Cu(II)-Silsesquioxanes as Secondary Building Units for Construction

Article pubs.acs.org/crystal

Cu(II)-Silsesquioxanes as Secondary Building Units for Construction of Coordination Polymers: A Case Study of Cesium-Containing Compounds Alexander A. Korlyukov,*,†,‡ Anna V. Vologzhanina,† Mikhail I. Buzin,† Nataliya V. Sergienko,† Boris G. Zavin,† and Aziz M. Muzafarov† †

A. N. Nesmeyanov Institute of Organoelement Compounds RAS, 28 Vavilova str., 119991 Moscow, Russian Federation Pirogov Russian National Research Medical University, 1 Ostrovityanova str., 117997 Moscow, Russian Federation

‡

S Supporting Information *

ABSTRACT: Five new bi- and trimetallic copper-organosilsesquioxanes {[VinSiO 2 ] 12 Cu 4 Cs 4 (BuOH) 2 (EtOH) 2 (MeOH)}·2BuOH (1), {[PhSiO2]12Cu4Cs2K2(1,4-dioxane)9(H2O)2}·3.4(1,4-dioxane) (2), {[PhSiO2]12Cu4Cs4(DMF)6}· 2DMF (3), {[MeSiO2]12Cu4Cs4(THF)4.5(MeOH)2(H2O)0.25} (4), and {[MeSiO 2 ] 24 Cu 10 Cs 6 (OH) 2 (THF) 4.2 (MeOH) 4.1 (H2O)0.7} (5) have been synthesized by an exchange reaction between discrete cage alkali,copper-siloxane and cesium chloride (1, 2) or cesium carbonate (4, 5) or by interaction of copperphenylsiloxane with cesium phenylsiloxanolate (3). While in 1−4 the alkali,copper-silsesquioxane cage remains stable during reaction procedures, complex 5 was obtained by unexpected dimerization of two cages. The neutral cages act with solvent molecules and neighboring cages as square (1, 3, 5), tetrahedral (4), or octahedral (2) nodes giving, respectively, the twoperiodic (2D) sql net, and the three-periodic (3D) dia or pcu nets. The roles of the cage structure, nature of metal atoms, and organic coating in the formation of one-, two-, and three-periodic coordination polymers are discussed in the example of newly synthesized and previously obtained alkali,copper-organosiloxanes and copper-organosiloxanes with sandwich or globular cage structures. What’s more, the charge distribution in crystals of 1−3 was analyzed by means of Bader’s Quantum Theory of Atomsin-Molecules approach giving evidence of relatively strong bonding between neighboring cages.

1. INTRODUCTION Reticular chemistry is understood as the rational design of compounds formed from finite secondary building units (SBU) joined by strong chemical bonds.1 Its basic assumption is that only a limited number of structures will be formed from a SBU with a given coordination number and a simple linker.2 A plethora of compounds prospective for application in heterogeneous catalysis, gas storage, and molecular separation were synthesized using this approach.3−6 The design of coordination networks with a desired net topology becomes possible, because the occurring nets have unequal probability of appearance7,8 and net topology depends on the local characteristics of a SBU.9−11 Among all frameworks, those based on polyoxometallate (POM) building blocks draw special attention as magnetic materials and catalysts because these contain polynuclear clusters of transition metals which act as an SBU in a resulting net and remain stable during reaction procedures. Thus, polynuclear building blocks for POM-based frameworks can be first obtained as a discrete unit and wellcharacterized; a library of building blocks and their properties can be collected; a predesigned polynuclear building block can be transferred to the POM-based framework.12 The main synthetic approaches to POM−metal−organic frameworks (MOFs) include interaction of transition metal ions or © XXXX American Chemical Society

complexes with POMs, typically, in the presence of organic linker. On the basis of mutual disposition of POM, transition metals and linkers, Cui and Xu suggested to separate all POM− MOFs into four groups (Figure 1).13 In our opinion, cage metallosiloxanes {[RSiO2]xMyM′z} are similar to POM building blocks because of the possibility to control the cage composition and structure, and the structure typically remains stable under exchange reactions.14−18 Moreover, these cages contain transition metal ions encapsulated so

Figure 1. Schematic representation of POM−MOFs connectivity proposed by Cui and Xu.13 Received: November 2, 2015 Revised: February 19, 2016

A

DOI: 10.1021/acs.cgd.5b01554 Cryst. Growth Des. XXXX, XXX, XXX−XXX

Crystal Growth & Design

Article

2. EXPERIMENTAL SECTION

that their number, disposition, and coordination environment can also be controlled. The family of metallosiloxanes is represented by compounds containing 2−11 transition metals, diverse organosiloxane cages, and various organic coatings. The most well characterized are coppersiloxanes with sandwich or globular structures (Figure 2) for which both good

2.1. General Methods and Materials. All chemicals of high purity grade were purchased from Sigma-Aldrich. All solvents were distilled prior to use. Elemental analyses were performed by the microanalytical service of the Nesmeyanov Institute of Organoelement Compounds RAS. Bimetallic organosiloxanes {[VinSiO2]12Cu4K4}· 6BuOH,34 {[PhSiO2]6Cu4K4[PhSiO2]6} ·8BuOH,35 {[MeSiO2]12Cu4Na4}·4EtOH·4MeOH,31 and copper-phenylsiloxane {[PhSiO2]6Cu6[PhSiO2]6}·6EtOH36 were obtained as reported earlier. Powder patterns were measured on a Bruker D8 Advance diffractometer at room temperature with a LynxEye detector and Ge (111) monochromator, λ(CuKα1) = 1.54060 Å, θ/2θ scan from 4° to 40°, stepsize 0.02°. TGA was done on a Derivatograph-C (MOM, Hungary) at a heating rate of 10 °C/min in air and in argon in the temperature range 25−800 °C. 2.2. Synthetic Procedures. Synthesis of 1 (Cesium,coppervinylsiloxane, {[VinSiO2]12Cu4Cs4(BuOH)2(EtOH)2(MeOH)}·2BuOH, globular) and 2 (Cesium,potassium,copper-phenylsiloxane, {[PhSiO2]12Cu4Cs2K2(1,4-dioxane)9(H2O)2}·3.4(1,4-dioxane), Sandwich-like). Compounds 1 and 2 were synthesized by a general procedure. The flask was charged with the corresponding potassium,copper-organosiloxane, cesium chloride CsCl and absolute ethanol; the reaction mixture was heated 3 h at reflux. The precipitate of potassium chloride KCl was separated. The filtrate was evaporated to dryness. The product was recovered by recrystallization. For 1: 0.72 g (0.47 mmol) of {[VinSiO2]12Cu4K4}·6BuOH and 0.36 g (2.16 mmol) of CsCl were dissolved at 80 mL of absolute ethanol and left heated at reflux for 3.0 h. The precipitate of potassium chloride KCl was separated. The filtrate was evaporated to dryness. The product was recovered by recrystallization from a methanol− ethanol mixture. A single crystal was selected for X-ray analysis. The other crystals were filtered off and dried in vacuo (10 Torr) at 120 °C to a constant weight. Yield is 65% (0.57 g). Found (%): C 15.42; H 1.99; Si 17.84; Cu 13.75; Cs 25.55. Calculated for {[VinSiO2]12Cu4Cs4(BuOH)2(EtOH)2(MeOH)}·2BuOH (%): C 24.01; H 3.96; Si 14.94; Cu 11.38; Cs 23.65. Calculated for {[VinSiO2]12Cu4Cs4} (%): C 15.74; H 1.98; Si 18.40; Cu 13.88; Cs 29.03. The difference between the theoretical and calculated compositions is attributed to the presence of the coordinated solvent molecules in the dried sample. For 2: 1.63 g (0.76 mmol) of {[PhSiO2]6Cu4K4[PhSiO2]6}·8BuOH and 0.57 g (3.35 mmol) CsCl were dissolved at 50 mL of absolute ethanol and left heated at reflux for 3.5 h. The precipitate of KCl and target product was filtered off and dissolved at 50 mL of 1,4-dioxane. The KCl was filtered from this mixture. Blue single crystals were obtained from the resulting solution. A single crystal was selected for X-ray analysis. The other crystals were filtered off and dried in vacuo (10 Torr) at 120 °C to a constant weight. Yield is 59% (1.01 g). Found (%): C 38.08; H 2.85; Si 13.75; Cu 12.06; Cs 12.20; K 2.70. Calculated for {[PhSiO2]12Cu4Cs2K2(1,4-dioxane)9(H2O)2}·3.4(1,4dioxane) (%): C 43.28; H 4.84; Si 9.97; Cu 7.59; Cs 7.89; K 2.31. Calculated for {[PhSiO2]6Cu4K2Cs2[PhSiO2]6} (%): C 38.49; H 2.69; Si 15.02; Cu 11.32; Cs 11.84; K 3.48. The difference between the theoretical and calculated compositions is attributed to the presence of the coordinated solvent molecules in the dried sample. Synthesis of 3 (Cesium,copper-phenylsiloxane {[PhSiO2]12Cu4Cs4(DMF)6}·2DMF, Sandwich-like). 3.18 g (13.22 mmol) of PhSi(OEt)3, 20 mL of methanol, 0.48 g (26.44 mmol) of water and 0.3 g (13.22 mmol) of metallic sodium were placed into a flask. After sodium dissolution 3.07 g (9.42 mmol) of Cs2CO3 was added to a reaction mixture and boiled for 20 min. The mixture was cooled, and the precipitate of Na2CO3 was filtered off. The solution was evaporated to dryness. The precipitate was dissolved at 20 mL of ethanol and 80 mL of DMF mixture. The mixture was brought to boil, 3.72 g (1.84 mmol) of {[PhSiO2]6Cu6[PhSiO2]6}·6EtOH was added, and the solution was turned from the heat. The filtrate was concentrated to precipitate. The precipitate was recrystallized from DMF solution. A single crystal was selected for X-ray analysis. The other crystals were filtered off and dried in vacuo (10 Torr) at 120 °C to a constant weight. Yield 67% (3.14 g). Found (%): C 35.72; H 2.63; Si 12.79; Cu 10.00; Cs 21.05. Calculated for {[PhSiO2]12Cu4Cs4(DMF)6}·2DMF (%): C 38.30; H

Figure 2. (a) Sandwich {[RSiO2]6Cu4Na4[RSiO2]6} and (b) globular {[RSiO2]12Cu4Na4} cage copper-siloxanes. Turquoise: Na, blue: Cu, red: O, orange tetrahedra: SiO3C. Carbon and hydrogen atoms are omitted.

catalytic19−21 and magnetic14,22−26 properties were confirmed. Molecular structure of the sandwich type contains two 12membered macrocyclic siloxanolate [RSiO2]6 ligands connected through a metal-containing ring metal cluster, while that of the globular type contains one 24-membered macrocyclic [RSiO2]12 ligand coordinated by metal atoms. Although the majority of metallosiloxanes have the discrete structure, one-, two-, and three-periodic structures are also published for some metallosiloxanes.16,19,27−32 It is worth mentioning that to date none of synthetic procedures aimed to obtain metal-organic frameworks based on metallosiloxanes (siloxane-MOF); thus, the effect of organic coating, structure, and composition of a metallosiloxane on its ability to form a siloxane-MOF is of great interest. The previously obtained siloxane-MOFs contain metallosiloxane cages connected through Cu−O or M′−O bonds, where M′ = Na+ or K+, and an oxygen atom belongs to another cage or a bridge ligand (similar to linkage depicted on Figure 1b,c). Thus, we decided to increase the size of an alkali ion and to use cesium salts aiming to obtain copper-containing siloxane-MOFs. In present paper we discuss the molecular and crystal structures of five novel bi- and trimetallic cesium,copperorganosiloxanes and the factors responsible for the formation of a coordination polymer structure. To reveal the effect of the coating of a copper-siloxane cage by hydrophobic organic substituents at the silicon atoms that hinder their further association into polymeric structure methyl-, vinyl-, and phenylsiloxanes were used, and data on previously reported copper-organosiloxanes were summarized by means of a topological approach to metal clusters and coordination networks. Periodic density functional theory (DFT) calculations and Bader’s Quantum Theory of Atoms-in-Molecules (QTAIM)33 calculated for simplified models of three cesiumcontaining crystal structures were applied to give deeper insight into chemical bonding in compounds under discussion. Thermogravimetric analysis (TGA) and multitemperature powder X-ray diffraction (XRD) measurements of a threeperiodic globular cesium,copper-methylsiloxane were carried out to study thermal stability of cesium,copper-alkylsiloxanes. B



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Table 1. Crystallographic Data and Refinement Parameters 1 empirical formula Fw color, habit crystal size (mm3) radiation (λ) a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) V (Å3) Z crystal system space group dcalc (g·cm−3) μ (mm−1) indep. refl. (Rint) obs. refl./restr./ param. R,a % [I > 2σ(I)] Rw,b % GOFc F(000) a

C45H83Cs4Cu4O31Si12 2242.99 blue, prism 0.18 × 0.16 × 0.13

2

3

17.1558(12) 19.4188(15) 26.0268(18) 90 90.111(2) 90 8670.7(11) 4 monoclinic P21/n 1.718 2.857 30419 (0.086) 14396/53/765

C116H148Cs2Cu4K2O47Si12 C48H58Cs2Cu2N4O16Si6 3229.60 1508.42 blue, prism blue, prism 0.21 × 0.10 × 0.09 0.16 × 0.15 × 0.11 Mo Kα (0.71073) 15.3167(9) 14.3950(6) 16.5175(9) 15.3307(12) 16.7429(9) 16.4977(7) 118.2910(10) 108.886(1) 106.4290(10) 115.381(1) 95.9930(10) 92.738(1) 3432.0(3) 3037.3 (3) 1 2 triclinic triclinic P1̅ P1̅ 1.563 1.649 1.383 2.065 20912 (0.034) 17778 (0.029) 14318/57/582 15471/0/711

0.093 0.308 1.05 4420

0.066 0.193 1.03 1652

0.024 0.061 1.04 1508

4

5

C32H80.5Cs4Cu4O30.75Si12 2080.34 blue, prism 0.32 × 0.26 × 0.19 21.717(3) 23.369(4) 15.071(2) 90 90 90 7648.5(19) 4 orthorhombic Pnma 1.807 3.231 11969 (0.108) 7300/12/407

C65.84H170Cs6Cu10O69Si24 4167.00 blue, prism 0.18 × 0.16 × 0.07 Cu Kα (1.54178) 22.0938(6) 26.5594(7) 14.7698(4) 90 90 90 8666.9(4) 2 orthorhombic Pbam 1.597 13.222 8045 (0.078) 7039/35/359

0.066 0.190 1.07 4090

0.085 0.242 1.06 4134

R = Σ||Fo| − |Fc||/Σ|Fo|. bRw = [Σ(w(Fo2 − Fc2)2)/Σ(w(Fo2))]1/2. cGOF = [Σw(Fo2 − Fc2)2/(Nobs − Nparam)]1/2.

3.59; Si 11.17; Cu 8.51; Cs 17.69. Calculated for {[PhSiO2]12Cu4Cs4}: C 35.56; H 2.48; Si 13.86; Cu 10.45; Cs 21.86. The difference between the theoretical and calculated compositions is attributed to the presence of the coordinated solvent molecules in the dried sample. Synthesis of 4 (Cesium,copper-methylsiloxane, {[MeSiO2]12Cu4Cs4(THF)4.5(MeOH)2(H2O)0.25}, globular) and 5 (Cesium,coppermethylsiloxane, {[MeSiO2]24Cu10Cs6(OH)2(THF)4.2(MeOH)4.1(H2O)0.7}). 3.87 g (3.1 mmol) of {[MeSiO2]12Cu4Na4}·4EtOH·4MeOH, 2.53 g (7.7 mmol) of Cs2CO3 and 75 mL of methanol were mixed in a flask. Reaction mixture was brought to boil and left stirring for 1 h. The precipitate of Na2CO3 was filtered off from the hot reaction mixture. Crystals obtained after cooling of solution were recrystallized. For 4: By recrystallization from saturated THF−MeOH mixture crystals of 4 were obtained within 24 h. A single crystal was selected from the sample for X-ray analysis. The other crystals were filtered off and dried in vacuo (10 Torr) at 120 °C to a constant weight. Yield 74% (5.7 g). Found (%): C 9.50; H 2.5; Si 18.73; Cu 14.69; Cs 29.40. Calculated for {[MeSiO2]12Cu4Cs4(THF)4.5(MeOH)2(H2O)0.25} (%): C 18.46; H 3.87; Si 16.15; Cu 12.30; Cs 25.57. Calculated for {[MeSiO2]12Cu4Cs4} (%): C 8.54; H 2.15; Si 19.98; Cu 15.06; Cs 31.51. The difference between the theoretical and calculated compositions is attributed to the presence of the coordinated solvent molecules in the dried sample. For 5: By recrystallization from dilute THF−MeOH solution, a mixture of amorphous precipitate and a small number of single crystals of 5 were obtained within a week. A single crystal was selected from the sample for X-ray analysis. The other precipitate was filtered off and dried in vacuo (10 Torr) at 120 °C to a constant weight. Elemental analysis of dried precipitate detected the experimental Si: Cu: Cs ratio equal to 3.00:1.06:1.02, which corresponds satisfactorily to the theoretical ratio of the elements at the {[MeSiO2]12Cu4Cs4} cage, equal to 3.0:1.0:1.0, and significantly differs from the theoretical ratio of the elements at the double-cage 5 {[MeSiO2]24Cu10Cs6(OH)2}, equal to 12:5:3. Thus, complex 5 is a side product of recrystallization from THF−MeOH solution, while the main product is similar in composition to complex 4. 2.3. Single-Crystal Structure. X-ray diffraction data were collected at T = 100 K with Bruker APEX II CCD diffractometer

using graphite monocromatted Mo Kα or Cu Kα radiation with multilayer optics. The structures were solved by the direct method and refined by full-matrix least-squares method against F2 of all data, using SHELXTL37 and OLEX238 software. The coordination environment of Cs atoms in 5 was completely disordered due to its proximity to m crystallographic plane, so it was impossible to reliably reveal the nature of coordinated molecules. A number of models describing the coordination environment of Cs atoms were tested, and one of them was chosen based on the R-values and atomic occupancies. To decrease the R-value, one of noncoordinated solvent molecules of unknown nature was excluded from refinement using the SQUEEZE procedure39 implemented into PLATON software. Non-hydrogen atoms in 1−5 were found on difference Fourier maps and refined with anisotropic displacement parameters (except for some substituents at silicon atoms (Ph, Vin) and disordered molecules of solvents which were refined isotropically). The positions of hydrogen atoms were calculated and included in refinement in isotropic approximation by the riding model with the Uiso(H) = 1.5Ueq(Ci) for methyl groups and 1.2Ueq(Cii) for other atoms, where Ueq(C) are equivalent thermal parameters of parent atoms. Hydrogen atoms of MeOH molecules in 5 were refined similarly to those of phenyl groups (Uiso(H) = 1.2Ueq(O)). For some molecules of alcohols (MeOH, EtOH, BuOH) and H2O the positions of hydrogen atoms of hydroxyl groups were not located. Anyway, these hydrogen atoms were included in composition presented in abstract and experimental part. Details of data collection and refinement are listed in Table 1. Molecular views of investigated compounds in representation of atoms with thermal ellipsoids are given as Supporting Information (Figures S1−S5). 2.4. Computational Details. The starting atomic configurations for quantum chemical calculations of the structures 1−3 were built using experimental atomic coordinates and cell parameters. Unfortunately, it was impossible to built suitable model for calculations of crystal structures 4 and 5 owing to the strong disorder of solvent molecules located close to mirror planes. The quantum chemical calculations of crystal structures 1−3 were carried out using the Vienna Ab-initio Simulation Package (VASP) 5.3.5 code.40−42 A conjugated gradient technique was used for optimizations of the atomic positions (started from experimental data) and minimization of C



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Figure 3. Cage cesium,copper-siloxanes 1−5 as SBUs. Carbon atoms of organic coating are omitted. Blue: Cu, violet: Cs, gray: K, orange polyhedra: SiO3C. Sum energy of coordination bonds between neighboring cages 1−3 estimated from PW−DFT calculations are given in kJ/mol. total energy. The projected augmented wave (PAW) method was applied to account for core electrons, while valence electrons were approximated by plane-wave (PW) expansion with 545 eV cutoffs. Exchange and correlation terms of total energy were described in terms of density functional theory (DFT). PBE exchange-correlation functional was used for this purpose.43 At a final step of our calculations, atomic displacements were less than 0.01 eV·Å−1, as well as energy variations were less than 10−3 eV. In order to carry out the topological analysis of electron density distribution function in 1−3 in terms of AIM theory the dense fast Fourier transformation (FFT) grid (stepsize ∼0.02 Å) were used. The latter were obtained by separate single point calculation of optimized geometry with small core PAWs for each atom type. The topological analysis of electron density distribution function was carried out using AIM programpart of ABINIT software package.44

{[VinSiO2 ]12 Cu4K4} + 4CsCl → {[VinSiO2 ]12 Cu4Cs4} + 4KCl

Similar reaction between cesium chloride and the sandwich potassium copper-phenylsiloxane afforded the trimetallic sandwich cesium,potassium,copper-phenylsiloxane 2: {[PhSiO2 ]6 Cu4K4[PhSiO2 ]6 } + 2CsCl → {[PhSiO2 ]6 Cu4K 2Cs 2[PhSiO2 ]6 } + 2KCl

Another synthetic approach to the sandwich bimetallic alkali,copper-phenylsiloxanes is interaction of copper-phenylsiloxane with alkaline silanolyates (M′ = Li, Na, K).45 Using this one-pot procedure we succeeded to obtain the bimetallic sandwich cesium,copper-phenylsiloxane 3:

3. RESULTS AND DISCUSSION 3.1. Synthesis. The standard route for the synthesis of the sodium (potassium) coppersiloxane salts is exchange reaction of the alkali metal organosilanolates with copper chloride:

6[PhSiO(OCs)]n + n{[PhSiO2 ]6 Cu6[PhSiO2 ]6 } → n{[PhSiO2 ]6 Cu4Cs4[PhSiO2 ]6 } + 1/n{[PhSiO2 ]6 Cu 2Cs 2}n

12/n[RSiO(OM)]n + 4CuCl 2

Exchange reaction between cesium carbonate and the globular sodium,copper-methylsiloxane afforded two cesium,coppermethylsiloxanes. Complex 4 was obtained by reaction:

→ {[RSiO2 ]6 Cu4M4[RSiO2 ]6 } + {[RSiO2 ]12 Cu4M4} + 8MCl

{[MeSiO2 ]12 Cu4Na4} + 2Cs 2CO3

where M = Na, K; R = Ph, Et, Vin, Me.16,18 According to our previous results we assumed that difference in solubility of the leaving and retained alkaline metal halides is the driving force of the exchange reaction. Using this assumption we succeed to obtain a number of lithium salts. Indeed, the solubility of lithium chloride is better than potassium and sodium chlorides. In our opinion, we can compare the solubility of any alkali metal salts in arbitrary solvents to evaluate the possibility of exchange reaction. According to reference literature, the solubility of cesium chlorides in ethanol is much better than that for potassium chloride. Thus, the globular cesium,copper-vinylsiloxane 1 was prepared by exchange reaction between cesium chloride and the globular potassium,copper-siloxane:

→ {[MeSiO2 ]12 Cu4Cs4} + 2Na 2CO3

Unexpected complex 5 of the {[MeSiO2]24Cu10Cs6(OH)2(THF)4.2(MeOH)4.1(H2O)0.7} composition is a side product of the same exchange reaction. It contains two globular {[MeSiO2 ] 12 Cu 5 Cs 3 } cages connected by Cu−OH−Cu bridges. Compound 5 is the first example of metallosiloxane cage dimerization that probably occurs during slow crystallization at presence of water or CO2/water at reaction mixture. 3.2. Molecular and Crystal Structures of Compounds. X-ray diffraction study confirmed that 1, 3, 4, and 5 are the bimetallic cesium,copper-organosiloxanes, whereas 2 contains both potassium and cesium as alkali atoms. Figure 3 represents D



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alkali,copper-organosiloxane cages in 1−5 and their closest neighboring. The metallosiloxane cage remained unchanged during reactions to obtain 1−4, although dimerization of two globular cages occurred for 5. Copper atoms are fourcoordinated and adopt square planar geometry with Cu−O distances as short as 1.886(4)−1.995(5) Å. Copper atoms form two Cu2O2 cycles situated in staggered conformation in globular cages of 1 and 4, and in eclipsed conformations in sandwich cages 2 and 3. In 5, a six-nuclear linear Cu6O10 cluster is present connected by bridge oxygen atoms of a methylsiloxane cycle and hydroxo-groups, and two Cu2O2 dimers in staggered conformation toward the Cu6O10 cluster. Alkaline atoms realize high coordination numbers: CN(Cs) = 6−7, 8, 6−9, 7, and 8 for 1−5 and CN(K) = 6 for 2. The Cs−O bonds vary from 2.95 to 3.92 Å; the longest interatomic Cs−O distances were revealed in globular 1, 4, and 5 with R = Vin and Me. Each alkaline atom coordinates four oxygen atoms of an organosiloxane as part of an alkali,copper-organosiloxane cage. The other oxygen atoms belong to solvent molecules or neighboring cages. Particularly, in 1 (R = Vin) and 4 (R = Me) Cs−O bonds between two metallosiloxane cages occur. Taking into account structural data one can propose that besides Cs− O bonds polymer structures of 1 and 4 are additionally stabilized by Cs−Vin and Cs−Me bonding between neighboring cages. Nevertheless, the bridge EtOH and MeOH molecules are found even in these complexes. In the rest of structures the copper-siloxane cages are assembled onto multiperiodic structures via bridge solvent molecules (1,4dioxane, DMF, and THF for 2, 3, and 5). As the result, all five compounds are coordination polymers, two-periodic (1, 3, 5) or three-periodic (2, 4). Thus, the large ionic radius of an alkali atom plays a lead role in the formation of a siloxane-MOF. We estimated the effect of an alkali ion on the geometry of the cage and obtained different structural trends for sandwich and globular cages. The undistorted sandwich anion is characterized by cylindrical symmetry, while the alkalicontaining anions adopt the form of an oblate cylinder (Figure 4). The least distorted geometry has the sodium-containing salt with refcode {OBIFAI} in the Cambridge Structural Database (CSD 1.16 Nov. 2013 Release46) (the average distance between opposite copper atoms is equal to 4.556 Å). The replacement of Na+ cation with K+ ones leads to pronounced distortion of anion shape. The average distance between opposite copper atoms in {YOLNOC}, where the sodium atoms are entirely replaced with potassium ones, is equal to 3.960 Å. In mixed salts the distortion of the anion shape can be even more pronounced. For instance, the distance between opposite copper atoms in mixed K,Li salt {IMAZIG} is the lowest one among the crystal structures containing sandwich anion (3.730 Å). In 3 the above Cu···Cu distance is decreased notably as compared to that in {YOLNOC} (3.682 Å). At the same time in mixed (K,Cs) salt the mean between opposite copper atoms even more decreased (3.561 Å). Thus, the increase of ionic radii of alkali metal leads to considerable distortion of cylindrical symmetry of sandwich cation. This tendency is the same as observed for the complexes with alkali-earth metals.17 In globular copper-siloxanes the distance between copper atoms and subtended oxygen atoms depends on the nature of alkali-metal cation and the substituent at the silicon atom. The shortest distance is observed in lithium salt {DIYGEY} (R = Vin). The configuration of two copper-siloxane moieties can be described as dished sheets. In turn, the shape of a whole copper-siloxane framework can be described as biconcave lens,

Figure 4. Overlaid {[RSiO2]6Cu4MxM′y[RSiO2]6} cages of OBIDOU (violet; M = Cu, x = 2, y = 0), IMAZEC (blue; M = Li, M′ = K, x = y = 2), XOCLEH (green; M = M′ = Na, x = y = 2), and 3 (red; M = M′ = Cs, x = y = 2): side view (a, c) and top view (b, d) as obtained with Mercury package.47

and the copper coordinate Cu−O bonds with subtend oxygen atoms are equal to 2.557 Å. The replacement of Vin to Ph ({DIYGAU}, M = Li) leads to an increase of the abovementioned average Cu···O distance to 2.886 Å. At the same time, in the sodium salts {DEQGEL} and {MEDVEW} (R = Vin) the mean Cu···O distances (3.485 and 3.392 Å) are increased by 0.8−0.9 Å as compared to {DIYGEY}. Similarly to lithium salts the replacement of Vin to Ph caused the increase of the Cu···O distance up to 3.960 Å, while the sodium salt {DODXAW} is characterized by the Cu···O distance (3.911 Å), which is only slightly less than for R = Ph. In 3 the mean Cu··· O distance is equal to 3.838 Å, which is considerably larger than in {DIYGEY}, {DEQGEL}, and {MEDVEW}. The coppersiloxane moieties became much closer to planar than in the case of lithium and sodium salts. 3.3. Topology of Copper-Organosiloxanes. To reveal effect of alkali ions, organic coating, and cage structure on the possibility of alkali,copper-organosiloxanes to act as SBU forming various coordination polymers, we analyzed crystal structures of 19 alkali,copper-organosiloxanes and 20 copperorganosiloxanes with sandwich or globular structures taken from the CSD (Table 2). Both discrete complexes, and one-, two-, and three-periodic nets were found among them. All copper-organosiloxanes have discrete structure, and many of them demonstrate the possibility of copper atoms to act with hard (THF, DMF, MeOH, BuOH, O2−) and soft (maoe = methyl 2-amino-2-oxoethanimidoate-N,N′; CN−, Hal−) Lewis bases to realize a [4 + 1] square pyramidal environment. Thus, one can expect that siloxane-MOFs connected by polydentate neutral N- and O-donor ligands through copper atoms can be constructed according to the scheme represented on Figure 1c. Complex {[PhSiO2]6Cu4Na4(1,4-dioxane)2[PhSiO2]6} is an example of a siloxane-MOF where 1,4-dioxane acts as a linker between sodium and copper atoms.19,48 Besides this complex, 11 other alkali,copper-siloxanes are also coordination polymers, E


a

F

Cu10Cs6 Cu8

Cu4Cs4

Cu8 Cu10 Cu4Li4 Cu4Na4

Cu4K4 Cu4K2Cs2 Cu4Cs4 Cu6

Me; Et; Ph Vin Ph Vin Ph Ph Et Ph Ph Me; Et; Ph Me; Et Me Vin; Ph Ph Me; Et; Vin Me Vin Me Me; Vin

R

MeOH, THF DMSO MeOH, THF THF, DMSO

DMF DMF EtOH; THF BuOH MeOH; BuOH

THF EtOH EtOH 1,4-dioxane DMF; BuOH BuOH THF DMF MeOH, EtOH; BuOH; DMF; DMEA; maoea; [Cu(CN)pyam]b

DMF; OH; bipy

L

maoe = methyl 2-amino-2-oxoethanimidoate-N,N′. bpyam = tris(pyrid-2-ylmethyl)amine.

{[RSiO2]8Cu8[RSiO2]8} {[RSiO2]10Cu10[RSiO2]10} {[RSiO2]12CumMn}

Cu4Li3.4Na0.6 Cu4Li2K2

{[RSiO2]6CumMn[RSiO2]6}

Cu4Na4

Cu6

{[RSiO2]5Cu6[RSiO2]5}

M

Table 2. Topological Classification of (Alkali,)Copper-Organosiloxanes

[4, this work] [1, this work] [5, this work] CIRYAF; CIRXUY; WAKTAE

IMAZOM IMAZEC IMAZIG CIRWUX; QIJKUR OBIFAI; XOCLEH YOLNOC [2, this work] [3, this work] GIBYEW; OBICOT; OBIDIO; OKUPIT; TOGYOD; VACJAL; VONNAN; XACZAD; XIFDOG01 OBIDOU; OBIDUA; WEHVAH XIFFAU DIYGAU; DIYGEY LARXOR02 DEQGEL; DODXAW; MEDVEW; OBIFIQ

LAZBUK; OBIDAG; OBIDEK; OBIFOW

Refcodes

3D (dia) 2D (sql) 2D (sql) 0D

0D 0D 0D 0D 3D (dia)

0D 0D 1D 2D (sql) 0D 1D 3D (pcu) 2D (sql) 0D

0D

periodicity (net)

Crystal Growth & Design Article



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tion polymers packed in the hxl (hexagonal rod packing, Figure 6) by means of hydrogen bonds between uncoordinated and coordinated solvate molecules.

connected by bridge bidentate (1,4-dioxane), bridge monodentate (ROH, DMF, THF, DMSO) ligands, or oxygen atoms of siloxanolate anion through alkali atoms. The globular alkali,copper-organosyloxanes can be regarded as pseudotetrahedral nodes with equivalent sites, while those compounds with a sandwich structure can act as linear “nodes” ({[VinSiO2]6Cu4Li2K2[VinSiO2]6} {IMAZIG} and {[EtSiO2]6Cu4K4[EtSiO2]6} {YOLNOC} are the examples), or squaretype nodes with nonequivalent diagonal vertices. The following uninodal two-periodic nets with maximum two kinds of edges can be obtained based on four-coordinated structure-building units: htb, kgm, and sql in terms of RCSR notation.49 The number of three-periodic nets satisfying these criteria is equal to 73. It is known that some of three-periodic7 and, especially, two-periodic8 networks occur more frequently than others. The sql net (Shubnikov tetragonal plane net) is the doubtless leader among two-periodic nets. The dia (diamond) net is the most widespread for three-periodic polymers based on tetrahedral SBUs, and nbo or cds (structure types of NbO and CdS, respectively) nets are expected for those based on square SBUs. Following the cluster simplification procedure7 implemented in the ToposPro package50 we determined that {[PhSiO2]6Cu4Na4[PhSiO2]6}, 1, 3, 5 are representatives of the sql net family (Figure 5), while

Figure 6. 1D polymers in the structures of (a, b) {[EtSiO2]6Cu4K4[EtSiO2]6} {YOLNOC} and (c,d) {[VinSiO2]6Cu4Li2K2[VinSiO2]6} {IMAZIG}: top (b, d) and bottom (a, c) view. Carbon and hydrogen atoms of terminal groups are omitted for clarity. Red: O, yellow: Si, blue: Cu, gray: Li, magenta: K.

To sum up, alkali,copper-organosiloxanes of the {[RSiO2]6Cu4M4[RSiO2]6} and {[RSiO2]12Cu4M4} composition can be regarded as building blocks of coordination polymers with easily available (tetra-coordinated) copper atoms. For both globular and sandwich complexes direct binding of two cages is possible, if an organosiloxane with short alkyl chain was used. The more bulky organosiloxane was used, the larger the size of an alkali cation should be to obtain a polymer; otherwise a polydentate linker should be applied. Thus, variation of the length of a linker and alkali nature would allow variation of distances between the centers of SBUs, and, hence, of pore diameters. Particularly, the shortest distance between cage centroids increases from 11.3 (4; M = Cs; R = Me) to 11.7 (DEQGEL; M = Na; R = Vin) to 11.9 (IMAZEG; M = K; R = Vin) to 12.4 (YOLNOC; M = K; R = Et) to 14.4 (3; M = Cs; R = Ph) to 15.3 (2; M = K, Cs; R = Ph) and to 18.4 Å (1; M = Cs; R = Vin). Disposition of four alkaline atoms allows emulating of both tetrahedral and square nodes. Taking into account that organosiloxane occupies not more than two-third of alkali coordination sphere, and, starting from potassium, high coordination numbers of these ions are expected, application of long linkers, heavy alkaline ions, and small alkyl chains might result in the formation of polymers, where the same SBU emulates a high-coordination node (CN(SBU) ≥ 6). At last, it is worth noting that organic coating probably prevents interpenetration of siloxane-MOFs. At the same time, the coating can be removed at heating of the sample, while an inorganic alkali,copper-siloxane framework is expected to remain unchanged (provided that monodentate linkers are used) giving a porous material. 3.4. Charge-Density Study of 1−3. To confirm the nature of bonding between various cages, estimate corresponding energies, and qualitatively compare alkali and copper ions as atoms available for interaction with linkers, a charge-density study of 1−3 was carried out. The details of computations are described in Experimental Section. The general scheme of

Figure 5. Cluster representation of coordination networks in 1−4 and underlying nets (black lines). Carbon and hydrogen atoms of terminal groups are omitted for clarity. Red: O, yellow: Si, blue: Cu, teal: Cs, magenta: K.

the family of globular {[RSiO2]12Cu4Na4} complexes (R = Me, Et, Vin) and 4 belong to dia nets (Figure 5). Surprisingly, 2 adopts a pcu net (primitive cubic lattice, Figure 5) because each cesium atom situated on the top belt of siloxane cage coordinates two bridge 1,4-dioxane molecules, and each potassium coordinates one bridge 1,4-dioxane molecule, so that the trimetallic cage becomes a pseudo-octahedral node, for which pcu net is the most widespread. The {[VinSiO2]6Cu 4 Li 2 K 2 [VinSiO 2 ]6 } {IMAZIG} and {[EtSiO 2 ]6 Cu 4 K 4 [EtSiO2]6} {YOLNOC} complexes are one-periodic coordinaG



Article

Table 3. Topological Parameters of ρ(r) for Selected Bonds in 1−3a compound

bond

d, Å

ρ(r), e·Å−3

∇2ρ(r), e·Å−5

Ve(r), a.u.

he(r), a.u.

Eint, kJ/mol

av.

1

Cu−O Cu···O Cu···H Cs−O Cs···C Cu−O Cu···H Cu−Cu Cs−O K−O Cu−O Cu···H Cu−Cu Cs−O Cs···Cs

1.887−2.008 3.821−3.898 2.760−2.903 2.950−3.565 3.395−3.844 1.910−2.001 2.776 3.148 3.052−3.289 2.700−3.111 1.914−1.991 2.975 3.159 2.960−3.697 3.904

0.64 0.01 0.04 0.11 0.06 0.64 0.05 0.04 0.11 0.14 1.02 0.04 0.03 0.11 0.07

10.03 0.17 0.39 1.49 0.65 10.22 0.41 0.24 1.54 2.14 13.76 0.48 0.41 1.52 0.94

−0.149 −0.001 −0.003 −0.012 −0.005 −0.149 −0.003 −0.002 −0.012 −0.016 −0.132 −0.003 −0.002 −0.012 −0.006

−0.022 < 0.001 < 0.001 0.002 0.001 −0.021 0.001 < 0.001 0.002 0.003 −0.075 0.001 0.001 0.002 0.002

194.8 0.8 3.4 15.9 5.9 195.7 3.8 2.5 15.5 21.8 193.2 3.8 3.4 15.5 8.0

16 4 2 22 5 8 1 1 8 6 8 1 1 15 1

2

3

For all bonds d is interatomic distance; ρ(r) is the ED at the bcp; ∇2ρ(r) is the corresponding Laplacian; Ve(r) − potential energy density; he(r) − local energy density; Eint is energy of a bond estimated as −1/2 Ve(r); Av. is the number of averaged values.

a

ligands) and a number of Cs···C interactions involving carbon atoms of vinyl groups. Their energies achieve 29.7, 28.1, 9.2, and 8.0 kJ/mol for, respectively, Cs−O, K−O, Cs···C, and Cs··· Cs interactions. Characteristics of ρ(r) for Cs−O bonds are close to those obtained theoretically for cesium butyratouranylate(VI).55 Although Eint (Cs−O) values are lower than the energies of Cu−O bonds, the sum energies of all Cs−O, Cs···C, and Cs···Cs interactions between neighboring cages achieve as much as 140.7 kJ/mol (Figure 3). Taking into account the energies of coordination bonds, one can propose that siloxane-MOF could also remain stable after removal of solvent molecules or even that porous alkali,copper-silicates with predesigned structure and composition can be constructed after thermal cleavage. Absence of interpenetration in a siloxane-MOF is an advantage for practical application of these materials as catalysts or molecular sieves. What is more, for siloxane-MOFs connected by direct interaction of metallosiloxane cages or by monodentate bridge ligands thermal cleavage of organic coating might keep the inorganic part of a siloxane-MOF unchanged giving inorganic molecular sieves of variable pore size. 3.5. Thermal Behavior. Thermal stability of siloxaneMOFs was attested on the example of complex 4 that has a three-periodic structure and does not contain bidentate organic linkers. The TGA curve for this sample was measured up to 800 °C (Figure 7). The mass loss takes place as a two-step smooth process. On the first stage the sample lost 7% of mass with lowmolecular compounds (probably, solvent molecules) in the broad temperature range (25−310 °C). Within 310−400 °C complex 4 decomposes, and the sample weight loss is equal to ∼8 and 7% in air and argon, respectively. In sum, the weight loss corresponds to thermal cleavage of solvent molecules and organic coating of the siloxane-MOF. The powder XRD data of sample 4 collected for the fresh sample, and after heating in air for 20 min at 100, 200, 300, and 400 °C (heating rate was 10 °C/min) are given in Figure 8. The sample heated to 400 °C contains no single peaks that can be assigned to any compound. For a fresh sample and a sample heated to 100, 200, and 300 °C the theoretically strongest peak at 2θ = 7° corresponding to the (011) plane is absent. Nevertheless, despite the loss of solvent molecules and organic coating at these temperatures, powder XRD patterns contain

topological analysis of PW−DFT electron densities follows the guideline reviewed in ref 51. Obtained theoretical electron densities for 1−3 were analyzed within the QTAIM33 approach. According to the QTAIM information on chemical bonding can be obtained analyzing the local minima, maxima, and saddle points of ρ(r) and its Laplacian. The electron density of a system of atoms typically exhibits bond paths (and related bond critical points, bcp) linking adjacent atoms to form the molecular graph. Results of PW−DFT calculations unambiguously indicate the presence of all expected bonds. Each copper atom coordinates four oxygen atoms of an organosiloxane anion and at least one additional contact. In globular 1 bcp’s were found between each copper atom and siloxanolate oxygen atom situated on the opposite site of the cage. Besides, Cu2 and Cu3 atoms form intramolecular Cu···H interactions with hydrogen atoms of butanol molecules. In sandwich 2 and 3 except Cu−O bonds, Cu−Cu interactions were found between the closest atoms of metallacycle and per one intramolecular Cu···H interaction. Average characteristics of ρ(r) at bcp’s corresponding to these interactions are listed in Table 3 (and full data are given at Supporting Information). On the basis of positive ∇2ρ(r) and negative he(r) values, the Cu−O bonds should be referred to highly polar bonds with strong ionic contribution. The Cu···O, Cu···H, and Cu−Cu bonds are the closed-shell interactions (∇2ρ(r) > 0; he(r) > 0). Using correlation formula Eint ≈ −1/2 Ve(r) proposed by Espinosa, Molins and Lecomte52 we obtained that the Cu−O bonds (193.2−195.7 kJ/mol) are much stronger than intramolecular Cu···O interactions (1.0 kJ/mol). At the same time, these values are comparable with Eint = 155.4−179.3, 168.9−196.9, and 1.7 kJ/mol obtained for Cu−O bonds between copper(II) atom and P,P′-diphenylmethylenediphosphinate,53 as well as intra- and interlayer bonding within Na2Cu(CO3)2.54 It is worth mentioning that the Cu···O interlayer contact in Na2Cu(CO3)2 with r(Cu···O) = 3.581 Å was proposed to be the reason for antiferromagnetic interlayer coupling; thus, analysis of magnetic properties of globular copper-organosiloxanes should also take Cu···O interactions into account. All interactions revealed by the QTAIM approach for alkali metals are also of the closed-shell type, here, ionic bonds. We found not only Cs−O and K−O bonds, but also a Cs···Cs interaction in 3 (for an atom pair connected by four bridge H



Article

alkali and copper atoms are able to act with linkers, hard Lewis bases are prone to act with alkali ions. A large radius of alkali ions and short-chain organic coating favors the appearance of two- and three-periodic architectures. This conclusion is supported by structures of five newly synthesized cesium,copper-organosiloxanes with globular and sandwich cage structures and an organic part varying from Me to Ph groups. The metallosiloxane cage is able to act not only with polydentate linkers, but also with bridge monodentate ligands or even to condense with neighboring cages (that is to our knowledge unprecedented for other MOFs based on polynuclear metalcontaining building blocks) to give a metallosilicate framework which can be predesigned. Stability of the resulting inorganic framework, steric restrictions for interpenetration, and the rich chemistry of discrete metallosiloxanes that can be used as building blocks are opening new ways for construction of MOFs based on polynuclear transition-metal containing synthones with potential application in catalysis, spinotronics, and magnetoelectronics.

Figure 7. TGA curves for complex 4 in air (1) and in argon (2) at a heating rate of 10 °C/min.

■

ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.5b01554. ORTEP view of independent part of 1−5, Rietveld refinement of the unit cell of complex 4 and its sample at RT and 300 °C, coordinates of optimized structures and the full list of ρ(r) characteristics for 1−3 (PDF)

Figure 8. Powder XRD patterns (λ = 1.54060 Å) of complex 4 for (a) fresh sample at ambient conditions, for a sample heated at (b) 100, (c) 200, (d) 300, and (e) 400 °C and (f) theoretically calculated pattern for 4 at −153 °C. The reflections corresponding to plains (111) + (200) (a−d, f) and (210) (d, f) are marked with a star.

Accession Codes

CCDC 1433680−1433683 and 1452791 and contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

individual peaks giving evidence of sample crystallinity. The positions of these peaks slightly change at heating as could be expected due to several reasons (texture, partial removal of solvent molecules, and thermal expansion of the unit cell). Rietveld refinement of the unit cell parameters of complex 4 for PXRD patterns at various temperatures allows us to conclude that the strongest peak for the fresh sample, and the sample heated to 100 and 200 °C corresponds to reflections from the (200) and (111) planes (these share a reflection at 2θ = 8.08 and 8.23° on theoretical pattern). Although this peak is also present in the XRD pattern of a sample heated to 300 °C, the strongest peak at this temperature corresponds to reflections from the (210) plane (see Figure S6, SI, for results of Rietveld refinement) All above-mentioned planes intersect the metallosiloxane cage; thus, powder XRD data confirm that mutual disposition of cages remains stable during thermal decomposition of the sample up to 310 °C, and only slight disorientation of these cages occurs expressed as broadening of peaks. Full decomposition of organic coating at 400 °C is accompanied by the loss of all peaks corresponding to mutual disposition of cages, and at the same time, no evidence of CuO, quartz or copper and cesium silicates presence was found. Thus, a three-periodic structure of the inorganic part of complex 4 might be kept even above 400 °C, although strongly disoriented. Thermal behavior of other copper-organosiloxanes and the effect of material periodicity on stability of these materials are under investigation now.

■

AUTHOR INFORMATION

Corresponding Author

*Fax: +7-499-135-50-85. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■ ■

ACKNOWLEDGMENTS The authors gratefully acknowledge support of the Russian Science Foundation (Project 14-23-00231). REFERENCES

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4. CONCLUSIONS In conclusion, the effect of nature of alkali metals and organic coating on the inclination of alkali,copper-organosiloxanes to form coordination polymers has been explored. Although both I



Article

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