Synthesis, structure and spectroscopic properties of some thiosemicarbazone complexes of platinum

Polyhedron 26 (2007) 2741–2748 www.elsevier.com/locate/poly

Synthesis, structure and spectroscopic properties of some thiosemicarbazone complexes of platinum Sarmistha Halder a, Ray J. Butcher b, Samaresh Bhattacharya a

a,*

Department of Chemistry, Inorganic Chemistry Section, Jadavpur University, Kolkata 700 032, India b Department of Chemistry, Howard University, Washington DC 20059, USA Received 8 December 2006; accepted 16 January 2007 Available online 9 February 2007

Abstract Reaction of salicyldehyde thiosemicarbazone (H2L1), 2-hydroxyacetophenone thiosemicarbazone (H2L2) and 2-hydroxynapthaldehyde thiosemicarbazone (H2L3) (general abbreviation H2L, where H2 stands for the two dissociable protons, one phenolic proton and one hydrazinic proton) with K2[PtCl4] afforded a family of polymeric complexes of type [{Pt(L)}n]. Reaction of the polymeric species with two monodentate ligands (D), viz. triphenylphosphine (PPh3) and 4-picoline (pic), yielded complexes of the type [Pt(L)(D)]. These mixedligand complexes were also obtained from the reaction of the thiosemicarbazones with [Pt(PPh3)2Cl2] and [Pt(pic)2Cl2]. The crystal structure of [Pt(PPh3)(L2)] has been determined. The thiosemicarbazone ligands are coordinated, via dissociation of the two protons, as dianionic tridentate O,N,S-donors. The [Pt(L)(D)] complexes show characteristic 1H NMR spectra and intense absorptions in the visible and ultraviolet region. They also fluoresce in the visible region at ambient temperature.  2007 Elsevier Ltd. All rights reserved. Keywords: Thiosemicarbazones; Platinum complexes; Crystal structure; Spectral properties

1. Introduction The chemistry of transition metal complexes of thiosemicarbazones has been receiving considerable attention, largely because of their bioinorganic relevance [1]. Such complexes are of particular importance due to their potentially beneficial biological (viz. antibacterial, antimalarial, antiviral and antitumor) activities [2]. Thiosemicarbazones usually bind to a metal ion, via dissociation of the hydrazinic proton, as a bidentate N,S-donor forming a five-membered chelate ring (1) [3]. If a third donor site (D) is incorporated in such ligands, linked to the carbonylic carbon via one or two intervening atoms, then D,N,S-tricoordination (2) normally takes place [4]. In addition to displaying D,N,S-tricoordination, such ligands are also known to bridge a second metal ion through the sulfur *

Corresponding author. Tel.: +91 33 2414 6223; fax: +91 33 2414 6584. E-mail address: [email protected] (S. Bhattacharya).

0277-5387/$ - see front matter  2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2007.01.048

[5]. It has also been observed that salicylaldehyde thiosemicarbazone, in spite of having the phenolic oxygen as a potential third donor site, coordinates as a bidentate N,Sdonor forming a rather unusual four-membered chelate ring (3) [6]. This mode of binding (3) has also been displayed by some other thiosemicarbazones [7]. Formation of such a chelate ring (3) by salicylaldehyde thiosemicarbazone, leaving some potential donor sites unused, has been successfully utilized for the synthesis of interesting polynuclear complexes [8]. This variable binding mode of thiosemicarbazones has encouraged us to explore their coordination chemistry further [6,8,9], and the present work has originated from this exploration. Herein we have chosen three potentially tridentate thiosemicarbazones, viz. thiosemicarbazones of salicylaldehyde, 2-hydroxyacetophenone and 2-hydroxynaphthaldehyde. These thiosemicarbazones are abbreviated in general as H2L, where H2 stands for the two dissociable protons, the phenolic proton and the hydrazinic proton. Individual ligand abbreviations

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S. Halder et al. / Polyhedron 26 (2007) 2741–2748

are shown in 4. To interact with the chosen thiosemicarbazones platinum has been selected as the metal. It may be mentioned here that although the chemistry of platinum complexes of some thiosemicarbazones has been studied [1,10], that of the chosen thiosemicarbazones (4) appears to have remained unexplored. Reaction of the three selected thiosemicarbazones (4) has been carried out with three platinum starting materials, viz. K2[PtCl4], [Pt(PPh3)2Cl2] and [Pt(pic)2Cl2] (pic = 4-picoline), which has afforded a family of interesting complexes containing the thiosemicarbazones coordinated in the tridentate fashion (5). The chemistry of these complexes is reported in this paper with special reference to their synthesis, structure and spectral properties. D M N

OH

M S

N

N

S

N

N

H

NH2

NH2 N

NH2

1

OH

OH N

N

NH2 H

N H

NH2 N

H

S 1

H2L (R = H)

S

H 2L 3

2

H2L (R = CH3) 4

O

O Pt

N R

Pt S

N

N

H NH2

2.2.1. [{Pt(L1)}n] To a solution of H2L1 (47 mg, 0.24 mmol) in hot methanol (30 mL) triethylamine (48 mg, 0.48 mmol) was added followed by K2[PtCl4] (100 mg, 0.24 mmol). The mixture was heated at reflux for 24 h. A brown precipitate settled down on cooling, which was collected by filtration, washed with methanol and dried in air to afford [{Pt(L1)}n] as a brown powder. Yield: 67 mg. Anal. Calc. for (C8H7N3SOPt)n: C, 24.67; H, 1.80; N, 10.79. Found: C, 24.78; H, 1.93; N, 10.82%. The [{Pt(L2)}n] and [{Pt(L3)}n] complexes were prepared by following the same above procedure using HL2 and HL3 instead of HL1. [{Pt(L2)}n]: Anal. Calc. for (C9H9N3SOPt)n: C, 26.87; H, 2.24; N, 10.45. Found: C, 27.38; H, 2.32; N, 10.49%. [{Pt(L3)}n]: Anal. Calc. for (C12H9N3SOPt)n: C, 32.88; H, 2.06; N, 9.59. Found: C, 32.14; H, 2.14; N, 9.63%.

S M 3

2

R

2.2. Synthesis

S N NH2

R = H, CH3 5

2. Experimental 2.1. Materials Chloroplatinic acid was purchased from Arora Matthey, Kolkata, India, and was converted to K2[PtCl4] [11]. Salicylaldehyde, 2-hydroxyacetophenone and 2-hydroxynaphthaldehyde were obtained from S.D. Fine-chem, Mumbai, India. Thiosemicarbazide was procured from Loba Chemie, Mumbai, India. All other chemicals and solvents were reagent grade commercial materials and were used as received. [Pt(PPh3)2Cl2] and [Pt(pic)2Cl2] were prepared from K2[PtCl4] by following reported procedures [12,13]. The thiosemicarbazone ligands (H2L1, H2L2 and H2L3) were prepared by condensing the respective aldehyde or ketone with thiosemicarbazide in hot ethanol.

2.2.2. [Pt(PPh3)(L1)] Method A: The polymeric [{Pt(L1)}n] species (100 mg) and triphenylphosphine (68 mg) were taken together in ethanol (30 mL) and the solution was refluxed for 24 h to yield a lemon yellow solution. Evaporation of this solution gave a yellow solid, which was subjected to purification by thin layer chromatography on a silica plate. With 1:40 acetonitrile–benzene as the eluant, a major yellow band separated, which was extracted with acetonitrile. Upon evaporation of the acetonitrile extract [Pt(PPh3)(L1)] was obtained as a crystalline yellow solid. Yield: 95 mg (57%). Method B: To a solution of H2L1 (20 mg, 0.10 mmol) in hot ethanol (30 mL) triethylamine (20 mg, 0.20 mmol) was added followed by cis-[Pt(PPh3)2Cl2] (100 mg, 0.13 mmol). The mixture was heated at reflux for 6 h to yield a lemon yellow solution. Evaporation of this solution gave a yellow solid, which was subjected to purification by thin layer chromatography on a silica plate. With 1:40 acetonitrile– benzene as the eluant, a major yellow band separated, which was extracted with acetonitrile. Upon evaporation of the acetonitrile extract [Pt(PPh3)(L1)] was obtained as a crystalline yellow solid. Yield: 49 mg (60%). Anal. Calc. for C26H22N3SOPt: C, 47.92; H, 3.38; N, 6.45. Found: C, 47.88; H, 3.42; N, 6.49%. 1H NMR in CDCl3, d ppm: 4.87 (s, 2H), 6.86 (t, 1H, J = 7.4 Hz), 6.75 (d, 1H, J = 8.5 Hz), 7.20–7.32* (2H), 7.39–7.77* (PPh3), 8.55 (d, 1H, J = 11.8 Hz). The [Pt(PPh3)(L2)] and [Pt(PPh3)(L3)] complexes were prepared by following the same above procedure using H2L2 and H2L3 instead of H2L1. [Pt(PPh3)(L2)]: Anal. Calc. for C27H24N3SOPt: C, 48.71; H, 3.61; N, 6.31. Found: C, 48.74; H, 3.66; N, 6.33%. 1H NMR in CDCl3, d ppm: 2.86 (s, 3H), 4.84 (s, 2H), 6.67–6.72* (2H), 7.17 (t, 1H, J = 7.5 Hz), 7.40–7.80 (PPh3 + 1H). [Pt(PPh3)(L3)]: Anal. Calc. for C27H24N3SOPt: C, 51.20; H, 3.41; N, 5.97. Found: C, 51.23; H, 3.88; N,

S. Halder et al. / Polyhedron 26 (2007) 2741–2748

6.92%. 1H NMR in CDCl3, d ppm: 4.87 (s, 2H), 6.86 (d, 1H, J = 9.1 Hz), 7.26 (t, 1H, J = 7.4 Hz), 7.61 (d, 1H, J = 9.2 Hz), 7.67 (d, 1H, J = 7.9 Hz), 7.42–7-.78* (PPh3 + 1H), 8.17 (d, 1H, J = 8.6 Hz), 9.57 (d, 1H, J = 11.8 Hz). 1

2.2.3. [Pt(pic)(L )] Method A: The polymeric [{Pt(L1)}n] species (100 mg) and 4-picoline (24 mg) were taken in ethanol (30 mL) and the solution was refluxed for 24 h to yield a yellow solution. Evaporation of this solution gave a yellow solid, which was subjected to purification by thin layer chromatography on a silica plate. With 1:20 acetonitrile–benzene as the eluant, a prominent yellow band separated, which was extracted with acetonitrile. Upon evaporation of the acetonitrile extract [Pt(pic)(L1)] was obtained as a crystalline yellow solid. Yield: 70 mg (56%). Method B: To a solution of H2L1 (44 mg, 0.22 mmol) in hot ethanol (30 mL) triethylamine (45 mg, 0.44 mmol) was added followed by [Pt(pic)2Cl2] (100 mg, 0.22 mmol). The mixture was heated at reflux for 24 h to yield a yellow solution. Evaporation of this solution gave a yellow solid, which was subjected to purification by thin layer chromatography on a silica plate. With 1:20 acetonitrile–benzene as the eluant, a major yellow band separated, which was extracted with acetonitrile. Upon evaporation of the acetonitrile extract [Pt(pic)(L1)] was obtained as a crystalline yellow solid. Yield: 54 mg (50%). Anal. Calc. for C14H14N4SOPt: C, 34.63; H, 2.89; N, 11.54. Found: C, 34.45; H, 2.74; N, 11.34%. 1H NMR in CDCl3, d ppm: 2.45 (s, 3H), 5.05 (s, 2H), 6.70–6.80* (2H), 7.13 (d, 1H, J = 6.2 Hz), 7.33–7.46* (3H), 8.30 (s, 1H), 8.81 (d, 2H, J = 6.2 Hz). The [Pt(pic)(L2)] and [Pt(pic)(L3)] complexes were prepared by following the same above procedure using H2L2 and H2L3 instead of H2L1. [Pt(pic)(L2)]: Anal. Calc. for C15H16N4SOPt: C, 36.36; H, 3.23; N, 11.32. Found: C, 36.54; H, 3.46; N, 11.13%. 1H NMR in CDCl3, d ppm: 2.43 (s, 3H), 2.81 (s, 3H), 4.93 (s, 2H), 6.74 (t, 1H, J = 7.5 Hz), 7.14 (d, 1H, J = 8.3 Hz), 7.22 (d, 2H, J = 5.6 Hz), 7.30 (t, 1H, J = 8.9 Hz), 7.81 (d, 1H, J = 8.3 Hz), 8.78 (d, 2H, J = 9.0 Hz). [Pt(pic)(L3)]: Anal. Calc. for C18H16N4SOPt: C, 40.67; H, 3.01; N, 10.54. Found: C, 40.43; H, 3.20; N, 10.32%. 1H NMR in CDCl3, d ppm: 2.47 (s, 3H), 4.94 (s, 2H), 7.24– 7.33* (4H), 7.53 (t, 1H, J = 7.5 Hz), 7.71–7.80* (2H), 8.14 (d, 1H, J = 8.5 Hz), 8.86 (d, 2H, J = 6.1 Hz), 9.30 (s, 1H).

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CDCl3 solutions on a Bruker Avance DPX 300 NMR spectrometer using TMS as the internal standard. 2.4. X-ray crystallography Single crystals of [Pt(PPh3)(L2)] were grown by slow evaporation of a solution of the complex in 1:1 dichloromethane–acetonitrile. Selected Crystal data and a summary of the data collection appear below, the full details are provided in Table S1. Data were collected on a Bruker SMART CCD diffractometer using graphite-monochro˚ ) by / and x scans. mated Mo Ka radiation (k = 0.71073 A X-ray data reduction, structure solution and refinement were done using the SHELXS-97 and SHELXL-97 packages [14]. The structure was solved by direct methods. Crystal data for C27H24N3OPSPt, M = 664.61, 0.55 · 0.35 · 0.12 mm3, monoclinic, space group P21/c, ˚ , b = 9.0719(6) A ˚ , c = 17.1657(13) A ˚, a = 15.7537(12) A 3 ˚ b = 94.61(10), V = 2445.3(3) A , Z = 4, Dcalc = 1.805 ˚ , T = 103 K, l = Mg m 3, F(000) = 1296, k = 0.71073 A 1 5.915 mm , 17 316 reflections collected, 5901 unique (Rint = 0.049). Final goodness-of-fit = 1.03, R1 = 0.0273, wR2 = 0.0625, R indices based on 4607 reflections with [I > 2r(I)].

3. Results and discussion 3.1. Syntheses and crystal structure The reaction of the selected thiosemicarbazones (H2L, 4) has been first carried out with K2[PtCl4] in refluxing methanol in the presence of triethylamine to afford a family of polymeric complexes of the type {Pt(L)}n in decent yields. Based on the composition of these complexes, as well as the +2 oxidation state of platinum in them, the thiosemicarbazones are believed to coordinate the metal center in the expected dianionic O,N,S-fashion (5). The fourth coordination position on platinum in the Pt(L) fragment is assumed to be taken up by the sulfur of a coordinated thiosemicarbazone belonging to another Pt(L) fragment (6). This bridging action of the thiosemicarbazone–sulfur, which is well documented in the literature [5], appears to be responsible for the formation of the polymeric species.

O Pt

2.3. Physical measurements

N R

Microanalyses (C, H, N) were performed using a Heraeus Carlo Erba 1108 elemental analyzer. IR spectra were obtained on a Shimadzu FTIR-8300 spectrometer with samples prepared as KBr pellets. Electronic spectra were recorded on a JASCO V-570 spectrophotometer. Emission spectra were recorded on a Perkin–Elmer LS55 Luminescence Spectrometer. 1H NMR spectra were recorded in

S N NH2

n

6

In order to explore the possibility of forming monomeric complexes by splitting the sulfur bridge in the {Pt(L)}n complexes, their reaction has been carried out with two

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S. Halder et al. / Polyhedron 26 (2007) 2741–2748

monodentate ligands (D), viz. 4-picoline (pic) and triphenylphosphine (PPh3). From each of these reactions a monomeric complex of type [Pt(L)(D)] (D = pic or PPh3) has been obtained. The same monomeric complexes have also been synthesized, in better yields, by the reaction of the thiosemicarbazones (4) with [Pt(PPh3)2Cl2] and [Pt(pic)2Cl2] in refluxing ethanol in the presence of triethylamine. The synthetic methods of all the complexes are illustrated in Scheme 1. The observed elemental (C, H, N) analytical data of all the complexes are consistent with their compositions. It appears from the formulation of the monomeric complexes that the thiosemicarbazones are serving as tridentate ligands in them. In order to authenticate the coordination mode of the thiosemicarbazones in these complexes, the structure of a representative member of this family of monomeric complexes, viz. [Pt(PPh3)(L2)], has been determined by X-ray crystallography. A selected view of the complex molecule is shown in Fig. 1 and some relevant bond parameters are listed in Table 1. The structure shows that the thiosemicarbazone ligand (L2) is coordinated to platinum in the expected tridentate fashion (5), forming a six- and a five-membered chelate ring with O–Pt–N and N–Pt–S bite angles of 92.48(12) and 85.09(9), respectively. A triphenylphosphine is coordinated to the metal center, which is trans to the nitrogen atom. Platinum is thus nested in a NOSP core, which is slightly distorted from an ideal square-planar geometry, as reflected in the bond parameters around the metal center. The Pt–N, Pt–O, Pt–P and Pt–S distances PPh3

O

[Pt(PPh3)2Cl2]

Pt

ethanol, reflux

N

S N

H

NH2 PPh3, ethanol reflux

OH N H

Pt–N(1) Pt–P Pt–S Pt–O Bond angles () N(1)–Pt–P O–Pt–S

2.038(3) 2.2598(11) 2.2575(10) 2.006(3)

175.57(9) 175.71(8)

C(7)–N(1) C(9)–N(3) C(1)–O N(1)–N(2) C(9)–S

1.320(5) 1.386(5) 1.325(5) 1.403(5) 1.742(4)

N(1)–Pt–O N(1)–Pt–S

92.48(12) 85.09(9)

are normal, as observed in other structurally characterized complexes of platinum containing these bonds [10,15]. Bond distances within the coordinated thiosemicarbazone are also usual [9c]. The absence of any solvent of crystallization in the crystal lattice indicates possible existence of non-covalent interaction(s) between the individual complex molecules. A closer look at the packing pattern of the crystal reveals that non-covalent interactions of two types, viz. C–H


S and N–H


p interactions, are active in the lattice,

Pt

MeOH

N

S N

H

N

H

Table 1 ˚ ) and angles () for [Pt(PPh3)(L2)] Selected bond distances (A ˚ Bond distances (A)

O

K2[PtCl4] NH2

Fig. 1. Structure of the [Pt(PPh3)(L2)] complex.

NH2

S

n

N ethanol reflux CH3 CH3 N

O

[Pt(pic)2Cl2]

Pt

ethanol, reflux

S

N H

N NH2

Scheme 1.

Fig. 2. Packing pattern in the lattice of the [Pt(PPh3)(L2)] complex.

S. Halder et al. / Polyhedron 26 (2007) 2741–2748

which is illustrated in Fig. 2. The sulfur of each coordinated thiosemicarbazone is hydrogen-bonded to two C–H fragments, a phenyl C–H of the PPh3 and a methyl C–H of the thiosemicarbazone belonging to two different neighboring complex molecules. One N–H fragment of the thiosemicarbazone is involved in hydrogen-bonding with the p-cloud over a C–C bond of the phenol fragment of an adjacent complex molecule in a g2-fashion. Each complex molecule is thus linked with three surrounding complex molecules through C–H


S and N–H


p interactions (Fig. 2), and this extended intermolecular hydrogen-bonding is responsible for holding the crystal together. All the monomeric [Pt(L)(D)] complexes are assumed to have similar structures to [Pt(PPh3)(L2)]. 3.2. Spectral properties 1

H NMR spectra of the [Pt(PPh3)(L)] complexes, recorded in CDCl3 solutions, show all the expected signals. The phenyl protons of the PPh3 ligands show broad signals within the range d 7.39–7.80 ppm. In the [Pt(PPh3)(L1)] and [Pt(PPh3)(L3)] complexes the azomethine proton signal of the coordinated thiosemicarbazone is observed at d 8.55 and 9.57 ppm, respectively. The methyl signal from the coordinated thiosemicarbazone in the [Pt(PPh3)(L2)] complex is observed at d 2.86 ppm. The NH2 signal of the coordinated thiosemicarbazone is observed around d 4.8 ppm. Most of the expected signals from the phenyl ring of the coordinated thiosemicarbazone are clearly observed in the aromatic region, whilst a few could not be detected due to their overlap with other signals. In the 1H NMR spectra of the [Pt(pic)(L)] complexes, the methyl signal of the coordinated picoline is observed around d 2.4 ppm. The NH2 signal of the coordinated thiosemicarbazone is observed around d 5.0 ppm and the azomethine proton signal (for L = L1 and L3) is observed between d 8.3–9.3 ppm. The methyl signal from the coordinated thiosemicarbazone in the [Pt(pic)(L2)] complex is observed at d 2.81 ppm. A distinct doublet observed near d 8.8 ppm in all the three [Pt(pic)(L)] complexes is attributable to the a-proton of picoline. The other aromatic proton signals are observed in the expected region as overlapping signals. The 1H NMR spectral data of the [Pt(PPh3)(L)] and [Pt(pic)(L)] complexes are therefore in well accordance with their compositions. Infrared spectra of all the [Pt(PPh3)(L)] complexes show many vibrations of different intensities in the 1600– 400 cm 1 region. Assignment of each individual band to a specific vibration has not been attempted. However, three strong bands displayed near 513, 694 and 743 cm 1 by each of these complexes are attributed to the coordinated PPh3 ligands. Comparison with the spectrum of the cis[Pt(PPh3)2Cl2] complex shows the presence of several new bands (e.g. near 1130, 1226, 1292, 1334, 1352, 1423, 1508, 1541 and 1560 cm 1) in the spectra of the [Pt(PPh3)(L)] complexes, which must be due to the presence of the coordinated thiosemicarbazone ligands. Besides an absence of

2745

the three diagnostic bands for the PPh3 ligands near 513, 694 and 743 cm 1, the infrared spectral properties of the [Pt(pic)(L)] complexes are qualitatively similar to that of the [Pt(PPh3)(L)] complexes. The [Pt(PPh3)(L)] and [Pt(pic)(L)] complexes are readily soluble in methanol, ethanol, acetone, acetonitrile, dichloromethane, chloroform, etc., producing intense lemon yellow solutions. Electronic spectra of these complexes have been recorded in dichloromethane solution. A selected spectrum is shown in Fig. 3 and spectral data are presented in Table 2. Each complex shows several intense absorptions in the visible and ultraviolet regions. The absorptions in the ultraviolet region are assignable to transitions within the ligand orbitals. To have an understanding of the nature of the transitions in the visible region, qualitative EHMO calculations have been performed [16] on computer generated models of the [Pt(PPh3)(L)] and [Pt(pic)(L)] complexes. The composition of selected molecular orbitals is given in Table 3 and a partial MO diagram for a representative [Pt(PPh3)(L)] complex is shown in Fig. 4. The partial

Fig. 3. (a) Electronic spectrum and (b) emission spectrum of [Pt(PPh3)(L3)] in dichloromethane solution.

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S. Halder et al. / Polyhedron 26 (2007) 2741–2748

Table 2 Electronic spectral data in dichloromethane solution kmax/nm (e/M

Compound 1

[Pt(PPh3)(L )] [Pt(PPh3)(L2)] 3

[Pt(PPh3)(L )] 1

[Pt(pic)(L )] 2

[Pt(pic)(L )] [Pt(pic)(L3)] a

1

Table 4 Emission spectral data for the complexes

cm 1)

Compound

416 (4800), 364 (9000), 306 (6500), 294 (7200), 262 (18 300)a 412 (4600), 364 (8300), 306 (6700)s, 292 (8300), 256 (21 300) 436 (9100), 413 (7400), 379 (9500), 327 (10 400), 317 (8400)s, 267 (32 300) 411s (2000), 364 (3800), 338 (3400), 310 (3500), 299 (3400), 253 (9100)s 417s (4500), 364 (10 100), 349s (8500)s, 311s (7300), 299 (7700) 245 (25 400) 438 (3600), 415 (3200), 380 (5200), 328 (4500), 317s (3800)s, 267 (15 800)

s, shoulder.

Table 3 Composition of selected molecular orbitals of the complexes Compound

Contributing fragments

% Contribution of fragments to HOMO

LUMO

Pt L1

64 32

95 (C@N, 52)

Pt L2

64 29

92 (C@N, 47)

[Pt(PPh3)(L )]

Pt L3

11 84

[Pt(pic)(L1)]

Pt L1 pic

64 32

2 73 (C@N, 43) 21

[Pt(pic)(L2)]

Pt L2 pic

55 34 2

2 44 (C@N, 24) 46

[Pt(pic)(L3)]b

Pt L3 pic

19 72

1

[Pt(PPh3)(L )] 2

[Pt(PPh3)(L )] 3 a

a b

The HOMO The HOMO

95 (C@N, 33)

90 (C@N, 31) 2

1 has 68% contribution from the metal. 1 has 67% contribution from the metal.

Emission data kmax (nm)

[Pt(PPh3)(L1)] [Pt(PPh3)(L2)] [Pt(PPh3)(L3)] [Pt(pic)(L1)] [Pt(pic)(L2)] [Pt(pic)(L3)]

Quantum yield (u)

Excitation

Emission

364 364 379 363 364 380

595, 432, 610, 600, 610, 610,

570, 445, 415s 415s 460, 422 565s, 430, 408 562, 432s, 415 432, 416

1.82 · 10 1.13 · 10 0.42 · 10 2.39 · 10 0.59 · 10 0.73 · 10

3 3 3 3 3 3

s, shoulder.

MO diagram for a representative [Pt(pic)(L)] complex is deposited as Fig. S1. In the case of the [Pt(PPh3)(L1)] and [Pt(PPh3)(L2)] complexes the highest occupied molecular orbital (HOMO) has major contributions from the platinum d orbitals. The lowest unoccupied molecular orbital (LUMO) is delocalized almost entirely on the thiosemicarbazone ligand, and is largely concentrated on the imine fragment. Therefore the lowest energy absorption near 410 nm is assignable to a charge-transfer transition taking place from the filled metal orbital (HOMO) to the vacant p*-orbital of the thiosemicarbazone ligand (LUMO). However, in case of the [Pt(PPh3)(L3)] complex the HOMO has only 11% metal character, while the LUMO is similar in nature as before. Hence the lowest energy absorption at 436 nm in this complex is assignable to a transition within the orbitals belonging primarily to L3. It may be mentioned in this context that this absorption is much more intense compared to the lowest energy transition in the earlier two complexes. It may also be noted here that there is a second absorption at 413 nm, which may be assigned to a charge-transfer transition taking place from the filled metal orbital (HOMO 1) to the vacant p*-orbital of the thiosemicarbazone ligand (LUMO). The electronic spectral properties, as well as features of the molecular orbitals, of the [Pt(pic)(L)] complexes are similar to those of the [Pt(PPh3)(L)] analogues. The intensities of the charge-transfer transitions in the visible region have tempted us to explore the luminescence properties of these complexes and this has been carried out in dichloromethane solution at ambient temperature (298 K). All six complexes display emission in the visible region using an excitation wavelength of 350 nm (Fig. 3). Quantum yields (/) of these emissions have been evaluated (Table 4) with reference to [Ru(bpy)3]Cl2 (/ = 0.028 at 298 K) [17]. 4. Conclusion

Fig. 4. Partial molecular orbital diagram of the [Pt(PPh3)(L2)] complex.

The present study shows that salicylaldehyde thiosemicarbazones and similar ligands can smoothly bind to platinum as tridentate O,N,S-donors yielding stable complexes, which not only show intense absorptions in the visible region but also fluoresce at room temperature.

S. Halder et al. / Polyhedron 26 (2007) 2741–2748

Acknowledgements Financial assistance received from the Council of Scientific and Industrial Research, New Delhi, India [Grant No. 01(1952)/04/EMR-II] is gratefully acknowledged. The authors thank Prof. Chittaranjan Sinha and Dr. Asok Nath Mandal of the Department of Chemistry, Jadavpur University, for their help with the florescence and NMR spectral measurements, respectively. Sarmistha Halder thanks the University Grants Commission, New Delhi, India, for her fellowship [Grant No. 10-2(5)2004(1)-E.U.II]. Appendix A. Supplementary material CCDC 626762 contains the supplementary crystallographic data for [Pt(PPh3)(L2)]. 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]. uk. Full details of data collection are provided in Table S1. A partial MO diagram of the [Pt(pic)(L1)] complex has been deposited as Fig. S1. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2007.01.048.

[3] [4]

[5]

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