Vibrational and Electronic Spectroscopy of Neutral Antimony

Article pubs.acs.org/JPCA

Vibrational and Electronic Spectroscopy of Neutral Antimony Coordination Compounds of the 1,3-Dithiole-2-thione-4,5-dithiolate (dmit) Laura J. S. Lopes,† Antonio C. O. Guerra,† Nadia M. Comerlato,† Cássia C. Turci,† and Glaucio B. Ferreira‡,* †

Instituto de Química, Universidade Federal do Rio de Janeiro, CP 68563, 21945-970 Rio de Janeiro, RJ, Brazil Instituto de Química, Departamento de Química Inorgânica, Universidade Federal Fluminense, 24020-150 Niterói, RJ, Brazil

‡

S Supporting Information *

ABSTRACT: The S 1s X-ray absorption near edge structure (XANES) and X-ray photoelectron spectra (XPS) of the neutral complexes [SbL(dmit)] (L = Br or I; dmit =1,3-dithiole-2thione-4,5-dithiolate) have been measured using tunable synchrotron radiation. The valence shell electronic excitation by ultraviolet−visible (UV−vis) spectroscopy and the infrared vibrational spectra are presented and analyzed. The UV−vis results lead to an assignment of bands at 400 nm as πSm → π*CS, where Sm is the thiolate sulfur. The corresponding S 1s → π*CS transition was identified at 2468.3 eV. Ab initio calculations, within the improved virtual orbital (IVO) method, carried out with the GSCF3 program, were applied to establish a complete and accurate spectral assignment. It has been the first attempt to apply such methodology for dmit coordination compounds, and very consistent results were obtained.

1. INTRODUCTION Inorganic complexes with 1,3-dithiole-2-thione-4,5-dithiolate (dmit) as a ligand have been studied extensively over the past decades because of their unusual properties.1 Particular attention has been paid to complexes with nickel and palladium2−4 due to their potential electrical conductivity and even superconductivity. Over the past decade a number of inorganic and organometallic complexes of Sb(III) and Sb(V) have also been characterized. In contrast to Sb(V), Sb(III) ionic complexes exhibit interanion Sb---S interactions that result in polymeric structures in the solid state. Examples of these are the bis-chelates [Q][Sb(dmit)2] (Q = NBu4+, PPN+, Ph4As+, Me3Te+, (C5H5)2Co+), which present the Sb(III) as pentaor hexacoordinated centers as a result of the Sb---S interanionic interactions.7−13 The same structural feature was observed in the neutral organometallic Sb(III) compound, [SbPh(dmit)(THF)]14 (THF = tetrahydrofuran). In this case, the Sb(III) centers acquire 6coordination as a consequence of two Sb---S intermolecular interactions and a Sb−O bond due to the coordination of one THF molecule. As reported by Comerlato and co-workers, the inorganic complex [SbBr(dmit)(THF)] presents a molecular structure very similar to its organometallic analog.15 However, for this inorganic compound, the oxygen from the THF is closer to the Sb(III) center than in the [SbPh(dmit)(THF)] complex. The greater electronegativity of Br compared to Ph acts as a more effective electron acceptor for electron donation from Sb, thereby resulting in a stronger bond with the oxygen. Despite such efforts to understand the structural features of this class of dithiolate complexes, only recently have detailed © 2012 American Chemical Society

experimental−theoretical studies been performed on the electronic structure of a series of Zn(II), Sb(III), and Bi(III) dmit chelates.5,6,11,16,17 Careful analysis of experimental infrared, Raman and UV−vis spectra of these compounds has been supported by density functional (DFT) calculations, although many details of the electronic structure still have to be investigated. Solomon and co-workers have been using sulfur K-shell (1s) photoabsorption spectroscopy to study the covalent character of transition metal−thiolate bonds, in both anionic chelates and bridging sulfides, which are models for biological systems.18−22 The assignments have been established using DFT calculations. X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge structure (XANES) spectroscopy, using synchrotron radiation, have also been used to study thiols chemisorbed on metallic surfaces.23−25 Finally, polypyrrole systems, doped with [NEt4]2[Ni(dmit)2], have been studied recently using innershell photofragmentation techniques at the S 1s edge.26,27 This work presents a detailed analysis of the electronic structure at valence and inner shells of two neutral dithiolates complexes, the [SbBr(dmit)] and [SbI(dmit)] compounds (Scheme 1). Infrared, UV−vis, and S 1s photoabsorption and photoemission spectroscopies have been used. Received: September 14, 2011 Revised: February 2, 2012 Published: February 3, 2012 2244

dx.doi.org/10.1021/jp208881g | J. Phys. Chem. A 2012, 116, 2244−2260

The Journal of Physical Chemistry A

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Scheme 1. Chemical Structures

The XPS measurements were carried out with 2800 eV energy, and a full width at half-maximum (fwhm) of the photons of 0.95 eV using the InSb(111) crystal. The XANES and XPS spectra were analyzed with a least-squares curve-fitting routine based on the BAN and Bgauss programs.30,31 Details are discussed in the Results and Discussion. 2.3. Calculations. The calculations were carried out for the neutral complexes, [SbL(dmit)] (L = Br or I), considering them as noninteracting independent units. The vibrational experimental spectra were evaluated individually. Although intermolecular interactions Sb---S and weaker S---S contacts in [SbBr(dmit)THF] and [NEt4][Sb(dmit)2] have been confirmed by Xray crystallography, these interactions were neglected in the present work, so the complexes were considered solely as isolated units.7−10,32−34 The same approach was employed in a previous theoretical work on vibrational and UV−vis spectroscopy of a series of anionic dmit and dmio (1,3-dithiole-2-one-4,5-dithiolate) complexes, and good agreement between calculated and experimental data5,6 was obtained. For geometry optimization, the density functional theory method (DFT-B3LYP) in the GAMESS program35 was used. For the spectroscopy calculations we have used the double-ζ 631++G** basis set in the carbon, sulfur, and bromine atoms. For the central metal and iodine atoms, the basis set was the SBKJC,36 with p and d functions. All calculations have been optimized from several initial geometries, to guarantee global minima energy structures. After this procedure, the vibrational calculations were performed. Prior experience with this approach and the B3LYP functional37 shows most of the frequencies are within 0.96 and 1.00 of experimental values. No imaginary modes were observed. All calculated normal modes were visualized using the graphical Chemcraft38 and Molekel 4.339 programs. The analysis of the molecular orbitals was carried out using the Mulliken and the coefficient square population analysis and a graphical analysis with the Chemcraft and GaussSum40 programs. The information of the molecular orbitals was also evaluated through the density of states (DOS) spectra and orbital overlap population (OPDOS), using the GaussSum program. Thus, the metal− sulfur and metal−halogen interactions in each molecular orbital were described as being bonding, nonbonding or antibonding. The calculations of the transition energies and the oscillator strengths in the UV−vis spectra of the optimized structures were carried out using the time-dependent (TD) method, implemented in Gaussian 0341 with the transition moment calculation based on DFT-B3LYP orbitals. Evaluation of the theoretical methods was accomplished using the first 40 lowest energy states. The analysis of the TD-B3LYP states and the spectra simulation were carried out with the GaussSum program, using Gaussian functions with half-widths of 3000 cm−1. The incorporation of the solvent effect in the TD method, using the polarizable conductor calculation model (CPCM), was also carried out with MeCN, MeOH, THF and DMSO. Ab initio calculations have also been performed to corroborate the analysis of the S 1s XANES spectra for antimony complexes. Kosugi’s GSCF3 package, which is highly optimized for calculating core-excited states,42 has been used. This program is based on the improved virtual orbital (IVO) method of Hunt and Goddard43 and explicitly includes the core hole in the Hartree− Fock Hamiltonian. The GSCF3 calculations have been carried out on the optimized structures using the Gaussian type extended basis sets of Huzinaga et al.44 A high-quality basis set has been used

2. EXPERIMENTAL AND COMPUTATIONAL STUDIES 2.1. Compounds. [SbBr(dmit)] and [SbI(dmit)] complexes were synthesized using a literature procedure that consists in reacting SbBr3 or SbI3 with Sn(CH3)2(dmit).15 The physical properties, melting point, and elemental analysis of both complexes are in agreement with published values.15 Elemental analysis (CHN) was carried out in a Perkin-Elmer CHN 2400 spectrometer. The melting points were measured on a Melt-temp-II apparatus. The following results were obtained: [SbBr(dmit)]; mp 242−244 °C. Anal. Found: C, 9.7%. Calc for SbBr(C3S5): C, 9.0%. [SbI(dmit)]; mp 237−239 °C. Anal. Found: C, 8.9%. Calc for SbI(C3S5): C, 8.1%. Infrared spectra between 4000 and 150 cm−1 of the compounds in CsI pellets were measured at room temperature, on a Nicolet Magna-IR 760 FTIR instrument, with 4 cm−1 resolution. The mirror velocity was 0.6 cm−1/s−1, 64 scans, beam splitter KBr, aperture of 90 and sample gain of 2.0 between 4000 and 600 cm−1. The same conditions were applied for wavelengths between 600 and 150 cm−1, except that 128 scans and a solid substrate beamsplitter were used. The UV−vis spectra of the neutral complexes were acquired between 900 and 190 nm using a Varian-Cary 1E spectrometer. The samples were measured in very dilute solutions [ca. 1.0 × 10−4 M] in acetonitrile (MeCN), tetrahydrofuran (THF), methanol (MeOH), and dimethyl sulfoxide (DMSO) using 1.0 cm quartz cells. The spectra were deconvoluted using Gaussian fits with the Origin 5.0 program. 2.2. XANES and XPS. The experiments have been performed using the Soft X-ray spectroscopy beamline (SXS)28 at the Brazilian National Laboratory of Synchrotron Light (LNLS, Campinas-São Paulo). The SXS double-crystal monochromator operates in the 790−4000 eV spectral range, with three pairs of crystals, Si(111), InSb(111), and beryl (1010). The best photon flux was obtained using the InSb(111) crystal with an energy resolution of ∼2.8 eV at 4 keV. The spot size on the sample was 2.0 × 3.0 mm2. The S 1s X-ray absorption near edge structure (XANES) spectra of the solid samples have been acquired for all compounds using the total electron yield (TEY) mode. The solid samples were placed on carbon sticky tape as a uniform thin powder layer. The energy calibration was carried out using the Mo L3 transition of molybdenum metal at 2520 eV.29 Multiple spectra were recorded to confirm reproducibility and to check the stability of the samples. The spectra presented here were recorded using acquisition methods which kept the dose at a level where no radiation damage or photodissociation occurred. Long range (0−1000 eV) X-ray photoelectron spectra (XPS) of the solid samples were measured to confirm their chemical composition. Fine scale S 1s spectra were also acquired. The energy calibration of these spectra was performed by taking the well-known value for the metallic silver M5,4 transition at 368/ 374 eV and for the metallic gold M4 transition at 2291 eV.29 The chamber pressure was 2 × 10−8 mbar during the measurements. 2245



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Table 1. Comparison of Calculated (ab Initio) and Experimental (X-ray) Geometrical Parameters (Å, deg) for [SbL(dmit)] (L = Br or I) geometric parameter (X = Br or I)

experimental15 [SbBr(dmit)]

theoretical [SbBr(dmit)]

theoretical [SbI(dmit)]

SbX (Å) SbS (Å) SC (Å) CS (Å) SSbS (deg) XSbS (deg)

2.5833(3) 2.4723(7)−2.4936(6) 1.719(3)−1.754(3) 1.671(3) 87.44(2) 96.566(19)−97.680(18)

2.5766 2.4852 1.7667−1.7693 1.6430 87.60 99.91

2.9148 2.4846 1.7655−1.7684 1.6420 87.42 102.22

Figure 1. Infrared spectra of [SbL(dmit)] (L = Br or I): (a) 4000−600 cm−1; (b) 600−150 cm−1.

for the core-excited molecule. A basis set of (3112121/5111/ 1*1*) has been used on the core excited sulfur atom (1*, ξd = 0.659 and 0.183), expanded from (5321/521). Additional tight functions, denoted by 1+, account for sulfur orbital contraction upon core hole creation.45 Without these tight basis functions the orbital relaxation energy upon core hole creation was not sufficiently taken into account and the excitation and ionization energies were overestimated. The contraction schemes were (31121211 + 1+/51111+/1 + 1+1*1*) with (1+, ξs = 182.0, 16.0; ξp = 33.0; ξd = 6.4, 2.1). A double-ζ basis set of (621/421) was used for carbon, (43321/4321/31/1*) with (ξd = 0.389) for the bromine atom, (43321/4321/31) with (ξd = 0.266) for the iodine atom, and (433321/43321/421/1*) with (ξd = 0.211) for the antimony atom. Diffuse functions were not included because Rydberg orbitals are not discussed in the present work. Simulated spectra have been generated from these calculations through a Gaussian line shape for each theoretical excitation, using the program SIMILE2.46 The full widths at halfmaximum have been set as follows: 3 eV for orbitals of eigenvalue ε < 0 eV, 10 eV for 0 eV < ε < 8 eV, and 20 eV for ε > 8 eV, which correspond to the estimate experimental resolution for the discrete transitions. This procedure also helps to simulate the shape of the experimental spectra, considering the

three different chemical environments for the sulfur in the studied dmit compounds: the thiolate (Sm), the thiole (Sa), and the thione (St) groups. The analysis of the molecular orbitals was carried out using a coefficient square population analysis and a graphical analysis with the G3PLOT program.42

3. RESULTS AND DISCUSSION 3.1. General Information. [SbL(dmit)] (L = Br or I) belong to the Cs symmetry group. The coordination chemistry of the chelating dmit is very vast and covers various metal ions, coordination geometries, and crystalline arrangements. The bis(dmit) anionic complexes of Zn(II)47,48 and Cu(II)49 present tetrahedral coordination spheres; quadratic plane for Ni(II),4 and octahedral or bipyramidal for compounds with Sb(III) and Bi(III).7 The crystalline packing for these systems is strongly influenced by the counterions and there are no interactions between the anion and the cation. However, in many complexes, there are very important interanionic π---π interactions and in the most of the cases they are responsible for the conduction properties in these materials. On the other hand, structural optimization of these complexes presents a nice theoretical−experimental correlation for the isolated units that define the intramolecular parameters. With this approach we were very successful in achieving a 2246



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Figure 2. Computed infrared spectra of [SbL(dmit)] (L = Br or I): (a) 4000−600 cm−1; (b) 600−150 cm−1.

Table 2. Observed and Calculated Frequencies of the [SbBr(dmit)] Complex experimental IR(s) (cm−1)

optimized IR (cm−1)

IIRa

symm

1435 1015 997

1522 1091 991

9.7646 420.4282 1.6465

a′ a′ a″

vCC vCS v(CeSa) + v(CeSm)

897

882

30.8365

a′

v(CeSm)

880

860

5.2969

a″

v(CtSa)

783

764

9.3524

a″

v(CeSa) + v(CeSm) + v(CtSa)

534

502

22.5990

a′

vCS + v(CtS)

463

469

17.2762

a′

ω(SCtS) + v(CeSa) + v(CeSm)

439

449

2.9133

a′

ω(SCtS)

392 369

415 387

0.2097 0.6998

a″ a″

τ(SCCS) ρ(SCtS) + τ(SCCS)

350

353

13.6357

a′

vSbS + σSCtS

328 317

345 343

22.8570 0.2591

a′ a″

vSbS + χ(SCCS) vSbS + σSaCeSm + σSCtSa

265 250 196 168

268 254 217 178 156 121 110 67 65 25

2.6281 0.4579 48.6263 7.5439 0.3594 2.0341 0.3001 0.4304 0.4949 0.4217

a″ a′ a′ a′ a″ a′ a″ a″ a′ a′

vSbS + σSCS χ(SCCS) vSbBr σ(SSbS) ρ(SCS) + ρ(SSbS) ω(SSbS) + χ(SCCS) τ(SSbS) + τ(SCCS) σ(SSbBr) + τ(SCCS) χ(SCCS) σ(SSbBr) + χ(SCS)

mode description (assignment)b

Calculated infrared intensities (IIR) are in Debye2 Å−2 amu−1. bν: stretching mode. σ: scissoring mode. τ: twisting mode. ω: wagging mode. ρ: rocking mode. χ: out-of-plane. Ce = carbon of ethylen, Ct = carbon of thione, Sm = sulfur of thiolate, Sa = sulfur of thiole, St = sulfur of thione.

a

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Table 3. Observed and Calculated Frequencies of the [SbI(dmit)] Complex experimental IR(s) (cm−1)

optimized IR (cm−1)

IIRa

symm

1430 1013 993

1514 1092 991

3.4434 430.8496 1.5736

a′ a′ a″

vCC vCS v(CeSa) + v(CeSm)

895

882

26.3056

a′

v(CeSm)

877

861

5.0033

a″

v(CtSa)

782

765

10.2607

a″

v(CeSa) + v(CeSm) + v(CtSa)

532

501

22.2787

a′

vCS + v(CtS)

391 463

475 468

0.0001 14.9928

a″ a′

τ(SCCS) ω(SCtS) + v(CeSa) + v(CeSm)

436

450

2.7920

a′

ω(SCtS)

366

393

0.8992

a″

ρ(SCtS) + σ(SCCS)

346

354

20.1187

a′

vSbS + σSCtS

316

345

0.2614

a″

vSbS + σSaCeSm + σSCtSa

325 263 248 165 154

343 266 258 176 159 154 110 110 63 55 21

31.0983 2.5584 0.3144 10.4624 30.4398 0.4151 0.0966 1.0728 0.9057 0.3530 0.4485

a′ a″ a′ a′ a′ a″ a″ a′ a′ a″ a′

vSbS + χ(SCCS) vSbS + σSCS χ(SCCS) σ(SSbS) vSbI ρ(SCS) + ρ(SSbS) τ(SSbS) + τ(SCCS) ω(SSbS) + χ(SCCS) χ(SCCS) σ(SSbI) + τ(SCCS) σ(SSbI) + χ(SCS)

mode description (assignment)b

Calculated infrared intensities (IIR) are in Debye2 Å−2 amu−1. bν: stretching mode. σ: scissoring mode. τ: twisting mode. ω: wagging mode. ρ: rocking mode. χ: out-of-plane. Ce = carbon of ethylene, Ct = carbon of thione, Sm = sulfur of thiolate, Sa = sulfur of thiole, St = sulfur of thione.

a

ture. The Sb−S and Sb−Br bonds obtained from the crystallographic data have been described correctly by the density functional method. Also, the dmit ligand structure has shown great theoretical−experimental agreement, with bond length deviations less than 0.02 Å. This comparison shows that the optimized geometry is valid even though the intermolecular interactions in the solid state were ignored in computing the geometry. Replacing bromine for the iodine atom does not change significantly the structural parameters of the Sb−S bonds and the coordination geometry around the Sb(III). These facts have been confirmed by typical vibrational normal mode analysis of the ligand structure. The Sb−S stretching modes in the far-infrared are described in sections 3.2 and 3.3. 3.2. Experimental Infrared Vibrational Spectra. The infrared vibrational spectra of solid [SbL(dmit)] (L = Br or I) are presented in Figure 1. The first statement from a preliminary analysis of the infrared spectra of the dmit neutral complexes is the direct observation of the main bands that characterize the dmit ligand in the 4000−600 cm−1 region. This case is simpler than that of [Q][Sb(dmit)2] and [Q][Bi(dmit)2] complexes because some vibrational modes of the organic cations do not overlap the dmit ones.7−9 Thus, in the neutral complexes, the bands at 1435 and 1015 cm−1 are the main bands that characterize the dmit ring and were assigned as CC and CS stretching. There is also a shoulder at 995 cm−1 and a band of smaller intensity at 897 cm−1, which are identified as CS stretching. Other bands of smaller intensity are identified, without any accurate information about assignment. In the farinfrared region bands at 463 cm−1 and between 350 and 250 cm−1 are assigned as a dmit ring breathing and SbS stretching modes, as previously assigned for the dmit salts.5,6

complete and precise assignment of the vibrational and electronic spectra for a group of bis- and tris(dmit) complexes of Zn(II), Sb(III), and Bi(III).6,11 The literature reports few attempts to assign the electronic spectra of main group dmit and dmt (1,2-dithiole-3-thione-4,5-dithiolate) compounds, [Q]2[M(dmit)2] and [Q]2[M(dmt)2] (M = Se, Te or Bi, Q = NBu4+, PPN+, Ph4As+, Me3Te+, (C5H5)2Co+), but this task was made without computational support.10 The π → π* transitions in the dmit complexes were reported to occur between 273 and 246 nm and the M(II) ← S charge-transfer bands between 390 and 300 nm. However, for the Na2(dmit) compound some intense bands at lower energy, between 473 and 402 nm, were assigned as the same π → π* transitions. The S 1s studies of compounds of the thiolate class include several transition metal complexes, but there is little spectral assignment information about them in the literature.23−25 The S 1s spectra of thiolates are characterized by an intense pre-edge structure, which has been interpreted using DFT as a means to estimate the degree of covalence of the metal-S bond.18 Although comparison to the spectroscopy of monothiolate systems helps, it is not sufficient to interpret the more complex dithiolates, as in the case of dmit complexes, because they contain three different chemical environments of the sulfur atoms: the thiolate, thiole, and thione groups. Detailed assignments of the inner shell electronic spectra of the dmit ligands in these Sb compounds, validated by theoretical studies, is the core of this work. First we have evaluated the quality of the computationally optimized geometric parameters for the neutral complexes by comparison with structures from X-ray diffraction studies.15 Table 1 presents the geometric parameters obtained with the DFT-B3LYP method, in comparison to the experimental struc2248



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Figure 3. UV−vis spectra (190−800 nm) of dilute (0.1 mM) solutions of [SbL(dmit)] (L = Br or I) in THF, DMSO, MeCN, and MeOH solvents.

Another important observation for the two neutral dmit complexes was the identification of several overtones and combination bands. For instance, for the [SbI(dmit)], the bands at 2026 and 1790 cm−1 were classified as the overtones of 1013 and 895 cm−1. The bands at 2006, 1895, 1361, and 1054 cm−1

were assigned as combination bands of 1013 and 993, 1430 and 463, 1013 and 346, and 895 and 165 cm−1, respectively. 3.3. Calculated Infrared Vibrational Spectra. On the basis of a Cartesian representation of the [SbL(dmit)] (L = Br or I) with Cs symmetry, 24 fundamental vibrational modes were 2249



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above the experimental data. Between 1080 and 500 cm−1, the modes were overestimated by 1−3%. Below 500 cm−1, the modes were overestimated between 4 and 10%. A correction factor was not applied to the computed frequencies because it could be misleading and induce mistakes in the assignments.5,6,11 The experimental and theoretical data displayed in Figures 1 and 2 and Tables 2 and 3 indicate there is a great similarity between the two neutral complexes. The deviation in some modes was limited to a few cm−1. Thus, fundamental modes of the ligand were completely characterized. For example, for the [SbBr(dmit)] compound, the CC stretching mode, calculated as 1522 cm−1, was compatible with other results already published with this methodology.6−11 The calculations for the neutral complexes, considering a single dmit ring, have the great advantage of simplifying the CS and CS stretching vibrational assignments in the region between 1100 and 600 cm−1. For the [SbL(dmit)] (L = Br or I) complexes, the intense vCS band was observed experimentally at 1013/1015 cm−1 and calculated at 1091 cm−1. There is a shoulder at 997/993 cm−1, characterized as vCS and calculated at 991 cm−1, also a group of vCS experimental bands at 897 and 783 cm−1, and a shoulder at 880 cm−1. These modes were precisely calculated by the DFT method and located at 882, 764, and 860 cm−1, respectively. Finally, the band assigned as a ring breathing mode at 532 cm−1, is in reasonable agreement with the computed value at 502 cm−1 Moreover, the calculated frequency of the SCtS out-of-dmit-planar angular deformation mode, which is coupled to the CS stretching, is very similar to the experimental value. These similarities confirm that when the halogen is replaced, there are no significant changes in the fingerprint region of the dmit ring. The bands between 350 and 240 cm−1 were similarly identified among the experimental bands for both complexes. Two bands of strong intensity at 328/325 and 265/262 cm−1 were compatible with the calculated modes at 345/343 and 268/265 cm−1. In this region, the SbS stretching modes are coupled to the angular deformation modes of the ring. This corroborates the structural similarity obtained from the theoretical calculations and suggests a coordination sphere with no significant changes caused by the substitution of the bromine by the iodine. In that way, this can be characterized according to the variation of the SbBr and SbI stretching. The spectrum of [SbBr(dmit)] presents an intense band at 196 cm−1, calculated at 217 cm−1 and assigned as a SbBr stretching. On the other hand, the [SbI(dmit)] complex presents an intense band at 154 cm−1, calculated at 159 cm−1 and assigned as a SbI stretching. 3.4. Experimental UV−vis Spectra. The UV−vis spectra of the neutral complexes are presented in Figure 3. They were acquired in acetonitrile (MeCN), tetrahydrofuran (THF), methanol (MeOH), and dimethyl sulfoxide (DMSO) solutions. The solutions of [SbL(dmit)] (L = Br or I) in MeCN, THF, or MeOH present intense yellow colors. The spectra do not exhibit significant bathochromic shifts. For example, a strong absorption occurs at 378/381/392 nm for [SbBr(dmit)] and at 377/386/404 nm for [SbI(dmit)] in THF/MeOH/MeCN. However, the DMSO spectra exhibit more pronounced bathochromic shifts, because strong absorption bands occur at 416 nm for [SbBr(dmit)] and at 415 nm for [SbI(dmit)]. Moreover, the spectra of both complexes do not present bands above 500 nm for THF/MeOH/MeCN/DMSO, except for the spectrum of [SbI(dmit)] in DMSO which has a weak, broad band between 400 and 600 nm.

Table 4. Deconvolution Parameters and Molar Absorptivity for the UV−Vis Spectra for [SbL(dmit)] (L = Br or I) in DMSO peak

area

center (nm)

1 2 3 4 5 6

75.1 10.9 13.0 39.7 12.4 2.5

415 340 284 254 230 194

1 2 3 4 5 6 7 8

5.43 1.93 80.0 16.9 16.9 47.2 14.2 2.81

539 498 415 348 288 254 228 194

width (nm)

height (au)

[SbBr(dmit)]a 66.99 1.0539 74.44 0.1376 55.83 0.2189 41.93 0.8889 19.21 0.6063 21.76 0.1089 [SbI(dmit)]b 205.37 0.0248 62.35 0.0291 67.60 1.1120 82.44 0.1931 61.74 0.2564 43.45 1.0216 20.43 0.6514 18.72 0.1409

abs (au)

ε (mol L−1 cm−1)

1.0693 0.1813 0.4661 0.9730 0.9417 0.0564

10693 1813 4661 9730 9417 564

0.0338 0.0689 1.1608 0.2886 0.4971 1.1463 1.0339 0.1459

338 689 11608 2886 4971 11463 10339 1459

a C = 1.00 × 10−4 M for [SbBr(dmit)]. bC = 1.00 × 10−4 M for [SbI(dmit)].

Table 5. Deconvolution Parameters and Molar Absorptivity for the UV−Vis Spectra for [SbL(dmit)] (L = Br or I) in THF peak

area

center (nm)

1 2 3 4 5 6 7

4.74 13.60 13.70 8.99 2.48 1.45 7.82

402 381 363 282 256 249 211

1 2 3 4 5 6 7

0.20 40.90 44.60 11.80 5.33 2.63 3.67

446 380 339 265 257 250 210

width (nm)

height (au)

[SbBr(dmit)]a 59.24 0.0752 39.32 0.3255 56.55 0.2280 76.73 0.1101 20.66 0.1128 10.88 0.1255 23.05 0.3187 [SbI(dmit)]b 26.14 7.2336 46.15 0.8320 90.45 0.4628 35.16 0.3165 13.66 0.3665 7.10 0.3472 20.79 0.3917

abs (au.)

ε (mol L−1 cm−1)

0.2790 0.5544 0.4420 0.1124 0.2442 0.2697 0.3290

5580 11088 8840 2248 4884 5394 6580

0.0195 1.1022 0.5544 0.5366 0.7246 0.7295 0.3877

195 11022 5544 5366 7246 7295 3877

C = 5.00 × 10−5 M for [SbBr(dmit)]. bC = 1.00 × 10−4 M for [SbI(dmit)]. a

defined and according to the irreducible representations; they are assigned as 14 a′ modes and 10 a″ modes, all of them active in the infrared region. The Hessian matrix calculated for the geometry of the neutral complexes in the Cs symmetry did not reveal imaginary frequencies. The results are presented in Tables 2 and 3, accompanied by assignments established from the graphical mode analysis and comparison of the computed and experimental vibrational spectra. The theoretical spectra are presented in Figure 2. Comparison shows that deviations of the DFT results from experiment are very small but depend on the type of mode. For example, the values for the CC stretching mode are 6% 2250



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Table 6. Koopmans’ Energy (eV), Mulliken Population Analysis, and Assignment for the Frontier Orbitals of [SbL(dmit)] (L = Br or I), with the B3LYP Methoda orbital

energy (eV)

[SbBr(dmit)] Mulliken population

orbital

energy

81 a′

0.72

σ*CtSaCe [Sa 39.1% (Ryd-s), Ct 32.1% (Ryd-s), Ce 22.1% (Ryd-s)]

67 a″

0.78

80 a″ 79 a′

0.63 0.47

Ryd-s, Ryd-p Ce 97.7% (σ*CC) σ*SCS [Sa 19.1% (Ryd-s), Ct 29.6% (Ryd-p), St 47.5% (Ryd-s)]

66 a″ 65 a′

0.60 0.44

78 a″

−0.09

σ*CtSaCe [Ce 32.8% (Ryd-s), Sa 31.0% (Ryd-s), Ct 20.4% (Ryd-s)]

64 a′

−0.07

77 a′ 76 a′

−0.21 −0.34

Ryd-s Ce 86.9% σ*SmCe [Sm 14.4% (3p), Ce 60.2% (2s, Ryd-s)]

63 a″ 62 a′

−0.28 −0.44

75 a″

−1.08

π*CC [Ce 56.4% (2p, Ryd-p), Sa 29.9% (Ryd-s, Ryd-p)]

61 a″

−1.17

74 a′

−1.44

σ*SaCt and σ*SaCe [Sa 15.8% (3p), Ct 62.7% (Ryd-s), Ce 13.2% (2s,2p) ]

60 a′

−1.53

73 a′

−2.38

59 a′

−2.48

72 a′

−2.47

π*CS [Ct 19.8% (2p), St 8.3% (3p)], Sb 20.0% (5p), Ce 21.3% (Ryd-s), Sm 17.6% (Ryd-s, Ryd-p) σ*Sb−Sm [Ce 29.3% (Ryd-s), Sb 25.8% (5p), Sm 25.8% (Ryd-s, Ryd-p)].

58 a′

−2.55

71 a′

−2.79

σ*SbBr [Sb 41.4% (5s, 5p), Br 14.8% (4p)]

57 a″

−3.35

[SbI(dmit)] Mulliken population σ*SaCe [Sa 16.1% (Ryd-s), Ce 75.4% (Ryd-s, Ryd-p)] Ryd-s, Ryd-p Ce 97.6% (σ*CC) σ*SCS [Sa 32.7% (Ryd-s), Ct 26.6% (Ryd-p), St 28.8% (Ryd-s)] σ*CtSaCe [Ce 39.1% (Ryd-s, Ryd-p), Sa 25.6% (Ryd-s), Ct 16.3% (Ryd-s)] Ryd-s Ce 90,6% σ*SmCe [Sm 12.6% (3p), Ce 58.3% (2s, Ryd-s)] π*CC [Ce 63.9% (2p, Ryd-p), Sa 26.0% (Ryd-s, Ryd-p)] σ*SaCt and σ*SaCe [Sa 16.9% (3p), Ct 61.4% (Ryd-s), Ce 16.0% (2p)] π*CS [Ct 26.0% (2p), St 11.7% (3p)], Sb 15.6% (5p), Ce 18.3% (Ryd-s) σ*Sb−Sm [Ce 32.6% (Ryd-s), Sb 29.4% (5s, 5p), Sm 24.1% (Ryd-s, Ryd-p)]. σ*SbSm [Sb 7.1% (5p), Sm 24.2% (Ryd-s)], Ce 66.3% (Ryd-s) σ*SbI [Sb 50.1% (5s, 5p), I 21.4% (5p)]

70 a″

−3.21

σ*SbSm [Sb 13.20% (5p), Sm 37.6% (Ryd-s)], Ce 48.3% (Ryd-s, 2s)

56 a′

−3.40

69 a′

−6.24

πSm 25.9% (3p), πSa 25.6% (3p), π CC 23.5% (2p), πSt 17.5% (3p)

55 a′

−6.32

πSm 24.8% (3p), πSa 24.9% (3p), πCC 22.8% (2p), πSt 17.9% (3p)

68 a″

−6.71

πSt 16.7% (3p), πCC 12.2% (2s,2p), Sa 51.1% (Ryd-s)

54 a″

−6.80

πSt 24.8% (3p), Ce 22.8% (Ryd-s, Ryd-p), Sa 24.9% (Ryd-s), Sm 17.9% (Ryd-s) ns I 71.2% (5p)

67 a′

−7.65

πSm 37.2% (3p), πSt 18.9% (3p), πSa 14.8% (3p), πCC 12.2% (2s,2p)

53 a′

−7.55

66 a″

−7.93

πSm 33.8% (3p), πSa 30.2% (3p), Ce 30.7% (Ryd-s)

52 a′

−7.61

65 a′ 64 a″

−8.17 −8.54

ns Br 39.2% (4p), Sb 13.20% (5s,5p), Sm 23.4% (Ryd-s) ns Br 5.5% (4p), Ce 79.8% (Ryd-s, 2s)

51 a″ 50 a″

−7.66 −8.13

63 a″

−9.17

πSm 17.2% (3p), πSa 19.9% (3p), Ce 61.8% (Ryd-s, 2s,2p)

49 a′

−8.85

62 a’

−9.19

σSbBr [Sb 32.7% (5p), Br 37.8% (4p)]

48 a″

−9.33

a

πSm 31.7% (3p), πSa 15.1% (3p), πSt 19.3% (3p), I 15.60% (5p) ns I 11.1% (4p), Ce 75.3% (Ryd-s, 2s) πSa 19.4% (3p), πSm 12.2% (3p), Ce 67.3% (Ryd-s) σSbI [Sb 42.0% (5p), I 19.7% (5p)] πSm 16.4% (3p), πSa 16.9% (3p), Ce 65.8% (Ryd-s)

Ce = carbon of ethylene, Ct = carbon of thione, Sm = sulfur of thiolate, Sa = sulfur of thiole, St = sulfur of thione.

Figure 3 presents the spectra acquired in methanol and acetonitrile. They have low resolution, caused by the low solubility of the compounds in these solvents. Consequently, it was not possible to obtain accurate molar absorptivity values. However, this problem was not observed for THF and DMSO. The spectra in THF and DMSO were analyzed with Gaussians curves. The fit parameters and the molar absorptivities (L mol−1 cm−1) are presented in Tables 4 and 5. The molar absorptivity values are compatible with the experimental values for dmit anionic complexes with antimony, bismuth, and zinc, regarding the πSm → π*CC transitions.16,17 The neutral complexes present simpler spectra compared to those of the anionic complexes, because a group of bands between 200 and 260 nm is better defined. However, the UV−vis spectra do depend on the solvent. For example, with DMSO, two bands between 240 and 260 nm are found. Instead, with THF, methanol, and MeCN, an envelope of bands of smaller intensity is observed. 3.5. Molecular Orbitals for the [SbL(dmit)] (L = Br and I). The ground states of both complexes present many occupied molecular orbitals, distributed between −6 and −10 eV below the vacuum level. Table 6 presents the DFT orbitals, the Koopmans’ energy, and the Mulliken analysis of the occupied orbitals. Plots of a low energy occupied and virtual frontier

orbitals in the ground state of [SbL(dmit)] (L = Br or I) are presented in Figure 4. The Koopmans’ energy difference between the highest occupied and the lowest unoccupied orbitals (the HOMO−LUMO gap) was 3.03 eV for [SbBr(dmit)] and 2.92 eV for [SbI(dmit)]. For both complexes, the HOMO orbital (a′ symmetry) is characterized by delocalized π/nonbonding sulfur thiolate (Sm) orbitals (∼25%) and thiole (Sa) orbitals (∼26%) with ∼24% πCC character. The LUMO orbitals reflect the chemical differences between these complexes. [SbBr(dmit)] has a σ*SbSm (a″) as its LUMO at −3.21 eV. For [SbI(dmit)], the LUMO is a σ*SbI (a′) at −3.40 eV whereas the σ*SbSm orbital is 50 meV higher at −3.35 eV. The energies of other orbitals are similar in the two compounds, with energy differences of the order of 0.1 eV. Also, using the diffuse functions, some virtual frontier orbitals have partial Rydberg character. The energy of π*CS (2.38/2.48 eV), σ*SaCt/σ*SaCe (1.44/1.53 eV), and π*CC (1.08/1.17 eV) are identified in the same order as in [Sb(dmit)2]− complex. Ground-state energy level diagrams for both species are presented in Figure 5. The analysis of the molecular orbitals obtained with the CPCM methodology, which incorporates the effect of different solvents, presents some differences of HOMO−LUMO energy in both complexes. However, the order of the orbitals does not 2251



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Figure 5. Self-consistent field (SCF) molecular orbital energy levels of the ground states of [SbL(dmit)] (L = Br or I), within the DFY method using the B3LYP functional.

Figure 4. Orbital representations at the B3LYP level of the HOMO, HOMO−1, LUMO, LUMO+1 [SbL(dmit)] (L = Br or I). The contour values of the orbitals are all 0.035 au.

Figure 6. SCF molecular orbital energy levels of [SbL(dmit)] (L = Br or I), within the DFT method using the B3LYP functional and the CPCM method to take into account the influence of the MeCN solvent.

change. This is clear from analysis of the molecular orbital diagram obtained with the calculation carried out in MeCN, as displayed in Figure 6. The variations in the HOMO−LUMO gap with different solvents was between 3.43 and 3.51 eV. Comparative diagrams are found in Figures 1S and 2S in the Supporting Information. From the final analysis obtained through the density of states it is now possible to rationalize the valence shell electronic structure of [SbL(dmit)] (L = Br and I). These results are shown in Table 7 and Figure 7, where the percentage of the contribution of Sb and halogen orbitals are indicated. The frontier occupied orbitals are dominated by the dmit ligand whereas the virtual orbitals are characterized by major contributions from both the metal and halogen atoms. Analysis of the Sb−L (L = Br or I) and Sb−dmit orbital overlap indicates the virtual orbitals are metal− ligand antibonding in nature, whereas the frontier occupied orbitals are primarily ligand in character. This analysis is important, especially to evaluate the nature of the electronic valence transitions. 3.6. Valence Electronic Transitions in [SbL(dmit)] (L = Br and I). Considering that the major interest was to elucidate the existence of charge-transfer spin-allowed transitions, the cal-

culations were restricted to singlet states. Forty singlet electronic excited states were calculated in the vacuum and with different solvents. All calculations were carried out with the ground-state SCF orbitals without symmetry, to obtain the oscillator strength between ground and excited states. The symmetry states, their energies, and the oscillator strengths of the main calculated transitions in the vacuum are presented in Table 8 for [SbL(dmit)] (L = Br and I). The results for the forty calculated excited states are displayed in Tables S1−S3 as Supporting Information, along with the detailed calculation descriptions. The simulated spectra in the vacuum, with different solvents, are presented in Figure 8. The characteristic multireference states observed in the anionic complexes are also found with lower intensity in the neutral complexes. This is confirmed by analysis of the coefficients of the dominant configuration. Comparing the experimental results to the calculated excitations, a variation around 50 nm was observed in the first large intensity band of the UV−vis spectra. This transition corresponds to the HOMO → LUMO+3 or πSm → π*CS transition. 2252



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Table 7. Koopmans’ Energy and Percentage of Antimony and Halogen in Density of States of [SbL(dmit)] (L = Br or I) with Cs Symmetry for the B3LYP Method [SbBr(dmit)] orbital 81 a′ 80 a″ 79 a′ 78 a′ 77 a″ 76 a′ 75 a″ 74 a′ 73 a′ 72 a′ 71 a′ 70 a″ 69 a′ 68 a″ 67 a′ 66 a″ 65 a′ 64 a″ 63 a″ 62 a′

L+11 L+10 L+9 L+8 L+7 L+6 L+5 L+4 L+3 L+2 L+1 LUMO HOMO H−1 H−2 H−3 H−4 H−5 H−6 H−7

[SbI(dmit)]

energy (eV)

% Sb

% Br

orbital

0.72 0.63 0.47 −0.09 −0.21 −0.34 −1.08 −1.44 −2.38 −2.47 −2.79 −3.21 −6.24 −6.71 −7.65 −7.93 −8.17 −8.54 −9.17 −9.19

0 0 0 0 1 15 1 0 23 44 50 76 2 1 2 0 17 2 3 32

41 10 10 0 1 1 0 1 0 2 23 3 2 0 8 9 67 78 0 38

67 a″ 66 a″ 65 a′ 64 a′ 63 a″ 62 a′ 61 a″ 60 a′ 59 a′ 58 a′ 57 a″ 56 a′ 55 a′ 54 a″ 53 a′ 52 a′ 51 a″ 50 a″ 49 a′ 48 a″

L+11 L+10 L+9 L+8 L+7 L+6 L+5 L+4 L+3 L+2 L+1 LUMO HOMO H−1 H−2 H−3 H−4 H−5 H−6 H−7

energy (eV)

% Sb

%I

0.78 0.60 0.44 −0.07 −0.28 −0.44 −1.17 −1.53 −2.48 −2.55 −3.35 −3.40 −6.32 −6.80 −7.55 −7.61 −7.66 −8.13 −8.85 −9.33

2 0 0 0 1 15 1 0 14 46 77 50 1 1 4 4 3 1 43 3

0 0 0 0 0 0 0 0 0 2 3 29 6 1 89 18 84 9 23 0

Figure 7. Molecular orbital diagram, density of states diagram (DOS) and orbital overlap population diagram (OPDOS) of the complexes [SbL(dmit)] (L = Br or I), computed with DFT using the B3LYP functional.

3.6.1. [SbBr(dmit)] UV−Vis. The results of the TD calculation in the vacuum, reported in Table 8, predict states of large oscillator strength, localized in two regions, at 348 and 250 nm, compatible with the deconvolution results presented in Figure 3. The first excitations, calculated at 444 and 396 nm, are πSm → σ*SbBr and πSm → σ*SbSm/πSm → π*CS transitions, but these were not observed in the experimental data, probably because they are overwhelmed by the larger molar absorptivity of the πSm → π*CS transition. Figure 8 presents the calculated UV−vis spectra in the four different solvents. There are bands in three different regions at 400, 320, and between 270 and 220 nm. These CPCM results do not depend on the solvent. The πSt → σ*SbSm transition predicted at 320 nm does not appear to have a counterpart in

the experimental data. Also, these calculations were not able to describe the bathochromic shift observed with DMSO for πSm → π*CS transition. However, compared with the vacuum, the energy of this transition is closer to the experimental data, with a difference of approximately 10 nm. 3.6.2. [SbI(dmit)] UV−Vis. The bands between 500 and 200 nm, obtained by deconvolution of the spectrum of [SbI(dmit)] in DMSO and THF solution, are presented in Tables 4 and 5. The calculated states and their respective oscillator strengths are displayed in Table 8. There are significant differences between the computed and experimental results. In the vacuum, three different regions are observed between 520 and 407 nm related to πSm → σ*SbI excitations, 349 nm, which is the πSm → π*CS transition, and the region between 275 and 245 nm, which describes 2253



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Table 8. Main Singlet Transition Energies (eV) and the Oscillator Strength from the Ground State of [SbL(dmit)] (L = Br or I), with the B3LYP Method SbBr(dmit)

SbI(dmit)

state

dominant configuration

wavelength (nm)

oscillator strength

state

dominant configuration

wavelength (nm)

oscillator strength

A′ A′

HOMO → L+1 (79%) HOMO → L+2 (25%) HOMO → L+3 (37%)

444 396

0.0096 0.0326

A′ A′

513 407

0.0420 0.0286

A′

348

0.2383

A′

A′

HOMO → L+2 (19%) HOMO → L+3 (41%) H−3 → LUMO (93%)

318

0.0228

A′

A′

H−4 → L+1 (82%)

279

0.0203

A′

A′

H−5 → LUMO (55%) H−4 → L+2 (18%) H−5 → L+1 (17%) H−3 → L+2 (20%) H−3 → L+3 (30%) H−2 → L+2 (19%) H−2 → L+3 (50%) H−1 → L+5 (18%) H−1 → L+5 (78%) H−3 → L+2 (20%) H−3 → L+3 (50%) H−6 → LUMO (42%) H−4 → L+2 (28%) H−7 → LUMO (72%) H−6 → LUMO (21%) H−4 → L+2 (39%) H−4 → L+3 (33%) H−4 → L+3 (17%) H−2 → L+4 (53%) H−6 → LUMO (19%) H−4 → L+3 (31%) H−2 → L+4 (33%) H−5 → L+2 (51%) H−5 → L+3 (22%) HOMO → L+8 (87%) H−8 → LUMO (20%) H−7 → L+1 (55%) H−2 → L+5 (47%) H−1 → L+6 (28%)

269

0.0140

A′

HOMO → LUMO (88%) H−1 → L+1 (34%) HOMO → L+2 (23%) HOMO → L+3 (34%) H−1 → L+1 (15%) HOMO → L+3 (48%) H−5 → L+1 (16%) H−4 → L+1 (18%) H−3 → LUMO (32%) H−3 → L+2 (21%) H−2 → L+2 (31%) H−3 → L+3 (60%)

266

0.0127

A″

260

0.0713

A′

250 249

0.0975 0.0250

A′ A′

243

0.0424

A″

239 237

0.0204 0.0286

A′ A′

235

0.0116

A′

235

0.0621

234

0.0167

229 217

0.0163 0.0823

215

0.0100

A″

A′

A′ A″ A′ A″ A′

A′ A′

A″ A′ A′ A″

the σSbI → σ*SbI, nsI → σ*SbSm, and πSm → π*CS transitions. The first region is not observed in the experimental results, except for the experimental spectrum in DMSO, which has two low molar absorptivity bands at 539 and 498 nm after deconvolution. The results from CPCM calculations, displayed in Figure 8, describe four regions. However, the intensities are overestimated when compared with the experimental data in Figure 3. The excitations at 477 and 324 nm are related to πSm → σ*SbI and πSt → σ*SbSm transitions. The πSm → π*CS transition at 393 nm presents a better experimental approximation, once this transition is observed experimentally between 381 and 415 nm. The final electronic transition assignments of the UV−vis spectra of both complexes, based on comparison of the experimental data and theoretical results, are summarized in Table 9. 3.7. S 1s Photoabsorption and Photoelectron Spectra. Panels a and c of Figure 9 present the XANES and XPS spectra

̀

0.2612

325

0.0457

275

0.1034

264

0.1081

257

0.0171

252

0.0367

HOMO → L+6 (72%) H−6 → LUMO (42%) HOMO → L+6 (17%) H−5 → L+3 (51%) HOMO → L+7 (16%) H−7 → L+1 (83%) HOMO → L+8 (92%)

248 245

0.0256 0.1042

244

0.0241

238 227

0.0111 0.0126

H−8 → L+1 (19%) H−6 → L+2 (56%)

224

0.0217

H−6 H−5 H−5 H−1

→ → → →

L+1 L+2 L+3 L+5

(17%) (29%) (22%) (91%)

of the sulfur K-shell of [SbL(dmit)] (L = Br or I) complexes. The low energy XPS long scan (0−1000 eV) spectra are represented in Figure 9b and show peaks related to all the elements present in these complexes: bromine (3d, 3p), iodine (4d, 4p, 4s, 3d, 3p), sulfur (2p, 2s), carbon (1s), and antimony (4d, 3d, 3p). The steady-state charging caused by the synchrotron light was 6 and 10 eV for [SbI(dmit)] and [SbBr(dmit)], respectively. This is due to secondary electrons and photons, which usually cause unwanted positive charging in poorly conducting samples or parts of surface heterostructures.50 This surface biasing can be explained by the balance between the electron loss from the surface by photoelectron and secondary emission and electron gain by conduction or acquisition of slow or thermal electrons from the vacuum. The steady-state charge can be minimized by using bombardment of the sample with low energy electrons (or ions) from an external unit (flood-gun).51,52 We decided to confirm this effect through indirect procedure, i.e., 2254



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Figure 8. Simulated UV−vis spectra (190−600 nm) of [SbL(dmit)] (L = Br or I), using the GaussSun program from the TD-B3LYP and TDB3LYP-CPCM calculations.

Table 9. Electronic Transitions Assignment for [SbL(dmit)] (L = Br or I)a SbBr(dmit)

SbI(dmit)

THF

DMSO

MeCN

MeOH

peak

nm

nm

nm

nm

1

402

2

381

415

404

3

363

340

320

4

282

284

5

256

254

THF assignment

peak

nm

πSm → σ*SbBr

1

381

πSm → π*CS

2

446

315

πSt → σ*SbSm

3

380

nsBr → σ*SbBr

4

240

πSm → π*CS

5

πSm → σ*SaCt σSbBr → σ*SbBr

DMSO

MeCN

MeOH

nm

nm

nm

assignment πSm → σ*SbI

539/498 415

392

386

πSm → π*CS

339

348

320

318

πSt → σ*SbSm

265

288

6

257

254

7 8

250 210

228 194

nsI → σ*SbI

πSm → π*CC

a

6

249

230

232

7

211

194

203

221

256 212

250

πSm → π*CS

220

σSbI → σ*SbI σSbI → σ*SbSm

Ce = carbon of ethylene, Ct = carbon of thione, Sm = sulfur of thiolate, Sa = sulfur of thiole, St = sulfur of thione.

mical environments of sulfur atoms in dmit. Two of these environments have IP values within 1 eV, whereas the IP of the third environment is lower by 3 to 4 eV. Unfortunately, the expected 2:2:1 ratio between thiole, thiolate, and thione was not observed in the peak areas determined by Gaussian line shape analysis. It is possible charging may cause broadening of the signals. In contrast to XPS where charging plays havoc with the spectral energy scales, the energy scales of XANES spectra are not affected by charging and thus, with valid calibration, correctly reflect the electronic structure of the S in the dmit ligands of these compounds. The XANES spectra exhibit five identifiable features. The sulfur ionization potential is estimated to be 2472 eV from that of atomic sulfur. This is close to the intense peak at 2471 eV. Features 4 and 5 are continuum resonance structures at 2477 and 2485 eV, whereas feature 3 is a weak shoulder just above the IP. The theoretical calculations of the XANES signals for each of three different chemical environments for the sulfur atoms give support to the assignments. 3.8. Simulation XANES. The theoretical core excitation results were obtained with compact basis functions. This allows evaluation of the energies and intensities of the characteristic core → valence transitions without applying diffuse functions, which are needed to investigate Rydberg and Rydberg-valence excited states, but which add a large degree of confusion,

using the Sb 3d5/2 binding energy of antimony sulfide at 529.5 eV as a reference.52 The experimental results indicated that the screening effect was significant. Thus, the ionization potential (IP) results are only used qualitatively. Photoelectron spectroscopy allows characterization through analysis of the line positions and the fine structures of the spectra.53,54 However, if the compounds are insulators, the spectra will present a charging effect. For example, the reported F 1s binding energy for polytetrafluoroethylene is incorrect as it is 210 eV above the expected theoretical value.55 Even metallic species can experience charging, if there are insulating oxides on the surface (e.g., Fe2O3 and SiO2).53 So, direct and indirect corrections for the experimental binding energies for XPS spectra experiments should be a routine procedure as a consequence of the charging effect in insulating materials. This effect was often observed for the dmit complexes during the XPS experiments. The conductivity values show that most of the dmit complexes salts are insulators or poor semiconductors, with the exception of Ni3+ and Pd2+ compounds.56 The same electrical behavior is observed for complexes of the main group elements such as In(III), Sb(III), and Bi(II).13,33,57,58 In this way, the charging effect observed for compounds like [SbL(dmit)] (L = Br or I) can be related to their electrical properties. The deconvoluted S 1s XPS spectra, presented in Figure 9c, indicate there are signals from each of the three different che2255



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Figure 9. XANES and XPS spectra of the [SbL(dmit)] (L = Br or I) dmit complexes. The S 1s XANES spectra recorded with total electron yield (TEY) detection (top). The experimental long scan (0−1000 eV) XPS spectra (center). The experimental S 1s XPS data (black line) and the shapes used to fit them (gray line) (bottom).

The main structure in the simulated XANES spectrum is the most intense feature observed for the experimental spectra, identified as the sum of S 1s → σ*CS transitions with the large presence on the thiole sulfur. The first experimental feature, a weak low energy shoulder below 2470 eV, is indicated to be S 1s → π*CS transitions, with some contribution from S 1s → σ*SbS transitions. The third feature on the high energy side of the main peak is consistent with the computed S 1s → σ*CS transition of the thiole. The S 1s → σ*SbBr transitions are present in the first and third features but are predicted to have low intensity. In the S 1s continuum the GSCF3 calculation predicts resonance structures with the character of p Rydberg orbitals of the antimony and sulfur atoms, as well as σ*CS orbitals. 3.8.2. [SbI(dmit)] XANES. The results for [SbI(dmit)], evaluated using the IVO method, are displayed in Table 5S (Supporting Information). The values are compatible with the results described for the [SbBr(dmit)] compound. However, there are some differences because the calculated ionization potential for thione sulfur in [SbI(dmit)] is smaller than that for [SbBr(dmit)]. Such variations are unexpected because there

especially in extended systems as in this work, such that spectral interpretation becomes very challenging. The theoretical results for the analysis of the S 1s transitions for the neutral dmit complexes, obtained with the IVO method, are presented in Tables 4S and 5S in the Supporting Information. The simulated spectra along with the principal orbital representations of the [SbL(dmit)] (L = Br or I) are presented in Figure 10. 3.8.1. [SbBr(dmit)] XANES. The results for the S 1s NEXAFS spectrum of [SbBr(dmit)], evaluated using the IVO method, are displayed in Table 4S (Supporting Information). The main experimental structure at 2471 eV, Figure 9, is compatible with the calculated oscillator strength and values described in Table 4S (Supporting Information). The calculations indicate this feature arises from the overlap of transitions in the three different chemical environments: thiole, thione, and thiolate, as shown in Figure 10. The IP calculations predict the thione sulfur IP (2470.9 eV) is significantly different from the thiole and thiolate IPs (2474.0 and 2474.9 eV), which are within 1 eV. This corroborates the qualitative XPS analysis and is consistent the observed asymmetrical profile. 2256



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Figure 10. Experimental and simulated S 1s spectra of the [SbL(dmit)] (L = Br or I) dmit complexes predicted by GSCF3 ab initio calculations. The individual S thione, S thiolate, and S thiole contributions as well as the sum are plotted. The curve fitting of the experimental spectra are indicated. Molecular orbital diagrams of the upper orbital of the strongest features are plotted.

are very small changes in the structure caused by the different halogen coordinated to the metal. As with [SbBr(dmit)], the lowest energy transition in [SbI(dmit)] is the S 1s → π*CS transition, as shown in Figure 10. The most intense feature is predicted to arise from overlap of transitions at the thiole and thiolate groups. The third feature is characterized by the S 1s → σ*CS transition of the thiole. The computed IP values at 2467.6, 2474.0, and 2474.9 eV, confirming there are three chemically distinct environments and the asymmetry observed for the XPS spectra. The final assignments of these complexes are displayed in Table 10. Results of a curve fitting procedure, based on deconvolution of peaks and steps in the spectra, are presented in Figure 10. The deconvolution process, applied to the experimental data (see parameters in Table 6S, Supporting Information), shows

Table 10. Energies (eV) and Assignments Proposed for the S 1s Spectra of [SbL(dmit)] (L = Br or I) no.

[SbBr(dmit)]

[SbI(dmit)]

assign.

1 (sh) 2 3 (sh) 4 5

2468.3 2470.9 2473.0 2477.0 2486.0

2469.3 2471.0 2472.9 2478.3 2483.8

π*CS + σ*SbL + σ*SbS σ*CS σ*CS(thiole) Ryd. S +Ryd Sb σ*CS

better results when Gaussian line widths are used to describe the line shapes of the peaks. This indicates either that the instrumental resolution, i.e., the monochromator resolution, dominates or that our features may be composed of several overlapping transitions. This last statement is consistent with 2257



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tional support. Thus, this work helps fulfill this gap in the literature. The theoretical−experimental comparison of the vibrational results confirms the structural characterization that resembles the geometric parameters of neutral complexes of dmit with antimony. The multiconfigurational nature of several excited states of [SbL(dmit)] (L = Br or I) involving intraligand transitions are also observed. The calculated results are in agreement with the experimental spectra, confirming the existence of some ligandto-metal charge-transfer (LMCT) transitions. Analyses of the transition energies, using the TD-B3LYP method, are in excellent agreement with the experimental results. The CI calculations allowed a more precise analysis than previously obtained from simple analysis of the frontier orbitals. CPCM calculations carried out on the solvated complexes describe the UV−vis energies with larger precision than vacuum calculations, although the spectral intensities were not properly reproduced by either approach. The S 1s XANES and UV−vis experimental spectra did not reveal any influence of the halogen coordinated to the metal. Although the XPS experimental data are qualitative due to strong charging, it was possible to identify three different chemical environments, which were confirmed by the computed IPs. This helped us to establish the resonance region in the XANES spectra.

the theoretical results and rationalizes the use of Gaussian functions in this work. Attempts to use Lorentzian functions to describe the measured profiles do not work properly, even though this function is appropriate to represent atomic states.59 The steplike features in the spectra are reasonably reproduced by error functions. Finally, the relative errors associated with the fits include contributions from the statistical precision of the data and the fitting model, treated as uncorrelated quantities and added in quadrature to give the total error. We have chosen not show the error bars, although we have estimated their values as 3 to 5% of the presented data. 3.9. Comparison between Inner and Valence Shell Spectroscopies. The ab initio calculations, involving the IVO methods, allowed us to assign the main experimental structures, especially those related to π*CS and σ*CS valence orbitals with small contribution from the metal or the halogen. For both spectral regions, the possible structural changes were not significant, given the similarity between the theoretical and experimental results. However, the comparison between valence and inner shell regions is not easy due to the large number of observed transitions in the valence shell. A possible connection between XANES and UV−vis spectra is discussed in the literature60 by comparison of valence and inner shell term values (TV) for corresponding final states. The term value is defined as the difference between the ionization potential (IP) of the originating orbital and the energy of the electronic excitation. Comparing to a computed ground state frozen orbital energy difference is usually inappropriate due to relaxation in the excited state, although the magnitude of the relaxation effect decreases with increasing molecular size and dimensionality.61−63 In addition, there is a large chance in absolute energies due to the electrostatic effect of the localized core hole.64 This can be accommodated within a “Z + 1” model. With those qualifying statements, we interpret the S 1s XANES spectrum as a replica of the density of unoccupied states (DOUS), with an overlap of three sets, due to the presence of three distint S chemical environments. Within this approach, one can compare the TV of an experimental feature with the binding energy (BE) of the probed unoccupied molecular orbital (UMO). However, not all compounds have this characteristic, and small changes in the chemical environment may cause large differences in the transition energies. This approach can be explored using the data obtained in this work and summarized in Table S7 (Supporting Information). Using the Koopmans energy of HOMO as an IP reference for the [SbL(dmit)] (L = Br or I), and the experimental transition energy of the UV−vis π*CS, as 380 nm or 3.27 eV, the calculated TV is 2.97 eV. On the other hand, the term values predicted from XANES calculations using IVO orbitals, or derived from the experimental XANES data and the estimated IP, are all larger than 4 eV. Thus, interpreting XANES as DOUS is not valid for the compounds of this work. This is clear by comparing Figures 9 and 10 with Figure 3S (Supporting Information).

■

ASSOCIATED CONTENT

S Supporting Information *

NEXAFS theoretical results and analysis of [SbL(dmit)] (L = Br or I): tables of transition energies, oscillator strengths, ionization potentials, term values, orbital sizes, transition characters, spectral assignments, and deconvolution paramaters; energy level and density of states diagrams. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS We are grateful to LNLS for the use of its facilities (SXS 5725/ 2006). We thank Professor Adam P. Hitchcock, from McMaster University, and Professor Stephen G. Urquhart, from University of Saskatchewan, for the use and help on the GSCF3 and SIMILE2 programs. We also acknowledge Professor Maria Luiza Rocco Duarte Pereira and Professor José Walkimar de Mesquita Carneiro for reading the text, and the Brazilian agencies CNPq and FAPERJ for financial support. We thank again Professor Adam P. Hitchcock for his insightful comments and suggestions for the final version of this paper.

■

4. CONCLUSIONS The combination of several techniques of inner and valenceshell spectroscopies allowed us to characterize the electronic and vibrational states of these neutral complexes. Inner-shell study is still relatively little developed especially for these types of large complexes. Most of the literature consists of presentation and discussion of experimental data without computa-

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Vibrational and Electronic Spectroscopy of Neutral Antimony

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