Article pubs.acs.org/IC
Optical Resolution, Determination of Absolute Configuration, and Photoracemization of cis-RuL2(CN)2 (L = 2,2′-Bipyridine and Its Analogues) Yusuke Aihara, Kyohei Sato, and Kazuteru Shinozaki* Department of Material Science, Graduate School of Nanobioscience, Yokohama City University, 22-2 Seto, Kanazawa-ku, Yokohama 236-0027, Japan S Supporting Information *
ABSTRACT: We synthesized neutral Ru(II) complexes cis-Ru(bpy)2(CN)2 (bpy = 2,2′-bipyridine), cis-Ru(dmb)2(CN)2 (dmb = 4,4′-dimethyl-2,2′-bipyridine), cis-Ru(dbb)2(CN)2 (dbb = 4,4′-di-tertbutyl-2,2′-bipyridine), and cis-Ru(phen)2(CN)2 (phen = 1,10-phenanthroline) and optically resolved them into respective enantiomers using high-performance liquid chromatography with a chiral column. The absolute configuration of enantiomer of cis-Ru(dbb)2(CN)2 was determined by an X-ray crystallography. Upon photoirradiation, the entire enantiomers of the complexes underwent the racemization with considerably slow rates (k = 1 × 10−6 to 1 × 10−5 s−1) and small quantum yields (ϕ = 1 × 10−6 to 1 × 10−5). The photoracemization was concluded to proceed via a five-coordinate pyramidal intermediate with the base plane composed of Ru, bidentate polypyridine, and two cyanides and the axial ligand of monodentate polypyridine. We derived the equations for photoracemization rate and quantum yield by a kinetics analysis of the photoracemization reaction that depended on polypyridine ligand, solvent, temperature, wavelength and intensity of irradiation light, and emission lifetime. From the temperature-dependent photoracemization reaction, the energy gap between 3MLCT (metal-to-ligand charge transfer) and 3 d−d* states was estimated as ΔE = 4000−5000 cm−1, and the energy of invisible 3d−d* state was estimated to be ca. 20 500 cm−1, which was in good agreement with that of [Ru(bpy)3]2+.
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INTRODUCTION Chiral transition-metal-complex catalysts have been frequently employed for the synthesis of fine chemicals.1 Not only the catalytic reaction but also the emission of chiral transition-metal complex has contributed to the development of chiral chemistry. For example, enantiomers of [Ru(bpy)3]2+ (bpy = 2,2′-bipyridine),2 fac-Ir(ppy)3 (ppyH = 2-phenylpyridine),3 Pt(pppb)Cl (pppbH = 1-pyridyl-3-(4,5-pinenopyridyl)benzene),4 and so on, have been expected to be applied their circularly polarized luminescence to a next-generation threedimensional organic light-emitting diode or security system. To use the transition-metal complexes practically, it is required to be highly stable toward racemization by heat and light. Therefore, it is worthy to investigate the photoracemization reaction of enantiomer to prevent the degradation of performance. Optical resolution for [Ru(bpy)3]2+ has been performed by many researchers using enantioselective synthesis, diastereomeric salt formation, chromatography, and so on.5 The diastereomeric salt formation was effective to separate Δ- and Λ-enantiomers for the divalent transition-metal-complex cation, while this procedure has not been applicable for neutral complex such as cis-Ru(bpy)2(CN)2. In this paper, we present © XXXX American Chemical Society
the optical resolution using high-performance liquid chromatography (HPLC) with a chiral column and the determination of absolute configuration for a series of neutral complexes of cisRu(bpy)2(CN)2, cis-Ru(dmb)2(CN)2 (dmb = 4,4′-dimethyl2,2′-bipyridine), cis-Ru(dbb)2(CN)2 (dbb = 4,4′-di-tert-butyl2,2′-bipyridine), and cis-Ru(phen)2(CN)2 (phen = 1,10phenanthroline). In addition, the photoracemization rates for enantiomers of these complexes, which are slower than that of [Ru(bpy)3]2+,6 are shown to depend on wavelength of irradiation light, solvent, temperature, and concentration of oxygen dissolved in solution. A detail kinetics analysis provides an elucidation of the variation in photoracemization rate. Furthermore, the structure of reaction intermediate is predicted by a density functional theory (DFT) calculation to reveal the photoracemization mechanism.
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EXPERIMENTAL SECTION
Materials. 2,2′-Bipyridine (bpy), 4,4′-dimethyl-2,2′-bipyridine (dmb), 1, 10-phenanthroline (phen), and 4,4′-di-tert-butyl-2,2′bipyridine (dbb) were purchased from Wako Chemical Inc. Received: March 29, 2016
A
DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry Syntheses of cis-RuL2(CN)2 (L = bpy, dmb, dbb, and phen).7 The dicyanoruthenium(II) complexes were prepared from RuL2Cl2 and NaCN by refluxing in an EtOH−H2O mixture. The complexes were purified by a silica gel column eluted with CHCl3 and recrystallized from EtOH. Caution! NaCN is highly toxic. Inhalation, ingestion, and skin contact should be avoided. cis-Ru(bpy)2(CN)2:7a electrospray ionization mass spectrometry (ESI-MS) 467(MH+), 1H NMR(400 MHz, δ CD3OD) 9.70 (2H, d, J = 5.2 Hz), 8.55 (2H, d, J = 7.9 Hz), 8.49 (2H, d, J = 8.1 Hz), 8.13 (2H, t, J = 7.7 Hz), 7.96 (2H, t, J = 7.8 Hz), 7.70 (2H, t, J = 5.7 Hz), 7.65 (2H, d, J = 4.7 Hz), 7.32 (2H, t, J = 5.5 Hz), Fourier transform infrared (FTIR; KBr) 2054 and 2068 cm−1 (CN stretching). This material was too hydroscopic to perform CHN elemental analysis. cis-Ru(dmb)2(CN)2:7b ESI-MS 523 (MH+), 1H NMR(400 MHz, δ CD3OD) 9.47 (2H, d, J = 7.1 Hz), 8.39 (2H, s), 8.32 (2H, s), 7.51 (2H, d, J = 5.0 Hz), 7.43 (2H, d, J = 5.6 Hz), 7.13 (2H, d, J = 7.1 Hz), 2.65 (6H, s), 2.47 (6H, s), FTIR (KBr) 2056 and 2072 cm−1 (CN stretching). Anal. Found for this material were C 53.62, H 5.55, N 12.07%. These analytical values could not be obtained only considering the water of crystallization. Since this material was purified by a silica-gel column eluted with CHCl3 and recrystallized from CHCl3−hexane, CHCl3 and hexane should be contained in the complex crystal. Considering the CHCl3 and hexane, calculated for Ru(dmb)2(CN)2·1.68H2O·0.68CHCl3·0.74C6H14 were C 53.62, H 5.55, N 12.07%. cis-Ru(dbb)2(CN)2:7b ESI-MS 691(MH+), 1 H NMR(400 MHz, δ CD3OD) 9.55 (2H, d, J = 5.8 Hz), 8.54 (2H, s), 8.48 (2H, s), 7.73 (2H, d, J = 6.0 Hz), 7.53 (2H, d, J = 5.8 Hz), 7.37 (2H, d, J = 5.6 Hz), 1.56 (18H, s), 1.39 (18H, s), FTIR (KBr), 2057 and 2074 cm−1 (CN stretching). Anal. Calcd for Ru(dbb)2(CN)2· 2H2O C 62.87, H 7.22, N 11.56; found C 62.87, H 7.26, N 11.75%. cisRu(phen)2(CN)2:7c ESI-MS 515(MH+), 1H NMR(400 MHz, δ CD3OD) 10.76 (2H, dd, J = 5.3, 1.2 Hz), 8.75 (2H, dd, J = 8.3, 1.1 Hz), 8.48 (2H, dd, J = 8.2, 1.2 Hz), 8.25 (2H, d, J = 9.0 Hz), 8.15 (2H, d, J = 9.0 Hz), 8.11 (2H, dd, J = 8.4, 5.3 Hz), 7.84 (2H, dd, J = 5.1, 1.3 Hz), 7.51 (2H, dd, J = 8.2, 5.2 Hz), FTIR (KBr) 2055 and 2067 cm−1 (CN stretching). Anal. Found for this material were C 53.80, H 3.39, N 14.10%. These analytical values could not be obtained only considering the water of crystallization. Since this material was purified by a silica-gel column eluted with CHCl3 and recrystallized from CHCl3−hexane, CHCl3 and hexane should be contained in the complex crystal. Considering the CHCl3 and hexane, calculated values for Ru(phen)2(CN)2·1.49H2O·0.43CHCl3·0.05C6H14 were C 53.80, H 3.39, N 14.10%. Measurements. UV−vis absorption spectra were measured by a JASCO V-530 or a Shimadzu MultiSpec-1500 spectrometer. Circular dichroism (CD) spectra were recorded on a JASCO J-820 spectropolarimeter. Emission spectra were recorded on a JASCO FP-6500 spectrofluorometer. Time profiles of emission detected by a Hamamatsu R-928 photomultiplier after a Nd:YAG laser excitation (532 nm) were recorded on a Tektronics TDS-350 digital oscilloscope. Emission lifetimes were evaluated by a least-squares fitting using the OriginPro7 package. FTIR spectra of Ru(II) complexes were measured by a JASCO FT/IR-4100. NMR spectra of these were measured by a Bruker Avance-400. ESI-MS spectra were measured by a Bruker Daltonics ESI-Ion Trap HCT ultra ETD II. Elemental analysis was performed on an Exeter Analytical CE440 elemental analyzer. Density Functional Theory and Time-Dependent Density Functional Theory Calculations. Geometry optimizations for Ru(II) complex were performed at the B3LYP8 level applying LanL2DZ9 for Ru, 6-31G(d,p)10 for C, H, and N on Gaussian 03.11 Oscillator strength ( f) and rotatory strength (R) were predicted for the geometry-optimized Ru(II) complexes by a time-dependent density functional theory (TD-DFT) calculation with the polarizable continuum model (PCM).12 General Procedures. Chemical actinometry was performed using K3[Fe(ox)3] as a standard referring to the literature.13 Monochromatic lights of 150 W Xe-lamp were used for the chemical actinometry, and light intensities were determined as 92.1 × 10−9 (at 290 nm), 1.44 × 10−9 (at 320 nm), 2.17 × 10−9 (at 380 nm), 2.82 × 10−9 (at 410 nm), 3.87 × 10−9 (at 440 nm), 4.62 × 10−9 (at 470 nm), and 6.73 × 10−9
einstein/s (at 500 nm), respectively. The entire light of the 150 W Xe lamp (>220 nm) was employed for the determination of photoracemization rates because of the low quantum yield. The optical resolution for rac-RuL2CN2 (L = bpy, dmb, dbb, and phen) was performed by means of HPLC using a Daicel CHIRALPAK IC (ϕ 4.6 × 250 mm). MeOH was used as the eluent for the HPLC. X-ray Crystallography. Single crystal of Δ-cis-Ru(dbb)2(CN)2, which was obtained from a minimal amount of CHCl3 solution of each enantiomer by slow diffusion of layered hexane, was mounted with a cryoloop and flash-cooled using a cold nitrogen stream. The X-ray diffraction data of Δ-cis-Ru(dbb)2(CN)2 were obtained at −150 ± 1 °C using a Rigaku VariMax diffractometer with a Saturn CCD detector. Data were processed using the CrystalClear14 or the ProcessAuto15 software packages, and numerical absorption corrections16 were applied. The compound crystallized in the orthorhombic crystal system, and the observed systematic absence of reflections was indicative of the enantiomorphic space group of P212121 (No. 19). The structure was solved by the direct method using SIR200817 program and refined on F 2 (with all independent reflections) using SHELXL2013 software package.18 All non-H atoms were refined anisotropically, and H atoms were introduced at the positions calculated theoretically and treated with riding models. All calculations were performed using the CrystalStructure software package.19 Molecular modeling in this manuscript was performed by VESTA 3.20
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RESULTS AND DISCUSSION Optical Resolution. The optical resolution for rac-cisRu(dbb)2(CN)2 (dbb = 4,4′-di-tert-butyl-2,2′-bipyridine) was performed using HPLC attached with a chiral column. Enantiomers were successfully separated from each other as shown in Figure 1, in which the distinct two peaks were
Figure 1. HPLC chromatogram for the optical resolution of cisRu(dbb)2(CN)2 in MeOH.
observed at retention times of ∼8 and 23 min, and the surface integrals of the peaks were identical to each other. Figure 2
Figure 2. UV−vis (top) and CD (bottom) spectra of cisRu(dbb)2(CN)2 enantiomers in MeOH. CD of the first eluted Λenantiomer is illustrated as a red line, and that of the second eluted Δenantiomer is a blue line. Oscillator strength ( f) and rotatory strength (R) predicted by TD-DFT PCM(MeOH) for Δ-cis-Ru(dbb)2(CN)2 are illustrated as green bars. B
DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Table 1. Crystal Data for Δ-cis-Ru(dbb)2(CN)2
shows UV−vis and CD spectra measured for the enantiomers. When normalized, UV−vis spectra for the enantiomers coincided with each other in the entire wavelength range, and CD spectra were also identical except for the sign of each spectrum. The large Cotton effects were observed at ∼290 nm in the CD spectra;21 the positive peak at 292 nm and the negative one at 279 nm were observed for the first eluted enantiomer, and vice versa for the second eluted. On the one hand, the determination of absolute configuration based on the exciton coupling for π−π* transition concluded that the Λenantiomer was first eluted and that the Δ-enantiomer was the next.22 On the other hand, CD corresponding to the metal-toligand charge transfer (MLCT) band at ∼450 nm was considerably small. Oscillator strengths (f values) and rotatory strengths (R values) for the singlet transitions were predicted by the TDDFT method for Δ-cis-Ru(dbb)2(CN)2, which had been geometrically optimized in advance. The calculation was performed on Gaussian0311 at the B3LYP level,8 and LanL2DZ9 for Ru and 6-31G(d,p)10 for C, N, and H were applied. Solvent effect on UV−vis and CD spectra in MeOH was taken into account of the prediction of f and R using the PCM.12 Results of the calculation were shown in Figure 2. The calculated UV−vis spectrum was in very good agreement with the experimental result. The large oscillator strength f = 0.1960 at 453.16 nm was mainly attributed to highest occupied molecular orbital−2 (HOMO−2; Ru d, CN π) → lowest unoccupied molecular orbital (LUMO; dbb π*) and HOMO− 1 (Ru d, CN π) → LUMO+1 (dbb π*) transitions (Table S5). HOMO−4 (CN π) → LUMO (dbb π*) and HOMO−3 (CN π) → LUMO+1 (dbb π*) transitions largely contributed to the large f = 0.8014 and R = +804 at 275.49 nm, and HOMO−4 (CN π) → LUMO+1 (dbb π*) and HOMO−3 (CN π) → LUMO (dbb π*) transitions f = 0.2610 and R = −766 at 277.38 nm. The calculation for Δ-enantiomer concluded a negative Cotton effect at the π−π* band. It was supported that the exciton coupling analysis was valid for the determination of absolute configuration of cis-Ru(dbb)2(CN)2. Unfortunately, the calculation for R in the MLCT region could not reproduce the experimental CD spectrum. cis-Ru(bpy) 2 (CN) 2 , cis-Ru(dmb) 2 (CN) 2 , and cis-Ru(phen)2(CN)2 were also optically resolved into enantiomers by the same procedure. The chromatograms were shown in Figure S1. For these complexes, however, the first eluted samples showed the negative CD at ∼290 nm and positive at ∼270 nm, and vice versa for the second eluted ones (Figures S2−S4). Both the exciton coupling analyses for the eluted samples and TD-DFT calculations concluded that the first eluted enantiomers were in Δ-configuration and that the second eluted ones were in Λ-configuration. Absolute Configuration Determined by X-ray Crystallography. To determine the absolute configuration of enantiomeric cis-Ru(dbb)2(CN)2, an X-ray crystallography was performed. For the second band of HPLC, a single crystal suitable for X-ray crystallography was prepared by the slow crystallization from CHCl 3 /hexane. A result of X-ray crystallography for the crystal is listed in Table 1. The chemical formula was determined to be Ru(dbb)2(CN)2·2CHCl3·C6H14 accompanied by CHCl3 and hexane as crystal solvents. The crystal system and space group were determined as orthorhombic and P212121 (No. 19), respectively. The crystal density was evaluated as ρ = 1.264 g/cm3. The bond lengths and angles of coordinated atoms to the Ru(II) center were
Δ-cis- Ru(dbb)2(CN)2·2CHCl3·C6H14 empirical formula crystal system space group temp (°C) a (Å) b (Å) c (Å) Z V (Å3) ρ (g/cm3) R1 wR2 Flack parameter
C46H64Cl6N6Ru orthorhombic P212121 (No. 19) −150 ± 1 15.2569(19) 18.666(3) 18.731(2) 4 5334.3(12) 1.264 0.0850 0.1998 0.03(3)
Ru1−N3 2.076(7) Å, Ru1−N4 2.112(7) Å, Ru1−N5 2.131(7) Å, Ru1−N6 2.070(7) Å, Ru1−C1 2.009(10) Å, Ru1−C2 2.000(10) Å, C1−Ru1−C2 90.5(4)°, N3−Ru1−N4 77.3(3)°, N5−Ru−N6 78.0(3)°, C1−Ru1−N4 172.9(3)°, C2−Ru1−C5 170.5(3)°, N3−Ru1−N6 173.3(3)°, which were in good agreement with the results of DFT calculation.23 Also these bond lengths and angles were very similar to those of rac-cisRu(dbb)2(CN)2·2H2O previously reported.24 A perspective drawing for the enantiomeric cis-Ru(dbb)2(CN)2 is shown in Figure 3. Since the Flack parameter was determined to be
Figure 3. A perspective drawing for Δ-cis-Ru(dbb)2(CN)2. The solvents of crystallization, CHCl3 and hexane, and H atoms were omitted for clarity. Thermal ellipsoids were shown at 50% probability.
0.03(3), this enantiomer was concluded to have the Δ configuration. This result was consistent with that determined by the π−π* exciton coupling analysis and TD-DFT calculation. Photoracemization. When irradiated the broad band of UV light of 150 W Xe lamp, the CD of Λ-cis-Ru(dbb)2(CN)2 solution decreased in intensity due to the photoracemization. During the photoreaction, isosbestic points at 310, 285, 260 nm were clearly observed in CD spectral change, whereas the UV− vis spectrum was not changed at all. Figure 4 shows the CD spectral change in the photoreaction observed for MeOH solution containing Λ-cis-Ru(dbb)2(CN)2. C
DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry [Λ] − [Δ] = ([Λ]0 − [Δ]0 )exp(− 2kt )
(1)
where [Λ] and [Δ] were the concentration of Λ- and Δenantiomers, respectively, and [Λ]0 and [Δ]0 denoted the initial concentrations of them before photoirradiation. When [Δ]0 = 0 Δε /Δε0 = ([Λ] − [Δ])/[Λ]0 = exp( −2kt )
(2)
was obtained, because [Λ] − [Δ] was proportional to CD intensity (Δε). A plot of ln(Δε/Δε0) versus t gave a good straight line for the photoracemization of Λ-cis-Ru(dbb)2(CN)2 (Figure 6). From the slope (−2k) of line, the photo-
Figure 4. Change in CD spectrum of Λ-cis-Ru(dbb)2(CN)2 in MeOH measured at every 3600 s during the UV-light irradiation. A 150 W Xe lamp (λ > 220 nm) was used for the photoracemization.
We determined quantum yield (ϕ) for the photoracemization reaction of Λ-cis-Ru(dbb)2(CN)2 in MeOH, which was evaluated as a ratio between molarity of Δ-enantiomer produced in photoreaction and that of photon irradiated to the solution containing Λ-enantiomer. The molarity of photon was estimated by means of the chemical actinometry using K3[Fe(ox)3] as a standard agent. This photoracemization was considerably inefficient; ϕ was almost 1 × 10−5, which was 10 times smaller than that of [Ru(bpy)3]2+ (ϕ = 1 × 10−4).7a Since ϕ was dependent on excitation wavelength, we measured a variation in ϕ using the monochromatic lights of which wavelengths were set at an interval of 30 nm from 290 to 500 nm. Figure 5 shows an action spectrum as a function of
Figure 6. A plot of ln(Δε/Δε0) vs t for the photoracemization of Λcis-Ru(dbb)2(CN)2 in air-saturated MeOH at room temperature. A 150 W Xe lamp (>220 nm) was used for the photoracemization.
racemization rate was determined as kair = 3.43 × 10−5 s−1 for an air-saturated MeOH solution, which was considerably smaller than that of [Ru(bpy)3]2+.6a Table 2 summarizes the photoracemization rate constants of the Ru(II) complexes accompanied by respective emission Table 2. Photoracemization Rate Constant (k), Emission Lifetime (τ), and Maximal Wavelength of Emission Spectruma (λem) of Λ-cis-RuL2(CN)2 (L = dbb, bpy, dmb, phen)
Figure 5. An action spectrum for the photoracemization (black ●) and UV−vis absorption spectrum (red line) of Λ-cis-Ru(dbb)2(CN)2 in MeOH.
kair × 105, s−1 kAr × 105, s−1 τair, ns τAr, ns λem, nm
irradiation wavelength for the photoracemization, which was very similar to the UV−vis spectrum of cis-Ru(dbb)2(CN)2. This result indicated that ϕ was closely related to the amount of excited molecule by photoexcitation. According to the Kasha’s rule,25 the excited molecules were considered to be populated in the lowest 3MLCT state regardless of whether directly excited to the state or relaxed from the higher excited states. Since the 3MLCT state possessed a long lifetime as ca. 400 ns,26 the racemization was suggested to proceed via the thermally activated 3d−d* state. In addition, the direct excitation to the1,3d−d* state could also contribute to the photoracemization. When excited to the 3d−d* state, the elongation of Ru−N and/or Ru−C bonds would trigger the rearrangement of chiral structure leading to photoracemization. Photoracemization Rate. The appearance of isosbestic points in CD change, and invariance in UV−vis spectrum during the photoreaction strongly suggested that the photoracemization was described as the following first-order kinetics.
dbb
bpy
dmb
phen
3.43 16.4 77.3 409 634
3.03 6.94 86.4 394 634
2.75 12.8 70.1 319 634
0.56 1.21 126 1530 623
a
In air-saturated (air) or Ar-saturated (Ar) MeOH at Room Temperature.
lifetime (τ). The photoracemization rates of Λ-cis-Ru(bpy) 2 (CN) 2 , Λ-cis-Ru(dmb) 2 (CN) 2 , and Λ-cis-Ru(dbb)2(CN)2 in Ar-saturated MeOH were determined as kAr = 6.94 × 10−5 s−1, 12.8 × 10−5 s−1, and 16.4 × 10−5 s−1, respectively. kAr for Λ-cis-RuL2(CN)2 were in the order of L = bpy < dmb < dbb. On the one hand, the electron-donating ability or steric hindrance of substituent was likely related to the order of photoracemization rate. On the other hand, the photoracemization rate of Λ-cis-Ru(phen)2(CN)2 was evaluated as kAr = 1.21 × 10−5 s−1, which was 5 times or more smaller than the others. The rigidity of ligand was considered to reduce the photoracemization rate. For Λ-cis-RuL2(CN)2 (L = bpy, dmb, dbb) in air-saturated MeOH, as listed in Table 2, the photoracemization rates (kair) were mostly the same as 3 × 10−5 s−1, which was twice−five times smaller than those in Arsaturated MeOH solution. Emission lifetimes of the complexes in air-saturated MeOH (τair) were roughly the same as 80 ns,
Λ‐cis‐Ru(dbb)2 (CN)2 ⇌ Δ‐cis‐Ru(dbb)2 (CN)2
Provided that the forward and reverse isomerization rates were the same as k, the difference in concentration between enantiomers was represented as D
DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Λ-cis-Ru(bpy)2(CN)2 and Λ-cis-Ru(phen)2(CN)2. Actually, however, the photoracemization rate for Λ-cis-Ru(phen)2(CN)2 was 5 times smaller than that of Λ-cisRu(bpy)2(CN)2. Therefore, we concluded that the photoracemization for Λ-cis-RuL2(CN)2 proceeded via the fivecoordinate intermediate. Since, as mentioned above, the bond length of Ru−C was shorter than that of Ru−N, the Ru−C bond was considered to be hardly broken compared with the Ru−N bond. If the Ru−C bond cleavage occurred to produce a five-coordinate intermediate with eliminating a CN− ligand, the recombination of CN− to generate the other enantiomer would hardly proceed because of the small formation constant for the monodentate ligand of CN−. In this case, on the one hand, a stable five-coordinate complex would be detected in UV−vis spectrum, and the photoracemization would be deviated from the first-order kinetics. On the other hand, even if the Ru−N bond was broken in the 3d−d* state, it would be easily repaired because of the large formation constant due to the chelate effect of N^N bidentate ligand. Especially the Ru−N bond recombination for the structurally rigid phen would be more rapid than that for bpy having the rotational freedom along the C−C bond. This consideration was consistent with the low quantum yield (ϕ) and slow reaction rate (k) for cisRu(phen)2(CN)2. Since the bond lengths of Ru1−N4 2.112(7) Å and Ru1−N5 2.131(7) Å were longer than those of Ru1−N3 2.076(7) Å and Ru1−N6 2.070(7) Å, it was suggested that the bond breaking for Ru1−N4 or Ru1−N5 was easier to occur than that for Ru1−N3 or Ru1−N6, where both N4 and N5 were located at the trans positions with respect to CN ligand, and N3 and N6 were at trans positions from each other. On the contrary, according to the Adamson’s rule28 for the bond cleavage by photoexciation, the pyridine moieties of ligand in the trans configuration relative to each other were considered to undergo the selective bond breaking. In the present case, not only the bond-breaking process but also the bond recombination and rearrangement of enantiomeric structure should be taken into account of the overall photoracemization reaction. For the racemization, it is necessary to produce the five-coordinate intermediate, which was achiral and able to become alternatively Δ and Λ enantiomers in the same probability. Geometry optimization in DFT calculation for fivecoordinate complex was performed to predict the structure of intermediate. In this calculation, the optimization was performed for a model five-coordinate complex, Ru(bpy) (py) (CN)2 (py = pyridine), in the triplet state to get a stable structure for five-coordinate complex without bond recombination.29 Two initial geometries employed for the calculation were shown in Figure 7, one having a vacant site on Ru at the trans position with respect to bpy (I1) and the other having it at the trans position with respect to CN (I2). And the optimized geometries were also illustrated as F1 and F2. The structure of F2 was similar to that of I2, while F1 was different in conformation from I1 (Tables S9 and S10). For F1, the angle N−Ru−CN(1) was bent by ca. 90°, and a squarepyramidal structure was shown around Ru(II) ion; Ru, CNs, and bpy were placed on a single plane, and py was the axial ligand. These results for model complex were preliminary but gave us the essential picture of the photoracemization mechanism for cis-Ru(bpy)2(CN)2 enantiomer. Excitation to 3 d−d* state caused the (N−)Ru−N bond breaking followed by the rearrangement of structure to produce the achiral fivecoordinate square-pyramid like F1. The intermediate F1 would
which were decreased from ca. 400 ns (τAr) due to the quenching by O2 dissolved in solution. Note that the quenching by O2 effectively retarded the photoracemization; that is, the racemization was strongly suggested to be initiated from 3 MLCT excited state. The photoracemization of Λ-cisRu(phen)2(CN)2 was decreased from kAr = 1.21 × 10−5 s−1 to kair = 0.560 × 10−5 s−1 by O2. Table 3 shows the photoracemization rates of Λ-cisRu(dbb)2(CN)2 obtained in Ar-saturated H2O/MeOH (1:1), Table 3. Photoracemization Rate Constant (k), Emission Lifetime (τ), and Maximal Wavelength of Emission Spectruma (λem) of Λ-cis-Ru(dbb)2(CN)2
kair × 105/s−1 kAr × 105/s−1 τair/ns τAr/ns λem/nm
H2O/ MeOHb
MeOH
EtOH
1-PrOH
1-BuOH
21.3 50.0 202 366 624
3.43 16.4 77.3 409 634
1.07 7.65 84.0 380 641
0.753 4.91 99.8 398 643
0.930 5.15 111 383 644
a
In air-saturated (air) or Ar-saturated (Ar) solvents at room temperature. b1:1 mixture.
MeOH, EtOH, 1-PrOH, and 1-BuOH, which were dependent on solvent and in the order H2O/MeOH (5.00 × 10−4 s−1) > MeOH (1.64 × 10−4 s−1) > EtOH (7.65 × 10−5 s−1) > 1-PrOH (4.91 × 10−5 s−1), and 1-BuOH (5.15 × 10−5 s−1). However, the emission lifetime (τAr) was not in this order but H2O/ MeOH (366 ns) < EtOH (380 ns) < 1-BuOH (383 ns) < 1PrOH (398 ns) < MeOH (409 ns). The small variation in τAr was ineffective to the change in racemization rate. The coordination ability of solvent was likely effective to increase kair and kAr. On the one hand, especially, both kair and kAr in H2O/MeOH were considerably larger than the others. On the other hand, peak wavelength of emission spectrum (λem) was in the order H2O/MeOH (624 nm) < MeOH (634 nm) < EtOH (641 nm) < 1-PrOH (643 nm) < 1-BuOH (644 nm). The slight increase in MLCT energy effectively contributed to the acceleration of the photoracemization rate. The photoracemization of Λ-cis-Ru(dbb)2(CN)2 was also accelerated with increasing temperature; the higher the temperature T, the steeper the slope of line as shown in Figure S5. The relationship between kair (and kAr) and T was well-described using the Arrhenius equation, which gave an activation energy of ΔE = 4320 cm−1. From these results, the thermal activation to the higher-lying 3d−d* state was suggested to be a key step of racemization. Photoracemization Mechanism. Alternative possible intermediates, which were interchangeable between Δ and Λ enantiomers, were anticipated; one was a trigonal-prismatic sixcooridnated complex in the twisted mechanism with no dissociation of ligand,27 and the other was a five-coordinate complex produced by bond-cleavage.26 In the present case, when the excited complex molecule was thermally activated to 3 d−d* state, it would undergo the elongation of Ru−N and/or Ru−C bonds. The further bond elongation would induce the bond cleavage to generate the five-coordinate complex but not the conformational change to trigonal-prismatic structure. The formation of five-coordinate complex as an intermediate was considered to be responsible for the acceleration of kair and kAr by solvent coordination. In the twisted mechanism, there should be no difference in the photoracemization rates between E
DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
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to alternative products of Δ and Λ in the ground state. The Λ enantiomer undergoes the same photoexcitation and subsequent reactions like Δ enantiomer. The rate equation for [Λ] − [Δ], which is the difference in concentration between Λ and Δ enantiomers and corresponds to the change in CD spectrum, is described as −
d ([Λ] − [Δ]) = (k 0 + k 0′)([Λ] − [Δ]) dt − k1([Λ*] − [Δ*]) − k nr([Λd] − [Δd ])
(3)
Since, in the present work, the continuous irradiation of broad spectrum of Xe lamp was employed for the photoracemization reaction, the steady state approximation can be applied to the rate equations for the species as follows.
Figure 7. Schematic illustration for the alternative geometry optimization of Ru(bpy) (py) (CN)2 predicted by a DFT calculation.
d d d ([Λ] + [Δ]) = ([Λ*] + [Λ*]) = ([Λd] + [Δd ]) dt dt dt d = [INT] = 0 (4) dt
possess alternative reaction paths to Δ- and Λ-enantiomers in the same provability. When the N−Ru−N bond was fixed to reform the six-coordinate complex with raising CN(1) from the Ru−CNs−bpy plane, the Δ-enantiomer would be selectively produced. And when raised CN(2) up, the Λ-enantiomer would be obtained. However, when a five-coordinate complex like F2 was produced by the (NC−)Ru−N bond-breaking, the broken bond would be readily fixed to reform the initial enantiomer without rearrangement. Consequently, the photoracemization was concluded to proceed via the five-coordinate intermediate after the bond-breaking of (N−)Ru−N. This consideration would be applied to the elucidation of photoracemization mechanism of [Ru(bpy)3]2+ as well. Kinetics Study of the Photoracemization Reaction. Given that the photoracemization rate was strongly dependent on excitation wavelength, excitation light intensity, solvent, O2 concentration, and temperature, we can propose the following photoracemization scheme (Scheme 1).
When introduced α = [Λ*]/[Λ] = [Δ*]/[Δ] and β = [Λd]/ [Λ] = [Δd]/[Δ], the following equation is obtained. −
d ([Λ] − [Δ]) = {(k 0 + k 0′) − αk1 − βk nr} dt ([Λ] − [Δ])
(5)
So, we obtain the photoracemization rate k as k = (k 0 + k 0′) − αk1 − βk nr
(6)
On the one hand, since the ratio β/α gives the thermal equilibrium constant [Λd]/[Λ*] = k2/k−2 = exp (−ΔE/kBT) (kB is the Boltzmann constant) between 3MLCT and 3d−d* excited states, k is described as the following equation.
Scheme 1. A Schematic Relaxation Process Including the Isomerization between Δ and Λ Enantiomers
k = (k 0 + k 0′)
k 3 exp( −ΔE /kBT ) k1 + (k nr + k 3)exp( −ΔE /kBT )
(7)
On the other hand, the emission lifetime for MLCT excited state (τL) is represented as 1 = k1 + (k nr + k 3)exp( −ΔE /kBT ) τL
(8)
Consequently, the photoracemization rate is described as k = (k 0 + k 0′)k 3τL exp( −ΔE /kBT )
(9)
This equation elucidates well the experimental results; that is, the variation in (k0 + k′0) corresponds to the dependence on excitation wavelength and excitation light intensity, the decrease of emission lifetime (τL) due to the quenching by O2 results in the decrease of photoracemization rate, and the term exp(−ΔE/kBT) represents the solvent effect on emission peak wavelength and the temperature dependence on photoracemization rate. When considered the half of molar ratio of [INT]/[Λ*]0 as a conversion yield ϕ′, the following equation is derived.
Δ and Λ denote both enantiomers in the ground state, respectively, Δ* and Λ* those in 3MLCT excited state, Δd and Λd those in the 3d−d* excited state, and INT the intermediate such as the five-coordinate complex. When Δ is excited with the rate constant k0, Δ* is produced by the direct photoexcitation to the 3MLCT state or the rapid internal conversion and intersystem crossing from higher excited state. Δ* deactivates to the ground state with the rate k1 accompanied by emission of luminescence. The long-lived Δ* can be thermally activated to the 3d−d* state with the rate k2 to produce Δd. And Δd would also be produced by direct excitation of Δ with k0′. Δd can go back to the MLCT state with k−2, deactivate to the ground state with knr, or undergo the bond breaking of Ru−N to produce the intermediate INT with k3. INT undergoes the bond recombination with k4, which leads
ϕ′ =
k 1 [INT] 1 [INT] = = 3 2 [Λ*]0 2 [Λ*] + [Λd] 2k4
exp( −ΔE /kBT ) 1 + exp( −ΔE /kBT ) F
(10) DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
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evaluated to be 4320−5050 cm−1 from the slopes of line, which were likely dependent on solvent. The sum of ΔE and 3MLCT energy (EMLCT) gave the 3d−d* state energy (Edd), which was estimated to be 20 090−20 670 cm−1. For [Ru(bpy)3]2+, ΔE was evaluated to be ∼4000 cm−1 by many researchers.30 Since EMLCT for [Ru(bpy)3]2+ was ∼16 700 cm−1 (λem = 600 nm), Edd was roughly estimated as 20 700 cm−1, which was in very good agreement with the present case. As listed in Table 4, (k0 + k0′) k3 was also dependent on solvent. However, k0 + k0′ closely related to absorption coefficient was independent of solvent because EMLCT was mostly the same except for the H2O/ MeOH mixture. Therefore, the contribution of k3 to the solvent dependence on photoracemization rate was concluded to be large; that is, the formation of five-coordinate intermediate was concluded to be the rate-determining step of photoracemization reaction. ΔEL Estimated by Emission Lifetime Measurement. The temperature-dependent lifetime measurements were conducted to the estimation of the energy of invisible 3d−d* state lying above the emitting 3MLCT state.31 The energy gap ΔEL evaluated in the lifetime measurement was listed in Table 5 along with k1 and knr + k3. Although k1 values evaluated were roughly the same as 2 × 10−6 s−1 for entire solvents, knr + k3 and ΔEL were varied in the range from 1 × 108 to 1 × 1011 s−1 and 1450−2880 cm−1, respectively, depending on solvent. ΔEL values were smaller than those evaluated in photoracemization reaction. The larger the rate knr + k3, the larger the energy gap ΔEL; that is, the large deactivation rate was likely compensated by the large ΔE, which should retard the thermal activation to 3 d−d* state. The 3d−d* state energy (Edd,L) estimated by the sum of EMLCT and ΔEL was also dependent on solvent. This result was strange for us, because 3d−d* state energy was considered to be hardly affected by solvent in the outer sphere of complex molecule. It was suggested that the contribution of fourth MLCT state should be taken into account for the relaxation from the 3MLCT state.
Since the energy gap between 3MLCT and 3d−d* states (ΔE) was obtained as 4320 cm−1 by measurement of temperaturedependent photoracemization rate, exp(−ΔE/kBT) is negligibly small compared with 1 at room temperature. So, we obtain a simple relation. ϕ′ =
k3 exp( −ΔE /kBT ) 2k4
(11)
From this equation, we can predict that ϕ′ is dependent on only T, k3, and k4 but not on excitation wavelength and light intensity (k0 and k0′). Since the action spectrum for the photoracemization is in good agreement with the UV−vis spectrum, the photoracemization yield ϕ is concluded to be described as a product of ϕ′ and absorption coefficient ε(λ). Figure 8 shows plots of ln(k/τL) versus 1/T for the photoracemization reaction of Λ-cis-Ru(dbb)2(CN)2 in various
Figure 8. Plots of ln(k/τL) vs 1/T for the photoracemization of Λ-cisRu(dbb)2(CN)2 in H2O/MeOH, MeOH, EtOH, 1-PrOH, and 1BuOH.
solvents. Since each plot showed a good straight line, eq 9 was concluded to be valid for the description of photoracemization of Λ-cis-Ru(dbb)2(CN)2. As listed in Table 4, ΔE values were Table 4. Kinetics Parameters Estimated by the Curve Fitting Using eq 9 for Temperature-Dependent Photoracemization of Λ-cis-Ru(dbb)2(CN)2 in Various Solvents
(k0 + k′0)k3, s−2 ΔE, cm−1 b ΔE, kJ mol−1 b EMLCT, cmc Edd, cm−1 d
H2O/ MeOHa
MeOH
1-PrOH
1-BuOH
17.3
12.9
13.5
14.4
15.5
4640 55.5
4320 51.7
4650 55.6
4890 58.5
5050 60.4
16 030 20 670
15 770 20 090
EtOH
15 600 20 250
15 550 20 440
■
CONCLUSION
We successfully performed the optical resolution of rac-cisRu(dbb)2(CN)2 into a pair of Δ and Λ enantiomers using HPLC with a chiral column and determined its absolute configuration by X-ray crystallography. The photoracemization of the Λ-enantiomer was investigated, and the rate and quantum yield were determined to be k = 1 × 10−6 to 1 × 10−5 s−1 and ϕ = 1 × 10−5, respectively. The photoracemization was dependent on excitation wavelength, solvent, temperature, and emission lifetime that were successfully elucidated by the detailed kinetics analysis. The DFT calculation provided the structure of five-coordinate intermediate for photoracemization, which was the square pyramid with the plane composed of Ru,
15 530 20 580
a
1:1 mixture. bEvaluated using eq 9. cEvaluated from emission peak wavelength. dSum of ΔE and EMLCT.
Table 5. Kinetics Parameters Estimated by the Curve Fitting Using eq 8 for Temperature-Dependent Emission Lifetime Measurement for Λ-cis-Ru(dbb)2(CN)2 in Various Solvents −1
k1 × 10 , s knr + k3 × 10−10, s−1 ΔEL, cm−1 b ΔEL, kJ mol−1 b EMLCT, cm−1 c Edd,L, cm−1 d 6
a
H2O/MeOHa
MeOH
EtOH
1-PrOH
1-BuOH
1.68 1.14 1930 14.3 16 030 17 960
2.21 12.4 2670 31.9 15 770 18 440
2.15 0.290 1830 21.9 15 600 17 430
2.34 19.7 2880 34.5 15 550 18 430
2.05 0.05 1450 17.3 15 530 16 980
1:1 mixture. bEvaluated using eq 8. cEvaluated from emission peak wavelength. dSum of ΔEL and EMLCT. G
DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
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D. W. J. Mol. Struct. 2008, 890, 75. (i) Lacour, J.; Torche-Haldimann, S.; Jodry, J. J. Chem. Commun. 1998, 1733. (j) Sun, P.; Krishnan, A.; Yadav, A.; Singh, S.; MacDonnell, F. M.; Armstrong, D. W. Inorg. Chem. 2007, 46, 10312. (k) Shelton, C. M.; Seaver, K. E.; Wheeler, J. F.; Kane-Maguire, N. A. P. Inorg. Chem. 1997, 36, 1532. (l) Jiang, C.; Tong, M.-Y.; Armstrong, D. W.; Perera, S.; Bao, Y.; MacDonnell, F. M. Chirality 2009, 21, 208. (6) (a) Nagao, N.; Mukaida, M.; Miki, E.; Mizumachi, K.; Ishimori, T. Bull. Chem. Soc. Jpn. 1994, 67, 2447. (b) Porter, G. B.; Sparks, R. H. J. Photochem. 1980, 13, 123. (c) Tachiyashiki, S.; Yamatera, H. Bull. Chem. Soc. Jpn. 1982, 55, 759. (7) (a) Rillema, D. P.; Allen, G.; Meyer, T. J.; Conrad, D. Inorg. Chem. 1983, 22, 1617. (b) Abe, T.; Shinozaki, K. Inorg. Chem. 2005, 44, 849. (c) Kitamura, N.; Sato, M.; Kim, H.-B.; Obata, R.; Tazuke, S. Inorg. Chem. 1988, 27, 651. (8) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (9) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (c) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (10) (a) Petersson, G. A.; Bennett, A.; Tensfeldt, T. G.; Al-Laham, M. A.; Shirley, W. A.; Mantzaris, J. J. Chem. Phys. 1988, 89, 2193. (b) Petersson, G. A.; Al-Laham, M. A. J. Chem. Phys. 1991, 94, 6081. (11) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc: Wallingford, CT, 2004. (12) Tomasi, J.; Mennucci, B.; Cammi, R. Chem. Rev. 2005, 105, 2999. (13) Kuhn, H. J.; Braslavsky, S. E.; Schmidt, R. Pure Appl. Chem. 1989, 61, 187. (14) CrystalClear; Rigaku Corporation: Tokyo, Japan, 2008. (15) PROCESS-AUTO; Rigaku Corporation: Tokyo, Japan, 1998. (16) NUMABS; Rigaku Corporation: Akishima, Tokyo, 1999. (17) Burla, M. C.; Caliandro, R.; Camalli, M.; Carrozzini, B.; Cascarano, G. L.; De Caro, L.; Giacovazzo, C.; Polidori, G.; Siliqi, D.; Spagna, R. J. Appl. Crystallogr. 2007, 40, 609. (18) Sheldrick, G. M. Acta Crystallogr., Sect. A: Found. Crystallogr. 2008, 64, 112. (19) CrystalStructure; Rigaku Corporation: Tokyo, Japan, 2000. (20) VESTA 3, Momma, K.; Izumi, F. J. Appl. Crystallogr. 2011, 44, 1272.10.1107/S0021889811038970 (21) (a) Braterman, P. S.; Noble, B. C.; Peacock, R. D. J. Phys. Chem. 1986, 90, 4913. (b) Ziegler, M.; von Zelewsky, A. Coord. Chem. Rev. 1998, 177, 257. (22) (a) Bosnich, B. Acc. Chem. Res. 1969, 2, 266. (b) Telfer, S. G.; McLean, T. M.; Waterland, M. R. Dalton Trans. 2011, 40, 3097. (c) Telfer, S. G.; Tajima, N.; Kuroda, R. J. Am. Chem. Soc. 2004, 126, 1408. (23) Bond lengths and bond angles of the optimized structure in C2 symmetry were predicted as follows: Ru−C 2.0139 Å, Ru−C′ 2.0139 Å, Ru−N 2.1697 Å, Ru−N′ 2.0940 Å, C−Ru−C′ 91.8265°, C−Ru−N 87.9112°, C−Ru−N′ 89.3693°, C′−Ru−N 170.4725°, C′−Ru−N′ 93.8757°,N−Ru−N′ 76.5990°. (24) Shinozaki, K.; Tenmyo, K.; Suzuki, T. RSC Adv. 2013, 3, 7579. (25) Kasha, M. Discuss. Faraday Soc. 1950, 9, 14.
dbb, and two CN ligands and the axial ligand of monodentate dbb. From the analysis of temperature dependence of racemization rate, the energy gap ΔE between 3MLCT and 3 d−d* states was determined to be 4500−5000 cm−1, which gave the invisible 3d−d* state energy of 20 100−20 700 cm−1. On the one hand, this estimation was in good agreement with that of [Ru(bpy)3]2+. On the other hand, ΔEL evaluated from the temperature-dependent emission lifetime provided ΔEL = 2000 cm−1, which was likely underestimated.
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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.6b00772. UV−vis, CD, oscillator strength (f), rotatory strength (R), temperature-dependent photoracemization rate, chromatograms for cis-Ru(bpy) 2 (CN) 2 , cis-Ru(dmb)2(CN)2, and cis-Ru(phen)2(CN)2, DFT and TDDFT calculation results for cis-Ru(bbb)2(CN)2, cisRu(bpy) 2 (CN) 2 , cis-Ru(dmb) 2 (CN) 2 , and cis-Ru(phen)2(CN)2, results of geometry optimization for cisRuL2(CN)2 (L = bpy, dmb, phen) and five-coordinate species Ru(bpy) (py) (CN), and the derivation of rate equations. (PDF) The result of X-ray crystallography of Δ-cis-Ru(dbb)2(CN)2·2CHCl3·C6H14. (CIF)
■
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Author Contributions
All authors contributed equally to this work. Notes
The authors declare no competing financial interest.
■ ■
ACKNOWLEDGMENTS We thank Prof. T. Suzuki (Okayama Univ.) for the measurement of X-ray crystallography. REFERENCES
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DOI: 10.1021/acs.inorgchem.6b00772 Inorg. Chem. XXXX, XXX, XXX−XXX
