Hydrothermal synthesis, structural determination, and thermal properties of 2-D cobalt- and nickel-based coordination polymers incorporating pendant-arm 3-pyridinecarboxylate ligands

Inorganica Chimica Acta 360 (2007) 599–606 www.elsevier.com/locate/ica

Hydrothermal synthesis, structural determination, and thermal properties of 2-D cobalt- and nickel-based coordination polymers incorporating pendant-arm 3-pyridinecarboxylate ligands David P. Martin, Caleb H. Springsteen, Robert L. LaDuca

*

Lyman Briggs School of Science and Department of Chemistry, Michigan State University, E-30 Holmes Hall, East Lansing, MI 48825, USA Received 29 June 2006; received in revised form 31 July 2006; accepted 1 August 2006 Available online 15 August 2006

Abstract Hydrothermal synthesis has afforded a family of four coordination polymers containing divalent nickel or cobalt and pendant-arm pyridylcarboxylate ligands. Utilizing 3-pyridylacetic acid and appropriate metal precursors produced [M(3-pyrac)2(H2O)2] phases (M = Co (1); M = Ni (2)), while 3-pyridylpropionic acid generated [M(3-pyrprop)2(H2O)2] coordination polymers (M = Co (3); M = Ni (4)). Single crystal X-ray diffraction revealed that 1–4 all display discrete 2-D layers with (4,4)-topology, anchored via bridging 3-pyridylcarboxylate ligands bearing monodentate carboxylate termini. Intralamellar hydrogen bonding between the aquo ligands and unligated carboxylate oxygen atoms is observed within 1–4. The pseudo 3-D structures of 1–4 are further assembled via stacking of individual neutral layers by interlayer hydrogen bonding. Thermal properties are also discussed.  2006 Elsevier B.V. All rights reserved. Keywords: Coordination polymer; Nickel; Cobalt; Pyridylcarboxylate; Crystal structure; Thermogravimetric analysis

1. Introduction Over the past decade there has been heightened interest in the synthesis and characterization of metal-organic coordination polymers due to their potential capabilities as gas storage substrates [1], molecular absorbents [2], ionexchange materials [3], catalysts [4], and optical materials [5]. For the construction of these inorganic/organic hybrid materials carboxylate ligands have proven to be an efficacious choice [6]. In addition to their ability to scaffold coordination polymer networks due to covalent interactions, their anionic nature provides charge balance, mitigating the possibility of small counteranion incorporation and resulting in open framework materials [1]. Carboxylate *

Corresponding author. E-mail address: [email protected] (R.L. LaDuca).

0020-1693/$ - see front matter  2006 Elsevier B.V. All rights reserved. doi:10.1016/j.ica.2006.08.006

and dicarboxylate ligands are also advantageous due to their ability to engage in diverse bonding modes within metal-bearing coordination polymers [7], permitting enormous structural diversity and the tailoring of physical properties such as magnetism [8] and porosity [1a,9]. The employment of multifunctional ligands bearing both anionic and neutral donor atoms such as nicotinate [10], isonicotinate [11], and various pyridinedicarboxylates [12], has resulted in the preparation of many functional coordination polymers, some with intriguing optical [10b,11b] or gas sorption properties [13]. Nevertheless, reports of coordination polymers based on pyridines bearing more flexible pendant-arm carboxylate substituents are less common [14]. Herein we report the hydrothermal synthesis, structural characterization, and thermal degradation behavior of a family of four cobalt and nickel coordination polymers incorporating the difunctional pendant-arm

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pyridylcarboxylate ligands 3-pyridylacetate (3-pyrac) and 3-pyridylpropionate (3-pyrprop). These materials all possess the general formula [M(L)2(H2O)2] (L = 3-pyrac, M = Co (1), M = Ni (2); L = 3-pyrprop, M = Co (3), M = Ni (4)), and form related 2-D layer structures that aggregate into 3-D via similar hydrogen bonding patterns. 2. Experimental 2.1. General considerations All starting materials were obtained commercially from Aldrich. Water was deionized above 3 MX in-house. Thermogravimetric analysis was performed on a TA Instruments TGA 2050 Thermogravimetric Analyzer with a heating rate of 10 C/min up to 900 C. Elemental Analysis was carried out using a Perkin Elmer 2400 Series II CHNS/O Analyzer. Analyses for different samples of 2– 4 were consistently slightly low for carbon content, presumably due to incomplete combustion. IR spectra were recorded on a Mattson Galaxy FTIR Series 3000 using KBr pellets. 2.2. Preparation of [Co(3-pyrac)2(H2O)2] (1) Co(SCN)2 (97 mg, 0.55 mmol) and 3-pyridylacetic acid hydrochloride (193 mg, 1.10 mmol), were placed into 10 mL distilled H2O in a 23 mL Teflon-lined Parr acid digestion bomb, and then 2.2 mL of a 1.0 M NaOH solution was added (2.2 mmol) via pipet. The bomb was sealed and heated at 120 C for 23 h, whereupon it was cooled slowly to 25 C. Light pink blocks of 1 (172 mg, 84% yield based on Co) were isolated after washing with distilled water and acetone and drying in air. Crystals of 1 were stable indefinitely in air. Anal. Calc. for C14H16CoN2O6 (1): C, 45.79; H, 4.39; N, 7.63. Found: C, 45.59; H, 4.35; N, 7.52%. IR (KBr, cm1): 3230 vbr, 1590 m, 1580 m, 1569 s, 1500 w, 1440 m, 1401 s, 1288 w, 1194 w, 1170 m, 1109 w, 1060 w, 1044 w, 952 w, 811 w, 750 m, 707 m, 615 m. 2.3. Preparation of [Ni(3-pyrac)2(H2O)2] (2) Ni(SCN)2 (97 mg, 0.556 mmol) and 3-pyridylacetic acid hydrochloride (193 mg, 1.11 mmol) were placed into 10 mL distilled H2O in a 23 mL Teflon-lined Parr acid digestion bomb, and 2.8 mL of 1.0 M NaOH solution (2.8 mmol) was added via pipet. The bomb was then sealed, heated at 120 C for 24 h, and cooled slowly to 25 C. Light blue blocks of 2 (171 mg, 85% yield based on Ni) were isolated after washing with distilled water and acetone and drying in air. Crystals of 2 were stable indefinitely in air. Anal. Calc. for C14H16NiN2O6 (2): C, 45.82; H, 4.39; N, 7.63; Found: C, 44.75; H, 4.56; N, 7.38%. IR (KBr, cm1): 3200 vbr, 1611 m, 1588 m, 1569 s, 1557 m, 1485 w, 1439 m, 1397 s, 1289 w, 1194 w, 1171 w, 1109 w, 1060 w, 1037 vw, 972 vw, 949 vw, 807 w, 757 m, 708 m, 662 w, 612 m.

2.4. Preparation of [Co(3-pyrprop)2(H2O)2] (3) Co(NO3)2 Æ 6H2O (162 mg, 0.55 mmol) and 3-pyridylpropionic acid (168 mg, 1.10 mmol), were placed into 10 mL distilled H2O in a 23 mL Teflon-lined Parr acid digestion bomb, and then 0.6 mL of a 1.0 M NaOH solution was added (0.60 mmol) via pipet. The bomb was sealed and heated at 120 C for 23 h, whereupon it was cooled slowly to 25 C. Light pink blocks of 3 (97 mg, 44% yield based on Co) were isolated after washing with distilled water and acetone and drying in air. Anal. Calc. for C16H20CoN2O6 (3): C, 48.62; H, 5.10; N, 7.09; Found: C, 47.78; H, 5.16; N, 7.02%. IR (KBr, cm1): 3200 s vbr, 1590 m, 1554 s, 1500 m, 1490 m, 1475 m, 1404 s, 1312 m, 1251 w, 1205 w, 1178 w, 1136 w, 1113 w, 1041 w, 968 m, 930 w, 857 m, 826 m, 790 m, 761 m, 715 s, 658 s, 612 s, 555 m, 478 w, 405 w.

2.5. Preparation of [Ni(3-pyrprop)2(H2O)2] (4) Light blue blocks of 4 (142 mg, 65% yield based on Ni) were prepared in a similar manner to 3 above, using 97 mg Ni(SCN)2 (0.556 mmol), 168 mg 3-pyridylpropionic acid (1.11 mmol), and 0.6 mL of 1.0 M NaOH solution (0.6 mmol). Anal. Calc. for C16H20NiN2O6 (4): C, 48.65; H, 5.10; N, 7.09; Found: C, 48.08; H, 5.03; N, 6.89%. IR (KBr, cm1): 3200 s vbr, 1603 m, 1558 s, 1480 m, 1435 m, 1404 s, 1312 m, 1253 w, 1213 w, 1178 w, 1136 w, 1113 w, 1041 w, 968 m, 930 w, 857 m, 826 m, 790 m, 765 m, 707 s, 631 s, 623 s, 547 w, 474 w, 424 w.

3. X-ray crystallography Light pink blocks of 1 and 3 (with dimensions 0.42 mm · 0.26 mm · 0.20 mm and 0.42 mm · 0.26 mm · 0.24 mm, respectively), and light blue blocks of 2 and 4 (with dimensions 0.40 mm · 0.26 mm · 0.20 mm and 0.26 mm · 0.20 mm · 0.14 mm, respectively), were subjected to single crystal X-ray diffraction at 173 K using a Bruker-AXS SMART 1k CCD instrument. Reflection data was acquired using graphite–monochromated Mo ˚ ). The data was integrated Ka radiation (k = 0.71073 A via SAINT [15]. Lorentz and polarization effect and empirical absorption corrections from W scan data were applied with SADABS [16]. The structures were solved using direct methods and refined on F2 using SHELXTL [17]. All nonhydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon atoms were placed in calculated positions and refined isotropically with a riding model. The hydrogen atoms belonging to the ligated water molecules in 1–4 were found via Fourier difference maps, then restrained at fixed positions and refined isotropically. Relevant crystallographic data for 1–4 is listed in Table 1.

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Table 1 Crystal and structure refinement data for 1–4 Data

1

2

3

4

Empirical formula Formula weight Collection T (K) ˚) k (A Crystal system Space group ˚) a (A ˚) b (A ˚) c (A a () b () c () ˚ 3) V (A Z Dcalc (g cm3) l (mm1) Minimum/maximum transmission h k l Ranges

C14H16CoN2O6 367.22 173 0.71073 monoclinic P21/n 9.1472(11) 8.6343(10) 9.5513(11) 90 106.167(2) 90 724.53(15) 2 1.683 1.219 0.882 12 6 h 6 12 11 6 k 6 12, 12 6 l 6 12 8034 1737 0.0184 112/3 0.0242 0.0221 0.0600 0.0590 0.342/0.301 1.107

C14H16N2NiO6 366.98 173 0.71073 monoclinic P21/n 9.1015(10) 8.5917(9) 9.4926(10) 90 105.860(2) 90 714.04(13) 2 1.709 1.394 0.89 12 6 h 6 12, 11 6 k 6 12, 12 6 l 6 12 8221 1714 0.0195 114/3 0.0258 0.0230 0.0584 0.0570 0.396/0.293 1.089

C16H20CoN2O6 395.28 173 0.71073 monoclinic P21/n 9.4783(11) 8.7574(10) 9.9503(12) 90 104.141(2) 90 800.90(16) 2 1.639 1.109 0.895 12 6 h 6 12, 11 6 k 6 11, 13 6 l 6 13 9044 1920 0.0162 121/3 0.0254 0.0240 0.0607 0.0610 0.364/0.306 1.089

C16H20N2NiO6 395.03 173 0.71073 monoclinic P21/n 9.4139(15) 8.7417(14) 9.9170(20) 90 104.051(2) 90 791.7(3) 2 1.657 1.263 0.85 12 6 h 6 12, 11 6 k 6 11, 13 6 l 6 13 9327 1938 0.1147 121/3 0.1333 0.0452 0.1313 0.0921 0.750/1.213 1.043

Total reflections Unique reflections Rint Parameters/restraints R1a (all data) R1 (I > 2r(I)) wR2b (all data) wR2 (I > 2r(I)) ˚ 3) Maximum/minimum residual (e/A Goodness-of-fit P P a R1 = iFoj  jFci/ jFoj. P P b 2 wR2 ¼ f½wðF o  F 2c Þ2 = ½wF 2o 2 g1=2 .

4. Results and discussion 4.1. Synthesis and spectral characterization Hydrothermal reaction of Co(SCN)2 or Ni(SCN)2 with 3-pyridylacetic acid hydrochloride in the presence of base afforded high yields of crystalline solids with a formulation of [M(3-pyrac)2(H2O)2] (M = Co (1); M = Ni (2), 3pyrac = 3-pyridylacetate). Utilizing 3-pyridylpropionic acid with Co(NO3)2 Æ 6H2O and Ni(SCN)2 generated good yields of crystalline [M(3-pyrprop)2(H2O)2] ( M = Co (3); M = Ni (4), 3-pyrprop = 3-pyridylpropionate). Although 1 and 3 could be prepared using CoCl2 Æ 6H2O, and 3 could also be obtained using Co(SCN)2, the crystalline products in these cases were contaminated by a significant amount of amorphous (as determined by powder XRD) gray powder, perhaps Co metal generated via reduction. No other tractable materials besides 1 or 3, either inorganic or organic, could be obtained from these reaction mixtures. The use of a more oxidizing anion in the optimized synthesis of 3 presumably ameliorates reductive side reactions. The infrared spectra of 1–4 were similar and fully consistent with their formulations. Broad bands at 3200 cm1 in all cases documented the presence of bound aquo ligands, with their breadth indicative of extensive hydrogen

bonding. Spectral features caused by a symmetric and symmetric carboxylate stretching modes were noted at 1570 cm1 and 1400 cm1, respectively. Sharper but weaker bands in the range of 1600 cm1 to 1200 cm1 were ascribed to stretching modes of the pyridyl rings [18]. Features corresponding to pyridyl ring puckering mechanisms were evident in the region between 600 cm1 and 800 cm1. 4.2. Structural description of [M(3-pyrac)2(H2O)2] coordination polymers (1 and 2) Single crystal X-ray diffraction revealed that 1 and 2 crystallize in the centrosymmetric monoclinic space group P21/n, with asymmetric units consisting of one metal ion on an inversion center, one pyridylcarboxylate ligand, and one aquo ligand (depicted in Fig. 1 for 1). The metal ions in 1 and 2 display slightly distorted octahedral coordination geometry, with both pyridyl nitrogen donors situated in a trans orientation; this is also the case for both carboxylate oxygen donors and for both aquo ligands. The bond lengths and angles in 1–2 are standard for octahedral coordination (Table 2), with the bond lengths for 2 slightly shorter, consistent with well-known ionic radius trends [19].

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D.P. Martin et al. / Inorganica Chimica Acta 360 (2007) 599–606 Table 2 ˚ ) and angle () data for 1 and 2 Selected bond distance (A 1 Co1–O1#1 Co1–O3 Co1–N1 O1–C7 O2–C7 O1#1–Co1–O1#2 O1#1–Co1–O3 O1#2–Co1–O3 O1#1–Co1–N1 O1#2–Co1–N1 O3–Co1–N1 O3#3–Co1–N1 N1#3–Co1–N1 C3–C6–C7 O1–C7–O2

2 2.0819(9) 2.1139(10) 2.1559(11) 1.2560(16) 1.2604(16) 180.00(6) 92.76(4) 87.24(6) 91.52(4) 88.48(4) 87.92(4) 92.08(4) 180.00(5) 114.62(11) 125.20(12)

Ni1–O1 Ni1–O3 Ni1–N1 O1–C7 O2–C7 O1–Ni1–O1#4 O1–Ni1–O3 O1#4–Ni1–O3 O1–Ni1–N1#4 O1#4 –Ni1–N1#4 O3–Ni1–N1#4 O3#4–Ni1–N1 N1#4–Ni1–N1 C3–C6–C7 O1–C7–O2

2.0581(10) 2.0811(10) 2.1033(12) 1.2555(17) 1.2613(17) 180.00 93.93(4) 86.97(4) 88.62(4) 91.38(4) 92.39(4) 87.61(4) 180.00 116.2(4) 125.33(13)

Fig. 1. Asymmetric unit of 1 with thermal ellipsoids plotted at 50% probability. The complete coordination sphere about Co is shown. Hydrogen atoms attached to carbon are omitted for clarity.

Symmetry related equivalents: (#1) x + 1/2, y + 1/2, z + 1/2, (#2) x + 3/2, y  1/2, z  1/2, (#3) x + 1, y, z, (#4) x + 1, y, z + 1.

Each 3-pyridylacetate subunit serves exclusively as an exobidentate N,O-l2-bridging ligand, with the carboxylate termini binding to metal ions in a monodentate fashion. This binding pattern replicates that seen in the family of 4-pyridylacetate (4-pyrac) 2-D coordination polymers [M(4-pyrac)2(H2O)2] (M = Mn, Co, Zn) previously reported by Du and co-workers [14a]. In contrast to nicotinate ligands, which have been observed to ligate cobalt and nickel ions in l2- and l3-fashions (sometimes in the

same structure) [14a], the 3-pyrac and 4-pyrac moieties to date have only been observed to bind to these metals in an exobidentate manner. As a result of the monodenticity of the carboxylate moiety, the C–Ounligated bond distances are longer than C–Oligated distances in 1 and 2. The threeatom planes defined by the carboxylate moieties are tilted by 60 with respect to the aromatic ring plane in 1 and 2 in order to accommodate coordination of the 3-pyrac ligands that bridge neighboring metal ions.

Fig. 2. View of a single 2-D neutral layer in 1. Hydrogen bonds between ligated water molecules and unligated carboxylate oxygen atoms are shown as dashed lines. Hydrogen atoms attached to carbon atoms have been omitted.

D.P. Martin et al. / Inorganica Chimica Acta 360 (2007) 599–606

Within these coordination polymers, each metal ion connects covalently to four others through the l2-3-pyrac ligands. Due to the trans disposition of the aquo ligands at each metal center, a grid-like layer with (4,4) topology is propagated (Fig. 2 for 1). Covalent bonding within the layers is supplemented by hydrogen bonding pathways. The unligated carboxylate oxygen atoms (O2) in 1 and 2 accept hydrogen bonds from the bound water molecules within each layer (via H3B), shown as dashed lines in Fig. 2. Metrical parameters for these supramolecular interactions are given in Table 3. The intralayer Co–Co separa˚ (through tions within each grid rhomboid in 1 are 8.63 A ˚ ˚ ligand), 8.63 A, and 14.95 A (both through space). The corresponding Ni–Ni distances in 2 are slightly shorter (8.57, ˚ ). The meta-disposition of the pendant8.59, and 14.83 A arm carboxylate in the 3-pyrac ligand causes an elongation of the through-ligand metal–metal separations within the rhomboid units in 1 and 2 as compared to those observed ˚; in the related [M(4-pyrac)2(H2O)2] solids (M = Co, 8.43 A ˚ M = Ni, 8.38 A) [14a]. Correspondingly, the rhomboid motifs in 1 and 2 are splayed into a more open morphology than those in Du’s phases, which manifest intralayer

Table 3 ˚ ) and angles () within 1–4 Hydrogen bonding contact distances (A D–H  A

d(H  A)


d(D  A)

O3–H3A  O2a O3–H3B  O2b

1.918 1.921

168.85 154.79

2.747 2.726

O3–H3A  O2c O3–H3B  O2

1.936 1.912

169.35 155.61

2.772 2.708

O3–H3A  O2 O3–H3B  O2c

1.878 1.992

157.32 176.37

2.673 2.844

O3–H3A  O2b O3–H3B  O2a

1.834 2.016

161.90 169.90

2.665 2.866

1

2

3

4

Symmetry related positions: (a) x + 1, y, z  1; (b) x  1/2, y + 1/2, z + 1/2; (c) x + 1/2, y  1/2, z + 1/2.

603

˚ and through-space metal–metal distances of 15.8 A ˚ 5.7 A. The 2-D layered motifs in 1 and 2 aggregate into 3-D via interlayer hydrogen bonding between H3A-type hydrogen atoms of the bound aquo ligands, and unligated carboxylate oxygen atoms (O2) in the adjacent layer, as depicted in Fig. 3. The interlayer Oaquo  Ocarboxylate hydrogen bond˚ for 1 and 2.77 A ˚ for 2. Each unliing distances are 2.74 A gated carboxylate oxygen atom therefore accepts two hydrogen bonds, one from an aquo ligand in the same layer and one from another aquo ligand in a neighboring layer. Thus the hydrogen bonding pattern in 1 and 2 stands in contrast to that observed in the [M(4-pyrac)2(H2O)2] polymers [14a], where hydrogen bonding occurs solely within these materials’ 2-D ruffled layers, promulgating their overall 3-D structure only via van der Waals-type interactions. As seen in Fig. 3, the four oxygen atoms involved in hydrogen bonding in 1 are oriented towards the interlamellar region, forming a ring-shaped hydrogen bonding pattern with the graph symbol R44 (8) [20]. The interlayer metal– ˚ in 1, 7.06 A ˚ in 2) are shorter than metal separations (7.09 A the corresponding intralayer M–M distances. These values are significantly shorter than those found in the 4-pyrac ˚ ), illustrative of the effect of interlayer hydrophases (7.8 A gen bonding in the present case. 4.3. Structural description of [M(3-pyrprop)2(H2O)2] coordination polymers (3 and 4) As seen in Fig. 4 for 3, the asymmetric units and coordination environments within [M(3-pyrprop)2(H2O)2] (M = Co (3); M = Ni (4)) are very similar to those of aforementioned 3-pyrac analogues with the exception of an extra methylene unit in the pendant-arm portion of the pyridylcarboxylate ligand. Bond lengths and angles are consistent with slightly distorted octahedral coordination geometries about divalent Co and Ni, and are listed in Table 4. As in the 3-pyrac congeners, these materials possess l2exobidentate pyridylcarboxylate ligands, whose carboxylate termini bind in a monodentate fashion. Each metal

Fig. 3. View of hydrogen bonded neighboring metal pyridylcarboxylate layers in 1. Hydrogen bonds between ligated water molecules and unligated carboxylate oxygen atoms are shown as dashed lines. Hydrogen atoms attached to carbon are omitted for clarity.

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Fig. 4. Asymmetric unit of 3 with thermal ellipsoids plotted at 50% probability. The complete coordination spheres about Co is shown. Hydrogen atoms attached to carbon are omitted for clarity.

ion in 3 and 4 is linked to four others through the 3-pyrprop ligands, once again constructing 2-D layers with (4,4)-topology (Fig. 5), in structures isomorphous to the previously reported coordination polymer [Cu(3-pyrprop)2(H2O)2] [14b]. In order to achieve this arrangement, the carboxylate termini within the 3-pyrprop ligands are tilted by 100 with respect to the pyridyl rings, a 40 greater twisting than is seen in the related 3-pyrac phases. This conformational twist is enhanced by the greater flexibility of the 3-pyrprop ligand, which manifests a gauchelike conformation (C4–C6–C7–C8 torsion angle of 81). The intralayer Oaquo  Ocarboxylate hydrogen bonding con˚ , versus 2.75 A ˚ tact distance is slightly shorter in 3 (2.67 A for 1, see Table 3 for full hydrogen bonding details). This diminution may be rationalized by the greater flexibility of the pendant-arm tether in the 3-pyrprop ligand, which permits the unligated carboxylate to approach the aquo ligand more closely, thereby optimizing the hydrogen

Table 4 ˚ ) and angle () data for 3 and 4 Selected bond distance (A 3 Co1–O1 Co1–O3 Co1–N1 O1–C8 O2–C8 O1–Co1–O1#1 O1–Co1–O3 O1#1–Co1–O3 O1–Co1–N1 O1#1–Co1–N1 O3–Co1–N1 O3#1–Co1–N1 N1#1–Co1–N1 C4–C6–C7 C8–C7–C6 O1–C7–O2

4 2.0988(10) 2.1257(10) 2.1557(11) 1.2577(16) 1.2689(16) 180.00(5) 86.61(4) 93.39(4) 90.72(4) 89.28(4) 88.71(4) 91.29(4) 180.00(5) 115.63(11) 118.09(11) 125.13(12)

Ni1–O1 Ni1–O3 Ni1–N1 O1–C8 O2–C8 O1#3–Ni1–O1#4 O1#3–Ni1–O3 O1#4–Ni1–O3 O1#3–Ni1–N1 O1#4–Ni1–N1#4 O3–Ni1–N1 O3#5–Ni1–N1 N1#5–Ni1–N1 C2–C6–C7 C8–C7–C6 O1–C8–O2

2.074(3) 2.093(3) 2.104(4) 1.251(5) 1.265(5) 180.00(16) 86.20(12) 93.80(12) 89.21(13) 90.79(13) 88.11(13) 91.89(13) 180.00(18) 116.2(4) 118.3(4) 125.4(4)

Symmetry related equivalents: (#1) x, y, z (#2) x + 1/2, y  1/2, z  1/2 (#3) x + 3/2, y  1/2, z  1/2 (#4) x  1/2, y + 1/2, z + 1/2 (#5) x + 1, y, z.

bonding interaction. It is possible that this type of intralayer supramolecular interaction plays a role in the formation of the 2-D layers within the family of coordination polymers 1–4. A comparison of the structures of 3 and 4 with coordination polymers incorporating the rigid-arm pyridylcarboxylate ligand 3-pyridylacrylate (3-pyracryl) highlights some dissimilarities and commonalities. Eliminating the flexibility of the pendant segment of the ligand enforces a 120 kink between the pyridine donor and carboxylate moiety. Nevertheless, the 3-pyridylacrylate ligand has been shown to adopt a wide variety of binding arrangements, from tandem bridging/chelating and unidentate modes in the 1-D coordination polymers [M(3-pyracryl)2(H2O)2] (M = Co, Ni) [14c], to bridging/chelating modes alone in the 2-D material [Cd(3-pyracryl)2(H2O)] Æ 2H2O [14d], and to bridging monodentate modes in the 3-D interpenetrated coordination polymers [Cu1.5(3-pyracryl)3(H2O)] Æ 1.5H2O [14d] and lower temperature polymorphs of [M(3-pyracryl)2(H2O)2] (M = Co, Ni) [14c]. These last two materials display the same bridging/monodentate-carboxylate binding mode as 3 and 4. Due to the extra methylene unit in their organic moieties, 3 and 4 display longer intralayer metal–metal contacts than those in 1 and 2. For 3, the through-ligand Co–Co ˚ , while the respective value for 4 contact distance is 8.83 A ˚ (both 0.2 A ˚ longer than in the 3-pyrac derivais 8.79 A ˚ and tives). The through-space contact distances (8.75 A ˚ for 3, 8.74 A ˚ and 15.24 A ˚ for 4) are increased by 15.33 A ˚ for the shorter contact, and 0.5 A ˚ for the longer 0.1 A contact, as compared with 1 and 2. As in 1 and 2, the 3-D structures of 3 and 4 are fomented by supramolecular hydrogen bonding between individual 2-D [M(3-pyrprop)2(H2O)2] neutral layers with a R44 (8) ring pattern (Fig. 6). In contrast with the shorter intralayer hydrogen bonding distances in the 3-pyrprop polymers, the interlayer supramolecular hydrogen bonding contacts are ˚ than in the corresponding longer in 3 and 4 by 0.1 A 3-pyrac coordination polymers. Both intralayer and interlayer hydrogen bonding modes, organic moiety conformational effects, the bridging/monodentate bonding mode of the pyridylcarboxylate ligands, and the coordination geometric preferences of the metal ions act synergistically to direct the formation of the structures of this family of coordination polymers during hydrothermal selfassembly. 4.4. Thermal analysis To ascertain the thermal stability of this class of materials, 3 and 4 were subjected to thermogravimetric analysis. Compound 3 exhibits dehydration with an onset temperature of 180 C (11.3% mass loss, 9.8% calculated for two bound water molecules), followed by decomposition beginning at 300 C due to ligand degradation (67.4% mass loss, 67.6% predicted for two 3-pyrac ligands). The final mass was consistent with the deposition of NiO (21.3%

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Fig. 5. View of a single 2-D neutral layer in 3. Hydrogen bonds between ligated water molecules and unligated carboxylate oxygen atoms are shown as dashed lines. Hydrogen atoms attached to carbon are omitted for clarity.

Fig. 6. View of hydrogen bonded neighboring metal pyridylcarboxylate layers in 3. Hydrogen bonds between ligated water molecules and unligated carboxylate oxygen atoms are shown as dashed lines. Hydrogen atoms attached to carbon are omitted for clarity.

mass remaining, 20.3% calculated). Compound 4 behaved similarly, with dehydration commencing at 170 C (11.1% mass loss observed, 9.1% calculated for two bound water molecules) and ligand degradation beginning at 300 C (70.0% mass loss, 70.2% predicted for two 3-pyrprop ligands). The final mass remnant (19.8% of the original) correlated with that predicted for NiO (18.9%).

structure direction. These materials exhibit reasonable thermal stability, with dehydration taking place only above 170 C. Further variance of donor-bearing flexible units on difunctional pyridylcarboxylate ligands is certain to promote different covalent and non-bonding interactions, thus accessing new coordination polymers with intriguing structures and properties.

5. Conclusion

Acknowledgements

In summation, four new 2-D lamellar coordination polymers based on pendant-arm pyridylcarboxylate ligands have been hydrothermally prepared, providing deeper elucidation of intra- and interlayer supramolecular hydrogen bonding pathways and their synergistic role in

The authors gratefully acknowledge Michigan State University for financial support of this work. We thank Dr. Ronald Supkowski (King’s College, Wilkes-Barre, PA, USA) for performing the thermogravimetric analyses, Dr. Rui Huang (MSU) for elemental analysis, and Ms Sub-

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hashree Mallika Krishnan and Mr. Niral Patel (MSU) for experimental assistance. C.H.S thanks the MSU Honors College for his undergraduate Professorial Assistanship. SDG. Appendix A. Supplementary data Crystallographic data (excluding structure factors) for 1–4 have been deposited with the Cambridge Crystallographic Data Centre with nos. 602160 through 602163. Copies of the data can be obtained free of charge via the Internet at http://www.ccdc.cam.ac.uk/conts/retrieving. html or by post at CCDC, 12 Union Road, Cambridge CB2 1EZ, UK (fax: 44 1223336033, e-mail: deposit@ccdc. cam.ac.uk). Supplementary data associated with this article can be found, in the online version, at doi:10.1016/ j.ica.2006.08.006. References [1] For example: (a) J.L.C. Roswell, O.M. Yaghi, Angew. Chem. Int. Ed. Engl. 44 (2005) 4670; (b) M. Kondo, T. Okubo, A. Asami, S.-I. Noro, T. Yoshitomi, S. Kitagawa, T. Ishii, H. Matsuzaka, Angew. Chem. Int. Ed. Engl. 38 (1999) 140; (c) A.C. Sudik, A.R. Millward, N.W. Ockwig, A.P. Cote, J. Kim, O.M. Yaghi, J. Am. Chem. Soc. 127 (2005) 7110. [2] For example: (a) J.S. Seo, D. Whang, H. Lee, S.I. Jun, J. Oh, Y.J. Jeon, K. Kim, Nature 404 (2000) 982; (b) A. Cingolani, S. Galli, N. Masciocchi, L. Pandolfo, C. Pettinari, A. Sironi, J. Am. Chem. Soc. 127 (2005) 6144. [3] For example: (a) Q.-R. Fang, G.-S. Zhu, M. Xue, J.-Y. Sun, S.-L. Qiu, Shi-Lun, Dalton Trans. (2006) 2399; (b) X.-M. Zhang, M.-L. Tong, H.K. Lee, X.-M. Chen, J. Solid State Chem. (2001) 118. [4] For example: (a) S.G. Baca, M.T. Reetz, R. Goddard, I.G. Filippova, Y.A. Simonov, M. Gdaniec, N. Gerbeleu, Polyhedron 25 (2006) 1215; (b) H. Han, S. Zhang, H. Hou, Y. Fan, Y. Zhu, Eur. J. Inorg. Chem. 8 (2006) 1594; (c) W. Mori, S. Takamizawa, C.N. Kato, T. Ohmura, T. Sato, Microporous and Mesoporous Materials 73 (2004) 15. [5] For example: (a) S. Zang, Y. Su, Y. Li, Z. Ni, Q. Meng, Inorg. Chem. 45 (2006) 174; (b) L. Wang, M. Yang, G. Li, Z. Shi, S. Feng, Inorg. Chem. 45 (2006) 2474. [6] M. Eddaoudi, H. Li, O.M. Yaghi, J. Am. Chem. Soc. 122 (2000) 1391. [7] M. Murugesu, R. Clerac, B. Pilawa, A. Mandel, C.E. Anson, A.K. Powell, Inorg. Chim. Acta 337 (2002) 328, and references contained therein.

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