Supramolecular Isomerism in a Cadmium Bis(N ... - ACS Publications

May 27, 2013 - (6) Herein, an example of the latter scenario is presented where supramolecular isomers (as different solvates) of a cadmium bis(N-hydr...
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Supramolecular Isomerism in a Cadmium Bis(N‑Hydroxyethyl, N‑isopropyldithiocarbamate) Compound: Physiochemical Characterization of Ball (n = 2) and Chain (n = ∞) Forms of {Cd[S2CN(iPr)CH2CH2OH]2·solvent}n Yee Seng Tan,† Anna L. Sudlow,‡ Kieran C. Molloy,‡ Yui Morishima,§ Kiyoshi Fujisawa,§ Wendy J. Jackson,∥ William Henderson,∥ Siti Nadiah Binti Abdul Halim,† Seik Weng Ng,†,⊥ and Edward R.T. Tiekink*,† †

Department of Chemistry, University of Malaya, 50603 Kuala Lumpur, Malaysia Department of Chemistry, University of Bath, Bath BA2 7AY, United Kingdom § College of Science, Department of Chemistry, Ibaraki University, 2-1-1 Bunkyo, Mito, Ibaraki 310-8512, Japan ∥ Department of Chemistry, University of Waikato, Private Bag 3105, Hamilton 3240, New Zealand ⊥ Chemistry Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, Saudi Arabia ‡

S Supporting Information *

ABSTRACT: Needles of [{Cd[S2CN(iPr)CH2CH2OH]2}3·MeCN]∞ (2) were harvested from a dry acetonitrile solution of Cd[S2CN(iPr)CH2CH2OH]2 after one or two days and proved to be a coordination polymer in which all dithiocarbamate ligands are μ2,κ2-tridentate, bridging two cadmium atoms and simultaneously chelating one of these. If the same solution was allowed to stand for at least several days, 2 is replaced by blocks comprising a supramolecular isomer of 2, dimeric 1, with formula {Cd[S2CN(iPr)CH2CH2OH]2}2·2H2O·2MeCN, and two ligands coordinating μ2,κ2 as in 2 and the other two purely κ2-chelating. The time dependency correlates with the pivotal role of water in driving the conversion of “chain” 2 to “ball” 1; crystals of 2 could not be isolated from “wet” acetonitrile. When each of 1 and 2 are dissolved in solution, they exhibit comparable spectroscopic attributes (1H, 13C, and 113Cd NMR and UV/vis), indicating the solution structures are the same. Both 1 and 2 are luminescent in the solid state with 1 being significantly brighter than 2. Greenockite CdS nanoparticles are generated by the thermal decomposition of both 1 and 2. isomers,8 as opposed to different supramolecular aggregates cocrystallized with solvent.6 Herein, an example of the latter scenario is presented where supramolecular isomers (as different solvates) of a cadmium bis(N-hydroxyethyl, Nisopropyldithiocarbamate) compound, {Cd[S 2 CN(iPr)CH2CH2OH]2·solvent}n, are described, i.e., ball (n = 2) and chain (n = ∞) forms. Here, the basic building block is considered to be Cd[S2CN(iPr)CH2CH2OH]2, which dimerizes to a ball (2) or polymerizes to a chain (1). The terminology “pseudo polymorphism” was rejected on the basis that the species feature different coordination geometries. In crystal engineering endeavors of 1,1-dithiolate ligands, of which dithiocarbamate is a prominent example, metal 1,1dithiolates are known to serve as reliable precursors of nanosized metal sulfide crystals, especially of the main group

1. INTRODUCTION In the construction of coordination polymers incorporating three-dimensional (3D) metal−organic frameworks (MOFs),1,2 the crystal engineer is faced with opportunities whereby systematic variation of a metal center’s oxidation state, coordination number, geometry, etc. can be tailored, along with a choice of ancillary ligand(s), to generate a rich diversity of structural motifs with potential applications ranging from gas storage to catalysis.3−5 One challenge facing practitioners in this area relates to the phenomenon of supramolecular isomerism (SI).6 Here, quite different supramolecular architectures can be constructed from the same molecular constituents. An early and dramatic example of this is found in the structure of copper(I) ethylimidazolate, where from a polar solvent (water) a triplestranded helix is formed by contrast to the zigzag polymer formed from the less polar solvent system (e.g. water/ cyclohexane).7 These are examples of solvent-induced supramolecular isomerism, and given the empirical formulas are the same, they can be classified as “genuine” supramolecular © XXXX American Chemical Society

Received: March 26, 2013 Revised: May 21, 2013

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elements.9−14 In particular, with relevance to this paper, CdS nanoparticles are in a size domain where quantum confinement effects dominate their electronic properties. This causes a blue shift to the band gap, through which the absorption/emission of radiation can be controlled, leading to applications as LEDs15 and in PV devices,16 as well as in catalysis.17 However, in the realm of coordination polymers, metal dithiocarbamates have not received nearly as much attention compared with the ubiquitous carboxylates.18 Reasons for this relate the strong chelating ability of the dithiocarbamate anion, due the significant contribution of the R2N(+)=CS2(2‑) canonical form to the overall electronic structure, indicating a reduced propensity for bridging metal centers, in particular of transition metals, and the relative insolubility of complexes with two or more dithiolate residues. One way of overcoming the reluctance of dithiocarbamates to link metal centers is to introduce functionality in the ligands, such as hydrogen bonding or charge, and heteroatoms capable of forming secondary interactions.19−26 In cases where the dithiocarbamate is functionalized with pyridyl groups, otherwise zero-dimensional entities can associate via secondary interactions (e.g., Hg···N) to form supramolecular chains.23 A prominent example among these studies is the formation of 3D-, two-dimensional (2D)-, and one-dimensional (1D)-supramolecular arrays based on hydrogen bonding, the presence of the solvent and correlated with the nature of R (i.e., R = CH2CH2OH, Me, or Et, in {Zn(S2CN(CH2CH2OH)R]2}2(4,4′-bipyridine) adducts.24 In this connection, and in recognition of the contribution of cadmium compounds27 and ligands with sulfur donors28 in crystal engineering, cadmium compounds of dithiocarbamates carrying hydroxyethyl groups have been investigated and in the course of these studies the title supramolecular isomers were isolated and characterized.

min. Powder X-ray diffraction (PXRD) data were recorded with a PANalytical Empyrean XRD system with Cu Kα1 radiation (λ = 1.54056 Å) in the 2θ range of 5 to 40° with a step size of 0.026°. Comparison between experimental and calculated (from CIFs) PXRD patterns was perfomed with X’Pert HighScore Plus.29 Positive-ion ESI mass spectra were recorded on a Bruker MicroTOF instrument that was periodically calibrated using a solution of sodium formate. Samples were prepared by agitating ca. 0.1 mg of sample in 1.5 mL of HPLC-grade acetonitrile. The resulting supernatant, after centrifugation, was introduced into the instrument using direct infusion at 3 μL/min. Routine spectra were recorded using a capillary exit voltage of 150 V. Identification of ions was facilitated by use of an instrumentbased isotope pattern calculator and molecular formula calculation software. Diffuse reflectance spectra at room temperature were recorded on a JASCO V-570 spectrophotometer equipped with an integrating sphere apparatus JASCO ISN-470. Luminescence spectra of solid samples of 1 and 2 at 83, 173, and 298 K were recorded in the range from 280−750 nm on a JASCO FP-6500 spectrofluorometer cooled using a liquid nitrogen cryostat (CoolSpeK) from Unisoku Scientific Instruments (Osaka, Japan). The samples for spectroscopy were prepared by finely grinding microcrystalline material into powders. Synthesis. All reactions were carried out under ambient conditions. Na[S2CN(iPr)CH2CH2OH)] was prepared by taking (2-isopropylamino) ethanol (11.45 mL, 0.1 mol) into acetone (100 mL) followed by stirring at ambient condition for 10 min to achieve an homogeneous solution. The solution was placed in an ice bath that was continuously stirred. Carbon disulfide (6.00 mL, 0.1 mol) was added dropwise, at which point the solution turned yellow. After complete addition, the yellow solution was stirred for an additional 10 min to ensure the reaction had completed. Sodium hydroxide solution (50% w/v, 7.95 mL, 0.1 mol) was added, and a white precipitate formed immediately. Acetone (100 mL) was added followed by five minutes of stirring. The precipitate was filtered by suction and dried on a hot plate (80 °C) overnight. Mp: 166.0−167.0 °C. IR (cm−1): 1447 m ν(C−N); 1165 s, 963 s ν(C−S). 1H NMR (d6-EtOH, ppm): δ 6.17 (sept, 2H, CH, 6.67 Hz), 4.09 (m, 2H, CH2O, 6.62 Hz), 3.88 (m, 2H, NCH2), 1.19 (d, 6H, CH3, 6.76 Hz), the signal due to OH overlaps with that of the solvent. 13C NMR (d6-EtOH, ppm): δ 214.2 (CS2), δ 62.3 (CH2O), δ 54.2 (NCH2), δ 49.6 (CH), δ 20.6 (CH3). To prepare the cadmium compound, an aqueous solution (50 mL) of CdCl2·21/2H2O (5.4024 g, 0.0236 mol) was added to an aqueous solution (50 mL) of Na[S 2 CN(iPr)CH2CH2OH] (10.0000 g, 0.0497 mol) in a 1:2.1 mol ratio. The white precipitate that appeared immediately was extracted with warm chloroform (40 °C). After cooling, the precipitate was filtered off and dried on a hot plate (80 °C) overnight (yield: 8.6590 g, 0.0185 mol, 78%). Elemental analysis: C, 30.61; H, 4.99; N, 5.77. C12H24CdN2O2S4 requires: C, 30.73; H, 5.16; N, 5.98%. 1H NMR (d6-DMSO, ppm): δ 5.21 (sept, 1H, CH, 6.49 Hz), 4.80 (t, 1H, OH, 5.18 Hz), 3.80−3.60 (m, 4H, NCH2CH2O), 1.17 (d, 6H, CH3, 6.64 Hz). 13C NMR (d6DMSO, ppm): δ 205.3 (CS2), δ 58.2 (CH2O), δ 56.6 (NCH2), δ 50.4 (CH), δ 19.9 (CH3). Noteworthy is the observation that no evidence for additional species in the NMR was found. See Results and Discussion for discussion of PXRD and TGA results.

2. EXPERIMENTAL SECTION Reagents were purchased as used as received: N-isopropyl ethanol amine (70% purity; Aldrich), carbon disulfide (99.9% purity; Merck), sodium hydroxide (99.00% Purity; R&M Chemicals), CdCl2·21/2H2O (99.0% purity; Friendemann Schmidt), acetone (>99.8% purity; Merck), and acetonitrile (>99.5% purity; Merck). Melting points were determined on a Krüss KSP1N melting point meter. Elemental analyses were performed on a PerkinElmer PE 2400 CHN Elemental Analyzer. 1H and 13C{1H} NMR spectra were recorded in d6-DMSO solution on a Bruker Avance 400 MHz NMR spectrometer with chemical shifts relative to tetramethylsilane as the internal reference; abbreviations for NMR assignments: s, singlet; d, doublet; t, triplet; sept, septet; m, multiplet. 113Cd{1H} NMR were measured on a Bruker Avance 500 MHz spectrometer using Me2Cd as a reference. The optical absorption spectra were measured in the range of 190−1100 nm on an Agilent Cary 60 UV−vis spectrophotometer. IR spectra were measured on a Perkin-Elmer Spectrum 400 FT mid-IR/far-IR spectrophotometer from 4000 to 400 cm−1. Photoluminescence (PL) measurements were carried out on an Agilent Varian Cary Eclipse Fluorescence Spectrophotometer using a Xenon flash lamp as the excitation source at room temperature. Thermogravimetric analyses were performed on a PerkinElmer TGA 4000 Thermogravimetric Analyzer in the range of 30−900 °C at a rate of 10 °C/min. DSC analyses were performed on a Perkin-Elmer DSC-6 differential scanning calorimeter in the range of 30 to 350 °C at the rate of 5 °C/ B

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Figure 1. (a) Centrosymmetric dimeric aggregate in 1 showing atom-labeling scheme. (b) Asymmetric unit (less acetonitrile molecule) in 2 showing atom labeling scheme. The atom indicated with an * is C13, and atoms C14, C18, and C30 are obscured. (c) The extended polymeric chain in 2 highlighting the edge shared polyhedra; all but O−H hydrogen atoms have been omitted. Solvent molecules have been omitted from all images.

NCH2CH2O), 2.06 (s, 3H, MeCN), 1.16 (d, 12H, CH3, 6.76 Hz). NMR data for 2 are identical, save for the integration for the MeCN protons, which is now 1H (i.e., one MeCN per three cadmium dithiocarbamate moieties). 13C NMR (d6DMSO, ppm): δ 205.2 (CS2), 118.1 (CH3CN), 58.3 (CH2O), 56.7 (NCH2), 50.5 (CH), 19.9 (CH3), 1.18 (CH3CN). 113Cd NMR (d6-DMSO, ppm): δ = −412.1 (1) and −415.5 (2). UV− vis (EtOH:MeCN 1/1 v/v) for 2 (absorption maxima for 1 were experimentally equivalent): λmax = 203 nm (ε = 72990 cm−1 M−1); 219 (ε = 48920); 261 (ε = 81520); and 283 (ε = 47660). Characterization of “unknown species” (see Results and Discussion). Colorless crystals with elemental analysis: C, 30.79; H, 5.20; N, 6.22. C24H48Cd2N4O4S8.0.5H2O·0.25ACN requires: C, 31.52; H, 5.22; N, 6.77%. See Results and Discussion for discussion of PXRD and TGA results. X-ray Crystallography. Single crystal X-ray diffraction data for a colorless block of 1 (0.09 × 0.25 × 0.27) were measured at 100(2) K on a Bruker SMART APEX CCD diffractometer using Mo Kα radiation (λ = 0.71073 Å) to a θmax of 27.5°. A multiscan absorption correction was applied.30a The structure was solved by direct methods (SHELXS9730b) and refined (anisotropic displacement parameters, H atoms in the riding model approximation, and a weighting scheme of the form w =

Subsequently, compounds 1 and 2 were obtained from recrystallization from HPLC-grade acetonitrile that had also been dried over 3 Å molecular sieves. Crystals of 2 appeared initially and after the egress of time and slow ingress of water, crystals of 1 were isolated. Crystals of 1 were harvested after 12 days and proved to have composition {Cd[S2CN(iPr)CH2CH2OH]2}2·2H2O·2MeCN. Mp: 158.0−158.4 °C. Elemental analysis: C, 31.61; H, 5.50; N, 7.71. C28H58Cd2N6O6S8 requires: C, 31.84; H, 5.54; N, 7.96 %. IR (cm−1): 1452 m ν(C−N); 1158 m, 968 s ν(C−S). The full IR spectrum is given in Figure S1a of the Supporting Information. Crystals of 2 were harvested after two days and have composition [{Cd[S2CN(iPr)CH2CH2OH]2}3·MeCN]∞. Mp: 159.5−159.9 °C. Elemental analysis: C, 31.27; H, 4.90; N, 6.68. C38H75Cd3N7O6S12 requires: C, 31.52; H, 5.22; N, 6.77%. IR (cm−1): 1446 m ν(C−N); 1158 m, 966 s ν(C−S). The full IR spectrum is given in Figure S1b of the Supporting Information. 1 H NMR spectroscopy conducted on crystals of 1 and 2 were indistinguishable in terms of chemical shifts and multiplicity, with the difference relating solely to the integration ratio for the occluded acetonitrile solvent; 13C NMR were indistinguishable. 1 H NMR (d6-DMSO, ppm) for 1: δ 5.20 (sept, 2H, CH, 6.67 Hz), 4.88 (t, 2H, OH, 5.52 Hz), 3.63−3.70 (m, 8H, C

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Table 1. Crystal Data and Refinement Details for 1 and 2 parameter

1

2

formula formula weight crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z density, g/cm3 (calculated) μ (mm−1) reflections collected independent reflections reflections with I ≥ 2σ(I) R (observed data) a, b in weighting scheme Rw (all data) Largest difference in peak and hole (e Å−3) CCDC deposition number

C24H48Cd2N4O4S82(C2H3N)·2(H2O) 1056.08 triclinic P1̅ 9.4391(2) 11.0392(2) 12.4895(3) 102.541(2) 103.201(2) 112.898(1) 1097.14(4) 1 1.598 1.393 10557 4999 4692 0.018 0.022, 0.297 0.046 0.39 and −0.29 930082

C36H72Cd3N6O6S12·C2H3N 1448.12 triclinic P1̅ 13.7236(5) 13.7779(4) 17.4104(5) 97.877(3) 96.072(3) 114.683(3) 2913.99(16) 2 1.650 1.561 49185 13440 10896 0.053 0.061, 18.675 0.153 2.91 and −1.65 930083

1/[σ2(Fo2) + aP2 + bP], where P = (Fo2 + 2Fc2)/3) with SHELXL97 on F2.30b Data for a pale yellow prism of 2 (0.03 × 0.05 × 0.35) were measured at 100(2) K on an Agilent Supernova dual diffractometer with an Atlas (Mo) detector (ω scan technique), using Mo Kα radiation (θmax = 27.6°). The data were reduced following standard procedures30c and refined as for 1. The crystal is a nonmerohedral twin. The twin domains were identified using PLATON,30d and the minor component was refined to 26.8(2)%. The structure is disordered in the N(iPr)CH2CH2OH residue of the S5dithiocarbamate ligand with two positions being resolved. Similar disorder was found for the methyl groups bound to the C34 atom. The anisotropic displacement parameters of chemically equivalent pairs of atoms were made equal, and these were tightly restrained to be nearly isotropic. For the affected residues, the N−C(thiocarbonyl) distance was restrained to 1.35 ± 0.01 Å, N−C(alkyl) distances to 1.45 ± 0.01 Å, C−O to 1.50 ± 0.01 Å, and C−C to 1.54 ± 0.01 Å. In the adopted model, the O4−H atom apparently does not participate in a hydrogen bond. As shown in Figure S2 of the Supporting Information, there is a row of three hydroxyl groups with two hydrogen bonds; it is likely that the three H atoms are disordered so that at any one instant, any two are engaged in hydrogen bonding. For 2, the maximum and minimum residual electron density peaks of 2.91 and 1.65 e Å−3, respectively, were located 1.15 and 1.35 Å from the H2 and Cd2 atoms, respectively. In the final refinement of 1, three reflections [i.e., (0 0 1), (5 −12 4), and (10 1 0)] were omitted, owing to poor agreement. The displacement ellipsoid diagrams in Figure 1 were drawn with ORTEP-3 for Windows30e at the 35% probability level and other crystallographic diagrams were drawn with DIAMOND.30f Crystal data and refinement details are collected in Table 1. Thermodecomposition under “Reaction under Autogenic Pressure at Elevated Temperatures” (RAPET).31 The thermal decomposition under RAPET conditions of 1 and 2 were carried out in a 5 mL closed vessel cell (method 1). The

cell was assembled from stainless steel Swagelok parts. A 1/2 in. union part was capped on both sides by standard plugs. For both these syntheses, 0.4 g of compound was introduced into the vessel at room temperature in nitrogen atmosphere of a glovebox. The filled cell was closed tightly with the other plug and then placed inside an iron pipe in the middle of the furnace. The temperature was raised to 400 °C at a rate of 10 °C per minute. The closed vessel reactor (Swagelok) was heated at 400 °C for 1 h, and then gradually cooled (1 °C/min) to room temperature, opened with the release of a little pressure, and the residual dark black powder was collected. TEM images were taken using a JEOL JEM 1200EXII transmission electron microscope, with an accelerating voltage of 120 kV, using an Oxford INCA Energy X-ray analyzer for EDX measurements. PXRD of the nanoparticles was performed on a Bruker D8 Powder diffractometer with Cu Kα1 radiation (λ = 1.54056 Å) in the 2θ range of 20 to 80°. Solvothermal Synthesis. Ethylene glycol (50 mL) was heated to reflux and in separate experiments 0.25 g of solid 1 and 2 added and the mixture stirred for 30 min (method II). Alternatively, 0.25 g of either 1 or 2 was added to ethylene glycol (50 mL), which had been brought to reflux before addition (method III). In both cases product isolation was the same: the solution was cooled to room temperature leaving a yellow precipitate, propan-2-ol (50 mL) added, and the solution centrifuged at 3000 rpm for 10 min. Te supernatant was discarded and the process repeated 3 times to obtain a clean yellow powder which was dried in vacuo.

3. RESULTS AND DISCUSSION Synthesis and Solution Characterization. The facile metathetical reaction between CdCl2·21/2H2O and 2 mol equiv of Na[S2CN(iPr)CH2CH2OH)] yielded the anticipated cadmium bis dithiocarbamate compound (based on 1H and 13C NMR) that was isolated in two crystalline forms, depending on the time of harvesting from its acetonitrile solution. Both forms, 1 and 2, were solvates, as detailed below, and crystallography D

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for ideal square pyramidal and trigonal pyramidal geometries, respectively.33 In this description, the Cd atom lies 0.6175(2) Å above the plane of the S1−S4 atoms (rms deviation = 0.0953 Å) in the direction of the transannular S4i atom. The structure of 1 is as anticipated, having a number of literature precedents.34a In fact, the structural chemistry of cadmium dithiocarbamates is remarkable for its lack of variety compared to other zinc triad 1,1-dithiolates for which 0, 1, 2 and 3D structures are known.34a Cadmium dialkyldithiocarbamate structures, for which 11 examples are known (see Table S1 for a listing), uniformly adopt structures akin to that described above for 1, including those with bulky Nsubstituents such as cyclohexyl34b and an example with CH2CH2OH.34c On this basis, the characterization of 2 as an unprecedented 1D coordination polymer (Figure 1, panels b and c) was quite unexpected. Needles of 2 were obtained from a dry acetonitrile solution and were harvested after only two days. The crystallographic asymmetric unit of 2 comprises three formula units of Cd[S2CN(iPr)CH2CH2OH]2 (Figure 1b and Table 2) and a solvent acetonitrile molecule. Unlike binuclear 1, where there is an equal number of bidentate κ2-chelating and μ2,κ2-tridentate dithiocarbamate ligands, in 2 each dithiocarbamate ligand is μ2,κ2-tridentate, bridging two Cd centers and chelating one of them. The six-coordinate coordination geometries at each of these metals are distorted but each is based on an octahedron so that the 1D polymer arises from a sequence of edge-shared octahedra (Figure 1c). The Cd−S bond lengths in 2 (Table 2) are in the range of 2.5808(16) to 2.7744(14) Å and are generally longer than the comparable bonds in 1, consistent with the higher coordination number in 2. The most notable feature of the crystal packing of 1 is the formation of supramolecular layers with zigzag topology in the (−1 0 1) plane (Figure 2a); geometric details for the hydrogen bonding are collected in Table 3. The layers arise as a result of the O−H···O hydrogen bonds between the O1-hydroxyl group and the water molecule via a centrosymmetric eight-membered {···HO···HO}2 synthon (Figure 2b). Two O2-hydroxyl groups are connected to this via O2−H···O1 hydrogen bonds, and two molecules of acetonitrile are similarly connected but via water− O1w−H···N hydrogen bonds. The layers stack (Figure S3 of the Supporting Information) with the major connections between them being of the type C−H···O and C−H···S (Table 3). Disorder in some of the hydroxyethyl groups precludes a detailed description of the crystal packing of 2. The structure can be described as comprising layers of supramolecular polymers in parallel to [1 0 1] connected into a 3D architecture by O−H···O bonds; additional stability is provided by C− H···O contacts (Table 3). A feature of the hydrogen bonding is the formation of a chain comprising four O−H···O hydrogen bonds, involving the O2 and O4−O6 atoms. The O1−H group forms a hydrogen bond with one component of the disordered O3 atom, while the second component of the latter forms an O−H···N hydrogen bond with the solvent acetonitrile molecule (see Figure 3). Crystallography. Compounds 1 and 2 can be isolated from the same covered (parafilm wax) solution, depending on the time of harvest. The initial solution was HPLC-grade acetonitrile that had been dried over 3 Å molecular sieves. In reference to Figure 4, which collects images of crystals grown between days 1 and 6 for one particular experiment, after 1 day, isolated needles of polymeric 2 appear and by day 2 these were

showed these to be supramolecular isomers. Proof that this was a phenomenon of the solid state was found in the homogeneity of the solution characterization for both species; see Experimental Section for data. Thus, 1H and 13C{1H} NMR of freshly dissolved 1 and 2 were identical and showed the expected resonances and integration, including evidence for the solvent acetonitrile molecules of crystallization established by X-ray crystallography (see below). 113Cd{1H} NMR spectra of 1 and 2 were also experimentally indistinguishable, confirming a common coordination number at the metal despite their differences in the solid state (see below). 113Cd NMR chemical shifts are dependent on a number of factors, including the electronegativity of the coordinated atoms and, crucially, the coordination number of the metal. In this regard, chemical shifts move upfield (more negative) with increasing coordination number. Data for 1 and 2 are very similar to those recorded for Cd[S2CN(H)R]2 at low concentration in THF, where the species present are believed to be six-coordinate Cd[S2CN(H)R]2·2THF (δ 113Cd, ca. −433 ppm, vs Me2Cd).32 However, the precise species present in solutions of 1 and 2 remains uncertain due to the use of the strongly coordinating DMSO as a solvent to aid dissolution, which may very well become involved in metal coordination as the structure of the coordination polymers is broken down. As for the 113Cd data, UV−vis data measured in acetonitrile:ethanol (1:1 v/v) for 1 and 2 matched. Single Crystal X-ray Crystallography. Recrystallization of Cd[S2CN(iPr)CH2CH2OH]2 from its HPLC acetonitrile solution by slow evaporation at room temperature over 12 days afforded 1 (i.e., {Cd[S2CN(iPr)CH2CH2OH]2}2·2H2O·2MeCN) as blocks. The molecular structure of 1 is shown in Figure 1a and selected geometric parameters are collected in Table 2. The Cd atom in 1 is Table 2. Selected Geometric Parameters (Å) for 1 and 2 parameter 1 Cd1−S1,S2 Cd1−S3,S4,S4i 2 Cd1−S1, S1ii, S2ii Cd1−S3,S4 Cd1−S5 Cd2−S3 Cd2−S5, S6 Cd2−S7, S8 Cd2−S10 Cd3−S7 Cd3−S9, S10 Cd3−S11, S11iii, S12iii

2.5708(4), 2.5843(4) 2.5792(4), 2.7428(4), 2.6207(4) 2.7297(15), 2.7266(14), 2.7198(16) 2.6903(15) 2.7568(15), 2.7039(15), 2.7744(14) 2.7075(14) 2.5808(16), 2.7527(15),

2.7167(16), 2.6419(17) 2.6338(16)

2.5927(17) 2.6136(17)

2.7558(15) 2.7052(14), 2.6316(15)

Symmetry operations, i: 2 − x, 2 − y, 1 − z; ii: 2 − x, 1 − y, 1 − z; and iii: 1 − x, 1 − y, 2 − z. a

coordinated by a symmetrically chelating S1,S2-dithiocarbamate ligand and asymmetrically by the S3,S4-ligand. A fifth coordination site is occupied by a S4 atom derived from a centrosymmetrically related molecule, leading to a dimeric aggregate. The tridentate bridging mode (μ2,κ2) of the S3,S4dithiocarbamate, via the S4 atom, accounts for the elongation of the bond lengths involving the S4 atom. The Cd coordination geometry is based on a distorted square pyramid, as evidenced in the value of τ = 0.16, which compares with τ = 0.00 and 1.00 E

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Figure 3. Crystal packing in 2: View in projection down the b axis of the unit cell contents. Hydrogen atoms not participating in the hydrogen bonding have been omitted. The O−H···O and C−H···O interactions are shown as orange and blue dashed lines, respectively.

these turn into the clusters from which thin plates may be discerned by day 4. Clusters change into blocks by day 6, corresponding to dimeric 1. The experiments are perfectly reproducible but sometimes the blocks formed as late as day 14, depending on the ambient laboratory conditions. Besides the different structure, the key difference between 1 and 2 is the presence of water molecules of solvation in 1. Given that it was not possible to isolate crystals of 2 from laboratory-grade acetonitrile, it is likely that it is water that is the driving force leading to 1; similarly, the deliberate addition of water to HPLC-grade acetonitrile did not produce needles 2 with blocks (1) appearing after two days. In short, in the absence of water crystals of the 2 form and upon standing, the mother liquor absorbs ambient moisture, leading to the formation of 1. Time-dependent supramolecular isomerism has precedents in the literature,35−37 including in the context of hydrothermal synthesis,38 but the present example appears to be dependent on the increasing concentration of water in the initially dry solvent. Powder X-ray diffraction (PXRD). PXRD performed on freshly isolated 1 and 2 revealed that the single-crystal

Figure 2. Crystal packing in 1: (a) Supramolecular layer in the bc plane; hydrogen atoms not participating in the hydrogen bonding have been omitted. (b) Detail of the O−H···O and O−H···N hydrogen bonding scheme; see Table 3 for symmetry operations. In both figures, the O−H···O and O−H···N hydrogen bonds are shown as orange and blue dashed lines, respectively.

sufficiently large for single-crystal X-ray analysis. On day 3, needles had grown and coalesced into larger aggregates and

Table 3. Geometric Characteristics (Å, deg) of the Intermolecular Interactions (A−H···B) Operating in the Crystal Structures of 1 and 2 A

H

B

H···B

O1 O2 O1w O1w C11 C14 C12

H1o H2o H1w H2w H11b H14c H12b

O1w O1 O1 N3 O2 O2 S1

1.89(2) 1.990(15) 2.04(2) 2.07(2) 2.59 2.58 2.85

O1 O2 O3′ O5 O6 C33

H1o H2o H3′o H5o H6o H33a

O3 O5 N7 O4 O4 O1

1.76 1.95 2.21 1.99 1.97 2.51

A···B

A−H···B

2.7141(19) 2.8110(18) 2.855(2) 2.909(2) 3.423(2) 3.353(2) 3.7251(17)

176(2) 173(2) 164(2) 176(2) 143 136 149

symmetry operation

1 x, y, 1+z x, y, −1+z 1-x, 1-y, 1-z x, y, −1+z 1-x, 2-y, -z x, −1+y, 1+z x, 1+y, z

2 2.550(16) 2.751(7) 2.831(16) 2.772(9) 2.769(10) 3.335(11) F

155 159 113 155 158 141

1+x, 1+y, z x, y, z x, y, z x, −1+y, z −1+x, −1+y, z −1+x, −1+y, z

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Figure 4. Images of crystal growth of 1 (blocks) and 2 (needles) starting from a solution of HPLC acetonitrile (0.5 g in 100 mL): (a) day 1, (b) day 3, (c) day 4 , and (d) day 6.

Figure 5. DTA (red trace), TGA (green), and DSC (blue) curves for (a) 1 and (b) 2.

pattern exhibited by 2. Analogous experiments on 2 showed the sample to be stable for at least 24 h (Figure S5b of the Supporting Information), with no evidence of conversion to 1. Microanalytical data (see Experimental) gave supporting evidence for the formulations of 1 and 2, and the IR spectra for each showed the expected absorptions for ν(C−N) and ν(C−S), see Figure S1 of the Supporting Information for spectra. Thermal Degradation. Freshly harvested samples of 1 and 2 were subjected to DTA, TGA, and DSC analysis. TGA of 1 (Figure 5a) shows evidence for a three-step decomposition to yield CdS, but the distinction between the first two steps is problematic. The combined weight loss of 11.1% (calc. 11.2%) between 41 and 116 °C corresponds to the loss of lattice water

structures measured at 100 K are representative of the bulk material at room temperature (r.t.) (Figure S4, panels a and b, of the Supporting Information). PXRD on crystals harvested on day 8 in one experiment (i.e., not containing needles or blocks, corresponding to the central cluster in Figure 4c) did not conform to either 1 or 2 (Figure S4c of the Supporting Information), indicating that the transformation of 2 to 1 proceeded via a third, thus far unknown, crystalline form. Both isolated samples of 1 and 2 became opaque with the passing of time due to the loss of solvent. Time-dependent PXRD on 1 showed that after 2 h the patterns matched, but discernible differences were noted after 12 h (Figure S5a of the Supporting Information) consistent with decomposition; PXRD after 12 h did not match with the G

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Figure 6. Representative TEM images of Greenockite CdS nanoparticles generated from (a−c) 1 and (d−f) 2, using methods I, II, and III, respectively.

and acetonitrile. The final step, 164 to 310 °C, left 29.2% consistent with a residue of CdS (cf. theoretical 27.4%). DSC of 1 (Figure 5a) shows two distinct events at 69 °C (onset-end: 48−84 °C; ΔHvap = 120.3 kJ/mol) and 99 °C (96−104 °C;

28.4 kJ/mol), which correspond to vaporization of the solvent molecules. The decomposition at 176.3 °C (168−183 °C) occurs by two distinguishable endothermic stages with an overall ΔHdecomposition = 226.3 kJ/mol. TGA of 2 (Figure 5b) H

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Figure 7. Diffuse reflectance spectra for (a) 1 and (c) 2 measured at 298 K. Solid-state emission spectra at 83 K for (b) 1 and (d) 2.

CH 2 CH 2 OH] 2 } 8 .2H 2 O,MeCN (or {Cd[S 2 CN(iPr)CH2CH2OH]2}2·0.5H2O,0.25 MeCN). This composition is plausible as it contains more water and less MeCN than the needles 2 from which it is derived. Further support for this hypothesis is found in the characterization of the material used in the original recrystallization for the acetonitrile solution. Elemental analysis, 1H and 13C NMR, of this initial material showed no evidence for the solvent. PXRD (Figure S9 of the Supporting Information) showed this be distinct from 1, 2, desolvated 1, desolvated 2, and the intermediate species. TGA similarly showed no evidence for the presence of solvent, as no weight loss was observed below 120 °C. On this basis, the original material could be construed as water-free, as are needles of 2, which crystallize out of dry acetonitrile, and with the advent of water, the intermediate forms, converting to “water-rich” blocks of 1. Investigations into this matter are ongoing, including the influence of other solvents. CdS Nanoparticle Generation. As mentioned in the introduction, metal 1,1-dithiolates, including cadmium dithiocarbamates, have proven to be effective single source precursors for CdS nanoparticle generation.9−14 In the present study, CdS nanoparticles have been prepared by RAPET31 and solvothermal methods. In the former, 1 and 2 were subjected to RAPET at 400 °C, left for 1 h and cooled at 1 °C/min (method I). Two variations on solvothermal synthesis were employed. In the first of these (method II), 1 and 2 were refluxed in ethylene glycol for 30 min. In other experiments, each of 1 and 2 were dropped into a refluxing solution of ethylene glycol and refluxed for 30 min (method III). EDS (Figure S10, panels a−f, of the Supporting Information) proved the presence of Cd and S, and PXRD (Figures S11, panels a−f, of the Supporting Information) showed patterns consistent with Greenockite, hexagonal CdS (PDF 89-2944). Representative TEM images

shows a two-step decomposition to yield a mass corresponding to CdS. Weight loss of 2.8% (calcd 2.8%) between 95 and 116 °C), corresponds to the loss of lattice acetonitrile. The final step between 153 and 308 °C left a weight 30.3% cf. theoretical 29.9%, again consistent with a residue of CdS. In the DSC (Figure 5b) the endotherm at 85.5 °C (75−93 °C; ΔHvap = 33.0 kJ/mol) corresponds to the loss of acetonitrile. The decomposition at 177 °C (170−183 °C) occurs by at least four distinguishable endothermic stages with an overall ΔHdecomposition = 336.1 kJ/mol. Stability of 1 and 2 (Figure S6, panels a and b, of the Supporting Information) with respect to solvent loss, was also monitored by TGA measured on samples at different times intervals. In the case of 1, TGA measured on fresh and a sample 2 h old were indistinguishable. After 12 h, the TGA showed evidence that a considerable amount of solvent had been lost. Analogous experiments on 2 showed that solvent loss was minimal up to 24 h. PXRD was also measured on desolvated 1 and 2 (Figure S7, panels a and b, of the Supporting Information), which showed that the respective original crystal structures did not persist after desolvation. On the Nature of Unknown “Intermediate” Material. Owing to the reproducibility of the experiments whereby 2 converted to 1, it was possible to harvest significant quantities of the intermediate species. Regrettably, the samples did not provide suitable crystals for structure determination. TGA, Figure S8 of the Supporting Information, on this material, based on the assumption that the final residue was CdS, showed a weight loss of 1.2% between 63 and 136 °C. The next weight loss to yield CdS was 69.3% in a two-step process between 136 and 300 °C. This is tentatively interpreted to indicate the presence of some solvent in the intermediate species. On the basis of TGA and elemental analysis, as well as the observation the PXRD of the unknown species is distinct from both 1 and 2, the formulation could be {Cd[S2CN(iPr)I

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for CdS nanoparticles formed for 1 and 2, using methods I−III, are collected in Figure 6. Discernible differences are noted in the nature of the CdS nanoparticles generated by the RAPET procedure. Those derived from 1 (Figure 6a) have hexagonal morphologies and range in size from 50 to 200 nm, whereas those from 2 (Figure 6d) are also hexagonal but with some showing a tendency to be triangular and the size range is 50−250 nm. The samples are black. As there is no evidence for carbon in the PXRD, carbon must be present in an amorphous form; no sign of core shell formation involving carbon was found. Method II led to CdS nanospheres with a diameter of 10 nm for 1 (Figure 6b) and nanocylinders for 2 [i.e., 10 × 20 nm (Figure 6e)], where, perhaps, the needle morphology of 2 appears to be exerting an influence in keeping with analogous studies of polymeric bismuth 1,1-dithiolates.11 Method III gave smaller spherical nanoparticles, around 5 nm in size in each case (Figure 6, panels c and f), with no difference discernible between samples at this size range. Greenockite are the normal CdS nanoparticles generated from cadmium dithiocarbamates,39−41 there being a sole example of a mixture of hexagonal and cubic forms derived from cadmium pyrrolidine dithiocarbamate.42 ESI Mass Spectra. Positive-ion ESI mass spectra were obtained for 1 and 2 in acetonitrile solution. Tentative assignments based on matching molecular weights and isotopic distribution patterns are summarized in Table S2 of the Supporting Information. While there may be some doubt as to the exact formulation in some of the assignments, in the spectrum of 1, ions corresponding to species having one, two, and three cadmium atoms are present. By contrast, the spectrum of 2 is considerably simpler, exhibiting ions due to only monocadmium-containing species. It is thought that the reduced solubility of polymeric 2 is the reason for the simpler spectrum. Photophysical Behavior. UV spectra of 1 and 2 recorded in acetonitrile:ethanol (1:1 v/v) solution (Experimental Section) showed three prominent absorptions at 219, 261, and 283 nm. When excited at these wavelengths in the same solution, experimentally equivalent emission spectra were obtained (Table S3 of the Supporting Information) (i.e., a result consistent with the other solution characterization data discussed above). Diffuse reflectance spectra on solids 1 and 2 were also measured and revealed a shoulder at 422 nm for 1, absent in the spectrum of 2, as well as a shift to lower energy of the band at 344 in 1 to 364 in 2 (Figure 7, panels a and c). On this basis, excitation wavelengths were chosen to be 220, 260, and 360 nm for solid-state luminescence spectroscopy recorded at 298, 173, and 83 K (Table S4 of the Supporting Information). Excitation of 1 at λ = 220 and 260 nm at all three temperatures revealed three and four emissions, respectively, which increased in intensity and had improved resolution with decreasing temperature. A similar response was observed for 2, but the absorptions were less intense and less well-resolved (Figure 7, panels b and d, and Table 4). A clear difference between the response of 1 and 2 was noted when the samples were excited at λ = 360 nm. For 1 at 298 K, a sharp absorption occurred at λem = 382, which increased in intensity as the temperature was reduced with additional bands at 572.5 nm (at 173 K) and at 571.5 and 473.5 nm (at 83 K) appearing. By contrast, the comparable response in 2 was much less intense, although additional features in the spectrum at 83 K were delineated above the background. The differences in emission spectra recorded in solution and in the solid state,

Table 4. Solid State Photophysical Data for 1 and 2 Measured at 83 K compound

λexcitation (nm)

λemission (nm)

1 2 1 2 1 2

220 612, 397, 311 260 624, 396, 314 360 567, 464.5, −

606, 384, 326, 299 622, 382, 316 572, 474, 382

most notably the absence of an emission at 520 nm present in solution but absent in the solid state when excited at λemission = 220 and 260 nm, reflect differences in structure between the two phases. The low-energy emissions between 570 and 620 nm are most likely due to sulfur-to-metal charge transfer (LMCT), while the high-energy absorptions are due to intraligand (IL) dithiocarbamate transitions.43,44

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CONCLUSIONS Isolation of crystals from a dry acetonitrile solution of the initial product “Cd[S2CN(iPr)CH2CH2OH]2” after one or two days yields a coordination polymer formulated as [{Cd[S2CN(iPr)CH2CH2OH]2}3·MeCN]∞ (2) in which all dithiocarbamate ligands are tridentate, bridging two cadmium atoms. Upon standing for at least several days, needles of 2 are replaced by blocks of a supramolecular isomer, dimeric 1, with formula {Cd[S2CN(iPr)CH2CH2OH]2}2·2H2O·2MeCN, where two ligands coordinate as in 2 and the other two ligands are chelating. The time dependency in the appearance of crystals indicates that “chain” 2 is replaced by the deposition of crystals of “ball” 1 in a process mediated by adventitious water; solution spectroscopy confirms that this is a solid-state effect. Both 1 and 2 are useful single source precursors for Greenockite CdS nanoparticles. Solid-state luminescence responses of 1 and 2 to excitation at three wavelengths are comparable but much better defined (intensity and resolution) for 1.

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ASSOCIATED CONTENT

S Supporting Information *

Crystallographic data for 1 and 2 in CIF format, solution and solid-state photophysical data, mass spectral data, packing diagrams, PXRD, TGA, and EDX. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +60 3 7967 6775. Fax: +60 3 7967 4193. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The Ministry of Higher Education (Malaysia) is thanked for funding crystal engineering studies through the High-Impact Research scheme (UM.C/HIR-MOHE/SC/03). K.F. thanks financial support from JSPS and from MEXT and from the Iwatani Naoji Foundation’s Research Grant.

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