Crystal Structure and Thermal Properties of Double-Complex Salts [M1(NH3)6][M2(C2O4)3] (M1, M2 = Co, Rh) and K3[Rh(NH3)6][Rh(C2O4)3]2∙6H2O

Here, seven new double-complex salts, [M1(NH3)6][M2(C2O4)3] (M1, M2 = Co, Rh) and K3[Rh(NH3)6][Rh(C2O4)3]2∙6H2O types, are synthesised. The crystal structure and composition of DCS (double-complex salts) are studied by SCXRD, XRD, CHN and IR methods. The complex salts of the [M1(NH3)6][M2(C2O4)3] (M1, M2 = Co, Rh) type can be crystallised both as a crystalline hydrate [M1(NH3)6][M2(C2O4)3]·3H2O (sp. gr. P-3) and as an anhydrous complex (sp. gr. P-1) depending on the synthesis conditions. The process of [Rh(NH3)6][Rh(C2O4)3] formation is significantly dependent on the synthesis temperature. At room temperature, a mixture is formed comprising [Rh(NH3)6][Rh(C2O4)3] and K3[Rh(NH3)6][Rh(C2O4)3]2∙6H2O, while the [Rh(NH3)6][Rh(C2O4)3] target product crystallises at elevated temperatures. The thermal behaviour of double-complex salts is studied by the STA, EGA-MS, IR and XRD methods. The complete decomposition of complex salts in helium and hydrogen atmospheres resulting in metals or CoxRh1−x solid solutions is achieved at temperatures of 320–450 °C.


Introduction
The platinum group metals are widely known to exhibit high catalytic activity in various chemical reactions. Thus, rhodium has found wide application as a catalyst, for example, a heterogeneous rhodium catalyst exhibits high catalytic activity, as well as excellent resistance to coking in the reaction of dry methane reforming [1,2]. However, due to the high cost of rhodium, its use on an industrial scale is limited [3]. One of the ways to solve this problem is to add base metals (Ni, Co, Fe) to rhodium. The catalyst containing rhodium and a base metal has high catalytic activity and stability due to the formation of a synergistic effect between these metals [4][5][6]. A large number of works are devoted to the study of the activity of alloys or solid solutions of cobalt and rhodium of different compositions, which have good catalytic properties, especially in organic synthesis reactions. Thus, bimetallic particles of Co 0.5 Rh 0.5 , obtained from Co 2 Rh 2 (CO) 12 , have demonstrated high catalytic activity in the amino-carbonylation reaction of alkynes in the presence of CO and amines [7]. In [8], the catalytic activity of the Co x Rh 100−x @SILP system (SILP-supported ionic liquid phase) in the hydrogenation reaction of substituted aromatic compounds has been investigated. In particular, it is noted that when the rhodium content in the sample is 75 at.% or more, the catalyst has a typical activity for metallic Rh and is able to completely hydrogenate aromatic substrates. The increase of the cobalt content in the alloy  at.%) results in a decrease in the activity in the arene hydrogenation, accompanied by an increase in the activity in the C=O bond hydrogenation. Co 30 Rh 70 @SILP and Co 25 Rh 75 @SILP samples are highly active, selective and stable with respect to hydrogenation of benzylideneacetone  3 ] from an aqueous solution at room temperature, a precipitate was deposited, which, according to XRD, is a mixture of two phases ( Figure 1). The first phase was isostructural to [Ir(NH 3 ) 6 ][Ir(C 2 O 4 ) 3 ] with space group P-1 [14], but with a shift of reflexes towards smaller angles, while the composition of the second phase was unknown. To determine the composition of the second phase, the mother liquor was evaporated in the air, while two kinds of crystals were obtained from the solution: the first one, according to XRD data, was [Rh(NH 3  pink substance on the surface of the sample. Most likely, Co(III) was restored to Co(II), and CoC2O4 was formed.

The Features of [Rh(NH3)6][Rh(C2O4)3] Synthesis
When synthesising [Rh(NH3)6][Rh(C2O4)3] from an aqueous solution at room temperature, a precipitate was deposited, which, according to XRD, is a mixture of two phases ( Figure 1). The first phase was isostructural to [Ir(NH3)6][Ir(C2O4)3] with space group P-1 [14], but with a shift of reflexes towards smaller angles, while the composition of the second phase was unknown. To determine the composition of the second phase, the mother liquor was evaporated in the air, while two kinds of crystals were obtained from the solution: the first one, according to XRD

Crystal Structures of Complex Compounds
The structure is an island, and it consists of isolated [Rh(NH3)6] 3+ cations and [Co(C2O4)3] 3− anions, both located in the common positions. [Rh(NH3)6] 3+ complex ions are octahedral. The Rh-N bond lengths are in the range of 2.052(2) to 2.087(2) Å, while the average length of 2.067(7) Å corresponds to the same ions in other structures [15,16]. The N-Rh-N angles are in the range of 88.00(10) to 92.07(11)°, which corresponds to octahedral angles with a maximum deviation of 2.1°. Cations are located in the common positions.
The [Co(C2O4)3] 3− complex ions have an airscrew-like form. The Co-O bond lengths are in the range of 1.889(2) to 1.900(2) Å, with an average distance of 1.894(2) Å. The   [15,16]. The N-Rh-N angles are in the range of 88.00(10) to 92.07(11) • , which corresponds to octahedral angles with a maximum deviation of 2.1 • . Cations are located in the common positions.
The [Co(C 2 O 4 ) 3 ] 3− complex ions have an airscrew-like form. The Co-O bond lengths are in the range of 1.889(2) to 1.900(2) Å, with an average distance of 1.894(2) Å. The chelate O-Co-O angles are in the range of 86.21 (8) to 88.26 (9) • . These geometric parameters of the anion correspond to those of the same anions in other structures [12][13][14] formed in the inert atmosphere.
Each complex ion is surrounded by eight neighbouring counter-ion complex species. The Co-Rh distances between the adjacent complex ions range from 5.1143(4) to 7.6708(5) Å.
Each complex ion is surrounded by eight neighbouring counter-ion complex species. The Co-Rh distances between the adjacent complex ions range from 5.1143(4) to 7.6708(5) Å. [Rh(NH3)6] 3+ and [Co(C2O4)3] 3− are hydrogen bonded. This packing crystallises in the structural mode, as in CsCl ( Figure 2).  These structures are formed by the columns of two types, with alternating cationanion particles directed along the c axis ( Figure 3). The columns of the first type, oriented along the c axis (0, 0, z), are formed due to the formation of hydrogen bonds between the ligands of the [M 1 (NH 3 ) 6 ] 3+ and [M 2 (C 2 O 4 ) 3 ] 3− complexes. The second type of columns are formed along the c axis, (2/3, 1/3, z) and (1/3, 2/3, z). The distance between metal atoms along the c axis is within 4 to 6 Å. The cation and anion are linked by a hydrogen bond, directly between the ligand atoms and between the ligand and water atoms. These columns are interconnected by a system of hydrogen bonds.
These structures are formed by the columns of two types, with alternating cation-anion particles directed along the c axis ( Figure 3). The columns of the first type, oriented along the c axis (0, 0, z), are formed due to the formation of hydrogen bonds between the ligands of the [M 1 (NH3)6] 3+ and [M 2 (C2O4)3] 3− complexes. The second type of columns are formed along the c axis, (2/3, 1/3, z) and (1/3, 2/3, z). The distance between metal atoms along the c axis is within 4 to 6 Å. The cation and anion are linked by a hydrogen bond, directly between the ligand atoms and between the ligand and water atoms. These columns are interconnected by a system of hydrogen bonds.  Each complex ion is surrounded by eight neighbouring counter-ions. In this case, the neighbours located along the c axis have the smallest distances, of about 5 Å, and the remaining distances, M 1 -M 2 , have an average distance of 7.4 to 7.6 Å. These structures have a CsCl structural motif with trigonal distortion.
The structure is isolated, consisting of the separate [Rh(NH 3   These layers are stabilised by hydrogen bonds between the lig es, as well as between the ligands and water.

The Decomposition of [Rh(NH3)6][Rh(C2O4)3] in Inert and Red
The thermal decomposition of the [Rh(NH3)6][Rh(C2O4)3] com mosphere (Figure 5b) takes place in the temperature range of 245 poorly resolved stages, accompanied by endothermic effects. The m are CO2, CO, NH3 and nitrogen. The total mass loss is 62.0%, and product is 38%. The overestimation of the mass of the final prod theoretical value (35.98%) can be explained by the formation of ins amorphous carbon during thermolysis in an inert atmosphere. Ac final product of thermolysis at 600 °C is metallic rhodium (sp. gr. V/Z = 13.7 Å 3 , crystallite size 5-6 nm):   Figure 5b) takes place in the temperature range of 245-315 • C through two poorly resolved stages, accompanied by endothermic effects. The main gaseous products are CO 2 , CO, NH 3 and nitrogen. The total mass loss is 62.0%, and the mass of the final product is 38%. The overestimation of the mass of the final product compared to the theoretical value (35.98%) can be explained by the formation of insignificant amounts of amorphous carbon during thermolysis in an inert atmosphere. According to XRD, the final product of thermolysis at 600 • C is metallic rhodium (sp. gr. Fm-3m, a = 3.802 Å, V/Z = 13.7 Å 3 , crystallite size 5-6 nm): In the reducing atmosphere, the complete decomposition of the [Rh(NH 3 (Figure 5a) occurs at a lower temperature and proceeds in the temperature range of 220-290 • C. The process of thermolysis, similar to the inert atmosphere, occurs through two poorly resolved stages, accompanied by endothermic effects. According to mass spectrometry, the main gaseous products are CO 2 , CO, N 2 and H 2 O. A slight mass loss of~1% is observed above 290 • C. The total mass loss is 64.4%, and the mass of the final product is 35.6%, which is in good agreement with the theoretical content of rhodium in the initial DCS (35.98%). According to XRD, the final product of thermolysis at 350 • C is metallic rhodium (sp. gr. Fm-3m, a = 3.802 Å, V/Z = 13.7 Å 3 , crystallite size 7-11 nm): In the reducing atmosphere, the complete decomposition of the [Rh(NH3)6][Rh(C2O4)3] complex (Figure 5a) occurs at a lower temperature and proceeds in the temperature range of 220-290 °C. The process of thermolysis, similar to the inert atmosphere, occurs through two poorly resolved stages, accompanied by endothermic effects. According to mass spectrometry, the main gaseous products are CO2, CO, N2 and H2O. A slight mass loss of ~1% is observed above 290 °C. The total mass loss is 64.4%, and the mass of the final product is 35.6%, which is in good agreement with the theoretical content of rhodium in the initial DCS (35.98%). According to XRD, the final product of thermolysis at 350 °C is metallic rhodium (sp. gr.

The Decomposition of [Co(NH 3 ) 6 ][Rh(C 2 O 4 ) 3 ] in Inert and Reducing Atmospheres
The thermal decomposition of the [Co(NH 3 ) 6 ][Rh(C 2 O 4 ) 3 ] complex salt in inert and reducing atmospheres occurs in a similar manner and proceeds in three stages (Figure 6a,b).  The first stage occurs in the temperature range of 200-260 °C, accompanied by an endothermic effect. In the mass spectrum of gaseous products, a significant increase in ionic currents from m/z = 18, 17, 16 and 15 (H2O and NH3), and a slight increase in ionic currents from m/z = 44, 28 and 14 (CO2 and N2) are observed. According to XRD data, the diffraction pattern of the intermediate product obtained at a temperature of 260 °C contains reflexes related to the CoC2O4•nH2O phases. Based on the above data, it can be assumed that the first stage is the removal of three ammonia molecules with simultaneous reduction of Co(III) to Co(II), and the formation of the CoC2O4•NH3 phase and some other phases, with the gross formula of "Rh(NH3)x(C2O4)y". Note that the formation of the CoC2O4•NH3 phase was also observed by the authors of [17,18] in their works devoted to the thermal decomposition of [Co(NH3)6]2(C2O4)3•4H2O. The mass loss at this stage is ~9.5%, which is in good agreement with the calculated value (9.6%). The scheme of this stage is as follows: with the gross formula of "Rh(NH 3 ) x (C 2 O 4 ) y ". Note that the formation of the CoC 2 O 4 ·NH 3 phase was also observed by the authors of [17,18] in their works devoted to the thermal decomposition of [Co(NH 3 ) 6 ] 2 (C 2 O 4 ) 3 ·4H 2 O. The mass loss at this stage is~9.5%, which is in good agreement with the calculated value (9.6%). The scheme of this stage is as follows: At the second stage of decomposition in the temperature range of 260-310 • C, further decomposition of the intermediate product formed at the first stage occurs. According to mass spectrometry, the main gaseous products are NH 3 , N 2 , CO, CO 2    The third stage of decomposition in reducing and inert atmospheres occurs in the temperature ranges of 310-375 °С and 310-460 °С, respectively. According to mass spectrometry, the main gaseous products are CO and CO2. The mass of the final product is 30.6%, which is consistent with the theoretical content of metals (30.65%) in the initial DCS. The equation of the process reaction is: Rh + CoC2O4 → 2Co0.5Rh0.5 + 2CO2.
The final decomposition product obtained in the hydrogen atmosphere at 500 °С, according to XRD data, is a mixture of 46.6% face-centred cubic (FCC) phase (sp. gr.  The third stage of decomposition in reducing and inert atmospheres occurs in the temperature ranges of 310-375 • C and 310-460 • C, respectively. According to mass spectrometry, the main gaseous products are CO and CO 2 . The mass of the final product is 30.6%, which is consistent with the theoretical content of metals (30.65%) in the initial DCS. The equation of the process reaction is: Rh + CoC 2 O 4 → 2Co 0.5 Rh 0.5 + 2CO 2 .

The Decomposition of [Rh(NH 3 ) 6 ][Co(C 2 O 4 ) 3 ] in Inert and Reducing Atmospheres
The decomposition of the [Rh(NH 3 ) 6 ][Co(C 2 O 4 ) 3 ] complex salt in inert and reducing atmospheres is almost identical (Figure 8a,b). The main difference is the lower temperature of the end of the thermolysis process in the reducing atmosphere (340 • C) compared to the inert (380 • C), as well as the release of greater amounts of ammonia at the third stage of decomposition. Let us consider in detail the decomposition of the complex in a reducing atmosphere.   The decomposition of the complex proceeds in three stages. The first stage, in the temperature range of 100-150 • C, is accompanied by a pronounced exothermic effect and leads to a mass loss of~5.5%. The main gaseous product is CO 2 . According to XRD, the intermediate product obtained at a temperature of 200 • C is a mixture of cobalt oxalate and the amorphous phase (Figure 9a) [Rh(NH3)6][Co(C2O4)3] ((a) in a hydrogen atmosphere, (b) in a helium atmosphere).
The decomposition of the complex proceeds in three stages. The first stage, in the temperature range of 100-150 °C, is accompanied by a pronounced exothermic effect and leads to a mass loss of ~5.5%. The main gaseous product is CO2. According to XRD, the intermediate product obtained at a temperature of 200 °C is a mixture of cobalt oxalate and the amorphous phase (Figure 9a). In this case, the IR spectrum of the intermediate product obtained at this temperature features the bands related to the vibrations of the [Rh(NH3)6] 3+ cation, as well as CoC2O4-related vibrations. Therefore, cobalt(II)-oxalate and the intermediate "   (Figure 9b). Therefore, a decomposition of "[Rh(NH 3 ) 6 ] 2 (C 2 O 4 ) 3 " to metallic rhodium can be assumed at the beginning of the stage. With subsequent heating, the decomposition of CoC 2 O 4 to cobalt occurs, and its introduction into the rhodium lattice takes place with the formation of Co x Rh 1−x solid solution. This assumption is confirmed by XRD data, since at 325 • C the reflexes corresponding to the CoC 2 O 4 phase and the Co 0.15 Rh 0.85 solid solution (sp. gr. Fm-3m, a = 3.776 Å, V/Z = 13.5 Å 3 , crystallite size 4-5 nm) containing a greater amount of rhodium are observed in the diffraction patterns. Then, it can be assumed that: The final decomposition product in the helium atmosphere at 900 • C is a mixture of FCC phase (sp. gr. Fm-3m, a = 3.693 Å, V/Z = 12.6 Å 3 , Co 0.5 Rh 0.5 , crystallite size 17-24 nm) and a small amount of HCP phase. The final decomposition product in the hydrogen atmosphere at 800 • C is a mixture of 14.7% HCP phase (sp. gr. P6 3 /mmc, a = 2.631, c = 4.209 Å, V/Z = 12.6 Å 3 , Co 0.5 Rh 0.5 , crystallite size 4-5 nm) and 85.3% FCC phase (sp. gr. Fm-3m, a = 3.692 Å, V/Z = 12.6 Å 3 , Co 0.5 Rh 0.5 , crystallite size 17-24 nm).  (Figure 10a,b).
The first stage of decomposition-the removal of crystallisation water moleculesoccurs in the temperature range of 90-180 • C and proceeds in several stages: 4.209 Å, V/Z = 12.6 Å 3 , Co0.5Rh0.5, crystallite size 4-5 nm) and 85.3% FCC phase (sp. gr. Fm-3m, a = 3.692 Å, V/Z = 12.6 Å 3 , Co0.5Rh0.5, crystallite size 17-24 nm).  The mass loss at this stage is~9.0%, which is in agreement with the calculated value (9.3%). At the next stage, in the temperature range of 200-280 • C, the decomposition of the anhydrous complex occurs with the formation of metallic rhodium and K 2 C 2 O 4 . The decomposition process takes place in the same temperature range as the DCS [Rh(NH 3  Note that the decomposition of the complex occurs in the same temperature range as that for [Rh(NH 3 ) 6 ][Rh(C 2 O 4 ) 3 ], with the release of the same gaseous products. However, due to the presence of potassium in the initial DCS, the final product, in addition to metallic rhodium, contains K 2 C 2 O 4 . At a temperature of 382 • C, an endothermic effect is observed on the DTA curve caused by the K 2 C 2 O 4 melting (according to the literature, the melting temperature of K 2 C 2 O 4 is 397 • C [19]). At the last decomposition stage at 500-570 • C, the K 2 C 2 O 4 decomposition occurs to K 2 CO 3 and CO (according to literature, the decomposition temperature of K 2 C 2 O 4 is over 500 • C [19]). The final product obtained at a temperature of 600 • C, according to XRD, is a mixture of metal Rh and K 2 CO 3 .
The thermal decomposition of the synthesised complex salts in helium and hydrogen atmospheres is quite similar, with the exception that in a reducing atmosphere, the temperatures of the beginning and end of the processes are slightly less. This is because hydrogen is also involved in the reduction of metals. After the dehydration process, the decomposition of [Rh(NH 3

The Synthesis of Compounds
The initial compounds, [Co(NH 3 ) 6 [15,[20][21][22]. The identification and single-phase composition of the initial compounds were confirmed by the SCXRD using the PDF2 database or the known monocrystalline data. Weighed amounts of [Rh(NH 3 ) 6 ]Cl 3 (0.0453 g, 0.1461 mmol) and K 3 [Rh(C 2 O 4 ) 3 ] (0.0704 g, 0.1455 mmol), in a molar ratio of 1:1, were dissolved in a minimum amount of water (3 mL and 5 mL, respectively). The resulting solutions were mixed, and immediately a pale-orange precipitate was obtained. The solution with the precipitate was boiled for 30 min, while the colour of the precipitate passed from a pale orange to a saturated orange colour. The precipitate solution was cooled to room temperature, filtered on a glass filter, washed with a small amount of water and dried in the air.

Characterisation
X-ray diffraction analysis of the samples was performed on a DRON-RM4 diffractometer (Cu-Kα radiation, a graphite monochromator using a reflected beam and a scintillation detector with amplitude discrimination, Brevetting, Saint Petersburg, Russia). The samples were prepared by applying a suspension in alcohol on the polished side of a fused quartz cuvette. A polycrystalline silicon sample (a = 5.4309 Å) prepared in the same way was used as an external reference sample. The diffraction patterns were recorded in a step-by-step mode in a 2θ angles range of 5 • -120 • .
X-ray phase analysis (XRD) of the thermolysis products was carried out in accordance with the data provided in the PDF file for pure substances [23]. Parameters of the metal phases were refined over the entire data array using the Powder Cell 2.4 application program [24]. The size of the crystallites in the resulting metal powders was estimated from the coherent scattering regions as a result of Fourier analysis of the profiles of single diffraction peaks using the WINFIT 1.2.1 software [25].
The composition of the obtained bimetallic solid solutions was determined using calibration curves of the volume per atom ratio (V/Z, where V is the volume of the unit cell, and Z is the number of structural units in it-atoms, in this case) depending on the concentration of one of the metals. The calibration curves were plotted from the experimental values of atomic volumes for single-phase solid solutions of known composition, provided in the references for Co-Rh systems [26][27][28][29].
The single-crystal X-ray diffraction data for 2277194 and 2277196 were collected on a X8APEX Bruker Nonius diffractometer, equipped with a 4K CCD area detector, and for 2277197, 2277195 and 2277193 on a Bruker D8 Venture diffractometer with a CMOS PHOTON III detector and an IµS 3.0 source (Montel mirror optics), using the graphite monochromatized MoKα radiation (λ= 0.71073 Å) at 150(2) K ( Table 2). The θ-and ωscan techniques were employed to measure the intensities. Absorption corrections were empirically applied using the SADABS program [30]. Structures were solved by the direct methods of the difference Fourier synthesis and further refined by the full-matrix least squares method using the SHELXTL package [31]. Atomic thermal parameters for nonhydrogen atoms were anisotropically refined. The positions of hydrogen atoms of amino groups were calculated corresponding to their geometrical conditions and refined using the riding model, while the water molecule's hydrogen atoms were refined in the isotropic approximation, with the distance constraint of about 0.96 Å, or unlocalised. For IR and elemental analysis, IR spectra were recorded on a Bruker Vertex 80V FTIR spectrometer in the range of 4000-400 cm −1 from pellets pressed with KBr. The attribution of IR spectral bands was conducted by comparison with the literature data [16].
Elemental (CHN) analysis was carried out with a Euro EA 3000 analyser. To analyse the metal content in the salts, weighted samples of the DCS (~50 mg) were placed in a tubular quartz reactor. Heating was performed in a split furnace at the rate of 10 K/min in a hydrogen atmosphere. After reaching the final temperature, samples were kept for 1 h, and then the hydrogen stream was switched off and the system was purged with helium for 0.5 h. Afterwards, the furnace was removed, and the reactor was allowed to cool to ambient temperature in a continuous helium stream. The product was then weighed.
For thermal analysis, the simultaneous TG-DTA/EGA-MS measurement was performed using an STA 449 F1 Jupiter thermal analyser connected to an QMS 403D Aëolos quadrupole mass spectrometer (NETZSCH, Selb, Germany). The spectrometer was connected online to a thermal analyser (STA) by a quartz capillary heated to 280 • C. The QMS was operated with an electron impact ioniser with the energy of 70 eV. The ion currents of the selected mass/charge (m/z) numbers were monitored in multiple-ion detection (MID) mode with the collection time of 0.1 s for each channel. The measurements were carried out in a helium-hydrogen mixture (10.0 vol.% H 2 ) or helium atmosphere in the temperature range of 30-500 • C, using the heating rate of 10 • C min −1 , the gas flow rate of 30 mL min −1 and opened Al 2 O 3 crucibles.
The processing of the experimental results was performed using the conventional Proteus Analysis software v.6.1.0 [32].

Conclusions
This paper presented the methods of synthesis of [M 1 (NH 3 ) 6 ][M 2 (C 2 O 4 ) 3 ] (M 1 , M 2 = Co, Rh) double-complex salts and the studies of their crystalline structure. It was shown that varying the synthesis temperature allowed to obtain isostructural compounds in the "cobalt-rhodium" rows, which in turn led to the possibility of the synthesis of continuous series of solid solutions based on double-complex salts in the [Rh(NH 3  O structure, which was not formed during boiling. Thermal properties in inert and reducing atmospheres were studied in detail, and a staged thermolysis mechanism was proposed. It was shown that complete decomposition of complex salts to metals or solid solutions was achieved at temperatures of 320-450 • C.