Structural Versatility in Copper(II) -Hydroxycarboxylate Complexes

E-36310 Vigo, Spain. ReceiVed October 9, 2007; ReVised Manuscript ReceiVed NoVember 19, 2007. ABSTRACT: Five new complexes, [Cu(L1)2(SS)2] (1), ...
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CRYSTAL GROWTH & DESIGN

Structural Versatility in Copper(II) r-Hydroxycarboxylate Complexes with the Twisted Ligands 4,4′-Dipyridyldisulfide and Bis(4-pyridylthio)methane: From Molecular Compounds to Two-Dimensional Coordination Polymers

2008 VOL. 8, NO. 3 995–1004

Rosa Carballo,*,† Berta Covelo,†,‡ Nuria Fernández-Hermida,† Emilia García-Martinez,† Ana Belén Lago,† and Ezequiel M. Vázquez-López† Departamento de Química Inorgánica, Facultade de Química, UniVersidade de Vigo, E-36310 Vigo, Spain, and Unidade Determinación Estructural e Proteómica, CACTI, UniVersidade de Vigo, E-36310 Vigo, Spain ReceiVed October 9, 2007; ReVised Manuscript ReceiVed NoVember 19, 2007

ABSTRACT: Five new complexes, [Cu(L1)2(SS)2] (1), [Cu(L1)2(SS)2] · H2O (1w1), [Cu2(L1)2(SCS)] · H2O (2w1), 1∞[Cu(L2)2(SS)] · 4H2O (3w1D), and 2∞[Cu(L2)2(SCS)] · 2H2O (4w1D) (where SS ) 4,4′-dipyridyldisulfide, SCS ) bis(4-pyridylthio)methane, L1 ) 2-methyllactate, and L2 ) glycolate), have been synthesized by the reaction of the SS and SCS ligands with the corresponding copper(II) R-hydroxycarboxylates (2-methyllactate and glycolate). The structures of the mononuclear compounds 1 and 1w1, which could be considered as pseudo-polymorphs, show that the solvent molecules are not innocent partners and play a key role in the supramolecular association. Compound 2w1 is a discrete dinuclear molecule in which the SCS ligand acts as bridge between the two copper(II) ions. In these three compounds it seems that the presence of the 2-methyllactate ligand prevents the formation of the expected coordination polymers through the twisted SS and SCS ligands. Compounds 3w1D and 4w1D, which both contain the glycolate ligand, are one- and two-dimensional (1D and 2D) coordination polymers, respectively. In the crystal lattices of the two compounds, two 1D water morphologies are hosted: a treelike chain for 3w1D and a staircase T4(2) tape for 4w1D. In both cases, the dehydration-rehydration processes are investigated. Introduction Research into crystal engineering follows two main directions: coordination networks and molecular materials.1 In the engineering of coordination networks the use of divergent polydentate ligand-metal coordination, as opposed to the traditional convergent coordination chemistry of chelating ligands, extends the coordination network through space in one-, two-, and threedimensional (1D, 2D, and 3D) architectures.2 On the other hand, in molecular crystal engineering the interactions of interest are mainly of the noncovalent type: van der Waals, hydrogen bonds, π-stacking, etc.3 A network material can also be constructed from a combination of coordination bond linkages, which give rise to low dimensionality coordination polymers, and noncovalent interactions to increase the dimensionality. As a consequence it is still a great challenge to predict the exact structure and composition of the assembly products built by coordination bonds and/or noncovalent interactions in crystal engineering. Another remarkable consequence of the intensive research efforts in this field is the possibility of obtaining structural information about discrete and infinite water clusters entrapped within organic synthetic hosts or within metal-organic frameworks.4 In this sense, if a host chain can provide hydrogenbond donors and/or acceptors, it may induce the free water molecules or discrete water clusters to extend with the chain to give 1D or 2D water aggregates.5 Our first aim with this work was to investigate the structural patterns in coordination polymers of copper(II) that increase their dimensionality through hydrogen bonding and/or other weak interactions. To achieve this aim we chose two N,N′-spacer ligands to produce polymerization through coordination bonds, * Corresponding author. Phone: +34 986 812 273. Fax: +34 986 813 798. E-mail: [email protected]. † Departamento de Química Inorgánica, Facultade de Química. ‡ Unidade Determinación Estructural e Proteómica, CACTI.

Scheme 1

that is, 4,4′-dipyridyldisulfide (SS) and bis(4-pyridylthio)methane (SCS) (Scheme 1), in combination with two R-hydroxycarboxylate ligands that are able to introduce weak interactions: 2-methyllactate (L1) and glycolate (L2) (Scheme 2). The introduction of sulfur atoms in the bridge between the two 4-pyridine groups in the spacer ligands restricts the number of possible conformers giving a twisted structure to the spacer ligand. As it has been known for over 50 years acyclic disulfides, like the SS ligand, exist as an equilibrium mixture of two enantiomeric conformers with different helicity6 (P- and Mforms) and idealized torsion angles (C-S-S-C) of 90°

10.1021/cg7009836 CCC: $40.75  2008 American Chemical Society Published on Web 01/23/2008

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Scheme 2

offer the appropriate environment to stabilize two different 1D water morphologies. Results and Discussion

(Scheme 1).7 Compounds of the type RSCH2SR, such as the SCS ligand, exist predominantly in the G+G+ form,8 where the R groups are on opposite sides of the S-C-S plane, and it is also possible to distinguish between two conformers with opposite helicity (Scheme 1). These characteristic twisted structures with implicit helicity in the spacer ligands are interesting in terms of chiral crystal engineering. However, although some metal complexes of these spacer ligands have been found containing chiral structural motifs, chiral crystals have not yet been produced. The second aim of this work was to provide a suitable environment to stabilize the aggregation of crystallization water molecules into discrete clusters or extended morphologies. In this sense, we have previously reported the stabilization of a 2D water morphology entrapped within the 2D coordination polymer [Cu(mal)(SS)] · 6H2O [mal ) malate(-2)].9 We describe here the synthesis and structural characterization of four types of compounds: a 1D coordination polymer, 1 2 ∞[Cu(L )2(SS)] · 4H2O (3w1D); a 2D coordination polymer that also uses the glycolate ligand for the polymerization, 2 2 1 ∞[Cu(L )2(SCS)] · 2H2O (4w1D); a dimeric molecule, [Cu2(L )4(SCS)] · H2O (2w1); and finally the anhydrous (1) and monohydrated (1w1) forms of the monomer [Cu(L1)2(SS)2] (Scheme 3). Surprisingly, the combination of the SS and SCS ligands with 2-methyllactate produces molecular complexes without crystallization water molecules or with isolated ones. When SS and SCS are combined with glycolate the degree of covalent polymerization is different, but in both cases the crystal hosts

Compounds 1, 1w1, 2w1, and 3w1D (Scheme 3) were prepared at room temperature by slow diffusion of ethanol solutions of the N,N′-ligands into aqueous solutions of the previously prepared copper(II) R-hydroxycarboxylate compounds. In the diffusion of SS into [Cu(L1)2(H2O)2] both 1 (in low yield) and 1w1 were obtained simultaneously as blue and green crystals, respectively. Attempts to prepare these compounds by reaction of the solutions at room temperature or under reflux afforded an insoluble mixture of powders, and the slow evaporation of the mother liquors led to a certain degree of decomposition of the N,N′-ligand. The preparation of 4w1D was only possible by solvothermal procedures at 120 °C, using [Cu(L2)2] or a mixture of CuCO3 · Cu(OH)2 and glycolic acid. The use of a higher temperature led to a powder mixture that also showed signs of N,N′-ligand decomposition. This mixture is currently undergoing further investigation. On using the copper(II) glycolate precursor coordination polymers (3w1D and 4w1D) were always obtained with the two N,N′-ligands, but with 2-methyllactate the SS ligand produced only monomers (1 and 1w1) and the SCS ligand (acting as bridge) gave rise to a binuclear species (2w1). The previously reported complexes with the SCS10 and the SS9,11,12 ligands are all coordination polymers with the exception of the mononuclear manganese compound: [Mn(dca)2(SS)2(H2O)2] [dca ) dicyanamide(-1)].12 Structural Studies Molecular Complexes 1, 1w1, and 2w1. Selected interatomic distances and angles and the main hydrogen bonds are listed in Tables 1 and 2, respectively. The coordination environments of the copper atoms are represented in Figure 1, together with the atom numbering scheme used, for compounds 1 and 2w1.

Scheme 3

Copper(II) R-Hydroxycarboxylate Complexes

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Table 1. Selected Interatomic Bond Lengths (Å) and Angles (°)a 1 Cu1-O11 Cu1-O13 Cu1-N1 Cu1-O21 Cu1-O12i Cu1-O23 Cu2-N2 Cu2-O31 Cu2-O33 Cu2-O41 Cu2-O43 O11-Cu1-N1i/N2 O11-Cu1-N1 O11-Cu1-O13 N1-Cu1-O13 O11-Cu1-O13i N1-Cu1-O13i O11-Cu1-O11i N1-Cu1-N1i/N2 O21-Cu1-N1 N1-Cu1-O12i O11-Cu1-O12i O13-Cu1-O13i O21-Cu1-O12i O21-Cu1-O11 O21-Cu1-N2 N2-Cu1-O12i O21-Cu1-O23 O11-Cu1-O23 O23-Cu1-N1 O21-Cu1-O13 O23-Cu1-O13 O31-Cu2-O41 O31-Cu2-O33 O41-Cu2-O33 O31-Cu2-N2 O41-Cu2-N2 O33-Cu2-N2 O31-Cu2-O43 O41-Cu2-O43 O33-Cu2-O43 N2-Cu2-O43 a

1.960(2) 2.278(2) 2.067(3)

1w1 1.9562(19) 2.277(2) 2.057(3)

2w1 1.935(2) 2.214(3) 2.014(3) 1.903(2)

3w1D

4w1D

1.998(4) 2.293(4) 2.017(5)

2.283(3)

1.975(3) 2.023(3) 1.897(2) 1.990(2) 1.928(2) 2.195(2) 88.65(11) 91.35(11) 76.43(9) 90.31(11) 103.57(9) 89.69(11)

91.65(9) 88.35(9) 76.88(8) 90.50(9) 103.12(8) 89.50(9)

95.02(10) 77.56(9) 99.18(11)

2.033(3) 1.959(3) 1.976(3) 2.010(3)

172.33(17) 89.21(16) 75.80(15) 98.65(17) 89.00(15) 97.02(17) 93.3(2) 89.2(3)

91.22(10)

95.18(13) 86.42(12)

178.21(15) 90.91(13) 90.90(13) 89.17(11)

157.9(2) 172.78(10)

175.99(12) 94.52(11) 88.17(13) 89.92(12)

81.72(10) 91.17(10) 158.99(11) 105.04(10) 101.76(12) 171.46(10) 81.10(10) 91.73(10) 90.67(11) 94.06(10) 155.60(11) 107.69(10) 77.93(9) 99.32(11) 105.06(11)

Symmetry code i: 1 - x, 1 - y, 1 - z; -x, 1 - y, -z; -x, y, 3/2 - z and x - 1, y, z for 1, 1w1, 3w1D, and 4w1D, respectively.

The structures of 1 and 1w1 are based on mononuclear neutral complexes [Cu(L1)2(SS)2], where L1 and SS are monoanionic O,O′-bidentate chelating and monodentate terminal ligands, respectively. Each one of the SS ligand in the complex presents opposite helicity. Each copper atom has a distorted elongated octahedral environment with the largest deviations from the ideal geometry in the chelate bite angle. The compounds exhibit the expected Jahn-Teller distortion; with four short metal-ligand bonds (Cu-N and Cu-carboxylato) and two long ones (Cu-hydroxyl). In both compounds the configuration around the metal atom can be described in the CPI system as OC-612,13 which has only been observed in some copper(II) R-hydroxycarboxylate coordination polymers with 4,4′-bipyridine14 and 1,2-bis(4-pyridyl)ethane.15 Compound 1 is anhydrous and 1w1 contains a disordered water molecule of crystallization in the asymmetric unit. The two mononuclear species have very similar interatomic bond lengths and angles, but some differences are detected in the intramolecular geometry of the SS ligands. Although the S-S bond distances are statistically equivalent in both compounds [2.033(2) Å in 1 and 2.030(1) Å in 1w1], significant differences are apparent in the values of the C-S-S-C torsion angles: 97.2(2)° in 1 and 83.9(2)° in 1w1. The former angle has the highest value found in reported metal complexes of the SS ligand,11,12 whereas the latter falls within the range usually observed in free aromatic disulfides16

and in metal complexes of SS ligand.11,12 Both complexes present weak C-H · · · S intramolecular hydrogen bonds with C · · · S distances and C-H · · · S angles in the ranges 3.21–3.31 Å and 111–113°, respectively. In compound 2w1 the asymmetric unit is composed of the neutral binuclear complex [Cu2(L1)4(SCS)] and a water molecule of crystallization. The two copper atoms are pentacoordinated, and the L1 ligand shows the same behavior as in 1 and 1w1. However, the SCS ligand acts as an N,N′-bis(monodentate) bridging ligand. The resulting coordination polyhedra are distorted square pyramids (Addison’s τ values:17 0.23 for Cu1 and 0.26 for Cu2). The bond distances are very similar for the two metallic centers, and in both cases one hydroxyl group of one of the anionic ligands is located at the apex of the square pyramid. In the SCS ligand the C-S-C-S torsion angles of 62.5(2)° and 59.8(2)° are smaller than those found in the free ligand and in the polymeric complexes [CuX2(SCS)2] (X ) Cl, Br), which have values in the range 63.2–76.9°.10 The Cu1 · · · Cu2 distance of 11.513(2) Å is also shorter than the distance between two adjacent copper(II) atoms linked by SCS in the previously reported halo-complexes.10 As expected, the 2-methyllactate ligand plays a key role in the supramolecular arrangement in the three molecular compounds. It can be seen from Figure 2 in compounds 1 and 1w1 moderate OH · · · OCO hydrogen bonds (Table 2) are formed

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Table 2. Main Hydrogen Bond Distances (Å) and Angles (°) d(D-H) d(H · · · A) d(D · · · A) ∠(DHA)

D-H · · · A a

1

1w1

O13-H13 · · · O12 C2-H2 · · · N2ii O13-H13 · · · O12i O1w · · · O12ii O1w · · · O12iii O23-H230 · · · O42i O43-H430 · · · O22ii O13-H130 · · · O32iii O33-H330 · · · O12iv O1w · · · O12iv C106-H10A · · · O11v C106-H10B · · · O41vi O13-H13 · · · O11i O1w · · · O2w O2w · · · O3w O3w · · · O3wii O1w · · · O12i O3w · · · O12i O13-H6A · · · O12i O23-H3A · · · O22ii O1w · · · O2w O1w · · · O2wiii O2w · · · O1wii O2w · · · O23 i

b

2w1c

3w1Dd

4w1De

0.81(4) 0.93 0.87(3)

1.87(4) 2.56 1.81(3)

0.76(3) 0.87(4) 0.75(4) 0.74(4)

1.92(4) 1.83(4) 2.10(4) 1.89(4)

0.97 0.97 0.82(6)

2.32 2.36 1.91(6)

0.76(5) 0.86(6)

2.10(5) 1.88(6)

2.681(3) 3.287(5) 2.683(3) 2.745(5) 2.737(6) 2.683(3) 2.682(4) 2.825(4) 2.617(3) 3.032(10) 3.162(4) 3.192(4) 2.734(5) 2.708(16) 2.61(2) 2.51(2) 2.777(6) 2.992(16) 2.828(5) 2.707(6) 2.524(9) 2.787(9) 2.903(8) 2.569(8)

176(4) 135.6 179(3) 173(4) 165(4) 164(4) 164(5) 145.3 143.9 174(6)

161(6) 160(6)

a i ) x - 1, y, z. ii ) -x, 1 - y, -z. b i ) x, y - 1, z; ii ) -x, 1 y, -z; iii ) x + 1, y - 1, z. c i ) x, y, z + 1; ii ) x - ½, ½ - y, z ½; iii ) x + ½, ½ - y, z + ½; iv ) x, y, z - 1; v ) 1 - x, -y, 2 - z; vi ) 1 - x, -y, 1 - z. d i ) x, y, z + 1; ii ) x - ½, ½ - y, z - ½; iii ) x + ½, ½ - y, z + ½; iv ) x, y, z - 1; v ) 1 - x, -y, 2 - z; vi ) 1 - x, -y, 1 - z. e i ) x - 1, y, z; ii ) x + 1, y, z; iii ) 1 - x, -y, -z.

Figure 1. Molecular structures of 1 (A) and 2w1 (B), showing the atom labelling of the coordinating atoms. Hydrogen atoms have been omitted for clarify.

between the 2-methyllactate OH groups and uncoordinated carboxylate atoms of neighboring molecules to give chains that run along the a-axis in 1 and along the b-axis in 1w1. The same kind of hydrogen bond in 2w1 associates each binuclear molecule with four neighboring ones (two of the same helicity in SCS and the other two of opposite helicity) (Figure 3A) leading to the formation of an undulating sheet-type structure, as shown in Figure 3B. The O · · · O distances are in the range 2.617–2.825 Å with almost linear O-H · · · O angles of 164–179°. The chains of 1 are interconnected by CH · · · N hydrogen bonds involving the uncoordinated N atoms of the SCS ligands

Figure 2. Molecular chains created by OH · · · OCO hydrogen bonds in 1 and 1w1.

(Figure 4A), generating a 2D association that is reinforced by π stacking interactions between the [N2/C8/C7/C6/C10/C9] rings of adjacent chains in a face-to-face mode with a centroidto-centroid distance of 3.729 Å and an interplanar dihedral angle of 0.02°, which is indicative of the parallelism of the aromatic rings. The most significant contact found between these sheets is a weak CHring · · · S hydrogen bond, with a C · · · S distance of 3.59 Å. In the monohydrated compound 1w1, the water molecule causes a different organization of the hydrogen bonded chains, which are similar to those in compound 1. In this case there are no CH · · · N hydrogen bonds or π-π interactions between the chains. The water molecule connects the chains, as shown in Figure 5A, through strong Ow · · · OCO hydrogen bonds (Table 2) involving the uncoordinated carboxylate atoms O12 of two neighboring chains. The 2D arrangement is reinforced by weak CH · · · Ow hydrogen bonds with one of the methyl groups of neighboring chains, with (C · · · Ow) distances of around 3.33 Å. These planar structures are themselves linked by C-H · · · π interactions18 between one methyl group of the 2-methyllactate ligand and the terminal ring of the SS ligand in such a way that each terminal ring is sandwiched between one methyl group and another terminal ring of an adjacent molecule, as represented in Figure 5B. The CHring · · · centroid distance is 3.504 Å with a C-H-centroid angle of 134.6° and the CHmet · · · centroid distance is 3.794 Å with a C-H-centroid angle of 112.8°. In 2w1 the undulating sheets are held together by CH · · · O hydrogen bonds (Table 2) between the -CH2- group of the SCS ligand and two metal-coordinated oxygen atoms (O11 and O41) belonging to two different molecules in an adjacent sheet (Figure 6A): in this way the hollow of a sheet is occupied by the convex section of the neighboring sheet. The C · · · O distances are 3.16 and 3.19 Å and the C-H · · · O angles are close to 145°. The resulting 3D network is further stabilized by π-π interactions (Figure 6A) between the aromatic rings of the units linked by the CH · · · O hydrogen bonds. The centroid-to-centroid distance is 3.531 Å for the two aromatic interactions, and the values of the dihedral angles (0.02 and 0.00°, respectively) are indicative of the parallelism of the rings. The water molecule of crystallization does not seem to take part in the supramolecular arrangement of the molecules and is only weakly hydrogen bonded (3.032 Å) to the uncoordinated oxygen atom O12. The 3D network down the c axis is shown in Figure 6B, with the water molecules trapped in the resulting voids of the supramolecular architecture of 2w1.

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Figure 3. (A) View of the OH · · · OCO hydrogen bonding pattern in the ac plane for 2w1. (B) View down the a-axis of the undulating sheet for 2w1.

Figure 4. (A) View in the bc plane of the 2D supramolecular arrangement due to CH · · · N hydrogen bonds in 1. (B) View of the packing of 1 in the bc plane.

Coordination Polymers 3w1D and 4w1D. The coordination environments of the copper atoms in compounds 3w1D and 4w1D are represented in Figure 7, together with the atom numbering scheme for coordinating atoms. The structures are based on polymeric neutral complexes [Cu(L2)2(SS)] or [Cu(L2)2(SCS)] and water molecules of crystallization. The L2 ligand presents different coordinative behavior in the two compounds: in 3w1D it acts as an O,O′-bidentate chelating ligand and in 4w1D as a monodentate ligand and also as a bis(monodentate) bridge, which creates a polymeric chain along the crystallographic a-axis. In both cases the SS and SCS ligands bridge between neighboring copper(II) atoms to form polymeric chains, which run along the crystallographic c-axis in 3w1D and along the

b-axis in 4w1D. As a result, 3w1D is a 1D coordination polymer, and 4w1D shows a 2D polymeric structure through coordination of the two ligands. In 3w1D the copper atom has a distorted, axially elongated octahedral environment showing the expected Jahn-Teller distortion in the long Cu-hydroxyl distances of 2.293(4) Å. The Cu · · · Cu distance across the SS ligand is 10.1694(12) Å, which coincides with the length of the unit cell along the c-axis. In 3w1D the configuration around the copper(II) ion can be described with the index [OC-6-33],13 which is usually found in R-hydroxycarboxylate complexes with coordinated water19 or N,N′-chelating aromatic diamines20 but is unusual for coordination polymers containing R-hydroxycarboxylate and N,N′-linear spacer ligands (either flexible or rigid).14,15 Several interesting comparisons between the polymer 3w1D and compounds 1 and 1w1 can be made: (i) the C-S-S-C torsion angle of 82.8(4)° in 3w1D is in the range found in 1w1 and in other metal complexes with SS11,12 but is again very different to the high value found in 1; (ii) in the mononuclear compounds the SS ligands are trans but in 3w1D they are cis, and this coordination arrangement defines a dissymmetric (chiral) coordination sphere (Λ-enantiomer in Figure 7A, top); (iii) Each Λ- or ∆- form of the complexes selects different helicity in the dissymmetric SS ligand (M- or P-). For instance, the chain shown in Figure 7A (bottom) is constituted by the combination of the Λ-coordination sphere and the M-conformation of the SS ligand. However, both kinds of chains are present in the crystal, as imposed by the centrosymmetric space group C2/c. The coordination polyhedron in 4w1D is an almost ideal square pyramid with an Addison’s τ value17 of 0.04. In the SCS ligand the values for the C-S-C-S torsion angles are 76.6(3) and 69.6(3)°, which are in the range reported for other polymeric metal complexes9 and higher than the value discussed previously for 2w1. Each sheet incorporates molecules of the SCS ligand with the same helicity. The Cu · · · Cu distance across the SCS ligand is 13.1241(14) Å, which is higher than that found in 2w1

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Figure 5. (A) View in the ab plane of the 2D organization generated by the Ow · · · OCO hydrogen bond in 1w1. (B) Molecular packing in 1w1 showing the CHring · · · π and CHmet · · · π interactions.

Figure 6. (A) View of the linkage through CH · · · O hydrogen bonds of the undulating sheets in 2w1. (B) Molecular packing in 2w1 (in the ab plane), with lattice water molecules shown in a space-filling representation.

but very close to that reported for the 1D coordination polymers [CuX2(SCS)2] (X ) Cl, Br).9 The distance between two adjacent copper(II) atoms linked by the glycolate ligand is 4.8474(6) Å.

In the crystal structure of 3w1D each chain interacts with two adjacent chains by means of strong hydrogen bonding interactions (Figure 8B) involving the hydroxyl group and the oxygen atoms of the carboxylate group. This particular OH · · · OCO hydrogen bond differs from those in the other compounds reported here because in this case the carboxylate oxygen involved in the interaction is coordinated. This interaction creates a 2D arrangement in which in the ab crystallographic plane (Figure 8D) shows a central edge containing the glycolate ligands connected by OH · · · OCO hydrogen bonds, with the SS ligands arranged in an alternating fashion at the two sides of this edge. The crystallization water molecules are inserted into the interlayer space (Figure 8A,D), and they are linked to one another to form 1D water chains through hydrogen bonding. The volume corresponding to these water channels is ca. 18.9% of the unit cell.21 The O1w and O2w molecules are located on the 2-fold rotation axis. As shown in Figure 8C, with O1w and equivalent molecules dangling at two sides, the water molecules are self-assembled into treelike chains along the c-axis. Thus, the water chains can be considered as “glue” between the 2D units that form a 3D supramolecular structure through hydrogen bonding interactions: one strong bond between the dangling molecule O1w and the uncoordinated carboxylate oxygen atom O12 and another weak bond between O3w and the same carboxylate oxygen (Table 3 and Figure 8C). The O2w molecule does not have any supramolecular interaction with the metal-organic layers. The water chains are separated far from each other (>10 Å) by the hydrophobic domains created by the SS ligand (Figure 8D). The OH · · · OCO hydrogen bonding interactions between the hydroxyl groups and the uncoordinated carboxylate oxygen atoms (Table 3) are also present in the structure of 4w1D, but in this case they are intralayer interactions that do not contribute to the increase in the dimensionality of the coordination polymer. The layers are linked in a 3D network by hydrogen bonding

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Figure 7. View of the coordination environment of copper ions and the coordination polymer in 3w1D (A) and 4w1D (B).

Figure 8. View of the 3D supramolecular structure in 3w1D: (A) down the a-axis with the undulating water chains represented in space filling fashion, (B) OH · · · OCO hydrogen bond linkage between chains, (C) “tree like water chain” with dangling molecules, (D) down the c-axis with the water chains in a space filling representation.

with the water molecules of crystallization (Figure 9), an interaction that connects sheets of opposite helicity. Closer examination of the packing reveals the presence of channels, approximately 8 × 7 Å wide, that run along the a-axis. The volume corresponding to these channels is ca. 11.3% of the unit cell.21 The two crystallographically independent free water molecules and their equivalents are associated by Ow · · · Ow hydrogen bonds in a tape of 4-membered rings with two shared water molecules; this arrangement can be defined as a T4(2) water tape (Figure 9). This water aggregate motif does not seem to be usual when compared with the T4(1) motif frequently

found in several metal-organic22 and organic hosts.23 The angle between the four-membered rings is 126.4°, which makes the water tape “staircase”-like (Figure 9). The O · · · O distance of the shared edge is 2.787(9) Å, which is intermediate between the other distances. A strong O2w · · · O23 hydrogen bond involving the hydroxyl group of the monodentate glycolate anchors the water tapes to the metal-organic framework. Thermal Analysis and X-ray Powder Diffraction for 3w1D and 4w1D. Analysis of the crystal structures of 3w1D and 4w1D shows that the water chains are located in channels that could allow the release of the water molecules. A TGA study

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Table 3. Crystal and Structure Refinement Data 1 empirical formula formula weight crystal system space group a (Å) b (Å) c (Å) R (°) β (°) γ (°) V (Å3) Z Fcalc (g cm-3) absorption coeff (mm-1) F(000) crystal size (mm3) θ range (°) reflections collected indep reflections (Rint) max/min transmission goodness-of-fit on F2 final R indices [I > 2σ(I)] final R indices (all data)

C28H30CuN4O6S4 710.34 triclinic P1j 5.7682(14) 9.561(2) 14.401(3) 85.600(5) 84.584(5) 73.549(4) 757.3(3) 2 1.558 1.046 367 0.28 × 0.16 × 0.11 1.42–27.99 4728 3303 (0.0259) 1.0000/0.7399 0.966 R1 ) 0.0465 wR2 ) 0.1222 R1 ) 0.0701 wR2 ) 0.1303

1w1

2w1

3w1D

4w1D

C28H32CuN4O7S4 728.36 monoclinic P21/n 11.7394(11) 5.7710(6) 23.827(2)

C27H38Cu2N2O13S2 789.79 monoclinic P21/n 12.834(2) 16.624(3) 16.302(3)

C14H22CuN2O10S2 506.00 monoclinic C2/c 9.4326(11) 23.545(3) 10.1694(12)

C15H20CuN2O8S2 483.99 monoclinic P21/n 4.8474(6) 25.814(3) 15.3589(18)

102.163(2)

94.421(4)

116.783(2)

92.348(2)

1578.0(3) 4 1.533 1.008 754 0.21 × 0.17 × 0.14 1.75–28.01 8976 3607 (0.0522) 1.0000/0.7873 0.812 R1 ) 0.0441 wR2 ) 0.0791 R1 ) 0.1061 wR2 ) 0.0919

3467.8(11) 4 1.513 1.410 1632 0.54 × 0.19 × 0.16 1.75–28.13 20427 7980 (0.0513) 1.0000/0.7762 0.869 R1 ) 0.0433 wR2 ) 0.0894 R1 ) 0.0924 wR2 ) 0.0994

2016.2(4) 4 1.667 1.346 1044 0.25 × 0.11 × 0.06 1.73–28.02 6033 2333 (0.0550) 1.0000/0.8649 1.136 R1 ) 0.0756 wR2 ) 0.1375 R1 ) 0.1165 wR2 ) 0.1501

1920.3(4) 4 1.674 1.401 996 0.20 × 0.18 × 0.17 1.54–28.08 12617 4621 (0.1070) 1.0000/0.7611 0.759 R1 ) 0.0518 wR2 ) 0.0775 R1 ) 0.1404 wR2 ) 0.0933

of 3w1D shows a weight loss of 9.47% from RT to 81 °C, which corresponds to 2.66 of the four free water molecules. The remaining water molecules are eventually lost at 138 °C. In an effort to confirm the TGA results, the original and the dehydrated samples of 3w1D were characterized by X-ray powder diffraction (XRPD) at room temperature (Figure 10). After water molecules were lost, the XRPD pattern shows significant differences compared with that of the original material, suggesting decomposition of the metal-organic framework. On leaving the dehydrated material for 5 days in a chamber saturated with water vapor, it completely reverted to the hydrated form (Figure 10C). A TGA study of 4w1D shows a weight loss of 1.80% from RT to 65 °C, which corresponds to 0.5 of the two free water molecules. The next weight loss occurred at 139 °C, and this

corresponds to loss of the remaining water molecules. At this temperature the metal-organic framework also begins to decompose. Powder X-ray diffractograms before and after the expulsion of water at 120 °C under vacuum reveal only marginal differences in the diffraction patterns and intensities. This finding suggests that the integrity of the host lattice is robust to the exclusion of water. The dehydrated sample was testified by the weight loss (8.07%; calcd. 7.43%). When a dehydrated sample is exposed to H2O vapor for 3 days, the hydrated form is obtained. Conclusions The structural analysis of five copper(II) R-hydroxycarboxylate complexes derived from the N,N′-twisted spacers 4,4′-

Figure 9. View of the crystal packing along the a direction of 4w1D, showing a fragment of the hydrogen-bonded tape of water molecules.

Copper(II) R-Hydroxycarboxylate Complexes

Crystal Growth & Design, Vol. 8, No. 3, 2008 1003

conformation of the SS ligand). In 4w1D molecules of the SCS ligand of the same helicity are incorporated in each sheet. Experimental Section

Figure 10. X-ray powder diffraction: (a) initial 3w1D, (b) dehydrated solid obtained on heating at 120 °C for 1 h, and (c) rehydrated solid, obtained by exposure of the hydrated solid to water vapor.

dipyridyldisulfide and bis(4-pyridylthio)methane revealed several interesting points, which are summarized as follows: (i) The flexibility of the two N,N′-twisted ligands in these complexes is restricted to a narrow range in the value of the torsion angle between 82 and 97° for 4,4′-dipyridyldisulfide and between 62 and 77° for bis(4-pyridylthio)methane. (ii) The dimensionality obtained through coordination of the resulting compounds seems to be dependent on the bulkiness of the R-hydroxycarboxylate ligand used. As a result, molecular complexes are obtained with 2-methyllactate and coordination polymers with the smaller glycolate, which also acts as a bridge between the copper ions in compound 4w1D. (iii) In the five compounds studied both R-hydroxycarboxylate ligands play a key role in the supramolecular organization through hydrogen bonding involving their hydroxyl and carboxylate functions. (iv) The molecular complexes show only a small degree of hydration and it was possible to isolate the same compound in both the anhydrous and monohydrated form. This clearly demonstrates the non-innocent role of the water of crystallization in the supramolecular organization. (v) The metal-organic frameworks in the 1D and 2D coordination polymers offer an appropriate environment to stabilize two 1D water morphologies: a treelike chain and a staircase-like T4(2) tape. Both metal-organic frameworks proved to be sufficiently robust to undergo reversible water release/uptake processes. (vi) In 3w1D monochiral elements form chains that show specific selectivity between the enantiomer corresponding to the coordination sphere and one of the two possible conformations of the SS ligand (i.e., Λ-coordination sphere with an M-

Materials and Physical Measurements. Bis(4-pyridylthio)methane was prepared as described previously.10 The other starting materials and solvents were obtained commercially and were used as supplied. Elemental analyses (C, H, N, S) were carried out with a Fisons EA1108 microanalyser. Melting points (m.p.) were measured with a Gallenkamp MBF-595 apparatus. IR spectra were recorded from KBr discs (4000–400 cm-1) on a Bruker Vector 22 spectrophotometer. A Shimadzu UV-3101PC spectrophotometer was used to obtain electronic spectra in the solid state. Magnetic susceptibility measurements were performed at room temperature using a Johnson Matthey. Alfa MSBMK1 Gouy balance. TGA analysis was carried out using a TA Instruments Hi-Res TGA2950 Thermobalance coupled to FT-IR Bruker Tensor 27 apparatus. Powder X-ray diffraction data were obtained using Cu-KR radiation and were collected on a Siemens D-5000 diffractometer over the range 5.0–45.0° in steps of 0.20° (2θ) with a count time per step of 5.0 s. Synthesis of the Precursors [Cu(L1)2(H2O)2] and [Cu(L2)2]. The precursors [Cu(L1)2(H2O)2] and [Cu(L2)2] were obtained by the reaction of Cu(AcO)2 · H2O and the corresponding R-hydroxycarboxylic acid (2methyllactic or glycolic acid) in 1:2 molar proportions in water. The resulting blue solutions were heated for 15 min and stirred at room temperature for several days. Blue crystalline products were obtained by slow concentration of the solution. These compounds were previously studied by Prout et al.24 Synthesis of the Complexes. [Cu(L1)2(SS)2] (1) and [Cu(L1)2(SS)2] · H2O (1w1). A yellow solution of 4,4′-dipyridyldisulfide (55 mg, 0.25 mmol) in 2 mL of ethanol was slowly added to a blue solution of [Cu(L1)2(H2O)2] (67 mg, 0.25 mmol) in 2 mL of water in a test tube. The reagents were allowed to mix slowly by diffusion to form blue and green crystals of 1 and 1w1, respectively. Data for (1): Yield: 8%. m.p. 171 °C. Anal. calc. for C28H30N4O6S4Cu: C 47.34, H 4.26, N 7.89, S 18.05%; Found: C 46.74, H 4.08, N 7.58, S 17.35%. IR (KBr, cm-1): 3445s,b; 1591vs, 1564s, 1402m, 1383s, 1178m, 824m, 799m, 714m, 507w. UV-vis (λmax, cm-1): 14660. µ (R.T.): 1.77 MB. Data for (1w1): Yield: 20%. m.p. 172 °C. Anal. calc. for C28H32N4O7S4Cu: C 46.17, H 4.43, N 7.69, S 16.61%; Found: C 46.01, H 4.22, N 7.79, S 17.02%. IR (KBr, cm-1): 3425vs, 1593vs, 1543sh, 1421m, 1388m, 829m, 715m, 505w. UV-vis (λmax, cm-1): 14600. µ (R.T.): 1.70 MB. [Cu2(L1)4(SCS)] · H2O (2w1). Compound 2w1 was synthesized using a similar procedure to that for 1 except that bis(4-pyridylthio)methane (59 mg, 0.25 mmol) was used instead of 4,4′-dipyridyldisulfide. The same compound was obtained when metal/ligand molar ratios of 1:2 and 2:1 were used. Yield: 86%. m.p. 200 °C (decomposition). Anal. calc. for C27H40S2O13N2Cu2: C 40.96, H 5.09, N 3.54, S 8.10%; Found: C 41.45, H 4.97, N 3.65, S 8.09%. IR (KBr, cm-1): 3445b, 1634s, 1599s, 1484m, 1382m, 1224m, 739m, 495m. UV-vis (λmax, cm-1): 11495. µ (R.T.): 1.85 MB. [Cu(L2)2(SS)].4H2O (3w1D). Compound 3w1D was synthesized using a procedure similar to that for 1 except that [Cu(L2)2] (54 mg, 0.25 mmol) was used instead of [Cu(L1)2(H2O)2]. Yield: 34%. m.p. 180 °C. Anal. calc. for C14H22S2O10N2Cu: C 33.23, H 4.38, N 5.54, S 12.67%; Found: C 34.01, H 3.28, N 5.71, S 13.19%. IR (KBr, cm-1): 3421vs,b; 3161s, 1599s, 1418m, 1354s, 1296m, 1061s, 824m, 716s, 509w. UVvis (λmax, cm-1): 14280. µ (R.T.): 1.65 MB. [Cu(L2)2(SCS)] · 2H2O (4w1D). A mixture of CuCO3Cu(OH)2 (230 mg, 1 mmol), glycolic acid (153 mg, 2 mmol), and bis(4-pyridylthio)methane (117 mg, 0.5 mmol) in water (10 mL) and ethanol (10 mL) was sealed in a Teflon-lined stainless steel autoclave and heated at 120 °C for 42 h under autogenous pressure and then cooled to room temperature immediately. Blue crystals were isolated in 35% yield after the slow evaporation of the resulting solution. Compound 4w1D was also obtained by a similar solvothermal procedure but using [Cu(L2)2] and bis(4-pyridylthio)methane in a 1:1 molar ratio in water (10 mL) and ethylene glycol (5 mL), heating at 120 °C for 4 h and then cooling to room temperature for 1 d. In this case blue crystals were isolated in 21% yield. m.p. 180 °C. Anal. calc. for C15H20S2O8N2Cu: C 37.22, H 4.17, N 5.79, S 13.25%; Found: C 37.93, H 3.70, N 5.77, S 13.68%.

1004 Crystal Growth & Design, Vol. 8, No. 3, 2008 IR (KBr, cm-1): 3406s, 1620s, 1590s, 1538m, 1487m, 1449m, 1228m, 732m, 505m. UV-vis (λmax, cm-1): 15100. µ (R.T.): 1.67 MB. Crystallography. Crystallographic data were collected on a Bruker Smart 1000 CCD diffractometer at 293 K using graphite monochromated Mo-KR radiation (λ ) 0.71073 Å) and were corrected for Lorentz and polarization effects. The frames were integrated with the Bruker SAINT25 software package, and the data were corrected for absorption using the program SADABS.26 The structures were solved by direct methods using the program SHELXS97.27 All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix leastsquares calculations on F2 using the program SHELXL97.28 Hydrogen atoms were inserted at calculated positions and constrained with isotropic thermal parameters, except for the hydrogen atoms of the hydroxyl groups which were located from a difference Fourier map and refined isotropically (in some cases the O-H distance was restrained with DFIX 0.9). The hydrogen atoms for the crystallization water molecules could not be located. Hydrogen-bonding was inferred by considering the O · · · O close contacts of water molecules involved (O · · · O distances less than 3.04 Å).29 Drawings were produced with MERCURY,30 and special computations for the crystal structure discussions were carried out with PLATON.21 Crystal data and structure refinement data are listed in Table 3. The structural data have been deposited with the Cambridge Crystallographic Data Centre (CCDC) with reference numbers 654587 (4w1D) 654588 (3w1D), 654589 (2w1), 654590 (1) and 654591 (1w1).

Acknowledgment. Financial support from ERDF (EU), MEC (Spain), and Xunta de Galicia (Spain) (research projects CTQ2006-05642/BQU and PGIDIT06-PXIB314373PR) are gratefully acknowledged. We thank Prof. A. Castiñeiras (Univ. Santiago de Compostela, Spain) for the use of facilities for the TGA and diffuse reflectance measurements.

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CG7009836