Facile Molecular Approach to Transparent Thin-Film Field-Effect

Article pubs.acs.org/cm

Facile Molecular Approach to Transparent Thin-Film Field-Effect Transistors with High-Performance Using New Homo- and Heteroleptic Indium(III)−Tin(II) Single-Source Precursors Kerim Samedov, Yilmaz Aksu, and Matthias Driess* Institute of Chemistry: Metalorganics and Inorganic Materials, Technische Universität Berlin, Sekr. C2, Strasse des 17 Juni 135, D-10623 Berlin, Germany S Supporting Information *

ABSTRACT: Various routes to new molecular bimetallic indium(III)− tin(II) alkoxides precursors for low-temperature synthesis of conducting tin-rich indium tin oxide were investigated. The studies include the facile syntheses, spectroscopic properties, and structural characterization of the new homo- and heterolepitic bimetallic alkoxides ( tBuO)Sn(μOtBu)2InX2 (X = OtBu (1), N(SiMe3)2 (2)) as well as the first heptanuclear bimetallic chlorides, that is, the oxo tert-butoxide clusters In3Sn4(OtBu)6Cl6O2+Cl− (3) and In3Sn4(OtBu)6Cl7O2*In3Sn4(OtBu)7Cl6O2 (4). To evaluate the suitability of 1 and 2 as single-source precursors (SSPs) for the preparation of tinrich indium tin oxide (ITO) thin films, their thermal degradation under dry synthetic air was investigated and the resulting materials were analyzed by a combination of different analytical techniques such as powder X-ray diffraction analysis (PXRD), transmission electron microscopy (TEM), FT-IR, scanning electron microscopy (SEM), energy-dispersive X-ray analysis EDX, elemental analysis, and inductively coupled plasma optical emission spectrometry (ICP-OES). Thermal treatment of 1 and 2 resulted in amorphous tin-rich ITO and crystalline ITO with traces of the cassiterite phase of tin dioxide. Incorporation of tin into the indium oxide lattice on atomic scale in crystalline ITO is evidenced by the increased lattice parameters extracted from the PXRD measurements. Results of the SEM-mapping show homogeneous distribution of indium and tin in the materials. The final In:Sn ratio of the as-prepared materials as determined by means of the ICP-OES analysis is close to that of the In:Sn ratio of 1:1 in the molecular precursors, thus confirming the suitability of 1 and 2 as SSPs to produce tin-rich ITO. Thin film field effect transistors were fabricated by spin-coating of Si-wafers with a solution of 2 in toluene and subsequent calcination under dry air. The as-prepared FETs from 2 exhibit excellent performance as shown by a field effect mobility of 3.7 × 10−1cm2V−1s at 350 °C and an Ion/off current ratio of 1 × 107. KEYWORDS: molecular precursors, metal oxides, semiconductors, alkoxides, nanocomposites

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deposition (CVD).13 All these processing approaches have apparent disadvantages with respect to the influence of the substrate structures, required high calcination temperatures, extent of control over materials’ properties and complexity of the equipment and expenses required. Alternatively, the sol−gel approach14 combined with spin- and dip-coating as well as several other wet-chemical deposition techniques15 described in literature were successfully employed in the preparation of ITO coatings. It is, however, noteworthy that the low abundance of indium and the resulting high price of the materials severely limit the possibilities of using ITO for large-scale industrial production thus prompting intensive efforts to develop new low-cost alternative materials in order to meet the growing demand. Given that generally, for both thin films and nanoscaled ITO

INTRODUCTION Seminconductor oxide materials such as zinc oxide, tin oxides, indium oxide, and tin-doped indium oxide (ITO) are essential materials for the optoelectronic industry, because they combine optical transparency in the visible region with a controllable electrical conductivity. The most widely used TCO in optoelectronics is tin-doped indium oxide (ITO, 5−15 mol % Sn).1 ITO combines the best to date available performance in terms of conductivity, transparency in the visible and reflectivity in the infrared region with high scratch resistance, thermal stability, chemical inertness, and good surface morphology.2 Because of these properties, today ITO is the state-of-the-art material in the flat panel display technology3 as well as the basic material of choice for a vast variety of other applications,1a,2 including functional glasses,4 photovoltaics,5 gas sensing,6 organic LEDs,7 and energy-efficient windows.8 ITO is usually employed in form of thin films deposited using various methods such as magnetron sputtering,9 reactive evaporation,10 spray pyrolysis,11 pulsed laser deposition,12 and chemical vapor © 2012 American Chemical Society

Received: February 15, 2012 Revised: April 19, 2012 Published: May 16, 2012 2078

dx.doi.org/10.1021/cm300510y | Chem. Mater. 2012, 24, 2078−2090

Chemistry of Materials

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indium(III)−tin(II) alkoxides, we probed whether the Lewis acid−base reaction outlined in Scheme 1 would lead to formation of the mixed heterobimetallic alkoxide 1.

powder, the preparation method and the tin concentration in the material strongly influence the optical and electrical properties, the development of new facile and efficient approaches to indium tin oxide materials with reduced indium content is highly desirable and of great commercial interest. Also, because the solubility of tin in the In2O3 lattice is known to depend on temperature,16 a low-temperature procedure for the preparation of ITO thin films with higher doping rates is necessary. Recently, we showed17 that following the singlesource precursor (SSP) concept, the incorporation of tin into ITO can be accomplished to a much higher degree than in various ITO materials known in the literature. Thermal degradation of the molecular precursor indium(I) tin(II) tritert-butoxide (ITBO) at 400 °C yields tin-rich ITO films displaying high conductivity and durable electro-optical performance. A literature survey revealed that to date there is only one further report18 on preparation of the tin-rich ITO nanomaterial with comparably high tin-incorporation ratio using a more drastic gas-phase synthetic approach. The single-source precursor (SSP) concept possesses a valuable bottom-up-approach for the preparation of mixedmetal oxide materials.19 This approach employs thermal degradation at relatively low temperatures of metastable organometallic compounds containing all elements necessary for the formation of the final solid material in a well-defined ratio, thus allowing a strict control over chemical composition and ensuring their high dispersion in the resulting material. In our group, a number of metal−organic single-source precursor (SSP) compounds has been synthesized, characterized, and successfully utilized for preparation of a wide range of different heterobimetallic oxide materials.17,20 Even though easily accessible, the ITBO precursor is extremely air- and moisture-sensitive and relatively labile, even under inert conditions; this prompted us to develop more reliable indium tin alkoxide-based alternatives suitable as SSPs for preparation of thin tin-rich ITO films. Because the intrinsic lability of ITBO is mainly due to the presence of indium in its less stable low oxidation state (1+), we assumed that indium tin alkoxides bearing indium in the common oxidation state (3+) will increase the stability of the system and decrease its air- and moisture-sensitivity thus being ideally suitable as SSPs for preparation of tin-rich ITO films. A literature survey revealed that there is only one report21 on synthesis and structural characterization of a heteroleptic indium(III)−tin(II) alkoxides by Veith et al. This encouraged us to explore new synthetic routes to synthesize new homo- and heteroleptic indium(III) tin(II) alkoxides. Here we report on the synthesis, spectroscopic characterization and X-ray crystal-structure analysis of the new homoand heteroleptic bimetallic indium(III)−tin(II) alkoxo compounds 1 and 2 as well as the first isomeric heptanuclear bimetallic indium tin oxo-alkoxide chloride clusters 3 and 4. We also assess the potential of 1 and 2 as suitable SSPs for deposition and low-temperature preparation of tin-rich ITO thin films.

Scheme 1. Synthesis of 1

In fact, reacting Sn(OtBu)2 and In(OtBu)3 in an equimolar ratio in THF and refluxing the reaction mixture for one hour leads to formation of 1 as evidenced by the appearance of a new resonance at δ = −201 ppm in the 119Sn-NMR spectrum (see the Supporting Information, SI-1c). Removal of all volatile components in vacuo yields 1 as a colorless solid in quantitative yield. Compound 1 is a thermally stable, low-melting and moderately air- and moisture-sensitive material that is very good soluble in hydrocarbons and nonprotic coordinating solvents. While 1 is indefinitely stable in solutions at room temperature under an inert atmosphere, it turns very slowly into yellowish oil upon prolonged storage as a solid. It has been fully characterized by multinuclear NMR (see the Supporting Information, SI-1a-c), IR, MS (see the Supporting Information SI-1d) and elemental analysis. The EI mass spectrum of 1 exhibits a intense peak with the characteristic tin isotope pattern at m/z = 527, assignable to the molecular ion. The room temperature 1H NMR and 13C NMR spectra of 1 in C 6 D6 solutions exhibit time-averaged resonances, indicating highly fluxional behavior. The 1H NMR spectrum displays 2 sharp resonances at δ = 1.39 and 1.45 ppm and a broad signal at 1.57 ppm in the integrated ratio of 4:1:5. Although the broad resonance can be assigned to the bridging tBuO ligands, the sharp sets of resonances are attributed to the terminal tBuO groups. The integration ratio can be rationalized by assuming the equilibrium between 1a and 1b as depicted in Scheme 2. This has been proven by Scheme 2. Assumed Equilibrium between 1a and 1b

variable temperature 1H NMR spectroscopy. The 119Sn NMR spectrum shows even at low temperature only one singlet, indicating that the difference of the chemical shifts of 1a and 1b are very small. The chemical shift at δ = −201 ppm is indicative of three-coordinate tin(II) species. In an analogous reactions we learned that sterically more hindered tin(II) compounds can also be employed to form tin(II)−indium(III) heteroaggerates. Thus, as outlined in Scheme 3, the reaction of In(OtBu)3 with Sn[N(SiMe3)2]2 in an equimolar ratio in THF lead to the desired compound 2 which could be isolated as colorless solid in almost quantitative yield. The constitution of 2 has been deduced from 1H-, 13C-, 29 Si- and 119Sn-NMR spectroscopy (see Experimental Section and the Supporting Information, SI-2a-d). Recrystallization of 2

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RESULTS AND DISCUSSION Synthesis and Properties of Compounds 1−4. The ability of different metal alkoxides to form even mixed heterometallic clusters is well-known.22 A vast number of molecular homo- and heterometallic alkoxides of various metals have been synthesized following the Lewis acid−base principle.23 Given our interest in the synthesis of bimetallic 2079



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NMR spectrum (see Supporting Information, SI-2c) at δ = 2.17 and 2.37 ppm, respectively. The 119Sn-NMR spectrum (see Supporting Information, SI-2d) exhibits a singlet at δ= −189 ppm, which is comparable to the chemical shift of δ = −201 ppm observed for 1 and falls into the range common for threecoordinated tin(II) species. The EI mass spectrum of 2 (see Supporting Information SI2e) displays an intensive fragment peak at m/z = 759, corresponding to the molecular-ion peak with the loss of one methyl group. The assigned composition of 2 was further corroborated by a satisfactory elemental analysis. Given that there is only a minor difference in the Lewis acidity of the metal centers [Sn(II) and In(III)] we were intrigued by the unexpected high stability of the Lewis donor− acceptor adducts 1 and 2. Thus, we set out to explore the reactivity of indium(III) tert-butoxide toward other tin(II) compounds. Sn(OtBu)3InCl2 · THF (3a) and its bromine homologue Sn(OtBu)3InBr2 · THF (3b) are to date the only two examples of heteroleptic indium(III)−tin(II) alkoxides reported in literature.21 They were accessible by salt metathesis reaction of Na2Sn2(OtBu)6 with InCl3 and InBr3, respectively, in refluxing THF. Following our protocol we probed whether the reaction of indium(III) tert-butoxide with tin(II) chloride in THF in the molar ratio of 1:1 leads via ligand exchange to 3a. The reaction was carried out at room temperature and gave a microcrystalline crude product. The latter is soluble in THF, scarcely soluble in aromatic but insoluble in nonaromatic hydrocarbons. Its 119Sn NMR spectrum in THF-d8 (see the Supporting Information SI-3c) displays one sharp resonance at δ = −300 ppm typical for the presence of three-coordinate tin(II) centers. The 1H NMR spectrum (see the Supporting Information SI-3a) exhibits a broad resonance at δ = 0.49 ppm in addition to the residual solvent peaks. Similarly, the 13C NMR spectrum consists of a set of signals (δ = 34.78 and 76.56 ppm) in the range typical for tert-butoxide groups (see the Supporting Information SI-3b). The solubility of the product and its NMR spectra are clearly different to those of 3a reported by Veith et al.21 In fact, a single-crystal X-ray analysis of crystals grown in saturated THF-toluene solutions revealed that the main product is a ion pair consisting of the novel heptanuclear indium(III)−tin(II) chloro-oxo-tert-butoxide cluster cation [In3Sn4(OtBu)6Cl6O2]+ and a chloride counterion. Because of the idealized C2v symmetry of the molecular cluster cation in 3 two different kinds of tert-butoxide groups and thus two sets of resonances in the 1H and 13C NMR spectra are expected. However, the observed spectra are far simpler, most likely because of the minor differences in the chemical environment of the tert-butoxide groups in the cationic cluster in 3. Any kind of fluxional behavior of 3 in solution could be ruled out based on the results of the solid- state 13C and 119Sn MAS NMR spectra of 3 (see the Supporting Information SI-3d and SI-3e). As expected, the yield of 3 can be improved by adjusting the stoichiometry of indium and tin components and using tin(II) tert-butoxide instead of the indium(III) tertbutoxide whose preparation is rather time-consuming and intricate. Monitoring the reaction progress by NMR spectroscopy clearly shows that 3 is formed as sole product through reaction of indium(III) chloride with tin(II) tert-butoxide in the molar ratio of 3:5 under otherwise identical conditions (Scheme 4, Figure 2). Compound 3 is a air- and moisturestable solid that could be obtained as single crystals upon recrystallization in saturated THF solutions in moderate 40% yield.


in hexane at −20 °C afforded single crystals suitable for a X-ray structural analysis. Accordingly, 2 is a heterometallic heteroleptic alkoxide amide with two bridging tBuO groups and terminal N(SiMe3)2 ligands at the In(III) atom (Figure 1). The

Figure 1. Molecular structure of 2. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Symmetry transformations used to generate equivalent atoms: I x, y, z; II −x + 1/2, −y, z + 1/2; III x + 1/2, −y + 1/2, −z; IV −x, y + 1/2, −z + 1/2; V −x, −y, −z; VI x − 1/2, y, −z − 1/2; VII −x − 1/2, y − 1/2, z; VIII x, −y − 1/2, z − 1/2.

formation of 2 implies a ligand exchange between In(OtBu)3 and Sn[N(SiMe3)2]2. Obviously 2 is the preferred product because the sterically more demanding N(SiMe3)2 groups occupy terminal positions. Similarly as 1, compound 2 is a colorless, low-melting and moderately air- and moisture-sensitive substance with a very good solubility in all common nonprotic organic solvents. In contrast to the spectroscopic features of 1 in solutions, which indicate highly fluxional behavior, the respective VT 1H and 13C NMR spectra show that 2 is rigid in solutions. The 1H spectrum consists of four distinct resonances in the integrated ratio of 2:2:1:2. While the upfield shifted signals at δ = 0.40 and 0.45 ppm with the integrated ratio of 2:2 lie in the range typically observed for N(SiMe3)2 ligands, thus indicating the presence of two different N(SiMe3)2 groups in the molecule, the two further downfield-shifted resonances at δ = 1.34 and 1.44 ppm with the integrated ratio 1:2 are located in the range characteristic of tBuO groups in different coordinating environments. Based on the integration ratio of 2:1 in the 1H spectrum of 2 and crystal structure data we assign the rather broad resonance at δ = 1.44 ppm to the bridging tBuO groups, whereas the sharp resonance at δ = 1.34 ppm is attributed to the terminal tBuO group at the tin(II) atom. The 13C NMR spectrum of 2 (see Supporting Information, SI-2b) shows three sets of signals corresponding to the N(SiMe3)2 methyl groups with the expected upfield shift of δ = 7.23 ppm and terminal as well as bridging tBuO groups further downfield. Typically, as in 1, the resonances of the primary and quaternary carbon atoms of the bridging tBuO groups are relatively broad. The presence of two different N(SiMe3)2 groups in the molecule is further corroborated by the appearance of two resonances in the 29Si 2080



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Figure 3. Molecular structure of 4 (1:1 mixture of 4a and 4b): Hydrogen atoms are omitted for clarity. An equatorial position at In3 was refined as equally occupied by a tert-butoxy group (O9tBu) and a chloro ligand (Cl7). The organic substituents have been shown for reasons of clarity only as thin lines. Thermal ellipsoids are drawn at the 50% probability level. Symmetry transformations used to generate equivalent atoms: I x, y, z; II −x, y, −z + 1/2; III x + 1/2, y + 1/2, z; IV −x + 1/2, y + 1/2, −z + 1/2; V −x, −y, −z; VI x, −y, z − 1/2; VII −x + 1/2, −y + 1/2, −z; VIII x + 1/2, −y + 1/2, z − 1/2.

Figure 2. Molecular structure of the ion pair 3. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. The organic substituents have been shown for reasons of clarity only as thin lines. Symmetry transformations used to generate equivalent atoms: I x, y, z; II −x + 1/2, y + 1/2, −z + 1/2; III −x, −y, −z; IV x − 1/2, −y− 1/2, z − 1/2.

of 2:2:3:6. Given the Cs symmetry of both components of 4 at least nine resonances would be expected. The significantly reduced number of observable signals is most likely due to the coincidence of some resonances because of very similar bonding features of 4a and 4b. Likewise, the 13C NMR spectrum of 4 displays four sets of resonances for the tertbutoxy groups. The structural integrity of the oxo-alkoxy-bridged heterobimetallic cores of 3 and 4, respectively, in solutions is proven by comparison of the respective solid state 13C and 119Sn MAS NMR spectra with the spectra obtained in solutions (see the Supporting Information SI-3c, SI-3e, SI-4c and SI-4e). Both compounds display similar chemical shifts in solutions (δ = −300 ppm for 3, δ = −449 ppm for 4) and in the solid state (δ= −282 ppm for 3, δ = −475 ppm for 4). Structural Elucidation of 2−4. The solid state structure of 2 is shown in Figure 1. Selected bond lengths and angles are given in the Supporting Information, SI-Table-1 and SI-Table2. Compound 2 crystallizes orthorhombic in the space group Pbca with eight molecules in the unit cell. The molecules of 2 possess a mirror plane passing through Sn1, In1, O1, N1, and N2 atoms (point group symmetry: Cs). The solid state structure of 2 shows an almost planar InO2Sn ring with two bridging tertbutoxide groups, two terminal hexamethylsilazanide ligands bonded to the distorted tetrahedral coordinate In atom, and a terminal tert-butoxide group bonded to the ψ-trigonal pyramidal coordinate Sn atom. The endocyclic In−O distances (In1−O2 = 213.8(3), In1−O3 = 214.2(3) pm) in 2 are little longer than the In−O bond lengths observed for other indium alkoxides. The Sn−O (Sn1−O2 = 215.6(3), Sn1−O3 = 215.4(3) pm) bond lengths in the InO2Sn ring in 2 are virtually equal. The endocyclic O2−In1−O3 angle in 2 is slightly smaller (73.8°) than those observed in homo- and heteroleptic homometallic In(III) alkoxides with a M2O2 ring,24 whereas the O−Sn−O angle (O2−Sn1−O3 = 73.2(1)°) is almost identical to those reported for homo- and heteroleptic homometallic Sn(II) alkoxides bearing a M2O2 ring.24c,d Accordingly, the In1−O1−Sn1 and In1−O2−Sn1 bond angles are essentially similar (106.13(12)° and 106.05(12)°). The

Interestingly, recrystallization of the crude product in tetrahydrofuran-toluene solutions led to small quantities of a side product as indicated by the appearance of an additional broad resonance at δ = −449 ppm in the 119Sn NMR spectrum. In order to characterize the aforementioned side product and to examine the influence of toluene on stability and formation of 3, the latter was redissolved in toluene. A white precipitate was filtered off, the filtrate concentrated and kept at −20 °C to give colorless plates of the new product 4 in 70% yield. Crystals of 4 dissolved in C6D6 reveal a broad resonance at δ = −449 ppm in the 119Sn NMR spectrum (see the Supporting Information SI4c) similar to the aforementioned resonance of the side product. The result of the X-ray structural analysis revealed that 4 is an equimolar mixture of the two neutral heptanuclear indium(III)−tin(II) oxo tert-butoxide clusters In3Sn4(OtBu)6Cl7O2 (4a) and In3Sn4(OtBu)7Cl6O2 (4b) (Scheme 5, Figure 3). Compound 4a is a constitutional isomer of 3. Attempts to separate 4a and 4b by fractional crystallization were unsuccessful. Scheme 5. Synthesis of 4

The composition of 4 was further corroborated by a satisfactory elemental analysis. The ambient-temperature 1H and 13C NMR spectra (see the Supporting Information SI-4a and SI-4b) are indicative of a stereochemically rigid behavior displaying the presence of several tert-butoxide groups in different coordinating environments. The room-temperature 1 H NMR spectrum exhibits four singlets, integrating in a ratio 2081



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are somewhat unusual since the distances between In1 and equatorial chloro ligands Cl1 and Cl5 are remarkably longer (In1−Cl2 = 259.2(1) and In1−Cl5 = 256.05(1) pm) than usually observed,26,24j whereas bond lengths between the In1 and axially positioned chloro atoms Cl3 and Cl4 are at the upper limit of the range (In1−Cl3 = 246.8(7) and In1−Cl4 = 248.28(1) pm). On the other hand, the In−μ4O distances (In1−O1 = 213.4 (3) pm and In1−O5 = 215.01(3) pm) are unexpectedly short being in the range typically observed for Inμ2O distances. The bonding parameters of the trigonalbipyramidal arrangement of the indium atoms in the InSn2O4 subunits flanking the In1 atom differ from each other only very little and are rather close to common values observed. The In− O bond lengths are in the expected range and clearly display the established tendency observed for In−OR < In−μ2OR < In−μ3OR < In−μ4O (In2−O3 = 209.3(3) and In2−O4 = 210.1(3) pm, whereas In2−O1 = 225.8(3) pm). While the O− Sn−O angles of the two trigonal pyramidally configured tin(II) centers present in the InSn2O4 subunit are virtually equal and fall nicely into the range usually observed for three-coordinated tin(II) species, the Sn−μ2−O distances are significantly larger (Sn1−O3 = Sn1−O3 = 217.2(3) pm and Sn2−O2 = 217.1(3), Sn2−O4 = 218.5(3) pm) than the Sn−O bond lengths found elsewhere in the literature27 (∼2.07 pm). Somewhat unusual are also the rather short Sn−μ4−O distances (Sn1−O1 = 214.7(3), Sn2−O1 = 214.0(3) pm) observed in the InSn2O4 subunits. A moderate distortion of the tetrahedral coordination sphere around the oxo ligands O1 and O5 is reflected in the bond angles (In1−O1−Sn1 = 120.34(1)°, In1−O1−Sn2 = 120.33(1) °, Sn1−O1−OIn2 = 99.95(2)°) deviating perceivably from the ideal tetrahedral angle. The solid state structure of 4 was established by X-ray crystal structure and the result is shown in Figure 3; the atomic numbering scheme and selected bond lengths and angles are given in the Supporting Information, SITable-4 and SI-Table-5. Compound 4 crystallizes monoclinic in the space group C2/ c. In contrast to the ionic nature of the solid state molecular structure of 3, 4 is a 1:1 mixture of the two cluster compounds 4a and 4b. Both consist of unprecedented neutral heptanuclear indium(III)−tin(II) oxo tert-butoxide clusters. Although in 4, similarly to 3, all tin atoms are present in an analogous distorted ψ-trigonal pyramidal environment (vide supra), in contrast to 3, the three indium atoms in 4 have each a distinct coordination environment. Whereas In1 is octahedral coordinated, In2 has a square pyramidal ligand surrounding, and In3 adopts distorted trigonal-bipyramidal coordination. Akin to 3, the octahedral coordinate In1 with two chloro atoms (Cl1 and Cl2) in the axial position and two μ4-O oxo ligands (O1 and O5) cis to each other in the equatorial position remains as central metal atom in 4. Unlike in 3, in 4, a terminal chloro ligand (Cl3) and with a 50% probability a bridging chloro ligand (Cl7) and a bridging tert-butoxide group, respectively, comprise the perfectly planar equatorial plane and complete the octahedral environment around In1. In1 is linked by the oxo ligand μ4-O5 to an InSn2O4 subunit, which is the second structural feature 3 and 4 have in common. As in 3, the indium atom in the InSn2O4 subunit possesses an identical (vide supra) but distinctly more distorted trigonal-bipyramidal environment. The intrinsic C2v symmetry of 3 is broken in 4 by the presence of another isomeric InSn2O4 subunit. Similarly as in 3, the central indium atom (In3) is out of the equatorial plane (∑ of angles of In 341.7(1)°) and slightly shifted toward the terminal

bridging oxygen atoms in 2 are almost trigonal-planar coordinated (354.4°) and thus close to sp2-hybridization. The Sn−O distances to the bridging oxygen atoms in 2 are essentially identical (∼ 200.0(3) pm). The angle between the plane comprising the InO2Sn ring (Sn1−In1 vector) and the terminal tert-butoxide ligand at the tin atom in 2 is relatively large (98.7°) and differs from the acute angle (∼ 86.4°) observed in homo- and heteroleptic homometallic In(III) and Sn(II) alkoxides with a M2O2 ring, respectively.24c,d This widening can be attributed to the increased repulsive interactions between the terminal ligands at the respective metal centers. The In−N bond lengths (In1−N1 = 208.1(3) pm, In1−N2 = 208.4(4) pm) are essentially equal and similar to the values observed for related indium amides.j24,25 The staggered conformation of the bridging tert-butoxide groups and the orientation of the terminal tert-butoxide group away from the InO2Sn plane reduces repulsive interactions in the molecule. The molecular structure of 3 is shown in Figure 2; selected bond lengths and angles are given in the Supporting Information, SI-Table-3 and SI-Table-5. The compound crystallizes monoclinic in the space group P21/n with four molecules per unit cell. It consists of an ion pair bearing a novel heptanuclear indium(III)-tin(II) oxo tertbutoxide cluster cation with approximately C2v symmetry and a chloride counterion. The two InSn2O4 subunits of the metaloxo core represent distorted cubane-like frameworks with a missing corner, bound symmetrically to the central octahedral coordinate indium atom (In1). The central In1 atom bears two chloro atoms (Cl3, Cl4) in axial positions and two chloro ligands (Cl2, Cl5), bridging In1 with In2 and In3, respectively, as well as the two aforementioned oxo ligands (μ4-O1, μ4-O5) comprising the planar equatorial plane. The InSn2O4 subunits consist each of an indium atom in a slightly distorted trigonalbipyramidal environment linked by an oxo ligand in the axial position and a bridging tert-butoxide group to one tin atom as well a second tert-butoxide group to another tin atom in equatorial positions. A bridging chloro atom as well as a further terminal chloro atom in the axial position complete the trigonal-bipyramidal environment of the respective indium atom in each InSn2O4 core. The tin atoms (Sn1, Sn2 and Sn3, Sn4) are both distorted trigonal-pyramidal coordinated consisting of oxo ligands and two bridging tert-butoxide groups. The oxo ligands O4 and O5 are each distorted tetrahedral coordinated by two tin atoms and two indium atoms, one of which belongs to the corresponding InSn2O4 subunit with the other one being the central indium atom In1. The octahedral arrangement around In1 is only slightly distorted from the ideal octahedral geometry as can be deduced from the bond angles. The indium atom In1 lies in the equatorial plane of the octahedron (∑ of angles of In1 360.0°) and the Cl3−In1−Cl4 angle deviates by less than 12° from 180°. The corresponding Cl4−In1−Cl2 (92.(9))°) and Cl4− In1−Cl5 (95.2(8)°) angles are a little obtuse deviating only slightly from 90°. The Cl4−In1−O5 (85.5(4)°) and Cl4−In1− O1 (85.9(2)°) angles are in turn somewhat acute and almost equal. They also deviate insignificantly from the ideally expected 90°. The same holds true for the O1−In1−Cl2 (81.33(8)°) and O5−In1−Cl5 (81.68(8)°) angles. Interestingly, in spite of larger covalent radius of the chlorine atoms (102 pm vs 66 pm for oxygen) the Cl2−In1−Cl5 angle (85.91(4)°) is distinctly smaller than the O1−In1−O5 (111.09(1)°) angle. The bond lengths within the octahedron 2082



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Figure 4. TGA and DTA curves for the degradation of 1 in dry synthetic air with a heating of rate 5 K min−1, starting from room temperature to 700 °C.

Figure 5. TGA and DTA curves for the degradation of 2 in dry synthetic air with a heating rate of 5 K min−1, starting from room temperature to 700 °C.

Cl6−In3−O8 = 116.22(1)°, Cl6−In3−O9 = 99.0(1)°) differ strongly from the anticipated angle of 90°. The In−Cl (In2−Cl4 = 239.6 pm, In2−Cl5 = 240.5 pm) and In−O (In2−O1 = 227.5 pm, In2−O2 = 211.3 pm, In2−O4 = 211.3 pm) bond lengths in the square pyramid around the In2 are comparable with those reported in literature.24 The somewhat elongated In2−O1 distance might be either caused by the μ4 coordination mode of the oxo ligand (O1) or a trans effect exerted by the trans covalently bound chloro atom (Cl4). The distortion of the square pyramidal geometry manifests

chloro ligand (Cl6). In contrast to 3, the deviation of the angular parameters from the ideal trigonal bipyramidal geometry in 4 is significantly larger. The O5−In3−Cl6 angle formed by the axially oriented atoms is 167.72(8)° (178.5(1)° in 3), whereas the ligands in the equatorial positions reveal smaller angles (O8−In3−O9 = 108.95(1)°, O9−In3−O7 = 104.79(1)°, O8−In3−Cl7 = 107.10(1)°, Cl7−In3−O7 = 112.57(1)°) than the expected value of 120°. Likewise, the respective Cl−In3−O angles (Cl6−In3−O7 = 100.16(1)°, 2083



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Figure 6. TEM images of (a) 1 and (b) 2, and (c) selected-area electron diffraction pattern of the material (tin-rich In2O3) prepared from the degradation of 1 and 2.

Figure 7. PXRD pattern of the material derived from the degradation of 2 in dry synthetic air with a heating rate of 5 K min−1 from room temperature to 700 °C.

1.55% between 240 and 580 °C, resulting in a final remaining mass of 41.01%. Apparently the degradation of 1 is essentially at least a twostep process, during which first the elimination of coordinated THF takes place resulting in a weight loss of 9.96% (10.74% calc.) followed by a rapid and complete oxidative combustion of the organic groups to carbon dioxide and water in the second step, resulting in a weight loss of ca. 47.30% (50.45% calcd). Assuming that the degradation of 1 takes place according to the scheme portrayed in the Supporting Information SI-5, and the remaining material consists solely of the metal oxides (SnO2, In2O3) as indicated by elemental analysis (C, H content

Facile Molecular Approach to Transparent Thin-Film Field-Effect

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