Ammonothermal Synthesis and Crystal Structures of Diamminetriamidodizinc Chloride [ Zn 2 ( NH 3 ) 2 ( NH 2 ) 3 ] Cl and Diamminemonoamidozinc Bromide [ Zn ( NH 3 ) 2 ( NH 2 ) ] Br

The treatment of excess zinc in the presence of ammonium chloride under ammonothermal conditions of 873 K and 97 MPa leads to diamminetriamidodizinc chloride [Zn2(NH3)2(NH2)3]Cl with a two-dimensionally μ-amido-interconnected substructure. Similar reaction conditions using ammonium bromide instead of the chloride (773 K, 230 MPa) result in diamminemonoamidozinc bromide [Zn(NH3)2(NH2)]Br with one-dimensional infinite μ-amido-bridged chains. Both compounds were obtained as colorless, very moisture sensitive crystals. Crystal structures and hydrogen bond schemes are analyzed. Raman spectroscopic data of the chloride are reported.


Introduction
Synthesis of high-quality nitride materials presents a challenge for various applications.In particular, semiconductor nitride materials are currently the focus of crystal growth, one example being GaN wafers as superior substrates for high-performance blue and white LEDs [1].A promising synthesis and crystal growth technique for such materials is the ammonothermal method, utilizing supercritical ammonia under either ammonoacidic or ammonobasic conditions.However, a number of unresolved issues remain, like the fundamental understanding of the chemistry of dissolution, materials transport and recrystallization processes.Additionally, the technique may not only provide superior GaN crystals, but further interesting nitride materials.
Recently, we have focused on the ammonothermal zinc nitride synthesis.In this respect, we have presented the ammonothermal synthesis and characterization of Zn(NH 3 ) 3 F 2 and Zn(NH 3 ) 2 F 2 , which show five-fold coordination at Zn [2].In contrast, the few further examples of ammoniates of zinc halides exclusively exhibit the tetrahedral environment of Zn, namely in [Zn(NH 3 ) 2 Cl 2 ], [Zn(NH 3 ) 2 Br 2 ] [3], [Zn(NH 3 ) 4 ]Br 2 and [Zn(NH 3 ) 4 ]I 2 [4], all obtained from reaction of either aqueous solutions or of the solid zinc halides with ammonia at ambient pressure.With [Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl and [Zn(NH 3 ) 2 (NH 2 )]Br, we present two ammineamidozinc halides, synthesized from nominally ammonoacidic conditions, which additionally show increased condensation within their cationic substructures.The formation of these compounds from supercritical ammonia may indicate their role as intermediates in a conceivable ammonothermal synthesis and crystal growth of the interesting semiconductor material Zn 3 N 2 analogously to the already commercially available ammonothermally grown GaN crystals [1].
Both title compounds were obtained in the colder zone of the autoclave, while additional Zn(NH 2 ) 2 was observed in the hot zone.According to literature, Zn(NH 2 ) 2 is insoluble in liquid ammonia at ambient conditions [8]; however, we have frequently observed that Zn reacts under various ammonoacidic as well as ammonobasic conditions to form the diamide and crystallizes in the colder zone of the autoclave in large crystals, indicating the ammonoamphoteric character of Zn and an enhanced solubility of the binary amide at elevated temperatures and pressures.However, it is apparently possible to adjust the solubility, respectively its temperature dependence, by the addition of halide ions and thus favor the formation of the supposedly less soluble title compounds in the zone with lower temperature.The deposition at the hotter or colder zone in the reaction vessel is usually dictated by the temperature dependence of the solubility.This temperature dependence can fundamentally change with the type of mineralizer and therefore with the nature of the dissolved species, as is well known, for example, for the ammonothermal synthesis of GaN [1].
The underlying thermodynamics governing these processes follow very similar principles to those that are well established for the so-called Chemical Vapor Transport [9].
It is interesting to note that we were only able to synthesize the title compounds in the presence of platinum used as liner material to minimize contact of the solution with the autoclave wall and thus minimize corrosion, known to be severe in ammonoacidic solutions at elevated temperatures [10].A catalytic action of both NH 4 Cl and Pt, for example, for the formation of alkali-and alkaline-earth metal amides from liquid ammonia is well established [11].

Crystal Structure Description
[Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl crystallizes in the chiral space group P2 1 2 1 2 with two formula units in the unit cell.A Flack parameter close to 1  2 indicates the presence of a racemic inversion twin.Table 1 gives further selected information on the crystal structure and its determination.Table 2 presents positional parameters and Table A1 anisotropic displacement parameters.In the crystal structure, the ammonia molecules, the amide and the chloride ions together form the motif of a hexagonal closed packing with stacking of hexagonal layers along [100].In addition, 1/6 of the tetrahedral voids exclusively built by ammonia molecules and amide groups are occupied by Zn.These tetrahedra are linked via amide vertices to layers 2  ∞ Zn(NH 3 )(NH 2 ) 3/2 1/2+ orientated within the a/b plane (Figure 1).As may be expected, the distances Zn-N to the amide groups with 199.1(5) pm and 201.2(6) pm are significantly shorter as compared to the distance to the NH 3 ligand (213.5(7)pm).Similar distance relations were earlier observed, e.g., in KNH 2 •2NH 3 [5] or [Cr 2 (NH 2 ) 3 (NH 3 ) 6 ]I 3 [6].Angles around Zn are in the range of 101.8 • to 116.4       Hydrogen atoms of the amide groups point towards the chloride ions located between the layers and form hydrogen bonds (Figure 2).Every chloride ion connects to twelve hydrogen atoms, where half of the hydrogen atoms belong to each neighboring layer.Table 4 summarizes donor-acceptor distances and angles.[Zn(NH3)2(NH2)]Br suffers from twinning by partial merohedry via two-fold rotation around the face diagonal [101] in space group P121/n1 setting.Due to application of the respective twin law, the reliability factors significantly drop (see the experimental part).Table 1 summarizes selected information for the final structure refinement, Table 5 gives positional parameters and Table A2 anisotropic displacement parameters.H-bonds (see Figure 3).Table 7 gives relevant donor-acceptor distances and angles.For both title compounds, there are no indications for a rotational disorder of the ammonia ligands, prohibited by an involvement in hydrogen bonding networks.Bromide ions interconnect three 1 ∞ Zn(NH 3 ) 2 (NH 2 ) 2/2 + zigzag chains each via eight H-bonds (see Figure 3).Table 7 gives relevant donor-acceptor distances and angles.For both title compounds, there are no indications for a rotational disorder of the ammonia ligands, prohibited by an involvement in hydrogen bonding networks.
In the range of 3167-3493 cm −1 , three asymmetric and three symmetric valence vibrations appear in agreement with the three crystallographic different nitrogen sites of ammonia and amide ligands.The broadening of this group of signals indicates the presence of hydrogen bonds of relevant strength, which is in perfect agreement with short NH . . .Cl distances (see above) [18,19].These hydrogen bonds are also the reason for a shift to smaller wave numbers as compared to the respective modes of the free ammonia molecule (3337 and 3450 cm −1 [20]).For ammoniates of zinc fluoride Zn(NH 3 ) 3 F 2 and Zn(NH 3 ) 2 F 2 (3093-3337 cm −1 [2]), a similar, but even larger shift was observed, due to the higher electronegativity of fluorine compared to chlorine, while the bromide [Zn(NH 3 ) 4 ]Br 2 (3194 cm −1 [4]) and the iodide [Zn(NH 3 ) 4 ]I 2 (3177 cm −1 [21]) exhibit smaller shifts in the symmetric valence mode.
The absence of signals above 3500 cm −1 indicates the absence of significant impurities of OH − and H 2 O, which should provoke a prominent mode at around 3700 cm −1 [23].

Synthesis
All ammonothermal syntheses were carried out in custom-built autoclaves from austenitic nickel-chromium-based alloy 718 with an inner volume of 97 mL and equipped with a high-pressure valve, a filling line, a rupture disc and a pressure transmitter (HBM P2VA1/5000 bar), which enables continuous pressure monitoring during synthesis [24].A platinum liner was introduced to minimize contact of the solution with the autoclave wall and thus reduce corrosion.A tubular furnace (LOBA, HTM Reetz GmbH, Berlin, Germany) in vertical position was used to heat the autoclave bodies.Due to unheated installations at the top of the vessel, a natural temperature gradient with a temperature difference of about 100 K developed, resulting in suitable convection for a chemical material transport.Additionally, the applied furnace temperature, which is referred to in the synthesis description, is about 150 K higher than the average reaction temperature [25].
Colorless transparent plate-like crystals [Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl with maximum size of several hundred µm were obtained from Zn and NH 4 Cl (molar ratio 10:1) under ammonothermal conditions (873 K, 97 MPa for 6 h, in 38 h cooling to RT) in the cold zone of the autoclave.In a similar reaction, [Zn(NH 3 ) 2 (NH 2 )]Br was synthesized from Zn and NH 4 Br (molar ratio 1:1.7; 773 K, 230 MPa for 12 h, 72 h cooling duration), and colorless crystals were collected from the cold zone.Both compounds are sensitive to moist air and immediately lose their transparency on contact.In the hot zone of the autoclaves, there was always well crystallized Zn(NH 2 ) 2 present along with small amounts of unreacted Zn.
Due to the hygroscopic nature of the reactants and products, all manipulations were carried out inside an argon filled glovebox (p(O 2 ) < 0.1 ppm).A known amount of ammonia was condensed into the autoclave with use of a tensieudiometer [26] for the simultaneous pressure and temperature measurement and a dry ice ethanol cooling bath (T = 198 K).After successful synthesis, the excess ammonia was vented and the autoclave was subsequently evacuated.

Diffraction Data Collection and Structure Refinements
Selected single crystals were isolated under Ar, sealed in glass capillaries and immediately mounted on a Bruker-Nonius Kappa-CCD diffractometer (Billerica, MA, USA).The chloride was measured at room temperature, while successful intensity data collection on the bromide was only possible if directly cooled to 100 K and subsequently measured (Mo-Kα radiation).Structure solution and refinements were carried out using the program package SHELXL97 [27].All atom positions except hydrogen were determined by direct methods and anisotropically refined.The hydrogen positions attached to amide groups of the complex amidozincate ions were refined using the riding model, constraining the interatomic N-H distances to 89(2) pm.The isotropic displacement factors U iso were restrained to 1.2/1.5 times the U iso of the nitrogen atom to which they are attached to.For absorption correction, a linear scaling was applied using the absorption correction coefficients 12.93 mm −1 ([Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl) and 11.17 mm −1 ([Zn(NH 3 ) 2 (NH 2 )]Br) [28].

Vibrational Spectroscopy
The solid state Raman spectrum of ([Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl was measured with a Horiba XploRa Raman spectrometer (Kyoto, Japan) coupled with a confocal polarization microscope (Olympus BX51, Tokio, Japan) employing a 638 nm solid state laser at a single crystal sealed in a glass capillary.

Conclusions
Zinc metal easily dissolves in ammonia, particularly in ammonoacidic or -basic solutions under ammonothermal conditions.From these solutions at increased temperatures, zinc amide crystallizes.However, quasi-ternary ammoniates of zinc amide halides can also be obtained.With increasing temperatures, condensation of the cationic ions in diamminetriamidodizinc chloride [Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl and diamminemonoamidozinc bromide [Zn(NH 3 ) 2 (NH 2 )]Br occurs via amide ions towards two-and three-dimensional moieties, which can be viewed as sections of the crystal structure of binary zinc nitride, Zn 3 N 2 .The formation of amides in solutions containing halide ions, hence nominally ammonoacidic conditions, might be understood by the large excess of Zn, providing an ammonobasic buffer solution Zn(NH 2 ) 2 /ZnX 2 in ammonia.Such investigations may pave the way to ammonothermal zinc nitride synthesis and crystal growth, a potential novel nitride semiconductor material for which currently no technique for bulk crystal production is known.

Table 1 .Figure 1 .
Figure 1.Section of the crystal structure of [Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl: Layers 2 ∞ Zn(NH 3 )(NH 2 ) 3/2 1/2+ built by occupation of tetrahedral voids exclusively formed by amide and ammonia molecules by Zn within the motif of an hcp of chloride (closed packed layers in stacking direction [100] are indicated by letters A and B), amide ions and ammonia molecules (yellow spheres: Cl, green spheres: N, red spheres: H, and blue spheres surrounded by grey tetrahedra: Zn).The hcp stacking motif is indicated by an anticuboctahedron and capital letters.

2 +
Very similar to [Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl, the ammonia molecules, amide and bromide ions in the crystal structure of [Zn(NH 3 ) 2 (NH 2 )]Br, form the motif of a hexagonal closed packing with stacking along [010] (Figure3).According to the composition, only 1/8 of the tetrahedral voids exclusively formed by two ammonia molecules and two amide ions are occupied by Zn.As discussed above for the chloride, the distances to the terminal ammonia ligands with 204(3) pm and 211(3) pm are significantly longer than those to the bridging amide ligands with 198.4(8) pm and 200.2(8) pm.Angles around Zn with 104 • -123.8 • deviate little from to the ideal tetrahedral angle.Table 6 collects selected interatomic distances and angles.Vertex-sharing via amide ligands leads to 1 ∞ Zn(NH 3 ) 2 (NH 2 ) 2/zigzag chains running along [010].According to the Liebau nomenclature, these chains are classified as unbranched zweier single-chains with molar ratio of n(Zn):n(N) = 1:3 [12].Once again, these chains may be viewed as sections from the three-dimensional crystal structure of binary zinc amide, formally broken up by the addition of HBr.Inorganics 2016, 4, 41 6 of 11 along [010] (Figure 3).According to the composition, only 1/8 of the tetrahedral voids exclusively formed by two ammonia molecules and two amide ions are occupied by Zn.As discussed above for the chloride, the distances to the terminal ammonia ligands with 204(3) pm and 211(3) pm are significantly longer than those to the bridging amide ligands with 198.4(8) pm and 200.2(8) pm.Angles around Zn with 104°-123.8°deviate little from to the ideal tetrahedral angle.Table 6 collects selected interatomic distances and angles.Vertex-sharing via amide ligands leads to along [010].According to the Liebau nomenclature, these chains are classified as unbranched zweier single-chains with molar ratio of n(Zn):n(N) = 1:3 [12].Once again, these chains may be viewed as sections from the three-dimensional crystal structure of binary zinc amide, formally broken up by the addition of HBr.

Figure 3 . 2 +
Figure 3.Sections of the crystal structure of [Zn(NH 3 ) 2 (NH 2 )]Br: (a) infinite 1 ∞ Zn(NH 3 ) 2 (NH 2 ) 2/2 + zigzag chains result from occupation of tetrahedral voids exclusively built by amide ions and ammonia molecules by Zn within the motif of an hcp formed by bromide and amide ions together with ammonia molecules (the hcp stacking motif is indicated by an anticuboctahedron and capital letters); (b) every bromide ion employs eight H-bonds and interconnects three chains (large red spheres: Cl, green spheres: N, small red spheres: H, and blue spheres surrounded by grey tetrahedra: Zn).

Table 4 .
Donor-acceptor distances (pm) and angles (deg.) in [Zn 2 (NH 3 ) 2 (NH 2 ) 3 ]Cl.NH 3 ) 2 (NH 2 )]Br suffers from twinning by partial merohedry via two-fold rotation around the face diagonal [101] in space group P12 1 /n1 setting.Due to application of the respective twin law, the reliability factors significantly drop (see the experimental part).Table1summarizes selected information for the final structure refinement, Table5gives positional parameters and TableA2anisotropic displacement parameters.