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Stereochemically non-rigid helical mercury(II) complexes
Inorganica Chimica Acta 362 (2009) 2487–2491
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Stereochemically non-rigid helical mercury(II) complexes Michael G.B. Drew a, Senjuti De b, Dipankar Datta b,* a b
School of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India
a r t i c l e
i n f o
Article history: Received 9 October 2008 Accepted 7 November 2008 Available online 18 November 2008 Keywords: Mercury Helical Spiral N ligands Crystal structures
a b s t r a c t Reactions of the 1:2 condensate (L) of benzil dihydrazone and 2-acetylpyridine with Hg(ClO4)2 xH2O and HgI2 yield yellow [HgL2](ClO4)2 (1) and HgLI2 (2), respectively. Homoleptic 1 is a 8-coordinate double helical complex with a Hg(II)N8 core crystallising in the space group Pbca with cell dimensions: a = 16.2250(3), b = 20.9563(7), c = 31.9886(11) Å. Complex 2 is a 4-coordinate single helical complex having a Hg(II)N2I2 core crystallising in the space group P21/n with cell dimensions a = 9.8011(3), b = 17.6736(6), c = 16.7123(6) Å and b = 95.760(3)o. In complex 1, the N-donor ligand L uses all of its binding sites to act as tetradentate. On the other hand, it acts as a bidentate N-donor ligand in 2 giving rise to a dangling part. From variable temperature 1H NMR studies both the complexes are found to be stereochemically non-rigid in solution. In the case of 2, the solution process involves wrapping up of the dangling part of L around the metal. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Results and discussion
A helical structure for a metallo organic complex was first proposed by Harries and McKenzie in 1969 in regard of a dinuclear copper(II) complex of bis(pyridinal)ethylenediamine [1]. It has gone largely unnoticed. The first crystal structure of a helical metal complex is possibly that of a Co(II) compound reported by Wester and Palenik in 1975 [2]. Systematic studies on helical complexes actually have started in the mid-eighties with the pioneering works of Lehn [3,4]. Examples of helical metallo organic compounds are now numerous. In all these cases the organic component (ligand) containing suitable donor atoms is non-helical. But the coordination properties of the metal ions direct the wrapping of the ligand around them in such a manner that an overall helical topology is assumed. For quite some time we have been interested in helical complexes [5–12]. A dinuclear Cu(I) helicate of ours [6] has subsequently been optically resolved by Wild and co-workers [13]. We have been working mainly with ligands which themselves are helical. Thereby we have been able to produce mononuclear double helical complexes; in some cases the metal ion is square planar. Lehn’s approach does not provide for mononuclear helical complexes, the possibility of which was not then considered. Herein we report two novel mononuclear helical complexes of mercury(II) generated by using a helical N-donor ligand.
Our ligand L here is derived from benzil dihydrazone which has a helical twist [14]. It is a 1:2 condensate of benzil dihydrazone and 2-acetylpyridine [15,16]. The N@C(Ph)–C(Ph)@N torsion angle which in benzil dihydrazone is 70o becomes 85.9(5)° in L. Thus L itself is helical.
* Corresponding author. Tel.: +91 33 24735374; fax: +91 33 24732805. E-mail address: [email protected] (D. Datta). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.11.008
N
Me Ph
Ph Me
N
N
N
N
N
ligand L
Reaction of L with Hg(ClO4)2 xH2O in 2:1 molar proportion in methanol at room temperature yields [HgL2](ClO4)2 (1). While L is reacted with HgI2 in equimolar proportion in dimethylformamide–methanol mixture at room temperature, we obtain HgLI2 (2). The crystal structure of 1 consists of a discrete [HgL2]2+cation (Fig. 1) together with two ClO4 anions. In the cation, the mercury atom is bonded to two ligands both acting as tetradentate. There is a wide range of Hg–N distances indicating that the structure is particularly strained. It is noteworthy that the Hg–N dimensions in one ligand N(11), N(18), N(23), N(30) at 2.48(1), 2.62(1), 2.78(1), 0 2.59(1) Å A, respectively, are longer than their counterparts in the other ligand N(41), N(48), N(53), N(60) at 2.35(1), 2.53(1), 0 2.57(1), 2.37(1) Å A. The torsion angles for each ligand are given in Table 1.
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Fig. 1. The structure of the cation in 1 with ellipsoids at 25% probability. For Hg–N distances, see text. Selected bond angles (°): N41–Hg1–N60 160.8(3), N41–Hg1–N11 118.2(3), N60–Hg1–N11 80.9(3), N41–Hg1–N48 65.7(2), N60–Hg1–N48 126.3(3), N11–Hg1–N48 74.0(2), N41–Hg1–N53 111.1(2), N60–Hg1–N53 66.2(2), N11–Hg1–N53 95.5(3), N48–Hg1–N53 70.0(3), N41–Hg1–N30 81.6(2), N60–Hg1–N30 79.3(3), N11–Hg1–N30 160.1(2), N48–Hg1–N30 120.0(2), N53–Hg1–N30 78.0(3), N41–Hg1–N18 74.1(2), N60–Hg1–N18 116.7(2), N11–Hg1–N18 63.0(2). 0
Table 1 Some torsion angles in the ligand backbones in 1 and 2. 1
N(11)–C(16)–C(17)–N(18) C(16)–C(17)–N(18)–C(19) C(17)–N(18)–N(19)–C(20) N(18)–N(19)–C(20)-C(21) N(19)–C(20)–C(21)–N(22) C(20)–C(21)–N(22)–N(23) C(21)–N(22)–N(23)–C(24) N(22)–N(23)–C(24)-C(25) N(23)–C(24)–C(25)–N(30)
2
Ligand 1
Ligand 2
7.7(1) 175.1(7) 137.3(8) 8.3(1) 84.9(1) 6.1(1) 154.1(8) 175.6(7) 27.1(1)
11.1(1) 177.7(7) 123.6(9) 8.4(1) 74.8(1) 0.2(1) 137.2(1) 176.7(8) 24.3(1)
11.0 78.2(5) 137.1(6) 3.4(8) 100.0(8) 2.1(8) 161.5(6) 173.7(5) 178.8(6)
The crystal structure of 2 is shown in Fig. 2 together with the atomic numbering scheme. Here the metal has a 4-coordinate tetrahedral environment being bonded to two iodine atoms at 0 atoms of the ligand L, N(11) 2.65(0), 2.65(0) Å A and two nitrogen 0 at 2.40(1) and N(18) at 2.50(1) Å A. The other potential donor nitro-
Fig. 2. The structure of 2 with ellipsoids at 50% probability. Selected bond distances (Å) and angles (°): Hg1–N11 2.40(1), Hg1–N18 2.50(1), Hg1–I1 2.65(0), Hg1–I2 2.65(0), N11–Hg1–N18 66.6(2), N11–Hg1–I1 112.6(2), N18–Hg1–I1 104.0(1), N11– Hg1–I2 97.7(1), N18–Hg1–I2 123.5(1), I1–Hg1–I2 131.05(2).
gen atoms are unbonded with Hg N distances >3.23 Å A. The distortions from tetrahedral geometry are primarily introduced by the small bite angle of the ligand, N(11)–Hg(1)–N(18) being 66.6(2)° which is concomitant with an extended I(1)–Hg(1)–I(2) angle of 131.05(2)°. The torsion angles in the ligand are given in Table 1 and show very little difference from those observed in the ligands acting as tetradentate donor atoms in 2. Clearly just a small increase in the N(19)–C(20)–C(21)–N(22) torsion angle is sufficient to remove N(22) from the coordination sphere; this torsion angle [100.0(8)o] is greater than that in the free ligand. It is also noteworthy that at the non-chelating part of the ligand, the N(23)– C(24)–C(25)–N(30) is trans rather than cis as is found in 2. Such trans configuration is found in the free ligand. The morphologies of both the complexes 1 and 2 are helical. While single helical nature of complex 2 is quite apparent from Fig. 2, the double helical nature of 1 can be discerned better in Fig. 3. These complexes are stereochemically non-rigid in solution as revealed by variable temperature 1H NMR studies.
Fig. 3. A space filling model of the cation in 1 showing its helical nature of the complex. The methyl and phenyl groups, and H atoms are removed for clarity. Colour code: N, black; Hg, white; C in one of the ligand strands is dark grey and in the other light grey.
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We have mentioned above that complex 1 is strained as demonstrated by the fact that it has two non-equivalent L fragments. The 1 H NMR of 1 in CD3CN at room temperature indicates that the two L moieties become magnetically equivalent in solution as only one singlet peak is observed for the methyl groups at 2.37 ppm (Fig. 4). It assumes D2 symmetry in solution. With the lowering of temperature, deviation from the D2 symmetry occurs and the methyl signal starts getting resolved into two singlets at 2.31 and 2.43 ppm, i.e. the solid state structure is approached as the temperature is lowered (Fig. 4) (actually, three methyl signals are expected; but there seems to be accidental coincidence of two of the peaks). Accordingly, the number of resonances in the aromatic region also increases with the lowering of temperature (Fig. 4). From the rela-
tive intensities of the methyl signals, the population of the solid state conformation in acetonitrile at 233 K is calculated to be 35%. The solvent polarity seems to have a role in this solution process, as only one methyl signal (singlet) is observed around 2.43 ppm in CD2Cl2 even down to 183 K. Since the HgI2 moiety is bound to one side of the potentially tetradentate L in 2 (Fig. 2), two separate singlets are expected for the two methyl groups in the 1H NMR spectrum of 2. Still only signal is
Fig. 4. Variable temperature 1H NMR spectra of 1 in CD3CN. Temperature in K: (a) 298; (b) 273; (c) 253; and (d) 233. The peak marked by S is due to the solvent.
Fig. 5. Variable temperature 1H NMR spectra of 2 in CD2Cl2. Temperature in K: (a) 298; (b) 273; (c) 233; (d) 213; and (e) 193.
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ber 6 for Hg(II) is uncommon and 8 is more so [21]. A few other examples of 8 coordinate Hg(II) complexes are known [22,23]. 4. Experimental 4.1. Materials and physical measurements Hg(ClO4)2 xH2O and HgI2 were purchased from Aldrich. L was synthesized as reported earlier [15]. Microanalyses were performed by a Perkin–Elmer 2400II elemental analyser. FTIR spectra (KBr) were recorded on a Shimadzu FTIR-8400S spectrometer, UV– VIS spectra were recorded in dichloromethane on a Perkin–Elmer Lambda 950 spectrophotometer and 1H NMR spectra (reference: TMS) on a Brucker AMX 300 spectrometer. 4.2. Computations The input model for 2 was taken from the crystal structure. The model of the 6-coordinate species involved in equilibrium (1) was built with the metal bonded to four nitrogen atoms in an approximately octahedral environment although no symmetry was imposed. Calculations were then carried out with the GAUSSIAN03 program [24]. Fig. 6. A gas phase structure of 2 as obtained from DFT calculations. Selected 0 calculated bond lengths (Å A): Hg1–I1 2.94, Hg1–I2 2.94, Hg1–N11 2.63, Hg1–N18 2.72, Hg1–N23 2.72, Hg1–N30 2.63.
observed for the methyl group at 2.23 ppm (Fig. 5) in CD2Cl2 at room temperature. Incidentally, the methyl signal in the free ligand L occurs at 2.36 ppm in CD2Cl2. As the temperature is lowered, the number of methyl signals remains one but the number of aromatic resonances decreases for 2 (Fig. 5) indicating generation of a species with higher symmetry. We believe, this is because of a solution equilibrium of the type (1) resulting in a 6-coordinate Hg(II) species where the ligand L I Hg
To a solution of Hg(ClO4)2 xH2O (80 mg) in methanol (5 ml) was added dropwise with stirring to a yellow solution of L (89 mg, 0.2 mmol) in methanol (20 ml). A shiny yellow compound started appearing immediately. Stirring was continued for 1 h. Then the compound was filtered, washed with a few drops of methanol and dried in air. Yield: 30 mg (50%). Light yellow single crystals were obtained from acetonitrile–toluene mixture. UV– VIS: kmax/nm (e/dm3 mol1 cm1): 293 (82 400), 228 (63 900). Anal. Calc. for C56H48Cl2O8HgN12: C, 51.46; H, 3.65; N, 13.42. Found: C, 51.59; H, 3.76; N, 13.33% 4.4. Synthesis of HgLI2 (2)
I
I
4.3. Synthesis of [HgL2](ClO4)2 (1)
Hg Hg
I
I
I
ð1Þ becomes tetradentate. The HgI2 fragment shuttles between the two bidentate ends of the ligand. At higher temperature the four coordinate species prevail. But at lower temperature the six coordinate species of higher symmetry predominates. Our DFT calculations at the B3LYP/LanL2DZ level shows that the four coordinate species (which is found in the solid state) has energy higher than that of the 6-coordinate one (Fig. 6) by only 12.3 kJ mol1 in the gas phase. Since the degrees of freedom are more for the 4-coordinate species which has a dangling part, the entropy factor offsets the difference in enthalpy of the two species (the 4-coordinate one and the 6-coordinate one) at higher temperature making the 4-coordinate structure prevalent in solution. 3. Concluding remarks Here we have described two mononuclear complexes of Hg(II) which are stereo-chemically non-rigid yet helical. Examples of such helical complexes of Hg(II), to the best of our knowledge, are not known in the literature. Further, while many helical complexes of Hg(II) are known [17–20], to date there is no example of a mononuclear helical complex of Hg(II). The coordination num-
To a solution of HgI2 (90 mg, 0.2 mmol) in dimethylformamide (5 ml) was added to a yellow solution of L (89 mg, 0.2 mmol) in methanol (30 ml) and stirred briefly. Then it was left for slow aerial evaporation. When the volume reduced (to 10 ml), the deposited yellow crystals, suitable for single crystal X-ray crystallography,
Table 2 Crystallographic data for the complexes 1 and 2.
Formula M Crystal system Space group Cell dimensions (Å, °) a b c b U (Å3) Z, Dcalc (g cm3) l (mm1) F(0 0 0) Unique reflections Observed reflections [I > 2r(I)] Parameters R1, wR2 (I > 2r(I)) R1, wR2 (all data) Largest peak/hole (e Å3)
1
2
C56H48N12Cl2O8Hg 1288.55 orthorhombic Pbca
C28H24N6I2Hg 898.92 monoclinic P21/n
16.2250(3) 20.9563(7) 31.9886(11) 90 10876.6(6) 8, 1.574 2.995 5168 15 784 10 897 716 0.1093, 0.1525 0.1580, 0.1660 1.84/3.25
9.8011(3) 17.6736(6) 16.7123(6) 95.760(3) 2880.30(17) 4, 2.073 7.515 1680 8114 5377 336 0.0493, 0.1028 0.0865, 0.1232 2.44/1.79
M.G.B. Drew et al. / Inorganica Chimica Acta 362 (2009) 2487–2491
were filtered, washed with a few drops of methanol and dried in air. Yield: 100 mg (55%). UV–VIS: kmax/nm (e/dm3 mol1 cm1): 290 (44 800), 229 (24 100). Anal. Calc. for C28H24I2HgN6: C, 37.38; H, 2.69; N, 9.34. Found: C, 37.49; H, 2.63; N, 9.23% 4.5. X-ray crystallography Crystal data for 1 and 2 were collected with Mo Ka radiation at 150 K using the Oxford Diffraction X-Calibur CCD System. The crystals were positioned at 50 mm from the CCD. Three hundred and twenty one frames were measured with a counting time of 10 s. Data analysis was carried out with the CRYSALIS program [25] to give 15784, 8114 independent reflections for 1 and 2, respectively. The structures were solved using direct methods with the SHELXS97 program [26]. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms bonded to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached. Absorption corrections were carried out using the ABSPACK program [27]. The structures were refined on F2 using SHELX97 [26]. Table 2 lists the important crystallographic data for the two complexes. Acknowledgments We thank EPSRC and the University of Reading for funds for the X-Calibur system. Appendix A. Supplementary data CCDC 652552 and 652553 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2008.11.008. References [1] c.m. harries, e.d. mckenzi, J. Chem. Soc. (A) (1969) 746. [2] D. Wester, G.J. Palenik, J. Chem. Soc., Chem. Commun. (1975) 74.
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[3] J.-M. Lehn, A. Rigault, J. Siegel, J. Harrowfield, B. Chevrier, D. Moras, Proc. Natl. Acad. Sci. USA 84 (1987) 2565. [4] J.-M. Lehn, Proc. Natl. Acad. Sci. USA 99 (2002) 4763. [5] M. Zimmer, D.A. Tocher, G.K. Patra, J.P. Naskar, D. Datta, Indian J. Chem. Sect. A 38 (1999) 1087. [6] P.K. Pal, S. Chowdhury, P. Purkayashta, D.A. Tocher, D. Datta, Inorg. Chem. Commun. 3 (2000) 585. [7] G.K. Patra, I. Goldberg, S.K. Chowdhury, B.C. Maiti, A. Sarkar, P.R. Bangal, S. Chakravorti, N. Chattopadhyay, D.A. Tocher, M.G.B. Drew, G. Mostafa, S. Chowdhury, D. Datta, New J. Chem. 25 (2001) 1371. [8] S. Chowdhury, P.B. Iveson, M.G.B. Drew, D.A. Tocher, D. Datta, New J. Chem. 27 (2003) 193. [9] S. Chowdhury, M.G.B. Drew, D. Datta, Indian J. Chem. Sect. A 43 (2004) 51. [10] M.G.B. Drew, D. Parui, S. De, J.P. Naskar, D. Datta, Eur. J. Inorg. Chem. (2006) 4026. [11] M.G.B. Drew, A. Mukherjee, S. De, S. Nag, D. Datta, Inorg. Chim. Acta 360 (2007) 3448. [12] M.G.B. Drew, S. De, D. Datta, Inorg. Chim. Acta 361 (2008) 2967. [13] N.C. Habermehl, P.M. Angus, N.L. Kilah, L. Noren, A.D. Rae, A.C. Willis, S.B. Wild, Inorg. Chem. 45 (2006) 1445. [14] S. De, S. Chowdhury, D.A. Tocher, D. Datta, CrystEngComm 8 (2006) 670. [15] P.K. Pal, S. Chowdhury, M.G.B. Drew, D. Datta, New J. Chem. 24 (2000) 931. [16] M.G.B. Drew, D. Parui, S. De, S. Chowdhury, D. Datta, New J. Chem. 31 (2007) 1763. [17] C. Piguet, G. Bernardinelli, G. Hopfgartner, Chem. Rev. 97 (1997) 2005–2062. [18] M. Albrecht, Chem. Rev. 101 (2000) 3457. [19] H.-P. Zhou, Y.-P. Tian, J.-Y. Wu, J.-Z. Zhang, D.-M. Li, Y.-M. Zhu, Z.-J. Hu, X.-T. Tao, M.-H. Jiang, Y. Xie, Eur. J. Inorg. Chem. (2005) 4976. [20] U.J. Scheele, S. Dechert, F. Meyer, Inorg. Chim. Acta 359 (2006) 4891. [21] F.A. Cotton, G. Wilkinson, Advanced Inorganic Chemistry, 5th ed., John Wiley & Sons, New York, 1998 (Chapter 16). [22] D.C. Bebout, D.E. Ehmann, J.C. Trinidad, K.K. Krahan, M.E. Kastner, D.A. Parish, Inorg. Chem. 36 (1997) 4257. [23] G.M. Cockrell, G. Zhang, D.G. VanDerver, R.P. Thumell, R.D. Hancock, J. Am. Chem. Soc. 130 (2008) 1420. [24] M.J. Frisch, G.W. Trucks, H.B. Schlegel, G.E. Scuseria, M.A. Robb, J.R. Cheeseman, J.A. Montgomery, Jr., T. Vreven, K.N. Kudin, J.C. Burant, J.M. Millam, S.S. Iyengar, J. Tomasi, V. Barone, B. Mennucci, M. Cossi, G. Scalmani, N. Rega, G.A. Petersson, H. Nakatsuji, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, M. Klene, X. Li, J.E. Knox, H.P. Hratchian, J.B. Cross, C. Adamo, J. Jaramillo, R. Gomperts, R.E. Stratmann, O. Yazyev, A.J. Austin, R. Cammi, C. Pomelli, J.W. Ochterski, P.Y. Ayala, K. Morokuma, G.A. Voth, P. Salvador, J.J. Dannenberg, V.G. Zakrzewski, S. Dapprich, A.D. Daniels, M.C. Strain, O. Farkas, D.K. Malick, A.D. Rabuck, K. Raghavachari, J.B. Foresman, J.V. Ortiz, Q. Cui, A.G. Baboul, S. Clifford, J. Cioslowski, B.B. Stefanov, G. Liu, A. Liashenko, P. Piskorz, I. Komaromi, R.L. Martin, D.J. Fox, T. Keith, M.A. Al-Laham, C.Y. Peng, A. Nanayakkara, M. Challacombe, P.M.W. Gill, B. Johnson, W. Chen, M.W. Wong,.C. Gonzalez, J.A. Pople, GAUSSIAN03, Revision B.03, Gaussian Inc., Pittsburgh, PA, 2003. [25] CRYSALIS Program, Oxford Diffraction Ltd., Oxford, UK, 2005. [26] G. Sheldrick, SHELXL-97, University of Göttingen, Göttingen, Germany, 1997. [27] ABSPACK Program, Oxford Diffraction Ltd., Oxford, UK, 2005.
Contents lists available at ScienceDirect
Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Stereochemically non-rigid helical mercury(II) complexes Michael G.B. Drew a, Senjuti De b, Dipankar Datta b,* a b
School of Chemistry, University of Reading, Whiteknights, Reading RG6 6AD, UK Department of Inorganic Chemistry, Indian Association for the Cultivation of Science, Calcutta 700 032, India
a r t i c l e
i n f o
Article history: Received 9 October 2008 Accepted 7 November 2008 Available online 18 November 2008 Keywords: Mercury Helical Spiral N ligands Crystal structures
a b s t r a c t Reactions of the 1:2 condensate (L) of benzil dihydrazone and 2-acetylpyridine with Hg(ClO4)2 xH2O and HgI2 yield yellow [HgL2](ClO4)2 (1) and HgLI2 (2), respectively. Homoleptic 1 is a 8-coordinate double helical complex with a Hg(II)N8 core crystallising in the space group Pbca with cell dimensions: a = 16.2250(3), b = 20.9563(7), c = 31.9886(11) Å. Complex 2 is a 4-coordinate single helical complex having a Hg(II)N2I2 core crystallising in the space group P21/n with cell dimensions a = 9.8011(3), b = 17.6736(6), c = 16.7123(6) Å and b = 95.760(3)o. In complex 1, the N-donor ligand L uses all of its binding sites to act as tetradentate. On the other hand, it acts as a bidentate N-donor ligand in 2 giving rise to a dangling part. From variable temperature 1H NMR studies both the complexes are found to be stereochemically non-rigid in solution. In the case of 2, the solution process involves wrapping up of the dangling part of L around the metal. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction
2. Results and discussion
A helical structure for a metallo organic complex was first proposed by Harries and McKenzie in 1969 in regard of a dinuclear copper(II) complex of bis(pyridinal)ethylenediamine [1]. It has gone largely unnoticed. The first crystal structure of a helical metal complex is possibly that of a Co(II) compound reported by Wester and Palenik in 1975 [2]. Systematic studies on helical complexes actually have started in the mid-eighties with the pioneering works of Lehn [3,4]. Examples of helical metallo organic compounds are now numerous. In all these cases the organic component (ligand) containing suitable donor atoms is non-helical. But the coordination properties of the metal ions direct the wrapping of the ligand around them in such a manner that an overall helical topology is assumed. For quite some time we have been interested in helical complexes [5–12]. A dinuclear Cu(I) helicate of ours [6] has subsequently been optically resolved by Wild and co-workers [13]. We have been working mainly with ligands which themselves are helical. Thereby we have been able to produce mononuclear double helical complexes; in some cases the metal ion is square planar. Lehn’s approach does not provide for mononuclear helical complexes, the possibility of which was not then considered. Herein we report two novel mononuclear helical complexes of mercury(II) generated by using a helical N-donor ligand.
Our ligand L here is derived from benzil dihydrazone which has a helical twist [14]. It is a 1:2 condensate of benzil dihydrazone and 2-acetylpyridine [15,16]. The N@C(Ph)–C(Ph)@N torsion angle which in benzil dihydrazone is 70o becomes 85.9(5)° in L. Thus L itself is helical.
* Corresponding author. Tel.: +91 33 24735374; fax: +91 33 24732805. E-mail address: [email protected] (D. Datta). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.11.008
N
Me Ph
Ph Me
N
N
N
N
N
ligand L
Reaction of L with Hg(ClO4)2 xH2O in 2:1 molar proportion in methanol at room temperature yields [HgL2](ClO4)2 (1). While L is reacted with HgI2 in equimolar proportion in dimethylformamide–methanol mixture at room temperature, we obtain HgLI2 (2). The crystal structure of 1 consists of a discrete [HgL2]2+cation (Fig. 1) together with two ClO4 anions. In the cation, the mercury atom is bonded to two ligands both acting as tetradentate. There is a wide range of Hg–N distances indicating that the structure is particularly strained. It is noteworthy that the Hg–N dimensions in one ligand N(11), N(18), N(23), N(30) at 2.48(1), 2.62(1), 2.78(1), 0 2.59(1) Å A, respectively, are longer than their counterparts in the other ligand N(41), N(48), N(53), N(60) at 2.35(1), 2.53(1), 0 2.57(1), 2.37(1) Å A. The torsion angles for each ligand are given in Table 1.
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Fig. 1. The structure of the cation in 1 with ellipsoids at 25% probability. For Hg–N distances, see text. Selected bond angles (°): N41–Hg1–N60 160.8(3), N41–Hg1–N11 118.2(3), N60–Hg1–N11 80.9(3), N41–Hg1–N48 65.7(2), N60–Hg1–N48 126.3(3), N11–Hg1–N48 74.0(2), N41–Hg1–N53 111.1(2), N60–Hg1–N53 66.2(2), N11–Hg1–N53 95.5(3), N48–Hg1–N53 70.0(3), N41–Hg1–N30 81.6(2), N60–Hg1–N30 79.3(3), N11–Hg1–N30 160.1(2), N48–Hg1–N30 120.0(2), N53–Hg1–N30 78.0(3), N41–Hg1–N18 74.1(2), N60–Hg1–N18 116.7(2), N11–Hg1–N18 63.0(2). 0
Table 1 Some torsion angles in the ligand backbones in 1 and 2. 1
N(11)–C(16)–C(17)–N(18) C(16)–C(17)–N(18)–C(19) C(17)–N(18)–N(19)–C(20) N(18)–N(19)–C(20)-C(21) N(19)–C(20)–C(21)–N(22) C(20)–C(21)–N(22)–N(23) C(21)–N(22)–N(23)–C(24) N(22)–N(23)–C(24)-C(25) N(23)–C(24)–C(25)–N(30)
2
Ligand 1
Ligand 2
7.7(1) 175.1(7) 137.3(8) 8.3(1) 84.9(1) 6.1(1) 154.1(8) 175.6(7) 27.1(1)
11.1(1) 177.7(7) 123.6(9) 8.4(1) 74.8(1) 0.2(1) 137.2(1) 176.7(8) 24.3(1)
11.0 78.2(5) 137.1(6) 3.4(8) 100.0(8) 2.1(8) 161.5(6) 173.7(5) 178.8(6)
The crystal structure of 2 is shown in Fig. 2 together with the atomic numbering scheme. Here the metal has a 4-coordinate tetrahedral environment being bonded to two iodine atoms at 0 atoms of the ligand L, N(11) 2.65(0), 2.65(0) Å A and two nitrogen 0 at 2.40(1) and N(18) at 2.50(1) Å A. The other potential donor nitro-
Fig. 2. The structure of 2 with ellipsoids at 50% probability. Selected bond distances (Å) and angles (°): Hg1–N11 2.40(1), Hg1–N18 2.50(1), Hg1–I1 2.65(0), Hg1–I2 2.65(0), N11–Hg1–N18 66.6(2), N11–Hg1–I1 112.6(2), N18–Hg1–I1 104.0(1), N11– Hg1–I2 97.7(1), N18–Hg1–I2 123.5(1), I1–Hg1–I2 131.05(2).
gen atoms are unbonded with Hg N distances >3.23 Å A. The distortions from tetrahedral geometry are primarily introduced by the small bite angle of the ligand, N(11)–Hg(1)–N(18) being 66.6(2)° which is concomitant with an extended I(1)–Hg(1)–I(2) angle of 131.05(2)°. The torsion angles in the ligand are given in Table 1 and show very little difference from those observed in the ligands acting as tetradentate donor atoms in 2. Clearly just a small increase in the N(19)–C(20)–C(21)–N(22) torsion angle is sufficient to remove N(22) from the coordination sphere; this torsion angle [100.0(8)o] is greater than that in the free ligand. It is also noteworthy that at the non-chelating part of the ligand, the N(23)– C(24)–C(25)–N(30) is trans rather than cis as is found in 2. Such trans configuration is found in the free ligand. The morphologies of both the complexes 1 and 2 are helical. While single helical nature of complex 2 is quite apparent from Fig. 2, the double helical nature of 1 can be discerned better in Fig. 3. These complexes are stereochemically non-rigid in solution as revealed by variable temperature 1H NMR studies.
Fig. 3. A space filling model of the cation in 1 showing its helical nature of the complex. The methyl and phenyl groups, and H atoms are removed for clarity. Colour code: N, black; Hg, white; C in one of the ligand strands is dark grey and in the other light grey.
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We have mentioned above that complex 1 is strained as demonstrated by the fact that it has two non-equivalent L fragments. The 1 H NMR of 1 in CD3CN at room temperature indicates that the two L moieties become magnetically equivalent in solution as only one singlet peak is observed for the methyl groups at 2.37 ppm (Fig. 4). It assumes D2 symmetry in solution. With the lowering of temperature, deviation from the D2 symmetry occurs and the methyl signal starts getting resolved into two singlets at 2.31 and 2.43 ppm, i.e. the solid state structure is approached as the temperature is lowered (Fig. 4) (actually, three methyl signals are expected; but there seems to be accidental coincidence of two of the peaks). Accordingly, the number of resonances in the aromatic region also increases with the lowering of temperature (Fig. 4). From the rela-
tive intensities of the methyl signals, the population of the solid state conformation in acetonitrile at 233 K is calculated to be 35%. The solvent polarity seems to have a role in this solution process, as only one methyl signal (singlet) is observed around 2.43 ppm in CD2Cl2 even down to 183 K. Since the HgI2 moiety is bound to one side of the potentially tetradentate L in 2 (Fig. 2), two separate singlets are expected for the two methyl groups in the 1H NMR spectrum of 2. Still only signal is
Fig. 4. Variable temperature 1H NMR spectra of 1 in CD3CN. Temperature in K: (a) 298; (b) 273; (c) 253; and (d) 233. The peak marked by S is due to the solvent.
Fig. 5. Variable temperature 1H NMR spectra of 2 in CD2Cl2. Temperature in K: (a) 298; (b) 273; (c) 233; (d) 213; and (e) 193.
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ber 6 for Hg(II) is uncommon and 8 is more so [21]. A few other examples of 8 coordinate Hg(II) complexes are known [22,23]. 4. Experimental 4.1. Materials and physical measurements Hg(ClO4)2 xH2O and HgI2 were purchased from Aldrich. L was synthesized as reported earlier [15]. Microanalyses were performed by a Perkin–Elmer 2400II elemental analyser. FTIR spectra (KBr) were recorded on a Shimadzu FTIR-8400S spectrometer, UV– VIS spectra were recorded in dichloromethane on a Perkin–Elmer Lambda 950 spectrophotometer and 1H NMR spectra (reference: TMS) on a Brucker AMX 300 spectrometer. 4.2. Computations The input model for 2 was taken from the crystal structure. The model of the 6-coordinate species involved in equilibrium (1) was built with the metal bonded to four nitrogen atoms in an approximately octahedral environment although no symmetry was imposed. Calculations were then carried out with the GAUSSIAN03 program [24]. Fig. 6. A gas phase structure of 2 as obtained from DFT calculations. Selected 0 calculated bond lengths (Å A): Hg1–I1 2.94, Hg1–I2 2.94, Hg1–N11 2.63, Hg1–N18 2.72, Hg1–N23 2.72, Hg1–N30 2.63.
observed for the methyl group at 2.23 ppm (Fig. 5) in CD2Cl2 at room temperature. Incidentally, the methyl signal in the free ligand L occurs at 2.36 ppm in CD2Cl2. As the temperature is lowered, the number of methyl signals remains one but the number of aromatic resonances decreases for 2 (Fig. 5) indicating generation of a species with higher symmetry. We believe, this is because of a solution equilibrium of the type (1) resulting in a 6-coordinate Hg(II) species where the ligand L I Hg
To a solution of Hg(ClO4)2 xH2O (80 mg) in methanol (5 ml) was added dropwise with stirring to a yellow solution of L (89 mg, 0.2 mmol) in methanol (20 ml). A shiny yellow compound started appearing immediately. Stirring was continued for 1 h. Then the compound was filtered, washed with a few drops of methanol and dried in air. Yield: 30 mg (50%). Light yellow single crystals were obtained from acetonitrile–toluene mixture. UV– VIS: kmax/nm (e/dm3 mol1 cm1): 293 (82 400), 228 (63 900). Anal. Calc. for C56H48Cl2O8HgN12: C, 51.46; H, 3.65; N, 13.42. Found: C, 51.59; H, 3.76; N, 13.33% 4.4. Synthesis of HgLI2 (2)
I
I
4.3. Synthesis of [HgL2](ClO4)2 (1)
Hg Hg
I
I
I
ð1Þ becomes tetradentate. The HgI2 fragment shuttles between the two bidentate ends of the ligand. At higher temperature the four coordinate species prevail. But at lower temperature the six coordinate species of higher symmetry predominates. Our DFT calculations at the B3LYP/LanL2DZ level shows that the four coordinate species (which is found in the solid state) has energy higher than that of the 6-coordinate one (Fig. 6) by only 12.3 kJ mol1 in the gas phase. Since the degrees of freedom are more for the 4-coordinate species which has a dangling part, the entropy factor offsets the difference in enthalpy of the two species (the 4-coordinate one and the 6-coordinate one) at higher temperature making the 4-coordinate structure prevalent in solution. 3. Concluding remarks Here we have described two mononuclear complexes of Hg(II) which are stereo-chemically non-rigid yet helical. Examples of such helical complexes of Hg(II), to the best of our knowledge, are not known in the literature. Further, while many helical complexes of Hg(II) are known [17–20], to date there is no example of a mononuclear helical complex of Hg(II). The coordination num-
To a solution of HgI2 (90 mg, 0.2 mmol) in dimethylformamide (5 ml) was added to a yellow solution of L (89 mg, 0.2 mmol) in methanol (30 ml) and stirred briefly. Then it was left for slow aerial evaporation. When the volume reduced (to 10 ml), the deposited yellow crystals, suitable for single crystal X-ray crystallography,
Table 2 Crystallographic data for the complexes 1 and 2.
Formula M Crystal system Space group Cell dimensions (Å, °) a b c b U (Å3) Z, Dcalc (g cm3) l (mm1) F(0 0 0) Unique reflections Observed reflections [I > 2r(I)] Parameters R1, wR2 (I > 2r(I)) R1, wR2 (all data) Largest peak/hole (e Å3)
1
2
C56H48N12Cl2O8Hg 1288.55 orthorhombic Pbca
C28H24N6I2Hg 898.92 monoclinic P21/n
16.2250(3) 20.9563(7) 31.9886(11) 90 10876.6(6) 8, 1.574 2.995 5168 15 784 10 897 716 0.1093, 0.1525 0.1580, 0.1660 1.84/3.25
9.8011(3) 17.6736(6) 16.7123(6) 95.760(3) 2880.30(17) 4, 2.073 7.515 1680 8114 5377 336 0.0493, 0.1028 0.0865, 0.1232 2.44/1.79
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were filtered, washed with a few drops of methanol and dried in air. Yield: 100 mg (55%). UV–VIS: kmax/nm (e/dm3 mol1 cm1): 290 (44 800), 229 (24 100). Anal. Calc. for C28H24I2HgN6: C, 37.38; H, 2.69; N, 9.34. Found: C, 37.49; H, 2.63; N, 9.23% 4.5. X-ray crystallography Crystal data for 1 and 2 were collected with Mo Ka radiation at 150 K using the Oxford Diffraction X-Calibur CCD System. The crystals were positioned at 50 mm from the CCD. Three hundred and twenty one frames were measured with a counting time of 10 s. Data analysis was carried out with the CRYSALIS program [25] to give 15784, 8114 independent reflections for 1 and 2, respectively. The structures were solved using direct methods with the SHELXS97 program [26]. The non-hydrogen atoms were refined with anisotropic thermal parameters. The hydrogen atoms bonded to carbon were included in geometric positions and given thermal parameters equivalent to 1.2 times those of the atom to which they were attached. Absorption corrections were carried out using the ABSPACK program [27]. The structures were refined on F2 using SHELX97 [26]. Table 2 lists the important crystallographic data for the two complexes. Acknowledgments We thank EPSRC and the University of Reading for funds for the X-Calibur system. Appendix A. Supplementary data CCDC 652552 and 652553 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2008.11.008. References [1] c.m. harries, e.d. mckenzi, J. Chem. Soc. (A) (1969) 746. [2] D. Wester, G.J. Palenik, J. Chem. Soc., Chem. Commun. (1975) 74.
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