Synthesis and spectroscopic studies of some chromium and molybdenum derivatives of bis-(acetylacetone)ethylenediimine ligand

Journal of Molecular Structure 1049 (2013) 7–12

Contents lists available at SciVerse ScienceDirect

Journal of Molecular Structure journal homepage: www.elsevier.com/locate/molstruc

Synthesis and spectroscopic studies of some chromium and molybdenum derivatives of bis-(acetylacetone) ethylenediimine ligand Ramadan M. Ramadan a, Laila H. Abdel-Rahman b,⇑, Mohamed Ismael b, Teraze A. Youssef a, Saadia A. Ali c a b c

Chemistry Department, Faculty of Science, Ain Shams University, Cairo, Egypt Chemistry Department, Faculty of Science, Sohag University, Sohag, Egypt Chemistry Department, University College for Girls, Ain Shams University, Cairo, Egypt

h i g h l i g h t s  Synthesis and characterizations of novel mono, and bi-nuclear molybdenum N2O2 Schiff bases complexes.  Synthesis and characterizations of novel chromium carbonyl N2O Schiff base complex.  Theoretical optimization of 3D geometry of the prepared complexes.  Calculating global chemical descriptors to predict the complexes’ relative reactivity.

a r t i c l e

i n f o

Article history: Received 20 March 2013 Received in revised form 19 May 2013 Accepted 11 June 2013 Available online 17 June 2013 Keywords: Chromium Molybdenum 3D structure Schiff base Photochemistry Thermal analysis

a b s t r a c t Interaction of [Cr(CO)6] with bis-(acetylacetone)ethylenediimine Schiff base, H2acacen, under reduced pressure resulted in the formation of [Cr(CO)3(H2acacen)] derivative. The Schiff base acted as a tridentate and coordinated the metal through the nitrogen of the azomethine groups and one hydroxyl group. Reaction of [Mo(CO)6] with H2acacen under sunlight irradiation in presence of air gave the oxo derivative [Mo2O6(H2acacen)2]. The ligand acted as a bidentate and coordinated the metal through the two imine groups. In presence of 2,20 -bipyridine (bpy), the reaction of [Mo(CO)6] with H2acacen gave [Mo2O6(bpy)(H2acacec)]. The structures of the reported complexes were proposed on the basis of spectroscopic studies. The proposed structures were also verified by theoretical calculations based on accurate DFT approximations. Moreover, the relative reactivity was estimated using chemical descriptors analysis. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Interest in the synthesis and investigation of transition metal complexes containing Schiff base ligands is retained to their importance as catalysts for many reactions [1–4]. Tetradentate Schiff base complexes are also important for designing model complexes related to synthetic and natural oxygen carrier [5]. Schiff bases obtained from condensation of ethylenediamine with b-diketones have been used as ligands for the complex formation with a variety of transition metals [6–15] and have found immense analytical applications, for example, in extracting traces of metals [16,17]. On the other hand, metal carbonyl derivatives play an

⇑ Corresponding author. E-mail address: [email protected] (L.H. Abdel-Rahman). 0022-2860/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.molstruc.2013.06.024

important role in many homogenous catalytic reactions such as hydrogenation, hydroformylation, carbonylation and oxygen transfer reactions [1]. Our interest in investigation of the reactions of Group 6 metal carbonyls with different Schiff bases containing N2O2 and NO2 donor sets has prompted us to study the reactions of [M(CO)6], M = Cr and Mo, with bis-(acetylacetone)ethylenediimine (H2acacen), Scheme 1, alone or in the presence of 2,20 bipyridine.

2. Experimental [Cr(CO)6], [Mo(CO)6], ethylenediamine and acetylacetone were purchased from Aldrich. All solvents were of analytical reagent grades and were purified by distillation before use using standard methods.

8

R.M. Ramadan et al. / Journal of Molecular Structure 1049 (2013) 7–12

CH 3 N

CH 3

N

Table 1 Elemental analysis and mass spectrometry data for the chromium and molybdenum complexes. Complex

OH CH 3

HO CH 3 [Cr(CO)3(H2acacen)]

Scheme 1. Structure of H2acacen ligand. [Mo2O6(H2acacen)2]

2.1. Synthesis of bis-(acetylacetone)ethylenediimine ligand Bis-(acetylacetone)ethylenediimine was prepared according to the literature procedure [18]. 1.2 g (2.0 mmol) of anhydrous ethylenediamine was added slowly to 4.0 g (4.0 mmol) of acetylacetone. Considerable heat was evolved during the reaction. The product solidified on cooling to give a straw-colored residue. Recrystallization from a hot mixture of ethyl acetate and dichloromethane (1:1) yielded colorless needle-like crystals. The crystals were dried under vacuum for several hours (63% yield). 2.2. Synthesis of [Cr(CO)3(H2acacen)]complex [Cr(CO)6] (0.10 g, 0.45 mmol) and H2acacen (0.11 g, 0.45 mmol) were mixed together in a sealed tube containing 25.0 ml THF. The mixture was degassed with one freeze–thaw cycle and then heated for 10 h at 60 °C. The color of the mixture was changed to brown. The reaction mixture was cooled and the solvent was removed on a vacuum line. The residue was washed several times by petroleum ether and then recrystallized from ethyl alcohol to give fine brown crystals. The complex was left to dry under vacuum line for several hours (yield 65%). 2.3. Synthesis of [Mo2O6(H2acacen)2] complex [Mo(CO)6] (0.10 g, 0.38 mmol) and H2acacen (0.09 g, 0.38 mmol) were dissolved in 25.0 ml THF. Photolysis of the mixture under sunlight irradiation for 8 h was performed. The color of the mixture was changed to brown and the solvent was removed on a vacuum line. The residue was washed several times by boiling petroleum ether and then recrystallized from hot THF. The complex was left to dry in vacuo for several hours (yield 80%). 2.4. Synthesis of [Mo2O6(bpy)(H2acacen)] complex Similar procedure was employed as used in the synthesis of [Mo2O6(H2acacen)2] using a mixture of equimolar ratio of [Mo(CO)6] (0.10 g, 0.38 mmol), H2acacen (0.09 g, 0.38 mmol) and bpy (0.06 g, 0.38 mmol) and a reaction period of 9 h. Yellowish brown fine crystals with a yield of 70% were obtained. 2.5. Instruments Infrared measurements (KBr pellets) were carried out on a Unicam- Mattson 1000 FT-IR spectrometer. 1H NMR measurements were performed on a Spectrospin-Bruker AC 200 MHz spectrometer. The samples were dissolved in DMSO, d6 using TMS as an internal standard. Electronic absorption spectra were measured on a Unicam UV2-300 UV–vis spectrophotometer. Thermogravimetric analyses (TG) were performed on a Shimadzu DT-50 thermal analyzer under nitrogen atmosphere with a heating rate of 10 °C/min. Elemental analyses were carried out on a Perkin–Elmer 2400 CHN elemental analyzer. Mass spectrometry measurements of the solid complexes were carried out on a Finnegan MAT SSQ 700 spectrom-

[Mo2O6(bpy)(H2acacen)]

Elemental analysis found (calc.)

Mass spectrometry

%C

%H

%N

Mol wt

m/z

39.30 (39.13) 38.95 (39.01) 39.30 (39.51)

4.41 (4.35) 5.25 (5.40) 4.50 (4.19)

6.20 (6.09) 7.54 (7.60) 8.50 (8.38)

360

276 [P3CO]+ 736 [P]+

736 668

512 [Pbpy]+

eter. Table 1 gives the elemental analyses and mass spectrometry data for the complexes. 3. Computational details All the calculations were performed using the hybrid density functional theory method B3LYP as implemented in the Gaussian 03 software package [19,20] The geometries were optimized using the standard double zeta plus polarization basis set 6-31G (d,p) for ligand atoms and effective core potential basis set LANL2DZ for Cr and Mo atoms. Through the calculations, all atoms of bis-(acetylacetone)ethylenediimine and bipyridine were used. The purpose of the quantum mechanics calculations validates the proposed 3D structure of the obtained complexes as well as to find out key factors of their activities. 4. Results and discussion The IR spectrum of the Schiff base H2acacen showed stretching frequencies due to the C@N, C@C and OH functional groups [29]. The presence of the OH bands in the IR spectrum indicates the existence of ligand in its enol form. This was confirmed by 1H NMR. It showed signals at 10.65 and 4.95 ppm due to O  H  N and@CHA, respectively. Reaction of chromium hexacarbonyl with H2acacen in THF under reduced pressure resulted in the formation of the tricarbonyl derivative [Cr(CO)3(H2acacen)]. The IR spectrum of the complex exhibited the ligand bands with the appropriate shifts due to complex formation (Table 2). The spectrum exhibited two OH bands where one of them was shifted to lower wavenumber indicating its participation in intra- and/or intermolecular hydrogen bonding. The other band is due to coordinated OH group. Interestingly, the OH coordinated to the metal without proton displacement. The t(C@N) stretching band was also found to be shifted in the spectrum of the complex which indicated the involvement of the azomethine group in coordination [27]. In addition, the IR spectrum of the complex showed a pattern of three CO bands in the terminal metal carbonyl region with symmetry of 2a1 + b1 [21]. The number and pattern of these CO bands concluded that they coordinated to chromium from one equatorial and two axial positions of an octahedral environment [22,23]. In addition, the IR spectrum showed the non-ligand CrAN and CrAO bands [27]. From the spectroscopic and elemental analyses data, it can be concluded that the ligand coordinated to the zero valent chromium species in a tridentate fashion (Scheme 2). Magnetic studies of the complex showed paramagnetic characteristics. The magnetic measurement of the solid complex at 298 K gave an effective magnetic moment value of 3.34 BM. This value is an intermediate value between the spin-only value for four unpaired electrons and the spin-only value for two unpaired electrons. This behavior may indicate that the chromium species existed in two mutually different electronic states (high spin and low spin) in thermal equilib-

9

R.M. Ramadan et al. / Journal of Molecular Structure 1049 (2013) 7–12 Table 2 Important IR, 1H NMR and kmax data for the H2acacen ligand and its chromium and molybdenum complexes. IR data (cm1)a

Compound

1

m(OH)

d(OH)

m(C@N)

m(M@O)

m(M–O–M)

(H2acacen)

3443(s)

1449(m)

1610(s)

–

–

[Cr(CO)3(H2acacen)]a

1434(m)

1596(s)

–

–

[Mo2O6(H2acacen)2]

3417(s) 3448(b) 3433(s)

1442(m)

1605(s)

940(s) 902(m)

760(m) 713(s)

[Mo2O6(bpy)(H2acacen)]

3410(s)

1442(s)

1601(s)

953(s) 883(m)

775(s) 717(s)

a

t(C@O): 2048(m), 1967(m) and 1921(m).

b

s, singlet; m, multiplet

H 3C

H O

CO

CO HO

Cr

kmax (nm) in DMSO

p–p⁄

n–p⁄

10.65 (s, O  H  N), 4.95 (s, @CHA), 3.73 (s, NACH2A), 1.93 (s, N@CACH3), 1.83 (s, C@CACH3) –

300

316

270

334

9.7 (OH), 7.7 (w, -ph), 4.9 (w, @CHA), 3.3 (s, NACH2A), 2.1 (m, N@CACH3), 9.4 (OH), 8.69–8.37 (m, -ph), 3.3 (s, NACH2A), 2.13 (w, N@CACH3)

280

330

278

320

OH

CH3

CH3

N CO N

CH3 O O O N Mo Mo O N O O H3C N

OH

N

H3C

CH3

H 3C

H NMR data (ppm)b

H3C

Scheme 2. Proposed structure of [Cr(CO)3(H2acacen)].

rium. Measurements of the magnetic susceptibilities at different temperatures showed a slight increase by increasing the temperature, which could support the findings. Reaction of molybdenum hexacarbonyl with H2acacen under reduced pressure yielded decomposed and unidentified product. Therefore, the interaction of [Mo(CO)6] with H2acacen was tried in presence of air. It was found that the reaction of [Mo(CO)6] with H2acacen ligand alone or in presence of bipyridine (bpy) in THF under sunlight photolysis in presence of air resulted in the formation of the binuclear complexes [Mo2O6(H2acacen)2] and [Mo2O6(bpy)(H2acacen)], respectively. The infrared spectrum of the [Mo2O6(H2acacen)2] complex displayed the ligand bands with the proper shifts indicating complex formation (Table 2) [29]. In addition, the IR spectrum of the complex exhibited two bands at 940 and 902 cm1 due to symmetric and asymmetric stretching vibrations of two Mo@O bonds in cis positions. Furthermore, another two bands at 760 and 713 cm1 were observed due to t(MoAOAMo) frequencies [24–27]. The IR spectrum of the complex also displayed a broad band due to t(OH) stretches. The presence of the OH groups was also confirmed by 1H NMR

OH

CH3 CH3

H3C OH

Scheme 3. The proposed structure of [Mo2O6(H2acacen)2].

spectroscopy. The OH signals displayed shifts to higher field (Table 3). The shifts in the 1H NMR signals could probably indicate the participation of the two OH groups in intra- and intermolecular hydrogen bonding. In addition, the IR spectrum showed the non-ligand MoAN and MoAO bands [27]. Scheme 3 shows the proposed structure for the complex according to the spectroscopic evidences. Similarly, the IR spectrum of the [Mo2O6(bpy)(H2acacen)] complex exhibited bands due to t(Mo@O) and t(MoAOAMo) stretches (Table 2). The [Mo2O6(bpy)(H2acacen)] complex also showed that the in-plane ring deformation bands of pyridine, d(py), at 656– 580 cm1 were shifted to higher frequency owing to bonding through the pyridyl nitrogen [28,29]. Furthermore, the 1H NMR

Table 3 Thermal analysis data for the chromium and molybdenum complexes. Complex

Decomposition step (K)

Weight loss (%)

Mol. Wt

Species eliminated

Solid residue (%)

[Cr(CO)3(H2acacen)]

323–573 573–745 323–765 765–1173 200–440 440–620 620–900

23.33 57.78 22.83 38.04 23.38 8.38 25.15

84.03 224.00 168.00 280.00 156.19 56.00 168.00

3 CO C12H20N2O C10H16O2 C14H24N4O2 C10H8N2 C2H4N2 C10H16O2

18.89

[Mo2O6(H2acacen)2] [Mo2O6(bpy)(H2acacen)]

39.13 43.09

10

R.M. Ramadan et al. / Journal of Molecular Structure 1049 (2013) 7–12

OH N

N O

Mo

O

O O H3C

O Mo N

O N

CH3 CH3

H3C

spectroscopy, where the OH signals were shifted to higher field. In addition, the IR spectrum of the complex displayed strong stretching bands due to t(C@N) stretches. These bands were shifted to lower wave numbers with respect to the free ligand confirming the coordination of the H2acacen ligand through its azomethine groups (Table 2). Magnetic measurements showed diamagnetic properties for the oxo molybdenum complexes. Accordingly, molybdenum may have + 6 formal oxidation states with d0 electronic configuration. Scheme 4 gives the proposed structure of [Mo2O6(bpy)(H2acacen)] complex. 4.1. UV–vis studies

OH Scheme 4. The proposed structure of [Mo2O6(bpy)(H2acacen)] complex.

spectrum of the complex showed a set of three multiplets due to the pyridyl moieties (Table 2); they exhibited shifts to lower field with respect to the corresponding signals of the free bipyridine [29]. A previous study showed that the interaction of [M(CO)6], M = Mo or Cr with salpenH2 in the presence of a secondary ligand L, L = bpy or dmbpy, gave the mixed ligand complexes with the formula [M(L)(salpenH2)]. The 1H NMR spectra of these complexes showed a similar set of multiplets due to bipyridine or 3,30 -dimethylbipyridine ligands with corresponding shifts with respect to free ligands [30]. The IR spectrum of the [Mo2O6(bpy)(H2acacen)] complex displayed bands due to t(OH) and d(OH) stretches (Table 2). The presence of the OH groups was also confirmed by 1H NMR

Fig. 1. Optimized 3D structure of H2acacen ligand.

The electronic absorption spectra of H2acacen and its complexes were studied in DMSO. The UV–vis spectra of the ligand displayed two absorption bands at 295 and 316 nm, which are assigned for the p–p⁄ and n–p⁄ transitions, respectively. In the complexes, a hypsochromic shift in the p–p⁄ electronic transitions was observed. On the other hand, the n–p⁄ electronic transitions exhibited bathochromic shifts (Table 2). 4.2. Thermogravimetric analysis The thermal studies of the reported chromium and molybdenum complexes provided further insight into the proposed structures. The thermal studies were carried out using thermogravimetric analysis (TG) technique. The decomposition mass losses were found in agreement with the formula weight of each complex proposed from the elemental analysis and the mass spectrometry, Table 1. The decomposition patterns were further confirmed from the theoretical calculations based on accurate DFT approximations. The decomposition ranges along with the corresponding weight losses are given in Table 3. The TGA plot of [Cr(CO)3(H2acacen)] complex displayed two decomposition steps in the temperature range 323–745 K. The first decomposition step which consisted of two overlapped and non–resolved decomposition steps occurred in the temperature range 323–573 K with a net weight loss of 23.33% which corresponds to the elimination of three CO molecules. The second decomposition step occurred in the temperature range 573–745 K with a net weight loss of 57.78% which corresponds to the elimination of C12H20N2O species to give chromium oxide residue with a net weight of 18.89% (Table 3). The decomposition sequences and steps confirm the proposed structure of the [Cr(CO)3(H2acacen)] complex. The [Mo2O6(H2acacen)2] complex displayed two decomposition steps. The first decomposition step occurred in the temperature range 323–765 K with a net weight loss of 22.83% and could be due to elimination of a C10H16O2 species. The second decomposi-

Fig. 2. Optimized 3D structure of [Cr(CO)3(H2acacen)] complex: (a) octahedral; (b) trigonal bipyramidal form.

R.M. Ramadan et al. / Journal of Molecular Structure 1049 (2013) 7–12

tion step occurred in the temperature range 765–1173 K with a net weight loss of 77.15%. This decomposition step corresponds to the elimination of C14H24N4O2 species to give finally a metallic oxide residue of MoO3 with a net weigh of 39.13% Table 3. The [Mo2O6(bpy)(H2acacen)] complex was found to decompose thermally in three steps. The first decomposition step occurred in the temperature range 200–440 K with a net weight loss of 23.38% corresponded to the elimination of C10H8N2 species. The second decomposition step occurred in the range 440–620 K with a net weight loss of 8.38% corresponds to the elimination of C2H4N2 species. The third decomposition step occurred in the temperature range 620–900 K with a weight loss of 25.15% which corresponds to the elimination of C10H16O2 species leaving finally a

11

metallic residue of MoO3 with a net weight of 43.09%. These results were in agreement with the proposed structure of the [Mo2O6(bpy)(H2acacen)] complex. 5. Chemical reactivity prediction To investigate the stereochemistry of the most stable complexes, we first focused on the structure of H2acacen molecule. More specifically, the orientation of its terminal function groups with respect to each other and with respect to the central ethyl moiety. Energetically stable model (Fig. 1) showed specific feature, where each hydroxyl group was oriented towards the nearest imine group to form a hydrogen bond. In addition, it can be ob-

Fig. 3. Optimized 3D structure of [Mo2O6(H2acacen)2] complex.

Fig. 4. Optimized 3D structure of [Mo2O6(bpy)(H2acacen)] complex.

12

R.M. Ramadan et al. / Journal of Molecular Structure 1049 (2013) 7–12

Table 4 The calculated HOMO, LUMO, energy gap (DE), chemical hardness (g), electronic chemical potential (l) and electrophilicity (x) of the reported complex. Model [Cr(CO)3(H2acacen)] [Mo2O6(H2acacen)2] [Mo2O6(bpy)(H2acacen)]

HOMO (eV)

LUMO (eV)

DE (eV)

g

l

x

(eV)

(eV)

(eV)

3.65 3.04 3.94

1.20 1.49 2.50

2.45 1.55 1.44

1.23 0.78 0.72

2.43 2.27 3.22

2.40 3.30 7.20

to change in the electron distribution or charge transfer. The electronic chemical potential is a measure of electronegativity of the molecule. The electrophilicity index, in turn, measures the propensity or the capacity of a species to accept electrons. Table 4 gives the computed chemical descriptors for all complexes under investigation. The results indicated that the chemical reactivity order of the complexes is [Mo2O6(bpy)(H2acacen)]  [Mo2O6(H2acacen)2] > [Cr(CO)3(H2acacen)]. References

served that the structure is not planned, however one hydroxyl group was bent away from the plane contains the ethyl, imine and the other hydroxyl groups in order to reduce the steric repulsion. The dihedral angle between the two planes was found to be 69.3°. This orientation suggested that the ligand may coordinate to the metal through the two imine nitrogen groups to act as a bidentate ligand or acts as a tridentate with the coordination of additional hydroxyl oxygen atom. For [Cr(CO)3(H2acacen)] complex, two proposed models were calculated with octahedral and trigonal bipyramidal geometries (Fig. 2a and b). Energetically octahedral model was found to be more stable by about 25.22 kcal/mol. This is consistent with the spectroscopic findings. The bonding between Cr and H2acacen was determined from the distance and angle analysis. The bond lengths for (CrAN) and (CrAO) are 1.89 Å and 2.01 Å, respectively. Optimized angles for (N1ACrAN2), (N1ACrAO) and (N2ACrAO) are 91.7°, 173.3° and 85.6°, respectively. The carbonyl groups bound to the central Cr forming a T-shape to complete the octahedral coordination. The two molybdenum complexes [Mo2O6(H2acacen)2] and [Mo2O6(bpy)(H2acacen)] were optimized at the same level of theory. Fig. 3 illustrates the minimum energy optimized structure of [Mo2O6(H2acacen)] complex. The significant feature of this structure is the distorted octahedral coordination of each Mo ion. To release the steric effect, the axial oxygen atoms were bent forming (O@Mo@O) angle of 120° with a cis configuration. The cis configuration is consistent with the order of the symmetric and asymmetric stretching frequencies in the IR spectrum of the complex. Electronic structural analysis reveals that [Mo2O6(H2acacen)2] complex has 1.55 eV energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). On the other hand, the [Mo2O6(bpy)(H2acacen)] complex structure was also found to have a distorted octahedral geometry (Fig. 4). The electronic structure of this complex has 1.44 eV energy gap, which is smaller energy gap relative to that of [Mo2O6(H2acacen)2] complex. This may explain the color change of the two complexes. Also, this value may give a first insight about the photochemical reactivity of these complexes. To predict the relative reactivity of the obtained complexes, global chemical reactivity descriptors (energy gap, chemical hardness, electronic chemical potential and electrophilicity) were calculated [31–33]. The physical meaning of these descriptors is defined as follows: Chemical hardness in a molecule refers to the resistance

[1] M. Kaushik, A. Singh, M. kumar, Eur. J. Chem. 3 (2012) 367. [2] A.K. Abu Al-Nasr, R.M. Ramadan, Spectrochim. Acta 105A (2013) 14. [3] M.A. Taher, S.E. Jarelnabbi, B.E. Bayoumi, S.M. El-Medani, R.M. Ramadan, Inter. J. Inorg. Chem. (2010), http://dx.doi.org/10.1155/2010/296215. [4] M.A. Taher, S.E. Jarelnabbi, A.G.M. Al-Sehemi, S.M. El-Medani, R.M. Ramadan, J. Coord. Chem. 62 (2009) 1293. [5] P.J. McCarthy, R.J. Hovey, K. Ueno, A.E. Martell, J. Am. Chem. Soc. 77 (1995) 5820. [6] H.G. Na, D.C. Lee, J.W. Lim, J.H. Choi, J.C. Byun, Y.C. Park, Polyhedron 21 (2002) 917. [7] S.A. Sallam, A.S. Orabi, B.A. El-Shetary, A. Lentz, Transition Met. Chem. 27 (2002) 447. [8] K. Kervinen, P. Lahtinen, T. Repo, M. Svahn, M. Leskela, Catal. Today 75 (2002) 183. [9] Y.P. Wang, C.P. Chai, R.M. Wang, Polym. Adv. Technol. 13 (2002) 11. [10] Y.P. Call, H.Z. Ma, B.S. Kang, C.Y.Su.W. Zhang, J. Sun, Y.L. Xiong, J. Organomet. Chem. 628 (2001) 99. [11] S. Chandra, K. Gupta, Indian J. Chem. 40A (2001) 775. [12] E.S. Ibrahim, S.A. Sallam, A.S. Orabi, B.A. El-Shetary, A. Lentz, Monatsh. Chem. 129 (1998) 159. [13] A. Böttcher, T. Takeuchi, K.I. Hardcastle, T.J. Meade, H.B. Gray, Inorg. Chem. 36 (1997) 2498. [14] S.A. Serron, C.M. Haar, S.P. Nolan, Organometallics 16 (1997) 5120. [15] E.B. Tjaden, D.C. Swenson, R.F. Jordan, J.L. Petersen, Organometallics 14 (1995) 371. [16] N. Chimpalee, D. Chimpalee, P.K. Eawpasert, D.T. Burns, Anal. Chim. Acta 408 (2000) 123. [17] N. Chimpalee, D. Chimpalee, S. Lohwithee, L. Nakwatchara, D.T. Burns, Anal. Chim. Acta 33 (1996) 253. [18] S. Özkar, D. Ülkü, L.T. Yıldırım, N. Biricik, B. Gümgüm, J. Mol. Struct. 688 (2004) 207. [19] M.J. Frisch et al., Gaussian 03, Revision C. 01, Gaussian, Inc., Wallingford, CT, 2004. [20] R. Ditchfield, W.J. Hehre, J.A. Pople, J. Chem. Phys. 54 (1971) 724. [21] K.V. Reddy, Symmetry and Spectroscopy of Molecules, New Age International, New Delhi, India, 1998. [22] D.Y. Sabry, T.A. Youssef, S.M. El-Medani, R.M. Ramadan, J. Coord. Chem. 56 (2003) 1375. [23] S.M. El-Medani, J. Coord. Chem. 57 (2004) 115. [24] A.A. Soliman, S.A. Ali, A. Orabi, Spectrochim. Acta 65A (2006) 841. [25] M.M.H. Khalil, H.A. Mohamed, S.M. El-Medani, R.M. Ramadan, Spectrochim. Acta 59 (2003) 341. [26] S. Partha, K. Wieghardt, Inorg. Chem. 26 (1987) 12. [27] K. Nakamoto, Infrared and Raman Spectra of Inorganic and Coordinated Compounds, fourth ed., Wiley, New York, 1986. [28] A.A. Abdel-Shafi, M.M.H. Khalil, H.H. Abdalla, R.M. Ramadan, Transition Met. Chem. 27 (2002) 69. [29] R.M. Silverstein, G.C. Bassler, T.C. Morrill, Spectrometric Identification of Organic Compounds, fourth ed., Wiley, New York, 1981. [30] T.A. Youssef, J. Coord. Chem. 61 (2008) 816. [31] K.D. Sen, D.M.P. Mingos, Structure and Bonding: Chemical Hardness, vol. 80, Springer, Berlin, 1993. [32] R.G. Parr, W. Yang, Density Functional Theory of Atoms and Molecules, Oxford University Press, Oxford, 1989. [33] P. Geerlings, F. de Proft, W. Langenaeker, Chem. Rev. 103 (2003) 1793.