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Ruthenium(II) Bipyridyl Complexes with Cyclometalated NHC Ligands David Schleicher,† Hendrik Leopold,† Horst Borrmann,‡ and Thomas Strassner*,† †

Physikalische Organische Chemie, Technische Universität Dresden, 01069 Dresden, Germany Max-Planck-Institut für Chemische Physik fester Stoffe, 01187 Dresden, Germany

‡

S Supporting Information *

ABSTRACT: We present the synthesis and characterization of novel cyclometalated ruthenium N-heterocyclic carbene (NHC) complexes of the general formula [Ru(C^C*)(bpy)2]PF6 (bpy = 2,2′-bipyridine), with the C^C* ligand being based on different 1phenylimidazoles. They were synthesized in a one-pot procedure starting from the corresponding p-cymene NHC complexes [Ru(C^C*)(p-cymene)Cl]. Their structural, spectroscopic, and electrochemical properties were investigated by NMR, X-ray, UV/vis, and CV, as well as density functional theory methods. Because of the stronger electron-donating carbene ligands, these complexes represent a new class of bisheteroleptic dyes with improved photophysical and electrochemical properties.

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INTRODUCTION Ruthenium complexes play an important role in Grätzel-type dye-sensitized solar cells (DSSCs), which were introduced over 25 years ago.1 A detailed overview on the principles and recent advances is given in reviews by Grätzel and others.2−7 Improving the long-term stability, cost-effectiveness, and overall efficacy of these cells has been the goal of recent research efforts. Important advances and developments are the use of perovskites instead of titania8 for the semiconductor anode, replacement of typical organic solvents like acetonitrile by ionic liquids,9−14 introduction of one-electron redox couples15−21 like CoII/CoIII, and platinum-free cathodes.22 Only some work has been devoted to the arguably most important part, the dye. These ruthenium complexes werefor many yearsalmost exclusively polypyridine complexes with NCS− auxiliary ligands like the well-known bidentate N3/N719 or the tridentate N749 (“black dye”). On the basis of these general motifs, a plethora of bis- or trisheteroleptic sensitizers has been developed, with the intention of improving the absorption properties23−32 (e.g., by enlarging the ligand’s πsystem), hydrophobicity4,33−40 (by adding alkyl chains), or efficient charge injection41−51 (by optimizing the number and nature of anchor groups). More than 15 years after the introduction of DSSCs and the corresponding ruthenium dyes, the group of van Koten published the first example52 of a ruthenium sensitizer with cyclometalating ligands, which opened a completely new area of research. Cyclometalating ligands replacing the thiocyanates have several advantages. First, the monodentate NCS− itself is quite labile and causes degeneration of the dye over the course of time.53−55 Therefore, substitution of two monodentate ligands by one chelating cyclometalating ligand (see Figure 1, © 2017 American Chemical Society

Figure 1. Evolution of ruthenium complexes with ligands bearing different donor atoms (see text for details).

left-hand side) improves the overall stability of the complexes. Second, the phenyl moieties in these cyclometalating ligands offer the possibility of various substitution patterns, which allows for a versatile modification of the energy of the highest occupied molecular orbital (HOMO) situated on the anionic ligand. The HOMO level plays a fundamental role in the overall efficiency of the sensitizer. On the one hand, the HOMO/lowest unoccupied molecular orbital (LUMO) gap should be small enough to enable excitation by visible or even near-infrared light. On the other hand, a (too) high-lying HOMO impedes or even prevents an efficient regeneration of the dye by the redox couple (EI−/I2·− = 0.8 V vs normal hydrogen electrode (NHE)56,57 or EI−/I3− = 0.4 V vs NHE26). Received: April 2, 2017 Published: June 6, 2017 7217

DOI: 10.1021/acs.inorgchem.7b00831 Inorg. Chem. 2017, 56, 7217−7229

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Inorganic Chemistry

(especially in protic solvents) slowly decompose by turning blue over the course of hours or days.98,99 The NMR spectra of the cyclometalated carbene complexes 4−6 (Figures S1−S6) show that for the η6-ligand the mirror symmetry along the axis of the substituents is lost, leading to four different signals for the aromatic ring and two different signals for the methyl groups of the isopropyl substituent. The reaction of the C^C* ligand with the ruthenium precursor leads to the disappearance of the imidazolium NCHN proton as well as one ortho (to the imidazole ring) proton of the phenyl ring, the three remaining aromatic protons show an asymmetric coupling pattern with a 3J/4J doublet of doublets for the C4H atom. The C* and C− carbon atoms are strongly deshielded, which leads to a downfield shift (160−200 ppm) of their signals in the 13C spectrum. We were able to obtain single crystals of complexes 5 and 6 by slow diffusion of pentane into a solution of the complex in THF (5) or evaporation of a solution of the complex in acetone (6). The solid-state structures of the complexes are shown in Figures 2 and 3. They show the typical piano-stool geometry,

These aspects of cyclometalated ruthenium dyes have since then been extensively studied by the groups of van Koten,58−60 Berlinguette,61−72 and others.73−87 Finally, in 2010 the groups of Chung88 and Li89 introduced a new dye architecture by replacing N-donors with Nheterocyclic carbenes (NHC; see Figure 1, right-hand side). NHCs are well-known for their significant (charge-neutral) donor strength and chemical stability. This makes them ideal ligands for ruthenium dyes for the reasons discussed above. Compared to the weaker N-donors they further destabilize the HOMO, thereby increasing its energy level with little effect on the LUMO energy,90 thus decreasing the HOMO/LUMO gap. In recent years, more and more studies regarding the use of NHC ligands in ruthenium sensitizers occurred in the literature.71,89,91−94 Our group has a long-standing interest in metal complexes bearing NHC ligands, especially complexes with C^C* ligands based on a phenylimidazole structure. This motif combines both a carbanionic and a carbene moiety in one bidentate ligand (see Figure 1, center), and we were interested in the applicability in ruthenium dyes. (Note that Yam and co-workers had published ruthenium bipyridyl complexes bearing orthometalated aminocarbene ligands95−97 even long before the previously mentioned examples. However, these complexes were never examined regarding their suitability for DSSCs, and no follow-up work was published.) Here we present the synthesis and characterization of ruthenium complexes of the general formula [Ru(C^C*) (bpy)2]+PF6− (see Figure 1, center). This new type of dyes is investigated regarding its suitability as sensitizer by spectroscopic, electrochemical, as well as density functional theory (DFT) methods.

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RESULTS AND DISCUSSION Synthesis and Characterization. On the basis of our experience with cyclometalated ruthenium NHC complexes bearing p-cymene as a relatively labile η6-ligand,98 we used these as starting material for the synthesis of the dyes. Starting from imidazolium salts 1−3, p-cymene complexes 4−6 were synthesized by transmetalation from the in situ formed silver(I) complex in a one-pot procedure to the widely used [Ru(pcymene)Cl2]2 dimer (Scheme 1). They are obtained in good yields as yellow solids and as such are stable under ambient conditions for months. All p-cymene C^C* complexes are readily soluble in polar organic solvents such as dichloromethane, chloroform, acetone, acetonitrile, tetrahydrofuran (THF), methanol, ethanol, dimethylformamide (DMF), and dimethyl sulfoxide (DMSO), but their clear yellow solutions

Figure 2. ORTEP representation of 5 in the solid state. Thermal ellipsoids are drawn at the 50% probability level; H atoms are omitted for clarity. Selected bond lengths [Å], angles, and dihedral angles [deg]: Ru(1)−Cl(1) 2.4364(5); Ru(1)−C(1) 2.0087(15); Ru(1)− C(5) 2.0740(18); Ru(1)−centroid 1.740; C(1)−Ru(1)−C(5) 76.68(7); C(1)−N(1)−C(4)−C(5) 0.74(19); C(1)−N(2)−C(11)− C(12) 58.0(2).

which leads to a distorted pseudotetrahedral coordination sphere around the ruthenium center. The isopropyl group of the p-cymene ligand is located next to the C^C* ligand. The bite angles of the C^C* ligands are relatively small with ∼77°, which results in a “yaw” distortion100 at the carbene atoms of 9−10°. Comparing the bond lengths of the carbene carbon atom and the anionic carbene atom to the metal center it was found that the Ru−C1 bond is slightly shorter due to π-back-donation. These structural findings are in good agreement with previously reported structures.98,99,101−105 Details of the solid-state structure determination can be found in Tables S1 and S2. The synthesis of the bisheteroleptic complexes is described in Scheme 2. The corresponding p-cymene complexes are reacted with 2.2 equiv of 2,2′-bipyridine (bpy) in a DMSO solution at high temperatures in the dark. The bidentate ligand bpy replaces the η6-arene ligand as well as the chlorido ligand, leading to octahedral complexes of the form [Ru(C^C*) (bpy)2]Cl (racemic mixture of Δ and Λ isomers). The NMR spectra are given in Figures S7−S20. These cationic complexes with chloride counterions give deeply violet aqueous solutions, where an anion exchange can

Scheme 1. Synthesis of Complexesa 4, 5, and 6

a

Numbers indicate NMR signal assignments. 7218


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Figure 3. ORTEP representation of 6 in the solid state. Thermal ellipsoids are drawn at the 50% probability level; H atoms and one solvent molecule (acetone) are omitted for clarity. Selected bond lengths [Å], angles, and dihedral angles [deg]: Ru(1)−Cl(1) 2.4379(12); Ru(1)−C(1) 2.025(5); Ru(1)−C(9) 2.059(4); Ru(1)− centroid 1.742; C(1)−Ru(1)−C(9) 76.50(18); C(1)−N(1)−C(8)− C(9) 0.1(5).

Figure 4. ORTEP representation of 12 (only Λ isomer shown) in the solid state. Thermal ellipsoids are drawn at the 50% probability level; H atoms, the anion, and one solvent molecule (dichloromethane) are omitted for clarity. Selected bond lengths [Å], angles and dihedral angles [deg]: Ru(1)−C(1) 2.013(5); Ru(1)−C(5) 2.056(5); Ru(1)− N(3) 2.107(3); Ru(1)−N(4) 2.058(4); Ru(1)−N(5) 2.137(4); Ru(1)−N(6) 2.073(4); C(1)−Ru(1)−C(5) 78.9(2); C(1)−N(1)− C(4)−C(5) −1.4(6); C(1)−N(2)−C(11)−C(12) −97.3(6).

be performed using NH4PF6. After two filtration steps over diatomaceous earth and basic alumina, respectively (with no column chromatography being necessary), the analytically pure complexes are obtained in moderate to good yields (47−86%). They are readily soluble in polar organic solvents such as alcohols, acetone, dichloromethane, DMF, or DMSO, giving intensely colored deep red to violet solutions. These complexes are air-stable in the solid state as well as in solution over the course of months. Deeply colored violet crystals suitable for a solid-state structure determination could be obtained by slow diffusion of diethyl ether in a dichloromethane solution of complexes 12 and 17 as well as slow evaporation from a solution of complex 16 in an acetone/water mixture. Figures 4−6 show the solidstate structures of these complexes. In all structures, each of the three bidentate ligands coordinates with a bite angle smaller than 80°, causing a distortion of the octahedral geometry around the ruthenium center. The “yaw” distortion at the carbene atoms is slightly larger compared to the p-cymene complexes, ranging from 11° to 13°. The Ru−C1 and Ru−C5 bond lengths reveal similar distances compared to the pcymene complexes. Furthermore, by comparing the distances to the nitrogen atoms N3−N6, a clear trans-influence of the

strongly donating carbene carbon atom and especially of the anionic carbon atom can be observed, causing an elongation by 0.03−0.05 Å for N(3) and 0.06−0.08 Å for N(5), respectively. These findings are in good accordance with previously published structures of cyclometalated phenylpyridine106,107 and aminocarbene95−97 analogues. Experimental details of the solid-state structure determination can be found in Tables S3− S5. Spectroscopic and Electrochemical Properties. Complexes 11−17 show strong absorptions over a broad range of the ultraviolet and the visible spectrum (up to ∼650 nm; see Figure 7 for UV/vis spectra). The absorption spectra for these complexes show a strong resemblance with the related compound [Ru(bpy)2(ppy)]+ at higher wavelengths (450− 650 nm), consisting of two broad bands at 475−500 nm and 550−575 nm. The electron-withdrawing groups in complexes 11, 13, and 17 cause a slight hypsochromic shift of the absorption bands by 30−50 nm. In contrast, electron-donating groups (complexes 12, 15, 16) seem to have little to no bathochromic effect on the absorption of these complexes. The additional aryl substituent on the imidazole moiety in complex

Scheme 2. Synthetic Route to the Bisheteroleptic Complexesa

a

Numbers indicate NMR signal assignments (complexes 7−10 were previously reported98). 7219


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Figure 5. ORTEP representation of 16 (only Λ isomer shown) in the solid state. Thermal ellipsoids are drawn at the 50% probability level; H atoms, the anion, and one solvent molecule (acetone) are omitted for clarity. Selected bond lengths [Å], angles and dihedral angles [deg]: Ru(1)−C(1) 2.021(5); Ru(1)−C(5) 2.060(4); Ru(1)−N(3) 2.095(4); Ru(1)−N(4) 2.055(4); Ru(1)−N(5) 2.117(4); Ru(1)− N(6) 2.065(4); C(1)−Ru(1)−C(5) 79.17(18); N(3)−Ru(1)−N(4) 77.96(14); N(5)−Ru(1)−N(6) 77.26(15); C(1)−N(1)−C(4)−C(5) −4.3(6).

Figure 7. UV/vis spectra of complexes 11−17 and of [Ru(bpy)3]2+ and [Ru(bpy)2(ppy)]+ for comparison (0.1 mM solutions in acetonitrile).

Section for details and Supporting Information for xyz coordinates). A first indication of the nature of the transitions at higher wavelengths is given by the frontier molecular orbitals (FMOs), which are very similar for complexes 11−17. Figure 8 shows the Kohn−Sham FMOs for the unsubstituted complex 14. For comparison and prediction of the potential use of these complexes in DSSCs we also calculated the corresponding sensitizer with carboxy groups at the bipyridine ligands, i.e., the 4,4′-dicarboxy-2,2′-bipyridine analogue (14_dcbp). The orbital plots clearly show that the HOMOs are located at the metal center as well as on the cyclometalating moiety. Moreover, the LUMOs lie almost exclusively on the bipyridine ligands, and the absorptions above 500 nm can be attributed to MLCT (Ru→bpy) transitions. In agreement with literature data from the groups of Berlinguette65 and Castellano108 this reflects the small influence of the substituents at the cyclometalating moiety (vide supra). The lowering of the LUMO by the electron-withdrawing carboxylate groups in 14_dcbp, however, leads to a significant bathochromic shift of more than 50 nm in the absorption spectrum (see Figure S22 for details). Figure 9 shows a comparison of the convoluted TD-DFT and the experimental absorption spectra of complex 14. Although the calculation clearly underestimates the wavelengths of the energetically lower transitions by as much as 25− 50 nm, it can provide useful insight into their nature by visualizing the corresponding natural transition orbitals (NTOs). Table S6 gives additional information on the six lowest transitions (with f ≥ 0.01) and their corresponding “hole” and “particle” NTOs. A clear directional electron flow from the metal and the C^C* ligand to the auxiliary ligands can be observed, which is an ideal prerequisite for the desired electron transfer from the dye to the TiO2 substrate. We also performed electrochemical measurements via cyclic voltammetry (see Figures S23−S30). All complexes with the exception of 17 show the expected reversible oxidation band of the RuII/RuIII oxidation. For complex 17, carrying a redoxactive nitro substituent, we observed a (quasi-)irreversible oxidation. The potential of the oxidation waves at 0.6−0.8 V versus NHE is very similar to that of previously published phenylpyridine analogues,65,106−108 whereas for the orthometalated aminocarbenes an irreversible oxidation wave at 1.6−1.7 V versus NHE was reported.96 The HOMO of these complexes is mainly located at the metal center, and it is therefore possible

Figure 6. ORTEP representation of 17 (only Δ isomer shown) in the solid state. Thermal ellipsoids are drawn at the 50% probability level; H atoms, the anion, and one solvent molecule (dichloromethane) are omitted for clarity. Selected bond lengths [Å], angles and dihedral angles [deg]: Ru(1)−C(1) 2.039(6); Ru(1)−C(5) 2.057(6); Ru(1)− N(3) 2.102(4); Ru(1)−N(4) 2.053(4); Ru(1)−N(5) 2.119(4); Ru(1)−N(6) 2.059(4); C(1)−Ru(1)−C(5) 78.6(2); N(3)−Ru(1)− N(4) 77.83(17); N(5)−Ru(1)−N(6) 77.3(2); C(1)−N(1)−C(4)− C(5) 0.8(7).

12 marginally shifts the absorption spectrum to longer wavelengths. In general, the absorption of these complexes strongly resembles that of similar phenylpyridine complexes, where comparable absorption bands up to ∼630−650 nm and only minor influences of the substituents on the cyclometalating moiety were observed.63,107 The emission of these complexes is generally very poor. We recorded emission spectra exemplary for complexes 11 and 14−16 in deaerated acetonitrile solution (λexc = 550 nm; see Figure S21). All complexes show a single weak and broad emission band in the range of 700−850 nm with maxima of ∼775 nm for 11 and 16 and ∼800 nm for 14 and 15. This again is in strong resemblance to the phenylpyridine106,108 analogues as well as the orthometalated aminocarbene95−97 systems. To gain further insight into the absorption processes, we performed time-dependent (TD) DFT calculations on the optimized structures (B3LYP/6-31G(d); see Experimental 7220


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Figure 8. DFT-calculated energy levels (in relation to the experimental conduction band energy and potential of the iodide redox couples in DSSCs26) and selected plots of the Kohn−Sham FMOs of complex 14 and the hypothetical sensitizer 14_dcpb.

Figure 9. Comparison of experimental (blue line) and calculated (orange hashed line) UV/vis spectra as well as calculated oscillator strengths and main transitions of complex 14 (see Experimental Section for details on measurements and calculations).

to compare the experimentally determined oxidation potential with the calculated HOMO energies (see Table 1). The DFT values are systematically too low by 0.1−0.15 V but otherwise show the same trends as the measured values. Electrondonating substituents like methyl (12) or methoxy (15) destabilize the HOMO and therefore raise its energy (note the inverse scale for values vs NHE). For electron-withdrawing substituents like an ester (11), nitrile (13), or nitro (17) group the opposite effect can be observed. This confirms that different substituents on the C^C* ligand are capable of tuning the absorption properties and energy levels in general. The position of the HOMO/LUMO energy levels in relation to the conduction band of the TiO2 (for efficient electron transfer into the semiconductor anode) and the potential of the redox mediator (e.g., iodide/triiodide or iodide/diiodide radical; for efficient dye regeneration) is of interest for the incorporation of this class of complexes into DSSCs. On the one hand, as can be seen for complex 14 from Figure 8 above,

Table 1. Electrochemical Properties of Complexes 11−17 and [Ru(bpy)2(ppy)]PF6 (B2P) E1/2[V] (ΔE[mV]) vs NHEa Ered2 11 12 13 14 15 16 17 B2P

−1.65 −1.70 −1.59 −1.65 −1.65 −1.67 −1.67 −1.62

(60) (101) (60) (63) (62) (88) (58) (67)

DFT

Ered1

Eox (RuII/RuIII)

EHOMOc [V]

−1.39 (61) −1.41 (67) −1.35 (66) −1.39 (62) −1.39 (60) −1.41 (64) −1.38b −1.36 (66)

0.69 (95) 0.60 (80) 0.75 (119) 0.62 (70) 0.62 (68) 0.67 (78) 0.80b 0.69 (75)

0.61 0.47 0.66 0.48 0.46 0.61 0.71 0.58

a

Recorded in MeCN (see Experimental Section for details) with Fc/ Fc+ as internal reference. b(Quasi-)irreversible. cDFT energies, converted to E vs NHE (see Experimental Section for details).

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overall: 77.8%, mp 264 °C). 1H NMR (600 MHz, DMSO-d6) δ 9.92 (m, 1H, NCHN), 8.41 (m, 1H, NCHCHN), 8.24−8.18 (m, 2H, CarH), 8.09 (m, 1H, NCHCHN), 7.99−7.92 (m, 2H, CarH), 4.37 (q, J = 7.1 Hz, 2H, OCH2CH3), 4.29 (q, J = 7.3 Hz, 2H, NCH2CH3), 1.52 (t, J = 7.3 Hz, 3H, OCH2CH3), 1.35 (t, J = 7.1 Hz, 3H, NCH2CH3). 13 C NMR (151 MHz, DMSO-d6) δ 164.7 (COOEt), 138.2 (CarCOOEt), 135.6 (CarN), 131.0 (2x CarH), 130.7 (NCHN), 123.3 (NCHCHN), 122.0 (2x CarH), 121.0 (NCHCHN), 61.4 (OCH2CH3), 45.0 (NCH2CH3), 14.8 (OCH2CH3), 14.2 (NCH2CH3). Anal. Calcd for C14H17BrN2O2 (M = 324.05 g/mol): C: 51.71, H: 5.27, N: 8.61, Found: C: 51.94, H: 5.37, N: 8.62% Synthesis of p-Cymene Complexes. Chloro[1-(4(Ethoxycarbonyl)phenyl-κC2)-3-ethylimidazolin-2-ylidene-κC2][η6-1methyl-4-(1-methylethyl)benzene] ruthenium(II) (4). 1-(4(Ethoxycarbonyl)phenyl)-3-ethylimidazolium bromide (130 mg, 0.4 mmol) (1), 122 mg (0.2 mmol) of [RuCl2(η6-p-cymene)]2, and 94 mg (0.4 mmol) of Ag2O were placed in a Schlenk tube, and 10 mL of CH2Cl2 were added. The resulting deep red suspension was stirred at room temperature under exclusion of light for 24 h, and the crude product was purified by column chromatography (SiO2; CH2Cl2/ MeOH 10:1; Rf = 0.9). The product fraction was after evaporation dissolved in little amounts (∼10 mL) of THF, and the yellow product was slowly precipitated by addition of pentane. (102 mg, 49.6%; mp 145 °C, dec >175 °C). 1H NMR (500 MHz, CDCl3) δ 8.82 (d, J = 1.7 Hz, 1H, C5H), 7.70 (dd, J = 8.1, 1.8 Hz, 1H, C4H), 7.38 (d, J = 2.1 Hz, 1H, C2H), 7.08 (d, J = 8.1 Hz, 1H, C3H), 7.03 (d, J = 2.1 Hz, 1H, C1H), 5.71 (d, J = 5.5 Hz, 1H, CarH(cym)), 5.63 (d, J = 5.5 Hz, 1H, CarH(cym)), 5.48 (d, J = 6.0 Hz, 1H, CarH(cym)), 5.35 (d, J = 5.8 Hz, 1H, CarH(cym)), 4.62 (dq, J = 14.7, 7.4 Hz, 1H, OCH2CH3), 4.48 (dq, J = 14.6, 7.3 Hz, 1H, OCH2CH3), 4.43−4.33 (m, 2H, NCH2CH3), 2.18−2.11 (m, 1H, CH(CH3)2), 2.07 (s, 3H, CarCH3(cym)), 1.63 (t, J = 7.3 Hz, 3H, NCH2CH3), 1.42 (t, J = 7.1 Hz, 3H, OCH2CH3), 0.88 (d, J = 6.9 Hz, 3H, CH(CH3)2), 0.70 (d, J = 6.9 Hz, 3H, CH(CH3)2). 13 C NMR (126 MHz, CDCl3) δ 189.1 (Ccarbene), 167.6 (COOEt), 162.8 (CRu), 149.5 (Cipso), 142.6 (Cipso), 125.9 (CarH), 125.2 (CarH), 120.5 (CarH), 115.0 (CarH), 110.6 (CarH), 105.8 (CarCH3(cym)), 99.0(Car(i-Pr) (cym)), 93.7 (CarH(cym)), 90.5 (CarH(cym)), 87.8 (C ar H(cym)), 84.0 (C ar H(cym)), 60.6 (OCH 2 CH 3 ), 45.8 (NCH2CH3), 31.1 (CH(CH3)2), 23.2 (CH(CH3)2), 21.8 (CH(CH3)2), 19.1 (CarCH3(cym)), 16.9 (OCH2CH3), 14.7 (NCH2CH3). Anal. Calcd for C24H29ClN2O2Ru·0.3 THF (M = 514.10 g/mol): C: 56.50, H: 5.91, N: 5.23, Found: C: 56.26, H: 6.01, N: 4.88% Chloro[1-(4-methylphenyl-κC2)-3-(4-methylphenyl)imidazolin-2ylidene-κC2][η6-1-methyl-4-(1-methylethyl)benzene] ruthenium(II) (5).

the LUMO is sufficiently high enough to ensure efficient electron transfer from the excited dye to the semiconductor. The HOMO of complex 14, on the other hand, is roughly on the same energy level as the exemplary iodide/triiodide redox couple and even higher than the iodide/diiodide radical redox couple. Therefore, the dye regeneration by this redox system could be hindered. However, the calculated values for the hypothetical sensitizer containing carboxylate anchor groups (14_dcbp) are significantly lower due to the electronwithdrawing effect of the carboxylate groups and should allow for efficient electron transfer to and from the dye.

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CONCLUSION We synthesized bipyridyl ruthenium complexes with cyclometalated NHC ligands in two reaction steps starting from the widely used ruthenium p-cymene dimer and substituted phenylimidazolium precursors. The investigation of optical and electronic properties by experimental and computational means revealed a potential applicability of this novel class of dyes as sensitizers in DSSCs. By changing the substituents, the C^C* ligand (in contrast to the standard SCN− ligands) offers a wide range of possibilities to further enhance the chemical and physical properties of the dyes, like the absorption profile (through extended π-systems or electron-withdrawing/-donating groups) or hydrophobicity (by introducing alkyl chains), which will be the subject of future studies.

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EXPERIMENTAL SECTION

General. The reactions throughout this study were performed employing standard Schlenk techniques. Solvents were of high purity (>99.5%); acetonitrile was dried in an MBraun Solvent Purification System prior to use. Dimethyl sulfoxide (p. a. 99.6%) was dried over activated molecular sieves 4 Å and degassed by freeze−pump−thaw cycles. The precursor 1,3-bis(4-methylphenyl)-1H-imidazolium chloride (2) was synthesized using literature procedures.109,110 The precursor 4-(1,1-dimethylethyl)-1-(4-cyanophenyl)-1-methyl-1H-imidazolium iodide (3) was synthesized following a literature procedure.111 The synthesis of the ruthenium cymene complexes 7− 10 is described elsewhere.98 All other chemicals are commercially available from common suppliers and used without further purification. 1H and 13C NMR spectra were recorded on a Bruker DRX 500 P or a Bruker ACS 600 spectrometer at 298 K. 1H and 13C spectra were referenced internally using the resonances of the solvent (1H: 7.26, 13C: 77.0 for CDCl3; 1H: 2.50, 13C: 39.43 for DMSO-d6; 1 H: 1.94, 13C: 1.32 for CD3CN). Shifts δ are given in parts per million downfield from tetramethylsilane, and coupling constants J are in hertz. Elemental analyses were performed on a Eurovector Hekatech EA3000 by the microanalytical laboratory of our institute. Melting points were determined on a hot stage microscope and are not corrected. UV/vis spectra were recorded on a PerkinElmer Lambda 25 as 0.1 mM solutions in acetonitrile. Emission spectra were recored on an Agilent Cary Eclipse spectrophotometer in deaerated acetonitrile solutions. 1-(4-(Ethoxycarbonyl)phenyl)-3-ethylimidazolium bromide (1). 1-(4-(Ethoxycarbonyl)phenyl)imidazole (0.86 g, 4 mmol) was dissolved in 10 mL of THF in an ACE pressure tube, and 0.33 mL (0.48 g, 4.4 mmol) of bromoethane was added. The reaction vessel was sealed, and the mixture was heated to 95 °C for 20 h. The reaction mixture was allowed to slowly cool, the white precipitate was filtered off, washed thoroughly with THF and diethyl ether, and dried in vacuo. (0.23 g, mp 262 °C). The combined filtrates were dried on a rotary evaporator; the residue was dissolved in 10 mL of THF and was again placed in an ACE pressure tube together with 0.5 mL (0.72 g, 6.67 mmol) of bromoethane and heated to 95 °C for another 60 h. The following workup gave a second product fraction. (0.78 g, yield

1,3-Di(4-methylphenyl)imidazolium chloride (427 mg, 1.5 mmol) (2), 459 mg (0.75 mmol) of [Ru(p-cymene)Cl2]2, and 351 mg (1.5 mmol) of Ag2O are placed in a Schlenk tube, and 35 mL of CH2Cl2 is added. The red-orange suspension is stirred (under the exclusion of light) for 2 h at room temperature and for 6 h at 40 °C. After further addition of 30 mg of Ag2O, the heating is stopped, and the mixture is stirred for another 40 h at room temperature. The solution is filtered via cannula, and the filtrate is concentrated in vacuo and dissolved in 75 mL of THF. The yellow to orange solution is filtered over basic Al2O3, the filtrate cautiously concentrated (until precipitation occurs), and pentane is added slowly (approximately same volume). The mixture is left to stand for 2 d in the refrigerator, where the product precipitates as yellow to orange crystals. The crystals usually contain residual solvent, which can be removed by dissolving them in CH2Cl2, concentrating the solution in vacuo, and subsequent drying at 50 °C. 7222


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Inorganic Chemistry (638 mg, 82%; dec >210 °C). 1H NMR (600 MHz, CDCl3) δ 7.98 (dd, J = 1.7, 0.8 Hz, 1H, C5H), 7.86 (d, J = 7.8 Hz, 2H, CarH(B1)), 7.44 (d, J = 2.0 Hz, 1H, C2H), 7.39−7.35 (m, CarH(B2)), 7.16 (d, J = 2.1 Hz, 1H, C1H), 7.02 (d, J = 7.7 Hz, 1H, C3H), 6.78 (ddd, J = 7.8, 1.8, 0.8 Hz, 1H, C4H), 5.25 (dd, J = 6.0, 1.2 Hz, 1H, CarH(cym)), 5.24 (dd, J = 6.0, 1.2 Hz, 1H, CarH(cym)), 4.70 (dd, J = 5.9, 1.1 Hz, 1H, CarH(cym)), 4.64 (dd, J = 5.8, 1.1 Hz, 1H, CarH(cym)), 2.51 (s, 3H, C ar CH 3 (B)), 2.37 (s, 2H, C ar CH 3 (A)), 2.05−2.00 (m, 1H, CH(CH3)2), 1.99 (s, 3H, CarCH3(cym)), 0.75 (d, J = 6.9 Hz, 3H, CH(CH3)2), 0.66 (d, J = 6.9 Hz, 3H, CH(CH3)2). 13C NMR (151 MHz, CDCl3) δ 187.7 (Ccarbene), 163.7 (CRu), 143.3 (CarN(A)), 142.2 (C5H), 138.6 (Cipso(B)), 138.5 (Cipso(B)), 133.9 (CarCH3(A)), 129.8 (2x CarH(B2)), 126.7 (2x CarH(B1)), 123.1 (C4H), 121.8 (C1H), 114.6 (C2H), 111.0 (C3H), 105.8 (CarCH3(cym)), 98.0 (CariPr(cym)), 91.3 (CarH(cym)), 90.5 (CarH(cym)), 89.3 (CarH(cym)), 85.5 (CarH(cym)), 31.1 (CH(CH3)2), 23.3 (CH(CH3)2), 21.7 (CH(CH 3 ) 2 ), 21.6 (C ar CH 3 (A)), 21.5 (C ar CH 3 (B)), 19.1 (CarCH3(cym)). Anal. Calcd for C27H29ClN2Ru (M = 518.11 g/ mol): C: 62.60, H: 5.64, N: 5.41, Found: C: 62.67, H: 5.61, N: 5.40% Chloro[4-(1,1-dimethylethyl)-1-(4-cyanophenyl-κC2)-3-ethylimidazolin-2-ylidene-κC 2 ][η 6 -1-methyl-4-(1-methylethyl)benzene] ruthenium(II) (6). 4-(1,1-Dimethylethyl)-1-(4-cyanophenyl)-1-methyl1H-imidazolium iodide (1102 mg, 3 mmol) (3), 919 mg (1.5 mmol) of [Ru(p-cymene)Cl2]2, and 1053 mg (4.5 mmol) of Ag2O are placed in a Schlenk tube, and 75 mL of CH2Cl2 is added. The red-orange suspension is stirred for 40 h at room temperature under the exclusion of light. The solvent is removed, the residue is dissolved in a mixture of CH2Cl2/MeOH 10:1, and the suspension is filtered over a pad of silica. After solvent evaporation, the crude product is dissolved in 50 mL of THF, and the yellow to orange solution is filtered over diatomaceous earth. The filtrate is cautiously concentrated (until precipitation occurs), and pentane is slowly added. The mixture is left to stand in the refrigerator for 2 d, where the product precipitates as a yellow solid (640 mg, 42%; dec >210 °C). 1H NMR (600 MHz, CDCl3) δ 8.38 (d, J = 1.6 Hz, 1H, C5H), 7.28−7.26 (m, 1H, C4H [partially obscured by residual solvent signal]), 7.04 (s, 1H, C2H), 7.03 (d, J = 7.8 Hz, 1H, C3H), 5.57−5.51 (m, 3H, CarH(cym)), 5.46 (dd, J = 6.0, 1.2 Hz, 1H, CarH(cym)), 4.31 (d, J = 1.0 Hz, 3H, NCH3), 2.13 (hept, J = 6.9 Hz, 1H, CH(CH3)2), 2.09 (s, 3H, CarCH3(cym)), 1.43 (d, J = 1.0 Hz, 9H, C(CH3)3), 0.87 (d, J = 6.7 Hz, 3H, CH(CH3)2), 0.73 (d, J = 6.8 Hz, 3H, CH(CH3)2). 13C NMR (151 MHz, CDCl3) δ 191.8 (Ccarbene), 164.7 (CRu), 149.6 (CarN), 144.3 (C5H), 143.6 (Cimt-Bu), 127.3 (C4H), 120.8 (CarCN), 110.7 (C2H), 110.6 (C3H), 107.0 (CarCN), 106.5 (CarCH3(cym)), 99.6 (CariPr(cym)), 92.9 (CarH(cym)), 90.6 (CarH(cym)), 88.4 (CarH(cym)), 85.4 (CarH(cym)), 38.2 (NCH3), 31.6 (C(CH3)3), 31.2 (CH(CH3)2), 29.5 (C(CH3)3 ), 23.2 (CH(CH3 )2), 21.8 (CH(CH3)2), 19.1 (CarCH3(cym)). Anal. Calcd for C25H30ClN3Ru (M = 509.12 g/ mol): C: 58.99, H: 5.94, N: 8.25, Found: C: 59.09 H: 5.72 N: 8.49% General Synthetic Procedure for the Synthesis of the Bisheteroleptic Bipyridyl Complexes. In a Schlenk tube 1 equiv of the corresponding p-cymene complex and 2.1 to 2.2 equiv of bipyridine are dissolved in small amounts of DMSO (degassed, dried over molecular sieves 4 Å) and heated to 140 °C for 20 h under the exclusion of light. The resulting deep purple solution is slowly cooled to room temperature and diluted with water to a five- to sevenfold volume. The deep purple aqueous solution containing the corresponding chloride complex is left to stand in the refrigerator overnight and subsequently filtered through a pad of diatomaceous earth to remove residual bipyridine. A solution of 1.25 equiv of ammonium hexafluorophosphate is then slowly added to the filtrate, and the resulting dark violet precipitate is filtered via cannula or a sintered glass frit. The solid is dried in vacuo and dissolved in little amounts of dichloromethane, and the solution is slowly filtered over a pad of basic aluminum oxide. Addition of diethyl ether to the deep violet filtrate and subsequent filtration gives the bisheteroleptic complexes as analytically pure hexafluorophosphate salts. (1-(4-(Ethoxycarbonyl)phenyl-κC2)-3-ethylimidazolin-2-ylideneκC2)-bis(2,2′-bipyridine-κN,κN′)ruthenium(II) (1+) hexafluorophosphate (11). The synthesis follows the general procedure using 257 mg

(0.5 mmol) of complex 4 and 172 mg (1.1 mmol) of bpy in 7 mL of DMSO. The precipitation is performed using 103 mg (0.625 mmol) of NH4PF6. (210 mg, 52%; mp 165−185 °C). 1H NMR (600 MHz, CD3CN) δ 8.39 (dt, J = 8.3, 1.1 Hz, 1H, CbpyH), 8.35 (dddd, J = 8.3, 6.7, 1.2, 0.7 Hz, 2H, CbpyH), 8.25 (ddd, J = 8.3, 1.4, 0.7 Hz, 1H, CbpyH), 8.02 (ddd, J = 5.7, 1.5, 0.8 Hz, 1H, CbpyH), 7.95−7.88 (m, 4H, CbpyH), 7.84−7.81 (m, 2H, CbpyH + C2H [d, J = 2.2 Hz]), 7.73 (ddd, J = 8.1, 7.5, 1.5 Hz, 1H, CbpyH), 7.54−7.50 (m, 2H, CbpyH + C4H [dd, 4 J = 1.9 Hz]), 7.36−7.33 (m, 2H, CbpyH + C3H [d, J = 8.3 Hz]), 7.31 (ddd, J = 7.5, 5.5, 1.2 Hz, 1H, CbpyH), 7.16 (ddt, J = 7.4, 5.7, 1.5 Hz, 2H, CbpyH), 7.09 (d, J = 2.2 Hz, 1H, C1H), 6.93 (d, J = 1.9 Hz, 1H, C5H), 4.10 (qd, J = 7.1, 3.2 Hz, 2H, OCH2CH3), 3.40 (qd, J = 7.3, 3.1 Hz, 2H, NCH2CH3), 1.18 (t, J = 7.1 Hz, 3H, OCH2CH3), 0.77 (t, J = 7.2 Hz, 3H, NCH2CH3). 13C NMR (151 MHz, CD3CN) δ 196.8 (Ccarbene), 175.3 (CRu), 168.0 (COOEt), 158.2 (Cipso,bpy), 157.4 (Cipso,bpy), 156.45 (Cipso,bpy), 156.4 (Cipso,bpy), 155.2 (CbpyH), 155.0 (CbpyH), 153.8 (CarN), 150.2 (CbpyH), 149.2 (CbpyH), 138.2 (C5H), 136.6 (CbpyH), 136.6 (CbpyH), 135.5 (CbpyH), 134.7 (CbpyH), 127.7 (CbpyH), 127.5 (CbpyH), 127.2 (CbpyH), 127.0 (CbpyH), 126.2 (CarCOOEt), 124.4 (C4H), 124.3 (CbpyH), 124.25 (CbpyH), 124.0 (CbpyH), 123.7 (CbpyH), 122.4 (C1H), 116.8 (C2H), 111.0 (C3H), 61.0 (OCH2CH3), 44.8 (NCH2CH3), 17.4 (OCH2CH3), 14.5 (NCH2CH3). Anal. Calcd for C34H31F6N6O2PRu (M = 802.12 g/ mol): C: 50.94, H: 3.90, N: 10.48, Found: C: 51.30, H: 4.05, N: 10.12%. (1-(4-Methylphenyl)-3-(4-methylphenyl-κC 2 )imidazolin-2-ylidene-κC2)-bis(2,2′-bipyridine-κN,κN′)ruthenium(II) (1+) hexafluorophosphate (12). The synthesis follows the general procedure using 259 mg (0.5 mmol) of complex 5 and 172 mg (1.1 mmol) of bpy in 6 mL of DMSO. The precipitation is performed using 103 mg (0.625 mmol) of NH4PF6. (250 mg, 62%; mp 200−205 °C, dec >350 °C). 1H NMR (600 MHz, CD3CN) δ 8.26 (dt, J = 8.2, 0.9 Hz, 1H, CbpyH), 8.24 (dt, J = 8.2, 1.1 Hz, 1H, CbpyH), 8.18 (ddd, J = 8.3, 1.4, 0.8 Hz, 1H, CbpyH), 8.14 (ddd, J = 5.7, 1.6, 0.8 Hz, 1H, CbpyH), 8.07−8.04 (m, 2H, CbpyH), 7.87 (d, J = 2.1 Hz, 1H, C2H), 7.86−7.83 (m, 1H, CbpyH)), 7.83−7.80 (m, 1H, CbpyH), 7.70 (ddd, J = 8.2, 7.5, 1.5 Hz, 1H, CbpyH), 7.59 (ddd, J = 8.2, 7.5, 1.6 Hz, 1H, CbpyH), 7.42 (ddd, J = 5.6, 1.6, 0.8 Hz, 1H, CbpyH), 7.27 (d, J = 7.8 Hz, 1H, C3H), 7.25−7.20 (m, 2H, CbpyH), 7.19 (ddd, J = 7.5, 5.7, 1.4 Hz, 1H, CbpyH), 7.12 (d, J = 2.1 Hz, 1H, C1H), 7.07 (ddd, J = 5.4, 1.6, 0.8 Hz, 1H, CbpyH), 6.75 (ddd, J = 7.5, 5.4, 1.2 Hz, 1H, CbpyH), 6.70−6.67 (m, 3H, C4H [obscured]+ CarH(B2)), 6.62 (d, J = 8.2 Hz, 2H, CarH(B1)), 6.14 (dd, J = 1.8, 0.7 Hz, 1H, C5H), 2.14 (s, 3H, CarCH3(B)), 1.99 (d, J = 0.7 Hz, 3H, CarCH3(A)). 13C NMR (151 MHz, CD3CN) δ 196.5 (Ccarbene), 175.1 (CRu), 158.5 (Cipso,bpy), 157.4 (Cipso,bpy), 156.2 (Cipso,bpy), 155.6 (Cipso,bpy), 155.0 (CbpyH), 154.9 (CbpyH), 149.7 (CbpyH), 148.9 (CbpyH), 146.8 (CarN(A)), 139.2 (CarCH3(B)), 138.4 (C5H), 138.2 (CarN(B)), 136.0 (CbpyH), 135.2 (CbpyH), 135.0 (CbpyH), 134.2 (CbpyH), 133.7 (CarCH3(A)), 130.3 (2x CarH(B2)), 127.4 (CbpyH), 127.0 (2x CarH(B1)), 127.0 (CbpyH), 126.8 (CbpyH), 126.4 (CbpyH), 124.1 (CbpyH), 124.05 (CbpyH), 124.0 (C1H), 123.4 (CbpyH), 123.3 (CbpyH), 122.6 (C4H), 116.3 (C2H), 111.5 (C3H), 21.4 (CarCH3(A)), 20.9 (CarCH3(B)). Anal. Calcd for C37H31F6N6PRu (M = 806.13 g/mol): C: 55.16, H: 3.88, N: 10.43, Found: C: 55.34, H: 3.70, N: 10.60% (1-(4-Cyanophenyl-κC2)-4-(1,1-dimethylethyl)-3-methylimidazolin-2-ylidene-κC2)-bis(2,2′-bipyridine-κN,κN′)ruthenium(II) (1+) hexafluorophosphate (13). Complex 6 (204 mg, 0.4 mmol) and 131 mg (0.84 mmol) of bpy are dissolved in 15 mL of MeCN, and the resulting mixture is stirred for 40 h under the exclusion of light. The volume of the solution is reduced to approximately one-third of its original volume before 10 mL of diethyl ether is added. The solvent is then removed via cannula, and the remaining red solid is dried in vacuo. The chloride complex is then dissolved in 10 mL of water, the deep red-violet solution is filtered over diatomaceous earth, and a solution of 68 mg (0.41 mmol) of NH4PF6 in 2 mL of water is slowly added. The crude product precipitates as a dark red-violet solid. After filtration of a DCM solution over basic alumina the analytically pure complex is obtained (260 mg, 82%; mp 210−225 °C). 1H NMR (600 MHz, CD3CN) δ 8.38 (dt, J = 8.3, 1.1 Hz, 1H, CbpyH), 8.37−8.34 (m, 7223


Article

Inorganic Chemistry

(1-(4-Bromophenyl-κC2)-3-methylimidazolin-2-ylidene-κC2)-bis(2,2′-bipyridine-κN,κN′)ruthenium(II) (1+) hexafluorophosphate (16). The synthesis follows the general procedure using 1014 mg (2 mmol) of complex 9 and 687 mg (4.4 mmol) of bpy in 15 mL of DMSO. The precipitation is performed using 412 mg (2.5 mmol) of NH4PF6 (1370 mg, 86%; mp 195−210 °C, dec >310 °C). 1H NMR (600 MHz, DMSO-d6) δ 8.70−8.64 (m, 3H, CbpyH), 8.59 (ddd, J = 8.4, 1.4, 0.8 Hz, 1H, CbpyH), 8.08 (d, J = 2.2 Hz, 1H, C2H), 8.05−7.98 (m, 2H, CbpyH), 7.97−7.92 (m, 2H, CbpyH), 7.91 (ddd, J = 5.7, 1.6, 0.8 Hz, 1H, CbpyH), 7.88 (ddd, J = 8.1, 7.6, 1.5 Hz, 1H, CbpyH), 7.83 (ddd, J = 5.5, 1.6, 0.8 Hz, 1H, CbpyH), 7.52−7.47 (m, 2H, CbpyH), 7.45 (ddd, J = 7.5, 5.5, 1.2 Hz, 1H, CbpyH), 7.41 (ddd, J = 7.4, 5.7, 1.4 Hz, 1H, CbpyH), 7.38 (d, J = 8.2 Hz, 1H, C3H), 7.34 (ddd, J = 7.4, 5.7, 1.4 Hz, 1H, CbpyH), 7.26 (d, J = 2.1 Hz, 1H, C1H), 6.94 (dd, J = 8.2, 2.2 Hz, 1H, C4H), 6.17 (d, J = 2.2 Hz, 1H, C5H), 2.98 (s, 3H, NCH3). 13C NMR (151 MHz, DMSO-d6) δ 194.0 (Ccarbene), 179.9 (CRu), 156.8 (Cipso,bpy), 155.9 (Cipso,bpy), 155.0 (Cipso,bpy), 154.9 (Cipso,bpy), 153.8 (CbpyH), 153.2 (CbpyH), 148.6 (CbpyH), 147.7 (CarN), 147.6 (CbpyH), 137.4 (C5H), 135.8 (CbpyH), 135.8 (CbpyH), 134.7 (CbpyH), 134.0 (CbpyH), 127.0 (CbpyH), 126.9 (CbpyH), 126.7 (CbpyH), 126.5 (CbpyH), 123.7 (CbpyH), 123.6 (CbpyH), 123.5 (CbpyH), 123.45 (C1H), 123.1 (CbpyH), 122.9 (C4H), 117.0 (CarBr), 115.6 (C2H), 112.6 (C3H), 34.9 (NCH3). Anal. Calcd for C30H24BrF6N6PRu (M = 793.99 g/ mol): C: 45.35, H: 3.04, N: 10.58, Found: C: 45.17, H: 2.95, N: 10.46% (1-Methyl-3-(4-nitrophenyl-κC2)imidazolin-2-ylidene-κC2)-bis(2,2′-bipyridine-κN,κN′)ruthenium(II) (1+) hexafluorophosphate (17). The synthesis follows the general procedure using 95 mg (0.2 mmol) of complex 10 and 69 mg (0.44 mmol) of bpy in 5 mL of DMSO. The precipitation is performed using 41 mg (0.25 mmol) of NH4PF6 (97 mg, 64%; mp 195−215 °C, dec >280 °C). 1H NMR (600 MHz, CD3CN) δ 8.40 (dt, J = 2.2, 1.1 Hz, 1H, CbpyH), 8.38 (dt, J = 2.4, 1.1 Hz, 1H, CbpyH), 8.36 (ddd, J = 8.3, 1.2, 0.7 Hz, 1H, CbpyH), 8.28 (ddd, J = 8.3, 1.5, 0.7 Hz, 1H, CbpyH), 8.00−7.96 (m, 2H, CbpyH), 7.94−7.89 (m, 3H, CbpyH), 7.85 (ddd, J = 8.2, 7.6, 1.6 Hz, 1H, CbpyH), 7.81 (d, J = 2.2 Hz, 1H, C2H), 7.78 (ddd, J = 8.1, 7.6, 1.5 Hz, 1H, CbpyH), 7.74 (dd, J = 8.5, 2.6 Hz, 1H, C4H), 7.57 (ddd, J = 5.5, 1.6, 0.8 Hz, 1H, CbpyH), 7.40 (d, J = 8.6 Hz, 1H, C3H), 7.39−7.37 (m, 1H, CbpyH), 7.33 (ddd, J = 7.5, 5.5, 1.2 Hz, 1H, CbpyH), 7.19 (dddd, J = 7.6, 6.6, 5.7, 1.4 Hz, 2H, CbpyH), 7.13 (d, J = 2.5 Hz, 1H, C5H), 7.04 (d, J = 2.2 Hz, 1H, C1H), 3.03 (s, 3H, NCH3). 13C NMR (151 MHz, CD3CN) δ 198.4 (Ccarbene), 178.7 (CRu), 158.2 (Cipso,bpy), 157.4 (Cipso,bpy), 156.3 (Cipso,bpy), 156.3 (Cipso,bpy), 155.7 (CarNim), 155.4 (CbpyH), 155.2 (CbpyH), 150.2 (CbpyH), 149.3 (CbpyH), 145.1 (CarNO2), 136.9 (CbpyH), 136.9 (CbpyH), 135.8 (CbpyH), 135.2 (CbpyH), 131.4 (C5H), 127.9 (CbpyH), 127.6 (CbpyH), 127.4 (CbpyH), 127.2 (CbpyH), 125.0 (C1H), 124.4 (CbpyH), 124.4 (CbpyH), 124.2 (CbpyH), 123.9 (CbpyH), 118.9 (C4H), 116.7 (C2H), 111.1 (C3H), 36.2 (NCH3). Anal. Calcd for C30H24F6N7O2PRu (M = 761.07 g/mol): C: 47.37, H: 3.18, N: 12.89, Found: C: 47.44, H: 3.20, N: 12.57% Cyclic Voltammetry. Electrochemical measurements were performed with a BioLogic SP-150 potentiostat in degassed, dry acetonitrile using a Pt counter electrode, a glassy carbon working electrode, and a Ag/Ag+ pseudo reference electrode. All complexes were measured as 0.5 mM solutions with the addition of 0.1 M (nBu)4ClO4 as supporting electrolyte at a sweep rate of 50 mV/s. Every measurement was internally referenced against the Fc/Fc+ redox couple, and potentials were converted to NHE via Fc/Fc+ versus NHE = 0.63 V.112 For visualization the EC-Lab software V11.01 and Origin 2016 were used. DFT Calculations. All calculations were performed using the Gaussian09113 package. The density functional hybrid model B3LYP114−119 was used together with the 6-31G(d)120,121 basis set. Ruthenium was described using a Hay-Wadt double-ζ basis set and ECP.122−124 No symmetry or internal coordinate constraints were applied during optimizations. All reported structures were verified as true minima by the absence of imaginary eigenvalues in the vibrational frequency analysis. Approximate free energies were obtained through thermochemical analysis, using the thermal correction to Gibbs free

2H, CbpyH), 8.28 (ddd, J = 8.3, 1.3, 0.7 Hz, 1H, CbpyH), 7.95−7.88 (m, 4H, CbpyH), 7.88−7.82 (m, 2H, CbpyH), 7.77 (ddd, J = 8.2, 7.6, 1.5 Hz, 1H, CbpyH), 7.55 (s, 1H, C2H), 7.46 (ddd, J = 5.5, 1.6, 0.8 Hz, 1H, CbpyH), 7.35−7.33 (m, 2H, CbpyH + C3H [J = 7.8 Hz]), 7.31 (ddd, J = 7.5, 5.5, 1.2 Hz, 1H, CbpyH), 7.23−7.18 (m, 3H, 2x CbpyH + C4H), 6.52 (dd, J = 1.8, 0.4 Hz, 1H, C5H), 3.20 (s, 3H, NCH3), 1.35 (s, 9H, C(CH3)3). 13C NMR (151 MHz, CD3CN) δ 199.5 (Ccarbene), 177.7 (CRu), 158.2 (Cipso,bpy), 157.5 (Cipso,bpy), 156.3 (Cipso,bpy), 156.2 (Cipso,bpy), 155.2 (CbpyH), 155.15 (CbpyH), 153.9 (CarN), 150.1 (CbpyH), 149.1 (CbpyH), 144.4 (Cimt-Bu), 140.2 (C5H), 136.8 (CbpyH), 136.7 (CbpyH), 135.5 (CbpyH), 135.0 (CbpyH), 127.9 (CbpyH), 127.6 (CbpyH), 127.4 (CbpyH), 127.0 (CbpyH), 126.7 (C4H), 124.4 (CbpyH), 124.4 (CbpyH), 124.2 (CbpyH), 123.9 (CarCN), 121.6 (CbpyH), 112.8 (C2H), 111.1 (C3H), 106.9 (CarCN), 36.1 (NCH3), 32.0 (C(CH3)3), 29.7 (C(CH3)3). Anal. Calcd for C35H32F6N7PRu (M = 797.14 g/mol): C: 52.76, H: 4.05, N: 12.31, Found: C: 52.87, H: 4.12, N: 11.92% (1-Methyl-3-(phenyl-κC2)imidazolin-2-ylidene-κC2)-bis(2,2′-bipyridine-κN,κN′)ruthenium(II) (1+) hexafluorophosphate (14). The synthesis follows the general procedure using 257 mg (0.6 mmol) of complex 7 and 206 mg (1.32 mmol) of bpy in 4 mL of DMSO. The precipitation is performed using 123 mg (0.75 mmol) of NH4PF6. The analytically pure complex is obtained after filtering over basic alumina twice (270 mg, 63%; mp 335 °C). 1H (600 MHz, CD3CN) δ 8.37 (dt, J = 8.3, 1.0 Hz, 1H, CbpyH), 8.33 (ddt, J = 8.2, 4.2, 0.9 Hz, 2H, CbpyH), 8.25 (dt, J = 8.2, 1.1 Hz, 1H, CbpyH), 8.02 (dddd, J = 5.7, 2.6, 1.5, 0.8 Hz, 2H, CbpyH), 7.92 (ddd, J = 5.5, 1.6, 0.9 Hz, 1H, CbpyH), 7.88 (dddd, J = 8.2, 7.5, 4.1, 1.6 Hz, 2H, CbpyH), 7.81 (ddd, J = 8.1, 7.5, 1.6 Hz, 1H, CbpyH), 7.74−7.69 (m, 2H [CbpyH + C2H as large d, J = 2.1 Hz]), 7.54 (ddd, J = 5.5, 1.6, 0.8 Hz, 1H, CbpyH), 7.30 (dddd, J = 7.4, 5.5, 1.9, 1.2 Hz, 2H, CbpyH), 7.27 (dd, J = 7.5, 1.1 Hz, 1H, C5H), 7.16 (dddd, J = 8.8, 7.3, 5.7, 1.4 Hz, 2H, CbpyH), 6.97 (d, J = 2.1 Hz, 1H, C1H), 6.84 (td, J = 7.5, 1.4 Hz, 1H, CphH(para)), 6.63 (td, J = 7.3, 1.2 Hz, 1H, C4H), 6.27 (ddd, J = 7.3, 1.4, 0.4 Hz, 1H, C3H), 3.00 (s, 3H, NCH3). 13C (151 MHz, CD3CN) δ 195.8 (Ccarbene), 174.9 (CRu), 158.4 (Cipso,bpy), 157.5 (Cipso,bpy), 156.4 (Cipso,bpy), 156.4 (Cipso,bpy), 155.2 (CbpyH), 154.8 (CbpyH), 150.1 (CbpyH), 149.5 (CarN), 149.2 (CbpyH), 137.4 (C3H), 136.3 (CbpyH), 136.2 (CbpyH), 135.1 (CbpyH), 134.1 (CbpyH), 127.5 (CbpyH), 127.5 (CbpyH), 127.0 (CbpyH), 126.8 (CbpyH), 124.8 (C4H), 124.2 (CbpyH), 124.1 (CbpyH), 124.0 (CbpyH), 123.8 (CbpyH), 123.6 (C1H), 122.0 (CphH(para)), 115.8 (C2H), 111.5 (C5H), 36.0 (NCH3). Anal. Calcd for C30H25F6N6PRu (M = 716.08 g/mol): C: 50.35, H: 3.52, N: 11.74, Found: C: 50.29 H: 3.32 N: 11.54% (1-(4-Methoxyphenyl-κC2)-3-methylimidazolin-2-ylidene-κC2)bis(2,2′-bipyridine-κN,κN′)ruthenium(II) (1+) hexafluorophosphate (15). The synthesis follows the general procedure using 275 mg (0.6 mmol) of complex 8 and 206 mg (1.32 mmol) of bpy in 5 mL of DMSO. The precipitation is performed using 123 mg (0.75 mmol) of NH4PF6 (210 mg, 47%; mp 175 °C, dec >310 °C). 1H (600 MHz, CD3CN) δ 8.37 (dt, J = 8.2, 1.1 Hz, 1H, CbpyH), 8.33 (dt, J = 8.2, 1.1 Hz, 2H, CbpyH), 8.25 (ddd, J = 8.3, 1.5, 0.7 Hz, 1H, CbpyH), 8.06 (ddd, J = 5.7, 1.6, 0.8 Hz, 1H, CbpyH), 8.03 (ddd, J = 5.7, 1.5, 0.8 Hz, 1H, CbpyH), 7.94 (ddd, J = 5.4, 1.6, 0.8 Hz, 1H, CbpyH), 7.88 (qd, J = 7.7, 1.5 Hz, 2H, CbpyH), 7.81 (ddd, J = 8.2, 7.5, 1.5 Hz, 1H, CbpyH), 7.73 (tdd, J = 8.1, 1.5, 0.6 Hz, 1H, CbpyH), 7.64 (d, J = 2.1 Hz, 1H, C2H), 7.55 (ddd, J = 5.6, 1.6, 0.8 Hz, 1H, CbpyH), 7.30 (dtd, J = 7.5, 5.5, 1.3 Hz, 2H, CbpyH), 7.19 (d, J = 8.3 Hz, 1H, C3H), 7.19−7.14 (m, 2H, CbpyH), 6.95 (d, J = 2.1 Hz, 1H, C1H), 6.37 (dd, J = 8.4, 2.7 Hz, 1H, C4H), 5.73 (d, J = 2.7 Hz, 1H, C5H), 3.49 (s, 3H, OCH3), 2.99 (s, 3H, NCH3). 13C (151 MHz, CD3CN) δ 194.2 (Ccarbene), 177.5 (CRu), 158.3 (Cipso,bpy), 157.5 (Cipso,bpy), 157.2 (CarO), 156.5 (Cipso,bpy), 156.4 (Cipso,bpy), 155.1 (CbpyH), 154.8 (CbpyH), 150.2 (CbpyH), 149.3 (CbpyH), 143.3 (CarN), 136.3 (CbpyH), 136.1 (CbpyH), 135.0 (CbpyH), 134.2 (CbpyH), 127.4 (2x CbpyH), 127.1 (CbpyH), 126.8 (CbpyH), 124.2 (CbpyH), 124.2 (CbpyH), 124.0 (CbpyH), 123.6 (CbpyH), 123.5 (C1H), 123.2 (C5H), 115.7 (C2H), 111.6 (C3H), 105.5 (C4H), 55.2 (OCH3), 35.9 (NCH3). Anal. Calcd for C31H27F6N6OPRu (M = 746.09 g/mol): C: 49.94, H: 3.65, N: 11.27, Found: C: 49.84 H: 3.75 N: 10.94% 7224


Article

Inorganic Chemistry energy as reported by Gaussian09. This takes into account zero-point effects, thermal enthalpy corrections, and entropy. The HOMO/ LUMO energies reported in this paper are obtained from single-point calculations incorporating a solvent model (CPCM,125,126 solvent = acetonitrile) on the optimized structures. To compare them with the experimental values from the cyclic voltammetry measurements (Table 1), the energy values were converted to E vs NHE via Eabs [eV] = −4.5 eV − eU (vs NHE) [V].127,128 For visualization GaussView,129 CYLview,130 and Avogadro131 were used. UV/vis spectra and electronic transitions were calculated using TD-DFT132 methods (singlet, 50 transitions, CPCM, solvent = acetonitrile). The construction of the spectra from calculated transitions and their corresponding oscillator strengths was performed following the methods of the tool SWIZARD29,30 by means of a Lorenz functional scheme: ε(λ) = ∑i

fi

0.25(Δ1/2)2

Δ1/2 (λ − λi)2 + 0.25(Δ1/2)2

NTOs of selected transitions for complex 14; xyzcoordinates of DFT-optimized structures for complexes 11−17, 14_dcbp, B2P; CV spectra of complexes 11−17 and B2P; NMR spectra of complexes 4−17 (PDF) Accession Codes

CCDC 1552201−1552205 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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with Δ1/2 = 30 nm. For

*E-mail: [email protected]. Fax: 49 351 46339679. Phone: 49 351 46338571.

visualization of the NTOs Chemissian 4.23 was used. X-ray Crystallography. Preliminary examination and data collection for single crystals of compounds 6 and 12 was performed on a Nonius κ-CCD area detecting system (FR590) using graphitemonochromated Mo Kα radiation (λ = 0.710 73 Å) with an Oxford Cryosystems cooling system at the window of a sealed fine-focus X-ray tube. The reflections were integrated. Raw data were corrected for Lorentz, polarization, decay, and absorption effects. The absorption correction was applied using SADABS.133 After merging, the independent reflections were used for all calculations. The structures were solved by a combination of direct methods134,135 and difference Fourier syntheses.136 All non-hydrogen atom positions were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions using the SHELXL riding model. Full-matrix least-squares refinements were performed by minimizing σw(Fo2 − Fc2)2 with the SHELXL-97130,137 weighting scheme and stopped at shift/err less than 0.001. Details of the structure determinations are given in the Supporting Information. Neutral-atom scattering factors for all atoms and anomalous dispersion corrections for the nonhydrogen atoms were taken from International Tables for Crystallography.138 All calculations were performed with the programs COLLECT,139 DIRAX,140 EVALCCD,141 SIR92,134 SIR97,135 SADABS,133 PLATON,142 and the SHELXL-97 package. For the visualization ORTEP3143,144 was used. Preliminary examination and data collection for single crystals of compounds 5, 16, and 17 was performed on a RIGAKU AFC7 diffraction system (Saturn 724+ CCD detector) equipped with a sealed X-ray tube using graphitemonochromated Mo Kα radiation (λ = 0.710 73 Å). Intensity data were extracted using the CRYSTALCLEAR program package.145 The reflections were merged and corrected from Lorentz, polarization, and decay effects, and an absorption correction was applied based on multiple scans. The structure was solved by a combination of direct methods146 with the aid of difference Fourier synthesis and was refined against all data using SHELXL-97.130,137 Hydrogen atoms were assigned to ideal positions using the SHELXL-97 riding model. All non-hydrogen atoms were refined with anisotropic displacement parameters. Full-matrix least-squares refinements were performed by minimizing ∑w(Fo2 − Fc2)2 with the SHELXL-97 weighting scheme. Neutral-atom scattering factors for all atoms and anomalous dispersion corrections for the non-hydrogen atoms were taken from the International Tables for Crystallography.138 All calculations were performed with the CRYSTALCLEAR program package,145,146 the SHELX-97 program package, PLATON,142 and WINGX.147 The images of the solid-state structures were generated with ORTEP3.143,144

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AUTHOR INFORMATION

Corresponding Author

ORCID

Thomas Strassner: 0000-0002-7648-457X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Center for Information Services and High Performance Computing (ZIH) of the Technical University Dresden for computation time and A. Tronnier for data collection and refinement of solid-state structure 6.

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REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.7b00831. 7225


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