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RHODIUM β-DIKETONATES

Rhodium acetylacetonate, Rh(acac)3

Rhodium acetylacetonate (rhodium 2,4-pentanedionate) Rh(acac)3 has vapor pressure ~0.5 kPa (~5mbar) at 498 K (225°C).

Rh(acac)3, is one of the rhodium complexes applied for the preparation of oxidation resistant Rh metal coatings. However, deposited Rh films reached of only some tens of nm thickness.

Rhodium trifluoroacetylacetonate, Rh(tfac)3

Rhodium trifluoroacetylacetonate, Rh(tfac)3, is other rhodium β-diketonate complex which may produce pure and adherent rhodium films in the presence of H2. However, the complexes containing the thfac ligand are thermally less stable than acac-containing complexes.

Rhodium tris-(2,2,6,6-tetramethyl-3,5-heptanedionate (rhodium (III) dipivaloylmethanate) Rh(thd)3

Synthesis of Rh(thd)3

Fig. Synthesis of Rh(thd)3

Rhodium tris-(2,2,6,6-tetramethyl- 3,5-heptanedionate (rhodium (III) dipivaloylmethanate) Rh(thd)3 was synthesized by the reaction of solution of Rh(NO3)3 in MeOH /H2O (1:1 vol.) with solution of thdH (20% excess) in MeOH, then the saturated water solution of NaHCO3 was added to the reaction mixture until the gas evolution (CO2) was not observed anymore (NaHCO3 is needed to increase the pH and move the equilibrium towards formation of Rh(thd)3 chelate). After stirring overnight at RT, the reaction mixture was extracted by n-heptane (x3 times), the solvent was distilled off and the residuals were recrystallized from toluene. (yield ~40%). [i]

[i] André Wolf, PhD Thesis, Universität Siegen, 1999, « Bestimmung der Löslichkeit von Metallchelaten in überkritischem Kohlendioxid », http://dokumentix.ub.uni-siegen.de/opus/volltexte/2006/170/pdf/wolf_andre.pdf

Characterisation of Rh(thd)3 – mass spectrometry, powder XRD

Table. Interpretation of FAB mass-spectrum for Rh[thd]3

Rh(thd)3 was characterized by mass spectrometry using fast atom borbardement (FAB) ionisation technique, in which the chelate Rh(thd)3 dissolved in 3-nitrobenzylalcohol was brought onto the the metal surface and bombarded with Xenon ions in mass-spectrometer, which were accelerated by the 6 kV potential difference. In this relatively mild technique the molecular fragments as well as low volatile and thermolabile molecules ionised. (f.e Rh(thd)3 molecular peak can be detected). In the mass-spectrum of Rh(thd)3 the molecular peak Rh(thd)3+ was found, as well as the peaks of molecular ions which underwent further thd ligand loss: Rh(thd)2+ and Rh(thd)+, and the peaks of protonised molecular ions: Rh(thd)3-H+, Rh(thd)2-H+ and Rh(thd) -H+.(relative intensities presented in the Table).[i]

Rh(thd)3 was analysed rentgenographically (powder XRD), but was found to be completely amorphous.

Solubility of Rh(thd)3 in supercritical CO2

Fig. Solubility of Rh(thd)3 in the supercritical CO2

The solubility of Rh(thd)3 in supercritical CO2 at different temperatures and pressures was investigated. The isotherms of solubility of Rh(thd)3 (at constant temperatures 60, 70, 80°C) demonstrate increase of solubility with increasing pressure (and related increase of CO2 density from r = 1,1 bis 1,96). The highest measured solubility 3,8 ×10-4 l/mol was obtained at 70 °C und reduced density 1,832. The Fig. shows, that at isotherm at 50°C has significantly smaller increase vs. pressure, compared to isotherms at 60 und 70 °C which show much higher increase of Rh(thd)3 solubility in CO2 with pressure. The isobars of Rh(thd)3 solubility at 150 bar pressure show decrease if solubility with increase of temperature, whereas at 200bar the increase of solubility vs. temperature is observed.[i]

Rh(thd)3 for Rh/ZrO2 films by laser CVD

Rh(thd)3 (combined with Zr(thd)4) was used as precursor for the preparation of rhodium-nanoparticle-dispersed zirconia (Rh/ZrO2) films laser chemical vapor deposition. The changes of microstructure of Rh/ZrO2 films vs. deposition conditions and heat treatment were studied. Rh nanoparticles precipitated from Rh(thd)3 on the surface of a (020)-oriented ZrO2 matrix having a feather-like structure at 1180 K (907°C) deposition temperature and Rh to Zr source molar ratio RRh/Zr= 0.05. As RRh(thd)3/Zr(thd)4 molar ratio increased from 0.01 to 0.10, the diameter of the Rh nanoparticles increased from 25 to 70 nm. The heat treatment at 873 K (600°C) in air for 10 h did not change the microstructure of the Rh nanoparticles and the feather-like structure of the ZrO2 matrix.[i]

[i] A. Honda, T. Kimura, A. Ito, T. Goto, Surf. Coat. Technology, 2012, Vol. 206, Iss. 11–12, p.3006–3010, « Rh-nanoparticle-dispersed ZrO2 films prepared by laser chemical vapor deposition », http://www.sciencedirect.com/science/article/pii/S0257897211012576

Rh(tfac)3 for Rh metal by MOCVD

Rh(tfac)3 was applied as precursor for the deposition of thin film of metal Rh by H2 reduction of precursor in the gas phase (thermal MOCVD). The Rh metal film deposition process could be conducted at atmospheric pressure and at temperatures as low as 250°C. Metal chelates like β-diketonates (like Rh(tfac)3) are well suited for metal plating from gas phase process due to their high volatility and ease of reduction by H2, moreover, the β-diketonate chelating agent ligand can be potentially recycled, as it is being regenerated by the reduction reaction. [i]

[i] R.L. Van Hemert, L.B. Spendlove, R. E. Sievers, J. Electrochem. Soc. 1965 vol. 112, iss. 11, 1123-1126, « Vapor Deposition of Metals by Hydrogen Reduction of Metal Chelates », doi: 10.1149/1.2423376, http://jes.ecsdl.org/content/112/11/1123.short

Rhodium tris-(hexafluoracetylacetonate) Rh(hfac)3

Rhodium hexafluoroacetylacetylacetonate Rh(hfac)3 is solid with melting point 114°C. It has been characterised by infrared spectrometry.[i]

[i] M.L. Morris, R.W. Moshier, R.E. Sievers - Inorganic Chemistry, 1963 – « Infrared Spectra of Metal Chelate Compounds of Hexafluoroacetylacetone »

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