DOI: 10.1021/cg901018s
Calcium and Magnesium Bicarboxylates Combined with Tetrathiafulvalene Moiety
2010, Vol. 10 2090–2095
Jin-Po Wang,† Zhe-Jun Lu,† Qin-Yu Zhu,*,†,‡ Ya-Ping Zhang,† Yu-Rong Qin,† Guo-Qing Bian, and Jie Dai*,†,‡ †
Department of Chemistry & Key Laboratory of Organic Synthesis of Jiangsu Province, Soochow University, Suzhou 215123, P. R. China, and ‡State Key Laboratory of Coordination Chemistry, Nanjing University, Nanjing 210093, P. R. China Received August 24, 2009; Revised Manuscript Received February 4, 2010
ABSTRACT: Four coordination compounds of calcium and magnesium with tetrathiafulvalene (TTF) bicarboxylate ligand have been synthesized and characterized by single-crystal analysis. As well-studied bicarboxylate complexes, the carboxylate groups in this ligand coordinate to alkaline earth metal ions in multidentate modes forming compounds 1-4. Calcium compounds [CaL(DMF)2] (1) and [Ca2L2(H2O)2(EtOH)] 3 MeOH (2) take a seven-coordinated sphere with a pentagonal bipyramid geometry, while magnesium compounds [Mg2L2(H2O)7] 3 EtOH 3 4H2O (3) and [Na2(DMF)2(H2O)9][MgL2(DMF)2] (4) are six-coordinated with an elongated octahedral geometry. The effect of metal ion radii on the coordination structure is clearly exhibited. The molecules of these alkalineearth metal complexes are assembled to a sandwiched structure with hydrophobic layers of a sulfur-rich TTF matrix and hydrophilic layer of the metal carboxylate. Equilibriums of the metal coordination, acid dissociation, and ligand oxidation have been studied in the solution systems of these compounds. At a neutral state of the TTF moiety, TTF-carboxylate is an appropriate ligand for Ca(II) ion, but when the TTF moiety is oxidized, it switches the alkaline earth metal ion to the solution.
Introduction Carboxylates are the most common and ubiquitous naturally occurring organic complexants, present in all the fractions of natural organic matter; therefore, their derivatives have received extensive attention in coordination chemistry and supermolecular chemistry. Recently, bicarboxylates or polycarboxylates have found a large field of application in the construction of coordination polymer arrays thanks to their diverse coordination modes and bridging ability as useful building blocks for porous networks with metal ions throughout the periodic system.1 The characteristics of hard ligands of carboxylates allow the ligands to be favored in the formation of complexes with alkaline earth metal ions with their ionic character.2 However, coordination chemistry of the poly carboxylate of alkaline earth metal in the widest sense has been largely undeveloped compared with transition metal coordination networks. Studies on alkaline earth metal complexes are meaningful to the interpretation of the biological functions and biochemical reactions with Ca2þ dependence,3 because calcium plays a key role in many biological and biochemical processes. Ion-exchange fibers containing carboxylate (-COOH) functional groups can be used in the removal of hardness from water.4 Most of the research in the coordination field on alkaline-earth metal carboxylates is about the equilibriums of the acid-base and metal coordination, or the solid-state crystal structure.5 The redox systems with alkaline-earth metal carboxylates have been largely undeveloped. As an excellent redox functional group, tetrathiafulvalene (TTF) has attracted great attention. In the field of coordination chemistry, bifunctional TTF derivatives are designed and synthesized in order to introduce a functional coordination moiety to the redox system, as shown by TTF-crown ethers,6 *To whom correspondence should be addressed. E-mail: zhuqinyu@ suda.edu.cn (Q.Y.Z.);
[email protected] (J.D.). pubs.acs.org/crystal
Published on Web 03/25/2010
TTF-py/pyrro derivatives,7 and so on. Although numerous coordination compounds or polymers have been reported using the strategy of carboxylate assembly, very few compounds have been reported and studied with TTF-carboxylate ligands.8,9 A hexaaquometal TTF salt,8a two dinuclear Rh complexes,8b and a Na supramolecular compound have been characterized crystallographically.9a We have been interested in the synthesis, structures, and solution chemistry of the metal complexes with TTF-derivatives, especially the TTF-COO ligand.9,10 As a consecutive work, we report herein a new series of coordination compounds of the TTF-bicarboxylate ligand with calcium and magnesium. Characteristics of the molecular arrangement, the cooperation of coordination, and acid-base equilibrium and redox behaviors have been studied to understand the effect of the TTF moiety. Experimental Section General Remarks. The ligand, 2,3-bis(carboxyl)-6,7-bimethylthiotetrathiafulvalene sodium salt, DMT-TTF-(CO2Na)2, were prepared according to the literature methods.9a,11 All the other reagents for syntheses and analyses are of analytical grade. The coordination reactions are not air sensitive, and therefore all the preparation were carried out under ambient condition. The IR spectra were recorded as KBr pellets on a Nicolet Magna 550 FT-IR spectrometer. Elemental analyses of C, H, and N were performed using an EA1110 elemental analyzer. Cyclic voltammetry (CV) experiments were performed on a CHI600 electrochemistry workstation in a three-electrode system, a Pt-plate working electrode, a Pt wire auxiliary electrode, and a saturated calomel electrode (SCE) as reference. Synthesis of Compounds. [CaL(DMF)2] (1). Ca(NO3)2 3 4H2O (11.8 mg, 0.05 mmol) was added to a solution of Na2L 3 3H2O (14.6 mg, 0.03 mmol) in DMF (3 mL) and H2O (3 mL) mixed solvent (1:1, v/v). After 0.5 h of stirring at room temperature, a precipitate appeared. Red crystals were obtained by self-evaporation of the filtrate under ambient conditions for two days (8.2 mg, yield 48% based on L). C16H20N2O6S6Ca (568.78): calcd: C 33.79, H r 2010 American Chemical Society
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Scheme 1. Structure of the Ligand L2-
Table 1. Crystallographic and Refinement Data for 1-3 formula formula weight crystal size (mm) crystal system space group a (A˚) b (A˚) c (A˚) a (°) β (°) r (°) V (A˚3) Z F(000) no. of unique no. of [I > 2.0σ(I)] goodness-of-fit (S) R1 [I > 2σ(I)] WR2 [I > 2σ(I)]
1
2
3
C16H20CaN2O6S6 568.78 0.65 0.20 0.11 triclinic P1 7.535(3) 9.586(4) 17.444(7) 99.266(8) 96.911(7) 99.438(3) 1212.7(8) 2 588 4405 3358 0.957 0.0356 0.0609
C23H26Ca2O12S12 959.32 0.40 0.33 0.06 triclinic P1 8.563(1) 9.016(1) 24.381(3) 95.23(1) 94.51(1) 103.15(2) 1815.6(4) 2 984 6611 4949 1.133 0.0685 0.1268
C22H40Mg2O20S12 1057.88 0.54 0.20 0.20 orthorhombic Pna21 13.026 (2) 35.454(4) 9.287(1) 90.00 90.00 90.00 4288.8(9) 4 2192 9478 8621 1.069 0.0646 0.1658
3.54, N 4.93%; found: C 34.13, H 4.65, N 4.70%. IR data (KBr pellet, cm-1): 1650vs, 1599vs, 1406s, 762w. [Ca2L2(H2O)2(EtOH)] 3 MeOH (2). Ca(NO3)2 3 4H2O (9.4 mg, 0.04 mmol) was added to a solution of Na2L 3 3H2O (19.3 mg, 0.04 mmol) in MeOH (2 mL), EtOH (1 mL), and H2O (3 mL) mixed solvent (1:1,v/v). Red crystals were obtained by a diffusing method in a glass tube (9.5 mg, yield 50% based on L). C23H26O12S12Ca2 (959.33): calcd: C 28.80, H 2.73%; found: C 28.98, H 3.20%. IR data (KBr pellet, cm-1): 1575vs, 1387vs, 1098vs, 1050w, 970w. [Mg2L2(H2O)7] 3 EtOH 3 4H2O (3). Mg(NO3)2 3 6H2O (51.2 mg, 0.2 mmol) in 2 mL of H2O was added slowly to a stirring aqueous solution of Na2L 3 3H2O (96.4 mg, 0.2 mmol, 4 mL H2O) and a precipitate appeared. DMF was dropwise added to the solution until the precipitate just dissolved. Then a red solution was obtained by filtration. Red crystals were obtained by adding a small quantity of EtOH to the filtrate and keeping it under ambient condition for three days (63.5 mg, yield 60% based on L). C22H40O20S12Mg2 (1057.88): calcd: C 24.98, H 3.81%; found: C 24.61, H 4.27%. IR data (KBr pellet, cm-1): 1608vs, 1371vs, 1168 m, 1094 m. [Na2(DMF)2(H2O)9][MgL2(DMF)2] (4). The crystals of 4 were prepared by a similar method used in the synthesis of 1, except that Ca(NO3)2 3 4H2O was replaced by Mg(NO3)2 3 6H2O. Red crystals were obtained by self-evaporation of the filtrate under ambient condition for three days (15.5 mg, yield 80% based on L). C32H58N4O21S12MgNa2 (1289.83): calcd: C 29.80, H 4.53, N 4.34%; found: C 30.45, H 4.66, N 4.05%. IR data (KBr pellet, cm-1): 1679vs, 1614vs, 1377s, 769w. X-ray Crystallographic Study. All measurements were carried out on a Rigaku Mercury CCD diffractometer at low temperature with graphite monochromated MoKR (λ = 0.71073 A˚) radiation. X-ray crystallographic data for compounds 1-4 were collected and processed using CrystalClear (Rigaku).12 The structures were solved by direct methods using SHELXS-97,13 and the refinements against all reflections of the compounds were performed using SHELXL-97.14 All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms were positioned with idealized geometry and refined with fixed isotropic displacement parameters, while the H atoms of H2O in 2 were located from the map. The H atoms of solvent in 3 and 4 were not dealt with for their disorder. Relevant crystal data, collection parameters, and refinement results of 1-3 can be found in Table 1. The data of 4 are not so good because of the serious disorder of the cation, but the fundamental
Figure 1. The molecular structure of [CaL(DMF)2] (1) and the inter-ions (Ca) linkage with selected labeling schemes, hydrogen atoms are omitted for clarity, (a) single oxygen (carboxyl) bridge, (b) carboxylic bridge. coordination mode and the structure of the anion are not in doubt. Considering the integrality of the coordination modes of the TTF carboxylate system, only the fundamental character of the structure is discussed. The crystallographic data of 4 are listed in Supporting Information. pH Titration. The pH titration experiments were carried out on a Mettler model FE20 potentiostat coupled to a LE438 combined electrode at constant temperature of 25 °C. The concentration of the ligand was 1.0 10-3 mol 3 dm-3 with 0.10 mol 3 dm-3 KNO3 in a DMF-water (1:1 in v/v) mixed solvent. In a typical experiment, the solution was treated with HNO3 in a molar quantity corresponding to 4 times the moles of the ligand. Titration was run by addition of 5-10 μL portions of standard 0.36 mol 3 dm-3 NaOH. Complexation equilibria were studied by means of titrations run under the same conditions, except that the starting solution also contained Ca(NO3)2 in a 1:1 molar ratio with respect to the ligand.
Results and Discussion Description of Structures of Metal Complexes 1-4. Calcium Compounds. Compound 1 crystallizes as a coordination polymer with formula [CaL(DMF)2]. The molecular structure and the inter-ions (Ca) linkage are shown in Figure 1. The coordination sphere of the metal is filled with seven oxygen atoms, in which five from the carboxylic oxygen of L and two
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Figure 4. Polymeric structure of 2: (a) The Ca(2) atoms are bridged by carboxylic groups along a axis forming a 1-D band structure. (b) The 2-D sandwich structure, in which an ionic Ca-O coordination layer is sandwiched by two sulfur-rich TTF layers.
Figure 2. The 1-D polymeric structure of 1 and the interchain’s short S 3 3 3 S contacts, (i) S(5) 3 3 3 S(50 ) 3.488 A˚ and (ii) S(5) 3 3 3 S(60 ) 3.530 A˚.
Figure 3. The fundamental structure of [Ca2L2(H2O)2(EtOH)] 3 MeOH (2) showing a four center’s cluster coordinated by oxygen atoms from TTF-carboxyl groups (L), water (w), and EtOH (sol) molecules.
from DMF molecules, resulting in a pentagonal bipyramid for the calcium ion. The Ca-O distances in axial coordination of O(2)L and O(6)DMF are shorter than those of equatorial bonds. There are two bridging modes for the Ca ion with the neighboring ions, single oxygen (carboxyl) bridge, and carboxylic bridge (Figure 1a,b). Linked by these bridges, the metal centers are connected to a one-dimensional (1-D) structure that is illustrated in Figure 2. The TTF moieties are distributed on two sides of the central Ca-O column and interact with those in neighboring column by short S 3 3 3 S contacts (3.488 and 3.530 A˚), forming a 2-D contacted framework. Compound 2 is also a calcium polymer with formula [Ca2L2(H2O)2(EtOH)] 3 MeOH. The fundamental structure of the inter-ions (Ca) linkage is shown in Figure 3, which is a four center’s cluster coordinated by oxygen atoms from carboxyl groups of four L and solvents, two water and one EtOH molecules. There are two symmetrically independent calcium centers, both of which take a seven-coordination sphere filled with oxygen atoms from these ligands. The Ca(2)O7 takes a pentagonal bipyramid with shorter axial coordination of
O(3)L and O(7)L, while the Ca(1)O7 is a distorted polyhedron. The Ca(2) centers are bridged by carboxylic groups along the a axis, forming a 1-D structure similar to compound 1 (Figure 4a). However, different from 1, these 1-D structures are further connected through carboxyl bridged Ca(1) atoms to form a 2-D polymeric sandwich structure in which an ionic Ca-O coordination layer is sandwiched by two sulfur-rich TTF layers (Figure 4b). Magnesium Compounds. Compound 3 has a 1D hydrated chain structure with formula [Mg2L2(H2O)7] 3 EtOH 3 4H2O. The asymmetric unit and the chain connection are shown in Figure 5. There are two symmetrically independent magnesium centers, showing octahedral geometry. The Mg(1) is coordinated by two ligands L, one in chelate mode and the other in bridge mode, and three water oxygens, O(9)-O(11). The Mg(2) is mainly a hydrated ion connected by O(5) and O(4) from two bridged carboxylic groups of two L ligands. The TTF moieties are arranged at the same side of the chain as a rope hung with things. Figure 6 shows the strong Ow-H 3 3 3 OL hydrogen bond interactions among the ropes to form a sandwich structure similar to compound 2. The formula of compound 4 is denoted as [Na2(DMF)2(H2O)9]-[MgL2(DMF)2]. The complex anion consists of a centrosymmetric center, where the magnesium ion is coordinated by two bideprotonated ligands and two DMF molecules and achieves an octahedral geometry (Figure 7). The cation is a dinuclear solvated ion of sodium with serious disorder (Supporting Information, Figure 1a). Coordination Modes of the TTF-bicarboxylate. Diverse coordination modes have been reported for the bicarboxylate system, especially the para-bicarboxylates, for example, the benzene-1,4-bicarboxylacetate ligand.1c Ligand L is an ortho-bicarboxylate ligand, which functions in diverse coordination modes in crystal structures of compounds 1-4. The results are summarized in Scheme 2, six coordination types for the ligand and six coordination modes for the caboxylate group. It is noticeable that all the calcium ions in 1 and 2 take a seven-coordinated sphere with pentagonal bipyramid geometry, while all the magnesium ions in 3 and 4 are six coordinated with an elongated octahedral geometry. The effect of ion radii on the coordination sphere is clearly exhibited. For this reason, the coordination modes of the ligand in calcium compounds are more complicated than those in magnesium compounds. Solvents with strong polarity such as DMF and water molecules are important cooperative ligands to compensate the coordination sphere (Figures 1 and 5) and stabilize the polymeric structure via hydrogen bonds (Figure 6a).
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Figure 5. The 1-D polymeric structure of [Mg2L2(H2O)7] 3 EtOH 3 4H2O (3) with selected labeling schemes; hydrogen atoms are omitted for clarity.
Figure 7. The anion structure of [Na2(DMF)2(H2O)9][MgL2(DMF)2] (4) with selected labeling schemes; hydrogen atoms and half part of the disordered DMF molecules are omitted for clarity. The molecular packing is shown in Supporting Information, Figure 1b.
Figure 6. (a) The strong Ow-H 3 3 3 OL hydrogen bond interactions among the sandwich structure of 3. (b) The sandwich structure in which an ionic Mg-O coordination layer is sandwiched by two sulfur-rich TTF layers in the structure of compound 3.
As the consequence, structures of these alkaline-earth metalcomplexes with TTF-bicarboxylates are characterized by an
extensive network of hydrophilic metal carboxylate layer sandwiched with hydrophobic layers of sulfur-rich TTF matrix. Description of Solution Chemistry of Metal Complexes 1-4. pH Titration Studies of the Metal Coordination. Equilibriums of proton transition and metal coordination for transition metal complexes have been studied for decades.15 This function of the present ligand has also been examined by means of pH titration in DMF-water (1:1 in v/v) mixed solvent. A solution containing H2L and HNO3 in a molar ratio of 1:4 was titrated by NaOH, and the result is shown in Figure 8 (titration curve 1). The curve 2 shows the same titration system but added 1 equiv of Ca(NO3)2. The first inflexion of the curves are all at about a = 5 (a denotes the mole equivalents of NaOH added vs that of the ligand), showing the existence of five equivalents free acid in the solutions, four from the added HNO3 and one from the bicarboxyl group. The data indicates that the ortho-bicarboxyl acid is a relatively strong acid that releases the first proton easily to form a stable anion of HL-. The structure of HL- has been reported previously by our group, in which there is a seven-membered ring stablized by an intramolecular hydrogen bond.9a The second step in the titration curve 1 (a ≈ 6) indicates the neutralization of the second proton of the ligand (equivalent point at about pH = 8.0). As a contrast, the second step of curve 2 shows a pH drop near pH = 6.0, indicating the formation of complex species, and that forces the HL- to release the proton at lower pH. The pH of equivalent point changes from 8.0 to 6.0 relates a phenomenon of metal ion-switched proton transfer. An
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Scheme 2. Coordination Modes of the Bicarboxylate Group Found in Compounds 1-4a
a (a) Bis-monodentate, bis-bridging, o-chelating (1); (b) monodentate, bridging, o-chelating (1); (c) chelating, bridging, o-chelating, monodentate (2); (d) monodentate, o-chelating (3); (e) bis-monodentate (3); (f) o-chelating (4).
Table 2. Data of the CV Measurements Na2L i ii iii
CaL
ER
EO
E1/2
ER
EO
E1/2
ΔE1/2
0.236 0.348 0.522
0.304 0.422 0.601
0.270 0.385 0.561
0.298 0.376 0.528
0.352 0.451 0.618
0.325 0.414 0.573
0.055 0.029 0.012
Scheme 3. Proposed Mechanisms of Coordination, Acid-Base and Redox Equilibriums of Ca(II)-TTF-COOH in DMF-Water Solution
Figure 8. The pH titration curves in DMF-water (1:1 in v/v) mixed solvent: (curve 1) a solution containing H2L (1.0 10-3 mol 3 dm-3) and HNO3 in a molar ratio of 1:4; (curve 2) the same titration system but added 1 equiv of Ca(NO3)2.
abrupt change of the curve was observed, that is attributed to the precipitation of the complex for its lower solubility (Figure 8 and Supporting Information, Figure 2). Cyclic Voltammetry. Redox systems of TTF-derivatives have received extensive attention for applications in ion sensors. Electrochemistry of calcium complex 2 was measured in the mixed solvent of DMF-water (in 1:1 volumes). The CV data of the sodium salt and the calcium complex in the range of 0.0-1.0 V vs SCE are listed in Table 2 (Supporting Information, Figure 3). Three couples of redox peaks are detected for both of them, corresponding to redox couples TTF/TTF•þ, E1/2(1), and E1/20 (1), and TTF•þ/ TTF2þ, E1/2(2). The peak of (ii) (E1/20 (1)) arises from the partial hydrolysis of the salt, a monoprotonated anion of the ligand in the presence of water.9a The ΔE1/2(1) of the alkaline earth metal calcium compound shifts about 55 mV to the positive by comparison with that of the alkaline metal (Na) salt, indicating the stronger coordination and electron withdraw properties of the alkaline earth ion (þ2). Therefore, the TTF-carboxylate is an appropriate ligand for Ca(II) ion. The ΔE1/2(2) of them are not significantly affected by the metal ions due to the dissociation of the Ca(II) ion when the TTF
unit is oxidized to more positive charge. That is, when the TTF moiety is oxidized, it switches the Ca ion to the solution. The proposed equilibriums are depicted in Scheme 3. Conclusion In summary, coordination compounds of calcium and magnesium with TTF-bicarboxylate ligand have been first synthesized. The ligand coordinates to ions of alkaline earth metals in multidentate modes forming compounds 1-4. The effect of ion radii on the coordination structure has been clearly observed that all the calcium ions in 1 and 2 take a seven-coordinated sphere with a pentagonal bipyramid geometry, while all the magnesium ions in 3 and 4 are sixcoordinated with an elongated octahedral geometry. The
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character of these alkaline earth metal complexes is novel in their assembled packing showing sandwiched hydrophobic layers of a sulfur-rich TTF matrix and hydrophilic layer of the ionic coordination layer. The formation of the ionic complexes can be controlled by adjusting the pH of solution based on the equilibrium of metal coordination and acid dissociation. At the neutral state of the TTF moiety, TTF-carboxylate is an appropriate ligand for the Ca(II) ion, but when the TTF moiety is oxidized, it switches the alkaline earth metal ion to the solution. This system with Ca(II) ion coordination and release might open the path to a redox-driven molecular machine capable of acting as a sponge of calcium ions. Acknowledgment. This work is supported by the NNS Foundation (20971092), the Program of Innovative Research Team of Soochow University, and the Doctoral Foundation of Soochow University. Supporting Information Available: Crystallographic data of 1-4 in CIF format. These materials and figures of molecular packing, CV are available free of charge via the Internet at http://pubs.acs. org.
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