Platinum(II) Dihalide Complexes with 9-Arsafluorenes: Effects of

Jun 27, 2017 - The X-ray data revealed that the platinum dihalide complexes with 1–5 were successfully constructed. Interestingly, although the plat...
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Platinum(II) Dihalide Complexes with 9‑Arsafluorenes: Effects of Ligand Modification on the Phosphorescent Properties Hiroaki Imoto,† Hiroshi Sasaki,† Susumu Tanaka,† Takashi Yumura,‡ and Kensuke Naka*,† †

Faculty of Molecular Chemistry and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan ‡ Faculty of Materials Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan S Supporting Information *

ABSTRACT: 9-Arsafluorenes, which can be easily and safely synthesized, are promising candidates as ligands for solid-state phosphorescent complexes due to their rigidity and bulkiness. The methyl group of 9methyl-9-arsafluorene was readily converted to various substituents, and the platinum(II) dihalide complexes with the obtained 9-arsafluorenes were synthesized. The geometry of the platinum(II) dihalide complexes was dependent on the induced substituent and the halogen. The trans isomers showed intense phosphorescence, while no emission was observed in the cis isomers. The mechanisms of the different emission behaviors were elucidated by diffuse reflection spectra and theoretical calculations. Among the emissive trans complexes, the substituent of the arsafluorene and the halogen played a key role in the emission intensity.

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INTRODUCTION Organoarsenic compounds are potentially attractive as ligands for transition metals. Since a lone pair of an arsenic atom has a diffuse orbital with s character, an arsenic atom has weak σ donation and π acceptance, steric bulkiness, and obscure coordination direction in comparison with a phosphorus atom.1 Such intrinsic natures of organoarsenic ligands sometimes produce highly functional transition-metal complexes: catalyst, luminescent material, and so on.2,3 Furthermore, a trivalent arsenic atom is much more stable in the air than a trivalent phosphorus atom, and thus organoarsenic ligands are easily handled.4,5 However, a lack of practical methods for the synthesis of organoarsenic compounds has prohibited systematic studies on their transition-metal complexes. That is, hazardous reagents, i.e., volatile and toxic precursors, must be employed in the conventional synthetic routes.6 Recently, we have developed practical synthetic methodologies for the construction of functional organoarsenic compounds. Arsenic radicals,7 electrophiles,4 and nucleophiles8 can be quantitatively generated in situ from cyclooligoarsines, which are prepared from nonvolatile inorganic precursors.9 Eventually, safe and facile As−C bond formation reactions have been attained by using these reactive species. Among them, the in situ generation of diiodoarsines is a powerful technique for the incorporation of arsenic atoms into π-conjugated backbones (Scheme 1a); arsafluorene,4 arsole,5 and dithienoarsole10 derivatives have been easily synthesized. We have been working on platinum(II) dihalide complexes with organoarsenic ligands and accumulated experimental results showing their intense solid-state phosphorescence at © XXXX American Chemical Society

Scheme 1. (a) Construction of Arsafluorene Skeleton and (b) Substituent Transformation on the Arsenic Atom

room temperature.11,12 Phosphorescent materials are crucially important in photocatalysts,13 sensitizers,14 organic light emitting diodes (OLED),15 and chemosensors.16 However, intense phosphorescence in the solid state at room temperature is still a challenging matter because excitons are readily deactivated via nonradiative pathways by molecular motions and/or intermolecular interactions under such conditions.17 To circumvent these deactivation processes, various kinds of ligands have been developed, but complicated molecular designs have been employed. In contrast to the conventional Special Issue: Tailoring the Optoelectronic Properties of Organometallic Compounds with Main Group Elements Received: March 29, 2017

A

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Organometallics molecular design, 9-phenyl-9-arsafluorene (1) is suitable for this end due to its bulkiness and rigidity, in spite of its simple structure. In platinum(II) dihalide complexes with 1 (trans[PtX2(1)2], X = Cl, Br, I), the ligand 1 can effectively freeze molecular motions and inhibit intermolecular interactions to achieve remarkably high quantum yield (up to 0.52) in the crystalline state at room temperature.12a Additionally, the solid samples of trans-[PtBr2(1)2] exhibited unique molecular recognition.12b These results motivated us to build a library of 9-arsafluorene derivatives for elucidating ligand effects on the phosphorescent properties. In a quite recent report, we have disclosed a synthetic route to obtain 9-arsafluorenes with various substituents on the arsenic atom.18 9-Methyl-9arsafluorene (2) is converted to 9-iodo- and 9-chloro-9arsafluorenes by the addition of iodine (I2) and iodine monochloride, respectively. Subsequent nucleophilic substitution gives various 9-arsafluorenes (Scheme 1b). This synthetic protocol is a strong tool to investigate structure−property relationships in the phosphorescent platinum(II) dihalide complexes with 9-arsafluorene ligands. In this work, we disclose the effects of ligand modification on the phosphorescence properties of platinum(II) dihalide complexes with 9-arsafluorenes. 9-Arsafluorene derivatives having various substituents on the arsenic atoms were prepared by employing an arsenic iodination reaction, and their platinum(II) dihalide complexes were synthesized. The molecular geometries and structural distortion of the obtained complexes were influenced by the ligands, including 9arsafluorenes and halogen atoms, resulting in different phosphorescent properties. X-ray crystallography, diffuse reflection spectra, and theoretical calculations support the elucidation of the mechanism of the difference.

Scheme 3. Synthesis of the Platinum(II) Dihalide Complexes

recrystallization. The chemical structures of the obtained complexes were confirmed by 1H and 13C{1H} NMR spectra and single-crystal X-ray diffraction. The geometries and isolated yields are summarized in Table 1. Table 1. Geometries and Isolated Yieldsa (in Parentheses) of the Obtained Complexes X

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RESULTS AND DISCUSSION Syntheses. Arsafluorene derivatives were synthesized from 2 by following the method in our report (Scheme 2).18 Iodine

L

Cl

Br

1b 2 3 4 5

trans (95) cis, trans (77) cis (68) cis, trans (58) cis (86)

trans (57) trans (65) cis (70) cis, trans (32) n.d.c

I trans trans trans trans trans

(77) (50) (60) (39) (90)

a

Scheme 2. Synthesis of Arsafluorene Ligands

Total isolated yields of cis and trans isomers in the mixture in percent. bCited from ref 12a. cRecrystallization failed because of the low solubility.

was added to a tetrahydrofuran (THF) solution of 2, and the reaction mixture was heated at 65 °C to generate 9-iodo-9arsafluorene in situ. Subsequently, substitution reactions with nbutyllithium, cyclohexylmagnesium bromide, and o-anisyllithium produced arsafluorene ligands 3−5, respectively. The chemical structures of the newly synthesized compounds were determined by 1H and 13C{1H} NMR spectra and highresolution mass spectra. Platinum(II) dihalide complexes with the arsafluorene ligands 1−5 were synthesized (Scheme 3). First, the arsafluorene ligands were reacted with cis-bis(benzonitrile)dichloroplatinum(II) (cis-[PtCl2(PhCN)2]) to obtain the platinum(II) dichloride complexes [PtCl2(L)2] (L = 1−5). The obtained dichloride complexes were converted to dibromide and diiodide complexes via reactions with the corresponding potassium halides. The purification of [PtX2(L)2] (X = Cl, Br, I, L = 1−5) was carried out by

Structural Analysis. The X-ray data revealed that the platinum dihalide complexes with 1−5 were successfully constructed. Interestingly, although the platinum precursor was cis-[PtCl2(PhCN)2], the geometries of the complexes were varied depending on the halogen and arsafluorene ligands. Heavier halogens offered crystals of trans isomers. Electrondonating substituents on the arsenic atom tend to give crystals of cis isomers. [PtCl2(3)2] and [PtCl2(5)2] adopted cis coordination in their crystalline states, and both of the cis and trans coordination crystals of [PtCl2(2)2] and [PtCl2(4)2] were obtained. In contrast, the crystals of [PtCl2(1)2] formed only trans coordination. The reason for the easy cis−trans isomerization in solutions is still unclear. As experimental evidence for the isomerization, in both cases of [PtCl2(2)2] and [PtCl2(4)2], CDCl3 solutions prepared from crystals of trans and cis isomers gave the same 1H NMR spectra, indicating that the cis−trans isomerization occurred in the solutions at room temperature.19 Additionally, when the crystals of cis-[PtCl2(2)2] were dissolved and subjected to recrystallization again, crystals of trans-[PtCl2(2)2] were obtained (for details, see the Experimental Section) and vice versa. The same experiment was applied to [PtCl2(4)2], and it was revealed that the cis and B

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Organometallics trans isomers can be interconverted. The detailed mechanism is under investigation and will be discussed in a future publication. Selected bond angles and torsion angles of the cis complexes are given in Table 2. The bite angles of the two arsafluorene

Table 3. Selected Bond Angles and Torsion Angles of Trans Complexes

Table 2. Selected Bond Angles and Torsion Angles of Cis Complexes

a

L

X

As−Pt−As (deg)

As−Pt−X (deg)

C−As−As−Ca (deg)

2 3 3 4 4 5

Cl Cl Br Cl Br Cl

95.97(2) 94.11(2) 92.75(4) 93.83(2) 93.79(5) 96.22(2)

176.42(4) 175.50(5), 176.01(5) 169.81(7), 173.25(6) 167.38(4), 174.24(4) 166.3(1), 174.4(1) 171.93(5), 176.93(5)

104.9(4) 67.7(7) 65(1) 66.2(5) 68(1) 176.8(6)

C denotes the α-carbon of the substituents on the arsenic atoms.

a b

ligands (As−Pt−As) were similar to each other (92.75− 96.22°). On the other hand, the As−Pt−X bond angles were affected by the steric hindrance of the ligands. Bulky ligands 3− 5 bent the linearity of the As−Pt−X angles, in comparison with the less hindered ligands 1 and 2. In addition, there is a huge difference in the C−As−As−C torsion angles (C denotes the αcarbon of the substituents on the arsenic atoms). In the case of 3 and 4, the torsion angles were relatively small (66.2−68°), and the two planes of the biphenyl moieties were intramolecularly overlapped. The ligand 2 slightly formed such an overlap in cis-[PtCl2(2)2] as well (torsion angle 104.9°). In contrast, in the case of cis-[PtCl2(5)2], each ligand coordinated oppositely; the biphenyl moiety of one ligand was overlapped with the anisyl group of the other. The structures of the trans complexes were similar to each other, except for trans-[PtI2(4)2] (Table 3). The trans complexes other than trans-[PtI2(4)2] adopted highly planar structures around the platinum atoms (As−Pt−As and I−As−I angles approximately 180°), and the two arsafluorene ligands are arranged upside down (C−As−As−C torsion angles approximately 180°). On the other hand, the planarity around the platinum atom in trans-[PtI2(4)2] was distorted, resulting in values of 172.60 and 175.22° for the As−Pt−As and I−As−I angles, respectively. In addition, the C−As−As−C torsion angle was 104.6°. Steric repulsion between the iodine and cyclohexyl group should cause such a large distortion. Optical Properties. For the measurements of the optical properties, solid samples of the complexes were prepared by recrystallization. Unfortunately, the cis and trans isomers of [PtBr2(4)2] were unable to be separated, and thus further investigation of [PtBr2(4)2] was not done here. Interestingly, the trans complexes showed solid-state emission even at room temperature, but no emission was observed in the cis complexes at room temperature or 77 K.20 To understand the different emission behaviors, initial electron transitions through photoexcitation were investigated by the diffuse reflection spectra of the obtained complexes in the solid state

L

X

As−Pt−As (deg)

X−Pt−X (deg)

C−As−As−Ca (deg)

b

1 1b 1b 2 2 2 3 4 4

Cl Br I Cl Br I I Cl Br

4 5

I I

179.21(4) 179.00(3) 179.55(4) 180.00(2) 180.00(3) 180.00(2) 180.00(2) 180.00(2) 180.00(4) 180.00(3) 172.60(3) 180.00(2)

179.45(8) 179.16(3) 179.54(3) 180.00(6) 180.00(4) 180.00(2) 180.00(1) 180.00(6) 180.00(9) 180.00(9) 175.22(2) 180.00(1)

175.9(4) 175.8(3) 179.5(5) 180.0(3) 180.0(4) 180.0(4) 180.0(3) 180.0(2) 180.0(4) 180.0(5) 104.8(3) 180.0(2)

C denotes the α-carbon of the substituents on the arsenic atoms. Cited from ref 12a.

with a Kubelka−Munk calibration (Figure 1). In the case of the trans complexes, the longest absorption maxima or shoulder peaks were observed around 460−510 nm. In contrast, the cis complexes had no peaks in these regions. TD-DFT calculations were performed to differentiate between the cis and trans isomers in terms of the electronic absorption spectra. As representative examples, cis-and trans[PtCl2(2)2] were selected. Figure S16 in the Supporting Information shows that the trans complex has a HOMO and LUMO similar to those found in our report.12a The HOMO consists of a 5d orbital on the Pt atom combined by 3p orbitals on Cl atoms in a π* fashion, whereas the LUMO has a σ* combination of 3p(Cl) orbitals and a 5d(Pt) orbital, together with π orbitals on ligands. The calculated HOMO−LUMO transition energy (426 nm) corresponds to that of the longest absorption in Figure 1. Although the HOMO−LUMO transition was forbidden in the molecular calculations, symmetry lowering in the solid state may allow the transition. 12a In contrast to the trans complex, more complicated characters were found in unoccupied orbitals in frontier orbital regions of the cis complex, although the HOMO is similar to that in the trans complex case. As mentioned above, unoccupied orbitals have orbital amplitudes in fluorene moieties, and therefore the cis orientation allows coupling between adjacent ligands through π−π interactions. Another characteristic of unoccupied orbitals is the appearance of σ* orbitals coming from interactions between 3p(Cl) orbitals and a 5d(Pt) orbital, similar to the LUMO in the trans complex. Both of the orbital characters are seen in the next and secondnext LUMOs (LUMO+1 and LUMO+2). Reflecting their frontier orbitals, the transition energy in the cis complex corresponding to the HOMO−LUMO transitions in the trans complex decreases in wavelength (367 nm). Thus, different trends between the cis- and trans-[PtCl2(2)2] complexes obtained from TD-DFT computations support the changes observed in diffuse reflection spectra (Figure 1). Our DFT C

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Figure 1. Diffuse reflection spectra (with Kubelka−Munk calibration) of the complexes with (a) 1,12a (b) 2, (c) 3, (d) 4, and (e) 5 in the solid state. Color scheme: dichloride (blue), dibromide (green), diiodide (red), trans (solid line), cis (dashed line).

complexes are phosphorescence; the lifetime τ was 0.24−25.5 μs (Figure S22 in the Supporting Information). Among the complexes, trans-[PtI2(1)2], trans-[PtI2(2)2], trans-[PtI2(3)2], trans-[PtCl2(4)2], and trans-[PtI2(5)2] showed remarkably efficient phosphorescence (Φ = 0.52, 0.39, 0.40, 0.44, and 0.46, respectively) in the solid state at room temperature, in comparison with conventionally reported platinum complexes.21 The quantum yield at 298 K tends to be higher as the halogen atoms become heavier, except for trans-[PtX2(4)2] (X = Cl, I). This is because larger halogen atoms effectively restricted molecular motions, which could deactivate the excitons via a nonradiative pathway. Curiously, in the case of trans-[PtI2(4)2], very weak emission (Φ = 0.01) was observed despite the existence of large iodine atoms. In comparison to the diiodide complex, the emission of trans-[PtCl2(4)2] was intense (Φ = 0.44). The X-ray diffraction data shown in Table 3 indicated that the structure of trans-[PtI2(4)2] was exceptionally distorted by the steric repulsion between the cyclohexyl groups and the iodine atoms. The cyclohexyl groups offered vacant spaces around the arsafluorene moieties to enable molecular motions, resulting in the weak emission. At low temperature (77 K), the molecular motions were frozen, and an intense emission of trans-[PtI2(4)2] was observed (Φ = 0.69). On the other hand, the dichloride complex trans-[PtCl2(4)2] probably has a good balance of the steric hindrance between the halogen atom and the arsafluorene ligand, resulting in the intense emission due to restriction of molecular motions.

studies preliminarily found that the two types of isomer structures lead to different frontier orbital features, as well as the electronic transitions. However, further discussion is still needed to clarify different emission properties between the cis and trans complexes, which would be governed by their excited triplet states. The emission properties of the trans complexes in their solid states are summarized in Table 4. The emission colors vary Table 4. Emission Properties of the Trans Complexes in the Solid State complex

a

298 K

77 K

L

X

λema (nm)

Φb

τc (μs)

λema (nm)

Φb

1d 1d 1d 2 2 2 3 4 4 5

Cl Br I Cl Br I I Cl I I

624 626 640 610 616 633 642 611 652 658

0.01 0.26 0.52 0.02 0.07 0.39 0.40 0.44 0.01 0.46

3.2 12.0 25.5 1.2 9.1, 1.3 15.4, 4.3 22.2 21.1 1.0, 0.24 22.0

603 618 624 600 600 612 629 588 631 633

0.53 0.88 0.73 0.51 0.86 0.88 0.84 0.88 0.69 0.96

Emission maxima. bAbsolute quantum yield. cEmission lifetime. Cited from ref 12a.

d

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from orange to red (610−658 nm) (Figure 2). Emission lifetime measurements implied that the emissions of the trans

CONCLUSION In summary, we have elucidated the effects of ligand modification on the phosphorescent properties of platinum(II) dihalide complexes with 9-arsafluorenes. 9-Arsafluorenes with various substituents at the 9-position were prepared through a practical synthetic route, and their platinum(II) dihalide complexes were readily synthesized. The geometry of the complexes was dependent on the ligands, including arsafluorene and halogen. The complexes adopted square-planar structures, but some complexes were distorted by the steric repulsion between the arsafluorene and the halogen. The trans isomers showed intense phosphorescence in the solid state even at room temperature due to the bulkiness and rigidity of arsafluorene ligands, but no emission was observed in the solid samples of the cis isomers at room temperature or 77 K. The different emission behaviors derived from the existence of the HOMO−LUMO transition, which is responsible for the initial electron transition toward the phosphorescence. Furthermore, among the emissive trans isomers, the emission efficiencies differed from each other. A large vacant space around the arsafluorene moieties induced molecular motions, resulting in

Figure 2. (a) PL spectra (excited at excitation maxima) and (b) photographs (excited at 365 nm) of the solid samples of trans[PtX2(L)2]. D

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A solution of a nucleophile (1.0 equiv) was added dropwise into the mixture at 0 °C, and the mixture was stirred overnight at room temperature. H2O was added to the reaction mixture, which was then extracted with CH2Cl2. The combined organic layers were dried over MgSO4, and after filtration, the volatiles were removed in vacuo. The residue was purified with column chromatography on silica gel to afford the title compound. 9-Butyl-9-arsafluorene (3). n-Butyllithium was used as a nucleophile. Yield: 63%. 1H NMR (CDCl3, 400 MHz): δ 7.88 (d, 2H, J = 8.0 Hz), 7.69 (d, 2H, J = 7.2 Hz), 7.41 (dt, 2H, J = 1.2 Hz, 7.2 Hz), 7.29 (dt, 2H, J = 1.2 Hz, 7.4 Hz), 1.68 (t, 2H, J = 7.2 Hz), 1.46− 1.39 (m, 2H), 1.36−1.28 (m, 2H), 0.82 (d, 3H, J = 7.2 Hz) ppm. 13 C{1H} NMR (CDCl3, 100 MHz): δ 146.4, 145.4, 131.1, 128.1, 127.1, 122.0, 30.1, 28.9, 24.5, 13.6 ppm. HRMS (FAB): calcd for C16H17As [M]+, 284.0546; found, 284.0553. 9-Cyclohexyl-9-arsafluorene (4). Cyclohexylmagnesium bromide was used as a nucleophile. Yield: 87%. 1H NMR (CDCl3, 400 MHz): δ 7.90 (d, J = 7.6 Hz, 2H), 7.68 (d, 2H, J = 7.2 Hz), 7.44 (t, 2H, J = 5.8 Hz), 7.31 (t, 2H, J = 6.0 Hz), 3.76−3.73 (m, 1H), 1.87−1.69 (m, 6H), 1.27−1.13 (m, 4H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): δ 145.8, 145.0, 131.5, 128.1, 127.0, 121.9, 41.2, 30.2, 27.5, 26.2 ppm. These data corresponded to those in the literature.18 9-o-Anisyl-9-arsafluorene (5). o-Anisyllithium was used as a nucleophile. Yield: 75%. 1H NMR (CDCl3, 400 MHz): δ 7.89 (d, 2H, J = 7.6 Hz), 7.85 (d, 2H, J = 8.0 Hz), 7.44 (t, 2H, J = 7.6 Hz), 7.33 (t, 2H, J = 7.2 Hz), 7.18 (t, 1H, J = 8.6 Hz), 6.86−6.81 (m, 2H), 6.68 (t, 2H, J = 7.6 Hz), 4.02 (s, 3H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): δ 161.1, 145.5, 144.8, 132.4, 130.9, 129.8, 128.5, 128.2, 127.4, 122.1, 121.4, 110.0, 55.6 ppm. These data corresponded to those in the literature.18 Synthesis of PtCl2(9-methyl-9-arsafluorene)2 ([PtCl2(2)2]). A PhCl (10 mL) solution of 2 (0.123 g, 0.509 mmol) and cis-[PtCl2(PhCN)2] (0.118 g, 0.250 mmol) was refluxed under N2 overnight. The solvent was removed in vacuo, and the residue was recrystallized with CH2Cl2 and MeOH to give cis-[PtCl2(2)2] (0.144 g, 0.192 mmol, 77%) as light yellow crystals. The obtained crystals of cis-[PtCl2(2)2] were dissolved in hot chlorobenzene, and the solution was rapidly cooled to room temperature to give yellow crystals of trans-[PtCl2(2)2] quantitatively. 1 H NMR (CDCl3, 400 MHz): δ 7.35−7.33 (m, 8H), 7.24 (d, 4H, J = 7.6 Hz), 7.07−7.03 (m, 4H), 1.75 (s, 6H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): δ 141.8, 133.5, 130.7, 130.3, 128.7, 122.3, 11.9 ppm. Synthesis of PtBr2(9-methyl-9-arsafluorene)2 ([PtBr2(2)2]). A PhCl (10 mL) solution of [PtCl2(2)2] (64.4 mg, 85.8 μmol), MIBK (1 mL), and KBr (0.214 g, 1.80 mmol) was refluxed under N2 overnight. Distilled water was added to the reaction mixture, which was then extracted with CH2Cl2. The solvent was removed in vacuo, and the residue was recrystallized with CH2Cl2 and hexane to give trans[PtBr2(2)2] as light yellow solids (46.8 mg, 55.8 μmol, 65%). 1H NMR (CDCl3, 400 MHz): δ 7.34−7.32 (m, 8H), 7.25−7.24 (m, 4H), 7.08− 7.02 (m, 4H), 1.87 (d, 3H, J = 8.8 Hz), 1.74 (d, 3H, J = 9.6 Hz) ppm. 13 C{1H} NMR was not measured because of the low solubility of the product. Synthesis of PtI2(9-methyl-9-arsafluorene)2 ([PtI2(2)2]). A PhCl (10 mL) solution of [PtCl2(2)2] (49.0 mg, 65.3 μmol), MIBK (1 mL), and KI (0.223 g, 1.34 mmol) was refluxed under N2 overnight. Distilled water (5 mL) was added to the reaction mixture, leading to a red precipitate in the organic layer. The precipitates were collected by filtration to give trans-[PtI2(2)2] as red solids (30.2 mg, 32.4 μmol, 50%). 1H NMR and 13C{1H} NMR were not measured because of the low solubility of the product. Synthesis of PtCl2(9-butyl-9-arsafluorene)2 ([PtCl2(3)2]). A PhCl (10 mL) solution of 3 (0.185 g, 0.652 mmol) and cis-[PtCl2(PhCN)2] (0.154 g, 32.6 μmol) was refluxed under N2 overnight. The solvent was removed in vacuo, and the residue was recrystallized with CH2Cl2 and MeOH to give cis-[PtCl2(3)2] as light yellow solids (0.184 g, 0.221 mmol, 68%). 1H NMR (CDCl3, 400 MHz): δ 7.33−7.20 (m, 8H), 7.21 (d, 4H, J = 7.4 Hz), 7.06−7.02 (m, 4H), 2.42−2.39 (m, 4H), 1.07 (m, 4H), 0.96−0.89 (m, 4H), 0.61 (t, 6H, J = 7.3 Hz) ppm. 13C{1H}

nonradiative deactivation of the excitons. It has been found that steric balance between the arsafluorene and the halogens should play an important role in highly efficient phosphorescence. This work has disclosed the crucial roles of the 9-position of the arsafluorene in the molecular geometry and packing structure of the complexes, which led to different emission behaviors. The present molecular design for the construction of highly phosphorescent platinum dihalide complexes with 9arsafluorenes should contribute to the development of luminescent materials. Investigation on the mechanism of cis−trans isomerization in solutions and development of other functional transition-metal complexes with organoarsenic ligands are under way.

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EXPERIMENTAL SECTION

Caution! Low-molecular-weight organoarsenic compounds are volatile, and it is necessary to avoid procedures generating volatile organoarsenic compounds. For safety, experiments should be performed in a fume hood. In addition, in a few cases, spontaneously flammable compounds are obtained as described in the literature,22 and thus fire prevention measures should be taken. Materials. cis-Bis(benzonitrile)dichloroplatinum (cis[PtCl2(PhCN)2]), cyclohexylmagnesium bromide (1.0 M in 2methyltetrahydrofuran), and were purchased from Sigma-Aldrich Co., Ltd. Chloroform, dichloromethane (CH 2Cl2 ), methanol (MeOH), chlorobenzene (PhCl), o-dichlorobenzene, hexane, methyl isobutyl ketone (MIBK), potassium bromide (KBr), potassium iodide (KI), and 2-bromoanisole were purchased from Nacalai Tesque, Inc. Other chemicals were purchased from Wako Pure Chemical Industry, Ltd. All commercially available chemicals were used without further purification. 9-Phenyl-9-arsafluorene (1) and 9-methyl-9-arsafluorene (2)4 and trans-[PtX2(1)2] (X = Cl, Br, I)12a were prepared by following the literature procedures. Measurements. 1H (400 MHz) and 13C{1H} (100 MHz) NMR spectra were recorded on a Bruker DPX-400 spectrometer. The samples were analyzed in CDCl3 using Me4Si as an internal standard. The following abbreviations are used; s, singlet; d, doublet; t, triplet; q, quartet; m, multiplet. High-resolution mass spectra (HRMS) were obtained on a JEOL JMS-SX102A spectrometer. The UV−vis spectra were recorded on a Jasco spectrophotometer (V-670 KKN). Emission spectra were obtained on an FP-8500 instrument (JASCO), and absolute PL quantum yields (Φ) were determined using a JASCO ILFC-847S instrument; the quantum yield of quinine sulfate reference was 0.52, which is in agreement with the literature value.23 Emission lifetimes were measured using a Quantaurus-Tau instrument (Hamamatsu Photonics). X-ray Crystallographic Data for Single-Crystalline Products. The single crystals were mounted on nylon loops. Intensity data were collected at room temperature on a Rigaku XtaLAB mini instrument with graphite-monochromated Mo Kα radiation. Readout was performed in the 0.073 mm pixel mode. The data were collected at room temperature to a maximum 2θ value of 55.0°. Data were processed by Crystal Clear.24 A numerical absorption correction was applied. The data were corrected for Lorentz and polarization effects. The structure was solved by direct methods25 and expanded using Fourier techniques. The non-hydrogen atoms were refined anisotropically. Hydrogen atoms were refined using the riding model. The final cycle of full-matrix least-squares refinement on F2 was based on observed reflections and variable parameters. All calculations were performed using the CrystalStructure25 crystallographic software package, except for refinement, which was performed using SHELXL2013.26 Crystal data and more information on X-ray data collection are summarized in Tables S1−S20 in the Supporting Information. Synthesis. General Procedure for the Synthesis of 9-Arsafluorenes 3−5. A THF solution of iodine (1.0 equiv) was added to a THF solution of 2 (1 equiv), and the mixture was stirred at 65 °C for 36 h. E

DOI: 10.1021/acs.organomet.7b00233 Organometallics XXXX, XXX, XXX−XXX

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Organometallics NMR (CDCl3, 100 MHz): δ 142.6, 132.3, 130.8, 130.5, 128.6, 122.1, 27.3, 25.9, 23.6, 13.3 ppm. Synthesis of PtBr2(9-butyl-9-arsafluorene)2 ([PtBr2(3)2]). A PhCl (5 mL) solution of [PtCl2(3)2] (51.2 mg, 61.4 μmol), MIBK (1 mL), and KBr (0.148 g, 1.24 mmol) was refluxed under N2 overnight. Distilled water (5 mL) was added to the reaction mixture, which was then extracted with CH2Cl2. The solvent was removed in vacuo, and the residue was recrystallized with CH2Cl2 and hexane to give cis[PtBr2(3)2] as light yellow solids (39.7 mg, 43.0 μmol, 70%). 1H NMR (CDCl3, 400 MHz): δ 7.30 (t, 8H, J = 4.0 Hz), 7.24−7.21 (m, 4H), 7.40 (m, 4H), 2.51−2.39 (m, 4H), 1.10−1.03 (m, 4H), 0.96−0.89 (m, 4H), 0.62 (m, 6H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): δ 142.64, 142.59, 132.5, 132.4, 132.5, 130.8, 130.75, 130.70, 130.5, 128.6, 122.1, 27.3, 27.2, 26.2, 25.9, 25.8, 23.6, 23.5, 13.3 ppm. Synthesis of PtI2(9-butyl-9-arsafluorene)2 ([PtI2(3)2]). A PhCl (5 mL) solution of [PtCl2(3)2] (53.6 mg, 64.2 μmol), MIBK, and KI (0.216 g, 1.30 mmol) was refluxed under N2 overnight. Distilled water (5 mL) was added to the reaction mixture, which was then extracted with CH2Cl2. The solvent was removed in vacuo, and the residue was recrystallized with CH2Cl2 and hexane to give trans-[PtI2(3)2] as red solids (39.3 mg, 38.6 μmol, 60%). 1H NMR (CDCl3, 400 MHz): δ 8.10 (d, 4H, J = 7.6 Hz), 7.88 (d, 4H, J = 7.6 Hz), 7.51 (t, 4H, J = 7.6 Hz), 7.39 (t, 4H, J = 7.4 Hz), 2.67−2.63 (m, 4H), 1.35−1.25 (m, 8H), 0.76 (t, 6H, J = 7.2 Hz) ppm. 13C{1H} NMR (CDCl3, 400 MHz): δ 143.1, 135.5, 133.7, 130.7, 127.6, 122.2, 31.4, 27.2, 23.9, 13.5 ppm. Synthesis of PtCl2(9-cyclohexyl-9-arsafluorene)2 ([PtCl2(4)2]). A PhCl (15 mL) solution of 4 (0.258 g, 0.830 mmol) and cis[PtCl2(PhCN)2] (0.195 g, 0.413 mmol) was refluxed under N2 overnight. The solvent was removed in vacuo, and the residue was recrystallized with CH2Cl2 and MeOH to give cis-[PtCl2(4)2] as light yellow solids (0.213 g, 0.240 mmol, 58%). The obtained crystals of cis[PtCl2(4)2] were subjected to recrystallization from CH2Cl2 and hexane, and the crystals of trans-[PtCl2(4)2] were obtained as yellow crystals quantitatively. 1H NMR (CDCl3, 400 MHz): δ 7.33−7.12 (m, 8H), 7.18 (d, 4H, J = 7.2 Hz), 7.05−7.02 (m, 4H), 3.53−3.45 (m, 2H, J = 7.6 Hz), 1.66 (d, 4H, J = 11.6 Hz), 1.43 (d, 6H, J = 10.8 Hz), 1.15 (q, 4H, J = 13.6 Hz), 0.78−0.67 (m, 2H), 0.55 (q, 4H, J = 12.0 Hz) ppm. 13C{1H} NMR (CDCl3, 100 MHz): δ 143.2, 131.2, 131.1, 130.4, 128.4, 121.9, 39.3, 27.7, 26.8, 25.6 ppm. Synthesis of PtBr2(9-cyclohexyl-9-arsafluorene)2 ([PtBr2(4)2]). A PhCl (10 mL) solution of [PtCl2(4)2] (44.0 mg, 54.0 μmol), MIBK (1 mL), and KBr (131.4 mg, 1.10 mmol) was refluxed under N2 overnight. Distilled water (5 mL) was added to the reaction mixture, which was then extracted with CH2Cl2. The solvent was removed in vacuo, and the residue was recrystallized with CH2Cl2 and hexane to give a mixture of cis- and trans-[PtBr2(4)2] as mixture of yellow and red solids (total: 17.1 mg, 17.5 μmol, 32%). Isolation of the isomers failed. 1H NMR (CDCl3, 400 MHz): δ 7.33−7.31 (m, 8H), 7.18 (d, 4H, J = 6.4 Hz), 7.05−7.02 (m, 4H), 3.46−3.75(m, 2H), 1.64 (d, 4H, J = 12.0 Hz), 1.43 (d, 6H, J = 11.6 Hz), 1.15 (q, 4H, J = 14.4 Hz) 0.88 (t, 2H, J = 6.8 Hz), 0.49 (m, 4H) ppm. 13C{1H} NMR (CDCl3, 100 MHz): δ 143.2, 131.2, 131.1, 130.3, 128.3, 121.9, 39.31, 27.7, 26.8, 25.7 ppm. Synthesis of PtI2(9-cyclohexyl-9-arsafluorene)2 ([PtI2(4)2]). A PhCl (10 mL) solution of [PtCl2(4)2] (42.6 mg, 52.2 μmol), MIBK (1 mL), and KI (0.176 g, 1.06 mmol) was refluxed under N2 overnight. Distilled water (5 mL) was added to the reaction mixture, which was then extracted with CH2Cl2. The solvent was removed in vacuo, and the residue was recrystallized with heated chlorobenzene to give trans[PtI2(4)2] as red solids (21.9 mg, 20.5 μmol, 39%). 1H NMR (CDCl3, 400 MHz): δ 8.13 (d, 4H, J = 6.8 Hz), 7.87 (d, 4H, J = 7.6 Hz), 7.50 (dt, 4H, J = 1.2 Hz, 7.6 Hz), 7.39 (dt, 4H, J = 1.2 Hz, 7.6 Hz), 3.43 (tt, 2H, J = 3.2 Hz, 12.8 Hz), 1.83 (d, 4H, J = 10.8 Hz), 1.60 (t, 6H, d, J = 14.8 Hz), 1.21 (q, 4H, J = 8.8 Hz), 0.97 (d, 6H, J = 12.4 Hz) ppm. 13 C{1H} NMR (CDCl3, 100 MHz): δ 143.7, 134.4, 134.1, 130.6, 127.4, 121.7, 43.0, 28.5, 27.2, 25.8 ppm. Synthesis of PtCl2(9-o-anisyl-9-arsafluorene)2 ([PtCl2(5)2]). A PhCl (10 mL) solution of 5 (0.196 g, 0.588 mmol) and cis-[PtCl2(PhCN)2] (0.139 g, 0.294 mmol) was refluxed under N2 overnight. The solvent was removed in vacuo, and the residue was subjected to

recrystallization from hot o-dichlorobenzene to give cis-[PtCl2(5)2] as yellow crystals (0.227 g, 0.252 mmol, 86%). 1H NMR and 13C{1H} NMR were not measured because of the low solubility of the product. Anal. Calcd for C38H30As2Cl2O2Pt: C, 48.84; H, 3.24. Found: C, 48.65; H, 3.36. Synthesis of PtI2(9-o-anisyl-9-arsafluorene)2 ([PtI2(5)2]). An odichlorobenzene (15 mL) solution of [PtCl2(5)2] (59.5 mg, 66.2 μmol), MIBK (1.5 mL), and KI (0.230g, 1.39 mmol) was heated to 140 °C under N2. The mixture was stirred at 140 °C overnight. Distilled water (5 mL) was added to the reaction mixture, which was then extracted with CH2Cl2. The solvent was removed in vacuo, and the residue was recrystallized with heated o-dichlorobenzene to give trans-[PtI2(5)2] as red solids (66.6 mg, 59.6 μmol, 90%). 1H NMR and 13 C{1H} NMR were not measured because of the low solubility of the product. Anal. Calcd for C38H30As2I2O2Pt: C, 40.85; H, 2.71. Found: C, 41.03; H, 2.75.

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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.organomet.7b00233. Synthetic procedures, spectral data, and single-crystal Xray diffraction data (PDF) Accession Codes

CCDC 1539988−1539990, 1539992−1539999, and 1540001 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_request@ccdc. cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*K.N.: tel, +81-75- 724-7534; e-mail, [email protected]. ORCID

Kensuke Naka: 0000-0002-4516-6296 Present Address

H.I.: Faculty of Materials Science and Engineering, Graduate School of Science and Technology, Kyoto Institute of Technology, Goshokaido-cho, Matsugasaki, Sakyo-ku, Kyoto 606-8585, Japan. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study is a part of a Grant-in-Aid for Scientific Research on Innovative Areas “New Polymeric Materials Based on ElementBlocks (No. 2401)” (No. 24102003) of The Ministry of Education, Culture, Sports, Science and Technology of Japan. T.Y. acknowledges a Grant-in-Aid for Scientific Research on the Innovative Area “Stimuli-responsive Chemical Species for the Creation of Fundamental Molecules (No. 2408)” (JSPS KAKENHI Grant Number JP15H00941 for T.Y.). H.I. acknowledges a JSPS KAKENHI Grant Number JP17H05369 (Coordination Asymmetry).

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DOI: 10.1021/acs.organomet.7b00233 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.7b00233 Organometallics XXXX, XXX, XXX−XXX