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Synthesis and spectral studies of cadmium(II) complexes derived from di-2-pyridyl ketone and N4-phenylsemicarbazide: First structural report of a cadmium(II) complex of semicarbazone
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Polyhedron 27 (2008) 1825–1831 www.elsevier.com/locate/poly
Synthesis and spectral studies of cadmium(II) complexes derived from di-2-pyridyl ketone and N4-phenylsemicarbazide: First structural report of a cadmium(II) complex of semicarbazone T.A. Reena, E.B. Seena, M.R. Prathapachandra Kurup * Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India Received 7 August 2007; accepted 18 February 2008 Available online 26 March 2008
Abstract A new semicarbazone (HL) based on di-2-pyridyl ketone and its three cadmium(II) complexes [CdL(CH3COO)]2 2CH3OH (1), Cd(HL)Br2 (2) and [Cd2L2N3]2 H2O (3) were synthesized and characterized by different physicochemical techniques. The complex, [CdL(CH3COO)]2 2CH3OH (1) is having a dimeric structure. In complexes 1 and 3, the ligand moieties are coordinated as monoanionic (L ) forms and in complex 2, the ligand is coordinated as neutral (HL) one. The coordination geometry around cadmium(II) in 1 is distorted octahedral, as obtained by X-ray diffraction studies. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Crystal structure; IR spectra; Cadmium(II) complex; Di-2-pyridyl ketone; Semicarbazone
1. Introduction Semicarbazones are compounds having the formula R2C@N–NH–(CO)–NH2 formally derived by condensation of aldehyde or ketone with semicarbazide. A great number of studies have been devoted to the search for derivatives of semicarbazides. In recent years there has been considerable interest in semicarbazones due to their wide spectrum of biological applications [1]. These materials have been used as drugs whose action is attributed to their ability to form metal complexes [2]. It was reported that many metal complexes of semicarbazones have antimicrobial and antitumor activities [3]. A number of aryl semicarbazones displayed anticonvulsant activity in the maximal estroschock (MES) and subcutaneous pentylenetetrazole (SCPTZ) screens when administered intraperitoneally to mice [4].
*
Corresponding author. Tel.: + 91 484 2575804; fax: +91 484 2577595. E-mail address: [email protected] (M.R. Prathapachandra Kurup).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.02.020
Semicarbazones are compounds with versatile structural features and can coordinate to the metal either as a neutral ligand or as a deprotonated anion. Semicarbazones exist in two tautomeric forms, keto (A) and enol (B) (Scheme 1). The coordination possibilities in the semicarbazones are increased if the substituents of the aldehyde or ketone include additional donor atoms. The p-delocalization and the configurational flexibility of their molecular chain can give rise to a great variety of coordination modes [5]. In some metal complexes, semicarbazone can act as a tridentate ligand with a donor atom apart from ketone/enol oxygen atom and azomethine nitrogen. Cadmium is well known to form complexes with acetates and carboxy-ligands to yield both charged and neutral compounds [6]. The possibility of forming structures with higher coordination numbers has resulted in the observation of unusual coordination geometries about the metal atom and formation of polymeric species [7]. Here we report three cadmium(II) complexes of N4-substituted semicarbazone ligand derived from di-2-pyridyl ketone and N4-phenylsemicarbazide. There are no previous
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T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831 O
OH R
N
R N
N
R
H
R
N N
R
N
R R
Keto (A)
R
Enol (B) Scheme 1.
structural reports on cadmium(II) complexes of a semicarbazone according to the best of our knowledge [5]. N
2. Experimental NH
2.1. Materials Di-2-pyridyl ketone (Aldrich) was used as received. CdBr2 4H2O, (CH3COO)2Cd 2H2O and NaN3 were commercial products of higher grade (Aldrich) and solvents were purified according to standard procedures. Elemental analyses were carried out using a vario EL III CHNS analyzer at SAIF, Kochi, India. Infrared spectra were recorded on a JASCO FT-IR-5300 spectrometer in the range 4000–400 cm 1 using KBr pellets. Electronic spectra were recorded on a Cary 5000 version 1.09 UV– Vis–NIR spectrophotometer using solutions in DMF. The 1H and 13C NMR spectra were recorded using Bruker DRX 500, with CDCl3 as solvent and TMS as standard at the Sophisticated Instruments Facility, Indian Institute of Science, Bangalore, India. 2.2. Synthesis of di-2-pyridyl ketone-N4-phenyl semicarbazone (HL) The semicarbazone was synthesized by the following method. A methanolic solution of di-2-pyridyl ketone (0.184 g, 1 mmol) was mixed with N4-phenylsemicarbazide (0.151 g, 1 mmol) in methanol and three drops of glacial acetic acid. The reaction mixture was refluxed for 2 h. On slow evaporation, colorless crystalline compound formed was filtered, washed with ether and recrystallized from ethanol and dried over P4O10 in vacuo. Melting point = 160 °C. 1H NMR (CDCl3, 400 MHz, d from TMS): 13.37 (s, N4H, 1H), 8.78 (d, C1H, 1H, J = 6.4 Hz), 8.66 (d, C11H, 1H, J = 7.2 Hz), 8.37 (s, N5H, 1H), 7.87 (t, C3H, 1H, J = 7), 7.79 (t, C9H, 1H, J = 6), 7.56 (d, C4H, 1H, J = 8), 7.51 (d, C8H, 1H, J = 8), 7.34 (m, C2,10,14,15,17,18, 6H), 7.07 (t, C16H, 1H, J = 7). 13C NMR (CDCl3, d from TMS): C(1), 148.68, C(2), 123.87, C(3), 136.81, C(4), 126.24, C(5), 151.96, C(6), 152.95, C(7), 148.0; C(8), 128.89, C(9), 137.98, C(10), 123.87, C(11), 140.28, C(12), 156.20, C(13), 137.98, C(14), 123.75, C(15), 123.38, C(16), 119.60, C(17), 123.38, C(18), 123.75 (see Fig. 1).
N NH
O
N
Fig. 1. Molecular structure of HL.
2.3. Synthesis of [CdL(CH3COO)]2 2CH3OH (1) A methanolic solution of HL (0.317 g, 1 mmol) and Cd(CH3COO)2 2H2O (0.266 g, 1 mmol) were mixed and heated under reflux for 5 h. On slow evaporation, yellow crystals were obtained, which were separated, washed with ether and dried over P4O10 in vacuo. 2.4. Synthesis of Cd(HL)Br2 (2) A methanolic solution of HL (0.317 g, 1 mmol) and CdBr2 4H2O (0.344 g, 1 mmol) were mixed and heated under reflux for 5 h. On slow evaporation, yellow coloured solid was resulted which were separated, washed with ether and dried over P4O10 in vacuo. 2.5. Synthesis of Cd2L2(N3)2 H2O (3) A methanolic solution of HL (0.317 g, 1 mmol) and Cd(CH3COO)2 2H2O (0.266 g, 1 mmol) were mixed and heated under reflux for 2 h and then added methanolic solution of NaN3 (0.065 g, 1 mmol). The resulting mixture was refluxed for 4 h. On slow evaporation yellow colored solid formed was separated, washed with ether and dried over P4O10 in vacuo. 2.6. X-ray crystallography Single crystals of compound 1 suitable for X-ray analysis were obtained from its methanolic solution and were found to be orthorhombic with a space group ‘pbca’. Xray diffraction measurements were carried out on CrysAlis CCD diffractometer with graphite-monochromated
T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
˚ ) radiation. The program CRYSALIS Mo Ka (k = 0.71073 A RED was used for data reduction and cell refinement [8]. The structure was solved by direct methods using SHELXS [9] and refined by full-matrix least-squares refinement on F2 using SHELXL [10]. The N–H hydrogen atoms were located from difference Fourier maps and refined isotropically. The remainder of the H-atoms were included in calculated positions and refined as riding atoms using default SHELXL parameters. The structure of the compound 1 was plotted using the program DIAMOND Version 3.1d [11] and PLATON [12]. The final refinement cycle was based on all 3610 independent reflections and 290 variables with R1 = 0.0301, wR2 = 0.0665. The crystallographic data along with the structural refinements are given in Table 1.
Table 1 Crystal data and structure [CdL(CH3COO)]2 2CH3OH
refinement
Empirical formula
C42H42Cd2N10O8
Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) ˚ 3) Volume (A Z Dcalc (g/cm3) Absorption coefficient F(0 0 0) Crystal size (mm) h Range for data collection (°) Index ranges
1039.68 120(2) 0.71073 orthorhombic ‘pbca’
Reflections collected Independent reflections [R(int)] Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data)
parameters
13.8648(3) 15.3588(5) 19.2468(4) 90 90 90 4098.54(18) 4 1.685 1.105 mm 1 2096 0.23 0.18 0.13 2.86–25 13 6 h 6 16, 18 6 k 6 15, 22 6 l 6 22 3610 290 [0.0361] full-matrix least-squares on F2 5041/0/313 1.065 R1 = 0.0301, wR2 = 0.0665 R1 = 0.0440, wR2 = 0.0725
for
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3. Results and discussion 3.1. Synthesis of complexes Compounds 1 and 2 were prepared by direct reaction between the ligand and the corresponding salts, while compound 3 was prepared by the replacement of the acetate in Cd(CH3COO)2 2H2O by the azide ion. Elemental analyses data (Table 2) of complexes 1 and 3 reveal that metal–ligand are in the ratio 1:1 and in complex 2, di-2pyridyl ketone-N4-phenyl semicarbazone (HL) interacts with Cd(II) ions in the presence of anion Br in the ratio 1:1. 3.2. Crystal structure of [CdL(CH3COO)]2 2CH3OH Fig. 2 shows the molecular structure of the binuclear centrosymmetric Cd(II) complex with the atom numbering scheme. Both Cd centers being hexacoordinated and the coordination around the Cd(II) ion can be best described as a distorted octahedron with a CdO3N3 chromophore. Each Cd atom is coordinated by azomethine nitrogen, oxygen atom from the principal ligand, two pyridyl nitrogens and two oxygen atoms from the acetate group. The acetate groups serve as bidentate ligands and the semicarbazones as tetradentate ligands. The two pyridyl nitrogens from the principal ligand are coordinated to two different Cd centers forming a cyclic type architecture (Fig. 3). The coordination of oxygen from the semicarbazone ligand occurs through deprotonation after enolization, confirmed by the partial single and double bond nature of C7–O1 ˚ and N3–C7 [1.368(4) A ˚ ] bond lengths [13,14]. [1.350(4)] A ˚ The C6–N2 and N2–N3 bond distances are 1.257(4) A ˚ respectively, which reveal extensive delocaland 1.338(3) A ization over the entire binuclear coordination framework. Table 3 shows the selected bond lengths and bond angles of compound 1. In the case of this Cd-semicarbazone complex, Cd–O ˚ ; Cd1–O2, bond distances are Cd1–O1, 2.249(2) A ˚ ; Cd1–O3, 2.434(2) A ˚ and Cd–N bond distances 2.7666 A ˚ ; Cd1–N2, 2.515(3) A ˚ and Cd1– are Cd1–N1, 2.168(3) A ˚ respectively. The Cd1–O1 bond distance is N5, 2.155(3) A slightly less than the other Cd–O bond distances reported for other octahedral Cd(II) complexes [15,16]. The Cd1– O2 and Cd1–O3 bond distances lie within the range of ˚] Cd–Ocarboxylate bond distances [2.209(2)–2.879(2) A
Table 2 Elemental analyses and color of Cd(II) complexes Compound
Color
Calc. (found) (%)
HL [CdL(CH3COO)]2 2CH3OH (1) [Cd(HL)Br2] (2) [CdLN3]2 (3)
yellow yellow yellow yellow
68.13 48.16 36.69 44.46
C
H (67.44) (48.52) (36.67) (45.06)
4.76 3.97 2.64 3.05
N (4.80) (4.07) (2.56) (3.15)
22.07 13.20 12.45 24.15
(22.14) (13.47) (11.80) (23.36)
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T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
Fig. 2. Structure and labeling diagram for compound 1 (hydrogen atoms and methanol molecules are omitted for clarity).
reported for Cd(II)–Ocarboxylate coordination polymers [17]. The Cd1–N2 bond distance is slightly larger than the other Cd–N bond distances, which supports the lack of significant out-of-plane p-bonding. The trans angle, O2–Cd1– N2 is 155.84(8)° indicating that the compound 1 has large distortion from octahedral geometry [18]. Other indication of distorted octahedral geometry is the bond angle of O1– Cd1–O3, 85.51(8)°. The unit cell packing diagram of the complex viewed along the b axis is given in Fig. 4. It can be observed that the molecules are packed in a zig-zag manner. The molecules in the unit cell are connected through intermolecular hydrogen bonding interactions involving the oxygen atom of the methanol molecule [O(4)–H(104)–O(3); O(4)–H(104), ˚; ˚; 0.81(4) A O(4) O(3), 2.726(4) A H(104) O(3), ˚ 1.92(4) A; O(4)–H(104) O3, 174(3)° and N4–H4N O4i ˚ ; N4 O4, [i = x, 1/2 + y, 3/2 z]; N4–H4N, 0.85(3) A ˚ ˚ 3.132(4) A; H4N O4, 2.30(3) A; N4–H4N O4, 166(3)°] (Fig. 5). Ring puckering analysis show that the ring Cg(3) consisting of atoms Cd1–N1–C5–C6–N2 adopts an envelope ˚ ] [19]. The diverse p–p stacking on Cd1 [QT = 0.1086 (22) A and C–H–p interactions give rise to polymeric chains in the unit cell. The shortest p–p interactions are perceived at ˚ for Cg(2)–Cg(3)ii [Cg(2) = Cd1–O1–C7–N3–N2; 3.1992 A ii = x, y, 2 z]. Two C–H–p interactions are present in the unit cell. They are C(11)–H(11)–Cg(2)iii [dH–Cg = ˚ ; iii = 1/2 + x, y, 3/2 z] and C(21)–H(21)–Cg(5)iii 2.94 A ˚ ; iii = x, 1/2 y, 1/2 + z]. The p–p, C–H– [dH–Cg = 2.93 A p and hydrogen bonding interactions contribute stability to the unit cell packing.
Fig. 3. Cyclic formation through cadmium centers.
Table 3 ˚) Selected bond lengths (A [CdL(CH3COO)]2 2CH3OH
and
bond
angles
(°)
for
Bond lengths
Bond angles
Cd(1)–N(5) 2.155(3) Cd(1)–N(1) 2.168(3) Cd(1)–O(1) 2.249(2) Cd(1)–O(3) 2.434(2) Cd(1)–N(2) 2.515(3) O(1)–C(7) 1.350(4) N(1)–C(1) 1.349(4) N(1)–C(5) 1.479(4) N(2)–C(6) 1.257(4) N(2)–N(3) 1.338(3) N(3)–C(7) 1.368(4) N(4)–C(8) 1.326(4) N(4)–C(7) 1.354(4) N(4)–H(4N) 0.85(3)
N(5)–Cd(1)–N(1) 116.23(9) N(5)–Cd(1)–O(1) 92.67(9) N(1)–Cd(1)–O(1) 132.65(9) N(5)–Cd(1)–O(3) 107.15(9) N(1)–Cd(1)–O(3) 116.82(9) N(5)–Cd(1)–N(2) 119.45(9) O(3)–Cd(1)–N(2) 123.35(8) C(7)–O(1)–Cd(1) 119.56(18) C(1)–N(1)–C(5) 125.7(3) C(6)–N(2)–N(3) 113.1(3) C(6)–N(2)–Cd(1) 120.7(2) N(3)–N(2)–Cd(1) 126.16(19) N(2)–N(3)–C(7) 100.2(3) O(1)–C(7)–N(3) 130.7(3)
Fig. 4. Unit cell packing diagram viewed along b axis.
T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
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Fig. 5. Intermolecular hydrogen bonding interactions through methanol molecule.
3.3. Spectroscopic studies The tentative assignments of the selected IR bands are listed in the Table 4. The spectrum of the ligand exhibits a medium band at 3369 cm 1 which is assigned to m(NH) vibration. The absence of m(NH) in the spectra of complexes 1 and 3 suggests that the ligand loses this proton on complexation, thus acting as uninegative ligand [20]. In complex 2, the presence of a band at 3251 cm 1 corresponding to m(NH) vibration indicates that the semicarbazone is coordinated in the neutral form. A band at 1718 cm 1 in the ligand has significant contribution from C@O stretching vibration and absence of such band in complexes 1 and 3 confirming the coordination through enolate oxygen. The presence of a band at 1656 cm 1 for complex 2 supports the keto form of the ligand in this complex. On coordination of the azomethine nitrogen, the IR stretching frequency m(C@N) at 1591 cm 1 shifts to 1605, 1597 and 1596 cm 1 for complexes 1, 2 and 3, respectively [21–23]. The spectra of the complexes exhibit a systematic shift in the position of m(N–N) bands in the region 1148– 1141 cm 1 and confirms the coordination of the azomethine nitrogen. The bands appearing at 457, 451 and 463 cm 1 which correspond to m(Cd–N) for complexes 1– 3 again confirm the coordination of the azomethine nitrogen [24]. Coordination of pyridyl nitrogen causes the outof-plane bending vibrational band to shift from 601 cm 1 to higher frequencies 622, 621 and 627 cm 1 for complexes 1–3 [25]. Presence of a band at 1515 and 1521 cm 1 for
complexes 1 and 3 are assigned to the newly formed m(N@C) after enolization [26]. The asymmetric and symmetric stretching vibrations of the acetate group appear at 1568 and 1437 cm 1, respectively, for complex 1 having the separation value Dm = 131 cm 1 suggests the presence of chelating acetate group linked with the metal center for the complex [27,28]. The azido complex 3 exhibits a strong band at 2052 cm 1 corresponding to ma(NNN) stretching vibration of the coordinating azido group. Electronic spectrum of the ligand shows an absorption maximum at 40000 cm 1 attributed to intra-ligand p ? p* transitions of the pyridyl ring and imine function of the semicarbazone moiety. The peak at 30770 cm 1 indicate n ? p* transition of the amide function, which is shifted to higher energy values upon complexation [23] (Table 5). The 1H NMR spectrum of the compound HL along with the assignments is given in Fig. 6. A sharp singlet, which integrate as one hydrogen at d = 13.37 ppm is assigned to the proton attached to the nitrogen atom N(4). The downfield shift of this proton is assigned to its
Table 5 Electronic spectral assignments for the ligand and its Cd(II) complexes Compound
p–p*
n–p*
MLCT
HL [CdL(CH3COO)]2 2CH3OH (1) [Cd(HL)Br2] (2) [CdLN3]2(3)
40 000 37 590 36 360 37 310
30 770 31 640 31 250 31 850
24 810 23 530 24 690
Table 4 IR spectral assignments for ligand and its Cd(II) complexes Compound
m(NH)
m(C@N)
m(CO)
HL [CdL(CH3COO)]2 2CH3OH (1) [Cd(HL)Br2] (2) [CdLN3]2 (3)
3369
1591 1605 1597 1596
1718
3257
m(N@C) 1515
1656 1521
m(N–N) 1129 1148 1147 1141
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T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
Fig. 6. 1H NMR spectrum of HL.
hydrogen bonding interaction with adjacent nitrogen atom N(1). Another singlet at d = 8.37 ppm is assigned to the N(5) proton. This downfield also explained with the hydrogen bonding interaction with nitrogen atom N(3). Hydrogen bonding decreases the electron density around the proton and thus moves the proton absorption to a lower field. Absence of any coupling interactions by N(4) H and N(5) H protons due to the lack of availability of protons on neighboring atoms render singlet peaks for the imine protons. Two doublets at d = 8.78 and 8.66 ppm are assigned to the C(1)H and C(11)H protons respectively. These signals are shifted to lower field due to electronic effect of the adjacent electronegative pyridyl nitrogens and the more downfield shift of C(1)H can be attributed to the increased charge density on N(1) resulted by its hydrogen bonding to N(4)H. The proton resonance of the two pyridyl rings also appear separately in the NMR spectrum, due to the intramolecular hydrogen bonding. There are two triplets at d = 7.87 and 7.79 ppm corresponding to similar protons on C(3) and C(9) of the two pyridyl rings, while the C(2) and C(10) protons resonances appear overlapped in the multiplet 7.345 ppm. Another two doublets at d = 7.56 and 7.51 ppm are assigned to C(4) and C(8) protons. The resonances for the C6H5-group appear as triplet at 7.04 ppm for para proton and as multiplets at 7.34 ppm for ortho and meta phenyl protons [29]. The 13C NMR spectrum provides direct information about the carbon skeleton of the compound HL. Assignment of different resonant peaks to respective carbon atoms is presented in Fig. 7. Considering the two pyridyl rings non-equivalent, resulting from the hydrogen-bonding inter-
Fig. 7.
13
C NMR spectrum of HL.
action, there are 15 unique carbon atoms in the molecule, which give a total of 15 different peaks in the 13C NMR spectrum. In both the pyridyl rings, the C(1) and C(11) carbon atoms adjacent to the more electronegative nitrogen atoms N(1) and N(2) are shifted farther downfield when compared to the neighboring carbon atoms. Also the carbon atoms at para position to the hetero atoms viz. C(3) and C(9) resonate at lower field values when compared to the meta positioned carbons C(2), C(4), C(8) and C(10). The 13C peaks are assigned as follows: C(1), 148.68 ppm; C(2), 123.87 ppm; C(3), 136.81 ppm; C(4), 126.24 ppm; C(5), 151.96; C(6), 152.95 ppm; C(7), 148.0 ppm C(8), 128.89 ppm; C(9), 137.01 ppm; C(10), 123.87 ppm; C(11), 140.28 ppm; C(12), 156.20 ppm; C(13), 137.98 ppm; C(14), 123.75 ppm; C(15), 123.38 ppm; C(16), 119.60 ppm; C(17), 123.38 ppm; C(18), 123,75 ppm. The non-protonated carbon atom at C(6) is found downfield in the spectrum (d = 152.95 ppm), effected by the magnetic interaction of two bulky pyridyl rings and p electron delocalization on the C(6)@N(3) bond. Similarly, the C(12) carbon atom resonance is also observed farthest downfield of 156.20 ppm resultant of the conjugative effect of the –N(3)–N(4)– C(O)–N(5)– semicarbazone skeleton. The NMR data for the complexes are not available because of poor solubility. Acknowledgements M.R.P. Kurup is thankful to the Council of Scientific & Industrial Research (CSIR), New Delhi, India for the financial assistance. The authors are thankful to the
T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
National Single Crystal X-ray diffraction Facility, IIT, Bombay, India for providing single crystal XRD data. Appendix A. Supplementary data CCDC 645825 contain the supplementary crystallographic data for 1. 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) 1223336-033, or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2008.02.020. References [1] D.X. West, S.B. Padhye, P.S. Sonawane, Struct. Bond. 76 (1991) 1. [2] S.N. Pandeya, J.P. Dimmock, Pharmazie 48 (1993) 659. [3] Q. Li, H. Tang, Y. Li, M. Wang, L. Wang, C. Xia, J. Inorg. Biochem. 78 (2000) 167. [4] J. Dimmock, K. Sidhu, S. Tumber, S. Basran, M. Chen, J. Quail, I. Rozas, D. Weaver, Eur. J. Med. Chem. 30 (1995) 303. [5] J.S. Casas, M.S. Garcia-Tasende, J. Sordo, Coord. Chem. Rev. 209 (2000) 197. [6] M.L. Post, J. Trotter, J. Chem. Soc., Dalton Trans. (1974) 674. [7] P.A. Prasad, S. Neeraj, S. Natarajan, C.N.R. Rao, J. Chem. Soc., Chem. Commun. (2000) 1251. [8] CRYSALIS CCD and CRYSALIS RED Versions 1.171.29.2 (CrysAlis171. NET), Oxford Diffraction Ltd., Abingdon, Oxfordshire, England, 2006. [9] G.M. Sheldrick, Acta Crystallogr., Sect. A 46 (1990) 467. [10] G.M. Sheldrick, SHELXS-97 Program for X-ray Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997.
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Polyhedron 27 (2008) 1825–1831 www.elsevier.com/locate/poly
Synthesis and spectral studies of cadmium(II) complexes derived from di-2-pyridyl ketone and N4-phenylsemicarbazide: First structural report of a cadmium(II) complex of semicarbazone T.A. Reena, E.B. Seena, M.R. Prathapachandra Kurup * Department of Applied Chemistry, Cochin University of Science and Technology, Kochi 682 022, Kerala, India Received 7 August 2007; accepted 18 February 2008 Available online 26 March 2008
Abstract A new semicarbazone (HL) based on di-2-pyridyl ketone and its three cadmium(II) complexes [CdL(CH3COO)]2 2CH3OH (1), Cd(HL)Br2 (2) and [Cd2L2N3]2 H2O (3) were synthesized and characterized by different physicochemical techniques. The complex, [CdL(CH3COO)]2 2CH3OH (1) is having a dimeric structure. In complexes 1 and 3, the ligand moieties are coordinated as monoanionic (L ) forms and in complex 2, the ligand is coordinated as neutral (HL) one. The coordination geometry around cadmium(II) in 1 is distorted octahedral, as obtained by X-ray diffraction studies. Ó 2008 Elsevier Ltd. All rights reserved. Keywords: Crystal structure; IR spectra; Cadmium(II) complex; Di-2-pyridyl ketone; Semicarbazone
1. Introduction Semicarbazones are compounds having the formula R2C@N–NH–(CO)–NH2 formally derived by condensation of aldehyde or ketone with semicarbazide. A great number of studies have been devoted to the search for derivatives of semicarbazides. In recent years there has been considerable interest in semicarbazones due to their wide spectrum of biological applications [1]. These materials have been used as drugs whose action is attributed to their ability to form metal complexes [2]. It was reported that many metal complexes of semicarbazones have antimicrobial and antitumor activities [3]. A number of aryl semicarbazones displayed anticonvulsant activity in the maximal estroschock (MES) and subcutaneous pentylenetetrazole (SCPTZ) screens when administered intraperitoneally to mice [4].
*
Corresponding author. Tel.: + 91 484 2575804; fax: +91 484 2577595. E-mail address: [email protected] (M.R. Prathapachandra Kurup).
0277-5387/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2008.02.020
Semicarbazones are compounds with versatile structural features and can coordinate to the metal either as a neutral ligand or as a deprotonated anion. Semicarbazones exist in two tautomeric forms, keto (A) and enol (B) (Scheme 1). The coordination possibilities in the semicarbazones are increased if the substituents of the aldehyde or ketone include additional donor atoms. The p-delocalization and the configurational flexibility of their molecular chain can give rise to a great variety of coordination modes [5]. In some metal complexes, semicarbazone can act as a tridentate ligand with a donor atom apart from ketone/enol oxygen atom and azomethine nitrogen. Cadmium is well known to form complexes with acetates and carboxy-ligands to yield both charged and neutral compounds [6]. The possibility of forming structures with higher coordination numbers has resulted in the observation of unusual coordination geometries about the metal atom and formation of polymeric species [7]. Here we report three cadmium(II) complexes of N4-substituted semicarbazone ligand derived from di-2-pyridyl ketone and N4-phenylsemicarbazide. There are no previous
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T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831 O
OH R
N
R N
N
R
H
R
N N
R
N
R R
Keto (A)
R
Enol (B) Scheme 1.
structural reports on cadmium(II) complexes of a semicarbazone according to the best of our knowledge [5]. N
2. Experimental NH
2.1. Materials Di-2-pyridyl ketone (Aldrich) was used as received. CdBr2 4H2O, (CH3COO)2Cd 2H2O and NaN3 were commercial products of higher grade (Aldrich) and solvents were purified according to standard procedures. Elemental analyses were carried out using a vario EL III CHNS analyzer at SAIF, Kochi, India. Infrared spectra were recorded on a JASCO FT-IR-5300 spectrometer in the range 4000–400 cm 1 using KBr pellets. Electronic spectra were recorded on a Cary 5000 version 1.09 UV– Vis–NIR spectrophotometer using solutions in DMF. The 1H and 13C NMR spectra were recorded using Bruker DRX 500, with CDCl3 as solvent and TMS as standard at the Sophisticated Instruments Facility, Indian Institute of Science, Bangalore, India. 2.2. Synthesis of di-2-pyridyl ketone-N4-phenyl semicarbazone (HL) The semicarbazone was synthesized by the following method. A methanolic solution of di-2-pyridyl ketone (0.184 g, 1 mmol) was mixed with N4-phenylsemicarbazide (0.151 g, 1 mmol) in methanol and three drops of glacial acetic acid. The reaction mixture was refluxed for 2 h. On slow evaporation, colorless crystalline compound formed was filtered, washed with ether and recrystallized from ethanol and dried over P4O10 in vacuo. Melting point = 160 °C. 1H NMR (CDCl3, 400 MHz, d from TMS): 13.37 (s, N4H, 1H), 8.78 (d, C1H, 1H, J = 6.4 Hz), 8.66 (d, C11H, 1H, J = 7.2 Hz), 8.37 (s, N5H, 1H), 7.87 (t, C3H, 1H, J = 7), 7.79 (t, C9H, 1H, J = 6), 7.56 (d, C4H, 1H, J = 8), 7.51 (d, C8H, 1H, J = 8), 7.34 (m, C2,10,14,15,17,18, 6H), 7.07 (t, C16H, 1H, J = 7). 13C NMR (CDCl3, d from TMS): C(1), 148.68, C(2), 123.87, C(3), 136.81, C(4), 126.24, C(5), 151.96, C(6), 152.95, C(7), 148.0; C(8), 128.89, C(9), 137.98, C(10), 123.87, C(11), 140.28, C(12), 156.20, C(13), 137.98, C(14), 123.75, C(15), 123.38, C(16), 119.60, C(17), 123.38, C(18), 123.75 (see Fig. 1).
N NH
O
N
Fig. 1. Molecular structure of HL.
2.3. Synthesis of [CdL(CH3COO)]2 2CH3OH (1) A methanolic solution of HL (0.317 g, 1 mmol) and Cd(CH3COO)2 2H2O (0.266 g, 1 mmol) were mixed and heated under reflux for 5 h. On slow evaporation, yellow crystals were obtained, which were separated, washed with ether and dried over P4O10 in vacuo. 2.4. Synthesis of Cd(HL)Br2 (2) A methanolic solution of HL (0.317 g, 1 mmol) and CdBr2 4H2O (0.344 g, 1 mmol) were mixed and heated under reflux for 5 h. On slow evaporation, yellow coloured solid was resulted which were separated, washed with ether and dried over P4O10 in vacuo. 2.5. Synthesis of Cd2L2(N3)2 H2O (3) A methanolic solution of HL (0.317 g, 1 mmol) and Cd(CH3COO)2 2H2O (0.266 g, 1 mmol) were mixed and heated under reflux for 2 h and then added methanolic solution of NaN3 (0.065 g, 1 mmol). The resulting mixture was refluxed for 4 h. On slow evaporation yellow colored solid formed was separated, washed with ether and dried over P4O10 in vacuo. 2.6. X-ray crystallography Single crystals of compound 1 suitable for X-ray analysis were obtained from its methanolic solution and were found to be orthorhombic with a space group ‘pbca’. Xray diffraction measurements were carried out on CrysAlis CCD diffractometer with graphite-monochromated
T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
˚ ) radiation. The program CRYSALIS Mo Ka (k = 0.71073 A RED was used for data reduction and cell refinement [8]. The structure was solved by direct methods using SHELXS [9] and refined by full-matrix least-squares refinement on F2 using SHELXL [10]. The N–H hydrogen atoms were located from difference Fourier maps and refined isotropically. The remainder of the H-atoms were included in calculated positions and refined as riding atoms using default SHELXL parameters. The structure of the compound 1 was plotted using the program DIAMOND Version 3.1d [11] and PLATON [12]. The final refinement cycle was based on all 3610 independent reflections and 290 variables with R1 = 0.0301, wR2 = 0.0665. The crystallographic data along with the structural refinements are given in Table 1.
Table 1 Crystal data and structure [CdL(CH3COO)]2 2CH3OH
refinement
Empirical formula
C42H42Cd2N10O8
Formula weight Temperature (K) ˚) Wavelength (A Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) ˚ 3) Volume (A Z Dcalc (g/cm3) Absorption coefficient F(0 0 0) Crystal size (mm) h Range for data collection (°) Index ranges
1039.68 120(2) 0.71073 orthorhombic ‘pbca’
Reflections collected Independent reflections [R(int)] Refinement method Data/restraints/parameters Goodness-of-fit on F2 Final R indices [I > 2r(I)] R indices (all data)
parameters
13.8648(3) 15.3588(5) 19.2468(4) 90 90 90 4098.54(18) 4 1.685 1.105 mm 1 2096 0.23 0.18 0.13 2.86–25 13 6 h 6 16, 18 6 k 6 15, 22 6 l 6 22 3610 290 [0.0361] full-matrix least-squares on F2 5041/0/313 1.065 R1 = 0.0301, wR2 = 0.0665 R1 = 0.0440, wR2 = 0.0725
for
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3. Results and discussion 3.1. Synthesis of complexes Compounds 1 and 2 were prepared by direct reaction between the ligand and the corresponding salts, while compound 3 was prepared by the replacement of the acetate in Cd(CH3COO)2 2H2O by the azide ion. Elemental analyses data (Table 2) of complexes 1 and 3 reveal that metal–ligand are in the ratio 1:1 and in complex 2, di-2pyridyl ketone-N4-phenyl semicarbazone (HL) interacts with Cd(II) ions in the presence of anion Br in the ratio 1:1. 3.2. Crystal structure of [CdL(CH3COO)]2 2CH3OH Fig. 2 shows the molecular structure of the binuclear centrosymmetric Cd(II) complex with the atom numbering scheme. Both Cd centers being hexacoordinated and the coordination around the Cd(II) ion can be best described as a distorted octahedron with a CdO3N3 chromophore. Each Cd atom is coordinated by azomethine nitrogen, oxygen atom from the principal ligand, two pyridyl nitrogens and two oxygen atoms from the acetate group. The acetate groups serve as bidentate ligands and the semicarbazones as tetradentate ligands. The two pyridyl nitrogens from the principal ligand are coordinated to two different Cd centers forming a cyclic type architecture (Fig. 3). The coordination of oxygen from the semicarbazone ligand occurs through deprotonation after enolization, confirmed by the partial single and double bond nature of C7–O1 ˚ and N3–C7 [1.368(4) A ˚ ] bond lengths [13,14]. [1.350(4)] A ˚ The C6–N2 and N2–N3 bond distances are 1.257(4) A ˚ respectively, which reveal extensive delocaland 1.338(3) A ization over the entire binuclear coordination framework. Table 3 shows the selected bond lengths and bond angles of compound 1. In the case of this Cd-semicarbazone complex, Cd–O ˚ ; Cd1–O2, bond distances are Cd1–O1, 2.249(2) A ˚ ; Cd1–O3, 2.434(2) A ˚ and Cd–N bond distances 2.7666 A ˚ ; Cd1–N2, 2.515(3) A ˚ and Cd1– are Cd1–N1, 2.168(3) A ˚ respectively. The Cd1–O1 bond distance is N5, 2.155(3) A slightly less than the other Cd–O bond distances reported for other octahedral Cd(II) complexes [15,16]. The Cd1– O2 and Cd1–O3 bond distances lie within the range of ˚] Cd–Ocarboxylate bond distances [2.209(2)–2.879(2) A
Table 2 Elemental analyses and color of Cd(II) complexes Compound
Color
Calc. (found) (%)
HL [CdL(CH3COO)]2 2CH3OH (1) [Cd(HL)Br2] (2) [CdLN3]2 (3)
yellow yellow yellow yellow
68.13 48.16 36.69 44.46
C
H (67.44) (48.52) (36.67) (45.06)
4.76 3.97 2.64 3.05
N (4.80) (4.07) (2.56) (3.15)
22.07 13.20 12.45 24.15
(22.14) (13.47) (11.80) (23.36)
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Fig. 2. Structure and labeling diagram for compound 1 (hydrogen atoms and methanol molecules are omitted for clarity).
reported for Cd(II)–Ocarboxylate coordination polymers [17]. The Cd1–N2 bond distance is slightly larger than the other Cd–N bond distances, which supports the lack of significant out-of-plane p-bonding. The trans angle, O2–Cd1– N2 is 155.84(8)° indicating that the compound 1 has large distortion from octahedral geometry [18]. Other indication of distorted octahedral geometry is the bond angle of O1– Cd1–O3, 85.51(8)°. The unit cell packing diagram of the complex viewed along the b axis is given in Fig. 4. It can be observed that the molecules are packed in a zig-zag manner. The molecules in the unit cell are connected through intermolecular hydrogen bonding interactions involving the oxygen atom of the methanol molecule [O(4)–H(104)–O(3); O(4)–H(104), ˚; ˚; 0.81(4) A O(4) O(3), 2.726(4) A H(104) O(3), ˚ 1.92(4) A; O(4)–H(104) O3, 174(3)° and N4–H4N O4i ˚ ; N4 O4, [i = x, 1/2 + y, 3/2 z]; N4–H4N, 0.85(3) A ˚ ˚ 3.132(4) A; H4N O4, 2.30(3) A; N4–H4N O4, 166(3)°] (Fig. 5). Ring puckering analysis show that the ring Cg(3) consisting of atoms Cd1–N1–C5–C6–N2 adopts an envelope ˚ ] [19]. The diverse p–p stacking on Cd1 [QT = 0.1086 (22) A and C–H–p interactions give rise to polymeric chains in the unit cell. The shortest p–p interactions are perceived at ˚ for Cg(2)–Cg(3)ii [Cg(2) = Cd1–O1–C7–N3–N2; 3.1992 A ii = x, y, 2 z]. Two C–H–p interactions are present in the unit cell. They are C(11)–H(11)–Cg(2)iii [dH–Cg = ˚ ; iii = 1/2 + x, y, 3/2 z] and C(21)–H(21)–Cg(5)iii 2.94 A ˚ ; iii = x, 1/2 y, 1/2 + z]. The p–p, C–H– [dH–Cg = 2.93 A p and hydrogen bonding interactions contribute stability to the unit cell packing.
Fig. 3. Cyclic formation through cadmium centers.
Table 3 ˚) Selected bond lengths (A [CdL(CH3COO)]2 2CH3OH
and
bond
angles
(°)
for
Bond lengths
Bond angles
Cd(1)–N(5) 2.155(3) Cd(1)–N(1) 2.168(3) Cd(1)–O(1) 2.249(2) Cd(1)–O(3) 2.434(2) Cd(1)–N(2) 2.515(3) O(1)–C(7) 1.350(4) N(1)–C(1) 1.349(4) N(1)–C(5) 1.479(4) N(2)–C(6) 1.257(4) N(2)–N(3) 1.338(3) N(3)–C(7) 1.368(4) N(4)–C(8) 1.326(4) N(4)–C(7) 1.354(4) N(4)–H(4N) 0.85(3)
N(5)–Cd(1)–N(1) 116.23(9) N(5)–Cd(1)–O(1) 92.67(9) N(1)–Cd(1)–O(1) 132.65(9) N(5)–Cd(1)–O(3) 107.15(9) N(1)–Cd(1)–O(3) 116.82(9) N(5)–Cd(1)–N(2) 119.45(9) O(3)–Cd(1)–N(2) 123.35(8) C(7)–O(1)–Cd(1) 119.56(18) C(1)–N(1)–C(5) 125.7(3) C(6)–N(2)–N(3) 113.1(3) C(6)–N(2)–Cd(1) 120.7(2) N(3)–N(2)–Cd(1) 126.16(19) N(2)–N(3)–C(7) 100.2(3) O(1)–C(7)–N(3) 130.7(3)
Fig. 4. Unit cell packing diagram viewed along b axis.
T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
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Fig. 5. Intermolecular hydrogen bonding interactions through methanol molecule.
3.3. Spectroscopic studies The tentative assignments of the selected IR bands are listed in the Table 4. The spectrum of the ligand exhibits a medium band at 3369 cm 1 which is assigned to m(NH) vibration. The absence of m(NH) in the spectra of complexes 1 and 3 suggests that the ligand loses this proton on complexation, thus acting as uninegative ligand [20]. In complex 2, the presence of a band at 3251 cm 1 corresponding to m(NH) vibration indicates that the semicarbazone is coordinated in the neutral form. A band at 1718 cm 1 in the ligand has significant contribution from C@O stretching vibration and absence of such band in complexes 1 and 3 confirming the coordination through enolate oxygen. The presence of a band at 1656 cm 1 for complex 2 supports the keto form of the ligand in this complex. On coordination of the azomethine nitrogen, the IR stretching frequency m(C@N) at 1591 cm 1 shifts to 1605, 1597 and 1596 cm 1 for complexes 1, 2 and 3, respectively [21–23]. The spectra of the complexes exhibit a systematic shift in the position of m(N–N) bands in the region 1148– 1141 cm 1 and confirms the coordination of the azomethine nitrogen. The bands appearing at 457, 451 and 463 cm 1 which correspond to m(Cd–N) for complexes 1– 3 again confirm the coordination of the azomethine nitrogen [24]. Coordination of pyridyl nitrogen causes the outof-plane bending vibrational band to shift from 601 cm 1 to higher frequencies 622, 621 and 627 cm 1 for complexes 1–3 [25]. Presence of a band at 1515 and 1521 cm 1 for
complexes 1 and 3 are assigned to the newly formed m(N@C) after enolization [26]. The asymmetric and symmetric stretching vibrations of the acetate group appear at 1568 and 1437 cm 1, respectively, for complex 1 having the separation value Dm = 131 cm 1 suggests the presence of chelating acetate group linked with the metal center for the complex [27,28]. The azido complex 3 exhibits a strong band at 2052 cm 1 corresponding to ma(NNN) stretching vibration of the coordinating azido group. Electronic spectrum of the ligand shows an absorption maximum at 40000 cm 1 attributed to intra-ligand p ? p* transitions of the pyridyl ring and imine function of the semicarbazone moiety. The peak at 30770 cm 1 indicate n ? p* transition of the amide function, which is shifted to higher energy values upon complexation [23] (Table 5). The 1H NMR spectrum of the compound HL along with the assignments is given in Fig. 6. A sharp singlet, which integrate as one hydrogen at d = 13.37 ppm is assigned to the proton attached to the nitrogen atom N(4). The downfield shift of this proton is assigned to its
Table 5 Electronic spectral assignments for the ligand and its Cd(II) complexes Compound
p–p*
n–p*
MLCT
HL [CdL(CH3COO)]2 2CH3OH (1) [Cd(HL)Br2] (2) [CdLN3]2(3)
40 000 37 590 36 360 37 310
30 770 31 640 31 250 31 850
24 810 23 530 24 690
Table 4 IR spectral assignments for ligand and its Cd(II) complexes Compound
m(NH)
m(C@N)
m(CO)
HL [CdL(CH3COO)]2 2CH3OH (1) [Cd(HL)Br2] (2) [CdLN3]2 (3)
3369
1591 1605 1597 1596
1718
3257
m(N@C) 1515
1656 1521
m(N–N) 1129 1148 1147 1141
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T.A. Reena et al. / Polyhedron 27 (2008) 1825–1831
Fig. 6. 1H NMR spectrum of HL.
hydrogen bonding interaction with adjacent nitrogen atom N(1). Another singlet at d = 8.37 ppm is assigned to the N(5) proton. This downfield also explained with the hydrogen bonding interaction with nitrogen atom N(3). Hydrogen bonding decreases the electron density around the proton and thus moves the proton absorption to a lower field. Absence of any coupling interactions by N(4) H and N(5) H protons due to the lack of availability of protons on neighboring atoms render singlet peaks for the imine protons. Two doublets at d = 8.78 and 8.66 ppm are assigned to the C(1)H and C(11)H protons respectively. These signals are shifted to lower field due to electronic effect of the adjacent electronegative pyridyl nitrogens and the more downfield shift of C(1)H can be attributed to the increased charge density on N(1) resulted by its hydrogen bonding to N(4)H. The proton resonance of the two pyridyl rings also appear separately in the NMR spectrum, due to the intramolecular hydrogen bonding. There are two triplets at d = 7.87 and 7.79 ppm corresponding to similar protons on C(3) and C(9) of the two pyridyl rings, while the C(2) and C(10) protons resonances appear overlapped in the multiplet 7.345 ppm. Another two doublets at d = 7.56 and 7.51 ppm are assigned to C(4) and C(8) protons. The resonances for the C6H5-group appear as triplet at 7.04 ppm for para proton and as multiplets at 7.34 ppm for ortho and meta phenyl protons [29]. The 13C NMR spectrum provides direct information about the carbon skeleton of the compound HL. Assignment of different resonant peaks to respective carbon atoms is presented in Fig. 7. Considering the two pyridyl rings non-equivalent, resulting from the hydrogen-bonding inter-
Fig. 7.
13
C NMR spectrum of HL.
action, there are 15 unique carbon atoms in the molecule, which give a total of 15 different peaks in the 13C NMR spectrum. In both the pyridyl rings, the C(1) and C(11) carbon atoms adjacent to the more electronegative nitrogen atoms N(1) and N(2) are shifted farther downfield when compared to the neighboring carbon atoms. Also the carbon atoms at para position to the hetero atoms viz. C(3) and C(9) resonate at lower field values when compared to the meta positioned carbons C(2), C(4), C(8) and C(10). The 13C peaks are assigned as follows: C(1), 148.68 ppm; C(2), 123.87 ppm; C(3), 136.81 ppm; C(4), 126.24 ppm; C(5), 151.96; C(6), 152.95 ppm; C(7), 148.0 ppm C(8), 128.89 ppm; C(9), 137.01 ppm; C(10), 123.87 ppm; C(11), 140.28 ppm; C(12), 156.20 ppm; C(13), 137.98 ppm; C(14), 123.75 ppm; C(15), 123.38 ppm; C(16), 119.60 ppm; C(17), 123.38 ppm; C(18), 123,75 ppm. The non-protonated carbon atom at C(6) is found downfield in the spectrum (d = 152.95 ppm), effected by the magnetic interaction of two bulky pyridyl rings and p electron delocalization on the C(6)@N(3) bond. Similarly, the C(12) carbon atom resonance is also observed farthest downfield of 156.20 ppm resultant of the conjugative effect of the –N(3)–N(4)– C(O)–N(5)– semicarbazone skeleton. The NMR data for the complexes are not available because of poor solubility. Acknowledgements M.R.P. Kurup is thankful to the Council of Scientific & Industrial Research (CSIR), New Delhi, India for the financial assistance. The authors are thankful to the
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National Single Crystal X-ray diffraction Facility, IIT, Bombay, India for providing single crystal XRD data. Appendix A. Supplementary data CCDC 645825 contain the supplementary crystallographic data for 1. 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) 1223336-033, or e-mail: [email protected]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.poly.2008.02.020. References [1] D.X. West, S.B. Padhye, P.S. Sonawane, Struct. Bond. 76 (1991) 1. [2] S.N. Pandeya, J.P. Dimmock, Pharmazie 48 (1993) 659. [3] Q. Li, H. Tang, Y. Li, M. Wang, L. Wang, C. Xia, J. Inorg. Biochem. 78 (2000) 167. [4] J. Dimmock, K. Sidhu, S. Tumber, S. Basran, M. Chen, J. Quail, I. Rozas, D. Weaver, Eur. J. Med. Chem. 30 (1995) 303. [5] J.S. Casas, M.S. Garcia-Tasende, J. Sordo, Coord. Chem. Rev. 209 (2000) 197. [6] M.L. Post, J. Trotter, J. Chem. Soc., Dalton Trans. (1974) 674. [7] P.A. Prasad, S. Neeraj, S. Natarajan, C.N.R. Rao, J. Chem. Soc., Chem. Commun. (2000) 1251. [8] CRYSALIS CCD and CRYSALIS RED Versions 1.171.29.2 (CrysAlis171. NET), Oxford Diffraction Ltd., Abingdon, Oxfordshire, England, 2006. [9] G.M. Sheldrick, Acta Crystallogr., Sect. A 46 (1990) 467. [10] G.M. Sheldrick, SHELXS-97 Program for X-ray Crystal Structure Refinement, University of Go¨ttingen, Germany, 1997.
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