Cite this: CrystEngComm, 2013, 15,7999

Conversions between dimeric and polymericketopiperazinediacetato complexes constructed bywater-layers3

Received 11th December 2012,Accepted 16th July 2013

DOI: 10.1039/c3ce26999j

www.rsc.org/crystengcomm

Yu-Chen Yang, Xue Jiang and Zhao-Hui Zhou*

Degradations of the layered polymeric products [M2(kpda)2(H2O)4]n?6nH2O (M = Co, 1; Ni, 2; Zn, 3; H2kpda

= ketopiperazinediacetic acid, C8H12N2O5) result in the formation of dimeric units [M2(kpda)2(H2O)6]?H2O

(M = Co, 4; Ni, 5; Zn, 6) in an ethanol solution, where the layered structures shrink from 11.23 Å to 6.22 Å.

This is attributed to the dehydration of interesting 2D water layers based on X-ray structural analysis. The

quasi-interconversion was monitored by IR, NMR measurements and product isolations, which involved

complicated hydrated–dehydrated reactions.

Introduction

Ethylenediaminetriacetic acid (H3ed3a = C8H14N2O6) is anindustrial intermediate for the preparation of chelatedsurfactant, which has unusual structural features, remarkablephysical and chemical properties.1–3 It is also a strongchelating agent for the preparations of a number of mono-nuclear complexes like [M(ed3a)X] [M = Co(III), Cr(III), Cu(II),Ti(IV) and X = mono- or bidentate ligand].4–9 Silver chelatedwith ethylenediaminetriacetic acid exhibits photosensitivityand antimicrobial activity even at very low silver levels.10 Ed3acould be cyclized to ketopiperazinediacetic acid under acidconditions (H2kpda = C8H12N2O5). A mononuclear complex[Pd(Hkpda)2] and two 1D complexes [M(kpda)(H2O)2]n?nH2O[M = Cu(II), Zn(II)] have been reported before.11

Water is the most important and indispensable compoundfor living beings, but it is still not a fully understood liquid.12

The structural information of small water clusters is the firststep towards understanding the behavior of bulk water.13 Avariety of water clusters have been found, but how the clusterslink to form a larger network is still challenging.14 Several 2Dwater layers have been observed in the solid state, whichprovide novel structural aspects and new insights into waterwith implications in biological environments.15,16 Here, we

report a series of 2D layered complexes, [M2(kpda)2(H2O)4]n?

6nH2O (M = Co, 1; Ni, 2; Zn, 3;), with new (H2O)20 2D waterlayers, which are composed of 4-membered and 20-memberedrings.

A number of factors can influence the construction ofcomplexes, such as the reactant stoichiometry, pH value,solvent and template agents, et al.17,18 Usually, high tempera-ture favors for the formation of high-dimensional compoundsthat are thermodynamically stable, while low temperaturefavors the formation of low-dimensional compounds.19 Severaldehydration transformations have been reported, where low-dimensional compounds were transformed to high-dimen-sional compounds. The hydration process led to the transfor-mation from high-dimensional to the low-dimensionalcompounds.20 Here, 1–3 show unusual temperature effectsfor the transformation of the related dimeric ketopiperazine-diacetato units [M2(kpda)2(H2O)6]?H2O (M = Co, 4; Ni, 5; Zn, 6)at heating conditions, and the process of transformation wastraced by IR and NMR techniques and product isolations.

Experimental

Materials and physical measurements

The chemicals used were analytical grade reagents. The pHvalue was measured by a potentiometric method with a digitalPHB-8 pH meter. Infrared spectra were recorded as Nujolmulls between KBr plates on a Nicolet 330 FT-IR spectrometer.Elemental analyses were performed with an EA 1110 elementalanalyzer. UV spectra were obtained from a Cary 5000 spectro-meter. Solution 13C{1H} NMR spectra were recorded in D2O ona Bruker AV 400 NMR spectrometer using DSS (sodium 2,2-dimethyl-2-silapentane-5-sulfonate) as an internal referenceand the spectra accumulated 10 000 times overnight. Solid

State Key Laboratory of Physical Chemistry of Solid Surfaces, College of Chemistry

and Chemical Engineering, Xiamen University, Xiamen, 361005, China.

E-mail: zhzhou@xmu.edu.cn; Fax: +86 592 2183047; Tel: +86 592 2184531

3 Electronic supplementary information (ESI) available: Experimental section;ORTEP figures of 2, 3, 5 and 6 (Fig. S1–S4); UV-vis spectra of 1, 3, 4 and 5 (Fig.S5); IR vibrational spectra (Fig. S6 and S7); thermogravimetric analysis of 1, 3, 5

(Fig. S8–S10); 13C NMR spectral data (in ppm) (Table S1); selected bonddistances and angles (Table S2); selected bond distances and angles in the waterlayers of 1 (Table S3); selected hydrogen bond distances and angles in 4 (TableS4). CCDC 881474–881479. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c3ce26999j

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state 13C NMR spectra were recorded on a Bruker AV 400 NMRspectrometer using cross polarization, magic angle spinning(13 kHz) and adamantane as a reference. Data collections of 1–6 were performed on an Oxford Gemini S Ultra system withgraphite monochromated Mo–Ka (l = 0.71073 Å) radiation at173 K. Adsorption corrections were applied using the CrysAlisprogram. The structures were solved by direct methods andrefined by full-matrix least-squares procedures with anisotro-pic thermal parameters for all of the non-hydrogen atomsusing SHELXL-97.21 Hydrogen atoms were located from adifference Fourier map and refined isotropically. Summary ofthe crystal data for complexes 1–6 is listed in Table 1.

Preparation of 1–6

Preparation of [M2(kpda)2(H2O)4]n?6nH2O (M = Co, 1; Ni, 2; Zn,3): CoCl2?6H2O (120 mg, 0.50 mmol) was added to a stirredsolution of ethylenediaminetriacetic acid (120 mg, 0.50 mmol)in water (8.0 mL). The pH value of the solution was adjusted to4.0–6.0 by 5.0 M potassium hydroxide. The mixture was stirredat room temperature for 1 h and left standing for several daysto give the red crystalline product of 1. The product wasisolated by filtration and dried in air. Yield: 96 mg (53%).Calcd. for C16H40N4O20Co2: C, 26.5; H, 5.6; N, 7.7. Found: C,26.5; H, 5.8; N, 7.6 (%). UV-vis (BaSO4) lmax: 225 and 491 nm.IR (KBr, cm21): 3418 s,br, 1632 s, 1607 s, 1508 w, 1435 m, 1394m, 1355 w, 1325 w, 1296 w, 1253 w, 1186 w, 1117 w, 1048 w,1015 w, 985 w, 916 w, 880 w, 741 w, 704 m, 533 w. Similarly,green crystalline product 2 was isolated with a yield of 4.0 mg(2.2%). Calcd. for C16H40N4O20Ni2: C, 26.5; H, 5.6; N, 7.7.Found: C, 26.4; H, 5.5; N, 7.8 (%). Colorless crystalline product3 was isolated with a yield of 116 mg (63%). Calcd. forC16H40N4O20Zn2: C, 26.0; H, 5.5; N, 7.6. Found: C, 26.0; H, 5.5;N, 7.5 (%). UV-vis (BaSO4) lmax: 210 nm. IR (KBr, cm21): 3431s,br, 1630 s, 1508 w, 1434 m, 1396 m, 1355 w, 1324 w, 1295 w,1250 w, 1190 w, 1117 w, 1050 w, 988 w, 914 w, 879 w, 737 w,704 m, 636 w, 528 w.

Preparation of [M2(kpda)2(H2O)6]?H2O (M = Co, 4; Ni, 5; Zn,6): CoCl2?6H2O (120 mg, 0.50 mmol) was added to a stirredsolution of ethylenediaminetriacetic acid (120 mg, 0.50 mmol)in water (5.0 mL). The pH value of the solution was adjusted to4.0–6.0 by 5.0 M potassium hydroxide with constant stirring.After 30 min, 5.0 mL of ethanol was added and the solutionsubsequently heated at 70 uC for 1 day. A pink crystallineproduct was isolated by filtration, washed and dried in air.Yield: 18 mg (11%). Calcd. for C16H34N4O17Co2: C, 28.6; H, 5.1;N, 8.3. Found: C, 28.1; H, 5.0; N, 8.1 (%). UV-vis (BaSO4) lmax:228 and 491 nm. IR (KBr, cm21): 3421 s,br, 1642 s, 1610 s, 1585s, 1512 m, 1432 m, 1398 s, 1358 m, 1398 m, 1358 m, 1320 m,1287 m, 1255 w, 1188 w, 1118 w, 1050 w, 1016 w, 991 w, 919 w,883 w, 711 w, 614 m, 532 w. Similarly, green crystalline product5 was isolated with a yield of 95 mg (57%). Calcd. forC16H34N4O17Ni2: C, 28.6; H, 5.1; N, 8.3. Found: C, 28.2; H, 5.1;N, 8.1 (%). UV-vis (BaSO4) lmax: 218, 386 and 646 nm. IR (KBr,cm21): 3418 s,br, 1645 s, 1610 s, 1585 s, 1513 m, 1433 m, 1399s, 1359 m, 1318 m, 1288 m, 1255 w, 1183 w, 1117 w, 991 w, 888m, 714 m, 617 w, 533 m, 405 w. Colorless crystalline product 6was obtained with a yield of 14 mg (8.2%). Calcd. forC16H34N4O17Zn2: C, 28.0; H, 5.0; N, 8.2. Found: C, 28.2; H,5.1; N, 8.1 (%). IR (KBr, cm21): 3417 s,br, 1642 s, 1613 s, 1582 s,

1513 w, 1432 m, 1399 m, 1358 m, 1317 w, 1287 w, 1256 w, 1188w, 1117 m, 1049 w, 1017 w, 991 w, 888 w, 855 w, 714 w, 617 w,533 w.

Results and discussion

Syntheses

Reactions of ethylenediaminetriacetic acid with metal chlor-ides proceeded in solution at a pH value of 5, which resulted inthe isolation of compounds 1–6 respectively. 1–3 wereobtained at room temperature, while 4–6 were obtained at 70uC. Heating conditions cleave the polymeric product into itsdimeric unit in ethanol solutions, while the dimeric unit couldbe transformed to its polymer in solution with heating for oneday, as shown in Scheme 1. Heating and solvent effects areconsidered as the driving force for the quasi-conversion. Theaddition of ethanol facilitates the dehydration of watermolecules in 1–3. Although the 2D network complexes withwater layers are the most stable structures, the conversionsfrom 1–3 to 4–6 are dehydrations, so the 2D networks will losesome water molecules under heating conditions.20

Scheme 1 Transformations between the ethylenediaminetriacetato and keto-piperazinediacetato complexes.

Fig. 1 ORTEP plot of the dimeric neutral unit in [Co2(kpda)2(H2O)4]n?6nH2O (1)at 30% probability levels.

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Crystal structure

Polymeric and dimeric cobalt, nickel and zinc complexes 1–3and 4–6 are isomorphous, respectively. The molecular struc-tures of the polymeric and dimeric products consist of asimilar dimeric neutral unit [M2(kpda)2(H2O)4], as shown inFig. 1 and 2 for 1 and 4, respectively. Those of 2, 3, 5 and 6 areshown in Fig. S1–S4, ESI.3 A chelated ring is formed bynitrogen and oxygen atoms of ketopiperazinediacetate, whichfurther couples with the other monodentate carboxy group ofkpda into a dimeric unit in 1–6. The asymmetric coordinationmode of the two acetates should be related to the a-carbonylgroup in the piperazine ring, which decreases the donorproperty of the nitrogen atom in the amide. The piperazinering in kpda is in a boat conformation. Compared with thedimeric structures of 4–6, the formation of polymeric

structures result in the longer M–O and M–N bond distancesin the kpda complexes, as shown in Table 2, while the averagebond distances of M–Owater in the polymeric units are shorterthan those in the dimeric units. Strong inter- and intramole-cular hydrogen bonds exist between the carboxy oxygen atomsand coordinated water molecules [O2w…O4 2.641(5) Å,O2w…O1, 2.724(5) Å for 1 and 2.696(5) Å for 4]. For thestructures of 1–3, the dimeric units are further connected bythe oxygen atoms of carboxy groups to generate infinite 2Dlayered structures, respectively, as shown in Fig. 3a, and form3D supramolecular structures by intermolecular hydrogenbonds.

The striking features of the structures of 1–3 are theformation of 2D water layers. Five coordinated and crystallizedwater molecules connect to each other by hydrogen bonds.Water molecules and their symmetric equivalent atoms arejoined together, forming a 2D water layer, as shown in Fig. 3d.The 2D water layer is composed of a unique (H2O)20 clusterwith symmetry, which can be regarded as an evolution of awater chain by adding water between the chains. Each O5wbwater molecule acts as donor and acceptor to connect the twoO3w and bridges the water chains into a 2D layer. Within thewater layer, two water molecules (O1w and O1wa) act as twohydrogen-bond donors with O4w and O4wa forming a cyclicwater tetramer.22 The average O…O distance of the (H2O)20

cluster is 2.778 Å. This is longer than the corresponding valuein ice Ih (2.759 Å)23 and shorter than in liquid water (2.854 Å),24

but within the values in the ice II phase (2.77–2.84 Å).25

Water clusters may play an important role in the stabiliza-tion of supramolecular systems both in solution and solidstate.26 The formation of the 2D water layer in 1 indicates thatthe intramolecular hydrogen bonds and the hydrogen-bond

Fig. 2 ORTEP plot of the dimeric neutral unit in [Co2(kpda)2(H2O)6]?H2O (4) at30% probability levels.

Table 1 Summary of the crystal data for complexes 1–6

1 2 3 4 5 6

Empirical formula C16H40N4O20Co2 C16H40N4O20Ni2 C16H40N4O20Zn2 C16H34N4O17Co2 C16H34N4O17Ni2 C16H34N4O17Zn2

Formula weight 726.38 725.94 739.26 672.33 671.89 685.21Crystal color Red Green Colorless Pink Green ColorlessCrystal system MonoclinicCell constantsa (Å) 9.8199(8) 9.726(4) 9.7930(7) 20.383(2) 20.210(1) 20.352(1)b (Å) 17.405(1) 17.322(5) 17.439(1) 6.2408(4) 6.2209(5) 6.2589(4)c (Å) 8.6142(6) 8.608(3) 8.6138(6) 20.297(2) 20.242(2) 20.321(1)b (u) 105.229(8) 105.16(4) 105.169(7) 105.286(8) 104.900(8) 105.162(7)V (Å3) 1420.6(2) 1399.7(9) 1419.8(2) 2490.5(3) 2459.4 (3) 2498.4(3)Space group P21/c C2/cFormula units/unit cell 2 4Dcalcd (g cm23) 1.698 1.722 1.729 1.793 1.815 1.822F (000) 756 760 768 1392 1400 1416Temp (K) 173Diffractometer Oxford Gemini CCDRadiation Mo–Ka (l = 0.71073 Å)Reflns. collected/unique 7443/3121 7210/3077 8169/3583 5721/2741 6434/2726 5879/2834Crystal size (mm) 0.10 6 0.06 6

0.020.10 6 0.10 60.02

0.20 6 0.20 60.10

0.20 6 0.20 60.10

0.25 6 0.20 60.20

0.08 6 0.03 60.02

Data/params. 3121/220 3077/ 220 3875/222 2741/198 2726/199 2834/199h range (u) 2.72 to 27.50 2.72 to 27.50 2.71 to 30.53 3.30 to 27.50 3.31 to 27.47 3.30 to 27.98GOF on F2 1.028 0.963 0.903 1.014 1.069 1.065R1, wR2 [I . 2s(I)] 0.070, 0.107 0.089, 0.103 0.056, 0.120 0.067, 0.112 0.058, 0.122 0.071, 0.097R1, wR2 (all data) 0.116, 0.123 0.180, 0.135 0.096, 0.154 0.107, 0.128 0.087, 0.139 0.112, 0.109Largest diff. peak and hole (e Å23) 0.614, 20.466 0.682, 20.628 0.564, 20.725 0.489, 20.512 0.809, 20.507 0.870, 20.871

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involving the carboxy oxygen atoms play a crucial role instabilizing the water layer and the host structure. Fig. 3edisplays the packing structure of compound 1, where the waterlayers act as a ‘‘glue’’ to connect the complex network. In orderto provide a stable structure, the geometry of the water layerexactly matches the host structure. When viewed along thec-axis, the wave-like water layers are sandwiched in the spaceof the wave-like complex networks.16

The new water phases in 1–3 provide further stability for theformation of polymeric structures at room temperature, whilethey decompose into their related dimeric structures withy10% yields under heating conditions in an ethanol solution.The newly formed dimer also contains a wave-like layerstructure connected by hydrogen bonds, as shown in Fig. 4.In this conversion, three water molecules between the[Co2(kpda)2(H2O)4]n units are lost and the distance betweenthe cobalt atoms in the different sheets is reduced from11.23 Å to 6.22 Å. The new dimeric products in 4–6 containisolated [M2(kpda)2(H2O)4]n units, which are linked by inter-molecular hydrogen bonding (Table S4, ESI3). The conversion

proceeds at low temperature, as revealed by the thermalanalysis of the 2D networks (Fig. S8 and S9, ESI3).

The conversion of [Zn2(kpda)2(H2O)4]n?6nH2O (3) to[Zn2(kpda)2(H2O)6]?H2O (6)

Solid IR experiments were used to trace the conversion of 3, asshown in Fig. 5. The other IR spectra are shown in Fig. S6 andS7, ESI.3 The polymeric structure in 3 is quasi-transformed to adimeric structure in 6 when heating at 70 uC. As timeincreases, the peaks around 1355, 1320, 1017, 991 and 617cm21 show obvious changes. The spectrum at 1.5 h is veryclose to that of the dimeric unit in 6, and the polymer 3 seemsto be converted to dimer 6 but it has mixed peaks around 900cm21 after heating, which suggests different dehydratedproducts after heating. In the thermal analysis, polymer 3loses water molecules in two steps. 3 loses eight watermolecules before 100 uC and loses the last two water moleculesbefore 200 uC. The thermal analysis proves that the dehydra-tion of 3 in the solid state is a complicated process.

Table 2 Comparisons of the bond distances between the coordinated carboxy, amine groups and water molecules for the kpda complexes

Compounds M–Ocarboxy M–N M–Owater

[Co2(kpda)2(H2O)4]n?6nH2O (1) 2.040(3), 2.049(3), 2.088(3) 2.205(4) 2.123(3), 2.147(3)[Ni2(kpda)2(H2O)4]n?6nH2O (2) 2.022(4), 2.027(5), 2.058(5) 2.136(6) 2.078(5), 2.103(5)[Zn2(kpda)2(H2O)4]n?6nH2O (3) 2.046(3), 2.054(3), 2.118(3) 2.210(4) 2.119(3), 2.141(3)[Co2(kpda)2(H2O)6]?H2O (4) 2.025(3), 2.052(3) 2.199(4) 2.084(3), 2.141(3), 2.214(3)[Ni2(kpda)2(H2O)6]?H2O (5) 2.011(3), 2.041(3) 2.140(4) 2.052(3), 2.107(3), 2.162(3)[Zn2(kpda)2(H2O)6]?H2O (6) 2.023(4), 2.053(3) 2.215(4) 2.058(4), 2.142(4), 2.273(4)Pd(Hkpda)2

11a 2.010(1) 2.048(2)[Cu(kpda)(H2O)2]n?nH2O11b 1.939(3), 1.993(3), 2.572(3) 2.046(3) 1.946(3), 2.358(3)[Zn(kpda)(H2O)2]n?nH2O (7)11b 2.051(3), 2.069(3), 2.434(3) 2.153(3) 2.046(3), 2.066(3)

Fig. 3 (a) 2D layered structure of [Co2(kpda)2(H2O)4]n?6nH2O (1) viewed from the c-axis. (b) 2D layered structure of 1 viewed from the a-axis. Hydrogen atoms areomitted for clarity. (c) 2D water layers in 1 viewed from the c-axis. (d) 2D water layers in 1 viewed from the a-axis. (e)View from the c-axis showing the interactionsbetween the water layers and the 2D polymeric units in 1.

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NMR spectra of [Zn2(kpda)2(H2O)4]n?6nH2O (3) and[Zn2(kpda)2(H2O)6]?H2O (6)

The NMR experiments show no band for the cobalt and nickelcomplexes due to the paramagnetic effect, while the solutionand solid state 13C NMR spectra of zinc complex 3 aredetectable, as shown in Fig. 6. The detailed data are comparedin Table S1, ESI.3 For the solid state 13C NMR spectrum of 3,obvious downfield shifts of 13C resonances are observedcompared with the free kpda ligand. The corresponding Dd

values are 3.1, 4.6, 1.1 ppm, respectively, for the a-CO2, a9-CO2

and –CLO groups. The obvious downfield shifts of the 13Cresonances are clear indications of the coordination of the twocarboxy oxygen atoms, while the carbonyl group shows nocoordination. Due to the low solubility, the solution spectrumof 3 was taken after heating for 20 min in a 70 uC water bath.For the solution 13C NMR spectrum of 3, obvious downfieldshifts of the coordinated carboxy groups are compared withthe free kpda ligand (Dd: 6.5, 4.2 ppm), which are the same asthose of [Zn(kpda)(H2O)2]n?nH2O (7)11b in solution. This

indicates that 2D coordination polymer 3 and[Zn(kpda)(H2O)2]n?nH2O (7) after heating have the samechemical composition in solution. The 2D water layers maydisappear and degradate into a 1D product or dimeric unit.The structures of polymeric and their dimeric units showsimilar degradation, while the 2D polymers are thermody-namic stable. Solid isolation from the heating solution showsthe further degradated product as the dimeric unit with a lowyield. The polymeric products are more easily separated bycrystallization.

Fig. 4 (a) 2D layered structure of [Co2(kpda)2(H2O)6]?H2O (4) viewed along the b-axis. (b) 2D layered structure of 4 viewed along c-axis.

Fig. 5 Qualitative IR tracing of the conversion from [Zn2(kpda)2(H2O)4]n?6nH2O(3) to [Zn2(kpda)2(H2O)6]?H2O (6) at different times.

Fig. 6 Solid and solution NMR spectra of the zinc ketopiperazinediacetatocomplexes.

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Conclusions

In summary, polymeric and dimeric kpda cobalt(II), nickel(II)and zinc(II) complexes are obtained at different temperatures,which are linked by asymmetric acetate groups of the kpdaligands. The formation of a (H2O)20 water layer givesadditional stability to the coordination polymer that couldbe isolated at room temperature. The water layer withhydrogen bonds plays an important role in the crystalstructure and the transformation from a 2D network to adimer. Under heating conditions in a ethanol solution, apartial dehydration process led to the conversion from a 2Dnetwork to a dimeric unit.

Acknowledgements

Financial contributions from the National Science Foundationof China (21073150), the Ministry of Science and Technologyof China (2010CB732303) and PCSIRT (No. IRT1036) aregratefully acknowledged.

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8004 | CrystEngComm, 2013, 15, 7999–8005 This journal is � The Royal Society of Chemistry 2013

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This journal is � The Royal Society of Chemistry 2013 CrystEngComm, 2013, 15, 7999–8005 | 8005

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