Multi-Centered Solid-Phase Quasi-Intramolecular Redox Reactions of [(Chlorido)Pentaamminecobalt(III)] Permanganate—An Easy Route to Prepare Phase Pure CoMn₂O₄ Spinel

Abstract

We synthesized and structurally characterized the previously unknown [Co(NH₃)₅Cl](MnO₄)₂ complex as the precursor of CoMn₂O₄. The complex was also deuterated, and its FT-IR, far-IR, low-temperature Raman and UV-VIS spectra were measured as well. The structure of the complex was solved by single-crystal X-ray diffraction and the 3D-hydrogen bonds were evaluated. The N-H…O-Mn hydrogen bonds act as redox centers to initiate a solid-phase quasi-intramolecular redox reaction even at 120 °C involving the Co(III) centers. The product is an amorphous material, which transforms into [Co(NH₃)₅Cl]Cl₂, NH₄NO₃, and a todorokite-like solid Co-Mn oxide on treatment with water. The insoluble residue may contain {Mn₄^IIIMn^IV₂O₁₂}_n⁴ⁿ⁻, {Mn₅^IIIMn^IVO₁₂}_n⁵ⁿ⁻ or {Mn^III₆O₁₂}_n⁶ⁿ⁻ frameworks, which can embed 2 × n (Co^II and/or Co^III) cations in their tunnels, respectively, and 4 × n ammonia ligands are coordinated to the cobalt cations. The decomposition intermediates decompose on further heating via a series of redox reactions, forming a solid Co^IIM^III₂O₄ spinel with an average size of 16.8 nm, and gaseous N₂, N₂O and Cl₂. The CoMn₂O₄ prepared in this reaction has photocatalytic activity in Congo red degradation with UV light. Its activity strongly depends on the synthesis conditions, e.g., Congo red was degraded 9 and 13 times faster in the presence of CoMn₂O₄ prepared at 550 °C (in air) or 420 °C (under N₂), respectively.

1. Introduction

Nanosized mixed transition metal-manganese oxides can be prepared by the controlled thermal decomposition of transition metal permanganate complexes that have reducing inorganic [1,2,3,4,5,6] or organic ligands [7,8]. For example, Mansouri et al. prepared a cobalt manganese oxide spinel (CoMn₂O₄) with excellent activity in the Fischer–Tropsch fuel synthesis, by the thermal decomposition of the [Co(NH₃)₄CO₃]MnO₄ complex [6,9]; however, [Co(NH₃)₄CO₃]MnO₄ contains Co and Mn in a 1:1 atomic ratio. Thus, the decomposition product is probably a mixed Co(Co_0.5Mn_1.5)O₄ spinel or a mixture of CoMn₂O₄ and Co₃O₄. To prepare a phase pure CoMn₂O₄ spinel, we attempted to prepare the [Co(NH₃)₆]Cl(MnO₄)₂ (compound 1-Mn) precursor (Co:Mn = 1:2), but despite the existing analogous perchlorate ([Co(NH₃)₆]Cl(ClO₄)₂) (compound 1-Cl) [10], our efforts were not successful. Alvisi [10] supposed that an unidentified chlorine-containing amminecobalt permanganate complex prepared by Klobb [11] was identical with compound 1-Mn, however, our repeated experiments showed that the mentioned compound was [Co(NH₃)₆]Cl₂(MnO₄) (compound 2-Mn) (Co:Mn = 1:1) [11]. Moreover, we were able to produce an amminecobalt(III) permanganate complex precursor with the expected Co:Mn = 1:2 ratio with the isolation of the formerly unknown [Co(NH₃)₅Cl](MnO₄)₂ complex (compound 3-Mn). This precursor complex was structurally and spectroscopically characterized and heated, as a result of which it transformed into CoMn₂O₄ in an expected quasi-intramolecular redox reaction between the oxidizing (Co^III and permanganate) and reducing (ammonia and chloride) components. The labels for the synthesized compounds and composites used in the paper are given in

Table 1. Labels of compounds.

Compound	Label
[Co(NH3)6]Cl(MnO4)2	1-Mn
[Co(NH3)6]Cl(ClO4)2	1-Cl
[Co(NH3)6]Cl2(MnO4)	2-Mn
[Co(NH3)5Cl](MnO4)2	3-Mn
[Co(NH3)5Cl](ClO4)2	3-Cl
[Co(NH3)5Cl])ReO4)2·nH2O	3-Re
[Co(NH3)5Cl]Cl2	4

.

2. Results and Discussion

2.1. Synthesis and Properties of Compound 3-Mn

[Chlorido(pentaammine)cobalt(III)] permanganate (compound 3-Mn) has not been prepared previously, although Krestov and Yatsimirksii [12] calculated its thermodynamical properties. The analogous perchlorate compound (Co(NH₃)₅Cl](ClO₄)₂ (compound 3-Cl) is known [13] and can easily be prepared by heating the [(aqua)(pentaammine)cobalt(III)] triperchlorate solution in 1 M HCl. Unfortunately, this reaction route cannot be used to prepare compound 3-Mn due to the spontaneous reaction between the permanganate ions and the HCl solution (chlorine evolution and Mn^II formation). Therefore, a chloride ion has to be introduced into the inner coordination sphere of the cobalt in the absence of a permanganate ion—for example, with the preparation of [Co(NH₃)₅Cl]Cl₂ (compound 4) [14]—and the outer sphere chloride ions have to be exchanged with permanganate ions, such as occurred with the perrhenate ions in the preparation of compound 3-Re [15,16]. A possible reaction route is the reaction of [Co(NH₃)₅Cl]SO₄ [14] with barium permanganate [17,18,19,20]; however, this method was not used due to the relatively low solubility of compound 3-Mn in water that causes problems in the separation of compound 3-M from the insoluble barium sulfate by-product. The reaction of compound 4 and an excess of NaMnO₄ in a 40% aq. solution, with subsequent cooling to +1 °C resulted in the formation of the compound 3-Mn as a reddish-purple precipitate with a 57% yield. Its powder X-ray diffractogram is given in Figure S1. Its solubility in water was 0.0089 M (3.71 g/L) and 0.0225 M (9.39 g/L) at 0 °C and 25 °C, respectively. It is insoluble in aliphatic hydrocarbons and such polar organic solvents like chloroform, dichloromethane, or benzene, but easily soluble in DMF, and decomposes in DMSO.

2.2. Structure of Compound 3-Mn

Dark violet single crystals of compound 3-Mn were selected from the precipitate formed during the synthesis. The powder X-ray diffractogram calculated from the SXRD data agreed very well with the experimentally determined powder X-ray diffractogram (Figure S1). Crystallographic data, details of the structure and the refinement results are listed in

Table 2. Crystal data of compound 3-M.

Empirical Formula	[Co(NH3)5Cl](MnO4)2	[Co(NH3)5Cl](ReO4)2·nH2O [15]
Formula Weight	417.4101 g·mol−1	679.95 g·mol−1
Crystal System	Orthorhombic	Orthorhombic
Space Group	Cmc21	Cmc21
Unit cell dimensions, Å	a = 14.2753 (7) b = 14.2816 (6) c = 12.2342 (5);	a = 14.9446 (3) b = 14.6562 (4) c = 12.2434 (4)
Z	8	8
Density (calcd.) (g·cm−3)	2.216	3.368
Temperature (K)	298	293
Volume (Å3)	2494.24 (19)	2681.68 (13)
R factor (%)	3.67	2.35

, together with the data obtained for the analog perrhenate complex (compound 3-Re) [15]. The structural features of compound 3-Mn are shown in Figure 1, Figure 2 and Figure 3. The detailed structural parameters, bond lengths and angles can be found in Tables S1–S3.

Compound 3-M is orthorhombic. Its space group is Cmc2₁ (Nr. 36). The coordination around the Co³⁺ cation is octahedral. The asymmetric unit contains two half complex [Co(NH₃)₅Cl]²⁺ cations and two permanganate anions (Figure 1). The central cobalt ions of the complex cations, the coordinated chloride anions, and some of the ammonia ligands sit on a mirror plane and therefore have half-site occupancy, so that the hydrogens of these ammonia molecules are disordered over two mirrored positions. The structural features of compound 3-Mn are shown in Figure 1, Figure 2 and Figure 3.

The axial Co-N bond distance (Co-N1 = 1.955 (6) Å) was shorter in cation A than the equatorial bond distances (1.964(4)–1.965(4) Å), whereas in cation B, the axial Co-N distance (Co-N7 = 1.964 (6) Å) was longer than the equatorial ones (1.950(4)–1.960 (6) Å). The Co-Cl bonds were almost equal (2.258 (2) and 2.255 (2) Å in cation A and B, respectively). The Co-Cl distances in the analogous perrhenate complex (Compound 3-Re) were found to be 2.270 (2) and 2.260 (2) Å, whereas the Co-N distances varied between 1.960 (5)–1.972 (6) and 1.948 (7)–1.988 (4) Å, respectively. Layers of the complex cations in the bc plane were embedded within permanganate layers (Figure 2).

The [Co(NH₃)₅Cl]²⁺ cation has five hydrogen donor groups and one hydrogen acceptor group. The arrangement of the cations in the crystal lattice is such that all ammonia hydrogens can find at least one acceptor (chloride or permanganate oxygen) (Figure 3, Table S3).

The strengths of N-H…O and N-H…Cl hydrogen bonds in compound 3-Mn were comparable with the hydrogen bond strengths of the analogous perrhenate complex (compound 3-Re) (N…O distances were between 2.86–3.23 Å and 2.91–3.29 Å, whereas N…Cl distances were between 2.98–3.60 Å and 3.33–3.53 Å for compound 3-Mn and compound 3-Re, respectively [15,16]).

2.3. Spectroscopic Properties of Compound 3-Mn

We analyzed the vibrational spectroscopic modes of compound 3-Mn by factor group analysis, considering the structure of [Co(NH₃)₅Cl](MnO₄)₂ to be composed of a Co–Cl group, five NH₃ molecules coordinated to Co (four and three crystallographic types at the special and trivial positions C_s and C₁, respectively), and two types of isolated MnO₄^– anions. The compound 3-Mn is orthorhombic (Cmc2₁, Z = 8), consisting of two types of positions only: general (with trivial symmetry, 1) and special (with symmetry m, yz plane). The unit cell is base-centered. Thus, the primitive cell contains only four motifs, meaning Z′ = 4 for the “spectroscopic cell”. This means that there are two quartets of permanganate anions, each at the position of trivial symmetry. The internal of the NH₃ molecules at the special and trivial symmetry positions (C_s and C₁) can be seen in Figure 4. The internal and external vibrations of the permanganate ions and Co-Cl groups (molecular symmetry C_∞v), or external vibrations for the NH₃ ligands in compound 3-Mn, are presented by Figures S2–S4.

2.3.1. Vibrational Modes of the Permanganate Anion

The vibrational modes of the permanganate ion were expected to appear in both the IR and Raman spectra as two series of singlet symmetric (ν₁), triplet antisymmetric (ν₃), doublet symmetric (ν₂) and triplet antisymmetric (ν₄) ones. The IR-forbidden ν₁ and ν₂ under T_d became IR active due to symmetry lowering (C_2v) (Figure 5).

The IR, Raman and far-IR spectra of compound 3-Mn are given in Figure 4, Figure 5, and Figure S5. The low-temperature Raman spectral range contains the permanganate and [CoN₅Cl]²⁺ skeleton vibrations of compound 3-Mn, which can be seen in Figure 6. The IR and Raman data assignments can be seen in Table 3, Table 4 and Table 5.

The low-temperature Raman spectra for compound 3-Mn were recorded in the range of 1000–200 cm⁻¹ (Figure 6).

Despite the forbidden nature of ν_s(Mn-O) and δ_s(Mn-O) modes in T_d, 2 and 2 × 2, weak singlet IR bands, respectively, were expected to appear due to the distortion of the MnO₄^– symmetry (C_2v) and the presence of two sets of permanganate ions in the structure; however, the IR spectrum contained one singlet band at 841 and one at 352 cm⁻¹, assigned as ν_s(Mn-O) and δ_s(MnO) modes. The band at 841 cm⁻¹ had a shoulder (Figure 5). The ν_s(Mn-O) mode was expected and found to be an intensive band in the Raman spectrum without splitting at room temperature and as two singlets at 123 K, whereas the ν_s(Mn-O) appeared as a singlet at room temperature and as a singlet with two shoulders in the low-temperature Raman spectra. The relatively high IR intensity of the ν_s(Mn-O) band can be attributed to the occurrence of a vibrational mode of the cation core [Co(NH₃)₅Cl]²⁺) located around this wavenumber and coinciding with the ν_s(Mn-O) band. Abbas [21] and Najar [22] assigned a band around ~844 cm⁻¹ to ν(Co-Cl), whereas Sacconi [23] assigned a band around 849 to ρ(NH₃) in the IR spectrum of [Co(NH₃)₅Cl]Cl₂ (compound 4). Thus, one of these modes might coincide with the band of ν_s(Mn-O) and play a role in the appearance of an intensive mixed IR band (a consequence of vibrational resonance). A detailed study of the skeletal vibrational modes [24,25,26] (see below), including the ν(Co-Cl) mode, showed that it was located in the far-IR range and not around 844 cm⁻¹. Thus, the coinciding component may only be the ρ(NH₃) mode. The low-temperature (123 K) Raman spectrum of compound 3-Mn showed two bands at 849 and 839 cm⁻¹ (Figure 6), with similar intensities. The ρ(NH₃) mode could not be seen in the Raman spectrum [25] but appeared in the IR spectrum of compound 4 [21,25]. Thus, the splitting of the singlet Raman band into two components can only be attributed to the appearance of the singlets of two kinds of permanganate ions located in the structure of compound 3-Mn. The lack of ρ(NH₃) band in the Raman spectrum and its role in the increase in the intensity of the mixed ν_s(Mn-O) + ρ(NH₃) IR band were confirmed by the deuteration of compound 3-Mn as well. The IR and Raman spectra of the deuterated compound 3-Mn (Figures S6–S8) showed that the ρ(NH₃) component of the mixed IR band was decomposed (the shape of the band at 841 cm⁻¹ was changed and a new band appeared at 646 cm⁻¹ (ρ(ND₃)), whereas the ν_s(Mn-O) band structure in the LT-Raman spectrum was not changed at all by deuteration. The antisymmetric stretching mode (ν_as(Mn-O) was a strong band in the IR and a weak one in the Raman spectra at room temperature (Table 3). The low-temperature Raman study (123 K, Figure 6) showed splitting of the band located around ~900 cm⁻¹ into six components, which corresponded to the two sets of triplet ν_as(Mn-O)(F₂) modes. There were no coinciding cationic modes in this spectral range.

Table 3. Assignments of permanganate ion vibrational modes in the IR and Raman spectra of compound 3-Mn.

Band/Assignation	Wavenumber/Raman Shift (cm−1)
	IR	Raman (785 nm)
		298 K	123 K
ν1 (MnO), νas (A) *	829	841	848, 838
ν3 (MnO), νas (F2)	895	903	928, 918sh, 913sh, 905sh, 899sh
ν4 (MnO), δas (F2)	379	389	396, 386, 383
ν2 (MnO), δs (E)	352	346	352sh, 358sh, 346

* Mixed band of ν₁ (Mn-O) and ρ(NH₃).

Table 3. Assignments of permanganate ion vibrational modes in the IR and Raman spectra of compound 3-Mn.

Band/Assignation	Wavenumber/Raman Shift (cm−1)
	IR	Raman (785 nm)
		298 K	123 K
ν1 (MnO), νas (A) *	829	841	848, 838
ν3 (MnO), νas (F2)	895	903	928, 918sh, 913sh, 905sh, 899sh
ν4 (MnO), δas (F2)	379	389	396, 386, 383
ν2 (MnO), δs (E)	352	346	352sh, 358sh, 346

* Mixed band of ν₁ (Mn-O) and ρ(NH₃).

The symmetric deformation mode (δ_s(Mn-O) was double degenerated, and according to this, 2 × 2 = 4 bands were expected both in the far-IR and Raman spectra. The far-IR and the LT-Raman spectra contained one and two bands at 381 and 386/396 cm⁻¹, respectively. The intensities of the two bands in the LT-Raman spectrum were almost equal, and the asymmetric shape of the bands showed that these consisted of more than one component. Since δ_{s s}(Mn-O) is a double degenerated mode, and there are two sets of permanganate ions in the structure, the two bands probably contain 2 components each. The wide band found at room temperature Raman spectra (Figure 6) split at 123 K. This may be attributed to the appearance of the two sets of permanganate ion modes and due to the splitting of the E symmetry mode. The antisymmetric ν_as(Mn-O) mode appeared in both the far-IR and the LT-Raman spectra around 349 (sh) and 346 cm⁻¹ (Figure 6 and Figure S5), respectively.

2.3.2. Ligand Vibrations

The correlation analysis of the cationic part of compound 3-Mn showed seven (4 in C_s and 3 in C₁) and two sets of vibrational modes belonging to ammonia molecules (Figure 4) and Co-Cl groups (Figure S3). The Co-N and Co-Cl modes were evaluated as skeletal modes, and the assignation of the vibrational modes of the ammonia ligands in compound 3-Mn and its deuterated form (Figures S6–S8) are given in Table 4.

Table 4. The IR and Raman spectra of the ammonia ligand in [Co(NH₃)₅Cl](MnO₄)₂ (compound 3-Mn) and its deuterated form.

Band/Assignation	Wavenumber (cm−1)
	Compound 3-Mn		Deuterated Compound 3-Mn
	IR, 298 K	Raman (785 nm), 123 K	IR, 298 K	Raman (785 nm), 123 K
νas (HNH)(DND)	3293	-	2444	-
νs (HNH)(DND)	3270	-	2319	-
δas(HNH)(DND)	1607	-	1153	-
δs(HNH)(DND)	1324, 1294 (m)	1352, 1311 (vw)	1015	-
ρ (NH3)(ND3)	829 (s)	-	646	-

Table 4. The IR and Raman spectra of the ammonia ligand in [Co(NH₃)₅Cl](MnO₄)₂ (compound 3-Mn) and its deuterated form.

Band/Assignation	Wavenumber (cm−1)
	Compound 3-Mn		Deuterated Compound 3-Mn
	IR, 298 K	Raman (785 nm), 123 K	IR, 298 K	Raman (785 nm), 123 K
νas (HNH)(DND)	3293	-	2444	-
νs (HNH)(DND)	3270	-	2319	-
δas(HNH)(DND)	1607	-	1153	-
δs(HNH)(DND)	1324, 1294 (m)	1352, 1311 (vw)	1015	-
ρ (NH3)(ND3)	829 (s)	-	646	-

The differences in the geometrical (axial and equatorial) and the seven crystallographic positions of ammonia molecules resulted in poorly resolved complex band systems for all kinds of N-H modes. The symmetric deformation mode ν_s (N-H) of compound 3-Mn showed two main types of ammonia ligated with different strengths to the central Co^III-ion. A relative bond strength parameter (ε) for the ammonia molecules in ammine complexes was defined by Grinberg [27,28]. The parameter (ε) was found to be 0.88 and 0.80 as the two types of coordinated ammonia molecules gave two different δ_s (N-H) bands in the IR spectrum of compound 3-Mn. Thus, the bond strength difference between the two main types of coordinated ammonia molecules in compound 3-Mn is ~10%. This difference may be attributed to the change in the strength of the Co-N bond (apical or equatorial position); however, the electron density change caused by this difference might be attributed to the difference between the hydrogen bond strength of each ammonia molecule, which can play a crucial role in the value of the Grinberg parameter [2,28].

The positions of N-H stretching and deformation N-H modes in the IR spectrum of compound 3-Mn did not give unambiguous information about the presence and strength of hydrogen bonds in the compound 3-Mn. The ρ (NH₃) rocking mode is the most sensitive vibrational mode to detect the presence of hydrogen bonds in ammonia complexes [23], but the ρ(NH₃) in the IR spectrum of the compound 3-Mn was a mixed band with ν_s (Mn-O). Therefore, we prepared the deuterated compound 3-Mn and determined the ρ (ND₃) band position (646 cm⁻¹). Using the ρ(NH₃)/ρ(ND₃) wavenumber ratio found for [Co(NH₃)₅X]X₂ (1.267) compounds [23], we calculated the position of ρ(NH₃) for compound 3-Mn and found it to be 818 cm⁻¹. This showed that the strength of the hydrogen bond in compound 3-Mn is weaker than in [Co(NH₃)₅Cl]Cl₂ (ρ(NH₃) = 849 cm⁻¹) and is between those found in [Co(NH₃)₅Br]Br₂ (ρ(NH₃) = 830 cm⁻¹) and [Co(NH₃)₅I]I₂ (ρ(NH₃) = 810 cm⁻¹) [23].

2.3.3. Skeletal Vibrations

The skeletal vibrations were evaluated by the normal coordinate analysis of the [Co(NH₃)₅Cl]²⁺ ion [24,25,26]. For the skeleton [MN₅X]²⁺, assuming C_4v symmetry, 11 normal modes were expected. The A₁ and E species were IR and Raman active, whereas the B₁ and B₂ species were only Raman active. The normal modes and band assignations for compound 3-M and its deuterated form are given in Table 5.

Table 5. Skeletal vibrational mode assignments in the IR and Raman spectra of compound 3-Mn and its deuterated form.

Species	Mode	Assignation	Compound 3-Mn		Deuterated Compound 3-Mn
			IR (298 K)	Raman (123 K)	IR (298 K)	Raman (123 K)
A1 (R IR)	ν1	ν (Co-Cl)	278, 272sh	288, 285	264	265
	ν2	νs(CoN4)	458	465	451	456
	ν3	ν (Co-N)axial	488	487	483	489
	ν4	π (Co-N4)	199	189sh	185	184
B1 (R)	ν5	νas (CoN4)	-	443	-	448
	ν6	π (Co-N4)	-	193sh	-	193sh
B2 (R)	ν7	δ (NCoN)	-	327	-	327
E (R, IR)	ν8	ν (Co-N)axial	505, 493 sh	496 sh	483	505
	ν9	δ (NCoN or CoN4 wag)	320	329	344 sh	347
	ν10	δ (NCoCl or CoN4 rock)	200	205	199 sh	203
	ν11	δ (NCoN or CoN4 in-plane blend)	259sh	265	264	265

Table 5. Skeletal vibrational mode assignments in the IR and Raman spectra of compound 3-Mn and its deuterated form.

Species	Mode	Assignation	Compound 3-Mn		Deuterated Compound 3-Mn
			IR (298 K)	Raman (123 K)	IR (298 K)	Raman (123 K)
A1 (R IR)	ν1	ν (Co-Cl)	278, 272sh	288, 285	264	265
	ν2	νs(CoN4)	458	465	451	456
	ν3	ν (Co-N)axial	488	487	483	489
	ν4	π (Co-N4)	199	189sh	185	184
B1 (R)	ν5	νas (CoN4)	-	443	-	448
	ν6	π (Co-N4)	-	193sh	-	193sh
B2 (R)	ν7	δ (NCoN)	-	327	-	327
E (R, IR)	ν8	ν (Co-N)axial	505, 493 sh	496 sh	483	505
	ν9	δ (NCoN or CoN4 wag)	320	329	344 sh	347
	ν10	δ (NCoCl or CoN4 rock)	200	205	199 sh	203
	ν11	δ (NCoN or CoN4 in-plane blend)	259sh	265	264	265

Based on the correlation analysis results (Figure S3), two sets of ν(Co-Cl) modes were expected and found at 278/272sh and 288/285 cm⁻¹ in the far-IR (Figure S5) and LT-Raman spectra (Figure 6), respectively. The slight shifts of these bands on deuteration (compound 3-Mn and its deuterated form) (average ν_H/ν_D = ~1.04) can be attributed to the coupling of the ν(Co-Cl) mode with other skeletal vibrational modes. The intensity of some bands was very weak but based on the data of normal coordinate analysis of the [Co(NH₃)₅Cl]²⁺ and [Co(ND₃)₅Cl]²⁺ skeletons, all bands were successfully assigned (Table 5).

2.4. UV-VIS Spectroscopy

The solid-phase UV-VIS spectrum of compound 3-Mn was recorded at room temperature, but the spectrum contained strongly overlapping bands both in the UV and visible region (Figure S9). The band at 217 nm may have belonged to both the CT band of the complex cation (electron transfer is from chlorine to the d_z2 orbital of cobalt (LMCT)) and the ¹A₁-¹T₂ (t₁-4t₂) transition of a permanganate ion as well. The Cl to Co band position for compound 4 and the ¹A₁-¹T₂ (t₁-4t₂) transition for KMnO₄ were found at 227 nm [8,29,30]. A wide band was observed with maximums at 249 and 266, which may have belonged to the ¹A₁-¹T₂ (3t₂-2e) and ¹T_2g-¹A_1g transition of the permanganate ion and the octahedral [Co(NH₃)₅Cl]²⁺ cation [29,30], respectively. The analogous bands were found at 259 and 274 nm for the KMnO₄ and compound 4, respectively [8,29,30]. Due to the purple color of the complex cation and the permanganate anion in compound 3-Mn, the visible region contained a very intensive band system between 500 and 600 nm. The permanganate ¹A₁-¹T₂ (t₁-2e) band for the KMnO₄ was found between 500 and 562 nm [8], and the doublet of the ¹T_1g-¹A_1g transition of the [Co(NH₃)₅Cl]₂SO₄ was found at 470 and 547 nm [29]. The maximums at 570 (permanganate) and 546 nm (complex cation) for compound 3-Mn probably belonged to these transitions. No split of the ¹T_1g-¹A_1g transition of the complex cation due to the difference between the contribution of the chloride ion (1Cl + 3NH₃) and ammonia (4NH₃) coordination was observed. The band at 685 nm probably belonged to the ¹A₁-¹T₁(t₁-2e) transition of permanganate ion. This transition was found at 720 nm for the KMnO₄ and 710 nm for the [Agpy₂]MnO₄ [8].

2.5. Thermal Decomposition of Compound 3-Mn

The thermal decomposition of [(chlorido)pentaamminecobalt(III)] permanganate (compound 3-Mn) was studied in an oxidative and inert atmosphere. There were no significant differences between the decomposition mechanisms in argon and air. The decomposition processes were somewhat more intensive in air than in argon, but the characteristic temperatures were almost the same. Based on the mass losses, powder XRD and the Co:Mn = 1:2 stoichiometry in compound 3-M, the final decomposition product above 400 °C was an oxide phase with the overall formula of CoMn₂O₄, with an average size of 16.8 nm (identified by powder XRD, and size determined by the Scherrer method (Figure 7)).

The thermal decomposition of compound 3-Mn started at 121 °C, onset (Figures S11 and S12). The mass loss in the first decomposition step was ~4.9%, which was somewhat more than the mass percent of one molecule of NH₃ (4.08%).

The IR study supported the notion that the ammonia molecules were bonded with various strengths (ε = 0.80 and 0.88, see above), which predicted a possible stepwise ammonia loss. The TG-MS study, however, unambiguously showed that in the first decomposition step, the leaving product was water (m/z = 18, H₂O⁺) and not ammonia (m/z = 17, NH₃⁺) (Figure 8 and Figure S11). The shoulder at around 125 °C on the ion curve m/z = 17 corresponded to the HO⁺ fragment ion of water. Moreover, the ammonia had a significant m/z = 16 peak but the ion intensity curve of m/z = 16 did not have any peak or shoulder at around 125 °C. Neither N₂ nor NO_x compounds were formed in this step. Compound 3-Mn is anhydrous. Thus, water may only be the reaction product of a solid-phase quasi-intramolecular redox reaction between ammonia as the hydrogen source and a species containing oxygen. Due to the inert atmosphere, the oxygen source may only be the permanganate ion. Thus, this redox reaction was centered on ammonia ligands and permanganate ions.

The IR spectra of the decomposition product formed from compound 3-Mn at 125 °C (Figure 9) showed that the permanganate ion IR bands almost completely disappeared. There were no signs of manganate (VI) or manganate (V) ions [17,19]. Thus, manganese valence in the decomposition product may only be +2, +3, or +4. The release of one water molecule left seven oxygen atoms remaining from the eight oxygen atoms of the two permanganate ions in compound 3-M, distributed among the possible redox products. The primary decomposition product was X-ray amorphous, but its IR (Figure 9) and far-IR spectra (Figure S12) showed the presence of an ammonium ion (shoulder at ~1680 cm⁻¹ (δ_as(N-H)), ν₂ + ν₄ combination band at ~2840 cm⁻¹) and nitrate ions (ν_as(N-O) ~1370 cm⁻¹). The N-H bands belonging to [Co^II(NH₃)₄]²⁺ ions were expected to appear around the same wavenumbers as the NH₃ bands of compound 4 [31]. The decomposition intermediate showed the band characteristics for todorokite and related mixed manganese oxide family members as containing Mn^III/Mn^IV species [32].

The first thermal decomposition step of the compound 3-Mn was also studied in refluxing toluene. The boiling temperature of toluene (110 °C) limits the maximal decomposition reaction temperature by absorbing the evolved reaction heat via the evaporation of toluene, which prevents overheating by the heat of the reaction. The solid phase formed in the thermal decomposition of compound 3-Mn under refluxing toluene in 1 h consisted of amorphous phases (Figure 10), which could be crystallized by heating above 400 °C with the formation of CoMn₂O₄ and gaseous (N₂, H₂O, NH₃, NH₄Cl) products. The crystallite size of the CoMn₂O₄ depended on the heating time and temperature, similarly to other spinels prepared from ammine complexes of transition metal permanganates [1,3,4,5].

Separating the amorphous reaction product into water-soluble and insoluble (oxide) parts and evaporating the aqueous leachate, we assigned the water-soluble components by powder XRD and IR as NH4NO3, [Co(NH3)5Cl]Cl2 (compound 4) and a small amount of NH4Cl (Figure 10). The NH4Cl was probably formed as a hydrolysis product of compound 4.

It is well known that ammonium nitrate forms in the solid-phase quasi-intramolecular redox reaction of the ammonia complexes of divalent metal permanganates with the formation of solid Mn (III) type spinel compounds [1,2,3,4,5]:

2NH₃ + 2MnO₄⁻ = NH₄NO₃ + H₂O + {Mn₂O₄}²⁻

Neutralizing one charge of the trivalent cobalt in compound 3-Mn with a chloride ion results in a formal “divalent complex cation”, and an analog reaction of this “divalent cation” can explain the formation of one mol of water and NH₄NO₃. The residual solid phase can be characterized with a formal {CoClMn₂O₄+ 3NH₃} composition. The material, valence, and charge balances for chlorine, nitrogen, hydrogen, and oxygen, together with the amount of the evolved water, and the lack of ammonia and oxygen evolution, resulted in the following reaction in the first decomposition step:

3[Co(NH₃)₅Cl](MnO₄)₂ = [Co(NH₃)₅Cl]Cl₂ + 3H₂O + 3NH₄NO₃ + {Co₂(NH₃)₄Mn₆O₁₂}

Since no gaseous ammonia was evolved, the ammonia molecules which were not oxidized to nitrate or protonated to ammonium ions, or were not coordinated in compound 4, could be fixed in a coordinated form in the cobalt and manganese-containing residual oxide phase. Depending on the valences of the Co ions (2 × III, 2 × II or II + III), the charge of Co cations can neutralize {Mn₆O₁₂}ⁿ⁻ units with six, four, or five negative charges, thus, according to this, the average valence of manganese was 18/6 (6Mn^III), 20/6 (4Mn^III and 2Mn^IV), or 21/6 (3Mn^III and 3Mn^IV), respectively. Although there was no sign of ammonia oxidation with Co^III in this decomposition step (the lack of N₂ formation confirmed it [33]), the Co^II and Mn^IV centers in the oxide phase might have been formed due to the valence distribution between the Co^III and Mn^III. This is a hidden solid-phase quasi-intramolecular redox reaction without mass loss and the formation of other assignable redox products.

The typical coordination number of Co^III in its ammonia complexes is 6, and Co(II) forms stable tetraammine complexes. Thus, the stoichiometry of Co:NH₃ suggested that one cobalt might be expected to be divalent and complexed with four ammonia molecules. Furthermore, the composition and IR spectrum of the decomposition product (Figure 8 and Figure S11) and the possible charges of the {Mn₆O₁₂}ⁿ⁻ (n may be 4, 5 or 6) unit strongly suggest that [Co₂(NH₃)₄Mn₆O₁₂] belongs to todorokite or a similar type of manganese oxide family [34]. Todorokite is constructed of triple chains of MnO₆ octahedra forming large tunnels with square cross-sections [34], which can incorporate as large cations as [Co(NH₃)_n]^2+/3+ (n = 2, 4 or 6) and free Co^II/^III cations as well.

XPS could not directly determine the Co^II/^III and Mn^II/^III ratio due to the ammonia loss of [Co₂(NH₃)₄Mn₆O₁₂] in a high vacuum, which led to chemical changes in the studied sample. Therefore, future ⁵⁷Co Mössbauer studies are planned to investigate the [Co₂(NH₃)₄Mn₆O₁₂] phase in more detail.

The first thermal decomposition step of compound 3-Mn was exothermic in both an oxidative and an inert atmosphere. The reaction heats were found to be 475.85 and 479.95 kJ/mol by DSC (Figure S14a,b) in pure O₂ and N₂, respectively. Thus, the outer oxygen source did not play any role in the first step of the decomposition process.

In the second decomposition step, the mass loss was ~28.6%, and above 300 °C, a slight mass loss of 6.5% was observed. The residual mass at 500 °C was 57.7%, which was somewhat more than the mass percent of the CoMn₂O₄ spinel (55.77%) (Figure S10).

The TG-MS curves showed simultaneous N₂O and H₂O formation (m/z = 44 and 18, respectively), which is typical in the thermal decomposition of ammonium nitrate:

NH₄NO₃ = N₂O + 2H₂O

Based on the N₂O/NO (m/z = 44 and 30, respectively) peak intensity ratios, the formation of NO may be attributed not only to the fragmentation of N₂O [35] but to another NO source as well. The formation of N₂ and ammonia as gaseous products (m/z = 28 and m/z = 17 or m/z = 16, respectively) may be attributed to the decomposition of compound 4 [33], but the mass loss showed that the [Co₂(NH₃)₄Mn₆O₁₂] oxide phase also lost ammonia. The intensity ratio of m/z = 18 (H₂O⁺) and m/z = 17 (OH⁺ and NH₃⁺) in this decomposition step was <1, which confirmed the ammonia evolution. Therefore, the decomposition product of compound 4 was NH₃, NH₄Cl, and N₂ [33]:

6 [Co(NH₃)₅Cl]Cl₂ = 6CoCl₂ + 6NH₄Cl + N₂ + 22NH₃

This process was a solid-phase quasi-intramolecular redox reaction centered on Co^III and the ammonia ligand. Ammonium chloride (HCl) could not be detected directly due to its fast reaction with the capillary column organic film layer. The reaction heat (Figure S13) was found to be −102.3 and −121.5 kJ/mol in O₂ and N₂, respectively. This shows that the oxygen gas had an influence on the reaction heat. A similar effect of the outer oxygen was observed in the thermal decomposition of [Ag(NH₃)₂ClO₄] [36]. That was attributed to the forming of NO (its formation was endothermic) in a higher amount than in an inert atmosphere. The second decomposition step of compound 3-Mn showed that the formation of NO (m/z = 30), and the presence of Co-Mn oxides can help the oxidation of ammonia with the outer oxygen into NO.

In the second decomposition step of compound 3-Mn (~180 °C), the previously formed NH₄NO₃, [Co(NH₃)₅Cl]Cl₂ and {Co₂(NH₃)₄Mn₆O₁₂} phases decomposed in simultaneous processes. The thermal decomposition temperature of [Co(NH₃)₅Cl]Cl₂ (compound 4) with N₂ and NH₃ formation (175 °C) [34] agreed well with the peak temperature of the second decomposition step (181 °C). The decomposition reaction of compound 4 may initiate the decomposition of both ammonium nitrate (catalyzed by the oxide phases present [37,38,39]) and {Co₂(NH₃)₄Mn₆O₁₂} simultaneously.

The third decomposition step around 340 °C was the reaction of the cobalt(II) chloride and [Co₂(NH₃)₄Mn₆O₁₂] intermediates with the formation of cobalt manganese oxide (CoMn₂O₄), which contained formally one Co^II and two Mn^III ions. Two manganese (IV) ions or two cobalt (III) ions can oxidize two chloride ions into chlorine according to the reaction:

CoCl₂ + {Co₂Mn₆O₁₂} = 3CoMn₂O₄ + Cl₂

The mass loss of the third decomposition step was slightly higher than the mass losses calculated for 1/3 Cl₂ due to the release of the residual ammonia. Still, the overall mass loss agreed well with the CoMn₂O₄ formation. Some amount of chlorine (m/z = 35 Cl⁺) was also detected, in the argon without NO₂ formation whereas in the air it was together with NO₂ formation (Figure S11). The decomposition process is summarized in Scheme 1. As Scheme 1 shows, the only solid final decomposition product was CoMn₂O₄. Thus, the temperature-controlled decomposition of compound 3-Mn is an easy way to prepare nanosized cobalt manganite with Co:Mn = 1:2 stoichiometry.

The stoichiometry of the third decomposition step requested the presence of two Mn^IV or two Co^III ions (or one Mn^III and one Co^III), which could oxidize two chloride ions into chlorine. Consequently, 4 Mn^III + 2Mn^IV + 2Co^II, 5Mn^III + Mn^IV + Co^III + Co^II, or 6Mn^III + 2Co^III ions (the oxidizing constituents are italicized) may have neutralized the negative charges of the framework oxygens in the [Co₂(NH₃)₄Mn₆O₁₂]. Thus, the {Co₂(NH₃)₄Mn₆O₁₂} intermediate phase consisted of todorokite-like {Mn₄^IIIMn^IV₂O₁₂}_n⁴ⁿ⁻, {Mn₅^IIIMn^IVO₁₂}_n⁵ⁿ⁻ or {Mn^III₆O₁₂}_n⁶ⁿ⁻ frameworks, which embedded 2 × n (Co^II and/or Co^III) cations in their tunnels, respectively. Ammonia ligands (4 × n) were coordinated to cobalt ions. The two cobalt ions may have been intercalated in the tunnels as one Co^II,III + one Co^II,III (NH₃)₄²⁺ ion, or two [Co^II,III(NH₃)₂]^2/3+ ions. Co^III typically forms hexacoordinated, whereas Co^II forms tetracoordinated ammonia complexes, thus, the most probable formula contained a tetraamminecobalt (II) cation or ammonia-bridged Co (III) structural units, but the structure of this decomposition intermediate requires further detailed investigation.

The last decomposition reaction was influenced by oxygen, which means that oxygen defects (CoMn₂O_4+δ) can be expected if the decomposition is completed in the presence of oxygen. Since the oxygen defects may play a role in the desired catalytic activity of Co-Mn oxides, the CoMn₂O₄ samples prepared in both an inert atmosphere and air were tested as photocatalysts in the UV degradation of Congo red.

2.6. Surface Characterization of the Thermal Decomposition Products of Compound 3-Mn and Their Photocatalytic Activity in Congo Red Degradation

Twice were the three decomposition intermediates and the two final decomposition products prepared by isothermal heating of compound 3-Mn at 125, 150, 220 and 420 °C under N_2, and at 125, 150, 300 and 550 °C in air. The morphology of the end product (spinel-like) obtained at 420 °C (N₂) and at 550 °C (air) was similar and can be seen in Figure 11.

The specific surface area of each intermediate and the final product was measured (

Table 6. The specific surface area of intermediates formed at different temperatures under air and N₂.

T °C (N2)	125	150	220	420
SSA (m2·g−1)	130	118	176	32
T °C (air)	125	150	300	500
SSA (m2·g−1)	22	23	194	23

SSA: specific surface area.

).

The specific surface area of the decomposition intermediates formed under N₂ changed irregularly, and the lowest surface area (~32 m²·g⁻¹) was found after crystallization and the sintering of the amorphous primary reaction products at 420 °C. Under an oxidizing atmosphere, there was an expansion in the bulk phase up to 300 °C, and then, the surface area decreased again due to crystallization and sintering at 550 °C (23 m²·g⁻¹).

The first intermediate was obtained at 125 °C under both atmospheres and the end products were produced at 420 °C under nitrogen and at 550 °C in air. We checked their potential as a photocatalyst in the degradation of organic dyes. Our findings are summarized in Figure 12 and the apparent decomposition rate for each intermediate can be seen in

Table 7. Photocatalytic parameters in the decomposition of methyl orange and Congo red with intermediates forming in different conditions.

Substrate	pH	Kapp/10−4·min−1	R2
Congo Red, 2·10−5 M, without catalyst	5.7	1.0	0.99
Congo red, 2·10−5 M, with compound 3-Mn at 125 °C in N2	5.7	5.0	0.93
Congo red, 2·10−5 M, with compound 3-Mn at 125 °C in air	5.7	18.0	0.89
Congo red, 2·10−5 M, with compound 3-Mn at 420 °C in N2	5.7	13.0	0.98
Congo red, 2·10−5 M, with compound 3-Mn at 550 °C in air	5.7	9.0	0.96

.

The photocatalyst candidates formed under an inert atmosphere and air during the thermal decomposition of compound 3-Mn showed activity in accelerating the decomposition rate of Congo red. The decomposition of methyl orange in our experiments did not reach any significant level. The end product (CoMn₂O₄) forming under N₂ had higher photoactivity (13 times faster degradation rate compared to the uncatalyzed process) in Congo red degradation by UV light than the intermediate [Co₂(NH₃)₄Mn₆Mn₂O₁₂]. The intermediate that formed in air at 125 °C accelerated the photodegradation of Congo red 18 times, whereas the final decomposition product (550 °C–air) resulted in only a 9-fold acceleration.

The differences in the chemical character and photocatalytic activity of samples prepared in an aerial and inert atmosphere can be attributed to the differences between the distribution and chemical environment of the low valence (Co^II, Mn^II) and oxidized (Co^III, Mn^III,IV) centers in the studied materials.

3. Materials and Methods

Chemical-grade basic cobalt carbonate, sodium permanganate (40% aq. soln.), ammonia and hydrochloric acid and chemicals containing the isotope (D₂O, DCl in D₂O, ND₄Cl, and ND₃ in D₂O solution) were supplied by Deuton-X Ltd., Érd, Hungary.

Synthesis of compound 4 [14]

Basic cobalt carbonate—20 g of CoCO₃.nCo(OH)₂·H₂O—was dissolved in a 1:1 HCl solution. Then the solution was filtered and chilled. A solution of 250 mL of cc. ammonia and 50 g of (NH₄)₂CO₃ in 250 mL of distilled water was added. Then, the mixture was oxidized for three hours in a stream of air. Then, 150 g of NH₄Cl was added to the previous solution and the mixture was evaporated until a syrup consistency was obtained. To drive off the CO₂, a dilute (10%) HCl solution was added. Then, to ensure the basicity of the reaction mixture, 10 mL of cc. ammonia was also added. The mixture was heated until all the solutions became clear, then 300 mL of cc. HCl was added, and the mixture was heated for 1 h. At the end of the process, the crude [Co(NH₃)₅Cl]Cl₂ was precipitated by cooling. The precipitate was filtered off, washed with 1:1 HCl until it was free of NH₄Cl, and then with alcohol until it was free of acid. To purify the crude salt, we dissolved 10 g of powdered crude [Co(NH₃)₅Cl]Cl₂ in 75 mL of distilled water and 50 mL of aqueous NH₃ (10%) in an Erlenmeyer flask covered with a watch glass under heating and stirring. Then, the deep-red solution was filtered and acidified with oxalic acid to be weakly acidic, and some additional (NH₄)₂C₂O₄ was added to complete the precipitation. The slurry was allowed to stand and the precipitated, [Co(NH₃)₅(H₂O)]₂(C₂O₄)₃·4H₂O was filtered off and washed with cold water and dried at room temperature. To obtain the pure compound 4, 20 g of [Co(NH₃)₅(H₂O)]₂(C₂O₄)₃·4H₂O was dissolved in 150 mL of diluted (2%) aq. ammonia solution in an ice bath, and the insoluble luteo oxalate, [Co(NH₃)₆]₂(C₂O₄)₃·4H₂O, was filtered off. The filtrate was precipitated in an ice bath with the addition of a 10% aq. HCl solution. The [chloridopentaamminecobalt(III)] chloride (compound 4) was filtered off, washed successively with alcohol, absolute alcohol and ether, and dried in air.

Synthesis of compound [Co(NH₃)₅Cl](MnO₄)₂ (compound 3-Mn)

To prepare compound 3-Mn, we dissolved 1.0 g of [Co(NH₃)₅Cl]Cl₂ in 150 mL of distilled water and 5 mL of a NaMnO₄ solution was added. Then the mixture was heated to 60 °C and stirred for 15 min. Finally, the mixture was cooled to 1 ^°C. The precipitate that formed was filtered off and dried at room temperature in a desiccator containing CaO. The yield was 57%.

Synthesis of deuterated compounds 4 and 3-Mn

The deuterated compound 4 could be prepared in three ways: (1) using anhydrous CoCl₂, ND₄Cl, and ND_3, and D₂O according to the method described in [24]; (2) via deuteration of [Co(NH₃)₅Cl]Cl₂ dissolved in D₂O and DCl and evaporated to dryness at room temperature in the presence of freshly prepared CaO; and (3) dissolving compound 4 in D₂O and freeze-drying the solution to obtain [Co(ND₃)₅(D₂O)]Cl₃, which was converted into deuterated compound 4 by heating in a high vacuum at 80 °C, according to Sacconi’s method [23]. All three ways gave the deuterated compound 4, but the deuteration of compound 4 with D₂O required a large amount of D₂O because of the low solubility of compound 4 in H₂O (D₂O) (5.2 g/L at 25 °C [40]). The deuteration process was repeated 4 times, then the [Co(ND₃)₅Cl]Cl₂ was dissolved in a minimum quantity of D₂O and reacted with 2 equiv. of KMnO₄ dissolved in D₂O at 60 °C. Then the mixture was cooled, and the deuterated compound 3-Mn was isolated as described for the non-deuterated 3-Mn.

Elemental Analysis

The cobalt and manganese content of the sample was determined by atomic emission spectroscopy with a Spectro Genesis ICP-OES (SPECTRO Analytical Instruments GmbH, Kleve, Germany) spectrometer. A multielement standard solution (Merck Chemicals GmbH, Darmstadt, Germany) was used for calibration.

The ammonia content of the starting compound and the ammonia/ammonium-ion content of the decomposition intermediates were determined by gravimetry as (NH₄)₂PtCl₆ (ammonia was liberated by an alkaline treatment and boiling). Due to the redox by-reactions that appeared on heating of the compound 3-M which produced NO_x gases, the absorption of the evolved ammonia in mineral acid solutions and titrating back the excess acid could not be used due to HNO₂ and HNO₃ formation.

The evolved ammonia/oxidized ammonia ratio was measured by the isotherm heating of the starting complex at a given temperature with the absorption of ammonia in the H₂PtCl₆ solution. The NO_x and N₂ gases did not give a precipitate with H₂PtCl₆, whereas the ammonia formed insoluble orange-colored (NH₄)₂PtCl₆.

Vibrational Spectroscopy

The far-IR and FT-IR spectra of the crystalline compound 3-Mn and its deuterated form were recorded in ATR (attenuated total reflection) mode on a BioRad-Digilab FTS-30-FIR and a Bruker Alpha IR spectrometer for the 400–40 and 4000–400 cm⁻¹ range, respectively. Raman measurement (298 and 123 K) of compound 3-Mn and its deuterated form were performed on a Horiba Jobin-Yvon LabRAM microspectrometer with an external 785 nm diode laser source (~80 mW) coupled to an Olympus BX-40 optical microscope. In the low-temperature measurements, a Linkam THMS600 temperature-controlled microscope stage was used. The laser beam was focused on an objective of 20×. In order to avoid the thermal degradation of compound 3-Mn, we used a D0.6 (123 K) and a D2 (298 K) intensity filter to decrease the laser power to 25% and 1%, respectively. The confocal hole of 1000 µm and monochromators with 950 groove mm⁻¹ grating were used for light dispersion. The spectral range of 2000–100 cm⁻¹ was selected with a resolution of 4 cm⁻¹. The exposure times were 60 s to produce intensive peaks.

The UV-VIS DRS (UV-VIS diffuse reflectance spectrum) of compound 3-Mn was measured at room temperature with a Jasco V-670 UV–VIS spectrophotometer equipped with a NV-470 integrating sphere (BaSO₄ standard as reference).

Scanning Electron Microscopy

A JEOL JSM-5500LV scanning electron microscope was used in the experiments. The specimens were fixed on sample holders (Cu/Zn alloy with a carbon tape) and sputtered with a conductive layer (Au/Pd) for imaging.

Powder X-ray diffractometry

A Philips PW-1050 Bragg-Brentano parafocusing goniometer was used, equipped with a Cu cathode operated at 40 kV and 35 mA with a secondary beam graphite monochromator and a proportional counter. Scans were recorded in step mode. The diffraction patterns were evaluated with a full profile fitting technique.

Single-crystal X-ray diffraction

A dark violet needle of [Co(NH₃)₅Cl](MnO₄)₂ (compound 3-Mn) was mounted on a loop. Single crystal X-ray diffraction measurements were carried out on an Agilent Supernova diffractometer equipped with a dual-sealed X-ray tube source, a kappa goniometer and a position-sensitive detector at room temperature. An Mo source was used in a hemisphere of the reciprocal space up to the resolution of 0.65 Å. Data was collected with the CrysAlis1 software. The structure was solved and refined with the SHELX software package2 from Olex23. The atomic positions were determined by direct methods, while hydrogen atomic positions were calculated from assumed geometries. Hydrogen atoms were included in the structure factor calculations, but they were not refined. The isotropic displacement parameters of the hydrogen atoms were approximated from the U(eq) value of the atom they were bonded to.

Thermal studies

Thermal data were collected with a modified TGS-2 thermobalance (Perkin Elmer, USA) coupled to a HiQuad quadrupole mass spectrometer (Pfeiffer Vacuum, Germany). Approximately 1 mg samples were measured in a platinum sample pan. Decomposition was followed from ambient temperature to 500 °C at a 20 °C min⁻¹ heating rate in argon or air as the carrier gas (flow rate = 140 cm³ min^–1). Selected significant ions between m/z = 2–88 were monitored in the selected ion monitoring (SIM) mode.

The other set of thermal data were collected with a TA Instruments SDT Q600 thermal analyzer coupled to a Hiden Analytical HPR20/QIC mass spectrometer. Decomposition was followed from room temperature to 500 °C at a heating rate of 5 °C min⁻¹ in argon as the carrier gas (flow rate = 50 cm³ min⁻¹) and at a heating rate of 2 °C min⁻¹ in air as the carrier gas (flow rate = 50 cm³ min⁻¹). The sample holder and the reference were the same alumina crucible. A sample mass of ~1 mg was mixed with Al₂O₃ to avoid an explosive decomposition of the sample. Selected ions between m/z = 1−83 were monitored in multiple ion detection modes (MIDs).

The non-isothermal DSC curves between −130 and 300 °C were recorded with a Perkin Elmer DSC 7 apparatus. The sample masses were between 3 and 5 mg and the samples were tested at a heating rate of 5 °C/min under a continuous nitrogen flow (20 cm³ min⁻¹) in an unsealed aluminum pan.

4. Conclusions

• An unknown cobalt(III) complex with chlorido and ammonia ligands, [Co(NH₃)₅Cl](MnO₄)₂ (compound 3-Mn), was synthesized. This complex is a precursor to nanosized Co-manganite with Co:Mn = 1:2 stoichiometry and an average size of 16.8 nm.
• The vibrational modes in compound 3-Mn were studied in detail and the overlapping bands were assigned via deuteration and low-temperature Raman measurements.
• The structure of compound 3-Mn was revealed with single-crystal X-ray diffraction and the 3D-hydrogen bonds were evaluated.
• The hydrogen bonds between the N-H hydrogen atoms and the Mn-O oxygen atoms acted as redox centers to initiate a solid-phase quasi-intramolecular redox reaction even at 120 °C involving the Co^III centers and permanganate ion as the oxidants and ammonia as a reductant. An amorphous precursor of [Co(NH₃)₅Cl]Cl₂, NH₄NO₃, and a todorokite-like compound contains {Mn₄^IIIMn^IV₂O₁₂}_n⁴ⁿ⁻, {Mn₅^IIIMn^IVO₁₂}_n⁵ⁿ⁻ or {Mn^III₆O₁₂}_n⁶ⁿ⁻ frameworks with 2 × n (Co^II and/or Co^III) cations, and 4 × n ammonia ligand in their tunnels.
• The decomposition intermediates formed at ~120 °C and decomposed on heating via a series of redox reactions with the formation of such redox products as Co^IIMn^III₂O₄ spinel, N₂, N₂O and Cl₂. 6) The only solid was the nanosized (16. 8 nm) CoMn₂O₄, with a Co:Mn = 1:2 stoichiometry.
• The thermal decomposition reaction of compound 3-Mn consisted of a series of solid-phase quasi-intramolecular redox reactions consisting of various redox pairs such as permanganate ions and ammonia ligands, Co(III) and Mn(III), Co(III) and ammonia, or nitrate and ammonium ions.
• The prepared CoMn₂O₄ spinel had photocatalytic activity in Congo red degradation with UV light. The activity of CoMn₂O₄ depended on its synthesis conditions such as temperature or atmosphere. Congo red degradation was 9 and 13 times faster in the presence of CoMn₂O₄ prepared at 520 °C (in air) or 450 °C (under N₂), respectively.