Crystal Structure and Band Gap Engineering in Polyoxometalate

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Crystal Structure and Band Gap Engineering in PolyoxometalateBased Inorganic−Organic Hybrids Soumyabrata Roy,† Sumanta Sarkar,† Jaysree Pan,‡ Umesh V. Waghmare,‡ R. Dhanya,§ Chandrabhas Narayana,§ and Sebastian C. Peter*,† †

New Chemistry Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India Theoretical Science Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India § Chemistry and Physics of Material Science Unit, Jawaharlal Nehru Centre for Advanced Scientific Research, Jakkur, Bangalore 560064, India ‡

S Supporting Information *

ABSTRACT: We have demonstrated engineering of the electronic band gap of the hybrid materials based on POMs (polyoxometalates), by controlling its structural complexity through variation in the conditions of synthesis. The pH- and temperature-dependent studies give a clear insight into how these experimental factors affect the overall hybrid structure and its properties. Our structural manipulations have been successful in effectively tuning the optical band gap and electronic band structure of this kind of hybrids, which can find many applications in the field of photovoltaic and semiconducting devices. We have also addressed a common crystallographic disorder observed in Keggin-ion (one type of heteropolyoxometalate [POMs])-based hybrid materials. Through a combination of crystallographic, spectroscopic, and theoretical analysis of four new POM-based hybrids synthesized with tactically varied reaction conditions, we trace the origin and nature of the disorder associated with it and the subtle local structural coordination involved in its core picture. While the crystallography yields a centrosymmetric structure with planar coordination of Si, our analysis with XPS, IR, and Raman spectroscopy reveals a tetrahedral coordination with broken inversion symmetry, corroborated by first-principles calculations.

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INTRODUCTION Polyoxometalates (POMs) make up a class of metal−oxy anion cluster containing early transition metals (Cr, V, Mo, W, Nb). Since the advent of the first member (Keggin ion) in this family,1 the field has evolved greatly starting from the structural aspects and growing to a wide range of applications. During the past two decades, the applications of POMs have increased enormously in new dimensions starting from materials design and ranging to biological sciences.2−5 Few important properties of POMs that are generally exploited for applications are (a) redox facile nature, (b) photoactive properties, (c) hydrolyzable protons giving rise to acidity, and (d) anionic and oxygen-rich nature. All of these properties combined with the rich chemistry of the transition metals render these compounds enormous potential for applications in catalysis (chemical, photochemical, electrochemical), single molecule magnets, molecular electronics, sensor design, nanoarchitecture, supramolecular chemistry, and medicinal and biological chemistry.6−12 Research activity on POM-based compounds has moved far ahead of its initial structural challenges and has now become a tool of complex synthetic routes used for applications in supramolecular and bioinorganic synthesis.7,9,13 However, the major drawback of using POMs in various applications is the © XXXX American Chemical Society

homogeneous mode of activity owing to its high solubility in all aqueous and polar solvents. To overcome this, scientists have tried to immobilize POMs using various methods out of which using organic ligands to form hybrids are of utmost importance because of the ease of synthesis and large tunability. The class of organic−inorganic hybrids involving POMs can be broadly divided into two parts based on the nature of bonding of the organic moiety with the POM unit. It can either be direct and noncovalent (through supramolecular interactions, where the ligand is in direct interactions with POMs) or indirect and covalent (through transition metals, where the ligand does not have direct interaction with POMs). Syntheses and structure of these hybrids are highly dependent on conditions such as pH,14,15 temperature, and duration of reaction owing to the high sensitivity of POMs and coordination modes of ligands toward these factors. This sensitivity, though sometimes challenging, gives us the control to manipulate the overall structure of the hybrids based on the type of application. Another structural issue that is frequently observed in POM (Keggin)-based organic hybrids16,17 is the disorder in the structure of the Keggin ion. Received: November 23, 2015

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DOI: 10.1021/acs.inorgchem.5b02718 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

of the solution from 5.48 at 26.7 °C to 9.6 at 26.5 °C. Glacial acetic acid (16 N) was then added dropwise to adjust the pH of the solution in between 4 and 5. To this solution was added 0.5 g (2.36 mmol) of Na2SiO3·5H2O followed by glacial acetic acid to adjust the pH between 4 and 5. The initial transparent solution became yellowish green. In the next step, 0.238 g (1.4 mmol) of CuCl2·2H2O and 0.27 g (1.73 mmol) of bpy were added to obtain a greenish blue solution of pH around 4.6. Conc HCl was added dropwise to adjust the pH close to 1. The reaction mixture was stirred for 1 h and was kept at three different temperatures of 160, 170, 180 °C for 5 days. Each reaction at the three temperatures was again carried out at pH 1.2, 2.2, 3.2, 4.2, 5.2. Crystals of 1a, 1b, 2, 3, 4 were obtained directly in the solution, separated, and washed with distilled water and acetone. 2.3. Single Crystal X-ray Diffraction (SCXRD). SCXRD data were collected on a Bruker Smart X2 APEX II CCD diffractometer having a normal focus, 2.4 kW sealed tube X-ray source with graphite monochromatic Mo Kα radiation (λ = 0.710 73 Å) operating at 50 kV and 30 mA, with ω scan mode. Suitable single crystals of the compounds 1a, 1b, 2, and 3 were mounted on a thin glass fiber with commercially available super glue. The program SAINT26 was used for integration of diffraction profiles, and absorption corrections were made with the SADABS27 program. All of the structures were solved by direct methods, and the refinements were done with full-matrix least-squares on F2 using SHELXL-2014,28 SHELXS 97,28 PLATON,29 WinGX system, version 1.80.05,30 and Olex2 software.31 All of the non-hydrogen atoms were refined anisotropically, and the hydrogen atoms were placed in their geometrically ideal positions except for water molecules. 2.4. Fourier Transform Infrared Spectroscopy (FT-IR). FT-IR spectra of the compounds were collected on powdered single crystals. The spectra were recorded on a Bruker IFS 66v/S using KBr as the reference. 2.5. UV−Vis Spectroscopy. Absorption spectra within the UV− vis−NIR region for the samples were recorded on a PerkinElmer Lambda750 UV instrument. To calculate the band gaps, the spectra were recorded in the reflectance mode on solid powdered samples. 2.6. Scanning Electron Microscopy (SEM)−Energy Dispersive X-ray Spectroscopy (EDAX). Single crystals of the compounds were loaded on carbon tape, taken on a brass stub. Leica 220i microscopy instrument (SEM) and Bruker 129 eV EDAX instrument (EDAX) were used for X-ray semimicroanalysis. The relative atomic ratios of Mo:O, Mo:O:Si, and Cu:N were calculated on the basis of the measurements taken on clean crystal surfaces. The values are in accordance with the ones obtained from the refinement of single crystal XRD data. The EDAX data of compound 2 confirmed the absence of Cu in the crystal, while that of compound 3 showed the presence of Cl in considerable amount in the system. 2.7. Magnetic Measurements. Magnetic measurement of the sample was carried out on a Quantum Design MPMS-SQUID magnetometer. The compound 1b was powdered and screened by PXRD to confirm phase purity before doing magnetic measurements. Temperature-dependent magnetic susceptibility data were collected for field cooled mode (FC) between 2 and 300 K with an applied field of 1000 Oe. 2.8. Raman Studies. Raman experiments were done in backscattering geometry using a custom built Raman spectrometer equipped with a 532 nm laser excitation. Unpolarized Raman spectra were obtained with a laser power of ∼2 mW at room temperature. 2.9. X-ray Photoelectron Spectroscopy. XPS spectra for the samples 1a and 1b were collected as a function of isochronal annealing (1 min) at increasingly high temperature up to 940 K (with XPS spectra being recorded on cooling to 300 K). 2.10. Computational Methods. Our first-principles calculations are based on density functional theory treating exchange correlation energy with a generalized gradient approximation of Perdew−Burke− Ernzerh functional (GGA (PBE)) as implemented with projected augmented wave (PAW)32,33 potential in Vienna ab initio simulation package (VASP).34,35 Kohn−Sham wave functions are expanded in a plane wave basis truncated with energy cutoff of 400 eV. In selfconsistent calculation of energy and its minimization for structural

This disorder at the core oxygen atoms of the Keggin ion was first mentioned by Evans and Pope18 in the year 1977. Although there have been many reports, the details of this crystallographic disorder and the factors affecting it have not been understood yet. Particularly, there have been no reports, to the best of our knowledge, on experimental and theoretical probing of the nature of this disorder and the true local symmetry of the heteroatom. Here we report four new hybrid compounds (1a, 1b, 2, and 3) containing Keggin POM (SiMo12O404−) and 4,4′-bipyridine with Cu as the transition metal linker. These four compounds were strategically synthesized to probe this disorder, and demonstrate how the band gap of these hybrids can be tuned with variation in their structural organization. Band gap engineering of different types of materials has been a decade-long research endeavor in the search for better systems with tunable electronic and optical properties for application in photovoltaics, semiconducting devices, and enhanced optoelectronics.19−22 The class of materials targeted/used and the different mechanisms/methods adopted for band gap engineering vary widely. While in some cases the mechanism adopted was simple alloying,20 doping,20,23 composition modulation,23 or making film composites, in others it was more subtle and complex like quantum confinement effect,22 donor−acceptor substitution,19 controlling bond rotation defects,21 and controlling lengths of eletron flow channels.24 Though it initially started with inorganic semiconductors, quickly it spread to different materials like nanocrystals,22 nanowires,23 carbon nanotubes,21 organic semiconductors, conjugated polymers,19 and recently organic− inorganic hybrids.25 The different hybrid compounds used for our study have been synthesized under hydrothermal conditions by varying pH and temperature, and it has been shown how pH can be used to control the complexity of the hybrid structures. All of the compounds have been characterized using single crystal Xray diffraction, IR spectroscopy, and elemental analysis. Temperature- and pH-dependent studies were carried out to show how these factors affect the crystal structure of the hybrids in terms of disorder of the POM and the incorporation of Cu (transition metal). From optical absorption measurement studies done to measure the band gap of these hybrids, we report a transition from the insulating to the semiconducting region with increasing complexity of the structure. The disorder in the POM involving geometry of core Si has been analyzed in detail through Raman and IR spectroscopic techniques and theoretical calculations and supported by XPS and magnetic measurements. Theoretical calculations have been extended to shed light on the electronic and magnetic properties of this class of hybrids.

2. EXPERIMENTAL SECTION 2.1. Materials. All of the chemicals were obtained from commercially available certified reagents and used without further purifications unless mentioned. Sodium molybdate dihydrate (Na2MoO4·2H2O, AR, 99%), sodium silicate pentahydrate (Na2SiO3· 5H2O, LR, 97%), cupric chloride dihydrate (CuCl2·2H2O, AR, 99.9%), conc HCl, and acetone (GC, 95%) were purchased from SDFCL, Mumbai. Glacial acetic acid (CH3COOH, 16 N, 99.99%) and 4,4′bipyridyl (bpy (C10H8N2), AR) were purchased from Sigma-Aldrich, and disodium hydrogen phosphate dihydrate (Na2HPO4·2H2O, AR) was purchased from Merck. 2.2. Synthesis. The hybrid materials were synthesized in situ under hydrothermal conditions using 50 mL Teflon lined stainless steel containers under autogenous pressure. The autoclaves were filled with 35 mL distilled water (70% volume capacity) followed by 0.5 g (2.07 mmol) of Na2MoO4·2H2O with stirring, which increases the initial pH B

DOI: 10.1021/acs.inorgchem.5b02718 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry Table 1. Products Formed at Different Reaction Conditions of pH and Temperaturea pH

cryst syst, space group

lattice params

type of POM

presence of Cu

MOF formed

160 °C

temp

reaction code I

1.2

170 °C

II III IV V VI VII

2.2 3.2 4.2 5.2 6.2 1.2

180 °C

VIII IX X XI XII

2.2 3.2 4.2 5.2 1.2

XIII XIV XV XVI

2.2 3.2 4.2 5.2

(a) triclinic, P1̅ (b) monoclinic, P2/n (c) monoclinic, C2/c triclinic, P1̅ triclinic, P1̅ tetragonal, I4/acd tetragonal, I4/acd Monoclinic, P2/c (a) triclinic, P1̅ (b) monoclinic, C2/c triclinic, P1̅ triclinic, P1̅ triclinic, P1 tetragonal, I4/acd (a) triclinic, P1̅ (b) triclinic, P1̅ (c) triclinic, P1̅ triclinic, P1̅ triclinic, P1̅ X tetragonal, I4/acd

a = 10.86 b = 11.81 c = 13.26 a = 11.54 b = 12.09 c = 17.33 a = 15.21 b = 18.20 c = 21.24 a = 10.89 b = 15.03 c = 15.23 a = 10.84 b = 15.08 c = 27.48 a = 14.17 b = 14.17 c = 38.58 a = 14.17 b = 14.17 c = 38.58 a = 3.78 b = 12.73 c = 11.49 a = 10.86 b = 11.81 c = 13.26 a = 15.21 b = 18.20 c = 21.24 a = 10.63 b = 13.24 c = 20.12 a = 10.88 b = 14.00 c = 14.21 a = 5.54 b = 5.71 c = 13.51 a = 14.17 b = 14.17 c = 38.58 a = 10.86 b = 11.81 c = 13.26 a = 10.63 b = 13.24 c = 20.12 a = 13.96 b = 14.22 c = 21.73 a = 10.88 b = 12.07 c = 12.50 a = 10.88 b = 12.07 c = 12.50 no crystal obtained a = 14.17 b = 14.17 c = 38.58

(a) disordered (b) disordered (c) normal disordered disordered no POM no POM no POM (a) disordered (b) normal disordered disordered no POM no POM (a) disordered (b) disordered (c) normal disordered disordered no POM no POM

yes no no yes yes yes yes yes yes no yes yes yes yes yes yes yes yes yes no yes

no no no no no yes yes yes no no no no yes yes no no no no no no yes

a At 160, 170, 180° C. At each temperature, reactions were done at 5 pH conditions, e.g., 1.2, 2.2, 3.2, 4.2, 5.2. All other reaction conditions were kept the same. Reagents: Na2MoO4, Na2SiO3, CuCl2, bpy. Duration: 5 days. Chamber vol: 50 mL. Solvent: water. The text in bold corresponds to the compounds discussed in the paper.

Table 2. Different Important Products Selected from pH- and Temperature-Dependent Study Correlating the Product Codes and Type of Compounds reaction code

pH of reaction

temp (°C)

Pdt

type

nature of hybrid

color of the crystal

Ia Ib Ic VIIa VIIb VIII XIIa XIIb XIIc

1.2 1.2 1.2 1.2 1.2 2.2 1.2 1.2 1.2

160 160 160 170 170 170 180 180 180

1b 2 4 1b 4 1a 1b 1a 3

type 1 type 2 type 4 type 1 type 4 type 1 type 1 type 1 type 3

disordered Keggin with Cu disordered Keggin without Cu normal Keggin without Cu disordered Keggin with Cu normal Keggin without Cu disordered Keggin with Cu disordered Keggin with Cu disordered Keggin with Cu normal Keggin with Cu

dark red light green dark green dark red dark green dark red dark red dark red dark red

relaxation, a 2 × 2 × 1 uniform mesh of k-points is used for sampling Brillouin zone integrals.

[(SiMo12O40)][Cu5Cl2(C10H8N2)5]·2H2O were obtained at 160 °C (Ib) and 180 °C (XIIc), respectively, both at pH = 1.2. The known compound 4 ([SiMo 12 O 40 ](H 2 4,4′-bpy) 2 · xH2O),36,37 synthesized for comparative study from reported procedure,37 formed both at 160 and 170 °C at pH = 1.2 (Ic, VIIb). The synthetic conditions for selected products (1a, 1b, 2, 3, 4) are listed in Table 2. The reactions led to the incorporation of transition metal Cu into the crystal structures of compounds 1a, 1b, and 3. In our previous report36 we found that the use of transition metal salt did not lead to any incorporation of the transition metal in the crystal structure, but the hybrid structure did not form in the absence of the salt. This fact can be attributed to the change in ionic strength of the solution upon the addition of CuCl2.37 Initially, at 180 °C and pH = 1.2, the reaction yielded dark red polymorphic crystals consisting of two phases: 1a and 1b. The compounds 2, 3, and 4 were obtained as light green small rhombic, dark red rectangular flat, and dark green rectangular crystals, respectively. Except the crystals of compound 2, which are brittle, all are chemically and mechanically stable. The compounds with Cu were insoluble in solvents such as H2O, EtOH (ethanol), DCM (dichloromethane), DMF (dimethylfor-

3. RESULT AND DISCUSSION 3.1. Reaction Chemistry. All of the compounds were synthesized in hydrothermal conditions under autogenous pressure starting from precursors Na2MoO4, Na2SiO3, CuCl2, and bpy (bpy = 4,4′-bipyridine). The reaction conditions (temperature and pH) were varied to synthesize four novel different types of hybrid structures. The effects of temperature and pH on the extent of complexity of hybrid structure were studied by carrying out the reactions at three different temperatures (160, 170, 180 °C) and five different pH (1.2, 2.2, 3.2, 4.2, 5.2) conditions. The summary of various reaction conditions of pH and temperature on the formation of the com po und s is given in Table 1. Com po un d 1 a [H3(SiMo12O40)2][Cu5(C10H8N2)7]·2H2O was obtained at (i) 170 °C and pH = 2.2, (ii) 180 °C and pH = 1.2 (VIII, XIIb), while 1b [(H2SiMo12O40)][Cu2(C10H8N2)3]·2H2O, was formed at (i) 160 °C, (ii) 170 °C, (iii) 180 °C, all at pH ∼ 1.2 (Ia, VIIa, XIIa). Compounds 2 [(H4SiMo12O40)][(C10H8N2)2]·2H2O and 3 C

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Figure 1. Graphical representation of the constituents of four different types of hybrid materials: 1a [H3(SiMo12O40)2][Cu5(C10H8N2)7]·2H2O, 1b [(H2SiMo12O40)]·[Cu2(C10H8N2)3]·2H2O, 2 [(H4SiMo12O40)][(C10H8N2)2]·2H2O, 3 [(SiMo12O40)][Cu5Cl2(C10H8N2)5]·2H2O, 4 [SiMo12O40]· [H2(4,4′-bpy)]2·xH2O.

Table 3. Crystal Data and Structure Refinement for Compounds 1a [H3(SiMo12O40)2][Cu5(C10H8N2)7]·2H2O, 1b [(H2SiMo12O40)][Cu2(C10H8N2)3]·2H2O, 2 [(H4SiMo12O40)][(C10H8N2)2]·2H2O, and 3 [(SiMo12O40)][Cu5Cl2(C10H8N2)5]· 2H2Oa param fw cryst syst space group, Z unit cell dimensions

V density (calcd) abs coeff F(000) cryst size θ range for data collection index ranges

reflns collected indep reflns completeness to θ = 25.00° data/restraints/params GOF final R indices [>2σ(I)] R indices [all data] largest diff peak and hole

1a

1b

2

5089.79 triclinic P1̅, 1 a = 10.8950(3) Å b = 13.5706(4) Å c = 20.6833(6) Å α = 85.8030(10)° β = 88.4890(10)° γ = 78.1060(10)° 2984.18(15) Å3 2.832 g/cm3 3.428 mm−1 2419 0.5 × 0.25 × 0.04 mm3 1.54−30.00° −15 ≤ h ≤ 15 −18 ≤ k ≤ 19 −28 ≤ l ≤ 29 44861 17227 [Rint = 0.0535] 99% 17227/0/928 1.037 Robs = 0.0674 wRobs = 0.1551 Rall = 0.1200 wRall = 0.1871 1.811 and −2.471 e Å−3

2453.05 triclinic P1̅, 1 a = 10.8540(3) Å b = 11.7897(3) Å c = 13.2814(3) Å α = 105.1840(10)° β = 102.8070(10)° γ = 114.8050(10)° 1378.15(6) Å3 2.956 g/cm3 3.520 mm−1 1164 0.39 × 0.14 × 0.08 mm3 2.74−30.00° −15 ≤ h ≤ 15 −16 ≤ k ≤ 16 −17 ≤ l ≤ 18 28563 7876 [Rint = 0.0373] 97.8% 7876/30/440 1.040 Robs = 0.0498 wRobs = 0.1074 Rall = 0.0639 wRall = 0.1205 1.586 and −1.388 e Å−3

2171.80 monoclinic P21/n, 2 a = 11.5292(4) Å b = 12.0812(5) Å c = 17.3184(7) Å β = 103.319(2)° 2347.34(16) Å3 3.073 g/cm3 3.238 mm−1 2052 0.23 × 0.13 × 0.01 mm3 2.57−25.00° −13 ≤ h ≤ 12 −14 ≤ k ≤ 14 −20 ≤ l ≤ 20 23967 4087 [Rint = 0.0622] 98.9% 4087/0/376 1.289 Robs = 0.0789 wRobs = 0.1572 Rall = 0.1021 wRall = 0.1678 0.898 and −0.849 e Å−3

3 3024.92 triclinic P1̅, 2 a = 13.9658(17) Å b = 14.2231(16) Å c = 21.732(2) Å α = 85.513(8)° β = 82.560(9)° γ = 63.280(12)° 3822.3(7) Å3 2.628 g/cm3 3.439 mm−1 2894 0.66 × 0.17 × 0.04 mm3 1.81−25.00° −16 ≤ h ≤ 16 −16 ≤ k ≤ 16 −15 ≤ l ≤ 25 28360 13467 [Rint = 0.0528] 99.9% 13467/30/1099 1.052 Robs = 0.0969 wRobs = 0.1681 Rall = 0.1942 wRall = 0.2082 1.605 and −1.323 e Å−3

Temperature −293 (2) K, wavelength −0.71073 Å, refinement method full-matrix least-squares on F2. R = ∑∥Fo| − |Fc∥/∑|Fo|, wR = {∑[w(|Fo|2 − |Fc|2)2]/∑[w(|Fo|4)]}1/2. a

mamide), DMSO (dimethyl sulfoxide), CHCl3 (chloroform), and CH3CN (acetonitrile). The compounds synthesized under the hydrothermal conditions consist of disordered POM, with Cu as the transition

metal and bpy as the organic ligand. The Keggin units in these types of hybrids often have a common crystallographic disorder, giving rise to apparently distorted Keggin ions. It was found that the POMs incorporated in 1a, 1b, and 2 are disordered while D

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Figure 2. Crystal structure of 1a {[H3(SiMo12O40)2][Cu5(C10H8N2)7]·2H2O}. Left: Crystal structure along a-direction. Right top: Arrangement of bpy units. Right bottom: Single POM unit.

Figure 3. Crystal structure of 1b {(H2SiMo12O40)][Cu2(C10H8N2)3]·2H2O}. Left: Crystal structure along a-direction. Right top: Arrangement of bpy units. Right bottom: Single POM unit.

increased. This is because of the less protonation of the bpy ligands at lower H+ concentration. The basicity over the nitrogen centers remains higher, and the ligands can easily coordinate to the Cu centers. This increase in incorporation of Cu leads to more complex structures at higher pH. At the highest temperature (180 °C), all the hybrid structures obtained were found to contain Cu. 3.2. Crystal Structure. The refinement parameters and other crystallographic details of the compounds 1a, 1b, 2, and 3 are listed in Table 3, and the important bond distances are given in Table S1. The crystal structures of all four compounds are shown in Figures 2−5. The compounds 1a and 1b crystallize in the triclinic system with space group P1̅. The general 3D structural arrangement in the case of the Cu incorporated systems (1a, 1b, 3) can be broken up into three units: (i) organic moiety, (ii) Cu linker, and (iii) POM as the anionic ligand. The bpy units coordinate to Cu atoms forming chains, which arrange

those of 3 and 4 are normal ordered structure. Thus, we have been successful in synthesizing four types of hybrid systems: disordered POM with (1a, 1b) and without Cu (2), normal POM with (3) and without (4) Cu. Compounds 1a, 1b, and 3 crystallize in the triclinic system with space group P1̅, while compounds 2 and 4 crystallize in the monoclinic space groups P2/n and C2/c, respectively. Figure 1 represents constituents used for the synthesis and the formation of hybrid structure under different pH values. These controlled studies reveal that the presence or absence of Cu does not affect the disorder in Keggin geometry as both types of Keggin (disordered and normal) could be tactically synthesized with and without incorporation of Cu in the structure. A slow transformation from normal to disordered structure was observed as the temperature increased from 160 to 180 °C. With an increase in pH (at all the three temperatures) and temperature of the reaction, the probability of Cu inclusion E

DOI: 10.1021/acs.inorgchem.5b02718 Inorg. Chem. XXXX, XXX, XXX−XXX

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Figure 4. Crystal structure of the compound 2 {(H4SiMo12O40)][(C10H8N2)2]·xH2O}. Left: Crystal structure along a-direction. Right top: Arrangement of bpy units. Right bottom: Single POM unit.

Figure 5. Crystal structure of compound 3 {(SiMo12O40)][Cu5Cl2(C10H8N2)5]·2H2O} along the c-direction. Left: Crystal structure along the cdirection. Right top: Arrangement of bpy units. Right bottom: Single POM unit.

structures, we can easily account for the shrinkage in the lattice parameters from 1a to 1b. The crystal structure of 1a and 1b consists of disordered POM linked by Cu with layers of bpy (Figures 2 and 3). The organic unit of compound 1a can be viewed as the repetition of stacked bunch of five different bpy layers along the b-axis. A diagonal bpy unit is intercalated between these bpy layers (Figure 2). In the case of 1b, the organic unit consists of two similar layers of stacked bpy chain with one sandwiched bpy moiety in between them. As this stacking of bpy layers occurs along the c-direction, the unit cell dimension along the c-axis is smaller in the case of 1b compared to 1a. On the other hand, the POM structures in both compounds are similar, having disordered α-Keggin structures. The average structure refined from the single crystal XRD data having the formula SiMo12O40 consists of eight half occupied

and propagate along various directions of the cell axes. These chains are again linked together by the anionic ligand POM, through Cu centers via weak interactions to form 3D structures. In the case of the compounds that are devoid of Cu (2 and 4), the hybrid structures are formed by supramolecular interactions between the POM units and organic ligands. The compounds 1a and 1b crystallize in the triclinic system with space group P1̅. The crystal structures of the compounds 1a and 1b are shown in Figures 2 and 3, respectively. Both hybrid structures are threedimensional, consisting of 1D chains composed of bpy and the Cu atom, which are linked to POMs by weak interactions through the Cu center (Figure S1a,b). The difference between the two compounds is in the arrangement of the bpy rings around the POM cluster. From the structural arrangement of the bpy rings in the two crystal F

DOI: 10.1021/acs.inorgchem.5b02718 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

Figure 6. (a) K-M function vs energy (eV) curve for the compounds 1a, 1b, and 3. The optical band gap obtained shown as dotted line. (b) Variation of color of crystals in different hybrid structures.

There are three types of 1D bpy chains present in compound 3, which propagate along the c-direction. Chain 1 contains a bpy− Cu1−bpy−Cu5−bpy linkage, and chain 2 is composed of only Cu2−bpy while chain 3 is composed of a bpy−Cu3−bpy−Cu4− bpy linkage. The Cu2 atom exists in the +2 oxidation state and is linked to two Cl atoms (Cl1, Cl2). There are five 1D chains present in the unit cell, which are linked together by Cl atoms along the b-direction to give 2D layers. These 2D layers are further connected by POM units, which resulted in the 3D hybrid structure. The important bond distances for the compound 3 are listed in Table S1. Compound 4, which has been synthesized from the reported literature36 for comparison, consists of normal Keggin (similar to compound 3) along with bpy, with no Cu incorporated into the crystal structure (similar to compound 2). Thus, the geometry of the Si in POM of 4 has tetrahedral geometry as in compound 3. 3.3. Band Gap Measurements from UV Studies. Band gap engineering in organic−inorganic perovskite solar cells44,45 has recently become a hot topic of research and has captivated many scientific minds worlwide. There are various techniques that have been used in this type of organic−inorganic solar cell for tuning their band gap properties like chemical tuning,46 interface engineering,31 cation doping, sterric effect, or bond angle tuning.47 The importance of this emerging field in the case of organic−inorganic hybrids triggered us to investigate the optical and elctronic properties of our promising materials. There has been a marked difference in the color of the crystals as we move from compounds incorporated with Cu to the compounds without Cu in the structure. We know that color in these hybrid materials can mainly arise from ligand to metal charge tranfer as d−d transitions and metal to metal charge transfer (as in case of mixed valent metal centers in POM clusters) are not possible in these cases. This observation indicated interesting band gap and electronic properties of these compounds and prompted us to see how those are changing with structural complexity. The diffuse reflectance spectra (DRS) were recorded on solid compounds. The reflectance spectra was then converted to the Kubelka−Munk function F = (1 − R)2/ 2R48 and plotted against energy in eV (Figure 6 and Figure S3). The band gap (Eg) was calculated from the slope change of the curve.49,50 The band gaps considerably decreased with the incorporation of Cu into the system. The constituent units of the hybrids (bpy (4.23 eV) and [SiMo12O40]4− (4.73 eV)) are already reported as insulators.51 In addition, the band gap reported for compound 4, which has no Cu linkage between the POM and ligand, falls in the semiconducting region with a band gap of 3.2 eV.36 With the incorporation of Cu into the structure,

core oxygen atoms. The Cu atoms in both the cases are connected to the oxygen atoms of POMs and are coordinated by the nitrogen of the bpy unit. The Cu−N (1.885(8)−1.913(6) Å), Cu−O (2.6−2.88 Å), and Mo−O (Ot, Ob, Oc: 1.642(4)− 2.472(11) Å) interactions (Figure S1a,b, Table S1) are within the acceptable ranges.38−43 Compound 1a has two types of α-Keggin POM clusters with different positions of Si atoms, one being at the corner of the unit cell (Si1:1, 0, 0) and another at the face center (Si2:1, 0.5, −0.5). The center of symmetry lies at the Si center in both POMs. There are three types of Cu in the unit cell (Figure S1a) (Cu1, Cu2 and Cu3): Cu1 is connected to three terminal oxygen atoms and two nitrogen atoms of two bpy. Cu2 is coordinated to two oxygen atoms of two different POMs (bridging oxygen O37 and terminal oxygen O15 of POM clusters) and two nitrogen atoms of two bpy units. Cu3 is also associated with a center of inversion like Si, having the coordinates 1, 0.5, 0 (special position). It is connected to two O18 (Ot) of two POM clusters on two opposite sides and two N5 atoms of two bpy units, which again coordinate from opposite sides. In the case of 1b, there is only one Si atom present at the (1, 0.5, 0) position in the center of a cell edge (Figure S2b), which contains a center of inversion. Only one type of Cu is present in the unit cell. The Cu is coordinated to two nitrogen atoms of two bpy units and two oxygen atoms (bridging O9 and terminal O20) of POM. The important bond distances of 1a and 1b have been supplied in Table S1 in the Supporting Information. Compound 2 crystallizes in the monoclinic system with space group P2/n. It is composed of disordered α-Keggin anion and bpy with two lattice water molecules per formula (Figure 4). The Si is present at the vertices of the unit cell and at the body center (0, 1, 0) accompanied by a center of symmetry, which is again coordinated by eight half occupied oxygen atoms like 1a and 1b (Figure S2c). Interestingly, no Cu was incorporated into the structure, which is probably due to reaction conditions of pH = 1 at 160 °C. The hybrid structure of 2 has been formed through supramolecular interactions (H-bonding) between the bpy rings and POM units. The hydrogen bonds are observed between the centers N2−O8 (2.906 Å), N2−O9 (3.099 Å), N1−O6 (2.988 Å), N1−O4 (2.836 Å). Compound 3 crystallizing in the triclinic system with space group P1̅ is composed of normal α-Keggin units, bpy as the organic moiety, and Cu and Cl as the linkers (Figure 5). The POM structure present in 3 is that of a normal Keggin anion having no disorder with tetrahedral Si at the center (Figure S2d). Crystallographically, five different types of Cu atoms (Figure S1c) and two different types of Cl are present in the unit cell. G

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Inorganic Chemistry Table 4. Variation in Color of Crystals and Optical Band Gap compd Bpy Keggin 4 1a 1b 3

color of crystal

chemical formulas

green dark red dark red dark red

4,4′- bpy [SiMo12O40]4− [(SiMo12O40)][(bpy)3]·xH2O [(SiMo12O40)2][Cu5(bpy)7]·2H2O [(SiMo12O40)][Cu2(bpy)3]·2H2O [(SiMo12O40)2][Cu10Cl4(bpy)10]·2H2O

presence of Cu

band gap (eV)

No Yes Yes Yes

4.23 4.73 3.2 1.83 1.78 1.62

Figure 7. Graphical representation of the crystallographic disorder present the hybrids 1a, 1b, and 2. Left top: Two adjacent Si centers having square planar geometry. Left bottom: Two adjacent Si centers having tetrahedral geometry. Right: The common average picture of either situation.

the band gap decreased further down to 1.6 eV in compound 3, 1.78 eV in 1b, and 1.83 eV in 1a (Table 4). It is interesting to note that, in the hybrids containing Cu, with more Cu incorporation in the structure, band gap can also be fine-tuned to lower values (in 1a, band gap 1.83 eV, Cu:POM ratio is 2.5:1 while in 3, band gap 1.6 eV, Cu:POM ratio is 5:1). The arrangement of the molecular POMs and ligands into 3D/2D structural array leads to the formation of band structure in these materials. These bands decrease the HOMO and LUMO energy gap through the formation of valence and conduction bands. Further reduction in band gap could be achieved by linking these units through transition metals like Cu. This reduction in band gap due to the presence of Cu can be explained on the basis of the band structure of these compounds at the Fermi level.52−54 The presence of Cu gives rise to new bands above the valence band structure, which should further decrease the gap between valence and conduction bands giving rise to these low band gap semiconductors. On the contrary, in the compounds which are devoid of Cu, the Fermi level will be pushed further down, and hence the band gap will increase. Theoretical calculations of compound 1a give the proof for this fact, as we can see in the presence of Cu 3d bands (the major contributor) just below the Fermi level (discussed in the theorectical section). Here, we can also relate the symmetry and the optical band gap of each system (Figure S4) as less symmetric systems are expected to have more complex structures giving rise to new valence and conduction bands. For example, as we move from more symmetrical POMs (generally cubic or tetragonal) to the less symmtetrical hybrids POM−bpy (monoclinic, compounds 2 and 4) and POM−Cu− bpy (triclinic, compounds 1a, 1b, and 3), the reduction in band gap is observed. The constituent transition metal and organic

moeity play a crucial role in the symmetry and engineering the band gap of the resulted products. We tried to find other systems with intermediate symmetry and reported band gaps to correlate the analogy better. However, there are almost no reports so far on band gap engineering in this type of hybrid, which makes this study even more unique and novel. Thus, our study opens the scope for synthesizing new low band gap semiconductors joining insulators into hybrid stuctures showing how the nature of the connectivity between the constituent units affecting the band gap in these types of hybrids. 3.4. Origin and Nature of the Disorder. In a normal αKeggin POM, the heteroatom at the center is tetrahedrally coordinated with four oxygen atoms while all of the 12 addenda atoms are hexacoordinated with four edge-shared oxygen atoms (Ob: bridges two Mo atoms), one terminal oxygen (Ot: terminally bonded to Mo), and one core oxygen (Oc: connects Mo with the heteroatom Si). In the case of a disordered Keggin ion (1a, 1b, and 2: Figures 2−4), the central heteroatom Si appears to be coordinated by eight half occupied oxygen atoms (Figure S2a−c), which reduces its effective coordination number to four. On the other hand, compounds 3 (Figure 5) and 4 have ordered Keggin structure with a tetrahedrally coordinated Si atom (Figure S2d,e). The eight half occupied oxygen atoms around the central hetero-Si atom (Figure S2a−c) can be either in square planar or in tetrahedral geometry. Although there have been reports on these types of disordered POMs in hybrid forms,16,17,55 the structural distortion in Si geometry of these POMs have never been discussed in detail, to the best of our knowledge. In all of these disordered structures, a pseudoinversion center (“i”) can invariably be found over the Si position. H

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Inorganic Chemistry Table 5. Major Peak Assignment (IR and Raman) for the Keggin57 Raman (cm‑1)

IR (cm‑1)

compd

νs(Mo−Ot)

νs(Mo−Ot−Cu)

νas(Si−O), νas(Mo−Ot) asymmetric coupling

νas(Si−O), νas(Mo−Ot) symmetric coupling

δ(O−Si−O)

1a 1b 2 3 4

975.9 971.4 977.7 975.1 976.8

968.1 962.2

986 987 98557 987 987

901 901 89957 899 902

607 608 60057 608 607

970.5

Figure 8. (a−e). Raman spectra showing the Mo−Ot sym str modes of 1a, 1b, 2, 3, 4. The peaks colored cyan and gray correspond to νs(Mo−Ot−Cu) and νs(Mo−Ot), respectively. (f) IR spectra of the compounds. The dotted lines show the peaks involving Si−O stretching vibrations.

It is well-known that Si exists as stable tetravalent state in the αKeggin POM and hence is tetracoordinated. The arrangement of eight partially occupied oxygen atoms around the atom Si is the result of a positional disorder in the system arising from the pseudo “i” (center of inversion) symmetry. This positional disorder in the system is static in nature as it is present even at low temperature (100 K, verified through single crystal XRD). The cubo-octaheral (where the coordinated atoms exist at the eight vertices of a cube) geometry around Si can arise in two ways: Either from the superimposition of one tetrahedrally coordinated Si in one unit cell with another Si tetrahedron (just oppositely oriented) in the next unit cell, or from the average structure of two square planar SiO4 orientations in two unit cells arranged at 90° to each other. The schematic representation of these two possibilities is demonstrated in Figure 7. If the Si geometry is planar (which comes directly from the symmetry operation “i”), the arrangement of the core oxygen atoms inside the metaloxyanion cluster leads to an unequal sharing of the core oxygen atoms among the Mo centers generating different molybdenum coordination environments in the POM unit. The Mo environments in POMs of 1a and 3 are given in Figure S5. Thus, this disorder in POM and the true Si geometry can be probed through spectroscopic analysis of the Mo−Ot bonds and Si−O bonds. Also, it was important to verify the oxidation state of the addenda Mo atoms to see if any variation owing to different coordination environment has occurred. 3.5. Raman and IR Spectroscopy. Vibrational spectroscopy can be used to probe the local symmetry of molecules in

inorganic/organic moieties and has played an important role to aid XRD in this aspect. Raman spectra were measured on single crystals of all of the compounds, while in the case of IR, spectra were measured on KBr pellets of the samples. Raman spectra of all of the compounds and IR spectra for compounds 1a, 1b, 3 are shown in Figures S6 and S7, respectively, in the Supporting Information. The spectra have contributions from the inorganic (POM) and organic moiety (Cu−bpy for the compounds 1a, 1b, and 3, see Figure S7, bpy for the compounds 2 and 4). The POM peaks were assigned (Supporting Information Tables S2−S4) using the previous reports56−59 on the Keggin structure.56,57 The POM peaks are present mostly below 1000 cm−1, and the peaks above 1000 cm−1 are found to be predominantly due to Cu− bpy/bpy vibrations.58,59 The major peaks of POM were found to be similar in all of the compounds, though POM vibrations in the Raman spectra of compounds 1b and 3 were very weak. Raman and IR band assignment for modes of all compounds are listed in Tables S2 and S3, respectively. The peak positions for modes involving Mo−Ot and Si−O of POM in all of the compounds are given in Table 5. The region from 900 to 1000 cm−1 in the Raman spectra is due to the asymmetric and symmetric stretching vibrations of Mo−Ot, which are pure uncoupled vibrations.57 The symmetric stretching can capture the changes in the bond length due to its direct correlation with the stretching vibrations. Figure 8a−e shows the Raman spectra in the symmetric stretching region of Mo−Ot. The single peak observed at ∼977 cm−1 for compounds 2 and 4 corresponding to Mo−Ot stretching was found to be split I

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Figure 9. Charge density plots of (a) nonmagnetic and (b) magnetic compound 1a (along the ab-plane) showing symmetry breaking due to Cu ion (specially Cu3, which is interacting with a bridging oxygen (O73) instead of terminal oxygen (O71/O80)).

position of Si−O stretching peaks in 1a, 1b, 3, and 4 remained at the same position (Figure S8). It was slightly shifted in the case of compound 4 due to the effect of the counterion.56 Absence of splitting/shift of these modes in IR, combined with the fact that no new peaks corresponding to Si−O vibrations appeared in the Raman spectra, implies that within a unit cell Si should be tetrahedrally coordinated.60 Hence, the cuboctahedron coordination of Si obtained from XRD should be considered as the superposition of two tetrahedral Si, rather than two square planar ones, of adjacent unit cells. 3.6. Density Functional Theory (DFT) Calculations. To further verify the information obtained from spectroscopy and understand the electronic properties of the hybrids, we have performed first-principles DFT calculations on compound 1a. Our calculations show that square planar geometry of (SiO4)4− does not correspond to an energy minimum. Structural relaxation of the POM structure embedded in the metal−organic matrix always leads to breaking of symmetry of the planar structure, and results in a lower energy and slightly distorted tetrahedral coordination. The energy difference between the two structures (unrelaxed with sq planar Si and relaxed with tetrahedral Si) is about 0.26 eV per atom which is equivalent to 3017 K temperature change. Thus, the planar structure of the

into two in the case of compounds 1 and 3. Initially, we thought this splitting was due to the disordered POM and affected M−Ot bonds, but later ruled out this assumption as it was observed both in disordered (1a, 1b) and ordered (3) structures. It was also worth noting that the Mo−Ot peak splits only in case of the structures with Cu (1a, 1b, 3), but not in 2, 4 (without Cu). This happened due to the electrostatic interaction between Cu and a few terminal oxygen atoms in 1 and 3, which decreases the bond stiffness of the Mo−Ot, thereby decreasing the stretching frequency. In order to get more detailed information on the weak Si−O modes in Raman, we have used IR spectroscopy. The Si−O modes are seen at ∼900 (s), 985 (w), and 600 (sh) cm−1. Except for the mode at 600 cm−1, other modes are mixed vibrations involving Mo−Ot, due to which they appear broad and asymmetric in shape.56 Any change in the local symmetry of Si should be reflected in these modes. The shoulder observed at 600 cm−1 (Figure S8) is very weak and is not considered for further discussion. If the SiO4 unit had assumed square planar structure (in 1a, 1b, and 2), we should expect a huge variation in the position of the peaks involving Si−O stretching from 3 and 4 where SiO4 is in tetrahedral geometry, even though a SiO4 unit within a POM cannot be equated to an isolated tetrahedron. The J

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Figure 10. Electronic density of states of both (a) nonmagnetic and (b) ferromagnetic compound 1a. Cu 3d orbitals color code: cyan line is dxy, magenta line is dyz, blue line is dzx, green line is dz2, and red line is dx2−y2.

(SiO4)4− polyhedra appears incompatible with the overall expected symmetry of POM. On the contrary, the POM structure with (SiO4)4− in tetrahedral geometry is found to be quite stable in our simulations. A close comparison of the disordered structure obtained from the single crystal X-ray diffraction and the stable structure obtained with DFT-based structural relaxation is shown in Figure S9. The tetrahedral geometry is expected from the octet rule that dictates the four electrons (3s23p2) in the outermost shell of Si to participate in bonding with four oxygen ions. In the square planar geometry oxygen ions are separated by 90° angles from one another, leading to a large strain as compared to the tetrahedral geometry (with O−Si−O angle of 109.5°), which makes the square planar coordination energetically expensive. In fact, very few complex ions are known to exhibit square planar coordination, and they all consist of a central ion with 4 bonded electrons and two lone pairs, which are stereochemically active occupying the space above and below the square plane, thus compensating for the stress in such a coordination. In the energetically stable structure of POM embedded in the metal−organic matrix, four oxygen atoms around Si form an almost perfect tetrahedral coordination. The O−Si−O bond angles vary from 109° to 109.8°. The slight deviation of (SiO4)4− ionic structure from a perfect tetrahedron (where O−Si−O bond angle is expected to be exactly 109.5°) can be understood from the electronic charge density distribution (Figure 9a), which shows that Cu ions close to the POM interact with the terminal oxygen ions contributing to the stability of the structure. However, we clearly see that one particular Cu ion (Cu3) strongly interacts with the bridging oxygen (O73) instead the terminal oxygen (O80/O71) (see Figure 9a). This disparity in the bonding of Cu atom with the oxygen atoms in the system breaks the symmetry, and distorts the Mo−O arrangement around (SiO4)4− complex ion slightly and leads to distortion of the tetrahedral geometry. There is a sharp peak in the electronic density of states at the Fermi level (Figure 10a), which is seen through atom projected density of states to be arising mainly from Cu 3d orbitals. Such a high density of states at the Fermi

level hints at a possible electronic or structural instability in the system, also relating to the activity of Cu in destabilizing Sicoordination. The optical band gap properties of these compounds have been studied which showed that the compounds (including 1a) are not metallic but semiconducting. Thus, we concluded that this instability might be coming from a magnetic contribution either inherent or induced in the system. In fact, experimental results concurred with our prediction, and incorporation of the magnetic factor in our calculation yielded better results in the electronic density of states calculations (Figure 10b). Our calculations with ferromagnetic ordering in the system show an even stronger disparity (compared to nonmagnetic system) in the interaction of Cu ions with oxygen ions (Figure 9b). Similar to the nonmagnetic structure, Cu3 interacts with bridging oxygen (O73). However, in the magnetically ordered structure, Cu4 ion has no interaction with the POM complex (in the center of the figure). Topological analysis and real-space partitioning of charge density reveals the disparity in the Bader charges (Table S5) of Cu ions. Charge of Cu3 is very close to the nominal charge of +1, while Cu4 has the largest deviation from the nominal charge of +1. Estimated magnetic moments (see Table S6) show that only Cu ions have significant magnetic moments, which is again in good agreement with our magnetic measurements. This is an important finding as it shows that Cu ions alone determine the magnetic nature of this material. Density of states of the ferromagnetic state shows that the sharp peak at Fermi level found in the nonmagnetic structure has now split (Figure 10b) and the Fermi energy now lies in the minimum rendering greater stability to the structure. Cu 3d and Mo 4d orbitals are the primary contributors to electronic states near Fermi level (Figures S10 and S11). Experimentally, this material exhibits a semiconducting gap (as observed in the optical band gap measurements), while our calculations give a kind of a pseudogap at the Fermi energy between the two peaks. This is likely due to the strong correlations of electrons in the d-orbitals and inclusion of on-site correlations (Hubbard U correction) may help in reproducing a nonzero gap. K

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Figure 11. (a) Magnetic susceptibility (χM) and inverse susceptibility for compound 1a within the temperature range 2−300 K. An increase in susceptibility below 25 K is probably due to weak ferromagnetic ordering. (b) M vs H plot measured at 2 and 300 K (shown in red and blue color). The inset shows the hysteresis loop owing to ferromagnetism.

by the density functional theory calculations on 1a when it yielded better results with incorporation of magnetism during the relaxation of the crystal structure. However, at 300 K, magnetization decreases with applied magnetic field, a typical behavior shown by diamagnetic systems. Although this data is apparently in contradiction with the theoretical calculations, it should be noted that the DFT calculation accounts for the electronic structure of a system at absolute temperature (0 K) and also confirms that, at room temperature (300 K), Cu moieties in the system are essentially monovalent in nature and might undergo valence transition to Cu2+ at low temperature under applied magnetic field. Thus, from magnetic property study we could account for the instability observed in DFT in the nonmagnetic calculation and understand the way to solve it. To check the oxidation states of all the Mo atoms in the POM units, XPS measurement was done on compounds 1a and 1b. The XPS was measured at room temperature (300 K) for the Mo 3d region (220−240 eV) and fitted to get two well-separated peaks (for 3d5/2 and 3d3/2) (Figure S14). It is clear that all of the Mo atoms in the POM units of both compounds (1a and 1b) exist in the common +6 state. The binding energies corresponding to each oxidation state are given in Table S8. The binding energy values and fwhm of the peaks match well with reported data for Mo 3d electrons.62−64 Bond valence sum61 values were calculated for all of the Mo centers in both of the compounds, which were also found to be close to +6. All of these data indicate that all Mo of POMs are in their +6 oxidation state having similar coordination environments. This in fact indirectly confirms that the Si at the center has tetrahedral geometry in both compounds. Mo and Cu are the probable magnetically active species in these hybrid structures. Mo being in its highest oxidation state of +6 is expected to be magnetically inactive leaving behind Cu as the only moiety to show magnetic properties.

On relaxing the structure (normally and magnetically), there has been a reduction in symmetry of the structure of compound 1a. It has reduced from P1̅ to P1. The structural changes before and after relaxation have been depicted in Figure S9. For being able to relax to tetrahedral geometry, the Si atoms had to get rid of the “i” located on it, which it did by shifting its positions slightly from the special crystallographic sites (100) and (1 − 0.5 0.5), Table S7. This induced further adjustments of atoms inside the cell, which led to a change in cell constants. When we compared the simulated PXRD patterns of the structures, before and after relaxation (Figure S12), a very interesting fact could be noticed. The peaks corresponding to the (100) (bc-plane perpendicular to a) and (0−11) (Table S7) planes interchanged positions in the PXRD patterns before and after relaxation, and also the splitting between them increased (Figure S12). Actually the (100) plane did not move (2θ = 8.3), while (0−11) shifted from 2θ = 8.18 to 8.52. Interestingly, both the planes pass through Si (Figure S13), and the main change in the unit cell occurred along the b-, c-axes and α-, γ-angles with the a-axis and β remaining the same. Thus, as there were only changes in the band c-axes, the plane cutting those axes, i.e., the (0−11) plane, changed its position while the other plane (100), which is parallel to the bc-plane, remained at the same position. The crystallographic values before and after relaxation along with the shifts in PXRD are listed in Table S7. 3.7. Magnetism and X-ray Photoelectron Spectroscopy (XPS). DFT calculations predicted the instability in the system, which led us to analyze the magnetic property of these hybrids at very low and room temperatures (2 and 300 K). To study the magnetic contribution, we have performed the magnetic measurements on compound 1a. The temperaturedependent magnetic studies were done on powdered crystalline sample of 1a in the range 2−300 K at an applied magnetic field of 1000 Oe. The variation of molar magnetic susceptibility (χM) and inverse susceptibility (χ−1) with temperature (T) has been depicted in Figure 11a. A magnetic moment (μeff) value of 1.67 μB/formula was obtained by fitting the χ−1 versus T plot from linear region (130−330 K). This magnetic moment can be attributed to the presence of Cu2+ in the structure at low temperatures which has a spin only value of 1.73 μB.61 However, the negative value of magnetic susceptibility at room temperature and low magnetic field can be explained as the valence fluctuation of Cu from +1 to +2 as the temperature decreases. In addition, the field-dependent magnetic measurement was performed at 2 K. The field-dependent magnetic moment plot (Figure 11b) shows a typical sigmoid curve with a weak hysteresis (coercive field, Hc = 60 Oe), which establishes the fact that the Cu2+ ions couple ferromagnetically at low temperature. This was confirmed

4. CONCLUDING REMARKS Thus, the work that has been presented in this paper has successfully demonstrated how variation in simple reaction conditions can be used to manipulate crystal engineering in POM-based inorganic−organic hybrids. Variation in simple parameters like pH and temperature of synthesis has been successfully utilized to develop various types of hybrids where we have arrived at low band gap semiconductors by joining insulators through a transition metal. The inclusion of Cu in the organic chain was found to play a crucial role in the stability of the hybrids and in the determination of the overall structural, electronic, optical, and magnetic properties of this material. We have also resolved some important aspects of structural L

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Inorganic Chemistry disordering in POM-based hybrids. This paper is the first, to the best of our knowledge, to discuss the common Keggin ion disorder in detail. The subtle structural coordination of the core Si atom has been properly resolved and shown to be of tetrahedral geometry, through analysis with a combination of spectroscopic techniques like Raman, IR, and XPS and theoretical calculations. Our work also signifies the way spectroscopy can be used to determine the local geometry of atoms in such complex hybrid synthesis. While giving valuable information regarding structure and electronic properties, our DFT calculations support stable tetrahedral geometry of (SiO4)4− in disordered POM-based hybrids. Finally, our identification of the synthetic strategies here to control the optical band gap of POM-based hybrid structures will find many applications in the field of semiconducting devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02718. The CCDC reference numbers for the compounds 1a, 1b, 2, and 3 are 1047478, 1047479, 1047480, and 1047481, respectively. Selected bond distances in all compounds studied; the assignments of peaks in Raman, IR, and XPS; magnetic data and structural parameters before and after relaxation in the DFT calculations; figures of coordination environments of Si, Mo and Cu; selected Raman, IR, and XPS spectra; and structure and simulated XRD patterns of compound 1a before and after relaxation in the DFT calculation (PDF) Crystallographic information for 1a (CIF) Crystallographic information for 1b (CIF) Crystallographic information for 2 (CIF) Crystallographic information for 3 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 080-22082998. Fax: 080-22082901. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Jawaharlal Nehru Centre for Advanced Scientific Research, Sheikh Saqr Laboratory, and Department of Science and Technology (DST), India, for financial support. S.R. and R.D. thank University Grants Commission, India, and S.S. thanks the Council of Scientific and Industrial Research (CSIR), India, for the research fellowship. U.V.W. acknowledges support from JC Bose National Fellowship, C.N. thanks Sheikh Saqr Laboratory, JNCASR, for Senior Fellowship and S.C.P. thanks DST for the Ramanujan fellowship (Grant SR/S2/RJN-24/ 2010). We are grateful to Prof. C.N.R. Rao for his constant support and encouragement.

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DOI: 10.1021/acs.inorgchem.5b02718 Inorg. Chem. XXXX, XXX, XXX−XXX