Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
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Synthesis, Structure, and Electronic Properties of Sn(CN2) and Sn4Cl2(CN2)3 Manuel Löber, Konstantin Dolabdjian, Markus Ströbele, Carl P. Romao, and Hans-Jürgen Meyer* Section of Solid State and Theoretical Inorganic Chemistry, Institute of Inorganic Chemistry, University of Tübingen, Auf der Morgenstelle 18, D-72076 Tübingen, Germany
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ABSTRACT: A solid-state metathesis reaction between equimolar amounts of Li2(CN2) and SnCl2 revealed the formation of two new compounds, Sn4Cl2(CN2)3 and Sn(CN2). Thermal analysis of this reaction indicated that Sn4Cl2(CN2)3 forms exothermically near 200 °C and subsequently transforms into Sn(CN2) at higher temperatures. The crystal structures of both compounds are presented. According to optical measurements and band structure calculations, Sn(CN2) can be considered as a semiconductor with a band gap on the order of 2 eV. The presence of Sn2+ ions in the structure of Sn(CN2) with a toroidally shaped lone pair is indicated by electron localization function calculations. The structure of Sn(CN2) is shown to be related to the structures of FeS2 and CaC2.
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INTRODUCTION The development of metal dinitridocarbonates, which assume the carbodiimide (NCN)2− or cyanamide (N−CN)2− moiety, has progressed to include compounds with rare earth,1,2 transition metal,3−7 and main group elements.8−12 These compounds are occasionally considered as pseudooxides with a spatially elongated (NCN)2− ion. Carbodiimides of the group IV elements are exemplified by Si(CN2)2,13 Pb(CN 2 ), 14 and the structurally related compounds APb2Cl3(CN2) (A = Li, Na, Ag).15 The structures and optical properties of several tetracyanamido-metallates with a pseudotetrahedrally shaped [T(CN2)4]n− ion (T = Si, Ge, Ga, Al) have been reported.16−19 However, this series of compounds is not continued for T = Sn and Pb. Tin carbodiimides can appear in two different oxidation states. Sn(IV) compounds have been reported as A2Sn(CN2)3 with A = Li or Na,20 and Sn(II) is present in the carbodiimideoxide Sn2O(CN2).21 Sn2O(CN2), like SnO, is a semiconductor with a reported band gap on the order of 1−2 eV. Tin atoms in the PbO-type crystal structure of α-SnO are surrounded with O2− in a square fashion, yielding a [SnO4] square pyramid; a similar Sn2+ environment is found in Sn2O(CN2). This typical arrangement is consistent with Sn2+ possessing a stereochemically active lone pair. The recently reported compound Sn2O(CN2) was originally obtained as a minority phase in the synthesis of Li2Sn(CN2)3, demonstrating that reactions in this system can produce divalent or tetravalent tin. A thermoanalytical study of the synthesis of Sn2O(CN2) revealed the presence of intermediate compounds during the reaction; one of these was proposed to be Sn4Cl2(CN2)3. A closer study of reactivity in the Li2(CN2)/ SnCl2 system now confirms the compound Sn4Cl2(CN2)3 as an © XXXX American Chemical Society
intermediate phase which also appears in the formation of the new compound Sn(CN2). The structures and properties of both compounds are the subject of this work.
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EXPERIMENTAL AND CALCULATION DETAILS
Synthesis of Sn(CN2). Li2(CN2)1 and SnCl2 (Sigma-Aldrich 99.999% ultradry) or SnF2 (Sigma-Aldrich 99%), respectively, were mixed in an agate mortar under a dry argon atmosphere (glovebox) in 1:1 molar ratio (total mass: 300 mg). The mixture with SnCl2 was sealed into a silica tube under vacuum, and the mixture with SnF2 was sealed into a niobium tube by arc-welding (and then fused into an evacuated silica ampule). Both reaction mixtures were heated in a crucible furnace up to 450 °C with a heating rate of 3 K min−1 and held at this temperature for 20 h before cooling to room temperature at 2 K min−1. Reaction products were washed with ethanol and dried at 80 °C in a drying oven. Synthesis of Sn4Cl2(CN2)3. Li2(CN2)1 and SnCl2 (Sigma-Aldrich 99.999% ultradry) were mixed in a 3:4 molar ratio in an agate mortar and sealed into a silica tube under vacuum. The mixture was heated in a crucible furnace to 250 °C with a heating rate of 3 K/min and kept at this temperature for 5 h before being cooled to room temperature at 3 K/min. The crystalline powder was removed from the ampule, washed with ethanol, and dried at 80 °C in air. Thermoanalytic Studies. Differential thermal analysis (DTA) was performed with a STA 449F3 Jupiter; Fa. Netzsch, Selb, Germany. Samples were fused into homemade silica containers and analyzed between room temperature and 500 °C with a heating and cooling rate of 2 K min−1. X-ray Powder Diffraction. X-ray powder diffraction (XRPD) was carried out with a powder diffractometer (STOE Darmstadt, STADIP, Ge-monochromator) using Cu Kα1 (λ = 1.540598 Å) radiation in Received: February 22, 2019
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DOI: 10.1021/acs.inorgchem.9b00527 Inorg. Chem. XXXX, XXX, XXX−XXX
Article
Inorganic Chemistry the range of 5 < 2θ < 120°. The recorded X-ray powder diffraction patterns were indexed, and the crystal structure was subsequently solved with the program EXPO2009.22 Structure refinement was performed using the Fullprof Suite.23 The structure refinement based on the preparation with SnCl2 showed few weak unmatched reflections of unknown origin. The refinement based on the synthesis of SnF2 revealed LiF and elemental Sn (
