MetalâIndolizine Zwitterion Complexes as a New Class of

Article pubs.acs.org/Organometallics

Metal−Indolizine Zwitterion Complexes as a New Class of Organometallic Material: a Spectroscopic and Theoretical Investigation Lai-Hon Chung,† Chi-Fung Yeung,† Dik-Lung Ma,‡ Chung-Hang Leung,§ and Chun-Yuen Wong*,† †

Department of Biology and Chemistry, City University of Hong Kong, Tat Chee Avenue, Kowloon, Hong Kong SAR, People’s Republic of China ‡ Department of Chemistry, Hong Kong Baptist University, Kowloon Tong, Hong Kong SAR, People’s Republic of China § State Key Laboratory of Quality Research in Chinese Medicine, Institute of Chinese Medical Sciences, University of Macau, Macao, People’s Republic of China S Supporting Information *

ABSTRACT: Indolizine zwitterion coordinated metal species have been commonly proposed as intermediates in the mechanisms of metal-catalyzed cycloisomerization of propargylic pyridines for indolizines. Yet, it is only recently that the first metal−indolizine complexes have been isolated by our group. Considering from the perspective of molecular materials, the πinteraction between the dπ(M) and the π-system of the indolizine skeleton in the electronic ground or excited states may allow charge delocalization and offer functionalities for optoelectronic applications. We herein report the synthesis and spectroscopic and theoretical investigations on two classes of Ru−indolizine zwitterion complexes. The synthetic strategy employed, i.e., cycloisomerization of propargylic pyridines, represents a general preparation method for stable metal−indolizine complexes. Indolizine zwitterions in this work have been found to exhibit strong trans effect. Spectroscopic studies on these complexes reveals the tunability of the π*(indolizine) level and its impact on the luminophores nearby. Overall, indolizine zwitterion represents a new class of organometallic ligand with high potential in the design of functional molecular electronic/photonic elements.

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INTRODUCTION Indolizine, a class of fused-ring N-heterocycle, is a commonly encountered structural motif in natural products and an important building block for pharmaceuticals.1 Derivatives of indolizine also exhibit rich luminescent properties and have been employed as emitter materials for light-emitting devices.2 In the past decades, a great progress has been made in the preparation of functionalized indolizines due to the discovery of metalcatalyzed cycloisomerization of propargylic pyridines.3 While these studies initiated a large number of follow-up development for the efficient synthesis of related N-heterocycles,4,5 studies on metalated indolizine species are rare, although they are believed to be important intermediates in the catalytic cycles. Considering from the perspective of molecular materials, metal−indolizine complexes would be an interesting class of organometallic material if there exists π-interaction between the dπ(M) and the π-system of the indolizine skeleton in the electronic ground or excited states, as such interactions would allow charge delocalization and offer functionalities for optoelectronic applications. Our group has been scrutinizing the metal−carbon bonding interactions in a variety of organometallic complexes including metal−alkynyl, −alkoxycarbene, −allenylidene, −carbonyl, −cyano, −isocyano, and −N-heterocyclic carbene complexes with both spectroscopic and theoretical © 2014 American Chemical Society

approaches and demonstrated that the photophysical properties derived from these complexes are strong functions of the metal− carbon bonding interactions.6 Very recently, we have communicated the isolation of the first metal−indolizine complexes (more specifically indolizine zwitterion complexes).7 Given that stable metal−indolizine complexes can be obtained and with the vision that they may possess interesting properties for photonic applications, it would be beneficial to probe the electronic transitions associated with [metal−indolizine] moieties and examine the metal−indolizine bonding interactions in both the ground and excited states. We herein report the synthesis and spectroscopic and theoretical studies on two series of ruthenium complexes bearing indolizine zwitterion as ligand: the first series [Ru([14]aneS4)(CH3CN)(indolizine)]2+ (1) bears the optically transparent 1,4,8,11-tetrathiacyclotetradecane ([14]aneS4) which allows investigation of any charge transfer transitions associated with the [Ru−indolizine] moieties, whereas the second series [Ru([9]aneS3)(bpy)(indolizine)]2+ (2) contains 2,2′-bipyridine (bpy) and another optically transparent thioether 1,4,7-trithiacyclononane ([9]aneS3), which can provide insight Received: April 9, 2014 Published: June 18, 2014 3443

dx.doi.org/10.1021/om5003705 | Organometallics 2014, 33, 3443−3452

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on the effect of the Ru−indolizine bonding interaction on the photophysics of a [Ru(bpy)] luminophore.

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RESULTS AND DISCUSSION Synthesis and Characterizations. Ruthenium−indolizine complexes bearing [14]aneS4 have been prepared by reacting [Ru([14]aneS4)Cl2] with propargylic pyridines in MeOH (Scheme 1). The chloride ligands for the resulting complexes Scheme 1. Synthetic Routes for 1 and 2

Figure 1. Perspective view of 1c as represented by 30% probability ellipsoids; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−C(1) 2.048(3), C(1)−C(2) 1.338(4), C(2)−N(1) 1.444(4), N(1)−C(3) 1.336(4), C(3)−C(4) 1.516(4), C(4)−O(1) 1.419(4), C(4)−C(5) 1.533(5), Ru−N(2) 2.145(3), Ru− C(1)−C(4) 126.9(2), Ru−C(1)−C(2) 126.9(2), C(2)−C(1)−C(4) 106.0(3).

trans- configuration. The Ru−C(1) distance (2.048(3) Å) is indicative of Ru−C single-bond character. The strong transinfluence of the indolizine ligand is revealed from the long Ru− Nacetonitrile distance (2.145(3) Å). Although the C(4) atom is sp3 hybridized, the metalated five-membered ring moiety is essentially coplanar with the quinoline moiety it fused with. The reactions between nonpyridine-substituted propargylic alcohols HCCC(OH)Ar2 and [Ru([14]aneS4)Cl2] in MeOH have also been studied in order to gain insight on the formation mechanism of Ru−indolizine complexes 1′, and Ru−methoxycarbene complexes [Ru([14]aneS4)(C(−OMe)(−CH CAr2))(Cl)]+ (3) have been isolated in high yield (Scheme 2). Scheme 2. Synthetic Route for 3

[Ru([14]aneS4)(indolizine)(Cl)]+ (1′) are substitutionally labile: signals corresponding to the chloride-ligated complex 1′ and the CH3CN-ligated complex [Ru([14]aneS4)(indolizine)(CH3CN)]2+ (1) have been observed by 1H NMR and ESI-MS spectra in an CH3CN solution of 1′. For the ease of characterization, all 1′ complexes have been converted to 1 by heating 1′ in CH3CN for 16 h. Another complex series [Ru([9]aneS3)(bpy)(indolizine)]2+ (2) have been prepared by reacting [Ru([9]aneS3)(bpy)(H2O)]2+ with propargylic pyridines in H2O. Both complex series 1 and 2 exhibit low-field 13C NMR signals at 181−187 ppm, which are characteristic for metalated carbon atoms. The molecular structure of 1c(ClO4)2 has been determined by X-ray crystallography, and the perspective view of the cation is depicted in Figure 1. The Ru atom adopts a distorted octahedral geometry with the indolizine and CH3CN coordinating in a

The molecular structure for 3c has also been determined by Xray crystallography (Figure 2). As the formation of these methoxycarbene species are well-established to be a result of nucleophilic attack by MeOH on the Cα of Ru−allenylidene species [RuCCCAr2],8 the isolation of 3 is indicative of the existence of Ru−vinylidene intermediates [RuCCH− C(OH)Ar2]. However, the fact that no Ru−methoxycarbene complexes have been detected in the synthesis of 1′ serves as an argument to discount pathways involving vinylidene intermedi3444


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Table 1. UV−Vis Absorption Data for 1 and 2 in CH3CNa at 298 K λmax/nm (εmax/dm3 mol−1 cm−1) 1a 1b 1c 2a 2b 2c a

257 (sh, 10380), 292 (sh, 3100), 426 (9220) 283 (sh, 4370), 306 (sh, 3350), 455 (10140) 283 (12810), 330 (sh, 4340), 492 (8590) 254 (sh, 13880), 292 (23700), 410 (8520), 466 (sh, 4810) 291 (29170), 431 (11520), 466 (sh, 9800) 291 (33580), 332 (sh, 8020), 463 (10300)

See Supporting Information for data obtained in MeOH and acetone.

Figure 2. Perspective view of 3c as represented by 50% probability ellipsoids; hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ru−C(1) 1.9167(14), Ru−Cl(1) 2.6143(5), C(1)−O(1) 1.327(2), C(1)−C(2) 1.469(2), C(2)−C(3) 1.349(3), Ru−C(1)−C(2) 121.45(13), Ru−C(1)−O(1) 117.96(12), C(2)− C(1)−O(1) 120.59(16).

ates. This echoes our previous experimental and theoretical investigations on the isolation of the first metal−indolizine complexes.7 With reference to the previous studies, a 5-endo-dig cyclization mechanism (Scheme 3) is suggested for the formation of Ru−indolizine species in this work. Scheme 3. Proposed Mechanism for the Cycloisomerization of Propargylic Pyridines into Metal−Indolizine Zwitterion Complexes in This Work

Figure 3. UV−vis absorption spectra of 1 and 2 in CH3CN at 298 K.

indolizine is increased, consistent with a dπ(RuII) → π*(indolizine) metal-to-ligand charge transfer (MLCT) transition assignment. This assignment is further supported by theoretical calculations: time-dependent density functional theory (TDDFT) calculations on 1 produced simulation spectra (Figure 4) not only resemble the corresponding experimental spectra, but the trend for the calculated lowest-energy transition (vertical transitions marked with * in Figure 4) also parallel those observed experimentally. The calculated vertical transitions with λ > 350 nm are summarized in Table 2, and Table 3 summarizes the compositions of the molecular orbitals (MOs) involved in the lowest-energy electronic transitions. For series 1, the calculated lowest-energy dipole allowed transitions mainly originate from the HOMO → LUMO transitions. The HOMOs have greater Ru contribution (46.7−49.7%) than that in LUMOs (2.4−3.3%), whereas the LUMOs have greater indolizine contribution (93.7−95.9%) than that in HOMOs (23.6−29.4%). Thus, the lowest-energy transitions for 1 are dπ(RuII) → π*(indolizine) MLCT in character. The electronic difference density plots for 1 in their lowest-energy excited states (Figure 5, generated by taking the difference between the excited state electron density and ground state electron density) also clearly show that electronic charge is depleted from the dπ(Ru) and accumulated at the π*(indolizine).

The difference in the trans-labilizing effect between indolizine and methoxycarbene ligands is worth discussing. The chlorides trans to the methoxycarbene ligands in 3 are substitutionally inert, whereas the chlorides trans to the indolizine ligand in 1′ are labile and can readily be substituted by CH3CN. Considering the trans effect/influence to be principally electrostatic in nature, ligands (say L in a linear [L−M−X] system) with stronger donating ability exhibit stronger trans effect/influence as they increase the electron density on M and thus leading to a stronger repulsive interaction between M and X.9 Given the facts that (i) the Ru−Cindolizine distance for 1c (2.048(3) Å) is significantly longer than the Ru−Ccarbene distance for 3c (1.917(2) Å), and (ii) the indolizine ligands exert stronger trans effect than methoxycarbene ligand, it can be deduced that the metalated C atom in 1 carry higher negative charge than that in 3. This is consistent with the indolizine zwitterion description depicted in Scheme 3. Spectroscopic, Electrochemical, and Theoretical Studies. The UV−visible absorption data for 1 and 2 are summarized in Table 1, and their spectra recorded in CH3CN are depicted in Figure 3. In series 1, each complex features an absorption band in the 340−600 nm region with εmax ≈ 1 × 104 dm3 mol−1 cm−1. These absorption bands red-shift in energy as the number of electron withdrawing N atom or the degree of conjugation on the 3445


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Table 3. Selected Molecular Orbital Compositions (%) for 1 from TD-DFT/COSMO Calculations 1a

1b

1c

1a 1b 1c

major contribution

435.6 464.5 520.1

0.2677 0.2947 0.2171

78% H→L, 19% H−1→L 71% H→L, 24% H−1→L 79% H→L, 18% H−1→L

indolizine

[14]aneS4

CH3CN

10.27 25.77 93.67 97.33 10.12 23.60 94.10 98.36 9.97 29.37 95.89 98.54

3.64 3.56 0.74 0.04 3.64 3.62 0.64 0.04 3.82 3.38 0.37 0.02

33.59 21.79 2.25 1.92 33.82 23.06 2.04 1.26 33.29 20.58 1.35 1.18

reveal that the visible absorptions are mixtures of several vertical transitions (Figure 6). The calculated vertical transitions with λ > 350 nm are summarized in Table 4, and the compositions of the MOs involved in these electronic transitions are summarized in Table 5. The HOMOs are majorly composed of Ru (43.7− 46.2%), while the LUMOs are dominated by bpy in 2a (68.5%) and indolizine in both 2b and 2c (90.3−95.6%). The electronic difference density plots for the three dominant vertical transitions (marked with Roman numbers I, II, and III in Figure 6) are depicted in Figure 7. In the case of 2a, the higher energy transitions I and II are dπ(RuII) → π*(indolizine) MLCT in nature, whereas the lowest energy transition III is almost dπ(RuII) → π*(bpy) MLCT in character. This is different from the case of 2c that its transition I is dπ(RuII) → π*(bpy) MLCT in character and the transitions II and III are dπ(RuII) → π*(indolizine) MLCT in nature. 2b can be regarded as an intermediate case, for which the transition III is a mixture of dπ(RuII) → π*(indolizine) and dπ(RuII) → π*(bpy) MLCT transitions. It is therefore clear that the nature of the lowest energy transition for complexes 2 is an interplay between the π*(indolizine) and the π*(bpy) levels; lowering the π*(indolizine) can effectively increase the dπ(RuII) → π*(indolizine) character in the lowest energy transition and

Table 2. Calculated Vertical Transition Energies (λ > 350 nm) for 1 from TD-DFT/COSMO Calculationsa oscillator strength

Ru 52.49 48.89 3.34 0.71 52.42 49.72 3.22 0.34 52.92 46.66 2.40 0.26

Figure 5. Electronic difference density plots for 1a, 1b, and 1c of the lowest-energy excited states (corresponding to the vertical transitions marked with * in Figure 4; isodensity value = 0.004 au).

Figure 4. Calculated absorption spectra for 1a, 1b, and 1c from TDDFT/COSMO calculations. Excitation energies and oscillator strength are shown by the blue vertical lines; spectra (in black) are convoluted with Gaussian function having full width half-maximum of 3000 cm−1. See Figure 5 for the electronic difference density plots for the vertical transitions marked with *.

λ/nm

MO HOMO−1 HOMO LUMO LUMO+1 HOMO−1 HOMO LUMO LUMO+1 HOMO−1 HOMO LUMO LUMO+1

Only excitations with oscillator strength >10−1 are listed; see Supporting Information for detailed data.

a

Complexes 2 also exhibit absorptions in the 400−600 nm region (Figure 3). However, a closer look reveals that the absorption profiles are not Gaussian in shape and can be resolved into two absorption bands. Because Ru(II) complexes bearing bipyridine-type ligands are known to display dπ(RuII) → π*(bipyridine) MLCT transitions in the visible region,10 the visible absorption bands for 2 may be attributed to a mixing of dπ(RuII) → π*(indolizine) MLCT and dπ(RuII) → π*(bpy) MLCT transitions. TD-DFT calculated absorption spectra for 2 3446


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Table 5. Selected Molecular Orbital Compositions (%) for 2 from TD-DFT/COSMO Calculations 2a

2b

2c

MO

Ru

indolizine

bpy

[9]aneS3

HOMO−2 HOMO−1 HOMO LUMO LUMO+1 LUMO+2 HOMO−2 HOMO−1 HOMO LUMO LUMO+1 LUMO+2 HOMO−2 HOMO−1 HOMO LUMO LUMO+1 LUMO+2

63.93 57.53 46.00 3.53 4.97 8.01 64.18 57.85 46.24 1.79 6.78 0.32 62.77 58.14 43.67 1.26 6.42 7.27

5.62 11.12 29.11 25.06 72.51 6.13 5.59 10.75 28.23 90.33 7.31 98.26 6.15 10.81 33.38 95.55 3.66 5.77

8.09 12.00 9.37 68.48 20.26 80.49 7.97 11.83 9.56 6.25 82.58 0.54 8.08 11.54 8.51 1.96 86.60 81.75

22.35 19.35 15.52 2.93 2.26 5.38 22.25 19.56 15.97 1.63 3.33 0.87 23.00 19.51 14.45 1.23 3.31 5.20

Supporting Information). As the HOMOs for the complexes are dominated by the Ru centers (∼40−50%), the oxidation waves for the complexes are assigned as Ru(II/III) couples. The oxidation potentials for complexes 2 are about 60−70 mV more positive than their corresponding complexes 1. Among all the complexes in this work, only 2a exhibits luminescence in several common organic solvents at ∼630 nm upon photoexcitation, and its quantum yields (Φ) and lifetimes (τ) at 298 K are ∼10−3 and ∼0.5 μs, respectively (Table 7, Figure 9); its emission blue-shifts to 605 nm and possesses much longer τ (∼9 μs) in glassy n-butyronitrile at 77K. The fact that no emission signal could be detected for complexes 1 suggests that the dπ(RuII) → π*(indolizine) MLCT absorptions do not lead to any radiative decay, and therefore the luminescent behavior of 2a is likely to be originated from a dπ(RuII) → π*(bpy) 3MLCT excited state. A reasonable explanation on the nonemissive behavior for 2b and 2c would be that their dπ(RuII) → π*(bpy) 3 MLCT excited states are not their lowest-energy excited states. To verify this proposition, the lowest-energy triplet states for 2 (denoted as 2*) have been optimized, and their spin density plots clearly reveal that 2a* is dπ(RuII) → π*(bpy) 3MLCT in nature, whereas 2b* and 2c* are dπ(RuII) → π*(indolizine) 3 MLCT in character (Figure 10). This is consistent with our previous spectroscopic studies that the lowest energy transition for complexes 2 is an interplay between the π*(indolizine) and

Figure 6. Calculated absorption spectra for 2a, 2b, and 2c from TDDFT/COSMO calculations. Excitation energies and oscillator strength are shown by the blue vertical lines; spectra (in black) are convoluted with Gaussian function having full width half-maximum of 3000 cm−1. See Figure 7 for the electronic difference density plots for the marked vertical transitions.

can make a pure dπ(RuII) → π*(indolizine) MLCT to be the lowest energy transition as an extreme case. Electrochemical data for all the complexes are summarized in Table 6. Cyclic voltammograms of 1c and 2c are depicted in Figure 8. Both 1 and 2 show reversible oxidation couples and irreversible reduction waves. The ease of oxidation for the complexes with the same auxiliary ligands is in the order c > a > b, which parallels the calculated energies for the HOMOs (see

Table 4. Calculated Vertical Transition Energies (λ > 350 nm) for 2 from TD-DFT/COSMO Calculationsa 2a

2b

2c

a

λ/nm (transition)a

oscillator strength

major contribution

446.4 (III) 408.2 (II) 394.8 (I) 444.4 (III) 420.3 (II) 415.8 (I) 493.0 (III) 444.9 (II) 437.8 (I)

0.1086 0.1582 0.0553 0.1564 0.0637 0.1190 0.1998 0.0321 0.0503

66% H→L, 24% H−1→L 76% H→L+1 66% H−2→L+1, 23% H−2→L 37% H→L, 36% H−1→L+1, 14% H→L+1 38% H→L+1, 24% H−2→L, 18% H→L, 15% H−2→L+1 70% H−2→L, 11% H→L+1, 10% H→L 81% H→L, 13% H→L+1 80% H−2→L, 15% H−2→L+1 58% H→L+1, 17% H−1→L+1

Only the three dominant vertical transitions (labeled as I, II, and III in Figure 6) are listed; see Supporting Information for detailed data. 3447


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Figure 8. Cyclic voltammograms for 1c and 2c (supporting electrolyte: 0.1 M [Bu4N]PF6 in CH3CN; 298 K; scan rate = 100 mV s−1).

Table 7. Emission Data for 2a solvent, temperature

λmax/nm

Φ

τ/μs

CH3CN, 298 Ka MeOH, 298 Ka Acetone, 298 Ka n-butyronitrile,77Kb

624 623 629 605, 659(sh)

1.17 × 10−3 1.36 × 10−3 2.00 × 10−3

0.41 0.35 0.53 8.74

Concentration =3 × 10−5 M, λex = 450 nm. bConcentration =3 × 10−4 M, λex = 450 nm.

a

Figure 7. Electronic difference density plots for 2a, 2b, and 2c of the lowest-energy excited states (corresponding to the vertical transitions marked with I, II, and III in Figure 6; isodensity value = 0.004 au).

Table 6. Electrochemical Data for 1 and 2a E1/2b/V vs Cp2Fe+/0 1a 1b 1c 2a 2b 2c

Figure 9. Excitation and emission spectra for 2a.

reduction

oxidation

−1.96c −1.59c −1.55c −1.86c −1.56c −1.48c

0.63 0.66 0.61 0.70 0.73 0.67

a

Supporting electrolyte: 0.1 M [Bu4N]PF6 in CH3CN. bE1/2 = (Epc + Epa)/2 at 298 K for reversible couples. cIrreversible; the recorded potential is the cathodic peak potential at scan rate of 100 mV s−1.

the π*(bpy) levels. Interestingly, 2a exhibits improved Φ and longer τ than its alkynyl analogue [Ru([9]aneS3)(bpy)(C CPh)]+ (Φ = 5.44 × 10−4, τ = 59 ns in degassed CH3CN),6e presumably because the indolizine zwitterion serves as a stronger σ-donor than alkynyl ligand and thus elevate the metal-centered 3 dd quenching state at a greater extent.

Figure 10. Löwdin spin density distribution in the lowest-energy triplet states for 2 (denoted as 2*) (surface isovalue = 0.004 au). Positive and negative spin densities are shown in pink and cyan, respectively.

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CONCLUSIONS In this work, we have reported the synthesis and spectroscopic and theoretical investigation on two classes of Ru−indolizine zwitterion complexes. Although metal−indolizine species are generally believed to be highly reactive and hard to be isolated/

probed, the synthetic strategy employed in this work, i.e., cycloisomerization of propargylic pyridines, represents a general preparation method for stable metal−indolizine complexes. The formation mechanism for these Ru−indolizine complexes is 3448


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10−AL, where A is the absorbance at the excitation wavelength and L is the optical path length. [Ru([14]aneS4)(CH3CN)(indolizine)](ClO4)2, 1(ClO4)2. A mixture of [Ru([14]aneS4)Cl2] (0.23 mmol) and propargylic pyridines (0.51 mmol) was refluxed in MeOH (30 mL) under argon for 2 h. Upon cooling to room temperature, MeOH was removed under vacuum and the resultant red or purple gel was dissolved in CH3CN (25 mL) and transferred to a Schlenk tube. The reaction mixture was then degassed and heated at 60 °C under argon for 16 h, during which the color of the reaction mixture turned to be yellow or red. After cooling to room temperature, a saturated aqueous NaClO4 solution (5 mL) was added and the CH3CN was removed under vacuum to give yellow or red solids. The solids were collected by suction filtration and washed with H2O (5 mL × 2) followed by EtOH/Et2O mixture (1:10 v/v, 10 mL × 3). The resultant solids were recrystallized by slow diffusion of Et2O into CH3CN solution of 1(ClO4)2 to yield brown or deep-red crystals. Complex 1a(ClO4)2. Yield 0.249 g, 63%. Anal. Calcd for C21H32N2S4RuCl2O9: C, 33.33; H, 4.26; N, 3.70. Found: C, 33.63; H, 4.57; N, 3.78. 1H NMR (400 MHz, CD3CN): δ 1.60 (s, 3H, CH3 on indolizinyl ligand), 1.56−1.71 (m, 2H, [14]aneS4), 2.55−2.68 (m, 2H, [14]aneS4), 2.78−2.93 (m, 8H, [14]aneS4), 2.94−3.06 (m, 2H, [14]aneS4), 3.12−3.42 (m, 6H, [14]aneS4), 3.71 (s, 1H, −OH), 6.87 (s, 1H, Hi), 7.60−7.69 (m, 1H, Hk), 7.84 (d, J = 7.6 Hz, 1H, Hm), 7.99 (t, J = 7.6 Hz, 1H, Hl), 8.28 (d, J = 5.8 Hz, 1H, Hj). 13C NMR (100.6 MHz, CD3CN): δ 26.16 (CH3 on indolizinyl ligand), 27.02, 34.02, 36.87, 37.33 (10 carbons on [14]aneS4, resolved with 1H−13C HSQC NMR experiment), 90.66 (C−OH), 121.69 (Cm), 126.92 (Ck), 129.08 (Ci), 131.78 (Cj), 139.23 (Cl), 165.20 (quaternary carbon), 181.03 (Ru−C). ESI-MS: m/z 618.0 [M − CH3CN − ClO4]+. Complex 1b(ClO4)2. Yield 0.194 g, 49%. Anal. Calcd for C20H31N3S4RuCl2O9: C, 31.70; H, 4.12; N, 5.55. Found: C, 31.42; H, 4.42; N, 5.44. 1H NMR (400 MHz, CD3CN): δ 1.63 (s, 3H, CH3 on indolizinyl ligand), 1.56−1.71 (m, 2H, [14]aneS4), 2.55−2.68 (m, 2H, [14]aneS4), 2.75−2.94 (m, 8H, [14]aneS4), 2.95−3.05 (m, 2H, [14]aneS4), 3.13−3.42 (m, 6H, [14]aneS4), 3.77 (s, 1H, −OH), 6.99 (s, 1H, Hi), 7.65 (dd, J = 6.2, 5.2 Hz, 1H, Hk), 8.48 (dd, J = 6.2, 1.8 Hz, 1H, Hj/Hl), 8.77 (dd, J = 5.2, 1.8 Hz, 1H, Hj/Hl). 13C NMR (100.6 MHz, CD3CN): δ 24.74 (CH3 on indolizinyl ligand), 27.02, 34.02, 34.04, 36.94, 37.35 (10 carbons on [14]aneS4, resolved with 1H−13C HSQC NMR experiment), 88.71 (C−OH), 122.25 (Ck), 127.80 (Ci), 137.86 (Cj/Cl), 156.13 (Cj/Cl), 172.22 (quaternary carbon), 185.85 (Ru−C). ESI-MS: m/z 619.0 [M − CH3CN − ClO4]+. Complex 1c(ClO4)2. Yield 0.227 g, 54%. Anal. Calcd for C25H34N2S4RuCl2O9: C, 37.22; H, 4.25; N, 3.47. Found: C, 37.12; H, 4.59; N, 3.50. 1H NMR (400 MHz, CD3CN): δ 1.66 (s, 3H, CH3 on indolizinyl ligand), 1.62−1.77 (m, 2H, [14]aneS4), 2.57−2.68 (m, 2H, [14]aneS4), 2.77−2.96 (m, 8H, [14]aneS4), 2.98−3.10 (m, 2H, [14]aneS4), 3.20−3.34 (m, 4H, [14]aneS4), 3.35−3.44 (m, 2H, [14]aneS4), 3.81 (s, 1H, −OH), 7.40 (s, 1H, Hi), 7.83−7.90 (m, 1H, Ho/Hp), 7.95 (d, J = 8.4 Hz, 1H, Hm/Hl), 8.06−8.12 (m, 1H, Ho/Hp), 8.18−8.24 (m, 1H, Hn/Hq), 8.33−8.40 (m, 1H, Hn/Hq), 8.60 (d, J = 8.4 Hz, 1H, Hm/Hl). 13C NMR (100.6 MHz, CD3CN): δ 25.97 (CH3 on indolizinyl ligand), 27.06, 34.11, 34.16, 36.94, 37.46 (10 carbons on [14]aneS4, resolved with 1H−13C HSQC NMR experiment), 92.31 (C−OH), 117.41 (Cm/Cl), 119.12 (Cn/Cq), 124.77 (Ci), 130.02 (Co/ Cp), 130.46 (quaternary carbon), 130.49 (Cn/Cq), 131.37 (quaternary carbon), 135.02 (Co/Cp), 140.16 (Cm/Cl), 168.19 (quaternary carbon), 181.91 (Ru−C). ESI-MS: m/z 666.2 [M − CH3CN − ClO4]+. [Ru([9]aneS3)(N∧N)(indolizine)](ClO4)2, 2(ClO4)2. A mixture of [Ru([9]aneS3)(N∧N)(H2O)](ClO4)2 (0.23 mmol) and propargylic pyridines (0.51 mmol) was refluxed in deionized H2O (25 mL) under argon for 2 h. Upon cooling to room temperature, the resultant orange solution was added saturated aqueous NaClO4 solution (5 mL) to give organe precipitates which were filtered and washed with water (5 mL × 2), EtOH (5 mL × 2) and finally with Et2O (10 mL × 3). The solids were recrystallized by slow diffusion from Et2O into CH3NO2 solutions of 2(ClO4)2 to give bright-orange crystals. Complex 2a(ClO4)2. Yield 0.208 g, 76%. Anal. Calcd for C25H29N3S3RuCl2O9: C, 38.31; H, 3.73; N, 5.36. Found: C, 38.05; H, 3.50; N, 5.25. 1H NMR (400 MHz, d6-acetone): δ 1.33 (s, 3H, CH3 on

consistent with a 5-endo-dig cyclization pathway. Acting as a ligand, indolizine zwitterions in this work have been found to possess strong trans effect as suggested by structural and substitutional reactivity studies. Spectroscopic studies on the Ru−indolizine complexes reveal the tunability of the π*(indolizine) level and its impact on luminophores nearby. Overall, indolizine zwitterion represents a class of new organometallic ligand with high potential in the design of molecular electronic/photonic elements. Applications of these ligands in polymeric organometallic conductor and emitter are currently under investigation in our laboratory.

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under an argon atmosphere using standard Schlenk techniques unless otherwise stated. All reagents were used as received, and solvents for reactions were purified by a PureSolv MD5 solvent purification system. [Ru([9]aneS3)(bpy)(H2O)](ClO4)211 and propargylic pyridines/alcohol3i were prepared in accordance with literature methods. 1H, 13C{1H}, DEPT-135, 1H−1H COSY, and 1H−13C HSQC NMR spectra were recorded on a Bruker 400 DRX FT-NMR spectrometer (Scheme 4).

Scheme 4. Labeling Scheme for H and C Atoms in the NMR Assignment

Peak positions were calibrated with solvent residue peaks as internal standard. Electrospray mass spectrometry was performed on a PESCIEX API 3000 triple quadrupole mass spectrometer. Elemental analyses were done on an Elementar Vario Micro Cube carbon− hydrogen−nitrogen elemental microanalyzer. UV−visible spectra were recorded on a Shimadzu UV-1700 spectrophotometer. Cyclic voltammetry was performed with a CH Instrument model 600C series electrochemical analyzer/workstation. All the electrochemical measurements were performed in CH3CN solution with [n-Bu4N]PF6 (0.1 M) as supporting electrolyte at room temperature. The glassy-carbon working electrode was polished with 0.05 μm alumina on a microcloth, sonicated for 5 min in deionized water, and rinsed with CH3CN before use. An Ag/AgNO3 (0.1 M in CH3CN) electrode was used as reference electrode, with a platinum wire as the counter electrode. All solutions were degassed with nitrogen before experiments. The E1/2 value of the ferrocenium/ferrocene couple (Cp2Fe+/0) measured in the same solution was used as an internal reference. Steady-state emission spectra were obtained on a Jobin Yvon Fluorolog-3-TCSPC spectrophotometer. Sample and standard solutions were degassed with at least three freeze−pump−thaw cycles. The emission quantum yields were measured by the method of Demas and Crosby12 with [Ru(bpy)3](PF6)2 in degassed CH3CN as standard (Φr = 0.062) and calculated by Φs = Φr(Br/Bs)(ns/nr)2(Ds/Dr), where the subscripts s and r refer to sample and reference standard solution, respectively, n is the refractive index of the solvents, D is the integrated intensity, and Φ is the luminescence quantum yield. The quantity B is calculated by B = 1− 3449


Organometallics

Article

on Ph, resolved with 1H−13C HSQC NMR experiment), 135.76 (Cβ), 139.16, 140.21, 142.55 (Cγ + 2 quaternary carbons on Ph), 298.66 (Ru− C). ESI-MS: m/z 627.5 [M − ClO4]+. Complex 3b(ClO4 ). Yield 0.182 g, 67%. Anal. Calcd for C26H32S4RuCl4O5: C, 39.25; H, 4.05; N, 0.00. Found: C, 39.28; H, 4.25; N, 0.18. 1H NMR (400 MHz, CD3NO2): δ 1.40−1.56 (m, 2H, [14]aneS4), 2.49−2.60 (m, 2H, [14]aneS4), 3.04−3.17 (m, 8H, [14]aneS4), 3.34−3.46 (m, 4H, [14]aneS4), 3.69−3.82 (m, 4H, [14]aneS4), 4.11 (s, 3H, CH3O−), 6.38 (s, 1H, H on Cβ), 7.12−7.18 (m, 2H, Ph), 7.38−7.47 (m, 6H, Ph). 13C NMR (100.6 MHz, CD3NO2): δ 27.06, 33.73, 37.89 (10 carbons on [14]aneS4), 66.51 (CH3O−), 130.02, 130.03, 131.77, 132.32 (8 carbons on Ph, resolved with 1H−13C HSQC NMR experiment), 136.35 (Cβ), 135.75, 136.48, 136.68, 138.50, 140.86 (Cγ + 4 quaternary carbons on Ph), 298.00 (Ru− C). ESI-MS: m/z 695.1 [M − ClO4]+. Complex 3c(ClO 4). Yield 0.190 g, 71%. Anal. Calcd for C28H38S4RuCl2O7: C, 42.74; H, 4.87; N, 0.00. Found: C, 42.49; H, 5.16; N, 0.15. 1H NMR (400 MHz, CD3NO2): δ 1.48−1.64 (m, 2H, [14]aneS4), 2.50−2.63 (m, 2H, [14]aneS4), 3.05−3.17 (m, 8H, [14]aneS4), 3.34−3.46 (m, 4H, [14]aneS4), 3.72−3.83 (m, 4H, [14]aneS4), 3.83 (s, 6H, CH3O-Ph), 3.94 (s, 3H, CH3O−), 6.31 (s, 1H, H on Cβ), 6.93−7.00 (m, 4H, Ph), 7.06−7.13 (m, 2H, Ph), 7.33− 7.41 (m, 2H, Ph). 13C NMR (100.6 MHz, CD3NO2): δ 27.17, 33.78, 37.87 (10 carbons on [14]aneS4), 56.18, 56.27 (CH3O-Ph), 66.22 (CH3O−), 115.10, 115.20, 131.76, 132.32 (8 carbons on Ph, resolved with 1H−13C HSQC NMR experiment), 133.35 (Cβ), 132.75, 134.84, 139.49, 161.88, 162.21 (Cγ + 4 quaternary carbons on Ph), 298.04 (Ru− C). ESI-MS: m/z 687.3 [M − ClO4]+. X-ray Crystallography. X-ray diffraction data for 1c(ClO4)2 and 3c(ClO4) were collected on an Oxford Diffraction Gemini S Ultra X-ray single crystal diffractometer with Mo Kα radiation (λ = 0.71073 Å) at 173 and 133 K, respectively. The data were processed using CrysAlis.13 The structures were solved by Patterson method and refined by fullmatrix least-squares based on F2 with program SHELXS-97 and SHELXL-9714 within WinGX.15 All non-hydrogen atoms were refined anisotropically in the final stage of least-squares refinement. The positions of H atoms were calculated based on riding mode with thermal parameters equal to 1.2 times that of the associated C atoms. CCDC 994131 (1c(ClO4)2) and 994132 (3c(ClO4)) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Computational Methodology. DFT calculations were performed on complexes series 1, 2 (S = 0, spin-restricted Kohn−Sham solutions), and 2* (S = 1, spin-unrestricted Kohn−Sham solutions) using the ORCA software package (version 2.9.1).16 Their electronic ground states were optimized using the PBE0 functional17 accompanied by (i) the zero-order regular approximation (ZORA)18 to account for relativistic effects and (ii) the conductor-like screening model (COSMO)19 to model solvation in CH3CN. The def2-SVP basis sets were used for the H, C, N, and O atoms, while the def2-TZVP(-f) basis sets were used for the Ru and S atoms.20 Auxiliary basis sets, used to expand the electron density in the calculations, were chosen to match the orbital basis sets.21 The combination of the resolution of the identity and the “chain of spheres exchange” algorithms (RIJCOSX)22 was used to accelerate all DFT and TD-DFT calculations. Tight SCF convergence criteria (1 × 10−8 Eh in energy, 1 × 10−7 Eh in the density charge, and 1 × 10−7 in the maximum element of the DIIS error vector) were used throughout. The vertical transition energies for these complexes in CH3CN were computed using TD-DFT method with the density functional and basis sets aforementioned.

indolizinyl ligand), 2.54−2.70 (m, 4H, [9]aneS3), 2.70−2.91 (m, 2H, [9]aneS3), 2.94−3.09 (m, 4H, [9]aneS3), 3.20−3.26 (m, 1H, [9]aneS3), 3.31−3.37 (m, 1H, [9]aneS3), 4.75 (s, br, 1H, −OH), 7.02 (s, 1H, Hi), 7.66−7.75 (m, 2H, Hb + Hg), 7.76−7.82 (m, 1H, Hk), 7.89−7.98 (m, 1H, Hm), 8.08−8.23 (m, 3H, Hc + Hf + Hl), 8.55−8.65 (m, 3H, Hd + He + Hj), 9.20 (d, J = 5.6 Hz, 1H, Ha/Hh), 9.33 (d, J = 5.6 Hz, 1H, Ha/Hh). 13C NMR (100.6 MHz, d6-acetone): δ 25.13 (CH3 on indolizinyl ligand), 30.44, 31.93, 32.80, 33.84, 35.53, 38.18 (6 carbons on [9]aneS3), 90.90 (C−OH), 121.87 (Cm), 124.27 (Cd + Ce, resolved with 1H−13C HSQC NMR experiment), 127.13 (Ck), 127.95, 128.23 (Cb + Cg), 128.71 (Ci), 133.20 (Cj), 138.32, 138.35 (Cc + Cf), 140.10 (Cl), 154.38, 154.45 (Ca + Ch), 157.48, 157.51 (2 quaternary carbons on bpy), 183.61 (Ru−C). ESI-MS: m/z 684.0 [M − ClO4]+. Complex 2b(ClO4)2. Yield 0.169 g, 61%. Anal. Calcd for C24H28N4S3RuCl2O9: C, 36.74; H, 3.60; N, 7.14. Found: C, 36.52; H, 3.89; N, 7.12. 1H NMR (400 MHz, CD3NO2): δ 1.29 (s, 3H, CH3 on indolizinyl ligand), 2.35−2.58 (m, 3H, [9]aneS3), 2.64−2.90 (m, 5H, [9]aneS3), 2.92−3.15 (m, 3H, [9]aneS3), 3.22−3.32 (m, 1H, [9]aneS3), 3.39 (s, 1H, −OH), 6.93 (s, 1H, Hi), 7.56−7.63 (m, 1H, Hb/Hg), 7.64−7.73 (m, 2H, Hb/Hg + Hk), 8.05−8.17 (m, 2H, Hc + Hf), 8.38−8.44 (m, 2H, Hd + He), 8.53 (dd, J = 6.0, 2.0 Hz, 1H, Hj/Hl), 8.81 (dd, J = 6.0, 2.0 Hz, 1H, Hj/Hl), 9.05−9.11 (m, 1H, Ha/Hh), 9.16−9.22 (m, 1H, Ha/Hh). 13C NMR (100.6 MHz, CD3NO2): δ 23.90 (CH3 on indolizinyl ligand), 30.40, 32.15, 33.10, 34.14, 35.59, 38.46 (6 carbons on [9]aneS3), 89.42 (C−OH), 122.70 (Ck), 124.61 (Cd + Ce, resolved with 1H−13C HSQC NMR experiment), 127.92 (Ci), 128.23, 128.54 (Cb + Cg), 138.87 (Cc + Cf, resolved with 1H−13C HSQC NMR experiment), 138.93 (Cj/Cl), 154.29, 154.55 (Ca + Ch), 157.66 (Cj/Cl), 157.76, 157.82 (2 quaternary carbons on bpy), 160.71 (quaternary carbon on indolizinyl ligand), 186.93 (Ru−C). ESI-MS: m/z 685.3 [M − ClO4]+. Complex 2c(ClO4) 2. Yield 0.207 g, 70%. Anal. Calcd for C29H31N3S3RuCl2O9: C, 41.78; H, 3.75; N, 5.04. Found: C, 41.52; H, 3.96; N, 5.04. 1H NMR (400 MHz, CD3NO2): δ 1.33 (s, 3H, CH3 on indolizinyl ligand), 2.43−2.62 (m, 3H, [9]aneS3), 2.70−2.87 (m, 5H, [9]aneS3), 2.97−3.13 (m, 3H, [9]aneS3), 3.19 (s, 1H, −OH), 3.22− 3.32 (m, 1H, [9]aneS3), 7.54 (s, 1H, Hi), 7.57−7.64 (m, 1H, Hb/Hg), 7.66−7.73 (m, 1H, Hb/Hg), 7.84−7.95 (m, 2H, Hc/Hf + Hl), 8.03−8.17 (m, 3H, Hn/Ho/Hp/Hq + Hc/Hf), 8.18−8.25 (m, 1H, Hd/He), 8.30− 8.46 (m, 3H, Hn/Ho/Hp/Hq + Hd/He), 8.60−8.67 (m, 1H, Hm), 9.15− 9.21 (m, 1H, Ha/Hh), 9.28−9.35 (m, 1H, Ha/Hh). 13C NMR (100.6 MHz, CD3NO2): δ 24.82 (CH3 on indolizinyl ligand), 30.85, 32.16, 32.72, 33.78, 35.92, 38.10 (6 carbons on [9]aneS3), 93.05 (C−OH), 117.60 (Cl), 119.15 (Cd/Ce), 124.54, 124.57 (Cn/Co/Cp/Cq × 2), 124.79 (Ci), 128.28, 128.39 (Cb + Cg), 130.51 (Cc/Cf), 130.91 (Cd/Ce), 131.06, 132.39 (2 quaternary carbons on indolizinyl ligand), 135.65, 138.75, 138.77 (Cn/Co/Cp/Cq × 2 + Cc/Cf, resolved with 1H−13C HSQC NMR experiment), 141.71 (Cm), 154.38, 154.41 (Ca + Ch), 157.79, 157.84 (2 quaternary carbons on bpy), 168.59 (quaternary carbon on indolizinyl ligand), 183.39 (Ru−C). ESI-MS: m/z 735.2 [M − ClO4]+. [Ru([14]aneS4)Cl(MeO-carbene)](ClO4), 3(ClO4). A mixture of [Ru([14]aneS4)Cl2] (0.15 mmol) and propargylic alcohols (0.33 mmol) was refluxed in MeOH (30 mL) under argon for 16 h. Exceptionally, longer time (48 h) is required for synthesis of 3c. The resultant deep-orange mixture was added saturated NaClO4 solution (5 mL) and MeOH was removed in vacuo. Orange solids precipitated during the process and were collected by suction filtration. The solids were washed with water (5 mL × 2), EtOH (10 mL), and Et2O (10 mL × 3) and recrystallized by diffusion of Et2O into CH3NO2 solution of 3(ClO4) to give orange or red crystals. Complex 3a(ClO 4 ). Yield 0.193 g, 78%. Anal. Calcd for C26H34S4RuCl2O5: C, 42.97; H, 4.72; N, 0.00. Found: C, 42.77; H, 4.76; N, 0.20. 1H NMR (400 MHz, CD3NO2): δ 1.41−1.59 (m, 2H, [14]aneS4), 2.48−2.60 (m, 2H, [14]aneS4), 3.04−3.15 (m, 8H, [14]aneS4), 3.34−3.44 (m, 4H, [14]aneS4), 3.71−3.82 (m, 4H, [14]aneS4), 4.07 (s, 3H, CH3O−), 6.37 (s, 1H, H on Cβ), 7.13−7.19 (m, 2H, Ph), 7.38−7.48 (m, 8H, Ph). 13C NMR (100.6 MHz, CD3NO2): δ 27.09, 33.73, 37.87 (10 carbons on [14]aneS4), 66.36 (CH3O−), 129.88, 129.92, 130.19, 130.24, 130.62, 130.81 (10 carbons

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ASSOCIATED CONTENT

S Supporting Information *

Spectroscopic data, theoretical calculation details, and crystallographic data (CIF, XYZ) for 1c(ClO4)2 and 3c(ClO4). This material is available free of charge via the Internet at http://pubs. acs.org. 3450


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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The work described in this paper was supported by grants from the Hong Kong Research Grants Council (project no. CityU 103911), City University of Hong Kong (project no. 7004022), and the Special Equipment Grant from the University Grants Committee of Hong Kong (SEG_CityU02). We are grateful to Dr. Shek-Man Yiu for X-ray diffraction data collection, and technical support from Wing-Chun Lee.

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