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Effect of pyridyl donor disposition and ligand flexibility on dimensionality in luminescent and nitrobenzene-detecting cadmium adamantanedicarboxylate coordination polymers
Inorganica Chimica Acta 451 (2016) 187–196
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Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica
Research paper
Effect of pyridyl donor disposition and ligand flexibility on dimensionality in luminescent and nitrobenzene-detecting cadmium adamantanedicarboxylate coordination polymers Cassi J. LaRose, Robert L. LaDuca ⇑ 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 23 May 2016 Received in revised form 11 July 2016 Accepted 15 July 2016 Available online 18 July 2016 Keywords: Cadmium Coordination polymer Crystal structure Luminescence Nitrobenzene detection
a b s t r a c t Hydrothermal reaction of cadmium nitrate, 1,3-adamantanedicarboxylic acid (adcH2), and a hydrogenbonding capable and conformationally flexible dipyridyl ligand has afforded five coordination polymers with differing dimensionalities and topologies. [Cd(adcH)2(4-bpmp)]n (1, 4-bpmp = bis(4-pyridylmethyl) piperazine) exhibits 1-D chain motifs with monodentate and protonated adcH ligands. {[Cd(adc)(4bpmp)(H2O)]4.5H2O}n (2) displays a non-interpenetrated 4-connected 658 cds network. [Cd4(adc)4(3-bpmp)(H2O)2]n (3, 3-bpmp = bis(3-pyridylmethyl)piperazine) displays a 3D tetranodal {4462}3{446482} topology network, derived from a primitive cubic net with regularly spaced pillar vacancies. {[Cd2(adc)2(4-bpmt)2]H2O}n (4, 4-bpmt = bis(4-pyridylmethyl)trimethylenedipiperidine) shows a decorated (4,4) grid 2D structure based on syn–syn bridged {Cd2(OCO)2} dinuclear clusters. [Cd (adcH)2(4-bpfp)]n (5, 4-bpfp = bis(4-pyridylformyl)piperazine) shows a 1-D chain structure similar to that of 1. Thermal and luminescent properties of these new phases are reported, along with surveys of their capability to serve as detectors for nitrobenzene. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The synthesis and characterization of crystalline coordination polymers remains an extremely fertile field of modern research in inorganic chemistry. In addition to their significant potential in hydrogen storage [1], molecular separation [2], CO2 sequestration [3], catalysis [4], and detection applications [5], the beauty of their structures and topologies continues to be a potent driving force for exploratory synthesis [6]. Except in a few limited systems, a priori structure and properties predictions for this class of materials remains a long-term goal [7]. Aromatic dicarboxylates have proven to be the most popular choice for the linking ligands required for the self-assembly of coordination polymers, as they provide both a rigid backbone for framework stability and the requisite charge balance. The inclusion of neutral dipyridyl coligands such as the rigid-rod tether 4,40 -bipyridine (bpy) or longer and kinked variants has not only enhanced the topological scope of this class of solid state compounds, but also produced some phases with intriguing multifunctional properties [8]. Cadmium coordination polymers containing both dicarboxylate and dipyridyl ligands, in particular, show useful luminescent and ⇑ Corresponding author. E-mail address: [email protected] (R.L. LaDuca). http://dx.doi.org/10.1016/j.ica.2016.07.027 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
nitroaromatic detection properties because of the visible light transmission window imbued by their closed shell d10 electronic configuration [9–11]. In comparison to cadmium coordination polymers containing aromatic dicarboxylate ligands, there have been very few reports of dual-ligand cadmium phases containing both dipyridyl ligands and 1,3-adamantanedicarboxylate (adc, Scheme 1) [12–13]. This aliphatic dicarboxylate ligand possesses structural rigidity comparable to the more commonly used aromatic ligands, and due to its lack of aromaticity would only afford luminescent behavior based on electronic transitions within the molecular orbital manifolds of any dipyridyl coligands. [Cd(adc) (dpp)]n (dpp = 1,3-di(4-pyridyl)propane) exhibits a 3-fold interpenetrated 66 diamondoid net [12], while {[Cd2(adc)2(dpp)3 (H2O)]28H2O}n displays a 1D nanotubular structure with 1D ? 2D supramolecular interdigitation fostered by pendant dpp side arms [13]. The latter phase stabilizes water molecule octamer clusters within its nanotubular incipient voids. An especially versatile dipyridyl ligand for instilling intriguing coordination polymer topologies has been bis(4-pyridylmethyl) piperazine (4-bpmp, Scheme 1), which can adopt linear or curled conformations in response to specific metal coordination or dicarboxylate binding requirements [14–17]. The easily accessible isomeric ligand bis(3-pyridylmethyl)piperazine (3-bpmp, Scheme 1) has afforded different topologies in the same metal dicarboxylate
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One instrument. Thermogravimetric analysis was performed on a TA Instruments Q50 thermal analyzer under flowing N2. 2.2. Preparation of [Cd(adcH)2(4-bpmp)]n (1)
Scheme 1. Ligands used in this study.
Cd(NO3)24H2O (86 mg, 0.28 mmol), 4-bpmp (74 mg, 0.28 mmol), and 1,3-adamantanedicarboxylic acid (62 mg, 0.28 mmol) were mixed with 10 mL of distilled H2O in a 23 mL Teflon-lined acid digestion bomb. Aqueous NaOH solution (0.55 mL, 1.0 M) was added to encourage deprotonation of the dicarboxylic acid. The bomb was sealed and heated in an oven at 120 °C for 72 h, and then was cooled slowly to 25 °C. Colorless crystals of 1 (164 mg, 71% yield based on the acid) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C40H50CdN4O8 1 calcd. C, 58.07; H, 6.09; N, 6.77%; found C, ~) = 2909 (m), 1710 (s), 1542 (w), 57.84; H, 6.08; N, 5.90%. IR (m 1484 (m), 1411 (s), 1276 (s), 1231 (m), 1103 (w), 879 (m), 782 (m), 760 (m), 702 (s) cm1. 2.3. Preparation of {[Cd(adc)(4-bpmp)(H2O)]4.5H2O}n (2)
coordination polymer system, due to the altered nitrogen donor disposition. For example, {[Cu(sip)(3-bpmpH)(H2O)]3H2O}n (sip = 5-sulfoisophthalate) and {[Cu(sip)(4-bpmpH)(H2O)]4H2O}n both possess (4,4) grid topologies, although the latter exhibits mutually inclined 2D + 2D ? 3D interpenetration. {[Cu(sip)(4bpmpH)]7.5H2O}n, which forms concomitantly with the latter phase, has an uncommon (3,6) triangular grid layer based on {Cu2(OCO)2} dimers [14]. In a pair of coordination polymers with the aliphatic glutarate (glu) ligand, 3-bpmp afforded a dimer-based 3D 41263 pcu primitive cubic topology in [Cu2(glu)2(3-bpmp)]n [15], while 4-bpmp afforded a rarer 3D 446108 mab self-penetrated topology in {[Cu(glu)(4-bpmp)]4H2O}n [16]. The longer, more flexible dipyridyl ligand, bis(4-pyridylmethyl)trimethylenedipiperidine (4-bpmt, Scheme 1), has not yet been utilized in coordination polymer self-assembly. A dipyridylamide derivative of 4-bpmp, bis(4-pyridylformyl)piperazine (4-bpfp), has afforded coordination polymers within intriguing loop-based topologies, for instance in the 1D + 1D ? 1D threaded-loop parallel interpenetration seen in [Zn2Cl2(terephthalate)(4-bpfp)]n [18]. In this contribution, we report five cadmium coordination polymers containing these longer dipyridyl ligands and 1,3-adamantanedicarboxylate: [Cd(adcH)2(4-bpmp)]n (1), {[Cd(adc)(4-bpmp) (H2O)]4.5H2O}n (2), [Cd4(adc)4(3-bpmp)(H2O)2]n (3), and {[Cd2(adc)2(4-bpmt)2]H2O}n (4), and [Cd(adcH)2(4-bpfp)]n (5). The specific dipyridyl ligand plays a predominant role in enforcing the topology and dimensionality of the resulting coordination polymer solids. Thermal and luminescent properties for these new materials, as well as their potential as nitrobenzene sensing substrates, are also probed. 2. Experimental section
The procedure for the synthesis of 1 was followed, but with the addition of 1.1 mL 1.0 M aqueous NaOH solution. Colorless crystals of 2 (145 mg, 75% yield based on Cd) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C28H47CdN4O10.5 2 calcd. C, 46.70; H, 6.57; N, 7.78%; found C, ~) = 2909 (m), 1710 (s), 1542 (w), 46.85; H, 6.36; N, 7.72%. IR (m 1484 (m), 1411 (s), 1276 (s), 1231 (m), 1103 (w), 879 (m), 782 (m), 760 (m), 702 (s) cm1. 2.4. Preparation of [Cd4(adc)4(3-bpmp)(H2O)2]n (3) The procedure for the synthesis of 1 was followed, with the substitution of 3-bpmp (73 mg, 0.27 mmol) as the dipyridyl component. Colorless crystals of 3 (71 mg, 62% yield based on Cd) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C64H80Cd4N4O18 3 calcd. C, 46.79; H, 4.91; N, ~) = 3350 (w), 2900 3.41%; found C, 46.82; H, 5.12; N, 3.28%. IR (m (w), 2854 (w), 2810 (w), 1609 (m), 1536 (s), 1453 (w), 1394 (s), 1311 (w), 1221 (m), 1159 (m), 1133 (m), 1061 (m), 1011 (s), 930 (w), 887 (w), 843 (s), 784 (s), 704 (s), 677 (w) cm1. 2.5. Preparation of {[Cd2(adc)2(4-bpmt)2]H2O}n (4) The procedure for the synthesis of 1 was followed, with the substitution of 4-bpmt (74 mg, 0.18 mmol) as the dipyridyl component. Colorless crystals of 4 (72 mg, 35% yield based on Cd) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C74H102Cd2N8O9 4 calcd. C, 60.36; H, 6.98; N, ~) = 2903 (m), 1610 7.61%; found C, 60.01; H, 6.53; N, 7.51%. IR (m (m), 1559 (s), 1536 (s), 1399 (s), 1309 (m), 1222 (m), 1124 (m), 1015 (m), 813 (m), 776 (m), 752 (m), 707 (s) cm1.
2.1. General considerations 2.6. Preparation of [Cd(adcH)2(4-bpfp)]n (5) Metal nitrates and 1,3-adamantanedicarboxylic acid were commercially obtained. The dipyridyl ligands bis(3-pyridylmethyl) piperazine (3-bpmp) and bis(4-pyridylmethyl)piperazine (4bpmp) were prepared by a literature procedure [19]. Bis(4-pyridylmethyl)trimethylenedipiperidine (4-bpmt) was prepared by a similar procedure, substituting trimethylenedipiperidine for piperazine. Bis(4-pyridylformyl)piperazine (4-bpfp) was prepared by a literature procedure [20]. Water was deionized above 3 MX–cm in-house. Elemental analysis was carried out using a Perkin Elmer 2400 Series II CHNS/O Analyzer. IR spectra were recorded on powdered samples using a Perkin Elmer Spectrum
The procedure for the synthesis of 1 was followed, with the substitution of 4-bpfp (84 mg, 0.28 mmol) as the dipyridyl component. The bomb was sealed and heated in an oven at 120 °C for 48 h, and then was cooled slowly to 25 °C. Colorless crystals of 5 (108 mg, 45% yield based on the acid) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C40H46CdN4O10 5 calcd. C, 56.18; H, 5.42; N, 6.55%; found C, 56.38; H, 5.19; N, ~) = 2909 (m), 1710 (s), 1542 (w), 1484 (m), 1411 (s), 6.65%. IR (m 1276 (s), 1231 (m), 1103 (w), 879 (m), 782 (m), 760 (m), 702 (s) cm1.
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modeled successfully with partial occupancies. Crystallographic details for 1–5 are given in Table 1.
2.7. Nitroaromatic detection studies A 10 mg ground sample of coordination polymer 1–5 was suspended in 5 mL ethanol in a volumetric flask, with immersion in an ultrasonic bath for 60 s to ensure an even dispersion. The fluorescence spectra were recorded with an excitation wavelength of 300 nm. A stock solution of nitrobenzene (1 104 M) in dimethyl sulfoxide was prepared. Aliquots of this stock solution (10 lL) were added sequentially to coordination polymer ethanol suspensions with sonication for 30 s after each addition. The emission spectra were measured before any analyte addition and after each aliquot of analyte solution. 3. X-ray crystallography Diffraction data for single crystals of 1–5 was collected on a Bruker-AXS SMART-CCD X-ray diffractometer using graphitemonochromated Mo-Ka radiation (k = 0.71073 Å). The data were processed via SAINT [21], and subjected to Lorentz effect, polarization effect and absorption corrections using SADABS [22]. The structures were solved using direct methods with SHELXTL [23] using the OLEX2 software suite [24]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon were placed in calculated positions and refined isotropically with a riding model. Hydrogen atoms bound to oxygen atoms were found via Fourier difference map where possible, and then restrained. The crystal of 1 was non-merohedrally twinned. The twin law was determined with CELL NOW [25], and diffraction data from both twins were used in the refinement. Disorder within one of the 4-bpmp ligands and the co-crystallized water molecules in 2 were
4. Results and discussion 4.1. Synthesis and infrared spectroscopy Compounds 1–5 were prepared cleanly by hydrothermal reaction of cadmium nitrate, 1,3-adamantanedicarboxylic acid, and the requisite dipyridyl ligand in the presence of aqueous base. Infrared spectra were consistent with the structural components as determined by single-crystal X-ray diffraction. A broad, weak band at 3300 cm1 for 1 signify the OAH bonds in pendant and protonated carboxylate moieties; similar bands for 3 and 4 signify the OAH stretching modes of the aqua ligands or unbound water molecules, respectively. Bands at 3000–3200 cm1 are assigned to CAH bond stretching modes in all cases. Asymmetric and symmetric CAO stretching modes of the adc ligand carboxylate groups are marked by slightly broadened bands at 1536 cm1 and 1394 cm1 (1), 1533 cm1 and 1400 cm1 (2), 1542 cm1 and 1411 cm1 (3), 1559 cm1 and 1399 cm1 (4), and 1517 cm1 and 1414 cm1 (5). The CAO stretching band for the protonated carboxylate in 1 and 5 is evident at 1710 cm1 in both cases. Sharper, medium intensity bands in the range of 1600 cm1 to 1200 cm1 are attributed to the stretching modes of the pyridyl rings of the dipyridyl ligands [26]. Features corresponding to pyridyl and aryl ring puckering are observed in the region between 800 cm1 and 600 cm1. The C@O stretching band for the carbonyl groups within the 4-bpfp ligands in 5 appears at 1640 cm1.
Table 1 Crystal and structure refinement data for 1–5. Data
1
2
3
4
5
Empirical Formula Formula Weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z D (g cm3) l (mm1) Crystal size (mm) Min./max. trans. hkl ranges
C40H50CdN4O8
C28H47CdN4O10.5
C64H80Cd4N4O18
C74H102Cd2N8O9
C40H46CdN4O10
827.24 Monoclinic C2/c 21.234(7) 13.534(4) 12.926(4) 90 91.541(3) 90 3713(2) 4 1.480 0.647 0.53 0.39 0.23
720.09 Monoclinic P21/n 10.9663(11) 30.592(3) 11.9322(12) 90 117.0101(10) 90 3566.4(6) 4 1.308 0.663 0.50 0.39 0.09
1642.92 Triclinic P1
1472.43 Triclinic P1
9.4458(8) 13.3008(12) 13.3173(12) 63.0562(8) 89.8702(9) 81.9432(9) 1473.3(2) 1 1.852 1.505 0.21 0.19 0.08
10.1946(11) 18.208(2) 20.672(2) 110.2800(12) 100.6636(13) 93.1708(14) 3507.3(7) 2 1.394 0.669 0.33 0.16 0.10
855.21 Monoclinic C2/c 26.669(3) 6.3618(8) 24.298(3) 90 115.8704(12) 90 3709.4(8) 4 1.531 0.655 0.29 0.19 0.06
0.89 25 6 h 6 25, 0 6 k 6 16, 0 6 l 6 15 24551 6382
0.9075 13 6 h 6 13, 36 6 k 6 36, 14 6 l 6 14 29439 6502
0.9218 11 6 h 6 11, 16 6 k 6 16, 16 6 l 6 16 15018 5380
0.9438 12 6 h 6 12, 21 6 k 6 21, 24 6 l 6 24 58575 12858
0.9168 32 6 h 6 32, 7 6 k 6 7, 29 6 l 6 29 14782 3415
0.0250 242 0.0246 0.0232 0.0612 0.0602 1.105/0.257
0.0357 410 0.0675 0.0596 0.1881 0.1796 1.254/–1.100
0.0284 407 0.0339 0.0284 0.0712 0.0671 1.220/–0.768
0.0443 841 0.0473 0.0339 0.0837 0.0757 1.589/–0.325
0.0384 250 0.0407 0.0317 0.0832 0.0775 0.679/–0.310
1.064
1.048
1.093
1.044
1.053
Total reflections Unique reflections R (int) Parameters R1a (all data) R1a (I > 2r(I)) wR2b (all data) wR2b (I > 2r(I)) Max/min residual (e/ Å3) G.O.F. a b
R1 = R||Fo| |Fc||/R|Fo|. wR2 = {R[w(F2o F2c )2]/R[wF2o]2}1/2.
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4.2. Structural description of [Cd(adcH)2(4-bpmp)]n (1) The asymmetric unit of compound 1 contains a divalent cadmium atom sited on a crystallographic 2-fold rotation axis, one singly protonated adc ligand, and half of a 4-bpmp ligand whose piperazinyl ring centroid sits on a crystallographic inversion center. Operation of the rotation axis affords a rather distorted octahedral {CdO4N2} coordination environment with cis disposed 4-bpmp pyridyl nitrogen atom donors (Fig. 1a). Two adcH ligands bind to cadmium with chelating carboxylate groups; the protonated carboxylate terminus does not bind to cadmium. Bond lengths and angles within the coordination environment are listed in Table S1. Dipodal 4-bpmp ligands connect neutral [Cd(adcH)2] fragments into extremely zig-zagged 1-D [Cd(adcH)2(4-bpmp)]n coordination polymer ribbons that are aligned parallel to the c crystal axis (Fig. 1b). The Cd Cd through-ligand distance within the ribbon motif is 16.234 Å; the closest through-space Cd Cd distance is 12.296 Å. The Cd Cd Cd angle within the ribbon motif subtends 46.9°. When viewed down the c axis, the ribbon motif makes an ‘‘H” shape (Fig. 1c). Neighboring [Cd(adcH)2(4-bpmp)]n ribbons aggregate into the 3-D crystal lattice of 1 by means of OAH N hydrogen bonding interactions (Table S2) between the protonated adcH carboxylate groups and the piperazinyl nitrogen atoms of the 4-bpmp ligands (Fig. S1). Each 4-bpmp piperazinyl ring accepts hydrogen bonds from two distinct ribbon motifs. Every [Cd (adcH)2(4-bpmp)]n ribbon motif in 1 engages in hydrogen bonding interactions with four other ribbons. 4.3. Structural description of {[Cd(adc)(4-bpmp)(H2O)]4.5H2O}n (2) The asymmetric unit of compound 2 contains a divalent cadmium atom, a fully deprotonated adc ligand, halves of two 4-bpmp ligands (one of which is disordered), an aqua ligand, and net four and one-half disordered water molecules. The cadmium atom displays a {CdO5N2} pentagonal bipyramidal geometry (Fig. 2a), with
pyridyl nitrogen donor atoms from two 4-bpmp ligands in the axial positions. In the equatorial plane lie chelating carboxylate groups from two adc ligands and the bound water molecule. Pertinent bond lengths and angles within the coordination environment of 2 are listed in Table S3. Bis(chelating) adc ligands connect cadmium atoms into [Cd (adc)(H2O)]n 1-D coordination polymer chain submotifs (Fig. 2b), with a CdCd through-ligand distance of 9.798(3) Å. These chain submotifs are connected into a 3-D [Cd(adc)(4-bpmp)(H2O)]n non-interpenetrated framework (Fig. 3a) by tethering 4-bpmp ligands, which span Cd Cd distances of 17.027(3) and 17.222 (2) Å. The longer distance is bridged by the crystallographically disordered 4-bpmp ligand. Considering all cadmium atoms as 4-connected nodes produces a 658 cds topology (Fig. 3b), according to TOPOS software [27]. Viewing the framework down the a crystal axis reveals the presence of large 1-D water-bearing channels. The water molecules of crystallization form hydrogen-bonded tapes that contain 3-, 4-, 5-, and 14-membered circuits (Fig. 4), anchored to the coordination polymer framework by donating hydrogen bonds to adc carboxylate oxygen atoms and 4-bpmp piperazinyl nitrogen atoms (Table S2). 4.4. Structural description of [Cd4(adc)4(3-bpmp)(H2O)2]n (3) The asymmetric unit of compound 3 contains two divalent cadmium atoms (Cd1, Cd2), two fully deprotonated adc ligands (adc-A, adc-B), one aqua ligand, and a half of a 3-bpmp ligand whose central piperazinyl ring is sited over a crystallographic inversion center. The cadmium atom Cd1 adopts a {CdO7} pentagonal bipyramidal geometry, with the aqua ligand in one of the axial sites. A carboxylate group from an adc-A ligand chelates to Cd1 in an equatorial site and the second axial position. The four remaining equatorial positions are filled by a chelating carboxylate group from a second adc-A ligand, a single carboxylate oxygen atom donor from a third adc-A ligand, and a single carboxylate
Fig. 1. (a) {CdO4N2} distorted octahedral coordination environment in 1. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S2. (b) [Cd(adcH)2(4-bpmp)]n ribbon in 1. (c) View down c of a single ribbon motif in 1.
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Fig. 2. (a) {CdO5N2} pentagonal bipyramidal coordination environment in 2. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S3. (b) [Cd(adc)(H2O)]n chain in 2.
Fig. 3. (a) 3-D [Cd(adc)(4-bpmp)(H2O)]n non-interpenetrated framework in 2. [Cd(adc)(H2O)]n chain submotifs are shown in red in the online version of this article. (b) Schematic perspective of the 658 cds topology of 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Hydrogen-bonded water tape located within 1D incipient channels in 2.
oxygen atom donor from an adc-B ligand. In contrast, the cadmium atom Cd2 adopts a distorted pentagonal monopyramidal {CdO5N} coordination environment in which a 3-bpmp pyridyl nitrogen donor atom rests in the apical position. The skewed pentagonal plane consists of a chelating carboxylate group from an adc-B ligand, single carboxylate oxygen atoms belonging to two other adc-B ligands, and a single carboxylate oxygen donor atom from
an adc-A ligand. The distinct coordination environments in 3 are depicted in Fig. 5. Bond lengths and angles within the distinct coordination environments are listed in Table S4. The adc-A ligands adopt a l4-j4-O,O0 :O00 ,O000 :O00 :O000 exotetradentate binding mode, while the adc-B ligands adopt a l4-j3-O:O00 ,O000 : O00 :O000 exotetradentate binding mode. All of the adc-A carboxylate oxygen atoms bind to one or more cadmium atoms, while one carboxylate oxygen atom of the each adc-B ligand remains unligated. Pairs of carboxylate oxygen atoms, from two adc-A ligands, form dinuclear {Cd1AOACd1AO} units with a Cd1 Cd1 through-space separation of 3.741(2) Å. Similarly, pairs of carboxylate oxygen atoms from two adc-B ligands construct dinuclear {Cd2AOACd2AO} units with a Cd2 Cd2 through-space separation of 3.735(2) Å. Chelating carboxylate groups from adc-A and adc-B assemble the dinuclear units into 1-D [Cd(OCO)]n chain submotifs aligned parallel to the b crystal axis (Fig. 6a), and also create {Cd1AOACd2AO} dinuclear units with a Cd Cd separation of 3.773(1) Å. Parallel 1-D [Cd(OCO)]n chain submotifs are anchored
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adc-A ligands, and adc-B ligands as 4-connected nodes, and treating the Cd2 atoms as 5-connected nodes due to the additional linkage via 3-bpmp ligands. An analysis performed with TOPOS [28] software revealed a new 4,5-connected {4462}3{446482} topology (Fig. 7b). This net can be derived from the more commonly encountered 6-connected 41263 primitive cubic pcu net by removing three out of every four interlayer pillars in a regular, repeating fashion.
4.5. Structural description of {[Cd2(adc)2(4-bpmt)2]H2O}n (4)
Fig. 5. Coordination environments in 3. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S4.
into [Cd2(adc)2(H2O)2]n layer motifs by the full span of the adc-A and adc-B ligands (Fig. 6b). Hydrogen bonding between the aqua ligands bound to Cd1 and the unligated carboxylate oxygen atoms of the adc-B ligands serve an ancillary structure stabilization role (Table S2). These [Cd2(adc)2(H2O)2]n layer motifs are oriented along the ab crystal planes and are pillared into a 3-D [Cd4(adc)4(3-bpmp) (H2O)2]n coordination polymer network (Fig. 7a) by dipodal anti conformation 3-bpmp ligands that link Cd2 atoms in adjacent layers with a Cd2 Cd2 through-ligand distance of 14.248 Å. The network topology of 3 was developed by considering the Cd1 atoms,
The asymmetric unit of compound 4 contains two cadmium atoms, two adc ligands, two 4-bpmt ligands, and one water molecule of crystallization. Both cadmium atoms adopt a distorted octahedral {CdO4N2} configuration, with pyridyl nitrogen donor atoms from two 4-bpmt ligands in nominal trans axial positions. The equatorial planes of the six-coordinate geometries consist of a chelating adc carboxylate terminus and two single oxygen donor atoms from two other adc ligands (Fig. 8a). Bond lengths and angles within the coordination environments of 4 are listed in Table S5. The crystallographically distinct adc ligands adopt an exotridentate l3-j3-O:O0 ,O00 ,O000 exotetradentate binding mode in which one carboxylate terminus chelates to a single cadmium atom, and the other carboxylate endgroup bridges two cadmium atoms. The bridging adc carboxylate groups create crystallographically distinct {Cd2(OCO)2} dimeric units with Cd Cd through-space distances of 3.969(3) and 3.809(3) Å for Cd1- and Cd2-based dimeric units, respectively. The full span of the adc ligands connects the dimeric units into [Cd2(adc)2]n ribbon motifs that are aligned parallel to the c crystal axis (Fig. 8b). These are pillared by 4-bpmt ligands into 2D [Cd2(adc)2(4-bpmt)2]n coordination polymer layers 0) crystal planes (Fig. 9). All of the arranged parallel to the (1 1 4-bpmt ligands have a slightly distorted anti-anti conformation
Fig. 6. (a) 1-D [Cd(OCO)]n chain submotifs in 3. (b) {[Cd4(adc)4(H2O)2]n layer motif in 3.
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Fig. 7. (a) 3-D [Cd4(adc)4(3-bpmp)(H2O)2]n coordination polymer network in 3. (b) Schematic representation of the underlying 4,5-connected {4462}3{446482} topology. In the online version of this article, the purple rods indicate the 3-bpmp linkers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. (a) Coordination environments in 4. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S5. (b) 1-D [Cd2(adc)2]n ribbon motif in 4.
about their central trimethylene moieties (torsion angles = 164.4 and 174.7°; 169.1 and 171.4°), and span Cd Cd distances of 22.435 Å. Water molecules of crystallization are held in small pockets within the layer motifs, anchored by hydrogen bonding donation to ligated adc carboxylate oxygen atoms (Table S2). Treating the Cd2(OCO)2} dimeric units as the connecting nodes results in the layer motif displaying a (4,4) grid topology. Adjacent [Cd2(adc)2(4-bpmt)2]n layers stack by means of CAH O interactions (C O distance = 3.226(4) Å) between 4-bpmt pyridyl rings and adc carboxylate oxygen atoms (Fig. S2). 4.6. Structural description of [Cd(adcH)2(4-bpfp)]n (5) The asymmetric unit of compound 5 contains a divalent cadmium atom sited on a crystallographic 2-fold rotation axis, one singly protonated adc ligand, and half of a 4-bpfp ligand whose piperazinyl ring centroid is located on a crystallographic inversion center. Operation of the rotation axis affords a rather distorted octahedral {CdO4N2} coordination environment with cis disposed 4-bpfp pyridyl nitrogen atom donors (Fig. 10). Two adcH ligands bind to cadmium with chelating carboxylate groups; the protonated carboxylate endgroups do not bind to cadmium. Bond lengths and angles within the coordination environment are listed in Table S6.
The 4-bpfp ligands connect neutral [Cd(adcH)2] fragments into zig-zagged 1-D [Cd(adcH)2(4-bpfp)]n coordination polymer chains that are aligned parallel to the [1 0 1] crystal direction (Fig. 11). The Cd Cd through-ligand distance within the chain motif is 16.62(1) Å; the Cd Cd Cd angle measures 109.4°. Thus the 1-D motif in 5 much more closely resembles a splayed-open straight chain than the tight ribbon motif in 1. Nevertheless, when viewed down [1 0 1], the chains in 5 do make an ‘‘H” shape similar to compound 1. Adjacent [Cd(adcH)2(4-bpfp)]n ribbons aggregate into the 3-D crystal lattice of 5 by means of OAH O hydrogen bonding interactions (Table S2) between the protonated adcH carboxylate groups and bound adc carboxylate oxygen atoms (Fig. S3). Due to amide resonance mechanisms, the piperazinyl nitrogen atoms within the 4-bpfp are no longer available for hydrogen bonding acceptance. As a result, the supramolecular interactions between ribbons of 5 differ substantially from those in the related 4-bpmp congener 1. 4.7. Thermal properties The mass of compound 1 remained stable until 190 °C, at which point ligand combustion occurred. Compound 2 underwent loss of its water molecules of crystallization between 25 and 225 °C, with a mass loss of 10.3% slightly less than the predicted
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Fig. 9. 2D {[Cd2(adc)2(4-bpmt)2]H2O}n layer motif in 4. In the online version of this article, the 1D [Cd2(adc)2]n ribbon motifs are depicted in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. (a) {CdO4N2} distorted octahedral coordination environment in 5. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S6. (b) [Cd(adcH)2(4-bpfp)]n ribbon in 5.
value of 11.5%; ligand combustion occurred above 225 °C. The mass of compound 3 remained stable until 185 °C. Compound 4 underwent loss of its co-crystallized water molecules around 140 °C, with a mass loss of 1.3% very close to that of the predicted value (1.2%). Ligand ejection occurred above 200 °C for 4 and above 240 °C for 5. The TGA traces for 1–5 are shown in Figs. S4–S8, respectively. 4.8. Luminescent properties and nitrobenzene detection Compounds 1–4 show ultraviolet emission in sonicated ethanol suspension (2 mg/mL) upon excitation with higher energy ultraviolet light (kex = 260 nm), with emission maxima at 294 nm in all cases. The emissive behavior is attributed to p–p⁄ transitions within the molecular orbital manifolds of the dipyridyl-type ligands bound to cadmium. Compound 5 did not show appreciable emission in ethanol suspension. Compounds 1–4 show capability for detection of nitrobenzene in ethanol suspension. Addition of small aliquots of nitrobenzene dissolved in DMSO (1 104 M) resulted in decrease of the emission intensity (Fig. 11). This loss
of emission signal is attributed to charge transfer from the excited states of the dipyridyl ligand molecular orbitals within the coordination polymer, to those of the adsorbed electron-withdrawing nitrobenzene substrate [5]. The intensity factor (Io–I)/Io (Io = maximum intensity in the absence of analyte, I = intensity after analyte addition) was recorded for the samples 1–4, in the presence of 10 lL of the stock nitrobenzene solution. For 1–4, the intensity factors were 0.34, 0.48, 0.68, and 0.41, respectively. Further decreases in emission intensity were observed upon administration of more analyte. When 60 lL of the stock nitrobenzene solution was added to suspensions of 1–4, the intensity factors reached values of 0.98, 0.98, 0.99, 0.97, respectively, thereby showing nearly complete quenching of emission. 5. Conclusions Cadmium coordination polymers containing the rigid yet aliphatic 1,3-adamantanedicarboxylate ligand show a dramatic difference in dimensionality and topology depending on the nature of a hydrogen-bonding capable dipyridyl coligand
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Fig. 11. Nitrobenzene detection plots for 1–4.
and the stoichiometric ratio. Using 4-bpmp, 1-D chain and 3-D 658 cds topology phases were obtained if stoichiometric ratios were varied. A similar 1-D chain was obtained employing the related dipyridyl ligand 4-bpfp, in 5. Use of the conformationally flexible 3-bpmp ligand produced a new 3D topology in 3, derived from the usual primitive cubic pcu framework but with regular ligand vacancies. The very long spanning ligand 4-bpmt afforded 4, a 2D phase constructed from connection of dinuclear units. All the structures in this study differ greatly in topology from the two previously reported cadmium adamantanedicarboxylate dipyridyl coordination polymers, which showed diamondoid or nanotubular structures. Coordination polymers 1–4 manifest functionality as detectors for nitrobenzene in ethanol suspension, with the greatest detection ability for the 3D phases 3 and 2, followed by the 2D phase 4. It is plausible that the ‘‘ligand vacancies” in the 3D net of 3 allows greater nitrobenzene uptake into the coordination polymer lattice. Compound 1, which is a 1D coordination polymer phase, showed more modest nitrobenzene uptake. Coordination polymer 5, also a 1D phase, showed very weak emission in ethanol suspension. It is also possible that adsorption of nitrobenzene into the 3D phases with tighter packing results in enhanced electron transfer to the analyte and therefore a greater decrease in emission.
Acknowledgements Funding for the work was provided by Michigan State University. We thank Dr. Rui Huang for elemental analysis. We thank Mr. Sultan Qiblawi for the synthesis of the 4-bpmt ligand. Appendix A. Supplementary data Additional molecular graphics and thermogravimetric analysis plots. Crystallographic data (including structure factors) for 1–5 have been deposited with the Cambridge Crystallographic Data Centre with Nos. 1432997, 1441833, 1432998, 1432999, and 1447426, respectively. 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, U.K. (Fax: 44–1223336033, Email: [email protected]). Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2016.07.027. References [1] (a) M.P. Suh, H.J. Park, T.K. Prasad, D. Lim, Chem. Rev. 112 (2012) 782; (b) L.J. Murray, M. Dinca, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294. and references therein.
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Research paper
Effect of pyridyl donor disposition and ligand flexibility on dimensionality in luminescent and nitrobenzene-detecting cadmium adamantanedicarboxylate coordination polymers Cassi J. LaRose, Robert L. LaDuca ⇑ 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 23 May 2016 Received in revised form 11 July 2016 Accepted 15 July 2016 Available online 18 July 2016 Keywords: Cadmium Coordination polymer Crystal structure Luminescence Nitrobenzene detection
a b s t r a c t Hydrothermal reaction of cadmium nitrate, 1,3-adamantanedicarboxylic acid (adcH2), and a hydrogenbonding capable and conformationally flexible dipyridyl ligand has afforded five coordination polymers with differing dimensionalities and topologies. [Cd(adcH)2(4-bpmp)]n (1, 4-bpmp = bis(4-pyridylmethyl) piperazine) exhibits 1-D chain motifs with monodentate and protonated adcH ligands. {[Cd(adc)(4bpmp)(H2O)]4.5H2O}n (2) displays a non-interpenetrated 4-connected 658 cds network. [Cd4(adc)4(3-bpmp)(H2O)2]n (3, 3-bpmp = bis(3-pyridylmethyl)piperazine) displays a 3D tetranodal {4462}3{446482} topology network, derived from a primitive cubic net with regularly spaced pillar vacancies. {[Cd2(adc)2(4-bpmt)2]H2O}n (4, 4-bpmt = bis(4-pyridylmethyl)trimethylenedipiperidine) shows a decorated (4,4) grid 2D structure based on syn–syn bridged {Cd2(OCO)2} dinuclear clusters. [Cd (adcH)2(4-bpfp)]n (5, 4-bpfp = bis(4-pyridylformyl)piperazine) shows a 1-D chain structure similar to that of 1. Thermal and luminescent properties of these new phases are reported, along with surveys of their capability to serve as detectors for nitrobenzene. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction The synthesis and characterization of crystalline coordination polymers remains an extremely fertile field of modern research in inorganic chemistry. In addition to their significant potential in hydrogen storage [1], molecular separation [2], CO2 sequestration [3], catalysis [4], and detection applications [5], the beauty of their structures and topologies continues to be a potent driving force for exploratory synthesis [6]. Except in a few limited systems, a priori structure and properties predictions for this class of materials remains a long-term goal [7]. Aromatic dicarboxylates have proven to be the most popular choice for the linking ligands required for the self-assembly of coordination polymers, as they provide both a rigid backbone for framework stability and the requisite charge balance. The inclusion of neutral dipyridyl coligands such as the rigid-rod tether 4,40 -bipyridine (bpy) or longer and kinked variants has not only enhanced the topological scope of this class of solid state compounds, but also produced some phases with intriguing multifunctional properties [8]. Cadmium coordination polymers containing both dicarboxylate and dipyridyl ligands, in particular, show useful luminescent and ⇑ Corresponding author. E-mail address: [email protected] (R.L. LaDuca). http://dx.doi.org/10.1016/j.ica.2016.07.027 0020-1693/Ó 2016 Elsevier B.V. All rights reserved.
nitroaromatic detection properties because of the visible light transmission window imbued by their closed shell d10 electronic configuration [9–11]. In comparison to cadmium coordination polymers containing aromatic dicarboxylate ligands, there have been very few reports of dual-ligand cadmium phases containing both dipyridyl ligands and 1,3-adamantanedicarboxylate (adc, Scheme 1) [12–13]. This aliphatic dicarboxylate ligand possesses structural rigidity comparable to the more commonly used aromatic ligands, and due to its lack of aromaticity would only afford luminescent behavior based on electronic transitions within the molecular orbital manifolds of any dipyridyl coligands. [Cd(adc) (dpp)]n (dpp = 1,3-di(4-pyridyl)propane) exhibits a 3-fold interpenetrated 66 diamondoid net [12], while {[Cd2(adc)2(dpp)3 (H2O)]28H2O}n displays a 1D nanotubular structure with 1D ? 2D supramolecular interdigitation fostered by pendant dpp side arms [13]. The latter phase stabilizes water molecule octamer clusters within its nanotubular incipient voids. An especially versatile dipyridyl ligand for instilling intriguing coordination polymer topologies has been bis(4-pyridylmethyl) piperazine (4-bpmp, Scheme 1), which can adopt linear or curled conformations in response to specific metal coordination or dicarboxylate binding requirements [14–17]. The easily accessible isomeric ligand bis(3-pyridylmethyl)piperazine (3-bpmp, Scheme 1) has afforded different topologies in the same metal dicarboxylate
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One instrument. Thermogravimetric analysis was performed on a TA Instruments Q50 thermal analyzer under flowing N2. 2.2. Preparation of [Cd(adcH)2(4-bpmp)]n (1)
Scheme 1. Ligands used in this study.
Cd(NO3)24H2O (86 mg, 0.28 mmol), 4-bpmp (74 mg, 0.28 mmol), and 1,3-adamantanedicarboxylic acid (62 mg, 0.28 mmol) were mixed with 10 mL of distilled H2O in a 23 mL Teflon-lined acid digestion bomb. Aqueous NaOH solution (0.55 mL, 1.0 M) was added to encourage deprotonation of the dicarboxylic acid. The bomb was sealed and heated in an oven at 120 °C for 72 h, and then was cooled slowly to 25 °C. Colorless crystals of 1 (164 mg, 71% yield based on the acid) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C40H50CdN4O8 1 calcd. C, 58.07; H, 6.09; N, 6.77%; found C, ~) = 2909 (m), 1710 (s), 1542 (w), 57.84; H, 6.08; N, 5.90%. IR (m 1484 (m), 1411 (s), 1276 (s), 1231 (m), 1103 (w), 879 (m), 782 (m), 760 (m), 702 (s) cm1. 2.3. Preparation of {[Cd(adc)(4-bpmp)(H2O)]4.5H2O}n (2)
coordination polymer system, due to the altered nitrogen donor disposition. For example, {[Cu(sip)(3-bpmpH)(H2O)]3H2O}n (sip = 5-sulfoisophthalate) and {[Cu(sip)(4-bpmpH)(H2O)]4H2O}n both possess (4,4) grid topologies, although the latter exhibits mutually inclined 2D + 2D ? 3D interpenetration. {[Cu(sip)(4bpmpH)]7.5H2O}n, which forms concomitantly with the latter phase, has an uncommon (3,6) triangular grid layer based on {Cu2(OCO)2} dimers [14]. In a pair of coordination polymers with the aliphatic glutarate (glu) ligand, 3-bpmp afforded a dimer-based 3D 41263 pcu primitive cubic topology in [Cu2(glu)2(3-bpmp)]n [15], while 4-bpmp afforded a rarer 3D 446108 mab self-penetrated topology in {[Cu(glu)(4-bpmp)]4H2O}n [16]. The longer, more flexible dipyridyl ligand, bis(4-pyridylmethyl)trimethylenedipiperidine (4-bpmt, Scheme 1), has not yet been utilized in coordination polymer self-assembly. A dipyridylamide derivative of 4-bpmp, bis(4-pyridylformyl)piperazine (4-bpfp), has afforded coordination polymers within intriguing loop-based topologies, for instance in the 1D + 1D ? 1D threaded-loop parallel interpenetration seen in [Zn2Cl2(terephthalate)(4-bpfp)]n [18]. In this contribution, we report five cadmium coordination polymers containing these longer dipyridyl ligands and 1,3-adamantanedicarboxylate: [Cd(adcH)2(4-bpmp)]n (1), {[Cd(adc)(4-bpmp) (H2O)]4.5H2O}n (2), [Cd4(adc)4(3-bpmp)(H2O)2]n (3), and {[Cd2(adc)2(4-bpmt)2]H2O}n (4), and [Cd(adcH)2(4-bpfp)]n (5). The specific dipyridyl ligand plays a predominant role in enforcing the topology and dimensionality of the resulting coordination polymer solids. Thermal and luminescent properties for these new materials, as well as their potential as nitrobenzene sensing substrates, are also probed. 2. Experimental section
The procedure for the synthesis of 1 was followed, but with the addition of 1.1 mL 1.0 M aqueous NaOH solution. Colorless crystals of 2 (145 mg, 75% yield based on Cd) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C28H47CdN4O10.5 2 calcd. C, 46.70; H, 6.57; N, 7.78%; found C, ~) = 2909 (m), 1710 (s), 1542 (w), 46.85; H, 6.36; N, 7.72%. IR (m 1484 (m), 1411 (s), 1276 (s), 1231 (m), 1103 (w), 879 (m), 782 (m), 760 (m), 702 (s) cm1. 2.4. Preparation of [Cd4(adc)4(3-bpmp)(H2O)2]n (3) The procedure for the synthesis of 1 was followed, with the substitution of 3-bpmp (73 mg, 0.27 mmol) as the dipyridyl component. Colorless crystals of 3 (71 mg, 62% yield based on Cd) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C64H80Cd4N4O18 3 calcd. C, 46.79; H, 4.91; N, ~) = 3350 (w), 2900 3.41%; found C, 46.82; H, 5.12; N, 3.28%. IR (m (w), 2854 (w), 2810 (w), 1609 (m), 1536 (s), 1453 (w), 1394 (s), 1311 (w), 1221 (m), 1159 (m), 1133 (m), 1061 (m), 1011 (s), 930 (w), 887 (w), 843 (s), 784 (s), 704 (s), 677 (w) cm1. 2.5. Preparation of {[Cd2(adc)2(4-bpmt)2]H2O}n (4) The procedure for the synthesis of 1 was followed, with the substitution of 4-bpmt (74 mg, 0.18 mmol) as the dipyridyl component. Colorless crystals of 4 (72 mg, 35% yield based on Cd) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C74H102Cd2N8O9 4 calcd. C, 60.36; H, 6.98; N, ~) = 2903 (m), 1610 7.61%; found C, 60.01; H, 6.53; N, 7.51%. IR (m (m), 1559 (s), 1536 (s), 1399 (s), 1309 (m), 1222 (m), 1124 (m), 1015 (m), 813 (m), 776 (m), 752 (m), 707 (s) cm1.
2.1. General considerations 2.6. Preparation of [Cd(adcH)2(4-bpfp)]n (5) Metal nitrates and 1,3-adamantanedicarboxylic acid were commercially obtained. The dipyridyl ligands bis(3-pyridylmethyl) piperazine (3-bpmp) and bis(4-pyridylmethyl)piperazine (4bpmp) were prepared by a literature procedure [19]. Bis(4-pyridylmethyl)trimethylenedipiperidine (4-bpmt) was prepared by a similar procedure, substituting trimethylenedipiperidine for piperazine. Bis(4-pyridylformyl)piperazine (4-bpfp) was prepared by a literature procedure [20]. Water was deionized above 3 MX–cm in-house. Elemental analysis was carried out using a Perkin Elmer 2400 Series II CHNS/O Analyzer. IR spectra were recorded on powdered samples using a Perkin Elmer Spectrum
The procedure for the synthesis of 1 was followed, with the substitution of 4-bpfp (84 mg, 0.28 mmol) as the dipyridyl component. The bomb was sealed and heated in an oven at 120 °C for 48 h, and then was cooled slowly to 25 °C. Colorless crystals of 5 (108 mg, 45% yield based on the acid) were isolated after washing with distilled water, ethanol, and acetone and drying in air. C40H46CdN4O10 5 calcd. C, 56.18; H, 5.42; N, 6.55%; found C, 56.38; H, 5.19; N, ~) = 2909 (m), 1710 (s), 1542 (w), 1484 (m), 1411 (s), 6.65%. IR (m 1276 (s), 1231 (m), 1103 (w), 879 (m), 782 (m), 760 (m), 702 (s) cm1.
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modeled successfully with partial occupancies. Crystallographic details for 1–5 are given in Table 1.
2.7. Nitroaromatic detection studies A 10 mg ground sample of coordination polymer 1–5 was suspended in 5 mL ethanol in a volumetric flask, with immersion in an ultrasonic bath for 60 s to ensure an even dispersion. The fluorescence spectra were recorded with an excitation wavelength of 300 nm. A stock solution of nitrobenzene (1 104 M) in dimethyl sulfoxide was prepared. Aliquots of this stock solution (10 lL) were added sequentially to coordination polymer ethanol suspensions with sonication for 30 s after each addition. The emission spectra were measured before any analyte addition and after each aliquot of analyte solution. 3. X-ray crystallography Diffraction data for single crystals of 1–5 was collected on a Bruker-AXS SMART-CCD X-ray diffractometer using graphitemonochromated Mo-Ka radiation (k = 0.71073 Å). The data were processed via SAINT [21], and subjected to Lorentz effect, polarization effect and absorption corrections using SADABS [22]. The structures were solved using direct methods with SHELXTL [23] using the OLEX2 software suite [24]. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms bound to carbon were placed in calculated positions and refined isotropically with a riding model. Hydrogen atoms bound to oxygen atoms were found via Fourier difference map where possible, and then restrained. The crystal of 1 was non-merohedrally twinned. The twin law was determined with CELL NOW [25], and diffraction data from both twins were used in the refinement. Disorder within one of the 4-bpmp ligands and the co-crystallized water molecules in 2 were
4. Results and discussion 4.1. Synthesis and infrared spectroscopy Compounds 1–5 were prepared cleanly by hydrothermal reaction of cadmium nitrate, 1,3-adamantanedicarboxylic acid, and the requisite dipyridyl ligand in the presence of aqueous base. Infrared spectra were consistent with the structural components as determined by single-crystal X-ray diffraction. A broad, weak band at 3300 cm1 for 1 signify the OAH bonds in pendant and protonated carboxylate moieties; similar bands for 3 and 4 signify the OAH stretching modes of the aqua ligands or unbound water molecules, respectively. Bands at 3000–3200 cm1 are assigned to CAH bond stretching modes in all cases. Asymmetric and symmetric CAO stretching modes of the adc ligand carboxylate groups are marked by slightly broadened bands at 1536 cm1 and 1394 cm1 (1), 1533 cm1 and 1400 cm1 (2), 1542 cm1 and 1411 cm1 (3), 1559 cm1 and 1399 cm1 (4), and 1517 cm1 and 1414 cm1 (5). The CAO stretching band for the protonated carboxylate in 1 and 5 is evident at 1710 cm1 in both cases. Sharper, medium intensity bands in the range of 1600 cm1 to 1200 cm1 are attributed to the stretching modes of the pyridyl rings of the dipyridyl ligands [26]. Features corresponding to pyridyl and aryl ring puckering are observed in the region between 800 cm1 and 600 cm1. The C@O stretching band for the carbonyl groups within the 4-bpfp ligands in 5 appears at 1640 cm1.
Table 1 Crystal and structure refinement data for 1–5. Data
1
2
3
4
5
Empirical Formula Formula Weight Crystal system Space group a (Å) b (Å) c (Å) a (°) b (°) c (°) V (Å3) Z D (g cm3) l (mm1) Crystal size (mm) Min./max. trans. hkl ranges
C40H50CdN4O8
C28H47CdN4O10.5
C64H80Cd4N4O18
C74H102Cd2N8O9
C40H46CdN4O10
827.24 Monoclinic C2/c 21.234(7) 13.534(4) 12.926(4) 90 91.541(3) 90 3713(2) 4 1.480 0.647 0.53 0.39 0.23
720.09 Monoclinic P21/n 10.9663(11) 30.592(3) 11.9322(12) 90 117.0101(10) 90 3566.4(6) 4 1.308 0.663 0.50 0.39 0.09
1642.92 Triclinic P1
1472.43 Triclinic P1
9.4458(8) 13.3008(12) 13.3173(12) 63.0562(8) 89.8702(9) 81.9432(9) 1473.3(2) 1 1.852 1.505 0.21 0.19 0.08
10.1946(11) 18.208(2) 20.672(2) 110.2800(12) 100.6636(13) 93.1708(14) 3507.3(7) 2 1.394 0.669 0.33 0.16 0.10
855.21 Monoclinic C2/c 26.669(3) 6.3618(8) 24.298(3) 90 115.8704(12) 90 3709.4(8) 4 1.531 0.655 0.29 0.19 0.06
0.89 25 6 h 6 25, 0 6 k 6 16, 0 6 l 6 15 24551 6382
0.9075 13 6 h 6 13, 36 6 k 6 36, 14 6 l 6 14 29439 6502
0.9218 11 6 h 6 11, 16 6 k 6 16, 16 6 l 6 16 15018 5380
0.9438 12 6 h 6 12, 21 6 k 6 21, 24 6 l 6 24 58575 12858
0.9168 32 6 h 6 32, 7 6 k 6 7, 29 6 l 6 29 14782 3415
0.0250 242 0.0246 0.0232 0.0612 0.0602 1.105/0.257
0.0357 410 0.0675 0.0596 0.1881 0.1796 1.254/–1.100
0.0284 407 0.0339 0.0284 0.0712 0.0671 1.220/–0.768
0.0443 841 0.0473 0.0339 0.0837 0.0757 1.589/–0.325
0.0384 250 0.0407 0.0317 0.0832 0.0775 0.679/–0.310
1.064
1.048
1.093
1.044
1.053
Total reflections Unique reflections R (int) Parameters R1a (all data) R1a (I > 2r(I)) wR2b (all data) wR2b (I > 2r(I)) Max/min residual (e/ Å3) G.O.F. a b
R1 = R||Fo| |Fc||/R|Fo|. wR2 = {R[w(F2o F2c )2]/R[wF2o]2}1/2.
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4.2. Structural description of [Cd(adcH)2(4-bpmp)]n (1) The asymmetric unit of compound 1 contains a divalent cadmium atom sited on a crystallographic 2-fold rotation axis, one singly protonated adc ligand, and half of a 4-bpmp ligand whose piperazinyl ring centroid sits on a crystallographic inversion center. Operation of the rotation axis affords a rather distorted octahedral {CdO4N2} coordination environment with cis disposed 4-bpmp pyridyl nitrogen atom donors (Fig. 1a). Two adcH ligands bind to cadmium with chelating carboxylate groups; the protonated carboxylate terminus does not bind to cadmium. Bond lengths and angles within the coordination environment are listed in Table S1. Dipodal 4-bpmp ligands connect neutral [Cd(adcH)2] fragments into extremely zig-zagged 1-D [Cd(adcH)2(4-bpmp)]n coordination polymer ribbons that are aligned parallel to the c crystal axis (Fig. 1b). The Cd Cd through-ligand distance within the ribbon motif is 16.234 Å; the closest through-space Cd Cd distance is 12.296 Å. The Cd Cd Cd angle within the ribbon motif subtends 46.9°. When viewed down the c axis, the ribbon motif makes an ‘‘H” shape (Fig. 1c). Neighboring [Cd(adcH)2(4-bpmp)]n ribbons aggregate into the 3-D crystal lattice of 1 by means of OAH N hydrogen bonding interactions (Table S2) between the protonated adcH carboxylate groups and the piperazinyl nitrogen atoms of the 4-bpmp ligands (Fig. S1). Each 4-bpmp piperazinyl ring accepts hydrogen bonds from two distinct ribbon motifs. Every [Cd (adcH)2(4-bpmp)]n ribbon motif in 1 engages in hydrogen bonding interactions with four other ribbons. 4.3. Structural description of {[Cd(adc)(4-bpmp)(H2O)]4.5H2O}n (2) The asymmetric unit of compound 2 contains a divalent cadmium atom, a fully deprotonated adc ligand, halves of two 4-bpmp ligands (one of which is disordered), an aqua ligand, and net four and one-half disordered water molecules. The cadmium atom displays a {CdO5N2} pentagonal bipyramidal geometry (Fig. 2a), with
pyridyl nitrogen donor atoms from two 4-bpmp ligands in the axial positions. In the equatorial plane lie chelating carboxylate groups from two adc ligands and the bound water molecule. Pertinent bond lengths and angles within the coordination environment of 2 are listed in Table S3. Bis(chelating) adc ligands connect cadmium atoms into [Cd (adc)(H2O)]n 1-D coordination polymer chain submotifs (Fig. 2b), with a CdCd through-ligand distance of 9.798(3) Å. These chain submotifs are connected into a 3-D [Cd(adc)(4-bpmp)(H2O)]n non-interpenetrated framework (Fig. 3a) by tethering 4-bpmp ligands, which span Cd Cd distances of 17.027(3) and 17.222 (2) Å. The longer distance is bridged by the crystallographically disordered 4-bpmp ligand. Considering all cadmium atoms as 4-connected nodes produces a 658 cds topology (Fig. 3b), according to TOPOS software [27]. Viewing the framework down the a crystal axis reveals the presence of large 1-D water-bearing channels. The water molecules of crystallization form hydrogen-bonded tapes that contain 3-, 4-, 5-, and 14-membered circuits (Fig. 4), anchored to the coordination polymer framework by donating hydrogen bonds to adc carboxylate oxygen atoms and 4-bpmp piperazinyl nitrogen atoms (Table S2). 4.4. Structural description of [Cd4(adc)4(3-bpmp)(H2O)2]n (3) The asymmetric unit of compound 3 contains two divalent cadmium atoms (Cd1, Cd2), two fully deprotonated adc ligands (adc-A, adc-B), one aqua ligand, and a half of a 3-bpmp ligand whose central piperazinyl ring is sited over a crystallographic inversion center. The cadmium atom Cd1 adopts a {CdO7} pentagonal bipyramidal geometry, with the aqua ligand in one of the axial sites. A carboxylate group from an adc-A ligand chelates to Cd1 in an equatorial site and the second axial position. The four remaining equatorial positions are filled by a chelating carboxylate group from a second adc-A ligand, a single carboxylate oxygen atom donor from a third adc-A ligand, and a single carboxylate
Fig. 1. (a) {CdO4N2} distorted octahedral coordination environment in 1. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S2. (b) [Cd(adcH)2(4-bpmp)]n ribbon in 1. (c) View down c of a single ribbon motif in 1.
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Fig. 2. (a) {CdO5N2} pentagonal bipyramidal coordination environment in 2. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S3. (b) [Cd(adc)(H2O)]n chain in 2.
Fig. 3. (a) 3-D [Cd(adc)(4-bpmp)(H2O)]n non-interpenetrated framework in 2. [Cd(adc)(H2O)]n chain submotifs are shown in red in the online version of this article. (b) Schematic perspective of the 658 cds topology of 2. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 4. Hydrogen-bonded water tape located within 1D incipient channels in 2.
oxygen atom donor from an adc-B ligand. In contrast, the cadmium atom Cd2 adopts a distorted pentagonal monopyramidal {CdO5N} coordination environment in which a 3-bpmp pyridyl nitrogen donor atom rests in the apical position. The skewed pentagonal plane consists of a chelating carboxylate group from an adc-B ligand, single carboxylate oxygen atoms belonging to two other adc-B ligands, and a single carboxylate oxygen donor atom from
an adc-A ligand. The distinct coordination environments in 3 are depicted in Fig. 5. Bond lengths and angles within the distinct coordination environments are listed in Table S4. The adc-A ligands adopt a l4-j4-O,O0 :O00 ,O000 :O00 :O000 exotetradentate binding mode, while the adc-B ligands adopt a l4-j3-O:O00 ,O000 : O00 :O000 exotetradentate binding mode. All of the adc-A carboxylate oxygen atoms bind to one or more cadmium atoms, while one carboxylate oxygen atom of the each adc-B ligand remains unligated. Pairs of carboxylate oxygen atoms, from two adc-A ligands, form dinuclear {Cd1AOACd1AO} units with a Cd1 Cd1 through-space separation of 3.741(2) Å. Similarly, pairs of carboxylate oxygen atoms from two adc-B ligands construct dinuclear {Cd2AOACd2AO} units with a Cd2 Cd2 through-space separation of 3.735(2) Å. Chelating carboxylate groups from adc-A and adc-B assemble the dinuclear units into 1-D [Cd(OCO)]n chain submotifs aligned parallel to the b crystal axis (Fig. 6a), and also create {Cd1AOACd2AO} dinuclear units with a Cd Cd separation of 3.773(1) Å. Parallel 1-D [Cd(OCO)]n chain submotifs are anchored
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adc-A ligands, and adc-B ligands as 4-connected nodes, and treating the Cd2 atoms as 5-connected nodes due to the additional linkage via 3-bpmp ligands. An analysis performed with TOPOS [28] software revealed a new 4,5-connected {4462}3{446482} topology (Fig. 7b). This net can be derived from the more commonly encountered 6-connected 41263 primitive cubic pcu net by removing three out of every four interlayer pillars in a regular, repeating fashion.
4.5. Structural description of {[Cd2(adc)2(4-bpmt)2]H2O}n (4)
Fig. 5. Coordination environments in 3. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S4.
into [Cd2(adc)2(H2O)2]n layer motifs by the full span of the adc-A and adc-B ligands (Fig. 6b). Hydrogen bonding between the aqua ligands bound to Cd1 and the unligated carboxylate oxygen atoms of the adc-B ligands serve an ancillary structure stabilization role (Table S2). These [Cd2(adc)2(H2O)2]n layer motifs are oriented along the ab crystal planes and are pillared into a 3-D [Cd4(adc)4(3-bpmp) (H2O)2]n coordination polymer network (Fig. 7a) by dipodal anti conformation 3-bpmp ligands that link Cd2 atoms in adjacent layers with a Cd2 Cd2 through-ligand distance of 14.248 Å. The network topology of 3 was developed by considering the Cd1 atoms,
The asymmetric unit of compound 4 contains two cadmium atoms, two adc ligands, two 4-bpmt ligands, and one water molecule of crystallization. Both cadmium atoms adopt a distorted octahedral {CdO4N2} configuration, with pyridyl nitrogen donor atoms from two 4-bpmt ligands in nominal trans axial positions. The equatorial planes of the six-coordinate geometries consist of a chelating adc carboxylate terminus and two single oxygen donor atoms from two other adc ligands (Fig. 8a). Bond lengths and angles within the coordination environments of 4 are listed in Table S5. The crystallographically distinct adc ligands adopt an exotridentate l3-j3-O:O0 ,O00 ,O000 exotetradentate binding mode in which one carboxylate terminus chelates to a single cadmium atom, and the other carboxylate endgroup bridges two cadmium atoms. The bridging adc carboxylate groups create crystallographically distinct {Cd2(OCO)2} dimeric units with Cd Cd through-space distances of 3.969(3) and 3.809(3) Å for Cd1- and Cd2-based dimeric units, respectively. The full span of the adc ligands connects the dimeric units into [Cd2(adc)2]n ribbon motifs that are aligned parallel to the c crystal axis (Fig. 8b). These are pillared by 4-bpmt ligands into 2D [Cd2(adc)2(4-bpmt)2]n coordination polymer layers 0) crystal planes (Fig. 9). All of the arranged parallel to the (1 1 4-bpmt ligands have a slightly distorted anti-anti conformation
Fig. 6. (a) 1-D [Cd(OCO)]n chain submotifs in 3. (b) {[Cd4(adc)4(H2O)2]n layer motif in 3.
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Fig. 7. (a) 3-D [Cd4(adc)4(3-bpmp)(H2O)2]n coordination polymer network in 3. (b) Schematic representation of the underlying 4,5-connected {4462}3{446482} topology. In the online version of this article, the purple rods indicate the 3-bpmp linkers. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 8. (a) Coordination environments in 4. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S5. (b) 1-D [Cd2(adc)2]n ribbon motif in 4.
about their central trimethylene moieties (torsion angles = 164.4 and 174.7°; 169.1 and 171.4°), and span Cd Cd distances of 22.435 Å. Water molecules of crystallization are held in small pockets within the layer motifs, anchored by hydrogen bonding donation to ligated adc carboxylate oxygen atoms (Table S2). Treating the Cd2(OCO)2} dimeric units as the connecting nodes results in the layer motif displaying a (4,4) grid topology. Adjacent [Cd2(adc)2(4-bpmt)2]n layers stack by means of CAH O interactions (C O distance = 3.226(4) Å) between 4-bpmt pyridyl rings and adc carboxylate oxygen atoms (Fig. S2). 4.6. Structural description of [Cd(adcH)2(4-bpfp)]n (5) The asymmetric unit of compound 5 contains a divalent cadmium atom sited on a crystallographic 2-fold rotation axis, one singly protonated adc ligand, and half of a 4-bpfp ligand whose piperazinyl ring centroid is located on a crystallographic inversion center. Operation of the rotation axis affords a rather distorted octahedral {CdO4N2} coordination environment with cis disposed 4-bpfp pyridyl nitrogen atom donors (Fig. 10). Two adcH ligands bind to cadmium with chelating carboxylate groups; the protonated carboxylate endgroups do not bind to cadmium. Bond lengths and angles within the coordination environment are listed in Table S6.
The 4-bpfp ligands connect neutral [Cd(adcH)2] fragments into zig-zagged 1-D [Cd(adcH)2(4-bpfp)]n coordination polymer chains that are aligned parallel to the [1 0 1] crystal direction (Fig. 11). The Cd Cd through-ligand distance within the chain motif is 16.62(1) Å; the Cd Cd Cd angle measures 109.4°. Thus the 1-D motif in 5 much more closely resembles a splayed-open straight chain than the tight ribbon motif in 1. Nevertheless, when viewed down [1 0 1], the chains in 5 do make an ‘‘H” shape similar to compound 1. Adjacent [Cd(adcH)2(4-bpfp)]n ribbons aggregate into the 3-D crystal lattice of 5 by means of OAH O hydrogen bonding interactions (Table S2) between the protonated adcH carboxylate groups and bound adc carboxylate oxygen atoms (Fig. S3). Due to amide resonance mechanisms, the piperazinyl nitrogen atoms within the 4-bpfp are no longer available for hydrogen bonding acceptance. As a result, the supramolecular interactions between ribbons of 5 differ substantially from those in the related 4-bpmp congener 1. 4.7. Thermal properties The mass of compound 1 remained stable until 190 °C, at which point ligand combustion occurred. Compound 2 underwent loss of its water molecules of crystallization between 25 and 225 °C, with a mass loss of 10.3% slightly less than the predicted
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Fig. 9. 2D {[Cd2(adc)2(4-bpmt)2]H2O}n layer motif in 4. In the online version of this article, the 1D [Cd2(adc)2]n ribbon motifs are depicted in red. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
Fig. 10. (a) {CdO4N2} distorted octahedral coordination environment in 5. Thermal ellipsoids are drawn at 50% probability. The symmetry codes are listed in Table S6. (b) [Cd(adcH)2(4-bpfp)]n ribbon in 5.
value of 11.5%; ligand combustion occurred above 225 °C. The mass of compound 3 remained stable until 185 °C. Compound 4 underwent loss of its co-crystallized water molecules around 140 °C, with a mass loss of 1.3% very close to that of the predicted value (1.2%). Ligand ejection occurred above 200 °C for 4 and above 240 °C for 5. The TGA traces for 1–5 are shown in Figs. S4–S8, respectively. 4.8. Luminescent properties and nitrobenzene detection Compounds 1–4 show ultraviolet emission in sonicated ethanol suspension (2 mg/mL) upon excitation with higher energy ultraviolet light (kex = 260 nm), with emission maxima at 294 nm in all cases. The emissive behavior is attributed to p–p⁄ transitions within the molecular orbital manifolds of the dipyridyl-type ligands bound to cadmium. Compound 5 did not show appreciable emission in ethanol suspension. Compounds 1–4 show capability for detection of nitrobenzene in ethanol suspension. Addition of small aliquots of nitrobenzene dissolved in DMSO (1 104 M) resulted in decrease of the emission intensity (Fig. 11). This loss
of emission signal is attributed to charge transfer from the excited states of the dipyridyl ligand molecular orbitals within the coordination polymer, to those of the adsorbed electron-withdrawing nitrobenzene substrate [5]. The intensity factor (Io–I)/Io (Io = maximum intensity in the absence of analyte, I = intensity after analyte addition) was recorded for the samples 1–4, in the presence of 10 lL of the stock nitrobenzene solution. For 1–4, the intensity factors were 0.34, 0.48, 0.68, and 0.41, respectively. Further decreases in emission intensity were observed upon administration of more analyte. When 60 lL of the stock nitrobenzene solution was added to suspensions of 1–4, the intensity factors reached values of 0.98, 0.98, 0.99, 0.97, respectively, thereby showing nearly complete quenching of emission. 5. Conclusions Cadmium coordination polymers containing the rigid yet aliphatic 1,3-adamantanedicarboxylate ligand show a dramatic difference in dimensionality and topology depending on the nature of a hydrogen-bonding capable dipyridyl coligand
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Fig. 11. Nitrobenzene detection plots for 1–4.
and the stoichiometric ratio. Using 4-bpmp, 1-D chain and 3-D 658 cds topology phases were obtained if stoichiometric ratios were varied. A similar 1-D chain was obtained employing the related dipyridyl ligand 4-bpfp, in 5. Use of the conformationally flexible 3-bpmp ligand produced a new 3D topology in 3, derived from the usual primitive cubic pcu framework but with regular ligand vacancies. The very long spanning ligand 4-bpmt afforded 4, a 2D phase constructed from connection of dinuclear units. All the structures in this study differ greatly in topology from the two previously reported cadmium adamantanedicarboxylate dipyridyl coordination polymers, which showed diamondoid or nanotubular structures. Coordination polymers 1–4 manifest functionality as detectors for nitrobenzene in ethanol suspension, with the greatest detection ability for the 3D phases 3 and 2, followed by the 2D phase 4. It is plausible that the ‘‘ligand vacancies” in the 3D net of 3 allows greater nitrobenzene uptake into the coordination polymer lattice. Compound 1, which is a 1D coordination polymer phase, showed more modest nitrobenzene uptake. Coordination polymer 5, also a 1D phase, showed very weak emission in ethanol suspension. It is also possible that adsorption of nitrobenzene into the 3D phases with tighter packing results in enhanced electron transfer to the analyte and therefore a greater decrease in emission.
Acknowledgements Funding for the work was provided by Michigan State University. We thank Dr. Rui Huang for elemental analysis. We thank Mr. Sultan Qiblawi for the synthesis of the 4-bpmt ligand. Appendix A. Supplementary data Additional molecular graphics and thermogravimetric analysis plots. Crystallographic data (including structure factors) for 1–5 have been deposited with the Cambridge Crystallographic Data Centre with Nos. 1432997, 1441833, 1432998, 1432999, and 1447426, respectively. 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, U.K. (Fax: 44–1223336033, Email: [email protected]). Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.ica.2016.07.027. References [1] (a) M.P. Suh, H.J. Park, T.K. Prasad, D. Lim, Chem. Rev. 112 (2012) 782; (b) L.J. Murray, M. Dinca, J.R. Long, Chem. Soc. Rev. 38 (2009) 1294. and references therein.
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