Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Synthesis and Structure of Sn14Cl6(CH2SiMe3)12: Toward Nanoclusters of 4‑Coordinate α‑Sn Andrew P. Purdy,*,† Raymond J. Butcher,‡ James P. Yesinowski,† Sean A. Fischer,† Daniel Gunlycke,† and Brian L. Chaloux† †
Chemistry Division, Code 6100, Naval Research Laboratory, Washington, DC 20375, United States Howard University, Chemistry Department, 525 College Street NW, Washington, DC 20059, United States
‡
S Supporting Information *
ABSTRACT: Orange crystals of a Sn14 cluster have been isolated in up to 22% yield from a reaction between Me3SiCH2SnCl3, SnCl4, and LiAlH4. The structure determined by single crystal X-ray diffraction shows three unique Sn atoms in a 6:6:2 ratio, with all Sn atoms 4coordinate, similar to the tetrahedral bonding in elemental gray Sn. The solid state 117Sn MAS NMR spectrum shows the three types of distinct Sn atoms in the expected 3:3:1 intensity ratio with respective chemical shifts of 87.9, −66.6, and −607.1 ppm relative to Me4Sn. The chemical shift of the two Sn atoms without ligands (bonded only to Sn), at −607.1 ppm, is the most upfield, and is the closest to the chemical shift, reported here, of bulk gray tin (−910 ppm). First-principles density functional theory calculations of the chemical shielding tensors corroborate this assignment. While the core coordination is distorted from the ideal tetrahedral arrangement in the diamond structure of gray tin, this Sn14 cluster, as the largest reported cluster with all 4-coordinate Sn, represents a major incremental step toward being able to prepare atomically precise nanoparticles of gray tin.
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INTRODUCTION Tin is unique in occurring as both a metal (the common form, β-Sn or white tin) as well as a semiconducting allotrope (α-Sn, or gray tin, stable below 13 °C).1 The metallic form is tetragonal, with 6-coordinate Sn, while the semiconducting form, gray tin, has the cubic diamond structure, in which all Sn atoms have perfect tetrahedral coordination. While bulk gray tin has a band gap of zero,1c quantum confinement induced by nanoscale dimensions is predicted to open the band gap for small particles into the THz, IR, or even into the visible region for the smallest nanoparticles or clusters.2 To date, the quantum confinement effect in gray tin has only been experimentally observed in 2D films.2c Thus, it would be highly desirable to be able to chemically synthesize an “atomically precise nanoparticle” (APN)3 of gray tin, a cluster that would contain a core of Sn atoms having the diamond lattice structure. In addition to the potential of offering a wide range of band gaps and resultant optical and electronic properties, such APNs of gray tin would offer unique opportunities to investigate nanoscale effects upon the thermodynamics and kinetics of solid−solid phase transformations.4 Although many molecular clusters of Sn are known, most of them are derived from Zintl ions, and consequently contain mostly metallic bonded and low valent Sn.5 Until now, the largest structurally characterized clusters with all 4-coordinate Sn include Sn(SnPh3)4 and various square, ring, prism, and cube arrangements with up to 10 Sn atoms.6 The tin atom has great structural flexibility, allowing clusters to accommodate © XXXX American Chemical Society
bulky ligands, accounting for the wide array of structures reported. In order to produce large clusters having the geometry characteristic of gray tin, less bulky ligands would be desirable. The covalent bonding of gray tin can be surmised to most resemble that in molecules with Sn(IV), such as organotin compounds, even though all Sn atoms in any allotrope of elemental Sn are by definition Sn(0).7 Thus, one rational approach to the synthesis of large gray tin clusters would involve condensation reactions between organo-substituted and other Sn(IV) compounds. Here, we report a 14 Sn atom cluster where all the Sn atoms are 4-coordinate. Although the atomic arrangement of the core does not exactly match that of gray tin, this synthesis is a significant step toward producing molecular gray-tin-like clusters that show quantum confinement effects. Indeed, for our pertinent cluster, our density functional theory (DFT) calculations predict a band gap of 1.87 eV,9 which is well within the visible spectrum.
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RESULTS AND DISCUSSION In an attempt to prepare gray tin clusters, a parallel approach was used in which Me3SiCH2SnCl3 was combined with a variety of Sn compounds and reducing agents in separate vials under an argon atmosphere. Orange crystals of Sn14Cl6(CH2SiMe3)12 (1) were isolated after several months as a minor product from a reaction between Me3SiCH2SnCl3,8 SnCl4, and Received: December 12, 2017
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DOI: 10.1021/acs.inorgchem.7b03092 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry LiAlH4 in a mixture of heptane, toluene, and THF.9 A single crystal from this reaction was used for the X-ray structure. The synthesis was then repeated multiple times by varying the quantities of the same reactants in separate vials. The formation of 1 seems to be most favored by a Cl− to H− ratio of 1.2−1.8, a large, but not overwhelming, excess of SnCl4, and by conducting the reaction in a tightly closed container from which gases do not escape. [Caution! This reaction can produce toxic SnH4 gas and organotin hydrides. Use due care.] Performing the reaction with stoichiometric reactant ratios assuming that 1 is the only product produced only tiny amounts of 1. The preparation was also scaled up with a yield of 22% with reactant ratios that are fairly well optimized.9 Samples from subsequent syntheses were used for the 117Sn MAS NMR, UV−vis, and XPS microprobe measurements. The crystal structure (Figure 1) has point group symmetry S6 (3̅) and contains three
Figure 2. Atomic arrangement in 1 with SiMe3 groups not shown. Sn1−Sn1A 2.8341(4) Å, Sn1−Sn2 2.80183(15), Sn2−Sn3 2.79867(18), Sn2−Sn3B 2.80390(17), Sn2−C11 2.1596(16), Sn3− C21 2.1503(16), Sn3−Cl(1) 2.4892(4), Sn2−Sn1−Sn2E 118.714(2)°, Sn2−Sn1−Sn1A 96.561(4), Sn1−Sn2−Sn3 96.113(4), Sn3−Sn2− Sn3B 94.077(4), Sn1−Sn2−Sn3B 103.780(5), Sn2−Sn3−Sn2C 105.228(6), C11−Sn2−Sn3 122.35(5), C11−Sn2−Sn1 119.24(5), C11−Sn2−Sn3B 116.57(5), C21−Sn3−Cl(1) 96.70(5), C21−Sn3− Sn2 122.97(5), Cl(1)−Sn3−Sn2 102.886(11), C21−Sn3−Sn2C 125.93(5), Cl(1)−Sn3−Sn2C 94.828(11).
and metalloid clusters which contain naked Sn atoms bonded only to other Sn atoms. By these definitions, 1 is formally a metalloid cluster, but has the greatest structural similarity to the regular clusters in that all Sn atoms are 4-coordinate and approximately tetrahedral. Perhaps a better terminology than “regular cluster” is “purely covalent-bonded cluster”, which would include 1, various polystannane rings, Sn(SnPh3)4, and the previously reported SnxRx cages. All the Sn−Sn bond distances in 1 are very near the value of 2.8108 Å in gray tin, and differ greatly from the much longer distances of 3.0162 and 3.1749 Å in metallic tin, which shows the bonding in 1 resembles that in α-Sn.12 Similar Sn−Sn distances are observed in other compounds with only 4-coordinate Sn. For example, the Sn−Sn bonds in the cyclic structure (PhSn)6 range from 2.775(1) to 2.784(1) Å,6b the Sn−Sn bonds in Sn(SnPh3)4 are 2.7983(6) Å,6a and those in the square ((Me3SiCH2)2Sn)4 range from 2.829(1) to 2.834(1) Å.6c Generally, bulkier ligands result in more steric crowding and thus slightly longer Sn−Sn bonds, and the vast majority of reported clusters with all 4-coordinate Sn have bulky ligands. A series of clusters with 2,6 substituted aromatic ligands shows this phenomenon well and have Sn−Sn distances from 2.839(2) to 3.367(1) Å.6d−g Clusters with hypersilyl or supersilyl ligands have even longer bonds. For example, the cube Sn8(Si(CMe3)3)6(NaTHF2)2 has Sn−Sn bond distances from 2.880(2) to 2.904(2) Å,6h and the prism (Sn(Si(CMe3)3)6 has Sn−Sn distances from 2.903(1) to 2.941(1) Å.6i In contrast, 1 appears to have little steric crowding, with only the bond between the central Sn1−Sn1A atoms being slightly longer than the bonds in gray tin. Clusters with Sn atoms of higher coordination (metal-like) tend to have longer bonds, as in Sn10[Si(SiMe3)3]6, where the bond lengths between 4coordinate Sn atoms are 2.855(1) Å and those between the metallically coordinated Sn atoms extend up to 3.142(1) Å.5g
Figure 1. Structure diagram of 1, with inequivalent atoms labeled. Atom ellipsoids are at the 30% level, and H atoms are not shown.
inequivalent Sn atoms. Sn1A is equivalent to Sn1 by inversion. Both core Sn1 atoms lie on the structure’s rotation axis. Multiple applications of the 6-fold improper rotations around this axis generate the 12 outer Sn atoms from the two inequivalent Sn atoms Sn2 and Sn3. The Sn2 atoms have one Me3SiCH2 ligand and are attached to two Sn3 atoms and one core Sn1 atom, while the Sn3 atoms are bonded to a Me3SiCH2 ligand, a Cl atom, and two Sn2 atoms. The Sn−Sn bond distances range from 2.79867(18) to 2.8341(4) Å, and the Sn−Sn−Sn angles range from 94.077(4)° to 118.715(2)°, i.e., within 13° of the ideal tetrahedral angle. This variation in the bond angles of the Sn core can be explained by the atom connectivity. Although the 4coordination at each Sn atom resembles that in gray tin, the Sn atoms in 1 are all part of fused 5-membered rings, each of which include Sn1, Sn1A, two Sn2’s, and one Sn3. In contrast, all angles in gray tin are 109.5°, with each atom part of a 6membered ring. An enlarged diagram of the Sn core of 1 is shown in Figure 2. All the Sn−C distances, averaging 2.1549 Å, are normal.10 Reported Sn clusters fall into three types as defined by Wiederkehr et al.:11 Zintl clusters derived from Zintl anions, regular clusters with equal numbers of Sn atoms and ligands, B
DOI: 10.1021/acs.inorgchem.7b03092 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry Clusters with unligated Zintl-derived Sn9n− anions tend to have Sn−Sn bond lengths similar to metallic Sn, and the Sn15R6 clusters have average Sn−Sn bond lengths of 3.15 Å.5f An unusual cluster with 4-coordinate, but ligand-free, Sn atoms has Sn−Sn distances of 2.9464(14)−3.32(7) Å, clearly in the metallic range.5i We were only able to observe proton spectra and 13C NMR spectra with poor signal-to-noise of 1 in solution, due to low solubility in C6H6 and THF, even when heated. However, we were successful in obtaining a single pulse 117Sn spectrum in the solid state with magic angle spinning (MAS) and without proton decoupling on a 4 mg sample, as shown in Figure 3.
frequency). For example, the cube cluster (C6H3Et2Sn)8 is at 44.3 ppm,6d the pentagonal prism (with the same ligands) is at −21.3 ppm,6f and the prism Sn8(2,6-Mes-C6H3)4 bearing extremely bulky ligands has signals very downfield of +483.1 and +751.7 ppm.6j A large upfield shift of the Sn1’s in 1 is to be expected based on the chemical shift of −1094 ppm for the central Sn in (Ph3Sn)4Sn, and −910 ppm for gray tin itself (which we measured from the 119Sn NMR spectrum under static conditions of a sample of gray tin).9 The 119Sn chemical shifts of Zintl-derived and metallic Sn clusters tend to be to upfield from gray tin, as in Sn94−, which is at −1230 ppm, and the Sn5 propellane [(C6H3Et2)2Sn]3Sn2, which has the organicligated Sn at +356 ppm and the central Sn atoms at −1751 ppm.6e A theoretical computation of the 117Sn absolute chemical shielding tensors in 1 was carried out using DFT methods, as well as for the chemical shift reference, Me4Sn.9 From these results, the calculated isotropic chemical shift values of 1 are −427, +112, and +77 ppm for Sn1, Sn2, and Sn3, respectively. The predicted shift of −427 ppm for Sn1 reproduces the upfield character of the chemical shift of Sn1. For the remaining two peaks, it is difficult to decide which of the two leftmost experimental peaks should be assigned to Sn2 or Sn3 based upon the theoretical predictions of isotropic chemical shifts. Differences in the calculated chemical shift anisotropies might help resolve this assignment ambiguity, since the middle peak in the experimental spectrum exhibits more intense spinning sidebands than the other two peaks, reflecting a larger chemical shift anisotropy. Relative spinning sideband intensities predicted from the DFT results9 are in qualitative, albeit not quantitative, agreement with a tentative assignment of the middle peak to Sn2 and the leftmost peak to Sn3.
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Figure 3. 117Sn MAS NMR spectrum at 178.17 MHz (11.7 T) of 1 at a spinning speed of 22.0 kHz. A 68° pulse was used (the 90° pulse width was 2.0 μs) with 2784 acquisitions (93 h total). The delay before acquisition was 8 μs, and an apodization corresponding to a linebroadening of 2.0 ppm was applied. Spinning sidebands are indicated with *.
CONCLUSIONS In conclusion, we have demonstrated a promising approach to the synthesis of molecular clusters of gray tin. Using the crystal structure determined by X-ray diffraction, DFT calculations on 1 yield a band gap (1.87 eV) and 117Sn chemical shifts that compare well with experimental results.9 Additionally, the 119Sn chemical shift of bulk gray tin reported here provides a valuable benchmark for assessing the similarity of electronic structure between bulk and cluster forms of gray tin by NMR. We believe that applying a similar synthetic approach to our synthesis of Sn14Cl6(CH2SiMe3)12 may enable the synthesis of larger clusters having band gaps in the THz or IR. With larger clusters, it may even be possible to study the effects of particle size, surface energies due to ligand binding, and quantum confinement on the stability of the allotropic forms of tin.
Three distinct peaks at isotropic chemical shifts of 87.9, −66.6, and −607.1 ppm were observed, along with several weak spinning sidebands. Sidebands were identified from a spectrum obtained at a 25 kHz spinning speed. A long recycle delay of 120 s was used because of the typically long T1 values observed for 117,119Sn NMR in solids. While 117Sn is not the usual isotope reported in the literature, chemical shifts for 117Sn and 119Sn are normally assumed to be equal,13b and we verified that is the case here.9 The intensity ratio of the peaks combined with their relevant sidebands is very close to the 3:3:1 ratio of unique Sn atoms in 1,9 but only the peak at −607 ppm can be definitively assigned to Sn1, while the assignment of the 87.9 ppm and −66.6 ppm peaks can be tentatively assigned to Sn2 and Sn3, respectively, based on the DFT results below. All the peaks are somewhat broader than typical for crystalline compounds, which we attribute to unresolved J couplings between tin nuclei (and protons).9 Proton MAS NMR shows the presence of solvent in the crystals,9 which may be present as inclusions, since the voids in the crystal structure are not large enough for heptane or toluene. The 119Sn chemical shifts in other reported clusters show a very wide range of values. As a general trend, Sn atoms with smaller Sn−Sn−Sn angles and longer Sn−Sn bonds usually have substantially downfield (higher frequency) chemical shifts, while Sn atoms without ligands are shifted upfield (to lower
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EXPERIMENTAL SECTION
General Comments. All reactions and manipulations were done inside a Vacuum-Atmospheres Dri-Lab under argon. All solvents were dried and distilled by standard methods (heptane, toluene − distillation from molten Na; THF from Na+OCPh2−; C6D6 from Na/K) prior to use. Flame-sealed NMR tubes were used for all solution NMR spectra, which were recorded on a Bruker Avance-300. The data for the single crystal structure were recorded on a Bruker Apex 2 diffractometer using Mo Kα radiation. Details of DFT calculations are given in the Supporting Information. Synthesis of Me3SiCH2SnCl3. In a similar manner to ref 8, SnCl2 (11.75 g, 62 mmol), 30 mL of THF, and Me3SiCH2Cl (18 mL, 129 mmol) were combined in a Pyrex test tube in a Newport Scientific Superpressure 2′′ OD/1′′ ID 316 SS reaction vessel. The vessel was heated in a furnace for 36 h at 230−250 °C where the external temperature of the pressure vessel sticking out of the furnace was C
DOI: 10.1021/acs.inorgchem.7b03092 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry 150−165 °C (presumably reflecting the actual internal temperature). Product was distilled twice under vacuum at 64−65 °C to afford 12.38 g of Me3SiCH2SnCl3 (64%). NMR (C6D6, 300 MHz): 1H (C6D5H = 7.15) −0.12 (SiMe3), 0.30 (CH2, 2J 1H-117Sn 67.4 Hz, 2J 1H-119Sn 70.6 Hz); 13C (C6D6 = 128) 0.08 (SiMe3), 16.32 (CH2, 1J 13C−117Sn 220.6 Hz, 1J 13C-119Sn 231.1 Hz); 119Sn 21.77 (HHLW = 300 Hz). Synthesis of 1. (a) To a screw cap vial under argon was added 0.25 mL of toluene, 0.20 mL of a 0.33 M solution of Me3SiCH2SnCl3 in heptane, 0.20 mL of a 0.25 M solution of SnCl4 in heptane, and 0.40 mL of a 0.25 M solution of LiAlH4 in THF. After several months, orange crystals of 1 formed, particularly at the surface of Sn metal crystals. Additional sets of test reactions, where the ratios of the reactants and the solvents used were systematically varied, are described in the Supporting Information. (b) A large scale prep of 1 was done using the reactant ratios identified in one of those test reactions (rxn A5 in the Supporting Information). Me3SiCH2SnCl3 (1.04 g, 3.33 mmol) and SnCl4 (0.325 g, 1.25 mmol) were mixed in 7 g of heptane in a Pyrex reaction tube, a solution of LiAlH4 (0.095 g, 2.50 mmol) in 2 g of THF was added slowly, and the Kontes Teflon valve was secured. After 1 day, orange crystals began to form, and after 3 months, the solids were separated from the liquids and washed several times with THF. The solids consisted of orange crystalline powder and metallic Sn. The largest chunks of Sn were mechanically removed, and the mixture was mixed with ether and 3 g of Hg to dissolve the Sn. It was swirled around occasionally over several days and the liquids were removed. After drying, 0.176 g of orange powder remained (22% isolated yield based on Me3SiCH2SnCl3). Several crystals were examined by single crystal X-ray diffraction and had the same unit cell as 1. The powder pattern of the product matched the peak positions for a calculated pattern of 1.9 (c) A prep was performed using stoichiometric reactant ratios designed to give 1, under the assumption that 1 is the only Sncontaining product. Me3SiCH2SnCl3 (1.00 g, 3.20 mmol) was mixed with SnCl4 (0.14 g, 0.537 mmol) in 30 mL of heptane in a reaction bulb equipped with a Kontes Teflon valve. LiAlH4 (0.10 g, 2.63 mmol) in 1 mL of THF was added and the reaction bulb was quickly closed. After 1 day, there was an orange solution and a brown mud-like solid. The bulb was sonicated for 15 min to break up the solids and let stand for 4 months. Then the reaction was filtered and washed with 25 mL of THF. Only 50 mg of solids was isolated on the filter frit. The powder pattern9 showed the presence of both 1 and elemental Sn. Synthesis of Gray Tin. Our gray tin sample was prepared by taking a disk of tin prepared by melting mossy tin (Baker), scraping the surface with a razor blade, and using a hammer to crush a small piece of an InSb wafer and drive it into the ductile white tin to serve as a nucleating agent. The disk was placed in a −25 °C freezer for several months with occasional shaking to bring the crumbly gray tin that formed into contact with remaining portions of the white tin disk, and the identity of the gray tin was verified by powder diffraction (JCPDS card 05-390). The (static) 119Sn NMR spectrum was obtained at 111.92 MHz (7.05T) on a Bruker Avance-300. Elemental analysis of 1 was performed by X-ray photoelectron spectroscopy (XPS). Sn, Si, C, and Cl were the only elements observed in the sample. Atomic ratios were evaluated by fitting the Sn 3d, Si 2p, C 1s, and Cl 2p regions and normalizing to Cl. Experimental (predicted): Sn, 2.0(8) (2.33); Si, 2.1(9) (2.00); C, 8.4(4) (8.00); Cl, 1 (1). Binding energies were consistent with the crystal structure: Sn 3d, 485.4 eV; Si 2p, 100.0 eV; C 1s, 283.7 eV, Cl 2p, 198.3 eV. The data are shown in the Supporting Information. Solid State MAS NMR. Magic angle spinning (MAS) NMR experiments were carried out on a Varian/Agilent DDRS 11.7 T NMR spectrometer (500 MHz 1H Larmor frequency) using a 3.2 mm tripleresonance probe operating at 178.2 MHz for 117Sn and 186.5 MHz for 119 Sn. Low-temperature operation was achieved using an FTS chiller to deliver cold nitrogen gas around the zirconia rotors and samples, with temperature measured by a thermocouple just at the entrance. The 117 Sn and 119Sn chemical shifts were referenced using a secondary reference of tetracyclohexyltin at room temperature, and converted to the primary tetramethyltin chemical shift reference by adding −97.3
ppm.14 A chemical shift using the Ξ scale (ratio of 119Sn frequency to H frequency of TMS) is also reported in the Supporting Information, as well as a discussion of experimental isotope effects showing that 117 Sn and 119Sn chemical shifts are essentially equivalent. For 117Sn, the 90° pulse length was 5.3 μs, and for 119Sn, the 90° pulse length was ca. 3 μs. Single pulse spectra with long relaxation delays (ca. 1 min) were used because of the long T1 values for the tin nuclei. No 1H decoupling was used for the spectra shown in the text and Supporting Information; high-power decoupling with an rf field strength of 50 kHz was seen to have only very minor effects upon the line widths of the 119 Sn peaks.9 Crystal data for 1: C48H132Cl6Si12Sn14, a = 23.6880(10) Å, b = 23.6880(10) Å, c = 15.8713(7) Å, α = 90°, β = 90°, γ = 120°, V = 7712.6(7) Å3, space group R3̅, Z = 3, T = 123(2) K, 28 626 reflections measured, 4654 unique [R(int) = 0.0297], final R1 = 0.0155, wR2 = 0.0312 for 4295 data [I > 2σ(I)]. 1
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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.7b03092. Additional experimental data and methods, and notes (PDF) Accession Codes
CCDC 1589223 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
[email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Andrew P. Purdy: 0000-0001-5274-0818 Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank Professor Chris Cahill and George Washington University for access to their diffractometer, and the Office of Naval Research for financial support.
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REFERENCES
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DOI: 10.1021/acs.inorgchem.7b03092 Inorg. Chem. XXXX, XXX, XXX−XXX
