Bis-salicylaldiminato zinc complexes: Syntheses, characterization and luminescent properties

Polyhedron 26 (2007) 5053–5060 www.elsevier.com/locate/poly

Bis-salicylaldiminato zinc complexes: Syntheses, characterization and luminescent properties Qing Su a, Qiao-Lin Wu a, Guang-Hua Li b, Xiao-Ming Liu a, Ying Mu

a,*

a

b

Key Laboratory for Supramolecular Structure and Materials of Ministry of Education, School of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, School of Chemistry, Jilin University, 2699 Qianjin Street, Changchun 130012, People’s Republic of China Received 14 February 2007; accepted 9 July 2007 Available online 21 August 2007

Abstract The syntheses of four four-coordinate zinc(II) complexes,

(Ar1N=HCAr2O)2Zn

(Ar1 = 7-(2,4-Me2)C9H4N, Ar2 = C6H4 (2a); Ar1 =

7-(2,4-Me2)C9H4N, Ar2 = 3,5-tBu2C6H2 (2b); Ar1 = 7-(2,4-Me2)C9H4N, Ar2 = 5-BrC6H3 (2c); Ar1 = 7-(2,4-Me2)C9H4N, Ar2 = 3,5Br2C6H2 (2d)) are described. Complexes 2a–2d were synthesized by the reaction of ZnEt2 with 2 equiv. of the corresponding ligand. Ligands 1a–1d and the corresponding zinc(II) complexes 2a–2d were characterized by elemental analysis, IR, 1H and 13C NMR spectroscopy. The molecular structure of complex 2b was determined by single crystal X-ray crystallography. Luminescent properties of the ligands 1a–1d and complexes 2a–2d in both solution and the solid state were studied.  2007 Elsevier Ltd. All rights reserved. Keywords: Luminescence; Quinoline; Salicylaldiminato; Spectra; Zinc

1. Introduction Organic and organometallic/coordination luminescent compounds have received intensive attention for decades due to their potential application in areas of chemical sensors [1] and optoelectronic devices [2]. A great number of metal complexes, such as Pt(II) [3], Zn(II) [4–7] and Zn(II)–Nd(III) [8], with Schiff base ligands have been synthesized and their luminescent properties have been explored. Particularly, Zn(II) complexes bearing salicylaldiminato ligands have been employed as blue, greenishwhite and red emitters in organic optoelectronics with better stability and efficiency [4a–4d,5]. In those zinc(II) complexes, the salicylaldiminato ligands are mainly salen

ligands with two N and two O donor atoms. However reports on the luminescent properties of bis(salicylaldiminato) zinc(II) complexes are limited. As far as luminescent materials are concerned, the emission color of a series of complexes can be tuned by modifying the ligands [9]. We have synthesized four new bis-salicylaldiminato zinc complexes with different substituents on the 3- and/or 5-position(s) of the salicylene ring and found that they are efficient fluorescent materials and the emission color in solution can be tuned in the range 503–532 nm by ligand modification. In this paper, we wish to report the syntheses, characterization and fluorescent properties of a number of zinc complexes with four new salicylaldiminato ligands:

(Ar1N=HCAr2O)2Zn *

Corresponding author. Tel.: +86 431 85168376; fax: +86 431 85193421. E-mail address: [email protected] (Y. Mu). 0277-5387/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.07.006

(Ar1 = 7-(2,4-Me2)C9H4N,

Ar2 =

C6H4 (2a); Ar1 = 7-(2,4-Me2)C9H4N, Ar2 = 3,5-tBu2C6H2 (2b); Ar1 = 7-(2,4-Me2)C9H4N, Ar2 = 5-BrC6H3 (2c); Ar1 = 7-(2,4-Me2)C9H4N, Ar2 = 3,5-Br2C6H2 (2d)).

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2. Experimental 2.1. General procedures All air-sensitive manipulations, unless stated otherwise, were performed using standard Schlenk techniques under an atmosphere of high-purity argon or glovebox techniques. Toluene and n-hexane were dried by refluxing over sodium and benzophenone and distilled under argon prior ˚ molecular sieves to use. C6D6 was dried over activated 4 A and vacuum-transferred to a sodium-mirrored air-free flask. CDCl3 was dried over CaH2 for 48 h and vacuumtransferred to an air-free flask. ZnEt2 was purchased from Aldrich and used as received. The 7-amino-2,4-dimethylquinoline [10], 3,5-di-tert-butyl-2-hydroxyl-benzaldehyde [11a], 5-bromo-2-hydroxyl-benzaldehyde [11b] and 3,5-dibromo-2-hydroxyl-benzaldehyde [11c] were synthesized according to the literature methods. NMR spectra were measured using a Varian Mercury-300 or Bruker AVANCE-500 NMR spectrometer. IR spectra were recorded on a Nicolet Impact 410 FTIR spectrometer using KBr pellets. The elemental analyses were performed on a Perkin–Elmer 2400 analyzer. UV–Vis absorption spectra were recorded on a UV-3100 spectrophotometer. Fluorescent measurements were carried out on a RF-5301PC spectrophotometer. Mass spectra were recorded on an 1100 LC/MS system (Agilent Technology Corporation). All melting points were determined by an X-5 micro-melting point apparatus and are uncorrected. 2.2. General procedure for the salicylaldimine ligands The condensation of salicylaldehydes with 1 equiv. of 7amino-2,4-dimethylquinoline was carried out by refluxing the two reactants in ethanol for 8 h. Upon cooling the resulting reaction solution to room temperature, a yellow or orange-red solid precipitated from the solution. The solid product was filtered and washed with cold ethanol and then dried in vacuo to give the desired salicylaldimine ligand in excellent yields. 2.3. Synthesis of C18H16N2O (1a) 2-Hydroxyl-benzaldehyde (2.10 mL, 18.6 mmol) and 7amino-2,4-dimethylquinoline (3.21 g, 18.6 mmol) in 35 ml of refluxing ethanol afforded 4.16 g of compound 1a as a yellowish solid. Yield: 81%. m.p.: 137–138.6 C. ES–MS (MeOH) m/z: 277.0 [(M+1)+]. Anal. Calc. for C18H16N2O (276.33): C, 78.24; H, 5.84; N, 10.14. Found: C, 78.36; H, 5.77; N, 10.10%. 1H NMR (300 MHz, CDCl3, 298.3 K): d (ppm) 2.70 (s, 3H, quinolyl-CH3), 2.75 (s, 3H, quinolylCH3), 6.97 (t, J = 7.5 Hz, 1H, aryl), 7.05 (d, J = 7.5 Hz, 1H, Ph-H), 7.15 (s, 1H, quinolyl-H), 7.39–7.44 (m, 2H, Ph-H), 7.51 (d, J = 8.7 Hz, 1H, quinolyl-H), 7.94 (s, 1H, quinolyl-H), 8.01 (d, J = 8.7 Hz, 1H, quinolyl-H), 8.77 (s, 1H, CH@N), 13.11 (s, 1H, OH).13C NMR (75.4 MHz, CDCl3, 298.3 K): d (ppm) 18.6 (quinolyl-CH3), 25.0 (quin-

olyl-CH3), 117.2, 118.3, 119.2, 121.3, 122.6, 124.8, 125.4, 132.5, 133.5, 144.5, 148.1, 149.3 (quinolyl-7-C), 159.5 (Ph-2-C), 161.2 (quinolyl-2-C), 163.7 (CH@N). IR (KBr, cm1): t 3377 (br) 3046 (w), 2984 (m), 2951 (m), 2921 (m), 1972 (w), 1926 (w), 1622 (s), 1605 (vs), 1574 (vs), 1514 (s), 1497 (vs), 1460 (s), 1372 (s), 1336 (m), 1284 (vs), 1259 (s), 1191 (vs), 1161 (vs), 1136 (s), 1030 (m), 1001 (w), 972 (m), 959 (w), 943 (m), 901 (m), 874 (vs), 826 (vs), 666 (m), 630 (m), 595 (w), 570 (w), 541 (w), 470 (m), 450 (w), 428 (w). 2.4. Synthesis of C26H32N2O (1b) Similarly, 3,5-di-tert-butyl-2-hydroxybenzaldehyde (1.72 g, 7.36 mmol) and 7-amino-2,4-dimethylquinoline (1.27 g, 7.36 mmol) in 25 ml of refluxing ethanol afforded 2.36 g of complex 1b as an orange solid. Yield: 85%. m.p.: 185.5–186.5 C. ES–MS (DMSO) m/z: 389.2 [(M+1)+]. Anal. Calc. for C26H32N2O (388.55): C, 80.37; H, 8.30; N, 7.21. Found: C, 80.29; H, 8.25; N, 7.23%. 1H NMR (300 MHz, CDCl3, 298.3 K): d (ppm) 1.34 (s, 9H, C(CH3)3), 1.50 (s, 9H, C(CH3)3), 2.70 (s, 3H, quinolylCH3), 2.75 (s, 3H, quinolyl-CH3), 7.15 (s, 1H, Ph-H), 7.25 (s, 1H, Ph-H), 7.49–7.54 (m, 2H, quinolyl-H), 7.94 (s, 1H, quinolyl-H), 7.99 (d, J = 8.7 Hz, 1H, quinolyl-H), 8.80 (s, 1H, CH@N), 13.56 (s, 1H, OH). 13C NMR (75.4 MHz, CDCl3, 298.3 K): d (ppm) 18.6 (quinolyl-CH3), 24.9 (quinolyl-CH3), 29.4 (C(CH3)3), 31.4 (C(CH3)3), 34.1 (C(CH3)3), 35.1 (C(CH3)3), 117.7, 118.3, 121.9, 122.4, 124.7, 125.2, 127.1, 128.5, 137.0, 140.8, 144.4, 148.4, 149.9 (quinolyl-7-C), 158.4 (Ph-2-C), 159.3 (quinolyl-2-C), 165.0 (CH@N). IR (KBr, cm1): t 3493 (br), 2960 (m), 2912 (w), 2868 (w), 1603 (m), 1580 (s), 1509 (w), 1472 (w), 1438 (m), 1392 (w), 1369 (w), 1361 (w), 1378 (w), 1273 (w), 1251 (w), 1193 (w), 1168 (m), 1139 (w), 1029 (w), 1004 (w), 962 (w), 933 (w), 888 (m), 862 (w), 838 (w), 809 (w), 764 (w), 726 (w), 662 (w), 641 (w), 601 (w), 572 (w), 524 (w), 497 (w). 2.5. Synthesis of C18H15BrN2O (1c) Similarly, 5-bromo-2-hydroxybenzaldehyde (0.92 g, 4.6 mmol) and 7-amino-2,4-dimethylquinoline (0.79 g, 4.6 mmol) in 35 ml of refluxing ethanol afforded 1.42 g of compound 1c as an orange solid. Yield: 87%. m.p.: 177.7–178.7 C. ES–MS (MeOH) m/z: 357.0 [(M+1)+]. Anal. Calc. for C18H15BrN2O (355.23): C, 60.86; H, 4.26; N, 7.89. Found: C, 60.92; H, 4.16; N, 7.78%. 1H NMR (300 MHz, CDCl3, 298.3 K): d (ppm) 2.72 (s, 3H, quinolyl-CH3), 2.79 (s, 3H, quinolyl-CH3), 6.95 (d, J = 8.7 Hz, 1H, Ph-H), 7.14 (s, 1H, Ph-H), 7.47–7.54 (m, 3H, quinolyl-H and Ph-H), 8.01–8.04 (m, 2H, quinolylH), 8.70 (s, 1H, CH@N), 13.10 (s, 1H, OH). 13C NMR (75.4 MHz, CDCl3, 298.3 K): d (ppm) 18.9 (quinolylCH3), 24.2 (quinolyl-CH3), 110.7, 117.2, 119.3, 120.5, 122.0, 122.8, 123.1, 125.2, 125.4, 125.6, 134.6, 136.3, 159.0 (Ph-2-C), 160.3 (quinolyl-2-C), 162.8 (CH@N). IR

Q. Su et al. / Polyhedron 26 (2007) 5053–5060

(KBr, cm1): t 3444 (br), 3038 (w), 2967 (w), 2917 (m), 1896 (w), 1717 (w), 1601 (vs), 1555 (vs), 1511 (m), 1476 (s), 1449 (m), 1412 (m), 1370 (s), 1340 (s), 1281 (s), 1212 (m), 1195 (s), 1167 (vs), 1130 (m), 1092 (w), 1072 (w), 1028 (w), 993 (w), 955 (w), 935 (w), 897 (s), 859 (s), 824 (vs), 770 (m), 743 (w), 691 (w), 674 (m), 656 (s), 623 (m), 595 (w), 540 (m), 465 (w), 441 (w). 2.6. Synthesis of C18H14Br2N2O (1d) Similarly, 3,5-di-bromo-2-hydroxybenzaldehyde (2.37 g, 8.46 mmol) and 7-amino-2,4-dimethylquinoline (1.46 g, 8.46 mmol) in 35 ml of refluxing ethanol afforded 3.01 g of compound 1d as an orange-red solid. Yield: 82%. m.p.: 174.7–175.7 C. ES–MS (DMSO) m/z: 434.9 [(M+1)+]. Anal. Calc. for C18H14Br2N2O (434.12): C, 49.80; H, 3.25; N, 6.45. Found: C, 49.73; H, 3.20; N, 6.54%. 1H NMR (300 MHz, CDCl3, 298.3 K): d (ppm) 2.72 (s, 3H, quinolyl-CH3), 2.78 (s, 3H, quinolyl-CH3), 7.47–7.54 (m, 3H, quinolyl-H and Ph-H), 8.01–8.04 (m, 2H, quinolyl-H), 7.10 (s, 1H, Ph-H), 7.13 (s, 1H, PhH), 7.36 (s, 1H, quinolyl-H), 7.78–7.89 (m, 2H, quinolyl-H), 8.05 (s, 1H, quinolyl-H), 8.70 (s, 1H, CH@N), 14.21 (s, 1H, OH). 13C NMR (75.4 MHz, d6-DMSO, 298.3 K): d (ppm) 18.5 (quinolyl-CH3), 24.7 (quinolylCH3), 109.4, 111.6, 117.8, 118.6, 118.9, 119.4, 119.9, 120.9, 122.7, 125.6, 126.4, 134.4, 137.6, 157.3 (Ph-2-C), 159.4 (quinolyl-2-C), 163.0 (CH@N). IR (KBr, cm1): t 3492 (br), 3069 (w), 2982 (w), 2956 (w), 2918 (w), 1917 (w), 1898 (w), 1753 (w), 1733 (w), 1720 (w), 1594 (vs), 1554 (s), 1513 (m), 1435 (s), 1370 (s), 1349 (m), 1339 (m), 1294 (m), 1272 (m), 1211 (m), 1197 (m), 1161 (vs), 1138 (s), 1029 (m), 1000 (m), 961 (m), 933 (m), 878 (s), 861 (s), 816 (s), 769 (w), 741 (m), 690 (m), 678 (w), 661 (w), 651 (m), 597 (w), 556 (w), 540 (w), 496 (w), 441 (w). 2.7. Synthesis of C36H30N4O2Zn (2a) To a solution of ligand 1a (0.30 g, 1.07 mmol) dissolved in 30 ml of dry toluene was added a solution of ZnEt2 (0.51 mmol) in toluene (5 ml) at 78 C with stirring. The reaction mixture was allowed to warm to room temperature for 2 h, and heated at 80 C for an additional 8 h. The solvent was removed under vacuum and the residue was washed with dry nhexane (3 · 20 ml). Then the crude product was further purified by recrystallization from toluene/nhexane (1:6 in v/v) to give yellowish-green plate crystals (0.27 g, 86%) m.p.: 263.3–264.2 C. Anal. Calc. for C36H30N4O2Zn (616.04): C, 70.19; H, 4.91; N, 9.09. Found: C, 70.13; H, 4.85; N, 9.14%. 1H NMR (300 MHz, CDCl3, 298.3 K): d (ppm) 2.57 (s, 3H, quinolyl-CH3), 2.59 (s, 3H, quinolyl-CH3), 6.78 (t, J = 6.9 Hz, 1H, Ph-H), 6.98 (d, J = 8.4 Hz, 1H, Ph-H), 7.05 (s, 1H, quinolyl-H), 7.22 (dd, J = 6 Hz, 1.8 Hz, 1H, quinolyl-H), 7.36–7.46 (m, 2H, Ph-H), 7.54 (s, 1H, quinolyl-H), 7.78 (d, J = 9 Hz, 1H, quinolyl-H), 8.57 (s, 1H, CH@N). 13C

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NMR (75.4 MHz, CDCl3, 298.3 K): d (ppm) 18.5 (quinolyl-CH3), 25.1 (quinolyl-CH3), 115.4, 118.4, 118.8, 121.3, 122.7, 123.8, 125.3, 125.4, 136.6, 136.9, 144.2, 148.3, 149.5, 159.7 (quinolyl-2-C), 170.1 (Ph-2-C), 171.7 (CH@N). IR (KBr, cm1): t 3039 (w), 3020 (w), 2953 (w), 2911 (w), 1604 (m), 1582 (m), 1531 (m), 1510 (w), 1462 (w), 1435 (w), 1392 (w), 1343 (w), 1315 (w), 1248 (w), 1187 (w), 1170 (w), 1170 (w), 1151 (w), 1125 (w), 1030 (w), 991 (w), 949 (w), 915 (w), 887 (w), 858 (w), 839 (w), 817 (w), 775 (w), 758 (w), 677 (w), 629 (w), 599 (w), 568 (w), 541 (w), 525 (w), 505 (w), 472 (w), 452 (w). 2.8. Synthesis of C52H62N4O2Zn (2b) Complex 2b was synthesized by the same procedures as those used for 2a with 1b (0.45 g, 1.17 mmol) as a starting material. Pure 2b (0.45 g, 91%) was obtained as a yellow powder by recrystallization from toluene/nhexane (1:6 in v/v). m.p.: 246.1–248 C. Anal. Calc. for C52H62N4O2Zn (840.46): C, 74.31; H, 7.44; N, 6.67. Found: C, 74.35; H, 7.36; N, 6.64%. 1H NMR (300 MHz, C6D6, 298.3 K): d (ppm) 1.34 (s, 9H, C(CH3)3), 1.77 (s, 9H, C(CH3)3), 1.94 (s, 3H, quinolyl-CH3), 2.39 (s, 3H, quinolyl-CH3), 6.45 (s, 1H, quinolyl-H), 6.78 (d, J = 2.7 Hz, 1H, Ph-H), 7.39 (d, J = 8.7 Hz, 1H, quinolyl-H), 7.58 (dd, J = 8.7 Hz, 2.4 Hz, 1H, quinolyl-H), 7.74 (d, J = 2.7 Hz, 1H, Ph-H), 7.86 (d, J = 2.1 Hz, 1H, quinolyl-H), 8.16 (s, 1H, CH@N). 13C NMR (75.4 MHz, C6D6, 298.3 K): d (ppm) 18.0 (quinolyl-CH3), 25.1 (quinolyl-CH3), 29.9 (C(CH3)3), 31.5 (C(CH3)3), 34.0 (C(CH3)3), 36.0 (C(CH3)3), 118.7, 119.7, 121.4, 122.3, 125.2, 125.4, 131.3, 131.6, 136.1, 142.3, 143.3, 149.4, 150.3 (Ph-2-C), 159.5 (quinolyl-2-C), 170.8 (CH@N). IR (KBr, cm1): t 2953 (s), 2912 (s), 2869 (m), 1590 (vs), 1550 (m), 1530 (vs), 1511 (s), 1460 (s), 1426 (vs), 1385 (s), 1361 (m), 1340 (m), 1325 (m), 1271 (m), 1254 (s), 1236 (m), 1218 (m), 1194 (s), 1164 (vs), 1133 (s), 1024 (w), 1002 (w), 988 (w), 963 (w), 940 (w), 914 (w), 879 (w), 830 (w), 816 (w), 788 (w), 772 (w), 746 (w), 681 (w), 662 (w), 637 (w), 595 (w), 571 (w), 539 (w), 511 (w), 485 (w). 2.9. Synthesis of C36H28Br2N4O2Zn (2c) Complex 2c was synthesized by the same procedures as those used for 2a with 1c (0.25 g, 0.71 mmol) as a starting material. Pure 2c (0.23 g, 83%) was obtained as a brownyellow powder by recrystallization from toluene/nhexane (1:6 in v/v). m.p.: 184.3–187.8 C. Anal. Calc. for C36H28Br2N4O2Zn (773.83): C, 55.88; H, 3.65; N, 7.24. Found: C, 55.79; H, 3.71; N, 7.27%. 1H NMR (300 MHz, CDCl3, 298.3 K): d (ppm) 2.58 (s, 3H, quinolyl-CH3), 2.59 (s, 3H, quinolyl-CH3), 6.86 (d, J = 8.7 Hz, 1H, PhH), 7.06 (s, 1H, Ph-H), 7.16 (d, J = 7.8 Hz, 1H, quinolylH), 7.23 (s, 1H, quinolyl-H), 7.42 (d, J = 8.7 Hz, 1H, quinolyl-H), 7.50 (s, 1H, quinolyl-H), 7.79 (d, J = 8.7 Hz, 1H, Ph-H), 8.40 (s, 1H, CH@N). 13C NMR (75.4 MHz, CDCl3,

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298.3 K): d (ppm) 18.5 (quinolyl-CH3), 25.2 (quinolylCH3), 118.8, 119.8, 120.7, 122.9, 125.5, 125.8, 128.2, 130.0, 137.8, 139.0, 144.2, 148.2, 148.8, 160.0 (quinolyl-2C), 168.9 (Ph-2-C), 170.4 (CH@N). IR (KBr, cm1): t 3033 (w), 2952 (m), 2912 (m), 2856 (w), 1602 (vs), 1515 (vs), 1456 (vs), 1414 (vs), 1381 (vs), 1335 (s), 1310 (vs), 1240 (m), 1156 (vs), 1073 (m), 1027 (m), 1006 (m), 984 (m), 963 (w), 943 (m), 922 (w), 876 (m), 826 (s), 787 (s), 748 (w), 713 (w), 682 (w), 652 (s), 605 (w), 540 (m), 525 (w), 504 (w), 471 (w), 453 (w). 2.10. Synthesis of C36H26Br4N4O2Zn (2d) Complex 2d was synthesized by the same procedures as those used for 2a with 1d (0.52 g, 1.19 mmol) as a starting material. Pure 2d (0.49 g, 88%) was obtained as a yellow powder by recrystallization from toluene/nhexane (1:6 in v/v). m.p.: 281.3–285 C. Anal. Calc. for C36H26Br4N4O2Zn (931.62): C, 46.41; H, 2.81; N, 6.01. Found: C, 46.52; H, 2.84; N, 5.90%. 1H NMR (300 MHz, d6-DMSO, 298.3 K): d (ppm) 2.56 (s, 3H, quinolyl-CH3), 2.61 (s, 3H, quinolyl-CH3), 7.21 (s, 1H, quinolyl-H), 7.71–7.72 (br, 3H, Ph-H and quinolyl-H), 7.90–7.95 (br, 2H, quinolyl-H), 8.65 (s, 1H, CH@N). 13C NMR (75.4 MHz, d6-DMSO, 298.3 K): d (ppm) 18.0 (quinolyl-CH3), 24.7 (quinolyl-CH3), 118.0, 120.4, 121.0, 122.2, 124.5, 125.3, 128.2, 128.9, 137.7, 138.1, 143.7, 147.7, 151.0, 158.9 (quinolyl-2-C), 164.5 (Ph-2-C), 168.4 (CH@N). IR (KBr, cm1): t 3059 (w), 2911 (w), 1752 (w), 1588 (s), 1558 (w), 1506 (m), 1428 (m), 1402 (m), 1372 (w), 1342 (w), 1307 (w), 1214 (w), 1196 (w), 1144 (s), 1119 (w), 1029 (w), 1001 (w), 932 (w), 868 (w), 815 (w), 751 (w), 713 (w), 682 (w), 651 (w), 643 (w), 633 (w), 621 (w), 611 (w), 600 (w), 590 (w), 580 (w), 570 (w), 559 (w), 548 (w), 539 (w), 528 (w), 517 (w), 507 (w), 496 (w), 487 (w), 476 (w), 465 (w), 454 (w), 444 (w), 433 (w), 423 (w), 413 (w), 403 (w). 2.11. X-ray structure determination of 2b

Table 1 Crystal data and structure refinement for complex 2b Data

2b

Formula Fw Temperature (K) Crystal system Space group ˚) a (A ˚) b (A ˚ cA

C53.50H65.50N4O2Zn 861.97 293(2) triclinic P 1 12.5327 (11) 14.8692 (13) 15.4913 (14) 77.809 (2) 85.060 (2) 69.277 (2) 2639.0 (4) 2 1.058 896 1.74–28.31 14 6 h 6 16, 16 6 k 6 19, 20 6 l 6 20 12965/115/553 1.030 Ra1 = 0.0743, wRb2 = 0.2136 Ra1 = 0.1304, wRb2 = 0.2662 0.820 and 0.505

a () b () c () ˚ 3) Volume (A Z Dcalc (Mg m3) F(0 0 0) h Range for data collection () Limiting indices

Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data) Largest difference in peak and hole (e A3) P P a R1 ¼ kF o j  jF c k= jF o j. P P b 2 2 2 wR2 ¼ ½ ½wðF o  F c Þ = ½wðF 2o Þ2 1=2 .

3. Results and discussion 3.1. Synthesis of the ligands Ligands 1a–1d were readily prepared by simple condensation of salicyaldehydes with 7-amino-2,4-dimethylquinoline in EtOH in high yields (Scheme 1). Compounds 1a–1d were characterized via 1H, 13C NMR and IR spectroscopies, as well as elemental analysis. The 1H NMR spectra of 1a–1d exhibit resonances at d = 8.77, 8.80, 8.70 and 8.70 ppm, respectively, for the CH imino protons, with R1

Single crystals of 2b suitable for X-ray structural analysis were obtained from a saturated solution of 2b in toluene/nhexane (1:6, v/v) at room temperature. Diffraction data were collected at 293 K on a Bruker Smart diffractometer equipped with graphite-monochromated Mo Ka ˚ ) for 2b. The structure was solved radiation (k = 0.71073 A by direct methods [12] and refined by full-matrix leastsquares on F2. Most non-hydrogen atoms, except those in the disordering groups, were refined anisotropically. All hydrogen atoms, except those on disordered carbon atoms, were calculated and their contributions in structural factor calculations were included. All calculations were performed using the SHELXTL crystallographic software packages [13]. Details of the crystal data, data collection and structure refinements are summarized in Table 1.

OH

OH R1

CHO

N

EtOH

N

R2 H2N

N

R1 = R2 = H, 1a R1 = R2 = tBu, 1b R1 = H, R2 = Br, 1c R1 = R2 = Br, 1d

R2

R2 N

ZnEt2

R1

N O Zn O N

R1 N

R2

R1 = R2 = H, 2a R1 = R2 = tBu, 2b R1 = H, R2 = Br, 2c R1 = R2 = Br, 2d

Scheme 1. Synthetic route for ligands 1a–1d and complexes 2a–2d.

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the corresponding 13C resonance at d = 163.7, 165.0, 162.8, 163.0 ppm, respectively. The OH resonances appear at characteristically low field shifts (d = 13.11 ppm for 1a, 13.56 ppm for 1b, 13.10 ppm for 1c and 14.21 ppm for 1d). The infrared absorption band of the imine C@N stretching vibration occurs in the region 1594–1605 cm1. 3.2. Synthesis of the zinc complexes The bis-salicylaldiminato zinc complexes 2a–2d were conveniently synthesized in higher yields (83–91%) by the reaction of ZnEt2 with two equivalents of the corresponding ligand by alkane elimination (Scheme 1). Complexes 2a–2d were always obtained when the molar ratio of ligand/ZnEt2 was 1:1 or 2:1 in toluene, which is due to the fact that the salicylaldiminato ligands are not sterically bulky enough to inhibit the formation of bis-ligand species [14]. Complexes 2a–2d have different solubilities in common solvents. Complexes 2a and 2c are well soluble in methylene chloride, diethyl ether and THF, moderately soluble in toluene, and sparingly soluble in saturated hydrocarbon solvents. Complex 2b is well soluble in common solvents. Complex 2d is soluble in DMSO (dimethyl sulfone), moderately soluble in methylene chloride and sparingly soluble in other common solvents. Complexes 2a–2d were all characterized by elemental analyses, 1H, 13C NMR and IR spectroscopies, and satisfactory analytical results were obtained. In the 1H NMR spectra of these complexes, the resonance for the CH imino protons in 2a (8.57 ppm), 2b (8.16 ppm), 2c (8.40 ppm) and 2d (8.65 ppm) shifts by about 0.2 ppm, 0.64 ppm, 0.30 ppm and 0.05 ppm, respectively, towards high field compared to the corresponding signals of the free ligands. However the resonances (171.7 ppm for 2a, 170.8 ppm for 2b, 170.4 ppm for 2c and 168.4 ppm for 2d) for the CH imino carbons in the 13C NMR spectra shift by about 8 ppm, 5.8 ppm, 7.6 ppm and 5.4 ppm, respectively, to low field in comparison with the corresponding ones of the free ligands. The O–H proton signals of the free ligands disappear in the 1H NMR and IR spectra of the complexes 2a– 2d, which indicates the formation of a Zn–O bond in these complexes. All the complexes are stable to air and moisture in the solid state and can be left in the air for several days without obvious decomposition. 3.3. The crystal structure of complex 2b Crystals of complex 2b suitable for X-ray crystal structure determination were grown from toluene/nhexane at room temperature. The ORTEP drawing of the molecular structure of 2b is shown in Fig. 1, top. Selected bond lengths and angles for complex 2b are given in Table 2. There are two nhexane solvent molecules in the unit cell of complex 2b, and the nhexane solvate exhibits molecular disorder. The high residual peaks in a difference fourier map with suitable geometry were refined isotropically with occupancies factors (0.25:0.25) for each nhexane moiety.

Fig. 1. Top: Molecular structure of complex 2b; bottom: Crystal packing diagram between two adjacent molecules of complex 2b showing the existence of the p–p stacking interaction in the solid state. (Thermal ellipsoids are drawn at 30% probability. All H atoms and the disordered n hexane solvate are omitted for clarity.) Table 2 ˚ ) and angles () for complex 2b Selected bond lengths (A Complex 2b Zn(1)–N(1) Zn(1)–N(3) Zn(1)–O(1) Zn(1)–O(2) N(1)–C(7) N(3)–C(33) N(1)–Zn(1)–O(1) N(1)–Zn (1)–O(2)

1.994(3) 2.016(3) 1.903(3) 1.901(3) 1.305(5) 1.298(5) 121.66(12) 118.79(13)

N(1)–Zn(1)–N(3) N(3)–Zn(1)–O(1) N(3)–Zn(1)–O(2) O(1)–Zn(1)–O(2) C(7)–N(1)–Zn(1) C(8)–N(1)–Zn(1)

110.98(13) 115.21(13) 95.65(12) 121.66(12) 120.9(3) 121.5(2)

C(33)–N(3)–Zn(1) C(34)–N(3)–Zn(1)

119.9(3) 123.1(3)

The X-ray analysis reveals that complex 2b adopts a distorted tetrahedral geometry with the metal center chelated by two bidentate ligands via the phenolate oxygen atom and imine nitrogen atom. The O–Zn–O angle in 2b (121.66(12)) is larger than those values (105–112.5) in the similar bis-salicylaldiminato zinc complexes 1

t

(NR2C7H5-x(R1)xO)2Zn [x = 1 or 2; R = Me, Bu, Cl, OMe;

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R2 = 2,6-iPr2C6H3] [14], while the N–Zn–N angle in 2b (110.98(13)) is smaller than those in the latter complexes (122.9–128.9). The two dihedral angles between the quinolyl ring and the six-membered chelating ring are 46.5 and 48.9, respectively. The two six-membered chelating rings are nearly planar with the zinc atoms lying 0.0454 and ˚ out of the plane, and they are closely perpendic0.0328 A ular to each other with the dihedral angle being 88.7. The imino C@N bonds in complex 2b retain their double ˚ . In addibond character, being 1.305(5) and 1.298(5) A tion, the X-ray diffraction analysis reveals that an intermolecular p–p interaction appears in the solid state of complex 2b, forming an antiparallel dimeric structure (Fig. 1, bottom). One quinolyl ring in complex 2b stacks together with the corresponding one in an adjacent mole˚ [15]. cule by the p–p interaction with a distance of 3.64 A Moreover, one thing that should be mentioned is that all the methyl groups of the tBu group at the 5-position of the salicylene ring in 2b display rotational disordering over two sites, with 50% occupancy factors for each site. The H-atoms are placed in calculated positions on the disordered atoms.

3.4. Luminescent property Table 3 summarizes the absorption and emission data for compounds 1a–1d and 2a–2d in solution and the solid state. As shown in Table 3, it can be seen that the four complexes 2a–2d show three main absorption bands similar to those in the ligands 1a–1d in the UV region. The electronic absorption spectra of those complexes in the maximum absorption wavelength are red-shifted relative to the ligands due to the perturbation of the intraligand p–p* transition of the salicylaldiminato unit by the metal atom. Fig. 2 shows the emission spectra of 1a–1d and 2a–2d in solution. And Fig. 3 shows the emission spectra of 1a–1d and 2a–2d in the solid state. In comparison with the corresponding free ligands (kmax = 476, 524, 478 and 486 nm, respectively, for 1a–1d), complexes 2a–2d in solution exhibit red-shifts with kmax = 503, 532, 514 and 515 nm, respectively. The coordinated zinc atom plays a dual role in the luminescence of 2a–2d as pointed out in the literature for coordination complexes [16]. First, the formation of covalent bonds between the Zn and O atoms via p-donation of

Table 3 Photoluminescent data for ligands 1a–1d and complexes 2a–2d Compound

Absorption (nm) e (dm3 mol1 cm1)

Excitation (k, nm)

Emission (kmax, nm)

Quantum yields (U)a

Conditions

1a

272(18847) 348(13659)

404

476

0.003

CH2Cl2, 298 K

404

536

432

524

393

553

390

478

390

558 486

1b

1c

277(41670), 328(28384), 363(24353) 272(27519), 329(17234), 357(18484)

1d

275(27906), 334(12169), 363(sh, 10503)

407

407

578

2a

277(44266), 323(29571), 407(18194)

407

503

407

493

428

532

428

519

438

514

438

535

420

515

420

526

2b

2c

2d

a

283(48358), 322(33104), 428(20292) 276(41691), 321(26805), 417(19710) 284(33089), 325(27703), 422(21188)

Determined using quinine sulfate in 0.1 M sulfuric acid as a standard.

solid, 298 K 0.004

CH2Cl2, 298 K

solid, 298 K 0.001

CH2Cl2, 298 K

solid, 298 K 0.001

CH2Cl2, 298 K

0.059

CH2Cl2, 298 K

solid, 298 K

solid, 298 K 0.151

CH2Cl2, 298 K

solid, 298 K 0.016

CH2Cl2, 298 K

solid, 298 K 0.005

CH2Cl2, 298 K

solid, 298 K

Q. Su et al. / Polyhedron 26 (2007) 5053–5060

Normalized Intensity (a.u.)

1.0

1a 1b 1c 1d 2a 2b 2c 2d

0.8

0.6

0.4

0.2

0.0 400

450

500

550

600

650

700

Wavelength (nm) Fig. 2. Emission spectra of ligands 1a–1d and complexes 2a–2d in CH2Cl2.

Normalized Intensity (a.u.)

1.0

1a 1b 1c 1d 2a 2b 2c 2d

0.8

0.6

0.2

0.0 500

550

600

650

coordination of the ligands to the zinc atom increases the rigidity of the molecule. The quantum yields of complexes 2c and 2d are lower than those of 2a and 2b due to the fact that the bromo atom can quench the fluorescence and results in luminescence decay. All compounds 1a–1d and 2a–2d emit bright fluorescence in the solid state at room temperature. Complexes 2a–2d exhibit a bright emission with kmax = 493, 519, 535 and 526 nm, respectively (kmax = 536, 553, 558 and 578 nm for ligands 1a–1d). The emission maxima of these complexes are blue-shifted compared to those of the corresponding free ligand in the solid state, which is probably due to the disappearance of the intramolecular hydrogen bonding in compounds 1a–1d after coordination with the zinc atom [17]. The emission maxima of complexes 2a and 2b in the solid state are blue-shifted compared to their corresponding emission maxima in solution, while the emission maxima of complexes 2c and 2d in the solid state are red-shifted compared to their corresponding emission maxima in solution. Blue shifting probably results from the diminished conjugated extent of the two complexes 2a and 2b since the free rotation of the quinolyl ring in these complexes is blocked in the solid state. Red shifting of emission maximum can be normally observed for most fluorescent compounds in the solid state, probably due to p–p stacking of the aromatic rings in the molecules. 4. Conclusions

0.4

450

5059

700

Wavelength (nm) Fig. 3. Emission spectra of ligands 1a–1d and complexes 2a–2d in the solid state.

a lone-pair electrons of the O atom to the Zn atom changes the emission energy, due to the lowering of the energy gap between p* and p. Second, the coordination of the ligands with the Zn atom increases the rigidity of the ligands, which can diminish the loss of energy via vibrational motions and increase the emission efficiency. The emission maximum of complexes 2b–2d in solution is red-shifted by about 29, 11 and 12 nm, respectively, compared to that of the complex 2a, which is possibly due to the fact that the electron-donating nature of the tert-butyl group and bromo group on the phenyl ring lowers the energy gap between p* and p. The quantum yields of all the compounds have been determined in solution. It was found that the quantum yields of the complexes are higher than those of the corresponding ligands, considering that the

A series of bis-salicylaldiminato zinc complexes have been synthesized and characterized. Our research results indicate that the chelating bidentate salicylaldiminato conjugated ligands are well suitable for synthesizing four-coordinate zinc complexes by the reaction of ZnEt2 with the corresponding ligands. These complexes produce bright fluorescence in both solution and the solid state, and the emission color in solution can be tuned by the 3- and 5position substituents on the salicylene rings of the ligands. The four-coordinate zinc complexes are a new class of luminescent material with potential applications in optoelectronic devices. 5. Supplementary material CCDC 634611 contains the supplementary crystallographic data for 2b. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. Acknowledgements This work was supported by the National Natural Science Foundation of China (No. 20674024) and the Ministry of Science and Technology of China (No. 2002CB6134003).

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