Photochemical CO2 Reduction Catalyzed by Phenanthroline

Article pubs.acs.org/IC

Photochemical CO2 Reduction Catalyzed by Phenanthroline Extended Tetramesityl Porphyrin Complexes Linked with a Rhenium(I) Tricarbonyl Unit Corinna Matlachowski,† Beatrice Braun,† Stefanie Tschierlei,*,‡,§ and Matthias Schwalbe*,† †

Institute of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany Institute of Physics, University of Rostock, Universitätsplatz 3, 18055 Rostock, Germany

‡

S Supporting Information *

ABSTRACT: A series of heterodinuclear complexes (M-1-Re) based on a phenanthroline (phen) extended tetramesityl porphyrin ligand (H2-1) has been prepared. The phen moiety of this ligand selectively coordinates a Re(I) tricarbonyl chloride unit, whereas the metal in the porphyrin moiety has been varied: namely, Cu, Pd, Zn, Co, or Fe was used. These dinuclear complexes were fully characterized by standard analytical methods. Additionally, a crystal structure of Cu-1-Re·5.5(C7H8)·0.5(C6H6) could be obtained, and extended time-resolved emission lifetime measurements were conducted. Furthermore, their ability to catalyze the photochemical reduction of CO2 to CO was investigated. Light-driven CO2 reduction experiments were performed in dimethylformamide (DMF) using triethylamine (TEA) as the sacrificial electron donor. The TONs (turnover numbers) of CO were determined and revealed a surprising catalytic activity that is obviously independent from the redox activity of the porphyrin metal. We have recently shown that the parent M1 compounds are active photocatalysts, but the catalytic activity was dependent on the redox activity of the porphyrin metal. In the case of the new heterodinuclear complexes M-1-Re reported in this study, the catalytic active center seems to be the Re(I) moiety and not the porphyrin. Surprisingly, Zn-1-Re proved to be the most active compound in this series showing a TONCO of 13 after 24 h of illumination using a >375 nm cutoff filter while all other compounds showed minimal activity under this condition.

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INTRODUCTION The concentration of the most abundant anthropogenic greenhouse gas, CO2, is continuously increasing in the atmosphere, mainly due to the burning of fossil fuels. This constant rise is expected to cause global climate changes with unpredictable consequences.1 One approach to attenuate the negative impact is to recycle CO2 from the atmosphere or the exhaust fumes of industrial plants and to transform it into a renewable fuel. This energy demanding process could be driven by sunlight, however, suitable photocatalysts are still lacking. Hence, research is focused on developing catalysts for the photochemical conversion of CO2 into valuable products such as CO. Carbon monoxide is one feedstock in the Fischer− Tropsch reaction, which produces liquid hydrocarbons.2 Besides heterogeneous CO2 to CO systems,3 several homogeneous systems4 have been developed. Complexes based on the Re(L)(CO)3Cl motif, where L can be 2,2′-bipyridine (bpy; bpy-Re), phenanthroline, or similar diimine ligands, are wellknown homogeneous catalysts for the light-driven CO2 reduction. Lehn, Ziessel, et al. were the first who investigated these complexes and found that CO is selectively formed during photolysis experiments (λ > 400 nm) with a turnover number (TONCO) of 48 after 4 h.5,6 In subsequent studies, the substitution of the chloride ligand with phosphorus ligands PR3 (e.g., P(OEt)3) increased the catalytic activity significantly.7 © XXXX American Chemical Society

Ishitani et al. improved Lehn’s system further by using two different entities for cooperative catalysis, whereby the CO2 photoreduction catalyst [Re(bpy)(CO)3(CH3CN)]+ shows enhanced catalytic activity in the presence of a [Re(4,4′(MeO)2bpy)(CO)3(P(OEt)3)]+ photosensitizer.8 Recently, Ishitani et al. reported the intramolecular attachment of one or more visible light absorbing Ru(bpy)22+ fragments to a Re(I) photocatalyst and obtained TONCO of up to 230.9,10 Furthermore, the group of Alberto investigated mononuclear Re(I) tricarbonyl complexes with various diimine ligands for their ability to catalyze CO2 reduction.11 Diimine ligands with a more extended π-system like dipyrido[3,2-a:2′,3′-c]phenazine (dppz) were found to decrease the photochemical activity compared to the parent bpy-Re catalyst. It was concluded that diimines with extended π-systems were detrimental to catalytic function, presumably due to lower energy π-accepting orbitals. There are only few examples of dinuclear complexes combining a porphyrin unit and a Re(I) bipyridine tricarbonyl unit.12−17 The porphyrin units, acting as photosensitizer, would represent a cheaper alternative than the ruthenium based chromophores. In most cases the bpy ligand of the Re(I) moiety is connected to one phenyl group in meso-position of Received: July 31, 2015

A

DOI: 10.1021/acs.inorgchem.5b01717 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Figure 1. Structure of M-1, M-1-Re, and bpy-Re with M = H2, Cu, Pd, Zn, Co, FeCl.

Figure 2. Molecular structure of Cu-1-Re·5.5(C7H8)·0.5(C6H6) (hydrogen atoms and solvent molecules were omitted for clarity).

catalyze the reduction of CO2.32−35 The groups of Fujita and Neta reported that the catalytic product CO is obtained with TONCO of 80 for Co-TPP and of 65 for FeCl-TPP, both after 200 h of illumination (λ > 320 nm).34,35 We recently published the synthesis of M-1 and M-1-Ru compounds (M = H2, Cu, Pd, Co, FeCl) based on the phenanthroline extended tetramesityl porphyrin ligand H2-1 (Figure 1), in which Ru represents a Ru(tbbpy)22+ fragment (tbbpy = 4,4′-di-tert-butyl-2,2′-bipyridine).36,37 The electron storage capability of the copper and palladium complexes could be shown by means of EPR spectroscopy, and all complexes were investigated concerning their ability to drive photochemical CO2 reduction. We revealed that near UV light is required for light-driven CO2 reduction through wavelength dependent photolysis experiments.37 A similar result was recently presented by Bonin et al.38 Furthermore, we have shown that the photocatalytic activity depends on the redox activity of the porphyrin metal. FeCl-1 and FeCl-1-Ru displayed the highest catalytic activity and demonstrated even higher TONsCO in our setup than the literature known compounds Co-TPP and FeCl-TPP. Herein, we replaced the ruthenium moiety by a Re(I) tricarbonyl chloride unit and, thus, a new type of M-1-Re complex with Re = Re(CO)3Cl (Figure 1) is presented. This is the first time that a metalloporphyrin and a Re(I) tricarbonyl unit are connected via a linker with a delocalized π-system. Beside the rhenium unit, the metalloporphyrin moiety can function alone as photocatalyst for CO2 reduction. Thus, the porphyrin unit might assist or hamper the overall catalytic activity depending on the competition for the photoelectron. Herein, the influence of different porphyrin metals (Zn, Cu, Pd, Co, FeCl) on the photocatalytic activity of the whole complex is investigated with wavelength- and time-dependent experiments. Our results show that cooperative catalysis is possible with the right choice of the porphyrin metal.

the porphyrin via an aliphatic amide bond. While the photophysics of these complexes were widely investigated by spectroscopic methods, there are only three studies concerning their photochemical CO2 reduction activity.13,16,17 Two studies used Zn-TPP (with TPP = tetraphenylporphyrin), which was connected to the Re(I) moiety via an amide bond, and either a chloride or a picoline ligand was coordinating the Re(I) tricarbonyl unit. The photocatalytic activity was relatively low; the highest TONsCO were reported to be around 30.16 The third study used Pd-TPP, which was also connected to the Re(I) moiety via an amide bond, and the regular chloride ligand was also exchanged by a picoline ligand at the Re(I) unit.13 Here, the highest TONCO reported was only 2. These last three compounds belong to the class of photochemical molecular devices (PMDs), which were developed to improve light absorption and overall catalytic activity for different photocatalytic reduction reactions (i.e., separation of tasks: light absorption−catalytic reaction).18,19 The linker in PMDs connecting the two moieties should have partial electron storage ability to accumulate electrons for the often multielectron reduction reactions, e.g., H2 evolution or CO2 reduction.20 Therefore, in the last 20 years several heterodinuclear complexes with ligands possessing an extended πsystem as linker were developed, like tetraazatetrapyridopentacene (tatpp)21−23 and tetrapyridophenazine (tpphz),24−26 and proved to be catalysts for photochemical proton reduction. However, CO2 reduction was hardly investigated with these systems. Crossley and co-workers used the tatpp motif to connect two porphyrinic units.27,28 Further groups combined a porphyrin unit with a phenanthroline moiety to allow the connection of two different metal centers,29−31 but no catalytic activity was tested. Mononuclear porphyrin compounds, e.g., M-TPP, containing redox-active metals like iron or cobalt are able to B


Article

Inorganic Chemistry Table 1. Selected Bond Lengths [Å] and Angles [deg] for Cu-1-Re·5.5(C7H8)·0.5(C6H6) bond lengths [Å] Re1−C69 Re1−C70 Re1−C71 Re1−N5 Re1−N6 Re1−Cl1 O1−C69 O2−C70 O3−C71 Cu1−N1 Cu1−N2 Cu1−N3 Cu1−N4

1.923(4) 1.927(5) 1.976(6) 2.183(3) 2.167(3) 2.4649(11) 1.155(5) 1.141(6) 1.045(6) 2.013(3) 1.989(3) 2.042(3) 1.992(3)

bond angles [deg] C69−Re1−C70 C69−Re1−C71 C70−Re1−C71 C69−Re1−N6 C70−Re1−N6 C71−Re1−N6 C69−Re1−N5 C70−Re1−N5 C71−Re1−N5 N6−Re1−N5 C69−Re1−Cl1 C70−Re1−Cl1 C71−Re1−Cl1

87.46(18) 88.63(19) 92.7(2) 96.70(16) 173.11(15) 92.87(17) 172.41(16) 100.10(15) 90.37(16) 75.83(11) 96.25(13) 90.67(14) 174.19(14)

O1−C69−Re1 O2−C70−Re1 O3−C71−Re1 Cl1−Re1−N5 Cl1−Re1−N6 N3−Cu1−N4 N3−Cu1−N1 N4−Cu1−N1 N2−Cu1−N3 N2−Cu1−N1 N4−Cu1−N2

176.9(4) 176.8(4) 174.8(5) 84.38(8) 83.44(8) 90.49(12) 178.59(11) 89.45(12) 90.04(11) 90.03(11) 179.09(12)

Figure 3. Comparison of the UV−vis spectra in DCM of (A) H2-1, H2-1-Re, Zn-1-Re, and bpy-Re; (B) H2-1-Re, Pd-1-Re, FeCl-1-Re, and Co-1Re.

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phenanthroline palladium-dichloride published by Liu et al.39 All copper−nitrogen distances are around 2.0 Å, which is comparable to those of Cu-TPP.40 Note that no crystal structure of a copper porphyrin with only one π-extended pyrrole ring is known in the literature. Ligand 1, consisting of the porphyrin and the phen fragment, is nearly perfectly planar. In Cu-1-Re·5.5(C7H8)·0.5(C6H6), the dihedral angle between the phen moiety and the modified pyrrole ring of the porphyrin moiety equals 6.92(19)°. This lies between the analogous angle in Zn-1 (14.35(13)°) and in the zinc tetraphenylporphyrinphenanthroline palladium-dichloride complex (3.43(13)°).36,39 The rhenium atom is situated in a distorted octahedral coordination geometry (see Figure 2). The bidentate phen ligand displays a N6−Re1−N5 bite angle of 75.83(11)° that is very similar to the corresponding angle found in (11,12difluorodipyrido(3,2-a:2′,3′-c)phenazine-N,N′)-rhenium tricarbonyl chloride [Re(dppzF2)] with 75.73(14)°41 or 74.6(3)° in bpy-Re.42 Consequently, the equatorial C−Re−N angles are larger than 90° [C69−Re1−N6, 96.70(16) Å; C70−Re1−N5, 100.10(15) Å] whereas the equatorial C−Re−C angle is again smaller than 90° [C69−Re1−C70, 87.46(18) Å], as was observed for Re(dppzF2) and bpy-Re too. The Re−N and Re− C distances in Cu-1-Re·5.5(C7H8)·0.5(C6H6) are also comparable to those found for Re(dppzF2) or bpy-Re. Spectroscopic Properties. The UV−vis spectrum of H2-1Re shows a small splitting of the Soret band as well as a slight red shift of the Q bands in comparison to the spectrum of H2-1 (Figure 3A). The spectra of M-1-Re with M = Zn, Cu, Pd, Co, and FeCl also depict typical porphyrin spectra with strong

RESULTS AND DISCUSSION Synthesis and Characterization. The synthesis of M-1 (M = H2, Cu, Pd, Zn, Co, or FeCl) was reported earlier.36,37 The subsequent reaction of M-1 with one equivalent of Re(CO)5Cl in toluene afforded M-1-Re in 65 to 96% yield (see Experimental Section). M-1-Re compounds were characterized by UV−vis spectroscopy, ESI-MS, elemental analysis, and IR spectroscopy. Additionally, single crystals of Cu-1-Re could be obtained that were suitable for an X-ray diffraction study. The diamagnetic complexes H2-1-Re, Pd-1-Re, and Zn-1-Re were characterized by 1H NMR spectroscopy, whereby the spectra are very similar (Figures S1−S3). Only the chemical shifts of the pyrrolic protons differ, but no clear trend can be observed in correlation to the central metal. Crystal Structure of Cu-1-Re·5.5(C7H8)·0.5(C6H6). Single crystals were obtained by slow evaporation of a solution of Cu1-Re in a mixture of toluene/benzene/hexane. Cu-1-Re· 5.5(C7H8)·0.5(C6H6) crystallizes in the triclinic centrosymmetric space group, P1̅. The coordination geometry of copper is square planar, and the copper atom resides nearly perfectly in the plane of the four porphyrinic nitrogen atoms (see Figure 2). All N−Cu−N angles are approximately 90° (selected bond lengths and angles are listed in Table 1). Furthermore, the copper−nitrogen distance to the substituted pyrrole [Cu1−N3 = 2.042(3) Å] and to the opposite one [Cu1−N1 = 2.013(3) Å] are slightly longer than the two remaining Cu−N bonds [1.989(3) Å and 1.992(3) Å], which was also found for the similar complexes Zn-136 and zinc tetraphenylporphyrinC


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Inorganic Chemistry absorption around 420 nm and weaker bands in the 500−650 nm region (Figure 3B, Tables S1 (DCM) and S2 (THF)). The MLCT from Re to the phenanthroline moiety of 1 (expected in the region of 370−430 nm;11,43 and found for bpy-Re at 401 nm (ε = 3600 mol/cm·L (in CH3CN)))44 is hidden by the broad and intense Soret band for all M-1-Re compounds. The influence of the phenazine part of ligand 1, which is very important in Re-dppz compounds,41 on the absorption properties is only marginal as the porphyrin unit is so overwhelming. Thus, no assignment of (ligand-based) energetic transitions to absorption features in the region 280−380 nm is possible at the moment. Furthermore, the Soret band of H2-1-Re is more intense and sharper compared to the other investigated M-1-Re complexes, which is in accordance with the reported M-1-Ru complexes.36,37 The Soret bands of M-1-Re are broadened compared to those of the parent M-1 compounds and split into at least two bands. This is observable especially for Co-1Re and FeCl-1-Re (Figure 3B); the Soret band of the latter one is even split into three peaks. Furthermore, the position of the Soret band of M-1-Re is not strongly influenced by the Re(I) moiety when compared with M-1 (Figure S7) pointing toward a rather weak electronic interaction between the porphyrin and Re(I) unit. Accordingly, the Q bands of H2-1-Re as well as those of Zn-1-Re and FeCl-1-Re are only slightly red-shifted in relation to the Q bands of the parent M-1 compounds. However, the opposite trend applies to Pd-1-Re, Cu-1-Re, and Co-1-Re, where the Q bands are slightly blue-shifted compared to the corresponding M-1 compounds (see Table S1). The shapes of the emission spectra of H2-1 and H2-1-Re in DCM are typical for porphyrin complexes12,45 and differ clearly from the emission behavior of the bpy-Re complex (Figure 4,

Table 2. Emission Properties of the Complexes M-1 and M1-Re with M = H2, Zn, Cu, Pd, Co, and FeCl in DCM Obtained by a Streak Camera Systema λem [nm] bpy-Re H2-1 Zn-1 Cu-1 Pd-1 H2-1-Re Zn-1-Re Cu-1-Re Pd-1-Re FeCl-1 FeCl-1-Re Co-1 Co-1-Re

612 660 610, 660 − 585 660 660, 685 668 592 − − − 529

720 720 − 638 726 726 731 659 − − − 692

τ [ns]

Φ

35.99 ± 0.31 10.35 ± 0.05 1.48 ± 0.02 − b 10.22 ± 0.07 1.34 ± 0.01 1.27 ± 0.14c 11.38 ± 3.01 b − − − 1.42 ± 0.01

0.013 0.080 0.028 − vw 0.082 0.031 vw vw − − − vw

a Differences between steady state and time-resolved measurements concerning the band localization are possible; see Table S1 for steady state measurements and Supporting Information for further information; vw, very weak; −, not detected. bFaster than detection limit. cAmplitudes are 0.54 (1.27 ns) and 0.5 (11.38 ns).

the Re center in H2-1-Re has only a slight impact on the location of the excited state. This becomes obvious since the two emission maxima of H2-1 at 655 and 726 nm (measured under steady state conditions) are only marginally red-shifted to 661 and 731 nm for H2-1-Re (Table S1). In contrast, there is a strong effect of the Zn center on the emission behavior: The emission of Zn-1, which is still porphyrin based, is clearly blueshifted to 615 and 673 nm. The quantum yield (3%) is lower and the lifetime (1.48 ns) is considerably shorter compared to H2-1. The obtained emission lifetime is in the same time range as measured for related Zn-TMP and zinc-rhenium dyads where the lifetime was assigned to the relaxation of the S1 state to the ground state after excitation of the S2 state followed by an internal conversion.17 In the complex Zn-1-Re the impact of the Re center is more pronounced than in H2-1-Re, because the emission of Zn-1-Re is shifted to higher wavelengths at 660 and 730 nm. Altogether, the Zn center has a strong influence on the excited states of Zn-1 and Zn-1-Re, which may be a possible explanation for the increased catalytic activity concerning CO2 reduction (Table 4). All other Re complexes display no emission under steady state conditions. Nevertheless, for the complexes Pd-1, Pd-1-Re, Cu-1-Re, and Co-1-Re very weak emission could be detected by a sensitive streak camera, which was used for the determination of the emission lifetimes. This discovery is in accordance with the literature, where Perutz et al. found that Pd-TPP-Re(bpy)(CO)3Br and even Pd-TPP emit under these conditions.13 As in the case of Zn as M the Pd center induces a distinct blue shift of the emission. The weak emission intensities in our case can probably be explained by the very fast relaxation processes of the corresponding excited states since the lifetimes for Pd-1 and Pd-1-Re are below the detection limit. In case of Cu-1-Re we observed two lifetimes with similar amplitudes. Dual emission is very likely because it was not possible to fit the data with just one time constant. However, without any further studies (like transient absorption spectroscopy) we cannot determine the transitions involved in these processes.

Figure 4. Steady state emission spectra of H2-1 and H2-1-Re as well as Zn-1 and Zn-1-Re in DCM in comparison to bpy-Re using an excitation wavelength of 410 nm.

Table S1, data in THF, see Supporting Information) and related Re complexes possessing a π-extended dipyrido[3,2a:2′,3′-c]phenazine ligand.41 The comparison of the quantum yields (Table 2) and the emission lifetimes supports this finding (Table 2, Figure S8). The quantum yields of H2-1 (8.0%) and H2-1-Re (8.2%) are higher compared to bpy-Re with only 1.3%. In addition, the emission lifetimes of these systems are much shorter, 10.35 ns (H2-1) and 10.22 ns (H2-1-Re), compared to bpy-Re with about 36 ns. Thus, the emission state is directly connected to the porphyrin unit, while the linked Re center has no influence on the radiative rates of the emission as also analyzed for other M-porphyrin-Re systems.12 Moreover, D


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Inorganic Chemistry Table 3. Redox Potentials of M-1 and M-1-Re Complexes in DCM and 0.1 M TBAP potential vs Fc/Fc+ [V] 0

Re /Re

I

36

H2-1 Zn-136 Cu-136 Pd-136 H2-1-Re Zn-1-Re Cu-1-Re Pd-1-Re Co-137 FeCl-137 Co-1-Re FeCl-1-Re

−2.04 −2.03 −2.04 −2.01 −2.25 −2.18 −2.02

Ep,ca Ep,c Ep,c Ep,c Ep,c Ep,c Ep,c

2−

−

1 /1 −2.01 −2.25 −2.21 −2.25 −1.82

−1.78 −1.47 −1.65 Ep,c −1.58 Ep,c (b)c

−

1 /1

1/1+

−1.74 −1.80 −1.78 −1.80 −1.57 −1.65 −1.62 −1.61 −1.25 −1.23 −1.27 Ep,c

0.59 0.44 0.62 0.67 0.66 0.44 0.60 0.72 0.51 0.72b 0.51 0.77b

−0.84 Ep,c −1.00 Ep,c (b)

1+/12+

ReI/ReII

0.70 0.89 1.13 0.76 0.89 1.17 0.79 1.15

1.02b 1.03 1.02 1.01

0.86 Ep,ab,d 1.03

a

Ep,c: potential for cathodic peak in the case of irreversible redox events. bTwo-electron processes due to overlapping events. cb = broad peak. dEp,a: potential for anodic peak in the case of irreversible redox events.

Electrochemistry. Electrochemical investigations were performed in dichloromethane solutions because the solubility of the compounds was not good enough in DMF, THF, or acetonitrile. During CV and square wave voltammetry (SQV) experiments we could not observe the formation of any precipitate on the electrodes. Furthermore, the CVs are not scan rate dependent and do not change over time. Hence, we do not have any evidence for immobilization of the compounds on the electrode. The electrochemistry of compounds M-1 was reported in detail in earlier publications36,37 and is only summarized in this discussion. M-1 compounds with M = H2, Zn, Cu, Pd exhibit two ligand-based reductions and two ligandcentered oxidations, except H2-1, which displays only one oxidation event at 0.59 V vs Fc/Fc+ (Table 3). The reduction potentials for M = Zn, Cu, and Pd are nearly independent from the metal center (around −1.8 and −2.2 V), whereas the oxidation potentials are shifted to higher values with increasing electronegativity of the porphyrin metal. Thus, the Zn complex is the easiest to oxidize (0.44 V) and the Pd complex the hardest (0.67 V). A similar trend is observed for the M-1-Re compounds with M = H2, Zn, Cu, Pd. They also exhibit two ligand based oxidation events that are accompanied by a third, Re-metal-centered oxidation event. In the case of H2-1-Re, the second oxidation event at 1.02 V vs Fc/Fc+ is actually a two electron process in which the second ligand-based oxidation occurs at almost the same potential as the ReI/ReII oxidation, which is confirmed by comparing the area underneath the redox events and SQV experiments. For the other M-1-Re (M = Zn, Cu, Pd) complexes, the ReI/ReII couple occurs at around 1.0 V and is basically independent of the metal in the porphyrin cavity. The ReI/ReII redox couple is very similar to the value found for Re(11,12-methyl-dipyrido[3,2-a:2′,3′-c]phenazine)(CO)3Cl (0.92 V) and for bpy-Re (1.03 V).41,46 The insensitivity of this potential to the different porphyrin metals indicates only weak electronic interaction between these two metal centers. Zn-1-Re, Cu-1-Re, and Pd-1-Re exhibit two reduction potentials (the CV of Zn-1-Re is shown exemplarily in Figure 5): The first is reversible, shifted to more positive values compared to the parent M-1 compounds, and is very likely ligand-based. The second irreversible reduction at around −2.0 V for each compound is assigned to the ReI/Re0 couple. The assignment to the rhenium metal center is made according to the irreversibility of the event and comparison with the complex

Figure 5. CV of Zn-1-Re in DCM obtained with a scan rate of 100 mV/s and 0.1 M TBAP as electrolyte. Note that the first reduction becomes reversible, when the CV has a returning potential of −1.8 V (inner CV trace).

Re(11,12-methyl-dipyrido[3,2-a:2′,3′-c]phenazine)(CO)3Cl, in which the second reduction event also occurs at −2.0 V.41,46 For bpy-Re, this reduction is found at −2.10 V (for the cathodic wave).47 However, a second ligand-based reduction, as it is found for the parent compounds M-1 (M = Zn, Cu, Pd), could not be observed for these three M-1-Re complexes in DCM, in contrast to H2-1-Re (Table 3). Here, the ligand is first reduced twice before an irreversible ReI reduction to Re0 occurs. The interpretation of the redox behavior of the cobalt and iron complexes is much more difficult due to the redox activity of the porphyrin metals. Co-1-Re shows three reduction events (Figure S4). The first irreversible one is assigned to be cobaltbased and the second (quasi-reversible) to be ligand-based, as it is found for Co-1.37 They only show small shifts after addition of the rhenium unit. Furthermore, we suggest that the third reduction corresponds to the ReI/Re0 couple, because it occurs at around −2.0 V, as it was found for other M-1-Re compounds. The first oxidation potential of Co-1-Re and Co-1 has the same value (Table 3) and is likely cobalt-centered, thus, it also seems to be insensitive to the rhenium complexation. The second irreversible oxidation of Co-1-Re E


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Inorganic Chemistry displays a two-electron process. This could be confirmed by comparing the peak area of this event with those of the other redox events. Hence, one ligand-centered oxidation overlays with the ReI/ReII couple. FeCl-1-Re also shows two oxidation events (Figures S5 and S6). The first oxidation of the parent FeCl-1 is assigned to be a two-electron process (the integration of this event in comparison to the second oxidation event is twice as high in the SQV).37 Here, one ligand-centered oxidation very likely overlays with the FeIII/FeIV couple. The same interpretation would apply for the first oxidation event of FeCl-1-Re (see Figure S6) although this oxidation event is slightly shifted to a more positive value by 50 mV in comparison with FeCl-1. The second oxidation of FeCl-1-Re could be assigned to the ReI/ ReII couple; it is in the same range as found for the other M-1Re complexes. A third (ligand-based) oxidation, as was found for FeCl-1, cannot be observed in DCM. An interpretation of the reduction events of FeCl-1-Re is not straightforward, because they are very broad and irreversible. Very likely there is a combination of metal and ligand-centered reduction events that occur at the major irreversible cathodic peaks at −1.00 and −1.58 V vs Fc/Fc+ (Figure S6). Based on the results of FeCl-1 and FeCl-1-Ru,36,37 the first big reduction event very likely involves two metal-centered reductions because the ligand should not be involved at these potentials. After the second big reduction event the oxidation state of the iron is supposedly zero and the ligand is probably reduced too. In summary, all metal centers have a noticeable influence on the redox properties of the investigated complexes. Unfortunately, these properties do not stand in any relation to their catalytic properties as outlined below. Photocatalytic CO2 Reduction. Photochemical CO2 reduction experiments were done in DMF with TEA (5%) as sacrificial electron donor under 1 atm of CO2. DMF was used due to solubility reasons, as the M-1 or M-1-Re compounds are sparingly soluble in water, acetonitrile, or THF, but show good solubility in DCM and reasonable solubility in DMF. Moreover, a 200 W high-pressure-mercury-vapor lamp was utilized for all catalytic experiments as a marked influence of the light source on CO2 reduction capability was observed before.37 This lamp was equipped with several long-wave pass cutoff filters for wavelength-dependent investigations. Further experimental details can be found in the Experimental Section. In an earlier study37 we observed that the catalysts M-1 showed basically no activity using a >375 nm cutoff filter but were active if the cutoff was lowered to 305 nm, as seen in Table 4. Photolysis experiments with Co-1-Re, H2-1-Re, CoTPP, and Co-1 (Figure 6A) over a 48 h period using the >305 nm filter showed promising initial activity for Co-1-Re and H21-Re, however, this began to slowly level off after 20 h. In contrast, Co-TPP and Co-1 showed no signs of decay after 48 h, with the Co-TPP being approximately twice as active as Co-1 and just slightly better than Co-1-Re and H2-1-Re after 48 h of illumination. At least initially the two Re complexes show superior and almost like behavior. This and the lack of noticeable decay in the Re-free Co−porphyrin complexes support that the catalytic activity of the Re complexes is at the Re center with some likely background CO production from the M-porphyrin center. In the absence of the Re center, this basal level of activity is evident as seen in the Co-TPP and Co-1 complexes, and shows that the latter catalytic center is far more stable.

Table 4. TONCO of M-1 and M-1-Re Complexes after 24 h of Photolysis (in DMF/TEA (5%), CO2 Atmosphere) TONCO catalyst

>305 nm cutoff filter

>375 nm cutoff filter

Co-137 Co-1-Re FeCl-137 FeCl-1-Re Cu-137 Cu-1-Re Pd-137 Pd-1-Re H2-137 H2-1-Re Zn-137 Zn-1-Re

3.2 7.4 7.6 7.4 0.7 7.3 1.0 12.1 0 8.0 0 12.5

0.1 0.6 0.1 0.6 0 0.3 0 0.4 0 0.8 0 12.8

Figure 6. (A) Time course of TONCO for H2-1-Re (red), Co-1-Re (magenta), Co-1 (turquoise), and Co-TPP (black) using the >305 nm cutoff filter. (B) TONCO after 24 h of photolysis for M-1-Re compounds depending on different cutoff filters.

Further wavelength dependent experiments were done for all M-1-Re compounds with M = H2, Cu, Pd, Zn, Co, or FeCl using cutoff filters at 305, 320, 350, and 375 nm (Figure 6B) under otherwise identical conditions. TONs were calculated after a 24 h irradiation time, and the headspace was analyzed by GC for CO and H2. Table 4 shows the TONCO for the complexes when irradiated with light with wavelengths above 305 and 375 nm, respectively. The data for the Re containing complexes is plotted in Figure 6B, and as can be seen the majority of these complexes behave similarly, with a maximal TON seen when the UV radiation is included and TON falling to near zero as the cutoff wavelength approaches 375 nm. FeCl1-Re, Cu-1-Re, Co-1-Re, and even H2-1-Re reach a similar TONCO of around 7.5 for the >305 nm filter after 24 h. The F


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Inorganic Chemistry

center should be the focus of further investigations. Of special interest should be the photophysical properties of the reduced species. At this point, some parallels to reaction mechanisms that have been discussed in the literature for the photochemical CO2 reduction with Re-based catalysts like the well-known bpy-Re complex can be drawn. All proposed mechanisms agree in the first step, which is the formation of the one-electronreduced species of bpy-Re. The free electron is thereby located on the bpy ligand.48,5 In one possible pathway, the oneelectron-reduced species might form an adduct with CO2 in a consecutive step that is then protonated and reduced again. This metal-carboxylate species releases H2O and CO upon an acid-promoted hydrolysis.49,50 The origin of the second electron, that is required for CO2 reduction, is unknown in this process. Ishitani, Fujita, and others did an important approach to resolve this problem.8,51 They revealed that the second electron might come from another one-electronreduced species and the reaction mechanism includes actually a CO2-bridged Re(I) dimer. Afterward the second oxygen atom is transferred to another CO2 molecule to form carbonate and CO is released. Both mechanisms are very likely to proceed during the photolysis in our experiments. At the beginning the pathway including the Re(I) dimer may dominate, because no proton source is available. And when the proton concentration increases due to TEA decomposition,52,53 the first pathway involving the metal-hydroxycarbonyl intermediate might be more important. In future studies we intend to isolate some of the intermediates to prove the reaction mechanism. The role of the porphyrin unit is uncertain, but it could be similar to the zinc porphyrin Re(I) bipyridine dyads developed by Perutz.16 In that case, the zinc porphyrin unit acts as lightharvesting unit in the visible region and an electron transfer occurs from the excited porphyrin S2 state to the Re(I) moiety.16 Charge recombination competes with the electron transfer quenching from TEA, and the lifetime of the charge separated state is relatively low. We can assume that the electron storage properties of ligand 1 assist in the accumulation of reduction equivalents in the bridging ligand that finally leads to a similar active species as the “simple” oneelectron-reduced species. We cannot exclude a competition for the photoelectron between the metal−porphyrin moiety and the rhenium moiety of M-1-Re. The basal level of CO2 reduction activity from the catalytically active M−porphyrins might in fact hamper higher efficiencies because of this competition. In the case of Zn-1-Re the assembly seems to work in full cooperation with the Re center. Thus, the influence of the different porphyrin metals on the catalytic activity as well as the wavelength dependent results are important findings. To the best of our knowledge, these investigations, also in an experimental manner, were not done before for any linked porphyrin−Re(I) system. Further photophysical investigations and long-term catalytic experiments are envisaged to shed light on the different photocatalytic intermediates and activities of M-1 and M-1-Re compounds and differences in the reaction mechanism.

catalytic activity of parent FeCl-1, Cu-1, Co-1, and H2-1 is very different though. H2-1 shows no activity at all while Cu-1 exhibits only a very small catalytic activity in contrast to Cu-1Re, whose TONCO is roughly 10 times higher. Co-1 is around half as active as Co-1-Re while FeCl-1 and FeCl-1-Re have a similar TONCO. This behavior indicates again that the dominant catalytically active center is the Re moiety. Pd-1-Re shows a strikingly different behavior in that the TONsCO are in the range of those of the other M-1-Re compounds for most of the cutoff filters used except when working with the >305 nm filter. Pd-1-Re is 1.5 times more active (TONCO = 12.1) than the other metal complexes, and furthermore, dihydrogen could be detected as a product of photocatalysis with a constant TONH2 of around 20 for all the cutoff filters used. This stands in contrast to the literature, where only one example for a covalently linked palladium porphyrin Re(I) bipyridine tricarbonyl dyad exists (both units are linked via an amide bond) and no hydrogen evolution was mentioned during CO2 reduction experiments.13 Nevertheless, it seems that in this case the palladium porphyrin moiety of Pd1-Re influences the hydrogen evolution ability, because no hydrogen evolution could be detected for all other M-1-Re compounds, and this agrees with literature reports.48 Zn-1-Re also differs from the other Re complexes and surprisingly shows equal activity (TONCO of around 12.5) whether or not the 305−375 nm region is included during irradiation. Thus, this is the only example for the M-1 and M-1Re compounds in which UV light is not necessary for the catalytic reduction of CO2 to CO. This is similar to the work of Perutz et al.16 that showed a TONCO within the same range after several hours of irradiation with visible light (λ > 520 nm) using the dinuclear Zn-TPP-(bpy)Re complex. From these data we see two distinct groups of catalysts in our work, those requiring UV irradiation and the single Zn-1-Re complex that does not. The exact cause for this difference is not immediately evident from the absorption or electrochemical data, but the emission behavior is different for Zn-1-Re, showing a porphyrin like emission spectrum.37 Furthermore, the Soret band for the Zn-1-Re complex is “unsplit” relative to that seen for the other metal complexes. These data suggest a stronger degree of electronic coupling between the M-porphyrin and Re unit for the majority of the rhenium compounds. As the various redox couples for the Zn-1-Re complex are unremarkable compared to other M-1-Re complexes, the energy of the porphyrin-based LUMO in Zn-1-Re must be very close in energy to the nonemissive LUMO of the other complexes. The ReI/Re0 couple has nearly the same electrochemical potential for the M-1-Re compounds with M = H2, Zn, Cu, and Pd. No conclusive statement can be made for FeCl-1-Re and Co-1-Re, because the reduction potentials could not be assigned reliably. Furthermore, there is no correlation between the emission lifetimes obtained after excitation at 388 nm and the catalytic activities (Table 2 and Figure 6). Although Pd-1-Re and Zn-1Re show a higher catalytic performance compared to other M1-Re complexes, they do not show remarkably different emission properties compared to their M-1 analogues, which have no or only a low catalytic activity. Introduction of the Re center has no influence on the emission lifetimes and only a strong influence in the Zn-1-Re complex can be observed that needs further investigation. The relaxation time of the emission seems not to be the rate-limiting step since fast processes like in Pd-1-Re still result in comparable activity. Thus, the electron transfer processes from the light harvesting to the catalytic

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CONCLUSIONS We have successfully developed the dinuclear complexes M-1Re with various metals in the porphyrin unit (M = H2, Zn, Cu, Pd, Co, and FeCl) for application in photocatalytic CO2 reduction. This interesting class of photochemical molecular devices consists of a phenanthroline-extended metal porphyrin G


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Inorganic Chemistry

pseudoreference electrode. All data were referenced versus the ferrocene/ferrocenium couple at the end of each measurement. CVs were collected at scan rates of 10−1000 mV/s. Square wave voltammograms were collected at an amplitude of 20 mV, step potential of 5 mV, and a frequency of 25 Hz. Irradiation experiments were performed in DMF with 5% TEA and with 5 × 10−5 M catalyst concentration. Fresh solutions were prepared before each experiment in Schlenk flasks with 5 mL of solution and 20 mL of headspace, and they were bubbled with CO2 for 20 min to remove argon. Photolysis was performed with a 200 W high-pressuremercury-vapor lamp using a water filter to absorb IR radiation and different long-wave pass cutoff filters to absorb wavelengths below the given values.37 The filters were purchased from LOT-QuantumDesign GmbH and had a sharp edge of Δ = ±3 nm. CO and H2 evolved were determined by gas chromatography (Shimadzu GC-17A with thermal conductivity detector and Resteks ShinCarbon packed column ST 80/ 100 (2 m, 1/8 in. o.d., 2 mm i.d.)) and quantified using a calibration curve. Measurements were done by manual injection of gas phase samples (250 μL) at least in duplicate. CO or H2 dissolved in the solvent was not considered, thus the TON of CO is only related to the amount detected in the gas phase above the solution. To estimate the amount of CO that was produced by other sources, like solvent decomposition, photolysis experiments were done without the catalyst under the same conditions and all turnover numbers of CO were corrected by these values. In these blank experiments, no CO was detected using λ > 350 nm cutoff filter and higher, while TONCO values obtained with the >305 nm filter were corrected by a value of 0.8. An experiment using 13CO2 was conducted to demonstrate that the CO released actually derives from the CO2 substrate and not from solvent decomposition. We were able to detect the product 13CO from the gas phase with help of GC−MS. However, the experiment also showed the presence of 12CO, which obviously results from the exchange of the coordinating 12CO ligands from the starting compound with the formed 13CO from the catalytic reaction. This exchange has been acknowledged earlier for rhenium-tricarbonyl-type catalysts.5,54 H2 could only be detected in major amounts in the case of Pd-1-Re. The error range for TONCO is estimated (based on multiple injections) to be around 10% and for hydrogen to be around 20%. The lowest detectable TON for H2 is around 6 with our experimental setup. General Procedure for the Synthesis of M-1-Re. M-1 (25 μmol) and Re(CO)5Cl (25 μmol) were dissolved in 15 mL of toluene and heated at reflux for 3 h. The solvent was subsequently removed under reduced pressure, and the product was purified by column chromatography. Impurities were first eluted with DCM, and the product was finally eluted with DCM:MeOH = 97:3. This method afforded M-1-Re as red to brown solids. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrin-tricarbonylchloro-rhenium(I) (H2-1-Re). Yield: 72%. Found: C, 65.60; H, 4.60; N, 8.39. Calcd for C71H58N8O3ClRe: C, 65.96; H, 4.52; N, 8.67. λmax (DCM)/nm: 429 (ε × 10−3/dm3 mol−1 cm−1 206.0), 534 (25.3), 604 (16.3). HR-ESI ([M+]): m/z found 1293.3953, calcd for C71H58N8O3ClRe 1293.3900. IR (CH2Cl2): νCO = 1898, 1920, 2023 cm−1. 1H NMR (300 MHz, CDCl3, 25 °C): δ = −2.34 (s, 2H), 1.78 (s, 6H), 1.86 (s, 6H), 1.91 (s, 6H), 1.92 (s, 6H), 2.65 (s, 6H), 2.88 (s, 6H), 7.32 (s, 4H), 7.47 (s, 2H), 7.50 (s, 2H), 7.98 (dd, J = 5.1 and 8.2 Hz, 2H), 8.28 (d, J = 4.8 Hz, 2H), 8.59 (s, 2H), 8.93 (d, J = 4.8 Hz, 2H), 9.22 (dd, J = 1.4 and 8.2 Hz, 2H), 9.46 (dd, J = 1.4 and 5.1 Hz, 2H). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 21.55, 21.66, 21.71, 21.79, 115.96, 119.81, 126.45, 127.71, 127.85, 128.00, 128.18, 128.37, 131.49, 134.15, 136.61, 136.73, 137.64, 137.78, 138.10, 138.16, 138.35, 138.64, 139.33, 139.37, 139.66, 139.97, 142.74, 148.46, 151.93, 153.50, 155.82, 189.65, 197.16. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-palladium(II)-tricarbonylchloro-rhenium(I) (Pd1-Re). Yield: 87%. Found: C, 60.71; H, 4.13; N, 7.59. Calcd for C71H56N8O3ClPdRe: C, 61.03; H, 4.04; N, 8.02. λmax (DCM)/nm: 412 (ε × 10−3/dm3 mol−1 cm−1 165.8), 452 (114.4), 556 (35.9), 589 (14.0). ESI ([M+]): m/z found 1397.0, calcd for C71H56N8O3ClPdRe 1396.4. IR (CH2Cl2): νCO = 1897, 1921, 2024 cm−1. 1H NMR (300

unit linked via the phen moiety with a Re(I) tricarbonyl chloride unit. Ligand 1 exhibits electron storage possibility that seems to be advantageous for photocatalytic reaction(s). In any case, the electrochemical properties of M-1-Re are not connected to their photocatalytic activity. We suggest that the Re(I) moiety is the catalytic active center, because H2-1-Re catalyzes the CO evolution whereas the parent H2-1 compound does not. Depending on the metal center in the porphyrin unit, the catalytic activity is hampered or induced in comparison with the M-1 compounds. Especially, Pd-1-Re and Zn-1-Re show a surprising wavelength-dependent catalytic activity. For example, Zn-1-Re has the highest TONCO of 12.8 under light excitation with illumination wavelengths above 375 nm, in contrast to all other M-1-Re complexes that are inactive under this condition. As concluded from the emission properties, a reason for these findings could be the strong impact of the Zn metal on the nature of the excited states located at the porphyrin unit, which needs further detailed investigations.

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EXPERIMENTAL SECTION

General Methods. Column chromatography was performed with silica gel purchased from VWR or Acros (Silica Gel 60, 230−400 μm mesh). All solvents (tetrahydrofuran (THF), toluene, dichloromethane (DCM), dimethylformamide (DMF), and methanol (MeOH)) and starting materials were obtained from Sigma-Aldrich or abcr. If necessary, solvents were dried employing an MBraun solvent purification system. Starting materials were used without further purification. DMF for photolysis experiments was fractionally distilled with benzene and water and dried with CaH2. Triethylamine for photocatalytic reactions was refluxed with KOH, filtered, and dried with CaH2. Compounds M-1 (with M = H2, Zn, Pd, Cu, FeCl, and Co) were prepared following literature procedures.36,37 1 H NMR spectra were recorded at ambient temperature on either a Bruker DPX-300 or AV-400 spectrometer. All spectra were referenced to tetramethylsilane (TMS) or deuterated chloroform (CDCl3) as an internal standard (measured values for δ are given in ppm and for J in Hz). Assignment of signals was done with the help of 2D experiments. Elemental analysis was performed by the microanalytical laboratory of the Institute of Chemistry at the Humboldt-Universität zu Berlin using a HEKAtech EURO 3000. ESI mass spectra were recorded by using Thermo Finnigan LCQ XP and LTQ FT instruments. Absorption spectroscopic measurements were obtained on DCM solutions of each compound using a Cary 100 UV−vis−NIR spectrometer from Varian employing the software Cary WinUV. SUPRASIL Quartz cells from Hellma Analytics with a 10 mm path length were used. Emission spectra were recorded on a Cary eclipse fluorescence spectrophotometer using SUPRASIL Quartz cells from Hellma Analytics with a 10 mm path length. Emission measurements were made on DCM solutions of the investigated compounds. The emission lifetimes were measured with a streak camera system (Streakscope C10627, Hamamatsu) under aerobic conditions. For the streak camera experiments the samples were excited with femtosecond pulses at 388 nm, generated by frequency doubling the output of a Ti:sapphire laser system (CPA 2001, Clark MXR). The samples were dissolved in dichloromethane and tetrahydrofuran (THF) of spectroscopic grade, and the optical density was below 0.1 at the excitation wavelength. IR data were collected on a Shimadzu FTIR 8400S spectrometer. The samples were dissolved in DCM and measured in cuvettes with KBr windows. All cyclic voltammograms (CV) were performed on DCM solutions containing 0.1 M NBu4PF6 (tetrabutylammonium hexafluorophosphate (TBAP)) and the corresponding compound under argon atmosphere at ambient temperature. A potentiostat/ galvanostat PGSTAT 101 from Metrohm was used to record cyclic and square wave voltammograms. A three compartment cell was outfitted with a glassy carbon button electrode as the working electrode, a platinum wire as the auxiliary electrode, and a Ag/AgPF6 H


Inorganic Chemistry

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MHz, CDCl3, 25 °C): δ = 1.77 (s, 6H), 1.85 (s, 6H), 1.90 (s, 12H), 2.65 (s, 6H), 2.88 (s, 6H), 7.31 (s, 4H), 7.46 (s, 2H), 7.49 (s, 2H), 7.98 (dd, J = 5.1 and 8.3 Hz, 2H), 8.65 (s, 2H), 8.70 (d, J = 4.9 Hz, 2H), 8.77 (d, J = 4.9 Hz, 2H), 9.20 (dd, J = 1.3 and 8.2 Hz, 2H), 9.47 (dd, J = 1.3 and 5.1 Hz, 2H). 13C NMR (75 MHz, CDCl3, 25 °C): δ = 21.50, 21.57, 21.67, 21.69, 117.80, 122.05, 126.49, 127.94, 127.97, 128.21, 128.42, 130.24, 130.28, 130.40, 131.27, 131.39, 136.68, 137.29, 137.67, 137.94, 138.13, 139.01, 139.18, 139.46, 140.96, 141.40, 142.63, 148.30, 148.50, 153.58, 193.84, 197.09. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-zinc(II)-tricarbonylchloro-rhenium(I) (Zn-1-Re). Yield: 96%. Found: C, 62.36; H, 4.20; N, 7.99. Calcd for C71H56N8O3ClZnRe: C, 62.87; H, 4.16; N, 8.26. λmax (DCM)/nm: 425 (ε × 10−3/dm3 mol−1 cm−1 178.1), 465 (94.5), 582 (28.4), 621 (8.3). ESI ([M+]): m/z found 1354.3, calcd for C71H56N8O3ClZnRe 1354.4. IR (CH2Cl2): νCO = 1901, 1918, 2024 cm−1. 1H NMR (300 MHz, CDCl3, 25 °C): δ = 1.76 (s, 6H), 1.83 (s, 6H), 1.87 (s, 6H), 1.88 (s, 6H), 2.63 (s, 6H), 2.86 (s, 6H), 7.28 (s, 4H), 7.44 (s, 2H), 7.47 (s, 2H), 7.96 (dd, J = 5.1 and 8.2 Hz, 2H), 8.68 (s, 2H), 8.78 (d, J = 4.6 Hz, 2H), 8.85 (d, J = 4.6 Hz, 2H), 9.25 (dd, J = 1.2 and 8.3 Hz, 2H), 9.44 (dd, J = 1.4 and 5.0 Hz, 2H). (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-copper(II)-tricarbonylchloro-rhenium(I) (Cu-1Re). Yield: 96%. Found: C, 63.32; H, 4.46; N, 7.77. Calcd for C71H56N8O3ClCuRe: C, 62.96; H, 4.17; N, 8.27. λmax (DCM)/nm: 419 (ε × 10−3/dm3 mol−1 cm−1 164.1), 462 (84.9), 576 (22.9), 614 (8.2). ESI ([M − Cl− + MeOH]): m/z found 1353.3, calcd for C72H63N8O4CuRe 1353.4. IR (CH2Cl2): νCO = 1898, 1921, 2023 cm−1. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-cobalt(II)-tricarbonylchloro-rhenium(I) (Co-1Re). Yield: 80%. Found: C, 63.03; H, 4.55; N, 7.80. Calcd for C71H56N8O3ClCoRe: C, 63.17; H, 4.18; N, 8.30. λmax (DCM)/nm: 406 (ε × 10−3/dm3 mol−1 cm−1 121.0), 450 (109.0), 571 (29.0). ESI ([M+]): m/z found 1349.3, calcd for C71H56N8O3ClCoRe 1349.4. IR (CH2Cl2): νCO = 1899, 1921, 2024 cm−1. (Pyrazo[5′,6′-e]-1′,10′-phenanthroline)[b]meso-tetramesitylporphyrinato-iron(III)chloride-tricarbonylchloro-rhenium(I) (FeCl-1-Re). After the purification by column chromatography, the product was dissolved in DCM and stirred over 7% HCl. The organic layer was separated, washed with water, and dried over Na2SO4. After removal of the solvent, FeCl-1-Re was obtained in 65% yield. Found: C, 61.30; H, 4.40; N, 7.80. Calcd for C71H56N8O3Cl2FeRe: C, 61.70; H, 4.08; N, 8.11. λmax (DCM)/nm: 376 (ε × 10−3/dm3 mol−1 cm−1 103.7), 421 (112.6), 446 (108.9), 531 (28.4), 735 (4.8). ESI ([MCl−]+): m/z found 1346.3, calcd for C71H56N8O3ClFeRe 1346.3. IR (CH2Cl2): νCO = 1899, 1921, 2024 cm−1. X-ray Crystallographic Details. Data collections were performed at 100 K with a Bruker D8 Venture area detector. The structure was solved by direct methods (SHELXS-97)55 and refined by full matrix least-squares procedures based on F2 with all measured reflections (SHELXL-2013).56 The SADABS program was used for multiscan absorption corrections. All non-hydrogen atoms were refined anisotropically. All hydrogen atom positions were introduced at their idealized positions and were refined using a riding model. Crystallographic data (excluding structure factors) for Cu-1-Re·5.5(C7H8)· 0.5(C6H6) have been deposited at the Cambridge Crystallographic Data Centre as supplementary publication no. 1057427 CCDC. These data can be obtained free of charge from The Cambridge Crystallographic Data Center via www.ccdc.cam.ac.uk/data_request/ cif. Crystallographic data for Cu-1-Re·5.5(C 7 H 8 )·0.5(C 6 H 6 ): C71H56ClCuN8O3Re, 5.5(C7H8), 0.5(C6H6), M = 1900.21 g/mol, brown crystals, space group triclinic P1̅, a = 13.0726(8) Å, b = 16.9489(10) Å, c = 23.2795(15) Å, α = 73.803(2)°, β = 79.629(2)°, γ = 70.847(2)°, V = 4656.7(5)Å3, Z = 2, T = 100(2) K, R1 = 0.0416, wR2 = 0.1078, GOF = 0.996.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b01717. NMR spectra, cyclic voltammograms, and absorption and emission properties (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*Fax: +49-30-2093-6966. Tel: +49-30-2093-7571. E-mail: [email protected]. *Tel: +49-711-685-64142. E-mail: [email protected]. Present Address §

Institute of Organic Chemistry, University of Stuttgart, Pfaffenwaldring 55, 70569 Stuttgart, Germany. Tel: +49-711685-64142. E-mail: [email protected].

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the German Science Foundation (DFG, SCHW 1454/4-2) for financial support, Stefan Lochbrunner from the University of Rostock for the possibility to use the equipment in his laboratory for lifetime measurements, and Merle Ohlsen for experimental support.

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REFERENCES

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