Polymorphs or solvates? Coordination of 3,5-dihydroxybenzoate to copper and zinc metal centers

Inorganica Chimica Acta 394 (2013) 729–740

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Polymorphs or solvates? Coordination of 3,5-dihydroxybenzoate to copper and zinc metal centers Linsheng Feng a, Zhichao Chen a, Matthias Zeller b, Rudy L. Luck a,⇑ a b

Department of Chemistry, Michigan Technological University, 1400 Townsend Drive, Houghton, MI 49931, United States Department of Chemistry, Youngstown State University, 1 University Plaza, Youngstown, OH 44555, United States

a r t i c l e

i n f o

Article history: Received 30 May 2012 Received in revised form 22 September 2012 Accepted 27 September 2012 Available online 12 October 2012 Keywords: Solvates 3,5-Dihydroxylbenzate ligand Coordination polymer Crystal structures Copper and zinc

a b s t r a c t Infinite polymeric 1D chains of [Cu(3,5-dhb)(pyridine)2]n (1a and 1b) and [Cu2(3,5-dhb)4]n (2) were prepared by reacting copper acetate hydrate with 3,5-dihydroxybenzoic acid in the molar ratios of pyridine to Cu(II) of 5:1 and 1.1:1, respectively. In contrast, a zinc monomer Zn(3,5-dhb)2(pyridine)2, (3), was obtained under similar reactant conditions to the synthesis of 1a, but with zinc acetate. In compounds 1, the copper centers are coordinated in a near octahedral fashion containing four 3,5-dhb ligands and two pyridine molecules. Interestingly, the Cu containing molecules in compound 1a share the same formula with compound 1b, but they are solvates with different conformations. The structure of 2 revealed a paddlewheel dicopper core bridged by four carboxylate groups in a syn,syn mode. Each copper center is coordinated in a square pyramidal fashion. In contrast, the geometry around the zinc center is distorted tetrahedral and consists of two monodentate 3,5-dhb and two pyridine ligands. Compounds 1–3 were characterized by elemental analyses, IR, UV–Vis and single crystal X-ray diffraction methods. Compounds 1a and 1b were further studied by thermogravimetric and differential scanning calorimetry (DSC) analyses and X-ray powder diffraction experiments. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction The rational design and synthesis of coordination polymers have drawn a great deal of interest in the past decades not only because of their interesting structural topologies, but also their potential industrial applications, such as in the field of magnetism, catalysis, gas storage and separation, photoluminescence, and shape-selective absorption [1–13]. The syntheses of coordination polymers proceed primarily via self-assembly of metal ions and functional ligands, constructed mainly through covalent bonds [14–17]. More recently, strategies combining covalent bonds and non-covalent interactions utilizing H-bonds and p–p stacking have been utilized [18–22]. Hydrogen bonds are commonly used as a structure directing force to construct coordination polymers as these materials may have better solubility and flexible structures due to the existence of weak interactions [23]. Although a lot of energy has been expended in exploring effective synthetic strategies for the preparation of coordination polymers with predictable structures [24–26] it is still a great challenge to predict the structures of coordination polymers at this stage. The structures of coordination polymers play a vital role in their potential applications and are greatly influenced by many factors, such as solvent ⇑ Corresponding author. Tel.: +1 906 487 2309; fax: +1 906 487 2061. E-mail address: [email protected] (R.L. Luck). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.09.036

systems, topologies of metal compounds and organic ligands, the ratio of metal to ligand, reaction temperature and the pH of the solution [24,27–31]. Among coordination polymers, carboxylate anions have been used as bridging ligands due to their various coordination modes, such as syn–syn, anti–anti, syn–anti [32–35] and mono–monodentate and mono–bidentate [34,36,37]. In contrast to the extensively studied polycarboxylates or nitrogen functionalized carboxylates ligands, the numbers of structures of metal coordination polymers bridged by the 3,5-dihydroxybenzoate ligand (3,5-dhb) are few, especially considering its binding abilities [38–40]. Compared to the paddlewheel shaped dinuclear copper benzoate [41], the compounds from the reaction of Cu2+ and 3,5dhb are much more complex due to the following reasons. First, 3,5-dhb can bind to the metal center as a monodentate or bidentate ligand through the carboxylate group. Furthermore, the hydroxyl groups can coordinate to the metal center resulting in different topologies of coordination polymers. Finally, diverse hydrogenbonding networks can be afforded via the active oxygen atoms. With the above considerations in mind, we studied the effect of differing concentration of pyridine (py) on the structures and properties of copper (octahedral) and zinc (tetrahedral) complexes ligated by 3,5-dhb. We found that the topologies of copper coordination polymers depend not only on the concentration of pyridine in solution but on the presence of solvent molecules as well. We report on three copper coordination polymers bridged by the

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3,5-dihydroxybenzoate ligand. Two of these are solvates consisting of [Cu(3,5-dhb)(pyridine)2CH2Cl2C4H8O2]n (1aCH2Cl2C4H8O2), and [Cu(3,5-dhb)2(pyridine)22CH2Cl2CH3OH]n (1b2CH2Cl2CH3OH), and the dimer [Cu2(3,5-dhb)43CH2Cl22CH3OH]n (23CH2Cl22CH3OH). Reacting zinc acetate with 3,5-dhb in the presence of excess pyridine afforded the monomeric Zn(3,5-dhb) 2 (pyridine) 2 1.25CH 2 Cl 2 0.75CH 3 OH (31.25CH 2 Cl 2 0.75CH 3 O). The syntheses and single crystal X-ray structure determinations of these compounds are presented and all exhibit 3-D networks based on an H-bonding framework. The different conformations of 1a and 1b were also investigated by X-ray powder diffraction and DSC measurements.

2.4. Synthesis and characterization of [Cu2(3,5-dhb)43CH2Cl22CH3 OH]n, (23CH2Cl22CH3OH) Compound 2 was prepared following a similarly scaled procedure to that for 1 except that the molar ratio between pyridine and copper acetate was 1.1:1. Yield: 75.7%. Blue crystals suitable for X-ray diffraction were obtained by allowing slow diffusion of dichloromethane into a methanol solution. IR (neat, cm1): 3303(m), 3079(w), 1608(w), 1575(w), 1541(s), 1486(s), 1447(m), 1371(s), 1299(s), 1216(w), 1153(vs), 1067(m), 1045(m), 1002(s), 954(w), 872(m), 852(m), 786(s), 753(vs), 695(vs), 678(m). The UV/Vis spectrum (THF) contains two bands at 302 nm (e = 10745 L mol1 cm1) and 670 nm (e = 411 L mol1 cm1).

2. Experimental 2.1. General methods Infrared spectra were obtained on a PerkinElmer Spectrum one FT-IR spectrometer. A PerkinElmer Lambda 35 UV/Vis Spectrometer was used for the UV/Vis spectrum. The DSC analyses were conducted under nitrogen on a Shimadzu DSC-50 analyzer. A Scintag XDS-2000 h/h diffractometer was used to collect powder diffraction X-ray data. Solvents were used as received from commercial suppliers. Most chemicals were purchased from Aldrich, and all chemicals were used as received. The elemental analyses were conducted by Galbraith Laboratories Inc., Knoxville, TN. 2.2. Synthesis and characterization of [Cu(3,5-dhb)(pyridine)2CH2Cl2 C4H8O2]n (1aCH2Cl2C4H8O2) Excess pyridine (0.393 g, 5.00 mmol) was added dropwise to a methanol solution (10 mL) of 3,5-dihydroxybenzoic acid (0.308 g, 2.00 mmol) under constant stirring. This mixture was then added to a methanol solution (15 ml) of copper acetate hydrate (0.200 g, 1.00 mmol). A blue solution was obtained after the reaction mixture was stirred for one hour at ambient temperature. A blue precipitate formed after the solution was concentrated using a rotary evaporator. This product was recrystallized from THF, filtered and dried under vacuum resulting in 0.29 g of 1. Yield: 79.0%. Blue crystals suitable for X-ray diffraction were obtained by allowing slow diffusion of dichloromethane into a THF solution of 1. Anal. Calcd. for Cu2C48H40O16N42CH2Cl22C4H8O2: C, 49.69; H, 4.31. Found: C, 49.74; H, 4.32%. IR (neat, cm1): 3396(m), 3079(w), 1607(w), 1576(w), 1541(s), 1487(s), 1448(m), 1416(s), 1372(s), 1298(s), 1276(w), 1156(vs), 1070(m), 1006(s), 857(m), 782(s), 753(vs), 736(m), 693(vs). 675(m). M. p.: 155–157 °C. The UV/Vis spectrum (THF) shows two bands have maxima at 303 nm (e = 10028 L mol1 cm1) and 670 nm (e = 543 L mol1 cm1). 2.3. Synthesis and characterization of [Cu(3,5-dhb)2(pyridine)22CH2 Cl2CH3OH]n (1b2CH2Cl2CH3OH) Crystals of compound 1b2CH2Cl2CH3OH were prepared following a similarly scaled procedure to that for 1 except the product was recrystallized by adding dichloromethane to a concentrated methanol solution of 1 with a final yield of 67.0%. IR (neat, cm1): 3511(w), 2973(w), 1608(w), 1578(w), 1545(s), 1487(m), 1449(m), 1369(vs), 1358(vs), 1299(s), 1220(w), 1155(vs), 1071(m), 1045(m), 1003(s), 954(w), 877(m), 858(m), 788(s), 753(vs), 694(vs). The UV/Vis spectrum (THF) shows two bands have maxima at 302 nm (e = 11048 L mol1 cm1) and 676 nm (e = 419 L mol1 cm1).

2.5. Synthesis and characterization of Zn(3,5-dhb)2(pyridine)21.25CH2 Cl20.75CH3OH (31.25CH2Cl20.75CH3OH) Compound 3 was obtained by following a similarly scaled procedure to that for 1, except that zinc acetate was used in place of copper acetate hydrate. Colorless crystals suitable for X-ray diffraction were obtained by diffusing dichloromethane into a methanol solution of 3. Yield: 76.0%. Anal. Calcd. for ZnC24H20O8N2CH3OH: C, 53.45; H, 4.27. Found: C, 53.60; H, 3.95%. Drying removed some of the solvent. IR (neat, cm1): 3296(m), 1610(m), 1577(w), 1553(s), 1488(w), 1448(s), 1402(vs), 1355(m), 1291(w), 1217(w), 1141(vs), 1069(m), 1047(m), 1000(s), 859(m), 787(m), 772(vs), 696(vs). The UV/Vis spectrum (THF) consists of a single band at 309 nm (e = 6599 L mol1 cm1).

2.6. X-ray crystallography Suitable crystals containing 1a and 3 were coated with epoxy resin and mounted on the tip of a thin glass fiber or in the capillary tube respectively. These measurements were obtained using an Enraf-Nonius Turbo CAD4 X-ray diffractometer at room temperature. Mo Ka radiation (k = 0.71073 Å) was used for data collection. A Bruker-Nonius SMART APEX CCD Diffractometer with Mo radiation (k = 0.71073 Å) was used for the measurement of crystals containing 1b and 2 at 100 K. Single crystals of compounds containing 1b and 2 were mounted on Mitegen micromesh supports using viscous oil flash-cooled to 100 K. Data were collected, unit cells determined, and the data integrated and corrected for absorption and other systematic errors using the Apex2 suite of programs [42]. All structures were solved by direct methods, refined by full matrix least squares against F2 with all reflections using SHELXL [43] and completed using previously published procedures [43–47]. In addition to these crystals containing ordered solvent molecules, disordered arrangements were also found. Complex 1a crystallized with a disordered CH2Cl2 over two positions (55(1):45(1)), 2 crystallized with a CH2Cl2 molecule arranged around an inversion point and complex 3 contained a disorder consisting of 75% MeOH and 25% CH2Cl2. There was no disorder within the metal containing complexes in all structures. The final models for crystals containing 1aCH2Cl2C4H8O2 and 31.25CH2Cl20.75CH3OH consisted of nonH atoms represented by anisotropic displacement parameters and all H-atoms were refined through constraints to the atoms they were bonded to. In those containing 1b2CH2Cl2CH3OH, the H atoms bonded to the O atoms were freely refined. For those crystals containing 23CH2Cl22CH3OH, the H atom attached to atom O2 was freely refined. Details of the data collection and refinement of the compounds are given in Table 1.

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L. Feng et al. / Inorganica Chimica Acta 394 (2013) 729–740 Table 1 Crystal data and structure refinement details of complexes 1aCH2Cl2C4H8O2, 1b2CH2Cl2CH3OH, 23CH2Cl22CH3OH and 31.25CH2Cl20.75CH3OH.

a b c d e f

Compound

1aCH2Cl2C4H8O2

1b2CH2Cl2CH3OH

23CH2Cl22CH3OH

31.25CH2Cl20.75CH3OH

Chemical formula

CuC24H20O8N2CH2Cl2C4H8O2

CuC24H20O8N22CH2Cl2CH3OH

Cu2C28H20O163CH2Cl22CH3OH

Formula weight Crystal size (mm) Crystal color Temperature (K) Crystal system Space group

701.00 0.33  0.28  0.25 blue 294(2) monoclinic C 2/c

729.85 0.40  0.40  0.40 blue 100(2) monoclinic P21/c

1058.40 0.20  0.15  0.05 blue 100(2) monoclinic P21/c

ZnC24H20O8 N21.25CH2Cl2 0.75CH3OH 660 0.4  0.20  0.20 colorless 294(2) triclinic

a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å)3 Z Dc (mg/m3) Absorption coefficient (mm1) F(0 0 0) Scan range h (°) No. of reflections measured No. of observed data, I > 2r(I) No. of parameters R1, wR2 [I > 2r (I)]a,b R1, wR2 (all data) Goodness of fit on F2

23.893(7) 15.673(6) 17.237(6) 90 98.39(3) 90 6386(4) 8 1.458 0.909 2888 1.56–24.98 5459 5613 433 4.32, 9.27c 8.58, 10.77 1.070

13.532(2) 15.229(3) 15.657(3) 90 105.488(3) 90 3109.4(10) 2 1.559 1.100 1492 1.90–30.42 28820 9091 409 4.08, 9.32e 6.06, 10.22 1.072

8.537(2) 11.292(3) 21.660(5) 90 101.569(8) 90 2045.6(9) 4 1.718 1.507 1072 1.92–30.31 14728 5859 304 5.79, 14.41d 8.80, 15.56 1.060

P1 11.236(4) 11.941(5) 12.956(3) 110.64(3) 98.48(2) 110.50(3) 1449.1(9) 2 1.513 1.130 670 1.77–22.49 3993 3763 392 5.75, 14.57f 9.21, 16.66 1.085

R1 = ||Fo|–|Fc||/|Fo|. wR2 = [[w(Fo2–Fc2)2]/[w(Fo2)2]]1/2. w = 1/[2(Fo2) + (0.0356P)2 + 12.28P]. w = 1/[2(Fo2) + (0.0667P)2 + 4.95P]. w = 1/[2(Fo2) + (0.0404P)2 + 2.04P]. w = 1/[2(Fo2) + (0.0840P)2 + 3.1702P], where P = (Fo2 + 2Fc2)/3.

3. Results and discussion 3.1. Synthesis We were interested in studying the 3-D structures that would be produced via hydrogen bonds using 3,5-dhb and also the effect of varying the concentration of pyridine in reactions with Cu(II) and Zn(II), Scheme 1. Cu(II) and Zn(II) paddle wheel structures, for example copper cymantrenylcarboxylates [48] and ‘‘Zn2 (COO)4’’ clusters within a 3-D pillared framework [49] are known to be produced with reactions between these metals and carboxylates. Two different Cu(II) complexes were afforded when different solvents were used. Of these, 1a and 1b constitute crystalline solvates where two different arrangements result depending on crystal growth conditions and presumably the incorporation of different solvent molecules. Compared to compounds 1a and 1b, compound 2 with a dinuclear paddlewheel topology was afforded when the smallest quantity of pyridine was used (npy/Cu(II) = 1.1:1).

For compounds 1a, 1b and 3, the pyridine was added in excess since we were using it as a base and a reactant. As is detailed below in an examination of their crystal structures, intricate and extended arrangements are afforded through hydrogen bonds. Compounds 1b and 2 were synthesized similarly except for different pyridine to Cu ratios. These compounds have similar powder X-ray diffraction spectra except that the diffraction peaks in 2 were broader. The stability of coordination polymers in aqueous media is important for industrial application and, unfortunately, studies revealed that both Cu polymers decomposed after stirring them in water for a few minutes. 3.2. Description of crystal structures Crystal data and details of structural refinement for compounds 1a, 1b, 2 and 3 are listed in Table 1. Listing of hydrogen bonds of 1a, 1b, 2 and 3 are given in Table 2.

Scheme 1. Schematic illustration of the formation of different structures using different ratios of pyridine to copper acetate done in CH2Cl2 with different crystallization solvents.

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Table 2 Hydrogen bond geometries for Compounds 1aCH2Cl2C4H8O2, 1b2CH2Cl2CH3OH, 23CH2Cl22CH3OH and 31.25CH2Cl20.75CH3OH. Compound

D–HA

d(D–H)/Å

d(HA)/Å

d(DA)/Å


Symmetry transformation

1a

O1–H10O8 O2–H20O2A O3–H30O20 O7–H70O50 O2A–H2A1O80

0.82 0.82 0.82 0.82 0.82

1.82 1.86 1.96 1.76 1.91

2.613(3) 2.639(4) 2.776(4) 2.579(3) 2.711(4)

163 157 173 174 164

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

O3–H31O80 O4–H41O20 O7–H71O6 O8–H81O90 O9–H91O20 O2–H2O60 O6–H6O70 O7–H7O80 O8–H8O21 O21–H21Cl20 O21–H21Cl10

0.71(3) 0.74(3) 0.82(3) 0.82(3) 0.79(3) 0.73(6) 0.84 0.84 0.84 0.84 0.84

2.04(3) 1.94(3) 1.70(3) 1.86(3) 1.93(3) 2.05(6) 1.90 1.82 1.79 2.73 2.75

2.754(2) 2.665(2) 2.520(2) 2.645(2) 2.717(2) 2.768(4) 2.741(2) 2.654(4) 2.621(5) 3.435(4) 3.346(4)

176(4) 166(4) 176(3) 160(3) 177(3) 166(6) 167(6) 169 173 142(5) 130(5)

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

O3–H3AO70 O4–H4AO60 O7–H7AO20 O8–H8AC130

0.82 0.82 0.82 0.82

1.96 1.94 1.82 2.09

2.781(8) 2.743(8) 2.630(7) 2.822(1)

174 165 171 147

x  1, y  1, z – 1 x, y + 2, z + 1 x + 1, y + 2, z + 1 x, y + 1, z + 1

1b

2

3

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

Fig. 1. Perspective view of the coordination environment of compound 1aCH2Cl2C4H8O2 with selected atomic numbering and atoms represented by spheres of arbritary size. The orientations of the disordered CH2Cl2 molecules are shown.

The molecular structure of 1a CH2Cl2C4H8O2 along with selected atom labeling is shown in Fig. 1. Coordination distances and angles are listed in Table 3. Compound 1a crystallizes in the space group C2/c and contains a disordered dichloromethane molecule (54:46% ratio) and a 2-hydroxy-tetrahydrofuran molecule which is a product of the decomposition of 2-hydroperoxotetrahydrofuran [50] in the asymmetric unit, see Fig. 1. The asymmetric units for 1a contains two divalent copper ions both located on different two fold axes, two complete 3,5-dhb ligands, and two pyridine ligands; one off the two fold axis and the other arranged on a two-fold axis. The two crystallographic independent copper centers are coordinated in a near octahedral manner and are bridged together by two hydroxyl oxygen atoms (O1, O7) and two monodentate carboxylate oxygen atoms (O4, O6) from two opposite 3,5-dhb ligands, as shown in Fig. 1. For each copper center, two trans pyridyl nitrogen atoms and two monodentate carboxylate oxygen atoms from two distinct 3,5-dhb ligands can be considered to be on the equatorial plane, and the hydroxyl

Table 3 Bond distances (Å) and bond angles (°) to the Cu Atom in 1aCH2Cl2C4H8O2. Cu(1)–O(6)

1.935(2)

Cu(2)–N(3)

2.010(4)

Bond distances Cu(1)–N(1) Cu(1)–O(7) Cu(2)–O(4)

2.029(3) 2.6045(7) 1.957(2)

Cu(2)–N(2) Cu(2)–O(1) N(1)a–Cu(1)–N(1)

2.023(4) 2.6341(9) 180.0(1)

Bond angles O(6)a–Cu(1)–O(6) O(6)–Cu(1)–N(1)a O(6)–Cu(1)–N(1)

180.0(14) 89.88(10) 90.12(10)

O(4)b–Cu(2)–O(4) O(4)–Cu(2)–N(3) O(4)–Cu(2)–N(2)

178.37(15) 90.82(8) 89.18(8)

Symmetry transformations used to generate equivalent atoms: ax + 1/2, y + 1/2, z; bx + 1, y, z + 3/2.

oxygen atoms occupy the longer apical sites. Two types of Cu–O bonds exist in the crystal structure of compound 1a; a shorter bond between Cu and monodentate carboxylate oxygen atoms with a distance of 1.935(2) Å and 1.957(2) Å for Cu1–O6 and Cu2–O4,

L. Feng et al. / Inorganica Chimica Acta 394 (2013) 729–740

respectively; and a longer bond length at 2.6045(7) Å and 2.6341(9) Å for Cu1–O7 and Cu2–O1 respectively due to Jahn– Teller like effects. The bond distances between Cu and the pyridyl nitrogen atoms are 2.029(3) Å, 2.010(4) and 2.023(4) Å for Cu1–N1, Cu2–N3 and Cu2–N2, respectively, and are not that significantly different. The bond angles are similar to those reported for octahedrally coordinated divalent copper ions [32]. Interestingly, while the planes of the pyridine ligands on Cu1 are coplanar, those on Cu2 are almost orthogonal at 73.6(5)°. If superimposed, the lines connecting N3–Cu2–N2 and N1–Cu1–(N1)’ are not collinear but are rotated at 33.6(5)°. The copper centers are bridged into infinite 1D coordination polymeric chains by the 3,5-dhb ligands with an intramolecular CuCu separation of 7.917(2) Å, Figs. 1 and 2. Various hydrogen bonds exist in the crystal structure of compound 1a, as listed in Table 2 and as shown in Fig. 2. The non-coordinated carboxylate oxygen atoms of the 3,5-dhb ligand are hydrogen bonded to the hydroxyl oxygen atoms of an adjacent of 3,5-dhb molecule with bond lengths of 2.579(3) Å and 2.613(3) Å for O5H–O7 and O8H–O1, respectively. Additionally, the non-coordinated carboxylate oxygen atom O8 and an uncoordinated hydroxyl oxygen atom O2 are hydrogen bonded to oxygen atom (O2A) of the solvent molecule, 2-hydroxyl-tetrahydrofuran with distances of O8H– O2A, 2.711(1) Å and O2–HO2A, 2.639(4) Å. These 1D [Cu2(3,5dhb)2(py)4]n chains are further connected into 2D coordination undulating polymeric sheets via O2H–O3 hydrogen bonding between uncoordinated hydroxyl oxygen atoms, 2.776(4) Å, as shown in the unit cell packing in Fig. S1. The shortest interchain CuCu distance in this structure is 13.808(5) Å. The coordination environment of compound 1b 2CH2Cl2CH3OH with selected atomic labeling is shown in Fig. 3 and coordination bond lengths and angles are listed in Table 4. This complex crystallized in the P21/c space group and contains one methanol and two dichloromethane molecules in the asymmetric unit. The crystallographically identical copper centers are coordinated in a distorted octahedral environment with two monodentate carboxylates O

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atoms (O1, O5) and two pyridyl N atoms (N1, N2) in the equatorial plane. The axial positions are occupied by two hydroxyl O atoms (O4, O7) trans to each other. The bond distances of Cu1–O1 and Cu1–O5 are 1.9520(15) Å and 1.9627(15) Å, respectively, which are similar to the corresponding bond lengths in compound 1a. The average Cu–N bond length is around 2.01 Å which is not significantly different to the equivalent bond length in compound 1a. Jahn–Teller like distortions render the bonds of Cu1–O4 and Cu1–O7 longer with distances of 2.6949(3) Å and 2.4389(14) Å, respectively. It is noteworthy that the Cu1–O7 bond is significantly shorter than the Cu1–O4 which may stem from the different coordination environment caused by solvent molecules. A small but still significant difference in the distances for the equivalent bonds in 1a was also noted. The two opposing 3,5-dhb ligands bridge the copper monomers together into an infinite polymeric chain, as shown in Fig. 3. In 1b, the trans pyridine ligands are twisted slightly with a C5–N1–N2–C10 torsion angle of 8(1)°. Here, the N1–Cu1–(N1)’ vector is at 39.5(5)° which is different than that of 33.6(5)° in 1a. The bond angles in 1b, Table 4, suggest a more distorted octahedral arrangement. The intrachain CuCu distance is 7.843(1) Å which is shorter than the CuCu separation in compound 1a. The 1D chains are further built into 2D networks via O3–HO8, O8–HO9, and O9– HO2 hydrogen bonds with distances of 2.754(2) Å, 2.645(2) Å, and 2.717(2) Å, respectively, Fig. 4. The packing diagram, Fig. S2, depicts that the 2D networks in 1b are arranged differently compared to that for 1a, Fig. S1. The shortest interchain CuCu distance is 13.532(2) Å which is also shorter than the corresponding distance in 1a. Similar to compound 1a, the O4–HO2, 2.665(2) Å, and O7–HO6, 2.520(2) Å hydrogen bonds afford stability to the monodentate carboxylate groups. The shorter hydrogen bonds in O4– HO2 compared to O3–HO8 are due to the fact that the shorter distances are obtained when the hydroxyl groups are binding to Cu centers. The oxygen atom (i.e., O9) of the methanol solvent serves a similar function to the 2-hydroxy-tetrahydrofuran in 1a and is hydrogen bonded to O2 and O8, see Table 2.

Fig. 2. Hydrogen bonds (dotted lines) in compound 1aCH2Cl2C4H8O2 with selected atomic numbering. Symmetry code (0 , 1/2 + x, ½  y, 1/2 + z; 00 , 1  x, y, 1.5  z). Thermal ellipsoids are drawn at the 30% probability level.

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Fig. 3. Coordination environment of compound 1b2CH2Cl2CH3OH with selected atomic numbering. Atoms are represented by spheres of arbitrary radii.

Table 4 Bond distances (Å) and bond angles (°) to the Cu atom for 1b 2CH2Cl2CH3OH. Bond distances Cu(1)–O(1) Cu(1)–O(5) Cu(1)–N(1)

1.9520(15) 1.9627(15) 2.0066(18)

Cu(1)–N(2) Cu(1)–O(7) Cu(1)–O(4)

2.0125(18) 2.4389(14) 2.6949(3)

Bond angles O(1)–Cu(1)–O(5) O(1)–Cu(1)–N(1) O(5)–Cu(1)–N(1) O(1)–Cu(1)–N(2) O(5)–Cu(1)–N(2)

179.46(6) 90.36(7) 89.96(7) 88.52(7) 91.13(7)

N(1)–Cu(1)–N(2) O(1)–Cu(1)–O(7) O(5)–Cu(1)–O(7) N(1)–Cu(1)–O(7) N(2)–Cu(1)–O(7)

174.75(7) 87.20(6) 93.18(6) 97.94(6) 87.13(6)

The TOPOS program package [51] was utilized to illustrate any topological differences between 1a and 1b. As is shown in Figs. S3 and S4 the Cu centers in both 1a and 1b are arranged in parallel planes in a similar manner. The results from the TOPOS calculation reveal that the Cu atoms in both polymers adopt 2-c uninodal net structures. The structure of compound 23CH2Cl22CH3OH with selected atomic labeling is shown in Fig. 5. Coordination bond lengths and angles are listed in Table 5. Compound 23CH2Cl22CH3OH crystallizes in the P1 space group and the asymmetric unit consists of the Cu(II) dimer, three dichloromethane and two methanol solvent molecules. Complex 2 is arranged around a crystallographic inversion center and consists of the commonly occurring paddlewheel shaped dinuclear Cu(II) unit bridged by four distinct 3,5-dhb ligands in a syn, syn manner. The CuCu distance in the dinuclear unit is 2.5784 (8) Å which is below the summed van der Waals radii for two copper atoms (2.8 Å) but is longer than the Cu–Cu distance in metallic copper (2.56 Å) [52,53]. This CuCu distance is in good agreement with other paddlewheeled CuCu lengths [52– 55]. If we ignore the Cu–Cu bonding contact, each Cu(II) center is pentacoordinated to four oxygen atoms from two distinct 3,5dhb ligands (O1, O5, O3, O4 atoms) with Cu–O distances ranging from 1.956(2) to 1.961(3) Å in the basal plane. Additionally, the apical position is occupied by an oxygen atom of the hydroxyl

group from neighboring 3,5-dhb (O2 atom) with a Cu–O distance of 2.224(3) Å. The atoms in the apical O–Cu–Cu–O fragment are not collinear but are slightly bent with an average angle of 164.73°. The dinuclear copper centers are bridged into 1D ribbon type chains that are oriented along the a axis by the hydroxyl O atom (O2) of the adjacent 3,5-dhb ligand with a CuCu atom separation of 8.537(2) Å, and interestingly, all of the hydroxyl O atoms in complex 2 are involved in H-bonding as displayed in Fig. 6. The lengths and angles for these H-bonds are listed in Table 2. One hydroxyl O atom (O8) forms hydrogen bonds with the neighboring hydroxyl O atom (O7) with an O8O7 distance of 2.654(4) Å. These H bonds result in the formation of 2D sheets by connecting the 1D chains of the same orientation. The interchain CuCu separation averages at 14.156 Å. Oxygen atoms (O8) also form hydrogen bonds with a molecule of methanol with an O8O21 distance of 2.620(5) Å. The hydroxyl O atom (O6) forms two hydrogen bonds: O2–HO6 average at 2.767(4) Å, and O6–HO7 at a distance of 2.742(4) Å. These hydrogen bonds further link the chains together and develop the 2D sheets into 3D networks, resulting in channels containing the interstitial molecules of solvation as shown in Fig. S5. An illustration of compound 31.25CH2Cl20.75CH3OH along with atomic labeling is shown in Fig. 7. Selected bond lengths and angles are given in Table 6. Compound 3v1.25CH2Cl20.75CH3OH crystallizes in the P 1 space group and the asymmetric unit consists of Zn(3,5dhb)2(py)2 and one ordered dichloromethane and one site co-occupied by a mixture which refined to 75% methanol and 25% dichloromethane. The zinc center is in a distorted tetrahedral environment and is bonded to two monodentate carboxylate O atoms from two distinct 3,5-dhb ligands and two pyridyl N atoms. The O–Zn–O and N–Zn–N bond angles are 107.5(2)° and 105.4(2)°, respectively. The bond angles for O–Zn–N range from 98.1(2) to 115.9(2). The bond lengths for Zn–O and Zn–N at 1.945(5) Å, 1.962(5) Å and 2.04(5) Å, 2.046(5) Å, respectively are significantly different and are comparable with those in other zinc compounds [56].

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Fig. 4. The nature of the H-bonds in 1b2CH2Cl2CH3OH. Symmetry code (0 , x, ½  y, 1/2 + z; 00 , 2  x, 1/2 + y, 1.5  z; 00 0 ’, 1 + x, y, z). Thermal ellipsoids are drawn at the 50% probability level, H atoms are represented by spheres of arbitrary radii and H-bond are indicated with dotted lines.

Fig. 5. Perspective view of the coordination environment of compound 23CH2Cl22CH3OH with atomic numbering of selected atoms represented by thermal ellipsoids drawn at the 25% probability level. Molecules of solvation are not shown.

An interesting aspect of the structure of compound 31.25CH2Cl2 0.75CH3OH is that all of the oxygen atoms on the 3,5-dhb ligand are involved in hydrogen bonding, as displayed in Fig. 8. One uncoordinated carboxylate oxygen atom O6 forms a hydrogen bond with a neighboring hydroxyl oxygen atom O4 with a O6O4 atom distance

of 2.7404(9) Å. Another uncoordinated carboxylate oxygen atom O2 forms a hydrogen bond with an oxygen atom of another 3,5-dhb ligand, O7 with an O2O7 atom distance of 2.6288(7) Å. The hydroxyl oxygen atom O7 also forms a hydrogen bond with another neighboring hydroxyl oxygen atom O3 with an O7O3 atom distance of

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Table 5 Bond distances (Å) and bond angles (°) to the Cu atom for 23CH2Cl22CH3OH. Cu(1)–Cu(1)a Bond distances Cu(1)–O(4) Cu(1)–O(3) Bond angles O(4)–Cu(1)–O(3) O(4)–Cu(1)–O(1) O(3)–Cu(1)–O(1) O(4)–Cu(1)–O(5) O(3)–Cu(1)–O(5) O(1)–Cu(1)–O(5) O(4)–Cu(1)–O(2) O(3)–Cu(1)-O(2)

2.5784(8)

Cu(1)–O(1)

1.960(3)

1.955(2) 1.960(2)

Cu(1)–O(5) Cu(1)–O(2)

1.961(3) 2.224(3)

170.09(10) 89.71(11) 89.24(11) 90.01(11) 89.35(11) 170.17(10) 83.72(10) 106.19(10)

O(1)–Cu(1)–O(2) O(5)–Cu(1)–O(2) O(4)–Cu(1)–Cu(1)a O(3)–Cu(1)–Cu(1)a O(1)–Cu(1)–Cu(1)a O(5)–Cu(1)–Cu(1)a O(2)–Cu(1)–Cu(1)a

97.55(11) 92.19(11) 81.70(7) 88.40(7) 86.89(7) 83.34(8) 164.74(7)

Symmetry transformations used to generate equivalent atoms: ax, y + 1, z.

2.779(1) Å. The hydroxyl O8 atom is involved with H-bonding to the Cl atom in a dichloromethane molecule (Cl3) at an O8Cl3 distance of 2.822(1) Å. A zinc dimer is thus formed via O7–HO2 hydrogen bonding with a ZnZn separation of 5.634(3) Å (bottom right of Fig. 8). The zinc dimers are further linked into an infinite 1D chain via O4–HO6 hydrogen bonds with the shortest ZnZn separation of 10.456(3) Å, as seen in Fig. 8. An infinite 2D sheet is built up by connecting the zinc chains via O3–HO7 hydrogen bonding with the pyridine ligands located between the chains. 3.3. Spectroscopic characterization The IR spectra of complexes 1a, 1b, 2 and 3 contain broad bands at 3396 cm1, 3303 cm1, 3511 cm1 and 3296 cm1, respectively,

which can be ascribed to O–H vibrational modes. The IR spectrum of 1a also contains diagnostic bands at 1541 cm1 and 1372 cm1 indicating the asymmetric and symmetric stretching mode for the carboxylate group. It has been suggested that the separation between the asymmetric and symmetric stretching frequency for monodentate carboxylate ligands is greater than 200 cm1 [57]. We expected that the difference between the asymmetric and symmetric carboxylate stretching band in compound 1a and 1b should be greater 200 cm1 since they contain monodentate carboxylate ligands. However, we observed a ‘psuedo-bridging’ arrangement formed via hydrogen bonding between the non-coordinated oxygen atom of the monodentate carboxylate ligand and other ligands or a solvent. In that case, a designated ‘‘symmetrical’’ structure still pertains for the monodentate carboxylate ligand [58,59]. For compound 1a, the formation of O1–HO8, and O7– HO5 hydrogen bonds between the non-coordinated carboxylate O atoms and the hydroxyl O atoms of the neighboring ligands renders the monodentate carboxylate groups arranged in a ‘‘symmetrical’’ mode, as seen in Fig. 2. This fact is reflected in the observed small separation between the asymmetric and symmetric carboxylate stretching in compound 1a. The separation of the asymmetric and symmetric COO frequency in compound 1b is 176 cm1 which is similar to that in compound 1a. For compound 2, the separation between the asymmetric and symmetric carboxylate group is at 170 cm1, which suggests a bidentate binding mode for the coordinated carboxylate groups. This IR result is in agreement with the crystallographic structural analyses detailed above. The asymmetric and symmetric stretching frequencies for the carboxylate groups in 3 appear at 1553 cm1 and 1355 cm1, respectively. This small separation between the asymmetric and symmetric stretch-

Fig. 6. The nature of the H-bonds in 23CH2Cl22CH3OH. Symmetry code (0 , 1  x, y, z; 00 , 1 + x, 1/2  y, 1/2 + z; 00 0 , 1  x, 1/2 + y, 1/2  z). Thermal ellipsoids are drawn at the 50% probability level, H atoms are represented by spheres of arbitrary radii and H-bond are indicated with dotted lines.

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Fig. 7. Perspective view of the coordination environment of compound 31.25CH2Cl20.75CH3OH with selected atomic labeling. Atoms are represented by spheres of arbitrary radii. Solvent molecules are not illustrated.

Table 6 Bond distances (Å) and bond angles (°) to the Zn atom for 31.25CH2Cl20.75CH3OH. Zn(1)–O(1) Bond distances Zn(1)–O(5) O(1)–Zn(1)–O(5) Bond angles O(1)–Zn(1)–N(2) O(5)–Zn(1)–N(2)

1.945(5)

Zn(1)–N(2)

2.022(5)

1.962(5) 107.5(2)

Zn(1)–N(1) O(1)–Zn(1)–N(1)

2.047(5) 113.7(2)

115.2(2) 115.9(2)

O(5)–Zn(1)–N(1) N(2)–Zn(1)–N(1)

98.1(2) 105.3(2)

ing frequencies of carboxylate ligand can also be ascribed to the existence of hydrogen bonds between the O atoms of 3,5-dhb and the O atoms of neighboring ligands, as shown in Fig. 8. In UV–Vis spectra, a broad band at about 670 nm was observed for compounds 1a, 1b, and 2, which can be ascribed to d–d chargetransfer bands in Cu(II) complexes [37,60,61]. Additionally, the spectra of compounds 1a, 1b, and 2 also contain strong bands at 302 nm that can be assigned to the ligand-to-metal-charge-transfer (LMCT) transition [61,62]. The LMCT transition band for compound 3 was observed at 309 nm. The fact that these solid state structures can be dissolved suggests that the lattice frameworks are not strong. 3.4. Can polymorphs be produced? Tentative experiments It is clear that 1aCH2Cl2C4H8O2 and 1b2CH2Cl2CH3OH constitute crystalline solvates but given the different conformation of the copper containing polymers and the continuing discussion regarding the definition of what constitutes a polymorph, [63–68] it was of some interest to see if the different arrangements (i.e., polymorphic forms) remained after the solvent molecules were removed. These experiments proved very difficult as unfortunately attempts

to produce more crystals of compound 1aCH2Cl2C4H8O2 were not successful. We therefore decided to conduct a more practical test which consisted of recrystallizing 1 in different solvent mixtures in order to see if after drying, different arrangements pertained as evident in X-ray powder diffraction measurements. As detailed in Chart 1, this consisted of preparing compound 1 followed by recrystallization using THF/CH2Cl2 and MeOH/CH2Cl2 solvent mixtures, resulting in what we suspected could be two polymorphs. The material labeled as 1c in Chart 1 was subsequently recrystallized with MeOH/CH2Cl2. Our thinking here was that if polymorphs could be produced by recrystallizing this substance under different solvent conditions, then we should be able to demonstrate the interconversion of these forms. The four samples were each dried under vacuum for sufficient periods to remove solvent and then had their FTIR spectra, Fig. S6 and X-ray powder diffraction Fig. S7 measured. In the fingerprint region of the FTIR spectra from 1700 to 600 cm1, the spectra appear identical apart from some peaks being more sharply defined. There are some small differences in the region from 3500 to 2500 cm1 which may indicate presence of residual solvent molecules. However, the powder X-ray diffraction patterns contained more definitive differences. The material recrystallized from THF/ CH2Cl2, i.e., 1c, labeled as compound (b) in Fig. S7 appeared of a different consistency and this may be responsible for the more amorphous X-ray pattern. This 1c sample, (b), consists of a main diffraction peak at 11.03° and several smaller peaks at 7.88° and between 20° and 25° (2h). This pattern is very different from that for (a), (c) and (d) as shown in Fig. S6. The material recrystallized from methanol, i.e., (c), contains a main doublet peak at 9.56 and 9.98° and several smaller peaks between 10° and 15°, and between 20° and 25° (2h). This is very close to freshly produced 1 which contains large peaks at 9.62° and 9.98° (2h). The X-ray pattern

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Fig. 8. Hydrogen bonds (dotted lines) in compound 31.25CH2Cl20.75CH3OH with selected atomic labeling. Symmetry code (0 , 1 + x, 1 + y, 1 + z; 00 , x, 2  y, 1  z; 1  x, 2  y, 1  z).

Compound 1

THF/CH2Cl2

1c

Methanol/CH2Cl2

Methanol/CH2Cl2

1

Chart 1. Recrystallization of 1 using different solvents.

reproduced as (d), which was obtained by taking the material used to produce trace (b) and re-crystallizing this from MeOH/CH2Cl2, is similar to that for (a) and (c). This contains a sharp peak at 9.65° and smaller peaks between 10° and 15°, and between 20° and 25° (2h). These measurements suggest that if freshly prepared 1 is subsequently re-crystallized from methanol and THF, different conformations (i.e., polymorphs) can be produced. However, these powder diffraction patterns do not match the simulated patterns from single crystal X-ray data, see Figs. S8 and S9. Unfortunately we were unable to reproduce the solvate 2-hydroxy-tetrahydrofuran in the powder synthesis and also the compounds were dried

00 0

,

before obtaining the powder spectra whereas the crystals contained solvent which would clearly alter the simulated patterns. Considering their preparation, IR and X-ray differences, we suspected there might still be residual solvent molecules sticking to the compound which were not removed by pumping under vacuum. This point was established by dissolving dried 1c in deuterated methanol and obtaining the 1H NMR spectrum which contained peaks assignable to THF. Methanol substitutes for the coordinated THF molecules and thus liberates them. The results of TGA and DSC measurements on complex 1 recrystallized under the aforementioned conditions are illustrated in Figs. 9 and 10, respectively. Recrystallizing compound 1 from a THF/CH2Cl2 solution produced material that changed color from blue to dark green over 120–150 °C and melted around 175 °C. In contrast, recrystallization from a methanol/CH2Cl2 solvent mixture resulted in a compound that began a slow color change at 140 °C followed by melting at 180 °C as observed visually with samples placed inside of a capillary tube heated in oil. In the TGA analysis, Fig. 9, the material which was recrystallized from the THF/CH2Cl2 solvent mixture displayed a 13.9% decrease in weight over 120–150 °C which could correspond to release of a THF molecule (calc. 12.0%). The material produced from the MeOH/CH2Cl2 recrystallization appears stable until heated to a temperature around 140 °C then has a 4.5% weight loss (calc. 5.7% for a methanol molecule) over 135–150 °C followed by further decomposition. Both

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Fig. 9. TGA plots of 1 recrystallized from THF/CH2Cl2/(left) and from MeOH/CH2Cl2/(right) both obtained at heating rates of 10 °C min1.

Fig. 10. DSC thermograms of 1 recrystallized from THF/CH2Cl2/(left) and from MeOH/CH2Cl2/(right) both obtained at heating rates of 10 °C min1.

compounds ended up as black residues (presumably CuO) when heated to 500 °C with residual weights of 15.1%. Differences in the two forms of the compound are also evident in the results from the DSC thermograms, Fig. 10. For 1 recrystallized from the THF/CH2Cl2 mixture, there is a big endothermic band (71.2 kJ/mol) over 110–150 °C followed by two more bands (23.3 and 40.2 kJ/mol) around 175 and 200 °C. The DSC thermogram for 1 recrystallized from the methanol/CH2Cl2 mixture differs both in peak position and shape. It contains a small endothermic peak (4.3 kJ/mol) at 140 °C followed by a large peak (58.5 kJ/ mol) at 180 °C and a medium sized one (45.1 kJ/mol) at 200 °C. It appears that the release of solvent molecules triggered further decomposition in both cases. The release of THF occurred at a lower and wider range of temperature and its corresponding DSC peak overlapped with further decomposition resulting in a big endothermic peak over 110–150 °C, followed by melting around 175 °C. However, methanol release occurred at a higher and narrower temperature which could be responsible for the small decrease in the TGA plot and the small endothermic peak in the DSC thermogram around 140 °C, followed by decomposition and melting around 180 °C. The total required heat before 180 °C for two materials differed probably because of the energy requirements for conformational changes and/or decomposition and the energy required to liberate THF. These results clearly indicate that we were unsuccessful in our attempts to produce classical polymorphs of compound 1 and the term ‘‘solvates’’ would be more appropriate.

solvents yielded compounds 1a, 1b, and 2. Compound 1a and 1b are solvates resulting from the different crystal growing solvent conditions, perhaps an example of solvent directed crystal growth. Compound 1a and 1b consisted of infinite polymeric chains formed by bridging copper centers together with the monodentate carboxylate and hydroxyl group of the 3,5-dhb ligand. 2D polymeric sheets were further formed via hydrogen bonds among the various oxygen atoms. Experiments designed to synthesized polymorphs of complex 1 were not conclusive. A paddlewheel copper dimer symmetrically bridged by four 3,5-dhb ligands was found in compound 2, which are further arranged into 1D ribbon type chains via the hydroxyl O atoms bridging at the apical sites of the Cu(II) dimers. Various hydrogen bonds incorporating remaining hydroxyl groups further develop the 1D chains into 2D sheets and 3D networks. For compound 3, the 3,5-dhb ligand coordinates to the zinc center as a monodentate ligand via the carboxylate group. The rich hydrogen bonds in 3 give rise to the formation of infinite 2D polymeric sheets. Acknowledgements The diffractometer used to collect data at low temperature was funded by NSF Grant 0087210, by Ohio Board of Regents Grant CAP-491 and by YSU. We thank Michigan Technological University for supporting this research. Appendix A. Supplementary material

4. Conclusions The reaction of 3,5-dihydroxybenzoic acid with copper acetate in the presence of different quantities of pyridine and in different

Tables of selected bond distances (Å) and bond angles (°) for 1aCH2Cl2C4H8O2, 1b2CH2Cl2CH3OH, 23CH2Cl22CH3OH and 31.25CH2Cl20.75CH3OH, Tables S1–S4, respectively. Unit cell

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packing diagrams for 1aCH2Cl2C4H8O2, 1b2CH2Cl2CH3OH, topological diagrams for 1b2CH2Cl2CH3OH and 23CH2Cl22CH3OH, and unit cell packing diagram for 23CH2Cl22CH3OH, Figs. S1–S5. FTIR and powder X-ray spectra of complex 1, Figs. S6 and S7, respectively. Simulated (a) and experimental (b) powder diffraction pattern patterns for 1aCH2Cl2 C4H8O2 and 1b2CH2Cl2CH3OH, Figs. S8 and S9, respectively. CCDC 853741–853744 contains the supplementary crystallographic data for this paper. 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 http:// dx.doi.org/10.1016/j.ica.2012.09.036.

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