J Chem Crystallogr (2010) 40:1175–1179 DOI 10.1007/s10870-010-9818-3
ORIGINAL PAPER
Crystal Structure of a Trinuclear Mercury(II) Cyanide Complex of Tetramethylthiourea, [{(Tetramethylthiourea)2 Hg(CN)2}2Hg(CN)2] Muhammad Altaf • Helen Stoeckli-Evans • Saeed Ahmad • Anvarhusein A. Isab • Abdul Rahman Al-Arfaj • Muhammad Riaz Malik Saqib Ali
•
Received: 5 January 2010 / Accepted: 22 May 2010 / Published online: 9 June 2010 Ó Springer Science+Business Media, LLC 2010
Abstract The title compound was prepared by reacting mercury(II) cyanide and tetramethylthiourea (Tmtu) in the molar ratio of 1:1.75. It was characterized by IR and NMR (1H and 13C) spectroscopy, and X-ray crystallography. The appearance of a band around 2,200 cm-1 in IR and a resonance around 145 ppm in 13C NMR indicated the binding of cyanide to mercury(II). The crystal structure of the title complex, [{(tetramethylthiourea)2Hg(CN)2}2Hg(CN)2] (1) consists of two independent [(Tmtu)2Hg(CN)2] moieties bridged by a Hg(CN)2 unit. The mercury atom in [(Tmtu)2Hg(CN)2] unit is coordinated to two thione sulfur atoms of Tmtu and to two cyanide carbon atoms in a distorted tetrahedral mode. Keywords Mercury(II) cyanide Tetramethylthiourea NMR X-ray structure
M. Altaf H. Stoeckli-Evans Institute of Physics, University of Neuchaˆtel, 2009 Neuchaˆtel, Switzerland S. Ahmad (&) Department of Chemistry, University of Engineering and Technology, Lahore 54890, Pakistan e-mail:
[email protected]
Introduction Mercury(II) halides or pseudohalides generally form 1:1 or 1:2 complexes with neutral nitrogen, phosphorus and sulfur donor ligands depending on the molar ratio of reactants and donor properties of the ligands [1–40]. The crystal structures of the 1:2 complexes usually consist of discrete monomeric molecules [5, 7, 13, 14, 23–40], while those of 1:1 often involve dimeric species [12, 17–23]. The metal center in majority of these complexes shows tetrahedral (somewhat distorted) coordination environment. Whereas numerous adducts of mercury halides with thiourea ligands have been structurally characterized [23–29, 35–40], the number of complexes defined with Hg(CN)2 is much more limited [29, 35]. We have been interested in studying the coordination chemistry of cyanide complexes of d10 metals such as silver(I) and gold(I) and in this regard we have already reported the spectroscopic and structural characterization of a number of silver(I)- and gold(I)-cyanide complexes of thiones and selenones [41–46]. With the aim of extending our knowledge on the coordination chemistry of metal cyanide systems, we report here the crystal structure of [{(tetramethylthiourea)2Hg(CN)2}2Hg(CN)2] (1), which presents a unique example of a Hg(CN)2 bridged mercury(II)–thione complex.
Experimental
A. A. Isab A. R. Al-Arfaj Department of Chemistry, King Fahd University of Petroleum and Minerals, Dhahran 31261, Saudi Arabia
Materials
M. R. Malik S. Ali Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan
Mercury(II) chloride (HgCl2) was obtained from Merck Chemical Company, Germany. Tetramethylthiourea (Tmtu) was purchased from Acros Organics, Belgium.
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Preparation of Complexes Mercury(II) cyanide, Hg(CN)2 was prepared by mixing the solutions of HgCl2 in methanol and KCN in water in the molar ratio of 1:2. The resulting colorless solution was concentrated by evaporation and the white crystalline product was separated by filtration. The title complex (1) was prepared by adding 1.75 equivalents of Tmtu in 10 ml water to a solution of Hg(CN)2 (0.25 g, 1.0 mmol) in 15 ml of methanol and stirring the solution for half an hour. White crystalline product was obtained from the resulting colorless solution, which was washed with methanol. Two other products were also isolated in the Tmtu–Hg(CN)2 system under different conditions. The complex (2), [(Tmtu)2Hg(CN)2]H2O was prepared by mixing methanolic solutions of Tmtu and Hg(CN)2 in 2:1 ratio and the crystals were obtained by adding 5 mL water to the resulting solution. For the preparation of 3, [(Tmtu)2Hg(CN)2]n2H2O, Tmtu was dissolved in water instead of methanol keeping the Tmtu to Hg(CN)2 molar ratio of 2:1.
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empirical absorption correction was applied using the MULscanABS routine in PLATON [49]; transmission factors: Tmin/Tmax = 0.015/0.028. Crystal data and details of the data collection are summarized in Table 1.
Results and Discussion IR and NMR Studies The IR spectroscopic vibration bands for the Tmtu and the mercury(II) complex are given in Table 2. The m(C=S) vibration, which occurs at 622 cm-1 for the free ligand is shifted towards lower frequency upon complexation, in accordance with the data observed for the other thione complexes [22, 31–38]. A sharp band at 2,145 cm-1 (due to CN stretch) indicated the presence of cyanide. The 1H and 13C chemical shifts of the complexes in DMSO-d6 are summarized in Table 3. In 1H NMR spectra, the appearance CH3 signal indicated the presence of Tmtu. In 13C NMR, the C=S resonance of Tmtu shifted upfield upon complexation compared to its position in the free
IR and NMR Measurements The solid-state IR spectra were recorded on a Perkin-Elmer FTIR 180 spectrophotometer using KBr pellets over the range 4,000–400 cm-1. The 1H NMR spectra in DMSO-d6 were obtained on Jeol JNM-LA 500 NMR spectrometer operating at a frequency of 500.00 MHz at 297 K using 0.10 M solution. The 13C NMR spectra were obtained at the frequency of 125.65 MHz with 1H broadband decoupling at 298 K. The spectral conditions were: 32 K data points, 0.967 s acquisition time, 1.00 s pulse delay and 45° pulse angle. The 13C chemical shifts were measured relative to TMS. X-ray Structure Determination Single crystal data collection for complex 1 was performed at 173 K (-100 °C) on a Stoe Mark II-Image Plate Diffraction System [47] equipped with a two-circle goniometer and using MoKa graphite monochromated radiation. The distance between the imaging plate and the sample was 100 mm. Diffraction data were collected using x rotation scans of 0–180° at / 0° and of 0–79° at / 90° with step Dx = 1.0°, exposures of 4 min per image and 2h range 2.29–59.53°. The structure of 1 was solved by Direct methods using the program SHELXS-97 [48]. The refinement and all further calculations were carried out using SHELXL-97 [48]. The H-atoms were included in calcu˚, lated positions and treated as riding atoms: N–H = 0.88 A ˚ C–H = 0.98–0.99 A and Uiso(H) = 1.2Ueq(parent N or C-atom). The non-H atoms were refined anisotropically, using weighted full-matrix least-squares on F2. An
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Table 1 Summery of crystal data and details of structure determination for 1 Crystal data Formula
C26H48Hg3N14S4
Formula weight
1286.83
Crystal system
Monoclinic
Space group ˚) a, b, c (A
P21/c
a, b, c (°) ˚ 3) V (A
90, 94.759(11), 90 2051.0(3)
10.4327(9), 13.6535(9), 14.4183(13)
Z
2
qcalc (g/cm3)
2.084
l (MoKa) (mm-1)
11.442
F(000)
1212
Crystal size (mm)
0.46 9 0.30 9 0.30
Data collection Temperature (K) ˚) k MoKa (A
0.71073
173(2)
h Min–max (°)
1.95–26.05
h, k, l limits
-12:12, -16:16, -17:17
Reflns: total, uniq. data, Rint
16003, 4003, 0.0476
Observed data [I [ 2r(I)]
3410
Absorption: Tmin/Tmax
0.015/0.028
Refinement Nref, Npar
4003, 222
R, wR2, S
0.0215, 0.0549, 1.008
˚ 3] Min. and max. resd. dens. [e/A 2
w = [r
(F2o)
2
? (0.0242P) ? 1.6454P]
-1.238, 0.511 -1
where P = (F2o ? 2F2c )/3
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Table 2 Melting point, and selected IR frequencies (cm-1) of Tmtu and its Hg(CN)2 complex Species
m.p. (°C)
t(C–N)
–
–
622
1491
–
614
1556
2155
–
Hg(CN)2 Tmtu [{(Tmtu)2Hg(CN)2}.2 Hg(CN)2]
t(C=S)
126–128
t(C:N) 2185
Table 3 1H and 13C NMR chemical shifts (in ppm) of various species in DMSO-d6 Species
d (1H)
d (13C)
CH3
[C=S
N–CH3
CN
Tmtu
3.08
193.42
42.05
–
[{(Tmtu)2Hg(CN)2}.2Hg(CN)2]
3.00
191.97
42.80
144.27
Hg(CN)2
–
–
145.25
state. The upfield shift is attributed to a lowering of C=S bond order upon coordination and a shift of N ? C electron density producing a partial double bond character in the C–N bond, as observed in other metal complexes of thiourea [31–38, 42–58]. In the CN region of the 13C NMR spectrum of title complex, a sharp singlet was observed around 145 ppm although there are two kinds of cyanide environments in the complex. This suggests that CN groups are permuting rapidly over all Hg atoms. This also suggests that the complex does not undergo dissociation in solution as it was observed for the analogous AuCN complexes. For gold complexes, two resonances were detected in the CN region; one for the CN carbon of LAuCN and the other due to [Au(CN)2]- [44–46]. This showed that they underwent disproportionation in solution according to Eq. (1); 2 LAuCN ½AuL2 þ þ AuðCNÞ2 ð1Þ The formation of ionic species was related to the very high stability constant of [Au(CN)2]- (logb of [Au(CN)2]- & 36) [44–46, 53]. Crystal Structure Description The molecular structure of 1 together with the atomic labeling scheme is shown in Fig. 1a. Selected bond lengths and angles are presented in Table 4. The complex is trinuclear consisting of two [(Tmtu)2Hg(CN)2] moieties bridged by a centrosymmetric Hg(CN)2 molecule via weak ˚ ; symmetry HgN interactions (Hg2N5i = 2.791(4) A operation (i) = 1 - x, -1/2 ? y, 1/2 - z) (Fig. 1b). The mercury(II) center, Hg(1) in [(Tmtu)2Hg(CN)2] is bound to two S atoms of Tmtu, and through the carbon atoms to two cyanides, adopting a distorted tetrahedral geometry. Three
Fig. 1 a A view of the molecular structure of compound 1, with displacement ellipsoids drawn at the 50% probability level [H-atoms have been omitted for clarity; symmetry operation (i) = -x, 1 - y, 1 - z]. b A view along the a-axis of the crystal packing of compound 1
of the six bond angles show significant distortion from tetrahedral geometry. The C12–Hg1–C11 angle has a value of 131.30(16)°, which is much larger than the tetrahedral value of 109.5°. This high angle is counterbalanced by the other bond angles; C11–Hg1–S2 and S1–Hg1–S2, whose values are 94.71(11)° and 95.49(4)°, respectively. The coordination environment around Hg(2) is square planar having Hg atom coordinated to four cyanide groups through carbon and nitrogen atoms. The planar geometry around mercury(II) has rarely been observed [1, 54]. The Hg1–C:N and Hg2–C:N angles of 177.2(4)° and 176.7(5)° respectively, are close to linear geometry. The Hg–S distances are comparable to the reported mean ´˚ distances of *2.6 A in other mercury(II) complexes of
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˚ ) and bond angles (°) for comTable 4 Selected bond distances (A pound 1 Bond distances Hg1–S1
2.5776(10)
Hg2–C13
2.041(4)
Hg1–S2
2.6858(10)
Hg2N5i
2.791(4)
Hg1–C11
2.121(4)
Hg1–C12
2.107(4)
Bond angles 95.49(4)
C13–Hg2–C13ii
180
S1–Hg1–C11
109.38(12)
N5iHg2N5iii
180
S1–Hg1–C12
109.24(12)
S2–Hg1–C11
94.71(11)
S1–Hg1–S2
S2–Hg1–C12 C11–Hg1–C12
109.88(12) 131.30(16)
Symmetry code: (i) 1 - x,-1/2 ? y,1/2 - z, (ii) 2 - x,-y,-z; (iii) 1 ? x,1/2 - y,-1/2 ? z
thiones [23–29, 34–40]. However, this distance is signifi´˚ cantly shorter than that in [(H4pymthH)2Hg(CN)2] (*2.9 A ) [35] (H4pymthH = 3,4,5,6-tetrahydropyrimidine-2-thione) showing the strong binding of sulfur to Hg(II). The Hg–C ´˚ distance of the Hg(CN)2 unit (2.041(4) A ) is found to be shorter than that in the [(Tmtu)2Hg(CN)2] moieties (2.107(4)– ˚´ ) indicating that there is a greater shift of p-electron 2.121(4) A density from Hg to CN carbon, when CN is present in a planar environment. The Hg–C distance is slightly longer than in the ´˚ structure of Hg(CN)2 (2.015(3) A ) [55]. The bridging i ˚ is comparable with that Hg2N5 distance of 2.791(4) A reported previously for a Hg(II) complex [56]. The crystal structures of the two other tetramethylthiourea complexes with Tmtu:Hg(CN)2 = 2:1 (2 and 3) were also determined [57, 58] but due to the poor quality of the crystals the data are unpublishable. However, the preliminary structure analyses show that complex 2 exists as independent [(Tmtu)2Hg(CN)2] molecules, while complex 3 is a polymeric chain structure consisting of [(Tmtu)Hg(CN)2] units. In both compounds the mercury atoms are coordinated to two sulfur atoms of Tmtu and two cyanides in distorted tetrahedral modes (average bond angle = 108.58°). This is the first study that suggests the possibility of formation of a variety of mercury(II) cyanide complexes in the presence of tetramethylthiourea depending upon reaction conditions. It describes the crystal structure of a novel mercury–thione complex, the most unique feature of which is the bridging Hg(CN)2 group between two mercury(II)– thione complexes.
Supplementary Material Supplementary crystallographic data of 1 (CCDC No. 734855) can be obtained free of charge via http://www.
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ccdc.cam.ac.uk/data_request/cif, by e-mailing
[email protected], or by contacting the Cambridge Crystallographic Data Center, 12 Union Road, Cambridge CB2 1EZ, UK. Acknowledgments Financial support from Pakistan Council for Science and Technology Islamabad, Pakistan, and from King Fahd University of Petroleum and Minerals, Dhahran, Saudi Arabia, Dhahran, Saudi Arabia is gratefully acknowledged.
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