Structural Diversities of Cobalt(II) Coordination Polymers with Citric

empirical formula, C6H8O8Co (1), C12H38O28Co3 (2), C12H30O24Co3 (3) .... for [Co(H2cit)(H2O)]n (1), [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n·6nH2O (2), ...
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CRYSTAL GROWTH & DESIGN

Structural Diversities of Cobalt(II) Coordination Polymers with Citric Acid

2005 VOL. 5, NO. 3 1109-1117

Zhao-Hui Zhou,* Yuan-Fu Deng, and Hui-Lin Wan State Key Laboratory for Physical Chemistry of Solid Surfaces and Department of Chemistry, College of Chemistry and Chemical Engineering, Xiamen University, Xiamen, 361005, People’s Republic of China Received November 1, 2004

ABSTRACT: The reactions of cobalt(II) ion with citric acid (C6H8O7 ) H4cit) have been studied in an acidic aqueous solution of pH 1-4, which result in the isolations of four new polymeric cobalt(II) citrate complexes: [Co(H2cit)(H2O)]n (1), [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2), [Co(H2O)6]n[Co(Hcit)(H2O)]2n‚2nH2O (3), and (NH4)2n[Co2(Hcit)2(H2O)2]n (4). The complexes have been characterized by spectroscopic and single crystal X-ray diffraction studies. The Co(II) ion in each complex exists in an octahedral coordination environment. The citrate ligand binds the Co(II) ion tridentately via its R-hydroxyl, R-carboxylate, and one of the β-carboxylate groups as a basic feature. The polymeric structures are constructed by the further coordination of R-carboxylate or β-carboxylate groups. Complex 1 forms a chiral helical chain running along the a-axis of the crystal via the two bridged bonded oxygen atoms of the R-carboxylate group, leaving the β-carboxylic acid group free and forming strong hydrogen bond. The dimeric cobalt(II) citrate unit [Co2(Hcit)2(H2O)4]2- in the complex 2 forms a one-dimensional polymeric chain through the coupling of long-arm β-carboxylate groups with the planar [Co(H2O)4] and [Co2(Hcit)2(H2O)4] units. The [Co(Hcit)(H2O)]unit in complex 3 forms an infinite chain along a 21 axis. Complex 4 forms a layered complex through the links of the new dimeric unit [Co2(Hcit)2(H2O)2] by the oxygen atoms of the β-carboxylate groups. Interconversions between 1, 3, and 4 are found to be pH- and counterion-dependent. Heating of coordination polymer 2 results in the irreversible formation of 3. The structural diversities of these cobalt(II) citrate polymers demonstrate that pH, counterions, and reaction temperature play essential roles in the formations of such one-dimensional and two-dimensional frameworks. Introduction Crystal engineering of coordination polymers has attracted great attention in recent years due to their potential as functional materials as well as their interesting compositions and topologies.1-3 Although the general and precise principles for controlling the solid structures of the target products still need to be established, many rational synthetic strategies have been brought forward and proved significant in the approach of design of the metal-based coordination polymers. The factors such as metal ions with different coordination geometry4 or radius,5 counteranions with different bulk6 or coordination ability,7 solvent,8 metal/ ligand ratio,9 and pH value10 have been found to highly influence the structural topologies of such coordination frameworks. Metal-carboxylato complexes have been of special interest because they not only form open framework structures resulting from the presence of the carboxylate function itself11 but also act as the precursors for the preparations of high chemical homogeneity and high surface area multicomponent oxides materials (perovskites, spinels, ferrites, polytitanates, et al.).12,13 Citric acid, a R-hydroxyl tricarboxylic acid, has been widely known for its abundance in physiological fluids and its chemical versatility toward transition metal ions. Although citric acid has been little employed in the synthesis of coordination polymers, it has been found that citrate displays a rich solution chemistry with Co(II) ions and different coordination modes in the isolation of cobalt(II) citrate complexes, such as M4[Co* To whom correspondence should be addressed. Tel: +86-5922184531. Fax: +86-592-2183047. E-mail: [email protected].

(Hcit)2]‚nH2O [M ) NH4, n ) 0 or M ) C(NH2)3, n ) 2],14 M2[Co2(Hcit)2(H2O)4]‚6H2O (M ) Na or K),15 and (NMe4)3Na{Co4(cit)4[Co(H2O)5]2}‚11H2O.16 To better understand the influence of pH, counterions, and temperature on the formation of cobalt citrate complexes in acidic solutions, herein, we report the isolations of four new polymeric cobalt(II) citrate complexes with the formulas of [Co(H2cit)(H2O)]n (1), [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2), [Co(H2O)6]n[Co(Hcit)(H2O)]2n‚2nH2O (3), and (NH4)2n[Co2(Hcit)2(H2O)2]n (4), respectively, with some minor different reaction conditions. The isolated cobalt(II) citrate complexes illustrate the coordination diversities of citrate as a ligand toward cobalt(II) ions and display different structural features. Experimental Section All of the manipulations were carried out in the open air. All of the chemicals were analytical reagents and were used without further purification. Nanopure quality water was used throughout this work. Infrared spectra were recorded as Nujol mulls between KBr plates using a Nicolet 360 Fourier transform infrared (FT-IR) spectrometer. Electronic spectra were obtained with a powder diffuse reflectance adsorption on a Cary 5000 UV-visible-NIR spectrophotometer using barium sulfate as a standard. Elemental analyses were performed using an EA 1110 elemental analyzer. Circular dichroism (CD) spectra were measured on a JASCO 810 spectrometer. Preparation of [Co(H2cit)(H2O)]n (1). CoCl2‚6H2O (2.14 g, 9 mmol) and citric acid monohydrate (1.89 g, 9 mmol) were dissolved in 5 mL of water. The pH of the reaction mixture was carefully adjusted to 2.0 by adding sodium hydroxide (4 M) and heated at 70° C for 10 h, and then, pink crystals precipitated. The products were collected, washed with water three times, and air-dried to give 1 (1.44 g, 60%). C and H elemental analyses for C6H8O8Co follow. Found (calcd): C, 27.1

10.1021/cg0496282 CCC: $30.25 © 2005 American Chemical Society Published on Web 02/12/2005

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Table 1. Crystal Data Summaries of Intensity Data Collections and Structure Refinements for [Co(H2cit)(H2O)]n (1), [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2), [Co(H2O)6]n[Co(Hcit)(H2O)]2n‚2nH2O (3), and (NH4)2n[Co2(Hcit)2(H2O)2]n (4) empirical formula formula weight crystal color crystal system a (Å) b (Å) c (Å) β (°) V (Å3) space group formula units/unit cell Dcalcd (g cm-3) F000 diffractometer/radiation reflections collected/unique/Rint data/restraints/parameters θ range (°) goodness-of-fit on F2 R1, wR2 [I > 2σ(I)] R1, wR2 (all data) largest diff. peak and hole (e Å-3) a

C6H8O8Co (1) 267.05

C12H38O28Co3 (2) 807.21

orthorhombic

monoclinic

C12H30O24Co3 (3) 735.15 red monoclinic

cell constants 11.204(1) 9.0540(7) 12.000(1) 6.6468(5) 12.014(1) 20.277(2) 116.253(2) 96.871(1) 827.1(1) 1437.7(4) 1211.5(2) P212121 P21/n P21/n 4 2 2 2.145 1.864 2.015 540 830 750 Smart Apex CCD/Mo KR (λ ) 0.7107 Å) 7073/1928/0.0589 8572/3322/0.0639 11171/2878/0.0879 1928/5/148 3322/20/241 2878/16/211 2.48-28.28 2.55-28.24 2.02-28.36 0.992 0.683 1.024 0.038, 0.071 0.040, 0.052 0.058, 0.119 0.042, 0.72 0.057, 0.080 0.089,0.134 0.591, -0.470 0.564, -0.491 0.666, -0.641 5.9131(4) 10.4071(9) 13.4411(7)

C12H22O16N2Co2 (4) 568.18 monoclinic 11.8034(9) 8.3749(6) 9.5543(7) 96.213(1) 938.9(1) P21/c 2 2.010 580 10274/2251/0.0940 2251/14/166 1.74-28.32 1.044 0.055, 0.122 0.077, 0.130 0.762, -0.511

R1 ) ∑||Fo| - |Fc||/∑(|Fo|), wR2 )∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2]1/2.

(27.0); H, 2.8 (3.0). UV-vis (BaSO4) λ max/nm: 305 (relative absorbance 0.018), 532 (0.017). Both are attributed to 4T1g(F) f 4T1g(P) split, while 680 (0.009) is attributed to 4T1g(F) f 4 A2g(F) split. IR (KBr, cm-1): 3484s, 3396s, 1734s, 1614vs, 1547vs, 1475w, 1419vs, 1394vs, 1327s, 1284s, 1260s, 1153m, 1074s, 890m, 582s, 545m. CD (KBr) λ max/nm: 230.8, 471.8, and 546. Preparation of [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2). Solid CoCl2‚6H2O (2.14 g, 9 mmol) was added slowly to an aqueous solution containing citric acid monohydrate (1.89 g, 9 mmol) in an ice bath. The pH of the reaction mixture was carefully adjusted to 4.0 by adding sodium hydroxide solution (4 M). The solution was stirred at 0 °C for 10 min and filtered. The filtrate was kept in the refrigerator for approximately 1 week to produce red crystals of 2 (1.21 g, 50%). C and H elemental analyses for C12H38O28Co3 follow. Found (calcd): C, 17.4 (17.8); H, 4.9 (4.7). UV-vis (BaSO4) λ max/nm: 520 (relative absorbance 0.041) is attributed to 4T1g(F) f 4T1g(P) split. IR (KBr, cm-1): 3455s, 3222b, 1618vs, 1578s, 1419s, 1394m, 1283m, 1064w, 825m, 758w, 615w, 569w. Preparation of [Co(H2O)6]n[Co(Hcit)(H2O)]2n‚2nH2O (3). CoCl2‚6H2O (2.14 g, 9 mmol) and citric acid monohydrate (1.89 g, 9 mmol) were dissolved in 5 mL of water. The pH of the reaction mixture was carefully adjusted to 4.0 by adding sodium hydroxide (4 M) and heated at 70° C for 10 h, and then, red crystals precipitated. The products were collected, washed with water three times, and air-dried to give 3 (0.88 g, 40%). C and H elemental analyses for C12H30O24Co3 follow. Found (calcd): C, (19.6); H, (4.1). UV-vis (BaSO4) λ max/nm: 522 (relative absorbance 0.038), is attributed to 4T1g(F) f 4T1g(P) split. IR (KBr, cm-1): 3427s, 3221b,, 1625vs, 1579s, 1421s, 1394m, 1355m, 1281m, 1061w, 841m, 687m, 622m, 569w. Preparation of (NH4)2n[Co2(Hcit)2(H2O)2]n (4). CoCl2‚ 6H2O (2.14 g, 9 mmol) and citric acid monohydrate (1.89 g, 9 mmol) were dissolved in 5 mL of water. The pH of the reaction mixture was carefully raised to 4.0 with ammonia (1:1) and was heated at 70 °C for 10 h. Subsequently, ethanol was added and the flask containing the solutions was placed at room temperature. Three days later, red crystalline material came out of the solution. The products were collected, washed with water, and air-dried to give 4 (1.79 g, 70%). C, H, and N elemental analyses for C12H22O16N2Co2 follow. Found (calcd): C, 25.5 (25.3); H, 4.2 (3.9); N, 4.5 (4.9). UV-vis (BaSO4) λ max/ nm: 520 (relative absorbance 0.013) is attributed to 4T1g(F) f 4T1g(P) split. IR (KBr, cm-1): 3425m, 3189s, 1636vs, 1605vs, 1435s, 1415s, 1386s 1299m, 1283m, 1252m, 1064m, 846w, 819w, 715w, 630w, 557w.

Interconversions Compounds 1 and 3. [Co(H2cit)(H2O)]n (1) (2.67 g 10 mmol) was suspended in 5 mL of water with continuous stirring. The pH was maintained at 4.0 with sodium hydroxide (4 M) until 1 was completely dissolved. The resulting solution was stirred for 10 h at 70 °C. Subsequently, ethanol was added and the flask containing the solutions was placed at room temperature. A week later, crystalline material came out of the solution, which was isolated by filtration and washed with water. Positive identification of the crystalline material as complex 3 came from the IR spectrum. The yield was 40% (0.98 g). Under similar reaction conditions, 3 (3.68 g, 5 mmol) was found to convert into 1 by adjusting the pH of the solution to 2.0 with dilute hydrochloric acid. The IR spectrum of the isolated crystals was identical to that of 1. The yield was 50% (1.34 g). Compounds 1 and 4. The same procedure above was followed for the transformation of 1 to 4, except that ammonia maintained the pH to 4.0. Positive identification of the crystalline material as complex 4 came from the IR spectrum. The yield of the conversion reaction was 70% (2.00 g). Under similar reaction conditions, 4 (2.84 g 10 mmol) was found to convert into compound 1 by adjusting the pH of the solution to 2.0 with dilute hydrochloric acid. The IR spectrum of the isolated crystals was identical to that of 1. The yield was 62% (1.66 g). Compounds 2 and 3. The crystals of [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2) (1.61 g, 2 mmol) were picked out and put in a beaker with the mother liquid at 70 °C. Two days later, the products that separated were showed to complex 3. Positive identification of the crystalline material as complex 3 came from the IR spectrum. The yield was 90% (1.32 g). X-ray Structural Determination. Crystals of 1-4 were measured on a Bruker Smart Apex CCD diffractometer with graphite monochromate Mo KR radiation (λ ) 0.71073 Å) at 296 K. The data were corrected for

Cobalt(II) Coordination Polymers with Citric Acid

Crystal Growth & Design, Vol. 5, No. 3, 2005 1111

Table 2. Selected Bond Distances (Å) and Selected Bond Angles (°) for [Co(H2cit)(H2O)]n (1), [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2), [Co(H2O)6]n[Co(Hcit)(H2O)]2n‚2nH2O (3), and (NH4)2n[Co2(Hcit)2(H2O)2]n (4) 1a Co(1)-O(1) Co(1)-O(4) Co(1)-O(3ii) O(1)-Co(1)-O(2) O(1)-Co(1)-O(1w) O(1)-Co(1)-O(3ii) O(2)-Co(1)-O(1w) O(4)-Co(1)-O(2i) O(4)-Co(1)-O(1w) O(1w)-Co(1)-O(3ii)

2.034(2) 2.099(2) 2.093(3) 75.0(1) 90.9(1) 90.4(1) 93.9(1) 94.89(9) 175.5(1) 91.1(1)

Co(1)-O(1) Co(1)-O(4) Co(1)-O(1w) Co(2)-O(7) Co(2)-O(4w) O(1)-Co(1)-O(2) O(1)-Co(1)-O(4) O(1)-Co(1)-O(6i) O(1)-Co(1)-O(1w) O(1)-Co(1)-O(2w) O(2)-Co(1)-O(4) O(2)-Co(1)-O(6ii) O(2)-Co(1)-O(1w) O(2)-Co(1)-O(2w) O(4)-Co(1)-O(6i) O(4)-Co(1)-O(1w) O(4)-Co(1)-O(2w)

2.155(2) 2.061(2) 2.064(2) 2.080(2) 2.048(2) 77.52(8) 84.50(8) 90.99(9) 99.88(9) 170.63(9) 88. 96(9) 86.44(9) 177.39(9) 94.37(9) 174.16(9) 91.02(9) 90.73(8)

Co(1)-O(1) Co(1)-O(4) Co(1)-O(7ii) Co(2)-O(2w) Co(2)-O(4w) O(1)-Co(1)-O(2) O(1)-Co(1)-O(4) O(1)-Co(1)-O(6i) O(1)-Co(1)-O(7ii) O(1)-Co(1)-O(1w) O(2)-Co(1)-O(4) O(2)-Co(1)-O(6i) O(2)-Co(1)-O(7ii) O(2)-Co(1)-O(1w) O(4)-Co(1)-O(6i) O(4)-Co(1)-O(7ii) O(4)-Co(1)-O(1w)

2.116(3) 2.074(3) 2.051(3) 2.100(3) 2.059(3) 76.9(1) 83.6(1) 102.2(1) 96.4(1) 168.2(1) 89.8(1) 88.4(1) 167.1(1) 95.9(1) 173.3(1) 100.5(1) 87.1(1)

Co(1)-O(1) Co(1)-O(4) Co(1)-O(1w) O(1)-Co(1)-O(2) O(1)-Co(1)-O(4) O(1)-Co(1)-O(5i) O(1)-Co(1)-O(6ii) O(1)-Co(1)-O(1w) O(2)-Co(1)-O(4) O(2)-Co(1)-O(5i) O(2)-Co(1)-O(6ii)

2.089(3) 2.101(3) 2.068(3) 75.7(1) 85.4(1) 105.6(1) 87.8(1) 164.5(1) 86.4(1) 172.8(1) 94.2(1)

Co(1)-O(2) Co(1)-O(2i) Co(1)-O(1w) O(1)-Co(1)-O(4) O(1)-Co(1)-O(2i) O(2)-Co(1)-O(4) O(2)-Co(1)-O(2i) O(4)-Co(1)-O(3ii) O(1W)-Co(1)-O(2i) O(2i)-Co(1)-O(3ii)

2.211(2) 2.070(2) 2.077(3) 87.6(1) 177.4(1) 81.70(9) 105.68(6) 93.1(1) 85.6(1) 89.1(1)

2b Co(1)-O(2) Co(1)-O(6i) Co(1)-O(2w) Co(2)-O(3w)

2.069(2) 2.089(2) 2.088(2) 2.105(2)

O(6i)-Co(1)-O(1w) O(6i)-Co(1)-O(2w) O(1w)-Co(1)-O(2w) O(7)-Co(2)-O(7ii) O(7)-Co(2)-O(3w) O(7)-Co(2)-O(4w) O(7ii)-Co(2)-O(3w) O(3w)-Co(2)-O(4w) O(3w)-Co(2)-O(3wii) O(3w)-Co(2)-O(4wii) O(4w)-Co(2)-O(7ii) O(4w)-Co(2)-O(4wii)

93.41(9) 93.20(8) 88.2(1) 180.00(1) 96.6(1) 91.80(9) 83.4(1) 87.7(1) 180.0(1) 92.3(1) 88.20(9) 180.0(1)

3c Co(1)-O(2) Co(1)-O(6i) Co(1)-O(1w) Co(2)-O(3w)

2.079(3) 2.085(3) 2.063(3) 2.091(3)

O(6i)-Co(1)-O(7ii) O(6i)-Co(1)-O(1w) O(7ii)-Co(1)-O(1w) O(2w)-Co(2)-O(3w) O(2w)-Co(2)-O(4w) O(2w)-Co(2)-O(2wiii) O(2w)-Co(2)-O(3wiii) O(2w)-Co(2)-O(4wiii) O(3w)-Co(2)-O(4w) O(3w)-Co(2)-O(4wiii) O(3w)-Co(2)-O(3wiii) O(4w)-Co(2)-O(4wiii)

82.2(1) 86.7(1) 92.3(1) 89.9(1) 87.8(1) 180.00(8) 90.1(1) 92.3(1) 91.4(1) 88.6(1) 180.0(2) 180.00(1)

Co(1)-O(2) Co(1)-O(5i) Co(1)-O(6ii) O(2)-Co(1)-O(1w) O(4)-Co(1)-O(5i) O(4)-Co(1)-O(6ii) O(4)-Co(1)-O(1w) O(5i)-Co(1)-O(6ii) O(1w)-Co(1)-O(5i) O(1w)-Co(1)-O(6ii)

2.073(3) 2.144(3) 2.074(3) 89.0(1) 87.0(1) 172.8(1) 91.7(1) 92.6(1) 89.4(1) 95.5(1)

4d

1 2 3 4

O1‚‚‚O5iii O1‚‚‚O6 O1‚‚‚O6 O1‚‚‚O5iv

2.600(3) 2.596(3) 2.763(4) 2.664(4)

Hydrogen Bondings 170(4) 154(3) 144(4) 171(5)

O6‚‚‚O5iv O4w‚‚‚O5ii O4w‚‚‚O5 O1w‚‚‚O3v

2.674(3) 2.710(3) 2.699(4) 2.691(3)

170(4) 180(4) 171(4) 172(5)

a Symmetry transformation for 1: (i) x + 1/2, y + 1/2, -z + 1; (ii) x + 1, y, z; (iii) -x + 1, y + 1/2, -z + 1/2; (iv) -x + 1/2, -y + 1, z + 1/2. b Symmetry transformation for 2: (i) x, -y, -z + 1; (ii) -x + 1/2, y - 1/2, 3/2 - z; (iv) -x + 1/2, y - 1/2 y - 1/2, -z + 1/2. c Symmetry transformation for 3: (i) -x - 1/2, -y - 1/2, -z + 1/2; (ii) x, y - 1, z; (iii) x + 1, -y + 1, -z + 1. d Symmetry transformation for 4: (i) -x, -y, -z; (ii) x, -y + 1/2, z + 1/2; (iii) -x, y - 1/2, -z - 1/2; (iv) -x, -y + 1/2, -z - 1/2; (v) x, -y - 1/2, z + 1/2.

Lorentz and polarization effects. An absorption correction was applied using the SADABS program.17 The structures were primarily solved by direct methods Shelxs-9718 and refined by full-matrix least-squares

procedures with anisotropiclly thermal parameters for all of the nonhydrogen atoms. All calculations were performed on a microcomputer using Shelxl-97.19 H atoms were located from a difference Fourier map and

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Scheme 1.

Zhou et al.

Syntheses and Interconversions of Cobalt Citrate Complexes

refined isotropically. Summaries of crystallographic data for 1-4 are given in Table 1. Selected bond distances and bond angles of the complexes are listed in Table 2. Results and Discussion Syntheses and Interconversions. Syntheses of complexes 1-4 were expediently carried out in aqueous solutions under slightly different conditions. In these specific reactions, the products do not depend on the ligand-to-metal ratio. That is, despite the fact that a 1:1 ligand/metal ratio was carried out in all of the reactions, complexes 1 and 4 have 1:1 ligand/metal compositions, while a 2:3 ligand/metal moiety was achieved for 2 and 3. Scheme 1 illustrates the sensitivity of the reactions toward pH value, temperature, and the base in the solutions. When pH < 3.0, only complex 1 was isolated. When pH ∼ 4.0, complexes 2 and 3 were isolated at 0 and 70 °C in the presence of sodium hydroxide, respectively. However, complex 4 was isolated at a pH ∼ 4 when ammonia was used. Use of various bases (NaOH or NH4OH) will not only supply the necessary counterions to balance the negative charges of the cobalt citrate complexes but also force the formations of one-dimensional (1D) chains or two-dimensional (2D) layered structures. Both of reactions 3 and 4 were conducted at the same pH, so the subtle difference in base proves to have a significant effect on the structure of the complex formed. As compared with the pH value and molar ratio, the sensitivity of the reactions of metal ions and citric acid toward temperature is not common. When the reaction of cobalt chloride and citric acid at 0 °C in the presence of sodium hydroxide proceeds, only complex 2 was separated, whereas at 70 °C, complex 3 was isolated solely. Furthermore, the irreversible conversion from 2 to 3 was observed when complex 2 was exposed to the temperature of 70 °C for 10 h. An effort was made to investigate the interconversions of the isolated complexes under different reaction conditions. In that case, 1 was suspended in water at 70 °C, and the pH was gradually adjusted to 4.0 using

Figure 1. (a) ORTEP plot of the unit in [Co(H2cit)(H2O)]n (1) with 30% thermal ellipsoids probability. (b) ORTEP plot of 1, depicting the helical chain propagating along the a-axis. Hydrogen atoms have been omitted for clarity.

NaOH and ammonia, and precipitation with alcohol led to the isolations of 3 and 4, respectively. Conversely, when the compounds 3 and 4 were suspended in water and the pH of the resulting solutions was decreased to 2.0 with the addition of dilute hydrochloric acid, both 3 and 4 are converted into 1. The four isolated complexes in the crystalline state are difficult to dissolve in water and common organic solvents, such as ethanol, acetone, et al., even after strong heating. The synthetic cobalt(II) citrate complexes exhibit various structural features among them.

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Figure 2. (a) ORTEP plot of the unit in [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2) with 30% thermal ellipsoids probability. The crystalline water molecules were omitted for clarity. (b) ORTEP plot of 2, depicting the 1D polymeric chain propagating along the c-axis. The crystalline water molecules and hydrogen atoms have been omitted for clarity.

The interconversions of the cobalt(II) citrate complexes provided significant insight into the linkage of cobalt(II) citrate complexes as viable patterns of the corresponding scheme in aqueous media, albeit the detailed processes of the interconversions are in further need of perusal. Description of the Crystal Structures. Singlecrystal X-ray structural analysis revealed that complex 1 crystallizes in a chiral space group P212121. On the basis of the analysis of the Flack parameter (-0.0061), it was found that the configuration of 1 is in ∆s. Each Co(II) ion in 1 exists in an octahedral environment, surrounded by citrate ligand and a water molecule (Figure 1a). Specifically, the six apexes of the oxygen coordination octahedron are occupied as follows: One citrate ligand employs both of the R-hydroxyl and the R-carboxylate oxygen atoms as well as one of its β-carboxylate oxygens to bind Co(II), thus occupying three coordination positions. Two additional citrate ligands from two adjacently located octahedral Co(II) ions are reaching out and coordinate to cobalt(II) through the oxygen atoms of their R-carboxylate groups, thus occupying two coordination sites. The remaining coordination site is occupied with a water molecule. The other uncoordinated β-carboxylic acid group of the citrate ligand is prontoned, and it participates in hydrogen bonding interaction [O6‚‚‚O5iv, 2.674(3) Å]. Moreover, the bonded oxygen atom O2 of the R-carboxylate group is further coordinated to the adjacent cobalt(II) ion, and the other oxygen atom O3 of this R-carboxylate group is also bonded strongly to another Co(II) ion. Adjacent molecules are linked by a screw axis to form a helical chain running along the a-axis of the

crystal (Figure 1b). Neighboring chains are consolidated into three-dimensional (3D) structures by hydrogen bonds. This coordination mode of the citrate ligand is also observed in [Cd(H2cit)(H2O)]n,20 which differs from that encountered in the other acidic citrate complexes, such as K2[VO2(H2cit)]2‚4H2O,21 Na2[MO2(H2cit)2]‚3H2O (M ) Mo, W),22 and M2[Mg(H2O)6][Ti(H2cit)3]2‚6H2O (M ) K, NH4),23 where the citrate ligands only bidenately coordinate to the metal ions via their deprontonated R-hydroxyl and R-carboxylate groups. As illustrated in Figure 2a,b, complex 2 comprises a trinuclear [Co(H2O)4][Co2(Hcit)2(H2O)4] repeating unit along the c-axis. The cobalt ions exist in two different types of environments. The Co1 occupies a general position, and Co2 lies on a crystallographic symmetric center. The triionized citrate ion serves as a tridenate ligand and coordinates with Co1 ion via its R-hydroxyl, R-carboxylate, and one of the β-carboxylate groups. While the other β-carboxylate group does not participate in the coordination to the same metal ion, instead, it acts as a bridged ligand to bind the other two cobalt(II) ions simultaneously. The center Co1 and Co2 ions are bridged via an oxygen atom O7 of this β-carboxylate group, resulting in a 5.080(2) Å Co1-Co2 separation. The oxygen atom O6 of this β-carboxylate forms a bridge between the trimers linking Co1 to Co1i, which is related to Co1 by 21 axis symmetry, resulting in a Co1Co1i distance of 5.348(2) Å. The coordination modes of the [Co2(Hcit)2(H2O)4]2- cyclic dimer in the trinuclear repeating unit are similar to those observed in the other dinuclear metal citrate complexes, such as M2[Ni2(Hcit)2(H2O)4]‚nH2O (M ) K, n ) 4; M ) NH4, n ) 2),24 M2[Co2(Hcit)2(H2O)4]‚6H2O (M ) K or Na).15 The six

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oxygen apexes around the Co1 ion consist of three oxygen atoms from R-hydroxyl (O1), R-carboxylate (O2), and the β-carboxylate groups (O4), respectively, two ciscoordination water molecules (O1w and O2w), as well as one of the β-carboxylate oxygen atom (O6i) from the other citrate ligand. The equatorial plane is defined with four oxygen atoms corresponding to the R-carboxylate oxygen atom (O2), the β-carboxylate oxygen atom (O4), the β-carboxylate oxygen atom (O6i) from the other citrate ligand, and one coordinated water molecule (O1w). None of the four atoms deviates more than 0.01 Å. The axial positions are occupied with the R-hydroxyl oxygen atom (O1) and one coordinated water molecule (O2w). The six apexes around Co2 ion are completed with two pairs of trans-coordination water molecules (O3w and O3wii; O4w and O4wii) and two oxygen atoms (O7 and O7i) from two different citrate ligands. The geometry mode of Co2 ion is in a normal octahedron. Complex 3 is isomorphous with the magnesium, manganese, and iron complexes of triionized citrate: [M(H2O)6]n[M(Hcit)(H2O)]2n‚2nH2O (M ) Mg, Mn, or Fe).25 The triionized citrate ion forms a tridentate chelate in which R-hydroxyl, R-carboxylate groups, and one of the β-carboxylate groups are coordinated to a single cobalt(II) ion, and the other β-carboxylate group is coordinated to the other cobalt(II) ions, which are related by a screw axis as shown in Figure 3a. The 1:1 complex of cobalt ion and triionized citrate forms an infinite chain of covalently linked units along a 21 axis of the unit cell, as presented in Figure 3b. The hexaquocobalt(II) counterions of this ionic structure that are located on centers of symmetry are linked by extensive hydrogen bonding to the covalent columns of cobalt citrate. Complex 4 consists of discrete ammonium cations and polymeric [Co2(Hcit)2(H2O)2] 2- anions. As shown in Figure 4a, the anion is a new dinuclear structure with two Co(II) ions linked by coordinated β-carboxylate oxygens in citrate ligand. Each citrate ligand is triply deprotonated, and as such, it coordinates to the Co(II) ion, thus giving rise to two distorted octahedral units within the dimer. In each octahedron, the citrate ligand binds in a tridentate fashion. It employs the R-hydroxyl and the R-carboxylate groups, as well as one of the β-carboxylate groups, to coordinate to the Co(II) ion. Furthermore, the other oxygen atom (O5) of the coordinated β-carboxylate group links Co1 and Co1i ions into a dimer, which results in a Co1-Co1i separation of 4.955(2) Å. The remaining β-carboxylate group does not participate in the coordination to the same Co(II) ion. Instead, it spans over to the other dimeric [Co2(Hcit)2(H2O)2]2- anion and coordinates to one of the Co(II) ions, which assembles the [Co2(Hcit)2(H2O)2]2- unit into a planar polymeric structure (Figure 4b). The planar polymeric structure is linked by hydrogen bonds involving the ammonium cations into a 3D structure. The coordination mode in the dimer of complex 4 differs from that in complex 2, where the citrate ligand employs its free β-carboxylate group to span over to the second Co(II) ion in the [Co2(Hcit)2(H2O)4]2- unit and links the [Co(H2O)4] unit. The six oxygen apexes around the Co(II) ion consist of three oxygen atoms from R-hydroxyl (O1), R-carboxylate (O2), and the β-carboxylate (O4)

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Figure 3. (a) ORTEP plot of the unit in [Co(H2O)6]n[Co(Hcit)(H2O)2n‚2nH2O (3) with 30% thermal ellipsoids probability. The [Co(H2O)6]2+ cations and crystalline water molecules were omitted for clarity. (b) ORTEP plot of 3, depicting the infinite chain of [Co(Hcit)(H2O)] along a 21 axis of the unit cell. Hydrogen atoms have been omitted for clarity.

groups, respectively, two β-carboxylate oxygen atoms (O5i and O6ii) from two different citrate ligands, as well as one water molecules (O1w). The oxygen atom (O6ii), the coordination water molecule (O1w), and the oxygen atoms (O1, O4) from R-hydroxyl and β-carboxylate groups occupy the equatorial plane, while the other oxygen atoms (O2, O5i) from R- and β-carboxylate groups occupy the axial positions.

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Figure 4. (a) ORTEP plot of the anion unit in (NH4)2n[Co2(Hcit)2(H2O)2]n (4) with 30% thermal ellipsoids probability. The NH4+ cations were omitted for clarity. (b) ORTEP plot of 4, depicting the 2D network. Hydrogen atoms and NH4+ cations have been omitted for clarity. Table 3. Comparisons of the Bond Distances (Å) in Cobalt Citrate Complexes complex

Co-O (Å) (hydroxyl)

Co-O (Å) (R-carboxylate)

Co-O (Å) (β-carboxylate)

(NH4)4[Co(Hcit)2] (5) Na2[Co2(Hcit)2(H2O)4]‚6H2O K2[Co2(Hcit)2(H2O)4]‚6H2O [Co3(cit)4(4,4′-bpy)4(H2O)2]‚4H2O [Co(Im)6][Co(Im)3(Hcit)]2‚4H2O [C(NH2)3]4[Co(Hcit)2]‚2H2O [Co(H2cit)(H2O)]n (1) [Co(H2O)4]n[Co2(Hcit)2(H2O)4]n‚6nH2O (2) [Co(H2O)6]n[Co(Hcit)(H2O)2n‚2nH2O (3) (NH4)2n[Co2(Hcit)2(H2O)2]n (4)

2.157(2) 2.169(2) 2.194(2) 2.180(2) 2.162(1) 2.124(5) 2.034(2) 2.155(2) 2.116(3) 2.089(2)

2.075(2) 2.044(2) 2.065(2) 2.050(3) 2.085(1) 2.051(5) 2.125(2)av 2.069(2) 2.079(3) 2.073(2)

2.051(2) 2.052(2) 2.044(2) 2.056(3)av 2.146(1) 2.092(5) 2.099(2) 2.075(2)av 2.070(3)av 2.106(1)av

Apart from the different coordination features in the four polymeric complexes, there are extensive networks of hydrogen bonding interactions in the crystal structures of these complexes. The most important hydrogen bonds are listed in Table 2. These aforementioned interactions, further affected by the involvement of the water molecules or the counterion, could very well act as important factors contributing to the stability of the complexes in three dimensions. Comparisons of the Co-O distances for these cobalt(II) citrate complexes are given in Table 3. Complex 1 shows the shortest Co-O (R-hydroxyl) bond and the longest Co-O (R-carboxylate) bond. The other Co-O distances in 2-4 are in the normal range, which are similar to those observed in the other cobalt(II) citrate complexes.14,15,26,27 Moreover, these Co-O distances are also comparable to those in the other cobalt(II) hydroxycarboxylate complexes. Such as [Co(S-Hmal)(H2O)2]n‚ 2nH2O [2.067(3)-2.136(3) Å],28 trans-[Co(R,S-H2mal)2(H2O)2]‚2H2O [2.066(1)-2.112(1) Å]29 (H3mal ) malic

ref 14a 15 15 26 27 14b this work

acid), [Co2(R,R-H2tart)2(H2O)2]n‚3nH2O [2.04(1)-2.16(1) Å] (H4tart ) tartaric acid),30 [Co(Hglyc)2]n [2.053(1)2.118(1) Å] (H2glyc ) glycolic acid),31 [Co(R,S-Hlact)2(H2O)2]‚H2O [2.018(3)-2.129(3) Å] (H2lact ) lactic acid), and [Co[CO2C(OH)(CH3)2]2(H2O)2] [2.023(2)-2.090(2) Å].32 The Co1-Co1i distance in 2 is 5.348(1) Å, which is very similar to those observed in K2[Co2(Hcit)2(H2O)4]‚6H2O [5.247(1) Å], Na2[Co2(Hcit)2(H2O)4]‚6H2O [5.361(1) Å],15 K2[Ni2(Hcit)2(H2O)4]‚6H2O [5.364(1) Å],24 and Co3(Hcit)2(4,4′-bpy)4(H2O)2‚4H2O [5.378(1) Å] (bpy ) 4,4′bipyridine)26 but is longer than that observed in 4 [4.955(2) Å]. Of all the angles formed in the octahedron of the three octahedral of the Co1 ions, the one worth nothing is the O1-Co1-O2 angle, which is 75.0(1), 77.52(8), 76.9(1), and 75.7(1)° in complexes 1-4, respectively. This small angle is similar to the corresponding angle in M2[Co2(Hcit)2(H2O)4]‚6H2O (M ) Na, K),15 and it denotes the distortion brought about by the small bite of the five-membered ring provided by R-hydroxyl and R-carboxylate groups.

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Scheme 2. Different Coordination Modes of Citrate Ligand in the Known Cobalt(II) Citrate Complexes14-16

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polymeric complexes, aids significantly in comprehending the chemical diversities of citric acid toward transition metal ion. Acknowledgment. We thank the Ministry of Science & Technology (001CB108906) and the National Science Foundation of China (20021002) for the generous support of this research. Supporting Information Available: X-ray crystallographic files in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

References

With the assessment of the structural features in the four isolated cobalt citrate complexes, some similarities are as followed: (i) The cobalt(II) ions in all complexes are octahedral coordination; (ii) the citrate ligand in all of the complexes has a basic tridentated coordination mode, that is, it coordinates to the metal ions via its R-hydroxyl, R-carboxylate, and one of the β-carboxylate groups; and (iii) the R-hydroxyl group of the citrate ligand retains its proton. The major differences among the complexes are worth noting; that is, the citrate ligand employs its R-carboxylate group as a bridging ligand to coordinate the cobalt ions in complex of 1; however, it employs its β-carboxylate group as a bridge ligand to coordinate cobalt(II) ions in complexes 2-4. The R-carboxylate group is a better bridging ligand than those of β-carboxylate groups in citrate at low pH. The β-carboxylate groups act as a better bridging ligands in weak acidic medium. Scheme 2 illustrates the different coordination modes of the citrate in the known cobalt(II) citrate complexes, which have been characterized structurally.14-16 Conclusions The molecular structures of these cobalt citrate coordination polymers are influenced by pH, reaction temperature, and counterions, forming 1D chains and 2D topology networks. Citrate displays coordination diversities toward the cobalt(II) ion in aqueous solution. The isolated 1D and 2D polymeric complexes and soluble monoclear and dinuclear cobalt(II) citrate complexes,14-16 as well as the results from the past solution studies of cobalt(II) citrate system,33 will provide the detailed spectroscopic and structural properties for the potential components related to biological media and the solution of cobalt(II) citrate precursor for the preparation of pertinent materials. Also, the interaction of cobalt(II) ion and citrate, resulting in the

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