Copper aromatic dicarboxylate complexes with a very long conformationally flexible dipyridyl ligand: Coordination polymers and a supramolecular nanotube

Inorganica Chimica Acta 428 (2015) 73–82

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Copper aromatic dicarboxylate complexes with a very long conformationally flexible dipyridyl ligand: Coordination polymers and a supramolecular nanotube Nathan H. Murray b, Emily M. Low a, Amy L. Pochodylo b, Robert L. LaDuca b,⇑ a b

Heritage High School, Rogers, AR 72756, USA Lyman Briggs College and Department of Chemistry, Michigan State University, East Lansing, MI 48825 USA

a r t i c l e

i n f o

Article history: Received 16 October 2014 Received in revised form 4 December 2014 Accepted 10 December 2014 Available online 2 February 2015 Keywords: Copper Crystal structure Coordination polymer Dicarboxylate Dipyridylamide

a b s t r a c t Hydrothermal reaction of copper nitrate and aromatic dicarboxylate precursors with the conformationally flexible and very long-spanning dipyridylamide ligand propane-1,3-diylbis(piperidine-4,1diyl)bis(pyridin-4-ylmethanone) (4-ppbp) has generated three coordination polymers and a threaded-loop supramolecular complex. {[Cu(tere)(4-ppbp)(H2O)]1.5H2O}n (1, tere = terephthalate) and {[Cu(iph)(4-ppbp)(H2O)]2H2O}n (2, iph = isophthalate) both display slab motifs built from interdigitated (4,4) grid layers. {[Cu2(phda)(phdaH)2(4-ppbp)2]6H2O}n (3, phda = 1,4-phenylenediacetate) possesses a 4-connected 658 cds network with self-penetration of 8-membered circuits. {[Cu2(phtH)2(4-ppbp)2(H2O)4](NO3)2H2O}n (4, pht = phthalate) manifests ring-like dinuclear coordination complex cations with anionic phtH ligands projecting into supramolecular tubes with an inner diameter of approximately 12 Å. Dehydration and thermal degradation properties of these new materials are discussed. We also report the crystal structure of the recrystallized dipyridylamide, 4-ppbp2H2O. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Current investigations into the preparation, structural characterization, and properties of divalent metal coordination polymers still continue to reveal new structure types and possible applications [1–6] even after two decades of intensive basic research efforts. The overwhelming majority of divalent metal coordination polymers are built from organic dicarboxylate ligands that can provide both the necessary charge balance and structural scaffolding for the self-assembly of stable frameworks. Divalent copper aromatic dicarboxylate coordination polymers display a veritable cornucopia of coordination polymer structural topologies [7], due to the ion’s flexible coordination preferences and tendency to form clusters. The specific donor dispositions and numerous possible binding modes of the dicarboxylate components prevents a priori structure prediction; the inclusion of neutral dipyridyl-type ligands results in an even wider scope of coordination polymer structural diversity with some cases displaying intriguing physical properties [8–17].

⇑ Corresponding author at: Lyman Briggs College, 919 East Shaw Lane, E-35 Holmes Hall, Michigan State University, East Lansing, MI 48825, USA. E-mail address: [email protected] (R.L. LaDuca). http://dx.doi.org/10.1016/j.ica.2014.12.026 0020-1693/Ó 2015 Elsevier B.V. All rights reserved.

Using the ortho-dicarboxylate phthalate ligand (pht, Scheme 1) and the common rigid rod 4,40 -bipyridine (4,40 -bpy) afforded the (4,4) grid layered coordination polymer {[Cu(pht)(4,40 -bpy) (H2O)2]2H2O}n [8], while the kinked tethering ligand 4,40 -dipyridylamine (dpa) generated the 1-D nanobarrel phase {[Cu2(pht)2 (dpa)]H2O}n [9]. Employing the long-spanning bis(4-pyridylformyl)piperazine (4-bpfp) as the neutral coligand afforded two different phases depending on synthetic conditions. [Cu3(4bpfp)(pht)2(OH)2]2H2O}n exhibited a complicated 3,4,4,4-connected tetranodal 3-D coordination polymeric framework [10], while [Cu2(pht)2(4-bpfp)(H2O)2]n showed binding of the 4-bpfp formyl oxygen atoms and a previously unreported binodal layer structure with (4.648)2(4264) topology [11]. The meta-dicarboxylate isophthalate ligand (iph) showed similar condition-dependent structural diversity with 4,40 -bpy, forming two different 2-D layered coordination polymers and a 1-D double chain compound depending on the pH of the reaction solution [12]. {[Cu(iph)(4bpfp)]2H2O}n possesses a (4,4) rectangular grid structure with an unusual ABCD stacking pattern along a 41 screw axis [13]. The twofold interpenetrated 3-D phase [Cu2(tere)2(4,40 -bpy)]n (tere = terephthalate, para-benzenedicarboxylate, Scheme 1) showed a shape memory effect depending on crystallite size [14]. There have been no reports to date of copper terephthalate coordination polymers incorporating 4-bpfp, but the related aromatic

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dipyridylamide N,N0 -bis-(3-pyridyl)isophthalamide (3-bpia) afforded either a metallogel or the corrugated sheet coordination polymer phase {[Cu(tere)(3-bpia)(H2O)2]2H2O}n depending on the solvent used during self-assembly [15]. Extension of the paradicarboxylate pendant arms in 1,4-phenylenedicarboxylate (phda, Scheme 1) resulted in a decrease in dimensionality compared to the related 3-D terephthalate phase in [Cu(phda)(4,40 -bpy)]2H2O}n, which exhibited a 2-D binodal (42678)(426) sheet topology [16]. {[Cu4(phda)3(OH)2(dpp)2]2H2O}n (dpp = 1,3-di(4-pyridyl)propane) manifests tetrameric {Cu4(OH)2}6+ kernels, connected into a very rare 8-connected self-penetrated ilc lattice with 4245.63 topology [17]. Unlike the ubiquitous rigid-rod tether 4,40 -bpy or the uncommonly encountered longer bispyridylamide 4-bpfp, the very longspanning conformationally flexible dipyridylamide propane-1,3diylbis(piperidine-4,1-diyl)bis(pyridin-4-ylmethanone) (4-ppbp, Scheme 1) has never been employed in the construction of coordination polymers to the best of our knowledge. Another potential advantage of this ligand besides its traditional dipodal pyridyl nitrogen atom donors and potential carbonyl oxygen atom donors is the hydrogen bonding capability at its central amide functional groups. Additionally, 4-ppbp has an ability to adopt ‘‘S’’, ‘‘L’’, or ‘‘U’’ conformations depending on its N  N  N  N dihedral angle (defined as w) locked into the solid-state structure. While a w angle of 0° would result in an idealized ‘‘U’’ conformation, a w angle of 180° would cause a ‘‘S’’ conformation. The ‘‘L’’ conformation would result from intermediate, skewed w angles. This enhanced conformational flexibility on the part of trimethylene group within the 4ppbp ligand could potentially facilitate different span lengths between adjacent metal centers, and exert a significant structure directing effect in the resulting coordination polymer. In this study we thus aimed to survey the effect of benzenedicarboxylate donor disposition and pendant arm length in a series of divalent copper coordination polymers containing the long-spanning 4-ppbp ligand. Herein we report the synthesis, structural and topological characterization, and preliminary physical property investigations of three new crystalline coordination polymers {[Cu(tere)(4ppbp)(H2O)]1.5H2O}n (1), {[Cu(iph)(4-ppbp)(H2O)]2H2O}n (2), {[Cu2(phda)(phdaH)2(4-ppbp)2]6H2O}n (3), and the complex cation salt {[Cu2(phtH)2(4-ppbp)2(H2O)4](NO3)2H2O}n (4).

synthesized according to a modification the published procedure for the preparation of 4-bpfp [18], via condensation of trimethylbipiperidine and isonicotinoyl chloride hydrochloride in dry pyridine with elimination of pyridinium chloride (Scheme 2). The hydrate 4-ppbp2H2O was formed by recrystallizing 4-ppbp from deionized water. Water was deionized above 3 MX-cm in-house. IR spectra were recorded on powdered samples using a Perkin Elmer Spectrum One DRIFT instrument. Elemental Analysis was carried out using a Perkin Elmer 2400 Series II CHNS/O Analyzer. Thermogravimetric analysis was performed on a TA Instruments high-resolution Q500 thermal analyzer under flowing N2. 2.2. Preparation of {[Cu(tere)(4-ppbp)(H2O)]1.5H2O}n (1) Cu(NO3)22.5H2O (65 mg, 0.28 mmol), potassium terephthalate (57 mg, 0.24 mmol) and 4-ppbp (116 mg, 0.28 mmol) were placed into 10 mL distilled H2O in a Teflon-lined acid digestion bomb. The bomb was sealed and heated in an oven at 120 °C for 120 h, and then cooled slowly to 25 °C. Blue crystals of 1 (134 mg, 80% yield) were isolated after washing with distilled water and acetone, and drying in air. Anal. Calc. for C33H41CuN4O8.5 1: C, 57.17; H, 5.96; N, 8.08. Found: C, 56.94; H, 5.87; N, 8.09%. IR (cm1): 2922(w), 2857(w), 1627(m), 1599(s), 1550(m), 1499(w), 1447(m), 1413(m), 1360(s), 1287(m), 1217(m), 1136(w), 1122(w), 1063(w), 1020(w), 979(w), 841(s), 816(m), 776(m), 743(s), 703(s), 655(m). 2.3. Preparation of {[Cu(iph)(4-ppbp)(H2O)]2H2O}n (2) Cu(NO3)22.5H2O (65 mg, 0.28 mmol), isophthalic acid (46 mg, 0.28 mmol) and 4-ppbp (116 mg, 0.28 mmol) and 0.50 mL of a 1.0 M NaOH solution were placed into 10 mL distilled H2O in a

2. Experimental 2.1. General considerations Copper nitrate, dicarboxylic acids, and ligand synthesis precursors were purchased commercially. Potassium terephthalate was prepared via the reaction of excess potassium hydroxide with terephthalic acid in ethanol. The dipyridyl ligand propane-1,3-diylbis(piperidine-4,1-diyl)bis(pyridin-4-ylmethanone) (4-ppbp) was

Scheme 2. Synthesis of 4-ppbp.

Scheme 1. Organic precursors used for coordination polymer synthesis.

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Teflon-lined acid digestion bomb. The bomb was sealed and heated in an oven at 120° C for 48 h, and then cooled slowly to 25 °C. Blue crystals of 2 (92 mg, 47% yield) were isolated after washing with distilled water and acetone, and drying in air. Anal. Calc. for C66H84Cu2N8O18 2: C, 56.44; H, 6.03; N, 7.97. Found: C, 56.96; H, 5.67; N, 8.36%. IR (cm1): 3406(w), 2930(w), 1628(s), 1613(s), 1561(m), 1499(w), 1469(m), 1448(m), 1437(m), 1412(s), 1392(m), 1367(s), 1323(m), 1279(s), 1228(w), 1213(w), 1158(w), 1120(w), 1093(w), 1061(w), 1029(w), 1008(w), 970(m), 957(w), 934(w), 890(w), 839(m), 764(m), 743(s), 717(s), 703(m).

2.4. Preparation of {[Cu2(phda)(phdaH)2(4-ppbp)2]6H2O}n (3) Cu(NO3)22.5H2O (86 mg, 0.37 mmol), 1,4-phenylenediacetic acid (72 mg, 0.37 mmol) and 4-ppbp (115 mg, 0.37 mmol) were placed into 10 mL distilled H2O in a Teflon-lined acid digestion bomb. The bomb was sealed and heated in an oven at 100 °C for 24 h, and then cooled slowly to 25 °C. Blue crystals of 3 (89 mg, 29% yield based on Cu) were isolated after washing with distilled water and acetone, and drying in air. Anal. Calc. for C80H102Cu2N8O22 3: C, 58.07; H, 6.21; N, 6.77. Found: C, 58.44; H, 6.05; N, 6.49%.

Table 1 Crystal and structure refinement data. Data

1

2

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) Minimum/maximum transmission hkl ranges

C33H41CuN4O8.5 693.24 triclinic  P1 10.8918(9) 12.0051(10) 13.5228(11) 78.457(1) 87.280(1) 67.674(1) 1601.8(2) 2 1.456 0.744 0.9486 13 6 h 6 13, 14 6 k 6 14, 16 6 l 6 16 26 564 5879 0.0309 431 0.0483 0.0407 0.1135 0.1084 0.545/0.897 1.072

C66H84Cu2N8O18 1404.49 monoclinic P21/n 17.8450(2) 10.0557(1) 37.8562(3) 90 96.273(1) 90 6752.39(11) 4 1.382 1.409 0.8505 18 6 h 6 21, 12 6 k 6 11, 46 6 l 6 46 43 950 12 204 0.0854 873 0.0953 0.0560 0.1735 0.1438 0.583/0.514

Total reflections Unique reflections Rint Parameters R1 (all data)a R1 (I > 2r(I))a wR2 (all data)b wR2 (I > 2r(I))b Max/min residual (e Å3) Goodness-of-fit (GOF) Data

3

4

4-ppbp2H2O

Empirical formula Formula weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z Dcalc (g cm3) l (mm1) Minimum/maximum transmission hkl ranges

C80H102Cu2N8O22 1654.78 triclinic  P1

C66H84Cu2N10O23 1512.52 triclinic  P1

7.4106(17) 10.898(3) 24.760(5) 96.993(4) 97.886(6) 92.198(4) 1963.0(7) 1 1.400 0.622 0.9380 8 6 h 6 8, 13 6 k 6 13, 29 6 l 6 29 32 473 7161 0.0543 524 0.0905 0.0627 0.1906 0.1686 0.887/1.253 1.058

6.8586(9) 15.800(2) 17.344(2) 110.834(2) 99.468(2) 93.209(2) 1719.3(4) 1 1.461 0.705 0.8803 8 6 h 6 8, 19 6 k 6 18, 20 6 l 6 20 25 504 6325 0.1074 466 0.1082 0.0535 0.1194 0.1012 0.513/0.527 1.009

C25H36N4O4 456.58 monoclinic P21/c 21.431(2) 6.6341(7) 18.695(2) 90 114.298(1) 90 2422.5(4) 4 1.252 0.086 0.9185 25 6 h 6 25, 8 6 k 6 8, 22 6 l 6 22 19 058 4436 0.0428 314 0.0574 0.0363 0.0811 0.0714 0.166/0.178 1.020

Total reflections Unique reflections Rint Parameters R1 (all data) a R1 (I > 2r(I)) a wR2 (all data) b wR2 (I > 2r(I)) b Maximum/minimum residual (e Å3) Goodness-of-fit (GOF) P P a R1 = ||Fo|  |Fc||/ |Fo|. P P b 2 wR2 = { [w(Fo  F2c )2]/ [wF2o]2}1/2.

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IR (cm1): 3300 (w, br), 2913(w), 1716(m), 1637(m), 1609(m), 1577(s), 1514(w), 1440(s), 1358(s), 1326(w), 1290(s), 1221(w), 1165(m), 1066(m), 969(m), 838(m), 793(m), 767(w), 704(s). 2.5. Preparation of {[Cu2(phtH)2(4-ppbp)2(H2O)4](NO3)2H2O}n (4) Cu(NO3)22.5H2O (65 mg, 0.28 mmol), phthalic acid (46 mg, 0.28 mmol) and 4-ppbp (116 mg, 0.28 mmol) and 0.50 mL of a 1.0 M NaOH solution were placed into 10 mL distilled H2O in a Teflon-lined acid digestion bomb. The bomb was sealed and heated in an oven at 120° C for 48 h, and then cooled slowly to 25 °C. Blue crystals of 4 (97 mg, 46% yield based on Cu) were isolated after washing with distilled water and acetone, and drying in air. Anal. Calc. for C66H84Cu2N10O23 4: C, 52.41; H, 5.60; N, 9.26. Found: C, 52.56; H, 5.36; N, 9.30%. IR (cm1): 3300 (w, br), 2928(w), 1683(w), 1625 (s) 1611(s), 1509(w), 1473(w), 1450(m), 1421(m), 1366(s), 1311(s), 1224(w), 1149(w), 1075(w), 1036(m), 975(w), 929(w), 835(m), 807(m), 762(s), 708(m), 690(s). 3. X-ray crystallography Single crystal X-ray diffraction was performed on single crystals of 1–4 with a Bruker-AXS ApexII CCD instrument at 173 K. Diffraction data was acquired using graphite-monochromated Mo Ka radiation (k = 0.71073 Å) for 1, 3, and 4, and with Cu Ka radiation (k = 1.54178 Å) for 2. The data was integrated via SAINT [19]. Lorentz and polarization effect and empirical absorption corrections were applied with SADABS [20]. The structures were solved using direct methods and refined on F2 using SHELXTL [21] subroutines within the OLEX2 crystallographic suite [22]. All non-hydrogen atoms were refined anisotropically. One of the piperidinyl rings within the 4ppbp ligand in the structure of 1 was disordered and modeled with a 75:25 ratio of partial occupancies. The trimethylene tether within the 4-ppbp ligand in the structure of 3 was disordered and was modeled with a 75:25 ratio of partial occupancies. Disordered water molecules of crystallization in 2 were treated with partial occupancies. Hydrogen atoms were placed in calculated positions and refined isotropically with a riding model. Relevant crystallographic data for 1–4 and 4-ppbp2H2O is listed in Table 1. 4. Results and discussion 4.1. Synthesis and infrared spectra The 4-ppbp ligand was easily prepared via the condensation of two molar equivalents of isonicotinoyl chloride hydrochloride and one molar equivalent of trimethylenepiperidine in dry pyridine solvent (Scheme 2). The final product was largely insoluble in pyridine, so it was filtered and then washed with cold water and dried in air. Compounds 1–4 were prepared cleanly by hydrothermal reaction of copper nitrate and 4-ppbp with potassium terephthal-

ate (for 1) or the requisite dicarboxylic acid and aqueous base (for 2–4). The infrared spectra of compounds 1–4 were consistent with structural components determined by single-crystal X-ray diffraction. Intense, slightly broadened asymmetric and symmetric C–O stretching bands were observed at 1599 and 1360 cm1 in 1, 1561 and 1367 cm1 in 2, 1577 and 1358 cm1 in 3, and 1611 and 1311 cm1 in 4. The C–O stretching band within the protonated phdaH carboxylate groups in 3 was observed at 1716 cm1. The C–O stretching band within the protonated phtH carboxylate groups and the N–O stretching band for the unligated nitrate anions in 4 were observed at 1683 and 1421 cm1, respectively. The C@O stretching bands within the 4-ppbp ligands were observed at 1628 cm1 in 1, 1629 cm1 in 2, 1637 cm1 in 3, and 1625 cm1 in 4. Sharper bands in the range of 1610 cm1 to 1300 cm1 were attributed to stretching modes of pyridyl rings within the 4-ppbp ligands and the aromatic rings of the dicarboxylate ligands [23]. Features corresponding to C–H bending and arene puckering within the pyridyl rings exist in the region between 900 and 650 cm1. Broad, weak spectral bands in the vicinity of 3000–3200 cm1 indicate the presence of bound and unbound water molecules. IR spectra for 1–4 are shown in Figs. S1 through S4, respectively. 4.2. Structural description of 4-ppbp2H2O In its crystalline state when co-crystallized with two water molecules per asymmetric unit, 4-ppbp adopts a twisted L-conformation with a w angle of 68.0° (Fig. 1). Water molecule pairs engage in hydrogen bonding donation to one of the terminal pyridyl nitrogen atoms. The 4-ppbp molecules are arranged into slab patterns by means of hydrogen bonding donation provided by the water molecule pairs, that span between a pyridyl ring in one 4-ppbp molecule and the carbonyl group of an amide moiety in another (Fig. S5). The supramolecular slabs stack in an ABAB pattern along the c crystal direction (Fig. S6). 4.3. Structural description of {[Cu(tere)(4-ppbp)(H2O)]1.5H2O}n (1) The asymmetric unit of compound 1 contains a divalent copper atom, a deprotonated tere ligand, a 4-ppbp ligand in a L-conformation, and aqua ligand, and two water molecules of crystallization. The coordination environment at copper is a slightly distorted {CuO3N2} square pyramid, with an Addison s factor of 0.027 [24] (Fig. 2). Pyridyl nitrogen donor atoms from two 4-ppbp ligands occupy a basal position and the apical site in a cis orientation; the aqua ligand occupies the basal site trans to the basal pyridyl donor. The other two trans basal sites are filled by carboxylate oxygen atoms from two tere ligands. Bond lengths and angles within the coordination sphere are listed in Table 2. Bis(monodentate) tere ligands connect copper atoms in 1-D [Cu(tere)(H2O)]n chain motifs oriented along [1 0 0], with a Cu  Cu

Fig. 1. Complete 4-ppbp molecule and water molecules of crystallization with thermal ellipsoids drawn at 50% probability.

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Fig. 2. Coordination environment in 1. Thermal ellipsoids are drawn at 50% probability.

Table 2 Selected bond distance (Å) and angle (°) data for 1. Cu1–O1 Cu1–N1 Cu1–N4#1 Cu1–O7 Cu1–O3#2 O1–Cu1–N1 O1–Cu1–N4#1 O1–Cu1–O7

1.9664(17) 2.044(2) 2.270(2) 1.9760(18) 1.9579(17) 90.65(8) 92.78(8) 87.96(7)

N1–Cu1–N4#1 O7–Cu1–N1 O7–Cu1–N4#1 O3#2–Cu1–O1 O3#2–Cu1–N1 O3#2–Cu1–N4#1 O3#2–Cu1–O7

91.29(8) 176.21(7) 92.30(8) 177.84(7) 90.55(8) 88.99(8) 90.72(7)

Symmetry transformations to generate equivalent atoms: #1 x  1, y, z + 1; #2 x + 1, y, z.

contact distance of 10.8918 Å representing the a lattice parameter. These are pillared into [Cu(tere)(4-ppbp)(H2O)]n (4,4) grid coordination polymer layers (Fig. 3a) by twisted L-conformation 4-ppbp ligands (w angle = 65.4°) that span axial and basal coordination sites on the linked copper atoms at a Cu  Cu distance of 16.96 Å; the layer motifs are oriented along the ac crystal plane. Pinched apertures within the grid measure 13.52 by 25.09 Å, with Cu  Cu  Cu angles of 52.8° and 127.2°. Within each layer motif the L-conformation 4-ppbp ligands all curl in the same direction. These engage in aromatic C–H  O interactions with carbonyl groups of 4-ppbp ligands in another layer in which the dipyridyl ligands curl in the opposite direction thereby forming double layer slab supramolecular motifs (Fig. 3b). Individual layers within the slab interleave but do not interpenetrate, with the 4-ppbp ligands from one layer projecting into apertures within the other. Isolated

water molecules are located within the slab motifs, fixed in place in the crystal by hydrogen bonding donation to unligated tere oxygen atoms and to 4-ppbp carbonyl groups. These occupy small incipient voids comprising 2.6% of the unit cell volume, according to PLATON [25]. Hydrogen bonding interactions between aqua ligands and unligated tere carboxylate oxygen atoms (Table S1) anchor the slab motifs together, which stack in an AAA pattern along the b crystal direction (Fig. S7). The corrugated grid topology of 1 is contrasted with the twofold interpenetrated primitive cubic type net seen in the previously reported copper terephthalate phase containing the rigid-rod linker bpy [14], but has some similarity to a related phase with a longer-spanning dipyridylamide [15]. 4.4. Structural description of {[Cu(iph)(4-ppbp)(H2O)]2H2O}n (2) The asymmetric unit of compound 2 contains a two divalent copper atoms (Cu1, Cu2), two iph ligands, two 4-ppbp ligands in slightly different S-conformations, two aqua ligands, and net four and half water molecules of crystallization. The coordination environments at Cu1 and Cu2 are both Jahn–Teller distorted {CuO4N2} octahedra (Fig. 4), in contrast with the square pyramidal coordination seen in the terephthalate derivative 1. Both Cu1 and Cu2 exhibit a similar arrangement of donor atoms. Pyridyl nitrogen donor atoms from two 4-ppbp ligands trans positions; the aqua ligand occupies the basal site trans to the basal pyridyl donor. An aqua ligand and an oxygen atom from a monodentate iph ligand fill cis coordination sites. The remaining two cis coordination sites are

Fig. 3. (a) [Cu(tere)(4-ppbp)(H2O)]n (4,4) grid coordination polymer layer in 1. (b) Side view of interdigitated double layer slab supramolecular motif in 1.

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Fig. 4. Coordination environment at Cu1 in 2. Thermal ellipsoids are drawn at 50% probability. The coordination environment at Cu2 is largely similar, but with a very slightly different 4-ppbp conformation.

Table 3 Selected bond distance (Å) and angle (°) data for 2. Cu1–O1 Cu1–O2 Cu1–O4#1 Cu1–O7 Cu1–N1 Cu1–N4#2 O1–Cu1–O2 O1–Cu1–O7 O1–Cu1–N1 O1–Cu1–N4#2 O4#1–Cu1–O1 O4#1–Cu1–O2 O4#1–Cu1–O7 O4#1–Cu1–N1 O4#1–Cu1–N4#2 O7–Cu1–O2 N1–Cu1–O2 N1–Cu1–O7 N1–Cu1–N4#2 N4#2–Cu1–O2 N4#2–Cu1–O7

1.992(3) 2.427(3) 1.941(3) 2.344(3) 2.038(3) 2.046(3) 58.93(10) 106.53(11) 88.10(12) 92.62(12) 158.43(12) 99.53(11) 95.02(12) 90.79(13) 88.79(13) 165.44(10) 89.92(12) 90.23(12) 179.04(14) 91.00(11) 88.94(12)

Cu2–O8 Cu2–O10#3 Cu2–O11#3 Cu2–O14 Cu2–N5 Cu2–N8#4 O8–Cu2–O10#3 O8–Cu2–O11#3 O8–Cu2–O14 O8–Cu2–N5 O8–Cu2–N8#4 O11#3–Cu2–O10#3 O11#3–Cu2–O14 O11#3–Cu2–N5 O11#3–Cu2–N8#4 O14–Cu2–O10#3 N5–Cu2–O10#3 N5–Cu2–O14 N5–Cu2–N8#4 N8#4–Cu2–O10#3 N8#4–Cu2–O14

1.935(3) 2.420(3) 2.013(3) 2.283(3) 2.032(3) 2.032(3) 100.51(11) 159.33(12) 95.34(12) 91.00(13) 87.87(13) 58.82(10) 105.31(11) 89.02(12) 91.50(12) 163.73(11) 90.63(11) 92.73(12) 178.09(13) 88.06(12) 88.91(12)

Symmetry transformations to generate equivalent atoms: #1 x, y + 1, z; #2 x + 1/2, y + 5/2, z + 1/2; #3 x, y  1, z; #4 x  1/2; y + 5/2, z  ½.

occupied by a chelating carboxylate group from another iph ligand. Bond lengths and angles within the coordination sphere are listed in Table 3.

All of the iph ligands in 2 adopt a monodentate/chelating binding mode, connecting copper atoms into 1-D [Cu(iph)(H2O)]n chain motifs oriented along b, with a Cu  Cu contact distance of 10.065 Å. These chains are pillared into [Cu(iph)(4-ppbp)(H2O)]n (4,4) grid coordination polymer layers (Fig. 5a) by twisted S-conformation 4-ppbp ligands (w angle = 161.3° or 164.6°). The slightly different 4-ppbp conformations result in the crystallographic distinction between Cu1 and Cu2 atoms, as they provide different Cu1  Cu1 (20.296 Å) and Cu2  Cu2 (20.560 Å) contact distances. Not unexpectedly, the larger w angle affords the longer Cu  Cu distance. Within the layer motifs of 2, the grid apertures are far less pinched than those in 1, due to the trans orientation of the 4-ppbp pyridyl donor atoms in 2. For the Cu1-containing layers, the Cu  Cu  Cu angles subtend angles of 80.6° and 99.4°, with Cu  Cu distances of 21.131 and 24.074 Å. For the Cu2-containing layers, the Cu  Cu  Cu angles subtend angles of 76.9° and 103.1°, with Cu  Cu distances of 20.738 and 24.852 Å. Adjacent layer motifs in 2 interdigitate to form dual layer supramolecular slabs (Fig. 5b), facilitated by water molecules of crystallization embedded within the slabs. These engage in hydrogen bonding between aqua ligands in one layer within the slab, and unligated iph carboxylate oxygen atoms in the other layer within the slab (Table S1). These occupy incipient voids comprising 7.9% of the unit cell volume. Neighboring dual layer slabs stack in an ABAB pattern along the a direction (Fig. S8), interacting via C– H  O interactions between 4-ppbp pyridyl ring carbon atoms in one slab and 4-ppbp carbonyl groups in another (C  O

Fig. 5. (a) [Cu(iph)(4-ppbp)(H2O)]n (4,4) grid coordination polymer layer in 2. (b) Side view of interdigitated double layer slab supramolecular motif in 2.

N.H. Murray et al. / Inorganica Chimica Acta 428 (2015) 73–82 Table 4 Selected bond distance (Å) and angle (°) data for 3. Cu1–O8 Cu1–O8#1 Cu1–N1 Cu1–N1#1 Cu2–O2#2 Cu2–O2#3 Cu2–O4 Cu2–O4#4 Cu2–N3#4 Cu2–N3 O8–Cu1–O8#1 O8–Cu1–N1#1 O8–Cu1–N1 O8#1–Cu1–N1 O8#1–Cu1–N1#1 N1–Cu1–N1#1

1.941(3) 1.941(3) 2.007(3) 2.007(3) 2.610(3) 2.610(3) 1.940(3) 1.940(3) 2.027(3) 2.027(3) 179.998(1) 87.84(13) 92.16(13) 87.84(13) 92.16(13) 180.00(9)

O2#3–Cu2–O2#2 O4–Cu2–O2#3 O4#4–Cu2–O2#2 O4#4–Cu2–O2#3 O4–Cu2–O2#2 O4#4–Cu2–O4 O4#4–Cu2–N3#4 O4#4–Cu2–N3 O4–Cu2–N3 O4–Cu2–N3#4 N3–Cu2–O2#2 N3#4–Cu2–O2#2 N3#4–Cu2–O2#3 N3–Cu2–O2#3 N3–Cu2–N3#4

180.00(12) 88.21(10) 88.21(10) 91.79(11) 91.79(11) 180.00(16) 89.48(12) 90.52(12) 89.48(12) 90.52(12) 93.32(12) 86.68(12) 93.32(12) 86.68(12) 180.00(10)

Symmetry transformations to generate equivalent atoms: #1 x + 4, y + 2, z; #2 x; y  1, z; #3 x, y + 2, z + 1; #4 –x, y + 2, z + 1.

distance = 3.052 Å). The structural topology of 2 is overall similar to that seen in a copper isophthalate phase containing the shorter dipyridylamide bpfp, however the previously reported bpfp-containing material has a different stacking pattern (ABCD) and does not exhibit any appreciable interdigitation [13]. 4.5. Structural description of {[Cu2(phda)(phdaH)2(4-ppbp)2]6H2O}n (3) The asymmetric unit of compound 3 contains two divalent copper atoms (Cu1, Cu2) on crystallographic inversion centers, half of a deprotonated phda ligand whose central aromatic ring is sited over an inversion center, a singly protonated phdaH ligand, a complete 4-ppbp ligand, and three water molecules of crystallization. The Cu1 atoms display a simple {CuN2O2} square planar coordination environment, where the trans nitrogen donor atoms belong to

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pyridyl rings within two different 4-ppbp ligands. Carboxylate oxygen atom donors from two deprotonated phda ligands occupy the other two trans positions. In contrast the Cu2 atoms possess a Jahn–Teller distorted {CuN2O4} octahedral coordination environment, with the elongated axial sites taken up by hydroxyl oxygen donors belonging to the protonated termini of the phdaH ligands. Trans carboxylate oxygen donors belonging to the unprotonated endgroups of the phdaH ligands are located in the equatorial plane, as are trans pyridyl nitrogen donors from two 4-ppbp ligands. Bond lengths and angles within the different coordination spheres are listed in Table 4. Coordination environments with thermal ellipsoids are shown in Fig. 6. The Cu1 atoms are linked into [Cu(phda)]n chain motifs by bis(monodentate) phda ligands in a splayed-open anti conformation (Fig. 7a), that span a Cu  Cu distance of 13.412 Å. These  1 0]. The Cu2 atoms are connected motifs are arranged parallel to [1 by pairs of anti conformation phdaH ligands to construct [Cu(phdaH)2]n chain motifs (Fig. 7b) in which the Cu  Cu distance is 10.898 Å; these chains are oriented parallel to the b crystal direction. The [Cu(phda)]n chain motifs based on Cu1 atoms and the [Cu(phdaH)2]n chain motifs built from Cu2 atoms are then connected into a 3-D {[Cu2(phda)(phdaH)2(4-ppbp)2]n coordination polymer network (Fig. 8) by S-conformation 4-ppbp ligands (w angle = 152.3°). These connect Cu1 and Cu2 atoms in different chain motifs with a Cu  Cu distance of 21.522 Å. The 3-D {[Cu2(phda)(phdaH)2(4-ppbp)2]n coordination polymer network in 3 can be simplified by invoking each Cu1 and Cu2 atom as a 4-connected node, with the pairs of phdaH ligands being treated as single linkers. According to TOPOS [26], this net possesses a 658 cds topology (Fig. 9a) with Hopf linkages within the Cu1and Cu2-based 8-membered circuits (Fig. 9b) resulting in a selfpenetrated network. Isolated water molecules of crystallization and water molecule pairs occupy small cavities comprising 6.1% of the unit cell volume. These are anchored to the 3-D coordination polymer net by hydrogen bonding donation to phda and phdaH

Fig. 6. Coordination environments in 3. Thermal ellipsoids are drawn at 50% probability.

Fig. 7. (a) [Cu(phda)]n chain motif based on Cu1 atoms in 3. (b) [Cu(phdaH)2]n chain motifs based on Cu2 atoms in 3.

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Fig. 8. {[Cu2(phda)(phdaH)2(4-ppbp)2]n 3-D coordination polymer network in 3.

Fig. 9. (a) Schematic perspective of the 658 cds topology of 3. (b) Close-up of self-penetration of 8-membered rings in the network of 3.

Table 5 Selected bond distance (Å) and angle (°) data for 4. Cu1–O10 Cu1–O11 Cu1–O3 Cu1–N1 Cu1–N4#1 O11–Cu1–O10 O11–Cu1–N1 O11–Cu1–N4#1

2.208(3) 1.991(2) 1.950(3) 1.993(3) 1.999(3) 94.42(10) 90.75(11) 87.24(11)

O3–Cu1–O10 O3–Cu1–O11 O3–Cu1–N1 O3–Cu1–N4#1 N1–Cu1–O10 N1–Cu1–N4#1 N4#1–Cu1–O10

98.22(11) 167.06(11) 91.90(11) 88.80(11) 90.41(12) 173.83(14) 95.55(12)

Symmetry transformation to generate equivalent atoms: #1 x + 2, y, z + 1.

carboxylate oxygen atoms and also 4-ppbp carbonyl groups (Table S1). 4.6. Structural description of {[Cu2(phtH)2(4ppbp)2(H2O)4](NO3)2H2O}n (4) The asymmetric unit of compound 4 contains a divalent copper atom, a singly protonated phtH ligand, a 4-ppbp ligand, two aqua ligands, an unligated nitrate counteranion and a water molecule of crystallization refined at half occupancy. The coordination environment is a distorted {CuN2O3} square pyramid (s = 0.117), with cis aqua ligands in apical and basal positions. Trans pyridyl nitrogen donor atoms from two 4-ppbp ligands lie in the basal plane, along with an oxygen donor atom from the unprotonated carboxylate group of a phtH ligand. Pertinent bond lengths and angles for 4 are listed in Table 5.

Fig. 10. Discrete [Cu2(phtH)2(4-ppbp)2(H2O)4]2+ cationic coordination complex in 4. Thermal ellipsoids are drawn at 50% probability.

The 4-ppbp ligands in 4 adopt a nearly perfect U-conformation (w angle = 9.9°), forming a [Cu2(4-ppbp)2] ring motif with a Cu  Cu distance of 12.336 Å. Singly protonated phtH ligands project towards the interior of the ring, resulting in discrete

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Fig. 11. Supramolecular tube motif in 4. Hydrogen bonding between protonated phtH carboxylate groups is shown as dashed lines.

[Cu2(phtH)2(4-ppbp)2(H2O)4]2+ cationic coordination complexes (Fig. 10) centered on large 44-membered rings. The protonated termini of the phtH ligands project slightly above and slightly below the plane of the ring, and the aqua ligands are oriented towards the exterior of the ring. The C  C contact distance between the central C atoms within the trimethylene regions of the 4-ppbp ligands is 15.843 Å. Neighboring [Cu2(phtH)2(4-ppbp)2(H2O)4]2+ complex cations are arranged along the [3 1 0] direction, anchored by hydrogen bonding (Table S1) between the protonated carboxylate groups of the phtH ligands into a supramolecular tube motif (Fig. 11). Water molecules of crystallization are also located within the nanotubular motifs, held by hydrogen bonding donation to the carbonyl groups of the 4-ppbp ligands. The unligated water molecules occupy very narrow incipient channels embedded inside the nanotubes, comprising 0.8% of the unit cell volume. Adjacent supramolecular tubes aggregate via nitrate counterions (Fig. S9), which accept hydrogen bonding from the aqua ligands within the complex cations (Table S1). These are also engaged in C–H  O interactions (C  O distance = 3.214 Å) mediated by piperazinyl rings within the 4-ppbp ligands. The supramolecular nanotubular structure of 4 contrasts very greatly with previously reported copper phthalate coordination polymers, from the simple (4,4) grid of the bpy derivative [8] to the complicated 2D [10] or 3D nets [11] formed with the shorter-spanning dipyridylamide 4-bpfp ligand. This variance is ascribed to the ability of the 4-ppbp ligand to adopt a long staple-like U-conformation, inaccessible to shorter dipyridines like bpy or even 4-bpfp.

corresponding well to the 6.5% calculated for six molar equivalents of water. Ligand ejection occurred above 215 °C. The final mass remnant of 10.2% at 600 °C matches reasonably well with a putative deposition of CuO (9.6% calc’d). Compound 4 underwent a 6.7% mass loss between 25 and 150 °C, corresponding to loss of bound and unligated water molecules (7.9% calc’d). Ligand ejection and counteranion loss occurred above 150 °C. The final mass remnant of 14.0% represents a likely mixture of CuO (11.6% calc’d) and CuCO3 (18.1% calc’d). Thermogravimetric analysis traces for 1–4 are shown in Figs. S10–S13. 5. Conclusions The conformationally flexible and very long-spanning dipyridylamide ligand 4-ppbp was employed for the first time in coordination polymer synthesis. A complicated set of synergistic interactions between 4-ppbp conformation, copper ion coordination geometry preference, and aromatic dicarboxylate donor disposition, pendant arm length, and binding mode dictated the overall topology. Shorter dicarboxylate arms appeared to result in interdigitated pairs of (4,4) rectangular grids, even though the 4-ppbp pyridyl nitrogen donor atoms bound to copper in either cis or trans orientation. In the case of protonated phthalate ligands, an intriguing 44-membered ring-shaped complex cation was obtained, which showed a U-shaped 4-ppbp conformation. Supramolecular tubes of these cations entrained water molecules of crystallization. It is expected that 4-ppbp and its geometric isomers will produce divalent metal coordination polymers with a wide topological scope; efforts in this direction are currently underway in our laboratory.

4.7. Thermogravimetric analysis To probe the decomposition behavior of the coordination polymers prepared in this study, thermogravimetric analysis was undertaken on polycrystalline samples of compounds 1–4. Compound 1 underwent dehydration in two stages between 25 and 245 °C, with a mass loss of 9.0% corresponding to ejection of three molar equivalents of water (7.8% calc’d). Above 245 °C, ligand combustion occurred. The final mass remnant of 13.8% corresponds to a plausible deposition of CuO (11.3% calc’d) with some admixed CuCO3 (17.6% calc’d). There was too little sample remaining to undertake PXRD to verify the phases present. Compound 2 underwent loss of its water molecules of crystallization between 25 and 65 °C, denoted by a mass loss of 5.2% (5.7% calc’d). The mass remained stable between 65 and 235 °C, whence ligand ejection occurred. The final mass remnant of 16.6% corresponds to a possible deposition of CuCO3 (17.4% calc’d). Compound 3 underwent dehydration between 25 and 190 °C, with a mass loss of 6.0%

Acknowledgments We acknowledge Lyman Briggs College of Michigan State University for funding this work. We thank Mr. Rui Huang for performing the elemental analyses and Mr. Andrew Latimer for helpful assistance. Appendix A. Supplementary material CCDC 1028269–1028272 and 1029209 contains the supplementary crystallographic data for this data for 1–4 and 4-ppbpH2O, respectively. 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.2014.12.026.

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