Synthesis of Organoplatinum Oligomers by Employing
N-Donor Bridges with Predesigned Geometry:
Structural and Photophysical Properties of Luminescent
Cyclometalated Platinum(II) Macrocycles
Siu-Wai Lai, Michael C. W. Chan, Kung-Kai Cheung, Shie-Ming Peng,
†and
Chi-Ming Che*
Department of Chemistry, The University of Hong Kong, Pokfulam Road, Hong Kong Received May 10, 1999
A series of luminescent di- and trimeric cyclometalated platinum(II) macrocycles, namely, [Pt(C-N)(N-N)]2(N-N ) pyrazolate, pz (2); 7-azaindolate, aza (3); C-N ) 2-(2′ -thienyl)-pyridyl, Thpy (a); 7,8-benzoquinolate, Bzqn (b); 2-phenyl-thienyl)-pyridyl, Phpy (c)) and [Pt(C-N)-(N-N)]3(N-N ) benzimidazolate, bzim (4); C-N ) Thpy (a) and Bzqn (b)), are synthesized in high yields (70-90%). The rigid, predefined coordination geometry of the pz, aza, and bzim bridging ligands ensure the efficient and selective assembly of the respective cyclic oligomers. The stacking arrangements in the crystal structures of 1a and 4a revealπ-π interactions between 2-(2′-thienyl)pyridyl moieties. The lowest energy absorption band in the UV-vis spectra is assigned to metal-to-ligand charge transfer (MLCT) transitions. The 298 K fluid emissions of complexes 2-4a bearing Thpy ligands are assigned to transitions with mixed MLCT and IL (intraligand) character. In crystalline form, the observed red-shift is attributed to excimeric emission arising fromπ-stacking in the solid state. In contrast, a number of Bzqn derivatives are nonemissive in CH2Cl2at 298 K. The energy of the lowest energy UV-vis and emissive bands for Thpy oligomers are red-shifted relative to the Bzqn and Phpy congeners. Significantly, Thpy complexes and especially theµ-pyrazolate species 2a (τo) 15.5 µs, φo) 0.18) emit with longer lifetimes and higher quantum yields.
Introduction
Multidentate nitrogen-donor ligands are extensively utilized for the assembly of cyclic supermolecules. Pyrazolyl,1,2 imidazolyl,3 and particularly pyridyl4-7 derivatives have been incorporated into the design and construction of novel oligometallic macrocycles and cage
complexes. Nevertheless, integration of photolumines-cent moieties into these potential molecular receptors is rarely undertaken, although this can confer chemical-sensing applications.8 In this context, cyclometalated platinum(II) complexes bearing substituted pyridyl and 2,2′-bipyridyl ligands are promising candidates because they display emissive metal-to-ligand charge transfer (MLCT) excited states in solution,9-14and these emis-sions are extremely environment-sensitive.15,16
In this work, our intention was to fabricate platinum-containing macrocycles by connecting cyclometalated
Pt-* Corresponding author. Fax: +852 2857 1586. E-mail: cmche@ hkucc.hku.hk.
†Department of Chemistry, National Taiwan University, Taipei,
Taiwan.
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Publication on Web 08/28/1999
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(II) luminophores with various bidentate N-donor link-ers. The pz (Hpz ) pyrazole), aza (Haza ) 7-azaindole), and bzim (Hbzim ) benzimidazole) ligands exhibit predefined yet different geometrical characteristics. Hence accurate design is anticipated to yield specific molecular architectures that are thermodynamically favored. A ligand-directed strategy for the synthesis of triplatinum macrocycles by employing bzim as a rigid nonlinear bridging motif has been communicated.17We now describe the preparation, crystal structures, and photophysics of luminescent platinum(II) cyclic dimers (bearing pz and aza) and trimers (bearing bzim) plus their monomeric precursors. By varying the N-donor bridges and the cyclometalating ligands, namely, Thpy (HThpy ) 2-(2′-thienyl)pyridine), Bzqn (HBzqn ) 7,8-benzoquinoline), and Phpy (HPhpy ) 2-phenylpyridine), we are able to assess trends and changes in emissive properties. Seminal spectroscopic studies by Gray and co-workers on d8-d8binuclear species containing µ-pyra-zolate ligands have appeared.18,19
Experimental Section
General Procedures. The following chemicals were
ob-tained from Aldrich Chemical Co. and used as received: K2
-PtCl4 (98%), 2-phenylpyridine (HPhpy), 7,8-benzoquinoline
(HBzqn), sodium hydride, pyrazole (Hpz), benzimidazole (Hbzim), and 7-azaindole (Haza). 2-(2′-Thienyl)pyridine (HTh-py) was purchased from Lancaster Chemical Co. [nBu
4
N][Pt-(Phpy)Cl2] (1c) was prepared by literature methods.20
Synthe-sis of 4a,b has been described previously.17Dichloromethane
for photophysics studies was washed with concentrated sul-furic acid, 10% sodium hydrogen carbonate, and water, dried by calcium chloride, and distilled over calcium hydride. All other solvents were purified according to conventional meth-ods.21
Fast atom bombardment (FAB) mass spectra were obtained on a Finnigan Mat 95 mass spectrometer.1H (300 MHz) and 13C (126 MHz) spectra were recorded on DPX 300 and 500
Bruker FT-NMR spectrometers, respectively, with chemical shifts (in ppm) relative to tetramethylsilane. Elemental analy-sis was performed by the Institute of Chemistry at the Chinese Academy of Sciences, Beijing. Infrared spectra were recorded as KBr plates on a BIO RAD FT-IR spectrometer. UV-vis spectra were recorded on a Perkin-Elmer Lambda 19 UV/vis spectrophotometer.
Emission and Lifetime Measurements. Steady-state
emission spectra were recorded on a SPEX 1681 Fluorolog-2 series F111AI spectrophotometer. Low-temperature (77 K) emission spectra for frozen and solid-state samples were recorded in 5 mm diameter quartz tubes, which were placed
in a liquid nitrogen Dewar equipped with quartz windows. Emission spectra were corrected for monochromator and photomultiplier efficiency and for xenon lamp stability. Details of emission quantum yield determinations by the method of Demas and Crosby22 have been given.11 Emission lifetime
measurements were performed with a Quanta Ray DCR-3 pulsed Nd:YAG laser system (pulse output 355 nm, 8 ns). The emission signals were detected by a Hamamatsu R928 pho-tomultiplier tube and recorded on a Tektronix model 2430 digital oscilloscope. Errors forλ values ((1 nm), τ ((10%), and φ ((10%) are estimated.
Synthesis. [Pt(Thpy)(HThpy)Cl], 1a. [nBu
4N]2[PtCl4] was
prepared in the organic phase of a phase-transfer metathesis reaction by mixing K2PtCl4(0.20 g, 0.48 mmol) in H2O (10 mL)
withnBu
4NCl (0.58 g, 2.09 mmol) in CH2Cl2(30 mL).20HThpy
(0.32 g, 1.99 mmol) in CH3OH (30 mL) was then added to the
CH2Cl2phase. The red solution was stirred at reflux for 48 h
until an orange solution was obtained. Upon removal of solvent, the orange oily solid was washed with hexane (3× 10 mL) and cold acetone (10 mL). Recrystallization by diffusion of diethyl ether into a CH2Cl2 solution afforded orange
crystals: yield 0.20 g, 74%. Anal. Calcd for C18H13N2S2PtCl:
C, 39.17; H, 2.37; N, 5.08. Found: C, 39.25; H, 2.24; N, 5.04. FAB-MS: m/z 552 [M+], 516 [M+- Cl]. IR (Nujol): ν ) 1603,
1549, 1516 cm-1.1H NMR (CD
2Cl2): 6.07 (d, 1H, J ) 4.8 Hz),
7.03 (m, 2H), 7.22-7.33 (m, 3H), 7.46 (d, 1H, J ) 5.0 Hz), 7.71-7.98 (m, 4H), 9.15 (d with broad 195Pt satellites, 1H, 3J HH ) 5.8 Hz, 3JPtH ) 47 Hz), 9.41 (d with broad 195Pt satellites, 1H,3J HH) 5.3 Hz,3JPtH) 42 Hz).13C{1H}NMR (CD2Cl2): 117.5, 119.8, 124.0, 126.5, 127.4, 128.4, 129.7, 130.1, 130.4, 138.3, 139.8, 141.2, 145.4, 151.4, 155.1, 156.0, 163.4.
[Pt(Thpy)(pz)]2, 2a. A mixture of pyrazole (0.03 g, 0.40
mmol) and excess NaH in THF (10 mL) under a N2atmosphere
was stirred for 30 min until the evolution of hydrogen ceased. The solution was filtered, added to 1a (0.20 g, 0.36 mmol) in CH2Cl2 (30 mL), and stirred under reflux for 30 h. The
resultant orange solution was concentrated to 5 mL, and addition of diethyl ether afforded an orange solid. Recrystal-lization by diffusion of diethyl ether into a dichloromethane solution afforded orange crystals: yield 0.11 g, 72%. Anal. Calcd for C24H18N6S2Pt2: C, 34.12; H, 2.15; N, 9.95. Found:
C, 33.92; H, 1.94; N, 10.00. FAB-MS: m/z 844 [M+], 777 [M+ - pz]. IR (Nujol): ν ) 1605, 1563, 1520 cm-1.1H NMR (CD 2 -Cl2): 6.45 (virtual t, 2H, H(4) of pz), 6.78-6.91 (m, 4H), 7.35-7.42 (m, 4H), 7.62-7.78 (m, 6H), 8.15 (d with v. broad195Pt satellites, 1H, 3J HH ) 5.7 Hz), 8.30 (d with v. broad 195Pt satellites, 1H,3J HH) 6.0 Hz).13C{1H}NMR (CDCl3): 105.7, 106.0, 106.1, 117.5, 117.6, 119.1, 119.4, 127.6, 127.8, 132.8, 133.0, 138.1, 138.4, 138.8, 138.9, 139.9, 140.1, 149.2, 149.3, 149.4, 150.1, 163.6, 163.9.
[Pt(Thpy)(aza)]2, 3a. The procedure for 2a was adopted
using 7-azaindole (0.04 g, 0.38 mmol), excess NaH, and 1a (0.20 g, 0.36 mmol) to afford red crystals: yield 0.13 g, 76%. Anal. Calcd for C32H22N6S2Pt2: C, 40.68; H, 2.35; N, 8.89.
Found: C, 40.43; H, 2.43; N, 8.63. FAB-MS: m/z 944 [M+],
827 [M+ - aza]. IR (Nujol): ν ) 1607, 1559 cm-1.1H NMR
(CD2Cl2): 6.23 (d, 1H, J ) 4.7 Hz), 6.33 (d, 1H, J ) 3.8 Hz),
6.62 (t, 1H, J ) 6.0 Hz), 6.75-7.35 (m, 10H), 7.53 (d, 1H, J ) 4.8 Hz), 7.60-7.90 (m, 6H), 8.11 (d with v. broad 195Pt
satellites, 1H, J ) 3.0 Hz), 9.71 (d with v. broad195Pt satellites,
1H, J ) 4.8 Hz). 13C{1H} NMR (DMSO-d
6): 99.6-101.1,
114.2-122.2, 125.1-133.2, 135.9-142.3, 146.3-146.8, 159.2, 159.6.
[nBu4N][Pt(Bzqn)Cl2], 1b. The procedure for 1a was
adopted using 7,8-benzoquinoline (0.36 g, 1.99 mmol), K2PtCl4
(0.20 g, 0.48 mmol), andnBu
4NCl (0.58 g, 2.09 mmol) to yield
yellow crystals: yield 0.16 g, 50%. Anal. Calcd for C29H44N2
-Cl2Pt: C, 50.73; H, 6.46; N, 4.08. Found: C, 50.96; H, 6.45; N,
3.78. FAB-MS (-ve): m/z 444 [M-]. IR (Nujol): ν ) 1621, 1592, (13) (a) Tse, M. C.; Cheung, K. K.; Chan, M. C. W.; Che, C. M. Chem.
Commun. 1998, 2295. (b) Lai, S. W.; Chan, M. C. W.; Cheung, K. K.; Che, C. M. Organometallics, in press.
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(15) Liu, H. Q.; Peng, S. M.; Che, C. M. J. Chem. Soc., Chem. Commun. 1995, 509.
(16) (a) Wu, L. Z.; Cheung, T. C.; Che, C. M.; Cheung, K. K.; Lam, M. H. W. Chem. Commun. 1998, 1127. (b) Wong, K. H.; Chan, M. C. W.; Che, C. M. Chem. Eur. J., in press.
(17) Lai, S. W.; Chan, M. C. W.; Peng, S. M.; Che, C. M. Angew. Chem., Int. Ed. 1999, 38, 669.
(18) Marshall, J. L.; Stobart, S. R.; Gray, H. B. J. Am. Chem. Soc. 1984, 106, 3027.
(19) Bailey, J. A.; Miskowski, V. M.; Gray, H. B. Inorg. Chem. 1993, 32, 369.
(20) Craig, C. A.; Garces, F. O.; Watts, R. J.; Palmans, R.; Frank, A. J. Coord. Chem. Rev. 1990, 97, 193.
(21) Perrin, D. D.; Armarego, W. L. F.; Perrin, D. R. Purification of
Laboratory Chemicals, 2nd ed.; Pergamon: Oxford, 1980. (22) Demas, J. N.; Crosby, G. A. J. Phys. Chem. 1971, 75, 991.
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1569 cm-1.1H NMR (CD 3CN): 0.91 (t, 12H, J ) 7.3 Hz, CH3 -(CH2)3), 1.28 (m, 8H, CH3(CH2)3), 1.54 (m, 8H, CH3(CH2)3), 3.06 (m, 8H, CH3(CH2)3), 7.43-7.83 (m, 5H), 8.06 (d with broad195Pt satellites, 1H,3J HH) 7.3 Hz,3JPtH) 43 Hz), 8.42 (d, 1H, J )
8.0 Hz), 9.99 (d with broad195Pt satellites, 1H,3J
HH) 5.5 Hz, 3J
PtH) 47 Hz).13C{1H}NMR (CD3CN): 13.6; 20.3; 24.4; 59.4
(CH3(CH2)3), 120.9, 122.3, 124.2, 127.5, 129.7, 129.8, 130.8,
134.3, 137.3, 140.0, 149.5, 156.6, 158.2.
[Pt(Bzqn)(pz)]2, 2b. The procedure for 2a was adopted
using pyrazole (0.02 g, 0.29 mmol), excess NaH, and 1b (0.18 g, 0.26 mmol). A yellow solution was obtained after stirring at reflux for 12 h under a N2atmosphere. Recrystallization
by slow evaporation from an acetone/DMF mixture yielded yellow crystals: yield 0.08 g, 72%. Anal. Calcd for C32H22N6
-Pt2: C, 43.64; H, 2.52; N, 9.54. Found: C, 43.45; H, 2.78; N, 9.67. FAB-MS: m/z 879 [M+], 812 [M+- pz]. IR (Nujol): ν ) 1621, 1594, 1568 cm-1.1H NMR (CD 2Cl2): 6.58 (virtual t, 2H, H(4) of pz), 7.33-7.57 (m, 10H), 7.74-7.80 (m, 4H), 7.94 (d, 2H, J ) 7.2 Hz), 8.31 (t, 2H, J ) 6.6 Hz), 8.67 (d with v. broad
195Pt satellites, 1H, J ) 4.9 Hz), 8.79 (d with v. broad195Pt
satellites, 1H, J ) 5.3 Hz).13C{1H}NMR (DMSO-d 6): 106.3, 106.4, 106.5, 121.2, 121.3, 121.9, 122.0, 123.2, 123.4, 123.5, 126.4, 128.7, 128.8, 129.0, 129.2, 129.4, 130.5, 132.9, 138.1, 138.3, 139.1, 141.4, 141.5, 141.8, 142.6, 148.1, 148.3, 156.0, 156.1.
[Pt(Bzqn)(aza)]2, 3b. The procedure for 2a was adopted
using 7-azaindole (0.04 g, 0.30 mmol), excess NaH, and 1b (0.18 g, 0.26 mmol). An orange solution was obtained after stirring at reflux for 12 h under a N2atmosphere.
Recrystal-lization by diffusion of diethyl ether into a dichloromethane solution yielded an orange microcrystalline solid: yield 0.09 g, 74%. Anal. Calcd for C40H26N6Pt2: C, 48.98; H, 2.67; N, 8.57.
Found: C, 49.13; H, 2.43; N, 8.30. FAB-MS: m/z 979 [M+], 862 [M+ - aza]. IR (Nujol): ν ) 1620, 1596, 1576 cm-1.1H
NMR (CD2Cl2): 5.83 (d, 1H, J ) 7.4 Hz), 6.67-7.36 (m, 15H),
7.46-7.81 (m, 5H), 8.10-8.25 (m, 3H), 8.36 (s, 1H), 9.12 (d with v. broad195Pt satellites, 1H, J ) 5.0 Hz).13C{1H}NMR
(DMSO-d6): 100.8-101.3, 101.7, 113.8, 114.4, 121.7-133.3,
135.6-140.2, 146.8, 151.6, 156.5.
[Pt(Phpy)(pz)]2, 2c. The procedure for 2b was adopted
using pyrazole (0.02 g, 0.32 mmol), excess NaH, and 1c (0.20 g, 0.30 mmol) to afford yellow crystals: yield 0.10 g, 80%. Anal. Calcd for C28H22N6Pt2: C, 40.39; H, 2.66; N, 10.09. Found: C,
40.69; H, 2,42; N, 10.00. FAB-MS: m/z 832 [M+], 765 [M+ -pz]. IR (Nujol): ν ) 1607, 1584, 1565 cm-1.1H NMR (DMSO-d6): 6.46 (virtual t, 2H, H(4) of pz), 6.89 (d, 1H, J ) 6.5 Hz), 6.96-7.07 (m, 5H), 7.30 (m, 2H), 7.58 (d, 1H, J ) 2.2 Hz), 7.69 (m, 4H), 7.80 (d, 1H, J ) 2.0 Hz), 8.04 (m, 4H), 8.27 (d, 1H, J ) 5.8 Hz), 8.41 (d, 1H, J ) 5.7 Hz).13C{1H}NMR (DMSO-d6): 106.1, 106.2, 106.3, 119.3, 119.4, 122.8, 122.9, 123.1, 123.2, 123.6, 123.7, 129.1, 133.0, 133.1, 137.7, 137.9, 138.6, 138.7, 139.3, 144.3, 145.0, 145.2, 145.3, 148.5, 148.7, 166.8, 167.0.
[Pt(Phpy)(aza)]2, 3c. The procedure for 2a was adopted
using 7-azaindole (0.04 g, 0.34 mmol), excess NaH, and 1c (0.20 g, 0.30 mmol) to afford red crystals: yield 0.11 g, 79%. Anal. Calcd for C36H26N6Pt2: C, 46.35; H, 2.81; N, 9.01.
Found: C, 46.46; H, 2.79; N, 9.14. FAB-MS: m/z 932 [M+], 815 [M+ - aza]. IR (Nujol): ν ) 1607, 1585, 1557 cm-1.1H NMR (CD2Cl2): 5.93 (m, 1H), 6.03 (d, 1H, J ) 7.4 Hz), 6.23 (m, 1H), 6.44-6.81 (m, 9H), 6.99 (d, 2H, J ) 7.4 Hz), 7.15-7.75 (m, 8H), 7.91-7.99 (m, 2H), 8.47 (d with v. broad195Pt satellites, 2H, J ) 5.1 Hz).13C{1H}NMR (DMSO-d 6): 100.1, 100.2, 101.3, 113.4, 117.7, 121.3-124.4, 128.1-131.0, 138.1-140.4, 142.5, 144.7, 149.3, 159.9.
X-ray crystallography. Crystals of 1a and 2a were grown
by vapor diffusion of diethyl ether into dichloromethane solutions. Crystal data and details of collection and refinement are summarized in Table 1.
For 1a, diffraction experiments were performed on a Rigaku AFC7R diffractometer (λ ) 0.71073 Å, ω-2θ scans). The
structure was solved by Patterson methods, expanded by
Fourier methods (PATTY23), and refined by full-matrix
least-squares using the software package TeXsan24 on a Silicon
Graphics Indy computer. One formula unit constitutes a crystallographic asymmetric unit. All 24 non-H atoms were refined anisotropically, and 13 H atoms at calculated positions were not refined.
For 2a, diffraction experiments were performed on a Nonius diffractometer (λ ) 0.71073 Å, θ/2θ scans). The structure was solved by direct methods and refined by least-squares treat-ment on F2using the NRCVAX program. One formula unit
constitutes a crystallographic asymmetric unit. All 40 non-H atoms were refined anisotropically, and 26 H atoms at calculated positions were not refined. The two 2-(2′ -thienyl)-pyridyl groups are disordered with NC(1), NC(2), NC(3), and NC(4) having half carbon and nitrogen atom occupancy and S(1), S(1′), S(2), S(2′), C(6), C(6′), C(7), C(7′), C(20), C(20′), C(21), and C(21′) also having half occupancy.
Results and Discussion
Synthesis and Characterization. A general syn-thetic route to bi- and triplatinum macrocycles is illustrated in Scheme 1. The monomeric precursors 1a-c (a, Thpy; b, Bzqn; c, Phpy) comprise two substi-tutionally labile ligands (HThpy and/or chloride). Treat-ment with stoichiometric amounts of deprotonated N-donor ligands in CH2Cl2/THF under a nitrogen at-mosphere afford cyclic dimers withµ-pz (denoted 2) and
µ-aza (denoted 3) linkers, while trimers are formed with µ-bzim groups (denoted 4). Syntheses of compounds
based on the [Pt(µ-pz)2Pt] core have been reported.2a-c All complexes are air- and moisture-stable at room temperature in solid and solution states.
These reactions to form macrocycles proceed in mod-erately high yields (70-90%). We have already estab-lished17 that the coordination geometry of the bzim ligand is suitable for the assembly of trimeric
metallo-(23) PATTY: Beurskens, P. R.; Admiraal, G.; Bosman, W. P.; Garcia-Granda, S.; Gould, R. O.; Smits, J. M. M.; Smykalla, C. The DIRDIF program system; Technical Report of the Crystallography Laboratory, University of Nijmegen: The Netherlands,1992.
(24) TeXsan: Crystal Structure Analysis Package; Molecular Struc-ture Corporation: The Woodlands, TX, 1985 and 1992.
Table 1. Crystal Data
1a 2a
formula C18H13N2S2PtCl C24H18N6S2Pt2
fw 551.98 844.75
color orange orange
crystal size, mm 0.30× 0.20 × 0.08 0.45× 0.25 × 0.15
crystal system monoclinic monoclinic
space group P21/n (No. 14) P21/c
a, Å 9.078(2) 17.215(2) b, Å 8.363(1) 9.204(1) c, Å 23.562(7) 16.735(2) β, deg 94.57(2) 116.466(8) V, Å3 1783.3(6) 2373.8(4) Z 4 4 Dc(g cm-3) 2.056 2.364 µ, cm-1 82.22 120.93 F(000) 1048 1567 T, K 301 295 2θmax, deg 50 50
no. of unique data 3381 4177
no. of obsd data 2666 [I > 3σ(I)] 2530 [I > 2σ(I)]
no. of variables 217 361 R,aR wb 0.037, 0.045 0.031, 0.027 residual F, e Å-3 +1.30, -0.94 +1.20, -0.62 aR )∑||F o| - |Fc||/∑|Fo|.bRw) [∑w(|Fo| - |Fc|)2/∑w|Fo|2]1/2.
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macrocycles, by coupling of three 150° edges (as for bzim) and three cis binding sites at square planar and octahedral metal centers (Scheme 2). Our unsuccessful attempts to synthesize [Pt(Phpy)(bzim)]3, which afforded insoluble, intractable solids, are therefore surprising. Nevertheless, the intrinsic binding geometry of the bidentate pz and aza ligands evidently ensure the efficient and selective formation of dimeric Pt(II) com-plexes. The positive FAB mass spectra of the cyclic dimers and trimers reveal clusters corresponding to the molecular ion, while no other oligomeric species are detected. The1H NMR spectra for all complexes contain distinctive low-field doublets with broad195Pt satellites (3J
PtH∼40 Hz) which are assigned to Hβof the
cyclo-metalating ligands. Variable-temperature 1H NMR studies (-60 to 40 °C in CD2Cl2) were performed on 4a to investigate the orientation of the bzim ligands in solution. No significant changes were detected, but the syn, anti, anti conformation in the solid state (see below) is unlikely to be maintained in solution.
Crystal Structures. In this work, the molecular structures of the mononuclear complex 1a and
µ-pyra-zolate dimer 2a (Figures 1 and 2, respectively) were
determined by X-ray crystallography (Table 2). We shall also elaborate on the solid-state structure of 4a.
The platinum atom in 1a resides in an approximately square planar environment and is coordinated to mono-dentate (pyridyl N) and bimono-dentate (pyridyl N and thienyl C) 2-(2′-thienyl)pyridyl ligands plus a chloride group (Figure 1). The Pt(1)-Cl(1) bond length of 2.402(2) Å is partially longer than that reported for Pt(bpy)Cl2 (2.306(2) Å)25due to the greater trans influence of the (25) Osborn, R. S.; Rogers, D. J. Chem. Soc., Dalton Trans. 1974, 1002.
Scheme 1. Synthetic Route toward Bi- and Trimetallic Macrocycles
Scheme 2. Ligand-Directed Strategy toward
Trimetallic Macrocycles
Figure 1. (Top) Perspective view of
[Pt(Thpy)(HThpy)-Cl], 1a, with 50% probability ellipsoids. (Bottom) Crystal
packing diagram showingπ-π stacking interactions.
Figure 2. Perspective view of [Pt(Thpy)(pz)]2, 2a, with
35% probability ellipsoids. Thpy ligands (NC(1-4), S(1′), S(2), C(6′), C(7′), C(20), C(21) atoms) are disordered.
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thienyl carbanion compared to the pyridyl N donor. The stacked arrangement of 1a monomers (bottom of Figure 1) indicatesπ-π interactions between adjacent
mono-dentate HThpy ligands, with interplanar separations of ca. 3.5 Å. The structure of the iodide congener has been reported.26
In 2a, two [Pt(Thpy)] moieties are bridged by two pyrazolate ligands in an exo-bidentate fashion (Figure 2). The central six-membered [Pt(µ-pz)2Pt] core is non-planar, with mean Pt-N(pz) distances of 2.04 Å and N-Pt-N angles of ca. 88° between the pz groups. The metal atoms are positioned at the vertexes of a boat conformation like in previously reported Pt(II)2b,cand Ir(I)27µ-pyrazolate dimers. The Pt(1)-Pt(2) separation of 3.4863(6) Å in 2a is comparable to that in the mono-(pyrazolyl) species [Pt2(tpy)2(µ-pz)](ClO4)3(3.432(1) Å),28 but is longer than the corresponding metal-metal distance in [PtCl(PPh2Me)(µ-3,5-Me2pz)]2(3.170(1) Å).2c Comparable bond lengths within the pyrazolate rings suggest substantial delocalization upon deprotonation. The bite angles of the cyclometalating Thpy ligands (79.1(4)° and 79.5(4)°) are comparable to that in 1a (80.4(3)°).
The “two-up, one-down” orientation of the bzim ligands in the molecular structure of 4a, which allows compari-sons with the partial cone conformation in calixarenes29 and the Pt(II)-incorporated analogue of calix[4]arene [Pt(en)(uracilate)]44+ (en ) 1,2-diaminoethane),30 has been described previously.17 From the partial crystal packing diagram (Figure 3),π-π interactions of ca. 3.5
Å between Thpy systems in adjacent molecules are apparent, and these are expected to influence the photophysical behavior of 4a in the solid state (see below).
Absorption and Emission Spectroscopy. The UV-visible spectral data of the Pt(II) complexes are listed in Table 3. Their emission data in fluid solution
and in crystalline state are given in Tables 4 and 5, respectively.
Complexes with Thpy Ligands. The UV-vis ab-sorption spectra of 2-4a in CH2Cl2 are comparable to that for the precursor 1a (Figure 4 for 1a and 2a). For example, the spectrum of 4a contains high-energy bands (λ < 360 nm) which are dominated by spin-allowed intraligand (1IL: π(Thpy) f π*(Thpy) with metal per-turbation) transitions.31 The moderately intense low-energy bands withλmaxin the range 409-421 nm and the notably weaker, structured absorptions atλmax 480-554 nm ( < 120 dm3mol-1cm-1) are assigned to the spin-allowed and spin-forbidden metal-to-ligand charge transfer (1MLCT and 3MLCT: (5d)Pt f π*(Thpy)) transitions, respectively.
(26) Giordano, T. J.; Rasmussen, P. G. Inorg. Chem. 1975, 14, 1628. (27) Coleman, A. W.; Eadie, D. T.; Stodart, S. R.; Zaworotko, M. J.; Atwood, J. L. J. Am. Chem. Soc. 1982, 104, 922.
(28) Bailey, J. A.; Gray, H. B. Acta Crystallogr. 1992, C48, 1420. (29) Gutsche, C. D. Calixarenes; Royal Society of Chemistry: Cam-bridge, 1989.
(30) Rauter, H.; Hillgeris, E. C.; Erxleben, A.; Lippert, B. J. Am. Chem. Soc. 1994, 116, 616.
(31) Kvam, P.-I.; Puzyk, M. V.; Cotlyr, V. S.; Balashev, K. P.; Songstad, J. Acta Chem. Scand. 1995, 49, 645.
Table 2. Selected Bond Lengths (Å) and Angles (deg) Complex 1a Pt(1)-Cl(1) 2.402(2) Pt(1)-C(7) 1.990(9) Pt(1)-N(1) 2.060(7) S(1)-C(6) 1.73(1) Pt(1)-N(2) 2.032(7) S(1)-C(9) 1.71(1) N(1)-Pt(1)-C(7) 80.4(3) Cl(1)-Pt(1)-N(2) 89.3(2) Cl(1)-Pt(1)-N(1) 97.3(2) Cl(1)-Pt(1)-C(7) 177.3(3) Complex 2a Pt(1)-NC(1) 1.995(9) Pt(2)-N(6) 2.031(8) Pt(1)-NC(2) 1.986(9) Pt(2)-N(8) 2.043(8) Pt(1)-N(5) 2.032(8) S(2)-C(17) 1.799(15) Pt(1)-N(7) 2.056(8) S(2)-C(18) 1.72(1) Pt(2)-NC(3) 2.017(9) S(1′)-C(5) 1.88(1) Pt(2)-NC(4) 2.017(8) S(1′)-C(8) 1.804(14) NC(1)-Pt(1)-NC(2) 79.1(4) NC(4)-Pt(2)-N(8) 96.3(3) NC(1)-Pt(1)-N(5) 96.6(4) N(6)-Pt(2)-N(8) 88.3(3) NC(2)-Pt(1)-N(7) 95.9(3) Pt(1)-N(5)-N(6) 122.8(6) N(5)-Pt(1)-N(7) 88.4(3) Pt(2)-N(6)-N(5) 120.8(6) NC(3)-Pt(2)-NC(4) 79.5(4) C(5)-S(1′)-C(8) 90.9(6) NC(3)-Pt(2)-N(6) 95.9(4) C(17)-S(2)-C(18) 92.5(6)
Figure 3. Partial packing diagram of [Pt(Thpy)(bzim)]3,
4a (π-π interactions indicated by asterisks).
Figure 4. UV-vis absorption spectra of 1a and 2a in
dichloromethane at 298 K.
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Structured emissions are observed for complexes 1-4a in CH2Cl2 at room temperature, with peak maxima in the range 556-561 (sh 577-581) and 602-606 (sh 628-638, 661-664) nm (Table 4). Minor solva-tochromic effects are detected for the emission energy, e.g., for 4a,λmax) 564 (C6H6), 560 (CH2Cl2), 562 (CH3 -CN), 567 nm (CH3OH). Self-quenching of the emission has been detected for 1-4a; in each case, a linear plot of 1/τ against complex concentration was obtained. For
example, kqvalues of 3.5× 107and 3.0× 107M-1s-1 have been obtained for 2a and 4a, respectively. Theµ-pz
complex 2a emits with a longer intrinsic lifetime (τo) 15.5µs) and a significantly higher quantum yield (0.18) than theµ-aza dimer 3a (τo) 3.1 µs, φo) 0.012) and the trimer 4a (τo ) 10.9 µs, φo ) 0.016). Indeed, the photoluminescent properties displayed by 2a are sub-stantially superior to those for previously reported binuclear Pt(II) luminophores, which are weakly emis-sive in solution at room temperature.19At 77 K, emis-sion bands are slightly blue-shifted toλmax553-557 (sh 573-578) and 600-602 (sh 624-662) nm. With refer-ence to earlier work31,32and the absence of any major solvatochromic effects (<10 nm), we suggest that these emissions are more appropriately assigned to transitions with mixed MLCT and IL character.
The solid-state emission spectrum of 4a at 298 K (Figure 5) contains three bands atλmax571, 617, and 664 (max) nm, all of which undergo nonexponential decay. Similarly, complexes 1-3a display multiple emissions (including low-energy bands at 651-665 nm) in crystalline form at 298 K, and these are generally red-shifted from the solution-state emission (Table 5). This can be attributed to excimeric emission arising from weakπ-π stacking of square planar Pt(II) species
(32) Maestri, M.; Sandrini, D.; Balzani, V.; Chassot, L.; Jolliet, P.; von Zelewsky, A. Chem. Phys. Lett. 1985, 122, 375.
Table 3. UV-Vis Absorption Data in Dichloromethane at 298 K
complex λmax/nm (/dm3mol-1cm-1)
[Pt(Thpy)(HThpy)Cl], 1a 262 (22300), 300 (23900), 329 (sh, 17600), 406 (4600), 419 (4550), 509 (60), 553 (40) [Pt(Thpy)(pz)]2, 2a 293 (39400), 354 (26800), 410 (sh, 3800), 512 (60), 555 (40) [Pt(Thpy)(aza)]2, 3a 292 (42900), 354 (22800), 474 (sh, 2100) [Pt(Thpy)(bzim)]3, 4a 277 (47000), 337 (22000), 357 (18300), 409 (5250), 421 (4800), 480 (110), 512 (100), 554 (70) [nBu 4N][Pt(Bzqn)Cl2], 1b 280 (28700), 358 (4350), 374 (4400), 434 (1300) [Pt(Bzqn)(pz)]2, 2b 286 (35100), 370 (12300), 388 (sh, 8850), 421 (sh, 3150), 465 (sh, 250) [Pt(Bzqn)(aza)]2, 3b 285 (34400), 380 (sh, 8700), 410 (sh, 4000) [Pt(Bzqn)(bzim)]3, 4b 271 (70000), 278 (73000), 358 (14000), 372 (14000), 413 (4000), 469 (sh, 440) [nBu 4N][Pt(Phpy)Cl2], 1c 259 (23700), 287 (15200), 331 (6300), 379 (4200), 426 (sh, 780), 490 (120) [Pt(Phpy)(pz)]2, 2c 257 (45600), 284 (24500), 329 (12200), 359 (11000), 406 (3350), 446 (sh, 310) [Pt(Phpy)(aza)]2, 3c 266 (33600), 290 (36200), 345 (sh, 17500), 410 (sh, 3450), 459 (sh, 1400)
Table 4. Emission Data in CH2Cl2(complex concentration 5× 10-5M)
complex 298 K: λmax/nm;το/µs; φο 77 K: λmax/nm
[Pt(Thpy)(HThpy)Cl], 1a 556 (max), 577 (sh), 602, 628 (sh), 661 (sh); 12.0; 0.19 553 (max), 573 (sh), 600, 624 (sh), 654 (sh) [Pt(Thpy)(pz)]2, 2a 559 (max), 581 (sh), 606, 632 (sh), 664 (sh); 15.5; 0.18 557 (max), 576 (sh), 602, 624 (sh), 655 (sh)
[Pt(Thpy)(aza)]2, 3a 561 (max), 579 (sh), 606, 638 (sh), 661 (sh); 3.1; 0.012 557 (max), 578 (sh), 602, 662 (sh)
[Pt(Thpy)(bzim)]3, 4a 560 (max), 580 (sh), 606, 636 (sh), 661 (sh); 10.9; 0.016 557 (max), 575 (sh), 601, 633 (sh), 656 (sh)
[nBu
4N][Pt(Bzqn)Cl2], 1b nonemissive 494, 532, 572, 620 (sh)
[Pt(Bzqn)(pz)]2, 2b 497, 535, 588 (sh); 1.6; 9.9× 10-3 486, 522, 560
[Pt(Bzqn)(aza)]2, 3b nonemissive 497, 530, 565
[Pt(Bzqn)(bzim)]3, 4b nonemissive nonemissive
[nBu
4N][Pt(Phpy)Cl2], 1c nonemissive 485 (max), 527, 557, 578, 604, 631 (sh), 668 (sh)
[Pt(Phpy)(pz)]2, 2c 487, 522, 556, 600 (sh); 1.3; 0.039 485 (max), 515, 544, 596 (sh)
[Pt(Phpy)(aza)]2, 3c 494, 520, 556, 601 (sh); 0.29; 4.5× 10-4 489, 523 (max), 557, 605 (sh)
Table 5. Solid-State Emission Data
complex 298 K: λmax/nm 77 K: λmax/nm
[Pt(Thpy)(HThpy)Cl], 1a 563, 581, 606 (max), 632, 651 574 (max), 598, 625, 652 (sh), 690 (sh) [Pt(Thpy)(pz)]2, 2a 570 (sh), 591 (sh), 616 (max), 665 577 (max), 595, 624, 651 (sh)
[Pt(Thpy)(aza)]2, 3a 556, 601 (max), 658 552 (max), 599, 677
[Pt(Thpy)(bzim)]3, 4a 571, 617, 664 (max) 579 (max), 603 (sh), 630, 659 (sh), 691 (sh)
[nBu
4N][Pt(Bzqn)Cl2], 1b 525 (sh), 542 (max), 591 (sh) 514 (max), 554, 597 (sh)
[Pt(Bzqn)(pz)]2, 2b 533, 568 (max), 633 534 (max), 568, 615 (sh)
[Pt(Bzqn)(aza)]2, 3b 550 (max), 592, 640 (sh) 517, 548 (max), 598, 632 (sh)
[Pt(Bzqn)(bzim)]3, 4b nonemissive 512 (max), 549, 593 (sh)
[nBu
4N][Pt(Phpy)Cl2], 1c 502, 536, 566 (sh) 498 (max), 515, 526, 536, 558, 569
[Pt(Phpy)(pz)]2, 2c 505, 539, 557 (sh) 506, 526, 549 (max), 596 (sh)
[Pt(Phpy)(aza)]2, 3c 490, 524, 555 493, 528, 563
Figure 5. Solid-state emission spectrum of 4a (λex) 380
nm) at 298 K (asterisk denotes instrumental artifact).
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in the solid state,33since these interactions are evident from the crystal structures of 1a (bottom of Figure 1) and 4a (Figure 3). Blue-shifts in emission maxima are exhibited upon cooling to 77 K. For example, the solid emission of 4a at 77 K shows bands atλmax579 (sh 603, 630, 659, 691) nm, which are higher in energy than the 664 nm emission at 298 K. These solid-state emissions are proposed to originate from3MLCT excited states. Complexes with Bzqn and Phpy Ligands. The UV-vis absorption spectra of 2-4b in CH2Cl2 are similar to that for 1b and the bis(cyclometalated) derivative [Pt(Bzqn)2].32High-energy absorption bands withλmax< 390 nm are ascribed to1IL transitions. The moderately intense bands atλmax410-421 nm and the significantly weaker low-energy absorptions (λmax> 450 nm) are assigned to 1MLCT and 3MLCT transitions, respectively.
In contrast to the Thpy complexes, a number of Bzqn derivatives are nonemissive in fluid solution. At room temperature, only the µ-pz species 2b is luminescent and displays structured emission at 497, 535, and 588 (sh) nm (τo) 1.6 µs, φo) 9.9 × 10-3). A blue-shifted 77 K luminescence at 486, 522, and 560 nm is observed, and we tentatively assign the emission as mixed3IL/3 -MLCT. Theµ-aza complex 3b shows structured emission at 77 K only, while theµ-bzim trimer 4b is nonemissive
at 298 and 77 K in solution. The solid-state emissions of 1-3b are structured at 298 K, and blue-shifts are again observed upon cooling to 77 K.
The UV-vis absorption spectrum of the previously described 1c20resembles those of 2c and 3c bearing the Phpy ligand, where1IL,1MLCT, and3MLCT transitions (λ < 370, λmax) 406-410, and λ > 440 nm, respectively) can be assigned. While the precursor 1c is nonemissive in solution at 298 K, 2c and 3c exhibit structured emission atλmax487-556 (sh 600) nm. A longer lifetime and greater quantum yield are again detected for the
µ-pz complex 2c (τo) 1.3 µs, φo) 0.039) compared to theµ-aza dimer 3c (τo) 0.29 µs, φo) 4.5 × 10-4).
Concluding Remarks. Our systematic photophysi-cal investigation has revealed several trends. First, complexes bearing the Thpy group and in particular
µ-pyrazolate species display superior luminescent
prop-erties. Second, the energy of the emissions and lowest energy UV-vis bands for Thpy oligomers are generally red-shifted relative to the Bzqn and Phpy congeners. This can be illustrated by the 77 K emission spectra of 2a-c in CH2Cl2 (Figure 6) and evidently reflects dif-ferences in the energies of the π* orbitals for the cyclometalating ligands. The relatively low-energy
ex-cited states for Thpy derivatives result in greater energy differences between the MLCT/IL and upper-lying non-emissive MC states. Hence such complexes, in particular [Pt(Thpy)(pz)]2(2a), exhibit improved emissive param-eters compared to known Pt(II) binuclear complexes and are potentially interesting for photoinduced energy transfer and sensing applications.
We have described herein a simple yet efficient methodology for the selective synthesis of di- and trimeric platinum(II) macrocycles. Like the recently reported examples of molecular rectangles,34the trime-tallic molecular pockets 4a and 4b have lower symmetry than tetrametallic square analogues.35While the cavity size of 4a and 4b pre-empts their employment as molecular hosts, this study has conceived a new direc-tion for development using cyclometalated platinum(II) luminophores.
Acknowledgment. We are grateful for financial support from the University of Hong Kong and the Research Grants Council of the Hong Kong SAR, China [HKU 7298/99P].
Supporting Information Available: Tables of crystal
data, atomic coordinates, calculated coordinates, anisotropic displacement parameters, and bond lengths and angles for 1a and 2a. This material is available free of charge via the Internet at http://pubs.acs.org.
OM990342H
(33) Houlding, V. H.; Miskowski, V. M. Coord. Chem. Rev. 1991, 111, 145.
(34) (a) Benkstein, K. D.; Hupp, J. T.; Stern, C. L. Inorg. Chem. 1998, 37, 5404. (b) Woessner, S. M.; Helms, J. B.; Shen, Y.; Sullivan, B. P. Inorg. Chem. 1998, 37, 5406.
(35) Hunter, C. A. Angew. Chem., Int. Ed. Engl. 1995, 34, 1079.
Figure 6. Normalized emission spectra of 2a-c (λex) 380
nm) in dichloromethane at 77 K.
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![Figure 1. (Top) Perspective view of [Pt(Thpy)(HThpy)- [Pt(Thpy)(HThpy)-Cl], 1a, with 50% probability ellipsoids](https://thumb-ap.123doks.com/thumbv2/9libinfo/8665835.195460/4.918.496.805.68.543/figure-perspective-thpy-hthpy-thpy-hthpy-probability-ellipsoids.webp)
