Alkyldiamine bis(selenocyanato)cadmium(II) complexes: Synthesis, 113Cd, 77Se, 15N and 13C NMR spectroscopy and X-ray structure of a 2D metal–organic framework

Polyhedron 30 (2011) 1262–1266

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Alkyldiamine bis(selenocyanato)cadmium(II) complexes: Synthesis, 113Cd, 77Se, 15N and 13C NMR spectroscopy and X-ray structure of a 2D metal–organic framework Mohammed Fettouhi, Bassem A. Al-Maythalony, M. Nasiruzzaman Shaikh, Mohammed I.M. Wazeer, Anvarhusein A. Isab ⇑ Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia

a r t i c l e

i n f o

Article history: Received 4 January 2011 Accepted 4 February 2011 Available online 23 February 2011 Keywords: Cadmium(II) complexes Selenocyanate Ethylenediamine Solid state NMR 113 Cd NMR X-ray structure

a b s t r a c t Complexes of cadmium(II)–selenocyanate with several alkyldiamine ligands have been synthesized and characterized by IR, 113Cd, 77Se, 15N and 13C NMR spectroscopy. The X-ray structure of the complex [Cd(SeCN)2-en] reveals two non-equivalent metal ion centers, both with a distorted octahedral geometry. The combined bridging modes of selenocyanate and ethylenediamine with the blocking mode of a chelating ethylenediamine generate a 2D metal–organic framework. Ó 2011 Elsevier Ltd. All rights reserved.

1. Introduction The high affinity of cadmium for the sulfhydryl groups in proteins makes it one of the most hazardous ecotoxicants [1–3]. The cadmium ion relatively readily substitutes for the zinc ion incorporated in metalloproteins and blocks zinc-containing enzymes. When taken up into the organism, cadmium accumulates in the liver and kidneys, and forms stable complexes with intracellular proteins. The latter circumstance causes a cumulative effect and an extremely slow removal of cadmium from the organism. Cadmium (even in extremely low concentrations) is able to trigger some forms of tumors and inhibit the DNA mismatch repair mechanism [4,5]. Therefore, the synthesis and study of complexes containing Cd(II) and its chemistry is important. It is also worth noting that the ability of cadmium to substitute the ‘spectroscopically silent’ metals in the active centers of metalloproteins is widely used in biological studies to obtain structural information by 113Cd NMR spectroscopy [6]. The study of Cd–ethylenediamine (en) complex formation dates back to 1945, where three complexes were shown to form in aqueous solution [7]. In addition, the equilibrium constants of the three complexes were determined by pH titration and polarography [7,8]. Cadmium complexes based on alkyldiamines and selenocyanate anions are very rare in the literature [9]. In this paper, we describe the synthesis and ⇑ Corresponding author. E-mail address: [email protected] (A.A. Isab). 0277-5387/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2011.02.008

characterization of new complexes of the empirical formula Cd(SeCN)2L where L is an bidentate alkyldiamine as shown below:

(where en = ethylenediamine, pn = 1,3-propylenediamine, bn = 1,4butylenediamine).

2. Experimental 2.1. Chemicals CdCl2, acetone, methanol, acetonitrile and DMSO-d6 were obtained from Fluka Chemical Co. KSe13CN was bought from Isotech Chemical Co., USA. 2.2. IR and solution NMR The IR spectra of the ligands and their Cd(II) complexes were performed on a Perkin–Elmer FTIR 180 spectrophotometer using KBr pellets over the range 4000–400 cm1 [10,11]. Selected IR data are shown in Table 1 and the solution NMR chemical shifts of the

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ligands along with the corresponding complexes are given in Table 2. 2.3. Solid state NMR studies Natural abundance 13C and 113Cd NMR spectra were obtained on a JEOL LAMBDA 500 spectrometer with basic resonance frequencies 125.65 and 110.85 MHz respectively, at 11.74 T magnetic field, at an ambient temperature of 25 °C. Samples were packed into 6 mm zirconia rotors. Cross polarization and high power decoupling were employed. A pulse delay of 7.0 s and a contact time of 5.0 ms were used for cadmium and carbon observations in the CPMAS experiments. The magic angle spinning rates were from 3000 to 5000 Hz. The cadmium chemical shifts were referenced using a secondary reference, Cd(NO3)24H2O, by settings the peak to 100 ppm relative to 0.1 M Cd(ClO4)2 with an ionic strength of 4.5, whose chemical shift was taken to be 0.0 ppm [12]. The same Cd(NO3)24H2O sample was used to set up the Hartmann–Hahn condition for the CPMAS experiments. 13C chemical shifts were referenced to TMS, by setting the high frequency isotropic peak of solid adamantane to 38.48 ppm. The center peaks were differentiated from the spinning side-bands by recording the spectra at two different spinning speeds. The 113Cd and 13C spectra containing spinning side-band manifolds were analyzed using the computer program WSOLIDS, developed at Dalhousie and Tubingen universities [13]. The calculated 13C and 113Cd isotropic chemical shifts and the shielding tensors are shown in Table 3. 2.4. Preparation of the complexes N,N0 -(ethane-1,2-diamine)-selenocyanocadmium(II), [Cd(SeCN)2en]: a solution of Cd(SeCN)2 (0.500 mmol) in warm methanol was mixed with a solution of ethylenediamine hydrochloric salt (0.500 mmol) in methanol, neutralized with a drop of diluted sodium hydroxide. The solid that formed was collected, washed and dried. The yield was 0.040 g (22%). N,N0 -(propane-1,3-diamine)-selenocyanocadmium(II), [(Cd(SeCN)2pn], and N,N0 -(butane-1,4-diamine)-selenocyanocadmium(II), [(Cd(SeCN)2-bn], were prepared in a similar way to [Cd(SeCN)2-en],

Table 1 IR frequencies (cm1) of the CN region in Cd(SeCN)2– alkyldiamine complexes. Species

m(CN) from Cd(SeCN)2

KSeCN Cd(SeCN)2 [Cd(SeCN)2-en] [Cd(SeCN)2-pn] [Cd(SeCN)2-bn]

2073a 2146(s), 2106(s) 2090(s), 2058(s) 2088(s) 2146, 2085(s)

a

Table 2 113 Cd, 77Se, 15N and 13C NMR chemical shifts of Cd(SeCN)2–alkyldiamine complexes in DMSO-d6. 113

Cd

77

Se

15

N

13

C

SeCN Cd(SeCN)2 en* [Cd(SeCN)2-en] pn* [Cd(SeCN)2-pn] bn* [Cd(SeCN)2-bn] * 15

N label ligands.

66.47

281.36

143.77

274.44

121.27

278.81

139.99

281.68

2.5. X-ray data collection and structure determination X-ray diffraction data were recorded on a Bruker-Axs SMART Apex system equipped with graphite monochromatized Mo Ka radiation (k = 0.71073 Å). The data were collected using SMART [14]. The data integration was performed using SAINT [15]. An empirical absorption correction was carried out using SADABS [16]. The structure was solved with direct methods and refined by full matrix least square methods based on F2, using the structure determination package SHELXTL [17] based on SHELX 97 [18]. Graphics were generated using ORTEP3 [19] and MERCURY [20]. Hydrogen atoms were included at calculated positions using a riding model. Crystallographic data are given in Table 4. Selected bond lengths and angles are given in Table 5. Complete bond lengths and bond angles, anisotropic thermal parameters and calculated hydrogen coordinates are deposited as supplementary materials. The molecular structure with the atomic labeling scheme is given in Fig. 1.

Table 3 Solid-state 113Cd isotropic chemical shifts (diso) and principle shielding tensors (rxx)a of Cd(SeCN)2 and [Cd(SeCN)2-en] complexes. Complex

Nucleus

diso

r11

r22

r33

Dr

g

Cd(SeCN)2

113

Cd 13 C

211.9 117.0

322

283

30

291

0.73

[Cd(SeCN)2-en]

113

309.9 316.6 149.2 43.7

358

333

257

88

0.415

Cd1 Cd2 13 C 13 C 113

a

Isotropic

shielding,

ri = (r11 + r22 + r33)/3;

C-1

C-2

45.05 40.12 39.86 42.68 42.15 41.32

37.33 29.79 31.18 28.46

CCDC deposit no. Empirical formula Formula weight T (K) Wavelength (Å) Crystal system Space group Unit cell dimensions a (Å) b (Å) c (Å) Volume (Å3) Z Dcalc (g cm3) Absorption coefficient (mm1) F(0 0 0) Crystal size (mm) h Range (°) Limiting indices

118.778 342.68 359.59 343.12 347.99 343.36 348.48

117.15 117.742 118.548

Dr = r33  0.5(r11 + r22);

g = 3(r22  r11)/2Dr.

Table 4 Crystal structure refinement data for the compound [Cd(SeCN)2-en].

Previous work [21].

Species

using 1 mmol of Cd(SeCN)2 and 1 mmol of propane-1,3diamine or butane-1,4-diamine instead, respectively. The yields were 0.30 g (90%) for [(Cd(SeCN)2-pn] and 0.20 g (60%) for [(Cd(SeCN)2-bn].

Maximum and minimum transmission Data/restraints/parameters Goodness-of-fit (GOF) on F2 Final R indices [I > 2r(I)] R indices (all data) Largest difference in peak and hole (e Å3)

783276 C4H8CdN4Se2 382.46 299(2) 0.71073 orthorhombic Pnma 11.677(1) 33.224(4) 7.6291(8) 2959.8(5) 12 2.575 9.540 2112 0.35  0.26  0.23 2.45–28.29 15 6 h 6 15 43 6 k 6 42 10 6 l 6 10 Tmin = 0.1351, Tmax = 0.2176 3684/0/157 1.079 R1 = 0.0292, wR2 = 0.0656 R1 = 0.0380, wR2 = 0.0683 0.798 and 0.770

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Table 5 Selected bond lengths (Å) and bond angles (°) for the compound [Cd(SeCN)2-en]. Bond lengths (Å) Cd1–N1 Cd1–N2 Cd1–N3i Cd1–N4 Cd2–N5

2.306(5) 2.344(4) 2.306(5) 2.379(4) 2.350(3)

Cd2–N6 Cd2–N7 Cd2–Se1 Cd2–Se2 Cd2–Se3

2.347(3) 2.333(4) 2.7665(6) 2.8596(6) 2.8949(5)

Bond angles (°) N1–Cd1–N3i N1–Cd1–N2 N3–Cd1–N2i N2–Cd1–N2ii N2–Cd1–N4ii N1–Cd1–N4 N3–Cd1–N4i N2–Cd1–N4 N4–Cd1–N4ii N7–Cd2–N6 N7–Cd2–N5 N6–Cd2–N5

172.2(2) 94.0(2) 91.9(1) 81.7(2) 174.1(1) 85.8(1) 88.8(1) 92.4(1) 93.6(2) 75.4(1) 88.5(1) 163.6(1)

N7–Cd2–Se1 N6–Cd2–Se1 N5–Cd2–Se1 N7–Cd2–Se2 N6–Cd2–Se2 N5–Cd2–Se2 Se1–Cd2–Se2 N7–Cd2–Se3 N6–Cd2–Se3 N5–Cd2–Se3 Se1–Cd2–Se3 Se2–Cd2–Se3

166.4(1) 92.21(8) 103.4(1) 92.5(1) 96.93(8) 86.9(1) 94.65(2) 94.4(1) 94.37(8) 83.4(1) 80.76(2) 167.97(2)

Symmetry operations: i = x, y, z + 1; ii = x, y + 1/2, z.

3. Results and discussion IR data of the synthesized complexes are given in Table 1. The CN vibrational frequency is lower for KSeCN [21] compared to Cd(SeCN)2. This indicates a strengthening of the CAN bond upon complexation with the cadmium ion. The electron donation from the selenium atom to the metal likely causes a decrease of the electron occupation of the antibonding pCN orbital. The complexation of Cd(SeCN)2 with alkyldiamine ligands shifts m(CN) to a lower wave number, likely due to electron donation from the diamine nitrogen atoms to the metal ion, leading to an increase of its basicity. The solution 113Cd NMR chemical shifts for Cd(SeCN)2 complexes with alkyldiamine ligands are given in Table 2. In the com-

Fig. 2. Packing scheme in the compound [Cd(SeCN)2-en], showing the 2D layers.

plex [Cd(SeCN)-en], the 113Cd isotropic shift moves downfield by about 100 ppm upon complexation with the diamine, compared to Cd(SeCN)2. According to the X-ray structure of the complex, there are two different cadmium environments (vide infra). However, we observed only a single Cd(II) peak in solution NMR, presumably due to DMSO competing with en ligands and a fast equilibrium between different species. Little effect was observed in the 77Se NMR chemical shifts for Cd(SeCN)2 before and after complexation with alkyldiamine ligands. The highest 77Se NMR downfield shift was observed for the en complex (7 ppm), that binds both in a bridging and chelating mode. A comparison of the 13C NMR chemical shifts for the synthesized complexes showed minor changes in resonances corresponding to cyanide after complexation, while an up-field change in the chemical shifts of the alkyldiamine ligands was observed in the range 1–7 ppm. The 15 N NMR chemical shifts show up-field shifts for the synthesized

Fig. 1. ORTEP plot showing the coordination spheres of the two metal ion centers in the compound [Cd(SeCN)2-en], together with the atomic numbering scheme. Ellipsoids are drawn at the 30% probability level (symmetry code: i = x  0.5, y, z + 0.5; ii = x, y, z + 1; iii = x, y + 0.5, z; iv = x + 0.5, y + 0.5, z + 0.5; v = x + 0.5, y, z + 0.5; vi = x, y, z  1).

M. Fettouhi et al. / Polyhedron 30 (2011) 1262–1266

Fig. 3. 113Cd CPMAS spectrum of [Cd(SeCN)2-en]. The isotropic peaks are denoted by ‘‘⁄’’.

complexes, compared to free alkyldiamine ligands resonances, in the range 5–17 ppm. These shifts indicate the increase in electron density at the nitrogen atom, in agreement with the literature [22]. The highest change was observed for the en ligand. The 13C and 113Cd solid-state NMR spectral data for complexes [Cd(SeCN)2] and [Cd(SeCN)2-en] are shown in Table 3. The 113Cd CPMAS spectrum of [Cd(SeCN)2-en] is shown in Fig. 3 and the isotropic peaks are marked with asterisks. The 113Cd isotropic peak of the [Cd(SeCN)2-en] complex moves downfield by 105 ppm upon complexation with the ethylenediamine ligand. The sharp peak is assigned to the cadmium (Cd2) bonded with Se as well as a N atom. The other center peak, which appears as a broad shoulder (Fig. 3), could be assigned to the cadmium (Cd1) which is surrounded by six nitrogen atoms from ethylenediamine as well from selenocyanate. A cadmium nucleus surrounded by six nitrogens may display extensive broadening. The 13C solid NMR chemical shift at 149 ppm is assigned to the selenocyanate carbon atom in [Cd(SeCN)2-en], which is 32 ppm downfield shifted upon complexation with ethylenediamine.

3.1. Crystal structure of the compound [Cd(SeCN)2-en] There are two metal ion centers in the asymmetric unit, both with a distorted octahedral geometry. The structure is depicted in Fig. 1 and the crystallographic data are shown in Tables 4 and 5. The first metal center, Cd1, has a coordination sphere central core of Cd[N6]. The metal ion is bonded to four selenocyanate ions through their nitrogen atoms, defining the basal plane, and two nitrogen atoms of two bridging ethylenediamine ligands are at the trans axial positions. Both Cd1 and the en ligand are located on a mirror plane [x, 1=4 , z]. The basal Cd–NSeCN bond lengths are in the range 2.344(4)–2.379(4) Å, which is significantly longer than the axial Cd–Nen bond (2.306(5) Å). The cisoid and transoid bond angles are in the ranges 81.7(2)–94.0(2)° and 172.2(2)–174.1(1)°, respectively, reflecting the extent of distortion from an ideal octahedral geometry. These values are consistent with those reported for other cadmium complexes [23–25]. The second metal ion center (Cd2) has a Cd[N3Se3] coordination sphere central core, in which the metal ion is bonded to two nitrogen atoms of a chelating ethylenediamine ligand, one nitrogen atom of a bridging selenocyanate, in addition to three selenium atoms of three other bridging selenocyanate anions. The Cd–Se and Cd–N bond distances are in the ranges 2.7665(6)–2.8949(5) Å and 2.333(4)–2.350(3) Å, respectively. The ethylenediamine chelate bite angle is 75.4(1)°, while the other cisoid bond angles are larger, being in the range 80.76(2)– 96.93(8)°. These geometrical data are similar to those found for

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the single hit obtained by searching in the CSD database [26] for a Cd[N3Se3] central core [24]. Within the Cd2[en(SeCN)4] fragment, intra-moiety hydrogen bonding takes place, involving two ethylenediamine hydrogen atoms and the nitrogen atoms of two trans selenocyanate ligands (Fig. 1). Such an interaction is likely contributing to the bent coordination mode of the two SeCN ligands involved. The relevant bond angles are C4–Se2–Cd2 89.2(1)° and C7–Se3–Cd2 94.1(1)°. Each Cd1 metal ion is connected to four Cd2 centers through the four basal SeCN bridging ligands, and to two other Cd1 centers through the two axial bridging ethylenediamine ligands in the c direction. Two pairs of cisoid Cd2 moieties are further interconnected by a selenocyanate bridge, generating 12-membered rings of [Cd3(SeCN)3] (Fig. 1). Each Cd2 center is also bridged by SeCN with two other Cd2 ions in a chain-like scheme running in the a direction. Two of these chains form due to the blocking effect of chelating ethylenediamine ligands, with side walls of semi-rectangular bricks propagating along the a direction, and the chains are vertically interconnected by [Cd(en)2]1 pillar chains in the c direction. This polymeric bonding mode generates a 2D framework parallel to the ac plane (Fig. 2). 4. Conclusion Several alkyldiamine selenocyanato cadmium(II) complexes have been synthesized and characterized. The X-ray structure of the [Cd(SeCN)2-en] complex reveals two metal ion centers, both with a distorted octahedral geometry, having Cd[N6] and Cd[N3Se3] coordination sphere center cores respectively. The 13Cd CPMAS spectrum was assigned accordingly. The combined bridging modes of selenocyanate and ethylenediamine, together with the blocking mode of a chelating ethylenediamine, generate a 2D metal–organic framework. Acknowledgments The authors would like to acknowledge the support by the Deanship of Scientific Research at King Fahd University of Petroleum and Minerals for funding this work through Project No. SB070010. Appendix A. Supplementary data CCDC 783276 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via http:// www.ccdc.cam.ac.uk/conts/retrieving.html, or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: (+44) 1223-336-033; or e-mail: [email protected]. References [1] P.D. Akrivos, Coord. Chem. Rev. 213 (2001) 181. [2] S. Ahmad, A.A. Isab, S. Ali, A.R. Al-Arfaj, Polyhedron 25 (2006) 1633. [3] A. Terrón, J.J. Fiol, A. García-Rasoa, M. Barceló-Oliver, V. Moreno, Coord. Chem. Rev. 251 (2007) 1973. [4] Y.H. Jin, A.B. Clark, R.J.C. Slebos, H. Al-Refai, J.A. Taylor, T.A. Kunkel, M.A. Resnick, D.A. Gordenin, Nat. Genet. 34 (2003) 326. [5] C.T. McMurray, J.A. Tainer, Nat. Genet. 34 (2003) 239. [6] M.F. Summers, Coord. Chem. Rev. 86 (1988) 43. [7] G.A. Carlson, J.P. McReynolds, F.H. Verhoek, J. Am. Chem. Soc. 67 (1945) 1334. [8] B.D. Douglas, H.A. Laitinen, J.C. Bailar, J. Am. Chem. Soc. 72 (1950) 2484. [9] P.P. Singh, A.K. Srivastava, S.B. Sharma, Ind. J. Chem. 14A (1976) 714. [10] M.I.M. Wazeer, A.A. Isab, M. Fettouhi, Polyhedron 26 (2007) 1725. [11] A.A. Isab, M.I.M. Wazeer, J. Coord. Chem. 58 (2005) 529. [12] P.G. Mennit, M.P. Shatlock, V.J. Bartuska, G.E. Maciel, J. Phys. Chem. 85 (1981) 2087. [13] K. Eichele, R.E. Wasylischen, W: Simulation Package, Version 1.4.4, Dalhousie University, Canada and University of Tubingen, Germany, 2001. [14] SMART APEX Software (5.05) for SMART APEX Detector, Bruker Axs. [15] SAINT Software (5.0) for SMART APEX Detector, Bruker Axs Inc.

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