Two Intermediates in Ammonothermal InN Crystal Growth: [In(NH3)5Cl]Cl2 and InF2(NH2)

Crystal Structure

[In(NH3)5Cl]Cl2 crystallizes in the orthorhombic space group Cmcm. Selected crystallographic parameters and refinement data are gathered in Table 1, for atomic coordinates and isotropic displacement parameters see Table 2. Further information on the structure determination and isotropic displacement parameters can be found in the supplementary material. [In(NH3)5Cl]Cl2 crystallizes as a distorted antitype of the K2[PtCl6] structure type and thus it can also be described as hierarchical variant of the CaF2 structure type. The crystal structure of [In(NH3)5Cl]Cl2 features one crystallographic site for In, two for Cl and three for N, arranged as isolated octahedral [In(NH3)5Cl(1)]2+ complex ions, which are surrounded by eight further isolated Cl(2)− in the second coordination sphere, realizing tetragonal prisms (Figure 2). Every second tetragonal prism is occupied by an [In(NH3)5Cl]2+ octahedron, while the others stay empty. The distortion of these tetragonal prisms from ideal cubic arrangement can be explained by the orientation of the [In(NH3)5Cl(1)]2+ octahedra, where the Cl(1) ligand is situated within the enlarged face. Consequently, the larger ionic radius of the chloride ligand as compared to NH3 and the electric repulsion between Cl(1)− and Cl(2)− appear as reasons for the distortion.

Table 1. Selected crystallographic parameters and refinement data of [In(NH3)5Cl]Cl2 and InF2(NH2).

Composition

[In(NH3)5Cl]Cl2

InF2(NH2)

Crystal system

Orthorhombic

Monoclinic

Space group

Cmcm

P21/n

a/pm

1046.3(1)

526.99(7)

b/pm

890.40(9)

523.50(5)

c/pm

1073.1(2)

920.7(2)

β/°

90

92.49(2)

Z

4

4

Density (calculated)/g cm−3

2.035

4.420

Volume/106 pm3

995.69(2)

253.76(5)

Index ranges hkl

−11≤h≤10, −11≤k≤13, −13≤l≤13

±7, ±6, ±12

θmax/°

54.12

56.52

F (000)

600

304

T/K

293

293

Diffractometer

STOE & Cie STADIVARI

STOE IPDS-I

μ(Mo-Kα1)/mm−1

3.11

9.07

Measured reflections/sym. independent

10196/603

3300/619

Rint/Rσ

0.0258/0.139

0.0518/0.0418

R1 with ∣Fo∣ >4σ(Fo)

0.0216

0.0281

R1/wR2/GooF

0.0273/0.0718/1.244

0.0388/0.0726/1.056

Remaining electron density/106 pm−3

0.57/−0.33

2.22/−1.38

Table 2. Atomic coordinates and isotropic displacement parameters (in 104 pm2) of [In(NH3)5Cl]Cl2.

Atom

Site

x/a

y/b

z/c

Ueq

In

4c

urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0001

0.20727(5)

urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0002

0.0240(2)

Cl(1)

4c

0

0.4288(2)

urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0003

0.0343(4)

N(1)

8f

urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0004

0.2093(5)

0.0416(5)

0.041(1)

N(2)

8g

0.2843(5)

0.2108(5)

urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0005

0.040(1)

N(3)

4c

urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0006

0.4614(8)

urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0007

0.040(2)

Cl(2)

8e

0.2087(1)

0

0

0.0389(4)

image

Section of the crystal structure of [In(NH3)5Cl]Cl2 with indicated unit cell axes. Dashed lines mark the distorted cubes from Cl(2), which are aligned in the second coordination sphere of In3+.

The interatomic distances In−N are found to be in the range of 223–225 pm (see Table 3), in good agreement with those distances in In(NH3)3Cl3=[In(NH3)4Cl2][In(NH3)2Cl4] (225–226 pm).11 In [Ga(NH3)5Cl]Cl2, which has strong structural relations to [In(NH3)5Cl]Cl2, the Ga−N distances are noticeably shorter: 206–208 pm.12, 18 The In−Cl(1) distance in [In(NH3)5Cl(1)]Cl2 with 248 pm is slightly shorter than the corresponding bond lengths in In(NH3)3Cl3, ranging from 250 to 254 pm. Unfortunately, we were unable to reliably determine the hydrogen positions for this title compound from the X-ray diffraction data.

Table 3. Selected interatomic distances (in pm) and angles (in deg.) in [In(NH3)5Cl]Cl2.

Distance

Distance

In−N(1)

2.236(5)

In−Cl(1)

2.480(2)

In−N(2)

2.257(5)

In−N(3)

2.262(7)

Angle

Angle

N(1)−In−N(1)

179.1(2)

N(1)−In−Cl(1)

90.5(1)

N(1)−In−N(2)

89.994(2)

N(2)−In−Cl(1)

90.8(1)

N(2)−In−N(2)

178.4(2)

N(3)−In−Cl(1)

180

N(1)−In−N(3)

89.6(1)

N(2)−In−N(3)

89.2(1)

Several pentaamminemonochloridometal dihalides are described in literature (space group type Pnma, with cell axes a≈2urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0008 , b, c≈urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0009 ), which all represent isotypes of [Ga(NH3)5Cl]Cl2.18, 19 Currently, the exclusively known isotype of [In(NH3)5Cl]Cl2 is [Al(NH3)5I]I2.20 Jacobs et al. argued that the occurrence of this particular arrangement for [Al(NH3)5I]I2 may either arise from differences in the hydrogen bonding system or be due to the ionic radii relations. However, since the ionic radius of In3+ in a six-fold coordination is significantly larger than these of Ir3+, Ru3+ and Rh3+, while the radii of Al3+ and Ga3+ are rather similar and smaller, the latter reason seems unlikely.21

The crystal structure of [In(NH3)5Cl]Cl2 shows strong structural similarities to the crystal structure of [Ga(NH3)5Cl]Cl2.12, 18 Both compounds crystallize in a distorted variant of the aristotype K2[PtCl6]. [M(NH3)5Cl]2+ octahedra (M=Ga, In) form the motif of a cubic close packing. The Cl(2)− anions, which are not directly connected to the metal ion, occupy all tetrahedral holes within this packing, by that realizing a distorted motif of a cubic primitive arrangement. However, the distortion of the Cl(2)− anion lattice in [In(NH3)5Cl]Cl2 differs from the distortion of the respective Cl(2)− anion lattice in Ga(NH3)5Cl]Cl2 due to the different relative orientations of the complex ions. In [In(NH3)5Cl]Cl2, the Cl(1) ligands of the complex ions point alternatingly either in [010] or the opposite urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0010 direction, while in Ga(NH3)5Cl]Cl2, they alternate in two orientations by a rotation of about 90° (pointing along [101], urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0011 and urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0012 , urn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0013 respectively of the orthorhombic unit cell, compare Figure 3).

image

Sections of the crystal structures of a) [In(NH3)5Cl]Cl2 orthogonal to the (001) plane and b) [Ga(NH3)5Cl]Cl2 orthogonal to the (010) plane.

The crystallographic relations of the two structures can be conveniently described in a group-subgroup scheme according to Bärnighausen.22 The aristotype K2[PtCl6] crystallizes in the cubic space group Fmurn:x-wiley:00442313:media:zaac202100052:zaac202100052-math-0014m. In a first symmetry reduction step removing the threefold rotational axis of the cubic system and choosing the smaller I-centered unit cell, a transition to the translationengleiche subgroup I4/mmm (t3) is performed. The left column of Figure 4 shows the further symmetry reductions leading to [In(NH3)5Cl]Cl2 via a transition to the klassengleiche subgroup Fmmm, choosing the larger F-centered unit cell again, and a t2 transition to the subgroup Cmcm. The steps in symmetry reduction leading to [Ga(NH3)5Cl]Cl2 from the aristotype deviate after the first step in an alternative k2 transition to the subgroup Immm (compare Figure 4 right column), followed by a translationengleiche transition with index 2 (t2) to the subgroup Pmmn. Finally, a further klassengleiche transition of index 2 (k2) leads to the subgroup Pnma by doubling one unit cell axis. Finally, the occupation of one ligand site in each model of reduced symmetry with the chloride ligand Cl(1) (Wyckoff site 4c) and the remaining one with ammonia in both cases results in the final crystal structure description.

image

Group-subgroup scheme according to Bärnighausen. Left: Structural relation of the aristotype K2[PtCl6] and the hettotype [In(NH3)5Cl]Cl2. Right: structural relation of the aristotype K2[PtCl6] and the hettotype [Ga(NH3)5Cl]Cl2.

Vibrational spectroscopy

IR and Raman spectra of [In(NH3)5Cl]Cl2 are depicted in Figure 5. In the IR spectrum, multiple signals in the range of 3133–3297 cm−1 can be observed, which can be assigned to the symmetric and antisymmetric ν(NH3) stretching vibrations. In gaseous ammonia, these vibrations can be observed at 3337 cm−1 and 3450 cm−1.23 In solid compounds, they can be shifted to lower frequencies due to hydrogen bonding. The magnitude of the shift depends on the strength of the hydrogen bonds, since stronger H⋅⋅⋅A bonds cause a weakening of the D−H bond.23 A splitting of the symmetric and the antisymmetric stretching vibrations into multiple signal arises from the different chemical environments.

image

Vibrational spectroscopy on [In(NH3)5Cl]Cl2; top: IR transmission spectrum, bottom: single crystal Raman spectrum (excitation wavelength: 532 nm).

In the region of 600–1700 cm−1, typical bands for the N−H deformation vibrations can be observed. Three types of deformation vibrations are present: the degenerate deformation vibrations δd(NH3), the symmetric deformation vibrations δs(NH3) and the rocking vibrations ρ(NH3). Observed modes in the Raman spectrum agree with those from IR. Additional signals at 403 and 213 cm−1 can be assigned to stretching or deformation modes of the complex ion. In literature, spectroscopic investigations of numerous pentaamminemonochloridometal dichlorides have been reported and accord with the vibrations observed in [In(NH3)5Cl]Cl2.24 A summary of all observable modes can be found in Table 4.

Table 4. Observed IR and Raman vibration modes (in cm−1) in [In(NH3)5Cl]Cl2.

Mode

IR

Raman

ν(NH3)

3297, 3189, 3133

3311, 3227, 3151

δd(NH3)

1601

1577

δs(NH3)

1248, 1201

1233

ρ(NH3)

716, 677

738

ν(In−N; In−Cl), δ(N,Cl−In−N)

403, 213

Thermal Analysis

In order to study the thermal decomposition of [In(NH3)5Cl]Cl2, a thermogravimetric measurement in inert atmosphere was conducted, which is depicted in the supporting information section. Between about 450 K and 550 K a decomposition with a mass loss of 20 % takes place. The release of the entire ammonia would lead to a calculated mass loss of 24.4 %. The discrepancy in mass loss is mostly attributed to some sample decomposition during sample transport and mounting, due to moisture sensitivity, as well as some premature ammonia loss during evacuation and purging of the thermal analyzer in order to remove traces of oxygen from the gas atmosphere. Furthermore, PXRD measurements of the sample revealed the presence of small amounts of elemental indium and indium oxide impurities. Above 700 K the remaining InCl3 starts to sublime, leaving above 1000 K about 20 % of the initial material behind. Earlier investigations of the decomposition of respective ternary gallium amides revealed GaN as a minor product.12 However, due to the metastable character of InN,15 small amounts of elemental indium and indium oxide are more likely to constitute the remains. A small recovered amount of the light-yellow decomposition product appeared yellowish and PXRD revealed indium oxide.

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