Synthesis and characterization of metal complexes of Calcein Blue: Formation of monomeric, ion pair and coordination polymeric structures

Inorganica Chimica Acta 362 (2009) 2189–2199

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Synthesis and characterization of metal complexes of Calcein Blue: Formation of monomeric, ion pair and coordination polymeric structures Wei Lee Leong, Jagadese J. Vittal * Department of Chemistry, National University of Singapore, 3 Science Drive 3, Singapore 117543, Singapore

a r t i c l e

i n f o

Article history: Received 6 August 2008 Accepted 22 September 2008 Available online 1 October 2008 Keywords: Calcein Blue Ion-pair complexes Hydrogen bonding Solid-state fluorescence

a b s t r a c t A series of metal complexes containing the 4-methylumbelliferone-8-methyleneiminodiacetic acid (H3muia, also named as Calcein Blue) has been synthesized and characterized. Complexes of Cu(II), Ni(II), Mn(II), Zn(II) and Mg(II) have been structurally characterized while Ca(II) and Al(III) complexes by elemental analysis and thermogravimetry. The Cu(II) and Ni(II) complexes are neutral and mononuclear in the solid state. Interestingly, the Mn(II), Zn(II) and Mg(II) muia complexes exist as ion-pairs containing hydrated or solvated metal cations and dimeric metal(II) muia anions. Owing to the presence of hydrogen-bond donors and acceptors in the ligand, hydrogen bonding interactions are dominant along with p– p stacking in their solid-state structures. The solid-state fluorescence studies indicate that the family of muia complexes exhibit comparable emission properties as in solution state, in which only main group and post-transition complexes show bright blue fluorescence while transition metal complexes do not fluoresce. Ó 2008 Elsevier B.V. All rights reserved.

1. Introduction Crystal engineering of coordination polymers and supramolecules have attracted lot of attention due to their potential applications as functional materials, as well as their fascinating architectures and topologies [1–7]. A successful strategy in the construction of such networks is to utilize appropriate multidentate ligands with flexible backbone that are capable of binding metal ions in different modes. One such class of ligands is the amino acid moiety in which both amino and carboxylate groups can take part in the supramolecular interactions. Considerable efforts have been made to study the coordination chemistry of various transition metal complexes derived from salicyaldehyde and amino acid Schiff base [8–14] and reduced Schiff base ligands [15–20]. Our laboratory has been interested in the coordination chemistry of Cu(II) and Zn(II) complexes containing reduced Schiff base ligands derived from substituted salicylaldehyde and amino acid for the construction of supramolecular network structures. X-ray crystal structures of these complexes revealed that the N-(2-hydroxybenzyl)-amino acid ligands mainly act as tridentate moiety, coordinating through the phenolato oxygen, amine nitrogen and carboxylate oxygen. The other exodentate carboxylate oxygen atom coordinating to metal ions intermolecularly is responsible for the fascinating supramolecular architectures [21,22]. We have successfully demonstrated that incorporation of yet another carboxylate group to * Corresponding author. Tel.: +65 6516 2975; fax: +65 6779 1691. E-mail address: [email protected] (J.J. Vittal). 0020-1693/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2008.09.049

the reduced Schiff base ligand, viz., N-(2-hydroxylbenzyl)-glutamic acid has led to an interesting 1D coordination polymer which encapsulated helical water channel [23]. Thus, it is promising that the additional functional group such as carboxylate is essential for furnishing structurally interesting coordination polymers by these ligands. Hence, we have chosen 4-methylumbelliferone-8-methyleneiminodiacetic acid (H3muia) or Calcein Blue which has two carboxylic acid groups as shown in Scheme 1. This commercially available ligand has coumarin group instead of phenyl group. The coumarin derivatives are advantageous as they show remarkable absorption and luminescence properties [24]. Furthermore, the presence of extra ring may induce the p–p interactions in the solid-state structures of the resulting complexes. Owing to the presence of coumarin group, Calcein Blue has been employed as an indicator in the EDTA titration of calcium and some transition metals based on its fluorescence properties [25–27]. However, this tetradentate ligand with flexible amino dicarboxylate functional group and multi-hydrogen bonding functionalities has never been exploited for its coordination behaviour in the solid state. While much research has been devoted to natural product chemistry, biological application [28,29] and photophysical properties [24] of organic coumarin derivatives, little information is available on solid-state structures of transition metal complexes of coumarin derivatives [30–42]. Recently, we reported the self assembly of ion-pair cobalt(II) complexes of Calcein Blue. In one-pot synthesis, concomitant self-assembled monomeric and dimeric ion-pair cobalt(II)

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O HO

O

O H3 C

N O OH OH

complexes have been isolated. Both solvated and hydrated cobalt(II) cation interacted with cobalt(II) anion through intermolecular hydrogen bondings. It has been found that solvents play an important part in the condensation of ion-pair complexes to pseudo-supramolecular isomer and led to a 1D coordination polymer [43]. Here in we have further explored the coordination chemistry of Calcein Blue and describe the synthesis, characterization, solid-state structures and fluorescence properties of Cu(II), Ni(II), Mn(II), Zn(II), Mg(II), Ca(II) and Al(III) complexes of the H3muia ligand.

Scheme 1. Molecular structure of Calcein Blue. Table 1 Selected hydrogen bond parameters for 1–6 D–H

d(D–H)

d(DA)

d(HA)

\DHA

A (symmetry)

Compound 1 O1–H1 O1S–H1S O2S–H2SB O3S–H3SA O3S–H3SB O8–H8A O8–H8B

0.68(5) 0.76(5) 1.20(3) 1.23(3) 1.22(3) 0.80(3) 0.78(4)

1.94(5) 2.11(5) 2.53(3) 1.41(4) 1.66(3) 1.85(3) 1.99(4)

2.615(5) 2.852(4) 3.430(5) 2.601(7) 2.854(7) 2.624(5) 2.740(4)

170(5) 164(5) 131(2) 159(3) 164(3) 163(3) 163(5)

O1S O5 (x, 2  y, z) O7 (x, 1/2 + y, 1/2  z) O2S O3 (x, 1 + y, z) O2S O3 (1  x, 1/2 + y, 1/2  z)

Compound 2 O1–H1 O8–H8A O8–H8B O9–H9A O9–H9B O10–H10A O10–H10B O11–H11A O11–H11B

0.86(3) 0.86(3) 0.86(4) 0.86(3) 0.85(3) 0.88(2) 0.89(3) 0.87(3) 0.87(2)

1.70(3) 1.77(3) 1.96(4) 1.96(3) 1.96(3) 1.91(3) 1.94(3) 1.90(3) 2.03(2)

2.558(4) 2.620(3) 2.806(3) 2.817(3) 2.793(3) 2.717(4) 2.815(3) 2.759(4) 2.897(4)

175(2) 169(4) 167(4) 179(4) 164(3) 153(3) 171(5) 171(4) 176(7)

O10 (1  x, 1/2 + y, 1/2  z) O2 (1/2 + x, y, 1/2  z) O5 (1 + x, y, z) O3 (1/2  x, 1/2 + y, z) O4 (1/2 + x, y, 1/2  z) O11 O3 O5 (x, 1/2 + y, 1/2  z) O7 (x, 1/2  y, 1/2 + z)

Compound 3 O8–H8A O8–H8B O9–H9B O10–H10A O10–H10B O11–H11A O11–H11B

0.83(5) 0.86(7) 0.71(5) 0.83(5) 0.91(5) 0.69(5) 0.75(7)

1.89(5) 1.88(6) 2.14(5) 1.85(5) 1.87(5) 2.02(5) 1.93(7)

2.718(4) 2.732(4) 2.813(8) 2.670(4) 2.773(4) 2.669(5) 2.653(5)

173(5) 170(6) 159(5) 169(5) 176(7) 172(5) 166(7)

O3 (1  x, 1/2 + y, 1/2  z) O7 (x, y, z) N1S O4 (1  x, 2  y, 1  z) O2 (x, 1 + y, z) O3 (1  x, 1  y, 1  z) O5 (1  x, 2  y, 1  z)

Compound 4 O8–H8A O9–H9A O10–H10A O10–H10B O11–H11A O11–H11B

0.85(5) 0.86(7) 0.91(7) 0.88(7) 0.82(7) 0.77(7)

1.94(5) 2.05(7) 1.76(7) 1.88(7) 1.86(7) 1.89(7)

2.735(4) 2.881(7) 2.673(4) 2.733(4) 2.679(4) 2.656(5)

156(5) 165(7) 175(7) 164(6) 178(8) 178(9)

O3 N2 O4 O2 O3 O5

Compound 5 O15–H15A O15–H15B O16–H16A O16–H16B O17–H17A O17–H17B O18–H18B O19–H19 O20–H20 O21–H21A O21–H21B O22–H22A O22–H22B O23–H23A O23–H23B O24–H24A O24–H24B

1.02(8) 0.91(8) 1.11(6) 0.85(8) 0.90(2) 0.90(4) 0.94(7) 0.94 0.94 0.90(5) 0.90(6) 0.91(6) 0.90(4) 0.89(7) 0.90(5) 0.91(10) 0.90(12)

1.68(8) 1.87(8) 2.27(8) 1.89(9) 2.04(5) 2.19(5) 1.75(7) 2.07 1.81 1.84(5) 1.83(6) 1.95(7) 2.10(3) 1.86(7) 2.33(5) 2.30(12) 2.22(10)

2.698(6) 2.756(6) 2.780(7) 2.736(7) 2.838(8) 3.065(9) 2.692(9) 2.648(6) 2.6334 2.731(7) 2.694(8) 2.849(7) 2.969(8) 2.731(8) 3.052(15) 2.903(17) 3.012(16)

179(9) 167(7) 106(4) 172(9) 148(7) 166(6) 175(8) 118 145 169(5) 160(7) 170(7) 161(6) 165(6) 137(6) 124(12) 146(11)

O4 O2 (1  x, 1  y, 1  z) O7 (x, y, 1  z) O11 (1 + x, y, z) O3 (1  x, 1  y, 1  z) O3 (1 + x, y, z) O5 O9 (1  x, 1  y, 2  z) O10 (1  x, 1  y, 2  z) O22 O23 O7 (1  x, y, 1  z) O14 (x, 1 + y, z) O10 (1  x, 1  y, 2  z) O24 (1  x, 1  y, 2  z) O14 O19

Compound 6 O8–H8A O8–H8B O9–H9 O10–H10A O10–H10B O11–H11A O12–H12B

0.899(10) 0.895(11) 0.894(14) 0.899(15) 0.905(15) 0.908(13) 0.901(14)

1.899(11) 1.990(12) 1.797(16) 1.85(2) 1.724(19) 2.011(14) 1.84(2)

2.761(3) 2.828(3) 2.652(3) 2.725(4) 2.619(4) 2.869(6) 2.625(7)

160.0(11) 155.3(14) 159(3) 164(2) 170(3) 157.0(12) 144(3)

O5 O3 O2 O3 O5 O4 O7

(1  x, 1/2 + y, 1/2  z) (1  x, 2  y, 1  z) (x, 1 + y, z) (1  x, 1  y, 1  z) (1  x, 2  y, 1  z)

(3/2  x, 1/2 + y, 1/2  z) (3/2  x, 1/2 + y, 1/2  z) (x, 1 + y, z) (x, 1 + y, z)

(1/2 + x, 1/2 + y, 1/2 + z)

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2. Results and discussion 2.1. Synthesis and physical characterization A series of metal complexes of H3muia have been synthesized in the moderate yield. Among these complexes, [Cu(Hmuia)(H2O)]CH3OH  2H2O (1), [Ni(Hmuia)(H2O)2]  2H2O (2), [Mn(H2O)6] (3), [Mg(H2O)6][Mg2(muia)2([Mn2(muia)2(H2O)2]  2CH3CN H2O)2]  2CH3CN (4), [Mn(H2O)4.5(CH3OH)1.5]2[{Mn2(muia)2}{Mn2(muia)2(H2O)2}]  5H2O (5) and [Zn(H2O)5][Zn2(muia)2(H2O)2] (6) were structurally characterized. Attempts to obtain suitable single crystal of Ca(II) and Al(III) complexes for diffraction study were unsuccessful. Therefore, complexes [Ca(H2O)6][Ca2(muia)2 (H2O)2]  2H2O (7) and [Al2(muia)2(H2O)2]CH3CN4H2O (8) were characterized by elemental analysis and thermogravimetry. In general, the single crystals of muia complexes were obtained by the slow evaporation or layering excess of acetonitrile over the solution mixture of metal salt and potassium salt of muia ligand in metal-to-ligand ratio 1:1 or 2:1. In all of these complexes, the IR absorption bands around 3430 cm1 confirm the presence of water molecules and this is further supported by the weight loss observed in thermogravimetry. Interestingly, in 3 and 4, sharp peaks 3615 cm1 indicate that the

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O–H bond are free from hydrogen bonding. These observations later have been confirmed by the solid-state structures of the corresponding complexes. Furthermore, the absorption bands in the region 1685–1726 cm1 are due to the asymmetric vibration of coordinated carboxylate group [mas(COO)] and the bands in the region 1388– 1396 cm1 may be attributed to the symmetric stretching vibration of carboxylate group [ms(COO)]. The Dm of 296–338 cm1 observed in all the complexes except 5 suggests the monodentate coordination mode of carboxylate and the same has been confirmed by the X-ray crystal structures. The observed Dm of 286 cm1 in 5 suggests the bridging carboxylate has been reflected which is proved by X-ray crystal structure. The bands around the 1298–1344 cm1 may be assigned to m(C–O) of phenolic group [44]. The UV–Vis absorption spectra for all the complexes exhibit a strong absorption band in the range of 336–356 nm which may be assigned to the p–p* transition of the ligand. The d–d transition bands are rather weak compared to the pronounced p–p* absorption band, thus determination of d–d transitions in 1 and 2 were recorded separately with higher concentration of the complexes. The d–d transition for 1 and 2 are 724 nm and 654 nm, respectively [45]. No d–d transition band was observed of 3 and 5 even in concentrated solution. The solid-state UV–Vis spectra of complexes show similar p–p* absorption band in the range of 337–374 nm suggesting that the complex structures do not change much in the solution state. The structural behaviour of all the complexes in methanol solution has been investigated by electrospray ionization mass spectroscopy (ESI-MS). In these complexes, there was only one major peak has been observed for [ML] in the negative scan mode. The [ML] peaks can be assigned to the mononuclear complexes of 1 (381.0) and 2 (376.2). ESI-MS data of complexes 3–6 indicate that the dimeric anions are dissociated into monomeric species with the m/z in the range of 373.2–382.4. Complexes 7 and 8 exhibit a predominant peak corresponding to [ML] species with water molecules. 2.2. Description of crystal structures

Fig. 1. A perspective view of 1 showing the (H2O)3 cluster.

The solid-state structures of complexes 1–6 were determined by X-ray crystallographic techniques. Selected hydrogen bond parameters are given in Table 1. In all these complexes, the amine N, phenolic O and both carboxylate O are coordinated to the metal

Fig. 2. A portion of the 2D structure present in the crystal structure of 1 is viewed from c-axis. The C–H hydrogen atoms are not shown for clarity.

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centre. Depend on the experimental conditions, phenolic O can be deprotonated and bridged two metal centres to form a dimeric structure.

Fig. 3. A perspective view of 2. Lattice water molecules are omitted for clarity.

[Cu(Hmuia)(H2O)]CH3OH + 2H2O (1). A perspective view of neutral complex 1 is shown in Fig. 1. The Cu(II) centre displays a distorted square pyramid geometry (s = 0.033) [46] in which an amine N (Cu(1)–N(1) = 1.995(2) Å), two carboxylate (Cu(1)– O(2) = 1.940(2) Å; Cu(1)–O(4) = 1.929(2) Å) and an aqua ligand (Cu(1)–O(8) = 1.935(2) Å) formed the base of a square while phenolic O (Cu(1)–O(1) = 2.510(3) Å) occupies apical position which remained protonated. It is noteworthy that O7 is not considered to coordinate to Cu1 since the distance of 2.948(3) Å is slightly longer than the sum of Van der Waals radii of Cu–O (2.9 Å) [47]. Two uncoordinated water and one methanol molecule filled up the crystal lattice. There is no p–p interaction between the coumarin rings as the closest C–C distance is 4.368 Å. The packing of 1 reveals extensive hydrogen bonding in the crystal lattice. The intermolecular hydrogen bonding between carboxylate and lattice water (O2S–H2SBO7 and O3S–H3SBO3) and carboxylate and aqua ligand (O8-H8BO3) formed a 1D hydrogen-bonded polymeric chain. In this chain, coordinated water and two lattice water molecules form a water trimer. Such water trimers are ubiquitous in the crystal structures [48–50]. The phenolic proton, a noncoordinating carboxyl oxygen (O5)

Fig. 4. (a) Hydrogen-bonded network of 2; (b) packing diagram of 2 showing the hydrogen bonding and p–p interactions.

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Fig. 5. Perspective view of ion-pair complex 3. Solvent molecules and C–H hydrogen atoms are omitted for clarity.

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and methanol solvate extends to form a 2D hydrogen-bonded polymeric structure in the ab plane as shown in Fig. 2. [Ni(Hmuia)(H2O)2]  2H2O (2). A perspective view of 2 is displayed in Fig. 3. The Ni(II) ion has a typical octahedral geometry. The amine N (Ni(1)–N(1) = 2.073(2) Å), carboxylate O (Ni(1)– O(2) = 2.031(2) Å), phenolic O (Ni(1)–O(1) = 2.099(2) Å) and aqua ligand (Ni(1)–O(9) = 2.026(2) Å) occupy the equatorial positions while carboxylate O (Ni(1)–O(4) = 2.035(2) Å) and aqua ligands (Ni(1)–O(8) = 2.048(2) Å) occupy axial sites. The phenolic O remains protonated. Two lattice water molecules are present in the crystal lattice. Both aqua ligands on Ni(II) centre are hydrogen bonded to adjacent O atoms of carboxylate groups as displayed in Fig. 4a. The hydrogen-bonded polymeric chains further interact with each other through weak p–p interactions between the neighbouring coumarin rings (see Fig. 4b). The disk-like coumarin rings are arranged in slip-stacked fashion with the closest C–C distance is 3.820 Å between C1 and C13 and the farthest distance is 3.932 Å between C2 and C14. Apart from the coordinated water, lattice water molecules also involved in interesting hydrogen bonding

Fig. 6. (a) Perspective view of the ion-pair complex 3 showing the interactions between cations and anions in bc-plane; (b) interactions between anions only. Solvent molecules and C–H hydrogen atoms are omitted for clarity.

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interactions. The carbonyl O7 and phenolic O1 of two neighbouring molecules are connected through O–HO hydrogen bondings by two lattice water molecules to furnish 1D hydrogen-bonded polymer in zig-zag fashion. Overall, 2 has a 3D hydrogen-bonded network structure, the connectivity of which cannot be described easily by any available model. [Mn(H2O)6][Mn2(muia)2(H2O)2]  2CH3CN (3) and [Mg(H2O)6] [Mg2(muia)2(H2O)2]  2CH3CN (4). Complexes 3 and 4 were synthesized by layering acetonitrile on the mixture of H3muia and Mn(CH3COO)2  4H2O or Mg(CH3COO)2  4H2O, respectively, in 1:1 ratio. Compounds 3 and 4 are isostructural with the previously reported ion-pair Co(II) complex, [Co(H2O)6][Co2(muia)2 (H2O)2]  2CH3CN [43]. A representative dimeric anion of 3 is shown in Fig. 5. Each metal centre adopts a distorted octahedral geometry in which amine N (Mn(1)–N(1) = 2.289(3) Å; Mg(1)– N(1) = 2.228(3) Å), aqua ligand (Mn(1)–O(8) = 2.171(3) Å; Mg(1)–O(8) = 2.065(4) Å), phenolic O (Mn(1)–O(1) = 2.211(2) Å; Mg(1)–O(1) = 2.095(3) Å) and bridging phenolic O (Mn(1)– O(1a) = 2,226(2) Å; Mg(1)–O(1a) = 2.044(3) Å) form an equatorial plane while two carboxylate O (Mn(1)–O(2) = 2.191(2) Å; Mn(1)– O(4) = 2.159(2) Å; Mg(1)–O(2) = 2.086(3) Å; Mg(1)–O(4) = 2.090(3) Å) occupy the axial positions. The phenolic O atoms bridge two metal(II) atoms asymmetrically to form a M2O2 ring and the centre of the plane serves as crystallographic inversion centre. Each unit of the dinuclear complex has two negative charge neutralized by a neighbouring hydrated cation [M(H2O)6]2+, which is shared by another two dimeric anions as shown in Fig. 6a. The carboxylate groups of the anionic dimers are hydrogen bonded to the aqua ligands of the hexaaqua metal cations and resulted in a sheet-like arrangement of alternating cation and anion units in bc-plane. Packing of anions reveals a 3D porous network channels parallel to a-axis through C@OH–OH interactions (Fig. 6b). The channels are occupied by [M(H2O)6]2+ cations and acetonitrile molecules. There is no direct contact between the cations. The void between two [M(H2O)6]2+ is filled with two acetonitrile molecules and the N atom of each acetonitrile is hydrogen bonded to O(9) through NHOH. Furthermore, there are weak p–p interactions between the slip-stacked coumarin rings of complexes 3 and 4. The closest C–C distance in the complex 3 is 3.776 Å (between C2 and C12) and the farthest C–C distance is 3.829 Å (between C4 and C5). As for the complex 4, the closest C–C distance is 3.756 Å (between C1 and C13) and farthest C–C distance is 3.810 Å (between C4 and C5). [Mn(H 2 O) 4.5 (CH 3 OH) 1.5 ] 2 [{Mn 2 (muia) 2 }{Mn 2 (muia) 2 (H 2 O) 2 }]  5H2O (5). By changing the metal-to-ligand ratio to 2:1, an ion-pair 1D coordination polymer of 5 was obtained, instead of discrete ionpair complex. The yellow block crystals of 5 can lose the solvent when separated from mother liquor and turn to dark brown powder when left in ambient atmosphere for one day. Fig. 7a shows the asymmetric unit of ion-pair polymer of 5. Interestingly, differing from other muia complexes, compound 5 exhibit two different types of coordination environment in the anion core. Both Mn1 and Mn2 adopt distorted octahedral geometry. Similar to the other muia ion pair complexes including 3, 4, 6 and Co(II) [43], the equatorial plane of Mn2 is comprised of amine N (Mn(2)– N(2) = 2.282(4) Å), aqua ligand (Mn(2)–O(21) = 2.147(4) Å), phenolic O (Mn(2)–O(8) = 2.185(4) Å) and bridging phenolic O (Mn(2)–O(8a) = 2.117(4) Å), while axial positions are occupied by two carboxylate O (Mn(2)–O(9) = 2.205(4) Å; Mn(2)–O(11) = 2.211(4) Å). For the Mn1 centre, instead of aqua ligand in the equatorial position, a neighbouring carboxylate (Mn(1)–O(12) = 2.150(4) Å) occupies the place and bridges Mn1 and Mn2 in bidentate fashion to form 1D coordination polymer. The connectivity between the two types of dimers in the anion is schematically illustrated in Fig. 7b. It is shown that the two dif-

ferent repeating dimeric units alternate in the 1D coordination polymeric chain, in which one with aqua ligand and another with bridging carboxylate. For the cation, instead of hexaaquacation as in 3 and 4, 5 is in distorted octahedral geometry with 4.5 aqua ligands and 1.5 methanol molecules. The solvated Mn(II) cations are hydrogen bonded to the anionic polymeric strand as shown in Fig. 8a. It is noticeable that the complex 5 does not exhibit the alternate arrangement of cations and anions in two dimension as observed in 3, 4 and cobalt ion-pair complexes [43]. In this connection, all solvent molecules at Mn(3) are hydrogen bonded to the adjacent carboxylate groups through complimentary O–HO interactions and form a 3D hydrogen-bonded network. The cations fall within the 1D anionic coordination polymeric strands and hold firmly through hydrogen bondings. Fig. 8b displays the position of cations in the anionic polymer layer structure. Furthermore, lattice water molecules positioned and sustained within the hydrogen-bonded network between cation and anionic polymer. [Zn(H2O)5][Zn2(muia)2(H2O)2] (6). Complex 6 also exists as an ion-pair complex which consists of a disordered pentaaqua cation and a dimeric anion. In the dimeric anion core with crystallographic 2-fold rotational symmetry and the coordination of Zn(II) centre with muia ligand is similar to the other complexes. The Zn(II) centre adopts distorted octahedral geometry with muia ligand coordinated through amine N (Zn(1)–N(1) = 2.1883 Å), phenolic O (Zn(1)–O(1) = 2.137(3) Å), bridging O atoms (Zn(1)– O(1a) = 2.035(2) Å) and two carboxylate O (Zn(1)– O(2) = 2.106(2) Å; Zn(1)–O(4) = 2.102(2) Å). The bond distance between Zn(1) and aqua ligand O(8) is 2.135(3) Å. However, unlike other ion-pair complexes, the dimeric structure is twisted as shown in Fig. 9. The pentaaqua Zn(II) cation has the crystallographic 2-fold axis going through Zn(2)–O(9) bond and exhibits distorted trigonal bipyramidal geometry (s = 0.88) [46]. The aqua ligands of O11 and O12 are disordered. It is noticeable that there

Fig. 7. (a) A perspective view of asymmetric unit of the anion in 5; (b) the schematic representation of the 1D polymeric anion in 5. The Mn(II) cation and solvent molecules are omitted for clarity.

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is no p–p stacking observed in 6, as compared with other aforementioned ion-pair complexes. Owing to the non-planar geometry of dimeric anion and trigonal bipyramidal geometry of pentaaquacation, the packing of 6 does not exhibit a stable and rigid hydrogen-bonded cation and anion alternate arrangement as observed in other ion-pair complexes 3, 4 and Co(II) [43]. The dimeric anions form a porous honeycomb-like hydrogen-bonded network structure in ab-plane with channels along c-axis as shown in Fig. 10a. The cavities are filled by [Zn(H2O)5]2+ cations as displayed in Fig. 10b. The detailed intermolecular interactions between the anions and cations are shown in Fig. 10c–e. The porous honeycomb-like hydrogen-bonded network is formed by dimeric anions through (O8–H8BO3) and (O8– H8AO5) interactions (see Fig. 10c). The interactions between the anions and cations in ab-plane are shown in Fig. 10d and e. Except one aqua ligand, the other ligands are hydrogen bonded to the car-

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boxylate oxygens in ab-plane. The ‘‘non-bonded” aqua ligand which is roughly along c-axis involved in the complimentary OH2O@C interactions with the carbonyl oxygen of the coumarin group. Although we could not confirm the solid-state structures of 7 and 8 by X-ray crystallography, it is assumed that 7 may have similar structure to 4, which is having hexaaqua cation ion-pair structure while 8 being most likely 1:1 neutral complex. The composition and solvent molecules were determined based on elemental and thermogravimetry data. 2.3. Solid-state fluorescence studies Coumarin compounds are well-established analytical reagents in the determination of metals [29]. Calcein Blue is one of the useful fluorescent indicators in metal ion titrations. Alkali earth and first-row transition metal complexes of Calcein Blue formed

Fig. 8. (a) Packing diagram of 5 showing hydrogen bonding interactions between anionic polymer and Mn(II) cations; (b) placement of Mn(II) cations within the anionic polymeric strands. All C–H hydrogen atoms and solvent molecules are omitted for clarity.

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in situ, have been studied for their fluorescence properties in solution. It was found that the free ligand exhibited a bright blue fluorescence. Upon addition of transition metals, the fluorescence of indicator were quenched. Addition of alkali earth metal ions to the indicator retained the fluorescence properties [25–27].

Fig. 11 shows the solid-state fluorescence spectra for free ligand and complexes 1–8 upon excitation at k = 350 nm. It has been observed that the free ligand exhibits emission around 395 nm in solid state which deviates from the reported emission about 450 nm in the solution state [25–27]. This may be attributed to the fact that the fluorescence behaviour of Calcein Blue is pH dependent and most of the fluorescence studies were performed in basic solution. Nevertheless, in this context, the Calcein Blue was examined in the neutral form and do not undergo hydrolysis. On the other hand, complexes 4, 6–8 exhibit strong blue emission with maxima ca. 420 nm. Interestingly, complex 7 exhibits red shift of 20 nm as compared to other complexes. The fluorescence properties of ligand are quenched for transition metal complexes 1–3 and 5. The results are in agreement with the reported solution studies [25–27]. As indicated from UV–Vis studies, the similar absorption spectra in the solution and solid state evidenced that the complex structures remain same in the solution state. Thus, it is reasonable that the complexes show comparable fluorescence properties in both solution and solid state. 3. Conclusion

Fig. 9. A perspective view of 5 with pentaaqua Zn(II) cation. The disorder at Zn2 is omitted for clarity.

Several main group and transition metal complexes of muia ligand have been synthesized and characterized by X-ray crystallography except for 7 and 8. The muia ligand coordinates to metal(II) ions through tetradentate mode. Depending on the experimental conditions, phenolic O can be deprotonated and bridged two metal centres to form a dimeric structure as M2O2. The Cu(II) and Ni(II) complexes display mononuclear structure while Mn(II), Mg(II) and

Fig. 10. (a) A packing diagram of anionic 6 down from c-axis showing honeycomb-like cavity; (b) perspective view of 6 showing cations in the honeycomb-like cavity with space filling model; (c) hydrogen bonding interactions between anions; (d) and (e) intermolecular interactions between anions and cations in ab-plane.

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4.3. Synthesis

Emission Intensity (a.u.)

free ligand

6

4.3.1. [Cu(Hmuia)(H2O)]  CH3OH  2H2O (1) To H3muia (0.064 g, 0.2 mmol) in H2O (2 mL) containing KOH (0.022 g, 0.4 mmol), Cu(ClO4)2  6H2O (0.156 g, 0.4 mmol) in MeOH (2 mL) was added. Blue block-like crystals obtained next day were then dried in air. Yield: 0.05 g (53%). Anal. Calc. for C16H23NO11Cu: C, 40.98; H, 4.94; N, 2.99. Found: C, 40.62; H, 4.28; N, 3.12%. IR (KBr, cm1): m(OH) 3535; masðCOO Þ 1726; msðCOO Þ 1388; m(C–O) 1304. Calcd. TG weight loss for CH3OH and H2O 18.3%; found 18.6%. ESI-MS [CuC15H13NO7] 381.4. UV–Vis (kmax (nm), e(M1 cm1)): (MeOH) p–p*, 334(9865), d–d transition, 724(53); (nujol mull) p–p*, 342.

8

7 4 1-3,5

400

450

500

550

600

Wavelength (nm) Fig. 11. Solid-state fluorescence spectra for complexes 1–8 upon excitation at k = 350 nm.

Zn(II) complexes exist as ion-pair complexes. By variation of reactant ratio, Mn(II) complex can be obtained as an ion-pair coordination polymer. The presence of various hydrogen-bond donors and acceptors in the ligand contributes to extensive hydrogen bonding interactions in the solid state of these complexes. Interestingly, in ion-pair complexes 3, 4 and 6, anionic metal-muia moieties construct 3D porous channels through hydrogen bonding and the complimentary metal cation entities fill the cavities. We have demonstrated that both intermolecular hydrogen bonding and p–p interactions play important roles in the solid-state structure of muia complexes. In this context, p–p stacking is mainly observed in ionpair complexes but not in mononuclear complexes, indicating that p–p stacking is important in the formation of ion-pair complexes. Solid-state fluorescence studies indicate that complexes of muia have the similar emission properties as in the solution state. Transition metal ions quench the fluorescence of muia while alkali earth and post-transition metal complexes of muia exhibit strong blue emission. 4. Experimental 4.1. Materials All chemicals were purchased from Aldrich and used without further purification. Reagents used for the physical measurements were of spectroscopic grade. 4.2. Physical measurements The elemental analyses were performed in the microanalytical laboratory, Department of Chemistry, National University of Singapore. 1H NMR and 13C NMR spectra were recorded on a Bruker ACF 300FT-NMR spectrometer operating in the quadrature mode at 300 MHz. The infrared spectra (KBr pellet) were recorded using an FTS165 Bio-Rad FTIR spectrophotometer in the range of 4000– 400 cm1. ESI mass spectra were recorded on a Finnigan MAT LCQ mass spectrometer using the syringe pump method. Solvent present in the compounds was determined using an SDT 2960 TGA thermal analyzer with a heating rate of 5 °C min1 from room temperature to 600 °C in a N2 atmosphere using a 5–10 mg sample per run. The electronic transmittance spectral data were recorded on a Shimadzu UV-2501 PC UV–Vis spectrophotometer in the wavelength of 300–800 nm using Nujol mulls and in MeOH solution. The d–d transition for all complexes is much weaker compared with the P–P* absorption band and thus was determined with higher concentration.

4.3.2. [Ni(Hmuia)(H2O)2]  2H2O (2) To H3muia (0.064 g, 0.2 mmol) in 0.1 M KOH (4 mL), Ni(CH3COO)2 4H2O (0.048 g, 0.2 mmol) in MeOH (2 mL) was added and stirred for 15 min. Green block crystals were obtained from the filtrate by slow evaporation. Yield: 0.06 g (67%). Anal. Calc. for C15H21NO11Ni: C, 40.03; H, 4.70; N, 3.11. Found: C, 39.71; H, 4.81; N, 3.20%. IR (KBr, cm1): m(OH) 3392; masðCOO Þ 1708; msðCOO Þ 1395; m(C–O) 1298. Calcd. TG weight loss for 4H2O 16.0%; found 15.5%. ESI-MS [NiC15H13NO7] 376.2. UV–Vis (kmax (nm), e(M1 cm1)): (methanol) p–p*, 356(11 920), d–d transition, 654(90); (nujol mull) p–p*, 346. 4.3.3. [Mn(H2O)6][Mn2(muia)2(H2O)2]  2CH3CN (3) To H3muia (0.064 g, 0.2 mmol) in 0.1 M KOH (4 mL), Mn(CH3COO)2  4H2O (0.048 g, 0.2 mmol) in MeOH (2 mL) was added and stirred for 15 min. Acetonitrile (15 mL) was layered on the resulting yellow clear solution. Yellow block crystals were obtained after one week. Yield: 0.012 g (18%). Anal. Calc. for desolvated compound C30H44N2O24Mn3: C, 36.71; H, 4.52; N, 2.85. Found: C, 36.52; H, 4.37; N, 2.82%. IR (KBr, cm1): m(free OH) 3611; m(OH) 3344; masðCOO Þ 1685; msðCOO Þ 1389; ms(C–O) 1310. Calcd. TG weight loss for 8 H2O 15.2%; found 16.1%. ESI-MS [MnC15H12NO7]- 373.2. UV–Vis (kmax (nm), e(M1 cm1)): (methanol) p–p*, 354(28 190); (nujol mull) p–p*, 374. 4.3.4. [Mg(H2O)6][Mg2(muia)2(H2O)2]  2CH3CN (4) To H3muia (0.064 g, 0.2 mmol) in 0.1 M KOH (4 mL), Mg(CH3COO)2  4H2O (0.044 g, 0.2 mmol) in H2O (2 mL) was added and stirred for 15 min. Acetonitrile (15 mL) was layered on the resulting colourless clear solution. Colourless block crystals were obtained after two weeks and then dried in air. Yield: 0.032 g (51%). Anal. Calc. for C34H46N4O22Mg3; C, 43.65; H, 4.96; N, 5.99. Found: C, 43.23; H, 5.68; N, 5.79%. 1H NMR (D2O): d 7.53 (d, 1H, Ar–H), 6.67 (d, 1H, Ar–H), 5.97 (s, 1H, Ar–H), 3.90 (s, 2H, –CH2NH), 3.26 (q, 4H, –NCH2), 2.39 (s, 3H, Ar–CH3), 2.06 (s, 3H, CH3CN). 13C NMR (D2O): d 178.74 (–COOH), 172.11, 165.74, 158.09, 153.54, 126.05, 117.81, 110.30, 108.93, 105.41 (Ar), 59.92 (–CH2COOH), 49.22 (–CH2N), 18.04 (Ar–CH3). IR (KBr, cm1): m(free OH) 3619; m(OH) 3443; masðCOO Þ 1709; msðCOO Þ 1389; ms(C–O) 1315. Calcd. TG weight loss for 8 H2O 16.9%; found 16.7%. ESI-MS [MgC15H12NO7] 382.3. UV– Vis (kmax (nm), e(M1 cm1)): (methanol) p–p*, 356(22 280); (nujol mull) p–p*, 345. 4.3.5. [Mn(H2O)4.5(CH3OH)1.5]2[{(Mn2(muia)2)}{(Mn2(muia)2 (H2O)2)}]  5H2O (5) To H3muia (0.064 g, 0.2 mmol) in 0.1 M KOH (4 mL), Mn(CH3COO)2  4H2O (0.096 g, 0.4 mmol) in MeOH (2 mL) was added and stirred for 15 min. Yellow block crystals were obtained next day. Yield: 0.048 g (72%). Anal. Calc. for C60H80N4O44Mn6: C, 38.11; H, 4.26; N, 2.96. Found: C, 37.62; H, 4.62; N, 2.86%. IR (KBr, cm1): m(OH) 3362; masðCOO Þ 1677; masðCOO Þ 1391; ms(C–O) 1310. Calcd. TG weight loss for 16H2O 15.3%; found 16.0%.

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ESI-MS [MnC15H12NO7] 373.2. UV–Vis (kmax (nm), e(M1 cm1)): (methanol) p–p*, 356(26 700); (nujol mull) p–p*, 367. 4.3.6. [Zn(H2O)5][Zn2(muia)2(H2O)2] (6) To H3muia (0.064 g, 0.2 mmol) in 0.1 M KOH (4 mL), Zn(CH3COO)2  4H2O (0.088 g, 0.4 mmol) in MeOH (2 mL) was added and stirred for 15 min. The solution mixture was left for slow evaporation. Colorless block crystals were obtained after one week and then dried in air. Yield: 0.045 g (70%). Anal. Calc. for C30H38N2O21Zn3: C, 37.58; H, 3.99; N, 2.92. Found: C, 37.97; H, 4.40; N, 3.08%. 1H NMR (D2O): d 7.58 (d, 1H, Ar–H), 6.74 (d, 1H, Ar–H), 6.03 (s, 1H, Ar–H), 3.98 (s, 2H, –CH2NH), 3.33 (q, 4H, – NCH2), 2.40 (s, 3H, Ar–CH3), 2.08 (s, 3H, CH3CN). IR (KBr, cm1): m(OH) 3427; masðCOO Þ 1689; masðCOO Þ 1389; ms(C–O) 1316. Calcd. TG weight loss for 7 H2O 13.2%; found 13.7%. ESI-MS [ZnC15H12NO7] 382.4. UV–Vis (kmax (nm), e(M1 cm1)): (methanol) p–p*, 350(7740); (nujol mull) p–p*, 365. 4.3.7. [Ca(H2O)6][Ca2(muia)2(H2O)2]  2H2O (7) To H3muia (0.064 g, 0.2 mmol) in in 0.1 M KOH (4 mL), Ca(NO3)2 4H2O (0.156 g, 0.2 mmol) in MeOH (2 mL) was added. Yellowish powder is obtained after one week. Yield: 0.036 g (57%). Anal. Calc. for Ca3C30H44N2O24: C, 38.46; H, 4.73; N, 2.99. Found: C, 38.36; H, 4.31; N, 3.13%. 1H NMR was not recorded due to the poor solubility of the compound. IR (KBr, cm1): m(OH) 3415; masðCOO Þ 1701; msðCOO Þ 1390; m(C–O) 1340. Calcd. TG weight loss for 10 H2O 19.2%; found 19.1%. ESI-MS [CaC15H13NO7+H2O] 376.4. UV–Vis (kmax (nm), e(M1 cm1)): (MeOH) p–p*, 374(6540); (nujol mull) p–p*, 343. 4.3.8. [Al2(muia)2(H2O)2]  CH3CN  4H2O (8) To H3muia (0.064 g, 0.2 mmol) in in 0.1 M KOH (4 mL), Al(NO3)3 9H2O (0.074 g, 0.2 mmol) in MeOH (2 mL) was added. Acetonitrile (15 mL) was layered on the resulting colourless clear solution. Colourless platy crystals were formed after one month, however the crystals do not have good diffraction quality. Yield: 0.032 g (57%). Anal. Calc. for Al2C32H39N3O20: C, 45.78; H, 4.68; N, 5.00. Found: C, 45.61; H, 4.29; N, 5.10%. 1H NMR (DMSO-d6): d 7.45 (d, 1H, Ar–H), 6.56 (d, 1H, Ar–H), 5.98 (s, 1H, Ar–H), 4.05 (s, 2H, –CH2NH), –NCH2 (not detected due to overlapping with water), 2.33 (s, 3H,

Ar–CH3). IR (KBr, cm1): m(OH) 3482; masðCOO Þ 1708; msðCOO Þ 1396; m(C–O) 1344. Calcd. TG weight loss for CH3CN and 6H2O 17.8%; found 17.6%. ESI-MS [AlC15H12NO7+H2O] 362.5. UV–Vis (kmax (nm), e(M1 cm1)): (MeOH) p–p*, 336(12 595); (nujol mull) p– p*, 337. 4.4. X-ray crystallography Single crystals X-ray diffraction measurements were carried out on a Bruker AXS APEX CCD diffractometer. Unit cell dimensions were obtained with least-squares refinements, and all structures were solved by direct methods. The program SMART was used for collecting frames of data, indexing reflections and determination of lattice parameters. SAINT [51] for integration of intensity of reflections and scaling; SADABS [52] was used for empirical absorption correction and SHELXTL [53] was used for space group determination, structure solution and least-squares refinements on F2. Selected crystallographic data and refinement details are displayed in Table 2. All the C–H hydrogen atoms were placed in their appropriate calculated positions using riding models. All the hydrogen atoms of the water molecules were located. Their positional parameters were refined using SADI and or DFIX options in SHELXTL along with either individual or common thermal parameters. Acknowledgements We thank the Ministry of Education, Singapore, for the financial support through the National University of Singapore (Grant No. R143–000-252–112). Ms. G.K. Tan and Prof. L.L. Koh of the X-ray facility are thanked for their help in X-ray crystallography. Appendix A. Supplementary material CCDC 696766, 696767, 696768, 696769, 696770 and 696771 contain the supplementary crystallographic data for 1–6. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.ica.2008.09.049.

Table 2 Crystal data and structure refinement details for complexes 1–6

Formula M T (K) Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z l (mm1) Dcalc (mg m3) Reflections collected Independent reflections Rint Goodness-of-fit on F2 Final R [I > 2r(I)], R1a wR2b a b

1

2

3

4

5

6

C16H23CuNO11 468.89 298(2) monoclinic P21/c 11.7256(8) 10.0987(7) 17.245(1) 90 109.758(2) 90 1921.8(2) 4 1.198 1.621 11 018 3388 0.0261 1.061 0.0460 0.1283

C15H21NNiO11 450.04 223(2) orthorhombic Pbca 7.6635(4) 14.9271(8) 30.942(2) 90 90 90 3539.5(3) 8 1.160 1.689 18 933 3104 0.0304 1.277 0.0418 0.0943

C34H46Mn3N4O22 1027.57 223(2) monoclinic P21/c 11.0327(4) 11.2261(4) 17.5320(7) 90 97.871(1) 90 2150.9(1) 2 0.955 1.587 12 397 3780 0.0228 1.141 0.0529 0.1214

C34H46Mg3N4O22 935.68 223(2) monoclinic P21/c 10.913(1) 11.223(1) 17.076(2) 90 95.372(3) 90 2082.2(4) 2 0.164 1.492 11 879 3662 0.0394 1.240 0.0805 0.1671

C63H92Mn6N4O47 1987.05 223(2) triclinic  P1

C30H38N2O21 Zn3 958.73 233(2) monoclinic C2/c 16.9154(9) 11.1158(6) 21.763(1) 90 108.670(1) 90 3876.8(4) 4 1.926 1.643 10 230 3313 0.0321 1.067 0.0519 0.0674

R1 = R||Fo|  |Fc||/R|Fo|. P P wR2 = ½ wðF 2o  F 2c Þ2 = wðF 2o Þ2 1=2 .

11.0083(7) 12.5277(9) 16.104(1) 91.777(2) 108.46(2) 100.321(2) 2063.3(2) 1 0.994 1.599 12 214 7255 0.0390 1.076 0.0731 0.1672

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