Synthesis and Characterization of the Amidomanganates Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄]

Abstract

We report the successful synthesis of Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄] from ammonothermal conditions at 723 K and pressures above 850 bar. Both compounds were obtained single phase according to powder X-ray diffraction. The crystal structures were determined by single crystal X-ray diffraction. For Rb₂[Mn(NH₂)₄] we have obtained the high-temperature phase. The structures are analyzed with respect to the earlier reported alkali metal amidomanganates. Upon heating in inert atmosphere Cs₂[Mn(NH₂)₄] decomposes to manganese nitrides. IR spectroscopic results are reported.

1. Introduction

Ammonothermal synthesis is regarded as an innovative and promising approach for synthesis and growth of free-standing crystals of functional nitrides such as AlN [1], GaN [2] and InN [3] or other group III–V semiconductor materials [4]. Compared to further established methods such as chemical vapor deposition (CVD) it provides an alternative, less wasteful technique [5,6]. While the main interest in ammonothermal systems is fueled by the search for new or improved nitride semiconductors, transition metal amides are also widely available by this technique, such as Zn or Mn compounds [7,8]. These compounds are regarded as intermediates in the ammonothermal synthesis of transition metal nitrides like Zn₃N₂ [9] and Mn₃N₂ [10,11] and thus give a clue on the chemical processes during ammonothermal formation.

Ammonothermal synthesis is a derivative from the better known hydrothermal method, which share several common characteristics. Both methods typically use the supercritical fluid range, requiring temperatures above $T_c=407.7$ K and pressures above $p_c=119$ bar for ammonothermal systems [12]. Since ammonia is similar in behavior to water, the acidic and basic media as known in aqueous solutions can also be achieved in liquid ammonia by adding ammonobases such as alkali metal amides ANH${}_2$ (A = Li – Cs) or ammonoacids such as ammonium halides NH${}_{4}X$ (X = F – I). The chemical nature of the intermediate and thus the solubility of any metal nitride and its temperature dependence are fundamentally affected by the nature of the mineralizer. While in an acidic milieu the product tends to precipitate in the colder zone of the autoclave, a basic milieu often causes products to crystallize in the hotter zone depending on the dissolved intermediate [13,14,15,16].

Recently, the successful synthesis of K₂[Mn(NH₂)₄] and Rb₂[Mn(NH₂)₄] by ball milling technique was reported [17]. Both compounds share the same chemical formula type as amidometalates like K₂[Zn(NH₂)₄] [18], Rb₂[Zn(NH₂)₄] [19] and Cs₂[Zn(NH₂)₄] [8], these compounds well-known as intermediates in ammonothermal systems with zinc and ammonia. Thus, they give a deeper insight into the processes of dissolution and crystallization under ammonothermal conditions. Furthermore, the amidomanganates Na₂[Mn(NH₂)₄] [7] and K₂[Mn(NH₂)₄] [19] were previously obtained by reaction of the transition metal with a corresponding mineralizer in (supercritical) liquid ammonia. Generally, when heating such amidometalates in inert atmosphere, a decomposition to the corresponding nitride is often observed, for example in the decomposition of Rb[Al(NH₂)₄] to AlN [20].

In a previous report [17], two different modifications of Rb₂[Mn(NH₂)₄] were observed, namely a low- and a high-temperature form. The low-temperature form with an orthorhombic structure in $Pbca$ was obtained via ball-milling of rubidium and manganese under gaseous ammonia. Upon heating, this modification undergoes a phase transition to a monoclinic form in $P{2}_1/c$ at 429 K before starting to decompose. Generally, alkali metal amidometalates, such as $A_2$[M(NH${}_2$)${}_4$] (A = Na – Cs, M = Mn, Zn) appear to be active in hydrogen storage and ammonia decomposition and formation [21], giving rise to useful applications. In this work, we describe the synthesis and crystal growth of the high-temperature modification of Rb₂[Mn(NH₂)₄] as well as novel Cs₂[Mn(NH₂)₄] from ammonothermal conditions and report crystal structures derived from single crystal X-ray diffraction, vibrational spectroscopy and thermal decomposition.

2. Materials and Methods

2.1. General Information

Due to air and moisture sensitivity, all handling of the starting materials and products was done in a glove box filled with argon (MBraun, Garching, Germany, p(O₂) < 0.1 ppm). Custom-made autoclaves (98 mL Volume, nickel-based alloy 718) [22] were used for reactions. One-side closed tubular furnaces (HTM Reetz GmbH, Berlin, Germany) caused a natural temperature gradient in the autoclave of about 100 K between the bottom and top of the reaction vessel. The different zones will be referred to as hot (bottom) and cold (top). Temperatures given at any point refer to the set temperature of the furnace, resulting in a lower actual temperature inside the autoclave, dependent on the one administered from outside [13]. To observe the maximum pressure inside the autoclave during the synthesis, the autoclave was fitted with a digital pressure transmitter (P2VA1/5000 and DA2510, HBM GmbH, Ismaning, Germany). In preparation, the starting materials were filled into the autoclave, which was then assembled, evacuated and flushed once with ammonia. Afterwards, ammonia was condensed into the reaction vessel using a dry-ice/ethanol cooling bath at approximately 200 K. NH₃ (99.999% anhydrous, Linde, Pullach, Germany) was purified from H₂O, CO₂ and O₂ to a grade less than one ppbV with the use of a MicroTorr MC400-720F gas purifier (Rainer Lammerz pure gas products, Hürth, Germany). A self-built tensi-eudiometer after Hüttig was used to precisely determine the amount of ammonia condensed into the autoclave [23].

Manganese powder (325 mesh, 99.95% metal basis, Alpha Aesar) was purchased and used as received. RbNH₂ and CsNH₂ were synthesized by reaction of the alkali metal in NH₃ at 383 K and 50 bar in 200 ml steel autoclaves (Carl Roth GmbH + Co. KG, Karlsruhe, Germany). The products were confirmed to be single phase by powder X-ray diffraction (PXRD).

2.2. Synthesis

Rb₂[Mn(NH₂)₄] was synthesized from manganese powder and RbNH₂ with the following procedure: The autoclave was loaded with Mn (200.4 mg, 3.648 mmol) and RbNH₂ (741.1 mg, 7.302 mmol), filled with ammonia (2.13 mol) to a total of 54% volume and then heated to 723 K with a heating rate of 1.43 K/min reaching a maximum pressure of 880 bar. The autoclave was kept at this temperature for a duration of 24 h and then cooled down to room temperature with 0.149 K/min. The product was collected as yellow transparent crystalline plates from the hot zone of the autoclave. PXRD of the product shows no traces of residue RbNH₂.

Cs₂[Mn(NH₂)₄] was obtained using the same procedure with manganese (154.3 mg, 2.809 mmol) and CsNH₂ (860.4 mg, 5.777 mmol) in ammonia (2.23 mol, 57% of total volume). A maximum pressure of $p_{max}$ = 1150 bar was recorded. Again yellow plates crystallized in the hot zone were obtained.

2.3. Characterisation

X-ray Diffraction: Single crystal X-ray diffraction data were measured on a Bruker-Nonius-Kappa-CCD with Mo$K_{\alpha 1}$ radiation ($\lambda =71.073$ pm). The obtained data were processed using the program SHELX-97 [24]. Additional investigations on the micro-crystalline powder were performed on a STADI-P (STOE & Cie GmbH, Darmstadt, Germany) equipped with a Mythen 1-K detector and Mo$K_{\alpha 1}$ radiation. Visualisation of the resulting structures was done with the program Diamond 4.0 (Crystal Impact GbR, Bonn, Germany). Hydrogen positions could not be unambiguously located during the refinements. Since the exact location of hydrogen does not give deeper insights, also no riding model was applied.

Thermal Analysis: Investigations on thermal behaviour were conducted on a STA 449 C (NETZSCH GmbH, Selb, Germany) with an SiC furnace in corundum crucibles under continuous Argon flow (Argon 5.0, Air Liquide, Düsseldorf, Germany). Measurement data were analysed with the corresponding Proteus program package (Netzsch, Selb, Germany). The reaction chamber was evacuated and flushed with argon three times prior to any measurement, and a correction for buoyancy was administered.

Spectroscopy: Infrared spectroscopy measurements were performed on micro-crystalline powder samples on a Nicolet iS5 (Thermo Fischer Scientific, Waltham, MA, USA).

3. Results and Discussion

3.1. Crystal Structures of HT-Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄]

Yellow, plate-like crystals of Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄] were obtained from manganese and the respective alkali metal amide in supercritical ammonia at a maximum temperature of 723 K and maximal pressures of 880 and 1150 bar, respectively. Both compounds were recovered from the hotter zone at the bottom of the autoclave interior, indicating a retrograde solubility, i.e., lower solubility at higher temperatures. While the cesium compound was not reported before, the crystals of Rb₂[Mn(NH₂)₄] revealed the monoclinic unit cell of the high-temperature modification as reported by Cao et al. [17]. The unit cell volume of the well-crystallized product from high-pressure synthesis presents significantly larger than the one from ball-milling (740.86(4) $\times {10}^6$ pm${}^3$ compared to 724.24(12) $\times {10}^6$ pm${}^3$), despite the higher temperature during structure determination of the latter. Additionally, the high-temperature form did not undergo a structural transition to the low-temperature modification upon moderate heating up to 453 K, irrespective of the reported transition temperature of 429 K. The high-temperature form is thus either stabilized by the larger crystal size as compared to microcrystalline powder, or by deviations in composition. All discussed crystal structures contain mutually isolated tetraamidomanganate ions, however, arranged differently as shown in Figure 1. The difference in volumes of the differently synthesized samples of high-temperature rubidium tetraamidomanganate may indicate a deficiency of ammonia in the ball-milled sample, possibly via di- and oligomerization of the complex ions as was earlier observed for tetraamidoaluminates [20]. No reduced occupation of any sites were observed during the structure refinements against single crystal diffraction intensity data of the title compounds, while the structure of the high-temperature phase of Rb₂[Mn(NH₂)₄] in literature was derived from temperature dependent powder diffraction data. Selected crystallographic data are gathered in

Table 1. Selected crystallographic data and information concerning structure determination for HT-Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄] from this work and comparison to literature data for the rubidium compound obtained from ball milling and subsequent phase transition from the respective low-temperature phase.

	Ref. [17]	HT-Rb2[Mn(NH2)4]	Cs2[Mn(NH2)4]
Crystal system	monoclinic	monoclinic	monoclinic
Space group	$P{2}_1/c$	$P{2}_1/c$	$P{2}_1/c$
a/pm	785.36(5)	779.45(3)	704.25(2)
b/pm	694.49(5)	702.04(2)	942.06(2)
c/pm	1384.11(11)	1408.48(5)	1396.04(4)
$\beta$/deg.	106.389(7)	106.004(2)	115.590(2)
V/${10}^6$ pm${}^3$	724.24(12)	740.86(4)	819.19(4)
Density/g cm${}^{-3}$		2.600	3.121
Z		4	4
F(000)		540	684
$hkl$ range		−10–9/−7–9/−16–18	$\pm 9$/$\pm 12$/$\pm 16$
2${\mathsf{\Theta }}_{max}$/deg.		54.93	54.99
$\mu$/mm${}^{-1}$		14.74	10.29
Refl. meas./independent		12333/1690	21819/1876
$R_1/w{R}_2/GooF$		0.0489/0.1028/1.091	0.0369/0.0840/1.106
$R_{int}$/$R_{\sigma }$		0.0748/0.0375	0.0497/0.0192
Refined parameters		65	65
Extinction coefficient		0.00340	0.00614
$T_{measurement}$/K	429	293	293
Largest electron		−0.70/0.67	−1.69/1.66
difference min/max/Å${}^{-3}$

. Fractional coordinates and isotropic displacement parameters are given in

Table 2. Fractional coordinates and isotropic displacement parameters $U_{eq}$ (in ${10}^4$ pm${}^3$) for HT-Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄].

Atom	Wyckoff Site	$\mathit{x}/\mathit{a}$	$\mathit{y}/\mathit{b}$	$\mathit{z}/\mathit{c}$	${\mathit{U}}_{\mathit{eq}}$
Rb(1)	$4e$	0.07762(6)	0.16605(7)	0.89877(4)	0.0412(2)
Rb(2)	$4e$	0.56877(6)	0.15528(7)	0.89187(3)	0.0399(2)
Mn	$4e$	0.70622(9)	0.19650(9)	0.65510(5)	0.0286(2)
N(1)	$4e$	0.4345(5)	0.2200(6)	0.5646(3)	0.0363(9)
N(2)	$4e$	0.8787(5)	0.2224(6)	0.5601(3)	0.0361(9)
N(3)	$4e$	0.7565(6)	0.0679(6)	0.2525(3)	0.0478(12)
N(4)	$4e$	0.2453(6)	0.0517(6)	0.2587(3)	0.0532(13)
Cs(1)	$4e$	0.04594(6)	0.03197(4)	0.35455(3)	0.0433(2)
Cs(2)	$4e$	0.34299(8)	0.15821(5)	0.10823(4)	0.0573(2)
Mn	$4e$	0.65076(13)	0.24799(9)	0.42175(7)	0.0325(2)
N(1)	$4e$	0.6317(8)	0.0223(5)	0.4028(4)	0.0401(11)
N(2)	$4e$	0.7880(9)	0.1695(7)	0.8243(5)	0.0558(15)
N(3)	$4e$	0.3435(9)	0.1677(6)	0.8648(5)	0.0565(15)
N(4)	$4e$	0.8481(9)	0.1940(6)	0.0827(5)	0.0507(13)

.

For Cs₂[Mn(NH₂)₄] a closer inspection of the packing concerning the [Mn(NH₂)₄]²⁻ complex anions reveals the motif of hexagonal closest packing along [010] (see Figure 2), also known from the isotypes Cs₂[Mg(NH₂)₄] [25] and Cs₂[Zn(NH₂)₄] [8]. HT-Rb₂[Mn(NH₂)₄] is an isotype of A2[Mg(NH₂)₄] with A = K, Rb [26], A2[Zn(NH₂)₄] with A = K, Rb [18,19], and K₂[Mn(NH₂)₄] [19]. Here, the tetraamidometalate ions similarly realize a hexagonal closest packing along [010], but strongly distorted.

In HT-Rb₂[Mn(NH₂)₄] the rubidium ions of both crystallographically distinct sites exhibit nearly identical eight-fold (7 + 1) coordination environments of amide groups with the shape of two-capped trigonal prisms. The larger cesium cations in Cs₂[Mn(NH₂)₄] realize larger polyhedra with nine and eleven amide groups surrounding Cs(1) and Cs(2), respectively. Selected interatomic distances and angles are gathered in

Table 3. Selected bond distances (in pm) and angles (in deg.) in HT-Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄].

Distance	Rb2[Mn(NH2)4]		Cs2[Mn(NH2)4]		Angle	Rb2[Mn(NH2)4]	Cs2[Mn(NH2)4]
	Rb(1)	Rb(2)	Cs(1)	Cs(2)
A–N	316.9(4)	300.5(4)	315.2(5)	333.5(7)	N(1)–Mn–N(2)	107.542(2)	108.174(1)
	318.2(4)	303.0(4)	325.9(5)	336.6(6)	N(1)–Mn–N(3)	107.435(1)	109.152(1)
	320.1(4)	311.8(4)	327.5(6)	344.1(5)	N(1)–Mn–N(4)	113.428(1)	111.297(1)
	322.9(5)	319.0(4)	329.8(6)	346.0(6)	N(2)–Mn–N(3)	107.734(1)	107.763(1)
	324.2(5)	322.6(5)	348.7(6)	363.3(7)	N(2)–Mn–N(4)	112.875(1)	106.921(1)
	330.9(5)	336.9(5)	360.1(6)	370.6(6)	N(3)–Mn–N(4)	107.558(1)	113.329(1)
	334.0(4)	339.1(5)	366.6(6)	373.7(6)
	395.1(1)	401.3(1)	369.4(6)	386.7(6)
			388.7(6)	387.6(7)
				407.5(1)
				415.6(1)
Mn–N(4)	209.8(4)		210.7(5)
Mn–N(3)	211.6(4)		211.3(5)
Mn–N(2)	215.0(4)		210.5(6)
Mn–N(1)	215.6(4)		213.9(5)

.

For both title compounds, the coordination polyhedra surrounding the alkali metal ions connect to columns. In HT-Rb₂[Mn(NH₂)₄] the monocapped trigonal prisms share the triangular faces, resulting in straight columns parallel to the a-axis as shown in Figure 3. These columns are further connected via edge-sharing to form a three-dimensional framework hosting the manganese ions in tetrahedral voids of the amide groups. In Cs₂[Mn(NH₂)₄] instead a three-dimensional network of polyhedra around the cesium sites, connected by face-, edge- and corner-sharing is realized. Similar to the rubidium compound, the manganese is found in tetrahedral voids (see Figure 3).

3.2. Comparison of Structures of Known Alkali Metal Amidomanganates

Currently five structure types for alkali metal amidometalates with the composition A2[M(NH₂)₄] and A = Na – Cs, M = Mg, Mn, Zn are known. As expected, the coordination number of the alkali metal ions with respect to amide ions monotonically increase from six for sodium up to eleven for cesium. Particularly for Rb₂[Mn(NH₂)₄], a low- and a high-temperature modification are known. In the high-temperature title phase, all rubidium ions are coordinated by only eight amide ions, while in the low-temperature phase they are surrounded by eight or nine amide groups [17]. Noteworthy, all structure types contain two crystallographic sites for the alkali metal ion. It is interesting to compare the coordination environments in the isostructural compounds K₂[M(NH₂)₄] with those of HT-Rb₂[M(NH₂)₄], both with M = Mg, Mn, Zn: In the potassium compounds, the coordination is best described as seven-fold, while due to the increased size of the rubidium ions and a shorter relative distance, one additional amide edge of a coordination polyhedron must be taken into account for both alkali metal sites.

As recently demonstrated for the three modifications of Ba[Ga(NH₂)₄]₂, which all realize coordination environments with eight amide groups for the barium ions, differences in coordination behaviour can be effectively described by extending the cation environments with the full tetrahedral [Mn(NH₂)₄]²⁻ complex anions [27]. In Figure 4, all these coordination environments are gathered. The full [Mn(NH₂)₄]²⁻ ions are shown, if they coordinate by two or three amide groups. If connected to the central alkali metal atom only by one amide group, the tetrahedral complex ion is omitted for clarity. A clear difference in local structures is found in the arrangements of the type of complex [Mn(NH₂)₄]²⁻ ions in the coordination of the alkali metal ions. With increasing alkali metal size, not only the coordination number increases, but in general, also the number of different complex ions in the direct neighborhood and particularly the number of bi- and tridentate coordinations of those. In Na₂[Mn(NH₂)₄] [7], two monodentate and two bidentatete tetrahedra are attached to each crystallographically distinct sodium ion in very similar arrangements. The triclinic form of K₂[Zn(NH₂)₄] [18], for which no manganese representative is known so far, realizes qualitatively the same situation on one potassium ion, while the second one interestingly carries only one bidentate complex ion but three monodentate ones. Since K₂[Mn(NH₂)₄] and the HT-Rb₂[Mn(NH₂)₄] both share the same crystal structure [19], the surrounding of the alkali metals is similar, but in HT-Rb₂[Mn(NH₂)₄] extended to a coordination with three bi- and two monodentate tetrahedra ligands on one and already four bidentate ligands at the second rubidium ion. However, the LT-form of Rb₂[Mn(NH₂)₄] already exhibits a tridentate connection additionally two bi- and one monodentate ones for one alkali metal site, while the other is still attached to three bidentate and two monodentate tetrahedra [17]. Lastly, in Cs₂[Mn(NH₂)₄], the Cs(1) site exhibits a total of three tetrahedra connecting bi- and three monodentate. The Cs(2) site even is coordinated by two three-, two bi- and only one monodentate tetrahedra.

3.3. Vibrational Spectroscopy

While Raman spectroscopy proved difficult due to decomposition, measurements with infrared radiation posed possible for both HT-Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄]. HT-Rb₂[Mn(NH₂)₄] shows four signals in total in between 3217 and 3342 cm${}^{-1}$, which can be assigned to NH₂^- valence vibrations $\nu$(N–H), in agreement with two groups of amide ions with very similar surroundings each in the crystal structure. Signals in the range of 1516–1549 cm${}^{-1}$ can be assigned to amide deformation modes $\delta$(H–N–H). Further signals below 600 cm${}^{-1}$ are probably caused by Mn–N lattice vibrations. Since these are numerous and ambiguous to assign, they were omitted from

Table 4. List of observed modes in the infrared spectra of HT-Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄]. All values are given in cm${}^{-1}$.

	HT-Rb2[Mn(NH2)4]	Cs2[Mn(NH2)4]
Valence modes
	3342	3297
	3327	3248
	3275	3236
	3217
Bending modes
	1549	1538
	1529	1525
	1516

.

For Cs₂[Mn(NH₂)₄], IR bands can be found within the same ranges: $\nu$(N–H) = 3236–3297 cm${}^{-1}$, $\delta$(H–N–H) = 1525–1538 cm${}^{-1}$. Due to even more similar environments of the four distinct amide groups, we observed only three valence modes compared to the rubidium compound as well as only two deformation modes.

All assignments of signals were done in accordance to data on similar compounds like NaNH₂, Rb[Al(NH₂)₄], Cs[Al(NH₂)₄] and Cs₂[Zn(NH₂)₄] [8,28,29].

3.4. Thermal Analysis

Thermal decomposition of Cs₂[Mn(NH₂)₄] was performed in inert atmosphere. The thermal gravimetry (TG) curve shows a total of three steps of decomposition. Similar to the decomposition of other amidometalates such as Na[Al(NH₂)₄] or Rb[Al(NH₂)₄] [20,30], ammonia is first released from the compound (3.8% of total mass) starting at a temperature of about 370 K. The difference to the expected weight loss of 5.9% for 4/3 equivalents of ammonia per formula unit might be explained with premature decomposition during evacuation of the thermal analyzer in order to remove air and humidity prior to filling with argon gas for measurement. In a second step of 56.1%, CsNH₂ evaporates (expected 77.4%). In a third and final step, manganese nitride Mn₃N₂ decomposes and manganese slowly evaporates, leaving traces of manganese oxide behind. The TG curve is shown in Figure 5. Contrary to observations of Cao et al. in the decomposition of Rb₂[Mn(NH₂)₄], CsNH₂ could not be identified after the first decomposition step at about 573 K by powder X-ray diffraction [17]. Instead, reflections were assigned to manganese oxide, MnO, and the manganese nitrides Mn₃N₂ and Mn₄N next to unknown phases. Upon further decomposition at higher temperatures, the signals due to Mn₃N₂ gradually disappear.

4. Conclusions

The successful synthesis and crystal growth of HT-Rb₂[Mn(NH₂)₄] and Cs₂[Mn(NH₂)₄] by ammonothermal method is reported. The structures of those compounds fit smoothly into the structural evolution of the known alkali metal amidomanganates(II) and related amidometalates(II) of Mg and Zn. The atomic arrangements exhibit rising coordination numbers and multidentate coordination of tetraamidometalate ions at the alkali metal ions with increasing radius. The amidomanganates expectedly produce binary manganese nitrides upon decomposition in inert atmosphere.