Synthesis, characterization, crystal structure, solvatochromism, fluorescence and electrochemical studies of new organometallic platinum complexes, kinetic investigation of oxidative addition reaction

Journal of Organometallic Chemistry 780 (2015) 34e42

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Synthesis, characterization, crystal structure, solvatochromism, fluorescence and electrochemical studies of new organometallic platinum complexes, kinetic investigation of oxidative addition reaction Bita Shafaatian*, Bahareh Heidari School of Chemistry, Damghan University, Damghan 3671641167, Iran

a r t i c l e i n f o

a b s t r a c t

Article history: Received 13 October 2014 Received in revised form 29 December 2014 Accepted 30 December 2014 Available online 8 January 2015

A new organoplatinum(II) complex containing 2,20 -biquinoline ligand (biq) was synthesized by the reaction of [Pt(p-MeC6H4)2(SMe2)2] with 2,20 -biquinoline in a 1:1 molar ratio. In this complex the ligand was coordinated to metal via the chelating nitrogen donor atoms. Also, platinum(IV) complex was obtained by the oxidative addition reaction of the methyl iodide with the platinum(II) complex in acetone. The platinum complexes have been found to possess 1:1 metal to ligand stoichiometry and the molar conductance data revealed that the metal complexes were non-electrolytes. The platinum(II) and platinum(IV) complexes exhibited square planar and octahedral coordination geometry, respectively. The emission spectra of the platinum(II) and platinum(IV) complexes were studied in dichloromethane. Furthermore, electrochemical properties of the metal complexes were investigated in dimethylformamide (DMF) solvent at 150 mV s1 scan rate. The ligand and metal complexes showed both reversible and irreversible processes at this scan rate. The complexes have been characterized by IR, 1H NMR, UV/Vis, elemental analysis and conductometry. The crystal structure of the platinum(II) complex containing 2,20 biquinoline has been determined by single crystal X-ray diffraction. Moreover, kinetic studies of the oxidative addition reaction of methyl iodide with the platinum(II) complex in different temperatures were investigated. It was indicated that the reaction occurred by the SN2 mechanism. The rate of reaction in two different solvents was compared and the activation parameters were determined. © 2015 Elsevier B.V. All rights reserved.

Keywords: Organoplatinum Platinum(II) complex 2,20 -biquinoline Kinetic

Introduction The Platinum-based compounds are widely used in the treatment of cancer [1]. Moreover, square planar platinum(II) complexes have been shown to display efficient phosphorescence over a broad range of energy [2]. With the appropriate molecular design, they have high thermal stability with profound optoelectronic applications [3]. In particular, platinum(II) complexes containing aromatic N-donor and/or cyclometalated ligands are well documented to display a variety of emissive excited states, including ligand field (LF), metal-to-ligand charge transfer (MLCT), ligand-to-ligand charge transfer (LLCT), intraligand (IL) p / p*, excimeric and oligomeric metal-metal-to-ligand charge transfer (MMLCT) states

* Corresponding author. Tel./fax: þ98 23 35235431. E-mail address: [email protected] (B. Shafaatian). http://dx.doi.org/10.1016/j.jorganchem.2014.12.037 0022-328X/© 2015 Elsevier B.V. All rights reserved.

[4]. Luminescent metal complexes have been attracting a surge of interest in the design of functional molecular materials with practical interests [5]. They are widely used as sensory materials for signaling studies [6] and phosphorescent dopants in organic lightemitting diodes (OLEDs) [7]. Through judicious choice of auxiliary ligands and metal ions, the photophysical properties of luminescent metal complexes could be systematically modified. Thus, the synthesis of organoplatinum(II) complexes have been important for the development of organometallic chemistry. The oxidative addition reaction represents one of the most fundamental processes in transition metal chemistry [8e10]. It plays an invaluable role in many synthetic and catalytic reactions, particularly in organic synthesis. Furthermore, two electron oxidation and reduction reactions between three oxidation states (0, þ2 and þ4) are an integral part of the chemistry of platinum [11]. Oxidative addition of the substrate XeY to a metal center (M) leads to an increase in the coordination number of the complex due

B. Shafaatian, B. Heidari / Journal of Organometallic Chemistry 780 (2015) 34e42

to the formation of two new bonds, MeX and MY, upon complete dissociation of the XeY bond. The addition of a molecule XeY to a square planar d8 platinum(II) complex to give an octahedral d6 platinum(IV) complex represents one of the classic example of such reactions. Oxidative addition and reductive elimination reactions of transition metal complexes form the basis of many catalytic systems [12e14]. The rich nucleophilic reactivity of square planar platinum(II) and palladium(II) complexes is well established. One of the most documented examples is the stepwise oxidative addition of alkyl halides to organoplatinum(II) complexes via SN2-type substitution at the sp3 carbon center [15]. In this mechanism, bimolecular (SN2type) oxidative addition, the metal center acts as a nucleophile. A characteristic feature of this process is the formation of a cationic intermediate. Formation of a metalecarbon bond usually proceeds through a CeH, CeC or CeX (X ¼ halogen atom) bond activation step. Synthesis of platinum complexes with chelating ligands containing N donor atoms have received considerable attention because of the possibility of their use as catalysts [16], antibacterial and anticancer agents [17]. On the other hand, oxidative addition and reductive elimination reactions of platinum complexes form the basis of many catalytic systems [18]. Thus, synthesis of new organoplatinum complexes with biological, electron transfer and optical characteristics can be very valuable. Complexes having these unique properties are considered to be applicable in different fields. In this paper we reported the synthesis of new organoplatinum(II) and organoplatinum(IV) complexes containing 2,20 biquinoline. These complexes were characterized by the FT-IR, 1H NMR, UV-Vis spectroscopy, elemental analysis, molar conductance and X-ray crystallography. Furthermore, electrochemical and emission behavior of these complexes were studied by cyclic voltammetry and fluorescence spectroscopy, respectively. The solvatochromism of platinum(II) complex in different solvents was also investigated. Moreover, kinetic studies of the oxidative addition reaction of methyl iodide with organoplatinum(II) complex were investigated and the temperature dependence of the rate constants were reported. From the temperature dependence of rate constants, the activation parameters were calculated according to Arrhenius and Eyring equations, respectively. The results indicated that the reaction occurred by the SN2 mechanism. Experimental section Materials and physical measurements All chemicals were reagent grade quality purchased from commercial sources and used as received. UV-Vis and fluorescence spectra were recorded on an Analytik Jena Specord 205 spectrophotometer and FP-6200 spectrofluorometer, respectively. FT-IR spectra were obtained as KBr pellets on a PerkineElmer spectrum RXI FT-IR spectrophotometer. 1H NMR spectra were recorded on a Bruker Avance DPX 400 MHz spectrophotometer, using TMS as an internal standard. The microanalyses were performed using a PerkineElmer 2400 series II. Melting points were determined on a Barnstead Electrothermal 9100. Conductivity was measured in dichloromethane solution (3  104 M) using a 712 conductometer (Metrohm). Cyclic voltammograms were performed using a Metrohm Autolab/PGSTAT302N system equipped with a three-compartment cell and a personal computer for data storage and processing. An Ag/AgCl (saturated KCl) reference electrode (Metrohm), a Pt-rod as counter electrode and a platinum disk electrode (i.d. ¼ 3 mm) as working electrode (Metrohm Pt-disk) were employed for the electrochemical studies. Voltammetric measurements were

35

performed at room temperature in DMF solution with 0.2 M sodium perchlorate as supporting electrolyte. Synthesis of starting material Complex cis-[Pt (p-MeC6H4)2(SMe2)2] was prepared by the literature methods [19]. Synthesis of [Pt(p-MeC6H4)2(biq)], 1 To a solution of cis-[Pt(p-MeC6H4)2(SMe2)2] (0.040 g, 0.078 mmol) in acetone (6 mL) was added a solution of biquinoline (0.020 g, 0.078 mmol) in acetone (6 mL) and the mixture was stirred for 2 h. The solvent was evaporated and a red precipitate product separated and washed twice with n-hexane. Then, the precipitate was dried in vacuum. (Yield: 84%), red, m.p: 160  C (decomp.), Anal. Calc. for [C32H26N2Pt] (1): (M.W: 633.654), C, 60.66; H, 4.14; N, 4.42%. Found: C, 60.09; H, 4.25; N, 4.17%; IR (KBr, y/cm1): (C]C) and (C]N) 1594sh, 1508sh, 1481s, 1431w, (CeH) 815sh, 800s, 746w, (MCl) 509w. 1H NMR (400 MHz, DMSO-d6, d/ ppm): 2.17 (6H, s, C6H5CH3), 6.66 (4H, d, 3J(HoHm) ¼ 7.60 Hz, 7.23 (d, 4H, 3J(HmHo) ¼ 8.00 Hz, 7.29 (3J(PtHo) ¼ 17.21 Hz), 8.88 (d, 2H, 0 3 J(H3H4) ¼ 8.40 Hz, H3 and H3 of biq), 8.63 (d, 2H, 3 4 3 4 40 J(H H ) ¼ 8.80 Hz, H and H of biq), 8.33 (dd, 2H, 0 3 J(H8H7) ¼ 8.80 Hz, 3J(H8H6) ¼ 8.80 Hz, H8 and H8 of biq), 7.86 (d, 0 2H, 3J(H5H6) ¼ 8.00 Hz, H5 and H5 of biq), 7.51 (t, 2H, 3J(H6H5), 0 3 J(H6H7) ¼ 16.0 Hz, H6 and H6 of biq). 7.61 (t, 2H, 3J(H7H6), 3 7 8 7 70 J(H H ) ¼ 16.81 Hz, H and H of biq). Synthesis of [Pt(Me)I(p-MeC6H4)2(biq)], 2 To a solution of [Pt(p-MeC6H4)2(biq)] (0.030 g, 0.047 mmol) in acetone (30 mL) was added an excess of MeI (1 mL). The reaction mixture was stirred for 5 h. An orangeeyellow solution was formed. Then, the solvent was evaporated and a yellow precipitate product separated and washed twice with n-hexane. The precipitate was dried in vacuum. (Yield: 71%), yellow, m.p: 130  C, Anal. Calc. for [C33H29N2IPt] (2): (M.W: 775.593), C, 51.10; H, 3.77; N, 3.61%. Found: C, 50.98; H, 3.63; N, 3.45%; IR (KBr, y/cm1): (C]C) and (C] N) 1616w, 1594vs, 1550w, 1509vs, 1488m, 1432m; (CeH) 868w, 825m, 804m, 746w, (MN) 489w, 510w. 1H NMR: d 2.25 (s, 3H, 2 J(PtH) ¼ 61.6 Hz, MeePt), 2.27 (s, 6H, ArCH3), 6.03 (d, 4H, 3 J(HoHm) ¼ 8.0 Hz, Hm of Ar), 7.30 (d, 4H, 3J(HmHo) ¼ 8.0 Hz, 3 J(PtHo) ¼ 39.2 Hz, Ho of Ar), 8.48 (d, 2H, 3J(H3H4) ¼ 8.80 Hz, H3 and 0 0 H3 of biq), 8.55 (d, 2H, 3J(H4H3) ¼ 7.80 Hz, H4 and H4 of biq), 8.25 0 3 8 7 3 8 6 8 (dd, 2H, J(H H ) ¼ 9.20 Hz, J(H H ) ¼ 8.80 Hz, H and H8 of biq), 3 5 6 5 50 7.89 (d, 2H, J(H H ) ¼ 8.00 Hz, H and H of biq), 7.63 (t, 2H, 0 3 J(H6H5), 3J(H6H7) ¼ 14.8 Hz, H6 and H6 of biq), 7.51 (t, 2H, 3J(H7H6), 3 7 8 7 70 J(H H ) ¼ 11.2 Hz, H and H of biq). Crystal structure determination and refinement The X-ray diffraction measurements were made on an STOE IPDS-2T diffractometer with graphite monochromated Mo-Ka radiation. For complex 1, brown needle shape crystal was chosen using a polarizing microscope and was mounted on a glass fiber which was used for data collection. Cell constants and orientation matrices for data collection were obtained by least-squares refinement of diffraction data from 4454 unique reflections. Data were collected to a maximum 2q value of 49.98 in a series of u scans in 1 oscillations and integrated using the Stoe X-AREA [20] software package. A numerical absorption correction was applied using X-RED [21] and X-SHAPE [22] software. The data were corrected for Lorentz and Polarizing effects. The structures were solved by direct methods [23] and subsequent difference Fourier maps and then refined on F2 by a full-matrix least-squares procedure using anisotropic displacement parameters [24]. The atomic factors were taken from the International Tables for X-ray Crystallography [25].

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All refinements were performed using the X-STEP32 crystallographic software package [26]. Results and discussion Synthesis and characterization of the compounds The platinum(II) complex containing macrocyclic chelating ligand was synthesized by the reaction of cis-[Pt(pMeC6H4)2(SMe2)2] with 2,20 -biquinoline in acetone, in 1:1 molar ratio and the red precipitate of the platinum(II) complex was obtained (Scheme 1). In order to synthesize the platinum(IV) complex, an excess of iodomethane was reacted with the platinum(II) complex 1 (Scheme 2). Single crystals of complex 1 were grown by slow diffusion of n-hexane into a saturated CH2Cl2 solution during 8 days at 4  C and its structure was determined by X-ray crystallography, whereas suitable single crystals could not be grown for complex 2. Based on the X-ray crystallography data, square planar geometry was suggested for the platinum(II) complex. In suggested structure, the chelating ligand was coordinated to the platinum via the nitrogen atoms. Moreover, based on the spectroscopy data, octahedral geometry was suggested for the platinum(IV) complex. The elemental analyses for the new complexes were in good agreement with the proposed composition. The complexes were collected in fair to good yields. Description of the molecular structure of 1 Brown crystals of 1 were obtained by slow diffusion of n-hexane into a saturated CH2Cl2 solution during 8 days at 4  C. Crystallographic data and parameters for the complex were summarized in Table 1. This Table reveals that 1 crystallize in the monoclinic (space group, P21/c) crystal system. The asymmetric unit of the complex consists of one crystallographically independent molecule which contain one Pt(II) atom, one 2,20 -biquinoline ligand and two toluene molecules. An ORTEP drawing of the complex 1 was presented in Fig. 1. As shown in Fig. 1, coordination geometry around platinum (II) ion can be described as slightly distorted square planar geometry. In complex 1, four coordination sites are occupied by two nitrogen from 2,20 -biquinoline ligand and two carbon atoms from two toluene molecules. Selected bond lengths and angles with their standard deviations for complex 1 were given in Table 2. Structural parameters which have shown in Table 2 are comparable with previously reported organometallic Pt(II) compounds containing bidentate nitrogen donor ligands [27,28]. Spectroscopy IR spectra In IR spectra, the two complexes displayed the characteristic peaks in the region of 1431e1616 cm1 which were due to the n(C]

C) and (C]N). The y(CeH) bands in these complexes appeared in the range of 746e868 cm1. Additional supports for the formation of the complexes were provided by the existence of weak intensity bands at 489 and 510 cm1 which were attributed to the formation of MN in the platinum(II) and platinum(IV) complexes, respectively. However, a detailed comparison with the uncoordinated ligands cannot be made. In the free ligand the nitrogen donor atoms are s-trans [29,30] in order to minimize steric repulsion between the meta protons in pyridine rings, but upon complexation they adopt an s-cis conformation [31,32]. 1

H NMR spectra The structural assignments were further supported by their 1H NMR spectra. 1H NMR spectra of the complexes were obtained in CHCl3-d1 at room temperature using TMS as the internal standard. In the 1H NMR spectra, the integral intensities of each signal were found to agree with the number of different types of protons presented in the platinum complexes. The most important peak in the platinum(II) complex is a singlet signal and its satellites due to the coupling of platinum with ortho protons of para-tolyl at 7.29 ppm confirmed the formation of this complex (Fig. 2). Investigation of the 1H NMR of the platinum(IV) clearly showed the existence of both cis and trans isomers. The most important peak is due to the Me group which appeared as a singlet signal at 2.25 ppm with its satellites for the trans isomer, whereas in the cis isomer of platinum(IV) a singlet signal and its satellites was observed at 1.27 ppm with relative intensity 1:4:1 and coupling constant different from the trans isomer. The singlet signal due to the methyls of para-tolyl groups were appeared at 2.27 and 2.36 ppm for the trans and cis isomers, respectively. Formation of cis and trans mixture due to the oxidative addition reactions was also previously reported by other researchers [33]. The oxidative addition reaction leads to the rapid formation of trans isomer around the platinum(IV) center as a kinetic product. This geometry is a consequence of the bulky groups in the complex. The cis isomer is formed by a competitive cis oxidative addition pathway, or it may be formed by trans oxidative addition with subsequent isomerization of the platinum(IV) complex to the thermodynamically more stable cis isomer [14]. Electronic spectra The absorption spectra of the platinum(II) and platinum(IV) complexes were investigated in CH2Cl2 and their absorption data were summarized in Table 3. The stronger and higher energy peaks was attributed to the p / p* transition of the biquinoline and benzene ring in para-tolyl groups, while the weaker and less energetic peak was assigned to the n / p* transition involving the promotion of the lone pair electron of nitrogen atoms of the ligand to the antibonding p orbital associated with the biquinoline ligand [34]. Each complex also exhibited a lower-energy band at substantially longer wavelengths than the absorption of the ligands,

Scheme 1. The synthetic route of the platinum(II) complex.

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Scheme 2. The synthetic route of the platinum(IV) complexes.

Table 1 Crystallographic and structure refinement data for complex 1. Empirical formula Formula weight Temperature (K) Wavelength (Å) Crystal system Space group Crystal size (mm3) a (Å) b (Å) c (Å) b ( ) Volume (Å3) Z Densitycalc (g cm1) q Ranges for data collection ( ) F (000) Absorption coefficient (mm1) Index ranges

C32H26N2Pt 633.63 120(2) 0.71073 Monoclinic P21/c 0.45  0.09  0.07 7.4023(15) 18.756(4) 19.651(5) 111.85(3) 2532.3(11) 4 1.662 2.44e24.99 1240 5.564 8  h  8 21  k  22 23  l  23 13,623 4454, (0.0819) 318/0 0.0435/0.0765 0.0722/0.0822 0.993 2.117, 1.206

Data collected Unique data, (Rint) Parameters/restrains Final R1/wR2a (I > 2s(I)) Final R1/wR2a (all data) Goodness-of-fit on F2 (S) Largest diff. peak and hole (e Å3) P P P P a R1 ¼ jjFo j  jFc jj= jFo j; wR2 ¼ ½ ðwðF02  Fc2 ÞÞ= wðFo2 Þ2 1=2 .

which were likely to arise from charge-transfer transitions involving the metal. The bands observed at 532 and 370 nm were due to the MLCT transitions in platinum(II) and platinum(IV) complexes, respectively [35].

Fluorescence spectral studies The emission spectra of the platinum(II) and platinum(IV) complexes (1 and 2) were investigated at room temperature (298 K) in dichloromethane solution. The emission spectrums of the platinum complexes were depicted in Fig. 3. The platinum(II) and platinum(IV) complexes were characterized by the emission

bands around 420 and 434 nm upon photo excitation at 340 and 370 nm, respectively. The emission observed in these complexes was assigned to the p / p* intraligand fluorescence. The platinum(IV) showed higher emission intensity than that of platinum(II) complex. Moreover, the heteroaromatic ligands each have lowenergy p* orbitals that can be involved in charge-transfer (CT) excited states of their respective complexes. Thus, for these complexes, the obtained broad emission band can also involve a variety of transitions such as intraligand charge-transfer (ILCT) and metalto-ligand charge-transfer (MLCT) [36]. These admixture transition energies are expected to be sensitive to solvent [37e39] and so the MLCT emission band has been characterized in the platinum complexes by the solvent dependence of the emission. A small change (ca. 6 nm) in the emission maximum was observed upon changing the solvent polarity. In these examples, the highest occupied molecular orbital (HOMO) may have orbital contributions from the metal ion, the polypyridyl moiety, the other ligands in the complex and/or mixtures thereof, but the lowest unoccupied molecular orbital (LUMO) in each case was localized on the heteroaromatic chelating ligand. Electrochemical studies of the platinum(II) and platinum(IV) complexes Cyclic voltammetry was performed with 2  103 M solutions of the complexes in DMF with 0.2 M sodium perchlorate as supporting electrolyte and scan rates 150 mV/s within the potential range 2.00 to 2.00 V. All solutions were nitrogen-purged prior to experiments. The cyclic voltammogram of platinum(II) complex, 1, was shown in Fig. 4. The cyclic voltammogram showed one reversible process at Epc ¼ 1.07 V and Epa ¼ 1.03 V due to the process of Pt(II) # Pt(0) with ipa/ipc ratio close to unity (ipa/ipc ¼ 0.97). One anodic wave was detected at Epa ¼ þ0.89 V which can be attributed to the oxidation of Pt(II) to Pt(IV). The cyclic voltammogram of platinum(IV) complex (Fig. 5) showed a quasi-reversible reduction peak at Epc ¼ 1.03 V and Epa ¼ 0.94 V which correspond to Pt(IV) # Pt(II). One anodic wave was also detected at Epa ¼ þ0.41 V which can be attributed to the oxidation of Pt(IV) to Pt(VI). It should be noted that the oxidation of platinum complex 2 was occurred at less positive

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Fig. 1. The labeled diagram of the platinum(II) complex (1). Thermal ellipsoids are at 50% probability level.

Table 2 Selected bond distances (Å) and bond angles (deg) for complex 1. Pt(1)eC(1) Pt(1)eC(8) C(1)ePt(1)eC(8) C(8)ePt(1)eN(2) C(8)ePt(1)eN(1)

1.986(8) 2.003(7) 82.2(3) 171.7(3) 102.2(3)

Pt(1)eN(1) Pt(1)eN(2) C(1)ePt(1)eN(2) C(1)ePt(1)eN(1) N(2)ePt(1)eN(1)

2.161(6) 2.152(5) 98.5(3) 172.9(3) 76.4(2)

potential than complex 1 due to the increasing of coordination number and the presence of methyl and iodide donor groups which facilitated the oxidation of platinum (IV). Furthermore, another anodic wave was detected at Epa ¼ þ0.70 V which can be attributed to partial oxidation of the ligand. Molar conductivity measurements The molar conductivity of 3  104 M complexes was measured in dichloromethane at room temperature. The molar conductivity values of both complexes were in the range of 1.68e3.21 U1 cm2 mol1, indicating the non-electrolytic nature of these complexes [40]. The very low conductivity value of complex 1 in DMSO can be explained with respect to the crystal structure of this compound which showed it didn't have any ionic structure. Solvatochromism The remarkable change of UV-Vis absorption characteristics induced by using different solvents can be regarded as solvatochromism. The high solubility of the platinum(II) complex in a variety of organic solvents allowed a detailed investigation of its solvatochromic behavior. Thus, in order to investigate the solvatochromism property of this complex, its absorption spectra were obtained at different solvents (methanol, ether, acetonitrile, toluene, acetone, dimethylformamid, chloroform and dichloromethane). Fig. 6 showed the position of charge transfer absorption

Table 3 Experimental photophysical data of Pt(II) and Pt(IV) complex with 5  105 M and 1.25  105 M in CH2Cl2, respectively. Complex

Fig. 2. 1H NMR spectrum (400 MHz) of the platinum(II) complex (a) and platinum(IV) complex (b) in CHCl3 (d1).

Pt(II) Pt(IV)

lemission (nm)

lmax (nm) p / p*

n / p*

MLCT

280 232

324 272

536 370

420 434

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Fig. 3. Fluorescence spectra of the platinum(II) and platinum(IV) complexes (1.25  105 M) in dichloromethane (lexc ¼ 340).

Fig. 6. Visible spectra (Solvatochromism).

of

the

platinum(II)

complex

in

different

solvents

indicative of a polar ground state and non polar excited state. The solvatochromic shifts in different solvents for the platinum(II) complex showed the energies of the absorption maxima for the charge-transfer to diimine band were fitted to a set of empirical solvent parameters devised by Cummings and Eisenberg [36].

Fig. 4. Cyclic voltammogram of platinum(II) complex initiated by the cathodic sweep at a Pt electrode of a solution of (2  103 M) in 0.2 M NaClO4/DMF (n ¼ 150 mV/s).

maxima of the platinum(II) complex was depend on the solvent and changed with various solvent compositions. It was observed that upon changing the solvent, the color of the complex solution changed dramatically from red to violet to pink. The differences in absorption maxima of the complex in solvents with different polarities were summarized in Table 4. The observed shift of band maxima to higher energy in solvents of increasing polarity was

Kinetics of oxidative addition of methyl iodide The oxidative addition of alkyl halides with square-planar complexes of platinum is often a key step in catalytic reactions in order to gain insights about the intrinsic properties of the reaction constituents [41,42]. Thus, the oxidative additions of MeI to [Pt(pC6H4)2(biq)], were studied using UV-Vis absorption spectroscopy to monitor the reaction. The disappearance of the MLCT band for the platinum complex 1 at lmax ¼ 535 and lmax ¼ 574 nm was used to monitor the reaction in acetone and toluene solvents, respectively. The intense red color of the platinum(II) complex changed to yellow color upon oxidation to platinum(IV) complex (Fig. 7). In this case, excess of methyl iodide was used and the reaction followed good first-order kinetics. The pseudo-first-order rate constants (kobs) can be calculated with curve fitting by using Microsoft Excell solver [43]. A typical of curve fitting plot in acetone and toluene was shown in Fig. 8. Graphs of these first-order rate constants against the concentrations of methyl iodide gave good straight line plots passing through the origin, showing a first-order dependence of the rate on the concentration of the MeI (Fig. 9). The overall second-order rate constants (k2) can be obtained from the slopes of these figures. The activation parameters were determined from the measurements of k2 at different temperatures, and the data were given in Table 5. Table 4 Absorption maxima for [Pt(p-tolyl)2(2,20 -biquinoline)] in various solvents.

Fig. 5. Cyclic voltammogram of platinum(IV) complexes initiated by the cathodic sweep at a Pt electrode of a solution of (2  103 M) in 0.2 M NaClO4/DMF (n ¼ 150 mV/ s).

Solvent

l (nm)

DMF Acetonitrile Methanol Acetone Chloroform CH2Cl2 Toluene Ether

532 510 496 538 538 536 576 570

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Fig. 7. Changes in the UVevisible spectrum during the reaction of [Pt(p-MeC6H4)2(biq.)] (3 mL, 2  104 M) with MeI in acetone (a) and (3 mL, 4  104 M) in toluene (b) at T ¼ 25  C, successive spectra recorded at intervals of 2 s.

Fig. 8. Curve fitting of absorbance vs time for the reaction of [Pt(p-MeC6H4)2(biq.)], with MeI in acetone (a) and toluene (b) at T ¼ 30  C.

The activation energy, Ea, was obtained from the Arrhenius equation as following:

Furthermore, DH# and DS# were obtained from the Eyring equation:

ln k2 ¼ ln A  Ea =RT

lnðk2 =TÞ ¼ lnðk0 =hÞ þ DS# =R  DH# =RT

The Arrehenius plots for acetone and toluene were shown in Fig. 10.

The Eyring plots were shown in Fig. 11 and the activation parameters were given in Table 5.

Fig. 9. Plots of first-order rate constants (lnkobs/s1) for the reaction of [Pt(p-MeC6H4)2(biq.)], with MeI in acetone at different temperatures vs ln[MeI], a ¼ 10, b ¼ 15, c ¼ 20, d ¼ 25, e ¼ 30 and f ¼ 35  C (a) and in toluene (b), a ¼ 15, b ¼ 20, c ¼ 25, d ¼ 30 and e ¼ 35  C.

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Table 5 Rate constants and activation parameters in acetone and toluene. Solvent

Acetone Toluene

k2 10  C

15  C

20  C

25  C

30  C

35  C

3.51 e

4.04 0.18

5.61 0.28

9.20 0.34

10.75 0.47

15.45 0.53

DH# (kJ mol1)

DS# (J K1 mol1)

Ea (kJ mol1)

42.39 37.31

85.07 128.73

44.84 39.78

Fig. 10. Arrhenius plot for the reaction of [Pt(p-MeC6H4)2(biq.)] with MeI in acetone (a) and toluene (b).

Fig. 11. Eyring plot for the reaction of [Pt(p-MeC6H4)2(biq.)] with MeI in acetone (a) and toluene (b).

As seen from Table 5, the rates of the reaction with MeI in toluene at different temperatures were slower than the rates of similar reactions in acetone by a factor of 3e10. In both of these solvents (polar and non polar), large obtained negative values of DS# indicated typical of oxidative addition reactions by the SN2 mechanism which involves nucleophilic attack of the metallic center to the methyl group of MeI and the formation of [Pt(Me)(pMeC6H4)2(biq)]þ cationic intermediate. This mechanism was wellestablished for the platinum(II) complexes from the previous works [44]. Conclusions In this study characterization by 1H NMR, FT-IR, UV-Vis spectroscopies, conductometry and elemental analyses supported the structure and purity of the platinum(II) and platinum(IV) complexes. Furthermore, the single crystal X-ray structure for the platinum(II) complex confirmed the square planar geometry of the Pt(II) center. Also, octahedral coordination geometry was suggested

for the Pt(IV) complex. The crystal structure showed that the 2,20 biquinoline was coordinated as a bidentate ligand through NN donor atoms. The two complexes have been shown to be effective emitters. Electrochemical properties of the two metal complexes were investigated in DMF solvent and they revealed reversible and irreversible processes at 150 mV s1 scan rate. Also, the solvatochromism of the platinum(II) complex was investigated and it was observed that its UV-Vis absorption spectrum changed with various solvent. Kinetic studies of the oxidative addition reaction of methyl iodide with platinum(II) complex in different temperatures indicated that the reaction occurred by the SN2 mechanism. The rate constants and activation parameters have been determined in different solvents and temperatures. The obtained results showed that the rate constants were sensitive to the polarity of the solvents. Acknowledgement We gratefully acknowledge the support of this work by Damghan University Research Council.

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