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DOI: 10.1039/D4NJ03084B (Paper) New J. Chem., 2024, Advance Article

Double complex salts based on tetraamminplatinum(II) and vanadyl(IV) bisoxalate for the preparation of bimetallic Pt–V alloys†

Sofia N. Vorobyeva*a, Zakhar V. Rudzisab, Taisiya S. Sukhikha, Evgeny Yu. Filatova, Pavel E. Plusnina, Vladimir A. Nadolinnya, Artem S. Bogomyakovc and Sergey V. Koreneva
aNikolaev Institute of Inorganic Chemistry, Siberian Branch of Russian Academy of Sciences, 3, Acad. Lavrentiev Ave., Novosibirsk 630090, Russia. E-mail: vorobyeva@niic.nsc.ru
bNovosibirsk State University, 1, Pirogova str., Novosibirsk 630090, Russia
cInternational Tomography Center, SB RAS, Institutskaya str., 3a, Novosibirsk, Russian Federation

Received 9th July 2024 , Accepted 15th August 2024

First published on 16th August 2024

Abstract

An oxalate complex of vanadyl(IV) (Bu4N)2[VO(C2O4)2] (I) was synthesized and used as a vanadium precursor. The synthesis and characterization of the double complex salts [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) and {[Pt(NH3)4][VO(C2O4)2]}n (IV) were carried out. The complexes obtained were characterized by IR and diffuse reflectance spectroscopy, single-crystal and powder X-ray diffraction, and EPR. The magnetic properties of I, III, and IV were also studied. The process of thermal decomposition of the synthesized complexes in reducing and inert atmospheres was studied, and the composition of the released gases was determined. Thermal decomposition of the salts in a hydrogen atmosphere at 600 °C resulted in the formation of a bimetallic solid solution, Pt0.6V0.4.

Introduction

The sustained interest of researchers in platinum metals is due to the wide possibilities of their use both individually and in the form of alloys with other metals. Research groups from different countries are studying bimetallic alloys and developing functional materials based on them. The need to reduce the cost of the materials used (reduction in the percentage of platinum metals) while maintaining or improving their functional properties contributes to the continuous search for new systems and new precursor compounds. Alloys of platinum group metals with refractory metals of the fifth group (vanadium, niobium and tantalum) attract special attention from researchers. Such alloys are promising in the creation of new structural materials for the needs of the space industry, and high-performance membranes for hydrogen energy.1–3

Nanoparticles of platinum–vanadium and platinum–niobium alloys are considered as promising electrocatalysts for fuel cells.4–8 Researchers have also noted the possibility of using Pt–V alloys as a coating material for jewelry that is resistant to damage.9 In other studies, Pt–V alloys were used as a crucible material to study the activity of vanadium oxide in molten silicates.10,11

The main problem, to which the attention of researchers has been directed in recent years, is the difficulty of obtaining such materials. At present, binary alloys of platinum metals with metals of group 5 are obtained through high-temperature alloying,12 atomization of the salt of one metal on another in reactors in a reducing atmosphere at elevated temperature,5 and annealing of oxide and metal compounds in a reducing atmosphere.13,14

The disadvantages of these methods include: the need to use high temperatures and pressures, and the uncertainty in the composition of the obtained particles. One of the promising ways to solve the above problem is the thermal decomposition of precursors containing two metals in their composition (e.g., double complex salts (DCS)). This method allows one to vary many parameters: central atoms, ligand environment, atmosphere and thermal decomposition regime, which makes it possible to obtain a variety of nanoscale metal and metal-oxide products with a set of different physicochemical properties.15–21

We have chosen oxalate complexes of vanadyl(IV) and their double salts with tetraammonium complexes of platinum(IV) as an object of study. Despite the fact that oxalate complexes of vanadyl(IV) have been known for quite a long time,22–24 examples of structural studies are very limited. They are represented by the following complexes: (NH4)2[VO(H2O)(C2O4)2]·H2O,25 and [VO(C2O4)(H2O)3],26 for the trans-[VO(H2O)(C2O4)2]2− anion,27,28 the cis-[VO(H2O)(C2O4)2]2− anion,25,29–31 and the dimeric cis-[(VO)2(H2O)2(C2O4)3]2− anion,32–36 [(VO)2(C2O4)5]6−.37 A vanadyl polymer complex of the composition (H3O+)4{(VO(C2O4)2)2}n(H2O) has been described in the literature, in which the vanadium centers are connected by bidentate and tetradentate oxalate ions.38 Vanadyl compounds are widely studied nowadays, e.g. organic hybrid polyoxovanadate clusters,39,40 three-dimensional (3-D) vanadoborates,41,42 and vanadium borophosphates,43 finding applications in various areas.

In this work, we present the synthesis and structure of monomeric vanadyl(IV) oxalate (Bu4N)2[VO(C2O4)2] (I) and polymeric vanadium(V) dioxovanadium(V) oxalate {Bu4N[VO2(C2O4)]}n (II) and double complex salts of the compositions [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) and {[Pt(NH3)4][VO(C2O4)2]}n (IV). The compounds containing vanadium(IV) were characterized by IR spectroscopy, elemental analysis, PRXD, EPR, and solid-state UV-vis and the magnetic susceptibility was studied. The thermal properties of the compounds were also studied with gas phase analysis in inert and reducing atmospheres, and the corresponding Pt–V alloy was prepared.

Experimental section

General information

All solvents were of analytical grade. All reactants were purchased from commercial suppliers and used as received.

IR spectra were recorded on a Scimitar FTS 2000 FTIR spectrometer in KBr tablets in the range of 400–4000 cm−1. Diffuse reflectance spectra were obtained on a Shimadzu UV-vis-NIR spectrometer UV-3101PC (200–800 nm). The spectra are reported as the Kubelka–Munk function, F(R) = (1 − R)2/2R, where R is the diffuse reflectance of the sample. UV/vis spectra were obtained on a SF-102 spectrophotometer in quartz cuvettes l = 1 cm.

Powder XRD

Powder X-ray diffraction studies of complex compounds and thermolysis products were carried out in the 2θ range of 5°–135° using a DRON-RM4 diffractometer (CuKα-radiation, graphite monochromator on the diffracted beam, ambient temperature). Registration of diffractograms was carried out in stepwise mode. 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.44 Parameters of the metal phases were refined over the entire data array using the Powder Cell 2.4 application program.45 The crystallite sizes of the metal phases were determined using the Scherrer equation (WINFIT 1.2.1).46 The compositions of solid solution phases were estimated based on the additivity of atomic volumes of pure metals.

Thermal analysis

Simultaneous thermal analysis (STA) including simultaneous thermogravimetric determinations (TG), differential scanning calorimetry (DSC) and mass spectrometric analysis of the separated gas (AVG-MS) were carried out on a NETZSCH STA 449F1 Jupiter instrument combined with a QMS 403D Aëolos quadrupole mass spectrometer. Experiments were performed under an inert (He) atmosphere and reducing atmosphere (helium–hydrogen mixture 90/10 vol%), 30 mL min−1. Al2O3 crucibles were used, heating rate 10 deg min−1 in the temperature range of 30–600 °C. The experimental data were processed using the Proteus analysis47 software package.

Elemental analysis was carried out using a CHNS analyzer vario MICRO cube using a standard technique.

Magnetic measurements

EPR spectra were measured using a Varian E-109 spectrometer in the X-band in the temperature range of 77 to 300 K. 2,2-Diphenyl-1-picrylhydrazyl (marked hereafter as DPPH, g = 2.0036) and copper sulphate pentahydrate were used as standards.

The magnetic susceptibility of the polycrystalline samples was measured with a quantum design MPMSXL SQUID magnetometer in the temperature range of 2–300 K with a magnetic field of up to 5 kOe. None of complexes exhibited any field dependence of molar magnetic susceptibility at low temperatures. Diamagnetic corrections were made using the Pascal constants. The effective magnetic moment was calculated as μeff(T) = [(3k/NAμB2)χT]1/2 ≈ (8χT)1/2.

X-ray crystal structures

Single-crystal XRD data for compounds (Bu4N)2[VO(C2O4)2] (I), {Bu4N[VO2(C2O4)]}n (II), [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III), and {[Pt(NH3)4][VO(C2O4)2]}n (IV) were collected at 150 K with a Bruker D8 Venture diffractometer with a CMOS PHOTON III detector and IμS 3.0 microfocus source (MoKα radiation (λ = 0.71073 Å), collimating Montel mirrors). The crystal structures were solved using the ShelXT48 and were refined using ShelXL49 programs with Olex2 GUI.50 Atomic displacement parameters for non-hydrogen atoms were refined anisotropically. Hydrogen atoms were placed geometrically. Crystallographic data, parameters of experiments and refinements of structures are given in Table S1 (ESI†). The main bond lengths for complex ions are given in Table 1. The structures of (Bu4N)2[VO(C2O4)2] (I), {Bu4N[VO2(C2O4)]}n (II), [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III), and {[Pt(NH3)4][VO(C2O4)2]}n (IV) were deposited to the Cambridge Crystallographic Data Centre (CCDC) as a supplementary publication, No. 2364915–2364917, and 2364932.
Table 1 Bond lengths and distances in compounds I, III, and IV
Atoms Distance, Å Atoms Distance, Å
(Bu4N)2[VO(C2O4)2] (I)
V–O(1) 1.5860(19) V⋯V 9.17
V–O(11) 1.9808(13)    
V–O(12) 1.9674(12)    
[Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III)
V–O(1) 1.6080(16) V–O(2) 2.0395(15)
V–O(11) 1.9985(15) V⋯Pt 4.70
V–O(12) 1.9881(16) V⋯V 7.28
V–O(21) 2.0413(15) Pt⋯Pt 7.84
V–O(22) 2.1743(15)    
{[Pt(NH3)4][VO(C2O4)2]}n (IV)
V–O(1) 1.601(14) V–O(22) 2.000(14)
V–O(11) 2.015(13) V–O(23) 1.997(13)
V–O(12) 1.979(14) V⋯Pt 5.44
V–O(21) 2.210(14) V⋯V 4.93
    Pt⋯Pt 5.29

SEM

SEM images were obtained on a Hitachi SU1000 FlexSEM II, scanning electron microscope (Hitachi, Tokyo, Japan), with a resolution of 4 nm, and a voltage of 0.3–20 kV. The specimens were appropriately fixed on a specimen stub with conductive tape.

Synthesis of (Bu4N)2[VO(C2O4)2] (I)

A mixture of 0.107 g (0.59 mmol) of V2O5 in 10 mL of distilled water and 0.376 g (3.0 mmol) of H2C2O4·2H2O in 10 mL of water was stirred at 90 °C until the precipitate of V2O5 dissolved and a clear orange-colored solution was formed. Further heating caused the solution to change color to blue, after which 6.19 g of 10% aqueous solution of (C4H9)4NOH (2.4 mmol) was gradually added. The reaction mixture was stirred at 90 °C for 1 hour, cooled to room temperature and left to evaporate. After 7 days, large emerald green crystals (Bu4N)2[VO(C2O4)2] (I) were filtered off and washed with distilled water and ethanol. Yield: 0.399 g (46.4%). Anal. calc. for VC36H72N2O9: C, 59.4; H, 9.97; N, 3.85. Found: C, 59.5; H, 9.9; N, 3.9%. IR (KBr, cm−1): 2960 s (ν(CH3)), 2873 s (ν(sp2CH2, CH3)), 1730 s + 1710 s (νa(C[double bond, length as m-dash]O)), 1685 s (νa(C[double bond, length as m-dash]O)), 1486 m, 1465 m + 1355 s (νs(C[double bond, length as m-dash]O) + ν(C–C)), 1224 s (ν(C–N)), 1152 w, 1033 w, 1000 s (ν(V[double bond, length as m-dash]O)), 911 s (νs(C–O) + δ(OCO)), 886 m, 801 s (δ(OCO) + ν(V–O)), 742 m, 738 m, 556 s, 480 w, 425 s.

Synthesis of {Bu4N[VO2(C2O4)]}n (II)

To a mixture of 0.175 g (0.962 mmol) of V2O5 in 10 mL of distilled water and 0.243 g (1.93 mmol) oxalic acid in 10 mL of distilled water was added 10% Bu4NOH solution to pH ≈ 6. The mixture was heated until the orange precipitate of vanadium oxide dissolved, the solution color changed to blue and the light yellow residue formed. Single crystals of {Bu4N[VO2(C2O4)]}n (II) suitable for X-ray single crystal analysis were obtained by recrystallization from distilled water. Yield: 0.037 g (9.38%). Anal. calc. for VC16H36NO6: C, 52.29; H, 8.78; N, 3.39. Found: C, 51.8; H, 9.1; N, 3.4%. IR (KBr, cm−1): 2960 s (ν(CH3)), 2871 m (ν(sp2CH2, CH3)), 1700 s (νa(C[double bond, length as m-dash]O)), 1622 s, 1487 m, 1380 m (νs(C[double bond, length as m-dash]O) + ν(C–C)), 1305 m(ν(C–N)), 1166 w, 1148 w, 1106 w, 1064 w, 925 s (ν(V[double bond, length as m-dash]O)), 910 s (νs(C–O) + δ(OCO)), 803 m (δ(OCO) + ν(V–O)), 483 w, 446 w, 414 m.

Since the polymer phase contains vanadium(V), further investigation of its physicochemical properties was not carried out.

Synthesis of [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III)

A mixture of 0.194 g (0.266 mmol) of (Bu4N)2[VO(C2O4)2] (I) in 1 mL of distilled water (dissolved at 40 °C) and 0.103 g (0.266 mmol) of [Pt(NH3)4](NO3)2 in 1 mL of distilled water was put in a flask over ethanol to undergo crystallization for 5 days. For this purpose, the beaker with the solution was placed in a sealed bucket filled with ¼ ethanol so that the level of the solvent outside did not exceed the level of the solution inside the beaker. Blue crystals of the complex [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) were filtered off and washed with water and ethanol. Yield: 0.137 g (91.9%). Anal. calc. for PtVC4H18N4O12: C, 8.6; H, 3.2; N, 10.0. Found: C, 8.6; H, 3.2; N, 10.5%. IR (KBr, cm−1): 3450 s (ν(O–H)), 3312 s (νas(N–H)), 3200 sh (νs(N–H)), 1724 s (νas(C[double bond, length as m-dash]O)), 1673 s (νas(C[double bond, length as m-dash]O) + δd(N–H)), 1403 s + 1352 s (νs(C–O) + ν(C–C)), 1320 s (δs(NH3)), 1292 s + 1259 s (νs(C–O) + δ(OCO)), 966 s (ν(V[double bond, length as m-dash]O)), 874 m (ρr(NH3)), 800 s (δ(OCO) + ν(V–O)), 548 m (ν(V–O)), 480 s (ν(Pt–N)), 390 s (ν(V–O)), 353 s (δ(OCO)), 246 s.

Synthesis of {[Pt(NH3)4][VO(C2O4)2]}n (IV)

A mixture of 0.368 g (0.506 mmol) of (Bu4N)2[VO(C2O4)2] (I) in 2 mL of distilled water (dissolved at 100 °C) and 0.196 g (0.506 mmol) of [Pt(NH3)4](NO3)2 in 2 mL of distilled water was put in a flask over ethanol to undergo crystallization for 5 days. For this purpose, the beaker with the solution was placed in a sealed bucket filled with ethanol so that the level of the solvent outside did not exceed the level of the solution inside the beaker. As a result, the formation of a blue fine crystalline precipitate was observed. Single crystals of {[Pt(NH3)4][VO(C2O4)2]}n (IV) suitable for X-ray single crystal analysis were obtained by recrystallization from distilled water. Yield: 0.152 g (59.3%). Anal. calc. for PtVC4H12N4O9: C, 9.5; H, 2.4; N, 11.1. Found: C, 9.6; H, 2.9; N, 11.5%. IR (KBr, cm−1): 3308 s νas(N–H), 1719 s(νas(C[double bond, length as m-dash]O)), 1687 s (νas(C[double bond, length as m-dash]O)), 1626 s, 1594 s (δd(NH3)), 1385 s (νs(C–O) + ν(C–C)), 1315 s (δs(NH3)), 1283 s + 1246 s (νs(C–O) + δ(OCO)), 962 s (ν(V[double bond, length as m-dash]O)), 874 m (ρr(NH3)), 800 + 785 s (δ(OCO) + ν(V–O)), 530 m (ν(V–O) + ν(C–C)), 460 m (ν(Pt–N)), 420 s, 395 s (ν(V–O) +), 353 s (δ(OCO)), 279 m, 246 m, 226 m, 203 m, 150 m, 104 m, 87 m.

Polymeric complex IV can be obtained from complex III by dehydration over P2O5 at room temperature for 7 days or by heating in a dessicator at 105 °C for 1 hour.

Results and discussion

Synthetic aspects

The synthesis of the complexes is shown in Scheme 1. The identity and purity of the complexes were established by elemental analysis, IR spectroscopy, single crystal and powder X-ray diffraction (ESI,† Fig. S13–S22).
image file: d4nj03084b-s1.tif
Scheme 1 Synthesis of complexes I, II, III, and IV.

A green colored vanadyl(IV) oxalate complex was obtained by adding an aqueous solution of Bu4NOH to an aqueous vanadyl oxalate solution obtained by interaction of V2O5 with oxalic acid solution under heating. Crystals of (Bu4N)2[VO(C2O4)2] (I) were isolated by evaporation of the resulting solution at room temperature. A bright yellow-colored polymeric oxalate dioxovanadium complex {Bu4N[VO2(C2O4)]}n (II) was obtained as a by-product in the synthesis of compound I under oxalic acid deficient conditions. Blue crystals of the double complex salt [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) are formed in the metathesis reaction between solutions of [Pt(NH3)4](NO3)2 and I in ethanol in the concentration range from 0.035 M to 0.150 M. Interestingly, in the course of the reaction the vanadyl ion coordinates with an additional water molecule, thus changing its coordination number from 5 to 6. The compound without an inner-sphere water molecule, blue crystalline phase {[Pt(NH3)4][VO(C2O4)2]}n (IV), was isolated from the reaction between [Pt(NH3)4](NO3)2 and I in a water/ethanol mixture, followed by recrystallization of the product from boiling water.

Crystal structure of (Bu4N)2[VO(C2O4)2] (I)

For convenience when discussing the features of the structure of the vanadium(IV) coordination center, hereinafter we will use the numbering of atoms presented in Scheme 2.
image file: d4nj03084b-s2.tif
Scheme 2 Numbering of oxygen atoms in the vanadium(IV) coordination environment in structures I, III, and IV (n = 1–2).

Compound I crystallizes in the space group C2/c. According to single-crystal X-ray diffraction (SCXRD) analysis, the structure of the compound belongs to the island type and is formed by tetrabutylammonium cations and [VO(C2O4)2]2− anions of a tetragonal-pyramidal metal environment (Fig. 1a). The structure of tetrabutylammonium dioxalatovanadyl(IV) is described for the first time, despite the fact that the reports on oxalatovanadyl salts were published almost 60 years ago.22 A similar geometry of the vanadyl coordination polyhedron, a distorted tetragonal pyramid, is observed in the structures of known vanadyl tartrate salts, both monomeric and dimeric,51–54 bis-benzylato di 2-propanolates of vanadyl,55,56 and perfluoropinacolates of vanadyl.57 In all the structures, the V[double bond, length as m-dash]O bond lengths are in the range of 1.58–1.62 Å, the V–O(organic ligand) distances are 1.90–2.03 Å, and the distance from the vanadium center to the plane formed by the donor oxygen atoms of the coordinated ligands varies from 0.50 to 0.63 Å. The coordination polyhedron of I follows the geometry features of this series (Table 1).


image file: d4nj03084b-f1.tif
Fig. 1 Representation of the structural units in compound (Bu4N)2[VO(C2O4)2] (I) (a), the nearest environment of the [VO(C2O4)2]2− anion in structure I (b).

The presence of bulky tetrabutylammonium as a counterion, as well as peculiar crystal packing (Fig. 1b, anions are isolated from each other by Bu4N+ cations, the shortest V⋯V distance is as large as 9.17 Å) make compound I soluble not only in water, but also in a number of organic polar solvents: in alcohol and in acetone, which significantly expands the synthetic possibilities of using the obtained complex as a source of vanadyl oxalate.

The IR spectrum of I shows the stretching vibrations of V[double bond, length as m-dash]O bonds at 1000 cm−1 and intensive bands at 1730, 1710, 1685, 1465, 1355 and 911 cm−1, characteristic of the oxalate ligand.

Crystal structure of {Bu4N[VO2(C2O4)]}n (II)

{Bu4N[VO2(C2O4)]}n (II) is crystallizing in the space group P21/n. In its structure, bisoxalatodioxovanadium(V) fragments are linked into chains via oxygen atoms of tetradentate oxalate ions (Fig. 2a).
image file: d4nj03084b-f2.tif
Fig. 2 The chain structure of {VO2(C2O4)}n anions of the compound {Bu4N[VO2(C2O4)]}n (II).

The tetrabutylammonium cations are located between the {VO2(C2O4)}n anion chains (Fig. 2b). The vanadium coordination polyhedron is a distorted octahedron with cis-arranged oxo ligands. The V[double bond, length as m-dash]O distances are larger by ca. 0.4 Å compared to I due to a steric effect. The V–OC2O4 distances are significantly different, with two being 2.03 Å (cf.) and two being 2.27 Å due to the trans-effect of the oxo-ligands. The shortest distance between vanadium centers in the polymer anions V⋯V is much smaller than in compound I (5.60 Å).

The IR spectrum of II exhibits the intense bands at 1700, 1622, 1380 and 910 cm−1, characteristic of the oxalate ligand. The strong absorption at 925 cm−1 associated with stretching vibrations of V[double bond, length as m-dash]O bonds, which has the lowest wavenumber among the compounds due to the different oxidation state of the V.

The polymer complex is poorly soluble even in boiling water. Since the resulting compound was obtained as a by-product and contains vanadium in the +5 oxidation degree, it was not investigated further.

Crystal structure of [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III)

In the double complex salt [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) structure the O2− and H2O ligands occupy cis-positions, forming with two chelate oxalates the distorted octahedral coordination polyhedron of V. Structures containing similar fragments [VO(H2O)(C2O4)2]2− of both cis- and trans-type are known in the literature, viz. trans-[VO(H2O)(C2O4)2]2−,27,28 cis-[VO(H2O)(C2O4)2]2−,25,29–31 and dimeric cis-[(VO)2(H2O)2(C2O4)3]2− anions.32–36 In the structure of III, V–OC2O4 distances vary in a wide range from 1.99 to 2.17 Å; the largest distance is observed for the O atom in the trans-position to the O2− ligand, i.e. the variation of the distances is conditioned by the trans-effect. The structure of III contains two crystallographically independent [Pt(NH3)4]2+ cations in the special positions, and the Pt atom exhibits conventional square planar coordination environment (Fig. 3). The anions are arranged in the crystal closer to each other than in the parent I (Table 1); they form layers that alternate with layers of the cations [Pt(NH3)4]2+. Hydrate molecules in the structure participate in hydrogen bonds with the conventional intermolecular distances Ow⋯OC2O4, Ow⋯Ow, and Ow⋯NNH3 of 2.85, 2.72, and 2.97 Å, correspondingly. Crystals of the double complex salt III are moderately soluble in hot water. The IR spectrum of III shows the stretching vibrations of V[double bond, length as m-dash]O bonds at 966 cm−1 and intense bands at 1724, 1403, 1352, 1292 and 1259 cm−1, characteristic of the oxalate ligand. The absorption at 3450 cm−1 associated with the O–H stretching of H2O molecules. The vibrations of ammine ligands are observed at 3312, 3200, 1320 and 874 cm−1.
image file: d4nj03084b-f3.tif
Fig. 3 Representation of the structural units of the compound [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) (a), and packing of the structural units in III along the a axis (b).

Crystal structure of [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (IV)

The crystalline phase {[Pt(NH3)4][VO(C2O4)2]}n (IV) has a polymeric structure, in which bisoxalatovanadyl(IV) anions are connected to each other in infinite chains by means of tridentate-coordinated bridging oxalate ions (Fig. 4).
image file: d4nj03084b-f4.tif
Fig. 4 The chain structure of {VO(C2O4)22−}n anions of the compound {[Pt(NH3)4][VO(C2O4)2]}n (IV).

The vanadium coordination polyhedron is a distorted octahedron; the V–OC2O4 distances vary in the same way as in compound III: the O atom in the trans-position toward the O2− ligand shows the longest distance (2.22 Å) with respect to the others (2.00 Å). The shortest distance between vanadium centers in the polymer anions V⋯V is 4.93 Å. Flat-square complex cations [Pt(NH3)4]2+ alternate with chains of anions {VO(C2O4)22−}n. The polymer complex is moderately soluble in boiling water. The IR spectrum of IV exhibits the intense bands at 1719, 1385, 1283 and 1246 cm−1, characteristic of the oxalate ligand. The strong absorption at 962 cm−1 is associated with stretching vibrations of the V[double bond, length as m-dash]O bonds. Note that for the hexacoordinated V ion in compounds III and IV, the wavenumber is lower than that for pentacoordinated V in compound I. The vibrations of ammine ligands are observed at 3308, 1594, 1315 and 874 cm−1.

A vanadyl polymer complex of the composition (H3O+)4{(VO(C2O4)2)2}n(H2O) has been described in the literature, in which the vanadium centers are connected by bidentate and tetradentate oxalate ions.38

Absorption properties

The data of diffuse reflectance spectra in the solid state and electronic absorption spectra in water solutions are generally interpreted according to the literature data for vanadyl(IV) complexes.23,24 The diffuse reflectance spectra for the initial complex (Bu4N)2[VO(C2O4)2] (I) differ significantly from the spectra of [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) and {[Pt(NH3)4][VO(C2O4)2]}n (IV) (Fig. 5) due to the different geometry of the vanadium center – distorted tetragonal pyramid (C2v) for I and distorted octahedron for III and IV.
image file: d4nj03084b-f5.tif
Fig. 5 Diffuse reflectance spectra: 1 – (Bu4N)2[VO(C2O4)2] (I), 2 – [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III), and 3 –{[Pt(NH3)4][VO(C2O4)2]}n (IV).

More specifically, the spectrum of compound I shows four absorption bands in the range from 300 to 700 nm, related to the d–d transitions of vanadyl ion in the geometry of the distorted tetragonal pyramid:58 two well separated bands at 360 nm (dxy → dz2) and 430 nm (dxy → dx2−y2), and two poorly separated bands at 560 nm (dxy → dyz) and 640 nm (dxy → dxz).

The spectra of complexes III and IV are very similar and contain three broad absorption bands in the same wavelength ranges: intense broad absorption at less than 400 nm (dxy → dz2 + charge transfer band), at 585 nm (dxy → dx2−y2) and at 790 nm (dxy → dxz, dyz).

According to ref. 23, for a vanadyl ion in an octahedral environment ([VO(H2O)5]2+) in water solution, three absorption bands corresponding to electronic transitions should be observed: 760 nm for dxy → dxz, dyz; 625 nm for dxy → dx2−y2 and 350–400 nm for dxy → dz2, but the last band is often masked by more intense charge transfer or intraligand bands. All electronic absorption spectra of the solutions of compounds I, III, and IV are similar (Fig. 6) and in good agreement with literature data, and two absorption bands are observed: λ1 = 796 nm corresponding to the transition dxy → dxz, dyz, λ2 = 605 nm corresponding to the transition dxy → dx2−y2 and intense absorption at λ < 400 nm, apparently associated with charge transfer and dxy → dz2.


image file: d4nj03084b-f6.tif
Fig. 6 Electronic absorption spectra: 1 – (Bu4N)2[VO(C2O4)2] (I) water solution (c = 0.0020 M, ε792nm1 = 27 l mol−1 cm−1, ε604nm2 = 11 l mol−1 cm−1), 2 – [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) (c = 0.0407 M, ε797nm1 = 47 l mol−1 cm−1, ε605nm2 = 20 l mol−1 cm−1), and 3 – {[Pt(NH3)4][VO(C2O4)2]}n (IV) (concentrated solution).

The spectra in the solution and in the solid phase for compounds III and IV are close, indicating similar geometry of the vanadium center. Whereas for compound (Bu4N)2[VO(C2O4)2] (I) a fundamental change in the spectrum is observed - instead of four bands we observe two bands coinciding in position with the absorption bands in compounds [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) and {[Pt(NH3)4][VO(C2O4)2]}n (IV). This change in the spectrum can be explained by a change in the geometry of the vanadium center - instead of a square pyramidal geometry it becomes distorted-octahedral due to water coordination during dissolution of the compound.

Magnetic properties

The paramagnetism of the complex (Bu4N)2[VO(C2O4)2] (I) (Fig. 7) is due to the center with an electron spin S = 1/2, which corresponds to the d1 state of the vanadium ion. The complex structure of the EPR spectrum is due to the hyperfine structure (HFS) from a single vanadium isotope 51V with nuclear momentum I = 7/2 with a natural abundance of 99.75% and g-factor anisotropy. Modeling of the experimental EPR spectrum of complex I using the WinEPR Simfoniya program showed that the experimental spectrum corresponds well to the simulated spectrum and is described by the spin-Hamiltonian:
Ĥ = gxxβHxSx + gyyβHySy + gzzβHzSz + A(V)xxIxSx + A(V)yyIySy + A(V)zzIzSz
with parameters: S = 1/2, gxx = gyy = 1.981(5), gzz = 1.945 and HFS from vanadium atom A(V)xx = A(V)yy = 6.2 mT и A(V)zz = 17.98 mT.
image file: d4nj03084b-f7.tif
Fig. 7 EPR spectra of (Bu4N)2[VO(C2O4)2] (I): a – experimental, and b – simulated.

The data of X-ray diffraction analysis show that in the structure of complex I, the distance between vanadium ions is R = 8.62 Å and, in the absence of chemical bonds between them, a well-resolved anisotropy of the vanadium superfine structure is observed.

The parameters of the EPR spectra of the complex (Bu4N)2[VO(C2O4)2] (I) are typical of other vanadyls with a square pyramid structure.59 The parameters of the EPR spectrum of the vanadium complex (oxidovanadium(IV) complex with 4,4′-di-tert-butyl-2,2′-bipyridine ligand) obtained in ref. 59 have the following values: gxx = gyy = 1.978, gzz = 1.945, Axx = Ayy = 6.5 mT, and Azz = 17.86 mT, and they are very close to the values obtained for the vanadyl complex I.

Due to the different structure of compounds III and IV with respect to I, their EPR spectra differ strongly. For the complex [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) the EPR spectrum is a single line with g-factor 1.997 and half width ΔH1/2 = 14.4 mT (Fig. 8).


image file: d4nj03084b-f8.tif
Fig. 8 EPR spectra of [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III): a – experimental, and b – simulated. Fragment of the structure of complex compound III with an indirect exchange interaction channel via π–π stacking between fragments 1 and 2.

The absence of the superfine structure from vanadium(IV) ion in the spectrum of complex III can be explained by the presence of exchange interaction between vanadium ions in this structure. The analysis of the crystal packing reveals the interaction between the oxalate ligands. Indeed, the interanionic distance between the electron-rich O and electron-deficient C fragments of the ligands is 3.1–3.6 Å, which can provide a channel for indirect exchange interaction between vanadium ions, with the corresponding distance V⋯V of R = 7.24 Å.

For the polymer structure {[Pt(NH3)4][VO(C2O4)2]}n (IV), the EPR spectrum is also a single symmetric line (ESI,† Fig. S23). Modeling of the spectrum from polymer structure IV showed that the line width and g-factor are different from that of sample III (Fig. 8). The half-width of the spectrum line is ΔH1/2 = 22.7 mT and the value of the g-factor is g = 1.987.

As in the case of complex III, the absence of resolved HFS from vanadium is explained by the presence of exchange interaction between vanadium ions. For this polymer structure IV, as well as for the case of complex III there is an exchange interaction between vanadium ions through neighbouring oxalate ligands, but in contrast to complex III, the planes of the ligands are rotated relative to each other by 121° and the distances between electron-rich O and electron-deficient C fragments are larger (3.4–4.8 Å). On the other hand, the contribution to the indirect exchange interaction can also be realized through –V–O–C–O– bridges. In any case, such channels provide a weaker indirect interaction between vanadium ions than for the complex {[Pt(NH3)4][VO(C2O4)2]}n (IV), which leads to a larger width of the exchange line in the EPR spectrum.

In accordance with the theoretical work,60 the effects of polarization of electronic states in the presence of intermolecular/interionic dispersion interactions should be observed at distances between interacting fragments of less than 5 Å. Such interactions result in the formation of supramolecular aggregates.61 Together with ionic and covalent bonds, they can provide channels for indirect exchange interactions. A similar effect of the formation of an indirect exchange interaction channel between paramagnetic ions via π–π stacking was observed in ref. 62.

The temperature dependencies of the effective magnetic moment (μeff) and inverse magnetic susceptibility (1/χ) for complexes I, III, and IV are shown in Fig. 9. The μeff values at 300 K are close to a theoretical one of 1.72μB for one V(IV) ion with a d1 configuration with g ∼ 1.99. The μeff values are practically unchanged when cooling and magnetic susceptibilities obey the Curie–Weiss law for the complexes. The best fit parameter values are: C = 0.365 K cm3 mol−1 and θ = −0.03 K for complex I, C = 0.358 K cm3 mol−1 and θ = −0.09 K for complex III and C = 0.317 K cm3 mol−1 and θ = −0.10 K for complex IV. The values of the Weiss constant θ are indicative of the presence of very weak antiferromagnetic exchange interactions between spins of vanadyl ions even in the case of complex IV with the polymeric structure.


image file: d4nj03084b-f9.tif
Fig. 9 The μeff(T) (●) and 1/χ(T) (■) dependencies for complexes (Bu4N)2[VO(C2O4)2] I (a), [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O III (b), and {[Pt(NH3)4][VO(C2O4)2]}n (IV). Solid lines are theoretical curves.

Thermal decomposition

The thermal decomposition of the [Bu4N][VO(C2O4)2] (I) complex under inert (Fig. 10) and reducing (ESI,† Fig. S24) atmospheres proceeds similarly. Upon heating to 200 °C, an endo-effect without mass loss is observed in the DTA curve due to the melting of the initial complex. The decomposition proceeds in one step in the temperature range of 220–380 °C. The total mass loss is 91.5%. The main gaseous products both under an inert and reducing atmosphere are CO2 (m/z = 44, 28, 12) and CO (m/z = 28, 12) as well as decomposition products of Bu4N+ (m/z = 26, 27, 29, 41, 43, etc.).
image file: d4nj03084b-f10.tif
Fig. 10 STA and EGA_MS curves for [Bu4N][VO(C2O4)2] (I) under an inert atmosphere (He), 10 K min−1.

The thermal decomposition of the [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) complex under inert (ESI,† Fig. S25) and reducing (Fig. 11) atmospheres, proceeds similarly in the temperature range of 55–550 °C in several steps. The first one in the interval 55–135 °C represents the removal of three water molecules (outer- and inner-sphere) and is accompanied by an endo-effect. The mass loss in this step is 8.5% under the inert and reducing atmospheres. In the temperature range of 200–380 °C, there are three poorly separated steps accompanied by endo-effects, leading to almost complete decomposition of the anhydrous complex. The mass loss at this stage is 42.5 and 43.4% for the inert and reducing atmospheres, correspondingly. The main gaseous products are ammonia (m/z = 17, 15), water (m/z = 18, 17), and carbon dioxide (m/z = 44, 28, 12). The last stage of decomposition proceeds at a temperature of 380–540 °C and is accompanied by a loss of 2.6 and 2.3% of mass. In the mass-spectrum of gaseous products at this stage, the release of insignificant amounts of CO is registered. The main difference between decomposition processes in the inert and reducing atmosphere is the ratio of the gaseous products released. In the latter case, a larger amount of ammonia and CO is released.


image file: d4nj03084b-f11.tif
Fig. 11 STA and EGA_MS curves for [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) under the reducing atmosphere, 10 K min−1.

The thermal decomposition of the {[Pt(NH3)4][VO(C2O4)2]}n (IV) complex under inert and reducing atmospheres proceeds in a similar manner (ESI,† Fig. S26 and S27). In the temperature range 100–440 °C, several poorly separated mass loss steps accompanied by endo effects are observed. At the same time, under a reducing atmosphere, a noticeable mass loss is observed at a lower temperature. The main gaseous products are water, nitrogen, carbon dioxide, and carbon monoxide. The ratio of gaseous products in this case also depends on the atmosphere of thermolysis. Under an inert atmosphere, there is predominantly a release of CO2 and CO, and under a reducing atmosphere, there is a larger release of water and ammonia. The total mass loss in both cases is 50%.

XRD and solid solutions

The final products of thermolysis of [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) and {[Pt(NH3)4][VO(C2O4)2]}n (IV) were investigated by XRD. The diffraction patterns of the obtained samples (Fig. 12) clearly show the peaks of the face-centered cubic (fcc) lattice phase shifted relative to the position of the metallic platinum reflexes, which indicates the formation of the PtxV1−x solid solution. The diffraction pattern also shows low-intensity peaks (at background level) related to the V2O3 phase. At the same time, the mass content of vanadium oxide in the sample is less than 1%, which may indicate the oxidation of finely dispersed vanadium during removal from the reactor to an air atmosphere. The background rise in the region of 20° 2θ on the diffraction pattern of the IV thermolysis product is due to the contribution of the quartz cuvette due to the small amount of the sample. The average crystallite size of the obtained samples, determined from the XRD data, is 8 nm for III and 6 nm for IV. According to analysis of the SEM images (ESI,† Fig. S29), the typical size of the particles is hundreds of nanometers.
image file: d4nj03084b-f12.tif
Fig. 12 XRD patterns of the III and IV thermolysis products obtained under a hydrogen atmosphere at 700 °C (heating rate 10 K min−1) and diffraction patterns of platinum and vanadium(III) oxide from the PDF powder database.

Refinement of the unit cell parameters of the fcc phase of the PtxV1−x solid solution gives the values: a = 3.879(3) Å, and V/Z = 14.6(1) Å3. Assuming a linear dependence of the atomic volume per structural unit on the metal content in the solid solution (Retgers rule), the calculated value of V/Z corresponds to the composition Pt0.6V0.4. According to the data from the phase diagram of the Pt–V system,63 this phase is metastable.

Conclusions

Methods for the synthesis of four new complexes (Bu4N)2[VO(C2O4)2] (I), {Bu4N[VO2(C2O4)]}n (II), [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III), and {[Pt(NH3)4][VO(C2O4)2]}n (IV) were developed in this work. IR and diffuse reflectance spectroscopy, single-crystal and powder X-ray diffraction, and elemental analysis were used to characterize these compounds. Compound I reveals a hyperfine structure (HFS) from a single vanadium isotope 51V, while complexes III and IV exhibit the absence of a resolved HFS from vanadium, which is explained by the presence of interanionic exchange interactions between vanadium ions. The temperature dependencies of the effective magnetic moment (μeff) for complexes I, III and IV are mainly consistent with the presence of isolated V(IV) ions with a d1 configuration.

The thermal decomposition processes of the complexes under inert and reducing atmospheres were studied. It was found that the process of thermal destruction of DCS III and IV under inert and reducing atmospheres proceeds in a similar manner. According to the PXRD data, the final products of the thermal decomposition of [Pt(NH3)4][VO(H2O)(C2O4)2]·2H2O (III) and {[Pt(NH3)4][VO(C2O4)2]}n (IV) under reducing atmospheres at temperatures of 600 °C are bimetallic solid solutions of Pt0.6V0.4. Thus, we presented a convenient low-temperature synthetic method for obtaining the bimetallic alloy that has a high potential for use in catalysis and in the development of new structural materials with improved characteristics.

Author contributions

The manuscript was written through contributions from all authors. Conceptualization: S. N. V. and S. V. K.; methodology: S. N. V., Z. V. R., T. S. S., E. Yu. F., P. E. P., V. A. N. and S. V. K.; formal analysis: A. S. B. and E. Yu. F.; investigation: S. N. V., Z. V. R., T. S. S., E. Yu. F., P. E. P., V. A. N. and A. S. B.; writing – original draft preparation: S. N. V., T. S. S., E. Yu. F., V. A. N., A. S. B. and K. S. V.; writing – review and editing: S. N. V., T. S. S. and K. S. V.; visualization: S. N. V., E. Yu. F., P. E. P., V. A. N., A. S. B. and S. V. K.; supervision: S. N. V. and K. S. V.; project administration: S. N. V. and K. S. V. All authors have given approval to the final version of the manuscript.

Data availability

Crystallographic data for the structures reported in this article have been deposited at the Cambridge Crystallographic Data Centre, under deposition numbers CCDC No. 2364915 (I), 2364916 (II), 2364917 (III), and 2364932 (IV). All other relevant data generated and analyzed during this study, which include experimental, spectroscopic and crystallographic data, are included in this article and its supplementary information. Source data are provided with this paper.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This research was funded by the Russian Science Foundation, grant number 24-23-00260. The authors thank the Ministry of Science and Higher Education of the Russian Federation for their support. The authors thank the XRD Facility of NIIC SB RAS and personally thank A. S. Sukhikh and D. V. Kochelakov V. N. for the X-ray diffraction data collection. In addition, the authors thank Multi-Access Chemical Research Centre SB RAS at the N. N. Vorozhtsov Novosibirsk Institute of Organic Chemistry SB RAS for harvesting the SEM data.

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Footnote

† Electronic supplementary information (ESI) available: IR spectra, structural data, EPR and TG data. CCDC 2364915 (I), 2364916 (II), 2364917 (III) and 2364932 (IV). For ESI and crystallographic data in CIF or other electronic format see DOI: https://doi.org/10.1039/d4nj03084b
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