Synthesis, structure and properties of mercury complexes with a new ...

Polyhedron 24 (2005) 671–677 www.elsevier.com/locate/poly

Synthesis, structure and properties of mercury complexes with a new extended tetrathiafulvalene 4,5-dithiolate ligand He-Rui Wen a, Jing-Lin Zuo a

a,*

, Thomas A. Scott b, Hong-Cai Zhou b, Xiao-Zeng You

a

Coordination Chemistry Institute, State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, China b Department of Chemistry and Biochemistry, Miami University, Oxford, OH 45056-1465, USA Received 21 October 2004; accepted 21 January 2005

Abstract A new extended tetrathiafulvalene (TTF) dithiolate ligand, benzotetrathiafulvalenedithiolate (btdt2
), together with mercury complexes based on it, have been synthesized and characterized by cyclic voltammetry, ESR, IR and magnetic susceptibility measurements. The crystal structure of (Me4N)2[Hg(btdt)2] Æ 4/3H2O Æ 2/3C4H9O (4) is determined, in which the central Hg atom is in an approximately tetrahedral coordination environment. Both ESR and cyclic voltammetry studies show that there is very little conjugating interaction between the two extended TTF dithiolate ligands. The neutral complex Hg(btdt)2 (5) is prepared from 4 by oxidation with iodine. The electrical conductivity of 5 was measured as compressed pellets and the room temperature conductivity is 0.0015 S cm
1. Magnetic susceptibility measurements reveal that complex 5 is a radical antiferromagnet. Ó 2005 Elsevier Ltd. All rights reserved. Keywords: Benzotetrathiafulvalenedithiolate; Mercury complexes; Crystal structure; Conductivity; Magnetic susceptibility

1. Introduction Metal complexes of 1,2-dithiolene ligands have been intensively studied as materials for molecular conductors and superconductors [1]. The crystal structure of these complexes reveal that intermolecular interaction through the sulfur–sulfur interatomic contact is the basic requirement for high conductivity. The incorporation of more heteroatoms, such as S or Se, into the periphery of the structure stabilizes the interactions via direct S  S or Se  Se overlap on the adjacent inter- and intra-stack ions. Recently metal complexes with extended tetrathiafulvalenedithiolene ligands have been of great interest because of their possible extended p-conjugation system and their better electrical conduction properties [2–14]. A few highly conducting molecule crystals of nickel, copper and gold complexes with extended TTF *

Corresponding author. Tel.: +86 25 8359 3893; fax: +86 25 8331 7761. E-mail address: [email protected] (J.-L. Zuo). 0277-5387/$ - see front matter Ó 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.poly.2005.01.015

ligands have been reported [4–6]. Since the basic idea on the design of a molecular conductor is to reduce the HOMO–LUMO energy gap in the molecule by extending the p conjugated system and improving the planarity [12], the synthesis of highly conjugated and planar ligands becomes the main target. However, in these metal complexes with extended TTF dithiolate ligands, the side groups on the TTF framework all are alkyl groups. As we know, the length and type of the side alkyl group on the TTF framework have great effect on the conducting properties of the resulted complexes [3,10]. Bulky alkyl groups on the TTF framework will lead to molecules with non-compact packing, which decrease the intermolecular reaction. In order to eliminate the alkyl group effect on the TTF framework and increase the planarity and p-conjugation of the extended TTF dithiolate, we report herein the synthesis of a new extended TTF dithiolate ligand, benzotetrathiafulvalenedithiolate, together with mercury complexes based on this ligand.

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H.-R. Wen et al. / Polyhedron 24 (2005) 671–677

2.2. Preparation of 2,3-bis(2-cyanoethylthio)benzotetrathiafulvalene (3)

2. Experimental 2.1. General procedures Reagent-grade tetrahydrofuran (THF) was purified and distilled with standard methods. Other solvents and chemicals were purchased from commercial sources and used as received. Schlenk techniques were used in carrying out manipulation under nitrogen atmosphere. Elemental analyses for C, H and N were performed on a Perkin–Elmer 240C analyzer. The ESR spectra were measured using a Bruker ER 200-D-SRC spectrometer. The IR spectra were taken on a Vector22 Bruker Spectrophotometer (400–4000 cm
1) with KBr pellets. NMR spectra were measured on a Bruker AM 500 spectrometer. Cyclic voltammetry data were recorded by an EG & G PAR Model 273 electrochemical analytical instrument. The magnetic susceptibility was measured using a Quantum Design MPMS-XL7 SQUID magnetometer at temperatures ranging from 1.8 to 300 K. The temperature dependence of the resistivity was measured by the conventional four-probe method using a copper wire attached with indium in the temperature range of 80–293 K. The btdt2
ligand and the related complexes were synthesized as shown in Scheme 1.

NH2

iso-C5H11ONO

COOH

2-Thioxo-1,3-benzodithiol (1) [15,16] and 5-bis(2-cyanoethylthio)-1,3-dithiole-2-one (2) [9] were synthesized as described in the literature. Under a nitrogen atmosphere, to the mixture of 1 (0.92 g, 5 mmol) and 2 (0.86 g, 3 mmol) was added 25 ml of freshly distilled P(OC2H5)3. The reaction mixture was heated at 110 °C for 1 h and then allowed to cool to room temperature. After 25 ml of methanol was added to the above suspension, on filtering an orange-red precipitate was collected and washed with methanol, which was then chromatographed on a silica gel column using dichloromethane as the eluent. Orange-yellow powder 3 was obtained. Yield: 0.72 g (56%, based on 2). Anal. Calc. for C16H12N2S6: C, 45.28; H, 2.83; N, 6.60; S, 45.28. Found: C, 45.21; H, 2.65; N, 6.88; S, 45.35%. IR (KBr cm
1): m 3446(w), 1567(w), 1441(m), 1426(s), 1408(s), 1320(w), 1281(m), 1118(m), 892(s), 770(m), 736(s), 675(w), 415(m). 1H NMR (CDCl3): d = 2.77 (t, 4H, CH2), 3.12 (t, 4H, CH2), 7.16 (m, 2H, Ph), 7.28 (d, 2H, Ph). 13C NMR (CDCl3): = 19.32, 31.72, 117.87, 122.43, 126.61, 128.39, 136.50. EI–MS: 424 (M+, 25.6), 370 (13), 286 (15), 196 (50), 152 (70), 108 (73), 54 (100).

S

H

S

OC5H11

S

S

S

CS2, iso-C5H11OH

S

1 S

S

S

S

S Zn

S

2-

S S

BrCH2CH2CN

Hg(OAc)2

SCH2CH2CN

S

SCH2CH2CN

O

S

S

S

2

S

S

SCH2CH2CN

S

SCH2CH2CN

(EtO)3P

S+ O S

S

S

SCH2CH2CN

S

S

SCH2CH2CN

3

Me4NOH

Hg2+

S

S

S

S

S

S

S

S

2-

S

S

S

S

S

S

S

S

S

S

S

S

Hg

4 I2

S

Hg S

S

S

5

Scheme 1.

H.-R. Wen et al. / Polyhedron 24 (2005) 671–677

2.3. Preparation of (Me4N)2[Hg(btdt)2] Æ 4/3H2O Æ 2/3C4H9O (4) Under a nitrogen atmosphere, a methanol solution of Me4NOH (0.45 ml, 25% (w/w), 1 mmol) was added to 3 (212 mg, 0.5 mmol) in 20 ml of THF at
5 °C. A red-orange precipitate was observed immediately and the reaction mixture was left stirring at the same temperature for 30 min. THF was removed in vacuo at 10 °C and the resulting pink solid was dissolved in 20 ml of methanol. Then Hg(OAc)2 (80 mg, 0.25 mmol) in 10 ml of methanol was added and a gold-yellow precipitate was observed immediately, the reaction mixture was left to stir at 0 °C for 12 h. The gold-yellow precipitate was filtered off, washed with methanol and ether, and dried in vacuo. Yield: 210 mg (86%). Anal. Calc. for C28H32N2S12Hg: C, 34.26; H, 3.26; N, 2.86; S, 39.16. Found: C, 34.16; H, 3.28; N, 3.02; S, 39.42%. IR (KBr cm
1): m 3442(w), 3018(w), 1598(m), 1566(m), 1479(s), 1445(s), 1433(m), 1118(w), 1022(w), 947(s), 864(m), 746(m), 674(w), 467(w). Air-stable gold-yellow plateshaped crystals of (Me4N)2[Hg(btdt)2] Æ 4/3H2O Æ 2/3C4H9O were obtained by slow diffusion of diethyl ether into a solution of the product in acetonitrile. 2.4. Preparation of [Hg(btdt)2] (5) Microcrystalline complex 5 was obtained by slow diffusion of iodine vapor into a DMF solution of complex 4 under nitrogen. The microcrystalline precipitate was filtered off, washed with DMF, acetonitrile, methanol and ether, and then dried in vacuo. Anal. Calc. for C20H8S12Hg: C, 28.83; H, 0.96; S, 46.12; N, 0. Found: C, 29.10; H, 0.91; S, 46.23; N, 0%. IR (KBr cm
1): m 1564(m), 1467(s), 1444(s), 1431(s), 1282(s), 1257(m), 1197(s), 1117(s), 1026(m), 959(m), 886(w), 770(m), 733(s), 674(m), 469(w).

673

Table 1 Summary of crystal and experimental data for complex 4 Formula Formula weight Crystal system Space group Unit cell dimensions ˚) a (A ˚) b (A ˚) c (A a (°) b (°) c (°) Z ˚ 3) V (A Dcalc (g cm
3) T (K) ˚) k (A l (mm
1) F (0 0 0) 2max(°) h k l Range

Collected Unique Parameters Goodness-of-fit R1 [I > 2r(I)] wR2 [I > 2r(I)] ˚
3) (D map) maximum/minimum (e A

C92H124N6O6S36Hg3 3165.90 orthorhombic Pbcn (no.60) 25.253(1) 16.910(1) 29.893(1) 90 90 90 4 12765.3(9) 1.647 213 0.71073 4.236 6328 52.0
31 6 h 6 25,
20 6 k 6 20,
36 6 l 6 35 78 278 12 563 645 0.911 0.0526 0.0921
1.866/1.745

ods using the program SHELXL-97. The positions of the metal atoms and their first coordination spheres were located from direct-methods E-maps; other non-hydrogen atoms were found in alternating difference Fourier syntheses and least-squares refinement cycles and, during the final cycles, refined anisotropically. Hydrogen atoms were placed in calculated positions and refined as riding atoms with a uniform value of Uiso. Crystallographic parameters and agreement factors are contained in Table 1.

3. X-ray crystallography The structure of complex 4, (Me4N)2[Hg(btdt)2] Æ 4/3H2O Æ 2/3C4H9O, was determined. The crystal was mounted in Infineum oil and placed in a dinitrogen cold stream on a Siemens (Bruker) SMART CCD-based diffractometer. Cell parameters were retrieved using SMART software and refined using SAINT on all observed reflections. Data were collected using the following strategy: 606 frames of 0.3° in with / = 0°, 435 frames of 0.3° in x with / = 90°, and 235 frames of 0.3° in x with / = 180°. An additional 50 frames of 0.3° in x with / = 0° were collected to allow for decay correction. The highly redundant data sets were reduced using SAINT and corrected for Lorentz and polarization effects. Absorption corrections were applied using SADABS supplied by Bruker. Structures were solved by direct meth-

4. Result and discussion 4.1. Synthesis The benzotetrathiafulvalenedithiolene was prepared similar to the literature method [9]. The synthetic procedure is shown in Scheme 1. Cyanoethyl was used as the protecting group in the cross-coupling reaction to synthesize the unsymmetrical TTF derivative. By silica gel column chromatography, using CH2Cl2 as eluent, 2, 3-bis(2-cyanoethylthio)-5,6-benzotetrathiafulvalene was obtained in high yield (56%). Under nitrogen atmosphere, the cyanoethyl group was removed by using Me4NOH as the base in THF at
5 °C, which generated the dithiolate. Then the dithiolate ligand reacted further

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H.-R. Wen et al. / Polyhedron 24 (2005) 671–677

with mercury(II) ions and resulted the isolation of the metal complex. The analytically pure neutral complex 5 was obtained by slow diffusion of iodine vapor into a DMF solution of complex 4 under nitrogen. The yielded microcrystalline powder sample of complex 5 is insoluble in common solvents. 4.2. Crystal structure of 4 There are one and half molecules of complex 4 in each asymmetric unit. The atom labeling scheme of one anion part of the solvated complex, (Me4N)2 [Hg(btdt)2] Æ 4/3H2O Æ 2/3C4H9O, is shown in Fig. 1. The selected bond lengths and angles are listed in Table 2. The mercury atom is in an approximately tetrahedral geometry. The dihedral angle between the planes [S1, S2, S3, S4, C10, C11] and [S7, S8, S9, S10, C20 C21] is 90.3°. In the five-membered ring containing the mercury atom, the average Hg–S and C@C bond lengths ˚ , respectively. The S–C bond are 2.532(2) and 1.352(9) A ˚ , which distance of the dithiolate are 1.732(6)–1.755(6) A are somewhat shorter than the S–C bond distance of the ˚ ). The bond angles two TTF units (1.745(7)–1.780(7) A of S(1)–Hg–S(2) and S(7)–Hg–S(8) are 89.02(5)° and 88.74(5)°, respectively, which are smaller than the idealized tetrahedral value. The dihedral angle between the planes [S7, S8, S9, S10, C20. C21, C22] and [S11, S12, C23–C29] is 132.1°, and the angle between the planes [S1, S2, S3, S4, C10, C11, C12] and [S5, S6, C13–C19] is 132.8°. The two TTF units in complex deviate from planarity due to the non-planar coordination of the central atom Hg and the packing effects of intermolecular adjacent TTF units. The packing diagram for complex 4 is shown in Fig. 2, the short intermolecular S  S dis˚ (S(5)  S(12)a, symmetry tances in crystal are 3.868 A ˚ (S(3)  S(12)a) operations: x, 1
y, 1.5 + z), 3.949 A a ˚ (S(5)  S(10) ), which are somewhat shorter and 3.956 A than that of the similar mercury complex with an extended TTF ligand [3]. 4.3. Cyclic voltammetry Cyclic voltammetry of the ligand 3 and complex 4 were carried out on a Macroscopic platinum-disc elec-

Table 2 ˚ ) and angles (°) for complex 4 Selected bond lengths (A Bond lengths Hg(1)–S(1) Hg(1)–S(7) S(1)–C(10) S(3)–C(10) S(4)–C(12) S(5)–C(14) S(6)–C(15) S(7)–C(21) S(9)–C(21) S(10)–C(20) S(11)–C(25) S(12)–C(23) C(10)–C(11) C(12)–C(13) C(14)–C(15) C(15)–C(16) C(25)–C(26) Bond angles S(1)–Hg(1)–S(2) S(1)–Hg(1)–S(8) S(2)–Hg(1)–S(8) Hg(1)–S(1)–C(10) Hg(1)–S(7)–C(21) S(1)–C(10)–C(11) S(1)–C(10)–S(3) S(8)–C(20)–C(21) S(7)–C(21)–S(9) S(3)–C(12)–S(4) S(5)  S(10)a S(5)  S(12)a

2.497(2) 2.553(1) 1.755(6) 1.767(6) 1.748(7) 1.780(7) 1.754(8) 1.745(7) 1.750(6) 1.774(6) 1.730(7) 1.770(7) 1.353(9) 1.352(8) 1.377(10) 1.387(10) 1.438(10) 88.98(5) 130.64(5) 117.68(6) 97.2(2) 96.3(2) 128.4(5) 114.1(4) 129.7(5) 114.0(4) 113.3(3) 3.956(3) 3.868(3)

Hg(1)–S(2) Hg(1)–S(8) S(2)–C(11) S(3)–C(12) S(4)–C(11) S(5)–C(13) S(6)–C(13) S(8)–C(20) S(9)–C(22) S(10)–C(22) S(11)–C(23) S(12)–C(24) C(20)–C(21) C(22)–C(23) C(24)–C(25) C(14)–C(19) C(24)–C(29) S(7)–Hg(1)–S(8) S(2)–Hg(1)–S(7) S(1)–Hg(1)–S(7) Hg(1)–S(8)–C(20) Hg(1)–S(2)–C(11) S(2)–C(11)–C(10) S(2)–C(11)–S(4) S(7)–C(21)–C(20) S(8)–C(20)–S(10) S(9)–C(22)–S(10) S(3)  S(12)a

2.563(2) 2.517(1) 1.732(6) 1.745(7) 1.749(6) 1.752(7) 1.750(7) 1.733(7) 1.757(7) 1.747(7) 1.745(7) 1.740(8) 1.352(9) 1.338(9) 1.405(10) 1.387(10) 1.380(10) 88.71(5) 113.01(5) 119.81(6) 96.9(2) 95.9(2) 129.5(5) 114.7(4) 128.4(5) 114.6(4) 113.3(3) 3.949(3)

Symmetry operations a: x, 1
y, 1.5 + z.

trode in DMF solution, and the reference electrode was Ag/AgCl (using 0.1 mol dm
3 (Bu4N)ClO4 as supporting electrolyte, 100 mV S
1). The ligand 3 shows a single electron oxidation peak at 0.8 V. Similar to the mercury complex with TTF ligand [3], a single oxidation peak at 0.48 V was observed for complex 4, indicating that the two TTF units in complex 4 were completely absent of interaction and independently oxidized to remove one electron with equal potential in DMF solvent. This is the reason that the monoanionic species of 4 is difficult to isolate.

Fig. 1. An ORTEP drawing of the dianion [Hg(btdt)2]2
showing the atom labelling. The counter ions and hydrogen atoms have been omitted for clarity.

H.-R. Wen et al. / Polyhedron 24 (2005) 671–677

3000

675

3200

3400

3600

3800

H Fig. 3. ESR spectrum of complex Hg(btdt)2 (5) at 110 K.

Fig. 2. Packing diagram for complex [Hg(btdt)2]2
showing the arrangement of the anion and the short intermolecular S  S contacts.

ESR spectrum. The result is in accord with other neutral mercury complexes with a TTF-based ligand [3].

4.4. ESR spectra

4.5. Magnetic properties

The ESR spectrum of the neutral complex 5 was measured in the solid state at 110 K. Complex 5 contained a singlet with a g value of 2.003 (Fig. 3). There is little conjugation interaction between the two linked TTF dithiolate units in this molecule since complex 5 has approximately tetrahedral coordination around the Hg atom, as shown in structural analyses. Each extended TTF dithiolate unit has an independent single electron radical, which accounts for the singlet observed in the

Magnetic susceptibility measurements for complex 5 were performed on a Quantum Design MPMS-XL7 SQUID magnetometer at temperatures ranging from 1.8 to 300 K. For complex 5, at the room temperature, vMT is 0.042 emu K mol
1, much smaller than the value (0.75) of two electrons radicals in a molecule sheet. As the temperature is lowered, the vMT value decreases al-

0.005 0.045 0.040

0.004 χMT / emu K mol

-1

0.035

χM / emu mol

-1

0.003

0.002

0.030 0.025 0.020 0.015 0.010 0.005

0.001

0

50

100

150

200

250

300

T/K

0.000 0

50

100

150

200

250

300

T/K Fig. 4. Temperature dependence of the vM and vMT (inset) for complex Hg(btdt)2 (5).

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H.-R. Wen et al. / Polyhedron 24 (2005) 671–677

most linearly to reach 0.008 emu K mol
1 at 2 K (Fig. 4), implying that an antiferromagnetic interaction dominates the magnetic properties of this complex over the whole temperature range. Since the absence of a conjugation interaction between the two TTF dithiolate groups of the intramolecule, due to the tetrahedral coordination around the central Hg atom, each TTF dithiolate group is viewed as an independent radical cation. This was confirmed by the ESR spectrum of complex 5 which contained a singlet with a g value of 2.003. Thus, the intramolecular spin coupling between the radicals is not the source leading to the strong antiferromagnetic properties of complex 5. It is suggested that the strong antiferromagnetic coupling results from intermolecular exchange, since the spin widely delocalizes in the individual TTF-dithiolate group and further leads to the effective intermolecular interaction. It is consistent with the ESR result exhibiting no coupling radical cation in the molecule. Using the mean-field expression of the Curie–Weiss temperature h = 2zj 0 S (S + 1)/3kB [17], the magnetic interaction zj 0 kB between the radicals can be estimated to be ca.
465.3 K (
322.9 cm
1) with S = 1/2, which supports the results mentioned above. 4.6. Electrical conductivities The electrical conductivities of neutral complex 5 were measured as compressed pellets over the range 80–293 K (Fig. 5), the room temperature conductivity is 0.0015 S cm
1 and the activation energy is 0.24 eV. Since the Hg atom is in an approximately tetrahedral environment, with two non-planar coordinating dithiolate ligands, only a very weak conjugation interaction between the two extended TTF dithiolate units exists in complex which together with the molecule
s non-compact packing lead to lower conductivity. From room

temperature to 125 K, the change in the value of resistivity (q) is very small. Below 125 K the value of q increases rapidly on lowering the temperature, indicating that complex 5 is a semiconductor. The work on the nickel analogs of this new tetrathiafulvalene 4,5-dithiolate ligand, and experiments aimed at highly conducting metal complexes and multi-functional materials are current under way in our laboratory.

Acknowledgments This work was supported by The Major State Basic Research Development Program (G2000077500), the National Natural Science Foundation of China (NSF20201006 and 90101028), the Petroleum Research Fund of the American Chemical Society (PRF 38794G3). The X-ray diffractometer is supported by NSF Grant EAR-0003201.

Appendix A. Supplementary data CCDC- 244536 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge at www.ccdc.cam.ac.uk/conts/retrieving.html or from the Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK [Fax: (internat.) +44-1223/336-033; e-mail: deposit @ccdc.cam.ac.uk]. Supplementary data associated with this article can be found, in the online version at doi:10.1016/j.poly.2005.01.015.

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5

8.0x10

5

ρ (Ωcm)

6.0x10

5

4.0x10

5

2.0x10

0.0

100

150

200

250

300

T (K) Fig. 5. Temperature dependence of the electrical resistivity of Hg(btdt)2 (5).

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