Highly Emissive Cycloplatinated(II) Complexes Obtained by the

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Highly Emissive Cycloplatinated(II) Complexes Obtained by the Chloride Abstraction from the Complex [Pt(ppy)(PPh3)(Cl)]: Employing Various Silver Salts Hamid R. Shahsavari,*,† Reza Babadi Aghakhanpour,† Mahshid Nikravesh,† John Ozdemir,‡ Mohsen Golbon Haghighi,§ Behrouz Notash,§ and M. Hassan Beyzavi*,‡ †

Department of Chemistry, Institute for Advanced Studies in Basic Sciences (IASBS), Zanjan 45137-66731, Iran Department of Chemistry and Biochemistry, University of Arkansas, Fayetteville, Arkansas 72701, United States § Department of Chemistry, Shahid Beheshti University, Evin, Tehran 19839-69411, Iran

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‡

S Supporting Information *

ABSTRACT: In the present investigation, the precursor complex [Pt(ppy)(PPh3)(Cl)], 3, ppy = 2-phenylpyridinyl, undergoes the chloride abstraction reaction using various AgX (X = PF6, BF4, NO3 and CH3COO) salts. Depending on the nature of anions in AgX salts (coordinating or noncoordinating), the products can be neutral or ionic. In the cases of NO3 and CH3COO, they can be coordinated to the Pt center so that the neutral complexes [Pt(ppy)(PPh3)(NO3)], 4a, and [Pt(ppy)(PPh3)(CH3COO)], 4b, are formed. In contrast, the ionic complexes [{Pt(ppy)(PPh3)(CH3CN)}PF6], 5a, and [{Pt(ppy)(PPh3)(CH3CN)}BF4], 5b, can be generated when the AgPF6 ([{Ag(CH3CN)4}PF6]) or AgBF4 ([{Ag(CH3CN)4}BF4]) salts are used in which the PF6 and BF4 stand as counteranions. In these two complexes, CH3CN fills the empty ligand position which can be present as solvent or ligand in the initial silver salts. The structures of the new complexes were accurately deduced from the multinuclear (1H, 31P{1H}, 195Pt{1H}) NMR spectroscopy and further authenticated by X-ray crystallography. Interestingly, the complexes are green emitters under various states and temperature conditions for which nonchelating L/X (PPh3/NO3 or PPh3/CH3COO) and L/L (PPh3/CH3CN) ancillary ligands exist in the structure of cycloplatinated(II) complexes. The photophysical properties of these new complexes, supported by density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations, were investigated by photoluminescence and UV−vis spectroscopies.

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INTRODUCTION In recent years, the chemistry of heteroleptic cycloplatinated(II) complexes has been investigated because of their photophysical properties.1−20 The photophysical properties of the heteroleptic cycloplatinated(II) complexes are highly tunable by modification of the ancillary ligands. This photophysical tunability makes cycloplatinated(II) complexes reliable phorphorescent emitters and makes them well-suited for various applications in light-emitting devices,21,22 photocatalytic processes, and hydrogen generation,23 dye-sensitized solar cells,24 biosensors, photoswitches,25−27 optoelectronic devices,28−30 photocatalysts,31 and chemical or biochemical sensors.32 © XXXX American Chemical Society

It is well-known that cyclometalated ligands which exhibit strong ligand field raise the energy of the d−d states to be energetically nonaccessible and thus decrease the nonradiative decay.33 However, there are many examples of cyclometalated compounds which are poor or nonemissive at room temperature. Although the functionalization of the cyclometalated moiety can be effective, exchange of the ancillary ligands seems to be the most straightforward method of increasing emission. Strength of the ligand field in the ancillary ligands plays a crucial role in determining the photophysical properties of the Received: July 4, 2018

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DOI: 10.1021/acs.organomet.8b00461 Organometallics XXXX, XXX, XXX−XXX

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Organometallics Scheme 1. Synthesis Route for the Formation of 4a,b and 5a,b

obtained. Despite the weak ligand fields for the NO3 and CH3COO ligands, the products have strong green luminescence emission. Under such conditions, the presence of PPh3 with strong ligand field seems essential for the compound to exhibit strong luminescence. However, two other silver salts ([{Ag(CH3CN)4}PF6]61 and [{Ag(CH3CN)4}BF4])62 with noncoordinating anions were reacted with the starting complex in the presence of acetonitrile (CH3CN) as the ligand in the initial silver salts. These reactions were performed by a different method using AgPF6 or AgBF4 in acetonitrile as a solvent with coordination capability. Both methods produced the cationic products with nonchelating L/L (PPh3/CH3CN) ancillary systems with PF6 or BF4 as counteranion.

final cyclometalated products.34 In the neutral cyclometalated complexes, the ancillary ligands can be in chelating (L∧X) or nonchelating (L/X) systems (L is neutral and X is anionic). So far, the anionic L∧X chelates like O∧O, N∧N, and N∧O systems have been used as the L∧X ancillary ligands in the structures of cyclometalated complexes.35−45 There are also some examples for the nonchelating L/X ancillary systems in the literature which proves that the complexes, bearing nonchelating L/X systems, can be appropriate phosphorescent emitters.17,46−54 In most L/X systems, the neutral L is either a phosphorus or carbon donor, while the X bears a wide range of anionic ligands like Me, C6F5, Cl, Br, I, and so on. In an interesting investigation by Gonzalez et al., various anionic X ligands such as CH3COO, CF3COO, I, Br, Cl, and F were employed in order to trigger the photophysical properties of a bis-cyclometalated Pt(IV) fragment.55 Abstraction of an anionic ligand on a transition metal center usually leads to the formation of cationic metal complexes.56 Probably, halide abstraction is the most common method of abstraction. Halide abstraction normally takes place by a salt containing a univalent metal (Ag, Tl) and a weakly coordinating counteranion. The product complex of halide abstraction drastically depends on the nature of this counteranion. Although anions like NO3, CH3COO, and CF3COO are weakly coordinating ligands, they are able to replace the halide ligand within the halide abstraction process. In the case of the anions with noncoordinating moieties (PF6, BF4, SbF6, AsF6), a cationic complex is produced in which the solvent molecule fills the empty position. In the presence of noncoordinating solvents like CH2Cl2, a very reactive unsaturated cationic complex is formed, exhibiting high electrophilicity. In a very rare investigation, PF6 is coordinated to Ir(III) center via one of its F atoms.57 In light of our previous investigations in the field of luminescent cycloplatinated(II) complexes,46−48,54,58,59 we have employed a halide abstraction process in order to make new cyclometalated platinum(II) complexes with weakly coordinating ligands and investigated their photophysical properties. The precursor complex [Pt(ppy)(PPh3)(Cl)], 3,60 ppy = 2-phenylpyridinyl, was reacted with two silver salts, i.e., AgNO3 and AgOOCCH3. For both of these silver salts, since the anions have coordination ability, they make nonchelating L/X (PPh3/NO3, PPh3/CH3COO) ancillary systems toward the cycloplatinated(II) moiety in which the neutral product was

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RESULTS AND DISCUSSION Synthesis and Characterization of the Complexes in Solid and Solution States. The synthesis route for the new complexes is displayed in Scheme 1. The chloride abstraction from the complex [Pt(ppy)(PPh3)(Cl)], 3, ppy = 2-phenylpyridinyl, was performed under dark conditions at room temperature. The treatment of 3 with AgNO3 and AgOOCCH3 led to formation of the complexes [Pt(ppy)(PPh3)(NO3)], 4a, and [Pt(ppy)(PPh3)(CH3COO)], 4b, respectively. In the obtained complexes, the chloride ligand is replaced by the introduced coordinating anions to produce the nonchelating L/X (PPh3/ NO3, PPh3/CH3COO) ancillary ligand systems. It was observed that the reactions proceeded with no isomerization at the Pt(II) center. In contrast, the halide abstraction reaction using [{Ag(CH3CN)4}PF6] and [{Ag(CH3CN)4}BF4] (containing noncoordinating anions) in CH2Cl2 resulted in the complexes [{Pt(ppy)(PPh3)(CH3CN)}PF6], 5a, and [{Pt(ppy)(PPh3)(CH3CN)}BF4], 5b, respectively, with the same cationic complex but different counteranions. For both complexes, the acetonitrile expectedly filled the empty ligand position with no isomerization at the Pt(II) center. Complexes 5a and 5b are also accessed through the reaction of AgPF6 and AgBF4 with CH3CN as the solvent. It should be noted that the reaction of 3 with AgPF6 and AgBF4 in CH2Cl2 (noncoordinating solvent) resulted in unstable products which could not be characterized. All the products were fully characterized using multinuclear (1H, 31P{1H}, 195Pt{1H}) NMR spectroscopy (Figures S1−S12). All the 195Pt{1H}NMR spectra contain a clear doublet arising from the coupling between platinum center and phosphorus B


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Organometallics

Figure 1. Perspective views of the complexes (a) 4a: Selected geometrical parameters (Å, °): Pt1−C1 1.996(9); Pt1−N1 2.073(8); Pt1−O1 2.130(7); Pt1−P1 2.246(3); C1−Pt1−N1 79.5(4); C1−Pt1−O1 169.3(4); C1−Pt1−P1 98.5(3); N1−Pt1−O1 89.9(3); N1−Pt1−P1 176.0(2); O1−Pt1−P1 92.1(2). (b) 4b: Selected geometrical parameters (Å, °): Pt1−C1 1.984(13); Pt1−N1 2.070(9); Pt1−O1 2.086(8); Pt1−P1 2.238(3); C1−Pt1−N1 82.6(4); C1−Pt1−O1 170.8(3); C1−Pt1−P1 97.0(3); N1−Pt1−O1 88.5(4); N1−Pt1−P1 179.2(3); O1−Pt1−P1 91.9(2). (c) 5a: Selected geometrical parameters (Å, °): Pt1−C1 2.04(2); Pt1−N1 2.074(19); Pt1−N2 2.078(17); Pt1−P1 2.234(6); C1−Pt1− N1 81.6(9); C1−Pt1−N2 175.2(9); C1−Pt1−P1 93.7(7); N1−Pt1−N2 93.8(7); N1−Pt1−P1 175.0(5); N2−Pt1−P1 90.9(5). Ellipsoids are drawn at the 50% probability level, and hydrogen atoms, PF6, and CH2Cl2 solvent molecules are omitted for clarity.

Figure 2. (a) Normalized absorption spectra for all the complexes in CH2Cl2 solutions. (b) Diffuse reflectance UV−vis spectra of all the complexes in solid state at room temperature.

atom of the PPh3 ligand. Also, the 31P{1H}NMR spectra for all the complexes exhibit a singlet flanked by platinum satellites confirming that PPh3 is directly connected to the Pt(II) center. The 31P{1H}NMR spectrum of 5a has an extra septet signal centered at δ = −144.4 ppm which is clearly attributed to the PF6 counteranion. In this way, all the 1HNMR spectra include various signals for the aromatic protons corresponding to the phenyl rings of PPh3 and protons of ppy ligand. Among them, the H2 and H9 protons of ppy ligand have platinum satellites, indicating that the ppy is in a chelated situation in all the cases. The 1HNMR spectra for 5a and 5b both have a singlet signal in the aliphatic region, being related to the methyl group of coordinated acetonitrile. The crystal structures of 4a, 4b, and 5a were solved by single-crystal X-ray crystallography method. ORTEP plots of these complexes are displayed in Figure 1, and selected bonds and angles are listed in the caption. Besides, the crystal packing views for 4a, 4b, and 5a are shown in Figures S13−S15, respectively. In all the complexes, four-coordinated Pt(II) center with

a distorted square planar geometry is seen, and ppy bite angle is much smaller than 90° (4b [82.58°] > 5a [80.52°] > 4a [79.48°]). The distances of Pt−C bond, being trans to the NO3, CH3COO, and CH3CN ligands, decrease in the sequence of 5a (2.041 Å) > 4a (1.997 Å) > 4b (1.984 Å). It means that the trans influence for the ligands trans to Pt−C bond is in the order of CH3CN > NO3 > CH3COO. The crystal packing of 4a contains various kinds of short contact for example intermolecular O···H interactions between NO3 ligand and different C−H bonds. In this context, the packing of 4b is simpler in relation to 4a, having some C−H···π interactions as the most important short contacts. Also, in the packing of 5a, the F atoms of PF6 counteranions are located at the one side of almost all the short contacts. Absorption Spectra. The UV−vis spectra of 4a, 4b, 5a, and 5b, shown in Figure 2a, were recorded in their CH2Cl2 solutions, and the related numerical data are summarized in Table 1. As can be seen, the UV−vis spectra of 5a and 5b are almost the same because of their identical cations which C


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Organometallics 3

MLCT. The nature of anion X (NO3 CH3COO) does not apparently influence the emission band shape or wavelength; however, it has a relatively considerable effect on the quantum efficiencies of the mentioned complexes (4a > 4b). These observations appropriately confirm the large involvement of cyclometalated ligand in the emissive states. It should be mentioned that no intermolecular interaction (Pt−Pt or π−π stacking) is observed in the emission bands. In the solid state at 77 K and frozen CH2Cl2, the emission bands of 4a and 4b appear at almost the same wavelength and shape with their corresponding room temperature bands, but being more intense as expected. On the basis of the band shapes, the 3 ILCT still forms the major character in the excited states in low temperatures. The emission band (solid, 298 K) for cationic complex 5a is slightly red-shifted in relation to those observed for 4a and 4b. This emission band similarly appears in the green area, having a structured band shape (3ILCT/3MLCT). Upon excitation at 365 nm, 5a affords an emission band at 490 nm with a vibronic progression at 525 nm and a shoulder at 560 nm. Meanwhile, 5b, having the same cationic complex as 5a but with a different couteranion, exhibits an emission band in the yellow region. This emission band is likely formed by a weaker structured emission band centered at 488 nm (green area) and a stronger unstructured band at 562 nm (yellow area) where the latter arises from intermolecular short contacts. In relation to the band at 488 nm, the band at 562 nm has a higher intensity so that its color prevails over the color of the band at 488 nm, and consequently, the emission color is observed to be yellow under irradiation. Regarding to that much lower quantum efficiency of 5b compared to 5a, it can be concluded that the effect of counteranions (PF6, BF4) is very determining and outstanding. This is spite of the absorption spectra wherein 5a and 5b have almost the same spectra. Similar to 4a and 4b, upon lowering the temperature, 5a and 5b give emission bands without any change in comparison to their corresponding room temperature bands. However, in the frozen solvent, 5a has a blueshift in relation to the solid state both in 298 and 77 K. This blueshift for 5b is more considerable than that for 5a. Probably the unstructured band, being related to the intermolecular short contacts, vanished, and the structured emission band remains so that the emission color for 5b in frozen solvent tends to green. The spectra of the PMMA (poly(methyl methacrylate)) films were recorded for all the complexes (Figure S16). For complexes 4a and 4b, the obtained values are almost close to the other states, having structured emission bands. However, the spectrum of the PMMA film of 5a is very close to that of its CH2Cl2 matrix at 77 K, showing a blue shift in relation to the corresponding solid state (at 298 or 77 K). Interestingly, in the case of 5b, the emission band of PMMA film becomes clearly well-structured and has a meaningful blueshift in relation to that of the other mediums. Theoretical Calculations. Due to their similarity, 4a and 5a were chosen for density functional theory (DFT) and timedependent DFT (TD-DFT) calculations to avoid verbosity. The available crystal structures for these complexes were used as input files for the software. The ground states for both of the complexes were optimized in CH2Cl2 and gas phase shown in Figure 4. Besides, the selected bond distances and angles for 4a and 5a are presented in Table S1. The energy levels and compositions of the metals and ligands for the selected molecular orbitals (MO) of 4a and 5a

Table 1. Numerical Absorption Data for All the Complexes in their CH2Cl2 Solutions (2.5 × 10−4 M) and Solid State at Room Temperature complex 4a 4b 5a 5b

absorption/nm (104 ε/M−1 cm−1) 364 (0.225), 327 (0.482), 382, 329, 289, 254; solid 377 (0.162), 327 (0.334), 390, 329, 286, 243; solid 357 (0.182), 327 (0.340), 423, 294, 257, 217; solid 356 (0.246), 326 (0.435), 382, 293, 254; solid

312 (0.482), 283 (0.805); CH2Cl2 286 (0.728); CH2Cl2 314 (0.373), 280 (0.692); CH2Cl2 314 (0.486), 281 (0.764); CH2Cl2

indicates that the nature of anion does not affect the absorptions in the molecule. However, the spectra of 4a and 4b are slightly different in relation to each other. Similar to our previous experiences, two absorption regions can be found in the spectra of all the compounds. In all the spectra, the region between 330 and 425 nm (lower energy bands) are majorly assigned as 1ILCT/1MLCT-based transitions. This region is dominated by intraligand charge transfer (1ILCT) character which is relatively perturbed by Pt(II) center so that it is mixed with metal-to-ligand charge transfer (1MLCT). These bands are used for excitation wavelengths in order to yield emissions. In the high-energy region (the wavelengths smaller than 330 nm), the majority is as expected related to the ligand-centered π → π* transitions in the cyclometalated ligand. These bands are normally more intense than those in the low energy regions. In the following, the diffuse reflectance spectra for all the complexes were recorded for their solid states which are depicted in Figure 2b. Spectra for 4a and 4b are very close to each other and to those for 5a and 5b as well. No intermolecular Pt−Pt or excimeric interactions can be observed in the diffuse reflectance UV−vis spectra for all the cases so that the solid spectra relatively resemble their corresponding solution spectra.47 Photoluminescence Spectra. Under irradiation of a hand-light UV lamp (366 nm), all the complexes, excluding 5b, exhibit highly green luminescence in solid state at 298 K, while the poor emission of 5b inclines to the yellow area. Logically, in all cases even 5b, upon freezing (77 K) the complexes the color of their emission become sharper which is related to the rigidochromism. Due to dynamic molecular motions, the complexes almost do not emit in their CH2Cl2 solutions (negligible emission), whereas by lowering the temperature down to 77 K, the frozen glasses become strongly luminous under UV light exposure. For all the compounds, the nature of emissions arises from a triplet excited state (phosphorescence) which is strongly proved by the obtained lifetime values in the microsecond domain. Figure 3 depicts all the normalized emission and excitation spectra under all states and temperature conditions and the corresponding numerical data are listed in Table 2. For all the compounds, the emission band wavelength and shape do not depend on the excitation wavelength in the wide range of 295−505 nm. For neutral complexes 4a and 4b, completely structured emission bands are observed in green region with tails to near orange region in solid state at 298 K. For both, the emission wavelength occurs on 480 nm with different vibronic progression at 512 and 508 nm for 4a and 4b, respectively. The structured bands normally shows a large amount of 3ILCT in the emissive state which is accompanied by small amount of D


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Organometallics

Figure 3. Normalized emission and excitation spectra together with photographic images for the complexes 4a, 4b, 5a, and 5b in (a) solid state at 298 K, (b) solid state at 77 K, and (c) CH2Cl2 glass state at 77 K.

are summarized in Table S2. The plots of frontier molecular orbitals, i.e., HOMO (highest occupied molecular orbital) and LUMO (lowest unoccupied molecular orbital), for 4a and 5a are displayed in Figure 5. Besides, the full coverage of the selected MOs, including “HOMO to HOMO−10” and “LUMO to LUMO+10”, for 4a and 5a are depicted in Figures S17 and S18, respectively. Similar to the previously studied cycloplatinated(II) complexes; for both of the complexes the HOMO is relatively localized on ppy ligand (54 and 65% for 4a and 5a, respectively) which is accompanied by a lower contribution of Pt center (43 and 32% for 4a and 5a, respectively) and negligible contribution of the other ligands. Meanwhile, the LUMO for these two complexes is predominantly centered on the ppy moiety in which Pt and the other ligands have no considerable

contributions. For both, the contribution of PPh3 remarkably increases in LUMOs above main LUMO and HOMOs lower than main HOMO. In the case of 4a, the contribution of NO3 ligands is too small in all the MO levels excluding the LUMO+5, LUMO+4, HOMO−1, HOMO−3, and HOMO−10. However, in 5a, the contribution of CH3CN in all the HOMOs is very poor, whereas it is almost on the increase from LUMO+6 to LUMO+10. In order to rationalize the obtained photophysical data, TD-DFT calculations were performed in CH2Cl2 solvent using the CPCM (conductor-like polarizable continuum model) solvent model. In CH2Cl2, TD-DFT calculations afford the predicted electronic transitions for 4a and 5a for which the transitions together with their assignments are listed in Tables S3 and S4, E


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Organometallics Table 2. Numerical Data for the Emission and Excitation Wavelengths of All the Complexes complex

4a

4b

5a

5b

λem/nm (λex/nm)

τ (μs)

Φ

480, 512max, 547sh (365), solid (298 K) 480, 513max, 545sh (365), solid (77 K) 475max, 506, 543sh (365), 5 × 10−5 M (77 K) 479, 512max,545sh (365), PMMA film (298 K) 480, 508max, 545sh (365), solid (298 K) 478, 507max, 543sh (365), solid (77 K) 480, 505max, 542sh (365), 5 × 10−5 M (77 K) 484, 517max, 547sh (365), PMMA film (298 K) 490, 525max, 560sh (365), solid (298 K) 493, 525max, 565sh (365), solid (77 K) 476, 507max, 546sh (365), 5 × 10−5 M (77 K) 478, 511max, 544sh (365), PMMA film (298 K) 488, 562max (365), solid (298 K) 489, 563max (365), solid (77 K) 490, 517max (365), 5 × 10−5 M (77 K) 477, 510max, 542sh (365), PMMA film (298 K)

4.27 9.63

0.69

3.21 5.61

9.11 16.63

1.47 3.39

0.71 0.33

0.51 0.79

0.88 0.07

0.29

respectively. Figure 6 also reveals the overlaid experimental absorbance spectrum and calculated TD-DFT bars for these two complexes. A good agreement is observed between the experimental spectra and the theoretical bars particularly in low-energy areas which have been chosen for the excitation wavelength to yield emission spectra. For both of the complexes, the first excited state (S0 → S1) is majorly attributed to the HOMO → LUMO transition, appearing at 362 and 354 nm for 4a and 5a, respectively. In addition, in both cases, the character of this transition is considerably related to the ILCT in ppy ligand with minor presence of MLCT. The mixed ILCT/MLCT (L = ppy and M = Pt) character is still the main character in the lower wavelengths (290 and 302 nm for 4a and 306, 303, and 282 nm for 5a). The experimental excitation wavelength span (295−505 nm) covers the areas with mixed ILCT/MLCT character in UV−vis spectra. In addition to the mixed ILCT/MLCT character, a marginal contribution of ML′CT (L′ = PPh3) character also exists for 4a and 5a. For 4a, at the wavelengths lower than 290 nm, the effective contribution of PPh3 (L′) accompanied by the small contribution of NO3 (L″) can be mostly observed in the L″L′CT, L′LCT, LL′CT, and ML′CT transitions. However, for the wavelengths lower than 282 nm in 5a, PPh3 (L′) efficiently contributes in

Figure 5. Frontier molecular orbitals for 4a and 5a.

the transitions (L′LCT, LL′CT, and ML′CT), while the CH3CN (L″) ligand never participates in the electronic transitions. This is because in all the HOMOs (starting points of transitions) and also LUMO to LUMO+5 (targets of transitions), the contribution of CH3CN is ignorable (see Tables S2 and S4). The contribution of CH3CN is only remarkable in the LUMO+6 to LUMO+10 levels which are not involved in the electronic transitions. In order to calculate the theoretical emission wavelength, the lowest-energy triplet states (T1) in the gas phase were optimized for 4a and 5a. The geometry of the excited state (T1) was observed to be close to that of ground state (S0) with minimal changes in bonds and angles. The theoretical emission wavelength is the energy gap between the lowest optimized T1 and S0 in gas phase. This value is calculated to be 2.628 and 2.634 eV for 4a and 5a which affords the theoretical emission wavelength of 472 and 471 nm, respectively. The calculated

Figure 4. Optimized structures of the complexes (a) 4a and (b) 5a in gas phase or CH2Cl2. Hydrogens were eliminated for clarity. F


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Organometallics

Figure 6. Overlaid experimental absorbance spectrum and calculated TD-DFT bars for the complexes (a) 4a and (b) 5a. purification. The microanalyses were performed using a vario EL CHNS elemental analyzer and also all the melting point values were measured by a Buchi 510. Multinuclear (1H, 31P{1H}, 195Pt{1H}) NMR spectra were recorded on a Bruker Avance DPX 400 MHz spectrometer at 298 K. All chemical shifts are reported in ppm (parts per million) relative to their corresponding external standards (SiMe4 for 1H, H3PO4 for 31P{1H}, and Na2PtCl6 for 195Pt{1H}) and all the coupling constants (J values) are given in Hz. UV−vis spectra were performed using a PerkinElmer Lambda 25 spectrophotometer. Diffuse reflectance UV−vis (DRUV) data of the pressed powders were recorded on UV/vis/NIR spectrometer Cary 5-E (Varian). Polymer films containing about 0.1 wt % of the Pt complexes were obtained by dissolving the emitter and polymer in dichloromethane and spin-coating the solutions onto quartz glass substrates. Poly(methyl methacrylate) (PMMA) films were examined under continuous flushing with argon. Photoluminescence spectra were recorded on a PerkinElmer LS45 fluorescence spectrometer at room and low temperatures and the lifetimes measured in phosphorimeter mode. The quantum yields of the complexes were measured using an integrating sphere. All AgX (X = PF6, BF4, NO3, and CH3COO) salts were purchased from commercial sources. The NMR labeling for the ppy moiety is shown in Scheme 2 for clarifying the chemical shift

wavelengths are quantitatively and qualitatively close to their related experimental values with a slight spectral blue shift (see Table 2 for the experimental emission wavelengths).

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CONCLUSION In summary, a series of new cycloplatinated(II) complexes were synthesized by chloride abstraction from the precursor complex of [Pt(ppy)(PPh3)(Cl)], 3, by applying the various AgX (X = PF6, BF4, NO3, and CH3COO) salts. As expected, reactions of 3 with AgNO3 and AgOOCCH3 salts resulted in the formation of neutral 4a and 4b, respectively. In fact, during the chloride abstraction, Cl ligand is replaced by NO3 or CH3COO due to the coordinating ability of these entering ligands. However, since the PF6 and BF4 are noncoordinating anions, the reaction between 3 and AgX (X = PF6 and BF4) results in the formation of products with the general formula of [{Pt(ppy)(PPh3)(CH3CN)}X]. Expectedly, the coordinating solvent, i.e., CH3CN is able to fill the empty position on Pt center. In noncoordinating solvents like CH2Cl2, the CH3CN ligands can be present in the initial silver salt when Ag salt itself is a four-coordinated complex with CH3CN ligands ([{Ag(CH3CN)4}X], X = PF6 and BF4). Full NMR characterization and X-ray crystallography vividly confirm the proposed structures of the new cycloplatinated(II) complexes. All the new complexes, excluding 5b, have shown to be strong green luminescent materials at room temperature. Complex 5b however is relatively poorly emissive in the yellow region. Complexes 4a and 4b exhibit the same emission bands (shape and wavelength) under all states and temperature conditions but with different quantum efficiencies. This is confirmed by the fact that the ppy as the chromophore ligand is jointly present in both of the complexes. In contrast, for complexes 5a and 5b with the same cationic complex and different counteranions, completely different emission wavelengths and efficiencies were observed. This is despite the fact that their UV−vis spectra are exactly the same. Due to this observation, it can be asserted that the counteranions (PF6 and BF4) never interfere in the electronic transitions whereas they efficiently affect emission through their different short contacts with the cationic complex.

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Scheme 2. NMR Labeling for the ppy Moiety

assignments. Platinum complexes cis-[PtCl2(DMSO)2], 1,60 [Pt(ppy)(DMSO)(Cl)], 2,60 [Pt(ppy)(PPh3)Cl], 3,60 and silver salts [{Ag(CH3CN)4}PF6]61 and [{Ag(CH3CN)4}BF4]62 were prepared as reported in literature. [Pt(ppy)(PPh3)(NO3)], 4a. To a solution of the complex [Pt(ppy)(PPh3)(Cl)], 3, (100 mg, 0.154 mmol) in CH2Cl2 (15 mL) was added AgNO3 (26.1 mg, 0.154 mmol). Under dark conditions and argon atmosphere, the resulting mixture was stirred at room temperature for 15 h. The reaction mixture was filtered on a Celite

EXPERIMENTAL SECTION

General Procedures and Materials. All the reactions were carried out in common solvents in the lab without any further G


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Organometallics

Under dark conditions and argon atmosphere, the resulting mixture was stirred at room temperature for 4 h. The reaction mixture was filtered on a Celite in order to separate the AgCl precipitate. The solvent of resulting transparent pale green solution was evaporated using the vacuum. After reducing the volume to 1 mL, 5 mL of n-hexane was added to precipitate the product. Yield: 84%; mp = 223 °C. Elem. Anal. Calcd for C31H26BF4N2PPt (739.41): C, 50.36; H, 3.54; N, 3.79. Found: C, 50.49; H, 3.58; N, 3.87. 1H NMR (400 MHz, CDCl3, 20 °C, δ): 9.08 [ddd, 3JHH = 5.3 Hz, 4JPH = 4.7 Hz, 4JHH = 1.0 Hz, 3JPtH = 29.1 Hz, 1 H, H2], 7.99 [td, 3JHH = 8.0 Hz, 4JHH = 1.3 Hz, 1H, H4], 7.71−7.82 [m, 8H, H3, H5, Ho of PPh3], 7.46−7.57 [m, 10H, H6, Hm, Hp of PPh3], 7.02 [td, 3JHH = 7.7 Hz, 4JHH = 1.0 Hz, 1H, H7], 6.54−6.60 [m, 2H, H8, H9], 1.84 [s, 3H, Me of CH3CN]. 31P{1H} NMR (162 MHz, CDCl3, 20 °C, δ): 21.5 [s, 1JPtP = 4096 Hz, 1P]. 195Pt{1H} NMR (86 MHz, CDCl3, 20 °C, δ): −4046.6 [d, 1JPtP = 4094 Hz, 1Pt]. Method B. To a solution of the complex [Pt(ppy)(PPh3)(Cl)], 3, (100 mg, 0.154 mmol) in acetonitrile (15 mL) was added AgBF4 (29.97 mg, 0.154 mmol). Under dark conditions and argon atmosphere, the resulting mixture was stirred at room temperature for 4 h. The reaction mixture was filtered on a Celite in order to separate the AgCl precipitate. The solvent of resulting transparent pale green solution was evaporated using the vacuum. After reducing the volume to 1 mL, 5 mL of n-hexane was added to precipitate the product. X-ray Structure Determinations. The X-ray diffraction measurements were carried out on STOE IPDS-2/2T diffractometer with graphite-monochromated Mo Kα radiation. All single crystals were mounted on a glass fiber and used for data collection. Cell constants and an orientation matrix for data collection were obtained by leastsquares refinement of the diffraction data. Diffraction data were collected in a series of ω scans in 1° oscillations and integrated using the Stoe X-AREA63 software package. A numerical absorption correction was applied using X-RED64 and X-SHAAPE65 software. The data were corrected for Lorentz and polarizing effects. The structures were solved by direct methods66 and subsequent difference Fourier maps and then refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters.67 Atomic factors are from the International Tables for X-ray Crystallography.68 All non-hydrogen atoms were refined with anisotropic displacement parameters. Hydrogen atoms were placed in ideal positions and refined as riding atoms with relative isotropic displacement parameters. All refinements were performed using the X-STEP32 program.69 Crystallographic and structure refinement data for 4a, 4b, and 5a are collected in Table S5. Computational Details. Density functional calculations were performed with the program suite Gaussian0370 using the B3LYP level of theory.71−73 The LANL2DZ basis set was chosen to describe Pt.74,75 The 6-31G(d) basis set was used for other atoms. The geometries of complexes were optimized by employing the DFT without imposing any symmetry constraints.

in order to separate the AgCl precipitate. The solvent of resulting transparent yellow solution was evaporated using the vacuum. After reducing the volume to 1 mL, 5 mL of n-hexane was added to precipitate the product. Yield: 86%; mp = 187 °C. Elem. Anal. Calcd for C29H23N2O3PPt (673.56): C, 51.71; H, 3.44; N, 4.16. Found: C, 51.58; H, 3.42; N, 4.21. 1H NMR (400 MHz, CDCl3, 20 °C, δ): 8.66 [ddd, 3JHH = 5.6 Hz, 4JPH = 4.3 Hz, 4JHH = 1.1 Hz, 3JPtH = 30.3 Hz, 1 H, H2], 7.92 [td, 3JHH = 8.1 Hz, 4JHH = 1.3 Hz, 1H, H4], 7.79−7.84 [m, 7H, H5, Ho of PPh3], 7.47−7.51 [m, 4H, H6, Hp of PPh3], 7.42 [td, 3JHH = 7.6 Hz, 4JPH = 1.9 Hz, 6H, Hm of PPh3], 7.30 [dddd, 3 JHH = 6.8 Hz, 3JHH = 7.5 Hz, 4JHH = 1.1 Hz, 5JPH = 1.3 Hz, 1H, H3], 6.97 [td, 3JHH = 7.9 Hz, 4JHH = 1.1 Hz, 1H, H7], 6.61 [dd, 3JHH = 7.9 Hz, 4JPH = 3.6 Hz, 3JPtH = 54.3 Hz, 1H, H9], 6.55 [td, 3JHH = 7.8 Hz, 4JHH = 1.2 Hz, 1H, H8]. 31P{1H} NMR (162 MHz, CDCl3, 20 °C, δ): 22.3 [s, 1JPtP = 4455 Hz, 1P]. 195Pt{1H} NMR (86 MHz, CDCl3, 20 °C, δ): −4068.6 [d, 1JPtP = 4451 Hz, 1Pt]. [Pt(ppy)(PPh3)(CH3COO)], 4b. To a solution of the complex [Pt(ppy)(PPh3)(Cl)], 3, (100 mg, 0.154 mmol) in CH2Cl2 (15 mL) was added AgCH3COO (25.7 mg, 0.154 mmol). Under dark conditions and argon atmosphere, the resulting mixture was stirred at room temperature for 7 h. The reaction mixture was filtered on a Celite in order to separate the AgCl precipitate. The solvent of resulting transparent yellow solution was evaporated using the vacuum. After reducing the volume to 1 mL, 5 mL of n-hexane was added to precipitate the product. Yield: 90%; mp = 173 °C. Elem. Anal. Calcd for C31H26N2O2PPt (670.60): C, 55.52; H, 3.91; N, 2.09. Found: C, 55.64; H, 3.87; N, 2.16. 1H NMR (400 MHz, CDCl3, 20 °C, δ): 8.71 [ddd, 3JHH = 5.1 Hz, 4JPH = 4.8 Hz, 4JHH = 1.3 Hz, 3 JPtH = 29.7 Hz, 1 H, H2], 7.84−7.89 [m, 7H, H4, Ho of PPh3], 7.78 [d, 3JHH = 8.3 Hz, 1H, H5], 7.35−7.48 [m, 10H, H6, Hm, Hp of PPh3], 7.25 [dddd, 3JHH = 6.2 Hz, 3JHH = 7.6 Hz, 4JHH = 1.0 Hz, 5JPH = 1.4 Hz, 1H, H3], 6.92 [td, 3JHH = 7.6 Hz, 4JHH = 1.3 Hz, 1H, H7], 6.64 [dd, 3JHH = 7.6 Hz, 4JPH = 4.1 Hz, 3JPtH = 53.6 Hz, 1H, H9], 6.53 [td, 3JHH = 7.6 Hz, 4JHH = 1.5 Hz, 1H, H8], 1.40 [s, 3H, Me of CH3COO]. 31P{1H} NMR (162 MHz, CDCl3, 20 °C, δ): 23.3 [s, 1JPtP = 4461 Hz, 1P]. 195Pt{1H} NMR (86 MHz, CDCl3, 20 °C, δ): −4016.3 [d, 1JPtP = 4468 Hz, 1Pt]. [Pt(ppy)(PPh3)(CH3CN)]PF6, 5a. Method A. To a solution of the complex [Pt(ppy)(PPh3)(Cl)], 3, (100 mg, 0.154 mmol) in CH2Cl2 (15 mL) was added [Ag(CH3CN)4]PF6 (64.2 mg, 0.154 mmol). Under dark conditions and argon atmosphere, the resulting mixture was stirred at room temperature for 3 h. The reaction mixture was filtered on a Celite in order to separate the AgCl precipitate. The solvent of resulting transparent green solution was evaporated using the vacuum. After reducing the volume to 1 mL, 5 mL of n-hexane was added to precipitate the product. Yield: 91%; mp = 241 °C. Elem. Anal. Calcd for C31H26F6N2P2Pt (797.57): C, 46.68; H, 3.29; N, 3.51. Found: C, 46.79; H, 3.31; N, 3.46. 1H NMR (400 MHz, CDCl3, 20 °C, δ): 8.91 [ddd, 3JHH = 5.2 Hz, 4JPH = 4.7 Hz, 4JHH = 1.1 Hz, 3 JPtH = 29.6 Hz, 1 H, H2], 8.03 [td, 3JHH = 8.1 Hz, 4JHH = 1.3 Hz, 1H, H4], 7.87 [d, 3JHH = 8.1 Hz, 1H, H5], 7.79 [dd, 3JHH = 7.4 Hz, 3JPH = 12.0 Hz, 6H, Ho of PPh3], 7.65 [dd, 3JHH = 6.7 Hz, 3JHH = 7.6 Hz, 4 JHH = 1.1 Hz, 1H, H3], 7.48−7.59 [m, 10H, H6, Hm, Hp of PPh3], 7.07 [td, 3JHH = 7.4 Hz, 4JHH = 1.1 Hz, 1H, H7], 6.59−6.64 [m, 2H, H8, H9], 1.85 [s, 3H, Me of CH3CN]. 31P{1H} NMR (162 MHz, CDCl3, 20 °C, δ): 20.5 [s, 1JPtP = 4053 Hz, 1P], −144.5 (septet, 1JPF = 715, 1P, P of PF6). 195Pt{1H} NMR (86 MHz, CDCl3, 20 °C, δ): −4230.1 [d, 1JPtP = 4061 Hz, 1Pt]. Method B. To a solution of the complex [Pt(ppy)(PPh3)(Cl)], 3, (100 mg, 0.154 mmol) in acetonitrile (15 mL) was added AgPF6 (38.93 mg, 0.154 mmol). Under dark conditions and argon atmosphere, the resulting mixture was stirred at room temperature for 3 h. The reaction mixture was filtered on a Celite in order to separate the AgCl precipitate. The solvent of resulting transparent green solution was evaporated using the vacuum. After reducing the volume to 1 mL, 5 mL of n-hexane was added to precipitate the product. [Pt(ppy)(PPh3)(CH3CN)]BF4, 5b. Method A. To a solution of the complex [Pt(ppy)(PPh3)(Cl)], 3, (100 mg, 0.154 mmol) in CH2Cl2 (15 mL) was added [Ag(CH3CN)4]BF4 (55.26 mg, 0.154 mmol).

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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.8b00461. NMR spectra, emission spectra, crystallographic and computational details (DOCX) Cartesian coordinates (XYZ) Accession Codes

CCDC 1853566−1853568 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033. H


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(12) Hofbeck, T.; Lam, Y. C.; Kalbác,̌ M.; Záliš, S.; Vlček, A., Jr; Yersin, H. Thermally Tunable Dual Emission of the d8−d8 Dimer [Pt2 (μ-P2O5 (BF2) 2) 4] 4−. Inorg. Chem. 2016, 55, 2441−2449. (13) Bossi, A.; Rausch, A. F.; Leitl, M. J.; Czerwieniec, R.; Whited, M. T.; Djurovich, P. I.; Yersin, H.; Thompson, M. E. Photophysical Properties of Cyclometalated Pt(II) Complexes: Counterintuitive Blue Shift in Emission with an Expanded Ligand π System. Inorg. Chem. 2013, 52, 12403−12415. (14) Chan, M. H.-Y.; Wong, H.-L.; Yam, V. W.-W. Synthesis and Photochromic Studies of Dithienylethene-Containing Cyclometalated Alkynylplatinum(II) 1, 3-Bis (N-alkylbenzimidazol-2′-yl) benzene Complexes. Inorg. Chem. 2016, 55, 5570−5577. (15) Liao, K.-Y.; Hsu, C.-W.; Chi, Y.; Hsu, M.-K.; Wu, S.-W.; Chang, C.-H.; Liu, S.-H.; Lee, G.-H.; Chou, P.-T.; Hu, Y.; et al. Pt (II) metal complexes tailored with a newly designed spiro-arranged tetradentate ligand; harnessing of charge-transfer phosphorescence and fabrication of sky blue and white OLEDs. Inorg. Chem. 2015, 54, 4029−4038. (16) Babadi Aghakhanpour, R.; Nabavizadeh, S. M.; Rashidi, M.; Kubicki, M. Luminescence properties of some monomeric and dimeric cycloplatinated(II) complexes containing biphosphine ligands. Dalton Trans. 2015, 44, 15829−15842. (17) Babadi Aghakhanpour, R.; Nabavizadeh, S. M.; Rashidi, M. Newly designed luminescent di-and tetra-nuclear double rollover cycloplatinated(II) complexes. J. Organomet. Chem. 2016, 819, 216− 227. (18) Baya, M.; Belío, Ú .; Forniés, J.; Martín, A.; Perálvarez, M.; Sicilia, V. Neutral benzoquinolate cyclometalated platinum(II) complexes as precursors in the preparation of luminescent Pt−Ag complexes. Inorg. Chim. Acta 2015, 424, 136−149. (19) Chatterjee, S.; Norton, A. E.; Edwards, M. K.; Peterson, J. M.; Taylor, S. D.; Bryan, S. A.; Andersen, A.; Govind, N.; AlbrechtSchmitt, T. E.; Connick, W. B.; Levitskaia, T. G. Highly Selective Colorimetric and Luminescence Response of a Square-Planar Platinum(II) Terpyridyl Complex to Aqueous TcO4−. Inorg. Chem. 2015, 54, 9914−9923. (20) Zhao, D.; Krause, J. A.; Connick, W. B. Platinum(II) Monomer and Dimer Complexes with a Bis(oxazolinyl)phenyl Pincer Ligand. Inorg. Chem. 2015, 54, 8339−8347. (21) Yang, S.; Meng, F.; Wu, X.; Yin, Z.; Liu, X.; You, C.; Wang, Y.; Su, S.; Zhu, W. Dinuclear platinum(II) complex dominated by a zigzag-type cyclometalated ligand: a new approach to realize highefficiency near infrared emission. J. Mater. Chem. C 2018, 6, 5769− 5777. (22) Chan, A. K.-W.; Ng, M.; Wong, Y.-C.; Chan, M.-Y.; Wong, W.T.; Yam, V. W.-W. Synthesis and Characterization of Luminescent Cyclometalated Platinum(II) Complexes with Tunable Emissive Colors and Studies of Their Application in Organic Memories and Organic Light-Emitting Devices. J. Am. Chem. Soc. 2017, 139, 10750− 10761. (23) Schneider, J.; Du, P.; Jarosz, P.; Lazarides, T.; Wang, X.; Brennessel, W. W.; Eisenberg, R. Cyclometalated 6-Phenyl-2, 2′bipyridyl (CNN) Platinum (II) Acetylide Complexes: Structure, Electrochemistry, Photophysics, and Oxidative-and ReductiveQuenching Studies. Inorg. Chem. 2009, 48, 4306−4316. (24) Pashaei, B.; Shahroosvand, H.; Graetzel, M.; Nazeeruddin, M. K. Influence of Ancillary Ligands in Dye-Sensitized Solar Cells. Chem. Rev. 2016, 116, 9485−9564. (25) Yam, V. W. W.; Tang, R. P. L.; Wong, K. M. C.; Lu, X. X.; Cheung, K. K.; Zhu, N. Syntheses, Electronic Absorption, Emission, and Ion-Binding Studies of Platinum (II) C∧ N∧ C and Terpyridyl Complexes Containing Crown Ether Pendants. Chem. - Eur. J. 2002, 8, 4066−4076. (26) Lanoë, P.-H.; Le Bozec, H.; Williams, J. G.; Fillaut, J.-L.; Guerchais, V. Cyclometallated platinum (II) complexes containing pyridyl-acetylide ligands: the selective influence of lead binding on luminescence. Dalton Trans. 2010, 39, 707−710. (27) Siu, P. K.; Lai, S. W.; Lu, W.; Zhu, N.; Che, C. M. A Diiminoplatinum (II) Complex of 4-Ethynylbenzo-15-crown-5 as a

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (H.R.S.). *E-mail: [email protected] (M.H.B.). ORCID

Hamid R. Shahsavari: 0000-0002-2579-2185 Mohsen Golbon Haghighi: 0000-0002-2422-9075 M. Hassan Beyzavi: 0000-0002-6415-8996 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Institute for Advanced Studies in Basic Sciences (IASBS) Research Council and the Iran National Science Foundation (Grant no. 95834232). M.H.B. gratefully acknowledges the financial support through the startup funds from the University of Arkansas. This paper is dedicated to Professor Piero Mastrorilli.

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