Size-Selective Vapochromic Platinum(II) Complex with a Sterically

Jun 24, 2011 - Demanding Pincer-Type N-Heterocyclic Carbene Ligand. Chen-Shiang Lee,. †. Rui Rui Zhuang,. †. Shahulhameed Sabiah,. †. Ju-Chun Wa...
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Size-Selective Vapochromic Platinum(II) Complex with a Sterically Demanding Pincer-Type N-Heterocyclic Carbene Ligand Chen-Shiang Lee,† Rui Rui Zhuang,† Shahulhameed Sabiah,† Ju-Chun Wang,‡ Wen-Shu Hwang,*,† and Ivan J. B. Lin† † ‡

Department of Chemistry, National Dong Hwa University, Hualien 974, Taiwan, Republic of China Department of Chemistry, Soochow University, Taipei 111, Taiwan, Republic of China

bS Supporting Information ABSTRACT: A pyridine-based pincer-type N-heterocyclic carbene complex of Pt(II), [Pt(II)(mesC∧N∧C)CO](PF6)2 (mesC∧N∧C = 2,6-bis(1-mestylimidazol2-ylidenyl)pyridine), displays size-selective vapochromic behavior upon exposure to small molecules with donor atoms. An acetone adduct showing a weak interaction between acetone and a CtO ligand is also structurally determined.

1. INTRODUCTION Over the past decades, the dependence of luminescence behavior on volatile organic compounds (VOCs) has received much attention; this is especially true for emissive transition metal complexes.1 In most cases, compounds with weak intermolecular metal metal interactions may change their optical properties in the presence of solvents through structural rearrangement.2 In addition, porous materials with sorption sites may allow the analyte to reach the chromophore to generate a vapochromic response.3 Despite the extensive applications of metal-NHC complexes in catalysis, reports of their photophysical properties are relatively rare.4 Among them, few examples displayed vapochromism under ambient conditions.3d Recently, we also reported a pincer-type Pt(II) NHC complex as host material that displayed a pronounced shift in emission color upon inclusion of water vapor (aquachromic photoluminescence).4d In this work, we report a new class of Pt(II)carbonyl complex with a C∧N∧C pincer NHC ligand bearing mesityl rings. The steric bulkiness and rigidity of the mesityl rings are expected to create a pocket, which allows small solvent molecules to cocrystallize in the crystal structure and gives rise to emissive behavior depending on the size and donor ability of the solvate. Furthermore, an acetone adduct was structurally characterized, which demonstrated the sequential interaction and reaction upon exposure of this pincer Pt(II) complex to small donor solvents. 2. RESULTS AND DISCUSSION Treatment of a mixture of 1 equiv of 6-bis(1-mestylimidazole3-yl)pyridine dibromide and K2PtCl4 with 1.1 equiv of Ag2O in r 2011 American Chemical Society

DMSO at 100 °C gave pure 2 in 91% yield (Scheme 1). The 13C NMR of 2 displayed a signal at 167.8 ppm corresponding to the carbenoid carbon bonded to Pt(II), while the 1H NMR spectra showed the disappearance of C2 H. Treatment of complex 2 with AgPF6 under CO gas resulted in the formation of complex 3. Complex 3 was characterized by IR spectroscopy; the carbonyl unit gives rise to an absorption at 2124 cm 1. This value is higher than those of the Pt(II) complexes with anionic pincer ligands,5 suggesting a weaker π-back-bonding to the CO ligand. Recrystallizion of 3 in acetone under an atmosphere of diethyl ether provided crystalline 3 3 2acetone suitable for single-crystal X-ray analysis. Alternatively, crystallization from a CH2Cl2/ hexane mixture gave 3 3 1/2CH2Cl2. A perspective view of 3 3 2acetone (Figure 1) shows clearly that the imidazole and the pyridine rings of the pincer ligand are almost coplanar. This C∧N∧C pincer and CtO form a square-planar geometry around the platinum center. The two carbenes are disposed trans to each other, and the pyridyl nitrogen is trans to the CO ligand. The bond lengths between the carbenoid carbons and the Pt center are 2.007(8) and 2.014(8) Å, while the separation from the carbonyl carbon to the Pt center is 1.889(8) Å. All the observed geometrical parameters are similar to those in analogous complexes.6 It is worthy to note that the bulky mesityl rings at the pincer ligand in this system are almost perpendicular to the heterocyclic rings (80.7(2)° and 90.0(2)°). The large torsions of Received: March 23, 2011 Published: June 24, 2011 3897

dx.doi.org/10.1021/om200253f | Organometallics 2011, 30, 3897–3900

Organometallics

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Scheme 1. Synthesis of the Pincer Pt(II) Complex 3

Figure 1. ORTEP drawing of complex 3 3 2acetone with atomic numbering scheme (30% probability level). Hydrogen atoms, anion, and cocrystallized acetone molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): C1 Pt1 = 2.007(8), C9 Pt1 = 2.014(8), N1 Pt1 = 1.974(6), C30 Pt1 = 1.889(8), C1 Pt1 C9 = 156.8(3)o, N1 Pt1 C30 = 179.4(3)o.

the mesityl rings lead to the formation of an H-shaped framework viewed sideways along the Pt CtO axis (Figure 2a); a packing diagram with the acetone solvate omitted is shown in Figure 2b. There are no π 3 3 3 π and Pt 3 3 3 Pt interactions in the crystal; the separation between the pincer ligands is ca. 8.113 Å, defined by the mean planes of the ligand; and the Pt Pt distance is 8.355(1) Å. Of the two acetone molecules in the lattice of 3 3 2acetone, one sits outside the pocket, and the other one sits in the pocket between the two mesityl rings, in which the acetone oxygen atom points to the Pt center with O 3 3 3 Pt and O 3 3 3 C (carbon monoxide) distances of 3.45 and 2.82 Å, respectively. The latter value is shorter than the corresponding van der Waals distance of 3.22 Å, suggesting the existence of a weak interaction between the acetone oxygen and the CtO carbon. Further supporting evidence for this weak interaction are that (i) the ν(CdO) of 1705 cm 1 for the solvated acetone is lower than that of the free acetone at 1715 cm 1 7 and (ii) the ν(CtO) value of 2110 cm 1 for 3 3 2acetone is lower than that of the value of 2125 cm 1 for 3. The solvate acetones in 3 3 2acetone evaporate slowly under ambient conditions. This process was monitored by powder X-ray diffraction. Diffractograms show that the sharp reflections of the crystalline solids change dramatically along with time and eventually become broad after 4 h, suggesting the deterioration of the crystallinity due to the loss of acetone molecules (Figure S1, Supporting Information). The IR spectrum of the final sample shows the disappearance of the ν(CdO) 1705 cm 1

band, confirming the total evaporation of the acetone molecules (Figure S2, Supporting Information). The cation of 3 3 1/2CH2Cl2 (depicted in Figure S3, Supporting Information) also adopts a square-planar geometry around the Pt center. Unlike 3 3 2acetone, the solvated CH2Cl2 molecules locate at a position away from the Pt(II) centers. Hence, there is no specific interaction between the CH2Cl2 molecule and Pt CtO. The crystal of this compound also deteriorates upon exposure to air and at a much faster rate than that of 3 3 2acetone. All compounds reported in this work are luminous (Figures 3). 3 3 2acetone, which shows a solvate Pt-CtO interaction, displays a bright featureless yellow emission at λmax 529 nm (λexc 390 nm). On the other hand, without a solvate Pt-CtO interaction, 3 3 1/2CH2Cl2 exhibits a blue luminescence at λmax 462 nm (λexc 390 nm) with fine structure. It appears that a solvate Pt-CtO interaction plays a role in the emission behavior of these compounds. A dispersed solid of 3 without solvate was employed to study its vapochromic nature. To prepare 3 without solvate, 3 3 1/2CH2Cl2 was crushed to a powder in an agate mortar and was then placed in a vacuum to remove the CH2Cl2; the absence of CH2Cl2 was confirmed by 1H NMR spectroscopy. 3, like 3 3 1/2CH2Cl2, exhibits a structured blue emission at 462 nm (Figure 3). When 3 was exposured to a stream of N2 gas saturated with small solvent molecules containing oxygen or nitrogen donor atoms, such as MeOH, H2O, THF, ether, DMF, or pyridine, a change of emission from blue to yellow occurs (Figure 4). Interestingly, small solvent molecules without a donor atom (CH2Cl2, chloroform, or benzene) do not change the emission spectrum of 3 (Figure 5). Furthermore, exposure to vapors of larger molecules with donor atoms such as benzaldehyde, cyclohexanol, cyclohexanone, and acetophenone also does not change the emission spectrum of 3 (Figure 5), suggesting that larger molecules do not interact with the PtCO moiety. The vapochromic behavior is reversible; while the blue emission of 3 changes to yellow upon exposure to small donor molecules, further exposure to small nondonor molecules switch the emission color back to blue and vice versa. Although the exact mechanism of the vaporchromism is still unclear, we believe that the interaction between the Pt CtO carbon and the oxygen or nitrogen donor atom causes the yellow emission. Larger molecules or molecules without a donor atom could not interact with the Pt CtO moiety, and hence the emission nature of 3 appears.

3. CONCLUSIONS To sum up, the emission of compound 3 is very sensitive to the presence of small donor molecules such as acetone, diethyl ether, THF, water, or methanol, which causes a change of emission from blue to yellow. The structure of 3 3 2acetone also provides a good example demonstrating an intermediate for CtO nucleophilic 3898

dx.doi.org/10.1021/om200253f |Organometallics 2011, 30, 3897–3900

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Figure 2. (a) Slot above/below the Pt(II) center in a space-filling drawing of complex 3 3 2acetone viewed from the CO ligand. (b) Packing diagram of complex 3 3 2acetone along the b axis. Acetone molecules are removed for clarity.

Figure 3. Emission spectra of 3, 3 3 2acetone, and 3 3 1/2CH2Cl2 at 300 K in the solid state (λexc = 390 nm without degas).

addition reaction. Accidentally, we put crystalline 3 3 2acetone in pure H2O for a week, and an unexpected Pt-CO2H complex, [MesCNCPt(COOH)]PF6], was obtained (Figure S4, Supporting Information). The formation of the hydroxycarbonyl ligand suggested that a water molecule could react with a CtO ligand.

Figure 4. Emission spectra of 3 upon exposure to solvent vapors (MeOH, H2O, THF, ether, DMF, pyridine) at 300 K in the solid state (λexc = 390 nm without degas).

4. EXPERIMENTAL SECTION General Procedures. All reactions were carried out in air. The solvents and reagents were purchased from general sources and were used without further purification. NMR spectra were recorded on a Bruker DX-300 NMR spectrometer (1H, 299.96 MHz; 13C, 75.43 MHz). Mass spectra were measured on a Micromass Platform II spectrometer. Elemental analyses were carried out using a Perkin-Elmer 2400, 2400II elemental analyzer. Emission spectra were obtained with an Aminco Bowman AD2 luminescent spectrofluorometer. Synthesis of 2. To a suspension of 2,6-bis(1-mestylimidazolium-3-yl)pyridine dibromide8 (0.73 g, 1.2 mmol) in 5 mL of DMSO were added Ag2O (0.32 g, 1.3 mmol) and K2PtCl4 (0.5 g, 1.2 mmol). The mixture was heated for 6 h at 100 °C in the dark to give an orange solution. Filtration of this solution into an equal amount of dichloromethane, followed by adding 100 mL of ether, immediately gave a light yellow precipitate. Collection of the precipitate and evaporation of solvent in vacuum yielded 2 as a light yellow powder (0.78 g, 91%). 1H NMR (DMSOd6): 8.66 (s, 2H, imidazole H), 8.59 (t, JH H = 8.4 Hz, 1H, py-H), 8.12

Figure 5. Emission spectra of 3 upon exposure to vapors (CHCl3, benzene, benzaldehyde, cyclohexanol, cyclohexanone, acetophenone) at 300 K in the solid state (λexc = 390 nm without degas). (d, JH H = 8.4 Hz, 2H, py-H), 7.73 (s, 2H, imidazole H), 6.93 (s, 4H, C6H2Me3), 2.28 (s, 6H, p-CH3), 1.96 (s, 12H, o-CH3) ppm. 13C NMR (DMSO-d6): 167.8 (NCN), 152.0 (2,6-pyridyl C), 146.0 (4-pyridyl C), 108.8 (3,5-pyridyl C), 139.1 and 119.8 (imidazole C), 125.9, 129.0, 134.4, 3899

dx.doi.org/10.1021/om200253f |Organometallics 2011, 30, 3897–3900

Organometallics 133.8 (mestyl C), 17.8 and 21.1(CH3) ppm. MALDI-MS: [M]+ = 678 (m/z). Anal. Calcd for C29H29Cl2N5Pt 3 1.5H2O: C 47.03, N 9.46, H 4.36. Found: C 46.81, N 9.46, H 4.75. Synthesis of 3 3 2acetone. CO gas was bubbled through a mixture of 2 (0.14 g, 0.2 mmol) and AgPF6 (0.1 g, 0.41 mmol) in acetone for 12 h to give a pale green solution. Removal of solvent in vacuum yielded 3 as a yellow powder (0.81 g, 85%). Recrystallization of 3 in acetone of diffusion diethyl ether provided crystalline 3 3 2acetone. 1H NMR (d6-acetone): 8.74 (s, 2H, imidazole H), 8.87 (t, JH H = 8.4 Hz, 1H, py-H of pincer ligand), 7.96 (s, 2H, imidazole H), 8.36 (d, JH H = 8.4 Hz, 2H, py-H), 7.13 (s, 4H, C6H2Me3), 2.32 (s, 6H, p-CH3), 2.09 (s, 12H, o-CH3) ppm. 13 C NMR (d6-acetone): 167.3 (NCN), 159.6 (CO), 152.3, 151.7, 141.6, 134.7, 133.4, 129.5, 125.5, 121.1, 110.7 (C of the pincer ligand), 20.2 (p-CH3), and 16.7 (o-CH3) ppm. Anal. Calcd for C36H41F12N5O3P2Pt: C 40.16, N 6.50, H 3.84. Found: C 40.12, N 6.55, H 3.91. X-ray Crystallographic Procedures. Single crystals of 3 3 2acetone and 3 3 1/2CH2Cl2 suitable for X-ray diffraction were covered with mineral oil, embedded in Araldite glue under nitrogen, mounted on the tips of glass fibers with epoxy resin, and subjected to X-ray diffraction analysis. Data were collected on a Bruker APEX II diffractometer, using graphitemonochromated Mo KR radiation (λ = 0.71073 Å).9 Data reduction was performed with SAINT,9 which corrects for Lorentz and polarization effects. Absorption corrections were performed using multiscan (SADABS).9 Structures were solved by the use of direct methods, and refinement was performed by the least-squares methods on F2 with the SHELXL-97 package.10 All H atoms were added in idealized positions. Large solvent-accessible voids in 3 3 1/2CH2Cl2 are observed, indicating that there might be more solvent molecules exiting from the cell. However, no suitable solvent molecules can be located and refined due to the flat difference map, which may be caused by disorder problems or by losing solvent during data collection. Therefore, a SQUEEZE/PLATON technique was applied to remove those unidentified solvent contributions.11 Crystal data for 3 3 2acetone: C36H41F12N5O3P2Pt, M = 1076.77, monoclinic, a = 32.657(2) Å, b = 8.3550(5) Å, c = 30.777(2) Å, β = 90.571(2)°, V = 8396.9(9) Å3, T = 296(2) K, space group C2/c, Z = 8, μ = 3.513 mm 1, 54 764 reflections measured, 10 921 independent reflections (Rint = 0.0501). The final R1 values were 0.0695 (I > 2σ(I)). The final wR(F2) values were 0.1473 (I > 2σ(I)). The final R1 values were 0.0749 (all data). The final wR(F2)) values were 0.1502 (all data). The goodness of fit on F2 was 0.952. CCDC number 794553. Crystal data for 3 3 1/2CH2Cl2: C61H60Cl2F24N10O2P4Pt2, M = 2006.15, monoclinic, a = 42.657(6) Å, b = 21.943(2) Å, c = 16.483(2) Å, β = 94.191(4)°, V = 15387(3) Å3, T = 296(2) K, space group C2/c, Z = 8, μ = 3.891 mm 1, 72 497 reflections measured, 13 560 independent reflections (Rint = 0.0879). The final R1 values were 0.0647 (I > 2σ(I)). The final wR(F2) values were 0.1506 (I > 2σ(I)). The final R1 values were 0.1198 (all data). The final wR(F2) values were 0.1712 (all data). The goodness of fit on F2 was 0.957. CCDC number 794554.

’ ASSOCIATED CONTENT

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’ ACKNOWLEDGMENT The authors acknowledge the National Science Council (Taiwan, ROC) and National Dong Hwa University for financial support. ’ REFERENCES (1) (a) Kato, M.; Omura, A.; Toshikawa, A.; Kishi, S.; Sugimoto, Y. Angew. Chem., Int. Ed. 2002, 41, 3183. (b) Kato, M. Bull. Chem. Soc. Jpn. 2007, 80, 287. (c) Ni, J.; Wu, Y. H.; Zhang, Xu.; Li, Bin.; Zhang, L. Y.; Chen, Z, N. Inorg. Chem. 2009, 48, 10202. (2) (a) Fernandez, E. J.; Lopez-de-Luzuriaga, J. M.; Monge, M.; Olmos, M. E.; Perez, J.; Laguna, A.; Mohamed, A. A.; Fackler, J. P., Jr. J. Am. Chem. Soc. 2003, 125, 2022. (b) Fernandez, E. J.; Lopez-deLuzuriaga, J. M.; Monge, M.; Montiel, M.; Olmos, M. E.; Perez, J.; Laguna, A.; Mendizabal, F.; Mohamed, A. A.; Fackler, J. P., Jr. Inorg. Chem. 2004, 43, 3573. (c) Cariati, E.; Bu, X.; Ford, P. C. Chem. Mater. 2000, 12, 3385. (d) Drew, S. M.; Smith, L. I.; McGee, K. A.; Mann, K. R. Chem. Mater. 2009, 21, 3117. (e) Wadas, T. J.; Wang, Q.-M.; Kim, Y.-j.; Flaschenreim, C.; Blanton, T. N.; Eisenberg, R. J. Am. Chem. Soc. 2004, 126, 16841. (f) Du, P. W.; Schneider, J.; Brennessel, W. W.; Eisenberg, R. Inorg. Chem. 2008, 47, 69. (g) Grove, L. J.; Rennekamp, J. M.; Jude, H.; Connick, W. B. J. Am. Chem. Soc. 2004, 126, 1594. (h) Yam, V. W.-W.; Wong, K. M.-C.; Zhu, N. Y. J. Am. Chem. Soc. 2002, 124, 6506. (3) (a) Evju, J. K.; Mann, K. R. Chem. Mater. 1999, 11, 1425. (b) Abe, T.; Shinozaki, K. Inorg. Chem. 2005, 44, 849. (c) Abe, T.; Suzuki, T.; Shinozaki, K. Inorg. Chem. 2010, 49, 1794. (d) Strasser, C, E; Catalano, V, J. J. Am. Chem. Soc. 2010, 132, 10009. (e) Das, S.; Bharadwaj, P. K. Cryst. Growth Des. 2007, 7, 1192. (4) (a) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3651. (b) Mercs, L.; Albrecht, M. Chem. Soc. Rev. 2010, 39, 1903. (c) Unger, Y; Meyer, D; Strassner, T. Dalton Trans 2010, 39, 4295. (d) Lee, C. S.; Sabiah, S.; Wang, J. C.; Hwang, W. S.; Lin, I. J. B. Organometallics 2010, 29, 286. (5) (a) Cave, G. W. V.; Fanizzi, F. P.; Deeth, R. J.; Errington, W.; Rourke, J. P. Organometallics 2000, 19, 1355. (b) Newman, C. P.; Cave, G. W. V.; Wong, M.; Errington, W.; Alcock, N. W.; Rourke, J. P. J. Chem. Soc., Dalton Trans. 2001, 2678. (6) (a) Newman, C. P.; Deeth, R. J.; Clarkson, G. J.; Rourke, J. P. Organometallics 2007, 26, 6225. (b) Han, Y.; Tan, G. K.; Huynh, H. V. Organometallics 2007, 26, 4612. (c) Fantasia, S.; Petersen, J. L.; Jacobsen, H.; Cavallo, L.; Nolan, S. P. Organometallics 2007, 26, 5880. (7) Acetone IR data: Neat sample: 1715 cm 1. Silverstein, R, M.; Webster, F. X. Spectrometric Identification of Organic Compounds, 6th ed.; John Wiley & Sons, Inc.: New York. (8) Loch, J. A.; Albrecht, M.; Peris, E.; Mata, J.; Faller, J. W.; Crabtree, R. H. Organometallics 2002, 21, 700. (9) APEX2, SAINT, and SADABS; Bruker AXS Inc.: Madison, WI., USA, 2004. (10) Sheldrick, G. M. Acta Crystallogr. Sect. A 2008, 64, 112. (11) Spek, A. L. J. Appl. Crystallogr. 2003, 36, 7.

bS

Supporting Information. Powder XRD and FTIR spectral changes of 3 3 2acetone. ORTEP drawing and packing diagram of complex 3 3 1/2CH2Cl2. Molecular structure of [MesCNCPt(COOH)]PF6 ]. X-ray crystallographic file of 3 3 2acetone, 3 3 1/2CH2Cl2, and [MesCNCPt(COOH)]PF6] in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Fax: +886-3-8632000. Tel: +886-3-8632001. E-mail: hws@ mail.ndhu.edu.tw. 3900

dx.doi.org/10.1021/om200253f |Organometallics 2011, 30, 3897–3900