong luminescence of these compounds can complicate measurement of good resonance Raman spectra.42 When using focused or pulsed excitation and making use of the relatively long-lived (200 ns) optically excited state, it has even been possible to record resonance Raman spectra of the oxidized complex with unsubstituted bipyridine [Re(X)(bpy)(CO)3] (X = halide) for the ground state as well as the excited state.43,44 Calculations have also been performed and calculated resonance Raman spectra for the ground state in good agreement with experiment have been reported.45 Nevertheless, Raman spectroscopy of the reduced states with the aim to investigate in how far first and second reduction takes place at the metal or the ligand and of postulated intermediates of the catalytic CO2 reduction has not yet been attempted. Such an investigation is advantageous, since Raman spectroscopy is a high-resolution method that affords local information about the electronic structure of the molecule. Therefore, Raman studies of Re complexes, which are proposed to be involved in the catalytic process, can provide information about the charge distribution over the bipyridine ligand and the metal ion, respectively, during 5 ACS Paragon Plus Environment


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CO2 reduction. For this purpose the combination of spectroscopy with quantum chemistry is of crucial importance.

Figure 1. Schematic overview of studied rhenium complexes. In this contribution, we attempt such a study and report Raman spectra of the Re complexes [Re(Cl)(bpy-R)(CO)3] (1), as well as of the doubly reduced compounds [Re(bpy-R)(CO)3] [K(18crown-6)] (3) and also of the mono-reduced complex [Re(Cl)(bpy-CF3)(CO)3][K(18-crown-6)] (2c), which have been obtained by reduction by KC8 and by using 18-crown-6 to encapsulate the potassium ion. We chose three different substituents at the bipyridine ligand (a: R = tBu, b: R = H, c: R = CF3, Figure 1) that vary in their electron-withdrawing capacity. Furthermore, the postulated intermediates [Re(COOH)(bpy)(CO)3] (4) and [Re(bpy)(CO)4]OTf (5, Tf = trifluoromethanesulfonyl) were examined by Raman spectroscopy. Additionally, we present a vibrational assignment of the observed bands based on comparison with quantum-chemical calculations.


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Experimental and Theoretical Methods Materials. Complexes 1a, 1b, 1c and 3a, 3b, 3c were synthesized according to a reported procedure for the synthesis of [Re(Cl)(bpy-R)(CO)3]46 or [Re(bpy-R)(CO)3] [K(18-crown-6)],29 respectively. Complex 2c was prepared as described in literature.9 4,4´-bis(trifluoromethyl)-2,2´bipyridine was synthesized as previously reported by Pd-catalyzed homocoupling47 and purified by vacuum sublimation. NMR data for 1c: 1H NMR (500.0 MHz, CD3CN): δ = 9.27 (d, JH,H = 6 Hz, 2 H, CH(6,6´)), 8.85 (s, 2 H, CH(3,3´)), 7.94 (dd, JH,H = 6 Hz, J = 1 Hz, 2 H, CH(5,5´)) ppm.

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C{1H} NMR

(125.7 MHz, CD3CN): δ = 198.1 (CO), 189.4 (CO), 157.3 (C), 155.5 (CH(6,6´)), 141.4 (q, JC,F = 35 Hz, C(4,4´)), 124.9 (q, JC,F = 4 Hz, CH(3,3´)), 123.1 (q, JC,F = 274 Hz, CF3), 122.3 (q, JC,F = 4 Hz, CH(5,5´)) ppm. 19

F NMR (470.5 MHz, CD3CN): δ = -65.5 (s) ppm. NMR data for 3c: 1H NMR (500.0 MHz, THF-d8):

δ = 8.81 (m, 2 H, CH(6,6´)), 7.62 (m, 2 H, CH(3,3´)), 5.47 (m, 2 H, CH(5,5´)), 3.56 (br s, 24 H, CH2 (18crown-6)) ppm. Complexes 439 and 515 were prepared according to literature. Raman measurements. Samples of 1a-c were prepared in MeCN, samples of 2c and 3a-c were prepared as THF solutions to concentrations of 5 - 25 mM. Complexes 1a, 4 and 5 were measured as solids. Raman spectra were recorded on a TriVista 555 triple monochromator equipped with a liquid-nitrogen-cooled Roper Scientific 400BR Excelon CCD camera, using Cobolt diode lasers at fixed wavelength; a diffraction grating in the excitation path was used to remove plasma lines from the laser. The samples were contained in either a quartz EPR tube within an EPR quartz finger dewar and cooled with liquid nitrogen or measured with an Oxford Cryostream 600 system at 100 K. The Raman light was collected in a 150° backscattering geometry using a 100 mm diameter f/1.5 lens and focused onto the entrance slit of a spectrograph with a 100 mm diameter f/4 lens. The scattered light was dispersed with gratings 7 ACS Paragon Plus Environment


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of 900 mm−1, 900 mm−1 and 1800 mm−1 at the three different stages, respectively. Slit widths at the first and third stages were set to 50 μm, giving rise to a spectral resolution at the CCD camera of about 0.8 cm−1. Spectra were collected for ~30 min for a given wavelength and spectral window; corresponding spectral windows were stitched together ensuring overlap that included at least one peak for normalization. Calibration of the Raman shifts has been achieved to an accuracy of 1 cm−1 by using Na2SO4 as well as the solvent signals as a reference. EPR measurements A continuous-wave EPR spectrum of 2c has been recorded at T = 10 K on a Bruker E500 Elexsys EPR spectrometer equipped with a ST9402 resonator. The microwave frequency amounted to 9.65 GHz. The microwave power was set to 0.2 mW and a modulation amplitude of 0.1 mT was used. Computational methods. All calculations were carried out using the ORCA quantum chemistry program.48 Geometry optimization and frequency analysis were performed with the BP86 functional at the level of density functional theory (DFT).49 The def2-SVP basis set was used for all atoms.50 A few tests were performed with basis sets of triple zeta quality, which confirmed that the def2-SVP basis set is sufficient for calculation of Raman spectra. The resolution of identity RI approximation has been employed to speed up calculation time.51,52 Scalar relativistic effects are included in zero order regular approach (ZORA).53,54 The Cartesian coordinates of all optimized structures are provided as supporting information. Vibrational frequencies and Raman intensities have been calculated from diagonalization of the Hessian matrix and from the polarizability tensor as implemented in ORCA.48 A Doppler broadening with a band width on half


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height of 10 cm-1 has been used to dress the calculated spectra. The accuracy of this method is typically 25 cm-1.55,56 Assignments are based on both comparison of experimental and calculated band positions and intensities. For calculations of the UV-Vis spectra, the CAM-B3LYP functional57 was used, after the structures were re-optimized by including solvent effects with the COSMO model.58 In order to characterize the location of the reducing equivalents, the single occupied natural orbital of the mono-reduced [Re(bpy-R)(CO)3] species has been used. This orbital is virtually identical to the highest occupied molecular orbital (HOMO) of the doubly reduced [Re(bpy-R)(CO)3]- complex. The C2,2´-N bond values are indicated as average of the carbon-nitrogen bond lengths in the crystal structure or calculated structure, respectively.



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Results and Discussion

Figure 2. Raman spectrum of [Re(Cl)(dtbpy)(CO)3] (1a) as a solid, excited at (a) λ = 355 nm, P = 25 mW, 100 K, (b) λ = 561 nm, P = 25 mW, (c) λ = 660 nm, P = 50 mW, and in frozen MeCN, excited at (d) λ = 355 nm, P = 15 mW, (e) λ = 416 nm, P = 30 mW; (f) calculated Raman spectrum for 1a. Solvent signals have been subtracted in (d) and (e).

Oxidized complexes The Raman spectrum of the improved catalyst, [Re(Cl)(dtbpy)(CO)3] (1a, dtbpy = 4,4'-di-tertbutyl bipyridine, R = tBu), as a solid is presented in Figure 2a and was measured with an 10 ACS Paragon Plus Environment

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excitation wavelength of 355 nm, which is close to the absorption maximum at 363 nm. In accordance with calculations of related complexes,13,59 this absorption is associated with a MLCT (metal-to-ligand charge transfer) and ligand-to-ligand charge transfer transition from a ReCl π* orbital into a π* orbital of the bipyridine moiety. When measured non-resonantly at wavelengths of 561 and 660 nm, the spectra (Figure 2b, c) retain the same structure with limited variation in the relative intensities of the bands. In frozen solution, similar spectra have been measured at 355 and 407 nm (Figure 2d,e), indicating the absence of strong interactions with the solvent. In line with expectations, at off-resonance excitation at 561 and 660 nm, the amplitudes of the bands in frozen solution decrease by multiple orders of magnitude to an extent that the spectrum is completely dominated by bands of the solvent. The relatively insignificant decrease of the amplitudes in the solid at off-resonant excitation (Figure 2b,c) may be related to the increased penetration depth of the light into the sample, which increases the number of excited molecules. The Raman spectrum of 1a includes bands at ν = 1618, 1544, 1491, 1322, 1293, 1272 and 1037 cm-1, which can be attributed to normal modes of the substituted bipyridine ligand. The bands are similar to those observed in the resonance Raman spectrum of [Ru(dtbpy)3](PF6)2 (ν = 1615, 1538, 1481, 1317, 1283, 1264, 1031 cm-1)60 and in the Raman spectrum of [Mo(Cl)(η3C3H5)(dtbpy)(CO)2] (ν = 1613, 1543, 1483, 1316 cm-1).61 Note, that bands at ν = 1609, 1552, 1491, 1426, 1321, 1001 cm-1 were observed for uncoordinated dtbpy,60 showing that coordination to a metal center changes the frequencies of particular bands (e.g. the one at about 1000 cm-1 by 35 cm-1). In the lower region of the spectrum of 1a an additional characteristic band at ν = 511 cm-1 is observed. 11 ACS Paragon Plus Environment


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In order to obtain specific assignments for all bands, the Raman spectra of 1a as well as of all other measured Re compounds were calculated. The calculated Raman spectrum of 1a is shown in Figure 2f. Calculated nuclear displacements for selected modes which give rise to strong Raman bands have been depicted in Table S1. According to the calculations, the band at ν = 511 cm-1 is assigned to two nearly degenerate normal modes, in which in particular the CO ligands and, to a lesser extent, the dtbpy ligand are involved. The observation of this band indicates that the Re complex is not damaged by the laser irradiation and that the carbonyl ligands have not been photo-dissociated. The vibrational analysis also confirms that the bands in the 1000 - 1650 cm-1 region involve predominantly C-C and C-N stretching and in-plane C-H bending modes of the dtbpy ligand. These modes are resonantly enhanced owing to the MLCT character of the transition which places an additional π* electron at the dtbpy moiety, thereby changing these CC and C-N bond lengths in the electronically excited state. The agreement between experiment and calculation is satisfactory: the calculated Raman spectrum under harmonic approximation correctly reflects the structure to within Δν = ±25 cm-1 and even the intensity distribution of the experimental spectrum. Only, the intensity of the band at ν = 1618 cm-1 (1611 + 1619 cm-1 in the calculation) is overestimated, whereas that of the band at ν = 1491 cm-1 (1477 cm-1 in the calculation) is slightly underestimated. The general agreement of the calculation and experiment confirms the accurate geometric and electronic structure of the used model, whose Cartesian coordinates are included in the SI. The calculated Re-N and Re-Cl bond lengths of 1a are only 0.01 Å longer than those determined by single-crystal X-ray analysis.10 The C2-C2´ bond distance of 1.479 Å and the average C2/2´-N bond length of 1.366 Å are in the range of experimentally observed distances: average distances in 12 ACS Paragon Plus Environment

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complexes with neutral bpy-R amount to 1.475 ± 0.015 Å for C2-C2´ and 1.355 ± 0.015 Å for C2/2´N41 and distances in the crystal structure of 1a amount to 1.487(5)/ 1.475(5) Å for C2-C2´ and an average value of 1.360 Å for C2/2´-N (two crystallographically independent molecules in the asymmetric unit).10 For comparison, the latter distances equal 1.493(5) Å for C2-C2´ and 1.326 Å for C2/2´-N in uncoordinated dtbpy.62 Attempts to measure the Raman spectrum of the manganese analogue of 1a, [Mn(Br)(dtbpy)(CO)3], which also exhibits a remarkable catalytic activity for electrochemical CO2 reduction in the presence of Brønsted acids,63,64 failed even at 77 K with excitation of 407 and 660 nm due to its light sensitivity.



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Figure 3. Raman spectra of oxidized complexes [Re(Cl)(bpy-R)(CO)3] (1a-c) in MeCN: (a) 1a, λ = 416 nm, P = 30 mW, (b) 1b, λ = 407 nm, P = 25 mW, (c) 1c, λ = 355 nm, P = 15 mW. (d-f) Calculated Raman spectra of oxidized complexes [Re(Cl)(bpy-R)(CO)3] (1a-c), respectively. Solvent signals are omitted for clarity in (a-c).

For the Raman spectrum of [Re(Cl)(bpy)(CO)3] (1b) an excitation wavelength of 407 nm was used, which induces a mixed MLCT/L’LCT transition (calculated HOMO-1 to LUMO transition: 456 nm; experimental absorption maximum: 370 nm65). Comparison of the Raman bands of 1a 14 ACS Paragon Plus Environment

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with those obtained for a solution of 1b in MeCN (Figure 3b, ν = 1605, 1564, 1495, 1320, 1284, 1036, 491 cm-1; literature data:43 1605, 1565, 1494, 1316, 1261, 1034 cm-1) reveals that the substitution of the bipyridine ligand with tBu groups changes the experimental spectrum only to a minor degree, in contrast to, e.g., exchange of a hydrogen atom for deuterium, which results in shifts up to 36 cm-1.66 The calculated normal modes of 1a and 1b that lead to strong bands are very similar in most cases (Table S2 Figure 3d, 3e): for example, the tBu group in 1a admixes to a bpy vibration, which gives rise to bands at 1006 and 1010 cm-1 for 1b, and this admixture leads to calculated bands at 1009 and 1014 cm-1. Experimentally, the corresponding bands occur at 1037 cm-1 for 1a and 1036 cm-1 for 1b, and differ only by 1-4 cm-1 in theory and experiment. As a further example, the dtbpy ligand in 1a leads to a band at ν = 1491 cm-1, whereas the corresponding normal mode of the bpy ligand in 1b results in a band at ν = 1495 cm-1. Slightly larger differences are present with respect to relative intensities. The band of 1b at ν = 1284 cm1

is of low intensity in relation to that at ν = 1320 cm-1, whereas this ratio is reversed in the

Raman spectrum of 1a. Still, the calculated Raman bands of [Re(Cl)(bpy)(CO)3] (1b) overall fit well with experimental data. The intensities of the experimental bands differ only slightly from the calculated intensities in the range from ν = 1450 to 1650 cm-1 (see Figure 3b, e). As such, our observation is in accordance with that of Strommen and Kincaid at al.66 who demonstrated for [Ru(bpy)3]Cl2 that the MLCT excitation, in contrast to excitation of a π – π* transition, strongly enhances the mode at ν = 1491 cm-1 in relation to that at ν = 1608 cm-1. For [Re(Cl)(bpy-CF3)(CO)3] (1c) in MeCN, characteristic bands at ν = 1629, 1563, 1484, 1438, 1343, 1275, 1030 cm-1 were detected (Figure 3c). Literature data of the CF3-substituted bpy or metal complexes bearing this ligand do not exist for comparison purposes. The band associated 15 ACS Paragon Plus Environment


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with the mixed CO and the bpy-CF3 vibration at about 500 cm-1 is very weak. As with the other complexes, the experimental and calculated spectra match well when the positions of the bands are examined; one particular discrepancy concerns the group of calculated bands at ν = 1316, 1320 and 1336 cm-1, which in experiment may coincide, leading to one observed band at ν = 1345 cm-1 (Table S2). Especially regarding the band at ν = 1629 cm-1, the calculated intensities differ from the experimental ones. In summary, the Raman spectra of all oxidized rhenium complexes 1a-c exhibit three strong bands at about ν = 1030, 1490, and 1555 cm-1, one weaker band at about ν = 1620 cm-1 and one to three moderate bands in the range from ν = 1250 to 1350 cm-1. The shift of the band from ν = 1037 (1a) to 1036 (1b) to 1030 cm-1 (1c) correlates well with the increased electron withdrawing potential of the bpy substitution and is reproduced by the calculations. The effect of the substituent is also detectable by the reduction of the ring-ring bond distance in the optimized structure going from R = tBu (C2-C2´: 1.479 Å) to R = H (C2-C2´: 1.476 Å) to R = CF3 (C2-C2´: 1.473 Å). Finally, as compared to 1a and 1b, the bpy-CF3 ligand in 1c possesses a more negative Mulliken charge, indicating that it withdraws more electron density from the {Re(Cl)(CO)3} fragment (Table S3), which is concomittant with a shortening of the C2-C2´ bond length.

Doubly reduced complexes Figure 4a (see also Figure S1) presents the Raman spectrum of [Re(dtbpy)(CO)3] [K(18-crown-6)] (3a) in THF with an excitation wavelength of 416 nm. For the anion, bands at ν = 1607, 1512, 1487, 1432, 1294, 1228, 1100, 1002, 587, 471 cm-1 have been detected. These 16 ACS Paragon Plus Environment

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frequenciesoriginate from nuclear displacements of the dtbpy ligand or the CO ligands (the two latter frequencies) and are notably different from those in the oxidized Re complex. In particular, the band at ν = 1037 cm-1 (νcalcd = 1012 cm-1) of 1a shifts to 1002 cm-1 (νcalcd = 993 cm1

) and its intensity is reduced as compared to the oxidized complex, both in experiment (Figure

4a) and calculation (Figure 4d). Furthermore, the normal modes giving rise to bands at ν = 1491 cm-1 and 1618 cm-1 in the spectra of 1a lead to bands at ν = 1512 and 1607 cm-1, respectively, in the spectra of 3a. The band at ν = 1294 cm-1 does not shift. The other characteristic bands of 1a and 3a are not directly comparable as is also apparent in Table S1. The observed changes in the Raman spectrum of 3a with respect to that of 1a reflect significant changes in the normal mode structure, which stem from reduction of the bpy entity. Again, the overall agreement of calculation with experiment is satisfying; in general, the calculated values are lower than the experimental ones by up to 18 cm-1. The experimental spectrum fits better to the calculated Raman spectrum of [Re(dtbpy)(CO)3]- than to the calculated spectra of free (dtbpy·)- and dtbpy2(Figure S2). The optimized structure of 3a reveals a square pyramidal coordination geometry at the rhenium atom, which is in accordance with the X-ray crystal structure analyses of [Re(bpy-R´)(CO)3]- (τ5 = 0.11-0.46).9 About 50% of the anionic charge is located at the dtbpy ligand according to the Mulliken population analysis (Table S3). The bond lengths in the bipyridine moiety differ significantly with respect to those of the oxidized state.41,67 These differences stem from the population of the π* LUMO of the bpy ligand upon reduction. This orbital is bonding for the C2C2´ bond and anti-bonding for the C2/2´-N bond (Figure 5). The bond distances in the optimized structure of 3a (1.418 Å for C2-C2´ and 1.415 Å for C2/2´-N) are more comparable to those found 17 ACS Paragon Plus Environment


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in the calculated structure of uncoordinated doubly reduced dtbpy (1.416 Å for C2-C2´ and 1.426 Å for C2/2´-N) than with those obtained for the uncoordinated mono-reduced dtbpy species (1.454 Å for C2-C2´ and 1.387 Å for C2/2´-N) or for 1a, but note that coordination of the bipyridine ligand to a metal center can further decrease the C2-C2´ bond length and increase the C2/2´-N bond lengths. Compared to the bond length in the crystal structure (3a: 1.37(1) Å for C2-C2´ and 1.41 Å for C2/2´-N;10 average distances in [Re(bpy-R´)(CO)3] [K(18-crown-6)] with R´= H, Me, OMe, Bu: 1.39 ± 0.02 Å for C2-C2´ and 1.40 ± 0.02 Å for C2/2´-N9) only the calculated C2-C2´ bond

t

distance is overestimated possibility reflecting a too low electron density on the ring system of the calculated anionic complex. A triplet and broken symmetry singlet state have also been considered, but calculations point out a closed-shell singlet state as the electronic ground state in accordance with literature.31,37 Given the closed-shell character of the wavefunction, the doubly reduced complex thus semantically has to be described as a ReI system with a bpy-R dianion rather than as an open-shell Re0 system antiferromagnetically coupled to a bpy radical anion. However, when examining the actual charge distribution by using the Mulliken spin population and the SOMO of the mono-reduced species [Re(dtbpy)(CO)3] (Figure 5) as an indicator, a distribution of 26 % at the metal atom and 63 % at dtbpy is found. Therefore the overall two electron reduction step going from 1a to 3a also takes place at the metal atom to a non-negligible extent, with about 0.5 reducing equivalents at Re and about 1.3 equivalents at the bidentate ligand. As such, the reducing equivalent at Re is in accordance with, for instance, FTIR data9 and X-ray spectroscopic results indicating that the Re actual (as opposed to formal) oxidation state in 3a is best described as Re0.31 In addition, when taking the Mulliken charge as


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an indicator, the calculations actually reveal that the charge associated with the bipyridine ligand is lowered by 1.00 ± 0.06 when going from 1 to 3 (Table S3).

Figure 4. Raman spectra of doubly reduced complexes [Re(bpy-R)(CO)3] [K(18-crown-6] (3a-c) in THF: (a) 3a, λ = 416 nm, P = 30 mW, (b) 3b, λ = 457 nm, P = 15 mW, (c) 3c, λ = 416 nm, P = 30 mW. (d-f) Calculated Raman spectra of [Re(bpy-R)(CO)3]- (3a-c), respectively. Solvent signals have been subtracted in (a-c). Bands marked with an asterisk are decreased by a factor of 2.5 for clarity.



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Figure 5. Singly occupied natural orbital (SOMO) of [Re(dtbpy)(CO)3]. The experimental spectrum of [Re(bpy)(CO)3][K(18-crown-6)] (3b) and the calculated spectrum of the corresponding anion are shown in Figure 4b and 4e. A 457 nm excitation leads to intense bands at ν = 1605/1590, 1508, 1471, 1360, 1283, 1220, 1013/996, 483, 380 cm-1. The bands overall agree satisfactorily with calculated data (ν = 1590/1579, 1490, 1455, 1297/1292, 1204, 1031/1011/991, 473, 374 cm-1). A particular mismatch concerns the calculated intensity of the band at ν = 1490 cm-1 which dominates the calculated spectrum. Compared to 3a (ν = 1607, 1512, 1487, 1294, 1228, 1002 cm-1) these bands are shifted by about ν = ±15 cm-1, illustrating the influence of the tBu substituent on the normal modes of bpy. Figure S3 represents the dependence of the spectrum on the excitation wavelength (355, 407, 457, 532 nm) and shows that bands at about ν = 1590, 1500, , 1285, 1220, 1010, 1000, 480, 380 cm-1 were observed in all four spectra. The changes upon different excitation concern the intensities in the range from ν = 1410 to 1610 cm-1 and derive from the presence of multiple CT excitations. It is known that complexes of the type ([Re(bpy-R’)(CO)3]-) exhibit a broad UV-Vis absorption at about 570 nm and a shoulder at about 370 nm (3b: 560, 367 (sh) nm; literature data: 560 nm for 20 ACS Paragon Plus Environment

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[Re(bpy)(CO)3]-14, 570, ~370 (sh) nm for [Re(dtbpy)(CO)3]-10, 582, 378 (sh) nm for [Re(bpyMe)(CO)3]-68). Calculations reveal that these absorptions arise from transitions with bpy π* π*, MLCT and LL´CT character (calculated electronic transitions: 568, 477, 447, 288 nm). The Raman spectrum of [Re(bpy-CF3)(CO)3] [K(18-crown-6)] (3c) upon 416 nm-excitation (Figure 4c; ν = 1558, 1521, 1494, 1449, 1349, 1288, 1223, 1017, 860cm-1) exhibits the same typical Raman bands which were observed for 3a and 3b (ν = 1521, 1288, 1223, 1017 cm-1), but differs in some aspects. The band of 3c with the largest shift is detected at a lower region and the strong band at ν = 1449 cm-1 does not have a corresponding intense spectral feature for 3a and 3b. Analogous to 3b, the agreement of the experimental spectrum with the calculated spectrum (Figure 4f) is moderate. Particularly, the calculated band at 1502 cm-1 is much more intense than the corresponding band in the experimental spectrum and the bands at 1170, 1172 and 1213 cm-1 may coincide in experiment at 1223 cm-1. Additional calculations indicate that the bands in the range of 1440 to 1560 cm-1 are sensitive to external influences. In particular, a new band at ν = 1523 cm-1 appears and the intensity of the band at ν = 1503 cm-1 decreases in the calculated spectrum, when a counter ion is included. Both experiment and calculation thus indicate that the Raman spectra of the reduced bpy moieties are more sensitive to changes induced by electronegative substituents than the Raman spectra of the oxidized bpy complexes. Mono-reduced complex and connection to CO2 reduction In order to verify the identity of [Re(Cl)(bpy-CF3)(CO)3] [K(18-crown-6)] (2c), an EPR spectrum (Figure S4) of the same sample as used for Raman spectroscopy was recorded, which evidences the presence of an organic radical, (bpy-CF3·)-, with negligible spin density at Re. The g-value of



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2.007 and the relatively narrow signal are similar to those found for the in situ electrolytically generated complex [Re(Br)(bpy)(CO)3]- (g = 2.002).26,27 Since calculations of the mono-reduced complex (vide supra) have indicated that the spin population at Re increases to about 25% upon halide loss, the observation of an organic radical with no resolved Re coupling at low temperature strongly indicates that the chlorine atom is still attached to the mono-reduced complex. Surprisingly, the Raman spectra of the mono-reduced complex (2c) are identical with those of the doubly reduced species, [Re(bpy-CF3)(CO)3] [K(18-crown-6)] (3c) (Figure 6), even at different excitation wavelengths.

Figure 6. Comparison of the experimental Raman spectrum of doubly reduced complex [Re(bpyCF3)(CO)3] [K(18-crown-6] (3c) (a: λ = 416 nm, P = 30 mW; b: λ = 355 nm, P = 23 mW) and monoreduced complex [Re(Cl)(bpy-CF3)(CO)3] [K(18-crown-6] (2c) (c: λ = 416 nm, P = 30 mW; d: λ = 355 nm, P = 23 mW) in THF. Solvent signals are omitted for clarity.


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Since the same bands were observed in the Raman spectra of 3c and 2c, the spectra indicate that the bpy-CF3 moieties have an identical degree of reduction. Calculation of the electronic structure and the Raman spectrum of 2c gave rise to a different total charge at bpy-CF3 by 0.3 reducing equivalent as compared to the structural model for 3c and the calculated Raman spectrum was in poor agreement with experiment. Because loss of chloride from 2c leads to a redistribution of reducing equivalents by the same amount, it is conceivable that the presence of counterions and weak interactions to solvent molecules, presently not included in the calculations, modulate the charge distribution by equal amounts. Theoretically, one-electron reduction of 1 gives rise to a lowered total charge at the bipyridine ligand of 0.75 ± 0.02 (Table S3), in line with the formal {ReI(bpy-R·)-} oxidation state. As stated before, second reduction leads to a closed-shell wave function, semantically only compatible with a ReI(bpy-R)2-} state. However, the actual charge distribution is such that the two reducing equivalents are approximately shared equally by Re and bpy. The postulated intermediate of the CO2 reduction, [Re(COOH)(bpy)(CO)3] (4), was independently synthesized via nucleophilic attack of hydroxide on [Re(bpy)(CO)4]OTf (5) and investigated by Raman spectroscopy. Its spectrum (Figure 7a) reveals bands attributed to a neutral bipyridine (ν = 1600, 1563/1548, 1492, 1319, 1033 cm-1), which, indeed, are very similar to those of 1b (Δν = ±5 cm-1). According to analysis of the calculated frequencies (cf. Figure 7b), normal modes with involvement of the COOH moiety lead to bands with negligible intensity in the Raman spectrum. In the lower region of the spectrum, two bands at ν = 510 and 487 cm-1 have been detected. The calculation shows that mainly the displacement of the two equatorial CO ligands leads to these bands (ν = 502, 499 cm-1). The good agreement of the experimental and calculated Raman 23 ACS Paragon Plus Environment


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spectra confirms the validity of the optimized structure of 4. The bipyridine ligand distances of 4 (1.472 Å for C2-C2´ and 1.370 Å for C2/2´-N) are in accordance with those of the optimized structure of 1b (1.476 Å for C2-C2´ and 1.368 Å for C2/2´-N), the Re-COOH bond length of 2.206 Å is similar to that in the X-ray structure of [Re(COOMe)(bpy)(CO)3] (2.198(8) Å)69 and the bipyridine ligand possesses a significantly more positive Mulliken charge (0.31) than that in 3b (0.49), demonstrating that the electron stored at the bipyridine ligand is involved in the activation of CO2 at the Re catalyst (Table S4). Our results are also in agreement with previous calculations regarding the CO2 reduction mechanism of [Re(Cl)(bpy)(CO)3].36–38 In particular, computations of Carter et al.37 demonstrated that upon binding of CO2 to the active catalyst to form a {Re-CO2} moiety significant electron density has been transferred to the CO2 ligand, mainly from the bipyridine ligand: firstly, the C2C2´ bond length in the optimized geometry of the CO2 adduct is elongated by 0.04 Å as compared to corresponding bond distance in the active species. Secondly, according the Mulliken charge distribution, the bipyridine ligand loses 0.66 electrons overall and thereof, 0.40 electrons are transferred to the CO2 molecule. Concomitant with the protonation of the CO2 moiety to yield the rhenium carboxylic acid, further electron transfer from the bipyridine ligand (by the equivalent of 0.45 electrons) occurs. In total, the change of the Mulliken charge residing on the bpy ligand during the entire catalysis process is 1.35,37 indicating that bipyridine is the source of at least one electron for the CO2 reduction. One possibility for the CO release process from the rhenium carboxylic acid is its initial protonation, giving the cationic species [Re(bpy)(CO)4]+ and H2O as byproduct.36,37 For this reason, the Raman spectrum of [Re(bpy)(CO)4]OTf (5) was measured (Figure 7c), too, and 24 ACS Paragon Plus Environment

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reveals bands at ν = 1605, 1564, 1497, 1320, 1037 cm-1 for the bidentate ligand at 355 nm excitation. Calculations (Figure 7d, Table S4) indicate the presence of neutral bpy, which is also obvious from experiments, since the bands of the formal neutral bpy ligand in 1b, 4 and 5 hardly differ (Δν = ±3 cm-1).

Figure 7. Raman spectrum of [Re(COOH)(bpy)(CO)3] (4) (a; λ = 355 nm, P = 10 mW) and [Re(bpy)(CO)4]OTf (5) as solid (c; λ = 355 nm, P = 2 mW), calculated Raman spectrum of 4 (b) and [Re(bpy)(CO)4]+ (d).

Conclusions Raman spectra of the complexes [Re(Cl)(bpy-R)(CO)3] (1) reveal that the bands in the 1000 1650 cm-1 region, which are attributed to modes of the bpy ligand, are resonantly enhanced upon excitation at about 400 nm and differ only to a minor degree depending on the character of the substituent. The general agreement of theory and experiment confirms the accurate 25 ACS Paragon Plus Environment


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description of the complexes by DFT calculations. First Raman spectra of the doubly reduced Re complexes 3 were presented. These complexes have received considerable attention owing to their role as key intermediates in CO2 reduction and their bands show significant shifts compared to those of 1 confirming the reduction of the bpy moiety. For [Re(bpy-R)(CO)3]-, the substituent of the bpy ligand influences the Raman spectrum slightly more compared to 1 and the agreement of the experimental and calculated data are good to moderate. For the monoreduced complex [Re(Cl)(bpy-CF3)(CO)3] [K(18-crown-6)] (2c) the same Raman bands were observed as for 3c indicating the same electronic structure of the bpy moiety. Overall, our study demonstrates that Raman spectroscopy in combination with quantumchemical calculations allows to obtain local information about geometric and electronic structure in catalysts with non-innocent ligands irrespective of the presence (2) or absence (3) of unpaired electrons. Our investigations point out a closed-shell singlet state for 3 giving rise to a formal {ReI(bpy-R)2−} moiety, but the actual charge distribution is comparable with a {Re0(bpyR•)−} species, where the first reduction effectively takes place at bpy-R and second redction occurs at the metal center. Note also that a striking similarity of the Raman spectrum of the doubly reduced complex 3b (355 nm, Figure S3a) with the Raman spectrum of the MLCT excited state of 1b (ν = 1548, 1503, 1424, 1285, 1218, 1020 cm-1 with 355 nm-excitation), which was presented by Wrighton et al.43, exists. The resemblance of resonance Raman spectra of complexes with an effectively monoreduced ligand to those of excited states of α-diimine metal complexes has been described before in literature43,44,70–73 and led to the assumption that the same actual oxidation state, i.e., a monoanionic α-diimine species, is present in both states; of course, the same α-diimine π* 26 ACS Paragon Plus Environment

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orbital is occupied upon mono reduction or excitation and similar intra-ligand bond length changes results in the same qualitative changes in the vibrational spectra.43,44,70–73 Thus, this similarity may be considered as an additional hint for the presence of the actual (as opposed to formal) mono-reduced bipyridine species in 3b. Regarding the CO2 reduction process, we demonstrated that the electron stored at the bpy ligand in 3 is involved in the activation of CO2, because the postulated intermediate [Re(COOH)(bpy)(CO)3](4), which is formed after CO2 binding and protonation of the CO2 ligand, exhibits a neutral bpy species in experiment and theory. Thus, during one catalytic cycle both bipyridine and rhenium participate in the reduction of CO2.

Supporting Information Overviews of characteristic normal modes of 1a-c and 3a, additional Raman spectra, EPR spectrum of 2c, Cartesian coordinates of all optimized structures, tables of Mulliken charge distributions, difference density plots for calculated optical transitions of 1a and 3a.

Author Information Corresponding Author: M. van Gastel, E-mail: [email protected]. Phone: +49 208 3063586. The authors declare no competing financial interest.



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Acknowledgment We thank D. Skerra for his help with the Ramam measurements. Dr. Th. Weyhermüller is acknowledged for helpful discussions. This work has been supported financially by the Max Planck Society.


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Raman Spectroscopy as a Method to Investigate Catalytic

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