Trigonal-Bipyramidal Coordination in First Ammoniates of ZnF2: ZnF2

Article pubs.acs.org/IC

Trigonal-Bipyramidal Coordination in First Ammoniates of ZnF2: ZnF2(NH3)3 and ZnF2(NH3)2 Theresia M. M. Richter,† Sylvain LeTonquesse,† Nicolas S. A. Alt,‡ Eberhard Schlücker,‡ and Rainer Niewa*,† †

Institute of Inorganic Chemistry, University of Stuttgart, 70569 Stuttgart, Germany Institute of Process Machinery and Systems Engineering, Friedrich-Alexander-University, 91058 Erlangen, Germany

‡

S Supporting Information *

ABSTRACT: Single crystals of ZnF2(NH3)3 and ZnF2(NH3)2 were obtained under ammonothermal conditions (250 °C, 196 MPa and 500 °C, 136 MPa). Upon thermal decomposition of both ZnF2(NH3)3 and ZnF2(NH3)2, a microcrystalline powder of ZnF2(NH3) was obtained. ZnF2(NH3)3 and ZnF2(NH3)2 represent probable intermediates in a conceivable ammonothermal synthesis of the semiconductor Zn3N2 and manifest a rare trigonal-bipyramidal coordination of F− and NH3 ligands around Zn2+ according to single-crystal X-ray diffraction. Thermal analysis of all three compounds showed not only ZnF2(NH3) but also ZnF2(NH3)2 to be decomposition intermediates of ZnF2(NH3)3 prior to the formation of ZnF2. All three compounds demonstrate hydrogen bonds, as indicated by the intensities and half-widths of the bands in the vibrational spectra and by short N−H···F distances in the crystal structures of ZnF2(NH3)3 and ZnF2(NH3)2. With ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3), we present the first ammoniates of ZnF2.

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neous reaction in ammonia as one homogeneous fluid close to or in the supercritical state, leading to nitrides, amides, imides, ammoniates, and non-nitrogen containing compounds”.12 Often, solvothermal and particularly ammonothermal crystal growth is carried out by chemical transport, driven by a temperature gradient. Mineralizers in ammonothermal synthesis are added to enhance the dissolution of the reactants, such as metals, through the formation of soluble complexes between the metal and NH2−, NH3, or halide ions (from mineralizers such as NH4F). Pure ammonia undergoes autoprotolysis, where a proton is transferred from the Brønsted acid NH3 to the Brønsted base NH3, resulting in amide ions NH2− and ammonium ions NH4+. The addition of a basic mineralizer, such as an alkali metal, an alkali metal amide, or an azide increases the NH2− concentration, whereas the addition of an acidic mineralizer, such as an ammonium halide NH4X (X = F, Cl, Br, I) or metal halide (e.g., GaCl3, ZnCl2) increases the NH4+ concentration. The formation of the soluble species depends strongly on the respective mineralizer, as do the material transport and solubility in general. Under certain conditions, intermediate compounds can be isolated. Specifically, under ammonobasic conditions, amidometalates are expected to be present, whereas under ammonoacidic conditions, mainly ammoniates of metal halides and amidometalates form. Zinc nitride is an intriguing semiconductor material with a narrow band gap of 0.9−1.2 eV,13−15 a carrier concentration of 1019−1020 cm−3, and an electron effective mass m* of 0.29 ±

INTRODUCTION Because of the successful growth of high-quality group III nitride semiconductors, such as GaN,1 and the beginning of their commercial application, ammonothermal synthesis has gained increasing research interest in recent years. There is a long tradition of solvothermal methods, dating back to Bunsen in 1839, who used sealed glass tubes with an integrated mercury manometer to carry out experiments with different condensed gases (e.g., HCN, NH3) at high pressures and high temperatures (200 °C, 15 MPa).2 Later, he demonstrated the suitability of this method for the growth of large crystals, obtaining millimeter-long BaCO3 and SrCO3 single crystals from aqueous solutions.3 Since then, an abundance of different solvothermal methods have been developed, for example, for αquartz from H2O,4 GaN from NH3,5 MnS from tetrahydrofuran,6 BN from C6H6,7 and poly(vinyl chloride) from CO2.8 Today, more than 3000 t of α-quartz single crystals is synthesized per year by commercial hydrothermal synthesis.9 Given the multitude of applied solvents and reaction conditions, it is not suprising that various definitions of solvothermal reactions have been developed that differ significantly. Rabenau’s definition for a hydrothermal system as “an aqueous medium over 100 °C and 1 bar”10 is widespread and was subsequently transferred to solvothermal conditions in general by adjusting the temperature to “the boiling point of the solvent”.11 Because most ammonothermal syntheses are carried out in a supercritical fluid (for pure ammonia, p ≥ 11.3 MPa, T ≥ 132 °C) and because the difference between a supercritical fluid and a fluid for which one physical quantity is slightly below the critical point is only marginal, we chose the following definition: “Ammonothermal synthesis is a heteroge© XXXX American Chemical Society

Received: December 11, 2015

A

DOI: 10.1021/acs.inorgchem.5b02837 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry

uncovered by the furnace and thus cooled by ambient air. This setup leads to a temperature gradient with a temperature difference of up to 100 K inside the autoclave. Such conditions are favorable for convection-driven chemical material transport. The constant heat loss at the cold end (head part) lowers the average temperature inside the autoclave. Thus, the temperature inside the autoclave is significantly lower than the furnace temperature. Pressure and temperature monitoring and comparison with literature data for pure ammonia led to a temperature difference of about 150 K.40 The following chemicals were used as purchased: ZnF2 (Fluka, technical, ∼99%), Zn (Alfa Aesar, 99.9% metal-basis), NH4F (Sigma-Aldrich, 99.99% metal-basis), and ammonia (99.999% anhydrous, Linde; purified with a MicroTorr MC400-720F apparatus for H2O, O2, and CO2 contamination of ≤1 ppbV). Because of the hygroscopic nature of the reactants and products, all handling was performed inside an argon-filled glovebox [p(O2) < 0.1 ppm]. A defined amount of ammonia was condensed in the autoclave by means of a tensieudiometer for simultaneous volume and pressure measurement.41 For this purpose, the autoclave body was placed into a dry ice/ ethanol cooling bath (T = −75 °C) and connected to the ammoniafilled tensieudiometer. After synthesis, the ammonia was removed from the autoclave by evaporation and subsequent evacuation with ∼10 mbar for about 5 s. Colorless transparent crystals of ZnF2(NH3)3 with a maximum length of 2 mm were obtained from ZnF2 (Fluka ID 96481, 97%) and supercritical ammonia under ammonothermal conditions of 250 °C (furnace temperature) and 197 MPa. According to powder X-ray diffraction, the sample was single-phase. Colorless transparent crystals of ZnF2(NH3)2 were obtained from the same starting materials at 500 °C and 136 MPa. The samples were collected from the cold upper zone of the autoclave and, therefore, were obtained by chemical material transport, as the reactant ZnF2 being loaded in the hot bottom zone of the reaction vessel. Both compounds could also be obtained from Zn and NH4F at similar temperatures and pressures. However, the higher level of oxygen impurities in NH4F compared to ZnF2, determined by elemental analysis through hot gas extraction, led to the use of ZnF2. Upon thermal decomposition at 86−112 °C of both ZnF2(NH3)3 and ZnF2(NH3)2, the same crystalline, currently unidentified compound was obtained. According to thermogravimetry, the composition corresponds to Zn(NH3)F2. The sample used for IR spectroscopy in this work was obtained by thermal decomposition of ZnF2(NH3)2 at 100 °C. Diffraction Data Collection. Powder X-ray diffraction patterns were collected on a STADI-P apparatus (STOE & Cie GmbH) equipped with a Mythen1K microstrip detector in transmission geometry using Mo Kα1 radiation (λ = 70.93 pm) at room temperature. The powder samples were fixed with grease between two foils, made of aluminum-coated biaxially oriented poly(ethylene terephtalate), to prevent contact and reaction with air. Single-crystal X-ray diffraction was conducted on a Bruker-Nonius Kappa-CCD diffractometer with a graphite monochromator and Mo Kα radiation (λ = 71.07 pm). The crystal structures were solved and refined with aid of the program package SHELX-2013 including XPREP.42 To solve the twinned structures, searches for additional symmetry and twin matrix were performed using PLATON.43 The crystal of ZnF2(NH3)3 is twinned by pseudomerohedry, with the twin law 1̅00 01̅0 001. The crystal of ZnF2(NH3)2 is also twinned by pseudomerohedry with the twin law 1̅00 01̅0 1̅01̅. For the ZnF2(NH3)3 data, all non-hydrogen atoms were located by direct methods and refined anisotropically. The hydrogen positions were refined from residual electron-density maxima, the isotropic displacement parameters were restrained to 1.2 times the Uiso value of the nitrogen atom to which they are attached. No constraints of position were applied, but the N−H distances were restraint to 0.89(2) Å. A linear scaling was applied using the Nonius software package. ZnF2(NH3)2 data were subjected to a numerical absorption using the X-SHAPE program package.44 The hydrogen positions were taken from residual electron-density maxima and were refined with a

0.05 m0 (for n-type Zn3N2 grown by radio-frequency molecular beam epitaxy13). In addition, it was successfully used as a precursor for the growth of p-type semiconducting ZnO, through nitrogen doping.16 Whereas microcrystalline powders can be easily obtained, for example, through the decomposition of Zn(NH2)2 at 330 °C or the reaction of Zn powder in an ammonia flow at 600 °C,17 no single-crystal growth of Zn3N2 is yet known. Thus, the material remains inaccessible for many applications. Ammonothermal synthesis is known to be a useful method for the synthesis of free-standing bulk nitride materials. Therefore, we believe it to be a conceivable synthetic route for obtaining single-crystalline Zn3N2. Whereas Zn2+ ions in tetrahedral (e.g., in Zn3N218) and octahedral (e.g., in ZnF219) environments are well-known, only a few examples of trigonal-bipyramidal coordination have been reported: For example, in [Zn(N(CH2CH2N(CH3)2)3)Br]Br, zinc is coordinated by one Br− ion (axial position) and four nitrogen atoms from one N(CH2CH2N(CH3)2)3 ligand (one axial and three equatorial positions).20 There are also compounds containing F − ligands contributing to the trigonal-bipyramidal coordination around Zn2+, such as [Zn2L(BF4)3(H2O)]BF4·CH3CN·H2O (with L = 6-hydroxymethylacryloyl-2-pyridinecarboxaldehyde), 2,2′-[2,2′-(2- methyl-4,6pyrimidinediyl)bis(1-methylhydrazone)], 21 and [ZnF(AmTAZ)]·solvent (with AmTAZ = 3-amino-1,2,4-triazole), where the F− ions occupy the axial positions and the nitrogen atoms (from three different ligands) occupy the equatorial positions.22 The herein-presented compounds diammine difluorido zinc(II) [ZnF2(NH3)2] and triammine difluorido zinc(II) [ZnF2(NH3)3] also manifest trigonal-bipyramidal coordination for Zn2+. Recently, a trigonal-bipyramidal environment of five NH3 ligands at Cu2+ in an ammoniate of copper(II) fluoride ([Cu(NH3)5]F2·NH3) was reported.23 Several ammoniates of metal fluorides are known. Some of them contain both F− and NH3 ligands coordinating the metal cation, as in [UF4(NH3)4]·NH3.24 So far, only a few ammoniates of zinc halides have been reported, namely, [Zn(NH3)2Cl2],25 [Zn(NH3)2Br2],25 [Zn(NH3)4]Br2,26 and [Zn(NH3)4]I2,26 all constituting tetrahedral coordination of zinc. From the ammonobasic milieu, a few alkali metal amidozincates have already been obtained, namely, Li4[Zn(NH2)4](NH2)2,27 Na2[Zn(NH2)4]·1/2NH3,28 K2[Zn(NH2)4] in two modifications,29−31 Rb2[Zn(NH2)4],32 and Cs2[Zn(NH2)4],33,34 all with tetrahedral coordination of zinc. Unlike the hydrates of ZnF2 {Zn(H2O)4F2,35 Zn(H2O)2F2,36 and [ZnF4](NH3OH)2,37 all with Zn in octahedral coordination}, no ammoniates of ZnF2 are yet known. However, the synthesis and NH3 decomposition pressures were reported for ZnF2· 2H2O·4NH3, ZnF2·2H2O·3NH3, and ZnF2·2H2O·1/2NH3.38 Here, we report the ammonothermal synthesis of the first ammoniates of zinc fluoride, namely, ZnF2(NH3)3 and ZnF2(NH3)2, from NH3 and ZnF2, as well as the synthesis of ZnF 2 (NH 3 ) through the thermal decomposition of ZnF2(NH3)3 and ZnF2(NH3)2.

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EXPERIMENTAL SECTION

Custom-built alloy 718 (austenitic nickel-chromium-based superalloy) autoclaves with a volume of 97 mL were used for the ammonothermal syntheses.39 The head part of the autoclaves includes a rupture disk, a pressure transmitter for continuous pressure monitoring during synthesis (HBM P2VA1/5000 bar), a filler pipe, and a high-pressure valve. Tubular furnaces (LOBA, HTM Reetz GmbH) in a vertical position served to heat the autoclave body, with the head part being B


Article

Inorganic Chemistry

Table 1. Selected Crystal Structure Data and Measurement Information from Single-Crystal X-ray Diffraction Data on ZnF2(NH3)3 and ZnF2(NH3)2 crystal system space group a (Å) b (Å) c (Å) β (deg) volume (Å3) density (calcd) (g/cm3) Z diffractometer radiation F(000) index ranges h/k/l 2θmax (deg) abs coeff μ (mm−1) abs correction reflns collected/independent Rint/Rσ structure solution, refinement R1 for 1907 |Fo| ≥ 4σ(Fo) R1/wR2/GOF (all data) params refined restraints extinction coeff BASF twin law ρ (106 pm−3) largest e− diff peak/hole

ZnF2(NH3)3

ZnF2(NH3)2

monoclinic P21/c 7.7180(4) 10.8614(5) 11.4777(3) 90.156(3) 962.15(7) 2.133 8 KAPPA CCD Mo Kα, λ = 71.073 pm 624 ±9/±13/±14 54.49 5.01 linear scaling Nonius software47 3960/2139 0.0328/0.0435 SHELX-201342 0.0277 0.0343/0.0648/0.991 165 18 0.0033(6) 0.112(9) 1̅00 01̅0 001 2 1.04/−0.55

monoclinic P21/m 7.5319(3) 4.2808(2) 12.0187(5) 108.267(3) 367.99(3) 2.481 4 KAPPA CCD Mo Kα, λ = 71.073 pm 272 ±9/from −4 to 5/±15 53.36 6.53 numerical X-SHAPE3244 7893/883 0.0599/0.0254 SHELX-201342 0.0170 0.0270/0.0456/1.068 83 8 0.008(1) 0.503(1) 1̅00 01̅0 001 2 0.66/−0.46

semifree approach; the isotropic displacement parameters were restrained to 1.2 times the Uiso value of the nitrogen atom to which they are attached. During refinement, the N−H distances were restraint to 0.89(2) Å. Further details on the crystal structure investigations are available from the Fachinformationszentrum Karlsruhe, D-76344 EggensteinLeopoldshafen, Germany (Fax: +49-7247-808-666, e-mail crysdata@ fiz-karlsruhe.de, URL http://www.fiz-informationsdienste.de/en/DB/ icsd/depot_anforderung.html) upon quoting depository number 430377 for ZnF2(NH3)3 and 430378 for ZnF2(NH3)2. Raman Spectroscopy. The solid-state Raman spectra were measured on single crystals at ambient conditions. Prior to measurement, the single crystals were sealed into glass capillaries under an argon atmosphere. The data were collected with a Horiba XploRa Raman spectrometer coupled with a confocal polarization microscope (Olympus BX51). For the excitation of Raman scattering for the entire ZnF2(NH3)2 Raman spectrum and for the ZnF2(NH3)3 spectrum from 4000 to 2000 cm−1, a solid-state laser with a wavelength of 638 nm was used. For the ZnF2(NH3)3 spectrum from 2000 to 100 cm−1, a solid-state laser with a wavelength of 532 nm was chosen. IR Spectroscopy. A Thermo Scientific Nicolet iS5 FT-IR spectrometer with a fast-recovery deuterated triglycine sulfate (DTGS) detector on an iD5 ATR unit (smart orbit) equipped with a diamond crystal was used to collect FT-IR spectra from powder samples under an argon atmosphere. Thermal Analysis. A NETZSCH STA 449C thermal analyzer (NETZSCH, Selb, Germany) was used to conduct combined thermogravimetry (TG) and differential thermal analysis (DTA). The measurements were carried out in flowing argon at heating and cooling rates of 10, 5, and 1 K/min. Prior to measurement, the sample chamber was flushed three times with argon. The measurements were corrected for buoyancy effects. For all measurements, a subsequent powder X-ray diffraction phase analysis of the residues was performed.

Single-phase ZnF2(NH3)2 samples were subjected to several measurements from ambient temperature to different maximum temperatures (Tmax = 135, 150, 800 °C). The measurement at 800 °C was carried out with a temperature ramp of 10 K/min. For the measurements up to 150 and 135 °C, a heating and cooling rate of 5 K/min was chosen to avoid unintentionally exceeding the desired maximum temperature and causing subsequent accidental further decomposition. Samples of ZnF2(NH3)2 with minor ZnF2(NH3)2 impurities were used in measurements from ambient temperature to 600 °C (Tmax = 80, 150, 600 °C). Again, to avoid overheating of the furnace and unintentional decomposition of the decomposition intermediates, the two low-temperature measurements at 80 and 150 °C were carried out at a temperature ramp of 1 K/min, whereas the measurement at 600 °C was conducted at 5 K/min. The difference in the decomposition temperature of ZnF2(NH3)2 between the decomposition of ZnF2(NH3)3 and ZnF2(NH3)2 might be due to the different heating rates and inaccuracies in the temperature calibration.

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RESULTS AND DISCUSSION Formation of ZnF2(NH3)3, ZnF2(NH3)2, and Zn(NH3)F2. Diammine difluorido zinc(II) [ZnF2(NH3)2] and triammine difluorido zinc(II) [ZnF2(NH3)3] were grown from Zn and NH4F or from ZnF2 in supercritical ammonia, next to each other in the temperature range of 225−600 °C at 91−293 MPa. The reaction was carried out in a high-pressure autoclave placed in a horizontal tubular furnace. This setup results in two differently heated zones in the autoclave exhibiting a temperature gradient with a temperature difference of up to 100 K inside the autoclave.45 The reactants were loaded in the hightemperature zone of the autoclave, and the products were C


Article

Inorganic Chemistry Table 2. Selected Distances (Å) and Angles (deg) in ZnF2(NH3)3 distance (Å) Zn(1) Zn(1) Zn(1) Zn(1) Zn(1)

−N(1) −N(2) −N(3) −F(1) −F(2)

2.034(3) 2.041(3) 2.063(3) 2.155(2) 2.068(2)

Zn(2) Zn(2) Zn(2) Zn(2) Zn(2)

−N(4) −N(5) −N(6) −F(3) −F(4)

2.012(3) 2.021(3) 2.047(3) 2.151(2) 2.155(2)

Zn(1) Zn(1) Zn(2) Zn(2) Zn Zn

−N d̅ −F d̅ −N d̅ −F d̅ −N d̅ −F d̅

2.046 2.112 2.027 2.153 2.037 2.132

angle (deg) N(1)−Zn(1)−N(2) N(1)−Zn(1)−N(3) N(2)−Zn(1)−N(3) N(1)−Zn(1)−F(1/2) N(2)−Zn(1)−F(1/2) N(3)−Zn(1)−F(1/2) F(1)−Zn(1)−F(2) N(4)−Zn(2)−N(5) N(4)−Zn(2)−N(6) N(5)−Zn(2)−N(6) N(4)−Zn(2)−F(3/4) N(5)−Zn(2)−F(3/4) N(6)−Zn(2)−F(3/4) F(3)−Zn(2)−F(4)

128.8(1) 116.3(1) 114.9(1) 87.63(9), 87.3(1) 86.89(9), 88.2(1) 98.36(9), 93.23(9) 168.41(7) 132.2(1) 113.5(1) 114.0(1) 88.00(9), 86.65(9) 87.37(9), 87.51(9) 102.01(9), 90.94(9) 167.04(7)

Table 3. Atomic Coordinates and Isotropic Displacement Parameters Uiso in ZnF2(NH3)3 Zn(1) Zn(2) F(1) F(2) F(3) F(4) N(1) H(1A) H(1B) H(1C) N(2) H(2A) H(2B) H(2C) N(3) H(3A) H(3B) H(3C) N(4) H(4A) H(4B) H(4C) N(5) H(5A) H(5B) H(5C) N(6) H(6A) H(6B) H(6C)

site

x/a

y/b

z/c

Uiso (Å2)

4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e 4e

0.12839(5) 0.62754(5) 0.8777(3) 0.1343(3) 0.3677(3) 0.6221(3) 0.3653(4) 0.419(5) 0.351(5) 0.425(4) 0.8893(4) 0.836(5) 0.893(6) 0.813(4) 0.1267(4) 0.216(4) 0.127(5) 0.034(4) 0.3890(4) 0.603(5) 0.317(4) 0.330(4) 0.8668(4) 0.921(4) 0.134(5) 0.924(5) 0.6262(4) 0.715(4) 0.631(5) 0.544(4)

0.16762(3) 0.10850(3) 0.0098(2) 0.1827(2) 0.0120(2) 0.1908(2) 0.1211(3) 0.192(2) 0.081(3) 0.075(3) 0.1261(3) 0.074(3) 0.092(3) 0.188(3) 0.2237(3) 0.170(3) 0.236(3) 0.177(3) 0.0439(3) 0.029(2) 0.027(3) 0.092(3) 0.0453(3) 0.034(4) 0.026(2) 0.101(3) 0.2214(3) 0.178(3) 0.220(3) 0.172(3)

0.11017(3) 0.33955(3) 0.8060(1) 0.4990(2) 0.8091(1) 0.5103(1) 0.0468(2) 0.039(3) 0.980(2) 0.096(3) 0.0447(2) 0.093(3) 0.975(2) 0.035(3) 0.7576(2) 0.761(3) 0.324(2) 0.750(3) 0.3748(2) 0.590(3) 0.317(2) 0.423(3) 0.3791(2) 0.315(2) 0.581(3) 0.420(3) 0.7629(2) 0.791(3) 0.683(2) 0.787(3)

0.0179(1) 0.0187(1) 0.0234(4) 0.0224(4) 0.0235(4) 0.0226(4) 0.0199(5) 1.2Uiso[N(1)] 1.2Uiso[N(1)] 1.2Uiso[N(1)] 0.0208(6) 1.2Uiso[N(2)] 1.2Uiso[N(2)] 1.2Uiso[N(2)] 0.0205(5) 1.2Uiso[N(3)] 1.2Uiso[N(3)] 1.2Uiso[N(3)] 0.021(1) 1.2Uiso[N(4)] 1.2Uiso[N(4)] 1.2Uiso[N(4)] 0.0195(5) 1.2Uiso[N(5)] 1.2Uiso[N(5)] 1.2Uiso[N(5)] 0.0202(5) 1.2Uiso[N(6)] 1.2Uiso[N(6)] 1.2Uiso[N(6)]

temperatures below 300 °C. The best results for single-phase products were obtained for ZnF2(NH3)3 at 250 °C and 197 MPa and for ZnF2(NH3)2 at 500 °C and 136 MPa. The pressure makes no obvious contribution to the formation equilibria but is crucial for the material transport and, therefore, for the purity of the products. ZnO is a frequent impurity,

collected from the cold and hot zones after reaction. The use of ZnF2 instead of NH4F and Zn yields a purer product with fewer ZnO impurities. This is why it was used for the presented measurements here. Different synthesis temperatures and pressures showed that ZnF2(NH3)2 is the main product at higher temperatures and ZnF2(NH3)3 is the major product at D


Article

Inorganic Chemistry which was exclusively found in the hot zone of the autoclave. Successful material transport of the target products to the cold zone and their growth therein is a convenient way to separate the products from the impurities in the hot zone. However, material transport with crystal growth in the cold zone occurs only at sufficiently high pressures for the applied temperature; for example, at 500 °C, material transport did not occur below 136 MPa. A certain minimum synthesis pressure is apparently necessary for the stabilization of the rare trigonal-bipyramidal coordination, because reactions of ZnF2 in ammonia flows of 60 and 100 sccm at 150 and 250 °C, respectively, did not yield any ammoniates but instead, according to powder X-ray diffraction, gave phase-pure, unreacted ZnF2. ZnF 2 (NH 3 ) was obtained from ZnF 2 (NH 3 ) 3 and ZnF2(NH3)2 through thermal decomposition in the temperature range of 86−112 °C in an argon atmosphere at ambient pressure. Both decomposition residues consisted of the same crystalline compound whose composition was determined by thermogravimetry to be Zn(NH3)F2. The unit cell could not be determined from the powder X-ray diffraction pattern (see Supporting Information). Crystal Structure of ZnF2(NH3)3. ZnF2(NH3)3 crystallizes in the monoclinic space group P21/c. Additional crystallographic details are summarized in Table 1, bond lengths and angles are provided in Table 2, atomic coordinates and isotropic displacement parameters are reported in Table 3, and hydrogen-bond details are listed in Table 4. Two

Figure 1. Sections of the crystal structure of ZnF2(NH3)3. Dashed lines, hydrogen bonds; large gray spheres, Zn; white spheres, N; large black spheres, F; small black spheres, H.

in [Zn(N(CH2CH2N(CH3)2)3)Br]Br, the halide Br− is located in the axial position, and the four nitrogen atoms from the quadridentate N(CH2CH2N(CH3)2)3 ligand are located in both axial (one N atom) and equatorial (three N atoms) positions. Trigonal-bipyramidal coordinations are rare and, in the present case, might be stabilized by the high-pressure conditions applied (i.e., 197 MPa). The six equatorial Zn−N distances of the two distinct [Zn(NH3)3F2] units cover the range from 2.012(3) to 2.063(3) Å and are significantly shorter than the distances to the four axial F− ligands, which are in the range from 2.068(2) to 2.155(2) Å. These distances are similar to those in Zn3N2 [d(Zn−N) = 1.996(1)−2.262(1) Å]18 and ZnF2 [d(Zn−F) = 2.026(9) Å],19 with Zn2+ located in tetrahedral and octahedral coordinations, respectively. The observation of longer distances to ligands in axial positions than in equatorial positions agrees with literature data for d10 metal cations in trigonal-bipyramidal coordination.46 Astonishingly, one Zn−F distance, namely, Zn(1)−F(2), at 2.068(2) Å, is significantly shorter than the other three Zn−F bond lengths, which are located in the narrow range of 2.151(2)−2.155(2) Å. A nitrogen atom rather than a fluorine atom at this site does not seem likely, because such an occupation leads to a dramatic decrease of the isotropic displacement parameter of the atom (by a factor of 10) and a significant increase in the reliability values during structure refinements. However, the short distance might be due to the nearly quadratic-pyramidal coordination of this position by one zinc and four hydrogen atoms, compared to the octahedral coordination for F(1) and F(3) (one zinc and five hydrogen atoms) (see Figure 1). F(4) also shows a coordination of five but a significantly longer Zn(2)−F(4) distance of 2.155(2) Å. These two 5-fold coordination environments differ in geometry, however, being quadratic-pyramidal for F(2) and trigonal-bipyramidal for F(4). The quadratic-pyramidal coordination of F(2) with strong hydrogen bonds apparently induces the short distance. The axial angles [∠(F−Zn−F) = 167.04(7)−168.41(7)°], the equatorial angles [∠(N−Zn−N) = 113.5(1)−132.2(1)°], and the angles between equatorial and axial ligands [∠(F−Zn−N) = 86.65(9)−102.01(9)°] are close to the ideal values of 180°, 120°, and 90°, respectively.

Table 4. Hydrogen-Bond Lengths and Angles in ZnF2(NH3)3 with F Acting as the Acceptor and N Acting as the Donora

N(1)−H(1A)···F(4) N(1)−H(1B)···F(3) N(1)−H(1C)···F(3) N(2)−H(2A)···F(3) N(2)−H(2B)···F(1) N(2)−H(2C)···F(4) N(3)−H(3A)···F(3) N(3)−H(3B)···F(2) N(3)−H(3C)···F(1) N(4)−H(4A)···F(4) N(4)−H(4B)···F(1) N(4)−H(4C)···F(2) N(5)−H(5A)···F(1) N(5)−H(5B)···F(2) N(5)−H(5C)···F(2) N(6)−H(6A)···F(1) N(6)−H(6B)···F(4) N(6)−H(6C)···F(3) a

d(N−H) (Å)

d(H···F) (Å)

∠NHF (deg)

d(N···F) (Å)

0.88(2) 0.89(2) 0.88(2) 0.90(2) 0.88(2) 0.90(2) 0.90(2) 0.88(2) 0.88(2) 0.89(2) 0.88(2) 0.89(2) 0.86(2) 0.90(2) 0.88(2) 0.89(2) 0.91(2) 0.87(2)

2.05(2) 2.10(2) 2.15(2) 2.15(2) 2.14(2) 2.00(2) 2.15(2) 2.10(2) 2.27(3) 1.99(2) 2.10(2) 2.00(2) 2.14(2) 1.95(2) 2.06(2) 2.23(2) 2.01(2) 2.23(2)

158(4) 166(4) 163(3) 160(3) 175(4) 173(4) 161(3) 166(3) 150(3) 172(4) 173(3) 161(3) 161(3) 179(3) 157(4) 153(3) 171(3) 163(4)

2.877(4) 2.975(3) 3.009(3) 3.002(3) 3.019(3) 2.892(3) 3.015(4) 2.952(3) 3.066(4) 2.871(3) 2.977(3) 2.861(3) 2.963(3) 2.845(3) 2.892(3) 3.048(3) 2.918(3) 3.073(5)

N−H bond length restrained to 0.89 ± 0.02 Å during refinement.

crystallographically different Zn2+ ions are present in ZnF2(NH3)3. Both are trigonal-bipyramidally coordinated by three NH3 (equatorial position) and two F− ligands (axial position), resulting in complex isolated [Zn(NH3)3F2] units (see Figure 1). This arrangement correlates with the observation that, for trigonal-bipyramidal coordination, stronger σ donors prefer the equatorial position for d10 central ions, with NH3 being the stronger σ ligand.46 NH3 represents a pure σ donor, whereas F− additionally acts a a weak π donor, realizing π backbonding in the axial position. For comparison, E


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Inorganic Chemistry

are in axial positions. This arrangement again correlates with the observation that, for trigonal-bipyramidal coordination, stronger σ donors, here, NH3, prefer the equatorial position for central ions with a d10 configuration.46 In accordance with literature data for d10 metal cations in trigonal-bipyramidal coordination,46 in ZnF2(NH3)2, the Zn−F distance to the axial F− ligands [d = 2.1418(6) Å] is longer than the distances to the ligands in equatorial positions, with d̅(Zn− N) = 2.014 Å and d̅(Zn−Fe) = 1.935 Å (see Table 6). As in ZnF2(NH3)3, the refined interatomic distances are similar to those in Zn3N218 and ZnF2.19 The axial angles [∠(F−Zn−F) = 175.9(9)° and 174.0(8)°], the equatorial angles [∠(N−Zn−N/ F) = 112.6(5)−132.3(3)°], and the angles between equatorial and axial ligands [∠(F−Zn−N/F) = 88.1(3)−92.9(4)°] are close to the ideal values of 180°, 120°, and 90°, respectively. The occupation of the sites by fluorine or nitrogen atoms has been done in accordance with the isotropic displacement parameters. Additionally, hydrogen atoms at the nitrogen atoms can be found in the electron density map and refined with a semifree approach. There is no indication for any dynamic disorder of ammonia molecules, as all hydrogen atoms are involved in N−H···F hydrogen bonds (see Figure 2). The hydrogen-bond details are listed in Table 7. Each hydrogen is connected by hydrogen bonding to one F− ion of a nearby e 1 a ∞[Zn(NH3)2F2/2F ] chain. All fluoride ions are acceptors of N−H···F hydrogen bonds, with five hydrogen bonds in the case of the equatorial F(1) and F(3) ligands and one hydrogen bond in the case of the axial F(2) and F(4) ligands. Axial F− ligands act as acceptors, connecting chains of symmetry-equivalent zinc ions. Chains of the crystallographically different Zn(1) and Zn(2) are connected through N−H···F hydrogen bonds between the equatorial ligands (Figure 2). The hydrogen atoms at N(1) and N(4) are all connected to equatorial F(1) and F(3) ions as well as two hydrogen atoms at N(2) and N(3), whereas the remaining hydrogen atoms at N(2) and N(3) realize single F···H-hydrogen bonds with the axial F(2) or F(4) ions. Additionally, the crystal structure also is twinned by pseudomerohedry. A 2-fold axis as a twin operation contributes to systematic absence violations, making the cell appear to be orthorhombic. Transformation of this orthorhombic cell to the apparent monoclinic one and application of the twin law 1̅00 010̅ 101 leads to a significant decrease of the R values. More information is available in the Supporting Information. Thermal Analysis. ZnF2(NH3)2 decomposes in a two-step process, with both steps located in a narrow temperature range (see Figure 4). The first mass loss starting at 86 °C (12.20 wt %) corresponds to the mass of one formula unit of ammonia (calculated as 12.39 wt %) through the formation of a crystalline decomposition intermediate, probably the monoammoniate Zn(NH3)F2. An endothermic signal in the DTA curve with an onset at 135 °C accompanies this step in the decomposition. The second mass loss (11.05 wt %) takes place from 168 to 237 °C and can be ascribed to the formation of ZnF2 through the release of another formula unit of ammonia. The second decomposition step is manifested by an endothermal DTA signal with an onset at 200 °C. According to the remaining weight (76.19 wt %, expected 75.22 wt %) and powder-X-ray-diffraction-pure ZnF2 remains in the crucible after measurement. The deammination process of ZnF2(NH3)3 with minor ZnF2(NH3)2 impurities occurs in two steps through the formation of two decomposition intermediates, namely,

Additional information about interatomic distances and angles is available from Table 2. The involvement of all hydrogen atoms in N−H···F hydrogen bonds (see Figure 1) implies the absence of dynamic disorder of the ammonia molecules. The hydrogen bonds connect the isolated [Zn(NH3)3F2] units to form a three-dimensional network (see Figure 1). The [Zn(NH3)3F2] units are arranged in zigzag chains alternating Zn(1) and Zn(2) connected through hydrogen bonds (see Figure 1). Within one zigzag chain, the [Zn(1)(NH3)3F2] units are exclusively linked to [Zn(2)(NH3)3F2] units and vice versa. However, between the zigzag chains, hydrogen bonds also connect crystallographically equivalent units. All hydrogen atoms are involved in one N−H···F hydrogen bond, and all fluoride ions are involved in four [F(2) and F(4)] or five [F(1) and F(3)] N−H···F hydrogen bonds. From Figure 1, it can be seen that the complex [Zn(NH3)3F2] units are linked to each neighboring [Zn(NH3)3F2] unit, providing one accepting atom and one donating atom. Furthermore, the crystal structure is twinned by pseudomerohedry. The twin operator is a 2-fold axis, which leads to systematic absence exceptions, indicating an apparent orthorhombic primitive cell. Integration in the monoclinic cell and application of the twin law 1̅00 01̅0 001 results in a considerable drop of the R values. More information is available in the Supporting Information. Crystal Structure of ZnF2(NH3)2. ZnF2(NH3)2 crystallizes in the monoclinic space group P21/m. Additional crystallographic details are available from Tables 1 and 5; bond lengths Table 5. Atomic Coordinates and Isotropic Displacement Parameters Uiso in ZnF2(NH3)2 Zn(1) Zn(2) F(1) F(2) F(3) F(4) N(1) H(1A) H(1B) N(2) H(2A) H(2B) N(3) H(3A) H(3B) N(4) H(4A) H(4B)

site

x/a

y/b

z/c

Uiso (Å2)

2e 2e 2e 2e 2e 2e 2e 2e 4f 2e 2e 4f 2e 2e 4f 2e 2e 4f

0.2085(1) 0.2092(1) 0.290(2) 0.800(2) 0.294(2) 0.792(2) 0.420(1) 0.39(2) 0.466(6) 0.932(2) 0.92(1) 0.890(7) 0.928(1) 0.90(1) 0.879(6) 0.420(2) 0.39(1) 0.499(5)

1/4 1/4 1/4 1/4 1/4 1/4 1/4 1/4 0.066(6) 1/4 1/4 0.064(6) 1/4 1/4 0.111(7) 1/4 1/4 0.118(8)

0.17155(9) 0.67183(9) 0.340(1) 0.835(1) 0.842(1) 0.338(1) 0.100(1) 0.022(2) 0.110(4) 0.098(1) 0.022(3) 0.131(4) 0.597(1) 0.522(2) 0.628(3) 0.605(1) 0.529(2) 0.640(3)

0.0179(3) 0.0178(2) 0.019(2) 0.027(2) 0.024(2) 0.024(2) 0.019(2) 1.2Uiso[N(1)] 1.2Uiso[N(1)] 0.029(3) 1.2Uiso[N(2)] 1.2Uiso[N(2)] 0.013(2) 1.2Uiso[N(3)] 1.2Uiso[N(3)] 0.021(2) 1.2Uiso[N(3)] 1.2Uiso[N(3)]

and angles are reported in Table 6. Zn2+ ions are coordinated by three F− and two NH3 ligands, manifesting a trigonalbipyramidal environment. Figure 2 illustrates the condensation of these trigonal bipyramids by axial fluoride ions, resulting in 1 a one-dimensional ∞ [Zn(NH3)2F2/2 Fe] chains (a, axial; e, equatorial). These chains are arranged along the [010] direction (see Figure 3). There are two crystallographically distinct Zn atoms present in the structure, each realizing its own unique infinite chain. In both trigonal-bipyramidal arrangements, the two NH3 ligands and one F− ligand are located in equatorial positions, and the two bridging F− ligands F


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Inorganic Chemistry Table 6. Selected Distances and Angles in ZnF2(NH3)2 distance (Å) Zn(1) Zn(1) Zn(1) Zn(1)

−N(1) −N(2) −Fa(2) −Fe(1)

2.03(1) 2.0(1) 2.1418(6) 1.93(1)

Zn(2) Zn(2) Zn(2) Zn(2) Zn(1) Zn(1) Zn(2) Zn(2) Zn Zn

−N(3) −N(4) −Fa(4) −Fa(3) −N d¯ −F d¯ −N d¯ −F d¯ −N d¯ −F d¯

2.024(9) 2.00(1) 2.1433(7) 1.94(1) 2.015 2.07 2.012 2.076 2.014 2.073

angle (deg) N(1)−Zn(1)−N(2) N(1)−Zn(1)−Fe(1) N(2)−Zn(1)−Fe(1) Fa(2)−Zn(1)−Fa(2) Fe(1)−Zn(1)−Fa(2) Fa(2)−Zn(1)−N(1) Fa(2)−Zn(1)−N(2) N(3)−Zn(2)−N(4) N(3)−Zn(2)−Fe(3) N(4)−Zn(2)−Fe(3) Fa(4)−Zn(2)−Fa(4) Fe(3)−Zn(2)−Fa(4) Fa(4)−Zn(2)−N(3) Fa(4)−Zn(2)−N(4) Zn(2)−Fa(4)−Zn(2) Zn(1)−Fa(2)−Zn(1)

2×

2×

131.4(4) 114.2(5) 114.4(5) 175.9(9) 92.0(4) 89.8(3) 88.6(4) 132.3(3) 115.1(5) 112.6(5) 174.0(8) 92.9(4) 89.5(4) 88.1(3) 174.0(8) 175.9(9)

1× 1× 1× 1× 2× 2× 2× 1× 1× 1× 1× 2× 2× 2× 1× 1×

Figure 2. Interconnection of 1∞[Zn(NH3)2Fa2/2Fe] chains through hydrogen bonds (dashed lines) in ZnF2(NH3)2: (a) between Zn(1) chains, (b) between Zn(1) and Zn(2) chains, and (c) between Zn(2) chains. Large gray spheres, Zn; white spheres, N; large black spheres, F; small black spheres, H.

Table 7. Hydrogen-Bond Lengths and Angles in ZnF2(NH3)2 with F Acting as the Acceptor and N Acting as the Donor

N(1)−H(1A)···F(3) N(1)−H(1B)···F(3) N(2)−H(2A)···F(2) N(2)−H(2B)···F(3) N(3)−H(3A)···F(4) N(3)−H(3B)···F(1) N(4)−H(4A)···F(1) N(4)−H(4B)···F(1)

d(N−H) (Å)

d(H···F) (Å)

∠NHF (deg)

d(N···F) (Å)

0.91(2) 0.86(2) 0.89(2) 0.87(2) 0.86(2) 0.85(2) 0.87(2) 0.84(2)

2.05(3) 2.18(3) 2.16(4) 2.17(3) 2.10(3) 2.11(2) 2.15(3) 2.20(3)

172(12) 151(4) 158(9) 152(5) 174(10) 165(4) 171(9) 156(4)

2.95(2) 2.96(1) 3.00(2) 2.97(1) 2.96(2) 2.94(1) 3.02(2) 2.98(1)

temperature, which causes the lower mass loss. Additionally, some ammonia loss during preparation was similarly observed for other 3d transition-metal halide ammoniates with low decomposition temperatures {e.g., [Fe(NH3)6]Cl248}. An endothermal DTA signal with an onset at 53 °C and a maximum at 88 °C coincides with this mass loss. The second deammination step leads to the same crystalline decomposition intermediate as in the ZnF2(NH3)2 decomposition, namely, the monoammoniate ZnF2(NH3) (mass loss of 10.38 wt % starting at 112 °C). The endothermal DTA signal accompanying this decomposition step starts at 130 °C and has its maximum at

Figure 3. Section of the crystal structure of ZnF2(NH3)2. View along infinite 1∞[Zn(NH3)2Fa2/2Fe] chains parallel to [010]. Large gray spheres, Zn; white spheres, N; large black spheres, F; small black spheres, H; dashed lines, hydrogen bonds; solid lines, unit cell.

ZnF2(NH3)2 and probably Zn(NH3)F2. The first mass loss starts at 79 °C and totals 5.3 wt %, which is significantly lower than the expected 11.05 wt % for one formula unit of NH3. According to powder X-ray phase analysis, single-phase ZnF2(NH3)2 is formed at the end of this step. The initial ZnF2(NH3)2 impurities do not decompose at this low G


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Inorganic Chemistry

Figure 4. DTA/TG measurement of ZnF2(NH3)2 in Ar atmosphere at a heating rate of 10 K/min.

Figure 6. IR spectra of ZnF2(NH3)3, ZnF2(NH3)2, and Zn(NH3)F2.

144 °C. A powder X-ray diffraction pattern of ZnF2(NH3) can be found in the Supporting Information. The third mass loss of 12.34 wt % corresponds to the loss of another formula unit of NH3 (calculated 11.05 wt %) at 172 °C. It is also accompanied by an endothermal DTA signal, beginning at 190 °C and peaking at 205 °C. After the measurement, ZnF2 remains in the crucible according to powder X-ray diffraction. However, the rest mass of 72 wt % is higher than the calculated mass of ZnF2 at 66.9 wt %, which is mainly due to the ZnF2(NH3)2 impurities present in the original sample. Vibrational Spectroscopy. Raman spectra of ZnF2(NH3)3 and ZnF2(NH3)2 single crystals and IR spectra of ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3) powders were recorded (see Figures 5 and 6). The IR data for ZnF2(NH3)3 must be

particularly in the low-frequency regions. Additionally, the contributions from both NH3 and F− lead to overlapping bands. The bands appear in four different regions: Three regions can be attributed to NH3 molecular vibrations [ν(NH) = 2868−3337 cm−1, δ(HNH) = 1223−1673 cm−1, and ρr(NH3) = 690−806 cm−1], and one region can be attributed to skeletal modes [ν(ZnN)/ν(ZnF) = 394−464 cm−1 and δ(NZnN)/δ(FZnN)/δ(FZnF) = 151−333 cm−1]. In all three compounds, the first region shows broad features attributed to ν(NH) stretching vibrations. Four and five different bands can be identified, because of the crystallographically different nitrogen sites [six in ZnF2(NH3)3 and four in ZnF2(NH3)2]. The splitting of νas(NH) and νs(NH) into at least two bands each indicates the presence of more than one distinct nitrogen site in the crystal structure of ZnF2(NH3) as well. There might be more bands in this region, especially in the spectra of ZnF2(NH3)3 and Zn(NH3)F2, that are not resolved because of the broad shape. These broad stretching vibrations indicate the presence of hydrogen bonds, in agreement with the short distances and large NH···F angles in the crystal structures and are shifted to lower wavenumbers [3159−3335 cm−1 for ZnF2(NH3)3, 3093−3337 cm−1 for ZnF2(NH3)2, and 3192−3336 cm−1 for Zn(NH3)F2] compared to νas(NH) and νs(NH) of the free NH3 molecule (3450 and 3337 cm−152) and in [Zn(NH3)4]I2 (3290, 3234.5, and 3197 cm−1 53), because of the strong interactions of NH3 and F− through hydrogen bonding. Asymmetric NH3 deformation modes occur in all spectra of ZnF2(NH3)3 (1614−1672 cm−1), ZnF2(NH3)2 (1673 cm−1), and ZnF2(NH3) (1463−1671 cm−1). No symmetric NH3 deformation modes were observed in the ZnF2(NH3)2 Raman spectrum. There is a shift to lower wavenumbers from the fluorides through the bromide to the iodide ([Zn(NH3)4]Br2, 1607 and 1246 cm−1 in IR;26 [Zn(NH3)4]I2, 1615, 1600, 1256, and 1242 cm−1 54), which can be explained by a decreasing hydrogen-bond strength with increasing ionic radius of the halide acceptor. Also, for ρr(NH3) rocking vibrations, a shift to lower wavenumbers was observed. This rocking vibration is absent in free ammonia and is reported to increase in intensity with increasing covalent character of the N−metal bond.55 The already mentioned weaker character of some of the hydrogen bonds in ZnF2(NH3)2 (690 cm−1) is a likely reason for the slight shift to lower wavenumbers of the rocking vibrations compared to

Figure 5. Raman spectra of ZnF2(NH3)3 and ZnF2(NH3)2.

considered carefully because of minor ZnF2(NH3)2 impurities in the measured sample. The assignments of the different modes are summarized in Table 8. The obtained data are in good agreement with the literature data for ZnF2,49,50 Zn(NH3)2Br2,51 [Zn(NH3)4]Br2,26 and [Zn(NH3)4]I2.26 Because of the lack of information about the shifts of the bands caused by hydrogen bonding, some assignments are difficult, H


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Inorganic Chemistry Table 8. Bands Occurring in the IR and Raman Spectra of ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3) at 22 °Ca ZnF2(NH3)3 Raman (cm−1) 151 177 207 216

(w) (m) (m) (m)

307 333 395 430 464

(m) (w) (m) (vs) (w)

745 (m) 806 (m)

1297 (m)

ZnF2(NH3)2 IR (cm−1)

Raman (cm−1) 154 173 206 216 219 309

Zn(NH3)F2 IR (cm−1)

396 (vs) 432 (s) 463 (w) 690 (s)

690 (s)

738 (m)

737 (s) 792 (m)

1252 (vs) 1269 1313 (w)

1255 (ss) 1278 (s) 1313 (m)

3159 3201 3259 3275

(m) (vs) (vs) (vs)

722 (vs) 759 (m) 797 (w) 1223 (m) 1242 (m)

1427 (vs)

1614 (m) 1628 (w) 1672 (m)

1673 (w)

3167 (m) 3192 (m) 3246 (m)

3151 (s) 3200 (vs) 3253 (m)

1463 1496 1616 1629 1671 3093

(m) (w) (m) (w) (m) (m)

3191 (m) 3243 (m)

1617 1638 (vs)

3192 (w)

3284 (w) 3302 (m) 3317 (m)

3305 (m) 3320 (m)

3335 (m)

assignment δ(NZnN), δ(FZnN), δ(FZnF) δ(NZnN), δ(FZnN), δ(FZnF)

(m) (w) (m) (w) (w) (w)

1407 (w)

1648 (m) 1672 (w)

IR (cm−1)

3337 (m)

3312 (w) 3336 (m)

ν(ZnN), ρ(ZnF) ν(ZnN), ρ(ZnF) ν(ZnN), ρ(ZnF) ρr(NH3), νs(ZnF) ρr(NH3) νas(ZnF) νas(ZnF) δs(HNH) δs(HNH) δs(HNH) δs(HNH) δs(HNH) δ(HNH) δ(HNH) δas(HNH) δas(HNH) δas(HNH) νs(NH) νs(NH) νs(NH) νas(NH) νas(NH) νas(NH) νas(NH) νas(NH) νas(NH)

Relative intensities indicated as very strong (vs), ≥90%; strong (s), 90−70%; medium (m), 70−30%; weak (w), 30−10%; and very weak (vw), ≤10%. a

ZnF2(NH3) (722 cm−1). In this region, ν(ZnF) lattice vibrations also contribute to the observed bands. The asymmetric stretching vibrations ν(ZnF) give rise to bands in the range of 737−806 cm−1 in the spectra of all three compounds. Also, strong bands of the symmetric stretching vibrations occur in the IR spectra of ZnF2(NH3)2 and ZnF2(NH3)3 at 690 cm−1. The skeletal modes appear at the lowest wavenumbers in two groups: the stretching vibrations ν(ZnN) and the deformation vibrations ρ(ZnF), δ(NZnN), δ(FZnN), and δ(FZnF). In the range from 395 to 464 cm−1, ν(ZnN) stretching modes appear, which fits the literature data (412−431 cm−1) for [Zn(NH3)4]I2.53,54 These bands overlap with the bands of the ρ(ZnF) deformation vibrations, appearing in ZnF2 in the range of 388− 451 cm−1.50 In the range from 206 to 333 cm−1, several bands appear in the Raman spectra of ZnF2(NH3)3 and ZnF2(NH3)2 that could not be clearly assigned. In this range, ρ(ZnN) deformation vibrations might occur. At 307−333 cm−1, the bands might be due to ν(ZnN) and ρ(ZnF) vibrations, shifted to lower wavenumbers due to hydrogen bonding. The bands at even lower wavenumbers might be related to bending vibrations δ(NZnN), δ(FZnN), and δ(FZnF), shifted to higher wave-

numbers compared to the literature data. The bending vibrations δ(FZnF) (103−151 cm−1) in ZnF250 are reported in the same range as the δ(NZnN) (157 cm−1) bending vibrations in [Zn(NH3)4]I2.53 No values have been reported for δ(FZnN) vibrations in the literature. However, they are expected to appear in the same region as the δ(NZnN) and δ(FZnF) vibrations. Thus, the recorded bands at 151−177 cm−1 can be assigned to all three deformation vibrations δ(NZnN), δ(FZnN), and δ(FZnF). No information about ZnF2(NH3) is available for this region, because of the lack of single crystals for Raman measurements and the lower limit of the IR spectrometer of 550 cm−1. For none of the three compounds were bands recorded above 3337 cm−1, which indicates the absence of OH− and H2O impurities, possibly substituting NH3 or F− ligands because typical ν(OH) bands appear at about 3700 cm−1.56

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CONCLUSIONS Knowledge about the synthesis conditions, crystal structure, physical and chemical properties, and vibrational spectra of intermediate compounds can help to develop a formation mechanism of a target compound and, therefore, to control and improve the synthesis conditions. In this sense, the growth and I


Article

Inorganic Chemistry

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characterization of intermediates in the ammonobasic and ammonoacidic syntheses of GaN have lit the way to an improvement of controlled GaN growth using different mineralizers.57,58 The isolation and characterization of possible intermediates in the synthesis Zn3N2 might enable similar insights in the future. The stabilization of the very rare trigonal-bipyramidal 1 coordinations of Zn in ∞ [Zn(NH 3 ) 2F 2/2 F] chains [in ZnF 2 (NH 3 ) 2 ] and isolated [Zn(NH 3 ) 3 F 2 ] units [in ZnF2(NH3)3] might be favored by the high-pressure conditions of up to 197 MPa during synthesis. ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3) represent the first reported ammoniates of ZnF2. The presence of these trigonal bipyramids in the crystal structure might indicate [Zn(NH3)3F2] and [Zn(NH3)2F2F]− to be transport-active species in a conceivable ammonothermal synthesis and crystal growth of Zn3N2 using NH4F as a mineralizer. This knowledge plays an important role in the understanding of the reaction and transport mechanism in the Zn−N system. Vibrational spectroscopy allows the assignment of various stretching and deformation modes of the NH3 group and the lattice. Thermal analysis of ZnF2(NH3)3 shows ZnF2(NH3)2 and ZnF2(NH3) to be decomposition intermediates prior to the re-formation of ZnF2.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.5b02837. Details about the twinned crystal structures and powder X-ray diffraction patterns of ZnF2(NH3)3, ZnF2(NH3)2, and ZnF2(NH3) (PDF) CIF data for ZnF2(NH3)3 and ZnF2(NH3)2 (CIF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +49 0 711 68564217. Fax: +49 0 711 68564241. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG) within the Forschergruppe FOR1600 “Chemie und Technologie der Ammonothermal-Synthese von Nitriden”. We thank Dr. Falk Lissner and Dr. Sabine Strobel for the single-crystal X-ray diffraction and William Clark for the thermal analysis (all at Universität Stuttgart).

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REFERENCES

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Trigonal-Bipyramidal Coordination in First Ammoniates of ZnF2: ZnF2

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