Influence of Mesoionic Carbenes on Electro-and Photoactive Ru and

Article pubs.acs.org/IC

Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

Influence of Mesoionic Carbenes on Electro- and Photoactive Ru and Os Complexes: A Combined (Spectro-)Electrochemical, Photochemical, and Computational Study Lisa Suntrup,† Felix Stein,† Gunter Hermann,‡ Merlin Kleoff,† Martin Kuss-Petermann,§ Johannes Klein,† Oliver S. Wenger,§ Jean Christophe Tremblay,‡ and Biprajit Sarkar*,† †

Institut für Chemie und Biochemie, Freie Universität Berlin, Fabeckstraße 34-36, 14195 Berlin, Germany Institut für Chemie und Biochemie, Freie Universität Berlin, Takustraße 3, 14195 Berlin, Germany § Department of Chemistry, University of Basel, St. Johanns-Ring 19, 4056 Basel, Switzerland

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‡

S Supporting Information *

ABSTRACT: In recent years, mesoionic carbenes (MICs) are finding increasing use as building blocks of electro- and photoactive metal complexes. We present here a series of RuII and OsII polypyridine complexes where one or two pyridyl moieties of the well-known tris(bipyridine) analogues are replaced by MICs. We probe the structural, electrochemical, UV−vis-NIR/electron paramagnetic resonance spectroelectrochemical, and photophysical properties of these complexes as a function of the number of MICs in them. Insights from theoretical studies are used to describe the electronic structures of the various redox states. Additionally, electron flux density calculations provide an idea of the flow of electron densities in the excited states of these molecules. This is the first time that such electron flux density calculations are used to probe the excited state properties of transition metal complexes. Our results conclusively prove that the incorporation of MICs into Ru/Os-polypyridyl complexes has a profound influence on the ground and the excited state redox potentials, the position of the emission bands, as well as on the lifetimes of the excited states. These observations might thus be useful for the generation of novel photocatalysts and photosensitizers for dye-sensitized-solar-cells based on MICs.

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of the ligand-based LUMO of the complex. 19 Other possibilities include the exchange of one or more of the bpy units for other bidentate ligands, with popular examples being phenanthroline, cyclometalating 2-phenylpyridine, and redox noninnocent systems.20−24 The latter are intriguing since they also allow for a strong energy change of the metal-based HOMO. Corroborating this approach, Chábera et al. recently presented a homoleptic iron(III) complex with bidentate MICs in which the excited state is a luminescent 2LMCT state, exhibiting a record-breaking lifetime of ∼100 ps.23,24 Parallel to these developments, some of us have been involved in systematic studies that involve the substitution of one or both pyridyl units of the bpy ligand with MICs to generate pyridyl-MIC or bi-MIC ligands. The influence of this substitution on the spectroscopic, electrochemical, and catalytic properties of several transition metal complexes (Figure 1) was investigated.25−29 We found that the incorporation of a 1,2,3-triazolylidene moiety into the chelate can significantly stabilize the oxidized or reduced species.29 On the basis of these results, we were

INTRODUCTION Metal complexes of mesoionic carbenes (MICs) are currently considered a privileged class of ligands for generating both intriguing electrochemical as well as photochemical properties.1−4 Immediate usage of such properties can be made in electrocatalytic processes, to generate new generations of photoredox catalysts and to make new dyes for dye-sensitized solar cells.5−14 Transition metal complexes based on polypyridyl ligands are the paragon of photosensitizers, with numerous reports available on their most famous representative [RuII(bpy)3]2+ (bpy = 2,2′-bipyridine). This complex shows very favorable properties, such as well-behaved redox steps, long-lived triplet metal-to-ligand charge transfer (3MLCT)15 states, as well as electrochemiluminescence (ECL).16 Some of the aforementioned properties make the application of this compound as a photoredox catalyst17 possible. As the optimization of these properties relies on achieving a delicate interplay between the metal and the ligand centered orbitals, several attempts aimed at improving and tuning of the above properties were undertaken.18 One viable approach that has been sought out in the past is the introduction of either electron-donating or -withdrawing groups on the bipyridine backbone to influence the energy © XXXX American Chemical Society

Received: September 7, 2018

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DOI: 10.1021/acs.inorgchem.8b02551 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

Scheme 1. Synthesis of p-Cymene Ruthenium and Osmium Complexes from the Triazolium Salts

Figure 1. Relative donor/acceptor strength of the ligand classes utilized in this work.

intrigued to expand the application of mesoionic carbene ligands to the [M(bpy)2(L)]2+ fragment (M = RuII, OsII, L = bidentante ligand, Chart 1) in order to elucidate the influence on the electrochemical properties and the excited state of the ruthenium compounds and how it translates to the higher homologue osmium. Chart 1. New Ruthenium(II) and Osmium(II) MICComplexes Presented in This Work

Information for details). The compound [(cym)Os(L2)Cl](PF6) was already described in 2015.33 The follow-up reaction with 2,2′-bipyridine was first attempted as described by the group of Albrecht in 2013.32 Although the reaction of the half-sandwich complexes with 2,2′-bipyridine in DMSO in the presence of silver hexafluorophosphate led to the formation of the desired product, the isolation of the pure compound proved to be rather difficult due to persistent side products. Judging from the spectroscopic data, we ascribe these to the formation of bis(methylthio)methane from DMSO (Figure S9). In the case of the pyridyltriazolylidene L2, we were also able to isolate the intermediate species coordinated with two DMSO molecules (Scheme 2) and grow single crystals suitable for X-ray diffraction. This structural motif was equivalent to one published by Albrecht and co-workers where the triazolylidene was connected to the pyridine moiety via the C2-carbon instead of the N1nitrogen.32 In order to simplify the purification, the reaction was performed in ethylene glycol at 150 °C, which led to decent yields after aqueous workup and column chromatography on aluminum oxide (Scheme 3). This approach gave the ruthenium complexes in good yields of ∼70%. In the case of the osmium compounds, the use of silver additives was not necessary,34 although the reaction occurred at a slower rate and for the bicarbene L1 and the pyridyl-carbene L3 in diminished yields of under 20%. Addition of silver hexafluorophosphate to facilitate the reaction led to a complex reaction mixture with presumably oxidized osmium species. To the best of our knowledge, these are the first examples of bitriazolylidenes incorporated into complexes of the type [M(bpy)2(L)]2+ and the first examples for osmium triazolylidene complexes of this type in general. The new complexes were characterized via 1H and 13C{1H} spectroscopy, mass spectrometry, elemental analysis, and, apart from [Os3]2+, through single crystal X-ray diffraction (see Tables S2 and S3 for crystallographic details). Figure 2 illustrates the expected octahedral coordination sphere of the compounds. The bond lengths and angles are within the expected range for these types of complexes, with the triazolylidene moiety showing similar lengths as for the respective p-cymene precursors.31,33 When comparing the chelating angles for the different substituents, only little deviations can be observed for

The comprehensive investigation presented herein covers the synthesis of five new ruthenium(II) and osmium(II) complexes, their characterization by spectroscopic and crystallographic methods, as well as cyclic voltammetry, UV− vis- and EPR-(spectro-)electrochemistry and time-dependent UV−vis-absorption and -emission spectroscopy. Density functional theory (DFT) calculations support the description of the electronic structures derived from the data to give a conclusive picture of the photoredox properties of the presented compounds. Additionally, calculations on the electron flux densities of the excited states are presented, which to the best of our knowledge is the first time that such a theoretical methodology is applied to transition metal complexes.

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SYNTHESIS The compounds were synthesized based on previously published routes: the ligands were coordinated to the respective p-cymene (cym) metal fragments via the transmetalation route that is commonly used for the generation of 1,2,3-triazol-5-ylidene complexes of 4d and 5d metals (Scheme 1). The ruthenium complexes had been previously described by our group in 2013 and 2016.30,31 We abstained from synthesizing the ruthenium complex using L3, since a methylderivative of the target compound [Ru(bpy)2(L3)]2+ has been described elsewhere.32 The osmium analogues [(cym)Os(LX)Cl](PF6) were synthesized accordingly, although the reaction time after addition of the osmium precursor [(cym)OsCl2]2 had to be increased from 2 to 5 days to achieve decent yields (see Experimental Section). All complexes were easily purified by column chromatography on aluminum oxide, and the newly synthesized complexes [(cym)Os(L1)Cl](PF6) and [(cym)Os(L2)Cl](PF6) were fully characterized by 1H and 13C{1H} spectroscopy, mass spectrometry, and, in the case of [(cym)Os(L1)Cl](PF6), by single crystal X-ray diffraction (see Supporting B


Article

Inorganic Chemistry Scheme 2. Left: Formation of a Ruthenium Polypyridyl DMSO Complex. Right: ORTEP Representation of [Ru(bpy)(L2)(dmso)2]2+a

a

Hydrogen atoms and anions are omitted for clarity. Ellipsoids are shown at the 50% probability level.

the bipyridine ligands and the wingtip phenyl rings are observed.

Scheme 3. Synthesis of the Polypyridyl Complexes

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ELECTROCHEMISTRY All compounds were submitted to cyclic voltammetry to elucidate the electrochemical behavior of the complexes, which can often provide indirect indications of the HOMO/LUMO gap of such systems. Ruthenium and osmium polypyridyl complexes usually display rich redox chemistry, and the influence of the substitution pattern on the pyridine rings is well-defined and was intensively studied in the last few decades.35−38 Table 1 summarizes the results of the measurements in anhydrous acetonitrile. The potentials for the homoleptic bipyridine complexes are listed for purposes of comparison. All complexes exhibit one reversible oxidation for the MII/ III M redox couple and two reversible reductions at the bipyridine moieties, as is illustrated exemplarily in Figure 3 (see Supporting Information for full electrochemical description). In all cases investigated, the first oxidation is shifted to less positive potentials by about 150−200 mV with each MIC moiety incorporated, as would be expected when introducing donating ligands. For the reduction, the bi-MIC coordination has a significant influence on the first reduction potential. The potentials for [Os2]2+ and [Os3]2+ confirm that the connectivity of pyridyl- and MIC-unit is not crucial for the oxidation potential since this is mostly governed by the donating effect of the ligand. The first reduction on the other hand is with a shift of almost 100 mV influenced by the overall π-accepting capacity of the system, with a 2-substitution over the N3-nitrogen, i.e., L2, of the triazolium moiety being more electron-poor, therefore easier to reduce than L3. It is noteworthy that the incorporation of the carbene units should influence the bipyridine-based reduction, especially when compared to the triazole analogoues reported by the Sarkar group in 2014, where the use of click-derived triazole ligands had virtually no effect on the reduction potential.41 The subsequent reductions show, depending on the nature of the carbene ligand, variations in the redox chemical behavior: for the bi-MIC ligand L1 we observed a spiking event at about −2.3 V vs FcH/FcH+ for both the respective ruthenium and osmium complexes (Figure 4), which might suggest adsorption of the reduced species to the electrode surface. This seems plausible due to the strong donating nature of this ligand that could induce the dissociation of a ligand arm upon reduction.42 Also, the occurrence of positively shifted reoxidations after the third reduction (as for example for [Ru2]2+ or [Os2]2+) points toward the dissociation of a single ligand arm from the negatively charged species. This behavior

Figure 2. ORTEP representations of complexes [Ru1]2+, [Ru2]2+ and [Os1]2+, [Os2]2+. Hydrogen atoms, solvent molecules, and anions are omitted for clarity. Thermal ellipsoids are shown at the 50% probability level.

the different ligand types and metal centers: in all cases, the N−M−N angles of the bipyridine chelate are about 77.5°, whereas the C−M−C angle is between 76.3° and 78.5°. For the complexes [Ru1]2+ and [Os1]2+, π−π interactions between C


Article

Inorganic Chemistry Table 1. Electrochemical Data, All Potentials Given vs. FcH/FcH+ (in MeCN/0.1 M NBu4PF6)a complex 2+

[Ru(bpy)3] [Ru1]2+ [Ru2]2+ [Os(bpy)3]2+ [Os1]2+ [Os2]2+ [Os3]2+

E1/2ox/V

E1/2red1/V

E1/2red2/V

Epred3/V

Epred4/V

Epred5/V

0.89 0.47 0.76 0.43 0.17 0.37 0.32

−1.73 −1.94 −1.74 −1.68 −1.82 −1.69 −1.78

−1.92 −2.20 −1.96 −1.95 −2.10 −1.88 −1.97

−2.16 −2.36 −2.22

−2.98 −2.46

−2.93

−2.30 −2.22 −2.18

−2.90 −2.50 −2.86

−2.91

a

Potentials for [Ru(bpy)3]Cl2 had been reported.38 Potentials for [Os(bpy)3]Cl239 were converted with FcH/FcH+ vs SCE = 0.40 V in MeCN/0.1 M NBu4PF6.40

Similar findings have been reported by Berlinguette and Schubert for heteroleptic ruthenium complexes bearing terpyridine analogues with MICs incorporated.44 For heteroleptic bipyridine and triazolylidene systems however, this has not been described yet. In all cases, the MLCT bands of the respective complexes decrease in intensity upon oxidation, depleting the metal-based HOMO. For the complex bearing the bi-MIC ligand [Ru1]2+, an additional weak band arises at about 725 nm, which we ascribe to an LMCT in accordance with spectroelectrochemical measurements of [Ru(bpy)3]2+ (see Figure 5).45 Upon reduction, the MLCT bands at 370 and 500 nm increase slightly in intensity and shift to lower energies.

Figure 3. Cyclic voltammogram of [Ru2]2+ in MeCN/0.1 M NBu4PF6, scan rate: 100 mV/s. The first oxidation and the first two reduction processes are reversible in all cases presented herein.

Figure 5. UV−vis-NIR absorption spectra of native [Ru1]2+ and the in situ generated oxidized [Ru1]3+ and reduced [Ru1]1+, measured in MeCN/0.1 M NBu4PF6. Figure 4. Cyclic voltammograms of the presented complexes in MeCN/0.1 M NBu4PF6. Graphs show the reversibility depending on the vertex potential.

When comparing the complexes [Ru1]3+ and [Ru2]3+, i.e., one or two MICs incorporated, the band related to the LMCT in the oxidized species is only observed for two MIC units, suggesting that the intensity of the transition is proportional to the donor strength of the ligand (Figure 6a). This idea could be supported in silico: spin population analysis of the respective oxidized species reveals the distribution of the spin density between the metal and the bi-MIC ligand. This is in accordance with the report of Brown et al.44 In the case of the pyridyl-MIC ligand, the spin is mostly located on the MIC unit, not the pyridine ring. EPR spectra of the one-electron oxidized forms of the ruthenium complexes were recorded in frozen glass of the respective acetonitrile solutions following electrolysis. The gvalues and the g-anisotropy (Figure 6) are generally compatible with a predominantly ruthenium-centered spin. For [Ru2]3+, the g∥ is unfortunately not resolved due to a poor signal-tonoise ratio (Figure 6c). Even though the parameters obtained from the spectra generally point toward a ruthenium-centered

has been previously described for ruthenium terpyridine systems and electron-donating ligand systems.36 The reversibility of the first oxidation and the first reduction was furthermore verified by UV−vis-NIR spectroelectrochemistry (for full description, see S17−S21). In the native form, the complexes exhibit MLCT bands in the region of 350−600 nm comparable to similar systems described elsewhere.41,43 Time-dependent density functional theory (TDDFT) calculations suggest that the ligand contribution to these transitions is fairly equally distributed between the three chelating ligands and can be described as d(Ru) → π*(bpy) and d(Ru) → π*(L) (see Figure S41). Looking at the molecular orbitals involved in the excitation of [Ru1]2+ and [Os1]2+, a slight contribution of a LLCT from the bi-MIC ligand to the delocalized system can be observed when compared to the pure MLCT of [Ru(bpy)3]2+ (vide infra, Figures 10 and S39). D


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Inorganic Chemistry

Figure 6. (a) UV−vis-NIR absorption spectra of in situ generated generated oxidized [Ru1]3+ and [Ru2]1+; (b) EPR spectrum at 103 K of [Ru1]3+ after electrolysis with g⊥ = 2.528, g∥ = 1.642, Δg = 0.886; (c) EPR spectrum at 95 K of [Ru2]3+ after electrolysis with g⊥ = 2.571 (green asterisk = organic radical, orange asterisk = cavity signal); all measurements were carried out in MeCN/0.1 M NBu4PF6.

vibrational progression corresponds to the ligand framework with ∼1350 and 1200 cm−1 for [Ru1]2+ and [Ru2]2+, respectively, and is comparable to similar systems.46,47 Table 2 summarizes the spectroscopic data obtained for the complexes. Quenching of the 3MLCT state can occur through nonradiative deactivation via a low-lying 3MC state that is thermally populated. The energy difference (ΔE) of the lowest 3 MLCT and that 3MC state is often correlated with the ligand field splitting in an octahedral geometry. Here, the strongly σdonating MIC ligands were anticipated to increase ΔE and to render this deactivation pathway less likely.48,49 The transient absorption spectroscopy reveals a decrease in the 3MLCT excited-state lifetime (τabs) for all MIC-containing complexes compared to their tris-bpy analogue at room temperature, although no correlation with the number of MIC units incorporated can be observed for the presented cases. At low temperature, the behavior changes again: complexes [Ru2]2+ and [Os2]2+, which showed the shortest lifetimes at room temperature, exhibit the longest lifetimes at 77 K. It is possible that this is due to a preferred thermal population of the 3MC state in these complexes, which is suppressed at low temperatures.49 For both metal centers, the incorporation of L2 leads to a decrease of more than 1 order of magnitude compared to the homoleptic bpy complexes. However, when comparing [Os2]2+ and [Os3]2+, which both bear a pyridylMIC ligand, the difference is again 1 order of magnitude. This shows again that the connectivity of the pyridyl- and MIC seems to have a strong influence on the electronic structure (vide supra). The quantum yields of the emission (Φem) at room temperature follow the same trend in that regard. The complexes coordinated with bi-MIC L1 exhibit similar E00 compared to the tris-bipyridine complexes, while pyridylMIC complexes have slightly higher 3MLCT energies. Using the potentials derived from cyclic voltammetry combined with the 3MLCT energy E00, the potentials for reductive and oxidative quenching of the 3MLCT excited state can be

spin, the observance of a signal with good intensity at 103 K for [Ru1]3+ and the differences in the intensity of the signals for the two cases (Figure 6b,c) are indirect indications of the involvement of the MIC ligands at the SOMO (see discussions on the theoretical results below).

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PHOTOPHYSICS Time-resolved absorption and emission spectroscopy were employed to further explore the influence of the MIC-ligand on the electronic structure. The steady-state absorption spectra (298 K) and respective emission spectra (77 K) are depicted in Figure 7: although the absorption maxima for the MLCT

Figure 7. Normalized absorption (solid line, MeCN, room temp.) and emission spectra (dashed line, butyronitrile, 77 K) of the presented complexes.

absorption are quite similar, a bathochromic shift of the emission can be observed with the increase of MIC units at 77 K. The emission spectra of the ruthenium complexes show a vibrational structure typical for polypyridine systems.46 The Table 2. Spectroscopic Data of the Presented Complexesa λem/nm

complex 2+

[Ru(bpy)3] [Ru1]2+ [Ru2]2+ [Os(bpy)3]2+ [Os1]2+ [Os2]2+ [Os3]2+

50

611 684 684 74351 792 782 764

τabs/ns 50

890, 301 125 6051 16 2.2 29

Φem/% 32

530

5.9 ± 0.2 1.43 ± 0.1 0.35 ± 0.02 0.551 0.13 ± 0.03 0.015 ± 0.003 0.22 ± 0.04 50

τem/μsb

E00/eV 43

2.10 2.09 2.17 1.6751 1.76 1.81 1.81

4.4 6.1 0.56 0.32 (31%), 0.95 (69%)c 0.88

a

Measured in dry and de-aerated MeCN at room temperature. bMeasured in deaerated butyronitrile at 77 K. cFitted as a biexponential decay. E


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Inorganic Chemistry estimated as shown in the general Latimer diagram (Table 3). The combination of all of the above-mentioned data is generally used to gauge the possible use as a photosensitizer or photocatalyst as illustrated in Figure 8. Table 3. Latimer-Diagram with Relevant Potentials for Photoexcitation, Given vs. SCE (in MeCN at room temp)a

complex

Ered/V

Eox/V

*Ered/V

*Eox

E00/eV

[Ru(bpy)3]2+ [Ru1]2+ [Ru2]2+ [Os(bpy)3]2+ [Os1]2+ [Os2]2+ [Os3]2+

−1.3443 −1.54 −1.34 −1.2839 −1.42 −1.29 −1.38

1.2743 0.87 1.16 0.8339 0.57 0.77 0.72

−0.83 −1.22 −1.01 −0.84 −1.19 −1.04 −1.09

0.76 0.55 0.83 0.39 0.34 0.52 0.43

2.1043 2.09 2.17 1.6750 1.76 1.81 1.81

Figure 9. Determination of the character of the first optical band at 406 nm for [Ru(bpy)3]2+. Top panel: the hole (gray) and particle (blue) densities obtained by NTO analysis are seen to be isotropic and strongly localized on the metal and the bipyridine ligands, respectively. Bottom panel: schematic representation of the transition and fractional charge transfer numbers calculated using eq 1. Upon optical excitation at 406 nm, the ruthenium center (circle) transfers electron density to the three bipyridine ligands (triangles labeled {L1, L2, L3}) isotropically.

a

Potentials converted with FcH/FcH+ vs SCE = 0.40 V in MeCN/0.1 M NBu4PF6.40

The numbers are reported in the cartoon in the bottom panel of Figure 9, which account for about 72% of the transition character. The metal-to-ligand character is seen to be isotropic toward all three bipyridine ligands. The particle/hole densities for the complex [Ru1]2+ are shown in Figure 10 for four components contributing about equally to the lowest optical band in the theoretical spectrum. Here again, the band is slightly blue-shifted with respect to the experimental value at 457 nm. For the two contributions at 448 and 427 nm, the particle is seen to be localized on the two bipyridine ligands (labeled L2 and L3), and the hole density is mostly localized on the metal center, with some contribution on the bi-MIC ligand L1 (labeled L1). The fractional charge transfer numbers reported in the bottom panel confirm the strong MLCT character toward the bipyridine ligands. It is to note that L1 is stabilized by π−π interactions with the bipyridine rings, and the structure of the complex is therefore slightly distorted (vide supra). The preference for either one of the L2 and L3 ligands in the two states leads to a small energy splitting and to a more pronounced role of bi-MIC L1 in the stabilization of the hole density for the band at 427 nm. With a fractional charge transfer number of 0.64, the band at 413 nm has the strongest MLCT character toward the bi-MIC (rectangle labeled L1), with most of the particle density located on the two central five-membered rings. The benzene rings of L1 again do not contribute to the character of this optical transition. The enhanced particle density on the new ligand renders this transition attractive for photoinduced catalysis on this site, provided the energetic ordering can be tuned such that it is found in a long-lived state at the bottom of the 3MLCT band. An undesirable feature of this contribution to the optical spectrum is the relatively large hole density located on the ligand, which could lead to rapid recombination with the

Figure 8. Potentials of different ruthenium and osmium complexes in acetonitrile for potential photocatalytic applications (data for the trisbipyridine analogue were published elsewhere).43,50

The results show that [Ru(bpy)3]2+ and [Ru1]2+ perform very differently in spite of a similar E00. The variation in the potentials of the osmium complexes is much more subtle compared to the ruthenium compounds. Nevertheless, Figure 8 illustrates the broad potential range for photocatalytic applications.

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DENSITY FUNCTIONAL THEORY Figure 9 shows the particle/hole densities for [Ru(bpy)3]2+ obtained by natural transition orbital analysis. The lowest optical band in the theoretical spectrum is composed of a single dominant contribution at 406 nm, slightly blue-shifted compared with the experimental value at 447 nm. The transition reveals a strong metal-to-ligand character, as indicated by the fractional charge transfer numbers calculated using eq 1. F


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Inorganic Chemistry

Figure 10. Determination of the character of the first optical band in the visual region for the [Ru1]2+ complex. The band is composed of four dominant transitions at 448, 427, 413, and 409 nm, depicted from left to right. Top panel: hole (gray) and particle (blue) densities. Bottom panel: schematic representation of the transition and fractional charge transfer numbers calculated using eq 1. See text for more details.

flux density yields spatially resolved information about the flow of electrons during the excitation process. This is complementary to the NTO analysis, which provides knowledge of the final static properties of the excited states. Knowledge of the electron reorganization can help to predict the sites that are likely to affect the excited-state properties of the complex upon chemical substitution. The electronic flux densities for the two most promising transitions are shown in Figure 12.

particle density. A better charge separation is observed on the new ligand for the transition at 409 nm, although the MLCT character favors electron transfer to the bipyridine ligands (respectively 0.26 and 0.37 for L2 and L3) over L1 (fractional charge transfer numbers: 0.08). In general, it can still be inferred that the two contributions at higher energy in the lowest-lying optical band will present the larger propensity to react on the new ligand. To confirm this hypothesis, the spin densities are depicted in Figure 11 for the oxidized species of both the [Ru(bpy)3]2+

Figure 12. Electronic flux densities for the transitions of [Ru1]2+ at (a) λ = 413 nm and (b) λ = 409 nm. The ligands are separated according to a Voronoi partitioning technique and color coded (L1 = green (L1), L2 = orange, L3 = red (both bpy)) to facilitate visualization.

Figure 11. Comparison of the spin density for the oxidized species of (a) [Ru(bpy)3]2+ and (b) [Ru1]2+. The red and green isosurfaces represent regions of density increase and depletion, respectively.

The vector plots of the electronic flux densities are colored according to the different ligands, which are partitioned using Voronoi tessellation of the observation grid. Contrary to the static picture proposed by the NTO, both transitions exhibit a large degree of π delocalization and conjugation over the whole complex, which extends from the bipyridine ligands L2 and L3 to the new ligand L1. It can be inferred that substitution in the 4-position of the pyridine with an electron donating group will influence mostly the contribution at λ = 409 nm. The likely outcome would be an energetic increase of the transition and possibly a larger MLCT character toward the new ligand L1. Similarly, substitution on the ortho-position of the pyridine-N is likely to affect the flow of electron in both transitions. Finally, it can be recognized that the electrons flow asymmetrically on ligand L1 and that the phenyl rings are not directly involved in the electronic transition. On the one hand, symmetric substitution on these phenyl units using either bulky substituents or electron donating/withdrawing groups is expected to affect mostly the energy of the transitions. This would allow isolation of these contributions

and the [Ru1]2+ compounds. In the former, the region of spin density enhancement is predominantly localized on the metal center, with some weak features appearing symmetrically at the anchoring atoms of the bipyridine ligands. Substitution by the bicarbene L1 has a drastic effect on the spin density, which is seen in panel (b) as being localized on the outer nitrogen atoms of the triazolylidene rings. This is a strong indication that these sites are likely to be reactive in a catalytic context under oxidizing conditions. Further, the spin density bears strong similarity with the contribution at 413 nm in the first optical band. This indicates that the reactive nitrogen sites on ligand L1 can potentially be efficiently activated by blue light. The other contributions at 448, 427, and 409 nm will provide competition for the light harvesting step. In order to improve the selectivity and the efficiency of the light-induced charge transfer, and potentially improve the ensuing catalytic activity of the new ligand therewith, it is instructive to look at the electronic flux densities for the optically active transitions at 409 and 413 nm. The electronic G


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Inorganic Chemistry

pseudoreference electrode. The ferrocene/ferrocenium couple was used as internal reference. UV/vis/NIR Spectroelectrochemistry. UV/vis/NIR spectra were recorded with an Avantes spectrometer consisting of a light source (AvaLight-DH-S-Bal), a UV/vis detector (AvaSpec-ULS2048), and an NIR detector (AvaSpec-NIR256-TEC). Spectroelectrochemical measurements were carried out in an optically transparent thinlayer electrochemical (OTTLE)56 cell (CaF2 windows) with a goldmesh working electrode, a platinum-mesh counter electrode, and a silver-foil pseudoreference electrode. Electron Paramagnetic Resonance. EPR spectra at X-band frequency (ca. 9.5 GHz) were obtained with a Magnettech MS-5000 benchtop EPR spectrometer equipped with a rectangular TE 102 cavity and TC HO4 temperature controller. The measurements were carried out in synthetic quartz glass tubes. For EPR spectroelectrochemistry, a three-electrode setup was employed using two Tefloncoated platinum wires (0.005″ bare, 0.008″ coated) as working and counter electrode and a Teflon-coated silver wire (0.005″ bare, 0.007″ coated) as pseudoreference electrode. Solutions of the complexes in acetonitrile were electrolyzed prior to data collection. Single-Crystal X-ray Diffraction. X-ray data were collected on a Bruker D8 Venture system at 100(2) K using graphite-monochromated MoΚα radiation (λα= 0.71073 Å). The strategy for the data collection was evaluated by using the APEX3 software. The data were collected by the standard omega + phi scan techniques and were scaled and reduced using Saint+ and SADABS software. The structures were solved by intrinsic phasing methods using SHELXT-2014/7. Structures were refined by full matrix least-squares using SHELXL-2014/7, refining on F2. Non-hydrogen atoms were refined anisotropically.57−61 In the cases of [Ru1](PF6)2 and [Os2](PF6)2, the contribution of disordered solvent molecules to the diffraction pattern was subtracted from the observed data by the “SQUEEZE” method as implemented in PLATON.62−65 CCDC 1571289 ([Ru1](PF6)2), 1817577 ([Ru2](PF6)2), 1571282 ([Ru(bpy)(L2)(DMSO)2](PF6)2), 1571290 ([Os1](PF6)2), 1571340 ([Os2](PF6)2), and 1571291 ([(cym)Os(L1)Cl](PF6)) contain the supplementary crystallographic data for this paper. Computational Details. The structure optimizations for the different metal complexes are performed using density functional theory (DFT) with the PBE0 functional66 and the def2-SVP basis set67,68 on all atoms. In addition, Grimme’s dispersion correction D3(BJ)69,70 and the continuum solvation model implemented in the COSMO-RS package71−74 are employed to include the influence of dispersion effects and the solvation effects of the acetonitrile solvent, respectively. The optical absorption spectra of the complexes are obtained from linear-response TDDFT calculations75 using the same PBE0 functional. All electronic structure calculations reported are performed with the TURBOMOLE program package.76 Natural transition orbitals (NTO)77 are calculated for each state contributing to the lowest energy absorption band in the visual region in order to reveal their associated excitonic character. The NTOs and the associated electronic current densities, as well as the spin densities are computed using the postprocessing toolbox ORBKIT.78−80 The particle and (ν) hole functions (φ(ν) p,i (rp) and φh,i (rh), respectively) of a given state ν are used to determine the state character as

from the less promising ones present in the band, which transfer mostly electrons to the bipyridines. On the other hand, unsymmetrical substitution on the same phenyl rings would offer a way of both controlling the energetics of the transitions and the electron flow in the new ligand upon light excitation. The asymmetry of the electron flow pattern implies that an electron withdrawing group placed on the phenyl ring to the right of the bicarbene L1 would potentially favor the flow toward this side and stabilize energetically the transition at 413 nm.

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CONCLUSION We presented the synthesis of five new triazolylidene complexes using ruthenium and osmium. These are the first examples of bi-MIC ligands incorporated in the [M(bpy)2(L)]2+ fragment and the first examples in general of osmium complexes of this type. Their properties were investigated via electrochemical and photophysical techniques and our studies reveal a significant contribution of the σdonating carbenes to the MLCT states that are observed for these compounds. This shows that the energy of the HOMO is tunable by the choice of the ligand, while maintaining excitedstate lifetimes of up to 300 ns and decent luminescence quantum yields. The comparison between ruthenium and osmium complexes reveals that the general effect of the ligand can be transferred between the higher homologue, suggesting a general effect of this type of ligand. TDDFT calculations support these results. Electron flux density calculations on the excited state were performed for the first time on such complexes, and this method can be used to visualize the flow of electrons and holes in the excited state. Future efforts will deal with introducing a higher number of mesoionic carbene ligands into such metal complexes and investigating their redox and photophysical properties.

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EXPERIMENTAL SECTION

General Remarks and Instrumentation. Unless otherwise noted, all reactions were carried out using standard Schlenk-line techniques under an inert atmosphere of argon (Linde, HiQ Argon 5.0, purity ≥99.999%). Compounds: [(cym)OsCl2]2,52 Ag2O,53 [H2L1](BF4)2,31 [HL2]BF4,31 [HL3]BF4,54 (cym)Ru(L1)Cl](PF6),31 (cym)Ru(L2)Cl](PF6),31 and (cym)Os(L3)Cl](PF6)33 were synthesized following published procedures. Commercially available chemicals were used without further purification. Dry DMF was available from Acros Organics (99.8% extra dry) and was used as received. Other dry solvents were available from MBRAUN MB-SPS800 solvent system. All solvents were degassed by standard techniques prior to use. Column chromatography was conducted using aluminum oxide (Aluminum Oxide neutral 90, Macherey-Nagel, 50−200 μm). 1 H NMR and 13C{1H} NMR were recorded on JEOL ECS 400 spectrometer and JEOL ECZ 400R spectrometer at 20 °C. Chemical shifts are reported in ppm (relative to the TMS signal) with reference to the residual solvent peaks.55 Multiplets are reported as follows: singlet (s), duplet (d), triplet (t), quartet (q), quintet (quint), septet (sept), and combinations thereof. Mass spectrometry was performed on an Agilent 6210 ESI-TOF. Electrochemistry. Cyclic voltammograms were recorded with a PAR VersaStat 4 potentiostat (Ametek) by working in anhydrous and degassed DMF (99.8% extra dry, Acros Organics) or acetonitrile (freshly distilled from phosphorus pentoxide, VWR) with 0.1 M NBu4PF6 (dried, >99.0%, electrochemical grade, Fluka) as electrolyte. Concentrations of the complexes were about 1 × 10−3 M. A threeelectrode setup was used with a glassy carbon working electrode, a coiled platinum wire as counter electrode, and a coiled silver wire as a

Pa(ν→) b =

∑ σi(ν)( ∫ φh,(νi )(rh)Pâ φh,(νi )(rh)drh) i

(∫φ

)

(ν) (r )P ̂ φ(ν)(r )drp p, i p b p, i p

(1)

̂ where P(ν) a→b is the fractional charge transfer number. The operator Pa/b is a Mulliken projector on the metal (M) or one of the ligands (Ln, n = is the weight of the particle/hole pair for state ν. {1, 2, 3}), and σ(ν) i Photophysical Measurements. Steady-state luminescence spectra and luminescence quantum yields were measured using a Fluorolog-3-22 instrument from Horiba Jobin-Yvon. Transient absorption measurements were performed using the LP920-KS H


Article

Inorganic Chemistry

solution of [Ru2](PF6)2 in acetone at 4 °C. 1H NMR (400 MHz, acetone-d6): δ (ppm) 8.82−8.73 (m, 3H, pyridine-H), 8.66−8.60 (m, 2H, pyridine-H), 8.36−8.32 (m, 2H, pyridine-H), 8.24−8.12 (m, 3H, pyridine-H), 8.09−7.97 (m, 3H, pyridine-H), 7.75 (t, J = 1.0 Hz, 1H, pyridine-H), 7.64−7.48 (m, 10H, phenyl-H), 7.20−7.17 (m, 1H, pyridine-H), 7.06−7.01 (m, 5H, pyridine-H). 13C{1H} NMR (100 MHz, acetone-d6): δ (ppm) 180.2 (all Ccarbene), 158.2, 157.8, 157.8, 156.7, 156.3, 154.0, 153.0, 152.9, 152.0, 150.7, 149.1, 141.1, 139.3, 138.5, 138.1, 137.6, 135.9, 132.2, 130.8, 130.4, 130.3, 129.5, 128.8, 128.7, 128.3, 127.8, 127.8, 127.6, 126.6, 125.2, 125.0, 124.5, 124.2, 115.8 (all Caryl). MS (HR-ESI): m/z [M − PF6]+ found 857.1288 calc. 857.1273. General Procedure for Osmium Bipyridine Complexes. In a 50 mL-Schlenk flask, the respective (cym)Os(L)Cl]PF6 (1 equiv) and bipyridine (2 equiv) were dissolved in degassed ethylene glycol (5 mL). The flask was capped and the mixture was heated to 150 °C for 3 days. The resulting black solution was treated with saturated aqueous KPF6 and extracted with DCM (3 × 20 mL). The organic phase was washed with water (5 × 50 mL) before it was dried (Na2SO4) and the solvent removed under reduced pressure. The crude product was purified by column chromatography (aluminum oxide, neutral, activated with 5 w% water, eluent: DCM/acetone/ MeOH v/v/v 100:0:0 → 100:15:0 → 100:15:2) to give the products as dark brown or green solids. [Os1](PF6)2. From [(cym)Os(L1)Cl](PF6) (0.15 mmol, 123.3 mg) and bipyridine (0.30 mmol, 46.8 mg) as a dark brown solid (19%, 0.03 mmol, 31.6 mg). Single crystals suitable for X-ray diffraction were grown by slow diffusion of n-hexane into a solution of [Os1](PF6)2 in acetone. 1H NMR (400 MHz, acetone-d6): δ (ppm) 8.55 (d, J = 5.5 Hz, 2H, pyridine-H), 8.50 (d, J = 8.1 Hz, 2H, pyridine-H), 8.25 (d, J = 8.1 Hz, 2H, pyridine-H), 7.92 (t, J = 7.6 Hz, 2H, pyridine-H), 7.55 (t, J = 7.5 Hz, 2H, pyridine-H), 7.44 (t, J = 6.3 Hz, 2H, pyridine-H), 7.23 (t, J = 7.5 Hz, 2H, aryl-H), 7.10 (t, J = 8.0 Hz, 4H, pyridine-H), 7.01−6.98 (m, 6H, aryl-H), 6.85 (t, J = 6.4 Hz, 4H, pyridine-H), 4.86 (s, 3H, N−CH3). 13C{1H} NMR (100 MHz, acetone-d6): δ (ppm) 169.7 (Ccarbene), 159.5, 158.0, 156.4, 148.9, 145.6, 139.4, 136.9, 136.2, 131.0, 130.2, 128.8, 128.2, 126.5, 124.8, 124.0 (all Caryl), 40.4 (N−CH3). MS (HR-ESI): m/z [M − PF6]+ found 965.2119 calc. 965.2063, [M − (PF6)2]2+ found 410.1231 calc. 410.1208. [Os2](PF6)2. From [(cym)Os(L2)Cl](PF6) (0.10 mmol, 80.0 mg) and bipyridine (0.20 mmol, 31.0 mg) as a dark green solid (46%, 0.05 mmol, 46.0 mg). Single crystals suitable for X-ray diffraction were grown by slow diffusion of n-hexane into a solution of [Os2](PF6)2 in acetone. 1H NMR (400 MHz, acetone-d6): δ (ppm) 8.78−8.71 (m, 3H, aryl-H), 8.64−8.60 (m, 2H, pyridine-H), 8.39−8.35 (m, 1H, pyridine-H), 8.16−7.94 (m, 6H, aryl-H), 7.80−7.78 (m, 1H, pyridineH), 7.64−7.44 (m, 11H, aryl-H), 7.20−7.15 (m, 1H, pyridine-H), 7.06−6.94 (m, 5H, aryl-H). 13C{1H} NMR (100 MHz, acetone-d6): δ (ppm) 163.2 (Ccarbene), 160.1, 160.0, 159.7, 158.3, 156.4, 156.1, 152.6, 152.3, 151.3, 149.6, 147.0, 140.1, 138.9, 137.6, 137.5, 137.0, 135.9, 132.2, 130.7, 130.4, 130.3, 129.6, 129.5, 129.3, 129.0, 128.3, 128.0, 127.6, 126.5, 125.5, 125.2, 124.9, 124.6, 115.5 (all Caryl). MS (HRESI): m/z [M − PF6]+ found 947.0991 calc. 947.0845, [M − (PF6)2]2+ found 401.0575 calc. 401.1099. [Os3](PF6)2. From [(cym)Os(L3)Cl](PF6) (0.09 mmol, 70.0 mg) and bipyridine (0.19 mmol, 29.4 mg) as a dark green solid (14%, 0.01 mmol, 11 mg). 1H NMR (400 MHz, acetone-d6): δ (ppm) 8.76−8.69 (m, 2H, pyridine-H), 8.63−8.58 (m, 2H, pyridine-H), 8.46−8.44 (m, 1H, pyridine-H), 8.36−8.33 (m, 1H, pyridine-H), 8.06−7.86 (m, 6H, pyridine-H), 7.69−7.67 (m, 1H, pyridine-H), 7.62−7.58 (m,1H, pyridine-H), 7.57−7.54 (m, 1H, pyridine-H), 7.54−7.50 (m, 1H, pyridine-H), 7.47−7.42 (m, 2H, pyridine-H), 7.34−7.29 (m, 2H, arylH), 7.25−7.15 (m, 3H, aryl-H), 7.13−7.10 (m, 2H, pyridine-H), 6.97−6.93 (m, 1H, pyridine-H), 4.85 (s, 3H, N−CH3) 13C{1H} NMR (100 MHz, acetone-d6): δ (ppm) 169.5 (Ccarbene), 160.1, 159.5, 158.3, 156.7, 154.1, 153.4, 152.3, 151.4, 151.1, 149.3, 139.4, 138.8, 138.2, 137.29, 137.2, 136.5, 131.0, 130.3, 129.4, 129.3, 128.8, 128.3, 126.4, 126.1, 125.4, 125.1, 124.8, 124.6, 122.7 (all Caryl), 39.6 (N-CH3). MS

spectrometer from Edinburgh Instruments equipped with a Quantel Brilliant b laser as an excitation source. Synthesis. General Procedure for p-Cymene Osmium Complexes. The complexes were synthesized following a similar procedure as was previously published.15 In a 100 mL-Schlenk flask, the respective triazolium salt (1.00 mmol, 1.0 equiv), silver oxide (3.50 mmol, 3.5 equiv), and potassium chloride (10.0 mmol, 10 equiv) were suspended in acetonitrile (10 mL) and stirred at room temperature and under exclusion of light for 2 days. Then, [(cym)OsCl2]2 (0.50 mmol, 0.5 equiv) was added, and the mixture was stirred for another 5 days. The mixture was then filtered over a pad of Celite, and the solvent was reduced to a few milliliters before aqueous KPF6 was added to induce precipitation of the crude product. This was then loaded onto a column (aluminum oxide, neutral, activated with 5 w% water, eluent: DCM/acetone v/v 10:0 → 10:1) to give the products as yellow to orange solids. [(cym)Os(L1)Cl](PF6). Yield: 61%, yellow solid. Single crystals suitable for X-ray diffraction were grown by slow diffusion of n-hexane into a solution of complex in DCM. 1H NMR (400 MHz, CDCl3): δ (ppm) 7.99−7.98 (m, 4H, phenyl-H), 7.70−7.64 (m, 6H, phenyl-H), 4.71 (d, J = 5.7 Hz, 2H, Cym-H), 4.63 (s, 6H, N−CH3), 4.56 (d, J = 5.7 Hz, 2H, Cym-H), 2.08−2.00 (s + p, 4H, Cym−CH3 + Cym−CH overlaid), 0.77 (d, J = 6.9 Hz, 6H, Cym−CH3). 13C{1H} NMR (100 MHz, CDCl3): δ (ppm) 161.0 (Ccarbene), 141.3, 138.3, 131.1, 129.7, 126.3, 81.2, 78.8 (Caryl), 40.0 (N−CH3), 31.2, 22.9, 18.3 (CCym‑alkyl). MS (HR-ESI): m/z [M − PF6]+ found 677.1900 calc. 677.1830. [(cym)Os(L2)Cl](PF6). Yield: 64%, orange solid. Single crystals suitable for X-ray diffraction were grown by slow diffusion of n-hexane into a solution of complex in DCM, with one molecule of DCM incorporated per asymmetric unit. 1H NMR (400 MHz, CDCl3): δ (ppm) 9.24 (d, J = 5.8 Hz, 1H, pyridine-H), 8.27 (d, J = 7.8 Hz, 1H, pyridine-H), 8.13 (td, J = 7.5, 1.4 Hz, 1H, pyridine-H), 7.64−7.48 (m, 11H, aryl-H), 5.62 (d, J = 5.7 Hz, 1H, Cym-H), 5.57 (d, J = 5.8 Hz, 1H, Cym-H), 5.52 (d, J = 5.8 Hz, 1H, Cym-H), 5.28 (d, J = 5.8 Hz, 1H, Cym-H), 2.51 (sept, J = 6.7 Hz, 1H, Cym−CH), 1.07 (d, J = 6.9 Hz, 3H, Cym−CH3), 1.02 (d, J = 6.8 Hz, 3H, Cym−CH3). 13C{1H} NMR (100 MHz, acetone-d6): δ (ppm) 158.0 (Ccarbene), 151.5, 147.6, 143.1, 135.7, 132.6, 131.6, 130.7, 130.1, 128.3, 127.5, 126.8, 125.9, 119.8, 115.3, 100.4, 98.0, 82.8, 79.7, 78.2, 74.8 (Caryl), 31.8, 22.9, 22.3, 18.7 (CCym‑alkyl). MS (HR-ESI): m/z [M − PF6]+ found 659.1699 calcd. 659.1612. General Procedure for Ruthenium Bipyridine Complexes. In a 50 mL-Schlenk flask, the respective (cym)Ru(L)Cl]PF6 (1 equiv), bipyridine (2 equiv), and AgPF6 (2 equiv) were dissolved in degassed ethylene glycol (5 mL). The flask was capped and the mixture was heated to 150 °C for 12 h. The resulting red solution was treated with saturated aqueous KPF6 and extracted with DCM (3 × 20 mL). The organic phase was washed with water (5 × 50 mL) before it was dried (Na2SO4) and the solvent removed under reduced pressure. The crude product was purified by column chromatography (aluminum oxide, neutral, activated with 5 w% water, eluent: DCM/acetone v/v 100:0 → 100:15) to give the products as red solids. [Ru1](PF6)2. From [(cym)Ru(L1)Cl](PF6) (0.14 mmol, 100 mg), bipyridine (0.29 mmol, 44.8 mg), and AgPF6 (0.29 mmol, 72.2 mg) as a dark red solid (73%, 0.10 mmol, 102 mg). Single crystals suitable for X-ray diffraction were grown by slow diffusion of diethyl ether into a solution of complex in acetone. 1H NMR (400 MHz, acetone-d6): δ (ppm) 8.53−8.49 (m, 4H, pyridine-H), 8.25 (d, J = 8.1 Hz, pyridineH), 8.09 (td, J = 7.9, 1.0 Hz, pyridine-H), 7.70 (td, J = 8.0, 1.1 Hz, pyridine-H), 7.53−7.50 (m, 2H, pyridine-H), 7.27−7.23 (m, 2H, pyridine-H), 7.16−7.09 (m, 6H, aryl-H), 7.03−7.01 (m, 4H, aryl-H), 6.95−6.92 (m, 2H, aryl-H), 4.87 (s, 6H, N−CH3). 13C{1H} NMR (100 MHz, acetone-d6): δ (ppm) 187.8 (all Ccarbene), 157.7, 156.2, 156.0, 149.7, 142.6, 139.3, 137.3, 137.1, 131.0, 130.1, 127.9, 127.5, 126.4, 124.4, 123.7 (all Caryl), 40.5 (N−CH3). MS (HR-ESI): m/z [M − PF6]+ found 875.1619 calc. 875.1419. [Ru2](PF6)2. From [(cym)Ru(L2)Cl](PF6) (0.06 mmol, 40.0 mg), bipyridine (0.11 mmol, 17.5 mg), and AgPF6 (0.11 mmol, 28.2 mg) as a red solid (71%, 0.04 mmol, 39.8 mg). Single crystals suitable for Xray diffraction were grown by slow diffusion of diethyl ether into a I


Article

Inorganic Chemistry (HR-ESI): m/z [M − PF6]+ found 885.1720 calc. 885.1688, [M − (PF6)2]2+ found 370.1060 calc. 370.1020.

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Bodipy Chromophore: Synthesis and Monitoring of Ligand Exchange Reactions. Organometallics 2017, 36, 1469−1478. (9) Hettmanczyk, L.; Spall, S. J. P.; Klenk, S.; van der Meer, M.; Hohloch, S.; Weinstein, J. A.; Sarkar, B. Structural, Electrochemical, and Photochemical Properties of Mono- and Digold(I) Complexes Containing Mesoionic Carbenes. Eur. J. Inorg. Chem. 2017, 2112− 2121. (10) Schulze, B.; Escudero, D.; Friebe, C.; Siebert, R.; Görls, H.; Köhn, U.; Altuntas, E.; Baumgaertel, A.; Hager, M. D.; Winter, A.; Dietzek, B.; Popp, J.; González, L.; Schubert, U. S. A heteroleptic bis(tridentate) ruthenium(II) complex of a click-derived abnormal carbene pincer ligand with potential for photosensitzer application. Chem. - Eur. J. 2011, 17, 5494−5498. (11) van der Meer, M.; Glais, E.; Siewert, I.; Sarkar, B. Electrocatalytic Dihydrogen Production with a Robust Mesoionic Pyridylcarbene Cobalt Catalyst. Angew. Chem., Int. Ed. 2015, 54, 13792−13795. (12) Hettmanczyk, L.; Suntrup, L.; Klenk, S.; Hoyer, C.; Sarkar, B. Heteromultimetallic Complexes with Redox-Active Mesoionic Carbenes: Control of Donor Properties and Redox-Induced Catalysis. Chem. - Eur. J. 2017, 23, 576−585. (13) Klenk, S.; Rupf, S.; Suntrup, L.; van der Meer, M.; Sarkar, B. The Power of Ferrocene, Mesoionic Carbenes, and Gold: RedoxSwitchable Catalysis. Organometallics 2017, 36, 2026−2035. (14) Vanicek, S.; Podewitz, M.; Stubbe, J.; Schulze, D.; Kopacka, H.; Wurst, K.; Müller, T.; Lippmann, P.; Haslinger, S.; Schottenberger, H.; et al. Highly Electrophilic, Catalytically Active and RedoxResponsive Cobaltoceniumyl and Ferrocenyl Triazolylidene Coinage Metal Complexes. Chem. - Eur. J. 2018, 24, 3742−3753. (15) Thompson, D. W.; Ito, A.; Meyer, T. J. [Ru(bpy)3]2+* and other remarkable metal-to-ligand charge transfer (MLCT) excited states. Pure Appl. Chem. 2013, 85, 1257. (16) Li, J.; Wang, E. Applications of tris(2,2’-bipyridyl)ruthenium(II) in electrochemiluminescence. Chem. Rec. 2012, 12, 177−187. (17) Zeitler, K. Photoredox catalysis with visible light. Angew. Chem., Int. Ed. 2009, 48, 9785−9789. (18) Campagna, S.; Puntoriero, F.; Nastasi, F.; Bergamini, G.; Balzani, V. Photochemistry and Photophysics of Coordination Compounds: Ruthenium. In Photochemistry and Photophysics of Coordination Compounds; Balzani, V., Campagna, S., Accorsi, G., Eds.; Topics in Current Chemistry 280−281; Springer: Berlin, NY, 2007; pp 117−214. (19) Elliott, C. M.; Hershenhart, E. J. Electrochemical and spectral investigations of ring-substituted bipyridine complexes of ruthenium. J. Am. Chem. Soc. 1982, 104, 7519−7526. (20) Glazer, E. C.; Magde, D.; Tor, Y. Ruthenium complexes that break the rules: Structural features controlling dual emission. J. Am. Chem. Soc. 2007, 129, 8544−8551. (21) Bomben, P. G.; Robson, K. C. D.; Sedach, P. A.; Berlinguette, C. P. On the viability of cyclometalated Ru(II) complexes for lightharvesting applications. Inorg. Chem. 2009, 48, 9631−9643. (22) Ngo, K. T.; Lee, N. A.; Pinnace, S. D.; Szalda, D. J.; Weber, R. T.; Rochford, J. Probing the Noninnocent π-Bonding Influence of NCarboxyamidoquinolate Ligands on the Light Harvesting and Redox Properties of Ruthenium Polypyridyl Complexes. Inorg. Chem. 2016, 55, 2460−2472. (23) Chábera, P.; Liu, Y.; Prakash, O.; Thyrhaug, E.; Nahhas, A. E.; Honarfar, A.; Essen, S.; Fredin, L. A.; Harlang, T. C. B.; Kjaer, K. S.; et al. A low-spin Fe(iii) complex with 100-ps ligand-to-metal charge transfer photoluminescence. Nature 2017, 543, 695−699. (24) Sarkar, B.; Suntrup, L. Illuminating Iron: Mesoionic Carbenes as Privileged Ligands in Photochemistry. Angew. Chem., Int. Ed. 2017, 56, 8938−8940. (25) Hohloch, S.; Suntrup, L.; Sarkar, B. Arene−Ruthenium(II) and − Iridium(III) Complexes with “Click”-Based Pyridyl-triazoles, Bistriazoles, and Chelating Abnormal Carbenes: Applications in Catalytic Transfer Hydrogenation of Nitrobenzene. Organometallics 2013, 32, 7376−7385.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b02551. NMR spectra; electrochemistry measurements; photophysical measurements; crystallographic details and molecular structures; density functional theory (PDF) Accession Codes

CCDC 1571282, 1571289−1571291, 1571340, and 1817577 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

ORCID

Gunter Hermann: 0000-0002-0705-2028 Oliver S. Wenger: 0000-0002-0739-0553 Biprajit Sarkar: 0000-0003-4887-7277 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Dr. Lukas Eugen Marsoner Steinkasserer and Dr. Vincent Pohl are kindly acknowledged for fruitful discussions regarding the theoretical description of the presented complexes. This work was supported by the Deutsche Forschungsgemeinschaft [DFG, Priority Program SPP 2102, “Light-controlled reactivity of metal complexes” (SA 1840/7-1)].

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REFERENCES

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