Chelating ability of a conjugated redox active tetrathiafulvalenyl-acetylacetonate ligand

Inorganic Chemistry Communications 10 (2007) 1172–1176 www.elsevier.com/locate/inoche

Chelating ability of a conjugated redox active tetrathiafulvalenyl-acetylacetonate ligand Nathalie Bellec, Julien Massue, Thierry Roisnel, Dominique Lorcy

*

Matie`re Condense´e et Syste`mes Electroactifs (MaCSE), UMR 6226 CNRS-Universite´ de Rennes 1, Campus de Beaulieu, Bat 10A, Case 1013, 35042 Rennes cedex, France Received 23 May 2007; accepted 4 July 2007 Available online 12 July 2007

Abstract A novel tetrathiafulvalene (TTF) bearing a conjugated b-diketone moiety (TTFacacH) has been synthesized and fully characterized. The chelating ability of its enolate anion (TTFacac) has been investigated with [MII(OAc)2 Æ xH2O] (OAc = acetate and M = Zn, Cu and Ni) leading to complexes, where the metal center is coordinated by two TTFacac. Modulation of the redox properties of the TTF can be achieved through the simple change of the two apical ligands which completed the octahedral coordination geometry. This redox active ligand shows promising features for the elaboration of hybrid organic–inorganic building blocks. Ó 2007 Elsevier B.V. All rights reserved. Keywords: Tetrathiafulvalene; Redox-active ligand; b-diketone; Cyclic voltammetry; X-ray diffraction

Various hybrid organic–inorganic architectures containing the electroactive tetrathiafulvalene (TTF) core have been investigated with the aim of creating multifunctional materials upon association of partially oxidized TTFs with inorganic counterions [1]. A novel trend towards such materials is to prepare hybrid TTF precursors where a coordinating group can be used as cement between the organic and inorganic moieties [2–12]. Depending on the metal, its number of free coordination sites, and the number of coordination functions on the TTF, modulation of the molecular architecture and the properties thereof can be envisioned. The straightforward strategy consists of substituting the TTF core with one or more coordination functionalities and to use this building block as an electroactive ligand with various metal ions. Within this frame, the use of dithiolates [2] and phosphines [3–6] as well as pyridine [7–10] and related ligands have received a great deal of attention as compared to b-diketonate ligands, which offer quite a unique chelating ability with a large *

Corresponding author. E-mail address: [email protected] (D. Lorcy).

1387-7003/$ - see front matter Ó 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.inoche.2007.07.003

variety of metal centers [13]. Along these lines, we recently described the synthesis of TTF 1 substituted by a thioacetylacetone function (TTFSacacH) and demonstrated the chelating ability of its acetylacetonate ions (TTFSacac) towards metal (II) centers (M = Ni and Zn) [11,14]. The presence of the sulfur atom bridging the TTF core to the acac chelating ligand introduces flexibility, and rotation along C–S bonds could lead to disorder and preclude any close interaction of the TTF cores due to their location in a plane almost perpendicular to the one formed by the acetylacetone substituent [11]. In order to overcome these structural issues, we focused our research on the synthesis of TTF acetylacetone where the ligating part is directly linked to the TTF and potentially coplanar with the redox active core. In this paper, we present the synthesis and the redox properties of the hitherto unknown 1-tetrathiafulvalenyl-1,3-butanedione (TTFacacH) 2. We also report our investigations on the coordinating ability of its acetylacetonate ions toward various metal (II) cations (M = Cu, Ni, Zn) and the structural investigation of the first complex with this class of ligand. Moreover, we also present the modulation of the redox properties of the TTF core

N. Bellec et al. / Inorganic Chemistry Communications 10 (2007) 1172–1176

through the alteration of the apical ligands coordinated to the metallic atom. RS

S

S

SMe

RS

S

S

S Me

1

Me

S

S

Me

S

S

Me O

2

Me

R = Me, nBu

Me

O

O

O

Several strategies used for the synthesis of aryl b-diketones were investigated in order to prepare the diketone TTF (TTFacacH) 2. Among them, only one allowed us to isolate the target molecule and is described in Scheme 1 [15]. It consists first in the synthesis of TTF 4 by reacting trimethyl methoxycarbonyl TTF (Me3TTFCO2Me) [16] with the carbanion of 3, generated by treating N-(1-methylethylidene) cyclohexanamine 3 [17]with LDA. An acidic hydrolysis transformed easily TTF 4 into TTF 2 in 95% yield after purification over silica gel chromatography. Slow evaporation of a dichloromethane solution of TTF 2 afforded single crystals suitable for X-ray structure analysis [18]. As shown in Fig. 1, X-ray crystallographic analysis revealed a planar structure for 2 with the acetylacetone substituent located in the same plane as the one formed by the TTF. Two possible conformations of the acacH group can be envisaged relative to the external C@C bond, either a s-trans (represented in Scheme 1) or a s-cis conformation actually obtained as can be seen on Fig. 1. The central ˚ ) is within the expected range C@C bond length (1.350(4) A for a neutral TTF. This dissymmetrical TTF 2 forms stacks along the c-axis with the acceptor group of one TTF core

Me Me

Me

S

S

2)Me3TTFCO2Me Me

S

S

1) LDA N 3

HCl

Me

S

S

H2O

Me

S

S

2

4

Me

facing the electron richest dithiole moiety of the neighbouring TTF in a head-to-tail organization (Fig. 2). The presence of the electron withdrawing group, acacH, influences the electron density of the TTF core as can be seen on the redox properties determined by cyclic voltammetry for TTFs 4 and 2. Two reversible oxidation waves at E1 = 0.25 and E2 = 0.76 V vs. SCE for 4 and E1 = 0.36 and E2 = 0.86 V vs. SCE for 2 were observed. A positive shift of about 100 mV on both processes for 2 when compared to 4 indicates the electron withdrawing effect of the acacH substituent. On the basis of our precedent studies concerning the coordination of TTFSacacH 1 towards metal (II) centers, we examined the chelating ability of 2 towards Cu, Zn and Ni(II) by adding a methanolic solution of half an equivalent of MII(OAc)2, 2H2O (OAc = acetate and M = Zn, Cu and Ni) to a THF solution of TTF 2. Addition of water to the medium allowed us to isolate purple powders whose analysis by HRMS proved to contain the metal-linked TTF dimers, M(TTFacac)2 (Scheme 2). According to the 1H NMR spectrum of the diamagnetic complex, Zn(TTFacac)2 two water molecules are also coordinated to the metal center to form the hexacoordinated Zn complex, [Zn(TTFacac)2 Æ (H2O)2]. It is possible to solvate the metal center differently by simply dissolving these complexes [M(TTFacac)2 Æ (H2O)2] in hot pyridine or in hot DMSO. The water molecules are then replaced by two molecules of the solvent used as evidenced by the 1H NMR spectra of the Zn complexes [Zn(TTFacac)2 Æ L2] (L = Pyridine, DMSO). This is also confirmed by the X-ray crystal structure analysis of [Zn(TTFacac)2 Æ (DMSO)2], [19] which crystallizes with 2 molecules of DMSO, where the Zn atom is chelated by two TTFacac in the equatorial plane with the axial positions of the hexacoordinated metal center occupied by

Me OH

N

Me Me O

O

Scheme 1. Synthesis of TTFacacH 2.

C19 C17 C1

S5

S9

C7

C8

C2 C3 C4

O18

C16 C14 C11

O15

C12 S6

S10

C13

Fig. 1. ORTEP view of TTF 2 showing the atom labelling (50% probability ellipsoids). Hydrogen atoms are omitted for clarity.

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Fig. 2. Packing mode of TTF 2 along the c-axis.

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N. Bellec et al. / Inorganic Chemistry Communications 10 (2007) 1172–1176

Me

S

S

S

S

Me

S

S

Me O

2

O

1) M(OAc)2,xH2O 2) L = H2O, DMSO, pyridine

Me Me O O L M L O O

Me

S

S

Me

Me

Me

M = Cu2+, Zn2+, Ni2+

Me

S

S

Me

S

S

Me

M(TTFacac)2L2

Scheme 2. Synthesis of M(TTFacac)2L2.

two DMSO molecules, leading to the octahedral coordinating geometry of Zn(II) (Fig. 3). Within this complex, the two ligands are trans coordinated to the metal center and remain perfectly planar with the C@O bond of the acac group in a s-trans conformation this time with the external C@C bond of the TTF. The shortest intermolecular S. . .S distance between two neighbouring complexes being equal ˚. to 3.643(5) A The redox behavior of the complexes was investigated by cyclic voltammetry in CH2Cl2 for [Zn(TTFacac)2L2] (L = Pyridine, H2O and DMSO) and [Ni(TTFacac)2 (pyridine)2] whereas for [M(TTFacac)2(H2O)2], (M = Zn, Ni, Cu) in THF as they are rather insoluble in the other usual organic solvents. The redox potentials are shown in Table 1 relative to SCE. All the complexes exhibit two reversible redox systems which correspond to the concomitant oxidation of the TTF cores, respectively to the bis radical cation and to the bis dication species. One can observe that deprotonation and complexation increases slightly the electron

C13

C21

S4

Zn1 O21

C12 S2

C11

O2 O1

C8 C9

C10

C5

S3

C4 C6

S1

S21

C22

C2 C3

C1

C7

Fig. 3. ORTEP view of [Zn(TTFacac)2(DMSO)2] in [Zn(TTFacac)2 Æ DMSO2], 2DMSO showing the atom labelling (50% probability ellipsoids). Hydrogen atoms are omitted for clarity.

Table 1 Redox potentials of the complexes, E in V vs. SCE, 0.1 M Bu4NPF6, Pt working electrode, (a) in THF, (b) in CH2Cl2 [Zn(TTFacac)2 Æ (H2O)2]a [Cu(TTFacac)2 Æ (H2O)2]a [Ni(TTFacac)2 Æ (H2O)2]a [Zn(TTFacac)2 Æ (H2O)2]b [Zn(TTFacac)2 Æ (DMSO)2]b [Zn(TTFacac)2 Æ (pyridine)2]b [Ni(TTFacac)2 Æ (pyridine)2]b

E1(DEp)

E2(DEp)

DE (mV)

0.46 0.47 0.45 0.33 0.32 0.29 0.27

0.72 0.75 0.72 0.77 0.76 0.76 0.75

260 280 270 440 440 470 480

(60) (40) (70) (90) (90) (80) (100)

(50) (40) (40) (50) (40) (40) (60)

donating character of the TTF cores and that the values of the redox potentials do not vary significantly in accordance with the nature of the metal. These features compare with our previous observation on the complexes derived from TTFSacacH 1[11]. Comparing the redox potential of [Zn(TTFacac)2L2] in CH2Cl2, the first oxidation potential appears to be more sensitive to the nature of the apical ligands as there is a 40 mV shift when the apical ligand changes from water to pyridine. This is probably due to the electron p-donating character of pyridine molecule compared to water molecule and to the fact that the electronic effect is rather well transmitted to the TTF by the Zn(acac)2 bridge. Therefore, it is possible to modulate the donating ability of the TTF by simply changing the apical ligands within these complexes. Another noticeable point is the width of the first oxidation wave (DE = 80–100 mV) which is larger than expected, especially when compared to the second oxidation wave (DE = 40–60 mV). This variance in the width of the oxidation waves indicates that the first wave is composed of two very close oxidation processes due to presumably the successive oxidation of each TTF into the cation radical and to the bis(cation radical). The second wave should then corresponds to the concomitant oxidation of bis(cation radical) into the bis(dication). This behaviour is reminiscent of our previous observation on dimeric TTF complexes, involving the thioacetylacetonate TTF ligand 1, with the general formula [M(TTFSacac)2(Pyridine)2] with M = Zn and Ni, where intramolecular interactions between the two donor moieties were electrochemically evidenced [11]. Surprisingly, the splitting observed here for the same metal centers, Zn and Ni, is not as important as in the case of

N. Bellec et al. / Inorganic Chemistry Communications 10 (2007) 1172–1176

[M(TTFSacac)2(Pyridine)2] even though the coordination function is directly linked to the TTF core [11]. Actually, among the various type of dimeric TTFs it has to be recalled that the splitting of the oxidation waves are extremely variable and are function of the type and the number of connections between the two TTFs [20]. The behavior observed here, is due to a different stability of the mixed valence species formed upon oxidation. Indeed, even if we thought that the electronic delocalization could be increased in [M(TTFacac)2 Æ L2] due to the planar structure obtained, it has to be recalled that the DE determined electrochemically does not solely reflect the extend of the interactions between two identical redox centers. In fact, the measured DE is the sum of several different energetical factors (statistical distribution, electrostatic repulsion, inductive factor etc.) [21]. UV–vis–NIR investigations were performed on [Zn(TTFacac)2 Æ (DMSO)2] complex after chemical oxidation of the complex by successive addition of NOSbF6in order to evaluate the extend of interaction. The addition of one equivalent of NOSbF6 to the solution leads to the appearance of a weak and broad band centered at 2600 nm which disappeared upon the addition of a second equivalent of oxidizing agent. This band is characteristic of the formation of a mixed valence species and demonstrates that the first redox wave is composed of two very close oxidation processes. The oxidation of one TTF has an influence on the oxidation of the other TTF moiety and generates a mixed valence species, indicating that the two TTF cores are interacting with each other. To summarize, we have succeeded in the synthesis of a novel TTF acetylacetone, where the conjugated b-diketone moiety is coplanar with the TTF core. The chelating ability of this redox-active ligand has been evidenced through its reaction with metal (II) salts leading to complexes, where the two TTF are coplanar. Tuning of the redox properties of the TTF can be achieved by changing the apical ligand of the hexacoordinated metal complexes simply by recrystallization in a solvent such as DMSO or pyridine. Furthermore, electrochemical studies and NIR spectroscopy have evidenced intramolecular interactions within these complexes. Electrocrystallizations are under progress in order to obtain the observed mixed valence species as crystalline solids. Appendix A. Supplementary material CCDC 648127 and 648128 contain 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]. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.inoche.2007.07.003.

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[18] Selected data for 2 1-(4 0 ,5,5 0 -trimethyl-2,2 0 -bi-1,3-dithol-4-yl)butane2,4-dione mp 138–140°C; 1H NMR (300 MHz, CDCl3) d 1.98 (s, 6H, CH3), 2.15 (s, 3H, CH3), 2.43 (s, 3H, CH3), 5.60 (s, 1H, @CH), 15.80 (s, 1H, OH); 13C NMR (50 MHz, CDCl3) d 14.1, 17.2, 25.3, 100.3, 112.1, 123.2, 123.4, 125.0, 145.0, 178.2, 191.7; IR (KBr): mCO = 1585 cm1; Elemental analysis for C13H14O2S4 calc. C, 47.27; H, 4.24; S, 38.79; Found: C, 47.03; H, 4.27; S, 38.73 ; HRMS calc. for C13H14O2S4329.9877 found 329.9877. Crystal data for 2: M = 330.48, ˚ , b = 7.6451(2) A ˚, monoclinic, space group C2, a = 27.7546(9) A ˚, ˚ 3, c = 7.1403(2) A b = 95.856(1)°, V = 1507.17(8) A Z = 4, T = 293(2)°, l = 0.624 mm1, Dc = 1.456 g mm3 (Mo Ka) 0.71073;

12543 reflections were collected with 3127 unique and 2803 with I > 2r(I) yielding R(F) = 0.0351, Rw(F2) = 0.0955 and GOF = 1.042. [19] Crystal data for Zn (TTFacac)2 Æ DMSO2, 2DMSO: M = 1036.83, ˚ , b = 10.8761(7) A ˚, triclinic, space group P1, a = 9.4000(5) A ˚ , a = 74.725(3)°, b = 68.829(2)°, c = 74.761(2)° V = c = 12.4232(7) A ˚ 3, Z = 1, T = 100(2)°, l = 1.152 mm1, Dc =! 1122.50(11) A 1.534 g mm3 (Mo Ka) 0.71073; 17704 reflections were collected with 5083 unique and 4212 with I > 2r(I) yielding R(F) = 0.029, Rw(F2) = 0.0742 and GOF = 1.039. [20] M. Iyoda, M. Hasegawa, Y. Miyake, Chem. Rev. 104 (2004) 5085. [21] R.J. Crutchley, Coordinat. Chem. Rev. 219–221 (2001) 125.