Palladium(II)-iodo-{1-alkyl-2-(arylazo)imidazole} complexes: Synthesis, structure, dynamics of photochromism and DFT computation

Polyhedron 85 (2015) 900–911

Contents lists available at ScienceDirect

Polyhedron journal homepage: www.elsevier.com/locate/poly

Palladium(II)-iodo-{1-alkyl-2-(arylazo)imidazole} complexes: Synthesis, structure, dynamics of photochromism and DFT computation Chandana Sen a, Suman Roy a, Tapan Kumar Mondal a, Rajib Ghosh b, Jahur A. Mondal b, Dipak K. Palit b, Chittaranjan Sinha a,⇑ a b

Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India Radiation & Photochemistry Division, Bhabha Atomic Research Centre, Mumbai 400085, India

a r t i c l e

i n f o

Article history: Received 15 February 2014 Accepted 27 September 2014 Available online 20 October 2014 Keywords: Palladium(II)-azoimidazole-iodide Photochromism Transient absorption spectra Activation energy DFT computation

a b s t r a c t [Pd(Raai-CnH2n+1)2I)]2[Pd2I6] (Raai-CnH2n+1 = 1-alkyl-2-(arylazo)imidazole) has been characterized by the spectral data (UV–Vis, FTIR, Mass, 1H NMR). The single crystal X-ray structure of [Pd(Meaai-C2H5)2I]2 [Pd2I6] shows that one Meaai-C2H5 acts as monodentate N(imidazolyl) donor, while the other one is bidentate N(imidazolyl), N(azo) chelator; the charge is neutralized with [Pd2I6]2 anion. UV light irradiation in DMF solution of the complexes show trans-to-cis isomerisation of Raai-CnH2n+1 about –N@N– bond. Quantum yields (/t?c) of trans-to-cis isomerisation of the complexes are lower than that of the free ligand data. This observation is consistent with femtosecond transient absorption results of [Pd(Haai-C10H21)2][Pd2I6], which suggest that the trans ? cis isomerization occurs in the monodentate azo-imidazole group and the bidentate azo-imidazole, because of chelation with Pd(II), does not exhibit photochromism. The isomerization proceeds with a time constant of 1.0 ps in acetonitrile, 0.6 ps in methanol, and 1.7 ps in ethylene glycol, which are comparable to those the free azo-imidazole ligand. In contrast, the reverse transformation, i.e. cis-to-trans, is carried out by thermal process and the activation energy (Ea) of cis-to-trans isomerisation of the complexes is lower than that of free ligand. The spectral property and photochromic efficiency have been explained by DFT computation of optimized geometry of the complex. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Heterocyclic compounds are widely distributed in nature and are essential to life in various ways like – catalysis, redox activity, photo activity, organic synthesis, medicines [1,2]. On appending Ar–N@N– to the backbone of N-heterocycle p-deficient azoheterocycles like arylazopyridine, arylazopyrimidine, arylazoimidazole, etc. have been synthesized [3–7]. Arylazopyridine undergoes light stimulated reversible trans–cis isomerization about –N@N– bond. During the isomerization process the properties like absorption, molecular dimensions and dipole moment show significant change [8–10]. They are useful in information storage, optical switching devices, surface relief gratings, nonlinear optics, etc. [11–17]. Fatigue and low durability are main problems of organic photochrome which could be minimized by coordination with metal ions. Towards the exploration of photo switching activity and metal ion binding the design of azoheterocycles become an exciting area ⇑ Corresponding author. Fax: +91 033 2413 7121. E-mail address: [email protected] (C. Sinha). http://dx.doi.org/10.1016/j.poly.2014.09.033 0277-5387/Ó 2014 Elsevier Ltd. All rights reserved.

for the last few years of research [3–17]. We have used 1-alkyl-2(arylazo)imidazole (Raai-CnH2n+1) to characterize coordination chemistry of transition and non-transition metal ions [18–22]. Azoimidazole derivatives like, 1-N-(ribofuranosyl)-2-phenylazoimidazole has shown trans–cis photoisomerization upon irradiation with UV light [23]. In our studies, 1-alkyl-2-(arylazo)imidazole (Raai-CnH2n+1) both free ligand and coordinated one exhibit photochromic activity. We have examined the effect of metal ion coordination, H+ addition, other external parameters (like innocent molecules, micelles, and reverse micelle) [24–37] on the photochromic activity. Palladium(II) complexes of Raai-CnH2n+1 have been published in different dimension [28,38–40] like – structure, redox, nucleophilic substitution reaction, metal assisted organic transformation such as C–N, C–O coupling, photochromism. In this work, we wish to report a newer type of isomer of palladium(II) complexes of Raai-CnH2n+1 along with the structural characterization and photochromism. Dynamics of photoisomerisation is also examined. The spectral properties have been explained by DFT computation of optimized geometry.

C. Sen et al. / Polyhedron 85 (2015) 900–911

2. Experimental 2.1. Materials PdCl2 was obtained from M/S Arrora Matthey, Kolkata, India. K2[PdI4] was prepared by adding KI solution in excess to PdCl2 in acetonitrile and refluxed around 30 min. A dark brown precipitate was obtained which was gradually washed with water and a pure K2[PdI4] was collected. 1-Alkyl-2-(arylazo)imidazoles (Raai-CnH2 n+1) were synthesized by reported procedure [27]. 1-Bromo-nalkanes (n-C4H9-1-Br, n-C6H13-1-Br, C8H17-1-Brand C10H21-1-Br) were purchased from Sigma–Aldrich and used as such. All other chemicals and solvents were reagent grade as received. 2.2. Physical measurements Microanalytical data (C, H, N) were collected on Perkin-Elmer 2400 CHNS/O elemental analyzer. Spectroscopic data were obtained using the following instruments: UV–Vis spectra from a Perkin Elmer Lambda 25 spectrophotometer; IR spectra (KBr disk, 4000–200 cm1) from a Perkin Elmer BX-1 FTIR spectrophotometer; photo-excitation has been carried out using a Perkin Elmer LS-55 spectrofluorimeter and 1HNMR spectra from a Bruker (AC) 300 MHz FTNMR spectrometer. ESI mass spectra were recorded on a micro mass Q-TOF mass spectrometer (Serial No. YA 263). 2.3. Synthesis of [Pd(Haai-C2H5)I2] (7a) 1-Methyl-2-(phenylazo)imidazole (Haai-C2H5, 50 mg, 0.25 mmol) in MeOH (10 ml) was added dropwise to ethylene glycol monomethyl ether (EGME) solution (5 ml) of K2[PdI4] (180 mg, 0.26 mmol) and stirred for 2 h. An orange crystalline compound was separated on slow evaporation in air. The precipitate was collected by filtration, washed with MeOH and dried over CaCl2 in vacuum. The purity of the product was checked by TLC test. Microanalytical data and other spectroscopic characterization suggested the composition [Pd(Haai-C2H5)I2] (Yield, 96 mg, 69%). Other complexes were prepared under identical conditions and the yield varied in the range 65–75%. Microanalytical data: Anal. Calc. for C11H12N4PdI2 (7a): C, 23.57; H, 2.16; N, 9.99. Found: C, 23.43; H, 2.23; N, 10.02%. m/z+ (ESI), 559.98. 1H NMR (300 MHz, DMSO-d6) d 7.75 (1H, bs, 4-H), 7.30 (1H, bs, 5-H), 8.01 (2H, d, 7.8 Hz, 7,11-H), 7.68 (2H, t, 7.5 Hz, 8,10-H), 7.35 (1H, t, 7.6 Hz, 9-H), 4.52 (2H, q, 6.8 Hz, 12CH2),1.54 (3H,t, 3.7 Hz, 13-CH3); FT-IR (KBr, m, cm1) m(N@N), 1375; m(C@N), 1619; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 368 (38.21), 370 (35.70), 445 (17.31), 487(12.68). Anal. Calc. for C12H14N4PdI2 (7b): C, 25.09; H, 2.45; N, 9.75. Found: C, 25.02; H, 2.44; N, 9.62%. m/z+ (ESI) 574.23. 1H NMR (300 MHz, DMSO-d6) d 7.85 (1H, bs, 4-H), 7.27 (1H, bs, 5-H), 8.2 (2H, d, 8.2 Hz, 7,11-H), 7.48 (2H, d, 7.9 Hz, 8,10-H), 2.40 (3H,s, 9-CH3), 4.53 (2H, q, 1.1 Hz, 12-CH2), 1.56 (3H, t, 3.7 Hz, 13-CH3); FT-IR (KBr, m cm1) m(N@N), 1382; m(C@N), 1599; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 370 (37.88), 381 (36.53), 452 (24.69), 486 (12.97). Anal. Calc. for C17H24N4PdI2 (8a): C, 31.67; H, 3.75; N, 8.69. Found: C, 31.60; H, 3.81; N, 8.72%. m/z+ (ESI) 644.32. 1H NMR (300 MHz, DMSO-d6) d 7.76 (1H, bs, 4-H), 7.33 (1H, bs, 5-H), 7.95 (2H, d, 8.1 Hz, 7,11-H), 7.72 (2H, t, 7.7 Hz, 8, 10-H), 7.63 (1H, t, 6.8 Hz, 9-H), 4.51 (2H, t, 0.8 Hz, 12-CH2), 1.29–1.46 (12H, m, 13-18H), 0.83 (3H, t, 3.5 Hz, 19-CH3); FT-IR (KBr, m cm1) m(N@N), 1384; m(C@N), 1588; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 365 (40.24), 382(40.36), 450 (20.21), 484 (13.25). Anal. Calc. for C11H12N4PdI2 (8b): C, 32.82; H, 3.98; N, 8.50. Found: C, 32.82; H, 3.92; N, 8.51%. m/z+ (ESI) 658.23. 1H NMR (300 MHz, DMSO-d6) d 7.77 (1H,

901

bs, 4H), 7.32 (1H, bs, 5H), 7.95 (2H, d, 6.0 Hz, 7,11-H), 7.47 (2H, d, 7.5 Hz, 8,10-H), 2.51 (3H, s, 9-CH3), 4.52 (2H, t, 0.9 Hz, 12-CH2), 1.24–1.82 (12H, m, 13–18-CH2), 0.87 (3H, t, 3.5 Hz, 19-CH3); FT-IR (KBr, m cm1) m(N@N), 1385; m(C@N), 1599; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 367 (38.88), 385 (44.40), 444 (19.62), 485 (13.04). 2.4. Synthesis of [Pd(Haai-C6H13)2I]2[Pd2I6] (10a) 1-Hexyl-2-(phenylazo)imidazole (Haai-C6H13, 23 mg, 0.055 mmol) in MeOH (10 ml) was added dropwise to EGME solution (5 ml) of K2[PdI4] (31.05 mg, 0.057 mmol), which was refluxed for 2 h. Brown–red precipitate appeared. The precipitate was collected by filtration, washed with cold MeOH and dried over CaCl2 in vacuum. The yield was 168 mg (64%). Other complexes were prepared under identical conditions and the yield varied in the range 60%–70%. Anal. Calc. for C40H40N16Pd4I8 (9a): C, 21.98; H, 1.85; N, 10.26. Found: C, 21.89; H, 1.92; N, 10.31%. m/z+ (ESI) 605.18. 1H NMR (300 MHz, DMSO-d6) d 7.82 (2H, bs, 4,40 -H), 7.22 (2H, bs, 5,50 -H), 7.99 (4H, d, 8.3 Hz, 7,70 ,11,110 -H), 7.48 (4H, t, 7.6 Hz, 8,80 ,10,100 H), 7.34 (2H, t, 7.1 Hz, 9,90 -H), 4.52 (6H, s, 12,120 -CH3); FT-IR (KBr, mcm1) m(N@N), 1373; m(C@N), 1580; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 368 (37.57), 378 (37.40), 451 (18.10), 482 (13.68). Anal. Calc. for C44H48N16Pd4I8 (9b): C, 23.57; H, 2.16; N, 9.99. Found: C, 23.39; H, 2.24; N, 9.87%. m/z+ (ESI) 633.47. 1H NMR (300 MHz, DMSO-d6) d 7.83 (2H, bs, 4,40 -H), 7.25 (2H, bs, 5,50 -H), 7.94 (4H, d, 8.2 Hz, 7,70 ,110 110 -H), 7.45 (4H, d, 7.8 Hz, 8,80 ,100 100 -H), 2.33 (6H, s, 9,90 -CH3), 4.50 (6H, s, 12,120 -H); FT-IR (KBr, m cm1) m(N = 141 N), 1382; m(C@N), 1598; UV–Vis (kmax, nm (e,103 M1 cm1) in DMF), 368 (45.10), 383 (42.64), 451 (19.30), 485 (14.10). Anal. Calc. for C44H48N16Pd4I8 (10a): C, 23.57; H, 2.16; N, 9.99. Found: C, 23.49; H, 2.10; N, 9.92%. m/z+ (ESI) 633.52. 1H NMR (300 MHz, DMSO-d6) d 7.71 (2H, bs, 4,40 H), 7.28 (2H, bs, 5,50 -H), 8.04 (4H, d, 8.0 Hz, 7,70 ,11,110 -H), 7.62 (4H, t, 7.2 Hz, 8,80 ,10,100 -H), 7.31 (2H, t, 6.9 Hz, 9,90 -H), 4.48 (4H, q, 1.2 Hz, 12,120 -H), 1.6 (6H, t, 3.6 Hz, 13,130 -CH3); FT-IR (KBr, m, cm1) m(N@N), 1370; m (C@N), 1624; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 366 (41.24), 377 (39.79), 448 (18.30), 488 (12.92). Anal. Calc. for C48H56N16Pd4I8 (10b): C, 25.09; H, 2.46; N, 9.76. Found: C, 25.02; H, 2.32; N, 9.61%. m/z+ 661.72. 1H NMR (300 MHz, DMSO-d6) d 7.82 (2H, bs, 4,40 -H), 7.23 (2H, bs, 5,50 -H), 7.98 (4H, d, 8.2 Hz, 7,70 ,11,110 -H), 7.43 (4H, d, 7.8 Hz, 8,80 ,10,100 H), 2.32 (6H, s, 9,90 -CH3), 4.49 (4H, q, 0.9 Hz, 12,120 -CH2), 1.52 (6H, t, 3.2 Hz; 13,130 -CH3); FT-IR (KBr, m, cm1) m(N@N), 1380; m(C@N), 1597; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 369 (39.89), 382 (39.78), 450 (22.79), 490 (13.47). Anal. Calc. for C52H64N16Pd4I8 (11a): C, 26.532; H, 2.74; N, 9.52. Found: C, 26.44; H, 2.67; N, 9.50%. m/z+ 689.48. 1H NMR (300 MHz, DMSO-d6) d 7.81 (2H, bs, 5.8 Hz, 4,40 -H), 7.25 (2H, bs, 5,50 -H), 8.01 (4H, d, 8.1 Hz, 7,70 ,11,110 H), 7.61 (4H, t, 7.2 Hz, 8,80 ,10,100 -H), 7.28 (2H, t, 6.9 Hz, 9,90 -H), 4.51 (4H, t, 0.8 Hz, 12,120 -CH2), 1.36–1.54 (8H, m, 13–14, 130 -140 H), 0.92 (6H, t, 2.9 Hz, 15,150 -H); FT-IR (KBr, m cm1) m(N@N), 1369; m(C@N), 1597; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 366 (43.00), 377 (43.05), 441 (20.41), 488 (13.51). Anal. Calc. for C56H72N16Pd4I8 (11b): C, 27.907; H, 3.011; N, 9.299. Found: C, 27.846; H, 3.102; N, 9.189%. m/z+ 717.46. 1H NMR (300 MHz, DMSO-d6) d 7.75 (2H, bs, 4,40 -H), 7.24 (2H, bs, 5,50 -H), 7.99 (4H, d, 7.8 Hz, 7,70 ,11,110 -H), 7.49 (4H, d, 7.40 Hz, 8,80 ,10,100 -H), 2.30 (6H, s, 9,90 -H), 4.46 (4H, t, 4.7 Hz, 12,120 -CH2), 1.36–1.55 (8H, m, 13–14, 130 -140 ,-164 CH2), 0.9 (6H, t, 2.8 Hz, 15,150 -CH3); FT-IR (KBr, m cm1) m(N@N), 1380; m(C@N), 1586; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 365 (39.71), 379 (39.85), 438 (19.72), 484 (14.03). Anal. Calc. for C60H80N16Pd4I8 (12a): C, 29.22; H, 3.27; N, 9.09. Found: C, 29.15; H, 3.18; N, 9.04%. m/z+ (ESI) 745.62 .1H NMR (300 MHz, DMSO-d6) d 7.74 (2H, bs; 4,40 -H),

902

C. Sen et al. / Polyhedron 85 (2015) 900–911

7.23 (2H, bs, 5,50 -H), 8.0 (4H, d, 8.0 Hz, 7,70 ,11,110 -H), 7.59 (4H, t, 7.6 Hz, 8,80 ,10,100 -H), 7.34 (2H, t, 6.9 Hz, 9,90 -H), 4.44 (4H, t, 3.8 Hz, 12,120 -CH2), 1.22–2.1 (16H, m, 13–16,130 -160 -CH2), 0.77 (6H, t, 1.2 Hz,17,170 -CH3); FT-IR (KBr, m cm1) m(N@N), 1370; m(C@N), 1650; UV–Vis (kmax, nm (e,103 M1 cm1) in DMF), 367 (44.8), 381 (39.97), 440 (19.94), 489 (13.62). Anal. Calc. for C64H88N16Pd4I8 (12b): C, 30.48; H, 3.52; N, 8.89. Found: C, 30.39; H, 3.58; N, 8.83%. m/z+ 773.47. 1H NMR (300 MHz, DMSO-d6) d 7.75 (2H, bs, 4,40 -H), 7.32 (2H, bs, 5,50 -H), 7.86 (4H, d, 8.0 Hz, 7,70 ,11,110 -H), 7.42 (4H, d,7.3 Hz, 8,80 ,10,100 -CH2), 2.40 (6H, s,9,90 -CH3), 4.43 (4H, t, 2.8 Hz, 12,120 -CH2), 1.23–1.82 (16H, m; 13–16, 130 -160 ,-CH2), 0.79 (6H, t; 1.06 Hz; 17,170 -CH3); FT-IR (KBr, m cm1) m(N@N), 1392; m(C@N), 1592; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 369 (40.61), 379 (39.77), 509 (12.75), 482 (12.85). Anal. Calc. for C68H96N16Pd4I8 (13a): C, 31.68; H, 3.75; N, 8.69. Found: C, 31.69; H, 3.77; N, 8.53%. m/z+ (ESI) 801.57 .1H NMR (300 MHz, DMSOd6) d 7.74 (2H, bs, 4,40 -H), 7.28 (2H, bs, 5,50 -H), 7.91 (4H, d; 7.9 Hz; 7,70 ,11,110 -H), 7.63 (4H, t, 7.6 Hz, 8,80 ,10,100 -H), 7.54 (2H, t, 7.1 Hz, 9,90 -H), 4.42 (4H, t, 3.8 Hz, 12,120 -CH2), 1.25–1.36 (24H, m, 13–18, 130 -180 ,-CH2), 0.81 (6H, t, 1.01 Hz, 19,190 -CH3); FT-IR (KBr, m cm1) m(N@N), 1382; m(C@N), 1598; UV–Vis (kmax, nm (e,103 M1 cm1) in DMF), 369 (41.04), 380 (41.32), 452 (20.22), 486 (13.64). Anal. Calc. for C72H104N16Pd4I8 (13b): C, 32.82; H, 3.98; N, 8.51. Found: C, 32.90; H, 3.96; N, 8.46%. m/z+ 829.56 .1H NMR (300 MHz, DMSO-d6 d 7.71 (2H, bs, 4,40 -H), 7.25 (2H, bs, 187 5,50 -H), 7.89 (4H, d, 7.1 Hz, 7,70 ,11,110 -H), 7.38 (4H, d, 7.2 Hz, 8,80 ,10,100 -H), 2.48 (6H, s, 9,90 -CH3), 4.46 (4H, t, 0.8 Hz, 12,120 CH2), 1.24–1.75 (24H, m, 13–18, 130 -180 -CH2), 0.85 (6H, t, 0.9 Hz, 19,190 -CH3); FT-IR (KBr, mcm1) m(N@N), 1381; m(C@N), 1596; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 368 (38.63), 381 (40.46), 445 (19.69), 487 (13.21). Anal. Calc. for C76H112N16Pd4I8 (14a): C, 33.925; H, 4.196; N, 8.329. Found: C, 33.85; H, 4.15; N, 8.27%. m/z+ (ESI) 857.85 .1H NMR (300 MHz, DMSO-d6) d 7.64 (2H, bs, 4,40 -H), 7.23 (2H, bs, 5,50 -H), 7.86 (4H, d, 7.8 Hz, 7,70 ,11,110 -H), 7.58 (4H, t, 7.3 Hz, 8,80 ,10,100 -H), 7.53 (2H, t, 6.8 Hz, 9,90 -H), 4.39 (4H, t, 3.8 Hz, 12,120 -CH2), 1.21–1.67 (32H, m, 13–20, 130 -200 -CH2), 0.79 (6H, t, 2.9 Hz, 21,210 -CH3); FT-IR (KBr, m cm1) m(N@N), 1460; m(C@N), 1653; UV–Vis (kmax, nm (e,103 M1 cm1) in DMF), 369 (39.12), 380 (40.31), 449 (20.01), 491 (13.07). Anal. Calc. for C80H120N16Pd4I8 (14b): C, 34.98; H, 4.40; N, 8.16. Found: C, 35.16; H, 4.24; N, 8.12%. m/z+ (ESI) 885.68 .1H NMR (300 MHz, DMSO-d6) d 7.59 (2H, bs, 4,40 H),7.27(2H, bs, 5,50 -H), 7.96 (4H, d, 7.8 Hz, 7,70 ,11,110 -H), 7.35 (4H, t, 7.31 Hz, 8,80 ,10,100 -H), 2.47 (6H, s, 9,90 -CH3), 4.45 (4H, t, 2.8 Hz, 12,120 -CH2), 1.14–1.68 (32H, m, 13–20, 130 -200 ,-CH2), 0.76 (6H, t, 0.9 Hz, 21,210 -CH3); FT-IR (KBr, m cm1) m(N@N), 1383; m(C@N), 1597; UV–Vis (kmax, nm (e, 103 M1 cm1) in DMF), 370 (40.23), 377 (41.84), 451 (18.28), 487 (12.69). 2.5. X-ray diffraction study [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) A suitable single crystal of [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) (0.11  0.15  0.19 mm) was mounted on a Siemens CCD diffractometer equipped with graphite monochromated Mo Ka (k = 0.71073 Å) radiation. The crystallographic data were shown in Table 1. The unit cell parameters and crystal-orientation matrices were determined by least squares refinements of all reflections in the hkl range 10 < h < 10; 16 < k < 16; 21 < l < 20. The intensity were corrected for Lorentz and polarisation effects and an empirical absorption correction were applied. Data were collected applying the condition I > 2r(I). The structure was solved by direct method and followed by successive Fourier and difference Fourier syntheses. Full matrix least squares refinements on F2 were carried out using SHELXL-97 [41] with anisotropic displacement parameters for all non-hydrogen atoms. Hydrogen atoms were constrained to ride on the respective carbon or nitrogen atoms with isotropic

Table 1 Summarized crystallographic data for [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b). Empirical formula Formula weight T (K) System Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z l(Mo Ka) (mm1) h range Dcalc (mg m3) Refine parameters Total reflections Unique reflections R1a [I > 2r(I)] wR2b Goodness-of-fit (GOF) on F2

C24H28IN8Pd, 0.5(I6Pd2) 1148.94 293 triclinic  P1 8.050(5) 12.770(5) 16.883(5) 100.857(5) 100.609(5) 90.148(5) 1674.1(13) 2 4.789 1.6–27.0 2.279 343 24 733 7273 0.029 0.0802 0.88

a

R = R|F0  Fc|/RF0. wR = [Rw(F20  F2c )/RwF40]1/2, P = (F20 + 2F2c )/3. b

w = 1/[r2(F20) + (0.0336P)2 + 1.1741P]

where

displacement parameters equal to 1.2 times the equivalent isotropic displacement of their parent atom in all cases. Complex neutral atom scattering factors were used throughout for all cases. All calculations were carried out using SHELXL 97 [41], SHELXS 97 [42], PLATON 99 [43], ORTEP-3 [44] program.

2.6. Photometric measurements The absorption spectra were taken with a Perkin Elmer Lambda 25 UV–Vis Spectrophotometer in a 1  1 cm quartz optical cell maintained at 25 °C with a Peltier thermostat. The light source of a PerkinElmer LS 55 spectrofluorimeter was used as an excitation light, with a slit width of 10 nm. An optical filter was used to cut off overtones when necessary. The absorption spectra of the cis isomers were obtained by extrapolation of the absorption spectra of a cis-rich mixture for which the composition is known from 1H NMR integration. Quantum yields (/) were obtained by measuring initial trans-to-cis isomerization rates (m) in a well-stirred solution within the above instrument using the equation, k = (/I0/V)(1  10Abs) where I0 is the photon flux at the front of the cell, V is the volume of the solution, and Abs is the initial absorbance at the irradiation wavelength. The value of I0 was obtained by using azobenzene (/ = 0.11 for 233 p–p⁄ excitation [45]) under the same irradiation conditions. The thermal cis-to-trans isomerisation rates were obtained by monitoring absorption changes intermittently for a cis-rich solution kept in the dark at constant temperatures (T) in the range from 298–313 K. The activation energy (Ea) and the frequency factor (A) were calculated from ln k = ln A  Ea/RT, where k is the measured rate constant, R is the gas constant, and T is temperature. The values of activation free energy (DG⁄) and activation entropy (DS⁄) were obtained through the relationships, DG⁄ = Ea  RT  TDS⁄ and DS⁄ = [ln A  1 – ln (kBT/h)/R] where kB and h are Boltzmann0 s and Planck0 s constants, respectively. Transient absorption spectral evolutions were investigated by using a femtosecond pump–probe spectrometer, the details of which has been described elsewhere [46]. Briefly, the femtosecond laser pulses (40 fs, 4 nJ/pulse, 800 nm) that were obtained from a self-mode-locked Ti-Sapphire oscillator were amplified in two stages using chirped-pulse amplification (CPA) technique. First,

C. Sen et al. / Polyhedron 85 (2015) 900–911

the low energy oscillator output was amplified in a regenerative amplifier and then in a two-pass amplifier. Both the amplifiers were pumped by a 20 W DPSS laser (Jade). The amplified output is of 40 fs laser pulse duration, 1.2 mJ/pulse, and 1 kHz repetition rate. Part of the 800 nm output was converted to 400 nm (energy  5 lJ/pulse) by focusing in a 0.5 mm thick BBO crystal (second harmonic generator), which was used as pump pulse for the subsequent photoexcitation of the samples. A small fraction of the fundamental output (energy  1 lJ/pulse) was focused in a 2 mm thick CaF2 window to generate white light continuum (450– 750 nm), which was used as a probe pulse to monitor the change in absorbance of the sample due to the pump pulse. Relative delay between pump and probe pulses was computer controlled by a high-precision delay rail. The polarization of the pump pulse was fixed at the magic angle (57.4°) with respect to that of the probe pulse. The solutions were flowing through a quartz cell of 1 mm optical path length. 2.7. Theoretical calculations The geometry optimization of the complexes [Pd(Meaai-C2H5)2I]2 [Pd2I6] (10b) were carried out using density functional theory (DFT) [47]. All calculations were carried out using the GAUSSIAN 03 program package [48] with the aid of the GAUSSVIEW visualization program [49]. For C, H, N, O the 6-31G (d) basis set at the B3LYP level were assigned, while for I, LanL2DZ [50] and for Pd, the SDD basis set with effective core potential was employed [51]. The vibration frequency calculations were performed to ensure that the optimized geometries represent the local minima and there are only positive eigenvalues. Vertical electronic excitations

903

based on B3LYP optimized geometries were computed using the time-dependent density functional theory (TD-DFT) formalism [52–54] in acetonitrile using conductor-like polarizable continuum model (CPCM) [55]. Gauss Sum was used to calculate the fractional contributions of various groups to each molecular orbital [56].

3. Results and discussion 3.1. Synthesis and formulation 1-Alkyl-2-(arylazo)imidazole (Raai-CnH2n+1 where n = 1 (1), 2 (2), 4 (3), 6 (4), 8 (5), 10 (6); R = H (a), Me (b); Scheme 1) has been reacted with K2[PdI4] in the mixture of methanol and ethylene glycol monomethyl ether (EGME). An orange crystalline compound is separated at room temperature. The complex has been characterized by microanalytical and spectroscopic data as [Pd(Raai-CnH2n+1)I2] (n = 2 (–C2H5) (7) and n = 8 (–C8H17) (8)). In second experiment, the orange suspension is refluxed which becomes clear and brown–red crystals are separated upon cooling and have been characterized as ionic complex of composition, [Pd(Raai-CnH2n+1)2I]2[Pd2I6] (9–14). There is also a possibility of separation of [Pd(Raai-CnH2n+1)2][PdI4] (15) (Scheme 1) which we have not been isolated in this experiment. The molar conductance data of the complexes, 9–14 suggest 2:1 conductivity (130– 150 X1 mol1 cm2) in DMF solution while the complexes, 7,8 are nonconducting. The structure of [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) has been established by single crystal X-ray diffraction study. The mass spectral fragmentation also supports the composition of the cationic fragment of the complexes.

Scheme 1. The complexes, [Pd(Raai-CnH2n+1)I2] (n = 2 (–C2H5) (7) and n = 8 (–C8H17) 8)), [Pd(Raai-CnH2n+1)2I][Pd2I6] (9–14) and [Pd(Raai-CnH2n+1)2][PdI4] (15) (Raai-CnH2n+1 where CnH2n+1 = CH3 (1/9), C2H5 (2/10), C4H9 (3/11),C6H13 (4/12), C8H17 (5/13), C10H21 (6/14); R = H (a), Me (b)).

904

C. Sen et al. / Polyhedron 85 (2015) 900–911

3.2. The Molecular structure The molecular structure of [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) is shown in Fig. 1 and the selected bond distances and bond angles are listed in Table 2. The crystal structure shows that the cationic complex unit, [Pd(Meaai-C2H5)2I]+ carries two types of coordination of Meaai-C2H5 – bidentate N(imidazolyl) and N(azo) chelator (abbreviated L1) and monodentate N(imidazolyl) donor (abbreviated L2) to Pd(II) along with a Pd–I bond [Pd(L1)(L2)I]2[Pd2I6]. The charge is neutralized by [Pd2I6]2. In cation, [Pd(MeaaiC2H5)2I]+, the bond lengths are Pd(1)–N(1), 2.007(3); Pd(1)–N(5), 1.984(4); Pd(1)–N(8), 2.1463(3); and Pd(1)–I(1), 2.550(5) Å (Table 2). A significant difference is observed in the azo (–N@N–) distances: N(3)–N(4), 1.251(5) and N(7)–N(8), 1.279(5) Å which maybe originated from different ligating behavior of two MeaaiC2H5. The elongation of N(7)–N(8) (belongs to ligand L1) than that of N(3)–N(4) (belongs to ligand L2) may be due to chelation of L1 which encourages dp(Pd) ? p⁄(L1) much better than monodentate coordinated L2–Pd (L2(imidazolyl-N), (N(1) ? Pd(II)). This is also ascertained from difference in bond distances of N(3)–C(1), 1.385(4) and N(7)–C(13), 1.357(4) Å. The chelate angle \N(5)– Pd(1)–N(8) is 77.23(14)° and other angles in the coordination sphere are \I(1)–Pd(1)–N(5), 93.00(10)°; \N(1)–Pd(1)–I(1), 88.64(9)° and, \N(1)–Pd(1)–N(8), 101.12(14)°. Thus, a distorted square–planar structure about Pd(II) is established. In Pd2I2 fragment of [Pd2I6]2 the iodide bridging carries \I(2)–Pd(2)–I(2a), 85.66(8)° and \Pd(2)–I(2)–Pd(2a), 94.34(6)° and has four terminal Pd–I bonds (Pd–I(2/2i), 2.579(3) Å and Pd–I(4/4i), 2.579(3) Å). The bridging iodide (I(3), I(3i)) shows the distances Pd(2)–I(3/3i), 2.595(4)/2.595(4) Å. The torsion angles are Pd(2)–I(3i)–Pd(2i)– I(2i), 178.83(1)° (isymmetry: 2  x, 2  y, 1  z) and Pd(2a)–I(2)– Pd(2)–I(2), 178.83(1)° which are in support of planarity of bridging dimer. Ionic motifs of the complex show hydrogen bonding interaction: C–H  I (bridged/terminal) and C–H  p type and have directed to form 1-D chain (Fig. 2). Terminal-I (I(2/2i) of [Pd2I6]2 is interacting with –CH2– hydrogen of N–CH2–CH3 of chelated Meaai-C2H5 (L1) (C(14)–H(14)  I(2): H(14)  I(2), 3.050(2) Å; C(14)  I(2), 3.901(5) Å; \C(14)–H(14)  I(2), 154.00°; symmetry, x,1 + y, z) while bridging-I shows bonding interaction with imidazolyl-H (C(22)–H(22A)  I(3): H(22A)  I(3), 3.010(2) Å; C(22)  I(3), 3.873(5) Å; \C(22)–H(22A)  I(3), 148.0°; symmetry,

Fig. 1.

ORTEP

Table 2 Selected bond lengths (Å) and angles (°) for the complex [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) with estimated standard deviations in parentheses. Bond distances (Å) Pd(1)–N(1) Pd(1)–N(5) Pd(1)–N(8) Pd(1)–I(1) N(3)–N(4) N(7)–N(8) N(1)–C(1) N(1)–C(2) N(2)–C(1) N(2)–C(10) N(5)–C(13) N(5)–C(14) N(6)–C(22) N(6)–C(13) Pd(2)–I(3) Pd(2)–I(3i) Pd(2i)–I(3) Pd(2i)–I(3i) Pd(2)–I(4) Pd(2i)–I(4i) i

Bond angles (°) 2.007(3) 1.984(4) 2.146(3) 2.550(5) 1.251(5) 1.279(5) 1.332(5) 1.371(5) 1.350(5) 1.474(6) 1.336(6) 1.363(6) 1.465(5) 1.347(5) 2.595(4) 2.595(4) 2.595(4) 2.595(4) 2.579(3) 2.579(3)

I(1)–Pd(1)–N(5) N(5)–Pd(1)–N(8) N(1)–Pd(1)–I(1) N(1)–Pd(1)–N(5) I(1)–Pd(1)–N(8) N(1)–Pd(1)–N(8) Pd(1)–N(8)–N(7) Pd(1)–N(8)–C(16) N(2)–C(1)–N(3) N(1)–C(1)–N(3) N(1)–C(1)–N(2) I(2)–Pd(2)–I(4) I(3)–Pd(2)–I(3i) I(2)–Pd(2)–I(3) I(4)–Pd(2)–I(3i) Pd(2)–I(3)–Pd(2i) Pd(2)–I(3i)–Pd(2i) I(3)–Pd(2i)–I(3i)

93.00(10) 77.23(14) 88.64(9) 178.36(14) 168.71(14) 101.12(14) 114.83(5) 130.38(2) 120.10(5) 129.28(6) 110.58(2) 73.79(6) 2.593(1) 91.39(8) 99.23(2) 94.34(7) 94.34(6) 85.66(8)

Symmetry: 2  x, 2  y, 1  z.

x,1 + y, z). The nonchelated Meaai-C2H5 (L2) of adjacent molecules undergo C–H  p interaction with –CH3 hydrogen of –N– CH2–CH3 (C(11)H3) in one L2 and p-tolyl ring (Cg4: C(4), C(5),C(6), C(7), C(8), C(9)) of L2 of adjacent molecules (C(11)– H(11c)  Cg4: H(11c)  Cg(4), 2.879(2) Å; C(11)  Cg4, 3.816(2) Å; \C(11)–H(11c)  Cg4, 165.79(3)°).Thus, a supramolecular 1D chain is propagated. 3.3. The spectral studies The infrared spectral bands of the complexes were assigned on comparing with the results of free ligand and the reported complexes [24,27]. Moderately intense stretching at 1560–1600 and 1340–1365 cm1 are due to m(C@N) and m(N = 347 N), respectively. A weak band at 260–280 cm1 348 is assigned to m(Pd–I). The absorption spectra of the complexes recorded in MeCN solution show three spectral transitions at 440–510, 375–385

view of [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) with atom labeling scheme (50%) probability thermal ellipsoid (isymmetry: 2  x, 2  y, 1  z).

C. Sen et al. / Polyhedron 85 (2015) 900–911

905

Fig. 2. 1D supramolecular chain constituted by C–H  I and C–H  p interactions of 10b.

and 365–370 nm, while free ligands show only two transitions at 360–380 and 270–300 nm. The molar absorption coefficient (e) in the UV region (<400 nm) is on the order of 104 M1 cm1 and are assigned to ligand centred (p–p⁄and n–p⁄) transitions. A tail extending 440–510 nm in the complexes may arise from the mixture of metal-to-ligand, dp(Pd) ? p⁄ (ligand) and p(I) ? p⁄ (ligand) charge transitions. The assignment is also supported by theoretical calculations (vide supra). The alkylation of imidazole is established by 1H NMR spectra. The atom numbering scheme used to assign 1H NMR spectra is given in Scheme 1. It is observed that d(N–H) at 10.30 ppm of Raai-H is disappeared and new signals of different spin–spin interaction pattern appear at 0.85–4.40 ppm – a quartet for –CH2– at 4.40 ppm, a triplet at 0.85 ppm for –CH3 group (in Raai-C2H5) and a multiplet for –(CH2)n– at 1.22–1.90 ppm for –N–CH2– (CH2)n-2–CH3. Imidazolyl 4- and 5-H appear as broad singlet at 7.24–7.28 and 7.14–7.17 ppm, respectively. Broadening may be due to rapid proton exchange between these imidazolyl protons. The aryl protons (7-H–11-H) are upfield shifted on going from C6H5–N@N– (a) to p-Me-C6H4–N@N– (b) which may be due to +I effect of –Me group. The 1H NMR spectra of two different class of compounds, [Pd(Raai-CnH2n+1)I2] (7,8) and, [Pd(Raai-CnH2n+1)2I] [Pd2I6] (9–14) are recorded in DMSO-d6solution and the signals are assigned unambiguously by spin–spin interaction, the effect of substitution therein. The protons assignment of [Pd(Raai-CnH2n+1)I2] (7,8) have been done on comparing with the free ligand data 0 and those of previously characterized [Pd(RaaiR )Cl2] complexes [57]. In [Pd(Raai-CnH2n+1)2I][Pd2I6] (9–14) two Raai-CnH2n+1 are 0 stereochemically different – one Raai-CnH2n+1 is bidentate N,N chelated and other Raai-CnH2n+1 is monodentate imidazolyl-N coordinated. However, the protons of two Raai-CnH2n+1 do not show significant difference in chemical shift data. The signals of imidazolyl protons (4, 5-H) in the spectra are shifted downfield compared to the spectra of free ligand, while aryl protons (7-H–13-H) do not move significantly. This has confirmed strong coordination of imidazolyl-N to Pd(II). 3.4. Photochromism The effect of light irradiation to the solution of the complexes has been examined by electronic spectral measurements. Upon irradiation of light at kmax to DMF solution of the complexes (9– 14), the change of absorption spectral characteristic is shown in Fig. 3. Intense peak at kmax decreases, which is accompanied by a slight increase at the tail portion of the spectrum around 525 nm until a stationary state is reached (Photostationary State I, PSS-I). Further irradiation at newly appeared longer wavelength peak reverses the course of the reaction slowly and the original spectrum is recovered up to a point, which is another Photostationary state (PSS-II) under irradiation at the longer wavelength peak [28]. The quantum yields of the trans-to-cis photoisomerisation are given in Table 3. It is observed that upon irradiation with UV

Fig. 3. Spectral changes of [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) in DMF upon repeated irradiation at 383 nm at 3 min interval at 25 °C. Inset figure shows the spectra of cis and trans isomers of [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b).

light trans-to-cis photoisomerisation proceeded and the cis molar ratio is reached to >80%. The absorption spectra of the trans-ligands have changed the isosbestic points upon excitation. The ligands and the complexes show little sign of degradation upon repeated irradiation at least up to 20 cycles in each case. The /t?c values are significantly dependent on the nature of substituents. The 1H NMR technique has been adopted to measure the percentage composition of the irradiated solution which supports the composition obtained from absorption spectra. Similar experiment to [Pd(RaaiCnH2n+1)I2] (7,8) does not show any significant change in absorption spectral characteristics. Thermal cis-to-trans isomerisation of the complexes was examined by UV–Vis spectroscopy in DMF at different temperatures, 298–313 K. The plots of ln(K/T) versus (1/T) in the range 298– 313 K gave a linear graph from which the activation energy was obtained Table 4 (Fig. 4). The Eas are lower in the complexes than that of free ligand values. Light irradiation may be associated with excitation to electronically high energy state which undergoes several molecular, electronic, vibrational and rotational changes followed by nonradiative deactivation. Mechanistic investigation by different spectroscopic routes [26–40,58,59] suggest the cleavage of M–N(azo) bond followed by bond rotation/flipping leading to isomerisation and formation of cis-isomer. In the complexes, [Pd(Raai-CnH2n+1)I2] (7,8) we do not observe photoisomerisation even after prolong exposure to UV light irradiation; however, [Pd(Raai-CnH2n+1)2I]2[Pd2I6] (9–14) show photoisomerisation. The structural difference is that Raai-CnH2n+1 in 7 and 8 is serving as 0 N,N chelating agent (diimine function, –N@N–C@N–) while in the complexes of 9–14 one of Raai-CnH2n+1 acts as monodentate

906

C. Sen et al. / Polyhedron 85 (2015) 900–911

Table 3 Results of photochromism, rate of conversion and quantum yields upon UV light irradiation to Raai-CnH2n+1 (in MeCN) and [Pd(Raai-CnH2n+1)2I][Pd2I6] (in DMF). Compounds  #

1a 1b# 2aW 2bW 3a§ 3b§ 4a§ 4b§ 5a§ 5b§ 6aD 6bD 9a 9b 10a 10b 11a 11b 12a 12b 13a 13b 14a 14b

kp;p (nm)

Isosbestic point (nm)

Rate of t ? c conversion  108 (s1)

/t?c

363 365 364 365 363 362 363 364 364 365 362 363 369 368 367 369 365 366 367 368 365 369 370 369

333, 331, 333, 328, 328, 327, 325, 323, 330, 324, 328, 326, 339, 332, 335, 337, 333, 335, 336, 335, 332, 337, 337, 335,

5.06 3.70 4.83 4.59 3.60 3.57 3.49 3.41 3.38 3.21 3.41 3.31 1.17 1.17 1.11 1.00 0.98 0.94 0.92 0.88 0.86 0.85 0.81 0.80

0.25 ± 0.03 0.218 ± 0.007 0.241 ± 0.012 0.206 ± 0.006 0.190 ± 0.004 0.187 ± 0.002 0.181 ± 0.003 0.179 ± 0.002 0.169 ± 0.001 0.165 ± 0.002 0.161 ± 0.001 0.179 ± 0.002 0.068 ± 0.002 0.064 ± 0.001 0.059 ± 0.001 0.054 ± 0.002 0.051 ± 0.001 0.049 ± 0.003 0.048 ± 0.002 0.047 ± 0.002 0.046 ± 0.001 0.045 ± 0.001 0.042 ± 0.003 0.041 ± 0.002

431 443 434 444 437 430 435 433 427 435 424 425 455 448 438 445 441 438 511 445 452 445 446 445

 

[Pd(Raai-CnH2n+1)I2] (7, 8) do not show any significant change in absorption spectral characterizations upon repeated irradiation at 360 nm. Ref. [24]. W Ref. [25]. § Ref. [58]. D Ref. [59]. #

N(imidazolyl) donor with a hanging –N@N–Ar group (Fig. 1). The chelate ring may be stable to light irradiation and does not cleave unlike the complexes of Zn(II), Cd(II), Hg(II) and Pb(II) [25–27,29–31]. Thus, dangling Ar–N@N– may be responsible to isomerisation upon excitation to light irradiation (Scheme 2). Increase in molar mass and volume reduces the rate of isomerisation which is reflected from the entropy of activation (DS⁄). The complexes show higher DS⁄ than that of free ligand data.

which capable charge transfer to p⁄ (azoimidazole) and may not sufficient to cleave Pd–N(azo) bonds (in case of 7 and 8) and hence, the isomerisation does not observed. Presence of monodentate Raai-CnH2n+1 in the complexes of 9–14 may perform isomerisation at excited state. In the excited state the photochrome may perform charge transition in a secondary (MLCT or XLCT) process which is responsible for deactivation of excited species and reduces the rate of isomerisation and quantum yields. This is observed, indeed (Table 3).

3.5. Electronic structure calculation and optical spectra 3.6. Transient absorption studies of [Pd(Haai-C10H1)2I][Pd2I6] (14a) The electronic structure calculation by DFT and TD-DFT method of the optimized structure, [Pd(Meaai-C2H5)2I]+ has been used to interpret the electronic spectroscopy and photophysical properties. The MOs carry four participating functions in different percentage contribution such as Pd, I, L1 (Meaai-C2H5 abbreviates L1 who acts as bidentate N(imidazolyl), N(azo) chelator) and L2 (Meaai-C2H5 abbreviates L2 who acts as monodentate N(imidazolyl) donor). In the occupied MOs, the contribution of palladium is significantly large in HOMO-3 (63%) and HOMO-7 (50%); iodide involves in the formation of HOMO-1 (79%), HOMO-2 (93%); chelated MeaaiC2H5 (L1) appears at HOMO (98%), HOMO-4 (88%) and HOMO-9 (92%); the nonchelated imidazolyl-N coordinated ligand L2 uses to form HOMO-6 (76%) and HOMO-8 (62%). Details of the composition of MOs are summarized in Supplementary material (Table S1). Contour plots of some of the MOs are given in Fig. 5. Two Meaai-C2H5 (L1 and L2) are associated with intraligand (ILCT), ligand-to-ligand (LLCT) charge transfer transitions along with I ? L1/L2(XLCT) and d(Pd) ? L1/L2 (MLCT) charge transitions. The calculation shows that significant intensity of transition may be due to the combination of different CT bands like ILCT, LLCT, MLCT and XLCT (Table 5). A theoretically calculated spectrum of 10b is compared with experimental spectrum in Fig. 6. The irradiation in the UV region may be responsible to p ? p⁄ transition. The MLCT or XLCTs are of lower energetic transition

The transient absorption spectral evolution of [Pd(Haai-C10H1)2I] [Pd2I6] (14a) in acetonitrile solution in 450–750 nm is shown in Fig. 7. The transient spectrum at 0.2 ps (i.e. at a time delay of 0.2 ps after the photoexcitation) shows negative absorbance below 500 nm due to bleach of ground-state absorption; and positive absorbance in 500–750 nm region due to Sn ? Sm transition (0 < n < m), known as excited state absorption (ESA). As the delay time (after photoexcitation) increases (e.g. from 0.2 to 2 ps), the absorbance in 525–750 nm region decreases and the negative absorbance (below 500 nm) increases. Since the complex is nonemissive, the increase of negative absorbance is assignable to the decrease of ESA in that region, which is overlapped with the ground-state bleach response. Comparison of these spectral evolutions that are primarily due to the azo-imidazole groups in [Pd(Haai-C10H1)2I][Pd2I6] (14a), with that of free azo-imidazole ligand (dynamics of free azo-imidazole were discussed in detail in our previous work [37] indicates that the azo-imidazole in 14a undergoes trans–cis isomerization on photoexcitation. Exponential fitting of the temporal profiles (as shown in Fig. 8 at selective wavelengths) provides a short time component (1 ± 0.1 ps) and a very long time component (a few tens of ps; not shown in Fig. 8). To get insight into the nature of the processes associated with the 1 ps component, we investigated the variation of the

907

C. Sen et al. / Polyhedron 85 (2015) 900–911 Table 4 Rate and activation parameters for cis ? trans thermal isomerisation of Raai-CnH2n+1 (in MeCN) and [Pd(Raai-CnH2n+1)2I][Pd2I6] (in DMF). Compounds 

Temp. (K)

Rate of thermal c ? t conversion  104 (s–1)

Ea (kJ mol1)

DH⁄ (kJ mol1)

1a#

298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298

0.22 0.4 0.88 2.75 0.73 1.20 2.60 3.70 0.33 0.5 0.97 1.80 0.64 0.98 1.87 3.40 3.62 5.10 10.5 19.0 6.01 9.02 18.04 32.01 4.20 6.20 13.20 23.50 6.09 11.21 19.01 37.00 4.40 7.60 13.20 27.30 7.11 10.21 21.01 42.00 22.92 38.98 54.96 88.67 23.45 39.76 56.34 89.79 2.01 3.22 3.82 4.45 2.32 2.99 3.88 4.89 2.15 2.75 3.55 4.31 2.99 3.86 4.92 5.88 2.82 3.43 4.22 5.57 2.89 3.67 4.66 5.59 2.73

79.0

77.05

77.1

87.57

85.03

38.84

96.60

86.87

84.33

47.83

98.58

87.63

87.08

40.20

97.06

88.20

85.66

43.27

98.9

88.47

85.93

37.96

97.53

91.73

89.19

30.09

98.36

92.11

89.57

25.12

98.38

93.38

90.85

23.83

98.13

93.66

91.12

19.45

97.07

71.67

69.48

81.94

92.14

70.19

70.13

83.47

90.12

37.25

39.79

134.07

80.75

36.20

38.74

137.12

80.63

33.80

36.34

145.76

80.87

32.72

35.26

146.57

80.04

32.29

34.84

148.76

80.28

31.88

34.42

149.71

80.15

31.19

33.74

152.81

1b#

2aW

2bW

3a§

3b§

4a§

4b§

5a§

5b§

6aD

6bD

9a

9b

10a

10b

11a

11b

12a

DS⁄ (J mol1 K1)

DG⁄c (kJ mol1) 100

80.42 (continued on next page)

908

C. Sen et al. / Polyhedron 85 (2015) 900–911

Table 4 (continued) Compounds 

303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313 298 303 308 313

12b

13a

13b

14a

14b

  #

W §

D

Temp. (K)

Rate of thermal c ? t conversion  104 (s–1) 3.21 3.99 4.25 3.55 4.71 5.43 6.59 3.49 4.46 5.41 6.21 3.28 3.97 4.87 5.68 3.44 4.13 5.01 5.97 3.21 3.88 4.58 5.17

Ea (kJ mol1)

DH⁄ (kJ mol1)

DS⁄ (J mol1 K1)

DG⁄c (kJ mol1)

28.49

31.03

159.22

79.67

27.32

29.86

163.34

79.76

26.19

28.73

167.80

79.99

26.10

28.64

167.74

79.83

22.24

24.78

181.16

80.13

[Pd(Raai-CnH2n+1)I2] (7,8) do not show any significant change in absorption spectral characterizations upon repeated irradiation at 360 nm. Ref. [24]. Ref. [25]. Ref. [58]. Ref. [59].

-13.1 -13.2

ln(K/T)

-13.3 -13.4 -13.5 -13.6 -13.7 -13.8 0.00320

0.00324

0.00328

0.00332

0.00336

-1

(1/T) K

Fig. 4. The Eyring plots of rate constants of cis-to-trans thermal isomerisation of [Pd(Haai-C10H21)2I]2[Pd2I6] (14a) in DMF at different temperatures.

life-time of that component in solvents of comparable polarities but of different viscosities (e.g. methanol (g = 0.55 cp; e = 32.7) and ethylene glycol (g = 17.3 cp; e = 37.7)). As shown in Fig. 9, in low viscous methanol, there is a single dominant relaxation component with a life time of 0.6 ps (similar to that in acetonitrile), whereas in high viscous ethylene glycol, there are two distinct decay components with life-times: 0.24 and 1.7 ps, respectively. The increase of the life-time (from 0.6 to 1.7 ps) from low viscous methanol to high viscous ethylene glycol suggests that the lifetime is associated with a torsional motion in 14a. It is likely that the torsional motion is associated with the rotation or inversion at the one of the Ns of the azo group, which results in trans–cis isomerization. Accordingly, the component with a life-time of 0.6 ps (in methanol) or 1.7 ps (in ethylene glycol) is assigned to the trans–cis isomerization of azo-imidazole group in 14a. The first

component in ethylene glycol (0.24 ps) is most likely arisen from the relaxation of higher excited electronic state (S2 state) to the S1 state, since 400 nm photoexcitation (used as pump for transient study) populates the higher excited state of 14a. In low viscous acetonitrile and methanol, the rate of isomerization (faster than that in ethylene glycol) is comparable to the rate of relaxation from the higher excited state. As a result, a single decay component has been observed in acetonitrile and methanol. Thus, the azo-imidazole in 14a, undergoes trans–cis isomerization on photoexcitation, and the time constant of the isomerization process is comparable to that of free azo-imidazole ligand [37]. Nevertheless, unlike in the free ligand, the ESA of 14a does not decay completely with in the first few tens of picosecond. In fact, there is no significant spectral evolution beyond 2 ps delay time. This is quite obvious, since the 2 ps spectrum is quite similar to that at longer delay time (e.g. 20 ps spectrum; Fig. 7). This result suggests that, even after 20 ps of photoexcitation, there is a significant population of excited azo-imidazole which does not undergo isomerization. In our previous work, it has been observed that bidentate azo-imidazole (Cu(II)-complex) does not undergo trans–cis isomerization on

Scheme 2. Proposed mechanism of E-to Z (trans-to-cis) photoisomerisation and vice versa.

909

C. Sen et al. / Polyhedron 85 (2015) 900–911

HOMO-3,E=-

HOMO-2,E=

-5.71 HOMO-1,E=

6.07eV;Pd,63%;I,07%;

eV;Pd,05%;I,93%

L1,09%;L2,21%

-5.58 HOMO,E=

eV;

-4.38

eV;L1,98%

Pd,15%;I,79%;L1,04 %

LUMO, E=-3.13 eV;

LUMO+1,E= -2.6 eV; LUMO+2,

L2,98%

L1,100%

E=-2.04 LUMO+3,E= -1.96

eV;Pd,25%;I,11%;L1

eV;

,60%;L2,04%

Pd,24%;I,11%;L1,61 %;L2,04%

Fig. 5. Contour plots for some selected molecular orbital of [PdI(L1)(L2)]+ (where L1 refers to chelated Meaai-C2H5, L2 represents nonchelated imidazolyl-N coordinated Meaai-C2H5).

Table 5 TD-DFT data of [PdI(L1)(L2)]+ (where L1 refers to chelated Meaai-C2H5; L2 refers to nonchelated imidazolyl-N coordinated Meaai-C2H5).  Excitation energy (eV)

Wavelength (nm)

f

Key transitions

Experimental transitions (nm)

Character

2.3455

528.61

0.7385

450

2.9990

413.41

0.0407

3.3471

370.42

0.1464

4.0010

309.88

0.1137

(42%) HOMO ? LUMO+3 (18%) HOMO ? LUMO+2 (16%) HOMO-1 ? LUMO+1 (43%) HOMO-2 ? LUMO+1 (17%) HOMO-4 ? LUMO (03%) HOMO-1 ? LUMO+1 (55%) HOMO-6 ? LUMO (05%) HOMO-6 ? LUMO+1 (44%) HOMO-7 ? LUMO+3 (3%) HOMO-12 ? LUMO+2 (26%) HOMO-7 ? LUMO+2

LLCT LLCT XLCT XLCT ILCT XLCT LLCT ILCT MLCT, LLCT LXCT MLCT

382

369 320

 

Transitions are LLCT (ligand-to-ligand CT) (pNN0 ) ? p⁄(NN0 ); ILCT (intraligand CT) (p(NN0 /N) ? p⁄(NN0 )); XLCT (halogen-to-ligand CT) (p(I) ? p⁄(NN0 )); MLCT (metal-toligand CT) dp(Pd) ? p⁄(NN0 ); MXCT (metal-to-halogen CT) (dp(Pd) ? p⁄(I)); LXCT (ligand-to-halogen) (p(NN0 ) ? p⁄(I)).

photoexcitation [37]. Thus, the transient spectral evolution of 14a reveals that, along with the presence of a isomerization component, there is another component which does not undergo isomerization on photoexcitation. As discussed in the X-ray crystallographic results of 14a, out of the two azo-imidazole ligands, one is bound to Pd(II) as a monodentate ligand and the other one as a bidentate ligand. We assign the transient spectral response

beyond 2 ps, which does not evolve appreciably even in the next 20 ps in acetonitrile, to the azo-imidazole group that is in chelate-form with Pd(II). Thus, the present transient absorption results revealed that the monodentate azo-imidazole in 14a can undergo trans–cis isomerization whereas, the bidentate azo-imidazole cannot undergo trans–cis isomerization, since the latter requires the breakage of one of the coordination to Pd(II).

910

C. Sen et al. / Polyhedron 85 (2015) 900–911 4

6x10

4

5x10

Experimental Theoretical

-1

-1

ε(dm mol cm )

4

4x10

4

3

3x10

4

2x10

4

1x10

0 300

400

500

600

700

800

900

Wavelength(nm) Fig. 6. Theoretically calculated spectrum of [Pd(Meaai-C2H5)2I]2[Pd2I6] (10b) is compared with experimental spectrum.

Fig. 9. Exponential fitting of the transient profiles of [Pd(Haai-C10H21)2I][Pd2I6] (14a) in methanol (blue) and ethylene glycol (red) solution at 620 nm. (Color online.)

4. Conclusion

Fig. 7. Transient absorption spectral evolution of [Pd(Haai-C10H21)2I][Pd2I6] (14a) in acetonitrile solution on 400 nm photoexcitation at different delay times (as mentioned in the graph panel).

1-Alkyl-2-(arylazo)imidazole complexes of palladium(II), [Pd(Raai-CnH2n+1)I2] (n = 2, 8)and [Pd(Raai-CnH2n+1)2I)]2[Pd2I6] (Raai-CnH2n+1: R = H, Me; n = 1, 2, 4, 6, 8, 10) are characterized by spectroscopic data. The structure elucidation has been confirmed by single crystal X-ray diffraction study. Photochromism of free ligand and the complexes are examined for trans-to-cis isomerisation under UV radiation. Transient absorption studies revealed that the monodentate azo-imidazole group in [Pd(Haai-C10H21)2I] [Pd2I6] undergoes trans–cis isomerization with a time constant comparable to that of the free ligand (e.g. 0.6 ps in methanol). In contrast, the bidentate azo-imidazole group in [Pd(Haai-CH3)2I] [Pd2I6] does not undergo trans–cis isomerization, since the latter requires the breakage of one of the coordination to Pd(II). The cis-to-trans isomerisation is thermally driven process. The activation energy (Eas) of the isomerisation reaction of the complexes is about to one third of the free ligand. The slow rate of isomerisation of the complexes may be due to higher rotor mass and volume than that of free ligands. Besides, p(I) ? p⁄(Raai-CnH2n+1) transitions may snatch out energy and reduce the rate of the photoisomerisation. DFT computation also supports the origin of spectra and photochromism. Acknowledgments Financial support from the Department of Science & Technology, West Bengal, Kolkata (228/1(10)/(Sanc.)/ST/P/S&T/9G-16/ 2012) and the Council of Scientific and Industrial Research (CSIR) (Sanction No. 01(2731)/13/EMR-II) New Delhi are gratefully acknowledged. One of us (Chandana Sen) thanks to the University Grant Commission (UGC) New Delhi for fellowship. Appendix A. Supplementary data

Fig. 8. Exponential fitting of the transient profiles of [Pd(Haai-C10H21)2I][Pd2I6] (14a) in acetonitrile solution at 620 and 460 nm.

CCDC 960692 contains the supplementary crystallographic data for compound [Pd(Meaai-C2H5)2I)]2[Pd2I6]. 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) 1223-336033; or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.poly.2014.09.033.

C. Sen et al. / Polyhedron 85 (2015) 900–911

References [1] A.R. Katritzky, C.W. Rees, A.J. Boulton, A. Mckillop (Eds.), Comprehensive Heterocyclic Chemistry, 33, 1984, 57. [2] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, fifth ed., John Wiley & Sons, New York, 1988. 489. [3] C.K. Pal, S. Chattopadhyay, C. Sinha, D. Bandyopadhyay, A. Chakravorty, Polyhedron 13 (1994) 999. [4] P.K. Santra, P. Byabrata, S. Chattopadhyay, C. Sinha, L.R. Falvello, Eur. J. Inorg. Chem. (2002) 1124. [5] T. Mathur, J. Dinda, P. Datta, G. Mostafa, T.-H. Lu, C. Sinha, Polyhedron 25 (2006) 2503. [6] P. Datta, D. Sardar, A.P. Mukhopadhyay, E. Lo9 pez-Torres, C.J. Pastor, C. Sinha, J. Organomet. Chem. 696 (2011) 488. [7] D. Sardar, P. Datta, R. Saha, P. Raghavaiah, C. Sinha, J. Organomet. Chem. 732 (2013) 109. [8] H. Durr, H. Bouas-Laurent (Eds.), Photochromism: Molecules and Systems, Elsevier, Amsterdam, 2003. [9] H.M.D. Bandarab, S.C. Burdette, Chem. Soc. Rev. 41 (2012) 809. [10] T. Schultz, J. Quenneville, B. Levine, A. Toniolo, T.J. Martínez, S. Lochbrunner, M. Schmitt, J.P. Shaffer, M.Z. Zgierski, A. Stolow, J. Am. Chem. Soc. 125 (2003) 8098. [11] M. Irie (Ed.), Photo-Reactive Materials for Ultrahigh Density Optical Memory, Elsevier, Amsterdam, 1994. [12] M. Irie, Chem. Rev. 100 (2000) 1683. [13] B. Feringa (Ed.), Molecular switches, Wiley-VCH, 2001. [14] V. Balzani, A. Credi, M. Venturi, Coord. Chem. Rev. 171 (1998) 3. [15] K. Ichimura, Chem. Rev. 100 (2000) 1847. [16] S. Spörlein, H. Carstens, H. Satzger, C. Renner, R. Behrendt, L. Moroder, T. Tavan, W. Zinth, J. Wachtveitl, Proc. Natl. Acad. Sci. U.S.A. 99 (2002) 7998–8002. [17] A.A. Beharry, G.A. Woolley, Chem. Soc. Rev. 40 (2011) 4422. [18] T.K. Misra, D. Das, C. Sinha, P.K. Ghosh, C.K. Pal, Inorg. Chem. 37 (1998) 1672. [19] R. Roy, P. Chattopadhyay, C. Sinha, S. Chattopadhyay, Polyhedron 15 (1996) 3361. [20] D. Das, A.K. Das, C. Sinha, Talanta 48 (1999) 1013. [21] P.K. Santra, T.K. Misra, D. Das, C. Sinha, A.M.Z. Slawin, J.D. Woollins, Polyhedron 18 (1999) 2869. [22] P.K. Santra, P. Byabartta, S. Chattopadhyay, L.R. Falvello, C. Sinha, Eur. J. Inorg. Chem. (2002) 1124. [23] M. Endo, K. Nakayama, Y. Kaida, T. Majima, Tetrahedron Lett. 44 (2003) 6903. [24] J. Otsuki, K. Suwa, K. Narutaki, C. Sinha, I. Yoshikawa, K. Araki, J. Phy, Chem. A 109 (2005) 8064. [25] K.K. Sarker, B.G. Chand, J. Cheng, T.-H. Lu, C. Sinha, Inorg. Chem. 46 (2007) 670. [26] K.K. Sarker, D. Sardar, K. Suwa, J. Otsuki, C. Sinha, Inorg. Chem. 46 (2007) 8291. [27] S. Saha (Halder), B.G. Chand, J. -S. Wu, T. -H. Lu, P. Raghavaiah, C. Sinha, Polyhedron 46 (2012) 81. [28] P. Pratihar, T.K. Mondal, A.K. Patra, C. Sinha, Inorg. Chem. 48 (2009) 2769. [29] K.K. Sarker, S. SahaHalder, D. Banerjee, T.K. Mondal, A.R. Paital, P.K. Nanda, P. Raghavaiah, C. Sinha, Inorg. Chim. Acta 363 (2010) 2955. 539. [30] D. Mallick, K.K. Sarker, P. Datta, T.K. Mondal, C. Sinha, Inorg. Chim. Acta 387 (2012) 352. [31] D. Mallick, K.K. Sarker, R. Saha, T.K. Mondal, C. Sinha, Polyhedron 54 (2013) 147.

[32] [33] [34] [35] [36] [37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48]

[49] [50] [51] [52] [53] [54] [55] [56] [57] [58] [59]

911

P. Gayen, C. Sinha, J. Lumin. 132 (2012) 2371. P. Gayen, C. Sinha, Spectrochim. Acta, Part A 98 (2012) 116–121. 544. P. Gayen, C. Sinha, Spectrochim. Acta, Part A 104 (2013) 477–485. 545. P. Gayen, K.K. Sarker, C. Sinha, Colloids Surf., A 429 (2013) 60. P. Gayen, C. Sinha, J. Indian Chem. Soc. 90 (2013) 751. J.A. Mondal, G. Saha, C. Sinha, D.K. Palit, Phys. Chem. Chem. Phys. 14 (2012) 13027. G.K. Rauth, A. Mahapatra, C. Sinha, Inorg. React. Mech. 4 (2002) 57. J. Dinda, P.K. Santra, C. Sinha, L.R. Falvello, J. Organomet. Chem. 629 (2001) 28. P. Pratihar, T.K. Mondal, P. Raghavaiah, C. Sinha, Inorg. Chim. Acta 363 (2010) 831. G.M. Sheldrick, SHELXL 97, Program for Crystal Structure Refinement, University of Göttingen, Germany, 1997. G.M. Sheldrick, SHELXS 97, Program for the Solution of Crystal Structure, University of Gottingen, Germany, 1997. A.L. Spek, PLATON, Molecular Geometry Program, University of Utrecht, The Netherlands, 1999. L.J. Farrugia, ORTEP-3 for window, J. Appl. Crystallogr. 30 (1997) 565. G. Zimmerman, L. Chow, U. Paik, J. Am. Chem. Soc. 80 (1958) 3528. C. Singh, B. Modak, J.A. Mondal, D.K. Palit, J. Phys. Chem. A 115 (2011) 8183. C. Lee, W. Yang, R.G. Parr, Phys. Rev. B 37 (1988) 785. GAUSSIAN 03, Revision D.01, M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, J. A. Jr. Montgomery, T. Vreven, K. N. Kudin, J. C. Burant, J. M. Millam, S. S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G. A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J. E. Knox, H. P. Hratchian, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, P. Y. Ayala, K. Morokuma, G. A. Voth, P. Salvador, J. J. Dannenberg, V. G. Zakrzewski, S. Dapprich, A. D. Daniels, M. C. Strain, O. Farkas, D. K. Malick, A. D. Rabuck, K. Raghavachari, J. B. Foresman, J. V. Ortiz, Q. Cui, A. G. Baboul, S. Clifford, J. Cioslowski, B. B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R. L. Martin, D. J. Fox, T. Keith, M. A. AlLaham, C. Y. Peng, A. Nanayakkara, M. Challacombe, P. M. W. Gill, B. Johnson, W. Chen, M. W. Wong, C. Gonzalez, and J. A. Pople, Gaussian Inc, Wallingford CT, 2004. GaussView3.0, Gaussian: Pittsburgh, PA. P.J. Hay, W.R. Wadt, J. Chem. Phys. 82 (1985) 270. P. Fuentealba, H. Preuss, H. Stoll, L.V. Szentpaly, Chem. Phys. Lett. 89 (1989) 418. R. Bauernschmitt, R. Ahlrichs, Chem. Phys. Lett. 256 (1996) 454. M.E. Casida, C. Jamorski, K.C. Casida, D.R. Salahub, J. Chem. Phys. 108 (1998) 4439. R.E. Stratmann, G.E. Scuseria, M.J. Frisch, J. Chem. Phys. 109 (1998) 8218. M. Cossi, N. Rega, G. Scalmani, V. Barone, J. Comput. Chem. 24 (2003) 669. N.M. O0 Boyle, A.L. Tenderholt, K.M. Langner, J. Comput. Chem. 29 (2008) 839. G.K. Rauth, S. Pal, D. Das, C. Sinha, Transition Met. Chem. 26 (2001) 679. D. Mallick, A. Nandi, S. Datta, K.K. Sarker, T.K. Mondal, C. Sinha, Polyhedron 31 (2012) 506. A. Nandi, C. Sen, D. Mallick, R.K. Sinha, C. Sinha, Adv. Mater. Phys. Chem. 3 (2013) 133.