Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins

Article pubs.acs.org/IC

Cite This: Inorg. Chem. 2018, 57, 2141−2148

Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins in Their Reduced States as Determined by a Combination of Resonance Raman Spectroscopy and Quantum Chemistry Christina Römelt,† Shengfa Ye,† Eckhard Bill,† Thomas Weyhermüller,† Maurice van Gastel,† and Frank Neese*,† †

Max Planck Institute for Chemical Energy Conversion, Stiftstrasse 34-36, 45470 Mülheim an der Ruhr, Germany S Supporting Information *

ABSTRACT: Iron tetraphenylporphyrins are prime candidates as catalysts for CO2 reduction. Yet, even after 40 years of research, fundamental questions about the electronic structure of their reduced states remain, in particular as to whether the reducing equivalents are stored at the iron center or at the porphyrin ligand. In this contribution, we address this question by a combination of resonance Raman spectroscopy and quantum chemistry. Analysis of the data allows for an unequivocal identification of the porphyrin as the redox active moiety. Additionally, determination of the spin state of iron is possible by comparing the characteristic shifts of spin and oxidation-state-sensitive marker bands in the Raman spectrum with calculations of planar porphyrin model structures.

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Scheme 1. Schematic Structure of FeTPP Including Nomenclature for the Carbon Atoms

INTRODUCTION Over the last 100 years, carbon-based fuels have found tremendous application in combustion processes, having led to an increase in the CO2 concentration in the atmosphere by 100 ppm (roughly 40%). At present, the CO2 concentration is expected to further increase. Owing to its abundance and contribution to the greenhouse effect, the reconversion of CO2 into building blocks (formaldehyde, formic acid, oxalic acid, methanol, CO) which can be used in further processes is of great importance. On the basis of its thermodynamic stability [ΔH(C + O2 → CO2) = −394 kJ/mol; ΔH(CO2 + C → 2CO) = +173 kJ/mol], the reductive activation of carbon dioxide has been one of the most challenging and most relevant topics in catalysis for the last 40 years. The electrochemical one-electron reduction of CO2 to the CO2•− radical anion, which is accompanied by a bending of the CO2 molecule, is an energetically unfavorable step that requires a large potential (E0′= −1.90 V) but can be greatly facilitated by coupling reduction to proton transfer.1,2 Iron porphyrins in their reduced, formal Fe(0) state, are known to act as catalysts in the reduction of CO2. In fact, in the presence of proton sources, iron tetraphenylporphyrins (FeTPPs, Scheme 1) exhibit one of the largest catalytic activities and turnover frequencies observed so far for homogeneous CO2 reduction catalysts.3−7 © 2018 American Chemical Society

High-valent heme systems for oxidative chemistry have been investigated extensively.8−10 Here, we focus on the reductive chemistry of FeTPP relevant for CO2 reduction and for which the low-valent states are relevant. Because reduction of [FeTPP] Received: November 29, 2017 Published: January 31, 2018 2141

DOI: 10.1021/acs.inorgchem.7b03018 Inorg. Chem. 2018, 57, 2141−2148

Article

Inorganic Chemistry

for a related porphyrin system showed that the saddled and ruffled distortions are associated with energy differences on the order of about 0.5 eV only.44 As such, the potential energy surface is thus quite flat toward these distortions. In turn, the flatness of the potential energy surface means that mainly the low-lying “breathing” vibrations will be sensitive to these distortions. The vibrations of relevance for this study are not expected to significantly change upon ruffling or saddling of the porphyrin moiety unless, perhaps, a different electronic configuration or spin coupling becomes lower in energy. This is also not likely to be the case because the line width of the bands in the recorded Raman spectra are of normal width and indicate a well-defined electronic ground state and spin coupling. Thus, we believe that planar structures suffice as models, and to not overcomplicate analysis, degrees of ruffling and saddling of the porphyrin were not considered. The procedure to calculate resonance Raman spectra is based on a theory formulated by Lee and Heller,45,46 and its implementation in ORCA has been described in detail previously.46,47 In essence, the method encompasses four steps: (1) calculation of the ground state normal modes through diagonalization of the Hessian matrix; (2) the calculation of electronic transitions relevant for the resonance Raman excitation, e.g., by time-dependent density functional theory (TD-DFT) calculations. Both transition energies and excited state gradients are calculated, (3) estimation of excited state displacements using the ground state Hessian and the excited state gradients, (4) simulation of the resonance Raman spectra via integration of Heller’s time dependent equations. The latter step scales linearly with the number of modes and is therefore strongly preferable to the direct calculation of multidimensional Franck−Condon factors. In practice, however, it is mostly the well-known limited accuracy of the TD-DFT method in step (2) that can give rise to significant discrepancies between experiment and theory, in particular for charge-transfer transitions, which includes the Soret band of TPP. It is important to adjust the laser wavelength used in the simulations such that it is in resonance with the calculated state, even if the latter is significantly off the experimental band position. An advantage of this approach to resonance Raman spectroscopy is that the excited state displacements can be treated as adjustable variables and fitted to the experimental spectra. Hence, in this case quantum chemistry only serves as an initial guess for the fitting procedure, which is usually good enough to allow unambiguous assignment of the experimental spectra.

can occur either at the metal or at the ligand, the exact nature of the electronic ground state of the reduced iron species [FeTPP]− and [FeTPP]2− has long been controversial. A variety of spectroscopic studies, including Mössbauer,11−16 electron paramagnetic resonance (EPR),16−19 nuclear magnetic resonance (NMR),17,20−22 X-ray,12,15,16 UV/vis,18,20,23,24 and resonance Raman24−33 experiments have been published, partially supporting different electronic ground state formulations.8−10 Additionally, so far, resonance Raman experiments have been performed in solution, giving rise to solvent dependency of the electronic ground states. [FeTPP] and [FeTPP]− are even known to change their spin state upon axial ligation of solvent molecules.24 Because an understanding of the electronic nature of catalytically active species is a crucial step toward understanding the catalytic mechanism and designing novel, even more effective and stable catalysts, we recently reported new spectroscopic insights into the nature of the electronic ground states of the reduced species using Mössbauer and XAS techniques combined with quantum chemical calculations of their spectroscopic properties.34 For [FeTPP]−, an Fe(II) d6 S = 1 spin state with an antiferromagnetically coupled TPP3−• radical was formulated.34 Upon further reduction, an electronic ground state best described as Fe(II) d6 S = 1 intermediate-spin antiferromagnetically coupled TPP4−•• diradical was found. In fact, in a recently studied related Fe-NO porphyrin system, repression of the Soret band was observed upon reduction, while EPR data showed that the ferrous character of iron was retained, thus indicative of a porphyrin reduction.35 Because the equilibrium bond distances are largely determined by the electronic structure, they are sensitive to whether reduction is metal- or ligand-centered. For example, the bond distances of the porphyrin are expected to increase more if reduction occurs into a ligand-centered π* orbital than into an iron 3d orbital. Bond distances are even sensitive to the spin coupling of unpaired electrons. As such, vibrational spectroscopy in combination with quantum chemistry provides an ideal tool for investigating the electronic structure of the reduced intermediates with focus on the porphyrin, whereas the previous Mössbauer investigation focused on Fe, and for unequivocal confirmation of the above assignments for [FeTPP]− and [FeTPP]2−. We do so by performing resonance Raman spectroscopy of the series Fe(III)ClTPP, [Fe(II)TPP(MeTHF)2], [Fe(II)TPP], [FeTPP]−, and [FeTPP]2− as solids, where spin- and oxidation-state-sensitive marker bands were identified and assigned with the assistance of computational resonance Raman spectroscopy. Also, we examine and discuss agreements and discrepancies of our assignment with literature data, in particular the in-depth experimental and theoretical study of FeClTPP by Lehnert et al.25

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RESULTS Fe(III)ClTPP. Experimental and calculated Raman and resonance Raman spectra for FeClTPP were previously reported by Lehnert et al.25 in 2006. The authors presented a complete assignment of all observed vibrations. In particular, for planar iron tetraphenylporphyrins, four marker bands were identified, which are known to show characteristic behavior upon either metal center or ligand reduction (vide infra). The laser wavelengths for calculating our resonance Raman profiles were determined by comparing experimental and calculated absorption spectra, as indicated by arrows in Figure 1. The most intense band of the calculated spectrum corresponds to the Soret band, the weaker bands are traditionally called Q-bands.48 The calculated spectrum was shifted by 2300 cm−1 such that it has optimal overlap with the bands in the experimental spectrum. Because the experimentally observed intensities for the Soret band and Q bands are dominated by π → π* transitions,48 the calculated electronic states with energy close to the experimental excitation energies and with π → π* character as identified by visualizing their difference density plots (see Supporting Information) were selected. However, the difference densities also identified a transition with chloride-to-iron charge transfer character that contributes to the Soret band, whose Raman spectrum resembles the experimental data and which has been assigned before by magnetic circular dichroism (MCD) spectroscopy by the Lehnert group.49 The excitation energies giving rise to calculated spectra

EXPERIMENTAL SECTION

All synthetic procedures, sample preparation, and detailed descriptions of resonance Raman experiments, calculations, and model geometries are given in the Supporting Information. All calculations were performed with the ORCA program.36 Geometry optimizations were carried out on model structures derived from experimental X-ray data13,16,37,38 and employ the BP86 functional39 together with the def2-TZVP basis set40,41 and the resolution of identity (RI) approximation. Counterions were not included in the calculation. No scaling factors are employed; the BP86 functional was chosen because it provides good optimized geometries, and a typical accuracy of the calculated vibrational frequencies amounts to ±25 cm−1.42,43 A few words are necessary with respect to using planar model structures. Porphyrins are known to harbor varying degrees of ruffled and saddled distortions dependent on their substituents. A recent evaluation by density functional theory (DFT) methods of the energetic landscape 2142


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band (Lehnert: exp. 1364 cm−1, calcd. 1379 cm−1; here: calcd. 1347 cm−1), which was assigned to a symmetric νsym(pyrrole halfring) vibration (see Supporting Information, paragraph 3, for a detailed visualization).25,52 Marker band D occurs at 1553 cm−1 (Lehnert: exp. 1557 cm−1, calcd. 1598 cm−1; here: calcd. 1547 cm−1) and corresponds to a combined ν(Cβ−Cβ) + νsym(Cα−Cm) + δsym(Cβ−H) vibration. Regrettably, the weak spin-state-sensitive band C of b1g symmetry observed by Lehnert, which was assigned to a mixed νasym(Cα−Cm) + νasym(Cα−Cβ) + δasym(Cβ−H) porphyrin core vibration, has negligible intensity in our resonance Raman spectra at all employed wavelengths. Therefore, only bands E, A, and D are further considered. Lehnert’s calculated bands mostly have a Raman shift (−4 to +41 cm−1) larger than that of the experimental bands due to the neglect of anharmonicity effects. Applying our combination of functional and basis set, we find that our calculated bands are mostly shifted to lower wavenumbers, the deviation from experimental values ranging from −5 to −18 wavenumbers. Assignment of our calculated bands to the experimentally observed marker bands can also be rationalized by their intensity pattern. Upon Soret excitation, only vibrational modes with A1g symmetry should be significantly enhanced. The marker bands E, A, and D are such totally symmetric modes, whereas band C transforms as B1g. Moreover, the intensity is largest for those normal modes that give rise to large changes to the excited-state geometry with respect to the ground state geometry, for which difference density plots are given for Soret excitation and Q-band excitation in the Supporting Information. Reduced Iron Tetraphenylporphyrins. After successfully validating experiment and theory with literature data for the markers bands E, A, and D in the oxidized state of Fe(III)ClTPP, these bands can now be used for elucidation of the electronic structure and spin state of the reduced FeTPP. The procedure of computation of the resonance Raman spectra is identical to that for the oxidized state (for comparison between experimental and calculated UV−vis spectra of the reduced state, see Supporting Information). Assignment of the calculated marker bands to the experimentally observed bands was aided by the fact that the intensity pattern changes when going from Soret to Q-band excitation. Figure 3 shows the experimental and calculated resonance Raman spectra for Soret excitation. [Fe(II)TPP(MeTHF)2]. The resonance Raman spectrum of [FeTPP(MeTHF)2] (Mössbauer parameters: δ = 0.99 mm/s, ΔEQ = 2.73 mm/s, indicative of high-spin, S = 2, Fe(II)) was recorded in frozen-solution, which was chosen here to ensure ligation of the axial solvent molecules because these weakly bound ligands might dissociate upon laser irradiation. As can be seen, the experimental and the calculated resonance Raman spectra are in excellent agreement (Figure 3a), especially in the region between 1200 and 1700 cm−1, regarding both the shift as well as the relative intensity (high intensity modes have A1g symmetry). The calculated frequency of marker band E (νbreathing(Fe−N)) shifts from 381 cm−1 for FeClTPP to 380 cm−1 for [FeTPP(MeTHF)2], thus staying almost constant despite the change in oxidation state of the metal center. The experimental Fe−N distances decrease from 2.071 Å for Fe(III)ClTPP to 2.056 Å for [Fe(II)TPP(THF)2].13,37 The calculated bond lengths follow this trend (2.096 Å for Fe(III)ClTPP to 2.07 Å for [Fe(II)TPP(THF)2]). In the experimental spectrum, two bands at 367 and 397 cm−1 can be seen. Also, a band seems to be present in between those two bands at roughly 382 cm−1. In the calculated spectrum, a weak band can also be seen next to marker band E at 398 cm−1, which corresponds to a pyrrole τpyr twist vibration. The normal mode

Figure 1. (a) Experimental (solvent: 2-MeTHF; inset: concentrated sample) and (b) calculated absorption spectra of Fe(III)ClTPP. The calculated spectrum (BP86 functional) was red-shifted by 2300 cm−1 to match the experimental spectrum. Laser energies (24 096 and 17 825 cm−1) and corresponding values used in the calculation (see text) are highlighted with arrows.

best resembling the experimental spectra are indicated with arrows in Figure 1b. The excitation energy for the Soret band amounts to 22 020 cm−1 (vertical excitation; 0,0 transition: 21 485 cm−1) that for the Q-band 14 900 cm−1 (vertical excitation; 0,0 transition: 12 085 cm−1). Figure 2 shows the experimental and calculated resonance Raman spectra of FeClTPP for Soret and Q-band excitations.

Figure 2. (a) Calculated and (b) experimental resonance Raman spectra upon Q-band excitation (561 nm, excitation energy in calculation: 12 085 cm−1) of Fe(III)ClTPP. (c and d) Raman spectra with excitation in the Soret band (415 nm, excitation energy in calculation: 21 485 cm−1). Spin-state-sensitive marker bands are labeled E, A, and D according to Lehnert.25

The oxidation state sensitive marker band E (in the nomenclature of Lehnert25) at 391 cm−1 (Lehnert:25 exp. 392 cm−1, calcd. 388 cm−1; here: calcd. 381 cm−1) was previously assigned to a porphyrin core deformation26,50,51 but was reassigned by Lehnert et al. to the totally symmetric νbreathing(Fe−N) vibration25 (for visualization of displacements, see Supporting Information). The experimental and calculated shifts for this band as well as its assignment are consistent with those reported by Lehnert. The experimentally observed vibrational band A at 1361 cm−1 corresponds to an oxidation- and spin-state sensitive marker 2143


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[Fe(II)TPP]. Resonance Raman experiments and calculations were further carried out for the planar, intermediate spin [Fe(II)TPP] (Figure 3b). Marker band E was calculated to lie at 380 cm−1. For Fe(III)ClTPP, it was found that the calculated intensities of marker bands A and D are in very good agreement with experiment, while marker band E shows significant deviations in intensity. This is due to the fact that in the experimental setup, the absorption bands are dominated by π → π* excitations, while LMCT excitations play only a minor role. When calculating the electronic states belonging to the Soret band, however, it was observed that the individual transitions consist of mixtures of LMCT and π → π* excitations. Thus, bands corresponding to Fe−N stretching vibrations (including marker band E) are calculated to have higher intensity than they would have upon pure π → π* excitation. For Fe(III) and Fe(II) high spin, the calculated Raman shifts of all bands were found to be lower than the experimentally observed shifts. Therefore, in the experimental spectrum of [FeTPP], the band with a shift of 394 cm−1 was here assigned to the symmetric Fe−N breathing vibration. Marker band A is characterized by a calculated shift of 1353 cm−1. Upon analysis, this band was found to be comprised of four separate vibrations with very similar Raman shifts. These different vibrations can be partially resolved in the experimental Soret excited spectrum and are seen well-resolved in the Q-band excited spectrum (see Supporting Information). On the basis of the calculated intensity patterns for these vibrations for both excitations and comparison with the experimentally found intensity pattern, the band 1360 cm−1 was assigned to marker band A. Marker band D exhibits a calculated frequency of 1559 cm−1. Here, the experimentally observed band at 1566 cm−1 is assigned due to its change in intensity when comparing Soret excitation to Q-band excitation. Also of note is that the ground state of [Fe(II)TPP] is calculated to be of Eg symmetry with a (dxy)2(dxzdyz)3(dz2)1 electronic configuration whereas [Fe(II)TPP(MeTHF)2] is of high spin state. Therefore, the differences in frequency of the marker bands A and D in these two systems (see Table 1), which are reproduced by the calculations, are additionally related to the change in spin state upon presence/absence of the axial ligands. [FeTPP]−. Owing to the method of preparation, the anionic complex [FeTPP]− is obtained as the sodium solvate complex [FeTPP][Na(THF)3]. This species is known to exist in an overall spin state of S = 1/2,16−18 giving three possible spin state formulations: Calculations were carried out for the pure planar [FeTPP]− as well as for the sodium solvate complex [FeTPP][Na(THF)3]. In all of the previously described theoretical investigations, unrestricted Kohn−Sham calculations were carried out. For [FeTPP]−, however, this leads to convergence of both a and b to the broken symmetry solution of calculation of c. A restricted open-shell Kohn−Sham calculation, on the other hand, leads to

Figure 3. Experimental and calculated resonance Raman spectra for (a) [Fe(II)TPP(MeTHF)2], (b) [Fe(II)TPP], (c) [FeTPP][Na(THF)3], and (d) [FeTPP][Na(THF)3]2 upon Soret excitation. The band marked with an asterisk is from the reducing agent (Na-anthracenide). Spectra upon Q-band excitation are included in the Supporting Information.

structure seems to be such that the Fe−N vibration mixes with other modes so that all bands in this region contain Fe−N character. Marker band A (νsym[pyrrole half-ring] vibration, normal mode corresponds to a superposition of Fe−N, Cα−Cβ, and Cα−N stretching frequencies) changes from 1347 cm−1 for Fe(III)ClTPP to 1331 cm−1 in the calculated spectrum. On the basis of the similarities in intensities of experimental and calculated spectra, this vibration is assigned to the experimentally observed band at 1342 cm−1. As compared to the oxidized state, this would mean a downshift of 16 cm−1 in the calculated spectra vs a downshift of 19 cm−1 for marker band A, which is in very good agreement. Marker band D is easily identified in the experimental spectrum. Its frequency is sensitive to C−C bond lengths of the porphyrin core; in particular, elongated Cβ−Cβ and Cα−Cm bonds are expected to lead to lower frequencies. The calculated value of D of 1531 cm−1 is lower by 16 cm−1 as compared to the oxidized state. In comparison, the experimental band at 1535 cm−1 shows a similar large intensity and a downshift of 18 cm−1. A complete overview over all experimental and calculated marker bands for all investigated species is given in Table 1.

Table 1. Experimental and Calculated Shifts of the Marker Bands of All Investigated Iron Tetraphenylporphyrin Speciesa band E [cm−1] species FeClTPP [FeTPP(MeTHF)2] [FeTPP] S = 1 [FeTPP]− d6 TPP• [FeTPP]2− d6 TPP•• [FeTPP]2− d8 S = 0 a

exp 391 394(3) 391(0) 390(−1)

band A [cm−1]

band D [cm−1]

calcd

exp

calcd

exp

calcd

381 380(−1) 390(9) 382(1) 372(−9) 386(5)

1361 1342(−19) 1360(−1) 1358(−3) 1357(−4)

1347 1331(−16) 1353(6) 1351(4) 1353(6) 1335(−12)

1553 1535(−18) 1566(13) 1538(−15) 1531(−22)

1547 1531(−16) 1559(12) 1549(2) 1523(−24) 1553(6)

Shifts with respect to FeClTPP are indicated in parentheses. 2144


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Inorganic Chemistry the iron d7 low-spin configuration a. By comparing the electronic energies of c and a, the difference in energies is found to be 311 kcal/mol, thus rendering the d7 low spin configuration (a) far less stable than the broken symmetry solution c, as well as explaining the unanimous convergence to c. Convergence to b by rotating orbital energies of the case a restricted open-shell Kohn−Sham calculation was attempted but not successful. For the broken symmetry calculation c, the Raman and resonance Raman spectra were then computed and compared to the experimental results (Figure 3c). Marker band E is calculated to lie at 382 cm−1. Upon excitation in the Soret band, a broad band has been observed with two maxima at around 377 and 391 cm−1. Upon Q-band excitation, only one vibration is observed experimentally, exhibiting a shift of 391 cm−1. Upon comparison with the calculated intensity patterns, it is here assigned to the corresponding marker band E. Compared to the experimental spectrum of Fe(III), this band shows identical shifts and follows the calculated trends. Marker band A is calculated to exhibit a shift of 1351 cm−1. In the calculation, this band does not seem to be resonance enhanced upon Soret excitation and only weakly enhanced upon Q-band excitation. On the basis of the changes in intensity when going from Soret to Q-band excitation and the intensity pattern of the neighboring peaks, it is here assigned to the experimentally observed band at 1358 cm−1. Marker band D was identified by its characteristic ν(Cβ−Cβ) + νsym(Cα−Cm) + δsym(Cβ−H) vibration and exhibits a frequency of 1549 cm−1. On the basis of the intensity pattern in this region and changes in intensity when going from Soret to Q-band excitation, it is here assigned to the experimentally observed vibration at 1538 cm−1. Upon comparison of the calculated shifts of pure [FeTPP]− and the sodium solvate complex [FeTPP][Na(THF)3], the frequencies of marker bands E, A, and D for the solvate complex were calculated to be larger by 4 cm−1. The relative shifts do not change. Furthermore, this small upshift does not affect the observed general trends when comparing calculated to experimental spectra and reduction trends. [FeTPP]2−. [FeTPP]2−/[FeTPP][Na(THF)3]2 is known to exist in an overall spin state of S = 0,16 giving three possible spin state formulations (see Scheme 3): For a, a spin-restricted Kohn−Sham calculation was performed, converging to the desired Fe(0) d8 low spin state. As has been already observed for the [FeTPP]− species, b converges to the broken symmetry scenario of c upon performing unrestricted Kohn−Sham calculations, as could perhaps be predicted by examining the spin-coupling scheme in Scheme 3b. Hence, convergence to b was not achieved. When the single point energies of c and a are compared, the difference in energy is found to be 6.6 kcal/mol, thus indicating that the iron d8 low-spin configuration a slightly less stable than that of c. The experimental absorption spectrum (see Supporting Information) shows a very distinctive splitting of the Soret band into two maxima at 27 905 cm−1 (360 nm) and 22 230 cm−1 (450 nm), which is in agreement with literature.18,20,23,24 The calculated absorption spectrum for configuration a (Fe(0) d8 low spin) shows only one strong band, which is assigned to the Soret band here. Also, the Q-band feature next to the Soret band is captured but exhibits very low intensity. The calculated absorption spectrum of c (Fe(II) d6 intermediate spin TPP diradical), shows two strong bands whose relative positions are in very good agreement with experiment. Furthermore, the Q-band next to the Soret band is well-reproduced by the calculation. Thus, the

calculated absorption spectrum of case c better resembles the experimentally observed spectrum than that of case a. Therefore, resonance Raman calculations were performed for c. Excitation energies of 457 nm (Soret) and 532 nm (Q-band) were used in the calculation to simulate experimental excitation at 415 nm (Soret) and 561 nm (Q-band). Marker band E is calculated to exhibit a shift of 386 cm−1 for a and 372 cm−1 for c. In the experimental spectrum, a band at 390 cm−1 is present, which is assigned here to band E. When calculating the resonance Raman spectrum for a, the shift of marker band A is calculated to be 1335 cm−1 as compared to 1353 cm−1 for c. Two experimentally observed bands are present with frequencies of 1342 and 1357 cm−1, respectively. Upon closer inspection of the calculated marker bands, the observed vibrational bands of both electronic configurations actually consists of two vibrations with marker band A being the one with the larger shift. Accordingly, in the experimental spectrum, the band with the larger shift of 1357 cm−1 is here assigned to marker band A. Because the calculated shifts for all other iron species are observed to be only slightly lower than the experimental values (−14 to −7 wavenumbers), the calculated shift for case c is in much better agreement with experiment than that for case a. Marker band D is computed to occur at 1553 cm−1 for case a and 1523 cm−1 for case c. In the experimental spectra, a very broad band, stretching from 1518 to 1570 cm−1 can be observed. In the calculations of both possible electronic configurations, the intensity of marker band D is fairly low (see Figure 3, 4). Therefore, in the experimental spectrum, this marker band can be assigned to the left-hand side shoulder of the observed vibrational band instead of its maximum.

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DISCUSSION Several resonance Raman studies of reduced iron tetraphenylporphyrins have been published. Because the reduced species were prepared in situ by electrochemical reduction, all of these spectra have been recorded in solution, employing solvents such as DMF,24,28 butyronitrile, or DMSO.27 Especially [FeTPP] and [FeTPP]−, however, can change spin state upon axial ligation of solvent molecules.24 Thus, the frequencies of the marker bands reported in literature may not always be accurate, in particular when the nature of the electronic ground state has not been specified. Still, experimentally, several decades of resonance Raman spectroscopy of metal porphyrins24,26−31,33 have established trends for the different marker bands upon reduction provided that the axial ligand does not change and no high-spin to lowspin/intermediate-spin crossover, or vice versa, occurs: (1) The frequency of marker band A is largely unaffected upon reduction of the macrocycle; (2) marker band A downshifts upon metal centered reduction; (3) marker band D significantly downshifts upon macrocycle reduction; and (4) marker band D is largely unaffected upon metal center reduction, provided the axial ligand does not change, To verify these trends and compare them with the observed trends for our measurements in the solid state, we decided to calculate the shifts of marker bands A and D for the ligand itself (H2TPP) upon one and two electron reduction, respectively (Table 2). The calculations confirm that both bands A and D follow the postulated trends upon macrocycle reduction. We then compared these shifts to our calculated as well as experimentally observed shifts for the [FeTPP] reduction series. First, it can be seen that the calculated trends upon macrocycle reduction to give TPP radicals are in very good agreement with the experimentally 2145


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Inorganic Chemistry Table 2. Calculated Frequencies (cm−1) of Marker Bands A and D upon H2TPP Reduction and Comparison to Calculated and Experimental Marker Bands for [FeTPP] Reductiona species

A (calcd)

H2TPP H2TPP− H2TPP2− [FeTPP] [FeTPP]− [FeTPP]2−

1355 1357 1352 1353 1351 1353

A (exp)

D (calcd)

D (exp)

1360 1358 1357

1543 1526 1500 1559 1549 1523

1566 1538 1531

with iron in the low-spin state. Unfortunately, calculation of resonance Raman excitation profiles was not possible for the restricted-open-shell Fe(I) d7 low-spin calculation. However, Raman and resonance Raman excitation profiles were obtainable for the [Fe(0)TPP]2− species with an Fe(0) d8 low spin configuration. The calculated frequencies of marker bands A and D (Table 1) show significant deviations from the experimentally observed frequencies and trends upon reduction, thus further supporting ligand reduction rather than reduction of the central iron. To provide further confirmation for ligand reduction, we next compared the results to the shifts which were obtained and reported in literature for ZnTPP reduction.28,33 ZnTPP, exhibiting an Zn(II) d10 configuration, is known to be ligand reduced to give the Zn(II) TPP radical anion. Table 3 shows a series of vibrational mode shifts upon ZnTPP reduction, among them the shifts of marker bands A and D. Next to these, our observed shifts for [FeTPP] reduction are shown. Assignment of the vibrations apart from the previously presented marker bands was performed with the aid of our calculations and visualization of the vibrational modes. For the majority of the investigated vibrations, the trends in shifts for the FeTPP species are in quite good agreement with the shifts observed for ZnTPP reduction and the formation of ligand radicals. Certain deviations in the shifts can be rationalized because the exact composition of the vibrational modes might differ upon going from Zn to Fe. Besides marker bands A and D, the spin-state-sensitive band E was identified for our [FeTPP] series. Band E was assigned to the symmetric νbreathing(Fe−N) vibration and is therefore sensitive to the Fe−N bond lengths of the complex. Lehnert et al.25 postulated (by comparing different metal porphyrins with each other) that with decreasing M−N distance, the frequencies of ν(M−N) are expected to increase. Here, the experimental Fe−N distances, as obtained by X-ray crystallographic analysis, decrease from 2.071 Å for Fe(III)ClTPP to 1.966 Å for Fe(II)TPP S = 1. The calculated bond lengths follow this trend (2.096 Å for Fe(III) vs 1.982 Å for Fe(II) S = 1). Thus, with decreasing bond length, the frequency of Fe−N is expected to increase. Indeed, an increase in shift can be observed, although being quite small (391 cm−1 for Fe(III) to 394 cm−1 for Fe(II) S = 1). In the series [FeTPP] → [FeTPP]− → [FeTPP]2−, the changes in Fe−N bond lengths determined by X-ray crystallographic analyses are almost negligible (experimental: 1.966 → 1.980 → 1.969 Å; calculated: 1.982 → 1.998 → 2.015 Å). The experimentally observed shifts in marker band E (394 → 391 → 390 cm−1), also being extremely small, thus agree with the similarity in bond lengths of the reduced species. The calculated shifts of band E (387 → 382 → 372 cm−1) show some deviations from the experimental ones, a fact that can be rationalized because the calculated Fe−N bond lengths are slightly higher than the experimentally observed bond lengths. It is expected that reduction of the iron center would lead

a

The calculations for [FeTPP] reduction are shown for the electronic configurations with reduced macrocycle.

observed shifts and therefore strongly support macrocycle rather than metal-centered reduction. As mentioned before, three conceivable spin states for [FeTPP]− and [FeTPP]2− were considered (cf. Schemes 2 and 3). For both Scheme 2. Considered Spin-Coupling Formulations for [FeTPP]− with Formal Labeling of the Frontier Iron 3d Orbitals and the TPP Gouterman Orbitals ex and ey

Scheme 3. Considered Schematic Spin-Coupling Formulations for [FeTPP]2− with Formal Labeling of the Frontier Iron 3d Orbitals and the TPP Gouterman Orbitals ex and ey

[FeTPP]− and [FeTPP]2−, calculations reached convergence for only two spin states. Of the two spin states, the energy of the Fe(II) d6 intermediate-spin state with a ligand radical (Scheme 2) or ligand diradical species (Scheme 3) was calculated to be significantly lower than the energy of the metal-reduced species

Table 3. Comparison of Selected Vibrational Modes (cm−1) of ZnTPP and FeTPP upon Reduction mode

ZnTPP

[ZnTPP]−

Δν

[FeTPP]

[FeTPP]−

[FeTPP]2−

Δν

ϕ(phenyl) ν(Cα−Cm) D(ν(Cα−Cm)) ν(Cα−Cβ) A(ν(Cα−N)) ν(Cm−Ph) ν(Cm−Ph)

1598 1547 1548 1360 1352 1272 1237

1595 1528 1532 1345 1351 1257 1231

−4 −19 −16 −15 −1 −15 −4

1587 1562 1566 1367 1360 1288 1234

1594 1552 1538 1342 1358 1268 1230

1594 1542 1531 1342 1357 1265 1231

+7/0 −10/−10 −28/−7 −25/0 −2/−1 −20/−3 −4/+1

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Inorganic Chemistry

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to somewhat elongated bond lengths and therefore more pronounced downshifts for marker band E. In summary, assignment of the spin- and oxidation-state sensitive marker bands E, A, and D of planar iron tetraphenylporphyrins in their reduced forms [FeTPP], [FeTPP]−, and [FeTPP]2− was accomplished by the aid of resonance Raman spectroscopy with excitation in the Soret and Q bands. The interpretation of the data was aided by quantum chemical calculations. On the basis of the observed trends upon reduction and comparison of our experimental and calculated frequencies with those of H2TPP and ZnTPP, macrocycle reduction was confirmed for all anionic [FeTPP] species. Whereas our previous Mössbauer study focused on Fe, Raman spectroscopy employed here allowed clearly the elucidation of the electronic ground state of the catalytically highly relevant [FeTPP]2− species. For [FeTPP]−, an Fe(II) d6, intermediate spin, iron center with an antiferromagnetically coupled TPP radical was found. For [FeTPP]2−, the electronic ground state concerns an Fe(II) d6 intermediate spin iron center antiferromagnetically coupled to a TPP diradical tetra-anion (see Scheme 4).

We hope our studies afford important requisites for a detailed understanding of the catalytic mechanism for CO2 reduction by these complexes.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b03018. Synthesis protocols, sample preparation, details of Raman calculations, normal mode graphics, differences densities, and Cartesian coordinates of the used model structures (PDF)

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REFERENCES

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Scheme 4. Experimental and Calculated Shifts of Markers Bands A and D upon Reduction from [FeTPP] to [FeTPP]2−, Exclusively Compatible with an [Fe(II)TPP••]2− Reduction

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Article

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; Tel.: +49 208 306 3656. ORCID

Shengfa Ye: 0000-0001-9747-1412 Thomas Weyhermüller: 0000-0002-0399-7999 Maurice van Gastel: 0000-0002-1547-6365 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge financial support of this work by the Max Planck Society. 2147


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Electronic Structure and Spin Multiplicity of Iron Tetraphenylporphyrins

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